Tesis sobre el tema "Metal doping"
Siga este enlace para ver otros tipos de publicaciones sobre el tema: Metal doping.
Crea una cita precisa en los estilos APA, MLA, Chicago, Harvard y otros
Consulte los 50 mejores tesis para su investigación sobre el tema "Metal doping".
Junto a cada fuente en la lista de referencias hay un botón "Agregar a la bibliografía". Pulsa este botón, y generaremos automáticamente la referencia bibliográfica para la obra elegida en el estilo de cita que necesites: APA, MLA, Harvard, Vancouver, Chicago, etc.
También puede descargar el texto completo de la publicación académica en formato pdf y leer en línea su resumen siempre que esté disponible en los metadatos.
Explore tesis sobre una amplia variedad de disciplinas y organice su bibliografía correctamente.
1
Crawford, Kevin G. "Surface transfer doping of diamond using transition metal oxides". Thesis, University of Glasgow, 2017. http://theses.gla.ac.uk/8561/.
Texto completoResumen
This thesis presents a body of work which advances the use of single crystal hydrogen terminated diamond as a semiconducting material. Surface transfer doping of intrinsic diamond is investigated, examining the current state of this technology and its limitations. New techniques for producing robust, thermally stable surface transfer doped diamond were achieved through use of transition metal oxides such as MoO3 and V2O5, as demonstrated experimentally by way of Hall measurement. Through use of these materials, thermal stability was greatly increased up to temperatures of at least 300oC. To achieve this higher temperature operation, encapsulation of MoO3 and V2O5 was found to be necessary in maintaining conductivity of the diamond surface due to suspected thermally-induced loss of hydrogen termination. Similarly, long term atmospheric stability is shown to necessitate annealing of the diamond surface prior to oxide deposition and for thinner layers of oxide, down to 10 nm, encapsulation of the oxide to isolate from atmosphere is shown to be required for increased stability. As well as the improvements in stability offered by these transition metal oxides, sheet resistance of the hydrogen terminated diamond surface was also greatly reduced. Carrier densities as high as ~7.5 ×1013 cm-2 were observed for MoO3-induced surface transfer doping, resulting in a low sheet resistance of ~ 3 kΩ/□. In parallel to the development of oxide acceptor materials, conditioning of the diamond surface was explored using Atomic Force Microscopy (AFM). Techniques for smoothing the surface after mechanical polishing were developed by way of RIE and ICP etching using both chlorine and oxygen mixtures. Surface roughness down to 2 angstroms was demonstrated, showing a significant improvement in roughness over mechanical polishing alone. Similarly, observed defects produced by polishing induced damage were removed through use of this etching strategy. The effects of varied plasma density during hydrogen termination was explored on etched surfaces, which produced higher quality hydrogen-terminated surfaces as verified by surface conductivity and AFM measurements. Finally, incorporation of MoO3 into a preliminary Field Effect Transistor (FET) device on diamond was attempted. Fabrication techniques to produce a FET device on hydrogen-terminated diamond is shown with preliminary results of MoO3 encapsulated devices. Insights into the fabrication of ohmic and gate contacts, incorporating MoO3, is also discussed.
2
Taub, Samuel. "Transition metal oxide doping of ceria-based solid solutions". Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/18845.
Texto completoResumen
The effects of low concentration Co, Cr and Mn oxide, singly and in combination, on the sintering and electrical properties of Ce0.9Gd0.1O1.95 (CGO) have been investigated with possible mechanisms suggested to explain this modified behaviour. The influence of these dopants on the densification kinetics of CGO were primarily investigated using constant heating rate dilatometry. Whilst low concentration Co and Mn-oxide were found to improve the sinterability of CGO, the addition of Cr-oxide was found to inhibit the densification kinetics of the material. The location and concentration of these dopants were investigated as a function of relative density using scanning transmission electron microscopy combined with energy dispersive x-ray mapping. All materials showed a gradual reduction in the grain boundary dopant concentration with sintering time, leading eventually to the formation of a second phase that was subsequently analysed by either electron energy loss spectroscopy or synchrotron x-ray powder diffraction. The improved densification of both the Co-doped and Mn-doped materials was believed to be related to an increased rate of lattice and grain boundary cation diffusion, associated with the segregation of the transition metal dopant to the grain boundary. In both cases the onset of rapid densification was correlated with the reduction of the transition metal cation leading to an increase in cerium interstitials, which are suggested to be the defects responsible for cerium diffusion. The inhibiting effects of Cr-addition were similarly related to changes in the defect chemistry, with the Cr ions creating a blocking effect that hindered the dominant grain boundary pathway for cation diffusion. The effects of these dopants on the electrical conductivity of CGO were examined using a combination of AC impedance spectroscopy and Hebb-Wagner polarisation measurements. Whilst Co-doping was found to enhance the specific grain boundary conductivity of CGO, the addition of either Cr or Mn resulted in an approximate 2 orders of magnitude decrease, even at dopant concentrations as low as 100 ppm. Despite these differences in ionic conductivity, both Co and Cr-doping were found to significantly enhance the electronic contribution to the conductivity along the boundaries, particularly within the p-type regime. The modified electrical behaviour was related to the formation of a continuous, transition metal-enriched grain boundary pathway and a change in the driving force for grain boundary Gd segregation, leading to a depletion of oxygen vacancies within the space charge regions and the consequent reduction of oxygen transport across the boundaries. The effects of this segregation were finally examined with mono-layer sensitivity using low energy ion scattering incorporating a novel method of self-standardisation. These analyses provided strong support for the conductivity mechanisms previously outlined.
3
Fei, Wenwen. "Au25(SR)18: Metal Doping, Ligand Exchange, and Fusion Reactions". Doctoral thesis, Università degli studi di Padova, 2019. http://hdl.handle.net/11577/3424837.
Texto completoResumen
In the past few years, the interest in the study of atomically precise metal nanoclusters has grown very significantly. Main reasons are the refined techniques nowadays available for controlling their structure and composition, their often-intriguing properties, and the possibility to tailor them for specific applications. This Thesis aims at providing new tools to synthesize, control, modify, and characterize thiolate-protected gold nanoclusters. The focus of the research is on the thiolate-protected Au25(SR)18 cluster, which is considered by many the true benchmark system for the study of atomically precise nanoclusters. Due to quantum-confinement effects, these nanoclusters have discrete electron-energy states, and this causes the emergence of molecular properties, such as a HOMO-LUMO gap, distinct optical and redox behavior, and magnetism. Additionally, the composition of the metal part and/or the capping monolayer can be modified to make the cluster exhibit specific, sometimes unexpected properties.
The goal of this research is to show that by performing controlled modifications of the cluster core and protecting monolayer, one can indeed introduce new properties, and thereby, explore new frontiers for possible applications of these nanosystems. From the viewpoint of the metal, Au25(SR)18 was modified by introducing one-single foreign metal atom. The synthetic, purification, modification, and characterization procedures were refined to explore new ways for achieving proper understanding of the structure of the doped molecular cluster. Particular emphasis has been put on the NMR characterization of the products, a still unexplored yet very powerful tool to localize the position of the doping metal. It is shown that the actual position of the doping metal changes depending on the element.
The effect of Au25 doping is then explored from the viewpoint of the generation and detection of singlet oxygen, which is an area of tremendous interest for the treatment of cancer via photodynamic therapy. Metal nanoclusters exhibit discrete optical transitions and have sufficiently long-lived triplet excited states. This makes them react quite efficiently with triplet ground-state oxygen to form singlet excited-state oxygen. Here we show that by proper tuning of the cluster composition (doping metals and ligands), these nanosystems can be made to exhibit the same singlet-oxygen photosensitization performance of systems currently used in the medical practice.
We discovered an intriguing transformation of Au25 core. This can be considered as a fusion reaction that consists in the spontaneous transformation of two Au25(SR)18 clusters to form Au38(SR)24, which is another benchmark gold nanocluster. The radical nature of Au25(SR)180 appears to play an important role in this bimolecular reaction that, importantly, does not require addition of exogenous thiols or other co-reactants. This is indeed a very unexpected result that could modify our view about the relative stability of molecular gold nanoclusters.
After exploring core modifications, we also investigated strategies to carry out chemical reactions, namely polymerization, directly on the cluster monolayer. Proper functionalization of the nanocluster, that is, capping the cluster with different thiolates, relies on the possibility of either preparing the cluster directly, starting from a mixture of appropriate thiols, or taking advantage of ligand-place exchange reactions, in which the native thiolates present in preformed clusters are partially exchanged with other thiols. In this Thesis, we have implemented experimental conditions for controlling ligand-place exchange reactions on Au25(SR)18 with the goal of introducing functional groups suitable to react with a specific monomer. After polymerization, a polylysine protected Au25 cluster could be prepared.
