摘要:

Russian Chemical Reviews 69 (1) 1 ± 34 (2000) Chemical principles of preparation of metal-oxide superconductors Yu D Tretyakov, E A Goodilin Contents I. Introduction II. Characteristic features of high-temperature superconductors III. Phase diagrams of high-temperature superconductors IV. Development of advanced methods for the synthesis of high-temperature superconducting materials V. Practical applications of high-temperature superconducting materials Abstract. The review deals with chemical aspects of the problem of preparation of a novel class of advanced materials, viz., high- temperature superconducting cuprates of complex chemical composition and structure. Investigations into these types of superconductors not only made it possible to reveal the `compo- sition ± treatment ± structure ± property' correlations, but also contributed significantly to the development of concepts of chemical interactions in complex oxide systems, high-temper- ature phase transformations, the nature of peritectic reactions, properties of cuprate melts and structural phase transitions.Particular attention is paid to the analysis of different experimen- tal results obtained in the studies of features of physicochemical processes occurring in complex metal-oxide systems. The bibliog- raphy includes 292 references. I. Introduction Complex cuprates with a high transition temperature to the superconducting state were discovered more than 12 years ago and called high-temperature superconductors (HTSC).1±5 The discovery of HTSC has initiated fundamental investigations into their crystal structure and physicochemical properties as well as the search for possibilities of practical use of these phases.6±24 From the chemical viewpoint, the history of superconductivity is a chain of discoveries of materials with more and more complicated structures and can be considered as specific `chem- ical evolution' of these materials from simple to complex ones (Fig. 1).This dates back to 1911 when the Dutch physicist H Kamerlingh-Onnes first obtained liquid helium. This break- through has made it possible to systematically investigate the properties of materials at temperatures close to absolute zero. Kamerlingh-Onnes had established 25 that mercury metal known as a `bad metal' completely loses electrical resistance at 4.2 K.In 1933, Meissner and Oxenfeld showed 26 that superconductors (SC) are simultaneously ideal diamagnetics, which means that they completely exclude the magnetic field lines from their own bulk. Yu D Tretyakov, E A Goodilin Department of Chemistry, MV Lomonosov Moscow State University, Leninskie Gory, 119899 Moscow, Fax (7-095) 939 09 98. Tel. (7-095) 939 20 74. E-mail: yudt@inorg.chem.msu.ru (Yu D Tretyakov), Tel. (7-095) 939 47 29. E-mail: goodilin@inorg.chem.msu.ru (E A Goodilin) Received 27 April 1999 Uspekhi Khimii 69 (1) 3 ± 40 (2000); translated by AMRaevsky #2000 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2000v069n01ABEH000526 124 18 28 Following the discovery of superconductivity, the common belief was that superconductors would find extensive practical applications.However, this is limited mainly because of a very low transition temperature to the superconducting state (the so-called critical temperature, Tc). In the mid-1980s, it has been possible to raise this temperature 2, 12, 27, 28 to 23.2K for Nb3Ge intermetal- lide, despite the fact that commonly accepted theories of super- conductivity (see, e.g.,26) casted scepticism concerning the possibility of attainment of this high-temperature barrier. In 1986, Bednorz andMuÈ ller 29 discovered that ceramics based on copper, lanthanum and barium oxides (La27xBaxCuO4) can 165K ~~ Tc /K The lowest air temperature recorded on Earth (1983) 160 Pressure 140 HgBa2Ca2Cu3O8 (May 1993) 120 Tl2Ba2Ca2Cu3O10 (February 1988) Bi2Sr2CaCu2O8 (January 1988) 100 YBa2Cu3O7 (February 1987) Liquid nitrogen temperature 80 La1.6Sr0.4CaCu2O6 (1988) 60 La27xBaxCuO4 40 January 1987 December 1986 December 1986 April 1986 Liquid neon temperature Nb3Ge Liquid hydrogen temperature Ba0.6K0.4BiO3 Nd27xCexCuO4 20 Nb ±Al ±Ge NbN NbO ~~ 180 Hg (1911) Nb Nb3Sn Liquid helium temperature 0 Years 1990 1970 1960 1950 1910 Complex oxides Intermetallides Metals Figure 1.Chronology of discoveries in the field of high-temperature superconductors.2undergo a transition to the superconducting state at 30 K. Mention should be made of studies by Lazarev, Kahan and Shaplygin 30 who synthesised complex cuprates of analogous composition in 1978 and of those by a group of researchers from France,31 carried out two years later.However, unfortunately, the electrical conductivity of these specimens was measured down to liquid nitrogen temperature (77 K) only, which precluded obser- vation of the effect of superconductivity. It should be emphasised that high-temperature superconduc- tivity was first observed for an oxide ceramic, which usually possesses dielectric or semiconducting properties, rather than for conventional intermetallides, organic or polymeric structures.26 This destroyed the psychological barrier to the synthesis of HTSC and allowed the synthesis of new generations of metal-oxide superconductors nearly simultaneously and over a short period.The brief history is as follows. In February 1987, using the idea of `chemical compression' for structure modification, Wu et al.32 prepared a superconducting ceramic based on barium, yttrium and copper oxides (YBa2Cu3O77x) with a critical temperature of 93 K, which is higher than the liquid nitrogen boiling temperature. In January 1988, Maeda et al.33 synthesised a series of compounds with composition Bi2Sr2Can71CunO2n+4 . Among them, the phase with n=3 had a Tc of 108 K. A month later, Sheng and Hermann 34 prepared a superconductor Tl2Ba2Ca2Cu3O10 with Tc=125 K. Finally, in 1993, Antipov, Putilin et al.35, 36 discov- ered a series of mercury-containing superconductors with compo- sition HgBa2Can71CunO2n+2+d (n=1 ± 6).Currently, the Hg1223-phase of this family has the highest Tc ( 135 K). It should be noted that at an external pressure of 3506103 atm the transition temperature increases to 164 K,12, 36 which is only 19K lower than the lowest temperature ever reached under natural conditions on the Earth. These are the main stages of `chemical evolution' of superconductors from mercury metal (4.2 K) to mercury-containing HTSC (164 K). Currently, about 50 individual layered superconducting cuprates are known 14 (Table 1). Sometimes, sensational informa- tion on the preparation of novel `copper-free' superconductors with Tc above room temperature is reported. `Copper-free' super- conductors have been known for some time; however, no high transition temperatures to the superconducting state have yet been observed for these compounds (the highest Tc values for copper- free superconductors were reported for Ba17xKxBiO3 and fuller- ene-based intercalation phase Cs3C60 37 (see Table 1).A line of investigations associated with attempts to synthesise `ecologically safe' HTSC free from heavy metals (Hg, Pb, Ba) deserves special mention. Oxycarbonate phases of Ca prepared under high pressure 38 can serve as an example of these HTSC. Thus, most HTSC are oxide phases with very complicated chemical composition. These compounds are extremely sensitive to the conditions for synthesis, heat treatment and operating conditions; they are often called `chemical' superconductors.14 Numerous studies were devoted to structural,12, 17, 18 physical,19 synthetic 2 ± 11, 13, 14, 20 ±22 and technological aspects 15, 16, 23, 24 of preparation of HTSC materials; however, only very few of them consider all these problems simultaneously.It should be noted that studies of the processes accompanying preparation of HTSC- materials by different methods not only allow the solution of a practically important problem of revealing the `composi- tion ± treatment ± structure ± properties' correlations, but also favours the progress of general chemical ideas of the nature of complex oxide systems, e.g., of the interaction between their components, the effect of mutual rare-earth (RE) element sub- stitution, high-temperature phase relations, the nature of peritec- tic reactions, the structure and properties of the cuprate melts, oxygen and cation nonstoichiometry of solid phases and struc- tural phase transitions.In this regard, the principles of the synthesis of metal-oxide superconductors are of considerable interest to chemical sciences. This review deals with a systematic consideration of fundamental physicochemical problems related to the preparation of HTSC-materials as well as with analysis of Table 1. Main types of high-temperature superconductors. Type of high-temperature superconducting phase La27xMxCuO4 (`La201'-phase, M=Ca, Sr, K, Na, Rb) La1.6Sr0.4CaCu2O6 Nd27xCexCuO4 (T-phase, electron superconductor) Nd2CuO47xFx (Nd,Sr,Ce)2CuO4 (T *-phase) (Y,R)Ba2Cu3+n/2O7+n/2 (`R123'-phase, R is RE element) YBa2Cu3O6F2 Bi2(Ca,Sr)n+1CunO2n+4 `Bi2201'-phase `Bi2212'-phase `Bi2223'-phase TlBa2Can71CunO2n+3 (`Tl1212'�`Tl1234'-phases, n=1±4) Tl2Ba2Can71CunO2n+3 HgBa2Can71CunO2n+2+z Hg2Ba2R17xCaxCu2O8 (see a) 70 35 Pb2Sr2Rn71Cun+1O2n+4+z (R is RE element, RE element+Ca) CuSr2YCu2O7 77 12, 14 AuBa2(Y17xCax)Cu2O7 (see a) 82 Sr0.7La0.3CuO2 (see a) 40 12, 14 Sr2CuO2F2+z 47 12, 14 Sr27xKxCuO2CO3 (see a) 23 38 Sr2CaCuO4+zCl27x (see a) 80 12, 14 (BO)Sr2Can71CunO2n+2 (see a) 100 12, 14 (Sr,Ca)n+1CunO2n+2 (see a) 100 12, 14 (Cu0.5C0.5)Ba2Can71Cu3nOz (see a) 124 12, 14 Ba2Can71CunO2n72+z (see a) 126 (n=3) 12, 14 MC60 (fullerides, M=Na, K) 34 37 Ba0.6K0.4BiO3 30 2 Sr0.5K0.5BiO3 (see a) 13 12, 14 RNi2B2C 23 12, 14 a Obtained under pressure.different experimental results obtained by researchers all over the world. II. Characteristic features of high-temperature superconductors 1. The crystal structure An analysis of the available data on the structure and composition of HTSC allows some generalisations to be made.14, 36 First, almost all HTSC are complex layered copper-containing oxides whose structure contains oxygen-deficient perovskite-like blocks (Fig. 2). The (Sr,Ca)CuO2 phase built of alternating blocks comprising alkaline-earth metal ions sandwiched with planar CuO2 layers 38 is considered as a prototype of the oxide HTSC. It is the CuO2 layers that are currently considered to be responsible for superconductivity in cuprates.In these layers, the copper atoms form a square network, whereas the oxygen atoms are placed on the lines connecting the copper sites. The electrons Yu D Tretyakov, E A Goodilin Ref. Tc /K 29 35 ± 45 18 18 58 23 27 18 35 18 up to 95 (n=0) 32 60 (n=1) 80 (n=2) 94 18 33 34 12 (n=1) 85 (n=2) 108 (n=3) 125 (n=4) 34 35 95 (n=1) 105 (n=2) 128 (n=3) 115 (n=4) 98 (n=1) 128 (n=2) 135 (n=3) 125 (n=4) 113 (n=5) 2 40 ± 70Chemical principles of preparation of metal-oxide superconductors CuO 1 BaO CuO2 YCuO2 BaO 2321 CuO OBa Cu Y HgOz 1 BaO 23 CuO2 Ca CuO2 Ca CuO2 2 BaO 1 HgOz Figure 2.Crystal structures of HTSC YBa2Cu3O7 (a), Bi2Sr2CaCu2O8 (b), HgBa2Ca2Cu3Oz (c). Dielectric (1 ) and oxygen-deficient (3) perovskite-like blocks containing superconducting CuO2 planes (2) are shown. of copper and oxygen atoms that form Cu7O bonds in such a layer (3dx27y2 and 2pxy electrons, respectively) are delocalised, which means that they belong not to one but to all of the atoms in the layer. Therefore compounds comprising CuO2 layers in their structure can possess a metallic-type conductivity. At temper- atures below the critical temperature, superconductivity arises Figure 3. Generalised physical phase diagram of a high-temperature superconductor depending on the relative concentration of charge carriers in the superconducting planes.Mark 1 on the abscissa axis corresponds to the state characterised by the maximum transition temperature to the superconducting state. a Ca CuO2 SrO BiO 1 BiO SrO 2 CuO2 32 Ca CuO2 SrO BiO 1 c BiO SrO CuO2 Cac b OHg Ba Cu Ca T /K 600 400 Optimum doping 200 Tc HTSC 0 1 Concentration of charge carriers Antiferromagnetic Underdoping Overdoping b a Ca Cu OBi Sr 3 upon doping of the CuO2 layers with an optimum amount of charge carriers (Fig. 3); this occurs upon ordering of oxygen atoms and vacancies after the high-temperature superconducting phase has achieved a certain oxygen stoichiometry, upon hetero- valent doping, upon applying an external pressure, etc.12, 16, 19 It was experimentally established that manifestation of super- conductivity requires that the formal oxidation state of copper in the CuO2 layers with collective electrons be somewhat different from +2 and lie within the range from +2.05 to +2.25 for hole superconductors (123-, Bi- and Tl-families) or from +1.8 to +1.9 for electron superconductors 2, 12 (phases of the Nd2CuO4 type). The Cu7O bond length in the layer is yet another important parameter determining the superconducting properties; this must be within the limits 0.190 ± 0.197 nm provided that the distance between the nearest copper atoms lies in the range 0.380 ± 0.394 nm.The copper atoms can also be bonded to the oxygen atoms in neighbouring layers; however, these bonds should be much longer and exceed 0.22 nm.In other words, the structure of super- conducting cuprates contains inequivalent Cu7O bonds, viz., strong in-plane bonds in each CuO2 layer and much weaker bonds directed perpendicular to these layers. As a consequence, such superconductors have a layered structure, whereas the frame- work-type complex copper oxides (perovskites CuBO3 with equivalent Cu7O bonds) possess no superconducting properties. Electrical neutrality of crystals requires the presence of other charge-compensating layers or the presence of dielectric layers between superconducting CuO2 planes. These interlayers consist of readily polarisable ions (e.g., Ca2+, Sr2+ and Ba2+ ions) which, along with the holes in the CuO2 layer, can form the Cooper pairs upon transition to the superconducting state.2 Most of the known superconductors are built of alternating CuO2 and BaO, SrO, TlO+, BiO+, Ca2+, Y3+, etc.layers. Changes in the number of CuO2 layers in the structure give rise to homologous series of compounds with a similar structure (see Fig. 2). In this case, the crystal structure will be stable if each constituent layer is commensurable with the layers lying above and below a given layer. This analysis of the genesis of the structure of different HTSC families is not only useful for finding a general approach to the description of the phenomenon of high-temperature supercon- ductivity, but also favours a targeted search for and chemical design of novel superconductors.2. Physical properties In most cases, the practical applications of superconductors are associated with their capability to resist in the superconducting state the destructive action of a current density of *105 A cm72 in magnetic fields from 2 to 10 T, while the first (ceramic) specimens of superconducting cuprates were characterised by more than modest critical current density 8, 10 of *1 ± 100 A cm72 (Fig. 4). Currently, synthesis of superconducting phases with the desired crystal structure and Tc above the liquid nitrogen temper- ature can be accomplished without difficulty. However, the attainment of high values for other important parameters, e.g., the critical current density Jc and its stability in an external magnetic field, faces great difficulties for a number of reasons.For instance, evolution from metallic to ceramic superconductors results in an increase in Tc and, at the same time, in a dramatic decrease in the coherence lengths of oxide superconductors 19 compared to those of low-temperature intermetallide supercon- ductors (0.2 nm and *2 nm, respectively). The fundamental physical parameters of high-temperature superconducting YBa2Cu3Oz phase are listed in Table 2 as examples. As a consequence, the grain boundary thickness in polycrystalline metallic superconductors is commensurable with the coherence length, which favours the appearance of efficient pinning centres and an increase in the critical current. On the contrary, the supercondtion current in oxide superconductors is to a great extent limited by processes occurring at grain boundaries; this is4 J /A cm72 Theoretical limit 108 106 7 5 6 104 43 2 102 1 100 104 H /G 103 102 10 Figure 4.Dependence of the stability of the critical current density on the external magnetic field at 77K for different classes of HTSC materials; (1) sintered ceramics; (2) Bi2212 tapes; (3) single crystals; bulk melt- solidified ceramics: (4) Y123; (5) Nd123; (6) Y123 doped with 235U and irradiated with slow neutrons and (7) thin films. the reason why the demands imposed on the state of the inter- crystallite boundary are so severe.9±11 The situation is compli- cated by the fact that, because of the specific layered structure, almost all high-temperature superconducting phases possess a very high crystallographic anisotropy of physical properties, which requires texturing of the polycrystalline material.11 Additionally, HTSC are type II superconductors.In an external magnetic field, they can exist in the mixed state where the magnetic flux partly penetrates the superconductor in the form of the so-called Abrikosov fluxoids.19 As a rule, the vortex lattice thus formed becomes more stable if extra pinning centres are generated. It is assumed that each pinning centre is a distortion of the superconductor structure (a structural defect), the size of which is comparable with the coherence length.11, 19 Generation of efficient pinning centres in high-temperature superconducting Table 2.The Ginsburg ± Landau parameters of the HTSC YBa2Cu3Oz- phases (Tc=92 K).19 BCS a Calculation Experiment Parameter Anisotropy k||ab k\ab 74 2750 245 7 7 7 7150 kav 7 Coherence length /nm z||ab z\ab 7 7 0.3 0.7 70.9 1.2 0.2 0.7 zav Landau's penetration depth /nm l||ab l\ab 89 550 160 7 7 7 7134 140 lav 95 ± 110 90 71.1 194.6 2.23 7777 1.0 2282.21 Critical magnetic field /mT Hc1(\ab) Hc1(||ab) 21 24 7 Hc1(0) 7dHc1/dT /mT K71 7(dHc2/dT)av /T K71 DC/gTc (g=18.2 mJ mol71 K72) a The Bardeen ± Cooper ± Schrieffer model. Yu D Tretyakov, E A Goodilin materials is necessary since the practical use ofHTSC in the heavy- current devices requires not only high critical current densities, but also enhanced current stability in external magnetic fields.3. Chemical behaviour The preparation of modern high-temperature superconducting materials encounters considerable difficulties due to the complex- ity of their chemical composition and structure (and, as a consequence, to the thermal and chemical instability).27 The fact that solid-phase (ceramic) synthesis of superconducting cuprates is simple and rapid, as well as giving a first preparation of defect- free (according to visual inspection) as-grown small single crystals only gave the deceptive impression that most problems had been left behind. However, conventional methods of solid-phase syn- thesis of HTSC appeared to be inefficient for the preparation of materials with the desired properties. Because of this, the develop- ment of melt solidification methods 6± 11 has begun.These methods are based on melt crystallisation under controlled conditions, which allows a complex treatment of the real struc- ture of the material. It should be noted that almost all HTSC are characterised by incongruent melting and their melts are multi- component heterogeneous (containing both gaseous and solid phase along with the melt) open systems (participating in oxygen exchange with environmental gases), shifted apart from thermo- dynamic equilibria. This can be exemplified in the family of high-temperature superconductors, viz., RBa2Cu3Oz phases (R123-phases, R is a RE element) studied in most detail.These complex cuprates are composed of such chemically different components as stoichio- metric (with respect to oxygen) high-melting oxides of alkaline- earth and RE elements (BaO and R2O3) and a low-melting `acidic' oxide of a transition metal (copper) in different oxidation states. As a result, the problem arises of oxygen nonstoichiometry, which requires consideration of the phase diagrams for quaternary R2O3 ± BaO ±CuOn±O2 systems with complicated `pO2 ± temperature ± composition' relations (especially in the region where the solid and liquid phases coexist) instead of conventional isothermal R2O37BaO7CuO phase diagrams. In addition, RE-element-for-barium substitution solid solutions can be formed because of rather large ionic radii of `light' RE elements (La, Pr, Nd, Sm, Eu, Gd); their equilibrium composition in the supersolidus region is determined by the composition of the melt equilibrated with the solid solution.8 This complicates further the general pattern of phase relations and makes optimisation of the processes of preparation of high-temperature superconducting materials more difficult.Thus, the main problems are (i) the formation of nonstoichio- metric (with respect to both oxygen and cations) solid solutions with a preset composition, the degree of chemical homogeneity (i.e., with a particular macroscopic and microscopic distribution of the solution components) and (ii) targeted formation of a real structure of high-temperature superconducting material which provides the desired set of `structurally sensitive' properties. Accompanying (and sometimes no less important) problems are the investigation of the stability of this type of solid solution including the study of metastable states of high-temperature superconducting phases and their low-temperature decomposi- tion,7, 8 the study of equilibrium phase diagrams of high-temper- ature superconducting systems,8±11 the effects of their prehistory and topochemical memory,39, 40 chemical degradation of HTSC,2, 22 procedures for the preparation of superconducting composites,2, 6, 13, 15, 16, 23, 24 etc.III. Phase diagrams of high-temperature superconductors Phase diagrams reflect the phase states and phase relations depending on the temperature, pressure and the concentrations of the components in the systems. Therefore, it is reasonable to begin the analysis of efficient methods for the synthesis of metal-Chemical principles of preparation of metal-oxide superconductors oxide superconductors with the most comprehensively and reliably studied R7Ba7Cu7O systems in order to demonstrate general principles of the use of phase diagrams for the preparation of high-temperature superconducting materials.Investigations into the phase diagrams of a promising and technologically important Nd7Ba7Cu7O system,7, 8, 16 which exhibits the salient features of the phase diagrams of cuprate superconductors in the supersolidus region seem to be the most interesting.1. Subsolidus phase relations a. Geometrical factors and phase stability Most superconducting compounds are thermodynamically stable only in narrow ranges of temperature and partial pressure of oxygen; many of them are metastable under storage and operating conditions. Therefore, one can hardly find high-temperature superconductors (especially, among the last-generation materi- als) free from dopants stabilising their structure. As a rule, superconducting complex oxides are solid solutions with fairly wide homogeneity regions and comprise several elements with close crystallochemical characteristics. Internal structural strain is naturally relieved due to the difference in the ionic radii of the constituent elements.2, 17, 18, 36 `Chemical' deformation of the crystal structure of superconductors due to doping, as well as that caused by external pressure, can lead to essential changes in the superconductivity parameters due to changes in the distances between the superconducting planes and dielectric blocks and/or to the charge redistribution between them.41 ± 45 Because of this, the consideration of geometrical factors seems to be of great importance.The following types of substitution affecting the properties of high-temperature superconducting phases and the phase relations in high-temperature superconducting systems can be distinguished, (i) variation of RE elements (R) in R7Ba7Cu7O systems; (ii) Bi/Pb substitution in bismuth- containing HTSC; (iii) Hg/Pb, Hg/Re and Ba/Sr substitutions in mercury-containing HTSC;36, 46, 47 (iv) substitutions of alkaline- earth cations in binary and ternary cuprates; and (v) fluorination of RBa2Cu3Oz7yFy phases.48 The simplest method for assessing the stability of oxide superconductors is based on the well-known Goldschmidt crite- rion for tolerance, according to which a perovskite-like structure ACuO3 is stable 2 if 0.8<t<1, where t à r 2 pÅÅÅÖrCu á rOÜ A á rO and r are the ionic radii of the elements constituting the crystal lattice.The use of different REE is the most prominent example and important method of chemical modification of HTSC, since the radii of R3+ cations with close chemical properties monotoni- cally decrease due to the effect of lanthanide contraction.Along with the differences in the magnetic moments of R3+ ions, energies of their stabilisation by the ligand crystal field and possible oxidation states (+4, +3 or +2), the above-mentioned geometrical factor provides an additional degree of freedom in the synthesis of superconducting phases. Currently, almost all RE analogues of YBa2Cu3Oz have been synthesised by isomorphous substitution of yttrium.7, 12, 18, 22 As has been mentioned above, the unit cell of the RBa2Cu3Oz phase can be represented as three perovskite-like blocks containing barium or yttrium ions at the centre (see Fig. 2). Phases of the RBa2Cu3Oz type are characterised by two oppositely directed effects,49 viz., by structure stabilisation towards simple oxides due to the coordination of barium ions and the formation of BaO10 polyhedra and by structure destabilisation owing to the formation of RO8, CuO4 and CuO5 polyhedra.Small ionic radii (R3+) of `heavy' REE and yttrium result in the `repulsion' of oxygen ions in a smaller polyhedron (RO8) and generation of steric hindrances. Almost no structure destabilisation effect is observed for `light' REE with the largest ionic radii (Nd7Gd) in the case of 5 isomorphous isovalent replacement of the yttrium ion by its RE analogues. The oxygen content, which affects both the concentration of the charge (hole) carriers in superconducting CuO2 planes and the average Cu7O bond length, is the second factor influencing the stability of high-temperature superconducting phases with the YBa2Cu3Oz structure.It is noteworthy that the estimates of both the lower and upper limits of thermal stability of the phases under consideration 50, 51 suggest that tetragonal non-superconducting modifications (RBa2Cu3O6) are more stable than the orthorhom- bic superconducting phases characterised by higher content of oxygen (RBa2Cu3O7) and `holes'. The decrease in the thermal stability of solid solutions with partial substitution of REEcations for barium cations in the structure, in which the average oxidation state of copper increases in parallel to the increase in the degree of heterovalent substitution, is an additional confirmation of this hypothesis. The two above-mentioned structure destabilisation factors should also be taken into account when analysing the `geo- metrical' stability of bismuth-containing superconducting cuprates (see Fig.2 b).52 In these systems, almost all the most important phases are non-stoichiometric compounds with respect to all of the components. According to the data reported,53 ± 55 the superconducting phase of the Bi2Sr2CaCu2Oz type (2212-phase) is Bi-excessive and (Ca+Sr)-deficient and the `ideal' 2 : 2 : 1 : 2 composition seems not to be a single phase. The reasons for the existence of such a wide homogeneity region can be 53 (i) isomorphous substitution of calcium ions for strontium ions 56 and, to a lesser extent, partial hetero-valent substitution of strontium and calcium ions for bismuth ions; (ii) the formation of intergrowth structures with higher and lower homologues containing different amounts of calcium and copper and (iii) exchange between the crystallographic positions of bismuth and copper atoms due to the packing faults of the layered structure.Compared to RBa2Cu3Oz phases in which the dielectric `barium' block exhibits a fairly high `geometrical' stability, the dielectric Bi2O2 blocks can additionally contribute to the structure destabi- lisation. A formula was proposed 52 for assessing the relative stabilisa- tion of bismuth-containing HTSC with extra oxygen and with lead ions which replace bismuth ions in the structure t t a y à 1 á á g nz0 0 0 0 á 2g nz0 x . d á b y Here t is the parameter of the criterion for `geometrical' stability of a bismuth-containing high-temperature superconducting phase doped with lead and containing extra oxygen, t0 is the analogous parameter for a `pure' bismuth-containing high-temperature superconducting phase, a, b and g are constants, n is the number of superconducting CuO2 planes , d is the amount of extra oxygen and x is the degree of lead-for-bismuth substitution.It was assumed 52 that the length of the `in-plane' Bi7O bond is determined by the relationship y=y0+ad+bx , whereas the Cu7O(2) bond length is determined by the relation- shipz=z07gh , where h is the concentration of `holes'. According to calculations, the highest stabilisation of superconducting structure (especially at large n, i.e., for higher members of the homologous series) can be achieved, as in the case of 123-phases, if the internal structural strain is relieved by doping the oxide matrix with cations with large ionic radii (in this case, by doping with lead ions in the Bi2O2 block).The virtually planar structure of the CuO2 layers 35, 36 is a possible reason for the record transition temperatures to the6superconducting state for mercury-containing HTSC. Therefore the geometrical factors can directly affect fundamental super- conducting characteristics of a system. Schematically, the struc- ture of HgBa2Can71CunO2n+2+d phases can be represented as (HgOd)(BaO)(CuO2) . [(Ca)(CuO2)]n71(BaO)(HgOd) layers stacked along the c axis of the unit cell.36 These compounds have primitive tetragonal unit cells in which the parameter c increases as the thickness n of the perovskite fragment c (A)=9.5+3.2(n71) increases.The coordination of the copper atoms in CuO2 layers varies from octahedral in the Hg1201-structure to tetragonal-pyramidal in the Hg1212-phase and to the square and tetragonal-pyramidal in the Hg1223-phase as the thickness of the perovskite fragment increases. The octahedra and tetragonal pyramids are appreciably elongated along the c axis due to Jahn ± Teller distortion. Long Cu7Obonds between the copper atom and the axial oxygen atom and, correspondingly, a very weak interaction between these atoms in the coordination polyhedra of copper are the salient features of the structure of mercury-containing HTSC. On the other hand, a successive increase in the thickness of perovskite fragments owing to the incorporation of additional CaCuO2 blocks results in compression of interatomic planar Cu7O distances in the structures of higher homologues.This can be interpreted as anisotropic `chemical' compression and results in the distortion of the CuO2 layers with an increase in n and in corrugation of their planarity. As a consequence, the highest homologues are characterised by lower transition temperatures to the superconducting state. This effect of anisotropic chemical pressure is several times stronger than that caused by external isotropic pressure.57 Therefore, it can be suggested that the use of non-traditional methods of chemical synthesis (e.g., layer-by- layer epitaxial deposition of thin films on the substrates with specially chosen parameters) rather than conventional isovalent cation substitution resulting in isotropic compression of the structure will be the most promising way for structure modifica- tion to achieve higher Tc.The use of these substrates can result in anisotropic deformation of the crystal lattice of a high-temper- ature superconducting phase.58 A limited set of cations which constitute high-temperature superconducting phases along with copper and oxygen atoms is a factor that restricts the search for new superconducting materials. As has been mentioned above, this is associated with the necessity of meeting the conditions of commensurability of interatomic distances and electrical neutrality.The possibility of varying the composition of the anionic sublattice appreciably extends the area of search; this has been exemplified successfully with F7 anions, the crystallochemical characteristics of which are close to those of O27 ions.48 Incorporation of fluorine into the structure of the non-superconducting oxide YBa2Cu3O6 results in its oxidation with the formation of the YBa2Cu3O6F2 phase with Tc=94K. In this case, several copper atoms in the structure have an octahedral environment consisting of four fluorine atoms and two axial oxygen atoms. In such a structure, the symmetry of the high- temperature superconducting phase remains tetragonal after oxidation because of the absence of ordering of the anions and oxygen vacancies in the CuOz plane of the dielectric (`barium') perovskite-like block.An increase in the coordination number of copper atoms is accompanied by strong Jahn ± Teller distortion of the Cu(O,F)6 octahedra, which is responsible for the appreciably larger parameter c of the fluorinated phase compared with that of its oxide analogue (12.8 ± 13.2 vs. 11.8A, respectively). Geometrical factors also affect the stability of the most important non-superconducting phases in the systems under consideration, in particular, that of binary and ternary cuprates containing RE and alkaline-earth elements.59 ± 64 For instance, the formation of `blue' phases of the Y2Cu2O5 type (the 202-phase) is characteristic of REE of the yttrium group, whereas `light' REE with large ionic radii form phases of the La2CuO4 type (the 201- phase).59 `Green' R2BaCuO5 phases with the framework struc- ture, obtained for almost all REE, are stable in air up to Yu D Tretyakov, E A Goodilin 1250 8C.8, 17, 59, 61 `Light' REE with large ionic radii form the `422-phases' with composition R472xBa2+2xCu27xO1072x (R=La, Nd),59, 61 which is associated with the formation of brown solid solutions rather than green substances at 0.154x40.25 for R=La and at 04x40.1 for R=Nd, the latter being characteristic of the Y2BaCuO5 phase.The changes in the ionic radii of alkaline-earth elements (Ba, Sr, Ca), whose binary cuprates play a special role in high-temper- ature superconducting systems, also result in essential changes in the phase diagrams.60, 61, 63 It is suggested that two cuprates with Ba :Cu ratios of 2 : 3 and 3 : 5 exist in the Cu-rich region of the phase diagram of the BaO7CuO system (see S.1).{ Like BaCuO2, these cuprates can formally be considered as compounds belong- ing to the BaO7CuO7`Cu2O3' system since they have a rather wide oxygen homogeneity region (see S.2).An increase in the temperature and a decrease in the pO2 results in decomposition of Ba2Cu3O5 and Ba3Cu5O8 with the liberation of CuO and the formation of BaCuO2 , which is stable in air up to *1015 8C (at this temperature, the compound melts with the evolution of oxygen). O O2 Barium cuprate Ba2CuO3 , which is thermally stable at high p 2 , is found in the Ba-rich region of the phase diagram of the BaO7CuO system.Cuprite BaCu2O2 containing monovalent copper is the only phase stable at reduced partial pressure of oxygen and(or) at high temperatures. The phase relations in the Ca7Cu7O system exhibit some specific features as compared with Sr7Cu7O and Ba7Cu7O systems, which is probably due to the impossibility of calcium ion, which has the smallest ionic radius among alkaline-earth metal cations, to stabilise CuI and CuIII.22, 23, 61 No stoichiometric quaternary compounds were found in the Sr7Ca7Cu7O system, whereas the stability of solid solutions of mixed strontium and calcium cuprates is determined by the temperature and partial pressure of oxygen, as in the case of the Ba7Cu7O system.At high pO2 and moderate temperatures, the cuprate Sr147xCaxCu24O41+d is stable. An increase in temperature and/or a decrease in p results in sequential formation of the cuprate phases Sr17xCaxCuO2 (1 : 1) and Sr27xCaxCuO3 (2 : 1) and the cuprite Sr17xCaxCu2O2+d (1 : 2). Perhaps the most spectacular example of changes in the phase relations caused by geometrical factors is represented by R7Ba7Cu7O systems. There are three main types of quasi- ternary isothermal cuts of the subsolidus phase diagrams of these systems.62 The simplest type (Fig. 5 a) is characterised by the presence of a `point' RBa2Cu3Oz phase formed by `heavy' REE of the yttrium subgroup with the smallest ionic radii (Y, Dy, Ho, Er, Tm, Yb, and Lu), though the phase relations for Yb and Lu can have some peculiarities due to relative instability of the 123- phases.63 Solid solutions of the R1+xBa27xCu3Oz type (Fig.5 b) are formed in systems with `light' REE with large ionic radii (La, Nd, Sm, Eu, and Gd). In these systems, the variation of the composition of non-superconducting (secondary) phases, which play a significant role in the processes of peritectic melting and crystallisation, and of the equilibrium tie-triangles depends on the ionic radii and nature of the REE. The third type of phase relations (Fig. 5 c), which differs fromthe two preceding cases in the absence of the phase of the 211/422-type,61 ± 63 is characteristic of the Pr7Ba7Cu7O system.In the Pr- and Ba-rich region, the Pr123ss phase is in equilibrium with the Pr(IV)-containing phase of the PrBaO3 type. No compounds with the 1 : 2 : 3 stoichiometric composition are formed in systems containing Ce and Tb for which the most characteristic oxidation state is +4.61 Systems Bi(Pb) ± Sr ± Ca ±Cu ± O, Tl ± Ba(Sr) ±Ca ± Cu ±O and Hg(Re,Pb) ± Ba(Sr) ±Ca ± Cu ±O are quinary or even septe- nary, therefore their phase diagrams are extremely com- { The letter S denotes Supplement which contains notes added in proof (the results reported when preparation of the manuscript was completed as well as the results obtained in recent studies and accepted for publication elsewhere).Chemical principles of preparation of metal-oxide superconductors a YO1.5 Y2BaO4 Y4Ba3O9 211 Y2Cu2O5 123 132 CuO BaO Ba2CuO3 BaCuO2 c PrO1.5 Pr2CuO4 PrBaO3 123ss CuO BaO BaCuO2 plex.53, 61, 64 ± 69 As a rule, the phase diagrams of these systems are only studied around the stability regions of the high-temperature superconducting phase or analysed using the corresponding software; the most important quasi-binary polythermal or quasi- ternary isothermal cuts (Fig.6) are also considered.64 The phase diagrams of the Tl7Ba7Ca7Cu7O and Hg7Ba7Ca7Cu7O systems (Fig. 7) exhibit strong depend- ences not only on the partial pressure of oxygen, which is typical of all high-temperature superconducting systems, but also on the total pressure (ptot) determining the partial pressure of the volatile metal component (mercury or thallium).67, 68 For this reason, conventional methods of preparation of thallium-containing HTSC include `as-filled' sintering, which provides the possibility of performing the reaction in an atmosphere of a gas enriched with thallium compounds.Moreover, mercury-containing SC are usually prepared in ampoules,35, 36, 65 at elevated external pres- sure 36, 57 or in autoclaves at a high pressure of an inert gas atmosphere (argon, nitrogen).68 However, it was reported 36 that even these methods also do not preclude the formation of mercury-deficient high-temperature superconducting phases with partly vacant mercury positions.66 The volatility of metal-containing components further com- plicates the problem of stabilisation of these structurally complex superconducting phases.If the doping of phases in the Bi7Sr7Ca7Cu7O system with lead oxides can be considered mostly as a convenient procedure for enhancing the formation dynamics of higher homologues of metal-oxide SC,53, 69 the search for possibilities of stabilisation of the structure of mercury- containing HTSC (e.g., by doping with rhenium,36, 68 lead and/or strontium 46, 47, 65 compounds) is of prime importance. b. The problem of low-temperature decomposition In early studies, metastability of superconducting phases was often considered as the condition for the existence of high- temperature superconductivity;1, 2 however, the results of mod- ern investigations do not confirm this assumption.Nevertheless, the problem of chemical metastability of high-temperature super- conducting phases still remains topical, since this can have both negative and positive consequences from the practical viewpoint. Generally, this is associated with the analysis of the evolution of states of a supersaturated (with respect to one or several compo- nents) solid solution under given conditions. 7 b a T /8C NdO1.5 L L+Ca2CuO3+(Sr,Ca)CuO2 2201+L+(Sr,Ca)CuO2 900 Nd2BaO4 Nd2CuO4 2212+Ca2CuO3+L 2212+2223 860 422 2223+L+Ca2CuO3 123ss 820 163 2201+2212 2212+Ca2CuO3+ CuO BaO Ba2CuO3 BaCuO2 780 +CuO ~~~~~~680 640 2201+Ca2CuO3+CuO 2223 2212 2201 b SrO O (Sr,Ca)4Bi2O6 Figure 5. Basic types of isother- mal sections of subsolidus phase diagrams of R7Ba7Cu7O sys- tems (at p 2=0.21 atm).R: (a) Y; (b) Nd; and (c) Pr. Sr6Bi2O9 Sr3Bi2O6 Sr2Bi2O5 Sr17xBiO2.57x 2302 2201 Sr2CuO3 4805 SrCuO2 SrBi2O4 Sr14Cu24O417x 119x5 b 2212 2223 L Bi2O3 CuO a g Ca2CuO3 Ca7Bi6O16 CaO Ca7Bi10O22 c SrO (Sr,Ca)4Bi2O6 Sr6Bi2O9 Sr3Bi2O6 Sr2Bi2O5 Sr17xBiO2.57x 2302 2201 Sr2CuO3 4805 SrCuO2 SrBi2O4 b Sr14Cu24O417x 119x5 L Bi2O3 CuO a g Ca7Bi6O16 Ca2CuO3 CaO Ca7Bi10O22 Figure 6. Types of phase diagrams of the Bi7Sr7Ca7Cu7O system (at pO2= 0.21 atm); (a) polythermal section along Bi2Sr2CuO6±Bi2Sr2Ca2Cu3O10; phase relations in a quasi-quaternary system Bi2O3 ± CaO ± SrO ±CuO at 850 (b) and 910 8C (c). Phase relations in systems containing `light' REE (La, Pr, Nd, Sm, Eu and Gd) are very complicated because of the formation of solid solutions of the LR1+xBa27xCu3Oz type; difficulties in synthesising several phases of the stoichiometric 123-composi-8 T /8C 940 2201+L 920 900 2201+2212 880 40 20 pO2 /atm7 5 3 5 tion containing REE (La, Nd and Sm) have also been reported.7, 8, 70 ± 79 It was shown that the homogeneity region of the Nd1+xBa27xCu3Oz solid solution becomes narrower near the composition NdBa2Cu3Oz as the temperature decreases 75 and near the composition Nd2BaCu3Oz.63, 76, 78 ± 81 In any case, the system becomes thermodynamically unstable as regards solid- phase decomposition in the region of subsolidus equilibria 82 (see S.3).This problem is of interest first of all owing to pronounced variation of the superconductivity parameters of the metastable R1+xBa27xCu3Oz solid solution depending on its state.Of great importance is also the degree of homogeneity of this solid solution, which depends on the preparation conditions and subsequent heat treatment. For instance, an increase in the parameter x of a solid solution homogeneous at the unit cell level can result in disordering of the oxygen sublattice in the basal plane due to the presence of additional amounts of REE7, 8, 76, 83 ± 90 and in a reduction of the Tc value. If a solid solution decomposes with the formation of a heterogeneous mixture of particles of different compositions upon heat treat- ment, a mechanical mixture of phases with individual character- istics will be detected. A nanocomposite in which chemical inhomogeneity exists at a level comparable with the size of tens of unit cells can exhibit a fairly high Tc and nonlinear field dependence of superconducting properties (see Fig.4). It was reported that macroscopic phase segregation can occur in an atmosphere of elevated partial pressure of oxygen as a result of decomposition of supersaturated solid solution with the formation of barium cuprate.76 The specific behaviour of single crystals on heat treatment 8 is most probably due to a fundamen- tally different mechanism, which manifests itself, in particular, in the formation of a quasi-periodic tweed structure and in quasi- periodic spatial modulations of the ratio of concentrations of the elements constituting the single crystal (the so-called `composi- tional waves').For instance, the Ba :Nd ratio in NdBa2Cu3Oz varies from 2.0 to 0.7 (Fig. 8). Spinodal decomposition of the a 2234 Compositions for single crystal growth 2212+L 2223+L 2212+ +2223 2223+Ca2CuO3 60 (CaO+CuO) (mol.%) c T /K 1300 1200 1100 17 pHg /atm 13 9 Figure 7. Phase Tl7Ba7Ca7Cu7O (a) and diagrams of Hg7Ba7Ca7Cu7O (b, c) systems; (a): L is the melt; 2201 is the Tl2Ba2CuOz-phase; 2212 is the Tl2Ba2CaCu2Oz- phase; 2223 is the Tl2Ba2Ca2Cu3Oz-phase; and 2234 is the Tl2Ba2Ca3Cu4Oz- phase ; (b): (1) pHg; (2) pO2; (3) pHgO; (4) pHg in an ampoule; (5) pHg in an ampoule at initial pO2=1 atm; (c): the surface corresponding to the HgBa2CuO4+y>Ba2CuO3+z+Hg (gas)+(17y7z)/2 O2 equilibrium on the calculated diagram.Ba/Nd 2 1.5 1.0 3 0.5 120 40 0 80 Distance /nm Figure 8. Quasi-periodic nanoscale fluctuations of the composition of NdBa2Cu3Oz single crystals after low-temperature treatment at pO2= 1 atm. The composition of crystal matrix without nanoscale fluctuations (1, open circles) and with nanoscale fluctuations (2, filled circles); (3) variations of composition that should be observed upon the formation of interphase boundaries between the particles of decomposition products and the crystal matrix into which they are incorporated. Pressure /atm Yu D Tretyakov, E A Goodilin b T /K 1000 1200 1400 100 HgO 1 2 3 1201 10 4 5 1 Ba2CuO3(solid)+HgO(gas)+ +Hg(gas)+0.5O2(gas) 1.0 103/T /K71 0.8 Nd1+xBa27xCu3Oz solid solution, i.e., its instability towards small fluctuations of x, resulting in phase demixing and eventu- ally in the formation of coherent domains with appreciably different chemical composition, was suggested to be a possible reason for these changes.8 Reduction of temperature enhances the driving force of decomposition; however, its rate can be low because of extremely slow diffusion of components (the max- imum rate of the process was experimentally observed at *500 8C, see also S.3).The possibility for spinodal decomposi- Uniform crystal 2.0 1Chemical principles of preparation of metal-oxide superconductors tion to occur was inferred from the experimental results obtained for the specimens prepared by zone melting 91 and large-grain ceramics;70, 92 it was also postulated (based on the data on the equilibrium phase diagrams 80) for a model of a regular-type solution with a high positive energy of interaction between the components (*40 kJ mol71).Structurally, the segregation of a Nd1+xBa27xCu3Oz solid solution can be represented as a local change in the occupation of the barium sites in the NdBa2Cu3Oz structure, which results in the ordering of Nd3+ and Ba2+ ions in the BaO layer. The macro- scopic picture of this phenomenon can be considered using the results of studies of the structure and properties of the Nd1+xBa27xCu3Oz solid solution with a different degree of substitution (x).76, 78, 89, 90 This substitution has two salient fea- tures.First, it is hetero-valent (Nd3+?Ba2+, the `hole' redis- tribution in the structure); second, this leads to the substitution of a smaller neodymium ion for larger barium ion (the ionic radii are 1.11 and 1.35A, respectively; internal `chemical pressure'). The entropy of mixing of an ideal solution based on the NdBa2Cu3Oz phase and a hypothetical compound NdNd2Cu3Oz should be maximum for the composition NdBaNdCu3Oz (the Nd213-phase) (Fig. 9). Therefore, from the viewpoint of the entropy factor, the formation of a continuous series of Nd1+xBa27xCu3Oz solid solutions with statistical distribution of Nd3+ and Ba2+ ions at the barium positions is the most favourable up to x=1.However, the thermal stability of such a solid solution decreases as the degree of substitution x increases, i.e., compression of the structure can result in a decrease in the formation energy of the crystal lattice due to the internal steric strain.76 ± 81 The formation of the solutions with x>1 is unfav- ourable both from the viewpoint of energy factors and because of the decrease in the contribution of the entropy of mixing. Such an approach makes it possible to consider the solutions with x=1 as a `boundary' between real and hypothetical solid solutions. If the contributions of the entropy and energy factors are close, a distortion (puckering) of planar fragments of the structure can occur at x&1, resulting in a new type of atomic ordering and in a reduction of the symmetry of the struc- ture.78, 89, 90 Despite the fact that such a structural transition results in a decrease in the entropy of the system, a gain in the free energy owing to the increased energy of the lattice formation can be obtained in the case of partial relief of internal strain caused by the `chemical pressure'.Therefore, different types of cation ordering can be observed within the homogeneity region of the Nd1+xBa27xCu3Oz solid solutions, which is also accompanied by changes in the oxygen sublattice. The results of investigations of the Nd1+xBa27xCu3Oz solid solution (04x40.9) by X-ray difraction (XRD) and Raman 7Ordering Ideal entropy of mixing Real entropy of mixing CuO2 Superstructure (distortions) Nd2BaCu3Oz NdNd2Cu3Oz NdBa2Cu3Oz CuO2 + Lattice energy of a real structure Ideal layered structure Lattice energy of an ideal structure Internal (`chemical') pressure Figure 9.The effect of chemical pressure on the change in the structure of Nd1+xBa27xCu3Oz solid solution. a a, b, c/3 /A 3.94 3.92 3.90 3.88 3.86 3.84 b The Raman shift /cm71 560 540 520 500 0.4 0.2 0 Figure 10. Results of investigations of Nd1+xBa27xCu3Oz solid solutions by X-ray diffraction (a) and Raman spectroscopy (b); (a): unit cell parameters: (1 and 4) a; (2 and 5) b; (3 and 6) c/3; (1 ± 3) quenching; (4 ± 6) oxidation; (b): (1 and 2) the Raman shift; (3 and 4) broadening of an apical oxygen vibration mode (b); (1 and 3) oxidation; (2 and 4) quenching.spectroscopy (RS) are shown in Fig. 10.89, 90 The range of variation of x may conventionally be divided into three intervals, viz., 04x40.3, 0.34x40.6 and 0.64x40.9.76 The specimens for which the degree of substitution x varies within the first interval have tetragonal or orthorhombic crystal lattices; the unit cell parameters a and c/3 (a<c/3) regularly decrease due to replacement of the Nd3+ ions of a smaller radius for the Ba2+ ions. The unit cell of the tetragonal phase (a=c/3) of the solid solution with x values within the second interval is characterised by an `isotropic' compression of the unit cell, which occurs as x increases.An orthorhombic distortion of the unit cell is observed for compositions within the third interval. This distortion is retained even in the high-temperature region.81, 89 It was established in structural studies 76, 78, 89, 90 that the 123- and 213-phases { are characterised by different types of ordering of the Nd3+ and Ba2+ cations, namely, `vertical' ordering with the formation of uniform cation layers (Fig. 11) occurs in the former, whereas `horizontal' superstructural ordering of barium ions and neodymium ions which replace the barium ions occurs in the latter. The appearance of a structure consisting of alternating chains of barium ions and those of neodymium ions that occupy the barium positions predetermines the ordering of oxygen ions.One of the oxygen positions along the shortest axis (b) in the high- temperature orthorhombic modification of the 213-phase is only partly occupied. Ordered rows of CuO5 pyramids could be formed { In the phase diagram, they are terminating points of the existence region of solid solutions. 9 123456 b /cm71 70 60 50 40 1234 30 20 x 0.8 0.610 a 123 336 Figure 11. Different types of cation ordering in the structure of Nd1+xBa27xCu3Oz solid solution; (a) vertical Nd/Ba ordering; (b) cation and anion disordering and (c) in-plane Nd/Ba ordering. The solid solution with composition x=0 and z=7 is denoted as `123', that of composition x=0.5 is denoted as `336' and that of composition x=1 and z>7 is denoted as `213'; symbol O* corresponds to the position with variable occupation by oxygen atoms.in the `former' [BaO] block in the ideal (most oxidised) structure; however, a more complicated superstructural ordering in the real low-temperature orthorhombic modification of the 213-phase is observed. The temperature dependence of the oxygen content has an apparent sigmoid shape, which probably reflects this structural transition (at 700 ± 720 8C in air 89) (see S.4). The solid solution of intermediate compositions (0.3<x< <0.6, the 336-phase) is tetragonal. Unlike the 123-phase, the `vertical ordering' in this phase is distorted and a fraction of additional neodymium ions occupies the barium positions. At the same time, `horizontal' ordering of cations in the barium positions does not occur yet in the 336-phase, which differentiates this phase from the 213-phase and results in disordering of the cationic sublattice.84 ± 90 A maximum on the curve of the dependence of apical oxygen mode broadening in the Raman spectra on the composition of solid solution (see Fig.10) corresponds to disor- dering of the oxygen sublattice.90 It should be noted that analogous structural transformations result in changes in the hole distribution,84, 87 which also pertains in greater degree of substitution in solid solutions. In this case the hole concentration in the superconducting CuO2 planes decreases (the average, `collective' charge of the plane decreases), whereas the average oxidation state of the copper atoms in the CuOy plane of the dielectric (Nd,Ba)O block increases (see S.5).Phase `separation' of HTSC is a common phenomenon, which should be taken into account when analysing the phase relations in all the systems in question. For instance, it was postulated 93 that nonuniformity of oxygen distribution in the Y123-phase at a microscopic level is due to the spinodal decomposition. Low- temperature annealing of a single-phase solid solution (Bi,Pb)2Sr2CaCu2Oz in the preparation of a bismuth-containing 2212-phase enriched with lead 94 resulted in its solid-phase decomposition with the formation of a lamellar nanostructure with alternating domains enriched with and depleted of lead. This decomposition should be extremely slow because of the low rate of cation diffusion, the necessity of overcoming the energy barriers associated with the formation of a solid-solid interface and elastic deformation of the matrix for phases with different molar volumes and crystal structure.8, 75, 82 In accordance with the Ostwald rule, the degree of supersaturation of a solid solution can be partially reduced in several intermediate steps, where a coherent or semi- coherent interface is formed in the matrix of the initial phase rather than requiring that the energy barrier to hetero-phase nucleation of a new phase is to be overcome.Differences in the lattice parameters of solid solutions of different compositions give rise to local structural strain.75 b c b a OCu Nd Ba CuO5 CuO4 Obviously, this problem requires studies of the initial stages of phase decomposition (`ageing' of the supersaturated solid solu- tion) and construction of the so-called TTT (Time ± Temper- ature ± Transformation) diagrams for different initial states of solid solutions (ceramics, single crystals, films) as in the case of many alloys used in metallurgy.82 Mention may be made that solid-phase decomposition of HTSC can significantly improve certain practically important characteristics; in particular, this can be used for targeted formation of microscopic structural defects capable of acting as efficient pinning centres.7, 8, 94 2.Phase relations in the supersolidus region a. The use of the Gibbs phase rule Experimental studies of phase relations in the supersolidus region and the development of realistic models of phase diagrams which fulfil the Gibbs phase rule are necessary for devising efficient methods for the preparation of high-temperature superconduct- ing materials.8, 9, 53, 61, 64, 74, 75, 95 Experimental data on complex cuprate systems which would allow reliable calculation of the phase diagrams are extremely insufficient.Often, experimental phase diagrams (especially in the supersolidus region) are reported as being `schematic' or `tenta- tive' ones, since they do not often fulfil the Gibbs phase rule. Nevertheless, several basic and doubtlessly reproducible charac- teristics (e.g., melting temperatures and eutectic compositions) that are of great importance can be distinguished.61, 95 The Y7Ba7Cu7O, Nd7Ba7Cu7O and Bi7Sr7Ca7Cu7O systems are a few examples of comprehensively studied high- temperature superconducting systems for which three-dimen- sional 78, 81, 96, 97 or multidimensional 64 models have been pro- posed for the description of phase relations.Numerous experimental reviews 13, 21, 22, 63, 100 ± 102 have been devoted to the investigation and description of the phase diagram of parent Y7Ba7Cu7O system containing the high-temperature superconducting Y123- phase characterised by a very narrow cation homogeneity region. In particular,*11 invariant reactions involving the melt (L) were found in the phase diagram of this system in the supersolidus region,96 e.g., YBa2Cu3Oz+BaCuO2+CuO (a ternary eutectic at 890 8C with the liquid phase composition Y: Ba :Cu^1.0 : 32.5 : 66.5), BaCuO2+CuO (a binary eutectic at 910 ± 920 8C, *69 mol.%± 71 mol.% of CuO), Yu D Tretyakov, E A Goodilin c O* 213 studies 96 ± 108 and L LChemical principles of preparation of metal-oxide superconductors Y2BaCuO5+L YBa2Cu3Oz+CuO composition phase liquid with (near 940 8C the Y: Ba :Cu^2.0 : 23.0 : 75.0), L BaCuO2 (at *1015 8C) and the peritectic decomposition reaction at 1000 ± 1015 8C (1) Y2BaCuO5+L.YBa2Cu3Oz However, it should be noted that the peritectic decomposition temperature (Tp) of the Y123-phase appreciably decreases at low partial pressure of oxygen 8, 13, 100, 107 and increases at high pressure of oxygen (1 ± 3000 atm), the phase relations in the system being changed in the latter case.108 In both cases, the mechanism of decomposition of the 123-phase is also changed.Therefore, the simplified consideration of reaction (1) is incorrect. The peritectic decomposition reaction should be written as (2) HTSC L+S+O2 , O O O where S and O2 denote the melt, `secondary' solid phases and the gaseous phase (oxygen), respectively. Therefore, the Y2O37BaO7CuO7O2 system should be considered as a quaternary system for which four phases (two solid phases, one liquid and one gaseous phase) are in equilibrium at the peritectic decomposition point of YBa2Cu3Oz. Obviously, such an approach makes it possible to fix the partial pressure of oxygen in the gas phase (p 2 =const).An assumption can be made which is useful in studying many high-temperature super- conducting systems that the gas phase consists of the only component (oxygen) and, therefore, the total pressure ptot=p 2 . This is incorrect in the case of thallium- and mercury-containing HTSC (see above). Hence, the transformation (2) can be condi- tionally considered monovariant at ptot=p 2 =const. In fact, a particular temperature of `melting' (decomposition) of the 123- phase at fixed partial pressure of oxygen in the gas phase is observed, since it is necessary to introduce an additional condition (the cation stoichiometry), which imposes restrictions on the composition of the solid phase which undergoes decomposition.This assumption is also valid if the decomposition of a solid solution follows the scheme 76 ± 81 Nd4Ba2Cu2O10 (or Nd2CuO4)+L+O2 . (3) Nd1+xBa27xCu3Oz A substitution solid solution virtually degenerates into the stoichiometric 123-phase (x?0) at temperatures near peritectic melting. If both the cation composition (x) of the phase that undergoes decomposition and the partial pressure of oxygen are fixed, the experimentally determined peritectic decomposition temperature should have a unique value. Interpretation of phase equilibria in R7Ba7Cu7O (R=Nd, Sm) systems would be incomplete without considering the recently reported data 81 which suggest the existence of a five- phase equilibrium in the Nd-rich corner of the phase diagram.(4) Nd2CuO4+L+O2 . Nd2Ba1Cu3Oz+CuO (see S.6). O2 2 Obviously, this equilibrium is invariant at fixed p Investigation of bismuth-containing high-temperature super- conducting systems also confirms the necessity of considering all restrictions imposed on the system and thus reduce the number of degrees of freedom. As in the case of Nd1+xBa27xCu3Oz system, the composition of phases that can be in equilibrium with the high- temperature superconducting phase at the point of peritectic decomposition, as well as the corresponding decomposition temperatures differ depending on the stoichiometric ratios of the cations constituting the high-temperature superconducting phase and on the pO value in the gas phase. Consequently, the peritectic decomposition products of the Bi2212-phase (Bi2+dSr27x..Ca1+xCu2Oz) usually include different bismuthates [CF= Bi2(Sr,Ca)3+nO6+n], alkaline-earth cuprates [AEC= (Sr,Ca)nCumOz] and a melt. Melting and crystallisation of 11 bismuth-containing composites are strongly affected by silver metal used as the sheath of `long-length' tapes; the solubility of silver metal in the melt can reach up to 4 at.%± 5 at. %.23, 53, 54, 64 Therefore, the process of peritectic decomposition in this system can be represented as a six-phase equilibrium. (5) Bi2212+Ag AEC+CF+L+O2 . Practically, it is important that the partial pressure of oxygen changes not only the composition, but also the composition- dependent morphology of particles of the AEC and CF phases which coexist with the melt 23, 54 and, hence, the resulting micro- structure of high-temperature superconducting tapes.It is obvious that a simplified analysis of the process (5), e.g., without consid- ering the effect of gaseous atmosphere (O2) or silver, can lead to failures in developing new procedures for the preparation of bismuth-containing HTSC. b. The surface of the liquidus A realistic model of a phase diagram of a system in the super- solidus region is constructed on the basis of determination of melt composition and using the results of an analysis of the composi- tion of the equilibrium solid phases. The solubility of REE oxides in cuprate melts was studied by differential thermal analysis (DTA), the dissolution-extraction method, quenching experi- ments, the `last droplet' method, high-temperature microscopy and liquid-phase epitaxy (LPE) and single-crystal growth experi- ments.78 ± 81, 109 ± 116 Unfortunately, in some instances the com- plexity of peritectic reactions, slow dynamics of establishing a true equilibrium with the gas atmosphere, supercooling of the melt, corrosion of crucibles used and contamination of the melt with impurities can make the results of DTA studies incorrect.The use of a direct melt sampling procedure during the quasi-equilibrium heating and cooling of melt in a temperature-controlled chamber made it possible to appreciably increase statistical representative- ness and reliability of the results.109, 111, 113, 114 Generally, the solubility of REE oxides in a melt increases with an increase in temperature and partial pressure of oxygen, as well as in the case of REE with large ionic radii and at high copper oxide content.109, 114 Tendencies of changes in the slope of the liquidus near Tp observed on going to REE with large ionic radii and on increasing pO2 are similar.Practically, of most interest are the solubility of REE oxides and changes in the slopes of the liquidus near Tp, which make possible the prediction of the features of crystallisation in the systems under consideration. For instance, the content of yttrium in the melt at the peritectic decomposition temperature of the Y123-phase increases from 0.4 at.