Thermo-Chemistry on a Chip

Dreaming of the potential of thermocouple devices? Well, perhaps not… but maybe you should. The October 11th issue of the respected British science journal Nature says there has been a major breakthrough recently in the world of thermoelectric materials.

Scientists funded by the Office of Naval Research and the Defense Advanced Research Projects Agency have taken a field that has stagnated for over forty years and come up with a very high efficiency thermocouple device that could someday make both freon-dependant refrigerators, as well as power generators, obsolete.

By passing a current through thousands of super-thin layers of two different semi-conducting materials, scientists at the Research Triangle Institute (RTI) in North Carolina can make something hotter or colder (depending on which way the current flows) over 20,000 times faster than anything we have today. In addition to the astonishing cooling applications of such a device, these thermoelectric materials could someday be used to convert heat into electrical energy in a far more efficient manner than is possible now.

ONR began funding research in thermoelectric materials in 1993 as part of a program to look for alternatives to freon-based cooling systems aboard Navy ships. The Navy had investigated thermoelectrics in the 1960s and experimental thermoelectric cooling modules had been put aboard the USS Dolphin in the 1970s. "The problem was that the materials just didn't have the efficiency needed to meet the cooling demands for wide-spread use in ship compartment cooling," comments John Pazik, ONR program manager on the research.

In the 1990s ONR set out to discover and understand the science that would lead to new thermoelectric materials with potentially higher efficiency. RTI was one of the first groups ONR supported. They had a unique idea to separate electrical transport from thermal transport through an artificially engineered material based on a semiconductor superlattice. Over the years they had to surmount many obstacles: first they had to develop a chemical vapor deposition method to make thin films with repeating structures only tens of angstroms thick. Next they had to measure the properties of the structure. Finally they had to apply what they'd discovered and make a prototype device.

This marks the beginning of a new era in thermoelectrics. Ultimately these new materials will be engineered into many devices-eventually into plug-in modules-all at an affordable price. The RTI group has cleared the first hurdle: demonstrating the scientific feasibility of engineering such a material into prototype devices. ONR is also supporting solid state synthesis approaches to produce bulk thermoelectric materials. "The potential of all this could be truly significant and broad-based," says Rama Venkatasubramanian of RTI.

"Stay tuned," adds Pazik.

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