Hierarchically Porous Mullite Ceramics Assembled from Ultrathin Nanosheets for High-Temperature Thermal Insulation

Mullite porous ceramics show exceptional promise for high-temperature insulation applications. However, the mechanical strength and thermal insulation performance of porous ceramics typically cannot be optimized simultaneously. Herein, we proposed a novel method to overcome this limitation by synthesizing hierarchically porous mullite ceramics assembled from ultrathin two-dimensional nanosheets. A chemical blowing method first synthesized ultrathin mullite nanosheets approximately 2–3 nm thick, which were subsequently assembled into a rigid hierarchical network by gel casting and freeze-drying. The influence of solid content and sintering temperature on the phase composition, microstructure, mechanical properties, and high-temperature thermal stability of porous ceramics was systematically investigated. The results indicate that the 20 wt.% sample sintered at 1200 °C exhibited the best performance. This optimal sample achieved a porosity of 90.75%, a compressive strength of 0.38 MPa, and excellent thermal insulation properties, including a low apparent thermal conductivity of 0.0756 W/(m·K) at room temperature and a back surface temperature of 253.2 °C after exposure to a 1200 °C flame. Crucially, this porous ceramic maintained structural stability up to 1500 °C. This nanosheet assembly strategy successfully reinforced the structural skeleton while inhibiting heat transfer. This strategy has great promise for the fabrication of lightweight porous ceramics designed for extreme environments.