Development of interfacial microstructure during cooling of a ZnO varistor material

Abstract

The development of interfacial microstructure during cooling of a ZnO varistor material from the sintering temperature has been studied by a combination of analytical electron microscopy and x-ray diffractometry. It was found that pyrochlore is the first Bi-rich phase to form from the liquid-phase sintering medium during cooling from the sintering temperature. When cooling at normal rates, pyrochlore forms between 940 and 740°C, and γ-Bi 2O3 forms at about 740°C. Quenching from higher temperatures results in the formation of δ-Bi2O3 from the Bi-rich intergranular liquid. The formation of barriers to electrical conduction at individual ZnO junctions was studied by use of microelectrodes. The barriers started to form while cooling between the temperatures of 1000 and 800°C. Quenching from higher temperatures did not result in varistor behavior, but barriers to electrical conduction could be created in such specimens by post-sintering heat treatments.