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-Co-x(x), (0less than or equal toxless than or equal to2) 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-H-2 system, two endothermic peaks were observed. One was the peak for the transformation from the gamma phase (LaNi5H6 hydride) to the beta phase (LaNi5H3 hydride). The other was the peak for the transformation from the beta phase to the alpha phase (solid solution). In the cooling run, one exothermic peak for the transformation from the alpha phase to the gamma phase was observed. In the range of x less than or equal to 0.5. PDSC curves similar to that of the LaNi5-H-2, system were observed. However, in the range of x greater than or equal to 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 gamma-beta transformation was higher than that for the beta-alpha transformation and the activation energy for the beta-alpha transformation has a maximum at the composition of about x =1. (C) 2003 Elsevier Ltd. All rights reserved.