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Carbon fibers (CFs) were prepared from polyacrylonitrile fibers pre-irradiated with γ-ray irradiation in air prior to heat treatment and then by continuous stabilization and carbonization. The structural changes of fibers during heat treatment and the mechanical properties of the resultant CFs were investigated using dynamic mechanical analysis, wide-angle X-ray diffraction, small-angle X-ray scattering, and universal testing machine. It is found that not only the stabilization time was shortened with the increasing absorbed doses, but also the preferred orientation of carbon layers in the CFs was increased initially with the increasing absorbed doses and then decreased. Consequently, the tensile strength and Young's modulus of CFs were increased by 13.2 and 2.6 % at an appropriate dose, respectively, in which skin-core structure in CFs was relieved according to SEM images. Hence, it is believed that irradiation technology-assisted stabilization has a significant potential in CF industry. [ABSTRACT FROM AUTHOR]

The effect of a different stretching stress at different heat treatment temperatures (HTT) on the structure and the mechanical properties of polyacrylonitrile (PAN)- and rayon-based carbon fibers was studied. The tensile strength increases first and then decreases with increasing stretching stress, whereas the Young's modulus of the fibers continuously increases. The behavior of PAN- and rayon-based carbon fibers is similar with increasing stretching stress, but the tensile strength of PAN fiber decreased while that of rayon fiber increased with increasing HTT, what is more, the latter have a considerable lower tensile strength and modulus for equivalent processing conditions. The structure of the fibers was investigated with X-ray diffraction. A continuous change toward a nanostructure with a higher order was observed, which explains the increase in the Young's modulus. For more complex dependence of the tensile strength on the processing conditions, a quantitative model to describe the effect of stretching stress at different HTT on preferred orientation degree and shear modulus is proposed. From the critical stress fracture of carbon fiber analysis, we can understand the different changes of tensile strength of both type fibers with stretching stress at different HTT. [ABSTRACT FROM AUTHOR]

The influence of heat treatment temperature from 1400 to 2840 °C and time from 1.2 to 12.0 min on the structure and mechanical properties of polyacrylonitrile carbon fibers was studied. It was observed that the Young’s modulus increased with increasing temperature and time, but the tensile strength exhibited different variation trends with the different processing methods. For a fixed time of 1.2 min, the strength dropped from 4.6 GPa at 1400 °C to 2.6 GPa at 2840 °C, (~43.5 %) as opposed to a 63.9 % increase in Young’s modulus. However, when the treatment time was increased to 6.0 min at 2500 °C, the tensile strength decreased only by 1.9 %, from 3.71 to 3.64 GPa, versus a nearly 20.0 % increase in Young’s modulus. The same situation was found for treatment at 2000 and 2700 °C. Raman spectroscopy and uniform stress model analysis show that the degree of covalent cross-linking between the graphene planes decreased as temperature increased, while it remained almost constant as treatment time was increased. It is believed that during heat treatment of a carbon fiber, the cross-linking collapses at the beginning but the crystalline size keeps growing with prolonging time, so the tensile strength decreases little with further heat treatment while tensile modulus keeps increasing. [ABSTRACT FROM AUTHOR]