Researchers at the National Institute of Standards and Technology (NIST) have developed a new device that can increase the conversion (A New Device to Convert Heat into Electricity) of heat to electricity. If perfected, the technology could help recover some of the energy lost in the United States at a rate of about $100 billion a year.
The new manufacturing process — developed by NIST researcher Kris Bertness and colleagues — involves embedding hundreds of thousands of microscopic columns of gallium nitride into a silicon wafer. The silicone layer is removed from the bottom of the wafer until only a thin layer remains.
The interaction between the pillars (A New Device to Convert Heat into Electricity)and the silicon sheet reduces the transfer of heat through the silicon, allowing the heat to be converted into electricity. Bertness and colleagues at the University of Colorado at Boulder reported the results online March 23 in Advanced Materials.
Once the manufacturing process is complete, a sheet of silicon can be wrapped around a steam or exhaust pipe to convert the heat into electricity that can power nearby devices or be delivered to the power grid. Another possible tool is a computer chip.
The NIST-University of Colorado study is based on a phenomenon first discovered by German scientist Thomas Seebeck. In the early 1820s, Seebeck was studying two wires, each made of a different material, which were joined at both ends to form a loop.
He discovered that when the two wires connected to each other were held at different temperatures, the nearby compass needle turned.
Other scientists soon discovered that the repulsion occurred because the temperature difference created a voltage between the two regions, causing water to flow from the hot region to the smooth and quiet.
The current creates a magnetic field that rotates the compass needle.
In theory, the so-called Seebeck effect can be a good way to recycle wasted energy. But there is a big obstacle.
A negative heat source must be created to maintain a temperature difference between two areas to conduct electricity efficiently enough to convert heat into large amounts of electricity.
For many things, however, thermal conductivity and electrical conductivity go together; a negative conductor of heat makes a negative conductor of electricity and vice versa.
In studying the physics of thermoelectric changes, the University of Colorado theorist Mahmoud Hussein discovered that these elements can be combined with a thin membrane that covers nanopillas – vertical columns of material no more than a few millionths of a meter n ‘long, about one-tenth of the thickness of human hair.
His research led to his partnership with Bertness.
Using nanopillars, Bertness, Hussein and their colleagues successfully decouple thermal conductivity from electricity in a sheet of silicon – a first for any material and a necessary step to improve the conversion of heat and electricity.
The researchers reduced the temperature of the silicon sheet by 21% without lowering its electrical conductivity or changing the Seebeck effect.
In silicon and other solids, the atoms are constrained by bonds and cannot move independently to transmit heat.
As a result, the movement of thermal energy takes the form of phonons – stimulating the collective vibrations of atoms.
Gallium nitride nanopillars and silicon sheets carry phonons, but those inside the nanopillars are standing waves, which are fixed by the walls of the small pillars in the same way that guitar strings are held in -shake at both ends.
The interaction between the phonons moving in the silicon sheet and the vibration in the nanopillars causes the sound to vibrate, making it 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 performs this function while leaving the electrical conductivity of the silicon sheet unchanged.
The team is currently working on an all-silicon structure with better geometry for energy recovery.
The researchers hope to demonstrate a high degree of heat and electricity to make their process more economical for industry.
Source: National Institute of Standards and Technology (NIST)
