Purification of Mullite by Reduction and Volatilization of Impurities

Mullite materials usually contain a residual glassy phase rich in SiO₂, which concentrates impurities as Na₂O, K₂O, Fe₂O₃, and other minority compounds. A suitable way to minimize this glassy phase is the reduction and volatilization of its components by calcination at high temperatures (1300–1450°C) in atmospheres with a very low partial pressure of O₂. Over 95% of the Na₂O, K₂O, and Fe₂O₃ in mullite can be removed in this way, leaving concentrations lower than 0.02% by weight. To avoid the degradation of mullite that occurs when the partial pressure of O₂ is too low, the material to be purified is covered with TiO₂ plates.     

Decomposition of Mullite

Free surfaces of 2:1 mullite (2Al₂O₃·SiO₂) specimens decomposed with the evolution of SiO and O₂ when they were heated at high temperatures under low partial pressures of O₂; this reaction was analyzed thermodynamically. In addition, bubbles were observed at internal interfaces between mullite and fused-SiO₂ diffusion couples. These bubbles, when formed at 1 atm ambient pressure between 1650° and 1800°C, resulted from reaction of Si particles and residual SiO₂-rich glass in the fused cast mullite.     

Oxidation and Volatilization of Silica Formers in Water Vapor

At high temperatures, SiC and Si₃N₄ react with water vapor to form a SiO₂ scale. SiO₂ scales also react with water vapor to form a volatile Si(OH)₄ species. These simultaneous reactions, one forming SiO₂ and the other removing SiO₂, are described by paralinear kinetics. A steady state, in which these reactions occur at the same rate, is eventually achieved. After steady state is achieved, the oxide found on the surface is a constant thickness, and recession of the underlying material occurs at a linear rate. The steady-state oxide thickness, the time to achieve steady state, and the steady-state recession rate can be described in terms of the rate constants for the oxidation and volatilization reactions. In addition, the oxide thickness, the time to achieve steady state, and the recession rate also can be determined from parameters that describe a water-vapor-containing environment. Accordingly, maps have been developed to show these steady-state conditions as a function of reaction rate constants, pressure, and gas velocity. These maps can be used to predict the behavior of SiO₂ formers in water-vapor-containing environments, such as combustion environments. Finally, these maps are used to explore the limits of the paralinear oxidation model for SiC and Si₃N₄.     

微硅粉低成本制备莫来石基陶瓷材料

以工业废弃物微硅粉和工业氧化铝为原料,采用原位反应烧结法制备莫来石基陶瓷材料,主要研究了烧结温度对莫来石的形成和莫来石基陶瓷的显微结构和性能的影响.结果 表明:当微硅粉和工业氧化铝的配比为32∶68时,在1550℃下进行烧结,可制得主晶相为莫来石基的陶瓷材料,其抗弯强度为66 MPa,显气孔率为24％,体积密度为2.25 g/cm3.微硅粉中的金属氧化物杂质使制备莫来石的烧结温度降低100℃.     

微硅粉原位合成莫来石制备高强度多孔陶瓷材料

本文用微硅粉和工业氧化铝粉为起始原料,马铃薯淀粉为造孔剂,通过原位反应烧结工艺制备了高强度多孔莫来石陶瓷材料.研究了不同烧结温度,不同淀粉含量对合成莫来石多孔陶瓷工艺影响.结果表明:以优化起始原料Al2 O3/SiO2摩尔比为3∶2.5,淀粉含量40 wt％,在1450℃温度下烧结样品孔隙率为36.07％、体积密度为2.02g/cm3时、热导率为1.40W· m-1 ·K-1,折强度达到79.91 MPa,莫来石纯度达95wt％.该工艺研究产品的研发为提高微硅粉附加值提供技术支撑.     

