| 其他摘要 | ABSTRACT
During the last fifteen years, great achievements have been obtained in the synthesis, characterizations, properties and applications of single-walled carbon nanotubes (SWNTs). However, whether the knowledge obtained from the quasi-one-dimensional system (infinitely long SWNTs) is applicable to ultrashort SWNT structures and three-dimensional SWNT arrays is unknown yet. In this dissertation, the structures and properties of ultrashort SWNTs and three-dimensional SWNT arrays are investigated comprehensively and systematically using multi-scale calculation methods including first principle, density functional theory (DFT), semi-empirical quantum method and the continuum model on the basis of the known knowledge of SWNTs.
Firstly, the geometries, standard heats of formation, molecular strain energies, hybridizations and chemical reactivities are studied, and it is found for both closed and open SWNTs that (1) supershort SWNTs with only one repeating unit are stable; (2) with the increment of tube length, the energy gaps (ε) of zigzag SWNTs decrease monotonically, while a regular fluctuation of ε has been found for armchair SWNTs, with the oscillation decreasing to zero asymptotically; (3) in ultrashort cases, the stability of zigzag SWNTs increases steadily with the increment of tube length, while for armchair SWNTs, it oscillates regularly and becomes stable; (4) the strain energy and hybridization of ultrashort armchair SWNTs are sensitive to the tube length, while in the case of zigzag SWNTs, they only depend on the tube diameter; (5) dramatic misalignment angles of π orbital are found. For SWNTs with close ends, (i) nonplanar strain energy is distributed mainly on fullerene caps; (ii) the matching degree between the fullerene cap and the tube body plays an important role in the hybridization of carbon atoms and the distribution of strain energies.
Secondly, the interaction between infinitely long SWNTs is studied and the calculated results are applied to three-dimensional SWNT arrays, and a self-similar array model is proposed and applied to the understanding of fractality, packing-dependent pores and mechanical properties of SWNTs. It is found that (1) van der Waals (vdW) interaction between SWNTs is mainly contributed by dispersion force, and vdW potential between any SWN-SWNT pair falls on the same curve when the energy and the distance are expressed in units of the well depth and equilibrium vdW gap, respectively; moreover, the vdW potential is mainly contributed by nearest SWNTs and effects from other ones can be neglected; (2) due to the vdW potential, SWNTs are apt to form three-dimensional arrays with a hierarchy structure, which can not be described by the closely-packing model, so a self-similar array model is proposed; (3) the self-similar array model is universal and can describe complex packing configurations of SWNTs using different packing units; (4) on the basis of the self-similar array model, (i) SWNT arrays present fractality, which is mainly from the hierarchy structure, and the fractal dimension is associated with the packing density, and these results are confirmed by nitrogen adsorption and small-angle X-ray scattering experiments. Due to the fractal effect, the surface area of SWNTs measured by the adsorbates with small cross-sectional areas is bigger than that by big adsorbate, which is consistent with the experimental results reported by Fujiwara et al. and Wei et al.; (ii) abundant and regular mesopores are introduced due to the self-similar array of SWNTs, among which inter-bundle pores and inter-array pores are distributed in the ranges of 2-8 nm and 20-120 nm, respectively, which explains the structural source of mesopores found in SWNT ropes and fibers; moreover, based on the diameters and relative volumes of inter-bundle pores and inter-array pores, tube diameter and bundle size are estimated, consistent with the results from high-resolution transmission electron microscopy. Accordingly, the key to control mesopores of SWNTs is to control the secondary packing of SWNTs; (iii) axial Young’s modulus of SWNT ropes decrease dramatically with the increment of their hierarchy levels, which is the reason why the strength of SWNT ropes is much lower than isolated SWNTs. In the micrometer scale, axial Young’s modulus of SWNTs is close to that of commercial carbon fibers, but their specific modulus is much higher, which is independent of the packing configurations.
Through the above research, knowledge of the relationship between the dimension of SWNTs and their properties, such as electronic structures, stability and chemical reactivity, is obtained, and through the self-similar array model, the hierarchy structure of SWNT arrays can be well described. These understandings are of great scientific and technical significance for the application of SWNTs in different scales.
Keywords: single-walled carbon nanotubes, ultrashort structure, three-dimensional arrays, self-similar array model, calculation and modelling |
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