hydrogen power: theoretical and engineering solutions

HYDROGEN POWER: THEORETICAL AND ENGINEERING SOLUTIONS Proceedings of the HYPOTHESIS II Symposium held in Grimstad, Norway, 18-22 August 1997

Edited by

T. 0. SAETRE Faculty of Engineering, Agder College, Grimstad, Norway

KLUWER ACADEMIC PUBLISHERS DORDRECHT I BOSTON I LONDON

THE EFFECT OF IMPROVEMENT HYDROGEN-SORPTION PROPERTIES IN MULTIPHASE POLYMETALLIC COMPOSITIONS

M.V .LOTOTSKY Institute of Mechanical Engineering Problems of National Ukrainian Academy of Sciences. 2110 Pozharsky St., Kharkov 310046, the Ukraine

1. Introduction

Traditionally laboratory researches of the hydrogen-sorption characteristics of hydrideforming materials (individual metals and the intermetallic compounds) are carried out using the samples which are homogeneous on their composition and structure. Thus high purity metals are used, compound's stoichiometry is maintained carefully, a homogenising annealing is done, etc. As a result, the «pure» experimental data which have been not complicated by influence of various side effects is received during consequent researches of interaction of obtained compositions with hydrogen [ 1,2]. Proceeding to industrial-scale hydride-forming alloys the above mentioned uniformity of composition and structure, as a rule, is not observed. From economic reasons the requirement to purity of initial metals here are lower, except for rigid limitations to the contents of non-metallic components reducing hydrogen-sorption capacity. Thus the additional importation of technological additives, for example, calcium is possible [3]. Besides, the industrial hydride-forming materials are usually cast alloys which were not subjected to a homogenising annealing. As a result, alongside with basic intermetallic phase, in commercial hydride-forming materials there are additional phases which can change hydrogen-sorption properties of an alloy on a comparison with laboratory samples of appropriate intermetallides. The contents of such impurity phases in a final yield can be up to 5-10 % [3]. From an 1ipplied point of view, the non-uniformity of composition and structure of hydride-forming materials has both positive and negative aspects. The latter mainly include the deviations in a behaviour of P-C-T-diagrams of industrial alloys from «ideal» for homogeneous laboratory samples - the plateau pressure varies and its slope increases, sorption-desorption hysteresis grows, etc. [2]. On the other hand, the inhomogeneous samples are often characterized by less severe activation conditions, best hydrogen absotption kinetics and, in a number of cases, the higher hydrogen sorption capacities, than their homogeneous laboratory analogs. For a number of applications (for example, hydrogen getters [4]) this circumstance is decisive for the benefit of use just of multiphase, inhomogeneous hydride-forming compositions. 343 T. 0. Saetre (ed.), Hydrogen Power: Theoretical and Engineering Solutions, 343-352. © 1998 Kluwer Academic Publishers. Printed in the Netherlands.

344 In the basis of improving hydrogen-sorption properties of such compos1t10ns, probably, the effect experimentally opened at the end of 60-th - middle of 70-th lies [57]. This effect consists that if hydride-forming metals are saturated with hydrogen at the presence of additional phases characterized by high speeds of interaction with gaseous hydrogen, the process of hydride formation takes place in softer conditions, with the best kinetics and, as a rule, results in reaching higher hydrogenation degrees, than in case of the same metals taken as individual. Up-to-date the rather large volume of experimental data on a manifestation of described effect in various hydride-forming alloys and compositions is accumulated. Therefore, in this work the attempt of systematization of the main literature data on this problem was undertaken with the purpose of determining a working hypothesis about the mechanism (mechanisms) of the effect and definition of possible ways of its further researches.

