Computers, light bulbs, and even people generate heat—energy that ends up being wasted. With a thermoelectric device, which converts heat to electricity and vice versa, you can harness that otherwise wasted energy.
Thermoelectric devices are touted for use in new and efficient refrigerators, and other cooling or heating machines. But present-day designs are not efficient enough for widespread commercial use or are made from rare materials that are expensive and harmful to the environment.
Researchers at the California Institute of Technology (Caltech) have developed a new type of material—made out of silicon, the second most abundant element in Earth's crust—that could lead to more efficient thermoelectric devices. The material—a type of nanomesh—is composed of a thin film with a grid-like arrangement of tiny holes. This unique design makes it difficult for heat to travel through the material, lowering its thermal conductivity to near silicon's theoretical limit. At the same time, the design allows electricity to flow as well as it does in unmodified silicon.
"In terms of controlling thermal conductivity, these are pretty sophisticated devices," says James Heath, the Elizabeth W. Gilloon Professor and professor of chemistry at Caltech, who led the work. A paper about the research will be published in the October issue of the journal Nature Nanotechnology.
A major strategy for making thermoelectric materials energy efficient is to lower the thermal conductivity without affecting the electrical conductivity, which is how well electricity can travel through the substance. Heath and his colleagues had previously accomplished this using silicon nanowires—wires of silicon that are 10 to 100 times narrower than those currently used in computer microchips. The nanowires work by impeding heat while allowing electrons to flow freely.
In any material, heat travels via phonons—quantized packets of vibration that are akin to photons, which are themselves quantized packets of light waves. As phonons zip along the material, they deliver heat from one point to another. Nanowires, because of their tiny sizes, have a lot of surface area relative to their volume. And since phonons scatter off surfaces and interfaces, it is harder for them to make it through a nanowire without bouncing astray. As a result, a nanowire resists heat flow but remains electrically conductive.
But creating narrower and narrower nanowires is effective only up to a point. If the nanowire is too small, it will have so much relative surface area that even electrons will scatter, causing the electrical conductivity to plummet and negating the thermoelectric benefits of phonon scattering.
To get around this problem, the Caltech team built a nanomesh material from a 22-nanometer-thick sheet of silicon. (One nanometer is a billionth of a meter.) The silicon sheet is converted into a mesh—similar to a tiny window screen—with a highly regular array of 11- or 16-nanometer-wide holes that are spaced just 34 nanometers apart.
Instead of scattering the phonons traveling through it, the nanomesh changes the way those phonons behave, essentially slowing them down. The properties of a particular material determine how fast phonons can go, and it turns out that—in silicon at least—the mesh structure lowers this speed limit. As far as the phonons are concerned, the nanomesh is no longer silicon at all. "The nanomesh no longer behaves in ways typical of silicon," says Slobodan Mitrovic, a postdoctoral scholar in chemistry at Caltech. Mitrovic and Caltech graduate student Jen-Kan Yu are the first authors on the Nature Nanotechnology paper.
When the researchers compared the nanomesh to the nanowires, they found that—despite having a much higher surface-area-to-volume ratio—the nanowires were still twice as thermally conductive as the nanomesh. The researchers suggest that the decrease in thermal conductivity seen in the nanomesh is indeed caused by the slowing down of phonons, and not by phonons scattering off the mesh's surface. The team also compared the nanomesh to a thin film and to a grid-like sheet of silicon with features roughly 100 times larger than the nanomesh; both the film and the grid had thermal conductivities about 10 times higher than that of the nanomesh.
Although the electrical conductivity of the nanomesh remained comparable to regular, bulk silicon, its thermal conductivity was reduced to near the theoretical lower limit for silicon. And the researchers say they can lower it even further. "Now that we've showed that we can slow the phonons down," Heath says, "who's to say we can't slow them down a lot more?"
The researchers are now experimenting with different materials and arrangements of holes in order to optimize their design. "One day, we might be able to engineer a material where you not only can slow the phonons down, but you can exclude the phonons that carry heat altogether," Mitrovic says. "That would be the ultimate goal."
The other authors on the paper, "Reduction of thermal conductivity in phononic nanomesh structures," are Caltech graduate students Douglas Tham and Joseph Varghese. The research was funded by the Department of Energy, the Intel Foundation, a Scholar Award from the King Abdullah University of Science and Technology, and the National Science Foundation.
Visit the Caltech Media Relations website at http://media.caltech.edu.
Kathy Svitil | EurekAlert!
Strange but true: Turning a material upside down can sometimes make it softer
20.10.2017 | Universitat Autonoma de Barcelona
Metallic nanoparticles will help to determine the percentage of volatile compounds
20.10.2017 | Lomonosov Moscow State University
University of Maryland researchers contribute to historic detection of gravitational waves and light created by event
On August 17, 2017, at 12:41:04 UTC, scientists made the first direct observation of a merger between two neutron stars--the dense, collapsed cores that remain...
Seven new papers describe the first-ever detection of light from a gravitational wave source. The event, caused by two neutron stars colliding and merging together, was dubbed GW170817 because it sent ripples through space-time that reached Earth on 2017 August 17. Around the world, hundreds of excited astronomers mobilized quickly and were able to observe the event using numerous telescopes, providing a wealth of new data.
Previous detections of gravitational waves have all involved the merger of two black holes, a feat that won the 2017 Nobel Prize in Physics earlier this month....
Material defects in end products can quickly result in failures in many areas of industry, and have a massive impact on the safe use of their products. This is why, in the field of quality assurance, intelligent, nondestructive sensor systems play a key role. They allow testing components and parts in a rapid and cost-efficient manner without destroying the actual product or changing its surface. Experts from the Fraunhofer IZFP in Saarbrücken will be presenting two exhibits at the Blechexpo in Stuttgart from 7–10 November 2017 that allow fast, reliable, and automated characterization of materials and detection of defects (Hall 5, Booth 5306).
When quality testing uses time-consuming destructive test methods, it can result in enormous costs due to damaging or destroying the products. And given that...
Using a new cooling technique MPQ scientists succeed at observing collisions in a dense beam of cold and slow dipolar molecules.
How do chemical reactions proceed at extremely low temperatures? The answer requires the investigation of molecular samples that are cold, dense, and slow at...
Scientists from the Max Planck Institute of Quantum Optics, using high precision laser spectroscopy of atomic hydrogen, confirm the surprisingly small value of the proton radius determined from muonic hydrogen.
It was one of the breakthroughs of the year 2010: Laser spectroscopy of muonic hydrogen resulted in a value for the proton charge radius that was significantly...
17.10.2017 | Event News
10.10.2017 | Event News
10.10.2017 | Event News
20.10.2017 | Information Technology
20.10.2017 | Materials Sciences
20.10.2017 | Interdisciplinary Research