Fabrication of ultralightweight, thermal insulation alumina scaffolds by a hybrid sol–gel/freeze‐casting approach

Freeze casting is an effective way to fabricate the porous ceramics with anisotropic and interconnected porous structures and can be used in various applications, such as filtration, adsorption, and insulation. However, the ceramic‐based scaffolds fabricated by freeze casting have the upper limit of porosity since excessively low solid content in the slurry will lead to structural instability and cracks formation in scaffolds. This study aims at combining the freeze casting and sol–gel method and developing a hybrid process to overcome the limitation of freeze casting. Besides, the effects of solid content and cooling rates on the microstructure of ultralightweight alumina scaffolds (ULASs) and their mechanical and thermal properties were investigated. Aluminum isopropoxide was selected as the precursor to conduct the hydrolysis reaction in acid environment and condensation process by heat treatment. The successfully synthesized alumina scaffolds have ultrahigh porosity (>90%), low bulk density (0.1240–0.2429 g/cm3), and low relative density (0.0314–0.0615). The anisotropic porous lamellar structure was evaluated by scanning electron microscope, µ‐CT, and mercury porosimeter. A lot of nanoscale pores are observed on the lamellae surfaces, forming the dual‐scale porous structure inside the scaffolds and contributing to higher specific surface area. The unique anisotropic structure, high porosity, and stable mechanical properties enable ULASs to deliver a low thermal conductivity of 0.2 W/m/K and large anisotropy in thermal properties, possessing great potential for thermal insulative materials. This sol–gel/freeze‐casting hybrid approach is believed to be extended to different material systems and provide promising potentials in fabricating the porous materials with various functionalities.