Study On Fabrication Of In-situ CNT/graphene Composite Film And Its Thermal Transport Properties

Microelectronics,aerospace and high-energy weapons have stimulated higher demands on thermal management materials of high thermal conductivities.Sp~2 hybrid carbon based nano-materials such as carbon nanotubes(CNTs)and graphene,are excellent thermal conductors due to their regular carbon six-membered ring lattices.The graphene film formed by stacking graphene sheets shows outstanding and anisotropic thermal properties,with high in-plane thermal conductivity but poor out-of-plane thermal conductivity.Since CNTs and graphene are consisting of similar unit cells,CNTs are proposed to enhance the out-of-plane thermal conductivity of graphene films in this work.Firstly,the preparation process of graphene film with high thermal conductivity was explored.The CNT/graphene composite film was prepared by solution mixing and in-situ growth,respectively.The thermal transporting mechanism of carbon nanotube/graphene composite film was studied by molecular dynamics(MD)calculation.Finally,the application of the CNT/graphene film on the heat dissipation of the LED chip was explored.The research contents mainly include:(1)The graphene oxide(GO)film was prepared by the casting process.It was found that the GO concentration and drying temperature showed a great influence on the GO film.The effects of GO pre-reduced by stepwise thermal reduction,HI acid reduction and hydrazine hydrate reduction methods were compared.The microstructures and thermal conductivities of the reduced graphene oxide(RGO)films were characterized.The results show that stepwise thermal reduction method is the most advantageous.The RGO film obtained by graphitization and compression treatment exhibits foldable performance and highly in-plane thermal conductivity of～928 W/mK.(2)A uniformly dispersed CNT/GO film was prepared by solution mixing method.A flexible CNT/RGO composite film with good structure was obtained by stepwise thermal reduction and graphitization.The reduced thermal conductivity suggests that CNTs in the CNT/GO film prepared by the blending method do not enhance the heat conduction across the graphene layers.Due to the incorporation of CNTs,the interface within the film increases and the graphene inter-layer distances were enlarged,which result in a decrease in both in-plane and out-of-plane thermal conductivities.(3)With the intercalation of polymer carbon source and metal catalyst,CNTs were in-situly growed in the confined inter-gallery spaces between graphene sheets through thermal anneal.The reaction mechanism and the microstructures of the in-situ growth of CNTs were studied.The investigation demonstrates that the CNTs are covalently bonded to graphene sheets.Pyrolysis ferrocene(Fc)can be used to prepare small particle catalysts with narrow size distribution range,which is beneficial for regulating the growth density of CNTs.By adjusting the amount of Fc and PMMA,the out-of-plane thermal conductivity is improved,while the in-plane high thermal conductivity is retained.When the GO,Fc and PMMA incorporation ratio is1:0.001:0.001,the in-plane thermal conductivity of the prepared CNT/RGO composite film is 933 W/mK,and the out-of-plane thermal conductivity is 5.41 W/mK.Compared with the RGO film,the out-of-plane thermal conductivity of the CNT/RGO composite film is increased by 32%.(4)In order to further explore the thermal transporting mechanisms,CNT/graphene composite/hybrid film molecular models with CNTs non-covalently and covalently bonded to the graphene sheets were both constructed.Then the heat transporting behaviors of the films were simulated by MD simulations.The simulation results show that CNTs combined by weak intermolecular forces cannot enhance the heat conduction across graphene layers.When the CNTs are covalently bonded to graphene sheets,the thermal resistance at joints is reduced to 9%of the joint without CNT linkers,which exhibits greatly enhancement in thermal conduction across the graphene layers.The increase of CNT numbers or diameters will enhance the heat conduction across graphene layers,but result in more in-plane phonon scattering.These two effects comprehensively determine the conduction resistance across the graphene layers.(5)By bonding the CNT-graphene composite film with thickness of 20?m to interface between LED baseplate and heat sink,the temperature of LED chip decreased by～6.7℃with full working load of 3 W,which showed that the composite film has great application potential in the thermal management field of electronic products.