High-Pressure-Sintering-Induced Microstructural Engineering for an Ultimate Phonon Scattering of Thermoelectric Half-Heusler Compounds

Effect of Heat Treatment Temperature on the Crystallization Behavior and Microstructural Evolution of Amorphous NbCo1.1Sn

Heat treatment-induced nanocrystallization of amorphous precursors is a promising method for nanostructuring half-Heusler compounds as it holds significant potential in the fabrication of intricate and customizable nanostructured materials. To fully exploit these advantages, a comprehensive understanding of the crystallization behavior of amorphous precursors under different crystallization conditions is crucial. In this study, we investigated the crystallization behavior of the amorphous NbCo1.1Sn alloy at elevated temperatures (783 and 893 K) using transmission electron microscopy and atom probe tomography. As a result, heat treatment at 893 K resulted in a significantly finer grain structure than heat treatment at 783 K owing to the higher nucleation rate at 893 K. At both temperatures, the predominant phase was a half-Heusler phase, whereas the Heusler phase, associated with Co diffusion, was exclusively observed at the specimen annealed at 893 K. The Debye-Callaway model supports that the lower lattice thermal conductivity of NbCo1.1Sn annealed at 893 K is primarily attributed to the formation of Heusler nanoprecipitates rather than a finer grain size. The experimental findings of this study provide valuable insights into the nanocrystallization of amorphous alloys for enhancing thermoelectric properties.

Geometrical Optimization and Thermal-Stability Characterization of Te-Free Thermoelectric Modules Based on MgAgSb/Mg3 (Bi,Sb)2

Solid-state thermoelectric (TE) technology is a promising approach to harvest low-grade waste heat (<573 K) and converts it to useful electricity in industrial and civilian settings. After decades of efforts in improving the figure-of-merit (zT) of TE materials, the development of advanced modules has started springing up in recent years. Although high-performance modules have been largely reported based on the successful material improvement, it remains less investigated how and whether the module-level designs can further increase the conversion efficiency. Herein, following the recent demonstration of a tellurium (Te)-free TE generator, an increase is demonstrated in the efficiency by reducing both the electrical and thermal energy losses through simply optimizing geometric factors of filling factor and leg-pair numbers. These module-level optimizations enable a record conversion efficiency of 8.2% under a ∆T ≈ 260 K, thus fulfilling 90% of the theoretical efficiency of the materials and solidly exceeding the Bi₂ Te₃ modules. Furthermore, module robustness against > 10 160 thermal cycles while preserving a relative efficiency of 95% is demonstrated. These findings highlight the importance of the optimization strategy at the module level and demonstrate the feasibility of using Te-free thermoelectric compounds to harvest the omnipresent low-grade heat.