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 Al(2)(SO(4))(3) and Al(OH)(3) used as alumina precursors. It was found that amorphous Al(2)O(3) from Al(2)(SO(4))(3) reacted with silica to form mullite in Na(2)SO(4) flux at 900-1100 degrees C, whereas gamma-Al(2)O(3) from Al(OH)(3) did not participate the synthesis reaction of mullite. The formation of tiny-sized amorphous Al(2)O(3) resulted from both free Al-cations in liquid and amorphous silica by means of Al(2)(SO(4))(3). In addition, mullite with 67 mol% Al(2)O(3) formed in 1/1, 2/1 and 3/1 mixtures while mullite with 72 mot% Al(2)O(3) formed in 4/1 mixture, when powder mixtures [Al(2)(SO(4))(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. (C) 2009 Elsevier B.V. All rights reserved.