Commercial Varistors Research Articles

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.

Electrical properties of structures based on varistor ceramics and polymer nanocomposites with carbon filler

The characteristics of a two-layer structure on the basis of the layers of varistor ceramics and polymeric PPTC nanocomposite being in thermal contact for the purpose of using it as a limiter of constant voltage and long-term varying electrical overvoltages are analyzed. A theoretical model of such a structure has been developed, and its main electrical characteristics are simulated. It is shown that the provision of the required output voltage limitation is performed by selecting the classification voltage of the varistor layer. The maximum current of the varistor layer required for heating the structure is determined by the intensity of heat transfer to the environment. It has established a satisfactory agreement between the theoretical and experimental electrical characteristics for the structure based on the layers used in commercial varistors and PPTC fuses.

Statistical evaluation of grain boundaries of varistors with modified bismuth oxide

AbstractOxide varistors are made of inhomogeneous material whose properties are determined by active grain boundaries. It is essential that in the microstructure of a varistor only active grain boundaries are present as only such boundaries are involved in the process of conduction. Commercial varistors are characterized by a microstructure with a large amount of electrically inactive areas which include zinc-antimony spinel, bismuth oxide, and pores. Studies on elimination of inactive grain boundaries, which are the intergranular areas rich in reaction products of varistor components and pores, lead to an improvement in the microstructure, thereby improving the electrical properties of the varistor. The results were evaluated using statistical methods, defining the percentage of active grain boundaries in the varistor. Statistical analysis showed that the best results were obtained for a bismuth oxide varistor doped with antimony oxide, containing nearly 100 % conductive grain boundaries in its body.

Progressive degradation of high voltage ZnO commercial varistors upon Fe2O3 doping

In the complex multicomponent formulation of standard commercial varistors used in high tension devices, those based in the ZnO–Bi2O3–Sb2O3 (ZBS) ternary system, the presence of iron is ordinarily prevented in any of its plausible sources: dopant, impurity and/or unwanted contamination. Otherwise the material undergoes a systematic degradation in its electrical performance which rapidly disallows it for practical uses. In order to understand the origin of such degradation phenomena, we have deliberately doped the ZBS varistor composition with different amounts of Fe2O3. The research here presented evidences that the prime reason for the varistor electrical ruination is the progressive incorporation of iron inside the spinel secondary grains, which eventually leads to the formation of a highly conductive skeleton of secondary phases with an unbearable level of leakage currents below the varistor breakdown voltage.

Zinc Oxide Doped Red Sea Egyptian Clay as a Varistor

Clay consists of about 50% SiO2 + 25% Al2O3 plus some other oxides with low abundance ratio like CaO, TiO2, Fe2O3, MgO and Mn2O3. Burning the clay at 700°C removes out the organic compounds and the mentioned oxides only remain. Our aim is to dope ZnO by the remaining oxides for varistors fabrication. Six types of clay were collected from Egyptian Red Sea Coast mines, after burning for 2 hrs the remaining oxides were mixed and ball milled with ZnO for 10 hrs according to the formula (100-Xn) ZnO +Xn, where n is the clay type and X is the ratio of the clay in grams, X takes the values 0.525 in 15 steps. Samples were pressed and sintered at three different temperatures (Tsin) 1200°C, 1300°C and 1400°C for 1 hr, and then studied via XRD, SEM, EDAX, J-E and C-V measurements. The obtained results were discussed in terms of the microstructure of the samples and the formation of Schottky barriers. Barrier height Ф, width W, interface state density Ns and donor state density Nd were calculated for the samples with highest nonlinearity α. It was found that the breakdown electric field Eo is related exponentially with the ratio of Zinc silicate phase whereas α scales with ratio of ZnO phase and the later increases linearly with Tsin. The electrical characteristics of the most promising sample were compared with those of ZnO samples doped pure SiO2 and Al2O3 obtained at the same preparation condition. This sample is comparable with the Japanese commercial varistor 5N220k Dc JVR.

Effect of Sb Addition on the Electrical Characteristics of Bi-Based ZnO Varistors Containing Zr and Y

The effects of adding Sb to a BiMnCoSiCrNiYZr-added ZnO varistor (with the same composition as a commercial varistor) on the varistor voltage, leakage current, and resistance to electrical degradation were investigated. Bi is incorporated in spinel particles, and δ-Bi2O3eventually disappears with the addition of small amounts of Bi, especially as the amount of Sb2O3added increased. Reduction in both the nonlinearity index and the amount of δ-Bi2O3for small amounts of added Bi with the addition of more than approximately 1.25 mol% Sb2O3demonstrates that Sb inhibits Bi2O3from forming deep interfacial impurity levels at the grain boundaries. The sample containing 1.2 mol% Bi2O3, 1.0 mol% ZrO2, 1.0 mol% Y2O3, and 1.5 mol% Sb2O3added exhibits a high varistor voltage (approximately 630 V/mm), high resistance to electrical degradation and low leakage current.

