Varistor Performance Research Articles

Enhancing the performance of SiC-based varistors through the use of SPS processing and fluxes

SiC varistors, which are used as surge arrestors in high power/high energy applications, often contain high levels of porosity (∼25 %) which can degrade varistor performance. Starting with SiC-clay varistors (∼43 µm grain size SiC) we employed spark plasma sintering (SPS) and flux additions (to enable liquid phase sintering) to reduce the level of porosity and understand its impact on the microstructure and the electrical properties. Sintering by SPS at 1200 °C for 5 min increased the density to 94 % theoretical, increasing energy density capability by almost a factor of 10 (to ∼1100 J cm−3), but the non-linear coefficient (α) fell by 40 % (to 3.5) and breakdown field (Eb) fell by 75 % (to ∼200 V cm−1). Varistors prepared with nepheline syenite as a flux (replacing clay) which were pressureless sintered at 1200 °C had maximum densities of 80 %. The samples in which all the clay was replaced by the flux exhibited exceptional energy density performance (over 10,000 J cm−3) but suffered reductions in other parameters. Using small grain size SiC (∼3 µm) and 40 % syenite flux yielded consistently superior properties: low power range α value 12.2, high power range α value 32.5, breakdown field ∼5880 (V cm−1) and very low leakage currents ∼10−3 (mA/cm2).

Comparison of ZnO-based varistor performance by doping Zn–Sb–O phase and Sb2O3 respectively

In this study, ZnO–Bi2O3–Cr2O3–MnO2–B2O3 (ZBCMB) varistors were made by doping Zn–Sb–O phase and Sb2O3 phase in low-temperature respectively. Compared with the Sb2O3 doped sample, the breakdown field strength (E1mA) was effectively reduced from 605.36 V/mm to 440.24 V/mm due to the doping of Zn–Sb–O phase. The trend of the nonlinear coefficient α is similar for Sb2O3 and Zn–Sb–O doping, although Zn–Sb–O doping leads to a higher α = 64. This is due to the fact that the preparation of Zn–Sb–O phase avoids the oxygen vacancy reduction effect caused by the direct reaction of Sb2O3 with Bi2O3. The doping of Zn–Sb–O effectively increases the concentration of adsorbed oxygen, which has a very large positive effect on the formation of interfacial states and the increase of the potential barrier height.

Electrothermal characteristics of zinc oxide varistors with different aging states after multiple lightning strikes based on Voronoi network finite element model

Multiple lightning strikes significantly degrade the electrical and thermal properties of zinc oxide (ZnO) varistors, affecting their protection capability. In order to quantitatively analyze the hazard of multiple lightning strikes on ZnO varistors, simulations at the microscopic level are required to account for the changes in macroscopic parameters. Unlike other published research, this paper presents an analytical approach to simulate the micro-level performance of ZnO varistors in different aging states. The details of the model construction, experimental platform, and computational results are thoroughly investigated. First, a Voronoi network finite element model for ZnO varistors is developed, and the required model parameters are presented. Next, an experimental testing platform is used to physically simulate multiple lightning strikes and perform parametric testing. Finally, the microscopic electrical and thermal properties of ZnO varistors under different aging states and pulse numbers are analyzed and compared. The results show that with an equivalent amount of energy per impact, triple-strike aging causes severe degradation for ZnO varistors. Compared to single-strike aging, triple-impact aging results in a 15.21 times higher quality coefficient and a 7.67 times higher 10 min temperature rise ratio.

Improvements in the Electronic Performance of ZnO-Based Varistors by Modifying the Manufacturing Process Parameters

Varistors processed from mixtures of certain metal oxides (as additives to the main component, zinc oxide, ZnO), called MOVs, represent the devices most used for overvoltage protection and are integrated into the construction of high-performance surge arresters. The manufacturing process of these powerful electronic devices is crucial for their electronic performance. For manufacturing temperature-related studies, we used two seven-varistor experimental series: one based on two added oxides and the other on five ones. The main goal of these series was to identify the suitable sintering temperature in the case of each chemical composition from the point of view of assessing the most important electric/electronic behavioral parameters. A simple study considering mass losses after the sintering process was carried out in order to provide a brief reference for the manufacturing engineers. Before performing these studies, each varistor was sintered at a different temperature. In order to draw a general set of conclusions about the impact of the sintering pressure on the main electrical and electronic performances, a second activity involved producing two additional smaller varistors series with similar chemical compositions (two main oxides and five main oxides as additives) all processed at two different sintering pressures 4900 N/cm2 and 9800 N/cm2. The electrical/electronic parameters considered for the assessment are the main current–voltage characteristics, the non-linearity logarithmic coefficient, and the normal operational leakage current. All electrical/electronic behavioral tests were performed according to the IEC standards and regulations for both types of varistor devices (seven different temperatures and two pressure values). We concluded that a sintering temperature of 1300 °C and a pressure of 4900 N/cm2 are optimal for both types of varistors (with two and five additives).

