Phase Equilibria, Oxygen Potentials, and Activities in the System Ni-Co-Si-O at 1373 K

In the system Ni-Co-Si-O, the compositions of coexisting phases participating in two-phase equilibria between the monoxide $(Ni_yCo_{1-y}O)$ and orthosilicate $(Ni_zCo_{1-z}Si_{0.5}O_2)$ crystalline solutions, and alloy $(Ni_xCo_{1-x})$ and orthosilicate phase, have been determined at 1373 K. The samples were equilibrated at 1373 K under argon gas, and the equilibrium compositions of the phases present after quenching were measured by electron probe microanalysis (EPMA). A solid-state cell incorporating yttria-stabilized zirconia as the solid electrolyte was used to measure the oxygen chemical potential corresponding to the three-phase equilibrium between the alloy, orthosilicate solution, and quartz as a function of alloy composition. From these measurements, activity-composition relationships in the alloy and orthosilicate solid solutions have been derived using a new method. Activities in the silicate solution have also been obtained independently using the mixing properties of the rock-salt solid solution available in the literature and the tie-line information obtained in this study. Both methods yield almost identical results. All three solid solution series-alloy, monoxide, and orthosilicate-exhibit mild positive derivations from ideality. The excess Gibbs free energy of mixing of the orthosilicate solid solution $(Ni_zCo_{1-z}Si_{0.5}O_2)$ at 1373 K can be represented by the relation $\Delta GE=(2110 \pm 30)^.z(1-z) J^. mol^{-1}