Effect of microstructures on the ionic conductivity of ceria-calcia oxides

OXYGEN ion conductors are used in oxygen sensors, electrochemical pumps of oxygen, and high-temperature fuel cells. Materials with a high ionic conductivity at low temperatures are desired especially for fuel-cell electrolytes in order to derive great electrical power. Although zirconia-based oxides have been most extensively investigated, for fuel-cell applications their conductivities are low. Cation-doped ceria possesses higher conductivity than the calcia- or yttria-doped zirconia; however, its electrochemical properties were not sufficiently investigated in a wide composition range with respect to the crystal structures or microstructures. Previously, the authors reported the ionic conductivity and ionic transference number of the system CeO/sub 2/-CaO. High ionic conductivity was maintained even when the calcia was added after the solubility limit was reached. This behavior is quite desirable since precise control of the composition is not necessary to obtain the ionic oxygen conductor. To clarify the behavior of ionic conduction in such an oxide mixture, the crystal structure and the microstructure of the system CeO/sub 2/-CaO were investigated in relation to the conductivity of the oxide