SnO₂-based varistors capable of withstanding surge current

Abstract

SnO₂-based varistor material was fabricated by doping of CoO, Nb₂O₅, Cr₂O₃, and Y₂O₃ or Ho₂O₃. The microstructure, nonlinear I-V characteristic and surge current performances have been investigated. The average grain boundary voltage of the obtained materials is close to that of ZnO varistors. Similar to Cr³⁺, Y³⁺ or Ho³⁺ acts as the acceptor in the grain boundaries and the depletion layers. Moderate co-doping Y₂O₃ or Ho₂O₃ with Cr₂O₃ is helpful to improve the electrical performances and to prevent poor densification. The obtained optimal samples with a diameter of about 14 mm have a nonlinear coefficient (α) of about 60, threshold electric field (E_b) of about 380 V/mm, and the withstanding peak current of 8/20 μs wave is about 2400 A. Higher surge current will cause failure but no visual damage happen. This kind of failure mode is helpful for safety use of varistors.