Effect of Co3O4-doping on the microstructure and electrical properties of novel ZnO–Cr2O3-based varistor ceramics

Novel ZnO–Cr2O3-based varistor ceramics have superior current-voltage (I–V) characteristics and, do not contain volatile, rare or toxic dopants. The results showed that doping with Co3O4 greatly affected the nonlinearity of the ZnO–Cr2O3-based ceramics, with the highest coefficient of nonlinearity α equal to 73, while the leakage current was lower than 0.2 μA/cm2. Such a varistor performance enhancement was explained by the substitution of Co2+ into the Zn sites leading to the formation of Zn interstitials and free charge carriers of ZnO grains as well as an increased concentration of electron-trapping interface states like adsorbed oxygen and consequently substantially increased electrostatic barriers at the grain boundaries. The optimal amount of added Co3O4 for high-performance ZnO–Cr2O3-based varistor ceramics was about 0.5 mol.%.

Superior dielectric and varistor properties of ZnO or SnO2 diffused calcium copper titanate ceramics

Sub-micrometer-sized (0.33 μm) polycrystalline powders of calcium copper titanate (CCTO) were synthesized via the solgel method. Compacted calcined powders were sintered at 900 °C/2h to obtain porous (<80% density) pellets. Subsequently, ZnO or SnO2 pastes (∼10 μm grain size) were smeared on either side of the pellets and sintered at 1100 °C/15 h, which yielded dense (>95% density) ceramics. X-ray structural investigations revealed the presence of ZnO or SnO2 traces in the bulk of the CCTO ceramics. Scanning electron microscopy and energy dispersive spectroscopic studies confirmed the diffusion and the segregation of these oxides at the grain boundaries. The dielectric and varistor properties of the ZnO or SnO2 diffused samples were found to be superior to that of pristine CCTO ceramics. For instance, the dielectric constant (ɛ′) of ZnO diffused ceramics exhibited a value as high as 2.4 × 104 (1 kHz, at room temperature) and a dielectric loss (D) of 0.059. Similarly, SnO2 diffused ceramics exhibited a dielectric constant of 2.7 × 104 (1 kHz, at room temperature) associated with a dielectric loss of 0.047. The figure of merit of varistor performance, i.e., nonlinear coefficient (α = 10.6), of the SnO2 diffused sample is significantly higher than that of ZnO diffused (α = 7.4) and pristine CCTO (α = 4.5) ceramics. The dielectric data obtained for both the pristine and ZnO or SnO2 diffused ceramics were rationalized by invoking Cole–Cole analysis. The thermal activation energy was estimated from the temperature-dependent dielectric data besides current (I)–voltage (V) characteristics. Equivalent circuit modeling of the Nyquist plots demonstrated that the inclusion of ZnO and SnO2 layers in CCTO ceramics remarkably improved the grain boundary resistance (Rgb) by 5-fold and 20-fold, respectively, which resulted in making CCTO a better dielectric. This methodology of fabricating ceramics via interfacial engineering could pave the way for obtaining superior CCTO ceramics associated with exotic functional properties.

Effect of ZnBi2O4 and Bi2O3 addition on the densification, microstructure, and varistor properties of ZnO varistors

In this study, 1 wt% Bi2O3 (1B), 1 wt% ZnBi2O4 (1BZ), and a composite (a mixture of 1 wt% Bi2O3 and various amounts (1-4 wt%) of ZnBi2O4 ,1B1BZ-1B4BZ) were added to ZnO varistors to investigate the effects of additives on the densification, microstructure, and varistor performance. The results showed that the addition of ZnBi2O4 can lower the densification temperature to about 850oC. When the additive was changed from 1 wt% Bi2O3 to 1 wt% ZnBi2O4, the α value increased from 42 to 54, the breakdown voltage increased from 775 V/mm to 1011 V/mm, and the leakage current decreased to 0.11 μA. Additions of ZnBi2O4 and transition metal cations as donor dopants for the ZnO varistors promote oxygen chemisorption at grain boundaries, resulting in greater α value and lower leakage currents. This suggests the addition of ZnBi2O4 can effectively promote densification and improve the varistor properties of ZnO varistors.

