| PAN-based high strength and high modulus carbon fiber is one of reinforcing materials of anisotropy. In the process of graphitiation, carbon atoms of carbon fibers rearrange under high temperature field, microstructure of carbon fibers change from two-dimensional turbostratic structure into a three-dimensional graphite structure. Carbon fibers are made of hexagonal graphite crystallites. The direction of stacking is perpendcular to the axis of fibers in priority, while the graphite layers are preferred to be oriented along the axis of fibers. The values of degree of orientation determine the modulus of carbon fibers, so studying the crystallite size, crystallinity and degree of orientation of carbon fibers is essential for improving the mechanical properties of carbon fibers.Firstly, the surface morphology and microstructure of five kinds of self-made PAN-based high strength and high modulus carbon fibers (HMCF-1, HMCF-2, HMCF-3, HMCF-4, HMCF-5) were investigated by scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and Raman spectroscopy, and were compared with those of MJ series and T800 carbon fibers. Secondly, graphite fibers with different performance were prepared at different graphitization condtions by using carbon fiber T800 of Japan (Toray) as raw material. Thirdly, we researched the effect of three process parameters which including graphitization temperature, graphitization time and stretching ratio to microstructure of carbon fibers in graphitization. Finally, effects of the conditions of graphitization on the mechanical properties and density were investigated to determine the optimal process conditions.The photos of SEM and HRTEM releaved that surface morphology of T800, MJ series and self-made high strength and high modulus carbon fibers had grooves which paralled the axis of fibers. There were less mechanical damages in the prosess of preparation of carbon fibers, because obvious holes, cracks, scratches and other defects were not found in the pictures. High strength and high modulus carbon fibers were composed of crystalline and amorphous regions. The regularity of graphite layers was in order of HMCF-5>HMCF-4>HMCF-3>HMCF-2>HMCF-1.The result of XRD and Raman showed that the values of d002, Lc, La of T800 were the minimum in those of all carbon fibers. From HMCF-1 to HMCF-5, the values of Lc, La10, La110 and the degree of graphitization and crystallinity increased, while the values of d002 and angle of orientation reduced. After comparing them with those of MJ series carbon fibers, we found that sizes of crystallite and degree of orientation of HMCF-4 and HMCF-5 were bigger than those of M55J, while the values of d002 were smaller than that of M55J.FWHMs of D peaks and G peaks, values of R reduced and the degree of graphitization increased with the increase of graphitization temperature. If the graphitization temperature incresed, the sizes of graphite crystallites, degree of crystallinity and orientation were gradually increasing, while the values of d002 and angle of orientation were gradually reduced. The increase of stretching ratio can helps the angle of orientation of graphite crystallites which paralled the axis of fibers decreasing and made the degree of orientation increasing. It made the values of d002 reducing, while the sizes of rayers of graphite streached along the axis of fibers in the tension. Sizes of graphite crystallite increased and values of d002 decreased with the prolonging of graphitization time.The analysis of properties of carbon fibers showed that the tensile strength decreased gradually with the increase of graphitization temperature, while the elastic modulus reduced. The increase of temperature of graphitization and prolonging graphitization time also made the diameters and linear density of carbon fibers reducing. The increase of stretching ratio was bound to reduce the diameters of carbon fibers, so linear densities redused. |
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