Zinc Oxide Varistors Research Articles

Optimization of technological processes in the manufacturability of varistors based on recycled ZnO product, with emphasis on environmental sustainability

The main objective described in this paper is the optimization of technological parameters in the production of varistors based on recycled zinc oxide (ZnO). The content of this paper builds on our previous research aimed at proving that hydrometallurgically recovered ZnO material, from electric arc furnace dust, is suitable for applications in these semiconductors, an issue which has received very little attention at present. The focus of this research also corresponds to and supports the visions and strategies of environmental sustainability and the circular economy. Samples of ZnO varistors manufactured for this purpose were analyzed by scanning electron microscopy and then the electrical parameters were measured and calculated. Based on the results of the microstructural analysis and electrical properties, prerequisites for the optimal adjustment of technological parameters such as sintering temperature and sintering time were derived, which will be the starting point for further research.

Effect of Co-Doping of Al3+, In3+, and Y3+ on the Electrical Properties of Zinc Oxide Varistors under Pre-Synthesizing BiSbO4.

Under the premise of using the solid-phase method to pre-sinter Bi2O3 and Sb2O3 into BiSbO4 as a substitute for equal amounts of Bi2O3 and Sb2O3 in the formula, the effects of co-doping with In(NO3)3, Al(NO3)3, and Y(NO3)3 on the microstructure and electrical properties of ZnO varistors were studied. The experimental results show that with an increase in In3+-doped molar concentration, the leakage current of the ZnO varistor shows a rapid decrease and then a slow increase trend. However, the nonlinear coefficient is the opposite of it. With the combined effect of the rare earth element Y3+, the average grain size is significantly reduced, which leads to an increase in the voltage gradient. At the same time, a certain amount of doped In3+ and Al3+ is dissolved into the grains, resulting in a decrease in grain resistance and thus a low level of residual voltage. The varistor with 0.6 mol% In3+, 0.1 mol% Al3+, and 0.9 mol% Y3+ doping ratios exhibits excellent overall performance. The nonlinear coefficient is 62.2, with the leakage current being 1.46 µA/cm2 and the voltage gradient being 558 V/mm, and the residual voltage ratio is 1.73. The prepared co-doped ZnO varistors will provide better protection for metal oxide surge arresters.

Electrothermal characteristics of zinc oxide varistors with different aging states after multiple lightning strikes based on Voronoi network finite element model

Multiple lightning strikes significantly degrade the electrical and thermal properties of zinc oxide (ZnO) varistors, affecting their protection capability. In order to quantitatively analyze the hazard of multiple lightning strikes on ZnO varistors, simulations at the microscopic level are required to account for the changes in macroscopic parameters. Unlike other published research, this paper presents an analytical approach to simulate the micro-level performance of ZnO varistors in different aging states. The details of the model construction, experimental platform, and computational results are thoroughly investigated. First, a Voronoi network finite element model for ZnO varistors is developed, and the required model parameters are presented. Next, an experimental testing platform is used to physically simulate multiple lightning strikes and perform parametric testing. Finally, the microscopic electrical and thermal properties of ZnO varistors under different aging states and pulse numbers are analyzed and compared. The results show that with an equivalent amount of energy per impact, triple-strike aging causes severe degradation for ZnO varistors. Compared to single-strike aging, triple-impact aging results in a 15.21 times higher quality coefficient and a 7.67 times higher 10 min temperature rise ratio.

The influence of conventional sintering on the microstructure and electrical properties of ZnO-based varistor

Zinc oxide varistors are frequently utilized as surge protection devices in power systems. ZnO based varistors' electrical properties depend on their microstructure, which may be altered by sintering. In this paper, we look at how conventional sintering affects the microstructure and electrical characteristics of ZnO-semiconductor varistors. ZnO-based varistors were made using a traditional solid-state reaction process and sintered for 2 hours at varying temperatures (1100 °C, 1200 °C, and 1300 °C). Scanning electron microscopy and X-ray diffraction were used to examine the microstructure and phase composition. A conventional test procedure was used to measure the electrical parameters of the varistors, such as the breakdown voltage, leakage current, and nonlinear coefficient. Traditional sintering has a considerable impact on microstructure and electrical properties.

