Fabrication and properties of freeze-cast mullite foams derived from coal-series kaolin

Hierarchically porous mullite foams were fabricated from coal-series kaolin and Al 2 O 3 slurries by freeze-casting using tert-butyl alcohol (TBA) and water as templates. TBA developed unidirectionally aligned pore channels along the freezing direction and water developed lamellar pores. The total porosity (60.2–83.4%), density (0.49–1.23   g/cm 3 ) and macro-pore size (10–50   µm) of mullite foams were tailored by controlling the solid content of the slurries. The mullitization reaction was completed at the sintering temperature of 1500   °C. The compressive strength of mullite foams along the freezing direction was 3.8–49.4   MPa. Lower porosity and higher compressive strength of sintered mullite foams were obtained using TBA as solvent compared to water. The thermal conductivity of low density and mechanically stable TBA templated porous mullite with 80.2% porosity was 0.18   W/m   K, indicting that the freeze-cast porous mullite was a promising refractory material for applications in thermal insulation .