4
Trapatseli, Maria. "Doping controlled resistive switching dynamics in transition metal oxide thin films". Thesis, University of Southampton, 2018. https://eprints.soton.ac.uk/423702/.
Texto completoResumen
Transition metal oxide thin films have attracted increasing attention due to their potential in non-volatile resistive random access memory (RRAM) devices, where such thin films are used as active layers in metal-insulator-metal (MIM) configurations. Titanium dioxide is one of the most celebrated oxides among the ones that exhibit resistive switching behaviour due to its wide band gap, high thermal stability, and high dielectric constant. RRAM devices with various materials as active layers, have demonstrated very fast switching performance but also huge potential for miniaturisation, which is the bottleneck of FLASH memory. Nevertheless, these devices very often suffer poor endurance, physical degradation, large variability of switching parameters and low yields. In most cases, the physical degradation stems from high electroforming and switching voltages. Doping of the active layer has been often employed to enhance the performance of RRAM devices, like endurance, OFF/ON ratio, forming voltages, etc. In this work, doping in TiO2-x RRAM devices was used to engineer the electroforming and switching thresholds so that device degradation and failure can be delayed or prevented. Al and Nb were selected with basic criteria the ionic radius and the oxidation state. The doped RRAM devices, showed improved switching performance compared to their undoped counterparts. Alternative approaches to doping were also investigated, like multilayer stacks comprising Al2O3-y and TiO2-x thin films. Furthermore, Al:TiO2-x/Nb:TiO2-x bilayer RRAM devices were fabricated, to prove whether a diode behaviour of the p-n interface inside the RRAM was feasible. The latest would be a particularly interesting finding towards active electronics.
5
PRADA, STEFANO. "Enhancing oxide surface reactivity by doping or nano-structuring". Doctoral thesis, Università degli Studi di Milano-Bicocca, 2014. http://hdl.handle.net/10281/50011.
Texto completoResumen
Wide band-gap simple oxides are rather inert materials, which found applications in heterogeneous catalysis mainly as supports for active metal nanoparticles. This thesis investigates tailored modifications of the oxide characteristics aimed at making these substrates more reactive in catalytic processes. In particular we are interested in engineering the charge transfer with supported metal catalysts in order to enhance their activity and selectivity. By using first principles calculations in the framework of the density functional theory, we have explored two main routes in this field: 1) nanostructuring, in particular nanothick oxide films supported on metals, and 2) doping of oxides with substitutional metal ions. After addressing methodological aspects related to the theoretical simulations of these materials, we have considered the role of oxide doping in optimizing the structural and electronic properties of supported gold adparticles; we have shown that depending on the dopant and the nature of the oxide it is possible to finely tune the shape and the charge state of adsorbed metal particle. Moreover we have combined oxide doping and nanostructuring in modifying the work function of metal substrates. By varying parameters like nature, position, and concentration of dopants within the metal-supported oxide films, it is possible in principle to modify the work function of the metallic support in a desired way.
6
Derk, Alan Richard. "Understanding and Controlling Light Alkane Reactivity on Metal Oxides| Optimization Through Doping". Thesis, University of California, Santa Barbara, 2015. http://pqdtopen.proquest.com/#viewpdf?dispub=3724768.
Texto completoResumen
Metal oxide catalysts have numerous industrial applications and have garnered research attention. Although oxides catalyze many important reactions, their yields to products are too low to be of economic value due to low conversion and/or low selectivity. For example, some oxides can catalyze the conversion of methane to intermediates or products that are liquefiable at yields no higher than 30%. With improved yield, such a process could help reduce the trillions of cubic feet of natural gas flared every year, saving billions of dollars and millions of tonnes of greenhouse gases. To this end, one goal of this work is to understand and improve the catalytic activity of oxides by substituting a small fraction of the cations of a "host oxide" with a different cation, a "dopant." This substitution disrupts chemical bonding at the surface of the host oxide, which can improve reactant and lattice oxygen activation where the reaction takes place. Another goal of this work is to combine catalysts with metal oxides reactants to improve thermodynamic limitations. Outstanding challenges for the study of doped metal oxide catalysts include (1) selection of dopants to ix synthesize within a host oxide and (2) understanding the nature of the surface of the doped oxide during reaction.
Herein, strongly coupled theoretical calculations and experimental techniques are employed to design, synthesize, characterize, and catalytically analyze doped oxide catalysts for the optimization of light alkane conversion processes. Density Functional Theory calculations are used to predict different energies believed to be involved in the reaction mechanism. These parameters offer valuable suggestions on which dopants may perform with highest yield and activity and why. Synthesis is accomplished using a combination of wet chemical techniques, suited specifically for the preparation of doped (rather than supported or mixed) metal oxide catalysts of high surface area and high reactivity. Characterization is paramount in any doped-oxide investigation to determine if the catalyst under reaction conditions is truly doped or merely small clusters of supported catalyst. With that goal, diffraction, X-ray, electron microscopies, infrared spectroscopy, and chemical probes are used to determine the nanoscopic nature of the catalysts. Additional novel measurement techniques, such as transient oxidation reaction spectroscopy, determined the nature of the active site's oxidation state.
7
Crowley, Kyle McKinley. "Electrical Characterization, Transport, and Doping Effects in Two-Dimensional Transition Metal Oxides". Case Western Reserve University School of Graduate Studies / OhioLINK, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=case1597327584506971.
Texto completo
8
Banerjee, Tanushree. "Impact of Nickel Doping on Hydrogen Storage in Porous Metal-Organic Frameworks". VCU Scholars Compass, 2010. http://scholarscompass.vcu.edu/etd/2265.
Texto completoResumen
A supply of clean, carbon neutral and sustainable energy is the most scientific and technical challenge that humanity is facing in the 21st century. Though there is enough fossil fuels available for a few centuries, their use would increase the level of CO2 in the atmosphere. This would lead to global warming and may pose serious threats such as rising of sea level, change in hydrological cycle, etc. Hence there is a need for an alternative source of fuel that is clean and sustainable. Among the many resources considered as an alternative power source, hydrogen is considered one of the most promising candidates. To use hydrogen commercially, appropriate hydrogen storage system is required. Various options to store hydrogen for onboard use include gaseous form in high-pressure tanks, liquid form in cryogenic conditions, solid form in chemical or metal hydrides, or by physisorption of hydrogen on porous materials. One of the emerging porous materials are metal-organic frameworks (MOFs) which provide several advantages over zeolites and carbon materials because the MOFs can be designed to possess variable pore size, dimensions, and metrics. In general, MOFs adsorb hydrogen through weak interactions such as London dispersion and electrostatic potential which lead to low binding enthalpies in the range of 4 to 10 kJ/mol. As a result, cryogenic conditions are required to store sufficient amounts of hydrogen inside MOFs. Up to date several MOFs have been designed and tested for hydrogen storage at variable temperature and pressure levels. The overall results thus far suggest that the use of MOFs for hydrogen storage without chemical and electronic modifications such as doping with electropositive metals or incorporating low density elements such as boron in the MOFs backbone will not yield practical storage media. Such modifications are required to meet gravimetric and volumetric constraints. With these considerations in mind, we have selected a Cr-based MOF (MIL-101; Cr(F,OH)-(H2O)2O[(O2C)-C6H4-(CO2)]3•nH2O (n ≈ 25)) to investigate the impact of nickel inclusion inside the pores of MIL-101 on its performance in hydrogen storage. MIL-101 has a very high Langmuir surface area (5900 m2/g) and two types of mesoporous cavities (2.7 and 3.4 nm) and exhibits exceptional chemical and thermal stabilities. Without any modifications, MIL-101 can store hydrogen reversibly with adsorption enthalpy of 10 kJ/mol which is the highest ever reported among MOFs. At 298 K and 86 bar, MIL-101 can store only 0.36 wt% of hydrogen. Further improvement of hydrogen storage to 5.5 wt% at 40 bar was achieved only at low temperatures (77.3 K). As reported in the literature, hydrogen storage could be improved by doping metals such as Pt. Doping is known to improve hydrogen storage by spillover mechanism and Kubas interaction. Hence we proposed that doping MIL-101 with a relatively light metal possessing large electron density could improve hydrogen adsorption. Preferential Ni doping of the MIL-101’s large cavities which usually do not contribute to hydrogen uptake is believed to improve hydrogen uptake by increasing the potential surface in those cavities. We have used incipient wetness impregnation method to dope MIL-101 with Ni nanoparticles (NPs) and investigated their effect on hydrogen uptake at 77.3 K and 298 K, at 1 bar. In addition, the impact of metal doping on the surface area and pore size distribution of the parent MIL-101 was addressed. Metal content and NPs size was investigated by ICP and TEM, respectively. Furthermore, crystallinity of the resulting doped samples was confirmed by Powder X-ray Diffraction (PXRD) technique. The results of our studies on the successful doping with Ni NPs and their impact on hydrogen adsorption are discussed.
