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New Material Cools to -100 °C

Photonics Spectra
Jun 2000
Dr. James P. Smith

Many applications await the development of efficient, inexpensive, low-temperature thermoelectric coolers. Current passing through a thermoelectric material carries away heat, cooling the material and its surroundings. Thermoelectric heat pumps based on bismuth telluride are in limited use for room-temperature applications such as cooling solid-state lasers, but they are not useful at the low temperatures needed to reduce infrared background radiation.

Michigan State University chemist Mercouri G. Kanatzidis has synthesized a new thermoelectric material, CsBi4Te6, that may extend the useful temperature range of these devices to -100 °C. His work, in collaboration with researchers from the University of Michigan in Ann Arbor and Northwestern University in Evanston, Ill., was reported in the Feb. 11 issue of Science.

"We wanted to get away from what we call the tyranny of bismuth telluride," Kanatzidis explained. He said that, until recently, most research had focused on optimizing that material. "We wanted to do something drastically different, so we introduced cesium to cause the bismuth telluride to reorganize into a new structure. The idea was to make the crystal unit cells larger and also make the material more ionic- or ceramiclike. This could reduce the thermal conductivity," he said. The ideal thermoelectric material would have the unlikely dual properties of high electrical conductivity and low thermal conductivity.

A thermoelectric heat pump consists of a series of alternating N-type and P-type thermoelectric materials, but the classic PN diode-junction model does not apply. In fact, the two materials may even be different chemicals. As electric current flows through the pump, both holes and electrons are removed from the connection between the two types, carrying away energy.

To date, Kanatzidis has produced only P-type CsBi4Te6 -- by doping with antimony iodide. His group is evaluating other dopants and materials. "We synthesize and test many new compounds, but only a few are promising," he said.

Kanatzidis explained that there are two issues to address with thermoelectric devices: "One is to cool as low as possible, without cumbersome equipment or liquid nitrogen. The other is to cool, not at a very low temperature, but cool enough to keep devices from burning up. Right now the bismuth telluride cools well at room temperatures." The new CsBi4Te6 material has its maximum cooling merit at 70 °C below room temperature.

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