Electrical Characteristics Of ZnO Varistors Research Articles

ZnO varistors with low leakage current and high stability arrester with Ga doping

The insulation level of power equipment in power system is based on the overvoltage protection level of metal oxide arrester represented by zinc oxide valve blade. Owing to its superior nonlinear voltage current characteristics and surge energy absorption capacity, ZnO varistor is widely used as the core component of power system arrester. The electrical characteristics of ZnO varistors are determined by their complex microstructures and grain boundary characteristics. Therefore, to further improve the insulation level of power grid equipment, doping is required to further improve the grain boundary characteristics of ZnO varistors. In order to obtain more stable ZnO varistors, the stability characteristics of Ga doped ZnO varistors are investigated. The microstructural and electrical characteristics of the obtained experimental samples are tested by scanning electron microscope, voltage current nonlinear characteristics, capacitance voltage characteristics, X-ray diffraction spectrum, energy spectrum scanning, dielectric loss, and AC acceleration aging. The experimental results show that with the further increase of gallium doping, gallium ions occupy the vacancies on the zinc oxide lattice, increasing the interface state density, and improving the Schottky barrier height. On the one hand, the leakage current density of ZnO varistor is reduced, on the other hand, the migration of free electrons in the depletion layer is suppressed, and the stability of ZnO varistor in the high charge rate environment is improved. Aluminum ions are dissolved into the ZnO lattice to generate a large number of free electrons, thereby reducing the resistivity of ZnO grains, which can effectively reduce the residual voltage ratio of ZnO varistor when large current passes through it. When the doping amount of Ga reaches 0.6%, the leakage current is 0.84 μA/cm<sup>2</sup>, the residual voltage ratio is 1.97, the nonlinear coefficient is 66, and the Schottky barrier height is 1.81 eV. At 115 ℃, AC accelerated aging voltages of 87% <i>E</i><sub>1 mA</sub>, 89% <i>E</i><sub>1 mA</sub> and 91% <i>E</i><sub>1 mA</sub> are applied to the test sample separately. The aging time is 1000 h, and the aging coefficients are 0.394, 0.437 and 0.550 separately. This research will help to further improve the protection level of zinc oxide surge arresters, achieving the deep limitation of grid overvoltage, and improving the security and stability of power systems.

Influence of Co2O3 doping on the microstructure and electrical properties of ZnO–Bi2O3-based varistors

In this work, the microstructure (for instance grain size and elemental distribution) and comprehensive electrical performances of ZnO–Bi2O3–Cr2O3–MnO2 varistors, with various amounts of Co2O3 prepared via the traditional solid-state sintering method were investigated. The influences of Co2O3 doping on the microstructure of the ZnO varistors were evaluated via X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectrometer (EDS), while the effects on the electrical performance of the ZnO varistors were analyzed via voltage source meter and inductance-capacitance-resistance meter. The experimental results indicate that Co2O3 doping can contribute to improve the electrical characteristics of ZnO varistors. The double Schottky barrier height φb and nonlinearity coefficient α was enhanced significantly. Meanwhile, Co2O3 doping can effectively reduce ZnO grain resistivity ρg and increase grain boundary resistivity ρgb. In this research, there was an optimal content of Co2O3 (0.5 mol%), the α of the ZnO varistors reaches the maximum value of 46.7 and the φb reaches the maximum value of 0.63 eV with the optimal content.

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The safe operation of power equipment largely depends on the overvoltage protection level of the arrester. The ZnO varistors are widely used as the core components of the arresters in power systems because of the excellent nonlinear volt-ampere characteristics. In order to study the electrical properties of ZnO varistors under different external electric fields from the microstructure, the method of first-principles based on density functional theory (DFT) is used, and structure of ZnO/<i>β</i>-Bi<sub>2</sub>O<sub>3</sub> interface containing zinc interstitial (Zn<sub>i</sub>) and oxygen vacancy (V<sub>o</sub>) defects is built. The results show that the V<sub>o</sub> defect migrates after full relaxation. The Zn<sub>i</sub> shifts to the interface under an external electric field. The interface energy increases rapidly after the electric field intensity has exceeded 0.1 V/Å, which means that the interaction force between the interfaces becomes larger, the distance between ZnO and <i>β</i>-Bi<sub>2</sub>O<sub>3</sub> layers decreases, and the conductivity increases rapidly. The differential charge density, work function and Bader charge analysis method are used to calculate the barrier height at the interface, which proves that the built-in electric field is an important cause ingredient responsible for the non-linear volt-ampere characteristics of ZnO varistors. The effects of atomic orbital energy level, trap energy level and energy gap on the macroscopic conductivity of ZnO varistors are analyzed by using the method of density of states analysis. In this work are analyzed the different electrical parameters of the ZnO/<i>β</i>-Bi<sub>2</sub>O<sub>3</sub> interface with aggregation defects by adjusting the intensity of the external electric field, and a new idea is provided for learning the electrical characteristics of ZnO varistors.

Grain‐Boundary and Thermally Stimulated Current Characteristics of Y2O3‐Doped ZnO Varistor

The doping of rare‐earth oxides can greatly improve the electrical characteristics of ZnO varistors. Thermally stimulated current (TSC) characteristic test, capacitance voltage (C–V) characteristic test, scanning electron microscope (SEM) test, and voltage current (V–I) test were carried out to study the influence of Y2O3 content on the electrical properties of ZnO varistors in this study. The results show that the grain size decreases while the voltage gradient increases as the Y2O3 content is increased. The reaction of Y2O3 with other additives leads to the decrease in grain‐boundary defects, which accounts for the decrement of barrier height, donor density, and surface state density. The trap level and trapped charge of ZnO varistors decrease as the Y2O3 content is increased from 0.3 to 0.9 mol%, which means the shallow traps inside ZnO varistors reduce, and the Y2O3 additive can greatly improve the TSC characteristic of ZnO varistors.

