| Polyacrylonitrile(PAN)based carbon fiber has a series of excellent properties,such as high strength and modulus,good heat and chemical resistance,and low density,etc.,which not only contains the inherent properties of carbon materials,but also has the flexibility and processability of the fiber materials.PAN-based carbon fiber is a good reinforcement for preparing high-performance composite materials,has important application value in the fields of aerospace and military,as well as automotive energy and wind energy industries,and attracts wide attention from researchers.In the preparation process of PAN-based carbon fiber,thermal oxidative stabilization(TOS)stage is not only an important link between precursor and carbon fiber,but also a key step to determine the final structure and properties of carbon fiber.Therefore,in-depth study of the evolution of chemical and physical structure of PAN fiber during TOS process and its relationship with mechanical properties is of great significance for obtaining the structure stable TOS fibers and further preparing high-performance carbon fibers.Carbon fiber is a material controlled by defects,the actual tensile strength(?7 GPa)of PAN-based carbon fiber is less than 5%of the theoretical tensile strength(?180 GPa),which is related to the carbon fiber defects such as micropores,cracks,skin-core structure,etc.Among them,the formation of skin-core structure is one of the important factors leading to the decline of mechanical properties of carbon fibers.The formation and development of skin-core structure of carbon fiber through the whole production process.In addition to the initial skin-core structure formed in the preparation of precursor,the radial heterogeneity of chemical structure and physical structure in the TOS stage is the main reason for the formation of skin-core structure.Therefore,improving the skin-core structure of TOS fiber is also of great significant to obtain stable structure of TOS fiber and improve the mechanical properties of carbon fiber.In this paper,based on Fourier transform infrared spectroscopy(FTIR),the group changes of cyclization,oxidation,dehydrogenation and crosslinking was quantitatively tracked by peak-fitting and second-derivative method,and the structural evolution were using the differential scanning calorimetry(DSC),thermogravimetric analyzer(TG),wide-angle X-ray diffractometer(WAXD),scanning electron microscopy(SEM),high-resolution Raman spectrometer(Raman),etc.,around the relationship between chemical reaction and microstructure of PAN fiber during TOS process;the effect of physical and chemical structure changes on mechanical properties of PAN fiber during TOS process;the structure and properties of PAN fiber at high-temperature stage of TOS process;and a method may reduce the skin-core structure of TOS fibers has been studied and discussed in detail.The main research contents and results are as follows:(1)The effect of chemical reactions(extent of cyclization,oxygen content,crosslinking content)on the crystal structure and micro-morphology of TOS fibers during the TOS process of freely shrinking PAN fiber in air at constant temperature of 180-260?were studied.The results show that the physical shrinkage of TOS fibers occurred mainly at lower temperature of 180-200?,and the thermal shrinkage of TOS fibers above 200?is caused by chemical reaction.The increase in the extent of cyclization is helpful to the occurrence of oxidation and crosslinking reaction and leading to the decreased of crystallinity and crystal size of TOS fibers.The occurrence of three chemical reactions during TOS process enhance the stability of the TOS fibers,resulting in the increase of the bulk density and the decrease of the diameter of the fibers,and the fracture morphology of the cross-section changes form the ductile to brittle.However,the extent of cyclization of TOS fibers is not the higher,the better.When the extent of cyclization is higher than 83%,the dense oxidation crosslinking structure is formed under the control of diffusion process,which lead to the obvious skin-core structure phenomenon on the cross-section of TOS fibers,and the elongation at break reduced,and the stability of the TOS fiber decreased.