Ceramic varistor fabricated by surface diffusion

摘要

A novel ceramic varistor was fabricated by the surface diffusion of grain boundary materials into the semiconductive ceramic substrate. (Ba/sub 0.8/Sr/sub 0.2/)(Ti/sub 0.9/Zr/sub 0.1/)O/sub 3/ (BSTZ) doped with 0.2 mol.% Nb/sub 2/O/sub 5/ was adopted as the ceramic substrate. The powder blends of LiCO/sub 2/ and Bi/sub 2/O/sub 3/, with molar ratios of Li/sub 2/CO/sub 3//Bi/sub 2/O/sub 3/ (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.% Nb/sub 2/O/sub 5/-doped BSTZ substrate at 1080 degrees C for 2 h, a varistor with a varistor constant of 6 and V/sub B/=28 V was fabricated. While diffusing Li/Bi=11/89 at 1100 degrees 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.