Low-voltage ZnO varistor fabricated by the solution-coating method

A novel solution nano-coating technique, by coating ZnO powder with a mixed solution of dopants, has been developed to produce high performance low-voltage ZnO varistors. The sintering temperature in the present route is about 50 °C lower than that in the conventional oxide mixing route. The microstructure and electrical characteristics were examined by XRD, SEM and dc power supply and the results showed that the specimens prepared by the solution-coating route have bigger grain sizes, more evenly distributed intergranular phases, higher densities and nonlinearity coefficients, lower breakdown fields and leakage currents than those from the conventional oxide mixing route. The improved current–voltage properties are attributed to the excellent performance of the nano-composite ZnO powder and the advantages of the solution nano-coating technique.     

Microstructure and electrical properties of Sc2O3-doped ZnO-Bi2O3-based varistor ceramics

The microstructure and electrical properties of ZnO-Bi₂O₃-based varistor ceramics doped with different Sc₂O₃ content sintered at 1100 °C were investigated. The results showed that the nonlinear coefficient of the varistor ceramics with Sc₂O₃ were in the range of 18-54, the threshold voltage in the range of 250-332 V/mm, the leakage current in the range of 0.1-23.0 μA, with addition of 0-1.00 mol% Sc₂O₃. The ZnO-Bi₂O₃-based varistor ceramics doped with Sc₂O₃ content of 0.12 mol% exhibited the highest nonlinearity, in which the nonlinear coefficient is 54, the threshold voltage and the leakage current is 278 V/mm and 2.9 μA, respectively. The results confirmed that doping with Sc₂O₃ was a very promising route for the production of the higher nonlinear coefficient of ZnO-Bi₂O₃-based varistor ceramics, and determining the proper amounts of addition of Sc₂O₃ was of great importance.     

Microstructure, electrical properties, and aging behavior of ZnO-Pr6O11-CoO-Cr2O3-Y2O3-Er2O3 varistor ceramics

The microstructure, electrical properties, and aging behavior of the ZnO-Pr₆O₁₁-CoO-Cr₂O₃-Y₂O₃-Er₂O₃ varistor ceramics were investigated for different contents of Er₂O₃. The microstructure consisted of ZnO grain and an intergranular layer (Pr, Y, and Er-rich phases) as a secondary phase. The increase of Er₂O₃ content decreased the average grain size and increased the sintered density. As the Er₂O₃ content increased, the breakdown field increased from 4206 V/cm to 5857 V/cm and the nonlinear coefficient increased from 32.6 to 48.6. The varistor ceramics added with 1.0 mol% Er₂O₃ exhibited excellent stability by exhibiting −0.2% in the variation rate of the breakdown field and −2.7% in the variation rate of the nonlinear coefficient for aging stress of 0.95 E_1 mA/150 °C/24 h.     

Microstructural and compositional aspects of ZnO-based varistor ceramics prepared by direct mixing of the constituent phases and high-energy milling

ZnO-based varistor samples were prepared by the direct mixing of the constituent phases (DMCP) and sintering at 1100 °C for 2 h. The influence of the starting powder mixture's composition – the amounts of the pre-reacted varistor compounds and their composition – and its preparation, either with or without mechano-chemical activation (MCA), on the microstructure, phase composition and electrical characteristics of the varistor samples was studied. It showed that MCA improved the density and microstructural homogeneity of the varistor samples. MCA strongly affected the grain growth: it enhanced the nucleation of inversion boundaries (IBs) in the ZnO grains and the IBs-induced grain-growth mechanism resulted in uniform grain growth and hence a microstructure with smaller ZnO grains and a narrower grain size distribution. The final phase composition of the samples prepared by the DMCP method mainly depended on the presence of varistor dopants that can prevent the formation of the pyrochlore phase, especially Cr₂O₃, while MCA can affect it mostly by providing a homogeneous distribution of those dopants. The DMCP varistor samples prepared with MCA had much better current–voltage characteristics than the samples of the same composition prepared from unactivated powders.     

