The Experts below are selected from a list of 11385 Experts worldwide ranked by ideXlab platform
Jinliang He - One of the best experts on this subject based on the ideXlab platform.
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effects of cobalt doping on the electrical characteristics of al doped zno Varistors
Materials Letters, 2010Co-Authors: Wangcheng Long, Jun Hu, Jinliang HeAbstract:Abstract The effects of cobalt additive on the electrical characteristics of the Al-doped ZnO Varistors are studied in this paper. The current–voltage characteristics of the varistor samples have been investigated in a range from small to large current. With the amount of the doped cobalt increased, the leakage currents of the Al-doped ZnO Varistors are inhibited, and their nonlinear coefficients increase remarkably as well. In addition, their breakdown electric fields increase in a small extent and their residual voltage ratios change slightly. Furthermore, the donor densities and the barrier heights of samples decrease with the cobalt content increased. The XRD patterns show that the Bi-rich phase and the willemite phase vary obviously in the varistor samples with various content of cobalt additive.
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zno Varistors with high voltage gradient and low leakage current by doping rare earth oxide
Science China-technological Sciences, 2008Co-Authors: Jinliang He, Jun HuAbstract:The surge arrester of 1000 kV gas-insulated substation (GIS) needs ZnO varistor with high voltage gradient to effectively improve the potential distribution along ZnO varistor column inside the metal-oxide surge arresters. In this paper, the electrical and structural parameters of ZnO Varistors are changed by doping with some rare-earth oxides, and the mechanism which leads these changes is discussed. When rare-earth oxide additives are added into ZnO Varistors, the growing speed is slowed down due to the stabilization of the new spinel phases formed in the grain-boundary by rare-earth oxide additives, then the size of ZnO grains is smaller, and the voltage gradient of varistor increases obviously. By adding suitable amount of oxides of metal Co and Mn, the leakage current can be effectively decreased and the nonlinearity coefficient increased. The novel ZnO varistor samples sintered with the optimal additives have a voltage gradient of 492 V/mm, and the nonlinearity coefficient of 76, but their leakage currents are only 1 μA.
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Microstructure Simulation on Puncturing Phenomenon of ZnO Varistor under High Current
2006 IEEE 8th International Conference on Properties & applications of Dielectric Materials, 2006Co-Authors: Jinliang He, Jun Hu, Qingheng ChenAbstract:ZnO Varistors are vulnerable to be destroyed under high current. The puncturing phenomenon of ZnO varistor under high current is simulated in a microstructure model. The Voronoi diagram can effectively simulate the actual microstructure of ZnO Varistors, from which the electrical topology relation of the nonlinear resistor network of ZnO varistor microstructure can be derived. The steepest descent method and updated Newton iterative method are used to analyze the big scale nonlinear resistor circuit. The simulated results express that there is serious current concentration and temperature concentration in ZnO varistor, which can explain the puncturing breakdown of ZnO varistor under high current
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Dopant Effects to Pores in ZnO Varistors
2006 IEEE 8th International Conference on Properties and applications of Dielectric Materials, 2006Co-Authors: Youping Tu, Jinliang He, Lijian Ding, Jun HuAbstract:Pore is an intrinsic property of ZnO varistor. There are two different shapes of pores in ZnO varistor samples, one is sphere shape, and another is irregular polyhedron. Pores are caused by air particles wrapped in initial varistor compact and grains-surrounded spaces during the sintering. By analyzing the pore phenomena in two different systems of ZnO Varistors, we observed the pore ratio decreases, pores with large size are formed, and the pores inside ZnO grains decreases with the increase of the amount of Al2O3 additives, which are caused by the pinning effect of spinel formed by additives. The pores are possibly related to grain growth inhibition by dragging effects.
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Electrical parameter statistic analysis and parallel coordination of ZnO Varistors in low-voltage protection devices
IEEE Transactions on Power Delivery, 2005Co-Authors: Jinliang He, Xidong Liang, Shuiming Chen, Rong Zeng, Zhicheng GuanAbstract:Analyzing the nonuniformity of electrical parameters of ZnO Varistors is useful to select and coordinate the ZnO Varistors in parallel operation and improve the protection capability of surge-protection device (SPD). The nonuniformity of 1-mA dc voltage and impulse residual voltage of commercial low-voltage ZnO Varistors were estimated by statistic analysis in this paper. The Minimum Gobble Distribution was used to describe the distribution of 1-mA dc voltage and impulse residual voltage, ZnO Varistors were verified conforming to the Minimum Gobble Distribution by the Kolmogorov-Smirnov test. The relationship between the impulse residual voltage and 1-mA dc voltage has a chaos phenomenon, and they have a close relative degree by t-test, the impulse residual voltage can be estimated by voltage ratio and 1-mA dc voltage. The nonuniform phenomena of electrical parameters of ZnO Varistors were simulated by Voronoi network. The current overload of ZnO varistor with reduced residual voltage is serious, in order to keep safe operation of SPDs, when we design a SPD with ZnO Varistors in parallel, a safe margin coefficient of discharge withstand current should be considered according to the actual relative standard deviation of surge residual voltages of used ZnO Varistors.
