单壁碳纳米管薄膜的致密化：细观离散元模拟与实验验证
Densification of Single-Walled Carbon Nanotube Films: Mesoscopic Distinct Element Method Simulations and Experimental Validation
AI总结：
本研究通过细观离散元模拟与实验验证，揭示了单轴压缩下CNT薄膜的超软致密化机制，发现约75%应变下的结构重组导致厚度显著减小，为调控薄膜性质提供了重要途径。
摘要：
从气溶胶化学沉积反应器中收集的纳米级单壁碳纳米管（CNT）薄膜因其广阔的应用前景而备受关注。这些原始薄膜的致密化提供了一种调控其力学、电学和光学性质的重要途径。为了阐明最终导致性质变化的微观结构重组机制，我们进行了大规模基于矢量的细观离散元方法模拟，并结合电子显微镜和光谱椭偏仪对通过滴涂挥发性液体处理制备的原始和致密化薄膜进行了表征。与显微镜观察结果一致，具有有限厚度的原始CNT薄膜被建模为自组装CNT网络，包含缠结的树枝状管束，其分支向下延伸至单个CNT。对薄膜在单轴压缩下的模拟揭示了一个延伸至约75%应变的超软致密化区域，该区域可能通过蒸发期间液体表面张力产生的表面张力作用得以实现。当卸载时，预压缩样品演变为均匀致密的薄膜，其厚度值取决于预压缩水平和样品微观结构。由我们的光谱椭偏仪证实的厚度显著减小归因于100 nm尺度上发生的潜在结构变化，包括最薄树枝状分支的拉链式闭合。
英文摘要：
Nanometer thin single-walled carbon nanotube (CNT) films collected from the aerosol chemical deposition reactors have gathered attention for their promising applications. Densification of these pristine films provides an important way to manipulate the mechanical, electronic, and optical properties. To elucidate the underlying microstructural level restructuring, which is ultimately responsible for the change in properties, we perform large scale vector-based mesoscopic distinct element method simulations in conjunction with electron microscopy and spectroscopic ellipsometry characterization of pristine and densified films by drop-cast volatile liquid processing. Matching the microscopy observations, pristine CNT films with finite thickness are modeled as self-assembled CNT networks comprising entangled dendritic bundles with branches extending down to individual CNTs. Simulations of the film under uniaxial compression uncover an ultra-soft densification regime extending to a ~75% strain, which is likely accessible with the surface tensional forces arising from liquid surface tension during the evaporation. When removing the loads, the pre-compressed samples evolve into homogeneously densified films with thickness values depending on both the pre-compression level and the sample microstructure. The significant reduction in thickness, confirmed by our spectroscopic ellipsometry, is attributed to the underlying structural changes occurring at the 100 nm scale, including the zipping of the thinnest dendritic branches.
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