The aim of this thesis is to investigate the effects of additives, reactive elements and impurities, on the lifetime of thermal barrier coatings. The thesis consists of a number of studies on interface adhesion, impurity diffusion, grain boundary sliding and cleavage processes and their impact on the mechanical behaviour of grain boundaries. The effects of additives and impurity on interface adhesion were elaborated by using total energy calculations, electron localization and density of states, and by looking into the atomic separations. The results of these calculations allow the assessment of atomic level contributions to changes in the adhesive trend. Formation of new bonds across the interface is determined to improve the adhesion in reactive element(RE)-doped structures. Breaking of the cross interface bonds and sulfur(S)-oxygen(O) repulsion is found responsible for the decreased adhesion after S segregation. Interstitial and vacancy mediated S diffusion and the effects of Hf and Pt on the diffusion rate of S in bulk NiAl are studied. Hf is shown to reduce the diffusion rate, and the preferred diffusion mechanism of S and the influence of Pt are revealed to be temperature dependent. Finally, the effects of reactive elements on alumina grain boundary strength are studied. Reactive elements are shown to improve both the sliding and cleavage resistance, and the analysis of atomic separations suggest an increased ductility after the addition of quadrivalent Hf and Zr to the alumina grain boundaries.

The maximum theoretical energy efficiency of fuel cells is much larger than those of the steam-power-turbine cycles that are currently used for generating electrical power. Similarly, direct hydrocarbon fuel cells, DHFCs, can theoretically be much more efficient than hydrogen fuel cells. Unfortunately the current densities (overall reaction rates) of DHFCs are substantially smaller than those of hydrogen fuel cells. The problem is that the exchange current density (catalytic reaction rate) is orders of magnitude smaller for DHFCs. Other work at the University of Ottawa has been directed toward the development of polymer electrolytes for DHFCs that operate above the boiling point of water, making corrosion rates much slower so that precious metal catalysts are not required. Propane (liquefied petroleum gas, LPG) was the hydrocarbon chosen for this research partly because infrastructure for its transportation and storage in rural areas already exists. In this work nickel based catalysts, an inexpensive replacement for the platinum based catalysts used in conventional fuel cells, were examined using density functional theory, DFT. The heats of propane adsorption for 3d metals, when plotted as a function of the number of 3d electrons in the metal atom, had the shape of a volcano plot, with the value for nickel being the peak value of the volcano plot. Also the C-H bond of the central carbon atom was longer for propane adsorbed on nickel than when adsorbed on any of the other metals, suggesting that the species adsorbed on nickel was less likely to desorb than those on other metals. The selectivity of the propyl radical reaction was examined. It was found that propyl radicals

By using two electrospray emitters containing different solutions (“dual-spray”) we have recently conducted in-source hydrogen/deuterium exchange (HDX) reactions and synthesized organometallic species. For dual-spray HDX reactions, peptide and protein solutions were electrosprayed through one emitter and the deuterating agent D2O through the secondary electrospray emitter. Clear shifts in isotope distributions indicated hydrogen-deuterium exchange occurring within the ion source. By ion mobility, simultaneous deuterium exchange for two isobaric species, the oxytocin monomer and dimer, was observed. Lysozyme has a linear relation between the charge state and the average number of exchanges, indicating that lysozyme becomes increasingly unfolded as the charge state increases. Based on deuterium uptake data and the lack of a temperature dependence, the dual-spray HDX reaction is thought to occur mostly in the gas phase. Tris(2,2’-bipyridine)ruthenium(II) and similar complexes containing the 1,10-phenanthroline ligand were formed by spraying a ligand solution and the ruthenium trichloride solution through two independent ESI emitters. This was confirmed by comparing ion mobility drift time, mass spectra, and CID fragmentation with the reference standard compounds. Tris(2,2’-bipyridine)iron(II), tris(1,10-phenantroline)iron(II) and mixed ligand complexes of iron(II) formed by dual-spray showed two additional hydrogens bonded to the complex. By CID, these unique gas phase complexes showed similar initial ligand loss to the reference standards however the secondary ligand loss showed dissimilar dissociation channels and energetics. Using DFT calculations, geometry optimizations for the [Fe(phen)3 + 2H]2+ complex and its fragment ions were done. After the initial ligand loss, the additional hydrogens are believed to transfer to the central iron atom. The relative energy of the dissociation channels showed good agreement with experimental breakdown curves.