% at pO2=0.01 atm (at 985 8C) to 0.6 at.% at pO2=0.21 atm (at 1005 8C) and to 0.7 at.% in pure oxygen (at 1030 8C), whereas the neodymium content at Tp=1086 8C (at 0.21 atm O2) is several times higher (3 at.%). The content of neodymium in melts with high concentration of copper oxide can differ from that of yttrium by an order of magnitude (*7 at.% Nd at 1065 8C in air for the melt with a Cu : Ba ratio of 6 : 1).109 Analysis of the solubility curves makes it possible to evaluate several important parameters, viz., the dissolution enthalpy of the solid phase (it is determined from the slope of the liquidus),109, 113 ± 115 the peritectic decomposition temperature of the solid phase (it is determined from the bending of the solubility curve, since the dissolution enthalpies of 123- and 211-phases are different),113, 114 the Gibbs standard energy of a peritectic reaction (when determining the solubility limits at different pO2 ).111 The dependences of solubility of REE oxides on the `geo- metrical' stability of R123-phases, which is determined by the ionic radius of REE, are shown in Fig.12.8, 63, 81, 113, 114 As can be seen, the thermal stability of phases, characterised by the peritectic decomposition temperature, and the thermodynamic stability associated with the dissolution enthalpy of the solid phase in a melt increase as the ionic radius of REE increases. The same reason is also responsible for an increase in the ability of the melt to exist in the supersaturated state, since the smaller the slope ofHo YbTm 300 250 200 DH /kJ mol71 e /K71 at.% 12 Er YDy Lu 150~~ 100 50 Perovskite stability region 0.80 0 Figure 12.Dependences of thermal (Tp) and thermodynamic (DH) stability of RBa2Cu3Oz-phases on the `geometrical' stability of their crystal lattices (t) (at pO2=0.21 atm); Tp is the peritectic temperature; DH is the dissolution enthalpy of R123 in the BaO :CuO=3 : 5 melt; e is the slope of the liquidus near Tp , which characterises the tendency to supersaturation; t is the Goldschmidt tolerance factor which reflects the `geometrical' stability of the lattice. the liquidus near the peritectic decomposition temperature, the higher the supersaturation at a given supercooling of the melt and the larger the amount of the substance that can be solidified.As has been mentioned above, these changes are explained by stabilisation of a defect perovskite-like lattice of 123-phases upon introduction of REE ions with large ionic radii (except for praseodymium, which drops out of the common dependence). Establishment of the composition of the solid phase that is in equilibrium with a given melt is the key problem in the analysis of the liquid phase composition. Obviously � and this is of prime importance� knowledge of the liquid phase composition permits control of the structure and properties of the solid solution that directly depend on its composition. Actually, according to the Gibbs phase rule, the number of degrees of freedom for the `R1+xBa27xCu3Oz+L+O2' equilibrium is 473+2=3.Therefore, at a fixed temperature and partial pressure of oxygen, the solid phase composition is unambiguously determined by the melt composition. The dependences, which in combination with the equilibrium phase diagram can be used for predicting the composition of a solid solution with the maximum substitution degree, which can be crystallised from a melt of prescribed composition at a fixed temperature and partial pressure of oxygen, are shown in Fig. 13.78 ± 81 Distribution of the neodymium oxide between the solid solution and the melt is the most sensitive parameter of the equilibrium under consideration.80, 81 The maximum on the curve of the content of neodymium oxide in the melt (see Fig. 13) is due to the influence of two oppositely directed factors, viz., an increase in the neodymium content in the melt on an increase in the temperature and simultaneous decrease in the solubility limit of neodymium on the shift of the melt composition towards the BaO-rich region where a more thermally stable, less- substituted solid solution turns to equilibrium.In fact, the curve under consideration is a projection of the spatial trajectory of the point of ternary equilibrium `Nd123ss ± 201 ± L' on the `temper- ature ± composition' plane. The equilibrium melt compositions of the solid solution depleted of and enriched with neodymium are strongly different and are shifted towards the region of Cu-rich compositions (Cu : Ba=7 : 6^5 : 3 for the 123-phase and Cu :Ba=16 : 3 for the 213-phase).Quantitative analysis of the effects of the liquidus curvature near the `123-phase ± melt' equilibrium showed that the substitution x^0 is achieved in the Yu D Tretyakov, E A Goodilin La Pr Nd Sm Nd123ss+201+L Nd (at.%); Cu/Ba, 1+x Gd Eu Tp /K 1 2 5 1400 1360 4 1320 3 1280 Nd213 1240 3 2 0.67 1.00 1200 0.86 0.96 0.25 0.57 0.70 Nd123 1 t 0.84 0.82 Nd123ss+422+L 1020 1040 1060 1080 T /8C O Figure 13. Temperature dependences of the Cu : Ba ratio (1), NdO1.5 content (2) in melt L and the (1+x) parameter (3) of the solid phase of the Nd1+xBa27xCu3Oz solution coexisting in two-phase `Nd1+xBa27xCu3Oz +L' equilibrium (at p 2= 0.21 atm). Numbers on curve (3) are the values of the parameter x.case of crystallisation from melts enriched with barium oxide and on lowering the crystallisation temperature.79 c. Characteristic features of polythermal sections Technologically, the quasi-binary system `Ba3Cu5O8' ± Nd4Ba2Cu2O10 (Fig. 14 a) is the most important since the compo- T /8C a 422+L L S 1080 Nd123+L 422+Nd123ss 1000 Nd123+011+L 0.125Nd4Ba2Cu2O10 920 Nd123+011+001 0.125`3BaCuO2+2CuO' 40 Nd (at.%) 30 10 20 0 b T /8C 1080 S 422+L422+201+L Hypothetical x=0, Nd123 L 422+L 201+L Nd123ss+ +201+L Nd123ss+ +422+L 1000 S2 M S1 Nd123ss M0 201+001+L Nd123ss+011 201+001+Nd123ss 920 0.25`Nd2CuO4+CuO' 0.5BaCuO2 2.0 1.5 1.0 0.5 0 71.0 70.5 x in Nd1+xBa27xCu3Oz Figure 14. Most practically important quasi-binary polythermal sections of the Nd7Ba7Cu7O system (at pO2=0.21 atm).Chemical principles of preparation of metal-oxide superconductors O sitional point of the 123-phase belongs to this section.This can significantly simplify the consideration of transformations in this system. Investigations into phase relations in the vicinity of the 123-phase showed that the region of two-phase `Nd123- phase ± liquid' equilibrium in air lies in the temperature range 970 ± 1090 8C (at p 2=0.21 atm).110 ± 113 The region of solid solutions based on the 123-phase belongs to another important BaCuO27`Nd2CuO47CuO' quasi-binary section (Fig. 14 b).63, 76, 78 Invariant transformations involving the liquid phase (MS1*1000 8C, M0S2, 990 8C) characterise the highest probability of synthesising the solutions with the mini- mum(S1) and maximum (S2) substitution degrees.However, if the reaction with the 213-phase is fairly well studied,78, 81 it is difficult to carry out quantitative studies of equilibria with the 123-phase because of the very large slope of the solidus in the vicinity of the (S1) point.75, 79 It should be noted that the appearance of a liquid in this section with the increase in the temperature near the 123- phase is explained by the melt field propagation towards the BaCuO2 phase. According to the reported data,78 ± 81 the increase in temperature shifts the composition of the melt which is in equilibrium with 123- and 422-phases away from the point corresponding to barium cuprate because of the extension of the two-phase `422+L' field.The maximum peritectic decomposi- tion temperature of the 123-phase (S, *1085 8C) is achieved for those compositions that somewhat deviate from the ideal 123- phase (partial substitution of Nd for Ba). This fact is mistakenly ignored in some communications. O The composition of a solid solution with the maximum peritectic decomposition temperature Tp returns almost com- pletely to the 123-composition only after appreciable reduction of the partial pressure of oxygen.75 For instance, the composition of the solid solution with the maximum Tp is characterised by x^0.1 in oxygen, x=0.05 in air and x^0.0 at p 2=1073 atm. No solid solution based on the 123-phase is present in the system above the maximum temperature of peritectic decomposition (the `422+L' field).Below this temperature, the solid solution region is `gripped' from both sidy the `422+Nd123ss+L' three- phase field. At 1060 8C, the `422 ± Nd123ss ± L' tie-triangle becomes geometrically impossible for compositions with x > 0.6 81 and a decrease in temperature leads to establishment of equilibrium between the most neodymium-rich solid solution, neodymium cuprate and the melt. The slope of the solidus changes correspondingly.80 As has been mentioned above, below 990 8C the solid solution is in equilibrium with other two solid phases (CuO and Nd2CuO4). Thus, this type of systems is characterised by the following qualitative features.1. The region of the solid solutions based on high-temperature superconducting phases can be rather extended. The problems of determining the composition of solutions with the lowest sub- stitution degree and of the existence of solutions with Ba2+ substitution for R3+ in R7Ba7Cu7O systems (see Refs 7, 63, 76, 117) are poorly studied. 2. The decomposition temperature of a solid solution can vary over a wide range depending on its composition. This is probably associated with the complicated character of the cation ordering within the homogeneity region. The shape of the solidus near the maximum Tp (a sharp or flattened maximum) can provide information on the type of the solid solution, daltonide or berthollide that is formed.results in a change in the O2 3. As a rule, the reduction of p shape of the homogeneity region of the solid solution. In particular, the decomposition temperature of the solid solution and the slope of the solidus are changed and a narrowing or (more rarely) broadening of the homogeneity region is observed. 4. The compositions of equilibrium phases in the supersolidus region are different, which depends on the composition of the solid solution based on the high-temperature superconductor. Particular emphasis should be placed on quantitative determina- tion of the equilibrium tie-lines in the `solid solution ± melt' two- 13 phase region. Each tie-line is unique; therefore, the choice of a particular compositional point on the phase diagram (all other conditions being constant) determines not only the possible type and weight ratio of the coexisting phases, as in the case of the Y7Ba7Cu7O system, but also the chemical compositions of these phases and, hence, the physical properties of the crystallising solid solution. Determination of the actual chemical compositions of the coexisting phases is also necessary to avoid the errors when constructing the phase diagrams. 5.As a rule, polythermal quasi-binary sections of high- temperature superconducting systems are characterised by cer- tain temperatures of invariant transformation (in this case, at about 1000 8C, see Fig. 14). Below this temperature, liquid phases disappear and the equilibrium between solid phases is established.6. The narrowing of the homogeneity region can occur below the above-mentioned temperatures. In other words, solid solu- tions with the highest (lowest) substitution degrees appear to be less stable at both higher and lower temperatures. Analysis of this temperature region is closely related to studies on the decom- position dynamics of supersaturated solid solutions, the forma- tion of a particular nanostructure and to changes in physical characteristics of the system. 3. Melt crystallisation mechanisms a. Single-crystal growth Peritectic decomposition is a salient feature of all oxide super- conductors, which is due to their multicomponent composition and structural complexity. Crystallisation involves two different phases (the crystal and the mother liquid/melt) separated by a mesophase layer, which is a thin layer of the melt immediately adjacent to the solid phase.8 The components are transferred from one phase (the `mother' or nutrient phase) to the other phase (crystal) as this interface moves.In turn, the rate of the process can be limited by the kinetics of deposition of the substance on the crystal face, diffusion of the substance from the surrounding phase as well as by dissipation of the latent heat of crystallisation. Crystallisation cannot proceed in the absence of the driving force (supersaturation, supercooling or metastability of homoge- neous `mother' phase). In the case of crystallisation from the solution or melt, the driving force is often represented as the relative supersaturation of the melt s=c1 ¡ c , c1 where c1 is the equilibrium (under given conditions) concentration in the melt of the component transferred to the solid phase and c is its running value. The region of supersaturated solution is divided into the region of metastability and the labile region.The interface between these regions corresponds to maximum supersaturation in the system that can be achieved until spontaneous crystallisa- tion begins.8 For the Y123-phase, this region is rather narrow (0.8 8C in air with respect to supercooling).118 Investigations of the primary crystallisation field (PCF) are important when considering single-crystal growth. Unfortu- nately, the PCF boundaries have not been reliably determined yet even for the Y123-phase, though it is obvious that the PCF does not include the melt of the 1 : 2 : 3 stoichiometric composi- tion.In different studies, the ratios of barium and copper oxides (Ba : Cu) for PCF were estimated at 17 : 83 ± 44 : 56, 24 : 76 ± 46 : 54, 18 : 82 ± 46 : 54 and 24 : 76 ± 42 : 58, whereas the yttrium content (at.% Y) varied in the range 3.0 ± 5.0, 1.0 ± 2.0, 2.0 ± 4.0 and 0.5 ± 3.5, respectively.13 Crystallisation of the Nd1+xBa27xCu3Oz solid solution with x>0.7 from the Ba :Cu=3 : 12 melt has been reported.109 The composition of the solid solution changed during isothermal annealing. Artifacts in the determination of the PCF (especially using thermal analysis) often arise because of the absence of local equilibrium with the gas phase; therefore, data on PCF should be interpreted carefully.In some instances, the results of determination of the position of the14 liquidus surface should be used instead. The most important problem is the search for chemical additives that extend the PCF and allow the single-crystal growth over wider temperature and concentration ranges. Most often, BaF2, Bi2 O3, B2O3, Ag and BaCl2 are used as such additives.13, 119 ± 121 If a crystal face contacts the metastable `mother' phase, it can grow. Several main growth mechanisms exist under conditions of fast diffusion of the components of the crystallising substance and the absence of problems with dissipation of the heat of crystal- lisation.8 Depending on the driving force of crystallisation, continuous growth (face deposition), `island' growth due to two- dimensional nucleation and spiral growth along the screw dis- locations can be observed.The last-named mechanism is the most probable for ordinary solution/melt supersaturations; the first two mechanisms are observed very rarely because of the high activation energy of the processes.8 The `Terrace-Ledge-Kink' (TLK) growth of the crystal due to the `sliding' of the crystallising layers of the moving terraces over the surface is also characteristic of high-temperature superconducting materials,8 whereas renucleation on the defects of the `two-dimensional angle' type (the so-called `Twin-Plane- Re-entrant-Edge' or TPRE growth) occurs much more rarely and was detected in high-temperature superconducting systems only recently (Fig. 15).81 In real systems, the common events are (i) crystal face formation as a result of complex interaction of several crystallisation centres, (ii) combination of several growth mecha- nisms, (iii) growth due to the formation of macroscopic spirals and a Centre of a spiral z /nm 400 y /nm 200 20 b 10 0 10 20 x /nm 0 Figure 15.The mechanism of crystal growth in high-temperature super- conducting systems (the Nd1.85Ba1.15Cu3Oz-phase) by renucleation on defects of the `dihedral angle' type (the so-called `Twin-Plane-Re-entrant- Edge' or TPRE growth);81 (a) general view of the growing crystal face: twins are crossed by micro- scopic cracks arisen after thermal shock on detaching the crystal from the melt surface; the crystallisation front of the `tail' of growth spirals that form a visually observed macroscopic spiral moving along the twinning direction at a higher velocity; (b) the structure of twins according to atomic force microscopy data; the twin boundaries are indicated by arrows.Yu D Tretyakov, E A Goodilin terraces, etc. Currently, the morphology of growth of the spirals of R123-phases and its dependence on the crystallographic indices of the crystal face,122, 123 the degree of supersaturation, temperature and the nature of REE are well studied.124 ± 126 b. Evolution of a polycrystalline system The behaviour of the peritectic melt } obeys general regularities of crystallisation of an individual crystallite from the homogeneous melt.At the same time, analysis of this system is rather compli- cated and should be performed with consideration of the inter- actions between the melt and both the properitectic and gas phases that coexist at the peritectic decomposition point of HTSC. Additionally, the fact that crystallisation of a peritectic melt results usually in the formation of a polycrystalline product should be taken into account. When polycrystalline HTSC are prepared using melt crystal- lisation techniques, their microstructure is formed under non- equilibrium dynamic conditions. Hence the problem arises of the search for methods of reproducible preparation of materials comprising well-shaped crystallites with an optimum size and optimum mutual orientation and sufficiently strong intercrystal- lite contacts.From this point of view, the most interesting is to analyse the regularities of the formation and evolution of an ensemble of crystallites at both small and large deviations of the system from the equilibrium state. Three types of crystallisation models of REE-containing cuprates are known, in which the interaction in the 211-phase ± melt subsystem is considered. These are phenomenological models of the interaction between the particles of secondary phases and the melt, computer models of this interaction and mathematical models of propagation of the crystallisation front of the high-temperature superconducting phase and changes in its morphology.The models of REE- containing melt crystallisation with inclusion of interactions in the `gas phase ± melt' subsystem are much less developed. Superconducting phases are mainly formed owing to dissolu- tion in the melt of `secondary' phase particles, followed by homogeneous formation of the high-temperature superconduct- ing phase in the melt.8 ± 11, 127 ± 130 In the vicinity of the surface of the particles of the 211-phase, the melt is enriched with yttrium and the crystallites of the YBa2Cu3Oz phase are formed near the surface of these particles rather than at the `211-phase ± melt' interface. Crystallisation is accompanied not only by `pushing' large particles of the 211-phase by the growing faces of crystallites of the 123-phase, but also by their trapping by the bulk of the growing crystallites.The formation of crystallites of the YBa2Cu3Oz-phase is preceded by an induction period, after which crystallisation proceeds in the bulk at an abnormally high rate. Different melt crystallisation models are presented in Table 3. Mention should be made of considerable interest in the computer simulation of crystallisation processes. Different com- puter models and simulation concepts are used (e.g., `cellular automata' and diffusion-in-melt models and the phase field concept for multiphase systems, see S.7). It can be assumed that solidification of a peritectic melt can proceed in different ways depending on the parameters of the initial state of the melt, which results in the appearance of three different crystallisation regions characterised by particular final microstructure of polycrystalline materials.127, 128 The provision of stable crystallisation of the superconducting phase owing to stabilisation of the nucleation process is the most important condition for the preparation of high-temperature superconducting materials with high output characteristics.The experimental proof of this statement was obtained in studies of the doping of melts with REE oxides.128, 129 The melt } Hereafter a `peritectic melt' is meant a system consisting of a liquid and a solid (high-temperature, properitectic) formed upon peritectic decompo- sition of the HTSC.Chemical principles of preparation of metal-oxide superconductors Table 3.Crystallisation mechanisms of peritectic melts. Model Heterogeneous nucleation Dissolution in melt and homogeneous nucleation Edge effects, entrapment of particles of the 211-phase by the crystallisation front Formation of `gaps' and displacement of the melt to grain boundaries on trapping the particles of the 211-phase `Pushing-and-trapping' of particles of the 211-phase by the crystallisation front a Dispersion of particles of the 211-phase by the moving crystallisa- tion front a The mechanism is determined by particle size, the energy of surface interaction and by the crystallisation rate. analysed contained*80% of Cu(I), which was confirmed by the results of quenching experiments and chemical analysis,55, 129 by BaCu2O2 observation using X-ray phase analysis 13, 129, 131 and by in situ measurements of the EXAFS spectra of the melt.132, 133 Cooling of the melt results in the recovery of the initial oxygen content, which occurs more easily with an increase in the partial pressure of oxygen in the gas phase.In principle, the transformation Cu(I)?Cu(II) provides a possibility for crystallisation of a non-superconducting phase of barium cuprate BaCuO2 to occur. This is favoured by the presence in the melt of the components necessary for the crystallisation of BaCuO2 and by the fact that the temperature of its crystallisation in air 96 and the crystallisation temperature of the YBa2Cu3Oz- phase are close. In other words, a `bifurcation'-like situation occurs in the 211-phase ± melt system, which can be followed by crystallisation of either yttrium ± barium cuprate YBa2Cu3Oz or barium cuprate BaCuO2.127 ± 129 It is this phenomenon that is Morphology of particles of a 211-phase any isotropic (spheres) highly disperse particles large particles 7anisotropic particles (needles) Proposed scheme of the interaction L 211 L Y3+ ** * ** * *** 211 ** * ** * ** * ** * ****** ** * 123 Y 123 L** * 211 ** * ** * ** *** * L ** * ** * ** * ** * 211 Y3+ 211 123 * L ** repulsive force 211 phaseviscous-flow state surface tension force matrix 123 211 L observed experimentally.In these cases, the direction of crystal- lisation depends on many factors (which are often ignored) such as the oxidation rate of the melt during its cooling, the rate of dissolution of the Y2BaCuO5-phase in the melt, the partial pressure of oxygen in the system, the gradients of both the temperature and oxygen pressure at the gas phase ± melt interface and, finally, the prehistory of the sample and its size.Because of this, small deviations from the empirically established conditions for the synthesis of the YBa2Cu3Oz phase immediately result in undesirable formation of non-superconducting cuprate phases even in high-density specimens and in dramatic deterioration of properties of the high-temperature superconducting material prepared.Crystallisation of the 123-phase can probably be the major process in the system if nucleation begins at a temperature which is higher than the crystallisation temperature of BaCuO2. Super- conducting layered yttrium ± barium cuprate is the only phase 15 Ref. Result of interaction a c 6 c a 6, 8 c h a 123 Y h2 h1 9, 11 211 c 211 L L 123 9, 11 211 * Ba ±Cu ±O * * * * * * * Y3+ melt 211 8 r1211 r2 matrix a r1<r2 9, 11 123 21116 whose specific crystal structure can be selectively affected by the doping of the melt with REE ions. This makes it possible to experimentally distinguish between the melting temperatures of binary non-superconducting and ternary superconducting cup- rates, thus performing targeted crystallisation of the high-temper- ature superconducting phase from a multicomponent melt.128, 129, 134, 135 The general pattern of the process can be changed dramati- cally upon a decrease in the partial pressure of oxygen in the gas phase.Oxidation becomes the limiting stage of crystallisation in an oxygen-deficient atmosphere.129, 135 Copper (Cu2+) ions formed upon melt oxidation are bound by the excess R3+ and Ba2+ ions to form the (R,Y)Ba2Cu3Oz-phase; however, no rapid crystallisation of the nonequilibrium product (binary barium cuprate) is observed. Thus it can be assumed 55 that the formation of the phase and microstructure of the material are strongly affected by three main factors. These are (i) the total flux of yttrium (REE) ions jY from the surface of the `green-phase' particles to the melt, which is mainly determined by dispersity of the particles and by the surface energy of the interface; (ii) the oxygen flux jO to the crystallising melt, which depends on both the partial pressure of oxygen in the gas phase and geometric characteristics of the specimen and (iii) the heat flux jQ evolved from the specimen during its cooling, which is determined by the heat treatment regime.Thus, the optimum procedure for preparation of high-quality high-temperature superconducting materials should take into account simultaneous and mutually dependent changes in these most important parameters of the process. 4. Oxygen nonstoichiometry of high-temperature superconducting phases The oxidation stage is of particular importance when preparing, e.g., the RBa2Cu3Oz superconductors with a wide oxygen homo- geneity region (z=0 ± 1).1, 2, 8, 13, 17, 22, 50, 100, 108 For bismuth-, thallium- and mercury-containing high-temperature supercon- ducting phases, the homogeneity region is much nar- rower.17, 56, 67, 69, 101 On the contrary, several high-temperature superconducting materials [e.g., the `electron'-type (Nd, Ce)2CuO4+y HTSC] require annealing in a reductive atmos- phere.2 In all cases, there are strong reasons for detailed inves- tigation of the oxygen nonstoichiometry of HTSC.Here, it is well to bear in mind the following. 1. The oxygen content is correlated with the concentration of charge carriers, whose optimum value provides the maximum Tc value (a Tc `dome').`Underdoped' (`underoxidised') or `over- doped' (`overoxidised') phases can have a much lower Tc (Fig. 16). In the general case, it is the concentration of the charge carriers localised on particular elements of the structure that determines the physical phase diagram of HTSC (see Fig. 3).16 2. Changes in the oxygen content can result in significant changes in the phase stability and in structural phase transitions. For instance, ferroelastic RBa2Cu3Oz-phases undergo an `order ± disorder' second order phase transition which is accom- panied by a tetragonal-to-orthorhombic distortion of the crystal lattice and simultaneous formation of extended twins.8, 22, 93, 126 The changes in thermodynamic characteristics of high-temper- ature superconducting phases with different oxygen content can result in the appearance of the low-temperature stability thresh- old,50, 51, 75, 106 phase separation in such solid solutions and in nonuniform oxygen distribution over the superconducting matrix.8, 93 O 3.Investigations of oxygen diffusion in high-temperature superconducting phases is not only of practical, but also of theoretical importance since the diffusion coefficient is a `struc- turally sensitive' quantity, which depends on the concentration of oxygen vacancies (and, hence, on specific features of the equili- brium p 2 ± T± z-diagram), their ordering (the crystal lattice symmetry of the high-temperature superconducting phase and second order phase transitions, effects of various types of a Tc /K 80 1 2 400 0.2 0.1 0.3 `Hole' concentrationc Tc /K 120 1 2 3 80 4 3.86 3.84 Figure 16. Dependence of the transition temperature to the superconduct- ing state on the `hole' concentration (a), oxygen nonstoichiometry (b) and unit cell parameters which are determined by the oxygen content (c) for the most abundant types of HTSC ; (a): YBa2Cu3Oz-phases (1) and (La,Sr)2CuO4+y (2); (b): Bi2Sr2CaCu2O8+z (1) and Bi2Sr2Ca2Cu3O10+z (2); (c): the HgBa2Can71CunO2n+2+z homologous phases, Hg1223 (1), Hg1212 (2), Hg1234 (3), Hg1245 (4) and Hg1201 (5).substitution in the structure), the energy parameters of different oxygen sites (the activation energy and the probability of exchange between the oxygen positions, etc.).A rather high anisotropy of the diffusion coefficients along the main crystallo- graphic directions can indicate a layered structure of the HTSC (two-dimensional diffusion in CuOy planes in which the vacancies are concentrated, etc.). Relatively large absolute values of the diffusion coefficients of high-temperature superconducting phases compared to those of other systems 136 ± 139 indicate that they are highly defective. Studies of the diffusion mechanism give useful information on the different types of the oxygen bonding in the structure.140 Finally, parameters of the oxygen diffusion can essentially depend on the size of grains of the high-temperature superconducting phase, the quality of their surfaces, specific features of the interface between the superconducting and gas phases, the presence of internal microscopic strain, etc.Thus, analysis of these dependences makes it possible to obtain infor- mation on the real structure of high-temperature superconducting materials. The results of the diffusion studies can be used for determination of thermodynamic properties of high-temperature superconducting phases. O a. Phase diagrams of RBa2Cu3Oz±O2 systems and the oxygen ordering processes The problem of oxygen nonstoichiometry in RBa2Cu3Oz7O2 systems and related phenomena are extremely complica- ted.13, 100, 141 ± 148 Different types of diagrams used to represent the oxygen nonstoichiometry in high-temperature superconduct- ing phases of the Y7Ba7Cu7O system are shown in Fig.17. The p 27T7x7z-diagram presented in Fig. 18 illustrates the effect of cation ordering on the `behaviour' of the oxygen sublattice in the Nd1+xBa27xCu3Oz solid solution.89 Obviously, the temperature, total pressure and partial pres- sure of oxygen in the system affect significantly the oxygen content in the HTSC-phase, peculiarities of oxygen ordering and symme- try of the crystal lattice of this phase, the stability of various homologues of the high-temperature superconductors (123-, 247- Yu D Tretyakov, E A Goodilin b Tc /K 2 120 100 1 80 0 0.1 0.2 0.3 z 5 a /A 3.8817 Chemical principles of preparation of metal-oxide superconductors 1 700 1100 900 O2 z T /8C p/atm 2 6.8 124 1000 S O 6.6 34567 100 T 6.4 S 123 L 8 91011 6.2 10 6.85 247 0 74 6.40 1 72 log pO (atm) 2 9 13 11 7 T /K c 104/T /K71 800 T(P4/mmm) OI(Pmmm) 600 a b z=7.0 and 6.85.`Heavy' REE of the yttrium subgroup with smaller ionic radii form 123-phases for which (Tc)max<90 K, which means that the maximum value is shifted towards lower oxygen content. For instance, (Tc)max=87 K at z=6.90 for YbBa2Cu3Oz. On the contrary, HTSC containing `light' REE are characterised by higher (Tc)max values at higher oxygen content. Probably, the (Tc)max values of NdBa2Cu3Oz with z=7 and its analogue LaBa2Cu3Oz with z>7.0 reach 96 K7, 8, 43, 86 and 98 ± 100 K,86 respectively. High oxidation degrees of these phases are achieved by heat treatment at low temperatures (350 8C) and high pressure of oxygen (100 ± 900 atm).7, 8, 92 The effect of ionic radii of REE on Tc of the 123-phases is explained 17, 42, 43 using the model of charge redistribution between the superconducting CuO2 planes and dielectric CuOy blocks in the structure under conditions of internal chemical pressure (followed by compression or expansion of the structure) which results from changes in the geometrical size of cations occupying the sites of REE ions.Analogous dependences are also known for other cuprate super- conductors and the optimum oxygen content is different for different HTSC homologues (see Fig. 16 c). 400 AF is an antiferromagnetic SC is a superconductor OII AF 200 anti-OI AF SC AF SC 0 z 6.2 6.6 0:5, Dl= 128pgG ES The formation of twins owing to relaxation of internal strain which appears upon orthorhombic distortion of a tetragonal unit cell in the course of oxygen diffusion into the grains of super- conductor is clear evidence of the ordering of oxygen atoms in the unsubstituted 123-phases. The distance Dl between the twin boundaries in the twin ensemble varies in accordance with the formula:149 where g is the specific energy of the twin boundary, G is the size of twinned grain, E is the elastic modulus of the material and S is the coefficient of orthorhombicity.Figure 17. Types of representation of the diagrams of equilibrium oxygen content in YBa2Cu3Oz±O2 systems; (a): pO2 ±T ± z-diagram; T/ K: (1) 573; (2) 623; (3) 673; (4) 723; (5) 773; (6) 823; (7) 873; (8) 923; (9) 973; (10) 1073; and (11) 1173; (b) phase relations at a high pressure of oxygen, the z value varies from 6.40 to 6.85; (c) calculated low-temperature diagram of oxygen content in the Y123- phase and different types of ordering of the oxygen sublattice. Thus, one can expect that the largest twins will be formed in single crystals and that the size of these defects will to a great extent depend on the elasticity and `degree of orthorhombicity' of the material used.Twinning results in microscopic cracking of melt-solidified ceramics. Perfect specimens can be obtained using both the single-crystal detwinning and twin-free crystal growth methods.150, 151 or 124-phases) and that of the high-temperature superconducting phase against peritectic and/or solid-phase decomposition. The highest Tc values are achieved at the optimum oxygen content in the high-temperature superconducting phase.148 The (Tc)max values for YBa2Cu3Oz are 93.5K at z=6.94 and 87K at z Orthorhombic II 123-phase Orthorhombic I 213-phase 7.2 Orthorhombic II 213-phase 7.0 Ortho- rhombic I 123-phase 6.8 Tetragonal `336'-phase 6.6 b.Diffusion of cations and anions Oxygen diffusion in high-temperature superconducting phases has been studied using different methods such as dynamic 152 and isothermal 153 thermogravimetry, electrochemical potentiome- try,143 O18/O17 tracer diffusion method,154 ± 156 determination of internal friction coefficients,155 in situ measurements of electrical resistivity,157 direct observations of the twinning front propaga- tion in single crystals,158, 159 method of isotope exchange with the gas phase 160, 161 and computer simulation using the cellular automata method.147 The diversity of methods used and factors affecting the oxygen diffusion explains a great scatter (several orders of magnitude) of results obtained by different groups of researchers.Nevertheless, mention may be made of several important regularities. 1. One must discriminate between the coefficients of self- Oxygen ordering 6.4 T /8C in the 213-phase Nd/Ba in-plane ordering Tetragonal 123-phase 400 Disordering of cationic sublattice (`336') diffusion Dself and the coefficients of chemical diffusion of oxygen Dchem in high-temperature superconducting phases related by a simple formula 157 x 600 0.8 Dchem=DselfF, 0.6 0.4 800 0.2 Oxygen disordering 1000 0 Nd/Ba vertical ordering in the 123-phase O Figure 18.Section of the pO2 ±T ± x ± z-diagram of the Nd1+xBa27x. .Cu3Oz±O2 system at p 2= 0.21 atm, which illustrates the effect of the character of cation ordering on the equilibrium oxygen content in this solid solution. where F is the so-called thermodynamic factor. Usually, the F value is nearly constant if the oxygen content z in the R123-phases lies in the range from 6 to 6.8; however, it increases rapidly as z approaches the value corresponding to the most oxidised state (z=7).As a rule, the Dchem value is 4 ± 5 orders of magnitude higher than that of Dself. 2. For polycrystalline materials based on the Y123-phase, the Dchem and Dself values vary in the ranges 1075± 10711 and 1079 ± 10713 cm2 s71, respectively. The activation energies of18 self-diffusion in the R123-phases decrease as the ionic radii of REE increase.143 3. For single crystals, the coefficient of self-diffusion of oxygen along the c axis at 400 8C (Dc=10716 ± 10717 cm2 s71) is six orders of magnitude lower than for polycrystalline samples (Dpoly), whereas the rate of diffusion along the b axis at 300 8C in detwinned crystals (Db=2610712 cm2 s71) is at least two orders of magnitude higher than that of diffusion along the a axis (Da=5610714 cm2 s71).154 Thus, self-diffusion in single crys- tals is highly anisotropic and proceeds almost completely in the ab plane [Dab=Dpoly=(104 ± 106)Dc , Db=100Da , Db=10Dab].At the same time, the coefficient of chemical diffusion for these types of single crystals lies in the range 1078 ± 1077 cm2 s71. 4. Second order phase transitions, occurring on ordering or disordering of the oxygen sublattice, must result in jumpwise changes in the diffusion coefficients. This is confirmed by the results obtained in the studies of diffusion coefficients of RBa2Cu3Oz (R=Y and Nd) phases over a wide range of temper- ature and partial pressure of oxygen.143 According to the reported data, the rate of oxygen diffusion in the orthorhombic phase is higher than that in the tetragonal phase despite the fact that the number of available oxygen vacancies in the latter is larger than in the former.Probably, this indicates changes in the elementary mechanism of the process. The role of mobility of the cationic sublattice is at least as important as, and in some instances even more important than, that of oxygen diffusion; however, it has been much less studied.136 ± 140 Migration of cations in single crystals of oxide superconductors follows ordinary diffusion mechanisms (vacancy-type, interstitial, dissociative, along dislocations and point defects); in polycrystalline HTSC, diffusion along the grain and pore surface, twin boundaries, etc., is additionally observed.140 Self-diffusion of Cu2+ in the YBa2Cu3Oz phase was studied by the tracer diffusion method.137 ± 139 The activation energy of this highly anisotropic process lies in the range 165 ± 255 kJ mol71 and the diffusion coefficient at 780 ± 900 8C lies within the limits *5610712 ± 10713 cm2 s71.The value of the diffusion coeffi- cient of copper at 600 8C found by extrapolation is estimated at *6 ± 9610714 cm2 s71. The bulkier Ba2+ ion has a lower coefficient of self-diffusion in the same temperature range (*10714± 5610715 cm2 s71).137 ± 139 The coefficient of self- diffusion of Y3+ ion in the Y123-phase varies from 10714 to a Nd123ss Nd422 100 mm Figure 19. Single crystals of HTSC; (a), general view of needle-shaped Nd1+xBa2-xCu3Oz `stalactitic' crystals grown under isothermal conditions by the `pellet-in-pellet' method with addition of a small amount of BaF2 (see S.8); (b) the largest (by the date of publication of this review) `pyramidal' NdBa2Cu3Oz HTSC-crystal grown by the modified Czochralski method and a polished experimental substrate for thin film deposition, obtained by sawing a `bulk' YBa2Cu3Oz crystal grown by using this method (published by courtesy of Professor Y Shiohara, SRL, ISTEC, Japan);114 (c) general view of plate-like mercury-containing HTSC- crystals obtained by ampoule synthesis using the contact pair method (the only Hg-based crystals with peak effect known to date, see S.9); a pronounced peak effect observed on the `critical current vs.external magnetic field' dependence is assumed to be due to double (Hg/Pb and Ba/Sr) substitution (published by courtesy of S R Lee,M V Lomonosov Moscow State University, Russian Federation). Yu D Tretyakov, E A Goodilin 10716 cm2 s71 in the temperature interval 780 ± 980 8C (the activation energy is 280 kJ mol71).137 ± 139 Therefore, self-diffusion of oxygen in high-temperature super- conducting phases proceeds at a rate which is 1 ± 3 orders of magnitude higher than that of self-diffusion of cations, especially in those regions where low-temperature oxidation of supercon- ductors occurs. This is first of all due to the differences in the atomic weights of the elements, the charge of their ions and structural features of high-temperature superconducting phases containing oxygen-deficient perovskite-like blocks.Taking into account different values of diffusion coefficients, the cationic framework can be considered virtually immobile when studying various low- temperature processes with participation of high-temperature superconducting phases (see also Section III.1.b and S.3). The final stage of preparation ofHTSC can be optimised using the data on the equilibrium oxygen content and the parameters of its diffusion in high-temperature superconducting phases (see above). In combination with specific features of phase relations described above, this opens additional opportunities of targeted design of high-temperature superconducting materials.IV. Development of advanced methods for the synthesis of high-temperature superconducting materials Preparation of high-temperature superconducting materials with desired characteristics is the most important problem that became topical immediately after the discovery of this class of complex cuprates. However, only a few types of high-temperature super- conducting materials such as highly homogeneous powders (used as intermediate products for the preparation of ceramics, coat- ings, silver-sheathed tapes), thin films and hetero-structures, large-grain ceramics and long-length composites can be used in such areas as power engineering, electronics and medicine. A general view of the main classes of materials based on HTSC- phases is shown in Figs 19 ± 21 (see S.8 ± S.10) 1.Chemical homogenisation and preparation of high-quality powders Early studies of high-temperature superconducting materials were carried out using the so-called ceramic method, which includes thorough mechanical mixing of oxides (in some instances, c b (Hg,Pb)(Sr,Ba)1223 Sr14Cu24O41 Pb,Cu-rich MgO (Hg,Pb)1223 150 mm Nd123 1 cmChemical principles of preparation of metal-oxide superconductors 1 2 Jc 4 3 1 Bi : Sr : Ca :Cu=2.1 : 2 : 1 : 1.95 10 mm Figure 20. A SEM graph of a cross section of a long-length HTSC- composite (a silver-sheathed tape fabricated by the `powder-in-tube' method) with optimised composition of the superconducting core; (1) silver sheath; (2) phase of the Bi2Sr2CaCu2O8 type; (3) inclusions of particles of Ca7Sr bismuthates; (4) inclusions of particles of Ca7Sr cuprates; Jc denotes the direction of current in the superconducting wire (published by courtesy of Professor E Hellstrom, ASC, UWM, USA).54 HoBa2Cu3O7 (100) La0.35Pr0.35Ca0.3MnO3 (100)5 nm Figure 21.A cross section of HoBa2Cu3Oz/(La, Pr, Ca)MnO3 epitaxial film grown on the LaAlO3 substrate (the 001 orientation) in the region of the HTSC± manganite interface according to high-resolution transmis- sion electron microscopy data (see S.10). The smooth atomic layers at the interface indicate chemical compatibility of HTSC cuprate and manganite with giant magnetoresistance effect (published by courtesy of Professor A R Kaul, M V Lomonosov Moscow State University, Russian Feder- ation).alkaline-earth metal oxides and carbonates) and repeated `calci- nation ± grinding' (beat-and-heat) cycles needed for the solid- phase reaction between the reagents to proceed.2±5, 14 This conventional method for the preparation of many types of structural and functional ceramics has essential drawbacks, the most important of which consists in long-term heat treatment because of rather large grain size and mixing inhomogeneity of the reagents (see S.11). Often, the growth of crystallites is uncontrol- lable, which results in both chemical and granulometric inhomo- geneity. In combination with anisotropy of crystallites constituting the HTSC, this results in irreproducibility of their electrical and magnetic properties.Almost all oxide superconduc- tors known to date are complex multicomponent chemical compounds. Distinctions in properties of the elements constitut- ing high-temperature superconducting phases due to their differ- ent positions in the Periodic Table preclude development of a unified procedure for the preparation of HTSC by the ceramic method. Publications concerning the development and application of the so-called chemical methods for the preparation of HTSC powder have been known since 1987.162 ± 172 These methods allow improvement of the product homogeneity owing to the achieve- ment of a nearly molecular level of component mixing in solution and its retention to a greater or lesser extent in subsequent stages of the synthesis.As a rule, the oxide powders thus obtained are 19 characterised by a rather large specific surface and, as a conse- quence, readily enter into solid-phase reactions and are active in sintering. Additionally, the efficiency of chemical methods of synthesis is also manifested in increased chemical homogeneity of ceramics.14 It is also appropriate to use chemical methods along with the most widely used melt solidification methods for the preparation of ceramics despite marked levelling of distinctions in the morphology of powders with different prehistory, because of complete or partial melting. The establishment of the nature of pinning centres requires materials with strictly specified content and distribution of impurities.The easiest way of doing this is to use chemical methods. Mention has been made 9, 129, 173 of the possibility of controlling the size of crystallites of the secondary phases through the so-called effect of topochemical memory observed when varying the chemical prehistory of speci- mens.39, 40 Of particular importance is preparation of high- quality, highly disperse `soft' powders for fabrication of long- length HTSC± metal composites (e.g., silver-sheathed tapes) by the `powder-in-tube' method.16, 23, 54, 55 The size of internal cross sections of such composites varies from several to several tens micrometres. Therefore, `coarse' and inhomogeneous powders (with broad distribution of the particle size) prepared by conven- tional ceramic method are unusable in this case.The coprecipitation methods are widely used for the prepara- tion of various types of ceramics. Therefore, it is not surprising that they were among the first chemical methods for the synthesis of HTSC powder.2, 29, 162 In some instances (in correctly designed experiments) it has been possible to prepare homogeneous disperse mixtures of salts with the predetermined cation ratio and to reproduce the results obtained. In an ideal case, the conditions under which precipitation of cations from a solution occurs simultaneously and at equal rates are optimum. Most of the methods used include precipitation of carbon-containing salts (oxalates and carbonates) the thermolysis of which is completed at 900 ± 950 8C.Unfortunately, the presence of carbon-containing salts should be considered as a drawback of this type of method because of the possibility of the formation of oxycarbonate phases.36, 100, 136 The citrate method 2, 171 is the most widely used of the different versions of the sol-gel methods. Closely related to this method is the method of polymerised complexes, which has been successfully developed in a number of research laboratories worldwide.163 In this case, the ability of a-hydroxy acids (e.g., citric acid) to form chelate complexes with metal ions is used. When heated to 100 ± 140 8C, such complexes react with polyfunctional alcohols (e.g., ethylene glycol) to give low-molecular-weight oligomers (esterification).Subsequent heating to 180 ± 200 8C results in their polymerisation and the formation of a viscous resin (gel) with uniformly distributed metal atoms. The gel can be decom- posed to give the oxide powder. The sol-gel method is rather simple and inexpensive since almost no complex equipment is needed (the method requires no centrifuging, filtering, washing, drying, etc.) and readily available nitrates are most often used as initial substances. The viscosity of the gel obtained can be controlled by varying the component ratio and the duration and temperature of polymerisation. Therefore, the sol-gel method is used for the preparation of not only powders, but also thick films, fibres and planar ceramics. Currently, the spray drying and spray pyrolysis techniques have received the widest acceptance among chemical methods for preparation of HTSC powders.162, 172 The latter method includes sonication of a mixture of salt solutions to obtain a mist with a droplet size of 0.5 ± 0.8 mm, the transfer by a carrier gas into a hot chamber where instantaneous (complete or partial) decomposi- tion of the droplets occurs.The oxide-salt product that is formed is collected on a filter at the outlet of the gas flow from the decomposition zone. The molecular level of mixing of components (most often, nitrate solutions) and nearly instantaneous dehydration and20 decomposition of microscopic spray droplets make it possible to prepare a homogeneous product. Repeated grinding and calcina- tion characteristic of the ceramic technology and responsible for contamination of the product and uncontrollable grain growth are excluded in this case.However, the powders obtained can be contaminated with the material of the drying chamber (at high temperatures, in the presence of free acid). Additionally, the spray pyrolysis technique requires careful purification of large volumes of the carrier gas (oxygen) from CO2 to avoid the formation of barium carbonate. The method of rapid expansion of supercritical solutions (RESS)169, 170 at elevated temperature and pressure is based on the phenomenon of an anomalous increase in the solubility of inorganic compounds in water (or in other solvents such as ammonia, carbon dioxide, xenon, etc.) above its critical temper- ature (in autoclave).When such solutions are expanded in a chamber at a reduced pressure and temperature, the solubilities of the solutes decrease substantially and these are isolated as ultradisperse particles (often, as an X-ray amorphous phase or metastable crystalline modifications). Oxide materials can be best prepared using aqueous supercritical solutions.169 The drawbacks of most chemical methods used for the synthesis of HTSC powders can be to a great extent compensated using cryochemical technology.162, 165 ± 168 This method consists in the preparation of highly disperse and highly homogeneous salt (and the oxide) precursors by fast freezing of finely disperse salt solutions (preparation of cryogranulates) followed by removal of water by sublimation. Experimental conditions should exclude physicochemical processes resulting in deterioration of chemical and granulometric homogeneity of the product, e.g., separation of sprayed microscopic drops into solvent-rich and solvent-depleted regions due to insufficiently high cooling rate, partial melting of cryogranulates and, hence, segregation of the components during the freeze-drying or on subsequent heat treatment of the freeze- dried product, etc.Therefore, the highest homogeneity of the product is often achieved by (i) spraying the solution over a massive metal plate cooled by liquid nitrogen,168 (ii) replacing (if possible) nitrate solutions by acetate 168, 174 or nitrate ± nitrite solutions and (iii) drying in a thin layer with slow heating to 125 8C in an argon stream 14, 168 and decomposing the dried salt product in a furnace preheated to the desired temperature.2 The use of highly disperse (tens of nanometres) and highly homogeneous precursors prepared by the cryochemical method makes it possible to accelerate substantially the phase formation and to prepare high-temperature superconducting phases that can hardly be synthesised by other methods. For instance, the Bi2223- phase was obtained 168 after annealing for 12 ± 16 h only (cf. 200 ± 300 h required when using the ceramic technology) at 750 8C and the Y124-phase was synthesised 168 after annealing in air at 815 8C (120 h). In the latter case, high-pressure stages (pO up to 100 atm) usually required to synthesise this phase were excluded.Considerable achievements have been made in the preparation of different homologues of mercury-containing HTSC by the ampoule method using cryochemical technology and other methods of chemical homogenisation.175 2. Thin film preparation The known technologies of thin film preparation can formally be divided into physical and chemical ones.2, 24, 176 The former comprise the most widely used methods of pulse laser deposition and magnetron sputtering of films including the transfer of the material of the dense and chemically homogeneous target in the form of microscopic clusters knocked out by a high-energy beam from the target onto a substrate. These methods make it possible to prepare high-quality thin films with record physical character- istics and to carry out a layer-by-layer synthesis of novel types of structures (structural design) by `assembling' the film literally at the atomic plane level.Yu D Tretyakov, E A Goodilin However, expensive physical methods are almost inapplicable for the preparation of large specimens. This is achieved by using such chemical methods as liquid phase epitaxy (LPE) 8, 112, 120 and chemical vapour deposition (CVD).24 However, despite the advantages of precipitation from a solution in a melt by LPE, this has not acquired wide acceptance to date and has been developed by only a few research groups.120 Among accepted methods for the preparation of thin high-temperature super- conducting films, deposition on single-crystal substrates of thermally decomposed products of highly volatile organometallic precursors (Metal-Organic Chemical Vapor Deposition or MOCVD) is the most interesting. Currently, the efforts of researchers from about 40 laborato- ries 14 including those of the biggest electronic companies are focused on the development and improvement of CVD technol- ogy for the preparation of HTSC. This fact, as well as information on large-scale applications of the CVD method for the prepara- tion of diverse epitaxial semiconducting films and oxide coatings suggest that this method can become one of the main methods for the preparation of high-temperature superconducting films in the future. The MOCVD method 2, 24 includes transfer of the metal constituents of films to the reactor as vapours of volatile organo- metallic compounds, mixing of the vapours with a gaseous oxidant and decomposition of the vapours in a hot-wall reactor or on a heated substrate followed by the formation of a HTSC film.Metal b-diketonates are most often used as volatile compounds (pre- cursors). Novel volatile organometallic compounds 2, 24, 177 ± 179 made it possible to improve substantially film characteristics and to extend the potentialities of the MOCVD method. TheMOCVDmethod makes it possible to prepare thin HTSC films with characteristics that are at least as good as those of films prepared by physical methods. The obvious advantages of this method include first of all its versatility as regards the composition of the materials obtained, the possibility of single- and double- sided deposition of films with homogeneous composition and the same thickness on substrates with complex shapes and large surface areas (including continuous film deposition on a long- length metallic substrate, e.g., a tape).176 Unlike physical methods of sputtering, in which high performance of equipment is achieved due to the high energy of the particle beam of the sputtered substance and is connected with the risk of disturbing the morphology and stoichiometry of the film thus formed, the performance of the MOCVD method is achieved by increasing the vapour pressure of the volatile component and/or by increas- ing the flow rate of the carrier gas.Other advantages of the MOCVD method are the possibility of achieving higher deposi- tion rates (up to several millimetres per hour) with retention of high quality of the film, the use of a flow-through setup operating at pressures of 1073 ± 1 atm instead of high-vacuum equipment, its simplicity and low cost compared to those required when using physical methods and, finally, `flexibility' of the process in the adjustment stage (primarily due to the smooth change in the vapour phase composition). A common problem of all methods of thin film preparation including the MOCVDmethod is `symbiotic' choice of substrates. The substrates should be highly chemically inert (to prevent contamination of high-temperature superconducting phase with alien components), cheap and readily available.Additionally, the substrate material must possess specific physical properties such as (i) rather small (< 2%) mismatch between the crystal lattice parameters and those of the deposited film to provide conditions for the epitaxial growth, (ii) close values of the thermal expansion coefficients (TEC) of the substrate and HTSC phase to avoid the formation of microscopic cracks in the film due to compression and, especially, tension caused by temperature variations; (iii) the absence of twinning-type phase transitions that can appreciably deteriorate the film morphology and (iv) low dielectric constant and dielectric loss tangent to provide the possibility of using the films in microelectronics and in microwave devices.Chemical principles of preparation of metal-oxide superconductors Unfortunately, almost none of the known substrates meets all the above-mentioned requirements (Fig.22). Usually, SrTiO3, NdGaO3 and LaAlO3 are used as substrates.179 Recently, large Y123 and Nd123 single crystals were used for homoepitaxy of the R123-phases of high-temperature superconducting films.180 How- ever, these single-crystal substrates are superconductors rather than dielectrics. Intrinsic in these substrates is the tetragonal-to- orthorhombic phase transition (tetra ± ortho-transition) accom- panied by the formation of a twin structure. Non-superconducting tetragonal solid solutions of the Pr1+xBa27xCu3Oz type, in which no twinning occurs, seem to be more promising.8 Dielectric Nd1.85Ba1.15Cu3Oz single crystals are characterised by a high degree of orthorhombicity, the absence of tetra ± ortho-transi- tion, close TEC values, high lattice parameter matching with high-temperature superconducting films of the R123-phases and a narrow range of oxygen nonstoichiometry.For these reasons, they are also promising for use as substrates.80, 81, 181 The second, chemical problem arising when using the MOCVO method is the control of the cation and anion stoi- chiometry of the films. The chemical vapour deposition is an incongruent process and depends on several factors such as the temperature, total pressure, partial pressures of oxygen, carbon dioxide and water (the oxidation products of the organic compo- nent of compounds used), their flow rates and distribution of the flows in the reactor and above the substrate, total composition and homogeneity of mixing of volatile components in the gas phase, etc. The most promising way of controlling the vapour composition is to carry out instantaneous evaporation of the mixture of volatile components from one source.This is achieved by using an aerosol obtained from the solution of organometallic compounds in organic solvent (e.g., diglyme) or by pulse evapo- ration of microscopic portions of a mixture of organometallic compounds fed using a belt-conveyer feeder.24 CO In the first case, the solvent vapour has a strong effect on the film deposition, namely, the thermal effect of solvent oxidation: a decrease in the partial pressure of oxygen and an increase in the partial pressure of carbon dioxide and water are observed.Using the results of thermodynamic analysis, it is possible to minimise the influence of these effects and to optimise the conditions for film deposition in the reactor with cold walls and induction heating of the substrate.2, 24, 179 At p 2 <1075 atm and T<800 8C, no barriers to the formation of the YBa2Cu3Oz- Thermal cracking Expansion Compression TEC (%) Disturbance of optimum growth conditions Ag 60 SrLaAlO4 40 MgO Nd123 20 Pr123 LaAlO3 LaGaO3 0 Nd1.85Ba1.15Cu3Oz Y123 NdGaO3 YSZ YAlO3 SrTiO3 NdAlO3 720 Al2O3 CaNdAlO4 SrLaAlO4 KTaO3 740 Si 760 6 12 18 24 Parameter mismatch with YBa2Cu3O6.9 (%) Figure 22.Properties of materials used as substrates for the deposition of high-temperature superconducting thin films. Cooling 21 phase are observed. Using an aerosol source, it is possible to attain high deposition rates and reproducibility of the composition and morphology of the R123- and Bi2212-films.177 The second method consists in placing small droplets of an organic solution containing volatile metal complexes in a prede- termined ratio on a fibre glass transport tape. The vapour formed on pulse heating of the tape in a vacuum chamber moves to the substrate. The amount of substances evaporated per pulse as well as the vapour composition can easily be varied from pulse to pulse.This method appeared to be very useful in the preparation of multilayered film structures of complex chemical composi- tion.182, 183 The third problem that arises only when using the MOCVD method consists in the necessity of a targeted search for highly volatile substances with reproducible volatility. The most serious problems are associated with barium transfer through the vapour phase. Because of coordinative unsaturation of Ba2+ ions and high ionicity of the barium ± oxygen bond, barium complexes are oligomerised in both the solid and vapour phases. Stabilisation of barium dipivaloylmethanate is achieved by using the adduct of dipivaloylmethanate with phenanthroline, which saturates the coordination sphere of barium, thus resulting in nearly quantita- tive transition of this complex to the vapour phase.2, 24 The fourth (technological) problem is associated with the necessity of designing the optimum film morphology.It was shown in the studies of the effect of various factors on the orientation of films that the introduction of an excess of bismuth and copper compounds in the course of growth of high-temper- ature superconducting Bi2212-films favours the formation of c-oriented films, whereas the introduction of an excess of an alkaline-earth metal results in a-oriented films.177 This is associ- ated with the fact that the presence of the excess of bismuth and copper results in the formation of an equilibrium liquid phase or, at least, in a shift of the compositional point towards the regions where a melt should be present.This increases the mobility of the film components and facilitates the formation of thermodynami- cally more favourable c-oriented films, whereas a-oriented films are formed under kinetically controlled conditions. This is confirmed by the LPE experiments.120 Generally, the morphol- ogy (polycrystallinity, planarity, film orientation, the presence of microscopic inclusions of non-superconducting phases, micro- scopic cracks, etc.) depends on numerous process factors that should be optimised thoroughly. The most topical problems associated with the development of the MOCVD technology and requiring practical solution are connected with deposition of RBa2Cu3Oz-films on bicrystalline substrates (SrTiO3, sapphire) and fabrication of devices based on the Josephson effect (logistors, SQUID magnetometers, etc.), preparation of high-temperature superconducting films using substrates with a large surface area (up to 70 mm in diameter), in situ double-sided deposition of films and attainment of high superconduction characteristics of thin high-temperature super- conducting films using conventional substrates (R-sapphire, Si) and high-quality CeO2 buffer layer.184 Recently, R123-films (R is the `light' RE element), the effects of stabilisation of metastable phases (see S.11) and an increase in the critical currents in solid solutions have attracted the attention of researchers.Yet another topical problem is the preparation of thick films with high current-carrying ability deposited on flexible tapes made of nickel metal and its alloys (the so-called `Rolling- Assistant Biaxially Textured Substrates' or RABiTS) preliminar- ily coated with a buffer layer.176 The CVD method is certain to play a key role in solving this problem.3. Large-grain ceramics and single crystals a. Characteristic features of materials based on large-grain ceramics Real structure. As a rule, the idea of designing any material is based on a concept of a real structure with several hierarchical levels.185, 186 The basic crystal structure determines the fundamen-22 tal properties of HTSC and is responsible for their structural organisation at the microscopic level. Individual crystallites, which are always imperfect and consist usually of smaller subcrystallites (mosaic blocks, coherent scattering domains) separated by extended defects, form an intermediate (meso- scopic) level.Ensembles of crystallites (grains, granules) and pores form the macroscopic level of structural organisation of HTSC. The current concepts on complicated structure of melt- processed specimens of superconducting ceramics are illustrated in Fig. 23.9, 11, 55, 187 ± 191 Ensembles of large pseudo-single-crystal domains (their size reachs 0.5 ± 5 cm depending on the precipitation conditions) separated by large-angle boundaries are the main microstructural motifs of large-grain ceramics. Each domain is a stack of thin (5 ± 50 mm) RBa2Cu3Oz-plates or lamellae rather than a true single crystal.The lamellae are characterised by the aspect ratio of *1000. The plates are oriented parallel to one another and separated by low-angle boundaries, which makes them `trans- parent' to the critical current. It should be noted that the real structure of melt-processed HTSC is characterised by the presence of various extended defects such as twin boundaries, ultradisperse inclusions of non-superconducting phases, different types of microscopic and macroscopic cracks, appeared due to low plasticity of the RBa2Cu3Oz-phase, and increased concentration of dislocations. In principle, all levels of structural organisation of HTSC should be taken into account. However, only the compo- sition, structure of boundaries and nature of various defects have been studied in detail,187 ± 191 though such parameters as porosity and density of ceramics as well as the size of crystallites and pores should also be considered.The real structure of melt-processed HTSC can be considered as a large-domain system with pronounced `collective' super- conducting properties resulting in very high integral supercon- ductivity characteristics due to the specific crystallisation mechanism of the peritectic melt.9, 11 For instance, an increase in the total density of specimens as a result of melt solidification eliminates the percolation problems characteristic of the current flow in the specimens obtained by solid-phase synthesis.