沥青基碳纤维是重要的工程碳纤维之一,在航空航天、国防军工等领域具有广泛应用。沥青基碳纤维制备流程长、影响因素多,归咎于对其制备工艺、反应机理及调控方法的认识不足,迄今工业化生产强度性能优异的沥青基碳纤维仍面临诸多困难。以碳纤维生产流程为主线,针对原料预处理、沥青前驱体调制、熔融纺丝、预氧化及炭化与石墨化的现状、方法、机理及调控进行深入探讨分析。沥青基碳纤维的原料有煤系、石油系、生物质系、高分子系和纯化合物,原料预处理方法主要有蒸馏和萃取;沥青前驱体的调制本质是以氢转移反应和自由基反应并行的液相炭化反应,包括环化、芳构化、低聚和缩聚等复杂的反应过程。芳烃低聚物在高温下缩聚形成稠环芳烃片状大分子的基本结构单元,基本结构单元经过热运动不断聚集,形成聚集体;由于范德华力作用使基本结构单元相互堆叠,形成平行堆积体,进而通过层积、堆积等过程形成基本构筑单元聚集体。基本构筑单元聚集体在不同反应条件下可以转变为长程无序、短程有序的难石墨化结构,也可以转变为长程有序、短程有序的易石墨化结构。以煤系、石油系和生物质系物质为原料调制沥青前驱体时,因其反应活性好,通常选用设备简单、操作容易、成本低廉的热缩聚法;而当以萘、甲基萘等纯化合物为原料时,因其反应活性差,需要较高的温度才能引发自由基反应,故通常采用催化合成法。熔融纺丝宏观上起到塑型的作用,通过改变纺丝参数和喷丝板的形状分别可以调节碳纤维的直径和形状;此外,熔融纺丝对中间相沥青前驱体的重排分子作用尤为明显,使得石墨微晶更易沿轴向一维排列形成有序的石墨晶体结构。为使沥青纤维在炭化过程中不发生融化,需要将沥青纤维由热塑性转变为热固性,即通过轻微的氧化反应,让沥青中的稠环芳烃分子经历氧化、缩合、分解和脱水,从而形成特定的交联结构,最终在纤维结构中生成一定量的含氧官能团。炭化是使预氧化后的纤维经过复杂的化学反应和结构转变形成具有炭材料分子结构特征的碳纤维。炭化过程中,石墨微晶发生重排,非碳原子被移除,碳元素含量进一步升高;经过石墨化后,石墨晶体结构更加完整、取向度更高,使碳纤维向石墨纤维转变,强度性能更加优异。另外,针对沥青基碳纤维的突破方向,提出了从分子维度定向可控制备的思路;并为碳纤维生产过程中耗时耗能的预氧化步骤的优化改进提供了具体方法。

Pitch-based carbon fiber is a crucial engineering material extensively utilized in aerospace, national defense, and military industries, etc. However, the lengthy preparation process and numerous influencing factors hinder the production of pitch-based carbon fiberwith exceptional mechanical performance. This work delved into the current state, methods, mechanisms, and regulation of raw materialpretreatment, pitch precursor preparation and control, melt - spinning, stabilization, carbonization, and graphitization to addressthese challenges comprehensively. It emphasized that the breakthrough direction for pitch-based carbon fiber lies in its molecular-level directional and controllable preparation. The raw materials for pitch-based carbon fiber include coal-based, petroleum-based, biomassbased, and polymer-based materials as well as pure compounds. Distillation and extraction are the common methods for the pretreatment ofraw materials. The preparation of pitch precursor is essentially a liquid-phase carbonization reaction that involves parallel hydrogen transferreactions and free radical reactions, including complex reaction processes such as cyclization, aromatization, oligomerization, and condensation. Aromatic oligomers condense at high temperatures to form the basic structural units of polycyclic aromatic hydrocarbon flakes,which continuously aggregate through thermal effect to form aggregates. The basic structural units stack together with the force of van derWaals, forming parallel stacking bodies to convert to basic building unit aggregates. The basic building unit aggregates can transform intolong-range disordered and short-range ordered non-graphitization structures under different reaction conditions, as well as into longrange ordered and short-range ordered graphitization structures. When the pitch precursors derive from coal-based, petroleum-based, andbiomass-based materials, the thermal condensation method with simple equipment, easy operation, and low cost is usually selected owingto their high reaction activity. In contrast, higher temperatures are required to initiate free radical reactions when pure compounds with lowreactivity such as naphthalene and methyl naphthalene are used as raw materials. Therefore, catalytic synthesis is usually used. Melt-spinning plays a role in shaping, and the diameter and shape of carbon fibers can be adjusted by changing the spinning parameters and theshape of the spinneret. Moreover, the molecular rearrangement effect of melt-spinning on the precursor of mesophase pitch is particularlysignificant, making it easier for graphite microcrystals to form ordered graphite crystal structures along the axial one-dimensional arrangement. To prevent the melting of pitch fibers during the carbonization process, it is necessary to transform the pitch fibers from thermoplastic to thermosetting, that is, through a slight oxidation reaction, the polycyclic aromatic hydrocarbon molecules in the pitch undergo oxidation, condensation, decomposition, and dehydration, forming a specific cross-linked structure, and ultimately generating a certain amountof oxygen-containing functional groups in the fiber structure. Carbonization is the formation of carbon fibers with molecular structural characteristics of carbon materials through complex chemical reactions and structural transformations of stabilized fibers. During the carbonization process, graphite microcrystals undergo rearrangement, non-carbon atoms are removed, and the carbon element content further increases. After graphitization, the graphite crystal structure becomes more complete and highly oriented, which transforms carbon fibers into graphite fibers and enhances their strength performance. In addition, it provides clear guidance for optimizing and improving time-consuming and energy-intensive stabilization steps in carbon fiber production.

0 引言

1 原料预处理

2 沥青前驱体调制

3 熔融纺丝

4 预氧化

5 炭化与石墨化

6 结语与展望