Abstract
In light of the present global concern surrounding energy resources and their environmental impact, there is a noticeable shift towards more sustainable energy generation methods, which aim to have enhanced performance and efficiency compared to more conventional methods. To fulfil increasing energy requirements while adhering to Net Zero policies, transitioning from environmentally high-impact energy sources to more environmentally-friendly alternatives is essential. Integrating or retrofitting power generation technologies with thermoelectric (TE) devices can enhance the overall power generation capability through the recovery of heat which would otherwise be radiated into the environment. Growing research indicates competitive performance between oxide-based and conventional TE materials. Nevertheless, a principal challenge for oxide materials is their larger thermal conductivity compared to conventional materials, particularly at lower temperatures, limiting their operating window to higher temperatures, often surpassing waste heat produced from industrial/transport sectors (523 - 723 K). Among the prospective TE oxide materials, strontium titanate (SrTiO3) shows particular promise; its structure allows for flexible doping, allowing for modulation of TE properties. Furthermore, the introduction of intrinsic defects, can also play a role in shaping performance. The introduction of extrinsic defects and interfaces has been found to decrease the thermal conductivity in literature. This thesis aims to investigate the defect chemistry of SrTiO3 and its Ruddlesden-Popper (RP) phases, Srn+1TinO3n+1 (n = 1 − 3), followed by the electronic structure, transport characteristics, and structural dynamics of SrTiO3 nanostructures and composites with polyaniline (PANI) and graphene nanoribbons (GNR). Chapter 1 outlines the current global concern surrounding energy and introduces TE materials, and generators, and their key theory. This is followed by a discussion on leading inorganic TE materials, including SrTiO3, and the aims of this thesis." Chapter 2 is the underlying theory and methodology used in this thesis. Moving forward, Chapter 3 is the first results chapter, this is a comprehensive examination of the defect Chemistry of SrTiO3 and its Ruddlesden-Popper (RP) phases, Srn+1TinO3n+1 (n = 1 − 3), using a self-contained large library of potential parameters, which can be potentially applied to an array of other oxides. An approximation is also introduced which does not depend on experimentally derived ionisation energies and electron affinities for doping which requires electronic compensation. The defect solution energies were found to align with previous computational literature (where reported), and the findings were contextualised using experimental evidence of doping SrTiO3 and its RP phases. Chapter 4 aims to provide an evaluation of the electronic and thermal transport properties of SrTiO3 nanostructures compared to bulk SrTiO3, namely, those with a unique stacking which leads to the formation of nanofilms characterised by Σ3{111} grain boundaries. The findings predict that varying the grain boundary In light of the present global concern surrounding energy resources and their environmental impact, there is a noticeable shift towards more sustainable energy generation methods, which aim to have enhanced performance and efficiency compared to more conventional methods. To fulfil increasing energy requirements while adhering to Net Zero policies, transitioning from environmentally high-impact energy sources to more environmentally-friendly alternatives is essential. Integrating or retrofitting power generation technologies with thermoelectric (TE) devices can enhance the overall power generation capability through the recovery of heat which would otherwise be radiated into the environment. Growing research indicates competitive performance between oxide-based and conventional TE materials. Nevertheless, a principal challenge for oxide materials is their larger thermal conductivity compared to conventional materials, particularly at lower temperatures, limiting their operating window to higher temperatures, often surpassing waste heat produced from industrial/transport sectors (523 - 723 K). Among the prospective TE oxide materials, strontium titanate (SrTiO3) shows particular promise; its structure allows for flexible doping, allowing for modulation of TE properties. Furthermore, the introduction of intrinsic defects, can also play a role in shaping performance. The introduction of extrinsic defects and interfaces has been found to decrease the thermal conductivity in literature. This thesis aims to investigate the defect chemistry of SrTiO3 and its Ruddlesden-Popper (RP) phases, Srn+1TinO3n+1 (n = 1 − 3), followed by the electronic structure, transport characteristics, and structural dynamics of SrTiO3 nanostructures and composites with polyaniline (PANI) and graphene nanoribbons (GNR). Chapter 1 outlines the current global concern surrounding energy and introduces TE materials, and generators, and their key theory. This is followed by a discussion on leading inorganic TE materials, including SrTiO3, and the aims of this thesis." Chapter 2 is the underlying theory and methodology used in this thesis. Moving forward, Chapter 3 is the first results chapter, this is a comprehensive examination of the defect Chemistry of SrTiO3 and its Ruddlesden-Popper (RP) phases, Srn+1TinO3n+1 (n = 1 − 3), using a self-contained large library of potential parameters, which can be potentially applied to an array of other oxides. An approximation is also introduced which does not depend on