Abstract. Data on hydrogenation of mechanical alloys and intermetallic compounds formed in the course of mechanical alloying and mechanical milling are generalised. It is shown that mechanochemical methods make it possible to solve a number of problems arising in the hydrogenation of metals and alloys and to synthesise novel hydrides and nanocomposite materials. The mechanism of hydrogenation of mechanical alloys is discussed.The bibliography includes 98 references. I. Introduction The diversity of practical tasks associated with the accumulation of hydrogen and the use of metal hydrides and intermetallic compounds (IMC) in various technologies stimulate the develop- ment of research directed to the search for novel metal ± hydrogen systems with definite structural, kinetic, and thermodynamical characteristics.This type of research consists of the use of alloys and solid solutions based on the known hydride-forming metals, IMC and composite materials obtained from hydride-forming components containing binders that are inert with respect to hydrogen.1 LaNi5, TiFe, and magnesium and its alloys are considered to be the most promising from the viewpoint of the accumulation of hydrogen.2, 3 These materials are traditionally prepared by alloying and by the methods of powder metallurgy.One substantial problem is the activation of these materials which are usually covered by a layer of oxides and hydroxides, thus preventing the chemisorption of hydrogen at the surface and resulting in long induction periods and low reaction rates in the initial hydrogenation.To achieve the highest reaction rate it is necessary to perform several cycles of hydrogenation and dehydrogenation (cycling), which result in the dispersion of the alloy and the development of its microstructure. The microstructure plays a significant role in further cycling and affects considerably the reaction rate and the amount of absorbed hydrogen.In addition, traditional methods of preparation of alloys are mainly restricted by thermodynamically stable compounds and solid solutions. Mechanochemical methods (mechanical alloying, mechanical milling, etc.) consist in mechanical treatment of metal powders in high-energy planetary ball mills. This treatment results mainly in plastic deformation of the material under conditions that fix its metastable state.These methods can be applied to the synthesis of metastable phases: amorphous phases,4 supersaturated solid solutions, non-stoichiometric intermetallic compounds,5 quasi- crystals,6, 7 and composites with different microstructure and composition including those with non-interacting components.8 These phases often exhibit unusual physicochemical properties and enhanced reactivity.8, 9 Mechanical alloying, as shown in the present review, can be a promising method for the preparation of materials with high reactivity with respect to hydrogen.II. Magnesium systems 1. Problems arising in the hydrogenation of magnesium and its alloys Magnesium and its alloys are promising materials for the accu- mulation of hydrogen.The stoichiometric hydrogen content in magnesium hydride is 7.6 mass% (Table 1), which exceeds its content in other known hydrides and also in gas cylinders.10, 11 However, the relatively high thermal stability of magnesium hydride (equilibrium hydrogen pressure of 0.1 MPa is achieved at *560 K), insufficiently high rates of hydrogenation of magne- sium and dehydrogenation of MgH2, long-lasting activation, and incomplete conversion of magnesium into the hydride (which decreases the hydrogen content to 5 mass %) have stimulated the appearance of research projects aimed at the possibility of improving the above-mentioned characteristics.It has been established to date that the formation of magne- sium hydride is connected with the processes of formation and growth of MgH2 nuclei.For the majority of metal ± hydrogen systems, the first stage of the interaction of hydrogen with a metal is characterised by the formation of a solid solution of hydrogen in a metal, in which nuclei of the hydride phase are crystallised.12 The formation of solid solutions of hydrogen is not typical of magnesium, or more precisely, their concentrations are low.13 ± 15 The hydride nuclei appear at the surface of magnesium particles; when the nuclei overlap, the reaction rate is controlled by diffusion processes.`Diffusion retardation' then occurs, which prevents the complete conversion of magnesium into the hydride. In the first cycle of hydrogenation, the extent of conversion does I G Konstanchuk, V V Boldyrev Institute of Solid State Chemistry and Processing of Mineral Raw Materials, Siberian Branch of the Russian Academy of Sciences, ul.Kutateladze 18, 630128 Novosibirsk, Russian Federation. Fax (7-383) 232 28 47. Tel. (7-383) 232 96 00 (I G Konstanchuk), (7-383) 232 15 50 (V V Boldyrev) E Yu Ivanov TOSOH SMD, 3515 Grove City Road, Grove City, OH 43123, USA. Fax (1-614) 875 00 31. Tel. (1-614) 875 79 12 Received 10 March 1997 Uspekhi Khimii 67 (1) 75 ± 86 (1998); translated byMG Ezernitskaya UDC 541.44+541.124 Interaction of alloys and intermetallic compounds obtained by mechanochemical methods with hydrogen I G Konstanchuk, E Yu Ivanov, V V Boldyrev Contents I.Introduction 69 II. Magnesium systems 69 III. Hydrogenation of FeTi and LaNi5 undergoing mechanical milling 76 IV. The direct formation of hydrides during mechanical milling 77 V.Conclusion 77 Russian Chemical Reviews 67 (1) 69 ± 79 (1998) #1998 Russian Academy of Sciences and Turpion Ltdnot usually exceed 0.9, in subsequent cycles it does not exceed 0.6 ± 0.7.16 ± 18 Kinetic characteristics of hydrogenation processes in the first cycle differ substantially from those in the later cycles.The surface layer of magnesium oxide, which is always present on the original metal particles before the first hydrogenation, inhibits the for- mation of magnesium hydride nuclei, because no dissociative adsorption of hydrogen on this layer occurs.19 Therefore, the rate of hydrogenation of magnesium in the first cycle is deter- mined by the processes of destruction of the oxide film and formation of a metallic surface with hydrogen chemisorption ability.19 ± 21 A model of the first cycle of hydrogenation of magnesium taking into account the dynamics of the destruction of the oxide film and kinetics of the nucleus growth has been proposed.22 During activation through cycling, exfoliation of the oxide film occurs. The initial reaction rate is determined by the dis- sociative chemisorption of hydrogen on a pure metal surface, and the nucleus growth occurs due to the surface diffusion within the layer of the chemisorbed hydrogen.23 An increase in the number of nuclei with an increase of DP (the difference between the pressure in the reactor and the hydrogen equilibrium pressure for magne- sium hydride at the temperature of the experiment), which is a driving force of the reaction, or the introduction of catalysts accelerates the initial stage of hydrogenation, leads to the faster formation of the continuous layer of the hydride, and to a change over to diffusion conditions at lower conversion rates.24 It was shown by experiments on the movement of the Kirkendal mark that hydrogen atoms or ions are the diffusing species.25 The rate of decomposition of magnesium hydride depends on the dynamics of the formation and growth of nuclei of magnesium metal.23 The desorption of hydrogen from the surface of the solid reaction product, metallic magnesium, is the limiting stage of the reaction; for this reason, the size of the Mg/MgH2 interface plays a significant role.Thus, the rate of magnesium hydrogenation in the first stage and the rate of decomposition of magnesium hydride are deter- mined by the processes of adsorption (or desorption) of hydrogen on the metal surface (in the first hydrogenation, by the formation of the metal surface), by the rates of formation and growth of nuclei of the reaction product, and after the creation of the