Developing sustainable, stable and high-performance thermoelectric mateirals for Lunar exploration and beyond

Kurzfassung

"Thermoelectric generators, which directly convert heat into electricity, are a cost-effective and promising technology to enhance the effective use of resources and promote environmental sustainability. In addition to terrestrial waste-heat recovery and solid-state cooling, thermoelectric technologies are well established in the space sector, where they have been used for reliable power generation in harsh environments using Radioisotope Thermoelectric Generators (RTGs), e.g. for deep space and planetary exploration (Voyager space probes, Mars rover Perseverance and Curiosity, and many more). Fully exploiting the capabilities of RTGs requires, however, the development of high-performance thermoelectric materials that remain functionally stable for long under application conditions. While commercially established materials such as bismuth telluride (Bi2Te3-based) offer excellent performance at near room temperature (up to 200-250 °C) [1], they are not optimized for the high-temperature operation of RTGs. Mg-based thermoelectric materials, such as Mg2(Si,Sn), operate at higher temperatures (up to 450-500°C) and are therefore a promising alternative.

Replacing BiTe thermoelectric materials with Mg-based alternatives could potentially double RTGs performance by exploiting a much larger operational temperature range. However, despite their excellent thermoelectric performance, Mg-based materials face one major challenge: the loss of magnesium by diffusion and sublimation [2]. Even minor loss of Mg can severely degrade the material’s performance, as it introduces charged defects that reduce charge carrier concentration and rapidly deteriorate the thermoelectric properties. To address this challenge, the thermal stability of these materials was systematically investigated through annealing experiments combined with transport measurements, microstructural analysis, and first-principles calculations. By understanding the underlying degradation mechanism, targeted stabilization strategies were developed and experimentally evaluated. In this work, two approaches are investigated: (i) fine-tuning the Mg content of the material to prevent incorporation of loosely bound excess Mg in the structure [3], and (ii) grain boundary engineering to inhibit Mg diffusion. While Mg stoichiometric control alone remains insufficient to fully stabilize doped materials, grain boundary engineering effectively slows down degradation, resulting in high-performance Mg-based thermoelectric materials with significantly enhanced stability."