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Structural Distortions Emerge from Nothing at the Nanoscale

Newly discovered phase helps explain materials' ability to convert waste heat to electricity

Story content courtesy of Brookhaven National Laboratory, US

Scientists have discovered a class of materials known to convert heat to electricity and vice versa behaves quite unexpectedly at the nanoscale in response to changes in temperature. The discovery is a new "opposite-direction" phase transition that helps explain the strong thermoelectric response of these materials. It may also help identify other useful thermoelectrics, and could further their application in capturing energy lost as heat, for example, in automotive and factory exhaust.

The scientific team includes the U.S. Department of Energy's (DOE) Brookhaven National Laboratory, Columbia University, Argonne National Laboratory, Los Alamos National Laboratory, Northwestern University, and the Swiss Federal Institute of Technology. The scientists were studying lead chalcogenides using newly available experimental techniques and theoretical approaches that allow them to "see" and model behavior of individual atoms at the nanoscale, or on the order of billionths of a meter. With those tools, they were able to observe subtle changes in atomic arrangements invisible to conventional probes of structure.

The scientists created an animation to illustrate the emergence of these displacements upon heating. In it, the displacements are represented by arrows to indicate the changing orientations of the atoms as they flip back and forth, or fluctuate, like tiny dipoles.

According to the scientists, it is this random flipping behavior that is key to the materials' ability to convert heat into electricity.

When one side of the material comes in contact with heat – e.g., in the exhaust system of a car - the gradient will cause charge carriers in the thermoelectric material (e.g., electrons) to diffuse from the hot side to the cold side. Capturing this thermally induced electric current could put the "waste" heat to use. The research may help scientists search for other thermoelectric materials with exceptional properties, since it links the good thermoelectric response to the existence of fluctuating dipoles.

Mr. Simon Billinge, PhD, a physicist at Brookhaven Lab and Columbia University's School of Engineering and Applied Science and a lead author on the Science paper, notes in the release, “Such studies of complex materials at the nanoscale hold the key to many of the transformative technological breakthroughs we seek to solve problems in energy, health, and the environment."

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