Solute segregation, electrical properties and defect thermodynamics of nanocrystalline TiO2 and CeO2

摘要

The thermodynamic and kinetic properties of nanocrystalline oxides, including nominally undoped TiO2 (anatase) and Pr-and Cu-doped CeO2, are reviewed. The electrical properties of nominally undoped nanocrystalline TiO2 and CeO2 differ 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 is observed at high oxygen partial pressures, whereas at low P(O-2), the electronic conductivity increases strongly with a P(O-2)(-1/2) dependence. Nanocrystalline CeO2, on the other hand, exhibits strongly enhanced oxygen nonstoichiometry and electronic conductivity over the whole P(O-2) range. Reduced defect formation energies at interface sites are proposed to be responsible for these properties. The apparent solubility of copper in nanocrystalline CeO2-Cu2O of about 10 mol% is much enhanced over that of coarse-grained ceria and is accommodated by segregation of copper to the grain boundaries. Nanocrystalline CeO2-PrOx, with up to 70 mol% PrOx, is found to be single phase. The oxygen deficiency in this system attains large values (x > 0.1) with evidence for vacancy ordering. The chemical diffusivities (approximate to 10(-6) cm(2)/s) and the low activation energy (approximate to0.3 eV) suggest short circuiting diffusion paths via interfaces. (C) 2000 Elsevier Science B.V. All rights reserved. [References: 28]