The Preparation Of Graphene-Based Composites And Their Thermal Performances

With the rapid development of science and technology,the thermal management system is applied to various fields including modern industry and electronic devices,and plays an important role in the dispersion,conversion and storage of thermal energy.Thermal management material is the core technical factor of thermal management system.Therefore,the design and preparation of effective thermal management material with excellent thermal conductivity has become one of the key issues to promote the development of science and technology.Among many thermal conductive materials,graphene is considered as an ideal candidate due to its high theoretical thermal conductivity(5300 W m-1 K-1)and excellent mechanical properties.However,in previous research,graphene sheets are usually dispersed randomly in composites,which causing large thermal resistance and low thermal conductivity.Fortunately,pre-construction of three-dimensional(3D)graphene structure effectively reduces the interface thermal resistance and contact thermal resistance,but the thermal conductivity is still far from the theoretical value.In order to further address these problems,this work mainly constructs an oriented 3D graphene network by freeze-casting method,and prepares corresponding phase change energy storage materials and heat dissipation materials.The main research contents are as follows:(1)To solve the problems of low thermal conductivity,high filling content and low solar-thermal conversion efficiency of traditional phase change composites,high-quality graphene aerogel with highly anisotropic structure were constructed by directional freezing method and the phase change energy storage material was prepared by vacuum-assisted impregnation method.First of all,the graphene network prepared by directional freezing method possesses an oriented structure,which effectively reduce the thermal resistance;What's more,polyimide was introduced to connect the adjoined graphene sheets as a solder during graphitization,which further reduce the edge phonon scattering of graphene and hence producing an effective path for thermal transmission;Finally,oriented 3D graphene structure endows composite with high thermal conductivity and solar-thermal conversion ability in the situation of remaining the high latent heat storage capacity.Through the test,the obtained phase change composite exhibits a high longitudinal thermal conductivity of 4.66 W m-1 K?1 with only 0.51 wt%graphene and an excellent thermal conductivity enhancement of 6085.2%,which ranks the highest at similar filling content among previously reported literature.Furthermore,the latent heat is even increased to 103.6%when compared to pure PCM due to nucleation of the network.Beyond that,the latent heat retention only decreases by 0.5%after 200 melting-solidification cycles,showing an excellent circulation performance.During illumination of 200 mW cm-2,the composites completed the charging period only in 1147 s,representing the excellent solar-thermal conversion capability.(2)To improve the thermal conductivity of graphene-based thermal conductive films,in this part,high-quality graphene aerogels were fabricated in advance by bidirectional-freezing,freeze-drying and graphitization treatment,followed by mechanical pressing process to obtain graphene films.To prove the advantages of this method,comparative samples were prepared by vacuum suction filtration and natural drying method.By contracting,it's verified that during the bidirectional freezing process,the graphene sheets were extruded by ice crystals to form an extremely thin and dense cell wall.Even during the graphitization process,the dense cell wall will not form bubbles which resulting from the removal of the oxygen-containing functional groups and hence causing a decrease in thermal conductivity.What's more,graphene sheets were compacted together to reduce contact thermal resistance,leading a better thermal and mechanical performance;In addition,the introduction of a small amount of polyimide can reduce the edge thermal resistance and further improve the thermal conductivity.Through the test,the graphene film exhibits a high thermal conductivity of 1053.4 W m-1 K-1,which is higher than comparison samples.In addition,the tensile strength of the film reaches at 27.3 MPa and exists excellent electrical conductivity.