用羧酸铝和硅溶胶制备莫来石纤维

采用金属铝粉、甲酸和醋酸为原料,以氯化铝为催化剂制备了羧酸铝溶液。在羧酸铝溶液中加入硅溶胶,制备了可纺的前躯体溶胶。凝胶纤维在1400℃煅烧后,主要物相为莫来石,有少量的氧化铝和氧化硅相,得到的纤维直径均匀,但纤维表面有较多的裂纹。     

Crystallization Kinetics of Mullite in Alumina–Silica Glass Fibers

The mechanism of mullite crystallization in Al₂O₃-SiO₂ glass fibers with compositions of 49 and 69 mass% Al₂O₃ was investigated using isothermal methods. The activation energies ( E _a) of the nucleation and nucleation-growth of mullite were obtained from the temperature dependence of the incubation times and rate constants of mullite formation, respectively. The formation of mullite in both glasses occurred in two stages. The amount of mullite increased very steeply in the first stage within very short firing time, whereas the increase was more gradual in the second stage. The E _a values for nucleation and nucleation-growth in the first stage were 864 and 1288 kJ/mol in the 49-mass%-Al₂O₃ glass fiber and 980 and 1138 kJ/mol in the 69-mass%-Al₂O₃ glass fiber, respectively. Arrhenius plots of the second-stage data showed that the slope ratios were different at temperatures below and above 1200°C. The E _a values at <1200°C were 1195 and 1099 kJ/mol in the 49- and 69-mass%-Al₂O₃ glass fibers, whereas those at >1200°C were 696 and 645 kJ/mol, respectively. Observation of the microstructures and crystallite-size data indicates that the crystallization of mullite proceeds via three different mechanisms: nucleation, nucleation–growth, and coalescence of mullite grains.     

Synthesis of Submicrometer Mullite Powder via High-Temperature Aerosol Decomposition

High-purity mullite powders (3Al₂O₃.2SiO₂) have been prepared using a high-temperature aerosol decomposition technique yielding submicrometer (0.6 μm average) particles of spherical morphology with no hard agglomerates using aluminum nitrate and fumed silica as precursors. Depending upon the reaction conditions used, the powders range from amorphous to crystalline with no evidence of secondary-phase formation. This mullite synthesis approach has the advantages of not requiring postsynthesis milling, the ability to use a wide range of precursor systems, and enhanced control over chemical homogeneity and particle size/shape.     

Mullite Formation by Endothermic Reaction of α-Alumina/Silica Microcomposite Particles

The formation of mullite was investigated using microcomposite powders which consist of α-alumina cores and amorphous silica coatings. Differential thermal analysis and X-ray diffraction showed that the mullitization reaction was endothermic. In contrast, mullite forms exothermically in samples prepared by sol-gel processing. The results are shown to be consistent with available thermodynamic data for mullite formation from different alumina and silica phases.     

Mullite Whiskers Grown from Erbia-Doped Aluminum Hydroxide–Silica Gel

Mullite whiskers and anisotropic grains that were derived from erbia-doped aluminum hydroxide–silica gel were studied. Firing 3.0-mol%-erbia-doped isostatically pressed pellets at 1600°C for 1.0–8.0 h resulted in a high surface concentration of mullite whiskers. Their c -axes were aligned preferentially along the pellet surface; the maximum length was 50 μm, and the maximum aspect ratio was 23. The pellet surface was fully covered by mullite whiskers, and small anisotropic grains with a low aspect ratio were observed in the bulk. The voids that were observed in the fracture surfaces were covered fully by mullite whiskers. The large number of voids resulted in an apparent density of 1.60 g/cm³ in the sintered pellets. The molar ratio of alumina to silica in the whiskers was in the range of 1.30–1.45 (an average value of 1.31), regardless of whether the alumina/silica powder compositions were mixed in a 3:2 or 2:1 ratio.     

硅溶胶对莫来石微球质隔热耐火材料性能的影响

以轻质莫来石微球为主要原料,以Al2 O3微粉为添加剂,在实验室研制了硅溶胶结合的莫来石质耐火材料,并分析了硅溶胶添加量对其力学性能、热学性能和显微结构的影响.结果表明:经1450℃ ×3 h处理后的试样,随着硅溶胶加入量的增加,结构更加致密、显气孔率下降、烧后冷态强度上升、线变化率由收缩变为膨胀、1400℃下耐压强度增加、导热系数上升,其中硅溶胶添加量为8％时,可制得各项性能最佳的隔热耐火材料.     

