The conductivities of bulk ceramic samples were investigated, after sintering at various

temperatures, and were related to the resultant microstructure. It was found that lower sintering temperatures (below 1400C) gave much higher overall direct-current conductivities: such as 0.007S/cm at 700C for a 4%Y2O3-containing sample which had been sintered at 1400C. Samples which were sintered at lower temperatures exhibited higher grain-boundary conductivities than did those sintered at the usual sintering temperature of 1500C. A model which involved non-resistive grain boundaries could be used to explain the present low grain-boundary resistivities in samples sintered at low temperatures. The samples were examined by means of scanning transmission electron microscopy, plus energy-dispersive X-ray and electron energy-loss spectroscopy. Most (over 90%) of the boundaries were found to be precipitate-free in the case of small-grained samples. A higher Y/O ratio was observed at all of these boundaries. The lower sintering temperatures suppressed grain growth and gave rise to small (less than 1μm) grain sizes. The finer grain size provided large grain-boundary areas for the precipitation of impurities and the segregation of solutes. Under such conditions, there were insufficient impurities present to form continuous precipitate layers at all of the boundaries. Consequently, ion-transport blocking-layers could not be complete over the boundaries. At the same time, there were insufficient YCe’ ions for all of the boundaries in fine-grained samples. Meanwhile, the mobility of the YCe’ ions was too low at low sintering temperatures to form well-developed space-charge regions at these boundaries. The 3 effects combined to impair some of most resistive mechanisms for ionic transport across boundaries.

Ionic Conductivities, Sintering Temperatures and Microstructures of Bulk Ceramic CeO2 Doped with Y2O3. C.Tian, S.W.Chan: Solid State Ionics, 2000, 134[1-2], 89-102