Varistor Discs Research Articles

Application of the Polynomial Chaos Expansion to the Uncertainty Propagation in Fault Transients in Nuclear Fusion Reactors: DTT TF Fast Current Discharge

Nuclear fusion reactors are composed of several complex components whose behavior may be not certain a priori. This uncertainty may have a significant impact on the evolution of fault transients in the machine, causing unexpected damage to its components. For this reason, a suitable method for the uncertainty propagation during those transients is required. The Monte Carlo method would be the reference option, but it is, in most of the cases, not applicable due to the large number of required, repeated simulations. In this context, the Polynomial Chaos Expansion has been considered as a valuable alternative. It allows us to create a surrogate model of the original one in terms of orthogonal polynomials. Then, the uncertainty quantification is performed repeatedly, relying on this much simpler and faster model. Using the fast current discharge in the Divertor Tokamak Test Toroidal Field (DTT TF) coils as a reference scenario, the following method has been applied: the uncertainty on the parameters of the Fast Discharge Unit (FDU) varistor disks is propagated to the simulated electrical and electromagnetic relevant effects. Eventually, two worst-case scenarios are analyzed from a thermal–hydraulic point of view with the 4C code, simulating a fast current discharge as a consequence of a coil quench. It has been demonstrated that the uncertainty on the inputs (varistor parameters) strongly propagates, leading to a wide range of possible scenarios in the case of accidental transients. This result underlines the necessity of taking into account and propagating all possible uncertainties in the design of a fusion reactor according to the Best Estimate Plus Uncertainty approach. The uncertainty propagation from input data to electrical, electromagnetic, and thermal hydraulic results, using surrogate models, is the first of its kind in the field of the modeling of superconducting magnets for nuclear fusion applications.

Dispersion of ZnO Nanocrystals Using Ammonium Polycarboxylate (AMP) for the Fabrication of ZnO-Based Ceramics

The effects of ammonium polycarboxylate (AMP) on the dispersion of zinc oxide nanopowders were investigated in this study. Good dispersibility and stability in ZnO suspension with a dispersant content of 0.5 wt.% at pH 10 was observed by field emission scanning electron microscopy, and was confirmed by sedimentation experiments. The adsorption state of AMP on the surface of nano-sized ZnO powder was examined by Fourier transform infrared spectroscopy. For the ZnO nanopowders with various AMP amounts in the alkaline region, a maximum absolute zeta potential of 55.8 mV was measured at pH 10, which correlated well with the saturation amount of adsorbed AMP. Compacted varistor discs were prepared from the nano-origin ZnO powders before and after AMP absorption, showing sintered densities of 97.6% and 98.4%, respectively. The advantages of AMP were clearly seen in grain structuring, in addition to the enhanced densification and homogeneous microstructures for obtaining a high nonlinear coefficient.

On the Computation of the Voltage Distribution along the Non-Linear Resistor of Gapless Metal Oxide Surge Arresters

The voltage distribution along the non-linear resistance of metal oxide surges is of great importance for their proper operation, since the non-uniform potential distribution results in higher thermal stresses of the varistor discs near the high voltage electrode, leading to a faster ageing of the discs at the top and, consequently, a downgrade in arrester effectiveness and reliability or even failures. The current work deals with the examination of the voltage distribution along the non-linear resistance of medium voltage metal oxide gapless surge arresters, using an appropriate computer tool, discussing configuration that improve the voltage distribution. Moreover, the impact of various factors on the voltage distribution is examined. The extracted results can contribute to the more efficient design of modern metal oxide gapless surge arresters, in an effort to ensure their reliable operation to protect the electrical equipment against lightning surges.

Performance Evaluation of Doped Titanium Oxide for Low‐Voltage Applications

Varistors are the electronic devices which are used in various industries to protect the electrical and electronic systems from sudden surges. In this research, the electrical properties of titanium dioxide (TiO2) doped with tantalum pentoxide (Ta2O5), tungsten trioxide (WO3), cobalt oxide (Co3O4), and bismuth oxide (Bi2O3) and fired at different temperatures were investigated for low‐voltage applications. The adequate amount of dopants at suitable sintering temperature had beneficial effect in improving the properties of TiO2. The relative density was found to be more than 97% of theoretical density when samples sintered between 1300°C and 1400°C for all composition compared to undoped samples. On the other hand, the addition of dopants enhanced hardness and compressive strength of varistor disks. The average grain size was also increased with the dopants system, making it suitable for low‐voltage application. Furthermore, the current–voltage characteristic of the TiO2 revealed a significantly high value of nonlinearity of 19.6. A high dielectric constant of 104 with minimum dissipation factor of 0.002852 at 1 kHz was also obtained, thereby making it suitable for low‐voltage application.

