Relationship Between Microstructure And Mechanical Property Of High Performance PAN-based Carbon Fibers

The structure of six kinds of Toray T-series carbon fibers and four kinds of domestic carbon fibers were characterized by using scanning electron microscope, X-ray diffractometer and high-resolution transmission electron microscope. The microstructure information was obtained including the fibril structure reflected by the surface groove morphology, the bond of graphite micro-crystal interlayer reflected by the shear fracture, the compactness reflected by the tensile fracture morphology, the graphite microcrystal size and degree of orientation, the turbostratic graphite strip structure. Then the correlation between organizational structure and mechanical properties of carbon fibers was analyzed.Comparing the structure and mechanical properties of six kinds of Toray T-series carbon fibers, the experimental results showed that the mechanical properties of carbon fibers were not determined by some microstructure characterization, but a comprehensive expression of all microstructure characterization. In the six kinds of carbon fibers, the mechanical properties of T1000G were the best, whose five microstructure parameters were all the best. The mechanical properties of T300were the worst, whose five microstructure parameters were all the worst. T700S and T700G had close mechanical properties. T700S had the similar monofilament diameter, shear fracture morphology to T700G, while T700S had smaller graphite microcrystal size and degree of orientation than T700G. In addition to tensile strength, T800S’s and T800H’s other mechanical properties were close, meanwhile they had similar monofilament diameter, shear fracture morphology, crystallinity and degree of orientation. However T800H were prepared by wet spinning process. So its surface morphology had many obvious grooves, meanwhile its graphite crystallite size was smaller.Difference of the microstructure and mechanical properties between domestic carbon fibers and Toray T-series carbon fibers was studied. It was found that not all of organizational structure of domestic carbon fibers were worse than that of Toray T-series carbon fibers. For instance, with smaller monofilament diameter, shear surface morphology of GT800A and GT800B were better than T800H, but worse than T800S. Fracture morphology of GT800A had several big broken stairs, which indicated its bond of graphite microcrystal interlayer was stronger. However fracture morphology of GT800B and T800S were both smoother, which indicated their bond of graphite microcrystal interlayer were weaker. GT800A and GT800B had smaller graphite crystallite size and less number of stack layers. Meanwhile the ordered arrangement in graphite layer, degree of orientation and turbostratic graphite strip size of T800H, T800S and GT800B were better than those of GT800A. These results confirmed the conclusion that the mechanical properties of carbon fibers were a comprehensive expression of all microstructure characterization. Among GT700, T700S and T700G, GT700had many deep grooves, the smallest graphite crystallite size, the lowest apparent crystallinity, lower ordered graphite arrangement and degree of orientation, poorer graphite crystallite regularity, which caused that GT700had poorer mechanical properties than T700S and T700G.Before using carbon fibers, some treatments were carried out to carbon fibers; moreover heat treatment approach played an important role generally. In this paper, GT800A and T800H were treated at different temperature, and then their microstructure was characterized. Results showed that the surface and cross-section morphology of carbon fibers didn’t change significantly after heat treatment. In the process of heat treatment, the microscopic graphite structure improved unceasingly, exhibiting a high temperature-driven self-assembly process. This was mainly reflected in the following aspects. Both d002and d100decreased; FWHMc and FWHMa decreased; Lc and La increased; the number of stack layers increased; the apparent crystallinity and the degree of orientation largened. Therefore, in order to obtain carbon fibers with appropriate graphite crystallite structure, the heat treatment temperature must be chosen appropriately.