DEFECT THERMODYNAMICS AND ELECTRICAL-PROPERTIES OF NANOCRYSTALLINE OXIDES - PURE AND DOPED CEO2

Electrical characterization of fully-dense, nanocrystalline CeO2 (10 n m grain diameter) of undoped and Gd-doped compositions illustrates the influence of size scale on defect formation thermodynamics and transp ort 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. Preferent ial oxygen vacancy formation at grain boundary sites is indicated loni cally conducting n-Ce0.74Gd0.26O1.87 exhibits no conductivity enhancem ent, indicating that oxygen vacancy conductivity is not significantly increased along grain boundaries. Lightly-doped Ce0.9846Gd0.0154O2-D w hich 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 s how that size reduction to the nanometer scale provides a new way to c ontrol stoichiometry and electronic conductivity in semiconducting oxi des. (C) 1997 Acta Metallurgica Inc.