Optimization of Flexible Nacre-Like Cellulose Nanofiber Films by a Covalent Overlapping Method: Excellent Thermal Conductivity and Superior Flame Resistance

Facing the explosive growth of heat flux in microelectronic equipment, advanced thermal management materials should not only ensure the safe and stable operation of equipment, but also have the ability to withstand fire risks. Carbon materials such as graphene are subject to many restrictions in use due to their inherent high conductivity. Hexagonal boron nitride (h-BN) is often used to blend with polymers to prepare flexible thermal management materials due to its excellent electrical insulation and thermal conductivity. However, its further application is limited by its insufficient flame resistance and limited improvement of thermal conductivity at low filling levels. In this paper, urea-assisted ball milling is used to achieve the amination of boron nitride nanosheets (BNNS) and black phosphorus (BP), which creates the covalent bond between the filler and the cellulose. With the overlapping between small-size BP and large-size BNNS, the thermal conductivity, flame resistance, and mechanical properties of the film are significantly enhanced. Accordingly, the cellulose nanofiber (CNF)-based film has a high thermal conductivity of 42.29 W m–1 K–1 at 50 wt % loading (40 wt % BNNS-NH2 and 10 wt % BP-NH2), which is 777% higher than that of pure CNF. In addition, the peak heat release rate and total heat release of CBP10 decrease by 80.3 and 64.7%, respectively, compared with pure CNF, and the residue is more complete and denser, indicating that the film can effectively reduce and delay the fire hazard.