Thermoelectricity: from heat to electricity

A lot of heat is lost in the conversion of energy. In fact, estimates put it at over 70%. However, in thermoelectric materials, such as those studied at the Institute of Solid State Physics at TU Wien, heat can be converted directly into electrical energy. This effect (the Seebeck effect) can be used in many applications in industry as well as in everyday life.

Recently, Ernst Bauer’s research team made an exciting discovery in a thermoelectric material consisting of iron, vanadium and aluminum (Fe₂VAL). The researchers recently published their results in nature communication†

The ideal thermoelectric

To achieve the largest possible energy conversion effect, researchers look for materials that meet a number of characteristics: They must have a large Seebeck effect, high electrical conductivity and low thermal conductivity. However, this is extremely difficult because these properties are interrelated and interdependent. Therefore, the researchers wondered what a material should physically look like in order to best fulfill all these conditions.

For example, physicists at TU Wien have succeeded in finding a new concept to resolve this contradiction and at the same time optimize all thermoelectric properties in one material. “In the so-called Anderson transition, a quantum phase transition from localized to mobile electron states, the conditions for the ideal thermoelectric state are met. This means that all conduction electrons have approximately the same energy,” reports Fabian Garmroudi, lead author of the study.

The Anderson transition occurs in semiconductors when impure atoms are added, tightly bonding their electrons. “Alike to ice floes in the sea, these are initially isolated from each other and cannot be entered. However, if the number of ice floes is large enough, you have a continuous connection through which you can cross the sea”, Fabian Garmroudi draws a comparison. With solids, this happens in a similar way: 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.

Atoms change places when it gets hot

Demonstrated in close collaboration with researchers from Sweden and Japan, as well as the University of Vienna, the Anderson transition was first associated with a significant change in thermoelectric properties. The team made the exciting discovery when they heated the material to very high temperatures, close to its melting point.

“At high temperatures, the atoms vibrate so strongly that they occasionally change their lattice position. Iron atoms are then located where vanadium atoms used to be. – called ‘quenching’, i.e. cooling quickly in a water bath,” reports Ernst Bauer. These irregular defects serve exactly the same purpose as the impurity atoms mentioned before, without the need to change the chemical composition of the material.

Energy conversion due to disorder

Many areas of solid state physics are interested in materials that are as pure as possible and have an ideal crystal structure. The reason: the regularity of the atoms simplifies a theoretical description of the physical properties. In the case of Fe₂However, VAL are the very imperfections that account for the bulk of thermoelectric performance. It has already been shown in neighboring disciplines that irregularities can be beneficial: “Basic research on quantum materials is a good example of this. Science has already shown that disorder is often the necessary spice in the ‘quantum soup'”, says Andrej Pustogow, a from the co-authors “Now this concept has also arrived in applied solid state research.”