Nanostructured Bulk Thermoelectrics


Thermoelectric (TE) technology has great potential for cooling and power generation in many applications, as its unique solid-state nature allows TE devices to be free from maintenance and emission, thus offering extraordinary reliability. However, advanced thermoelectrics face several challenges for large-scale application, including but not limited to a paucity of durable, high-performance thermoelectric materials and inadequate translation of these materials into devices. Dr. Ran He's research group has been addressing these challenges by applying the following complementary multi-pronged strategies: 1) machine learning-assisted selection of promising thermoelectric materials; 2) understanding electron and phonon transport properties; 3) non-equilibrium synthesis for high-performance thermoelectrics; and 4) bulk thermoelectric modules for power generation and cooling. Our ultimate goal is to generate a new paradigm for the realization of non-toxic, durable, high-performance, and robust thermoelectric materials and modules using scalable processing approaches, thereby significantly advancing the sustainability of thermoelectric technology for cooling and power generation applications.


Dr. Ran He

Head of Research Group "Nanostructured Bulk Thermoelectrics "

Room:     D 1E.06
Phone:   +49 351 4659 337


Recent Highlights

Bulk thermoelectric modules for power generation and cooling

P. Ying, R. He, J. Mao, Q. Zhang, H. Reith, J. Sui, Z. Ren, K. Nielsch, G. Schierning, Nature Communications 12, 1121 (2021)
P. Ying, H. Reith, K. Nielsch, R. He, Small 18 (24), 2201183 (2022)
P. Ying, L. Wilkens, H. Reith, N. Pérez Rodríguez, X. Hong, Q. Lu, C. Hess, K. Nielsch, R. He, Energy & Environmental Science, 15, 2557-2566 (2022).

The applications of thermoelectric technology around room temperature are monopolized by bismuth telluride. 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 Mg3Sb0.6Bi1.4 for near-room-temperature applications. 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 < 10% loss of the output power after thermal cycling for ∼32000 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.

R. He, T. Zhu, Y. Wang, U. Wolff, J.-C. Jaud, A. Sotnikov, P. Potapov, D. Wolf, P. Ying, M. Wood, Z. Liu, L. Feng, N. Perez Rodriguez, G. J. Snyder, J. C. Grossman, K. Nielsch, G. Schierning, Engergy & Environmental Science (2020), 13, 5165-5176.

It’s of great importance to unveil the electronic and phononic transport features as well as the individual mechanisms in scattering them. This is especially true for certain thermoelectric materials such as half-Heusler (HH) compounds due to their great applicational potentials. Through a combination of experimental and first-principle approaches, we have shown that point-defect scattering has been the major effective mechanism for phonon scattering other than the intrinsic phonon–phonon interaction for HH compounds. Induced by the charge-compensation effect, the formation of Co/4d Frenkel point defects is responsible for the drastic reduction of lattice thermal conductivity in ZrCoSb. Our work systematically depicts the phonon scattering profile of HH compounds and illuminates subsequent material optimizations.

T. Zhu, R. He, S. Gong, T. Xie, P. Gorai, K. Nielsch, G. C. Grossman, Energy & Environmental Science, 2021, 14, 3559-3566.

Out of about 105 synthesized inorganic materials in the Inorganic Crystal Structure Database (ICSD), only less than 5% of materials have documented thermal conductivity. In our work, predictions of the lattice thermal conductivity through machine learning have been realized of known inorganic materials in the ICSD. We have employed a high-throughput approach and identified rare-earth chalcogenides as promising candidates with measured zT exceeding 1.0 and stable at higher than 1000 K. This work demonstrated the strength of high-throughput synthesis in finding materials with intrinsically low lattice thermal conductivity that are promising for thermoelectrics.

R. He, T. Zhu, P. Ying, J. Chen, L. Giebeler, U. Kühn, J. C. Grossman, Y. Wang, K. Nielsch, Small 17 (13), 2102045 (2021)
A. Bahrami, P. Ying, U. Wolff, N. Peréz Rodríguez, G. Schierning, K. Nielsch, R. He, ACS Appl. Mater. Interfaces 13 (32), 38561-38568 (2021)

Preventing phonon transport remains one of the most challenging tasks to improve the thermoelectric performance of certain materials such as half-Heusler compounds. On the other hand, established strategies such as alloying or grain boundary refinement have almost exhausted their potential to further reduce the lattice thermal conductivity (κL). It is still unclear how to improve the phononic scattering of materials. We show that the thermal conductivity can be drastically reduced by non-equilibrium synthesis by using two strategies that are unconventional for thermoelectric studies, including cryogenic milling and high-pressure sintering. A maximum κL reduction of 83% was achieved at room temperature with a relative density greater than 95%. These results uncovered the phonon transport properties of half-Heusler compounds under unconventional microstructures, showing the potential of high-pressure compaction and cryogenic milling to advance the performance of thermoelectric materials.

Current projects

Reduced Lattice Thermal Conductivity for Improved Thermoelectric Performance through Non-equilibrium Processing
Dr. Ran He (DFG, Project number 453261231), since 2021

Microstructural Modification and Interface Optimization for High-Performance GeTe-based Thermoelectric Materials and Modules
Dr. Xiao Xu (AvH Research Fellowship), since 2023