摘要:

Russian Chemical Reviews 71 (10) 789 ± 835 (2002) Applications of NMR tomography to mass transfer studies I V Koptyug, R Z Sagdeev Contents I. Introduction II. Sorption into porous materials III. Swelling of polymers and coal during sorption of solvents IV. Drying V. Diffusion VI. Application of magnetic resonance imaging to flow studies VII. Supplement Abstract. tomog- NMR of applications physicochemical Modern Modern physicochemical applications of NMR tomog- raphy, the to (MRI), imaging resonance magnetic or raphy, or magnetic resonance imaging (MRI), to the investigation investigation of mass transfer processes are described. The mass transfer of mass transfer processes are described. The mass transfer involved swelling solutes, and fluids gases, of sorption the in involved in the sorption of gases, fluids and solutes, swelling of of polymers is materials various of drying and coal and polymers and coal and drying of various materials is considered.considered. Various and anisotropic isotropic, studying of methods Various methods of studying isotropic, anisotropic and restricted restricted diffusion to approaches The discussed. are media porous in diffusion in porous media are discussed. The approaches to the the investigation gases fluids, of flow the and filtration of investigation of filtration and the flow of fluids, gases and and granulated bibliography The presented. are materials granulated materials are presented. The bibliography includes includes 373 references 373 references. I. Introduction This review concerns a number of the most widely used modern physicochemical applications of the method of NMR tomogra- phy, or magnetic resonance imaging (MRI), which mainly con- cern the studies in the field of the transfer (transport, flow) of fluids, gases and granulated materials.The studies devoted to the obtaining of spectral data and the NMR experiments carried out without spatial resolution are covered fragmentarily. The excep- tion are the investigations involving the use of the pulsed magnetic field gradients for studies of the spatial molecular displacements due to diffusion, to the flow of fluids and other types of motion. These communications were covered in the review because of their great value and also for yet another reason, namely, because the corresponding experiments are often considered as the MRI studies in the molecular displacement space rather than in the conventional coordinate space.The title field of research is so broad that the published data are covered only partially. The salient features of the MRI techniques and their applications to studies of liquid-containing materials have been reported in a recent review.1 I V Koptyug, R Z Sagdeev International Tomography Centre, Siberian Branch of the Russian Academy of Sciences, ul. Institutskaya 3A, 630090 Novosibirsk, Russian Federation. Fax (7-383) 233 13 99. Tel. (7-383) 233 35 61. E-mail: koptyug@tomo.nsc.ru (I V Koptyug) Tel. (7-383) 233 14 48. E-mail:itc@tomo.nsc.ru (R Z Sagdeev) Received 20 June 2002 Uspekhi Khimii 71 (10) 899 ± 949 (2002); translated by AMRaevsky #2002 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2002v071n10ABEH000743 789 789 792 795 798 805 816 II.Sorption into porous materials 1. General remarks Sorption has found a very broad spectrum of technological applications including, in particular, the drying and separation of chemical substances. Transport of fluids and gases through porous materials also plays an important role in catalysis. Perme- ability of building materials is critical to the development and use of engineering protective structures and containers for nuclear waste. Water and brine ingress into building materials can cause their accelerated degradation following different mechanisms including freezing, uptake of ions and corrosion and affects the gas permeability and thermal conductivity of materials. The problem of how the structure of the pore space influences the transport processes in porous media is also topical.Applica- tion of the MRI technique to studies of this interrelation offers considerable prospects, since the method allows investigation of both structural and transport features for the same sample under the same conditions. Mass transfer can be strongly affected by not only microscopic inhomogeneities with the characteristic sizes lying in the pore size length scale, but also by macroscopic structural inhomogeneities inside porous materials including those due to specific features of the preparation procedure; there- fore, MRI is widely used in the studies of transport processes. Most modern building materials, as well as soil and many catalysts contain large amounts of paramagnetic ions, predom- inantly, Fe ions, which are responsible for appreciable shortening of the spin relaxation times of the fluids confined within the pores of such materials and for the appearance of local inhomogeneities of the magnetic field due to different magnetic susceptibilities of the chemical substances at the interfaces (if the experiments are carried out in strong magnetic fields).For instance, the T2 time of water present in the fired-clay red brick containing a large amount of iron was found to lie between 180 and 360 ms. 2±4 Correspond- ingly, the MRI studies are often carried out in relatively weak fields (<1 ± 2 T) using strong amplitudes of the applied field gradients and appropriate experimental techniques that provide a short effective echo time.5, 6 Large variations of the fluid content in the sample usually cause substantial changes in the relaxation times (see Section IV.2 in Ref.1). Short T2 times make a complete elimination of the relaxation weighting of MRI images virtually impossible; there- fore, variations of the signal amplitudes are not in proportion to variations of the fluid content in the sample. In addition, adsorp- tion can be accompanied by pronounced variation of the temper-790 ature, which also affects the relaxation time values. Therefore, quantitative measurements require the use of some references and calibration curves. Sorption of water usually causes large variations of the dielectric losses in the sample, while the presence of paramagnetic ions affects the electrical conductivity of the sample.This can be responsible for substantial variations of the sensitivity of the NMR spectrometer as a result of detuning the resonant circuit, which additionally complicates the quantitative interpretation of experimental data and in some instances makes it ambiguous due to nonmonotonic variation in the signal amplitudes upon a monotonic change in the water content.7, 8 For this reason, a number of experiments were carried out on a tailor-made instru- ment designed to study the processes accompanied by substantial variations of the water content in the samples.3 2. Sorption studies Investigations of one-dimensional (1D) capillary sorption of water into different building material bars showed 9 that transport of water can be described using a nonlinear 1D diffusion equation in which the effective capillary diffusion coefficient, D(C), depends on the water concentration, C.For gypsum and mortar bars 9 and for some limestone rock bars 9, 10 the D value exponen- tially increases with the local concentration of water. Sorption of water into cylindrical hardened mortar samples 11 is described by an analogous equation with exponential dependence of D on C. Transfer of water in the sample dried at 105 8C was found to occur much faster than in the sample dried under milder conditions by extraction in isopropyl alcohol followed by drying at 40 8C. This can be due to the formation of a system of interconnected larger pores in the high-temperature drying.Shortening of the T2 time of the adsorbed water in the region behind the adsorption front with time indicates a gradual redistribution of water from capillary pores into smaller gel pores that are formed around the cement grains upon hardening of the mortar.11 Capillary sorption of water into different materials is characterised by the presence of a sharp front of water. In the case of 1D diffusion the front moves in proportion to the square root of time and the amount of the adsorbed water behaves analogously.5, 9 ± 11 Adsorption of D2O into samples with large initial water content was studied using the 2D NMR signals.5 Often, the results of the sorption dynamics studies are pre- sented as the C=C(x/t1/2) dependence using the Boltzmann coordinate transformation.In these coordinates, e.g., the 1D water distribution profiles obtained 2± 4 for water in the fired- clay red brick at different times that passed after the beginning of the adsorption process fall on the same curve.4 Numerical simulation of adsorption using the 1D diffusion equation and exponential dependence of D on the water content D=D0exp(bC), where b is the fitting parameter, led to good agreement between the calculated and experimental data. Absorption of aqueous NaCl solution into a cylindrical calcium silicate sample was studied by simultaneously recording the 1D water distribution profiles and sodium concentration profiles.12 Similarly to the data obtained for water, the sodium concentration profiles fall on the same curve in the C(x/t1/2) coordinates.However, the system exhibits no sharp sodium concentration front. Virtually no sodium is present in the vicinity of the water front; sodium falls far short of the water front, being adsorbed into the pore space. It was recognised that the profiles obtained using the 23Na NMR signals correspond to the sodium ions present in solution, whereas the concentration of the adsorbed sodium can be found from the adsorption isotherm measured in that study. The results obtained for sodium cannot be described by the diffusion equation in the C(x/t1/2) coordinates even assuming that the equilibrium local concentration of the adsorbed sodium (determined from the adsorption isotherm) is reached following an exponential dependence some time after the I V Koptyug, R Z Sagdeev beginning of the process rather than instantaneously.All calcu- lations predict the presence of a more or less sharp sodium concentration front, which contradicts the experimental results.12 Good agreement between the calculated and experimental data for water was obtained using an exponential dependence of D on C. To reduce the water permeability of building materials, they are impregnated with some agents based on hydrophobic poly- mers and other compounds. An impregnation study of a hardened mortar sample with 5% solution of alkyltrialkoxysilane in meth- anol showed 6 that the impregnating agent penetrates only a short distance (1.5 mmfrom the surface). After drying the sample water poured on its surface did not penetrate the material, in contrast to non-impregnated blank sample.Water uptake by hardened cement samples containing both clay (sodium aluminosilicate, 2%) and polyvinyl alcohol (4%) additives was much smaller as compared to the samples containing only one additive.13 This behaviour was explained by the formation of a hydrophobic material (as a result of cross-linking of the aluminosilicate particles and alcohol molecules), which fills the pores, thus preventing water ingress. MRI allows visualisation of the spatial distribution of water and hydrocarbons in rock samples, which is necessary for the development of efficient procedures for improved petroleum recovery from petroleum pools based on the results of numerical simulation of the fluid diffusion in rocks (see also Section V). In particular, adsorption of water vapour into sandstone cores of various compositions, porosities and permeabilities has been studied.14 The 1D water distribution profiles recorded at relatively low saturations allowed visualisation of the ingress of water vapour into the core through its open end.Simulation of the results obtained was carried out based on the assumption of simultaneous diffusion transport of water vapour and the adsorbed water with inclusion of the local equilibrium between them and led to good agreement between the calculated and experimental data. Assuming an exponential dependence of the vapour pressure on the concentration of the adsorbed water, it was found that D exponentially increases with an increase in the water content in the sample until the vapour filtration threshold is reached at times longer than 100 h.At even longer times, the distribution of water along the core axis became relatively homogeneous and varied only slightly upon further keeping the sample over a water bath, which was due to blocking of narrow inter-pore channels with the liquid that prevented free diffusion of vapour. Carrying out a series of experiments at different temper- atures allowed the determination of the activation energy for surface diffusion.14 Selective water sorbents (SWS) 15 ± 17 represent porous matri- ces impregnated with hygroscopic salts. The ability of such materials to efficiently absorb water vapour and alcohol vapours and release a great deal of energy offers considerable prospects for versatile applications of these materials, in particular, for dehu- midification of gases, in heat pumps and in chemical heat accumulators. Adsorption of water vapours by individual CaCl2-impregnated silica gel granules exposed to a humid air stream leads to the appearance of a sharp front of the adsorbed water inside the granule 7, 8, 18 (Fig.1). Therefore, the presence of salt inside the millimetre-size granules makes capillary transport inefficient. The limiting stage of the process is transport of the adsorbed water into the interior of the granule. The results of numerical simulation of the process using the diffusion equation are in satisfactory agreement with the experimental data if one assumes an exponential increase in D with an increase in the water content.8, 18, 19 The use of CuCl2 allows one to follow the salt redistribution as a result of successive sorption ± drying cycles for the granules prepared with inhomogeneous salt distribution.7, 18 Granules with large initial water content were characterised by the absence of a sharp water front during the adsorption process.18 Sorption of water vapour by a cylindrical pack of SWS granules exposed to a humid air stream has been studied.7, 8Applications of NMR tomography to mass transfer studies 2 1 6 5 10 9 14 13 b 0 70.2 70.4 Distance /cm Figure 1.Visualisation of the adsorption of water vapours into a cylindrical silica gel granule impregnated with CaCl2 (granule diameter: 6 mm). Sixteen successively recorded two-dimensional images (acquisition time: *15 min per image; the dark ring corresponds to the region containing the adsorbed water) (a); one-dimensional water distribution profiles along the diameter of the granule (acquisition time: 34 s per profile, the time taken to complete the experiment: nearly 5 h) (b).Mention has been made that numerical simulation of the longi- tudinal movement of the sorbed water front requires the inclusion of (i) the effect of intragranular transport on the adsorption efficiency, (ii) flow dispersion effects in the porous medium and (iii) the character of the adsorption isotherm. Adsorption of water vapours by individual SWS granules and pellets and by SWS packs under vacuum has also been reported.8, 20 It was found that transport of adsorbed water under vacuum is, by and large, much more efficient; however, it is to a great extent determined by the granule (pellet) composition and morphology.Interest in mass transfer in zeolites is due to the wide use of these materials in separation processes, drying and catalytic reactions. MRI was used for studying adsorption of different gases and vapours by zeolites. The movement of the adsorbed a 4 3 8 7 12 11 16 15 Time 0.4 0.2 791 water front into the interior of the adsorbent (small crystals of zeolite 4A) has been studied in detail.21, 22 The shape of the front and the character of its movement were found to be strongly dependent on the relative efficiency of transport in the intercrys- talline space and intracrystalline transport within the adsorbent layer. For tiny crystals (several micrometres in size) one can expect that the limiting stage of transport process is the diffusion of water vapour in the intercrystalline space, while the equilibrium between the adsorbate concentration in the crystal and the vapour pressure above the surface of the crystal is described by the adsorption isotherm.In this case the process can be described using one nonlinear diffusion equation while the diffusion coefficient, D, depends on the amount of the adsorbed water and is expressed by a combination of constant diffusion coefficients of the water vapour and adsorbate inside individual crystals. When exponen- tial adsorption isotherm is used, the D value increases as the water content increases and can be represented as the sum of a constant and an exponential term.21, 23 Analogous experiments were car- ried out at different temperatures for two contacting layers comprised of 3 mm zeolite 4A granules with different initial water content.23 Simulation of the results obtained showed that a decrease in the water content and an increase in temperature make the role of intragranular transport more significant.Equi- libration of two contacting layers saturated with the light and heavy water occurs at a high vapour pressure and much faster, while the apparent (constant) diffusion coefficients of the adsorbed water and water vapour are somewhat higher than the corresponding values obtained in the equilibration experiments with a water-saturated layer contacting a dry layer. This difference becomes significant at higher temperatures and is probably due to the drawbacks of the mathematical model employed to describe the transport process. In particular, the intracrystalline vapour diffusion coefficient can also be strongly dependent on the vapour concentration.For instance, adsorption of n-hexane by a NaX zeolite layer is accompanied by a decrease in the intracrystalline diffusion coefficient with an increase in the adsorbed n-hexane.24 In the beginning of adsorption process the concentration of adsorbate in the surface layer rapidly reaches a maximum value, which is followed by the movement of the sharp concentration front into the interior of the adsorbent.22 The concentration front is likely to move proportionally to the square root of time. In the later stages of the process the concentration front becomes some- what diffuse and a wing appears in the region of low adsorbate concentration.The T1 times obtained for water in the vicinity of and behind the adsorption front are different, which was associ- ated with the changes in the mobility of water molecules that cannot be instantaneously located inside zeolite cavities.22 The gradual movement of a sharp adsorption front was also observed in studies of the adsorption of water or butane vapours by a layer comprised of tiny crystals (4 mm in diameter) of zeolite NaX (Ref. 25) and of the adsorption of n-hexane vapours by zeolite NaX layer.24, 26 In contrast to this, experiments with zeolite NaX crystals 40 mm in diameter showed that intercrystalline transport is no longer the limiting factor and that butane remains uniformly distributed within the layer, whereas the overall amount of the adsorbed butane gradually increases.25 The MRI technique can be used for studies of adsorption from solutions.For instance, the chemical shift of 1,3,5-tri-tert-butyl- benzene adsorbed on g-Al2O3 differs from that of the bulk phase of 1,3,5-tri-tert-butylbenzene. This allowed the determination of the variation of the adsorbate concentration with time and estimation of the effective diffusion coefficient for a CDCl3-filled granule from the NMR signal intensities.27 Moreover, the chem- ical shift of the adsorbate can be pore size dependent. For instance, NMR studies of the samples of a carbon adsorbent characterised by a broad multimodal pore size distribution (from micropores to macropores) and saturated with solutions of acetonitrile, dichloromethane or benzene in CCl4 revealed that the NMR spectra exhibit a superposition of several broad lines, each of them corresponding to the fluid confined inside pores of792 different size.The chemical shift difference between the pore- confined and bulk-phase fluids varied from 1.4 ppm (large pores) to 9.6 ppm (small pores) for the three compounds studied. This indicates that the difference in the chemical shifts of the molecules inside pores of different size is due to different diamagnetic susceptibilities in the vicinity of the interfaces rather than donor ± acceptor interactions. The `dips' in the pore size distribu- tion plot and slow inter-pore molecular exchange are two pre- requisites for this conclusion. The linewidth is determined by intra-pore processes and is temperature independent in the range between 20 and 60 8C.From the surface areas of the signals in the NMR spectrum one can determine the fraction of the adsorbate inside the pores of different size (determination of absolute values requires the use of other measuring techniques). The method allows investigations of the role played by the pores of different size in the course of adsorption. As the adsorbate concentration increases from 0.2 to 1.2 mol litre71, the total amount of the adsorbate inside the micro- and mesopores is `saturated', whereas the amount of the adsorbate in the large pores increases. Hence, preferred adsorption in micropores occurs initially. The T1 and T2 relaxation times of the peaks corresponding to different chemical shifts are also different.The T2 times of the fluids confined in micropores remain constant as the adsorbate concentration varies, since the micropores are completely filled at a concen- tration of 0.2 mol litre71. At the same time, the T2 times of the fluids confined in larger pores are lengthened as the concentration of adsorbate increases, since the fraction of the adsorbed mole- cules in these pores decreases. The amount of adsorbate in meso- and macropores increases with time, which is accompanied by displacement of the solvent molecules from the pores. The results obtained allowed construction of the isotherms of acetonitrile and benzene adsorption from CCl4 separately for large, small and intermediate pores in the sample.Variation of the adsorbate concentration is accompanied by some variation of the chemical shifts of the NMR signals, which is consistent with their assign- ment to pores of different size.28 Transport of fluids in soil is critical to soil contamination studies. Equilibration of soil and water after placing a sample of dried soil (humus) in water has been studied over a period of 50 days.27 2D spin density maps and the T2 time maps of water were obtained. The sample was found to be inhomogeneous and characterised by a bimodal T2 distribution and by the shift of the short-T2 mode towards the short-time region. This is probably due to either a gradual filling of micropores with water or to gelation. III. Swelling of polymers and coal during sorption of solvents 1.General remarks The interactions between polymers and fluids or gases accompa- nied by swelling and dissolution of the polymers are of great practical value for fabrication, processing and use of polymeric materials as well as in the physical chemistry of coal. Swelling of polymers is often accompanied by gelation. The polymer ± solvent systems can exhibit various types of mass transfer. If the rate of solvent diffusion into the polymer is much lower than the segmental relaxation rate of the polymer, the diffusive transport of the solvent is usually observed, which obeys Fick's laws (the so-called Case I transport). This type of transport is characterised by a gradual change in the solvent concentration along the direction of the solvent diffusion, movement of a diffuse solvent concentration front in proportion to the square root of time (t1/2) and by nearly constant spin ± spin relaxation time, T2, of the solvent in the solvent-filled regions of the polymer.A quite different behaviour is characteristic of Case II transport, which occurs if the solvent diffusion rate is higher than the segmental relaxation rate of the polymer. In this case, a clearly visible front of the solvent permeating into the polymer is observed, the solvent concentration front moves linearly with time, the solvent concen- tration behind the front is constant and the T2 time is shortened I V Koptyug, R Z Sagdeev with the distance from the polymer surface contacting the solvent. The contour of the solvent front in 2D images matches that of the outer surface of the sample, through which the solvent ingress occurs, whereas in the case of Fickian transport the solvent contour acquires a circular shape after passage of a short time.Fickian transport is usually observed in amorphous polymers characterised by the glass transition temperature lying below the temperature of the experiments. In contrast to this, realisation of Case II transport requires that the polymer be (initially) in the glassy state. Swelling of a polymer is usually accompanied by a decrease in the glass transition temperature, Tg, which can lead to transition of the polymer to the rubbery state. In addition to the two limiting cases, an intermediate type of transport (Case III transport) occurs. Often, the character of transport and its features can be revealed only using spatially resolved measure- ments. Solvent transport into the interior of the polymer is often accompanied by cleavage of hydrogen bonds, weakening of other intermolecular interactions and by disentangling and moving apart the polymer chains.Infinite swelling leads to dissolution of the polymer. Impregnation of polymeric materials with various solvents can be studied either by periodically interrupting the process to acquire MRI images or using real-time monitoring. The use of conventional techniques employed in the liquid-phase NMR allows selective monitoring of solvent transport,29 whereas the signal of the solid matrix is often not observed due to short T2 relaxation times. However, sometimes the mobility of the macro- molecules becomes so high due to swelling that MRI imaging of the polymer is possible without using solid-state NMR techni- ques.30, 31 It should be noted that MRI allows one to obtain separate images of the solvent and the polymer, as well as the images of individual solvents if their mixtures are used in the experiments (see Section III.1 in Ref.1). Swelling of polymers and coal in solvents occurs at relatively low rates; therefore, corre- sponding experiments may take times from tens of minutes to tens of days. 2. Swelling of polymers and coal Periodic recording of the 1D acetone distributiion profiles in a vulcanised rubber sheet revealed 32 a Fickian transport of the liquid. The acetone front moves from the surface into the interior of the sample in proportion to the square root of time (t1/2), since the glass transition temperature of the polymer under study is below the room temperature (the experiment was carried out at room temperature).Nevertheless, no substantial dispersion of the acetone front was observed, which indicated an (exponential) increase in the diffusion coefficient with an increase in the acetone concentration (C) in the polymer. Transport of boiling water in a nylon-6,6 block also obeys Fick's laws (the results obtained were described using an exponentially increasing dependence of D on C).33 More recently,34 transport of water in nylon-6,6 was studied at different temperatures between 20 and 100 8C; in this case, a linear dependence of D on C was used. The activation energy, Ea, for water diffusion in the polymer, extracted from the experimen- tal data, was found to be 15 kcal mol71 .This value is only slightly dependent on the water content. The temporal evolution of both the water distribution profiles and the spin relaxation times of water was described by the two-phase model, which implies the presence of two types of water molecules differing in degree of binding to the polymer. An increase in pressure caused an increase in both the diffusion coefficient and the maximum content of water in the polymer.34 Analysis of the 2D MRI images of radial transport of 1,4- dioxane in a poly(vinyl chloride) rod showed 35 that the system is characterised by Case II transport and the movement of the 1,4- dioxane front into the interior of the rod is proportional to t 0.91.At the same time, the swelling-induced variation of the rod diameter was proportional to t1/2, since the increase in the volume occurs in the polymer region that is behind the liquid front and isApplications of NMR tomography to mass transfer studies in the rubbery state for which transport of liquids obeys Fick's laws.35 The solvent region behind the front is characterised by a constant spin density and by lengthening of the T2 time with the distance from the surface. A more complex picture was observed for swelling of a bisphenol A polycarbonate rod in acetone. The movement of the solvent front was proportional to t1/2, thus indicating Fickian transport. The solvent concentration decreased from the surface towards the core of the rod, while the T2 time was nearly constant.At the same time, the 2D images exhibited a contracting, high-intensity ring-shaped zone of the NMR signal around the core; the concentration and the T2 time (mobility) of the solvent inside this zone were higher than outside.35 This was associated with crystallisation of the polymer, accompanied by the displacement of the solvent into the interior of the sample, which causes the solvent front to move in proportion to the square root of time. Crystallisation of the sample leads to the appearance of wedge-like cracks oriented from the surface into the interior of the cylinder; on further swelling, stress cracking is developed and the cracks become larger. The butadiene rubber ± toluene,36 isobutyl