ZnO Varistor Ceramics Prepared With Tape-casting And The Reduction-reoxidation Method

ZnO varistors are widely used in voltage protection and instantaneous voltage surge absorbing owing to its properties of high nonlinear performance, quick response and large discharge current capability. the inner electrode of multilayer chips varistor is mainly Pt or Ag/Pb alloy of which price are very high, because the base metal can't be co-sintered with ZnO ceramic for the sintering temperature is higher than the melt point of base metal.In this paper, preparing ZnO varistors with the reduction-reoxidation method allows to avoid the oxidation of base metals, which means samples was sintered in reducing atmosphere at high temperature firstly and then oxidized in air at a lower temperatureFirstly, we study the feasibility of preparing ZnO varistors with the tape-casting and reduction-reoxidation method. The ceramics with varistor characteristics are successfully prepared by adding BN into ZnO-Bi2O3 material with the tape-casting and reduction-reoxidation method. Samples which are sintered at 950?in N2 and re-oxidized at750? in air shows a good characteristics where ? =23.1, JL=0.5 ? A/mm2. By comparing sintering characteristics, electrical characteristics and microstructure of samples sintered in different atmosphere,we explain the process and principle of the reduction and re-oxidation method by studying the change of the atomic defects of ceramic.On this basis, characteristics of ZnO ceramic are optimized by changing the sintering temperature at reducing atmosphere and re-oxidized temperature at air. We find that the varistor voltage and nonlinear characteristics decrease while the donor concentration and leakage current increase with the temperature increasing, by comparing the electrical characteristics, the J-E curve, the impedance analysis, donor concentration and density of interface state of samples sintered in N2 at different temperature and re-oxidized at 750 ?.The samples sintered at 900? and re-oxidized at 750? shows good properties while ?is 27.3 and JL is 0.01 ? A/mm2 By comparing the varsitor characteristics, electrical properties, backscattered SEM and EDAX energy spectrum results of samples sintered in N2 or N2-H2 at 950 ? and re-oxidized at different temperature, we find that the varistor characteristics shows great improvement when the re-oxidized temperature beyond the melting point of Bi2O3. After being re-oxidized at 800 ?,the samples sintered in N2 at950 ? shows a great viristor characteristics while the ? is 40 and the JL is 0.01 ?A/mm2,while the nonlinear characteristics of samples sintered in N2-H2 at 950 ? is 12 and JL is about 8?A/mm~2.