Simultaneous Exfoliation And Flame-Retardant Functionalization Of Hexagonal Boron Nitride And Its Epoxy-based Thermal Conductive Composites

The integration and miniaturization of current electronic equipment lead to a sharp increase in the heat density per unit area,which makes them face more thermal failure and fire risk than before.In recent years,polymeric materials have gained attention as packaging materials because of their light weight,low cost and flow processing.However,the intrinsic high flammability and low thermal conductivity（TC）significantly limit their applicability in microelectronic packaging and other new fields.Hexagonal boron nitride（h BN）has shown great application potential in the preparation of highly flame-retardant polymer-based thermally conductive composites,due to its high thermal conductivity,excellent thermal stability,and superior antioxidation ability.However,the strong interlayer force and surface inertness of h BN make it difficult to achieve effective peeling and flame retardant functionalization.Accordingly,in this paper,the ball milling process as the h BN exfoliating method to simultaneously achieve the edge doping of the flame-retardant groups and use it as a high-performance filler to prepare EP-based polymer composites with high thermal conductivity,high flame retardancy and electrical insulation properties.the main work are as follows:（1）h BN was simultaneously exfoliated and flame-retardant functionalized into Boron nitride nanosheets（BNNS）via one-step ball milling process based on the synergetic effect of mechanical shear and chemical peeling of ammonium phosphate and sodium hydroxide.Using it as a multifunctional micro/nano filler and epoxy resin（EP）as polymer matrix,EP/BNNS composites with high flame retardancy and thermal conductivity were prepared through the process of"solution blending-programmed curing".The dramatical enhancement in flame retardancy was mainly attributed to the catalytic charring effect and physical barrier action of flame-retardant functionalized BNNS,led to the formation of a compact and robust char layer during combustion to protect the underlying polymer.The flame retardant functionalised BNNS exhibits excellent catalytic charring and physical barrier effects,forming a dense and hard char layer that effectively reduces the exchange of internal decomposition volatiles and external heat between the gas/solid phases of the composite,preventing the spillage of volatile products and enhancing the flame retardancy of the EP/BNNS composites.Simultaneously,due to uniform dispersion and strong interfacial adhesion,the incorporation of BNNS not only increased the thermal conduction paths by increasing specific surface area,but also reduced the interfacial thermal resistance（Rb）caused by phonon scattering,leading to an enhancement in the TC of EP/BNNS composites.（2）By optimising the ball milling process,[bmim][PF₆]ionic liquid（IL）was chosen as the abrasive to buffer the strong collisions and impacts generated during the ball milling process,while obtaining micron-scale flame-retardant functionalized boron nitride nanosheets（BNNS@IL）.Furthermore,a highly oriented and flexible EP/BNNS@IL composite film is prepared using a cyclic layer-by-layer coating method,thus building interconnected BNNS thermal conductivity pathways and forming an effective physical barrier,giving it high anisotropic thermal conductivity,flame retardancy and good mechanical properties.The composite film obtained via cyclic layer-by-layer coating method,BNNS@IL is highly oriented along the in-plane in the EP matrix to form an overlapping filler network,which effectively improves the heat conduction path,reduces the interface thermal resistance caused by phonon scattering,and realizes the composite high in-plane thermal conductivity of the material(8.3 W m^-1K^-1)Simultaneously,the addition of flame-retardant functionalized BNNS@IL catalyzes the formation of a dense and complete char layer during the combustion,combining the physical barrier formed by the highly oriented BNNS@IL,effectively reduces the penetration of external heat and oxygen into the interior,while inhibiting the diffusion of internal decomposition products（fuel）to the exterior,thus improving the flame retardancy of the EP/BNNS@IL composite film.This paper is promising to provide a viable technical route and theoretical guidance for the efficient exfoliation and simultaneous functionalization of BNNS,as well as the preparation of high performance thermally conductive and flame retardant composites.