The Effect Of Carbonization Stretching On The Structure And Properties Of Mesophase Pitch-based Carbon Fibers

Mesophase pitch-based carbon fibers have excellent performance such as high modulus,high thermal conductivity,low bulk density,and low thermal expansion coefficient.They are widely used in aerospace,automotive industry,new energy technology,sport equipment and other fields.The physical properties of carbon fibers are closely related to the internal microstructure(such as graphite microcrystallite size,graphite layer orientation and structural defects,etc.),and are greatly affected by high temperature treatment processes.The carbonization stage is a critical period for the formation of turbostratic structure inside the carbon fibers.The adjustment and control of the fiber structure(such as stretching treatment)during the carbonization process is very vital for the subsequent development of graphite microcrystalline structure,thereby improving the final physical properties of resulting carbon fibers.In this thesis,two different carbonization methods(i.e.,batch carbonization stretching and continuous carbonization drawing)were used to treat mesophase pitch-based low-temperature heat-treated fiber bundles(about 20?m in diameter,?250 filaments,and around 1 Km in length).The effects of process parameters such as carbonization temperature,time and stretching force on the microcrystalline structure evolution of mesophase pitch-based carbon fibers,and their mechanical properties,electrical conductivity and thermal conductivity,were mainly investigated.The main conclusions of this research work are as follows:(1)For batch carbonization stretching on fixed-length fiber bundles,it is found that applying appropriate tension on the fiber bundles during the carbonization process helps to improve the internal structure of carbon fibers.The graphite-like microcrystals develop well.The interlayer spacing d₀₀₂ reduces and the microcrystalline size increases,the micropores inside the fibers become small and narrow,and the ordered degree of the graphite-like microcrystals along the fiber axis increases,thereby improving the tensile mechanical properties and electrical-thermal conductivity of the carbon fibers.However,the stretching force during the carbonization process should not be too large,otherwise it would cause more defects in the fibers(such as excessive stress leads to fiber breakage and increases splitting angle),which will reduce the physical properties of carbon fibers.(2)In the batch carbonization process,a"one-stage"constant tension treatment at the temperature range of 400?1000?was adopted on the fiber bundles,and the tensile strength of carbonized fibers increases first and then decreases with the stretching force(0?4%).The mechanical properties of the fibers obtained by 2%stretching force are better:the graphitization degree of 3000?-graphitized fibers is87.21%,the microcrystalline orientation degree is 85.74%,the tensile strength and tensile modulus are 1.32 and 422.11 GPa,and the axial electrical resistivity and thermal conductivity are 2.06??�m and 635.93 W/(m�K).Based on the thermal decomposition behavior and scientific law of mesophase pitch and its derived fibers,a"two-stage"variable tension heat treatment was adopted,i.e,1%and 2%tensions were applied in the low temperature stage(400?700?)and high temperature stage(700?1000?),respectively.Compared with the"one-stage"stretching carbonization,the"two-stage"is more conducive to the growth of the fiber crystalline structure and the improvement of fiber physical properties.The final graphitization degree of the obtained mesophase pitch-based graphite fibers reaches 91.86%,and the microcrystalline orientation degree is 85.94%,the tensile strength and tensile modulus are 1.39 GPa and 435.72 GPa,the axial electrical resistivity and thermal conductivity are 1.99??�m and 653.64 W/(m�K).(3)In the continuous carbonization stretching process for the long filaments,compared to the use of constant temperature in each temperature zone(fixing other conditions such as the stretching ratio),gradual heating(i.e.,the temperatures of the four continuous temperature zones of the long carbonization furnace were set as 500-580-680-780?)is more conducive to the increase of fiber microcrystallite size and orderly stacking of microcrystals,thereby enhancing the mechanical properties and electrical-thermal conductivity of carbon fibers.The longer the carbonization time,the larger the microcrystallite size of the obtained carbon fibers,the better the orientation of the graphite-like sheets along the fiber axis.Therefore,the physical properties(such as tensile strength,tensile modulus,and electrical-thermal conductivity)of carbon fibers are significantly improved,such as the mechanical properties of the graphitized fibers after carbonization for 40 minutes are about 10%higher than those of the corresponding 4-minute-carbonization sample.When the carbonization stretching ratio is 0.965,the physical properties of the obtained carbon fibers are better,and the stretching ratio should be appropriate,otherwise the physical properties of the fibers would decrease.The fiber bundles was subjected to firstly gradual heating from 500 to780�C at a hot-stretching ratio of 0.965 for 40 minutes,and then 1000�C carbonization and 3000�C graphitization treatment,the tensile strength and tensile modulus of the obtained mesophase pitch-based graphite fibers increase to 1.59 GPa and 444.06 GPa,and the axial electrical resistivity and thermal conductivity are 1.66??�m and 768.95 W/(m�K).(4)Stretching treatment on the mesophase pitch-based fiber bundles during the low-temperature carbonization stage exerts a significantly effect on the control of internal structure of carbon fibers.It can effectively promote the development of the microcrystalline structure of the mesophase pitch-based carbon fibers,and reduce the crystal plane spacing of graphite-like microcrystals.The microcrystallite size increases,the stacking order of the microcrystals improves,the microcrystalline orientation along the fiber axis becomes better,the original wrinkles,distortions and defects on the graphite-like layers are weakened,furthermore,the micropores inside the fibers become thinner and narrower,which significantly improves the final carbon(graphite)fibers'tensile mechanical properties and electrical-thermal conduction performance.