Fuel cells represent a promising technology for clean power generation because they convert chemical energy (fuel) into electrical energy with high efficiency and low-to-none emission of pollutants. Direct ethanol fuel cells (DEFCs) have several advantages compared to the most studied hydrogen and methanol fuel cells. First and foremost, ethanol is a non-toxic liquid, which lowers the investment of handling facilities because the current infrastructure for gasoline can be largely used. Second, ethanol can be conveniently produced from biomass, hence is carbon neutral which mitigates increasing atmospheric CO2. Last but not least, if completely oxidized to CO2, ethanol has a higher energy density than methanol since it can deliver 12 electrons per molecule. The almost exclusive oxidation to acetic acid overshadows the attractiveness of DEFCs considerably, as the energy density is divided by 3. The standard potential of acetic acid formation indicates that a reaction path including acetic acid, leads to inevitable potential losses of about 0.4 V (difference between ideal potential for CO2 and acetic acid "production"). The development of alkaline DEFCs had also been hampered by the lack of stable and efficient anion exchange membranes. Fortunately, this challenge has been well tackled in recent years,8,9 making the development of alkaline fuel cells (AFCs) which are of particular technological interest due to their simple designs and ability to operate at low temperatures (25-100 °C). In alkaline conditions, the kinetic of both the cathodic oxygen reduction and the anodic ethanol oxidation is facilitated. Furthermore, the expensive Pt catalyst can be replaced by the lower-cost and more active transition metals such as Pd. The main objectives of this project are: i) to provide detailed fundamental understanding of ethanol oxidation reaction on transition metal surfaces in alkaline media, ii) to propose the best rational catalyst design strategies to cleave the C–C bond during ethanol electrooxidation. To achieve these goals two methodologies are used, i.e., in-situ identification of ethanol electrooxidation products using polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and mechanistic investigation using computational studies in the framework of density functional theory (DFT). The PM-IRRAS technique was advanced in this project to the level of distinguishing electrooxidation products at the surface of the nanoparticles (electrode) and in the bulk-phase of the electrolyte. This new PM-IRRAS utility makes it possible to detect molecules such as CO2 which desorbs from the catalyst surface as soon as they are formed. The DFT insights in this project, provides an explanation as to why it is difficult to break the C–C bond in ethanol and is used for screening the top candidate metals for further studies.

The focus of this thesis is on C-O bonds activation by transition metal atoms. Lignin is a potential alternative energy resource, but currently is an underused biomass species because of its highly branched structure. To aid in better understanding this species, the oxidative cleavage of the Cβ-O bond in an archetypal arylglycerol β-aryl ether (β–O–4 Linkage) model compound of lignin with late 3d, 4d, and 5d metals was investigated. Methoxyethane was utilized as a model molecule to study the activation of the C-O bond. Binding enthalpies (ΔHb), enthalpy formations (ΔH) and activation enthalpies (ΔH‡) have been studied at 298K to learn the energetic properties in the C-O bond cleavage in methoxyethane. Density functional theory (DFT) has become a common choice for the transition metal containing systems. It is important to select suitable functionals for the target reactions, especially for systems with degeneracies that lead to static correlation effects. A set of 26 density functionals including eight GGA, six meta-GGA, six hybrid-GGA, and six hybrid-meta-GGA were applied in order to investigate the performance of different types of density functionals for transition metal catalyzed C-O bond cleavage. A CR-CCSD(T)/aug-cc-pVTZ was used to calibrate the performance of different density functionals.

abstract: Losses in commercial microwave dielectrics arise from spin excitations in paramagnetic transition metal dopants, at least at reduced temperatures. The magnitude of the loss tangent can be altered by orders of magnitude through the application of an external magnetic field. The goal of this thesis is to produce “smart” dielectrics that can be switched “on” or “off” at small magnetic fields while investigating the influence of transition metal dopants on the dielectric, magnetic, and structural properties. A proof of principle demonstration of a resonator that can switch from a high-Q “on state” to a low-Q “off state” at reduced temperatures is demonstrated in (Al1-xFex)2O3 and La(Al1-xFex)O3. The Fe3+ ions are in a high spin state (S=5/2) and undergo electron paramagnetic resonance absorption transitions that increase the microwave loss of the system. Transitions occur between mJ states with a corresponding change in the angular momentum, J, by ±ħ (i.e., ΔmJ=±1) at small magnetic fields. The paramagnetic ions also have an influence on the dielectric and magnetic properties, which I explore in these systems along with another low loss complex perovskite material, Ca[(Al1-xFex)1/2Nb1/2]O3. I describe what constitutes an optimal microwave loss switchable material induced from EPR transitions and the mechanisms associated with the key properties. As a first step to modeling the properties of high-performance microwave host lattices and ultimately their performance at microwave frequencies, a first-principles approach is used to determine the structural phase stability of various complex perovskites with a range of tolerance factors at 0 K and finite temperatures. By understanding the correct structural phases of these complex perovskites, the temperature coefficient of resonant frequency can be better predicted. A strong understanding of these parameters is expected to open the possibility to produce new types of high-performance switchable filters, time domain MIMO’s, multiplexers, and demultiplexers. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2020

