Structural characterization and mixed conductivity of TiO2-doped ceria stabilized tetragonal zirconia

By using X-ray diffraction lattice parameter measurements and Raman spectroscopy studies, the solid solubility limits of titania in ceria tetragonal zirconia solid solutions (12 mol% CeO2) have been established. Electrical properties of the mixed conductor TiO2–Ce–TZP containing 5 and 10 mol% TiO2 were measured at the 300 °C–900 °C temperature range. The total electrical conductivity in air of the TiO2–Ce–TZP ceramics decreases with titania content due to Ti-Vö associations, which depresses the mobility of the oxygen vacancies during the conduction process. The electrical behaviour of undoped-Ce–TZP solid solutions as a function of the pO2 and temperature showed that in oxidising conditions and up to an oxygen partial pressure of 10−9 Pa the materials were nearly pO2 independent. Under lower oxygen partial pressure the total electrical conductivity increases according to pO2–1/6 indicating that the n-type conduction in Ce–TZP is due to the electron hopping between Ce4+ and Ce3+ cations. In Ce–TZP containing 5 and 10 mol% titania the electrical behaviour was very different mainly under the most reducing conditions. Above 800 °C and with decreasing oxygen partial pressure below 10−9 Pa, the theoretical correlation for electronic conduction was near to n ∝ pO2−1/4, which indicates that the conduction process is controlled by the Ce′Zr and Ti′Zr defect concentrations, by hopping of the electrons between Ce4+ and Ce3+ and Ti4+ and Ti3+ via a small polaron hopping mechanism.