The Effects of Cu Dopant on the Microstructure and Non-Ohmic Electrical Properties of ZnO Varistors

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.

Effect of Simultaneous Addition of Zr and Y on Electrical Characteristics of ZnO Varistors

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.

산화아연소자의 성형공정에 따른 전기적 특성과 성능평가

This paper presents the electrical characteristics with manufacturing process and performance evaluation of high performance zinc oxide varistors. ZnO varistors were fabricated with typical ceramic production methods with different thickness and the structural and electrical characteristics of ZnO varistors were investigated. All varistors exhibited high density, which was in the range of . In the electrical properties, the reference voltage increased in the range of with increasing their thickness and the residual voltage exhibited the same trends as the reference voltage. In the long duration current impulse withstand test, E-2 and F-1 samples failed at the two and four shots of impulse current, respectively, but E-1 and F-2 samples survived 18 shots during the test. Before and after this test, the variation ratio of residual voltage of E-1 and F-2 samples were -0.34 % and 0.05 %, respectively, which were in the acceptance range of 5 %. According to the results of tests, it is thought that if the fabrication process such as insulating coating, sintering condition, and soldering method is improved, these ZnO varistors would be possible to apply to the station class arresters in the near future.

The Formation of Defect Complexes in a ZnO Grain Boundary

The microscopic properties a ZnO grain boundary containing extrinsic point defects are studied using a density functional computational approach. The results show that the grain boundary acts as a sink for native defects, such as the zinc vacancy and the oxygen interstitial, and also for bismuth substitutional impurities. The defects tend to accumulate at under-coordinated sites in the boundary core and prefer to form small clusters. In particular the segregation of Bi promotes the formation of the other native defects by lowering their formation energies in the boundary. Individually, the native defects and the Bi impurity do not produce deep interface states in the band gap which are electrically active. However, when the defects cluster to form a BiZn-VZn-Oi complex, new gap states are created of acceptor type. It is suggested that these new states are caused by defect interactions which compensate one another resulting in the depletion of an occupied impurity state and new bond formation. The results are discussed in terms of the Schottky barrier model commonly used to describe the electrical characteristics of ZnO varistors.

Electrical properties of ZnO-based varistors prepared by combustion synthesis

Ceramic varistors are generally produced by the oxide mixing method or by chemical methods, such as sol–gel, precipitation and others. Chemical methods produce powders that are highly reactive, allowing for increased microstructural homogeneity and control of grain growth during sintering, which is essential for good varistor performance. The purpose of this work was to study the electrical characteristics of ZnO varistors produced from stoichiometric mixtures of water-soluble metal nitrates, such as precursor cations and urea as a fuel. This method, called combustion synthesis, stands out for its simplicity (a single-step reaction), purity, chemical homogeneity, and the high reactivity of the precursor powder. After sintering at 1050 °C using this method, varistors with a non-linear coefficient of 40 and the lowest leakage were obtained.

Effects of processing and valence states of metal oxides on electrical properties of ZnO varistors

The electrical characteristics of ZnO varistors with various valence states of manganese and cobalt were studied. The electrical properties of varistors are discussed in terms of defects produced by the additions. Compositions with lower trap densities have the characteristics of higher non-linear coefficient, higher voltage ratio and lower leakage current. The electrical properties are improved by increasing the sintering temperature and time due to a better tunnelling effect. It is also found that the depression angle and relaxation time decrease as a function of increasing sintering time, and the depression angle decreases and relaxation time increases with increasing sintering temperature. This indicates an enhancement in the degree of uniformity in trapping conductance and the trap emission/capture rate when the sintering time increases. The dependence of the depression angle and relaxation time on the composition are also presented.

The Effects of Valence States of Manganese on Electrical Properties of ZnO Varistors

The electrical characteristics of ZnO varistors with various valence states of manganese were studied. The composition containing low valence state of manganese exhibits low donor concentration, trap density and barrier height. The compositions with lower trap density have the characteristics of higher nonlinear coefficient, higher voltage ratio and lower leakage current. The electrical properties were improved by increasing the sintering temperature and time. It is also found that the depression angle decreases as a function of the increasing sintering time and temperature. The relaxation time for τ3 decreases with increasing sintering time. It indicates an enhancement in the degree of uniformity in trapping conductance and the trap emission/capture rate when sintering time increases. The dependence of the depression angle and relaxation time on the composition have been also presented. On the other hand, the dependence between the electrical properties and wetting behavior of intergranular phase is investigated.

THEORY OF CONDUCTION OF NONLINEAR ZnO CERAMICS (II)——APPLICATION OF MODEL OF CONDUCTION

The model of conduction proposed in the previous paper is treated quantitatively. This model is suitable for the explanation of V-I curve, C-V(DG) curve, and degradation phenomena of ZnO varistors. In addition, this model can also predict the influence of the microstrueture parameter and the elec trical characteristics parameter of grain-grain contact on the electrical characteristics of ZnO varistors. It provides a basis of the theory for materials design of ZnO varistors.