Synthesis of Crystalline Mullite Fine Powders at Low Temperature by Hydrothermal Treatment of Mixed Alkoxide

Hydrothermal conditions for the synthesis of mullite powders from mixed alkoxide containing A12O3: SiO2=3: 2 (mole ratio) were investigated at 300~600°C under 10~l00 MPa for 2~168 h. Above 20 MPa, fine crystalline mullite powders were obtained at 600°C within 2 h. To produce homogeneous mullite powders, the hydrothermal treatments below 30 MPa at 600°C were preferable, because hydralsite(2 Al2O3 · 2 SiO2 · H2O) and AS(H)-II (5 A12O3 · 2 SiO2 ·n H2O) were produced prior to mullite above 30 MPa at 600°C. In the produce prepared at 600°C under 20 MPa for 2 h, accicular particles of 100 nm in length and 20 nm in width were observed under the transmission electron microscope (TEM). The particles grew up to more than 1 εm in length and 20 nm in width at 600°C under 20 MPa for 168 h. The energy dispersive spectrometry (EDS) proved that the prepared particles had the constant Al2O3/SiO2 mole ratio(=3/2). The synthesized particles was revealed to be mullite by IR absoption method.