Low-dimensional Mechanical And Thermal Properties Of The Molecular Dynamics Study

In this thesis we have studied the mechanical and thermal properties of low dimensional systems, such as carbon nanotubes, Ni nanowires, by using classical molecular dynamics simulations. We also studied the nano-friction of carbon nanotubes.The first is the structure and phase transitions of (6,6), (8,8), (10,10), (12,12), (15,15), (24,0), and (30,0) single-wall carbon nanotube bundles under external pressure. Under pressure, the cross section of the tube changes from circle to well hexagonal or ellipse, dependent on the different symmetry of the tube. With increasing the pressure, all carbon nanotubes undergo a structural phase transition, with the cross section collapsed to elongated ellipse. The transition pressure is dependent on not only the tube radius but also the tube symmetry. For the tubes with T₂¹C_6h or T₂¹C_3h symmetries, tubes can be well matched each other, making the transition pressure larger. We also studied the total energy and the enthalpy of the different structure after the transition, and found that the parallel structure is more favorable than the herringbone structure.The second is the hydrogen storage in carbon materials under pressure. In this study we calculated the free energy change before and after the storage to answer the question whether it is possible to store hydrogen inside graphite and carbon nanotubes. We found the different storage mechanism for graphite and carbon nanotubes. In the graphite system, the covalent bonds between carbon atoms doesn't contribute to the total energy change, while in carbon nanotubes, the curvature effect makes the total energy change large enough to overcome the TΔS term. We also found the curvature effect becomes more important when decreasing the tube radius. Thus it is possible to store hydrogen inside carbon nanotubes, for example, the storage can be higher than 5.2 wt% inside (15,15) carbon nanotube under 7.0 GPa.The third is the study of the nano-friction in the sliding and rotation of double-wall carbon nanotubes. In this study we used a more accurate Kolmogorov-Crespi potential to describe the corrugation of the nanotubes. We found that the speed plateaus and jumps are ubiquitous in nanotubes. The reason for the speed plateaus is the negative differential friction, similar to the idea of negative differential resistance. We also found that some special vibrational modes are excited at the speed plateaus.The fourth is the thermal conductivity of low dimensional systems. We have studied three different methods for the calculation of thermal conductivity. In our two studies, the thermal conductivity of carbon nanotubes are calculated. We found a high thermal conductivity at 100 K by using the direct method, and found a strong finite-size effect. By using the homogeneous nonequilibrium molecular dynamics method, we found a different behavior for the dependence of the thermal conductivity on the external friction F_e for carbon nanotubes and UO₂.Furthermore, in Chapter 1 we give a general description of carbon nanotubes on the history and properties and a short introduction of nano-friction. In Chapter 2 we introduce the classical molecular dynamics method, including the basic ideas, atomic potentials, and different constant temperature/pressure methods.