Effect of two-step sintering on rare earth (RE = Y2O3, Pr6O11) doped ZnO–Bi2O3 varistors processed from ‘nano-precursor’ powders

Nano size ZnO–Bi₂O₃ varistor precursor powders containing Y₂O₃ and Pr₆O₁₁ rare earth dopants were prepared by low temperature refluxing at 80 °C. Effect of rare earth dopants, densification by two-step sintering, evolution of microstructures and their influence on varistor properties were investigated. Chemically synthesized nano- precursor varistor powders produced controlled grain size in two-step sintering in which the average sintered ZnO grain size was reduced to at least half compared to the conventionally processed ZnO–Bi₂O₃ varistors. The study revealed that such grain size reduction is highly beneficial to attain enhanced varistor properties.     

微波烧结氧化锌压敏电阻的致密化和晶粒生长

研究了微波烧结的ZnO压敏电阻的致密化和生长动力学, 微波烧结温度从900～1200℃, 保温时间从20min～2h. 研究表明, 微波烧结ZnO压敏电阻的物相组成和传统烧结的样品没有区别; 微波烧结有助于样品的致密化, 并降低致密化温度. 随着烧结温度的升高, 致密化和反致密化作用共同影响样品的密度, 其中Bi的挥发是主要影响因素. 微波烧结ZnO压敏电阻的晶粒生长动力学指数为2.9～3.4, 生长激活能为225kJ/mol, 传统烧结的ZnO压敏电阻的晶粒生长动力学指数为3.6～4.2, 生长激活能为363kJ/mol. 液相Bi₂O₃、尖晶石相和微波的“非热效应”是影响微波烧结ZnO压敏电阻陶瓷晶粒生长的主要因素.     

Effects of SiO2 and Cr2O3 on the formation process of ZnO varistors

The effects of SiO₂ and Cr₂O₃ on the formation process of ZnO varistors were investigated. Prior to formation of the Zn_2.33Sb_0.67O₄ spinel phase (Sp-phase), a spinel-like phase forms. However, this phase does not control the varistor microstructure. The Sp-phase and the Bi₂O₃-phase which were formed by the decomposition of the Bi₂(Zn_4/3Sb_2/3)O₆ pyrochlore phase played important parts in the control of the varistor microstructure. That is, the Bi₂O₃ phase produced in the reaction promotes the initial sintering of the varistor and the Sp-phase inhibits the ZnO grain growth. In this reaction, SiO₂ and Cr₂O₃ play a role in decreasing the decomposition temperature of the pyrochlore phase. Decreasing the decomposition temperature below 900° C (where ZnO grain growth begins) leads to the inhibition of ZnO grain growth.     

Effects of spinel phase formation in the calcination process on the characteristics of ZnO–glass varistors

Ceramic varistors based on ZnO with lead zinc borosilicate glass were prepared in order to study the effects of various calcination processes on the formation of spinel phase during the sintering process, including the effects of different temperatures and soaking times. A ZnO–glass sample was prepared using powder calcined at 600°C for 10 h then sintered at 1250°C for 1 h; this sample possessed the highest non-linear coefficient, breakdown voltage and non-linear resistance as well as the lowest leakage current. The optimum amount of Zn7Sb2O12 spinel phase, formed in the calcination process, that can inhibit ZnO grain growth in the subsequent sintering plays an important role in the grain size distribution and stability of ZnO–glass ceramic varistors. Uniform distribution of the grain size obtained from suitable calcination processes was an important microstructural parameter in achieving a good device stability of ZnO–glass varistors. The dynamic resistance and the non-linear resistance of the ZnO–glass varistor, correlated with the average grain size, were proposed to describe insulating characteristics of the varistor samples. Increases in these two parameters, created by decreasing the grain size, enhances the sample clamping voltage during the surge impact and the sample breakdown voltage. This revised version was published online in July 2006 with corrections to the Cover Date.     

