Microstructure and property of porous mullites with a whiskers framework obtained by a sol-gel process

Porous mullites with a whiskers framework and high porosities were fabricated by the reaction sintering (1100 to 1600 °C, 1 h, in an airtight container) of an aerogel block shaped by the sol-gel transition of a mullite precursor composed of SiO2 sol, Al2O3 and AlF3 powders (as reaction catalyst). The effect of heating temperatures on porosity, whisker formation, microstructure feature and compressive strength of the porous mullites was determined by XRD, SEM and compressive test. The results indicate that after heating at temperatures from 1100 to 1600 °C, the porosities of the mullites varied within the range of 84.1-80.2%. The whiskers in the framework well lap-jointed each other to form the large space and became elongated and smooth at high temperatures due to the accelerated vapor-solid reaction rate. A maximum compressive strength of 16.1 MPa was obtained for the whiskers framework heated at 1600 °C; this strength was attributed to the strong bonding among the smooth whiskers.