`Pseudo- single-crystalline domains' are united to form macroscopic aggre- gates that (potentially) can carry heavy critical currents. Finally, a large number of structural defects (inclusions, dislocations, low- angle boundaries) favours the appearance of new pinning centres.6 Unfortunately, only the `lamellar' level of the structure of melt- processed materials is formed spontaneously and is characterised by high superconducting parameters. The remaining hierarchical levels of the structure can be formed only by using special techniques. Methods of generation of new pinning centres. Generation of efficient pinning centres of the magnetic flux, which make it possible to increase substantially the critical current density Jc inside the grains, is the most important stage of the preparation of materials with a high repulsion force.The methods of solving this problem can arbitrarily be divided into physical and chemical ones. Currently, the highest Jc values have been obtained using samples of superconducting ceramics irradiated with high-energy particles. Irradiation of samples with neutrons or heavy ions to produce cluster-type radiation defects (tracks) of size 20 ± 100 nm was found to be the most efficient.192, 193 In this case, the Jc value increases in proportion to the irradiation dose. However, this type of treatment encounters considerable technological difficulties. Additionally, it has such drawbacks as residual activity of materials and the possibility of radiation-induced degradation of samples if the radiation doses exceed an optimum.More promising are chemical methods which make it possible to introduce non-superconducting highly disperse inclusions in the matrix of the 123-phase.194 ± 203 Particles of the Y2BaCuO5- phase, an intermediate crystallisation product, are most often used as inclusions.194 ± 197 The results obtained in the studies of pinning 6, 9, 11 suggest that changes in Jc depend on the content and Yu D Tretyakov, E A Goodilin Pores Lamellae Cracks Twins 1 mm Macroscopic Domains 1 cm Dislocations Ceramics Mesoscopic Inclusions 1 mm Microscopic 1 nm Unit cell Nanoscale compositional fluctuations Atomic rows Figure 23. Macroscopic (1073± 1072 m), mesoscopic (1076± 1073 m) and microscopic (10710 ±1076 m) levels of real structure of large-grain ceramics.dispersity of the Y2BaCuO5-phase. It is noteworthy that particles of this phase are not the pinning centres themselves and that the major contribution is made by the 123-phase ± 211-phase interface and the interfacial defects. Introduction of zirconates, titanates, stannates, nickel metal, uranium-235 oxide (its radioactive decay leads to `internal' irradiation of the superconducting matrix) also results in generation of new pinning centres (see Fig. 4). Types of intercrystallite boundaries. Generation of efficient pinning centres plays an important role in the preparation of high- temperature superconducting materials characterised by high values of the transport critical currents.However, in this case the perfection of intercrystallite boundaries and their `transparency' to the critical current are the most important points. As has been mentioned above, the oxide superconductors are characterised by abnormally low coherence lengths (x). Because of this, two types of links are formed between the crystallites, viz., strong links (ordinary intergrain phase contacts similar to intercrystallite `necks') and weak links which, in the general case, are the breaks of the phase continuity. The extension of weak links is comparable with x values. The reasons for the phase discontinuity can be the formation of both the domains with local changes in stoichiom- etry and amorphous domains on intercrystallite boundaries, or the appearance of microscopic cracks, as well as high crystallo- graphic anisotropy of HTSC and spatial disordering of crystal- lites.204 ± 206 In accordance with the formation mechanism of the 123- phase, the ensemble of growing crystallites formed in the early stages of melt crystallisation consists of c-oriented nuclei with disordered orentations in the ab plane.Growth of particles of suchChemical principles of preparation of metal-oxide superconductors an ensemble and their coalescence result in the formation of large- angle boundaries that are parallel to the c axis and the tilt boundaries that are perpendicular to the c axis. The best trans- port characteristics will be obtained if the current will flow through both types of boundaries without a considerable loss. Based on these grounds, a `brick wall' model of the transport current flow through high-temperature superconducting materi- als was proposed.207 According to this model, the tilt boundaries enabling the maximum surface area of intercrystallite contacts play an important role in the formation of high transport current.However, the `brick wall' model does not consider the current flow through large-angle boundaries along the ab plane. Accord- ing to the `railway switch' model,208 there are both large-angle intergrain boundaries and a rather large relative amount of intercrystallite links `transparent' to the current flow, which favours low current loss. These links responsible for the current flow through intercrystallite boundaries and the current flow through the tilt boundaries necessary for obtaining the maximum surface area of intercrystallite contacts form a three-dimensional superconducting network.Such a mechanism is most character- istic of bismuth-containing HTSC. Probably, it can also be postulated for yttrium-containing polycrystalline superconduc- tors.Thus, the density of the critical current that flows through intercrystallite boundaries is mainly determined by mutual spatial orientation of their constituent crystallites. Nevertheless, it was assumed 209 that in the general case the Jc values are only dependent on the surface area of the `strongly linked' areas of intercrystallite boundary.Obviously, the probability for two oriented (textured) crystallites to be `strongly linked' is much higher than for disordered grains. However, it was reported 209 that high Jc values can also be retained in the latter (unfavourable) case. The results of recent studies 176, 210 indicate the presence of high-angle-but-low-energy (HABLE) boundaries, thus confirm- ing this conclusion. This means that probability factors come into play when considering the problem of a critical transport current in bulk specimens.9, 55, 128, 129 b. Methods of preparation Large-grain ceramics cannot be a chemically and structurally homogeneous material. It is a composite whose practically important superconducting properties are superior to those of Regime(grad T ) dT/dt (grad T ) dT/dt O dp 2 /dt dT/dL dcR3á /dL Figure 24.Melt solidification methods of preparation of large-grain ceramics. Conditions Phase R123 ceramics O p 2=const MTG (S Jin, 1987) m-MTG(LPP) (K Salama, 1989) Ag BaZrO3 pO2 <1 Pt/CeO2 PDMG (N Ogawa, 1991) T=const IMC (Y Idemoto, 1990) pO2=const ZM (P McGinn, 1988) LPRP (D Willer, 1992) pO2=const T=const CGMG (M Morita, 1990) Nonequlibrium state of a precursor + seed crystal MIA (M Lees, 1992) TSMG (K Sawano, 1991) SDS, Bridgman (K Salama, 1994) VGF (M Ulrich, 1993) CRT GEORGE (B Soylu, 1995) (G Schmitz, 1997) 23 highly homogeneous specimens and single crystals. Therefore, the development of melt solidification processes revolutionised the production technology of high-temperature superconducting ceramics since it opened a new way to the practical use of superconducting ceramic materials.Minimisation of the inclusion size of secondary phases. The modern classification of melt solidification procedures for prepa- ration of HTSC and the most important details including the chemical type of the starting precursor, mechanical prehistory and the heat treatment regime are presented in Fig. 24. Some trends in the development of melt solidification methods can be traced, which made possible the preparation of materials with high superconducting characteristics. These are:7±11 (1) a step-by-step change in the regime of high-temperature treatment in order to increase the degree of nonequilibrium in all stages of the process; (2) the use of methods of synthesis resulting in more homoge- neous and disperse starting oxide powders; (3) the use of precursors in different initial states and the introduction of various additives.The first trend is associated mainly with attempts to shorten the duration of treatment of samples at maximum temperatures. This made it possible to prevent the rapid growth of particles of secondary phases at temperatures above the peritectic temper- ature, which was probably a reason for passing from the classical `Melt-Textured-Growth' (MTG) method to its modified version, viz., `Liquid Phase Processing' (LPP). The `Quenched-Melt-Growth' (QMG) method involves a chemically induced increase in the degree of nonequilibrium of the system.In this case, the 211-phase is formed in the region of thermodynamic stability as a result of a fast `downward' inter- action of Y2O3 with the melt rather than due to relatively slow `upward' decomposition of YBa2Cu3Oz. Experiments on super- fast cooling of droplets of a high-temperatureY2O3+L melt in an evacuated vertical metallic tube 211 are probably one of the most successful implementations of this method. TheQMGmethod has an important alternative, the doping of melts with, e.g., platinum 212 (the `Platinum-Doped-Melt- Growth' or PDMG technique) and cerium dioxide.213 In this case, the formation and decomposition of not-too-stable Pt- containing complex oxides (Ba4CuPt2O9, R2Ba2CuPt2O8, R2Ba3Cu2PtO10, etc.) not only affects the nucleation processes, 211+BaCuIIO 100+BaCuIO (quenching) QMG (M Murakami, 1989) OCMG (S Yoo, 1994) MPMG (H Fujimoto, 1990) SLMG (D Shi, 1993) CUSP (A Endo, 1996) QDR (V Selvamanickam, 1994) PMP (L Zhou, 1991)24 but also retards the growth of individual faces of crystallites of the 211-phase.This results in a change in their shapes and sizes and prevents coalescence of the crystallites into larger aggregates.213 Several methods such as the `Melt-Powder-Melt-Growth' (MPMG) and `Powder-Melt-Process' (PMP) involve additional grinding of both the starting reagents and intermediate products in order to increase their dispersity and homogeneity of mix- ing.6, 214Asuccessful attempt has been performed at using a highly homogeneous mixture of barium cuprate and copper oxide with yttrium oxide, which simulates the phase composition of the specimens prepared by the QMG method (the so-called `Solid- Liquid-Melt-Growth', or SLMG method).197 Yet another, basi- cally new modification of this method is the directional recrystal- lisation of amorphised quenched melt at temperatures *100 8C below the peritectic decomposition temperature of the 123-phase (the so-called `Quench-and-Directional-Recrystallisation' or QDR method).This allows a rather fast (over 3 ± 5 min) forma- tion of the 123-phase and ultradisperse `green' phase and prepa- ration of highly textured superconducting ceramics using conventional zone melting technique with reduced hot zone temperature.215 Apositive effect of the above-mentioned innovations becomes more understandable when considering numerous experimental data indicating that the particle size of the 211-phase depends on prehistory of the system, despite the extreme conditions for the synthesis.9, 173, 216, 217 This is probably due to the possibility of intermediate formation of a superheated metastable (congruent) melt of the 123-phase.Numerous structural defects (grain boun- daries mostly) accumulated mainly in the fine crystalline material facilitate strongly the decomposition of the congruent melt into a melt and the 211-phase. However, the excess `green' phase can inhibit the growth of faces of the 123-phase on sintering, thus resulting in a more fine-grained structure.217 The addition of Pt or CeO2 changes the surface energy at the 211-phase ± melt interface and results in the formation of the 211-phase with another morphology (needle-shaped particles).In accordance with the mechanism considered above, this facilitates dispersion of aniso- tropic particles of the 211-phase by the moving crystallisation front and, finally, the formation of more finely disperse inclusions of the 211-phase in the 123-phase matrix (see Table 3). The possibility of controlling the composition of composites by extracting the excess of BaCuO2 from stoichiometric specimens of the YBa2Cu3Oz-phase using porous Y2BaCuO5-substrates (the so-called `Liquid-Phase-Removal-Process' or LPRP method) was also reported.200 Thus, the analysis of the published data 9, 11, 55 shows that an increase in the dispersity and uniformity of distribution of the particles of secondary phases is simultaneously one of the major lines of modification of the `melt solidification processes' and a criterion for evolution of these methods. The 211-phase has a complex effect on the microstructural and functional character- istics of specimens.This phase is responsible for completeness of crystallisation, strength of materials, morphology of the super- conductor grains and the appearance of new pinning centres. Eventually, controllable modification and variation of the above- mentioned processes and parameters results in an essential improvement of functional characteristics of the materials obtained.The role of gas atmosphere. As has been mentioned above, gas exchange with the environment plays an essential role in the preparation of high-temperature superconducting materials. By varying the partial pressure of oxygen it is possible to solve several important problems. First is the problem of reduction of the crystallisation temperature and provision of compatibility of the melt with low-melting substrates. The second problem is associ- ated with modification of procedures for creation of supersatura- tion by smooth variation of the partial pressure of oxygen, which can favour both controllability of the process and a decrease in the amount of impurities in the final product.Third, prevention of bubbling and deformation of melt-processed material. Finally, the Yu D Tretyakov, E A Goodilin fourth problem is the control of both the width of the homoge- neity region and the cation ordering in the solid solutions based on the 123-phase. O The effect of partial pressure of oxygen on the processes of preparation of high-temperature superconducting materials has been little studied.13, 218 ± 222 A decrease in the temperature of peritectic melting of the 123-phase at reduced oxygen content and the formation of low-melting eutectics 220 containing Cu(I) (at 770 ± 800 8C) was observed.218, 219 This made it possible to lower the growth temperature of single crystals of the 123-phase (down to*910 8C at p 2=56102± 26104 Pa) 220 and to prepare thick films on a silver substrate [Tm(Ag)&960 8C] as well as rather dense bulky polycrystalline specimens.13, 221, 222 An original syn- thetic method consisting of isothermal crystallisation accompa- nied by a slow increase in the partial pressure of oxygen has been reported.218, 220 In this case, the process begins at a low partial pressure of oxygen and is completed in pure oxygen (the so-called `Isothermal Melt Crystallisation' of IMC method).Obviously, the density of superconducting materials should be maximum. However, the specimens prepared by melt solidifica- tion processes change their linear dimensions and shapes during oxygen exchange,9, 11, 55, 129 which results in an increase in their volumes and in the formation of a well-developed pore system which can hardly be removed.This problem is usually considered as applied to bismuth-containing high-temperature superconduct- ing silver-sheathed tapes.23 Unfortunately, almost no studies on this effect in high-temperature superconducting R7Ba7Cu7O materials have been reported, though it also plays a negative role, especially in the fabrication of materials with high critical trans- port currents. Experiments have shown that the main factors affecting the oxygen exchange between a sample and a gas phase are the phase composition, chemical homogeneity, relative con- tent of Cu(I) in the solid phase, partial pressure of oxygen and the procedure used for compacting the sample.9, 129 Currently, the 123-phases containing `light' RE metals (La, Nd, Sm and Eu) are often synthesised using the single-crystal growth and preparation of melt-processed ceramics in an inert atmosphere.This is first of all due to the necessity of decreasing the degree of REE substitution for Ba in solid solutions in order to increase the transition temperature to the superconducting state.7, 8 Formation of single-domain structure. Taking into account the grain boundary models considered above, it can be assumed that the most favourable way of fabricating the HTSC capable of carrying strong critical transport currents is to prepare textured materials.223 ± 231 Attempts at preparing superconducting ceramics with oriented (plate- or rod-like) structure were based on conventional texturing procedures such as slow cooling in a uniform temperature field (grad T=0), slow cooling in a temper- ature gradient field (grad T>0) without moving a specimen (versions of the Bridgman method: `Vertical Gradient Freezing' or VGF, `Seeded Directional Solidification' or SDS), a gradient crystallisation with a moving hot zone (the `Zone Melting' or ZM method) and the use of seed crystals (the `Top-Seeded-Melt- Growth' or TSMG method).Non-traditional methods, e.g., crystallisation along the concentration gradient (the so-called `Constitutional-Gradient-Melt-Growth' (CGMG) and `Geomet- rically-Organised-Growth-Evaluation' (GEORGE) processes) are also used. Introduction of artificial nucleation centres of the 123-phase is an efficient method for controlling the nucleation.Such centres are formed by introducing relatively large seed crystals of REE- containing analogues of the 123-phase with higher peritectic decomposition temperature.223 Usually, Sm123 (Tp&1050 8C) and Nd123 (Tp&1085 8C) phases are used as seed crystals. As a rule, the seed crystals are placed on top of a dense substrate (a pellet or a rod). Then the melt ± crystallisation cycle is performed using the temperature gradient or movement of the hot zone along the sample. The seed crystal with higher melting temperature initiates the growth of the desired phase along the direction ofChemical principles of preparation of metal-oxide superconductors propagation of the crystallisation front, which results in the formation of giant pseudo-single-crystal domains whose dimen- sions are comparable with those of the sample.7, 8, 224 ± 226 It is noteworthy that the orientation of pseudo-single crystals thus formed virtually coincides with that of the seed crystal, therefore the growth direction of the 123-phase is readily control- lable.This is probably due to the epitaxial character of the process on the faces of the seed crystal, the unit cell parameters of which are close to those of the desired 123-phase. Often, the observed formation of differently oriented domains indicates complexity of the actual growth mechanism.8 It was proposed to coat the `non- working' surface of the seed-crystallising specimens with com- pounds that form the Yb123-phase with a lower peritectic decomposition temperature, thus preventing crystallisation on the lateral surface of the pellet.225 Single crystals of magnesium oxide and strontium titanate were also proposed as seed crystals; however, no epitaxy was observed because of the chemical reaction between the melt and seed crystals and the formation of a buffer layer of the reaction products, so the efficiency of using such seed crystals decreases drastically.6, 7 If the seed crystals are introduced at the beginning of melt crystallisation in the course of its cooling, it is possible to obtain large-grain specimens of high quality.The `Constant-Undercool- ing-Solidification-Processing' (CUSP) method,8, 226 consisting in isothermal crystallisation of the 123-phase after introduction of a seed crystal into the hot zone, is a modification of the above- mentioned approach.Unfortunately, it is difficult to introduce a large number of oriented seed crystals anisotropically distributed in the bulk of the sample, which is necessary for the corresponding texturing of a high-temperature superconducting material (the `Composite Reaction Texturing' or CRT method).227 Only a few studies concerning the introduction of multiple seed crystals either into the bulk or on the surface of specimens have been reported so far.228 Magnetically induced alignment (MIA) 229, 230 is performed using REE atoms with large magnetic moments (Gd, Dy and Ho).The degree of magnetically induced texturing of the specimens prepared even by ordinary sintering increases in proportion to the applied magnetic field. Combination of this approach with crystallisation from the melt increases its efficiency, which reaches an optimum value in magnetic fields higher than 1 T.230 Mention should be made of the original CGMG method in which crystallisation is driven by the gradient of the REE concentration in the melt, resulting from varying the concentra- tion of REE (e.g., Yb and Y) with different peritectic decom- position temperatures.231 It is believed that this method can serve as a basis for the development of a process of preparation of long- length textured materials including tapes sheathed with highly oriented 123-phase.228 Chemical modification and generation of efficient pinning centres.The `Oxygen-Controlled-Melt-Growth' (OCMG) method is a new method for the preparation of high-temperature superconducting materials at reduced partial pressure of oxygen (0.1 mol.%± 1 mol.% O2).7, 232 It is based on the ability of REE with the largest ionic radii (in particular, Nd, Sm, Eu and Gd) to form solid solutions of theR1+xBa27xCu3Oz type. Crystallisation from the melt at reduced partial pressure of oxygen results in a considerable decrease in the degree of barium substitution and in appreciable increase in the transition temperature to the super- conducting state (up to 95 ± 96 K).7, 8, 76, 86, 88 This is also con- nected 88 with possible cation ordering in the crystal lattice, e.g., with the formation of pairs of neodymium ions in the barium positions, which results in the ordering of the oxygen sublattice.At the same time, domains with fluctuations of chemical compo- sition can appear in such a superconducting matrix. These domains are efficient pinning centres because of the strong suppression of superconductivity therein in nonzero magnetic fields, which results in the peak effect (see Fig. 4).7, 8 The advantage of the pinning centres thus generated over point defects consists in higher efficiency of modulations of 25 chemical composition of the structure at relatively high temper- atures (the liquid nitrogen temperature) corresponding to the operating mode of high-temperature superconducting materials.The formation of these types of pinning centres that were not observed for the Y123-phases is the major advantage of the OCMG method, which allows the preparation of superconduct- ing materials with record characteristics.7 Several reasons for the appearance of pinning centres are O2 discussed in different models.7, 8, 76, 232 ± 234 The first of them is connected with random fluctuations of the Nd: Ba ratio in the superconducting matrix due to local fluctuations of temperature, p and the melt composition during the growth of the pseudo- single-crystal domains (for the OCMG method). The next reason is the formation of a quasi-ordered nanostructure as a result of spinodal decomposition of the solid solution caused by post- crystallisation annealing.Third, it is the formation of clusters, oxygen vacancies and twin boundaries in the case of nonuniform oxygen distribution in the superconducting matrix. Next, the pinning centres can appear due to the formation of highly disperse inclusions of the R422-phases and redistribution of cations between the matrix and these non-superconducting inclusions. Finally, antistructural defects produced in the course of (probable) mutual exchange of Nd3+ and Ba2+ ions between the corresponding crystallographic positions can also result in the appearance of pinning centres (see S.12). The peak effect observed in all R123-phases including R=Y is associated with the presence of local oxygen-deficient regions characterised by lower Tc as compared to that of the entire matrix.233, 234 It is assumed 233 that this effect increases due to the presence of impurities (e.g., the components of the crucible) which reduce the oxygen mobility and favour its nonuniform distribu- tion.It was hypothesised 93 that nonuniformity of the oxygen distribution in the materials based on the Y123-phases can be due to the spinodal decomposition of the solution formed by the YBa2Cu3O6 and YBa2Cu3O7 phases. An alternative model 7, 8, 91, 92 relating the peak effect in the materials based on the R123-phases (R=Nd, Sm, Eu, Gd) to the fluctuations of the cation composition does not contradict the preceding model since it considers variations of the cation composition as a primary phenomenon responsible for a side effect, viz., nonuniform distribution of oxygen. The possibility was reported 7, 8, 92 of controlling the peak effect by different parameters of the process, such as the post-crystallisation anneal- ing temperature, oxidation degree of the specimens and introduc- tion of a `cocktail' of REE dopants.c. Methods of single-crystal growth Unlike the production technology of polycrystalline materials, single-crystal growth is focused on growth of chemically and structurally homogeneous crystals of a specified size, shape, composition and controllable low level of defects and impurities. Meeting these requirements allows the crystals to be used in basic or applied investigations, e.g., in structural analysis,17, 235, 236 spectroscopy,237, 238 studies of oxygen diffusion,154, 157atomic force microscopy,7, 8, 12 measurements of fundamental physical constants,19 etc.8, 239 Anisotropy of the rate of crystal growth along crystallo- graphic directions depends in a complex manner on the nature of REE and conditions of crystallisation.8 This can be due to the crystallographic anisotropy, different face energies and different mechanisms of the face growth.In turn, the growth anisotropy causes changes in the morphology and shape of the entire crystal. For instance, the crystals grown by spontaneous crystallisation are usually thin plates oriented perpendicular to the h001i direction,126, 240, 241 since the growth rate of the {100} faces is approximately five times higher than that of the {001} faces at high cooling rates (kinetic control).If the rate of melt cooling decreases down to 0.5 ± 1.0 K h71, thick prisms-parallelepipeds or even isometric crystals are formed.241 The {101}, {011} and {111} faces grow under these conditions along with the {100},26 {010} and {001} faces, which indicates a quasi-equilibrium crystal growth (thermodynamic control).126 The method of top-seeded crystal pulling from supercooled melt is characterised by a rather low supersaturation.8, 114, 239, 242 Such factors as the melt hydrodynamics and the distribution of temperature and concentration near the growing single crystal begin playing a significant role in this case (see S.13).By controlling the pulling rate and taking into account the growth anisotropy it is possible to obtain various `bulk' single crystals, e.g., pyramidal crystals with expanded bases, large isometric crystals with a small angle of inclination of edges, pyramidal crystals with `concaved' bottom face and cylindrical crys- tals.8, 114, 239, 243 Other crystallisation methods allow the obtain- ing of needle-shaped and plate-like single crystals and crystals in the form of parallelepipeds as well.244 ± 249 Thus, modern crystal- lisation methods make it possible to grow the HTSC crystals of any desired shape. Crystallisation from melts is strongly affected by the nature of RE elements. In particular, introduction of `light' REE can cause a chain of interrelated `domino'-like changes in the characteristics of the R123-phases.The chain is as follows: an increase in the `geometrical' stability ? an increase in the thermal and thermo- dynamic stability ? an increase in both the peritectic decompo- sition temperature and dissolution enthalpy of the R123-phases in melts ? an increase in the solubility of REE and a decrease in the slope of the liquidus near the peritectic decomposition temper- ature, a decrease in the viscosity and acceleration of diffusion of the components in the melt. As a result, the crystallisation rate of the R123-phases containing `light' REE (especially, Nd and Sm) should be higher than that of the same phases containing `heavy' REE (Y, Yb, etc.) at a given supercooling degree of the melt, which favours the preparation of larger Nd123, Sm123, etc., crystals.114 The predicted regularities of the growth rate of 123- phases with different REE are observed experimentally, which can be seen from comparison of the growth rates along the direction of pulling the crystals grown by the modified Czochralski method:8, 114 Y123 air Nd123 air 0.24 Pr123 air 0.1 Y123 oxygen 0.075 ± 0.108 0.16 Phase Atmosphere Growth rate /mm h71 The growth rate of the crystals of a mixed (Y,R)123-phase was increased by doping the melt with samarium and neodymium. 114 Unfortunately, the advantages of using `light' REE to assist single-crystal growth and preparation of `melt-processed' ceramics are combined with problems in controlling the chemical composition of products due to cation nonstoichiometry of the 123-phases containing `light' RE elements.A simplified consid- eration of crystallisation in the frame of the Y2BaCuO5 ± Ba3Cu5O8 quasibinary system is possible only for the `point' Y123-phase (Fig. 25). In the case of systems whose phase diagrams contain regions where solid solutions can exist,8 the C compositional point moves from the melt region L or a 422 ± L (PrBaO3 ±L) two-phase region of the peritectic melt (at T=Tb), which contains particles of `secondary phases' at elevated temper- atures, to the R123ss ±L two-phase region where the composi- tions of equilibrium solid [c(ss)] and liquid [c(L)] phases are determined at Ts<Tb (Tb>Tp) by tie-lines.The ratio of the coefficients of cation diffusion in the melt plays a particularly important role in the establishment of an interrelation between the true compositions of the liquid and solid phases in the case of steady-state crystal growth. Obviously, the close values of these coefficients favour the choice of a tie-line that is the closest to the equilibrium tie-line. If the diffusion coefficient of a given cation is lower than those of other cations, the true tie- line is shifted with respect to the equilibrium tie-line.8 As a rule, the differences between the diffusion coefficients are not too large and the use of equilibrium phase diagrams makes it possible to correctly (quantitatively) predict the compositions of phases that crystallise under given conditions.Yu D Tretyakov, E A Goodilin R2BaCuO5 Different types of high- temperature phases PrBaO3 L Tb c(L)(Tb) L Tp RO1.5 C8 Composi- tional point 123 Ts c(ss) CuO Solid phase composition c(L)(Ts) `035' BaCuO2 BaO Figure 25. Supersolidus phase diagrams and single-crystal growth of HTSC. Analysis of the data presented in Table 4 indicates that the volume of single crystals of the R123-phases can be appreciably increased and their perfection can be essentially improved 250 using two modifications of the Czochralski method [the `Solute- Rich-Liquid-Crystal-Pulling' (SRL-CP) and `Top-Seeded-Solu- tion-Growth' (TSSG) procedures].8 This is a universal method since it allows one to grow large single crystals of almost all R123- phases (R=Y, Nd, Sm, Pr) and (R1R2)Ba2Cu3Oz (R1=Y, R2=Sm, Nd) solid solutions 8, 80, 81, 114, 251 as well as Nd1+xBa27xCu3Oz, Pr1+xBa27xCu3Oz, Nd1+xBa27xCu37y..GayOz, YBa2Cu3-yZnyOz phases, etc. According to the reported data,252 superconducting crystals of the Pr123-phase can be obtained by the `Top-Seeded-Floating-Zone' (TSFZ) procedure 8 at reduced partial pressure of oxygen (see S.14). Considerable attention has been paid recently on the develop- ment of growth methods of single-crystal high-temperature super- conducting whiskers.76, 244, 253 ± 256 The growth of whiskers in the systems with regions of solid solutions of the Bi ± Sr ± Ca ± Cu ±O 254 ± 256 and R± Ba ±Cu ±O (R=Sm,244 Nd76) types was reported.It is known that this type of crystals possesses unique electrophysical 253 and mechanical 257 properties. 4. Preparation of long-length composites Almost all the HTSCmaterials considered above (except for single crystals) are composites comprising the superconducting matrix and non-superconducting phases that determine specific proper- ties and areas of application of a particular composite. For instance, highly diperse powders prepared by the chemical homogenisation methods are usually a mixture of different phases and dopants. The components of a film ± substrate pair affect strongly each other. Large-grain ceramics are also a composite containing `secondary phases' as inclusions in the superconducting matrix.High-temperature superconducting silver-sheathed tapes are a typical example of industrial superconducting composites.16, 23, 54 To prevent thermoresistive instability of such tapes, they are fabricated as multifilament wires in which disappearance of superconductivity due to a local overheating above Tc will cause the current to flow round the resistive area through adjacent superconducting cores. Usually, superconducting wires are made of bismuth- or (more rarely) thallium-containing HTSC which possess higher plasticity and can be textured much easier than theTable 4. The progress in the preparation of single crystals of the RBa2Cu3Oz-phases.8, 114, 126, 239 ± 256 Single-crystal growth regime R flux Spontaneous crystallisation (flux growth) `YBa4Cu10Ox' Y *4 at.% Y, 30 at.% Ba, 66 at.% Cu Y: Ba :Cu=1 : 6 : 18 Y: Ba :Cu=1 : 18 : 45 64% Y123 ± 36%(7BaCuO2+11CuO) 0.13(1/6)YBa2Cu3Oy ± 0.87(0.28BaO+ +0.72CuO)+1 mass% BaF2 KCl : NaCl (1 : 1)+5 mass% AgNO3 1.5 mol.% Y123, 2CuO .BaCuO2 10 mol.% R123, flux of an eutectic Y, Pr composition flux of an eutectic composition Nd Floating zone melting Nd : Ba : Cu=1 : 4 : 6 ± 1 : 29 : 66 La, Nd, Sm Nd : Ba : Cu=1 : 7.6 : 21.5 Nd Top-seeded solution growth by crystal pulling (a modified Czochralski method) 0.15(1/6)YBa2Cu3Oy ± 0.85(0.3BaCuO2± Al2 O3 Y ± 0.7CuO) Y, Sm, 211+0 : 3 : 5, 5 : 36 : 59 Nd 211+0 : 3 : 5 Y 211+0 : 3 : 5, 5 : 36 : 59 (*0.84 ± 1.98) at.% Nd, Nd (28.49 ± 28.16) at.% Ba, (70.67 ± 69.86) at.% Cu Nd : Ba : Cu=0 : 0.78 : 1 a Values in parentheses refer to the height of the pyramidal crystal. pO /atm crucible 2 7 0.21 0.21 Al2O3 Al2O3 with 0.21 polished walls 0.21 ZrO2/Y2O3 0.21 Y2O3 0.21 Al2O3 0.21 Al2O3 5 .1073?0.2 ZrO2/Y2O3 BaZrO3, 0.21 density 98.5% 0.05 ZrO2/Y2O3 1072± 1074 77 0.001 0.21 Y2O3 0.21 Y2O3,1100 mm 0.21 1.0 Y2O3 0.21 SnO2 0.21 Nd2O3 vcool cooling /8 8C h71 C / 7 4 ± 15 1010 ± 960 0.5 1000 ± 970 0.1 ± 0.4 0.3 ± 0.8 990 ± 950 1050 ± 970 0.5 1000 ± 900 2 1030 ± 850 1for 50 h 910 1005 ± 950 0.7 1030 ± 950 7 7 7 361.561.5 7 7 3632 1 for 17 days 930 7 1000 1002 ± 997 77 1015 1030 ± 960 0.5 1058 ± 1070 for 67 h Tc /K Note Crystal size a mm6mm6mm 7 7 46460.5 5656 7 2 88 56461.5 90 56360.7 93.2 4.364.363.6 91 2061060.5 93 1261061 8666 7 2 90 56560.1 92 26260.5 91 46464 94.8 90 91 (single-crystal 94 grains) 94 86665 90 868 (5) 89 14614614 (angle 92 of inclination of faces <258) 19.8619.5 (16.5) 92.7 2261563 92 24624 (21) 95 Ref.249 126 251 temperature oscillations 7 246 241 temperature gradient 7 119 245 T=9108C=const 220 7 248 7 126 hot zone movement at 0.4 ± 1 mm h71 8 hot zone movement at 0.46 ± 0.50 mm h71 110 118 crucible rotation at 2 ± 8 rpm pulling rate 0.2 mm h71, seed crystal 8 rotation at 120 rpm pulling rate 0 ± 0.14 mm h71, 8 seed rotation at 120 rpm pulling rate 0.05 mm h71, 114 seed rotation at 70 ± 120 rpm, cooling in N2 251 crucible rotation at 15 rpm, doping by Ga pulling rate 0.1 ± 0.25 mm h71, 114 seed rotation at 70 ± 120 rpm28 Table 5.Chemical compatibility of phases and the superconduction characteristics of the composites based on Bi2Sr2CaCu2O8 -phase doped with non- superconducting oxides.264 ± 270 Phase in the 2212-matrix containing A Solubility/mol A/mol 2212 Element (A) composition in a melt in 2212 T /8C 1300 1300 1300 1300 900 <0.05 <0.05 <0.05 <0.1 0.05 <0.05 <0.05 <0.03 <0.05 <0.03 Zr Hf Sn Mg Al 900 0.8 0.15 Ga 900 <0.1 <0.05 In SrZrO3 SrHfO3 Sr17xCaxSnO3 (x&0.1) Mg17xCuxO (x&0.1) Sr1.7Ca1.3Al2O6 BiSr1.5Ca0.5Al2Oz Sr1.3Ca1.7Ga2O6 Bi0.2SrCa0.8Ga2Oz Bi1.7Sr2.3Ca0.6Cu1.6Ga0.4Oz Sr17xCaxIn2O4 (x&0.55) a DM(60 K, 10 mT)/DM(5 K, 1 T), where DM is the width of a hysteresis loop; this value for the undoped phase equals 0.01.materials based on the R123-phases.258 ± 263 Additionally, rather low melting temperatures of bismuth-containing HTSC provide a wider choice of metals and alloys for fabricating the sheath. Silver is one of the most widely used materials for the outer sheath. It is plastic and relatively available, has high electrical and heat conductivity, causes no decrease in the transition temper- ature to the superconducting state of HTSC and serves as a specific membrane for oxygen exchange between the supercon- ducting cores inside the tape and the gas atmosphere outside the tape.This system is closed as regards the mass transfer of bismuth, calcium, strontium and copper oxides. In addition, the walls of silver-sheathed tapes can favour nucleation and oriented grain growth near the surface areas of HTSC adjacent to the sheath,23, 54, 263 especially in the case of tapes with planar (ribbons) or pseudo-1D (multifilamentary wires) geometry. Often, the strength characteristics of the sheath are improved using the effect of dispersion hardening, e.g., due to the formation of the MgO microcrystals in the silver matrix upon oxidation of silver ± magnesium alloys by atmospheric oxygen. The essential feature required of superconducting silver- sheathed tapes is that they be capable of carrying heavy currents.This first of all requires the attainment of perfect mutual orientation of highly anisotropic superconducting crystallites and exclusion of weak links between them. The negative effect of `secondary phases' and gas evolution on peritectic melting or high- temperature annealing, which can cause a nonuniform current flow through the cross section of the tape, local violation of the optimum orientation of the HTSC grains and even shut off the orifice of the wire of size 1 ± 30 mm, should also be prevented. The optimum microstructure is usually formed by varying the composition, external gas atmosphere and thermal annealing regime. This requires a detailed knowledge of the phase dia- grams.53, 54, 69, 263 To attain high transport characteristics of high-temperature superconducting tapes, it is also appropriate to use the methods of chemical homogenisation for preparation of highly disperse chemically and granulometrically homogeneous carbon-free powders for filling the silver sheathes that should undergo a plastic deformation.The methods used in the fabrica- tion of superconducting silver-sheathed tapes should include (i) modern processes for the preparation of composites (`powder-in- tube', `tube-in-tube', `rod-in-tube' methods, etc.), (ii) thermome- chanical treatment (repeated drawing and rolling) resulting in elongation of the silver sheath filled with powder by a factor of several tens, in an appreciable increase in the density of the superconducting cores inside the tape and in forced orientation of the HTSC grains along the deformation axis and (iii) low- temperature vacuum annealing to optimise the oxygen content inside the tape and to prevent bubbling.Thus, it is now possible to fabricate tapes with a micro- structure close to the desired one using modern processes and Yu D Tretyakov, E A Goodilin Pinning a Tc /K (2212) state size /mm in 2212 0.092 70.074 0.021 0.026 between 2212-grains 7 in 2212, agglomerated 93 93 in 2212, agglomerated 90 92 in 2212, agglomerated 90 7 7 0.016 80 7 7 7 in 2212, agglomerated 82 0.27 7 0.2 1.0 2 ± 5 2 ±5 20 107 7 0.565 7 77 production methods. However, the fundamental problem associ- ated with a dramatic decrease in the critical currents of bismuth- containing HTSC (compared to other types of HTSC and first of all, to R123-phases) on raising the temperature or an increase in external magnetic field (see Fig.4) remains unsolved. Therefore, the operating temperatures of bismuth-containing tapes are lowered to the liquid hydrogen temperature (20 K), despite the fact that their Tc values (90 ± 110 K) are higher than the boiling point of a cheap coolant, liquid nitrogen (77 K), which is effectively used with other types of HTSC. The most promising way of increasing the critical current density for bismuth-containing high-temperature superconduct- ing materials is associated with the preparation of composites with uniformly distributed microscopic inclusions of non-supercon- ducting phases formed by the constituents of the system or by introducing `alien' phases compatible with the HTSC.264 ± 270 The main characteristics of the composites thus prepared (homologues of the 2212- and 2223-bismuth-containing HTSC) are presented in Table 5.As can be seen, the most pronounced positive effect is observed with zirconates and stannates, although there are many phases compatible with the high-temperature superconducting matrix which do not result in appreciable decrease in its Tc and do not suppress the HTSC formation dynamics. At the same time, measurements of physical properties indicate an increased stabil- ity of superconducting characteristics of the composites.270 Obviously, this promising investigation line will be developed in the future.V. Practical applications of high-temperature superconducting materials The possibility of practical applications of high-temperature superconducting materials has opened fresh opportunities for microelectronics, medicine, production of efficient energy stor- age and power transmission systems and industry as a whole.7, 8, 15, 16 The use of high-temperature superconducting films allowed production of small-scale systems belonging to a new generation of communication devices including electromagnetic screens, modulators, antennas, commutators, filters of microwave and pulse signals, multilayered film hetero-structures including dielec- tric, ferroelectric and normal metal layers in addition to the HTSC layers.Such films made it possible to develop bolometers operat- ing in the millimetre, submillimetre and infrared bands, basic circuit arrangements of superfast computers, ultrasensitive tomo- graphs and diagnostic devices capable of responding even to changes in the mental condition of a man (measuring devices based on the Josephson effect). High-temperature superconducting tapes with high super- conducting characteristics can actually be used in practice forChemical principles of preparation of metal-oxide superconductors production of super-power magnets and lines for nondissipative transmission of electrical power. Currently, silver-sheathed tapes can be manufactured commercially.Large-scale production and wide use of these tapes are still limited by high cost. Nevertheless, many small-scale systems and test lines are already at work. The prospects of using HTSC based on the 123-phase are associated with the possibility of production of bulky items of a rather simple shape using the most suitable and operationally convenient methods. Such items can be divided into two classes. The first of them comprises the specimens characterised by a high ability to shield the external magnetic field or to be repelled by the field. This ability can be characterised by the so-called levitation force, which is dependent on the intracrystallite critical current density. The other class comprises the specimens of high-tempera- ture superconducting materials with high transport (intercrystal- lite) current.In the future, such ceramic devices can find practical use in the production of permanent magnets with `frozen-in' magnetic flux, magnetic repulsion trains (the MAGLEV proj- ect), flying wheels, bearings rotating virtually without friction, efficient and economical motors, super-power generators and transformers, magnetic separators, superconducting relays, fast current limiters, powerful nondissipative electrical current leads, medical tomographs, powerful magnetic systems for thermonu- clear fusion and accelerators of elementary particles (new-gener- ation Tocamac) and, finally, magnetohydrodynamic generators. Preparation of substrates in thin film technology and micro- electronics seems to be the most realistic practical use of large single crystals of HTSC.Such substrates should be characterised by small mismatch between the unit cell parameters and those of materials of the films, have close TEC values and favour epitaxial growth of films. Additionally, all crystallochemical and thermo- mechanical parameters of high-temperature superconducting single-crystal substrates can be precisely adjusted by using solid solutions with different types of substitution of the yttrium and barium positions. Supplement S.1. The presence of a metastable orthorhombic BaCu3O4 phase [a=10.986(3), b=5.503(1), c=3.923(1)A] was found in thin films of the R123-phase.271 Stabilisation of this phase requires the formation of a coherent interface between the phase and sur- rounding high-temperature superconducting matrix.The BaCu3O4 phase decomposes if the film thickness exceeds *0.3 mm. S.2. Discrepancies in determination of the limits of oxygen nonstoichiometry in BaCuO2+z can be due to the formation of oxycarbonates [e.g., Ba44Cu48(CO3)6O87.9, see Aranda and Att- feld 272] or to an ambiguous assignment of the specimens with other cation compositions to this phase.273, 274 A reliable thermo- dynamic assessment of the Ba7Cu7O system was reported.273 The oxygen nonstoichiometry in the Ba2CuO3-, BaCuO2- and Ba2Cu3O5-phases was also discussed.274 ¡¾ 276 S.3. The low-temperature decomposition of NdBa2Cu3Oz was considered.277 It was established that degradation of supercon- ducting properties of high-quality polycrystalline specimens (Tc=94K) in the temperature range 450 ¡¾ 750 8C occurs non- linearly depending on the `ageing' temperature and duration of low-temperature isothermal annealing.Prolonged annealing at 400 ¡¾ 600 8C for 450 h in air can result only in a slight decrease in Tc after complete oxidation of the sample (Tc=89 K), whereas the annealing at 700 8C for 165 h brings about a decrease in Tc by 11 K. S.4. The first reliable determination of the structure of a low- temperature (oxidised) modification of the Nd1.9Ba1.1Cu3Oz- phase using the data of X-ray, electron and neutron diffraction experiments and IR and Raman spectroscopy was reported recently.278 29 Studies carried out by neutron diffraction, differential scan- ning calorimetry, Raman and MoE ssbauer spectroscopy and EXAFS have confirmed that, unlike the NdBa2Cu3Oz-phase, oxidation of the Nd1.9Ba1.1Cu3Oz-phase can occur owing to `one-dimensional' oxygen diffusion along the b axis of the structure.This is accompanied by a second order phase transition resulting in the appearance of an inversion centre (the high- temperature modification has the Amm2 space group vs. the Ammm group for the low-temperature modification 278), order- ing of the oxygen sublattice and coupled zigzag-shaped displace- ments of copper atoms from the `ideal' positions at the centres of perovskite-like structural blocks, which is most likely due to `compression' of the structure along the long axis c caused by introduction of extra oxygen atoms.279 S.5.Discussion of the charge transfer between the super- conducting planes and dielectric block using the results obtained by X-ray structural analysis and Raman spectroscopy can also be found elsewhere.278, 280 S.6. At a given partial pressure of oxygen, the compositions with the minimum x(S1) and maximum x(S2) substitution degrees in the Nd1+xBa27xCu3Oz solid solution are fixed, because they correspond to the invariant quinary equilibria (the number of degrees of freedom is 475+271=0) Nd1+x(S1)Ba27x(S1)Cu3Oz+BaCuO2 Nd4Ba2Cu2O10+L+O2 , Nd1+x(S Nd2CuO4+L+O2 . 2)Ba27x(S2)Cu3Oz+CuO O The corresponding phase transformation temperatures are also fixed, which provides the optimum conditions for achieving the above-mentioned substitution degrees.For instance, it was experimentally established that a temperature range between 985 and 1030 8C is optimum for preparation of the NdBa2Cu3Oz phase with Tc^94 K (at p 2=0.21 atm).281 S.7. One of the most promising and intensively developing universal approaches to computer simulation of crystallisation of HTSC systems is the phase field concept for multiphase systems (PFCMS).282 In the classical nucleation models, the liquid ¡¾ solid interface is assumed to be infinitely thin. The PFCMS model considers a liquid ¡¾ solid interface of finite thickness. Obviously, this is a great stride toward deriving a fundamental generalisation of crystallisation mechanisms.An additional continuous function (the phase field) is also introduced in this model, which plays the role of the order parameter used for identification of the regions of the liquid and solid states. In the simplest case (one solid phase in melt), the phase field can be described using a function p(x,t) whose values vary between 0 and 1. If p(x,t)=1, then at a point x and at instant t the system is in the solid state, whereas p(x,t)=0 corresponds to the liquid. The liquid ¡¾ solid interface is described by all p(x,t) values between 0 and 1. The following equation for the phase field function was derived 282 by minimising the density of the free energy of the system tOp Ox; tUa e 52 pOx,tU ¢§ f 0O pU a mOp,T,cU. This equation includes most of the physically important melt solidification parameters such as the (free) energy gradient e; the crystallisation front propagation velocity (by means of the orientation-dependent function t, which describes an anisotropic or facet growth); the derivative f 0( p) of the potential determining the phase boundary stability under equilibrium conditions, as well as the driving force m(p,T,c) of the process, which is proportional to the degree of deviation from thermodynamic equilibrium and should include the temperature dependence T(x,t) and the concentration profile of the distribution of constituents (c).The necessity of inclusion of the function c is explained by considering the diffusion processes occurring in the system (e.g., the dissolu- tion rate of the solid phases in the melt).30 S.8.The mechanism of the needle-shaped crystal growth under isothermal conditions remains unclear as yet. At the same time, this phenomenon is not observed for those REE of the yttrium subgroup whose R123-phases do not form the bar- ium/REE substitution solid solutions. It was also estab- lished 281 that the reason for growth of the needle-shaped crystals is closely related to the features of the phase diagram of the Nd7Ba7Cu7O system, in particular, to the increased deviation of the composition of the equilibrium phase of the Nd1+xBa27xCu3Oz solid solution towards x>0 which occurs on raising the temperature. In this case, the phase with x=0 decomposes on heating with the formation of a solid solution with x^0.1, a non-superconducting Nd4Ba2Cu2O10 phase and a melt L.At the initial moment, the composition of the liquid phase L can differ from the equilibrium composition, thus providing the driving force for the growth of Nd1+xBa27xCu3Oz and Nd4Ba2Cu2O10 crystals not only under isothermal conditions, but (probably) also even with increasing temperature (mention may be made here that single crystals of HTSC are usually obtained by cooling of melts). S.9. Single-phase specimens of mercury-containing HTSC and especially mercury-containing crystals can hardly be obtained because of extremely high volatility of mercury and its com- pounds. Therefore, the preparation of the crystals of the most promising Hg1223-homologue with unique physical character- istics 283 by the ampoule synthesis is of considerable interest.Single crystals of HgBa2Can71CunOz (n<4) with Tc ^130K were synthesised by spontaneous crystallisation from PbO or BaCuO2 ¡¾CuO flux in a crucible made of stabilised zirconium dioxide (crystallisation temperature 930<T<1070 8C) in an autoclave at an Ar pressure of (10 ¡¾ 15)6103 atm.284, 285 Intro- duction of rhenium in order to stabilise the structure makes it also possible to grow single crystals of Hg0.75Re0.22Ba2Ca5Cu6O15 (Tc=100 K) and Hg17xRexBa2Ca6Cu7O16+4x+z (Tc=84K) homologues.68 S.10. The uniqueness of the HTSC¡¾ manganite hetero-struc- ture 286 shown in Fig. 21 is that it comprises two promising and intensively studied phases with nonlinear electrophysical proper- ties, viz., the superconducting cuprate phase and the manganite phase exhibiting a giant magnetoresistance effect.Such hetero- structures are promising for the use in microelectronics. A microstrip resonator can be considered as an example of devices manufactured using conventional elements (HTSC-films and a dielectric substrate). In this case, the double-sided deposi- tion of a superconducting film on the single-crystal substrate was carried out by CVD methods.287 S.11. Phases of the RBa2Cu3Oz type can be synthesised over a period of about 10 to 30 min at 500 ¡¾ 550 8C using a mechanoac- tivated decomposite mixture, i.e., a mixture of copper-free and copper-rich reagents (Y4Ba3O9 and CuO) compacted at 250 8C.288 S.12.Recently, the formation of antisite defects in the barium and neodymium sublattices in the fully oxidised NdBa2Cu3O6.9 phase with a low transition temperature to the superconducting state was found by X-ray spectral and X-ray structural analysis and by Raman spectroscopy.279 S.13. A cuprate melt considered as a high-temperature solvent is characterised by a density from 4.8 to 5.4 g cm73. The minimum density is observed for an eutectic composition 28 mol.% BaO : 72 mol.% CuO.289 Such melts have similar temperature dependences of the surface tension and viscosities which increase as the BaO concentration increases.289 Above 955 8C, the surface tension coefficient of the melt lies in the range *0.3 ¡¾ 0.6 N m71.The calculated radii of the viscous flow units suggest that the melts have a `slightly ionic' nature and a low aggregation degree of the ions in the melt. The melts exhibit a mixed (mostly electron-type) conductivity. Two factors should be considered when discussing the limiting stages of single-crystal growth from a liquid (in particular, by the modified Czochralski method). First, this is the steady-state Yu D Tretyakov, E A Goodilin growth of the single crystal. This requires meeting the condition for the balance of the amount of R2O3 in the flux and that necessary for the deposition of the R123-phase on a crystal face, written over the liquid ¡¾ solid interface. Second, crystallisation can be characterised by relatively high near-boundary supersaturation with respect to R2O3 (in the so-called boundary layer) due to kinetically hindered crystal face growth.The following approx- imate formula for the crystal face growth rate (u), which includes the diffusion of Y3+ ions in the melt, was proposed 8 under some assumptions as a first approximation. u a D2=3 s 1:6v1=6Oc123 ¢§ c L o1=2OTb ¢§ TpU=m211 L a OTp ¢§ TiU=m123 L . iU L L s Here, DL is the coefficient of bulk diffusion from the Y2O3 source phase (Y2BaCuO5) contacting the melt at the temperature Tb to the face of the YBa2Cu3Oz crystal (the peritectic decomposition temperature Tp) that grows from the melt at the temperature Ti ; o is the angular velocity of crystal rotation; m211 and m123 are the slopes of the tangent to the liquidus above and below Tp; n is the kinematic viscosity of the melt; and c123 and ci are the concen- trations of Y3+ ions in the solid phase (crystal) and in the melt near the growing crystal, respectively. The maximum crystal face growth rate that can be achieved in the absence of kinetic complications is umax ^0.36 mm h71.However, it is 3 times higher than that observed experimentally (0.108 mm h71). This indicates that the growth of single crystals of the Y123-phase using the modified Czochralski method is likely to be controlled not only by diffusion of the components through the melt, but also by the interphase kinetics of deposition of the Y123-phase. The hydrodynamic conditions of the experiments on the growth of large HTSC-crystals and their effects on the morpho- logical features of RBa2Cu3Oz crystals have been analysed in detail using the generalised `dimensionless analysis' approach.290 Growth of a crystal by the modified Czochralski method is characterised by an increase in the diameter of the crystal with time, which continuously changes the conditions for crystal growth.Generally, the relative contribution of convection due to (i) the density difference between the melts with different temper- atures (natural convection) and (ii) rotation of the crystal (forced convection) can be assessed as the ratio of the Reynolds and Grasshof numbers. The dimensionless temperature Y a Ti ¢§ Ttop Tb ¢§ Ttop of the `crystal ¡¾ melt' contact surface (Ti) and the top region of the crucible (Ttop), normalised to the overall temperature gradient between the bottom (Tb) and top regions of the crucible can be estimated Y= (from experimental data) as 0.47Re0.028Pr0.041Gr0.019Dsc0.064+0.10 (Dsc is the geometrical factor).According to this expression, the steady-state crystal growth requires a precise control over the rotation velocity taking into account the increase in the crystal diameter during the growth. Otherwise, a crucible of a large diameter should be used. S.14. 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ISSN:0036-021X    

