Microstructure and properties of low-voltage ZnO varistor ceramics

The influences of pre-firing process and sintering temperature on the microstructure and electrical properties of ZnO-Bi₂O₃-TiO₂-Sb₂O₃-MnO₂-CoO low-voltage varistor ceramics were studied. Especially, twin boundaries and exaggerated grain growth (EGG) were observed here, and the mechanism of EGG was discussed. It seems that the formation of twin boundaries has a correlation with Sb₂O₃ content and sintering temperature, and that twin boundaries have a great influence in grain growth of ZnO.     

Combustion synthesized ZnO powders for varistor ceramics

Commercial ZnO varistor ceramics are multicomponent, with minor amounts of added oxides that play important roles, both in the strict electrical sense and for the control of the microstructure. The present work describes the straightforward combustion synthesis of pure and doped ZnO powders from stoichiometric mixtures of the relevant water soluble metal nitrates as cation precursors and urea as fuel. The mixtures were ignited at 500°C resulting in a dry, very fine powder. The as-prepared combustion products, characterized by XRD, SEM, TEM and BET, show high specific surface area, have very small particle sizes and are crystalline, with atomic level homogeneity. Implications on sintering and electrical behaviour are discussed.     

Microstructural development of ZnO varistor during reactive liquid phase sintering

The microstructural evolution, grain growth and densification for the varistor systems ZnO-Bi₂O₃ (ZB), ZnO-Bi₂O₃-Sb₂O₃ (ZBS), ZnO-Bi₂O₃-Sb₂O₃-MnO-Cr₂O₃-CoO (ZBSCCM) were studied using constant heating rate sintering, scanning electron microscopy (SEM) andin situ phase formation measurement by high temperature X-ray diffraction (HT-XRD). The results showed that the densifying process is controlled by the formation and decomposition of the Zn₂Bi₃Sb₃O₁₄ pyrochlore (PY) phase for the ZBS and ZBSCCM systems. The addition of transition metals (ZBSCCM system) alters the formation and decomposition reaction temperatures of the pyrochlore phase and the morphology of the Zn₇Sb₂O₁₂ spinel phase. Thus, the spinel grains act as inclusions and decrease the ZnO grain growth rate. Spinel grain growth kinetics in the ZBSCCM system showed ann value of 2.6, and SEM and HT-XRD results indicate two grain growth mechanisms based on coalescence and Ostwald ripening.     

稀土氧化物Pr2O3掺杂对高压ZnO压敏电阻性能的影响

采用低温固相化学反应法制备了Pr2O3掺杂的ZnO纳米复合粉体,并用此粉体在不同烧结温度下制备了高压ZnO压敏电阻.采用X射线衍射、比表面测试、透射电镜、扫描电镜等手段对制备的ZnO纳米复合粉体及高压ZnO压敏电阻进行了表征,并与未掺杂ZnO压敏电阻进行了对比研究,探讨了稀土氧化物Pr2O3掺杂对高压ZnO压敏电阻电性能的影响机制.结果表明:较低的烧结温度(1030～1130℃)时,掺杂的稀土氧化物Pr2O3偏析于ZnO晶界中,有活化晶界、促使晶粒生长的作用;同时,Pr2O3掺杂导致1080℃烧结的ZnO压敏陶瓷体中晶体相互交织形成晶界织构,比未掺杂的更均匀和致密,这有助于高压ZnO压敏电阻晶界性能的改善,从而提高其综合电性能.当烧结温度为1080℃时,Pr2O3掺杂的高压ZnO压敏电阻的综合电性能最佳:电位梯度为864.39 V/mm,非线性系数为28.75,漏电流为35 μA.     

多元施主掺杂对直流ZnO压敏陶瓷结构与电气性能的影响EI北大核心CSCD

采用传统的陶瓷烧结工艺制备B_(2)O_(3),In_(2)O_(3),Al_(2)O_(3)多元施主掺杂的直流ZnO压敏陶瓷样品,考察不同掺杂比(0.1%~0.4%,摩尔分数)的B_(2)O_(3)对直流ZnO压敏陶瓷样品微观结构和电气性能的影响。利用X射线衍射仪、扫描电子显微镜、能量色散X射线光谱及数字源表等分别对样品的物相、微观形貌、成分及电性能进行表征。结果表明,多元施主掺杂剂(Al_(2)O_(3),In_(2)O_(3)和B_(2)O_(3))的共掺杂明显改善直流ZnO压敏陶瓷的综合性能,其中,Al_(3)+提高样品的电导率,降低样品的残压比;In^(3+)通过钉扎效应限制晶粒的生长,改善样品的电压梯度;B^(3+)的掺杂增加样品的表面态密度,提高势垒高度并有效抑制泄漏电流的增加。B_(2)O_(3)掺杂量为0.3%时,样品的综合性能最优:电压梯度为486 V/mm,泄漏电流密度为0.58μA/cm^(2),非线性系数为85,残压比为1.55。     