Nanofillers in ZnO based materials: a ‘smart’ technique for developing miniaturized high energy field varistors

The present work encompasses the strategic design, synthesis and development of high energy field ZnO varistors. The key aspect of the study involves the chemical synthesis of ZnO based nanorod nanofillers by a pH selective precipitation and reflux method. High field varistors were developed out of these crystalline nanofillers by employing them as fillers to the micron sized commercial varistor powder. A systematic investigation of the microstructural changes of the nanofiller added varistor samples were carried out under step-sintering and the normal sintering conditions. Comparative studies were also performed with the nano and micro counterparts. The influence of nanofillers in the varistor powder packing, grain structure refinement, grain size reduction and on the I–V properties were analyzed under the step-sintering conditions. Based on the performance analysis, the current study foresees ample opportunities for miniaturizing varistors with at least half the size of currently available varistors.

Effects of Sb, Zr, and Y addition on the electrical characteristics of Bi-based ZnO varistors

The effects of adding Sb, Zr, and Y to a Bi–Mn–Co–Si–Cr–Ni-added ZnO varistor (with the same composition as a commercial varistor) on the varistor voltage, leakage current, and resistance to electrical degradation were investigated. Both the varistor voltage and the nonlinearity index of the voltage–current characteristics a before electrical degradation for samples added 1.0 mol% ZrO2 and 1.0 mol% Y2O3 were maximized at approximately 1.0 mol% Sb2O3 and decreased rapidly when the amount of Sb2O3 exceeded more than about 1.0 mol%. Samples added 1.0 mol% ZrO2, and 1.0 mol% Y2O3, and less than 0.75 mol% Sb2O3 had a comparable leakage current to samples added neither ZrO2 nor Y2O3. Sb is an important element that is necessary to obtain a high varistor voltage, a low leakage current, and an excellent resistance to electrical degradation. However, the same varistor voltage can be obtained by adding Y and reducing the amount of Sb2O3 added.

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

The effect of simultaneously adding Zr and Y to Bi–Mn–Co–Sb–Si–Cr–Ni-added ZnO varistors (having the same composition as a commercial varistor) on the varistor voltage, leakage current, and resistance to electrical degradation were investigated. Varistor voltage increased with increasing amount of Y for addition of 0–2 mol % Zr. On the other hand, the nonlinear coefficient α prior to electrical degradation changed very little on the addition of both Y and Zr. With the addition of approximately 1 mol% Zr, the leakage current decreased with increasing amount of Y added. A ZnO varistor with a varistor voltage of approximately 600 V/m, a low leakage current, and excellent resistance to electrical degradation was fabricated by adding approximately 2 mol% Y and approximately 1 mol% Zr.

Synthesis of novel core-shell nanocomposites for fabricating high breakdown voltage ZnO varistors

A novel varistor-composite synthesis for the fabrication of high breakdown voltage ZnO varistors has been successfully developed. It involves coating ZnO nanoparticles with SiO2 through the hydrolysis of tetraethyl orthosilicate (TEOS) and the subsequent coating of additives on the core of the SiO2-coated ZnO particles, producing a double core-shell structure. The effect of the amount of TEOS used on the morphology of the synthesized composites and the microstructure, as well as the electrical property of the sintered samples from the double core-shell composites are fully studied. Such materials, with the appropriate amount of TEOS added (Si4+/Zn2+ = 0.15 moles) enables the superior electrical performance of ZnO varistors: high breakdown voltage (VB = 1123 V mm−1), excellent nonlinear coefficient (α = 42.5) and low leakage current (IL = 8.5 μA), compared with current commercial varistors and the sample from single core-shell composites. The high performance is attributed to the smaller ZnO grain size and perfect additive homogeneity. The SiO2-coating provides a better way to control the varistor microstructure through the formation of a nearly circular distribution of blocky Zn2SiO4 around the ZnO grains.