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。     

燃烧法制备ZnO纳米粉体及Pr系ZnO压敏陶瓷

以Zn(NO32)和NH2CONH2为原料,通过燃烧法合成纳米ZnO粉体。用X射线衍射、扫描电镜、比表面积分析手段对所制备粉体的性能进行了表征。结果表明,制备的纳米氧化锌为纯相六角纤锌矿结构。在三种反应模式中,当φ=0.85(贫燃比,SCS)时,晶粒尺寸为41.5nm,其比表面积为0.849m2/g;当φ=1.67(化学计量比,VCS)时,晶粒尺寸为36.6nm,其比表面积为0.516m2/g;当φ=2.8(富燃比,SHS)时,晶粒尺寸为30.5nm,其比表面积为4.068m2/g。燃烧法制备的Pr系ZnO压敏陶瓷在1250℃烧结2h后,其电性能优异,晶粒尺寸约为3.5μm,压敏电压(VlmA)为5470V/cm,非线性系数(α)为27.84,漏电流(IL)为11.5μA。     

Microstructural and electrical properties of varistors prepared from coated ZnO nanopowders

This paper describes a solution-based technique for fabrication of varistor grade composite nanopowders. The method consists of coating major varistor dopants on the surface of the ZnO nanoparticles. As a result, a homogenous mixture of dopants and ZnO nanoparticles will be achieved. TEM results indicated that a composite layer of dopants with the average particle size of 9 nm on the surface of ZnO nanoparticles has been successfully prepared. Sintering of the coated powders was performed in temperatures as low as 850 °C and final specimens with average particle size of 900 nm and density of 98.5% were achieved. In comparison to conventional mixing, varistors prepared from coated nanopowders exhibited superior electrical properties and microstructure homogeneity. The improvement of electrical properties can be attributed to small grain size, homogenous distribution of dopants and elimination of large Bi-Pockets. In addition, the processing route of schottky barrier formation is quite different from what is generally considered as the method of barrier formation in ZnO grain boundaries.     

ZnO压敏陶瓷的微波烧结

对用纳米粉体制备的ZnO压敏生坯进行了微波烧结,通过XRD、SEM分析和电性能测试,与普通烧结比较,微波烧结可使ZnO压敏材料快速成瓷,显著缩短烧结时间;在相同晶粒尺寸下,微波烧结温度更低,瓷体更致密;并能获得较好电性能．微波烧结为ZnO压敏陶瓷材料制备提供了一条新的、高效节能的途径．     

Electrical properties of ZnO-based varistors prepared by combustion synthesis

Ceramic varistors are generally produced by the oxide mixing method or by chemical methods, such as sol–gel, precipitation and others. Chemical methods produce powders that are highly reactive, allowing for increased microstructural homogeneity and control of grain growth during sintering, which is essential for good varistor performance. The purpose of this work was to study the electrical characteristics of ZnO varistors produced from stoichiometric mixtures of water-soluble metal nitrates, such as precursor cations and urea as a fuel. This method, called combustion synthesis, stands out for its simplicity (a single-step reaction), purity, chemical homogeneity, and the high reactivity of the precursor powder. After sintering at 1050 °C using this method, varistors with a non-linear coefficient of 40 and the lowest leakage were obtained.     

Microstructure and electrical properties of Pr<Subscript>6</Subscript>O<Subscript>11</Subscript> doped ZnO-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-based varistors

The microstructure and electrical properties of ZnO-Bi₂O₃-based varistor ceramics doped by Pr₆O₁₁ in the content range of 0–5.49 wt% were investigated at different sintering temperatures (1,100, 1,150, 1,175, 1,200 °C). The increase of sintering temperature leads to more dense ceramics, which increases the nonlinear property, whereas it decreases the voltage-gradient and leakage current. With increasing Pr₆O₁₁ content, the breakdown voltage increases because of the decreases of ZnO grain size. The improvement of non linear coefficient together with the decrease of leakage current are related to the uniformly distribution of secondary phases along the grain boundaries of the ZnO. The varistors sintered at 1,175 °C with the 3.37 wt% Pr₆O₁₁ doping possess the best electrical properties: the varistor voltage, nonlinear coefficient, and leakage current are 340 V/mm, 46 and 0.63 μA, respectively.     

