Hydriding properties of LaNi3 and CaNi3 and their substitutes with PuNi3-type structure

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Abstract

The PuNi3-type intermetallic compounds LaNi3, CaNi3, La0.5Ca0.5Ni3, LaCaMgNi9, La0.5Ca1.5MgNi9, CaTiMgNi9, LaCaMgNi6Al3 and LaCaMgNi6Mn3 have been prepared using a powder–metallurgy–sintering method. The hydrogenation behaviour of these materials has been studied through the gas–solid reaction. The as-prepared compounds were easily activated at room temperature under a hydrogen pressure of 3.3 MPa. The pressure–composition–temperature (PCT) curves show a single plateau region with the exception of LaNi3-H, which shows no plateau, and La0.5Ca1.5MgNi9-H, which shows two plateaus. All of these alloys can absorb/desorb hydrogen by 1.8 wt.% under the conditions studied. X-ray diffraction (XRD) analysis reveals that LaNi3H4.5 is in the amorphous state, and the other hydrides are accompanied by different expansions of the unit cell volume of the host alloy.

Introduction

Intermetallic compounds for reversible hydrogen absorption/desorption have been the subject of extensive research for about 30 years [1], [2], [3], [4]. According to differences in the crystal structure, the intermetallic compounds AxBy (A is a stable hydride-forming element and B is an unstable hydride-forming element) can mainly be classified into the forms: AB5 (CaCu5 type), AB3 (PuNi3 or CeNi3 type), A2B7 (Ce2Ni7 type), AB2 (MgCu2 or MgZn2 Laves phase), AB (CsCl type) and Mg-based alloys [5]. The AB5 and AB2 alloys have seen commercial application in the area of nickel-metal hydride batteries [6], [7]. However, the hydrogen storage capacity of AB5 alloys is <1.4 wt.% and this is not enough to apply them for various uses. The catalytic activity of AB2 alloys is still rather low and thus these alloys need surface treatment [8]. In addition, this kind of alloy usually contains the element vanadium, which is expensive and causes serious environmental pollution. Therefore, the demand for hydrogen storage materials with high capacity, low cost and environmentally friendly behaviour has increased.

The structure of AB3 compounds is closely related to the AB5 and AB2 structures [9], [10]. The AB3 structure contains a long-range stacking arrangement of which one-third is AB5-like and two-thirds AB2-like. Previous work reported by Oesterreicher et al. [11], [12] showed that LaNi3 and CaNi3 react rapidly with hydrogen under ambient conditions to form LaNi3H5 and CaNi3H4.6, respectively. Takeshita et al. [13], [14] studied the hydrogen absorption characteristics of RT3 phases (R=Dy, Ho, Er, Tb, Gd; T=Fe or Co), and their results showed that the hydrogen storage capacity of the RT3 phases exceeds those for the RCo5 phase and the well-known hydrogen absorber LaNi5. Recently, a structural observation of the RMg2Ni9 (R=Y, Ca, rare earth) system by Kadir et al. [15], [16] revealed that the substituted AB3 compounds are promising candidates for reversible hydrogen storage. It is expected that the AB3 phase will be more receptive to hydrogen than its AB5 counterpart because it contains the AB2 unit. Also, for the AB3 phase it is possible to combine the elements Mg, Ca, Ti and rare earth in the A site due to its special crystal structure. Although there are some studies on the hydrogen storage of AB3 compounds, the results are not complete. The purpose of the present work is to report our study on the hydriding properties of LaNi3 and CaNi3 and their substitutes. This may provide useful information on the generalization of AB3-type metal hydrides.

Section snippets

Compounds

All compounds with nominal compositions LaNi3, CaNi3, La0.5Ca0.5Ni3, LaCaMgNi9, La0.5Ca1.5MgNi9, CaTiMgNi9, LaCaMgNi6Al3 and LaCaMgNi6Mn3 were prepared using a powder–metallurgy–sintering method. The starting materials such as the mother alloys LaNi5, LaNi2, CaNi5, CaNi2 and MgNi2, and the metal elements Mg, Ti, Al, Mn and Ni, were powders (<76 μm) and supplied by Ergenics or Japan Metal and Chemical Co. The X-ray powder diffraction patterns for the mother alloys were consistent with data

Structural analysis

Fig. 1 shows the diffractometer intensity profiles for the intermetallic compounds prepared above. It can be seen that, except for a few small peaks due to the impurity phases, all the other peaks could be attributed to a hexagonal base structure with unit cell dimensions as summarized in Table 1. The lattice parameters for the alloys LaNi3 and CaNi3 agree well with those reported previously [18], [19], indicating that the sintering method is an effective way to prepare the AB3 phase. As an

Conclusions

AB3 intermetallic compounds (A is La or Ca with partial replacement by Mg or Ti, and B is Ni with partial replacement by Al or Mn), having the PuNi3-type structure, have been synthesized. Hydrides of these compounds have been prepared by hydrogen absorption at temperatures from 10 to 40°C and under a hydrogen pressure of 3.3 MPa. The maximum hydrogen content (H/M) is: 1.12, 1.10, 1.10, 1.10, 1.08, 1.00, 0.99 and 1.08, respectively, for LaNi3, CaNi3, La0.5Ca0.5Ni3, LaCaMgNi9, La0.5Ca1.5MgNi9,

Acknowledgements

The New Energy and Industrial Technology Development Organization (NEDO) supports this work.

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