Ultra-Lightweight, Highly Formable, and Thermally Anisotropic Polyimide Aerogel Composites Prepared by Unidirectional Freeze Drying for Thermal Protection Applications

Developing lightweight materials that combine low drying shrinkage with highly anisotropic thermal conductivity remains a key issue that must be addressed to meet the special requirements of integrally formed nose cones applied in hypersonic aircraft, which must achieve efficient windward (i.e., axial) insulation and enable heat dissipation perpendicular to the windward direction (i.e., radial) simultaneously. The present work employs unidirectional freeze-drying combined with molecular bonding engineering strategies to enhance the anisotropic structure of the polyimide (PI). By innovatively introducing a trace amount (only 1 wt%) of aminated glass fibers (GF) to construct multiple robust structural anchors, the network structure is significantly reinforced, yielding a high-performance PI/GF aerogel composite to address this challenge. The PI/GF aerogel composite exhibits an ultra-low density of 0.01 g/cm 3 and a drying shrinkage of only 1.75%. The axial compressive strength of the PI/GF aerogel composite materials reaches up to102.1 KPa. Finally, the PI/GF aerogel composites demonstrate significantly lower thermal diffusivity perpendicular to the freezing direction (radial direction, 0.601–3.680 mm 2 /s) than that observed parallel to the freezing direction (axial direction, 2.074–13.182 mm 2 /s) owing to pronounced morphological anisotropy, where a lamellar porous structure in the radial direction contributes to its superior thermal insulation performance. These results demonstrate that the proposed composites represent advanced thermal protection materials appropriate for hypersonic aircraft.