9
Buta, Sarah H. (Sarah Hume) 1972. "A first principles investigation of transitional metal doping in lithium battery cathode materials". Thesis, Massachusetts Institute of Technology, 1999. http://hdl.handle.net/1721.1/9550.
Texto completoResumen
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1999.Includes bibliographical references (p. 77-82).
The goal of this work is to understand the properties of mixed-metal intercalation oxides. Using first-principles methods, the effect of doping on the mixing, energetic, and voltage properties as well as the phase diagrams of lithium transition-metal oxides for lithium battery cathode materials was investigated. The effect of doping on the phase separation tendencies of layered transition-metal oxides was examined and it was found that for normal processing temperatures, Al is miscible in layered transition metal oxides (LiMO2) for five of the eight first-row transition metals studied. Temperature-composition phase diagrams for both Li(Al,Co)O2 and Li(Al,Cr)O2 were calculated. In these two systems, Al-doping is limited above 600°C by the formation of [gamma]-LiA1O2 and at very low temperatures owing to the existence of a miscibility gap. Reduced solubility is expected in the layered phase above 600°C for all oxides which have substantial solubility with LiA1O2 due to the formation of yLiAlO2. The effect of transition-metal doping on the average voltage properties in Mn-based spinets was calculated and the large increase in average voltage found experimentally was reproduced. A detailed analysis on the layered structure Li(Al,Co)O2 was performed, studying the energetics of different lithium sites and the effect of short-range clustering on the shape of the voltage curve. Though the average voltage is raised by Al substitution, the unexpected stability of sites with a few Al nearest neighbors leads to an initial decrease in voltage. For the Al-doped LiCoO2 system, a step in the voltage curve is found only for micro-segregated materials. When the Al and Co ions are randomly distributed in a solid solution, the voltage curve shows a continuous, gradual slope. The effect of oxygen defects in the Li(Al,Co)O2 system was investigated. A model for the effect of oxygen vacancies on the free energy of doped layered oxides was created by combining an ideal gas approximation and first-principles energy defect calculations. The results qualitatively confirm experimental studies on oxygen release in lithium battery materials.
by Sarah H. Buta.
S.M.
10
Wang, Junwei. "Chemical doping of metal oxide nanomaterials and characterization of their physical-chemical properties". Case Western Reserve University School of Graduate Studies / OhioLINK, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=case1333829935.
Texto completo
11
Ola, Oluwafunmilola. "Effect of metal doping and supports on TiO₂-based catalysts for CO₂ photoreduction". Thesis, Heriot-Watt University, 2014. http://hdl.handle.net/10399/2820.
Texto completoResumen
CO₂ photoreduction into fuels has the potential to reduce future dependence on fossil fuels. This work has examined the effects of metal ion doping on the properties and performance of TiO₂ for CO₂ reduction in different photoreactor configurations. Metal (Pd/Rh, Ni, Cu, V, Cr and Co) doped TiO₂ photcatalysts synthesized by the sol-gel method were suspended or immobilized onto monoliths threaded with optical fibers or quartz plates. Doping with Cu, V and Cr facilitated anatase to rutile phase transformation, while Ni and Co doping inhibited transformation to rutile. All the metal atoms were found to be replacing some of the Ti atoms in the cystal lattice of TiO₂ during synthesis, thus causing a shift to longer wavelengths. Metal doping can considerably enhance the photocatalytic activity of TiO₂ for CO₂ reduction to fuels under UV or visible light irradiation when H₂O was used as a reductant. The activities of the best photocatalysts (lwt%Cu-TiO₂ and lwt%Co-TiO₂) were 67 times higher than pure TiO₂. More importantly, the conversion rate, 79.95μmol/gcath achieved using the quartz plate reactor was near one order of magnitude higher than other reactor configurations due to better accessibility of the catalytic surface to photons and the reactants during photocatalytic reaction.
12
Jayasinghe, Ashini Shamindra. "Exploring the unique water properties of metal-organic nanotubes". Diss., University of Iowa, 2017. https://ir.uiowa.edu/etd/5521.
Texto completoResumen
Metal-organic nanotubular (MON) materials have garnered significant attention in the recent years not only due to the aesthetic architecture but also due to the interesting chemical and physical properties that have been reported for these compounds. The number of MONs reported in the literature are limited compared to metal organic frameworks due to synthetic challenges and difficulties in crystal engineering. These types of materials are of interest given their one-dimensional channels that lead to their potential application in advanced membrane technologies.
In Forbes group, a uranium-based metal-organic nanotube (UMON) was synthesized using zwitterionic like iminodiacetic acid (IDA) as the ligand. IDA ligand chelates to the U(VI) metal center in a tridentate fashion and doubly protonated IDA linker connects the neighboring uranyl moieties until it forms hexameric macrocycles. These macrocycles stack into a nanotubular array due to supramolecular interactions. Single crystal X-ray diffraction studies displayed there are two crystallographically unique water molecules that can be removed reversibly at 37 °C. UMON indicated selectivity to water, the selectivity of this material was analyzed using solvents with different polarities, sizes, and shapes. In the current body of work, dehydrated UMON crystallites were exposed to these solvents (in liquid and vapor phase) and studied using TGA coupled FTIR set up, confirming the highly selective nature of UMON. Kinetic studies were also conducted using an in-house built vapor adsorption setup confirmed the water uptake rate of the nanotube depends on the humidity of the environment. Uptake rates were estimated using a simple kinetic model and indicated enhanced hydration compared to other porous materials. One of the hypotheses regarding the interesting properties of UMON is that the uranium metal center might play a central role in the selectivity of this material. To test this hypothesis, a similar uranium based metal-organic nanotube containing 2,6-pyridine dicarboxylic acid (UPDC) as the ligand was synthesized and its properties were compared to that of the UMON material. UPDC did display some selectivity based upon size exclusion but did not exhibit the same selectivity to water that is observed for UMON. Different transition metals were also incorporated into the nanotubular structures to determine the influence of dopants on the observable properties. Only small amounts of transition metal dopants were incorporated into the structure, but it increased the stability under high humid environment. Attempts to incorporate transition metal dopants in the UPDC led to the formation of novel chain structures.
13
Randell, Heather Eve. "Applications of stress from boron doping and other challenges in silicon technology". [Gainesville, Fla.] : University of Florida, 2005. http://purl.fcla.edu/fcla/etd/UFE0010292.
Texto completo
14
Luo, Ming. "Transition-metal ions in II-VI semiconductors ZnSe and ZnTe /". Morgantown, W. Va. : [West Virginia University Libraries], 2006. https://eidr.wvu.edu/etd/documentdata.eTD?documentid=4630.
Texto completoResumen
Thesis (Ph. D.)--West Virginia University, 2006.Title from document title page. Document formatted into pages; contains xiv, 141 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 135-141).
15
Fu, Jie. "Advanced doping techniques and dehydrogenation properties of transition metal-doped LiAlH 4 for fuel cell systems". Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-159024.
