Ten cation contaminants (Al, Ga, Co, Fe, Mg, Zn, Zr, Ca, Sr, Ba) were investigated with regard to their effects upon the electrical properties, microstructures, and discoloration of conductive TiO2 ceramics. It was found that Al, Ga, Co, Fe and Mg caused discoloration and increased the electrical resistivity by a factor of 104 to 106 in Nb‐doped TiO2 ceramics. The other dopants did not introduce such changes in TiO2. The electrical properties, microstructures, and discoloration were measured in specimens of AlxNb0.007Ti0.993−xO2 with 0 ≤ x ≤ 0.01. When the Al content exceeded a critical value, ranging from 0.48% at 1400C to 0.25% at 1200C, the electrical resistivities and grain size increased rapidly, and the specimen was discolored from the original black to an ivory white color. Color boundary migration induced by Al diffusion in Nb‐doped TiO2 was quantitatively measured. From the kinetics of the boundary migration, the Al diffusivity was calculated to be given  by:

D(cm2/s) = 2.67 x 100exp[-53.3(kcal/mol)/RT]

at 1200 to 1400C. The rapid diffusion of the small cations, namely Al, Ga, Co, Fe, and Mg, results from an interstitial diffusion mechanism. However, other cations, having a radius larger than the interstitial channel (∼0.77Å radius), cannot diffuse by this mechanism. Defect reactions were proposed to explain the increase in the electrical resistivity and microstructural changes due to Al diffusion. These defect reactions also showed that the problem of acceptor contamination cannot be avoided by adding an excess quantity of donor dopant if the solubility of the donor was much less than that of the acceptor contaminant.

Effects of Cation Contaminants in Conductive TiO2 Ceramics. Yan, M.F., Rhodes, W.W.: Journal of Applied Physics, 1982, 53[12], 8809-18