Te-Huan Liu, a postdoc in MIT’s department of mechanical engineering, said: “We’ve found we can push the boundaries of this nanostructured material in a way that makes topological materials a good thermoelectric material, more so than conventional semiconductors like silicon.
“In the end, this could be a clean-energy way to help us use a heat source to generate electricity, which will lessen our release of carbon dioxide.”
When a thermoelectric material is exposed to a temperature gradient electrons in that material start to flow, thus generating an electric current.
The study found that some topological materials can be made into efficient thermoelectric devices through nanostructuring. It is thought that a reduced thermal conductivity is responsible for the thermoelectric advantage topological materials possess, but it is unclear how increased efficiency connects with the material’s inherent, topological properties.
By studying the performance of a particular topological material known as tin telluride, Liu and his team found that its electron characteristics have a significant impact on their mean free paths.
They plotted the range of electron energies against the associated mean free paths, finding that the outcome was very different to most conventional semiconductors. The results suggested that electrons with higher energy have a shorter mean free path, while lower-energy electrons usually have a longer one.
The team also discovered that the electron energy effected the material’s ability to conduct electricity, or generate a flow of electrons, under a temperature gradient. Specifically, they found that lower-energy electrons have a negative impact on the generation of a voltage difference. These certain electrons also have longer mean free paths, so they can be scattered by grain boundaries more intensively than higher-energy electrons.
MIT then explored the effects of changing tin telluride’s individual grain size. They found that when they decreased the diameter to 10nm, the contribution from higher-energy electrons raised. It also produced three times the amount of electricity when compared with the larger grains.
Liu said: “In our simulations, we found we can shrink a topological material’s grain size much more than previously thought, and based on this concept, we can increase its efficiency.”
Other topological materials are yet to be explored in this way, but if the ideal grain size can be determined for each one, Liu believes it may be a better alternative to producing clean energy in the future.
