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University of Texas at Austin engineers & physicists first to measure thermo-electric power

University Of Texas At Austin : 05 February, 2004  (Technical Article)
An important and previously elusive property of nano-materials is now measurable, thanks to a method devised by University of Texas at Austin engineers and physicists.
According to an article in the Feb. 6 issue of Science, the researchers developed a way to measure the heat- and voltage-carrying capacity of novel nano-materials that could revolutionize cooling systems and other energy-consuming devices.

The new method promises to tremendously advance work toward cleaner, quieter refrigeration and power generation units, as well as work toward improved semiconductors.

The measurement has proven difficult for researchers because of the tiny size of nano-materials. They are built from atomic components of about a billionth of a meter in size.

“This is the first time we’ve been able to precisely measure the thermoelectric power of nanostructured materials,” says Dr. Li Shi, assistant professor of mechanical engineering at The University of Texas at Austin and the paper’s co-author. “Materials scientists are quite excited about this.

“Up to now theoretical calculations have only predicted that nanostructured materials can have wonderful thermoelectric properties far superior to materials currently in use, but it has been difficult to verify the prediction because of the lack of good measurement techniques and instruments,” says Shi.

Shi and C. K. Shih, a physics professor at The University of Texas at Austin and co-author of the Science paper, developed a new instrument using a scanning tunneling microscope that takes measurements in ultra-high vacuum. Using a semiconductor sample, they attached a heater wire to raise the sample’s temperature above room temperature.

When the extremely sharp, room-temperature STM tip touched the nano-scale sample, the tip could detect their difference in temperature at the contact point. The temperature gradient generated a thermoelectric voltage directly related to the sample’s thermoelectric power, allowing a precise measurement of that power to a nano-scale resolution, an unprecedented achievement.

Engineers have been working since the 1950s to create better solid-state, thermoelectric refrigeration and power generation units. The most pervasive and efficient refrigeration technology uses compression-based systems like those in building and vehicle air-conditioning units relying on Freon, which is toxic. By contrast, the less efficient, but more environmentally friendly thermoelectric-based refrigeration systems now available realize direct energy conversion between heat and electricity without moving mechanical components and hazardous working fluids. Although significantly more inefficient, they continue to be used in smaller systems such as devices to transport blood in hospitals, temperature controls in infrared night-vision cameras and high-end car seats.

However, the energy efficiency of thermoelectric devices is very low, below 10 percent of the Carnot efficiency (the maximum possible efficiency). In comparison, kitchen refrigerators show a Carnot efficiency of 30 percent and high-volume air conditioning systems have even much higher efficiencies.

“If engineers could increase the efficiency of thermoelectric devices to 30 percent of the Carnot efficiency, they would have many more applications and open up a large consumer market,” says Shi.

In the past 10 years, engineers have created two new types of promising nanostructured thermoelectric materials for cooling systems. Thin film superlattices theoretically could reach 30 percent of maximum efficiency and nanowires could potentially reach much higher efficiencies. However, until their thermoelectric properties can be measured precisely, the efficiencies of these materials remain unproven, says Shi.

In addition to its applications for developing cooling systems, Shi will also use this new method in semiconductor development. The researchers showed their new technique can determine with nanometer spatial resolution the concentration of impurity dopants across semiconductor junctions. Dopant profiling has remained a prominent characterization issue challenging the semiconductor industry for many years. Doping occurs when known impurities are added to semiconductors to achieve certain extrinsic properties.

The research was supported by Shi’s National Science Foundation CAREER award, and Shih’s awards from the NSF Division of Materials Research and the Texas Advanced Technology Program, in collaboration with electrical engineers from Massachusetts Institute of Technology and the University of California at Berkeley. Shi and Shih are both faculty fellows of The University of Texas at Austin’s Center for Nano- and Molecular Science and Technology. Other paper co-authors from The University of Texas at Austin were Ho-Ki Lyeo, a physics graduate student, and A. A. Khajetoorians, a graduate student with the Texas Materials Institute.
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