ZnO Varistors Research Articles

Effect of Ni2O3 Doping on the Electrical Properties and Intrinsic Defect Concentration of ZnO Varistor Ceramics

With the development of high voltage transmission, there is an urgent need to develop ZnO varistor ceramics with high anti-aging properties. The key is to manipulate the intrinsic defect concentration of ZnO varistor ceramics precisely. In this paper, ZnO varistor ceramics doped with different contents of Ni2O3 are taken to study the relationship between the microstructure and electrical properties, and the effect of ZnO varistor ceramics doped with different contents on intrinsic defect concentration is also considered. The results show that, best electrical performance is shown when the content of Ni2O3 is 1.2mol%, the electrical breakdown field E1mA is 356 V/mm, the nonlinear coefficient reaches 32, and the leakage current IL is 3.4 . While the amount of the doped Ni2O3 is more 0.8mol%, a new phase of Co2Cr5Sb5O4 phase is observed from X-ray diffraction. The SEM micrographs showed that the average grain size decreased monotonously from 14.56 m to 5.73 m with the amount of the doped Ni2O3 increased. According to the results of dielectric spectroscopy, the intrinsic defect concentration increased with the contents of the doped Ni2O3 increased. The increase of Zinc interstitial is much greater than that of Oxygen vacancies, which is harmful to Long-term aging characteristics of ZnO varistor ceramics.

Coupled Simulation of Current Flow and Residual Thermal Stress in ZnO Varistors

An electromechanically coupled modeling framework for the simulation of electric current flow in ZnO varistors is developed. The model is based on an equivalent circuit representation of the varistor microstructure with grain boundaries represented by nonlinear resistor branches of the circuit. This approach extends on previous circuit models by including the effect of mechanical stress on grain boundary conductivity. In this article, we consider, in particular, the effect of residual thermal stress in polycrystalline structures on electrical conductivity. The distribution of this stress throughout the material is computed by finite element method (FEM) simulations. Then, each grain boundary conductivity is determined by applying a self-consistent model for the trapped interface charge induced by piezoelectric polarization. Finally, the electric current flow patterns and the bulk conductivity of the material are computed using the nonlinear circuit model. The simulated I–V characteristics reveal a significant sensitivity of the electrical conductivity to internal thermal stress within the material. Furthermore, for realistic ZnO microstructures, the simulations demonstrate the effect of current concentration along thin conducting paths depending on the mechanical stress condition of the material.

Segregation and properties at curved vs straight (000) inversion boundaries in piezotronic ZnO bicrystals

AbstractTEM and SEM investigations of ZnO bicrystal interfaces were undertaken with an aim to study the correlation of local grain‐boundary structure, segregation, and electrical transport perpendicular to the interface. To this end, varistor‐like ZnO bicrystals with piezotronic characteristics were chosen with (000)║(000) tail‐to‐tail orientation with respect to the c‐axis. In order to contrast different local grain‐boundary structures with different coherency and segregation of bismuth, but identical macroscopic polarization state, two complementary processing techniques were applied. A diffusion‐bonded bicrystal with an intermediate thin film containing Zn–Bi–Co–O provided a straight interface as reference. In contrast, a ZnO bicrystal prepared by epitaxial solid‐state transformation was manufactured by bonding two ZnO single crystals with a 100 µm thick polycrystalline ZnO varistor material with a typical dopant composition including bismuth and cobalt. This structure was annealed to the point that a bicrystal was formed with the varistor concentration at the boundary, which was strongly curved due to the polycrystalline microstructure still providing a shadow image at the interface. The results highlight a distinct correlation between local interfacial morphology, degree of segregation of bismuth, and degree of nonlinearity of the electrical transport across the interface.

