If there’s one thing we humans are good at, it’s producing heat. Significant amounts, and in many cases most of the energy we generate and put into our systems we lose as heat, whether it be our appliances, our transportation, our factories, even our electrical grid.

“Waste heat is everywhere,” said UC Santa Barbara mechanical engineering professor Bolin Liao, who specializes in thermal science and renewable energy. “Our power plants, our car exhaust pipes — there are so many places where we create excess heat waste.”

For the moment, we’re fairly limited as to how we can make the most out of this dissipating heat. But Liao and UCSB colleagues, alongside collaborators from Ohio State University and University of Hong Kong are making headway toward putting that heat to use, with a first-time comprehensive characterization of the thermoelectric properties of high-quality cadmium arsenide thin films.

“If we could harvest that waste heat then that would be fantastic,” he said. “That would really increase our energy efficiency and it’s also a really sustainable energy source.”

The team’s research is published in the journal Advanced Materials.

A better thermoelectric material
“To obtain high efficiency, we need the material to conduct electricity well, conduct heat poorly and generate a high voltage for a given temperature difference,” Liao said. Poor heat conduction minimizes heat dissipation while maintaining a temperature difference across the material, resulting in an electric current enhanced by the material’s high-performing electric conductivity. The voltage resulting from a temperature gradient is known as the Seebeck effect.

This combination of electrical and thermal transport properties is ideal but, according to Liao, “very hard to achieve in practice.”

Enter cadmium arsenide (Cd3As2), a Dirac semimetal with promising transport properties, in particular, a low thermal conductivity and high electron mobility.

“We were pretty excited about this material, and we thought ‘okay, this is really a combination of these two great properties,” Liao said. “But there is only one problem.

“This problem was that in addition to good electric conduction and poor thermal conduction, you also need this material to be able to generate enough voltage under a temperature gradient.” As a semimetal, cadmium arsenide is excellent at conducting electricity very rapidly, but it only generates a very small Seebeck voltage. To create a useful voltage, Liao explained, one would need to open up a band gap.

“You want this material to have a certain energy range where the electrons cannot conduct. That’s called a band gap,” he said. Because of the gap, which essentially blocks the free flow of electrons, enough electrical “pressure” (a.k.a. voltage) can build up as a response to a temperature difference across the material. In bulk cadmium arsenide crystals, there is no band gap.