Oxidation protection of carbon-carbon composites by sol-gel ceramic coatings

Carbon-carbon composites were coated with Calcium Magnesium Zirconium Phosphate (CMZP) and Mg-doped Al₂TiO₅ sol-gels in order to form an oxygen barrier at temperatures above 650°C. The coatings were applied using a dipping technique and controlled drying procedures. It was determined that 10 coats of CMZP sol-gel fired every other coat created crack free coatings and oxidation protection. Mg-doped AlTi₂O₅ coatings produced were unsuccessful as oxidation barriers.     

Diphasic xerogels: I. Ceramic-metal composites

The sol-gel process has been extended to the preparation of new diphasic xerogels leading to new hybrid ceramic-metal materials. The final component compositions were Al₂O₃, SiO₂, ZrO₂ as oxides and Cu, Pt, Sn, and Ni as metals.The xerogels as examined by XRD, SEM, TEM, consisted or a noncrystalline (in a few cases, microcrystalline) ceramic matrix with small metallic islands (5–50 nm) and micro and macro pores. The fine structure of these materials can be controlled by both preparation steps: the gelation parameters and subsequent thermal treatment.     

Investigation on the pressure infiltration of sol-gel processed textile ceramic matrix composites

A pressure infiltration apparatus was used to fabricate textile ceramic matrix composites using sol-gel processing with added solid particles. A parabolic rate kinetics model for the infiltration process has been developed and experimentally verified. Darcy's law and a global permeability of the whole material system were adopted in the model. Experiments were conducted using both a 3-d angle interlock and a 2-d woven carbon fibre preform, and a silica sol containing silica particles of submicrometre sizes. The thickness of the particle compaction layer inside the 3-d preform is proportional to the square root of the processing time. The total infiltration time was inversely proportional to the constant processing pressure and increased significantly with reducing the solid particle size by a factor of two. The addition of a non-ionic surfactant reduced the surface energy which resulted in higher composite green density values of up to 90%. Sol-gel processing with added solid particles proved to be a viable method to fabricate textile ceramic matrix composites with a higher density and improved properties.     

Synthesis of CaCu3Ti4O12 ceramic via a sol-gel method

The novel nano-ultrafine powders for the preparation of CaCu₃Ti₄O₁₂ ceramic were prepared by the sol-gel method and citrate auto-ignition method. The obtained precursor powders were pressed, sintered at 1000 °C to fabricate microcrystal CaCu₃Ti₄O₁₂ ceramic. The microcrystalline phase of CaCu₃Ti₄O₁₂ was confirmed by X-ray powder diffraction (XRD). The morphology and size of the grains of the powders and ceramics under different heat treatments were observed using scanning electron microscopy (SEM). The relative dielectric constant of the ceramic sintered at 1000 °C was measured with a magnitude of more than 10⁴ at room temperature, which was approaching to those of Pb-containing complex perovskite ceramics, and the loss tangent was less than 0.20 in a broad frequency region. The relative dielectric constant and loss tangent were also compared with that of CaCu₃Ti₄O₁₂ ceramic prepared by other reported methods.     

溶胶凝胶法制备磷酸锆钠陶瓷材料

以Zr(NO3)4.5H2O、NH4H2PO4和NaNO3为原材料,采用溶胶凝胶法合成了磷酸锆钠(NaZr2(PO4)3)陶瓷粉体,采用TG-DTA,XRD和SEM测试方法对粉体性能进行了表征,研究了烧结温度和pH值等反应条件对粉体合成的影响.结果表明,要获纯的磷酸锆钠粉体,合成温度需控制在800～1 000℃之间,其中最合适合成温度为900℃左右.前驱体pH值对合成温度基本没有影响,但会影响产物微观形貌.当合成温度为900℃,pH值为9时可以制备出团聚较少、颗粒均匀、分散性较好且平均粒径为100nm的磷酸锆钠粉体.     

Hardness recovery of ceramic coated aluminium matrix composites using thermal-shock resistant sol-gel silica coatings

Sol-gel silica coatings increase wear and corrosion resistance of aluminium matrix composites, but a heat treatment stage at temperatures as high as 500 °C is usually required to consolidate the ceramic coating. Under these thermal conditions, sol-gel ceramic coatings tend to crack and the aluminium matrix hardness strongly reduces. Present paper describes how the combination of a certain sol-gel procedure accompanied with quenching treatment in silicone oil allows recovering most of the hardness of the substrate, while keeping undamaged the sol-gel silica coating deposited.     

