Varistor Ceramics Research Articles

Impact of the particle size on the non-Ohmic properties of doped ZnO ceramic varistor materials synthesized via mechanosynthesis

The effects of nano-ZnO particle size on the microstructure and non-Ohmic electrical properties of a varistor system doped with 0.025 mol% Bi2O3, 0.100 mol% Nb2O5, 0.500 mol% V2O5, and 2.000 mol% MnO2 (ZnO-balance) were investigated. The study compared ZnO from two suppliers, one with a particle size of <5 μm from Aldrich Inc., and ZnO from NANOQEM, which was processed using mechanosynthesis at various power levels (X), where X represents the mechanosynthesis power applied at values of X = 1, 70, 139, 209, 417, 626, 834, 1042, and 1251 W/kg. X-ray diffraction (XRD) and scanning electron microscopy (SEM) revealed that the microstructure consisted of wurtzite-type ZnO with a uniform grain size distribution, which decreased as the ZnO particle size decreased, along with a homogeneous distribution of dopants. Rietveld analysis confirmed that mechanosynthesis did not alter the ZnO lattice parameters. A promising nonlinearity coefficient (α), with high reproducibility between 15 and 35 (and a peak value of 46.8 at 139 W/kg), and breakdown voltages of up to 4700 V/cm were achieved for low to moderate milling energies. Additionally, low leakage currents, ranging from 0.0407 to 0.0840 mA/cm2, were measured within the same milling energy range.

Effects of sintering temperature on microstructure and varistor performances of ZnO-SrCO3-Co2O3 ceramics

Sintering temperature is decisive for optimizing the grain boundary environment to obtain the high performance ZnO based varistor ceramic. In this work, the impacts of sintering temperature on microstructure and electrical properties of ternary Zn-Sr-Co varistor were investigated. It was found that the distribution of Sr, the critical factor to form grain boundary, was heavily sensitive to sintering temperature. The considerable Sr ions precipitated at grain boundaries and formed the clusters of SrZnO2 while the sintering temperature increases from 1150 °C to 1190 °C. Besides, the precipitation of Sr led to the large segregation of Co at grain boundaries. The enrichment behavior of Sr and Co contributed to the optimization of grain boundaries, resulting in the enhanced barrier height. As a result, the excellent nonlinear current-voltage performances, i.e., the high nonlinear coefficient of 56.47 and the low leakage current density of 0.73 μA/cm2 were obtained in the ternary ZnO-SrCO3-Co2O3 varistor sintered at 1190 °C. However, the grain boundary environment would be destroyed by the excessive temperature of 1210 °C, resulting in the degradation of grain boundary barrier and especially a surge in the leakage current IL from 0.73 to 92.19 μA/cm2. In addition, varying sintering temperature has the important effects on impedance and dielectric properties of the ZnO-SrCO3-Co2O3 varistors. The findings provide new perspectives for developing the high-performance ternary ZnO-SrCO3-Co2O3 varistor ceramics by optimizing the initial grain boundary environment at different sintering temperatures.

Effect of CaCO3, SrCO3 and BaCO3 additions on the microstructure and electrical characteristics of SnO2-based varistor ceramics

The ceramic varistors SnO2–Co3O4–Nb2O5–Cr2O3 with the addition of CaCO3, SrCO3 or BaCO3 were sintered at temperatures 1250 and 1350°C, then their structures and electrical properties were studied. In all samples, the SnO2 cassiterite phase with a rutile-type structure and the Co2SnO4 phase with a spinel-type structure were observed. All materials exhibited a highly nonlinear dependence of the current density on the electric field with the nonlinearity coefficient 24–50. When alkaline earth metal carbonates were added, the grain size of all samples decreased and the electric field E1 increased. The addition of CaCO3 lead to the decrease of the low-field electrical conductivity of varistors. The lowest low-field conductivities 4·10-13 and 6·10-13 Ω-1 cm-1 were found in the samples with CaCO3 addition baked at 1250 and 1350°C, respectively. The observed effect is due to the increase of the potential barrier height at the SnO2 grain boundaries by 7 and 13%, respectively, and the decrease of the grain size by 26 and 28% compared to SnO2-based varistor ceramics without CaCO3 addition.