不同烧结工艺对TiO2基压敏陶瓷电性能的影响

分别采取四种不同的工艺过程,按照配方TiO2+0.3mol%(SrCO3+Bi2O3+SiO2)+0.075mol%Ta2O5制备四种试样.通过压敏电压、非线性系数、复阻抗特性、伏安特性、势垒高度、介电频率特性和损耗频率特性的测定,研究了一次烧成、粉料预烧及预烧方式对TiO2基压敏陶瓷电性能的不同影响.结果表明,采取一次烧成工艺制备的试样具有压敏电压较低(E10mA=7.9 V·mm-1)、介电常数较大(εra=5.88×104)等特性,并从理论上对此作出一定的分析.     

粉料埋烧的TiO2瓷半导化和电学非线性研究

基于一次烧结工艺，采用埋烧和裸烧两种方法制备组分相同(Sr，Bi，Si，Ta)掺杂的TiO2陶瓷。通过复阻抗特性、晶粒电阻、晶界电阻、压敏电压、非线性系数及势垒高度的测定，研究埋烧与裸烧工艺对TiO2陶瓷的半导化以及电学非线性特性的影响。结果表明，采取粉料埋烧有利于晶粒半导化，降低压敏电压。     

Hybrid processing and anisotropic sintering shrinkage in textured ZnO ceramics

Abstract

We have studied the combined effects of the templated grain growth and magnetic alignment processes on sintering, anisotropic sintering shrinkage, microstructure development and texture in ZnO ceramics. Suspensions of 0–10 vol % ZnO template particles were slip cast in a 12 T rotating magnetic field. Sintering and texture characteristics were investigated via thermomechanical analysis and electron backscatter diffraction, respectively. Sintering as well as texture characteristics depend on template concentration. For the studied ZnO system, there is a critical template concentration (2 vol % in this study) above which densification is limited by the templates owing to constrained sintering. Below this limit, the densification is enhanced and the anisotropic shrinkage is reduced, which is attributed to densifying characteristics of the templates.     

Effect of long-term electrical degradation on the distribution of donor impurities in ZnO varistor ceramics

The effect of long-term electrical degradation on the distribution of donor impurities in ZnO varistor ceramics and their electrical properties was examined. The results are consistent with the views that, in the initial stages of the process, the degradation is due mainly to local Joule heating at intergranular barriers, while at longer degradation times, the major mechanism of degradation is the redistribution of donors in the bulk of ZnO grains owing to their migration in the electric field of the barriers.     

The effect of sintering temperature on varistor properties of (Pr, Co, Cr, Y, Al)-doped ZnO ceramics

The effect of sintering temperature on varistor properties of (Pr, Co, Cr, Y, Al)-doped ZnO ceramics was investigated in the range of 1280-1350 °C. The increase of sintering temperature increased the average grain size (4.8-12.9 μm), whereas the sintered density was not nearly affected by sintering temperature. The breakdown field decreased over a wide range from 4610 to 1513 V/cm with the increase of sintering temperature. The nonlinear coefficient of this system was nearly constant in the range of 44-47 regardless of sintering temperature. The donor density decreased in the range of 1.51-1.32 × 10¹⁸/cm³ and the barrier height decreased from 1.40 to 0.92 eV with the increase of sintering temperature.     

烧结温度对TiO2压敏陶瓷非线性和介电性质的影响

基于一次烧结工艺,通过改变烧结温度,制备5种组分相同、(Sr,Bi,Si,Ta)掺杂的TiO2陶瓷试样.借助于伏安特性、介电频率特性、损耗频率特性及非线性系数的测定,研究烧结温度对TiO2基压敏陶瓷压敏和介电性质的影响.结果表明,在1200～1400℃范围内,随着烧结温度的降低,陶瓷的压敏电压降低、介电常数增大,同时非线性系数有所减小.兼顾陶瓷压敏和介电特性,烧结温度选择1350℃为宜.     