出版商:RSC    

年代:2000

数据来源: RSC

摘要:

Russian Chemical Reviews 69 (1) 35 ± 52 (2000) Methods for preparation of carbon nanotubes E G Rakov Contents I. Introduction II. Electric-arc synthesis III. Laser-assisted synthesis IV. Other methods of graphite vaporisation V. Pyrolysis of hydrocarbons and decomposition of CO VI. Growth of nanotubes by decomposition of metal carbides VII. Other methods VIII. Purification and opening of nanotubes IX. Conclusion Abstract. The most important methods of synthesis and purifica- tion of carbon nanotubes, a new form of material, are described. The prospects for increasing the scale of preparation processes and for more extensive application of nanotubes are evaluated. The bibliography includes 282 references. I. Introduction Immediately after they had been discovered in 1991,1 carbon nanotubes (nanotubulenes, hereinafter, NT) attracted so much attention of scientists of various professions that the discoverer, Iijima, soon became and still remains one of the most cited researchers in the field of nano-sized materials.The reason for this interest is not the unusual structure of these compounds, as in the case of fullerenes (although this also plays a certain role) but rather the prospects that are opened by the properties of NT for materials science. The journal Fullerene Science and Technology, having the word `technology' in its title, was founded in 1992; during the decade which has passed since the discovery of fullerenes (by 1995), 150 patents for their use were granted in the USA;2 reviews dealing with commercialisation of the manufacture and use of fullerenes appeared;3 however, there are still no data on the large- scale implementation of any patented method or, even more so, on the appearance of a new line of research in the materials science.The situation with NT is absolutely different: as early as 1992 ± 1993, the main fields of their potential application took shape and design of the first prototypes of future devices started. Some applications of NT were inherited from fullerenes (electro- des for chemical cells, safe sources of hydrogen in transport devices, optical filters), other applications coincide with those of carbon fibres (high-strength composites), and some fields of application are due to the unique properties of NT themselves (semiconductor devices, field emitters, probes of tunnelling micro- scopes, `quantum wires').It is clear that this is by no means the full E G Rakov D I Mendeleev University of Chemical Technology of Russia, Miusskaya pl. 9, 125047 Moscow, Russian Federation. Fax (7-095) 490 75 23. Tel. (7-095) 948 54 67 Received 18 May 1999 Uspekhi Khimii 69 (1) 41 ± 60 (2000); translated by Z P Bobkova #2000 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2000v069n01ABEH000531 35 36 40 41 42 45 45 46 47 list of currently known applications of NT. It is not without reason that a recent review 4 is entitled `Nanotubes: a revolution in materials science and electronics'. As Buchachenko has noted vividly, carbon nanotubes made it possible `to pass from fine words to impressive deeds'.5 Certainly, fullerenes, too, are finding and will be finding ap- plication (the rapidly progressing chemistry of fullerenes is now at an early stage of development); however, NT as materials have already clearly separated from their three-dimensional relatives. Defect-free carbon NT are formed as a result of rolling of planar atomic network graphite sheets (graphenes) into seamless cylinders with diameters of *1 to 120 ± 150 nm and with lengths of up to hundreds micrometers. There exist three forms of NT: achiral NT of the `armchair' type (two sides of each hexagon are oriented perpendicular to the NT axis), achiral `zigzag' NT (two sides of each hexagon are parallel to the NT axis) and chiral or helical NT (each pair of the hexagon sides is arranged at an angle other than 0 or 90 8 relative to the NT axis).The structures of NT are usually described by means of two indices, n and m, which are related unambiguously to the NT diameter (d ) and the chiral angle (y, characterising the deviation from the `zigzag' configuration and ranging from 0 to 30 8) (Fig. 1). d à a 3Ön2 á m2 á mnÜ , pÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅÅ p where a are the interatomic distances in the planar network, (5,0) (0,0) Zigzag y a1 a2 (5,3) (0,5) (5,5) Armchair Figure 1. Scheme illustrating the structure of idealised NT.36 y a arctan ¢§2n a m 3AAAAAA m p A . Thus achiral NT of the `armchair' type are characterised by indices (n, n), those of the `zigzag' type have indices (n, 0) and chiral ones are described by (n, m).Nanotubes can be single- or multi-walled; the number of walls is theoretically unlimited but normally it does not exceed a few dozen. The distances between the neighbouring shells are close to the interlayer spacing in graphite (0.34 nm); thus, the smallest diameter of carbon NT is *0.7 nm. The diameter of the second and subsequent coaxial atomic layers is stipulated by the diameter of the innermost layer. In this connection, the structure of NT resembles that of onion fullerenes; if the inner shell is C60 , the second shell is C240 , the third one is C540, etc. The spatial accordance of the structures of NT layers, i.e.retention of interatomic distances close to 0.34 nm is possible only provided that the chiral angle changes on passing from layer to layer. The specific structural features of NT can hamper their use as materials �¢ synthesis gives NT with different structures. Multi- walled NT are much more diversified; therefore, more uniform single-walled NT, which, in addition, usually contain fewer defects, are preferred for the design of functional materials. In most cases, the tips of NT are covered by hemispherical or conical caps, which contain not only hexagons but also pentagons, in which the configuration of the carbon atoms is less stable. These caps are somewhat more chemically reactive than the lateral surfaces. The hemispherical caps resemble `halves' of fullerene molecules.Thus the NT (10,10), isolated rather frequently, are capped by halves of spherical C240 fullerene. Synthesis of NT can give a tough precipitate consisting of sintered NT, a somewhat softer precipitate of densely stacked NT, a rubbery material consisting of entangled NT (`paper,' `mats'), a three-dimensional network of long fibres (`lace collar', `web'), or a textured material consisting of parallel or nearly parallel (but arranged at a distance from one another) NT on a substrate (`forest'). Structures looking like a `sea-urchin' and NT as helices (see below) have also been described. Nanotubes tend to form relatively stable aggregates (they are referred to as bundles or `ropes'), in which the axes of individual NTare parallel to one another, the shortest distance between them being *0.32 nm (Fig.2). These aggregates arise due to van der Waals forces. Figure 2. Schematic structure of bundles of single-walled NT. In the majority of cases, the synthesis of carbon NT is accompanied by the formation of other carbon modifications �¢ fullerenes, nanoparticles and amorphous carbon. The yields of NT and side products are determined by the conditions of synthesis; the removal of side products is an important stage in the synthesis of NT. Good prospects for the design of new functional and struc- tural materials are opened by modification of carbon NT, which can be accomplished by the following methods: (1) filling of the inner cavities of NT with substances which alter their electronic, magnetic or mechanical properties; (2) `grafting' of functional groups to the tips of NT; (3) replacement of some of the carbon atoms in NT by atoms of other elements; (4) partial or complete cleavage of double bonds on the lateral surfaces of NT by adding one or another reagent; E G Rakov (5) intercalationsertion) of `guest' atoms or molecules into the intertubular space of NT bundles.In addition, NT can be used as templates in the template syntheses of nanotubular or nano-rod materials for various purposes. The scope of this review is restricted to the methods of synthesis of NT because these methods are important for the development of studies and projects related to NT.They should lay the grounds for the future commercial manufacture. Attention is focussed on recent studies. The publication in 1999 of a Russian review devoted to the structure, electronic properties and morphology of NT6 facili- tated the performance of this task. Earlier Russian reviews dealing with the mechanisms of the formation of fullerenes, nanoparticles and NT7, 8 were also useful; however, they are somewhat less significant because lots of newer studies have appeared. Apart from carbon NT, those containing B and N atoms are also briefly considered in this review. These NT can be synthesised either from carbon NT or simultaneously with them. II. Electric-arc synthesis It is known that an electric arc develops a temperature of up to 4000 8C, its `burning' being accompanied by transfer of a substance between the electrodes.In 1990, electric arc-synthesis with graphite electrodes under an atmosphere of inert gases was first used to prepare fullerenes in relatively large amounts.9 It is among the products of arc synthesis that Iijima discovered carbon NT in 1991.1 Subsequently, this procedure has been modified and used in many laboratories of different countries. 1. Principles of the method The first synthesis of NT in relatively large (gram) amounts was also performed in Japan.10 The electric-arc synthesis was carried out under a helium atmosphere using graphite electrodes �¢ an anode 8 mm in diameter and a cathode 12 mm in diameter �¢ located at a distance of less than 1 mm from each other.The current in the arc reached 100 A (current density *150 A cm72) and the voltage was 10 to 35 V. The rate of the growth of the cathode deposit was *1 mm min71. Some of the graphite evaporated from the anode turned into carbon black and soot, which were deposited on the walls of the reaction chamber, and some was deposited at the cathode. The outer solid layer of the cathode deposit contained sintered NT and nanoparticles, which could not be separated. The purity and yield of NT depended appreciably on the helium pressure�¢the formation ofNTstarted at *13 kPa, and at 66 ¡¾ 332 kPa, the cathode deposit contained only NT and nanoparticles. The optimum pressure was 67 kPa; in this case, *75% of the electrode material consumed was depos- ited at the cathode, the yield of NT, which were accumulated in the inner, black, relatively soft part of the deposit being*25%. The yield of NT attained in an argon flow was much lower than that in a helium flow.Analysis of the results of the first studies on the structure and synthesis of NT10 ¡¾ 14 allowed Ebbesen 15, 16 to note some charac- teristic features. The material formed has a hierarchical structure, in which tens or hundreds of individual multi-walled NT with diameters of 2 ¡¾ 20 nm and nearly equal lengths (microns or tens of microns) are joined into regularly organised bundles resembling ropes. These ropes are combined into fibres with a diameter of*50 mm, while the fibres are joined into threads having even greater diameters (about a millimeter) and visible with the naked eye.The larger the bundle, the more disordered can it be. The formation of columns *50 mm in diameter consisting of NT and arranged parallel to the electrode axis has been reported. 17 This study is notable due to the setup used; it had movable vertical electrodes with a diameter of 19 mm, which (as well as the arc area) could be cooled during the process. (In the conventional arc syntheses, the cathode and anode diameters are different, the anode diameter being smaller than the cathodeMethods for preparation of carbon nanotubes diameter.) Using this setup controlled by a computer, the researchers were able to deposit *90% of the substance at the cathode and to obtain thus high-quality multi-walled NT.The current in the arc of this setup reached 250 ± 300 A, although the current density at the beginning of the process was even lower than the standard value and amounted to 70 ± 80 A cm72. Upon annealing in air at 650 8C, the columns were etched being replaced by channels surrounded by entangled NT. An increase in the electrode diameter normally causes sinter- ing and cracking of NT.Adense greyish product is accumulated in the inner part of the cathode deposit, instead of the easily extractable black material. This phenomenon was initially explained by insufficiently high temperature of the synthesis;18 however, the assumption that the temperature was, conversely, too high seems more reasonable.17 The decrease in the anode diameter from 12.7 to 8.0 mm with the cathode diameter (25.4 mm) and current density (140 A cm72) remaining the same increased the yield of NT.An increase in the pressure of helium (which is believed to quench NT) from 6.6 to 101 kPa also resulted in an almost proportional increase in the rate of growth of the deposit.18 However, accord- ing to Ando and Iijima,19 variation of the yield ofNTas a function of gas pressure passes through a maximum, the position of which corresponds to 7 kPa in the case of Ar orCH4 and to 3 kPa for He. In the opinion of Wang et al.,18 the discharge used in the synthesis of NT is quasi-continuous and has a characteristic interruption frequency of*8 Hz. The discharge is initiated between the closest sections of the electrodes and, after some amount of graphite has been evaporated from the anode (which extends the discharge), it `jumps' to the neighbouring section and thus becomes shorter.Travelling of the arc over the electrode surface changes presum- ably the sites of localisation of NT. Lozovik et al.7 believe that an arc discharge between graphite electrodes has two modes of operation, a noisy and a quiet one, transition from one mode to the other being induced by changes in the current density. A high current density and a low inert gas pressure (noisy mode) result in the predominant formation of fullerenes, while nanoparticles and NT are formed predominantly at a relatively low current density and a high pressure (quiet mode).A thorough investigation of the cross-sections of bundles obtained by the arc method showed that NT do not have, as a rule, a cylindrical shape.20 The cross-sections of NT look like polyhedra or ellipses with numerous defects including edge dislocations. The structures of NT are intermediate between the `Russian doll' model (coaxial seamless cylinders) and the `folded carpet' (`scroll') model. In earlier studies, either model has been preferred, although the methods used in the investigations did not allow one to reveal the differences between them.21 Moreover, it was found that the `scroll' model is too regular to describe many NT and, in some cases, the `papier-mache' model (separate fragments applied onto one another) should be accepted (Fig.3).22 Some disordering of the structure is due to the fact thatNTare formed in the arc under non-equilibrium conditions and to the stress arising upon joining of individual NT into bundles. The presence of a large number of defects in multi-walled NT obtained in an electric arc is confirmed by experiments on their intercala- tion.23, 24 It has been suggested 25 that bundles are formed under the influence of an electric field, their length, diameter and the manner of laying being dependent on the electric field strength. However, in reality, bundles can be formed even without electric fields. Nevertheless, it is beyond doubt that the field has an influence on the morphology of the products of arc synthesis.The yield and shape of NT formed in an arc discharge plasma depend not only on the main arc discharge variables (the voltage between the electrodes, the current strength and density, temper- ature of the plasma) and parameters related to the rge 37 Figure 3. Structure models of multi-walled NT. (a) `russian doll'; (b) `scroll'; (c) `papier-mache'. variables (the pressure and composition of the inert or reacting gas) but also on the gas flow rate, on the dimensions of the reaction chamber, and on the duration and the scale of the process. The configuration of cooling devices if they are present (determines the size and shape of temperature fields) and the power of cooling devices (determines the heat removal), the nature and purity of the electrode materials and several other parameters that can hardly be quantitatively evaluated are also signifi- cant.10, 15 ± 19, 26 The main parameters governing the yield of NT � the rates of NT growth and quenching � depend on numerous variables which are far from being fully taken into account in experimental studies.This accounts for some contra- dictions, hampers comparison of the results and often makes them poorly reproducible. The absence of theoretical models (related to the setup geometry), which makes scaling impossible, might be due to the same reasons. The mechanism of the formation ofNTin an arc discharge has not yet been unambiguously elucidated, although it is discussed in many studies. There exist two main opposing models.According to one model, growth of NT advances due to the addition of carbon atoms or fragments from the vapour phase to the dangling bonds at the tips of open NT, whereas according to the other model, they add to the topological defects in the caps of closed NT. In recent years, preference is given to the former model, because the occurrence of `edge-to-edge' (`lip ± lip') interactions has been proved. They prevent the formation of caps in multi- walled NTdue to the formation of `fluctuating' (closure ± rupture) C7C bonds at the tips of two or three neighbouring coaxial NT.27, 28 The role of the electric field in the mechanism of the formation of NT is obviously overstated in some publications. It has been shown 16, 29 that this role is far from being paramount, the NT growth involving simultaneously charged (C+ etc.) and neutral (C2 etc.) carbon species.The difference between the main carbon sources might account for the formation of two types of con- densed products, NT and polyhedral particles. It is also unlikely that the tips of NT growing at the cathode in an arc discharge are opened due to strong electric fields.30 The experimental studies considered above describe the preparation of multi-walled NT. The formation of single-walled NT was first observed by Ajayan and Iijima,31 but this study has not been adequately appreciated, later studies being regarded as the pioneering ones.32, 33 Both groups of researchers cited 32, 33 performed the synthesis in the presence of catalytic additives.This opened up a new chapter in the history of NT. 2. The influence of catalytic additives The pioneering studies 32, 33 somewhat contradict each other in details but coincide in the main point stating that the introduction of small amounts (1% ± 2%) of transition metals such as Fe, Co, Ni or their mixtures into the graphite anode influences appreci- ably the shape and the yield of NT and, in some cases, also localisation of the NT-containing product in the reaction chamber. Subsequently, admixtures of Li, Cu, Ag, Zn, Cd, B, Al, In, Y, La, lanthanides, Si, Ge, Sn, Ti, Hf, Pb, Sb, Bi, S, Se, Cr, W, Mn, Ru, Pd, Pt, mixtures of two metals or of a metal with a nonmetal and several carbides and oxides were tested.28, 34 ± 37 It was found that vaporisation of an anode containing Co, Co7Ni, Co7Y, Co7Fe, Ni, Ni7Y, Ni7Lu and Ni7Fe gives deposits looking38 like a lace collar or a soft belt being formed around the cathode deposit.They contain single-walled NT mixed with amorphous carbon and metal particles, the NT having diameters of 1.2 ± 1.4 nm and being joined into more or less ordered bundles. The tips of single-walled NT are closed and contain no metal particles. Upon evaporation of graphite anodes with Ni7Y and Co7Y admixtures, the content of single-walled NT in the deposits reaches 70%± 90%. 37 The yield of NT also markedly increases when a Co ± Pt mixture is used. 38 Some catalysts (Cu; Cu with Ni, Pt, Yor Fe; Ni; Ni with Y, Lu or Fe) caused the formation of a `web', which hung between the cathode and the walls of the chamber. This web, as well as the deposit on the chamber walls, contained fullerenes, amorphous carbon, flat graphite particles and small amounts of single-walled NT.In some cases, a rubber-like deposit was formed on the walls; it could be detached as pieces or ribbons (Fig. 4).39 3 1 2 5 4 7 6 8 9 10 13 11 12 14 16 15 Figure 4. Setup for electric arc synthesis and localisation of various products; (1) to vacuum pump; (2) filter; (3) viewing window; (4) cathode; (5) nozzle for the removal of the cooling water; (6) graphite electrodes; (7) web-like precipitate; (8) cathode deposit; (9) rubber-like deposit around the cathode; (10) filling material (a mixture of powdered graphite with a metal); (11) fullerene- and NT-containing soot on the walls; (12) anode; (13) nozzle for the supply of cooling water; (14) voltmeter; (15) helium inlet; (16) power supply.The addition of sulfur to aCo catalyst also resulted in a greater amount of the web-like product and higher NT yields and in a quite noticeable scatter in the NT diameters (from 1 to 6 nm).40 Evaporation of Co and S simultaneously with graphite changed the shapes of the resulting products and led to the evolution of multi-walled NT in the middle of the cathode deposit, multi- and single-walledNTtogether with many other particles in the rubber- like deposit around the cathode, and single-walled NT in the web- like material.39 Kiang et al.40 believe that the multi- and single-walled NT are formed via different mechanisms and that Co and S induce the formation of single-walled NT directly in the gas phase and prevent the NT tips from closing.Since sulfur itself does not catalyse the formation of NT, its role seems to reduce to the stabilisation of dangling bonds. The addition of Bi and Pb also causes an increase in the NT diameter. The carbon ring structures formed in the gas phase are believed to play an important role in the mechanism of formation of single-walled NT. The most stable ring structures are those containing 10 to 40 carbon atoms, which form ComCn clusters together with Co atoms. It is these clusters that function as catalysts, while S, Bi and Pb stabilise the ring structures.41 The results of studies on the catalysed arc synthesis of single- walled NT are summarised in Table 1.42 It can be seen that the diameter of NT obtained in this way varies from 0.6 to 6 nm.E G Rakov Table 1. Characteristics of NT prepared by arc discharge synthesis in the presence of catalysts.42 Composition of crystallites a Diameter of NT /nm Catalyst average limits >0.6 >0.6 0.7 ± 1.6 0.80, 1.05 b 0.6 ± 1.3 0.7 ± 0.8 0.9 ± 2.4 1.3, 1.5 b 0.6 ± 1.8 7 1.2 ± 1.5 7 0.6 ± 1.3 0.7 ± 0.8 0.9 ± 3.1 1.7 1.3 ± 1.8 1.2 ± 1.3 1.0 ± 6.0 1.3, 1.5 b Fe Fe Co Co Ni Ni Fe+Ni Fe+Ni Co+Ni Co+S 0.8 ± 5.0 1.2, 1.5 b 7 Fe3C 7Co Co in a graphite shell Ni in polyhedral particles 7777Co and Co in polyhedral particles 7 0.7 ± 4.07 7 *2 1.1 ± 1.7 7 Co+Bi Co+Pb Co+Pt Y CoPt YC2 in polyhedral particles Cu in polyhedral particles 1 ± 4 >2 Cu None 77 7 a The crystallites are located inside NT or polyhedral particles (see Section III.3).b The curve for the distribution of NT over diameters exhibits two maxima. The mechanism of the catalytic action of metals in the formation of single-walled NT implies adsorption of carbon atoms on the surface of metal particles and their free migration over the surface to the base of a growing NT.43 It was shown experimentally that the catalytic reaction can follow either of two routes, depending on the size of the metal catalyst particles.If the particle size (average diameter) is several tens of nanometers, which is much greater than the NT diameter (*1 nm)ny closed NT sprout from one particle. If the particle size is not larger than the NT diameter, the particle moves together with the growing NT tip. The results of molecular dynamics modelling 43 confirmed the probability of the former pathway, involving migration of adsorbed carbon atoms to the NT base and the `root' growth of NT. It should be borne in mind, however, that this model is of limited utility. It does not take into account the fact that NT formed in arc synthesis contain lots of defects and that a stream of carbon atoms directed towards growing NT has the greatest intensity near the NT tips.In addition, the conditions of vapour condensation in different sections of a setup, as well as in different setups are nonequivalent, which results in the formation of particles with dissimilar morphologies. Nevertheless, the modelling made it possible to explain why multi-walled NT are not formed in the presence of catalytic additives (this would require cooperative processes, associated with consistent interaction of numerous particles), why single- walled NT always have relatively small diameters (the growth of NT is initiated on the surface bulges of metal particles and the diameter of these bulges is small with respect to the height), and why mixed catalysts are often more efficient than one-component ones (this is due to the enhancement of carbon adsorption, to the change in the activation energy of the NT growth, and to the formation of a surface with a large number of bulges).Apparently, single-walled NT are even more prone to form bundles, which are combined in `ropes' with a two-dimensional triangular crystal lattice.37, 44, 45 Journier and Bernier,26 who surveyed studies on catalytic arc synthesis, have rightly stated that the yield and the shape of NT can differ substantially even when the same metal is used, depending on the metal concentration, pressure of the inert gas, current strength (or density), and the geometry of the arc setup.Methods for preparation of carbon nanotubes Aspecial role in the arc synthesis ofNTis played by boron, the introduction of which as B, B2O3 orBNinduces the formation of a large number of well graphitised long (up to 20 mm) NT with boron-containing caps.46 In this case, B4C crystals, large full- erenes and BC3 nanotubes are produced in addition to carbon NT.The presence of boron in the gas phase decreases appreciably the concentration of C2 fragments in the plasma zone and the contents of fullerenes in the wall deposit.47 Since the C2 clusters are the main source of condensed structures, the introduction of boron can change the mechanism of the formation of NT. Note that the arc setup used in the study cited 47 differed only slightly in geometry and electric parameters from the conventional setups used for the synthesis of carbon NT [anode and cathode diameters 6 mm (a cathode with a diameter of 200 mm was also tested), distance between the electrodes 1 mm, current 80 or 150 A, voltage 25 or 40 V, respectively, gas pressure 27 ± 67 kPa].In some studies the formation of carbon NT, separate areas of which were made of BN has been noted. Under certain conditions, for example, upon evaporation of BN or mixtures of B or BN with graphite, boron carbides, nitride, and carbonitrides can form `self-contained' NT.48 ± 51 The use of HfB2 (see Refs 52, 53) or ZrB2 (see 54, 55) electrodes for the electric arc synthesis in a stream of N2 results in the formation of BN nanotubes. If BC4N serves as the electrode material,56, 57 NT containing B, C and N (or BN) are produced; when the electrodes are made of graphite with the addition of BN,48 the process yields NT containing BC2N and BC3 .However, in the latter case, the composition of NT is nonuniform along the length of the tubes. Unique multi-walled NT have been prepared by Suenaga et al.,58 who also used HfB2 and graphite as the electrode materials; several inner shells consisted of carbon, several central shells (embracing the carbon shells) were made of BN, and some outer shells were again carbon. The mechanism of formation of these NT (they evolved together with polyhedral particles, which also contained layers with different compositions) is totally unclear. The inner layers might grow upon diffusion from an NT tip; however, the probability of this process is low. Since the electronic properties of C and BN layers are different, this result brought about the idea of designing electronic devices with radial hetero- junctions.Studies on the catalytic synthesis have much in common with the studies on the synthesis of NT the inner cavity of which is filled with one or another substance. 3. Preparation of filled nanotubes Filling of the inner cavity of NT (encapsulation) is a route to the development of a great number of novel nanomaterials of various classes and functions (materials with specific electronic, magnetic, optical, or mechanical properties, catalysts, sorbents). Some of them could be used as a type of `NT ± filler' nanocomposite, while other require that the carbon shell be removed (burned out). Particular attention is devoted to the preparation of `quantum wires', i.e., conducting materials with a diameter of several nanometers, whose conductivity is close to truly one-dimensional conductivity.The possibility of using NT and nano-rods in nanotechnological devices is being discussed. The first study on filling NT was performed in 1993.59 Mixtures of NT with Pb were annealed in air at *400 8C. In the presence of O2, the molten metal opened the tips of NT, removed the hemispherical caps and was sucked into the inner cavities of the NT by capillary action. However, it has been reported 60 that lead is able to fill NT only at pressures of 1000 to 10 000 atm; hence, under the conditions described above, NT were filled by lead compounds rather than by lead metal.Indeed, it has been shown that heating of a mixture of Pb3O4 with NT for 9 h at 700 8C results in opening and filling of the NT.61 Encapsulation of molten V2O5 , PbO, Bi2O3 (see Refs 62, 63), MoO3 (see Ref. 64) and AgNO3 (see Refs 65, 66) into open NT has also been reported. Even before the discovery of NT, the arc method has been used to prepare endohedral fullerenes 67 and nanoparticles encap- 39 sulated into a multilayered graphite shells.68, 69 Therefore, it comes as no surprise that filling of NT can occur directly during the arc synthesis. Ordinary and filled NT are obtained under different conditions, in particular, filled NT are formed at lower temperatures (1000 to 2000 8C). Yttrium carbide was the first substance detected inside NT prepared in an arc.70 ± 72 According to an estimate,35 yttrium is encapsulated into NT more easily than many other metals, although this feature can hardly be related to the enthalpy of formation of YC2 .Later, carbides of other lanthanides and Mn have also been encapsulated into NT.73 ± 75 Carbides have been formed when metal-doped graphite anodes have been used because carbides are the species most stable in the presence of carbon. However, in some cases, elementary substances have also been detected inside NT, for example, Mn,76 Cu, Ge 77 ± 79 and even small amounts of Y.72 These elements either do not form stable carbides or are liberated in an arc discharge under an H2 atmosphere. Thus copper forms no stable carbides and does not catalyse the formation of NT or fullerenes; therefore, the mechanism of its encapsulation differs from the mechanism of insertion of other metals.78 Evaporation of a graphite anode in a hydrogen plasma in the presence of copper gives rise to polycyclic aromatic hydro- carbons, which form graphite shells around Cu particles.78, 79 The formation of filled NT upon joint evaporation of pyrene and Cu or Ge (this was done using a Cu anode with a cavity filled with pyrene and aWcathode) 80 was considered 78, 79 as direct evidence supporting the above statement.Metals with different volatilities show different behaviour during encapsulation.81 Some substances fill long NT giving continuous crystals with lengths of up to 1 mm (Cr, Ni, Sm, Gd, Dy, Yb, S, Se, Ge, Sb); other substances fill only short NT (Al, Bi, Te), and some other elements form inclusions separated from one another inside NT (Co, Fe, Pd).36, 82 ± 85 In a study of encapsula- tion of high-melting transition metal carbides (NbC, TaC, MoC), an exceptionally stable face-centred form of the carbide MoC was discovered. 85 Nanotubes containing B4C in the inner cavity have also been prepared. 86 The operation conditions used in these studies 70 ± 83 did not differ much from those used for the synthesis of NT.Thus Setlur et al.78 used electrodes with a diameter of 10 mm located at a distance of 0.25 to 2.0 mm; the current was 100 A, the voltage was 20 V and the gas pressure ranged from 1.3 to 6.7 kPa.In another series of studies,82 ± 84 the corresponding parameters were 9 mm, 1 mm, 100 ± 110 A, 20 ± 30 V and 60 kPa. Sulfur plays a special role in the formation of filled NT.84 Sulfur can be contained in a graphite anode as a minor impurity; it promotes filling of NT with other elements. In some cases, sulfur enters NT as a sulfide but more frequently it is not present in the filling material. This effect of sulfur has been explained 84 by the fact that sulfur forms clusters with carbon in the gas phase; this ensures transportation of sulfur to the growing NT, where it promotes graphitisation of NT and reconstruction of a metal surface, resulting in the exposure of catalytically more active crystallographic planes. At the final stages of the process, sulfur is removed from the graphite shell of the NT. These results are similar to the data reported in earlier publications.39, 40 In some cases, sulfur suppresses the formation of single-walled NT.The technique of filling NT under the conditions of arc synthesis suffers from a severe drawback �this process is hardly controllable. The yield of filled NT and the composition, structure and morphology of encapsulated substances cannot be governed in the majority of cases. 4. Modifications of arc synthesis The use of an alternating-current arc and electrodes with identical diameters changes substantially the pattern of the process, namely, the products are formed on the chamber walls rather than at the electrodes, new forms of NT being produced together with the known ones.87, 88 The yields and forms of some products40 also depend on whether the electrodes are mounted in the vertical or horizontal position.A series of publications 19, 89 ± 91 deal with the formation of multi-walled NT under a methane atmosphere. This process differs from the conventional one (performed under an inert gas) in the fact that, under some conditions, the formation of NT is not accompanied by evolution of fullerenes or nanoparticles. At high partial pressures of methane, relatively thick multi-walled NT are formed and at low pressures (1 ± 3 kPa), thin and long multi- walled NT are produced. The optimal conditions for the forma- tion of thin NT for the vertical mounting of electrodes with a diameter of 6 mm are the following: methane pressure 2.7 kPa, current 30 A.Later, the authors of the above series of studies arrived at the conclusion that, since CH4 decomposes in an arc to give C2H2 and H2 , the synthesis of NT should better be performed in a stream of H2 (see Refs 92 ± 95).{ Some advantages of using H2 have been mentioned above in the consideration of copper encapsulation. A certain role in the formation of NT can also be attributed to the high heat capacity of H2 , owing to which it can efficiently quench NT.96 When the process is performed at `plasma' temperatures, the fact that the energy of ionisation (the first ionisation potential) of H2 is half that of He also becomes significant. Rather long NT are formed even at an H2 pressure of 7 kPa; their yield at a pressure of 13 kPa is comparable with the yield attained in the atmosphere of He at 66 kPa.Many NT prepared in a flow of H2 have a narrow inner channel; they are open and usually high-quality NT (having few nanoparticles on the outer surface). The thinnest and longest multi-walled NT were prepared at H2 pressures of 6.7 ± 11 kPa. The arc synthesis in an atmosphere of H2 differs appreciably from the synthesis in inert or hydrocarbon media because the temperature developed in the atmosphere of H2 is higher. This provided grounds for assuming that C+ ions participate in the formation of NT in anH2 medium.92 It is also assumed that either H2 molecules or H atoms add to the dangling bonds of growing NT and thus prevent closure of these bonds and, in addition, eliminate the possibility of the formation of amorphous carbon.92 The addition of H2 is evidenced by the fact that an increase in the partial pressure of H2 results in higher yields of open NT.The mechanism involved may be similar to the mechanism of the formation of diamond from the gas phase. Particles of various natures are produced; however, under a hydrogen atmosphere, the most reactive ones are completely or partially gasified (etched). Yokomichi et al.97 have attempted to prepare NT in a CF4 medium. It has been assumed that the F atoms formed upon decomposition of CF4 would attach to the dangling bonds and prevent them from closing, thus influencing the process of NT formation. This assumption largely proved to be correct; in any case, no fullerenes were formed together with NT.The synthesis was carried out using a setup with electrodes 5 mmin diameter at a current of 20 ± 60 A and a CF4 pressure ranging from 2.7 to 53 kPa. Under the optimal conditions (20 ± 40 A, 6.7 ± 13.3 kPa), multi-walled NT were synthesised with an outer and inner diameters of 20 and 5 nm, respectively; they did not differ much from the NT obtained in a flow of He. The researchers cited 97 studied the influence of the main parameters of the process (medium, gas pressure and current) on the yield of NT and polyhedral particles and on the length of NT and compared their data with those reported in other publica- tions; the results were summarised in a table (Table 2).In particular, it follows from this table that the processes occurring in an atmosphere of CF4 and CH4 are fairly similar. However, the data of Table 2 should be treated with caution because this way of presenting the information does not show, for example, that in { Subsequently, the same conclusions have been drawn by other inves- tigators.96 E G Rakov Table 2. Influence of the increase in the gas pressure and in the current on the main characteristics of electric-arc synthesis of NT under atmospheres of various gases.97 Length of NT Medium Yield of NT Yield of nano- particles decreases does not change does not change does not change increases does not change increases does not change decreases increases decreases decreases CF4 He CH4 H2 some cases, the plots for the variation of the yield vs current pass through a maximum.Koprinarov et al.98 have attempted to devise a continuous electric arc process and to make up for the consumption of graphite due to evaporation by supplying CH4 . For this pur- pose, they used the `reverse method', i.e. the area of the anode in the setup used was an order of magnitude greater than the area of the cathode. The products of electric-arc synthesis were found to contain fullerenes, polyhedral particles and single-walled and multi-walled NT. The nanotubes had different lengths and structures. Some of them were partly single-walled and partly multi-walled structures; this indicates that various sections of NT grew at different rates.The `reverse method' offers no advantages over the conventional method. Carbonised coal can be used as the electrode material instead of graphite;99 however, the presence of non-volatile impurities causes contamination of NT; hence, in the majority of studies, high-purity graphite is the material of choice. The main drawback of the arc method is low output. This is demonstrated by a publication 37 in which synthesis in aHe flow in the presence of Ni and Y introduced in a graphite anode gave only 2 g of NT over a period of 2 min. After that, the setup had to be disassembled to replace electrodes and take the produc out. III. Laser-assisted synthesis Fullerenes were first obtained by the laser method in 1985; however, only 10 years later, this method was employed to prepare NT.100, 101 The first setup was a 50-cm quartz tube with a diameter of 2.5 cm containing a graphite rod 1.25 cm in diameter arranged along the tube axis.The tube was evacuated and simultaneously heated to 1200 8C. After that, Ar was supplied (pressure 66.5 kPa, linear gas velocity 0.2 ± 2.0 cm s71). The target was exposed to a laser beam with a wavelength of 532 nm (Nd laser), a pulse frequency of 10 Hz, a pulse power of 250 mJ, and a pulse duration of 10 ns. The surface of the target was scanned by a laser spot with a diameter of 3 or 6 mm. The products of evaporation (multi-walled NT and nanoparticles) were collected on a cooled copper finger, on the walls of the tube, and on the reverse side of the graphite target (Fig.5). 1 4 3 2 Figure 5. Scheme of the laser setup; (1) furnace heated to 1200 8C; (2) neodymium laser; (3) graphite target; (4) water-cooled collector.Methods for preparation of carbon nanotubes The daily output of the first setup was up to 80 mg of a nanomaterial containing NT. The process had to be terminated because the tube was clogged near the target with a web-like material which also contained NT. By using a tube with a diameter of 3.8 cm, all other conditions remaining almost the same, another group of researchers attained a higher yield of a rubbery material containing single-walled NT. In this case, the output was 200 mg per experiment (3 ± 5 h).102 It was noted that distribution of the NT over diameters depends on the radiation wavelength (either 532 or 1064 nm).102 An increase in the diameter of the reaction tube to 5 cm was even more effective.This setup can afford *1 g of the material (containing 60%± 90% of NT) per 24 h.103 Subsequently, one laser was replaced by two lasers (wave- lengths 532 and 1064 nm), which emitted alternately, the interval between the pulses being 42 ns, and the power of a pulse was increased to 490 and 550 mJ, respectively.103 This made it possible to `knock down' the plums formed. Subsequently the tube diameter was doubled once again; in addition, alternation of the side of the target exposed to radiation and beam scanning were used.As a consequence, the yield of products containing 40%± 50% of NT reached 20 g during 48 h of continuous operation.103 Yudasaka et al.104 used a modified setup with two targets, one made of a graphite powder and the other made of a metal or an alloy. Study of the influence of the pressure of Ar and the furnace temperature on the yield and properties of the laser ablation product has shown that at pressures below 13 kPa only amor- phous carbon is formed, while NT appear in the products, together with amorphous carbon, above 26 kPa.105 An increase in the furnace temperature from 780 to 1050 8C resulted, in the presence of catalysts, in an increase in the average diameter of single-walled NT from 0.8 to 1.51 nm, although the yield of NT at low temperatures was relatively low.106 The yield and the shape of NT prepared by the laser method are determined by fewer parameters than those of the NT obtained by the electric-arc synthesis.Perhaps, this is why markedly higher yields of NT have been attained in the laser- assisted synthesis. The temperature of the area from which graphite is evaporated and the temperature gradient in the gas phase near this area can be considered to be the crucial factors. Unfortunately, in the experiments described above these param- eters have been determined insufficiently definitely if at all (the measurements were hampered by the beam scanning over the target surface). Instead, properties influencing the temperature of evaporation were measured (the furnace temperature, laser radiation power, diameter of the light spot on the target, the velocity of the spot migration, the gas pressure, and the gas flow rate).The introduction into graphite of small amounts of catalysts led to the formation of single-walled NT, which, unlike those obtained in arc, were only slightly covered by amorphous carbon particles.101, 107 Mixtures of Co with Ni (0.6 at.% each) and of Co with Pt (0.6 at.% and 0.2 at.%) proved to be the best catalysts. When these mixtures were used, the yield of single-walled NT exceeded 70%, which is tens or hundreds times higher than the yields attained using individual metals as the catalysts. High yields have also been obtained with mixtures of Ni with Pt.100 The use of a mixture of two noble metals, Rh and Pd, mixed with graphite in a laser-assisted synthesis afforded single-walled NTwith diameters of 1.0 ± 1.5 nm.108 Only mixtures of Cu with Ni were found to be substantially less active than copper itself.100 The mechanism of the catalytic formation of single-walled NT upon laser ablation proposed by Thess et al.107 was defined as the `scooter' mechanism.According to this mechanism, individual atoms of Ni, Co or other catalytically active metals are adsorbed on the open tips of the curved Cn (n450) graphene fragments and `scoot' around the tips, thus promoting removal of all carbon structures except for the energetically preferred ones. 41 Clusters comprising 10 to 100 closed-tip nanotubes formed `ropes', 10 to 50 of which composed bundles.A more detailed study of the `ropes' showed that they are polycrystals, the sizes of separate grains ranging from 10 to 100 nm (the typical size is 10 to 20 nm); their cross-sections are oval and the predominant aspect ratio is*3.109 Some of the NT are clustered into 5 ± 15 nm thick `ropes' forming rings with diameters of 300 ± 500 nm.110 The products of laser-assisted synthesis were found to contain an interesting type of NT, so-called `nano-peas', which are chains composed of spherical C60 molecules located inside single-walled NT with a diameter of 1.3 ± 1.4 nm.111 The laser method also makes it possible to prepare NT from BN;112 ± 115 however, the experimental conditions employed in this case differed sharply from those used to prepare carbon NT; specifically, high and superhigh pressures were used.The process induced by laser heating of hexagonal BN in a diamond anvil at a high pressure under an atmosphere of N2 has an interesting feature, namely, cubic BN appears at the bottom of the NT formed.115 The mechanism of formation of NT from BN upon laser evaporation at superhigh pressures might include a stage of surface diffusion of molecules from the bottom of an NT to its tip, which implies restriction of the NT length.116 Laser ablation of a complex target (C, Si, BN) at near- atmospheric pressure accompanied by chemical reactions can be used to synthesise axial NT. This type of NT with diameters of several tens of nanometers and with lengths of up to 50 mm contained a core of b-SiC, an intermediate layer of amorphous SiO and an outer shell of BN and C.117 The study cited aimed at designing coaxial nano-sized electronic devices with heterojunc- tions.In addition to the neodymium laser, a CO2 laser has also been tested for the synthesis of carbon NT.118 IV. Other methods of graphite vaporisation The conclusion that fullerenes and NT are formed in the vapour phase, irrespective of the way used to vaporise graphite or another form of carbon, was drawn fairly long ago and has been confirmed in a number of publications. Thus Ebbesen et al.119 proved, using isotope dilution, that NT (as well as fullerenes) are formed exactly from carbon vapour. Apart from the arc- and laser-induced evaporation of graphite considered above, NT are also prepared using resistivity vaporisation and vaporisation induced by elec- tron or ion beams or by sunlight. 1.Resistivity vaporisation Heating a 0.5-mm graphite foil by Joule heat in vacuo (1076 Pa) and cooling the resulting vapour to730 8C resulted in NT being deposited on the surface of single-crystalline graphite.120 ± 123 The rate of deposition of NT with a diameter of 1 ± 4 nm was 0.5 As71. The NT were capped and combined in arrays. This method permits the preparation of particles having diverse shapes � single-walled and multi-walled NT, NT bundles and nano-cones, the yield of single-walled NT ranging from several percent to 80%± 90%. 2. Electron- or ion-beam vaporisation The first experiments on the preparation of NT by electron-beam vaporisation of high-purity (99.99%) reactor grade graphite in vacuo (1073 Pa) and deposition on various substrates (Si, quartz, graphite, ceramics, anodised Al) were performed in Russia back in 1992.124 The resulting condensate was a 0.01 ± 10-mm thick film consisting of NT with a diameter of *1 nm.Separate NT were joined in fibres*5 nmin diameter, while the fibres were clustered in cables with a diameter of 10 ± 30 nm. The tubular texture was retained throughout the whole film thickness; moreover, variation of the angle between the direction of the stream of carbon particles and the substrate surface resulted in the formation of inclined42 textures.Kosakovskaya et al.124 measured a large number of film characteristics and noted anisotropy of film properties. Nanotubes have also been formed on bombardment of high- purity graphite with Ar+ ions with an energy of 60 keV in a high vacuum at a normal angle of incidence.125 Polyhedral nano- particles were isolated together with NT. Exposure of hexagonalBNto an electron beam gave NT;126 as in the study described above,115 in this case, too, the formation of seeds of cubic boron nitride at an initial stage was assumed. The irradiation with electrons with a current density of 10 ± 20 A cm72 at a voltage of 300 kV resulted in annealing of most of defects and in almost complete ordering of theBNwalls of the NT.112 An electron beam with a current density of 150 Acm72 at an accelerating voltage of 400 kV ensured the formation of multi-walled coaxial BN clusters.127 Irradiation of boron-containing carbon with electrons gave carbon NT doped with boron.128 The role of the gas phase in this process and in other processes described above is obscure; therefore, the term `vaporisation' is used only conditionally. 