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Luo, Kun. "Cation ordered and anion-vacancy ordered perovskite materials." Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:f36a3f97-70b1-4ab6-819b-d400341a4558.
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The investigation in this thesis focuses on the synthesis of cation-ordered perovskite phases by introducing anion vacancies into the structure. Complex cation-ordered phases Ba2YMO5 and Ba3YM2O7.5 (M = Fe, Co) have been synthesized using ceramic or citrate gel methods under flowing argon. Close inspection reveals that the structures are constructed from Y2M2O102 basic units which consist of two YO6 octahedra and two MO4 tetrahedra in a rock-salt type arrangement. In the structure of Ba2YMO5 (M = Fe, Co), the neighbouring Y2M2O102 units are connected with an equivalent one in the yz-plane with YO6 octahedra sharing an apex. In the structure of Ba3YM2O7.5 (M = Fe, Co), the basic units are connected to each other by the M2O7 dimers via a chain of Y – O – M – O – M – O – Y bonds. Complex cation ordering can be achieved by carefully controlling the anion vacancies and selecting the cations with different ionic radii. The anion vacancies present in Ba2YMO5 (M = Fe, Co) (space group P21/n) allow the intercalation of anions like O2- and F- into the lattice. The fluorination of Ba2YCoO5 leads to the formation of a new orthorhombic phase Ba2YCoO5F0.42 (space group Pbnm) in which the inserted fluoride ions are distributed in a disordered manner. In contrast, the topochemical oxidation of Ba2YFeO5 leads to the formation of a new orthorhombic phase Ba2YFeO5.5 (space group Pb21m), in which Fe4+ centres are located in 4-coordinate tetrahedral sites and 5-coordinate pyramidal sites, respectively. The polar structure of Ba2YFeO5.5 is confirmed by the observation of second-harmonic generation activity and pyroelectric behaviour. Ba2YFeO5.5 also exhibits a combination of ferromagnetic and antiferromagnetic behaviours at low temperature. LaCa2Fe2GaO8 adopts a six-layer structure consisting of an OOTLOOTR stacking sequence of layers of (Fe/Ga)O6 octahedra (O) and (Fe/Ga)O4 tetrahedra (T), related to that of the four-layer brownmillerite structure (space group Pbma). The chains of tetrahedra in the structure of LaCa2Fe2GaO8 exhibit a cooperative twisting distortion in which the twisting direction of the chains of tetrahedra alternates in adjacent tetrahedral layers. LaxSr2-xCoGaO5+δ (0.5 < x < 1) adopts brownmillerite structures which consist of octahedral and tetrahedral layers with mixed valence of Co2+/Co3+. The members with x = 0.5, 0.6 and 0.7 adopt structures with I2mb space group symmetry, in which all the tetrahedra twist in the same direction. The members with x = 0.8, 0.9 and 1.0 adopt structures with Imma space group symmetry, in which the chains of the tetrahedra twist in a disordered manner. A change in the Co3+ spin state from high spin (HS) to low spin (LS) is observed as the La/Sr ratio increases. The change of the Co3+ spin state can be rationalized on the basis of internal chemical pressure.
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THANNEERU, RANJITH. "VACANCY ENGINEERED DOPED AND UNDOPED NANOCRYSTALLINE RARE EARTH OXIDE PARTICLES FOR HIGH TEMPERATURE OXIDATION RESISTANT COATING." Master's thesis, University of Central Florida, 2007. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/3986.
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Rare earth oxides with trivalent lattice dopants have been of great interest to researchers in the recent years due to its potential applications in catalysis and high temperature protective coatings. The ability to store oxygen in rare earths is the basis for catalysis because of the ability to change valence states which causes the presence of intrinsic oxygen vacancies in the crystal lattice. Although, several doped-rare earth oxide systems in micron scale have been investigated, the doping effect in cerium oxide nanoparticles with well characterized particle size has not been studied. The doping of ceria at that small size can be very beneficial to further improve its catalytic properties and alter the high temperature phases in alloy systems. Cost effective room temperature chemical methods are used in the current work to synthesize uniformly distributed undoped and doped (dopants: La, Nd, Sm, Gd, Y and Yb) rare earth oxide nanoparticles. In the present study, the variation of the properties in nanocrystalline ceria (NC) synthesized by microemulsion method is studied as a function of dopant size and its concentration. To further understand, the role of dopant (cation) size on the oxygen vacancy concentration, doped nanocrystalline oxide powders were analyzed by Raman Spectroscopy, X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS). XRD studies showed that lattice parameter change in nanocrystalline oxide by doping trivalent rare earth elements is largely depending on size of trivalent ions. It showed that by doping larger cations (Gd3+ and Y3+) compare to Ce3+ causes lattice expansion where as smaller cations (Yb3+) leads to lattice contraction. It also showed that the lattice expansion or contraction is directly proportional to dopant concentration. The results of Raman Spectroscopy showed that the correlation length decreases resulting in increase in oxygen vacancies for larger trivalent dopants (Sm3+, Gd3+ and Y3+). However, the correlation length increases resulting in decrease in oxygen vacancies for smaller trivalent dopants (Yb3+) compare to nanocrystalline ceria. These nanostructured oxides are further applied to develop high temperature oxidation resistance coatings for austenitic steels. The present study investigates the role of oxygen vacancies in the performance of high temperature oxidation resistance as a function of various trivalent dopants and dopant concentration. NC and La3+ doped nanocrystalline ceria (LDN) particles were coated on AISI 304 stainless steels (SS) and exposed to 1243K in dry air for longer duration and subjected to cycling. The results are further compared with that of micro-ceria (MC) coatings. The coated samples showed 90% improvement in oxidation resistance compared to uncoated and MC coated steels as seen from the SEM cross-sectional studies. XRD analysis showed the presence of chromia in both NC and 20 LDN samples which is absent in uncoated steels. From SIMS depth profiles, Fe, Ni depletion zones are observed in presence of LDN coated sample indicating diffusion through the oxide layer. The role of oxygen vacancies in the nanoceria coatings on the early formation of protective chromia layer is discussed and compared to its micron counterpart. This study helps in understanding the role of oxygen vacancies to protect austenitic stainless steel at high temperature and confirms the oxygen inward diffusion rather cation outward diffusion in rare earth oxide coatings. It also gives an idea to identify the type of dopant and its concentration in nanocrystalline cerium oxide which supplies the critical oxygen partial pressure required at high temperature to form primarily impervious chromia layer.M.S.M.S.E.
