Creating Electricity with Caged Atoms

At the Vienna University of Technology, a new class of thermoelectric materials has been discovered.

Due to a surprising physical effect they can be used to create electricity more efficiently.

A lot of energy is wasted when machines turn hot, unnecessarily heating up their environment. Some of this thermal energy could be harvested using thermoelectric materials; they create electric current when they are used to bridge hot and cold objects. At the Vienna University of Technology (TU Vienna), a new and considerably more efficient class of thermoelectric materials can now be produced. It is the material’s very special crystal structure that does the trick, in connection with an astonishing new physical effect; in countless tiny cages within the crystal, cerium atoms are enclosed. These trapped magnetic atoms are constantly rattling the bars of their cage, and this rattling seems to be responsible for the material’s exceptionally favourable properties.

Cerium Cages from the Mirror Oven

“Clathrates” is the technical term for crystals, in which host atoms are enclosed in cage-like spaces. “These clathrates show remarkable thermal properties”, says Professor Silke Bühler-Paschen (TU Vienna). The exact behaviour of the material depends on the interaction between the trapped atoms and the cage surrounding them. “We came up with the idea to trap cerium atoms, because their magnetic properties promised particularly interesting kinds of interaction”, explains Bühler-Paschen.

For a long time, this task seemed impossible. All earlier attempts to incorporate magnetic atoms such as the rare-earth metal cerium into the clathrate structures failed. With the help of a sophisticated crystal growth technique in a mirror oven, Professor Andrey Prokofiev (TU Vienna) has now succeeded in creating clathrates made of barium, silicon and gold, encapsulating single cerium atoms.

Electricity from Temperature Differences

The thermoelectric properties of the novel material have been tested. Thermoelectrics work when they connect something hot with something cold: “The thermal motion of the electrons in the material depends on the temperature”, explains 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.”

Experiments show that the cerium atoms increase the material’s thermopower by 50%, so a much higher voltage can be obtained. Furthermore, the thermal conductivity of clathrates is very low. This is also important, because otherwise the temperatures on either side would equilibrate, and no voltage would remain.

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