Structure and properties of metal hydrides prepared by mechanical alloying

Our research examines the structure and reversible hydrogen storage capacity of alloys based on the LaNi5 intermetallic. The alloys are prepared by mechanical alloying (MA), a technique particularly useful when alloying LaNi5 with low melting point elements such as tin and calcium. In LaNi5-ySny, x-ray diffraction and Rietveld analysis show that tin preferentially occupies the Ni(3g) sites in the LaNi5 structure, and the unit cell volume increases linearly with tin content to a maximum tin solubility of 7.33 atomic percent (LaNi4.56Sn0.44). The addition of tin to LaNi5 causes (a) a logarithmic decrease in the plateau pressures for hydrogen absorption and desorption, which is consistent with the corresponding increase in the volume of the LaNi5 unit cell; (b) a decrease in the hysteresis between the pressures for hydride formation and decomposition, which is in agreement with a recent theoretical model for the effect; and (c) a linear decrease in the hydrogen storage capacity. Effect (c) is explained by a rigid-band model whereby electrons donated by the tin atoms occupy holes in the 3d band of LaNi5, which could otherwise be occupied by electrons donated by the hydrogen atoms. Thermodynamic van't Hoff analysis for these alloys show an increase in hydride formation enthalpy and no change in entropy with increasing tin concentration. LaNi5 with calcium additions shows enhanced kinetics of hydrogen absorption/desorption. The powder particles prepared by MA have a larger surface area than particles of the same overall size prepared by arc casting. All LaNi5-based alloys prepared by MA in an inert environment require no activation for hydrogen absorption and suffer less comminution upon hydriding/dehydriding.