A Study on Guiding Principles to Predict the Properties of Hydrogen Absorbing Compounds with Emphasis on Their Positron Lifetime

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Alloys based on the intermetallic phase, LaNi_5 (the hexagonal CaCu_5-type structure) have been used as negative electrode materials of rechargeable nickel-metal hydride batteries. In order to develop rechargeable nickel-metal hydride batteries with higher performance than that of batteries using LaNi_5 as their negative electrode materials, a new intermetallic compound with a higher hydrogen storage capacity than that of LaNi_5 with reasonable resistance to powdering after cycles of hydriding-dehydriding is required to be discovered.The stabilities of metal hydrides are closely associated with the radii of interstitial holes in their lattices and the valence of constituent elements. Positron life time is sensitive to both the radii of interstitial holes and valence of constituent elements. One of the two aims of this study is to study the correlation between hydrogen desorption pressure and positron life time and that between unit cell volume and positron lifetime. The other aim is to … More clarify the deformation modes in LaNi_5 as basic knowledge to control the powdering of the compound.Measurements of positron life time in LaNi_5, LaNi_<5-X>Al_X and MmNi_<5-X>Al_X results in the following findings : (1) no structural vacancies exist in LaNi_5 and thus hydrogen atoms trapped at interstitial holes, (2) the positron lifetime in LaNi_<5-X>Al_X and MmNi_<5-X>Al_X is proportional to the size of their unit cell, (3) the plateau pressure of LaNi_<5-X>Al_X and MmNi_<5-X>Al_X is proportional to the positron lifetime in these compounds, (4) thus, positron lifetime measurements can be used to develop new hydrogen absorbing compounds.Deformation experiments of LaNi_5 at high temperatures result in the following findings : (1) LaNi_5 can be deformed at temperatures higher than 950ﾟC,(2) yield stress rapidly decreases with increasing temperature, (3) dislocations with Burgers vectors of [0001] and 1/3 <1120> are observed in specimens deformed at temperatures higher than 950ﾟC,(4)slip plane for the former type of dislocations is {1010}. However, that for the latter type of dislocations has yet to be determined. Less