This crystal structure was built using alternating layers of silicon dioxide (the basis of the dielectric layers in most microchips) and a polymer material. The resulting two-component material successfully reflected phonons — vibrational waves that are the carriers of ordinary heat or sound, depending on their frequency. In this case, the phonons were in the gigahertz range — in other words, low-level heat.
Edwin L. Thomas, head of MIT’s Department of Materials Science and Engineering and the Morris Cohen Professor of Materials Science and Engineering, was a co-author of a new paper, published on March 10 in the journal Nano Letters, that describes this creation of phononic crystals in the hypersonic range (that is, above the frequency range of sound, and thus can be considered in the range of heat).
Phonons may sometimes be thought of as particles, and sometimes as vibrational waves, analogous to the dual wave and particle nature of light. Physically, the phonons are manifested as a wave of density variation passing through a material, like the wave of compression that travels along a child’s Slinky toy when you stretch it out and give one end a shove.
Thomas says phonons, which exist in all solids, are usually a nuisance that must be disposed of with cooling systems. They have been “denigrated and ignored, but they could be the future star attraction if we can train them to do tricks for us.” Among other things, this could lead to highly efficient ways of scavenging heat that is now wasted, in everything from computers and cell phones to cars and power plants, in order to produce electricity. This latest research, funded by the National Science Foundation and its German equivalent, DFG, is still at the level of simple tricks, he says: “It’s a step on the path.”
Phonons can be controlled through manufactured crystal-like structures. In this latest research, Thomas and his colleagues in Germany and Greece fabricated a “one-dimensional periodic” crystal structure, which means that although the material has three dimensions, its regularly varying molecular structure — in this case, alternating layers of two different materials — only varies along one direction, like a stack of vanilla and chocolate ice cream where the layers alternate. So if you look at a single layer, there’s just a uniform color, but if you drill through the stack, you find regularly alternating layers. When the spacing between similar layers matches the wavelength of the phonons, those phonons are blocked and reflected back.
The phonons that are reflected from this newly developed material are in the range of low-frequency heat (since anything above absolute zero, or minus 273 degrees Celsius, is considered heat, which is just due to the movement of vibrational waves). Hence, this reflector currently only works at sub-freezing temperatures. Further work on decreasing the thickness of the layers could bring them closer to the range of a theoretical “perfect insulator” that could block heat of a certain frequency range in an ordinary room-temperature environment. And this could open up a host of potential applications.
No material is ever going to be perfect, but even a material that reflects back a very high percentage of heat could be a big improvement over present insulators. For example, a shell of such material could be used to maintain the temperature in a package of delicate research instruments in a frigid environment.
How far off are such applications? “It’s close, if you don’t worry about price,” Thomas says — which may be the case for some uses such as spacecraft, or instruments deployed in Antarctica. And as the technology develops and as production gets scaled up, prices could eventually come down far enough to enable more widespread applications.
Ihab El-Kady, a researcher at Sandia National Laboratories, says that while much current research on phonons involves the creation of two- or three-dimensional crystals, which may have greater long-term applications, there are some advantages to studying one-dimensional crystals as Thomas and his co-authors did in this case. “One-D systems are still preferable for their ease of fabrication, and can offer particular insight into the basic physical mechanics of phononic crystals,” he says. “In that light, this paper represents a novel and insightful tool” for analyzing fundamental wave phenomena, as well as the interactions between phonons and other particles such as photons.
Most early work on phonons dealt with sound-wave frequencies, which can be manipulated using larger crystal structures, but advances in nanotechnology have made it possible to create materials with structures small enough to handle the high-frequency, short-wavelength phonons associated with heat.
The best way to understand the enormous potential of devices that control phonons is by comparing them to devices that control electrons and photons, says Thomas. He explains that our growing understanding of electrons and photons — which carry electricity and light, respectively — has led to decades of technological innovation, including the invention of lasers, transistors, photovoltaic cells and microchips. These basic inventions, in turn, made possible most of the devices that define modern life, including cell phones, computers, DVD players and flat-screen TVs. Now there are a lot of people trying to understand phonons, he says, which could lead to a similar proliferation of new — and impossible to predict — technologies.
As a result, Thomas says, the field of phononics “has the potential to rocket off.”