How to Treat Heat Like Light

A new technique makes it possible to manipulate heat as if it were light, an advance that could improve thermoelectric devices, create thermal diodes or achieve thermal cloaking.

The method developed by MIT research scientist Martin Maldovan is based on engineered materials consisting of nanostructured semiconductor alloy crystals. Heat, like sound, is a vibration of matter. Such vibrations can be thought of as a stream of phonons — a kind of “virtual particle” that is analogous to the photons that carry light. The new approach is similar to recently developed photonic crystals that control the passage of light; phononic crystals do the same for sound.

By tuning the spacing of the tiny gaps present in these materials, the size of the openings can be matched to the wavelength of the heat phonons.

Thermal lattices, shown here, are one possible application of the newly developed thermocrystals from MIT. In these structures, where precisely spaced air gaps (dark circles) control the flow of heat, thermal energy can be "pinned" in place by defects introduced into the structure (colored areas). Courtesy of Martin Maldovan.

“It’s a completely new way to manipulate heat,” said Maldovan, of MIT’s department of materials science and engineering. Heat differs from sound, he explains, in the frequency of its vibrations: Sound waves consist of a single lower frequency (up to the kilohertz range), while heat spans a range of higher frequencies (in the terahertz range).

To manipulate heat using the technique developed for sound, Maldovan first had to reduce the number and frequencies of heat phonons, bringing them closer to the sound range, or “hypersonic heat,” he said.

“The first thing we did is reduce the number of frequencies of heat, and we made them lower,” bringing these frequencies down into the boundary zone between heat and sound, Maldovan said. Making alloys of silicon that incorporate germanium nanoparticles of a particular size range accomplished this lowering of frequency.

Reducing the frequency range was also achieved by making a series of thin films of the material, so that phonon scattering would take place at the boundaries. This concentrated most of the heat phonons within a relatively narrow “window” of frequencies. More than 40 percent of the total heat flow was concentrated within a hypersonic range of 100 to 300 GHz, with most of the phonons aligning in a narrow beam.

This new beam of narrow-frequency phonons can be manipulated using phononic crystals, or thermocrystals, similar to those developed to control sound phonons.

These thermocrystals could be used in a variety of applications to control the flow of heat in a single direction. Possibilities include better thermoelectric devices, which convert temperature differences into electricity, thermal diodes for heat flow in energy-efficient buildings, and thermal cloaking.

The findings were reported in Physical Review Letters (doi: 10.1103/PhysRevLett.110.025902).

For more information, visit: www.mit.edu