木质素基碳纤维高取向度类石墨化碳结晶结构的调控与形成机制

The poor mechanical properties of lignin based carbon fibers were fundamentally caused by small amount and less order of analogously graphitic carbon crystal structure. Mixing with crystal materials in previous research could not change the carbon structure with low crystalline and less orientation in lignin, inducing less improvement of mechanical properties. The transformation from carbon skeleton of lignin into analogously graphitic carbon crystal structure might be achieved by catalytic graphitization of metal catalyst. In current research, lignin based carbon fibers were prepared from soda lignin mixed with metal catalyst by melt-spinning and thermalstabilization as well as carbonization. The project would focus on controllable preparation of analogously graphitic carbon crystal structure of lignin based carbon fibers by modulating the key parameters. It will establish the structure-property relationship between analogously graphitic carbon crystal structure and mechanical properties. Besides, the formation mechanism of analogously graphitic carbon crystal structure can also be investigated. Finally, the unified system of crystal carbon structure, mechanical properties and formation mechanism will be constructed. Prospective results of this project will provide the scientific evidence for the design and preparation of lignin-based carbon fibers with high mechanical properties, and are useful to the development of lignin-based carbon fibers reinforced composite materials.

类石墨化碳结晶结构少且无序排列是木质素基碳纤维力学性能低下的根本原因。目前采用的与结晶结构物质共混方法未能改变木质素碳骨架在碳化后低结晶、无取向的状态，力学性能提高有限。金属催化剂的催化石墨化性质有望实现木质素碳骨架向类石墨化碳结晶结构转变。本项目以碱木质素为原料，与金属催化剂均匀混合，经熔融纺丝、热稳定化和碳化处理制备木质素基碳纤维。重点通过调节制备工艺参数实现木质素基碳纤维类石墨化碳结晶结构的可控制备；建立类石墨化碳结晶结构与力学性能之间的构效关系；揭示类石墨化碳结晶结构的形成机制，最终构建结构与性能统一的“碳结晶结构—力学性能—形成机制”关联系统。本项目预期成果将为高力学性能木质素基碳纤维的设计与制备提供科学依据，对其增强复合材料的开发具有重要意义。

类石墨化碳结晶结构少且无序排列是造成木质素基碳纤维力学性能相对低下的根本原因，限制其广泛应用。以往采用的与结晶结构物质共混方法未能改变木质素碳骨架在碳化后低结晶、无取向的状态，力学性能提高有限。基于金属催化剂催化石墨化的性质，本项目提出催化剂在碳化过程催化木质素中碳结构向类石墨化碳结晶结构生长排列的方法，重点研究不同金属催化剂对木质素基纤维成形、热稳定化和碳化过程的影响，探讨木质素在形成木质素基碳纤维时类石墨化碳结晶结构生长排列的特征，分析类石墨化碳结晶结构对木质素基碳纤维力学性能的影响并建立它们之间的构效关系，揭示力学性能优良的木质素基碳纤维类石墨化碳结晶结构的演变规律，阐明木质素基纤维生长类石墨化碳结晶结构的形成机理。结合精准的化学测定技术和先进的仪器分析手段，本项目对纤维材料的各方面性能进行了表征，结果表明，化合物型金属催化剂优于氧化物型金属催化剂和单质型金属催化剂，在其催化后可以在木质素基碳纤维中明显观察到碳微晶尺寸更大、排列更有序的类石墨化碳结晶结构。添加催化剂均可在不同程度上提高碳纤维的力学性能，且在化合物型金属催化剂条件下提高的效果更明显；但过量催化剂的添加会使碳纤维中出现团簇现象，导致碳纤维在受拉力时更容易断裂，反而降低力学性能；碳结构的结晶尺寸与碳纤维抗拉强度和弹性模量呈正相关，并且后者与晶面尺寸的相关性更好。碳化过程中，木质素在低温时断裂醚键产生自由基，继而形成小分子气体和水；木质素中的芳香环结构在中高温时相互结合形成多稠苯环结构，并随温度升高逐渐向碳六环网状结构转变；碳六环网状结构的La和Lc方向的尺寸在高温碳化时均出现增加，尤其是在La方向上的尺寸和层间距增加较为明显，并在恒温阶段基本保持不变。本项目的成果可为高力学性能木质素基碳纤维的设计与制备提供科学依据，对其增强复合材料的开发具有重要意义。