continuous layer of MgH2 in the hydrogenation, by the rate of diffusion of hydrogen through the hydride.Hence, changes in the kinetics of hydrogenation and dehydrogenation can be achieved by controlling these stages. In the majority of studies dealing with the improvement of kinetic and thermodynamic characteristics of magnesium in the interaction with hydrogen, its alloys with other metals have been used. However, it should be noted that in the case of magnesium, the choice of these metals is rather limited for the enthalpy of mixing of magnesium with many elements of the Periodic Table is positive.To date, only two magnesium intermetallic compounds are known that absorb hydrogen reversibly, viz., Mg2Ni and Mg51Zn20, and the corresponding hydrides, MgNiH4 (Ref. 26) and Mg51Zn20Hy (y = 90 ± 95).27 The hydride Mg2NiH4 is regarded as a promising one for the accumulation of hydrogen, because it interacts with hydrogen at a high rate at temperatures of about 473 ± 573K and PH2 '0.5 ± 1.0 MPa,28 although its hydro- gen capacity is less than that of magnesium (see Table 1).At the first stage of hydrogenation of Mg2Ni under the conditions ensuring fast heat removal and at rather high hydrogen pressures, chemisorption of hydrogen on the surface of IMC with an effective activation energy of 14.00.8 kJ mol71 is the limiting stage.29 However, due to high rates of hydrogenation and dehy- drogenation, the processes of heat and mass transfer play a significant role and may become limiting, especially if this IMC is used on a technological scale.The creation of composite materials containing inert metallic fillers is the way to solve the problems of heat and mass transfer,30 but this further decreases the hydrogen capacity of an accumulator.The interaction of IMC of magnesium and of some other elements, such as rare earth metals, Cu, Al, Ca, with hydrogen leads to the decomposition of the intermetallic compound to form a mixture of two hydrides or a hydride and a metal (or another IMC).This process was termed hydrogenolysis,31, 32 in general it can be represented by the equation AxBy+ m 2 H2 xAHm7n+yBHn , where A, B are elements constituting the IMC; n=0 if the second element does not give a hydride. Hydrogenolysis can be reversible, as in the case of Mg2Cu,33 and irreversible, as for most IMC of magnesium with rare earth metals and Al.11, 32, 34 ± 40 The reversibility is determined by the relationship between the values of the heat of formation of the binary hydride and the initial IMC.The mechanism of hydro- genolysis has not been studied in detail. No complete separation of hydrides or a hydride and a metal (IMC) resulted from hydro- genolysis occurs. The material obtained is composed of particles with a developed interface between its constituents 35, 36 which determines the reactivity of these particles in further hydrogena- tion ± dehydrogenation cycles. Of the systems that undergo hydrogenolysis, the main atten- tion has been paid to IMC of magnesium with rare earth metals.Hydrogenolysis results in a non-stoichiometric hydride LnHx, which plays the role of a `hydrogen pump', which facilitates the delivery of hydrogen to the magnesium surface and thus accel- erates hydrogenation.35 ± 37, 41 However, no significant enhance- ment of the kinetics of dehydrogenation of these reaction mixtures occurs in comparison with pure MgH2.34 ± 36, 40 In order to accelerate dehydrogenation, it is necessary to introduce elements that facilitate the processes of hydrogen desorption and the formation of the metallic phase nuclei, for example, d-met- als.34, 37, 42 ± 44 These additives form a separate phase as a result of cycling. After hydrogenolysis the material is a virtually multi- phase system, its features being typical of those for multi-phase alloys.Table 1. Characteristics of hydrogen storage systems.10, 11 Method Hydrogen content Hydride density Accumulated energy Hydrogenation of storage /g cm± 3 rate (mass %) /1022 at cm± 3 /kJ g71 /kJ cm73 H2, gas 1.2a 0.9 7 142.7 2.13 7 20 MPa, 300 K H2, liquid 412 a 4.2 7 142.7 9.96 7 LaNi5H6 1.4 5.5 6.2 2.0 13.04 High TiFeH2 1.9 6.3 5.5 2.7 14.93 " MgH2 7.6 6.6 1.5 10.8 15.64 Low MgNiH4 3.6 5.8 2.7 5.1 13.75 High Gasoline 7 7 7 48 38 7 a With account of the mass of the storage system. 70 I G Konstanchuk, E Yu Ivanov, V V BoldyrevTwo- and multi-phase systems are alloys of magnesium with other elements, the composition of which does not correspond to the stoichiometric composition of intermetallic compounds. These can be alloys with Ni, Cu, rare earth, and some other metals. Hydrogenation of alloys of magnesium with rare earth metals yields a mixture of binary hydrides, as in the case of hydro- genolysis of IMC.The compositions formed differ, first of all, in specific features of the microstructure and in the size of the magnesium/lanthanide hydride interface. In the decomposition of a non-stoichiometric alloy, the interface is, as a rule, smaller than that in hydrogenolysis of IMC, which results in a decrease in the rates of the interaction of these systems with hydrogen.The alloys Mg± Ni and Mg± Cu are two-phase systems containing magnesium and IMC Mg2Ni or Mg2Cu. Hydrogena- tion of these alloys can be carried out in two versions: under conditions where both phases are hydrogenated and under con- ditions where only magnesium is hydrogenated and the interme- tallic compound serves only as a catalyst.However, in both cases dissociative hydrogen adsorption takes place on clusters of the d- element resulting from segregation occurring in the course of hydrogenation ± dehydrogenation.{ Studies of the surface of alloys by ferromagnetic resonance,45 X-ray photoelectron and Auger spectroscopy 46, 47 gave reason to adopt a model of the structure of the surface of the alloy and the scheme of the initial stage of hydrogenation, which accounts for the catalytic activity of transition metals.47 ± 49 The essence of the model is as follows: 1.Activation of the material in hydrogenation ± dehydro- genation cycles leads to dispersion of the sample, that is, to the formation of a new clean surface. 2. In the interaction with traces of oxygen and water, clusters of a d-element are formed on this surface (the so-called oxidative segregation). 3. Dissociative chemisorption of hydrogen takes place on the clusters of the d-element. 4. The hydrogen atoms formed diffuse into the bulk of the material along the metal/oxide interface . The course of the reaction, as was mentioned above, depends largely on the size of the magnesium/catalyst interface and accordingly, on the size of magnesium particles in the alloy.The size of magnesium crystallites determines the degree of its conversion into the hydride. The process of nucleation is accel- erated under the action of catalytic additives, which leads to faster formation of a continuous layer of the hydride and to a change to the diffusion control of the reaction.24 In the case of larger magnesium particles, these processes slow down and practically completely stop the reaction long before complete conversion.The portion of magnesium that does not enter the reaction decreases and the hydrogen content increases with a decrease in the size of particles in the alloy. Hence, the main problems in the creation of two- and multi- component materials for accumulation of hydrogen are, first, the choice of a catalyst and determination of its optimum concen- tration at which the maximum hydrogen content is achieved over a given time and second, the creation of the microstructure of the sample ensuring the maximum magnesium/catalyst interface.In addition, the problem of activation exists for all the alloys. The rate of the first hydrogenation is determined by the rate of the rupture of the oxide film, by the rates of the formation and growth of magnesium hydride nuclei.The effect of additives reduces most commonly to a decrease in the induction period and to an increase in the rate of the first hydrogenation as