The Evolution Of Composition And Microstructure Of Polyacrylonitrile-based Carbon Fibers During High Temperature Treatment And Their Effect On Thermal Oxidation Properties

The PAN-based carbon fiber was treated continuously at temperaturesfrom1300℃to2300℃in this thesis. The evolution of composition andmicrostructure of carbon fibers was investigated. By selecting these carbonfibers with different composition and the microstructure, their oxidationproperties were investigated and compared. The correlation of compositionand microstructure with oxidation property was established.The resultsshowed:With the increase of heat treatment temperature up to1900℃, the contentN element of the carbon fiber decreased continuously,which results inincreasing of C element content. At treatment temperature higher than1900℃, nearly all N in the fiber is expelled, resulting nearly100%Ccontent. By expelling of N in the fiber, the graphitization degree increasesgradually, and the crystalline increases simultaneously, and because of the effect of N element, there was a inflection point as the increase ofgraphitization degree. After all N in the fiber expelled, the graphitizationdegree increases more rapidly.The microcrystalline size La and Lc were alsoincreased simultaneously, and that showed that graphite microcrystallinegrowed with all directions. and also the distance between graphite sheetsd002decreased, which means the graphite sheets pile up closely.Carbon fibers with different composition and microstructure, which weretreated at different heat treatment temperature, were used for investigatingthermal oxidation. By analyzing the TG data, two oxidative reaction processidentified: low oxidative temperature of500℃, weight gain of carbon fiberis observed at initial stage, then weight loss of CF happens. while at highoxidative temperature of600℃, continuous weight loss of CF with oxidationtime is observed.At low oxidative temperature of500℃,the exist of N in the CF increasesthe reaction activity of carbon with oxygen. the higher the N content in thefiber, the more the oxygen containing groups in the surface of the fiber.There is little effect of the microstructure on reactivity of C with oxygen forcarbon fibers with no N left.At high oxidative temperature of600℃, more weight loss is observed forcarbon fiber with higher N content, while less weight loss is observed for CFwith higher graphite. In the process of thermal oxidation, the graphitizationand the microstructure size decreases with oxidation. For CF higher N content, more decreases of graphitization degree was observed, while for CFwith no N, no obvious difference in decrease rate of graphitization degree andmicrocrystalline size is found.