Renewal of energy recovery: a new way to efficiently convert waste heat into electricity

Nanopillars efficiently convert heat energy into electricity

Illustration of nanopillars used in a new project to efficiently convert heat energy into electricity. Credit: S. Kelley/NIST

A team from NIST and the University of Colorado Boulder has developed a new device using nanopillars of gallium nitride on silicon that significantly improves the conversion of heat to electricity. This could potentially reclaim large amounts of wasted thermal energy, benefiting industries and power grids.

Researchers at the National Institute of Standards and Technology (NIST) have fabricated a new device that could dramatically increase the conversion of heat to electricity. If perfected, the technology could help recover some of the heat energy that is wasted in the United States at a rate of about $100 billion a 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 onto a silicon wafer. The layers of silicon are then stripped from the underside of the wafer until only a thin sheet of material remains. The interaction between the pillars and the silicon sheet slows the heat transport in the silicon, allowing more heat to be converted into electric current. Bertness and her collaborators at the University of Colorado Boulder recently reported the findings in the journal Advanced material.

Once the manufacturing method was perfected, the silicon sheets could be wrapped around steam or exhaust pipes to convert heat emissions into electricity that could power nearby devices or be fed into an electrical grid. Another potential application would be cooling computer chips.

By growing nanopillars on top of a silicon membrane, NIST scientists and their colleagues reduced heat conduction by 21 percent without reducing electrical conductivity, an achievement that could dramatically increase the conversion of thermal energy into electrical energy. In solids, thermal energy is carried by phonons, periodic vibrations of atoms in a crystal lattice. Some vibrations of the phonons in the membrane resonate with those in the nanopillars, acting to slow down heat transfer. Basically, nanopillars don’t slow down the movement of electrons, so electrical conductivity remains high, creating a superior thermoelectric material. Credit: S. Kelley/NIST

The NIST-University of Colorado study is based on a curious phenomenon first discovered by German physicist Thomas Seebeck. In the early 1820s, Seebeck was studying two wires, each of a different material, which were joined at either end to form a loop. He observed that when the two junctions connecting the wires were held at different temperatures, a nearby compass needle deflected. Other scientists quickly realized that the deflection occurred because the temperature difference induced a voltage between the two regions, causing current to flow from the hotter region to the cooler one. The current created a magnetic field which deflected the compass needle.

In theory, the so-called Seebeck effect could be an ideal way to recycle thermal energy that would otherwise be lost. But there was a big obstacle. A material must conduct heat poorly to maintain a temperature difference between two regions, but conduct electricity extremely well to convert heat into a significant amount of electrical energy. For most substances, however, thermal conductivity and electrical conductivity go hand in hand; a bad conductor of heat is a bad conductor of electricity and vice versa.

Studying the physics of thermoelectric conversion, theorist Mahmoud Hussein of the University of Colorado has discovered that these properties could be decoupled in a thin membrane covered with nanopillars, columns of material no more than a few millionths of a meter long, or about one-tenth the thickness of a human hair. The discovery of him led to the collaboration with Bertness.

Using nanopillars, Bertness, Hussein and their colleagues were able to decouple thermal conductivity from electrical conductivity in silicon foil, a first for any material and a milestone in enabling efficient conversion of heat into electrical energy. The researchers reduced the thermal conductivity of the silicon sheet by 21% without lowering its electrical conductivity or changing the Seebeck effect.

In silicon and other solids, the atoms are bound by bonds and cannot move freely to transmit heat. Consequently, the transport of thermal energy takes the form of phonons that move collective vibrations of the atoms. Both the gallium nitride nanopillars and the silicon foil carry phonons, but those inside the nanopillars are standing waves, blocked by the walls of the tiny columns much like a vibrating guitar string is held steady at both ends.

The interaction between the phonons traveling in the silicon sheet and the vibrations in the nanopillars slow down the traveling phonons, making it more difficult for heat to pass through the material. This reduces the thermal conductivity, thereby increasing the temperature difference from one end to the other. Equally important, the phonon interaction accomplishes this feat while leaving the electrical conductivity of the silicon sheet unchanged.

The team is now working on structures fabricated entirely in silicon and with improved geometry for thermoelectric heat recovery. The researchers expect to demonstrate a high enough heat-to-electricity conversion rate to make their technique economically viable for industry.

Reference: Thermal and Electrical Properties of Semiconductors Decoupled from Localized Phonon Resonances by Bryan T. Spann, Joel C. Weber, Matt D. Brubaker, Todd E. Harvey, Lina Yang, Hossein Honarvar, Chia-Nien Tsai, Andrew C. Treglia, Minhyea Lee, Mahmoud I. Hussein and Kris A. Bertness, March 23, 2023, Advanced material.
DOI: 10.1002/adma.202209779

This research was funded in part by the Department of Energy’s Advanced Research Projects Agency-Energy.

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