The electrical properties and d.c. degradation characteristics of silver doped ZnO varistors

Abstract

ZnO-Bi₂O₃-based ceramic varistors containing up to 2500 p.p.m. Ag₂O were prepared by the mixed oxide route. The products were characterized in terms of current-voltage (I–V) behaviour, capacitance-voltage (C-V) behaviour and d.c. degradation behaviour. Additions of silver did not significantly affect ZnO grain growth or sintered density, but decreased the non-linearity exponent, a, and increased the breakdown voltage. Silver-doped specimens exhibited an unusual d.c. degradation behaviour, with the leakage current initially decreasing before reaching a stable value. For doping levels ≥ 500 p.p.m. Ag₂O, the change in breakdown voltage was positive for both forward and reverse directions. Silver doping appeared to increase the Schottky barrier height under conditions of continuous d.c. stress and improve the degradation behaviour. The optimum levels of Ag₂O addition were in the range 500–1000 p.p.m.