Varistor Materials Research Articles

Linear resistance limiting the current of intergranular barriers in varistor ceramics

Method for measuring the linear resistivity ρlin, which affects the volt-ampere characteristic (VAC) nonlinearity of varistor materials in a high current region is proposed. For ZnO based varistor ceramics the dependence of ρlin on amplitude density Jmax of current pulse is studied in the range 5 A cm-2 – 4600 A cm-2. At Jmax < 200 A cm-2 a sharp decrease of ρlin with increasing Jmax takes place. This behavior indicates a non-uniform distribution of high current over the cross section of ceramics sample. At Jmax > 1000 A cm-2 ρlin does not depend on Jmax and has a value of 0.49 – 0.50 Ohm cm corresponding to the volume resistivity ρg of ZnO grains. The decrease in nonlinearity of VAC at high currents is due to the formation of conductive paths of electric current between the electrodes of a sample. Along such paths, all intergranular double Schottky barriers are in a reversible electrical breakdown mode, probably due to impact ionization. The current in these paths is limited by small linear volume resistance of the grains. A spread of grain sizes is the reason for existence of current paths with different numbers of contacts between grains and therefore with different breakdown voltages. At voltage increases, the sequential formation of parallel-connected conductive paths occurs. This explains the inhomogeneous distribution of high current over the cross section of a sample at Jmax < 200 A cm-2. At Jmax > 1000 A cm-2 almost all current paths in a sample are switched on and the value of ρlin limiting the barriers current is close to the volume resistivity ρg of grains.

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.

Quality Assessment of Low Voltage Surge Arresters

Users expect reliable operation of the surge arrester during overvoltages, which may originate from a switching process or a lightning discharge. The necessary conditions to guarantee these expectations are: appropriate construction of the surge arrester, its production being maintained in accordance with technical standards, and a positive results of the type test. The recipient, especially when purchasing large quantities of the product, can check the final quality of the product through selective testing of samples from the market in order to find the best supplier. This manuscript specifies the basic parameters of low-voltage (LV) surge arresters of four manufacturers. Basic electrical parameters, i.e. leakage current and reference voltage before and after the discharge current of 8 μs /20 μs, were determined for 10 randomly selected surge arresters of each manufacturer. Moreover one sample of each varistor batches was aged by 50 simulated lightning strikes. The obtained results of structural analysis indicate that the crystal structure of the varistor materials changes after ageing process, affecting its electrical properties. The proposed research methodology can be employed as a basis for forecasting the stability of the arrester parameters under operating conditions.

Volt-ampere characteristic of the double Schottky barrier

The volt-ampere characteristic (I-V characteristic) of the double Schottky barrier located in the contact region of ZnO grains of zinc oxide based varistor ceramics is calculated using the mechanism of the above-barrier electron emission. I-V characteristic is symmetric to the polarity of the voltage U. At U > 2kBT/e (kB is the Boltzmann constant, T is the absolute temperature, e is electron charge) the electric current is saturated. The contact of ZnO grains with a double Schottky barrier behaves like an electrical circuit consisting of two oppositely connected Schottky diodes. A small maximum possible decrease in the height of the double Schottky barrier in an electric field ~ 0.7kBT ≈ 0.018 eV does not allow explaining the high nonlinearity of I – V characteristic of varistor materials by the above-barrier electron emission. The most probable cause of nonlinearity is the tunnel emission of electrons and impact ionization.

Novel Ultrahigh-Performance ZnO-Based Varistor Ceramics.

The nonlinear response of a material to an external stimulus is vital in fundamental science and technical applications. The power-law current–voltage relationship of a varistor is one such example. An excellent example of such behavior is the power-law current–voltage relationship exhibited by Bi2O3-doped ZnO varistor ceramics, which are the cornerstone of commercial varistor materials for overvoltage protection. Here, we report on a sustainable, ZnO-based varistor ceramic, without the volatile Bi2O3, that is based on Cr2O3 as the varistor former and oxides of Ca, Co, and Sb as the performance enhancers. The material has an ultrahigh α of up to 219, a low IL of less than 0.2 μA/cm2, and a high Eb of up to 925 V/mm, making it superior to state-of-the-art varistor ceramics. The results provide insights into the design of materials with specific characteristics by tailoring states at the grain boundaries. The discovery of this ZnO-Cr2O3-type varistor ceramic represents a major breakthrough in the field of varistors for overvoltage protection and could drastically affect the world market for overvoltage protection.