Enhanced electrical and photoluminescence properties of BiSbO4-doped CaCu3Ti4O12 ceramics by modifying grain boundary response

CaCu3Ti4O12-xwt%BiSbO4 ceramics (CCTO-xwt%BSO, x = 0, 1, 2, 3) were prepared by solid-state reaction method. The microstructure, dielectric properties, varistor properties, photoluminescence properties of CCTO-xwt%BSO ceramics were studied in this work. Results showed that all samples formed CaCu3Ti4O12 (CCTO) single phase. Doping BiSbO4 (BSO) restrained the abnormal grain growth and increased the grain boundary density of ceramics. The introduction of BSO led to the increase of the grain boundary resistance, reducing the dielectric loss and enhancing the temperature stability of dielectric properties. The nonlinear electrical characteristics are enhanced with proper concentration of BSO. And the improved varistor performance with breakdown electric field of ∼3.98–34.6 and nonlinear coefficient of ∼1.49–2.96 are obtained for CCTO-xwt%BSO samples. In addition, the photoluminescent emission of the samples is enhanced with the addition of appropriate equivalent BSO, showing the potential applications in novel devices with photoluminescent/electrical multifunctional properties.

Optimising the performance of SiC-based varistors through composition and microstructure control

SiC varistors are employed as surge arrestors in high power/high energy niche applications. Using a model formulation based on 50 % SiC and 50 % clay plus graphite, the effects of SiC grain size, composition and graphite content were investigated. Disc and toroidal samples were sintered at 1130 °C under a reducing atmosphere; products were ∼70–75 % dense. The main phases detected by SEM and XRD were SiC, SiO2 (quartz), mullite, graphite and porosity. With increasing graphite content (zero to 11 wt%) the non-linear coefficient (α) decreased from 5.9 to 2.7, the breakdown field Eb decreased from 3346 to 36 V cm−1, and bulk electrical resistivity fell by three orders of magnitude. As SiC grain size reduced from 120 μm to 10 μm, non-linear coefficients (α) almost doubled (3.7–6.3) and breakdown field (Eb) increased by an order of magnitude (226–2656 V cm−1) as a result of increasing numbers of resistive grain boundaries between the device electrodes. The impurity content of SiC grains had a modest impact on electrical properties. The resistance of grain boundary regions was typically one to two orders of magnitude larger than that of grain cores.

ОСОБЕННОСТИ ЭКСПЛУАТАЦИИ И КОНТРОЛЯ МОЛНИЕЗАЩИТЫ СИСТЕМЫ ЭЛЕКТРОСНАБЖЕНИЯ

The issue of the operation and control of the performance of valve dischargers and varistors in the electricity system zipper protection system is considered

Effectively enhanced comprehensive electrical performance of ZnO varistors by a fast combinatorial refinement method

We demonstrated a rapid refinement method consisting of ball milling and sand milling, which greatly improved the milling efficiency, the dispersion of the suspension and the electrical properties of the ZnO varistors. The effects of the refinement processes on the size of the doping additive oxides, the microstructure and the comprehensive electrical properties, especially the degradation behaviors of the ZnO varistors were investigated. Compared with the traditional single ball milling production mode, the additive refinement process combined 6 h ball milling with 15 min sand milling greatly improved the milling efficiency by 6.68 times, with better electrical properties that the leakage current decreased from 3.46 to 1.54 μA, the clamping voltage ratio decreased from 1.99 to 1.96, and the non-linear coefficient increased from 29.1 to 47.

Varistor performance of a SnO2-based system doped with low content of SrTiO3

The search and design of new SnO2-ceramics with non-linear current-voltage behavior is quite an important task due to the possibility of their practical application as varistors, devices for the protection of electrical circuits against voltage surges. This work reports the doping impact of the low content of SrTiO3 (0, 0.05, 0.1, 0.2, and 0.4 mol%) on the microstructure and non-ohmic properties of the SnO2-Co3O4-Cr2O3-Nb2O5 system which is frequently used in varistor design. It is found that the SrTiO3-addition results in the increase of activation energy of electrical conduction and the decrease of the leakage current in obtained materials compared with undoped ceramics. The reported materials with SrTiO3-addition possess a smaller average grain size, higher porosity, and their density is lower than that of unmodified SnO2-Co3O4-Cr2O3-Nb2O5 ceramics. The highest non-linear coefficient (α = 17.24) is observed for material with 0.05 mol% SrTiO3 while for undoped ceramics it reaches only 2.68. Energy dispersive spectroscopy (EDS) revealed agglomeration of SrTiO3 which was observed in materials with 0.1–0.4 mol% of SrTiO3. The obtained results are discussed in terms of the known barrier model for varistors based on the information from dc and ac electrical measurements, scanning electron microscopy, X-ray diffraction patterns, and EDS-data.

