The thermodynamic and kinetic properties of nanocrystalline oxides, including nominally undoped TiO2 (anatase) and Pr- and Cu-doped CeO2, were studied. It was found that the electrical properties of nominally undoped nanocrystalline TiO2 and CeO2 differed from those of conventional microcrystalline materials, due to a greatly reduced specific grain-boundary impedance and enthalpy of reduction. In the case of TiO2, an unusual domain of ionic conductivity was observed at high O partial pressures while, at low O partial pressures, the electronic conductivity increased markedly with the reciprocal of the square root of the O partial pressure. Nanocrystalline CeO2 exhibited a markedly The thermodynamic and kinetic properties of nanocrystalline oxides, including nominally undoped TiO2 (anatase) and Pr- and Cu-doped CeO2, were studied. It was found that the electrical properties of nominally undoped nanocrystalline TiO2 and CeO2 differed from those of conventional microcrystalline materials, due to a greatly reduced specific grain-boundary impedance and enthalpy of reduction. In the case of TiO2, an unusual domain of ionic conductivity was observed at high O partial pressures while, at low O partial pressures, the electronic conductivity increased markedly with the reciprocal of the square root of the O partial pressure. Nanocrystalline CeO2 exhibited a markedly enhanced O non-stoichiometry and electronic conductivity over the entire range of O partial pressures. Reduced defect-formation energies at interface sites were proposed to be responsible for these properties. The apparent solubility of Cu in nanocrystalline

CeO2-10%Cu2O was greatly enhanced over that of coarse-grained ceria, and was accommodated by the segregation of Cu to the grain boundaries. Nanocrystalline CeO2, with up to 70mol%PrO2, was found to be single-phase. The O deficiency in this system attained values greater than 0.1; with evidence of vacancy ordering. Chemical diffusivities of the order of 10-6cm2/s, and an associated low (0.3eV) activation energy, suggested the occurrence of short-circuit diffusion via interfaces.

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