Short duration current impulse waveform effects on the degradation and energy absorption capability of zinc oxide varistors

Short duration current impulse waveform effects on the degradation and energy absorption capability of zinc oxide varistors

Study of the behavior of low voltage ZnO varistors against very fast transient overvoltages (VFTO)

• Complete VFTO(s) generation and measurement system based on (MPC) and (TLT) technologies. • Five commercial ZnO varistors of the same reference were used in the characterization. • The influence of the applied overvoltage front time on the capacitive effect of the varistor is confirmed. To study the behavior of low voltage ZnO varistors against Very Fast Transient Overvoltage (VFTO), the authors have developed a complete VFTO(s) generation and measurement system based on magnetic pulse compression (MPC) and Transmission Line Transformer (TLT). This system has an output voltage of 24 kV, rise time of 32 ns, and a bandwidth of 10 MHz. The measurement system comprises a damped resistive voltage divider with an impedance of 1000 MΩ and a maximum bandwidth of 10 MHz. The experiment was performed with five commercial zinc oxide varistors with a nominal voltage of 150 V, each one was subjected to VFTO-type voltage pulses and, by Analysis of Variance (ANOVA), a single varistor was selected to perform the case study. This work presents an experimental methodology to observe the response of low voltage ZnO varistors against VFTO. This response is the delay of the residual voltage to the applied discharge current and indicates the poor performance of these devices against VFTOs. Our methodological proposal consists of a series of laboratory tests that will allow manufacturers and consumers to take corrective measures in case of malfunction, damage of the varistor material, or permanent degradation of the device.

The electrical model and harmonic characteristics of ZnO varistor considering unipoilar impulse aging polarity effect

The frequent and high amplitude unipolar impulse currents during the process of switching-on AC filter banks can accelerate the aging of arresters in some ultrahigh-voltage direct-current (UHVDC) and HVDC transmission systems, and result in polarity effect of zinc-oxide (ZnO) varistors. Hence, an improved ZnO varistor electrical model considering the polarity effect caused by unipolar impulse aging was proposed under power frequency voltage. Accelerated aging test under a negative polarity 15/35 µs impulse current was conducted on ZnO varistor samples to validate the proposed model. Furthermore, the current waveform and harmonic characteristics of the leakage current and its resistive component were simulated. Comparative analysis revealed that the simulation results were generally consistent with the test results. Based on the results, it's inferred that the variation of even-order harmonics are attributed to the polarity effect, and can be considered as one of the aging criterion of arresters.

Effects of Dispersant on Electrical Properties of Zinc-Oxide Varistors and Methods to Improve Protection Performance

Effects of Dispersant on Electrical Properties of Zinc-Oxide Varistors and Methods to Improve Protection Performance

Synthesis and characterization of ZnO based varistor ceramics: effect of sintering temperatures

PurposeThe purpose of this paper is to investigate the effects of the sintering temperature on the microstructural, morphological and electrical characteristics of Zinc oxide (ZnO)-based varistors.Design/methodology/approachThis study used a conventional method to design and produce ZnO varistors by sintering ZnO powder with small amounts of various metal oxides. Furthermore, the effect of sintering temperature on varistor properties of (Bi, Co, Cr, Sb, Mn)-doped ZnO ceramics was investigated in the range of 1280–1350 °C.FindingsThe obtained results showed an EB value of 2109.79 V/cm, a Vgb value of 0.831 V and a nonlinear coefficient (α) value of 19.91 for sample sintered at temperature of 1300 °C. In addition, the low value of tan δ at low frequency range confirmed that the grain boundaries created in 1300 °C sintering temperature were obviously good.Originality/valueBased on the previous research on the ZnO-based varistors, a thorough study was carried out on these components to improve their electrical characteristics. Thus, it is necessary that those varistors have low leakage current and low value of dissipation factor to ensure their good quality. High breakdown fields and nonlinearity coefficients are also required in such kind of components. The effect of sintering temperature on the varistor properties of the new compositions (zinc, bismuth, manganese, chrome, cobalt, antimony and silicon oxides)-doped ZnO ceramics was studied in the range of 1280–1350 °C. Also, the microstructure and the phase evolution of the samples sintered at various temperatures (1280 °C, 1300 °C, 1320 °C and 1350 °C) were investigated according to X-ray diffraction and scanning electron microscope measurements.