Zr, Y, Sb添加のZnOバリスタの課電劣化への影響

The effects of amount of Sb added to Bi-Mn-Co-Sb-Si-Cr-Ni-Zr-added ZnO varistors on the tolerance characteristics of electrical degradation were investigated. Furthermore, the effects of addition of both Zr and Y to Bi-Mn-Co-Sb-Si-Cr-Ni-added ZnO varistors (the same composition as one of commercial varistors) on the varistor voltage and leakage current were investigated. It was found that addition of Sb enhances the tolerance characteristics to the electrical degradation and the amount of Sb2O3 addition of approximately 1 mol% is the optimal. When the amount of added Sb increased, the amount of existence of a Zn2.33Sb0.67O4-type spinel particle increased, because the amount of existence of Zr compound increased similarly. It was suggested that Zr compound was also related to the electrical degradation characteristic. Varistor voltage decreases a little when the amount of added Zr increases in simultaneous addition of Zr and Y. On the other hand, the value of nonlinear index α hardly changed. Without Zr, the increase in the leakage current accompanying the increase in the amount of added Y, which has been reported conventionally, was not found in this study. However, adding Zr of approximately 0.2 - 0.45 mol%, the leakage current increased by adding Y more than 1 mol% and adding Zr of approximately 0.6 mol%, it decreased.

Effects of Zr and Y Addition on the Electrical Degradation Characteristics of ZnO Varistors

The effects of the addition of Y or both Y and Zr to Bi-Mn-Co-Sb-Si-Cr-Ni-added ZnO varistors (the same composition as one of commercial varistors) on the varistor voltage and the tolerance characteristics of electrical degradation were investigated. The tolerance characteristic of electrical degradation was deteriorated rapidly by addition of Y of more than approximately 1 mol%. The degree of deterioration for addition of both Zr and Y was smaller than that for addition of only Y. Moreover, the number of formed spinel (Zn2.33Sb0.67O4)-type particles monotonically increased with increasing amount of Y. On the other hand, the number of formed willemite (Zn2SiO4)-type particles decreased by addition of Y of more than approximately 1 mol%. It was suggested that the deterioration of the tolerance characteristics of electrical degradation by the addition of Y is probably caused by an increase in the number of willemite-type particles. Moreover, it was suggested that the reduction in the average ZnO grain size due to the addition of Y was a major factor in the increased varistor voltage and the ZnO grain growth was inhibited by the formation of an unknown compound, of which main ingredients are Y, Zn, and Sb or Y, Zr, Zn, and Sb, after adding Y. The varistor voltage of a varistor with 2 mol% added Y increased by approximately 50% compared to a varistor with no Y added. A ZnO varistor with a varistor voltage of approximately 600 V/mm and excellent tolerance characteristics of electrical degradation was fabricated by adding approximately 1.25 mol% Y and approximately 0.45 mol% Zr to a Bi-Mn-Co-Sb-Si-Cr-Ni-added ZnO varistor.

First Study of Oxygen Diffusion in a ZnO - Based Commercial Varistor

Oxygen diffusion coefficients were determined in a commercial ZnO-based varistor by means of the gas-solid exchange method using the isotope 18O as the oxygen tracer. The diffusion annealings were performed at 892, 942, 992 and 1092oC, in an Ar + 18O2 atmosphere under an oxygen partial pressure of 0.2 atm. After the diffusion annealings, the 18O diffusion profiles were established by secondary ion mass spectrometry (SIMS). The results show an increase of the oxygen diffusion in the varistor, both in bulk and in grain boundaries, when compared to the oxygen diffusion in undoped ZnO. The increase of the oxygen bulk diffusion in the varistor agrees with an interstitial mechanism for the oxygen diffusion. The results also show that the grain boundary is a fast path for the oxygen diffusion in the varistor. However, the oxygen diffusion in the grain boundaries of the varistor seems to depend on several chemical and microstructural parameters and does not allow a simple explanation.

High performance ZnO varistors prepared from nanocrystalline precursors for miniaturised electronic devices

An industrially viable solution-based processing route using minimal amounts of solvent has been used to prepare bulk quantity nanopowders (average particle size 15 ± 3 nm) for the fabrication of ZnO varistors. The xerogels, calcined powders and sintered materials were fully characterised. The preparation of varistors from nanopowders has been optimised by studying the effect of temperature on grain growth, densification and breakdown voltage. The varistors are prepared by sintering at 1050 °C for 2 hours, a temperature that is significantly lower than that used in the current industrial process. Highly dense varistor discs prepared from the sintered material produce devices, with a breakdown voltage 85% higher than that of commercial varistors, making them suitable for use in miniaturised electronic circuitry. Improved performance of these materials has been attributed to the small grain size and better dispersion of additives on ZnO grains.

