Thermoelements: World record material supplies sensors with “green” power

Energy efficiency is one of the most crucial tasks that we face in the 21st century. This effort also includes the use of immense amounts of waste heat—on a large and small scale. “Contradictory” materials play an important role in the process.

Most processes used in everyday life and industry produce waste heat that generally vanishes without being put to any good use. The temperatures of this heat are generally too low for energy recovery purposes. For a long time, one of the most promising options has been thermoelectric materials that are capable of generating power from small differences in thermal energy. The underlying principle is known as the Seebeck effect. This principle shows how a difference in temperature on both ends of particular materials can produce electrical voltage.

Of course, such electricity can’t power a factory. But for radio sensors on machines, small and efficient thermoelements could generate sufficient amounts of power from thermal energy. The advantages are obvious: no cables, no battery replacements, autonomy and permanent availability.

Thermoelements with “contradictory” material

This description makes it all sound so simple. But the reality of the process is highly complex. This is not just the case because the electronics have to “handle” electronics with mV currents. Furthermore, the material for the thermoelements has to meet demands that actually do not go together.

On the one hand, it should conduct electricity well, but thermal energy poorly. The problem: Good conductors of electricity are generally good conductors of thermal energy as well. This is not the case with a mishmash of materials made up of iron, vanadium, tungsten and aluminum that researchers at the Christian Doppler Lab for Thermoelectricity at the TU Wien (Vienna University of Technology) have come across. The atoms in this group are generally arranged in a strict, orderly manner, and the distance between the atoms of the individual materials is equally large. This results in a completely regular crystal structure.

But this structure changes radically when a thin layer of the material is applied to silicon. The atoms then take on a cubic shape, one that is characterized by a body-centered arrangement. This results in a completely random distribution of the various types of atoms.

This mix of regular and irregular patterns of atom order produces very little electrical resistance. At the same time, though, it disrupts the lattice vibrations that handle the job of transporting thermal energy. The thermal conductivity declines, and the temperature difference that is essential for power generation exists for a longer period of time.

The amount of electricity that can be generated is expressed by the ZT figure of merit: The higher the figure, the better the thermoelectric properties. Thermoelectric materials that have been measured thus far have ZT levels of about 2.5 to 2.8. The completely new material in use at the TU Wien boosts this level markedly, all the way up to 5 to 6.

Original publication: B. Hinterleitner et al., Thermoelectric performance of a metastable thin-film Heusler alloy, Nature (2019).