Microstructural engineering of ZnO-based varistor ceramics

In ceramic materials, special boundaries play the key role in crystal growth. They introduce an abrupt structural and chemical anisotropy, which is readily reflected in an unusual microstructure evolution, whereas their local structure affects the physical properties of polycrystalline materials. These effects, however, can be exploited to tailor the electronic and optical properties of the materials, as demonstrated in this review. The presented topic is related to a preparatory stage of phase transformations, manifested through the evolution of chemically induced structural faults. In non-centrosymmetric structure of ZnO, inversion boundaries (IBs) are the most common type of planar faults that is triggered by the addition of the specific IB-forming dopants (Sb₂O₃, SnO₂, TiO₂). In addition to conventional TEM techniques, new methods were developed to resolve crystallography and atomic-scale chemistry of IBs. The absolute orientation of the polar c-axes on both sides of an IB was determined by micro-diffraction, providing the most reliable identification of crystal polarity in non-centrosymmetric crystals. To determine sub-monolayer quantities of dopants on the IB, we developed a special technique of analytical electron microscopy using concentric electron probe (CEP) in EDS or EELS mode, providing more accurate and precise results than any other technique. Knowing the local crystal chemistry of IBs, we were able to design experiments to identify their formation mechanism. IBs nucleate in the early stage of grain growth as a dopant-rich topotaxial 2D reaction product on Zn-terminated surfaces of ZnO grains. Soon after their nucleation, ZnO is epitaxially grown on the inherent 2D phase in an inverted orientation, which effectively starts to dictate anisotropic growth of the infected crystallite. In very short time, the grains with IBs dominate the entire microstructure via IB-induced exaggerated grain growth mechanism. This phenomenon was used to design physical properties of ZnO-based varistor ceramics, whereas the bottom-up approach demonstrated here provides the basic tool for microstructural engineering of functional materials in virtually any system that is prone to the formation of special boundaries.     

稀土氧化物Pr2O3掺杂对高压ZnO压敏电阻性能的影响

采用低温固相化学反应法制备了Pr₂O₃掺杂的ZnO纳米复合粉体, 并用此粉体在不同烧结温度下制备了高压ZnO压敏电阻。采用X射线衍射、 比表面测试、 透射电镜、 扫描电镜等手段对制备的ZnO纳米复合粉体及高压ZnO压敏电阻进行了表征, 并与未掺杂ZnO压敏电阻进行了对比研究, 探讨了稀土氧化物Pr₂O₃掺杂对高压ZnO压敏电阻电性能的影响机制。结果表明: 较低的烧结温度(1030～1130 ℃)时, 掺杂的稀土氧化物Pr₂O₃偏析于ZnO晶界中, 有活化晶界、 促使晶粒生长的作用; 同时, Pr₂O₃掺杂导致1080 ℃烧结的ZnO压敏陶瓷体中晶体相互交织形成晶界织构, 比未掺杂的更均匀和致密, 这有助于高压ZnO压敏电阻晶界性能的改善, 从而提高其综合电性能。当烧结温度为1080 ℃时, Pr₂O₃掺杂的高压ZnO压敏电阻的综合电性能最佳: 电位梯度为864.39 V/mm, 非线性系数为28.75, 漏电流为35 μA。     

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

Journal of Materials Science: Materials in Electronics - Gd2O3-doped ZnO varistor ceramics were successfully prepared through the solid reaction sintering route. The influence of Gd2O3 additive on...     

Improvement of electrical properties of V2O5 modified ZnO ceramics by Mn-doping for varistor applications

The dependence of microstructure, electrical properties, dielectric characteristics, and stability of conduction characteristics in ternary ZnO–V₂O₅–Mn₃O₄ system on the amount of Mn₃O₄ present in them was investigated. For all compositions studied, the microstructure of the ternary ZnO–V₂O₅–Mn₃O₄ system consisted of mainly ZnO grains and Zn₃(VO₄)₂ as a secondary phase. The incorporation of Mn₃O₄ to the binary ZnO–V₂O₅ system was found to restrict abnormal grain growth of ZnO. The breakdown field in the electric field–current density characteristics increased from 175 to 4,635 V/cm with the increase of Mn₃O₄ amount. The ternary system doped with 0.5 mol% Mn₃O₄ exhibited the highest non-ohmic properties, in which the non-ohmic coefficient is 22.4 and the leakage current density is 0.22 mA/cm². Furthermore, the sample doped with 0.5 mol% Mn₃O₄ was found to possess 0.43 × 10¹⁸/cm³ in donor density and 2.66 eV in barrier height.