Effect of Milling Time on the Physical and Mechanical Properties of Celsian-Mullite Composites Synthesized from Coal Fly
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Effect of Milling Time on the Physical and Mechanical Properties of Celsian-Mullite Composites Synthesized from Coal Fly Ash Jorge López-Cuevas1, David Long-González2, Carlos A. Gutiérrez-Chavarría1 1 Cinvestav-Saltillo, Carr. Saltillo-Monterrey, km 13.5, Ramos Arizpe, Coahuila, México, 25900 2 RHI REFMEX, S.A. de C.V., Carr. Saltillo-Monterrey, Km. 9, Ramos Arizpe, Coahuila, México, 25900 ABSTRACT Four Celsian (Ba0.75Sr0.25Al2Si2O8)/Mullite (Al6Si2O13) composites, with potential structural applications at high temperatures, are synthesized from coal fly ash (byproduct of a Mexican coal-burning power plant, constituted mainly by SiO2 and Al2O3). Nominal Celsian/Mullite weight ratios studied are 80/20, 60/40, 40/60 and 20/80. Mullite is synthesized separately at 1600ºC/2h and then mixed with a Celsian precursor mixture previously calcined at 900°C/5h. During this process the Celsian phase is formed by a solid state reaction at 1100-1400ºC/5h. Prior to this, the reacting mixture is milled in a planetary mill during 1 or 2h and then compacted by uniaxial and cold isostatic pressing. The microstructure and phase composition of the synthesized composites are characterized by X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM/EDS). Their dynamic Young’s modulus is measured by an ultrasonic technique, and their mechanical strength is evaluated from flexural tests carried out at room temperature. The expected phases are obtained in all cases, although with some differences with respect to their expected relative proportions, according to the studied nominal compositions. In general, the longest milling time employed produced samples with the largest degree of crystallinity and density, as well as with the best microstructural characteristics and mechanical properties. INTRODUCTION During the last ten years, ceramic composites based on barium aluminosilicate (BAS, BaAl2Si2O8) have attracted a great deal of attention for structural applications at high temperatures. The BAS phase, especially in its monoclinic polymorphic form known as Celsian or Monocelsian, possesses good mechanical properties, low thermal expansion coefficient, high resistance to oxidation, high resistance to reduction and high resistance to attack by molten slags. It also has stable dielectric properties and good resistance to thermal shock [1-3]. Celsian is highly refractory due to its high melting point (1760°C), it is stable below 1590°C and it is useful as a matrix for ceramic composites [4,5]. BAS also has a hexagonal polymorphic form known as Hexacelsian, which tends to form before Celsian and frequently remains in a metastable state at low temperatures [6]. Hexacelsian is undesirable due to its relatively high thermal expansion coefficient and because it undergoes a conversion into an orthorhombic form at ~300°C, which is associated with a volume change of ~3-4%. This causes microcracking in the material, negatively affecting its final mechanical properties [3,7,8]. Partial substitution of BaO by SrO in BAS promotes the formation of Ba1-x
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