Einhorn, Maud; (2021) Theoretical insights of novel mixed-anion thermoelectrics. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The ever increasing global demand for energy, coupled with the impending climate emergency has driven an influx of research into new renewable energy sources and methods to increase energy efficiency. Wasted thermal energy results in a significant increase in energy consumption of processes spanning domestic, energy generation and transportation applications, and remains a largely unexploited source of energy. Thermoelectric materials present a clean and reliable method of converting thermal energy to electricity with no waste products of moving parts, and provide an opportunity for energy generation via waste-heat harvesting. Unfortunately, the majority of leading thermoelectric materials consist of toxic and non-earth abundant elements, including tellurium and lead. For these reasons, there is therefore a significant impetus to identify novel alternative thermoelectric materials, which exhibit both high thermoelectric efficiencies and alleviate sustainability concerns. The thermoelectric properties of a material are influenced by the underlying thermal and charge transport properties, with excellent charge transport and poor thermal transport required for a high thermoelectric efficiency. The thermoelectric figure of merit ZT is the most commonly used metric for assessing the thermoelectric efficiency of a material, and can be simply calculated from the Seebeck coefficient, conductivity and thermal conductivity of a material. In this thesis, an investigation of the thermal and charge transport properties of four mixed-anion compounds, LaZnOP, LaZnOAs, YZnOP, and YZnOAs will be performed using first principles calculations. These materials are of interest, as they possess properties which are commonly indicative of high-ZT thermoelectric materials, including a layered crystal structure, and are predominantly composed of earth-abundant and non-toxic elements. Throughout this work, first-principles density functional theory (DFT) calculations will be used to investigate the structural, lattice dynamical and charge transport properties of these four materials. This will ultimately provide an insight into the intrinsic properties which contribute to the thermoelectric performance of these systems. Special attention is focussed on methods to improve ZT, including an investigation of the nanostructuring potential of the compounds. The intrinsic defect chemistry of one compound, LaZnOP, is explored using hybrid-density functional theory, with a focus on assessing the doping potential of this material. This work aims to guide experimental researchers interested in these or similar compounds in the field of thermoelectrics.

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