Two stage sintering behaviour of Er2O3 doped high performance ZnO varistors

High performance Er₂O₃ doped ZnO-V₂O₅ based varistors were prepared by two stage sintering (TSS) method at T₁ = 1050 °C for 1/6 h and T₂ = 750 and 800 °C for 2.5, 10, 20 and 40 h duration. The microstructural features and electrical properties were studied by XRD, SEM, EDS and a high voltage source unit. The combined effect of TSS and Er₂O₃ doping is very beneficial to restrain the grain coarsening of varistors. The effect of grain size reduction on nonlinear electrical properties was investigated. The results showed that superior electrical properties were obtained in doped samples via TSS than single stage one. A typical fine-grained (1.7 μm, 97.5 %TD) varistor with sufficiently high nonlinear exponent (α = 154), breakdown field (E_B = 15.2 kV cm^?1) and reasonably low leakage current density (J_L = 0.13 mA cm^-2) could be prepared with 0.5 mol.% Er₂O₃ doping at an optimum sintering condition of T₁/T₂ = 1050/750 °C for 40 h.     

Indium tailors the leakage current and voltage gradient of multiple dopant-based ZnO varistors

In the present study, the effect of indium doping on the micro-characteristics and electrical properties of ZnO varistors co-doped with Al₂O₃ and Y₂O₃ were determined. Scanning electron microscopy, current-voltage testing in a range from small to large current, capacitance-voltage testing, and X-ray diffraction pattern testing were conducted. The results show that both the residual voltage ratio and the leakage current of sintered ZnO varistors decrease and then increase as the indium dopant increases at a given aluminum and yttrium content. The nonlinear coefficient shows an inverse relationship. In addition, the voltage gradient of the samples increases as the indium dopant increases. The sintered ZnO varistor samples with 0.02 mol% indium, 0.2 mol% aluminum, and 0.9 mol% yttrium show the optimal performance, exhibiting a 1-mA residual voltage of 448 V/mm, a leakage current of 0.69 µA/cm², a nonlinear coefficient of 76, and a residual voltage ratio of 1.58. This study has great significance for improving the protective effects of surge protection devices assembled with ZnO varistors and the stability of power systems.     

不同方法合成掺杂ZnO粉体制备ZnO压敏电阻

分别采用低温固相化学法和共沉淀法合成掺杂ZnO粉体,并用这两种粉体在不同温度下烧结制备了ZnO压敏电阻。借助XRD、SEM、TEM、BET等检测手段对粉体产物的性能进行了表征,采用XRD、SEM等手段对ZnO压敏陶瓷的物相、结构进行了分析,并对两种方法制备的粉体及压敏电阻的性能进行了比较研究。结果表明:采用低温固相化学法合成的粉体平均粒径为23.95 nm,用其制备ZnO压敏电阻的最佳烧结温度是1 080℃,其电位梯度为791.64 V/mm,非线性系数是24.36;采用共沉淀法合成的粉体平均粒径为188 nm,用其制备ZnO压敏电阻的最佳烧结温度是1 130℃,其电位梯度为330.99 V/mm、非线性系数是19.70,低温固相化学法制备的ZnO压敏电阻性能优于共沉淀法制备的ZnO压敏电阻。     

Preparation and electrical properties of multilayer ZnO varistors with water-based tape casting

Multilayer ZnO varistors were prepared by water-based tape casting with water-soluble acrylic as binders. Zeta potentials of the doped ZnO suspensions as a function of pH with and without dispersant were measured. Viscosity measurements were used to find the optimum dispersant concentration needed to prepare a stable slurry. Viscosity properties of the tape casting slurry were investigated. The results showed that aqueous acrylic binders have shear thinning properties suitable for tape casting of ceramic powders. Scanning electron microscopy (SEM) studies revealed that the green sheets have a smooth defect-free surface and that the multilayer varistor (MLV) ceramics prepared by water-based tape casting have a fine grain microstructure with a uniform grain size and dopant distribution. The multilayer ZnO varistors prepared by water-based tape casting display comparable good electrical properties to those prepared by solvent-based tape casting. This is believed to be attributed to the well dispersed water-based slurry, which makes more uniform dopant distribution throughout the multilayer ZnO varistors. Therefore, water-based tape casting is suitable for the manufacture of high performance multilayer ZnO varistors.     

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.     

