The effects of hydriding-dehydriding cycles on the plateau pressures and van't Hoff plots for PdNi alloys

Abstract

Pd and its alloys that form hydride phases exhibit hysteresis. It has been observed that if PdNi alloys have been hydrided and then dehydrided a number of times, “cycled”, at moderate temperatures, the hysteresis is reduced. After the Pd 0.91Ni 0.09 alloy had been cycled until its isotherms no longer changed, isotherms at other temperatures were measured. It was found for the cycled alloy that hysteresis is appreciable at 273 K and small at 343 K. The van't Hoff plots are affected by this changing hysteresis and consequently the derived thermodynamic parameters are also affected. If, instead of employing an alloy that had been cycled, virgin alloys, i.e. annealed ones, are employed for each temperature, the hysteresis is more constant and the resulting van't Hoff plots are more linear. The enthalpy has also been measured by reaction calorimetry for comparison with the values from the van't Hoff plots.