Abstract
Abstract Admixtures of tetraethoxysilane (TEOS) and aluminium sec.-butylate (AlOBu) with stoichiometric 3Al2O3.2SiO2 mullite composition were used for the syntheses. Depending on the amount of H2O, the velocity of the hydrolysis process and the pH of the solvent, three different types of temperature-induced mullite formation processes are observed. Type I mullite precursors: produced by slow hydrolysis with very little H2O. From ≈350°C and up to ≈900°C these precursors are non-crystalline and show a homogeneous mixture on an atomic level. Above this temperature limit the precursors transform to Al2O3-rich mullite and non-crystalline SiO2. Type II mullite precursors: produced by rapid hydrolysis with excess H2O in a very basic environment (pH > 10). Above ≈350°C these precursors are more phasic, consisting of relatively large crystalline γ-Al2O3, and of non-crystalline SiO2-rich areas. Mullite formation is observed at ≳1200°C. Type III mullite precursors: produced by rapid hydrolysis with excess H2O in a moderately basic environment (pH ≤ 10). From ≈350°C and up to ≈900°C these precursors are non-crystalline. Above this temperature limit the precursors gradually transform to γ-Al2O3 and non-crystalline SiO2-rich areas. Mullite formation is observed at ≳1200°C.
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