Microstructural design of thermoelectric materials
Thermoelectric materials can convert largely untapped heat energy sources, e.g. geothermal or industrial waste heat, into sustainable electricity. Despite their high potential, efficient thermoelectrics are rare. High thermoelectric conversion efficiency requires high electrical conductivity (σ) but low thermal conductivity (κ), a rare combination in materials. Using materials science principles, these transport properties can be tailored by microstructural defects.
High grain-boundary density in thermoelectrics can suppress κ, but often yields a lower σ, which compromises the overall efficiency. Hence, we identify grain boundary engineering to obtain high σ as a promising way to optimize the performance. We employ atomic-resolution microscopy and spectroscopy to investigate the grain boundary phases, in order to understand their impact on electrical transport. In our pioneering work, grain boundary phases with HCP-stacking are identified in a NbFeSb half-Heusler (FCC) thermoelectric [1]. The grain boundary electrical conductivity is greatly improved as the grain boundary defect phase transitions from FeSb-enriched to a TiSb-enriched defect phase. Building on this concept, we have designed InSb as a grain boundary dopant to selectively improve the grain boundary conductivity, as In has practically no solubility in NbFeSb [2, 3]. We have also demonstrated that grain boundaries with dopant segregation can serve as conductive pathways for electrical transport, reaping both benefits of high σ and low κ for fine-grained TiCoSb thermoelectrics [4].
Figure 1: The carrier mobility as a function of grain size and composition. EBSD maps of Nb0.95Ti0.05FeSb and Nb0.80Ti0.20FeSb revealing the different grain sizes. In addition, high resolution STEM image and APT data of the grain boundary complexion are shown. Data from [1].
Figure 1: The carrier mobility as a function of grain size and composition. EBSD maps of Nb0.95Ti0.05FeSb and Nb0.80Ti0.20FeSb revealing the different grain sizes. In addition, high resolution STEM image and APT data of the grain boundary complexion are shown. Data from [1].
Beside grain boundaries, we also investigate the effect of dislocations, stacking faults, and precipitates on the transport properties. An important aspect is how the microstructure evolves during thermoelectric operation at elevated temperatures. We employ correlative microscopy to study processes such as Ostwald ripening [5] and crystallization [6], as well as in situ microscopy to understand mechanisms behind dynamic changes in carrier concentrations [7] and dislocation-phonon scattering [8].
Project publications
1.
Bueno Villoro, R.; Zavanelli, D.; Jung, C.; Mattlat, D. A.; Naderloo, R. H.; Pérez, N. A.; Nielsch, K.; Snyder, G. J.; Scheu, C.; He, R.et al.; Zhang, S.: Grain Boundary Phases in NbFeSb Half-Heusler Alloys: A New Avenue to Tune Transport Properties of Thermoelectric Materials. Advanced Energy Materials 13 (13), 2204321 (2023)
Jung, C.; Zhang, S.; Cheng, N.; Scheu, C.; Yi, S.-H.; Choi, P.-P.: Effect of Heat Treatment Temperature on the Crystallization Behavior and Microstructural Evolution of Amorphous NbCo1.1Sn. ACS Applied Materials and Interfaces 15 (39), pp. 46064 - 46073 (2023)
Max Planck scientists design a process that merges metal extraction, alloying and processing into one single, eco-friendly step. Their results are now published in the journal Nature.
Start of a collaborative research project on the sustainable production of manganese and its alloys being funded by European Union with 7 million euros