Thermoelectric power factor of nanocomposite materials from two-dimensional quantum transport simulations

TL;DR

This study uses advanced quantum transport simulations to analyze how nanoinclusions in nanocomposites affect thermoelectric power factors, revealing conditions for optimizing thermoelectric efficiency without degrading power output.

Contribution

It introduces a detailed 2D quantum transport simulation approach to evaluate nanocomposite thermoelectric properties, highlighting how nanoinclusion density influences power factor retention.

Findings

Nanoinclusions cause only moderate power factor improvements due to weak energy filtering.

Power factor can be independent of nanoinclusion density under certain conditions.

Materials with high nanoinclusion density can achieve low thermal conductivity and high ZT.

Abstract

Nanocomposites are promising candidates for the next generation of thermoelectric materials since they exhibit extremely low thermal conductivities as a result of phonon scattering on the boundaries of the various material phases. The nanoinclusions, however, should not degrade the thermoelectric power factor, and ideally should increase it, so that benefits to the ZT figure of merit can be achieved. In this work we employ the Non-Equilibrium Greens Function (NEGF) quantum transport method to calculate the electronic and thermoelectric coefficients of materials embedded with nanoinclusions. For computational effectiveness we consider two-dimensional nanoribbon geometries, however, the method includes the details of geometry, electron-phonon interactions, quantisation, tunneling, and the ballistic to diffusive nature of transport, all combined in a unified approach. This makes it a convenient and accurate way to understand electronic and thermoelectric transport in nanomaterials, beyond semiclassical approximations, and beyond approximations that deal with the complexities of the geometry. We show that the presence of nanoinclusions within a matrix material offers opportunities for only weak energy filtering, significantly lower in comparison to superlattices, and thus only moderate power factor improvements. However, we describe how such nanocomposites can be optimised to limit degradation in the thermoelectric power factor and elaborate on the conditions that achieve the aforementioned mild improvements. Importantly, we show that under certain conditions, the power factor is independent of the density of nanoinclusions, meaning that materials with large nanoinclusion densities which provide very low thermal conductivities, can also retain large power factors and result in large ZT figures of merit.