a study at the massachusetts institute of technology (mit) in cambridge, mass. has identified the underlying property that makes topological materials potentially more efficient thermoelectric materials than conventional semiconductors such as silicon.

mit researchers, looking for ways to turn heat into electricity, find efficient possibilities in certain topological materials. (christine daniloff/mit)

according to an article on the mit website, scientists had previously believed that nanostructuring, synthesizing a material by patterning its features at the nanoscale, could produce efficient thermoelectric materials from topological materials because of their reduced thermal conductivity at the nanoscale.

the challenge for scientists was understanding the connection between the enhanced thermoelectric efficiency and the topological materials’ properties. mit researchers worked with tin telluride, which is known as a good thermoelectric material to try and find an answer.

“the team aimed to understand the effect of nanostructuring on tin telluride’s thermoelectric performance, by simulating the way electrons travel through the material,” the article explained. “to characterize electron transport, scientists often use a measurement called the ‘mean free path,’ or the average distance an electron with a given energy would freely travel within a material before being scattered by various objects or defects in that material.”

it added, “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.”

the simulations show that the material’s ability to generate electricity under a temperature gradient is dependent on electron energy. lower-energy electrons have longer mean free paths and could be scattered by grain boundaries. by decreasing the diameter of an average grain to 10 nanometers, researchers saw increased activity from higher-energy electrons.

“with smaller grain sizes,” the article continued, “higher-energy electrons contribute much more to the material’s electrical conduction than lower-energy electrons, as they have shorter mean free paths and are less likely to scatter against grain boundaries. this results in a larger voltage difference that can be generated.”

at 10 nanometers, tin telluride produced three times as much electricity than with larger grain sizes.

the research was recently published in the proceedings of the national academy of sciences (pnas). the abstract read:

“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.”