EFFECT OF ANTIMONY OXIDE STOICHIOMETRY ON THE NONLINEARITY OF ZINC-OXIDE VARISTOR CERAMICS

The effect of antimony oxide at higher concentrations (> 2 mol%) and v ariable valence states of Sb on the nonlinearity of ZnO varistor ceram ics has been investigated. Simplified compositions containing 92.5ZnO + 3Bi(2)O(3) + 2.5Co(3)+O-4 + 2Sb(2)O(5) (mol%) show nonlinearity coef ficients (alpha) up to 65. Ceramic formulations derived from Sb2O5 bri ng about higher alpha than those with Sb2O3 or Sb2O4, provided the con centration of Sb2O5 is greater than or equal to 2 mol%. The secondary phase is predominantly antimony spinel, Zn7Sb2O12. Formation and invol vement of a liquid phase during sintering is indicated from the micros tructure studies. Energy-dispersive X-ray analysis shows that Sb is di stributed more in the grain boundaries and within the secondary phase. Admittance spectroscopy, capacitance-voltage analyses, dielectric dis persion and the grain boundaries and within the secondary phase. Admit tance spectroscopy, capacitance-voltage analyses, dielectric dispersio n and electron paramagnetic resonance show that the observed defect st ales and the type of traps in ZnO + Bi2O3 + Co3O4 ceramics remain unal tered whereas the trap density increased with the addition of antimony . The method of formulation of the ceramics by way of higher oxygen co ntent of the additives is critical in attaining high nonlinearity. Thi s 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 dep letion layer is retained during sintering as a result of the high vale nce state of cobalt, as evidenced from the electron paramagnetic reson ance spectroscopic results.