compared to analogous characteristics for pure magnesium,17, 24, 50, 51 however, the dura- tion of the first cycle remains too high. 2. The interaction of mechanical alloys of magnesium with hydrogen A new approach to the creation of two- and multi-component systems interacting with hydrogen has been proposed.52 ± 55 It is based on the use of mechanical alloying. In the initial stage of processing of a mixture of metal powders (in *5 min) in centrifugal planetary mills, composites are formed with a consid- erable interface similar to those resulting from hydrogenolysis of IMC, but the hydrogenolysis stage is excluded.These composites were termed mechanical alloys (MA). A typical (rather porous) structure of a particle of a mechanical alloy with uniformly distributed particles of a metal catalyst within the magnesium matrix is shown in Fig. 1.55 Wide angle X-ray scattering (WAXS) of these materials did not reveal the formation of either interme- tallic compounds or solid solutions.The processes of hydrogenation and dehydrogenation have been studied for the systems Mg± Ce;56 Mg± Fe, Mg± Co, Mg± Ti, Mg± V, Mg± Nb, Mg± Cr, Mg± C, Mg± Si;52 ± 54, 57 ± 59 Mg± Ni;29, 52, 53, 60 ± 63 Mg± TiO2;64 ± 66 Mg±V2O5, Mg± Cr2O3;66 Mg±MnO2; Mg± Fe2O3, Mg± NiO;67 Mg±MmM05 (MM05 is IMC of the type LaNi5, Mm is a mischmetall, M0 is Ni with admixtures of Al, Co, and Mn);68, 69 Mg± FeTi,70 etc.Thus, mechanical alloying allows the considerable extension of the range of systems under study including mixtures that could not be alloyed by traditional methods, such as Mg± Fe, Mg± Ti, Mg± Nb, and Mg± Cr with positive enthalpy of mixing of the components, or mixtures of magnesium with oxides.It is also very difficult to prepare from the latter composites with homogeneous distribution of the components by traditional alloying methods. The additives to magnesium used in mechanical alloys differ in their nature. Thus the above-mentioned additives can be divided into six groups. 1. Ni forms with magnesium the intermetallic compound Mg2Ni, which can absorb and liberate hydrogen. 2. Ce, Nb, and Ti form hydrides CeH3, NbH2, and TiH2, which can serve as `hydrogen pumps' due to the change in stoichiometry. 3. MmM0 and FeTi are intermetallic compounds, which absorb hydrogen reversibly under milder conditions compared to magnesium. 4. Fe, Co, and Cr are metal catalysts, which do not form hydrides themselves under the conditions studied.{ Segregation processes occur due to the presence of traces of oxygen and water in the hydrogen. Figure 1. A particle of mechanical alloy Mg± Ni at a fracture; the image in the Ni Ka radiation. Interaction of alloys and intermetallic compounds obtained by mechanochemical methods with hydrogen 715. Si and C form with magnesium compounds with predom- inantly covalent bonds. 6. Metal oxides. All the types of catalytic additives accelerate remarkably the first hydrogenation as compared to that of pure magnesium 52, 67 (Fig. 2). The character of the action of additives (except for Si and C) seems to be the same and consists in the acceleration of the stage of chemisorption of hydrogen on the surface of a catalyst.It is of note that mechanical alloys Mg± Ni are hydrogenated during the first cycle faster than the corresponding ordinary alloys Mg± Ni (Fig. 3).61 The rate of the first hydrogenation depends on the nature and concentration of the catalyst, but in almost all the systems examined, the reaction begins at the maximum rate (see Figs 2, 3). This means that the surface of the catalyst in mechan- ical alloys, unlike that in traditional alloys, is accessible to hydro- gen.X-ray photoelectron spectroscopic studies 29, 71 of the surface of MA have shown that it is covered with a layer of organic impurities and oxygen-containing magnesium compounds. The thickness of this layer is small, and it does not seem to affect the hydrogenation process. The profiles of concentration of the elements for magnesium MA containing 50 mass% Ni are shown in Fig. 4. The surface layer (3 ± 5 nm) contained neither metallic magnesium, which appeared at a depth of 40 nm, nor nickel, which appeared at a depth of *4 ± 10 nm in the oxidised state and at a depth of 40 nm in the metallic state. The presence of oxygen throughout the entire layer provides evidence for the destruction of the surface MgO film and its dispersion in the matrix.Carbon-containing impurities also penetrate into the bulk upon mechanical alloying. The chemical state of the elements in the near-surface layer of MA differs significantly from that in the original metals. The contour of the nickel 2p3/2 line at a depth of 40 nm corresponds to metallic nickel and to the nickel implanted into an inert matrix as isolated atoms.In addition, charge transfer from magnesium to non-bonding levels of the nickel atoms has been observed. In this case, a portion of magnesium is in the state to which the formal oxidation degree of +1 can be ascribed. The chemical state of oxygen also differs from that in the original metals and MgO, which might be due to defectiveness of the surface layer of MA.29, 71 The nickel atoms resulting from the chemical interaction between Mg and Ni in the course of mechanical alloying bear an excessive electron density and are active catalysts of hydrogen dissociation.The nickel atoms are, probably, present not only in the surface layer, but also in the bulk of the sample forming a 0 50 100 150 t /min 1.0 2.0 [H] (mass %) b 1 2 3 10 20 t /h 2.0 4.0 [H] (mass %) a 1 2 3 5 4 7 6 8 Figure 2.Kinetics of the first hydrogenation of magnesium and its mechanical alloys. (a): T=625K and PH2=1.5 MPa; (1) Mg+5 mass%of Co; (2) Mg+ 5 mass% of Nb; (3) Mg + 5 mass% of Fe; (4) Mg + 5 mass% of C; (5) Mg+8 mass%of Ce; (6) Mg+5 mass%of Ti; (7) Mg+5 mass% of Si; (8) metallic magnesium, particle size is 20 mm; (b): T = 615 K, PH2 = 1.5 MPa; (1) Mg + 8 mass% of Fe; (2) Mg + 10 mass%of Fe2O3; (3) Mg+10 mass%of NiO. 0 10 20 t /min 0.2 0.4 0.6 1 2 3 4 5 a Figure 3.Kinetics of the first hydrogenation of mechanical alloys of magnesium with nickel. [Ni] (mass %): (1) 60, (2) 53, (3) 35, (4) 1; (1 ± 4) at PH2 = 1.8 MPa, T = 583 ± 593 K; (5) ordinary alloy of Mg with 25 mass% of Ni at PH2 = 2.1 MPa, T=673 K.[Mg, Ni, O, C] (at.%) 15 10 5 0.2 0.1 Ni/Mg 0 100 200 h /nm 5 3 4 2 1 Figure 4. The dependence of the chemical composition on the etching depth (h) for a mechanical alloy of Mg with 50 mass%of Ni. (1) Mg, (2) Ni, (3) O, (4) C, (5) Ni/Mg. 72 I G Konstanchuk, E Yu Ivanov, V V Boldyrevsupersaturated solid solution around the nickel particles upon dispersion of nickel within the matrix.Other MA of magnesium with metals and IMC, even those having a positive enthalpy of mixing, seem to have a similar struc- ture. This can be indirectly proved by the fact that the initial hydro- genation rate of mechanical alloys Mg± Fe and Mg± Ni depends linearly on the square of the concentration of the catalyst inMA.59 Hence, disordering of the surface layer, the rupture of the oxide film, and dispersion of the catalyst take place in mechanical alloys.Disordering of the surface layer makes the catalyst accessible to hydrogen, which is the main reason for the rapid first hydrogenation of MA as compared to that of alloys and mixtures of the same composition.29, 71 It should be noted that this surface structure is quite stable over a long period: the initial rate of hydrogenation of MA that have been stored in air for 9 ± 12 months did not differ from that of freshly prepared samples.29 The high reactivity (with respect to hydrogen) of mechanical alloys of magnesium with transition metal oxides may also be accounted for by the presence of metallic particles and partially reduced metal oxides at the surface formed in the course of mechanical processing.64 ± 67 The efficiency of the action of metal oxides as catalysts depends on the ease of their reduction.66 For the majority of systems examined (except for Mg± Ce and, under special experimental conditions that