Microstructure and electrical properties of ZnO-ZrO2-Bi2O3-M3O4 (M=Co, Mn) varistors

Phase evolution, microstructure and the electrical properties of ZrO2-added pyrochlore-free ZnO-Bi2O3-M3O4 (M=Co, Mn) varistors have been studied as functions of ZrO2 content up to 10 vol% and the sintering temperature between 900 and 1300 degreesC. Zirconia remained as intergranular second phase particles up to 1100 degreesC, which retarded densification and inhibited the grain growth of ZnO. At higher temperatures, on the contrary, ZrO2 particles began to be entrapped in ZnO grains and irreversibly transform froin monoclinic to stable cubic phase dissolving transition metal ions. The grain size of ZnO decreased with increasing ZrO2 content, and increased with the increase of the sintering temperature. Accordingly breakdown voltage changed with both ZrO2 content-and the sintering temperature as was expected. Nonlinear coefficient (a), depended primarily on,the sintering temperature: it increased to > 40 up to 1000 degreesC, and significantly decreased to <30 at higher temperatures, probably due to the volatilization of Bi2O3. While the specimens sintered at 1200 degreesC or above had relatively high leakage current (I-L) and large clamping ratio (C-R), those with ZrO2 content of 0.5-5.0 vol% and sintered below 1200 degreesC revealed low I-L of less than or equal to20 muA/cm(2) alnd C-R well below 2.0. In spite that varistor characteristics of Zr02-added system could not match t ose t those of commercial ZnO varistors its low temperature sinterability and ease of breakdown voltage control via ZrO2 content without a serious loss of its figures of merit are worth noticing, particularly for multi-layered chip varistor (MLV) application. (C) 2003 Elsevier Ltd. All rights reserved.