Effect of mechanical grinding under Ar and H2 atmospheres on structural and hydriding properties in LaNi5

The effects of mechanical grinding (MG) under argon and hydrogen gas atmospheres on the hydrogen storage properties of a LaNi, alloys were studied in detail. During MG under Ar atmosphere, a crystallite size reaches a similar to20 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 LaNli(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)-LaNi5 under H-2 atmosphere, a nanocrystalline LaNi5H0.15 and an amorphous phase coexist when the grinding time is less than 180 min. For much longer RMG times than 180 min, the nanostructured LaNi5H0.15 phase disappears and the remaining amorphous phase dissociates into nanocrystalline Ni+amorpous LaNiy-H-z (y<5). The P-C isotherm indicates no plateau for the LaNi5 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. (C) 2002 Elsevier Science B.V. All rights reserved.