溶胶-凝胶法制备纳米近红外高反射率黑色陶瓷颜料

以硝酸铁、硝酸铬、柠檬酸、乙二醇为原料,采用溶胶-凝胶法制备了纳米近红外高反射率黑色陶瓷颜料,探讨了Fe/Cr摩尔比、水解温度、溶液pH值、金属离子浓度、煅烧温度等因素对溶胶-凝胶过程及样品性能的影响。结果表明,在Fe/Cr摩尔比为0.50,水解温度70℃,溶液pH值=3～4,金属离子浓度为2.5～3.5mol/L,煅烧温度为900℃时,制备的颜料主晶相为Cr1.3Fe0.7O3,平均颗粒尺寸为50～150nm,在700～2500nm波段内的平均反射率为77.58%。     

溶胶-凝胶法制备适用于还原性气氛下烧结的钛酸钡基超细粉体

以醋酸钡、钛酸四丁酯和冰醋酸为主要原料,采用溶胶-凝胶方法制备出适用于还原气氛下烧结的超细钛酸钡基陶瓷粉体.利用差热分析研究了前驱体的热转变过程.利用XRD分析了在不同烧结温度下,粉体的相结构.利用透射电镜研究了温度对粉体粒度和形貌的影响.实验结果表明,所得干凝胶在600℃度煅烧后就可以得到均匀且均相的掺杂改性钛酸钡粉体.在700℃度煅烧1h后,就可以得到形貌规则,粒度为100nm左右的高纯掺杂改性钛酸钡粉体.     

Structural ceramic composites

Major recent advances: Near-stoichiometric, small-diameter SiC fibers with excellent properties are now available. A new polycrystalline oxide fiber offers significantly improved temperature limits. Oxide fiber coatings to substitute for C and BN for deflecting cracks and promoting distributed damage appear feasible. Two show substantial low-temperature plasticity in constrained loading. Properties of SiC–BN–SiC and coatingless oxide composites have been improved markedly, but combustion atmospheres have proven more problematic than thought for the former.     

Preparation and properties of KNbO3 via the sol-gel method

Sol-gel processing of a solution of niobium and potassium ethoxides in methanol was used to prepare polycrystalline potassium niobate ceramic discs. Gel powders completely crystallized after only 2 h at 600 ° C. Crystallized powders were hot-pressed at pressures less than half of that required in conventional methods. Densities of up to 99% were achieved without encountering the common problem associated with the evaporation of the potassium oxide and subsequent formation of a stable second phase. The variation of dielectric constant with temperature was measured up to several degrees above the Curie temperature (ã420b ° C). Dielectric constants of dense samples were found to be more than one and one-half times greater than that of their single crystal counterparts.     

基于扩展有限元的陶瓷复合材料多重增韧机制

为了提高陶瓷材料的断裂韧性和可靠度,改善材料抵御破坏的能力,将优化的多重增韧机制应用到氧化铝基陶瓷材料的开发中。相变增韧机制可以耗散部分能量,降低裂纹尖端处的应力集中程度,阻止或延缓裂纹扩展速率。当增强相分布较为合理、材料的致密度较高时,裂纹偏转与桥接增韧机制可以有效地削弱裂纹扩展动力,提高材料的断裂韧性。利用扩展有限元（X-FEM）手段讨论了裂纹扩展问题,为分析陶瓷复合材料的多重增韧机制提供了新思路。     

纳米陶瓷、复相陶瓷及纳米复相陶瓷

纳米陶瓷、复相陶瓷及纳米复相陶瓷的研究是纳米材料研究领域中的一个重要部分，综述了其发展过程和研究动态，阐述了纳米复相陶瓷的优异性能、产生机理及应用前景，展望了纳米复相陶瓷的发展趋势．     

Application of sol-gel processing to ceramic development

The essential features and advantages of the process are described. Application in the preparation of a range of ceramic materials is emphasised.     

溶胶-凝胶法制备尖晶石型锰酸锂的研究进展

尖晶石型锰酸锂具有独特的三维锂离子扩散通道,作为锂离子电池正极材料具有良好的充放电循环性能,且具有原材料价格便宜,对环境无污染等优点,使其成为最有希望替代LiCoO2的正极材料。尖晶石型LiMn2O4的制备方法成为近年来研究的重点,其中溶胶-凝胶法具有突出的优越性。简述了溶胶-凝胶法的基本原理,综述了溶胶-凝胶法在锰酸锂的制备及改性上的研究进展。     

Hybrid ceramic matrix composites

A model hybrid glass-matrix composite has been studied. The system investigated was Corning Code 1723 glass matrix (an alkaline earth aluminosilicate glass) with silicon carbide whiskers and Nicalon^® fibres. It was found that a 10 wt % whisker loading of the matrix gave optimum composite properties. The optimized hybrid composite, when compared to an optimized non-hybrid composite, showed increases in microcrack yield stress from 330 to 650 MPa, interlaminar shear strength from 47 to 130 MPa, and transverse strength from 12 to 50 MPa, while the ultimate strength decreased from 965 to 900 MPa.     

