Nov 15 2019
Thermoelectric materials are capable of changing heat into electrical energy. This can be attributed to the so-called Seebeck effect—that is, when both ends of the thermoelectric material have a temperature difference, an electrical voltage can be produced and the current can begin to flow.
Professor Ernst Bauer in the lab. Image Credit: © TU Wien.
The amount of electrical energy that can be produced at a specified temperature difference is quantified by what is known as the ZT value—a material with a higher ZT value will have better thermoelectric properties.
So far, the best thermoelectrics were quantified at ZT values of about 2.5 to 2.8. At TU Wien (Vienna), researchers have currently succeeded in creating an entirely new material whose ZT value ranges between 5 and 6. The material is a thin layer of aluminum, iron, tungsten, and vanadium applied to a silicon crystal.
The novel material developed by the researchers is so effective that it can be used for providing energy to sensors or even tiny computer processors. The researchers can produce their own electricity from temperature variations instead of linking tiny electrical devices to cables. This innovative material has been recently described in the Nature journal.
Electricity and Temperature
A good thermoelectric material must show a strong Seebeck effect, and it has to meet two important requirements that are difficult to reconcile. On the one hand, it should conduct electricity as well as possible; on the other hand, it should transport heat as poorly as possible. This is a challenge because electrical conductivity and thermal conductivity are usually closely related.
Ernst Bauer, Professor, Institute of Solid State Physics, TU Wien
In the past few years, different thermoelectric materials meant for different applications have been investigated at the Christian Doppler Laboratory for Thermoelectricity. Ernst Bauer established this laboratory at TU Wien in 2013.
This study has currently resulted in the discovery of a specifically remarkable material, which is a mixture of aluminum, iron, tungsten, and vanadium.
“The atoms in this material are usually arranged in a strictly regular pattern in a so-called face-centered cubic lattice,” added Ernst Bauer. “The distance between two iron atoms is always the same, and the same is true for the other types of atoms. The whole crystal is therefore completely regular.”
But upon applying a thin layer of the material to silicon, a remarkable phenomenon occurs—that is, the structure changes drastically. Even though the atoms still create a cubic pattern, they are presently organized in a space-centered structure, while the distribution of the diverse types of atoms becomes entirely haphazard.
“Two iron atoms may sit next to each other, the places next to them may be occupied by vanadium or aluminum, and there is no longer any rule that dictates where the next iron atom is to be found in the crystal,” explained Bauer.
This combination of irregularity and regularity of the arrangement of atoms also alters the electronic structure. It is this structure that determines the way electrons move in the solid.
The electrical charge moves through the material in a special way, so that it is protected from scattering processes. The portions of charge travelling through the material are referred to as Weyl Fermions.
Ernst Bauer, Professor, Institute of Solid State Physics, TU Wien
In this manner, a very low electrical resistance is obtained.
By contrast, lattice vibrations that transport heat from high-temperature places to low-temperature places are impeded by the irregularities present in the crystal structure. As a consequence, thermal conductivity reduces.
This is crucial if electrical energy has to be produced permanently from a temperature variation. This is because the thermoelectric effect would come to a halt when temperature variations equilibrate very rapidly, and the whole material immediately has the same temperature everywhere.
Electricity for the Internet of Things
Of course, such a thin layer cannot 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.
Ernst Bauer, Professor, Institute of Solid State Physics, TU Wien
The need for such types of small-scale generators is increasing rapidly—in the “Internet of Things,” an increasing number of devices are connected together online, allowing them to automatically coordinate their behavior with one another. This is specifically promising for upcoming production plants, where a single machine needs to react dynamically to another machine.
“If you need a large number of sensors in a factory, you can’t wire all of them together. It’s much smarter for the sensors to be able to generate their own power using a small thermoelectric device,” concluded Bauer.