Molecular Dynamics Simulation Study On Mechanical Behaviors Of Carbon Nanotubes

Since carbon nanotube (CNT) was first discovered by lijima in1991, due to itsunique molecular structure, many studies have been done to achieve excellentmechanical, electrical and chemical properties. The carbon nanotube has been drawnmore and more attention because of its outstanding properties and widely potentialapplications. However, the lack of experiment devices slows down the developmentof nanotubes. Therefore, some computational methods have been developed in orderto predict mechanical behaviors of carbon nanotubes. Molecular dynamics (MD)method not only can describe the deformation of nanoscale material and the physicalphenomenon evolution accurately, but also can extract the physical and mechanicalparameters of nano-materials. We focus on MD simulations for single-walled carbonnanotubes (SWCNTs) by using LAMMPS and theoretical analysis with relateddynamics theory. The following conclusions can be drawn from the present study:(1). A theoretical framework is developed for predicting the fracture strength andthe fracture strain as a function of temperature and strain rate based on Eyring’stheory and Bailey’s principle. We also investigate the mechanical properties of CNTsunder various temperatures, strain rates and moistures by using the MD method. Bothof the theoretical and numerical results show that the fracture strength and fracturestrain increase with the increasing strain-rate with the increasing strain-rate. Theresults also indicate that all of the fracture strength, fracture strain decrease rapidlywith the increasing temperature.(2). A theoretical framework is developed for the fracture strength of CNT asremoved atoms based on Quantized Fracture Mechanics (QFM) theory. Effects of thedefects in the CNTs on the mechanical properties are also investigated based on theMD simulations. Both of the theoretical and numerical results show that the fracturestrength of CNT with defects increase with the increasing strain-rate. The results alsoshow that the QFM theory has many limits and shortages, which only can give aaccurate prediction for the crack tip along the circumferential direction.(3). Moisture dependence of the mechanical properties of carbon nanotubes isstudies via MD simulations. The results indicate that the young modulus, fracturestrength and fracture strain of the CNTs under moisture are smaller than those in vacuum. When C-C bonds are destroyed, water molecules gather round to the placethat C-C bonds broken and the hydrogen atoms of water molecule are attracted by freecarbon atoms to form C-H bonds, thus accelerating the fracture of CNTs.(4). Based on Tersoff potential，a molecular dynamics simulation is performed forpredicting the fracture strength and the fracture strain as a function of temperature forcarbon nanotube bundles (CNTBs). The results show that with the increasingtemperature, both the fracture strength and fracture strain decrease significantly butYoung’s modulus changes slightly. At high temperatures some C atoms of one tube areattracted by the C atoms of another tube to form a new C-C bond, thus acceleratingthe fracture of CNTBs. When unloading of CNTB is completed, some new C-C bondsform again in the fracture zones, but the interaction potential energy of the CNTB isless than its initial energy. In the reloading stage, the tensile stress is less than that inthe first loading stage but more than that in the unloading stage due to some new C-Cbonds formed when the unloading is completed.(5). The effect of external electric field on the structure stability of CNT is studiesvia MD simulations. As the diameter of CNTs increases, a stronger applied electricfield is required for tensile failure. When the diameter greater than2nm, the appliedelectric field strength converges to-14V/nm. However, a weaker applied electric fieldis required for compression buckling as the diameter of CNTs increases. When thediameter greater than1.6nm, the applied electric field strength converges to4V/nm.Due to the very small radius of the tube，the change in strained energy is quitedifferent from that of the other CNTs.