2. The review of experimental data for various hydride-forming materials

In this section we shall consider experimental observations of the effect of improving of hydrogen-sorption properties in multiphase hydride-forming compositions. The greatest number of activities on the given line was carried out using compositions on the base of magnesium and zirconium. Therefore, we have born the data devoted to these compositions in separate subsections (2.1 and 2.2). The subsection 2.3 is devoted to compositions based on other, less systematised by corresponding experimental data metals or intermetallides. 2.1. MAGNESIUM-BASED COMPOSITIONS In the literature there are enough many data under the hydriding characteristics of a magnesium and compositions based thereon. The rather full review of these data as of a 1985 was made by Selvam e.a. [8]. The more late information can be found in the papers included into the book [ l]. The main shortage constraining creation of hydrogen accumulators based on MgH 2 are the low speeds of interaction of pure metal with hydrogen, especially at the first hydriding. According to the data of the different authors surveyed in the work [9], time of a complete hydriding of magnesium (the first cycle) at the temperature of 613 Kand pressure up to 3 MPa makes from 6 till 336 hours. Such significant divergences are probably caused by high sensitivity of reaction of magnesium with hydrogen to purity, particle size and surface state of initial metal, conditions of its activation, purity of hydrogen used, etc. The reaction can be accelerated by increase of temperature and pressure (650-720 K, 10-20 MPa) in a combination with application of gas-phase catalysts (iodine vapours, carbon tetrachloride, etc.) and milling of the solid during the process [ 10, 11]. As to repeated hydridings of a pure magnesium, here the data of the different authors are more reproducible: at hydrogen pressure of 2-4 MPa and temperature of 550-670 K the time of 80-90% hydriding of a magnesium makes from 2 till 20 hours (the reason of discrepancies is, probably, the same, as for the first hydriding), then the reaction practically stops [ 12-15].

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t

345 In the case of hydriding of polymetal compositions based on magnesium the dynamics of hydrogen sorption is cardinally improved. Still Reilly and Wiswall [5,6] indicated that the hydride-forming intermetallides Mg2Cu and Mg2Ni present alongside with a phase of a magnesium in its alloys with copper and nickel catalyse the hydriding of magnesium. The similar effect was discovered a little earlier by Mikheeva and her colleagues. [16,17]. They showed a possibility of the formation of magnesium hydride during interaction of alloys Ce-Mg and Ce-Mg-Al with gaseous hydrogen in extremely soft conditions - at room temperature and atmospheric pressure, without any preliminary activating processing. At this, the cast alloys showed the more good hydrogen-sorption characteristics (best hydriding dynamics and higher hydrogen sorption capacities), than annealed ones [ 16]. In the Figure 1 the most typical results illustrating change of hydrogen sorption dynamics for magnesium in B polymetal Compositions based mass.% H - - - - - - - - - - - - - - - - - - - ~~~ - - - - - - thereon are presented. The ap7 propriate characteristics of the I I 6 samples, as well as their hydridI I ing conditions are given in the 5 I Table 1. As it can be seen from the data 4 of Fig. I, the introduction of conventional gas-phase catalysts 3 (CC1 4) into hydrogen (the curve 2) essentially accelerates the 2 hydriding of magnesium (curve 1), however hydrogen absorption dynamics still remains slow. o~~~==;=:~~::;:=;::~:,____,-,-,,TTTTr~ The speed of hydrogen absorp0.1 1D t,min 100 1000 tion is increased in some extend Figure I. Dynamics of hydrogen sorption by when the alloying components magnesium and magnesium-based compositions are introduced into magnesium, within the limits of formation of the diluted solid solution (curve 3). The authors [24] note that with increasing the concentration of a solid solution of an alloying element (Al, Ga, In) in magnesium the effect of improving hydrogen sorption dynamics disappears. However, in the case of highly-concentrated solid solutions containing hydride-forming components, it is exhibited again even in the more pronounced form. It is illustrated by the data [14] obtained for industrial magnesium alloy of a type MA-15 (curve 4). The alloys of this type are characterised by a mesh size of a grain (due to the presence of zirconium), besides they, depending on a mode of heat treatment, can represent either over-saturated magnesium solid solutions, or inclusions of small quantities of intermetallic phases into magnesium matrix [25]. The effect of improving hydrogen-sorption properties have much more pronounced character if one adds to magnesium the hydride-forming intermetallides characterized by high hydrogen sorption properties. It is illustrated with curve 5, which corresponds to compacted mechanical mixture of magnesium with 20 % LaNi5 . Here is practically observed a complete hydriding of a magnesium up to MgH 2 for 30-60 minutes [18,19].

