ZnO-based Varistor Ceramics Research Articles

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

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.

Rapid microwave sintering of functional electroceramic materials

Rapid microwave sintering processes with heating rates of up to 300 °C/min and zero isothermal hold have been implemented using a 5 kW 24 GHz gyrotron system for high-temperature microwave processing of materials. ZnO-based varistor ceramics, BaTiO3/SrTiO3 dielectric ceramics and Gd:CeO2 ceramics for solid electrolyte applications have been sintered to densities of up to 96% of the theoretical value. Using in situ optical dilatometry, correlation between the development of thermal instability under intense volumetric microwave heating and the early onset of densification has been revealed. The influence of the absorbed microwave power on densification and grain growth has been studied by comparing direct and susceptor-assisted microwave heating processes. The possibility of tailoring the microstructure and functional properties of the obtained materials by choosing optimal regimes of rapid microwave sintering is discussed.

Advances in understanding of damage formation during laser-assisted milling of ZnO-based varistor ceramics

Zinc oxide (ZnO)-based varistor ceramic is a hard-to-machine material. Its microstructure, developed for its electrical properties, results in extremely low fracture toughness. Therefore, the current state-of-the-art in machining this material is lapping. In the present work, laser-assisted milling (LAMill) of ZnO ceramics was evaluated with a systematic approach to allow for new insights and advance the understanding of damage formation during machining of this material. A machining strategy to machine ceramic workpieces along the edge was studied. Additionally, a new transient analytical thermal model was developed to accurately predict laser-induced temperatures near the edge of the workpiece without excessive use of computational power, and the model predictions were experimentally verified. A novel approach to evidence machining-induced damage with the mathematical description of the ideal machined surface was also designed and tested. It was found that at higher temperatures the propagation of intergranular fractures is inhibited. Another mechanism based on the material porosity and contributing to more instances of intergranular crack initiation has been identified. To reliably predict the extent of damage on the machined surface, the analysis has been extended by covering the effects of the two identified influential process parameters: feed per tooth, fz, and the temperature of the to-be-cut material, T. By adjusting these parameters, surface damage was successfully controlled and the extent of damage was minimized at fz = 0.18 mm and T = 380 °C, achieving a 44 % reduction compared to the conventional process with the same machining parameters.

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.

A Comprehensive Study on the Damage Formation During Laser-Assisted Milling of Zno-Based Varistor Ceramics

A Comprehensive Study on the Damage Formation During Laser-Assisted Milling of Zno-Based Varistor Ceramics

Microstructure and Electrical Properties of In2O3, La2O3 and Ga2O3 Doped ZnO-Bi2O3-MnO2-SiO2-TiO2 Varistor Ceramics

ZnO-Bi2O3-MnO2-SiO2-TiO2 varistor ceramics with addition of X2O3 (X = In, La, Ga) were manufactured through the conventional solid-state method and their phase transformation, microstructure, band gap, and electrical properties were investigated by X-ray diffractometer, Scanning electron microscopy, Fourier transform infrared spectroscopy, and solid UV absorption spectrometry, respectively. The results show that ZnO-Bi2O3-MnO2-SiO2-TiO2-0.15 mol% X2O3 ceramics can be successfully produced at a very low temperature of 875 °C. When X2O3, especially Ga2O3, is doped into ZnO varistors, the grain sizes of specimens are refined. The grain size of ZnO-based varistor ceramics with Ga2O3 doping decreases obviously from 4.31 to 3.10 μm, and the voltage gradient increases from 763.1 to 1047.5 V mm−1. The nonlinear coefficient of ZnO-based varistor ceramics with In2O3 doping increases from 26.5 to 36.7.

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.

Low temperature sintering and characterization of 0.25–1 mol% Bi2O3 doped ZnBiMnNbO based varistor ceramics

The effect of Bi2O3 on the microstructure and properties of ZnO-based varistor ceramics sintered around 900 °C has not been fully understood. Therefore, ZnO (balanced)-x Bi2O3–2%MnCO3-0.1%Nb2O5 (x = 0.25, 0.5, 0.75 and 1%, all in molar ratio) varistor ceramics were prepared by solid-state sintering at 850–900 °C. Then, the effect of varying Bi2O3 concentration on the microstructure and nonlinear electrical properties of the as fabricated ceramic was studied by using XRD, SEM, EDS and conventional electric field-current density (E-J) test. The Results show that the Mn and Nb doping is the main reason for making Bi2O3 derived liquid phase capable of densifying the ZnBiMnNbO varistor ceramic at 850–900 °C. The sintered ceramics consist of ZnO and other three secondary phases:Bi2O3, Mn3O4 and Bi5Nb3O15. The increase of Bi2O3 can improve the grain growth of the ceramic up to 0.75mol%. The further addition of Bi2O3 slightly inhibits the growth of ZnO grains. As a result, the breakdown voltage first decreases from 633.83 to 403.63 V/mm with the increase of Bi2O3 up to 0.75mol%, then increases slightly to 439.71 V/mm with the further increase of Bi2O3 to 1mol%. The nonlinear coefficient reaches its maximum value (58.68) at 0.5 mol% Bi2O3 in the ceramic sintered at 900 °C. Meanwhile, the leakage current density is as low as 3.16 μA/cm2. This study provides a good reference for producing the ZnO-based varistors at low costs.

