Mullite formation from xerogels of (0.84–2.2) Al2O3·1SiO2
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Xerogels of (0.84–2.2) Al2O3·1SiO2 prepared by chemical coprecipitation of Al(NO3)3·9H2O and Si(OC2H5)4 experience three thermal reaction paths for mullite formation. Those with pseudoboehmite are found to form mullite via the paths of either Al-Si spinel → mullite transformation or γ-Al2O3 → θ-Al2O3 + amorphous SiO2 → mullite, depending upon the ratios of Al2O3/SiO2. Higher SiO2 content may prefer the former reaction. Xerogels composed of bayerite form mullite via the route η-Al2O3 → γ-Al2O3 + amorphous SiO2 → mullite. Mullite thus formed exhibits a different crystal size, being 20–25 nm for that obtained from pseudoboehmite and around 37 nm for bayerite. The highest yield of mullite formation may be achieved with xerogels of pseudoboehmite with the stoichiometric mullite compositions, 3Al2O3·SiO2.
KeywordsPolymer SiO2 Al2O3 Crystal Size Reaction Path
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