Preparation of porous mullite ceramics consisting of needle-like mullite whiskers

Two steps were involved in the preparation of the porous mullite ceramics: preparation of porous green body by foaming method combined with gelcasting, using alumina, silica and AlF3 as raw materials and synthesis of needle-like mullite whiskers by vapor-phase reaction. Owing to the special pore structure of foaming method, the needle-like mullite crystals had enough space to develop with good structure. Most of the needle-like mullite crystals have diameters around 1.5 μm and lengths of >80 μm. By adjusting the amount of foaming agent, the open porosity varied from 66.3 to 85.5%, and the compressive strength and flexural strength decreased evidently, from 23.4 ± 1.8 to 4.9 ± 0.4 MPa and 9.7 ± 0.9 to 1.0 ± 0.1 MPa.