Papers by Keyword: ZnO Varistor

Abstract: The SnO₂ doping can increase the nonlinear coefficients of ZnO varistors, the highest α value is achieved at the 0.75 wt% doping level samples sintered at 1150^oC. The improved I-V nonlinearity may be attributed to the donor behavior of Sn⁴⁺ ions. The SnO₂ doping can enhance the ZnO grain growth that lower the breakdown field of doped samples.

Abstract: ZnO-Pr₆O₁₁-Co₃O₄-Cr₂O₃-SnO₂ varistors with different doping levels of Pr₆O₁₁ (0.25-2 mol%) were prepared at 1300 °C with conventional ceramic processing, and the effect of Pr₆O₁₁ doping on the microstructure and electrical properties of the varistor materials were investigated. The results indicated that the doped Pr₆O₁₁ basically existed at the boundary of ZnO grains in the varistor ceramics, and SnO₂ might enter into the lattice of ZnO grains or precipitate in reaction with Pr₆O₁₁ into Pr₂Sn₂O₇ at the gain boundaries particularly where there were three or more ZnO grains. The analysis of scanning electron microscopy further revealed that Pr₆O₁₁ doping would inhibit the growth of ZnO grains, resulting in decreasing ZnO grain size with increasing doping level of Pr₆O₁₁. The measured electric-field/current-density characteristics of the samples showed that the varistor voltage increased with increasing doping level of Pr₆O₁₁ when the doping level was no more than 1.5 mol%, and the nonlinear coefficient of the varistors increased with increasing doping level of Pr₆O₁₁ up to no more than 1.0 mol% in the varistors, respectively.

Abstract: The Cu₂O addition deteriorates the electrical properties of ZnO varistors, which is a good agreement with similar findings on Ag₂O additions. The best fitted impedance analysis reveals that the Cu₂O addition increases grain resistance and lowers the grain boundary resistance, results in low nonlinearity and higher leakage current of ZnO varistors.

Abstract: Thermally stimulated current (TSC) is a simple and effective test technique to study the thermal activated charge, electron trap and activation energy of dielectric and semiconducting materials. It is well known that the addition of glass frits can improve the degradation property of ZnO varistors. An activation energy of 0.45 eV has been found on the ZnO varistors with/without glass frits after the DC degradation by using TSC method. The tested quantity of thermal activated charges, Q_TSC, may represent the numbers of migrated zinc interstitials. ZnO visitors doped with glass frits have less Q_TSC. Experiment indicates that TSC test is a convenient method to evaluate the degradation behavior of ZnO varistors.

Abstract: The effect of WO₃ doping on microstructural and electrical properties of ZnO-Pr₆O₁₁ based varistor materials was investigated. The doped WO₃ plays a role of inhibitor in ZnO grain growth, resulting in decreased average grain size from 2.68 to 1.68 μm with increasing doping level of WO₃ from 0 to 0.5 mol%. When the doping level of WO₃ was lower than 0.05 mol%, the nonlinear current-voltage characteristics of the obtained varistors could be improved significantly with increasing amount of WO₃ doped. But when the doping level of WO₃ became higher, their nonlinear current-voltage performance would be dramatically deteriorated when more WO₃ was doped. The optimum nonlinear coefficient, varistor voltage, and leakage current of the samples were about 13.71, 710 V/mm and 13 μA/cm², respectively, when the doping level of WO₃ was in the range from 0.03 to 0.05 mol%.

Abstract: The grain-boundary and its electrical characteristics in SrCoO₃doped ZnO varistors were studied. The grain-boundary around ZnO-grain is probably composed of SrCoO₃, and its electrical behavior is clearly different from two conventional types of Bi-and Pr-based ZnO varitors. The non-linearity and characteristic behavior could be explained by considering the n-p-n hetero-structure at the grain boundary. SrCoO₃ in the grain-boundary region should play crucial roles of not only the appearance of non-liner property but also the formation of different hetero-structure from double Schottky-barrier model on conventional varistors.

Abstract: The effects of the addition of tin oxide (SnO₂) and yttrium oxide (Y₂O₃) to bismuth (Bi)-manganese (Mn)-cobalt (Co)-silicon (Si)-chromium (Cr)-nickel (Ni)-added zinc oxide (ZnO) varistors (a basic varistor) on the varistor voltage, resistance to electrical degradation, and leakage current were investigated. The addition of SnO₂ increased both the varistor voltage and the resistance to electrical degradation. However, simultaneous addition of both SnO₂ and Y₂O₃ increased the varistor voltage but the resistance to electrical degradation deteriorated. ZnO varistors with varistor voltage over approximately 520 V/mm, excellent resistance to electrical degradation, and low leakage current could be obtained by adding SnO₂ with SnO₂-to-ZnO molar ratio of approximately 1:10 to the basic varistor.

Abstract: The microstructural and electrical properties of ZnO-Pr₆O₁₁-based ceramics fabricated with 97.5mol% ZnO + 0.5mol% Pr₆O₁₁ + 1.0mol% Co₃O₄ + 0.5mol% Cr₂O₃ + 0.5mol% MNO₃ (M=Li, Na, K, or Rb) were investigated. Scanning electron microscopy analysis revealed that the doping of alkali ions would inhibit the growth of ZnO grains in the as-prepared ZnO-Pr₆O₁₁-based ceramics, in which the addition of K⁺ ion showed the strongest effect. Through the analysis of electric field vs current density characteristics, it was found that the doping of Li⁺ ion into ZnO-Pr₆O₁₁-based ceramics would change the varistor into ohmic resistor, but the addition of Na⁺, K⁺ or Rb⁺ ion could improve the nonlinearity of the varistors, in which the addition of K⁺ ion resulted in the strongest improving effect.

Abstract: t is well known that nanostructured materials have relevant influences in properties behavior that can be achieved when compared with conventional materials. In this study is proposed an investigation of the electrical and microstructural properties of zinc oxide based varistors prepared with nanostructured zinc oxide powder obtained by a thermal evaporation process. Zinc oxide powder morphology was investigated by scanning and transmission electron microscopy (SEM and TEM, respectively) and the specific surface area evaluated by adsorption of N₂. The varistors were prepared by the mixture of typical dopants with zinc oxide powders in a ball mill. The surface area of zinc oxide powder used was 17.4 m²/g with tetra-needle like morphology. After powder mixture process it was observed by TEM micrographs that most of the tetrapod shaped zinc oxide broke into needles well mixed with dopant particles. The compressed powders were sintering at 1050, 1150 and 1250°C for 1.5 h and densification over 94% were achieved in all tested temperatures. Preliminary electrical characterization reveals that nanostructured zinc oxide compositions have interesting varistor properties.

Abstract: A novel fabricated technique, by feeding two sets of different ZnO formulations powder in a die by parts, molded only once to produce layered structure(including layer A and layer B) low-voltage ZnO varistor. The samples are examined by using energy dispersive X-ray spectroscopy (EDS), electron probe microanalysis (EPMA), scanning electron microscope (SEM) and DC electrical measurements. EDS and EPMA data indicate that doped elements only exists in layer A, The results of SEM indicate that secondary phases are formed at grain boundaries in layer A, not found in layer B. It is found that the electrical properties of low-voltage varistor are improved without reducing thickness and changing energy absorption capabilities. The higher nonlinearity coefficients, lower breakdown fields and leakage currents of layered structure low-voltage ZnO varistor, as compared to those of ZnO varistor fabricated from the conventional route. The improved current-voltage properties are attributed to the band structure difference in both sides grains, due to the different ion concentration and species in both sides of grain boundary. Layered structure varistor also has more simpler prepared technology than multilayer chip varistor.