346 The best hydrogen-sorption properties have magnesium alloys contammg rare earth metals. They are represented by magnesium-rich intermetallides (RMg2 , R 2Mg 17 , R5Mg4 i, RMg 12 etc.) which disproportionate yield MgH 2 and RH 3 during the interaction with gaseous hydrogen (curve 6) [20,21]. In other cases these are multiphase alloys containing, apart from a phase of magnesium, intermetallic phases which, as a rule, are hydride-forming ones. Such compositions have the highest hydrogen sorption dynamics (curve 7), and hydrogen absorption takes place even at room temperature. T able 1 HlYdn'd'mg charactenstics o f ma1mesmm an d compos1t10ns b ase d there on Curve No. (Fig.l) I 2

3 4

5

6

7

Sample composition Component Mg Mg

Mgo_99Jno 01 Mg-94.11 Zn-4.01 La-1.24 Cd-0.52 Zr-0.12 (mass.%) Mg-80 LaNis-20 (mass.%) RMg12 (R-La,Ce) Mg1sY6Ni19

Phase Mg Mg

Hydriding conditions T,E P,MPa 683 40 683 40

Mg (solid solution) 543 Mg (solid solution) 613 The traces of intermetallie phases

80 30

Mg+LaNis

618

30

RMg12 (initial and after 598 dehydriding in vacuum at T>700 K); Mg+RH2 Mg+Mg2Ni+ YNh 473

30

30

Notes

Ref.

[14] Hydrogen contained the (14] admixture of CCl4 (-2%) Alloy [15] Industrial magnesium [14] alloy of the type MA-15

Compacted mechanical mixture of Mg and LaNis powders Alloy; phase compositi on hydriding of products is MgH2+RH3 Alloy; phase compositi on of hydriding is products MgH2+Mg2NiH4+ YH2

[18, 19] (20, 21] (22, 23]

The main explanations of the mechanism of improving hydriding characteristics of magnesium-based compositions are reduced to surface deoxidation at the boundaries of magnesium grains with catalysing hydride phase [26], facilitation of internal diffusion of hydrogen due to increasing the concentration of lattice defects in the solid phases [24], or its external diffusion via microcracks and grains boundaries [9]. In some works it is also mentioned about the role of hydrogen state in course of improving magnesium hydrogen-sorption properties by the metal components. So, the availability of a hydrideforming intermetallide in the system Mg-Mg2 Cu [26] results that to the Mg-Mg 2Cu interface from the side ofMg2 Cu monatomic hydrogen is continuously supplied. The latter can also be formed during dissociative adsorption of hydrogen molecules on clusters of transition metals - this makes it's possible to explain the acceleration of hydriding in mixtures Mg + LaNi 5 [ 18, 19] and in «mechanical alloys» of magnesium with various metals [27]. The fine illustration of this mechanism is described in [10] where the synthesis of magnesium hydride from a shaving of pure metal and active hydrogen which was obtained immediately during the synthesis by interaction of zinc with hydrochloric acid. The hydriding of magnesium in this case takes place at room temperature and atmospheric pressure.

347 2.2. ZIRCONIUM-BASED COMPOSITIONS The reaction of gaseous hy* T=295K o T=625K • T=200-250 K drogen with a zirconium with formation of ZrH 2 takes place in severe conditions: pressure a_zr +ZrV2 V+ZrV2 •• till 7-10 MPa and temperature '' of the order 500 /(, Reaction •O : a:zr ZrV, \ : duration thus makes some ' v hours [28, 29]. At the same ' ' 100c ' ' : ' ' time, the hydriding of major* •. 'oo ; ity of zirconium intermetal0 • • lides, as a rule, takes place • already at room temperature and hydrogen pressure of the order 0.1 MPa. Thus the hy100 o 20 40 60 80 driding speed is extremely high - in the case of ZrV 2 it is Figure 2. Hydriding characteristics of Zr-V alloys 2-3 order of magnitude higher [10,31] than for individual zirconium at 500-600 K [29]. In zirconium-based alloys and compositions where hydride-forming intermetallides are formed the hydriding proceeds practically in the same (or little bit more rigid) conditions, as in case of appropriate intermetallic compounds. Thus, apart from hydriding of an intermetallide, it is observed practically complete hydriding of a zirconium with ZrH 2 formation. So, according to data [30] the compacted mechanical mixture of zirconium with 20 % of ZrV 2 absorbs hydrogen with quantitative formation of ZrH 2 at P = 1 MPa and room temperature for 3 2 hours, and at the same pressure and T = 523 K only for 15 minutes. The Figure 2 illustrates a behaviour of Zr-V alloys during their 2 hydriding. The ordinate axis represents the time of complete hydriding of an alloy at P = 3-4 MPa. It can be seen in Fig.2 that the alloys of the composition a-Zr + ZrV 2 have still best hydrogen sorption dynamics [29] than appropriate mixtures [30]. The similar effect of improving the hydrogen-sorption properties is exhibited in alloys of a 0.1 10 100 1000 system Zr-V-Fe (a-Zr + t. min ) Zr (V,Fe) [31, 32]. 2 Figure 3. Hydrogen sorption dynamics at At modifying of alloys Zr-V P=0.05 MPa by the samples of alloys Zrss V40 Fe 5 and Zr-V-Fe by the oxide compo(1) and ZrssY 40 Fes + 3% B203 (2) nents they, apart from a-Zr and

