WEB Synthesis of mullite whiskers in Na2SO4 flux
Mullite (Al6Si2O13) is a well-known alumosilicate material with superior thermal and chemical properties, such as high melting point, good creep resistance, low thermal expansion, and an excellent mechanical property even at 1300°C. The solid-state synthesis of mullite whiskers from alumina and silica precursors needs very high temperatures (~1600°C) . In contrast, mullite can be obtained at much lower temperature (~ 1000°C) by flux method. However, the flux method reported in the previous studies led to the formation of side phases such as alumina and/or silica in the final product despite the stoichiometric ratio of Al and Si in precursor materials . The reason could be related to the limited solubility of alumina and silica in salt flux. To overcome this issue, AlF3 can be used as an additive leading to increased solubility of oxides, which in turn favours the mullite formation.
In the present work, we report the synthesis of mullite whiskers from Al2(SO4)3 and SiO2 at 1000°C by flux method using Na2SO4 flux with AlF3. The obtained mullite whiskers are characterised by XRD, SEM, and TGA techniques. The results reveal that powder product obtained by flux method without or with a small amount of AlF3 (10 mol. %) contains mullite whiskers with unreacted alumina as a side phase. When 25 mol% of AlF3 is added, only mullite whiskers are formed and no alumina side phase is observed. Moreover, the morphology of mullite whiskers (i.e. the length and diameter) is found to be influenced by the AlF3 amount used in the synthesis.
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