Microstructural Evolution Of PAN Precursor Fibers During Preoxidation And Carbonization

The excellent properties of PAN-based carbon fibers are determined by their structure. The structure and morphology of carbon fibers gradually form during the preparation process, and are correlated with the technique, the structure of PAN precursor fibers and preoxidized fibers. Therefore, in order to obtain high quality carbon fibers, it is important to fully understand the structure of carbon fibers, master the influence of the structure of PAN precursor fibers and preoxidized fibers on carbon fibers, and deeply study the correlation between structure and properties. In this paper several technologies, such as Fourier transform infrared spectroscopy (FTIR), elemental analyzer, wide angle X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were used to systemically investigate the evolution of crystal structure, chemical structure, microstructure and properties of wet-spinning and dry-jet wet-spinning PAN precursor fibers during proxidation and carbonization process, analysis the structural change of carbon fibers during graphitization, and discuss the correlation between the structure and properties.The crystallinity of wet-spinning PAN precursor fibers gradually reduces during preoxidation process, while it gradually increases during carbonization process. Because preoxidation reactions firstly take place in amorphous regions and then transits to crystal regions, crystallite size (L_c) firstly increases below 235℃, and gradually decreases above 235℃. During carbonization, d₀₀₂ firstly increases below 700℃, and gradually decreases above 700℃. L_c firstly decreases, then increases again with the carbonization temperature rising.At the initial stage of preoxidation, there is little variation in C, H, N, O content. At the later stage, due to the acute oxidation reaction, the content of O increases obviously along with the gradual decrease of C, H and N. During carbonization, O and H contents of preoxidized fibers obviously decreases below 700℃, and gradually decreases above 700℃. While N content gradually decreases below 700℃, and obviously decreases above 700℃, as a result, carbon fibers with 94.81% of C content is formed. The bulk density gradually increases during preoxidation, while the variation of linear density is complex. Influenced by many factors, linear density firstly decreases, then increases, and at last decreases again. The bulk density obviously increases between 275 and 700℃, and between 700 and 1000℃, corresponding to the gradual decrease of linear density. The bulk density gradual increases between 500 and 600℃, and between 1000 and 1200℃, corresponding to the gradual decrease of linear density.FESEM study on surface morphology and fracture morphology of wet-spinning and dry-jet wet-spinning fibers shows that the grooves exist on the surface of wet-spinning precursor fibers. During preoxidation and carbonization process, the grooves combine with each other. Dry-jet wet-spinning precursor fibers have a smooth surface. The tensile fracture morphologies of wet-spinning PAN precursor fibers transform from tough fracture to brittle fracture throughout the whole preparation process. The diameter of fibrils of PAN precursor fibers gradually decrease, along with the closer bonding between fibrils. Deeply investigation was carried out on the structure of PAN precursor fibers, preoxidation fibers and carbon fibers by TEM. The results show that the structure of PAN precursor fibers is inhomogeneous and has skin-core structure. With the process of preoxidation reaction, the core texture grows thick. After low temperature and high temperature carbonization, the compactness and stability in skin and core region are improved.Selected-area electron diffraction study on the formation and evolution of two-dimensional turbostratic graphite shows that during carbonization, the dispersion of amorphous diffractive ring gradually decreases, and orientation diffraction arc of (002) crystal planes and polycrystalline diffraction ring of (100) and (110) crystal planes appear. With the rising of carbonization temperature, the intensity of orientation diffraction arc of (002) crystal planes strengthens along with the arc length shortening, and more over the intensities of polycrystalline diffraction ring of (100) and (110) crystal planes strengthen, and halo disturbance of diffraction ring gradually decreases. Above results indicate that during carbonization, graphite lamella closes up further, the orientation of graphite lamella along the axis gradually increases, and the crystallinity of graphite crystallite increases.HRTEM study on the microstructural evolution of PAN precursor fibers during the formation of carbon fibers shows that the HRTEM microstructure of PAN precursor fibers is quasicrystal structure, which is composed of amorphous region and relative ordered region. There have no apparent interfaces between amorphous region and