rubber ± toluene 37 and methylsiloxane rubber ± hexafluorobenzene 36 systems exhibit Fickian transport while the poly(ethyl methacrylate) ± methanol and bituminous coal ± pyridine systems are characterised by Case II transport (a sharp solvent front is clearly visible in the corresponding 2D MRI images and moves at a constant veloc- ity).36, 37 It was found that, due to attenuation of the signals characterised by short T2 times, the solvent front in the MRI images can appear to be sharper than that which actually exists.36 A relatively simple model proposed for the description of the kinetics of the process couples the solvent diffusion and the polymer network relaxation upon transition from the glassy state to the rubbery state.36 The experimentally measured solvent front velocities were used for calculations of the constant solvent diffusion coefficient in the glassy polymer and the polymer relaxation rate constant.The poly(methyl methacrylate) ± metha- nol system exhibits Case II transport, which is probably due to the presence of the initial Fickian induction period.38 The 2D images of poly(methyl methacrylate) rods partially saturated with deuter- omethanol (CH3OD) or acetone and the 2D maps of both the spin relaxation time distribution and the diffusion coefficient of the solvent were also obtained using the NMR signal of the solvent.39 The presence of a sharp solvent front and a constant solvent concentration in the region behind the front are indicative of Case II transport. When moving from the glassy core into the interior of the swollen region, the T1 and T2 times and the diffusion coefficients of both solvents increase and then reach a plateau.This behaviour can be due to gradual increase in the polymer mobility along the polymer chain that is fastened at one end (in the glassy domain of the sample) and to the effect of the polymer mobility on the mobility of the solvent molecules. In practice, the behaviour of the system can strongly depend on the presence of residual amounts of the monomer or water in the polymer. For instance, an increase in the initial water content in poly(methyl methacrylate) increased the diffusion rate of meth- anol during the absorption process. If the water content exceeds 1 mass %, a passage from the Case II transport to Fickian trans- port of methanol occurs.40 The absorption ± desorption cycling of water and methanol followed by drying of the polymer also accelerate the diffusion of methanol in poly(methyl methacrylate) and the diffusion kinetics alters from Case II diffusion to Fickian diffusion.In all cases the initial period of absorption was characterised by a specific type of methanol diffusion, intermedi- ate between the Fickian diffusion and Case II diffusion.40 Studies of the swelling of a poly(vinyl chloride) rod in toluene for 15 ± 50 h revealed 41, 42 a sharp solvent front characteristic of Case II diffusion. The solvent front contour matched the cross- sectional shape (round or square) of the sample. When toluene penetrated into the rectangular rod through two oppositely lying surfaces, two parallel solvent fronts moved to meet each other.42 At the same time the high-density polyethylene ± n-pentane system 793 exhibited a diffuse circular n-pentane front 41 typical of Fickian diffusion.The sample of stretched poly(vinyl chloride) used in yet another study 41 was prepared as follows. A poly(vinyl chloride) pipe was heated above the glass transition temperature of the polymer and expanded in a radial fashion. Then, slices were cut from the pipe, parallel to the pipe symmetry axis. Investigation 41 revealed strong anisotropy of solvent diffusion during the swelling process. In this case, the diffusion of toluene or trichloroethylene in the polymer occurred only in the direction corresponding to the radial direction for the initial pipe and was accompanied by elongation of the sample in the same direction and by contraction of the sample along the axis orthogonal to the direction of solvent diffusion.The anisotropy of the properties of the polymer was found to be hard to detect by high-resolution solid-state NMR techniques irrespective of the presence or absence of solvent in the sample. Swelling of a copolymer of 2-hydroxyethyl methacrylate H (HEMA) and tetrahydrofurfuryl methacrylate (THFMA) in water has been studied.29 The diffusion coefficient of water (DH2O) in polyHEMA is (1.5 ± 2)61077 cm2 s71; in the copoly- mer the D 2O value decreases with an increase in the THFMA content due to the lower polarity of THFMA. The NMR images obtained revealed a Fickian diffusion and the peaks correspond- ing to increased content of water in the region between the diffusion front and the glassy core of the sample at the mole fractions of HEMA greater than 0.6.The peaks are thought to be due to the formation of small cracks appeared in peripheral regions of the glassy core owing to stress induced by swelling of the outer part of the sample; the cracks are filled with water. Further swelling of the polymer leads to closure of the cracks, while the region with the increased water content moves through the sample along with the diffusion front. As the diffusion front reaches the core of the sample, the peaks in the MRI images disappear along with the glassy core. The character of diffusion for the same polymer ± solvent pair can change appreciably with temperature. For instance, the diffusion of methanol in poly(methyl methacrylate) is Fickian at temperatures above 60 8C and Case II diffusion at temperature below 30 8C, whereas in the temperature range between 30 and 60 8C one can expect a combination of the two diffusion mecha- nisms.43, 44 The character of diffusion can also strongly depend on the degree of cross-linking of the polymer.For instance, the diffusion of dioxane in a material prepared by polymerisation of styrene with divinylbenzene (DVB) additives can obey Fick's law (5% DVB), be Case II diffusion (1% DVB) or exhibit an inter- mediate diffusion type (2.5% DVB).45 An increase in the degree of cross-linking of the polymer chains is accompanied by substantial reduction of the velocity of the movement of the dioxane front into the interior of the polymer sample. Ageing of polymers strongly affects the diffusion processes.This was shown,46 in particular, in the study of swelling of natural rubber exposed to high-temperature ageing at 170 8C inC6D6. At the ageing times of the order of 50 ± 100 min, the diffusion and swelling rates in the surface layer, in which the ageing occurs most intensely, increase. At prolonged ageing, radical polymerisation in the presence of oxygen induces cross-linking of the polymer chains, which precludes the changes in the size of the sample due to swelling, but has no effect on diffusion. The anisotropy of the polymer structure strongly affects its swelling behaviour, e.g., the change in the size of the sample. Swelling of a polymer proceeds differently when a mixture of two or more liquids is used instead of the individual liquids.Swelling of a bisphenol A polycarbonate rod in a mixture of acetone and methanol (a good and a poor solvent for the rod material, respectively) has been studied using the difference between the chemical shifts of the solvents.35 It was shown that the presence of methanol reduces the rate of acetone diffusion in the polymer, whereas the diffusion coefficient of methanol increases in the presence of acetone. Analogous studies of the diffusion of individual components of acetone ± methanol mix-794 tures with different component ratios were carried out using deuterated compounds.47 A square-root-of-time dependence of the movement of the solvent front and a constant T2 time of acetone in the region behind the solvent front were interpreted as manifestations of Fickian diffusion.In mixtures with the mole fraction of acetone being less than 50% both solvents diffuse with the same rate, whereas diffusion of acetone in the mixtures with higher content of this component occurs faster. Swelling of a cylindrical vulcanised rubber sample in an acetone ± methanol mixture has been studied based on large difference between the chemical shifts of the mixture components.48 It was found that both liquids diffuse in the mixture with the same rate equal to 80% and 250% of the diffusion rates of benzene and acetone, respec- tively, in the case of single-component impregnation. The creation and recording double-quantum coherence of the nuclear spins allowed one to follow isooctane diffusion in identical cylindrical sample impregnated with cyclohexane.48 Swelling studies of cross- linked polystyrene in C6F6 revealed the presence of the absorption front.However, no clearly visible absorption front was observed for polystyrene initially saturated with C6H6; structural inhomo- geneities favoured diffusion transport of liquid.49 A polymer gel based on poly(N-isopropylacrylamide) and H2O/D2O with CH3OD additive, in which cross-linking of the polymer chains occurs in the presence of different amounts of N,N0-methylenebisacrylamide, is known to have the lower critical solution temperature.46 As the temperature exceeds the critical value or as the concentration of the organic component of the solvent increases, the gel undergoes a transition from the swollen to the `compressed' state.This behaviour can be used for fabrica- tion of automatic chemomechanical valves. The 2DNMR images of a cylindrical sample revealed a decrease in the diameter of the sample with an increase in the temperature of water. The process was accompanied by the change in the T2-contrast due to the variation of the mobility of the polymer and water molecules. Experiments with aD2O±CH3ODmixture showed that the initial stage of CH3OD diffusion in the polymer gel obeyed Fick's laws. Since the degree of swelling of the polymer gel depends on the local component ratio in the solvent, this causes some changes in the polymer gel structure, followed by deviation of the diffusion mechanism from Fickian diffusion. Sometimes the formation of a surface layer was observed, which slowed down or precluded further ingress of the solvent into the polymer gel.Not only the solvent behaviour, but also the changes in the properties of polymers have been studied. In particular, a study of the swelling of the peroxide vulcanised natural rubber in dec- ane 21, 50 revealed a correlation between the T2 time of the polymer and the local concentration of decane in the sample. An empirical relationship was suggested to describe the correlation. It was also found that the diffusion coefficient of decane exponentially increases as its local concentration increases. The study 51 of swelling of poly(ethylene oxide) in water showed that both diffusion of water in the swollen hydrogel and the swelling process are diffusion controlled and not accompanied by the change in the overall volume of the polymer and water.It was also found that monitoring simultaneous ingress of H2O and D2O into the polymer from the opposite ends of the sample allows one to observe the redistribution of the light and heavy water in the sample even after the two solvent fronts meet. The use of CD2Cl2 as solvent permitted the observation of the 1H NMRsignal of the polymer prepared by cross-linking the liquid-crystalline nematic prepolymer chains.46 The 1H NMR signal of the polymer was shown to increase with the degree of swelling of the sample. In this case, solvent diffusion along the polymer chains occurs much faster than lateral diffusion, the latter being jumpwise. Sodium polyacrylate is a hydrophilic polymer capable of absorbing large amounts of water (several hundred times its own mass).52 The 1DH2O profiles obtained in the course of swelling of the polymer in water indicate a Fickian diffusion of water.Swelling was accompanied by shortening of the T2 time of water due to gelation. By recording the NMR signal of the polymer it I V Koptyug, R Z Sagdeev was shown that the swelling of the polymer followed by gelation occurred nearly two times faster than the movement of the water front into the interior of the polymer. It was found that distilled water diffuses faster than water containing salts and other impurities due to different chemical potential gradients. Swelling of polymers occurs not only when the polymers contact liquids, but also when they are exposed to vapours. For instance, the ionic conductivity of polymer electrolytes (Pb,Zn)(CF3SO3)2(PEO)n [PEO is poly(ethylene oxide)] is enhanced by several orders of magnitude upon adsorption of water vapours.53 The 1D spin density and T2 profiles for the adsorbed water and for the water-soaked polymer were obtained.In a number of experiments the contributions of water and polymer were separated using D2O. The experiments confirmed that the adsorbed water exhibits a Fickian diffusion despite the presence of a sharp front. At the same time, the results obtained using the NMR signal of the polymer are quite different, namely, the signal amplitude is constant in the region behind the water front, while the T2 time is shortened along the direction from the adsorbing face into the interior of the sample.The apparent velocity of the H2O front movement was much higher than in the case ofD2O; this was explained by different dynamic properties of water and the polymer. An amorphous film with high content of poly(ethylene oxide) contained crystalline domains of the pure polymer with characteristic sizes from 500 to 800 mm. The 2D NMR images of the films totally exposed to water vapours showed that adsorption of water by the sample occurred due to the amorphous phase and caused a transition of the crystalline domains to the amorphous state.53 Similar discrepancies between the results obtained using the NMR signals of the solvent and polymer were also observed during adsorption of acetone vapours by poly(vinyl chloride) samples.54 In this case, the solvent exhibited a Case II diffusion with a constant acetone concentration behind the liquid front; however, a `Fickian precursor', that is, a long wing of low intensity and substantial extension into the interior of the polymer, was observed ahead of the solvent front.Variation of the signal behind the liquid front was rationalised by gradual softening of the swollen polymer. Another possible explanation is low segmental mobility of the polymer chain in the glassy region of the sample and gradual increase in this parameter with an increase in the depth of penetration into the rubbery swollen region.39 The acetone and polymer signals were distinguished 54 using both deuterated acetone and the difference in the T2 times. Despite the fact that the samples studied were synthesised in the United Kingdom while studied in Germany, it was stated that no errors were introduced.The charge carrier distribution in the polyelec- trolyte-based electrochemical cells has been studied.55 Analysis of the profiles acquired using the NMR signals of polymers characterised by high segmental mobility [experiments on adsorption of the deuterochloroform or carbon tetrachloride vapours by poly(methyl methacrylate) or polystyrene bars] revealed 56 a Fickian diffusion in these systems. This conclusion is based on the assumption of proportionality of the local concentration of the mobile polymer to the local concentration of the adsorbed solvent. However, the T1 profiles showed that the T1 time of the polymer varies along the length of the sample even after establishment of homogeneous swelling after long-term impregnation with the liquid. This was interpreted as a conse- quence of the polymer mobility gradient.Water diffusion is of great importance for food polymers since the presence of water affects the quality and the shelf life of food products. Studies of amylose pellets prepared from corn starch revealed 57 a Fickian diffusion of water in the sample exposed to water vapours and a Case II diffusion of water in the polymer sample contacting liquid water; in addition, polymer swelling in the latter case occurred much faster than in the former. The signal amplitudes were converted into actual water content using an empirical calibration relationship.Applications of NMR tomography to mass transfer studies The 2D NMR images obtained in the course of swelling of bituminous coal in deuterated solvents (pyridine, acetone) exhib- ited 58 a gradual increase in the concentration of proton-contain- ing substances in solution, which was attributed to the removal of loosely bound solvent molecules from coal.The NMR signal of protons inside the coal sample was detected only after 48 h monitoring, thus reflecting the appearance of mobile proton- containing components. A sample of another type of coal exhib- ited conventional swelling; however, only the NMR signal of the surrounding liquid was observed.58 In a number of studies theNMRimages of polymers and coals partially saturated with a certain solvent were presented `as is' without reporting the swelling dynamics data. Rod-like samples of bisphenol ± acetone epoxy resin reinforced with a glass fibre swell in water at 93 8C in different manners depending on the character of the curing agent used in the sample preparation.59 The differ- ence is first of all due to the character of the microcracking of the sample.The distribution of water in the samples remains inho- mogeneous even after swelling over a period of 90 days. The surface swelling occurs mainly in the vicinity of the glass fibre bundles.59 The 2D images of the signal intensity and T2 time distributions obtained for a rod-like sample of polybutadiene rubber completely saturated with 1,4-dioxane 60 revealed inhomo- geneity of the polymer structure, which is due to, e.g., poor mixing of the components or nonuniform heating in the course of the preparation and vulcanisation of the polymer.Saturation of a sample of natural rubber with hot water allowed visualisation of pores connected to the outer surface while the NMR images of a sample of vulcanised rubber completely saturated with cyclo- hexane allowed one to find the regions characterised by increased degree of cross-linking of the macromolecular chains.61 In spite of very broad NMR signals (300 ± 800 Hz), the B1- imaging technique 1 was successfully employed to acquire the 2D images of cellulose exposed to acetic acid vapours for 48 h and polystyrene swollen in liquid n-pentane for 72 h.42 The images of coal samples swollen in acetone, DMSO or pyridine over a period of several hours allowed studies of the coal morphology and its changes caused by swelling.62 It was found that permeation of solvents into coal was essentially inhomogeneous.The presence and arrangement of microcracks, cavities, mineral inclusions and other coal structure peculiarities was found using pyridine and pyridine-d6.63 Studies of solvent desorption from a polymer are also of interest. If swelling of a polymer exhibits a Case II diffusion, one can expect a transition of the polymer from the rubbery state to the glassy state in the course of desorption, which must affect the efficiency of solvent transport. The dynamics of changes in the NMR images of a poly(methyl methacrylate) rod in cyclohex- ane-d12 at 30 8C (the sample was preliminarily kept in CH3OD at the same temperature) has been studied.64 During the first 60 min period the concentration of methanol in the surface layer of the sample substantially decreased and then varied only slightly over a period of 100 h.The results obtained suggested that the near- surface layer of the polymer undergoes a glass transition faster than other regions of the sample, which strongly hampers trans- port of the desorbing methanol. Studies of infinite swelling resulting in dissolution of polymers are of interest from the standpoint of, e.g., the development of controlled drug release systems based on dissolution of polymeric supports. For instance, water diffusion in a polyvinyl alcohol pellet (the molecular mass of the polymer lies between 35 740 and 133 000) causes a transition of the polymer to the rubbery state under normal conditions.65 The 1D water content profiles exhibit a steep liquid front, thus indicating a relaxation-controlled (Case II) transport. For the rubbery domain, the water distribu- tion profiles have a plateau and suggest a low diffusion resistance.As the molecular mass of the polymer increases, the diffusion rate decreases. Numerical simulation showed that the diffusion coef- ficient of water decreases with the distance from the pellet surface, since restricted mobility of the polymer chains in the vicinity of the 795 glassy core hampers transport of water. As time passes, the diffusion coefficient in the vicinity of the pellet surface contacting the solid surface increases while remains constant near the free surface contacting the solvent. A gel layer surrounding the pellet reduces the rates of dissolution and outward diffusion of drugs.66, 67 Swelling of a hydroxypropylmethylcellulose (HPMC) pellet in water was char- acterised 66 by gradual permeation of water into the pellet and by swelling-induced movement of the interface.The penetration depth of water is a function of the square root of time, t1/2. The T2 relaxation times of water were converted into the 1D polymer concentration profiles along the direction of swelling using the results of NMR calibration experiments. After swelling for 37 h the pellets became four-fold thicker. The regions with the HPMC content greater than 30% remained solid; reduction of the HPMS fraction to 10%± 30% was accompanied by gelation, while in the regions with the HPMC content less than 10% the polymer slowly dissolved and diffused into the bulk water phase.Osmosis-based substance release mechanisms were found to act for water in a silicon matrix containing NH4F.68 In the presence of salt the T1 and T2 times of water are shortened and the ability of the cylindrical polymer sample to absorb water is enhanced considerably. Water diffusion in the matrix was accom- panied by the formation of concentrated brine droplets that grew under the action of osmotic pressure. The process was monitored over a period of three months. The spatial distribution of water in a cylindrical sample containing 1% NH4F was more or less homogeneous during the observation time. At 5% NH4F, after swelling for seven days the content of water in the inner region of the sample was lower than in the outer region.After swelling for 35 days at the same concentration ofNH4F the content of water in the outer region of the sample decreased. This is due to the development of stress cracking in the matrix and to release of the content of brine `pools'. Cracking of the sample was initiated at the surface and gradually moved into the interior. Poly(hydroxyalkanoates) form a family of semicrystalline biopolymers. They are of interest because of biocompatibility and the ability to undergo biodegradation under the action of enzymes released by microorganisms. Enzymatic degradation of thin polymer films (0.2 mm) has been studied.69 Since the crystal- line domains do not contribute to the NMR signal, the signals observed in the experiments carried out at 50 ± 60 8C after wash- ing and drying the sample corresponded to the spin density in the amorphous domains.At the beginning of the process the content of amorphous polymer exponentially decreased and after some time became a linear function of time. In the deeper stages of the process enzymatic degradation of both the amorphous and the crystalline polymer occurred, which was confirmed by comparing the results of NMR imaging experiments and mass loss measure- ments. IV. Drying 1. General remarks Drying has found numerous technological applications. For instance, quality of cement and concrete articles depends on the water content in set and dried materials. Drying is used in film technologies (adhesives, binders, paints, coatings, etc.), synthesis of catalysts and treatment of food products and vegetable materials.Violation of the drying regimes is often accompanied by deformation and cracking of articles and coatings. Because of this, the development of methods for controlling the water content in the samples in the course of the drying process is of great importance. MRI allows one to obtain necessary information and real-time monitoring makes it reliable. The spin relaxation times of water in many porous materials are short due to the presence of paramagnetic impurities, reduc- tion of molecular mobility owing to bindingH2Omolecules and to the interactions of water molecules with the developed surface of796 solids. This situation occurs, in particular, in the studies of cement and concrete. For instance, the relaxation times of hydrated white Portland cement are as follows: T2=320 ms, T 2 =170 ms and T1=1.2 ms.70 Therefore, quantitative studies require the use of solid-state NMR techniques.71 ± 77 For instance, the use of strong gradients of the stray fields of a superconducting magnet in the STRAFI technique 1 allows the recording of the proton signals even for the hydrated and gel phases during hydration of cement.78 The single-point imaging (SPI) technique is also prom- ising,1, 70 though these methods do not allow the imaging of the whole amount of water in hydrates 78, 79 [this water has a T 2 time of 20 ms (Ref.79)]. Samples tens or hundreds of micrometres thick are studied using the GARField device, which produces a homo- geneous magnetic field in the plane of the sample and a strong static gradient in an orthogonal direction.80, 81 The advantage of the design is that the sample is placed on the surface of the probe rather than inside it.It should be noted that the relaxation times are usually shortened as the water content decreases; therefore, the signal amplitude is not proportional to the actual water content in the sample. Often, the signal amplitudes are corrected using some calibrations obtained by independent methods of water content determination.82 This allows one to exclude, in particular, the influence of other factors on the amplitude of the NMR signal. For instance, large variations of the water content in the sample cause detuning of the resonant circuit and change the sensitivity of the NMR probe. 2. Drying of materials and films As mentioned above, MRI allows one to monitor the processes associated with the presence of water in cement, mortar and concrete.Hydration of cement during the first 48 h after the preparation of the cement paste has been studied by periodically recording the 1D water content profiles along the axis of the sample placed in a glass ampoule.78 The experiments showed that water displaced from the bulk of the sample is present above the cement surface and the linear dimensions of the sample are reduced owing to shrinkage. The appearance of hydrates caused appreciable shortening of the T2 time of a fraction of the H2O molecules; therefore, even with an echo time of 30 ms the signal amplitude is attenuated and does not correspond to the actual content of water.Gypsum and lime additives cause a decrease in the degree of shrinkage of the samples and a faster attenuation of the signal amplitude. The drying of cement 70, 79, 83 and concrete 79, 83, 84 after long- term moist curing has been studied. The samples were of different compositions and had different initial water content, the drying time being also different. During the first 24 h of drying, the water content in the sample decreased appreciably in the region near the open surface and to a lesser extent along the full length of the sample up to its closed end (due to capillary flow).70 Capillary transport plays a more important role in the samples with high initial water content, since in this case the amount of hydrate formed is insufficient for filling the volume that was initially filled with water.83 As the water-to-cement mass ratio decreased to 0.3 ± 0.4, almost the whole amount of water was involved in the hydrate formation.For longer moist curing periods the porosity and permeability of the samples decreased, thus substantially reducing the efficiency of capillary transport of water in the course of drying.79, 83 In the deeper stages of the drying process, capillary transport of water changed to diffusion transport, which was accompanied by the movement of the drying front into the interior of the sample.83, 84 Compared to cement, the loss of water in concrete occurred faster in the early stages of the drying process, while in deeper stages the loss of water occurred more slowly. The presence of large aggregate particles in concrete prevented the material from cracking under the action of the surface tension forces due to the presence of the residual water in the pores of size 2.5 to 50 nm.I V Koptyug, R Z Sagdeev Interpretation and quantitative analysis of the data on the drying of cement and concrete first of all require detailed analysis of the relaxation behaviour of water in order to elucidate the correspondence between the NMR data and the actual water content in the sample. Probably, sometimes the signal amplitude reflects the distribution of only that fraction of water, which can be removed by drying,79 and does not reflect the water in the gel pores.84 For concrete samples moist cured for a long time (28 and 90 days), variation of the amplitude of the MRI profile was proportional to the change in the water content, whereas for the samples moist cured for no longer than 24 h this proportionality was observed only after drying over a period of a week.79 This is probably due to the presence of at least three types of water molecules characterised by different (and variable) T1 and T2 times and, hence, by different (and variable) relative contribu- tions.At the same time, in some instances the T2 time is independent of the water content, i.e., the amplitude of the MRI profile is proportional to the water content.70 When the T1 times are analysed using the stretched exponent t (1) Mz(t)=M0 a 1 ¡ 2 exp ¡ T1

