【作者】 葛曷一；

【导师】 王成国；

【作者基本信息】 山东大学 ， 材料学， 2007， 博士

【摘要】 本文以扫描电子显微镜（SEM）、透射电子显微镜（TEM）和高分辨透射电子显微镜（HRTEM）为主要表征手段，并以傅立叶红外吸收光谱（FT-IR）、X射线衍射仪（XRD）、热分析（DTA／TG、DSC／TG）等测试技术相辅助，系统地研究了湿法纺聚丙烯腈（PAN）纤维结构成形机理、预氧化纤维的结构演变、碳化过程纤维的结构演变、共聚单体和工艺条件对纤维微观结构的影响以及PAN纤维的缺陷形成与遗传。初生纤维在凝固浴中的双扩散致使纤维外层和内部致密化程度不同，呈现出皮芯结构。表皮层的结构致密，芯部较疏松，含有孔洞。在凝固浴浓度较高的情况下，适当提高凝固浴温度可使初生纤维的截面趋于圆形，芯部结构趋于致密，结晶量增加。致密化工艺采用温度150℃、时间60s并结合饱和蒸汽压力0.50MPa、蒸汽温度151.7℃的蒸汽牵伸工艺，可以较大程度消除内部孔洞缺陷，提高原丝的均质性与密度。XRD表明，纤维的晶态结构在初生纤维中已基本形成，牵伸、干燥致密化等过程是在基本的晶态结构上提高分子链的有序性，以减少晶格畸变并提高纤维的结晶度。自制原丝的结晶度、取向度和强度高于日本旭化成原丝，但由前者制得的碳纤维性能却不及后者，说明非晶态结构对碳纤维性能也有较大影响。SEM和TEM分析表明，初生纤维的皮芯结构会遗传至PAN原丝。PAN原丝的皮芯结构有多层组织，最外面是一层极薄的表皮，向内依次为表层、内层和芯部。表皮为片层结构，薄且致密，其分子链结晶度高、晶粒细小且沿轴向取向度好。柱形表层的层状结构中晶粒分布均匀，沿轴向定向较好，每层的厚度约1.5um-2.0um。内层也存在层状结构，但分层较模糊。由表皮至内层，层状结构的厚度逐渐增加。原丝芯部组织较疏松，结晶度较低，微晶杂乱无章且较粗大。根据上述表征结果与分析，建立了PAN原丝的结构模型。比较自制原丝和日本旭化成原丝的DTA曲线，后者放热峰温度宽并且有分峰现象，是形成理想的梯形分子结构的主要原因之一。通过FT-IR分析发现，环化反应在较低温度下就开始发生，环化反应先于或优于脱氢反应，侧基氰基团在预氧化阶段逐渐转变成环状结构排列，分子链逐渐发展形成为芳环结构。预氧化纤维O元素含量和密度变化趋势与红外光谱的相对环化率η的变化趋势相同，表明预氧化温度对化学反应的程度有重要的控制作用。XRD分析显示，当温度低于200℃时，氧化、环化反应较弱，但晶区附近非晶区部分的分子链结构能够发生重组，向有序化转变，PAN纤维的分子链晶区有序结构的完善程度提高。结合FT-IR分析可知，预氧化反应首先在非晶区中发生。预氧化纤维的皮芯多层结构与原丝的皮芯结构具有密切的相关性，它由原丝的特定结构经过复杂的化学反应演变而来。预氧化纤维表皮由紧密堆积的片层构成，与纤维轴几乎垂直，皮层呈柱形层状结构，含有尚未环化的PAN晶粒。芯部的层状结构变得十分模糊、不规则并带有微孔。由表皮至芯部，层状结构的厚度逐渐增加，纤维结构的有序性和致密性逐步降低。HRTEM照片显示，预氧化纤维内部存在许多球形结构由原丝的类同结构演变而来，球形颗粒的外层是完全非晶组织，围绕球心呈层状环形结构，而芯部存在部分未完全预氧化的晶区。这种结构表明，PAN原丝的晶粒尺寸及均匀性将直接影响预氧化的程度以及预氧化纤维的组织结构，细晶化是制备优质原丝以及高性能碳纤维的关键因素之一。同时，PAN原丝中分子链的取向性将直接影响预氧化纤维的分子链排列。FT-IR分析表明，预氧化阶段中未环化的—C≡N基团在中温碳化阶段继续反应，中温碳化脱H反应的比例大于脱N反应。预氧化反应形成的N六元环结构经过中温碳化脱H后，再在高温碳化下进一步脱H、脱N形成C的六元环结构，其中碳的含量高达94.54％。SEM和TEM研究表明，碳纤维的皮芯结构与原丝、预氧化纤维的结构极其类似。表皮由许多类似鱼鳞的片层堆叠而成，几乎与纤维轴垂直，结构致密，而芯部结构疏松，含有孔洞。表皮由含有许多纳米级微晶的石墨层平面构成，沿纤维轴向排列有序，微晶沿纤维轴取向好。碳纤维这种皮芯微观结构的非均质性是由原丝和预氧化纤维结构遗传而来。HRTEM研究表明，PAN基碳纤维的分子结构中存在条带结构和球形结构。条带结构的碳层与PAN原丝中的PAN链条带结构状态十分相似。球形结构中的碳网面围绕球心排列，与PAN预氧化纤维的球形结构类似，表明在PAN原丝至碳纤维的演变过程中纤维分子链结构的变化具有密切的相关性。由XRD分析可知，T-700碳纤维和自制碳纤维的002衍射峰都比较宽，但前者的002衍射峰的强度明显高于后者。T-700碳纤维的层面间距d₀₀₂小，石墨网堆叠厚度Lc大以及结晶度高是其拉伸强度高的结构因素。原丝至碳纤维的表面缺陷具有遗传性，如沟槽、划伤和孔洞等。改进聚合及纤维生产工艺、提高生产环境的洁净度及设备的制造精度可有效地减少表面缺陷。原丝至碳纤维的内部缺陷具有极强的相关性，如皮芯结构、芯部疏松和孔洞。通过调整凝固阶段工艺参数能够增大初生纤维皮层比例，提高芯部致密性；通过改善致密化和蒸汽牵伸工艺可以减少内部孔洞；预氧化阶段中采用合适温度和时间使氧的扩散更充分可增大预氧化纤维皮层比例。PAN原丝至碳纤维结构转化的相关性与缺陷形成和遗传的研究表明，提高纤维结构的均质性，增加表皮和皮层厚度，降低芯部比例和缺陷，优化PAN原丝和预氧化纤维的分子链结构以及提高碳纤维碳层的取向和有序堆叠程度是获得高性能碳纤维的必要条件。此外，以（NH₄）₂IA作为第二共聚单体改性PAN的研究证实，P[AN-co-（NH₄）₂IA]较好的亲水性可使其在纺丝工艺中的牵伸倍数提高到12倍。P[AN-co-（NH₄）₂IA]原丝表面缺陷少、芯部结构较致密、结晶度高、晶粒定向程度高。原丝较优的微观结构使P[AN-co-（NH₄）₂IA]在预氧化工艺中的牵伸率提高到10％。同时，（NH₄）₂IA能显著降低纤维预氧化过程中的放热量，减少了预氧化纤维内部热量的蓄积，抑制PAN大分子主链的断裂。最终制得的P[AN-co-（NH₄）₂IA]基碳纤维的拉伸强度为3810MPa，其内部存在许多微晶择优取向的条带结构。更多还原

【Abstract】 In this paper, the structural evolution and interrelationship from PAN nascent fibers to carbon fibers was systematically investigated by means of scanning electron microscopy （SEM）, transmission electron microscopy （TEM） and high resolution transmission electron microscopy （HRTEM）. Fourier transform infrared spectroscopy（FTIR）, X-ray diffraction（XRD）, differential scanning calorimetry （DSC） /were also been used for fibers characterization.In PAN wet-spinning process, the counterdiffusion of the solvent DMSO and coagulation reagent H₂O caused the obvious skin-core structure of PAN nascent fibers, which had compact skin and loose core with pores. It has been found that the temperatures of coagulation baths would greatly influence the cross section and skin-core texture of PAN fiber. With high concentration of coagulation baths, higher temperature increased the rate of counterdiffusion, which led to