SOFTWARE FOR MEASURING OF DIFFERENTIAL RESISTANCE BY SCANNING OF DIGITAL IMAGES OF ANALOG OSCILLOGRAMS

A program for measuring the differential resistance of varistor materials by scanning digital images of analog oscillograms is described. The program was created using the languages HTML, CSS, JavaScript and Canvas technology. The software environment for its execution are Internet browsers. The program can be used to measure linear resistance, which limits the nonlinearity of current-voltage characteristics of varistor materials at high electric currents.

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.

Znx-1CuxMn2O4 Spinels; Synthesis, Structural Characterization and Electrical Evaluation

This work presents the structural characterization and electrical evaluation of Znx-1CuxMn2O4 spinels, which are materials presented as secondary phases into the varistor ceramic systems. Samples were analyzed by X-ray diffraction, solid-state nuclear magnetic resonance, infrared spectroscopy, scanning electron microscopy and impedance spectroscopy. Although, the addition of copper to the ZnMn2O4 spinel did not produce morphological changes, the structure and electrical behaviors changed considerably. Structurally, copper addition induced the formation of partial inverse spinels, and its addition increases significantly the electrical conductivity. Therefore, the formation of Znx-1CuxMn2O4 spinels, as secondary phases into the varistor materials, may compromise significantly the varistor efficiency.
 
 Ceramic, Impedance Spectroscopy, spinel, Varistor, XRD.

The influence of yttrium-ions on the optical and electrical behavior of PVA polymeric films

The polyvinyl alcohol (PVA) films doped with various wt% of yttrium nitrate salt were synthesized by standard solution casting technique in order to examine the functional electrical and optical properties. The degree of the crystalline structure that evaluated in the films were investigated by x-ray diffraction and Fourier transform infrared spectroscopy measurements. In addition, the surface images have been obtained via scanning electron microscopy (SEM). The optical parameters have been calculated from UV–visible-NIR transmittance spectroscopy. Also, the dielectric constant measurement and the DC resistance that are arisen in the PVA films with different wt% of Y3+-ions have been done. All the samples show semi-crystalline phases. The average size, of the cluster Y3+-ions in the SEM images, increase to 1.63 μm for PVA/37 wt% Y3+-sample. Therefore, the optical absorption of this sample is higher than the others. The value of the energy gap decrease from 5.11 eV to 4.47 eV for PVA/0.037 wt% Y3+ and PVA/37 wt% Y3+-samples, respectively. The nonlinear current—voltage behavior, at high applied voltage, of the polymeric films, is observed with different values of slope, which is a characteristic of the varistor materials. So, these films could be used in different applications like optoelectronic and varistor devices.

New Technical Parameters and Operational Improvements of the Metal Oxide Varistors Manufacturing Process

At the beginning, this article details the manufacturing procedures for varistor materials. Starting from the initial composition of two large series of varistors (those with two additive oxides and those with five additive oxides), there is a major overview of the main stages of the technological process and the equipment used for the production, emphasizing the technological changes that were made. The article continues with the study of the influence of the sintering pressure and the sintering temperature on the electrical properties of the varistor materials made before. There were two experimental series of 7 varistors, one series based on 2 additive oxides and one based on 5 additive oxides. Each varistor of these series was sintered at another temperature, the fundamental purpose being to determine an optimal sintering temperature for each chemical composition. A second activity consisted of manufacturing two more series of varistors with the same chemical composition (2 oxides and 5 additive oxides), which were sintered at two different pressures, for having a set of conclusions on the influence of sintering pressure on the electric performances. All conclusions are underlying a new process for manufacturing metal oxide based varistors.

Modeling of current distribution in zinc oxide varistors using Voronoi network and finite element method

Nonuniform current distribution inside varistor ceramics is a key factor influencing its performance and failures. Therefore understanding, modeling and predicting of current distribution in varistor ceramics is of crucial significance. This paper proposes a numerical model for simulation of nonuniform electric current distribution inside zinc oxide varistors. A numerical model is based on physical modeling of the varistor's grain-structured geometry presented by Voronoi network using finite element method (FEM) simulation. The presented method is solving complete electric field inside the modeled geometry and therefore provides a more physically accurate approach for better understanding and predicting nonuniform current distribution in the varistor. In order to properly establish a FEM model a novel approach in defining grain boundary characteristic is proposed. Thus, a macroscopic model of the varistor microstructure has been developed and the grain micro-junction boundary characteristic has been derived. The simulation results of nonuniform current distribution in a varistor agree well with measurement results for a typical ZnO varistor. The presented model enables investigation of influences of varistor geometry (shapes, sizes) and material properties on the current distribution. A new mathematical expression for varistor I–V characteristic based on Lambert function is proposed.