A C Caballero - One of the best experts on this subject based on the ideXlab platform.
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Varistors based in the zno bi2o3 system microstructure control and properties
Journal of The European Ceramic Society, 2007Co-Authors: M Peiteado, J F Fernandez, A C CaballeroAbstract:The present contribution deals with the development of an empirical approach that will describe the evolution of the breakdown voltage in ZnO-based Varistors as a function of the sintering variables. This relationship is described through the kinetic analysis of the grain growth process. It is found that for a certain range of sintering conditions the varistor breakdown voltage could be calculated from the average size that zinc oxide grains achieve, provided that the varistor microstructure evolves in a gradual and controlled manner and the distribution of the electrical interphases remains stable.
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compact shape as a relevant parameter for sintering zno bi2o3 based Varistors
Journal of The European Ceramic Society, 2004Co-Authors: M A De La Rubia, M Peiteado, J F Fernandez, A C CaballeroAbstract:Abstract Ceramic Varistors based on the ZnO–Bi2O3–Sb2O3 system, are usually sintered at 1150–1200 °C in the presence of a Bi-rich liquid phase that forms at 700–800 °C. Bismuth is critical for the formation of electrically active grain boundaries, and the Bi-rich liquid phase plays a major role in the grain growth kinetics. Therefore, uncontrolled bismuth oxide vaporization during sintering can heavily deteriorate the electrical response of the varistor, and the area–volume ratio of the green compact becomes a relevant parameter, along with the sintering temperature and soaking time. In the present work, we have studied the microstructure and the current–voltage characteristics of ceramic Varistors with different area–volume ratios and sintering cycles. We find that, depending on the sintering cycle, there exists a value for the area–volume ratio that leads to a strong deterioration of the varistor electrical properties.
Tseungyuen Tsen - One of the best experts on this subject based on the ideXlab platform.
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electrical properties of multilayer chip zno Varistors in a moist air environment
Journal of the American Ceramic Society, 1994Co-Authors: Tseungyuen TsenAbstract:Multilayer-chip Varistors were prepared using the ZnO-glass system by applying tape casting technology. The effect of processing conditions on the multilayer ceramic microstructure and electrical properties was studied. The location, size, and density of the pores within the multilayer structure were examined by scanning electron microscopy and the displacement method using distilled water. The experimental results showed that the electrical properties of a multiplayerchip varistor could be influenced substantially by the porosities associated with environmental moisture in the range ≅15%–95% relative humidity at 25°C. Such an effect can cause substantial current leakage as well as the separation of the I–V curves into four regions, i.e., pre-breakdown, linear, breakdown, and upturn. A proposed pore model and equivalent circuit for the multilayer-chip ZnO Varistors. have been simulated with a commercial varistor and a serial resistance to demonstrate the observed I–V phenomena.
Volker Hinrichsen - One of the best experts on this subject based on the ideXlab platform.
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recent experimental findings on the single and multi impulse energy handling capability of metal oxide Varistors for use in high voltage surge arresters
IEEE Transactions on Power Delivery, 2014Co-Authors: Maximilian Nikolaus Tuczek, Volker HinrichsenAbstract:This contribution discusses different aspects of impulse energy handling capability of metal-oxide (MO) Varistors used in high-voltage surge arresters. The investigations were performed on more than 3000 MO Varistors of different manufacturers worldwide. It can be confirmed that the single-impulse energy handling capability increases with increasing current density of the energy impact, a finding that was also published earlier but based on a very limited number of test samples. In addition, the impact of the MO varistor diameter is discussed. Thereafter, the differentiation of single and multi-impulse energy handling capability is addressed. Finally, investigations with a repeated energy injection (up to 100 prestresses per MO varistor at time intervals that allow the MO Varistors to cool down to ambient temperature) are reported. At none of the investigated repeated energy stresses, any change of the V-I characteristic in the leakage current region was observed that would affect the arrester's overall thermal stability.
