NEW EXPERIMENTS ON PRESSURE HYSTERESIS IN LANI5-HX

We report here a series of experiments in which parameters such as the measurement time, H aliquot size and degree of thermal isolation of the sample are systematically varied. The hysteresis of a well-cycled sample trends down to a steady value for cycle times longer than a few days, even when viewed on a logarithmic time scale, suggesting that the absorption plateau should the thought of as a sequence of local energy minima cor-responding to metastable equilibrium states. The same conclusion has been drawn from experiments in which very small pressure excursions are used to drive the metal hydride into the region between the absorption and desorption plateaux, where linear, reversible behaviour is found. We have also studied the absorption of large versus small aliquots of H, and are currently investigating the effects of the temperature rise during absorption. The reduction of hysteresis achieved by increasing the defect concentration appears to far outweigh any increase in hysteresis due to the energy lost in their creation.