Redistribution of electronic clouds causes a lattice instability and freezes the flow of heat in highly efficient tin selenide. The crystal lattice adopts a distorted state in which the chemical bonds are stretched into an accordion-like configuration, and makes an excellent thermoelectric because heat propagation is thwarted. Credit: Oak Ridge National Laboratory, U.S. Dept. of Energy
Engines, laptops and power plants generate waste heat. Thermoelectric materials, which convert temperature gradients to electricity and vice versa, can recover some of that heat and improve energy efficiency. Scientists have explored the fundamental physics of the world’s best thermoelectric material — tin selenide – using neutron scattering and computer simulations. Their work may aid energy sustainability and design of materials that convert heat into electricity.
“We performed the first comprehensive measurements of atomic vibrations in this important new thermoelectric material,” said Olivier Delaire. “We discovered the origin of its very low thermal conductivity, which leads to its high efficiency.” It turns out unusual atomic vibrations help prevent “heat leaks,” maximizing the conversion into electricity.
Through the Seebeck effect, thermoelectric devices produce a voltage and generate electric current when a temperature differential is maintained. Or, when powered with an external electricity source, the devices can actively pump the heat out for refrigeration applications.
To preserve a usable temperature gradient, thermoelectric materials need to be good conductors of electricity but bad conductors of heat. In 2014 researchers at Northwestern University discovered that tin selenide, which is inexpensive, could be the world’s most efficient thermoelectric material.
The ORNL researchers observed atomic vibrations that underpin heat flow–called phonons–and tried to understand their origins in terms of electronic structure and chemistry.
“What we found is that this particular phonon mode is the one that’s unstable, that ‘freezes,'” Delaire said. “If you cool down the material, it goes from undistorted to distorted, and when you heat it up the distortion goes away. That is the atomic mechanism behind the freezing in of this particular phonon mode.”
Knowledge the team gained may aid efforts to control thermal transport in a wide range of energy-related technologies, including thermal barrier coatings, nuclear fuels and high-power electronics.
The key to tin selenide’s high efficiency was revealed through exploring the dynamics of atoms in the crystal lattice. In a harmonic system, waves of atomic vibrations can propagate freely. Many waves, carrying a lot of heat, can travel through the material without sensing each other. In an anharmonic system, in contrast, atomic vibration waves feel a viscous friction against each other. The friction creates a sort of slush that prevents heat propagation, much like the vibration dampers in a vehicle’s shock absorbers. Tin selenide at the temperatures tested was strongly anharmonic: The phonon waves were strongly damped and the heat was well contained, so the temperature gradient could be preserved.
“With simulations we showed the strong underlying anharmonicity stems from a bonding instability,” Delaire said. Below a phase transition of 810 kelvin (about 540 degrees C or 1000 degrees F), electronic orbitals spontaneously reorganize and the lattice assumes an accordion structure. Phonons feel this instability, which damps the oscillations–making tin selenide an outstanding thermoelectric material.
“Out of all the energy that goes into the U.S. economy every year, 60% is lost in the form of waste heat,” Delaire said. “If you can recapture even a small fraction, you can have a big impact.” Thermoelectric materials can support sustainable energy. Thermoelectric materials can be placed under solar panels, where a temperature difference can generate electricity cheaply.
Thermoelectric materials still need to reach higher efficiencies for widespread application, but recent discoveries like understanding the dynamics of tin selenide have achieved big steps in that direction. They have already been big successes in niches including very long-lasting space batteries developed by NASA and DOE. http://www.newswise.com/articles/molecular-accordion-drives-thermoelectric-behavior-in-promising-material
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