Hydrogenation and dehydrogenation behavior of LaNi_5-x Co_x (x = 0, 0.25, 2) Alloys studied by pressure differential scanning calorimetry

The hydrogenation and dehydrogenation behavior of LaNi₅, LaNi_4.75 Co_0.25 and LaNi₃Co₂ was studied by the pressure differential scanning calorimetry (PDSC) at the hydrogen pressure range of 1 to 5 MPa in the temperature range from 323 to 473 K with the heating and cooling rates of 2 to 30 K min^-1. In the heating run of the hydride of LaNi₅, two endothermic peaks were observed. One was the peak for the transformation from the γ phase (full hydride LaNi₅H₆) to the β phase (hydride LaNi₅H₃). 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. These endothermic and exothermic peaks shifted to higher temperatures with the increase in hydrogen pressure. In the heating and cooling runs of the LaNi_4.75Co_0.25-H₂ system the PDSC curves similar to those of the LaNi₅-H₂ system were observed. However, in the heating run of the hydride of LaNi₃Co₂ only one endothermic peak was observed. Using Ozawa's method, the activation energies for dehydrogenation of the hydrides were estimated. The activation energy for the γ-β transformation was higher than that for the β-α transformation. Substitution of cobalt for a part of nickel in LaNi₅ increased the activation energies for the phase transformations.