Equilibria in the hydrogen-intermetallics systems with high dissociation pressure

Journal of

ALL(3Y5 AND COM~3UND5 ELSEVIER

Journal of Alloys and Compounds 231 (1995) 498-502

Equilibria in the hydrogen-intermetallics systems with high dissociation pressure V . Z . M o r d k o v i c h a, Y u . K . B a i c h t o k b, N.V. D u d a k o v a b, E . I . M a z u s b, V . E M o r d o v i n b ~Yoshimura ~'-Electron Materials Project, ERA TO, JRDC, c/o Matsushita Research Institute Tokyo, Inc., Higashimita 3-10-1, Tama-ku, Kawasaki, Japan bGIAP Institute, Zemlyanoy Val 50, Moscow 109815, Russia

Abstract

Equilibria in the hydrogen-intermetallics systems with high dissociation pressure (Pd ~>0.5 MPa at room temperature) were studied in a wide pressure range from 0.1 to 40 MPa. The survey has covered CexLa j xNi5 yAly-H2, CexLa l_xNi4Co-H2 and Til_~Zr~Crt 8_yFey-H2 systems. The CexLa l_xNi5 yAly-H 2 systems are characterized by the critical temperatures being lower than those of CexLat_~Ni4Co-H z at the same x. A metastable hydride phase occurs in the Ce~La~_~Ni5 ymly-H2 beyond the critical point for a narrow range of alloy compositions (x0.5 and y0.1). The hexagonal Laves phases Ti~ _~ZrxCr~.~_yFey formed the hydrides with the highest desorption pressure among the three systems studied. Their interaction with hydrogen, however, induces phase transition to cubic phase. The transition proceeds at an appreciable rate at temperatures higher than 100°C. The peculiarities of the systems studied are discussed in terms of possible applications in hydrogen/hydride technologies. Keywords: Equilibria; Hydrogen intermetallics; Pressure

1. Introduction The hydrogen-intermetallics systems have been under detailed investigation since their amazing property of absorbing hydrogen reversibly at moderate temperatures and pressures was discovered [1-3]. Different branches of the hydrogen/hydride technology demand a variety of the hydrogen-intermetallics systems. Chemical industry applications, in particular, demand intermetallic hydrides to be characterized by high dissociation pressure Pd at room temperature (0.5 MPa and higher) and fast Pd growth with increasing temperature [4,5]. There are two groups of intermetallic compounds which could provide the desirable properties: AB 5 ( A ~ L a , partially substituted with other rare earth metals or Ca; Bw~-Ni, partially substituted with Co, Mn, A1 etc.) and Laves phases, Though the hydrogen-intermetallics systems of the AB 5 group have been studied much more thoroughly than other systems, most publications cover pressures no higher than 1 MPa. All such systems may be characterized by the occurrence of only one hydride phase (so called fl-phase) containing up to about one hydrogen atom per atom of metal, e.g. AB5H67. 0925-8388/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0925-8388(95)01865-4

Laves phases may be characterized as AB 2 or AB intermetallic compounds which are formed at definite r A / r B ratios only, where r A and r B are the atomic radii of the constituents A and B, respectively. AB 2 phases occur at r A / r ~ ~ 1.22 ( r A / r B -~ 1.10-1.40 for handbook radii values) [6]. If the radii ratio exceeds the mentioned limit, a half of A atoms are substituted with B and, as a result, the AB 5 phase is formed [6]. Intermetallics ZrNi 2 ( r z r / r N i = 1.29) and LaNi 5 (rLa/rNi = 1.51) may be taken as examples. The correlation between the number of outer electrons per atom n e and the structural type of AB 2 Laves phase was analysed in Ref. [7] in detail for A = Ti, Zr, Y, La; B =V, Cr, Mn, Fe, Co, Cu. The following rule was drawn: for n e < 5.4 titanium does not form Laves phase but zirconium forms cubic phase C15 (MgZn 2 structural type); for ne = 5.4-7 both titanium and zirconium form hexagonal phase C14 (MgCu 2 structural type); for n e > 7 both titanium and zirconium form cubic phase C15 (MgZn 2 structural type). It is important to note for the subject of the present work that TiCr 2 (n e = 5.33) lies on the boundary of C15 and C14 phase stability ranges. Among Laves phases with relatively high Pj, various modifications of TiMnl.s_ 2 have attracted most atten-