Department of Mechanical, Materials and Aerospace Engineering;
Engineering and Computer Science
Materials Science & Engr MSMSE
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Iwata, Tatsuya. "Study on Resistive Switching Phenomenon in Metal Oxides for Nonvolatile Memory." 京都大学 (Kyoto University), 2014. http://hdl.handle.net/2433/188598.
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Nishi, Yusuke. "Nonpolar Resistive Switching Based on Quantized Conductance in Transition Metal Oxides." Kyoto University, 2019. http://hdl.handle.net/2433/242544.
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Maiti, Debtanu. "Defect Laden Metal Oxides and Oxynitrides for Sustainable Low Temperature Carbon Dioxide Conversion to Fuel Feedstocks." Scholar Commons, 2018. https://scholarcommons.usf.edu/etd/7694.
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The current energy and environmental scenario in the world demands acute attention on sustainable repurposing of waste CO2 to high value hydrocarbons that not only addresses the CO2 mitigation problem, but also provides pathways for a closed loop synthetic carbon cycle. Difference in the scales of global CO2 emissions (about 40 Gtpa, 2017) and the carbon capture and sequestration (CCS) facilities (estimated cumulative 40 Mtpa, 2018) provokes active research on this topic. Solar thermochemical (STC) and visible light photocatalysis are two of the most promising routes that have garnered attention for this purpose. While STC has the advantages of high CO2 conversion rates, it operates at high temperatures (more than 1000 °C) limiting its industrial implementation. Photocatalysis, on the contrary, is plagued by the poor quantum efficiency and conversion rates, although its exhibits the benefits of low temperature operation. Thus, any significant progress towards low temperature STC and visible light photocatalytic CO2 reduction is a giant leap towards a greener and sustainable energy solution. This dissertation is an effort towards improving both the STC and photocatalytic CO2 reduction.
Reverse water gas shift - chemical looping (RWGS-CL) is a modified STC approach that has the potential for low temperature CO2 conversion. RWGS-CL process uses mixed metal oxides like perovskite oxides (ABO3) for the conversion to CO, a potential feedstock for subsequent hydrocarbon production. Generation of oxygen vacancy defects on these perovskite oxides is a key step of RWGS-CL and thus, oxygen vacancy formation energy has been found to be a key descriptor for this process. Using density functional theory based calculations, this intrinsic material property has been used towards rational design of better catalysts. Highest rate of CO2 conversion at the low temperatures of 450 °C was demonstrated by earth abundant perovskite oxide via RWGS-CL. This low temperature and stable CO2 conversion process enables thermal integration with subsequent Fischer Tropsch processes for the hydrogenation of CO to hydrocarbons. Parallel to the developments on materials discovery, another crucial parameter that deserves attention is the surface termination effects of the perovskite oxides. Hence, the site specificity of the bulk and surface oxygen vacancies have been probed in detail towards elucidating the CO2 conversion performance over these materials. In the view of recent progress on the growth of selective crystal facets and terminations, this study opens new avenues for enhanced CO2 conversion performance not only through bulk composition variation, but also via exposing desired crystal facets.
Type-II semiconductor heterojunctions (staggered type) are promising candidates for efficient photocatalytic reactions, not only because of their capabilities of electronic density of states tuning, but also their ability to segregate the excited electrons and holes into different materials thereby restricting exciton recombination. Metal oxynitride heterojunctions have recently demonstrated promising activity on visible light water splitting. Elucidating the structure-function relationships for these materials can pave the way towards designing better CO2 conversion photocatalysts. This dissertation focuses on unravelling the roles of material composition, anion vacancy defects and lattice strain towards modulating the electronic density of states of lateral and vertical heterojunctions of (ZnO)X(AlN)1-X and (ZnO)X(GaN)1-X. The heterojunctions consist of periodic potential wells that allows for restricting interlayer charge transport. Increased ZnO concentration was explicitly shown to decrease the band gap due to N 2p and Zn-3d repulsion. Biaxial and vertical compressive strain effected increased band gap while tensile strain reduced the same. Oxygen vacancies was found to have different effect on the electronic state of the materials. When present in charged state (+2), it promotes mid gap state formation, while in neutral state it revealed increased electronic densities near the valence band and conduction band edges. These fundamental site specific material property tuning insights are essential for designing better photocatalysts for future.
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Peng, Yung-Kang. "Surface mapping of faceted metal oxides by chemical probe-assisted NMR for catalytic applications." Thesis, University of Oxford, 2017. https://ora.ox.ac.uk/objects/uuid:7b56021f-71fb-437b-8c6b-0569705ef68e.
