MICROSTRUCTURE AND IONIC-CONDUCTIVITY OF FREEZE-DRIED YTTRIA-DOPED ZIRCONIA

The relationship between the microstructure and ionic conductivity of 9 mol% yttria-doped zirconia is deduced from a comparative study per;f ormed on polycrystalline samples prepared either from commercial powde r (sample Z(C)) or from freeze-dried powder (sample Z(F)). The grain b oundary ionic conductivity of the Z(F) samples increases with the sint ering temperature and this effect is due both to an increase in grain size and to a decrease in the number of glassy triple points. Furtherm ore, the grain boundary conductivity of the Z(F) sample is 30 times hi gher than that of the Z(C) sample sintered in the same conditions and with the same grain size. From the microstructural characterizations, it is concluded that this effect is due to the poor microstructure of the Z(C) sample and in particular to the presence of a glassy film on a large number of grain boundaries. On the centrally, the microstructu re of the Z(F) samples is cleaner and more homogeneous with larger len s-shaped glassy pockets at triple points and no evidence for continuou s boundary films. In spite of the differences in the Z(F) and Z(C) mic rostructure the activation energy for the grain boundary conductivity is the same; this is consistent with a partially-wetted grain boundary model in which conductivity occurs across unwetted grains in direct c ontact. A comparison with recent work on other Y2O3-compositions shows excellent agreement. (C) 1998 Elsevier Science Limited All rights res erved.