Exploring a new way to convert heat into electricity

Researchers at the National Institute of Standards and Technology (NIST) have created a new device that can dramatically enhance the conversion of heat into electricity. If the technology is perfected, it could help recover some of the heat energy that is wasted in the United States at a rate of about $100 billion each year.

The new fabrication technique, developed by NIST researcher Kris Bertness and her collaborators, involves depositing hundreds of thousands of microscopic columns of gallium nitride on top of a silicon wafer. The layers of silicone are then removed from the underside of the wafer until only a thin layer of the material remains.

The interaction between the pillars and the silicon sheet slows heat transfer in the silicon, allowing more heat to be converted into electric current. Bertens and her collaborators at the University of Colorado Boulder reported the results online March 23 in the Advanced materials.

Once the manufacturing method is perfected, silicon sheets can be wrapped around steam or exhaust pipes to convert heat emissions into electricity that can power nearby appliances or be connected to an electricity grid. Another potential application is cooling computer chips.

The NIST-University of Colorado study is based on a strange phenomenon first discovered by German physicist Thomas Seebeck. In the early 1820s, Seebeck was studying two metal wires, each made of a different material, that were joined at both ends to form a loop.

He noticed that when the two crosses connecting the wires were kept at different temperatures, the adjacent compass needle deflected. Other scientists soon realized that the anomaly occurred because the difference in temperature caused a voltage between the two regions, causing current to flow from the hotter region to the cooler region. The current created a magnetic field that deflected the compass needle.

In theory, the so-called Seebeck effect could be an ideal way to recycle thermal energy that would otherwise be wasted. But there was a big hitch. A material must conduct heat poorly in order to maintain a temperature difference between two areas while conducting electricity very well to convert heat into a large amount of electrical energy. However, for most materials, thermal conductivity and electrical conductivity go hand in hand; A poor thermal conductor leads to a poor electrical conductor and vice versa.

In studying the physics of thermoelectric conversion, theorist Mahmoud Hussein of the University of Colorado discovered that these properties could be separated in a thin film covered with nanopillars—columns of material just a few millionths of a meter, or about one. One-tenth the thickness of a human hair. His discovery led to a collaboration with Bertens.

Using nanostructures, Bertens and colleagues have succeeded in separating thermal conductivity from electrical conductivity in a silicon sheet—a first for any material and a milestone for enabling the efficient conversion of heat into electrical energy. The researchers reduced the thermal conductivity of the silicon sheets by 21% without lowering the electrical conductivity or altering the Seebeck effect.

In silicon and other solids, the atoms are bound by bonds and cannot move freely to transfer heat. As a result, the transfer of thermal energy takes the form of phonons – the moving collective vibrations of atoms. Both gallium nitride nanosheets and silicon sheets carry phonons, but the waves inside the nanopillars are standing waves, anchored to the walls of the tiny columns in the same way a vibrating guitar string is attached at either end.

The interaction between phonons traveling in the silicon sheet and vibrations in the nanotubes slows down the traveling phonons, making it difficult for heat to pass through the material. This reduces thermal conductivity, thus increasing the temperature difference from one end to the other. Equally important, the phonon interaction achieves this feat while leaving the electrical conductivity of the silicon sheets unaltered.

The team is now working on structures made entirely of silicon and with better geometry for thermoelectric heat recovery. The researchers expect to show a conversion rate from heat to electricity high enough to make their technology economically viable for industry.