Effect of Liquid Phase and Vaporization on the Formation of Microstructure of Pr Doped ZnO Varistor

Our previous works and our recent data were summarized to discuss the effect of liquid phase formation and vaporization of the components on the densification, grain growth and change of the microstructure of Pr doped ZnO ceramics in air. In the ZnO-Pr₂O₃ binary system, eutectic liquid forms at 1382± 5°C° and significant vaporization of the components occurred above the eutectic temperature. Below eutectic temperature (1350°C), only 0.1 mol % of Pr doping into ZnO brought about the grain growth of ZnO, however further addition of Pr brought about the suppression of the grain growth. Comparing the grain size distribution at the surface and inside part of Pr doped ZnO ceramics, it was clarified that wider grain size distribution was observed at the surface than inside part. At the temperature just below the eutectic (1370°C), abnormal grain growth was also observed. Depth profile of Pr content indicated that no concentration gradient was observed below eutectic temperature (1350°C), however above eutectic temperature (1500°C), condensation of Pr was observed at the surface. Grain growth rate as well as weight loss rate were drastically increased between 1350 and 1370°C, which suggested that formation of liquid phase accelerate the grain growth rate and weight loss rate. Consequently the microstructure of Pr doped ZnO ceramics was formed by both effects on the formation of liquid phase and on the vaporization of the components.