Enhanced Thermal Conductivity of Epoxy Nanocomposites via Aligned Boron Nitride Nanosheets

Abstract

Background: Thermal management remains a critical bottleneck in high-power electronic devices, where heat dissipation through polymer encapsulants and substrates limits performance and reliability. Hexagonal boron nitride (h-BN) nanosheets are among the most promising thermally conductive fillers for polymer composites owing to their high intrinsic thermal conductivity (~400 W/mK in-plane), electrical insulation, and chemical stability. However, achieving high through-plane thermal conductivity in BN/polymer composites has proven difficult because randomly oriented nanosheets create tortuous thermal pathways.

Methods: We developed a magnetic-field-assisted alignment technique to orient surface-functionalized h-BN nanosheets within an epoxy matrix during curing. BN nanosheets were first functionalized with Fe₃O₄ nanoparticles via a sol-gel process to impart magnetic responsiveness. Composites were prepared at filler loadings of 5 to 30 vol% and cured under a 0.5 T magnetic field to achieve preferential alignment along the through-plane direction. Thermal conductivity was measured using the laser flash method in both the alignment and transverse directions, and filler orientation was quantified through X-ray diffraction pole figure analysis and scanning electron microscopy of cross-sections.

Results: Aligned composites achieved a through-plane thermal conductivity of 5.2 W/mK at 30 vol% BN loading, representing a 320% enhancement over randomly oriented composites at the same filler fraction (1.24 W/mK) and a 26-fold increase over neat epoxy (0.2 W/mK). The thermal conductivity anisotropy ratio (through-plane to in-plane) reached 3.8, confirming effective alignment. Pole figure analysis indicated a Herman orientation factor of 0.82, corresponding to strong preferential alignment. Importantly, the glass transition temperature of the epoxy matrix was maintained above 140 °C across all filler loadings, confirming that the alignment process did not compromise the thermal stability of the host polymer.

Conclusions: Magnetic-field-assisted alignment of BN nanosheets offers a scalable route to high through-plane thermal conductivity in polymer nanocomposites while preserving the processability and electrical insulation of the epoxy matrix. These materials show strong potential as next-generation thermal interface materials for power electronics, LED packaging, and battery thermal management systems.

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