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Overvoltage protection of inverter-fed drives with the help of energy Varistors - Dimensioning rules for consideration of different cable types
2009 Twenty-Fourth Annual IEEE Applied Power Electronics Conference and Exposition, 2009Co-Authors: A. Rocks, Volker HinrichsenAbstract:This contribution deals with overvoltage protection in inverter-fed drives by Varistors. Several kinds of overvoltage in combination with very low rise times impose extreme dielectric stress on the insulation of electrical machines. With Varistors it is possible to decrease these overvoltages in combination with an extended rise time of the impulses occurring at the machine terminal. During this service the varistor has to withstand the power losses and has to dissipate heat into the surrounding medium. The operation mode of the varistor strongly depends on the basic parameters like pulse frequency, rise time of the impulses at the inverter, applied cable, size of the motor etc. Here correlations between the voltage-current characteristic (U-I characteristic) of the varistor, the maximum occurring voltage steepness and the applied cable type are given in order to be able to carry out detailed dimensioning for different application cases.
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Effect of Varistors for overvoltage protection in inverter-fed drives on the voltage distribution in the stator winding
Conference Record of IEEE International Symposium on Electrical Insulation, 2008Co-Authors: A. Rocks, Volker HinrichsenAbstract:The use of Varistors for overvoltage protection in inverter-fed drives can be a promising alternative to common solutions like filters. In this contribution the varistor's influence on the voltage distribution in the stator winding is presented under certain aspects such as cable type and varistor type. The results of different measurements are shown and the main aspects are discussed. It is shown voltage amplitudes and voltage distribution can be affected positively to increase life time of the winding insulation of the machine.
Lihong Cheng - One of the best experts on this subject based on the ideXlab platform.
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electrical properties of al2o3 doped zno Varistors prepared by sol gel process for device miniaturization
Ceramics International, 2012Co-Authors: Lihong Cheng, Lei Meng, Liaoying Zheng, Guorong Li, Yan Gu, Fuping Zhang, Zhijun XuAbstract:Abstract Al 2 O 3 -doped ZnO Varistors with high performance have been fabricated with nano composite powders, which were prepared by the sol–gel method. The xerogels, calcined powders and sintered material ceramics were fully characterized. The electrical properties and grain boundary characteristics were measured. The average grain size decreased from 4.4 μm to 3.0 μm and the breakdown voltage increased from 720 V/cm to 1160 V/cm as the amount of Al 2 O 3 increased from 0.0 wt.% to 0.40 wt.%. 2 ms square-wave test indicates that the energy capability was better in a sol–gel derived ZnO varistor than a conventional sample and was enhanced by Al 2 O 3 doping. Improved electrical properties of sol–gel derived ZnO Varistors is attributed to the smaller grain size and more uniform distribution of additives. Al 2 O 3 -doped ZnO varistor obtained from sol–gel process is useful for device miniaturization.
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synthesis of novel core shell nanocomposites for fabricating high breakdown voltage zno Varistors
Journal of Materials Chemistry, 2011Co-Authors: Lei Meng, Liaoying Zheng, Lihong Cheng, Guorong Li, L Huang, Yan Gu, Fuping ZhangAbstract:A novel varistor-composite synthesis for the fabrication of high breakdown voltage ZnO Varistors has been successfully developed. It involves coating ZnO nanoparticles with SiO2 through the hydrolysis of tetraethyl orthosilicate (TEOS) and the subsequent coating of additives on the core of the SiO2-coated ZnO particles, producing a double core-shell structure. The effect of the amount of TEOS used on the morphology of the synthesized composites and the microstructure, as well as the electrical property of the sintered samples from the double core-shell composites are fully studied. Such materials, with the appropriate amount of TEOS added (Si4+/Zn2+ = 0.15 moles) enables the superior electrical performance of ZnO Varistors: high breakdown voltage (VB = 1123 V mm−1), excellent nonlinear coefficient (α = 42.5) and low leakage current (IL = 8.5 μA), compared with current commercial Varistors and the sample from single core-shell composites. The high performance is attributed to the smaller ZnO grain size and perfect additive homogeneity. The SiO2-coating provides a better way to control the varistor microstructure through the formation of a nearly circular distribution of blocky Zn2SiO4 around the ZnO grains.
