The Research Of Surface Structure Of Polyacrylonitrile-based Carbon Fibers

Surface structure of carbon fibers, include physical morphology, carbonstructure and Oxygen containing chemical structure, have great impact on theinterface of composites. The association of the three strucures has little studied,especially the effect of morphology and surface carbon structure on thegeneration and evolution of oxygen containing chemical structure. This part ofresearch is very important to the study of surface structure and the surfacetreatment of carbon fibers.In the thesis, the carbon fibers with different carbon structure wereprepared through the high temperature heat treatment. The differentmorphology was prepared by wet spinning and dry-wet spinning. Then all thecarbon fibers were treated by the method of electrochemical oxidation. Theevolution of carbon structure was investigated by XRD and Raman spectra.The morphology was observed by SEM and AFM. The O/C and oxygencontaining functional groups on the surface of carbon fibers were researchedby the XPS. Then we analyse the effect of the carbon structure andmorphology on the electrochemical oxidation behavior and the evolution ofthe oxygen containing chemical structure. The results show that: The O/C on carbon fibers’ surface has increased after electrochemicaloxidation. The increased amplitude decreases as the graphitization degree rises.For the same current density, the O/C decreased with the graphitization degree.These changes have different trend when the G=1.47more or less， WhenG<1.47，the changes are much bigger, however when G≥1.47, the differencesare little. Through the Raman spectra, the change of the carbon structure of thecarbon fibers surface can explain it. Generaly, the SP2carbon would increasewith the increases of graphitization degree, the order of the graphite layersincreases, while the edge and the low symmetrical carbon reduces. But whenG<1.47, the process of graphitization has another step of element remove. Theelement removal will increase the defects, so it has better oxidation activity.For the carbon fibers of G≥1.47, the only process of carbon structuralrearrangements and microcrystalline graphite grew rapidly and orderly, makethe graphite structure more perfect, so the oxidation activity of carbon fiber isextremely low. That’s the reasons why the characteristics of oxidation andoxygen containing chemical structure formation and evolution in the twostages are different.The O/C on the carbon fibers with grooves were much bigger than thatwith smooth surface when treated in the same current density. The analysisshows that the grooves of the carbon fibers made the current densityconcentrated in the Convex parts, so the Convex part was preferentiallyoxidized, then the valley part was difficult to be oxidized. This made the groove-carbon fibers had less oxidation areas than smooth-carbon fibers. Sowhen the carbon structure had little difference, the morphology made theelectrochemical oxidation different. The smooth-carbon fibers had betteroxidation effect, they had higher oxidation degree. In addition, the effect ofthe presence of the this effect can also cause convex part was easy to excessiveoxidation, but the valley part had insufficient oxidation, so oxidation of thecarbon fibers’ surface was not uniform and oxidation degree was not easy tobe controled.