Thermal conductivity of 2D nano-structured boron nitride (BN) and its composites with polymers

High thermal conductivity, structural stability, good mechanical and anti-oxidant properties makes hexagonal boron nitride (h-BN) a promising functional filler for polymers to produce composite materials where excellent thermal management is required, such as in electronic devices. Theoretical studies have revealed that two dimensional (2D) BN has higher thermal conductivity (up to 400 Wm−1 K−1, in-plane) than bulk h-BN due to a reduction in phonon-phonon scattering when scaling down the thickness of the material. For this reason, 2D boron nitride nanosheets (BNNS) are gaining intense interest since they could be utilised in the design of composite materials with excellent efficiency to dissipate heat. Various methods have been explored to produce 2D BNNS including mechanical and chemical exfoliation of pristine bulk BN, chemical reaction, chemical vapour deposition (CVD) and electron irradiation. To facilitate the dispersion of BNNS in polymers, different functionalization strategies have been applied for surface-treatment of BNNS. In this review, the different synthesis approaches adopted for BNNS are compared and the effects of BNNS dispersion on the thermal conduction of polymers are discussed. The factors influencing the mechanism of thermal conduction such as materials crystallinity, filler geometry, filler surface functionalization and alignment, filler/matrix interface and processing conditions are discussed. Some perspectives and future directions on how to generate high thermally conductive composites of BNNS and polymer are proposed.