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Thermo-shifts – Veerle Keppens

Veerle Keppens

by Laura Buenning

Imagine being able to generate thermoelectric power from a temperature gradient as small as the difference between our internal and external body temperatures and then use it to operate implantable devices that monitor, diagnose, or administer therapeutic health care on missions into deep space.

Thermoelectricity refers either to the way temperature differences between two sides of a material can be used to produce electricity or, to its reverse, the way an electrical current passed through a material creates a temperature difference between the two sides—which is useful for solid-state refrigeration or heating without combustion or moving parts.

Desktop DeviceAs promising as this sounds, Veerle Keppens says efficient thermoelectric materials require the unusual combination of poor thermal conductivity and good electrical conductivity.

“Typically, materials with good electrical conductivity also have good thermal conductivity,” she says.

Electrons alone carry electricity; both electrons and phonons (the atomic vibrations from atoms in a crystal’s lattice structure) carry heat. So, a good thermoelectric material is efficient at scattering phonons, which impedes the transfer of heat from one side of a material to the other while allowing the electrons to pass right on through.

Working in tandem, Keppens’ JDRD team, with PhD student Lindsay VanBebber and an LDRD team led by Olivier Delaire of ORNL’s Materials Science and Technology Division, are examining the influence ferroelectric properties have on lattice dynamics—in other words, how these properties affect the atoms moving within the crystal. Their goal is to gain a better understanding of the microscopic origins of suppressed thermal conductivity.

The two teams will explore how a material’s ferroelectric ability to spontaneously polarize under specific conditions correlates with thermoelectric performance.

Temperature Chart“Ferroelectric material as a whole is neutral; there’s no actual charge,” Keppens says. “But the way the tiny units inside are distributed makes it a little more negative on one end and a little more positive on the other. This can only happen if there’s a lack of a certain symmetry in the structure.

“In a perfect crystal at absolute zero degrees, the atoms occupy well-defined positions. Raise the temperature and the atoms start to move, some differently from others. Because of that you can create peculiar lattice dynamics, depending on the structure and the atoms you put into a material.”

For their study, the JDRD team is growing single crystals of Pb1-xSnxTe (lead, tin, telluride)—a new combination, cousin to lead-telluride materials, foremost in thermoelectric power generation for applications above room temperature. They will use Resonant Ultrasound Spectroscopy (RUS) to measure the crystal’s physical response to ultrasonic signals at varying temperatures. Keppens says RUS should show when structural changes actually occur in the material—information that will add insight into the mechanisms linking lattice dynamics with suppressed thermal conductivity.

JDRD project:
Ferroelectric instabilities in thermoelectric materials
Veerle Keppens, UT Department of Materials Science and Engineering

LDRD project:
Improving energy efficiency in thermoelectric materials by integrating neutron scattering with supercomputing and modeling
Oliver Delaire, ORNL Materials Science and Technology Division

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