Preparation And Thermal Protection Of High Thermal Conductivity C/C Composites

Recently,carbon/carbon composites are the most promising thermal management materials owing to its low density,low coefficient of thermal expansion,excellent mechanical properties and high thermal conductivity.However,the anisotropy,crystallite size,and orientation of graphite crystals are different,which results in obvious difference of thermal conductivity in carbon materials.Thus,to controll the size,orientation and continuity of graphite crystallites is the key to improving the highly preferred orientation thermal conductivity of carbon materials.In this paper,3D CFMP/C and 3D CFPPAN/C composites were fabricated by preform weaving,chemical vapor infiltration/deposition(CVI/CVD)and liquid phase impregnation pyrolysis(PIP)and graphitization using CFMP and CFPAN as reinforcements.Following,using CFMP and CFPAN as reinforcements,and mesophase pitch as binder,1D CFMP/C and 1D CFPPAN/C composites were also prepared by a hot-press process,as well as further carbonization,chemical vapor infiltration/deposition(CVI/CVD)and graphitization treatments.Subsequently,using blocky Si and SiO2 powder as raw materilas,the SiC coated 3D C/C and 1D C/C composites were prepared by a chemical vapor phase reaction process,and the 1D SiC/C/C and 3D SiC/C/C composites exihibit a SiC coating of dense and proper thickness.The microstructure,mechanical properties,thermal conductivity properties,oxidation properties and ablation properties of C/C composites were characterized by scanning electron microscopy(SEM),polarized light microscopy(PLM)and X-ray diffraction(XRD)analyses as well as,laser thermal conductivity meter and electronic universal material testing machine measurements.Compared to CFPAN(graphitization at 2250 ?),the graphite crystallite of CFMP(graphitization at 3000 ?)in 3D CFMP/C composites is more perfect and shows a higher orientation,higher graphitization degree(80.22%)as well as thermal conductivity(?800 W·m-1K-1).The graphite crystallite,graphitization degree(31.11%)of CFPAN(graphitization at 2250 ?)are lower.The crystal structure of 3D CFMP/C composite(Lc=12.36 nm,La=35.42 nm)is more integrit than the 3D CFPPAN/C composite(Lc=8.24 nm,La=21.41 nm),resulting in a higher graphitization degree of 3D CFMP/C composite(71.75%)than the 3D CFPPAN/C composite(22.89%).The thermal diffusivities and thermal conductivities of the two composites exhibit obvious anisotropy((X-Y)direction>Z direction).3D CFMP/C composite with a relative perfect graphite crystallite shows a higher thermal conductivity of X-Y direction at room temperature(220 W·m-1·K-1)than that of 3D CFPAN/C composite(36.13 W·m-1·K-1).However,the tensile strength of CFPAN(3.25 GPa)is higher than the CFMP(2.0 GPa).The interface of carbon fiber and carbon matrix in the two composites shows a proper strength.Thus,the bending fractures of both C/C composites are pseudoplastic fractures.Besides,the bending strength of 3D CFPPAN/C composite(146.01 MPa)is higher than that of 3D CFMP/C composite(99.15 MPa)due to the higher strength of carbon fiber in 3D CFPAN/C composite.No cracks could be observed between pyrolytic carbon and carbon fiber in 3D CFPAN/C composite,resulting from a relative consistent shrinkage of pyrolytic carbon and carbon fiber in the 3D CFPAN/C composite.However,a clear interface between pyrolytic carbon and carbon fiber in 3D CFMP/C composite could be seen,which is due to the big shrinkage difference of pyrolytic carbon and carbon fiber in the 3D CFMP/C composite.The anti-oxidation properties of 3D CFPAN/C composite are better than that of 3D CFMP/C composite due to a less physical defects of interface in the 3D CFPAN/C composite.The anti-oxidation properties of 3D SiC/CFPAN/C and 3D SiC/CFMP/C composites are approach after oxidation for 1 h at 1400?.However,the 3D SiC/CFMP/C composite(-0.5 ?m/s,0.061 mg/(cm2s))exihibits a better mass ablation rate and linear ablation rate than the 3D SiC/CFpAN/C composite(18.3 ?m/s,0.46 mg/(cm2s)),implying that the higher thermal conductivity of 3D SiC/CFMP/C composite is beneficial to the thermal transfer.In 1D C/C composites,the graphitization degree(76.1%)and microcrystalline size(La=55.55 nm,L,=26.46 nm)of 1D CFMP/C composite are higher than the 1D CFPAN/C composite(55%),(La=36.23 nm,Lc=11 nm),respectively.The thermal diffusivities and thermal conductivities of the two 1D C/C composites exhibit obvious anisotropy,which is the higher thermal conductivity of parallel to the carbon fiber than perpendicular to the fiber.After heat-treating at 3000 ?,the thermal conductivities of 1D CFMP/C and 1D CFPAN/C composites are 23.21 W·m-1·-1(perpendicular to the fiber)and 528.56W·m-1·K-1(parallel to the fiber)as well as 7.01 W·m-1·K-1(perpendicular to the fiber)and 154.96 W·m-1·K-1(parallel to fiber),respectively.It can be seen that the thermal conductivities of the former are obvious higher than those of the latter.Although the CFMP and CFPAN have an approach strength after heat-treating at 3000 ?,the bonding strength of carbon fiber and pyrocarbon in ID CFMP/C composite is better than 1D CFPAN/C composite,resulting in a higher bend strength of 1D CFMP/C composite.No obvious cracks were observed between pyrolytic carbon and carbon fiber in 1D CFPAN/C and 1D CFMP/C composites,which is due to the consistent shrinkage of pyrolytic carbon and carbon fiber during heating-treatment,1D CFMP/C composite possesses a relative perfect crystal structure,which results in a better oxidation properties of 1D CFMP/C composite.1D SiC/CFPAN/C composite exhibits a better oxidation properties than the 1D SiC/CFMP/C composite,which is due to a bigger thickness of SiC coating of 1D SiC/CFPPAN/C composite.However,after plasma flame ablation for 360s at 2300 0C,the 1D SiC/CFMP/C composite(0.101 mg/(cm2s),0.28 ?m/s)exihibits a better mass ablation rate and linear ablation rate than the 1D SiC/CFPAN/C composite(0.117mg/(cm2s),0.83 ?m/s),which is due to the higher thermal transfer of 1D SiC/CFMP/C composite.