will be considered below, Mg± Fe and Mg± Co), magnesium hydride (the second phase, catalyst) is the product of the first hydrogenation.The degree of conversion of magnesium into the hydride, as in two- phase systems, in these MA depends on their microstructures.A model of a microstructure reflecting qualitatively the initial stage of hydrogenation is presented in Fig. 5: MgH2 is formed around the catalyst particles. It is difficult for hydrogen to penetrate through the layer of hydride; that is why the reaction slows down with an increase in the thickness of the layer.The thickness of the hydride layer, which depends on the distance between particles of the catalyst, decreases with an increase in the concentration of the catalyst, and the degree of conversion also increases. However, the hydrogen content (mass %) in the alloy decreases due to the large mass of catalyst. It has been found experimentally 59, 63 that the optimum catalyst content was in the range of 2 mass%± 25 mass %.In subsequent cycles, MA retain high rates of hydrogenation and dehydrogenation (Figs 6, 7) and rather high hydrogen capacity.52 In the case of the Mg± Ni mechanical alloy, the intermetallic compound Mg2Ni is formed after 3 ± 4 cycles of hydrogenation ± dehydrogenation, mainly according to the equation 61 2MgH2+Ni =Mg2Ni +2H2 .Further, kinetic characteristics of the system do not differ from those of the ordinary two-phase system Mg± Ni or the interme- tallic compound Mg2Ni.29, 63 Kinetic characteristics of the mechanical alloy Mg± Ce in the cycling are also comparable with those of hydrogenation and dehydrogenation obtained in the hydrogenolysis of Mg± CeHx mixtures (Fig. 8).56 This shows that the size of the interface inMA has the same order of magnitude as the interface in the mixtures obtained in the hydrogenolysis. As for MA composed of thermodynamically immiscible components, one could expect rapid destruction of a sample in repeating hydrogenation ± dehydrogenation cycles due to exfolia- tion of the system. However, these systems appeared to be rather stable in cycling.A mechanical alloy Mg± Fe retains its hydrogen capacity for *70 cycles, and the rate of hydrogenation even Mg Ni Ni Ni Ni a Solid solution of Ni in Mg b MgH2 Mg± 5 mass%of Ni Mg± 55 mass%of Ni Figure 5. A model of the microstructure of mechanical alloy Mg± Ni (a) and the illustration of the effect of the Ni concentration on the degree of conversion (b). 0 20 40 t /min 2 4 [H] (mass %) 1 23 4 5 6 7 8 Figure 6. Kinetics of hydrogenation for the fifth cycle, PH2 = 1.5 MPa, T=625 K. (1) Mg, particle size is 20 mm, mechanical alloy; (2)Mg+5 mass%of Nb, (3) Mg+5 mass%of Fe, Co or Ni, (4) Mg+5 mass%of Ti, (7) Mg+ 5 mass% of Si, (8) Mg + 50 mass% of C; (5) intermetallic compound Mg2Ni; (6) Mg, particle size is equal to the size of particles of the mechanical alloy (P=0.7 MPa, T=583 K). 2 4 0 20 40 t /min [H] (mass %) 1 2 3 4 5 6 Figure 7. Kinetics of dehydrogenation, P=0.1 MPa. Mechanical alloy: (1) Mg + 5 mass% of Ni, T = 615 K; (2) Mg + 5 mass% of Fe, T = 625 K; (3) Mg + 5 mass% of Nb, T = 625 K; (4) Mg+5 mass% of Ti, T = 625 K; (6) Mg + 5 mass% of Si, T= 621 K; (5) Mg, particle size is 20 mm, T=625 K. Interaction of alloys and intermetallic compounds obtained by mechanochemical methods with hydrogen 73gradually increases;59 a mechanical alloy Mg± TiO2 retains its kinetic characteristics at least up to the 39th cycle 64 (there are no data on longer cycling). It was shown 59, 72 that complex competing processes includ- ing sintering and dispersion of the sample occur in a Mg± Fe system in the hydrogenation ± dehydrogenation.Hydrogen under- goes dissociation on iron and it further diffuses into the bulk of the sample along the interface and grain boundaries to form an Mg/Fe adsorption layer at the interface in which rather high mobility of both hydrogen and metals is observed. Some experi- ments on the annealing of a contact pair Mg/Fe using a micro- probe analysis have shown that the principal possibility is for iron to diffuse along the surface of magnesium in a hydrogen atmos- phere.59, 72 This favours an increase in the Mg/Fe contact area and in the hydrogenation rate.When a definite hydrogen concentra- tion in the adsorption layer at the interface Mg/Fe is achieved, a ternary hydride of the composition Mg2FeH6 crystallises at a noticeable rate, which was observed at T5615 K.At hydrogen pressures exceeding the equilibrium pressure over MgH2, catalytic hydrogenation of magnesium occurs in parallel, often at a higher rate; the synthesis of Mg2FeH6 can also occur according to the equation 2MgH2+Fe+H2 Mg2FeH6 . However, since the equilibrium hydrogen pressure forMg2FeH6 is lower than that for magnesium hydride 57 (Fig. 9), the synthesis of the latter was successfully accomplished directly from the ele- ments. This reaction has been described in detail.59, 72 Analogous processes seem to occur in the hydrogenation of a mechanical alloy Mg± Co. This system does not belong to immiscible ones; however, for this system only one intermetallic compound MgCo2 is known, which does not absorb hydrogen under ordinary hydrogenation conditions. In the hydrogenation of mechanical alloys in this system, two hydrides, viz., Mg2CoHx (x=4 ± 5) and Mg3CoH5 , were detected 58, 59 which, like Mg2FeH6 , can be synthesised from the elements (without the intermediate stage of formation of magnesium hydride).Decom- position of ternary magnesium ± cobalt hydrides gave a previously unknown IMC with composition close to Mg2Co and having a cubic structure.This compound interacts with hydrogen even at room temperature to form solid solutions and at elevated temper- atures to form ternary hydrides (Table 2). It should be noted that ternary magnesium ± iron and magne- sium ± cobalt hydrides could not be detected for a long time due to the lack of hydride-forming IMC in the systems Mg± Fe and Mg± Co.Yvon et al.73 considered the hydride Mg2NiH4 as a complex compound composed of the Mg2+ cation and the [NiH4]47 anion. Assuming that the 18 electron rule, which determines the content of a ligand (H in this case), is valid for these complexes, they have predicted the possibility that hydrides Mg2FeH6 and Mg2CoH5 exist. Research by these authors in 1984 ± 1985 aimed at synthesising these hydrides was success- ful.74, 75 At the same period, the hydrides were found in the hydrogenation of mechanical alloys.76 The lack of a method for the preparation of a material with a large Mg/Fe and Mg/Co interface caused the authors 74, 75 to use a mixture of pressed metal powders.The reaction had to be conducted for weeks under drastic conditions, nevertheless, complete conversion of these mixtures into the hydrides was not achieved.Neither the second hydride Mg3CoH5 existing in the Mg± Co system nor the inter- metallic compound Mg2Co could be isolated and identified. The magnesium ± iron and magnesium ± cobalt ternary hydrides were examined as catalysts for the hydrogenation of unsaturated hydrocarbons. These catalysts manifested catalytic activity comparable with that of catalysts based on platinum- group metals, and high selectivity in processes of partial reduction 0 30 t /min [H] (%) 50 4 3 2 1 Figure 8.Kinetics of hydrogenation at T=603K and PH2=1.9 MPa. (1) mechanical alloy Ce + 25Mg; (2) CeMg12; mixture: (3) Mg + 10 mass%of LaNi5; (4) Ce+12Mg. T /K 1073 (1/T) /K71 1.4 1.5 1.6 600 650 700 1 0 71 1 2 0.2 1.0 2.0 3.0 P /MPa ln (P /MPa) Figure 9.Temperature dependence of equilibrium pressures of hydrogen over the hydride phases MgH2 (1) and Mg2FeH6 (2). Table 2. Crystallographic data for novel ternary hydrides and intermetallic compounds. Compound Unit cell Cell parameters /nm Ref. Mg2FeH6 Cubic, a=0.6443 74 Fm3m a=0.64420.0002 59 Mg2CoHx , Tetragonal a=0.44770.0001, 59 (low-tempera- c=0.66120.0001, ture modifica- a=0.4480(2) 75 tion) a c=0.6619(3) Mg2CoHx , Cubic a=0.644 59 (high-tempera at 480K ture modifica- tion) Mg3CoH5 Orthorhombic a=0.4675+ 0.0002, 