Rahim, Warda; (2021) Computationally Driven Discovery of Complex Oxide Materials for Thermoelectric Applications. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

With the increasing challenges of global warming and depleting reserves of fossil fuels, there is an ever-growing demand for clean and renewable sources of energy. Thermoelectric generators can convert waste thermal energy into useful electricity, and represent a promising route to renewable energy generation for a wide range of industries.The dimensionless figure of merit, ZT, is used to determine the efficiency of a thermoelectric material. It depends on both electronic and thermal properties, with a material considered promising if its ZT exceeds ∼1. Despite over 50 years of development, the champion thermoelectric materials, such as PbTe and Bi2Te3, show lacklustre performance and contain toxic (Pb) or costly and rare (Te) elements. Recently, oxide materials have become popular due to their chemical inertness, earth-abundance and reduced costs. However, the majority of oxides have low thermoelectric efficiency, primarily because of their high lattice conductivities.This has fuelled significant research effort toward the search for structure types with low lattice thermal conductivities. In this thesis, we examine two families of oxide compounds for their suitability and potential as thermoelectrics. Ab initio methods are used to provide useful insight into their electronic and thermal transport properties. Initially, in the first part of the thesis, a first-principles polymorph exploration method, based on phonon mode mapping, is discussed and used to resolve the structural debate surrounding a complex ternary oxide. In the second part of the thesis, we explore the thermoelectric properties of this complex oxide, and extend further investigation into a class of mixed-anion semiconductors. There is great emphasis on understanding the microscopic origins of lattice thermal conductivity due to its critical role in maintaining the temperature gradient and flow of charge carriers in a thermoelectric device. Throughout this work, we aim to provide guiding principles to experimental researchers for identifying novel oxide materials for high-performance thermoelectric applications.

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