Characterization of Doped Titanium Oxide Varistor Powder Processed with Latex Binder

In the processing of semiconducting electro ceramic materials the use of appropriate binder can lead to improve characteristics of powder which will reduce density gradient in the green body during compaction. The binder system is anticipated to have an influence on the grain growth and microstructure which in turn affect the electrical performance. To investigate this phenomenon, TiO2 powder doped with tungsten oxide (WO3), silicon oxide (SiO2) and bismuth oxide (Bi2O3) was processed with and without various percent of latex binder. The prepared samples were characterized by evaluating the physical properties like green density, fired density, axial and radial shrinkage, micro hardness and compressive strength. It was observed that TiO2 processed with latex binder exhibited higher green density and less axial and radial shrinkage compared to powder processed without binder. However, the binder level did not have any significant influence on the other characteristics of varistor discs. SEM analysis indicated that the average grain size was also influenced by the percent of binder used and but the higher average grain size was obtained for discs without any binder.

Physical Characterization and Microstructure Evaluation of Titanium Dioxide Semiconductor Discs Processed with Binders

The processing of TiO2 semiconducting materials has a long route with numerous contributing variables. The binder plays an important role in the development of this electro-ceramic material. Higher density and strength of a green body are critical not only for handling at its green state but also to avoid the causes of flaw formation during sintering. Use of appropriate binder can lead to improve characteristics of powder which will reduce density gradient in the green body during compaction. Enhanced green properties can consequently lead to higher fired strength of the varistor discs. The binder system is anticipated to have an influence on the grain growth and microstructure which in turn affect the electrical performance. To investigate this phenomenon, TiO2 powder doped with 0.2% of Ta2O5 was processed with and without binder polyvinyl alcohol. The prepared samples were characterized by evaluating the physical properties like green density, fired density, axial and radial shrinkage, micro hardness and compressive strength. It was observed that TiO2 processed with binder exhibited better properties compared to powder processed without binder. The higher percent of binder enhanced the properties further. XRD revealed that the diffraction angle peak was similar to that of pure TiO2. SEM analysis indicated that the average grain size was larger for discs prepared with higher percent of binder. It is anticipated that the larger grain size will improve varistor properties in terms of low voltage applications.

Sintering of surfactant modified ZnO–Bi2O3 based varistor nanopowders

Abstract Surfactant modified nano-origin ZnO–Bi2O3 varistor powder was prepared in presence of cetyltrimethyl ammonium bromide (CTAB) surfactant through an aqueous reflux reaction at 100 °C. The compacted varistor discs made from the nano-origin powders were subjected to step-sintering, microwave sintering and solid-state sintering. The influences of CTAB in different sintering methods were analyzed from the densification characteristics, evolution of sintered microstructures and associated varistor properties (I–V). The conventional solid-state sintering produced 96% theoretical sintered dense samples at 1100 °C. The step and microwave sintered samples showed 93% and 99% sintered densities, respectively, with controlled microstructures having grain sizes in the range of 2–6 μm at the given conditions. The CTAB advantages were clearly seen in grain structuring and grain boundary properties, in addition to the enhanced densification and homogenous microstructures for obtaining high breakdown voltage and non-linearity coefficient.

Improvement in electrical stability of ZnO varistors by infiltration of molten Bi 2O 3

The infiltration of molten Bi 2O 3 into ZnO varistors was carried out. The sintered varistor disks doped with Bi 2O 3 were painted by Bi 2O 3 paste on the surfaces and then were reheat-treated. The DC degradation test was carried out to demonstrate the electrical stability of varistors. The influence of the Bi 2O 3 infiltration on the electrical nonlinear characteristics and stability of varistors was analyzed by capacitance–voltage ( C– V) measurements and impedance spectroscopy. The results showed that the Bi 2O 3 infiltration process at temperatures from 800 to 900 °C reduced the donor density in ZnO grains and inhibited the decrease of grain boundary resistance of varistors caused by degradation. The samples infiltrated with Bi 2O 3 at 850 and 900 °C exhibited an excellent electrical stability and maintained relatively high nonlinear characteristics. The enhancement in the electrical stability may be attributed to the diffusion of oxygen into the body of ceramics facilitated by the infiltration of molten Bi 2O 3.