Barrier formation of single junctions with oxidation in SrCoO<sub>3</sub>-doped ZnO varistors sintered in a reducing atmosphere

Barrier formation of single junctions with oxidation in SrCoO<sub>3</sub>-doped ZnO varistors sintered in a reducing atmosphere

Influence of CaO doping on phase, microstructure, electrical and dielectric properties of ZnO varistors

In present study, impact of CaO doping on microstructure and phase electrical properties of a ZnO varistor is being reported. The doped ZnO nanopowders were prepared by solution combustion followed by conventional sintering. The ZnO particle size of powders decreased from 22 to 17 nm with CaO doping was noted. Decrease in grain size of the sintered samples from 4.54 to 1.19 μm with CaO doping was observed. This is attributed to Ca4Bi6O13 and Ca0.89Bi3.11O5.58 secondary phase formations apart from pyrochlore and spinel phases. A breakdown field as high as 21 kVcm−1 in X = 1.00 sample was obtained. The Coefficient of nonlinearity (α) decreased from 95 to 42 is due to decrease in carrier concentrations. Increase in resistivity with CaO doping and decrease in resistivity beyond X = 2.5 were noted. The electric modulus plots yielded two relaxations at a temperature in the range 100–300 °C and frequency in the range 0.1 Hz-1MHz. Activation energy (Ea) decreased from 0.65 to 0.50 eV for a region I and from 0.89 to 0.75 eV for region II with CaO doping was observed.

Improving the protective effect of surge arresters by optimizing the electrical property of ZnO varistors

In the present research, we studied ZnO varistors having multiple dopants and optimized their microstructure and electrical properties by tailoring the sintering process. At certain concentrations of the rare-earth elements yttrium and indium, the voltage gradient decreased from 683.7 to 293.0V/mm as the sintering temperature was increased from 1160 to 1320°C. In addition, the nonlinear coefficient first increased and then decreased as sintering temperature was increased. When sintered at 1240°C, the multiple doped ZnO varistors samples showed excellent performance, exhibiting a voltage gradient of 520V/mm, a residual voltage ratio of 1.55, and a density of 63.7A/cm2 under a current waveform of 8/20μs, and a nonlinear coefficient of 74.5. These optimized properties will be of great utility in the manufacture of metal oxide arresters. When the high-performance varistors were used in surge arresters of ultra-high voltage grids, the overvoltage level along the transmission line and in the substation were reduced by 31.14% and 33.53%, respectively. This combination of characteristics will dramatically improve the protective effect of surge arresters for deeply suppressing over-voltages in power systems, significantly reduce the insulation requirement and cost of power apparatus as well as greatly increase the safety of power systems.

Dependence of Properties of High Voltage Zinc Oxide Varistor from Antimony and Nickel Oxides

Studies were carried out to reduce the leakage current of high-voltage ZnO varistor ceramics by varying Sb2O3 and NiO content in a ZnO-Bi2O3-Sb2O3-Al2O3-Co3O4-NiO. It was found that the ceramic composition (wt %): ZnO 80, Bi2O3 5.83, Sb2O3 2.62, A12O3 4.66, Co2O3 3.80, NiO 3.09, has a minimum leakage current density Iout ≤ 0.1 μA cm-2, breakdown voltage Ub = 4.1kV mm-1, nonlinearity coefficient α = 65. The resulting ceramics are promising for the production of varistors with high stability of performance.

Effect of Pr6O11 doping on the microstructure and electrical properties of ZnO varistors

ZnO-Pr6O11 based varistors have anticipated application prospect in electrical system, but the study of the detailed microstructure is still poor. In this paper, the effect of Pr6O11 doping on the microstructure (such as element distributions and grain boundary characteristics) and electrical properties of the ZnO-Pr6O11–Co2O3–Cr2O3–Y2O3 varistors manufactured by the conventional solid-state sintering procedure was studied detailedly. Without Pr6O11 doping, the elements have the similar distributions in the interior and grain boundaries of ZnO phase, and the double Schottky barrier cannot be formed. With Pr6O11 doping, Pr6O11 liquid phase dissolving a large number of Zn, Cr, and Y forms during sintering and Pr6O11 solid phase forms in the ZnO grain boundaries during cooling, so the double Schottky barriers form. The voltage gradients E1 mA and nonlinearity coefficients α of the samples with Pr6O11 contents of 0.25–1 mol% are much larger than those of the sample without Pr6O11 doping, and the values of α and E1 mA increase with Pr6O11 content increasing from 0 to 1 mol%. The values of α and E1 mA simultaneously depend on the double Schottky barrier height ϕb and the CPE exponent αCPE. Finally, the ZnO varistor with a voltage gradient of 333.6 V/mm and a nonlinearity coefficient of 24.4 was obtained by Pr6O11 doping of 1 mol%. The detailed study of the microstructure and related mechanism is of great importance for preparing ZnO-Pr6O11 based varistors with excellent electrical properties.

