Performance And Microstructure Of Carbon Fibers In The Liquid Phase Oxidation

The properties of carbon fibers have close relationshiop with itsstructure.In some strong oxidation environment,the structure andproperties may be changed. The changes of the properties andmicrostructure of carbon fibers made in our laboratory were studied instrong oxidation agent, concentrated nitric acid and the mixtures ofconcentrated sulfuric acid and concentrated nitric acid. The resultsshowed that when carbon fibers were oxidized by concentrated nitric acid,tensile strength of carbon fibers increased than that of the un-treatedsample, while modulus had little change. When the carbon fibers areoxidized by the mixture, the strength and the modulus of carbon fiberswere decreased with increase of oxidation time, and decreased rapidlywhen the volume fraction of concentrated sulfuric acid.We study themicrostructure of carbon fibers treated in strong oxidation agent by meansof XRD, Raman spectroscopy, SEM and HRTEM analysis. The resultsshow that the oxidation of concentrated nitric acid didn’t damagemicrocrystalline structure of graphite layers, which primarily affect the microscopic size parallel to the fiber axis. The microcrystalline ofgraphite layers become smaller when treated; and the oxidation ofconcentrated sulfuric acid destroyed the microcrystalline structure ofcarbon fibers, which make fracture between microcrystalline layers, atlast the morphology of fibers completely disappeared. Oxidationconcentrated sulfuric acid mainly chang the thickness of stacking fiberlayers. Through the density of the treated samples, the elementalcomposition analysis and TG analysis,we study the mechanism of liquidphase oxidation:the affect of oxidation is primary concentrated nitricacid,but it can only word on fiber surface and to the internal structure it isalmost not worked.But when adding the concentrated sulfuric acid,itmakes sulfonic acid groups introduction to the hexagonal carbon networklayers.One reason is the high value of sulfonic acid group let oxidationagent diffusing the internal fibers become easier. Another one because ofits very strong suction power, resulting the activity of based carbon atomsin around greatly strengthened,which more easily to damage thetrapezoidal structure of fibers, eventually leading to disappearance of thefiber morphology.