3. Sunlight-induced vaporisation The studies dealing with the use of solar concentrators for the preparation of NT have been briefly surveyed in a publication.26 These studies were carried out only in France.Asetup producing a temperature of about 3000K at the focus was developed. Evaporation of pure graphite gave only a small amount of soot, whereas vaporisation of a graphite powder with catalysts gave rise to NT.The type of NT formed (single-walled, multi-walled or bamboo-like) and the amount of impurities were governed by the catalyst and the gas pressure in the process, i.e., by the same parameters as in the case of electric-arc and laser-assisted syntheses, described above and studied much more extensively. In the presence of Co, single-walled NT with a diameter of 1 ± 2 nm were found in the soot deposit and NT bundles with diameters of more than 20 nm containing no admixture of amorphous carbon were found in the web-like deposit. V. Pyrolysis of hydrocarbons and decomposition of CO 1. Catalytic pyrolysis Catalytic pyrolysis of hydrocarbons had been used to prepare carbon fibres even before the discovery of NT and fullerenes.Nanotubes were first synthesised by this method in 1993.129 The course of the process is influenced by the temperature, the overall pressure, the initial hydrocarbon used and its partial pressure, the nature and characteristics of the catalyst (first of all, the particle size, which could determine the diameter of the NT), and the nature of the catalyst support. The pyrolysis can result in layers of amorphous carbon or graphite formed around catalyst particles, carbon fibres and multi-walled or single-walled NT.26 The length of NT and the degree of their coverage by amorphous carbon particles depend on the duration of the process. a. Pyrolysis of acetylene Acetylene is used most often for pyrolysis. Jose -Yacama n et al.129 carried out acetylene pyrolysis at 500 8C and atmospheric pres- sure over a graphite catalyst containing 2.5% of Fe.The concen- tration of acetylene in the diluent (N2) was 9%. It was noted that graphite particles are formed initially around the Fe particles and this is followed by growth of NT with diameters of 5 ± 20 nm and lengths of up to 50 mm. Nine different catalysts (Fe, Co, Ni and Cu supported on three materials � flake graphite, SiO2 or HY zeolite) have been assayed;130, 131 the effects of the flow rate, the temperature and duration of the pyrolysis on the yield and quality of NT have been studied. An increase in the temperature from 500 to 800 8C influences only slightly the length of NT but increases the yield of amorphous carbon (amorphous carbon constitutes up to 50% of the deposit even under the optimal conditions).Longer E G Rakov pyrolysis times result in higher relative yields and greater lengths and diameters of NT. The replacement of the inert diluting gas by H2 barely influences the growth of NT. In all cases, the structure of NT was defective. A graphite coating was formed around the catalyst particles and multi-walled NT were `extruded' from it. Testing of Fe catalysts prepared by various procedures showed 132 that Fe supported on SiO2 provides the best results. The greatest yield ofNTwas attained when C2H2 was pyrolysed at 700 8C. The outer and inner diameters ofNTwere, on the average, 10 ± 20 and 5 ± 8 nm, respectively. As a further development of the study cited,132 pyrolysis of C2H2 over Fe particles in mesoporous SiO2 has been carried out.133 The catalyst was prepared by hydrolysis of tetraethoxysi- lane in a solution of Fe(NO3)3 followed by reduction with an H27N2 mixture at 550 8C.The diameter of NT (*30 nm) obtained at 700 8C from an acetylene ± nitrogen gas mixture (9% of C2H2) was nearly equal to the diameter of pores in SiO2 . It has been assumed that NT with a smaller diameter can also be obtained in this way. Each NT contained 40 to 50 shells and was located at a distance of *100 nm from the closest neighbours; thus, the deposit looked like a `forest' consisting of nanotube `stems' parallel to one another. The rate of the growth of NT was about 25 mm h71.In the initial stages, NT were free from the amorphous carbon impurity; however, as the duration of the pyrolysis and, correspondingly, the length of NT increased (the NT grew to a length of 50 ± 100 mm over a period of 2 ± 5 h), the `stems' bent and amorphous carbon appeared on their surface. The main obstacle hampering the conducting of larger-scale process is the difficulty of manufacturing substrates with large dimensions because they are subject to shrinkage and cracking. An improved method for the synthesis of a supported Fe catalyst has been proposed.134, 135 The method was based on the deposition of a film of a Fe(NO3)3 -containing gel onto a 30 ± 50 mmquartz plate, the removal of excess water and other solvents and drying at 80 8C.As this was done, the gel cracked into pieces with an area of 5 ± 20 mm2 each. These pieces were calcined in vacuo and then the iron was reduced to give small (5 ± 50 nm) iron particles distributed uniformly over the surface. Pyrolysis of a C2H2±N2 mixture containing 9% of acetylene at 600 8C and a pressure of 24 kPa in the presence of this catalyst made it possible to obtain NT in very high yields. The outer diameter of NT amounted to 30 ± 40 nm and the inner diameter was 10 ± 15 nm. They consisted of 10 ± 30 coaxial layers. Individ- ual NT were located at distances of *100 nm from one another. The rate of the growth was 30 ± 40 mm h71 and the length of NT (attained in 48 h) was *2 mm. The greatest area coated by NT was 15 mm2. No formation of polyhedral particles was detected.The layer of NT was easily separable from the substrate. A mechanism consisting in the NT growth on the side of a free tip, which is covered by a catalyst particle, has been proposed for this process.135 The micrographs of NT obtained by the trans- mission electron microscopy, which are presented in a study cited,135 show an irorticle at the tip of the NT and a top- shaped cavity at theNTbase. These results are at variance with the data of Jose -Yacama n et al.129 It has been considered initially that the reaction mechanism should include the formation and decomposition (upon supersaturation) of `active' metal carbides. This mechanism was consistent with the fact that the growth of NT, which has stopped after cooling of the carbonised catalyst, is resumed after the subsequent heating.The root growth of NT can proceed by a mechanism described previously and assuming the surface diffusion of carbon atoms to the bulges on the catalyst surface;43 in this case, dehydrogenation of C2H2 on the catalyst surface to give H2 rather than mere adsorption of carbon can be the first stage of pyrolysis. A mechanism involving immersion of small Cn clusters into the caps of NT has also been considered. It should be stated that catalytic pyrolysis of the same hydrocarbon (C2H2) present in the same concentration in N2 (9%) at similar temperatures (500 and 600 8C) can follow absolutely differentMethods for preparation of carbon nanotubes mechanisms depending on the pressure in the system, the method used to prepare the catalyst and other factors.Pyrolysis of C2H2 over Fe-containing catalysts supported on various materials including SiO2 has been considered in several other publications.136 ± 139 It was noted that, in addition to the NT described above, helical and even branched NT can be formed. Apart from iron, cobalt also exhibits high activity in the pyrolytic decomposition of C2H2.136 The materials used as supports for this metal include SiO2 , 137, 140, 141 NaY zeolite 137 and Al2O3 .142 The procedure for the preparation of Co catalysts, in particular, the pH of precipitation of the metal salt from a solution, influences appreciably the quality of NT.141 Biro et al.143 have classified NT into two groups: `raft-like arrays' with a diameter of 1 nm and single ones with a diameter of 10 nm.Remarkable NT with a coil structure were also isolated upon pyrolytic decomposition of C2H2 over Co. Nanotubes of this type have been studied in detail.144 It was shown by high-resolution electron microscopy that the coils consist of several straight sections, the junctions of which can contain disordered areas. The convolutions closely adjoin one another, the NT forming them being `flattened', i.e. compressed along the coil axis. A certain catalytic activity in the formation of NT from C2H2 is displayed by Ni andMnoxides, while platinum metals proved to be poor catalysts.136 However, a catalyst promoting the formation of very long NT was prepared from Pt5(CNC8H9)10.Catalyst particles can be introduced into a support by impregnating it with aqueous solutions of salts followed by decomposition of the salts, by chemical vapour deposition using organometallic compounds, or by electrodeposition. The size of metal particles can range from several nanometers to tens of nanometers. Zeolites, especially NaY zeolite, make good supports for the catalysts of acetylene pyrolysis. 137 The synthesis described in Refs 133 ± 135 should actually be categorised as a template synthesis. Membranes prepared by anodic oxidation of Al are also good templates for the catalytic synthesis of NT.139 These membranes contain parallel closely packed hexagonal channels (pores) running throughout the whole bulk.By varying the conditions of anodic treatment, one can control the pore diameter (2 ± 500 nm), the membrane thickness (50 ± 500 mm) and the pore density (107 ±1012 cm72). The Al2O3 membranes are transparent and stable up to a temperature of at least 1000 8C; the chemical properties of their surface can change upon the addition of hydroxy groups. Recently a template synthesis of NT by pyrolysis of C2H2 in the presence of a mixed catalyst has been performed.145 Commer- cial zeolite samples were impregnated with aqueous solutions of Co and V acetates and used in the process immediately after drying. The pyrolysis was carried out at 700 8C. This procedure yielded well graphitised and relatively thin NT (10 ± 12 shells, outer diameter 8 ± 10 nm, inner diameter 2.5 ± 2.8 nm).The role of the vanadium additive is unknown; the mechanism proposed in the study cited 145 and involving the transition of metal com- pounds to the gas phase seems doubtful. b. Pyrolysis of methane, ethylene and propylene It has been noted in a publication 137 devoted to the pyrolysis of acetylene that the use of methane, ethylene or propylene instead of acetylene either does not give NT or gives them in low yields. However, in some studies, conditions have been selected under which pyrolysis of these compounds does yield NT. Thus a deposit consisting of NT to an extent of 90% was obtained from CH4 at 600 8C in the presence of the NixMg17xO catalyst, prepared from Ni and MgO, in which some Ni atoms were located on the catalyst surface as aggregates.146, 147 The synthesis of NT with a length of >20 mm from a CH4±H2 mixture in the presence of Fe particles has been reported.148 An interesting point in this study 148 is that NT were synthesised using a microwave setup (Fig.6), widely employed for the preparation of diamond films. However, the 43 1 2 3 4 6 5 7 89 10 Figure 6. Setup for plasma-activated chemical deposition from the gas phase. The pressure of the mixture of CH4 and H2 at the inlet is 0.13 Pa, the substrate temperature is 370 ± 950 8C; (1) microwave antenna; (2) quartz window; (3) gas inlet; (4) viewing window; (5) plasma discharge; (6) to the optical emission spectrometer; (7) substrate; (8) an energy-sensitive graphite stand; (9) drive to a stepper motor; (10) to a vacuum pump.synthesis of single-walled NT carried out at Stanford University (USA) appears the most impressive.149, 150 In this study, methods of nanotechnology were employed. The catalyst was applied to a silicon plate as micron-sized islets. To this end, square holes were made in a poly(methyl methacrylate) film by electron-beam lithography and then the catalyst precursor was deposited onto the film from a methanol solution containing Fe(NO3)3 , MoO2(acac)2 and Al2O3 particles. Subsequently the solvent and the film were removed, the catalyst was heated in argon, and, finally, pyrolysis of CH4 was carried out. During pyrolysis, very straight NT with diameters of 1 ± 3 nm and lengths of up to 20 mmcontaining no topological defects were formed on the catalyst islets over a period of 10 min.Some NT served as bridges between neighbouring catalyst islets and thus formed electric contacts. An increase in the process duration to several hours resulted in thicker NT. This method is rather straightforward and easily reproducible; it can be used to prepare NT on substrates having large areas. The CoSi2 formed upon the deposition of cobalt onto silicon also catalyses the formation of NT from CH4 .151 The use of ethylene for the template catalytic synthesis of single-walled NT on porous Al2O3 plates has been described.152, 153 Pyrolysis of propylene 132, 137, 139 and pyrolysis of polyethylene over a Ni catalyst have been reported.154, 155 c.Pyrolysis of benzene and other aromatic compounds Benzene can also serve as the initial hydrocarbon for the synthesis of NT; this synthesis can be carried out using the equipment designed for the preparation of carbon fibres fromC6H6 .156 It was noted that the growth of NT and carbon fibres follow different mechanisms; indeed, catalyst particles were detected at the tips of the fibres, whereas the NT usually had conical caps free from inclusions. The preparation of NT from a C6H6±H2 mixture over an Fe catalyst requires as a rule lower partial pressures of C6H6 and lower temperatures than the synthesis of fibres. In some cases, the growth of NT is replaced by the growth of solid fibres. Solid fibres and NT prepared from the same hydrocarbon in single-type equipment differ markedly in mechanical properties � the fibres are easily broken on bending, whereas NT exhibit flexibility and elasticity.Pyrolysis of C6H6 over Co supported on SiO2 affos simultaneously bent and helical NT and fibres,157 the ratio of the yields of NT and fibers being determined by the size of the catalyst particles and the composition of the reaction mixture. A catalyst containing a higher proportion of Co (5 mass %) and consisting of larger particles (11 nm) results in a higher proportion of fibres than a catalyst with a lower metal content (0.5 mass %) and a44 medium particle size (7 nm). The replacement of the gas diluent H2 by N2 increases the yield of NT.The helical NT resulting from the pyrolysis of C6H6 differ from those formed upon the pyrolysis ofC2H2 (see above 144); they are multi-walled NT, the distance between the shells being intermediate between the corresponding distances in ordinary and turbostratic graphites. The shells themselves are largely disordered.157 Among other reasons, the formation of such structures might be due to the addition of hydrogen (the diluent of benzene) to the dangling bonds of NT. However, the catalyst particles exert a greater influence. This hypothesis is consistent with the model proposed by Amelinckx et al.,158 which assumes that the growth ofNTprogresses on the side of a free tip having an encapsulated catalyst particle. Deposits of NT with a definite texture can be obtained from gaseous C6H6.Thus pyrolysis of C6H6 on a profiled substrate with scratched parallel grooves gives rise toNT arranged parallel to the substrate surface and growing from the upper edges to the centres of the grooves.159 It was suggested that irregularities on the upper edges of the grooves exhibit enhanced catalytic activity. A similar expedient has been used in the pyrolysis of 2-amino- 4,6-dichloro-s-triazine.160, 161 A 10- to 100-nm thick Co film was sprayed onto a SiO2 plate. Then 1 to 20 mm wide tracks spaced 100 ± 200 mmapart were etched on the substrate using a laser. The substrate was placed in a furnace the coating downwards and pyrolysis was carried out at 950 8C. This gave rise to NT bundles having a strictly identical length and very close outer diameters (30 ± 50 nm).The NT obtained in this way contained up to 5% of nitrogen. When the plate was placed in the furnace the coating upwards, the situation changed sharply; strongly twisted NT were formed.161 This outcome has not yet been adequately interpreted. The researchers believe that etching of the Co film gives rise to energy-saturated metal or metal oxide clusters, which are depos- ited along the track edges. Hydrogen chloride, arising upon decomposition of triazine, can convert Co into volatile CoCl2 and thus influence the catalytically active particles at the tips of NT. The growth of NT is terminated when the catalyst species have lost activity or have been completely evaporated as the chloride.Pyrolysis of tris(aminotriazine) (melamine) and cyanuric acid over etched Co, Ni, or Fe films occurred in a similar way. A mixture of gaseous phenylacetylene and thiophene with He 162 and a mixture of 2-methyl-1,20-dinaphthyl ketone with He 163 in the presence of a Ni catalyst have also been used for the pyrolytic synthesis of NT. It is noted in both studies that at the first stage of pyrolysis, Ni particles are coated with a graphite layer; this is followed by the nucleation and growth of NT. The temperature of choice is 700 8C. Yudasaka et al.163 proposed an original procedure for the preparation of the catalyst. They sputtered a metal film on a quartz glass substrate and heated it in vacuo. The size of the Ni particles thus formed was determined by the thickness of the deposited film; in the optimal case, it was 20 ± 30 nm.It has been concluded 162 that NT are `extruded' from Ni3C particles; Yudasaka et al.163 noted an important role of the shape of Ni nanoparticles. 2. Pyrolysis in the presence of a `floating catalyst' A conventional supported catalyst is coated, sooner or later, by a layer of the products of pyrolysis of hydrocarbons and is thus deactivated. The introduction into the system of catalyst precur- sors as volatile compounds which decompose to give catalytically active species directly in the reaction area could, in principle, permit one to avoid the catalyst deactivation and to bring the process of pyrolysis closer to a continuous process.There are only a few publications in which this approach has been employed. Thus decomposition of Ni phthalocyanine at 700 and 800 8C has resulted in the synthesis of NT containing some nitrogen.164 These NT were multi-walled particles with large (*200 nm), small (10 nm) or variable diameters, their length reaching 6 mm. It has been assumed 164 that the mechanism of E G Rakov the NT growth includes encirclement (encapsulation) of Ni particles with a carbon layer, graphitisation of this layer, the change in the shape of Ni particles, the appearance of seeds and the growth of NT. The Ni particle inside theNTcan either migrate from the substrate to the growing tip or remain in a particular section of the NT, forming a node point. Decomposition of Fe and Co phthalocyanines is also accom- panied by the formation of NT, whereas Cu phthalocyanine gives neither NT nor graphitised particles.Ferrocene has been used as the `floating catalyst' in the pyrolysis of thiophene.165 Unlike the study cited above,164 in this case, single-walled NT were produced. The process was carried out at 1100 ± 1200 8C for 1 to 30 min. This yielded a large amount of long and relatively thick `ropes' and ribbons coloured silvery black. Some of the ribbons were semitransparent and not attached to the surface. The longest `ropes' were 3 ± 4 cm long and had a diameter of 0.1 mm, while the ribbons were up to several millimeters wide. Each `rope' and each ribbon consisted of several thousands of vaguely oriented threads, comprised by bundles of well oriented closely packed single-walled NT.The diameter of the bundles ranged from several to forty nanometers, the average diameter of the NT being 1.7 nm. It was found that the bundles are formed in the reaction area and carried away with the gas stream; outside the area they stick together forming `ropes' or ribbons. Pyrolysis of ferrocene and its mixtures with C2H2 has also been studied.166 3. Decomposition of CO Thermal decomposition of CO (disproportionation to CO2 and carbon) differs sharply from the pyrolysis of hydrocarbons from the thermodynamic viewpoint; at atmospheric pressure and low temperatures (300 ± 750 K), the equilibrium yield of carbon is nearly quantitative, while at higher temperatures and lower pressures, it diminishes.Conversely, the yield of carbon in the pyrolysis of C2H2 and CH4 increases as the temperature rises and the pressure is reduced and approaches a quantitative yield at 1250 ± 1500 K. In this respect, carbon monoxide may seem to be a less convenient starting compound for the synthesis of NT. How- ever, for kinetic reasons, a hydrocarbon is more difficult to heat to a temperature above 800 ± 900 8C (before it is brought in contact with the catalyst) than CO; hence, carbon monoxide offers some advantages. The first study 167 on the catalytic decomposition of CO to give NT was carried out in 1995, i.e. much later than NT were synthesised by other methods. Decomposition of a CO±CO2 gas mixture (20% of CO) was carried out at 500 8Cover the Ni/Al2O3 catalyst.A graphitised shell was formed around the Ni particle; nanotubes protruded out of this shell. The introduction of hydro- gen into the gas mixture changed appreciably the morphology of the deposit. Thermal decomposition of neat CO over a Ni ± Co catalyst supported on Al2O3 was performed at 1200 8C at a pressure somewhat higher than atmospheric. 168 This process gave rise to single-walled NT. The preparation of an active molybdenum catalyst used for the synthesis of NT from CO was an important achievement. The catalyst was prepared by impregnation of alumina with a methanol solution of bis(acetylacetonato)dioxomolybdenum fol- lowed by heating to 200 8C.The diameter of NT formed in the presence of this catalyst was 1 ± 5 nm. Catalyst particles with sizes of several nanometers were detected on the tips of many of the NT. This provided grounds for proposing a mechanism of the growth, which was called the `skull-cap' mechanism. According to this mechanism, a catalyst particle promotes dehydrogenation of the hydrocarbon molecules deposited on the particle from the gas phase. Carbon diffuses to the open tip of the NT (where the catalyst particle is located) over the catalyst surface or through the bulk and is embedded in the NT structure. It is the coatingMethods for preparation of carbon nanotubes consisting of chemisorbed carbon atoms on the surface of the catalyst particle that is referred to as `skull-cap'.It prevents the formation of dangling bonds. Micrographs of single-walled NT with catalyst particles located on the open tip and having a size equal to the NT diameter were reported.168 Testing of the Ni ± MgO, 146, 169 Co ±MgO170 and Ni ± AlPO4 169 systems as catalysts for decomposition of CO gave positive results. Not only multi-walled NT147 but also single-walled NT containing no deposit of amorphous carbon on the external surface have been synthesised from CO. None of the proposed mechanisms 171 is able to explain the influence of the nature of the initial gas on the morphology of NT and other structures. 4. Synthesis of nanotubes containing B ±C± N, C±N and B±N It has been noted above that the catalytic pyrolysis of triazine gives rise to nitrogen-containing NT.160 The pyrolysis of CH3CN.BCl3 at 900 ± 1000 8C over a Co powder afforded fibres andNTof the composition BxCyNz with various morphologies.172 Nanotubes of the composition C38N, the properties of which differed markedly from those of carbon NT, were obtained from pyridine in the presence of a Co catalyst; the nanotubes BC28N were synthesised from the adduct (CH3)3N.BH3. 173 All the nanotubes thus prepared were multi-walled. Since the yield of NT in the pyrolysis of the adduct NH3 .BH3 was very low, it was proposed that coating of metal particles with graphite is a necessary initial stage in the formation of boron- and nitrogen- containing NT. The data available to date are still inadequate for drawing definite conclusions about the reaction mechanisms, although the shape of some NT provided grounds for speaking about the unusual phenomenon of periodic motion of the catalyst particles located inside the NT along the NT axis.172 Previously a similar mechanism had been described for purely carbon NT with a bamboo-like structure.174 VI.Growth of nanotubes by decomposition of metal carbides When performing arc synthesis, researchers have noticed that particles of catalysts (carbides) are first covered by an envelope of several graphitised layers, which serve as the source for the growth of NT.175 The mechanism of formation of such a structure (which has been termed `sea-urchin') includes the formation of super- saturated solutions of carbon in a metal or metal carbide, subsequent segregation of carbon from these solutions and the `root' growth of NT.`Sea-urchins' were formed on particles of YC2 , 71, 175 LaC2 , 176 GdC2 , 177 Ni3C, 178 TiC 179 and other carbides. Heating of silicon carbonitride under a static atmosphere of N2 at 1400 8C or in a flow of N2 at 1850 8C resulted in the synthesis of multi-walled NT with a diameter of 10 ± 25 nm and with a length of up to 1 mm.180 According to a publication which appeared almost simultaneously,181 laser ablation of a-SiC resulted in sublimation of silicon and the formation of NT on its surface. Soon this process was even more simplified; in particular, laser treatment was replaced by resistivity heating of powdered 182 or single-crystalline 183 silicon carbide.Heating of powdered silicon carbide in vacuo at 1600 ± 1700 8C allowed preparation of NT on a relatively large area over a period of 10 ± 15 min. These NT grew at right angles to the external surface of the powder; they were substantially shorter than those obtained by the arc method but longer than those prepared by the laser ablation of SiC. Even more uniform NT (`a forest') were synthesised at a temperature of 1700 8C and a pressure of 1.361072 Pa on a 36560.34 mm plate. They grew to a length of 0.15 mm over a period of 30 min and were distinguished by a fairly regular mutual orientation. Apparently, this procedure is the simplest way of producing cathodes for field electron emitters.45 Small amounts of NT were formed in Fe7Ni and Fe7Ni7Co alloys containing carbon 184, 185 and in complex solid solutions containing Fe3C. 186 During subsequent studies, this procedure could be modified for the preparation of NT- containing composites possessing specific properties. VII. Other methods 1. Synthesis in the flame Combustion of hydrocarbons in oxygen is employed for the synthesis of diamonds (see, for example, Ref. 187) and full- erenes.188 Nanotubes can also be prepared in this way.189 It is of interest that the C:O ratios, which were equal to 1.06, 1.07 and 0.86 ± 1.00 for the combustion of C2H2, C2H4 and C6H6, respec- tively, are close to the optimal values for the synthesis of diamond. However, these processes were carried out under different conditions � in the synthesis of diamond, the inner cone of the flame was directed at a substrate cooled to a particular temper- ature, whereas in the preparation of NT, catalysts were used.The data on the synthesis of NT in flames are too scarce to analyse them, as has been done for the synthesis of diamonds (see, for example, Refs 190, 191). It is possible that NT, like diamonds, can be synthesised using any carbon compound, no matter what other chemical elements (halogens, sulfur, nitrogen, phosphorus, silicon, boron, etc.) are contained in the compound. 2. Electrolysis of molten salts Nanotubes can also be synthesised without participation of a gas phase, namely, in ionic salt melts. The electrolysis of molten LiCl at temperatures above 600 8C in a cell with carbon anode and cathode causes a heavy erosion of the cathode and appearance of sludge in the melt, which can be washed by water and toluene after cooling the melt.192 In addition to spheroidal and polyhedral particles, multi-walled (2 ± 10 walls) NT with a diameter of 2 ± 10 nm were detected in the sludge.The current density has a strong influence on the yield of NT. The yield and the quality of NT also depend substantially on the temperature of the melt and the nature of the salt used (LiCl, NaCl or KCl).193 The electrolysis should not be carried out for a long time because the sludge can short circuit the electrodes. The electrolysis of a LiCl ± SnCl2 melt resulted in the synthesis of NT filled with b-Sn, i.e.a nanowire.85 3. Chlorination of carbides and other methods According to a publication,194 back in the 1960s, multi-walled NT were obtained by chlorination of SiC and TaC at 800 ± 850 8C; the electron micrograph of these NT was published in 1978 in a book.195 Presumably, the coke that had been prepared using a graphite electric heater (with a temperature of at least 2500 8C) and employed for the synthesis of carbides, served as the source of NT.There also exist other methods for the synthesis of NT, for example, pyrolysis of the powdered polymer prepared by poly- esterification of oxalic acid and ethylene glycol,26 and the reaction of metallic Cs with nanoporous carbon, prepared from a mixture of polyfurfuryl alcohol and polethylene glycol, carried out at 50 8C.196 A method for the synthesis of NT similar to that described in Ref.148, has been proposed by KuÈ ttel et al. 197 The researchers prepared NT in a microwave discharge using a setup for the plasma synthesis of diamond coatings and a CH4 (2%) ±H2 (98%) mixture at a pressure of 0.4 kPa; Ni or Fe islets were applied preliminarily on the substrate, the temperature of the substrate being only slightly higher than the optimum temperature for the synthesis of diamonds (900 ± 1000 8C). The deposit had a `spa- ghetti' structure and consisted of NT with a diameter of 20 ± 60 nm and a length of up to 100 mm entangled with onenother and firmly attached to the substrate. The catalyst particles remained near the NT `roots' during the synthesis.46 The same researchers were able to perform the synthesis ofNT using the `classical' setup used for chemical vapour deposition of diamonds, namely a hot-filament reactor.Since setups similar to that used by KuÈ ttel et al. 197 are widely employed in dozens of laboratories in many countries and do not need to be modified for switching to the preparation of NT, further development of these methods for the synthesis of NT may be expected. Recently, syntheses ofNTby heating carbon black with boron at 2200 8C and by detonation of 2,4,6-triazidotriazine have been described.198 An interesting feature of the latter method (explosive synthesis) is that it permits preparation of NT with a record- breaking inner diameter equal to 80 ± 120 nm (although they are formed in a very low yield).VIII. Purification and opening of nanotubes Detailed investigaton into the properties of NT and some of their practical applications require individual and uniform open-tipNT containing no impurities. However, they are usually obtained as bundles consisting of NT with different lengths, capped at one tip and contaminated with impurities. Therefore, techniques for purification and opening of NT are as important as the methods used to synthesise them. Unlike fullerenes, neither individual NT nor, all the more so, bundles are soluble in any solvent, which complicates the problem of their purification. Nevertheless, several techniques which permit more or less successful purification of NT have been proposed.The methods are based on the following characteristic features of NT. The materials that typically contaminate NT (fullerenes, polyhedral graphitised particles, amorphous carbon) are more reactive than NT and some of them (for example, fullerenes) are soluble in organic solvents. Sections ofNTwith a higher density of defects also exhibit higher reactivity than defect-free NT. This refers first of all to the caps at the NT tips, which contain not only six-membered but also five-membered carbon rings. The carbon atoms in these rings are more reactive. The sections on the lateral surfaces of bent NT possess similar properties because kinks would be impossible without insertion of either five-membered (positive curvature) or seven-membered (negative curvature) carbon rings in a net consisting of standard six-membered rings.Finally, enhanced reactivity is also characteristic of the atoms at the edge dislocations of NT (the `scroll' or `papier-mache' structure), in which dangling bonds are concentrated together with defects arising upon replacement of carbon atoms by atoms of other elements. Multi-walled NT, which are normally more defective than single-walled ones, are more reactive, whereas NT annealed at high temperatures (annealing leads to elimination of defects) are less reactive. Nanotubes filled with metals or metal carbides differ in density from empty NT. The methods used to purify nanotubes can be divided into three groups�chemical, physicochemical and mechanical.1. Chemical methods The simplest method for opening NT is selective oxidation of the caps, which can be carried out by gases, melts or aqueous solutions. The gaseous oxidants used for this purpose are O2 (see Ref. 62), air,199 CO2 (see Ref. 200) and oxygen plasma.201 The maximum rates of oxidation with air are attained at 420 8C for C60, at 645 8C for graphite and at 695 8C for NT or nano- particles.199 Oxidation with O2 or with air proceeds most efficiently at 650 ± 750 8C. Even at 550 8C, a gas flow containing 1% of O2 causes disordering of the outer shells of NT and formation of wells with a diameter of 2 ± 10 nm on their surface; as a consequence, thin NT swell up.202 At 750 8C, gasification readily occurs at the sites of cracks, defects or deformations.E G Rakov After opening of NT, the oxidation slows down, whereas oxidation of nanoparticles goes on until they are completely eliminated.203 However, complete elimination of nanoparticles requires that more than 99% of the initial material be oxidised. Meanwhile, even after 95% oxidation, a sample contains only 10%± 20% of the initial NT. This is due to the fact that not only caps but also lateral walls of the NT are oxidised. As this takes place, multi-walled NT become thinner and some of them are completely gasified. To purify multi-walled NT in air, heating by IR radiation can be used.204 In particular, a 0.1-mm thick spongy paste with an area of *10 mm2 consisting of multi-walled NT was obtained in this way from the products of arc synthesis in an H2 flow.In some cases, it is sufficient to heat the material in air at 500 ± 600 8C for 30 min.205 Purification can also be carried out in hydrogen plasma.201 However, in practice, purification of multi-walled NT from amorphous carbon is carried out by treatment in an H2±N2 mixture at 900 8C.137 The method of oxidation in melts 59, 61 has not been further developed. In the case of single-walled NT, oxidation in aqueous solutions is apparently the most important method. The most frequently encountered procedure is refluxing in concentrated (60% ± 70%) HNO3 . 109, 203, 206 ± 208 The products of catalytic synthesis are freed simultaneously from the inevitable metal impurities.After refluxing for 4.5 h, the loss of mass in the material obtained by the arc method was less than 2%; as a result, 80% of NT became open.206 Refluxing of 10 g of the initial material produced by the laser ablation method in 1 litre of a 2 ± 3 M solution of HNO3 for 45 h resulted in a 70% loss of weight.108 Apart from treatment with nitric acid or after this treatment, NT are made to react with mixtures of HNO3 with H2SO4 and of H2SO4 withH2O2 (see Ref. 103). In some cases,H3IO5 and HClO4 (see Ref. 209) or solutions based on concentrated HCl 210 have been used instead of HNO3. Other compounds used as oxidants include H2O2, K2Cr2O7 , KMnO4 , solutions of Ru and Os chlorides in a solution of NaIO3, etc.Some of the oxidants (acidic solutions of H2O2 and K2Cr2O7) proved to be much less selective and efficient than HNO3 , some other (solutions of KMnO4 containing MnO2 or CrO3) exhibited medium activity, and a group of reagents (solutions of Ru and Os chlorides in a solution of NaIO3) exhibited a very high reactivity and ensured opening of up to 80% ±90% of NT at 100 8C.211, 212 Oxidation in solutions can be combined with filling of NT (for this purpose, a soluble metal salt is added to the solution 206, 210) or with chemisorption of metals on the NT surface.213 Chemisorp- tion is due to the fact that upon treatment with an acid, the surface of NT is coated with the acidic groups COOH, which can react with metal ions. The quantity of the Pd2+ ions absorbed was found to be strictly correlated with the concentration of acidic groups on closed or open NT.213 An especially high density of acidic groups is attained by using a mixture ofH2SO4 withHNO3 ; these groups promote deposition of finely dispersed metal clusters on the NT surface (see, for example, Ref.214). The data on wetting, filling and coating of NT have been briefly surveyed in a publication.215 Foreign particles can be removed using organic solvents, for example, toluene, carbon disulfide and other solvents, while metal particles can be removed with the aid of acids. In a pioneering study dealing with purification of NT,216 chemical modification of NT by grafting dichlorocarbene to the graphene wall at a double bond has been proposed.In a later publication,217 preparation of soluble NT has been reported. The researchers attached a long-chain amide in place of the carboxy groups at the tips of cut single-walled NT. This was attained by treatment of NT with thionyl chloride (70 8C, 24 h) and then with octadecylamine (90 ± 100 8C, 96 h). The product was readily soluble in chloroform, dichloromethane, aromatic compoundsMethods for preparation of carbon nanotubes and CS2 . These studies open up new prospects for the develop- ment of more facile methods for the pification, separation, study, description and application of NT.218 2. Physicochemical and mechanical methods In recent years, a number of physicochemical and mechanical techniques for the purification of NT have been proposed.They are briefly described below. A physicochemical method, chroma- tography, has been used to purify both multi-walled 219 and single- walled NT.220 The initial NT are dispersed in aqueous media, the dispersions being stabilised by adding surfactants. After separa- tion of the material into fractions using a column with a porous glass packing (with an average diameter of pores of 300 nm), NT are isolated from the individual fractions by centrifuging. Thus not only impurities are removed but, in addition, NT of different lengths are separated. `Cutting' of NT into 100 ± 300-nm long sections has been reported.221 The foundations of an electrophoretic method for the purifi- cation of NT (as a suspension in isopropyl alcohol) have been outlined in a study.222 Various mechanical procedures for the purification of NT are known.They include sonication, microfiltration and centrifug- ing.103, 199, 206 ± 208, 223 ± 225 Many of these procedures are labour- consuming and include large numbers of stages; in some cases, they are used only in combination with chemical methods. Sonication increases the density of defects, especially, in multi- walled NT. The ability of individual NT to form bundles (`ropes') manifests itself not only during the synthesis but also in the purification � in some cases, the purified material contains bundles with greater diameters than the initial material. Under some conditions of sonication in acids, the bundles of NT assume a ring shape with a diameter of 0.25 ± 0.55 mm.226 IX. Conclusion Particular examples of combined strategies for the purifica- tion of single-walled NT can be found in the literature.103, 208 Methods for the synthesis of NT are fewer in number, judging by reviews, 3, 7 than methods for the synthesis of fullerenes. Never- theless, the main range of these methods has already taken shape and now the question is what of these methods are best suited for the large-scale synthesis of NT. Different methods can only be compared conventionally because in most cases, the necessary characteristics are not fully available or not reported at all. First of all, methods that require vacuum or elevated pressures should be distinguished from the procedures of synthesis at ambient pressure.The former are less productive. The vaporisation of graphite induced by electron or ion beams and the use of diamond anvils also imposes some restrictions on the productivity. It is also expedient to classify as a separate group those methods that cannot be easily made continuous, for example, electrolysis or explosive synthesis. Many investigators agree in the opinion that the most widely used electric-arc and laser-ablation methods are applicable only on a laboratory scale, whereas pyrolysis should become the most important commercial method.131, 156, 165, 227 This method is similar in many respects to the pyrolytic method long employed for the production of carbon fibres both in the presence and in the absence of catalysts.The necessary equipment is relatively simple. It is the pyrolytic method that was used to prepare the longest known thread-like bundles of NT and macroscopic ribbons (`mats') of interwoven NT.165 The advantages of the pyrolytic method are manifested especially clearly in the production of ordered structures by deposition of NT onto a smooth substrate with a supported catalyst or onto a porous template. Neither arc nor laser methods can give such structures. Mechanical expedients have been developed which allow laying of NT of any origin parallel to one another (for example, dispersion in a matrix made of a polymeric 47 resin followed by cutting thin slices of the composite 228 or microfiltration of a suspension in ethanol through specific ceramic filters followed by transfer onto a plastic surface 229).However, they are all much more complicated and much less elegant than catalytic pyrolysis and cannot in full measure compete with it. Ordered structures fromNTprepared by the pyrolytic method are best suited for template synthesis, i.e. application of metals, for example, Ni or Co on their surface,142, 230, 231 for deposition of oxides,63 carbides 232 ± 235 or gallium nitride,236 for the preparation of catalysts, sorbents, membranes for electrochemical cells,152 field emitters and artificial muscles (actuators). Pyrolysis of hydrocarbons in flame 188 and in plasma 196 are also promising because both methods are facile and can be implemented in a continuous mode.Diamond coatings are already prepared on an industrial scale by means of plasma devices; the principal (perhaps, the only) condition for using them for the synthesis of NT is the presence of an appropriate catalyst on the substrate. However, it may happen that NT meant for particular purposes will be prepared by other methods which have not yet been tested, for example, in plasma jet devices. However, the use of solar energy concentrators for the preparation of NT can hardly be expected in Russia, because the intensity of solar radiation in Russia is relatively low and the level of development of this method does not surpass that for other methods of heating.237 Studies of carbon NT have provoked interest in the prepara- tion of nanotubes consisting of other inorganic compounds.Tubular structures are possible for a fairly large number of inorganic compounds of various classes.238 Studies on the syn- thesis of NT from BCx , BN, BCxNy and CNx are only at the beginning; however, in this case, too, pyrolytic methods would apparently prove to be the most convenient. The problem of the synthesis of NT with identical geometries might be solved using the method proposed by Smalley, namely, selection of homogeneous seed NT and their further growth by pyrolysis of hydrocarbons (http://www.dtic.mil/dusdst/agenda/ agenda31999.html). A complicated synthetic task would be routine preparation of NT with reproducible heterojunctions (variation of diameter, chirality or chemical composition); some enthusiasts believe that this could serve as the basis for the nascent nanoelectronics.The design of nanomechanical devices in which NT are expected to serve as important and abundant parts will also require great efforts. * * * Since this manuscript was prepared for the publication, new journal papers and Internet sites devoted to the synthesis of carbon NT have appeared (or become available). Thus the mechanism of formation of carbon NT, nanowires and nanoparticles in an electric arc burning under a hydrogen atmosphere has been described. The participation of polycyclic aromatic hydrocarbons in the synthesis of NT has been con- firmed. A direct correlation between the ability of H2 to remove graphenes from an anode and the yield of NT has been eluci- dated.239 It was found that only three platinum metals, Rh, Pd and Pt, induce the formation of single-walled NT in an electric arc, whereas mixtures of metals (Ru ± Pd, Rh ± Pd, Ru ± Rh, Ru ± Pt and Pd ± Pt) exhibit low or zero activity; only a Rh ± Pt mixture (1 : 1 or 5 : 2) is active in this process.240 At a helium pressure of 78 kPa over a Rh ± Pt catalyst, NT with approximately equal diameters (1.280.07 nm) were formed.A10 ± 12 fold decrease in the helium pressure resulted in the growth of nanotubes of different diameters (0.7 ± 1.3 nm).A novel method for the synthesis of NT in a pulsed arc setup has been developed.241 The highest yield of NT was attained in a48 flow of Ar or Kr at temperatures of >1000 8C and pulse duration of >1 ms.Interesting results concerning growing of NT by the electric- arc method have been obtained in a study by Chang et al.,242 who used an anode with a diameter of 6 mm and a 10-mm thick disk 30 mm in diameter as a cathode. Both electrodes were intensely cooled with water and the electrolysis was carried out in a self- sustained mode (interelectrode distanc3 ± 4 mm, voltage 20 V, current 55 ± 65 A, helium pressure 67 kPa). The cathode deposit was formed on an area with a diameter close to the anode diameter. The morphology of the deposit differed appreciably from the standard morphology. A bed of chaotically entangled slightly bent NT (diameter 10 ± 40 nm, length >30 mm), the axes of which were oriented randomly, was located under a thin tough shell. These NT had no defects, contained no amorphous carbon impurity and could be purified from the nanoparticle impurity by merely keeping in air (850 8C, 1 h).The intense cooling of the electrodes and gases near the cathode as well as the large distance between the electrodes, which influences the mode of the current passage, is assumed to be largely responsible for the sharp change in the morphology and properties of the arc synthesis products. A detailed study of some parameters of the laser-induced thermal synthesis of NT has been reported.243, 244 In particular, it was found that the amount of carbon being vaporised from the target is determined by the intensity of laser radiation, while the course of the chemical reaction yielding NT is governed by the temperature in the furnace.The rate of evaporation of catalysts (Ni and Co) is affected by both parameters. The laser radiation intensity has a slight influence on the diameter of NT, while a decrease in the furnace temperature results in smaller diame- ters.243 The increase in the delay between single laser pulses from 0.1 to 120 s has almost no influence on the structure of single- walled NT but decreases the yield of the web-like product.244 It has been shown theoretically 245 that the use of shorter pulses (picoseconds instead of nanoseconds) and higher frequencies (tens of megahertz) brings the synthesis of NT closer to a quasi- continuous mode.It has been shown 246 that the laser-assisted synthesis of NT in the presence of Ni and Co can also be performed in a flow of N2 . The application of a continuous-wave CO2 laser to the synthesis of NT without additional furnace heating of the target is a method new in principle.247 In this study, a vertical setup was used with a rotating target moving along its axis (a rod 6 mm in diameter) and made of a mixture of graphite with a catalyst. A hot zone with a diameter of*1 cm was formed around the laser spot; this resulted in local heating of the gas flow and prevented the plasma from cooling too fast. The synthesis was carried out under Ar. When the radiation power was 250 Wand the diameter of the laser spot was *1 mm, the vaporisation rate reached 200 mg h71.The yield of single-walled NT in the presence of Ni ±Y or Ni ±Co catalysts was*20% based on the amount of the vaporised material. French investigators have successfully tested a prototype of a 1000-kW solar concentrator for the synthesis of NT (http://www.uiuc.edu/cnrs/Cnrspresse/en352a3.htm). The formation of NT on irradiation of graphite with high- energy Ne+, Kr+ and Xe+ ions has been studied.248 Numerous publications in 1998 ± 1999 were devoted to the pyrolytic synthesis of NT. For instance, the influence of the substrate on the catalytic pyrolysis of C2H2 in the presence of Co or Fe has been considered. 249 The influence of the natures of the catalyst (Fe2O3 , CoO, NiO or their mixtures) and the support (Al2O3 or SiO2 with a specific surface area of 100 and 300 m2 g71, respectively) on the formation of single-walled NT in the pyrolysis of pure CH4 has been studied.250 Relying on the fact that the tips of growing NT contained no catalyst particles, the researchers 250 concluded that the process followed a `root' growth mechanism. The highest yield of individual NT was attained with the Fe2O3/ Al2O3 catalyst. E G Rakov The pyrolysis ofmetallocenes and Fe(CO)5 mixed withC2H2 or C6H6 has been studied.251, 252 It was found that the content of the amorphous carbon impurity can be diminished by introducing H2 into the gas phase. The conduct of the pyrolysis of C2H2 by means of a hot filament with plasma activation permitted the substrate temper- ature to be decreased to 650 8C.253, 254 Yet another novelty was the use of an NH3 admixture, which was found to function as a catalyst.The diameter of the NT produced in this way was dictated by the thickness of the Ni film sprayed preliminarily onto a glass substrate and varied from 20 to 100 nm. The growth rate was several times higher than that attained in earlier studies (see, for example, Ref. 133) and was equal to 120 mm h71. Pyrolysis of naphthalene vapour in the presence of Cr(CO)6 vapour with `anode field activation' was performed for the first time.255 The process at a voltage of 4 ± 6 kV, an anode temper- ature of 1100 ± 1200 8C and a vapour pressure of*8 Pa afforded carbon NT with Cr nano-rods protruding from their inner cavity.These rods had a strictly invariant diameter (*10 nm) along the full length (up to 0.5 mm). The attempts to accomplish a similar process with Mo(CO)6 or W(CO)6 failed. Yet another modification of the pyrolytic synthesis of NT includes two-stage heating in vacuo (first at 350 ± 450 8C and then at 500 ± 800 8C) of tripropylamine introduced in the channels of AlPO4-5 single crystals.256 The NT thus formed had equal diameters and equal lengths; however, they proved to be unstable with respect to HCl. Nanotubes consisting of a new promising material, carbon nitride, have been prepared on a graphite substrate from a C2H2±N2 mixture in a microwave plasma using a bias poten- tial.257 A peculiar method for the synthesis of B,N-containing NT is based on the replacement of C atoms in carbon NT by B and N atoms upon the reaction of NT with B2O3 vapour diluted with N2 .258 The preparation of multi-walled NT filled with molten UCl4 or its mixtures with KCl has been described.259 A method for purification of NT (prepared by the pyrolysis of C2H2 over Co introduced into NaY zeolite) has been described, which involves dissolution of impurities in hydrofluoric acid and subsequent oxidation of NT with a solution of KMnO4 or air.260 Afairly comprehensive study 261 is devoted to the behaviour of NT treated with a mixture of concentrated HNO3 and H2SO4 . The presence of acid functional groups on the surface of these NT makes it possible to prepare viscoelastic gels; on drying, solid materials or films with specific structures and properties can be obtained.Under particular conditions, the dispersions can be converted into a material reminiscent of liquid crystals. Fullerenes have been synthesised by irradiation of C6H6±O2 orC6H6±N2Omixtures with aCO2 laser in the presence of SF6.262 This method might also enable the synthesis of NT. The preparation of Langmuir ± Blodgett films from matrix- diluted single-walled NT and application of the films onto various surfaces using micelle-like aggregates has been reported.263 Soluble carbon NT coated with poly(phenylacetylene) have been synthesised.264 Many of the studies mentioned above were reported at the conference on commercialisation of the achievements in the large- scale production of carbon NT (USA, Washington, April 1999; http://www.knowledgefoundation.com/carbon.html). The pros- pects for the investigation and use of NT-based materials were also discussed at the conference of the Materials Research Society (USA, Boston, December 1998), which has been described in a brief review.197 Studies on the application of NT in electronics were surveyed by Johnson at the International Conference on Solid-State Circuits (USA, San-Francisco, January 1999; http:// www.eet.com/story/OEG19990217S0045). Several new promising fields of application of NT have appeared quite recently. The greatest interest was aroused by the development of a prototype of artificial muscles, actuators, activated by application of a low electric potential, which wasMethods for preparation of carbon nanotubes reported by an international group of scientists 265 ± 268 (see also M Fox Infoseek News May 21, 1999; http://www.mpg.de/news99/ news26_99.html; and http://www.msnpc.com/news/271534.asp).A new route to the design of NT-based parts of electronic devices (amplifiers, switches, logical elements) was discovered.269 The possibility of isotope separation with the aid of NT270 and of usingNTfor nanolithography 271 was demonstrated. An electrode manufactured from a single NT was developed and tested.272, 273 Recent reviews and issues of journals devoted to NT are also worth mentioning.274 ± 282 References 1. S Iijima Nature (London) 354 56 (1991) 2.Carbon 33 1011 (1995) 3. J C Withers, R O Loutfy, T P Lowe Fullerene Sci. Technol. 5 1 (1997) 4. 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ISSN:0036-021X    

