The Preparation of Three-dimensional Graphene Networks for Thermally Conductive Composites

Author:An Fei

Supervisor:yu zhong zhen


Degree Year:2018





With the integration and miniaturization of electronic devices,timely dissipation of excess heat during their highly efficient running becomes a great challenge,which deeply influences the performance and lifetime of electronic.Thermal interface materials(TIMs),composed of polymer and thermally conductive fillers,are highly required to solve the problem of excess heat dissipation.Compared to conventional thermally conductive fillers,such as metal nanoparticles,boron nitride platelets and carbon nanotubes,graphene is a highly promising candidate as its ultrahigh in-plane thermal conductivity of~5300 W m-1 K-1.However,its polymer based composites usually exhibit less satisfactory thermal conductivities,which are mainly caused not only by the undesired thermal resistance between loosely contacted graphene sheets but also the low content of graphene fillers.To solve this problem,construction of continuous graphene networks with high density before compounding with polymers has been confirmed to be an effective way.Besides,the quality of graphene building blocks is also vital for ultimate thermal conductivity of graphene composites.Therefore,we prepared three-dimentional graphene networks with high density through hydrothermal reduction followed by air-drying.After annealed at high temperature,the high-quality graphene networks were compounded with polymers and the resultant composites exhibited excellent thermal conductivity,which could be applied in thermal management as well as solar-thermal conversion and storage.1.The preparation of graphene/boron nitride hybrid aerogels for anisotropically thermally conductive epoxy composites:Although graphene aerogels have great potentials in preparing heat dissipation composites on the basis of their continuous thermally conducting networks,their low density and isotropic architecture hinder further improvement of the thermal conductivity of their composites.Herein,highly anisotropic graphene/boron nitride(BN)hybrid aerogels with a long-range ordered architecture and moderate density are prepared for the first time by hydrothermally treating the suspension of graphene oxide sheets and BN nanoplatelets,air-drying the resultant hydrogels,and thermally annealing the highly anisotropic aerogels at 2000℃.During the hydrothermal treatment,the chemically reduced graphene oxide(RGO)sheets are self-assembled into the highly anisotropic and long-range ordered network,while the BN nanoplatelets are distributed between the aligned RGO sheets to prevent the excessive volume shrinkage of the aerogel and retain its anisotropic porous structure with high porosity during the air-drying.Furthermore,the RGO/BN hybrid aerogel is thermally annealed at 2000℃ to fully remove the residual oxygen-containing groups and heal the defects on its RGO component.The thermally annealed hybrid aerogel is highly efficient in enhancing the thermal conductivity of epoxy resin,and the resultant composite exhibits an ultrahigh through-plane thermal conductivity of 11.01 W m-1K-1 with an excellent thermal conductivity enhancement of 277%.2.The preparation of graphene hybrid aerogels with high quality for anisotropically thermally conductive epoxy composites:Although graphene based thermal interface materials(TIMs)have great potentials in removing excess heat generated during highly efficient running of electronic devices,their practical applications are usually limited by their unsatisfactory thermal conductions,which are mainly caused by unsatisfactory dispersion and distribution,low loading and low quality of graphene sheets,as well as the thermal interfacial resistance between graphene sheets and polymer matrix.Herein,we develop vertically aligned graphene hybrid aerogels(GHAs)with high density by hydrothermal reduction of graphene oxide in the presence of high-quality graphene nanoplatelets(GNPs)followed by air-drying.The reduced graphene oxide sheets play an important role in constructing a vertically aligned interconnection network for accommodating GNPs during the hydrothermal reduction process,while the incorporated GNPs not only make the thermal conductance network denser but also prevent excessive shrinkage of the aerogels during air-drying.More critically,graphitization of GHAs at 2800℃ removes the residual oxygen-containing groups and heals the defects of their reduced graphene oxide component,leading to high-quality graphene aerogels.The resultant vertically aligned high-quality graphene porous architecture with high density as an ideal thermal conductance network of TIMs is thus highly efficient in improving thermal conductivity of its epoxy composite,which exhibits an ultrahigh through-plane thermal conductivity of 35.5 W m-1 K-1 at a graphene loading of 19.0 vol%.The excellent thermally conductive performance makes the annealed GHA/epoxy composites suitable for the thermal management.3.The preparation of graphene aerogels with high quality for isotropically thermally conductive phase change composites:solar energy,a common renewable energy,can be collected by phase change materials(PCMs)through solar-thermal conversion and storage.However,traditional PCMs exhibit low thermal conductivity and poor shape stability,which results in low heat transfer rate and thus restricts the efficiency of solar-thermal conversion and storage.Herein,we prepared high density graphene aerogels with isotropic structure through hydrothermal reduction of the suspension containing graphene oxide and graphene nanoplatelets(GNPs)followed by air-drying.The reduced graphene oxide constructs the isotropic three-dimentional networks to accommodate GNPs,while GNPs reinforce the networks to avoid excessive shrinkage during air-drying.After annealed at 2800℃,high quality graphene aerogesl were obtained by removing residual oxygen functional groups and restoring the lattice defects.These high quality aerogels endow octadecanol PCMs with high thermal conductivity of 9.5 W m-1 K-1 at a graphene loading of 13 wt%.Meanwhile,these PCMs exhibit excellent shape stability and high latent heat of fusion(209 J g-1).Furthermore,they could effectively convert solar energy into thermal energy,which is stored as latent heat,with extraordinary solar-thermal efficiency of 89.2%.