Thermodynamic properties and the degradation of ZrNiHx at elevated temperatures

The hydrogen absorption and desorption behavior of ZrNi was measured over the temperature range from 525 K to 700 K. The isotherms demonstrate that the (beta + gamma) two-phase region extends to these temperatures, in contrast to a recently published phase diagram. The hysteresis ratio of the absorption to desorption plateau pressures decreases with temperature and approaches unity just above 700 K. The pressure-composition data were reproducible, even at temperatures greater than 680 K, provided the isotherm measurements were completed within a few hours. If the hydrided ZrNiHx sample was held above 680 K for more extended periods? significant changes in the pressure and isotherms were observed, The extent of these changes is dependent on both the temperature and hydrogen content. Only minor changes were seen after 24 h at 700 K for x < 1.3, but the degradation as reflected by the isotherms was substantial above 680 K for x > 1.5. X-ray diffraction studies of several ZrNiHx samples revealed that the degradation in the isotherms can be attributed to the formation of ZrHx and Ni via disproportionation.