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The radial microstructural in carbon fibers is important for their mechanical properties. Microstructures at different locations in the radial direction produced by heat treatments from 1350 to 2400 °C and their relationship to the mechanical properties were investigated by Raman spectroscopy, elemental analysis, X-ray diffraction, high-resolution transmission electron microscopy and mechanical testing. Results indicated that the differences of the shift and full widths at half maxima (FWHM) of the Raman G and D bands at different points in the radial direction from the skin to the core, showed minima while the tensile strength of the fibers was a maximum at 1700 °C heat treatment temperature. The maximum tensile strength of carbon fibers at 1700°C was ascribed to the smallest skin-core variation at this temperature. The FWHM of the G and D bands from the same points in the radial direction decreased with heat treatment temperature, indicating an increased uniformity of the vibration modes of both G and D bands The sizes and the orientation degree of the crystallites increased with heat treatment temperature, leading to an increase of tensile modulus. Non-carbon elements were preferentially released, and crystallites were preferentially grown and orientated in the skin region compared to those in the core due to the preferential heat absorption in the skin, leading to the lower values of FWHM of G and D bands in the skin than in the core. The Raman shift of the G band had a maximum at 1700 °C while that of D band decreased with heat treatment temperature at the same radial points of the fibers. The fact that the release rate of non-carbon elements increased with heat treatment temperature below 1700 °C could be responsible for the increased G band shift below 1700 °C. With a continuous decrease in the non-carbon element content, this effect was lessened and the improvement of the perfection of the graphite structure became dominant above 1700 °C.

The reaction behavior of carbon fiber in electrochemical grafting is related to its electrode potential. In this paper, carbon fiber and metals with different electrode potentials were used to form combined electrodes to regulate the electrode potential of carbon fiber. The results showed that galvanic coupling was formed in the combined anode when the potential difference between carbon fiber and the metal (Δϕ = ϕCF0 - ϕmetal) was higher than 0.05 V. The electrode potential of carbon fiber was reduced due to cathodic polarization. The electrode potential of carbon fiber after galvanic coupling was proportional to the self-corrosion potential of metals. By applying the electrode potential regulation of carbon fiber in the electrochemical grafting of poly(glycidyl methacrylate) onto the carbon fiber surface, the grafting effect was significantly improved with the decrease of the electrode potential of carbon fibers. The grafting amount of carbon fibers increased from 0.83 to 69.86% as the electrode potential of carbon fibers dropped from 0.55 to -0.72 V. Consequently, the interfacial shear strength of the carbon fiber composite was remarkably promoted from 47.59 to 81.41 MPa, increasing by 71.07%.

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 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.