Further analysis of high-voltage ZnO varistor prepared from novel chemically aided method

Based on previous work, this study further investigated the high-voltage varistor materials prepared from chemically aided method. Microstructural characterization and electrical measurement results showed that the sample possessed more homogeneous microstructure and ultrahigh drop voltage per grain boundary (Vgb) than the conventional sample. The obtained ultrahigh value of Vgb is connected with the much lower donor concentration (Nd) and density of interface states (NIS) of the samples, which resulted from the increased number of active grain boundaries per unit volume. The improved homogeneous microstructure, together with the smaller grain size and narrower size distribution are essential to obtain high-voltage ZnO varistor.

Effect of SrO on the electrical barrier formation and microstructure of TiO2 varistors

TiO2-based varistor materials with SrO dopants were prepared by the mechanical mixed oxides synthesis technique using conventional sintering at 1400 °C. IV and microstructural characterization were performed. The composition range of 0.50–2.00 mol % SrO was studied and compared to pure TiO2. Experimental evidence shows that small amounts of SrO improve the nonlinear properties of the samples significantly. Optimal varistor characteristics α = 5.50 and Eb = 345 V/cm, were obtained with 1.00 mol % SrO-doped TiO2. SrO in larger amounts causes the formation of precipitates of the SrTiO3 layer on the microstructure, being deleterious to the electrical properties. Therefore, dopants such as SrO or TiO2 play a special role in the morphology of the grain boundary and nonlinear response of these materials. An atomic defect model based on the double barrier Shottky type can be adopted to explain the formation of electrical barriers in TiO2 grain boundaries.

A charge-based deep level transient spectroscopy measurement system and characterization of a ZnO-based varistor and a Fe-doped SrTiO3 dielectric

A charge-based deep level transient spectroscopy (Q-DLTS) method is applied to provide insights into the electronic behavior near grain boundaries and may provide new insights into mechanisms such as fatigue, degradation, dielectric aging, and dielectric breakdown. Here, we tested the Q-DLTS in both a ZnO varistor material and Fe-doped SrTiO3 materials. Comparisons are made to other data on ZnO varistors, and we obtain very good agreement for the energy levels. The status of deep traps in Fe-doped SrTiO3 dielectrics has been investigated where the relaxation was contrasted in a single crystal and polycrystalline ceramic materials. The relaxation is only observable in the polycrystalline materials, and was absent in single crystal Fe-doped crystals indicating that the deep traps originating from the Schottky barriers at the grain boundaries provide the DLTS signals. The energy associated with this grain boundary trap was found to be 1.26 eV.

Varistor piezotronics: Mechanically tuned conductivity in varistors

The piezoelectric effect of ZnO has been investigated recently with the goal to modify metal/semiconductor Schottky-barriers and p-n-junctions by application of mechanical stress. This research area called “piezotronics” is so far focused on nano structured ZnO wires. At the same time, ZnO varistor materials are already widely utilized and may benefit from a piezotronic approach. In this instance, the grain boundary potential barriers in the ceramic can be tuned by mechanical stress. Polycrystalline varistors exhibit huge changes of resistivity upon applied electrical and mechanical fields and therefore offer descriptive model systems to study the piezotronic effect. If the influence of temperature is contemplated, our current mechanistic understanding can be interrogated and corroborated. In this paper, we present a physical model based on parallel conducting pathways. This affords qualitative and semi-quantitative rationalization of temperature dependent electrical properties. The investigations demonstrate that narrow conductive pathways contribute to the overall current, which becomes increasingly conductive with application of mechanical stress due to lowering of the barrier height. Rising temperature increases the thermionic current through the rest of the material with higher average potential barriers, which are hardly affected by the piezoelectric effect. Hence, relative changes in resistance due to application of stress are higher at low temperature.