circular cross section, reduction of defects in the core and higher crystallization. Suitable collapse process and steam drawing process could make the fibers more homogeneous. XRD showed that the crystal structure was initially formed in nascent fibers. The regularity of molecular chains and crystallization were gradually improved by the latter processes. The crystallization, the orientation and the tensile strength of self-made precursor fibers were higher than those of Japanese fibers. However, the properties of the resultant self-made carbon fibers were lower than the latter, which might be accounted for the influence of the non-crystal structure. The images of SEM and TEM indicated that the skin-core structure of PAN precursor fibers was inherited from that of nascent fibers. PAN precursor fibers were composed of four parts. The sheet-like skin, which was compact and homogeneous, had high crystallization and highly oriented structure. The core with low crystallization and some voids was loose, somewhat disorderly and unsystematic. Moreover, the precursor fiber had a pillar-like layered structure along the fiber axis. The average thickness of each layer increased gradually from the skin to the endothecium. Meanwhile, a structural model of PAN precursor fibers has been built.The Japanese precursor fibers had broader exothermic regime and double separated exothermic peaks, which might lead to better cyclization reaction and structure. The result of FTIR spectra of PAN fibers and the stabilized fibers revealed that cyclization reaction took place before dehydrogenation reaction.—C（?）N groups were gradually transformed to cyclization structure. The changes of oxygen element content and density of the fibers in the stabilization process consisted with relative cyclization index （η）. When the temperature was lower than 200℃, oxidation reaction and cyclization reaction were not intense. However, the chains of amorphous phase near crystal area became flexible, which started vibrating, rotating and could form new crystallites. Combing FT-IR analysis, cyclization reaction firstly took place in amorphous phase.The images of SEM and TEM revealed that PAN stabilized fibers had the analogical structure as PAN precursor fibers. The cross section was also skin-core morphology, with a concentric circles structure. The white outmost layer presented the thin dense skin, followed by the cortex, the endothecium and the core. The skin consisted of stacked sheets, which were almost perpendicular to the fiber axis. The cortex had the same pillar-like layered structure as that of PAN precursor fibers. In the core, the structure became disorderly with microvoids. This indicated that the compactness and regularity decreased from the skin to the core. Moreover, there was an indistinct interface between the cortex and the endothecium. The structure of the cortex, which was more regular than that of the endothecium, had some oriented crystallites caused by those unstabilized PAN crystallites. The images of HRTEM showed that many global particles existed in the stabilized fibers. The outer of global particles were amorphous phase, which was annular structure. The inner of global particles had some crystallite areas, which was not completely stabilized. Therefore, The size of crystallites in PAN precursor fibers would severely influence the structure of stabilized fibers. Diminishing the size of crystallites in PAN precursor fibers is a key factor for high performance carbon