Grain Resistivity and Conduction in Metal Oxide Varistors

Abstract

Metal oxide varistors are ceramic devices with highly nonlinear current-voltage characteristics similar to back-to-back Zener diodes.1−4 Typical I-V characteristics for a metal oxide varistor at 77 K and for a small range of temperatures near 300 K are shown in Fig. 1. They are produced by sintering ZnO powder together with small amounts (1 to 10 mole %) of other oxide additives. The resultant structure, which can be idealized by the “block model” shown in Fig. 2, is comprised of semiconducting n-type ZnO grains of dimension 10 to 20 μm surrounded by insulating barriers at the ZnO grain boundaries. These varistors have proved useful in a variety of applications particularly as high-quality voltage surge suppressors.5,6