Abstract
In the practical application of carbon nanotubes (CNTs), some serious problems greatly hinder the theoretical high-performance of CNTs, such as their disordered distribution and agglomeration. One possible route of overcoming these problems is applying individual CNTs as building blocks to fabricate CNT-based networks. Junctions are the most important nanostructures of CNT-based networks by connecting two or more CNTs with covalent chemical bonds. Various types of geometry, such as T-, X-, and Y-shaped CNT junctions, have been achieved in experiments and analysed in the theoretical studies. Compared with the van der Waals (vdW) interactions, covalent junctions can provide more robust connections between individual CNTs, resulting in great enhancement of their mechanical performance.Super carbon nanotube (SCNT) is a CNT-based network fabricated by the hierarchical assembly of single-walled CNTs. According to the bio-inspired studies, hierarchical fabrication is a common material assembly strategy to obtain high-performance materials, which is hierarchically repeating the same geometric unit to fabricate the self-similar structures from nano-scope to macro-scope. With the hierarchical structure, SCNTs have shown some superior performances in terms of the mechanical, electrical and thermal properties. Moreover, the hierarchical structure enables the designable properties of SCNTs for various application purposes.
Apart from that, the network geometry makes SCNTs perfect reinforcements for the polymer matrix. With the introduction of junctions, the network structure may improve the integrity of the CNT reinforcements, as well as the interfacial performance between the polymer matrix and the reinforcements. With the tubular-like geometry and large surface area, SCNTs could be promising candidates for various applications, such as nano-transportation, absorption and filtration. The designable feature also widens the applicable scope of SCNT. Therefore, it is necessary to have systematic investigations on SCNTs and have a better understanding of the geometric effect on different properties.
In this thesis, the mechanical properties of SCNTs and relevant polymer nanocomposites are explored, which represent the typical characteristics of the CNT-based honeycomb networks. A series of atomistic simulations are carried out to reveal the fundamental mechanisms behind the performances from the atomistic view. There are four major parts of the work in this thesis: (1) The tensile deformation and fracture process of SCNTs are investigated via molecular dynamics simulations. Hierarchical deformation mechanism of SCNTs is revealed by characterizing the detailed structure evolution, which can explain the geometry effect on mechanical properties. (2) Reversibility and radial deformation characteristics of SCNTs are investigated based on the molecular dynamics simulations. The criterion of reversibility is revealed to depend on new-appearing defects. With the proper structure design, the radial shrinkage of SCNT can reach up to 66% and even larger. The specific rules on chirality and CNT length are concluded. Moreover, the variations of the Poisson’s ratios within the reversible ranges are found to reflect the deformation modes of hierarchical structures. (3) The influence of defects on the bearing capacities of SCNTs is studied through a series of MD simulations. Bearing capacities of SCNTs are controlled by different impact factors, including defect number, location, continuity, and arranging direction. The reduction of bearing capacities is essentially controlled by the force concentration degree induced by the defects, which can be generally predicted by the static force analysis. (4) The potentials of CNT-based network on reinforcing polymer materials are explored by evaluating the influence of connected-CNT junction on the mechanical performance of polymer matrix. From the evolution of energy and morphology, lower mobility and better interfacial performance are the major contributions of connected-CNT reinforcements, especially for lower temperatures and stiffer polymer matrix.
| Date of Award | 3 Sept 2018 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Xiaoqiao HE (Supervisor) |