Resonant Thermal Transport in Nanophononic Metamaterials

Thermoelectric materials allow heat to be converted directly into electricity or, conversely, to be absorbed or rejected when electrical power is provided. A key requirement for a thermoelectric material is to exhibit a low thermal conductivity while simultaneously possessing a high power factor, which is proportional to the electrical conductivity. The challenge, however, is that these two property targets are coupled and are at odds with each other in common materials. This natural constraint has limited the performance of thermoelectric materials and impeded their industrial proliferation. In this chapter, we present a recently proposed material concept, termed nanophononic metamaterial, which overcomes this historical constraint. One promising configuration of a nanophononic metamaterial consists of a silicon membrane with inherently attached local resonators in the form of nanopillars distributed on the surface. The nanopillar local resonances, or vibrons, hybridize with the underlying phonon waves carrying the heat in the membrane, which leads to significant reductions in the phonon group velocities and to mode localizations within the nanopillars. These two wave-based phenomena, supplemented by reductions in phonon lifetimes, together cause a reduction in the lattice thermal conductivity along the base membrane. Since the nanopillars are located external to the main body of the membrane, changes to the electronic band structure and electron scattering are both minimized – thus a negligible effect on the generation and flow of electrons is expected. This novel nanostructure-induced mechanism therefore has all the ingredients to enable thermoelectric energy conversion at record high performance while using a lowcost and practical base material such as silicon. This chapter includes lattice dynamics- and molecular dynamics-based analyses of the underlying physical processes stemming from the presence of the local resonances and discusses how the nature of resonant thermal transport is shaped particularly by the wave-based mechanism of phonon-vibron coupling.