A study on co-firing between Zn0.9Mg0.1TiO3 and ZnO-based multilayer varistor

A semi-conducting ZnO-based multilayer varistor (MLV) is cofired with a passivation layer with Zn0.9Mg0.1TiO3 (ZMT) composition to prevent ZnO-based MLV from over-nickel coating during nickel plating. The cofiring results show that no de-lamination between ZMT and ZnO can be found, suggesting good co-firing compatibility between ZMT and ZnO though the anisotropic densification of ZMT is noted. However, the microstructure and electrical properties of ZnO based MLV is greatly influenced since ZMT is cofired with ZnO-based MLV. Reduction of grain size of ZnO-based MLV from 5.2 to 3.7 μm that is presumably attributed to constraining sintering of ZnO-based MLV by ZMT is observed after cofring ZMT. Simultaneously, the reduction of grain size of ZMT covered ZnO-based MLV results in a decrease of capacitance and in an increase of breakdown voltage. On the other hand, a decrease of non-linear coefficient and an increase of leakage current of ZMT covered ZnO-based MLV are observed as well. The results are associated with the change of slope of I–V curve for ZMT covered ZnO-based MLV due to the formation of a semi-conducting Zn2TiO4 phase, which is resulted from the diffusion of titanium ion into the matrix of ZnO-based MLV during co-firing.