ITHACA, N.Y. -- It's possible that one day all the cooling power ofa noisy, bulky household refrigerator will be available on a small devicethat is lightweight and has no moving parts. And the same device, whengiven a heat source like a car's exhaust pipe, could be used to generateelectricity.
Such thermoelectric devices already exist in consumer products likeplug-in auto beverage coolers, where energy efficiency is less importantthan portability and low weight. The challenge facing researchers is tofind new materials that could bring the technology to the next level inwhich the efficiency would rival that of conventional coolants in airconditioners as well as refrigerators. Also in the future might beminiature cooling devices directly on computer chips.
"As we increase the efficiency of thermoelectric devices, we createanother tool in the arsenal for choosing the most efficient way to dothings. You can think of some applications pretty quickly, and others wouldcome up once the technology is available," said Francis DiSalvo, professorof chemistry and chemical biology, who is attempting to develop newthermoelectric materials. DiSalvo described the status of research in thefield in a recent issue of "Science".
In conventional cooling devices, heat is carried away by a workingfluid, such as a chlorofluorocarbon, which involves the moving parts thatcause most equipment breakdowns, environmental damage and bulkiness. Inthermoelectric devices, the "working fluid" is electrical current that runsthrough a junction between differently doped semiconductors and pulls heataway from that junction, producing cooling without any moving parts.
Current thermoelectric materials operate at roughly 10 percent ofCarnot efficiency, the theoretical maximum allowed by the laws ofthermodynamics, compared with about 30 percent for an average householdrefrigerator. The theory behind thermoelectric devices has been around formore than 40 years, but current materials don't rival the efficiency ofcompressor-based devices. "The theory is not specific enough to say, 'Ifyou could make this kind of material -- that is, this composition withatoms in a particular crystalline arrangement -- that will give you thehigh thermoelectric efficiency.' We have to go find the materials by anempirical process, test them one at a time and say, 'Is our understandinggood enough that we can predict from what we're learning today about what'sthe next best thing to try to synthesize after that?'" DiSalvo said.
The search, funded by the U.S. Office of Naval Research, is complexbecause, according to DiSalvo, with the exception of designing organicmolecules based on carbon, the ability to predict the composition andstructure of materials made from three or more elements is completelylacking. "For most of the elements in the periodic table, we don't knowwhat will happen when we put them together. If we knew how to do that, thenwe could calculate from the structure what the thermoelectric propertiesmight be like. And the theory is good enough now that the results would befairly accurate," he said.
The search is focused on uniform bulk materials, which can beprepared in large amounts by traditional synthetic methods, and oncompositionally modulated films, which require expensive nanofabrication.Bulk materials, the object of DiSalvo's research, primarily haveapplications in large devices like home refrigeration and recovering powerfrom car heat exhaust, while modulated film research might be applicable toniche markets like on-chip cooling.
Researchers know they need a material with low thermal conductivityand high electrical conductivity, which has led them to look at compoundsof heavy elements like lead, antimony, bismuth and tellurium. "We have somecompounds that looked promising based on platinum. No way you're going tobuild devices like that out of platinum -- way too expensive," DiSalvosaid. But, he said, "if we can do the proof of principle, then we're offand running; we've got our foot in the door."
DiSalvo's article, "Thermoelectric Cooling and Power Generation,"appeared in the July 30 issue of "Science".
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