Phase relations of Zr(Fe_xMn_(1-x))_2-H_2 systems studied by XRD and isothermal-calorimetric measurement

摘要

Zr(Fe_xMn_(1-x))_2 alloys and their hydrides with x = 0, 0.2, 0.5, 0.7 and 0.8 were characterized by X-ray diffraction (XRD) under nitrogen and calorimetry under hydrogen where isotherms were measured at the same time. The hexagonal alloys (x = 0, 0.2 and 0.5) did not change crystal structure type on hydriding but significantly expanded. The alloys with x = 0.7 and 0.8, the XRD peaks from which were well indexed as cubic, did not expand on hydriding. Plots of differential heats on hydriding showed presence of phase transition through endothermic concaves all over the studied x values. Estimated transition heats monotonously decreased with the iron content, which might relate to decreasing volume expansion. Equilibrium pressures below 100 kPa at 298 K essentially increased with the iron content, but Zr(Fe_(0.2)Mn_(0.8))_2 showed lower equilibrium pressures than ZrMn_2 in a limited hydrogen concentration range. This was because the alpha -> beta transition of Zr(Fe_(0.2)Mn_(0.8))_2 occurred at a higher hydrogen concentration, which varied the order of the pressures.