ISSN:0036-021X    

出版商:RSC    

年代:2002

数据来源: RSC

摘要:

Russian Chemical Reviews 71 (10) 837 ± 851 (2002) Electrocatalysis on polymer-modified electrodes B I Podlovchenko, V N Andreev Contents I. Introduction II. Modification of electrode surfaces by polymers III. Electrochemical reactions on polymer-modified electrodes IV. Electrodes coated with polymeric films with incorporated metal-catalyst particles V. Conclusion Abstract. onto films polymeric applying for procedures The The procedures for applying polymeric films onto conductive supports and doping them with active microparticles conductive supports and doping them with active microparticles of metal-catalysts or functional redox groups are considered. of metal-catalysts or functional redox groups are considered. General principles of the electrocatalytic action of these systems General principles of the electrocatalytic action of these systems are discussed.Experimental results on the electrocatalysis on are discussed. Experimental results on the electrocatalysis on electrodes modified by polymeric films are analysed. Attention is electrodes modified by polymeric films are analysed. Attention is drawn to systems involving particles of platinum-group metals drawn to systems involving particles of platinum-group metals incorporated into polymeric matrices. The properties of electrodes incorporated into polymeric matrices. The properties of electrodes modified by ion-conducting and electron-conducting polymers modified by ion-conducting and electron-conducting polymers are compared. Certain problems to be solved in future studies of are compared.Certain problems to be solved in future studies of the electrode systems under discussion are formulated. The the electrode systems under discussion are formulated. The bibliography includes 265 references bibliography includes 265 references. I. Introduction The development of new types of electrode materials pertains to important problems of electrochemistry. The electrodes the sur- faces of which have been formed with involvement of polymeric films are the promising subject of studies aimed at solving this problem.{ The attention of scientists to these electrode materials is determined by the fact that their adsorption, electrochemical, photoelectrochemical, corrosion and electrocatalytic properties can be changed by varying their surface composition and struc- ture.The progress in solving this problem requires the knowledge of the structure and electrochemical properties of the support ± modifying layer system and an insight into the mechanisms of the processes in this system. Polymers used in electrochemical systems may be divided into two groups, viz., ion-exchange (without electron conduction) and electron-conducting polymers. The polymeric films with solely ionic conduction applied onto conductive supports should be, as a rule, functionalised by redox groups to find catalytic (electro- catalytic) applications (this is the reason for the frequently used term `redox polymers'). The latter act as mediators, so that the B I Podlovchenko Department of Chemistry, M V Lomonosov Moscow State University, Leninskie Gory, 119992 Moscow, Russian Federation.Fax (7-095) 939 01 71. Tel. (7-095) 939 40 27. E-mail: podlov@elch.chem.msu.ru V N Andreev A N Frumkin Institute of Electrochemistry, Russian Academy of Sciences, Leninsky prosp. 31, 119991 Moscow, Russian Federation. Fax (7-095) 952 08 46. Tel. (7-095) 955 46 97 Received 24 July 2002 Uspekhi Khimii 71 (10) 950 ± 966 (2002); translated by T Ya Safonova #2002 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2002v071n10ABEH000672 837 838 841 843 847 polymer ± support system can accelerate certain electrode proc- esses. The structures of electron-conducting organic polymers are characterised by the presence of conjugated bonds between electroactive groups in the chains, which is the reason for their high electron conductance in a certain potential range.The possibilities of the mediator catalysis by electron-conducting polymers can substantially be widened by additional introduction in them of functional groups through chemical bonding or Coulomb interaction between multicharge ions sorbed from a solution and charges of the polymeric matrix. The use of polymeric films as the matrices for classical electrocatalysts, viz., platinum-group metals, opens up wide prospects. The development of such systems increases substan- tially the number of electrode processes where the use of polymers can be very effective. Particularly, this will allow the use of M± polymer ± support systems (M is the metal-catalyst) in the electrooxidation of hydrogen and simple organic compounds, electroreduction of oxygen, etc., which attracts the keenest attention in the context of the fuel cell problem.It is those chemical reactions, which are accompanied by strong adsorption (chemisorption) of initial and/or intermediate species, that are frequently called `electrocatalytic' in the electrochemical litera- ture.1, 2 At the same time, in many studies, the term `electro- catalysis' is used in a more broad sense, embracing acceleration of electrode reactions and/or enhancement of their selectivity due to the mediator catalysis. This review surveys the current directions of research in the field of using polymers for fabricating highly active electrodes.The main advances and problems in this field of electrochemistry are considered. Despite the abundance of reviews on electro- chemistry of polymers (e.g., see Refs 3 ± 16), quite a limited number of publications generalising the results on electrocatalysis on polymer-coated electrodes are available: certain reports com- prehensively discussed the problems of synthesis, structure and physicochemical and electrocatalytic properties of a single poly- mer or a group of polymers similar in composition (e.g., Refs 9, 11, 12), other reviews delving deeply into the theory of electro- chemistry of polymers and the principles of their use for electro- catalysis merely listed the electrode systems and electrocatalytic processes studied (e.g., see Refs 6, 13, 16).{ In the scientific literature, these electrode systems are shortly called the polymer-modified electrodes, because it is by the polymeric coatings that their electrochemical behaviour is often determined virtually totally. It is not without purpose that along with the term `polyaniline-modified electrodes', the term `polyaniline electrode' is frequently used.838 This review cannot also pretend to comprehensively cover all the problems on the subject and give an exhaustive bibliography, otherwise this would contain over thousand references. The questions concerned with the main procedures of the preparation of electrodes with polymeric coatings, the principles of using them for the catalysis of electrochemical processes, the specific and/or general features of the behaviour of systems with ion-exchange and electron-conducting coatings, the main developments and problems in the promotion of a number of important electrode reactions are considered.Among the latter reactions, preference is given to the processes occurring in aqueous solutions and vital for the development of fuel cells and electrosynthesis. Nafion and poly(vinylpyridine) are chosen as the typical representatives of ion-exchange polymers, and polyaniline and polypyrrole repre- sent the electron-conducting polymers. Attention is drawn to the systems with electrocatalytically active surfaces formed by incor- poration of microparticles of noble metals into polymeric matri- ces.An attempt is undertaken to analyse the reasons for such a wide scatter and discrepancy in the experimental results on the electrocatalytic activity of these systems. II. Modification of electrode surfaces by polymers Of the various methods for applying polymeric coatings onto conductive supports, here we consider only those used most frequently in fundamental studies for fabricating the support ± polymeric matrix systems. As a rule, films of ion-exchange polymers are `attached' to the surface by chemical bonding or through adsorption interaction.17 ± 32 Both chemical and electro- chemical methods are used for synthesising conducting polymeric films on the surfaces of solid supports.3, 7, 9, 14, 33 Solubility of the polymer is the important factor determining the adhesion of the film.1. Methods of application of modifying polymeric coatings a. The immersion method The immersion method is based on the exposure of an electrode (support) to a polymer solution for a certain time sufficient for the adsorption of molecules and the formation of a film on the surface.17 ± 21 Due to chemisorption and low solubility of coatings formed in the solutions used, the films obtained by this method well adhere to the surface. The formation of coatings can proceed as a result of chemical reactions of the polycondensation type. A typical example is the formation of polymeric layers from organosilanes X3SiR (X=Cl, OCH3, etc., R is the substituent involving a certain functional group) on oxidised surfaces.14, 20, 21 The formation of molecular networks occurs due to a sequence of reactions presented by the following scheme: X X Z OH+X Si R Z O Si X+HX, R X X X Z O Si OH+HX, Z O Si X+H2O R R X X Si R Z O Si OH+XX R X X R+HX, Si Z O Si O X R where Z is the supporting surface.B I Podlovchenko, V N Andreev b. `Drop evaporation' The method of drop evaporation was used for modifying elect- rodes of small areas. Dropwise application of several microlitres of a polymer solution of a given concentration followed by evaporation of the solvent allowed one to obtain films well characterised with respect to both the amount of the polymer and its composition.22 ± 25 The structural uniformity of the films depended to the greater extent on the evaporation procedure, namely, the latter should occur sufficiently slowly.The composi- tion of the original film (including its functional groups) can be changed by treating it with the corresponding reagent using the same method of `drop evaporation'. Thus the stability of a poly(ethylene imine) film in acidic solutions is increased upon treatment with a solution of dibromobutane in propan-2-ol, which resulted in cross-linking of the polymer 26 ± 28 and a decrease in its solubility in aqueous solutions with low pH. c. Oxidative and reductive deposition Solubilities of certain compounds in their oxidised and reduced forms are different. This can be used for the deposition of modifying (including polymeric) films on electrodes.29 ± 32 For example, it was found 29, 30 that viologens with more than four carbon atoms in the alkyl substituent and certain polymeric viologens form stable films of radical cations upon their chemical or electrochemical reduction.The same principle was also applied for the deposition of poly(vinylferrocene) films.31, 32 The methods discussed of deposition of polymeric coatings and their versions were used for the deposition of redox polymers and, in certain cases, for modifying electrodes by electron-con- ducting polymers. However, the common practice for the prepa- ration of electron-conducting polymeric films is electrochemical polymerisation. 2. Redox polymers as the mediators of redox processes Ion-conducting polymeric films with incorporated redox groups were used as the charge carriers from the substrates in a solution to the support.7, 10, 14, 17 ± 20 If the substrate is oxidised or reduced faster on electrodes modified by redox polymers as compared with unmodified electrodes (support), one can speak of the catalytic effect of the film.A scheme in Fig. 1 illustrates the process of charge transfer in the reduction of species A to species B (A+e7?B) through redox centres (Red/Ox) in the polymeric film.In the general case, the reduction rate is determined by the rates of the following processes: diffusion of species A from the solution bulk to the film surface, the reaction B+Ox A+Red in the film surface layer, the charge transfer in the film and the reaction Red.Ox+e7 6 8 6 Aspecific and the most important process in the systems of the type discussed above is the charge transport in the film, which is characterised by the formal diffusion coefficient Dt. Despite the absence of the self-conduction of the polymer, the charge transfer through the film is possible by either physical diffusion of Red and/or Ox in the film or the step-by-step electron exchange between neighbouring redox centres (see Fig. 1).34 ± 41 These processes are accompanied by the motion of polymeric chains, the solvent and counter-ions. The redox centres frequently repre- sented polycharge ions [Fe(CN)3¡=4¡, Mo(CN)3¡=4¡, IrCl2¡=3¡, etc.] which are strongly bound to ion-exchange polymers due to the Coulomb interaction.Figure 2 exemplifies the voltammograms of a smooth plati- num electrode and the electrode modified by a poly(ethyleneElectrocatalysis on polymer-modified electrodes FilmRed Ox Red Ox Dt Figure 1. Ascheme of reduction of a substance on the electrode modified by a redox-polymer film. DA is the diffusion coefficient of A particles in solution; cA is the concentration of A in the solution bulk; Dt is the formal coefficient of charge diffusion in the polymeric film. imine) film.{ It is seen that before the introduction of a redox group into the ion-exchange polymer (curves 1 ), the system was characterised by a very low polarisation capacity I/u (lower than that of the support, i.e., smooth platinum).With the accumulation of Fe(CN)637/47 ions in the film, the system acquires the gradually increasing redox activity due to the following reaction: Fe(CN)4¡ 6 . Fe(CN)3¡ 6 +e7 I /mA 3 1.0 2 1 0 0.5 1.0 1 2 71.0 3 Figure 2. Cyclic voltammograms of a platinum electrode modified by a poly(ethylene imine) film. Measured in solutions: (1) 0.5M H2SO4; (2) 0.2mM K3Fe(CN)6+0.5M H2SO4 every 5 min (2) and 60 min (3) after the immersion of the electrode into solution. The potential scan rate u=50 mV s71. 3. Electrochemical polymerisation Synthesis of polymeric films by electrochemical polymerisation shows a number of advantages over the method of chemical polymerisation. First, the films are formed directly on the elec- trode surface, and the reaction product, viz., an electroactive film, exhibits a high conductivity. Second, the electrosynthesis provides a good current efficiency and a strict stoichiometry of the process; hence, the films of desired masses and thicknesses can be obtained.Third, the properties of the polymeric film can be controlled in the course of the synthesis. Possible electrochemical and chemical { Hereinafter, the potential values E are shown with respect to the reversible hydrogen electrode in the same solution, unless otherwise specified. Electrode (e7) Diffusion layer Solution B Ox cA Red A DA E /V 839 reactions occurring during the syntheses of polyaniline 3, 4, 42, 43 and polypyrrole 11, 12, 43 ± 46 were described in detail in the liter- ature. Electrochemical properties of the support ± electron-conduct- ing polymer system depend strongly on the conditions of the polymer synthesis.Below, we consider the effects of some of these conditions.3 ± 9, 11, 42, 43, 47, 48 a. The effect of the support material In the synthesis of electron-conducting polymers, different mate- rials were used as the supports, viz., platinum,49 ± 53 gold,53 ± 55 metal oxides,49, 55, 56 glassy carbon,51, 52 pyrographite,53 stainless steel 49, 57 and iron 58 ± 61 for polyaniline; noble metals,62 conduc- tive glass,43, 62 carbon materials,11, 63 aluminium, tantalum and copper,62 ± 66 steels 63 ± 67 and semiconductors 68 for polypyrrole. From these studies, it can be concluded that the nature of the support material had virtually no effect on the synthesis and properties of conductive polymeric films with the thicknesses exceeding tens of monolayers.The exception is provided by the metals with surface oxide films.11, 45 On the other hand, the formation of first layers initiated by the formation of radical cations 3, 47 is determined largely by the nature and the structure of the support. However, in such systems the electrochemical effects caused, on the one hand, by the adsorption-induced surface modification and, on the other hand, directly by the polymeric film can hardly be separated. The process of film formation was shown to be `sensitive' to the solution composition (see below); hence, to obtain reprodu- cible results, it is desirable that the synthesis (not too lengthy) is carried out in a three-electrode cell with separate anodic and cathodic compartments.b. The effect of the polarisation mode Films of conducting polymers used in electrocatalysis are mainly prepared by electrooxidation of monomers (the anodic synthesis). This synthesis always proceeds at a potential exceeding those of anodic doping of polymers. The anodic doping is the reversible oxidation of a polymer resulting in its transition from the state of an insulator or semiconductor into the conductive state.3 ± 11, 13, 47, 69 The electroneutrality of the oxidised polymer with positive charges on certain parts of its chain is achieved by incorporation of counterions, i.e., anions into the polymeric matrix (P) (P)xá n (A¡)x , (P)n +xA¡ ¡ xe¡ where n is the degree of polymerisation.The x/n ratio characterises the degree of doping of the polymer. Dedoping (cathodic reduc- tion) of an anodically doped polymer results in the elimination of charges in its chains and the dissociation of the anions to the solution. The doping ± dedoping processes are accompanied by I /mA 0.60 E /V 1.0 0.5 70.6 Figure 3. A cyclic voltammogram of a glassy carbon electrode modified by a polyaniline film.840 redox reactions, which determine the redox activity of electron- conducting polymers. The intrinsic redox acitivity of a polyaniline film (Fig. 3) is manifesting itself in the vicinity of two pairs of anodic ± cathodic peaks in the cyclic voltammogram (at *0.45 and *0.95 V).In the potential range between these peaks, the film exhibited a high electron conductivity (up to 105 S cm71). The films can be synthesised using different modes, viz., potentiostatic, galvanostatic, potentiodynamic and pulsed modes. These modes were used in the synthesis of electron- conducting films of polyaniline 43, 50, 70 ± 78 and polypyr- role.11, 70, 79 ± 82 An analysis of the experimental results obtained allowed the following conclusions to be drawn. The number of aniline molecules incorporated into the poly- meric film was roughly proportional to the charge passed through the system during the synthesis. The mass of the polymeric film on the electrode, as a rule, was proportional to the charge consumed in its doping.However, the slopes of the corresponding linear dependences vary depending on the synthetic mode. The results of measurements of these dependences for polyani- line have shown that, for a galvanostatic synthesis, the degree of doping decreases with a decrease in the current density. Under potentiostatic conditions, the maximum degree of doping was reached at the potential of 0.9 V. In the potential cycling mode, the dependence of the degree of doping on the potential scan rate also demonstrated a maximum.50 The following explanation to the regularities observed was proposed.50 At low current densities, the film growth involves a small number of nuclei, which can result in a polymeric structure with a low redox activity.Probably this is the reason for the decrease in the redox activity observed for the films synthesised at the potentials below 0.9 V. As the potential increased above 0.9 V, the redox activity of films decreased probably due to the degra- dation processes } in the polymeric films.51, 83 ± 85 The decrease in the redox activity of polyaniline at high potential scan rates was probably due to either the finite rate of recombination of radical cations or the adsorption of the monomer on the polymer.47 Syntheses under galvanostatic (the current density> 0.1 mA cm72) and potentiostatic (the potential>1.0 V) condi- tions were accompanied by noticeable exfoliation of polymeric films formed.Of different deposition modes used, the cyclic potential scanning allowed polyaniline of the maximum redox activity to be obtained.50, 51 Films synthesised under such conditions also exhibited better adhesion to the electrode. The foregoing mainly concerns the films with thicknesses of*300 ± 400 nm. c. The effect of solution composition } Physicochemical properties of polypyrrole 43, 46, 80, 81, 86, 87 and polyaniline 88 ± 114 films were shown to depend on the solution composition. This can be illustrated by several examples. The polymerisation rate of aniline depends on the nature of an acid (supporting electrolyte) in the order HCl<HClO4< HBF4<HF<H3PO4<H2SO4.47 The effect of organic additives, benzene and its derivatives serving as examples,99 ± 101 is defined by the character of their interactions with the polymer during the film growth, viz., on the possibilities of formation of chemical bonds with elements of the polymer chain and of co-polymerisation.Thus in the presence of phenol, the synthesis of polyaniline decelerated and, for a certain phenol/aniline concentration ratio (1 : 2) in solution, the synthesis stopped. The presence of phenol in the deposition solution enhanced by 15% the susceptibility of polyaniline to doping and } Irreversible redox processes which occur during potential cycling. } This review considers the systems using water as the solvent. Hence, the term `solution composition' means the monomer concentration and concentrations and compositions of the supporting electrolyte and differ- ent additives.B I Podlovchenko, V N Andreev prevented its degradation. The presence of phthalic acid in solution increased the porosity of the polymeric film, which was accompanied, on the one hand, by the increase in its susceptibility to doping and, on the other hand, a decrease in its stability towards degradation. The additions of salicylic acid had the opposite effect. According to the results of other studies,50, 51 no polymeric chains were formed in a two-dimensional adsorption layer at the polymerisation potential. For a polymer to form, its monomer should be present in solution in a certain concentration. For example, for polyaniline, this concentration was 51074 mol litre71 (see Refs 75, 76).Probably, the monomer can be adsorbed on the polymer and get incorporated into it under certain conditions. of d. Stability of film characteristics Keen attention was drawn to the problem of stability of electro- chemical parameters modified electro- des.70, 72, 73, 84, 85, 94, 95, 99 ± 101, 115 For acidic media, cyclic voltammograms measured on films synthesised on supports by the electrochemical polymerisation method were shown to be stable only in a certain potential range. The extension of the potential cycling range into anodic and cathodic directions entailed a decrease in the area under the peak of polymer doping in the voltammogram and caused irreversible changes in certain characteristics of the polymer, e.g., its con- ductivity.The limits of the potential range where a film exhibited stable electrochemical properties depended of many factors, e.g., on the electrolyte composition. These changes in the film characteristics were caused by the degradation of polymers. According to the features of the changes occurring in the polymeric film, two main types of degradation could be singled out.96, 97 For one of them, the specific doping ability of the polymer tended to decrease, whereas its mass remained constant; for the other, both the specific doping ability and the polymer mass decreased. Such processes of severe degradation resulting in disintegration of the film were also observed in the course of the polymer synthesis. For high potentials of the synthesis, the growth of the film bulk was accompanied by its exfoliation.The radioactive tracer technique proved to be effective for studying polymer degradation.94, 96, 115 Using this method, it became possible to confirm the existence of the degradation types mentioned above and show that different electrochemical synthetic modes produced polyaniline films with different stabil- ities towards oxidative degradation. The differential radioactive tracer method has shown that the processes of oxidative degradation of polyaniline films with thicknesses <500 nm involve the whole mass of the polymer on the electrode. At the potentials of 0.7 ± 0.75 V in 0.5MH2SO4, the third degradation type named `mild degradation' was observed.97 It was shown that under these conditions, the polymer mass and its susceptibility to doping (the area under doping peaks) remained constant, chemical bonds of certain polymer molecules with the surface were broken with retention of the electrochemical bonding due to the interaction between polymeric chains.e. Composite polymeric films Composite materials containing nonconducting and ion-conduct- ing polymers along with electron-conducting ones were proposed to improve unsatisfactory mechanical characteristics of conduct- ing polymeric films (polyaniline, polypyrrole). Many