Investigation of dielectric relaxation and degradation behavior of two-step sintered ZnO varistors

In this study, ZnO varistors using sub-micron varistor powders by two-step sintering were fabricated in contrast to conventional sintering. The microstructures, electrical properties, and the degradation behavior under a severe dc electric field and a high temperature were systematically studied. The dielectric spectroscopy, including impedance and electric modulus, was also measured. The results showed that two-step sintering would effectively decrease the grain size, uniform grain size distribution and strengthen densification. Two-step sintered ZnO varistors (TSZV) acquired better electrical properties with the following sintering schedule: rising up to 1150 °C, immediately down to 850 °C, and soaking for 10 h. For this two-step sintered sample, the breakdown field was 402.26 V/mm, the leakage current was 0.98 μA/cm2, and the nonlinear coefficient was 60.88. With the help of electric modulus spectroscopy from −110 to 200 °C, three dielectric relaxation peaks with activation energy of 0.21–0.23 eV, 0.31–0.41 eV, and 0.71–0.78 eV originated from zinc interstitial, oxygen vacancy and interface states, respectively were detected and a peak with an activation energy of 0.60 eV originated from DC conductance was also detected for the TSZV samples and the conventional sintered sample. The differences in dielectric relaxation suggested different defect trap states at the grain boundary inside ZnO varistors. It was also found that TSZV were more likely to degrade when compared with the conventional sintered varistor, and some explanations were given for the worse aging stability of two-step sintering.

Breakdown phenomenon of ZnO varistors caused by non-uniform distribution of internal pores

Here, we analyzed the internal structure of ZnO varistors with X-ray micro-CT technology and showed that the porosity and pore volume at the radial edge is considerably higher than at other regions. Experimental results showed that the breakdown phenomenon usually occurs near the outer edge of circular varistors. The distribution of impulse currents applied to concentric ring electrodes indicated that the current ratio at the edge region increased as the total current increased. The pores are insulating under low electric fields and the current through the pores is almost zero. Hence, the voltage gradient at the edge region is higher than that elsewhere. Additionally, the simulation results were consistent with this assumption. The amounts of open pore defects, which showed a conductive state after break down, increased, and the proportion of the current flow through the pores increased accordingly, which lead to a local temperature rise and breakdown.

Hydrometallurgical Recycling of Electric Arc Furnace Dust

The current global primary zinc stocks are estimated to last for approximately 20 years. For this reason, the recycling and processing of waste (especially steelmaking dust) containing zinc are currently necessary, and this will remain the case in the future. The main objective, we deal with in this article is the experimental research on electric arc furnace dust (EAFD) compounds which may potentially result in the commercial manufacture of products in the electrotechnical industry. In our work we focus on the production of ZnO-based varistors used in surge protection devices (SPD), which are the current trend in the world. ZnO, a resultant product of hydrometallurgical recycling of zinc-containing dust produced by the remelting of steel waste in an EAF, was used for this research. From the resulting product containing 79.04% Zn, five varistor samples were prepared. Subsequently, their electrical characteristics were measured and calculated on these samples. From the measured electrical parameters, the non-linear characteristic typical of voltage-dependent resistors–varistors is evident. Based on the results of the electrical characteristics, we conclude that the ZnO product, which is the output of hydrometallurgical recycling of EAFD, is a potentially suitable material for use in the production of ZnO varistors in the future.