The Degradation Phenomena of SLS Glass Doped ZnO Based Varistor Ceramics

A stability loss of varistor is subjected to any of variety extreme conditions such different stresses AC or DC, temperature, pulse, pressure, humidity, etc. This causes degradation phenomenon and leads to thermal runaway or failure of varistor. The investigation of degradation phenomena is an important subject for prolonging their lives. In this study, we investigate the degradation process of ZnO1-x-CoO2.0-SLSx varistor ceramics where x= 0.5, 1.0, 1.5, and 2.0 mol % by using conventional solid state method at sintering temperature of 1100 °C for 2 hour. The stability of their nonlinear properties was investigated under prolonged application of DC electric field at different SLS glass concentration. The degradation process was speeded up by applying direct current (DC) 85 % of electric field for 12 hours at temperature of 85 °C. A concentration of 2 mol % SLS glass doped ZnO varistor shows the best degradation properties as its nonlinear coefficient has increased by 3.56 %, the breakdown field has increased by 3.85 %, and the leakage current density (JL ) increased by 2.40 % in comparison to its initial value.

Synthesis by Hydrothermal Treatment of ZnO-Based Varistors Doped with Rare Earth Oxides and Their Characterization by Impedance Spectroscopy

ZnO-based ceramic varistors have shown excellent electrical and dielectric properties due to their characteristics microstructures represented by the arrangement of their grains and grain boundaries that allow the absorption and flow of energy when subjected to an electrical surge. Their properties and characteristics depend on their chemical compositions and processing routes. Typical processing routes involve several stages of grinding and precalcination—which are time consuming processes. Because of this, this study proposes a simpler and cheaper alternative route for processing ceramic varistors. The alternative process proposed is the mixing of the precursor oxides by means of a hydrothermal treatment. The characteristics and properties of the synthesized ceramic varistors were evaluated by means of scanning electron microscopy, X-ray diffraction and impedance spectroscopy, considering the effect of the addition of rare earth oxides (La2O3, CeO2 and Nd2O3). The results showed that the mixing of the oxides through hydrothermal treatment produces ceramic varistors with characteristics and properties similar to those obtained by other processing routes. Furthermore, it was observed that the addition of rare earth oxides affects the characteristics and properties of the ceramic varistor depending on the type of rare earth oxide added, its concentration and ionic radius.

Effects of pre-calcination and sintering temperature on the microstructure and electrical properties of ZnO-based varistor ceramics

The electrical properties of ZnO-based varistors are closely dependent on the microstructure of ZnO ceramics which is sensitive to the sintering process. In this study, the effects of pre-calcination and sintering temperature on the microstructure and electrical properties of ZnO-based varistors were investigated. The patterns of X-ray diffractometry (XRD) and the morphologies of scanning electron microscopy (SEM) showed that the secondary phase distributed along the boundaries of ZnO grains. After pre-calcined at 800 °C and then sintered at 950 °C, ZnO-based varistors exhibited the optimal electrical properties, of which the breakdown field is 1602 V/mm and the nonlinear coefficient is 37.4.

Cold sintering ZnO based varistor ceramics with controlled grain growth to realize superior breakdown electric field

Controlling the grain growth and grain boundary morphology is of great importance in the manipulation of electrical properties of electro-ceramics. However, it has been a challenge to achieve dense varistor ceramics with grain sizes in submicrons and nanometers using conventional thermal sintering at high temperatures. Here we present a strategy to fabricate dense ZnO based ceramics with controlled grain growth and thin grain boundaries using cold sintering process (CSP). With CSP, the sintering temperature of ZnO based ceramics dramatically drops from 1100 °C to 300 °C. The Bi2O3, Mn2O3, and CoO dopants suppress the grain growth of ZnO under CSP conditions, and Bi-rich intergranular films (2−5 nm) can be observed along grain boundaries. The cold sintered ZnO-Bi2O3-Mn2O3-CoO ceramic shows a non-linear coefficient of 33.5, and a superior breakdown electric field of 3550 V/mm. This work thus demonstrates that CSP is a promising technique for designing new submicron-/nano-ceramics with superior performances.

