Hysteresis in the palladium + hydrogen system

Abstract

Hysteresis in the relationship between the uptake of hydrogen by palladium sponge and the equilibrium hydrogen pressure has been studied at 120°C. The boundary curves of the hysteresis loop have been obtained together with several absorption and desorption scanning curves. The main loop is analyzed in terms of Lacher’s (1937 a ) statistical mechanical theory of hydrogen in palladium : the α - and β -phases are characterized by different parameters in his equation. This implies that the energies of absorption of a hydrogen atom, and the interactions between absorbed protons are different in the two phases. No single equation of the form deduced by Lacher exists which represents the system throughout the whole range of hydrogen concentrations. The generally accepted interpretation of earlier absorption data seems to be incorrect, and doubt is thrown upon the possibility of the existence of a reversible transition path between the two phases. By combining our data with a knowledge of the critical point of the α-β -phase change, the points of onset of the phase transitions are calculated as a function of temperature and compared with the values obtained by other workers. The co-existence of both α - and β -phases over a range of pressures is probably associated with the large volume change accompanying the transition and a tentative theory is put forward which accounts for the ranges of co-existence of the two phases both in absorption and desorption. The shapes of the scanning curves are shown to be generally consistent with the predictions of the domain theory of hysteresis.