[Editors] MIT: Thermoelectric materials are one key to energy savings

[Elizabeth Thomson]

MIT News Office
Massachusetts Institute of Technology
Room 11-400
77 Massachusetts Avenue
Cambridge, MA  02139-4307
Phone: 617-253-2700
http://web.mit.edu/newsoffice/www

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MIT: Thermoelectric materials are one key to energy savings

--Researchers jumpstart old field with new approach

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For Immediate Release
TUESDAY, NOV. 20, 2007
Contact: Elizabeth A. Thomson, MIT News Office -- Phone: 617-258-5402  
-- Email: thomson at mit.edu

PHOTO AVAILABLE

CAMBRIDGE, Mass. - 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.

--END-

Written by David Chandler, MIT News Office