Preparation And Investigation Of ZnO Varistor Ceramics For LTCC Technology

ZnO varistor ceramics were widely used in household appliances,communication,transportation,military industry,high-voltage electrical power transmission and so on because of the high nonlinear,good surge absorption,fast response speed and small leakage current.The preparation and research of ZnO varistor ceramic used in LTCC technology was the critical foundation to replace the traditional silver-palladium electrode with pure silver electrode in MLCC field.Therefore,in order to lower the sintering temperature of the ceramics to apply to MLCC field,the preparation and research of ZnO varistor ceramics were carried out from three ways:adjusting sintering system,doping sintering aids and pre-synthesizing Bi₂O₃ and Sb₂O₃ powder.?1?The sintering schedule of ceramics was adjusted to optimize the microstructure and improve the properties of ceramics.Samples were prepared though the multi-steps sintering with different turning point temperatures?750°C,800°C and 850°C?before sintering temperature.Then the samples were compared with the samples prepared by one-step sintering.The sintering temperature was 950°C.The results showed that the multi-steps sintering samples had higher breakdown voltage,more compact microstructure,the grain size distribution were more uniform.And multi-steps sintering increased the grain boundary barrier height and the depletion layer width.Samples obtained the best performance whose turning point temperatures was 800°C:the breakdown voltage was 883.0 V/mm,the nonlinear coefficient was 10.1,the grain size was 3.4?m,the density was 5.52 g/cm³.?2?The effect of SnO₂-doping and SnO₂-B₂O₃-Si O₂-doping was discussed on lowering the sintering temperature and improving performance of ZnO ceramics.It was found that the SnO₂-B₂O₃-Si O₂-doped samples showed the smallest average grain size and the microstructure uniformity was the best.The viscosity of bismuth-rich liquid phase and the second phase increased,and the density and uniformity of the sample were improved by SnO₂-B₂O₃-Si O₂-doping.The interface state density and the barrier height of samples sintered at 950°C were increased with the variety of sintering aids.SnO₂-B₂O₃-Si O₂-doped samples sintered at 910°C had better comprehensive property:breakdown voltage of 1040 V/mm,nonlinear coefficient of 6.6,average grain size of2.0?m,density of 5.37 g/cm³.?3?Bi₂O₃ and Sb₂O₃ were pre-synthesized at 700°C with a mixture of 0.5:1,0.7:1,1:1,1.3:1,1.6:1.Then the pre-synthesis powders replaced some of Bi₂O₃ and Sb₂O₃ to prepare ZnO varistor ceramics to study the effect of pre-synthesis Bi₂O₃ and Sb₂O₃ on improving the electrical properties and microstructure of ceramics.The result showed that the samples with Bi₂O₃ and Sb₂O₃ pretreated had higher density,smaller average grain size and more uniform microstructure.Because the volatilization of Bi₂O₃ and Sb₂O₃ were decreased with the powders pre-synthesis.The sample whose mass ratio of Bi₂O₃ and Sb₂O₃was 1.6:1 sintered at 910°C obtained better comprehensive performance:voltage of 1215.7 V/mm,leakage current less than 0.1?m,nonlinear coefficient of 12.2,average grain size of 2.3?m,density of 5.38 g/cm ³.