Preparation And Doping Modification Of TiO 2 -based Varistor Ceramics

TiO₂-based varistor ceramics simultaneously have varistor and dielectric properties. A promising prospect is presented in low-voltage protection because they have low breakdown voltage, high dielectric constant and the preparation process is simple. But the nonlinear coefficient is low, which inhibits their application in industry. In order to increase the nonlinear coefficient and improve the varistor properties of TiO₂ varistor ceramics, the further studies are needed for researchers. In this paper, the donors, acceptors and sintering additives were doped to improve the varistor properties. The main contents and conclusions are as follows:1. The effects of Ta2O5 on the microstructure and properties of TiO₂-based varistor ceramics were investigated. The results showed that the secondary phases were detected in this system. The appropriate amount of Ta2O5 could effectively increase conductivity and help the formation of grain boundary barriers to improve the varistor properties. Optimized electrical properties were shown by a sample doped with 0.50 mol% Ta2O5, having a breakdown voltage of 14.90 V/mm, a nonlinear coefficient of 4.48, a dielectric constant of 9.68×104 and a dielectric loss of 0.36.2. The effects of MnO₂ on the TiO₂ varistor ceramics were studied. Analysis of microstructures showed that small amounts of MnO₂ could promote the growth of grain, but the influences of different MnO₂ contents on the grain size uniformity were different. The appropriate amounts of MnO₂ could increase the interface state density and the grain boundary barrier height which could improve the varistor properties. The microstructure of samples doped with 0.3 mol% MnO₂ was more uniform. The samples doped with 0.3 mol% MnO₂ showed optimal electrical properties: the varistor electric field E1 m A=4.95 V/mm, the nonlinear coefficient ?=5.07, dielectric constant ?r=11.09×104 and dielectric loss tan?=0.1618.3. The effects of Pr₆O₁₁ on the microstructure and electrical properties of TiO₂-based varistor ceramics were investigated. The results showed that the secondary phases could be detected in this system and the peak positions slightly shifted to the lower angle as shown in XRD patterns with the increase of Pr₆O₁₁ content. A appropraite amount of Pr₆O₁₁ could promote the growth of grain, which could lower the breakdown voltage and increase the dielectric constant. Because the ionic radius is bigger for Pr than that of Ti, Pr tended to segregation at the grain boundary to increase the grain boundary barrier height. Optimal electrical properties were obtained for TiO₂ varistor ceramics doped with 0.50 mol% Pr₆O₁₁: the varistor electric field E1 m A=5.13 V/mm, the nonlinear coefficient ?=5.43 and dielectric constant ?r=1.34×105.4. The properties of TiO₂-based varistor ceramics doped with different amounts of Ca CO3 were studied. The results showed that the amounts of Ca CO3 could effect the constituents and contents of the secondary phases. A appropriate amount of Ca CO3 could promote the growth of grains, which could result in a decrease in the breakdown voltage. However, the nonlinear coefficient was also reduced. TiO₂ varistor ceramics doped with 0.50 mol% of Ca CO3 possessed optimal electrical properties, which had a low varistor electric field of 2.07 V/mm, a high nonlinear coefficient of 3.35, a high dielectric constant of 2.6×105 and a low dielectric loss of 0.23.5. The effects of the sintering additives Bi₂O₃ on the microstructure and properties of TiO₂-based varistors were investigated. The addition of Bi₂O₃ could effectively reduce the sintering temperature of materials. The electrical results showed that TiO₂ varistor ceramics doped with 0.10 mol% Bi₂O₃ exhibited good properties with a low breakdown voltage of 8.70 V/mm, a high nonlinear coefficient of 4.93, a high relative dielectric constant 4.12×104 and a low tan? of 0.24 at 1 k Hz.