Ceramic varistor fabricated by surface diffusion

A novel ceramic varistor was fabricated by the surface diffusion of grain boundary materials into the semiconductive ceramic substrate. (Ba_0.8Sr_0.2)(Ti_0.9Zr_0.1)O ₃ (BSTZ) doped with 0.2 mol.% Nb₂O₅ was adopted as the ceramic substrate. The powder blends of LiCO₂ and Bi₂O₃, with molar ratios of Li₂CO ₃/Bi₂O₃ (Li/Bi)=1/1 and 11/89 were adopted as the grain boundary material for surface diffusion. Better varistor characteristics were obtained by diffusing Li/Bi=11/89 into the BSTZ substrate. By surface diffusing Li/Bi=1/1 into 0.2 mol.% Nb₂ O₅-doped BSTZ substrate at 1080°C for 2 h, a varistor with a varistor constant of 6 and V_B=28 V was fabricated. While diffusing Li/Bi=11/89 at 1100°C into the same substrate, the varistor voltage was also 28 V with a higher varistor constant of 14. The average breakdown voltage per grain boundary of Li/Bi=11/89 diffused BSTZ was about 0.4 V/grain boundary, which is much smaller than that of ZnO ceramic varistor. A model similar to the n-C-i-C-n model is adopted to explain the results obtained