Carbon fibers produced from PAN (polyacrylonitrile) as a precursor are generally subjected to the three heat treatment processes of stabilization and carbonization followed by graphitization. The fibers that had been stabilized in the first process were carbonized in a high magnetic field imposed parallel to fiber axis at the temperature of 1445K and graphitized without magnetic field at 2273K. The tensile strength of these treated fibers was increased in 15-31% in comparison with that of the fibers treated in no magnetic field. The reason why the imposition of the magnetic field could improve the strength of fibers has been studied through the methods such as a microscopic observation of fiber surfaces, a statistical Weibull analysis, an X-ray diffraction and a Raman spectroscopy. By using of the observed and measured results, the mechanism of strengthening of PAN-based carbon fibers by imposing the high magnetic field has been discussed on the basis of an intermolecular cross-linking reaction in terms of a radical pair theory in a magnetic field.
