Abstract
Micro-structure and fiber compressive strength evolution in a series of poly(acrylonitrile) (PAN)-based carbon fibers at various stages of the carbonizing process were studied. The compressive strengths of the fibers were determined by the tensile recoil test method. Wide angle and small angle X-ray scattering (WAXS, SAXS) and low voltage high resolution scanning electron microscopy (LVHRSEM) as well as thermogravimetric-mass spectral analysis (TGMS) were used to determine the fiber micro-structures. The compressive strength improves through the stabilization and carbonization stages and reaches a maximum at the early stage of graphitization. Further treatments at higher temperatures reduce the fiber compressive strength. The changes observed in the fiber compressive strength were compared with the micro-structure and structure-property relationships were determined. Two parameters, based on the Euler buckling equation, namely, material's stiffness and geometrical stability derived from the X-ray scattering intensities, were utilized to estimate the buckling stability of the fibers. Initial increase of fiber compressive strength was primarily due to the conversion of the PAN fiber into a much stiffer graphite layered structure. At later stages of the process where the conversion was completed, the loss of geometrical stability of the fiber played a dominant role to reduce the fiber compressive strength.