Shear-assisted Production Of Few-layer Boron Nitride Nanosheets By Supercritical CO₂ Exfoliation And Its Use For Thermally Conductive Composites

Layered nanomaterials with their intriguing properties are of great research interest nowadays.Hexagonal boron nitride nanosheet?BNNS?,called white graphene,is one of the primary two-dimensional nanomaterial,which is an analogue of graphene,and possesses various attractive properties,such as high intrinsic thermal conductivity and electrical insulation properties.Therefore,BNNS have broad applications ranging from electronics to energy storage.However,large-scale production of BNNS remains a challenge due to the low yield of current established methods.In this paper,the preparation of BNNS has been realized by a shear-assisted supercritical CO₂ exfoliation process,during which supercritical CO₂ intercalates and diffuses between boron nitride layers,and then the exfoliation of BN layers is obtained in the rapid depressurization process by overcoming the van der Waals forces.What's more,the method is green,pollution-free and easy to be amplified.In this paper,the particle size and layer number of BNNS are characterized in detail.It is found that the produced BNNS is well-dispersed in isopropyl alcohol?IPA?with a higher extinction coefficient compared with the bulk BN.Based on the preparation of BNNS by supercritical CO₂ exfoliation,the application of BNNS in the field of thermal materials is further investigated.The experimental results show that the thermal conductivity of BNNS/Epoxy is nearly 130%higher than that of pure Epoxy.Furthermore,we fabricate a three-dimensional?3D?filler by in-situ growth of CNTs on the surface of BNNS?BNNS/CNT?by chemical vapor deposition method,whose thermal conductivity enhancement is as high as 162%compared with that of BNNS/Epoxy.Meanwhile,the BNNS/CNT/Epoxy remains a high electrical resistivity of more than 1 Mohm·cm,which is important to avoid short-circuit in electronics.The tensile strength of the composite increases nearly 4 times with the increase of CVD time.These experimental results indicate that the fabricated BNNS/CNT has great potential for reinforcement of composites with wide applications.