Synthesis of hierarchically porous mullite ceramics with improved thermal insulation via foam-gelcasting combined with pore former addition

To further improve the thermal insulation performance of porous mullite ceramics used in important industrial sectors, a combined foam-gelcasting and pore-former addition approach was investigated in this work, by which hierarchical porous mullite ceramics with excellent properties, in particular, thermal insulation property, were prepared. Both mesopores (2-50 nm) and macropores (117.8-202.7 m) were formed in porous mullite ceramics resultant from 2 h firing at 1300 degrees C with various amounts of submicron-sized CaCO3 pore former. The former mainly arose from the decomposition of CaCO3, and the latter from the foam-gelcasting process. The porous samples prepared with CaCO3 addition had low linear shrinkage of 2.35-4.83%, high porosity of 72.98-79.07% and high compressive strength of 5.52-14.82 MPa. Most importantly, they also exhibited a very low thermal-conductivity, e.g. 0.114 W m(-1) K-1 at 200 degrees C, which was much lower than in the cases of their counterparts prepared via the conventional foam-gelcasting route.