| During the pre-oxidation process,the PAN fiber exhibits a unique radial structure due to the influence of heat mechanism and environmental oxygen diffusion kinetics This difference in the radial direction will cause the cracking and reorganization reaction in the carbonization process also have a radial difference,so that the final formed carbon fiber has a certain radial structure difference,which was known as the carbon fiber sheath-core structure.Optimizing the radial distribution of the pre-oxygen structure and constructing the relationship between the pre-oxygen structure and the carbon fiber structure have a guiding significance to the preparation of high-performance carbon fibers.This paper using optical density method,Raman spectroscopy,nuclear magnetic,thermal weightlessness analysis and other different research methods,through controlling the pre-oxidation reaction rate by temperature to preparing pre-oxygen fibers with different radial structures.This paper also using the process simulation of ultra-fine fibers,reasonably describing the structural characteristics of the different regions of the pre-oxidized fiber radial by the model fiber structure,and in further exploring the evolution characteristics of the pre-oxidized fiber radial structure during high temperature treatment,and eventually building the relationship between the pre-oxygen structure and the carbon fiber structure.This paper’s research results shows that:1.The pre-oxidation reaction of the PAN fiber will cause the fiber to have a traditional sheath-core structure,in which the skin portion is a relatively high reaction area and the core portion is a relatively low reaction area.According to the reaction process of pre-oxidation,the pre-oxidation process can be divided into three stages early stage,middle stage and end stage.Compared with the conventional pre-oxidation reaction rate,overall increasing the pre-oxidation reaction rate of PAN fibers not only improves the overall pre-oxidation reaction degree of the fiber,but also promotes the expansion of the relatively high reaction region to the deeper fiber region.In addition,the fiber’s apparent skin-to-core ratio is increased,and the degree of pre-oxidation reaction in the fiber skin area increased faster,and the difference in the degree of pre-oxidation between the fiber skin core will become larger.The excessively high rate of pre-oxidation reaction increased the content of oxygen-containing structures in the fiber,and reduced the fiber thermal stability.2.Increasing the pre-oxidation reaction rate of PAN fiber in the mid-term,makes that the difference between the sheath and the core is not significantly increased,but the pre-oxidation degree of each area in the radial direction of the fiber is effectively improved,and the thermal stability of the fiber is improved.Compared with the middle stage,increasing the pre-oxidation reaction rate in the early stage can more effectively increase the pre-oxidation degree of the fiber skin portion,rather than the pre-oxidation degree of the core portion.The pre-oxidation of the fiber skin is more sufficient while the oxygen-containing structure increases less,this will improves the thermal stability of the pre-oxygen fiber.3.The pre-oxidized fiber skin reacts with oxygen when fully contacted,which promotes the cyclic dehydrogenation of the PAN fiber,and generates a large amount of SP2 carbon and a conjugated structure containing-C=O,which improves the degree of pre-oxidation of the fiber skin and decreases the degree of regularity and deepens the color.The oxygen content of the fiber core is very small,and the pre-oxidation is insufficient,then the PAN macromolecule only undergoes a cyclization reaction and a small amount of dehydrogenation reaction.There is no oxygen-containing structure,and there is PAN molecules without pre-oxidation reaction,it has a relatively single structure.4.The radial distribution of the pre-oxidation degree of the pre-oxidized fiber can be inherited into the carbon fiber after carbonization.The fiber with a high pre-oxidation degree after carbonization has a relatively high degree of graphitization.Because the pre-oxidation fiber sheath has more oxidized cyclization and dehydrogenation structures,and the crosslinking polycondensation reaction is more likely to occur,so the increase rate of the g value of the sheath during carbonization of the preoxygen fiber is higher than that of the core.The g value of the core is increased relatively slow so that formed a "core convergence" phenomenon.The distribution of the PAN fiber pre-oxygen structure is also closely related to the performance of the carbon fiber.Moderately increasing the initial reaction rate of the pre-oxidation,not only improving the radial distribution of the pre-oxidized structure,but also effectively controlled the oxygen-containing structure content of pre-oxidation fiber,which greatly improved the tensile properties of the final carbon fiber.Only the initial temperature of pre-oxidation was increased from 200℃ to 210℃,the graphitization degree(g value)of the skin and core of the pre-oxidized fiber increased from 0.395 and 0.388 to 0.399 and 0.389,respectively,and the skin-core difference increased from 1.8%to 2.6%;Increasing the terminal temperature in the pre-oxidation by 5-10℃,the g values of the skin and core increased to 0.407 and 0.391,respectively,and the difference between the skin and core was 4.1%,and the difference between the skin and core increased.After carbonization at 1350℃,the carbon value of the carbon fiber whose pre-oxygenation starting temperature was changed increased from 0.479 and 0.471 to 0.485 and 0.473,respectively,and the difference in g value of the sheath and core increased from 1.7%to 2.5%.The tensile strength of the fiber is increased from 5.28GPa to 5.50GPa,and the modulus is increased from 273GPa to 279GPa.The carbon fiber sheath core g value of the overall increase in pre-oxygenation rate increased to 0.407 and 0.391,and the difference between the sheath and core was 4.4%.Under the combined influence of the higher oxygenated structure of the pre-oxygenated fiber,the tensile strength and modulus of the carbon fiber decreased to 5.21GPa and 265GPa. |
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