Mitigating Contact‐Constricted Thermal Bottlenecks by Magnetically Programmable hBN Bridge Fillers in Al ₂ O ₃ /Epoxy Composites

Thermally conductive polymer composites often suffer from limited heat transport due to contact constriction in spherical particulate networks. To address this challenge, we report a magnetically programmable bridge‐filler architecture that enhances thermal pathways in electrically insulating epoxy systems. In this design, spherical Al 2 O 3 serves as a process‐friendly primary filler framework, while Fe 3 O 4 ‐functionalized hexagonal boron nitride (hBN@Fe 3 O 4 ) flakes act as magnetically addressable bridge fillers that connect neighboring alumina particles and can be oriented along the through‐plane direction under an external magnetic field. At a fixed total filler loading of 40 vol%, the Al 2 O 3 :hBN@Fe 3 O 4 ratio of 8:2 provides the most effective balance between bridge‐network formation and rheological freedom for alignment, yielding a through‐plane thermal conductivity of 2.02 W m − 1 K − 1 , 72.8% higher than that of the corresponding Al 2 O 3 ‐only composite. Finite‐element modeling reveals that the hybrid aligned architecture alleviates point‐contact‐dominated thermal bottlenecks by redistributing heat flux through more continuous bridge‐mediated pathways. The optimized composite retains high volume resistivity (4.4 × 10 1 5 Ω cm) and lower dielectric constant and loss than the Al 2 O 3 composite, while package‐level thermal tests confirm faster heat transfer. These results establish a generalizable design principle for directionally efficient thermal management in electrically insulating composites with relevance to advanced electronic packaging.