Electron beam irradiation enhanced varistor properties in ZnO nanowire

Through a combination of atomic and electronic structure characterization studies based on aberration-corrected transmission electron microscopy and varistor property tests, we quantitatively investigated the growth process and oxygen vacancy generation in ZnO quantum dots induced by irradiating ZnO nanowires with high-energy electron beams. These processes are associated with improved varistor performance in single-crystalline ZnO nanowires. Quantitative strain measurements revealed the formation of a strong tensile strain of up to 4.4% in the region of the ZnO quantum dots. Electron energy loss spectroscopy demonstrated a rapid increase in oxygen vacancies in ZnO under electron beam irradiation. These two major changes greatly decreased carrier transport, resulting in a 34% reduction in leakage current after irradiation at a beam voltage of 2 MeV. These experimental results suggest that ZnO is an excellent semiconductor material and shows promise for practical application in electronics.

Revisiting the effects of Co 2 O 3 on multiscale defect structures and relevant electrical properties in ZnO varistors

Element doping is an effective method to improve the performance of ZnO varistors. Previous studies mainly focused on the variation of microstructures and Schottky barriers. In this study, the effects of Co dopant on electrical properties are investigated from the aspect of multiscale defect structures, including intrinsic point defects, the heterogeneous interface of depletion/intergranular layers, and interface states at grain boundaries. Combining with analysis of phase composition and energy dispersive spectroscopy, it is found that Co tends to dissolve into ZnO grains when slightly doped. It substitutes Zn2+ with the same valence and affects little on densities of donors. Segregation of Co at grain boundaries would result in the formation of spinel phase Co(Co4/3Sb2/3)O4 and transformation of the intergranular phase from α-Bi2O3 to δ-Bi2O3. Meanwhile, densities of point defects are indirectly affected by oxygen ambient during sintering, resulting in abnormal variation of grain resistivity. And interface states are enhanced, leading to improved barriers at grain boundaries. Therefore, reduced leakage current, enhanced grain resistivity, and improved non-linear coefficient in Co-doped ZnO varistor blocks are understood from the underlying multiple defect structures. This presents a potential approach to explore short-term performance and long-term stability of ZnO varistors from the aspect of defect responses.

Comparison between the use of Co3O4 or CoO on microstructure and electrical properties in a varistor system based on SnO2

This comparative study pursues to clarify the differences between the addition of CoO or Co3O4 as a dopant on the non-linear electrical behavior and microstructure characteristics of a varistor system based on SnO2. The samples were fabricated according to the conventional ceramic processing using the nominal composition: (99.99-X)%SnO2 – 0.05%Cr2O3 – 0.05%Sb2O5 – X%CoO/Co3O4 where X = 0, 1, 3 and 5 mol%. The TGA-DTA analysis revealed essential differences in the thermal stability of CoO and Co3O4 during the sintering process, being CoO the oxide that induced the most visible changes. SEM micrographs, supported by EDS and XRD, showed remarkable grain growth even with a slight addition of cobalt oxide in comparison with the reference sample. The use of CoO holds the average grain size (6.7 μm), composition (SnO2), and shape homogeneity. In the case of Co3O4, a decreasing tendency in the average grain size and an increase in the residual porosity due to the in-situ formation of Co2SnO4 phase is observed. The electrical analysis revealed similarities on the alpha value for all molar percentages of both cobalt oxides: ∼9.6 and ∼8.1 units for CoO and Co3O4, respectively. In contrast, the breakdown electrical field shows notable differences: 1.08–1.16 kV cm−1 for CoO and 1.08–2.46 kV cm−1 for Co3O4. The evidence of this study confirms previous findings and suggests the existence of an essential role for the phase transitions of cobalt oxide on the final performance of SnO2-based varistors.

High voltage gradient and low leakage current SnO2 varistor ceramics doped with Y2O3 and Nb2O5

SnO2 varistors are a type of ceramic material with strong heat dissipation and nonlinearity. To improve the overall nonlinear electrical properties of SnO2 varistor ceramics, we studied the electrical properties and microstructure of SnO2 varistors doped with different contents of Nb2O5 at a given concentration of Y2O3. We studied the electrical characteristics of the obtained experimental samples by scanning electron microscope imaging, voltage-current and capacitance-voltage measurements, and X-ray diffraction. When the SnO2 varistor ceramic samples were fabricated with 0.006 mol% niobium and 0.05 mol% yttrium, excellent electrical performance was achieved with a voltage gradient of 832.4 V/mm, leakage current of 4.31 μA/cm2, and nonlinearity coefficient of 45. This study will help guide efforts to strengthen the electrical performance of SnO2 varistors.

