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3 adding on properties of SiC porous ceramics
Published in Domenico Lombardo, Ke Wang, Advances in Materials Science and Engineering, 2021
T.Y. Yu, H. Li, M.E. Ma, Y.N. Sun, X.Y. Deng
Mullite is a ceramic material with good high-temperature strength and thermal expansion coefficient similar to that of SiC porous ceramics. An appropriate amount of mullite can effectively improve the thermal shock resistance and creep resistance of SiC porous ceramics. The addition of potassium carbonate can promote the formation of mullite phase. Potassium carbonate decomposes into potassium oxide and carbon dioxide at high temperature. Carbon dioxide escapes in the form of gas. The change of alumina mole content in mullite is closely related to the content of potassium oxide in the sample. K atoms are conducive to the increase of liquid phase, and keep the amorphous state of liquid phase, as well as make the reaction of Si-O structure more active, thereby promoting the formation of mullite phase [6–10].
Thermodynamic Aspects of Phase Stability
Published in Mary Anne White, Physical Properties of Materials, 2018
The binary phase diagram for SiO2 (quartz) and Al2O3 shows several interesting features. Pure SiO2 crystallizes from the melt at 1735 °C as cristobalite. At 1475 °C, cristobalite undergoes a first-order phase transition to another quartz polymorph characterized by its small (often triplet) crystals, tridymite. Aluminum oxide crystallizes at 2020 °C to produce corundum, a very hard mineral often used as an abrasive. In the SiO2/Al2O3 phase diagram, at 1540 °C there is a eutectic. There is one compound in the binary phase diagram, and this is mullite, a rare clay mineral of disputed empirical formula, most likely Al6SiO11. Mullite melts incongruently at 1805 °C. Sketch and label the SiO2/Al2O3 binary phase diagram (with the abscissa as mass% Al2O3). Al6SiO11 is 83.5 mass% in A12O3.
Advances in Fabrication of Functionally Graded Materials
Published in T. S. Srivatsan, T. S. Sudarshan, K. Manigandan, Manufacturing Techniques for Materials, 2018
Jin et al. (2005) developed mullite/Mo FGM using powder metallurgy. Mullite and Mo have almost similar thermal expansion coefficients (αMull = 5.13 × 10−6/K, αMo = 5.75 × 10−6/K at 1000°C). Because of this special attribute, the residual thermal stress induced during the sintering process is reduced. It was found that the variation of mechanical properties followed the Voigt model, whereas the variation of the thermal properties tended to follow the Reuss model. The fabricated graded materials reflect better thermal shock resistance than the monolithic mullite. In Figure 12.6a, the layer boundaries were observed clearly, which shows that the structures of the graded materials were macroscopically inhomogeneous due to the graded distribution of the composition. Figure 12.6b depicts the microstructural distribution of the concentration of Mo.
Kinetics and phase transformation of low-calcium coal fly ash (CFA) under hydrofluoric acid leaching
Published in International Journal of Coal Preparation and Utilization, 2023
Yang Chen, YiMiao Nie, ZhanJing Zhao, Hua Zhang, Shuxian Liu, Ling Wang, Long Wang, Wang Sen
When the hydrofluoric acid concentration was 20%, some mullite was dissolved and a new crystalline phase (ralstonite) formed. In the crystal structure of mullite, the most basic structural units are silicon-oxygen tetrahedrons [SiO4] and aluminum oxygen tetrahedrons [AlO4], which form double chains without arrangement along the c-axis, and are connected by octahedrons [AlO6]. The octahedrons [AlO6] in the structure play a supporting role in stabilizing the skeleton. With the different ratios of aluminum to silicon, there are periodic oxygen vacancies, resulting in more vacancies at the lattices and looser structures. The process of acid leaching of mullite is similar to that of quartz, in which the element replaces Si in an isomorphic form, thus reducing the energy of each bridging oxygen bond. However, aluminum in mullite structures exists in the form of [AlO6] octahedrons, which play a supporting role in stabilizing the skeleton, leading to the strong stability of mullite. This also explains why mullite dissolves only at higher concentrations of hydrofluoric acid than quartz.
Effect of Al(OH)3 addition on densification mechanism and properties of reaction-sintered mullite-corundum composite ceramics
Published in Journal of Asian Ceramic Societies, 2022
Zhenying Liu, Nan Xie, Hanxin Zhang, Shouwu Huang, Chongmei Wu, Shuhuan He, Jinbo Zhu, Yin Liu
Mullite is a nonstoichiometric compound whose composition generally lies between 3Al2O3 · 2SiO2 and 2Al2O3·SiO2. It possesses the advantages of high chemical stability, low thermal expansion coefficient, excellent chemical resistance, superior thermal shock resistance and good creep resistance [15–17]. Thus, preparing mullite ceramics from coal gangue is essential for developing value-added products. Some researchers have attempted to use coal gangue to replace traditionally pure materials during the fabrication of mullite-based ceramics to reduce costs. For example, Ji et al. [18] found that mullite ceramics could be fabricated from coal gangue by adding γ-Al2O3 and bauxite as aluminum sources, rendering excellent performance. Liu et al. [19] used coal gangue and high alumina waste to produce ceramics with excellent properties and realized the resource utilization of coal gangue. Hao et al. [20] reported that low-density ceramic proppants could be successfully prepared from coal gangue and flint clay.
Microwave heat treatment on kyanite for mullite formation and the preparation of zirconia toughened mullite
Published in Journal of Microwave Power and Electromagnetic Energy, 2022
Satya Sai Srikant, Raghupatruni Bhima Rao
Mullite is an important and valuable rare mineral consisting of alumino-silicate (3Al2O3·2SiO2). It is widely used in whiteware, porcelains, high temperature insulating and refractory ceramic industries because it has high melting point (max 3438 °F), low density (2.8 mg/m3), high resistivity (> 1013 Ω cm), insoluble in most acids, resistance to abrasion, high thermal stability, high resistance to oxidation process, low thermal conductivity (between 1.9 and 6 Wm−1/°K), low thermal expansion coefficient (4.5 × 10−6 K−1), low dielectric constant (5.8 @ 1 MHz), good mechanical properties at high temperature low thermal conductivity and good chemical stability (Schneider and Eberhard 1990; Tripathi and Banerjee 1998; Serra et al. 2016). Generally, mullite being occasionally found as a rare mineral in nature, and present deposits are insufficient to supply a growing market to new applications encountered. The mullite (3Al2O3·2SiO2) is the combination product of the reaction of alumino-silicates at high temperatures and low-pressure conditions, which contain 71.8% of alumina and 28.2% of silica (Tripathi and Banerjee 1998).