Texto completoResumen
Hydrogen is an efficient, carbon-free and safe energy carrier. However, its compact and weight-efficient storage is an ongoing subject for research and development. Among the intensively investigated hydrogen storage materials, lithium aluminum hydride (LiAlH4) is an attractive candidate because of its high theoretical hydrogen density (volumetric: 96.7g H2/l material; gravimetric: 10.6 wt.%-H2) in combination with rather low decomposition temperatures (onset temperature <100°C after doping). Although the reversible dehydrogenation of LiAlH4 must be carried out with the help of organic solvent, LiAlH4 can serve as single-use hydrogen storage material for various special applications, for example, hydrogen fuel cell systems. This thesis deals with transition metal (TM)-doped LiAlH4 aiming at tailored dehydrogenation properties. The crystal structure and morphology of TM-doped LiAlH4 is characterized by XRD and SEM respectively. The positive effects of four dopants (NiCl2, TiCl3, ZrCl4 and TiCl4) on promoting the dehydrogenation kinetics of LiAlH4 are systematically studied by thermal analysis. Based on the state of each TM chloride (solid or liquid), three low-energy-input doping methods (1. ball-milling at low rotation speed; 2. manual grinding or magnetic stirring; 3. magnetic stirring in ethyl ether) are compared in order to prepare LiAlH4 with the maximum amount of hydrogen release in combination with fast dehydrogenation kinetics. The dehydrogenation properties of the TM-doped LiAlH4 powders are measured under isothermal conditions at 80°C at a H2 pressure of 1 bar, which is within the operating temperature range of proton exchange membrane (PEM) fuel cells, aiming at applications where the exhaust heat of the fuel cell is used to trigger the dehydrogenation of the hydrogen storage material. Furthermore, the mid-term dehydrogenation behavior of TM-doped LiAlH4 was monitored up to a few months in order to test its mid-term storability. In addition, the pelletization of TM-doped LiAlH4 is investigated aiming at a higher volumetric hydrogen storage capacity. The effects of compaction pressure, temperature and the H2 back-pressure on the dehydrogenation properties of TM-doped LiAlH4 pellets are systematically studied. Moreover, the volume change through dehydrogenation and the short-term storage of the TM-doped LiAlH4 pellets are discussed in view of practical applications for PEM fuel cell systemsWasserstoff ist ein effizienter, kohlenstofffreier und sicherer Energieträger. Jedoch die kompakte und gewichtseffiziente Speicherung ist ein permanentes Forschungs- und Entwicklungsthema. Unter den intensiv untersuchten Materialien für die Wasserstoffspeicherung ist aufgrund der hohen theoretischen Speicherdichte (volumetrisch: 96,7 g H2/L, gravimetrisch: 10.6 Gew.%-H2) in Kombination mit sehr niedrigen Zersetzungstemperaturen (Anfangstemperatur < 100°C nach Dotierung) Lithium Aluminiumhydrid (LiAlH4) ein vielversprechender Kandidat. Obwohl die reversible Dehydrierung von LiAlH4 mit Hilfe von organischen Lösungsmitteln durchgeführt werden muss, kann LiAlH4-Pulver als Einweg-Speichermaterial für verschiedene Anwendungen dienen, beispielsweise für Wasserstoff/Brennstoffzellensysteme. Diese Doktorarbeit beschäftigt sich mit LiAlH4 dotiert mit Übergangsmetall, mit dem Ziel maßgeschneiderte Dehydrierungseigenschaften zu erreichen. Die Kristallstruktur und die Morphologie der mit Übergangsmetallen dotierten LiAlH4-Pulver wurden mit Röntgenbeugung (XRD) und Rasterelektronenmikroskopie (REM) charakterisiert. Weiterhin wurde der positive Effekt der Dotanden auf die reaktionsfördernde Dehydrierung von LiAlH4 systematisch mit Hilfe thermoanalytischer Methoden untersucht. Für jedes Übergangsmetall, welches in Form von Übergangsmetallchloriden vorlag, wurden drei Dotierungsmethoden mit niedrigem Energieeintrag (Kugelmahlen mit geringer Rotations-geschwindigkeit, manuelles Schleifen/Magnetrühren, Magnetrühren mit Ethylether) verglichen, um LiAlH4-Pulver mit einer maximalen Wasserstofffreisetzungsmenge in Kombination mit einer schnellen Dehydrierungskinetik zu erzielen. Die Dehydrierung des dotierten LiAlH4-Pulvers wurde unter isothermen Bedingungen bei 80°C und einem H2-Druck von 1 bar gemessen, was im Bereich der Betriebstemperatur von PEM-Brennstoffzellen (Proton Exchange Membran) liegt. Dadurch sollen Anwendungen anvisiert werden, bei denen die entstehende Abwärme der Brennstoffzelle genutzt wird, um die Dehydrierung des Wasserstoffspeichermaterials auszulösen. Zudem wurde das Dehydrierungsverhalten des dotierten LiAlH4 bis zu einigen Monaten kontrolliert, um die mittelfristige Haltbarkeit zu testen. Weiterhin wurde die Pelletierung des mit Übergangsmetallen dotierten LiAlH4 mit dem Ziel untersucht, eine hohe volumetrische Speicherkapazität zu erreichen. Der Einfluss des Pressdrucks, der Dehydrierungstemperatur und des H2-Gegendrucks auf die Dehydrierungseigenschaften der mit Übergangsmetallen dotierten LiAlH4-Presslinge wurde systematisch analysiert. Außerdem wird die Volumenveränderung durch die Dehydrierung und die Kurzzeitspeicherung der mit Übergangsmetallen dotierten LiAlH4-Presslinge im Hinblick auf praktische Anwendungen unter Nutzung der Brennstoffzelle diskutiert
16
Khanyile, Sfiso Zwelisha. "Silicon nanowires by metal-assisted chemical etching and its incorporation into hybrid solar cells". University of Western Cape, 2021. http://hdl.handle.net/11394/8340.
Texto completoResumen
Philosophiae Doctor - PhDThe rapid increase in global energy demand in recent decades coupled with the adverse environmental impact of conventional fuels has led to a high demand for alternative energy sources that are sustainable and efficient. Renewable solar energy technologies have received huge attention in recent decades with the aim of producing highly efficient, safe, flexible and robust solar cells to withstand harsh weather conditions. c-Si has been the material of choice in the development of conventional inorganic solar cells owing to it superior properties, abundance and higher efficiencies. However, the associated high costs of Si processing for solar cells have led to a gravitation towards alternative organic solar cells which are cheaper and easy to process even though they suffer from stability and durability challenges. In this work, combination of both inorganic and organic materials to form hybrid solar cells is one of the approaches adopted in order to address the challenges faced by solar cell development.
17
Grüne, Philipp. "Structure and reactivity of metal and semiconductor clusters in the gas phase". Berlin mbv, Mensch-und-Buch-Verl, 2009. http://d-nb.info/995894353/04.
Texto completo
18
Lee, Edwin Wendell II. "Growth and Nb-doping of MoS2 towards novel 2D/3D heterojunction bipolar transistors". The Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1480686917234143.
Texto completo
19
SAVINO, UMBERTO. "Physical properties of metal-oxide surfaces for CO2 valorisation". Doctoral thesis, Politecnico di Torino, 2020. http://hdl.handle.net/11583/2823956.
Texto completo
20
Meroueh, Laureen. "Effects of doping and microstructural variables on hydrogen generated via aluminum-water reactions enabled by a liquid metal". Thesis, Massachusetts Institute of Technology, 2020. https://hdl.handle.net/1721.1/129067.
Texto completoResumen
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2020Cataloged from student-submitted PDF of thesis.
Includes bibliographical references (pages [110]-127).
Hydrogen has the potential to replace fossil fuels in numerous industrial sectors, considering its high energy density, ability to be used within our existing power or heating infrastructure, and lack of CO₂ emissions upon conversion of hydrogen's chemical energy into electricity. However, 96% of hydrogen is currently produced through steam methane reformation, which emits ~12 tons of CO₂ for every 1 ton of hydrogen produced. Consequently, hydrogen production accounts for roughly 830 million tons of annual global CO₂ emissions. Additionally, hydrogen storage can be impractical and expensive. The aluminum-water reaction presents itself as a hydrogen storage and generation solution. Without a passive oxide layer, aluminum will react with water to produce emission-free hydrogen, on-demand.
We enable the reaction by harnessing eutectic gallium-indium (eGaIn), an ambient temperature liquid metal that permeates through aluminum grain boundaries, disrupting its passive oxide layer and inhibiting passivation of its grain surfaces. The focus of this work is on the investigation of the underlying aluminum-water reaction mechanism in the presence of eGaIn and on understanding the effects of using scrap aluminum (i.e. doped aluminum) as feedstock. Surprisingly, experiments show that silicon doping has a tremendous accelerating effect on the aluminum-water reaction in the presence of eGaIn. In combination with grain size manipulation, Si-doping can increase hydrogen evolution rates by two orders of magnitude compared to pure aluminum. Doping with magnesium significantly retards the aluminum-water reaction, resulting in relatively steady hydrogen evolution rates. It is also shown that eGaIn permeates through aluminum as a line dislocation front.
