Breathing new life into an old idea, MIT Institute
Professor Mildred S. Dresselhaus and co-workers are
developing innovative materials for controlling
temperatures that could lead to substantial energy
savings by allowing more efficient car engines,
photovoltaic cells and electronic devices.
Novel thermoelectric materials have already
resulted in a new consumer product: a simple,
efficient way of cooling car seats in hot climates.
The devices, similar to the more-familiar car seat
heaters, provide comfort directly to the individual
rather than cooling the entire car, saving on
air-conditioning and energy costs.
The research is based on the principle of
thermoelectric cooling and heating, which was first
discovered in the early 19th century and was advanced
into some practical applications in the 1960s by MIT
professor (and former president) Paul Gray, among
others.
Dresselhaus and colleagues are now applying
nanotechnology and other cutting-edge technologies to
the field. She'll describe her work toward better
thermoelectric materials in an invited talk on Monday,
Nov. 26 at the annual meeting of the Materials
Research Society in Boston.
Thermoelectric devices are based on the fact that
when certain materials are heated, they generate a
significant electrical voltage. Conversely, when a
voltage is applied to them, they become hotter on one
side, and colder on the other. The process works with
a variety of materials, and especially well with
semiconductors -- the materials from which computer
chips are made. But it always had one big drawback: it
is very inefficient.
The fundamental problem in creating efficient
thermoelectric materials is that they need to be very
good at conducting electricity, but not heat. That
way, one end of the apparatus can get hot while the
other remains cold, instead of the material quickly
equalizing the temperature. In most materials,
electrical and thermal conductivity go hand in hand.
So researchers had to find ways of modifying materials
to separate the two properties.
The key to making it more practical, Dresselhaus
explains, was in creating engineered semiconductor
materials in which tiny patterns have been created to
alter the materials' behavior. This might include
embedding nanoscale particles or wires in a matrix of
another material. These nanoscale structures -- just a
few billionths of a meter across -- interfere with the
flow of heat, while allowing electricity to flow
freely. "Making a nanostructure allows you to
independently control these qualities," Dresselhaus
says.
She and her MIT collaborators started working on
these developments in the 1990s, and soon drew
interest from the US Navy because of the potential for
making quieter submarines (power generation and air
conditioning are some of the noisiest functions on
existing subs). "From that research, we came up with a
lot of new materials that nobody had looked into,"
Dresselhaus says.
After some early work conducted with Ted Harman of
MIT Lincoln Labs, Harman, Dresselhaus, and her student
Lyndon Hicks published an experimental paper on the
new materials in the mid 1990s. "People saw that paper
and the field started," she says. "Now there are
conferences devoted to it."
Her work in finding new thermoelectric materials,
including a collaboration with MIT professor of
Mechanical Engineering Gang Chen, invigorated the
field, and now there are real applications like seat
coolers in cars. Last year, a small company in
California sold a million of the units worldwide.
Other Potential Applications
The same principle can be used to design cooling
systems that could be built right into microchips,
reducing or eliminating the need for separate cooling
systems and improving their efficiency.
The technology could also be used in cars to make
the engines themselves more efficient. In conventional
cars, about 80 percent of the fuel's energy is wasted
as heat. Thermoelectric systems could perhaps be used
to generate electricity directly from this wasted
heat. Because the amount of fuel used for
transportation is such a huge part of the world's
energy use, even a small percentage improvement in
efficiency can have a great impact, Dresselhaus
explains. "It's very practical," she says, "and the
car companies are getting interested."
The same materials might also play a role in
improving the efficiency of photovoltaic cells,
harnessing some of the sun's heat as well as its light
to make electricity. The key will be finding materials
that have the right properties but are not too
expensive to produce.
Dresselhaus and colleagues are continuing to probe
the thermoelectric properties of a variety of
semiconductor materials and nanostructures such as
superlattices and quantum dots. Her research on
thermoelectric materials is presently sponsored by
NASA.
Source:-Massachusetts
Institute of Technology
Published on
30th November 2007
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