Hydride formation in MgZrFe1.4Cr0.6 composite material

Journal of Alloys and Compounds, 209 (1994) 117-124 JALCOM 1055

117

Hydride formation in Mg-ZrFel.4Cro.6 composite material Zhou Ye Department of Physics, Uppsala University, Box 530, S-75121 Uppsala (Sweden)

L.C. E r i c k s o n Department of Technology, Materials Science Division, Uppsala University, Box 534, S-75121 Uppsala (Sweden)

B. H j 6 r v a r s s o n * Department of Physics, Uppsala University, Box 530, S-75121 Uppsala (Sweden) (Received October 22, 1993)

Abstract In the present work, we report on hydride formation in an Mg-ZrFel.4Cr0.6 composite. The thermodynamic properties, the hydrogen absorption and desorption kinetics at different temperatures, the cycling properties, and the resistance to oxidation were examined. The two phases of the composite, i.e. Mg and the alloy, retained their original thermodynamic properties upon repeated cycling. The hydrogen absorption and desorption in the composite were rapid, even at moderate temperatures (150 °C). The Mg particles were found to be partially covered by the alloy. The alloy islands acted as a catalyser for the dissociation and recombination of hydrogen atoms. Furthermore, the alloy overlayer protected the Mg from oxidation during sorption cycles and exposure to the atmosphere. The hydrogen absorption and desorption were determined by standard volumetric methods. Structural and chemical analyses were performed by X-ray diffraction, scanning electron microscopy, energydispersive X-ray spectroscopy, transmission electron microscopy and selected-area diffraction.

1. Introduction

The interest in Mg as a hydrogen-storage material originates in the high hydrogen weight per cent (7.6%) of the metal hydride. However, the high temperature required for cycling excludes all but a few practical applications. During the last few decades, there has been much effort to improve the hydriding and dehydriding properties of Mg-based storage materials: (i) alloying Mg with other elements to form, for example, MgzNi [1], MgzCu [2], Mg-La [3] and Mg solid solution alloys, such as Mg-Ag, and Mg-In [4]; (ii) surface treatment to improve the surface reaction rate of hydrogen by coating the surface with Ni and Pd [5-7], and etching Mg powder with HC1 solution [8]. Another approach to increasing the surface activity is to prepare Mg-based composites such as Mg-MgzNi [9] and Mg-LaNi5 [10]. MgzNi reacts slowly with hydrogen and new phases, such as Mg2Ni and LaMglz, are formed in the Mg-LaNi5 composite. This makes these composites rather poor candidates for applications. *Author to whom correspondence should be addressed.

Mandal et al. [11] investigated hydride formation in an Mg-40wt.%FeTi(Mn) composite. These composites were shown readily to absorb and desorb hydrogen at room temperature, and the hydrogen-storage capacity was found to be 3.5 wt.%. In this article, we present results from investigations on the composite Mg-50wt.%ZrFeL4Cro.6. None of the elements in the ZrFeL4Cr0.6 forms an alloy with Mg, which implies that the hydriding properties of the alloy and the Mg in the composite could be separated and studied without a memory effect.

2. Experimental details

The Mg-50wt.%ZrFel.4Cro.6 composite was prepared from Mg powder (purity of 99.8% and maximum particle size of 50 /xm) and ZrFel.4Cro.6 alloy powder using a common powder metallurgy method. The ZrFel.4Cro.6 alloy was prepared from Fe chips (99.9%), Zr sponge (99%) and Cr pieces (99.5%), by non-consumable arc melting on a water-cooled Cu hearth in a purified Ar atmosphere. The Ar atmosphere was purified by melting a Ti button and allowing it to react with the residual oxygen and water vapour.

