ZnO Varistor Ceramics Research Articles

Investigation of Nonlinearity in ZnO Varistor Ceramics Based on Defect Characterization

Investigation of Nonlinearity in ZnO Varistor Ceramics Based on Defect Characterization

Power loss transition of stable ZnO varistor ceramics: Role of oxygen adsorption on the stability of interface states at the grain boundary

Highly stable ZnO varistor ceramics with steadily decreasing power loss have been put into applications in electrical and electronic systems for overvoltage protections, even with the absence of general understandings on their aging behaviors. In this paper, we investigated their aging nature via conducting comparative dc aging experiments both in air and in nitrogen, during which variations of electrical properties and interface properties were measured and analyzed. Notably, continuously increasing power loss with severe electrical degradation was observed for the sample aged in nitrogen. The power loss transition was discovered to be closely related to the consumption of oxygen absorption at the grain boundary, which could, however, remain constant for the sample aged in air. The density of interface states, which is crucial for pinning the potential barrier, was proved to decrease in nitrogen, but keep stable in air. Therefore, it is concluded that the oxygen absorption at the grain boundary is significant for the stability of interface states, which further correlates to the long-term stability of modern stable ZnO varistor ceramics.

Influence of Calcium Element on Microstructure and Electrical Properties of ZnO Varistor with Lesser Dopants

AbstractIn this paper, the influence of Ca‐doping on microstructure and electrical characteristics of ZnO varistor ceramics is studied. With the increase of the doping concentration of calcium, the ZnO ceramics show strong directional growth along the Ca2MnO3.5 phase and the intensity of the X‐ray diffraction peaks of the Ca2MnO3.5 phase increase obviously, which indicates that the phase structure of ZnO ceramics is significantly affected by the calcium concentration. The average grain size of 0, 1, 2, and 3 mol% Ca‐doped ZnO ceramics is 7.2, 5.2, 4.8, and 4.2 µm, respectively, confirming the doping of Ca restrained the growth of grain in ceramics. The varistor ceramics doped with 1 mol% Ca have a density of 4.37 g cm−3, with a nonlinear coefficient of ∼41 at room temperature. They can be used to protect the electrical circuits from over voltages in the range of 280–460 V. Moreover, the ZnO ceramics show good temperature stability and the nonlinear coefficient changes slightly when the temperature changes from 25 to 150 °C; especially for varistor ceramics with 2 mol% Ca, the nonlinear coefficient is stable at about 37.

Effect of Nb2O5 on ZnBiMnO varistor ceramic prepared by solid-state sintering at 850°C

Nb 2 O 5 is a commonly used donor dopant for ZnO-based varistor ceramics, but its effect, especially on the low-temperature sintered ZnO varistor ceramics, is not fully understood. To provide a possible answer to this problem, ZnO–Bi 2 O 3 –MnCO 3 (ZnBiMnO) based varistor ceramics with 0.05%–0.3% (in mole ratio) Nb 2 O 5 were fabricated by solid-state sintering at 850 °C for 3 h. Their microstructure and nonlinear electrical properties were studied by XRD, SEM and the standard current-voltage (I–V) tests to reveal the effect of Nb 2 O 5 . With the increase of Nb 2 O 5 from 0.05 mol% to 0.3 mol%, more solid Bi 5 Nb 3 O 15 inter-granular particles form within the ceramic during sintering, thereby decreasing the Bi-rich liquid phase. As a result, the average size of ZnO grain decreases from 4.35 μm to 1.67 μm. This microstructural change leads to the increase of the breakdown voltage in the range of 821 V/mm to 1851 V/mm. The ZnBiMnO varistor ceramic with 0.1 mol% Nb 2 O 5 shows the best nonlinear properties. The optimum nonlinear coefficient is 35.81, the breakdown voltage is 907.51 V/mm, and the leakage current is 7.72 μA/cm 2 . The result of this study provides a promising candidate material for manufacturing the multilayered low-voltage varistor that may use Ag, Ni or even Cu as the inner electrodes.