Influence of TiO2 doping on the grain growth and electrical properties of ZnO–Cr2O3-based varistor ceramics

The influence of TiO2 doping in the range 0–1 mol% on the microstructure and electrical characteristics of ZnO–Cr2O3-based varistor ceramics was studied. Exaggerated grain growth is observed owing to the presence of TiO2, which triggers the formation of inversion boundaries (IBs) in only a limited number of grains. TiO2 acts as the controller of ZnO grain growth, adjusting the grain size of the ZnO from 5.6 to 9.3 μm, and thus controlling the breakdown voltage. Meanwhile, the TiO2 doping decreased the height of the Schottky barrier, resulting in a lower nonlinear coefficient and a higher leakage current. The results are important for the control of the ZnO grain size with ZnO–Cr2O3-based varistors (having different nominal breakdown voltages) to obtain a suitable thickness for applications.

Impacts of SiO2 on phase composition, microstructure, and electrical properties of ZnO-SrCO3-Co2O3 ceramics

ZnO-SrCO3-Co2O3-SiO2 varistor ceramics were fabricated using the solid state method. Structural, morphological characterizations and varistor characteristics of samples were systematically studied. Average grain size is gradually reduced from 13.74 to 6.67 μm, resulting in a simultaneous increase of breakdown gradient E1 mA from 157.14 to 339.29 V mm−1. An increase of doping amount of SiO2 from 0 to 0.50 mol% contributes to the positive influences on double Schottky barrier of ZnO-SrCO3-Co2O3 varistors. The sample doped by 0.50 mol% SiO2 exhibits the highest height of double Schottky barrier φB of 2.99 eV resulting in the high nonlinear coefficient α equal to 76.83 and small value of leakage current density IL less than 0.29 μA cm−2 among the samples, which may be attributed to the solid solution reactions of SiO2 and Co2O3 in grain boundaries. The fabrication of ZnO-SrCO3-Co2O3-SiO2 represents the low-cost, sustainable development and environmental friendliness, providing a high performance candidate material for dominant ZnO-Bi2O3 based varistor ceramics.

Effects of Tm2O3 and Bi2O3 Co-Doped with Equal Content of Nb2O5 on the Properties of TiO2-Based Varistor Ceramics

Effects of Tm2O3 and Bi2O3 Co-Doped with Equal Content of Nb2O5 on the Properties of TiO2-Based Varistor Ceramics

Effect of Bi-Er-O addition on the electrical properties of ZnBiCoMnSb based varistor

In order to restrain the breakdown voltage gradient E1 mA at a low level and simultaneously improve nonlinear coefficient α, the rare earth element Er was doped into ZnBiCoMnSb based varistor ceramic in the form of Bi-Er-O phase. As a result, the excellent electrical performances including low E1 mA of 190 V/mm, high α of 55.6 as well as low leakage current IL of 2.5 μA/cm2 are obtained in 3 wt% Bi-Er-O phase modified ZnBiCoMnSb based varistor ceramic. It is found that the concentration of Er at grain boundaries is slightly higher than that in ZnO grains, while it is uniformly distributed throughout the bulk ceramic. This characteristic of element distribution mainly contributes to the enhancement of α by forming interface state densities and deep bulk traps at grain boundaries, rather than E1 mA. The reported herein strategy to restrain E1 mA and improve α at the same time on ZnBiCoMnSb based varistor ceramic can act as a reference for further fabrication of low-E1 mA varistor ceramics with excellent nonlinear performances by rare earth elements doping.

Effects of In2O3 doping on the microstructure and electrical properties of ZnO–V2O5–Nb2O5 varistor ceramics

To understand the effects of In2O3 on the microstructure, grain size dispersion, density and nonlinear electrical characteristics, ZnO–V2O5–Nb2O5 varistor ceramics with 0.02, 0.05 and 0.1 mol.% In2O3 were fabricated via sintering at 950–975 °C for 1 h. The sintered samples were evaluated by x-ray diffraction, scanning electron microscopy, density measurements and electrical measurements through a voltage source meter. In2O3 addition improves relative density to ≥99 % and acts as an efficient grain growth inhibitor, resulting in a narrower grain dispersion and finer ZnO grains due to the formation of In–V–O intergranular phases. The breakdown potential (E1mA) increases sharply to 7.49 ± 0.3 kV/cm (from 1.66 ± 0.1 kV/cm) as a result of grain size reduction to 2.1 ± 0.1 μm (from 3.89 ± 0.2 μm) with In concentration of 0.1 mol.% (from 0.02 mol.%). Doping of In2O3 improves the nonlinear exponent and reduces the leakage current density as a direct consequence of enhanced Schottky barrier height.