These discoveries demonstrate that doping, grain refining and grain coarsening offer latitudes in the engineering of aluminum microstructures to tune hydrogen generation rates across three orders of magnitude and to tune the reaction efficiency. Using the results of this work, I provide a guide to the design/selection of aluminum for controllable hydrogen generation according to application. Lastly, while the corrosion of aluminum and its commercial alloys has been historically studied, results of this work show that the redox behavior of aluminum in the presence of eGaIn strays from what can be understood within the classical corrosion (galvanic theory) framework.
by Laureen Meroueh.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Mechanical Engineering
21
Nakayama, Ryo. "Exploration into an Innovative Science of Hydrogen Functional Materials Using Low-temperature Ion Beam Irradiation". Kyoto University, 2019. http://hdl.handle.net/2433/236604.
Texto completo
22
Peleckis, Germanas. "Studies on diluted oxide magnetic semiconductors for spin electronic applications". Access electronically, 2006. http://www.library.uow.edu.au/adt-NWU/public/adt-NWU20070821.145447/index.html.
Texto completo
23
Vinogradov, Nikolay. "Controlling Electronic and Geometrical Structure of Honeycomb-Lattice Materials Supported on Metal Substrates : Graphene and Hexagonal Boron Nitride". Doctoral thesis, Uppsala universitet, Institutionen för fysik och astronomi, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-194089.
Texto completoResumen
The present thesis is focused on various methods of controlling electronic and geometrical structure of two-dimensional overlayers adsorbed on metal surfaces exemplified by graphene and hexagonal boron nitride (h-BN) grown on transition metal (TM) substrates. Combining synchrotron-radiation-based spectroscopic and various microscopic techniques with in situ sample preparation, we are able to trace the evolution of overlayer electronic and geometrical properties in overlayer/substrate systems, as well as changes of interfacial interaction in the latter.It is shown that hydrogen uptake by graphene/TM substrate strongly depends on the interfacial interaction between substrate and graphene, and on the geometrical structure of graphene. An energy gap opening in the electronic structure of graphene on TM substrates upon patterned adsorption of atomic species is demonstrated for the case of atomic oxygen adsorption on graphene/TM’s (≥0.35 eV for graphene/Ir(111)). A non-uniform character of adsorption in this case – patterned adsorption of atomic oxygen on graphene/Ir(111) due to the graphene height modulation is verified. A moderate oxidation of graphene/Ir(111) is found largely reversible. Contrary, oxidation of h-BN/Ir(111) results in replacing nitrogen atoms in the h-BN lattice with oxygen and irreversible formation of the B2O3 oxide-like structure. Pronounced hole doping (p-doping) of graphene upon intercalation with active agents – halogens or halides – is demonstrated, the level of the doping is dependent on the agent electronegativity. Hole concentration in graphene on Ir(111) intercalated with Cl and Br/AlBr3 is as high as ~2×1013 cm-2 and ~9×1012 cm-2, respectively. Unusual periodic wavy structures are reported for h-BN and graphene grown on Fe(110) surface. The h-BN monolayer on Fe(110) is periodically corrugated in a wavy fashion with an astonishing degree of long-range order, periodicity of 2.6 nm, and the corrugation amplitude of ~0.8 Å. The wavy pattern results from a strong chemical bonding between h-BN and Fe in combination with a lattice mismatch in either [11 ̅1] or [111 ̅] direction of the Fe(110) surface. Two primary orientations of h-BN on Fe(110) can be observed corresponding to the possible directions of lattice match between h-BN and Fe(110). Chemical vapor deposition (CVD) formation of graphene on iron is a formidable task because of high carbon solubility in iron and pronounced reactivity of the latter, favoring iron carbide formation. However, growth of graphene on epitaxial iron films can be realized by CVD at relatively low temperatures, and the formation of carbides can be avoided in excess of the carbon-containing precursors. The resulting graphene monolayer creates a periodically corrugated pattern on Fe(110): it is modulated in one dimension forming long waves with a period of ~4 nm parallel to the [001] direction of the substrate, with an additional height modulation along the wave crests. The novel 1D templates based on h-BN and graphene adsorbed on iron can possibly find an application in 1D nanopatterning. The possibility for growing high-quality graphene on iron substrate can be useful for the low-cost industrial-scale graphene production.
24
Xing, Shihui. "Rational design of bi-transition metal oxide electrocatalysts for hydrogen and oxygen evolutions". Thesis, Queensland University of Technology, 2021. https://eprints.qut.edu.au/209307/1/Shihui_Xing_Thesis.pdf.
Texto completoResumen
This thesis mainly focuses on the rational design and preparation of bi-transition metal oxide materials for high-performance electrochemical catalysis, such as hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). To address the challenges of sluggish kinetics and large overpotentials in HER and OER, the effective strategy of morphology engineering, introducing a secondary metal element and supporting on carbon-based materials were carried out and discussed.
25
Almeida, Cristiane Gomes. "Síntese, caracterização e avaliação da atividade fotocatalítica de BiTa(Nb)O4 dopados com cromo e molibdênio na geração de hidrogênio". Instituto de Química, 2013. http://repositorio.ufba.br/ri/handle/ri/16253.
Texto completoResumen
Submitted by Ana Hilda Fonseca (anahilda@ufba.br) on 2014-10-03T15:33:10Z
No. of bitstreams: 1
Dissertação Cristiane.pdf: 9154051 bytes, checksum: aa1fa030f760c514e3fc02dbacb83066 (MD5)Approved for entry into archive by Fatima Cleômenis Botelho Maria (botelho@ufba.br) on 2014-10-03T15:55:32Z (GMT) No. of bitstreams: 1 Dissertação Cristiane.pdf: 9154051 bytes, checksum: aa1fa030f760c514e3fc02dbacb83066 (MD5)
Made available in DSpace on 2014-10-03T15:55:32Z (GMT). No. of bitstreams: 1 Dissertação Cristiane.pdf: 9154051 bytes, checksum: aa1fa030f760c514e3fc02dbacb83066 (MD5)
Hidrogênio obtido a partir de água é uma atrativa fonte de energia, visto que pode ser produzido por fontes renováveis e inesgotáveis, e é não poluente. Atualmente, a principal forma de produção de hidrogênio é através da reforma catalítica de gás natural; porém, um novo processo baseado na fotólise da água com auxílio de semicondutores fotocatalíticos é considerado uma alternativa promissora. Diversos materiais já foram testados e outros desenvolvidos com o intuito de aumentar a atividade fotocatalítica na decomposição da água, dentre estes estão o niobato e o tantalato de bismuto (BiNbO4 e BiTaO4), fotocatalíticamente ativos quando irradiados com luz ultravioleta. No entanto, é possível que modificações químicas e nas dimensões de partículas resultem em melhor atividade. O método citrato de precursores poliméricos foi empregado para preparar BiTaO4 e BiNbO4 com a determinação da temperatura ideal de polimerização empregando técnicas termoanalitícas, e um estudo exploratório da temperatura mínima de calcinação foi realizado para obtenção da fase pura desejada, confirmada por difratometria de raios X. A fim de ativar os semicondutores com radiação visível foi realizada a dopagem desses semicondutores com íons de metais de transição, Cr(III) e Mo(V), com concentrações que variaram de 1 – 4% (mol/mol), e um estudo de como as energias de band gap sofreram modificações. Foi observado que a atividade fotocatalítica dos óxidos dopados com molibdênio ou cromo, nas condições avaliadas, é fortemente influenciada pelos teores dos metais e a amostra que apresentou melhor atividade fotocatalítica na geração de hidrogênio foi BiTaO4 dopado com Cr(III) a 2%. Os resultados desse trabalho podem contribuir para o desenvolvimento de sistemas fotoquímicos eficientes empregados na produção fotocatalítica de hidrogênio, utilizando matérias- primas abundantes, renováveis e ambientalmente amigáveis, como água e luz solar.