Effect of DC ageing on the NiO-doped ZnO varistor ceramics

Effect of DC ageing on the NiO-doped ZnO varistor ceramics

Electrical and microstructural evaluation of ZnO varistor ceramics with different CaSiO3 contents

In the present study, the impact of CaSiO3 addition on microstructure and electrical characteristics of a ZnO-based varistor system was investigated. The ceramic materials were prepared by solid-state reaction route by varying the CaSiO3 concentrations (0, 3, 6, 9, 12, and 15 wt%) and sintered at 1300 °C. Characterization of the pelletized samples was carried out by density measurement, scanning electron microscopy, energy dispersive X-ray, X-ray diffraction, and current density-electric field characteristics measurement. The growth of ZnO grain was restrained with the introduction of CaSiO3; the average grain size decreases from 18.09 to 10.85 μm, whereas their relative density increases up to 99.79%. Structural results indicated the existence of the CaSiO3-rich phase without any other secondary phase formation. It is also revealed that different CaSiO3 contents significantly affect the electrical properties, prominently in the sample with 12 wt% CaSiO3 addition. The maximal value of the nonlinear coefficient, breakdown voltage, and barrier height acquired in this work were 4.41, 0.80 V/mm, and 0.691 eV, respectively, along with a sharp decline in leakage current density to 373.93 μA/cm2. Further increase in CaSiO3 content (15 wt%) eventually caused a deterioration in varistor behavior.

Effect of Zn7Sb2O12 pre-synthesis on microstructure and properties of ZnO varistor ceramics

In this work, the Zn7Sb2O12 spinel phase is pre-synthesized at 980, 1000, 1020, 1040, and 1060 °C by mixing ZnO and Sb2O3 in a molar ratio of 7:1. ZnO varistors are prepared by replacing Sb2O3 with the Zn7Sb2O12 spinel phase. The effect of the Zn7Sb2O12 spinel on the electrical properties and microstructure of ZnO varistors is studied. The results show that Sb2O3 can produce free electrons and reduce the number of oxygen vacancies (VO⋅⋅) and grain size during the reaction process. The ZnO varistors prepared by replacing Sb2O3 with the Zn7Sb2O12 spinel phase have full grain growth and high density. In addition, ZnO varistors prepared by the Zn7Sb2O12 spinel phase replacing Sb2O3 can not only reduce the sintering temperature by 60 °C but also reduce the reverse voltage gradient rate of ZnO varistors from 7.85% to 1.2% on the basis of stable electrical properties and improve the impact stability of ZnO varistors. This method has the advantages of no change in formula composition and being a simple process and can provide ideas for the preparation of multilayer chip ZnO varistors.

Evolution of Intrinsic and Extrinsic Electron Traps at Grain Boundary during Sintering ZnO Based Varistor Ceramics.

In this paper, evolution of microstructures, electrical properties and defects of the double Schottky barrier during the sintering process were investigated by quenching ZnO varistor ceramics at different sintering stages. It was found that morphology of the samples changed little when the temperature was below 800 °C. Remarkable enhancement of the Schottky barrier height and electrical properties took place in the temperature range between 600 °C and 800 °C. The Bi-rich intergranular layer changed from β phase to α phase. The interfacial relaxation at depletion/intergranular layers became detectable in the samples. Meanwhile, a distinct relaxation loss peak from electron trapping of interface states was observed instead of two dispersed ones. It indicated that the differences among the Schottky barriers in ZnO varistor ceramics became smaller with the process of sintering, which was also supported by the admittance spectra. In addition, oxygen vacancy was found more sensitive to the sintering process than zinc interstitial. The results could provide guidance for fine manipulating the Schottky barrier and its underlying defect structures by optimizing sintering process.

Simultaneously Enhanced Potential Gradient and Nonlinearity of ZnO Varistor Ceramics by MnO Doping with Nano-Sized ZnO Powders.

ZnO varistor ceramics with a high potential gradient, as well as a high nonlinear coefficient, were reported and analyzed in this paper. With the use of nano-sized ZnO powders, the average grain size was reduced to about 2.6 μm, which successfully raised the potential gradient to 1172 V/mm. Moreover, the nonlinear coefficient increased to 48, and the leakage current was decreased to 8.4 μA/cm2 by doping a moderate amount of MnO (0.9 mol%). This was proven to be caused by the high Schottky barrier height formed at the grain boundary, where the Mn element segregated and, consequently, led to the increased density of interface states. Therefore, this could be considered as a potential method to simultaneously enhance the potential gradient and the nonlinear coefficient of ZnO varistor ceramics.

