Electrical properties of niobium-doped titanium dioxide. 2. Equilibration kinetics

The present work reports isothermal gas/solid equilibration kinetics for Nb-doped TiO₂ (0.65 atom %) at elevated temperatures (1073-1298 K) within narrow ranges of oxygen activity spanning between 10^-13 Pa and 75 kPa. The equilibration kinetics were monitored using electrical conductivity measurements. The kinetic data were used to determine the chemical diffusion coefficient (D_chem). D_chem as a function of p(O ₂) exhibits a complex dependence, which is considered in terms of defect disorder and the related concentrations of electronic charge carriers. The activation energy of D_chem in the p(O₂) range 10 Pa < p(O₂) < 75 kPa varies in the range 88.0-98.2 kJ/mol. It is important to note that the chemical diffusion coefficient in strongly reduced conditions [p(O₂) = 10^-9 Pa] exhibits a negative temperature dependence of D_chem (-67.2 kJ/mol). This finding indicates that under these conditions transport in a chemical potential gradient is consistent with metallic charge transport.