Papers by Keyword: Varistor

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Authors: Rabab Khalid Sendi, Shahrom Mahmud, Amna Sirelkhatim
Abstract: Zinc oxide-based varistors are semiconductor ceramics. Their excellent nonlinear electrical behaviors are induced from their grain boundaries and depend on their microstructural characteristics. From a theoretical aspect, finer primary particles with narrow size distributions provide better electrical and optical properties. Thus, these properties are related to the morphology and size of ZnO grains. In this study, ZnO-Bi2O3-Mn2O3 varistors fabricated from ZnO micro-and nanoparticle powders are prepared via conventional ceramic processing. The effects of ZnO particle size and different annealing ambients on the properties of ZnO varistors are also investigated. The strong solid-state reaction during sintering may be attributed to the high surface area of the 20 nm ZnO nanoparticles that promote strong surface reaction. The annealing process also improves grain crystallinity, as shown in the decrease in intrinsic compressive stress based on the X-ray diffraction lattice constant and full-width at half-maximum (FWHM) data. The reduced particle size significantly influences the electrical properties, showing a sharp drop in the breakdown voltage. Thus, the ZnO nanoparticles can be used to manufacture of varistors with superior properties and lower breakdown voltage compared with commercial ones.
Authors: Jun Liu, Jin Liang He, Jun Hu, Wang Chen Long, Feng Chao Luo
Abstract: ZnO varistors are multicomponent polycrystalline ceramics with highly nonlinear current-voltage characteristics and surge energy absorption capabilities. The voltage gradient of the ZnO varistor is inversely proportional to its average grain size. Recently, the rare-earth oxides have been reported as growth inhibitor of ZnO grains to obtain high voltage gradient. Although the dopant of rare-earth oxides can remarkably enhance the voltage gradients of varistor samples, their leakage currents and nonlinear coefficients deteriorate at the same time. In this paper, the sintering effects on electrical characteristics and admittance spectroscopies of ZnO varistor samples doped with yttrium oxides were investigated. Samples were fabricated under different sintering temperatures, including 1000 °C, 1100 °C, 1200 °C and 1300 °C. Then, the electrical characteristics and admittance spectroscopies of these varistor samples were measured. The measured current-voltage results behave special U-type curves related to sintering temperature. Furthermore, the admittance spectroscopy of these samples revealed the sintering effects on the intrinsic defects of ZnO varistors.
Authors: Rabab Khalid Sendi, Ayman Munshi
Abstract: In this study, ZnO-Bi2O3-Mn2O3 varistors fabricated from the nanoparticle size of ZnO powder and doped with borosilicate frit were prepared via the conventional ceramic processing method. The influence of different borosilicate frit concentrations (0-3.0 mol%) on the sintering, microstructure enhancement, and nonlinear behavior of the ZnO-Bi2O3-Mn2O3 system was investigated. Results show that the borosilicate frit was liquefied to improve the density of the ceramic during sintering and found to have significant effects on the ZnO varistors, especially on enhancing grain growth even at a low doping concentration of only 0.5% mol. The strong solid-state reaction in the varistor made from 20 nm ZnO powder during sintering may be attributed to the high surface area of the 20 nm ZnO nanoparticles. X-ray diffraction analysis indicated that the addition of borosilicate frit to the ZnO-Bi2O3-Mn2O3 varistor system results in the formation of Zn4O(BO2)6 during sintering if too much borosilicate frit was added (over 0.5% mol). Borosilicate frit doping also significantly influenced the electrical properties of the varistor with a marked drop in the breakdown voltage from 545 V to 188 V with increase of borosilicate frit doping concentration. The resistivity also experienced a dramatic drop from 535.7 kΩ.cm to 133.5 kΩ.cm with increase of borosilicate frit doping contents. Therefore, borosilicate frit doping can be used to control the structural properties and breakdown voltage of ZnO-Bi2O3-Mn2O3 varistor system fabricated from 20 nm ZnO powder.
Authors: Samarpita Roy, Tapatee Kundu Roy, Debdulal Das
Abstract: This study examines the effect of Er2O3 addition (0 to 2.0 mol.%) on microstructure and electrical characteristics of ZnO-based varistor ceramics prepared by high energy ball milling and sintering at 1200 °C for 1 h. With increasing Er2O3 content, the densification process diminishes marginally, but the grain size reduces dramatically due to the formation of Er-rich phases that inhibit grain coarsening specifically when the amount of Er2O3 addition is above 0.1 mol.%. The ZnO-based varistor modified with incorporation of 0.5 mol.% Er2O3 exhibits excellent varistor properties, since the breakdown field is increased from 1324 to 2320 V cm-1 and the nonlinear coefficient is enhanced from 23.1 to 27.4 when compared with the un-added ones.