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Semiconductive metal oxides are of great importance in environmental remediation and electronics because of their ability to generate charge carriers when excited with appropriate energy. The electronic structure, light absorption and charge transport properties have made the transition metal oxides an attractive material as photocatalyst. Recently, facet-engineering by morphology control has been intensively studied as an efficient approach to further enhance their photocatalytic performance. However, various processing steps and post-treatments used in the preparation of facet-engineered particles may generate different surface active sites which may affect their photocatalysis. Moreover, many traditional techniques (PL, EPR and XPS) used for materials characterization (oxygen vacancy, hydroxyl group, cation, etc.) are not truly surface specific but analyzing a range from surface few layers to bulk. Accordingly, they can only provide very limited information on chemical states of the surface active features and their distribution among facets, causing difficulties to unambiguously correlate facet-dependent results with activity. As a result, this often leads to different interpretations amongst researchers during the past decades. As the publications of titanium and zinc ranked top two among studies of first row of transition oxides in the past decades, this thesis will firstly review on the disagreements generated among researchers when they correlated the performance of ZnO and TiO2 with their facet activities based on traditional techniques. As there are shortcomings of these techniques in producing truly facet-dependent features, some results can be misleading and with no cross-literature comparison. To address these issues, we have developed a new technique "probe-molecule-assisted NMR" which allows a genuine differentiation of surface active sites from various facets. This surface-fingerprint technique has been demonstrated to provide both qualitative (chemical shift) and quantitative (peak intensity) information on the concentration and distribution of truly surface features among facets. In light of the new technique, this thesis will revisit the facet-dependent photocatalytic properties and shed light on these issues.
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Shojaee, Kambiz. "Fundamental aspects of ammonia oxidation on cobalt oxide catalysts." Thesis, The University of Sydney, 2014. http://hdl.handle.net/2123/13657.
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The current thesis deals with the ammonia oxidation on cobalt oxide catalyst at the molecular level. The catalytic oxidation of ammonia to NO is crucial in the industrial process of nitric acid production. Cobalt oxide catalysts are being used together with platinum gauzes to reduce the production cost and emission of greenhouse gas N2O. However, the fundamentals of ammonia oxidation on cobalt oxides are not known. This thesis aims to provide insights into our fundamental understanding of ammonia oxidation on Co3O4 surfaces. The performance of cobalt oxide catalysts in the oxidation of NH3 strongly depends upon the exposed surface terminations. Results indicate that different surfaces of Co3O4 behave markedly differently in oxidative reactions due to the difference in binding energy and O recombination energies and oxygen vacancy formation. Overall, NH3 oxidation follows stepwise dehydrogenative route (NH3* → NH2* → NH* → N*) on Co3O4 surfaces. Desorption of lattice products results in the formation of O vacancy sites opening the way for a Mars-van Krevelen mechanism. The successive dehydrogenation of ammonia preferably occurs on the surfaces exposing active lattice O sites. Removal of active lattice O sites from the Co3O4 surfaces in the form of products results in the surface reduction. If the rate of reduction is faster than that of re-oxidation, a CoO-like phase might form. The formation of CoO in Co3O4 catalysts during NH3 oxidation not only reduces the NH3 conversion but also alters the selectivity towards N2 rather than NO due to weak ability of lattice O at the CoO surface to assist the hydrogen abstraction process. A surface with a lower oxygen vacancy formation energy and a higher binding energy of hydrogen exhibits a higher activity towards ammonia oxidation to NO.
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Stokes, Stephen J. "Atomistic modelling studies of fluorite- and perovskite-based oxide materials." Thesis, University of Bath, 2010. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.527142.
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Fast oxide-ion and proton conductors are the subject of considerable research due to their technological applications in sensors, ceramic membranes and solid oxide fuel cells (SOFCs). This thesis describes the use of computer modelling techniques to study point defects, dopants and clustering effects in fluorite-and perovskitetype ion conductors with potential SOFC applications. Bi2O3 related phases are being developed with the objective of high oxide-ion conductivities at lower operating temperatures than 1000°C, as in current generation SOFC electrolytes. Doped Bi2O3 phases have shown promise as materials capable of accomplishing this goal. First, the Y-doped phase, Bi3YO6, has been investigated including the ordering of intrinsic vacancies. The defect and dopant characteristics of Bi3YO6 have been examined and show that a highly mobile oxygen sub-lattice exists in this material. A preliminary structural modelling study of a new Re-doped Bi2O3 phase was also undertaken. A comprehensive investigation of the proton-conducting perovskites BaZrO3, BaPrO3 and BaThO3 is then presented. Our results suggest that intrinsic atomic disorder in BaZrO3 and BaThO3 is unlikely, but reduction of Pr4+ in BaPrO3 is favourable. The water incorporation energy is found to be less exothermic for BaZrO3 than for BaPrO3 and BaThO3, but in all cases the results suggest that the proton concentration would decrease with increasing temperature, in accord with experimental data. The high binding energies for all the dopant-OH pair clusters in BaPrO3 and BaThO3 suggest strong proton trapping effects. Finally, a study of multiferroic BiFeO3 is presented, in which the defect, dopant and migration properties of this highly topical phase are investigated. The reduction process involving the formation of oxygen vacancies and Fe2+ is the most favourable redox process. In addition, the results suggest that oxide-ion migration is anisotropic within this system.
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Umeda, Yuji. "Rational design of dielectric oxide materials through first-principles calculations and machine-learning technique." Doctoral thesis, Kyoto University, 2020. http://hdl.handle.net/2433/245844.
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京都大学0048
新制・課程博士
博士(工学)
甲第22159号
工博第4663号
新制||工||1727(附属図書館)
京都大学大学院工学研究科材料工学専攻
(主査)教授 田中 功, 教授 中村 裕之, 教授 邑瀬 邦明
学位規則第4条第1項該当
Doctor of Philosophy (Engineering)
Kyoto University
DFAM
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Agarwal, Sahil. "Defect Studies In Metals, Alloys, and Oxides By Positron Annihilation Spectroscopy and Related Techniques." Bowling Green State University / OhioLINK, 2021. http://rave.ohiolink.edu/etdc/view?acc_num=bgsu1626713209028374.