The Effect of Sintering Temperature on the Microstructure and Electrical Properties of ZnO–Bi2O3 Varistor Ceramics

The Effect of Sintering Temperature on the Microstructure and Electrical Properties of ZnO–Bi2O3 Varistor Ceramics

Influence mechanism of Ni2O3 doping on leakage current of SnO2 varistor ceramics

AbstractThe doping of Ni2O3 has significantly enhanced the electrical properties and density of SnO2–Co3O4–Cr2O3–Nb2O5 varistor ceramics. The leakage current density is reduced to 4.73 µA/cm2, the nonlinear coefficient is 37, and the voltage gradient is 549 V/mm. The undoped varistor ceramics exhibited a considerably higher leakage current of 48.04 µA/cm2. The performance improvement comes from alterations in grain boundary parameters. The doping of an appropriate amount of Ni improves the grain boundary barrier of SnO2 varistor ceramics and makes samples obtain higher grain boundary resistance, which hinders the leakage current path and results in excellent nonohmic characteristics. The stable grain boundary barrier structure dominated by Ni enables the sample to obtain stable power loss performance under long‐term direct current (DC) bias.

Elaboration and electrical characterization of ZnO-based varistor ceramics in different sintering temperatures

Elaboration and electrical characterization of ZnO-based varistor ceramics in different sintering temperatures

An Ultralow Concentration of Cr2O3 Dopants‐Driven Lower Temperature Sintering ZnO‐Based Varistor Ceramics

Low working voltage‐driven ZnO‐based varistor ceramics play an important role in the multilayer chip varistors, which require a low sintering temperature of ZnO varistor for its low energy consumption. Herein, a remarkable reduction of the sintering temperature from the usual 1100–1300 °C to 950 °C is successfully achieves in the ZnO ceramics via a certain 0.05 mol% of Cr2O3 dopants. The underlying mechanism is found to be involved with the formation of basal‐plane inversion boundaries between the ZnO grains, which can promote the rapid grain growth within the ceramics. Furthermore, the ZnO varistors with 0.05 mol% Cr2O3 dopant exhibit excellent performance. A low breakdown voltage of 416 V mm−1, a high nonlinear coefficient of 39, and a low leakage current of 3.4 μA are obtained simultaneously. This work presents an effective and promising approach for the cost‐efficient preparation of high‐performance ZnO‐based varistors, which have particular significance for the application of multilayer chip varistors with low working voltage.

Structure modification and electrical properties by Mn2O3 dopant addition to SnO2 varistor system

This work aims to study the effect of Mn2O3 additions on the electrical properties and microstructure characteristics of a SnO2-based ceramic varistor system doped additionally with Sb2O5, Cr2O3, and CoO. The specimens were prepared using conventional ceramic processing and homogenized by a high-energy ball milling system using the following composition: (98.99-X) % SnO2 – 0.05% Cr2O3 – 0.05% Sb2O5 – 1.00% CoO – X% Mn2O3 where X = 0.00, 0.05, 0.10, 0.20, 0.50 and 1% mol. Characterization by TG-DSC/DTA, XRD, XPS, and SEM/EDS, and the proposed chemical and defect-formation reactions allowed the conclusion that Mn2O3 additions produce alterations of the microstructure consisting of the in situ formation of the spinel Co2MnO4 secondary phase and modification of the potential barriers in the intergranular regions, where, in addition, oxygen vacancies are formed. With the 0.05 mol % Mn2O3, the grain size (average in the range of 2–3 μm) drops by 20 %, thus augmenting the grain boundaries. Altogether, this leads to a decrease in the nonlinearity coefficient (α) and an advantageous displacement of the breakdown electric field (EB). The 0.05 mol % Mn2O3 specimen compares favorably with the reference material by surpassing the EB value by a factor of 8. Elucidation of Mn2+ and Co2+ in 1 mol % Mn2O3 specimens by XPS suggests the role of MnO and CoO as intermediate phases, essential for Co2MnO4 formation. The pathway for in situ formation of Co2MnO4 is set forth based on the above characterization techniques in conjunction with proposed chemical and defect-formation reactions.

