The thermodynamic properties of cobalt-palladium solid solutions

Abstract

The thermodynamic activities of cobalt in palladium, relative to pure f.c.c. ferromagnetic cobalt, have been determined from e.m.f. measurements on solid-electrolyte oxygen concentration cells over the temperature range 800–1100°C. The activity of cobalt in palladium exhibits positive departures from ideal solution behavior in cobalt-rich alloys and negative departures in palladium-rich alloys. The activities of palladium, deduced from the experimental data, indicate negative departures from ideality at all compositions. Cobalt-palladium alloys were of particular interest because their unusually high Curie temperatures permitted compositions up to 40 at. % palladium to be studied in the ferromagnetic state. The experimental data were of sufficient precision that the ferromagnetic-paramagnetic transition could be detected in alloys containing nominally 20 and 30 at. % palladium. The relative integral molar excess entropies, determined from the changes in the partial molar free energies of mixing as a function of temperature, are positive for all the compositions studied. The heats of mixing are endothermic for cobalt-rich compositions and exothermic for palladium-rich compositions. An extrapolation of the experimental values of the partial molar free energies of cobalt to lower temperatures, suggests the presence of a miscibility gap below 600°C at high cobalt concentrations.