[

~r:

348 Laves phase Zr(V,Fe) 2, include oxygen-containing hydride-forming intermetallide T]Zr3(V,Fe)30. Such modified alloys have the higher hydrogen-sorption characteristics (Fig. 3) intensively absorbing hydrogen at room temperature and pressure lower than atmospheric one [32,33]. The similar behaviour is exhibited by oxygen-modified alloys Zr-Fe having the phase composition of a-Zr+ Zr2Fe + T]-Zr2Fe 40 [34, 35]. The improvement of hydrogen-sorption properties in zirconium-based multiphase compositions can be explained by a number of factors, first of all: • Hydrogen activation during its sorption-desorption by hydride-forming intermetallic phases [32, 36]. • Change of surface state for difficult to hydriding phase a-Zr - either its deoxidation or, in the case of oxygen-containing alloys, formation of oxygen-containing intermetallic layers permeable for hydrogen [32]. • The availability of a developed grid of microcracks providing the intensive hydrogen entry into the bulk of an alloy [35]. 2.3.THE OTHER COMPOSITIONS The effect being considered in this work, apart from compositions based on Mg and Zr, is peculiar to a lot of similar compositions in which the individual metal or intermetallic compound acts as a «difficult to hydriding» component. The adding of hydride-forming intermetallides to vanadium or niobium [7,29,30,37,38] allows to obtain VH 2 or NbH 2 at room temperature and hydrogen pressure of several MPa during less than 1 hour. The relation of time of a hydriding to vanadium contents in alloys V+ZrV 2 is shown in Figure 2. At the same time the dihydrides of pure vanadium and niobium can be synthesized only at reduced (200-270 K) temperatures, the time of a hydriding thus makes about 10 days [7, 10,29]. It is marked in [3 7] that with increase of vanadium purity the speed of its hydriding is essentially decelerated. In a number of works devoted to interaction of titanium-based alloys and compositions with gaseous hydrogen was shown that the presence of hydride-f()rming intermetallides essentially increases hydriding speed and depth. Examples ar,e the alloys of a system Ti-V-Ni in the region of phase compositions ~-Ti+ Ti 2Ni [39,40], Ti-V-Fe (~-Ti + A. 1-Ti(V,Fe) 2) [41], mechanical mixtures of alloys Ti-V-M (where M - Cr, Mn, Fe, Co, Ni, Cu and Al) with LaNi 5 [42], etc. In [40] the original and rather probable explanation of acceleration ofa hydriding in alloys ~-Ti+TizNi is given: the increase ofTi 2Ni volume during hydrogen absorption results in mechanical destruction of a sample and increases the total reaction surface. In some works [30] it is mentioned about catalytic activity of hydride-forming intermetallic compounds in the processes of obtaining ScH 2 and YH 3 from individual R.E.M. and gaseous hydrogen. The effect of improving hydrogen-sorption properties is exhibited also for some hydride-forming intermetallides. As a rule, it is observed on mechanical mixtures of intermetallic compounds one of which is a highly effective reversible hydrogen absorbent. Examples of such compositions are TiFe+LaNi 5 [43, 44], Ti 08 Zr 02 Cr 0 .8Mn 12+ LaNi 5 [45], etc. In such mixtures the «difficult to hydriding» component is characterised by high speeds of hydrogen sorption - desorption even during the first hydriding cycle, at less hard preliminary activation conditions.

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349 In a number of cases the hydride-forming intermetallides as component parts of multiphase alloys containing other intermetallic hydride-forming phases which are characterized by highly effective hydride formation, show much higher degrees of a hydriding, than observed in single-phase samples. So, for 11-phases in alloys a-Zr+ZrV 2+11Zr3 V30 and a-Zr+Zr2 Fe+ZrFe2+11-Zr2 Fe 40 the degree ofhydriding determined proceeding from known concentration relations of hydrides lattice periods, was more than twice above achieved at a hydriding single-phase 11-Zr3V30 and 11-Zr2Fe 40 [32-35].

