A study on hydrogen-induced amorphization in C15 Laves phase DyNi_2 under different hydrogen pressures

摘要

Four exothermic reactions of (1) hydrogen absorption forming crystalline (o) and amorphous (a-) DyNi_2H_x (2) the transformation of c-DyNi_2H_x to a-DyNi_2H_x, i.e., hydrogen-induced amorphization (HIA), (3) the precipitation of DyNi_5 in a-DyNi_2H_x, and (4) the decomposition of the remaining amorphous hydride into DyNi_5 and BiF_3-type DyH_3 were observed sequentially when the C15 Laves phase DyNi_2 was heated using a pressure differential scanning calorimeter (PDSC) at 2.0 MPa H_2 or higher hydrogen pressures. On the other hand, (1) the formation of c- and a-DyNi_2H_x, (2) partial HIA and (3) the decomposition of the mixture of a- and c-DyNi_2H, into DyNi_5 and DyH_3 (or DyH_2) were observed below 2.0 MPa H_2. Simultaneous formations of c- and a-DyNi_2H_x at the first exothermic peak were explained by the local temperature increment resulting from the low activation energy for hydrogen absorption forming c-DyNi_2H_x. On the other hand, the formation of DyNi_5 during the decomposition was explainable in consideration of the large formation enthalpy of DyNi_2. Furthermore, the mechanism of the phase transformations of DyNi_2 heated in a hydrogen atmosphere was discussed on the basis of the experimental results such as the pressure dependence of Tp and T_p/T_m (the peak temperature/the melting point of Di_2Ni, the activation energy and the enthalpy change AH.