Super-suppression of long wavelength phonons in constricted nanoporous geometries

TL;DR

This paper demonstrates that specific nanoporous geometries with constrictions can super-suppress long-wavelength phonons, significantly reducing thermal conductivity beyond traditional predictions, using advanced simulation techniques.

Contribution

It introduces a novel nanoporous design that traps long-wavelength phonons, achieving super-suppression of heat transport not accounted for by Matthiessen's rule.

Findings

Long-wavelength phonons are effectively trapped in constricted nanoporous geometries.

Thermal conductivity is reduced below Matthiessen's rule predictions.

Super-suppression results from heat trapping and anticorrelated heat currents.

Abstract

In a typical semiconductor material, the majority of heat is carried by long wavelength, long mean-free-path phonons. Nanostructuring strategies to reduce thermal conductivity, a promising direction in the field of thermoelectrics, place scattering centers of size and spatial separation comparable to the mean-free-paths of the dominant phonons to selectively scatter them. The resultant thermal conductivity is in most cases well predicted using Matthiessens rule. In general, however, long wavelength phonons are not as effectively scattered as the rest of the phonon spectrum. In this work, using large-scale Molecular Dynamics simulations, Non-Equilibrium Greens Function simulations, and Monte Carlo simulations, we show that specific nanoporous geometries, which create narrow constrictions in the passage of phonons, lead to anticorrelated heat currents in the phonon spectrum. This results in super-suppression of long-wavelength phonons due to heat trapping, and reductions in the thermal conductivity well below what is predicted by Matthiessens rule.