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Fine mullite powders were prepared from molecular-designed Al, Si-double alkoxide. The change in the lattice parameters of the mullite powders with different compositions was consistent with the phase diagram proposed by Prochazka et al., showing the high purity and homogeneity of the resultant mullite powders prepared in this study. The surface areas of the resultant mullite powders with different compositions decreased with increasing calcination temperatures, depending on the compositions. However, fine mullite powder with a stoichiometric composition maintained high surface area even after high-temperature calcination. The change in the surface area could explain from the crystallization behavior of precursors, which was affected by the compositions. Precursor powders with SiO2-rich composition crystallized into mullite at relatively low temperature, resulting in the formation of amorphous SiO2-rich phase which promoted the grain growth during calcination by the viscous sintering. On the other hand, precursor powders with Al2O3-rich composition mullitized, via ultra-fine intermediate spinel and/or alumina, leading to the high surface areas at lower temperatures. However, the surface areas of the Al2O3-rich precursor powders abruptly decreased by the calcination above 1100°C, due to the grain growth with the aid of the active ultra-fine alumina. As a result, only the precursor powder with a stoichiometric composition showed high surface area after high-temperature calcination.

Crystallization Behavior and Change in Surface Area of Alkoxide-Derived Mullite Precursor Powders with Different Compositions