Phase Identification and Electrical Properties in ZnO–Glass Varistors

Ceramic varistors based on ZnO with lead zinc borosilicate glass instead of Bi₂O₃ were prepared. The effect of sintering conditions on the electrical properties was studied by sintering samples at various temperatures and cooling them at different rates. The sample sintered at 1250°C for 1 h, then furnace cooled, possessed the best electrical properties, as characterized by the highest nonlinear coefficient, lowest leakage current, and lowest degradation. The microstructure and crystal structure of the glass phase of ZnO–glass varistors were examined by means of scanning electron microscopy, transmission electron microscopy, and powder X-ray diffractometry. The glass phase was originally amorphous, but crystallized as an intergranular layer in the sintered and furnace-cooled samples. This crystallized phase was a zinc borate phase (5ZnO·2B₂O₃), which was identified by X-ray diffractometry, transmission electron microscopy, and Auger electron spectroscopy. The zinc borate phase at the grain boundary of ZnO–glass samples enhanced the nonohmic characteristics of the ceramic varistors.