Hydrothermal synthesis and low temperature conduction properties of substituted ceria ceramics

Rare earth or alkaline earth oxide-substituted ceria (CeO₂) powders were successfully prepared via hydrothermal synthetic route. Using these powders after freeze-drying, highly dense ceramics of Ce_1-xM_xO₂ with M = Ca²⁺, Sr²⁺, Sm³⁺, Gd³⁺ and Y³⁺ and x = 0.2 were prepared by sintering at much lower temperatures than 1700 °C, common for CeO₂ preparation by conventional ceramic techniques. For example, 20 mol% samaria-substituted ceria with 98% of the theoretical density was sintered at 1450 °C. The conductivities determined by an ac impedance method were approximately 100 times higher than that of yttria-stabilized zirconia (YSZ). The conductivity was optimal with Sm³⁺ substitution (σ_{(210 °C}) ~10^-6 S/cm). The analysis of the impedance spectra indicate that the observed impedance at the relatively low temperatures of measurement (300-600 °C) is predominantly dependent on grain boundary effects.