relative ordered region. Amorphous structure is observed in preoxidized fibers. When preoxidized fibers are carbonized, with the process of pyrogenation, changes of the structure take place. The amount and size of aromatic plane continues to increase, carbon planes widen and improve further, and the orientation of graphite lamella along the axis gradually increases. Above phenomena represent in detail that the length of single graphite lamella gradually increases, the size of graphite crystallite (L_c and L_a) increases, amorphous texture between graphite lamella decreases, as a result the crystallinity of graphite crystallite increases. The continuity of adjacent graphite lamella gradually increases, graphite lamella close up further and the interplanar spacing decreases, and two-dimensional turbostratic graphite structure is formed gradually, which guarantees high tensile strength for carbon fibers.XRD, elemental composition, fracture morphology, HRTEM microstructure were investigated by comparing self-produced carbon fibers with Toray T700 carbon fibers. The results indicate that Toray T700 carbon fibers have better completeness of crystal structure, better graphitization degree, more graphitization stack layers and higher orientation. The C content of Toray T700 carbon fibers is higher than that of self-produced carbon fibers, so in order to obtain high performance carbon fibers, the temperature of high temperature carbonization needs to be selected according to various technologies. T700 carbon fibers have lesser structural defects, such as holes, small openings and disorder of graphitization layers. As a result, structural compactness and homogenization are the main technology to improve tensile strength of carbon fibers.After being graphitized at 2350℃, the microstructure of Toray T300 carbon fibers transforms from two-dimensional turbostratic graphite structure to three-dimensional order graphite structure. The N, H and O content of fibers are further released, and the wt. % of C content increases from 95.04% to 99.27%. The number of the C six-membered ring increases, and graphite crystallites grow up, representing the increase of L_c, L_a, and crystallinity. The orientation degree of graphite crystallites along fiber axis improves, which makes the interplanar spacing shorten to 0.3359 nm, and the graphitization degree increase. Above-mentioned changes of structure result in the increase of the young's modulus, the decrease of tensile strength and elongation at break. After graphitization, carbon fibers convert into brittle materials completely. More structural weaknesses exist in graphite fibers, which make the tensile strength of T300 carbon fibers decrease after graphitization.The bulk density of dry-jet wet-spinning PAN precursor fibers gradually increases in the formation of carbon fibers, while the variation of linear density is complex. Influenced by many factors, linear density firstly increases, decreases, then increases, decreases, and at last increases, decreases again. Because of the decrease of orientation degree of fibers at the initial stage of preoxidation (below 220℃), the diffraction peak intensity weakens, and moreover, the crystallite size decreases. Because preoxidation reactions firstly take place in amorphous regions at the temperature range of 220-230℃, quasicrystal structure grows up, crystallite size increases, and the crystallinity increases. With the rising of preoxidation temperature, cyclization reaction spreads to crystal regions, as a result, ordered regions transit to amorphous regions, crystallite size and the crystallinity both decrease. During carbonization process, along with the release of non-carbon elements, C atoms form C basal planes through cyclization and condensation polymerization reaction, the combined force between C atoms strengthens. Consequently L_c and crystallinity increase.HRTEM study on the microstructural evolution of dry-jet wet spinning PAN precursor fibers during preoxidation and carbonization indicates that The HRTEM microstructure of dry-jet wet-spinning is quasicrystal structure, which is composed of amorphous region and relative ordered region. There have no apparent interfaces between amorphous region and relative ordered region. During preoxidation process, along with the transformation from PAN linear molecular chains to planar ladder structure, ordered regions gradually transit to amorphous regions. During carbonization process, carbon basal planes form, and with the carbonization temperature increasing, carbon basal planes orient along the fiber axis.Compared with wet-spinning PAN precursor fibers and the resultant carbon fibers, dry-jet wet spinning PAN precursor fibers and carbon fibers with high density, high crystallinity, uniform structure and smooth surface guarantee high tensile strength for carbon fibers. Compared with wet-spinning, dry-jet wet spinning is a relatively better spinning technology for producing carbon fibers with high performance.