Improvement of current withstand capacity of high gradient ZnO varistors

The effects of the dispersant and the inorganic high resistance coating on the microstructure and the electrical properties of the ZnO varistors, especially on the current withstand capacity, were studied. The results showed that under the premise of adequate slurry dispersion, when the dispersant content was the minimum, the porosity of the varistor was the minimum, and the current withstand capacity was up to 24 A/cm² in 2 ms square waveform impulse test. By coating a high resistance layer with high proportion of spinel forming elements, the content of spinel near the side of the varistor ceramic was increased, thus increasing the resistivity and alleviating the current impact near the edge. The current withstand capacity of 2 ms square waveform impulse was upgraded to 32 A/cm².     

Processing and characterizations of flash sintered ZnO-Bi2O3-MnO2 varistor ceramics under different electric fields

The dense ZnO-Bi₂O₃-MnO₂ (ZBM) varistors were prepared by flash sintering under electric fields ranging from 200 V/cm to 400 V/cm at constant heating rate (CHR) and constant furnace temperature (CFT), respectively. The structure and electrical properties of the ZBM varistors were studied via the XRD, SEM and a DC parameter instrument. The onset temperature and incubation time of flash sintering decrease with increasing electric field. The effects of the maximum limiting current on the density of the samples were also investigated. The results showed that the density of samples increase with the increasing current values. The improved electrical characteristics were obtained during constant furnace temperature flash sintering. The ZBM varistor ceramics exhibited superior comprehensive electrical characteristics under a field of 250 V/cm, in which the nonlinear coefficient is 26.4, the threshold voltage and the leakage current is 466 V/mm and 12.32 μA, respectively.     

Effect of NiO doping on grain growth and electrical properties of ZnO-based varistors

The effects of NiO content on the grain growth and the electrical properties of the ZnO varistors were studied. The effect on the grain growth was characterized by scanning electron microscopy, energy dispersive spectroscopy and X-ray diffractometer, while the effects on the electrical properties were investigated by varistor tester, pulse surge tester and precision impedance analyzer. The results showed that the growth of the (0002) plane of the ZnO varistors was inhibited by doping NiO, which improved the electrical properties of the varistors, especially the surge shock stability. The sample doped with 1.56 mol% NiO exhibited excellent electrical properties with breakdown voltage gradient of 184.0 V/mm, nonlinear coefficient of 72.7, leakage current of 0.37 µA, clamp voltage ratio of 2.20 under 8/20 μs waveform impact of 20 kA and change rates of positive and negative breakdown voltages of < 4% under 20 times impacts of 20 kA.     

High‐performance varistors prepared by hot‐dipping tin oxide thin films in Sb2O3 powder: Influence of temperature

A series of SnO_x–Sb₂O₃ thin film varistors were fabricated through hot‐dipping tin oxide films deposited by radio‐frequency magnetron sputtering in Sb₂O₃ powder at varied temperatures in air. With the increase in hot‐dipping temperature (HDT) from 200°C to 600°C, the nonlinear coefficient (α) of the samples increased first and then decreased, reaching the maximum at 500°C, which was mainly determined by the completeness of high‐resistant Sb₂O₃ layer at tin oxide grain boundary and the chemical composition of tin oxide films. Correspondingly, the leakage current (I_L) decreased first and increased later. The breakdown electric field (E_100 mA) decreased constantly with increasing HDT. The SnO_x–Sb₂O₃ film varistors prepared at 500°C exhibited the optimum nonlinear properties with the maximum α of 10.88, the minimum I_L of 36.3 mA/cm², and an E_100mA of 0.0188 V/nm. The obtained nanoscaled film varistors would be promising in electrical/electronic devices working in low voltage.     

掺杂TiO_2制备低压ZnO压敏陶瓷

主要研究了晶粒助长剂TiO2、烧成条件等对ZnO低压压敏陶瓷电性能的影响。结果表明:TiO2的掺入能显著促进晶粒生长,降低压敏电压V1mA,但掺入量超过一定值后一方面会生成阻止晶粒继续生长的晶界反应层,促使压敏电压又有所升高,另一方面会生成钛酸铋立方相而引起富铋晶界相含量的减少,导致非线性特性下降。提高烧成温度可以促进晶粒生长,降低压敏电压,但温度过高时由于铋的挥发加剧,利用提高烧成温度降低压敏电压会引起非线性特性和漏流等性能的劣化。     

