Grain-Size Dependence of Electrolytic Properties in 25 at.% Yttrium Doped Ceria Solid Electrolytes

The grain size dependence of electrolytic properties in 25 at.% yttrium doped ceria was investigated. The sintered bodies were prepared from fine Y_0.25Ce_0.75O_1.875 powders synthesized by carbonate co-precipitation method. By using sintering temperature in range of 950deg-1400degC, the average grain size of electrolytes varied from 90 nm to 0.9 mum. The conductivity of samples with different grain size was determined by dc three-point measurements at 400degC-600degC. Then the activation energy was calculated from the data of electrical conductivity. Grain-size dependence was observed on the plots of conductivity vs. average grain size. As the grain size decreased from 0.9 mum to 0.3 mum, the conductivity decreased and the activation energy increased. This result was consistent with previous reports and could be explained by the space-charge-layer model. However, the conductivity increased while the grain size decreased from 0.3 mum to 90 nm. Correspondingly, the activation energy decreased. To clarify the mechanism of this phenomenon, the microstructure of samples was observed using TEM. Nano-sized domains inside the grains were observed on the high resolution images. Their size and amount were reduced as the grain size decreased. It is possible that the abnormal increase in conductivity at fine grain size (<0.3 mum) was partly contributed by the changes in nano-sized domains