Influence of impurities on the oxygen activity-dependent variation of the oxygen content of a commercial, CaO-doped ZrO2

A thermogravimetric study has been performed on the variation of the oxygen content of (partially stabilized) zirconia, doped with 10 mol% CaO and with Fe2O3 and TiO2 in varying concentrations, and of a commercial, CaO-doped zirconia, ZR23. The variation of the oxygen content was investigated as a function of the oxygen activity at 1300 °C and in the case of the commercial material also at 1200 and 1350 °C. The oxygen activity range considered was approximately between about 10−12 and 0.5. In all cases, it was observed that the oxygen content increased with increasing oxygen activity, in (Ca0.1Zr0.9)O1.9 more than in pure zirconia, in TiO2- or Fe2O3-doped (Ca0.1Zr0.9)O1.9 more than in pure (Ca0.1Zr0.9)O1.9 and in the commercial, CaO-doped zirconia similar as in (Ca0.1Zr0.9)O1.9 with concentrations of TiO2 and Fe2O3 very similar to those present in the commercial material. The results obtained indicate that the largest contribution to the variation of the oxygen content in the commercial, CaO-doped zirconia investigated is from the oxidation of Fe2+ to Fe3+ ions. A smaller, but significant contribution to the variation of the oxygen content is related to valence state changes of titanium ions.