The structural evolvement of polyacrylonitrile (PAN) oxidized fiber during the carbonization process and the microstructure of carbon fiber were investigated by the combination of DSC/TG

FT-IR

XRD

elemental analysis

SEM

TEM and HRTEM.The results showed that remnant —C≡N groups continued cyclization reaction in the pre-carbonization process.N hexahydric rings gradually dehydrogenized and denitrogenized.Then they formed C hexahydric rings during the carbonization process.T-700 carbon fiber had high tensile strength due to its smaller d002

thicker Lc and higher crystallinity.The microstructure of carbon fiber was composed of 4 parts from skin to core.The high density of sheets packing in the outer skin and the loose structure in the core could be noted.The compact skin of carbon fiber containing lots of nano-crystallites consisted of stacked carbon sheets.The skin-core structure of carbon fiber was evolved from PAN precursor and oxidized fiber.There were strip microstructure and globular microstructure in carbon fiber.The strip microstructure of carbon layers resembled that of PAN molecular chains in precursor fiber.The globular microstructure of carbon layers encircled the center

which was similar to that of the oxidized fiber.It indicated that the microstructure of molecular chains had significant structural interrelationship.