59 b=0.8073+0.0003, c=1.00910.0003, Z=4 `Mg2Co' Cubic a=1.143+ 0.001 58, 59 a The temperature of a reversible phase transition of Mg2CoHx from the low-temperature modification to the high-temperature one is 470 K, DH for the phase transition is*2 kJ mol± 1.58, 59 74 I G Konstanchuk, E Yu Ivanov, V V Boldyrevof acetylene derivatives and compounds with several double bonds (Table 3).59, 77 Hydrogenation and dehydrogenation of MA containing, for example, two or more IMC can be more complicated.Depending on the composition of MA, cycling can result in the interaction between the components, their decomposition, and the formation of other phases, which can be both hydride-forming and hydride- non-forming.Thus a study of hydrogenation of a MA containing 66.6 mass%of La2Mg17 and 33.3 mass%of LaNi5 has shown 78 that several hydrogenation ± dehydrogenation cycles result in their decomposition and the formation of Mg, Mg2Ni, and LaHx (x=2.00 ± 2.99).Then, depending on the temperature and hydro- gen pressure used in hydrogenation, hydrogen is absorbed either by magnesium or by magnesium and Mg2Ni, and LaHx serves as a `hydrogen pump'. After 20 cycles, the sample was a conglomerate of all the phases with a granule size of *0.2 mm. Although these MA have lower hydrogen capacity as compared to that of pure La5Mg17, the rates of their hydrogenation and dehydrogenation are higher in the first cycles and depend, other conditions being the same, on the duration and intensity of mechanical processing.Thus MA obtained under the most drastic conditions (at an acceleration of 130 m s72 for 25 min) absorbs 3.5 mass% of hydrogen (90% of full capacity) in less than 1 min at 523K and liberates the same amount in 6 min, whereas under the same conditions it takes 2.5 h for pure La5Mg17 to absorb 4.9 mass% of hydrogen (also 90% of its full capacity) and 3 h to desorb it.78 This was explained 78 by the small size ofMAparticles (*10 mm), their multi-phase composition and catalytic activity of one or two components.It was thus concluded that a solution of the problem of improvement of the kinetic characteristics of hydrogenation and dehydrogenation lies in the creation of analogous composite materials. 3. Hydrogenation of Mg2Ni obtained by mechanical alloying and mechanical milling The intermetallic compound Mg2Ni interacts with hydrogen at a high rate at 473 ± 573K and a hydrogen pressures of 0.5 ± 1.0 MPa; however, a problem of activation exists. From 10 to 20 cycles of hydrogenation ± dehydrogenation are necessary for activation 79 in the course of which dispersion of the material and the formation of a metallic surface occur.These difficulties can be avoided if Mg2Ni is subjected to mechanical milling or is synthes- ised by mechanical alloying using the same procedure as for the synthesis of MA but increasing the duration of the treatment.It was shown 80, 81 that mechanical alloying of a mixture of Mg and Ni powders gives nanocrystalline Mg2Ni with the size of crystal- lites of about 20 ± 30 nm. This material interacts rapidly with hydrogen, and reproducible rates of hydrogenation are achieved even by the second cycle (Fig. 10). If a small amount of palladium (less than 1 mass%) is added to the reaction mixture during mechanical alloying, the thus modified Mg2Ni absorbs hydrogen at room temperature (Fig. 11).80 ± 83 An analogous result was obtained without addition of palladium.84 However, in this case hydrogenation of Mg2Ni was performed immediately after its mechanical milling without exposure to air (Fig. 12). Mechanical milling implies practically the same procedure as mechanical alloying except that the initial material is not a mixture of metal powders but an intermetallic compound obtained by traditional alloying.As is shown by Fig. 12, the reaction rate and the amount of absorbed hydrogen increase as the duration of mechanical Table 3. Catalytic characteristics of ternary hydrides and traditional catalysts.59 Catalyst Degree of Selectivity (%) conversion (%) Compound to be hydrogenated is acetylene Mg2CoHx 44 100 (with respect to C2H4) Pd/Al2O3 21 90 Compound to be hydrogenated is butadiene Mg2FeH6 51 ± 55 99.2 ± 99.7 Pt ± S/Al2O3 23 100 Pd ± S/Al2O3 36 72 10 30 50 t /min 1.0 2.0 3.0 1 2 x 0 Figure 10.Kinetics of hydrogenation of nanocrystalline Mg2Ni prepared by mechanical alloying at T=573 K, PH2=1.16 MPa; (1) the first cycle; (2) the second and subsequent cycles.The x values in the formula Mg2NiHx are shown in the y-axis. 10 30 50 t /min 2.0 1.5 1.0 0.5 0 x Figure 11. Kinetics of hydrogen sorption by nanocrystalline Mg2Ni modified with palladium during mechanical alloying (room temperature, PH2=1.2 MPa, without activation). The x values in the formula Mg2NiHx are shown in the y-axis. 0.2 0.1 0 H/M 0 0.5 1073 t /s 4 3 2 1 Figure 12.Kinetics of hydrogen sorption for Mg2Ni (285 K, PH2= 2 MPa) after its mechanical milling in an argon atmosphere for 30 (1), 5 (2), 1 min (3), and without milling (4). The ratio of hydrogen atoms to metal atoms in Mg2NiHx is shown in the y-axis. Interaction of alloys and intermetallic compounds obtained by mechanochemical methods with hydrogen 75milling is increased, which can be due to gradual dispersion of the sample and the formation of pure metal surface during this milling.However, at room temperature hydrogenation does not go to completion; the hydrogen content corresponds to the formula Mg2NiHx (x&2). This might be connected with the fact that as the temperature decreases, following the formation of a hydride layer on the surface of particles, the diffusion of hydrogen through this layer, the rate of which can be small at room temperature, becomes the limiting stage.The `diffusion retardation' occurs similarly to that observed in the hydrogena- tion of magnesium. Moreover, at T<515K Mg2NiH4 exists as a low-temperature modification, the structure of which is the monoclinic distortion of a cubic high-temperature modification.85 The latter normally results from hydrogenation at temperatures above the temperature of phase transition.The explanation of this fact calls for additional studies. III. Hydrogenation of FeTi and LaNi5 undergoing mechanical milling As compared to magnesium systems, LaNi5 and FeTi are low- temperature accumulators of hydrogen. They absorb hydrogen at room temperature.However, commercial polycrystalline samples covered with oxygen-containing compounds require, as a rule, preliminary activation. For instance, the procedure for activation of FeTi consists of heating the sample in a vacuum to 673 ± 723K and subsequent annealing in hydrogen at a pressure of*0.7 MPa followed by cooling to room temperature and increase in the hydrogen pressure to 3.5 ± 6.5 MPa.86 This procedure is repeated several times so that hydrogenation is reproducible. Activation of LaNi5 proceeds much more easily than in the case of FeTi, but this is still required and consists of a prolonged exposure of the sample at high hydrogen pressure (about 5.0 MPa) at room 87 or elevated (*623 K) temperatures in vac- uum.88 Mechanical milling of these intermetallic compounds in ball mills allows omission or simplification of the activation proce- dure.The kinetic curves of hydrogen sorption for mechanically milled FeTi are presented in Fig. 13.84 After mechanical milling, contact of the sample with air was excluded. It is seen that in this case hydrogenation starts at the maximum rate, and the degree of conversion depends on the duration of the treatment.If the sample was exposed to air after mechanical milling and prior to hydro- genation, activation was necessary, but it occurred much more easily than for ordinary alloys: it is sufficient to heat the sample at 673K in vacuum for 30 min.89 If FeTi is modified with a small amount of palladium, which is added during mechanical milling, neither annealing of the sample nor preliminary exposure of the sample to hydrogen are required.This sample immediately absorbs hydrogen at room temper- ature.81, 83, 