Hydrogen obtained from water is an attractive energy source, since it can be produced by renewable and inexhaustible primary source, besides it is not pollutant. Nowadays, the principal form of hydrogen production is through catalytic reform of natural gas; although, a new process based on water splitting, with the support of photocatalytic semiconductors, is considered a promising alternative. Many materials have already been tested and developed with the intention of increasing the photocatalytic activity for water splitting, in which are present bismuth niobates and tantalates (BiNbO4 e BiTaO4), photocatalyticly active under UV light irradiation. However, it is possible that chemical modifications and modifications in particle dimensions result in better activity. The citrate method of polymeric precursors was used to prepare BiTaO4 e BiNbO4 with the determination of the ideal temperature of polymerization using thermoanalytical techniques, and an exploratory study of the minimal temperature of calcination was used to obtain a pure desired phase, confirmed by X rays diffractometry. In order to activate the semiconductors with visible radiation, a doping of these semiconductors with ions of transition metals, Cr(III) and Mo(V), with concentrations that ranged from 1 – 4% (mol/mol), and a study of how the band gap energies suffered modifications. The results suggest that the type and the concentration of metal doping exercise great influence on the crystalline structure and morphology of the oxides. While Cr(III) favors a beta phase (triclinic), Mo(V) favors an alpha phase (orthorhombic). Also, the presence of Cr(III) induces the formation of materials with more homogeneous morphologies and size of particles smaller than the obtained in presence of Mo(V). It was observed that a photocatalytic activity of the doped oxides with Molybdenum or Chromium, in the evaluated conditions, is strongly influenced by the metal concentrations and the sample that presented the best photocatalytic activity in hydrogen generation was BiTaO4 doped with Cr(III) at 2%. The experimental ratio H2/CO2, in this case, is much higher than the theoretical, suggesting that the isopropyl alcohol is not completely mineralized. The results of this work can contribute for the development of efficient photochemical systems applied in the photocatalytic production of hydrogen, using raw material in abundance, renewable and environmentally friendly like water and solar light.
26
Paniagua, Barrantes Sergio. "Interfacial engineering of transparent electrodes and nanoparticles with phosphonic acids and metal-organic dopants for organic electronic applications". Diss., Georgia Institute of Technology, 2013. http://hdl.handle.net/1853/52920.
Texto completoResumen
This thesis focuses on understanding the chemistry involved in a variety of surface modification reactions, both on metal oxides and graphene. In this work, the rates of chemisorption of a prototypical phosphonic acid on ITO under several processing protocols are measured using XPS to determine the optimal procedure. UPS is used to track the dependence of the electronic structure of the system, specifically of the work function and position of the valence band maximum on coverage. Phosphonic acid monolayers with appropriate tail groups can also be used to initiate chemistry from surfaces, which has potential for building layers of organic-electronic devices, including organic solar cells and capacitors. The growth of non-conjugated polymers from BaTiO₃ nanoparticles using a facile ATRP technique is studied via solution-phase and solid-state techniques to determine its applicability to make matrix-free composites for hybrid dielectrics. In addition, the surface chemistry involved in Kumada Catalyst-Transfer to grow polythiophene derivatives from ITO is examined via XPS. Finally, the newly emerged alternative for replacement of ITO as transparent electrode, graphene, is n- and p-doped using redox-active, solution-processable metal-organics, which increased its conductivity and allowed the work function to be tuned over a range of 1.8 eV. The systems are characterized in a systematic study, and the results are promising for future applications of graphene.
27
Brazda, Petr. "Preparation of Fe2O3/SiO2 nanocomposites from molecular precursor by the sol-gel method and the doping of iron (III) by trivalent metal cations". Strasbourg, 2009. http://www.theses.fr/2009STRA6289.
Texto completoResumen
Les travaux que nous présentons dans cette thèse se concentrent sur la préparation de nanocomposite de silice sol-gel amorphe comme matrice de synthèse de nanocristaux de epsilon-Fe2O3 purs. La souplesse du procédé de synthèse nous a permis d’envisager et de réaliser la substitution partielle et raisonnée du fer par l’aluminium et gallium. Pour cela nous avons élaboré les nanocomposites Al0. 25Fe1. 75O3/SiO2, Al0. 50Fe1. 50O3/SiO2, Al0. 75Fe1. 25O3/SiO2, Ga0. 25Fe1. 75O3/SiO2, Ga0. 50Fe1. 50O3/SiO2 and Ga0. 75Fe1. 25O3/SiO2. Nous avons pu grâce à la maitrise de la synthèse du précurseur et des traitements thermiques raisonnés déterminer les conditions optimales permettant de préparer presque pure une phase métastable de l’oxyde de fer, la phase e-Fe2O3. La majorité d'échantillons substitués n'a pas contenu d'autre que la phase epsilon. Les mesures de spectroscopie par RMN à 4. 2K nous ont permis de voir sous un autre angle les attributions des sous spectres de spectroscopie Mössbauer et d’expliquer plus finement le lien entre données structurales et propriétés magnétiques de epsilon-Fe2O3Presented work is focused on preparation of nanocomposite of epsilon-Fe2O3 crystallites embedded in amorphous SiO2 matrix using new sol-gel technique. Another aim of our study was to prepare epsilon iron(III) oxide with part of iron atoms substituted by diamagnetic aluminium or gallium atoms. From this reason nanocomposites of compositions of Al0. 25Fe1. 75O3/SiO2, Al0. 50Fe1. 50O3/SiO2, Al0. 75Fe1. 25O3/SiO2, Ga0. 25Fe1. 75O3/SiO2, Ga0. 50Fe1. 50O3/SiO2 and Ga0. 75Fe1. 25O3/SiO2 were prepared. Thanks to the optimal conditions, the non-substituted samples prepared were very pure. The majority of substituted samples did not contain other than epsilon phase. The anisotropy of hyperfine fields at 4. 2 K observed by NMR spectroscopy led to an alternative assignment of the subspectra in Mössbauer spectrum of epsilon phase. Fine structure of Mössbauer spectra induced by the substitution permitted a deeper understanding of the relationship between structure and magnetic properties of epsilon-Fe2O3
28
Ramamurti, Rahul. "Synthesis of Diamond Thin Films for Applications in High Temperature Electronics". University of Cincinnati / OhioLINK, 2006. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1145630189.
Texto completo
29
Elhag, Sami. "Chemically Modified Metal Oxide Nanostructures Electrodes for Sensing and Energy Conversion". Doctoral thesis, Linköpings universitet, Institutionen för teknik och naturvetenskap, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-134275.
Texto completoResumen
The goal of this thesis is the development of scalable, low cost synthesis of metal oxide nanostructures based electrodes and to correlate the chemical modifications with their energy conversion performance. Methods in energy conversion in this thesis have focused on two aspects; a potentiometric chemical sensor was used to determine the analytical concentration of some components of the analyte solution such as dopamine, glucose and glutamate molecules. The second aspect is to fabricate a photo-electrochemical (PEC) cell. The biocompatibility, excellent electro-catalytic activities and fast electron transfer kinetics accompanied with a high surface area to volume ratio; are properties of some metal oxide nanostructures that of a potential for their use in energy conversion. Furthermore, metal oxide nanostructures based electrode can effectively be improved by the physical or a chemical modification of electrode surface. Among these metal oxide nanostructures are cobalt oxide (Co3O4), zinc oxide (ZnO), and bismuth-zincvanadate (BiZn2VO6) have all been studied in this thesis. Metal oxide nanostructures based electrodes are fabricated on gold-coated glass substrate by low temperature (< 100 0C) wet chemicalapproach. X-ray diffraction, x-ray photoelectron spectroscopy and scanning electron microscopy were used to characterize the electrodes while ultraviolet-visible absorption and photoluminescence were used to investigate the optical properties of the nanostructures. The resultant modified electrodes were tested for their performance as chemical sensors and for their efficiency in PEC activities. Efficient chemically modified electrodes were demonstrated through doping with organic additives like anionic, nonionic or cationic surfactants. The organic additives are showing a crucial role in the growth process of metal oxide nanocrystals and hence can beused to control the morphology. These organic additives act also as impurities that would significantly change the conductivity of the electrodes. However, no organic compounds dependence was observed to modify the crystallographic structure. The findings in this thesis indicate the importance of the use of controlled nanostructures morphology for developing efficient functional materials.
30
Li, Xianlong. "Insight into complex systems in heterogeneous catalysis : ab initio study of solid supported metal clusters and the effects of oxygen doping on catalytic activity". Thesis, Queen's University Belfast, 2017. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.727421.
Texto completoResumen
Both of CO oxidation and water-gas-shift(WGS) reactions in the heterogeneous catalysis were investigated using density functional theory(DFT). Pd/CeO2, Au/TiO2, Pt oxide clusters and Pd were used as the reaction catalysts. For CO oxidation, the L-H, Mv-K mechanisms were determined over these catalysts. For WGS reaction, both of carboxyl and redox pathways were investigated. The clusters and surfaces with global stable structures are predicted by using crystal structure analysis by particle swarm optimization(CALYPSO).
31
Janzen, Oliver. "Untersuchungen zur Wechselwirkung von Buckminsterfullerenen mit Siliziumoberflächen und zur Dotierung von Metall/Silizium-Grenzflächen mit Buckminsterfullerenen Interaction between Buckminsterfullerenes and Silicon Surfaces and Interface Doping of Metal/Silicon Interfaces with Buckminsterfullerenes". Gerhard-Mercator-Universitaet Duisburg, 2001. http://www.ub.uni-duisburg.de/ETD-db/theses/available/duett-05182001-091725/.
