Abstract
As a carbon material, helical carbon nanotubes (HCNTs) show excellent mechanical, electrical, thermal and wave absorption properties due to its special three-dimensional spring structures. The superior properties make HCNTs suitable for mechanical sensors, nanoelectromechanical systems, supercapacitor, micro/nano spring, hydrogen storage and reinforcement materials in composites. Through theoretical and experimental studies, HCNTs show superior mechanical properties, such as excellent ductility, large elastic limit, and great energy absorption ability. Moreover, the properties of HCNTs are dependent on the geometries, such as the pitch length, helix angle and the tube diameter. Since the non-hexagonal defects are essential for the construction of HCNTs based on the observation in experiments and theoretical properties, the geometrical properties can be adjusted by changing the number, position or the type of introducing defects in HCNTs.Furthermore, HCNTs are desirable nanofillers in the composites due to its 3D
spring structures which can provide large contact area and strong physical interlock between matrix and HCNTs. The HCNTs can help to improve the ductility and the energy absorption of composites. The mechanical properties of composites can also be adjusted by introducing the skillfully designed HCNTs with different geometries. Hence, it is necessary to exhaustively investigate the effect of geometries on the properties of HCNTs.
In this thesis, the effect of defect distribution and pitch length of HCNTs fabricated by different construction methods on their tensile and compressive mechanical properties is comprehensively studied. Twelve examples of HCNTs with various defect distribution are constructed according to three widely used methods based on two parent toroidal carbon nanotubes (TCNTs). Their mechanical performances are studied by using molecular dynamic (MD) simulations through tensile and compressive tests. The results reveal that the spring constant, ductility, strength, gravimetric energy density and deformation process obviously depend on the defect distribution, indicating that the mechanical properties of HCNTs can be enhanced greatly by skillfully introducing defect distribution for specific purposes. Remarkably, the tensile stiffness (1.89~68.13 nN/nm) and ductility (84%~794.8%) can be adjusted by changing the defect positions and geometries of HCNTs. The force-strain curves of HCNTs under tensile loads show sawtooth-like patterns and the oscillations of force-strain curves can be controlled by the defect distribution to produce the high energy absorbing capacity. Furthermore, all HCNTs show excellent reversibility under compressive loads, and energy storage densities ranging from 53.31 to 859.67 J/g are achieved by adjusting the defect distribution and pitch length. The findings are meaningful for scientists to design HCNTs with high performance.
Furthermore, defects are inevitably introduced during the manufacturing process of carbon nanotubes, which significantly impact the performance of the carbon nanotubes. Hence, six samples of defective HCNTs with vacancy and Stone-Wales defects are constructed by introducing the defects in the different positions. The results show obvious decrease of the spring constant and elastic limit of defected HCNTs, which results in the lower energy storage ability during the elastic range compared with the perfect HCNTs. However, the defected HCNTs exhibit better ductility (138.9%) and higher absorbing ability (1539.93 J/g) during the fracture process since introduced defects change the deformation pattern. Furthermore, among the defected HCNTs, the stiffness (1.48 nN/nm~1.93 nN/nm), elastic limit (75.2%~88.7%), ductility (108.5%~138.9%) and deformation pattern can be adjusted by changing the position or the type of defects. This study firstly provides the insight into the effects of Stone-Wales (SW) and vacancy defects on the mechanical properties of HCNTs, and the obtained results are meaningful for designing HCNTs with specified properties by introducing defects.
Inspired by the high performance of entwined structures in the nature, such as DNA molecular, plant twining stems and vining plants, it is expected that the entwined HCNTs can also possess superior mechanical properties. Herein, the entwined HCNTs bundles (EHCNT) with one to six strands are constructed based on the single HCNTs. The effect of the number of strands and the temperature on the mechanical properties of EHCNTs are investigated through MD simulations. The results show that the increase of the number of entwined HCNTs will reduce the elastic limit and EHCNTs have the better ductility than single HCNTs. By contrast, the entwined HCNTs with 4 strands possess the largest strength of 3.559 GPa. Moreover, the mechanical properties of EHCNTs with 4-strands decrease with the increase of the temperature. And the fracture process of EHCNTs with 4 strands under different temperature also shows different pattern where the necking phenomenon and interaction between the strands are observed under high temperature. The results are meaningful for researchers to fabricate HCNT bundles with tailored properties for diverse applications.
The HCNT with different geometrical properties are constructed and incorporated into the nanocomposites for the investigation of anti-crack mechanism. The interfacial mechanical properties of the nanocomposites reinforced with straight carbon nanotubes and various types of HCNTs are investigated through the pullout of HCNT in the crack propagation by using molecular dynamics. The results show that the pullout force of HCNT is much higher than that of CNT because the physical interlock between HCNTs and matrices is much stronger than the van der Waals (vdW) interactions between CNTs and matrices. Remarkably, HCNTs with the large pitch length can not only effectively prevent the initiation of breakages but also hinder the growth of cracks while HCNT with small diameter and tube radius cannot even effectively prevent the initiation of cracks which is similar to the straight CNT. Moreover, the shear resistance of HCNTs increases with the increase of the helix angle and remains a high level when the helix angle reaches the critical value. However, the HCNTs with small helix angle and large diameter can carry out more polymer chains while the snake-like HCNT and HCNT with small diameter and helix angle almost cannot carry out any polymer chain during the pullout process, which show similar interfacial properties to the straight CNT.
| Date of Award | 30 Oct 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Xiaoqiao HE (Supervisor) |