Physical Mechanics Of Large Deformation In Carbon Nanotubes At High Temperatures And In Applied Fields

Due to their unique one-dimenstional structure, excellent physical, chemical and mechanical properties, carbon nanotubes (CNTs) are widely believed to be the ideal key materials for farbricating new generation high performance nano electromechanical devices. Exploration of the physical mechanics of large deformation in carbon nanotubes has been one of the central fundamental areas in carbon nanotube research. In this dissertation, molecular dynamics simulations based on Tersoff-Brenner potential, density functional theory (DFT) and first principle calculations have been used to study the large deformation properties of carbon nantubes in high temperatures, strong electric field and under bending manipulations, the following results and conclusions are obtained.1, Using molecular dynamics simulations, we studied the recent experiment finding of high temperature superplasticity of single-walled carbon nanotubes at high temperatures, it is noticed that previous theoretical explanation of this phenomenon is against the physical reality, the as-proposed nucleation and propagation of very few defects do not follow high temperature statistical principle and tend leading to localized failure of CNTs. Our results show that homogeneous nucleation of widely distributed topological defects near the elastic limit are essential to superelongation at high temperatures, it impedes the localization of defect distribution in subsequent tensile process, the random interaction and propagation of these defects produces the superelongation privior to its eventual broken. At low temperatures, due to the lack of sufficient thermal energy for activating large amount of topological defects, CNTs primarily show brittle fracture behavior. We found the superelongation behavior are closely related to the diameter of CNTs, for small diameter CNTs, defect nucleation prone to be localized which makes superelongation unfavored, while for large diameter CNTs, superelongation happens more frequently. Our results corrected the inappropriated understanding of the underlying physics in early literatures, we reveal for the first time the importance of wide-distributed defects for superelongation of carbon nanotubes at high temperature and agree well with the experiment.Our simulation within wide temperature range show that high strain tensile ductility can be realized in a broad temperature domain (generally over 500K), but the ability of being plastically stretched goes down with decreasing temperature, while the elastic limit increases when temperature is decreasing. Based on this finding and by noticing of the Joule Heating procedure in experiment, we proposed a mechanism of enhancing plastic elongation of CNTs by gradual decreasing of system temperature, molecular dynamics simulations demonstrated this strategy, and are in accordance with experiment observation.2, We studied the physical mechanical properties of multiwalled carbon nanotubes (MWCNTs) at high temperatures, it is found that interlayer sp3 bonds are formed during the heating procedure, the capability of forming the sp3 bonds increases with temperature. For small diameter CNTs, they have very high intrinsic strain energy which makes them less stable, therefore the first interlayer bonding form inbetween the inner layers of a MWCNT, for bilayer graphene that has an infinitely low curvature, although interlayer sp3 bondings can form during the heating procedure, these bonds are not very stable and will disappear under external perturbation. Tensile testing show that the tensile strength is not noticeably reduced because these interlayer bonds do not lead to the abrupt damage of the MWCNT structure. Meanwhile, due to the enhanced loading transfer capability between different layers of the MWCNT assisted by the interlayer bondings, we propose a new strategy for farbricating new carbon nanotube based ultra-strong nano fibers or composites through high temperature heating treatment.Our first principles calculations demonstrate that, regardless of the starting metallic or semiconducting nature of the intrinsic MWCNTs, introducing of interlayer sp3 bonds can eventually turn them into semiconductors, the transition from graphite-like band structure to diamond like band structure opens the energy gap around Fermi level. We propose a procedure of producing all-semiconducting carbon nanotubes based on the above findigs, it is expected that this scheme would accelerate the application of CNTs in semiconducting industry in the near future.3, Based on Density Functional Theory, we studied the electromechanical behavior of finite length SWCNTs under electric field. Our results show that due to the electricstrictive effect, electronic structures of carbon nanotubes can be significantly modified with increasing electric field strength and are chirality dependent. Electronic orbital charge density of armchair SWCNTs is found to be more sensitive to electric field than zigzag SWCNTs, while capped SWCNTs are least sensitive, implying that open ended SWCNTs are better for using as field emission sources, this is in accordance with experiment conclusions. In addition, we find that the electric dipole moment of SWCNT scales linearly with electric field strength if no electrostrictive effect is considered, however, electrostrictive deformation enhances the magnitude of dipole moment and the resultant relationship is nonlinear.4, We used molecular dynamics simulations in combination with atomic force microscopy experiment studies, a"dual-size effect"and two distinct buckling modes of CNTs under bend loading are identified. These two buckling modes correspond to the"abrupt"and"gradual"increase trend of the height at the buckling position with respect to the bending angle. Simulation results show that the abrupt buckling modes originate from the sudden release of strain energy due to the instability of SWCNTs at the critical buckling curvature. While the gradual buckling modes primarily correspond to the buckling of MWCNTs, due to the strong Van der Waals constraint in radial direction from the inner walls of a MWCNT, the strain energy from the outer layer of the MWCNT can not be released effectively and the inner layers as well, this leads to the layer by layer buckling of the MWCNT from outer wall to the inner wall.Simulations on the post-buckling behavior show that due to the stress concentration, sp3 bondings are formed at the buckling position. Interlayer sp3 bonds are easier to form in bent MWCNTs because they have strong Van der Waals interactions between adjacent layers, the sp3 bonds are mostly formed at the high curvature area. For SWCNTs, smaller sized (length and diameter) SWCNTs can form sp3 bonds easier than the larger size counterparts because the average strain energy per atom is larger than that in the large size SWCNTs, while large size SWCNTs show better flexibility that can release the strain energy effectively and are less able to form sp3 bonds.5,Following the recent advances in graphene research, we studied the thermal stability and mechanical properties of recent experimently identified new material: partially unzipped carbon nanotubes. Our molecular dynamics simulations show that due to the dangling bonds located at the edge of the graphene nano ribbons, their stability decreases with increasing temperature. These dangling bonds rebonded to form hexagonal structure and thus seamlessly roll back to nanotube structure, the rolling speed increases with temperture.Tensile test shows that both armchair and zigzag structures are brittle at room temperature with Young's modulus of around 700 GPa. The elastic limit of armchair structure is higher than that of zigzag structure due to the different response of bond angle change to tensile strain. The fracture locate is close to the junction area of nanotube and nano ribbon. Zigzag structure tend to exerience plastic elongation mode when temperature increases over 400 K. Our results offer important reference for the potential application of these new materials.