Oxygen permeability of mixed-conducting Ce0.8Tb0.2O2−δ membranes: Effects of ceramic microstructure and sintering temperature

Nanopowder of Ce0.8Tb0.2O2−δ, a fluorite-type mixed conductor stable in wide p(O2) range, was successfully synthesised by the hydrothermal method. Dense ceramics sintered at relatively low temperatures (900 and 1200 °C), by employing minor additions of cobalt oxide sintering additive (2 mol%), were compared to those sintered at 1500 °C without additives. The effect of the sintering temperature on oxygen transport was analysed in O2 and N2 atmospheres. Oxide-ion transference numbers were determined by the modified electromotive force (EMF) method under oxygen/air and nitrogen/air gradients, showing positive temperature dependencies for all sintering temperatures. The partial ionic and electronic conductivities, calculated from the transference numbers and total conductivity, both increase with cobalt additions. A detailed transmission electron microscopy (TEM) study shows a grain boundary location of the cobalt sintering additive at the lowest sintering temperature, 900 °C. On increasing sintering temperature the grain-boundary concentration of cobalt is depleted, leading to the presence of segregated grains of cobalt oxide. This factor is shown to be highly relevant with respect to measured oxygen permeation fluxes. Ce0.8Tb0.2O2−δ ceramics sintered at 900 °C show significantly higher oxygen permeation, related to improved surface exchange due to the grain-boundary enrichment of cobalt and larger grain-boundary area.