Preparation and characterization of mullite whiskers from silica fume by using a low temperature molten salt method

Mullite whiskers were prepared from silica fume in molten Al2(SO4)3–Na2SO4 mixture salts at low temperatures. The resulting mullite whiskers, as well as the nucleation and growth mechanism in the molten environment, have been investigated by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and thermogravimetric analysis (TG–DTA) techniques. XRD studies showed that the materials obtained were orthorhombic mullite. SEM, TEM and HRTEM results revealed that the mullite whiskers were single crystal fibers with diameters ranging from 30 to 150 nm and lengths of over several microns. According to thermodynamic analysis, mullite phase might be spontaneously formed in molten salts as the temperature reached the decomposition temperature of aluminum sulfate (1023 K). Moreover, the mullite crystals grew along [1 1 1] crystal plane firstly and developed into fibrous microstructure finally.

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► Mullite whiskers were prepared from silica fume in molten Al2(SO4)3–Na2SO4 mixture salts system.
► The experimental results show a mullitization temperature of 800 °C, which is the lowest reported mullite whiskers formation temperature via molten salt method.
► TEM and HRTEM studies show that mullite crystal grow along the [1 1 1] crystal plane continuously and develop into fibrous microstructure finally.
► The thermodynamic data indicate that mullite phase may spontaneously form when the temperature is up to the decomposition temperature of aluminum sulfate (1023 K).