A study was made of the effect of the microstructure and local chemistry of grain boundaries upon the ionic conductivity of 9mol%Y2O3-stabilized ZrO2 and Al2O3 composites. Tests were performed on samples which were prepared from 2 batches of Y2O3-stabilized ZrO2 powder that contained 1 or 1.6wt%SiO2. Conductivity measurements showed that the grain boundary conductivity increased with sintering temperature, and cooling rate after sintering, or when the amount of Si in the ceramic decreased. Additions of Al2O3 led to a decrease in the grain boundary conductivity of samples which contained 1wt%SiO2. The grain boundary conductivity went through a maximum in highly Si-contaminated materials. These results were combined with transmission electron microscopic X-ray microanalysis, which had revealed large gradients in the Al/Si concentration ratio in the grains, near to the second phase and in glassy precipitates. This suggested the existence of a competitive effect between insulating Al2O3 particles, and a strong interaction between Al2O3 and SiO2 which removed it from grain-boundary locations. Other analyses showed that Si and Y segregated near to the interfaces. The results suggested the occurrence of a kinetic de-mixing process, and permitted an explanation of the beneficial effect of a faster cooling rate following sintering.

Kinetic Demixing and Grain Boundary Conductivity of Yttria-Doped Zirconia - Experimental Observations. A.Rizea, C.Petot, G.Petot-Ervas, M.J.Graham, G.I.Sproule: Ionics, 2001, 7[1-2], 72-80