Grain size-dependent electrical conductivity of polycrystalline cerium oxide I. Experiments

The electrical conductivity of polycrystalline cerium oxide was investigate d in the nanometer and micrometer size range. Nanocrystalline samples of di fferent grain size were prepared by uniaxial hot-pressing of nanocrystallin e powder at various temperatures and pressures. Additional annealing at hig h temperatures was employed in order to obtain microcrystalline samples. An equivalent-circuit analysis of ac-impedance spectra based on the brick-lay er model was performed and the apparent bulk conductivity determined. The e ffect of a variation in temperature or oxygen partial pressure revealed the rather different nature of the electrical transport properties in the nano - and microcrystalline materials. Nanocrystalline cerium oxide exhibited el ectronic conductivity under conditions at which, microcrystalline samples s howed impurity-controlled ionic conductivity. The electronic conductivity o f nanocrystalline samples was larger than the intrinsic electronic conducti vity of pure single crystalline cerium oxide and was increasing with decrea sing grain size. The experimental results: were analyzed according to the d efect chemistry of cerium oxide and consequences of a space charge effect o n the partial electronic and ionic conductivity in polycrystalline cerium o xide will be discussed. (C) 2001 Elsevier Science B.V. All rights reserved.