| The mechanical properties of Polyacrylonitrile (PAN)-based carbon fiber are closely related with their microstructure. Microcrystalline structure is the foundation of carbon fiber strength, but the micro-defects, such as surface defects, and internal micro-voids has greatly reduced the strength of the fiber. Therefore, In the perspective of micro-defects, study and master the formation and evolution of the microstructure during carbon fiber production process, to make reasonable production process, as well as the preparation of high performance PAN-based carbon fiber, has significance and application value. In this paper, the testing technology, such as field emission scanning electron microscopy (FESEM), synchrotron radiation SAXS and WAXS were mainly employed, the formation and evolution of the fiber surface and cross section topography, micro-voids, and microcrystalline structure, etc. were studied, throughout the whole production process of PAN precursor molding, pre-oxidation, carbonation; And according to the different shapes of the micro-voids in PAN fibers at different stages, low orientation micro-voids structure model and hige orientation micro-voids structure model are made,Guinier theory and Ruland theory are adopted to process SAXS data,which effectively characterizing the variation of micro-voids structure.The differences in micro-defects and microcrystalline structure and its effects on the mechanical properties of Japan Toray high-strength carbon fiber T300, T700S, T800H and T1000G were systematically studied. The surface topography shows that the surface of T300and T800H carbon fibers have obvious stripes and grooves; the diameter of T300is the biggest and has the most surface defects; the diameter of T800H gets smaller, the stripes and grooves become regular and size reduces, surface defects also get fewer. The surface of T700S T1000G is smooth, but nanometer scale stripes and grooves can still be seen under high resolution; the diameter of T1000G is the smallest, which surface is smoother and has less defects. The datas of SAXS and WAXS show that the quantity and size distribution width of internal voids in T300are maximum, the degree of crystallinity is minimum, the micro-voids orientation and crystal orientation are the worst, so its mechanical properties is the lowest; With the improvement of fiber strength, the size of micro-voids is decreased, the aspect ratio becomes large, the orientation of micro-voids increases;at the same time crystallinity, grain size, crystal orientation also improves.By comprehensive comparison we found that the crystallite structure parameters of the four high-strength carbon fibers are within a certain scale, compared with the difference of micro-defects, the crystallite structure is not the key to impact the fiber mechanical properties;Thus, reducing micro-defects (surface defects and internal defects) is the most direct way to increase the strength of carbon fiber at present.The formation and evolution of the micro-defects and microcrystalline structure during primary coagulation bath, the gradient coagulation bath, washing bath, collapsing, drawing in vapor, etc. processes under quasi-dry jet wet spinning, are detailed studied, it has a guiding role to improve the PAN fiber structure and improve the performance of carbon fiber. We found that the nascent fibers have less surface defects, smaller micro-voids size, and moderate crystallinity, under a lower coagulation bath temperature; Raising the draft ratio, can significantly reduce the fiber denier, decrease the size of micro-voids, improve the uniformity and compactness of fibers, and improve the crystallinity. Under multi-step gradient coagulation bath drawing, the denier of fibers gets finer, surface becomes more regular and smoother, the number and size of micro-voids gradually reduce and become smaller, the aspect ratio increases, the orientation is also improved. Especially3YU drawing in hot air makes network skeleton structure of fibers inside disappear, and turn to a bundle fibrils aligned along the fiber axis, fibers get more dense; After4YU the crystallinity, crystal orientation degree and grain size of fibers have increased dramatically. With the washing time increasing, the fiber occurs radial expansion, axial shrinkage and the diameter becomes bigger, closely spaced fibril separated into small bundle, surface stripes get thinner, grooves become shallower. Part of the fibrils inversely separates and forms the skeleton network structure; the number and size of micro-voids inner fibers have substantially increased, the orientation and density of fiber get deteriorated; High temperature water washing makes molecular chains disoriented, crystallinity and orientation of the fibers decline.Being collapsed one circle, the water swelling in fibers sharply volatilize, the fiber diameters get significantly smaller, and at the same time, the number and size of micro-voids inner fibers greatly reduce, the aspect ratio and the orientation