Effect of antimony oxide stoichiometry on the nonlinearity of zinc oxide varistor ceramics

The effect of antimony oxide at higher concentrations (> 2 mol%) and variable valence states of Sb on the nonlinearity of ZnO varistor ceramics has been investigated. Simplified compositions containing 92.5ZnO + 3Bi(2)O(3) + 2.5Co(3)+O-4 + 2Sb(2)O(5) (mol%) show nonlinearity coefficients (alpha) up to 65. Ceramic formulations derived from Sb2O5 bring about higher alpha than those with Sb2O3 or Sb2O4, provided the concentration of Sb2O5 is greater than or equal to 2 mol%. The secondary phase is predominantly antimony spinel, Zn7Sb2O12. Formation and involvement of a liquid phase during sintering is indicated from the microstructure studies. Energy-dispersive X-ray analysis shows that Sb is distributed more in the grain boundaries and within the secondary phase. Admittance spectroscopy, capacitance-voltage analyses, dielectric dispersion and the grain boundaries and within the secondary phase. Admittance spectroscopy, capacitance-voltage analyses, dielectric dispersion and electron paramagnetic resonance show that the observed defect stales and the type of traps in ZnO + Bi2O3 + Co3O4 ceramics remain unaltered whereas the trap density increased with the addition of antimony. The method of formulation of the ceramics by way of higher oxygen content of the additives is critical in attaining high nonlinearity. This can be explained on the basis of formation of the depletion layer at the presintering stage itself, because of the surface states arising out of the chemisorbed oxygen from the incipient liquid phase. The depletion layer is retained during sintering as a result of the high valence state of cobalt, as evidenced from the electron paramagnetic resonance spectroscopic results.