Construction of master sintering curves to understand the effect of Na addition on ZnO-based varistors

This work describes the effect of a small sodium addition on the densification of Bi2O3-based ZnO varistors. The characterisation was performed using both the master sintering curve tool and the analyses of structural and microstructural characteristics of the materials. It was found that sodium-free varistor material exhibited very low activation energy for densification (149.2kJ/mol). This was explained by the liquid phase promoting diffusion through the enhanced reaction between ZnO and Bi2O3. For sodium-doped material, the calculated activation energy was much higher (292.5kJ/mol) and was very close to the one obtained for pure ZnO, which is often related to the grain boundary solid-state diffusion. It was shown that a small sodium addition prevented Bi2O3 from satisfactorily reacting with ZnO. Finally, a mechanism was proposed to explain how a slight sodium amount altered the reactivity between Bi2O3 and the ZnO matrix, resulting in some major changes in the final microstructures.

Improvement in the non-linear electrical characteristics of the SnO2·Co2O3·Ta2O5 varistor material with Pr6O11 additive

The electrical and microstructural properties of SnO2-based varistor ceramic sample were improved by addition of Pr6O11. It was found that the introduction of Pr6O11 can lead to a great improvement in the threshold voltage and the non-linear electrical properties of SnO2-based varistors. As the amount of Pr6O11 increased from 0.00 to 0.5 mol%, the mean grain size decreases from 16.64 to 7.58 μm, the relative dielectric constant (at 1 kHz) increases from 1243.2 to 4534.6, the non-linear coefficient increases from 15.44 to 18.25 and the break down electrical field increases from 275.3 to 880.5 V/mm. The structure-property relationship is discussed systematically.

Investigations into Recycling Zinc from Used Metal Oxide Varistors via pH Selective Leaching: Characterization, Leaching, and Residue Analysis.

Metal oxide varistors (MOVs) are a type of resistor with significantly nonlinear current-voltage characteristics commonly used in power lines to protect against overvoltages. If a proper recycling plan is developed MOVs can be an excellent source of secondary zinc because they contain over 90 weight percent zinc oxide. The oxides of antimony, bismuth, and to a lesser degree cobalt, manganese, and nickel are also present in varistors. Characterization of the MOV showed that cobalt, nickel, and manganese were not present in the varistor material at concentrations greater than one weight percent. This investigation determined whether a pH selective dissolution (leaching) process can be utilized as a starting point for hydrometallurgical recycling of the zinc in MOVs. This investigation showed it was possible to selectively leach zinc from the MOV without coleaching of bismuth and antimony by selecting a suitable pH, mainly higher than 3 for acids investigated. It was not possible to leach zinc without coleaching of manganese, cobalt, and nickel. It can be concluded from results obtained with the acids used, acetic, hydrochloric, nitric, and sulfuric, that sulfate leaching produced the most desirable results with respect to zinc leaching and it is also used extensively in industrial zinc production.

Effect of Pr<sub>6</sub>O<sub>11</sub> Doping on the Microstructural and Electrical Properties of ZnO-Pr<sub>6</sub>O<sub>11</sub>-Co<sub>3</sub>O<sub>4</sub>-Cr<sub>2</sub>O<sub>3</sub>-SnO<sub>2</sub> Varistors

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.

English

Varistors are the most efficient devices for the protection of power transmission lines and electronic devices against excessive transient surges. The current trends that require varistors with more functions have led to research to develop new ceramic varistor material and titanium dioxide (TiO2) is the emerging new material. In this research nanosized TiO2 was taken as the base materials to which a small amount of impurities was added as dopants for enhancing the properties. Ta2O5, WO3, Bi2O3 and Co3O4 were used as dopants where the percentage of Bi2O3 and Co3O4 was fixed; 0.5% and 0.2% respectively. Samples were prepared with different weight percentages of dopants and sintered at a constant temperature of 1300 C with 2 hour’s holding time to observe the effect of these dopants on physical and mechanical properties of nanosized TiO2. Analysis was undertaken to evaluate the properties of the nanosized TiO2, such as green density, fired density, fired strength, axial and radial shrinkage. It was found that with the composition of 98.1% TiO2+0.7% Ta2O5+0.5% WO3+0.2%Co3O4+0.5%Bi2O3 have a significant influence on the physical and mechanical properties, which is anticipated to improve the performance of varistor. With the improvement in the physical and mechanical properties of TiO2, the composite material has potential application in low voltage application devices.