Effect of impulse current degradation on the electrical properties and dielectric relaxations of ZnO-based ceramic varistors

The effects of impulse current degradation on the electrical properties and dielectric relaxations of zinc oxide (ZnO) ceramic varistors are systematically investigated in this work. The J-E characteristics show that the breakdown field E1mA (electric field at 1mA/cm2) increases, stabilizes, and then decreases with increasing impulse degradation, while the nonlinear coefficient α drops significantly over the entire degradation process. The DC conduction mechanism is consistent with a Schottky thermionic emission and the impulse degradation decreases the barrier height from 1.05 to 0.48 eV. It is also found that the electrical properties of the degraded samples can be partly recovered after removing the impulse degradation. SEM, XPS observations of the microstructure show increased voids at the grain boundaries and the content of chemisorbed oxygen is greatly reduced for degraded samples. Dielectric relaxations are characterized over a wide frequency and temperature range. Two dielectric relaxation peaks with activation energies of 0.25 and 0.37 eV are observed in 253 K, which do not change with impulse degradation. Another two dielectric relaxation with activation energies of 0.72 and 0.83 eV are characterized by δe'/δω spectra over 423–473 K. The activation energy of one of the latter peaks decreases from 0.83 to 0.76 eV with impulse current degradation and may be related to interfacial states at grain boundaries. A schematic energy band diagram is proposed to interpret the degraded process of the ceramic varistors.

Using semi-finished Ce-La extracts as the sole RE source to synthesize high-performance ZnVMnNbO varistor ceramics

Semi-finished Ce-La extracts of a local Rare Earth (RE) ore refining factory were heat-decomposed to oxides and used as the sole RE source to fabricate high-performance ZnO-V2O5-MnCO3-Nb2O5 (ZnVMnNbO) varistor ceramics. The effect of varying the concentration of this RE dopant within the range of 0–0.25mol% on the microstructure and nonlinearity of the ceramic sintered at 850–950°C was studied by XRD, SEM, EDS, EBSD and I-V test. Besides of ZnO main phase, Ce(La)VO4 solid solution and ZnMn2O4 were identified as the minor secondary phases along with ZnV2O4, Zn3(VO4)2. The average ZnO grain size decreased from 6.35 to 3.37µm in the ceramic samples sintered at 900°C with the increase of Ce-La dopant from 0 to 0.25mol%. The breakdown field was thereby elevated from 3320 to 5590V/cm and the homogeneity of the obtained microstructure was enhanced as well. Only 0.1mol% of such Ce-La dopant was sufficient to improve the nonlinear coefficient of the ZnVMnNbO based varistor ceramics sintered at 875°C to 48.4. The further addition of the RE dopant or the increase of sintering temperature from 875°C to 950°C all decreased the nonlinearity. In general, our result indicated the semi-finished Ce-La extracts can be a feasible and effective candidate to replace pure RE oxides for fabricating high-performance ZnO based varistor ceramics at a lower cost.

Highly nonlinear varistors fabricated by hot-dipping tin oxide thin films in Ta2O5 powder at different temperatures

This work proposed a smart approach to fabricate highly nonlinear SnOx-Ta2O5 thin film varistors. During processing, tin oxide films in nanoscaled thickness were firstly deposited onto electrically conducting silicon wafers through radio frequency magnetron sputtering a sintered tin oxide ceramic target; and then, the obtained films were hot-dipped in Ta2O5 powder in a Muffle oven for 60min at different temperatures. With the hot-dipping temperature increased from 300 to 800°C, the nonlinear coefficient of the samples increased first and then decreased, reaching the maximum of 16.8 at 600°C, which is a record in all the reported SnO2-based varistors and comparable with those of the most extensively investigated and commonly commercially applied ZnO-based ceramic varistors. Correspondingly, the leakage current decreased first and then increased, obtaining the minimum of 4.1mA/cm2. Besides, the breakdown electric field increased first and then decreased, presenting the maximum of 0.0313V/nm. Such nanoscaled thin film varistors would be promising in electrical and electronic devices working in low-voltage.

