One way to reduce the lattice thermal conductivity of solids is to induce additional phonon surface scattering through nanostructures. However, how phonons interact with boundaries, especially at the atomic level, is not well understood. In this work, we performed two-dimensional atomistic wave packet simulations to investigate the phonon surface interaction. Emphasis has been given to the angular-resolved phonon reflection at smooth, periodically rough, and amorphous surfaces. We found that the acoustic phonon reflection at a smooth surface is not simply specular. Mode conversion can occur after reflection, and the detailed energy distribution after reflection will dependent on surface condition and polarization of incident phonon. At periodically rough surfaces, the reflected wave packet distribution does not follow the well-known Ziman's model, but shows a nonmonotonic dependence on the depth of surface roughness. When an amorphous layer is attached to the surface, the incident wave packet will be absorbed by the amorphous region, and results in quite diffusive reflection. Our results clearly show that the commonly used specular-diffusive model is not enough to describe the phonon reflection at a periodically rough surface, while an amorphous layer can induce strong diffusive reflection. This work provides a careful analysis of phonon reflection at a surface with different morphology, which is important to a better understanding of thermal transport in various nanostructures.
Thermal boundary resistance (TBR) plays an important role in the thermal conduction of carbon nanotube (CNT)-based materials and CNT networks (e.g., thin films, arrays, and aerogels). Although individual CNTs have extremely high thermal conductivity, interfacial resistances can dominate the overall resistance and largely influence their thermal performance. Using molecular dynamics simulations, we systematically study the interfacial thermal conductance (ITC, the inverse of TBR) of multi-walled carbon nanotube (MWNT)-substrate interfaces and MWNT-MWNT junctions, and compare the CNT-CNT junctions with graphene-graphene junctions. The results show that for CNTs with the diameter of a few nanometers, the total ITCs first decrease and then stabilize with the increase of the number of walls, mainly due to the changes of mechanical strength and adhesive energy. Increasing the CNT diameter leads to a larger total ITC and it is mainly due to a larger contact area. The area normalized ITC of CNT-CNT junctions increases and then saturates with the number of walls, and it behaves non-monotonically with the diameter. Furthermore, a trapezoidal model of multi-layer graphene-graphene junctions is used to explain the number of wall dependence of ITC. We also find that with the same adhesive energy, total ITCs of CNT-CNT junctions and graphene-graphene junctions are similar, which allows us to roughly estimate ITCs of CNT-CNT junctions without performing numerical simulations.
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