Effect of mechanical grinding under Ar and H ₂ atmospheres on structural and hydriding properties in LaNi ₅

The effects of mechanical grinding (MG) under argon and hydrogen gas atmospheres on the hydrogen storage properties of a LaNi ₅ alloys were studied in detail. During MG under Ar atmosphere, a crystallite size reaches a ∼20 nm in grinding time of 60 min and reduces to approximately half this size after 600 min without any dissociation. The pressure-composition isotherm (P-C) in LaNi ₅ at 293 K indicates an increase in hydrogen in zero offset region (trapping site region), a lowering of plateau pressure and a narrowing of the width of the pressure plateau by MG. On the other hand, in reactive MG (RMG)-LaNi ₅ under H ₂ atmosphere, a nanocrystalline LaNi ₅H _0.15 and an amorphous phase coexist when the grinding time is less than 180 min. For much longer RMG times than 180 min, the nanostructured LaNi ₅H _0.15 phase disappears and the remaining amorphous phase dissociates into nanocrystalline Ni+amorpous LaNi _yH _z (y<5). The P-C isotherm indicates no plateau for the LaNi ₅ produced by RMG longer than 60 min and the hydriding properties become worse and worse with increasing RMG times. From the above results, we conclude that the hydriding properties cannot be improved by structural modifications in systems containing metals with a strong affinity for hydrogen like rare earth metals.