Polyimide Scaffolds Loaded with Ternary Multidimensional Fillers via Directional Freeze-Casting for Enhanced Thermal Management and Flame Retardancy

Constructing efficient thermally conductive networks at low filler loading while maintaining electrical insulation and flame retardancy remains a major challenge for polymer-based thermal interface materials (TIMs). Herein, a three-dimensional (3D) anisotropic polyimide (PI) scaffold containing ternary multidimensional fillers, including hydroxylated boron nitride nanosheets (OH-BN), KH550-modified alumina particles (KH550-Al 2 O 3 ), and tannic acid-coated carbon fibers (CF@TA), was fabricated via directional freeze-casting and subsequent thermal imidization. The aligned scaffold was then infiltrated with silicone rubber (SR) to obtain anisotropic thermally conductive composites. The directional freezing process induced the confined assembly of 0D/1D/2D fillers into an interconnected lamellar architecture, forming a “point–line–plane” thermal conduction network that effectively reduced interfacial thermal resistance and facilitated phonon transport. Owing to the synergistic effect of multidimensional fillers and the anisotropic PI framework, the optimized PBAC2/SR composite achieved a through-plane thermal conductivity of 2.41 W/(m·K) at a low filler loading of only 4.7 wt %, corresponding to an anisotropy ratio of 1.66. Infrared thermal imaging and high-power LED tests further demonstrated its superior heat dissipation capability compared with nondirectionally structured counterparts. In addition, the composite exhibited excellent thermal reliability, maintaining stable thermal conductivity after 1000 thermal cycles and prolonged thermal aging. Benefiting from the carbonized PI framework and thermally stable inorganic fillers, PBAC2/SR achieved a limiting oxygen index (LOI) of 31.2% and a UL-94 V-0 rating while preserving a high volume resistivity of 9.74 × 10 13 Ω·cm. This work provides an effective strategy for constructing lightweight, electrically insulating, flame-retardant, and highly thermally conductive polymer composites for advanced thermal management applications.