| Spinning process is the forming stage of precursor of PAN-based carbon fiber, pre-oxidized process is the transition stage of the preparation of PAN-based carbon fiber, and carbonized process is the key to significant increase of strength of carbon fiber. In order to further optimize the preparation technology and obtain carbon fibers with excellent performance, it is important to carry out deeply study on the evolution of short-range ordered structure and crystalline structure of fibers during the preparation of PAN-based carbon fiber. In this paper, several characterization methods,such as radial distribution function (RDF) based on X-ray diffractometer (XRD) using Mo target, XRD using Cu target, elemental analysis (EA), density testing, scanning electron microscopy (SEM), fourier transform infrared spectroscopy (FTIR) were used. The data processing of RDF based on XRD using Mo target was investigated, and the short-range structural evolution of fibers was investigated quantitatively during the preparation of PAN-based carbon fiber according to the resulting RDF curves. The crystalline structure and crystalline orientation of fibers were investigated systemically by XRD using Cu target.The data processing method of RDF was acquired. The calculating program was written on Interactive Data Language (IDL) platform for materials containing C, H, O, N elements. The experimental results obtained through the calculating program were compared with the theoretical value of graphite to validate its reliability, which laid the foundation for further analysis of short-range ordered structure of fibers and different carbon materials.The evolution of short-range ordered structure of fibers during spinning process was studied by RDF. The changes of the neighbor distances (0.1-0.8nm) of fibers were not obvious at various stages during the preparation of PAN fiber, as follows: r1≈0.151nm; r2≈0.261nm; r3≈0.323nm; r4≈0.403nm; r5≈0.495nm; r6≈0.641nm.The changes of the neighbor interchain distances of fibers were not obvious. The nearest neighbor interchain distance of PAN molecular was about0.64nm and second-nearest neighbor interchain distance of PAN molecular was about1.25nm. The size of the ordered region gradually increased along the fiber axis during the spinning process, from0.523nm to0.645nm. The change of ordered region was not obvious along the radial direction, with an ordered region size of about1.25nm. The nearest atomic coordination numbers of fibers were similar at various stages. The average atomic displacements decreased gradually during the spinning process and the degree of short-range order of fiber increased gradually.The evolution of short-range ordered structure of pre-oxidized fibers during pre-oxidized process was studied by RDF. When the temperature was below220℃, the change of the nearest neighbor distance was not obvious, which was close to that of PAN precursor fiber. When the temperature was higher than245℃, the nearest neighbor distance of fiber decreased gradually, from0.151nm to0.145nm, which was still longer than the nearest atomic distance of graphite (0.142nm). The second-nearest neighbor atomic distance also decreased gradually, and was close to that of graphite (0.247nm). The third-nearest neighbor distance increased gradually. For the final pre-oxidized fiber, the third and the fourth peaks combined into a broad peak. When the temperature was245℃, the change of short-range ordered structure was obvious.245℃was the critical temperature of short-range ordered structure. With the rising of the pre-oxidation temperature, the size of ordered region gradually decreased and the change for the nearest atomic coordination numbers of fibers was not obvious. The average atomic displacements increased gradually and the disorder of pre-oxidized fiber increased gradually.The evolution of short-range structure of PAN-based carbonized fibers was studied. When the temperature was600℃, the planar six-member ring structure appeared. When the temperature was higher than600℃, the carbonized temperature had small influence on atomic arrangement, while it affected the order of the short-range structure. With the increase of temperature, the ordered region size slightly increased and the carbonized fiber was ordered in the range of2～3planar six-member ring along the fiber direction. While the change of ordered region was not obvious perpendicular to the fiber direction, and the fiber was regular arrayed within around1.1nm. The change was not obvious for the nearest atomic coordination numbers of fibers. The average atomic displacements decreased gradually and the order of carbonized fiber increased gradually.In order to reveal the atomic scale structure of PAN-based carbon fiber, the short-range structural differences of PAN-based carbon fiber, carbon black, multi-walled carbon nanotube and graphite were compared by RDF. The nearest neighbor distances of four materials were all about0.142nm. The second-nearest neighbor distance and the third-nearest neighbor distance of PAN-based carbon fiber were longer than those of graphite. The ordered domain size of PAN-based carbon fiber was obviously smaller than that of multi-walled carbon nanotube and graphite, and greater than that of carbon black. Therefore, there was still order for PAN-based carbon fiber compared with carbon black. The nearest atomic coordination numbers of four materials were about3. The atomic average displacement of PAN-based carbon fiber was larger than multi-walled carbon nano-tube and graphite, but smaller than carbon black. The ordering of PAN-based carbon fiber was between carbon black and graphite.The evolution of crystalline structure and orientation in crystalline region of fibers during spinning process was investigated systemically. The crystal grain size and crystallinity of fibers increased gradually during the preparation of PAN fiber, but the transition rules were different. The crystallinity increased obviously after the air drawing and collapsing. The crystal grain size increased obviously after the collapsing and drawing in vapor. The change of interplanar spacing was not obvious. The air drawing and collapsing were the key stages to increase the crystallinity of PAN fiber. The collapsing and drawing in vapor were the key stages to increase the crystal grain size of PAN fiber. The orientations in crystalline region could be obtained qualitatively by the two dimensional orientation patterns of PAN fibers in the equatorial direction. The orientation in crystalline region increased obviously after the air drawing and drawing in vapor, while the orientation decreased slightly after heat setting.Fiber A (made by dry-jet wet spinning), fiber B (made by quasi dry-jet wet spinning) and fiber C (made by wet spinning) were studied respectively. The surface of fiber A the smoothest, then followed by fiber B. The grooves could be observed clearly on the surface of fiber C. The change of interplanar spacing was not obvious. The crystal grain size, the crystallinity, the orientation in crystalline region and tensile strength of fibers were all:A>B>C in turn.The crystalline structural evolution of fibers was studied by XRD during the pre-oxidized and carbonized process. The diffraction peaks located at2θ≈17°and29°became weaker during the pre-oxidized process.When the pre-oxidized temperature was245℃, a new peak located at2θ≈25.5°as observed, which was generated by new ordered structure of fibers. When the temperature was265℃, the diffraction peaks located at2θ≈17°and29°disappeared completely and the diffraction peak located at2θ≈25.5°got stronger. The crystal grain size and crystallinity increased first, and then decreased.The change of interplanar spacing was not obvious. The orientation in crystalline region weakened gradually with the increase of pre-oxidized temperature. When the pre-oxidized temperature was lower than220℃, orientation weakened slowly, and when the pre-oxidized temperature was higher than245℃, orientation weakened obviously. When the temperature reached265℃, the amorphous transition finished basically. When the temperature was265℃, the orientation in crystalline region of fiber disappeared. During the carbonized process, the diffraction peaks at θ≈25.5°increased gradually. With the increase of carbonized temperature, the crystal grain size and the crystallinity increased gradually and the interplanar spacing was similar. The orientation in crystalline region linearly increased gradually. |
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