Papers by Keyword: ZnO Varistor

Abstract: B2O3-doped ZnO-Pr6O11 based varistor ceramics were fabricated by sintering samples at 1350 °Cfor 2h with conventional ceramic processing method. The microstructure and electrical properties of the as-prepared samples were investigated. X-ray diffraction analysis showed that after the addition of B2O3 with the amount designed in this study, no new phase was examined in the detection limit. Through scanning electron microscopy it was found that the doping of B2O3 can promote the growth of ZnO grains, and the sintering of the samples. The result of electrical properties indicated that the nonlinear coefficient and varistor voltage of the samples could be improved to some extent with appropriate doping amount of B2O3, resulting in the highest nonlinear coefficient 6.7, lowest leakage current 329 μA/cm2, and highest varistor voltage 92.4 V/mm, respectively.

Abstract: The doping effects of Cu on the microstructure and non-ohmic electrical properties of ZnO varistors were studied. Addition of Cu₂O can enhance the ZnO grain growth during sintering. The SEM and EDS results revealed that the added Cu mainly distributed in the grain boundary and spinel phases of ZnO varistors. The Cu₂O addition increased the both of grain and grain boundary resistances. However it decreased the non-ohmic electrical characteristics of ZnO varistors, which is a good agreement with similar findings on Ag₂O additions, but contrasts to the reports of good non-ohmic electrical property which found on binary Cu doped ZnO varistors.

Abstract: 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.

Abstract: The non-linear properties provided from ZnO + ACoO₃ ceramics (A=Ca,Sr and Ba) and their application to MLCV for ESD-suppression were studied. Their ceramics showed excellent non-linear I-V properties in lower voltage region. Especially, ZnO + SrCoO₃ ceramics intrinsically had about 20 % lower Vgb than those of Pr and Bi-based ZnO varistors. MLCV using ZnO + SrCoO₃ ceramics demonstrated extremely low-voltage property of V_1mA= 5.6 V with maintaining excellent non-linearity and reliability performance against ESD.

Abstract: The electrical properties of ZnO-Pr6O11-based varistors, which composed of (98.5-x)mol% ZnO +0.5mol% Pr6O11 +1.0mol% Co3O4 +xmol% Cr2O3 (x=0.0, 0.25, 0.5, 1.0, 2.0), were investigated as a function of sintering temperature in the range of 1250-1400°C for 2 h. At the same sintering temperature, the nonlinear exponents increased with the increase of Cr2O3 doping amount no more than 0.5mol%, but decreased with more Cr2O3 doped. With the same doping level of Cr2O3, the samples' varistor voltage decreased with the sintering temperature increased; and the samples' nonlinear exponents increased first with the sintering temperature increased to 1300 °C, but decreased when the temperature increased further. Conclusively, the electrical properties of ZnO-Pr6O11-Co3O4- Cr2O3 varistors were strongly affected by the sintering temperature and the doping level of Cr2O3, and the samples, doped with 0.5 mol% Cr2O3 and sintered at 1300 °C, exhibited the optimum nonlinear performance.

Abstract: With the goal of fabricating low-breakdown-voltage varistors, the effect of adding Ba to ZnO varistors on the ZnO grain size was investigated. Grain growth of ZnO could be markedly promoted by adding both Ba and Bi. The maximum grain size was approximately 150 μm and the minimum varistor voltage was approximately 12 V/mm. However, it had relatively poor tolerance characteristics for electrical degradation. It is speculated that when adding both Ba and Bi to a Mn–Co-added ZnO varistor, it is necessary to form the molten phases of Ba and Bi to promote grain growth of ZnO. It is also conjectured that the growth of ZnO grains is not promoted when Ba and Bi do not coexist in the molten phase because Ba forms compounds with Mn independently with the addition of small amounts of Bi.

Abstract: The effects of the addition of Y or both Y and Zr to Bi-Mn-Co-Sb-Si-Cr-Ni-added ZnO varistors on the varistor voltage and the tolerance characteristics of electrical degradation were investigated. The deterioration of the tolerance characteristics of electrical degradation by the addition of Y was probably caused by an increase in the number of willemite (Zn2SiO4)-type particles or a decrease in the number of spinel (Zn2.33Sb0.67O4)-type particles, but this deterioration was reduced by adding Zr. Moreover, the reduction in the average ZnO grain size due to the addition of Y was a major factor in the increased varistor voltage, and the ZnO grain growth was inhibited by the formation of an un-known compound after adding Y. The varistor voltage of a varistor with 2 mol% added Y increased by approximately 50% compared to a varistor with no Y added.

Abstract: The additive of Al(NO3)3 was doped into ZnO varistors in order to reduce their residual voltages. Some of doped Al3+ enter the ZnO grains and infill into the vacancies of Zn2+ ions in the crystal lattice. Then, the resistances of ZnO grains decrease, which finally results in lower residual voltages of varistor samples. However, when most of Zn2+ vacancies inside the ZnO crystal lattice are filled with Al3+, redundant Al3+ ions inside ZnO grains will increase the grains’ resistances contrarily. In this paper, ZnO and Al(NO3)3 binary ceramics sintered at 1300 °C for 2, 4 and 8 hours were studied. The J-E curves of the investigated samples exhibit weak non-linear characteristics which may be due to the formation of grain boundary. Moreover, The resistivity dependence of ZnO ceramics on Al addition behaves as a U-type curve and the lowest resistivity of ZnO ceramics is obtained with doping 0.25mol% Al3+.

Abstract: The preparation and characterization of ZnO-Pr6O11-Co3O4-TiO2 (ZPCT) based varistor materials with different doping levels of TiO2 and Pr6O11 were investigated. The results reveal that: (1) TiO2 is an important additive, acting as an inhibitor of ZnO grain growth. The doping of appropriate amount of TiO2 can significantly improve the nonlinear properties and decreases the leakage current of the varistors, achieving a relatively high nonlinear exponent and low leakage current with 1.0 mol% TiO2 doped. (2) The oxide of Pr6O11 microstructurally plays the role of inhibition in grain growth. The doping of appropriate amount of Pr6O11 can improve the nonlinear property, and decrease the leakage currents of the varistors, acquiring the optimum results with 1.5 mol% Pr6O11 doped.

Abstract: The additive of Al(NO3)3 has been doped into ZnO varistors in order to reduce their residual voltages. However, the leakage currents of samples always increase at the same time. Generally, it is recognized that some of doped Al3+ ions enter the ZnO grains and reduce their resistivity, which results in lower residual voltages of varistor samples. On the other hand, the remnant Al3+ ions appear in the grain boundaries and also reduce their resistivity, which results in larger leakage currents. In this paper, the electrical properties of ZnO varistor samples with various amounts of Al(NO3)3 dopant were measured. The experimental data are compared with the numerical simulation results, which reveals that the increased leakage currents of ZnO varistors with Al(NO3)3 dopant are not only due to the decreased resistivity of grain boundaries, but also the increased donor density of ZnO grains.