High Performance Structural Modulation Of Carbon Fibers With Low Carbon Content

The excellent mechanical properties of polyacrylonitrile（PAN）-based carbon fibers come from their unique intrinsic structure.When the structure of carbon fiber is in the state of"turbostratic graphite"structure,carbon fibers exhibit high tensile strength and general tensile modulus.When the structure of carbon fiber is in the state of graphite-like structure with better regularity,carbon fibers exhibit high modulus and high strength with high modulus.Design methods to adjust the mechanical properties of carbon fibers have been one of the most important research interests of many scholars for a long time.Under the traditional process,the formation of graphite structure supporting the high performance of carbon fibers requires at least 92%of the carbon content,for which a carbonization temperature of at least 1300°C is required.Under this temperature,PAN molecules undergo complex chemical reactions and structural evolution,carbon fibers form a characteristic structure of"turbostratic graphite"which is conducive to its high mechanical properties.But the complex composition and structural evolution make the yield is only about 52%from precursors to carbon fibers.A large number of functional groups and structural groups are cleaved and removed,which not only leads to low utilization of precursor and high preparation cost,but also causes environmental pollution.PAN molecules undergo a series of complex reactions such as cyclization of cyano,dehydrogenation and cross-linking under the coupled heat/oxygen environment（also called pre-oxidation process）to generate a multi-structure system of PAN pre-oxidized fibers with linear and cyclized structures,conjugated and non-conjugated structures,and oxidized and non-oxidized structures coexisting,which becomes the basis of the ultimate structure of carbon fibers.And the radial/axial distribution of multi-structures in PAN pre-oxidized fibers determines the unique radial structure distribution and tensile properties of carbon fibers.By regulating the structural composition of PAN precursor during the pre-oxidation process,the structural recombination efficiency of PAN fibers during the carbonization process will be effectively improved,which is expected to obtain more"turbostratic graphite"when relatively low carbonation cleavage occurs.By homogenizing the radial structure during the preparation of PAN precursors,The radial distribution of PAN pre-oxidized structure can be optimized,so that the content of"turbostratic graphite"structure in PAN carbon fibers can be further improved,which makes PAN carbon fibers exhibit higher mechanical properties at lower carbon content.The paper uses the continuous pre-oxidation and carbonization experimental device,combined with ¹³C-NMR,Raman,XRD,elemental analysis and other testing methods.The effect of temperature,temperature/tension and heat/oxygen coupling on the composition-structure of PAN fibers during the pre-oxidation was investigated.The formation pattern of PAN pre-oxidized structure and its influence on the formation of"turbostratic graphite"structure of carbon fiber were revealed.In addition,the radial distribution of the multi-structures of PAN pre-oxidized fibers and its effect on the content of the"turbostratic graphite"structure of carbon fibers were investigated by homogenizing the radial structure of PAN precursors.The results show that:（1）Temperature is an important factor in the structural recombination and refinement of the"turbostratic graphite"structure of carbon fibers.The best mechanical properties of carbon fibers were achieved when the carbonization temperature was 1400°C,with a tensile strength of 4.50 GPa and a tensile modulus of 261 GPa.The carbon content in the fibers was accumulated to96.46%,but the carbonization yield was only 52.57%.With the decrease of carbonization temperature from 1400°C to 1000°C,The perfection of graphitized and microcrystalline structures of carbon fibers was weakened,the degree of orientation was reduced,so that the mechanical properties of the fibers gradually decreased.When the pre-oxidized fibers was carbonized at the temperature of 1000℃,the carbon fibers showed the worst mechanical properties,with a tensile strength of only 2.16 GPa and a tensile modulus of only 174 GPa,while the carbon content was reduced from 96.46%to 78.94%.（2）Traditional pre-oxidation process adjustment,including pre-oxidation tension and temperature,can optimize the cyclized structural orientation,characteristic structure and"skin-core structure"of the pre-oxidized fibers to a certain extent,which can further affects the orientation,"turbostratic graphite"structure and radial structure distribution of carbon fibers.The mechanical properties of carbon fibers with low carbon content can be certainly optimized,but the effect is limited,with a maximum tensile strength of 3.54 GPa and a maximum tensile modulus of 188 GPa.（3）The secondary cyclization of pre-oxidized fibers was used based on the conventional optimization of the pre-oxidized structure of PAN fibers.Without increasing the oxidation structure in the pre-oxidized fibers,the formation of the cyclized structure and the dehydrogenation reaction were further promoted,thus affecting the content of"turbostratic graphite"of carbon fibers and optimizing the mechanical properties.The tensile strength of the low carbon content carbon fibers was further increased to 3.75 GPa,while the tensile modulus did not change significantly.At the same time,the carbonization yield of the fibers was gradually increased from 61.80%to63.24%.（4）By optimizing the homogenization of the radial structure of the PAN precursor,the inhomogeneity of the pre-oxidized structure of the PAN fibers can be effectively reduced.The low carbon content carbon fibers prepared by1000°C not only has a higher degree of internal graphitization,with the I_G/I_Dof the core increasing from 0.481 to 0.509,but also the distribution of the graphitized structure in the radial direction is significantly optimized.The improved of distribution of radial structure facilitated an increase in the tensile strength of the carbon fibers after carbonization at 1000°C from 3.14 GPa to5.19 Gpa,an increase of 65.29%,while the tensile modulus increased from189 GPa to 229 GPa,a growth rate of 21.16%,corresponding to a carbon content of 84.57%.The aim of high performance of carbon fibers with low carbon content was achieved.