High Voltage Varistors Research Articles

A comparative study between the addition of nano and micro-particles of Co3O4 on the electrical and microstructural properties of a ceramic system based on SnO2

A comparative study between the addition of Co3O4 micro-particles and nano-particles as densifying dopant of a SnO2 based varistor system was conducted. The ceramic composition was (99.9-X) %SnO2–X %Co3O4–0.05 %Cr2O3–0.05 %Nb2O5 where X = 0, 0.5, 1.0, 2.0 and 4.0 mol%. Two particle sizes of Co3O4 were used (~5 µm and ~50 nm). The addition of 0.5 mol% of Co3O4 nano-particles promoted an increase of grain size of sintered samples up to 7.9 µm, that is, the maximum value among all variations. Characterization techniques such as TGA, DTA, XRD, and Rietveld analysis revealed a decrease of 16 ºC in the formation temperature of Co2SnO4 as well as an increase of 2.6 wt% in the amount of said phase with the use of 4.0 mol% of Co3O4 nano-particles in comparison with micro-particles. Statistical analysis indicated that the addition of nano-particles of Co3O4 yield better repeatability on densification of ceramic samples. Residual porosity also was decreased. Electrical breakdown and non-linear coefficient values correspond to a non-ohmic behavior with potential application on manufacture of high voltage varistors. The findings of this work can be used as a reference for conducting a later study to improve the electrical properties or even to lower the sintering temperature.

Synthesis of SiO2-Coated Core–Shell ZnO Composites for Preparing High-Voltage Varistors

Monodispersed ZnO composite microspheres were successfully prepared by a facile ultrasound irradiation method. Then, the uniform core–shell structured composites were synthesized through the hydrolysis of tetraethyl orthosilicate on the surface of the ZnO composite microspheres. Microstructural studies of the as-obtained powders were carried out using the techniques of the x-ray powder diffraction, field emission scanning electron microscopy and transmission electron microscopy with energy dispersive x-ray spectroscopy. The results show that the pink ZnO composite powders as the core were spherical structures with the size of approximately 100 nm, and the SiO2 shell was fully coated on the surface of the core. On the basis of these results, the effect of SiO2 content on the thickness of the synthesized composites and microstructure, as well as the electrical properties of the ZnO varistors sintered in air at 1150°C for 2 h, were fully studied. In particular, the ZnO varistor prepared with the appropriate amount of the SiO2 coating (∼40 nm) leads to a superior electrical performance with the high breakdown voltage of 418 V mm−1 and an excellent nonlinear coefficient of 70.7, compared with the varistors obtained without the SiO2 coating. The high performance is attributed to the smaller and more homogeneous ZnO grains obtained via the SiO2 coating.

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.

Preparation of doped ZnO nanopowders by refluxing method and applications in high voltage varistors

The doped ZnO nanopowders were successfully prepared via simple hydrolysis followed by refluxing route in alcohol medium. Several analytic techniques such as XRD, TEM and SEM were used to make characterizations of the as-synthesized samples. The influence of refluxing time on the morphology and average size of the products was investigated. Rod-like doped ZnO powders were easily prepared by refluxing for 5 h. With the elevation of the refluxing time to 20 h, nearly monodispersed sphere-like ZnO nanoparticles can be obtained and the diameter is about 30–50 nm. The results show that refluxing time plays an important role in the formation and oriented growth of ZnO nanocrystals. The powders were consolidated into dense varistors discs by compaction, sintering and evaluated for their electrical characteristics. The varistors can be produced possessing a nonlinear coefficient of ~56, a breakdown voltage of ~7.09 kV/cm.

Preparation of high-voltage ZnO varistor ceramics

We have examined the effect of dopant concentration on the properties of ZnO varistor ceramics and determined the composition and conditions of the synthesis of ceramic powders and sintering of ceramics at a temperature of 925°C. We obtained a breakdown voltage of 2.8–3.0 kV/mm and nonlinearity coefficient in the range 48–55.

