The Study On Low-Temperature Preparation And Properties Of Zn-Bi Based Varistor Ceramics

ZnO-Bi₂O₃ based varistors have been widely used in electronic equipment and electric power system due to their excellent non-ohmic characteristics,which are the most popularly researched varistor system.Multilayer chip varistors have been gradually concerned because of its merits of miniaturization,high stability,low-voltage.In order to replace the pure Pd pole by pure silver,the sintering temperature of varistors must be decreased underlying its excellent varistor performance.In this work,effects of several dopants on varistor performance and defect conditions in grain boundary of ZnO-Bi₂O₃based varistors were investigated.The main contents are as follows:1.The effect of low temperature sintering on phase composition and electric al properties of the ZnO-Bi₂O₃-Sb₂O₃-Co₂O₃-MnO₂?ZBSCM?ceramics had been investigated.The samples sintered at 900°C showed good compachness(?>0.95?_{T D})and excellent nonlinear properties:?=84.47,E_{1m A}=458.33 V/mm,I_L<0.1?A.At the same time,the interface state density increased up to 17.53×10¹⁶ m^-2,which makes significant barrier height 5.01 eV for ZBSCM ceramics.2.The effects of Cr₂O₃ on the varistor performance and breakdown voltage of ZnO-Bi₂O₃-Sb₂O₃-Co₂O₃-MnO₂ ceramic were investigated.Results show that 0.5 mol%Cr₂O₃ will lead to a transformation of secondary phases from Cr Bi₁₈O₃₀ and Co₂Cr_0.5Sb_0.5O₄ to MnCr₂O₄,which effectively modifies nonlinear coefficient as reaching at the peak value of 80.71.At the same time,breakdown voltage decreased to 310.68V/mm,barrier height increased up to 3.88 eV,and the leakage current was lower than 0.1?A.3.SiO₂,as dopant,was chose to investigate the electrical performance of the quinary system ZnO-Bi₂O₃-TiO₂-Co₂O₃-MnO₂ as dopant.The results show that ceramics can be sintered at 880°C with excellent varistor performance.SiO₂ acts as a controller in ZnO grain growth,decreasing average grain size gradually from 3.67 to 1.92?m,which in turn results in an increase in breakdown voltage from 358.11 to 1080 V/mm.O n the other hand,SiO₂ has a significant influence on the defect structure and component distribution at grain boundary regions.When SiO₂ content incerases from 0 to 4 wt%,the value of the interface state density increases sharply.As the same time,the electrical properties are improved gradually,and reach an optimized value with the nonlinear coefficient up to 54.18,the barrier height up to 2.90 eV,and the leakage current down to 0.193?A/cm².In addition,the content of SiO₂ has a primary influence on resistance of grain boundary and a slight effect on that of grain according the impedance spectroscopy.4.The varistor performance of ZnO-Bi₂O₃-MnO₂ ternary system doped with different amounts of SiO₂ was investigated.The varistor performance of optimized system was significantly improved.Electrical properties were all effectively modified when doped with2 wt%SiO₂.As a result,the highest barrier height of 4.74 e V is obtained,which contributes to the highest nonlinear coefficient of 73.68.Moreover,all the samples show low leakage current of lower than 0.1?A.Comparatively,its varistor performance is much better than that of the quinary system ZnO-Bi₂O₃-TiO₂-Co₂O₃-MnO₂ underlying lower content of SiO₂.5.The effect of synthetic multi-phase Bi-Cr-O on nonlinear electrical properties of ZnO-Bi₂O₃-MnO₂-SiO₂ varistor have been investigated.Results show that Bi-Co-O ccan modify the transportation of the absorbed oxygen in grain boundary layers and then improve nonlinear electrical properties.Samples doped with 8 wt%shows the excellent electrical properties:the nonlinear coefficient increased up to 45.97 corresponding to high barrier height of 1.23eV and leakage current decreased to 0.5?A.