Rapid microwave sintering of zinc oxide-based varistor ceramics

Samples of ZnO + Bi₂O₃ + Sb₂O₃ varistor ceramics were microwave sintered using gyrotron systems operating at a frequency of 24 GHz. The microwave power was automatically regulated to implement heating at a constant heating rate of 10–130 °C/min up to a temperature of 1100–1300 °C with no isothermal hold. The final sintered density of the samples was 95–96 % of the theoretical value. Manifestations of the thermal instability associated with the liquid phase formation were observed at a temperature of about 600 °C. The estimated volumetrically absorbed power density at the onset of instability was ≥20 W/cm³, and the temperature difference measured between the center and periphery of the samples reached 200 °C. Correlation has been revealed between the thermal instability occurrence and the shift of densification curves towards lower temperatures. A mechanism underlying enhanced densification in electromagnetic field-assisted sintering processes is suggested.     

Interface states and MWS polarization contributions to the dielectric response of low voltage ZnO varistor

The main purpose of this work is to study the dielectric response of commercial low voltage ZnO varistors by means of dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarization currents (TSDC) in a wide temperature range. Four relaxation processes have been studied here and all of them demonstrate Cole-Davidson behaviour. The first two faster relaxation mechanisms are known processes in ZnO varistors and those are related to the ZnO bulk traps. The next one faster relaxation mechanism attributed to the MWS polarization which should be related to the intergranular Bi-rich microregions. The remaining slower relaxation mechanism is associated to the grain boundaries interfaces. Based on the block model for the ZnO varistor a gradual reduction in the total depletion width is observed at a temperature about 330 K, which can be considered that is due to the gradual decrease of the interface states density at this temperature region.     

Sodium impurities in ZnO-Bi2O3-Sb2O3 based varistors

Sodium impurities are frequently present in the raw oxides that constitute the complex formulation of ZnO-Bi₂O₃-Sb₂O₃ based varistors. But actually little is known about their effect on the microstructure and the electrical response of these materials. This is the main goal of the present contribution and according to the obtained results an excessive presence of this alkaline impurity can lead to a compositional change in the Bi-rich skeleton of the varistor microstructure. As a consequence the grain growth kinetics, the densification rate and the characteristic non-linear I-V response of these electroceramics are seriously affected.     

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.     

Cr2O3 doping in ZnO–0.5 mol% V2O5 varistor ceramics

The effects of the amount of Cr₂O₃ (0.5–4 mol%) on the microstructure and the electrical properties have been studied in a binary ZnO–0.5 mol% V₂O₅ system. The microstructure of the samples consists mainly of ZnO grains with ZnCr₂O₄ and α-Zn₃(VO₄)₂ as the minority secondary phases. The addition of Cr₂O₃ is found to be effective in controlling the abnormal ZnO grain growth often found in V₂O₅-doped ZnO ceramic system, and a more uniform microstructure can be obtained. The varistor performance is also improved as observed from the increase in the non-linear coefficient α of the Cr₂O₃-doped ZnO–V₂O₅ samples. The α value is found to increase with the amount of Cr₂O₃ for up to 3 mol% Cr₂O₃ content. Further increase in Cr₂O₃ is found to cause a decrease in the α value. The highest α value of 28.9 is obtained for the ZnO–0.5 mol% V₂O₅–3 mol% Cr₂O₃ sample.     

Combustion synthesis of doped nanocrystalline ZnO powders for varistors applications

Doped nanocrystalline ZnO powders in the size range between 15 and 250 nm were synthesized by chemical combustion method. The powders were characterized for their physical, structural and chemical properties by BET, X-ray diffraction, FESEM, TEM and XPS. These powders were consolidated into dense varistors discs by compaction, sintering and evaluated for their I-V characteristics. Post-calcinations of these powders were found to have great influence on the green density and sinterability. The formations of phases after sintering were confirmed by XRD analysis and EDX. The varistor properties have been studied for different calcination temperatures and compositions. Breakdown voltage as high as 9.5 kV/cm and coefficient of nonlinearity 134 were obtained. Leakage current density was found to be ∼1.29 μA/cm² for a specific composition and condition. These studies demonstrate the feasibility of one step synthesis of doped ZnO nanopowder and their consolidation into ZnO fine grain varistor exhibiting improved performance.     

