Unlocking the full potential of MgAgSb by unravelling the interrelation of phase constitution and thermoelectric properties

Kurzfassung

"Thermoelectric generators (TEG) focus on harnessing the conversion of heat into electricity, holding promise for sustainable energy solutions and space power supply. Magnesium silver antimonide (MgAgSb) stands out among materials, exhibiting high performance from room temperature to 573 K. α-MgAgSb shows attractive thermoelectric (TE) properties and TEG lab prototypes have been successfully fabricated[1], but its sensitivity to secondary phases poses challenges[2], impacting the performance. A long-standing problem of this material system is its sensitivity to minute differences in effective composition and synthesis parameters, resulting in greatly differing TE properties and phase constitution in the micro- and nano-scale. We have investigated MgAgSb samples fabricated by mechanical alloying involving two high energy ball milling runs and two sintering steps. The samples have been synthesized with small differences in synthesis parameters, resulting in different effective compositions, and small (<3 at%), but varying amount of secondary phases. For those we have found recently a clear correlation between effective composition, the content of secondary phases and the resulting thermoelectric properties[2]. However, the employed SEM/EDS and XRD analysis is limited with respect to compositional and spatial resolution, preventing the development of a microstructure-based understanding of the material properties. By combined microstructural measurements using focused ion beam (FIB) equipped with wavelength dispersive X-ray spectroscopy (WDS), FIB-tomography, scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy (STEM-EDS) and electron diffraction, we identified fundamental differences between samples. Depending on synthesis conditions, we find a highly dispersed Ag3Sb phase with a typical size < 50 nm throughout some samples, invisible to XRD and SEM, which is identified to be highly detrimental to the TE properties. Similarly, Mg3Sb2 secondary phase appears to be surrounded by cracks, most likely influencing the resulting material performance. Analysis of the TE properties employing the Boltzmann formalism reveals strongly varying effective scattering constants depending on secondary phase type and content. Our approach reveals secondary phases to be a key player in this system and sets the foundation for synthesis upscaling and technological maturity of this material."