Orientation-Dependent Thermal and Mechanical Properties of 2D Boron Nitride Nanoplatelet Foams via Freeze-Drying

Anisotropic two-dimensional (2D) materials, such as boron nitride nanoplatelets (BNNP), are excellent polymer reinforcement candidates due to their superior and tailorable thermal and mechanical properties. A significant challenge with integrating nanosized 2D fillers in polymer matrices is their agglomeration tendency, which has detrimental effects on the nanocomposite's properties. Freeze-drying enables the construction of free-standing 2D material networks to be used as composite nanofillers. In this study, ultralight BNNP lamellar foams (0.05 g/cm(3) dense and 97% porous) were fabricated via freeze-drying. The BNNP foams presented highly anisotropic thermal and mechanical behavior due to their lamellar morphology. The thermal conductivity of the foam along its lamellar walls is 0.31 W/(mK), (4x greater), while it is 0.08 W/(mK) throughout the pores. The anisotropy in the thermal properties of 2D foams is modeled. The mechanical behavior of BNNP foams was studied via quasi-static and cyclic nanoindentation. The lamellar foams are stronger (10x) along the wall direction (11.3 MPa) than across the walls (1.3 MPa). Mechanical properties were modeled by using the Gibson and Ashby model for cellular materials. A protocol is established to design 2D material foams with different concentrations and particle sizes as composite nanofillers with tailorable thermal and mechanical properties for thermal management applications.