Sintering and varistor characteristics of ZnO-Bi2O3 ceramics

Abstract

The sintering behavior of binary ZnO ceramics (containing 0.01–1 mol% of added Bi₂O₃) showed that densification in the initial stage of sintering and grain growth of the ZnO matrix are enhanced in the presence of a liquid Bi₂O₃ phase. Exaggerated grain growth of ZnO was observed above 1100 °C and in mixtures containing as low as 0.05 mol % Bi₂O₃. Volatilization loss of Bi₂O₃ during sintering affected the microstructure of the sintered ceramics and resulted in the loss of varistor property and return to Ohmic behavior and high conductivity (∼1 Ω⁻¹ cm⁻¹). A simple microstructural model which predicts that the minimum content of Bi₂O₃ required to maintain varistor behavior is proportional to the thickness of intergranular layer and inversely proportional to the average grain size of the ZnO.