Fabrication of mullite ceramics with ultrahigh porosity by gel freeze drying

Porous mullite (3Al(2)O(3).2SiO(2)) ceramics with an open porosity up to 92.9% were fabricated by a gel freeze-drying process. An alumina (Al2O3) gel mixed with ultrafine silica (SiO2) was frozen and sublimation of ice crystals was carried out by drying the frozen body under a low pressure. Porous mullite ceramics were prepared in air at 1400 degrees-1600 degrees C due to the mullitization between Al2O3 and SiO2. A complex and porous microstructure was formed, where large dentritic pores with a pore size of similar to 100 mu m contained small cellular pores of 1-10 mu m on their internal walls. Owing to the complete mullitization, a relatively high-compressive strength of 1.52 MPa was obtained at an open porosity of 88.6%.