Linear resistance limiting the current of intergranular barriers in varistor ceramics

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

Microstructure and nonlinear electrical properties in Na2CO3-ZnO varistor ceramics

Herein, (1-x)ZnO + xNa2CO3 (x = 0, 0.5, 1.0, and 1.5 mol%) varistor ceramics were prepared via conventional solid-state sintering at a low sintering temperature of 900 ℃. The microstructure and electrical properties of these varistor ceramics were systematically investigated as a function of Na2CO3 concentration. Results revealed that the prepared varistor ceramics exhibit the hexagonal structure without a secondary phase. The increase of Na2CO3 concentration promotes the ZnO grain growth. Owing to an increasement in the barrier height at the grain boundaries, the nonlinear electrical properties of the ceramics considerably improve with increasing Na2CO3 concentration. The optimal composition of 98.5 mol%ZnO + 1.5 mol%Na2CO3 corresponds to a nonlinear coefficient of 3.55, leakage current density of 450 μA/cm2, breakdown strength of 241.2 V/mm, and barrier height of 0.57 eV. Moreover, the dielectric properties of the varistor ceramics are considerably affected by Na2CO3 concentration.

Effect of Mn on the ZnBiMnO varistor ceramics sintered at a low-temperature of 875 °C

The effect of Mn on ZnO–Bi2O3 (ZnBiO) based varistor ceramics sintered below 900 °C has not been fully understood due to the conventional ZnBiO-based varistor ceramics usually being sintered at temperatures higher than 900 °C. Therefore, (99.5-x) ZnO+0.5Bi2O3+xMnCO3 (x = 0, 1, 2, and 3; all in mole ratio) ceramics were prepared by solid-state sintering at 875 °C for 3 h to study the effect of Mn on the microstructure, electrical nonlinear properties and their stability within the range of 25–100 °C. The results show that doping MnCO3 results in the formation of Mn3O4 and Bi12MnO20 secondary phases within the sintered samples. Mn3O4 mainly exists at grain boundary as particle phase, restricting the growth of ZnO grains during sintering. The presence of Bi12MnO20 indicates MnCO3 may enhance the sintering of ZnBiMnO ceramics by increasing the amount of Bi-rich liquid phase. MnCO3 effectively improves the electrical nonlinear properties of ZnBiMnO varistor ceramic and their stability against the variations in ambient temperature. The 2 mol% MnCO3 doped ZnBiMnO varistor ceramic shows the best electrical nonlinearity, with a nonlinear coefficient, a breakdown voltage and a leakage current density of 53.61, 503.11 V/mm, and 2.85 μA/cm2 respectively. This study may improve our understanding of the effect of Mn on the low-temperature sintered ZnBiO based varistor ceramics, and provide a promising candidate material for manufacturing low-cost ZnO based varistor.

Modeling of voltage-limiting kinetics in two-layer varistor–posistor structures

PurposeThe purpose of this study is to model the dependences of the output voltage, temperature, current and electrical power dissipation of a voltage limiter based on a two-layer varistor–posistor structure on time and analysis the influence of operating modes and design parameters of such a limiter on these characteristics.Design/methodology/approachThe behavior of the limiting voltage, temperature and other parameters of the voltage limiter when an input constant overvoltage is applied is studied by the simulation method. The voltage limiter was a two-layer construction. One layer was a zinc oxide ceramic varistor. The second layer was a posistor polymer composite with a nanocarbon filler of PolySwitch technology.FindingsThe output voltage across the varistor layer decreases and reaches some fixed value related to its breakdown voltage after applying a constant overvoltage to the structure over time. The temperature of the structure increases to some steady state value, while the current decreases significantly. The amplitude of the transient current pulse increases, its duration and energy of the transient process decrease with increasing overvoltage. An increase in the internal resistance of the overvoltage source can cause a decrease in the amplitude and an increase in the duration of transient currents.Originality/valueThe ranges of values for the activation energy of conduction of the varistor layer in weak electric fields, the intensity of heat exchange between the structure under study and the environment are determined to ensure the stable operation of this structure as a voltage limiter. The results obtained make it possible to select the necessary parameters of the indicated structures to ensure the required operating modes of the voltage limiter for various applications.

Investigation of Nonlinearity in ZnO Varistor Ceramics Based on Defect Characterization

Investigation of Nonlinearity in ZnO Varistor Ceramics Based on Defect Characterization

Improving Stability and Low Leakage Current of ZnO Varistors Ceramics

Improving Stability and Low Leakage Current of ZnO Varistors Ceramics

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.%.