The microstructures of three ZnO varistor materials with different Bi2O3contents have been evaluated by analytical electron microscopy in combination with xhyphen;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 Bihyphen;rich phases, which lies along the triple junctions of the ZnO grains, increases with increasing Bi2O3content, The conductivity of this network is strongly influenced by its internal microstructure. It was found that increased volume fractions of dgr;hyphen;Bi2O3and less interpenetration between agr;hyphen;Bi2O and dgr;hyphen;Bi2O3increases 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 Bihyphen;rich amorphous films. The current/voltage characteristics of heterojunctions between ZnO and intergranular Bi2O3were asymmetrical with respect to the polarity of the applied voltage. It was found that agr;hyphen;Bi2O3and dgr;hyphen;Bi2O3give rise to different breakdowns for electrons traveling from the Bi2O3into an adjacent ZnO grain.
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