Abstract
The microstructures of three ZnO varistor materials with different Bi2O3 contents have been evaluated by analytical electron microscopy in combination with x-ray diffractometry. The results have been correlated to microelectrode measurements, where breakdown voltages of individual ZnO junctions were measured, and also to current/voltage characteristics of bulk specimens. The volume fraction of the continuous intergranular network of Bi-rich phases, which lies along the triple junctions of the ZnO grains, increases with increasing Bi2O3 content, The conductivity of this network is strongly influenced by its internal microstructure. It was found that increased volume fractions of δ-Bi2O3 and less interpenetration between α-Bi2O and δ-Bi2O3 increases the conductivity of the network. Individual ZnO/ZnO grain boundaries exhibited breakdowns at 3.2 and 3.6 V, depending upon whether they contained segregated Bi atoms or thin Bi-rich amorphous films. The current/voltage characteristics of heterojunctions between ZnO and intergranular Bi2O3 were asymmetrical with respect to the polarity of the applied voltage. It was found that α-Bi2O3 and δ-Bi2O3 give rise to different breakdowns for electrons traveling from the Bi2O3 into an adjacent ZnO grain.
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