Microstructure, electrical properties, and dc aging characteristics of Tb4O7-doped ZnO-based varistors

The microstructure, electrical properties, and dc-accelerated aging characteristics of Tb₄O₇-doped ZnO-based varistors were investigated for different Tb₄O₇ amounts and sintering temperatures. The sintered density increased with increasing Tb₄O₇ amount and sintering temperature. The average grain size decreased with increasing Tb₄O₇ amount and increased with increasing sintering temperature. The varistor voltage and nonlinear coefficient increased with increasing Tb₄O₇ amount and decreased with increasing sintering temperature. The stability was worse with increasing Tb₄O₇ amount for the varistors sintered at 1,300 °C. The 0.5 mol% Tb₄O₇-doped varistors sintered at 1,350 °C exhibited a good stability for dc-accelerated aging stress of 0.95 V _{1 mA}/150 °C/24 h.     

Microstructural and electrical characteristics of SiO2 doped ZnO-Bi2O3 varistors

Varistors in the new system ZnO-Bi₂O₃-SiO₂ were prepared through conventional ceramic processing route. The effect of sintering temperature and time (0·5 h to 2 h between 1000° and 1250°C) on the microstructure and current/voltage characteristics of the varistors of the new system were investigated and the results were compared with those of ZnO-Bi₂O₃ system varistors prepared. An increase in nonlinear coefficient (α) value was observed in the SiO₂ added varistors. The microstructure and the phase of the varistors were examined by means of SEM and XRD. The Zn₂SiO₄ spinel phase was found to be present in the intergranular region. The grain growth exponent was determined to be 2·5±0·2 and the activation energy for the ZnO grain growth was estimated to be 251±11 kJ/mol. These values were compared with those estimated for ZnO-Bi₂O₃ system varistors.     

Preparation of Barium Strontium Titanate Ceramic by Sol-Gel Method and Microwave Sintering

The (Ba,Sr)TiO₃ amorphous gel was prepared by sol-gel process and calcined in the 2.45-GHz multimode microwave furnace to synthesize (Ba,Sr)TiO₃ nanopowder. The calcination temperature of the (Ba,Sr)TiO₃ ceramic powders that convert the material into prevoskite phase can be reduced from 1100°C to 900°C, the nanopowder displays the highest sinterability. Using a new kind of insulator materials made of MgAl₂O₄–LaCrO₃, the crack-free and dense (Ba_0.80Sr_0.20)TiO₃ ceramics with fine grain size (<1 µm) were prepared by microwave sintering at 1310°C for 15 min. The fine (Ba,Sr)TiO₃ ceramics sintered by microwave sintering technique display lower dielectric loss than that of conventional samples, indicating a reduction of the influence of defects with the microwave process.     

Nanostructured constituents of ZnO-based varistors prepared by mechanical attrition

Nanostructured powders, with crystallite size of ZnO, spinel and γ-Bi₂O₃ phases below 100 nm, were obtained by the mechanical attrition of ZnO varistor powders prepared by method of directed synthesis of the constituent phases. Powders were further sintered and the resulting varistors showed excellent electrical characteristics with the non-linearity coefficients reaching 45, low values of the leakage current and density reaching 99% of the theoretical value.     

Nonlinear electrical properties of ZnO varistors fast-fired by using millimeter-wave sintering process

Fast firing of Bi₂O₃-based ZnO varistor materials, which includes zero minutes soaking at 1100°C with 120°C/min heating and 145°C/min cooling rate, was made possible using millimeter-wave sintering (mS) technique. The overall sintering time of the process is less than 18 minutes, and the varistor characteristics obtained are = 38, J _L = 5.55 × 10^–6 A/cm² and V _bk = 600 V/mm, whereas the intrinsic parameters of the materials are _b = 2.84 eV, N _d = 1.85 × 10²⁴ m^–3 and N _s = 7.02 × 10¹¹ cm^–2. By contrast, conventional sintering (cS) process needs higher sintering temperature (1200°C), longer soaking time (60 min) and slower ramping rate (30°C/min) to obtain ZnO materials with the same marvelous nonlinear properties as those prepared by mS-process. Moreover, millimeter-wave sintering (24 GHz, mS) process enhances the densification kinetics and grain growth behavior more efficiently than the microwave sintering (2.45 GHz, S) process, resulting in better varistor characteristics for ZnO materials. However, sintering by millimeter-wave for too long period induces overfiring of the samples, which results in a density reversion phenomenon. Such a phenomenon leads to the decrease in surface state (N _s) and the potential barrier height (_b), which are presumed to be the mechanism leading to the degradation of ZnO materials' nonlinear properties.     

