INFLUENCE OF ALKALI IONS IN ENHANCING THE NONLINEARITY OF ZNO-BI2O3-CO3O4 VARISTOR CERAMICS

Zinc oxide ceramic varistors with simplified compositions of ZnO+Bi2O3 +Co3O4+M(2)O (M=K or Na) show nonlinearity coefficients (alpha) of 40- 75. The electron paramagnetic resonance spectra and optical reflectanc e spectra show that there is a direct interdependence between the oxid ation state of transition metals and the alkali ions. The X-ray photoe lectron spectra indicate that the alkali ions preserve a higher oxidat ion state of cobalt, Co(III), in the grain boundary regions than in th e grain interiors having more Co(II). Admittance spectroscopy shows th at, while the nature of traps remains unaltered, the trap density incr eases with the concentration of alkali ions near the interface. The ob served defect states are associated with the grain bulk than with the grain boundary interfaces, as indicated by the isothermal capacitance transient signals.