Synthesis of ZnO based nanopowders via a non-hydrolytic sol gel technique and their densification behaviour and varistor properties

Hexagonal nanocrystalline varistor grade ZnO particles with size 50 nm and the specific surface area of 28 m2/g have been prepared by non-aqueous gelation technique involving diethylene glycol and triethanolamine. The as-prepared varistor nanopowders were analyzed with the support of XRD, TG/DTA, FTIR, TMA, SEM and TEM. Varistor discs were fabricated by pressing and their densification was studied at 850, 950, 1050 and 1150?C. The evolution of varistor microstructures, extent of grain growth and the influence of microstructure on the I-V properties were explored and presented. .

Characterization and Performance Evaluation of Zinc Oxide Varistor Blocks Processed by Latex Binders

The processing of ZnO varistor has a long route with numerous contributing variables. The binder plays an important role in the fabrication of this electro-ceramic device. Higher density and strength of a green body are critical not only for handling at its green state but also to avoid the causes of flaw formation during sintering. Use of appropriate binder can lead to improved characteristics of powder, which will reduce density gradient in the green body during compaction. Enhanced green properties can consequently lead to higher fired strength of the varistor discs. The binder system is anticipated to have an influence on the grain growth and microstructure which in turn affect the electrical performance parameters of the discs such as I–V characteristics, watt-loss behavior, and energy absorption capability, etc. To investigate this phenomenon, several commercial latex binders and their blends have been studied and compared with the conventional plasticized polyvinyl alcohol. A positive effect was observed in the context of physical properties and electrical performance of ZnO varistors. Factorial analysis has revealed that the two input parameters (binder content and solid concentration) influence the electrical properties such as electric field intensity, non-linear co-efficient, energy absorption capability, and watt-loss of ZnO varistor.

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.

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.

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.

Study of the effect of latex binder on the green and fired properties of ZnO varistor discs and estimation of Weibull parameters

The formation of green compact body is a major processing operation for manufacturing ZnO varistors. The density and strength of the green body is crucial not only for proper handling but also for avoiding introduction of further flaws through sintering. The green body may have a wide range of flaw sizes, resulting in corresponding variation of green strength. The green density and strength and consequently the fired strength of the varistor discs can be enhanced by proper selection of the binder, thereby, reducing flaw distribution. Shrinkage of the fired discs can also be reduced with discs of increased green density. Several commercial latex binders and their blends have been studied and compared with the conventional plasticized polyvinyl alcohol for the production of zinc-oxide varistor discs. It was found that discs with improved green density, green and fired strength and less shrinkage were achievable by the use of latex binder. Moreover, estimation of Weibull parameters showed that by using latex binder green and fired discs with more uniform distribution of strength can be produced.

The effect of processing conditions on varistors prepared from nanocrystalline ZnO

Nanoparticles of ZnO were prepared by the reaction of ethanolic solutions of zinc acetate and oxalic acid followed by drying (80 °C) and calcination (500 °C). Subsequently varistor materials were fabricated from this nanoparticular ZnO via two separate routes:- a) from a “core shell” material using metal salts as additives; b) by using a conventional solid state mixing of metal oxides. Sintering (1050 °C) and subsequent electrical studies were carried out for each of these samples and they were compared with commercial varistor samples prepared under similar conditions. “Core shell” type varistor material showed considerably higher breakdown voltage (Vc = 850 ± 30 V mm−1) as compared to a sample prepared by mixing with metal oxides (Vc = 683 ± 30 V mm−1) or commercial varistor discs (Vc = 507 ± 30 V mm−1). The high breakdown voltage obtained is attributed to the formation of more varistor-active grain boundaries per unit area.

