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Tuesday, 19 November 2019

New material breaks all records for thermoelectric power generation


More and more technologies are using thermoelectricity to supply energy. Thermoelectric energy comes from the conversion of heat into electricity through temperature differences. However, the usual thermoelectric materials generate a relatively small amount of energy. But recently, Austrian physicists have developed a brand new thermoelectric material breaking all records of the amount of energy generated. Such a material could equip the sensors and processors to self-power.

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Thermoelectric materials can convert heat into electrical energy. This is due to the Seebeck effect: if there is a temperature difference between the two ends of such a material, an electrical voltage can be generated and the current can begin to flow. The amount of electrical energy that can be generated at a given temperature difference is measured by the so-called ZT value: the higher the ZT value of a material, the better its thermoelectric properties.

The best thermoelectric materials to date have been measured at ZT values ​​of about 2.5 to 2.8. The physicists of the Technical University of Vienna have succeeded in developing a brand new material with a ZT value of 5 to 6. It is a thin layer of iron, vanadium, tungsten and aluminum applied on a silicon crystal.

The new material is so efficient that it could be used to provide power to sensors or even small computer processors. Instead of connecting small electrical devices to cables, they could generate their own electricity from temperature differences. The study was published in the journal Nature .

A combination of iron, vanadium, tungsten and aluminum

" A good thermoelectric material must have a strong Seebeck effect and must meet two important requirements, which are difficult to reconcile, " says physicist Ernst Bauer of the Institute of Solid Physics at TU Wien. " On the one hand, he must conduct electricity as well as possible; on the other hand, it must conduct the heat as little as possible. This is a challenge because electrical conductivity and thermal conductivity are usually closely related .

At the Christian Doppler laboratory for thermoelectricity, created by Ernst Bauer at TU Wien in 2013, various thermoelectric materials for different applications have been studied in recent years. This research has now revealed a particularly remarkable material: a combination of iron, vanadium, tungsten and aluminum.

Atomic structure of the new material. Credits: B. Hinterleitner et al. 2019

The atoms of this material are generally arranged in a strictly regular manner in a face-centered cubic lattice. The distance between two iron atoms is always the same, and the same goes for the other types of atoms. The whole crystal is therefore perfectly regular, "explains Bauer.

Weyl fermions and low thermal conductivity in crystalline structure

However, when a thin layer of material is applied to silicon, something amazing happens: the structure changes dramatically. Although atoms always form a cubic pattern, they are now arranged in a space-centered structure, and the distribution of different types of atoms becomes completely random.

" Two iron atoms can be assembled next to each other, adjacent sites can be filled with vanadium or aluminum, and there is no longer a rule dictating the location of the next iron atom in the crystal ".

This mixture of regularity and irregularity of the atomic arrangement also modifies the electronic structure, which determines the way in which the electrons move in the solid. " The electric charge moves in a particular way in the material in order to protect it from diffusion processes .  The charges passing through the material are called Weyl Fermions . In this way, a very low electrical resistance is obtained.

Graphics showing the thermoelectric properties of the material. Credits: B. Hinterleitner et al. 2019

On the other hand, the vibrations of the network, which carry the heat from the high temperature zones to the low temperature zones, are inhibited by the irregularities of the crystalline structure. As a result, the thermal conductivity decreases. This is important if the electrical energy has to be generated permanently from a temperature difference - because if the temperature differences could be balanced very quickly and all the material had the same temperature everywhere, the thermoelectric effect 'stop.

Equip interconnected technologies with standalone power

Of course, such a thin layer can not generate a particularly large amount of energy, but it has the advantage of being extremely compact and adaptable. We want to use it to provide energy for sensors and small electronic applications . "

The demand for such small-scale generators is growing rapidly: in the "Internet of Things", more and more devices are connected together online, so they automatically coordinate their behavior with each other. This is particularly promising for future production plants, where one machine must react dynamically to another.

" If you need a lot of sensors in a plant, you can not connect them together. It's much smarter for the sensors to be able to generate their own power using a small thermoelectric device, "concludes Bauer.

Bibliography:

Article: Thermoelectric performance of a metastable thin-film Heusler alloy

Authors: B. Hinterleitner, I. Knapp, E. Bauer

Nature (2019)

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