Effects of SiO2/Cr2O3 ratios on microstructures and electrical properties of high voltage gradient ZnO varistors

The effect of various proportions of SiO2 and Cr2O3 doping on the micro-characteristics and the electrical properties of the ZnO varistors were investigated. As the SiO2/Cr2O3 ratio increased, the voltage gradient increased and then decreased, the nonlinear coefficient α increased, while the leakage current showed the opposite trend, meanwhile the residual voltage ratio was almost stable. When the SiO2/Cr2O3 ratio was 1.7 mol% vs. 0.6 mol%, the ZnO varistors sintered at 1155 °C exhibited excellent comprehensive electrical properties with voltage gradient of 329.9 V/mm, nonlinear coefficient of 65.1, leakage current of 0 µA, residual voltage ratio of 1.663, and aging starting power of 0.72 W. The varistors were in good condition after being struck 18 times of 250 A square wave energy pulse impact, showing a great potential in the industrial application of power distribution systems.

Synthesis of the Composite Additive Fine Powders for ZnO Varistor by Low-Temperature Combustion

The composite oxide additive containing Mn, Ni, Cr and Co elements for the ZnO varistor was successfully synthesized through the low-temperature combustion synthesis approach. The combustion reaction was carried out using nitrate mixtures of Mn, Ni, Cr and Co as oxidants and hydrazine as fuel. Herein, hydrazine acts as a bidentate bridge ligand, and it forms a complex with metal nitrate. Controlling the pH value of reactant solution by adding the hydrazine had a considerable effect on the combustion characteristics of the precursor and the final synthesized powder properties. The results of the XRD, FT-IR, SEM and TEM analysis indicated that the suitable pH value for synthesizing the composite additive powder with fine grain size and good crystallinity was 7. The precursor prepared at [Formula: see text] exhibited steady and slow combustion characteristics, and its yield rate achieved ca. 100%. With this method, the composite additive fine powder suitable for improvement of the electrical characteristics of ZnO varistor can be synthesized simultaneously on a large scale.

Structural and electrical properties of zno varistor with different particle size for initial oxides materials


 
 
 We report here structural, electrical and dielectric properties of ZnO varistors prepared with two different particle sizes for initial starting oxides materials (5 µm and 200 nm). It is found that the particle size of ZnO does not influence the hexagonal wurtzite structure of ZnO, while the lattice parameters, crystalline diameter, grain size and Zn-O bond length are affected. The nonlinear coefficient, breakdown field and barrier height are decreased from 18.6, 1580 V/cm and 1.153 eV for ZnO micro to 410 V/cm, 7.26 and 0.692 eV for ZnO nano. While, residual voltage and electrical conductivity of upturn region are increased from 2.08 and 2.38x10-5 (Ω.cm)-1 to 4.55 and 3.03x10-5 (Ω.cm)-1. The electrical conductivity increases by increasing temperature for both varistors, and it is higher for ZnO nano than that of ZnO micro. The character of electrical conductivity against temperature is divided into three different regions over the temperature intervals as follows; (300 K ≤ T ≤ 420 K), (420 K ≤ T ≤ 580 K) and (580 K ≤ T ≤ 620 K), respectively. The activation energy is increased in the first region from 0.141 eV for ZnO micro to 0.183 eV for ZnO nano and it is kept nearly constant in the other two regions. On the other hand, the average conductivity deduced through dielectric measurements is increased from 2.54x10-7 (Ω.cm)-1 for ZnO micro to 49x10-7 (Ω.cm)-1. Similar behavior is obtained for the conductivities of grains and grain boundaries. The dielectric constant decreases as the frequency increases for both varistors, and it is higher for ZnO nano than that of ZnO micro. These results are discussed in terms of free excited energy and strength of link between grains of these varistors.
 
 

Electrical properties and microstructure of zno varistor with high surge protective characteristics

Electrical properties and microstructure of zno varistor with high surge protective characteristics

Impulse current withstanding capability of ZnO varistors with influence of varistor size

