Highly thermal conductive, anisotropically heat-transferred, mechanically flexible composite film by assembly of boron nitride nanosheets for thermal management

Fabricating thermally conductive yet electrical insulated composite films faces dilemmas of ineffective exfoliation of boron nitride (BN) platelets, unsatisfactory thermal conductivity (TC) and poor anisotropy ratio. Herein, few-layered and functionalized boron nitride nanosheets (BNNSs) were effectively exfoliated from BN platelets via eco-friendly biomolecule-assisted exfoliation. Then, BNNS/ethylene-vinyl acetate copolymer (EVA) composite films with the laminated structure were achieved by the green and scalable vacuum-assisted self-assembly. The as-prepared BNNS/EVA composite film showed superior in-plane TC of 13.2 W/mK and strong anisotropy ratio of ~2500% at the BNNS loading of 50 wt%. It was mainly ascribed that the highly oriented BNNSs formed effectively thermally conductive pathways for heat transfer. Additionally, the oxygen-containing functional groups of BNNSs improved interfacial interaction with the EVA matrix and reduced phonon scattering. Thermal interface resistance of the 50 wt% BNNS/EVA film was reduced by 68% compared to the 50 wt% BN/EVA counterpart. Furthermore, the BNNS/EVA films exhibited an attractive flexibility and TC reliability. The retention ratio of in-plane TC was 98% after repetitive bending, 95% after repeated tensile test, and 97% after heating/cooling cycles. The obtained results offer valuable fundamentals to fabricate high-performance thermally conductive polymer composites as advanced thermal management materials.