Thermoelectric power is becoming a feasible alternative for clean energy with the discovery of new materials - and a new layered crystal containing rhenium and silicon could usher the future of these devices.

Thermoelectric power generators create electricity from heat generated by other processes, which makes them an appealing method for cutting down carbon footprint from existing industrial and automotive processes. However, the technology is largely limited by the problem of regulating temperature, especially on the hot side. Existing materials get too hot over time, causing the device to fail.

Thermoelectric Seebeck power module
(Photo: Gerardtv via Wikimedia Commons)
Picture of a Thermoelectric Seebeck module (w:en: Thermoelectric generator), apparently manufactured by TECTEG MFR

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One solution that works around this limitation is the use of transverse thermoelectric devices. This class of devices creates a current that runs perpendicular to the conductive medium, so it only requires contacts at the cold end of the generator. While the technology is promising, it remains inefficient and impractical for real-world applications - until now.

A research team from Ohio State University found the new material, a layered crystal made up of rhenium and silicon, that could be the "gold standard" for transverse thermoelectric devices. They presented their findings in the report "Highly efficient transverse thermoelectric devices with Re4Si7 crystals†," appearing in the journal Energy and Environmental Science.

A Feasible Cornerstone for Transverse Thermoelectric Devices

The Ohio State University researchers were able to demonstrate how the layered crystal acts as an efficient transverse thermoelectric device thanks to its rare property. It can simultaneously facilitate the movement of both positive and negative charges. This way, either charge independently moves instead of conventional cases where they run parallel to each other and create the electric current by zig-zagging across the contacts.

Researchers fabricated a sample of the layered crystal about two inches long and used it in a thermoelectric generator. Additionally, they found out that when the crystal is titled towards a specific angle within the generator, it generates an unprecedented amount of power for this class of devices.

"We showed that these materials are as effective as conventional thermoelectric generator technology, but overcome its major disadvantages," said Joshua Goldberger, co-author of the paper and a professor of chemistry and biochemistry, in a news release from Ohio State.

Goldberger additionally explains that their study marks the first instance where this class of devices has been proven feasible. The layered crystal promises to be as good as commercially available materials but much simpler and more reliable.

The Potential of Harnessing Waste Heat

Waste heat is often unattended and allowed to escape into the environment, whether in the form of exhaust pipes in cars or discharge from smokestacks. Joseph Heremans, a co-author in the study and a professor of mechanical and aerospace engineering, stresses the importance of waste heat and the attempts to harness them as reusable energy - an achievement made by their research team.

Conventional materials only conduct one type of charge, and thermoelectric devices are usually made from different materials. These involve complex processes to create contacts that must be both efficient and resistant to increasing temperatures.

The Ohio State news release explains that two years ago, the same research team found unexpected behavior in a compound that allowed electrons and holes to run along separate directions. The online electronics portal All About Circuits explains the electrons and holes as the particle and the void pair in solid-state theory responsible for the negative and positive charge, respectively. It led them to find other possible materials that exhibit similar properties.

Researchers additionally explained that a generator based on this layered crystal could be used virtually anywhere with waste heat. The crystal they used in the study was limited by the size of the furnace where it was grown.


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