A series of ZnO-based varistors, containing 0.5wt%Bi2O3 and 0.5wt%Mn2O3, was prepared by using a conventional mixed-oxide route and sintered at 950 to 1300C. All of the samples exhibited varistor behavior. However, as the sintering temperature was increased from 950 to 1300C, the non-linearity coefficient decreased from 22 to 3. Deep-level transient spectroscopic studies of the varistors showed that the main active electron-trap migrated to shallower levels within the band-gap as the sintering temperature was increased. At the lowest sintering temperature, where the non-linearity coefficient attained its highest values, a second shallower trap was also activated.

The Effect of Sintering Temperature on the Development of Grain Boundary Traps in Zinc Oxide Based Varistors. C.Leach, K.Vernon-Parry: Journal of Materials Science, 2006, 41[12], 3815-9