Effect of annealing on phase structure and degradation of a zinc oxide varistor with Si-additive

The effect of annealing of phase structure and degradation of the current-voltage characteristics for a ZnO varistor with Si-additive has been investigated. By means of X-ray diffraction, analytical electron microscopy and thermal analysis, it is found that a phase transition from delta-Bi2O3 (with dissolved Si), to Bi24Si2O40 (bismuth silicate) took place in the ZnO varistor after annealing at 470 degrees C. This transition is associated with a volume contraction, no notable thermal effect, a high transition rate and reversibility. It is calculated by crystallography that the volume contraction of the phase transition is 6.02%. The transition of the Bi-rich phase results in decreased levels of degradation and improved stability of the ZnO varistor. (C) 1996 Elsevier Science Limited.