Effect of mechanical grinding under argon and hydrogen atmospheres on structural and hydriding properties of LaNi₅

Effect of mechanical grinding (MG) under argon and hydrogen atmospheres on structural and hydriding properties were investigated in detail. In MG-LaNi₅ under argon atmosphere, the grain size reaches approx. 20 nm for MG times of 60 min and almost remains unchanged for more longer MG times. The pressure-composition isotherm (P-C) in LaNi₅ at room temperature indicates an enhancement of hydrogen solubility, a lowering of plateau pressure and a narrowing of width of plateau by MG like FeTi but not Mg₂Ni. On the other hand, in reactive MG (RMG)-LaNi₅ under hydrogen atmosphere, a nanocrystallized LaNi₅H_0.15 and amorphous phases coexist within 180 min of grinding time. For RMG times longer than 180 min, the nanostructured LaNi₅H_0.15 phase disappears and the remaining amorphous-LaNi₅H_x dissociates into nano-Ni+LaNi_yH_z(y<5). The P-C isotherm indicates no-plateau in the LaNi₅ produced by RMG longer than 60 min and the hydriding properties become worse and worse with increasing RMG times. From these results, we conclude that the hydriding properties could not be improved by structural modifications in the system containing metals with strong affinity for hydrogen like rare earth metals.