Preparation Of Graphene From Graphite By Supercritical CO₂ Exfoliation And Its Application In Thermal Management

Graphene is a two-dimensional material with a single layer of carbon atoms,and has many excellent properties.Therefore,the study of graphene has attracted wide attention since the advent of graphene.Graphene has the highest value of thermal conductivity among all materials,which makes it favorable for the thermal management applications.However,the methods of preparation of graphene,including mechanical exfoliation method,chemical vapor deposition method,graphite oxide reduction method and solvent exfoliation method,cannot prepare high quality graphene in large quantities,thus hindering the progress of the application of graphene.In addition,the current researches on graphene thermal conductivity materials mainly concentrate in the pure graphene film and graphene/polymer composites.While the thermal conductivity of pure graphene film with high thermal conductivity is too fragile for applications and the low thermal conductivity graphene/polymer composite can hardly meet the requirement of thermal dissipation.Therefore,it is of great significance to find an effective way to prepare massive high quality graphene,then to fabricate graphene-based composite with both high thermal conductivity and excellent mechanical properties.In this work,a lightweight and flexible composite paper has been successfully fabricated by incorporating nanofibrillated cellulose?NFC?with graphene prepared by the exfoliation of expanded graphite using shear-assisted super critical carbon dioxide.This exfoliation method is found effective to prepare high quality graphene with the yield of few layer graphene higher than 95%.The X-ray diffraction and Raman spectroscopy results show that the exfoliation process causes few damages to the structure of graphene,which indicates the prepared graphene is of high quality.The graphene is partially oxidized by the mixed acid to increase its solubility in water.The Raman spectra and thermogravimetric curves of the graphene samples before and after acid treatment show that there are some defects introduced by the acid treatment.In this paper,cellulose nanofibers are prepared from Kraft paper.The diameter of nanofibers is between 2 and 50 nm and the length is several micrometers.The flexible composite thermal film was prepared by a filtration process.The experimental results show that the mechanical properties of graphene films are greatly improved with the combination of nanofibers.The tensile strength of pure graphene film has the value under 5 MPa.While the mechanical strength of composite film with cellulose content of 25 wt%reaches up to 40.1 MPa.In addition,the composite films have excellent thermal conductivities.The in-plane thermal conductivity of the composite film with graphene content of 10 wt%is12.4 W m^-1 K^-1,which is much higher than that of the traditional graphene polymer composite.Our findings provide a promising candidate,manufactured by a facile and low-cost process at industrial scale,for lateral heat dissipation applications.