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Kang, Yuhong. "Mechanisms, Conditions and Applications of Filament Formation and Rupture in Resistive Memories." Diss., Virginia Tech, 2015. http://hdl.handle.net/10919/77593.
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Resistive random access memory (RRAM), based on a two-terminal resistive switching device with a switching element sandwiched between two electrodes, has been an attractive candidate to replace flash memory owing to its simple structure, excellent scaling potential, low power consumption, high switching speed, and good retention and endurance properties. However, due to the current limited understanding of the device mechanism, RRAMs research are still facing several issues and challenges including instability of operation parameters, the relatively high reset current, the limited retention and the unsatisfactory endurance.
In this study, we investigated the switching mechanisms, conditions and applications of oxygen vacancy (Vo) filament formation in resistive memories. By studying the behavior of conductive Vo nanofilaments in several metal/oxide/metal resistive devices of various thicknesses of oxides, a resulting model supported by the data postulates that there are two distinct modes of creating oxygen vacancies: i) a conventional bulk mode creation, and ii) surface mode of creating oxygen vacancies at the active metal-dielectric interface. A further investigation of conduction mechanism for the Vo CF only based memories is conducted through insertion of a thin layer of titanium into a Pt/ Ta2O5/Pt structure to form a Pt/Ti/ Ta2O5/Pt device. A space charge limited (SCL) conduction model is used to explain the experimental data regarding SET process at low voltage ranges. The evidence for existence of composite copper/oxygen vacancy nanofilaments is presented. The innovative use of hybrid Vo/Cu nanofilament will potentially overcome high forming voltage and gas accumulation issues. A resistive floating electrode device (RFED) is designed to allow the generation of current/voltage pulses that can be controlled by three independent technology parameters. Our recent research has demonstrated that in a Cu/TaOx/Pt resistive device multiple Cu conductive nanofilaments can be formed and ruptured successively. Near the end of the study, quantized and partial quantized conductance is observed at room temperature in metal-insulator-metal structures with graphene submicron-sized nanoplatelets embedded in a 3-hexylthiophene (P3HT) polymer layer. As an organic memory, the device exhibits reliable memory operation with an ON/OFF ratio of more than 10.Ph. D.
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Ali, Fawad. "Investigation of metal oxides thin films developed by PVD system for perovskite solar cells." Thesis, Queensland University of Technology, 2019. https://eprints.qut.edu.au/127139/1/Fawad_Ali_Thesis.pdf.
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This research presents thin film deposition and characterization of metal oxides using industrially viable Physical Vapour Deposition (PVD) techniques. The research examines low temperature processed electron and hole transport metal oxides for high performance and stable perovskite solar cells. The physical, chemical, optical and electronic properties of the films were investigated and their device performance has been evaluated. The performance of the device improved and the materials cost reduced by replacing the expansive organic materials with more stable inorganic metal oxides.
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Girdauskaite, Egle. "Thermodynamische und kinetische Untersuchungen zum Sauerstoffaustausch in perowskitischen Mischoxiden auf Basis von Ferriten und Cobaltiten." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2007. http://nbn-resolving.de/urn:nbn:de:swb:14-1195658113234-25483.
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Oxidkeramische Materialien sind zunehmend von praktischem Interesse für neue Technologien, die in Brennstoffzellen, Sensoren und Ionentransport-Membranen Anwendung finden. Einige dieser Oxide mit Perowskitstruktur ABO3 zeigen hohe Ionen- und Elektronenleitung, ausreichende chemische Stabilität sowie thermisch-mechanische Eigenschaften, wie sie für die Anwendung als Sauerstofftransportmembran benötigt werden. Oxidionentransport erfolgt über einen Oxidionen-Leerstellenmechanismus. Die charakteristische Schwierigkeit für die Anwendung solcher Materialien besteht aber darin, dass die gestellten Forderungen wie hoher Ionentransport und hohe Stabilität sich diametral gegenüberstehen. In dieser Arbeit wurde eine systematische Untersuchung der Beziehungen zwischen Zusammensetzung, Struktur und Stöchiometrie der ferritischer und cobaltitischer Mischoxide und den Transporteigenschaften sowie der thermischen Ausdehnung durchgeführt. Erstmalig wurden thermodynamische und kinetische Parameter von Reihen von Oxiden in einem weiten Bereich von Temperatur und Sauerstoffpartialdruck systematisch bestimmt. Aus den Ergebnissen konnten Empfehlungen gegeben werden für die Zusammensetzung von Perowskitoxiden, die zum Aufbau von Sauerstofftransportmembranen unter bestimmten pO2/T-Bedingungen geeignet sind.
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He, Chenwei. "Experimental study of the interaction of vacancy defects with Y, O and Ti solutes to better understand their roles in the nanoparticles formation in ODS steels." Thesis, Orléans, 2014. http://www.theses.fr/2014ORLE2057/document.
