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 partic ipating in two-phase equilibria between the monoxide (NiyCo1-yO) and o rthosilicate (NizCo1-zSi0.5O2) crystalline solutions, and alloy (NixCo 1-x) and orthosilicate phase, have been determined at 1373 K, The samp les mere equilibrated at 1373 K under argon gas, and the equilibrium c ompositions of the phases present after quenching were measured by ele ctron probe microanalysis (EPMA), A solid-state cell incorporating ytt ria-stabilized zirconia as the solid electrolyte was used to measure t he oxygen chemical potential corresponding to the three-phase equilibr ium between the alloy, orthosilicate solution, and quartz as a functio n 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 ro ck-salt solid solution available in the literature and the tie-line in formation obtained in this study, Both methods yield almost identical results, All three solid solution series-alloy, monoxide, and orthosil icate-exhibit mild positive derivations from ideality, The excess Gibb s free energy of mixing of the orthosilicate solid solution (NizCo1-zS i0.5O2) at 1373 K can be represented by the relation.