氢氟酸挥发法测定石英砂中二氧化硅含量方法改进

由于经典氢氟酸挥发法测石英砂中SiO2含量采用铂制器皿,使方法应用在一定程度上受到限制。选用聚四氟乙烯坩埚和瓷坩埚联合方法代替铂坩埚测定SiO2含量,测定的结果与使用铂坩埚法相符。     

高岭土热分解期间莫来石形成的两个阶段

采用X射线技术研究了英国高岭土在加热时的结构变化，用计算机Rietveld程序计算了莫来石和方石英的晶格参数、应变值以及晶体尺寸的变化。这些结果表明了高岭土反应期间莫来石形成的两个独立阶段。讨论了在莫来石形成过程中其成分的变化。     

莫来石—钛酸铝复相陶瓷的高温强度与热分解

本文介绍了莫来石－钛酸铝复相陶瓷的制备工艺及其主要性能，重点对它的高温强度、热分解及抗热震性等进行了讨论并提出了我们的见解。     

Low-temperature Processing Routes for Fabrication of Open-cell Mullite Foams by Silica Nanoparticles Derived from Rice Husk

Silicon - In this study, rice husk (RH) upcycled as silica nanoparticles (nSiO2), was used as one of the raw materials for fabrication of open-cell mullite foams. To synthesis of nSiO2, RH was...     

高铝粉煤灰中莫来石及硅酸盐玻璃相的热分解过程

研究了高铝粉煤灰-Na2CO3体系中莫来石(3Al2O3·2SiO2)和硅酸盐玻璃相在焙烧过程中可能发生的化学反应,并以热力学计算为指导,研究了物料配比、焙烧温度、焙烧时间对反应的影响.实验结果表明:在Na2CO3与粉煤灰的质量比为1.0、焙烧温度为880℃、恒温1h时,粉煤灰中莫来石(3Al2O3·2SiO2)和硅酸盐玻璃相的分解率可达到98%以上.对粉煤灰中莫来石及硅酸盐玻璃相的热分解过程的动力学研究结果表明,该过程符合CrankGinstling-Braunshtein方程,由实验结果计算,其表观活化能为149.19kJ·mol-1.     

稻壳灰为硅源固相烧结法合成莫来石

通过固相烧结的方法,用无机铝盐和稻壳灰按莫来石的理论配比3Al2O3-2SiO2配制并较低温合成的莫来石。实验分别讨论了不同铝盐、合成温度和保温时间的条件下合成结果,通过XRD表征并分析,得出以硫酸铝和500℃稻壳灰为原料,在1300℃下保温4 h合成的莫来石纯度和峰强度均相对较高,达到了莫来石一般应用的要求。     

Decomposition and Activation of Pt-Dendrimer Nanocomposites on a Silica Support

Zero valent platinum nanoparticles were stabilized in solution by the use of poly(amido)amine dendrimers and were subsequently deposited onto a porous silica support. The resulting materials were subjected to various thermal treatments in oxidizing, reducing, and inert environments, in order to remove the surrounding polymer and expose the Pt metal sites to gas phase reagents. The materials were characterized at several different stages during this process via Fourier-Transform Infrared (FTIR) spectroscopy and Transmission Electron Microscopy (TEM). The results suggest that the dendrimer decomposition occurs at its mono-substituted amide groups and begins at relatively low temperatures (～50 °C). The presence of oxygen in the gas phase and the Pt particles in the Pt-dendrimer nanocomposite accelerate this process. Oxidation at 425 °C was the most successful temperature for removing the dendrimer fragments from the Pt surface, rendering the Pt sites most accessible for carbon monoxide adsorption. Limited sintering of the Pt particles is observed under these conditions, as well as during subsequent reduction steps, necessary to yield the metallic form of Pt.     

Corrosion of Mullite by Molten Salts

The interaction of molten salts of different Na₂O activities and mullite is examined with furnace and burner tests. The more-acidic molten salts form small amounts of Al₂O₃; the more-basic molten salts form various Na₂O–Al₂O₃–SiO₂ compounds. The results are interpreted using the Na₂O–Al₂O₃–SiO₂ ternary phase diagram, and some possible diffusion paths are discussed. The generally higher melting points of Na₂O–Al₂O₃–SiO₂ compounds lead to better behavior of mullite in molten salts, as compared to SiO₂-protected ceramics such as SiC. Mullite-coated SiC is discussed, and the corrosion behavior is evaluated.