The effect mechanism of functionalization on thermal conductivity of boron nitride nanosheets/paraffin composites

Functionalizations of boron nitride nanosheets (BNNS) are found to be useful for improving the thermal conductivity of materials. However, the mechanism responsible for the enhancement is still open. In this paper, classic molecular dynamic simulations and theoretical analysis model based on the effective medium theory were adopted to investigate the effect mechanism of three types of functionalizations, including C-doping, CH3-grafting and the single-vacancy, on the thermal conductivity of BNNS/paraffin composites. Research results showed that the functionalizations would produce the phonon impurity scattering, which deteriorated the thermal transport of BNNS. However, it could promote the phonon coupling between in-plane and out-of-plane of BNNS, which will boost the energy propagation between paraffin and BNNS and eventually enhance the interface thermal conductance. Under the interaction of the two opposite phenomena, the C-doping functionalization shows to be the most effective method to enhance the thermal conductivity of composites. Comparing with pristine BNNS (p-BNNS), there was a critical size for the thermal conductivity enhancement of functionalized BNNS/paraffin composites. The critical size of the C-doping could reach to 9.1 μm for the functionalized ratio of 2.5%. With the size of 2 μm of the C-doping BNNS, the thermal conductivity of composites could be enhanced by 30% compared with the p-BNNS/paraffin and 10 times compared with paraffin. The current research provides fundamental suggestions for the selections of fillers to enhance the thermal conductivity of matrix.