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A microscope image of the tungsten photonic crystal structure reveals the precise uniform spacing of cavities formed in the material, which are tuned to specific wavelengths of light. Image: Y.X. Yeng et al.
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A
team of Massachusetts Institute of Technology (MIT) researchers has developed a
way of making a high-temperature version of a kind of materials called photonic
crystals, using metals such as tungsten or tantalum. The new materials—which
can operate at temperatures up to 1,200 C—could find a wide variety of
applications powering portable electronic devices, spacecraft to probe deep
space, and new infrared light emitters that could be used as chemical detectors
and sensors.
Compared
to earlier attempts to make high-temperature photonic crystals, the new
approach is "higher performance, simpler, robust, and amenable to inexpensive
large-scale production," says Ivan Celanovic ScD '06, senior author of a paper
describing the work in the Proceedings of the National Academy of Sciences.
The paper was co-authored by MIT professors John Joannopoulos and Marin
Soljačić, graduate students Yi Xiang Yeng and Walker Chen, affiliate Michael
Ghebrebrhan and former postdoc Peter Bermel.
These
new high-temperature, 2D photonic crystals can be fabricated almost entirely
using standard microfabrication techniques and existing equipment for
manufacturing computer chips, says Celanovic, a research engineer at MIT’s
Institute for Soldier Nanotechnologies.
While
there are natural photonic crystals—such as opals, whose iridescent colors
result from a layered structure with a scale comparable to wavelengths of
visible light—the current work involved a nanoengineered material tailored for
the infrared range. All photonic crystals have a lattice of one kind of
material interspersed with open spaces or a complementary material, so that
they selectively allow certain wavelengths of light to pass through while
others are absorbed. When used as emitters, they can selectively radiate
certain wavelengths while strongly suppressing others.
Photonic
crystals that can operate at very high temperatures could open up a suite of
potential applications, including devices for solar-thermal conversion or
solar-chemical conversion, radioisotope-powered devices, hydrocarbon-powered
generators, or components to wring energy from waste heat at powerplants or
industrial facilities. But there have been many obstacles to creating such
materials: The high temperatures can lead to evaporation, diffusion, corrosion,
cracking, melting, or rapid chemical reactions of the crystals' nanostructures.
To overcome these challenges, the MIT team used computationally guided design
to create a structure from high-purity tungsten, using a geometry specifically
designed to avoid damage when the material is heated.
NASA
has taken an interest in the research because of its potential to provide
long-term power for deep-space missions that cannot rely on solar power. These
missions typically use radioisotope thermal generators (RTGs), which harness
the power of a small amount of radioactive material. For example, the new
Curiosity rover scheduled to arrive at Mars this summer uses an RTG system; it
will be able to operate continuously for many years, unlike solar-powered
rovers that have to hunker down for the winter when solar power is
insufficient.
Other
potential applications include more efficient ways of powering portable
electronic devices. Instead of batteries, these devices could run on
thermophotovoltaic generators that produce electricity from heat that is
chemically generated by microreactors, from a fuel such as butane. For a given
weight and size, such systems could allow these devices to run up to 10 times
longer than they do with existing batteries, Celanovic says.
Shawn
Lin, a professor of physics at Rensselaer Polytechnic Institute who specializes
in future chip-making technology, says that research on thermal radiation at
high temperatures "continues to challenge our scientific understanding of the
various emission processes at sub-wavelength scales, and our technological
capability." Lin, who was not involved in this work, adds, "This particular 2D
tungsten photonic crystal is quite unique, as it is easier to fabricate and
also very robust against high-temperature operation. This photonic-crystal
design should find important application in solar-thermal energy-conversion
systems."
While
it's always hard to predict how long it will take for advances in basic science
to lead to commercial products, Celanovic says he and his colleagues are
already working on system integration and testing applications. There could be
products based on this technology in as little as two years, he says, and most
likely within five years.
In
addition to producing power, the same photonic crystal can be used to produce
precisely tuned wavelengths of infrared light. This could enable highly
accurate spectroscopic analysis of materials and lead to sensitive chemical
detectors, he says.
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