First-Principles Investigation of Entropy-stabilized and Double Half-Heusler Alloys for Thermoelectric Applications

Abstract

This thesis presents two projects centered on the computational design and analysis of thermoelectric materials derived from half-Heusler and double half-Heusler compounds. The research is motivated by the growing need to harvest waste heat from diverse sources such as automobile exhausts, industrial furnaces, and domestic heating systems through thermoelectric energy conversion. While half-Heusler alloys offer advantages including mechanical robustness, high-temperature stability, and favorable Seebeck coefficients, their performance is significantly hindered by inherently high lattice thermal conductivity, which limits overall thermoelectric efficiency. To address this limitation, the first project explores entropy stabilization (chapter 3), whereas the second investigates double half-Heusler engineering (chapter 4) as potential pathways for performance enhancement. In chapter 1, the performance of thermoelectric devices is examined in detail. The chapter also includes a review of the literature on various classes of thermoelectric materials, highlighting their thermoelectric performance. Major part of chapter 2 describes Density Functional Theory (DFT), a fundamental and widely adopted computational framework in contemporary materials science for addressing the many-electron problem. The discussion covers the theoretical underpinnings of DFT, outlining its formalism as well as the approximations typically applied in practical simulations. The chapter also talks about the Boltzmann transport theory, which forms the theoretical foundation for the analysis of electronic transport properties presented in this work. Finally, the Debye–Callaway model is described as a tool for estimating lattice thermal conductivity, offering insights into phonon-mediated heat transport. In chapter 3, the mixed half-Heusler compound ZrHfCoNiSnSb is proposed and investigated through first-principles calculations. The study is motivated by the idea of combining two parent HH alloys to introduce mass disorder, thereby lowering lattice thermal conductivity while preserving favorable electronic transport characteristics. This chapter systematically examines the structural, electronic, thermodynamic, and transport properties of the mixed compound. The contribution of configurational entropy to stabilizing the mixed phase at elevated temperatures is highlighted, and the thermoelectric performance of the proposed material is compared with that of its parent alloys. In chapter 4, the thermoelectric properties of two quaternary double half-Heusler compounds, Nb2Co2InSb and Nb2Co2GaSb, are examined through theoretical calculations. These materials are designed by modifying the parent ternary half-Heusler NbCoSn, replacing Sn with In/Ga and Sb, with the goal of maintaining advantageous electronic characteristics while markedly lowering the lattice thermal conductivity (kL) via mass disorder. The investigation considers multiple structural arrangements, including two ordered phases and two Special Quasi-random Structures (SQSs), and assesses their structural, dynamical, electronic, and thermoelectric properties. Findings reveal a pronounced decrease in kL, nearly fivefold compared to NbCoSn, resulting in enhanced figures of merit (zT) reaching up to 2.0 at 1000 K. The study reveals that Nb2Co2GaSb achieves optimal performance in its ordered configuration, whereas Nb2Co2InSb attains superior thermoelectric efficiency in the disordered SQS phase. The final summary and outlook chapter of this thesis presents a consolidated overview of the key findings and the broader significance of the research carried out on these thermoelectric materials. It recaps the principal outcomes achieved in the course of this work and provides a forward-looking view on potential future research directions aimed at advancing the field of thermoelectric materials.