Mullite Precursor Synthesis in Aqueous Conditions: Dependence of Mullite Crystallization and Grain Size and Morphology o
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Ungyu Paik Department of Ceramic Engineering, Hanyang University, Seoul 133-791, Korea (Received 28 May 2003; accepted 30 December 2003)
The synthesis behavior of mullite in an aqueous system was investigated. Two kinds of aluminosilicate precursor sols were prepared by the dissolution of two kinds of salt (aluminum chloride hexahydrate, AlCl3·6H2O, and aluminum sulfate with 14–18 molecules of water, Al2[SO4]3·14–18H2O) into the mixture of colloidal silica sol. The effects of the solution pH, which was controlled by addition of ammonium hydroxide (NH4OH), and the type of aluminum salt on the mullite crystallization and the grain size and morphology in the sintered materials are discussed in this paper. It has been discovered that in an aqueous system, the mullitization temperature strongly depends on the solubility and decomposition temperature of the aluminum salt used and that the pH of the initial sol impacts the grain size and morphology of mullite, in addition to the homogeneity of mixture. Mullite is synthesized at 850 °C without resorting to the addition of an organic additive or nonaqueous solvent. I. INTRODUCTION
It is well known that crystallization of monophasic mullite occurs at temperatures around 980 °C, and that crystallization of diphasic mullite occurs at temperatures above 1250 °C.1–4 The synthetic behavior of mullite depends on the chemical homogeneity of the precursor used in the sol-gel and co-precipitation methods, resulting in monophasic or diphasic mullite.5 Other factors influencing the formation of the various phases of mullite are: the temperature, the starting material, the solution pH, the Al/Si molar ratio, and the preparative method.6–10 In the preparation of aluminosilicate gels, Si(OCH3)4 and Si(OC2H5)4 are generally used as the silica precursor, and aluminum isopropoxide [Al(i-OC3H7)3], aluminum ethoxide [Al(OC2H5)3], and aluminum tri-sec-butoxide [Al(OC4H9)3], which contain alkoxide groups, have been used as the alumina precursors.11–17 These materials are quite difficult to handle due to long reaction time and low solubility in water. Therefore, a mutual solvent, such as alcohol, is usually used as a homogenizing agent, because of the immiscibility between water and alkoxides. Aelion et al.18,19 have investigated the hydrolysis of Tetraethylorthosilicate (TEOS) under acidic and basic conditions using several co-solvents: methanol, ethanol, and dioxane. Even though a method for preparing fine a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0147 J. Mater. Res., Vol. 19, No. 4, Apr 2004
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pure mullite powder in an aqueous system has been reported20,21 using fumed silica as the silicon source and aqueous solutions of ammonium bisulphate and aluminum sulfate as the aluminum salt source, the mullitization temperature was similar to previous works.1–4 In addition, the general synthesis behavior in aqueous systems has not been discussed much. In our previous studies, the effects of
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