degree along the fiber axis of micro-voids also increases, all that made fibers more densificaion. Being collapsed five circles, the size and number of micro-voids are largely reduced; but at the tenth circle the effect of collapsing weakened. With increasing collapsing time, Crystallinity, grain size and crystal orientation of the fibers increase steadily.Under a fixed drawing ratio, improving vapor pressure, activity of macromolecular chain segments can be further enhanced, fibrils edges become smoother. As the vapor pressure reachs0.3MPa, the fiber diameter substantially reduces, the major axis of micro-voids is elongated, the minor axis dimension is greatly reduced, the aspect ratio and the degree of orientation are increased; the vapor pressure being up to0.4Mpa, micro-voids parameters can still be improved in a certain degree; but the effect of improving the pore parameters by vapor pressure of0.5MPa, because of the limitation of drawing ratio, significantly reduced. The microcrystal structure parameters of fibers such as crystallinity, orientation degree, etc., with the vapor pressure increasing, grow steadily.Under the same pre-oxidation process, the evolution of micro-defects and microcrystal structure of two different PAN fibers AO and BO are systematically studied. With the pre-oxidation process carrying out, shallow grooves along the fiber axis gradually emerge in the smooth surface of AO fiber, while the grooves in the surface of BO fiber become shallow gradually. With the rising of pre-oxidation temperature, fiber toughness gets lower, fracture morphology gradually becomes granular structure and skin-core structure in fibers began to appear in the stage of high-temperature pre-oxidation, especially the skin-core structure in A10fibers is more serious. During the pre-oxidation process, the Variation of micro-voids in AO is similar to BO fiber, the radial average size of micro-voids becomes larger firstly and then smaller, axial average size increases gradually, orientation degree along the fiber axis gradually improves after a first slight deterioration, the relative volume of micro-voids increases gradually, A10fiber generates more and larger micro-voids than B10fiber. In the initial pre-oxidation stage, the deterioration of macromolecular chain orientation reduces crystallinity, but grain size grows slightly;In the middle of the pre-oxidation, the crystalline region starts cyclization reaction, Crystallinity and grain size begin to decline; In the late pre-oxidation, cyclization reaction and oxidation reaction becomes severe, crystallinity, grain size and orientation degree of fibers are obviously reduced, and a new structure generates; while the Heat resistant trapezoidal structure in A10is less perfect than that in B10fiber.A10and B10pre-oxidation fiber are carbonized at500-1000℃, the test data show that the micro-defects and microcrystal structure of pre-oxidation fibers have a strong hereditary during carbonization.With the increasing of carbonization temperature, the fibril becomes finer and more dense, fiber diameter gets smaller, and surface quality of fibers improves, but the original surface characteristics still remains. SAXS data shows that the micro-voids inner fibers are genetic in the process of carbonization and size increases with temperature rising; More micro-voids are formed with the removal of non-carbon elements. Compared with the B10fibers, the size and the number increment of micro-voids inner A10fiber with severe skin-core structure is larger, the size distribution is wider, A1000fiber even forms big voids more than1μm and other defects; With the temperature increasing, graphite microcrystalline of fiber A10and fiber B10gradually get perfect, the orientation degree increasing, the difference of microcrystalline structure parameter between A1000fiber and B1000fiber is small. A1000fiber strength is1.52Gpa, significantly lower than2.68GPa of B1000fiber; The difference in tensile strength mainly caused by macro-voids in A1000fiber, a few macro-voids have much larger effect on the mechanical properties of the fiber than a large number of nano-scale micro-voids.An in-situ online system that detecting microstructure of fibers on synchrotron radiation facility is developed. The system using remote computer control, by the means of synchrotron radiation SAXS/WAXS, the relationships between process and microstructure of fibers can be characterized with the range of900℃. By debugging and testing, the system can test the structure changes of carbon fiber, aramid fiber and uhmwpe fiber in-situ online under different temperture, different strain or different atmosphere. According to the requirements of experiment, the functions such as temperature, drawing tension, fiber feeding speed and protective atmosphere can be set. Given the advantages of the synchrotron radiation, the system provides conditions characterizing the microstructureonline of pan fibers in-situ online. |
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