Effect of erbium-calcium manganite doping on microstructure and electrical properties of zinc oxide based varistor ceramics

In this research, (80-x) mol% zinc oxide with 20 mol% CaMnO 3 as the additive and (x) mol% erbium oxide Er 2 O 3 as the doping material where x = 0.5, 1.0 and 1.5. Aims of this work are to elucidate the effects of doping material on microstructure ZnO and nonlinear characteristics of ZnO based varistor ceramics. A compound comprising ZnO-CaMnO 3 -Er 2 O 3 underwent pre-sintering at 500 °C for 4 hours and sintering process at temperature of 1100 °C, 1200 °C and 1300 °C with the soaking time of 1.5 hours and 4.5 hours. X-ray diffraction patterns confirmed the presence of CaMnO 3 spinal and additional phases containing Er in the synthesized compounds. The disappeared of hydroxyl and carbonyl functional group was observed at 3600-3650 cm-1, 1421 cm-1 and 875 cm-1 respectively by Fourier Transform Infrared spectroscopy to indicate the starting formation oxide metal upon calcinations.

Influence of Cr2O3 on electrical and microstructural characteristics of ZnO–V2O5–MnCO3–Nb2O5 varistor ceramics

In order to clarify the controversy on whether Cr2O3 can enhance the nonlinearity of ZnO based varistor ceramics, ZnO–V2O5–MnCO3–Nb2O5 (ZVMN) ceramics with 0-0.5 mol% Cr2O3 were synthesized by traditional mixing oxide procedure ended by sintering at 900 °C for 3 h. Then XRD, FESEM, EDS and V–I tests were used to investigate the influence of Cr2O3 on electrical and microstructural characteristics of the obtained ceramics. The result shows ZVMN ceramics with Cr2O3 addition consisted of Zn3(VO4)2, ZnV2O4 and ZnCr2O4 as the minor secondary crystalline phases and ZnO as the main phase. The average size of ZnO grains decreased monotonously with the increase of Cr2O3 concentration from 0 to 0.5 mol%, which in turn led to a monotonous elevation of the breakdown field from 354 to 1032 V/mm. The nonlinearity coefficient of ZVMN ceramics increased to its maximum value of 41.0 at 0.1 mol% Cr2O3, while the breakdown field and leak current density were 415 V/mm and 95.0 μA/cm2 respectively. The comparison of this study with those of peers suggests Cr2O3 is more likely to enhance the nonlinearity of ZnO–V2O5 based varistor ceramics than it does in the case in ZnO–Bi2O3 system, wherein co-doping Sb2O3 may be a necessary step to secure an overall enhancing effect of Cr2O3 on the nonlinearity.

Effect of co-doping Y2O3–La2O3 on microstructure and electrical properties of ZnO-based varistor ceramics prepared from nanosize ZnO powder

ZnO-based varistor ceramics doped with fixed La2O3 and different Y2O3 have been fabricated by the conventional solid-state reaction route. Phase composition, microstructure, and electrical properties were studied by X-ray diffractometry (XRD), scanning electron microscopy (SEM), and direct current electrical measurement. The Y2O3 phase, Y-rich phase and La-rich phase were found to distribute along the grain boundaries of predominant ZnO grains from XRD and SEM analyses. The average grain size of the primary phases was calculated to be in the range of 3.80 to 4.17 μm. The varistor ceramic doped with 0.1 mol% La2O3 and 0.5 mol% Y2O3 sintered at 1100 °C for 2 h exhibited high comprehensive electrical properties with the breakdown field of 1055 V/mm, the nonlinear coefficient of 65.5 and the leakage current of 0.6 μA. The results illustrated that co-doping La2O3 and Y2O3 and employing the ZnO powder in nanometer size may be a promising route to obtain varistor ceramics with excellent electrical properties.

Reverse manipulation of intrinsic point defects in ZnO-based varistor ceramics through Zr-stabilized high ionic conducting βIII-Bi2O3 intergranular phase

Intrinsic point defect structure plays a crucial role in functional ceramics with a grain-grain boundary microstructure. In the present study, a novel method of reversely manipulating intrinsic point defects (oxygen vacancy, Vo and zinc interstitial, Zni) in ZnO-based varistor ceramics is proposed, which makes use of Zr-stabilized high ionic conducting βIII-Bi2O3 intergranular phase. It is found that Zr-doping not only modifies the grain growth by the formation of secondary Zr-rich phase, but also influences the intrinsic point defect structure via the stabilized βIII-Bi2O3 phase, resulting in reduced Vo density but increased Zni density. The reverse manipulation is unambiguously demonstrated by broadband dielectric spectroscopy and further confirmed by the accelerated ageing experiment. The proposed intrinsic point defect dynamics unveil an important but usually neglected function of the dopant that has little solid solubility in ZnO grains, which opens up a promising way to tailor the material property.