The thermodynamic and kinetic properties of nanocrystalline oxides, including nominally undoped TiO2 (anatase), were reviewed. The electrical properties of nominally undoped nanocrystalline TiO2 differed from conventional microcrystalline materials due to a greatly reduced specific grain boundary impedance and enthalpy of reduction. In TiO2, an uncommon domain of ionic conductivity was observed at high oxygen partial pressures whereas, at low P(O2), the electronic conductivity increased strongly with a P(O2)-1/2 dependence. Nanocrystalline CeO2, on the other hand, exhibited strongly enhanced oxygen non-stoichiometry and electronic conductivity over the whole P(O2) range. Reduced defect formation energies at interface sites were suggested to be responsible for these properties. The apparent solubility of copper in nanocrystalline CeO2-Cu2O of about 10mol% was much enhanced over that of coarse-grained ceria and was accommodated by segregation of copper to the grain boundaries. Nanocrystalline CeO2-PrOx, with up to 70mol%PrOx, was found to be single-phase. The oxygen deficiency in this system attained large values (x > 0.1) with evidence for vacancy ordering. The chemical diffusivities (≈10-6cm2/s) and the low activation energy (≈0.3eV) suggested short circuiting diffusion paths via interfaces.

Solute Segregation, Electrical Properties and Defect Thermodynamics of Nanocrystalline TiO2 and CeO2. Knauth, P., Tuller, H.L.: Solid State Ionics, 2000, 136-137, 1215-24

Figure 5

Ionic Conductivity of TiO2