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.     

Low-temperature sintering of zinc oxide varistors

High-field varistors in the system ZnO-CoO-MnO-Bi₂O₃ were fabricated using powders prepared by a previously developed coprecipitation process. Following calcination, the powders were compacted and densified by conventional pressureless sintering at temperatures below 750° C in air, The effects of sample green density, sintering temperature, and grain-growth inhibitor on densification and microstructure development were investigated. Addition of aluminium at the 125 p.p.m level was used to inhibit grain growth. Samples with densities >0.98 theoretical and grain sizes <1m were fabricated by high-pressure cold-isostatic pressing followed by sintering at 730° C. For comparison, typical commercial varistor devices have grain sizes of about 20 m and switching fields of approximately 2 kV cm^–1 after sintering at 1200 to 1400° C. As a result of the fine grain size, our high-field varistors had switching fields of 45 kV cm^–1 at a current density of 10 A cm^–2. Consistent with earlier work on extremely high-density varistors (>0.98 theoretical) prepared from similar powders, nonlinearity coefficients of about 10 were measured for current densities between 2.5 and 10 A cm^–2.     

Sintering of tin oxide using zinc oxide as a densification aid

The physicochemical electronic characteristics of SnO₂ render it useful in many technical applications, including ceramic varistors, stable electrodes used in electric glass-melting furnaces and electrometallurgy of aluminum, transparent windows and chemical sensors. The use of ZnO as a sintering aid was explored in this study to obtain SnO₂ as a dense ceramic. Compacts were obtained by mechanical mixing of oxides, isostatic pressing at 210 MPa and sintering in situ inside a dilatometer at heating rates of 10°C/min. The grain size and microstructure were investigated by scanning and transmission electron microscopy (SEM/TEM). The phases and chemical composition were analyzed by energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The results indicated that ZnO acts as a densification aid for SnO₂, improving its grain growth with additions of up to 2 mol%. ZnO forms a solid solution with SnO₂ up to 1 mol%, above which SnZnO₃ precipitates in the grain boundary, potentially inhibiting shrinkage and grain growth.     

Electrically active grain boundaries in ZnO varistors by liquid-infiltration method

The varistors were fabricated by spreading a thin layer of Pr₆O₁₁ powder paste on the surface of ZnO pellets and heating to various temperatures (1200–1400 °C) and times (0–60 min). Higher heat-treatment temperatures and/or times resulted in progressively higher breakdown voltages. Eventually the devices became varistor, which was attributed to the formation of a liquid (ZnO-PrO_x) layer between the grains. Microstructures of cross-sections of wetting pellets have shown that the infiltration rate was increased with the amount of Co₃O₄ and heat-treatment temperature. In addition, on the basis of the small variations of the varistor properties per grain boundary (e.g., threshold voltage, donor concentration, and barrier height), the number of active grain boundaries are believed to be increased when the samples were heat-treated above the liquid-phase temperature.     

Electrical characteristics and reheat-treatment effects in a ZnO varistor fabricated by two-stage heat-treatment

Conventional ZnO varistors are generally fabricated by sintering ZnO powder mixed with additives such as Bi₂O₃, Sb₂O₃, Cr₂O₃, Co₂O₃, and MnO₂. To reduce abnormal grain growth and change in electrical characteristics in the conventional ZnO varistors caused by volatilization of Bi₂O₃, the ZnO powder with all additive oxides except Bi₂O₃ was pressed into disc form and sintered. The disc was then painted with metal oxide paste containing Bi₂O₃ and again fired. The ZnO varistor fabricated by this process, i.e. a two-stage heat-treatment process, showed typical non-linearI-V characteristics with higher breakdown voltage exceeding 800 V mm^–1. It was also observed that the non-linearI-V coefficient change rate, , in the ZnO varistor due to reheat-treatment is almost linearly proportional to the sintered density.