Enhanced Thermoelectric Performance of Polycrystalline Si0.8Ge0.2 Alloys through the Addition of Nanoscale Porosity

TL;DR

This study shows that adding nanoscale porosity to polycrystalline Si0.8Ge0.2 alloys can modestly improve thermoelectric performance by reducing phonon conductivity and optimizing carrier concentration, despite previous assumptions about such structures.

Contribution

The paper demonstrates, through modeling and experiments, that nanoscale porosity can enhance thermoelectric efficiency in polycrystalline SiGe alloys, challenging prior beliefs.

Findings

Nanoscale porosity reduces phonon thermal conductivity.

Optimal carrier concentration is significantly lowered.

ZT can be modestly increased with porosity and grain boundary engineering.

Abstract

Engineering materials to include nanoscale porosity or other nanoscale structures has become a well-established strategy for enhancing the thermoelectric performance of dielectrics. However, the approach is only considered beneficial for materials where the intrinsic phonon mean-free path is much longer than that of the charge carriers. As such, the approach would not be expected to provide significant performance gains in polycrystalline semiconducting alloys, such as SixGe1-x, where mass disorder and grains provide strong phonon scattering. In this manuscript, we demonstrate that the addition of nanoscale porosity to even ultrafine-grained Si0.8Ge0.2 may be worthwhile. The semiclassical Boltzmann transport equation was used to model electrical and phonon transport in polycrystalline Si0.8Ge0.2 containing prismatic pores perpendicular to the transport current. The models are free of tuning parameters and were validated against experimental data. The models reveal that a combination of pores and grain boundaries suppresses phonon conductivity to a magnitude comparable with the electronic thermal conductivity. In this regime, ZT can be further enhanced by reducing carrier concentration to the electrical and electronic thermal conductivity and simultaneously increasing thermopower. Although increases in ZT are modest, the optimal carrier concentration is significantly lowered, meaning semiconductors need not be so strongly supersaturated with dopants.