studies are aimed at the development of such composite materi- als.43, 105, 112, 116 ± 129 These were synthesised in one or two steps. In one-step method, the electrode is immersed into a solution containing a matrix polymer and the monomer of a conducting polymer, and the electrosynthesis is carried out in this solution.Alternatively, a layer of an inert `matrix' polymer which usually represents a nonconducting (ion-conducting) polymer with good mechanical properties, e.g., Nafion, is formed in the first step. InElectrocatalysis on polymer-modified electrodes the second step, the electrode covered with the matrix polymer layer is immersed into a monomer solution, and the conducting polymer is synthesised. In the course of electropolymerisation, particles of the matrix polymer become incorporated into the conducting polymer. For the details of the syntheses of composite materials see Ref. 43. It should be borne in mind that, in many cases, the formation of a composite implies the appearance of new qualities rather than simple `summation' of the properties of its components.An illustration to the issues mentioned above is provided by the results obtained using radioactive-tracer and electrochemical methods for the formation of polyaniline ± Nafion composite films on platinum and glassy carbon.128, 129 Particularly, for Nafion-covered electrodes, it was shown that aniline is first accumulated in the porous matrix polymer and only later trans- formed into polyaniline (the template synthesis 130, 131). The degradation of such a film proceeds more slowly; hence, a substantially smaller amount of polyaniline fragments is removed from the electrode.This can be associated with the complex and branched structure of the polyaniline formed in the porous Nafion matrix. III. Electrochemical reactions on polymer- modified electrodes 1. Films of nonconducting polymers Ion-exchange polymeric films with incorporated redox systems are promising for electrochemistry of organic substances. Their use often made it possible not only to decrease the electrolysis overvoltage but also to enhance the selectivity of the main electrode reaction.132, 133 Thus manganese dioxide is known to selectively oxidise allyl alcohol to the corresponding aldehyde leaving the double bond intact. It was shown that the introduction of theMn2+/MnO2 pair into the Nafion film provides the efficient electrolysis of cinnamyl alcohol to b-phenylacrolein.134 The current efficiency in aldehyde increased from *70% to *82% with the changeover from the Pt ± Nafion system to the Mn, Pt ± Nafion one.3 Figure 4 shows voltammograms of a glassy carbon electrode in an aqueous cysteine solution in the absence and in the presence of the Nafion ± Os(bipy)2á=3á (bipy is bipyridyl) film on the surface. As can be seen, the diffusion current of cysteine electro- oxidation reached its limiting value in the second case.135 Dimethylferrocene immobilised together with glucose oxidase in an ion-exchange film on a graphite support was used as the charge-carrier from the reduced enzyme (obtained as a result of glucose oxidation) to the conducting support.136 In this system too, the limiting diffusion currents of glucose oxidation were reached, which allowed an amperometric sensor for glucose to be developed.The oxidation of Fe2+ cations to Fe3+ carried out on a cross- linked poly(4-vinylpyridine) (PVP) and PVP-poly(a-lysine) films containing the IrCl3¡ 6 /IrCl26 ¡ pair 137, 138 can be cited as the E /V (Ag/AgCl) 0 0.85 1 2 mA 2 Figure 4. Voltammograms of cysteine oxidation on a rotating disk electrode of glassy carbon (1) in the absence and (2) in the presence on the surface of a Nafion film with the Os(bipy)2+/3+ redox system;135 u=1 mV s71; cysteine concentration is 1.28 mol litre71, pH 4. 841 example of the mediator catalysis for inorganic reactions. These studies have also theoretically discussed the possibility of estimat- ing the contributions by the mass transfer, the kinetics of interactions of particles and the charge transport in the films into the general mechanism of the electrode process involving a mediator.2. Electron-conducting polymeric coatings To date, a considerable number of electrochemical reactions which occur on electrodes with electron-conducting films were studied. However, the occurence or the absence of the catalytic (electrocatalytic) effect for a particular process was debated more often than in the case of ion-exchange polymers. Apparently, this is largely caused by different conditions of film preparation and different compositions of supporting electrolytes used, because, as was shown above (see Section II.3), these factors strongly affect the physicochemical and electrochemical properties of electron- conducting coatings.The ambiguous interpretation of these results was due to the lack of a theory of electrocatalytic effects by such coatings, which would encompass the whole diversity of the occurring processes.6, 10 A typical electrocatalytic reaction studied on electrodes modi- fied by conducting polymers is the oxygen electroreduction. Studies of this process on a gold electrode covered with a polypyrrole film have established the promoting effect by the polymer.139, 140 Using the method of the rotating ring ± disc electrode, the authors of these studies have shown that this effect is due to an increase in the selectivity to the oxygen electro- reduction to give water.It was also concluded 139, 140 that polypyrrole films are per- meable to oxygen and the reaction mainly proceeds at the metal/ polymer interface. In another study,141 no promoting effect was observed for polypyrrole doped with perchlorate anions. The promoting effect of polyaniline films on the oxygen electroreduction on glassy carbon electrodes in dilute acidic solutions was observed.142, 143 It was noted that in the course of this process the electrochemical characteristics of polyaniline remained stable for a long time, i.e., the polymer did not degrade. Studies of oxygen reduction on polyaniline and Nafion films have revealed the promoting effects of polymers on the electro- chemical reaction.124 The problems of organic reactions on electrodes modified by polymeric films were discussed.144 ± 152 Particularly, the electro- oxidation of formic acid on polymer-modified platinum and gold electrodes was described in most detail.144 ± 147, 152 The promoting effect of a platinum-supported polyaniline film on this process in the potential range below 0.5 V was found.146 Presumably, the polymer prevented accumulation of the products of strong adsorption of formic acid on the electrode, which thus increases the number of active sites for the oxidation reaction.By combining radioactive-tracer and electrochemical meth- ods, the promoting effect of polyaniline films on the steady-state oxidation of formic acid on platinum and gold electrodes was also confirmed.144, 145 It was shown experimentally that the amount of products of strongly adsorbed formic acid on a polyaniline-film modified electrode was higher as compared with unmodified electrodes (Fig.5). This result qualitatively agrees with the con- clusions made earlier.146 However, it was found 144, 145 that an eightfold increase in the polymer-film thickness did not change the amount of strong-adsorption products accumulated on the elec- trode, which contradicts the conclusions drawn in Ref. 146. At the same time, the authors of these studies 144 ± 146 have agreed on the location of the oxidation process, viz., that the reaction occurs on platinum areas unoccupied by the polymeric film. This conclusion was confirmed experimentally.144, 145 It was shown that if the polyaniline-free platinum areas are filled with CO adsorption products, then virtually no HCOOH oxidation occurs.The promoting effect was explained 144, 145 by assuming that polymers catalyse the oxidation of weakly bound adsorption products of formic acid just as it was shown in earlier studies842 1010 G /mol cm72 420 60 Figure 5. Kinetics of HCOOH adsorption at a potential of 0.2 V from a 1072M HCOOH+0.5M H2SO4 solution on (1) platinum electrode and (2) platinum electrode modified by a polyaniline film. Measurements were carried out by a radioactive tracer method.144 (e.g., see Ref. 153) dealing with oxidation of organic substances on platinum modified by thallium adatoms.The electrooxidation of methanol on polyaniline-modified platinum was also shown to be accompanied by a substantial decrease in the amount of products of strongly adsorbed methanol on the electrode; however, in this case, the polymer inhibited the electrooxidation.154 Thus, as would be expected, the nature of the polymer effect (promotion, inhibition) on the oxidation process depends on the nature of the organic substance to be oxidised. Polyaniline films were shown 149 to catalyse the electrooxida- tion of hydroquinone in a wide potential range. This effect was also confirmed later.155 ± 162 Figure 6 shows polarisation curves of electrooxidation of hydroquinone and reduction of benzoquinone on a film-free gold electrode and that covered with a polyaniline film.The half-wave potentials of these processes were shown to shift in the cathodic direction for the hydroquinone oxidation and in the anodic direction for the benzoquinone reduction. It was found 161 that the exchange current of hydroquinone oxidation increased by two orders of magnitude on freshly prepared polyani- line films (supported on platinum). Most scientists associated the 100 mA 3 0 Figure 6. Cyclic voltammogram of (2) Au and (1, 3) Au ± polyaniline system in (3) 0.5M H2SO4 and (1, 2) p-benzoquinone solution (1072 mol litre71) in 0.5M H2SO4.159 Potential scan rate is 50 mV s71. 1 2120 t /min 1 2 600 E /mV (Ag/AgCl) B I Podlovchenko, V N Andreev catalytic effect with the adsorption interaction between the poly- aniline film and these compounds.159 ± 161 Some convincing evidence (based on experimental results) for the specific adsorp- tion of hydroquinone on the emeraldine form of polyaniline has been obtained.162 This case allows us to speak of `classical' electrocatalysis directly by the conducting polymer.The effect of the polyaniline film morphology on the rate of hydroquinone oxidation was assumed 162 to be associated with the changes in the film surface areas involved in the adsorption and oxidation of the substrate. Alkyl derivatives of polyaniline behaved like polyani- line in the benzoquinone ± hydroquinone reaction.163 ± 166 The differences in the kinetics of the hydroquinone ± benzoquinone reaction on polyaniline and poly(ortho-phenylenediamine) films were explained 159 by the different distribution of equilibrium potentials at the polyaniline/solution interfaces. The processes discussed above occurred in the potential range where the film was the electron conductor.In earlier studies (e.g., see Ref. 167), it was assumed that in the `nonconduction' potential range, electrode reactions should be entirely inhibited. However, this assumption turned out to be incorrect. Thus the reactions of hydrogen oxidation and oxygen reduction were shown to proceed (even if at much slower rates) in the potential region of `non- conducting' films as well.168 These processes were found to be strongly affected by the support nature. On nonconducting polyaniline films, the processes of reduction of oxygen 169 and naphthoquinonesulfonate 163 were observed to occur.To explain the occurrence of these processes, their rate-dtermining steps were assumed to be located at the metal/solution (due to porous structures of the films) and/or metal/polymer (for the mediator catalysis) interfaces 168 and the oxidant was supposed to inject positive charges into the polymeric matrix.6, 163 Moreover, it should be borne in mind that sufficiently thin films (4500 nm) across which the potential drop is small were used most often,170 and that the relaxation from the conducting state to the non- conducting one is sufficiently slow.171 As is seen from the above examples, to date there is no agreement on the reasons for the appearance of catalytic effects in the systems containing electron-conducting films.6, 172 Some authors favoured the mediator mechanism, the other (without any strong evidence) assumed the potential change in such systems to be associated only with the potential drop across the polymer/ electrolyte interface.An analysis of thermodynamic equilibria in the metal ± polymer ± electrolyte system at different interfaces has shown 172 that in the general case the potential drops across all the interfaces involved should be taken into account. For a more accurate elucidation of the role the polymer plays, additional information is required, particularly, on the exact location of the reaction and its rate-determining step. However, it is very difficult to obtain this information for electrodes with polymeric coatings.Considerable progress has been achieved in modifying elec- tron-conducting polymers by different redox groups and catalytic centres such as porphyrins, phthalocyanines, transition-metal complexes, biological molecules, etc.6, 169, 173 ± 182 In most of such systems, the mechanism of mediator catalysis was realised. Thus it was found 152 that electroreduction of oxygen on glassy carbon can be accelerated by covering it with a polyaniline film containing anions of heteropolyacids (SiW12O40)47 and (PW12O40)37. The possibility of using polythiophene and polypyrrole films modified by heteropolyacid anions for catalysing this process was also considered.183 ± 187 On a polyaniline+cobalt tetra-2,3-pyridine- porphyrin (CoTPP) composite film, the oxygen reduction poten- tial shifted to more positive values as compared with its reduction potential on the electrosorbed CoTPP.169 It is of note that even an increase in the currents was observed in the transition of polyani- line into the reduced state.It was shown 186, 187 that modification of a glassy-carbon electrode surface by an [Os(bipy)(PVP)Cl]Cl coating [PVP is poly(vinylpyridine)] allowed the limiting diffusion currents of nitrite electroreduction to be reached. This makes this system promising for the development of a sensor for nitrite anions.Electrocatalysis on polymer-modified electrodes IV. Electrodes coated with polymeric films with incorporated metal-catalyst particles From the foregoing it follows that modifying polymeric films play the role of charge carriers in electrocatalysis, and their catalytic activities manifest themselves in the processes free from destruc- tion of substrate molecules.Platinum-group metals are the best electrocatalysts (catalysts) for the processes which involve cleav- age of interatomic bonds (e.g., oxidation of hydrogen, organic substances, etc.). It is possible to prepare highly dispersed deposits of these metals by incorporating them into polymeric matrices. A specific feature of the systems thus obtained is that the polymeric matrix determines the conditions for the metal phase development. The properties of the matrix (film) itself can be changed by choosing a particular polymer and varying the conditions of the surface film formation.Keen attention was drawn to the processes of deposition of platinum-group metals in ion-exchange polymeric matri- ces,188 ± 201 because the latter are stable in different media and can be used as the solid polymeric electrolytes in electrochemical reactors. It was proposed in many studies to prepare deposits of platinum-group metals using electron-conducting polymers where the formation conditions and the behaviour of metal particles can differ substantially from those in ion-exchange films.54, 154, 202 ± 222 This can result in a three-dimensional structure representing a conducting matrix with sufficiently uniformly distributed catalyst microparticles.The porous structure of such a matrix can make the metal-catalyst accessible for reactants and provide the exit of the products to solution. For these systems, the practically important effect of the increase in the electrocatalytic activity calculated either per the geometrical surface of the electrode or per a mass unit of the noble metal could be achieved mainly due to an increase in the surface area of the deposit. However, of prime scientific interest is to increase the activity per the surface unit of the metal catalyst as compared with the compact metal. This can be associated with the structural peculiarities of microdeposits (e.g., the size effect 1), the interaction of metal particles with the polymeric matrix and the changes in the reactant activities in the porous space of the matrix. In theoretical studies, great attention was drawn to the electronic structure of microdeposits.Due to the high work function values for metals of the platinum group 223 as compared with other materials, it could be expected that the electrons would be partially transferred from the matrix to the microdeposit and, as a consequence, the mean electron density in the deposit would increase. However, such an effect was detectable only where the sizes of microdeposit particles were commensurable with the thickness of the electrical double layer. According to the esti- mates,224, 225 even in the absence of their interaction with the matrix, the Fermi energy in small particles (the size d43 nm) should substantially differ from that in the compact metal. 1.Dispersion of particles of platinum-group metals in polymeric matrices Different chemical procedures were developed for introducing metal particles into polymeric matrices.194, 226 ± 231 For example, deposits of Pt, Pt+Ru, and Pt+Sn were obtained by reducing the corresponding salt solutions by reactants diffused from the other side of a membrane.226, 227 Membranes were often impreg- nated with solutions of platinum-group metal salts and then immersed into the reducing solution.228, 229 However, those methods which use the electrodeposition of metals are most convenient (especially, for films) for incorporat- ing metal particles into polymeric matrices.188 ± 193, 195 ± 222 By varying the potential, current and deposition time, one can control the accumulation rate and the amount of the metal deposited.Such methods can be divided into two groups, viz., with single- and two-step deposition. In the former case, the film is first formed (by a chemical or electrochemical method), and then the metal is electrodeposited in it (often on it). In the latter method, the film is formed electrochemically from a solution containing both a monomer capable of electropolymerisation and a salt of a metal- catalyst. By varying the electrodeposition conditions (supporting electrolyte concentration, components concentrations, modes of current and potential variations, etc.) it is possible to prepare films with diverse morphologies and various contents and depth- distributions of metal particles.Two-step deposition was used for both ion-exchange and electron-conducting films. In the films devoid of intrinsic electron conduction [e.g., Nafion and poly(vinylpyridine)], the formation of metal particles begins from the support, and their further growth occurs along the pores `from one particle to another' with retention of contacts between the particles (Fig. 7). Gradu- ally, the electrodeposition process reaches the film surface (this stage is not shown in the figure), which can occur before the whole porous space is filled, as a result of the difference in the rates of formation and growth of films in different pores. Later, the metal layers are deposited on the film surface, and this process is only indirectly controlled by the film.Evidently, the specific behaviour of metal particles incorporated into the polymeric matrix should be judged based on the analysis of the behaviour of systems in which the metal deposit, at least, its greatest part, has not yet `escaped' to the film surface. According to the literature data, as might be expected, the mechanism of deposit growth and the sizes of metal particles formed strongly depend on the modes of the support polarisation. During the formation of metal particles in electron-conduct- ing polymers, the metal-catalyst can be deposited directly onto the polymer. Though it was suggested 220 that during the deposition of metals on an electron-conducting polymer in the potential range where the polymer is dedoped and assumed 9, 13 to become `non- conducting', the mechanism of metal deposition in the films remains virtually the same as for ion-exchange polymers (see Fig.7 a). However, it was found 211, 222 that the deposition of a metal directly on the polymer can also occur in the potential range of its `nonconducting' state. Using the AFM method for the a Metal particles Glassy carbon b Metal particles Glassy carbon Figure 7. A scheme of accumulation of metal particles in a polymeric matrix formed on a glassy carbon support for (a) ion-exchange and (b) electron-conducting polymers. 843 Ion-exchange polymer Electron-conducting polymer844 palladium deposition in sufficiently thick polymeric matrices where the metal volume was an order of magnitude lower than that of the porous space (estimated from the results of Ref.232), palladium particles were observed on the film surface at the deposition potential of 0.14 V, i.e., in the range where polyaniline is nonconducting. The appearance of metal particles was explained by the `residual' conductivity which is retained after the polymer is dedoped 233 and ensures sufficiently small potential drops across thin (*500 nm) polymeric films. The results of layer-by-layer Auger-spectroscopic studies 211 of a polyaniline film { with palladium particles have shown that deposition of palladium preferentially occurs in the external regions of the film. Figure 7 b shows the model for deposition of metal particles into an electron-conducting film.In this case, the contact between metal particles is not necessary: the most pronounced manifes- tation of the catalytic activity can be expected at the potentials of high conductivity of the polymer (especially, for thick films). A single-step method of preparation of composite M± elec- tron-conducting polymer films was used less frequently. Potential cycling in the range from sufficiently high positive values at which the monomer is preferentially polymerised, to the potentials close to that of the reversible hydrogen electrode, at which the metal is preferentially deposited, made it possible to provide a largely uniform distribution of metal-catalyst particles throughout the film.211, 222 Moreover, the formation of substantial amounts of metal on the film was ruled out. However, the single-step synthesis by periodic application of sufficiently high potential values to the electrode led to noticeable degradation of the polymer even during the formation of the electrocatalytic system,220 and the degrada- tion became more pronounced in the presence of particles of platinum-group metals.222 The use of polymeric matrices (especially those of ion- exchange polymers) made it possible to obtain ultradispersed deposits of platinum-group metals (with the grain diameter of 3 ± 5 nm) with larger specific surface areas.As a rule, the most dispersed deposits were formed at small metal-to-polymer mass ratios, i.e., where the deposit fraction remained within the polymeric matrix is substantial.195, 200 For large amounts of deposited metals and/or small film thicknesses, the specific surfaces substantially decreased and, as would be expected, often approached those of individual metal depos- its.200, 201, 203, 219, 233 ± 235 At attempt was made to deposit platinum in preformed polyaniline films from aqueous H2PtCl6 solutions.154 The form in which platinum is accumulated in the film was concluded to be strongly dependent on the lower limit of the potential cycling interval, viz., in the 0.1 ± 0.7 V range, it was accumulated as two- and four-charged ions, whereas in the 0 ± 0.7 V interval, platinum metal particles prevailed. A comparison of the data shown in Table 1 allows one to assess the possibilities of increasing the catalyst surface area by using polymeric matrices. The table shows the maximum values of specific surface areas for several metals of the platinum group electrodeposited into Nafion and poly(vinylpyridine) (the results 196 ± 199, 220 were averaged) and similar values for electro- deposits of these metals on platinum.The greatest effect was observed for palladium. It is important that the polymeric matrix also favoured the stabilisation of particles. This made it possible, for example, to observe the size effect, viz., a sharp decrease in the amount of absorbed hydrogen, for palladium particles of sizes <7 nm [poly(vinylpyridine) matrix].198 The attempts to achieve such a great development of the metal-catalyst surface area for electron-conducting polymeric films have failed.219, 120, 222 This indirectly confirmed that the metal deposit can grow on the film surface even for small metal- to-polymer mass ratios.