Preparation And Properties Of Surface-modified Graphite Films Reinforced C/C Composites

As a carbon material with low density and excellent hightemperature mechanical strength,C/C composites have been widely used in aircraft brakes,rocket nozzles,space shuttle and related fields.With the dramatic development of space exploration technology,the electronic equipment in the aerospace vehicle are gradually developing towards light weight,miniaturization and high performance.They will inevitably generate excessive heat during the operation.In addition,the aircraft will also experience the test of high temperature when passing through the atmosphere,which poses a serious challenge to the heat dissipation system,and also causes higher demands for the thermal conductivity of C/C composites.Because of complete crystal structure and excellent thermalconductivity,high thermal conductivity graphite film is regarded as a promising reinforcement.However,the weak bond between the graphite film and the matrix is a problem that affects the properties of the composites.In this paper,defects were introduced on the surface of the graphite films by high-temperature etching treatment firstly.Then the graphite films reinforced C/C composites were prepared by the impregnation-carbonization process and the influence of surface etching on the graphite films and microstructure of C/C composites was studied.In addition,the influence of graphite film volume fraction on the thermal and mechanical performance of the composites were also studied.First of all,it was found in the research of the high temperature airetching that oxygen selectively etched the graphene sheet along a certain direction and introduced hexagonal pit-shaped defects into the surface of the graphite film.Oxygen also preferentially reacted with the unstable carbon atoms at the grain boundary and disordered elongated channels will be formed on the surface of the graphite film.Moreover,under the same temperature,the increase of the etching time increased the size of the defects.At the same time,high-temperature air etching did not introduce oxygen-containing functional groups into the graphite film but reduced the intensity of the(002)diffraction peak in the XRD pattern due to the introduction of defects.When the temperature of air increases to 700°C and above,the resistivity of the graphite film will increase due to the increase of aggravated structural damage.Subsequently,with the pristine graphite films reinforcement,C/Ccomposites were prepared by the impregnation-carbonization process.The results show that the chemically inert surface of the graphite films made it impossible to form an effective interfacial bond with the carbon matrix,there were many pores at the interface between the two.As the volume fractions of graphite films increased from 0%to 45%,the density of the composite showed a decreasing trend.The maximum flexural strength decreased fast and the fracture mode gradually changed from brittle fracture to ductile fracture.However,because the graphite films are effective thermal conductivity enhancers,the thermal conductivity of the C/C composite with a graphite films volume fraction of 45%was increased by 152%compared to the sample without graphite films.The infrared thermal images showed that when there were no graphite films in the composites,the heat would be accumulated at the heating end and cannot be effectively transferred.Moreover,the temperature difference between the heating end and the cold end was large.However,after adding the graphite films,the heat transfer efficiency of the composites was higher and the surface temperature was more uniform.The temperature difference between the two ends was smaller.Finally,with the surface-modified graphite films after being treatedat 650°C,C/C composites were prepared.The results show that the surface-modified graphite films formed an effective interfacial bond with the carbon matrix.The formation of the stronger bond may be attributed to the rivets,which were originated from the pitch.In the impregnation process,the pitch infiltrated into the defects on the surface of the graphite films and formed a large number of micro rivets after solidification and carbonization.Therefore,these rivets strengthened the interface bond.Moreover,the microstructure at the interface shows that the pore diameter at the interface of the modified graphite films reinforced composites is smaller,and continuous carbon matrix interface can be observed.Among the composites with the same volume fractions of graphite films,the density of the composite prepared with graphite films after being treated at 650°C for 90 min increased to 1.4189g/cm~3,and the flexural strength increased to 213.59 MPa.In addition,pitch rivets could be used as channels for heat transfer and reduce the thermal resistance of interfacial contact between layers.Therefore,the heat transfer efficiency of the composites in the vertical direction has been improved.In summary,the high-temperature air etching treatment is a convenient,fast and effective surface modification method,which can effectively enhance the surface roughness of the graphite films and the interface bonding with the carbon matrix.Therefore,it can further improve the macroscopic thermal conductivity and mechanical properties of the composites and provides a basis for further exploration and application of high thermal conductivity C/C composites.