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第21571号 / 理博第4478号 / 新制||理||1642(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)准教授 菅沼 秀夫, 教授 永江 知文, 教授 田中 貴浩 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

In the pursuit of enhancing the electronic properties of transparent p-type semiconductors, this work uses density functional theory to study the effects of doping tin monoxide with nitrogen, antimony, yttrium and lanthanum. An overview of the theoretical concepts and a detailed description of the methods employed are given, including a discussion about the correction scheme for charged defects proposed by Freysoldt and others [Freysoldt 2009]. Analysis of the formation energies of the defects points out that nitrogen substitutes an oxygen atom and does not provide charge carriers. On the other hand, antimony, yttrium, and lanthanum substitute a tin atom and donate n-type carriers. Study of the band structure and density of states indicates that yttrium and lanthanum improves the hole mobility. Present results are in good agreement with available experimental works and help to improve the understanding on how to engineer transparent p-type materials with higher hole mobilities.

Gaseous pollution has become a global issue and its presence above certain limits is hazardous to human health and environment. Detection of such gases is an immediate need and researchers around the world are trying to solve this problem. Metal oxides are being used as sensing materials for a long time, but a high operating temperature limits applications in many areas. On the other hand, two-dimensional (2D) materials with high surface-to-volume ratio and chemical stability are promising candidates in the field of gas sensing. This includes monolayer transition metal dichalcogenides, such as MoS2 and WS2, which are direct band gap materials. While few layer transition metal dichalcogenides are indirect band gap materials, they are easier to synthesize than monolayers. Therefore, it is important to understand whether few layer transition metal dichalcogenides possess the same sensing behavior as the corresponding monolayers. For this reason the first part of this dissertation compares the sensing behavior of monolayer and few layer MoS2 and WS2. Two dimensional hexagonal boron nitride is a highly stable structural analogue of graphene. However, its insulating behavior with large band gap is not suitable for sensing. Recently, monolayer Si2BN has been proposed to exist. As the presence of Si makes this material reactive, the second part of this dissertation addresses its application as sensing material. In the _nal part of this dissertation, in search of a metal free, non-toxic, and earth abundant sensor material, further structural analogues of graphene are considered, namely monolayer C3N, monolayer C3Si, and monolayer C6BN. In particular, different theoretical approaches for studying the sensing performance of materials are compared to each other.

Functional materials are essential to human welfare and to provide foundations for emerging industries. As an alternative route to experimental materials discovery, computational materials designs are playing an increasingly significant role in the whole discovery process. In this work, we use an in-house developed python utility: PAOFLOW, which generates finite basis Hamiltonians from the projection of first principles plane-wave pseudopotential wavefunctions on pseudo atomic orbitals(PAO) for post-process calculation on various properties such as the band structures, density of states, complex dielectric constants, diffusive and anomalous spin and charge transport coefficients. In particular, we calculated the dielectric function of Sr-, Pb-, and Bi-substituted BaSnO3 over wide concentration ranges. Together with some high-throughput experimental study, our result indicates the importance of considering the mixed-valence nature and clustering effects upon substitution of BaSnO3 with Pb and Bi. We also studied two prototype ferroelectric rashba semiconductors, GeTe and SnTe, and found the spin Hall conductivity(SHC) can be large either in ferroelectric or paraelectric structure phase. Upon doping, the polar displacements in GeTe can be sustained up to a critical hole concentration while the tiny distortions in SnTe vanish at a minimal level of doping. Moreover, we investigated the sensitivity of two dimensional group-IV monochalcogenides to external strain and doping, which reveal for the first time giant intrinsic SHC in these materials, providing a new route for the design of highly tunable spintronics devices based on two-dimensional materials.