The role of silica in enhancing the nonlinearity coefficients by modifying the trap states of zinc oxide ceramic varistors

Zinc oxide ceramic varistors with simplified compositions ZnO + Bi2O3 + Co3O4 + Sb2O5 + SiO2 show nonlinear current-voltage characteristics with alpha in the range 45-130. Addition of SiO2 enhances the nonlinearity tremendously and the concentration range of SiO2 needed for high alpha(> 100) is around 3 mol%. Formation and involvement of a liquid phase during sintering is indicated from the microstructure studies. Energy dispersive x-ray analyses (EDAX) show that Si is distributed more in the grain boundaries and within the secondary phase than in the grain interiors. X-ray diffraction and infrared spectroscopy studies indicate the formation of zinc silicate, Zn2SiO4. The capacitance-voltage measurements show that the donor density (N-d), barrier height (Phi), interface density (N-s) and depletion width (W) are comparable to those of the ceramics without SiO2. Capacitance above the breakdown voltages shows negative values, indicative of oscillatory charge redistribution involving multivalent states of Co and the shallower interface states. Multiple trapping relaxations are evident from the complex-plane capacitance studies. The admittance spectroscopy data show that the type of trap state changes with the addition of SiO2, accompanied by a considerable increase in the density of traps.