Novel SnO2 ceramic surge absorbers for low voltage applications

To make SnO2–CoO–Nb2O5–Cr2O3–Y2O3–Bi2O3 ceramics suitable for low voltage varistor applications, the amount of Bi2O3 addition to a well-known SnO2-based composition for high voltage varistors was optimized. The addition of 0.15mol% Bi2O3 has resulted in a single phase ceramic material with the average grain size of 30μm and electrical properties useful for low voltage varistors. The obtained SnO2 low voltage varistors exhibit excellent electrical properties as surge absorbers.

Ultra-high breakdown field varistors prepared from individually synthesized nanoprecursors

Previously synthesized nanocrystalline ZnO doped with cobalt and manganese, Bi2O3 and Sb2O3 were the precursors for high voltage varistors preparation. Varistor ceramics were sintered at 750°C, 770°C or 900°C for 1h. An amount of 30 mass% of secondary phases and sintering temperature of 770°C were found to be optimal for obtaining varistor with prominent properties. Achieved density was 97% of the theoretical value. Post-sintering phase composition included ZnO, γ-Bi2O3 and pyrochlore-type with Zn2Bi3Sb3O14 nominal formula. Fine grained microstructure was accompanied with good phase distribution and excellent electrical characteristics: extraordinary high breakdown field of 3200Vmm−1, high nonlinearity coefficients of 71 and 51 in 0.1–1 and 1–10mAcm−2 current density ranges, respectively, and low leakage current of 7.7μAcm−2.

Testing of high voltage surge protection devices for use in liquid argon TPC detectors

In this paper we demonstrate the capability of high voltage varistors and gas discharge tube arrestors for use as surge protection devices in liquid argon time projection chamber detectors. The insulating and clamping behavior of each type of device is characterized in air (room temperature), and liquid argon (90 K), and their robustness under high voltage and high energy surges in cryogenic conditions is verified. The protection of vulnerable components in liquid argon during a 150 kV high voltage discharge is also demonstrated. Each device is tested for argon contamination and light emission effects, and both are constrained to levels where no significant impact upon liquid argon time projection chamber functionality is expected. Both devices investigated are shown to be suitable for HV surge protection applications in cryogenic detectors.

Microstructure control to reduce leakage current of medium and high voltage ceramic varistors based on doped ZnO

The leakage current (If) and the non-linearity coefficient (α) are crucial parameters in varistors. This work, deals with the optimization of the electrical characteristics of medium and high voltage varistors based on the ZnO–Bi2O3–Sb2O3 system. First, the aim was to allow the formation and the stabilization of the spinel phase by reducing the heating rate. To do so, the right sintering temperature and dwell time have to be chosen to obtain a breakdown field suitable for industrial requirements. During cooling, the spinel phase can return to a pyrochlore phase, which contributes to the increase of leakage current. In a second part, this reaction was prevented by faster cooling in an appropriate temperature range. The fine-tuning of both the heating and cooling phases leads to a significant decrease of the leakage current. Moreover, the value of the non-linearity coefficient was increased by 80%, due to better and more homogeneous wetting of the ZnO grains by the Bi-rich phase.

Tunable Nanodielectric Composites

This paper presents a progress update with the development of nanodielectric composites with electric field tunability for various high energy, high power electrical applications. It is demonstrated that nonlinear electrical/dielectric properties can be achieved via the nanostructure and interface engineering. A high level summary was given on the progress achieved as well as challenges remaining in nanodielectric engineering towards high energy density capacitors for energy storage and conversion, nonlinear dielectrics for tunable device, and high voltage varistor for surge suppression.

ZnO Varistor Defective Gd and Pr Ions

To the best of our knowledge the difference in the magnetic properties of ions like Gd (7.94 μB) and Pr (3.58 μB) on the electrical properties of ZnO-varistor have not been studied. To clarify this effect, the binary systems, ZnO-Gd2O3, ZnO-Pr6O11and the ternary system ZnO-Gd2O3-Pr6O11were studied. The ceramics were prepared according to the chemical formula; (100-X) mol% ZnO - (X) mol% Y, where X = 0.5, 1, 2, 3.5, 5, 6, 7.5, 9 and 15, Y is Gd2O3, Pr6O11or Gd2O3 +Pr6O11. The samples have been sintered at 1200°C, 1250°C, 1300°C, 1350°C and 1400°C for 1 hr. X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray (EDAX), (JxE), (CxV) and Electron Spin Resonance (ESR) have been studied and discussed. Utilizing J-E characteristics, the non-linearity coefficient (α), leakage current (Lg) and breakdown electric field (Eo) have been determined. In the present study, the effect of the magnetic properties on the non-linearity parameters was not clear and the non-linearity characteristics mainly depend upon the microstructure of the system. The ternary system (94%ZnO-4%Pr6O11- 2%Gd2O3) is superior to the binary systems, and can readily serve as a high-voltage varistor.

