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

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