Molecular Dynamics study of Phonon Transport in crystal/aerogel nanocomposites

Growing demands in alternative energy generation have accelerated efforts to develop efficient thermoelectric materials. Because silicon has a large Seebeck coefficient, is easily doped to be both n-type and p-type, and has high Earth abundance, research has focused on methods for reducing the lattice thermal conductivity of this material through nanostructuring techniques. In this work, Reverse Non-Equilibrium Molecular Dynamics (RNEMD) is applied to study the thermal conductivity of silicon-silica aerogel nanocomposites. Using RNEMD, it has been demonstrated that embedding the silicon nanowire in a silica aerogel results in significant reductions to the lattice thermal conductivity of the nanowire (κ_NW). Simulation results indicated that, for a smooth nanowire with radius and length of 38.01Å, and 130.32Å, respectively, κ_NW was reduced from 2.16 to 1.8 W/m.K by embedding the Si-NW in a silica aerogel with porosity of 69.09%; Replacing the smooth nanowire with a rough nanowire also resulted in reduction in κ_NW (from 1.76 to 1.32 W/m.K). These results cannot be explained by the Diffuse Mismatch Model of phonon transport at interfaces. The impact of the geometry of this system on phonon lifetimes will be discussed using simple Lennard-Jones solids. The results of this work suggest a means for independently optimizing electronic and phononic properties of nanostructures.