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/cm3 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/(m·K), (4× greater), while it is 0.08 W/(m·K) 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 (10×) 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.