90 Isotherms of `pressure ± composition' at room temperature without preliminary activation of samples were obtained 91 for amorphous and nanocrystalline FeTi formed upon mechanical alloying. It was shown that the shape of the isotherms is very `sensitive' to the microstructure of the samples and changes significantly after their annealing.The relaxation of structural defects and mechanical strains upon annealing leads to a decrease in the solubility of hydrogen in both cases and to a change in the length and slope of the plateau in the isotherm of nanocrystalline FeTi. The shape of the adsorption isotherm of the initial nano- crystalline powder indicates the significant contribution of an amorphous component.This fact together with the electron microscopy and WAXS data allowed the authors 91 to draw the following conclusion: the microstructure of particles of nano- crystalline FeTi formed upon mechanical alloying is nanocrystal- lites with a highly disordered intercrystallite region; the portion of this structure in the bulk material is 20%± 30%.It should be emphasised that these data give direct evidence that the degree of disorder and mechanical strains affect the character of the hydro- gen absorption in the sample. During preliminary activation, which is required for hydrogenation of intermetallic compounds not doped with palladium the relaxation processes have, as a rule, already occurred, which prevents the unambiguous interpretation of the results.Analogous kinetic data have been obtained for LaNi5.81, 83 The kinetic curves for hydrogenation of mechanically milled samples of LaNi5�one of which was hydrogenated immediately after mechanical milling, another was kept in air for a long period, and the third was prepared with the addition of palladium � are presented in Fig. 14. Hence, mechanical milling of intermetallic compounds Mg2Ni, FeTi, and LaNi5 yields a metallic surface, on which dissociative adsorption of hydrogen occurs. However, upon con- tact of these samples with air the surface is rapidly oxidised and the kinetic characteristics of hydrogenation are impaired. Modifica- tion with palladium (one of the best catalysts for the dissociative adsorption of hydrogen) during mechanical milling of these intermetallic compounds allows hydrogenation to be carried out at room temperature without preliminary activation. 0 0.5 1073 t /s 0 0.5 H/M 1 2 3 4 Figure 13. Kinetics of hydrogen sorption for FeTi (283 K, PH2=2 MPa) after its mechanical processing in an argon atmosphere for 24 (1), 3 h (2), 30 min (3), and without processing (4).The ratio of hydrogen atoms to metal atoms in FeTiHx is shown in the y-axis. 5 4 3 2 1 0 20 40 60 80 t /min 1 2 3 x Figure 14. Kinetics of hydrogenation of LaNi5 milled mechanically at T=313K and PH2=1.5 MPa; (1) the sample kept in air for several months; (2) the sample immediately after mechanical activation; (3) the sample modified with palladium. The x values in the formula LaNi3Hx are shown in the y-axis. 76 I G Konstanchuk, E Yu Ivanov, V V BoldyrevIV. The direct formation of hydrides during mechanical milling The formation of metal hydrides is possible during mechanical milling.92 ± 96 To this end, mechanical milling is run in a hydrogen atmosphere at elevated pressure. In this way, binary hydrides of the compositions TiH1.9, ZrH1.66 and MgH2 have been prepared and characterised by WAXS analysis, differential thermal analy- sis, and electron microscopy.92 ± 95 It is remarkable that magne- sium usually absorbs hydrogen in noticeable amounts only at temperatures above 573 K, whereas hydrogenation concurrent with mechanical milling yields magnesium hydride without heat- ing.To date it remains unclear which factor is determining in the formation of hydrides during mechanical milling in a hydrogen atmosphere. It might be crushing, plastic deformation, the for- mation of a fresh metal surface, local increase in temperature upon collision of grinding bodies, etc. Most likely, all these processes contribute to the formation of hydrides. Mechanochemical processes are known to be non-equilibrium ones and often occur via metastable states.This is the case in the hydrogenation of ZrNi during its mechanical milling in a hydro- gen atmosphere.96 This process includes the stages differing from those in the traditional hydrogenation, namely, the consecutive formation of the hydride phases ZrNiH and/or ZrNiH3 followed by their amorphisation, and then the decomposition of the amorphous phase to ZrH2 and metastable hydride a-Zr1 ± dNiHx.As a result, composite particles composed of amorphous (a-Zr1 ± dNiHx) and crystalline (a-ZrNiH3 and/or ZrH2) hydride phases are formed according to the hydrogen pressure used, which varies from 0.1 to 1.0 MPa. Particles resulting from mechanical processing in a hydrogen atmosphere appeared to be much smaller in size (0.5 ± 1.0 mmat a hydrogen pressure of 1.0 MPa) than those obtained under similar conditions in an argon atmosphere (50 mm) due to the higher fragility of the hydride phases as compared to that of ZrNi.Taking this into account, it was concluded 96 that mechanical processing in a hydrogen atmosphere can be a promising method for the creation of hydrogen-accumulating materials with a very large interface between the amorphous and crystalline phases and for the creation of hard magnets with anisotropic nanosized grains, which can be obtained by consecutive hydrogenation, amorphisation, and dehydrogenation with simultaneous crystal- lisation.When mechanical milling of Mg2Ni was carried out in a hydrogen atmosphere at a pressure of 1.0 MPa in a `Fritsch P7' ball planetary mill at a rotation rate of 400 rev min71, the sorption of hydrogen was also detected.97 According to the WAXS data, the samples obtained corresponded to the a-phase, a solid solution of hydrogen in the intermetallic compound Mg2NiH0.3.The latter is usually formed in the initial stages of hydrogenation and at higher hydrogen concentrations, the for- mation of Mg2NiH4 begins.98 However, upon hydrogenation during mechanical milling, the amount of hydrogen absorbed exceeded significantly that corresponding to the formula Mg2NiH0.3.For instance, after processing for 80 h this amount achieved 1.6 mass% (Mg2NiH1.8) without noticeable changes in X-ray patterns. In this case, a correlation between the value of specific surface of the sample and the hydrogen content in the sample was observed (Fig. 15). The authors 97 associate this fact not only with the disintegration of the sample, but also with the formation of a nanocrystalline composite material during mechanical milling composed of crystalline particles with this phase disordered at the grain interface where hydrogen can occupy more sites that in the crystalline phase.The thickness of this disordered phase might be only several nanometers and is not observed in X-ray patterns. In our opinion, the data given in Ref. 97 do not rule out the principal possibility for the formation of the hydride Mg2NiH4 during mechanical milling since the investigations were performed in a low-energy `Fritsch P7' apparatus.Under more drastic conditions (more powerful mills, higher hydrogen pressure), hydrogenation can occur to reach the stage of formation of the ternary hydride, which, probably, would be a metastable phase. In any case, the formation of a solid supersaturated solution of hydrogen in Mg2Ni at the grain interface 97 indicates that the processes of traditional hydrogenation and hydrogenation during mechanical milling differ in the initial stages.V. Conclusion The above review of published data on the interaction between hydrogen and mechanical alloys and intermetallic compounds undergoing mechanical milling allows the following conclusions to be drawn. Mechanical alloying is a promising method for the prepara- tion of materials with high reactivity with respect to hydrogen.As compared to traditional methods for the preparation of alloys and intermetallic compounds, mechanical