Hierarchically porous aluminum borate whisker-based ceramics via freeze-gel casting and in situ reaction

Porous aluminum borate ceramics are ideal candidates for high-temperature thermal insulation applications. In this work, lightweight, high strength and thermal insulating aluminum borate whisker-based porous ceramics with hierarchical pore structure were fabricated by the combination of freeze-gel casting and in situ reaction. The effects of B/Al ratio on the morphology of aluminum borate whiskers and the addition amount of agar on the pore structure and properties of aluminum borate whisker-based porous ceramics were investigated. The aluminum borate whisker-based porous ceramics exhibited a typical hierarchical pore structure, including macro pores formed by the growth of ice crystal in the freeze casting process and micro pores in pore walls formed by the overlapped aluminum borate whiskers generated from the in situ reaction of B 4 C and Al 2 O 3 . Results shows when the B/Al molar ratio was 6:9 and the addition amount of agar was 0.4 ​g, the resultant aluminum borate whisker-based porous ceramics shows a low density (0.20 ​g/cm 3 ), high porosity (93.5 ​%), high compressive strength (4.09 ​MPa) and low thermal conductivity (0.072 ​W/(m·K)), which can be regarded as a promising high temperature thermal insulation structural material.