V.Z. Mordkovich et al. / Journal of Alloys and Compounds 231 (1995) 498-502

tion [7-10]. The TiMnl.5_z-H 2 systems, however, are characterized by slow Pet growth with increasing temperature. Another Laves phase TiCr 2 seemed to be more promising [11,12]. TiCr2,. as is mentioned above, lies on the boundary of C15 and C14 phase stability ranges. Hence, it is possible to obtain both hexagonal C14 (homogeneity range TiCrl.Ts-TiCr~.95, thermodynamically stable at the temperatures higher than 850-1150°C) and cubic C15 (homogeneity range TiCrl.71-TiCrl.92, thermodynamically stable at the temperatures being lower than 850-1150°C)" It was shown in Refs. [11,12] that both hexagonal and cubic modifications of TiCr 2 reveal very similar hydriding properties. Sorption/desorption isotherms have at 195 K two plateaus which correspond to two hydride phases (distorted cubic TiCrl.sH2. 6 at 0.2 MPa and TiCrl.8H3. 6 at 5 MPa; hexagonal TiCrl.9H2. 5 at 0.02 MPa and TiCrl.gH3. 5 at 2 MPa). Both plateaus in both systems shrink dramatically with temperature increase. At 223 K one plateau is observed only, at 273 K no plateau is observed. Both hexagonal and cubic phase may be characterized as almost hysteresis-free in interaction with hydrogen. The purpose of the present work was to investigate several hydrogen-intermetallics systems with high Pd, based on AB 5 (CexLal_xNis_yAly-H 2 and CexLa~_,NiaCo-H2) and TiCr2-based Laves phase (Til_xZGCr~.8_yFey-H:) at pressures up to 40 MPa.

2. Experimental details Samples of intermetallic compounds were prepared from elements. The arc furnace with copper watercooled bottom provided a quenching rate of around 50 K s -~. The alloy composition was examined by chemical analysis. As-cast samples were annealed in vacuum at 1000°C for 1 week. X-ray diffraction analysis was performed in inert atmosphere using a DRON-3M diffractometer with Cu Ka irradiation. All other manipulations with actiwtted samples were carried out under inert atmosphere as well. As is known, equilibria in metal-hydrogen systems are commonly investigated by the method which is defined as the "classic" method in the present paper. The "classic" method includes only isotherms to be studied by the well-known Sieverts technique, i.e. first preliminary activation of a sample, next constant-temperature conditioning by thermostat and finally both sorption and desorption procedures to be carried out at a constant temperature T 1. Sorption-desorption procedures are controlled usually by both continuous recording of pressure and measurement of the volume of the sorbed-desorbed portions of the hydrogen. The

499

T1

1

~ ~ o.~ E

/ ~/ ~,-//

.~_°

j To

v

I P2 pressure

~-

I P1 >

Fig. 1. Schematic curves obtained from "classic" (---) and "alternative" ( ) experiments for studying the equilibria in hydrogen-

intermetallicssystems.

maximum pressure of the sorption-desorption procedure is denoted as P1 (Fig. 1). In addition to "classic" experiments which were carried out in each case, another experimental method to be defined as the "alternative" method was used in this paper. The "alternative" experiment includes preliminary activation of a sample, next its saturation by hydrogen at room temperature T O and pressure P2, followed by heating in a closed volume up to T~ (this process leads to a pressure rise to P~) and finally an isothermal desorption procedure at T1. Isobaric heating (P1 = P2) instead of polythermic heating was used too. Our experiments were carried out in the pressure range from 1 × 10 -2 to 40 MPa and in the isotherm temperature interval from 206 to 473 K. The "alternative" method is of interest because it simulates the reaction route which is taken during various processes in the hydrogen/hydride technology [4,5]. It becomes possible to observe not only the pressure hysteresis usually studied in "classic" experiments but also phenomena related to the approach to equilibrium from the unusual side.

3. Results and discussion 3.1. A B s - H 2 systems

Desorption isotherms for the Ce0.sLa0.sNia.98A10.02H 2 and Ce0.TLa0.3Ni4.98A10.02-H2 systems at 293 K and 373 K are shown in Fig. 2. The open symbols correspond to the experimental points obtained by the

V.Z. Mordkovich et al. / Journal of Alloys and Compounds 231 (1995) 498-502

500

,~

9 :~,~ ,,_~ - ~ ~ - - ~-, - - ~- - - e~- - -~

"~ 10

2-

~ .~-~-~ ~

~ ~ /

~0.~ 0

'

2

4

~

6

8

H/AB s

Fig. 2.

