Investigation On The Relationship Between Microstructure And Properties Of High Performance Carbon Fiber

Carbon fibers as an important high-performance structural and functional material have many unique properties,which has been widely applied in the construction,aerospace,transportation,medical,energy development,leisure and other civil industries.In recent years,considerable progress has been made in the industrial production and scientific research of carbon fibers in our country,whereas the scientific research and industrialization in the field of high-performance carbon fibers are still imperative.Given the domestic overcapacity for low-end carbon fibers is on the horizon,newer and higher requirements for carbon fiber are putting forward by the downstream market.Overcoming the technical bottlenecks and making new progress in the domestic research will be the only way for our carbon fibers industry.Carbon fibers are mainly obtained through the pyrolysis of organic precursors,according to the types of precursors,there are polyacrylonitrile,pitch and rayon.PAN/Pitch-based carbon fibers is mainly prepared from acrylonitrile/petroleum pitch or coal tar pitch through polymerization/asphalt,spinning,pre-oxidation,carbonization and graphitization,among which carbonization and graphitization process are a critical process of crystallization and rearrangement of graphite sheets in carbon fibers,as well as to refine the crystalline structure.High-strength carbon fibers were graphitized at high temperature?>2200??can obtain high-modulus and high-strength high-modulus carbon fibers.With increase of carbon fibers modulus,the strength loss is obvious when preparing high modulus process.However,high strength and high modulus carbon fibers has the high tensile strength and high tensile modulus.The correlations between microstructure of carbon fibers and the process parameters are important for controlling the structure,properties,and quality.Research and regulation of its microstructure is a hotspot in this field.In this dissertation,we characterized the high performance carbon fibers and discussed the relationship between microstructure and performance of high strength and high modulus PAN/Pitch-based carbon fibers based on the investigated systematically.The relationship between microstructure and thermal conductivity of mesophase pitch graphite fibers were investigated.The main results of the present dissertation are as follows:?1?The structural characteristics and their correlations with properties of high strength and medium modulus carbon fiber:sires of carbon fibers,say similar in modulus but different in tensile strength,we selected and characterized,research the effect of microstructure on properties.The results showed that:As the size of graphite microcrystalline decreases,the layer spacing d_?002?increases profit the crystalline structure connected the amorphous area was arranged orderly along axis and disorderly along radial axis in carbon fibers,these structural features gave large elongation and high strength to carbon fibers.Due to the rearrangement of the structure,the densification in the fiber interior is improved and the transverse shrinkage of the fiber occurs,therefore the relaxation of residual strain changed the graphite layers from a wrinkled and distorted morphology to a straight and smooth one.This work could provide a theoretical support on structure control in high-strength and medium modulus carbon fiber preparation.?2?Relationship between microstructure and thermal conductivity of mesophase pitch-based graphite fiber:In this work,The microcrystalline structure and microvoids in carbon fibers were investigated by using laser micro-Raman spectroscopy?Raman?,X-ray diffraction?XRD?,Scanning electron microscopy and small X-ray scattering?SAXS?.The crystalline sizes?L_a and L_c?increased and interlayer spacing(d_?002?)decreased with increasing of graphitization temperature,the phonon mean free path also increasing.In addition,with the increased of microcrystalline sizes,the preferred orientation angle of graphite fiber along the fibers axis decreases,the fiber changes to a highly ordered ribbon structure and the preferred orientation along the fiber axis is significantly enhanced,which provides important conditions for the fiber with high thermal conductivity.The pore structure of the mesophase pitch-based graphite fibers has undergone the transformation from large number of micropores to a small number of large-sized pores and the arrangement develops to a high degree of order which is very important for improving the tensile modulus of carbon fiber.This work could provide a theoretical support for understanding the relationship between graphite microcrystalline structure and thermal conductivity.