Fully-stabilized cubic zirconia is a good candidate for solid electrilyte because of high electrical conductivity. but the enhancement of sinterablity and mechamical properties are required in that it has microstructural characteristics.
Therefore this study was performed on two different ways. First, we have studied the effect of microstructural change including grain size, second phase and grain boundary segregation on the electrical properties of $ZrO_2$ - 9.3mol% $Y_2O_3$ added by $Bi_2O_3$ for enhancing sinteribility. Secondly, we investigated high temperature thermal aging characterisitics of $ZrO_2$ - 3mol% $Y_2O_3$ added by $Al_2O_3$ and $CeO_2$ for the improvement of low temperature degradation.
Relative densities near 95% and fine-grained microstructure could be achieved at sintering temperatures as low as 1350 to 1400℃ as reported elsewhere. Monoclinic second phases and $Y^{3+}$, $Bi^{3+}$ segregation at grain boundary were also observed. It has been reported that small addition of $Bi_2O_3$ decreased the electrical conductivity by about several times. We found that the electrical conductivity could be increased by the change of grain boundary structure, while nonconducting second phases were formed. 0.7~2.3 mol% $Bi_2O_3$ gave an increase of about 1.4 times higher in electrical conductivity than undoped one due to grain boundary cleaning effects. Grain boundary cleaning effect was achieved by the formation of highly conductive intergrannular contact region which originated fron nonwetting liquid. This facts were analyzed by the correlations between microstructural models, electrical equivalent circutis and corresponding impedance spectra.
We proposed that the critical grain size of tetragonal zirconia was about 0.5㎛ for prohibiting low temperature thermal degradation. This critical grain size is nearlu achieved by normal ceramic processing. Therefore we investigated the effedt of $Al_2O_3$ and $CeO_2$ addition on the low temperature thermal degradation. the addition of 2 mol% $CeO_2$ effectively improved low temperature thermal degradation by the supression of tetragonal to monoclinic transition. But the addition of $Al_2O_3$ was not so effective because grain size was increased for increased sintering temperature. High temperature thermal degradation mechanism of TZP was analyzed by thermal aging kinetics. The degradation mechanism of TZP for the initial stage of thermal annealing was inferred defect ordering process. $CeO_2$-containing TZP showed the best resistance to high temperature thermal degradation. It had the smallest coefficient of time dependence of intragrain resistivity although it had the largest coefficient of time dependence of grain boundary resistyvity. The ratio of grain boundary resistivity to intragrain resistivity was small in compared with pure TZP and $Al_2O_3$-containing TZP and therefore time-dependent change of grain boundary resistivity could be neglected.
