High-Performance Ceramics V

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Authors: Zhi Ping Zheng, Shu Ping Gong, Shu Fen Cheng, Dong Xiang Zhou, Yun Xiang Hu, Huan Liu
Abstract: Semiconducting barium titanate powderss with an average particle size less than 40 nm were used for tape casting. Green tapes with a thickness of 100-500 μm were tape-cast from aqueous slurry of barium titanate powders using polyvinyl alcohol (PVA) as binder and polyelectrolyte ammonium salt as dispersant. The solid loading, viscosity and rheological properties of the ceramic slurries were investigated. Slurries with low viscosity were obtained when the concentration of dispersant was 1.5-2.0 wt% (based on ceramic powders weight) with fixed 75 wt% solid loading. The highest solid loading and optimum dispersion were determined through the proper viscosity to cast. Microstructure and PTC effect of the ceramic chips were studied as a function of firing temperature. Yttrium ions were homogeneously diffused into barium lattices at a low temperature of 1240 °C, and fine-grained PTC ceramics with grain size of 1-2 μm were developed successfully. These results can be explained by the higher rate of reactivity of finer powders and stable behavior of the suspension.
Authors: F. Long, Wei Min Wang, Jian Jun Li, Zheng Guang Zou
Abstract: CuIn1-xGaxSe2 (CIGS) precursor films were fabricated on Mo foils by one-step electrodeposition in water and alcohol solutions. The precursor films were annealed in Ar atmosphere at 450°C to synthesize the polycrystalline thin films. The current density vs. potential curves of Cu2+, In3+, Ga3+ and Se4+ was studied by cyclic voltammetry. The compositions of CIGS, were analyzed by energy dispersive X-ray spectrum. The morphology and phase structure of films was characterized by scanning electron microscopy and X-ray diffraction, respectively.
Authors: Zhong Zhou Yi, Wei Liu, Guo Wei Zhang, Ying Li, Zhi Peng Xie
Abstract: One-dimensional ZnO nanopowder was prepared by chemistry liquid-phase method with polyethylene glycol (PEG) as additive in [Zn(OH)4]2- premonitory solution. TEM results showed that the nanopowder with the diameter of 30~50nm has the structure such as the good crystal state and well-defined crystallographic surfaces. The UV–Vis absorption revealed that the ZnO nanopowder have strong absorption in the UV region. The function of polyethylene glycol and the effects of solvent conditions were discussed.
Authors: Zhi Jian Peng, Cheng Biao Wang, Ludwig J. Gauckler, He Zhuo Miao
Abstract: Three series of ZnO-based materials with different doping levels were prepared. The correlation between the composition and microstructure, and the roles of main dopants, Bi2O3 and Sb2O3, in the sintering behaviors were proposed. Both Bi2O3 and Sb2O3 evaporated at 1115°C, but the amount of them, in which bismuth is the majority, is not significant. Bi2O3 functioned mainly as liquid during sintering to promote the sintering of ZnO, but it doesn’t mean the materials will be denser. The bismuth-rich phase retracted into small pores during cooling, leaving the big pores as voids at room temperature. More Bi2O3 added would result in less increase in material densities and dramatic decrease in relative densities, and a little bit increase in grain sizes of matrix ZnO. Sb2O3 would react with ZnO matrix into spinel, Zn7Sb2O12, which will pin at the grain boundary of ZnO to control the ZnO grain growth. The more Sb2O3 added, the smaller the grain sizes of ZnO. Appropriate amount of Sb2O3 added will yield denser materials.
Authors: You Ping Tu, Feng Chao Luo, Jin Liang He, Jun Hu
Abstract: The average intercept length and related corrective method are proposed to calculate the average grain boundary number. The average value of barrier voltage by direct measurement to single grain boundary is 3.3V in literatures, but from our prepared ZnO varistor samples, the value of the average barrier voltage by the average grain size is only 2.3 V. It is in the range from 3.24 to 3.38 V by the proposed method, which is very close to the direct measured value 3.3V of the single grain boundary. The introduced method to calculate average grain size has an obvious effect to correct the calculated barrier voltage to near the actual one by direct measurement to single grain boundary.
