Microstructure characterization of PAN preoxidation fibers prepared with radio frequency plasma

The microstructures of preoxidation fibers prepared with radio frequency plasma were charac- terized in terms of micron, nano and atomic scales through scanning electron microscopy (SEM), high resolving transmission electron microscopy (HRTEM), scanning tunneling microscopy (STM), X-ray diffrac- tion (XRD), etc. The polyacrylonitrile (PAN) precur- sors were first soaked in the oxygen-enriched solvent and polarized in radio frequency electric field, and then were oxidized in the atmosphere of oxygen plasma. The morphology of SEM at micron scales shows that the wrinkles on the surface of preoxida- tion fibers prepared with plasma are shallower, and the surfaces are more tidy and smoother than the unsoaked samples prepared with usual electric fur- nace, and the uneven radial structure is improved. The results of XRD calculation show that the graph- itization degree and microcrystalline size get larger and the interlayer spacing gets smaller. Also, the lattice stripe and edge of bedding plane (002) can be observed from HRTEM at nano scales. From STM images at nano and atom scales, the microfibrils were found to be composed of ultrafibrils that closely twined and arranged, forming the left spiral structures spreading to fiber axis, and the carbon atoms on the surface of microcrystalline were found to have the trend of directional arrangement. All the above characterization results indicate that the plasma method effectively makes the internal and external oxidation of PAN precursors at the same level, so that the radial structure difference of preoxidation fibers is reduced.