The traditional method of thinking in nanoscale materials is that smaller is better, but researchers at the University of Maryland’s A. James Clark School of Engineering are challenging traditional thermoelectric theory. Their discoveries may change the way engineers develop future thermoelectric devices.

The Maryland researchers are testing new thermoelectric devices on cars, capturing the waste heat from the exhaust and recycling it back into the electrical system to help run systems such as the air conditioning, lights or stereo.

A material that responds to a change in temperature by generating electric potential, or vice versa, exhibits what is known as the thermoelectric effect. Thermoelectric devices can generate electricity when heated by an external source or quickly cool or heat their environment when powered with electricity.

However, thus far, thermoelectric devices have been limited to niche markets because their efficiency is still too low.

“The goal of our work is to design thermoelectric materials that convert energy from one form to another more efficiently so we can promote the widespread use of products that recycle waste heat and effectively reduce our consumption of fossil fuels,” said Jane Cornett, a graduate student at the University of Maryland’s Department of Materials Science and Engineering.

To address the problem, Cornett and assistant professor Oded Rabin of the Department of Materials Science and Engineering and Institute for Research in Electronics and Applied Physics had to challenge some popular theories.

“Previous models told us that the use of nanomaterials at small dimensions would lead to an improvement in power- generation efficiency,” Cornett said. “The models also predicted that the smaller the nanostructure, the more significant the improvement would be. In practice, people weren’t seeing the gains they thought they should when they designed thermoelectric devices with nanoscale components, which indicated to us that there might be an issue with the interpretation of the original models.”

Cornett and Rabin have presented a revised thermoelectric performance model, which confirms that smaller is not always better. Using advanced computer modeling to investigate the potential of thermoelectric nanowires about 1,000 times thinner than a human hair, they demonstrate that in the set of the tiniest nanowires, measuring 17 nanometers or less in radius, decreasing their radii does not result in the increased thermoelectric performance previous models predicted. In nanowires above 17 nanometers in radius, however, an improvement is seen as the radius increases.

“The surprising behavior in the larger size range demonstrates that a different physical mechanism, which was overlooked in previous models, is dominant,” Cornett said.

“People were looking for solutions in the wrong places,” Rabin said. “We’ve created a better understanding of how to search for the best new materials.”

Thermoelectric devices are currently used in a few consumer products, including refrigerators and computers. They could eliminate the need for fluorocarbon refrigerants, giving rise to fluid- and compressor-free cooling systems that pose fewer hazards. Furthermore, they could be useful in more complex systems and aid in efforts to develop for a smarter, more efficient grid.