Superior performing nano‐enabled metal oxide varistors

AbstractCommercial metal oxide varistor devices are subject to nonuniformity, defects and coarse grain size, low working voltage, high leakage current, and mechanical cracking issues. This work tempted to make use of nanopowders and alternative sintering processes to achieve uniform microstructure and superior varistor performance. These nano‐enabled compositions exhibit lower sintering temperatures (650‐1050°C), smaller grain sizes (0.5‐2 μm), enhanced nonlinear coefficient (30‐130) in DC I‐V characteristics, higher varistor voltages (240‐2500 V/mm), and low leakage current (<1 μA/cm2). Conventional sintering offers highest nonlinearity. Spark plasma sintering and microwave sintering techniques show advantages in reducing grain sizes and sintering temperatures. Spark plasma sintering is particularly effective in achieving better varistor performance for the nano‐enabled MOV compositions after a post‐SPS sintering for sufficient oxygen diffusion. Microwave sintering needs higher sintering temperatures and additional research for the nano‐enabled MOV compositions to offer equivalent electrical properties.

Stable electrical properties of ZnO varistor ceramics with multiple additives against the AC accelerated aging process

The aging performance of zinc oxide (ZnO) varistors doped with aluminum and indium was investigated. Alternating current (AC) accelerated aging voltages of 0.85U1mA, 0.90U1mA and 0.95U1mA were applied to the doped varistors at a temperature of 115 °C for more than 1000 h. The electrical properties of the varistors before and after the aging process were evaluated by measuring their current–voltage, capacitance–voltage, and dielectric characteristics. The results showed that the ZnO varistor samples exhibited stable electrical properties under the applied aging conditions. Degradation rate coefficients (Kt) of 0.03, 0.05 and 0.06 were obtained at AC accelerated aging voltages of 0.85U1mA, 0.90U1mA and 0.95U1mA, respectively. The results indicated that the varistor ceramics should be able to operate safely and continuously under the given conditions for more than 110 years, which meets the requirements of surge arresters in power systems. These findings are important for improving the aging performance of ZnO varistors and the general security and stability of power systems.

Influence of Sintering Temperature and ZrO2 Dopants on the Microstructure and Electrical Properties of Zinc Oxide Varistors

This paper investigated the effects of sintering temperature on the microstructure and electrical properties of ZrO 2 -doped zinc oxide (ZnO) varistor ceramics. The results show that, the additive ZrO 2 exists as independent second phase between ZnO grains, which can limit the growth of ZnO grains and improve the voltage gradient. With the increasing of ZrO 2 , the content of extrinsic elements (Mn, Sb, Co, Cr) in the grain boundary layers tends to increase first and then decease. When ZrO 2 content is more than 1.0mol%, the electrical performance of ZnO varistors decreases sharply. With the increasing of sintering temperature, the ZnO grain size increases and the voltage gradient decreases. When the sintering temperature is larger than 1200°C, more monoclinic ZrO 2 phase transformed into cubic phase, and more micropores are generated, causing the non-linear coefficient to decrease and the residual voltage ratio and leakage current to increase. With a sintering temperature of 1150°C and a ZrO 2 content of 1.0mol%, the ZnO varistors can reach the overall optimum electric performance, exhibiting a breakdown voltage of E 1mA = 420V/mm, a nonlinear coefficient of α = 58, a residual voltage ratio of CR = 1.87, and a leakage current of I L = 4μA. The studies in this paper can give reference for the development of high quality ZnO arresters.

Influence of thermal treatment temperature on high-performance varistors prepared by hot-dipping tin oxide thin films in Nb2O5 powder

SnOx-Nb2O5 thin film varistors were prepared by hot-dipping oxygen-deficient tin oxide films in Nb2O5 powder in air, and the influence of hot-dipping temperature (HDT) on the varistor performance of the samples was systematically explored. When the HDT increased from 300 to 700 °C, the nonlinear coefficient of the samples raised first and then dropped down, reaching the maximum of 14.73 at 500 °C, and the breakdown electric field exhibited a similar variation trend, gaining the peak value of 0.0201 V/nm at this temperature. Correspondingly, the leakage current decreased first and then increased with increasing HDT, reaching the minimum of 17.1 mA/cm2 at 500 °C. Besides, it was proposed that a grain-boundary defect barrier model was responsible for the nonlinear behavior of the obtained SnOx-Nb2O5 film varistors. This high-performance thin film varistor with nanoscaled thickness might be much promising in nano-devices or devices working in low voltage.