Texto completoResumen
The present work investigates the interaction between buckminsterfullerene molecules (C60) and silicon surfaces as well as the influence of buckminsterfullerenes on metal/silicon interfaces. The knowledge about the mechanism of surface/molecule interaction between the technologically important semiconductor Si and C60 molecules may lead to new applications of fullerenes. In this context the present study compares the adsorption and the growth mechanisms as well as the desorption of C60 molecules which were evaporated onto well-prepared Si(111)-7x7-, Si(111):H-1x1- and Si(111):Ag-(sqrt(3) x sqrt(3))R30°-surfaces using Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) under ultra-high vacuum conditions. The electronic structure of C60-covered Si(111)-7x7-, Si(111)-1x1-, Si(111):H-1x1-and Si(111):Ag-(sqrt(3) x sqrt(3))R30°-surfaces was investigated using ultraviolet and X-ray photoelectron spectroscopy (UPS, XPS). From these experiments the molecule/surface interaction mechanisms can be identified. Crystalline C60 is a new semiconductor material. Therefore, evaporating C60 onto Si surfaces builds up a semiconductor heterostructure. The electronic properties of this heterostructure are characterized by the band discontinuities at the C60/Si interface. Using UPS and XPS the valence-band discontinuity at this semiconductor/semiconductor interface were determined. Additionally, metal/Si contacts were produced on initially clean silicon surfaces which were covered with distinct amounts of C60 before Ag-, Pb- or Pd-contacts were evaporated. The transport properties of these contacts were studied by current-voltage-chracteristics to determine the influence of C60-layers on the Schottky barrier heights of the metal/Si contacts.
32
Escobar, Diego. "Investigation of ZrNi, ZrMn2 and Zn(BH4)2 Metal/Complex Hydrides for Hydrogen Storage". Scholar Commons, 2007. https://scholarcommons.usf.edu/etd/701.
Texto completoResumen
The demand for efficient and clean fuel alternatives has been increasing in recent years and is expected to become more pronounced in the future. Utilization of hydrogen as a fuel is one of the most promising energy resources due to its easy production, abundance, regeneration and not creation of greenhouse gases during its combustion. Although gaseous hydrogen has a very high energy content per unit weight, its volumetric energy density is rather low. The large scale use of hydrogen as a fuel crucially depends on the development of compact hydrogen storage materials with a high mass content of hydrogen relative to total mass and to volume.
Certain metals and alloys are capable of reversibly absorbing large amounts of hydrogen to form metal hydrides. They exhibit the highest volumetric densities of hydrogen and are very promising for hydrogen storage because of their efficiency, cost and safety. Some of the metal hydride families can also be used in hydrogen compressors.
The objective of this work is to investigate the synthesis and characterization behavior of intermetallic alloys (ZrMn2, ZrNi) for hydrogen compression and of complex hydrides (Zn(BH4)2 ) for on-board hydrogen storage. An overview of hydrogen as a fuel and its storage means is provided, synthesis and characterization methods of metal hydrides are presented and the effect of mechanical milling and the catalytic doping of metal/complex hydrides are investigated in detail. The hydrogen storage alloys (hydrides) are extensively characterized using various analytical tools such as: XRD, SEM, EDS, TCD, FTIR and GC/MS. The thermal (heat flow and weight loss) and volumetric (storage capacity, kinetics, cycle life, etc) analysis have been carried out via DSC/TGA and high pressure PCT apparatus. Finally conclusions and recommendations for future work are provided to improve the absorption/desorption cycle of hydrogen storage in the compounds under investigation.
33
Krishnan, Bharat. "DEVELOPMENT OF SIMULATION FRAMEWORK FOR THE ANALYSIS OF NON-IDEAL EFFECTS IN DOPING PROFILE MEASUREMENT USING CAPACITANCE ? VOLTAGE TECHNIQUE". MSSTATE, 2005. http://sun.library.msstate.edu/ETD-db/theses/available/etd-04082005-092339/.
Texto completoResumen
Silicon Carbide devices are proving to be most promising for high power and high-temperature application in recent times. Efficient and accurate characterization of the device characteristics is key to the fabrication of high quality devices and reproduction of the quality of the devices fabricated. Capacitance-Voltage profiling is one of the most commonly used techniques to measure the doping profiles of semiconductors. However, interpretation of C-V profiling in the presence of traps in the material becomes complicated. Various complications arising from compensation between donors and acceptors, partial ionization of dopants and presence of deep level impurities could yield anomalous measured profile. Silicon Carbide being a wide bandgap semiconductor, many impurities commonly found such as Boron and Aluminum are not completely ionized at Room temperature. This leads to complications in calculating doping profiles when the trap levels are deeper. Other complications arising due to series resistance effect and diode edge effect may also affect the measured profile. Accounting for these complications may be difficult by mere observation of the measured profile. Simulation can be an excellent tool to extract parameters of interest from experimental results that are influenced by non-ideal effects. Fitting of the experimentally obtained data with simulated profile using specific models may be a useful technique to quantitatively account for the deviations from the actual profiles.
34
König, Andreas. "Charge-Density Waves and Collective Dynamics in the Transition-Metal Dichalcogenides: An Electron Energy-Loss Study". Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2013. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-126887.
Texto completoResumen
In this thesis, we present a detailed investigation of the electronic properties of particular transition-metal dichalcogenides. Applying electron-energy loss spectroscopy, the connection between the negative plasmon dispersion of tantalum diselenide and the occurrence of a charge-density wave state (CDW) in this compound as well as related materials is observed. Our studies include doping experiments with alkali metal addition altering the charge density of the compounds. This is known to suppress the CDW. We show that it further changes the plasmon dispersion from negative to positive slope. To estimate the doping rate of the investigated tantalum diselenide samples, a density functional theory approach is introduced, giving reliable results for a quantitative analysis of our findings. We refer to a theoretical model to describe the connection of the charge ordering and the plasmon dynamics. Investigations of the non-CDW compound niobium disulfide give further insights into the proposed interaction. Experimental results are further evaluated by a Kramers-Kronig-analysis. A structural analysis, by means of elastic electron scattering, shows the CDW to be suppressed upon doping, giving space for an emerging superstructure related to the introduced K atomsIn der vorliegenden Arbeit wird eine detaillierte Untersuchung der elektronischen Eigenschaften von ausgewählten Übergangsmetall-Dichalcogeniden präsentiert. Unter Anwendung von Elektronenenergieverlust-Spektroskopie wird die Verbindung der negativen Plasmomendispersion in Tantaldiselenid zum Auftreten eines Ladungsdichtewelle-Zustands (CDW) in diesem und in verwandten Materialien untersucht. Die Untersuchungen schließen Dotierungsexperimente mit dem Zusatz von Alkalimetallen ein, die die Ladungsdichte der Proben beeinflussen. Einerseits unterdrückt dies die CDW. Es wird außerdem gezeigt, dass sich der Anstieg der Plasmonendispersion von negativ zu positiv ändert. Ein Dichtefunktional-Theorie-Zugang zur Abschätzung der Dotierungsraten der untersuchten Tantaldiselenid-Proben wird genutzt, um verlässliche Ergebnisse für die quantitative Analyse unserer Messungen zu erhalten. Ein theoretisches Modell wird einbezogen, welches die Verbindung der Ladungsordung zur kollektiven Anregung der Ladungsdichte beschreibt, Untersuchungen der nicht-CDW Substanz Niobdisulfid geben weitere Einblicke in die Verbindung der beiden Phänomene. Die experimentellen Resultate werden weiterhin mit einer Kramers-Kronig-Analyse ausgewertet. Strukturelle Untersuchungen mit elastischer Elektronenstreuung zeigen, wie die CDW unterdrückt wird und einer auftauchenden Überstruktur, verursacht von den interkalierten K-Atomen, Raum gibt
35
Fu, Jie [Verfasser], Bernd [Akademischer Betreuer] Kieback y Rüdiger [Akademischer Betreuer] Lange. "Advanced doping techniques and dehydrogenation properties of transition metal-doped LiAlH 4 for fuel cell systems / Jie Fu. Gutachter: Bernd Kieback ; Rüdiger Lange. Betreuer: Bernd Kieback". Dresden : Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2015. http://d-nb.info/1069092673/34.