Discussions on Nonuniformity of Energy Absorption Capabilities of ZnO Varistors

The energy absorption capability is a key parameter for ZnO varistors. This paper discusses the nonuniformity of energy absorption capabilities of ZnO varistors. With experiment results, we show that energy absorption capabilities of ZnO varistors can be highly scattered, for different commercial varistors, and for the same varistors under diverse current waves. We explain this nonuniformity phenomenon through numerical simulations, based on the microscopic circuit and heat transfer models of ZnO varistors. How the applied current, the nonuniformity of microstructure, and the nonuniformity of microstructural electrical and thermophysical characteristics influence the energy absorption capabilities of ZnO varistors is also analyzed.

Caracterização e perspectivas de uso de concentrados de óxidos de terras-raras para fabricação de cerâmicas varistoras à base de óxido de zinco

Este trabalho tem por objetivo verificar a adequabilidade do uso de concentrados de óxidos de terras-raras (COTR) para a fabricação de varistores à base de óxido de zinco. COTR e misturas produzidas em laboratório foram utilizados na fabricação de varistores à base de óxido de zinco (ZnO) segundo o método cerâmico. Análises granulométricas, térmicas, química por fluorescência de raios-X e de fases por difração de raios-X foram empregadas na caracterização dos COTR. A caracterização microestrutural dos varistores produzidos foi feita a partir dos resultados de microscopia eletrônica de varredura, espectroscopia de raios-X por dispersão de energia e densimetria, e a caracterização elétrica teve por base os dados de tensão-corrente e tensão-capacitância. Os resultados obtidos mostram que existem perspectivas para a utilização de COTR na fabricação de cerâmicas varistoras à base de ZnO, as quais dependem, essencialmente, da natureza das impurezas ou do grau de refino químico dos COTR empregados. Varistores com coeficientes de não-linearidade entre 12 e 40 foram obtidos, alguns com valores de corrente de fuga comparáveis àqueles característicos de varistores comerciais.

Effect of step sintering on breakdown voltage of varistors prepared from nanomaterials by sol gel route

ZnO varistor materials were prepared by a sol gel route with subsequent drying and calcination. Varistor discs fabricated from these materials were subjected to a two step sintering schedule. Therefore in a typical experiment, the samples were heated to 1000°C, then allowed to cool for over 30 min to 900°C and held there for 6 h. The results were compared with commercial varistor samples sintered in a similar way. Considerably higher breakdown voltages were obtained for the varistors made from nanosample (1192±30 V mm−1) compared with the commercial samples (723±30 V mm−1) sintered under the same experimental conditions. The sintered materials were characterised by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and density measurements.

Microstructural analysis of varistors prepared from nanosize ZnO

ZnO nanoparticles were prepared by a solid state pyrolysis reaction of zinc acetate dihydrate and oxalic acid dihydrate at 500°C. The course of reaction at various temperatures was followed by XRD. Subsequently varistors were fabricated from this nano-ZnO material by solid state mixing with various oxide additives and sintering to 1050°C. The microstructure of the sintered material was studied using XRD, field emission SEM (FESEM), and EDX, and ZnO grains, bismuth rich regions and spinel phases were identified. Discs made from oxide doped nanoZnO show considerably higher breakdown voltage (656 ± 30 V mm-1) compared to those prepared from micrometre sized ZnO (410 ± 30 V mm-1) and commercial varistors (454 ± 30 V mm-1). However, varistors made from the nano-ZnO show very low densification and high leakage current, making them unsuitable for device fabrication.

