Improvement in electrical stability of ZnO varistors by infiltration of molten Bi2O3

The infiltration of molten Bi2O3 into ZnO varistors was carried out. The sintered varistor disks doped with Bi2O3 were painted by Bi2O3 paste on the surfaces and then were reheat-treated. The DC degradation test was carried out to demonstrate the electrical stability of varistors. The influence of the Bi2O3 infiltration on the electrical nonlinear characteristics and stability of varistors was analyzed by capacitance–voltage (C–V) measurements and impedance spectroscopy. The results showed that the Bi2O3 infiltration process at temperatures from 800 to 900 °C reduced the donor density in ZnO grains and inhibited the decrease of grain boundary resistance of varistors caused by degradation. The samples infiltrated with Bi2O3 at 850 and 900 °C exhibited an excellent electrical stability and maintained relatively high nonlinear characteristics. The enhancement in the electrical stability may be attributed to the diffusion of oxygen into the body of ceramics facilitated by the infiltration of molten Bi2O3.