Phase equilibra in the systems V2O3-VO2 and VO2-V2O5

The phase equilibria in the systems V2O3-VO2 and VO2-V2O5 were established by varying the oxygen partial pressures from 10-3 to 10-12 atm in the former system at the temperatures from 1307 to 1700K and in the latter from 773 to 934K, respectively. The stability regions of the vanadium oxides with respect to the oxygen partial pressures were determined by means of quenching, thermogravimetric and EMF methods. The oxygen partial pressures were measured using a solid electrolyte cell composed of (ZrO2)sub(0.85)(CaO)sub(0.15). Eight phases, V2O3, V3O5, V4O7, V5O9, V6O11, V7O13, V8O15 and VO2 were found to be stable at 1307K. The phases, V7O13 and V8O15, and V7O13, V8O15 and V5O9, however, could not be observed at 1600 and 1700K, respectively. The four phases, VO2, V6O13, V3O7 and V2O5, existed in stable at the temperatures from 773 to 934K. The only phases of V2O3 and V6O11 had extensive homogeneous range due to nonstoichiometry. The pressure-composition isotherm may be explained in terms of metal vacancies. The tentative phase diagrams with isobaric lines of oxygen partial pressures in respective of the systems V2O3-VO2 and VO2-V2O5 were determined. The standard free energy, enthalpy and entropy data of the oxidation reactions between the intermediate oxide phases were evaluated on the basis of the equilibrium oxygen partial pressures. (auth.)