Phonon thermal conductivity in nanolaminated composite metals via molecular dynamics

Abstract

We use nonequilibrium molecular dynamics to characterize the phonon contribution to thermal conduction of Al nanostructures and the role of interfaces in metallic nanocomposites. We characterize the lattice thermal conductivity of pure Al samples as a function of size and temperature from which we obtain, using kinetic theory, the temperature dependence of the phonon mean free path. We also calculated the thermal conductivity of AlAl* and AlNi nanolaminate composites (where Al* differs from Al only in its mass) for various periodic sizes and compositions as well as the associated interfacial thermal resistivities (ITRs). We find that simple, additive models provide good estimates of the thermal conductivities of the nanocomposites in terms of those of the individual components and interfaces if size effects on the behavior of the individual components are considered. The additive models provide important insight to the decrease in thermal conductivity of the nanolaminates as their periodicity (thickness of a bilayer) is reduced to a size comparable with the phonon mean free path and break down when this characteristic size is reduced further. At this point the system can be regarded as homogeneous and the conductivity increases with decreasing periodicity of the laminates. We also observe that the ITR depends on the direction of the heat flux; this is the first molecular level characterization of such thermal diode behavior in a realistic three dimensional material.