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Les conditions sévères de fonctionnement des réacteurs du futur, Génération-IV, -haute température et fortes irradiations-nécessitent le développement de matériaux adaptés. Les aciers ODS (Oxide Dispersion Strengthened) sont des candidats très compétitifs pour le gainage du combustible en raison de leurs excellentes propriétés de gonflement et de fluage. Ces atouts majeurs sont induits par la fine dispersion de nanoparticules d’oxydes (Y, O, Ti) obtenue par co-broyage de poudres de la matrice et d’oxyde dont les conditions sont à optimiser pour maitriser la distribution en taille et composition de ces nanoparticules. Dans l’objectif de mieux comprendre le mécanisme de formation de ces nanoparticules à l’échelle atomique, la présente thèse met à profit l’utilisation de la spectroscopie d’annihilation de positons (PAS) et de la spectrométrie de masse d’ions secondaires pour étudier l’interaction des défauts lacunaires avec des solutés Y, O et Ti et évaluer leur rôle dans la formation des nanoparticules. Les irradiations avec des ions He ont été effectuées pour révéler les propriétés des défauts lacunaires et les implantations d’Y, Ti, O ont été réalisées pour étudier les interactions de ces éléments Y, Ti, O avec les lacunes dans la matrice de fer. La distribution des défauts en profondeur indique la présence de défauts lacunaires avec une taille plus petite dans la région où la concentration d’Y, Ti, O est la plus élevée. Cet effet est plus prononcé pour O, Y et Ti respectivement. Il est expliqué par la formation de V-X (X=O, Y, Ti) complexes qui réduisent la probabilité de la mobilité et d’agglomération des défauts lacunaires. Les recuits des échantillons implantés Y et O révèlent que des complexes O-lacune sont mobiles à température ambiante, et que l’yttrium ne diffuse pas jusqu’à 550°C alors que des complexes Y-lacunes sont encore détectés comme cela est attendu par des résultats théoriques. Un modèle des premières étapes de la nucléation des nanoparticules est proposé en utilisant les résultats obtenus dans cette thèseThe severe operating conditions of the future nuclear reactor, Generation-IV, -high temperature and high irradiation damage-, require the adapted materials development. Oxide-dispersion strengthened (ODS) alloy is one of the most potential candidates expected to be used for fuel cladding material because of their outstanding swelling and creep properties. Their excellent properties are induced by the fine dispersion of oxide nanoparticles (Y, O, Ti), obtained by mechanical alloying of steel and oxide powders and which has to be better mastered. But the atomic scale clustering mechanism of these nanoparticles is not yet cleared. In this context, the present thesis using positron annihilation spectroscopy (PAS) and secondary ion mass spectrometry (SIMS) sheds light on the interaction of vacancy defects with Y, O and Ti solutes to better understand their roles in the nanoparticles formation. The He irradiations have been performed to reveal the vacancy defects properties and Y, Ti, O implantations realized to study the Y, Ti, O-vacancy interactions in bcc Fe matrix. In all cases, the defects depth distribution shows a lower size of vacancy defects in the region where the concentration of the incident ions Y, Ti and O is the highest. This effect of the ions on the damage formation is more pronounced for respectively O, Y and Ti. It is explained by the formation of V-X (X=O, Y, Ti) complexes which reduce the mobility and agglomeration probability of the vacancy defects. The annealing of the Y and O implanted samples reveals that some O-vacancy complexes are mobile at room temperature and Y doesn’t diffuse up to 550°C whilst Y-vacancy complexes remain as it is expected from theory. A model of the first steps of the ODS nanoparticles nucleation is proposed by using the results obtained in this thesis
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Yousefian, Pedram. "Pore Formation in Aluminum Castings: Theoretical Calculations and the Extrinsic Effect of Entrained Surface Oxide Films." UNF Digital Commons, 2017. https://digitalcommons.unf.edu/etd/761.
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Aluminum alloy castings are being integrated increasingly into automotive and aerospace assemblies due to their extraordinary properties, especially high strength-to-density ratio. To produce high quality castings, it is necessary to understand the mechanisms of the formation of defects, specifically pores and inclusion, in aluminum. There have been numerous studies on pore formation during solidification which lead to hot tearing and/or reduction in mechanical properties. However, a comprehensive study that correlates pore formation theory with in situ observations and modeling assumptions from the literature as well as experimental observations in not available. The present study is motivated to fill this gap.
An in-depth discussion of pore formation is presented in this study by first reinterpreting in situ observations reported in the literature as well as assumptions commonly made to model pore formation in aluminum castings. The physics of pore formation is reviewed through theoretical fracture pressure calculations based on classical nucleation theory (i) for homogeneous and heterogeneous nucleation, and (ii) with and without dissolved gas, i.e., hydrogen. Based on the fracture pressure for aluminum, critical pore size and corresponding probability of vacancies clustering to form the critical-size pore have been calculated by using thermodynamic data reported in the literature. Calculations show that it is impossible for a pore to nucleate either homogeneously or heterogeneously in aluminum, even with dissolved hydrogen. The formation of pores in aluminum castings can only be explained by inflation of entrained surface oxide films entrained during prior damage to liquid aluminum (bifilms) under reduced pressure and/or with dissolved gas, which involves only growth, avoiding any nucleation problem. This mechanism is consistent with reinterpretations of in situ observations as well as assumptions made in the literature to model pore formation.
To determine whether damage to liquid aluminum by entrainment of surface oxides can be observed and measured, Reduced Pressure Tests (RPT) have been conducted by using high quality, continuously cast A356.0 aluminum alloys ingots. Analyses of RPT samples via micro-computer tomography (μ-CT) scanning have demonstrated that number of pores and volume fraction of pore in aluminum casting increased by raising the pouring height (i.e., velocity of the liquid). Moreover, pore size distributions were observed to be lognormal, consistent with the literature.
Cross-sections of RPT samples have been investigated via scanning electron microscopy. In all cases, the presence of oxygen was detected inside, around and between the pores. The existence of oxide films inside all pores indicates that oxide films act as initiation sites for pores and hydrogen only assist to growth of pores. For the first time, the pore formation is reconciled with physical metallurgy principles, supported by observations of oxide films in aluminum castings. Results clearly indicate that pores are extrinsic defects and can be eliminated by careful design of the entire melting and casting process.
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Li, Hongfei. "Density functional simulations of defect behavior in oxides for applications in MOSFET and resistive memory." Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/274924.
