turns heat to light
Technology Research News
Controlling the range of light wavelengths
a material emits makes for more efficient lights. There's a lot of room
for improvement in this area.
The tungsten filaments commonly used in lightbulbs, for instance, are
notoriously inefficient, emitting only five to ten percent of the energy
they use as light, and producing enough heat, or infrared radiation, to
burn the skin of anyone unfortunate enough to touch a lightbulb that has
been on for more than a few seconds.
Researchers from Sandia National Laboratories and Iowa State University
have found a way to structure a tungsten filament so that, instead of
emitting radiation made up of a broad mix of light and heat wavelengths,
it emits 60 percent of the energy it receives in a relatively narrow band
The wavelengths of visible light measure from about 400 to 700 nanometers
from crest to crest, while heat wavelengths range from 700 to one million
nanometers. A nanometer is one millionth of a millimeter.
Concentrating wavelengths paves the way for lights that emit more visible
wavelengths than heat, drastically improving their efficiency.
The researchers' material, a type of photonic crystal, could also improve
the efficiency of thermophotovoltaic devices, which convert heat to electricity,
according to James Fleming, a researcher at Sandia National Laboratories.
Thermophotovoltaic devices convert heat to electricity the same way solar
cells convert visible light to electricity. They produce electricity when
photons from incoming light of certain wavelengths knock electrons loose
from the semiconductor material that makes up the bulk of the device.
The Sandia photonic crystal promises to boost efficiencies by shifting
a broad swath of wavelengths that make up the heat from a heat source
to a narrower band of the optimal wavelengths for thermophotovoltaic cells
to convert to electricity. When the researchers fed the properties of
their material into a mathematical model of thermophotovoltaics they found
that it could boost the efficiency of a thermophotovoltaic device to 51
percent, according to Fleming. By comparison, today's most efficient infrared
emitters come in at just under 13 percent.
The researchers current prototype emits heat, but it is possible to shrink
the crystal structure so that the narrow band it emits is within the wavelengths
of visible light, said Fleming. "The structure needs to be shrunk by roughly
a factor of eight to get into the visible" spectrum, he said.
The photonic crystals are made of tiny bars lined up like Lincoln logs
at regular distances and angles. This artificial crystal latticework allows
only certain wavelengths to pass through, and can also control the direction
of those wavelengths. The material can be made using the same processes
companies use to make computer chips, said Fleming.
These lattices are essentially the photonic counterparts to semiconductors.
They control photon flow in a similar way to how computer chips control
electron flow, said Fleming.
The researchers built the photonic crystal structure in silicon, then
removed some of the silicon and exposed the structure to a chemical vapor
to coat it with tungsten.
The discovery that this particular structure concentrated wavelengths
was accidental, said Fleming. "The structure appears to be able to modify
the range of wavelengths emitted by the filament. The odd emissive behavior
was... not predicted. We came upon it in the course of other, related
work," he said.
The researchers have not worked out the details of how the effect happens,
said Fleming. "We need to better develop the theory behind the effect,"
The work is novel, said Eli Yablonovitch, a professor of electrical engineering
at the University of California at Los Angeles. "It's... probably the
first application of photonic crystals to the energy industry," he said.
The effect has the potential to increase the efficiency of small-scale
devices that convert heat to electricity, he said. "It could lead to small
portable electric generators that run on... fuel and that would produce
electricity very efficiently. In general it's competitive with many other
methods of producing electricity, but it's efficient even in a small unit,"
The research could lead to practical products in about five years, said
Fleming. In terms of gaining a full understanding of the effect, "we should
have a good idea of what is happening in about two years," he said. In
addition, "there are... niche markets for efficient infrared sources which
could benefit from what we have already demonstrated," he said.
Fleming's research colleagues were Shawn Y. Lin, Ihab El-Kady and Rana
Biswas from Sandia and Kai-Ming Ho from Iowa State University. They published
the research in the May 2, 2002 issue of the journal Nature. The research
was funded by Sandia.
Timeline: 5 years
TRN Categories: Materials Science and Engineering; Optical
Computing, Optoelectronics and Photonics
Story Type: News
Related Elements: Technical paper, "All-Metallic Three-dimensional
Photonic Crystals with a Large Infrared Bandgap," Nature, May 2, 2002.
29/June 5, 2002
Crystal turns heat to
fuels atom lasers
Groups key to network
Reverb keeps secrets
safe and sound
Research News Roundup
Research Watch blog
View from the High Ground Q&A
How It Works
News | Blog
Buy an ad link