More power from waste heat

April 29, 2023

(Nanowerk News) When fossil fuels, but also biofuels, are burned, a large amount of energy is lost as waste heat. Thermoelectric materials can convert this heat into electricity, but are not efficient enough for technical applications. A team from the Institut Max Planck für Eisenforschung have now improved the efficiency of thermoelectric materials by elucidating the effect of microstructure on materials and optimizing material properties by adding titanium. The chemical and atomic arrangement of the grain boundary phase determines the transport of electrons through the grain boundary. The titanium-rich grain boundary phase provides a conductive pathway (left) whereas the iron-rich grain boundary phase is resistive to electrons (right). (Image: R. Bueno Villoro, Max-Planck-Institut für Eisenforschung)

The climate crisis is forcing us not only to phase out fossil fuels, but also to conserve energy. Especially where fossil fuels cannot yet be replaced so quickly, they should at least be used efficiently – for example, by generating electricity from waste heat of industrial plants or energy-intensive power plants.

Currently, around 17 percent of the energy used in European industry is lost as waste heat. It can be utilized with the help of thermoelectric materials. In such a thermoelectric, an electric voltage is generated when exposed to a temperature difference. However, today’s thermoelectrics are not efficient enough to be used on a large industrial scale.

A research team led by the Düsseldorf-based Max Planck Institut für Eisenforschung has now succeeded in optimizing the thermoelectric, as the material is known in technical jargon, and thus approaching industrial use.

The team publishes its findings in a journal Advanced Energy Materials (“Grain boundary phases in NbFeSb half-Heusler alloys: A new avenue for adapting the transport properties of thermoelectric materials”).

The team studied an alloy of niobium, iron and antimony that converts waste heat into electricity at temperatures ranging from 70 to over 700 degrees Celsius with an efficiency of eight percent – ​​making the alloy currently one of the most efficient thermoelectrics. Only materials made from bismuth and tellurium achieve similar values. However, bismuth telluride is only suitable for use at relatively low temperatures and is less mechanically stable than thermoelectrics made from niobium, iron and antimony. In addition, the constituents are less available.

Titanium increases electrical conductivity

To further improve the thermoelectric efficiency of niobium, iron and antimony, the researchers focused on their microstructure. Like most metals, thermoelectric materials consist of tiny crystals. The composition and structure of the grains, as well as the properties of the spaces between them, known as grain boundaries, are critical to the thermal and electrical conductivities of thermoelectric materials.

Previous research has shown that grain boundaries reduce the thermal and electrical conductivity of materials. For the highest possible efficiency, the thermal conductivity must be as low as possible so that heat, i.e. energy, remains within the material. The electrical conductivity, however, must be high to convert as much heat into electricity as possible.

The goal of the team from the Max Planck Institut für Eisenforschung, Northwestern University (USA) and the Leibniz Institute for Solid State and Materials Research Dresden was to optimize grain boundaries in such a way that only thermal conductivity is reduced, but not electrical conductivity.

“We used scanning transmission electron microscopy and atomic probes to study the microstructure of the alloy down to the atomic level,” said Ruben Bueno Villoro, a doctoral student at the Max Planck Institut für Eisenforschung. “Our analysis has shown that grain boundaries need to be optimized to improve electrical and thermal properties.”

“The smaller the grains in a material, the higher the number of grain boundaries and the poorer the electrical conductivity,” explained Siyuan Zhang, project leader in the same research group. “It makes no sense to increase the grain size in the material, because larger grains will increase the thermal conductivity and we will lose heat and energy. Therefore, we have to find a way to increase the electrical conductivity even though the small grains.”

The researchers solved the problem by enriching the material with titanium, which, among other things, accumulates at grain boundaries and increases electrical conductivity. In this way, they increased the alloy’s thermoelectric efficiency by up to 40 percent. However, for practical applications, efficiency still needs to be significantly improved.

Next step: selective enrichment of titanium at grain boundaries

Now the research team is analyzing ways to selectively add titanium to just the grain boundaries without enriching the entire material with titanium. This strategy saves costs and largely preserves the original chemical composition of the thermoelectric material. Current research shows how functional properties can be related to the atomic structure of a material to specifically optimize certain properties.

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