Salting-Out Effect Protected Ball-Milling Exfoliation of Hexagonal Boron Nitride and the Scale Laws on the Thermal Conductivities of the Exfoliated Nanosheet Assembled Composite Films

Herein, we discovered that using the mixed aqueous solution of hydroxyethyl cellulose (HEC) and sodium hydroxide (NaOH) as ball-milling medium of hexagonal boron nitride (h-BN) could exfoliate larger lateral size boron nitride nanosheets (BNNSs) than using HEC solution alone due to the salting-out effects of Na + to HEC that aggregated the polymer molecules and in turn enhanced the impact resistance to milling balls. Through centrifugal separation and reduced ball-to-h-BN ratios, different lateral size BNNSs were obtained and further assembled into films with aramid nanofibers (ANFs) to investigate their thermal conduction performances. The results indicated that the larger lateral size BNNS-based films show higher thermal conductivities (TCs) owing to the reduced phonon scattering at boundaries, but BNNSs with overlarge lateral sizes are difficult to be stacked into compact microstructures and in turn result in the much lowered TC for the assembled film. Meanwhile, the interfacial interactions and BNNS defects also have some influences on TCs of the composites. Overall, this study provides a novel route to the fabrication of large-area BNNSs and reveals their scale laws on thermal conduction, and the fabricated ANF/BNNS heat spreader has a high TC of 31.88 W·m –1 ·K –1, which is promising for use in thermal management of high-power electronic devices and batteries.