低压ZnO压敏电阻中Bi2O3-TiO2相变过程的研究

在最高温度950℃、1050℃、1150℃、1250℃4个温度下,采用2种配方:98.3%ZnO-0.7%Bi2O3-1.0%TiO2(摩尔分数),相应的无TiO2配方99.3%ZnO-0.7%Bi2O3(摩尔分数),分别制备样品进行对比研究。发现前者在950℃和1050℃的烧结温度下,没有形成明显Bi2O3偏聚的晶界;在1150℃和1250℃下,却形成了清晰的富铋晶界。而后者在950℃、1050℃、1150℃、1250℃4个温度下均形成了非常明显且清晰可辨的ZnO晶粒和晶界。通过XRD及相对峰强分析,发现了Bi2O3和TiO2在低压ZnO压敏电阻中的相变过程,并对其在烧结过程中的作用提出新的解释。     

Breakdown phenomenon of ZnO varistors caused by non-uniform distribution of internal pores

Here, we analyzed the internal structure of ZnO varistors with X-ray micro-CT technology and showed that the porosity and pore volume at the radial edge is considerably higher than at other regions. Experimental results showed that the breakdown phenomenon usually occurs near the outer edge of circular varistors. The distribution of impulse currents applied to concentric ring electrodes indicated that the current ratio at the edge region increased as the total current increased. The pores are insulating under low electric fields and the current through the pores is almost zero. Hence, the voltage gradient at the edge region is higher than that elsewhere. Additionally, the simulation results were consistent with this assumption. The amounts of open pore defects, which showed a conductive state after break down, increased, and the proportion of the current flow through the pores increased accordingly, which lead to a local temperature rise and breakdown.     

The degradation behavior of high-voltage SnO2 based varistors sintered at different temperatures

In this research, for the first time, the stability of SnO₂ based varistor ceramics sintered in the range of 1250–1350 °C against DC-accelerated aging and impulse surge current tests, was systematically studied. Microstructural study of the sintered samples by XRD and FESEM indicated that the sintering temperature only affects densification and grain size, while phase composition remains intact. With the increase of sintering temperature from 1250 °C to 1350 °C, the mean grain size increased from 1.6 to 8 μm. The maximum nonlinear coefficient of 50 and the minimum leakage current density of 1.5 μA/cm² were obtained in the sample sintered at 1300 °C. The breakdown electric field decreased from 800 V/mm to 270 V/mm, when sintering temperature increased from 1250 °C to 1350 °C. The samples sintered at 1250 °C did not show stability against neither of DC-accelerated aging and impulse current tests. The varistors sintered at 1300 °C exhibited the excellent resistance to DC-accelerated aging degradation, while ceramics sintered at 1350 °C showed the best resistance to impulse current degradation.     

Band structure and thermoelectric properties of inkjet printed ZnO and ZnFe2O4 thin films

The band structure and thermoelectric properties of inkjet printed ZnO and ZnFe₂O₄ thin films have been investigated. The bulk pellets were prepared by a solid-state method and thin films were deposited using an inkjet printing method. Multiple print cycles were required to fabricate homogeneous films and the composition of the thin films can be varied by varying the relative amounts of liquid deposited. It was possible to obtain high thermoelectric properties of ZnO by controlling the ratios of dopant added and the temperature of the heat treatments. XRD analysis showed that the fabricated samples have a wurtzite structure and an additional ZnAl₂O₄ phase was formed with increasing Al content and sintering temperature. It was found that the band gap of Al doped ZnO becomes smaller with increasing Al content and thus the electrical conductivity of Al_x doped ZnO (x=0.04) thin films showed the highest electrical conductivity (114.10 S/cm). The ZnFe₂O₄ samples were compared against the ZnO samples. The formation of single phase cubic spinel structure of the sintered ZnFe₂O₄ samples was found and confirmed by X-ray diffraction technique. Secondary phase Fe₂O₃ was also detected for compositions with Zn (x≤0.4). Finally, we want to report that the electrical conductivity of Zn_xFe_3−xO₄ was lower than the conductivity of the Al-doped ZnO.     

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 850^oC. 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.     