Isothermal depolarization current spectroscopy of localized states in metal oxide varistors

A new method for the processing of time-dependent isothermal depolarization current in oxide varistors is proposed. It involves (i) a new model for kinetics of the depolarization current, which is related to the parameters of the localized states, (ii) an account of the distribution of the relaxation time (which takes place due to the structure inhomogeneity of ceramics) using the Kohlrausch–Williams–Watts stretched exponential function and (iii) the determination of the most probable relaxation time. The proposed method was tested using ZnO high-voltage varistors. The ionization energy (0.06 and 0.16 eV) and the electron capture cross-section (∼10−17 and ∼10−16 cm2) of the bulk donor states in two ZnO varistor ceramics were estimated.

Analysis of High-Voltage ZnO Varistor Prepared from a Novel Chemically Aided Method

High-voltage varistor materials were prepared using an ultrafine additives precursor obtained from the precipitation method. Microstructural and electrical studies showed that the sample exhibited a smaller average grain size (d=4.5 μm) and a considerably higher breakdown voltage (Eb=1610±30 V/mm) as compared with the conventional one (d=6.5 μm, Eb=400±30 V/mm). The high voltage is attributed to the improvement in microstructure homogeneity and the formation of more active grain boundaries per unit volume. This novel route may explore the commercial possibility of device miniaturization.

HIGH POTENTIAL GRADIENT OF Y2O3-DOPED THICK FILM VARISTORS

The electrical and microstructural properties of a series of ZnO -based thick film varistors (TFVs) doped with 0.00, 0.02, 0.04, 0.06, 0.08, and 0.10 mol% Y 2 O 3 were studied. It was found that sample doped with 0.08 mol% Y 2 O 3 showed the highest potential gradient of 3159.4 V/mm with a leakage current of 36.4 μA and a nonlinear exponent of 13.1. The ZnO grain size decreased with increasing Y 2 O 3 content, which was the origin for the increase in potential gradient. Raman spectra results showed that the tensile stress increased linearly with Y 2 O 3 doping. Larger tensile stress was considered to result from the lattice distortion and inevitably influenced the grain boundary characteristics. While the Y 2 O 3 doping concentration was beyond 0.08 mol%, the effect of residual stress on electrical properties was much more remarkable than that of grain size, leading potential gradient to be weakened. As a result, high potential gradient of ZnO -based TFVs could be obtained with Y 2 O 3 doping concentration of 0.08 mol% and qualified as excellent candidates for high voltage varistor application.

Influence of compaction manufacturing process on the physical and electrical characteristics of high-voltage varistor

The zinc oxide varistor was originally used as surge protection in electronic circuits. Its energy handling capability was extensively developed by many companies for use in high-voltage surge arresters. Although varistor technology is well known, there is a need for continuous improvement in the varistor manufacturing process in order to achieve a reliable product that meets the requirements of the various international standards. This project focuses directly on the optimization of dry pressing techniques in the manufacturing process of high-voltage varistors in order to attain uniform density, low porosity and uniform distribution of secondary intergranular phase within the grains. The microstructural characteristics of varistor discs pressed in a double-action press machine (70 MPa) before (green) and after sintering are similar to those pressed under isostatic pressure, indicating the efficiency of double-action pressing. The breakdown electric field at 10 mA and nonlinearity coefficient (α) remain practically constant with increasing compaction pressure, increasing only in samples pressed above 70 MPa.

Nonlinear electrical characteristics of SnO2∙CuO ceramics with different donors

The effects of various donors such as Nb, Sb, Ta, and V on the densification and nonlinear current-density J–electrical-field E relations of tin oxide ceramics are investigated. The room-temperature resistivity ρ and the three vital varistor parameters, the nonlinear coefficient α, breakdown electrical field EB, and leakage current density JL, are studied as a function of donor concentration. Minor donors make highly resistive SnO2∙CuO ceramics nonlinear or conductive. The optimum doping samples with tantalum or niobium show promising properties for high-voltage varistor application.

