Molten salt synthesis of mullite whiskers from various alumina precursors

Different reaction processes were observed in the preparation of mullite whiskers by molten salt synthesis (MSS) when Al2(SO4)3 and Al(OH)3 used as alumina precursors. It was found that amorphous Al2O3 from Al2(SO4)3 reacted with silica to form mullite in Na2SO4 flux at 900–1100 °C, whereas γ-Al2O3 from Al(OH)3 did not participate the synthesis reaction of mullite. The formation of tiny-sized amorphous Al2O3 resulted from both free Al-cations in liquid and amorphous silica by means of Al2(SO4)3. In addition, mullite with 67 mol% Al2O3 formed in 1/1, 2/1 and 3/1 mixtures while mullite with 72 mol% Al2O3 formed in 4/1 mixture, when powder mixtures [Al2(SO4)3 as alumina precursor] with Al/Si atomic ratios of 1/1, 2/1, 3/1 and 4/1 were used. The formation of Al-rich mullite was ascribed to the low synthesizing temperature by MSS.