Abstract:

The demand for renewable energy is increasing as fossil fuel resources are finite. Thermoelectric materials offer a path to recovering waste heat from industrial processes, vehicles, and even the human body by converting it directly into electricity for remote sensors, medical devices, solid-state cooling, and deep-space missions, for example. However, the existing thermoelectric materials achieve only moderate conversion efficiency and suffer from high cost and limited performance at high operating temperatures.

Magnetic materials are traditionally not considered for thermoelectric applications because of their modest thermoelectric performance, low Curie or Néel temperature, and phase-stability issues. This thesis demonstrates that magnetic semiconductors both in bulk and two-dimensional forms can serve as efficient thermoelectric materials. It investigates how the electronic states, magnetic ordering, and phonon scattering can be controlled together to increase the power factor while simultaneously reducing the lattice thermal conductivity, which is essential for achieving a high thermoelectric figure of merit. Spin-polarized first-principles electronic structure calculations combined with semiclassical Boltzmann transport theory, applied to the electrons and phonons, are employed to study magnetic semiconductors, including ferromagnets, antiferromagnets, and altermagnets.

A series of magnetic semiconductors is addressed to quantify how the magnetic ordering influences the thermoelectric performance. Antiferromagnetic CsMnBi exhibits strong phonon-phonon scattering, which leads to ultralow lattice thermal conductivity and, consequently, a high thermoelectric figure of merit below its Néel temperature, making it suitable for room-temperature waste-heat recovery. The altermagnet V₂SeTeO has a narrow band gap, high Néel temperature, and strong lattice anharmonicity, which together yield promising thermoelectric performance for low-temperature (300-500 K) waste-heat recovery. FeCl₂ shows a semimetal-to-semiconductor transition under hydrostatic pressure, which results in a high power factor and low lattice thermal conductivity, together leading to suitability for medium-temperature (500-900 K) waste-heat recovery. Finally, a comparative study of the antiferromagnetic and ferromagnetic states of MnIn₂Te₄ in its tetragonal and orthorhombic phases shows that nearly degenerate conduction band valleys and a low lattice thermal conductivity of the orthorhombic phase are promising for the medium-temperature range.