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 (Al 2 O 3 ) gel mixed with ultrafine silica (SiO 2 ) 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°–1600°C due to the mullitization between Al 2 O 3 and SiO 2 . A complex and porous microstructure was formed, where large dentritic pores with a pore size of ∼100 μm contained small cellular pores of 1–10 μm 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%.