Preparation Of Naphthalene Mesophase Based Carbon Fibers And Carbon Fiber/Carbon Composites With High Thermal Conductivity

With the continuous development of science and technology,all kinds of high-tech industries have higher and higher requirements for heat dissipation materials.Traditional conductivity materials have been unable to meet the growing demands for heat dissipation.Pitch-based carbon fibers and their carbon/carbon(C/C)composites have excellent properties including light weight,high thermal conductivity,high modulus and low thermal expansion,so C/C composites have become a research hotspot in the field of heat management because of these unique performance advantages.The microstructure of carbon materials has an important influence on their performance.Moreover,the final microstructure of pitch-based carbon fibers and their C/C composites largely depend on the properties of raw materials.Therefore,the effect of pitch molecular structures on the structure and performance of their carbon fibers was studied and the effect of carbon fibers and matrixes on the structure and performance of C/C composites was further discussed.The main research contents are as follows:(1)High-performance carbon fibers were prepared from naphthalene-based mesophase pitches synthesized by HF/BF₃ catalytic one-step method(AR-MP)and AlCl₃ catalytic two-step method(N-MP),respectively.The effect of naphthalene-based mesophase pitches with different molecular structures on the structure and performance of their carbon fibers was studied.AR-MP shows a semi-rigid rodlike molecular configuration and contains more naphthenic structures and methyl side chains.The pre-oxidized fibers derived from AR-MP show a better carbon layer orientation,thus their graphitized fibers have higher thermal conductivity of 716 W/(m·K).N-MP with higher aromaticity possesses a rigid disclike molecular configuration.Therefore,N-MP based graphitized fibers have higher tensile strength of 3.47 GPa due to its carbon fibers with fewer defects during the subsequent heat treatment process.(1)P25 carbon fibers,HNU-1 carbonized fibers and HNU-2 graphitized fibers were used as reinforcements.Three kinds of one-dimensional C/C composites are prepared by CVI and pitch impregnation.The influence of carbon fibers on the structure and performance of C/C composites was studied.The mechanical property and thermal conductivity of C/C composite are low due to the small crystallite size and low strength of P25.HNU-1 fibers shrink and split during heat treatment.The mechanical property and thermal conductivity of C/C composite decrease due to the formation of pores at the interface between the fibers and matrix carbon.HNU-2fibers are tightly coupled with matrix carbon and have a larger crystallite size,so the thermal conductivity and bending strength of C/C composite can reach 715 W/(m·K)and 190.2 MPa,respectively.(3)The low-temperature carbonized pitch-based carbon fibers(X,Y direction)and high-strength PAN based carbon fibers(z direction)were woven into a three dimensional preforms,which were used to prepare three-dimensional C/C composites by hot-pressing,mesophase pitch impregnation and coal pitch high pressure impregnation.The influence of carbon fibers and matrix carbon on the structure and performance of C/C composites was studied.The untreated fibers break during the hot-pressing process due to their low strength.The heat transfer channels and the main load bearing parts of the composite reduce,so the mechanical property and thermal conductivity of the composite decrease.However,the strength of carbonized fiber is higher,which ensures the continuity and compactness of the C/C composite,so the thermal conductivity and bending strength of C/C composite can reach 400W/(m·K)and 90.4 MPa.The high content of disordered carbon in isotropic pitch carbon reduces the thermal conductivity of C/C composite.However,the crystallite size of C/C composite is larger due to the higher graphitization degree of the mesophase pitch carbon,so the thermal conductivity can reach 362 W/(m·K).