MIT engineers just made it a lot easier for heat to be turned in to electricity. The team took traditional thermoelectric devices and reinvented them using "topological" materials.
A topological insulator are materials that insulate the interior but support electron movement on an object's surface. For postdoc Te-Huan Liu of MIT's mechanical engineering department, the unique physics of topological materials are exactly what his team used to make the discovery.
"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," he said in an interview with MIT. "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."
Thermoelectric devices are currently used to power relatively low-power applications. They work for oil pipeline sensors, on nearly all space probes in recent years, in automotive thermoelectric generators to boost fuel efficiency, and even on some minifridges. They even can be found in power plants in order to convert excess waste heat into additional electric power. But the findings of Liu and the team could increase the energy produced by thermal three times more than what is traditionally thought possible.
But how does it work? When one end of traditional thermoelectric materials are heated and the other side cooled, electrons flow from the hot to the cold end and generate electric current. The bigger that temperature difference, the higher the current. The amoung of energy generated also depends on the properties of the material itself.
However, previous research showed that topological matierals can actually be nanostrucutred and patterned to enhance its ability to ultimately increase current. Liu and his team wanted to see exactly how much of that boost came from the topological matieral itself and how much stemmed from how it could be restructured.
In order to get those answers, Liu studied the performance of tin telluride -- one of the best thermoelectric topological matierals. The semiconductor is often alloyed with lead which is used in infrared detectors.
In order to measure the nanostructure against the natural performance, the team measured the average distance an electron with a given energy would travel in a material before being scattered by defects in said material. It's a commonly used process called the "mean free path."
The team ultimately found that the smaller the grain size of a material, electrons with higher energy conduct more electrical current as they're less likely to be scattered. Thus, there's a bigger room for voltage improvements. The best scenario the researchers found? Decreasing tin telluride's grain size to just 10 nanometers gave them three times the amount of electricity that would've happened with a larger grain.
According to the research:
"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."
“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,” Liu said.
Ultimately, Liu and the team said that this discovery could help engineers craft smarter devices that use energy more effectively and don't waste anything -- not even heat output.