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Comprehensive phonon “map” offers direction for engineering new thermoelectric devices

Thu, 01/09/2014 - 7:48am

Olivier Delaire, researcher in Oak Ridge National Laboratory’s Materials Science Div., at the Spallation Neutron Source’s Wide-Angular Range Chopper Spectrometer. Using neutrons to map heat as it propagates through materials, Delaire and his research group are studying how to design better thermoelectric devices that convert a temperature difference into an electric voltage.If you’ve ever been stuck in traffic on a hot, sunny afternoon, you might have noticed the rippling effect caused by the release of even hotter exhaust fumes. If so, you’ve watched opportunity drift away.

Automobiles, power plants, laptops and many other machines produce heat when they operate. Waste heat is an unavoidable energy loss, a tradeoff in order to produce the kind of energy for which the machine is intended. However, this heat could be partly recycled into electrical energy through thermoelectric technology, which converts a temperature difference into an electric voltage.

The greater the temperature difference, the more efficient the thermoelectric conversion process, so the best thermoelectric materials have low thermal conductivity—making them good heat insulators—but good electrical properties for delivering a voltage.

One potential application for thermoelectric devices is to recover energy for a car’s electrical systems by installing thermoelectric modules in the car exhaust pipe, where hot exhaust gases and cold air from the outside collide. Such installations are most efficient when they maximize the temperature gradient (and the voltage output) by blocking heat from leaking through the thermoelectric materials comprising the device.

To understand how to design better thermoelectric materials, researchers are using neutron scattering at the Spallation Neutron Source (SNS) and the High Flux Isotope Reactor (HFIR) at the U.S. Dept. of Energy (DOE)’s Oak Ridge National Laboratory (ORNL) to study how silver antimony telluride is able to effectively prevent heat from propagating through it on the microscopic level. Heat in materials is carried by quantized sound waves, known to scientists as phonons. By mapping the phonons and their interactions with the atomic architecture of silver antimony telluride, researchers in ORNL’s Quantum Condensed Matter and Materials Science groups discovered that a complex structure of nanoscale domains improved the thermoelectric properties of this compound.

“We wanted to understand how the surprisingly low thermal conductivity arises in the material in the first place,” said Olivier Delaire, a researcher in the Materials Science Div. and former Clifford Shull Fellow in the Neutron Sciences Directorate. “The microscopic underpinning of how heat travels through matter is of great interest for devising new energy-saving technologies.”

Scientists have known that phonons—by which heat propagates through a material—can be affected by the material’s nanostructure. If the nanostructure raises enough roadblocks for phonons as they move through a material, they will frequently be scattered, and the material will have low thermal or heat conductivity. If, on the other hand, phonons encounter few obstructions, they will conduct heat more readily and shrink the temperature gradient between the hotter and cooler regions of the material, which is necessary to generate an electric voltage.

“We want these phonons to be scattered a lot so they are not conveying as much heat and the thermal conductivity goes down,” Delaire said.

Delaire and his research group, including Jie Ma, the lead coauthor of the resulting paper published in Nature Nanotechnology, knew that silver antimony telluride exhibited a lower thermal conductivity than more familiar thermoelectric compounds like lead telluride, which they had previously studied.

“One of the compounds often used in thermoelectric applications is lead telluride. In fact, it helps power the Mars Curiosity rover,” Delaire said. “Lead telluride is often efficient, but this silver compound we studied has an even lower thermal conductivity—three times lower at room temperature.”

To understand what makes silver antimony telluride more efficient than its lead counterpart, researchers needed to map the propagation of phonons across the crystal lattice of the silver antimony telluride compound. Because phonons travel in a range of wavelengths, directions, and velocities—many of which affect thermal conductivity—they would have to make extensive measurements.

“We needed the sum contributions of the phonons, so it was an ambitious study and took significant effort to gather all the data,” Delaire said. “This is the first time we’ve done a full mapping of the thermal conductivity of a compound like this.”

To do so, they used an arsenal of neutron scattering instruments that measure a variety of energies and momentums.

Sampling silver antimony telluridecrystals grown by scientists in ORNL’s Correlated Electrons Materials group, the team mapped phonon excitations with inelastic neutron scattering instruments at SNS—the Wide Angular-Range Chopper and Cold Neutron Chopper spectrometers—and the triple-axis spectrometers at HFIR. Further characterization was performed with resonant ultrasound spectroscopy at the Univ. of Tennessee and with electron microscopy at the Massachusetts Institute of Technology.

“What we saw was a network of two lattices,” Delaire said. “One lattice is defined by tellurium atoms, while silver and antimony share the other lattice on which they form ordered nanodomains.”

Based on their knowledge of the compound, Delaire’s group expected the tellurium lattice to be uniformly organized, but they wanted to know how the other two elements would be organized. Observations of the second lattice showed patches of silver and antimony atoms with different patterns, revealing that nanoscale domains with local orders were increasing the complexity of the nanostructure and scattering phonons.

“It was not known if these two species were randomly scattered or if they adopted a configuration,” Delaire said. “We found it’s a little in between. In some regions they’re ordered one way, in another region, they order a different way.”

The interfaces, or connections, between different local ordering arrangements is where phonons are scattered, revealing how the microscopic structure of silver antimony telluride blocks phonons more effectively than a compound like lead telluride, which only has two elements and lacks nanoscale domains.

“The additional complexity of silver and antimony ordering further suppresses the thermal conductivity and increases the compound’s thermoelectric capability,” Delaire said.

A similar nanoscale structure could be replicated in other materials, leading to increasingly effective thermoelectric devices for heat recovery technologies.

“Now we’re developing modeling tools to help us do this systematically with other materials,” Delaire said.

Models would help industries to develop new materials. Delaire said auto manufacturers are already implementing programs to recover heat from exhaust pipes, but heat could also be recovered from factories and power plants, or even by concentrating heat from the sun through solar thermoelectric devices.

Source: Oak Ridge National Laboratory

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