Mechanical properties of palladium and palladium hydride

Abstract

Interest in the mechanical properties of the Pd-H system stems from its role as a model metal hydride as well as from its potential practical utility. The attractiveness of palladium hydride can be attributed, in part, to the high hydrogen mobility in Pd and the relative lack of absorption barriers such as stable surface oxides. However, despite extensive attention to the thermodynamic and kinetic behavior of the system, little has been reported on the mechanical properties of the hydride. The lack of mechanical property data for the hydride can be understood given the thermodynamics of the system. PdH is a weakly exothermic hydride and consequently, it requires a hydrogen overpressure to maintain the stability of the hydride phase. The importance of mechanical property measurements is made clear by considering how Pd responds to the solid solution ({alpha}) {yields} hydride ({beta}) phase transformation at low temperatures. This paper shows that near room temperature, the Pd-H system has a miscibility gap within which both phases co-exist. Thus the transformation can occur across a constant pressure plateau and the two-phase field can exist over a large range of hydrogen concentration from 0.03 to {approx}0.60. Both the {alpha}- and {beta}-phase are f.c.c. structures. However,more » the formation of the {beta}-phase results in {approx}10% volume expansion. This volume expansion causes the severe deformation of the host metal, which can be manifested in the change in shape of Pd foils after hydriding. Such observations promote the need to quantify the mechanical behavior of the system in order to understand the role of deformation in the formation of the hydride.« less