Mullite whiskers derived from coal fly ash

Alumina-deficient (Al2O3/SiO2 = 1.12, molar ratio), orthorhombic mullite whiskers with a diameter of 0.6–1.8 μm (aspect ratio >30) have been manufactured by firing compacts of coal fly ash and NH4Al(SO4)2·12H2O powders, with a small addition of NaH2PO4·2H2O, at 1300 °C for 10 h. The manufacturing process, the morphology, and structure of the whiskers are described.