Influence of microstructure on oxide ionic conductivity in doped CeO2 electrolytes

Doped ceria (CeO2) compounds are fluorite type oxides, which show oxide ionic conductivity higher than yttria stabilized zirconia, in oxidizing atmospheres. As a consequence of this, considerable interest has been shown in application of these materials for `low (500 degrees-650 degrees C)' temperature operation of solid oxide fuel cells (SOFCs). In this study, some rare earth (eg. Gd, Sm, and Dy) doped CeO2 nano-powders were synthesized via a carbonate co-precipitation method. Fluorite-type solid solution were able to be formed at low temperature, such as 400 degrees C and dense sintered bodies were subsequently fabricated in the temperature ranging from 1000 degrees to 1450 degrees C by conventional sintering (CS) method. To develop high quality solid electrolytes, the microstructure at the atomic level of these doped CeO2 solid electrolytes were examined using transmission electron microscopy (TEM). The specimens obtained by CS had continuous and large micro-domains with a distorted pyrochlore structure or related structure, within each grain. We conclude that the conducting properties in these doped CeO2 systems are strongly influenced by the micro-domain size in the grain. To minimize the micro-domain size, spark plasma sintering (SPS) was examined. SPS has not been used to fabricate dense sintered bodies of doped CeO2 electrolytes, previously; carbon from the graphite dies penetrates the specimens and inhibits densification. To overcome this challenge, and to be able to produce dense sintered bodies of doped CeO2 of a grain size that minimizes the microdomain growth, a combination of SPS and CS methods were examined. Using this combined method we report that we were able to produce fully dense specimens with improved conductivity. This is correlated with a reduction in the size of the micro-domains. Consequently we conclude that the control of micro-domain size within the grain structure is a key component in the successful design of electrolyte materials with improved conductivity.