Effect of Mechanical Activation on the Synthesis of Ba-Celsian and Sr-Celsian using Precursor Mixtures Containing Coal F
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Effect of Mechanical Activation on the Synthesis of Ba-Celsian and Sr-Celsian using Precursor Mixtures Containing Coal Fly Ash Claudia M. Lopez-Badillo1, Jorge López-Cuevas2, Carlos A. Gutiérrez-Chavarría2, José L. Rodríguez-Galicia2, Elia M. Múzquiz-Ramos1. 1 Facultad de Ciencias Químicas, Universidad Autónoma de Coahuila, Blvd. V. Carranza y José Cárdenas, 25280, Saltillo, Coahuila, México. 2 CINVESTAV-IPN, Unidad Saltillo, Parque Industrial Saltillo-Ramos Arizpe, Industria Metalúrgica No. 1062, 25900 Ramos Arizpe, Coahuila, México. ABSTRACT BaAl2Si2O8 and SrAl2Si2O8 were synthesized by solid-state reaction of stoichiometric mixtures of either BaCO3 or SrCO3 with coal fly ash and Al2O3. The mixtures were mechanically activated in an attrition mill for up to 12 h and then reaction-sintered at 900-1300 °C, aiming to promote the formation of BaAl2Si2O8 and SrAl2Si2O8 as well as the conversion from their hexagonal (Hexacelsian) into their monoclinic (Celsian) forms, which is associated with improved mechanical properties in the sintered materials. Especially in the case of SrAl2Si2O8, the formation of Celsian was favored at relatively low sintering temperatures by increasing milling time. Although only the SrAl2Si2O8 composition was fully converted into Celsian, the Hexacelsian to Celsian conversions obtained for the mechanically-activated BaAl2Si2O8 composition were significantly higher than those previously reported in the literature for this compound. This could be attributed to the use of coal fly ash as raw material, which contains mineralizers that promote the mentioned conversion. INTRODUCTION BaAl2Si2O8 and SrAl2Si2O8 are refractory materials with melting points of 1760 oC [1] and 1650 oC [2], respectively. Their monoclinic polymorphic forms have low thermal expansion coefficients [2, 3], and are also characterized by high resistance to oxidation and reduction, as well as to the attack by alkaline reagents [4], low dielectric constant [1, 5], and good resistance to thermal shock [6]. Since both BaAl2Si2O8 and SrAl2Si2O8 show very similar polymorphism [79], the Celsian (or Monocelsian) and Hexacelsian terms commonly refer to their monoclinic and hexagonal polymorphic forms, respectively. Hexacelsian tends to be the first phase forming at high temperatures, tending to remain metastably at low temperatures. Besides, during cooling down to relatively low temperatures it undergoes a reversible transformation into an orthorhombic form, which has high thermal expansion coefficient and is associated with a considerable volume expansion, leading to the generation of cracks and fractures in the materials, significantly reducing their final mechanical properties. The main aim of this work was to study the effect of mechanical activation combined with the use of coal fly ash (SiO2 and Al2O3-rich byproduct of coal-burning power generation plants) as raw material, on the Hexacelsian to Celsian conversion during the synthesis of BaAl2Si2O8 and SrAl2Si2O8. It was intended to take advantage of the known fact that mechanical
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