Study Of Micrifying Graphite Microcrystalline Size Of Polyacrylonitrile Based Carbon Fibers

Polyacrylonitrile(PAN)based carbon fibers are used in many fields because of the excellent mechanical properties.The actual strength of carbon fibers is much smaller than the theoretical strength.It is found that the graphite microcrystalline is critical for the high tensile strength of carbon fibers.Larger microcrystalline size means larger void defect and less crosslinking.And all of these can cause low tensile strength of carbon fibers.Studies have proved suitable decrease of microcrystalline size is helpful to improve strength of fibers.We should study the formation mechanism of microcrystalline during production process of PAN based carbon fibers to adjust microcrystalline size by changing technological conditions,and get high performance carbon fibers finally.In this paper,interval carbonization and continuous carbonization were used to study the effect of tension and carbonization rate on microcrystalline structure of PAN based carbon fibers.The properties of microcrystalline structure were characterized by X-ray diffractometer(XRD)and Ranman spectrum.The relationship between microcrystalline structure and mechanical properties was analyzed.The microcrystalline structures of the fibers produced under different tension of low-temperature carbonization were analyzed and it was found that the microcrystalline length(La)and microcrystalline stack thickness(Lc)did not grow up at the same time.The formation mechanism of microcrystalline was analyzed according to the relationship between microcrystalline structure and mechanical properties.The macromolecules in the stabilization fibers are ladder-structures and they orientated along with axial direction.Reactions of non-carbon atoms desorption occurred between macromolecules lines and formed carbon-net during low-temperature carbonization process.At the same time,the carbon-net stacked and microcrystalline formed.The initial microcrystalline was small and there were many ladder-structures macromolecules lines at the edge.When the temperature rose,the ladder-structures transformed to carbon-net plane.At the end of low-temperature carbonization,there were many carbon-net tapes at the edge of microcrystalline.The carbon-net grew up and stacked during high-temperature carbonization process and the crosslinking graphite microcrystalline formed.The tension of low-temperature carbonization could influence the reactions between macromolecules lines and stacking to change the structure of carbon fibers.When the tension increased from 250 c N to 450 c N,the degree of orientation stayed constant nearly,but the microcrystalline size first decreased and then increased.The trend of tensile strength changing was opposite.The tension of high-temperature carbonization changed the microcrystalline structure by influencing the movement of carbon-net and microcrystalline.When the force increased from 500 c N to 700 c N,the trend of strength changing which was accordant with the degree of orientation was first increasing and then decreasing.Under the condition of high tension,larger microcrystalline mean larger void defect and less crosslink,and it would inhibit tensile strength.The effect of carbonization rate on the degree of orientation was not obvious.The microcrystalline size first decreased and then increased when the rate increased from 15m/h to 35m/h and the mechanical properties showed the same trend.On the whole,the microcrystalline was more structured and the mechanical properties were better when carbonization rate was lower.Therefore,it is effective measure to change tension of low-temperature carbonization and high-temperature carbonization and carbonization rate for micrifying microcrystalline size of carbon fibers.