Institute for Metallic Materials - Recent Highlights

M. Gusenbauer et al., Journal of Applied Physics 129, 093902 (2021)

MnAl-C is a prominent candidate for the replacement of rare earth magnets with a moderate energy density product. Crystallographic defects have a strong effect on magnetization properties. In this work, we show the influence of twinning defects in the nanometer regime on the quality of the magnet. Standard micromagnetic simulations and computations of the saddle point configuration for magnetization reversal highlight the importance of optimizing the fraction of and reducing the width of crystallographic twin defects. Switching field distributions and the maximum possible coercive field for ideal microstructures without defects are estimated using a reduced order micromagnetic model.

The replacement of the scarce element Tellurium makes the devices cheaper while remaining the performance

More than 60% of the energy generated by burning fossil fuels is dissipated as waste heat, of which more than half is low-grade heat with temperatures below 300°C. Effective harnessing this low-grade heat to generate electricity is vital for alleviating the burden on the energy supply and reducing the emission of greenhouse gases. Thermoelectric technology stands out owing to its solid-state nature, which guarantees an ultra-long operational lifetime and is particularly attractive for heat-to-electricity conversion. The broader applicability of thermoelectric technology relies on the availability of high-performance materials and modules that operate efficiently below 300°C.

For more than 50 years, the commercial thermoelectric modules have relied on bismuth-telluride-based compounds because of their unmatched thermoelectric properties at temperatures associated with low-grade heat. However, the wider applicability of bismuth-telluride modules is severely limited by the scarcity of Tellurium with a concentration of <0.001 ppm in the Earth’s crust and an annual production of less than 500 metric tons. Therefore, it is imperative to develop thermoelectric modules from other, more abundant materials while retaining high performance in the low temperature range (<300°C).

Researchers from the Leibniz-Institute for Solid State and Materials Research Dresden, in collaboration with Prof. Zhifeng Ren at the Texas Center for Superconductivity (TcSUH) at the University of Houston, now developed for the first time a highly efficient Tellurium-free thermoelectric generator based on Magnesium-Antimony compounds, by using a simple, versatile, and thus scalable processing routine. These new thermoelectric generators achieve an efficiency of 7.0% at a temperature difference of 250°C and thus even exceed the efficiency of commercial bismuth telluride-based thermoelectric generators (~5.2%). This work marks a feasible, sustainable alternative to Bismuth-telluride-based thermoelectric modules and will spur a wider application of thermoelectric technology in converting low-grade heat to electricity and thermoelectric coolers.

This work is supported by the strategic project at IFW Dresden on “Wireless sensor devices for high temperature applications” and the Alexander von Humboldt Foundation.

Original publication: Ying, P., He, R., Mao, J. et al. Towards tellurium-free thermoelectric modules for power generation from low-grade heat. Nat Commun12, 1121 (2021).

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Stefan Schwabe, Robert Niemann, Anja Backen, Daniel Wolf, Christine Damm, Tina Walter, Hanuš Seiner, Oleg Heczko, Kornelius Nielsch, and Sebastian Fähler
Adv. Funct. Mater.  2005715 (2020)
(DOI: 10.1002/adfm.202005715)

Martensitic materials show a complex, hierarchical microstructure containing structural domains separated by various types of twin boundaries. Several concepts exist to describe this microstructure on each length scale, however, there is no comprehensive approach bridging the whole range from the nano- up to the macroscopic scale. Here, it is described for a Ni-Mn-based Heusler alloy how this hierarchical microstructure is built from scratch with just one key parameter: the tetragonal distortion of the basic building block at the atomic level. Based on this initial block, five successive levels of nested building blocks are introduced. At each level, a larger building block is formed by twinning the preceding one to minimize the relevant energy contributions locally. This naturally explains the coexistence of different types of twin boundaries. The scale-bridging approach of nested building blocks is compared with experiments in real and reciprocal space. The approach of nested building blocks is versatile as it can be applied to the broad class of functional materials exhibiting diffusionless transformations.

R. He et al., Energy & Environmental Science, 2020, 13, 5165
DOI: 10.1039/D0EE03014G

Half-Heusler (HH) compounds are among the most promising thermoelectric (TE) materials for large-scale applications due to their superior applicability such as high power factor, excellent mechanical and thermal reliability, and non-toxicity. Their only drawback is the remaining-high lattice thermal conductivity. Various mechanisms were reported with claimed effectiveness to enhance the phonon scattering of HH compounds including grain-boundary scattering, phase separation, and electron-phonon interaction. In this work, however, we show that point-defect scattering has been the dominant mechanism for phonon scattering other than the intrinsic phonon-phonon interaction for ZrCoSb and possibly many other HH compounds. Induced by the charge-compensation effect, the formation of Co/4d Frenkel point defect is responsible for the drastic reduction of lattice thermal conductivity in ZrCoSb1-xSnx. Our work systematically depicts the phonon scattering profile of HH compounds and illuminates subsequent material optimizations.

Anja Waske, Daniel Dzekan, Kai Sellschopp, Dietmar Berger, Alexander Stork, Kornelius Nielsch & Sebastian Fähler
Nature Energy 4, pages 68–74 (2019)
DOI: 10.1038/s41560-018-0306-x

To date, there are very few technologies available for the conversion of low-temperature waste heat into electricity. Thermomagnetic generators are one approach proposed more than a century ago. Such devices are based on a cyclic change of magnetization with temperature. This switches a magnetic flux and, according to Faraday’s law, induces a voltage. Here we demonstrate that guiding the magnetic flux with an appropriate topology of the magnetic circuit improves the performance of thermomagnetic generators by orders of magnitude. Through a combination of experiments and simulations, we show that a pretzel-like topology results in a sign reversal of the magnetic flux. This avoids the drawbacks of previous designs, namely, magnetic stray fields, hysteresis and complex geometries of the thermomagnetic material. Our demonstrator, which is based on magnetocaloric plates, illustrates that this solid-state energy conversion technology presents a key step towards becoming competitive with thermoelectrics for energy harvesting near room temperature.

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