Characterization of Temperature-Dependent Mechanical Properties of Half-Heusler Thermoelectric Material and Microstructural Evolution of Reactively-Brazed Half-Heusler/Incusil Aba/Copper Interfaces

Thermoelectric materials, which have the capability of converting heat into electricity and vice-versa, provide a possible solution for harvesting waste heat from conventional energy conversion processes. Development of economically viable thermoelectric devices for large-scale applications warrants high figure-of-merit, a quantity that characterizes the performance of thermoelectric materials, as well as mechanical robustness and chemical stability of the device and its components. This work determines the hardness and elastic properties of commonly-used thermoelectric materials through nanoindentation and atomic force microscopy. Based on these results, p-type thermoelectric half-Heusler, Hf0.44Zr0.44Ti0.12CoSb0.8Sn0.2, was selected for a comprehensive mechanical characterization according to ASTM standards. This study probed its flexure and fracture behavior at ambient and elevated temperatures chosen accoding to the operational temperature range half-Heusler-based thermoelectric devices and reported the temperature-dependent bending strength and SENB plane strain fracture toughness of the material. Microstructural characterization of fractured surfaces allowed for establishment of fracture mechanism and microstructure-strength correlation for half-Heusler. Additionally, the work investigated elastic anisotropy of the micron-sized grain p-type half-Heusler microstructure through nanoindentation combined with electron backscatter diffraction, offering the first experimental reporting of the elasticity tensor for p-type half-Heusler. It also reported other elastic properties computed from the elasticity tensor and the results were validated with those obtained for a simpler TiCoSb half-Heusler from first principle calculations as reported in the literature. Finally, given that the thermoelectric modules fail primarily at the thermoelectric leg/electrode interfaces near the hot side, as reported in numerous studies, the last part of this work assessed ceramic thermoelectric/braze interfaces in reactively-brazed half-Heusler/Incusil ABA/copper joints. Electron backscatter diffraction and energy dispersive spectroscopy allowed for microstructural characterization of these ceramic/braze interfaces processed under varying brazing times with the primary goal assessing the chemical stability of these ceramic/braze interfaces and identifying optimal brazing parameters for development of robust interfaces for thermoelectric modules.