How to freeze heat conduction

February 21, 2019, Vienna University of Technology
The atom in the cage can oscillate in two different directions. Credit: Vienna University of Technology

Physicists have discovered a new effect, which makes it possible to create excellent thermal insulators which conduct electricity. Such materials can be used to convert waste heat into electrical energy.

Every day we lose valuable energy in the form of —in technical devices at home, but also in large energy systems. Part of it could be recovered with the help of the "thermoelectric effect." The from a hot device to the cold environment can be directly converted into electrical power. To achieve that, however, materials with very special properties are required. They have to be good electrical conductors, but bad thermal conductors – two requirements which are difficult to reconcile.

Researchers all over the world are looking for such materials. Certain materials with a cage-like structure have proved particularly promising, for example clathrates, which are studied at TU Wien. Now, after elaborate investigations, a remarkable effect has been demonstrated, which can explain the particularly low thermal conductivity of these materials.

Prison cells for atoms

"Clathrates are crystals with a very special structure," explains Professor Silke Bühler-Paschen from the Institute of Solid State Physics at the Vienna University of Technology. "Their contains tiny cages in which are locked up. These can oscillate back and forth in their single cell, without seeing much of the rest of the crystal."

Heat in a solid is present in the form of vibrations of its atoms. When a crystal is heated, the vibrations get stronger until, at some point, the bonds between the atoms are broken and the crystal melts. "There are two types of vibrations," says Silke Bühler-Paschen. "If neighboring atoms are strongly bound together, then the vibration of one atom can be directly transferred to its neighbors and a heat wave spreads through the material. The stronger the coupling between the atoms, the faster the propagation of the wave and the greater the heat conduction. However, if an atom is only very weakly bound to its neighbors, just like the atom sitting in the clathrate cage, then it is largely independent of the others and the heat wave is extremely slow."

Prof. Silke Bühler-Paschen. Credit: Vienna University of Technology
New effect: The Kondo-like phonon scattering

As part of his dissertation with Silke Bühler-Paschen, Matthias Ikeda found out that it is due to a certain interaction between these two kinds of heat wave that clathrates are such good thermal insulators. Matthias Ikeda performed precise and extensive measurements. Series of , each one with slightly different properties, were produced at TU Wien and carefully measured. "In the end, we were able to prove what nobody wanted to believe us at first: there is a hitherto unknown physical effect that suppresses the thermal conductivity—we call it Kondo-like phonon scattering," says Matthias Ikeda.

Due to the crystal structure, an atom in the clathrate cage vibrates preferentially in two specific directions. "When a heat wave arrives, it can—for a short time—enter a kind of bound state with such a vibration. The heat wave changes the oscillation direction of the atom in the clathrate cage," says Silke Bühler-Paschen. "This process slows down the , and so the heat conductivity is decreased. Even though clathrates conduct electricity, they are good thermal insulators."

Better material for thermoelectrics

This is exactly the combination of material properties which is required in order to use the thermoelectric effect on an industrial scale. Something hot is connected to something cold using the right material, and the energy flow in between can be directly converted into electricity. On the one hand, the material must conduct electrical current, but on the one hand, it should not equilibrate the temperatures by conducting the too quickly, otherwise the effect can no longer be used.

"The project was very time-consuming, in addition to numerous experiments, extensive computer simulations had to be developed in order to understand the quantum physical processes behind this effect," says Silke Bühler-Paschen. "But it was worth it: With our concept of Kondo-like phonon scattering, it is now much easier to understand the behavior of clathrates and therefore we can work more purposefully to find the most efficient for thermoelectric applications."

Explore further: Creating electricity with caged atoms

More information: M. S. Ikeda et al. Kondo-like phonon scattering in thermoelectric clathrates, Nature Communications (2019). DOI: 10.1038/s41467-019-08685-1

Related Stories

Creating electricity with caged atoms

September 22, 2013

Clathrates are crystals consisting of tiny cages in which single atoms can be enclosed. These atoms significantly alter the material properties of the crystal. By trapping cerium atoms in a clathrate, scientists at the Vienna ...

Recommended for you

ATLAS experiment observes light scattering off light

March 20, 2019

Light-by-light scattering is a very rare phenomenon in which two photons interact, producing another pair of photons. This process was among the earliest predictions of quantum electrodynamics (QED), the quantum theory of ...

How heavy elements come about in the universe

March 19, 2019

Heavy elements are produced during stellar explosion or on the surfaces of neutron stars through the capture of hydrogen nuclei (protons). This occurs at extremely high temperatures, but at relatively low energies. An international ...

Trembling aspen leaves could save future Mars rovers

March 18, 2019

Researchers at the University of Warwick have been inspired by the unique movement of trembling aspen leaves, to devise an energy harvesting mechanism that could power weather sensors in hostile environments and could even ...

Quantum sensing method measures minuscule magnetic fields

March 15, 2019

A new way of measuring atomic-scale magnetic fields with great precision, not only up and down but sideways as well, has been developed by researchers at MIT. The new tool could be useful in applications as diverse as mapping ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.