Effect of Sintering Temperature on the Microstructure and Electrical Properties of Zno Varistors

Sintering schedule has important effect on the microstructure and electrical properties of ZnO varistors, but the study of sintering temperature is deficient. In this paper, the effect of sintering temperature on the microstructural evolution and electrical properties of ZnO-Bi 2 O 3 based varistors was studied detailedly. Sintered at 900 °C, polyhedral Bi-rich phase Bi 3.73 Sb 0.27 O 6.0+ x distributes in the ZnO grain boundaries leading to the double Schottky barrier cannot be formed effectively. With the sintering temperature increases from 900 to 1000 °C, the liquefaction of Bi-rich phase occurs leading to the beneficiation of Mn and Cr in the Bi-rich phase Bi 3.73 Sb 0.27 O 6.0+ x , and lamellar Bi-rich phase is formed in the ZnO grain boundaries during cooling. Excess O and enriched Mn 4+ in Bi-rich phase Bi 3.73 Sb 0.27 O 6.0+ x serving as oxidizing agent will accept e - , which improves the double Schottky barrier. The thickness of the lamellar Bi-rich phase increases with increasing sintering temperature, but the uniformity gets worse, leading to the deterioration of the microstructure and electrical properties at the sintering temperature large than 1000 °C . Ion diffusion is stronger for the samples sintered at higher temperature leading to more obvious polarization and loss peak. Finally, the ZnO varistors sintered at 1000 °C for 2 h with a nonlinear coefficient of 43.4±2.3, a voltage gradient of 451±4 V/mm, and a leakage current density of 4.5±0.4 μA/cm 2 were obtained, which show an optimum microstructural uniformity and reproducibility of the sample preparation. The intensive study about sintering temperature has important significance for the preparation of ZnO varistors with excellent microstructural uniformity and electrical properties