Influence of grain size on the bulk and grain boundary ion conduction behavior in gadolinia-doped ceria

Fine-grained (∼ 0.2 µm) gadolinia-doped (12 mol.% Gd2O3) ceria pellets with density above 95% of theoretical density were produced by sintering the solution-combustion synthesized powder at 1200 °C. Some of these pellets were fired at 1400 and 1500 °C to introduce microstructural changes by grain growth. Impedance measurement of the samples was carried out in oxidizing atmosphere at different temperatures in the range of 225 to 1000 °C. The measured impedance data were analyzed through impedance plots and impedance spectra to understand the influence of grain size on the bulk and the grain boundary ion conduction behavior. In the low temperature measurements, a significant increase in the total conductivity is observed as the grain size becomes finer. As the grain size decreases the bulk conductivity is found to decrease and grain boundary conductivity is found to increase. Analysis of the impedance spectra indicates that the relaxation frequencies in the bulk and the grain boundary are influenced by grain size.