0925-8388/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved SSD1 0925-8388(93)01055-9

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Zhou Ye et al. / Hydride formation in Mg-ZrFel.4Cro.6

After initial melting, the ingots were inverted several times to ensure homogeneity. The alloy was finely powdered by repeated hydrogen loading-unloading cycles. The alloy powder was mixed with Mg powder to 50 wt.%, and compressed under a pressure of about 850 MPa to form pellets 10 mm in diameter and 5 mm thick. Then, the pellets were sintered at 973 K for 1 h under an Ar gas pressure of 0.1 MPa. The activation was carried out at 623 K and under a hydrogen pressure of 4 MPa. After several hydrogen sorption cycles, the absorption and the desorption rates were measured at several temperatures and under different hydrogen pressures. The rates and the composition were measured using a conventional volumetric method. The surface topography and morphology of the composite particles were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Prior to SEM analysis, some of the samples were coated with C. This procedure was carried out to improve the electrical conductivity, so reducing the charge build-up in the Mg hydride, which is an insulator. The particles were incorporated into a C film for TEM analysis. Energy-dispersive spectroscopy (EDS) was used for elemental analysis of the particles, and selected-area diffraction (SAD) and X-ray diffraction (XRD) were employed for phase analysis.

3. Results

3.1. Hydrogen absorption and desorption of the composite 3.1.1. Activation and thermodynamic properties of the composite The composite was activated at 623 K under 4.0 MPa (40 bar) for 4 h. The activation of the composite was accomplished at a lower temperature with a shorter time than those for pure Mg powder, which was used as a raw material for the composite (673 K and 4.0 MPa for 6 h). The two phases of the composite absorb and desorb hydrogen under different conditions. When the composite is exposed to hydrogen at moderate temperatures (523 K and 1 MPa), only the Mg forms a hydride. After cooling to room temperature under the same pressure, a hydride is also formed in the alloy. In the desorption process the alloy hydride, i.e. ZrFel.4Cro.6n2.6, releases hydrogen first, which occurs at about 320 K under 0.1 MPa, and the Mg hydride releases hydrogen at temperatures around 570 K at the same pressure. In Fig. 1, the In P vs. 1/T plot of pure Mg and Mg in the composite is displayed. As seen in the figure, the thermodynamic properties of Mg in the composite are the same as those for pure Mg. We did not observe

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(--). any change in the hydriding and dehydriding properties of the Zr-Fe-Cr alloy in the composite (hydride, ZrFea.aCro.6H2.6; A n = - 2 7 . 1 kJ mo1-1 Hz; desorption plateau pressure at 298 K, 0.1 MPa), which is the same result as obtained by Fujii et al. [12]. The composite can absorb a large amount of hydrogen, since both components can form a hydride. If Mg is loaded for a long enough time to form MgH2, the resulting hydrogen weight per cent in the composite would be around 4.45. In the actual experiments, only partial hydride formation was achieved within a reasonable time-scale. After loading the composite for 1 h, the average hydrogen content was 3.0 wt.%, which yields an average composition MgH1.2.

3.1.2. Absorption and desorption kinetics of Mg in the composite The absorption kinetics of Mg powder and Mg in the composite are displayed in Fig. 2. The kinetics of hydride formation for Mg in the composite form were found to be much faster than the kinetics for pure Mg. Initially, the reaction rate is very fast for the composite, even at 443 K (see Fig. 3). The desorption kinetics in Mg powder and in the Mg composite, at several temperatures and pressures, are shown in Fig. 4. Hydrogen in the Mg part of the composite desorbs much faster initially than in pure Mg at any temperature. Furthermore, the rate-controlling step for desorption is changed. In Mg, the hydrogen uptake is limited by the nucleation rate and growth, however, in the Mg composite, the uptake rate is found to be diffusion limited. 3.1.3. Cycling properties and oxidation of the composite Figure 5 shows the hydriding kinetics of Mg in the composite after activation, 25 hydriding and dehydriding

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Fig. 2. Absorption kinetics of hydrogen in Mg (O, 623 K; O, 573 K) and Mg composite (Q, 623 K; &, 573 K). The initial hydride formation is more rapid for the composite. At [H]/[Mg] ratios above approximately 0.6, the rate for hydride formation in Mg is higher.

