Development of longitudinal mesoscopic textures in mesophase pitch-based carbon fibers through heat treatment Original Research Article

Development of mesoscopic texture in the longitudinal surface of mesophase pitch-based carbon fibers through heat treatment was characterized in as-spun, stabilized, carbonized and graphitized fibers using a high resolution scanning electron microscope (HR-SEM). The basic structural units on the longitudinal surface of the mesophase pitch-based graphitized fibers defined as the pleat and fibril were observed both from perpendicular and tilted views. No pleat and fibril were observable on the surface of as-spun and stabilized fiber. However, they were found on the surface of extracted as-spun fiber. The pleats and fibrils appeared on the longitudinal surface of the mesophase pitch-based carbonized fiber heat treated at 700°C for 1 hour. They maintained basically the same shape and size even after the graphitization at 2400°C. Their contours became more clear and the gaps between fibrils became deeper owing to shrinkage due to the graphitization. The pleat unit and fibril appearing on the surface of extracted as-spun fiber were basically maintained almost the same in shape and size after carbonization at 700 and 1000°C for 1 hour. The carbonization yield decreased to 78% in the carbonized fiber heat treated at 700°C and it stayed almost constant up to carbonization at 1200°C. Atomic ratios also drastically decreased at this carbonization temperature region. The pleats and fibrils appeared on the surface of carbonized fiber heat treated at 700°C as a result of shrinkage of the soluble fraction and volatilization of light gases during the heat treatment. The carbonization leads to the shrinkage of the soluble fraction in the as-spun fiber, following the shape of the oriented insoluble fraction during spinning with minor change of the latter fraction. The development and formation mechanism of nanoscale structural units on the longitudinal surface of carbon fiber were discussed in terms of the microdomain in the liquid crystal mesophase pitch, its orientation during spinning and the appearance by heat treatment owing to the shrinkage.