Mitigating Contact-Constricted Thermal Bottlenecks by Magnetically Programmable hBN Bridge Fillers in Al2O3/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 Al2O3 serves as a process-friendly primary filler framework, while Fe3O4-functionalized hexagonal boron nitride (hBN@Fe3O4) 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 Al2O3:hBN@Fe3O4 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 Al2O3-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 × 101 5 Ω cm) and lower dielectric constant and loss than the Al2O3 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.