Research team at NIST led by Kris Bertness is exploring a novel heat to electricity conversion method and has developed a device for the same. Once this technology is perfected, it would convert heat to electricity which would otherwise get wasted. The research is based on the discovery of German physicist Thomas Seebeck. This team has also incorporated the discovery made by Mahmoud Hussein in his thermoelectric conversion study.
Researchers at the National Institute of Standards and Technology (NIST) have designed a novel device that, if perfect, could successfully convert heat into electricity. This technology has the potential to recover the wasted heat energy in the United States, which is estimated to be wasted at a staggering rate of approximately $100 billion per year.
Kris Bertness, a researcher at NIST, and her team created this technique. It involves depositing thousands of microscopic columns of gallium nitride on top of a silicon wafer. The silicon is subsequently peeled off of the wafer’s underside layer by layer until only a thin sheet is left. More heat may be converted to electric current because of the interaction between the pillars and the silicon sheet, which slows down heat transfer in the material.
The silicon sheets might be wrapped around steam or exhaust pipes once the fabrication process is developed for heat conversion emissions into electricity that could be used to power nearby devices or be sent to the power grid. Cooling computer chips would be another possible potential use.
The German physicist Thomas Seebeck was the first to notice a peculiar behavior that served as the basis for the NIST-University of Colorado study. Early in the 1820s, Seebeck began investigating a loop formed by two metal wires of various compositions linked at both ends. He noticed that as the temperatures of the two junctions holding the wires together varied, a nearby compass needle would deflect.
Other researchers found out that the temperature difference between the two areas developed a voltage and led to current that caused the deflection. The magnetic field produced by the current caused the compass needle to be redirected.
The Seebeck effect, in theory to convert heat to electricity, might be the best method for reusing heat energy that would otherwise be wasted. But there was a significant roadblock.
To maintain a temperature difference, a material should have low heat conductivity and high electrical conductivity to generate a substantial amount of electrical energy. The majority of materials, however, exhibit both electrical and heat conductivity; a poor electrical conductor also exhibits poor heat conductivity, and vice versa.
Mahmoud Hussein of the University of Colorado, during his research on the physics of thermoelectric conversion discovered that these properties could be separated in a thin membrane covered with nanopillars—standing columns of material no longer than a few millionths of a meter, or roughly one-tenth the thickness of a human hair. His discovery inspired the team and Bertness to work on this technology.
With the help of the nanopillars, Bertness, Hussein, and their colleagues were able to successfully decouple the silicon sheet’s heat conductivity from its electrical conductivity. It is a first for any material and a significant step towards making efficient heat to electricity conversion possible.
Without affecting its electrical conductivity or altering the Seebeck effect, the researchers managed to reduce the silicon sheet’s heat conductivity by 21%.
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Atoms are bound together by bonds and are unable to move freely to conduct heat in silicon and other materials. As a result, the movement of heat energy manifests as moving phonons, which are the collective atoms’ vibrations. Both the silicon sheet and the gallium nitride nanopillars carry phonons; however, the phonons within the nanopillars are standing waves that are held stationary at both ends by the walls of the tiny columns.
This interaction between phonons and vibrations slows the traveling phonons, making it harder for the heat to pass the material. As a result, thermal conductivity is reduced and temperature difference from one end to another is increased. During all this, there is no change in the electrical conductivity of the silicon sheet.
Now the team is working on solely silicon structures and following a better geometry for thermoelectric heat recovery. They have expectations of demonstrating a high rate to convert heat to electricity. The conversion rate ensures that the technique becomes economically viable for the industry.
Source: NIST Press Release