P. Knauth et Hl. Tuller, Solute segregation, electrical properties and defect thermodynamics of nanocrystalline TiO2 and CeO2, SOL ST ION, 136, 2000, pp. 1215-1224
The thermodynamic and kinetic properties of nanocrystalline oxides, includi
ng nominally undoped TiO2 (anatase) and Pr-and Cu-doped CeO2, are reviewed.
The electrical properties of nominally undoped nanocrystalline TiO2 and Ce
O2 differ from conventional microcrystalline materials due to a greatly red
uced 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 st
rongly with a P(O-2)(-1/2) dependence. Nanocrystalline CeO2, on the other h
and, exhibits strongly enhanced oxygen nonstoichiometry and electronic cond
uctivity over the whole P(O-2) range. Reduced defect formation energies at
interface sites are proposed to be responsible for these properties. The ap
parent solubility of copper in nanocrystalline CeO2-Cu2O of about 10 mol% i
s much enhanced over that of coarse-grained ceria and is accommodated by se
gregation of copper to the grain boundaries. Nanocrystalline CeO2-PrOx, wit
h 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 order
ing. The chemical diffusivities (approximate to 10(-6) cm(2)/s) and the low
activation energy (approximate to0.3 eV) suggest short circuiting diffusio
n paths via interfaces. (C) 2000 Elsevier Science B.V. All rights reserved.