Sol-Gel Synthesis of Mullite Starting from Different Inorganic Precursors

Using silicotetraetilortosilicate (TEOS) mixed with aluminum tri-sec-butoxide (TSBAI) or aluminum cloaures mullite ceramics were created by the sol-gel method. The quantities used of each substance were those that led to obtain stoichiometric mullite (3Al2O3·2SiO2). The experimental methodology used for obtaining mullite consisted in: sol-gel synthesis of precursor materials, isothermal treatment of those materials, and characterization of resulting materials. In order to determine the advance of reactions during mullite formation, isothermal treatments between 300°C and 1600°C were performed, keeping the samples at each temperature during 4？h. From XRD results, it may be said that precursor powders originally amorphous start to crystallize in Al2O3 and SiO2 at 1200°C, and the mullite formation starts at 1200°C, with being completed at 1600°C. The use of TSBAI favors the formation of mullite crystals at lower temperature. From SEM observations a microstructure that presents primary mullite with randomly oriented grains of secondary mullite with acicular shapes and sizes that range between 1.25 and 1.50？ m long may be determined. 1. Introduction Materials from the SiO2-Al2O3 system play an important role in the development of traditional and advanced ceramics. Mullite, a material obtainable at atmospheric pressures, is part of this system. Mullite is a rare mineral not found in abundant quantities in nature; although most of traditional ceramics have it as part of their final composition for mineral aluminosilicates are regularly used for its manufacturing. The importance of mullite lies in its good mechanical, thermal, chemical, and electrical properties which remain under elevated temperatures (about 1500°C) [1–3]. Conventionally, mullite is produced by high temperature calcination of mixtures of SiO2 and Al2O3 [4]. The activation of energy for ion diffusion that takes place through the network of energy requires high temperatures; therefore, high sintering temperatures are required (>1700°C) to obtain dense bodies of mullite [5]. Moreover, the sol-gel process enables the production of amorphous and polycrystalline materials with special characteristics starting from submicron powders of high purity [6–8]. Its usefulness lies in the fact that it requires low temperatures to obtain dense bodies compared to traditional manufacturing methods by fusion [6–8]. One of the many uses of mullite takes place in the electronics industry where it is used as substrate [1, 3]. A substrate is the support on which an electronic circuit, consisting of conductors,