Authors: Marek Andrzej Kojdecki, Witold Mielcarek, Krystyna Prociów, Joanna Warycha
Abstract: Varistors obtained by sintering zinc oxide with additives were investigated. In this work the crystalline microstructure of ZnO varistor body is studied by X-ray diffraction method, before and after additional annealing. The crystalline microstructure is characterised by the prevalent crystallite shape, the volume-weighted crystallite size distribution, the second-order crystalline lattice strain distribution and the derivative quantities. The evolution of these characteristics, particularly of bimodal size distribution, elucidate some details of the formation of crystalline zinc oxide and provide additional arguments for explaining the change of the electric characteristics of varistors induced by annealing.
Authors: Yukio Sato, Fumiyasu Oba, Masatada Yodogawa, Takahisa Yamamoto, Yuichi Ikuhara
Abstract: Our recent studies on current-voltage characteristic and grain boundary structure in ZnO bicrystals are reviewed in this paper. All types of undoped ZnO bicrystals showed Ohmic characteristics. This indicates that nonlinear current-voltage characteristic cannot be generated solely by the atomic disarrangement in undoped ZnO. On the other hand, co-doped ZnO bicrystals showed nonlinear current-voltage characteristics, which depended on the types of grain boundary. It is considered that the nonlinearity depends on the Pr concentrations at the respective grain boundaries.
Authors: Jian Feng Zhu, Guo Quan Qi, Hai Bo Yang, Fen Wang
Abstract: ZnO-based varistor ceramics were prepared by sintering ZnO powders with minor additives that were synthesized by standard ball milling, high energy ball milling for 10 h, and pyrogenic decomposition of nitrate. The microscopic structure and composition were analyzed by SEM and XRD respectively, and the electrical properties were investigated by a DC parameter instrument for varistors. Influences of composite additives on composition, microstructure, and electrical properties of ZnO-based varistors were studied. The ZnO varistor with optimum performance is prepared by the pyrogenic decomposition method and it has an average grain size of 5 µm and possesses a varistor voltage of about 561 V/mm, non linear coefficient of 75, and leakage current of 0.45 µA.
Authors: Qun Qin, Tian Guo Wang, Wen Jun Zhang
Abstract: WO3-based capacitor-varistor ceramics doped with Er2O3 were prepared and the microstructures and nonlinear electrical properties were investigated. The results show that there exist second phase Er10W2O21 on the surface of WO3 grains. Doping Er2O3 in WO3 ceramic can inhibit the grain growth. A small quantity of Er2O3 can significantly improve nonlinear properties of the samples. The permittivity of doped samples was higher than that of the undoped, and the high permittivity makes Er2O3-doped WO3 ceramics be applicable as a kind of capacitor-varistor materials.
Authors: Guo Zhong Zang, Jin Feng Wang, Hong Cun Chen, Wen Bin Su, Chun Ming Wang, Peng Qi, Bao Quan Ming
Abstract: The effect of Gd2O3 on the electrical properties of (Co, Nb)-doped SnO2 varistors was investigated. It was found that the nonlinear coefficient presents a peak of α = 30 for the sample doped with 1.5mol% Gd2O3. The increase of the breakdown electrical field from 325V/mm to 1560V/mm with increasing Gd2O3 concentration is mainly attributed to the decrease of the grain size. The decrease of relative density and resistance of grain boundary indicate that Gd2O3 should be a two-sided dopant and the nonlinear coefficient peak was explained. To illustrate the grain-boundary barrier formation of (Gd, Co, Nb) doped SnO2 varistors, a modified defect barrier model was introduced, in which the negatively charged acceptors substituting for Sn ions should not be located at the grain interfaces instead at SnO2 lattice sites of depletion layers.
Authors: Tian Guo Wang, Qun Qin, Wen Jun Zhang
Abstract: The nonlinear electrical behavior and dielectric properties of WO3-based ceramics with various La2O3 contents have been investigated. Breakdown voltages Eb of WO3 doped with La2O3 are lower than that of undoped WO3, indicating that the dopant can reduce the breakdown voltage. The dielectric constant of doped samples is higher than that of undoped samples, and the high dielectric constant makes them suitable as capacitor-varistor materials. The theory defects in the crystal lattice was introduced to explain the nonlinear electricial behavior of the La2O3-doped WO3 ceramics. In view of these electrical characteristics, the WO3 ceramic doped with La2O3 is a viable candidate for capacitor-varistor functional devices.
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