Computer Simulation Of The Mechanical Property Of Carbon Nanotubes And Bi-Crystal Metal Interfaces

The basic concepts and major techniques of molecular dynamics simulation method are explained in detail in this dissertation. Some issues on nano-mechanics, including the role of interlayer van der Waals interaction in the deformation of double-walled carbon nanotubes, the buckling behavior of metal-filled single-walled carbon nanotubes, and the influence of crystalline imperfections on the mechanical property of bi-crystal interfaces, are investigated using molecular dynamics simulations cooperated with proper interatomic potential functions.Since their discovery in 1991 by Iijima, carbon nanotubes (CNTs) have attracted much attention due to their excellent properties in many aspects. Using different methods, including experimental techniques, theoretical models and computer simulations, a vast investigation has been conducted on the property of CNTs. By molecular dynamics simulations, the influence of interlayer van der Waals interaction on the mechanical behavior of double-walled carbon nanotubes (DWCNTs) is examined in the present dissertation. First, buckling behavior of armchair and zigzag DWCNTs with abnormal interlayer distances is simulated under axial compression. Results show that different interlayer distance leads to different van der Waals force between two walls, while different van der Waals force results in different critical buckling strain. Some novel DWCNTs with abnormal interlayer distances have better compressive stability than the normal DWCNTs whose distance between two walls is 0.34nm. Furthermore, the effects of van der Waals force are different on the mechanical deformation of DWCNTs with different chiralities.An axial strain may induce torsion in single- and double-walled chiral CNTs. There are two critical strains when the chiral single-walled carbon nanotube (SWCNT) is subject to compression. The CNT begins to untwist after the first critical strain value until the torsion completely vanishes at another critical strain value. Due to the interlayer van der Waals interactions, the torsional deformation of the outer tube is dramatically altered under both tension and compression. The two critical strains appear when tension but not compression is applied. The torsion of the inner tubes has a strong dependence on the change of tube radius, while the behavior of the outer tubes is insensitive to the variation of tube radius.The introduction of metals into CNTs may significantly alter their conducting, electronic and mechanical properties and create intriguing multifunctional nanodevices. These metal-filled CNTs have potential applications in heterogeneous catalysis, nanodevices, electromagnetic wave absorption, high-density magnetic data storage devices and sensors for magnetic force microscopy, etc. Under axial compression, the buckling deformation of SWCNTs partially or completely filled by both pure metal and alloys is investigated, with consideration of its dependence on several issues such as the filling ratio, the geometrical parameters of a tube (radius, length and chirality), and the composition and initial distribution of the alloys. Due to the restriction from tube wall, the internal metal atoms aggregate into clusters and can form a concentric layered structure along the tube axis. For partially-filled tubes, the edge of a metal cluster can be considered as an imperfection and softens the tube, while the constraints exerted by metal atoms on the tube tend to stiffen the tube and increase the critical strain. It is the competition between these two factors that determines the overall change of the critical strains. Furthermore, the buckling mode of filled SWCNTs is also different with that of the corresponding hollow tubes. By comparing the value of the critical strains, it can be concluded that the SWCNT completely filled with Ni atoms is more stabile than that fully filled with either Cu or Pt atoms, while the Pt-filled one has a better stability during the post-buckling deformation.The behavior of metal-filled tubes is more complicated than that of empty ones due to the accommodation of the internal metal atoms. The dependence of the critical strain on the tube length can be roughly divided into four different linear stages, and is accompanied by a transition of the buckling mode from local to global. It is the competition between the evolution of the structure of metal atoms and the variation of the tube length that determines the critical strain. Local buckling mode with lobes on the tube walls often occurs in shorter tubes, which is similar to the buckling of thin shells at the macro scale. In contrast, longer tubes tend to buckle sideways as a whole, just like the global buckling of an Euler rod. For a group of completely filled armchair tubes with the same length but different radii, the critical strain first decreases linearly with the increase of tube radius. Subsequently there exists a rather wide range of tube length within which the critical strain fluctuates and has a weak dependence on tube radius, which differs from the observation for empty tubes, that is, the critical strain decreases monotonically with the increase of the tube radius. It is the competition between the effect of tube radius and the constraints exerted by metals on the tube wall that determines the overall variation of the critical strain with tube radius. Compared with a counterpart zigzag tube with the same radius and length, a completely filled armchair tube has a lower critical strain but can be easily strengthened with the incorporation of the internal metal atoms.In the initial equilibrium state of SWCNTs completely filled with bi-metallic alloys, the arrangement of internal metal atoms changes with the variation of the number fractions of component metals. A spiral arrangement is usually observed in the Ni-Cu filled system when the number fraction of nickel atoms is less than 0.8. However, straight lines are the most favorable fashions in both Ni-Cu and Ni-Pt filled SWCNTs with single atomic chains at the center of tubes in nickel-riched systems. Moreover, the alloy-filled SWCNT can have a much higher critical strain than the tube filled with any pure component metal in certain ranges of the number fraction of nickel atoms. A strong dependence of the buckling deformation on the initial random distribution of the encapsulated alloy metals is also presented. It is the competition between species, number fraction, and initial random distribution of the internal metal atoms that determines the critical strain of an alloy-filled tube. The buckling deformation in these SWCNTs completely filled by alloys is mainly accompanied by local collapses on the tube walls.For materials at nano-scale, the influence of interfaces is amplified due to the increased fraction of atoms located at or near interfaces. Based on the understanding of the interatomic potential between metal atoms in the metal-filled CNTs, the mechanical property of metallic interfaces is investigated. The role of several kinds of defects (point vacancies, line vacancies and cracks) during the tensile deformation of both Cu/Cu and Cu/Al interfaces, which are used as model systems of homogeneous and heterogeneous metal interfaces respectively, is explored using molecular dynamics simulations and the embedded-atom potential functions.For bi-crystal copper interface system, the rippling nature of the joint may facilitate local stress concentration and make the interface region a weak link. Crystalline defects can have a strong influence on the mechanical response of bi-crystal interfaces. The sensitivity of mechanical properties of an interface system to a defect type can be different under tension than under shear. Some defect topologies are found to improve, instead of degrade, certain properties (e.g. strength and fracture strain) of a bi-crystal interface system.For the diffusion bonding between copper and aluminum, the heterogeneous joint at the interface region is also a weak link. The mechanical behavior of the interface has a strong dependence on the type and topology of defects. Interfacial ductile fracture is the main mechanical failure mode, which is accompanied by the nucleation and growth of voids near the interfacial region. In some cases of a defective interface, the nucleation of partial dislocations is observed.