Magnets could be used to control both heat and sound, according to new research from Ohio State University.
Magnetic fields approximately as powerful as those in MRIs were found to reduce thermal energy running through a semiconductor by around 12 percent, according to researchers. This new study reveals properties of sound waves that were previously unknown to physicists.
Phonons are excitations in the atoms or molecules present in condensed matter. Acoustic phonons are able to transmit both heat and sound, and this study was the first time these excitations were found to react to magnetic fields.
The experiment studied semiconductors cooled to 450 degrees below zero Fahrenheit before being exposed to seven-Tesla magnets, similar to those found in laboratories and hospitals. A standard image is taken at 1.5 Tesla, while a 7 Tesla image uses advantages in signal-to-noise ratio and image contrast and resolution to get a higher-resolution image, although the benefits can only be realized if the correct coils exist to capture the images.
Heat is a measure of the vibrations of atoms and molecules in a material, and sound consists of pressure waves in a given medium, such as air or water. When a person speaks, it is vibrations in vocal cords that create waves in the air, which we interpret as sound. These two properties may seem to be independent of one another, but they are similar on the smallest level. Because of this, any process that alters one should be able to affect the other.
"This adds a new dimension to our understanding of acoustic waves. We've shown that we can steer heat magnetically. With a strong enough magnetic field, we should be able to steer sound waves, too," Joseph Heremans, a mechanical engineer at Ohio State, said.
Using a powerful magnet, it may be possible to control heat in nonmagnetic materials, such as wood, plastic and concrete. However, materials that exhibit magnetic properties, including iron, would not be affected by phonons since they transfer a great deal of heat through electrons, overwhelming the effect.
Indium antimonide was used to create a semiconductor shaped like a minuscule tuning fork, with one tine having a diameter of one-sixth of an inch, and the other just one-quarter that size.
Phonons going through the narrow side slow down due to collisions, compared with those in the other half of the fork. Temperature changes were recorded in each tine, and the difference in speed between the two sides was recorded. When the magnet was active, flow of thermal energy through the larger side was found to be 12 percent lower than when the magnetic field was shut off. Researchers believe the phonons were more likely to experience collisions with the magnet on than with it off, reducing the flow.
The powerful magnet and low temperatures needed to produce this effect will likely keep this new discovery from being used for any practical purpose, at least in the near future.
Discovery of this property of phonons was published in the journal Nature Materials.
