Water-induced degradation in zinc oxide-based ceramic varistors

Abstract

Water-induced degradation in ZnO-based ceramic varistors is reported. A comparison experiment was conducted in which some varistors were immersed in a 0.05M NaOH solution for some time while other varistors were placed in a 0.05M NaOH solution to evolve hydrogen on their silver electrode by electrolysis of water. No degradation occurred to those samples that were immersed in the NaOH solution; the leakage currents increased by orders of magnitude and the switch voltage remained unchanged in the samples treated by electrolysis of water. These results indicate that the degradation resulted from the reduction reaction of hydrogen atoms generated by electrolysis of water rather than the permeation of water into the ceramics. Because water can be formed on varistors by condensation of aqueous vapor in air and varistors are usually operated under electric fields, it is proposed that the reduction reaction of hydrogen atoms generated by electrolysis of water may be an important origin for the degradation in ZnO varistors.