Alloy Hybrid Materials as Cost-Effective Thermoelectrics

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An international team led by Fabian Garmroudi has developed thermoelectric materials that could compete with conventional, expensive, and unstable materials.

Thermoelectric materials enable the direct conversion of heat into electrical energy. This makes them particularly attractive for the emerging "Internet of Things," such as for the autonomous energy supply of microsensors and other tiny electronic components. To make the materials even more efficient, lattice vibrations must be curbed while increasing the mobility of the electrons—a hurdle where research has often struggled.

An international team led by Fabian Garmroudi succeeded in developing hybrid materials using a new method that achieve both goals—reduced coherence of lattice vibrations with increased mobility of charge carriers. The key: a mixture of two materials with fundamentally different mechanical but similar electronic properties. The work was recently published in the renowned journal Nature Communications.

In thermoelectric materials, we primarily try to suppress heat transport through lattice vibrations, as they do not contribute to energy conversion.

Fabian Garmroudi

New properties Through New Material Combination

Good thermoelectric materials are those that conduct electricity well on one hand but transport heat as poorly as possible on the other hand—a seeming contradiction, as good electrical conductors are usually also good thermal conductors.

"Heat is transferred in solid matter both by electrically conductive charge carriers and by vibrations of the atoms in the crystal lattice. In thermoelectric materials, we primarily try to suppress heat transport through lattice vibrations, as they do not contribute to energy conversion," explains lead author Fabian Garmroudi, who earned his PhD at TU Wien and now works as a Director's Postdoctoral Fellow at Los Alamos National Laboratory (USA). In recent decades, materials research has developed sophisticated methods to design thermoelectric materials with extremely low thermal conductivity.

"Supported by the Lions Award, I was able to develop new hybrid materials with exceptional thermoelectric properties at the National Institute for Materials Science in Japan," Garmroudi recalls his research stay in Tsukuba (Japan), which he completed during his work at TU Wien. Specifically, a powder alloy of iron, vanadium, tantalum, and aluminum (Fe2V0.95Ta0.1Al0.95) was mixed with a powder of bismuth and antimony (Bi0.9Sb0.1) and pressed into a compact material under high pressure and temperature. Due to their differing chemical and mechanical properties, the two components do not mix on an atomic level. Instead, the BiSb material preferentially deposits at the micrometer-sized interfaces between the crystals of the FeVTaAl alloy.

Heat and Charge Transport are Decoupled

The lattice structures of the two materials, and thus their quantum mechanically allowed lattice vibrations, are so different that heat vibrations cannot simply transfer from one crystal to the other. Therefore, heat transport is significantly impeded at the interfaces. At the same time, the movement of charge carriers remains unhindered due to the similar electronic structure and is even significantly accelerated along the interfaces. The reason: the BiSb material forms a so-called topological insulator phase there—a special class of quantum materials that are insulating in the interior but allow nearly lossless charge transport on the surface.

Through this targeted decoupling of heat and charge transport, the team managed to increase the material's efficiency by more than 100%. "This brings us a big step closer to our goal of developing a thermoelectric material that can compete with commercially available compounds based on bismuth telluride," says Garmroudi. The latter was developed in the 1950s and remains the gold standard in thermoelectrics to this day. The major advantage of the new hybrid materials: they are significantly more stable and cost-effective.

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