Defect thermodynamics and electrical properties of nanocrystalline oxides: Pure and doped CeO2

Electrical characterization of fully-dense, nanocrystalline CeO2 (10 nm grain diameter) of undoped and Gd-doped compositions illustrates the influence of size scale on defect formation thermodynamics and transport properties. In undoped n-CeO2, the heat of reduction is less than one-half the value for conventional polycrystals and single crystals, and the electronic conductivity is correspondingly enhanced. Preferential oxygen vacancy formation at grain boundary sites is indicated lonically conducting n-Ce0.74Gd0.26O1.87 exhibits no conductivity enhancement, indicating that oxygen vacancy conductivity is not significantly increased along grain boundaries. Lightly-doped Ce0.9846Gd0.0154O2-D which is normally an ionic conductor becomes electronically conducting at nanocrystalline grain size. In all compositions, reduction of grain size also results in a lower resistance per grain boundary, which is attributed to size-dependent grain boundary segregation. The results show that size reduction to the nanometer scale provides a new way to control stoichiometry and electronic conductivity in semiconducting oxides. (C) 1997 Acta Metallurgica Inc.