Desorption

isotherms for

hydrogen-intermetallics:

©,

Ceo.sLao.sNi4.9~Aloo2, "classic"; Q, Ceo.sLao.sNi4.~sAlo.o2, "alternative"; A, Ceo.7Lao.3Ni4.98Alo.o2."classic"; A, Ce,,.TLao.3Ni4.9.Al0.,,2. " a l t e r n a t i v e " ; - - - , 293 K ; - - , 373 K.

"classic" method. The experimental points determined by the "alternative" method are denoted by the full symbols. These data were chosen for presentation in Fig. 2 because they form the most characteristic pattern. One can see that the 373 K "classic" plateau length is larger than the 293 K value for Ce0.sLa0.sNi4.98A10.o2 and there is no plateau at T~ = 373 K for Ceo.vLa0.3Ni4.98Alo.o2.An increase in cerium content in Ce~Lal_~Nis_yAly alloys leads to a dramatic decrease in the plateau length during the rise in temperature. In other words, the critical temperature decreases with increase in x. For example, our experiments have established the critical temperature value to be only 360 _+ 10 K in the case of Ce0.TLao.3Ni5 and near 480 K in the case of LaNi 5. A similar decline in the critical temperature with increase in x was established in the case of Ce~La~_~Ni4Co too. The critical temperature, however, decreases more slowly than it does in the case of CexLa~_~Nis_yAly (see Table 1).

One can see from Fig. 2 that "alternative" saturation results in an increase in the solubility of hydrogen in Ce o sLa 0 5Ni 4 98A10 02 and Ceo 7La 0 3Ni4 9sAlo 02 at . . . the . "alternative" . . . .isotherm . 373 K. .Accordingly, differs from the "classic" isotherm. It was essential that varying the maximum saturation pressure P~ in the range from 4 to 40 MPa did not influence the experimental results. The "classic" and "alternative" procedures were repeated in different sequences up to six times; all the desorption points were reproduced with an accuracy of about 3%. The X-ray analysis showed that the beforeexperiment and after-experiment samples both contained only the intermetallic phase AB 5. The phenomena described cannot be defined by the usual hysteresis. Indeed, the hysteresis would cause only the opposite result, namely a fuller saturation by hydrogen at higher temperatures, i.e. a larger plateau length in "classic" isotherms. Sorption and desorption plateau pressures for several systems are shown in Table 1. One can see from Table 1 that actual plateau hysteresis does not exceed 30%. It seems that the most likely interpretation of the phenomena observed is hydride phase formation in a metastable state at supercritical temperatures and pressures. The present investigation has established that the difference between "classic" and "alternative" isotherms occurs only in CexLa ~_XNi5 yAly-H 2 systems for a narrow range of alloy compositions (x/> 0.5 and y ~< 0.1) at temperatures 323 K to 423 K. The results may be presented as the three-dimensional diagram plotted in Fig. 3. One can see a surface limited by configuration points according to an intermetallics-H 2 equilibrium. The points A, B, C, D, E, F and G are located at this surface. The temperature T c is the critical temperature. The other surface, which is limited by the heavy line IHKLMNI, corresponds to the metastable state. It is apparent that the

Table 1 Characterization of several CexLa t xNis_yAl.-H~ and Ce~La 1 xNi4Co-H2 systems Intermetallics

Critical temperature T c (K)

Lamination temperature T L (K)

Middle plateau pressure (MPa) obtained by the "classic" method 293 K

LaNi4.6AIII.4 LaNi 5 Ceo.2Lao.~Nia.9~Alo.o2 Ceo.sLao.sNi 5 Ceo 6Lao.4Ni 5 Ceo 7Lao.3Ni4.~sAlo.o2 Ceo 7Lao 3Ni4.95Alo.o5 Ceo.TLao.3Ni4.6Al,. 4 Ceo 2Lao.8Ni4Co Ceo 5Lao.sNi4Co Ceo 6Lao.4Ni4Co