High‐performance ZnO varistor ceramics prepared by arc‐induced flash sintering with low energy consumption at room temperature

The development of ultra-high voltage transmission lines requires a ZnO arrester with excellent electrical response and high voltage gradient. Compared with conventional preparation methods, flash sintering allows fast production of novel high-performance ZnO varistor ceramics with low energy consumption and controlled grain growth. Herein, ZnO varistor ceramics were prepared using the flash sintering method at an air pressure of 21 kPa for 60 s at 25°C. The results showed that the flash-sintered samples had fine grains and reduced loss of volatile elements. Moreover, the voltage gradient, leakage current density, and non-linear coefficient of the flash-sintered varistor ceramics were significantly improved compared with those of conventionally sintered samples. Owing to the short preparation time and absence of a heating process, the energy consumption of the proposed flash sintering method was significantly reduced to 10% of that by conventional sintering. The proposed method is a promising approach for the preparation of ZnO arresters with excellent electrical properties and serves as an effective method that realises energy saving and emission reduction.

Enhancement in the long-term stability of ZnO varistor ceramics against DC aging by controlling intergranular phases

Herein, to enhance the long-term stability, ZnO varistor ceramics mixed with Bi2O3, Sb2O3, MnCO3, Co2O3, SiO2, and different concentrations of NiO (with 0, 0.3, 0.7, 1, 1.3, 1.6 mol%) were grown for controlling the Bi2O3 intergranular phase as α-Bi2O3 using a well-known ceramic process technology. The microstructure and crystalline phases were investigated using scanning electron microscopy (SEM), X-ray energy dispersive spectrometry (EDS), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The electrical properties were evaluated using a high-voltage DC power supply, current pulse generator, oscilloscope, and impedance analyzer. Results from XRD patterns and TEM indicated that the NiO dopant enables the transition of δ- to α-Bi2O3 phase. Current density-electric field measurements revealed that NiO-doped ZnO ceramics have an increased breakdown field and decreased leakage current density. During DC aging, the power loss-aging time curves of NiO-doped ZnO ceramics with increased α-Bi2O3 phase showed a downward trend, indicating increased stability. The current research has revealed that improving the electrical stability of polycrystalline ceramics via intergranular phase control might be feasible for the emerging DC power-based aging applications.

Simultaneously improving the electrical properties and long-term stability of ZnO varistor ceramics by reversely manipulating intrinsic point defects

Excellent electrical properties and the improved long-term stability of ZnO varistor ceramics were simultaneously achieved by doping NiO. The microstructural features were investigated using X-ray diffractometer, scanning electron microscopy, and energy dispersive spectroscopy, while the intrinsic point defects were characterized using frequency domain dielectric spectroscopy and verified by photoluminescence and Raman spectra. The results indicated that in the ZnO varistor ceramics, a reverse manipulation of donor point defects, i.e., suppressing mobile zinc interstitial but increasing stable oxygen vacancy, was achieved. The long-term stability of NiO-doped ZnO ceramics was improved via a decrease in zinc interstitial density, with a degradation rate of 0.064 μA cm−2 h−0.5. Meanwhile, due to an increase in oxygen vacancy density, the excellent nonlinear current–voltage performance, i.e., a high nonlinear coefficient (72.9), low leakage current density (0.08 μA cm−2), and low grain resistivity (13.43 × 10−3 Ω m), was maintained. The findings of this study provide a possible method for developing high-performance ZnO varistor ceramics by manipulating point defects.

Influence of Gd2O3 on phase, microstructure, and electrical properties of ZnO varistor ceramics

Influence of Gd2O3 on phase, microstructure, and electrical properties of ZnO varistor ceramics

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.

Effect of sintering process on the electrical properties and microstructure of Ca-doped ZnO varistor ceramics

The electrical properties of Ca-doped ZnO varistor ceramics are affected by sintering temperature. The samples exhibit excellent electrical properties and microstructure at a sintering temperature of 1050 °C. The ZnO varistor ceramics sintered at 950 °C have the best voltage gradient value, but their nonlinearity and leakage current are very poor. At different sintering temperatures, the grain boundary behavior of ZnO varistor ceramics is different. The grain boundary barrier increases with the increase in the surface state concentration, and it reaches the optimal value at the sintering temperature of 1050 °C. The electrical properties of the samples at the sintering temperature of 1050 °C are as follows: breakdown voltage E1mA = 529 V/mm, leakage current JL = 0.13 μA/cm2, and nonlinearity coefficient α = 92.

