High-field ZnO varistors: microstructures and properties

Zinc oxide varistors capable of supporting a high electric field before breakdown have been developed by use of sol-gel processing. Fine powders produced by the sol-gel technique can be fabricated into varistors by either hot pressing or pressureless sintering. Several model compositions were studied, including ZnO plus three additives and ZnO plus five additives. Sol-gel-derived materials were compared with conventional materials of the same nominal composition, and sintering was compared with hot pressing. Microstructural analysis and electrical measurements suggest that the benefits derived from sol-gel processing are related to the fine, uniform ZnO grain size and possibly to a more nearly optimum distribution of additives.