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Design and Study on Properties of High Voltage Gradient ZnO Varistor

Author: QiuPei
Tutor: LuZhenYa
School: South China University of Technology
Course: Microelectronics and Solid State Electronics
Keywords: ZnO varistor sol-gel barrier height activation energy orthogonal experiment
CLC: TM54
Type: Master's thesis
Year: 2011
Downloads: 96
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Abstract


ZnO varistors with highly nonlinear U-I characteristic, excellent surge withstanding capability are widely used to protect electrical circuits in the fields of electrical and electronics. To increase the breakdown voltage and surge withstanding capability, ZnO nanopowder prepared by sol-gel method is applied in ZnO varistor. Also, some additives, sintering temperature, barrier height and activation energy of ZnO varistor samples are studied in this paper.The effects of additives and sintering temperature on the electrical properties of ZnO varistor are studied by using orthogonal experiment. In orthogonal experiment, the various factors are doping quantities of Bi2O3、Co3O4、Sb2O3 and sintering temperature. In the range of the setting values, the optimum value of factors is gotten through orthogonal experiment. And the electrical properties of ZnO varistor prepared under optimum condition are as follow: nonlinear coefficientα=49; breakdown voltage V1mA=178V/mm; leakage current IL=0.4μA, and the capacitance C= 766pF.ZnO nanopowder is prepared by sol-gel method, and four kinds of nanopowder are prepared by changing the calcining temperature of the precursor. With the inspection of SEM, the average size and the range of size distribution of ZnO nanoparticles increase as the calcining temperature increasing. The smallest average size of ZnO particles is about 40nm as the precursor calcined at 500℃. The electrical properties of varistors prepared by different ZnO nanopowder are also studied. The results show that breakdown voltage and leakage current decrease, but nonlinear coefficient and residual voltage ratio increase. Comparing to the varistors fabricated by ZnO sub-micro-powder prepared by plasma method and commercial ZnO micro-powder, varistors fabricated by ZnO nano-powder prepared by sol-gel method appear much higher breakdown voltage and better surge current withstanding capability.In the semiconductor Schottky Barrier model, barrier height has a decisive effect on the electrical properties （such as nonlinear coefficient） of ZnO varistor. The barrier height of two kinds of varistors is respectively estimated in the presence of the expression of field enhanced thermal emission current by measuring the U-I characteristic at different temperature. And the result shows that varistor with higher nonlinear coefficient fabricated by ZnO sub-micro-powder appears bigger barrier height than varistor with lower nonlinear coefficient fabricated by ZnO nano-powder.By atomic defect model, the depletion layer consists of tow components: a stable component consisting of spatially fixed positively charged ions such as VO +, VO 2+and a metastable component consisting of mobile positively charged Zn i-, Zn i2-interstitials. The intrinsic donor defect structure of ZnO varistors is studied by measuring the dielectric spectroscopy characteristic in a wide temperature range. Tow loss peaks originating from electronic relaxation of oxygen vacancy and zinc interstitial, respectively. And the activation energy for electronic relaxation of oxygen vacancy and zinc interstitial are about 0.37eV and 0.23eV, respectively. Besides, the corresponding activation energies of varistors fabricated by different ZnO powder are almost the same.

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CLC: > Industrial Technology > Electrotechnical > Electrical > Resistor,potentiometer
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