Ionic conductivities, sintering temperatures and microstructures of bulk ceramic CeO2 doped with Y2O3

Ionic conductivities of CeO2:Y2O3 bulk ceramic were investigated with diffe rent sintering temperatures and correlated with the resulted microstructure . Lower sintering temperatures (T less than or equal to 1400 degreesC) were found to give much higher overall DC conductivities (e.g. sigma (DC) simil ar to7 X 10(-3) S/cm at 700 degreesC for a 4% Y2O3-doped sample sintered at 1400 degreesC). The samples sintered at lower temperatures showed higher g rain boundary conductivities than those sintered at the traditional sinteri ng temperature, 1500 degreesC, The model involving non-resistive grain boun daries can be employed to explain the lower grain boundary resistivities in our samples of low sintering temperatures. These samples were examined by scanning transmission electron microscopy (STEM) with energy dispersive X-r ay (EDX) and electron energy loss spectroscopy (EELS). Most of the boundari es (> 90%) were found precipitate-free in the small grain samples, and a hi gher Y/O ratio was observed at all these boundaries examined. The lower sin tering temperatures suppress grain growth giving rise to small grain size ( below 1 mum) The finer grain size provides large grain boundary areas for i mpurities to precipitate and solutes to segregate. Under such condition, th ere are insufficient impurities to form continuous precipitate layers at al l boundaries, such as ion transport blocking layers at boundaries are not f ully formed. At the same time, there are insufficient Y'(Ce) ions for all t he boundaries in the fine grain samples while the mobility of the Y'(Ce) io ns is low at low sintering temperatures to form well developed space charge d regions at these boundaries to abate transboundary ionic transport. These three combined effects have abated some of most resistive mechanisms for i onic transport across boundaries. (C) 2000 Elsevier Science B.V. All rights reserved.