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Absorption time (rain) Fig. 3. Absorption kinetics for the Mg composite at 0.9 MPa: l , 623 K; O, 573 K; O, 523 K; O, 473 K; O, 423 K. cycles, and exposure to air for one week. As can be seen in the graph, the composite is only moderately affected by the cycling and exposure to air. This implies good cycling properties and only minor passivation as a consequence of exposure to air. These properties are of major importance for applications. 3.2. S E M and E D S analyses

The motivation for using SEM was twofold: first, to investigate the size distribution of the particles in the composite powder, and secondly, to establish the spatial relationship between the alloy and the Mg hydride formed. A representative picture of the composite particles is shown in Fig. 6(a). The particles ranged in

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size from 5 to 65 /zm, with an average size of 20/xm. The surface topography of the sintered material is shown in Fig. 6(b). Qualitative analysis was performed using the X-ray analysis facilities (EDS) on the SEM apparatus to determine the distribution of the constituents of the different particles. The spot size was roughly 1-2/~m, with a penetration depth ranging from 0.1 to 2.0/xm. A typical hydrided composite particle, roughly 20 /xm across, is shown in Fig. 7(a).

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Zhou Ye et al. / Hydride formation in Mg-ZrFel.~Cro.6

3.3. TEM analysis Microstructural analysis of the hydrided composite particles was performed using TEM combined with EDS analysis and SAD for element and phase analyses. A high resolution scanning transmission electron microscopy (STEM) image detailing the topography of the tiny islands of Zr-Fe-Cr alloy covering a hydrided composite particle is shown in Fig. 9(a). TEM images showing the microstructures of typical alloy particles are presented in Figs. 9(b) and 9(c). The particles are composed of what appear to be small crystallites of Zr, Fe and Cr. The electron diffraction (SAD) pattern shown in the upper comer of Fig. 9(b), typical of a crystalline material, was taken through the thin edge of an alloy particle, corresponding to the lower half of the figure. Hexagonal close-packed (h.c.p.) Zr, together with some Cr, were the main elements detected in this region. The C coating across the center of the micrograph should be noted. Figure 9(c) shows the microstructure of an alloy crystallite at a still higher magnification.

4. Discussion

Fig. 6. (a) Overview (SEM) of hydrided composite particles, illustrating size distributions, and (b) surface view of sintered material (SEM).

The accuracy of EDS analysis in determining element distributions is limited by so-called particle effects, caused by the size and shape of small particles [13], on the path of the X-rays. A cross-section was taken from the hydrided composite particles (see Fig. 7(b)) to avoid this problem. The dark region seen in the figure is Mg, while the lighter regions shown are small particles of the Zr-Fe-Cr alloy phase. Mg was also detected underneath the alloy islands, as shown by the EDS spectra in Figs. 7(d) and 7(e). An SEM image and spectra for a particle of the dehydrided composite are shown in Fig. 8. The particle shown in the figure was not covered to the same extent with the Zr-Fe-Cr alloy particles as was the case for the particle displayed in Fig. 7(a). This sample was not coated with C, because these particles did not charge up to the same extent upon electron bombardment as did the hydrided particles. The spatial correlation between the charge build-up on the Mg hydride and the alloy islands was strong.

According to the binary alloy phase diagrams of Mg-Zr, Mg--Cr and Mg-Fe [14], the elements Cr and Fe do not form any new phase with Mg. Only a dilute solid solution of Zr in Mg (solubility of 3.8 wt.% at 923 K and 0.3 wt.% at 573 K) is formed. This was verified by the XRD results; only diffraction peaks originating from Mg and Zr-Fe-Cr phases were observed. As there is no formation of new Mg intermetallic compounds, it is reasonable that the thermodynamic properties of the composite remain the same after thermal cycling. It is well established that the slow hydrogen uptake of Mg at moderate temperatures arises from an activation barrier for dissociation and/or surface hydride formation. In the present study, the Mg powder in the composite was partially covered by the Zr-Fe-Cr alloy. This is seen in the SEM and TEM results (see Figs. 7 and 9). The hydrogen molecule can decompose at the alloy surface and diffuse into the underlying Mg matrix. Therefore, the dissociation on the Mg surface is effectively bypassed by the more efficient dissociation on the alloy surface. Thus, the composite can be regarded as having the surface properties of the alloy, and consisting of two bulk regions corresponding to Mg and the Zr-Fe-Cr alloy. After diffusing to the Mg-alloy interface, the hydrogen is effectively trapped in the near-interface region as MgH2 [6]. The Mg hydride is formed in a nucleation growth process, which initially occurs at the interface. This step we call stage I of hydride formation. As the

