Papers by Keyword: Varistor

Abstract: A stability loss of varistor is subjected to any of variety extreme conditions such different stresses AC or DC, temperature, pulse, pressure, humidity, etc. This causes degradation phenomenon and leads to thermal runaway or failure of varistor. The investigation of degradation phenomena is an important subject for prolonging their lives. In this study, we investigate the degradation process of ZnO_1-x-CoO_2.0-SLS_x varistor ceramics where x= 0.5, 1.0, 1.5, and 2.0 mol % by using conventional solid state method at sintering temperature of 1100 °C for 2 hour. The stability of their nonlinear properties was investigated under prolonged application of DC electric field at different SLS glass concentration. The degradation process was speeded up by applying direct current (DC) 85 % of electric field for 12 hours at temperature of 85 °C. A concentration of 2 mol % SLS glass doped ZnO varistor shows the best degradation properties as its nonlinear coefficient has increased by 3.56 %, the breakdown field has increased by 3.85 %, and the leakage current density (J_L ) increased by 2.40 % in comparison to its initial value.

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.

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.

Abstract: Varistor with TiO₂ as the base and SiO₂, WO₃ and Bi₂O₃ as dopants were investigated to create a low voltage varistor. The physical, mechanical, microstructural and electrical properties were studied where the concentration of SiO₂ was varied but the concentration of WO₃ and Bi₂O₃ was fixed. Physical and mechanical characterization consisted of green and fired density, axial and radial shrinkage and Vickers Hardness. Electrical evaluation on the other hand consisted of non-linear coefficient, breakdown voltage, power loss and clamping ratio. Non-linear coefficient of 2.16, very low breakdown voltage of 5.538V/cm and minimal power loss of 0.0124mW was achieved. It was found that optimum results were achieved with 98.3% TiO₂, 0.7% SiO₂, 0.5%Bi₂O₃ and 0.5%WO₃.

Abstract: TiO₂ was added in Zn-Bi based varistors to enhance the grain growth of ceramics and the effects of TiO₂ addition on the microstructure, I-V, flow and aging characteristics of Zn-Bi varistors were studied. The voltage-gradient of samples decreased to minimum value of 30.6 V/mm when the TiO₂ content was 0.6 mol%, but increased with further increasing TiO₂ content. The variation trend of average grain size was just the opposite of the voltage-gradient. 0.6 mol% TiO₂ added samples have a more unify microstructure and excellent three parameter values with I_L=5.7μA, E_1mA=30.6V/mm, α=25.8. After Dc aging at 125 ^oC for 7h, the voltage variation of samples is very slow (~-4.1%).

Abstract: We had described and discussed the testing methods of varistor tester about DC reference current, varistor voltage, leakage current, voltage ratio, and provided the corresponding test data,which has proved that the methods are feasible.

Abstract: Current-voltage (I-V) and impedance spectroscopy are employed to investigate the polarity of TiO₂ varistor. It is found that TiO₂ varistor shows typical polarity behavior, i.e., the I-V curve is asymmetric, the impedance slightly increases, and the capacitance slightly decreases when the bias voltage reverses. The effects of measuring frequency on impedance spectroscopy are systematically investigated. The results indicate that TiO₂ varistor behaves considerable polarity under low sweeping frequency. High sweeping frequency (over 10³ Hz) counteracts the polarity behavior. A model of micro-polarized zone is tentatively proposed to clarify the polarity behavior of TiO₂ varistor.

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.

Abstract: In the process of the piezoresistive sensor design, the placement of varistor has a great influence on the sensitivity of the sensor.In this paper, three theoretical models of piezoresistive sensor are established.The stress simulation analysis of the different varistor path way placed on the cantilever beam is conducted in Ansys software. The sensitivities of the axis and transverse direction are calcuated and compared by Matlab compiler. The results show that the placement of varistor which is parallel to the cantilever beam is the most optimal choice.

Abstract: The ZnO varistor ceramics were fabricated through microwave sintering at 800~1150°C using ZnO powder doped and undoped TiO₂ prepared by planetary ball mill. And the effects of milling time, sintering temperature and doping on microwave sintering technique have been studied. The result shows that the powder is more homogeneously and with smaller particle size with the increasing of milling time, therefore the electric properties is improved. However the particle size and the electric properties tend towards stability after 20h milling time. The density and electric properties increase with the increasing of sintering temperature, but decrease sharply at 1100°C, so the best sintering temperature is 1100°C. Doping TiO₂ improves the electric properties of ZnO, however the electric properties decrease with more than 2.5% doping.