Hydriding and dehydriding processes of LaNi 5− xCo x (x=0–2) alloys under hydrogen pressure of 1–5 MPa

Abstract

The hydriding and dehydriding behavior of LaNi 5− x Co x (0⩽ x⩽2) was studied by the pressure differential scanning calorimetry (PDSC) at the hydrogen pressure range of 1–5 MPa in the temperature range from 323 to 573 K. In the heating run of the LaNi 5–H 2 system, two endothermic peaks were observed. One was the peak for the transformation from the γ phase (LaNi 5H 6 hydride) to the β phase (LaNi 5H 3 hydride). The other was the peak for the transformation from the β phase to the α phase (solid solution). In the cooling run, one exothermic peak for the transformation from the α phase to the γ phase was observed. In the range of x⩽0.5, PDSC curves similar to that of the LaNi 5–H 2 system were observed. However, in the range of x⩾1, one endothermic and one exothermic peaks were observed in the heating and cooling runs, respectively. Using Ozawa's method, the activation energies for the dehydriding processes were estimated. The activation energy for the γ–β transformation was higher than that for the β–α transformation and the activation energy for the β–α transformation has a maximum at the composition of about x=1.