Thermo material may replace heat sinks, fans in electronic gear

EAST LANSING, Mich. — A new thermoelectric material that becomes cool when conducting electricity promises to replace heat sinks and fans in electronic equipment. For a 20 percent premium in manufacturing chips, the patented material may increase their speed 100 percent by dropping a circuit's internal temperature as much as 200C below current operating temperatures, its inventor says.

"Today people use bismuth and tellurium alloys to make thermoelectric materials that can drive temperatures down 50K below the ambient, but by adding cesium we can drive the temperature down from 60 to 100 or even 200," said professor Mercouri Kanatzidis, the material's inventor here at Michigan State University. Tests of p-n junctions made with the new material showed a maximum thermoelectric performance at 225K.

The material could enable applications in cooling chips and boards in computers. These do not routinely take advantage of the achievable 50 drop, because in most cases 50 is not a large enough temperature swing to justify the extra expense, according to Kanatzidis.

"You add about 20 percent to the chips' manufacturing costs to cool them with a thermoelectric material, so the current 50 is not enough of a drop to completely eliminate heat sinks and fans, though some CCD [charge-coupled device] detectors in digital cameras use the thermoelectric effect to help cool them. Our material is better, though, and will open up new applications because it drops the temperature as much as 200 below the ambient — that low a temperature not only cools the chip, but could make it run 100 percent faster," said Kanatzidis.

The thermoelectric effect has been used since the 1950s mainly in compressorless refrigerators in recreational vehicles. Unlike compression-based refrigerators, the thermoelectric junctions can be easily shrunk to smaller sizes, making them ideal refrigeration devices for microelectronics. With applications of Kanatzidis' material, such refrigerators should be able to rival conventional compressor-based units in terms of performance.

The thermoelectric effect occurs at a p-n junction, although for conventional semiconductor materials, it is too small to be useful. When current flows through a p-n junction, both electrons and holes flow away from the interface. The particles carry charge and conduct a certain amount of heat away from the junction, cooling it.

To significantly enhance the effect, it is necessary to find a material that is both a good conductor and thermally insulating. Normally, a good electrical conductor is also a good thermal conductor. Without the thermally insulating capability, however, heat would simply flow back into the p-n junction as fast as it is removed, so that there would be no net drop in temperature.

The researchers looked at a variant of a promising thermoelectric material by substituting cesium for potassium in the compound. Unexpectedly, when the cesium-bismuth-tellurium compound was formed, some of the bismuth-tellurium units that were expected did not form. The resulting material has a novel organization in which ribbons of bismuth-tellurium groups are separated by cesium ions. The compound conducts more strongly in the direction of the ribbons, making it an anisotropic conductor.

Thermoelectric materials are used for niche applications where highly accurate temperature control is required. With a properly designed thermoelectric cooling element, nearly anything can be cooled to within one hundredth of a degree. This high accuracy makes them useful for cooling lasers, where stable temperatures are required to ensure that constant performance parameters are achieved.

Continuous cooling

As for cooling chips and electronic equipment, Kanatzidis believes his material can be sandwiched with traditional bismuth-tellurium alloys to provide continuous cooling capability from ambient temperatures down to 200K below normal operating temperature.

The patented cesium-bismuth-tellurium material was discovered in the Exploratory Solid State Chemistry department of Michigan State University and was funded by the Office of Naval Research and the Defense Advanced Research Projects Agency in a joint program exclusively aimed at discovering exotic new thermoelectric properties.

Kanatzidis collaborated with researcher Duck-Young Chung and professor Tim Hogan at MSU, researchers Ctirad Uher and Marina Bastea at the University of Michigan, and researchers Carl Kannawurf, Paul Brazis and Melissa Rocci-Lane at Northwestern University. Those researchers are working jointly to improve the current cesium-bismuth-tellurium material and find others that operate in even lower temperature windows or that are more efficient.