In simulations, MIT researchers found that tin telluride's electron characteristics have a significant impact on their mean free paths. They plotted tin telluride's range of electron energies against the associated mean free paths, and found the resulting graph looked very different than those for most conventional semiconductors. Specifically, for tin telluride and possibly other topological materials, the results suggest that electrons with higher energy have a shorter mean free path, while lower-energy electrons usually possess a longer mean free path.
Researchers found that decreasing tin telluride's average grain size to about 10 nanometers produced three times the amount of electricity that the material would have produced with larger grains.
Liu says that while the results are based on simulations, researchers can achieve similar performance by synthesizing tin telluride and other topological materials, and adjusting their grain size using a nanostructuring technique. Other researchers have suggested that shrinking a material's grain size might increase its thermoelectric performance, but Liu says they have mostly assumed that the ideal size would be much larger than 10 nanometers.
Using ab initio simulations, we uncover the electron mean-free-path (MFP) spectrum in Dirac material and specifically show how the thermoelectric efficiency can greatly benefit from a distinct, monotonically decreasing trend of electron MFPs arising from the linear energy-momentum dispersion implied by the Dirac band topology. In the past, it was generally assumed that for the nanostructuring approach to be effective, one should design nanostructures to have characteristic length larger than the electron MFP but smaller than the phonon MFP to reduce thermal conductivity. Our results show that enhancement in thermoelectric performance can be achieved in Dirac materials even when they are smaller than the electron MFP by selectively filtering out long-MFP electrons that are harmful to the Seebeck coefficient.
Recent advancements in thermoelectric materials have largely benefited from various approaches, including band engineering and defect optimization, among which the nanostructuring technique presents a promising way to improve the thermoelectric figure of merit (zT) by means of reducing the characteristic length of the nanostructure, which relies on the belief that phonons' mean free paths (MFPs) are typically much longer than electrons'.