{ The film was deposited by the two-step method, and palladium was deposited in the potential range of `nonconducting' polyaniline. B I Podlovchenko, V N Andreev Table 1. Maximum specific real surface areas (m2 g71) for deposits obtained for metal ± polymer ± glassy carbon systems (S1) and metal electrodeposits on platinum (S2).196 ± 199, 220 Polymer Metal S2 S1 18 ± 25 Pt 10 ± 15 Pd 100 80 100 90 60 30 ± 40 poly(vinylpyridine) Nafion poly(vinylpyridine) Nafion poly(vinylpyridine) Rh However, for palladium microdeposits in polyaniline, the specific surface areas of 35 ± 45 m2 g71 were reached (see Refs 220, 222).2. Determination of the real surface area of metal-catalyst particles in polymeric matrices The determination of the real surface area of metal-catalyst particles incorporated into polymeric matrices presented a serious problem for comparing correctly the specific electrocatalytic activities of deposits. Unfortunately, many studies of the electro- catalytic activity of metal-catalyst ± polymeric matrix systems did not pay due attention to this problem and did not separate the contributions to the electrocatalytic activity into those caused by the surface development and by the changes in the intrinsic specific activity of the incorporated particles.54, 154, 212 ± 218 Determination of specific surface areas of metal deposits in polymeric films S (m2 g71) also presented independent interest for estimating the degree of dispersion of the metal-catalyst.The use of a certain model allows the average sizes of metal particles to be calculated roughly from the S values. The modern physical methods such as the scanning tunnelling microscopy and atomic force spectroscopy made it possible not only to assess more accurately the average size of particles but also to determine their size distribution.236 However, the use of these methods for characterising the particles located inside the polymeric matrix is associated with considerable experimental difficulties.The method for estimating the real surface areas from the hydrogen adsorption, which is commonly accepted for platinum group metals,237, 238 often gave insufficiently correct results and in many cases could not be used at all. Thus the `hydrogen regions' of metal particles in polymeric matrices can be deformed due to the small sizes of these particles, their adsorption interaction with the polymeric matrix and the changes in the pseudo-capacity of the support.197, 239, 240 In systems with electron-conducting polymeric matrices such as polyaniline and polypyrrole, additional difficul- ties arose due to the severe overlapping of the potential ranges of hydrogen adsorption ± desorption and those of the intrinsic redox activity of the polymer.154, 219, 220 The method for determination of the surface area by hydrogen adsorption cannot be used in principle for palladium particles, because the hydrogen absorp- tion in the Pd(H) a-phase substantially exceeds the hydrogen adsorption.241, 242 Many of the difficulties noted above could be avoided if the real surface area was estimated by the under- potential deposition (upd) of copper.196, 220, 241 ± 243 In the Pt ± poly(vinylpyridine) system (a glassy carbon support), a 10% deviation between the specific surface areas of platinum deter- mined from hydrogen adsorption and upd of copper appeared only for S>80 m2 g71.The use of copper adatoms for deter- mination of the real surface area of palladium particles deposited into poly(vinylpyridine) and Nafion films proved to be effec- tive.198, 199 At present, this is the most correct electrochemical method for determination of real surface areas of palladium particles incorporated into polymeric matrices.Calculations of real surface areas of palladium electrodes by the oxygen adsorp- tion 241, 243 proved to be useless, because the polarisation of electrodes to high anodic potentials often caused irreversible changes in theM± polymer ± support systems.83, 85, 98, 115, 196Electrocatalysis on polymer-modified electrodes It was found 219 that due to many similar charges being consumed in the removal of hydrogen adatoms from incorporated platinum particles and in the polyaniline film oxidation, the real surface areas estimated in parallel experiments from the hydrogen adsorption differed twofold even for the platinum-to-polymer mass ratio of *70.According to experimental results,220 well reproducible real surface areas estimated from upd of copper could be obtained for platinum and palladium particles incorpo- rated into polyaniline when the metal-to-polymer mass ratio was *0.2. The formation of monolayer coverages by adatoms (Cu, Ag, etc.) with the subsequent removal of their ions from the solution bulk, allowed the surface areas of metal-catalysts to be determined by suppressing the hydrogen adsorption. To determine the surface area of platinum particles in a polyaniline matrix, it was proposed 177 to use the CO adsorption. However, depending on the structures formed by adsorbed CO (bridge or linear), the real surface areas estimated can differ twofold.Moreover, carbon monoxide can be sorbed by the polyaniline film itself.221 The possibilities of using physical methods for the estimation of the sizes of particles incorporated into polymeric matrices could be remarkably increased, if the film could be separated from the support. Thus composite films of polypyrrole with palladium particles were deposited on plates of In7Ti oxides and then separated from them and analysed using a transmission electron microscope.244 The average size of palladium particles was found to be*5 nm. 3. Electrode processes on polymeric films with incorporated particles of metal-catalysts The results on the electrode processes on polymeric films with incorporated metal-catalyst particles are discrepant and unequal with respect to their quality and scientific importance.245 ± 265 This often means not only the quantitative discrepancies, but also concerns qualitative deviations in experimentally observed effects.This was caused by a number of factors, the main of which are as follows: (1) the difference in the procedures used for synthesising the metal-catalyst ± polymeric matrix systems; (2) the wide scatter in the metal-to-polymer mass ratios; this can cause substantial differences in the behaviour of electrodes with essentially similar metal-catalysts and polymers (see Section IV.1); (3) different conditions of measuring polarisation curves;{ (4) uncertainty associated with the steady-state criteria; (5) the low `sample-to-sample' reproducibility of the electro- chemical reaction rates (like electrooxidation of organic substan- ces) even for individual electrodes of platinum-group metals;} (6) in most studies, the specific currents were expressed per the geometrical surface area of the electrode, which dramatically reduced the fundamental importance of the results (see Section IV.2).The difficulties mentioned above were caused by the compli- cated nature of both the electrode systems themselves and the electrocatalytic processes on them, as well as by different factors which could promote or inhibit the electrode reactions. Hence, the attempts to explain the results obtained for the modified electro- des under consideration have yet involved only phenomenological { Unfortunately, the authors of the overwhelming majority of studies used the potentiodynamic method for recording relatively fast polarisation curves.As a rule, this involved measuring non-steady-state currents, the interpretation of which was especially difficult due to the uncertainty in the surface coverages by original particles, their products, solution ions, etc. } Naturally, as one passes to more complicated M± polymeric matrix systems, the reproducibility of the results is reduced. Hence, the compa- rative estimates of the activity based only on the experimentally observed two- to three-fold change in the currents should be considered with caution.845 interpretations. In connection with the aforesaid, when choosing the data for the discussion, we focused attention on the results of those studies in which the measurements were carried out under steady-state conditions on electrodes with small metal-to-polymer mass ratios and which provided the estimates of the metal-phase surface area. The literature data on the methanol and formic acid electrooxidation and oxygen electroreduction in acidic solutions well illustrate the variety of experimental results on the electro- catalysis by particles of platinum-group metals deposited in/on polymeric coatings (Table 2). Most of the studies in which the promotion of the methanol electrooxidation was detected, measured non-steady-state cur- rents calculated per unit of the geometrical surface area (with several exceptions 196, 200, 201, 213, 219, 222, 247), i.e., considered the gross effect.(Sometimes, the oxidation rate increased only two- to three-fold, e.g., in Refs 219 and 247.) When the effect of Nafion and poly(vinylpyridine) matrices on the behaviour of platinum particles was studied for extremely small amounts of the metal deposited (no more than 21 mg cm72), it was found 196, 200 that the specific rates of CH3OH electro- oxidation (under steady-state conditions) decreased as compared with the oxidation rate on platinised platinum (Pt/Pt). A similar effect of the Nafion matrix for the platinum mass <60 mg cm72 was mentioned.201 In the Pt ± poly(vinylpyridine) ± glassy carbon system, with an increase in the metal-phase dispersion, the rate of CH3OH electrooxidation decreased.196, 200 At the same time, for platinum deposits incorporated into Nafion films with the platinum mass <60 mg cm72, higher steady-state specific currents of CH3OH electrooxidation as compared with the currents on Pt/Pt were found.200 The increase was explained by the changes in the binding energy of the products of CH3OH adsorption with the surface due to the changes in the deposit structure.Inasmuch as small-mass deposits are much more dispersed as compared with large-mass ones, one can speak of the qualitative agreement between different estimates 196, 200, 201 of the effect of the degree of platinum dispersion on the methanol electrooxidation rate.For the electrooxidation of formic acid on platinum particles incorporated into poly(vinylpyridine) and Nafion, it was found that the specific rates increased with an increase in the degree of metal-phase dispersion.196, 200 Moreover, the activity of highly dispersed platinum deposits (530 m2 g71) was higher as com- pared with electrodeposits of platinum on platinum. A similar effect was greater for the Pt ± Nafion ± glassy carbon system (Fig. 8).199 The results discussed above show that the changes in the activity of metal particles incorporated into polymeric matrices largely depended on the nature of the substrate. Thus the effects of the degree of dispersion were found to be opposite for the E /V 1 2 3 0.2 0.1 5 6 7log i (A cm72) Figure 8.Stationary polarisation curves for electrooxidation of formic acid in the Pd/Pt (1), Pd ± Nafion ± glassy carbon (2) and Pd ± poly- aniline ± glassy carbon (3) systems in a 0.5M HCOOH+0.5M H2SO4 solution.199, 222846 Table 2. Results on the electrooxidation of methanol and formic acid and electroreduction of oxygen in theM± polymer ± support systems. M± polymer ± support system a Pt ± polyaniline ± glassy carbon Pt ± polypyrrole ± platinum Pt ± polypyrrole ± glassy carbon Pt ± polypyrrole ± gold Pt ± Nafion/polyaniline ± glassy carbon c Pt ± polyaniline ± platinum Pt ± Nafion ± glassy carbon Pt ± polyaniline ± glassy carbon (Pt+Pb) ± polyaniline ± glassy carbon Pt ± polyaniline ± platinum Pt ± polypyrrole ± platinum Pt ± polypyrrole ± glassy carbon Pt ± polypyrrole ± gold Pt ± Nafion ± glassy carbon Pt ± polyaniline ± glassy carbon Pt ± polyaniline ± gold Pt ± polypyrrole ± SnO2/glassy carbon c Pt ± Nafion ± glassy carbon Pt ± poly(vinylpyridine) ± glassy carbon Pt ± polypyrrole ± glassy carbon d Pt ± poly(vinylpyridine) ± glassy carbon Pd ± Nafion ± glassy carbon Pt ± polyaniline ± glassy carbon e Pd ± polyaniline ± glassy carbon e Pd ± polyaniline ± glassy carbon Pt ± polypyrrole ± (Pt, Ni) a Electrodeposition was used for incorporating metal into polymers unless otherwise specified.b Signs `plus' and `minus' designate the presence or absence (as assessed by the authors of the cited paper) of the catalytic effect; in the parenthesis, the surface area type (geometrical or real) used for calculating the currents is shown.c Chemical deposition was also used. d Three methods were used. e One-step method. methanol and formic acid electrooxidation processes. A similar phenomenon was also mentioned for other catalytic systems, viz., not only for systems containing Pt particles, but also for those with Rh, Pd and Ir (see references in Ref. 240). This can be explained by assuming that particles involved in methanol electro- oxidation occupy the greater number of surface sites than the sites required for the adsorption of HCOOH or CH3OH.240 On the whole, the specific steady-state currents of methanol electrooxidation on platinum particles deposited in/on polyaniline films were found to be higher than the currents of its oxidation on Pt/Pt particles.219, 222 However, a substantial catalytic effect was observed 219 only for the platinum mass>60 mg cm72.Inasmuch as only very thin films were studied (35 ± 50 nm), it can be assumed that the effect was caused by the peculiarities of the structures of Substrate Film Mcontent thickness /mm /mg cm72 2 ± 51 3 ± 4 CH3OH 7 7 O2 7 7 O2 7 7 O2 11 ± 31 1.4 ± 9.3 O2 7 7 HCOOH 4 ± 55 0.5 CH3OH CH3OH 7 25 ± 750 7 7 CH3OH 7 7 HCOOH HCOOH 7 47 45 ± 131 CH3OH 7 57 CH3OH 7 2 ± 1000 5 CH3OH 0.3 0.3 CH3OH HCOOH 77 7 7 CH3OH 13.1 0.08 ± 0.16 O2 4 ± 21 4 ± 21 1.0 1.0 CH3OH HCOOH 5 ± 20 1.0 CH3OH HCOOH 200 ± 1000 0.5 ± 10 CH3OH 5 ± 120 5 ± 120 1.0 1.0 CH3OH HCOOH HCOOH 1.6 ± 6.5 4 ± 6 62 ± 76 62 ± 76 62 ± 76 5 ± 30 0.25 ± 0.3 0.25 ± 0.3 0.25 ± 0.3 0.25 ± 0.3 CH3OH CO HCOOH HCOOH HCOOH 8 ± 30 0.25 ± 1.5 100 1.0 CH3OH platinum deposits formed on the surface of a polyaniline film (see Section IV.1).The higher electrocatalytic activity of platinum particles in a polyaniline matrix as compared with Pt/Pt particles was also found 222 for the electrooxidation of formic acid and carbon monoxide. For CO, the effect was explained by the growth of its concentration in the polymeric film. The fact that CO is sorbed by the film was confirmed by a special study.221 For platinum dispersed in polyaniline, the effect of promotion of formic acid electrooxidation was apparently of the same nature as for the aforementioned (see Section III.2) increase in its electrooxidation rate on platinum electrodes observed upon covering them with polyaniline films.Polarisation curves non-steady-state steady-state non-steady-state "steady-state, non-steady-state the same "steady-state "non-steady-state "non-steady-state, steady-state the same steady-state, non-steady-state the same ""steady-state ""non-steady-state steady-state """""non-steady-state B I Podlovchenko, V N Andreev Ref. Catalytic effect b 202 +(geometrical) +(geometrical) 129 129 129 +(geometrical) 126 7(geometrical) 213 + 265 +(geometrical) 219 219 +(real) +(real) 252 252 +(geometrical) +(geometrical) 255 +(geometrical) 264 +(geometrical) 201 +(real) 154 +(geometrical) +(geometrical) 54 +(geometrical) 263 +(geometrical) 200 200 7(real) +(real) 200 7(real) +(real) 246 +(geometrical) 196 196 7(real) +(real) 199 +(real) 222 +(real) +(real) +(real) +(real) 222 170 +(real) 247 +(geometrical)Electrocatalysis on polymer-modified electrodes The greatest increase in the specific rates of formic acid electrooxidation was found for palladium particles incorporated into polyaniline (see Fig.8). Moreover, the steady-state currents increased more than by an order of magnitude when both single- 222 and two-step 170 methods were used for synthesising Pd ± polyaniline ± glassy carbon systems.It is remarkable that the slope of polarisation curves remained virtually unchanged, although the process occurred in the potential range of polyaniline `nonconduction'. This suggests, first, that the mechanism of HCOOH electrooxidation of palladium remains unchanged 248 and, second, that the electrode reaction rate can remain virtually unaffected by the ohmic potential drops in sufficiently thin films of electron-conducting polymers in the potential ranges of their `nonconduction' (see also Section III.2). According to the literature data, electrodes modified by composite Pt ± polyaniline coatings can also be used for the oxidation of propan-2-ol,249 ethylene glycol,178, 250 and D-glu- cose 251 (the latter can also be oxidised on a Pr ± polypyrrole coating 218) at moderate anodic potentials.However, the authors of these studies have used the potentiodynamic method for measuring voltammograms and have not estimated the real sur- face area of the metal phase. Hence, even if certain `electro- catalytic effects' were found in these studies, this is more likely to be attributed to the surface development of the metal-catalyst. A minor increase in the specific steady-state currents was found for ethanol electrooxidation on platinum particles incorporated into a polyaniline matrix by the chemical deposition procedure.229 Most authors associated the promoting effect of polymeric matrices on the electrooxidation of simple organic substances at moderate potentials (before the onset of oxygen adsorption) either with the deceleration of the appearance on the surface of metal- catalyst of strongly chemisorbed species, which are the `poison' for the current-controlling reaction, or/and with an increase in the amount of weakly bound species.245 The `parallel mechanism' of electrooxidation of organic compounds, which was first proposed in Refs 253, 254 for the reaction of methanol electrooxidation have gained substantial recognition.Inasmuch as, as a rule, the sizes of metal-catalyst particles in polymeric matrices exceed 5 nm, the `pure' size effect can hardly be considered. It is most probable that the catalytic effect is caused by the changes in the structure and/or the composition of the surface. Moreover, the changes in the latter can be caused not only by the adsorption of polymeric molecules and their fragments but also by the adsorbed impurities which could get into the metal phase from the deposition solution.197, 222 The surface composi- tion was shown to strongly affect the formation of adsorbed OH groups, which play an important role in electrooxidation of organic compounds.245 Only few interpretations of the results took into account that the activities of water and substrate molecules in the pores of the M± polymer ± support system can differ from those in the solution bulk.194, 222 However, the difference in these characteristics is quite probable, because, according to experimental results, e.g., for Nafion 194 and polyaniline,232 the polymers have channels with the mean size of *1 nm, i.e., approaching the thickness of the electrical double layer.Evidently, the effect of the triple metal/ polymer/solution interface on the kinetics of electrocatalytic processes should also be taken into account. A new in principle interpretation of the promotion of formic acid electrooxidation in the Pt ± polyaniline ± glassy carbon sys- tem was given for the case where the incorporation of platinum into the film was accomplished by the potential cycling in the 0.1 ± 0.7 V range.154 It was assumed 154 that under these condi- tions platinum is preferentially accumulated as complex ions (Pt2+ and Pt4+) which catalyse the process.The effect was unsteady and decreased in time very quickly, because the complex ions were removed from the film being exchanged for solution ions. Methanol was not electrooxidised on such platinum com- plexes which confirms again the substantial differences between the electrooxidation mechanisms of methanol and formic acid. 847 The electrocatalytic acitivity ofM± polymeric matrix systems in electrooxidation of organic compounds could be enhanced (as in the case of individual metal-catalysts 240, 245) by the deposition of adatoms of foreign metals (e.g., see Refs 251, 256, 257). The high currents of hydrogen evolution on composite elec- trodes Pt ± polymer [poly(vinylpyridine), Nafion, polyaniline] ± glassy carbon and Rh ± poly(vinylpyridine) ± glassy carbon could be obtained for very small amounts of metal-catalysts (*20 mg cm72).200, 222, 234 For small overvoltages, the parame- ters of polarisation curves with the currents calculated per 1 cm2 of the geometrical surface area turned out to be similar to those for compact metal electrodes for which the hydrogen evolution was controlled by diffusion.258, 259 Some of these results 200, 222 correlated well with the results of studies 189, 202 which showed that the `hydrogen reaction exchange currents' in the Pt ± polyaniline and Pt ± Nafion systems virtually coincide with those on smooth platinum.However, it is more likely that the currents measured in the latter studies 189, 202 were also of the diffusion nature rather than of the kinetic nature.It was found 200, 222, 234 that with an increase in the over- voltage, the hydrogen evolution current on Pt ± polymer ± glassy carbon electrodes became smaller than those observed on smooth platinum. This was explained by the earlier transition to the mixed-kinetics region. The potentials of the transition depended on the polymer structure; for the polymers under consideration they changed in the following order: polyaniline>poly(vinylpyr- idine)>Nafion. Indeed, this points to the decrease in the real exchange currents of the hydrogen reaction in composite electro- des due to the adsorption interaction between the metal-catalyst particles and the polymer and/or its destruction products.200, 222 The results obtained for the Pt ± Nafion ± glassy carbon system, namely, that the diffusion currents estimated per 1 cm2 of the geometrical surface area exceeded the specific currents on smooth platinum, are of special interest.200 This was explained by the more effective stirring of the near-electrode layer by hydrogen bubbles on the hydrophobised surface.Anumber of studies 126, 260 ± 263 have mentioned a possibility in principle of oxygen electroreduction on platinum particles incor- porated into polyaniline or polypyrrole, but did not provide any correct comparison with the activity of platinum in this reaction. A lower activity was reported 126 for platinum deposited on a mixed polyaniline ± Nafion film as compared with compact plat- inum.This was explained by the peculiarities of the formation of metal-catalyst deposits on such a film. Other scientists assumed the oxygen electroreduction rate to be largely controlled by the oxygen diffusion in the polymeric matrix. On the whole, the sufficiently fast degradation of electron-conducting polymers in the presence of oxygen and the catalysis of this process by incorporated particles of platinum-group metals were shown 96, 97, 115, 200, 222 to have an adverse effect on the stability of properties of electrocatalytic systems involving these polymers and used in the oxygen electrode. V. Conclusion The results shown above suggest that the research carried out to date with polymer-modified electrodes is largely aimed at the elucidation of interrelations between the properties of support materials, polymeric coatings and incorporated metal-catalyst particles and/or certain functional groups.The fact that polymeric materials cannot provide the destructive chemisorption of sub- strate molecules gives the grounds to expect that the systems with polymeric matrices containing microparticles of catalytically active metals are more promising for a number of electrode reactions (particularly, those in fuel cells). Inasmuch as the properties of polymeric matrices and micro- particles are interrelated, the theoretical concepts describing the dependence of fine particles on the properties of phases in contact with them are of special importance. These studies are directly connected with the development of concepts on the structure of848 different interfaces in the systems studied, including the three- phase interfaces, viz., support/polymer/solution and polymer/ metal-catalyst/solution.Taking into account the polymer ± sol- vent interaction and possible changes in the activities of solvent and substrate molecules in the polymeric matrix also appears to be very important. The cited examples of polymer-containing electrode systems comprise only a small fraction of those available in the literature. However, at present, the development of theoretical concepts should be aimed at more comprehensive and deep experimental research to reveal more distinctly the catalytic effects for electrode reactions rather than at the increase in the number of systems studied.For this purpose, not only the complex use of modern experimental techniques (physical, physicochemical, analytical, etc.) but also the modernisation of experimental methods (see Section IV.3) are necessary. This review only mentions certain examples of the use of polymer-covered electrodes in practice. 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ISSN:0036-021X    

