Hydriding and dehydriding processes of LaNi5−xCox (x=0–2) alloys under hydrogen pressure of 1–5 MPa

The hydriding and dehydriding behavior of LaNi5−xCox (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 LaNi5–H2 system, two endothermic peaks were observed. One was the peak for the transformation from the γ phase (LaNi5H6 hydride) to the β phase (LaNi5H3 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 LaNi5–H2 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 Ozawaʹs 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.