Papers by Keyword: Varistors

Abstract: (1-x)SnO₂-xZn₂SnO₄ composite ceramics were prepared by traditional ceramic processing and the varistor, dielectric properties were investigated. With increasing Zn₂SnO₄ content, the breakdown electrical field E_B and nonlinear coefficient α reaches the minimum of 6.9 V/mm and 2.5 at x=0.206, respectively. In the dielectric spectra, the relative dielectric constant ε_r exhibits strong frequency dependent character and at 40 Hz, ε_r for the sample of x=0.206 reaches a maximum as high as 3×10⁴. In the frequency region lower than 1 kHz, accompanied by the sharp increase of dielectric loss at 40 Hz, ε_r is depressed and a dielectric peak is presented in the spectra with increasing bias voltage. In the low electrical current range of 1.37-20 μA, The barrier height φ_B about 1.0 eV are obtained and it is found that φ_B decreases with increasing measuring current for each sample. Based on the results, the varistor behavior with high dielectric constant is explained by the Schottky barriers at grain boundaries.

Abstract: The addition of different dopants affects the densification and electrical properties of TiO₂ based varistor ceramics. The nonlinear current (I) and voltage (V) characteristics of titanium dioxide are examined when doped with small quantities (0.5-2 at.%) of strontium oxide. This paper discusses the electrical properties of such an SrO doped TiO₂ system, and demonstrates that some combinations produce electrical properties suitable for use as low voltage varistors. The high value of the nonlinear coefficient (α) (6.6), the breakdown field strength (Eb) (328 V/cm) and the leakage current (Ir) (0.22 mA/cm²) obtained in a system newly doped with SrO, are all adequate properties for application in low voltage varistors.

Abstract: Multilayer ZnO varistors were prepared by aqueous gel tape casting with water-soluble acrylamide as binder. 0.8wt% PAA dispersant was found to be the optimum concentration needed to prepare stable slurry. Plasticizer glycerol has a positive effect on the fluidity of the suspension and oxygen anti-polymerizing inhibitor PEG 2000 deteriorated the fluidity. The addition of 15wt. % PEG2000 eliminates the surface exfoliation absolutely due to the oxygenation of ether units. The solid loading of the slurry was about 71wt% compared to the custom acrylic formulation binder 60wt%. The multilayer ZnO varistors prepared by aqueous gel tape casting display comparable good electrical properties to those prepared by water-based tape casting using custom acrylic formulation binder which is attributed to the high solid loading of slurry.

Abstract: SnO₂-based varistors were successfully fabricated from the mixed powders, SnO₂, Co₂O₃, Nb₂O₅ and Cr₂O₃. The effects of sintering temperature (1250, 1300, 1350 and 1400 °C) on the microstructure and electrical properties were investigated. The results reveal that the grain size increases with increasing the sintering temperature, and the breakdown electrical filed decreases gradually. When the sintering temperature was 1300 °C, the nonlinear coefficient of the as fabricated SnO₂ based varistors presents the maximum of 27. Meanwhile, the leakage current possesses the minimum of 4.5 µA.

Abstract: The microstructure and electrical properties of ZnO-Bi2O3-based varistor ceramics prepared by the high-energy ball milling were studied. The varistor ceramics samples were characterized by XRD and SEM analysis, as well as by dc electrical measurements, such as the nonlinearity coefficients, leakage current and threshold voltage. The best electrical characteristics were found in sample by the high-energy ball milling 1 h and sintered at 1000 °C for 2 h, which exhibited the threshold voltage was 457 V/mm, the nonlinear coefficient was 59.3 and the leakage current was 1.18 μA.

Abstract: ZnO varistors were synthesized with ZnO powders and intensity milled composite additives as raw materials. The effect of milling time of the composite additives on the grain boundary phase distribution as well as the electrical properties was investigated in detail. The results show that ball milling composite additives have a significant effect on the ZnO varistors. The samples derived from ball milled composite additives possess the smaller size and more uniform distribution of the second phases, which improved the electrical properties obviously. The optimal ZnO varistor samples were obtained by ball milling 15 h for the composite additives, which possess average ZnO crystalline grain size of about 4 µm, the gradient voltage V_1mA of 454 V/mm, the leakage current I_L of 0.12 µA, and the nonlinear coefficient α of 55.