Electrical properties and AC degradation characteristics of low voltage ZnO varistors doped with Nd2O3

The electrical properties and degradation characteristics of low voltage ZnO varistors were investigated as a function of Nd₂O₃ content. The varistor ceramics with 0.03 mol% Nd₂O₃ sintered at 1250 °C were far more densified than those with 0.06, 0.09 and 0.12 mol% Nd₂O₃. The addition of Nd₂O₃ to the low voltage ZnO varistors greatly improved the current–voltage characteristics; the nonlinear coefficient of varistors increase from 12.2 to 34.6 with increasing Nd₂O₃ content. The samples with 0.03 mol% Nd₂O₃ showed excellent stability due to high density and relatively good V–I characteristics, with the nonlinear coefficient of 22.5 and the leakage current of 9.6 μA. Their variation rate of varistor voltage and nonlinear coefficient and leakage current were −4.7%, −5.4%, 18.3%, respectively, under AC degradation stress (1.0 V_1 mA/125 °C/24 h).     

Sb2O3掺杂对ZnO压敏陶瓷晶界特性和电性能的影响

制备了掺有Sb2O3不同掺杂量ZnO压敏陶瓷样品,采用扫描电镜对样品进行显微结构分析,研究了Sb2O3掺杂浓度对ZnO压缩电阻显微结构和性能的影响,测量了样品的电性能,由样品C－V特性的测量计算出晶界参数,并由此讨论了陶瓷性能与晶界特性的相关性。研究发现,在ZnO压敏陶瓷样品中掺杂适量的Sb2O3可以提高ZnO压敏陶瓷样品的非线性性能,但当Sb2O3的摩尔分数超过0．088％时,电性能反而优化,这是因为Sb2O3掺杂浓度不同会引起晶界势垒高度、施主浓度与陷阱密度的变化,因此Sb2O3掺杂量要控制在适当的范围内。     

Effect of low H2 concentrations on the current-voltage characteristic of ZnO gas sensor

Abstract

Abstract

A self-heated ZnO gas sensor was prepared through thermal oxidation of Zn metal for 30 and 60?min in oxygen atmosphere. The XRD results confirmed the conversion of Zn metal to ZnO. The current-voltage measurements showed changes in forward current in the presence of hydrogen gas, with the concentration ranging from 40 to 160?ppm. A better resolution of current response due to H₂ concentrations was obtained for the samples with 30?min oxidation, although both samples showed enhanced response with the applied voltage. The barrier height of both samples was found to decrease with H₂ concentrations, thus aiding in the increase in current response.     

Electrical transport properties of M2FeV3O11 (M=Mg,Zn, Pb,Co, Ni) ceramics

Multicomponent oxide systems have been widely studied in the last few decades and can be used as cathode materials in high-energy cells. However, the electrical characteristics have not yet been fully disclosed. We report the electrical conductivity, thermoelectric power, the I-V characteristics, conductance and dielectric spectroscopy measurements made for M₂FeV₃O₁₁ (M=Mg, Zn, Pb, Co, Ni) ceramics. This multicomponent oxide system was found to show semiconducting properties strongly thermally activated above room temperature, n-type conduction at higher temperatures, higher conductance for the ceramics containing Co²⁺, Ni²⁺ and Mg²⁺ ions as well as a strong dependence of relative dielectric constant and loss tangent on temperature and frequency. Moreover, the transition metal ions, which have unfilled 3d-shells strongly affected polarization and conductivity of the ceramics, while the effect of porosity could be neglected. These effects are discussed in terms of microstructure, thermal activation of charge carriers, small polarons as well as the Maxwell–Wagner polarization.     

Effect of ZnO doping on structural,dielectric, ferroelectric and piezoelectric properties of BaZr0.1Ti0.9O3 ceramics

This study provides a fundamental understanding of structural, dielectric, ferroelectric and piezoelectric properties of bare and ZnO-doped BaZr_0.1Ti_0.9O₃ (BZT) solid solutions synthesized using mechanochemical activation technique. Structural investigation has been carried out using XRD patterns of the synthesized compositions by Rietveld refinement method. It confirms the formation of tetragonal phase with P4mm space group for ZnO doping up to 2.5 wt%, while in 5.0 wt% ZnO-doped sample, BZT and ZnO are distributed as individual phases of tetragonal (P4mm) and hexagonal (P6₃mc), respectively. Microstructural analysis shows that average grain size increases quite rapidly with the increase of ZnO content. Detailed analysis of dielectric constant as a function of temperature for the prepared samples shows that the frequency independent dielectric constant maximum shifts to lower temperature with increase in ZnO doping. The ferroelectric ordering is confirmed using P-E loop measurements. The piezoelectric constant of the synthesized specimens were found to decrease with increasing ZnO content.