<500 480 460 420 380 360 380 380 500 460 460

360 360 340 360 -

373 K

Absorption

Desorption

Absorption

Desorption

0.05 0.20 0.49 1.2 2.0 2.1 1.6 0.32 0.50 1.1 2.0

0.03 0.18 0.42 1.0 1.4 1.6 1.3 0.27 0.44 1.1 1.6

0.70 2.0 5.2 10.0 12.1 12.7 3.9 5.2 10.1 12.2

0.61 1.9 5.1 9.6 11.7 12.4 3.7 5.1 9.6 11.9

v.z. Mordkovich et al. / Journal of Alloys and Compounds 231 (1995) 498-502

~

~

H

t

~

T

K l =

I'P0

501

tially higher than the upper limit of existence of the metastable state. Comparing the peculiarities of CexLa ~ xNis_yAlyH 2 and CexLaa_xNi4Co-H 2 systems in terms of possible applications in hydrogen/hydride technologies one can conclude that both are appropriate for the technologies demanding high output pressures. The use of CexLa l_xNis_yAly , however, requires the metastable state formation to be taken into account. 3.2. 171 xZrx Cri.8_yFey-n2 systems

A I%,%1

TL

rc

B crv%l

x----~Fig. 3. Three-dimensional pressure-temperature-hydrogen content diagram for the intermetallics-hydrogen system which contains a metastable hydride phase,

metastable state exists both at supercritical and partially at subcritical conditions. It is possible to follow the routes of stable-state and metastable-state formation using the diagram plotted in Fig. 3. For example, let us consider the formation of the two states at T 1 and P1. In the "classic" experiment the sample is heated preliminarily in vacuum from T O to T~ ( A - * B ) and then it is saturated at T 1 by raising the hydrogen pressure to P~ (B ~ I ~ C). As a result the equilibrium state is created in the form of a supercritical solid solution of hydrogen in intermetallics, In the "alternative" experiment the sample is preliminarily saturated at TO by raising the hydrogen pressure to P~ (A ~ G--* F---~ E) and then it is heated to T 1 at a constant pressure. On heating, the configuration point moves along the route (L---~K) on the metastable surface IHKLMNI instead of the equilibrium route (L---~D---~C). Point L is the lamination point. The lamination point is determined as the point of a boundary line where lamination of a phase surface into equilibrium and metastable surfaces occurs. As a result the metastable state is created in the form of a solid solution of hydrogen in the hydride phase. The values of critical temperatures T c and lamination temperatures T L are listed in Table 1 for several systems, Analysing the reasons defining the limitation of the metastable-state-bearing conditions by a narrow interval of CexLa ~ ~Nis_yAly alloy compositions (x I> 0.5 and y ~<0.1) it may be noted that there is a relationship between these conditions and the critical parameters in hydrogen-intermetalhcs systems. Indeed, all the systems with T c > 423 K do not form the metastable state (see Table 1). In particular, the critical temperatures in all the Ce~Lal_xNiaCo-H 2 systems are essen-

IMC in series Ti~_xZrxCrl.s_yFey where x = 0-0.2; y = 0-0.8 were studied. Taking into consideration the correlation between the number of outer electrons per atom n e and the structural type of AB 2 Laves phase occurred it was supposed that partial iron substitution for chromium may stabilize the hexagonal C14 phase. The number of outer electrons per atom n e is as follows for some IMC: TiCr~. 8, 5.29; TiCrl.6Fe0. 2, 5.43; TiCrFe0.8, 5.86. As is mentioned above, ne =5.4 is a boundary between C15 and C14 phase stability ranges. Thus one could expect a rise in C14 phase stability in the case of iron-substituted compounds. Partial zirconium substitution of titanium is to control pressure of the supposed plateau. All the alloys prepared are characterized by the stoichiometry ABe. 8 which corresponds to the stability ranges of both cubic and hexagonal phases. X-ray diffraction analysis of the as-cast quenched buttons showed that they contained the hexagonal C14 Laves phase only, including the case of TiCr~.8. After annealing Til_xZrxCrl.8_yFey (y > 0) at 1000°C for 1 week the samples remained single-phase, which is evidence of the expected rise in C14 phase stability. In contrast, annealing TiCrl. 8 at 1000°C for 1 week led to a mixture of C14 and C15 phases in approximately 1:1 proportion. The study of interaction of these two-phase alloys with hydrogen gave results which are very similar to those published in Ref. [11 ]. Since longer annealing was not available in this work further experiments were done witn hexagonal TiCr,. 8 only. The equilibrium isotherms TiCr~.8(C14)-H 2 at 206 K, 230 K, 255 K and 273 K agree well with Ref. [12] in absolute values of hydrogen solubility (TiCrl.sHz. 8 at 273 K and TiCrl.sn3. 5 at 206 K). The plateaus, however, are not as well-shaped as in Ref. [12]. This may be caused by the difference in techniques for hexagonal phase preparation used in our work and in Ref. [12]. Typical isotherms are displayed in Fig. 4 for the Ti0.9Zr0.1Crl.4Fe0.z-H 2 system. There is one very sloped plateau, in contrast with the two plateaus in the TiCrl.8(C14)-H 2 system.

