Mullite whiskers were prepared from Si powders in molten Al2(SO4)3-Na2SO4 mixture salts with different Al/Si molar ratio (R) of raw materials. The resulting mullite whiskers, had been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and infrared spectroscopy analysis (FT-IR). Mullite phase was obtained in molten salts when the temperature reached at 850 °C. SEM and EDS results revealed that two kinds of microstructures were formed in the final product, Al-rich mullite pellets and clusters of tiny mullite crystals, and the content of Al-rich mullite pellets increased with the higher Al/Si molar ratio (R) adopted in raw materials. A new oxidation-dissolution mechanism was proposed to explain mullite whiskers growth. According to thermodynamic analysis, mullite phase might be spontaneously formed as the temperature reached the decomposition temperature of aluminum sulfate (1023 K).
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