Microstructure And Electrical Conductivity Of Polyacrylonitrile(PAN)-based Carbon Fibers During Carbonization

The preparation of polyacrylonitrile(PAN)-based carbon fiber mainly includes pre-oxidation and carbonization.The carbonization process is divided into low-temperature carbonization and high-temperature carbonization,which is usually carried out in an inert atmosphere of 400-1600°C.The chemical structure has changed greatly,such as the tensile modulus,thermal conductivity and electrical conductivity of carbon fibers.Different macroscopic properties of carbon fibers also indicate changes in their internal microstructures.In-depth study of the microstructure and properties of PAN-based carbon fibers during and after the production process of finished carbon fibers is crucial for improving the quality of carbon fibers.Therefore,in this paper,the microstructure and microstructure of carbon fibers with different moduli were studied by scanning electron microscopy(SEM),X-ray diffraction(XRD),Raman spectroscopy(Raman),element testing and resistivity testing during the carbonization process.The evolution of resistivity.The two parts of the work are researches on the microstructure and electrical properties,the first part is about different carbonization temperatures,and the second part is about different modulus.(1)A selection of carbon fibers with different tensile moduli from 230 GPa to 270 GPa.The results show that with the increase of the heat treatment temperature,the carbon content of the fiber increases rapidly,and the resistivity of the carbon fiber decreases with the decrease of the graphite interlayer spacing d₀₀₂,from 3.19×10^-5?·m to 2.12×10^-5?.m;during carbonization,the tensile strength of carbon fibers first increases and then decreases,which is due to the formation of an irregular turbostratic graphite structure.As the temperature increases,the graphite interlayer spacing decreases and the grain thickness gradually increases,the tensile modulus also increases gradually.Raman results show that the strength of D-Line is weakened,the strength of G-Line is increased,and the ratio of I_D/I_Gdecreases from 3.39 to 1.99.The chemical reaction converts sp³ hybrid carbon into sp²hybrid carbon atoms and the crystallite size becomes smaller.(2)A series of carbon fibers with different mechanical properties were selected,with tensile modulus ranging from 230 GPa to 270 GPa,and resistivity tests,X-ray diffraction and Raman spectroscopy tests were carried out on them.It can be obtained through experiments that the resistivity of carbon fibers ranges from The 2.76×10^-5?·m of 230 GPa gradually decreased to 2.2×10^-5?·m of 270 GPa.According to the XRD structure parameters,the interlayer spacing of graphite structure decreased from 0.35 to 0.346,and the stack thickness of graphite crystallites decreased from 1.54 to 1.51..It is shown that the carbon fiber resistivity also decreases as the crystallite structure becomes compact,which indicates that the graphite crystallites gradually become ordered from the turbine-like carbon structure,and the graphite planes will shrink.The mechanical properties of carbon fibers are related to the microscopic parameters of the degree of graphitization.The degree of graphitization is closely related to the microcrystalline structure of the fibers,and its effect on the tensile modulus is similar to that of the microcrystalline structure.As the degree of graphitization increases,the tensile modulus decreases in an approximately linear manner,and the enhancement of the degree of graphitization is beneficial to obtain high modulus.The resistivity of carbon fiber is also related to the modulus.The degree of graphitization is gradually ordered,and the sp² carbon structure is gradually formed.At the same time,with the gradual improvement of the graphitic carbon layer structure,the range of electron movement becomes larger,and the conductivity becomes better.decreasing gradually.The effects of crystal structure on tensile strength and elongation at break are similar,and the above data all indicate that the tensile modulus of carbon fibers is directly related to the degree of perfection of its crystal structure.To obtain high-performance carbon fiber,one of the ways is to improve its crystal structure parameters and perfect the ordered stacking of its disordered graphite structure.