Thermocrystal: an excellent idea to control the direction of heat by nanoparticle alloy

An MIT scientist has developed a technique that provides a new way of manipulating heat, allowing it to be controlled much as light waves can be manipulated by lenses and mirrors.

The approach relies on engineered materials consisting of nanostructured semiconductor alloy crystals. Heat is a vibration of matter, a vibration of the atomic lattice of a material like sound. Such vibrations can also be thought of as a stream of phonons, which is equivalent to the photons that carry light. The new approach is similar to recently developed photonic crystals that can control the passage of light.

The spacing of tiny gaps in these materials is tuned to match the wavelength of the heat phonons. It’s a completely new way to manipulate heat. Heat differs from sound in the frequency of its vibrations: Sound waves consist of lower frequencies (up to the kilohertz range, or thousands of vibrations per second), while heat arises from higher frequencies (in the terahertz range, or trillions of vibrations per second).

In order to apply the techniques already developed to manipulate sound, first step was to reduce the frequency of the heat phonons, bringing it closer to the sound range. Phonons for sound can travel for kilometres, but phonons of heat only travel for nanometers. That’s why we couldn’t hear heat even with ears.
Heat also spans a wide range of frequencies, while sound spans a single frequency. To get rid of the problem, the first thing to do is to reduce the number of frequencies of hea, bringing these frequencies down into the boundary zone between heat and sound. Making alloys of silicon that incorporate nanoparticles of germanium in a particular size range accomplished this lowering of frequency, scientist found.

Reducing the range of frequencies was also accomplished by making a series of thin films of the material, so that scattering of phonons would take place at the boundaries. This ends up concentrating most of the heat phonons within a relatively narrow window of frequencies.

Following the application of these techniques, more than 40 percent of the total heat flow is concentrated within a hypersonic range and most of the phonons align in a narrow beam, instead of moving in every direction.
As a result, this beam of narrow-frequency phonons can be manipulated using phononic crystals similar to those developed to control sound phonons. Because these crystals are now being used to control heat instead, these are referred to as thermocrystals, a new category of materials.

These thermocrystals might have a wide range of applications, including in improved thermoelectric devices, which convert differences of temperature into electricity. Such devices transmit electricity freely while strictly controlling the flow of heat.

Most conventional materials allow heat to travel in all directions, like ripples expanding outward from a pebble dropped in a pond; thermocrystals could instead produce the equivalent of those ripples only moving out in a single direction. The crystals could also be used to create thermal diodes; materials in which heat can pass in one direction, but not in the reverse direction. Such a one-way heat flow could be useful in energy-efficient buildings in hot and cold climates.

Other variations of the material could be used to focus heat to concentrate it in a small area. Another intriguing possibility is thermal cloaking, materials that prevent detection of heatto shield objects from detection by visible light or microwaves.

For further reading: http://prl.aps.org/pdf/PRL/v110/i2/e025902