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The atom in the cage can oscillate in two different directions. Credit: Vienna University of Technology
Physicists have discovered a new effect that creates excellent thermal insulators that conduct electricity. Such materials can be used to convert lost heat into electrical energy.
Every day, we lose valuable energy in the form of waste heat, whether in domestic appliances or large energy systems. Part of it could be recovered with the help of "thermoelectric effect". The heat flow of a hot appliance in a cold environment can be directly converted into electrical energy. To achieve this, however, materials with very special properties are needed. They must be good electrical conductors, but bad thermal conductors – two requirements difficult to reconcile.
Researchers around the world are looking for such materials. Some cage-shaped materials have been shown to be particularly promising, eg clathrates, studied at TU Wien. Now, after extensive research, a remarkable effect has been demonstrated, which may explain the particularly low thermal conductivity of these materials.
Prison cells for atoms
"Clathrates are crystals with a very special structure," says Professor Silke Bühler-Paschen of the Institute of Solid Physics of the ZPS of the Vienna University of Technology. "Their crystal lattice contains tiny cages in which individual atoms are locked in. These atoms can oscillate in their cells without seeing the rest of the crystal."
The heat in a solid is present in the form of vibrations of its atoms. When a crystal is heated, the vibrations become stronger until, at a given moment, the bonds between the atoms break and the crystal melts. "There are two types of vibrations," explains Silke Bühler-Paschen. "If the neighboring atoms are strongly linked, the vibration of an atom can be directly transferred to its neighbors and a heat wave propagates through the material.The stronger the coupling between the atoms, the greater the propagation of the atom. Wave is fast and over the However, if an atom is only very weakly bound to its neighbors, just like the atom sitting in the clathrate cage, it is largely independent of others and the heat wave is extremely slow. "
Silke Bühler-Paschen. Credit: Vienna University of Technology
New effect: phonon scattering at Kondo
As part of his dissertation with Silke Bühler-Paschen, Matthias Ikeda discovered that it is due to some interaction between these two types of heat waves that clathrates are so good thermal insulators. Matthias Ikeda has made precise and detailed measurements. Series of crystals, each with slightly different properties, were produced at TU Wien and carefully measured. "In the end, we were able to prove that nobody wanted to believe us at first: there is a previously unknown physical effect that suppresses thermal conductivity – we call it Kondo type phonon scattering," says Matthias Ikeda. .
Because of the crystal structure, an atom in the clathrate cage preferably vibrates in two specific directions. "When a heat wave arrives, it can – for a short time – enter a kind of state linked with such a vibration.The heat wave changes the direction of oscillation of the atom in the clathrate cage, "explains Silke Bühler-Paschen. "This process slows down the heat wave, which decreases the thermal conductivity.Although the clathrates conduct electricity, they are good thermal insulators."
Better material for thermoelectric
It is precisely the combination of material properties that is needed to use the thermoelectric effect on the industrial scale. Something hot is connected to something cold using the right material, and the flow of energy between the two can be directly converted into electricity. On the one hand, the material must conduct the electric current, but on the other hand, it does not have to balance the temperatures by driving the heat too quickly, otherwise the effect can no longer be used.
"The project was very time consuming, and in addition to many experiments, many computer simulations had to be developed to understand the quantum physical processes that underlie this effect," said Silke Bühler-Paschen. "But it was worth it: with our concept of Kondo type phonon scattering, it is now much easier to understand the behavior of clathrates and we can work more efficiently to find the most efficient materials for thermoelectric applications. "
Explore further:
Create electricity with atoms in a cage
More information:
Mr. S. Ikeda et al. Diffusion of Kondo-type phonons in thermoelectric clathrates, Nature Communications (2019). DOI: 10.1038 / s41467-019-08685-1
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