Problem found; problem mitigated: overcoming chemical instability in Mg-based thermoelectric materials

Devices from thermoelectric materials can directly convert heat flows into electrical energy powering autonomous sensors or providing reliable electrical power supply in remote areas, as successfully demonstrated e.g. by the Voyager space probes or the Mars rovers Curiosity and Perseverance. On the other hand, they can also be employed for maintenance-free, seamlessly adjustable and scalable thermal management solutions, e.g. for fuel cells or optoelectronic systems. To unlock further applications of thermoelectric materials alternatives to state-of-the-art bismuth telluride are required. Magnesium-based TE materials like MgAgSb, Mg3(Sb,Bi)2 and Mg2(Si,Sn) are among the most promising candidates due to excellent performance, low cost, and environmental compatibility. However, functional stability under application conditions is an indispensable requirement, which proves to be a significant challenge for many high- performance materials, Mg-based ones in particular. For those, loss of volatile Mg along grain boundaries as well as demixing are the main challenges. Taking Mg2(Si,Sn) as example, we’ll discuss how to derive material transformation mechanisms from readily available experimental data, compare their effect on material transport properties and analyze the influence of environmental parameters (temperature, atmosphere) on the material degradation rate. From this understanding strategies against material instability can be derived effectively and evaluated experimentally. Focussing on Mg loss as the most relevant degradation mechanism we’ll discuss the following approaches: i) controlling the Mg vapor pressure, ii) fine-tuning the Mg content of the material to avoid loosely bound Mg, iii) coating, and iv) grain boundary engineering to stop Mg diffusion. Employing these, material degradation is reduced by several orders of magnitude, resulting in high-performance materials with enhanced stability.