SINTERING AND MICROSTRUCTURAL DEVELOPMENT OF METAL-OXIDE VARISTOR CERAMICS

Anomalous retardation of the sintering kinetics of ZnO with addition of Bi4Ti3O12 (BIT) and a low voltage varistor composition containing ZnO, BIT, Co3O4 and Mn2O3 in the temperature interval between 780 and 850-degrees-C was investigated. On the basis of transmission electron microscopic analysis of diffusion couples the retardation of sintering kinetics was ascribed to surface diffusion of Bi and Ti along ZnO grain surfaces. Liquid phase formation resulting from chemical reactions between the components triggers exaggerated grain growth. Nucleation and growth of anomalous grains is influenced by the presence of inversion boundaries in ZnO grains.