The hydriding and dehydriding kinetics of some LaNi 5−χAl χ alloys

Abstract

In this study, kinetics measurements were performed on alloys of the general composition LaNi 5−χAl χ ( X=0, 0.1, 0.2, 0.3). It is known that replacing a portion of the Ni in LaNi 5 with aluminum drastically lowers the plateau pressure of the hydride and it was of interest to determine what, if any, effect this might have on the rates of hydrogen uptake and release. In all cases, the measurements were taken in the two phase (α-β) region under isothermal and isobaric conditions. A heat buffer, which consisted of 95 wt.% aluminum powder, was used to maintain isothermal conditions. Constant pressure driving forces were achieved by setting the ratio of the applied pressure to the equilibrium plateau pressure to the same value in each of the experiments. Prior to analysis, each sample was subjected to an identical number of hydriding- dehydriding cycles to assure that the surface was clean and that a consistent particle size distribution was achieved. Under these conditions, the reaction rates decreased with increasing aluminum content. Since an increase in the aluminum content of the LaNi 5−χAl χ-H system increased the hydride stability but decreased the reaction rate, it was concluded that reaction rates are inversely proportional to hydride stability.