V.Z. Mordkovich et al. / Journal of Alloys and Compounds 231 (1995) 498-502

502

t - o - - 423 K

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.................................!..................................... " ~ ' : ~ - . . - - . - - : - - * . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -, !i ...............................

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Fig. 5. Total hydrogen sorption capacity in TiCr, ~ vFey (O) and " -- capacity was Ti0.sZro2Cr~. 8 yFe~. (0) vs. the iron content y. The measured at the temperature 293 K and pressure 6 MPa.

Another important observation for IMC with y < 0.4 should be noted: heating these IMC in hydrogen at temperatures > 100°C (such heating is a natural part of each isotherm investigation) led to a sharp drop in the hydrogen absorption capacity. The residual capacity is 5-6-times lower than the initial as a result of 5 h heating at 100°C. Reactivation did not result in the restoration of properties. The X-ray diffraction analysis showed that heating in hydrogen leads to formation of cubic, hydride phase together with hexagonal hydride phase. The cubic hydride phase decomposed to cubic Laves phase and hydrogen only after heating in vacuum at temperatures around 600°C. Thus, heating IMC with y < 0.4 in hydrogen at the temperatures > 100°C induces the start of phase transition from hexagonal to cubic form. The phase mixture formed is characterized by very low hydrogen sorption capacity, which agrees well with the results

capacity decreases linearly with increase in the number of d-electrons in IMC.

[11,12]. One can conclude that the increase in n e to 5.4...5.6 which was reached by partial substitution of chromium up to y < 0.4 is not enough to stabilize the

[4] v.z. Mordkovich, N.N. Korostyshevsky, Yu.K. Baichtok and M.H. Sosna, Khim. Prom., 8 (1992)489. [5] v.z. Mordkovich, Yu.K. Baichtok, N.N. Korostyshevsky and

H/AB,.8

Fig. 4. Desorption isotherms for Tio.gZro.~Crl.4Feo.a-H 2.

hexagonal Laves phase enough.

Further increase of y does stabilize the hexagonal phase and prevents phase transition to the cubic form but leads to a decrease in total hydrogen sorption capacity (see Fig. 5). The decrease in total hydrogen sorption capacity with the rise in y may be explained by a decrease of the average atomic radius and, hence, expansion of the space where hydrogen atoms enter. Indeed, rFe = 1.26 A and rcr = 1.27 A. Another consid-

eration which supports this explanation is that partial zirconium substitution for titanium (ra-i = 1.46 ,~ and rzr = 1.607 ,~) results in a hydrogen capacity increase. Another possible explanation may lie in the increase in the number of d-electrons with the rise in y, since it is usually supposed [13] that total hydrogen sorption

4. Conclusion One can conclude that the Til_xZrxCrl.8_yFey alloys are not promising in terms of use in hydrogen/hydride technologies owing to the strong dependence linking the stability of hexagonal Laves phase and the total hydrogen sorption capacity.

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M.H. Sosna, in D.L. Block and T. Nejat Veziroglu (eds.), Hydrogen Energy Progress X. Proceedings of the lOth World Hydrogen Energy Conference, Cocoa Beach, Florida, USA, 20-24 June 1994, Vol. 2, Int. Ass. Hydr. Energy, Cocoa Beach,

1994, p. 1029. [6] M.Yu.Teslyuk, Metallic Compounds with Laves Phase Struclures, Metallurgia, Moscow, 1969. [7] 0. Bernauer, J. Topler, D. Noreus, R. Hempelmann and D. Richter, Int. J. Hydrogen Energy, 14 (1989) 187. [8] o. Bernauer and C. Halene, J. Less-Common Met., 131 (1987)

213. [9] Y. Osumi, H. Suzuki, A. Kato, T. Sugioka and T. Fujita, J. Less-Common Met., 89 (1983)257. [10] Y. Gamo,Y. Moriwaki,N. Yanagihara and T. Iwaki, J. Less-

Common Met., 89 (1983) 495. [11] J.R. Johnson and J.J. Reilly, lnorg. Chem., 17 (1978) 3103. [12] J.R. Johnson, J. Less-Common Met., 73 (1980) 345. [13] J.J. Reilly,Z. Phys. Chem. Neue Folge, 117(1979) 155.