Michael Parzer, Fabian Garmroudi and Alexander Riss (left to right) in their laboratory at TU Wien. Image: TU Wien.
Michael Parzer, Fabian Garmroudi and Alexander Riss (left to right) in their laboratory at TU Wien. Image: TU Wien.

To efficiently convert heat to electricity, researchers are looking for thermoelectric materials with a number of characteristics: they should have a large Seebeck effect, high electrical conductivity and low thermal conductivity. However, this is extremely difficult to achieve because these properties are interrelated and interdependent.

Physicists at the Vienna University of Technology (TU Wien) in Austria have succeeded in finding a new way to resolve this contradiction and optimize all thermoelectric properties in one material at the same time.

“At the so-called Anderson transition, a quantum phase transition from localized to mobile electron states, the conditions for the ideal thermoelectric are met,” said Fabian Garmroudi, first author of a paper on this work in Nature Communications. “This means that all conduction electrons have approximately the same energy.”

The Anderson transition occurs in semiconductors when impurity atoms are added, strongly binding their electrons. “Analogous to ice floes in the sea, these are initially isolated from each other and cannot be stepped on,” Garmroudi explained. “However, if the number of ice floes is large enough, you have a continuous connection through which you can cross the sea.”

This happens in a similar way in solids: if the number of impurity atoms exceeds a critical value, the electrons can suddenly move freely from one atom to another, and electricity can flow.

The TU Wien researchers demonstrated the importance of the Anderson transition in close collaboration with researchers from Sweden and Japan, as well as from the University of Vienna. For the first time, they linked this transition to a significant change in thermoelectric properties. The team made the exciting discovery when they heated a thermoelectric material made from iron, vanadium and aluminum (Fe2VAl) to very high temperatures, close to the melting point.

“At high temperatures, the atoms vibrate so strongly that they occasionally swap their lattice positions. For example, iron atoms are then located where vanadium atoms were before,” said co-author Ernst Bauer. “We succeeded in freezing this ‘atomic confusion’, which occurs at high temperatures, by so-called ‘quenching’, that is, rapid cooling in a water bath.” These irregular defects serve exactly the same purpose as impurity atoms, removing the need to change the chemical composition of the material.

In many research areas of solid-state physics, the materials being studied need to be as pure as possible and have an ideal crystal structure, because the regularity of the atoms simplifies a theoretical description of the physical properties. In the case of Fe2VAl, however, it is precisely the imperfections that account for most of the thermoelectric performance. It has also already been shown in neighbouring disciplines that irregularities can be advantageous.

“Basic research on quantum materials is a good example of this,” said co-author Andrej Pustogow. “There, science has already been able to show that disorder is often the necessary spice in the ‘quantum soup’. Now this concept has also arrived in applied solid-state research.”

This story is adapted from material from TU Wien, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.