Simulations performed by a team of researchers at the National Tsing Hua University show that highly efficient thermoelectric devices could be made from nanoribbons of zirconium pentatelluride and halfnium pentatelluride (ZrTe₅ and HfTe₅). This approach, published recently in Nano Energy, could be used to create the next generation of thermoelectric devices using these materials.

Thermoelectric materials convert a temperature gradient into electrical power. Commercially, thermoelectric systems have been used to cool military equipment deployed in the field, such as night vision goggles and to turn waste heat generated in exhaust pipes of cars into electricity to improve their fuel efficiency. Their efficiency is measured by the dimensionless thermoelectric figure of merit, zT. At a given temperature (T), zT is based on three factors: the thermopower or Seebeck coefficient, the electrical resistivity (or conductivity), and the thermal conductivity of the lattice and charge carriers.

“For decades, the main reason hindering the progress [in thermoelectric materials] is the trade-off relations between thermoelectric parameters,” says Te-Hsein Wang, first author of the Nano Energy publication. For example, increasing carrier concentration could be a way to increase zT, but, in many materials, an increase in carrier concentration leads to an accompanying decrease in the thermopower.

Nanoengineering is one potential way to circumvent these trade-offs in thermoelectric materials. Two influential  articles, both co-authored by L.D. Hicks and M.S. Dresselhaus in 1993, revolutionized the field; their work led to the creation of thermoelectric materials that are “nanostructured in the bulk, meaning they are made with extremely fine grains on the order of hundreds of nanometers,” says Joseph Heremans, a professor working on thermoelectric materials at The Ohio State University. As a result, zT of the thermoelectric materials increased from below one to almost two.

In 2014, further theoretical work suggested that making two-dimensional topological quantum materials that were tens of nanometers wide and tens of microns long could lead to substantial increases in thermoelectric efficiency. The work suggested that these geometries of topological insulators could have edge states with anomalously large Seebeck coefficients and consequently yielding much higher zT.

In the recent Nano Energy publication, Wang and co-author Horng-Tay Jeng demonstrate that nanoribbons of two specific topological insulators, ZrTe₅ and HfTe₅, may have thermoelectric efficiencies that are not limited by these traditional trade-offs. Their results show that in ZrTe₅, zT greater than 10 is possible.  

“We believe the unconventional thermoelectric behaviors and high zT should be seen [not just] in ZrTe₅,” Wang says. Calculations for HfTe₅ yielded similar high predictions for zT.

Wang also says that their research group intends to experimentally verify these thermoelectric properties predicted by their theoretical results. However, fabricating nanoribbons with the predicted thermoelectric properties might prove challenging. According to Heremans, “In order to have topologically protected edge or surface states, you need a solid where the interaction of the free electron with the atoms is dominated by spin orbit interactions.” Because the spin orbit interactions dominate, the bonding interactions between the atoms are much weaker. The weak bonding can lead to impurities or vacancies within the crystal structure of the nanoribbon, which will scatter electrons in the protected edge states. To make further gains in zT, Heremans suggests exploring spin-based systems and new thermoelectric materials.

Both researchers are also interested in improving the performance of current thermoelectric materials and devices. “To date, thermoelectric devices have only succeeded in niche applications due to their low efficiency,” Wang says. According to Heremans, one such possibility for expanding the market for thermoelectric materials could be deep cryogenic cooling, below liquid nitrogen temperatures.  

Read the article in Nano Energy.