alloying is sub- stantially less time- and energy-consuming. There is no need for high temperatures. Depending on the apparatus design, the process of mechanical alloying takes from several minutes to several hours to yield homogeneous materials or composites with a sufficiently uniform phase distribution, while in the case raditional alloying, as a rule, special prolonged homogenising annealing is necessary.This is especially important for obtaining alloys of metals differing considerably in their specific densities. Mechanical alloys formed in the early stages of mechanical alloying are metastable systems with a large interface where the `atomic' contact of metals occurs and their chemical interaction is possible.This interaction consists of both electron transfer from one element to another thus increasing the catalytic activity of the metal-catalyst and the reduction of oxides used as the second phase in magnesium systems thus creating centres of hydrogen adsorption. Ahighly disordered specific structure of theMAsurface with a catalyst dispersed in the form of atoms in the near-surface layer is responsible for the high hydrogenation rate in the first cycle.This structure is stable to oxygen and the operation with the sample in air does not require any special conditions. The possibility to vary the microstructure during mechanical alloying allows the reaction rate and hydrogen capacity of MA to be varied.The method of mechanical alloying provides a possibility to create composites from non-interacting components, which favours the synthesis of novel hydrides and IMC in the reaction of these MA with hydrogen. [H] (mass %) 1 2 3 4 s /m2 g71 0.5 1.0 1.5 0 1 2 3 4 5 Figure 15. The correlation between the specific surface of the samples (s) prepared by hydrogenation of Mg2Ni during mechanical milling and the hydrogen content in the samples.t /min: (1) 5, (2) 15, (3) 60, (4) 300, (5) 7800. Interaction of alloys and intermetallic compounds obtained by mechanochemical methods with hydrogen 77IV. The direct formation of hydrides during mechanical milling The formation of metal hydrides is possible during mechanical milling.92 ± 96 To this end, mechanical milling is run in a hydrogen atmosphere at elevated pressure.In this way, binary hydrides of the compositions TiH1.9, ZrH1.66 and MgH2 have been prepared and characterised by WAXS analysis, differential thermal analy- sis, and electron microscopy.92 ± 95 It is remarkable that magne- sium usually absorbs hydrogen in noticeable amounts only at temperatures above 573 K, whereas hydrogenation concurrent with mechanical milling yields magnesium hydride without heat- ing.To date it remains unclear which factor is determining in the formation of hydrides during mechanical milling in a hydrogen atmosphere. It might be crushing, plastic deformation, the for- mation of a fresh metal surface, local increase in temperature upon collision of grinding bodies, etc.Most likely, all these processes contribute to the formation of hydrides. Mechanochemical processes are known to be non-equilibrium ones and often occur via metastable states. This is the case in the hydrogenation of ZrNi during its mechanical milling in a hydro- gen atmosphere.96 This process includes the stages differing from those in the traditional hydrogenation, namely, the consecutive formation of the hydride phases ZrNiH and/or ZrNiH3 followed by their amorphisation, and then the decomposition of the amorphous phase to ZrH2 and metastable hydride a-Zr1 ± dNiHx.As a result, composite particles composed of amorphous (a-Zr1 ± dNiHx) and crystalline (a-ZrNiH3 and/or ZrH2) hydride phases are formed according to the hydrogen pressure used, which varies from 0.1 to 1.0 MPa.Particles resulting from mechanical processing in a hydrogen atmosphere appeared to be much smaller in size (0.5 ± 1.0 mmat a hydrogen pressure of 1.0 MPa) than those obtained under similar conditions in an argon atmosphere (50 mm) due to the higher fragility of the hydride phases as compared to that of ZrNi. Taking this into account, it was concluded 96 that mechanical processing in a hydrogen atmosphere can be a promising method for the creation of hydrogen-accumulating materials with a very large interface between the amorphous and crystalline phases and for the creation of hard magnets with anisotropic nanosized grains, which can be obtained by consecutive hydrogenation, amorphisation, and dehydrogenation with simultaneous crystal- lisation. When mechanical milling of Mg2Ni was carried out in a hydrogen atmosphere at a pressure of 1.0 MPa in a `Fritsch P7' ball planetary mill at a rotation rate of 400 rev min71, the sorption of hydrogen was also detected.97 According to the WAXS data, the samples obtained corresponded to the a-phase, a solid solution of hydrogen in the intermetallic compound Mg2NiH0.3.The latter is usually formed in the initial stages of hydrogenation and at higher hydrogen concentrations, the for- mation of Mg2NiH4 begins.98 However, upon hydrogenation during mechanical milling, the amount of hydrogen absorbed exceeded significantly that corresponding to the formula Mg2NiH0.3. For instance, after processing for 80 h this amount achieved 1.6 mass% (Mg2NiH1.8) without noticeable changes in X-ray patterns.In this case, a correlation between the value of specific surface of the sample and the hydrogen content in the sample was observed (Fig. 15). The authors 97 associate this fact not only with the disintegration of the sample, but also with the formation of a nanocrystalline composite material during mechanical milling composed of crystalline particles with this phase disordered at the grain interface where hydrogen can occupy more sites that in the crystalline phase.The thickness of this disordered phase might be only several nanometers and is not observed in X-ray patterns. In our opinion, the data given in Ref. 97 do not rule out the principal possibility for the formation of the hydride Mg2NiH4 during mechanical milling since the investigations were performed in a low-energy `Fritsch P7' apparatus.Under more drastic conditions (more powerful mills, higher hydrogen pressure), hydrogenation can occur to reach the stage of formation of the ternary hydride, which, probably, would be a metastable phase. In any case, the formation of a solid supersaturated solution of hydrogen in Mg2Ni at the grain interface 97 indicates that the processes of traditional hydrogenation and hydrogenation during mechanical milling differ in the initial stages.V. Conclusion The above review of published data on the interaction between hydrogen and mechanical alloys and intermetallic compounds undergoing mechanical milling allows the following conclusions to be drawn. Mechanical alloying is a promising method for the prepara- tion of materials with high reactivity with respect to hydrogen.As compared to traditional methods for the preparation of alloys and intermetallic compounds, mechanical alloying is sub- stantially less time- and energy-consuming. There is no need for high temperatures. Depending on the apparatus design, the process of mechanical alloying takes from several minutes to several hours to yield homogeneous materials or composites with a sufficiently uniform phase distribution, while in the case of traditional alloying, as a rule, special prolonged homogenising annealing is necessary.This is especially important for obtaining alloys of metals differing considerably in their specific densities. Mechanical alloys formed in the early stages of mechanical alloying are metastable systems with a large interface where the `atomic' contact of metals occurs and their chemical interaction is possible.This interaction consists of both electron transfer from one element to another thus increasing the catalytic activity of the metal-catalyst and the reduction of oxides used as the second phase in magnesium systems thus creating centres of hydrogen adsorption.Ahighly disordered specific