ECO-friendly alkaline-earth silicides for THERMoelectric applications – ECOTHERM

As part of the ECOTHERM project, the use of abundant elements such as silicon and strontium address ecological challenges. The non congruent alloy α-SrSi2 of these two elements exhibited modest performance up to 150°C, and a multi-scale approach could significantly improve it. To achieve this, two approaches were used:

• Mechanochemistry: This technique uses mechanical forces to induce chemical reactions and provoke structural changes in the material. It is thus possible to obtain, on the one hand, alloys with no melting point and, on the other hand, nanostructured alloys.

• Melt Spinning: This technique allows for rapid cooling of liquid alloys at rates up to 107 °C/s. It enables the formation of metastable phases and induces particular micro-/nano-structures.

The obtained materials were shaped by rapid sintering to densify them and characterized both structurally and for their thermoelectric performance.

The multi-scale approach developed in this project optimized the performance of the α-SrSi2 alloy by 70% through microstructure control and by optimizing the substitution rate. Through a combination of experimentation and theory, it was also shown that performance could be further improved through double substitution. While modest, the performance achieved in the ECOTHERM project already suggests potential applications at near-room temperature, as the elements used are cost-effective enough to be competitive.

The results have been repeatedly published in high-impact peer-reviewed scientific journals and presented at numerous international and national thermoelectricity conferences (such as ICT, ECT, WFJ, JNTE) and more general conferences (AFC, SFC, NanoMat, European Silicon Days, ISIEM).

Looking for new green energy sources is one of the most fundamental issues to sustain economic growth and reduce greenhouse gas emissions. Since the Kyoto protocol, the signatory countries have made strong efforts and renewable energy now represents a significant and rapidly growing share of total energy supply (19.3% in 2017). Among them, thermoelectricity appears as a possible alternative by recovering wasted energy and particularly wasted heat. Thermoelectric materials are well known for many decades but their widespread use in the industrial landscape is limited by their low power/cost-weight ratio and also because most of the used materials contain lead or very toxic tellurium. Therefore, the challenge of ECOTHERM is to develop new thermoelectric materials using cleaner resources with improved performances.

ECOTHERM aims to study and develop new alkaline earth silicides for thermoelectric applications. The project focuses more particularly on an alkaline earth silicide that has been poorly studied in the literature and has a high potential for optimization. An approach combining the study of the solubility and stability domains of ternary and quaternary systems as well as the use of non-equilibrium metallurgical processes will aim to optimize the charge carrier concentration and to reduce the thermal conductivity. The design of such alloys with non-equilibrium technics could provide thermoelectric materials with unprecedented properties.