GRAIN-BOUNDARY RESISTIVITY IN ZIRCONIA-BASED MATERIALS - EFFECT OF SINTERING TEMPERATURES AND IMPURITIES

The role of grain boundary phases and their influence on grain boundary resistivity in ZrO2-based electrolyte systems have been investigated by impedance spectroscopy and microstructural analysis. The effect of sintering temperature on the grain boundary and intragrain resistivity of seven ZrO2-Y2O3 systems varying in impurity and Y2O3 contents has been described. For each system six to eight different sintering temperatures between 1200 and 1700 degrees C were used. Changes in the intragrain resistivity with increasing sintering temperature were small and related mainly to densification of the ceramic. The grain boundary resistivity exhibited a complex behaviour which was different for different ceramics and varied with the impurity content. The grain boundary resistivity versus sintering temperature plots showed inflection or peaks. This type of behaviour has been attributed to the dynamic nature of the grain boundary phase (composition, location and wetting properties all of which change with the sintering temperature and atmosphere, cooling rates from the sintering temperature and subsequent heat treatments) and the amount and type of impurities in the starting powders. The activation energy for oxygen-ion conduction across grain boundaries was independent of the impurity content or nature of the glassy phase. It has been shown that the oxygen-ion conduction across grain boundaries takes place through direct grain to grain contact rather than through the intermediate grain boundary glassy phase which has an extremely low oxygen-ion conductivity.