出版商:RSC    

年代:2002

数据来源: RSC

摘要:

Russian Chemical Reviews 71 (10) 853 ± 866 (2002) Thermal stability of nanomaterials R A Andrievskii Contents I. Introduction II. The key types, structure, and general characteristics of nanomaterials III. The main processes in nanomaterials during heat treatment IV. Other stability aspects of nanostructures V. Conclusion Abstract. growth, (grain phenomena and processes main The The main processes and phenomena (grain growth, phase in on) so and diffusion, relaxation, transformations, phase transformations, relaxation, diffusion, and so on) in nano- nano- materials considered. are treatment heat to subjected materials subjected to heat treatment are considered. Experimen- Experimen- tal stability thermal the on views theoretical and data tal data and theoretical views on the thermal stability of of nanomaterials is Attention generalised.and analysed are nanomaterials are analysed and generalised. Attention is drawn drawn to unsolved problems. The bibliography includes 152 references to unsolved problems. The bibliography includes 152 references. I. Introduction At present, the concept of nanomaterials (NM), the development of which has been substantially stimulated by a number of publications,1 ±3 is widely known. This is indicated, for example, by the fact that representative all-Russian and international conferences and seminars dealing with nanomaterials have been regularly held for many years; this subject is also actively discussed at the Symposia of the American Materials Research Society (MRS) and the NATO Advanced Study Institute (NATO ASI); monographs, collections of works, and reviews have been pub- lished (see, for example, Refs 4 ± 24).The main emphasis in the above-mentioned studies 1± 3 was put on the crucial role of numerous interfaces existing in NM. The properties of solids containing such interfaces can be changed appreciably both by structural and electronic modification and by using new doping resources irrespective of the type of bond, size factors, etc. When the grain or crystallite size (L) is several nanometers, the proportion of interfaces in the total bulk of the material is at least *50%. This value was approximately esti- mated from the 3s/L ratio, where s is the width of the near- boundary region; for a reasonable value s&1 nm, a proportion of 50% is attained when L&6 nm.Gleiter and coworkers 1 ±3 proposed a method for producing nanomaterials, which included preparation of ultradisperse powders by vaporisation ± condensa- tion and subsequent in situ compaction at high pressures. This procedure gave disk-like specimens 10 ± 20 mm in diameter with a height of up to 0.1 ± 0.5 mm. In order to obtain high-purity specimens, it is necessary to eliminate contact of the highly reactive powders with the environment (although these specimens R A Andrievskii Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region, Russian Federation. Fax (7-096) 514 32 44. Tel. (7-096) 522 35 77.E-mail: ara@icp.ac.ru Received 4 April 2002 Uspekhi Khimii 71 (10) 967 ± 981 (2002); translated by Z P Bobkova #2002 Russian Academy of Sciences and Turpion Ltd DOI 10.1070/RC2002v071n10ABEH000723 853 854 856 863 863 usually contain up to 10%± 20% of residual porosity). However, initially, this fact was ignored and gave rise to some erroneous inferences (see, for example, Ref. 21). This method for the preparation of NM was assimilated by researchers in many countries, first of all, in the USA (Siegel's group in the Argonne National laboratory 25 and research groups in some other universities); this triggered an avalanche-like accumulation of data concerning the properties of NM. The first Russian session devoted to the research of properties of consolidated NM was held in 1984 and called the first All- Union Conference on the Physical Chemistry of Ultradisperse Systems.{ In an earlier study of NM published in 1983,27 it was shown that compaction of ultradisperse nickel powders at high pressures and moderate temperatures furnishesNMwith a crystallite size of about *60 nm with a hardness exceeding that of usual polycrys- talline nickel by a large factor.Unfortunately, the publications cited 26, 27 have passed almost unnoticed by both Russian and foreign specialists. Meanwhile, these studies 27 demonstrated one more time, independently of the results obtained by Gleiter and coworkers,1 ±3 the validity of the known Hall ± Petch relationship, which relates strength (hardness) to the grain size (H*L71/2) in the nano-size range, and the expediency of extending this relation- ship to NM.At present, the greatest L value is taken to be *100 nm.17, 20, 21 In view of the relatively small dimensions of NM crystallites (and, correspondingly, the numerous interfaces they contain), it becomes clear why it is important to study the nanostructure evolution, which is accompanied by changes in the properties of NM, upon heat treatment or other types of treatment. Naturally, these items attract attention of researchers; various aspects of the problem of NM stability have been considered in a number of papers, reviews, and monographs (see, for example publica- tions 14, 18, 20 ± 22, 28 ± 31); however, generally, this item does not appear to be adequately studied.In terms of the fields of application, one can distinguish the following main functional types ofNMfor which thermal stability is especially significant: tool materials and materials for high- temperature friction units; thermal and diffusion barrier compo- nents; high-temperature semiconductors, catalysts, and sensors; porous, emission and filter component parts; and refractory materials of various types. The knowledge of thermal stability patterns is also important for NM manufacture technology. In particular, when selecting the optimal conditions for superplas- { Proceedings of this conference were published only in 1987.26854 ticity and when developing the conditions for controlled crystal- lisation from the amorphous state and for plasma sputtering of nanostructured coatings and high-energy methods for consolida- tion of ultradisperse powders, specialists should take into account conditions for nanostructure maintenance.The information out- lined above accounts for both the theoretical and practical interest in the problems of thermal stability of NM. II. The key types, structure, and general characteristics of nanomaterials The concept of NM introduced by Gleiter and coworkers,1 ±3 mainly in relation to metal-like materials, has now substantially expanded. Twenty years later, Gleiter 22 considered a broader spectrum of NM, apart from the above-mentioned objects, namely, semiconductors, polymers, diamond-like and supramo- Table 1.Key methods for the preparation of compact NM. Preparation technique Powder technology Severe plastic deformation Controlled crystallisation from the amorphous state Film technology Layer-shaped Rod-shaped Equiaxed Figure 1. Sketch of single-phase (a) and multiphase NM with a statistical (b, c) and matrix (d ) distribution of structural components.22 (b) identical interfaces; (c) non-identical interfaces. Main types Gleiter method (gas-phase deposition and compaction) electric-discharge sintering hot pressure treatment high static and dynamic pressures at ambient and high temperatures equal-channel angular pressing high-pressure torsion pressure treatment of multilayer composites phase cold hardening normal and high pressures chemical vapour deposition (CVD) physical vapour deposition (PVD) electrodeposition sol ± gel processes a b c d lecular structures, and biomaterials.This can be supplemented by ceramics, catalysts, nanoporous materials, and a large number of nanocarbon and, generally, nanotubular structures; the scope of investigations of these materials is swiftly extending. TheNMlisted above have been prepared by different methods and have different types of microstructure. Table 1 and Fig. 1 present data concerning compact and nonpolymeric NM.21, 22 Classification according to the composition, distribution, and the shape of structural components implies three kinds of structural shapes (layer-shaped, rod-shaped, and equiaxed forms) and four sorts regarding the chemical composition and component distri- bution, namely, single-phase compositions, statistical multiphase compositions with either identical or non-identical interfaces (i.e., the possibility of segregation is taken into account), and matrix compositions. The number of structural types can be greater due to mixed variants, the presence of porosity and so on.One should R A Andrievskii Objects elements, alloys, compounds metals and alloys amorphous materials elements, alloys, compoundsThermal stability of nanomaterials also take into account the conditional character of the classifica- tion of production processes because the borders between partic- ular technological expedients are often blurred.When comparing the thermal stability of different NM, one must bear in mind that the diversity of production techniques and types of macro- and microstructures has an influence on the NM behaviour during heating. The most frequently encountered structural types are single- phase and multiphase matrix and statistical objects, rod-shaped and multilayer structures (so-called superlattices); the latter type is a 5 nm b NbN TiN NbN 5 nm c Figure 2. Photomicrographs of the NM structure: (a) Ti(N,B) film prepared by magnetron sputtering;32 (b) multilayer TiN/NbN film pre- pared by magnetron sputtering (the overall thickness of individual monolayers is 4.6 nm);33 (c) `Finemet' magnetic alloy prepared by crystallisation from the amorphous state followed by annealing at 500 8C (t=1 h); the arrows mark the grain boundaries.34 typical of films.Figure 2 shows examples of some typical struc- tures. High-resolution transmission electron microscopy is espe- cially effective for elucidating theNMstructure.32 ± 34 Attention is drawn by the difference between the structures and thicknesses of interfaces for NM prepared by different procedures (see, for example, Figs 2 a and 2 c). Various opinions have been expressed concerning the nature of interfaces in NM (see, for example, Ref. 21); however, it is clear that the diversity of production methods also leaves a mark on the nature of the interfaces. X-Ray diffraction methods, which allow one to estimate the anisotropy of grain size and to obtain information on structure distortion, are also widely used to study nanostructure (this has been done most comprehensively in investigating the effect of annealing on the rutile nanostructure 35).It is evident that the vast majority of NM, except for supra- molecular structures (which was first noticed in the review by Gleiter 22), are non-equilibrium in their nature.Asystem can move away from equilibrium for several reasons. As regards NM, the most important of these reasons is the abundance of interfaces (grain and phase boundaries and triple junctions), which gives rise to excess free surface energy. Figure 3 sketches a triple junction and Fig. 4 shows the dependences of the overall fraction of interfaces and the fractions of proper grain boundaries and triple junctions on the grain size.{ Grain boundary Triple junction s Figure 3.Triple junction formed by grains shaped like tetrahedral dodecahedra. 1072 { Accordting to estimates,36 the overall fraction of interfaces amounts to V1=17[(L7s)/L]3*3s/L, the fraction of proper grain boundaries is V2=[3s(L7s)2]/L3 and the fraction of triple junctions is, correspond- ingly, V3=V17V2. Volume fraction 100 1 2 1071 3 100 101 Grain size /nm Figure 4. Effect of the grain size on the overall fraction of interfaces (1) and on the fractions of grain boundaries (2) and triple junctions (3) (boundary width 1 nm).36 855 103 102856 As noted above, for grain sizes of less than 5 ± 6 nm, these fractions are rather high and, hence, the excess free surface energy is also rather high; the fraction of boundary segregations also increases.Note that the values of the interfacial and boundary surface energy can differ from those for usual coarse-crystalline materials; however, reliable experimental data are scarce (see, for example, Ref. 21). It is worth noting that the presence of free space in virtually non-porous NM has been noted in many publications. For example, this was found in densimetric measurements for nanocrystalline nickel 37 and gold 38 specimens (Table 2) and in a study of positron annihilation in palladium.39 A possible constit- uent of the free space is the discontinuity of the grain boundaries. The NM structure often contains non-equilibrium phases; therefore, phase equilibrium diagrams can markedly vary. Nano- materials produced using film technology and high-energy treat- ment (static and dynamic high pressures, intensive plastic flow, etc., see Table 1) occur inevitably under residual stress and contain crystal structure defects; this also influences their energy state.Table 2. Density of nanocrystalline nickel 37 and gold 38 specimens (gt is the theoretical density). L /nm Specimen Preparation method gt /g cm73 g /g cm73 8.897 Nickel 111 182 19 19.299 Gold 8.340.02 8.320.02 19.00.5 pulse electrodeposition magnetron sputtering Thus, heat treatment, exposure to force fields or radiation, or long-term operation of NM even at usual temperatures induce recrystallisation, segregation, homogenisation and relaxation processes; phase transitions; phase formation and destruction; amorphisation, sintering, and nanopore (nanocapillary) closing.For a long period, these processes have been the subjects of numerous studies related to material science and the physics and chemistry of solids. For example, the theoretical views on the stability of colloids and thin films have been surveyed in a monograph.40 However, the mechanisms of many of these proc- esses as applied to NM have not been adequately studied. Currently, it is still difficult to differentiate between them and to evaluate their role but, generally, almost all of the above phenom- ena have an appreciable effect on the properties and the service performance of NM.In the limiting case, degradation of NM is possible, although in many cases, an additional heat treatment of NM proves useful for stabilising their properties. III. The main processes in nanomaterials during heat treatment 1. Grain growth a. Experimental data The experimental curves that illustrate the effect of temperature and duration of the annealing on grain growth in various types of NM are shown in Figs 5 ± 12.41 ± 53 The general regularities based on the data presented here and other data on grain growth in nanocrystalline specimens are outlined below. 1. The lognormal or normal grain size distributions remain virtually the same for the initial and annealed specimens.Numer- ous kinetic studies gave diverse results. For example, in the general case, a power law holds for grain growth (see Fig. 5) but it is often rather difficult to prefer either a relation like L2*t or some other (L1; 3; 4*t).41 Using experimental results (isothermal annealing) for NbAl3 nanocrystals prepared by mechanical doping 47 and for Fe33Zr67 and (Fe,Co)33Zr67 alloys prepared by crystallisation from the amorphous state,48 the relationship L3*t was found. L /nm 50 30 10 0 Figure 5. Kinetics of the grain growth in nanocrystalline copper pre- pared with sliding friction.41 T/K: (1) 575, (2) 625, (3) 675. L /nm 55 45 35 250 4 8 12 16t/days Figure 6. Kinetics of the grain growth in nanocrystalline copper pre- pared by compacting an ultradisperse powder in a vacuum.42 Porosity (%): (1) 7, (2) 4, (3) 3; T=298 K.L /nm 80 60 40 s 200 Figure 7. Kinetics of nanograin growth during annealing of RuAl powders prepared by mechanosynthesis.43 T/K: (1) 873, (2) 973, (3) 1073, (4) 1173, (5) 1273. 40 20 1 2 0 R A Andrievskii 321 60 103t /s 3 2 1 54321 3 4 t /hThermal stability of nanomaterials L /nm 80 1 35 2 40 4 0 800 400 T /8C Figure 8. Effect of temperature (t=1 h) on the nanograin growth in iron (1) and iron ¡¾ aluminium alloy (2 ¡¾ 5) powders prepared by mechano- synthesis with attritor milling under nitrogen (1 ¡¾ 4) or argon (5) at low temperatures.44 Aluminium content in the alloy (mass %): (2) 2.6; (3) 10; (4) 10 (milling for a longer period); (5) 10.The exponential quantity n can depend not only on the object of investigation but also on the temperature range (1/n= 0.05 ¡¾ 0.5).20, 22 The experimental data obtained in kinetic meas- urements of the growth of nanograins during annealing of ruthenium monoaluminide powders were found to be in good correspondence with the following expression: max (1) ; a exp ¢§ 2qt L2max max L2OtU ¢§ L2 L2O0U ¢§ L2 where indices t and 0 refer to the current and initial time, Lmax is the maximum grain size at the given annealing temperature dictated by factors restricting the grain growth (for example, boundary segregations) (see Fig. 7), and q is a kinetic constant.43 In a study of the grain growth (L0*8 nm) in iron powders treated in a high-energy attritor and annealed at 350 ¡¾ 600 8C in a dynamic vacuum (*1073 Pa, annealing time from 5 min to 142 h), a dependence of the exponential quantity on temperature was found and the correspondence of the experimental kinetic data to both the dependence of the *t n type and relation of type (1) was established.29 d /nm L /nm 1 200 40 150 30 2 20 100 50 10 0 0 1 5 0 15 20t/min Figure 9.Effect of the duration of deposition from a [Cd(CH3COO)2 . 2H2O+H2NCSNH2+(HOCH2CH2)3N+NH4OH] solution at 75 8C on the CdS film thickness (1) and crystallite size (2).45 a N/N0 (%) 30 10 0.05 673 1073 T /K b N/N0 (%) 30 10 0.05 673 1073 T /K c N/N0 (%) 30 10 0.05 673 1073 T /K d N/N0 (%) 30 10 0.05 673 1073 T /K Figure 10.Statistical size distributions of crystallites in ZrN films (a, b), a ZrN+TiN 20-layer film (c) and a doped (Zr,Ti)N film (d ) annealed at different temperatures.46 Film thickness /mm: (a, c) 2, (b) 0.1, (d) 1. 