Inversion boundary induced grain growth in TiO 2 or Sb 2O 3 doped ZnO-based varistor ceramics

In low-voltage varistor ceramics, the phase equilibrium and the temperature of liquid-phase formation are defined by the TiO 2/Bi 2O 3 ratio. The selection of a composition with an appropriate TiO 2/Bi 2O 3 ratio and the correct heating rate is important for the processing of low-voltage varistor ceramics. The total amount of added Bi 2O 3 is important as the grain growth is slowed down by a larger amount of Bi 2O 3-rich liquid phase at the grain boundaries. Exaggerated grain growth in low-voltage varistor ceramics is related to the occurrence of the liquid phase and the presence of TiO 2 which triggers the formation of inversion boundaries (IBs) in only a limited number of grains, and as a result the final microstructure is coarse grained. The Zn 2TiO 4 spinel phase only affects grain growth in compositions with a TiO 2/Bi 2O 3 ratio higher than 1.5. In high-voltage varistor ceramics, just a small amounts of Sb 2O 3 trigger the formation of IBs in practically every ZnO grain, and in compositions with a Sb 2O 3/Bi 2O 3 ratio lower than 1, grain growth that is controlled entirely by an IBs-induced grain growth mechanism results in a fine-grained microstructure. The spinel phase interferes with the grain growth only at higher Sb 2O 3/Bi 2O 3 ratios.

Cerâmicas eletrônicas à base de SnO2 e TiO2

Este trabalho mostra o avanço de pesquisas realizadas em cerâmicas eletrônicas à base de SnO2 e TiO2. A adição de diferentes dopantes, bem como a realização de tratamentos térmicos em atmosfera oxidante e redutora, influenciam na densificação, tamanho médio de grãos e nas propriedades elétricas da cerâmica varistora à base de SnO2. Resultados obtidos por medidas de espectroscopia de impedância mostram as variações de altura e largura da barreira de potencial em função da atmosfera de tratamento térmico. Altos valores de coeficiente de não-linearidade foram obtidos para os sistemas à base de SnO2, característicos de varistores de alta tensão tal como os varistores comerciais de ZnO. Neste trabalho, também serão apresentados estudos de cerâmicas policristalinas à base de TiO2 e (Sn,Ti)O2, as quais apresentam resposta elétrica I-V não linear típica de sistemas varistores de baixa tensão. Todos estes sistemas são potencialmente promissores à aplicação como varistor.

Finite element modelling of the electrical impulse induced fracture of a high voltage varistor

In testing and in service, varistors are subjected to very short (μs range) high current pulses. Due to the inertia effects that appear on rapid Joule heating dynamic stress waves are generated, which can cause brittle failure. An analytical solution for the one-dimensional case was presented recently by Vojta and Clarke. In this work a full three-dimensional analysis of an axisymmetrical varistor has been performed using Finite Element Simulation. The reflections of the stress waves from the bases and the shell of a varistor and their interference are analyzed. The resulting stress field and its development with time is much more complex than in the 1D case. The aspect ratio of the varistor has been shown to have a strong influence on the amplitude of the mechanical stresses and can be varied to minimize the maximum stress level reached. Damping has been considered but found to be negligible in realistic cases.

Novel route for preparation of high voltage varistor powder

A novel technique for preparing ZnO varistor powder is described. It involves preparation of gel by adding citric acid to nitrate solution of varistor constituents. The gel on subsequent calcination forms submicron reactive varistor powder that exhibits the homogeneity and stoichiometric control associated with the chemical methods of preparation. SEM and BET surface area measurements show the resultant powder to comprise crystallite of submicron size. High field varistors with good nonlinear coefficient result from sintering at 1150 °C for 1 hour. The high value of nonlinear coefficient of the varistor sample can be attributed to the more uniform distribution of dopants and formation of more varistor active grain boundaries. The electrical characteristics of the citrate gel derived varistor are compared to those of a varistor sample prepared through the urea route.