Heat current anticorrelation effects leading to thermal conductivity reduction in nanoporous Si

TL;DR

This paper uncovers an anticorrelated phonon scattering effect in nanoporous silicon that significantly reduces thermal conductivity beyond boundary scattering effects, with potential for thermal management applications.

Contribution

It introduces a novel wave-vector dependent anticorrelation mechanism in phonon scattering that further decreases thermal conductivity in nanoporous silicon.

Findings

Up to 80% reduction in thermal conductivity due to anticorrelation effect.

Heat trapping between large pores with narrow necks causes phonon reflections.

The effect is wave-vector dependent at low temperatures.

Abstract

Prevailing nanostructuring strategies focus on increasing phonon scattering and reducing the mean-free-path of phonons across the spectrum. In nanoporous Si materials, for example, boundary scattering reduces thermal conductivity drastically. In this work, we identify an unusual anticorrelated specular phonon scattering effect which can result in additional reductions in thermal conductivity of up to ~ 80% for specific nanoporous geometries. We further find evidence that this effect has its origin in heat trapping between large pores with narrow necks. As the heat becomes trapped between the pores, phonons undergo multiple specular reflections such that their contribution to the thermal conductivity is partly undone. We find this effect to be wave-vector dependent at low temperatures. We use large-scale molecular dynamics simulations, wave packet analysis, as well as an analytical model to illustrate the anticorrelation effect, evaluate its impact on thermal conductivity, and detail how it can be controlled to manipulate phonon transport in nanoporous materials.