Efficient, Reliable Refrigerators, Air Conditioners Closer to Reality

EMBARGOED FOR RELEASE: Monday, March 24, 10:30 a.m., Central Time

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.

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." It also will be featured in a press conference on Monday, March 24, at 9:00 a.m., at the Morial Convention Center, Room 280.

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

#13464 Released 03/27/2003

EMBARGOED FOR RELEASE: Monday, March 24, 10:30 a.m., Central TimeIEC 232 Thin-film superlattice thermoelectric devicesRama Venkatasubramanian, Center for Thermoelectrics Research, Research Triangle Institute, 3040 Cornwallis Road, Research Triangle Park, NC 27709

Thin-film nano-structured materials offer the potential to dramatically enhance the performance of thermoelectrics, thereby offering new capabilities, ranging from CFC-free refrigeration to portable electric power sources, replacing batteries, to thermochemistry-on-a-chip. We demonstrated [1] a significant enhancement in thermoelectric figure-of-merit (ZT) at 300K - about 2.4 in 1nm/5nm p-type Bi2Te3/Sb2Te3 superlattice structures and recently, about 1.7 to 1.9 in 1nm/4nm n-type Bi2Te3/Bi2Te3-xSex superlattices, using the concept of phonon-blocking electron-transmitting superlattice structures. The phonon blocking arises from a complex localization-like behavior for phonons in nano-structured superlattices and the electron transmission is facilitated by optimal choice of band-offsets in these semiconductor heterostructures. More recently [2], Harman et. al has demonstrated a significant ZT enhancement in PbTe/PbTeSe quantum-dot superlattices, over PbTe-based materials, using similar reduction in thermal conductivity with nanostructures. The thin-film devices, resulting from microelectronic processing, allow high cooling power densities to be achieved for a variety of applications, with potential localized active-cooling power densities approaching 700 W/cm2. In addition to high-performance and power densities, these thin-film microdevices are also extremely fast acting, within ~10 to 20 microsec and about a factor of 23,000 faster than bulk thermoelectric devices. These results set the stage for a wide range of applications for the superlattice thin-film thermoelectric technology. Our technical progress in some of the near-term applications will be presented. [1] Nature, 413, 597-602 (2001); [2] Science, 297, 2297-2299 (2002)

EMBARGOED FOR RELEASE: Monday, March 24, 10:30 a.m., Central TimeIEC 232 Thin-film superlattice thermoelectric devices

*Briefly explain in lay language what you have done, why it is significant and its implications, particularly to the general public.

We have developed nanoscale engineered thermoelectric materials that represents a breakthrough compared to the state-of-the-art for the last 40 years. In addition to potentially providing a factor of two to three improvement in efficiency, it can lead to high cooling power densities, a factor of 23,000 better than state-of-the-art in cooling/heating speed, and a factor of 40,000 reduction in materials usage for the same functionality.

This thin-film nanoscale materials advancement has implications in the use of solid-state deices for thermal-to-electrical energy conversion, refrigeration, electronics (like microprocessors) thermal management, as well as in high-speed cooling/heating for applications in photonic switching and biotech like DNA studies and protein manipulation. If we are fully successful, we can imagine the possibility of replacing most of the mechanical refrigerators and air-conditioning systems with CFC-free,0solid-state, no-moving-parts (therefore reliable) electronic heat pump technology.

* How new is this work and how does it differ from that of others who may be doing similar research?

This work is about 15 months old. The initial materials breakthrough and some initial device results were reported in the journal Nature in October, 2001. Since then, we have made considerable progress in transitioning the early results to practical devices. Some of this will be presented at the ACS.

Our work reprsents the first materials advancement near room temperature. The only other work that has shown similar advancement near 300K is that of a recent publication by Ted Harman in Science, Sep. 2002.

* Corresponding author's name and business title or position:

Dr. Rama VenkatasubramanianResearch Director

* Work department:

Center for Thermoelectrics Research

* Business address including organization:

Research Triangle Institute3040 Cornwallis RoadResearch Triangle Park, NC 27709