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Defects in the functional oxides play an important role in electronic devices like metal oxide semiconductor field effect transistors (MOSFETs) and resistive random-access memories (ReRAMs). The continuous scaling of CMOS has brought the Si MOSFET to its physical technology limit and the replacement of Si channel with Ge channel is required. However, the performance of Ge MOSFETs suffers from Ge/oxide interface quality and reliability problems, which originates from the charge traps and defect states in the oxide or at the Ge/oxide interface. The sub-oxide layers composed of GeII states at the Ge/GeO2 interface seems unavoidable with normal passivation methods like hydrogen treatment, which has poor electrical properties and is related to the reliability problem. On the other hand, ReRAM works by formation and rupture of O vacancy conducting filaments, while how this process happens in atomic scale remains unclear. In this thesis, density functional theory is applied to investigate the defect behaviours in oxides to address existing issues in these electronic devices. In chapter 3, the amorphous atomic structure of doped GeO2 and Ge/GeO2 interface networks are investigated to explain the improved MOSFET reliability observed in experiments. The reliability improvement has been attributed to the passivation of valence alternation pair (VAP) type O deficiency defects by doped rare earth metals. In chapter 4, the oxidation mechanism of GeO2 is investigated by transition state simulation of the intrinsic defect diffusion in the network. It is proposed that GeO2 is oxidized from the Ge substrate through lattice O interstitial diffusion, which is different from SiO2 which is oxidized by O2 molecule diffusion. This new mechanism fully explains the strange isotope tracer experimental results in the literature. In chapter 5, the Fermi level pinning effect is explored for metal semiconductor electrical contacts in Ge MOSFETs. It is found that germanides show much weaker Fermi level pinning than normal metal on top of Ge, which is well explained by the interfacial dangling bond states. These results are important to tune Schottky barrier heights (SBHs) for n-type contacts on Ge for use on Ge high mobility substrates in future CMOS devices. In chapter 6, we investigate the surface and subsurface O vacancy defects in three kinds of stable TiO2 surfaces. The low formation energy under O poor conditions and the +2 charge state being the most stable O vacancy are beneficial to the formation and rupture of conducting filament in ReRAM, which makes TiO2 a good candidate for ReRAM materials. In chapter 7, we investigate hydrogen behaviour in amorphous ZnO. It is found that hydrogen exists as hydrogen pairs trapped at oxygen vacancies and forms Zn-H bonds. This is different from that in c-ZnO, where H acts as shallow donors. The O vacancy/2H complex defect has got defect states in the lower gap region, which is proposed to be the origin of the negative bias light induced stress instability.
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SARTORETTI, ENRICO. "Doped ceria nanostructures for the oxidation of pollutants: investigations into the role of defect sites." Doctoral thesis, Politecnico di Torino, 2021. http://hdl.handle.net/11583/2910076.
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Guttman, Jeremy. "Polymer-based Tunnel Diodes Fabricated using Ultra-thin, ALD Deposited, Interfacial Films." The Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1469125487.
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Vitoux, Laura. "Etude électrochimique et structurale du système NaxMoO2." Thesis, Bordeaux, 2016. http://www.theses.fr/2016BORD0362/document.
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Ce travail de thèse présente l’étude du diagramme de phase des oxydes lamellaires NaxMoO2dans le cadre de la recherche de nouveaux matériaux. L’identification des transitionsstructurales au cours de l’intercalation et désintercalation électrochimique du sodium dans lesdomaines de composition ½ ≤ x ≤ 1 et ¼ < x ≤ ½ a été faite par électrochimie combinée à ladiffraction des rayons X in situ. Il a été montré que le profil très accidenté de la courbegalvanostatique résulte de multiples réarrangements structuraux au cours du cyclage.Notamment l’existence de nombreuses phases NaxMoO2 particulières a été mise en évidence,pour lesquelles des mises en ordre des ions sodium et des atomes de molybdène sont attendues.Des composés Na~1/2MoO2, Na~2/3MoO2 et NaMoO2 ont été synthétisés ex situ par voieélectrochimique ou chimique et leur caractérisation révèle des arrangements structurauxcomplexes, tel que des chaînes de clusters de molybdène dans les feuillets [MoO2] de NaMoO2This work concerns the investigation of the phase diagram of sodium layered oxides NaxMoO2in the search of new materials. Structural transitions upon sodium electrochemical(de)intercalation were studied by electrochemistry combined with in situ X-ray diffraction forcompositions ½ ≤ x ≤ 1 et ¼ < x ≤ ½. It was shown that the very undulating aspect of theelectrochemical curve results from multiple structural rerarrangements upon cycling. Especiallynumerous NaxMoO2 specific phases have been evidenced, for which sodium/vacancy orderingsas well as the formation of Mo-Mo bonds are expected. Na~1/2MoO2, Na~2/3MoO2 et NaMoO2compounds have been (electro)chemically synthesized and their structural characterizationreveals complex structures, such as chains of diamond-like molybdenum clusters in NaMoO2
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Er-Rakho, Lahcen. "Oxydes de cuivre a valence mixte : perovskites deficitaires en oxygene." Caen, 1987. http://www.theses.fr/1987CAEN2036.
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Mise en evidence et etude de deux grandes familles de composes dans le systeme la::(2)o::(3)-ao-cuo(a=ca,ba,sr) : la::(2-x)ba::(1-x)cu::(1-x/2)o::(5-x)(ln=la,nd) ou cu est essentiellement au degre d'oxydation 2 et une seconde famille caracterisee par la valence mixte du cuivre, les quantites de cuivre 3 pouvant atteindre dans certains cas 40%. Tous ces oxydes ont en commum leur appartenance a la structure perovskite. Proprietes electriques et magnetiques
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Karapetrova, Euguenia. "Factors influencing the crystallization, phase and oxygen vacancy concentration in zirconia." Thesis, 1997. http://hdl.handle.net/1957/34055.