Effect of silica doping on the densification and grain growth in zinc oxide

The ability of silica (SiO₂) in controlling the densification and grain growth behavior of nano crystalline zinc oxide (ZnO) has been systematically studied. It has been observed that SiO₂ acts as a sintering inhibitor in the ZnO–SiO₂ system up to 4 wt.% limiting value beyond which densification behavior of the system remains almost unchanged, especially above 1100 °C. The addition of SiO₂ to ZnO retards grain growth which in turn results a finer ultimate grain size as compared to the undoped ZnO. However, stabilization in grain size occurs at ≥4 wt.% SiO₂ addition. It has been observed that SiO₂ incorporation changes the grain growth mechanism up to 4 wt.% addition, beyond which no remarkable changes was noticed. The grain growth (n) shows distinctly different slopes as a function of sintering time for the SiO₂ doped ZnO systems than undoped ZnO. The different slopes tend to indicate that different diffusion mechanisms and probably the formation of a secondary phase (Zn–Si–O) at the grain boundary control the densification and grain growth. The thermal expansion coefficient of the system has been found to decrease substantially beyond 4 wt.% SiO₂ addition to ZnO.     

Micro four-point probe investigation of individual ZnO grain boundaries in a varistor ceramic

The varistor effect in ZnO ceramics is triggered by the behavior of the grain boundaries. To understand this effect in detail we have electrically characterized individual grain boundaries. Here, we apply the micro four-point probe method to measure the electrical properties of individual grain boundaries in a polycrystalline ZnO varistor ceramic with a mean grain size of 10 μm. The investigation revealed a wide spread in electrical properties, like nonlinearty exponents from 10 to 150 and switching voltages from 2.3 V to 3.6 V.     

Investigation of grain boundaries influence on dielectric properties in fine-grained BaTiO3 ceramics without the core–shell structure

Grain boundaries of the Ca-doped and fine-grained BaTiO₃ (BT) ceramics were investigated in order to understand the role of grain boundaries, using a high-resolution transmission electron microscope (TEM) analyses, X-ray diffraction (XRD) and chemical etching analyses. Electrical properties and complex impedance spectroscopy of multilayer ceramic capacitors (MLCs) using Ca-doped BT were also examined to investigate with reference to the roles of grain boundaries. Doped elements were peculiarly enriched at the grain boundaries and tetragonality of the BT ceramics recovered significantly after grain boundaries were etched. It is confirmed that the grain boundaries have a significant influence in stabilizing the temperature dependence of the dielectric properties, and that the residual stress is caused by grain boundaries with the grain growth inhibited during sintering. In addition, the high reliability of BT ceramics without the core–shell structure is considered to be due to the high resistivity of the grain boundaries.     

Hot pressing of nanocrystalline zinc oxide compacts: Densification and grain growth during sintering

Sintering behavior of nanocrystalline zinc oxide (ZnO) powder compacts using hot pressing method was investigated. The sintering conditions (temperature and total time) and results (density and grain size) of two-step sintering (TSS), conventional sintering (CS) and hot pressing (HP) methods were compared. The HP technique versus CS was shown to be a superior method to obtain higher final density (99%), lower sintering temperature, shorter total sintering time and rather fine grain size. The maximum density achieved via HP, TSS and CS methods were 99%, 98.3% and 97%, respectively. The final grain size of samples obtained by HP was greater than that of TSS method. However, the ultra-prolonged sintering total time and the lower final density (88 ks and 98.3%) are the drawbacks of TSS in comparison with the faster HP (17 ks and 99%) method.     