Through injecting 8/20 and 30/60 μs impulse currents to five kinds of ZnO varistors, the influence of the varistor size on the single impulse current withstanding capability of a ZnO varistor is investigated in this article. A comprehensive damage criterion of a varistor consisting of three aspects, namely, mechanical failure by visual inspection, abnormal saltation in voltage or current waveform, and over 5% variation in reference voltage or leakage current >50 μA is proposed here. The diameters of the ZnO varistor under test vary from 42 mm to 60 mm, and the heights vary from 22 mm to 31 mm. The obtained results show that, under the same impulse current waveform, the mean voltage and the mean energy density of ZnO varistor increase approximately, proportionally with increasing amplitude of impulse current density. Besides, the voltage withstanding capability per unit height of varistor varies from 4.408 to 4.940 kV/cm under 8/20 μs impulse current, and from 4.256 to 5.148 kV/cm under 30/60 μs impulse current. The current withstanding capability per unit area of the varistor varies from 2.134 to 3.842 kA/cm2 under 8/20 μs impulse current, and from 1.738 to 3.490 kA/cm2 under 30/60 μs impulse current. Moreover, the energy withstanding capability per unit volume of the varistor ranges from 206 to 421 J/cm3 under 8/20 μs impulse current, and from 311 to 752 J/cm3 under 30/60 μs impulse current. Furthermore, due to the fact that nonuniformity of the polycrystalline‐sintered body of ZnO varistors increases with the increasing varistor area, the normalized parameters, including the mean failure voltage, the failure current density, and the failure energy density, decrease with increasing varistor diameters. The normalized parameters proposed here are more effective ones to consider the effect of sizes on the withstanding capacity than the failure current, the failure voltage, and the failure energy. © 2019 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Understanding the validity of impedance and modulus spectroscopy on exploring electrical heterogeneity in dielectric ceramics

Combined modulus and impedance spectra are widely employed to explore electrical inhomogeneity and carriers' behaviors in dielectric ceramics based on equivalent circuit. However, discrepancies are found between practical dielectric responses and widely proposed equivalent circuits. Taking ZnO varistor ceramics as an example, a low-frequency dielectric relaxation, which can be detected in practical dielectric spectroscopy, is overlooked in simulated dielectric spectroscopy based on the proposed equivalent circuit according to modulus and impedance spectra. Therefore, equivalent circuits are frequently incomplete because the real low-frequency dielectric response is unable to be characterized from them. The problem originates from debatable understanding of frequency responses in modulus and impedance spectra. The low-frequency peak in modulus spectroscopy is proved originating from DC conductance instead of a real dielectric relaxation and the involvement of DC conductance component makes a low-frequency dielectric relaxation unable to be characterized in modulus spectroscopy. Therefore, improved dielectric spectroscopy eliminating the component of DC conductance is proposed and a clear peak corresponding to the low-frequency dielectric relaxation appears. In addition, a modified equivalent circuit which is in accordance with practical dielectric responses in not only modulus and impedance spectra but also dielectric spectroscopy is presented.

An Experimental Study of the Failure Mode of ZnO Varistors Under Multiple Lightning Strokes

In this study, in order to explore the failure mode of ZnO varistors under multiple lightning strokes, a five-pulse 8/20 μs nominal lightning current with pulse intervals of 50 ms was applied to ZnO varistors. Scanning electron microscopy (SEM) and X-ray diffractometry (XRD) were used to analyze the microstructure of the material. The failure processes of ZnO varistors caused by multiple lightning impulse currents were described. The performance changes of ZnO varistors after multiple lightning impulses were analyzed from both macro and micro perspectives. According to the results of this study’s experiments, the macroscopic failure mode of ZnO varistors after multiple lightning impulses involved the rapid deterioration of the electrical parameters with the increase of the number of impulse groups, until destruction occurred by side-corner cracking. The microstructural examination indicated that, after the multiple lightning strokes, the proportion of Bi in the crystal phases was altered, the grain size of the ZnO varistors became smaller, and the white intergranular phase (Bi-rich grain boundary layer) increased significantly. The failure mechanism was thermal damage and grain boundary structure damage caused by temperature gradient thermal stress, generated by multiple lightning currents.

Investigation of microstructure of ZnO varistors taken from surge arrester counters

Investigation of microstructure of ZnO varistors taken from surge arrester counters

Dynamic Breakdown of ZnO Varistor Ceramics under Pulsed Electric Field

Dynamic Breakdown of ZnO Varistor Ceramics under Pulsed Electric Field