Limitations of zT as a Figure of Merit for Nanostructured Thermoelectric Materials

TL;DR

This study uses numerical simulations to show that nanostructuring can increase zT in thermoelectric materials, but the actual cooling performance may not improve proportionally, highlighting limitations of zT as a figure of merit.

Contribution

It demonstrates that zT may not reliably predict thermoelectric cooling performance in nanostructured materials with active grain boundaries.

Findings

Nanostructuring can increase zT with energy barriers at grain boundaries.

Maximum cooling temperature does not scale with zT as expected.

Electrically inactive grain boundaries yield lower zT but better cooling performance.

Abstract

A numerical study of thermoelectric nanocomposites is presented. Thermoelectric properties as a function of average grain size or nanoparticle density are examined by simulating the measurements as they would be done experimentally. In accordance with previous theory and experimental results, we find that the Seebeck coefficient, power factor and figure of merit, zT, can be increased by nanostructuring when energy barriers exist around the grain boundaries or embedded nanoparticles. When we simulate the performance of a thermoelectric cooler with the same material, however, we find that the maximum temperature difference is much less than expected from the given zT. The same nanocomposite without electrically active grain boundaries has a lower measured zT but a higher maximum cooling temperature. The physical reason for these results is explained. The results illustrate the limitations of zT as a figure of merit for nanocomposites with electrically active grain boundaries and suggest that the ability to enhance the electrical performance of thermoelectric materials by nanostructuring is limited.