Research and Development: Applications

Thermoelectric materials have been at the center of much research in the last decade, due to their potential for the production of green energy. The search and optimization of novel thermoelectric materials is crucial to make related devices competitive and efficient, and can therefore have a substantial impact on our society. The goal of this project is to explore the applicability of recently synthesized half-Heusler compounds as XRuAs and XFeSb, where X = V, Nb or Ta, for thermoelectric applications. By means of first-principles electronic structure calculations, we intend to examine the electronic structure, the nature of the chemical bonding, the lattice vibrations, and finally the thermoelectric properties of these materials under various conditions. During the previous year, we have investigated the physical properties of these materials when varying carrier concentration and external pressure [1]. In this new project, instead, we intend to investigate the role of defects, which have been suggested to be crucial in affecting the mismatch between theory and experiment observed in analogous systems [2]. Vacancies, interstitial defects, and antisites will all be considered to obtain a comprehensive overview of their likelyhood during growth, as well as their role on the optical and thermoelectric properties. The main tool to be used for our investigation will be density functional theory (DFT), as well as its extensions for correcting the deficiencies of approximated exchange-correlation functionals, as e.g. DFT+U. These techniques, which are widely employed in computational materials science, were proven to be suitable for describing Heusler and half-Heusler compounds in the study of thermoelectricity [1]. The analysis of the thermoelectric properties will be carried out by using a combination of DFT and semi-classical Boltzmann theory, as typically done in this field [3]. The lattice
thermal conductivity will be estimated via Slack’s equation, which provides a more convenient alternative to the solution of the full Boltzman transport equation [1]. The final goal of this project is to find materials that do not only have good thermoelectric properties in ideal conditions, but also are sufficiently stable to the formation of defects to be usable in real life applications.

[1] M. Yazdani-Kachoei, S. Li, W. Sun, and I. Di Marco; "Role of volume change on the physics of thermoelectric half-Heusler compounds"; under consideration in Physical Review Materials.

[2] Y. Zheng, et al.; "Defect engineering in thermoelectric materials: what have we learned?"; Chemical Society Reviews 50, 9022 (2021).

[3] G. K. H. Madsen, J. Carrete, and M. J. Verstraete; "BoltzTraP2, a program for interpolating band structures and calculating semi-classical transport coefficients"; Computer Physics Communications 231, 140 (2018).