Thermodynamic Study of Solid Co–Ga Alloys by E.M.F. Measurements Using the Solid Electrolyte

E.M.F. of a galvanic cell with the solid electrolyte (ZrO2+CaO) was measured to determine the activities of gallium in the temperature range of 800∼1000°C and the composition range of 0∼58.9 at% Ga for solid Co–Ga alloys, and thermodynamic functions were derived from the results. The cells used were as follows:Pt, Re, Ga, Ga2O3/ZrO2(+CaO)/Fe, FexO, PtPt, Re, Co–Ga alloy, Ga2O3/ZrO2(+CaO)/Fe, FexO, Pt.Activities of gallium showed large negative deviations from Raoult’s law in the whole composition range investigated. Activity changes with composition in cobalt-rich α solid solution were small, but very large in the β(CoGa) phase. Activities of cobalt showed very small positive deviations from Raoult’s law in the α solid solution, but very large negative ones in the β phase. Activities of gallium and free energies of formation showed larger values than those of Ni–Ga alloys. The solubilities of gallium in cobalt in the temperature range of 800∼1000°C were found to be larger than those shown in the phase diagram of Elliott. Standard free energy of formation of Ga2O3(s) was shown byΔGGa2O30=−260760+78.6T(°K)±400cal/molin the temperature range of 720∼1030°C.