出版商:RSC    

年代:2000

数据来源: RSC

摘要:

Russian Chemical Reviews 69 (1) 53 ± 80 (2000) Hybrid polymer-inorganic nanocomposites A D Pomogailo Contents I. Introduction II. Preparation of hybrid nanocomposites by the sol-gel method III. Preparation of nanohybrid multimetallic materials by the sol-gel method IV. Preparation of template synthetic nanobiocomposites by the sol-gel method V. Intercalation of polymers into porous and layered nanostructures VI. Metal chalcogenide ± polymer inclusion nanocomposites VII. Metallopolymeric Langmuir ± Blodgett films �self-organised hybrid nanocomposites VIII. Major fields of application of hybrid nanocomposites IX. Conclusion Abstract. Approaches to the preparation of organic ± inorganic nanocomposites are considered from a unified viewpoint for the first time.The major problems in the development of this new line of research in materials technology, which has arisen on the border of the science of polymers, colloid chemistry and physical chemistry of the ultradisperse state, are discussed. The main methods for the formation of composite materials and poly- mer ± inorganic cross-linked hybrids with interpenetrating net- works are analysed. Primary attention is given to sol-gel procedures for their preparation, including template processes, which occur under conditions of strict stereochemical orientation of reactants, intercalation of monomers and polymers into porous and layered matrices and their intracrystalline and post-interca- lation transformations. Methods for the synthesis and properties of metallopolymeric polymolecular Langmuir ± Blodgett films, which are peculiar supramolecular ensembles incorporating nano- sized metal-containing particles, are discussed.The generality of the processes of formation of organic ± inorganic nanocomposites in living and nonliving natural objects is demonstrated and the major fields of application of nanocomposites are considered. The bibliography includes 566 references. I. Introduction The science dealing with nanocomposites, which belong to the class of composite materials characterised by nanometer sizes of their structural elements (particles of metals, metalloids, their oxides, chalcogenides, etc.), has arisen in recent years on the border of different areas of knowledge.Apparently, the term `nanocomposites' was proposed for the first time by Theng in 1970.1 In the literature, the terms `hybrid nanocomposites,' `nanohybrids' and `nanostructural composites' are generally used and the terms `metallomatrix composites,' `monophase hybrids,' etc. are more rarely used for describing materials A D Pomogailo Institute for Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region, Russian Federation. Fax (7-096) 515 35 88. Tel. (7-096) 524 50 20. E-mail: adpomog@icp.ac.ru Received 10 January 1999 Uspekhi Khimii 69 (1) 60 ± 89 (2000); translated by T N Safonova #2000 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2000v069n01ABEH000506 53 54 59 63 64 68 69 72 74 consisting of an organic phase (polymer) and a nanodispersed mineral phase. If a polymer of biological origin serves as a component or a precursor of these polymer ± inorganic materi- als, the term `nanobiocomposites' is used.An organic phase can capture metal-containing particles into a `trap' of a peculiar kind, viz., into an oxopolymeric network or a polymeric unit with appropriate parameters. Silicon, aluminium, titanium, zirconium, vanadium and molybdenum oxides, glasses, clays, layered silicates and zeolites, metal phosphates and chalco- genides, iron oxychloride and graphite are used as inorganic precursors. Zeolites (molecular sieves) are of particular interest since methods for control over their pore sizes are well known.Not only carbon-chain but also organoelement (generally, orga- nosilicon) polymers are used as a polymeric component. In the resulting nanocomposite materials, distances between networks and layers formed by polymeric and inorganic ingredients and sizes of particles formed, including metal-containing particles, are nanometric.2, 3 Generally, hybrid nanocomposites exhibit a synergism of the properties of the initial components. These compounds are characterised by an enhanced mechanical strength and thermal stability and provide optimum heat transfer.4, 5 In metallomatrix composites, the strength and hardness of soft metals, for example, of aluminium, are enhanced. These materials possess good thermochemical, rheological, electrical and optical properties.6 ±8 These compounds are used as chromatographic carriers, mem- brane materials,9 optical and magnetic materials, components of polymeric compositions, carriers and catalysts of various reac- tions (see Section VIII).Currently, numerous procedures for the preparation of nano- composite materials are available. Recently, the major synthetic approaches (evaporation of elemental metal with its deposition on polymeric matrices, plasma-induced polymerisation, vacuum evaporation of metals, thermal decompositions of precursors in the presence of polymers and reduction of metal ions using different procedures, including electrochemical, etc.) have been surveyed in reviews.2, 3 However, the uniform distribution of ingredients is generally difficult to achieve when hybrid nano- composites are prepared with the use of the above-mentioned procedures resulting in the nonuniformity of the properties of the material. The following three principal procedures are most commonly employed: (1) the sol-gel method; (2) intercalation of polymers and nanoparticles into layered structures (including54 polymerisation in situ) with the use of approaches applied in the chemistry of intracrystalline structures (`guest ± host' systems); (3) a combination of polymerisation and formation of nanosized particles, which provides homogeneous dispersion of an inorganic component (which is often a specific filler) in a polymeric matrix.In the last decade, these procedures, particularly, the first and second methods, have received primary attention.The properties and procedures for the synthesis of metallopolymeric Lang- muir ± Blodgett films are being extensively studied. The results of investigations in this field are summarised in a number of monographs and reviews 10 ± 34 as well as in special issues of journals.35 ± 37 Note that nanocomposites containing not only synthetic but also natural molecules, including biologically active macromolecules, can be conveniently prepared using the above procedures. These procedures have been used for constructing biosensors, enzyme electrodes and other materials based on enzymes. These materials have already found use in biochemis- try, medicine, biotechnology and the technology of environmental control.In the present review, procedures for the preparation of hybrid organic ± inorganic nanocomposites are considered from a unified viewpoint and the generality of the processes of their formation in living and nonliving natural objects is demonstrated. Thanks to the help of colleagues from different countries who responded to my requests and sent copies of their latest articles, the review surveys fresh results and the data under consideration were generally published within the last decade. II. Preparation of hybrid nanocomposites by the sol-gel method From the ecological viewpoint, optimum procedures for the preparation of composite materials are those without outflow, in particular, the sol-gel method (sol-gel or spin-on-glass process).The latter method allows one to exclude numerous stages of washing because compounds, which are used as the initial compounds, do not contaminate the final product. The sol-gel method is a convenient procedure for the preparation of dispersed materials, which are often called ceramers. This method is based on polymerisation reactions of inorganic compounds (formation of metallooxo polymers in solutions) and involves the following stages: (1) preparation of a solution, (2) formation of a gel, (3) drying and (4) thermal treatment. It is commopractice to use metal alkoxides M(OR)n (M=Si, Ti, Zr, VO, Zn, Al, Sn, Ce, Mo, W, lanthanides, etc.; R=Alk or Ar), which are hydrolysed upon addition of water, as the starting compounds.The reactions are carried out in organic solvents. Subsequent polymerisation (condensation) affords a gel. For example, when n=4 M(OH)4+4ROH, M(OR)4+4H2O(MO2)m+2mH2O. mM(OH)4 This method is sometimes called `a one-stage procedure' because both reactions, viz., hydrolysis and condensation, are catalysed by the same compound (by acid, base or, sometimes, by a nucleophilic catalyst, such as NaF).14, 38 These reactions with the participation of Si(OR)4 have long been known. Apparently, the preparation of monodisperse TiO2 particles by hydrolysis of alkoxides has been described for the first time by Barringer and Bowen.39, 40 It is needless to say that the real process is much more complicated and occurs according to a multiple-route mecha- nism.Generally, metal oxoalkoxides MOn(OR)m, including poly- nuclear compounds, are formed as intermediates. Thus hydrolysis of Ti(OR)4 proceeds in two stages,27, 41 viz., nucleophilic replace- ment of the alkoxy group by the hydroxy group and condensation in the course of which oxo and hydroxo bridges are formed. It was demonstrated that the rates of these reactions are comparable.42 The TixOy(OR)4x72y compounds were isolated and character- ised.43 Generally, the controlled synthesis of hybrid nanocompo- A D Pomogailo sites based on zirconium alkoxysilanes and alkoxides proceeds according to Scheme 1. Scheme 1 1. Formation of complex 1: OR3 Zr CC(Me) C(Me)COOH CH2. Zr(OR3)4+CH2 OO OR3 1 2. Hydrolysis: H+, H2O R1 Si OR2 R1 Si OH+R2OH.MeOH 3. Condensation to form latent water: 2 R1 Si OH R1 Si O Si R1+H2O. 4. Addition of complex 1 and removal of free and latent water by condensation: +2R3OH. Zr O Zr 2 Zr OR3+H2O 5. Addition of a photoinitiator: block Transparent liquid thin films The following reaction conditions are of great importance: the use of catalysts, including polymeric catalysts (for example, polystyrenesulfonic acid 44); the nature of the alkoxy group and of metal [for example, Ti(OBu)4 is hydrolysed almost 150 times more slowly than Ti(OEt)4 (see Ref. 45)]; the use of alkoxides of a mixed type, particularly, with chelate ligands (b-diketonate, a- or b-hydroxy acid, polyol, etc.); the degree of association of alk- oxides {for example, in the case of [Ti(OEt)4]m m=2 or 3}; the formation of oxo or alkoxo cluster structures [for example, Ti18O22(OBu)26(acac)2] in the course of hydrolysis.The reactivity of alkoxides MIV(OR)4 increases in the series Si Sn and Ti<Zr<Ce.46 The ionic radius of the M atom (0.04 and 0.06 nm; and 0.064, 0.087 and 0.102 nm, respectively), its coordi- nation number (4 and 6; and 6, 7 and 8, respectively) and the degree of its nonsaturation (the difference between the coordina- tion number and the valence; 0 and 2; and 2, 3 and 4, respectively) increase in the same order.47 However, theH2O: M(OR)4 ratio (g) is of prime importance. Thus in the case of VO(OPrn)3, a homogeneous transparent gel with an alkoxide polymer network is formed in n-propanol if g=3, whereas if g>100, the resulting gel attains a quite different structure, which prevents the forma- tion of an inclusion compound. Besides, the formation of a gel is affected by the nature of the medium, the concentration of the initial solution of an alkoxide, the presence of a stabiliser and the reaction temperature.48 ± 50 The thermal effects of hydrolysis of Ti(OR)4 in ROH increase when g=0.2 ± 1, whereupon they remain virtually unchanged.This corresponds to the thermal effect of replacement of one alkoxy group by the hydroxy group.51 The solid phase with composition TiOx(OBu)472x . yBuOH was obtained upon hydrolysis of Ti(OBu)4 in butyl alcohol through intermediate formation of TiO(OBu)2.45, 51 The x and y values increase as the concentration of titanium alkoxide in a solution increases.Unfortunately, the effect of the nature of the solvent (generally, alcohols) on these processes has received little attention in experimental studies. It is known that hydroxy-containing compounds (for example, AlkOH, HOH or R3SiOH) can take an active part in trans- esterification in the systems M(OR1)4+nHOR2 M(OR1)47n(OR2)n+nHOR1.Hybrid polymer-inorganic nanocomposites The nature of metal and the ligand in alkoxides affects substan- tially these reactions.52 ± 54 Thus Ti(OPri)4 and OV(OPri)3, which are often used as cocondensates in the preparation of nano- composites by the sol-gel method,55 act as catalysts in trans- esterification of tetramethoxy- and tetraethoxysilanes (TMOS and TEOS, respectively).Therefore, the sol-gel process involves hydrolysis, polymer- isation (chemically controlled condensation) of a gel precursor, nucleation and growth of particles followed by their agglomer- ation.56, 57 TMOS and TEOS are most often used as precursors. They form silica gel structures (`host') about a dopant (`guest'), thus creating a specific cage trap. Nucleation proceeds through formation of a polynuclear complex. Its concentration increases until slight supersaturation, which is determined by the solubility of the complex, is achieved. From this time on, new nuclei do not form and only the available nuclei grow. To put it differently, these processes are analogous to the processes of formation of nanosized particles in polymers.2 In the stage of formation of a gel (gelatination), it can be impregnated with ions of different metals.The resulting oxopolymers have structures of ultrathin porous networks with pore sizes of 1 ± 10 nm similar to the structures of zeolites. Their specific surface (Sspec) varies in the range of 130 ± 1260 m2 g71 depending on the conditions of the synthesis, and the bulk density is 0.05 ± 0.10 g cm73. Thus molecular sieves based on silicon oxides, which are characterised by different mesopore structures and Sspec of *1000 m2 g71, were prepared by the sol-gel method in the presence of surfactants.58, 59 The specific surface of the powder prepared by hydrolysis of Ti(OEt)4 was 200 ± 300 m2 g71; Sspec of hydrolysis products of Ti(OBu)4 strongly depends on the concentration of the initial solution of alkoxide and attains 490 m2 g71 at the concentration of 1.0 mol litre71.45 Concentrated aggregatively stable hydrosols (containing up to 40 mass% of TiO2) can be prepared 60 by secondary dispersion of xerogels formed after drying of the sol (the TiO2 content is * 85 mass%) at 100 8C for 2 h.The sol contains crystalline TiO2 particles of an average size *3 nm. These particles are a mixture of two polymorphic modifications of TiO2, viz., rutile and anatase, with the latter slightly predominat- ing. The third modification (brookite) is formed much more rarely. In these systems, single crystals of the anatase phase are often metastable.Compounds containing labile vinyl, methacry- late, epoxy and other groups can act as cross-linking agents (see Scheme 1). The conditions of drying in the course of which volatile components are removed determine the texture of the product. Thus coarse-dispersion xerogels can form upon pro- longed drying in air due to coalescence of gel particles. Under conditions precluding the action of capillary forces, highly dispersed aerogels are formed. The formation of the structure and texture of the product is completed in the stage of thermal treatment.10, 61 ± 65 The sol-gel method with the use of highly dispersed silica gel in a polar solvent is applied in microencapsulation of photochromic fluorescent compounds,66 scintillators 17 and porphyrins.A sol- gel matrix for encapsulation is prepared by simultaneous hydrol- ysis of Si(OEt)4 ± Zr(OBu)4 and other alkoxides.67 ± 69 Thermally stable materials, which are used in nonlinear optics, can be prepared by introducing, for example, azo dyes into TMOS by the sol-gel method.70 Although these systems (except for bio- logically active systems; see Section IV) are not considered in this review, they are structurally similar to nanocomposites and are prepared according to analogous procedures. Thus the introduc- tion of methyl methacrylate (MMA), in which an organic dye of the perylene type was dissolved, into a xerogel followed by thermal or UV polymerisation, afforded a coimpregnated SiO2 ± poly- methyl methacrylate ± dye system.71, 72 The technique for subli- mation dehydration of impregnated gels was developed.73 This technique allows one to prevent redistribution of labile compo- nents in the matrix in the course of drying.Sol-gel materials are classified by a mode of formation and types of bonds between organic, organometallic and inorganic 55 components.17 Organic groups introduced into R1n Si(OR2)47n serve two functions,74, 75 viz., modify a network (ORMOSIL, organically modified silicates) and form a network (ORMOCER, organically modified ceramics). However, this classification has not yet received ample recognition. Low- percentage and high-percentage composites contain 2 vol.%± 30 vol.% and up to 45 vol.%± 75 vol.% of ceramics, respectively. Thus a material under the name `rubber ormosil' (based on TEOS and polydimethylsiloxane with Mw=1700) contains more than 70 vol.%of an inorganic component.76, 77 Hybrid nanocomposites are primarily classified by the type of interphase interactions between components.Thus nanocompo- sites whose macrostructures are determined by the presence of van der Waals, hydrogen or hydrophilic ± hydrophobic interactions are known. These are, for example, amorphous inorganic nano- composites, which are prepared from silicon-, titanium-, alumi- nium- or zirconium-containing oxopolymers formed in situ in a medium of a soluble organic polymer. Formally, this process consists in homogeneous formation of a nanosized filler in a medium of a polymer. Poly(n-butyl acrylate), polyphosphazene, polyvinylpyrrolidone (PVPr), poly(N,N-dimethylacrylamide), etc.were prepared according to this procedure.78 ± 82 An alter- native procedure involves the insertion of a polymer (or its precursor) into an oxogel, which is formed by mixing of metal alkoxides or by their impregnation into pores of an oxide xerogel network. Either organic molecules or polymers are inserted as `guests' into nanocomposites of another type. The one-stage formation of oxogels and polymers is exempli- fied by sol-gel polymerisation (including electrochemical polymer- isation) of the so-called silanised monomers, viz., N-[3-(trimethoxysilyl)propyl]pyrrole,83, 84 2,5-bis(trimethoxy- silyl)thiophene 85 and trimethoxysilylferrocene derivatives.86 Hybrid network composites in which organic and inorganic components are linked through strong covalent or ionic bonds have received primary attention. These networks are generally prepared according to two procedures: (1) secondary networks are formed in primary networks, which are functionalised as required; (2) two different networks (including interpenetrating networks) are simultaneously formed from molecular precursors of a different nature, which enter into different reactions (poly- addition, polycondensation, metathesis polymerisation, hydroly- sis ± condensation, etc.).This line of investigation is in a primitive state of development and reliable examples of realisation of this approach are few in number.87 ± 89 Initially, it was attempted to use solutions of polymers.However, the formation of a gel was accompanied by uncontrolled phase separation to yield a hetero- geneous material. More homogeneous sol-gel materials were prepared 90 by acid-catalysed hydrolysis simultaneously with condensation of HO(PES)OH [PES is (polyethersulfone)] and TEOS or TMOS in DMF. SiO(PES)OH+ROH SiOR+HO(PES)OH Inorganic and polymeric components are linked via deep interactions. Thus poly(ethersulfone) chains are linked through 7SiO27 units. Their condensation affords nanoparticles (Fig. 1).91 Bonding of TEOS, diethoxydimethylsilane or their mixture with sulfo groups of perfluoroalkylsulfonate ionomers (under the commercial name Nafion) in which the SO3H+ groups form clusters with sizes 3 ± 5 nm affords a hybrid heterogeneous material Nafion/SiO2 (Nafion/ORMOSIL).92 ± 96 Studies by dif- ferential scanning calorimetry demonstrated that a broad endo- thermic peak of the initial NafionH+ at 215 8C was shifted to the lower-temperature region as the ratio of the components of the reaction mixture was changed.This is associated with condensa- tion between the sulfo groups of Nafion and the silanol groups. In the course of the sol-gel process with the participation of TEOS and the styrene ± maleic anhydride copolymer in the presence of a56 Nanoparticle HO OH Si Linkage O O O Si OH HO Si PES OH O OH O O Si O Si O OH OH OH Figure 1. Formation of a hybrid nanocomposite in the reaction of PES with bridging7SiO27groups.binding agent, viz., (3-aminopropyl)triethoxylsilane, a covalent bond between polymeric and inorganic components was also formed.97 In this case, particles with sizes<20 nm were formed. H2N(CH2)3Si(OEt)3 CHPhCH2CHCH TEOS, H2O, NH4OH O O Si C C O O CHPhCH2CHCH O O SiO Si O HO2C C(O)NH(CH2)3Si Si O O Si O A series of monolithic hybrid materials containing 23 vol.%± 100 vol.% of an inorganic component were prepared by the sol-gel method starting from SiO2 and the copolymer of methyl methacrylate with 3-(trimethoxysilyl)propyl methacryl- ate.98, 99 Polyimide composite materials containing nanosized SiO2 or TiO2 particles possess the high mechanical strength due to their ability to form three-dimensional inorganic networks.100, 101 Polyimide ± polysilsesquioxane { composites were prepared, for example, by condensation of 1,1-bis(4-aminophenyl)-2,2,2-tri- fluoro-1-phenylethane with a derivative of pyromellitic anhy- dride and aminophenyltrimethoxysilane (simultaneously with imidisation at 250 ± 350 8C).104 ± 107 In this case, films of hybrid nanocomposites containing 32% ±70% of homogeneously dis- { Polysilsesquioxanes (PSO) belong to a class of three-dimensional organosilicon oligomers of the general formula (RSiO1.5)n containing the inorganic cubic Si8O12 nucleus.These compounds have polyhedral structures with different symmetries and are key intermediates in the formation of various nanocomposites (see, for example, Ref. 102). Thus the synthesis and properties of hyperelastic materials, which have been prepared by copolymerisation of 4-methylstyrene and oligomeric silses- quioxane macromers, have been reported recently.103 O Ph CO2Et Cl + NH2 H2N Cl CF3 EtO2C O O CO2Et Ph (EtO)3Si R HN NH EtO2C CF3 O ; B is a base.R=(CH2)m, A D Pomogailo persed SiO2 with particle sizes of 0.5 ± 7 nm were obtained (Scheme 2). Polyamide ± SiO2 compositions were prepared in situ by suspending SiO2 nanoparticles (which were sometimes prelimi- narily modified with aminobutyric acid) in a polymerised e-caproamide at high temperature.108, 109 This approach provides homogeneous distribution of SiO2 owing to which the mechanical properties of nanocomposites are improved, the glass transition temperature rises and the rate of crystallisation is increased.Analogous nanocomposites were also prepared 17 based on analogues of polyimides, viz., polyoxazolines, including those containing triethoxysilane groups.110 ± 112 Metathesis polymerisation accompanied by opening of the ring and free-radical addition of cyclic alcohols appears to be the optimum procedure for the synthesis of hybrid SiO2 ± polymer nanocomposites exhibiting the minimum shrinkage.31, 113 ± 115 The synchronous formation of interpenetrating networks occurs as a result of competitive reactions of polymerisation and hydrolysis of silicon alkoxide under the action of the nucleophilic catalyst NaF followed by condensation. O O H2O, NaF O(CH2)2OH O(CH2)2O Si 7SiO2 4 CHMe CH2 SiO2 interpenetrating network O HO(CH2)2O n O O H2O, NaF 7SiO2 CH2OH CH2O Si 4 O SiO2 interpenetrating network CH2OH n It is known that sols are thermodynamically unstable systems with high surface free energies.It is necessary to stabilise these systems by special techniques, for example, by controlling pH of the system. When the sol-gel method is used, nanosized particles or their precursors are stabilised through adsorption of special monomeric molecules on the sol surface.116 ± 119 Thus carboxylic acids (including polymeric acids 106, 120) are strongly bound to the surface of SiO2, ZrO2, TiO2 or Al2O3 particles. Amines are specifically adsorbed on metallic Pd or Au particles.Thiols are even more specific reagents for Au. Controlled hydrolysis with the use of bifunctional molecules containing a double bond along with hydrolysed silane or ZrOR groups affords the corresponding precursors (see Scheme 1). These precursors incorporate ZrO2 particles of size*2 nm, which can undergo copolymerisation, for example, with methacrylic acid (MAA). The latter simultaneously acts as a surface modifier due to the presence of a double bond. Scheme 2 H2NRSi(OEt)3, B O EtO2C NH R Si(OEt)3 PSO, H2O, D NH hybrid nanocomposite CO2Et n OHybrid polymer-inorganic nanocomposites Alkoxide Zr(OR)4 is readily hydrolysed. Direct hydrolysis of the latter affords a precipitate of ZrO2 . nH2O, which is unsuitable for the preparation of a homogeneous composite material.In strongly acidic media, a flocculent hydrated gel layer is present on the surface of these particles. This layer contributes significantly to the stability towards aggregation.121 It is believed 122 that this layer makes it possible to perform repeated peptisation of ZrO2 from a xerogel. When bound to MAA, Zr(OR)4 is hydrolysed more slowly and hence well-dispersed nanosized ZrO2 particles are formed in the presence of latent water. Silanes modified with MAA undergo copolymerisation. In this case, the resulting silicon-containing polymer is used as a matrix material for nano- sized zirconium particles (Fig. 2).76 A convenient procedure for the synthesis of these nanocomposites involves dispersion copoly- merisation of monomers containing trialkoxysilyl groups.123 ± 127 Thus statistical copolymerisation of silicon ± magnesium mono- mers affords beads of sizes 100 ± 500 nm from which organic compounds are removed upon heating to 1000 8C in the presence of wet air.The sizes of the final particles are*100 nm.128 C(Me)COO(CH2)3Si(OMe)3+ CH2 H2O polymer precursor 1000 8C AIBN is an initiator [2,20-azobis(isobutyronitrile)]. A nanocomposite material was prepared starting from meth- acryloxypropyltrimethoxysilane.76 The reaction of equimolar amounts of Zr(OR)4 and MAA yielded a product, which was mixed with silane. The latter was preliminarily hydrolysed and condensed in the presence of 0.5 MHCl, and thenH2Owas added. A photoinitiator and alcohol were introduced into the resulting mixture to control the viscosity and then a support was submerged in the mixture thus depositing a thin film of a photosensitive material.Hybrid nanocomposite materials are prepared from metal- containing monomers, such as alkoxy derivatives of Ti and V of the general formula Ti(OR1)3(OR2) 129 or OV(OR1)37n(OR2)n,130 respectively, where R2OH is ethylene glycol monomethacrylate, 2-hydroxyethyl methacrylate, furfuryl alcohol, 1,1-dimethylpent- 4-en-2-yn-1-ol, propargyl alcohol or other unsaturated alcohols, Si O Si O O O O O Si monomer Si Figure 2. Use of functionalised silanes for cross-linking of organic and inorganic components of the system. C(Me)COO]2Mg [CH2 MeOH, AIBN 65 8C fine ceramic powder C Si COC Si O C O C CO (CH2)3 O C O C C Si Si 57 which are able to undergo polymerisation.Radical polymer- isation of metal-containing monomers affords polymers with side chains containing alkoxy groups of titanium 131 or vana- dium.132 The reactions of MAA with alkoxy derivatives of titanium yield metal-containing monomers [CH2=C(Me)COO]. .Ti(OR)3 (R=Et,CH2But or But).133 Based on these compounds, nontoxic, self-polished and anticontamination coatings were prepared by free-radical copolymerisation with MMA or by etherification of the resulting polymer and titanium tetraalkox- ide.134 Polytitanosiloxanes are prepared in one stage by simultaneous controlled hydrolysis (actually, cocondensation) of Si(OEt)4 and Ti(OPri)2(acac)2.134 In this case, a ladder polymer containing Si7O7Si and Si7O7Ti units is formed.Their ratio depends on the conditions of the synthesis and can reach 10 : 1, respec- tively. This ratio determines the time of gel formation and the possibility of formation of fiber ceramics upon annealing of the material (500 ± 900 8C).135 The structures, the mechanism of growth and the properties of siloxane composites containing the silicon, titanium and mixed titanium-silicon phases (including those prepared in situ) were considered in more detail in the review.136 The sol-gel method was used 137 for the synthesis of tita- nium ± silicon nanocomposites based on perfluorosulfonate ion- omers as well as for the preparation of nanosized particles with sizes 10 ± 50 nm [from Ti(OR)4 and Zr(OR)4] incorporated into matrices of a copolymer of styrene and 4-vinylphenol,138 etc. These approaches can also be used for the activation of surface polymers, for example, perfluorinated polymers [such as polyte- trafluoroethylene (PTFE) or copolymers of tetrafluoroethylene with hexafluoropropylene] by their treatment with SiCl4 followed by controlled hydrolysis.139, 140 Metal (primarily, titanium and silicon) alkoxides are used as cross-linking reagents for many natural polymers (polysaccha- rides, cellulose derivatives, vegetable oils, etc.141, 142).These polymers contain highly active hydroxy groups, which can form oxopolymers in situ. Organic networks are also formed in the reactions of low-molecular-weight compounds containing several hydroxy groups (for example, anthracenobisresorcinol and its derivatives) with (PriO)2TiCl2.143 Cocondensation of macromers containing trialkoxysilyl groups with the best known polymers [modified polystyrene (PS), polyoxazoline, polyimide, polyethylene glycol (PEG), poly- etherketones, polymethyl methacrylate and derivatives of poly- tetramethylene oxide] is a convenient procedure for the preparation of telechelic polymer networks.106, 144 ± 150 The insertion of highly dispersed SiO2 particles into a polymerised system is also widely used for the preparation of nanocomposite materials.Although a survey of this problem is beyond the scope of the present review, let us consider several most prominent, in our opinion, examples.In aqueous media, spherical SiO2 particles possessing a high specific surface act as peculiar dispersing agents. In this manner, particles of a rasp- berry-shaped composite with sizes 100 ± 300 nm were formed in polyaniline ± SiO2 or polypyrrole ± SiO2 systems.151 ± 156 In the course of polymerisation initiated by the redox FeCl3 .6H2O±(NH4)2S2O8 system, not only spherical but also fiber-shaped SiO2 particles were introduced into polypyrrole formed.157 The particle sizes vary within 80 ± 200 nm as the SiO2 : polypyrrole ratio changes. Composites based on SnO2, which are used in lithography, are prepared by oxidation and cross-linking in films.158, 159 Tin- containing polymers (for example, poly(4-[(trimethylstan- nyl)methyl]styrene;160 its synthesis was reported in Refs 161 and 162) are used as precursors of SnO2.Photoirradiation of polymers followed by their pyrolysis in air results in simultaneous formation of SnO2 particles and a polymer network in which the polymers are incorporated. In this case, the C7Sn bonds are cleaved to form radicals of the benzyl and trimethylstannyl types. The former58 react with atmospheric oxygen on film surfaces. The latter undergo recombination to form networks. Hybrid nanocomposites based on nanosized Au163 and TiO2 particles 164 were also prepared using reverse micelles. A surfac- tant (for example, didodecyldimethylammonium bromide) was added to toluene with the resulting formation of revrerse micelles into which AuCl3 and a precursor gel, viz., TEOS, were intro- duced.The LiBH4 ±THF system was used as a reducing agent. Subsequent hydrolysis and condensation afforded Au particles impregnated into the wet gel. Their sizes depend on the H2O:Si ratio and the concentration of the surfactant. After immobilisa- tion on the surface of highly dispersed SiO2 with the use of bifunctional aminosilane (MeO)3Si(CH2)3NH(CH2)2NH2 (see the review 17), colloidal gold particles were stabilised with dodec- anethiol.165 Subsequent treatment of these particles with thiol resulted in the cleavage of bonds with SiO2 and the particles became labile. Repeated immobilisation ± stabilisation events led to the growth of two-dimensional highly ordered *10 nm thick layers in which particles are linked together (Fig.3). It is best to prepare reverse micelles with the use of sodium bis(2-ethylhexyl)sulfosuccinate (SBES).166 For example, Ti(OPri)4 was dissolved in reverse micelles, which were formed from isooctane and SBES and contained a calculated amount of water. In this case, the alkoxide slowly diffused into micelles and then was hydrolysed and condensed in these micelles as in microreactors. This procedure was used for the preparation of TiO2 nanoparticles, which were extracted and dispersed in a solution of fluorinated polyimide. When a polymer ± TiO2 com- posite was deposited on a glass plate followed by thermal treatment (at 300 8C under an atmosphere of N2 for 30 min), an optical film waveguide was formed. Therefore, one of the advantages of the preparation of these nanocomposites in a solution of a polymer is the possibility of formation of trans- parent films (in this case, yellow titanium complexes are not formed).The thermal stability of the above-described materials (4% of TiO2) is almost identical to that of the initial polymer. This indicates that the thermal characteristics of the `host' polymer remain virtually unchanged upon impregnation of TiO2. In some cases, simple mixing of ingredients, viz., a polyimide solution and a sol-gel precursor, which leads to phase separation, is sufficient to form a polymer ± inorganic nanocomposite. Compounds containing epoxysilane groups can form net- works as well.167 ± 170 Thus alkoxysilane (2) is a binder providing the compatibility of TEOS with polyacids.167, 168 H2C CHCH2O(CH2)3Si(OMe)3 2 O Adsorbed layerAu particle H2N NH2 NH NHSi Si O O O O O O SiO2 substrateA B Figure 3.Preparation of hybrid materials with the use of reverse micelles: (A) immobilisation of colloidal Au particles; (B) exchange of alkanethiol molecules and cleavage of the Au7NH2 bonds; (C) rapid regeneration of the layer on complete binding of Au particles. BunS SBun BunSH HSBun HSBun BunSHH2N NH2 NH NHSi Si O O O O O O SiO2 substrate A D Pomogailo The component compositions of hybrid nanocomposites, which were prepared by the gel-sol method based on SiO2 and an ethylene oxide ± epichlorohydrin copolymer, were 71 : 29 ± 29 : 71.Acid condensation of TEOS in a THF solution afforded an inorganic network.171, 172 These networks can also be prepared with the use of alkoxy derivatives of aluminium. The morphology and fractal structures of these materials were studied. It is known 173 that the structures of aggregates are characterised by the fractal dimension D (14D43), which may have a meaning of an exponent in the equation relating the weight of theMparticles to their characteristic size R M&RD. For surface fractals with the uniform density, D=3. The self- similarity or scale invariance of the fractal aggregate implies that the structure of the aggregate, on the average, remains unchanged on the interval as extended as is wished and any its portion is similar to the aggregate as a whole.When the surface is dealt with, the D value equal to 2 corresponds to the smooth surface and the D value equal to 3 corresponds to the maximum roughness. The fractal dimensions of polymeric and colloidal samples of the TiO2 aerogel with high values of Sspec (prepared by the sol-gel method and dried with CO2 in the supercritical state), which were calculated by different methods, were compared. These values are virtually identical (2.6 ± 2.8). The fractal surfaces of colloidal particles are only slightly less regular than those of polymeric aerogels.174 This indicates that at the nanosize level, the morphol- ogy is determined in the early stages of the process and depends only slightly on the conditions of the synthesis, whereas on the mesolevel, it is the conditions of the synthesis that have a determining effect on the morphology.Studies of the condensation of TEOS in a polysiloxane network demonstrated 14, 136 that fractal models typical of solu- tions were not observed upon matrix polymerisation. Owing to large molecular weights of the matrix and the polymer formed, the entropy of the mixture decreases resulting in phase separation. Particular conditions are required to prevent phase separation in the system. However, phase separation always occurs when the concentration of SiO2 is >5%, which enhances the mechanical strength of composites. When TiO2 nanoparticles were precipi- tated in situ, the maximum bulk domains (up to5%of filling) were formed.These nanocomposites rank below systems with SiO2 in strengths.175 Of special note is the use of the sol-gel method for performing `template syntheses.' In this case, nanocomposites are formed through `assembly' from components under conditions of a strictly defined stereochemical orientation of the reagents. These conditions and primarily a small reaction volume (due to which reacting molecules can approach each other more closely than in SBun BunS SBun BunS NH2 H2N BunS SBun NH NHSi Si O O O O O O Binding of Au with thioalkane molecules SiO2 substrateCHybrid polymer-inorganic nanocomposites solutions or in the solid phase) promote the so-called `mild' template synthesis in the course of which the `assembly' occurs under normal conditions (sometimes even at room temperature).For example, this procedure was used 176 for the preparation of tubular semiconducting nanostructures based on TiO2 [from Ti(OPri)4], commercial template membranes based on Al2O3 with the pore diameters of 22 and 200 nm177 and fibrous structures based on ZnO (from zinc acetate 178) and WO3.179 These materials are excellent photocatalysts. The template syn- thesis is described in more detail below (Section V). The template sol-gel syntheses in fibres, nanotubes or micro- and nanoporous membranes were surveyed in detail in the review.180 This proce- dure was used for the preparation of fibrous electrode materials based on V2O5 181 and semiconducting oxide materials, such as MnO2,182 Co3O4,183 ZnO, WO3, etc.The template synthesis of tubular nanostructural nanocomposites of the semiconduc- tor ± conductor type (as nanotubes or nanowires) was per- formed 176 starting from TiO2 in Al2O3 pores followed by polymerisation of monomers.184, 185 III. Preparation of nanohybrid multimetallic materials by the sol-gel method Nanocomposites based on heterometallic ceramics, for example, on perovskite with the ABO3 structure, can be synthesised by the sol-gel method. These materials (primarily, film and epitaxially oriented materials) possess specific ferro-, piezo- and pyroelectric properties and are widely used in electronics and optoelec- tronics.186 The naturally occurring mineral perovskite CaTiO3 has the pseudocubic crystal lattice.Perovskites, for example PbTiO3, are generally prepared by calcination of a mixture of PbO and TiO2, which is ground using a vibrational mill, at temperatures higher than 600 8C (synthesis exsitu). However, PbO is toxic and the presence of its phase in the final product is undesirable. The sol-gel technique for the preparation of perov- skite PbTiO3 is free from these drawbacks.187 A mixture of Ti(OPri)4, Pb(AcO)2 .3H2O, ethylene glycol and citric acid taken in a ratio of 1 : 1 : 40 : 10, respectively, was stirred at 50 8C. Then the resulting citrate metal complexes were subjected to polymer- isation at 130 8C and to pyrolysis at 300 8C. The resulting powdered precursor was calcined in air at 400 ± 600 8C for 2 h to obtain thin PbTiO3 films, which retained the properties of a bulk material.Different versions of the sol-gel syntheses of PbTiO3 were described in the literature in detail. Thus polymeric films, which were prepared by mixing BaTiO3 with solutions or melts of polymers, were studied.188, 189 However, in this case particles were nonhomogeneously distributed and their agglomeration occurred. An alternative hydrothermal procedure for the syn- thesis of these nanocomposites was proposed.190 According to this procedure, BaTiO3 particles prepared from Ba(OH)2 and titanium diisopropoxide bis(ethylacetoacetate) at temperatures higher than 100 8C were grown in a polymeric matrix (in a polybutadiene ± polystyrene block copolymer, etc.191). These materials can poten- tially serve as components of conducting compositions (see, for example, Ref.192). Structurally homogeneous BaTiO3 nanocom- posites can only be prepared using the sol-gel method.193 One of versions of this procedure involves solid-phase polymerisation of organometallic precursors followed by pyrolysis.194 C(Me)COOH Ba+Ti(OPri)4+CH2 200 8C BaTi [OOCC(Me) {BaTi [OOCC(Me)CH2]6}n CH2]6 600 8C BaCO3+TiO2 BaTiO3 Polymerisation was also used for the preparation 195 of BaSnO3 with the perovskite structure. The synthesis was carried out analogously to the synthesis of PbTiO3 (see above) but at a different temperature and with the use of BaCO3, SnCl2 . 4.5H2O, ethylene glycol and citric acid taken in a ratio of 1 : 1 : 40 : 10, respectively, as the starting compounds.Other multicomponent 59 ceramics of the perovskite type, viz., SrTiO3, NdAlO3, SrBi2Ta2O9 and superconducting ceramics YBa2Cu3O77d, were also prepared by the sol-gel method (see below). The application of this method to the synthesis of these materials (the use of metallochelates and decomposition of organometallic precursors) was described in more detail in the study.196 In the present review, let us note only that single-phase thin KTiOPO4 films, which exhibit remarkable optical properties and high thermal stability, can be prepared according to this procedure; Ti(OEt)4, KOEt and different sources of phosphorus [(EtO)2P(O)(OH), (EtO)P(O)(OH)2, P(OH)2Me, P(OMe)3, etc.], among which (BunO)2P(O)(OH) is an optimum source, are used as precursors.197 Heteropolymetallates of the type of Keggin's acids H3PW12O40 and H4SiW12O40, which are incorporated into organic ± inorganic structures, may be assigned to the group of polymetallic nanocomposites.17, 117 These materials possess good electrochemical properties and can be potentially used in the holography.Nanocomposites based on these materials can be prepared according to two procedures, viz., by mixing polymetal- lates (spheres of diameter*1 nm) with TEOS [taken in a ratio of (2 ± 6) : 10] and tetraethylene glycol or by introducing polymetal- lates into organosilanes. Specific clusters, viz., oxometallates containing W7O7Si units, which are inserted into the side chain of the polymer, were synthesised by the reactions of substituted trichlorosilanes RSiCl3 (R=CH2=CH, CH2=CHCH2, CH2 =C(Me)CH2 or CH=CHPh) with the polyanion K4SiW11O39. In this case, cluster-containing monomers with composition [SiW11O40(SiR2)2]47 were formed.17 Polymerisation was carried out under the action of radical initiators.The products were characterised by different methods, including 13C and 29Si NMR spectroscopy. The yield of the polymer and the chain length depend on the reactivities of unsaturated groups R and increase in the series CH2CH CH2=CHCH2<CH2=C(Me)CH2< <CH=CHPh. The structure of this polymer is schematically shown in Fig. 4. The syntheses of oxoclusters of transition metals Nb4(m2-OMc)4(m2-O)4(OPri)8, Ti6O4(OEt)8(OMc)8 (OMc is meth- acrylate) Zr10(m4-O)2(m3-O)4(m3-OH)4(m2-OPrn)8.and .(OPrn)10(AllylAc)6 (AllylAc is allyl acetoacetate) 198 by con- trolled hydrolysis of different metal alkoxides in the presence of complex-forming agents (OMc or AllylAc) were also reported. These compounds consist of the metal oxide nucleus and periph- eral ligands, which can undergo polymerisation. The latter are responsible for the formation of a cluster network. The construc- tion of stable ensembles of nanoblocks requires that clusters possess rather high lability and high reactivity. For example, the thermodynamical stability of titanium oxo clusters and their resistance to hydrolysis depend both on the size and the oxo(alkoxo) group : titanium ratio.With the aim of retaining Cluster Polymer Figure 4. Structure of the hybrid polymer-inorganic material based on the {SiW11O40[Si(CHPh=CH)2]}47 polyanion.60 their molecular structures, polymerisation should be performed in anhydrous organic solvents.199 The syntheses of three-dimensional oxovanadium borophos- phates with open frameworks 200 ± 202 and polyboroazinylamines, which are precursors of metal-containing matrix composites containing metal borides and nitrides,203 are also available. The possibilities of modification of materials prepared by the sol-gel method with different transition metals are being exten- sively studied (see, for example, Refs 204 and 205). For example, mesoporous materials M41S were prepared based on molecular sieves NCM-41 or MCM-48 modified with solutions of NH4VO3 or ZrOCl2 in a mixture with polyoxoethylene stearate. Solutions of NH4VO3 or ZrOCl2 acted as precursors of transition metal ions.206 Nanoparticles (50 ± 60 nm) of cobalt carbide immobilised in a silicon matrix were obtained.207 H H H H H Co(CO)4 O O Si Si O OO Si Si O OO Si O Si O H Si O Si O H Co2(CO)8 300 8C H Si Si O Si O HO O O Si O H H O O OO Si Si Si OO Si H H H Co(CO)4 Co2C.SiO2 Mesopores of MCM-41 matrices were modified 208 with the trans-[Co(en)2Cl2]+ or [Co(en)3]3+ complexes (en is ethylene- diamine) stabilised by ammonium salts. Hydrolytic condensation of cyclopentadienyltitanium chlor- ide with silsesquioxane (the product of incomplete condensation of cyclopentyltrichlorosilane) followed by calcination of titanium- containing oligosilsesquioxane afforded microporous tita- nium ± silicon oxides with Sspec of up to 780 m2 g71.209 In the stage of sol-gel polymerisation, metal clusters can be introduced into nanocomposites.Thus the trihydrosilyl group was inserted into a cluster with a known composition, viz., (m3-HC)Co3(CO)9, and then it was subjected to heterocondensation.210 CSi(OH)3 Si(OMe)4, H2O, DMF Co(CO)3 (OC)3Co NH3, cat. Co(CO)3 H2 SiO2(xerogel) . SiCCo3(CO)9 500 8C, 2 h SiO2(xerogel) . xCo2C In the resulting xerogel, the unimodal distribution of the particles was observed. The sizes of the particles were 10 ± 46 nm (the average diameter was 25 nm).The Os3, PtSn, Fe2P, Co2P and Ni2P nanoclusters can be introduced into a composite analo- gously (by incorporation into a complex with a bifunctional ligand containing the reactive alkoxy- or hydroxysilyl group).211 The insertion of Pd2+ ions,212 Cu2+ ions,213 etc. into poly(ethyl- trimethylsilane) formed was described. The structures of organo- metallic ± inorganic diblock copolymers, in particular, the struc- ture of the ferrocenyldimethylsilane ± dimethylsiloxane block copolymer, were studied.214 In a hexane solution, the latter copolymer exists as long rod-shaped cylindrical micelles in which the iron-containing nucleus is placed in a `case' (crown) formed from polydimethylsiloxane blocks.The micelle of this type comprises up to 2000 polymeric molecules and decomposes under the action of ultrasonics to form short cylinders containing *700 molecules. To perform heterogenisation of metal complex catalysts (see Section VIII), metal compounds are bound to an inorganic matrix A D Pomogailo through a bidentate phosphorus-containing ligand of the [Ph2P(CH2)2CO2]EOx(OR)372x type (E=Ti or Zr; R=Alk).215 Thus the reactions of such ligands with tungsten carbonyl afforded products W(CO)5[Ph2PXEOx(OR)372x], which are potential precursors for the sol-gel synthesis. Cocondensation of these precursors (analogous to the above-described condensation of metal clusters with TMOS) gave 216 xerogel nanocomposites with composition SiO2 (xerogel) .xMyPz 216 ± 218 (M=Ru, Ga or In, y=z=1;M=Pt, y=1, z=2;M=Fe, Co, Ni or Rh, y=2, z=1; M=Cd, y=3, z=1; M=Pd, y=5, z=2) and even bimetallic SiO2 (xerogel) . Zn(Cd)GeP2 composites.217 Immobili- sation of the [Z6-PhSi(OMe)3] . Cr(CO)3 complex in a highly dispersed amorphous xerogel (Sspec=500 ± 1800 m2 g71) in the course of sol-gel polymerisation (NH4F catalyst) made it possible to prepare a single-phase hybrid of inorganic oxide and an organic network polymer containing (1,4-phenylene)siloxane bridges of the `nucleus ± shell' type.219 Vacuum thermolysis (393 K, 24 h, <1 Torr) of this composite even at 389K resulted in elimination of carbonyl groups to form zero-valent chromium nanoparticles of sizes 1.0 ± 10.0 nm in pores of the xerogel (Fig.5). Under normal conditions, thermal elimination of carbonyl groups of the composite proceeded only at 400 8C. In the latter case, chromium in the low oxidation state was formed.220 O O O O Si O O Si (CO)3Cr O O Si O O O O Si Si 1 Si Si O O O O O Si Si O O O O 2 O O Si O Cr(CO)3 Figure 5. Structure of a single-phase hybrid of the `nucleus ± shell' type; (1) a shell, (2) a nanoparticle of zero-valent chromium. Tricarbonylmolybdenum groups were introduced into the resulting polysilane according to the polymer-analogous conver- sion method. For this purpose, poly(methylphenylsilane) was treated with the Mo(CO)3Py3 complex.221 Nanolayer silanisation of inorganic surfaces of the TiO2, Al2O3 or Fe2O3 types, including the introduction of Fe2O3 into mesopores of silicates,222 CaCO3, BaSO4, etc., is widely used.Generally,223 ± 226 a thin layer of silanol derivatives and a dime- thylsiloxane oil are deposited on inorganic surfaces and heated to 250 ± 280 8C. In this case, siloxane bonds are cleaved, the terminal groups of the polymer chain react with hydroxy groups of inorganic surfaces and the chains are cross-linked through methylene and siloxane bridges or oligomers. As a result, particles, which are stable to hydrolysis and extraction with different solvents, are formed. For example, Zr(OBut)4 modifies the aluminium surface.227 Zirconium groups are linked upon treatment with the poly(ethylene ± acrylic acid) copolymer (PE-co-PAA) containing 5 mass%of units of acrylic acid.228Hybrid polymer-inorganic nanocomposites ButO OBut Zr Zr OBut OBut OButPE-co-PAA O O OH OH OH Zr(OBut)4 O O O HO OH O O O O Zr O O O ZrO O O O The adsorption properties of these materials are changed due to formation of a hydrophobic layer and the material becomes moisture-stable. When Ti and Au layers 5 and 50 nm thick, respectively, were deposted on the activated polydimethylsilox- ane surface, ordered structures were formed.229, 230 The coating of the surface of inorganic particles (including metals or their oxides) is often used in the technology of powder preparation to form structures of the `nucleus ± shell' type, which possess additional useful properties.Thus SnO2 or In2O3 particles are coated with chelate polymers swelling in water (for example, those containing In3+7SiO2 or In3+7Sn2+7SiO2 as well as Ag+), which were preliminarily subjected to appropriate treat- ment. These particles exhibit the surface conductivity and are used for the preparation of thermally stable conducting films possess- ing antistatic properties, etc.231 ± 233 Therefore, several procedures for the insertion of a polymer into an inorganic material by the sol-gel method are available. Thus silanes containing two different functional groups can be used. One of these groups reacts with a monomeric unit of the macromolecule and the second group reacts with the sol-gel precursor resulting in the formation of a bond between the side group of the polymer and the sol-gel network.Sometimes special binding agents are additionally added. For example, the surface of highly dispersed SiO2 was modified with aminobutyric acid and the resulting product was dried, dispersed in e-caproamide and subjected to initiated polymerisation at 90 8C.234 If the concen- tration of SiO2 particles is 45 mass% (the distance between the particles is 50 ± 110 nm), they are homogeneously distributed in the composite. At concentrations of >10 mass %, aggregation of particles starts. Layered nanohybrid materials (see Section V) were prepared 235 by the reactions of Zn(OH)2, ZnO or a Zn/Al alloy with carboxylic acids or their oxychlorides. The `guest' :`host' ratio in these composites varies from 0.75 : 0.25 to 0.9 : 0.1.The morphology of the particles changes from fibrous to platelet depending on the nature of the reacting components, the interlayer distance increasing in the range of 1.61 ± 2.01 nm. Recently, the sol-gel procedures for the preparation of ceramics based on alumoxanes, viz., carboxylates of the general formula [Al(O)x(OH)y(OOCR)z]n, have been analysed. 236 At the stage of formation, the M(acac)n complex (M=Ca2+, Mn2+, Y3+, etc.) can be additionally introduced into ceramics.237, 238 Metallic Co and Fe and their alloys, which were formed upon reduction of the corresponding nitrates with hydrogen, were dispersed in an aluminosilicate matrix. The matrix was synthes- ised by the sol-gel method from (BuO)2AlOSi(OEt)3 and possesses dielectric properties.In this case, nanocomposites with the particle size of*20 nm possessing magnetic properties were obtained 239 (Fig. 6). One of new trends today in the field under consideration is to use carbon dioxide in the supercritical state as a solvent in the course of sol-gel polymerisation or in the stage of extraction of the resulting product. To perform the sol-gel process according to this procedure, the initial alkoxysilane is mixed with anhydrous 99% HCO2H. Then polymerisation of the mixture is carried out in 61 b a Mesopores Micropores Metal Co2+, Fe3+ 2 nm 20 nm Figure 6. Diffusion and adsorption of metal ions in pores of the `host' and the structure of the composite before (a) and after (b) thermal treatment.supercritical CO2 in an autoclave (40 8C, 41.4 ± 55.2 MPa). The formation of the gel proceeds over 12 h and then CO2 is slowly removed (8 h). This is favourable for the formation of highly porous monolithic composites with the meso- and macroarchi- tecture of the pores.240 Components, which sharply enhance the ability of SiO2 to adsorb water, are also introduced at the stage of formation. For example, selective water adsorbents, such as CaCl2 and LiBr, bind up to 53 mass%of H2O.241, 242 Colloidal dispersions of trimetallic Au7Pb7Cd particles (the diameter is 33 nm) containing the gold nucleus with a 18 nmthick lead mantle are formed by g-irradiation of salts of the correspond- ing metals.243 In this case (when nanocomposites contain three or more different metals, i.e., they are multimetallic nanohybrids), studies of the structures of these particles is a challenging task.At the same time, these materials have already found use as precursors in the production of superconducting ceramics, spe- cial multicomponent steels, etc. When traditional approaches are employed, a polymer either is formed in a previously prepared inorganic matrix or is inserted into the latter. Multimetallic nanocomposites are formed either in a polymeric matrix in situ or simultaneously with the latter. It is known that standard methods for the preparation of blends in the production of high-Tc superconducting ceramics (direct mixing of oxides, carbonates, oxalates, nitrates and other metal salts) give rather poorly reproducible results.This is due not only to the heterogeneity of grinding and mixing of the solid initial components but also due to complex physicochemical and mechanochemical conversions that occur in the course of the preparation of samples. As a result, microheterogeneities appear and different phases, including nonconducting phases, are formed resulting in low-quality high-Tc superconducting ceramics with a blurred superconducting transition (Meissner effect). A structur- ally homogeneous superconducting ceramic of the YBa3Cu4O8 type was formed when the initial components [for example, Y(NO3)3 . 6H2O, Ba(NO3)2 and Cu(NO3)2 . 2H2O] were mixed on the molecular level (in solution).After evaporation of the solvent (congruent evaporation), a homogeneous dispersion of a precur- sor of a high-Tc superconducting ceramic was obtained. With the aim of preparing composite materials, either a high-Tc super- conducting ceramic is introduced into a polymeric matrix or this ceramic is prepared in the presence of a polymeric matrix. For example, in the first case, a dispersed high-Tc superconducting phase, viz., YBa2Cu3O77d (ceramic Y-123), (PbxBi17x)2Ca2. .Sr2Cu3Oy 244 or Tl2Ba2Ca2Cu3Oy 245 with Tc *90, 110 and 125 K, respectively, was introduced into a polymeric polychloro- trifluoroethylene, polyvinylchloride or rubber matrix. The ceramic content is 50% ± 75%. The optimum compositions combine advantages of a high-Tc superconductor (high Tc, good magnetic properties and magnetic levitation) and of polymers (mechanical strength, flexibility, simplicity of processing of products and resistance to atmospheric effects, aggressive media, etc.).In addition, the preparation of compositions allows one to eliminate drawbacks of a high-Tc superconducting ceramics, viz.,62 the high porosity, the friability, the susceptibility to degradation, etc. However, rather large (micrometer) ceramic particles are inserted into a polymer according to this procedure. In the second case, polymeric matrices containing metal ions dispersed on the molecular level are used (including the prepara- tion of nanosized particles in the course of polymerisation or polycondensation).For example, citrates of different metals in aqueous solutions of acrylamide are polymerised in the presence of radical initiators and chain carriers (N,N,N0,N0-tetraethylene- diamine) to form a gelatinous phase.246 In this case, metal cations were trapped into a polyacrylamide gel much as occurs in the sol- gel synthesis. After calcination, ultrathin multicomponent oxide powders, such as YBa2Cu3O77x or LaAlO3, were obtained. The formation of the YBa2Cu3O77x ceramics was performed with the use of acrylic or methacrylic acid as chelating ligands 247 or with the use of polymer ± salt solutions based on polymeric alcohols or acids, PEG or PVPr.248 ± 250 The sizes of all salt crystals formed in the presence of any one polymer are substantially smaller than those obtained upon crystallisation of pure salt solutions.This is associated with complex formation in solutions as well as with adsorption of macromolecules on nuclei of crystallisation although it is not inconceivable that recrystallisation occurs simultaneously due to the presence of the concentration gradient of the components within the thickness of the film. Homogeneous films are most often formed because their formation occurs not only on the surface but also in the volume. It is essential that the final sizes of crystallites of the ceramics in the resulting powders should be approximately equal to their sizes in films. The polymer-analogous conversion method was used for the preparation of the Y3+, Ba2+ and Cu2+ complexes with poly- methacrylic acid 251, 252 and of the YBC chelates with poly- amides 253, 254 [gelatinisation with polyvinyl alcohol (PVA) can also be used].255 The superconducting ceramics based on these metallopolymers has Tc=80 ± 92 K, and the critical current density (Jc) is 150 ± 160 A cm72. Not only powders but also films and fibres can be prepared from these ceramics. (For this purpose, YBC-epoxy composites prepared from epoxide poly- mers are also suitable.256) For example, long (>200 cm) fibres 1 mm in diameter were formed from thermoplastic gels based on Y-123 ceramics and PVA.257 Calcined fibres have Tc=92 ± 94 K.These fibres form materials characterised by different Jc values depending on the degree of saponification (DS) and the content of the Y-123 ceramics in these materials.258 ± 260 The critical current density has the minimum value when DS=67 mol.%and the maximum value (Jc=3.56104 A cm72, 77 K) when DS=81 mol.%. This parameter is also affected by the conditions of treatment (calcination and pyrolysis) of samples, which is associated with the peculiarities of the distribution of the ceramics over fibres. Nanocomposite materials based on high-Tc superconductors are also prepared by polymerisation of acrylic acid in a mixture with aqueous solutions of Y3+ nitrate and Ba2+ and Cu2+ acetates.247 One of the most promising approaches to the polymer synthesis of high-Tc superconductors involves copolymerisation of metal-containing monomers.261 If metal ions are introduced into molecules of the monomer before copolymerisation, these ions are distributed in the polymer more uniformly.For this purpose, acrylates or acrylamide complexes of Y3+, Ba2+ or Cu2+ were mixed in a molar ratio of 1 : 2 : 3, dissolved in a minimum amount of methanol, dried and subjected to solid- phase copolymerisation.262 Acrylamide complexes of Bi3+, Pb2+, Sr2+ , Ca2+ and Cu2+ taken in a molar ratio of 2 : 0.3 : 2 : 2 : 3 were treated according to the same procedure (copolymerisation was carried out in concentrated aqueous solutions).263 The organic phase was calcined out from the resulting metal-containing copolymers. The properties of these high-Tc superconducting ceramics are shown in Fig. 7. Analysis of the temperature dependence of the electrical resistance (R) demonstrated that these ceramics behave as metals at T<Tc, a R(T)/R(300) 0.8 0.40 100 b 0.8 0.40 150 50 Figure 7.Temperature dependence of the electrical resistance and the magnetic susceptibility of high-Tc superconducting ceramics: (a) ceramics from a copolymer of Y, Ba and Cu acrylates; (b) ceramics prepared by spontaneous copolymerisation of acrylamide complexes of Bi, Ca, Sr, Pb and Cu. although Tc of these ceramics are not as high as those of single crystals. It is known that the properties of polycrystalline samples in the normal state depend to a greater extent on the quality of intergrain contacts than on the structure of grains as such. The widths of superconducting transitions in the range of 0.1 ± 0.9 of the value corresponding to complete resistance drop is no more than 2 ± 3 K, which is close to the characteristics of the best samples of high-Tc superconducting ceramics.The character of the temperature dependence of the magnetic susceptibility (w) is also indicative of the sharp transition and of the presence of the only superconducting phase. In this case, the complete volume of diamagnetic shielding of the phase, which is estimated from a lead reference compound, reaches 100%. The characteristics of super- conducting bismuth cuprates are of particular importance. The reproducible synthesis of the latter is a challenging task. Gen- erally, these compounds contain admixtures of a phase with Tc=85 K due to the microheterogeneity of the ceramics and the successive phase transitions 2201!2212!2223. Products pre- pared by spontaneous copolymerisation are well reproducible single-phase samples with Tc =110K and a critical current density of up to 240 A cm72.Apparently, this approach can also be used for the preparation of other single-phase materials in the nanocrystalline state. The preparation of multicomponent steels of the M50 type (alloys containing 4.0 mass %, 4.5 mass %, 1.0 mass% and 0.8 mass% of Cr, Mo, V and C, respectively, and Fe as the remainder) may become an important field of application of nanostructurised polymetallic materials. These steels are primar- ily used in aircraft construction for the preparation of supporting bearings, in gas turbine engines, etc.264, 265 Generally, M50 steels contain substantial amounts of micron-sized carbon particles, which initiate the formation of fatigue cracks in the material of bearings.The mechanical properties of these structurised materi- als would be expected to be improved as sizes of grains deposited on defects decrease (healing of microcracks); PVPr is used as a polymeric surfactant; Fe(CO)5, Cr(EtxC6H67x)2, Mo(EtxC6H67x)2 (x=0 ± 4) and V(CO)6 serve as precursors of the M50 steel. The process is carried out in dry decalin under ultrasonic irradiation (sonochemical synthesis). After removal of the solvent and the gaseous phase, colloidal particles have an average diameter of 7 nm and exist as a homogeneous alloy.Another procedure involves reduction of precursors of the M50 A D Pomogailo w (rel. u.) 71.0 0 T /K 200 71.0 0 250 T /KHybrid polymer-inorganic nanocomposites steel (FeCl3, MoCl3, CrCl3 or VCl3) with lithium triethylborohy- dride in THF followed by removal of solid LiCl by sublimation in vacuo at high temperature. Probably, these approaches will also be useful in the design of magnetic materials, for example, those based on polymetallic nanosized particles of the LaSrCrxNi17xO4+d type, etc. Thus a procedure for the prepara- tion of the La17xSrxMnO3 ceramics (x=0, 0.2, 0.4 or 0.6) has been proposed recently. 266 This procedure involves preliminary polymer synthesis consisting in binding of metal nitrates with a gel precursor from PEG (Mw=20 000) taken in certain ratios.Autoignition occurred at 300 8C and NO3 groups served as an oxidising agent. Subsequent thermal treatment of the combustion product afforded a homogeneous perovskite phase (see above). Its morphology and Sspec (1 ± 7 m2 g71; the sizes of crystallites were 24 ± 150 nm) depended on the reagent ratio and reaction con- ditions. A ceramic with cadmium and lanthanum additives with composition Pb0.85Cd0.05La0.10Ti0.975O3 was prepared by the sol- gel method from metal acetates and Ti(OBu)4.267 Thin silicon films can be coated with this material in the stage of concentration of the solution (before drying). Subsequent thermolysis is carried out according to a standard procedure. IV. Preparation of template synthetic nanobiocomposites by the sol-gel method The sol-gel method provides considerable possibilities for the preparation of a series of nanohybrid materials in which bio- logically active macromolecules can be encapsulated at the stage of formation of ceramics, glasses and other inorganic composites (see, for example, Refs 268 and 269).The biological objects can be enzymes, catalytic antibodies, noncatalytic proteins, polynucleic acids and microbial, plant and animal cells 270 ± 272 used in biocatalysis, for immunodiagnostics, as biooptical agents, vari- ous adsorbents, etc.268, 273 ± 275 Biomolecules (active enzymes) were encapsulated into a sol- gel matrix for the first time in 1990.276 About 40 different types of hybrid bioceramic materials whose inorganic matrices contain silicon, titanium and zirconium oxides, TiO2 ± cellulose compo- sites, etc.were described.277 Recently, bioceramic sensors, solid electrolytes, electrochemical biosensors, etc. have been surveyed in a review.278 Moderate temperatures and mild conditions of hydrolysis and polymerisation of monomeric alkoxides make it possible to trap proteins in the stage of formation of the matrix, thus preventing their denaturation. The high stability of the biomolecules trapped, the inertness, the large specific surface, the porosity and the optical transparency of the matrix facilitate heterogenisation because of which the sol-gel method of immobi- lisation is very attractive. Below are considered the principal approaches.Proteins, such as copper ± zinc superoxide dismutase, cyto- chrome, myoglobin, hemoglobin and bacteriorhodopsin, were encapsulated into a porous silica gel matrix prepared by the sol- gel method. The matrix effectively retains these biomolecules without loss in their enzymatic activity and changes in their spectral properties,279 does not preclude the approach of small molecules to the reaction centre and does not hinder transport of the reaction products. Heterogenisation of glucose oxidase and peroxidase, which are used as active solid-phase elements of glucososensors, was carried out according to the same proce- dure. The spectral characteristics of a gel containing oxalate oxidase and peroxidase are changed upon storage in aqueous solutions of oxalic acid.Antibodies bound in such a manner are used in medicine, immunochromatography, immunosensors, etc. For example, immunoglobulins which are trapped retain their ability to bind external antigens from solutions.280 Antibodies 14D9 incorpo- rated in sol-gel matrices catalyse different reactions, including hydrolysis of cyclic acetals, ketals, epoxides, etc.281 The ability of the sol-gel matrix containing 10% of PEG and antiatrazine antibodies to bind atrazine was studied.282 This matrix recognises 63 and binds atrazine and widely distributed herbicides in solutions. In this case, neither leaching of antibodies nor nonspecific physical sorption of atrazine on a ceramic matrix occur. A decrease in the activity was not observed for at least 2 months, while the activity in solutions decreased by 40% in this period.In addition, the use of the sol-gel method excludes the necessity of purifying immunoglobulin. It should be emphasised that these materials have other advantages, such as the enhanced thermal and pH stability, the possibility to monitor easily the enzymatic reactions both in pores and in the body of the matrix by spectral methods, the convenience of storage, the possibility of the repeated use, etc.283 Enzymes, which act as bioreactors (biodoped polymers), were immobilised according to the above-described procedure.125 ± 127 For this purpose, chemically active terminal groups of enzymes and active bonds of dopants of the ceramics, for example of Sn7Cl, were used.284 O O O SnCl2 RSH RSSnO Ti O ClSnO Ti O HO Ti O 7HCl 7HCl O O O R is an enzyme.For example, alcohol dehydrogenase was immobilised into nanotubes of TiO2, which was prepared by template synthesis, according to this mechanism. This enzyme retained its ability to oxidise ethanol for more than 4 days (NAD+ cofactor, phosphate buffer, pH 8).180 Since nanotubes are open at both ends, this configuration allows one to use them as a flow reactor. These examples are large in number. They involve also covalent binding of antibodies with the aim of functionalising sol-gel films.280 Of considerable interest are procedures for the modification of matrices (including surfaces of metal oxides,285 silicates, siloxanes, hybrid sol-gel polymers) with different polyols (most often with glycerol) and their ethers followed by binding of trypsin, cyto- chrome c, superoxide dismutase, glucose oxidase, phospholi- pase D, etc.286 Bioencapsulants exhibit activity, which is almost identical to that of nonimmobilised enzymes.Besides, the former possess the high stability and exhibit the substantially better properties than materials prepared by the standard sol-gel method. Vinyl groups are grafted to the surface of SiO2 particles (by treating aminosilica gel with acryloyl chloride) and copoly- merisation of the resulting product with acrylamide and acrylic acid or N,N-dimethylaminopropylacrylamide is performed in the presence of a cross-linking agent and glucose oxidase.287 After removal of the protein from these nanocomposites, a molecular impression (template) remains.This template recognises glucose oxidases in protein mixtures.288 The so-called template approach based on the formation of porous silicate films about a specific reagent, which is then removed, using the sol-gel method was described. Template films prepared based on 3-hydroxytyr- amine 289 have the higher affinity for this substrate than for structurally related molecules, including serotonin, dihydroxy- naphthalene, etc., and do not possess the affinity for negatively charged or large molecules, including the Tyr-Gly-Gly peptide. Apparently, the porosity of the film as well as steric, hydrophobic and electrostatic interactions play an important role in controlled penetration of different molecules through this film.The formation of an organised material by the sol-gel method can be performed according to four procedures:290 (1) formation of self-assembling organic templates (transcription syntheses); (2) cooperative assembly of ensembles, viz., template and build- ing blocks (synergistic syntheses); (3) morphosynthesis in which organised nonlinear chemical environments, viz., reaction fields (static, reorganisation, transition) are used for the construction of models;291 (4) combination of these methods (integrated syn- thesis). This strategy (reaction ensemble ? replication ? meta- morphism) resembles the general scheme of mineralisation.It can be exemplified by template-directed syntheses of ordered meso-64 1 mm Figure 8. Electron micrograph of a coccus-like coat of the one-celled algae Emiliania huxleyi.292 forms and organoclays, microframework structures, including those with the use of bacterial templates. In particular, a coat of the one-celled algae Emiliania huxleyi consists of radially arranged hammer-shaped single crystals of calcite, which are crystallo- graphically oriented and have a specific species form. The crystallographic orientation is retained in fossil casts, which is indicative of the control over nucleation on the molecular level (Fig. 8). This fact is most pronounced in the reproduction of hierarchic macrostructurised organised silica gels. Multicellular fibres from Bacillus subtilis (Fig. 9) can serve as an example of scale organic 3D-templates.292 Nanosized magnetic (Fe3O4), semiconducting (CdS) or TiO2 particles, which has been prelimi- narily prepared, were introduced into macroscopic filaments upon swelling in colloids.293 Dried fibres (the diameter was 500 nm) were coated with a thin (30 ± 70 nm) layer of aggregated colloidal particles.Negatively charged magnetic colloids exhibit the good permeability and reproduce the bacterial superstructure. Neutral CdS particles partially penetrate into the filament, but their major portion remains on the surface of the filament. Positively charged TiO2 sols form only a surface coating. Removal of a bacterial component from a magnetic composite upon heating afforded structural collapse. SiO2 sol 600 8C H2O A B C Figure 9.Scheme of formation of an organised macroporous SiO2 structure with the use of bacterial templates, viz., bacterial filaments with the structure of multicellular fibers (A), mineralisation of interfibre spaces of filaments (B) and the formation of macroporous replica upon drying (C).To summarise, nanohybrid materials are widespread and are of considerable importance both in nonliving and living nature.2, 268, 273 Binding and decomposition of different organic compounds, the cycles of compounds and energy in nature and the formation of many useful minerals occur with the participation of nanohybrid materials. Outstanding possibilities are opened up in the field of the design of molecular-organised self-assembled systems, including nanobiomaterials.The protein environment is favourable to the preparation of complicatedly organised (both in form and composition) products. For example, perovskites (BaTiO3, SrTiO3 or NaNbO3) whose synthetic analogues are considered in Section III can be involved in processes of bioag- gregation.294 It is believed 295, 296 that finely monodispersed A D Pomogailo precursors of high-Tc superconducting ceramics could be formed based on bacterial objects. V. Intercalation of polymers into porous and layered nanostructures Sometimes products prepared by the sol-gel method or by intercalation can be distinguished only by their past history. No less than 5000 studies dealing with problems of intercalation of organic, organometallic and inorganic compounds as well as with the properties of the resulting products were published up to 1994 inclusive,297 though the major body of information has been obtained during the last 5 ± 10 years.Natural layered silicates, the so-called smectites (or smectite clays) are most often used for the preparation of nanocomposites under consideration. Their structures, selected physicochemical properties and the nature of the active surface are considered in a review 298 and monographs.20, 299 These are very widespread minerals to which hectorite and montmorillonite [AlMg(OH)2(O)4]57[Si2O3]4+[Na,nH2O]+ (MMN) with struc- tures of the mica type belong. Crystals of MMN consist of alternating tetrahedral silicate layers of the cations condensed with negatively charged octahedral layers of metal oxides.How- ever, unlike mica, the cations in these minerals are readily exchanged for other ions, including transition metal ions. The exchange occurs predominantly in layers containing solvated sodium cations. Intracrystal cavities can swell when they are filled with organic solvents. The degree of swelling depends on the nature of the cations within the layers and on the negative charge density on the silicate layers. Aluminosilicates and magne- sosilicates belong to clays whose nature is determined by the type of the octahedral metal. Within the lattice, Mg2+ and Al3+ can isomorphically replace Al3+ and Si4+, respectively.The cations are hydrated and the layers readily swell and accommodate large cations or a substantial amount of water. In addition, most of inorganic oxides contain surface hydroxy groups, which actively bind metal ions. Crystalline rigid `host' matrices with a controlled system of percolation nanometer-sized pores, which can be occupied by atomic or molecular `guest' structures (clusters, nanosized particles, inorganic coordination polymers of the CdS type, large molecules of the C60 fullerene type and polymer units synthesised in situ 20, 300 ± 303) are rather well studied. Thus inter- calation of buckminster fullerene functionalised with ethylene- diamine into a mica-like silicate of the fluorohectorite type has been described.300 Many procedures for the introduction of polymers in `host' matrices are available.These hybrid nano- composites are of technological interest because their properties are substantially improved compared to those of materials filled according to a standard procedure.304 ± 306 For example, these materials prepared by thermal synthesis are generally metastable (in particular, due to flaking of the polymer from the inorganic component). The properties of intercalated hybrid nanocomposites are determined by a number of characteristics, particularly, by the sizes of their inner open-pore systems. These properties were considered in many reviews (see, for example Refs 20 and 307 ± 311). Intercalation systems are divided into two types according to their architecture and properties.309 The systems of the first type are characterised by the presence of rigid pores with a constant volume, the parallel isolation of lattice channels and the interrelation between the channels of the network.The location, the concentration and the spatial distribution of `guests' are governed by the topology, the chemical nature and the reactivity of the inner surface of the `host'. The matrix of the latter can be additionally dispersed and functionalised or long-chain surfac- tants can be intercalated into this matrix. In addition, the choice of `guests' is limited by the minimum size of cross-linked channels, which leads to selective intercalation into induced spatial matrices (of the types of molecular sieves or membrane films).Systems of the second type are characterised by a low dimensionality of theHybrid polymer-inorganic nanocomposites `host' lattice, i.e., by the structures of the `layer' or `chain' types. As a result, the pores are `flexible' and their sizes can be adapted to the size of the `guest.' In layered systems, the basal (intralayer) space comprises*5 nm. The thickness of the layer, for example, in perovskite varies in the range of 0.5 ± 2.2 nm. From the viewpoint of the electronic properties, the matrix lattice of the `guest' may have no effect on the intercalation ± deintercalation process (this is true for lattices of dielectrics, viz., of zeolites, layered aluminosilicates, metallophosphates, etc., the intercala- tion behaviour of which is determined primarily by the acid ± base and exchange properties). `Guest' lattices possessing the electronic conductivity (semiconductors and metals) are a special case.Upon intercalation, redox reactions with electron (ion) transfer occur in these materials resulting in a substantial change in the physical properties of the `host' matrix. The inclusion of monomer molecules into pores of a `host' followed by their controlled conversions into polymeric, oligo- meric or hybrid-sandwich products (postintercalation conver- sions in situ) belong to the most interesting group of intracrystalline chemical reactions. This field has been rather well studied (see, for example, Refs 312 ± 314). Intercalation of the appropriate monomer causes delamination and dispersion of `host' layers, which is accompanied by polymerisation to form a linear or cross-linked polymeric matrix.This procedure was used for the insertion of polyesters,315 PVPr 316 ± 318 and poly(ethylene oxide) (PEO) 319 into silicates and for the insertion of PVPr,320 polyimide 321, 322 and poly(e-caprolactam) 305, 306 into kaolin. (When polymerisation was performed within kaolin, the resulting composites were nondelaminated products.) The most interesting hybrid nanocomposites were prepared from polyconjugated conducting polymers (which are often considered as materials for the preparation of battery plate), such as polyaniline,323, 324 poly(2-ethylaniline), poly(p-phenylene), polythiophene, polypyr- role 323, 325 or pyrolysed polyacrylonitrile 326 in different matri- ces.327 ± 329 In this case, a great diversity of procedures can be used, viz., the enclosure of a polymer into a gel,330 the use of salt solutions of the corresponding polymers [for example, polyaniline hydrochloride in an acid-methanolic solution or a sulfonium salt of poly(p-phenylene) soluble in many solvents], etc.The results of early studies in this field were summarised in the review.297 Thus oxidative polymerisation of pyrrole, dithiophene,331, 332 tetrahy- drofuran 333 and aniline 334 in an FeOCl lattice was described. Intercalated aniline forms hydrogen bonds with chlorine atoms of the lattice, and polymerisation occurs along the (101) lattice diagonal.However, the conductivity of the resulting polymer is somewhat decreased. This lattice appeared to be suitable for oxidative polymerisation of aniline introduced from an aprotic solvent.331, 335, 336 The molecular formula of the resulting product was (C6H5NH2)0.28FeOCl.