Varistor Behavior Research Articles

Effect of additive particle size on degradation behavior of ZnO varistors under high current surges

Effect of additive particle size on degradation behavior of ZnO varistors under high current surges

Role of ZnMn2O4 phase in formation of varistor characteristics in ZnO:Mn ceramics

The samples ZnO:Mn were prepared using the conventional solid-state technique. To dope them with manganese, we used water solutions of MnSO4 and MnCl2. The properties inherent to both types of the obtained ceramics have been compared. It was found that the former demonstrated nonlinear current-voltage characteristics, whereas those of the latter were, in fact, linear. The analysis of EPR, diffuse reflectance and Raman spectra obtained for prepared ceramics allowed concluding that, in the samples doped with MnSO4, formation of Mn-related phase, namely, ZnMn2O4 spinel occurred at ZnO grain boundaries under sintering. It has been ascertained that a thin layer of this substance separates adjacent ZnO grains, which provides appearance of the back-to-back Schottky barriers at grain boundaries and “varistor behavior” of current-voltage characteristics.

Effects of Dopants and Processing Parameters on the Properties of ZnO-V2O5-Based Varistors Prepared by Powder Metallurgy: A Review.

This article reviews the progress in developing ZnO-V2O5-based metal oxide varistors (MOVs) using powder metallurgy (PM) techniques. The aim is to create new, advanced ceramic materials for MOVs with comparable or superior functional properties to ZnO-Bi2O3 varistors using fewer dopants. The survey emphasizes the importance of a homogeneous microstructure and desirable varistor properties, such as high nonlinearity (α), low leakage current density (JL), high energy absorption capability, reduced power loss, and stability for reliable MOVs. This study investigates the effect of V2O5 and MO additives on the microstructure, electrical and dielectric properties, and aging behavior of ZnO-based varistors. The findings show that MOVs with 0.25-2 mol.% V2O5 and MO additives sintered in air over 800 °C contain a primary phase of ZnO with a hexagonal wurtzite structure and several secondary phases that impact the MOV performance. The MO additives, such as Bi2O3, In2O3, Sb2O3, transition element oxides, and rare earth oxides, act as ZnO grain growth inhibitors and enhance the density, microstructure homogeneity, and nonlinearity. Refinement of the microstructure of MOVs and consolidation under appropriate PM conditions improve their electrical properties (JL ≤ 0.2 mA/cm2, α of 22-153) and stability. The review recommends further developing and investigating large-sized MOVs from the ZnO-V2O5 systems using these techniques.

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.

Electrical and microstructural evaluation of ZnO varistor ceramics with different CaSiO3 contents

In the present study, the impact of CaSiO3 addition on microstructure and electrical characteristics of a ZnO-based varistor system was investigated. The ceramic materials were prepared by solid-state reaction route by varying the CaSiO3 concentrations (0, 3, 6, 9, 12, and 15 wt%) and sintered at 1300 °C. Characterization of the pelletized samples was carried out by density measurement, scanning electron microscopy, energy dispersive X-ray, X-ray diffraction, and current density-electric field characteristics measurement. The growth of ZnO grain was restrained with the introduction of CaSiO3; the average grain size decreases from 18.09 to 10.85 μm, whereas their relative density increases up to 99.79%. Structural results indicated the existence of the CaSiO3-rich phase without any other secondary phase formation. It is also revealed that different CaSiO3 contents significantly affect the electrical properties, prominently in the sample with 12 wt% CaSiO3 addition. The maximal value of the nonlinear coefficient, breakdown voltage, and barrier height acquired in this work were 4.41, 0.80 V/mm, and 0.691 eV, respectively, along with a sharp decline in leakage current density to 373.93 μA/cm2. Further increase in CaSiO3 content (15 wt%) eventually caused a deterioration in varistor behavior.

Development of multifunctional electronic-ceramics with piezoelectric effect and varistor behavior

In this paper, electronic ceramics with multiple functions are studied for the first time and prepared by the traditional solid-phase sintering method. These electronic ceramics were composed by a serious of Pb(Ni1/3Nb2/3)0.55-(ZrxTi1-x)0.45O3 (PNN-PZT) (x = 0.3, 0.285, 0.27 and 0.255) piezoceramics raw materials and the commercial ZnO–Bi2O3 based varistor raw materials. The results show that they exhibit the piezoelectric effect and varistor behavior and that they can produce a kinetic response to a particular critical applied field. The multifunctional ceramics present both piezoelectric effect and varistor behavior simultaneously in a single sample. Two kinds of crystallographic phases in the samples are found, namely, perovskite and wurtzite, based on the XRD, EDS and SEM results. For all multifunctional ceramics, the piezoelectric constants d33 values are over 150 pC/N, potential gradients E1mA values are higher than 1100 V/mm, and nonlinear coefficients α values are over 10. The potential gradient values of multifunctional ceramics are much higher than that of commercially used varistors, and these samples present special hysteresis loops.