The effect of lithium oxide on the threshold field of ZnO varistors

Lithium oxide in form of Li₂CO₃ solution is added with contents of 0–200 ppm to two ZnO-based varistors standard formulations, once with Sb₂O₃ and the other without. According to Li₂CO₃ concentration, both threshold field and energy absorption capability evolution are studied. It is found that with the benefit of antimony, the lithium allows reaching high threshold field but concomitantly, low energy absorption capability. Without antimony, threshold fields up to 300 V/mm are attained, associated with a fair energy absorption capability. With 100 ppm of Li₂CO₃, optimum couple of values (315 V/mm; 115 J/cm³) is achieved. With 200 ppm of Li₂CO₃, threshold field exceeds 500 V/mm but energy absorption capability falls below 50 J/cm³. Correlations with SEM microstructures observations suggest that lithium increases voltage barrier height by decreasing donor density and that spinel phases (Zn₇Sb₂O₁₂) have detrimental effects on the electrical absorption capability by limiting the density of current, reducing the effective current path from one ZnO grain to another.     

Electrical and dielectric characteristics of erbium-added ZnO–V2O5-based varistor ceramics

This paper focuses on that the erbium-added ZnO–V₂O₅-based ceramics are attained at a sintering temperature as low as 875 °C. The effect of Er₂O₃ addition on microstructure, electrical properties, and dielectric characteristics has been investigated. Increasing the amount of Er₂O₃ slightly increased the densities of sintered pellets in the range of 5.52–5.59 g/cm³. The increase in the amount of Er₂O₃ increased the breakdown field from 6991 to 7408 V/cm up to 0.1 mol%, whereas a further addition decreased it. The sample added with 0.1 mol% Er₂O₃ exhibited the highest nonlinear coefficient (α=55) and the sample added with 0.25 mol% Er₂O₃ exhibited the lowest nonlinear coefficient (α=14). The donor concentration increased from 2.92×10¹⁷ to 8.48×10¹⁷ cm⁻³ with an increase in the amount of Er₂O₃.     

Inversion boundary induced grain growth in TiO2 or Sb2O3 doped ZnO-based varistor ceramics

In low-voltage varistor ceramics, the phase equilibrium and the temperature of liquid-phase formation are defined by the TiO₂/Bi₂O₃ ratio. The selection of a composition with an appropriate TiO₂/Bi₂O₃ ratio and the correct heating rate is important for the processing of low-voltage varistor ceramics. The total amount of added Bi₂O₃ is important as the grain growth is slowed down by a larger amount of Bi₂O₃-rich liquid phase at the grain boundaries. Exaggerated grain growth in low-voltage varistor ceramics is related to the occurrence of the liquid phase and the presence of TiO₂ which triggers the formation of inversion boundaries (IBs) in only a limited number of grains, and as a result the final microstructure is coarse grained. The Zn₂TiO₄ spinel phase only affects grain growth in compositions with a TiO₂/Bi₂O₃ ratio higher than 1.5. In high-voltage varistor ceramics, just a small amounts of Sb₂O₃ trigger the formation of IBs in practically every ZnO grain, and in compositions with a Sb₂O₃/Bi₂O₃ ratio lower than 1, grain growth that is controlled entirely by an IBs-induced grain growth mechanism results in a fine-grained microstructure. The spinel phase interferes with the grain growth only at higher Sb₂O₃/Bi₂O₃ ratios.     

A novel economical grain boundary engineered ultra-high performance ZnO varistor with lesser dopants

A varistor having ultra-high performance was developed from doped ZnO nanopowders using a novel composition consisting of only three (Bi, Ca and Co oxides) dopants. Improved varistor properties were obtained (breakdown field (E_b) 27.5?±?5?kVcm^?1, coefficient of nonlinearity (α) 72?±?3 and leakage current density (L_c) 1.5?±?0.06?μAcm^?2) which are attributed to the small grain size and grain boundary engineering by phases such as Ca₄Bi₆O₁₃ and Ca_0.89Bi_3.11O_5.56 along with Co⁺² doping in the ZnO lattice. Complex impedance data indicated three relaxations at 25?°C and two relaxations at high temperature (>100?°C). The complex impedance data were fitted into two parallel RC model to extract electrical properties. Two stages of activation energy for DC conductivity were observed in these varistor samples where region I (<150?°C) is found to be due to shallow traps and region II (<225?°C) is due to deep traps. The novel composition is useful for commercial exploitation in wide range of surge protection applications.