Influence of testing method on mechanical properties of ceramic matrix composites

Mechanical properties of a monolithic zircon and zircon-matrix composites reinforced with silicon carbide monofilaments and/or whiskers were measured in three-point flexure and uniaxial tension modes to study the influence of testing methods on mechanical behaviour. A number of composite characteristics, such as the first-matrix cracking stress and strain, the ultimate composite strength and strain, and the modulus were obtained from the load-deflection behaviour in flexure and tension tests. The results indicated that the modulus values and the qualitative dependence of mechanical properties on composite parameters were similar in flexure and tension tests. In contrast, all of the other mechanical properties of the monolithic and composites were different in tests performed in flexure and tension modes. Typically, the first-matrix cracking stress and strain were higher in flexure tests than in tension tests, and these stress and strain values were independent of the filament-matrix interfacial properties. Similarly, the ultimate strengths of the monolithic and composites were higher in flexure than in tension, and these strengths were independent of interfacial properties. Therefore, the mechanical properties of composites obtained in flexure should not be used for a quantitative comparison with the predictions of micromechanical models, which are derived under the assumption of a uniform tensile stress. However, the flexure data are perfectly valid in demonstrating the qualitative dependence of mechanical properties on composite parameters.     

Preparation, characterization and photocatalytic property of nanosized K-Ta mixed oxides via a sol-gel method

Pyrochlore-type K₂Ta₂O₆ compound cannot be obtained by a conventional solid-state reaction. Here, we report the synthesis of K₂Ta₂O₆ and tetragonal tungsten bronze (TTB)-type K₆Ta_10.8O₃₀ nanopowders using a facile and low-cost sol-gel method at low temperature. The as-prepared samples were characterized by XRD, TG-DSC, FESEM, UV-vis diffuse reflectance spectrum and XPS. The compound K₂Ta₂O₆ was formed at 700 °C and reached complete crystallization at 750 °C, and decomposed into pure K₆Ta_10.8O₃₀ with the volatilization of K₂O at 900 °C. The transformation of K₂Ta₂O₆ to K₆Ta_10.8O₃₀ is reported for the first time. The band gap of the pure K₂Ta₂O₆ and K₆Ta_10.8O₃₀ samples was found to be about 4.16 and 3.63 eV, respectively. The as-prepared K₂Ta₂O₆ sample exhibited an efficient photocatalytic activity for the decomposition of azo dye acid red G (ARG) and 4-nitrophenol (4-NP, a typical persistent organic pollutant) solution under UV irradiation. According to the experimental results, the mechanism of the photocatalytic reaction over K₂Ta₂O₆ was discussed.     

Fracture criterion of composites with a ceramic matrix

Asymptotic averaging and certain hypotheses are used to determine the structure of the stress-intensity factors and propose a mechanical fracture criterion for microscopically uniform, isotropic, linearly elastic composites with a periodic macrostructure and residual stresses. Use of the criterion makes it possible to optimize the composition of the composite. A comparison of theoretical and experimental data shows that they agree well.Translated from Problemy Prochnosti, No. 10, pp. 30–40, October, 1993.     

Preparation of acetylcellulose/silica composites by sol-gel method and their mechanical properties

Acetylcellulose (AC)/silica composites were prepared by sol-gel method in an attempt to realize materials with Young's moduli and bending strengths similar to those of cortical bones. Si(OCH₃)₄ (TMOS)-AC-H₂O-HNO₃-tetrahydrofuran-CH₃OC₂H₄OH solutions were allowed to be gelled, where AC/TMOS mole ratios were defined for AC monomers, and the gels were dried at 30–70^∘C to obtain composites. The composites prepared from solutions of mole ratios of AC/TMOS = 0.5 and 1.0 were composed of micrometer-sized particles rich in silica surrounded by the matrix rich in AC. The composites from solutions of AC/TMOS = 2.0, on the other hand, the interface between the particles and the matrix was much less distinct. All the composite samples showed good machinability, which could be cut into 5 mm × 2 mm × 4.4 mm rectangular specimens by an electric saw without cracking or fracture. The specimens were subjected to three-point bending test, where the degree of plastic deformation and the fracture strain increased, and Young's modulus and bending strength decreased with increasing AC/TMOS ratio in the starting solutions. When the gels were dried at higher temperatures, Young's modulus and strength increased. Young's modulus and bending strength could be varied in the range of 1.8–3.9 GPa and 48–100 MPa, respectively, by varying the AC/TMOS ratio in solutions and the drying temperature.