Unveiling the role of radial heterogeneity in determining the tensile strength of PAN-based carbon fibers: A multiscale structural characterization approach

Advancing the mechanical performance of high-performance carbon fibers (CFs) necessitates a deep understanding of their multiscale structure-property relationships. This study systematically characterizes three PAN-based CFs (N-CF1, N-CF2, N-CF3) with varying tensile strengths to elucidate the impact of radial structural heterogeneity on mechanical properties. Advanced techniques including synchrotron-based wide-angle X-ray scattering (WAXS), microbeam WAXS (μ-WAXS), spherical aberration-corrected transmission electron microscopy (SA-TEM), electron energy loss spectroscopy (EELS), Raman spectroscopy, small-angle X-ray scattering (SAXS), and positron annihilation lifetime spectroscopy (PALS) have been employed to assess crystallite size, orientation, skin-core microcrystalline evolution, radial distribution of sp² hybridized carbon, and micropore morphology across different length scales. Results demonstrate that N-CF1, with its homogeneous skin-core structure, high graphitization, dense and aligned microcrystallites, and minimal defect density, exhibits superior tensile strength. N-CF2, while less crystalline, maintains a relatively balanced radial structure with higher sp² carbon content, facilitating effective stress transfer despite partial disorder. Conversely, N-CF3 displays localized high crystallinity but suffers from severe radial heterogeneity and disrupted internal structure, leading to compromised mechanical performance. These findings highlight the critical role of radial structural uniformity in determining the tensile properties of CFs, outweighing the influence of average crystallinity.