R. B. Villoro, D. Zavanelli, C. Jung, D. A. Mattlat, R. Hatami Naderloo, N. Pérez, K. Nielsch, G. J. Snyder, C. Scheu, R. He, S. Zhang, Advanced Energy Materials 13, 2204321 (2023).
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Many thermoelectric materials benefit from complex microstructures. Grain boundaries (GBs) in nanocrystalline thermoelectrics cause desirable reduction in the thermal conductivity by scattering phonons, but often lead to unwanted loss in the electrical conductivity by scattering charge carriers. Therefore, modifying GBs to suppress their electrical resistivity plays a pivotal role in the enhancement of thermoelectric performance, zT. In this work, different characteristics of GB phases in Ti-doped NbFeSb half-Heusler compounds are revealed using a combination of scanning transmission electron microscopy and atom probe tomography. The GB phases adopt a hexagonal close-packed lattice, which is structurally distinct from the half-Heusler grains. Enrichment of Fe is found at GBs in Nb_0.95Ti_0.05FeSb, but accumulation of Ti dopants at GBs in Nb_0.80Ti_0.20FeSb, correlating to the bad and good electrical conductivity of the respective GBs. Such resistive to conductive GB phase transition opens up new design space to decouple the intertwined electronic and phononic transport in thermoelectric materials.

Y. Lee, M. Martini, T. Confalone, S. Shokri, C. N. Saggau, D. Wolf, G. Gu, K. Watanabe, T. Tanigucchi, D. Montemurro, V. M. Vinokur, K. Nielsch, N. Poccia, Advanced Materials 35, 2209135 (2023).
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High-temperature cuprate superconductors based van der Waals (vdW) heterostructures hold high technological promise. One of the obstacles hindering their progress is the detrimental effect of disorder on the properties of the vdW-devices-based Josephson junctions (JJs). Here, a new method of fabricating twisted vdW heterostructures made of Bi₂Sr₂CuCa₂O_8+δ, crucially improving the JJ characteristics and pushing them up to those of the intrinsic JJs in bulk samples, is reported. The method combines cryogenic stacking using a solvent-free stencil mask technique and covering the interface by insulating hexagonal boron nitride crystals. Despite the high-vacuum condition down to 10⁻⁶ mbar in the evaporation chamber, the interface appears to be protected from water molecules during the in situ metal deposition only when fully encapsulated. Comparing the current–voltage curves of encapsulated and unencapsulated interfaces, it is revealed that the encapsulated interfaces’ characteristics are crucially improved, so that the corresponding JJs demonstrate high critical currents and sharpness of the superconducting transition comparable to those of the intrinsic JJs. Finally, it is shown that the encapsulated heterostructures are more stable over time.

Q. Zhang, K. Deng, L. Wilkens, H. Reith, K. Nielsch,  Nature Electronics 5, 333 -347 (2022).
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Sustainable energy harvesting and efficient thermal management are required for the development of highly integrated electronic devices, the Internet of Things, and flexible and wearable technology. Micro-thermoelectric devices, which are capable of generating electricity from waste heat or using electricity to generate local cooling, are a promising solution. The devices have, in particular, a smaller leg cross-section and height than their commercial, macroscopic counterparts and can thus offer a faster response, higher resolution and greater power density. They can also be integrated with multifunctional microelectronic devices. Here we review the development of micro-thermoelectric devices. We examine progress in device design, integration, characterization and performance, and explore potential applications in cooling, power generation and sensing. We also analyse the key challenges that need to be addressed to create high-performance devices and realize the full commercial potential of the technology.

P. Ying, L. Wilkens, H. Reith, N. Perez Rodriguez, X. Hong, Q. Lu, C. Hess, K. Nielsch, R. He, Energy & Environmental Science 15, 2557-2566 (2022).
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The applications of thermoelectric (TE) technology around room temperature are monopolized by bismuth telluride (Bi₂Te₃). However, due to the toxicity and scarcity of tellurium (Te), it is vital to develop a next-generation technology to mitigate the potential bottleneck in raw material supply for a sustainable future. Hereby, we develop a Te-free n-type compound Mg₃Sb_0.6Bi_1.4 for near-room-temperature applications. A higher sintering temperature of up to 1073 K is found to be beneficial for reducing the electrical resistivity, but only if Mg is heavily overcompensated in the initial stoichiometry. The optimizations of processing and doping yield a high average zT of 1.1 in between 300 K and 573 K. Together with the p-type MgAgSb, we demonstrate module-level conversion efficiencies of 3% and 8.5% under temperature differences of 75 K and 260 K, respectively, and concomitantly a maximum cooling of 72 K when the module is used as a cooler. Besides, the module displays exceptional thermal robustness with a o 10% loss of the output power after thermal cycling for B32000 times between 323 K and 500 K. These proof-of-principle demonstrations will pave the way for robust, high-performance, and sustainable solid-state power generation and cooling to substitute highly scarce and toxic Bi₂Te₃