A current-controlled flash sintering processing leading to dense and fine-grained typical multi-element ZnO varistor ceramics

Comparing to conventional sintering, the densification, microstructure and electric properties of typical multi-element ZnO-Bi2O3-based varistor ceramics on different constant furnace temperatures (850–1000 °C) were studied at electric field of 250 V/cm by flash sintering. The flash-sintered specimens of near full densification could be obtained within 35 s, which had finer grain size and more homogenous microstructure. The effect of different limited current on densification was investigated in flash sintering process. The results showed the relative densities of specimens from 89.9% to 92.0%, 95.5% while limited current increased from 1.0 A to 1.5 A, 2.0 A. The 950 °C flash-sintered specimen with the relative density of 98.0% had the highest threshold voltage of 656 V/mm with the nonlinear coefficient of 41.9. The dielectric loss of flash-sintered specimens had the value lower than 0.2. Thus, typical multi-element ZnO varistor ceramics could show uniform microstructure and excellent electric properties by constant furnace temperature flash sintering.

Nonlinearity Characteristics of Low-Voltage Barium Titanate Based Zinc Oxide Varistor Ceramics Modified by Cobalt Dopants

Application of ZnO varistor at low voltage has increased significantly due to the high demands of low-voltage electronics with high nonlinearity characteristics and low leakage current. The varistor ceramics were developed via solid-state reaction method and the resultant sample was analyzed by means of SEM, EDS and XRD. The nonlinearity characteristics of ZnO varistor ceramics for different contents of cobalt oxide (Co3O4) at a given barium titanate (BaTiO3) amount were analyzed based on the J-E characteristics measurement. The increased value of nonlinear coefficient (α) equal to 4.8 was exhibited by the sample made with 12 wt.% BaTiO3 additive. As the concentration of dopant (Co3O4) incorporated was increased from 0.5 to 1.5 wt.%, the varistor voltage limit decreased from 8.9 V/mm to 7.0 V/mm, respectively. Additionally, the barrier height increased from 0.88 to 0.98 eV for 0.0 wt.% to 1.0 wt.% of Co3O4 concentration, respectively. The highest α of 7.2 was obtained at 0.5 wt.% Co3O4 and decreased with further doping content due to to the reduction of barrier height caused by the variation of electronic state at the grain boundaries.

A unique tuning effect of Mg on grain boundaries and grains of ZnO varistor ceramics

The effect of Mg doping on the structure and the comprehensive electrical properties of the ZnO varistors were systematically studied by comparing the doping methods of soluble salt and metal oxide. For the first time, we found that Mg as a neutral doping element could not only improve the nonlinearity but also decreased the residual voltage ratio, due to the unique tuning effect of Mg on the grain boundaries and the grains of the ZnO varistors. By appropriate Mg doping, the varistor showed excellent comprehensive electrical performance with breakdown voltage gradient of 197.1 V mm−1, nonlinear coefficient of 88.4, leakage current of 0.8 μA, residual voltage ratio of 1.68 under 5 kA 8/20 μs surges, current carrying capacity of 18 shocks of 2 ms rectangular impulse current of 400 A, and the minimum power loss of 0.15 W at 135 °C under 85 % U1mA/√2 AC voltage.

Simultaneously enhanced electrical stability and nonlinearity in ZnO varistor ceramics: Role of Si-stabilized δ-Bi2O3 phase

In the present study, simultaneously enhanced electrical stability (low degradation rate of 8.0 × 10−3 mA∙ h1/2) and high nonlinear coefficient of 56 were obtained in ZnO varistors by doping SiO2. To clarify the mechanism of enhanced properties, comprehensive microscopic analyses were studied. Particularly, the intrinsic point defects were quantitatively characterized for the first time. Results showed that the densities of zinc interstitials (Zni) and oxygen vacancies (Vo) were dramatically decreased, resulting in enhanced stability. Besides, reduced Zni and Vo decreased the total donor density, contributing to the improved barrier height and thus leading to enhanced nonlinearity. Combined with XRD and SEM results, it is deduced that such reduced Zni and Vo are attributed to the Si-stabilized high oxygen conducting δ-Bi2O3 phase. Furthermore, this elucidated mechanism, which has been long neglected in Si-doped varistors, may provide valuable insights into further developing high-performance ZnO varistors.

B2O3- and Y2O3-doped ZnO varistor ceramics: Enhanced voltage gradient and nonlinear properties for UHV

In this paper, the effects of boron and yttrium co-doping on the electrical properties of ZnO varistors were determined. The presence of both B3+ and Y3+ co-dopant improved the electrical performance of varistor ceramics. The nonlinearity coefficient α and potential barrier ϕb were increased noticeably. The nonlinearity coefficient α was 73 and ϕb was 2.75 eV, at the same time, a voltage gradient of 425 V/mm and leakage current of 0.9 μA/cm2 were obtained. This work provided a way to enhance the protective effects of ZnO varistor arresters for ultra-high voltage power transmission systems.