Gradient structure design of zinc oxide varistor microsphere composites for efficient electric field grading

Both the functionally graded material and the nonlinear conductive material can be used for grading electric fields. If a nonlinear conductive material with a gradient distribution can be designed according to the electric field distribution characteristics, better control of the electric field distribution will be achieved. In this paper, we realize switching field graded design of zinc oxide (ZnO) varistor microsphere composites in an epoxy resin matrix. Chopped glass fibers were added to a ZnO varistor microsphere/epoxy binary system to form a three-dimensional network. The volume fraction of glass fibers was altered to modify the density of the three-dimensional networks. Under the influence of gravity, the three-dimensional network composed of glass fiber realized layer-by-layer inhibition of the sedimentation of ZnO varistor microspheres, thereby creating a gradient distribution of the ZnO varistor microspheres. Thus, we realized gradient distribution design of the adaptive composites composed ZnO varistor microsphere.

Effects of boron oxide addition on electrical properties of yttrium-doped bismuth-based zinc oxide varistors

The optimal amount of B2O3 (0.75 mol%) drastically improved the electrical properties of Y-doped varistors, reducing the leakage current to 0.8 μA/cm2 while maintaining exceptionally long-term stability against DC electrical degradation and a varistor voltage around 900 V/mm. Boron ions were found with Bi ions in Y-compounds, which contributed to formation of a Bi-rich glass phase containing B at grain boundaries when the B2O3 content was above 0.75 mol%. The change in microstructure affected the donor density and barrier structure, leading to a change in electrical properties. The Bi-rich phase first reduced the Co concentration in the ZnO grains, thereby decreasing the donor density ND. When large amount of B2O3 was added, some Co remained in ZnO grains, causing an increase in ND. A new analytical method using the differential resistance RD for the nonlinear voltage–current density relation was proposed to clarify the effects of B2O3 addition. This method separates the three differential resistance RDi values (i = 1, 2, 3) and was used to analyze the corresponding conduction process in forward-biased region I, reverse-biased region III, and interface region II of the energy band. The RD1 and RD2 values exhibited junction diode-like behavior in regions I and III. The value of RD2 that correlated with αasMAX and the long-term stability of the varistor and RD3 may be related to region II.

Comparative Characterization of Different Cutting Strategies for the Sintered ZnO Electroceramics

Sintered zinc oxide (ZnO) ceramic is a fragile and difficult-to-cut material, so finishing operations demand handling cautious and accurate surface tolerances by polishing, grinding, or machining. The conventional machining methods based on grinding and lapping offer limited productivity and high scalability; therefore, their incapacity to prepare tight tolerances usually end up with uncontrolled edge chipping and rough surfaces in the final products. This study investigates microstructural features with surface roughness in a comparative mode for conventional milling and abrasive waterjet cutting (AWJ). Edge topography and roughness maps are presented in this study to weigh the benefits of AWJ cutting over the conventional material removal methods by altering the feed rates. The porosity analysis implies that the differences during the multi-channel processing of varistors, which tend to alter the microstructure, should in turn exhibit a different response during cutting. The surface roughness, edge contours, and porosity generation due to shear forces are interpreted with the help of 3D optical and electron microscopy. The results demonstrate that the surface microstructure can have a noteworthy impact on the machining/cutting characteristics and functionality, and in addition, mechanical properties of ZnO varistors can fluctuate with non-uniform microstructures.