Authors: Xiao Zhong Huang, Han Dong Liu, Xin Xing
Abstract: By means of high temperature solid-state reaction process, tetra-pod ZnO whisker (T-ZnOw) was doped with Fe(NO3)3. The doping induced the lattice distortion of the T-ZnOw, and improved its electrical properties significantly. After doping, the volume resistivity of the T-ZnOw decreased from 1010 &·cm to 103 &·cm, and its dielectric constant changed correspondingly.
Authors: Jin Liang He, Jun Hu, Feng Chao Luo
Abstract: The presence of twins in ZnO varistors raises interesting questions concerning the potential role of those electrically distinctive twin boundaries in the characteristics of ZnO varistors. The paper uses Voronoi networks to simulate the microstructures of ZnO varistors. The relation between the relative standard deviation of grain size and the ratio of twin number to grain number is obtained. It indicates that the relative standard deviation decreases with the increasing of the twin ratio. On the other hand, ZnO varistor samples with different content of Al2O3 additive were prepared to gain different twin ratios. The simulation has the same conclusion as that obtained from experiments. The probable mechanism of ZnO grain growth inhibition by twins is that the twins increase the mobility viscosity of ZnO grains and grain boundaries, and drag ZnO grains and liquid grain boundaries during the sintering course, then the grain growth is inhibited, and the microstructure becomes more uniform.
Authors: Chien Chen Diao, Shih Feng Chien, Cheng Fu Yang, Hsien Wu Chan, Ying Chung Chen, Ho Hua Chung
Abstract: Most commercial ZnO varistors containing Bi2O3 exhibit excellent varistor properties, but they have a few drawbacks due to Bi2O3 having high volatility and reactivity and higher sintering temperature. In this study, V2O5 is added as the varistor forming oxide to lower down the sintering temperature of ZnO varistors for the further developing the chip Varistor array by using the Ag as the inner electrode. It is found that the sintering temperature of V2O5-added ZnO will be lower down to about 600oC. But the addition of V2O5 has no improvement in the electrical characteristics, and they need many additives to obtain the high performance. For that, the nonlinear properties in these Varistors can be improved by incorporation of some oxide additives. Different MnOx oxides (MnO, MnO2, and Mn3O4) and CoOx oxides (CoO and Co3O4)are used as the minor oxide additives. The influences of different MnOx and CoOx oxides on the nonlinear voltage–current density (V-I) characteristics and the nonlinear exponent (α) of V2O5-ZnO varistor will be well developed.
Authors: Feng Hua Liu, Gao Jie Xu, Lei Duan, Ya Li Li, Ping Cui
Abstract: B2O3 doped ZnO-Bi2O3-Sb2O3-based varistors were fabricated by conventional solid state reaction method. The structure and electrical properties were investigated by XRD, SEM and electrical measurements. The grain size obviously increases with increasing B2O3 content, while the content of Zn7Sb2O12 spinel on the grain boundaries gradually decreases, which implies that B2O3 doping inhibits the growth of Zn7Sb2O12 spinel. The density (ρ) of ZnO varistors increases with increasing B2O3 content (x) and reaches the maximum at x = 0.4 mol%. The sample with x ≈ 0.6 mol% sintered at 1150 °C exhibits the best performance, with nonlinear coefficient of 48 and leakage current of 4 μA.
Authors: Jian Feng Zhu, Ji Qiang Gao, Fen Wang, Ping Chen
Abstract: The influence of the amount of Pr6O11 additions on the microstructure and electrical properties of varistors ceramics in the ZnO-Bi2O3 system was investigated. Samples with a low level of Pr6O11 (0.1wt %) have high microstructural homogeneity, which enhances the nonlinear coefficient greatly, and decreases the leakage current without change of voltage ratio. When the Pr6O11 content reached 7wt%, the ZnO grain growth was restricted and the threshold voltage was improved from 275v/mm to 440v/mm. The additive of Pr6O11 changed the process of creating spinel phase, which came from the decomposition of pyrochlore phase. This type of small size phase has more dragging force on the ZnO crystal, which make the whole materials more uniform and compact.

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