Electrical and dielectric properties of Na-doped ZnO varistors

ZnO-based varistors have been extensively used for the protection of devices and equipment against voltage surges because of their exceptional nonlinearity in current-voltage characteristics and the ability to absorb large amounts of energy. Since the composition and fabrication process are complicated, impurities are inevitable in the varistors. Sodium impurities are frequently present in the raw oxides that constitute the complex formulation of ZnO-Bi
2
O
3
-based varistors. The main goal of present paper is investigating the influences of sodium impurities (<0.1 wt%) on the electrical and dielectric properties of ZnO-based varistors. The results show that with the increase of Na amount, the varistor breakdown field remains stable at first and then begins to decrease from ∼210V/mm to 200V/mm; the residual voltage ratio reaches to the lowest value when the Na amount is at 0.0005∼0.0020 wt%. The relaxation loss peaks of samples at low temperature decrease at first and then increase which are closely related to variation of residual voltage ratio. It turned out that sodium impurity will influence the intrinsic defects in ZnO-based varistors, and the performances of samples are improved with trace of sodium impurity.