| The excellent properties of PAN-based carbon fibers are determined by their structure.The structure and morphology of carbon fibers are gradually formed during the preparation process,and are correlated with the technique,the structure of PAN precursor fibers and pre-oxidized fibers.Therefore,in order to obtain high quality carbon fibers,it is important to fully understand the structure evolution of PAN fibers,master their influence of the structure of PAN precursor fibers and pre-oxidized fibers on carbon fibers,and deeply study the correlation between structure and properties.First,the melt-spun PAN fiber and three kinds of commercial PAN fiber were studied for comparing the difference of their structure and performance by some measurement methods,such as Fourier transform infrared spectroscopy(FTIR),scanning electron microscope(SEM),X-ray diffraction(XRD),differential scanning calorimetry(DSC),tension testing,and so on.The results showed that the surface morphology of melt-PAN fibers was relatively smooth and without rough grooves,and the cross-section was round without skin-core structure.The crystallinity and the tensile strength of the melt-spun PAN fibers were 62.36%and 6.14cN/dtex respectively.And the extent of reaction was 0.18 of the melt-spun PAN fibers,which decreased the thermal stabilization initial temperature,made the exothermic peak broader,lowered the exothermic reaction energy.The thermal shrinkage of melt-spun PAN fibers was low and moderate,which could make the structure of pre-oxidized fibers more stable.Secondly,in an air atmosphere and a certain temperature gradient,as far as was concerned the melt-spun PAN fiber,the effects of the tension and air flow upon structure and properties of pre-oxidized fiber were studied by Fourier transform infrared spectroscopy(FTIR),differential scanning calorimetry(DSC),X-ray difraction(XRD),small-angle X-scattering methods,elemental analysis and density testing and so on.When a certain amount of tension was applied,the crystal orientation and dimensions of the fiber were decreased,while the pore structure of the fiber was affected.The porosity and average pore diameter had decreased to some extent,thus affecting the diffusion of oxygen.So the tension was over a certain extent,the pre-oxidation was not sufficient,the oxidation did not form a perfect cross-linked structure and the fibers during carbonization were severely damaged,resulting in decrease in the mechanical properties of the final fiber.Therefore,the pre-oxidation process should be strictly controlled conditions,considering its impact on the final properties of carbon fiber.The impact of air flow on the pre-oxidation process was more complicated.The crystal structure of pre-oxidation fiber was hardly affected,gas flow was desirable to promote cyclization reaction in a certain range,and the oxygen content of the oxidized fiber density increased as the gas flow increasing,but the relationship between the gas flow and the strength of the final carbon fiber showed the excessive oxygen content and the density was not conducive to carbonation.Finally,the low-temperature carbonization process of pre-oxidized fibers was studied.Heating rates that were too slow or too fast were unfavorable for carbonization.When the heating rate was too slow weight loss was so large that was not conducive to the production of high-strength carbon fiber.While too fast heating rate would cause the surface voids and other defects.When the heating rate reached to 15?/min the weight loss of pre-oxidized fibers was lowest.As the annealing temperature rising,the impurities on the surface of the fiber were reduced.Micro voids appeared on the surface at 800?.Results of small-angle X-scattering methods showed that the size of pores became smaller and the number increased from PAN fibers to low temperature carbon fibers.As the carbonization temperature rose the degree of graphitization was increased and the graphite plane size became smaller. |
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