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Turning wasted heat into a power source

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Mercouri Kanatzidis envisions a refrigerator that not only would keep the Maytag repairman pining by a silent phone, but could put him out of business altogether.

Gone would be the noisy compressors, the environmentally dubious coolants, and the dust bunnies under the cooling coils. Instead, says the chemistry professor at Michigan State University, the unit would rely on electricity flowing through specially designed semiconductors to keep the inside of the icebox chilled. Those same semiconductors also could be used to convert wasted heat in auto exhaust pipes, power-plant smokestacks, or other sources into valuable electricity.

The problem: So far, it's been hard to develop semiconductors in big enough chunks and with the right characteristics to turn these hopes into affordable hardware that works.

Now, Dr. Kanatzidis and colleagues have hit on a semiconductor recipe that appears to move those hopes closer to reality.

The material is a combination of silver, lead, antimony, and tellurium. And the thermoelectric equivalent of a football quarterback's rating - its "figure of merit," or ZT - appears to be the highest yet achieved for bulk materials at high temperatures. In other words, the material seems to be more efficient at converting heat into electricity than any other similar material.

Indeed, its rating appears achingly close to being competitive with current power-generation and cooling technologies, scientists believe.

This stems not just from the material's relatively high figure of merit, but also from the way the silver and antimony arrange themselves at such a minuscule scale, where sizes are measured in a few billionths of a meter.

That's encouraging, says Arun Majumdar, a mechanical engineer at the University of California in Berkeley who specializes in energy conversion and transport at such small scales. Heating and cooling technologies using materials with a ZT of 3 or more begin to close the cost gap with conventional technologies, he points out. Until now, bulk materials reached ZTs in a range of only 0.6 to 1.0. Kanatzidis's team pushed their material to a ZT of 2.2.

For all the high-tech approaches to forming the new materials, the principle behind what they do is simple, researchers say. Join a pair of wires made from different materials, apply an electric current, and one of the two wires will heat, while the other cools.

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