fibers.Study on the pre-carbonized fibers and carbon fibers by FT-IR suggested that remnant—C（?）N groups continued cyclization reaction in pre-carbonization process, where the ratio of dehydrogenation reaction was higher than denitrogenation reaction. The N hexahydric rings gradually dehydrogenized, denitrogenized and formed C hexahydric rings in carbonized process. XRD analysis indicated that the smaller d₀₀₂, thicker Lc and higher crystallization of T-700 carbon fibers led to the higher tensile strength. The images of SEM and TEM showed that the microstnicture of the carbon fiber could be also represented as a microcomposite, which was composed of four parts from the skin to the core. The high density of sheets packing in its outer skin and the loose structure in the core should be noted. The compact skin of carbon fiber consisted of stacked carbon layers, forming small coherent units （nano-crystallites） in the stacking direction. The stacking direction of layers is preferentially perpendicular to the fiber axis. There were also some pillar-like layered structures forming the cortex of carbon fiber. With the complicated chemical reactions in the stabilization and carbonization processes, the space between layers decreased gradually from precursor fibers to carbon fibers.HRTEM analysis suggested that there were strip microstructure and global microstnicture in PAN-based carbon fibers. The strip microstructure of carbon layers resembled that of PAN molecular chains in precursor fibers. The global microstructure of carbon layers encircled the center, which was similar to that of the stabilized fibers. This indicated that the microstructure of PAN molecular chains had significant structural interrelationship.Surface defects of fibers have transmissibility, such as grooves, scratches and holes, which can be greatly reduced through ameliorating manufacture processes, circumstance and equipment precision. Interior defects have close interrelationship, such as skin-core multilayer morphology, loose core and holes, which can be weakened by adjusting coagulation process, the temperature and time of stabilization process.From the study on the structural interrelation and heredity of defects, it can be concluded that alleviating the defects, such as skin-core morphology, loose core and deliberately optimizing molecular chain structures and improving the orientation of stacked carbon layers are essential to obtain high strength carbon fibers.Acrylonitrile-ammonium itaconate copolymer {P[AN-co-（NH₄）₂IA]} was fabricated by free-radical solution copolymenzation of acrylonitrile （AN） and ammonium itaconate [（NH₄）₂IA]. Due to preferable hydrophilicity and facilitating cyclization reaction in stabilization process, P[AN-co-（NH₄）₂IA ] fibers could increase the draw-ratio in the spinning process and in the stabilization process to attenuate some property-limiting facts in precursor fibers and carbon fibers. The optimized final draw-ratio （12 folds） in the spinning process and stretching ratio （10%） in the stabilization process were introduced. Moreover, （NH₄）₂IA could significantly help in reducing the initiation temperature and heat evolved, broadening exothermic peak and restraining the main molecular chains of PAN from rupture during stabilization process. The tensile strength of P[AN-co-（NH₄）₂IA]-based carbon fibers could reach 3810 MPa. The improvement was due to fewer surface defects, better interior morphology, higher degree of orientation and graphitization.更多还原