structure of theMAsurface with a catalyst dispersed in the form of atoms in the near-surface layer is responsible for the high hydrogenation rate in the first cycle. This structure is stable to oxygen and the operation with the sample in air does not require any special conditions.The possibility to vary the microstructure during mechanical alloying allows the reaction rate and hydrogen capacity of MA to be varied. The method of mechanical alloying provides a possibility to create composites from non-interacting components, which favours the synthesis of novel hydrides and IMC in the reaction of these MA with hydrogen. [H] (mass %) 1 2 3 4 s /m2 g71 0.5 1.0 1.5 0 1 2 3 4 5 Figure 15.The correlation between the specific surface of the samples (s) prepared by hydrogenation of Mg2Ni during mechanical milling and the hydrogen content in the samples. t /min: (1) 5, (2) 15, (3) 60, (4) 300, (5) 7800. Interaction of alloys and intermetallic compounds obtained by mechanochemical methods with hydrogen 77IV. The direct formation of hydrides during mechanical milling The formation of metal hydrides is possible during mechanical milling.92 ± 96 To this end, mechanical milling is run in a hydrogen atmosphere at elevated pressure. In this way, binary hydrides of the compositions TiH1.9, ZrH1.66 and MgH2 have been prepared and characterised by WAXS analysis, differential thermal analy- sis, and electron microscopy.92 ± 95 It is remarkable that magne- sium usually absorbs hydrogen in noticeable amounts only at temperatures above 573 K, whereas hydrogenation concurrent with mechanical milling yields magnesium hydride without heat- ing.To date it remains unclear which factor is determining in the formation of hydrides during mechanical milling in a hydrogen atmosphere. It might be crushing, plastic deformation, the for- mation of a fresh metal surface, local increase in temperature upon collision of grinding bodies, etc. Most likely, all these processes contribute to the formation of hydrides.Mechanochemical processes are known to be non-equilibrium ones and often occur via metastable states. This is the case in the hydrogenation of ZrNi during its mechanical milling in a hydro- gen atmosphere.96 This process includes the stages differing from those in the traditional hydrogenation, namely, the consecutive formation of the hydride phases ZrNiH and/or ZrNiH3 followed by their amorphisation, and then the decomposition of the amorphous phase to ZrH2 and metastable hydride a-Zr1 ± dNiHx.As a result, composite particles composed of amorphous (a-Zr1 ± dNiHx) and crystalline (a-ZrNiH3 and/or ZrH2) hydride phases are formed according to the hydrogen pressure used, which varies from 0.1 to 1.0 MPa.Particles resulting from mechanical processing in a hydrogen atmosphere appeared to be much smaller in size (0.5 ± 1.0 mmat a hydrogen pressure of 1.0 MPa) than those obtained under similar conditions in an argon atmosphere (50 mm) due to the higher fragility of the hydride phases as compared to that of ZrNi.Taking this into account, it was concluded 96 that mechanical processing in a hydrogen atmosphere can be a promising method for the creation of hydrogen-accumulating materials with a very large interface between the amorphous and crystalline phases and for the creation of hard magnets with anisotropic nanosized grains, which can be obtained by consecutive hydrogenation, amorphisation, and dehydrogenation with simultaneous crystal- lisation.When mechanical milling of Mg2Ni was carried out in a hydrogen atmosphere at a pressure of 1.0 MPa in a `Fritsch P7' ball planetary mill at a rotation rate of 400 rev min71, the sorption of hydrogen was also detected.97 According to the WAXS data, the samples obtained corresponded to the a-phase, a solid solution of hydrogen in the intermetallic compound Mg2NiH0.3.The latter is usually formed in the initial stages of hydrogenation and at higher hydrogen concentrations, the for- mation of Mg2NiH4 begins.98 However, upon hydrogenation during mechanical milling, the amount of hydrogen absorbed exceeded significantly that corresponding to the formula Mg2NiH0.3.For instance, after processing for 80 h this amount achieved 1.6 mass% (Mg2NiH1.8) without noticeable changes in X-ray patterns. In this case, a correlation between the value of specific surface of the sample and the hydrogen content in the sample was observed (Fig. 15). The authors 97 associate this fact not only with the disintegration of the sample, but also with the formation of a nanocrystalline composite material during mechanical milling composed of crystalline particles with this phase disordered at the grain interface where hydrogen can occupy more sites that in the crystalline phase. The thickness of this disordered phase might be only several nanometers and is not observed in X-ray patterns.In our opinion, the data given in Ref. 97 do not rule out the principal possibility for the formation of the hydride Mg2NiH4 during mechanical milling since the investigations were performed in a low-energy `Fritsch P7' apparatus. Under more drastic conditions (more powerful mills, higher hydrogen pressure), hydrogenation can occur to reach the stage of formation of the ternary hydride, which, probably, would be a metastable phase. In any case, the formation of a solid supersaturated solution of hydrogen in Mg2Ni at the grain interface 97 indicates that the processes of traditional hydrogenation and hydrogenation during mechanical milling differ in the initial stages. V. Conclusion The above review of published data on the interaction between hydrogen and mechanical alloys and intermetallic compounds undergoing mechanical milling allows the following conclusions to be drawn. Mechanical alloying is a promising method for the prepara- tion of materials with high reactivity with respect to hydrogen. As compared to traditional methods for the preparation of alloys and intermetallic compounds, mechanical alloying is sub- stantially less time- and energy-consuming. There is no need for high temperatures. Depending on the apparatus design, the process of mechanical alloying takes from several minutes to several hours to yield homogeneous materials or composites with a sufficiently uniform phase distribution, while in the case of traditional alloying, as a rule, special prolonged homogenising annealing is necessary. This is especially important for obtaining alloys of metals differing considerably in their specific densities. Mechanical alloys formed in the early stages of mechanical alloying are metastable systems with a large interface where the `atomic' contact of metals occurs and their chemical interaction is possible. This interaction consists of both electron transfer from one element to another thus increasing the catalytic activity of the metal-catalyst and the reduction of oxides used as the second phase in magnesium systems thus creating centres of hydrogen adsorption. Ahighly disordered specific structure of theMAsurface with a catalyst dispersed in the form of atoms in the near-surface layer is responsible for the high hydrogenation rate in the first cycle. This structure is stable to oxygen and the operation with the sample in air does not require any special conditions. The possibility to vary the microstructure during mechanical alloying allows the reaction rate and hydrogen capacity of MA to be varied. The method of mechanical alloying provides a possibility to create composites from non-interacting components, which favours the synthesis of novel hydrides and IMC in the reaction of these MA with hydrogen. [H] (mass %) 1 2 3 4 s /m2 g71 0.5 1.0 1.5 0 1 2 3 4 5 Figure 15. The correlation between the specific surface of the samples (s) prepared by hydrogenation of Mg2Ni during mechanical milling and the hydrogen content in the samples. t /min: (1) 5, (2) 15, (3) 60, (4) 300, (5) 7800. Interaction of alloys and intermetallic compounds obtained by mechanochemical methods with hydrogen 77