(Nanowerk News) Researchers at the National Institute of Standards and Technology (NIST) have created a new device that can 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 US at a rate of about $100 billion every year.
The new fabrication technique – developed by NIST researcher Kris Bertness and his collaborators – involves depositing hundreds of thousands of microscopic columns of gallium nitride on a silicon wafer. The silicon layer is then removed from the bottom of the wafer until only a thin sheet of material remains. The interaction between the pillars and the silicon sheet slows down the transport of heat in the silicon, allowing more of the heat to be converted into an electric current.
Bertness and his collaborators at the University of Colorado Boulder report the findings in Advanced Materials (“Thermal and Electrical Properties of Semiconductors Are Separated by Local Phonon Resonance”).
Once fabrication methods are perfected, silicon sheets can be wrapped around steam or exhaust pipes to convert the heat emissions into electricity that can power nearby devices or be sent to the electrical 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 studied two metal cables, each made of a different material, that were joined at both ends to form a loop. He observed that when the two junctions connecting the wires were kept at different temperatures, nearby compass needles were deflected.
Other scientists soon realized that the deflection occurs because a temperature difference induces a voltage between two regions, causing current to flow from the hotter region to the colder region. The current creates a magnetic field that deflects the compass needle.
In theory, the so-called Seebeck effect could be an ideal way to recycle heat energy that would otherwise be lost. But there is a big obstacle. A material must conduct heat poorly to maintain a temperature difference between two regions but conduct electricity very well to convert large amounts of heat into electrical energy. However, for most substances, thermal conductivity and electrical conductivity go hand in hand; a poor conductor of heat makes a poor conductor of electricity 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 thin membranes covered with nanopillars — standing columns of material no more than one millionth of a meter long, or about one-tenth the thickness of a human hair. . His findings led to a collaboration with Bertness.
Using the nanopillars, Bertness, Hussein and their colleagues managed to separate the thermal conductivity from the electrical conductivity in silicon sheets — a first for any material and a milestone for enabling the efficient conversion of heat to electrical energy. The researchers reduced the heat conductivity of the silicon sheets by 21% without decreasing their 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 – the collective vibrations of atoms in motion. Both the gallium nitride nanopillars and the silicon sheet carry phonons, but what’s inside the nanopillars are standing waves, pinned on by the walls of the tiny columns much like the way a vibrating guitar string is held fixed at either end.
The interaction between phonons traveling on the silicon sheet and vibrations in the nanopillars slows the phonons’ travel, 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, phonon interactions accomplish this feat while leaving the electrical conductivity of the silicon sheet unchanged.
The team is now working on a structure made entirely of silicon and with an improved geometry for thermoelectric heat recovery. The researchers hope to demonstrate a high enough heat-to-electricity conversion rate to make their technique economically viable for industry.