Thermal rectification in nanosized model systems: A molecular dynamics approach

The thermal conductivity in a set of mass-graded nanosized model systems has been studied by nonequilibrium molecular dynamics (MD) simulations in order to understand the phenomenon of thermal rectification that has been detected in externally mass-loaded nanotubes. We have found that the preferred direction of the heat transport in mass-graded nanotubes occurs from light to heavy atoms while the opposite direction of the heat transfer is observed in anharmonic mass-graded single-file chains. Mass-graded polyacetylenelike chains behave like single-file chains as long as the mass gradient is held by the backbone atoms. The thermal rectification in nanotubes with a gradient in the bond force constant (k(r)) has been studied too. They are characterized by a preferred heat transfer from the region with large k(r) to the domain with small k(r). Thermal rectification has been studied also in planar and three-dimensional mass-graded systems where the heat flow follows a preferred direction, similar to that observed in nanotubes. Additionally, a more realistic system has been implemented. Here, a different number of carbon nanotubes have been grafted on both sides of a graphene sheet. We have found that the transfer of the vibrational energy, as well as the generation of low-energy modes at atoms with large masses, is responsible for the sign of the thermal rectification. Its origin has been rationalized with the help of (projected) vibrational density of states. On the basis of the present MD simulations we suggest a possible design of materials showing a strong preference for the heat transfer into one direction.