High-temperature axial stress evolution mechanism of polyacrylonitrile-based carbon fiber

Temperature and stretching are important factors in the high-temperature treatment of carbon fiber. The axial stress during carbon-fiber high-temperature treatment affects its ability to stretch. The high-temperature axial stress evolution mechanism of polyacrylonitrile-based carbon fiber was studied through in situ tension tests, Raman spectroscopy, X-ray diffractometry, elemental analysis, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, thermal expansion coefficient tests, and density methods. The high-temperature axial stress evolution of polyacrylonitrile-based carbon fiber involved three stages: rapid increase, rapid decrease, and relaxation. The highest stress and relaxation temperatures of the polyacrylonitrile-based carbon fiber were 1600°C and 1950°C, respectively. The main factors that affected the fiber axial stress included carbon-structure rearrangement and the effect of thermal expansion and cold shrinkage on fiber length. During the first stage ( T < 1600°C), carbon-structure rearrangement after nitrogen atom removal increased the fiber axial stress. In the second stage (1600 ⩽ T ⩽ 1950°C), the difference in the thermal expansion of fibers that entered the graphite furnace and the cold shrinkage of fibers that exited the graphite furnace increased gradually, which resulted in a decrease in fiber axial stress by up to 1950°C, where the fiber relaxed and the third stage ( T > 1950°C) began. The difference between expansion and shrinkage increased significantly, which increased fiber relaxation. Carbon fibers with fewer nitrogen atoms and more regular structures had a lower axial stress during high-temperature treatment, but the trend and characteristic temperature remained unchanged. The corresponding fiber high-temperature maximum stretching ratio and axial stress showed opposite trends below 1950°C. The ability to stretch the carbon fiber increased above 1950°C, which differed from the axial stress relaxation.