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Study on Low Temperature Preparation and Properties of Tio2,V2O5 Doping ZnO-Bi2O3-based Low Voltage Varistor Ceramics

Author: JuJinHua
Tutor: WangHua
School: Guilin University of Electronic Science and Technology
Keywords: varistor ceramics microstructures voltage gradient nonlinear coefficient leakage current density
CLC: TM283
Type: Master's thesis
Year: 2011
Downloads: 49
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Owing to highly nonlinear coefficient, fast response, low leakage current and lowresidual voltage, ZnO varistor ceramics are widely used in the field of overvoltageprotection systems from electricity to electronics. Preparing the low-voltage ZnO varistorceramics at low sintering temperature has attracted lots of attention both at home andabroad, which has prominent meaning in the future. In this research, Ti doped to reduce thevaristor voltage of the Zn-Bi-based varistor ceramics by solid reaction process, theoptimum technologic conditions were obtained and the properties were characterized. ThenV doped to reduce the sintering temperature by solid reaction process, the optimumtechnologic conditions were obtained and the properties were characterized. Finally, theZn-Bi-Ti-V-based varistor ceramics were prepared by solution coating method comparedwith solid reaction process.Researches on preparing Zn-Bi-based varistor ceramics by solid reaction processshowed that: TiO2 helped the growth of ZnO grain when TiO2 doping amounts was 0 to 3.0mol%, and ZnO grain size increased with TiO2 content increasing. While TiO2 dopingamounts was too high, TiO2 would react with ZnO to form Zn2TiO4 of spinel phase, whichwould distribute at the grain boundaries and inhibit the growth of ZnO grains. When theheating rate of 2 /min, holding at 1050 for 2 h, the varistor with 1.0 mol% TiO2exhibited the best properties, which had uniform grain size of 35μm, density of 5.44 g/cm3,relative density of 97 %, the lowest threshold voltage 21.6 V/mm, the highest nonlinearcoefficient 33 and the lowest leakage current density 0.020μA/mm2. Impedancespectroscopy analysis indicated that TiO2 doping had a stronger effect on the grainboundary of varistor ceramics and no effect on the grains. Impedance spectroscopy in thelow frequency region showed that the grain boundary impedance was the main contributorto ceramic resistance. At the same time, it indicated that the equivalent circuit of thesamples adopted R(RC)(RC) model.Based on Ti doping Zn-Bi-based varistor ceramics, Zn-Bi-Ti-based varistor dopedwith V were prepared by solid reaction process. When the sintering temperature was toolow, the size of ZnO grain was too small and uneven. The sintering temperature was toohigh, which made some of the grains grow abnormally and heterogeneously. Therefore, theoptimum sintering temperature was 980 . With V2O5 doping amounts of 0.005 0.20mol%, heating rate of 5 /min, holding at 980 for 4 h, the varistor with 0.02 mol%V2O5 exhibited the lowest threshold voltage was 27.2 V/mm. The varistor with 0.01 mol% V2O5 exhibited the best properties with the threshold voltage 31.1 V/mm, the leakagecurrent density 0.02μA/mm2 and the maximum nonlinear coefficient 25. The minimumdonor concentration Nd and the minimum density of interface states Ns were 0.810×1018cm-3 and 2.632×1012 cm-2. The maximum depletion layer width was 32.49 nm.Meanwhile the higher barrier height B is 0.908 eV.Finally, Zn-Bi-Ti-V-based varistor ceramics were prepared by solution coatingtechnique, with the voltage gradient 64.8 V/mm, the leakage current density 0.03μA/mm2,and the nonlinear coefficient 19.6, which were slightly worse then the solid reactionprocess. The main reason was that the average gain size of powers prepared by solutioncoating method was 2.54μm, and specific surface area was 3.74 m2/c.c, but the averagegain size of powers prepared by solid reaction process was 2.00μm, and specific surfacearea was 3.98 m2/c.c.. The smaller the particle size, the larger the surface area reactionsystem. The reaction interface and a corresponding increase in proliferation ofcross-section, so solid state reaction rate increases, resulting to the better properties thansolution coating method under the same sintering conditions.

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