Microstructure and ionic conductivity of freeze-dried yttria-doped zirconia

Abstract

The relationship between the microstructure and ionic conductivity of 9mol% yttria-doped zirconia is deduced from a comparative study performed on polycrystalline samples prepared either from commercial powder (sample Z C) or from freeze-dried powder (sample Z F). The grain boundary ionic conductivity of the Z F samples increases with the sintering temperature and this effect is due both to an increase in grain size and to a decrease in the number of glassy triple points. Furthermore, the grain boundary conductivity of the Z F sample is 30 times higher 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 contrary, the microstructure of the Zp samples is cleaner and more homogeneous with larger lens-shaped glassy pockets at triple points and no evidence for continuous boundary films. In spite of the differences in the Z F and Z C microstructure 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 contact. A comparison with recent work on other Y 2O 3-compositions shows excellent agreement.