Thermal lattices. Image courtesy of the researchers
Posted on 01/11/2013 10:55:49 AM PST by Red Badger
FULL TITLE: New approach using nanoparticle alloys allows heat to be focused or reflected just like electromagnetic waves
An MIT researcher 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—technically, a vibration of the atomic lattice of a material—just as sound is. Such vibrations can also 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 can control the passage of light, and phononic crystals that can do the same for sound. The spacing of tiny gaps in these materials is tuned to match the wavelength of the heat phonons, explains Martin Maldovan, a research scientist in MIT's Department of Materials Science and Engineering and author of a paper on the new findings published Jan. 11 in the journal Physical Review Letters. "It's a completely new way to manipulate heat," Maldovan says. Heat differs from sound, he explains, 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, Maldovan's first step was to reduce the frequency of the heat phonons, bringing it closer to the sound range. He describes this as "hypersonic heat." "Phonons for sound can travel for kilometers," Maldovan says—which is why it's possible to hear noises from very far away. "But phonons of heat only travel for nanometers [billionths of a meter]. That's why you could't hear heat even with ears responding to terahertz frequencies." Heat also spans a wide range of frequencies, he says, while sound spans a single frequency. So, to address that, Maldovan says, "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. Making alloys of silicon that incorporate nanoparticles of germanium in a particular size range accomplished this lowering of frequency, he says. 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 of 100 to 300 gigahertz, 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, Maldovan refers to them as "thermocrystals," a new category of materials. These thermocrystals might have a wide range of applications, he suggests, including in improved thermoelectric devices, which convert differences of temperature into electricity. Such devices transmit electricity freely while strictly controlling the flow of heat—tasks that the thermocrystals could accomplish very effectively, Maldovan says. 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, Maldovan says. 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—much like focusing light with a lens—to concentrate it in a small area. Another intriguing possibility is thermal cloaking, Maldovan says: materials that prevent detection of heat, just as recently developed metamaterials can create "invisibility cloaks" to shield objects from detection by visible light or microwaves. Journal reference: Physical Review Letters search and more info website Provided by Massachusetts Institute of Technology
Thermal lattices. Image courtesy of the researchers
This could pave the way for making devices that convert heat DIRECTLY into electricity without needing turbines and steam etc....
This would be a great material to put BEHIND solar cells and would mop up the extra heat and then channel it into thermo-electric converters.
You might also be able to use it to increase the efficiency of things like stiring engines for use in regenerative electric cars that don’t suck.
Or crowd control. Selectively turn individuals into “popped corn”.
So in the future we'll have horrible 'heat weapons' - - and we'll be able to heat our homes with something similar to a radio on AAA batteries?
Transmit freely? Like Tesla wanted to broadcast electricity..?
This has enormous possibilities in cooling your home as well. Imagine a roof that could channel all the sun’s heat into an underground tank of water or a wall that could channel and release that heat in the winter..........
No, the writer meant that the devices turn heat directly into electricity, without the need for an intermediary step.........
Heat is a vibration of matter—technically, a vibration of the atomic lattice of a material—just as sound is. Such vibrations can also 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 can control the passage of light, and phononic crystals that can do the same for sound. The spacing of tiny gaps in these materials is tuned to match the wavelength of the heat phonons, explains Martin Maldovan, a research scientist in MIT's Department of Materials Science and Engineering and author of a paper on the new findings published Jan. 11 in the journal Physical Review Letters.
"It's a completely new way to manipulate heat," Maldovan says. Heat differs from sound, he explains, 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, Maldovan's first step was to reduce the frequency of the heat phonons, bringing it closer to the sound range. He describes this as "hypersonic heat." "Phonons for sound can travel for kilometers," Maldovan says—which is why it's possible to hear noises from very far away. "
But phonons of heat only travel for nanometers [billionths of a meter]. That's why you could't hear heat even with ears responding to terahertz frequencies." Heat also spans a wide range of frequencies, he says, while sound spans a single frequency. So, to address that, Maldovan says, "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.
Making alloys of silicon that incorporate nanoparticles of germanium in a particular size range accomplished this lowering of frequency, he says. 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 of 100 to 300 gigahertz, 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, Maldovan refers to them as "thermocrystals," a new category of materials.
These thermocrystals might have a wide range of applications, he suggests, including in improved thermoelectric devices, which convert differences of temperature into electricity. Such devices transmit electricity freely while strictly controlling the flow of heat—tasks that the thermocrystals could accomplish very effectively, Maldovan says. 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, Maldovan says.
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—much like focusing light with a lens—to concentrate it in a small area. Another intriguing possibility is thermal cloaking, Maldovan says: materials that prevent detection of heat, just as recently developed metamaterials can create "invisibility cloaks" to shield objects from detection by visible light or microwaves.
Journal reference: Physical Review Letters search and more info website Provided by Massachusetts Institute of Technology
Excellent application. An elderly friend of mine had several patents on Stirling engines. I was amazed at how his engines directly converted solar rays to mechanical energy to drive an engine (via a parabolic dish). Unfortunately he passed away before seeing this new development.
Devices already exist that convert heat directly to electricity. Well, actually, temperature differential, hot on one side, cool on the other.
I’ve been looking into the practicality of designing an array of TEG units to fit onto the exterior flue of a woodstove. Looks like about $5,000 worth of them would make a cabin minimally habitable so long as there was a fire in the woodstove. No moving parts, some have been in use for 200,000 hours without failing.
Minimally habitable meaning well pump, refrigeration, a few interior and exterior lights, and a few interior electrical receptacles with enoug current to power a few small appliances or a laptop,
I’m not well-versed enough to be able to consrtuct the thing myself, so the cost would go up considerably, to hire expertise in something that is currently rather obscure.
It won’t be for long, though. There are already small camp stoves on the market with a TEG unit that powers a small fan to aid combustion, with “waste” heat powering a USB port for recharging handheld devices such as cell phones, LED flashlights, etc. If you want to see one, do a web search for the Bio Lite camp stove.
Could this finally allow researchers to build working fusion reactors?
I also like the idea of cloaking yourself, equipment, perishables, etc., against the damaging effects of heat. Very cool! (Pun intended.)
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