Computational study of thermoelectric properties of two Heusler alloys and a wide gap semiconductor

Abstract

Thermoelectric materials are a fascinating class of materials that possess the unique ability to directly convert heat energy into electrical energy and vice versa. In recent times, they have attracted significant attention due to their potential for addressing important energy challenges. They have a wide range of applications, including waste heat recovery, power generation, refrigeration, and temperature sensing. By harnessing waste heat from industrial processes or automotive exhaust systems, thermoelectric materials can convert it into useful electrical power, thereby improving energy efficiency. Apart from experimental techniques, computational materials design plays a crucial role in the discovery of new thermoelectric materials. This thesis focuses on employing computational tools such as density functional theory, semiclassical Boltzmann theory, and many-body electron-phonon coupling to investigate key aspects of computational research in thermoelectric materials. Through these computational methods, we aim to gain a deeper understanding of the thermoelectric properties of Heusler alloys and wide gap semiconductor. By utilizing density functional theory, we analyze the electronic structure and calculate electron transport properties. The semiclassical Boltzmann theory allows us to study the transport of electrons and phonons, providing insights into the thermal conductivity and electrical conductivity of materials. Furthermore, by considering the many-body electron-phonon coupling, we explore the intricate interplay between electrons and lattice vibrations in ionic materials, in particular, shedding light on the transport properties. Through these computational approaches, we contribute to the advancement of computational materials design in the field of thermoelectric. In the first part of the thesis, we make use of computational tools to systematically study the effect of In doping on the thermoelectric properties of full Heusler Fe 2 VGa. We aim to deploy size disorder scattering by substituting larger atom In at place of Ga. We show that doping of In significantly enhances the Seebeck coefficient by suitably modifying the bands near the Fermi level and reduces the lattice thermal conductivity. However, In doped Fe 2 VGa alloys do not enhance the figure of merit of Fe 2 VGa, Fe 2 VIn is predicted to be the better thermoelectri material than Fe 2 VGa. Apart from this, we study the role of Sn doping onto the electronic properties of defective NbCoSb to make out the experimentally measured transport properties of Nb 0.8+δ CoSb 1-x Sn x where δ≤0.05 and x≤0.1. We find that Sn doping does not cause any significant change to the electronic band structure, apart from giving holes. The excess Nb in the sample causes suitable modification to the electronic band structure ,apart from giving free carries and in turn enhances the electrical conductivity and reduces the Seebeck coefficient. Overall, the Sn doping in presence of excess Nb controls the concentration of free electrons and enhances the thermoelectric performance of the sample. The second part of this thesis focuses on a critical examination of the validation of certain approximations commonly employed in transport properties calculation. To accurately predict the transport properties of new materials, computation of the carrier relaxation time is crucial. Typically, this is done using deformation potential theory, which considers the coupling between carriers and acoustic phonons. However, our research demonstrates that in ionic solids, the coupling between charge carriers and optical phonons is considerably strong. Neglecting this coupling when computing relaxation times leads to significant quantitative errors and also results in incorrect qualitative trends. Our findings emphasize the importance of considering the strong interaction between charge carriers and optical phonons in order to reliably predict the relaxation times and accurately describe the transport properties of materials.