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In order to achieve a better understanding of the processes that occur during
formation and sintering of zirconia, various chemical and physical techniques were
used. Along with Perturbation Angular Correlation spectroscopy, that allowed us to
investigate microscopic properties inside the nanometer-size zirconia grains, such
techniques as Scanning Electron Microscopy and X-ray diffraction were used for
determining the size of particles before and after sintering, and Neutron Activation
Analysis was employed for measuring the impurity levels in zirconia powders.
By controlling the initial conditions and heat treatment of the powders, we
investigated the dependence of formation of the charged defects on the existing
molecular structure and morphology of zirconia particles.
During the study, it was discovered that at low temperature the PAC frequencies of
tetragonal zirconia behave very similarly for all materials that were used in this study.
If stabilization is achieved by heavy doping, there are shifts and line-broadening due to
the presence of dopants but no obvious differences in the essential physics. One
material included in this group is Nb-doped zirconia that has no oxygen vacancies. It
was concluded that there are no detectable oxygen vacancies in our pure or lightly
doped tetragonal zirconia powders before they are heated into the temperature region
where sintering occurs.
Vacancies are incorporated as the samples are heated above 1050��C, the
temperature at which sintering becomes important. The oxygen vacancies in samples
that have been heated to 1200��C remain when cooled. We see no vacancy
concentration dependence on the atmosphere for samples not doped with +5 valent
elements in order to reduce the vacancy density at 1200��C. In several instances,
samples that had been heated to a maximum temperature of 1050��C or 1100��C
contained a vacancy density that was small (<100 ppm) but measurable. A reduced
oxygen pressure increased the oxygen vacancy density by a measurable amount in
these samples. Samples that are tetragonal at 800��C are well-sintered after being
heated to 1200��C. Samples that are monoclinic below 1170��C are very poorly sintered
at 1200��C and contain few vacancies. Flowing Cl in the system as the samples are
sintering retards the densification of the grains. These samples had the smallest density
of oxygen vacancies.Graduation date: 1998
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Lin, Yu-shiang, and 林裕翔. "The oxygen vacancy and spin polarization of cuprous oxide nanoparticle." Thesis, 2007. http://ndltd.ncl.edu.tw/handle/50981798243992667228.
Full textAbstract:
碩士國立中央大學
物理研究所
95
In studying the magnetic properties of cuprous oxide nanoparticle. We got a group of M-H curve measurements of cuprous oxide nanoparticle and discovered hysteresis at 1.8 K and 5 K. Then we pressed the cuprous oxide sample to control the interparticle separation, and dicussed the effect of interparticle interaction. The range of compacting density (CD) of particles is from 3 % to 77 % . There are two magnetic components in cuprous oxide nanoparticle system. We infer that one component is surface spin polarization and the other is magnetic characters of Cu2O0.9nanoparticle. The saturated magnetization (Ms) of spin polarization increases as the interparticle separation is decreased. The behavior can be taken the form of magnetic dipole-dipole interaction. After pressing the sample,we never discovered hysteresis again and diamagnetic properties was revealed.
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Huang, Cheng-Yen, and 黃丞言. "Studies of Oxygen Vacancy Defects and Magnetism in Co-Doped Cerium Oxide Nanoparticles." Thesis, 2012. http://ndltd.ncl.edu.tw/handle/45604922345335992055.
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Lee, Sangheon 1978. "First principles-based atomistic modeling of the structural properties of silicon-oxide nanomaterials." Thesis, 2010. http://hdl.handle.net/2152/ETD-UT-2010-08-1963.
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We have developed continuous random network (CRN) model based Metropolis Monte Carlo simulation tools which are capable of predicting the structural properties of amorphous semiconductor and oxide materials as well as their interface. To bolster the reliability of the CRN model, we have developed force fields based on gradient corrected density functional theory (DFT) calculations. Our in-house CRN-MMC tools have been massively parallelized, which allows us to create fairly large model structures within a reasonable computational time. Using the integrated CRN-MMC tools, we have elucidated the complex growth and structure of self-interstitial and vacancy clusters in silicon and the effect of strain on the structure and stability of the defect clusters. Our work for vacancy clusters suggests that small vacancy defects exclusively favor fourfold-coordination thermodynamically with no significant kinetic limitation rather than void-like structure formation, which has widely been adapted to explain the behavior and properties of vacancy defects. Our results also highlight the identification of stable high-symmetry fourfold-coordinated V₁₂ and V₃₂ clusters that could be expected to exist to a large extent in a vacancy rich region although its direct characterization appears impractical at present. Our work for self-interstitial clusters provides the first theoretical support for earlier experiments which suggest a shape transition from compact to elongated structures around n = 10. When the cluster size is smaller than 10, the stable I₄ and I₈ compact clusters are found to inhibit the formation of elongated defects, whereas the newly discovered fourfold-coordinated I₁₂ state is found to serve as an effective nucleation center for large extended defects. Our CRN-MMC approach also enabled us to elucidate the underlying mechanisms of synthesis and manipulation of Si rich insulators as well as the fundamental understanding of the relationship between the atomic structure and properties. We developed a valence force field based on a modified Keating model for the structure and energetics of amorphous Si rich oxide materials. In particular, our work emphasizes the importance of correctly describing the wide Si-O-Si angle distribution. Our work also suggests that the relative rigidity between Si and SiO₂ matrices is critical in determination of the Si/SiO₂ interface structure. The present potential model coupled with the CRN-MMC method can be used to create structural models (free of coordination defects) for complex a-SiO[subscript x]-based materials, which will further allow thorough studies of the properties of these materials.text
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Yu, Shen-Wei, and 俞勝為. "Evaluation for Oxygen Vacancy on Co-Ce-Y Composite Oxides and Steam Reforming of Ethanol." Thesis, 2014. http://ndltd.ncl.edu.tw/handle/78827177806809738326.