Abstract: TiO₂-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 Er₂TiO₃ on the surface of TiO₂ grains. The grain size was found to decrease with increasing Er₂O₃ content. The addition of rare earth oxide Er₂O₃ leads to increase the nonlinear coefficient and the breakdown voltage. It was found that the nonlinear coefficient presents a peak of α = 4.5 for the sample doped with 1.1 mol% Er₂O₃, which isconsistent with the highest grain boundary in the composition. In order to illustrate the role of grain boundary barriers for TiO₂-Ta₂O₅-Er₂O₃ varistors, a grian boundary defect barrier model was introduced.

Abstract: TiO2 varistors doped with 0.1 mol% Ta and different concentrations of CeO2 were obtained by ceramic sintering processing at 1400 °C. The effect of CeO2 on the nonlinear electrical behavior and dielectric properties of the Ta2O5-doped TiO2 ceramics were investigated. The nonlinear current (I)-voltage (V) characteristics of TiO2 are examined when doped with small quantities (0.1-0.9 mol%) of CeO2. It is found that CeO2 affects the electrical properties and the dielectric properties of the TiO2-based varistors. The samples have the nonlinear coefficients (α) values of (3.0-5.0), breakdown voltages (10-30 V/mm) and ultrahigh dielectric constants which is up to 105. A small quantities of CeO2 can improve the nonlinear properties of the samples significantly. It was found that an optimal doping composition of 99.4 mol% TiO2 - 0.1 mol% Ta2O5 - 0.30 mol% CeO2 was obtained with low breakdown voltage of 14.2 V/mm, high nonlinear constant of 4.5 , an ultrahigh electrical permittivity of 8.381.22×105 (measured at 1 kHz) and low tanδ of 0.32, which is consistent with the highest grain boundary barriers of the ceramics. The theory of defects in the crystal lattice was introduced to explain the nonlinear electrical behavior of the CeO2-doped TiO2-based varistor ceramics.

Abstract: Grain growth in ZnO ceramics doped with 0.01 and 0.02 mol.% Bi2O3 and Sb2O3 in amounts appropriate for Sb2O3/Bi2O3 ratios of 0.8, 1.0 and 1.2, sintered at 1200oC for 2 and 10 hours, was investigated. Grain growth is promoted by a sufficient amount of the Bi2O3 liquid phase at the grain boundaries and also by the presence of IBs in the ZnO grains. While the doping of ZnO with such small amounts of Bi2O3 caused the exaggerated growth of some grains, the addition of Sb2O3 resulted, via the IBs-induced grain-growth mechanism, in uniform grain growth and the presence of IBs in most of the ZnO grains. The formation of the pyrochlore phase bounds the Bi2O3 and Sb2O3, which affects, depending on the Sb2O3/Bi2O3 ratio, the occurrence of the Bi2O3 liquid phase and also the amount of available Sb2O3 for the nucleation of IBs in the ZnO grains during the early stages of sintering. As a result, it influences the grain growth.

Abstract: The influence of La2O3 and CeO2 on a new class of polystalline ceramics with electrical properties based on TiO2 was investigated. The content of added La and Ce is in 1 at%, while that of the other addition is always maintained at a constant value. The disks were sintered at 13800C for 4 hours. La2O3 was found to precipitate at the grain boundaries, probably inducing electronic interface states that can trap charges at the TiO2-TiO2 interface and lead to significantly increase the nonlinear behavior. The values for α (» 4.2) and V1mA(» 26 V) support that finding. It was found CeO2 that as a dopant did not strongly influence the nonlinear values of the systems as did La2O3, indicating that it could have formed a solid-state solution with TiO2 in the grains, but did not segregate sufficiently at the grain boundaries. In present work, impedance analyzer, X-ray spectrometer, and scanning electron microscope (SEM) were used.