Large-scale molecular dynamics investigation of geometrical features in nanoporous Si

TL;DR

This study uses large-scale molecular dynamics simulations to systematically analyze how various geometrical features of nanoporous silicon influence its thermal conductivity, revealing that phonon pathway reduction is the key factor.

Contribution

It provides an exhaustive atomistic investigation of geometrical effects on thermal transport in nanoporous silicon, highlighting the dominant role of phonon pathway reduction.

Findings

Phonon pathway reduction is the primary factor in thermal conductivity decrease.

Geometrical parameters like pore size and distribution significantly influence thermal transport.

Surface-to-volume ratio and porosity also affect thermal conductivity but less than phonon pathways.

Abstract

Nanoporous materials are of broad interest for various applications, in particular advanced thermoelectric materials. The introduction of nanoscale porosity, even at modest levels, has been known to drastically reduce a materials thermal conductivity, in some cases even below its amorphous limit, thereby significantly increasing its thermoelectric figure of merit, ZT. The details of the important attributes that drive these large reductions, however, are not yet clear. In this work, we employ large-scale equilibrium molecular dynamics to perform an exhaustive atomistic-scale investigation of the effect of porosity on thermal transport in nanoporous bulk silicon. Thermal transport is computed for over 50 different geometries, spanning a large number of geometrical degrees of freedom, such as cylindrical pores and voids, different porosities, diameters, neck sizes, pore/void numbers, and surface-to-volume ratios, placed in ordered fashion, or fully disordered. We thus quantify and compare the most important parameters that determine the thermal conductivity reductions in nanoporous materials. Ultimately, we find that, even at the nanoscale, the effect of merely reducing the line-of-sight of phonons, i.e. the clear pathways that phonons can utilize during transport, plays the most crucial role in reducing the thermal conductivity in nanoporous materials, beyond other metrics such as porosity and surface/boundary scattering.