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Mechanical Engineering professor Li Shi and a team of researchers in the Cockrell School of Engineering have discovered unique atomic motion in a promising thermoelectric material. The discovery will enable them to manipulate the material for use in thermoelectric generators. These systems will recover waste heat in car engines to deliver increased fuel efficiency.

The thermoelectric material investigated by the mechanical engineers and materials scientists is called higher manganese silicides (HMS), which is nontoxic and synthesized from naturally abundant elements -- unlike alternatives such as lead telluride and silicon germanium. The UT Austin research was published in an article in titled "Twisting phonons in complex crystals with quasi-one-dimensional substructures."

HMS's thermoelectric efficiency is a result of its low thermal conductivity. The material remains stable at very high temperatures, corresponding to the temperature of peak thermoelectric efficiency. In order to determine how the thermal conductivity of the material might be lowered to make it even more efficient, the UT Austin team and collaborators performed experiments on an HMS crystal grown by Materials Science and Engineering graduate student Xi Chen and Mechanical Engineering professor Jianshi Zhou.

In one test, called inelastic neutron scattering, Chen and Mechanical Engineering graduate student Annie Weathers worked with materials scientists at Oak Ridge National Laboratory to bombard the crystal structure with neutrons. The angles and energies at which the neutrons scattered off of the crystal enabled the researchers to search for a "twisting" motion of helical silicon atom chains in HMS. This twisting motion was predicted by theoretical calculations carried out in collaborations with theorists in France and at Cornell University.

The team determined that the twisting motion plays an important role in the low thermal conductivity of HMS. These findings will propel the researchers' ongoing efforts in suppressing HMS's thermal conductivity further by reducing the grain size, so it can be optimally manipulated for integration in the thermoelectric generator they are designing.

 

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