Evaluation of Chip Formation Mechanisms in the Turning of Sintered ZnO Electro-Ceramics

The sintered zinc oxide (ZnO) electro-ceramics are a brittle class of hard-to-cut materials such that shaping them with the post-finishing operations necessitates careful handling and precision machining. The conventional machining approach using the grinding and lapping processes represents limited productivity, an inability to produce the required geometries and frequent uncontrolled chipping of the edges of the final products. This study thus investigates the turning performance of dense sintered ZnO varistors and chip formations to obtain the parametric range (cutting mechanism) which causes the chipping or the trans-granular/sudden failure in these brittle materials. With the analysis of the cutting tool vibration in relation to the machining parameters (f and VC), the vibration-induced chipping correlations are made and interlinked with the occurrence of grain pull-out during the turning operation. The results show that the reflected vibratory motion of the tools is directly correlated with the chip formation mechanisms in the turning of ZnO ceramics and thus provide robust measurements for quality assurance in final products.

Measurement of the piezotronic effect on single grain boundaries in zinc oxide Varistors

In this work we propose a new way of applying biaxial strain on an electrically conductive material to investigate stress-induced change of properties, namely the piezotronic effect, at a microscopic level. A piezoelectric stage is actuated to certain biaxial compressive strain levels. The microsection of the sample attached to it experiences the same amount of strain. The external strain translates to inner stresses in the material which can be controlled with high accuracy.This method was applied on zinc oxide-based varistor ceramics which is known as a functional material with a pronounced piezotronic effect on its highly non-linear current-voltage characteristics. By setting up a micro four-point probe measurement it was possible to record the current-voltage characteristics at single grain boundaries at certain stress levels. The results lead to an understanding how mechanical stresses modify the overall electrical behavior of zinc oxide varistors.

A modelling approach to describe the DC current-voltage behaviour of low-voltage zinc oxide varistors

Zinc oxide varistors are among the most important surge arresters. The current-voltage (I–V) behaviour of these devices can be approximated by a model of electrical networks of grain boundaries arranged in 3D space, where nonlinear resistors mimic the effective grain boundary behaviour of the varistor microstructure. A simplified version of the network model, namely parallel circuits of chains of serially connected resistors of different length (path model), is derived from a fully 3D model of the polycrystalline microstructure. A comparison of the path model with a complex resistor network based on the 3D model demonstrates that the network of highly nonlinear resistors has a very similar DC I–V characteristic as parallel paths of these resistors connected in series. The path model, whose parameters were adapted from microscopic I–V measurements, is able to reproduce the macroscopic I–V behaviour of real low-voltage zinc oxide varistors.

Nonlinearity Characteristics of Low-Voltage Barium Titanate Based Zinc Oxide Varistor Ceramics Modified by Cobalt Dopants

Application of ZnO varistor at low voltage has increased significantly due to the high demands of low-voltage electronics with high nonlinearity characteristics and low leakage current. The varistor ceramics were developed via solid-state reaction method and the resultant sample was analyzed by means of SEM, EDS and XRD. The nonlinearity characteristics of ZnO varistor ceramics for different contents of cobalt oxide (Co3O4) at a given barium titanate (BaTiO3) amount were analyzed based on the J-E characteristics measurement. The increased value of nonlinear coefficient (α) equal to 4.8 was exhibited by the sample made with 12 wt.% BaTiO3 additive. As the concentration of dopant (Co3O4) incorporated was increased from 0.5 to 1.5 wt.%, the varistor voltage limit decreased from 8.9 V/mm to 7.0 V/mm, respectively. Additionally, the barrier height increased from 0.88 to 0.98 eV for 0.0 wt.% to 1.0 wt.% of Co3O4 concentration, respectively. The highest α of 7.2 was obtained at 0.5 wt.% Co3O4 and decreased with further doping content due to to the reduction of barrier height caused by the variation of electronic state at the grain boundaries.

