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

Abstract

Thermoelectric generators (TEG) convert heat into electricity, holding promise for sustainable energy solutions and space power supply. Currently, they are mainly based on bismuth, tellurium, lead or germanium, which are expensive, toxic and rare elements. However, new Mg-based TE materials have recently shown great promise for a next generation of thermoelectric generators, with applications between room temperature and 300 °C. The low weight, abundance and good mechanical properties combined with high performances make Mg-based thermoelectric generators suitable for energy harvesting on earth as well as in a future lunar habitat. α-MgAgSb shows attractive thermoelectric (TE) properties and TEG lab prototypes have been successfully fabricated, but its sensitivity to secondary phases poses challenges, resulting in greatly differing TE properties and impacting device performance and transfer into real-world applications. We have investigated MgAgSb samples fabricated by mechanical alloying and hot pressing. The samples have been synthesized with small differences in synthesis parameters, resulting in different effective compositions, and small (< 3 at%), but varying amounts of secondary phases. SEM/EDS and XRD analyses, along with transport property measurements, were employed to establish an initial correlation between effective composition, the content of secondary phases and the resulting thermoelectric properties. However, the limited compositional and spatial resolution of those techniques inhibits the development of a microstructure-based understanding of the material properties. To overcome these limitations, scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy and selected area electron diffraction were used, enabling us to identify fundamental differences between samples. For some synthesis conditions we find a highly dispersed Ag3Sb phase with a typical size < 50 nm throughout the samples, invisible to XRD and SEM, which is identified to be highly detrimental to the TE properties. A systematic statistical analysis of the TE properties reveals strongly varying performance degradation depending on secondary phase type and content, with binary Sb-related phases most detrimental. This allows for the identification of favorable synthesis conditions for upscaling, where secondary phases might not be completely avoidable.