Nanotech Drives TE Boost

Nanotechnology has been used to crush and then reconstitute a classic semiconductor used in industry and research, dramatically improving its ability to conduct electricity but not heat. The work could pave the way for a new generation of thermoelectric (TE) products -- from semiconductors and air conditioners to car exhaust systems and solar power technology -- that run cleaner.

The low-cost approach devised by a team of researchers at Boston College (BC) and the Massachusetts Institute of Technology involves building tiny alloy nanostructures that can serve as micro-coolers and power generators. The researchers said that in addition to being inexpensive, their method will likely result in practical, near-term enhancements to make products consume less energy or capture energy that would otherwise be wasted.

A cross-section of nanocrystalline bismuth antimony telluride grains, as viewed through a transmission electron microscope. Colors highlight the features of each grain of the semiconductor alloy in bulk form. A team of researchers from Boston College and MIT produced a major increase in thermoelectric efficiency after using nanotechnology to structure the material, which is commonly used in industry and research. (Image courtesy Boston College, MIT and GMZ Inc.)

The findings represent a key milestone in the quest to harness the thermoelectric effect, which has baffled scientists since its discovery in the early 19th century. The effect refers to certain materials that can convert heat into electricity and vice versa. But most of these materials also conduct heat, so their temperature equalizes quickly. In order to improve efficiency, scientists have sought materials that will conduct electricity but not heat.

By crushing bismuth antimony telluride into a nanoscopic dust and then reconstituting it in bulk form, the researchers at BC and MIT produced a 40 percent increase in the thermoelectric efficiency of the material, a semiconductor alloy that has been commonly used in commercial devices since the 1950s.

The grains and irregularities of the reconstituted alloy dramatically slowed the passage of phonons through the material, radically transforming the thermoelectric performance by blocking heat flow while allowing the electrical flow. Phonons, a quantum mode of vibration, play a key role because they are the primary means by which heat conduction takes place in insulating solids.

The achievement marks the first such gain in a half-century using the cost-effective material that functions at room temperatures and up to 250 °C. The success using the relatively inexpensive and environmentally friendly alloy means the discovery can quickly be applied to a range of uses, leading to higher cooling and power-generation efficiency.

“By using nanotechnology, we have found a way to improve an old material by breaking it up and then rebuilding it in a composite of nanostructures in bulk form,” said BC physicist Zhifeng Ren, one of the project's leaders. “This method is low cost and can be scaled for mass production. This represents an exciting opportunity to improve the performance of thermoelectric materials in a cost-effective manner.”

This simple closed circuit module was used by Boston College and MIT researchers to confirm how a reengineered semiconductor alloy, in bulk form, achieved a significant improvement in performance. The work represents a milestone in thermoelectric research that has numerous applications for everyday products to help them run more efficiently. (Photo courtesy Science/AAAS)

“These thermoelectric materials are already used in many applications, but this better material can have a bigger impact,” said Gang Chen, the Warren and Towneley Rohsenow Professor of Mechanical Engineering at MIT and another project leader.

In addition to Ren and six researchers at his BC lab, the international team involved MIT researchers, including Chen and institute professor Mildred S. Dresselhaus; research scientist Bed Poudel at GMZ Energy Inc., a Newton, Mass.-based company formed by Ren, Chen, and CEO Mike Clary; as well as BC visiting professor Junming Liu, a physicist from Nanjing University in China.

Thermoelectric materials have been used by NASA to generate power for far-away spacecraft. These materials have been used by specialty automobile seat makers to keep drivers cool during the summer. The auto industry has been experimenting with ways to use thermoelectric materials to convert waste heat from a car exhaust systems into electric current to help power vehicles.

The research, details of which were published yesterday in the online version of the journal Science, was supported by the Department of Energy and the National Science Foundation.

For more information, visit: www.mit.edu or www.bc.edu