Effects of sintering additives on the mixed transport properties of ceria-based materials under reducing conditions

Commercial Ce0.8Gd0.2O2-delta nanopowders and alternative precursors synthesized by a freeze-drying method were used to obtain samples with and without Co addition as a sintering agent. Two percent Co-doped samples were sintered at 1150degreesC and 1500degreesC and undoped samples were sintered at 1500degreesC or 1600degreesC to obtain samples with relative densities in the range 92-94%. The total conductivity and the relative roles of bulk and grain boundary conductivity were studied by impedance spectroscopy. These results demonstrated that additions of Co play a very significant effect on the grain boundary behaviour, which is spoiled when the sintering temperature is excessive. Significant differences in grain boundary behaviour were also found between samples prepared from different precursor powders. The electronic conductivity of these materials was evaluated by an ion-blocking method and revealed that samples containing Co additions as a sintering additive possess somewhat lower n-type conductivity under identical conditions of temperature and oxygen partial pressure. The differences tend to vanish when these Co-doped samples are fired at high temperatures (1500degreesC).