The characteristics of ZnO varistor depend strongly on the atmosphere, especially oxygen, during the fabrication process. The non-ohmic coefficient, alpha, decreases when the sample was heated in vacuum while it increased when heated in air. The behavior of oxygen which was important to improve 'varistor characteristic' was examined by the tracer (18O) method. The depth profile and distribution of the tracer were determined by means of SIMS. The diffusivity of oxygen in varistor was higher than that in pure ZnO. The high rate may be due to the presence of the secondary phase composed of a key compound such as Bi2O3. Therefore, the segregated phase of the key compound at a triple point played an important role to enhance the oxygen diffusion.

Enhanced Diffusion of Oxygen in ZnO Varistor. Fujitsu, S., Toyoda, H., Yanagida, H.: Journal of the Ceramic Society of Japan, 1988, 96[2], 119-23