A linearized MOV model-based method for fault location on off-terminal series capacitor bank-compensated transmission line using one-end current

This paper proposes a method for fault location on single and double-circuits series capacitor bank-compensated transmission line (SCTL). A common situation for installing the series capacitor bank (SCB) is to place it some point away from the terminal substations. One-end measurement-based fault locators are favorable for many utilities due to simplicity and less cost especially if they are accurate enough. For impedance-based one-end fault locators on SCTL, the current passing in the SCB is unknown for about 50% of faults. This prevents the prediction of the challenging metal oxide varistor (MOV) behavior that is inevitable for a one-end method. Herein, the nonlinear characteristics of the MOV protecting the SCB are simplified by a linearized simple yet precise model. This makes it possible to express the SCB-MOV characteristics as a linearized function of the far-end current for faults upstream the SCB keeping the nonlinear characteristics of the MOV. This eventually enables to estimate the fault current contribution of the far-end. Hence, an analytical fault distance estimation algorithm is derived for both single and parallel transmission lines using the sequence-networks modeling of the system. Beside simplicity and robustness, the proposed method is found to be accurate enough in locating a big number of faults under a variety of conditions.

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.

High non-linear electrical properties of Li3xCo7–4xSb2+xO12 a new ceramic varistor

In this work, a new varistor with a high non-linear J-E electrical characteristic (α = 54; EB = 9753 V/cm) is reported, synthesized from the binary system Co7Sb2O12-LiCoSbO4 with general formula Li3xCo7–4xSb2+xO12. This new material is a single-phase iso-structural to Co7Sb2O12 spinel. It was shown that the electrical properties improved with increasing the x value in the solid solution. Li3xCo7–4xSb2+xO12 varistor behaviour is explained by the presence of defects in the crystal structure of Co7Sb2O12 when Li+ and Sb5+ are added, which appears to be responsible for the formation of Schottky type potential barriers at the grain boundaries. Also, it is reported the effect of the sintering temperature on the electrical properties of the varistors. In addition, sub-solidus compatibility relations on the ternary system of Li2O-CoO-Sb2O5, up to>50% Li2O, and the binary phase diagram Co7Sb2O12-LiCoSbO4 have been determined, where an extensive solid solution Li3xCo7–4xSb2+xO12: 0.2 ≤ x ≤ 0.7, was formed. The lattice parameter of Li3xCo7–4xSb2+xO12 solid solution exhibit a discontinuity indicating a change in the substitution mechanism or symmetry of the solid solution. XPS analysis showed how Co2p spectra change as Li+ substitutes Co2+ into the structure and how this affects the linearity of the lattice parameter a.

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.

DC Overload Behavior of Low-Voltage Varistor-Based Surge Protective Devices

This letter provides evidence of a peculiar behavior of metal-oxide varistors under DC overvoltages. It stresses the need for extending the V–I characteristic of varistors in the time domain so as to accurately predict the dynamic performance of varistor-based surge protective devices under temporary overvoltages. Observations and mathematical formulation of DC overload behavior of metal-oxide varistors may have significant implications in energy absorption requirements for low-voltage surge protective devices and unlocks possible monitoring strategies in emerging DC distribution smart grids.

High improvement of degradation behavior of ZnO varistors under high current surges by appropriate Sb2O3 doping

The effects of Sb2O3 content on the microstructures and electrical properties of the ZnO varistors, especially on the degradation behavior under pulse current stress were studied. The results showed that the degradation behavior was effectively improved by doping appropriate amount of Sb2O3, which attributed to the homogenized microstructure and the compression of the interstitial void. The change rates of positive and negative breakdown voltage gradients of samples doped with 0.92 mol% Sb2O3 under 20 * 20 kA + 2 * 30 kA surges were -1.76 % and 1.56 % respectively, which was one of the best levels reported in the previous literatures. In addition, the sample exhibited excellent comprehensive electrical properties, with breakdown voltage gradient of 207.74 V mm−1, nonlinear coefficient of 119.1, leakage current density of 1.03 × 10-2 μA cm-2, and clamping voltage ratio of 2.17 under 20 kA surge, making it a promising candidate for surge protector devices.

