碳纳米管与石墨烯都是碳原子以sp2杂化轨道构筑的单分子结构。因其独特的结构与优异的力、热和电学等性质，成为当前材料学与凝聚态物理领域的研究热点。碳纳米结构的一个潜在用途是作为构筑单元组装成宏观材料，将碳纳米结构的优异性能发挥到宏观水平。目前已经发展了许多自下而上的方法制备碳纳米结构宏观材料，其中最重要的两种材料形态是纤维和薄膜。经过数十年的努力，虽然对碳纳米结构宏观材料的研究取得了许多重要进展，但是它们的若干关键性能与其构筑单元相比还存在着巨大差距。例如，碳纳米管纤维和石墨烯薄膜的拉伸强度都比其构筑单元 (碳纳米管和石墨烯) 的拉伸强度低1-2数量级。理解材料的微结构与性能之间的关联是材料设计的关键，本文使用实验、模拟与理论分析相结合的方法，研究了以碳纳米管纤维和石墨烯薄膜为代表的碳纳米结构宏观材料的微观结构与宏观性能之间的关联，主要内容如下：(1) 建立了多尺度力学理论模型。基于碳纳米管纤维和石墨烯薄膜的实验表征，提出三级微结构的划分。以碳纳米管纤维为研究对象，将缺陷、界面、卷曲等多级微结构特征引入，建立了相应的不同尺度力学模型。理论分析结果揭示了造成碳纳米管纤维拉伸强度下降的三个关键因素：碳纳米结构缺陷、界面间不充分载荷传递、微结构的低密度与低取向特征，并基于取向有序化、成束化、致密化给出了制备高强度碳纳米管纤维的工艺优化方向。(2) 揭示微结构对力学性能影响。基于力学模型与第一性原理计算，研究了石墨烯薄膜层间交联修饰对于石墨烯片层面内与层间力学性能的双重作用。分析了层间交联、片层尺寸等微结构对石墨烯薄膜力学性能的影响，给出了不同条件下石墨烯薄膜界面修饰所需的最优交联密度，并预报了石墨烯薄膜拉伸强度、杨氏模量等力学性能的上限。(3) 探索力学载荷对材料微结构的调控方法。褶皱与层状结构是石墨烯薄膜的两个微结构特征，对于石墨烯薄膜的性能与功能起着关键作用。实验发现褶皱结构赋予石墨烯薄膜负泊松比行为，以及湿加工预应力可以调控褶皱结构，进而强化石墨烯薄膜的杨氏模量等力学性能。基于力学模型与第一性原理计算进行材料设计，发现应变调控层间距的有效方法，有望用于选择性传质等领域。
Carbon nanotube and graphene, the sp2 nanostructures of carbon atoms, have received widespread attention from materials science and solid state physics due to their unique structures and fascinating mechanical, thermal and electrical properties. To harness the exceptional attributes of these carbon nanostructures, many bottom-up technologies have been developed to assembly individual carbon nanostructures into macroscopically ordered materials, of which macroscopic fibers and films are two of the most representative forms. However, after several decades of intensive work, some key performances of the macroscopic carbon nanostructured fibers and films still have huge gap with their building blocks (carbon nanotubes and graphene), and are even not competitive with commercial macroscopic materials, such as carbon fiber. In this thesis, theoretical analysis, experimental characterization, and numerical simulations have been used to investigate the mechanics and microstructures of macroscopic carbon nanostructured fibers and films. The results of the thesis work are summarized below.First, a multi-scale model is developed based on the experimentally characterized hierarchical structures of macroscopic carbon nanostructured materials. Taking the carbon nanotube fiber as a model system, we break down the microstructural complexity of carbon nanotube fibers into three levels: primary (individual carbon nanotubes), secondary (bundles with closely-packed carbon nanotubes) and tertiary (the fiber as an assembly of the bundles), and developed theoretical models to characterize the reduction of the tensile strength across these levels. Three key factors including defects in nanotubes, insufficient load transfer in nanotube bundles, wavy and loose morphology in fibers are given, which is inspirative for design/improvement on high strength carbon nanotube fibers. In addition, similar three-level hierarchical structures are also found in the graphene films and fibers, so the model can be generalized to study the mechanics and structures of graphene layer-by-layer materials.Second, the effect of key microstructures on the mechanical performance of carbon nanostructured is explored. The mechanical properties of macroscopic carbon nanostructured fibers and films are often tuned by surface functionalization that cross-links the building blocks in the assembly. Taking the graphene film as an example, combining first-principles calculations and continuum-mechanics-based model analysis, we find that the functionalization weakens the intrinsic mechanical resistance of graphene, while it enhances interlayer load transfer through cross-linking. There is optimum crosslinking densities or concentrations of the surface functional groups that maximize the overall tensile stiffness, tensile strength and strain to failure of graphene-derived layer-by-layer assemblies, arising from the competition between intralayer and interlayer load-bearing mechanisms, as defined by the type of functionalization and size of graphene sheets. Our work quantifies the ultimate mechanical performance of graphene films, on condition that their microstructures and functionalization could be adequately controlled in the fabrication process.Third, the methods of tailoring the microstructures by mechanical loads are probed. Wrinkled and layered microstructures are the key hierarchical structures of graphene film, and precisely controlling these microstructures plays a critical role in its functional applications. For the wrinkled microstructures, we demonstrate that it endows the graphene film with remarkable negative Poisson’s ratio, and the wrinkled structures can be tuned by the pre-stress method. For the layered microstructures, bio-inspired graphene films with sheets cross-linked by aligned covalent bonds are proposed in design to endow a controlled interlayer spacing while preserving structural and mechanical stabilities by prohibiting swelling. First-principles calculations and continuum mechanics based model analysis are combined to explore the feasibility of this protocol, by considering the microstructures of graphene films that have recently been demonstrated to offer exceptional performance in selective mass transport.
