Thick film ZnO based varistors prepared by screen printing
Miguel Angel de la Rubia LópezM. PeiteadoJ.F. FernándezA. C. CaballeroJanez HolcSilvo DrnovšekDanjela KuščerS. MačekM. Kosec
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Zinc oxide (ZnO) varistors exhibit a voltage upturn at higher current densities which reduces their effectiveness as over-voltage protection devices. This effect has been attributed to a limiting series impedance offered by the ZnO grains. Based on literature data, the varistors were made for this investigation wherein the grain impedance has been reduced by doping with Al2O3 or Ga2O3. The high current surge and high frequency impedance data combined with microstructural investigation indicated an improved varistor nonlinearity via donor doping. A simple defect model developed for a pure ZnO crystal has been used to explain the doping behavior of the varistor grain.
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ZnO varistors have an excellent non-linearity and a large surge-energy absorption capability. For these reasons, the ZnO varistors are widely used to protect electrical/electronic circuits from an abnormal surge and/or noise signal. In order to obtain the low-voltage varistor with randomly distributed large seed grain within bulk, the ZnO varistors are made by a new three-composition seed grain method. From the I-V characteristics of the fabricated varistors, the knee voltage of varistors with the seed grain 5wt% is 5 V. We may also present a carrier oscillation properties for the low-voltage varistor fabricated by the new method of three-composition seed grain.
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ZnO varistors produced by current technology have excellent non-linear voltage dependent characteristics. Therefore, they are suitable for use as a surge protection device of electric and electronic equipment. The operating life and the stability of installed arrester is affected by several factors, such as overvoltages having different duration and amplitude and increased ambient temperature. Moreover, the above factors act simultaneously and mutually influence each other. The article deals with the changes of electrical properties of low-voltage ZnO based varistor caused by injection of current pulses with a defined shape and energy. Comparison of changes in electrical properties is based on the observation of shift in dependence of the DC voltage and the corresponding current. Moreover, changes of nonlinearity coefficient and the absorbed energy have been observed. It was found that repeated injection of current pulses results in a significant change in electrical characteristics of the ZnO varistor. Total destruction of the varistor is demonstrated.
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The effect of simultaneously adding Zr and Y to Bi–Mn–Co–Sb–Si–Cr–Ni-added ZnO varistors (having the same composition as a commercial varistor) on the varistor voltage, leakage current, and resistance to electrical degradation were investigated. Varistor voltage increased with increasing amount of Y for addition of 0–2 mol % Zr. On the other hand, the nonlinear coefficient α prior to electrical degradation changed very little on the addition of both Y and Zr. With the addition of approximately 1 mol% Zr, the leakage current decreased with increasing amount of Y added. A ZnO varistor with a varistor voltage of approximately 600 V/m, a low leakage current, and excellent resistance to electrical degradation was fabricated by adding approximately 2 mol% Y and approximately 1 mol% Zr.
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Most commercial ZnO varistors containing Bi2O3 exhibit excellent varistor properties, but they have a few drawbacks due to Bi2O3 having high volatility and reactivity and higher sintering temperature. In this study, V2O5 is added as the varistor forming oxide to lower down the sintering temperature of ZnO varistors for the further developing the chip Varistor array by using the Ag as the inner electrode. It is found that the sintering temperature of V2O5-added ZnO will be lower down to about 600oC. But the addition of V2O5 has no improvement in the electrical characteristics, and they need many additives to obtain the high performance. For that, the nonlinear properties in these Varistors can be improved by incorporation of some oxide additives. Different MnOx oxides (MnO, MnO2, and Mn3O4) and CoOx oxides (CoO and Co3O4)are used as the minor oxide additives. The influences of different MnOx and CoOx oxides on the nonlinear voltage–current density (V-I) characteristics and the nonlinear exponent (α) of V2O5-ZnO varistor will be well developed.
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Degradation of potential barriers in ZnO-based varistors due to the electrostatic discharge (ESD) was investigated using scanning probe microanalysis and capacitance-time and isothermal capacitance transient spectroscopies. Pr6O11-ZnO (Pr-ZnO) varistors exhibit excellent ESD withstand capability compared with Bi2O3-ZnO (Bi-ZnO) varistors. After the application of ESD, asymmetrically degraded double Schottky barriers were observed in both Pr-ZnO and Bi-ZnO varistors, and the Schottky barrier in Bi-ZnO was found to be destroyed. The potential barriers of both types of varistors can respond to an ESD pulse, whose rise time is ∼1 ns, but after application of the ESD pulse, the Bi-ZnO varistor takes more time to recover its initial capacitance than does the Pr-ZnO varistor. Such difference in the transient behaviors of potential barriers is attributed to differences in the energy and distribution of interfacial states of Pr-ZnO and Bi-ZnO varistors. Experimental results clearly indicated a strong correlation between the transient behaviors of potential barriers and the ESD withstand capabilities of ZnO-based varistors.
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Degradation
Transient voltage suppressor
Transient (computer programming)
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Zinc Oxide (ZnO) surge arresters are used as a protective device for transient overvoltages. This is mainly due to the high nonlinearity of voltage-current characteristics of ZnO varistor. The electrical characteristics of ZnO varistor is determined by its microstructure. This paper reviews about the ZnO varistor for surge arrester applications. The paper starts with the concept of nonlinear characteristics and microstructure of the ZnO varistor. The fabrication method and characterization of varistors are discussed. The effects of various additives and sintering process are explained.
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Presented are the composition, process, properties and applications of the materials for the low voltage varistor (ZnO, BaTiO3, TiO3, WO3 systems), the high voltage varistors (ZnO and SnO2 systems) and the capacitance-voltage varistor (SiTiO3 and TiO2 systems), etc. The problems in research and production of the varistor in China and the solutions are discussed.
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ZnO-based varistors are frequently employed in voltage surge mitigation applications due to their distinctive nonlinear current-voltage properties. The addition of TiO2 to ZnO varistors has been found to increase their electrical characteristics, such as the nonlinear coefficient and breakdown voltage. A theoretical investigation is undertaken in this work to evaluate the impact of TiO2 on the electrical properties of ZnO-based varistors. The findings reveal that TiO2 can operate as a pinning agent, inhibiting ZnO grain development and improving varistor uniformity. As a result, the nonlinear coefficient and breakdown voltage of the varistor rise. Furthermore, TiO2 can form secondary phases with ZnO, which can improve the electrical characteristics of the varistor even further. The results of this study provide a theoretical understanding of the effects of TiO2 on the electrical characteristics of ZnO-based varistors, and they can be used to design varistors with improved electrical properties.
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