Failure of high power varistor ceramic components

Ceramic varistors are electronic components which have a sharp change (over several orders of magnitude) of their electrical resistance at a well-defined voltage (switching voltage). Starting at low voltages, the resistance is large. But by exceeding the switching voltage the resistance drops dramatically and the component acts as a good conductor. Mounted parallel to a consumer, they are used as protection devices against over voltage loading. In the technical routine yearly billions of varistors are used to protect power lines, transformation stations, electronic devices, microelectronic systems or even LEDs.In service large temperature differences may come into existence, which cause severe mechanical stresses that even may destroy the components. The basic principles of the varistor behaviour are explained and examples of mechanical failure are discussed.

Effect of high Al2O3 content on the microstructure and electrical properties of Co- and Ta-doped SnO2 varistors

Varistor behavior shows significant differences when the addition levels of different dopants like In2O3, Cr2O3, and Al2O3 are changed, thus stimulating current investigations on the SnO2–Co3O4–Ta2O5 ceramic system. In this contribution, the influence of high additions of Al2O3 on the microstructure, structure and the electrical properties of SnO2–Co3O4–Ta2O5 varistors ([98.95-X]%SnO2–1% Co3O4–X% Al2O3–0.05% Ta2O5, where X = 0, 0.05, 0.1, 1 or 2 mol%) is investigated. Characterization techniques such as thermal analysis, scanning electron microscopy and X-ray diffraction with Rietveld refinement were used for specimen analysis. The endothermic peaks in the ceramic system containing Al2O3 additions between 0.05 and 1% are ascribed to the formation of the Co2SnO4 and CoAl2O4 spinel type phases. Doping the ceramic system with 1 and 2 mol% Al2O3 leads to the formation of 1.163 and 3.449%, respectively, of the spinel phase Al2CoO4, which acts as a grain growth inhibitor because grain size decreases in about 16% for both addition levels. The apparent grains homogeneity and narrowest monomodal grain size distribution for the specimens with 2 mol% Al2O3 confirm the inhibitory role. With the lowest level of Al2O3 (0.05 mol%) the nonlinearity coefficient reaches a maximum, after which it decreases and fades at the highest alumina level. A remarkable decrease of about 50% in the leakage current from the reference specimen´s value to that of the one with 0.05 mol% Al2O3 concurrently with an increase in about 40% in the nonlinearity coefficient favors the potential use of alumina in the SnO2–Co3O4–Ta2O5 ceramic system.

Varistor behavior in a ternary system based on SnO2 doped with a hexavalent donor: SnO2-MnO2-WO3

Pentavalent metals have been used as donors in tin dioxide, giving it varistor characteristics when associated with densifiers such as CoO or MnO2. However, attempts to obtain varistor behavior in SnO2-based ternary systems using hexavalent donors such as W6+ have been unsuccessful to date, leading to typical linear behavior. Here it is shown that one can achieve nonlinear behavior in this kind of system, given a suitable choice of densifier (MnO2). In order to get insights into the role of WO3 content on the potential barriers responsible for varistor properties, a combination of experimental and theoretical techniques was used. Measurements revealed two minor but relevant phases, Mn3O4, and MnWO4, whose presence was explained by theoretical calculations and whose amount was estimated as a function of WO3 content. This estimate revealed a correlation between Mn3O4 amount and varistor properties, interpreted with basis on Mn defects at SnO2/Mn3O4 interfaces. WO3 content was varied to optimize the varistor properties, resulting in nonlinear coefficient α≈ 6, breakdown electric field Eb> 10 kV/cm and leakage current Il≈ 200 μA. This finding is expected to stimulate further investigations on SnO2-based ternary systems containing hexavalent oxides, especially using other densifiers, such as CoO.

Investigation of nonlinear electrical properties of ZnO/PPy nanocomposite and its application as a low-voltage varistor

The present study reports the electrical characteristics of a low voltage ZnO/Polypyrrole nanocomposite varistor which was fabricated as a thin disk based on zinc oxide with different weight percentages of polypyrrole as additive using a cold pressing method. Current-voltage characteristics and nonlinear coefficients of the ZnO/Polypyrrole nanocomposite thin disks were studied in direct current mode. Results showed good varistor behavior of prepared ZnO/Polypyrrole nanocomposite. According to the results, ZnO/Polypyrrole nanocomposite thin disk varistors can be utilized to preserve circuits from 35 V up to 350 V over voltages. I-V characteristics of nanocomposite samples showed that by increasing zinc oxide content from 98.52% to 99.01%, the breakdown voltage of ZnO/Polypyrrole nanocomposite varistor increased whereas its nonlinear coefficient decreased. However, more increase in ZnO content of nanocomposite caused to the reduction of its breakdown voltage and increase of nonlinear coefficient. The results obtained from SEM micrographs indicated that by increasing ZnO content, both the grain numbers and their morphologies are changed which is caused the different effects in the grain boundaries, I-V characteristics, and varistor properties of nanocomposites. As a very important result, in this experimental work, an ideal varistor with sufficient low breakdown voltage and acceptable high nonlinear coefficient was obtained.

