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Thin Film Blows Hot and Cold

Photonics Spectra
Feb 2002
Hank Hogan

Some like it hot, some like it cold, but everybody likes it efficient. That's why a new thin-film thermoelectric material is attracting attention. It responds 23,000 times faster than the material in bulk and offers hundreds of times the cooling power density.

The thin film is more than twice as effective at pumping heat as any known substance: A 1-cm2 device based on the material would offer 700 W of cooling at a temperature gradient of 58 °F. That could lead to thin-film thermoelectric devices with applications in photonics, communications and computing, including spot cooling or heating of electrical components and better infrared instruments, explained Rama Venkatasubramanian, head of the group at Research Triangle Institute in Research Triangle, N.C.

Thermoelectric materials comprise two conductors in contact with each other. This arrangement converts heat to electricity and vice versa. For 40 years, the best figure of merit (ZT) -- a measure of thermoelectric performance -- for a material has been approximately 1 at room temperature. The new material displays a maximum room temperature ZT of 2.4.

Venkatasubramanian attributed this to the film's construction. Made of alternating 10- to 50-Å-thick layers of bismuth telluride and antimony telluride, the material is a thin-film semiconductor superlattice. Its regular, gridlike array blocks phonons that transport heat from the hot side to the cold. The material simultaneously allows the free movement of electrons that transport heat from the cold side to the hot. Thus, it enhances the active pumping of heat while reducing undesirable parasitic flow.

As with other semiconductors, there are two types of the new material. Currently, the p-type is more efficient, with a ZT of 2.4. The n-type displays a ZT of about 1.4 at room temperature. Venkatasubramanian said that both types are necessary to construct practical thermoelectric modules, but that the average ZT needs to be only 1.5 to be useful in many refrigeration applications.

Photonic components could benefit from the speed and power density achievable with the new material. Another application might be powering wireless devices by converting waste heat into electricity. The institute is forming commercial partnerships to exploit the possibilities.

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