Advancing the understanding of the microstructure-property relationship in non-toxic and cost-effective thermoelectric Heusler compounds

• Verbesserung des Verständnisses des Mikrostruktur-Eigenschafts-Verhältnis bei nicht-toxischen und kostengünstigen thermoelektrischen Heusler-Verbindungen

Gomell, Leonie Marie; Raabe, Dierk (Thesis advisor); Scheu, Christina (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022

Abstract

Thermoelectricity describes the direct conversion of temperature gradients into voltage and vice versa. If thermoelectric generators recycle waste heat, the devices can contribute to sustainable energy production. Sustainability is only achieved if materials are found which contain non-toxic, earth-abundant elements and exhibit a high energy conversion efficiency. An example of promising thermoelectric materials is Fe2VAl, which convinces by its low cost and non-toxicity. Yet, the thermal conductivity of Fe2VAl is too high for efficient energy conversion. Thus, reducing the thermal transport by adding phonon scattering centers into the material's microstructure is essential. Finding, understanding, and producing the "perfect'' scattering centers, which scatter phonons but not electrons, is the main goal to increase the performance of Fe2VAl. Phonons can be scattered by point defects, dislocations, and grain boundaries. Their density, chemical composition, and structural properties influence phonon and electron transport in mostly undocumented ways. The aim of this thesis is to address this knowledge gap. To manufacture materials with defect-rich microstructures, two different processes are selected based on their ability for fast quenching: melt spinning and laser surface remelting. Laser surface remelting allows adjusting the solidification conditions by changing the remelting parameters. In this way, the microstructure can be manipulated and controlled. For example, the remelting speed controls the grain size, while the remelting environment controls the defect composition. A combination of advanced microscopy techniques is required for an in-depth characterization of defects (i.e. dislocations, grain boundaries, and precipitates) from the near-atomic to the millimeter scale. This characterization includes but is not limited to (i) the acquisition of crystallographic texture by electron backscatter diffraction, (ii) the visualization of the defect distribution and density by electron channeling contrast imaging, and (iii) the determination of the local chemical composition of defects by atom probe tomography. Using this scale-bridging approach, the heterogeneity of defect distributions is captured while precise insights into the local structure and chemistry are also provided. Then, by combining the microstructural knowledge with local measurements of the electrical and thermal transport properties, direct proof of the influence of microstructural features is found. The correlative, cross-scale approach shows the importance of integrating microstructural features in thermoelectric materials. Since their effect depends on the structure and chemistry of the defects, a thorough microstructural analysis is essential. Only then can defects be targeted to improve thermoelectric performance. Finally, flexible synthesis methods, such as laser surface remelting, can specifically create the desired defects and thus increase the efficiency of thermoelectric energy generation.

Institutions

• Division of Materials Science and Engineering [520000]
• Chair of Materials Physics and Institute for Physical Metallurgy and Materials Physics [523110]