857 0.10 L /nm 0.10 L /nm 0.10 L /nm 0.10 L /nm60 40 200 0.10 70.1 70.2 70.3 70.4 9000 7000 5000 3000 1000200 Figure 12. Effect of the annealing temperature on the electrical resistivity at 4.2Kr4.2K (a) during temperature increase (1) and decrease (2); temperature gradient Dr/DT (b); microhardness (c); degree of ordering (d ), internal distortions (e); and crystallite size ( f ) of nanocrystalline Ni3Al.18 Microhardness /GPa Dr/DT /mO cm K71 ** 35 Microhardness /MPa r4.2K /mO cm 858 30 25 20 15 100 50 800 Figure 11.Change in the microhardness of TiN/AlN multilayer films after annealing in an argon atmosphere for 2 h.53 The total thicknesses of TiN and AlN individual layers are 16 (1) and 2.9 nm (2); the total film thickness is 300 nm. R A Andrievskii 12 1000 T /8C 2. When NM are kept for a long time even at room temper- ature, growth of grains takes place.22, 42, 49, 50 This so-called anomalous growth of grains occurs if the grain size distribution is inhomogeneous (in this case, large grains serve as seeds for the anomalous growth).22 The activation energies for grain growth in NMand for the grain-boundary diffusion are close.21, 22 Measure- ments over a broad temperature range showed an increase in the activation energy for boundary growth with temperature rise; for example, for RuAl nanocrystals, the activation energies were found to be 39, *72 and 213.5 kJ mol71 for temperatures of 873 ± 1073, 1073 ± 1173 and 1073 ± 1273 K, respectively.This dependence could be due to the progressive effect of the boundary segregation temperature on the retardation of grain growth.43 The activation energy for grain growth in iron at low temperatures is 125 kJ mol71 (which is close to the activation energy of the boundary self-diffusion in iron), while at higher temperatures, this value is 248 kJ mol71 (which is approximately equal to the activation energy of bulk self-diffusion).29 These results can be a d 0.8 12 0.6 0.4 0.20 b e Degree of ordering (rel.u.) Internal distortions (%) Crystallite size /nm 0.6 0.5 0.4 0.3 0.2 0.10 f c 160 120 80 400 T /K 600 1000 T /K 600 1000 200Thermal stability of nanomaterials explained by assuming different growth mechanisms for low- and high-temperature ranges.3. The low-temperature milling of Fe ± Al alloys reduces the tendency of the resulting NM for recrystallisation, which is attributed to the formation of fine FeAl2O4 particles and boun- dary segregations (see Fig. 8). The temperatures of the onset of active grain growth are mainly correlated with the melting points. In the presence of interstitial impurities (oxygen, nitrogen, etc.) and with formation of oxides and nitrides in the system, the grain growth starts at higher temperatures.This was confirmed for the Ag ± O,49 Ti ±N50 and Mo±N51 systems. Their thermal stability was demonstrated in relation to two-phase nanosystems, Cu ± Ag and TiN ± Si3N4.49, 52 4. In film specimens, the size of crystallites is much smaller than the film thickness but is correlated with the latter (see Figs 9, 10).Adecrease in the film thickness promotes the growth of grains (see Fig. 10); in multilayer films, thermal stability increases with a decrease in the thickness of layers (Fig. 11).53 5. It was noted in several publications (see, for example, the studies 21, 22, 53, 54) that the thermal stability of grains increases as the grain size diminishes.6. The growth of grains is accompanied by not only changes in structural parameters such as microdistortions, the degree of ordering, etc., but also by changes in physicochemical properties of NM (Fig. 12).18 The features noted above were established using electron microscopy, diffraction and calorimetric techniques. b. The models and theoretical concepts The problem of maintenance of the nanostructure ofNMhas been discussed in several reviews and original studies (see, for example, the publications 20 ± 22, 28, 30, 31, 50 ± 56). The main factors considered to favour the production of thermally stable NM include the presence of nano- and micropores, boundary segregations and two- or multi-phase nanostructures, a decrease in the grain- boundary surface energy, the formation of supersaturated solid solutions, an appropriate grain morphology, and homogeneity of the grain size distribution.The stagnation of grain growth in nanocrystalline films and ultradisperse powders has been dis- cussed in the literature.57 The research into grain boundaries and, generally, interfaces has been the subject of numerous publications (including mono- graphs, see for example, Refs 58 ± 62). However, these works barely touch upon the specificity of nano-objects. In recent years, the structures of grain boundaries have been calculated and the NM properties and the probability of vacancy generation during boundary migration have been predicted using molecular dynamics techniques (see for example, the studies 21, 22, 28, 63 ± 69).In particular, it was shown that one-component NM can be simulated by two-phase models (the grain body and grain boun- daries). In estimating the properties that depend little on the structure, this approach provided satisfactory results.21, 22 The contribution of triple junctions to the excess energy of NM was found to be rather weighty.67 The molecular dynamics simulation proved to be useful for confirming the generation of vacancies upon grain boundary migration.66 This process is related to elimination of the free space existing as interface discontinuity; the `injection' of vacan- cies inside the grain body should increase the free energy of the system and hamper the boundary migration. An increase in the vacancy concentration during recrystallisation of ultradisperse powders has been recorded experimentally by positron-annihila- tion analysis.68 1=3 (2) , The following relation was obtained for the critical size of crystallites:69, 70 Lc 2 à 24NkTZÖbsdÜ2 mD whereNis the number of atoms in unit volume; k is the Boltzmann constant; Z is the coordination number; b is the excess relative free 859 volume of the grain boundaries; d is the distance between the vacancy sources and sinks; m is the boundary mobility; and D is the self-diffusion coefficient.When L0<Lc, excessive vacancies retard the migration of grain boundaries, and when L0>Lc, the grains grow in the usual way, for example, in conformity with the law (L2*t).If the number of dislocations as vacancy sinks is relatively low, the grain boundaries or film surfaces mainly act as the sinks. In this case, (3) Lc 2 à 24NkTZÖbsÜ2 mD , where Lc/2&d. For example, for aluminium (m= 2610714 m4 J71 s71; T=273 K; D = 1.3610717 m2 s71), the Lc value amounts to *100 nm, i.e., this is a typical value for an NM structure.70 When using relation (2) [or (3)], one should take into account the estimates of the typical grain sizes in NM made in a publication.71 In crystallites with dimensions of less than L*, (4) L à aEb , 2tPN where a is a coefficient dependent on the dislocation geometry and ranging from 0.1 to 10, E is the shear modulus, b is Burgers vector, and tPN is the Peierls ± Nabarro stress, the existence of disloca- tions is unlikely.For example, for linear edge dislocations, the following L* values (nm) were found for some metals 71 and for titanium nitride:32 TiN *1 Ni 10 Al 10 Cu 25 Fe 2 When Lc<L*, relation (2) should be used, while for Lc>L*, the use of formula (3) is preferred. However in both cases, one should bear in mind that these relations are approximations. Yet another interesting conclusion concerning triple junctions has been drawn in a study of grain boundary migration. Triple junctions can be considered as a sort of impurity at interfaces but they mainly play an important role at relatively low temperatures. It has been shown for zinc and aluminium 72 ± 74 that the migration velocity of grain boundaries with triple junctions is lower than that for grain boundaries without the junctions at low temperatures.This is due to different activation energies for migration of these boundaries. A temperature rise entails an inversion: the bounda- ries with junctions become more mobile and the grain growth is controlled by the grain boundary migration. In the case of 99.995% pure zinc, the upper limit of the predominant influence of triple junctions is *380 8C (tilt boundary grains in the h1010i system) and*430 8C (tilt boundary grains in the h1020i system). For 99.999% pure aluminium, these temperatures were found to be *400 8C (tilt boundary grains in the h111i system) and *510 8C (tilt boundary grains in the h110i system). The retarding role of triple junctions should be enhanced by a decrease in the grain size (see Fig.4). Analysis of the variation of the areas of two-dimensional grains showed that, with allowance made for triple junctions, the velocity of grain boundary migra- tion is proportional to the grain perimeter, but the pattern of dependence on topological parameters would be more com- plex }.75 However, the general features of switching of the mech- anism of the triple junction effect on grain growth and the temperature ranges in which each particular mechanism is effec- tive have not yet been adequately studied. In investigating the grain growth in metallic films, a thermo- dynamic approach taking into account tensile stresses has proved to be useful.76 According to estimates, the boundary migration and injection of vacancies inside the grains create conditions } Number of triple junctions per grain (m); when m=6, grains exist in an equilibrium form; when m<6, smaller grains disappear and when m>6, larger grains grow (anomalous growth situation).860 where recrystallisation is thermodynamically unfavourable and the grain growth is retarded.The incubation period depends on the grain size (or the distance between the sinks) and on the temperature (5) tinc à ÖbsdÜ2NkT , gDL0ceq where g is the grain-boundary surface energy, L0 is the initial grain size, ceq is the equilibrium concentration of vacancies.For example, for T/TM&0.3 and L0=d&10 nm, the tinc value would be *107 s, i.e., *150 days. From general considerations, this appears reasonable, although somewhat overestimated. One should bear in mind that relations (2) ± (5) are approximations and also the fact that the residual stresses in the crystalline films are not necessarily tensile but can also be compressive. The thermodynamic aspects of the segregation effects have been considered in several studies (see, for example, the publica- tions 21, 22, 28, 56, 77, 78). The patterns of adsorption at crystallite boundaries have been studied since long ago. It is noted in monograph 59 that a thermodynamic theory that regards the interfacial areas as a separate phase was proposed by Zhukhovit- skii back in 1944.The same study presents the thermodynamic relations determining the activity of a component in the interfacial layer as a function of its bulk concentration. Well-known are also Arkharov's studies in which the attention is focused on the role of boundary segregations in the formation of the properties of metal alloys. The general thermodynamic aspects of boundary segregations in metallic and ceramic alloys have been considered by Ishida;79 as applied to NM, this topic was discussed in a number of publica- tions.55, 56, 77, 78, 80 An important result obtained in these works is as follows: calculations performed in different approximations (Langmuir ± McLean and Fowler ± Guggenheim) showed that the free Gibbs energy (G) varies non-monotonically } as a function of the crystallite size, the minimum G being located in the nano-size region (Fig.13). Hence, the temperature rise in doped nano- crystals may produce situations where further increase in the grain size becomes thermodynamically unfavourable. Nanocrystalline nickel alloys with a minor content of phos- phorus (2 at.% ± 5 at.%) were used to demonstrate the possibility G /kJ mol71 7654321 40 L /nm 80 60 20 Figure 13. Gibbs free energy for a binary nanostructured alloy with an average concentration of 5 at.% at T=600K vs. crystallite size (for a standard polycrystal, G=4 kJ mol71).77 }We would like to note incidentally that this non-monotonic variation is correlated with the views 81 of singular points reflecting the topological transitions in the nanocrystalline state.R A Andrievskii [P] (at.%) 25 20 15 10 50 30 20 10 40 Distance /nm Figure 14. Concentration profile of phosphorus in a triple junction for a nickel ± phosphorus alloy (3.6 at.% P) after heating to 400 8C at a rate of 5 K min71 (see Ref. 82). The alloy containing 25 at.% P corresponds to nickel phosphide (Ni3P). of Ni3P crystallisation via boundary segregations (Fig. 14).56, 82 It was found 55, 83 that during the annealing of palladium ± zirco- nium alloys (grain size 10 ± 25 nm), the grain boundaries became enriched in zirconium.55, 83 In the Pd90Zr10 alloy, the grain size increased after annealing at 350 8C, whereas in the Pd80Zr20 alloy, no increase in the grain size was noted even after annealing at 500 8C, which was attributed to greater enrichment of grain boundaries in zirconium in this alloy.This fact was determined based on the change in the lattice spacing. Experimental investigations of the boundary segregations in NM make use of unique analytical equipment (such as atomic tomograph, atomic force microscope) as well as advanced inves- tigation methods (for example, electron energy loss spectroscopy and so on).82 Interesting features of iron segregation in nano- crystalline iron ± yttrium alloys have been followed.14, 28 The possibility of crystallisation at compound boundaries due to the inflow of surface active elements has been considered theoretically by Rabkin,84 who noted the retarding effect of particles (so-called pinning effect) and pointed to low probability of complete stoppage of boundary migration.Nevertheless, stabilisation of the nanograin growth has been observed by many researchers [for example, during annealing of RuAl (see Fig. 7), iron or aluminium powders after milling in an attri- tor 29, 43, 85]. Evidently, to take into account numerous factors that govern the evolution of grain ensembles [such as boundary migration, diffusional segregation, phase crystallisation,84 vacancy injection by moving grains, the possible relaxation of the residual stresses, size effects as applied to the intergrain surface energy and integral free energy (see Fig. 13), and so on], it is necessary to perform a versatile computer simulation of the main processes occurring during heat treatment of NM.An attempt at Monte Carlo simulation of some of these factors has been reported.86 2. Phase transformations The presence inNMof a nanocrystalline structure, supersaturated solid solutions or unusual crystalline phases is due to non- equilibrium conditions of NM preparation. Experimental and theoretical studies of the thermal stability of these objects are few, although thermodynamic characteristics and the features of phase diagrams as applied to nanoparticles have been the subject of investigation of many researchers (see, for example, Refs 87 ± 89). However, phase transitions in compact NM (diffusiveness, shifts, etc.) have not been studied systemati- cally.Several attempts have been undertaken to carry out rough calculations of state diagrams with allowance for the componentThermal stability of nanomaterials dispersity (see, for example, Refs 87, 90 ¡¾ 93). Using the relations for the partial free energies of the system in the liquid and solid states (for the latter, the excess component of the boundary surface energy was taken into account), in the simple approxima- tion of regular solutions, relations were derived that reflect the influence of the component dispersity on, for example, the decrease in the eutectic temperature DTE: (6) DTE a DG2 Rlnx ,DG2 (7) , DTE a RlnxE ¢§ DSM2 DG1 (8) , DTE a RlnaO1 ¢§ xEU=O1 ¢§ xUa ¢§ DSM1 where Gi is the contribution of the excess boundary surface energy per mole, DGi=6Vi si/Li (Vi is the molar volume), si is the grain- boundary surface tension, x and xE are the limiting solubility and eutectic concentrations; DSMi is the melting entropy.92 These relations are equivalent, their practical value being dependent on the presence of particular data for components 1 and 2.Expressions (6) ¡¾ (8) lead to a size dependence of the type DTE*1/L. The calculated DTE and TE values for the TiC ¡¾ TiB2, TiN ¡¾ TiB2 and TiN ¡¾ AlN systems are summarised in Tables 3 and 4. It can be seen that a tangible decrease in TE can start when the dimensions of the dispersed component are several tens of nanometers, although one should remember that the calculations are rough.With more complicated approximations, the estimates cannot be brought to a numerical solution. The available thermo- dynamic information is inadequate for determining the solubility regions in the phase diagrams. The phase stability in multilayer films has been considered in terms of various statistical thermodynamic approaches. 