The use of adsorption method to preparation of polyaniline/ZnO nanocomposite varistor

ZnO nanosheets were synthesized using a cost effective and simple precipitation method. Polyaniline/zinc oxide nanocomposites in various weight ratios were produced by the synthesis of polyaniline as a conducting polymer followed by adsorption of polymer chains on the ZnO nanosheets. UV–Vis spectroscopy, X-ray diffraction patterns, and SEM technique were utilized to characterize the nanocomposite structure. The polyaniline/ZnO nanocomposites prepared in various weight percent of polyaniline and ZnO nanosheets were pressed as thin disks at a pressure of 60 MPa and their usage electrical varistors were studied. The current–voltage characteristics of thin polyaniline/ZnO nanocomposites disk varistors were investigated. The results demonstrated good varistor behavior of polyaniline/ZnO nanocomposite disks so that they can be used to protect the electrical circuits from over voltages in the range of 250–550 V. The efficiency of ZnO percent on the varistors properties of polyaniline/ZnO nanocomposites disks was experimented. According to the results, the nanocomposites sample containing 99.01 wt% of ZnO showed minimum breakdown voltage (250 V) and highest nonlinear coefficient (4.28) and also the lifetime of this sample is high. From the UV–Vis spectra recorded for polyaniline/ZnO nanocomposites samples with various ZnO percent, it was found that absorption and light absorption band gap decreased by reducing ZnO content in nanocomposites samples. The band gap in the best sample is 3 eV.

THE ROLE OF POLYPYRROLE IN ELECTRICAL PROPERTIES OF ZNO-POLYMER COMPOSITE VARISTORS

Composite films on the base of ZnO with different weight percentages of polypyrrole have been prepared using the hot pressing method and their current-voltage characteristics have been studied. Results show that the films have nonlinear varistor behavior and can be used to protect sensitive circuits from 110V up to 350V overvoltages. It is found that the higher the content of polypyrrole in the varistor, the lower the breakdown voltage. Samples with higher polypyrrole content have less nonlinearity. The interesting result is that, unlike semiconductor-polyaniline based varistors, varistor hysteresis decreases through increasing polypyrrole content. Results have been analyzed with respect to SEM micrographs and XRD patterns of the samples. Energy gaps of varistors are evaluated using their UV spectra analysis regarding Tauc relation, which show that increasing polypyrrole content in varistors causes their absorption to increase and the corresponding band gap to decrease.

Effect of interfacial charge relaxation on conducting behavior of ZnO varistors under time varying electric fields

The conducting mechanism of ZnO varistors under the steady state has been well documented, but the corresponding time-dependent property was rarely studied. In this work, we proposed a physical model for analyzing the conducting behavior of a single ZnO grain boundary under pure ac fields. The established model mainly deals with the time-delay effect of the interfacial charge and its influence on the barrier height as well as the conducting process, which leads to a phase shift of the conduction current in advance of the applied voltage and an enhancement of the effective capacitance. To validate the model, the nonlinear conducting characteristics of ZnO varistors under pure ac fields were measured and the result agreed well with the model.

Dynamic characteristics and cascaded coordination of limiting‐type SPDs under subsequent negative strokes

Subsequent negative impulses are steep and common in lightning flashes. Under such currents, the dynamic behaviour of metal-oxide varistors (MOVs) is expected to be significant and would affect the coordination of cascaded surge protective devices (SPDs). This study experimentally investigates the dynamic behaviour of MOVs under steep currents. Based on the dynamic characteristics, simulations are conducted to investigate the coordination of two-cascaded limiting-type SPDs with different cascaded combinations, different separation distances, and different loads under subsequent strokes with 0.25/100 μs current waveform. Meanwhile, the results are compared with that under 8/20 and 10/350 μs currents. Experiments are performed to investigate SPD coordination performance under subsequent strokes. The results show that voltages on MOVs of low-voltage SPDs increase about 6.8-18.7% under steep currents, compared with that under 8/20 μs current. Thanks to the significantly changed dynamic characteristics, the cascaded SPDs could more easily realise effective coordination under subsequent strokes, compared with that under 8/20 and 10/350 μs currents. Moreover, effective coordination can be achieved in large ranges of separation distances and loads under subsequent strokes. The SPD combinations with clamping voltage of upstream SPD a little higher than downstream SPD have the best behaviour for lightning protection in low-voltage systems.