Preparation Of Pitch-based Carbon Fibers With High Thermal Conductivity And Their Application In Three-dimensional Carbon/carbon Composites

Mesophase pitch based carbon fibers and their composite materials possessing characteristic of low density,low thermal expansion coefficient.It has excellent performance in conduction,heat conduction and mechanical properties.It has become a leader in the field of material application.It has great potential in military industry,civil field and so on.The ultimate performance of mesophase pitch based carbon fiber depends largely on its microstructure features,such as graphite layer orientation and order arrangement,graphite microcrystal parameters and structural defects.However,the microstructure of carbon fiber is directly related to the evolution of chemical structure in the process of pre-oxidation and the evolution of crystallization in the process of carbonization.Therefore,in this paper the evolution of chemical structure in the process of pre-oxidation and the evolution of crystallization in the process of carbonization and graphitization were studied,and the correlation between the final carbon fiber microstructure and its thermally conductive and mechanical properties were also investigated.Meanwhile,three-dimensional high thermal conductivity C/C composites based on mesophase pitch based carbon fibers were prepared and the effect of matrix carbon on thermal conductivity was discussed.The main conclusions of the study are as follows:1.The evolution of the chemical structure the fibers in the pre-oxidation process and the microstructure of corresponding graphite fibers were revealed by FT-IR,TG-MS and EDS analyses.The results show that the diffusion and reaction of oxygen determine the process and uniformity of pre-oxidation process.The uniformity of the oxygen distribution and the oxidation degree in the fiber are the fundamental reasons that affect the microstructure and properties of the carbon fiber.After the low temperature oxidation for long time,the transverse section of the fiber is homogeneous and the radial oxygen distribution gradient is small.After the high temperature oxidation for short time,the core and shell structure is easy to be formed and the radial oxygen distribution gradient is large.2.During the carbonization process,the dehydrogenation and deoxygenation process were carried out in the pitch with the formation of water,hydrogen,methane,carbon monoxide and carbon dioxide,leading to eventually formation of a lamellar graphite microcrystalline structure.After low temperature carbonization at 450℃,the heating rate of carbonization has little effect on the structure and properties of the final graphite fiber at the carbonization temperature of 500-1000℃,which provides a technical basis for shortening the carbonization time.3.The results show that the optimized pre-oxidation conditions for prepared pitch fiber are at 240oC for 6 h in air.The growth and development of graphite microcrystals and the orientation and ordered stacking of carbon layers is mainly affected by carbonization and graphitization process.The results show that after preoxidation at 240oC in air for 6h and low-temperature carbonization at 450oC,and subsequently graphitization at 3000oC,the tensile strength and Young’s modulus of carbon fibers can reach to 2.54 GPa and 650 GPa respectively with the resistivity of1.3μΩ?m.The thermal conductivity of graphite fiber can reach to 862.6 W/m?K by empirical formula conversion.Furthermore,drawing has a significant effect on the mechanical properties and thermal conductivity of graphitized fibers.After drafting,the tensile strength and Young’s modulus of graphite fibers at 2600 C are increased by24.2%and 11%respectively,and the thermal conductivity of graphite fibers at 3000 C is increased from 846W/m K to 962.6 W/m.K.4.The electrical and thermal conductivity of the obtained three-dimensional C/C composites showed anisotropy along the directions of X-Y and Z.The thermal diffusion coefficient of 3D C/C composite is about 159.3 m~2/s and 145.9 m~2/s in X and Y directions,and the corresponding thermal conductivity is about 234 W/m?K and 203.6W/m?K,respectively.The thermal diffusion coefficient in the Z direction of 3D C/C composite is 30.5 m~2/s,and the corresponding thermal conductivity is only 41.5 W/m?K.The mechanical properties of the obtained three-dimensional C/C composites were significantly higher than that of a two-dimensional C/C composite.The bending strength in the X and Y directions was as high as 176.9 and 158.3 MPa,respectively.The influence of matrix carbon on the thermal conductivity of 3D C/C composite materials was explained by the percolation theory in the material domain.