New hybrid materials as efficient thermoelectrics

Thermoelectric materials enable the direct conversion of heat into electrical energy. This makes them particularly attractive for the emerging “Internet of Things”, for example for the autonomous energy supply of microsensors and other tiny electronic components. In order to make the materials more efficient, at the same time heat transport via the lattice vibrations must be suppressed and the mobility of the electrons increased – a hurdle that has often hindered research until now.

An international team led by Fabian Garmroudi has now succeeded in using a new method to develop hybrid materials that achieve both goals – reduced coherence of the lattice vibrations and increased mobility of the 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.

New properties through a new combination of materials

Good thermoelectric materials are those that conduct electricity well on the one hand, but transport heat as poorly as possible on the other – an apparent contradiction, as good electrical conductors are generally also good conductors of heat.

"In solid matter, heat is transferred both by mobile charge carriers and by vibrations of the atoms in the crystal lattice. In thermoelectric materials, we mainly try to suppress heat transport through the lattice vibrations, as they do not contribute to energy conversion," explains first author Fabian Garmroudi, who obtained his doctorate at TU Wien and is now working 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 at the National Institute for Materials Science in Japan that exhibit exceptional thermoelectric properties,” recalls Garmroudi of his research stay in Tsukuba (Japan), which he completed as part of his work at TU Wien. Specifically, powder of an alloy of iron, vanadium, tantalum and aluminum (Fe₂V_0.95Ta_0.1Al_0.95) was mixed with a powder of bismuth and antimony (Bi_0.9Sb_0.1) and pressed into a compact material under high pressure and temperature. Due to their different chemical and mechanical properties, however, the two components do not mix at an atomic level. Instead, the BiSb material is preferentially deposited at the micrometer-sized interfaces between the crystals of the FeVTaAl alloy.

Decoupling heat and charge transport

The lattice structures of the two materials, and therefore also their quantum mechanically permitted lattice vibrations, are so different that thermal vibrations cannot simply be transferred from one crystal to the other. Heat transfer is therefore strongly inhibited at the interfaces. At the same time, the movement of the 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 – a special class of quantum materials that are insulating on the inside but enable almost loss-free charge transport on the surface.

This targeted decoupling of heat and charge transport enabled the team to increase the efficiency of the material 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 back in the 1950s and is still considered the gold standard of thermoelectrics today. The big advantage of the new hybrid materials is that they are significantly more stable and also cheaper.