Phase equilibria in the cobalt oxide-copper oxide system

Thermodynamic properties were determined for the system cobalt oxide-copper oxide by means of an electromotive force (EMF) measurement technique, using galvanic cells with calcia-stabilized zirconia (CSZ) as the solid electrolyte and with air as the reference electrode according to the following schemes: CuO, Cu2O \ CSZ \ air and CoO-CuO, Cu2O \ CSZ \ air for composition variables y = X-Cu/(X-Co + X-Cu) equal to 0.05, 0.15, 0.25, 0.35, 0.45, 0.667, and 0.8; and within the temperature interval 1200-1350 K. Thermodynamic properties calculated directly from EMF values were combined with the available literature data on phase equilibria, and thermodynamic properties of solid phases in the Co-Cu-O system were assessed. Both terminal solid solutions, (Co,Cu)O and (Cu,Co)O, were described by a sublattice model with Redlich-Kister excess term. The interaction parameters for both (Co,Cu)O and (Cu,Co)O solid solutions and the Gibbs energy of formation for the intermediate phase Cu2CoO3 were obtained. The Gibbs energies of fictive end-members: monoclinic CoO and CuO with rock salt structure were derived as well. The phase diagrams were calculated using the assessed thermodynamic parameters. The (T, y) phase diagram was calculated for existence under ambient air. The property diagrams log(10)P(O-2) versus composition and activity of CuO versus composition were calculated at 1273 K. The results of our calculations were in a good agreement with available experimental data.