(2)The influence of the chemical structure and physical structure changes of the PAN fibers with constant draft in air during TOS process on the mechanical properties of TOS fibers was studied,and the main factors affecting the mechanical properties of TOS fibers were clarified.The results showed that the tensile strength of TOS fiber decreased with the increase of TOS temperature and time,which is not only related to the cyclization reaction,but also relative to the destruction of crystal structure by heating at high temperature.The crystal structure is more likely transform to amorphization at high temperature,which lead to decrease of crystallinity and crystal size.The occurrence of chemical reaction and the amorphous transformation of crystal structure during the TOS process makes the relaxation of molecular chain and the increase of flexibility,which lead to the increased of elongation at break.When the extent of cyclization is higher,the oxidation reaction is controlled by the diffusion process and it is difficult to further diffuse the oxygen in the air from skin to the core.Although the bulk density of the fibers continues to increase,the oxidation crosslinking structure mainly accumulated in the skin layer of the fiber,which increased the rigidity of macromolecular chain and reduced the elongation at break.(3)The TOS method with continuous heating was used to study the structural changes of PAN fibers during the TOS process at 200-300?under constant draft in air,especially the structure and property of high-temperature TOS stage at 270-300?.Through the analysis of the influence of high-temperature TOS fiber on the skin-core structure of low-temperature carbonization and high-temperature carbonization fiber clear the temperature range required to obtain stable TOS fibers.The results show that the extent of chemical reaction increases with the increase of TOS temperature.In the early TOS stage,the cyclization reaction is mainly reaction,and the change trend of the dehydrogenation index is similar to the cyclization reaction.Oxidation reaction mainly occurred in the late TOS stage,and the crosslinking structure gradually increased after 250?.In the high-temperature TOS stage at 270-300?,the cyclization reaction is basically completed,the oxidation reaction is controlled by the diffusion process,and the oxygen content increased slowly;the continuous increase of dehydrogenation index in this stage may be related to the removal of non-carbon elements at high-temperature(The TGA results showed that PAN precursor began to occur the obvious thermal weight loss phenomenon at 285?).After low-temperature and high-temperature carbonization,the grooves on the surface of high-temperature TOS fibers become smooth gradually.The high-temperature TOS fibers at 285?and 300?showed obvious skin-core structure after high-temperature carbonization at1350?,and the diameter of the carbonized fibers decreased to 5?m.The results indicated that the TOS fibers have unstable structure when the TOS temperature above 285?,and the carbonization fibers occurred the obvious structural defect.Therefore,the TOS temperature in the range of 200-285?is more conducive to obtain stable structure of TOS fiber.(4)PAN precursor was pretreated under nitrogen to study the effect of the extent of cyclization of pretreated fibers on the chemical structure,crystal structure and skin-core structure of TOS fibers.The results showed that pretreatment was beneficial to increase the extent of cyclization,while inhibited the oxidation reaction during TOS process.As the increase of cyclization degree,the oxygen content and crystal size of TOS fibers decreased and the crystal interplanar spacing increased.Combining with the XRD,Raman and SEM analysis,the untreated fiber SF0 has an obvious skin-core structure,and the skin layer of the fiber has a dense structure with more oxidative crosslinking structure.On the one hand,the structure prevented oxygen from diffusing into the core from skin layer.On the other hand,it also prevented the heat of fiber core from being dissipated in time,resulting in a hole in the fiber core appeared;As the increase of the cyclization degree of pretreated fibers,the oxidation crosslinking structure decreased in the fiber skin layer,and the interplanar spacing increased,which reduced the barrier of the diffusion from the skin to the core,which is conducive to the evolution of sp~2 hybrid carbon in the core,and reduce the fact that the radial distribution of oxygen content is uneven to a certain extent.When the cyclization degree of the pretreated fiber reached 40%,TOS fiber SF5 releases a part of the heat under the action of the cyclization reaction;in addition,the increase in the interplanar spacing of the fiber is conducive to the diffusion of oxygen from skin to the core,resulting in the exothermic position of the oxidative crosslinking reaction closer to the core of fiber,the concentration of heat release results in the melting in the core of fiber and caused a small hole.Therefore,nitrogen pretreatment with a certain extent of cyclization(10-35%)before TOS process can reduce the skin-core structure of TOS fibers.However,when the cyclization degree of pretreated fiber exceeds 35%,the TOS fiber has structural defects and the stability is reduced. |
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