Turning heat into electricity
Researchers at MIT have discovered a way to increase the efficiency of thermoelectric devices threefold, using topological (insulating) materials. While past work has suggested that topological materials may serve as efficient thermoelectric systems, there has been little understanding as to how electrons in such materials would travel in response to temperature differences in order to produce a thermoelectric effect.
In a paper published in the Proceedings of the National Academy of Sciences, the researchers identify the underlying property that makes certain topological materials a potentially more efficient thermoelectric material, compared to existing devices.
‘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,’ says Te-Huan Liu, a postdoc in MIT’s Department of Mechanical Engineering.
Nanostructuring is a technique scientists use to synthesise a material by patterning its features at the scale of nanometers. Scientists have thought that topological materials’ thermoelectric advantage comes from a reduced thermal conductivity in their nanostructures.
Nanostructured materials resemble a patchwork of tiny crystals, each with borders, known as grain boundaries, that separate one crystal from another. When electrons encounter these boundaries, they tend to scatter in various ways. Electrons with long mean free paths will scatter strongly, like bullets ricocheting off a wall, while electrons with shorter mean free paths are much less affected.
But it is unclear how this enhancement in efficiency connects with the material’s inherent, topological properties.
When a thermoelectric material is exposed to a temperature gradient — for example, one end is heated, while the other is cooled — electrons in that material start to flow from the hot end to the cold end, generating an electric current. The larger the temperature difference, the more electric current is produced, and the more power is generated. The amount of energy that can be generated depends on the particular transport properties of the electrons in a given material.
To try and answer this question, Liu and his colleagues studied the thermoelectric performance of tin telluride, a topological material that is known to be a good thermoelectric material. The electrons in tin telluride also exhibit peculiar properties that mimic a class of topological materials known as Dirac materials.
The team aimed to understand the effect of nanostructuring on tin telluride’s thermoelectric performance, by simulating the way electrons travel through the material. To characterise 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.
In their simulations, the 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.
Going one step further in their simulations, the team played with the size of tin telluride’s individual grains to see whether this had any effect on the flow of electrons under a temperature gradient.
They found that decreasing tin telluride’s average grain size to about 10nm produced three times the amount of electricity that the material would have produced with larger grains.
Tin telluride is just one example of many topological materials that have yet to be explored. If researchers can determine the ideal grain size for each of these materials, Liu says topological materials may soon be a viable, more efficient alternative to producing clean energy.