experimentally derived ionisation energies and electron affinities for doping which requires electronic compensation. The defect solution energies were found to align with previous computational literature (where reported), and the findings were contextualised using experimental evidence of doping SrTiO3 and its RP phases. Chapter 4 aims to provide an evaluation of the electronic and thermal transport properties of SrTiO3 nanostructures compared to bulk SrTiO3, namely, those with a unique stacking which leads to the formation of nanofilms characterised by Σ3{111} grain boundaries. The findings predict that varying the grain boundary (GB) density and the connectivity of the TiO6 octahedra leads to changes in the electronic and thermal transport properties. Furthermore, potential property anisotropy is observed for the nanofilm consisting of entirely face-sharing TiO6 octahedra, AB. The next chapter aims to build upon Chapter 4, by exploring the TE performance of composite materials which incorporate SrTiO3 with polyaniline and graphene nanoribbons (Chapter 5). The interaction between graphene nanoribbons (GNRs) and polyaniline (PANI) with the {100} SrTiO3 surface was considered, and the electronic structure, transport characteristics, and structural dynamics were analysed. A strong thermodynamic favourability for GNRs and PANI to adsorb on the SrTiO3 surface was shown, along with the co-adsorption of GNR and PANI introducing semi-metallic character into the electronic structure. Examination of the phonon spectra revealed a marked decrease in the phonon group velocities within the composites. The integration of PANI and/or GNRs is predicted to result in a substantial 72–88% drop in the lattice thermal conductivity compared to SrTiO3. Finally, we conclude with conclusions and future work in Chapter 6.Growing research indicates competitive performance between oxide-based and conventional TE materials. Nevertheless, a principal challenge for oxide materials is their larger thermal conductivity compared to conventional materials, particularly at lower temperatures, limiting their operating window to higher temperatures, often surpassing waste heat produced from industrial/transport sectors (523 - 723 K). Among the prospective TE oxide materials, strontium titanate (SrTiO3) shows particular promise; its structure allows for flexible doping, allowing for modulation of TE properties. Furthermore, the introduction of intrinsic defects, can also play a role in shaping performance. The introduction of extrinsic defects and interfaces has been found to decrease the thermal conductivity in literature. This thesis aims to investigate the defect chemistry of SrTiO3 and its Ruddlesden-Popper (RP) phases, Srn+1TinO3n+1 (n = 1 − 3), followed by the electronic structure, transport characteristics, and structural dynamics of SrTiO3 nanostructures and composites with polyaniline (PANI) and graphene nanoribbons (GNR).
Chapter 1 outlines the current global concern surrounding energy and introduces TE materials, and generators, and their key theory. This is followed by a discussion on leading inorganic TE materials, including SrTiO3, and the aims of this thesis." Chapter 2 is the underlying theory and methodology used in this thesis. Moving forward, Chapter 3 is the first results chapter, this is a comprehensive examination of the defect Chemistry of SrTiO3 and its Ruddlesden Popper (RP) phases, Srn+1TinO3n+1 (n = 1 − 3), using a self-contained large library of potential parameters, which can be potentially applied to an array of other oxides. An approximation is also introduced which does not depend on experimentally derived ionisation energies and electron affinities for doping which requires electronic compensation. The defect solution energies were found to align with previous computational literature (where reported), and the findings were contextualised using experimental evidence of doping SrTiO3 and its RP phases.
Chapter 4 aims to provide an evaluation of the electronic and thermal transport properties of SrTiO3 nanostructures compared to bulk SrTiO3, namely, those with a unique stacking which leads to the formation of nanofilms characterised by Σ3{111} grain boundaries. The findings predict that varying the grain boundary (GB) density and the connectivity of the TiO6 octahedra leads to changes in the electronic and thermal transport properties. Furthermore, potential property anisotropy is observed for the nanofilm consisting of entirely face-sharing TiO6 octahedra, AB.
The next chapter aims to build upon Chapter 4, by exploring the TE performance of composite materials which incorporate SrTiO3 with polyaniline and graphene nanoribbons (Chapter 5). The interaction between graphene nanoribbons (GNRs) and polyaniline (PANI) with the {100} SrTiO3 surface was considered, and the electronic structure, transport characteristics, and structural dynamics were analysed. A strong thermodynamic favourability for GNRs and PANI to adsorb on the SrTiO3 surface was shown, along with the co-adsorption of GNR and PANI introducing semi-metallic character into the electronic structure. Examination of the phonon spectra revealed a marked decrease in the phonon group velocities within the composites. The integration of PANI and/or GNRs is predicted to result in a substantial 72–88% drop in the lattice thermal conductivity compared to SrTiO3. Finally, we conclude with conclusions and future work in Chapter 6.
| Date of Award | 18 Jan 2024 |
|---|---|
| Original language | English |
| Sponsors | EPSRC-DTP competition 2018-19 (EP/R513234/1) |
| Supervisor | Marco Molinari (Main Supervisor) & Lisa Gillie (Co-Supervisor) |