{ The zigzag polymer chains with Mw=6100, which are comparable with the FeOCl lattice, are extended along the direction of the `host' crystal and along the { The method consisting in quantitative removal of a polymer from an inorganic `host,' its repeated dissolution and investigation could be an important technique for the analysis of polymers in these hybrid materials. However, although studies on the extraction of polymers from layered nanocomposites have been reported,6, 336, 337, 338 this field is poorly under- stood.Recently, rapid and quantitative extraction of PEO from the Kx(C2H4O)4M17x/2PS2 composite (M= Mn or Cd) with an aqueous tetraethylammonium salt under normal conditions has been carried out.339 This process was analysed in detail and the kinetic regularities were revealed. NH O2 6x NH2+xH3PMo12O40 NH 65 hydrogen bonds between the NH groups and the chlorine atoms of the layers of the lattice. The Fe2+ : Fe3+ ratio is *1 : 9. The polymeric intercalate behaves as a p-type semiconductor (the conductivity of a single crystal is 1.561072 S cm71). Its pro- longed oxidation in air afforded a mixture of polyaniline and b-FeOOH. Recently,340 it was proposed to use the a-RuCl3 matrix, which is structurally similar to FeCl3, for the preparation of lamellar nanocomposites of this type.The product of oxidative polymerisation of aniline in this matrix has the composition (C6H5NH2)xRuCl3 (0.5<x<0.6). Its conductivity at room temperature is *2 S cm71, which is more than three orders of magnitude higher than that of the initial a-RuCl3. Postintercalation polymerisation of aniline was carried out in air at 1308 in phosphate layers of a-Zr(HPO4)2 .H2O,341 ± 343 VOPO4,344 HUO2PO4 . 4H2O,342, 345 layered acidic zirco- nium ± copper phosphates,19 layered double hydroxides 346 as well as in layers of HM1M2O6 .H2O (M1=Nb or Ta; M2=Mo or W347 ± 349), molybdenum sulfide,350 porous zeolites of the mordenite type,351, 352 MCM-41,351 in layers of perovskite,353 mica-like silicates,354 etc.355 The resulting products are generally dielectrics.Polyoxometallates as `host' matrices can serve as ideal models of `molecular batteries' because they contain a small number (generally 6 ± 8) of W or Mo atoms linked through oxygen bridges. Their spatial and electronic structures were adequately characterised.356 Chemical (electrochemical) polymer- isation of pyrrole in Keggin's structures H3PM12O40 (M=W or Mo) was described.357, 358 Sometimes additional chemical reac- tions are carried out with the aim of converting these products into nanocomposites. For example, (Bu4N)5Na3[(1,5-COD)M . . P2W15Nb3O62] (M=Ir or Rh; 1,5-COD is 1,5-cyclooctadiene) was reduced with hydrogen in acetone.359, 360 These hybrid nano- composites can be completed with phosphomolybdate anions (Scheme 3).The composition of the product corresponds to the formula C6H5N. (PMo12O40)0.116. Photochemical polymerisation of a diacetylene, viz., 3,5-octadiyne, in layers of Mg, Mn or Zn phosphates was also reported.311 It can be suggested that under the appropriate conditions, monomers occupy virtually the entire space of pores or the whole interlayer space. Subsequent oxidative polymerisation (`redox- intercalation polymerisation') 351 was carried out in the presence of molecular oxygen (as an electron acceptor) and a redox-active `host' (a catalyst of electron transfer). In this connection, layered silicates containing metal ions, which initiate polymerisation of an intercalated monomer, are of particular interest. Thus when Na+ ions in hectorite are replaced by Cu2+ (sometimes by Fe3+), styrene undergoes polymerisation 163 both in pores and on the surface.The polymer has a `brush-like' structure, which indicates that the inorganic surface possesses an orienting effect. This effect decreases as the chain grows and moves away from the surface. N-Vinylcarbazole is polymerised in MMN at 64 8C (in a benzene solution at 50 8C) due to the presence of cations in MMN.361, 362 The thickness of the intercalated layer of `guest ± host' particles is 3310 nm and the conductivity of this nanocomposite (1076 S cm71) is 10 orders of magnitude higher than that of polyvinylcarbazole.Intercalated polystyrene exists in two forms.175 One of them possesses the enhanced rigidity due apparently to a higher degree of ordering of the polymer. The improvement in physicochemical properties of intercalated polystyrene was observed also in the model PS ±MMN system.363 Redox intercalation polymerisation of aniline, pyrrole and dithiophene with the use of a V2O5 xerogel affords polyconjugated Scheme 3 NH+ NH+ x [PMo12O40]37 + NH NH x66 anisotropic polymers.364 ± 366 Intercalation is accompanied by polymerisation. In this case, the chain growth initially occurs within the xerogel and is associated with transport of molecular oxygen.The V2O5 xerogel acts as a catalyst.Aconducting polymer with different polyaniline :V2O5 ratios is formed with the partic- ipation of mixed-valence (V4+/V5+) `host' lamellae ordered along one direction.The material consists of alternating layers of vanadium oxide and the polymer. Physicochemical studies dem- onstrated that polyaniline was formed as a salt. Ageing of the material in air facilitates partial oxidation of the inorganic frame- work and oxidative binding of polyaniline in the intralamellar space. The resulting polymer is `frozen' due to the formation of a NH_OV hydrogen bond. A detailed analysis 351, 367 of the insertion of aniline, its competitive polymerisation into MoO3 and the formation of the nanocomposite (C6H5NH2)0.24 .MoO3 structures, including those with the use of a low-temperature intercalation procedure,367 was carried out.It was found that polyaniline chains expand layers and change the potential surface thus decreasing the polarisability of the lattice.} The polyaniline chains intercalated in MoO3 oxidise (NH4)2S2O8.197 Oxidative polymerisation of aniline, pyrrole or thiophene monomers inter- calated in layered aluminosilicates affords highly oriented `host ± guest' layers. It was found 32 that radiation polymerisation of acylonitrile and acrylic acid in MMN inclusion compounds afforded stereoregular (which are most likely syndiotactic) poly- mers. Let us consider the most interesting procedures for the synthesis of one-dimensional nanocomposites based on intercala- tion into a matrix. An aqueous Au colloid (the particle size was 12 nm) stabilised with citric acid was introduced into pores of a Al2O3 filtration membrane (the average pore diameter was 20 nm) and oxidative polymerisation of pyrrole was performed in situ in these pores by adding an aqueous Fe(ClO4)3 solution.369, 370 After dissolution of the membrane in 0.5 M KOH, a suspension of one- dimensional colloid-polypyrrole nanostructures was obtained (Fig.10). Au nanoparticles Al2O3 membrane Polypyrrole Fe(ClO4)3, NH Figure 10. Template synthesis of one-dimensional Au ± polypyrrole nano- structures in a Al2O3 membrane. To construct solid light-sensitive solar cells, TiO2 particles of size *20 nm (prepared from TiCl3, pH 2.5) are obtained at the surface of a photochemical electrode. Simultaneously, polypyr- role, which is formed upon electrochemical polymerisation of pyrrole adsorbed in electrode pores, precipitates on these par- ticles.371, 372 These structurally controlled `templates' are ana- logues of self-assembling supramolecular ensembles.Note that the procedure for the preparation of supramolecular systems for molecular recognition comprising chromophores, semiconduc- tors and cluster aggregates, which perform special optical and electronic functions, is analogous to the above-considered prepa- ration of nanobiocomposites.311, 373, 374 One-dimensional parallel polyaniline chains prepared in situ can be encapsulated into structures with wide pores and large (the } Recently, the geometric structures and vibrational properties of com- plexes of polyenes with aluminium atoms as a constituent were studied by the ab initio quantum-chemical method in the Hartree ± Fock approxima- tion.368 A D Pomogailo diameter is*3 nm) channels.In this case, the chains take a fibre shape and exhibit microwave conductivity.351, 375, 376 An alternative to polymerisation in situ involves direct intercalation of macromolecules into layered structures, most often into silicates. The insertion of polymer molecules into layered `host' lattices is of interest from different points of view. First, the construction of organic ± inorganic polylayered compo- sites becomes possible. Second, the intercalation physical chem- istry by itself and the role of intercalation in the fact that the system gains electronic conductivity (for example, in the con- struction of reversible electrodes 308) or improved physicomechan- ical properties (nylon-layered silicate nanocomposites,377, 378 hybrid epoxide-clay composites,322 nanomaterials based on hec- torite and polyaniline, polythiophene or polypyrrole ,379 etc.) are of interest.The insertion of PEO into layers of mica-like sheet silicates in the reactions of a melt of the polymer with the `host' Na+- or NHá4 -exchange lattice is one of a few examples of direct inter- calation of polymers.380 Poly(ethylene oxide) is also inserted into lamellar networks of V2O5 . nH2O, MoO3,337, 338, 381 ± 383 MnPS3, CdPS3 ,381, 384 etc. Thus an aqueous solution of PEO (Mw=10 5) reacts with an aqueous gel of V2O5 .nH2O to form a composite after removal of water. In this case, the interlayer distance increases from 1.155 to 1.32 nm. Alkali metal ions react with PEO to form inclusion compounds, which can also be inserted into layers of ionic silicates, for example, intoMMN.The distance between the layers of the PEO± Li+ salt complexes intercalated in MMN is 0.8 nm, and the PEO chain adopts a slightly strained helicoid conformation. Poly(ethylene oxide) ± LiX systems find wide application as polymeric electrodes and solid electrolytes. However, these ques- tions are beyond the scope of the present review and were considered in detail, for example, in the monographs.385 ± 387 These systems are rather often used in combination with ceramic fillers (of the LiAlO2, Al2O3, TiO2, etc.types). Let us give only one example.388 Thus 10% of nanosized TiO2 (13 nm) or Al2O3 (5.8 nm) particles were dispersed in acetonitrile with LiClO4 and then PEO was added (the LiClO4 :PEO molar ratio was 1 : 8). The conductivity of the resulting polymer-inorganic nanocomposite is 1074 and 1075 S cm71 at 50 and 30 8C, respectively. Recently,389 the preparation of a series of new hybrid polymer-inorganic electrolytes, which contain the aluminium atom in the main chain and Li+ ions, by cocondensation of LiAl(OR)2H2 with organosilicon precursors has been reported. The intercalation of polymers 390 (PS,391 PEO,380 polypropy- lene,392 ± 394 etc.) from their melts into a silicate lattice is also a promising procedure.A molecular-dynamic model, theoretical calculations and the kinetics of formation of these intercalates were considered in the study.390 A new line of investigation involves direct hydrothermal crystallisation of silicate layers from a polymer gel.395, 396 This approach extends the range of intercalated polymers because it allows one to use compounds, which do not contain functional groups. Activation of surfaces, layers and `host' pores, including activation under the action of ultrasonics, is of importance in the processes under consideration. Thus Ni particles (10 ± 40 nm) are deposited on submicrospherical silica gel.397 When a solution of Ni(CO)4 in decalin was irradiated with ultrasonics, the Ni7C bonds were cleaved simultaneously with activation of the SiO2 surface (the removal of adsorbed water, the cleavage of Si7O7Si bonds and the formation of free Si7O bonds).An alternative procedure involves the reaction of metallic nickel with activated silanol groups of the surface to form Si7O7Ni bonds. These centres serve as nuclei for further growth of Ni particles. Amorphous superparamagnetic Ni clusters were converted at 400 8C into a crystalline ferromagnetic material. This mechanism is realised to some extent for other dehydrated surfaces as well. For example, Al2O3 and metal carbonyls M(CO)n form Al7O7M structures. 398 In such a manner gold nanosized particles were stabilised in a silica gel monolith.399 ThermolysisHybrid polymer-inorganic nanocomposites of rhodium carbonyl on the Al2O3 surface afforded small Rh clusters.400 Nanocomposites self-assembled as (M/P)n multilayers (M and P are nanosized oppositely charged layers of an inorganic component and a polymer, respectively) are worthy of special notice.These nanocomposites possess a variety of valuable properties, which makes it possible to use them, in particular, in the optical engineering for the production of displays.401 ± 407 A great variety of procedures for the layer-by-layer assembly of polyelectrolytes and clays,408 ± 413 lamellar zirconium phos- phates 414 and colloidal metal particles 415, 416 were proposed. Using anionic MMN and positively charged poly(diallylmethyl- ammonium chloride) (PDAMAC) as an example, the mechanism of formation of these materials on the surface of nanoplates from glass, quartz, silver, gold and even Teflon was considered in detail and the processes of defect formation were studied.417 Successive submersion of plates into a solution of the P component or in a suspension of M leads to an increase in the number of layers (n).Each event is accompanied by an increase in the thickness of the P and M layers by 1.6 and 2.5 nm, respectively. The formation of multilayers involves several stages. First, adsorption on the substrate surface occurs, which is determined by electrostatic and van der Walls interactions. In this case, the structural hierarchy of the M layers provides limitless possibilities of their application as templates for a great variety of molecules and clusters,24, 418 ± 422 which can be inserted between swollen layers or inside the M plates and can also be located on their surface.Second, theMcomponent is strongly and irreversibly (which was confirmed by physicochemical studies) adsorbed on the oppo- sitely charged polymeric electrolyte to form a very closely packed plane-oriented layer. Irregular M layers cannot provide complete coating of intercalated layers due to which overlapped stacks are formed. Interphase irregularities, which are independent of the nature of the substrate, exceed the thickness of the M/P layer. The control over the process can be performed by applying an external stress to the system in the course of self-assembly.In practice, a wide variety of these procedures were realised. For example, the PDAMAC polyelectrolyte and SiO2 nanoparticles (the diameter was 25 nm) were successively deposited on the surface of a polystyrene latex. As a result of adsorption, a three-component polylayered material is formed (Fig. 11) in which the thickness of one, two and four SiO2 ± polymer layers are 60, 120 and 240 nm, respectively. The method of self-assembling of multilayers was also used for the preparation of nickel phthalocyaninetetrasulfo- nate ±PDAMAC nanocomposites in which the average thickness of the M/P layer was 1.05 nm.423 1 2 3 2, 3, ... Figure 11. Scheme of self-assembly of a polylayer composite based on PS latex, SiO2 and PDAMAC; (1) adsorption of polyelectrolyte, (2) adsorp- tion of SiO2, (3) adsorption of a polycation.Layered nanostructures were prepared 424 with the use of liquid-crystalline polymers containing ionic groups; MMN or hydrotalcite [AlMg2(OH)2(OH)4]+[0.5 CO3, OH, Cl]7 was used as an inorganic component. According to different estimates, the average thickness of the M/P pair is 4.9 nm. It is believed that the electrostatic self-assembly promotes tight contacts between the components and the establishment of this molecular organisation, which allows one to prepare new types of liquid-crystalline structures with unusual properties. Apparently, the layer-by- layer assembly of aluminosilicate ± polyelectrolyte nanocomposites is a promising procedure for the design of new membrane materials.425 In this review, hybrid nanocomposites, which were prepared, for example, by emulsion or gas-phase 67 polymerisation of traditional monomers (most often, styrene, MMA, aniline, ethylene, propylene, etc.) in the presence of different organophilic minerals, are not considered although these approaches are undoubtedly of interest by themselves (see, for example, Ref.426). Thus emulsion polymerisation of the epoxy prepolymer (a product of condensation of equimolar amounts of bisphenol A with epichlorohydrin) in the presence of Na+±MMN was accompanied by the insertion of the polymer into the basal space of the smectite clay, which expanded from 0.96 to 1.64 nm.427 A micelle with the monomer rather than the monomer as such was inserted into the body of the `host,' which swelled in aqueous media.The resulting composite exhibits a higher thermal stability than that of the corresponding homopol- ymer. A hydrogel containing 3.5 mass% of MMN,428 which possesses the lower critical solution temperature (*32 8C) than that of the nonmodified product, was prepared according to the same procedure (copolymerisation of N-isopropylacrylamide and N,N0-methylenebisacrylamide as a cross-linking agent in aqueous suspensions of MMN).429 Layered graphite inclusion compounds (LGC) were studied in detail. Graphite can be considered as an aromatic macromolecule. The number of aromatic rings in this molecule is *1000 and the period of identity (the distance between the parallel planes) is 0.335 nm.Since the bonds between the parallel carbon layers in graphite are virtually absent (the energy of the interlayer inter- action is only 16.8 J mol71), monomolecular layers of different compounds, including metal ions, can be inserted into graphite to form layered (laminated) compounds.430 Graphite inclusion compounds are generally prepared by the reactions of graphite with vapours or solutions of metals in strongly ionising solvents, with low-boiling chlorides or with cationic metal complexes. These compounds are divided into products of the first, second and subsequent stages of insertion depending on the number of carbon layers separating two adjacent layers of the metal inserted. The nature of bonds in these LGC depends on the nature of the metal.For example, in the cases of Fe, Co, Ni,Mnand Cu, van der Waals interactions exist. Sometimes when the p-electron density of graphite is transferred to the inserted metal layer, the carbon network of graphite becomes a peculiar polymeric ligand. In the case of alkali metals, this bond is formed as a result of electron transfer from the metal atoms to the conduction band of the adjacent graphite layer, i.e., due to electrostatic interactions between the positively charged metal ions and free electrons of the conduction band of graphite (this problem was considered in more detail in Ref. 431). In the course of reduction of inserted metal ions, they may partially leave layered stacks and reduction may occur on the outer surface of graphite.In such a manner nanosized particles, for example, of titanium are inserted into the imperfect lattice of graphite.432 Many metals are inserted into graphite under high pressure in combination with shift deformation.433 Nanosized cobalt particles inserted into the graphite lattice according to this procedure exhibit the unusual magnetic and thermal properties.434 Therefore, intercalation chemistry provides virtually limitless possibilities for the construction of hybrid nanocomposites. Presently, the major effects attendant on the processes of for- mation of these materials are revealed. The structural organisa- tion and the principal properties of these materials have been studied. The number of studies in this field is gradually increasing, which allows one to predict the design of materials of new types.For example, the synthesis of nanocomposites of a new interesting class, viz., of hybrid materials containing azamacrocycles (gallium phosphates, etc.), has been reported recently.435 A start has been made on studies on intercalation (by the ionic exchange method) of nitrate groups of biomolecules, viz., of nucleosidemono- phosphates and DNA, into layered double hydroxides of the Mg2Al(NO3)2 .H2O type (`anionic clays').436 Nanocomposites containing metal chalcogenide as a structural element (`host') are of special interest.68 VI. Metal chalcogenide ¡À polymer inclusion nanocomposites Metal chalcogenide ¡À polymer inclusion nanocomposites are rather well studied.The results of studies were surveyed in the monographs.437, 438 Apparently, intercalation in the CdS structure has received much attention.439 Film composites are most generally prepared with the use of a solution of the corresponding precursor followed by the synthesis in situ.440 For example, Zr(OPr)4 in propanol was hydrolysed with an aqueous solution of acetic acid, and solutions of cadmium acetate and ammonium thiocyanate were added.441 The composition was coated onto a glass support and treated as in the case of the sol-gel method (see Section II). The cryochemical treatment of gels impregnated with Cd2+, Pb2+ or Zn2+ salts followed by sulfiding with hydrogen sulfide in the gaseous phase, afforded nanoparticles of sulfides with sizes from 3 ¡À 7 to 30 ¡À 100 nm in a SiO2 matrix.73 The formation of the dispersed CuS phase in a polyacrylic acid ¡À polyvinyl alcohol polymeric matrix proceeds through the stages of formation of sulfur- containing copper complexes, associates, amorphous clusters and, finally, crystalline CuS nanoparticles.442 The specific struc- ture of the interfacial layer and limitations imposed by diffusion in the swollen polymeric matrix are responsible for the characteristic features of formation of small (*4 nm) particles, the character of their distribution in the matrix and the aggregative stability.If the anion is readily polarised and the cation possesses strong polarising properties, the compounds with composition MX2 have layered structures.The distinguishing feature of these layered materials (of the MoS2 or TaS2 types) characterised by the low charge density of the layer is their ability to decompose into nanosized building blocks under appropriate conditions and to form colloidal solutions. In molybdenum dichalcogenide, a layer of molybdenum atoms is located between layers of chalcogen atoms, and packs are formed from these triple layers. In this case, bonds within the packs are substantially stronger than those between two packs linked only through van der Waals forces.443 The exfoliation procedure has been developed in sufficient detail.444, 445 Plastic superconducting electromagnetic materials can be prepared based on nanocomposites.446 Intercalation of PVPr, PEO and PEG from aqueous solutions into monolayers of suspended NbSe2 [it is known that the latter is of one the best superconductors among layered dichalcogenides (Tc=7.2 K)] was reported.447 The compositions and the selected properties of the resulting nanocomposites are given in Table 1. xH2O, LixNbSe2 NbSe2+xLiBH4 7xBH3, 70.5xH2 7xLiOH, 70.5xH2 polymer polymer ¡À NbSe2 NbSe2 (monolayer) A polymer is inserted into the WS2 phase according to the same mechanism.448 a 5 A 5 n�º mu�� Figure 12. Electron microphotograph of a `nanowire' (a) and `nanocable' (b). A D Pomogailo Table 1. Properties of intercalated polymer7NbSe2 nanocomposites. Composite Tc /K Conductivity /S cm71 d /nm Thermal stability in N2 /8C 7.1 6.5 7.0 140 250 240 310 224 233 2.40 1.96 1.88 (PVPr)0.14NbS2 (PEO)0.94NbS2 (PEG)0.80NbS2 In the case of delamination, the insertion of a polymer can occur according to the `host ¡À guest' mechanism.Then interca- lated systems are deposited by removing the solvent or by increasing the concentration of the electrolyte. Direct intercala- tion of polyalanine into the interlayer space of MoS2 (d=1.037 nm) through formation of colloidal suspensions has been described. 351 The polypyrrole ¡ÀMoS2 nanocomposite, which is a product of oxidative polymerisation in situ under kinetically restricted con- ditions, is a p-type conductor. Its electronic conductivity is three orders of magnitude higher than that of the initial MoS2 compound.449 Intercalation of PEO into a delaminated suspen- sion of TiS2 or MoS2 was carried out.450 In aqueous solutions, nanocomposites of linear polyethyleneimine in layers of MoS2, MoSe2, TiS2 or MPS3 (M=Mn or Cd)451 as well as composites of polyethyleneimine or poly(styrene 4-sulfonate) in TiO2 ¡À PbS layers possessing semiconducting properties were prepared.452 Nanocrystals of semiconductors based on metal chalco- genides immobilised into polymeric matrices exhibit luminescent properties.This is primarily true for nanocrystals of CdSe,453, 454 CdS ¡À Ag,455 ZnS and ZnS ¡À CuS.456, 457 Composites based on ZnS ¡À CuS, which contain crystals with dimensions *2 nm in a polymeric matrix, were prepared by copolymerisation of acrylates of the corresponding metals with styrene followed by treatment of the solution of the resulting copolymer with hydrogen sulfide in chloroform.458 These composites possess good photo- and electro- luminescent properties.It should be noted that a precipitate did not form from the resulting almost colourless organosol even after one year. Metal chalcogenides with more complex layered structures, for example, PbNb2S5 and SmNb2S4, can also be decomposed and subjected to intercalation.459 In polar solvents, one-dimensional `host' phases form colloidal systems with MMo3Se3 (M=Li, Na or In). These systems contain monodisperse negatively charged condensed cluster chains (Mo3Se¡¦3 )n. The structures of these systems (which are also called pseudo-one-dimensional metals) have been described in sufficient detail.460 They are of interest in the design of materials with nanowire (or molecular-wire) mor- phology.For this purpose, block radical polymerisation of dilute (1073¡À 1074 mol litre71) solutions of these `rigid rods' in a solvating monomer (vinylene carbonate) is performed in the presence of a cross-linking agent.461, 462 The system rapidly solidifies, and the polymeric matrix provides capture, association b 100 nmHybrid polymer-inorganic nanocomposites and isolation of the inorganic phase. The nancomposite contains individual isotropic nanowires 0.6 ± 2 nm in diameter and 5 ± 10 nm in length. Polymerisation of more concentrated solu- tions (1072 mol litre71) affords nanocomposites as oriented multiwires (`nanocables') 2 ± 4 nm in diameter and *500 ± 1500 nm in length.Each `nanocable' contains 5 ± 20 `nanowires' (Fig. 12 a, b). The molecular weight of the inorganic chain (wire) is estimated at *105. The conductivity is 102 ± 103 S cm71, i.e., it is approximately equal to the conduc- tivity of the (LiMo3Se3)n film. Therefore, intercalation of polymers into the interlayer space of chalcogenides is an extensively and fruitfully developing field of the technology for the preparation of nanocomposites, although many problems, particularly, those associated with the mecha- nism of insertion and `guest ± host' interactions are still not completely understood. In conclusion, let us give several more examples of these nanocomposites.Polymeric films based on nanosized particles (10 ± 16 nm) of chalcogenides ZnS,463 as well as of Cu2S7CdS7ZnS and different polymers, CdS 464 and b-cyclodextrin were described. The inner diameter of the cavity of the latter is 1.53 nm,465 which is substantially smaller than the sizes of the nanosized particles. Nevertheless, monodisperse complex structures with the participation of b-cyclodextrin are formed. It was suggested 464 that these structures are formed as a result of binding of monodisperse complexes with excess sulfide anions and cadmium cations to form CdS ± b-cyclodextrin ± S27 aggregates. VII. Metallopolymeric Langmuir ± Blodgett films �self-organised hybrid nanocomposites Metal-containing nanoparticles inserted into Langmuir ± Blodgett films (LBF) belong to yet another promising class of materials for the preparation of nanocomposites. Procedures for the synthesis of self-organised inorganic component ± surfactant composites, which have been developed in recent years, made it possible to prepare new two-dimensional composite materi- als.58, 466, 467 Generally, the sizes of the elements of these materials are no more than 2 ± 10 nm.Polymolecular LBF are used for the preparation of highly oriented ultrathin films possessing the special properties, which are determined by their supramolecular structures. Different sensor groups or their precursors possessing nonlinear optical properties can be inserted into these self-organised layers (in some cases, metal complexes, clusters or nanosized particles are inserted).Finally, LBF are used for modelling surface and biological processes (see, for example, Ref. 468). Supramolecular ensembles with mesosized periodicity (intermediates between materials of atomic and macroscopic sizes) are subjects of supra- molecular chemistry, viz., constructional chemistry of intermolec- ular bonds. Although this line of investigation is still in a primitive state of progress, its methodology has already been developed. The technique for the preparation of Langmuir ± Blodgett films was modified, when applied to this methodology. Heterogeneous polar nanolayers are prepared on the surface of a subphase (deionised water) at a given constant surface pressure p, which has the dimensionalitymN m71.The pressure is measured using a Wilhelmy balance in a specially designed apparatus with two- section baths controlled by microprocessor electronic units and a computer with the use of the constant automated monitoring of the technological process.469 Successive transfer (by methods of Langmuir ± Blodgett's vertical lift or Langmuir ± Scheffer's hori- zontal lift) of these layers in the liquid-crystalline state to a solid support allows one to design sufficiently complex molecular planar structures possessing different properties. Metal particles in LBF can exist as a `two-dimensional gas' (if their concentration on the surfacel and they do not interact with each other).Three-dimensional states, viz., the `gaseous' (the distance between the molecules is substantially larger that their sizes), `liquid' (the 69 distance between the molecules is comparable with their sizes), `liquid-crystalline' (the lability of the molecule in the plane of the monolayer is retained) and `solid' states, are formed as the two- dimensional gas is compressed. Two procedures for the formation of nanosized particles in these films are distinguished. The first of them is based on a combination of the principles of colloid chemistry and self- organisation and growth of monolayers.470 ± 472 In this case, the formation of nanosized particles (for example, by chemical and photochemical reduction of aqueous solutions of metal salts) is performed in the presence of stabilisation agents and components forming LBF.The resulting layers act as specific templates.472 ± 474 This approach is also of interest as regards studies of biomimetic processes of mineralisation, including studies with the use of the sol-gel method considered above. The second procedure consists in deposition of LBF on the surface of stabilised nanosized particles and insertion of these particles into polylayers (with the participation of functional groups), including layers of an inorganic nature. For example, LBF based on amphiphilic Ru2+ complexes was immobilised on a monolayer of hectorite.475 In this case, lamellar films and polylayers were formed. Langmuir ± Blodgett films with a speci- fied organisation either were formed directly on the surface of colloidal particles or were formed on the water surface and then were transferred to a support of nanosized particles with the use of the Langmuir ± Blodgett technique.476 ± 479 Thus a gold hydrosol stabilised with 4-carboxythiophene was immobilised on a mono- layer of octadecylamine by the electrostatic method.480 The charge on the film was controlled by changing pH.Multilayer films with different densities of the Au ± octadecylamine clusters with dimen- sions 103 nm can be prepared according to this procedure; the films contain from 2 to 20 monolayers (the surface pressure in the monolayer is 25 mN m71 and the film area (A) is 0.37 nm2 mol- ecule71). Apparently, the method of immobilisation of nanosized particles on LBF has a number of advantages over the chemical insertion of metal ions into LBF followed by assembly of clusters, viz., the deformation of films is excluded, ordering of the lamellar phase is not disrupted and various nanosized particles, including bi- and polymetallic particles, can be used by choosing appropri- ate hydrosol mixtures.When particles are immobilised on LBF, not only adsorption or chemical interactions occur, but also a higher recognition level is achieved. Examples are provided by self-organisation of monolayers of alkylsiloxanes, fatty acids, dialkyl sulfides or thiols on surfaces of Al, Au, SiO2, etc.481, 482 Thus two processes occur on the mosaic Au ±Al2O3 surface, viz., recognition of the `own' support (selective adsorption) and self- organisation of diphilic thiol (on Au regions of the support) and acid (on Al2O3 of the support).However, the majority of studies were devoted to self- organised hybrid nanocomposites based on mononuclear com- plexes (in particular, on CdS), procedures for the assembly of nanosized particles from these complexes, investigations of the quantum dimensional effects typical of semiconducting nano- particles, and practical applications of these nanocompo- sites.483, 484 It was demonstrated that a dispersion of CdS particles (2.65 ± 3.4 nm) stabilised with dodecylbenzenesulfonic acid in CHCl3 spreads over a water surface to form stable monolayers of nanosized particles.485, 486 According to the p ±A compression isotherms, an increase in the p value leads to the transition from the `gaseous' state to a close-packed monolayer of particles and, finally, to a polylayer.The resulting LBF are characterised by A= 0.65 ± 1.1 nm2 particle71, which is close to the corresponding values for the hexagonally packed rigid spheres (0.608 ± 0.887 nm2 particle71). The transfer of these monolayers to a solid support by the Langmuir ± Blodgett method produces polylayers of dimensionally quantised CdS clusters. Their optical density increases linearly as the number of monolayers transferred increases. The formation of nanosized semiconducting particles in LBF in situ by the reactions of metal ions with H2S or Na2S has been70 described (see, for example, Refs 487 ± 489).For example, sulfid- ing of layers of cadmium, zinc or lead behenates (C21H43COO)2M afforded films 100 nm thick (34 layers) containing sulfides of these metals.490, 491 The films are anisotropic and the resulting nonspherical nanosized particles (the diameter is 5 ± 10 nm) form the so-called cluster layers possessing pores (the thickness of the layer composed of the clusters is 1.12 nm). The formation of CdSe nanoparticles upon treatment of films of cadmium arachidate (C19H31COO)2Cd with a H2Se vapour occurs in the interlamellar space of films in the solid phase and is accompanied by their substantial deformation and even by disruption of the lamellar structure.492 Multilayer LBF are rather often prepared from cadmium stearate,493 magnesium stearate 494 and a-Fe2O3.495 It was found that self-organised structures were formed in hydrophobic layers of stearic acid as films of silver stearate (8 ± 14 layers).The film was transferred to electrodes (p=25 mN m71) and was electro- chemically reduced in a neutral or acidic solution to form two- dimensional Ag clusters (the diameter was 20 ± 30 nm).496 It was found that these films also contained Ag clusters of the sandwich type. Self-organised metal-containing LBF ensembles are rather often used for the modification of the electrochemical properties of electrode surfaces on the molecular level.497, 498 These ensem- bles are exemplified by self-assembling LBF based on the C8H17C6H4N=NC6H4O7(CH2)3COOH and X(CH2)2SH (X=OH, COOH) compounds linked though a hydrogen bond and deposited on Au.These systems are electrochemically stable and their behaviour is predictable and reproducible. One of the promising approaches of electrochemical synthesis involves the formation of two-dimensional (Langmuir) mono- layers of nanosized particles under monolayer matrices of surfac- tants, which are present on the surface of an electrolytic solution, O O S OOO Fe OOO SO S OOO Fe OOO SO 7H2O H2O O O S OOO Fe OOO SO S OOO Fe OOO SO 1 layer (dehydrated) n is a number of carbon atoms in a surfactant. OH2 H2O OH2 Fe H2OOOHO 2 OH2 OSO O OS S OOO H2 O OOO H2 O OH2 H2O OH2 H2O H2O OH2 OH2 H2O Fe OH2 OOO Fe OH2 OOO S S O O OS OOO OH2OH2 H2O Fe OH2 H2O OH2 1 layer (hydrated) n=10, 12, 14, 16, 18 in the course of kinetically controlled electroreduction with the use of an electrochemical circulating cell.499, 500 Two-dimensional aggregates of silver particles are formed only when a monolayer of a surfactant carries a negative charge.Of special note is the possibility of the use of the LBF technique, which involves controlled precipitation and hydrolysis of iron salts in surfactant layers,501 for the preparation of nanomaterials of a new type. The thickness of the layer is determined by the concentrations of Fe2+ and Fe3+ salts as well as of their oxides in aqueous solutions and by the redox Fe2+.Fe3+ equilibrium (oxidation by H2O2).The properties of self-assembling iron ± surfactant nanocomposites (n is a number of carbon atoms in the surfactant molecule) contain- ing 1, 2, 3 or 6 layers of iron oxide depend on their sizes. These composites exhibit the superferromagnetism and occupy the first step in the hierarchy of nanocomposite magnetic materials (Scheme 4). Magnetic LBF can be prepared based on heterobimetallic oxo complexes, for example, Cr3+7Fe2+7Cr3+,502 or using sulfid- ing of bimetallic Pb7Cd or Zn7Cd stearates.503 Classical LBF can be prepared with the use of not only low- molecular-weight but also polymeric systems. In this case, with the aim of imparting the hydrophobicity to the chain, thus providing the formation of monolayers, the chain is functionalised either using polymer-analogous conversions or by grafting side chains, which can respond to the external factors.As in the case of the preparation of nanocomposites of other types, polymerisation of functional monomers and their copolymerisation with monomers, which serve as spacer units, are also used. In principle, diphilic LBF based on polymers have advantages over polymolecular films based on low-molecular-weight compounds, if for no other reason than their higher stability. These materials support the required density of a monolayer upon formation of a film (an Surfactant+Fe(II) salt OFeO O O Fe OOO OOO S S O O O O O S S S OOO OOO OOO Fe Fe Fe O O O O O O Fe Fe O O O O O O Fe Fe O Fe OOO OOO S S OOSO O O O O S S O OOO OO O Fe Fe O O O 3 layers, n=16, 18 H2O2 O O O O OFe O O O O OFe S S O O O O O S S S OOO OO O OOO Fe O O Fe Fe O O O O O Fe Fe O O O O Fe O OFeO O O OFeO OFe Fe O O O O Fe Fe O O O O Fe OOFeO OO O Fe O O O OS S OOSO O O O O S S O OOO OO O Fe Fe O O O 6 layers, n=10, 12, 14 A D Pomogailo Scheme 4Hybrid polymer-inorganic nanocomposites equilibrium pressure of spreading of monolayers is a more rigorous criterion for their thermodynamical stability).To solve some applied problems, it is necessary to introduce functional groups of different types into LBF monolayers. For example, components of a redox pair can serve as such functional groups. However, serious limitations emerge when employing low-molec- ular-weight reagents because diphilic compounds are not mixed at the molecular level and phase separation occurs.The use of polymers allows one to solve this problem rather readily. The effect of the composition and the structure of polymers and copolymers as well as of the external conditions on the formation and properties of organised polymeric monolayers and LBF on surfaces of liquids and solids were analysed in detail in a review. 503 The formation of LBF was performed with the use of polymeric metal complexes based on porphyrins and phthalocyanines.504, 505 For example, the diphilic properties can be imparted to polymers by copolymerisation of long-chain a-alkenes with maleic acid, maleic anhydride and other compounds 506 because the carboxyl group is a convenient fragment for binding a metal complex.When reagents, such as 4-aminomethylpyridine, are used for the opening of the anhydride ring, groups capable of providing donor ± acceptor bonding of metals possessing coordination vacancies are formed in the polymer. This procedure was used for binding tris(phenanthrolino)iron(II) sulfate or bis(salicyl- idenoaminopropyl)aminocobalt(II) to diphilic copolymers, viz., to copolymers of maleic acid ± pyridinemonoamide with octa- decene or maleic acid ± picolinemonoamide with N,N-diocta- decylacrylamide.507 The dependence of the surface tension p on the area of the monolayer per molecule (A) at the air ± water interface is shown in Fig. 13. The type of isotherm for the film formation is determined by the concentration of a metal complex because its dimensions are substantially larger compared to the area of monomeric units (it is assumed that a cubic complex molecule is located nearly parallel to the water surface). At concentrations of the complex higher than 1077 mol litre71, this fact is of considerable importance in the organisation of a monolayer. After compression (p=25 mN m71), a stable homo- geneous monolayer is formed. However, the stability is lost at concentrations of*1074 mol litre71. An important point is that one monolayer as a component of a multilayer system can contain metal complexes of two different types.Their bonding by two functional groups possessing oppo- site properties prevents phase separation. Recently, the preparation of mono- and polylayer LBF based on a copolyimide the chain of which contains carbazole groups (as an electron donor) and copper phthalocyanine has been reported.508 The stack molecular organisation of the layers and p /mN m71 50 30 5 1 2 3 4 100 1.0 0.5 A /nm2 molecule71 Figure 13. Isotherm of the film formation of maleic acid ± pyridinemonoamide ± octadecene copolymers on different sub- phases: (1) an aqueous solution of an alkali with pH 10, (2 ± 5) solutions of [bis(salicylideneaminopropyl)amine]cobalt(II) with the concentration/ mol litre71: (2) 1077, (3) 1076, (4) 1075 and (5) 1074.71 small intermolecular distances in these LBF ensure the high lability of the charge and impart the good photoconducting properties to LBF.509 Regular ensembles of nanocomposites organised in LBF miltilayers are also constructed through electrostatic interactions between charged sol nanoparticles dispersed in a subphase and charged monolayers of the surface (for example, between anionic nanoparticles and a cationic polyelectrolyte).408 This procedure was used for the synthesis of regular nanowire Tl2O3 ± PVC ± arachidic acid composites (PVC is polyvinylcarbazole) based on cross-linked polycationic PVC, which was prepared by electrochemical polymerisation of N-vinylcarbazole in the pres- ence of NaClO4, and a sol of thallium oxide (n-type semiconduc- tor).510 The resulting polymeric layer (2.7 nm) serves as a peculiar template coated with regularly organised Tl2O3 particles (3.2 nm).The resulting 5.5 nm thick monolayers can be transferred layer- by-layer to the hydrophobic surface (p=25 mN m71). Evi- dently, this method will gain wide acceptance because electro- chemical polymerisation allows one to produce a large number of positively charged conducting polymers, for example, polyaniline, polypyrrole, polythiophene, etc.511 Apparently, this mechanism is also realised in the formation of self-organised layers of TiO2 on SO2-functionalised polymer surfaces 512 as well as in the preparation of ordered TiO2 layers on poly(sodium 4-styrenesulfonate) (PSS).TiO2 particles (*3 nm), which were prepared by acid hydrolysis of TiCl4, formed organised layered structures on surfaces of polymers of the cationic type, viz., on superthin (*1 nm) PSS or PDA- MAC.513 Optically transparent LBF organised on the molecular level and containing up to 120 layers (60 bilayers) were formed on surfaces of various substrates, viz., metal, silicon or a polymer cleaned with a 5% N-2-(2-aminoethyl)-3-aminopropyltrimethoxy- silane solution. The thickness of the bilayer was estimated at 3.6 nm. It is expected that this procedure will allow one to realise various combinations of materials with semiconducting metal ± dielectric structures containing nanosized units of the p-n-, p-n-p-, n-p-n-, etc.types. Recently, polylayer (2 ± 12 layers) films of hybrid nanocomposites with the N!Cd coordination bond have been prepared by the reaction of poly(4-vinylpyridine) (P4VP) with nanosized particles of cadmium sulfide.514 Nanocomposites of yet another type, viz., clusters in a Langmuir monolayer, are of interest for the design of metal-film materials used in the electronics as well as for the modelling of structures of fixed catalysts. For example, the reaction of Os3(CO)11(NCCH3) with self-organised layers of (3-mercapto- propyl)trimethoxysilane on a gold surface resulted in the disrup- tion of the well-organised thiol surface to form cluster aggregates (the diameter was 1.0 ± 2.2 nm).515 Layer-by-layer adsorption of one- and two-dimensional complexes of the [Cd4L4]8+ type, which are characterised by the octahedral coordination, on monomo- lecular poly(ethyleneimine hydrocloride) and polystyrenesulfo- nate films on silicon can be considered to be an efficient method for the preparation of new metal-containing supramolecular thin films.516 Particular attention is given to metal clusters in LBF deposited on the surface of highly oriented pyrolytic graphite (HOPG).Their application allows one to solve many catalytic problems as well as to prepare stable reproducible tunnel nanostructures. The Rh4(CO)12 and (NEt4)2[Pt12(CO)24]2+ clusters sorbed on HOPG from solutions of organic solvents were studied by scanning tunnel microscopy.517 Under laser irradiation, these clusters underwent decarbonylation and transformation to highly dispersed Pt crystallites of dimensions 1.860.5 nm bound to the graphite surface.518 The procedure for the deposition of isolated `naked' silver clusters on the HOPG surface with the topography charac- terised by planar dimensions of 3 ± 5 nmand the height of 2 ± 3 nm and the technique for the `assembly' of their ligand shell from PPh3 were developed.519 The voltammetric characteristics of individual clusters on the surface of freshly cleaved HOPG were measured.The insertion of the clusters into monolayers of stearic72 acid and their transfer to the HOPG surface also afforded cluster- containing LBF.520 In the resulting monolayer, the cluster molecules form an ordered two-dimensional lattice.By this means the problem of fixing of clusters on the surface was solved and the reproducible one-electron mode of tunneling was realised at room temperature. The regularities of the insertion of cluster molecules (Pd3, Pt5 and Pd10) into LBF and their voltammetric characteristics were analysed in the study.521 A multilayer LBF was transferred from magnesium stearate (from a monolayer of stearic acid to the surface of aqueous solutions of MgCl2) to the HOPG surface in the disrupted state; the surface per molecule is 0.15 nm2, p=36 mN m71 (see Ref. 494). The above-considered examples demonstrate that metal- containing Langmuir ± Blodgett films, including those based on nanosized particles and clusters, are of considerable interest for the preparation of hybrid organised nanocomposites.VIII. Major fields of application of hybrid nanocomposites When considering synthetic problems, we have already drawn attention to the properties of the resulting nanocomposites and to the possibilities of their practical application. Undoubtedly, these problems merit detailed consideration. In the present review, only the major problems are briefly analysed. Hybrid nanocomposites are primarily used for the prepara- tion of plastic materials possessing semiconducting and super- conducting properties. Among them are nanowires on polymeric matrices, films with special properties,309 and different-purpose ceramics, including membranes, luminophors, antireflection and reflecting coatings on optical units, carriers and catalysts, rein- forcing agents for plastics and rubbers, binders, adsorbents for pharmaceutic and cosmetic industry, etc.116, 522 ± 527 The recent review 278 devoted to the use of materials prepared by the sol-gel method in electrochemistry includes more than 300 references.Optical waveguides were constructed based on hybrid polymer- inorganic nanomaterials, which were prepared by the same procedure and possess improved thermal and mechanical proper- ties compared to the starting polymers.164, 528, 529 Nanoparticles, for example SiO2, TiO2, CdS, CaCO3 and BaSO4, including those formed by the sol-gel method, are used as specific fillers, because the properties of materials are notice- ably improved at a substantially lower concentration in a matrix than those prepared with the use of standard fillers.Thus when fillers are inserted into composites by mechanical dispersion using mixing equipment (the size particles is*1 m m), it is necessary that up to 40 mass%± 50 mass% of the filler should be taken for 100 mass%of polyisopropylene to attain the required reinforcing effect, whereas the same effect is achieved by the insertion of only 0.6 mass%± 0.8 mass% of a filler in situ.530 Highly filled (up to 75%) composite materials based on nanosized particles of the high-Tc superconductor Tl2Ba2Ca2Cu3Oy (Tc=125 K) and poly- chlorotrifluoroethylene possess improved physicomechanical and thermal characteristics and stability to atmospheric moisture.531 These materials can be used for the design of cryoelectronic instruments, levitation equipment and magnetic screens.Recently, efforts have been made to describe the viscoelastic properties of a composite taking into account the specificity of interactions between segments of the macromolecules and the active centres of the nanoparticles. The Kerner equation, which relates the elastic modulus of a composite upon simple extension to the portion j of the inorganic phase, was modified 532 when applied to systems with strong interactions between nanoparticles and a polymer. Based on scaling theory, the equilibrium mechan- ical properties of a complex of colloidal particles and macro- molecules some segments of which are adsorbed on active centres on the surface of particles (which are `polynodes' of a network of the nanocomposite) were described.533 Under deformation and changes of the temperature, some segments leave the surface and the fragment of the macromolecular chain linking the surfaces of A D Pomogailo two particles is increased by the same number of the segments.If this fact is taken into account, two nontrivial conclusions can be made: (1) the elastic properties of the composite are not only determined by the volume fraction of filling but are also inversely proportional to the linear size of the particles; (2) the shift modulus of the nanocomposite, unlike the shift modulus of nonfilled polymeric networks, does not approach zero as the temperature is extrapolated to zero.These conclusions agree well with the results of studies of the relaxation mechanical properties of the composite as well as the dependence of its viscosity on the molecular and structural parameters, such as the energy of interaction between the segment of the macromolecule and the active centre of the nanoparticle, the number of segments of the macromolecule, their size, the j value, the particle size (the diameter is430 nm) and the temperature.533 Let us briefly consider the conducting properties of hybrid nanocomposites. These properties are manifested only with particular inorganic component : polymer ratios when current- conducting channels of fractal metal-containing clusters are formed in a polymeric matrix for one reason or other.The highest conductivity is achieved when the composite is converted into a network of interrelated current-conducting chains, i.e., it has a percolation structure (see, for example, Ref. 534). To put it differently, critical concentrations of the filler jcr (the percolation threshold) exist above which (j>jcr) the conductivity sharply increases. With a knowledge of the percolation threshold, the minimum necessary filling of conducting composites can be predicted. For example, jcr for epoxysilicon resin filled with spherical particles of dispersed nickel is 0.25 and the critical parameter (Xcr) determined by the number of bonds at conducting nodes in the lattice of the solid is 0.30.535 The conductivity of metallopolymeric nanocomposites is substantially affected by the dispersity of an inorganic compo- nent.Different nanocomposites are characterised by different relationships between the conductivity and the metal content. The percolation threshold of composites based on layered poly- pyromellitimide films containing inserted silver particles is attained when the filler content is >9 mass %. At the same time, when silver nanosized particles (10 ± 15 nm), which are prepared in the course of thermolysis of a prepolymer [a solution of silver acetate in poly(pyromellitamide acid)], are uniformly distributed over a film, the composite does not take on the conducting properties at the same filler content. The dielectric characteristics of films (s=10715± 10712 S m71) are retained at a high level, which is in many respects associated with the presence of a substantial fraction of dielectric polymeric interlayers between conducting particles of the filler.It is possible to enhance the conductivity of polymeric composites as a result of the formation of a filler as `nucleus' (conductor or dielectric) ± `shell' (conductor) constructions. This is of practical interest primarily in the technology of the produc- tion of glues and varnishes. For example, the conductivity of dielectric SnO2 particles coated with a silver layer (8 vol.%) using thermal treatment of a precursor, viz., an Ag(I)-containing polymer, is substantially increased (s=161073 S m71), whereas s=261077 S m71 for a mechanical mixture of SnO2 and Ag powders (16 vol.%).536 Particular attention has been given to multicomponent metal- containing hybrid nanocomposites, which find use in the produc- tion of electrode materials for galvanic cells,537 high-Tc super- conducting ceramics, etc. Materials prepared by the sol-gel method are used as piezoelectric ceramic fillers in acoustic converters as well as in the medical diagnostics. Thus coatings with dense homogeneous microstructures possessing the enhanced mechanical strength and improved piezoelectric char- acteristics were formed by sheet rolling (413 K, 20 ± 30 MPa) of polymer ± ceramic mixtures of polyvinylidene fluoride with PbTiO3 (up to 65 vol. %).538Hybrid polymer-inorganic nanocomposites The preparation of modifying thin (3 ± 7 nm) magnetic coat- ings by electrochemical methods has considerable promise.In particular, organosilanes are coated with a film containing a-Fe (70 at.%of Fe), magnetite Fe3O4 and an admixture of Fe2O3.539 Sometimes magnetically active particles are introduced into different gels, viz., into silicon,540 polyacrylamide,541 poly(ethylene oxide) 542 etc. gels.543 To improve the electrophysical and magnetic characteristics of nanocomposites, ferroplastics are often formed in magnetic fields. In this case, magnetic orientation occurs. Thus neodymium ferrite is prepared in fields with the intensity of 361074 ± 361073 T and then it is subjected to the mechanochemical action (pressing,541 treatment between Bridgman's anvils 543 or a combination of these procedures 544).This makes it possible to produce magnetically filled matrices with oriented chain struc- tures, which are used in the techniques for the production of varnishes and films and are of considerable interest as elements of systems for data recording. The formation of coatings under conditions of hardening in a variable magnetic field makes it possible to prevent precipitation of nanoparticles, which are either concentrated closer to the film surface or uniformly distributed throughout the body of the film. These materials are used as drug carriers as well as for the magnetic recording of data, magneto- separation and the preparation of composites possessing the magneto-optical properties and magnetic liquids (for example, oligoorganosiloxanes).Organometallic ferromagnetics based on polymetalloorganosiloxanes possess peculiar magnetic proper- ties.545, 546 Hybrid polymer-inorganic nanocomposites are characterised by a unique combination of the optical and semiconducting properties associated with the sensitivity of plasmon vibrations (the frequency, the intensity, the shape and the width of the band) not only to the nature of the matrix and the morphology of the nanocomposite but also to the particle size. The character of interactions between the electronic and atomic subsystems changes substantially as the linear sizes of semiconductor par- ticles decrease to values comparable with the electron wave- lengths, which is manifested in the quantum dimensional effects, viz., in nonlinear optical effects, doubling of the frequency of the incident radiation (generation of the second harmonic), the `blue shift' of the exciton absorption band, etc.The ability of these nanocomposites to form films and the ease of their treatment make it possible to use them for the preparation of dispersing optical elements, band-pass light filters and other high-quality thin-filmed coatings (only *5 ± 20 nm thick), which are used in the optoelectronics. For example, the production of modern integrated circuits is based on the so-called planar technology combining processes of the nanolithography (the formation of nanosized surface figures as lines and dots) and the etching.With the aim of decreasing the sizes of elements of optoelectronic integrated circuits to <100 nm, new techniques of the lithogra- phy (in particular, electron-ray, ion-beam and X-ray) as well as new procedures for the dry etching (plasma-chemical, reactive ionic, etc.) are used. Almost all materials of this kind are characterised by a nonlinearity of the optical properties manifested in a substantial strengthening of the local field of the light wave. The latter is quantitatively characterised by the third-order susceptibility, the nonlinear indices of refraction and the nonlinear absorption coefficient. These effects are widely used in spectroscopic practice (local nonperturbing methods of diagnostics, electron-optical image converters, etc.).This is particularly true of sol-gel glasses based on CdS, nanohybrid composites based on polymers and SiO2 (or V2O5), Langmuir ± Blodgett films, etc.547 If the sizes of semiconducting nanosized particles in matrices are much smaller than the wavelength of the exciting field ( l/20), the nonlinear optical properties manifest themselves in the fact that `quantum points' appear in nanocomposites (quantum-point poly- mers).548 ± 550 There is a definite relationship between the wave- length of the exciting radiation and the sizes of nanoparticles. 73 Nanocomposites of this type can be used as active layers of light- emitting diodes.551 In the previous sections, procedures for the insertion of transition metal ions into polymers in the stage of the sol-gel synthesis or intercalation have already been mentioned.The materials thus obtained serve, for example, for the preparation of coloured light guides, which have a broad spectrum of applications, viz., from storage elements to highly sensitive detectors. Let us consider the use of hybrid nanocomposites in catalytic, most often hydrogenation, processes. Under conditions of for- mation of catalysts and hydrogen adsorption, dislocations are fixed at interblock boundaries of polymer-stabilised nano- particles. These particles have highly active surfaces and devel- oped internal structures. A polymeric matrix hinders smoothing of numerous defects upon catalyst ageing. Of prime importance in the catalysis is a knowledge of the surface composition and Sspec and the possibility to control these parameters.The inherent size of nanosized particles is comparable with the molecular sizes of the compound subjected to catalytic conversion. This fact is responsible for the characteristic features of the kinetics and the mechanism of the reactions with the participation of nanosized particles. In particular, the high efficiency of colloid-metallic catalysts in multiple-electron processes results from the fact that these catalysts are `reservoirs,' which can readily incorporate electrons. Polymer-stabilised metal nanoparticles are primarily of theo- retical interest as excellent models for studying the influence of the dimensional effects on the catalytic activity.Using these particles as examples, many concepts of the fundamental theories of catalysis can be verified. In addition, since nanocomposites are involved both in homogeneous and heterogeneous catalytic processes, they provide an additional possibility of revealing the relationships between homogeneous, heterogeneous and enzyme catalysis.552 In a number of characteristics, these nanocomposites are similar to homogeneous catalysts although they also inherit the principal features of heterogeneous contacts. Thus huge Pd561 clusters can be considered as a bridge between homogeneous and heterogeneous catalysts.553 Composites based on nanosized par- ticles of platinum and other metals, which are incorporated into glassy carbon matrices, can be assigned to the same type of catalysts.554 Of particular interest are catalysts based on organic ± inorganic hybrid materials in which catalytically active metals incorporated into an oxide network are dispersed.For example, highly dispersed heterogeneous catalysts in a SiO2 network prepared from substituted alkoxysilanes are known (see the review 555). NH3, H2O MYn+nAXSi(OR)3+xSi(OR)4 H2, D 7ROH O2, D MOy . (x+n)SiO2 Ym(AXSi3/2)n . xSiO2 M. (x+n)SiO2 Protective polymeric coating prevents poisoning of a metal- lopolymeric catalyst by potential catalytic poisons of the dibenzo- thiophene type. Nanocomposites exhibit the high catalytic activity in elimi- nation of hydrogen under the action of visible light 556 [for example, the ethylenediaminetetraacetate (electron donor) ±Ru2+(bipy)3 (photocatalyst) ± methyl viologen (electron acceptor) system].Substantial success has been achieved in charge separation as well as in the design of efficient photocatalysts with the use of semiconducting nanomaterials (primarily, based on TiO2) prepared by the sol-gel method (see the reviews 557, 558). The additional introduction of transition metal ions (for example, of copper 559, 560) in the stage of formation of these materials substantially enhances the efficiency of the photocatalytic reac- tion. These systems exhibit high activity also in low-temperature oxidation of CO.561, 56274 The use of polymer-protected nanoparticles can contribute new specific features to catalysis.A platinum sol as well as Pt on Al2O3 are the well-known enantioselective catalysts of hydro- genation processes.563 The potential possibilities of the use of hybrid nanocomposites in catalysis are far from all realised.564 IX. Conclusion To summarise, a great diversity of aspects of the chemistry of nanocomposites is progressing extensively. In the coming years, procedures for the synthesis of multicomponent materials based on ecologically pure solid-state reactions will be developed. Apparently, the use of alcoholates or oxoalcoholates of nontradi- tional metals, for example,565 of Ta5+ or Nb5+, in sol-gel procedures will have promise. This refers equally to new inter- calation systems. Procedures for the preparation of hybrid materials of the `network-in-network' type will be studied in more detail. The mechanism of their phase separation and the structures of the resulting nanocomposites will be elucidated.These materials will be formed with the use of monomers, which are traditionally used for the preparation of network polymers, including interpenetrating networks. A search for the optimum modes of formation of new types of lattices (`host' and 'guest' components in intercalated nanocomposites) is still in progress and studies of intracrystalline `host ± guest' interactions and their effect on the electronic properties of complex systems are being continued. For this purpose, it is necessary to reveal principal mechanisms of the orientation effect of pores and interlayer formations on the crystallinity and the stereoregularity of the polymer formed, which is one of the fundamental problems of physical chemistry of polymers and composite materials by itself.The approaches to the preparation of organic-inorganic nano- composites surveyed in this review may be useful in the synthesis of different single-phase crystalline nanometal-ceramic products with complex compositions, for example, the MgM2O4 ± a-Al2O3 systems (M=Al or Cr) (<10 nm). These products can also be synthesised with the use of Grignard reagents. Theoretical studies of the nature of bonds between metals (Fe, Ni, Cu and Ag) and the surface oxygen anion in cluster models started by Johnson and Pepper 566 will help in the understanding of the structures of these materials.The use of LBF, which incorporate isolated cluster molecules or nanosized particles, in molecular electronics may have considerable promise. Finally, the nanophase materials technology requires the appropriate scientific support, i.e., the construction of new instruments as well as the development of techniques for working with nanosized materials. The generality of many processes occurring both in the living and nonliving nature can give impetus to studies of nanobiocom- posites. In this review, a reasonably fair, though incomplete, picture of applications of nanobiocomposites to studies of organised matter and the preparation of hybrid bioceramic nanomaterials is given. 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ISSN:0036-021X    

