Effects of spinel phase formation in the calcination process on the characteristics of ZnO–glass varistors

Ceramic varistors based on ZnO with lead zinc borosilicate glass were prepared in order to study the effects of various calcination processes on the formation of spinel phase during the sintering process, including the effects of different temperatures and soaking times. A ZnO–glass sample was prepared using powder calcined at 600°C for 10 h then sintered at 1250°C for 1 h; this sample possessed the highest non-linear coefficient, breakdown voltage and non-linear resistance as well as the lowest leakage current. The optimum amount of Zn7Sb2O12 spinel phase, formed in the calcination process, that can inhibit ZnO grain growth in the subsequent sintering plays an important role in the grain size distribution and stability of ZnO–glass ceramic varistors. Uniform distribution of the grain size obtained from suitable calcination processes was an important microstructural parameter in achieving a good device stability of ZnO–glass varistors. The dynamic resistance and the non-linear resistance of the ZnO–glass varistor, correlated with the average grain size, were proposed to describe insulating characteristics of the varistor samples. Increases in these two parameters, created by decreasing the grain size, enhances the sample clamping voltage during the surge impact and the sample breakdown voltage.