Published on September 30th, 2013 | by James Ayre8
New Thermoelectric Material Created With “Caged” Atoms
A promising new thermoelectric material has been created by researchers at the Vienna University of Technology via the utilization of magnetic “cages” — the new material’s structure, essentially, consists of countless tiny cages with cerium atoms enclosed within them. These caged cerium atoms “rattle” the bars of the cage, greatly adding to the effectiveness of the material, creating some the material’s nearly unique properties. This new material represents a considerably more efficient class of thermoelectric materials than those that were previously known of, according to the researchers.
The new research/findings are worth noting because of the vast potential for thermoelectric materials with regard to waste-heat recovery — as a great deal of energy is lost by machines, via waste heat, thermoelectric materials could vastly improve the energy efficiency of a number of important technologies, converting this waste heat into electricity.
“These clathrates show remarkable thermal properties,” explains Professor Silke Bühler-Paschen (TU Vienna). “We came up with the idea to trap cerium atoms, because their magnetic properties promised particularly interesting kinds of interaction.”
The Vienna University of Technology provides more:
“Clathrates” is the technical term for crystals, in which host atoms are enclosed in cage-like spaces. The exact behaviour of the material depends on the interaction between the trapped atoms and the cage surrounding them. 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. (But now) with the help of a sophisticated crystal growth technique in a mirror oven, Professor Andrey Prokofiev (TU Vienna) has succeeded in creating clathrates made of barium, silicon and gold, encapsulating single cerium atoms.
(And now) the thermoelectric properties of the novel material have been tested. 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.
“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.”
“The reason for these remarkably good material properties seem to lie in a special kind of electron-electron correlation — the so-called Kondo effect,” continues Bühler-Paschen. “The electrons of the cerium atom are quantum mechanically linked to the atoms of the crystal. Actually, the Kondo effect is known from low temperature physics, close to absolute zero temperature. But surprisingly, these quantum mechanical correlations also play an important role in the novel clathrate materials, even at a temperature of hundreds of degrees Celsius. The rattling of the trapped cerium atoms becomes stronger as the temperature increases. This rattling stabilizes the Kondo effect at high temperatures. We are observing the world’s hottest Kondo effect.”
The researchers are now investigating possible replacements for the gold that they used — by replacing the (relatively) expensive gold with cheaper options, the material could be made much more commercially attractive. The cerium could potentially be replaced with cheaper options, as well.