Effects of addition of chromium and/or nickel oxides on the electrical characteristics of yttrium oxide-doped high-voltage zinc oxide varistors

The effects of Cr2O3 and/or NiO addition to Y2O3-doped Bi-based high-voltage ZnO varistors were investigated, including crystal structure and interface states analysis. Incorporating 0.35 mol% Cr2O3 in Y-doped varistors increased the single-grain varistor voltage VNNGB from 2.6 to 3.3 V and decreased the leakage current density by a factor of 40, from 2 × 10−5 to 5 × 10-7 A/cm2. The nonlinearity index α before and after degradation increased from 21 to 35. 1.2 mol% NiO increased VNNGB to 4 V. Highest varistor voltage was 1500 V for 0.3 mol% NiO. Resistance to electrical degradation improved with optimal amounts of Cr or Ni by reduction of grain boundary oxygen vacancies. Though the donor density was changed prominently by addition of Cr and/or Ni, changes in the barrier height were suppressed by the change in interface state density. As a result, the change in VNNGB was related mainly to the empty interface state under no bias.

Simulation research on space charge characteristics of zinc oxide varistor

Zinc oxide (ZnO) lightning arresters have good protection performance and have been widely used in power systems. ZnO varistor is the core component of the arrester, and its electrical performance determines the overvoltage protection level of the arrester. ZnO varistor will produce space charge accumulation phenomenon under the action of electric field. In order to study the space charge characteristics of ZnO varistors more deeply, a bipolar carrier transport model considering the change of grain boundary structure was established, and the law of carrier migration in ZnO grain boundaries was simulated. From the three aspects of pressure time, ambient temperature, and depletion layer density, the space charge distribution inside the grain boundary, Schottky barrier height, and grain boundary leakage current are systematically studied.

A case study on an explosion accident of a 110 kV porcelain housed MOA

A metal oxide arrester (MOA) accident occurred in a tropical coastal area is studied in this paper. The control test and disassembly analysis for the MOAs in the accident group and the control group were conducted. It was considered that the varistors of the MOA were deteriorating. Although the performance of the zinc oxide (ZnO) varistor is qualified, its side insulation glaze has defects. The leakage current under U1mA and 0.75 U1mA have exceeded the standard requirements due to the decrease of insulation performance. An electrothermal model of the MOA was constructed. The root-mean-square (RMS) value of the voltage across the varistor column and the RMS of leakage current flowing through the varistor were obtained through simulation. The heating power was calculated as the heat source input of the thermal circuit model to calculate the temperature of the varistor. It was verified that the electrolyte deposited on the surface of the porcelain house due to salt spray formed a dry area after being affected by moisture, because of the uneven potential distribution, the radial current flowing to the varistors through the coupling capacitor on the surface of the porcelain house increased sharply, and it caused the varistor to generate a large amount of heat in extreme condition. The side insulation glaze of the varistor was damaged by frequent heating effect. Eventually, the internal flashover occurred in the MOA, and the internal pressure rose sharply then broke through the explosion-proof film, causing an accident.

Multiscale defect responses in understanding degradation in zinc oxide varistor ceramics

Dielectric responses of multiscale defects, including electron trapping behaviours of intrinsic point defects in grains, heterogeneous interface of depletion/intergranular layers, and dynamic responses of interface states at grain boundaries, were investigated in ZnO varistor ceramics via the DC ageing process. The dielectric relaxation peaks corresponding to zinc interstitials and oxygen vacancies increased, revealing that the donor densities gradually increase with ageing time. Dynamic response of the interface states, which were observed to undergo significant dc conductance, using traditional dielectric spectroscopy, is unveiled based on an optimised (∂ε'/∂lnω)/ε′ spectroscopy. The energy level of interface states monotonously decreased while the relaxation peak value increased, corresponding to decreased interface states with degradation time. Collectively, the degradation of Schottky barrier during DC ageing process is discussed based on multiscale defect responses with the help of the (∂ε'/∂lnω)/ε′ spectroscopy.