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’. Pushing the nanostructure sizes down to the length scale dictated by electron MFPs, however, has hitherto been overlooked as it inevitably sacrifices electrical conduction. Here we report through ab initio simulations that Dirac material can overcome this limitation. The monotonically decreasing trend of the electron MFP allows filtering of long-MFP electrons that are detrimental to the Seebeck coefficient, leading to a dramatically enhanced power factor. Using SnTe as a material platform, we uncover this MFP filtering effect as arising from its unique nonparabolic Dirac band dispersion. Room-temperature zT can be enhanced by nearly a factor of 3 if one designs nanostructures with grain sizes of ∼10 nm. Our work broadens the scope of the nanostructuring approach for improving the thermoelectric performance, especially for materials with topologically nontrivial electronic dynamics.
Brian Wang is a Futurist Thought Leader and a popular Science blogger with 1 million readers per month. His blog Nextbigfuture.com is ranked #1 Science News Blog. It covers many disruptive technology and trends including Space, Robotics, Artificial Intelligence, Medicine, Anti-aging Biotechnology, and Nanotechnology.
Known for identifying cutting edge technologies, he is currently a Co-Founder of a startup and fundraiser for high potential early-stage companies. He is the Head of Research for Allocations for deep technology investments and an Angel Investor at Space Angels.
A frequent speaker at corporations, he has been a TEDx speaker, a Singularity University speaker and guest at numerous interviews for radio and podcasts. He is open to public speaking and advising engagements.