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rate of growth occurring in stage I declines (after approximately 2 min, with the [H]/[Mg] ratio in the range 0,4-0.6), stage II of hydride formation becomes more prominent. This stage can consist of two processes; transport of hydrogen through the completed hydride layer in the near-(Mg-alloy)interface region, and the surface and grain boundary diffusion of hydrogen. The rate for the second stage is much slower than that for stage I uptake. The transport of hydrogen through the hydride layer is expected to be retarded because the diffusion of hydrogen in MgH2 is normally regarded as being extremely slow. The transport of hydrogen on the surface (including grain boundaries) is, for geometrical reasons, less efficient; however, the jump rate

Fig. 7. Typical hydrided composite particle: (a) overview of particle (SEM); (b) cross-section (SEM); (c), (d), (e) EDS spectra of points marked in (b).

is unknown, which excludes any quantitative comparison of the two possible channels. Generally, the thickness of the hydride layer is approximately 0.1-1 /~m [6, 7, 15]. The particle size distribution ranged from 5 to 65 /zm, with an average size of 20/~m and standard deviation of 14/~m. However, the effective size is much smaller, because the particles are irregular in shape and, therefore, can be regarded as an ensemble of smaller particles. Consequently, this will be reflected in the scattering in the average hydride composition. For example, if a hydride (MgH2) layer 0.1/~m thick is formed on a spherical Mg particle with a diameter of 1 /zm, the average composition will be MgHo.72. As the penetration depth of the hydride

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Zhou Ye et al. / Hydride formation in Mg-ZrFet.4Cro. 6

constant, p the molar density of the host material, and r the mean radius of the absorbing particles. Plots of 1-3(1-a')2/a+2(1-a ') as a function of the exposure time, at several temperatures, are given in Fig. 10. As can be seen in the figure, the hydrogen uptake in stage II is well described by this model at all temperatures. Therefore, we conclude that the hydrogen uptake in the second stage is diffusion controlled. From the slope, we obtain an Arrhenius plot of the diffusion coefficient vs. the reciprocal temperature, as seen in Fig. 11. The activation energy for the ratelimiting step (diffusion) in the composite is found to be 48.5 kJ mo1-1 (0.5 eV). This activation energy is only one-half of that found for pure Mg powder [14]. If the rate-limiting step is the diffusion in Mg, the decrease in the activation energy for the diffusion would be expected to be due to defects and disorder, which 0.07

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are residual in the Mg matrix after cracking and powdering. Another possibility could be the solid solution of Zr which leads to the deformation of the Mg crystal structure and/or the formation of defects. It is reasonable to assume that the hydrogen diffuses more rapidly in a disordered matrix, as a result of a lowering of the effective activation energy. As the dissociation of hydrogen occurs preferentially at the alloy surface and thereafter is transported to the underlying Mg, the oxidation of the Mg surface does not affect the hydrogen uptake of the composite. The alloy surface does not form stable oxides and, therefore, is relatively insensitive to air exposure. Thus, the interface between the alloy and Mg is protected by the overlayer and the absorption channel is unaffected. As for the dissociation, the alloy has a catalytic effect on the recombination of hydrogen, as can be seen in Fig. 4. The rate-limiting step for recombination is altered in both form and magnitude. The rate of the desorption from Mg goes through a maximum which is typical for a nucleation growth and decay process. No such maximum is seen for the composite, in which the desorption is diffusion limited. Therefore, the rate of desorption is monotonically decreasing. The hydrogen uptake of the composite is not optimal. The Mg in the composite is only partially hydrided, mainly for morphological reasons. At [H]/[Mg] ratios above 0.6, the hydriding rate of Mg in the composite form is slower than that for Mg powder (see Fig. 1). The slowing down is probably linked to the existence of relatively large Mg particles, which are only partially hydrided. The hydride is only formed in the region close to the alloy-Mg interface. By decreasing the particle size, more complete hydride formation would be achieved. Furthermore, if the particles could be covered to a larger extent with the alloy, the kinetics as well as the total uptake would be improved.

