STUDY OF THERMAL ENERGY TRANSPORT BETWEEN HYDROGEN GAS MOLECULES AND A SINGLE-WALL CARBON NANOTUBE USING MOLECULAR DYNAMICS SIMULATIONS

摘要

The focus of the current research is the investigation and characterization of the energy transport between a (10,10) single-wall carbon nanotube (SWCNT) and surrounding molecular hydrogen gas using molecular dynamics (MD) simulations. The MD simulations use Tersoff-Brenner hydrocarbon potential for C-C, C-H, and H-H bonding interactions and the conventional Lennard-Jones potential for soft-sphere gas-CNT collision dynamics of H-H and H-C non-bonding van der Waals interactions. A simulation cell with periodic boundary conditions is created for 1200 carbon atoms in an armchair nanotube configuration and three distinct gas densities corresponding to 252, 500, and 1,000 H_2 molecules in the same volume. The MD simulation runs are performed with time steps of 0.1 fs and the total simulation times of 40 ps. The simulations are initialized by setting the gas species and CNT at two different temperatures. Initial gas temperatures range from 2000K to 4000K, whereas the carbon nanotube is held at 300K. After the equilibrium temperatures of the CNT and the gas molecules are achieved, the external constraints to maintain the temperature are removed and the thermal energy transport between the two is studied. The kinetic energy exchange between the nanotube and the surrounding gas is monitored to study thermal energy transport over the duration of the simulation. A parameter is proposed, the coefficient of thermal energy transfer (CTET), to characterize the thermal transport properties of the modeled systems based on parameters governing the transport process, thus mimicking the conventional heat transfer coefficient. Values for CTET vary directly with gas density and range from 50 MW/m~2K to 250MW/m~2K, showing that gas density has a significant impact with higher density corresponding to higher collision rates and higher rates of energy transfer. In contrast, the gas temperature has a lower impact on CTET, with colder gas providing in certain cases a slightly lower value for the coefficient. In order to validate the MD simulations, the time-series data of molecular vibrations of the CNT is converted to a vibrational frequency spectrum through FFT. The characteristic frequencies obtained on the spectra of isolated SWCNT and H_2 simulations are compared against the known natural frequencies of the CNT phonon modes and vibrational modes of H_2 molecules. The comparison is quite favorable.