Influence of Nonuniformity of ZnO Varistors on Their Energy Absorption Capability

A simple thermo-mechanical model is applied to evaluate the influence of the nonuniformity of ZnO varistor disks used in surge arresters on their energy handling capability. Puncture is the dominating failure mode for slightly nonuniform disks, but cracking becomes more likely as the degree of nonuniformities increases. It is shown that minimization of the chance of a failure of varistor disks at high-current pulses can be achieved by adjusting their resistivity in the upturn region of the I-V characteristic. Simulation of the behavior of varistor disks under high-current 4/10 /spl mu/s pulses required by the ANSI standard tests shows that these tests provide very little information about the actual energy handling capability of the disks. This conclusion suggests that alternate test methods should be developed and included in the relevant standards.

Influence of nonuniformity of ZnO varistors on their energy absorption capability

A simple thermo-mechanical model is applied to evaluate the influence of the nonuniformity of ZnO varistor disks used in surge arresters on their energy handling capability. Puncture is the dominating failure mode for slightly nonuniform disks, but cracking becomes more likely as the degree of nonuniformities increases. It is shown that minimization of the chance of a failure of varistor disks at high-current pulses can be achieved by adjusting their resistivity in the upturn region of the I-V characteristic. Simulation of the behavior of varistor disks under high-current 4/10 /spl mu/s pulses required by the ANSI standard tests shows that these tests provide very little information about the actual energy handling capability of the disks. This conclusion suggests that alternate test methods should be developed and included in the relevant standards.

Temperature behaviour of ZnO varistors before and after post sintering heat treatment

The temperature behaviour of ZnO varistor discs has been examined. Criteria have been found that predominantly determine the thermal behaviour of a ZnO varistor disc. In addition, it has been found that a post sintering heat treatment can change the temperature behaviour. The majority of the tempered samples are in agreement with established models. However, one varistor disc, consisting of a commercial varistor mixture, appeared to be deviant. The tempered samples of this mixture showed an improved thermal behaviour, i.e., a significantly lower increase of the leakage current with increasing temperature.

Failure modes and energy absorption capability of ZnO varistors

Thermal and mechanical behavior of ZnO varistors used in surge arresters is simulated for current pulses of various magnitude and duration. Time dependence of the temperature profile and of the distribution of thermal stresses is computed by solving heat transfer equations for varistor disks with nonuniform electrical properties. The simulations identify failure processes and determine energy absorption capability of varistor elements. The results are in excellent agreement with experiment and provide fundamental explanation of the energy handling dependence upon current surge intensity and duration.

Nonuniform Heating in Zinc Oxide Varistors Studied by Infrared Imaging and Computer Simulation

State‐of‐the‐art infrared (IR) thermal imaging was used to monitor the heating of ZnO varistors by electrical transients. On a macroscopic scale (e.g., 10 mm), heating in large varistor blocks (i.e., diameter of 42 mm) was found to be the greatest near the block edges and to be approximately radially symmetric in blocks fabricated at a low aspect ratio. In blocks fabricated at a higher aspect ratio, the heating was less symmetric, presumably because uniform properties are more difficult to achieve. Nonuniform heating in large blocks can be attributed to processing‐induced variations in the electrical properties of the blocks. On an intermediate size scale (e.g., 1 mm), the heating in small varistor disks (e.g., diameter of 10 mm) was observed to be most intense along localized electrical paths. The high electrical conductivity of these paths originates from the statistical fluctuations in properties that inevitably occur in polycrystalline materials. On a microscopic scale (e.g., 10 μm), the heating in thin varistor slices (e.g., thickness of 100 μm) was observed to be localized in strings of tiny hot spots. The hot spots occur at the grain boundaries in a conducting path, where the potential is decreased across Schottky‐type barriers and the heat is generated. The experimentally observed heating is interpreted by applying transport theory and using computer simulations. It is shown that, on the scale of the grain size, the heat transfer is too fast to permit temperature differences that could cause a varistor failure. Current localization and nonuniform heating on an intermediate size scale can have a microstructural origin (e.g., statistical fluctuations of grain sizes and grain‐boundary properties). However, these are shown to be significant only in small varistors, whereas destructive failures (puncture and cracking) of large varistor blocks can be caused only by nonuniform heating on a macroscopic scale.