High temperature thermodynamics of dilute solutions of hydrogen in palladium–copper alloys

Abstract

The thermodynamic properties of dilute solutions of hydrogen and deuterium in palladium-copper alloys with copper contents of 10, 25, and 40 at.% have been determined by a calorimetric-equilibrium method at 555 K. The results are compared (a) with corresponding data for hydrogen in palladium–silver at the same temperature and (b) with equilibrium data for palladium–copper measured near room temperature. (1) The partial excess entropies of hydrogen in dilute solutions in palladium–copper alloys are reduced significantly compared to pure palladium. In palladium–copper this effect is only about one-half that in palladium-silver. Even so, it is believed to reflect in both systems a nonrandom distribution of the hydrogen atoms among the available interstitial sites. (2) It is shown that to a first approximation the dissolved hydrogen atoms may be considered to be isotropic, three-dimensional Einstein oscillators. In this approximation the observed differences between hydrogen and deuterium may be used to calculate the vibrational frequency of hydrogen. For palladium–copper these differences do not vary with copper content within experimental error. This points towards ?H being essentially independent of alloy composition. This behavior contrasts with that of hydrogen in palladium–silver, for which Picard et al. [J. Chem. Phys. 70, 2710 (1979)] recently found increasing values of ?H with increasing silver content. It is suggested that the different behavior of ?H in large measure may account for the observed difference between the concentration dependence of the excess entropy of hydrogen in palladium–copper and in palladium–silver.