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The Mg-50wt.%ZrFel.4Cro.6 composite is found to be a stable and relatively high performance hydrogenstorage material. The components in the alloy do not form intermetallic compounds with Mg, which is of importance for the long-term stability of the composite. Because the alloy also absorbs hydrogen, the composite has a high hydrogen content (up to 3.2 wt.%), though the Mg is only partially hydrided. Mg in the composite was found to absorb and desorb hydrogen very fast initially, even at moderate temperatures. The kinetic properties are only modestly affected by repeated hydrogen sorption cycles and exposure to the atmosphere. The kinetics could be further improved

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Zhou Ye et al. / Hydride formation in Mg-ZrFel.4Cro.6

by decreasing the particle size, combined with a higher degree of coverage of the alloy overlayer. Mg in the composite form could be completely hydrided when the particle size of the powder was smaller than the mean nucleation range of the hydride formation. The resulting weight per cent would increase to 4.45.

References 1 J.J. Reilly and R.H. Wiswall, Jr., Inorg. Chem., 7 (1968) 2245. 2 J.J. ReiUy and R.H. Wiswall, Jr., Inorg. Chem., 6 (1967) 2220. 3 S. Yajima, H. Kayano, H. Tama, Z Less-Common Met., 55 (1977) 139. 4 D.L. Douglas, in A.F. Andresen and A.J. Maeland (eds.), Proc. Int. Syrup. on Hydrides for Energy Storage 9, 1977, Pergamon, Oxford, 1978, p. 151.

5 F.G. Eisenberg, D.A. Zagnoll and J.J. Sheridan, III, J. LessCommon Met., 74 (1980) 323. 6 J. Ryd6n, B. Hj6rvarsson, T. Ericsson, E. Karlsson, A. Krozer and B. Kasemo, J. Less-Common Met., 152 (1989) 195. 7 F. Stillesj6, S. Olafsson, B. Hj6rvarsson and E. Karlsson, Z. Phys. Chem., in press. 8 H.Y. Zhu, C.P. Chen, Y.Q. Lei, J. Wu and Q.D. Wang, J. Less-Common Met., 172-174 (1991) 873. 9 J.M. Boulet and N. G6rard, J. Less-Common Met., 89 (1981) 151. 10 H. Nagai, H. Tomizawa, T. Ogasawara and K.I. Shoji, J. LessCommon Met., 157 (1990) 15. 11 P. Mandal, K. Dutta, K. Ramakrishna, K. Sapru and O.N. Srivastava, J. Less-Common Met., 184 (1992) 1. 12 H. Fujii, S. Orimo, K. Yamamoto, K. Yoshimoto and T. Ogasawara, J. Less-Common Met., 175 (1991) 243-257. 13 J.I. Goldstein, D.E. Newbury, P. Echlin, D.C. Joy, C.E. Fiori and E. Lifshin, Scanning Electron Microscopy and X-ray Microanatysis, Plenum, New York, 1981. 14 C.M. Stander, Z. Phys. Chem. Neue Polge, 104 (1977) 229. 15 B. Vilgeholm, J. Kj611er, B. Larsen and A.S. Pedersen, J. Less-Common Met., 89 (1983) 135.