Microstructure and Crystal Phases of Praseodymium Oxides in Zinc Oxide Varistor Ceramics

The effect of sintering temperature on the microstructure and crystal phases of the intergranular praseodymium oxides in ZnO varistor ceramics was investigated using transmission electron microscopy and high-resolution electron microscopy. The ZnO grains were three-dimensionally separated from the intergranular praseodymium oxides. On the basis of microdiffraction analyses of the intergranular layer, the phase transformation from fcc-Pr₆O₁₁ into hcp-Pr₂O₃ was found when the sintering temperature increased from 1300° to 1350°C. The defect reaction equation and the decrease of donor concentration with increasing sintering temperature can verify the certainty of phase transition during the liquid-phase sintering observed by transmission elecron microscopy. Additionally, on the basis of the small variations of the breakdown voltage per grain boundary, the number of active grain boundaries is not a dominant factor for the donor concentration dependence on the sintering temperature.