Full textAbstract:
碩士國防大學理工學院
化學工程碩士班
102
The effect of accessible oxygen on the steam reforming of ethanol (SRE) over Co3O4-CeO2 and Co3O4-CeO2-Y2O3 catalysts was investigated. Both equal molar ratio of Co3O4-CeO2 catalysts were prepared by hydrothermal co-precipitation (H) and hydrothermal ultrasonic-assisted co-precipitation (UH) methods, and finally the UH method was applied to prepare 2.5, 5, and 10 wt % of yttrium-doped Co3O4-CeO2 catalysts, respectively. All catalysts were characterized through X-ray diffraction (XRD), temperature programmed reduction/temperature programmed oxidation (TPR/TPO), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HR-TEM), elemental analysis (EA) and CO2 - temperature programmed desorption (CO2-TPD) techniques at various stages. The results indicated that the incorporation of cobalt ion into the ceria lattice could increase the dispersion of ceria, oxygen vacancies and promote the oxygen-storing and releasing capability of ceria, especially over the catalyst prepared by ultrasonic-assisted method. The accessible oxygen played an important role on the SRE reaction and resistant to carbon deposition. The Co-Ce(UH) catalyst was more active and selective, i.e., ethanol conversion achieved complete and hydrogen selectivity (SH2) approached 90% at 400 C. The high oxygen storage capacity (OSC) and high accessible oxygen for the Co-Ce(UH) catalyst allowed oxidation/gasification of deposited carbon as soon as it formed, and less coke was detected. However, cobalt ions were not easily incorporated into CeO2 lattice with less formation of oxygen vacancies when the yttrium was added, which led to decrease of activity and more formation of carbonaceous deposits.
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Jiu-XingHuang and 黃久倖. "Influence of oxygen vacancy reservoirs on resistive switching characteristics of oxide based resistance switching memories." Thesis, 2016. http://ndltd.ncl.edu.tw/handle/33367771155989874222.
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Roh, Bumwook Macdonald Digby D. "Defect properties of anodic oxide films on titanium and impact of oxygen vacancy on oxygen electrode reactions." 2007. http://etda.libraries.psu.edu/theses/approved/WorldWideIndex/ETD-1878/index.html.
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McCrate, Joseph Michael. "Characterization of low density oxide surface sites using fluorescent probes." 2013. http://hdl.handle.net/2152/23078.
Full textAbstract:
Low density surface sites are believed to play an important role in processes occurring on oxide surfaces, including catalysis and particle and film nucleation. However, our understanding of the role and chemical nature of such sites play in these processes is limited by the inability to experimentally detect minority surface sites in many oxide systems. The research performed for this dissertation is focused on developing a surface science technique utilizing fluorescent molecules to titrate specific surface sites on planar fused silica surfaces in an ultra-high vacuum (UHV) environment. High sensitivity (low detection limit) is achieved by using derivatives of perylene, a high quantum yield fluorophore. High specificity is attained by employing perylene derivatives with functional groups designed to react chemically with and titrate various sites. In addition to titrating the well-studied hydroxyl sites with perylene-3-methanol (density ~ 10¹⁴ cm⁻²), which is used to establish the technique, the detection of strained siloxane sites (~ 10¹² cm⁻²), ) with perylene-3-methanamine and oxygen vacancy sites (~ 10¹¹ cm⁻²), ) with 3-vinyl perylene is demonstrated. Particle nucleation on oxides is suspected to involve defects that trap adatoms and form critical nuclei. Using this technique, the possible role strained siloxane and oxygen vacancy sites play in trapping adatoms during the nucleation of Ge nanoparticles on silica surfaces is examined.text
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Lin, YungHsun, and 林詠勛. "Construction of the Carrier Concentration(Oxygen-Vacancy and Cation-Substitution) Separation Model & Metal Doping Effect on the Carrier Concentration in Conductive Oxides." Thesis, 2011. http://ndltd.ncl.edu.tw/handle/02238829690685739233.
Full textAbstract:
博士國立中央大學
化學工程與材料工程研究所
99
In this thesis, we would propose a method which can decouple and estimate the oxygen-vacancy concentration and the cation-substitution concentration in an extrinsic metal oxide thin film separately. Also, we further propose how the properties of the dopant affect the cation-substitution reaction and the oxygen-vacancy concentration by observing the annealing ambient effects. In chapter 3, we propose the method which could estimate the oxygen-vacancy concentration and the cation-substitution concentration by measuring the carrier concentration of the thin film, which is annealed in different oxygen partial pressure. In chapter 4, we would discuss that how the difference of the electronegativity and the electron configuration would govern the cation-substitution reaction and the oxygen-vacancy formation reaction n. And how do oxygen-partial pressure and annealing temperature affect on the oxygen-vacancy concentration and the cation-substitution concentration. We found that the electronegativity would affect the cation-substitution concentration when the thin films were annealed at different oxygen pressure. And the electron configuration would affect the band diagram and affect the oxygen-vacancy formation energy. Besides the basic electrical properties, we also study the optical properties of ITO based thin films. In chapter 5, we found that the as-deposited ITO/Al structure would absorb particular light (400 nm- 600 nm) after thermal treatment. It is because the Al diffusion in ITO would locate at the interstitial site and serve as a defect level. Yet, we found that the pre-annealed ITO thin film is a good diffusion barrier layer which can retard the Al diffusion. On the other hand, by measuring the transmittance of the M:ITO thin films, we found that the transmittance of the extrinsic metal oxide in the near-UV region could be enhanced by doping the ternary metal. The transmittance of ITO thin film at near-UV region (380 nm) could be enhanced 22 % after doping Ti. Thus, after applying the Ti:ITO thin film on near-UV LED as the current spreading layer, the light output power of the near-UV LED is enhanced 52.1 %.