The mirror oven at TU Vienna, in which the novel material was created

Creating electricity with caged atoms

Scientists have created a new class of highly efficient materials that transform excess heat in machines into useful electricity.

Thermoelectric materials create electric current when they are used to bridge hot and cold objects and researchers at the Vienna University of Technology (TU Vienna) have devised a new method for producing an even more efficient class of the materials.

The new material is a “clathrate” – the technical term for crystals where host atoms are enclosed in cage-like spaces. In this particular material the caged atoms are magnetic cerium atoms and it is the constant rattling of the bars of their cage that seems to be responsible for the material’s exceptional properties.

“These clathrates show remarkable thermal properties,” said Professor Silke Bühler-Paschen. “We came up with the idea to trap cerium atoms, because their magnetic properties promised particularly interesting kinds of interaction.”

Clathrates: crystals enclosing single atoms Earlier attempts to incorporate magnetic atoms such as the rare-earth metal cerium into the clathrate structures failed, but with the help of a sophisticated crystal growth technique in a mirror oven, Professor Andrey Prokofiev has now succeeded in creating clathrates made of barium, silicon and gold, encapsulating single cerium atoms.

The thermoelectric properties of the novel material have been tested and the team has found the cerium atoms increase the material’s thermopower by 50 per cent, so a much higher voltage can be obtained.

“The thermal motion of the electrons in the material depends on the temperature,” said Bühler-Paschen. “On the hot side, there is more thermal motion than on the cold side, so the electrons diffuse towards the colder region. Therefore, a voltage is created between the two sides of the thermoelectric material.”

Furthermore, the thermal conductivity of clathrates is very low, an important feature as otherwise the temperatures on either side would equilibrate and no voltage would remain.

“The reason for these remarkably good material properties seem to lie in a special kind of electron-electron correlation; the so-called Kondo effect,” Bühler-Paschen said.

The electrons of the cerium atom are quantum mechanically linked to the atoms of the crystal, and while the Kondo effect is generally linked to low temperature physics, these quantum mechanical correlations also play an important role in the novel clathrate materials even at a temperature of hundreds of degrees Celcius.

“The rattling of the trapped cerium atoms becomes stronger as the temperature increases,” said Bühler-Paschen. “This rattling stabilizes the Kondo effect at high temperatures. We are observing the world’s hottest Kondo effect.”

The research team at TU Vienna will now try to achieve the same effect with different types of clathrates, with the expensive gold possibly substituted by other metals such as copper in order to make the material commercially more attractive.

Instead of cerium, a cheaper mixture of several rare-earth elements could be used. There are high hopes that such designer clathrates can be technologically applied in the future, to turn industrial waste heat into valuable electrical energy.

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