94, 95 The presence of metastable structures is especially typical of NM prepared by mechanosynthesis or by film techniques. The Table 3. DTE values in TiC(TiN) ¡¾ TiB2 and TiB2 ¡¾ TiC(TiN) pseudo- binary systems with different degrees of dispersity of one of the compo- nents.92 L /nm DTE /K TiB2 ¡¾ TiC(TiN) TiC(TiN) ¡¾ TiB2 200 100 20 10 *35 *70 *350 *700 *45 *90 *450 *900 Note.In the polycrystalline state, TE for the TiC ¡¾ TiB2 system is 2790 K; for the TiN ¡¾ TiB2 system, 2870 K. Table 4. TE values for the TiN ¡¾ AlN pseudo-binary systems with different degrees of component dispersity (for macrodisperse specimens, TE=2715 K).91 L /nm Dispersed component TE /K TiN AlN 200 100 20 200 100 TiN, AlN 20 200 100 2660 2610 2160 2710 2690 2565 2650 2590 2110 20 861 formation of supersaturated solid solutions and compounds was detected in the Fe ¡¾ Cu, Fe ¡¾ Ni, Fe ¡¾ Ti, Fe ¡¾ Al, W¡¾ Cu, Ni ¡¾ Al, TiN ¡¾ TiB2, and TiN ¡¾ AlN systems, etc.14, 44, 50, 95 ¡¾ 104 Detailed study of the Fe ¡¾ Cu and Fe ¡¾ Ni systems (crystallite site 3 ¡¾ 50 nm) 96, 98, 99, 104 showed a pronounced increase in the solu- bility of the components in each other.Whereas iron dissolution in copper increases the thermal stability, the reverse process has virtually no influence on the grain growth parameters; two-phase Fe+Cu compositions were found to be less stable than single- phase ones. In multilayer films (superlattices), cubic structures are often formed instead of hexagonal ones due to epitaxy (examples are aluminium and niobium nitrides with the NaCl structure in combination with cubic titanium and zirconium nitrides;30, 50 and titanium with FCC structure in multilayer titanium ¡¾ alumi- nium films 105). Magnetron sputtering with both single-phase and two-phase TiB2+(0%, 25%, 50%, 75% or 100%) TiN targets at moderate deposition temperatures (*1508C) resulted in the synthesis of single-phase nanostructured boride ¡¾ nitride films of the following composition: Ti(B0.73N0.2O0.05C0.02)1.56 and Ti(B0.56N0.29O0.05..C0.1)1.32 with a hexagonal structure and Ti(B0.34N0.49. .O0.12C0.05)1.49 and Ti(N0.6O0.2C0.2)1.58 with a cubic structure. The composition formulae were determined using Auger analysis. Judging by these formulae, these films were essentially non- stoichiometric because the homogeneity regions of titanium diboride and nitride in the equilibrium state differ from these values (TiB1.89 ¡¾ 2.0 and TiN0.7 ¡¾ 1.2). However, the solubility of interstitial elements in films is rather high; therefore, fairly broad homogeneity regions appear in the state diagram of the TiB2 ¡¾ TiN system.102, 103 The equilibrium state diagram of the pseudo-binary TiB2 ¡¾ TiN system (see Table 3) is an eutectic type diagram, the mutual solubility of the components in this system being very low.A similar situation is found in the TiN ¡¾ AlN and NbN¡¾ AlN systems. While the equilibrium solubility of Al in TiN at 1000 8C equals *2 at.%, the cubic equiatomic phase (Ti,Al)N occurs in nanostructured films even at room temperature.30 Thus, the pattern of state diagrams of nanocrystalline systems { and the homogeneity regions of individual compounds substantially change. Study of the effect of annealing at 700 and 1000 8C (t=15 min) on the structure of Ti(B,N) films did not reveal decomposition: the films remained phase-homogeneous and the grain size changed insignificantly (Table 5).106 Some changes in the microhardness will be discussed below.An extensive study of the effect of annealing in the temper- ature range from 850 to 1000 8C on the structure and composition of and the chemical bonding in plasmochemical SiCN films has been reported.108 Using electron microscopy and X-ray diffrac- tion analysis and resorting to the data of infrared, Raman and photoelectron spectra of the films, it was shown that high-temper- ature annealing leads to a decrease in the amorphous matrix and yields oblong oriented a-Si3N4 nanocrystals (in the amorphous matrix of the initial films, 2 ¡¾ 3 nm crystallites were detected). Interesting examples of spinodal decomposition of nanostruc- tured solid solutions, TiN ¡¾ ZrN and TiN ¡¾ AlN, have been reported.30, 50, 109, 110 The dependences presented in Fig.15 allow one to follow the influence of the annealing temperature on the change in the microhardness of films of individual titanium and zirconium nitrides and multilayer (TiN ¡¾ ZrN) and doped [(Ti,Zr)N] films.110 It can be seen that starting from T>1000 8C, the microhardness of doped films tends to increase with an increase in the annealing temperature; this is due to evolution of the ultradisperse products of spinodal decomposi- { Unlike the equilibrium state diagrams of usual polycrystalline objects, in the case of nanocrystalline systems, it is appropriate to speak of quasi- equilibrium diagrams to mean their lability and possible variation with time.862 Table 5.Effect of annealing on the average size (L /nm) and the microhardness (HM /GPa) a of crystallites in boride ± nitride films.106 Microhardness /GPa Target TiB2 TiB2+25%TiN TiB2+50%TiN TiB2+75%TiN TiN 35 ± 39 32 ± 38 38 ± 45 32 ± 42 33 ± 40 a TheHMvalues refer to a procedure 107 largely eliminating the effects of the substrate and film thickness; i.e., here,HM is the microhardness of the proper films. Figure 15. Effect of the annealing temperature on the microhardness of monolayer ZrN (1) and TiN (2), 10-layer ZrN/TiN (3), doped (Zr,Ti)N (4) and 20-layer ZrN/TiN (5) films.110 tion. These films provide an example of so-called `smart' materials whose properties not only deteriorate during operation but can also be upgraded to a certain extent.Annealing of the (Ti,Al)N ± Si3N4 nanocomposition at differ- ent temperatures showed a microhardness maximum and a grain size minimum at 800 8C, which should also be attributed to spinodal decomposition of the (Ti,Al)N phase; a substantial decrease in the microhardness and an increase in the crystallite size is observed after annealing at 1200 8C (t=30 min in an atmosphere of 90 vol.% N2+10 vol.% H2).111 The nanocrystalline cubic phase (Ti,Al)N is highly stable against oxidation.103, 112 Therefore, it is not accidental that one of the (Ti,Al)N-based layers is highly resistant to change in industrial and pilot multilayer superhard nitride films.113 3.Relaxation and diffusion Relaxation processes in nanocrystalline compounds were ana- lysed in the review;114 the diffusion processes in nitride films have been characterised.30 If anNMhas been prepared by, for example, severe plastic deformation or vapour quenching and represents a disordered solid solution, a temperature rise restores the short- range order, establishes a long-range order and induces the growth of grains. These regularities were followed for the Ni3Al interme- tallic compound in calorimetric experiments.114, 115 However, the use of differential calorimeters does not always permit detection of processes related to the relaxation of internal microstresses during low-temperature annealing. The relaxation of residual stresses can be detected in X-ray diffraction studies based on line narrowing and splitting into doublets.Relaxation is believed to precede the grain growth.114 Turning back to the data of Table 5, we would like to note that some decrease in the microhardness during annealing of boride ± nitride films can, apparently, be attributed Initial film L HM L HM L HM 3 ±5 2.31.1 2.91.1 41 ± 46 40 ± 46 46 ± 52 53 ± 59 36 ± 40 5.44.0 9.98.8 5 44 4 36 3 28 21 20 200 600 1000 T /8C R A Andrievskii T0=1000 8C T0=700 8C 74.41.4 3.71.2 40 ± 46 40 ± 45 47 ± 54 52 ± 58 38 ± 42 773.41.9 7.93.8 10.27.0 8.14.4 13.06.6 to relaxation of the residual compressive stresses, where the grain size changed insignificantly.The residual stresses in nanostructured nitride and other films caused by differences in the linear thermal expansion coefficients and elastic properties in the film ± substrate system or in multi- layer films can be rather pronounced. For example, in nitride films, the residual compressive stresses can reach 10 GPa and they can contribute noticeably to the hindering of dislocation and fracture migration (and, correspondingly, to the increase in hard- ness).30 Stress relaxation processes consisting in migration, redis- tribution and annihilation of various defects as well as grain boundary migration are known to be diffusional processes. The activation energy of stress relaxation (E /kJ atom71) and the temperature ranges of this process for the films of some refractory compounds are given below 30 CrN Ti(C,N) TiN TiN Compound 240 ± 670 200 ± 300 450 ± 900 200 ± 400 250 ± 430 *120 400 ± 900 *200 T /8C E /kJ atom71 The given E values are similar in magnitude to the activation energies of nitrogen self-diffusion in nitrides (which is, more likely, a boundary rather than a bulk process).116 Comparison of the residual stress relaxation in titanium nitride and carbide ± nitride showed that an equal level of compressive stresses in carbide ± nitride films is attained at temperatures *200 8C higher than in TiN; therefore, the high-temperature hardness of Ti(C,N) is higher.30 The relaxation processes and the evolution of nanostructures in metals, alloys and intermetallic compounds (Fe, Cu, Ni; Al, Fe, Cu alloys; Ni3Al, see Fig. 12) subjected to severe plastic deforma- tion have been discussed in detail in a monograph.18 However, the mechanism of stress relaxation in NM as a whole is not entirely clear.The information concerning heterodiffusion processes is important for the analysis of the thermal stability of multilayer films. A study of the mutual diffusion in single-crystalline multi- layer TiN/NbN films (the thicknesses of separate layers are 4.4 and 12.3 nm for a total thickness of *1 mm; isothermal main- tenance at 750 ± 875 8C, non-isothermal heating to 1200 8C) revealed the predominant diffusion of titanium.117 The activation energies (Q /kJ per atom) for different temperature ranges have been determined.30, 117 T /8C Q 885 ± 930 430 830 ± 875 250 <830 115 The last Q value is similar to the activation energy for the self- diffusion of metal atoms in transition metal carbides and nitrides.116 According to estimates,117 the duration of homoge- nisation in multilayer TiN/NbN films and, correspondingly, the working life of this nanocomposite with *2 nm thick layers is *10 h at 750 8C or *2 h at 850 8C.To study heterodiffusion in multilayer films, one should employ high-precision X-ray diffrac- tion and electron-microscopy procedures and also use powerful sources such as synchrotron radiation. The versatile capabilities ofThermal stability of nanomaterials X-ray diffraction for the assessment of InxGa17x films with one *10 nm thick quantum well have been demonstrated.118 A study of the thermal stability of multilayer Mo/NbN films (the metal has a body-centred cubic lattice and the nitride has a face-centred cubic lattice) showed that the formation of the tetragonal (Mo,Nb)N phase starts after maintenance for 3 h at 1000 8C.119 The diffusion of oxygen in films determines the stability of films against oxidation.The abundance of interfaces in NM creates conditions for substantial oxygen streams. Characteristic features of oxidation of nanostructured films have been ana- lysed.30, 103, 112, 113, 120 The resistance of nitride films against oxi- dation is markedly increased by chromium or yttrium additives which `lock up' the crystallite boundaries.113 Amorphous films have demonstrated both a poorer oxidation resistance (800 nm TiN films) 120 and a higher scale resistance (2 ± 3 mm AlN ± TiB2 and AlN ± TiB2 ± SiC films) 112 compared to the same character- istics of crystalline films or bulk specimens of the same composi- tion.IV. Other stability aspects of nanostructures For some types of NM (for example, for nanoporous, nano- semiconducting and nanocarbon materials), studies of thermal stability have started only recently (see, for example, Refs 121 ± 126). It should also be noted that nanoparticle-related problems have been barely considered above, because, in view of the multiaspect character of agglomeration and sintering proc- esses, these topics require separate consideration.We would only like to note that the sintering of nanoparticles and the grain growth in NM have much in common. Figure 16 shows the temperature dependence of the compaction during sintering of ultradisperse powders of metals and metal-like substances and also ionic and covalent compounds.127 Whereas active sintering (compaction) of metals and metal-like or ionic compounds starts at homological temperatures (0.3 ± 0.5)Tm , the powders of cova- lent compounds (B, Si, Si3N4) are actively sintered only at T5(0.8 ± 0.85)Tm, i.e., at much higher temperatures. Aluminium nitride, as an ionic-and-covalent compound, occupies an inter- mediate position. This difference is due to specific features of the diffusion mobility in objects with different types of chemical bonds.In all probability, the temperatures of the onset of the 0.9 0.7 0.5 0.6 0.4 Figure 16. Temperature dependence of compaction during sintering of various ultradisperse powders.125 (1) Si, (2) Si3N4, (3) B, (4) AlN, (5) WC, (6) ZrC, (7) MgO, (8) Mo, (9) CaF2, (10) Ni. Relative density 12345678910 0.8 T/Tm 863 active grain growth in these materials should lie within the same ranges. The aspects of NM thermal stability considered above are far from exhausting the general problem of nanostructure stability. For example, the NM stability is appreciably affected by various fields (force, radiation, etc.). In particular, the grain growth upon superplastic deformation of fine-grained ceramic materials has been considered theoretically taking into account the additional driving forces for migration.128 The microstructure changes induced by NM implantation or irradiation have been the subject of only a few studies (see, for example, Refs 129 ± 131).The appearance of nanoclusters in metal alloys (Fe ± Ni, Cu ± Ni, Fe ± Cr, etc.) at high levels of radiation damage has been noted.132 The formation of nanocrystalline diamonds (L=2 ± 70 nm) was detected on graphite irradiation with Ar+ ions (the fluence was *1022 cm72).133 Amorphisation of nanocrystalline zirconium oxide on exposure to radiation by high-energy Xe2+ and Ne+ ions has been reported;131 meanwhile, gold nanoparticles with dimensions of 3 nm remained crystalline after irradiation.It is noteworthy that NM based on ZrO2 are under vigorous research now (see, for example, studies 134, 135). The behaviour of irradiated NM is an extensive field for further research. It should be emphasised that the stability of NM largely depends on the process used for their manufacture. A number of techniques such as high-pressure processes, sintering with con- trolled heating rates, pulse sintering, pulse modes of electro- deposition, moderate substrate temperatures during film deposition, the formation of multiphase structures and boundary segregations and some other are widely used in the preparation of NM (see, for example, the studies 24, 31, 52, 136 ± 142). However, problems of deliberate selection of conditions for the production of the initial NM with a narrow nanocrystallite size distribution, the optimal content of doping or impurity elements for decreasing the grain-boundary surface energy or for the formation of barrier layers, and many other problems are far from being solved.It is not accidental, therefore, that the Japanese national program `Material Nanotechnology' pays considerable attention to the development ofNMwith optimal structures and to the prevention of anomalous grain growth phenomena.143 V. Conclusion When generally evaluating of the state-of-the-art research into the thermal stability of NM, we would like to stress the following points. On the one hand, there exist quite a few experimental and theoretical facts demonstrating the possibility of production of thermally stable NM; on the other hand, examples of anomalous grain growth in NM are documented. Unfortunately, our knowledge is still inadequate to unambig- uously explain many facts and to compare the efficiency of various methods for nanostructure maintenance, say nothing of the possibility of making predictions. Note that the latter depends not only on the theoretical works and availability of reliable experimental information but also on the general level of the nanostructure theory.For example, it is not entirely clear what are the grain dimensions (of phase components) down to which the classical thermodynamics remain valid. The point is that objects with crystallite dimensions of *1 ± 2 nm or even less are already involved inNMinvestigation and application (see Table 5 and the studies 32, 103, 106, 144 ± 146).A*1 nm grain of titanium nitride with a lattice spacing of*0.424 nm contains*8 (*23) unit cells, i.e., in essence, it is a cluster-consolidated object (unlike cluster- aggregated materials 16, 147). In this connection, the points such as the effect of the grain boundary curvature on its energy parameters, the applicability of the concepts of statistical thermo- dynamics or thermodynamics of irreversible processes, the type of excessive thermodynamic functions, the energy characteristics of triple junctions, and so on have been scarcely studied so far. These aspects have to be thoroughly analysed to be used in estimations.864 Note that research into the determination of the energy character- istics of the surfaces of isolated clusters, cluster aggregates and colloids is also in progress.148 ± 150 In the long-term American forecasts (for 10 ± 15 years) con- cerning the arrival of nanotechnological products on the world market, nanomaterials occupy a leading position � of the putative total volume of 1 trillion US dollars, a sum of 340 billion US dollars is predicted for NM.151, 152 It is clear that substantia- tion of the operation capacity of NM, including thermal stability characterisation, is among the most important problems facing the science of nanostructured materials.This work was supported by the `Science for Peace' (SfP 973529) and `Integratsiya' programmes.References 1. H Gleiter, in Deformation of Polycrystals. 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ISSN:0036-021X    

出版商:RSC    

年代:2002

数据来源: RSC