Effect of Fe2O3 on Compressive Strength and Microstructure of Porous Acicular Mullite

Porous acicular mullite was fabricated at 1300 °C starting from Al2O3 and mixture of SiO2 and MoO3 obtained by previous oxidation of waste MoSi2. It was found that the presence of MoO3 favors formation of acicular (prism-like) mullite grains with sharp edges. The effect of addition of Fe2O3 (4–12 wt.%) on phase composition, compressive strength, thermal conductivity and microstructure was studied. The addition of Fe2O3 improved the compressive strength from approximately 25 MPa in pure mullite to about 76 MPa in samples containing 12 wt.% Fe2O3, while the open porosity decreased from 55.4% to 51.8%. The presence of Fe2O3 caused a decrease in mullite formation temperature owing to the formation of liquid phase and accelerated diffusion. The solubility of iron oxide in mullite lattice was between 8 and 12 wt.% Fe2O3. The incorporated iron ions also promoted the rounding of sharp edges in prismatic mullite grains, leading to a reduced specific surface area of 0.55 m2/g in the sample with 12 wt.% Fe2O3. The thermal conductivity of mullite increased with addition of 12 wt.% Fe2O3 reaching value of 1.17 W/m·K.