3. Discussion and Conclusions

3.1. MECHANISMS OF THE EFFECT The results presented in Section 2 give the basis to assert that the effect of improving hydrogen-sorption properties in multiphase hydride-forming compositions has universal character and it is peculiar to various compositions which essentially differ on a chemical nature of a basic hydride-forming component (Mg, Zr, Ti, V, Nb, rare earth metals, etc.). The analysis of the adduced data allows to allocate some possible mechanisms which can underlie the effect being considered.

3.1.1. Hydrogen activation in gas phase In a series of works [26,29, etc.] a catalytic effect of hydride-forming intermetallides on hydrogenation of difficult to hydriding metals is explained by appearance of monatomic hydrogen formed either as a result of the H2 molecules dissociation on active metal clusters, or during hydrogen output from the bulk of intermetallic hydride. The elementary stage of hydrogen dissociative chemisorption is thereby bypassed. This stage is often limiting one in the total hydriding process [9]. In our opinion, it is necessary to treat little bit wider this mechanism, as appearance in a near-surface layer of a gas phase of hydrogen particles in thermodynamically non-equilibrium state. Apart from H atoms, it there can be hydrogen atomic or molecular ions, as well as exited atoms and molecules. The appearance of such particles is a result of hydrogen sorptive activation [36,46]. It can explain practically all cases of improving the hydrogen-sorption characteristics in multiphase polymetal compositions. In this respect large interest is represented by cross correlation between improving hydrogen-sorption properties of zirconium multiphase alloys and «degree» of hydrogen activation by them [32]. 3.1. 2. Surface effects The improving hydrogen-sorption properties can also occur in consequence of change of surface state of difficult to hydriding phase. If this phase is characterized by the formation of protective oxide layers, then the surface deoxidation, or other effects causing to destruction of such layers can take place in multiphase alloys [8,26,32]. Besides, in a number of cases the transformation of a surface oxide to hydrogen-permeable oxygencontaining intermetallic phase [32] is possible.

350 This mechanism has more particular character, being exhibited only in alloys under phase boundaries. At the same time, for components forming on air strong surface oxide layers (first of all it concerns to magnesium), its contribution can be determining. 3.1. 3. Microstructure factors It is necessary to relate to the microstructure factors promoting the hydriding of multiphase alloys the availability (or appearance during hydriding) ofmicrocracks in the bulk of an alloy, as well as increasing the concentration of lattice defects. It facilitates hydrogen supply into hydride-forming material that, in tum, results in acceleration of nucleation and growth of hydride phases grains, especially when the rate-limiting factor becomes internal diffusion of hydrogen [8,9,24,35,40]. This mechanism can take place only when the hydride-forming material is either the cast alloy, or polymetal composition obtained by a sintering. At the same time, probably just it is responsible for differences in the hydriding characteristics of cast and annealed samples.

3.2. THE PERSPECTIVES OF FURTHER INVESTIGATIONS Despite of an abundance of a published experimental material on study of the effect of improving hydrogen-sorption properties in multiphase polymetallic compositions, the data about its nature in many respects remain not clear. The attempts of explanation of the effect considered in the previous subsection, as a rule, affect particular cases and do not give convincing explanation of the mechanism of the effect as a whole. The exception represents the mechanism 3.1.l (hydrogen sorptive activation) which under our opinion is exhibited in all hydride-forming compositions, including mechanical mixtures of various metals and intermetallides. However, its contribution into total effect can be rather different in various compositions, and even the attempt of its rough estimation can become a subject for further, already systematic, researches. As a first step in this direction it is possible to recommend comparative study of phase-structural and hydrogensorption properties for multiphase polymetallic compositions in the following series: the multiphase alloy ~ the mixtures of phases composing an alloy, ~ the same mixtures with inert filler ~ individual phases. In addition, it would be also interesting to carry out parallel measurements of energy-state characteristics of hydrogen during its reversible interaction with samples of these compositions. The appropriate express-testing technique based on measurement of the electrotransfer characteristics in a gas phase of «hydride-forming material - hydrogen» systems, is developed in our laboratory [32,36,46]. On the basis of results obtained it can be essentially extended the existing conception about a nature of considered effect, and the general principles of development of highly effective hydrogen absorbents for various metal-hydride technological systems can be determined - beginning from hydrogen accumulators and finishing hydrogen getters and metal-hydride systems for increasing the efficiency of the physical-energy installations

351

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