Nonlinear electrical behaviour of the WO3-based system

Varistors can limit high transient voltage surges and can repeatedly endure such surges without being destroyed, therefore they are usually used to protect electronic circuits from voltage pulse shocks. The most important property of a varistor is its nonlinear voltage–current characteristic. This can be expressed by the equation I = K V α , where α is the coefficient of non-linearity, the essential parameter to scale the nonlinearity. Commercial varistors used in protection systems are based on SiC or ZnO. Varistors based on SiC have low coefficients (of non-linearity) [1]. ZnO varistors exhibit high coefficients (of nonlinearity), but the degradation problem of ZnO varistors has not been resolved [2, 3]. While efforts to improve the temperature stability of ZnO varistors are being made, the search for new varistor materials is ongoing. In 1995, Pianaro found a new varistor material, (Co, Nb)-doped SnO2, which is single phase with the rutile structure [1]. In 1999, Kim et al. found a one-step-air-fired SrTiO3-based ceramic which had varistor characteristics [4]. In 2000, Wang found that only one oxide (Sb2O3)-doped TiO2 ceramics shows varistor behavior [5]. Following Wang, in 2002, Su found another TiO2 varistor, doped with only one oxide (WO3) [6]. In this letter, the processing of a new WO3-based varistor system, WO3·Na2CO3·CuO/CdO/Bi2O3/ Sb2O3, as well as its nonlinear properties (electrical field as a function of current density) and dielectric properties are described. The raw chemicals used in this study were analytical grades of WO3 (99%), Na2CO3 (99.8%), CuO (98%), CdO (99%), Bi2O3 (99%), Sb2O3 (99.9%) and obtained from Shanghai Chemical Company. The composites investigated in the present work contain a molar ratio of WO3:Na2CO3:X = 96.5:0.5:3, where X = CuO, CdO and WO3:Na2CO3:Y = 98:0.5:1.5, where Y = Bi2O3, Sb2O3. Varistors were obtained by conventional ceramic processing. The mixed raw chemicals were milled in nylon pot for 15 h with ZrO2 balls and some distilled water, dried, mixed with 0.5 wt% PVA binder and pressed into disks 15 mm in diameter and 1.5 mm in thickness at 180 MPa. After burning out the PVA binder at 650 ◦C, the disks were sintered in air at 1000 ◦C for 60 min and then slowly cooled to room temperature. During sintering, compacts were surrounded with powders of matching compositions and covered with crucibles to reduce evaporation. The frequency dependence of the relative dielectric constant and complex impedance spectra were obtained using an impedance analyzer (Agilent 4294A). For electrical characterization of current density as a function of applied electrical field, a semiconductor I–V grapher (QT2) was used. The coefficient of non-linearity α was obtained from

New ZnSnO3-based varistor system

Semiconducting ceramics are widely used in electrical industries involving mobile communications, computers, signal processing, power transport, and control systems, because of their unique and useful electrical characteristics [1]. Varistors which are generally used to protect electronic circuits from voltage shock can sense and limit high transient voltage surges and can repeatedly endure such surges without being destroyed. The most important property of a varistor is its nonlinear voltage–current characteristic. It can be expressed by the equation I = K V α . The α coefficient gives the degree of nonlinearity and the constant K depends on the microstructure and is related to the electrical resistivity of the material. Commercial varistors used in protection systems are based on SiC or on ZnO. Varistors based on SiC have low nonlinearity coefficients [2–4] and ZnO varistors exhibit high nonlinear coefficients, but the degradation problem of ZnO varistors has not been resolved [5–7]. Therefore, the efforts to find new varistor materials have been ongoing. In 1983, N. Yumaka, M. Masuyama found that SrTiO3-based ceramics made by a two-step process had varistor characteristics [8]. In 1995, S. A. Pianaro found a new varistor material [4], (Co,Nb)-doped SnO2, which has only a single phase, rutile structure. In 2000, J. F. Wang found one oxide (Sb2O3)-doped TiO2 ceramic to have varistor behavior [9]. Following Wang, W. B. Su found, in 2002, another TiO2 varistor doped with only one oxide (WO3) [10]. In this work, we found a new varistor material, (Nb,Si)-doped ZnSnO3, and investigated the effects of SiO2 on the properties of the (Nb,Si)-doped ZnSnO3 varistor. The materials used were analytical grades of SnO2 (99.5%), Nb2O5 (99.5%), ZnO (99.5%), and SiO2 (90.0%). The compositions were SnO2 + ZnO + 0.2%Nb2O5 + x%SiO2 in molar terms, where x = 0, 0.25, 0.4, 0.5, 1.0. The varistors were prepared by conventional ceramic processing. The mixed raw chemicals were milled in a nylon kettle with ZrO2 balls and some distilled water, dried, mixed with 0.6% weight of PVA binder and pressed into disks 15 mm in diameter and 1.5 mm in thickness at 160 MPa. The disks were sintered at 1427 ◦C for an hour and cooled to room temperature after burning out the PVA binder at 650 ◦C. To measure the electrical properties, silver electrodes were made on both surfaces of the sintered disks. For microstructure characterization, the surfaces of the samples were observed by scanning electron microscopy (SEM) and the phases were analyzed by X-ray diffraction (XRD). For electrical characterization of current density versus applied electrical field, an I–V grapher (QT2) was used. The complex impedance dependent on frequency is measured using an Agilent 4294A impedance analyzer. According to the XRD analysis (Fig. 1), no apparent second phase was observed and SnO2 and ZnO should be synthesized according to the following reaction [11]: