【作者】 高学平；

【导师】 朱波；

【作者基本信息】 山东大学 ， 材料学， 2014， 博士

【摘要】 碳纤维的力学性能与其微观结构息息相关,微晶结构是碳纤维强度的基石,而微缺陷结构如纤维表面裂纹、内部微孔等大大降低了纤维的强度；因此以微缺陷为视角,研究并掌握碳纤维生产过程中微缺陷与微晶结构的形成与演变规律,对制定合理的生产工艺,以及制备高性能PAN基碳纤维具有重要应用价值。本文主要采用场发射扫描电镜(FESEM)、同步辐射X射线小角散射(SAXS)与广角散射(WAXS)等测试技术,对PAN纤维在纺丝过程、预氧化过程及碳化过程中表面与断面形貌、内部微孔等微缺陷与微晶结构的形成与演变进行了基础性研究,并根据不同阶段PAN纤维内部微孔的形态差异,建立了低取向微孔结构模型和高取向微孔结构模型,并结合Guinier理论与Ruland理论对数据进行处理,有效表征了纤维内部微孔的变化规律。本文首先对日本东丽T系列PAN基碳纤维T300、T700S、T800H和T1000G的微缺陷与微晶结构以及与力学性能的相关性进行了对比研究。表面形貌显示T300与T800H表面有明显的条纹和沟槽；其中T300直径最大,表面缺陷最多；T800H直径较小,表面条纹与沟槽规整、尺寸小,表面划痕、裂纹等缺陷少。T700S与T1000G表面光滑,但高倍下仍可观察到在数十纳米尺度的条纹与沟槽；T1000G直径最小,表面更加光滑、缺陷最少。SAXS与WAXS数据显示T300内部微孔数量多、尺寸最大、分布宽,微孔取向度、结晶度与结晶取向度都是最差的,所以其力学性能最低。从T300到T1000G,随着强度提高,碳纤维内部微孔尺寸减小,长径比增大,微孔取向度提高；同时结晶度、晶粒尺寸、结晶取向度也有提高。综合对比发现,四种碳纤维的微晶结构参数都在一定的尺度范围内,和微缺陷参数差异相比,其变化较小,不是提高纤维力学性能的关键所在；因此,减少纤维微缺陷的尺寸与数量是目前提高碳纤维拉伸强度的最直接途径。本文在准干喷湿纺条件下,对PAN原丝微缺陷及微晶结构在初级凝固浴、梯度凝固浴、水洗、致密化、蒸汽牵伸等工艺过程中形成与演变规律进行了系统研究。研究发现在较低的初级凝固浴温度下(30℃),可制备表面缺陷少、微孔尺寸小,结晶度适中的初生纤维；提高初级凝固浴牵伸倍率(三倍),可显著降低纤维纤度,减小微孔尺寸,提高纤维的均匀与致密性,并增加纤维的结晶度。经多级凝固浴牵伸后,纤维的纤度更细,纤维表面更加规整、光滑。纤维的内部微孔缺陷减少,尺寸变小,长径比增大,取向性提高。尤其是300℃热管牵伸使纤维内部的网络骨架结构消失,转变为沿纤维轴向排列的束状原纤结构,纤维组织更加致密；经第四级凝固浴后,纤维的结晶度,晶体取向度、晶粒尺寸都有大幅增加。采用5级水洗脱除纤维内残留的溶剂DMSO,但随着水洗时间增加,纤维发生轴向收缩、径向膨胀,纤维直径变大,紧密排列的大原纤板条束分离为细小的板条束,表面条纹变细,沟槽变浅；内部原纤逆分离,形成骨架网络结构,纤维内部微孔尺寸与数量都大幅增加,造成纤维的取向度、致密性变差；同时,高温水洗使纤维大分子链产生解取向,造成纤维的结晶度与取向度不断下降。致密化1圈时,纤维内溶胀的水分急剧挥发,纤维直径显著变细,同时纤维内部微孔的尺寸与数量大幅减小,微孔的长径比和沿纤维轴向的取向度都有提高,纤维变致密；致密化5圈时,微孔的尺寸与数量仍变化较大；但在致密化10圈时,微孔结构变化量较小,致密化效果减弱。延长致密化时间,纤维的结晶度、晶体取向度以及晶粒尺寸都不断增加。在固定牵伸倍率下,提高蒸汽压力,可进一步增强大分子链段的活动能力,表面原纤边缘更加圆滑。在蒸汽压为0.3MPa时,纤维直径大幅减小,微孔沿轴向被拉长,而短轴尺寸变小,微孔的长径比、取向度都增加；蒸汽压为0.4MPa时,纤维微孔结构参数仍有明显改善；蒸汽压为0.5MPa时,由于牵伸倍率的限制,微孔参数改善效果明显减弱。纤维的结晶度、取向度等参数,随蒸气压力的提高,一直稳定增加。对两种不同PAN原丝A0和B0进行了相同的预氧化热处理,并对纤维微缺陷及微晶结构的演变规律进行了对比研究。在预氧化过程中,A0原丝光滑的表面逐渐出现沿纤维轴向的条纹与沟槽,而B0原丝表面粗大的沟槽逐渐变浅。随着预氧化温度的提高,纤维韧性降低,断口形貌逐渐变为颗粒状结构,并在高温预氧化阶段出现皮芯结构,尤其是A10皮芯结构严重。A0与B0两纤维内部微孔变化趋势相同,微孔的径向平均尺寸先变大后变小,轴向平均尺寸先变小后变大,取向度先略有变差后又逐渐提高,微孔相对体积逐渐增加,但A10生成了更多、更大尺寸的微孔,微孔尺寸分布较宽。预氧化初期,纤维解取向造成结晶度降低,但晶粒尺寸略有长大；预氧化中期,晶区开始氧化、环化,纤维结晶度下降,晶粒尺寸减小：预氧化后期,氧化、环化反应剧烈,纤维结晶度、晶粒尺寸和取向度都明显减小,并生成新的结构,其中A10耐热梯形结构的转变不如B10完善。对A10与B10两预氧化纤维进行了500-1000℃的碳化实验,数据显示碳化纤维的微缺陷及微晶结构具有很强的遗传性。随碳化温度提高,纤维直径变细,纤维表面保留原特征,但表面质量提高；SAXS数据显示预氧化纤维内部微孔会遗传到碳化纤维中,并随温度升高尺寸增加；随着非碳元素的脱除,会产生更多微孔。与B10相比,皮芯结构严重的A10纤维微缺陷的尺寸与数量增幅较大,尺寸分布更宽,甚至出现了超过1μm的大孔缺陷；随碳化温度的升高,两纤维的石墨微晶逐渐完善,取向度提高,两纤维的微晶结构差异较小。A1000碳化纤维的强度为1.52Gpa,明显低于B1000碳化纤维的2.68GPa；拉伸强度的差异主要由大孔缺陷造成的,数量极少的大孔对纤维力学性能的影响远大于大量纳米级微孔。为了更好研究工艺参数与PAN纤维微结构的关联性,研制了一套同步辐射原位在线检测纤维微结构的系统,该系统可实现室温至900℃连续升温热处理,温度控制精度为±2℃；冷却循环水套保证了炉外壁的热辐射温度小于100℃；采用张力伺服控制系统,实现了走丝与张力的精确控制；气氛保护系统,满足了纤维碳化试验的要求。该系统所有参数的设置,都实现了远程电脑控制。借助同步辐射WAXS/SAXS大装置,可实时、原位在线跟踪纤维微结构的演化过程,为纤维微结构的原位在线表征提供了技术平台。更多还原

【Abstract】 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.更多还原