Adjustment of thermoelectric materials for efficient power generation

In times when energy is scarce and sustainable forms of energy production are explored, thermoelectric materials for power generation are being considered to transform waste heat into electricity. However, for this transformation to be more efficient and therefore usable on an industrial scale, a better understanding of the functional and structural properties of materials is needed.

A team of researchers led by the Max-Planck-Institut für Eisenforschung (MPIE) has now been able to fine-tune the microstructure of a promising new thermoelectric material for efficient energy conversion. The team published their results in the journal advanced energetic materials.

Tuning of thermoelectric properties through grain boundary engineering

Previous research has shown that the structure and composition of the grain boundaries are crucial for the electrical and thermal conductivity of thermoelectric materials. Grain boundaries generally reduce both the thermal and electrical conductivity of the material, whereas it is desirable to have low thermal but high electrical conductivity.

The aim of the researchers at MPIE, Northwestern University (USA) and the Leibniz Institute for Solid State and Materials Research in Dresden (Germany) was to modify the grain boundaries so that only thermal conductivity is reduced, as long as its electrical conductivity was kept high. They used an intermetallic compound of Ti-doped Heusler NbFeSb medium, a recently developed but promising thermoelectric alloy.

It has excellent thermoelectric properties at medium and high temperatures, good thermal and mechanical robustness, and its elements are abundant on earth and benign.

“We used advanced characterization techniques such as scanning transmission electron microscopy and atom probe tomography to reveal the microstructure of alloys down to the atomic scale. Our analysis showed that the chemistry and atomic arrangement of grain boundaries can be tuned to design electronic and thermal transport properties”, says Rubén Bueno Villoro, doctoral researcher in the independent research group “Nanoanalítica e Interfaces” of the MPIE and first author of the publication.

Since the grain size is small, the larger number of grain boundaries significantly reduces the electrical conductivity. “By doping the alloy with titanium, we found that the grain boundaries become rich in titanium and no longer strong, so we can fully utilize the beneficial low thermal conductivity provided by the small grain size,” explains Dr. Siyuan Zhang, project leader in the same research group and corresponding author of the publication.

After demonstrating the grain boundary engineering strategy, the researchers are exploring new ways to selectively dope grain boundaries. By relating functional properties to the atomic structures of critical microstructure features such as grain boundaries, the research team is developing new design principles for materials critical to a sustainable future.