Molecular dynamics simulation of nanoscale thermal conduction and vibrational cooling in a crystalline naphthalene cluster

A molecular dynamics simulation of crystalline naphthalene is used to study nanometer scale thermal transport in solids. One molecule in a cluster of 75 is heated to a large initial temperature and then allowed to cool. Stochastic boundary conditions which preserve the time averaged volume of the cluster are used. The excess translational and librational energy of the hot molecule is lost within 1 ps. The excess vibrational energy is lost on the 100 ps time scale. Translational and librational energy propagates rapidly throughout the cluster at velocities which are comparable to the speed of sound. Despite the far slower rate of vibrational energy loss from the hot molecule, the growth of vibrational energy occurs uniformly on the other molecules in the cluster. Therefore intermolecular vibrational energy transfer occurs primarily via an indirect mechanism. Vibrational excitations are first converted into translational and librational excitations, which propagate throughout the cluster and then excite vibrations on distant molecules via multiphonon up pumping. Examination of the molecular neighbors shows that intermolecular transfer of mechanical energy can be anisotropic, since the hot molecule can only transfer energy where it contacts atoms on adjacent molecules. Energy transfer along the b- and c-crystallographic axes is more efficient than along the a axis. The most efficient energy transfer is in the direction of two of the four nearest neighbors. Transient hot spots are produced on these neighboring molecules. The implications of this anisotropic conduction for the propagation of thermal reactions, e.g., the decomposition of high explosives, are discussed briefly.