Synthesis and Characterization of Cordierite, Mullite and Cordierite-Mullite Ceramic Materials using Coal Fly Ash as Raw
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.3
Synthesis and Characterization of Cordierite, Mullite and Cordierite-Mullite Ceramic Materials using Coal Fly Ash as Raw Material J. López-Cuevas*, E. Interial-Orejón, C.A. Gutiérrez-Chavarría, and J.C. Rendón-Ángeles CINVESTAV-IPN, Unidad Saltillo, Calle Industria Metalúrgica No. 1062, Parque Industrial Saltillo Ramos Arizpe, Ramos Arizpe, Coahuila, México, CP 25900
Abstract
Cordierite (Mg2Al4Si5O18), Mullite (Al4+2xSi2-2xO10-x) and Cordierite-Mullite ceramic materials were obtained from a stoichiometric mixture of coal fly ash (CFA) as a source of SiO2 and Al2O3, plus high-purity MgO and Al2O3. The starting stoichiometric mixtures were homogenized, and then uniaxially pressed, cold isostatically pressed, and sintered at 12001600 °C for 2-5 h. The sintered materials were characterized by X-ray diffraction, scanning electron microscopy, Vickers microhardness, density and four-point flexural strength. In general, the desired phases tended to form in the composites at temperatures of 1350 or 1400 °C, with a considerable amount of glassy phase developing from 3 h onwards at one of those two temperatures, depending on the composite composition. The microstructure of the composites consisted of a matrix of Cordierite and interwoven needles of Mullite. The bulk density decreased, while the flexural strength and the Vickers microhardness increased with increasing nominal content of Mullite in the composites. A synergistic effect taking place between Cordierite and Mullite enhances the mechanical properties of the composites. *Corresponding author; Tel. +52 844 4389600; E-mail address: [email protected]
Introduction Cordierite (Mg2Al4Si5O18) is the most important ternary intermediate phase in the system MgO-Al2O3-SiO2. It possesses a hexagonal high-temperature form known as Indialite, as well as an orthorhombic low-temperature form. It melts incongruently at 1460 °C, forming Mullite plus a liquid phase. It is a good electrical insulator at high temperatures and has a very low thermal expansion coefficient, which confers to it an excellent thermal shock resistance [1]. Some of its main applications are multilayer electronic circuits, substrates for catalytic converters and furnace refractories, among many others. Mullite is the only intermediate phase formed in the system SiO2-Al2O3. It is a solid solution formed between these two components, and its chemical composition can be represented by the formula Al4+2xSi2-2xO10-x, where x varies from 0.17 to 0.59, corresponding to the number of oxygen atoms missing per unit cell [2]. It is considered that its stoichiometric composition corresponds to the chemical formula Al 6Si2O13. Mullite is also characterized by a high thermal shock resistance, which is attributed to a morphology of interwoven needles that it frequently forms. It melts congruently at 1850 3865
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