出版商:RSC    

年代:2000

数据来源: RSC

摘要:

Russian Chemical Reviews 69 (1) 81 ± 94 (2000) Thermal transformations of organic compounds of divalent sulfur MG Voronkov, E N Deryagina Contents I. Introduction II. Thiols III. Sulfides IV. Hydrodisulfides V. Disulfides VI. Conclusion Abstract. Published data on thermolysis of organic compounds of divalent sulfur and on thermal synthesis involving these com- pounds are summarised. The bibliography includes 201 referen- ces. I. Introduction Among the great variety of the ways of formation and chemical transformations of organic compounds of divalent sulfur, the thermal reactions, i.e., the processes which occur at elevated temperatures in the absence of catalysts or promoters, take a special place. Such reactions proceeding in the earth's crust over millions of years have led to the formation of sulfur-bearing components of combustible fossils.Mankind has dealt with the thermal reactions of sulfurous compounds from time immemorial, whenever a sulfur-containing material of organic origin is subjected to thermal treatment. All combustible fossils, viz., oil, bituminous and brown coals, oil-shales and natural gas, contain some amount of sulfur compounds. Therefore, high-temperature reactions of formation and thermal transformations of organosulfur compounds accom- pany virtually all processes of thermal fossil treatment. These are, for instance, the distillation and cracking of high-boiling oil fractions at temperatures from 300 to 800 8C and dry-distillation of bituminous and brown coals, oil-shales and peat at 450 ± 1000 8C.Thermal reactions of organosulfur compounds were studied in the very early stages of the development of chemistry as a science. However, the detailed exploration of these processes was started only in the past century. The interest in thermal reactions of particular sulfur-contain- ing organic compounds and in high-temperature syntheses involv- ing these compounds is caused by the necessity to solve a number of problems, the most important of which are listed below. 1. The utilisation of sulfur-containig wastes in oil-refining, gas-, coal- and pulp-processing industries. MG Voronkov, E N Deryagina Irkutsk Institute of Chemistry, Siberian Branch of the Russian Academy of Sciences, ul.Favorskogo 1, 664033 Irkutsk, Russian Federation. Fax (7-395) 239 60 46. Tel. (7-395) 246 24 00 (M G Voronkov), (7-395) 251 14 34. E-mail: vlad@iroch.irk.ru (E N Deryagina) Received 12 October 1999 Uspekhi Khimii 69 (1) 90 ± 104 (2000); translated by S V Chapyshev #2000 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2000v069n01ABEH000542 81 81 83 90 90 92 2. The development of thermal methods for the synthesis of organic and organosulfur compounds which are difficult to obtain alternatively. 3. Mechanistic studies of free-radical thermal reactions of sulfur-containing organic compounds occurring with involvement of free radicals. The development of methods for the generation of thiyl radicals, the evaluation of their reactivities and possible utilisation in high-temperature synthesis.4. The generation of labile sulfur-containing intermediates, e.g., thioformaldehyde, ethylenethiol, thioketones, sulfines, sul- fenes, sulfenic acids, dialkylsilanethiols, etc., using the flash pyrolysis technique and the development of methods for the detection of such intermediates. The present review is devoted to the thermal transformations of organic compounds of divalent sulfur, except for thiones, and to the latest achievements in the field of high-temperature syn- thesis involving these compounds. Earlier, only a few papers 1, 2 with a brief survey of the literature on these issues have been published. II. Thiols 1. Alkanethiols Pyrolysis of alkanethiols in the gas phase at temperatures above 425 8C yields hydrogen sulfide and alkenes as the major prod- ucts.3, 4 The degree of decomposition of alkanethiols increases with an increase in the reaction time and temperature.5 Aliphatic hydrocarbons do not react with alkanethiols or products of their decomposition.Benzene accelerates thermolysis of alkanethiols, whereas carbon dioxide or hydrogen inhibit this process. Higher homologues of alkanethiols are somewhat less stable than the lower ones. Linear alkanethioles are more stable than their branched isomers,6 especially at higher temperatures (475 8C). Hydrogen sulfide, hydrogen and methane are the pyrolysis products of methanethiol.5 Dialkyl sulfides, thiophene, hydrogen sulfide and elementary sulfur have been found among the thermolysis products of other alkanethiols.4 The decomposition is catalysed by metal sulfides and inhibited by cyclohexene.7 Since in both cases the ratio of the surface to the volume of a reactor does not affect the reaction rate,8 a free-radical chain mechanism for the thermolysis of alkanethiols has been suggested.6, 9 (1) CnH2n+1SH CnH2n+1 + SH,82 (2) H2S + CnH2nSH , CnH2n+1SH + SH (3) CnH2nSH CnH2n +SH.The main process is accompanied by parallel reactions (4) and (5). (4) CnH2nSH+CnH2n+2 , CnH2n+1SH+CnH2n+1 (5) CnH2n+SH. CnH2nSH The thermal decomposition of n- and sec-butanethiols in a flow system follows a molecular mechanism involving the synchronous abstraction of hydrogen from the a-carbon atom via a cyclic transition state.10 (6) H2S+MeCH2CH CH2 .MeCH2CH2CH SH H The decomposition of 1,1-dimethylethanethiol under the same conditions follows a free-radical mechanism.9 (7) Me3C+SH , Me3CSH (8) Me3CSH+Me3C Me2(CH2)CSH+Me3CH , (9) Me2C CH2+SH. Me2(CH2)CSH The activation energy for reaction (8) was found to be 7 kcal mol71. The pyrolysis of isomeric butanethiols in a static system occurs as a homolytic process in accordance with Eqns (1) ± (8).8 The thermal decomposition of cyclohexanethiol at 420 ± 470 8C and under reduced pressure leads to the formation of hydrogen sulfide and cyclohexene.11 Hydrogen sulfide accel- erates this process, while cyclohexene retards it. In the presence of sulfur dioxide or over activated carbon at 425 8C, cyclohexane- thiol is converted into benzenethiol.12 a-Toluenethiol decomposes more readily than the lower alkanethiols yielding hydrogen sulfide and 1,2-diphenylethane.4±6 Ethanedithiol is converted into thiir- ane and hydrogen sulfide on heating in vacuo.13 Most of the data available suggest a free-radical mechanism of alkanethiol decomposition, the initial step of which involves the cleavage of the C7S bond and generation of an .SH radical.The activation energy of this reaction presumably equals the energy of the C7S bond dissociation. Owing to the generation of thiyl radicals, high-temperature reactions of alkanethiols with aryl halides can take place yielding the corresponding arenethiols.14 2. Arenethiols Decomposition of benzenethiol begins at about 200 8C.15 At 530 ± 600 8C, benzenethiol, its derivatives and naphthalenethiol are converted into the corresponding diaryl sulfides.16 These reactions are catalysed by copper sulfide, cadmium sulfide and activated carbon; in the presence of these catalysts conversions occur at lower temperatures (300 ± 500 8C).17, 18 The cis ± trans- isomerisation of the CArS7H bond takes place on flash vacuum pyrolysis of benzenethiols at 70 ± 600 8C.19 Under reduced pres- sure of ozone, benzenethiol decomposes at 30 ± 125 8C to form oxygen, carbon dioxide, sulfur dioxide and polymeric products depositing on the reactor walls.20 Benzenethiol reacts with acetylene at 550 ± 590 8C to give benzo[b]thiophene. A similar reaction with propargyl alcohol at 450 ± 630 8C yields benzo[b]thiophene (1) and 3-methylben- zo[b]thiophene.It is supposed that pyrolysis of benzenethiol involves the generation of thiyl radicals, which add to the triple bonds of the substrates.21 PhS+H PhSH PhS+HC CH 7H S S 1 M G Voronkov, E N Deryagina CH2OH PhS+HC CCH2OH 7H S CH2OH CH2OH PhS SPh S S Me 7PhSOH H S 1 7PhSOMe Gas-phase reactions of benzenethiol with chlorobenzene and its para-substituted derivatives 2a ± d at 500 ± 600 8C lead to symmetrical (3a) and unsymmetrical diaryl sulfides (3b ± d).22, 23 p-XC6H4SPh+HCl 3a ± d p-XC6H4Cl+PhSH 2a ± d X=H(a), Me (b), OMe (c), Cl (d). Analogous diaryl sulfides are formed in the gas-phase reac- tions of para-substituted arenethiols with chlorobenzene at 500 ± 620 8C.p-XC6H4SPh+HCl . p-XC6H4SH+PhCl The Arrhenius rate constants for these reactions have been determined.23, 24 nÄ k /litre mol71 s71 HMe OMe Cl 3.8961010 exp (7149.90/RT) 3.486109 exp (7140.20/RT) 4.206108 exp (7116.57/RT) 7.7661013 exp (7206.1/RT) Benzenethiol reacts in the gas phase at 400 ± 530 8C with 1,2- dichloroethene (a 3 : 1 mixture of E- and Z-isomers) to give b-chlorovinyl phenyl sulfide (a 1 : 1 mixture of E- and Z-isomers) in 84% yield.25 The reaction of benzenethiol with b-bromostyrene in the liquid phase at 140 ± 160 8C leads to phenyl b-styryl sulfide.26 At 125 ± 215 8C, benzenethiol readily reacts with 2- and 4-bromophenols, 1-bromo-2-naphthol, 1-bromonaphthalene and 1-bromoanthracene to form the corresponding aryl phenyl sul- fides.27 o-Chlorobenzenethiol and o-chloromethylbenzenethiol elimi- nate hydrogen chloride on flash vacuum pyrolysis.19 At 700 8C in vacuum, hydroxymethylbenzenethiol is converted into benzo- thiete (4).28 CH2OH 7H2O S SH 4 (68%) When a mixture of toluenethiol and 35S-labelled ditolyl disulfide is heated, isotope exchange between thiyl radicals occurs.This process is accelerated by UV irradiation (Scheme 1). The reaction rate does not depend on the thiol concentration and is determined by the proneness of a disulfide to generate thiyl radicals as a result of S7S bond cleavage.29 Scheme 1 2 RS*, RS* *SR RS*H+SR, RS*+HSR RS*+SR RS* SR.Thermal transformations of organic compounds of divalent sulfur 7-Aminobenzo[b]thiophene and benzothiazole are the major products in the gas-phase reaction of 2-aminobenzenethiol with acetylene at 450 ± 600 8C.The maximum yields of these products (39% and 41%, respectively) are achieved at 600 8C.30 NH2 NH2 HC CH 7H S SH S 7H NH2 NH2 NH SCH CH 7Me SCH CH2 N S Pyrolysis of sodium 2-bromobenzenethiolate or its derivatives results in thianthrene or a 1 : 1 mixture of the corresponding isomeric thianthrenes 5 and 6, thus suggesting the intermediate generation of short-lived thiirene 7.31, 32 SNa S X X Br 7 S S X+X X X S 6 S 5 X=H, Me. Flash vacuum pyrolysis of 2-mercaptobenzoic acid at 610 8C gives rise to benzothiet-2-one (8) and 1,4-dihydrofulvenethione (9).33 O CO2H C S + S 7H2O SH 9 8 Similarly, 3-mercapto-2-naphthoic acid is converted into naphtho[2,3-b]thiet-2-one (10) on flash vacuum pyrolysis at 840 8C.34 O CO2H S 7H2O SH 10 Naphthalene-1-thiol reacts with acetylene at 410 ± 700 8C to form naphtho[1,2-b]thiophene (11) and 1-thiaphenalene (12) in 47% and 60% maximum yields, respectively.35 SH S HC CH 7H S SCH CH 11 S 12 83 3.Hetarenethiols The gas-phase reaction of 2-thiophenethiol (13) with acetylene at 500 ± 600 8C affords thieno[2,3-b]thiophene (14) together with bis(2-thienyl) sulfide (15), thiophene and benzo[b]thiophene.36 The complex character of transformations of 2-thiophenethiol is caused by the initial cleavage of the S7Hand C7S bonds and the generation of radicals 16 and 17, which either react with acetylene or recombine.HC CH S S S S 16 14 S17 16 SH S 13 S S S 15 HC CH S 1 17 7HS H + S S S 4,4,5,5,6,6-Hexafluorocyclopenta[b]thiophene (18) is the main co-pyrolysis product of 2-thiophenethiol with tetrafluoroethylene at 620 ± 640 8C; 4,4,5,5,6,6,7,7-octafluorocyclohexa[b]thiophene (19) is also formed.37 F2C CF2 F F + 620 ± 640 8C SH S S S 19 18 The reaction mechanism involves the generation of difluoro- carbene from tetrafluoroethylene, the replacement of the thiol group in 2-thiophenethiol by this carbene and the subsequent addition of the C-centered radical thus formed to tetrafluoro- ethylene. The product 19 is formed due to the elimination of the SH group from the parent thiol and the reaction of thienyl radical with tetrafluoroethylene. Bromothiophenethiols are unstable and decompose on distil- lation with elimination of hydrogen bromide and hydrogen sulfide.38 III.Sulfides Thermal decomposition of organic sulfides usually involves cleavage of the C7S bonds. The strength of this bond is different in different compounds. In dialkyl sulfides, either only the 3p- orbitals 39 or the hybrid sp- and spd-orbitals 40, 41 are involved in the formation of the C7S s-bond. The sulfur atom in sulfides can exhibit both electron-donating and electron-withdrawing proper- ties. The latter is true when electron-withdrawing substituents are present in the a-positions to the sulfur atom.In the absence of such substituents the electron-withdrawing properties of the sulfur atom are not so pronounced, since its 3d-orbitals in the ground state are diffuse and the energy of their occupation by an electron cannot be compensated by the energy of covalent bond formation. The presence of electron-withdrawing substituents in sulfides favours the localisation of a positive charge on the sulfur atom, thus weakening the C7S bond. The lone electron pairs of the sulfur atoms in sulfides can conjugate with the p-system of a multiple bond or an aromatic ring (contribution of the structures with the 2pp ± 3pp bond). Owing to this effect, vinyl and aryl sulfides are rather weak electron donors and are characterised by high strengths of their C7S bonds.84 1.Dialkyl sulfides and their derivatives Studies on the pyrolysis of dialkyl sulfides were begun in 1928 with the aim of elucidation of the course of transformations of sulfur- containing components of raw oil and oil products upon refin- ing.42 It was found that diethyl sulfide decomposes at 500 8C to form considerable amounts of hydrogen sulfide, gaseous hydro- carbons (38 mass%) and ethanethiol (10 mass %).42, 43 Diisopen- tyl sulfide is much more stable. The products of its pyrolysis at 500 8C contain mostly the recovered diisopentyl sulfide, 3-meth- ylbutane-1-thiol and thiophene derivatives. Among the gaseous products, alkanes (64%), alkenes (30%), hydrogen sulfide and hydrogen (0.5%) have been found.Dibutyl sulfide decomposes at 500 8C rather rapidly to form hydrogen sulfide and thiols; the thermolysis is accelerated in the presence of benzene and decel- erated in the presence of n-heptane. In a vacuum, dibutyl sulfide decomposes at 380 ± 410 8C yielding ethylene and polysulfides.44 Isobutylene, hydrogen sulfide, isobutane, 1,1-dimethyl- ethanethiol and sulfur are the pyrolysis products of di(tert-butyl) sulfide in a static system at 25 ± 250 Torr and 360 ± 413 8C. In the presence of cyclohexane, the yield of the thiol is sharply increased, while the yield of hydrogen sulfide is diminished several-fold and sulfur is not formed at all. In a flow system at 446 ± 466 8C, small amounts of ethane, ethylene, propane, propylene and butene are also formed.45 Thermal stabilities of most dialkyl sulfides both in pure form and in solutions of Surakhansk kerosene (fraction with b.p. 180 ± 200 8C) have been determined.46 The decomposition of dialkyl sulfides in the gas phase at 400 ± 800 8C to yield hydrogen sulfide and the corresponding alkenes follows a free-radical mechanism involving the intermedi- ate formation of alkanethiols (Scheme 2).42, 47 ± 49 Scheme 2 R2(Me)CS+R2(Me)C , R2(Me)CSC(Me)R2 HR R2C CH2, R2(Me)CSH R2(Me)CS 7H2S 7R R2(Me)CC(Me)R2. 2R2(Me)C If the pyrolysis of alkanethiols is carried out in vacuum (0.1 Torr), the yield of hydrogen sulfide is quantitative.Both unsymmetrical dialkyl sulfides and those with branched aliphatic chains are less thermally stable than symmetrical di(n-alkyl) sulfides.Dialkyl sulfides bearing methyl groups in the b-position are converted into the corresponding alkenes and hydrogen sulfide on flash vacuum pyrolysis at about 1000 K. 1270K H2C S+H2S+CH4+CS2 , Me2S EtSMe 1100K H2C CH2+H2C S+H2S+CH4+CS2. All derivatives with an a-methyl group and thiirane are more thermally stable and decompose at higher temperatures.50 Thiirane is the most stable sulfide despite the strain in its molecule. It forms ethylene, acetylene and hydrogen sulfide at 1270 K.501270K H2C CH2+CS2+HC CH . S Anew reaction pathway in the thermal decomposition of di(n- alkyl) sulfide leading to ethylenethiol and its further conversion into thiophene was found later.51, 52 Most likely, ethylenethiol is generated from dimethyl sulfide and other dialkyl sulfides according to a scheme common for all dialkyl sulfides through the initial formation of thiirane and its subsequent isomerisation.H2C CHS , RCH2SCH2R H2C CHSH 7H 72R S M G Voronkov, E N Deryagina 2H2C CHS 7H2S S Thiophene is formed from butyl sulfide mostly as the result of an intramolecular cyclisation of the butenethiyl radical 20. Bu+SBu Bu2S CHS MeCH2CH2CH2S 7H2 7H2 MeCH2CH 20 7H2 S S The gas-phase reactions of dialkyl sulfides with chlorobenzene or 2-chlorothiophene lead to benzo[b]thiophene (1) and thieno- thiophenes, respectively.52 ± 55PhSCH CH2 H2C CHS+PhCl H 7HCl 7H2 S 1 S H + +H2C CHS Cl S S S S 7HCl, 7H2 The reaction of dimethyl sulfide with vinyl chloride in the gas phase at 430 ± 450 8C is accompanied by the formation of 1,1- bis(methylthio)ethane in 30% yield.56 H2C CHCl +Me2S H2C CHSMe +MeCl, CH2.H2C C(SMe)2+H2C H2C CHSMe+Me2S Thermolysis of aliphatic sulfides in non-polar solvents (heptane, nonane, octane, decalin) under pressure occurs at 140 ± 250 8C. In these reactions, sulfides and thiols can exchange their RS groups in accordance with the Scheme 1. Highest sulfides and lowest thiols have higher exchange rates.48, 57 Decomposition of aliphatic sulfides at 300 ± 400 8C in melts of ZnCl27HCl, ZnCl27LiCl, ZnCl27SnCl2, SnCl27KCl, ZnCl27KCl7NaCl, CuCl7KCl7NaCl eutectics leads to a mixture of alkanes, C2±C4 alkenes and hydrogen.Complexes of the (Pri2S)3(ZnCl2)2 type are formed as intermediate products. In zinc-containing eutectics, sulfides decompose faster to form predominantly alkenes the structures of which correspond to those of the alkyl chains in the starting sulfides. Sulfur is liberated as ZnS, which is then dissolved in the melt.58 Thermal decomposition of benzyl methyl sulfide and substi- tuted 1,2-bis(benzylthio)ethanes proceeds according to the Scheme 2.5, 59 Polyethylene sulfide decomposes at 230 8C to form hydrogen sulfide, ethane and a small amount of ethanethiol. The activation energy for this reaction corresponds to the energy of a homolytic cleavage of the C7S bond in macromolecular chains of poly- ethylene sulfide.60 Thermolysis of bis(2-ethoxyethyl) sulfide, 1,1-bis(ethyl- thio)ethane and 1,3-bis(propylthio)propane involves the initial cleavage of the C7S bonds and occurs in accordance with reactions (1) ± (4).The dissociation energies of the C7S bonds in these compounds have been calculated.61 Heating of dialkyl dithioacetals to 130 8C results in the elimination of a thiol and the formation of the corresponding vinyl sulfide.62 ± 64 SR1 C C CR2+HSR1 C C CHR2 SR1 SR1 R1=Alk, Ar; R2=Alk.Thermal transformations of organic compounds of divalent sulfur The main process is accompanied by the 1,2-migration of the thiyl group and the formation of the saturated sulfide 21 accord- ing the following scheme.65 ± 68 ButCHCH2SR2 21 SR1 But(Me)C(SR1)SR2 ButCHMe+ButC CH2+R2SH SR1 SR1 Hexakis(alkylthio)- and tetrakis(methylthio)bis(trimethyl- silyl)ethanes undergo thermolysis at 110 8C to form the corre- sponding ethylene derivatives 22 as the result of the alkylthiyl radical elimination.69 (RS)3CC(SR)3 C(SR)2+RS , (RS)3CC(SR)2 (RS)2C 7RS 22 2RS RSSR , Me3SiC(SMe)2C(SMe)2SiMe3 7MeS Me3SiC(SMe)2C(SMe)SiMe3 MeS 7MeSSMe Me3Si(MeS)C C(SMe)SiMe3 .Vacuum pyrolysis of tetrakis(trifluoromethylthio)methane at 400 8C and 1 ± 1.5 Torr leads mainly to bis(trifluoromethyl) trithiocarbonate together with small amounts of bis(trifluoro- methyl) sulfide, bis(trifluoromethyl) disulfide and tetrakis(tri- fluoromethylthio)ethylene. The latter is formed as the major product on pyrolysis of tetrakis(trifluoromethylthio)methane at 400 8C under atmospheric pressure. The reaction mechanism involves the formation of free radicals and carbenes.70 a (CF3S)2C S +CF3SCF3 (CF3S)4C b (CF3S)2C C(SCF3)2+CF3SSCF3 (a) 400 8C, 1 Torr; (b) 400 8C, 760 Torr.Hexakis(trifluoromethylthio)ethane undergoes the C7C bond dissociation producing tris(trifluoromethylthio)methyl rad- icals even at 30 8C.71 2-Chloroethyl methyl sulfide eliminates hydrogen chloride at 360 ± 400 8C.72 The rate of its decomposition at 400 8C is 234-fold higher than that for ethyl chloride or ethylene chlorohydrin.72 MeSCH CH2+HCl . MeSCH2CH2Cl 2-Alkylthioethyl acetates eliminate acetic acid at 460 ± 465 8C to form the corresponding alkyl vinyl sulfides.73 MeCO2H+H2C CHSR MeCO2CH2CH2SR R=Me, Bun, Bui, Bn.2-Alkylthio-1-methylethyl acetates are converted into a mix- ture of 1- and 3-(alkylthio)propenes at 500 8C. 3-Isomers are predominant at 400 8C and short contact times.74 MeCO2CH(Me)CH2SR MeCO2H+H2C CHCH2SR+MeCH CHSR . Gas-phase thermolysis of alkyl and cycloalkyl allyl sulfides at 400 ± 500 8C is accompanied by a molecular rearrangement involving either four- or six-membered cyclic transition states and leading to the corresponding alkenes and thioaldehyde.75 Upon flash pyrolysis at 600 ± 650 8C, allyl sulfides 23 elimi- nate propene and are converted into the corresponding thiocar- bonyl compounds.76, 77 S R1R2C CHMe . R1R2C S+H2C H 23 Thioacrolein and thiobenzaldehyde have been found in the pyrolysis products of diallyl and allyl benzyl sulfides.76, 78 1,3- Bis(allylthio)propane decomposes at 520 8C to form unstable compounds which are rapidly converted into a polymer built of 7S(CH2)3S7 units.79 The extent of isomerisation of a cis- rotamer of methyl vinyl sulfide into the gauche-rotamer at 200 ± 600 8C is 20%.80 2.Vinyl and ethynyl sulfides Thermolysis of tert-butyl 1-propenyl sulfide at 340 ± 390 8C proceeds tenfold faster than that of di(tert-butyl) sulfide and is not inhibited by cyclohexene. Isobutene, isobutane and propenyl- thiyl trimer are the reaction products.45 Heating of allyl 2,2-dichlorovinyl sulfide (24) at 160 ± 180 8C and of allyl 1,2,2-trichlorovinyl sulfide (25) at 100 ± 120 8C results in unusual rearrangements leading to 1,2-dichloropenta-1,4-diene and a mixture of 2,3-dichloro-5-chloromethyl-4,5-dihydrothio- phene and 3,5,6-trichloro-3,4-dihydropyran-2H-thione, respec- tively.The mechanism of the reactions involves a [3,3]- sigmatropic rearrangement and the formation of thiirane inter- mediates.81 S Cl Cl 24 S Cl Cl S Cl Cl Cl 25 Cl Cl ClS Gas-phase pyrolysis of divinyl sulfide at 450 ± 560 8Cgives rise to thiophene in 40% yield.82 At 440 ± 460 8C, 2-chlorovinyl sulfide is converted into 2- and 3-chlorothiophene predominantly through the involvement of the trans-isomer in this heterocycli- sation.83 XCH CHSCH X=H, Cl. Thermolysis of di(1-propenyl) sulfide (26) in the gas phase at 460 ± 540 C leads to propenethiol, thiophene, methylthiophenes and 2-ethylthiophene.Heterocycles are formed as the result of the intramolecular cyclisation of the sulfide, involving the intermedi- ate formation of radicals 27 and 28.84 MeCH CHSCH 26 CH2CH CHSCH CHMe+MeCH CHSH +CH2 27 27 S 28 Upon pyrolysis (580 8C) under an atmosphere of nitrogen, alkyl b-styryl sulfides undergo intramolecular heterocyclisation to 85 Cl7 +SS Cl Cl Cl Cl Cl 7SCl S Cl7 +S Cl Cl Cl Cl Cl Cl Cl + Cl S ClH2C Cl S X + CHX 7HX S X S 26 MeCH CHS+CH CHMe CHMe CHMe, 7Et S 7H Et S CHMe Me 7Me +Me S S86 give the corresponding benzo[b]thiophene derivatives in 50%± 70% yields.85 R2 R2 7R1 R1S R3 R3 S R2 R2 7H R3 R3 S S R1, R2, R3=Alk, Ph. 2-Alkoxy(acyloxy)-3-alkyl(aryl)thiocyclobutadienes are con- verted into the corresponding dienes on heating to 350 8C as the result of the cleavage of the C(1) ± C(4) bond in the rings.86 H2C C(SR2)C(OR1) CH2 SR2 R1O R1, R2=Me, Ph, CO2Me, 4-ClC6H4, 4-MeOC6H4, COMe.Pyrolysis of 3,4-bis(methylthio)cyclobutene-1,2-dione at 450 8C leads to the ring opening with the formation of bis(methylthio)acetylene and CO. At 550 8C, hydrogen sulfide and methanethiol are also formed in this reaction.87 O MeS 72CO MeSC CSMe . O MeS Allyl and propargyl 1,1-dicyano-2-ethoxyvinyl sulfides undergo the Claisen rearrangement on distillation being con- verted into the corresponding O-ethyl thiocarboxylates 29.88 EtOC(S)C(CN)2R (CN)2C C(OEt)SR 29 R=CH2CH=CH2, CH2C:CH.On heating, ketene allyl thioacetals 30 are converted into a-allyl dithiocarboxylates.89 ± 91 XYC XYC(R2)C(S)SR1 C(SR1)SR2 30 X, Y=CO2Me, CO2Et, CN, Ac, Bz etc.; R1=All, Alk; R2=All. Ketene allenyl thioacetals undergo the Claisen rearrangement on heating to 80 ± 100 8C to form ethyl acetylenedithio- carboxylates, while ketene ethynyl thioacetals are converted at 100 ± 140 8C into dithiocarboxylates containing the allene frag- ment.92, 93 In some cases, the latter undergo partial or complete heterocyclisation on distillation to form 2H-thiopyrans or sub- stituted thiophenes. C(R2)CH(R1)CSEt C R1CH C(SEt)SCH2C CR2 CH2 S R1 R2 R1=Ph, R2=Me H+, D Me SEt SR2 R1 R1=R2=H; R1=Ph, R2=H Et3N, D SEt S Vacuum pyrolysis of tert-butynyl ethynyl sulfide at 500 8C results in the formation of thioketene (31), which is more stable than its tautomers, viz., thiirane and ethylthiol.94 CSBut [HC CS+But] HC [HC CSH CMe2 .S]+H2C H2C C 31 M G Voronkov, E N Deryagina On heating to 250 8C, dibenzyl sulfide is converted into stilbene with liberation of hydrogen sulfide. The latter partly reduces stilbene to toluene. This produces sulfur which reacts with stilbene to give tetraphenylthiophene.95 3. Aryl and hetaryl sulfides Pyrolysis of a mixture of methyl phenyl sulfide and toluene vapours at 600 8C leads to methane, benzenethiol and dibenzyl. This free-radical reaction was proved to follow a monomolecular mechanism.96 PhS+Me , PhSMe PhS+PhMe PhSH+PhCH2, Me+PhMe CH4+PhCH2, PhCH2CH2Ph.2 PhCH2 The kinetics of the thermal decomposition of methyl phenyl sulfide and benzyl methyl sulfide at low pressure was studied, and the dissociation energies of the PhS7Me and PhCH27SMe bonds were found to be 67.5 and 59.4 kcal mol71, respectively.97 The enthalpies of the formation of radicals MeS. and PhS. are 34.2 and 56.8 kcal mol71, respectively.97 Heating of benzyl phenyl sulfide to 250 ± 270 8C leads to a mixture of liquid and gaseous products, viz., toluene, 1,2-diphe- nylethane, biphenyl, diphenylmethane, stilbene, benzenethiol, tetraphenylthiophene, benzaldehyde, hydrogen sulfide and car- bon dioxide. The yield of benzenethiol reaches 31%.98 Flash pyrolysis of benzyl phenyl sulfide in vacuum at 800 8C results in the formation of 1,2-diphenylethane and diphenyl disulfide, which are the recombination products of benzyl and phenthiyl radicals, respectively.99 1,2-Bis(phenylthio)ethane and its derivatives decompose at 350 8C to form benzenethiol, diphenyl disulfide and the corre- sponding alkene.The reaction occurs stereospecifically as a trans- elimination. Radicals 32a and 32b are converted into the corre- sponding butenes much more rapidly than they isomerise.100 Me PhS SPh PhS Me C C C C Me H Me 7PhS Me 7PhS H H H Me 32a SPh PhS H PhS 32a C C C C Me H 7PhS H Me Me H Me 32b7PhS Me Me Decomposition of hexakis(phenylthio)ethane begins at 100 8C to generate tris(phenylthio)methyl radicals detectable by EPR spectroscopy.101 ± 104 The cleavage of the C7C bond in hexakis(phenylthio)ethane is mostly caused by steric factors (repulsion of the ArS substituents).The disproportionation of the (ArS)3C. radicals is responsible for the formation of the major reaction products.101100 8C (ArS)3C (ArS)3CC(SAr)3 RH (ArS)3CH ArS (ArS)4C (ArS)2C (ArS)2C C(SAr)2+ArSC(S)Ar 7ArS On heating to 200 8C, allyl aryl sulfides undergo the thio- Claisen rearrangement.105Thermal transformations of organic compounds of divalent sulfur CH2CH CH2 SH Me Me SCH2CH CH2 Allyl 2-benzofuryl and allyl 2-benzothienyl sulfides behave similarly on heating. In the latter case, the rearrangement product undergoes further heterocyclisation.106, 107 The thioallyl rear- rangement usually precedes the thio-Claisen rearrangement in allyl aryl sulfides.Mechanisms involving the intermediacy of free radicals or a four-membered transition state have been considered for these rearrangements.108 PhS hn or D PhS SPh PhS PhS +PhS SPh +PhS a PhS SPh b+ Ph 7S g On flash vacuum pyrolysis, allyl phenyl sulfide undergoes initial cleavage of its C7SPh bond resulting in the formation of phenthiyl and allyl radicals and their subsequent dimerisation.50 PhS+CH2CH CH2 CH2 PhSCH2CH CHCH2)2 (CH2 (PhS)2 The gas-phase reactions of alkyl phenyl and allyl phenyl sulfides with acetylene at 480 ± 600 8C occur selectively yielding benzo[b]thiophene (1) as the addition product of phenthiyl radicals to the triple bond of acetylene.109 PhS+R PhSR CH PhS+HC CH CH 7H S S 1 R=Me, Et, Pri, All.Benzothiophene (1) was obtained in the highest yield starting from allyl phenyl sulfide, and this reaction occurs at lower temperature. At 600 8C, vinyl phenyl sulfide undergoes an intra- molecular cyclisation to form benzothiophene.110 Pyrolysis of bis(phenylthio)acetylene at 560 8C leads to diphenyl sulfide, benzenethiol, thianthrene (33) and benzothio- phene (1). The reaction involves the generation and subsequent transformations of phenyl and phenylthioethynylthiyl radicals.111 In the presence of chlorobenzene, diphenyl sulfide is formed upon its reaction with phenthiyl radicals.RH 1 CSPh .C 7PhS. 7R PhSC CSPh RH CSPh SC 1 7R,7S 7Ph. S 7H2 PhS S33 RH PhSH 7R Diaryl sulfides differ from other diorganyl sulfides in that they possess rather strongC7S bonds. These sulfides are usually stable at 400 ± 600 8C, while at higher temperature they undergo intra- molecular cyclisations. Thus at 700 ± 750 8C, diphenyl sulfide, phenyl o-tolyl sulfide and 2-hydroxyphenyl phenyl sulfide are 87 converted into dibenzothiophene, thioxanthene and 4-hydroxy- dibenzothiophene, respectively.112 Dithienyl sulfide is less stable: it decomposes at 500 ± 600 8C to give thiophene, isomeric thio- phenethiols and bithienyls and also isomerises into 2,3-dithienyl sulfide. These conversions result from the addition of thiyl radicals to the double bonds of thiophene and to the sulfur atom of sulfides.113 4.Sulfenyl halides and sulfonium salts Heating of alkanesulfenyl chlorides to 100 8C affords alkyl chlorides in low yield.114 Flash pyrolysis of methanesulfenyl chloride at 590 8C leads to thioformaldehyde.115 Benzenesulfenyl chloride decomposes at 100 8C to form chlorobenzene, diphenyl disulfide and sulfur monochloride. At 90 8C, 4-methyl- and 4-methoxy-benzenesulfenyl chlorides eliminate sulfur to yield the corresponding para-substituted chlorobenzenes.114 Phenyldiarylsulfonium halides undergo complete thermolysis at 250 8C yielding a mixture of aryl halides and diaryl sulfides containing all possible combinations of the aryl substituents in their molecules.A pair 2,5-Me2C6H3X and 4-MeC6H4SPh is formed in the highest yield upon pyrolysis of (2,5-dimethyl- phenyl)(phenyl)(p-tolyl)sulfonium chloride.116 5. Cyclic sulfides Thiirane (ethylene sulfide) is the simplest three-membered cyclic sulfide with a thermodynamically unfavourable three-centre dipolar atomic system which is, however, stabilised by the efficient involvement of the non-bonding orbitals of the heter- oatom into the frontal interactions.117 A characteristic feature of thiirane and its derivatives is their capacity to split the sulfur atom off yielding the corresponding alkene. The conditions required for this reaction, which can sometimes occur at temperatures below 0 8C, depend on the nature of substituents in the thiirane ring.As usual, the substituents exerting the 7E-effect and many p-donating systems facilitate the desulfuration. Thiirane itself decomposes completely in the gas phase at 250 8C into ethylene and elementary sulfur. This is a first-order homolytic reaction the activation energy of which is 40.2 kcal mol71. Since this value is lower than the enthalpies of the direct elimination of the sulfur atom from the thiirane ring or of the cleavage of this ring, both these processes can be ruled out from mechanistic considerations.Areaction mechanism involving the intermediacy of the excited thiirane has been suggested.118, 119 D S S S S + S2+2C2H4 . This mechanism is supported by the complete stereospecificity of the low-temperature thermolysis of cis- and trans-2,3-dime- thylthiiranes.118 Gas-phase pyrolysis of thiirane at 500 ± 600 8C results in the formation of hydrogen sulfide, ethylenethiol, thiophene, benzo- thiophene and hydrocarbons C1±C4 (Ref.120). Alkyl-, cyclo- alkyl-, 2-aryl- and 2,3-diaryl-thiiranes eliminate sulfur on distillation or moderate heating.121 ± 124 Only tetraarylthiiranes decompose at higher temperature (180 ± 215 8C) to form tetraaryl- ethylenes.125, 126 CAr2 Ar2C Ar2C CAr2 7S S Ar=Ph, 4-MeOC6H4, 4-BrC6H4, 4-Me2NC6H4 etc. 3,3-Dichloro-2,2-diphenylthiirane undergoes a similar reac- tion.127 Thermal transformations of 3-bromo- and 3,3-dibromo-2,2- diphenylthiiranes are accompanied by intramolecular cyclisations of the initially generated thiyl radicals without elimination of sulfur.12788 Br Ph X C BrXC C CPh2 7Br S Ph S Ph X Ph C C 7H S Ph X S X=H, Br.3-Chloro-2,2-diphenyl-3-X-thiiranes also cyclise into the cor- responding benzothiophenes on heating.128, 129 Tetrafluorothiirane is distinguished by its high thermal stability. Its partial conversion into difluorothioformaldehyde, tetrafluoroethylene and a number of other products takes place only at 430 8C.130 On thermolysis at 340 8C, cis- and trans-divinylthiiranes undergo the Cope rearrangement into 4,5-dihydrothiepane (34).131 S S34 +PhH+ 34+ S S + + Dihydrothiophene, thiophene and cis- and trans-1-phenyl- butadienes are the thermolysis products of cis- and trans-2-vinyl- 3-phenylthiiranes at 360 ± 400 8C.131 When the concentration of thiirane is relatively high, a bimolecular mechanism predo- minates.132 With a decrease in the concentration of thiirane in the reaction, monomolecular desulfurisation becomes the main process.Pyrolysis of cis- (35a) and trans-2,3-diethynylthiiranes (35b) in a solution (toluene-d8, 100 8C) yields only sulfur-free alkenes. If the pyrolysis of cis-isomer 35a is carried out in a flow system, thienocyclobutadiene is formed in 6% yield along with alkenes. This diene dimerises on standing in benzene solution. C CH H S C CH H H C CH C CH H35a + 7S H C CH C CH H HC C H S HC C H 35b S S S Thus, the data presented here show that in the thermal decomposition of thiirane and its derivatives at temperatures up to 200 ± 350 8C the elimination of sulfur is the main process.The isomerisation of the thiirane ring into an ethylenethiol form is observed only for bromine-substituted diphenylthiiranes. Divinylthiiranes also undergo intramolecular rearrangements. The high-temperature (up to 1000 8C) thermolysis of thiiranes results mostly in the cleavage of the C7S bond in the thiirane ring and its partial isomerisation into ethylenethiol rather than in elimination of sulfur. Unstable biradicals and thiyl radicals thereby formed undergo further decomposition into hydrogen sulfide, hydrocarbons and thiophene.120 M G Voronkov, E N Deryagina Four-membered sulfur-containing heterocycles are rather unstable. Thietane (trimethylene sulfide) decomposes on heating to 95 8C.Its thermal destruction in vacuum (250 8C, 10 Torr) leads to an oily polymer and small amounts of hydrogen sulfide, ethylene, ethane, propene and propane.117, 131 Thioformaldehyde is the product of the flash pyrolysis of thietane at 1000 8C.133 Photolysis of thietane, 3-ethyl-2-propylthietane and 3-methyl- thietane in the gas phase, solution and glassy matrices was shown 134 to involve the cleavage of the C7S bonds and the formation of 1,4-biradicals. Derivatives of thiete (thiacyclo- butene) undergo polymerisation even at room temperature. They are relatively stable only in solution.135 Five-membered sulfur-containing heterocycles are more sta- ble. Thiolane is stable below 450 8C.The extent of its decom- position at 500 8C is 10% and that at 550 8C is 60%; the products formed include thiophene, hydrogen sulfide and hydrocarbons. 1-Thiaindan decomposes at 450 8C into hydrogen sulfide and thiols.136 Thermal decomposition of 2,5-dihydrothiophene at 600 8C leads to thiophene and hydrogen in the ratio of 1 : 1 along with hydrogen sulfide and polymeric products. Hydrogen is liberated in this reaction synchronously with other products.119, 137 Benzo[b]thiophen-2(3H)-one is converted into benzothiete on flash vacuum pyrolysis (700 8C, 0.05 Torr).138 O 7CO S S (98%) 2,3-Dihydrothiophene-2,3-diones and their analogues elimi- nate CO on flash vacuum pyrolysis to form the corresponding thietones.139 Thiophene is stable up to 800 8C, while at 850 8C it decom- poses to give a mixture of isomeric bithienyls, benzothiophene, phenylthiophene (7% ± 8% overall yield) and gaseous prod- ucts.140 In a flow-recycling system, the yields of isomeric bithien- yls, benzothiophene and phenylthiophene are 30%, 20% and 10%, respectively.Hydrogen sulfide accelerates the thermal transformation of thiophene into isomeric bithienyls, among which 2,30- and 3,30-isomers predominate (95% total yield).141 Hydrogen sulfide, thiophene and benzene are the main pyrolysis products of 2-methylthiophene at 825 8C. Aromatic compounds are formed in this reaction as the result of condensa- tions of intermediate radicals and carbenes.142 2,3-Dichlorothiophene undergoes the condensation into tet- rachloro-3,30-bithienyl at 560 ± 570 8C.143S Cl Cl Cl S Cl Cl S Cl Vacuum pyrolysis of 1-(2-thienyl)buta-1,3-diene at 525 ± 575 8C and 0.1 ± 1.0 Torr leads to 4,5-dihydro- benzo[b]thiophene.144 S S (70%) 2-Chlorophenyl(2-thienyl)dichlorosilane undergoes hetero- cyclisation at 480 ± 700 8C.145 Cl 7HCl S Si S Si Cl Cl Cl Cl Vacuum thermolysis of thiophene-2,3-dicarboxylic anhydride at 500 8C (1 ± 5 Torr) in a flow system leads to annelated thio- phenes, which can be considered as analogues of anthraquinone and fluorenone.146Thermal transformations of organic compounds of divalent sulfur O O S O + S S S S O O O Six-membered sulfur-containing heterocycles are less stable than thiophene and its derivatives.Under an argon atmosphere, dihydro-2H-thiopyrans undergo rearrangements into the corre- sponding thiophenes and dihydrothiophene on heating to 260 8C.147 R2 R2 R2 R2 R2 R2 + + R3 S R3 S R3 R1 R1 R12 CH S R4 R2 R2R3+R12 CHCHR12 +R12 CH2+ R4 S R12 CH Fused dihydro-2H-thiopyrans undergo a retro-Diels ± Alder reaction on flash vacuum pyrolysis.148 S R1 R2 R3 500 8C R3 R4 + 1072 Torr R1 R2 R4 S Tetrasubstituted 4-thiapyrones rearrange into the corre- sponding 2-thiapyrones on heating to 200 ± 250 8C.149 On flash vacuum pyrolysis, the polynuclear cyclic sulfide 36 eliminates the sulfur atom to form a cyclophane-like hydro- carbon.150 650 8C, 1072 Torr S 7S 36 Benzothiophene and dibenzothiophene are fairly stable in the gas phase.Both the aromatic and thiophene rings are hydro- genated in the reaction of benzothiophene with atomic hydrogen at 150 8C. At 560 8C, only the thiophene ring is hydrogenated providing vinyl phenyl sulfide.151, 152 Octachloro-2,3-dihydroben- zothiophene is converted into hexachlorobenzothiophene at 200 ± 250 8C.153 Dibenzothiophene does not decompose up to 900 8C. Thian- threne decomposes partially at 550 8C to form dibenzothiophene and benzene. Due to the high stability of these heterocycles, not all sulfur from cyclic sulfides is lost on pyrolysis of coals.154 The direction of the gas-phase thermolysis of dithiacycloal- kanes and alkenes depends on the nature of the heterocycle. The vacuum thermolysis of 1,3-dithianes 77 and -dithietanes 155, 156 at 300 ± 750 8C leads to their cleavage into starting monomers, viz., thioaldehydes and thioketenes.S RHC 2RCH S, S S C RCH S+H2C H2C CHR S CHR S R=H, Ph. S R1 R1 S C COS+ O R2 R2 S R1=CN: R2=CN, COMe, COOEt. 89 Bis(trifluoromethyl) thioketene is obtained on pyrolysis of its dimer 37 at 650 8C; it can be stored in a sealed tube for several months.157 Hydrogen sulfide, tetrakis(trifluoromethyl)allene and perfluoroisobutylene are the other products of this reaction. F3C S F3C CF3 650 8C S . C CF3 F3C F3C S 37 Thioketenes were characterised by photoelectron spectro- scopy (PES) and isolated in the form of dimers or adducts of 1,3- dipolar cycloaddition reactions.Dicyanothioketene is formed in the vacuum pyrolysis of 2-(4- oxo-1,3-dithietane-2-ylidene)malononitrile (470 8C, 0.05 Torr). However, this thioketene appeared to be extremely unstable and was characterised only by PES.158 NC NC S S . C COS+ O S NC NC Dithiaspirocycloalkanes with five- and six-membered rings undergo pyrolysis at 600 8C to give cyclopentanethione or cyclo- hexanethione.159 Ethylenedithioketal of quadricyclanone under- goes extensive fragmentation at 200 8C to form benzene and hydrogen sulfide.160 On flash pyrolysis, 1,3-dithiol-2-one (38) and its derivatives lose CO to give the corresponding dithietes.161 S S O 7CO S S38 Dithiete and ethylene are formed in the flash vacuum pyrolysis of 2,3-dihydrodithiine at 700 8C.161 S S 700 8C CH2 .+CH2 S S Flash pyrolysis of 1,2-diphenyl-5,6-dihydro-1,4-dithiine at 720 8C (0.1 Torr) involves the elimination of ethylene and leads to polymers which are converted into tetraphenyldithiine on heating in decalin.162 1,2-Dithietes or a-dithiones are presumably the intermediate products in these reactions. Ph Ph S S Ph S 190 8C S Ph Ph Ph S S Ph Ph S Ph Ph S The main direction in the flash vacuum thermolysis of 1,4- dithiines at 700 8C involves the extrusion of sulfur and the formation of thiophene.161 S S S Substituted 1,4-dithiines eliminate sulfur even on heating to only 200 8C.163 ± 165 1,3,5-Trithiane and its derivatives decompose to the starting monomers on pyrolysis. Thus thioformaldehyde is formed from trithiane at 600 8C in vacuum.166, 167 S 3H2C S .S S Pyrolysis of 2,4,6-trimethyl-1,3,5-trithiane at 600 8C in vac- uum leads to monomeric thioacetaldehyde, which has a lifetime of90 10 s;166, 167 at 450 ± 600 8C and normal pressure; ethylenethiol and thiophene are formed. Thiophene is obtained as the result of the initial isomerisation of thioacetaldehyde and subsequent intermo- lecular condensation of ethylenethiol.168 Me Me S MeCH S CHSH CH2 S S S 7H2S, 7H2 Me Vacuum pyrolysis of hexamethyltrithiane at 500 8C leads to thioacetone, while at 900 8C it yields thioketene, which results from the thermal decomposition of thioacetone.169 Under ordi- nary conditions, monomeric thioacetone is stable for only a few minutes.Me Me S Me Me Me2C S H2C C S+CH4 . S S Me Me The fact that thiobenzaldehyde trimer and high-molecular- mass thiobenzaldehyde polymer decompose at 150 8C producing sulfur and stilbene from which tetraphenylthiophene is formed at 250 8C was established back in the past century.170, 171 Ph Ph S CHPh PhCH [PhCH S]3 7S 7H2S Ph Ph S Heating of thioacetophenone trimer with rapid distillation of the thermolysis products yields monomeric thioacetophenone. The latter, as well as its trimer, is converted into 2,4-diphenyl- thiophene, stilbene and ethylbenzene on heating.172 IV. Hydrodisulfides Hydrodisulfides (RSSH) are unstable compounds and are often formed as intermediates in the thermolysis of organic disulfides.Thus the thermal decomposition of di(tert-butyl) disulfide appa- rently results in the initial formation of tert-butyl hydrodisulfide, which then rapidly decomposes into isobutene and H2S2.173 [ButSSH] ButSSBut CH2 . Me2C 7HSSH Diphenylmethyl dihydrodisulfide (39) decomposes at 120 8C to form diphenylmethane, bis(diphenylmethyl) tetrasulfide (40) and hydrogen sulfide. The reaction involves the initial generation of thiyl radicals 41, which further react with the starting hydro- disulfide according to a free-radical chain mechanism.174 Ph2CHSSH 39 Ph2CHS+HS , 41 39+41 Ph2CHSH +Ph2CHSS , 39+HS H2S+Ph2CHSS , 2Ph2CHSS Ph2CHSSSSCHPh2, 40 39 Ph2CH+SSH, Ph2CH+RSSH Ph2CH2+RSS . V. Disulfides Organic disulfides are less stable than the corresponding sulfides and therefore readily undergo diverse transformations and react with organic compounds under the liquid- and gas-phase ther- molysis conditions.M G Voronkov, E N Deryagina 1. Dialkyl disulfides 2S2).50 Decomposition of dialkyl disulfides begins at temperatures approximately 200 ± 250 8C lower than those required for the thermolysis of the corresponding sulfides.45 Temperatures at which dialkyl disulfides decompose completely were determined by PES and decrease with an increase in the length and the branching of an alkyl chain in disulfides (from 1100 K for Me2S2 to 690 K for But The corresponding thiols, alkyl sulfides, hydrogen sulfide, saturated and unsaturated hydrocarbons are the main products in the gas-phase pyrolysis of dialkyl disulfides at 500 8C.Also, a small amount of thiophene is formed.49 At higher temperatures (580 ± 610 8C), carbon disulfide is the major liquid product (23.5% yield) in the pyrolysis of dimethyl disulfide. From diethyl disulfide under the same conditions, carbon disulfide is formed in only 9.5% yield.175 Flash vacuum pyrolysis of dimethyl disulfide at 300 ± 700 8C leads to methanethiol and thioformaldehyde, while at 900 8C it mainly yields carbon disulfide.77, 176 The addition of methanethiol to the reaction does not affect its rate, whereas hydrogen sulfide substantially accelerates the pyrolysis of dimethyl disulfide.177 Thioformaldehyde and methanethiol were detected and characterised by PES. Both molecular and radical mechanisms are assumed to be responsible for their forma- tion.47, 178 ± 180 S+MeSH H2C MeSSMe 2 MeS H2S+C2H4+S The reactions of thiyl radicals with the starting disulfide lead to its complete decomposition.Vacuum pyrolysis of diethyl disulfide occurs in a similar way.178 Flash pyrolysis of diisopropyl disulfide presumably occurs through the stage of formation of isopropyl hydrodisulfide (42), since more stable propane-2-thiol was not found among the pyrolysis products.50 MeCH PriSSPri CH2 , 7HSSH [PriSSH] 42 H2S + S . HSSH A liquid product obtained upon gas-phase pyrolysis of di(n-butyl) disulfide at 500 ± 550 8C represents pure thiophene (40% ±60% yield). Gaseous hydrocarbons C1±C4 and hydrogen sulfide are the other products.181 The reaction mechanism involves the thermal generation of butylthiyl radicals and their intramolecular cyclisation into dihydrothiophene with its further dehydrogenation.137, 182 2BunS , BunSSBun S Me S Me 7H MeCH2CH CHSH 7H2 7H2 S S Gas-phase pyrolysis of di(tert-butyl) disulfide at 330 ± 400 8C in both a flow and a static systems (60 ± 212 Torr) leads to isobutene and tert-butyl hydrosulfide.The latter then decom- poses into sulfur and hydrogen sulfide on the reactor walls.183 But CH2 CMe2 H ButSSH CMe2, CH2 S S 7HSSH S But SBut HSSH H2S + S. Carbon disulfide, hydrogen sulfide, acetylene and methane are formed at 1127 8C.45 Since the C7S bond in di(tert-butyl) disulfide is weaker than the S7S bond, thiyl radicals are not formed. The initial cleavage of the C7S bond can only induce a short chain process ButSSBut ButSS+Me3C ,Thermal transformations of organic compounds of divalent sulfur ButSS+ButSSBut ButSSH+ButSSCMe2CH2, CMe2, ButSS+CH2 ButSSCMe2CH2 CH2 2Me3C CMe2+CHMe3.The formation of isobutane (5% and 13% yields in a flow and a static systems, respectively) confirms this scheme.45 Thermal decomposition of bis(diphenylmethyl) disulfide at 150 ± 200 8C leads to thiobenzophenone, diphenylmethane and sulfur. The reaction rate is increased with an increase in temper- ature.184 Ph2CH2+Ph2C S + S. Ph2CHSSCHPh2 Diallyl disulfide eliminates sulfur on flash pyrolysis. The allyl radical formed in this reaction was isolated in an argon matrix and characterised by UV, IR and EPR spectroscopy.185 750 8C S S 7S2 Bis(triphenylmethyl) polysulfides decompose on refluxing in benzene to form hexaphenylethane and sulfur.The thermal stability of Ph3CSnCPh3 is decreased with the change in n as follows: 1>3>4>2 (Ref. 186) 2Ph3C+Sn, Ph3CSnCPh3 Ph3CCPh3. 2Ph3C 2. Unsaturated, aromatic and heteroaromatic disulfides Bis(1-methylthiopropenyl) (43a) or bis(1-methylthio-2-phenyl- vinyl) disulfides (43b) cyclise into tetra-substituted thiophenes at 100 ± 200 8C. The reactions involve a [3,3]-sigmatropic Cope rearrangement of compounds 43a,b into the corresponding dithio esters 44a,b and the elimination of hydrogen sulfide from the latter.187, 188 SMe SMe R R R S R S R S S 7H2S R MeS SMe S SMe 43a,b SMe 44a,b R=Me (a), Ph (b).Benzenethiol and thianthrene are formed from diphenyl disulfide on heating to 260 ± 300 8C in an inert atmosphere. This reaction follows a free-radical mechanism.189 S PhSH+ 2 PhS PhSSPh S S 2 S Analogous products are formed in the thermolysis of bis(4- methylphenyl) disulfide. The formation of phenthiyl radicals in the vacuum pyrolysis of diphenyl and 2,20-dinaphthyl disulfides (400 ± 700 8C) was proved by EPR spectroscopy (in a liquid nitrogen matrix).190 Thermolysis of diphenyl disulfide at 180 ± 250 8C in an atmosphere of hydrogen sulfide leads to benzenethiol and diphenyl sulfide. The maximum yield of benzenethiol (52%) is achieved at 180 8C, while diphenyl sulfide is the major reaction product (82%) at 250 8C.Benzenethiol and diphenyl sulfide result from two parallel reactions.191 2 PhS , PhSSPh PhSH+HS , PhS+H2S PhSPh+S, PhSSPh 91 2 HS. S+H2S The liberation of sulfur in the thermolysis of diphenyl disulfide predominates at high temperatures and is apparently induced by hydrogen sulfide. Thermolysis of diaryl disulfides in refluxing tetralin results in the formation of the corresponding arenethiols and 1,2,3,4- tetrahydro-1-arylthionaphthenes.192 SC6H4X-4 4-XC6H4SSC6H4X-4+ 4-XC6H4SH+ X=Cl, OMe, Me. The following reaction mechanism has been suggested 192 k1 2 ArS , ArSSAr k2 ArSH+R , ArS+RH k3 ArSR, ArS+R k4 products, R+R k5 ArS+products, ArS+ArSSAr k6 ArS+products R+ArSSAr RH is tetralin, R.is a radical derived from tetralin. The reaction is induced by arylthiyl radicals generated on the thermal dissociation of the S7S bond in diaryl disulfides. The arylthiyl radicals abstract the a-hydrogen from tetralin producing tetralinyl radicals, which react either with thiyl or other tetralinyl radicals. Based on the results of kinetic and UV studies, an assumption that arylthiyl radicals are p-radicals has been put forward.192 Diphenyl disulfides with substituents in the para-positions of the benzene rings decompose more rapidly. The order in the change in the reaction rates depending on the nature of substitu- ents (MeO>Cl^Me>H) correlates with the stability of the respective thiyl radicals.The latter is increased with the strength- ening of the resonance interaction between the substituent and the benzene ring. Di(2-thienyl) disulfide eliminates sulfur to form di(2-thienyl) sulfide on heating to 130 ± 180 8C in an atmosphere of hydrogen sulfide.193 3. Thermal reactions involving disulfides Thiyl radicals generated on thermal decomposition of dialkyl disulfides can substitute the halogen atom at the sp2-hybridised carbon atom and add to the triple bond of acetylene and its derivatives. These reactions predominantly yield heterocyclic compounds of the thiophene series. The gas-phase reaction of 2-chlorothiophene with diethyl polysulfides at 560 8C leads to a 4 : 1 mixture of thieno[2,3- b]thiophene (46) and thieno[3,2-b]thiophene (47) (in up to 33% overall yield).Thieno[2,3-b]thiophene is formed as the result of the addition of vinylthiyl radicals 45 generated from diethyl polysul- fides to 2-chlorothiophene and subsequent heterocyclisation 55 2 EtS , EtSSEt EtS+EtSSEt EtSH+MeCHSSEt, MeCHSSEt CHS , EtSH+CH2 45Cl +45 7Cl S S S Cl92 . 7H2 S S S 46 S Both isomeric thienothiophenes are formed in parallel addi- tion reactions of vinylthiyl radicals to thiophene. The latter is obtained on reduction of 2-chlorothiophene with hydrogen sulfide EtSH CH2+H2S, CH2 +HCl+S, +H2S S S Cl S +46. +45 7H2 S S 47 Chlorobenezene reacts with dialkyl disulfides in the gas phase in a similar way.194 Aryl bromides effectively react with a diphenyl disulfide ± hydrogen sulfide system at 125 ± 240 8C to form aryl phenyl sulfides (49% ± 84% yields).195 The reactions occur at a lower temperature if a p-donating substituent is present in the benzene ring of aryl bromides.H2S 2 ArBr+PhSSPh 2 ArSPh +2 HBr+S Ar=Ph, 2-HOC6H4, 4-HOC6H4, 4-EtC6H4, 4-CNC6H4, 1-naphthyl, 2-hydroxy-1-naphthyl.Phenthiyl radicals generated from diphenyl disulfide substi- tute the bromine atom in aryl bromides, thus inducing the reactions. These radicals are generated again in the reaction chain propagation stage, playing also a role as hydrogen atom acceptors from hydrogen sulfide.195 2 PhS , PhSSPh PhSH+HS , PhS+H2S SPh H2S, PhSH R Br+PhS R 7HBr Br RC6H4SPh+PhS+HS . Dialkyl polysulfides react with acetylene in the gas phase at 470 ± 520 8C to form thiophene (up to 60% yield) and isomeric thienothiophenes.Thiophene arises as the addition product of vinylthiyl radicals 45, generated on the thermolysis of dialkyl polysulfides, to acetylene.196 CHSCH CH 45 +CH CH CH2 7H S Isomeric thienothiophenes 46 and 47 are formed due to the addition of vinylthiyl radicals 45 to thiophene. The so-called `disulfide oil' representing a mixture of dimethyl, diethyl and ethyl methyl disulfides can be used instead of diethyl disulfide in the reaction with acetylene. On pyrolysis, all these disulfides generate vinylthiyl radicals 45, which then are trapped by acetylene.175 However, the yield of thiophene in this case is lower than that obtained from pure diethyl disulfide.197 If a mixture of diethyl disulfide and diethyl selenide is used in the reaction with acetylene, the yield of thiophene is increased up to 92%.198 The reaction of 2-chlorothiophene with a diethyl disulfide ± acetylene system at 650 ± 700 8C leads to thiophene and thieno[2,3-b]thiophene (46).199 Thiophene results from the reac- tion of acetylene with the vinylthiyl radicals 45.Thieno[2,3- b]thiophene (46) is formed due to the reaction of acetylene with 2-thienylthiyl radicals 16 generated in situ from 2-chlorothiophene and hydrogen sulfide (the thermolysis product of diethyl disul- fide). Acetylene is a more efficient scavenger of thiyl radicals than 2-chlorothiophene; its addition suppresses the competing reaction leading to di(2-thienyl) sulfide.199 M G Voronkov, E N Deryagina The gas-phase reaction of dimethyl di- or trisulfide with propargyl alcohol at 500 8C leads to 1,2-dithiol-3-one (48).200 In this reaction, propargyl alcohol acts as a scavenger of MeSS.radicals Me +SSMe, MeSSMe MeS+SSMe, MeSSSMe O CCH2OH MeS MeSS+HC CCH2OH 7CH4 CHSSMe SS . 48 (25%) Dimethylthiophenes (a mixture of 3,4-, 2,3- and 2,4-isomers) are the main products in the gas-phase reaction of dimethyl di- or trisulfide with a mixture of 1,2- and 1,3-dimethylenecyclobutanes at 450 ± 500 8C.201 The reaction involves cyclobutane ring-open- ing by methylthiyl radicals. Me2Sn + Me Me Me Me + + Me Me S S S n=2, 3. 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