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 curr ent-voltage characteristics for a ZnO varistor with Si-additive has be en investigated. By means of X-ray diffraction, analytical electron mi croscopy and thermal analysis, it is found that a phase transition fro m delta-Bi2O3 (with dissolved Si), to Bi24Si2O40 (bismuth silicate) to ok place in the ZnO varistor after annealing at 470 degrees C. This tr ansition is associated with a volume contraction, no notable thermal e ffect, a high transition rate and reversibility. It is calculated by c rystallography 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 E lsevier Science Limited.