Public Release: 

More efficient and reliable refrigerators and air conditioners a step closer to reality

American Chemical Society

NEW ORLEANS, March 27 -- Scientists at Research Triangle Institute in North Carolina have created the world's first working device that uses nanometer-scale materials to convert electric power into cooling or heating, or heat into electricity. Among its many potential applications, the development could eventually lead to more reliable solid-state refrigerators and air conditioners, and more efficient and compact power sources, according to the researchers.

The device -- based on a significant technology advance in thermoelectrics achieved by the RTI laboratory just eighteen months ago -- was described today at the 225th national meeting of the American Chemical Society, the world's largest scientific society, in New Orleans.

The prototype device, about the size of a large postage stamp, consists of a special semiconductor chip encased between two thin translucent crystals. The chip itself contains about 1,000 layers of nano-scale films of thermoelectric materials that can pump heat or generate power with unprecedented efficiency, according to team leader Rama Venkatasubramanian, Ph.D.

"This is the first nano-scale material-based device that can achieve a cooling effect suitable for everyday functions like refrigeration or power production," Venkatasubramanian says. Prior demonstrations of nano-materials have been done for "electronic functions such as signal amplification in a transistor," he adds.

When tested in the laboratory for its ability as a heat pump, the device cooled a block of solid steel from 79 degrees to 64 degrees in about two minutes, much faster than a conventional refrigerator can cool, says Venkatasubramanian. The performance of such devices is approaching the cooling efficiency of current thermoelectric devices, but in much smaller packages. Ongoing improvements in the device can increase its efficiency by two to three times, he added.

"By creating the first useful superlattice device, we've shown that these superlattices are robust enough to withstand very intricate device manufacturing. This portends well for the day when these superlattices could be used in many different applications."

"If we are fully successful, we can imagine the possibility of replacing most of the mechanical refrigerators and air-conditioning systems with CFC-free, solid-state, no-moving parts, and therefore reliable, electronic heat pump technology," Venkatasubramanian says.

The first applications of the new devices are likely to be as tiny heat pumps that can spot-cool microprocessors or communication lasers, according to Venkatasubramanian. The next applications might be in microscopic cooling and heating to regulate localized temperature changes on DNA microarrays, he says.

The prototype device is made of atomically precise superlattices -- stacks of very thin films of two alternating semiconductors (bismuth telluride and antimony telluride). Each film layer is only a few tens-of-a-billionth of a centimeter thick and contains from a few to about two-dozen layers of atoms.

The prototype devices that utilize these superlattices, known as thermocouples, are based on an old concept -- running electricity through two dissimilar conductors to set up a heat pump without any moving parts. Essentially, the current pushes heat toward one end of the circuit, thereby cooling the other end. A related phenomenon can also be used to turn heat into electricity.

Because they have no or very few moving parts, such thermocouples are very reliable but have always been considered to be very inefficient. Thermocouples are currently used in applications where reliability is critical, such as mini power packs for deep space probes or precise temperature control of lasers used in fiber-optic communication. They are also used in a limited range of consumer products, including climate-controlled car seats and picnic coolers that can be powered from a car battery.

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The Defense Advanced Research Projects Agency and the Office of Naval Research provided funding for this research. The paper on this research, IEC 232, will be presented at 9:35 a.m., Thursday, March 27, at the Morial Convention Center, Room 392, during the symposium, "Nanotechnology and the Environment."

Rama Venkatasubramanian, Ph.D., is the research director of the Center for Thermoelectrics Research at Research Triangle Institute in Research Triangle Park, N.C.

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