Texto completo
36
Cabral, Erika Dias. "Estudos de propriedades elétricas e magnéticas em nanoestruturas de GaMnAs de uso em spintrônica". Universidade do Estado do Rio de Janeiro, 2009. http://www.bdtd.uerj.br/tde_busca/arquivo.php?codArquivo=1462.
Texto completoResumen
Coordenação de Aperfeiçoamento de Pessoal de Nível SuperiorConselho Nacional de Desenvolvimento Científico e Tecnológico
Um estudo da interação entre desordem e polarização de spin no GaMnAs ajuda a compreender a natureza dos estados, estendidos ou localizados, bem como as consequências para as transições observadas sobre as propriedades de transporte e às mudanças na ordem magnética de momentos magnéticos localizados em sítios de Mn. Este estudo pressupõe a ocorrência de uma banda impureza com baixas concentrações de Mn, que merge na banda valência no caso de concentrações mais elevadas. A abordagem teórica, baseada em um formalismo de espalhamento múltiplo auto-consistente, através do cálculo da função de Green para buracos determina-se a função densidade espectral no nível de Fermi. A escolha de uma figura de mérito, com base na largura e sobre a posição do máximo da função densidade espectral no espaço recíproco, leva a um diagrama de fase que determina o caráter metálicos ou não metálicos da amostra. Também é possível identificar a mobility edge, e como consequência, a densidade efetiva de portadores livre. Uma amostra é definida pelo par de parâmetros independentes, a concentração de Mn e a densidade buraco. As melhores amostras, aquelas com os maiores valores de mérito, tem uma relação entre a densidade de buracos estendidos e a concentração de Mn aproximadamente de 0.3, muito próximo do raio de 10-25% observada entre as amostras reais produzidos com a mais alta temperatura de transição. Além disso, a relação entre essas concentrações de Mn correspondente as transições metal-não-metal e não-metal-metal que é de aproximadamente 2.4, muito próximo do valor 2.1 da amostras reais. Uma interpretação da ocorrência de ferromagnetismo com alta temperatura de transição em GaMnAs é dada como uma consequência da interação entre o mecanismo de interação assistido por estados localizados e interações indiretas assistida pelo estdos buraco estendidos. Portadores mediando magnetismo em semicondutores mostram diferenças importantes e potencialmente úteis a diferenças de magnetismo em metais tais como o luz- ou voltagem elétrica -controlando ferromagnetismo. Motivado por experiências reportadas em poços quânticos de GaAs com uma dopagem delta de Mn com altas temperaturas Currier (temperatura de transição) mais elevadas do que em bulk de (Ga, Mn)As, nós exploramos teoricamente a viabilidade do campo elétrico controlar ferromagnetismo em poços quânticos. Nós calculamos auto-consistentemente a interação de troca indireta em Mn-Mn íons e aplicamos a simulação Monte Carlo para encontrar transição temperatura Tc. A nossa abordagem permite-nos estudar sistematicamente os efeitos de confinamento quântico e da posição da camada magnética de Mn e Tc, que vai além da aproximação do campo médio. Nós comparamos nossos resultados com os resultados experimentais e sugerimos caminho para o melhor controle do ferromagnetismo.
A study of the interplay between disorder and spin polarization in the diluted magnetic semiconductor GaMnAs helps to understand the character of states, extended or localized. This study assumes the occurrence of an impurity band which merges into the valence band at higher concentrations. The theoretical approach, based on a self-consistent multiple scattering formalism, determines the spectral density function at the Fermi level from the calculation of the hole's Green's functions. A choice of a figure of merit, based on the width and on the position of the maximum of the spectral density function in the reciprocal space, leads to a phase diagram determining the metallic or non-metallic character of the sample. It is also possible to identify mobility edges and, in consequence, the density of effectively free carriers. A model sample is defined by the pair of independent parameters, Mn concentration and hole density. The best samples, those with the highest figures of merit, have a ratio between the extended hole density and the Mn concentration near 0.3, very close to ratio of 10-30% observed among the real samples produced with the highest transition temperatures. Also, the ratio between those Mn concentrations corresponding to the metal-to-non-metal and non-metal-to-metal transitions is approximately 2.4, very close to the value 2.1 inferred from the real samples. An interpretation of the occurrence of high transition temperature ferromagnetism in GaM-nAs is given as a consequence of the interplay between interaction mechanism assisted by localized states and indirect interactions assisted by extended hole states. Carrier mediated magnetism in semiconductors show important and potentially useful differences from magnetism in metals,such as light- or bias-controlled ferromagnetism. Motivated by experiments reporting higher Currier temperatures in GaAs quantum wells with Mn-delta doping than the ones observed in bulk (Ga,Mn)As, we explore theoretically the feasibility of bias-controlled ferromagnetism these systems. We calculate self-consistently indirect Mn-Mn exchange interaction, and apply Monte Carlo approach to find transition temperature Tc. Our approach allows us to systematically study the eects of quantum confinement and the position of Mn layer on magnetic ordering and Tc, beyond mean field approximation. We compare our findings with the experimental results and suggest paths toward improving the control of ferromagnetism.
37
Djongkah, Cissillia Young Chemical Sciences & Engineering Faculty of Engineering UNSW. "Effect of Pt and Ag metals to the degradation of trichloroethylene, ethylene, ethane, and toluene by gas phase photocatalysis". Awarded by:University of New South Wales. School of Chemical Sciences and Engineering, 2006. http://handle.unsw.edu.au/1959.4/26227.
Texto completoResumen
The photocatalytic oxidation of trichloroethylene (TCE), ethylene. ethane and toluene on TiO2, Pt/TiO2 and Ag/TiO2 were investigated in a dedicated reactor set-up operated at room temperature and ambient pressure condition. The gas phase experiments were carried out for both single and binary mixtures of these chemicals to identify the role of Pt and Ag metallisation in the photocatalytic oxidation of different contaminants. In a single contaminant system, the presence of Pt enhanced the oxidation of ethylene, ethane and toluene but detrimental to the oxidation of TCE. In the oxidation of ethylene, Pt enhanced the oxidation by acting as catalyst and as electron sink. However, in ethane oxidation, the enhancement was solely associated to the ability of Pt to act as electron sink. The detrimental effect observed in TCE oxidation was attributed to Pt and Cl interaction, which formed a persistent inorganic chlorine species decreasing the overall Pt/TiO2 photocatalyst performance. Interestingly, Ag did not show any significant effect to the oxidation of any single system degradation. In binary system degradation, where TCE and another organic compound either ethylene, ethane or toluene were degraded simultaneously, Pt always caused a detrimental effect due to its strong interaction with Cl. However, the presence of Ag and Cl gives a more synergetic effect. Ag was found to provide sites to temporarily trap chlorine radicals as AgCl. Under illumination, electrons transferred from Cl to Ag forming chlorine radicals that could react with the surface contaminant enhancing its breakdown and mineralization.
38
Simonov, Konstantin. "Effect of Substrate on Bottom-Up Fabrication and Electronic Properties of Graphene Nanoribbons". Doctoral thesis, Uppsala universitet, Molekyl- och kondenserade materiens fysik, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-295884.
Texto completoResumen
Taking into account the technological demand for the controlled preparation of atomically precise graphene nanoribbons (GNRs) with well-defined properties, the present thesis is focused on the investigation of the role of the underlying metal substrate in the process of building GNRs using bottom-up strategy and on the changes in the electronic structure of GNRs induced by the GNR-metal interaction. The combination of surface sensitive synchrotron-radiation-based spectroscopic techniques and scanning tunneling microscopy with in situ sample preparation allowed to trace evolution of the structural and electronic properties of the investigated systems. Significant impact of the substrate activity on the growth dynamics of armchair GNRs of width N = 7 (7-AGNRs) prepared on inert Au(111) and active Cu(111) was demonstrated. It was shown that unlike inert Au(111) substrate, the mechanism of GNRs formation on Ag(111) and Cu(111) includes the formation of organometallic intermediates based on the carbon-metal-carbon bonds. Experiments performed on Cu(111) and Cu(110), showed that a change of the balance between molecular diffusion and intermolecular interaction significantly affects the on-surface reaction mechanism making it impossible to grow GNRs on Cu(110). It was demonstrated that deposition of metals on spatially aligned GNRs prepared on stepped Au(788) substrate allows to investigate GNR-metal interaction using angle-resolved photoelectron spectroscopy. In particular intercalation of one monolayer of copper beneath 7-AGNRs leads to significant electron injection into the nanoribbons, indicating that charge doping by metal contacts must be taken into account when designing GNR/electrode systems. Alloying of intercalated copper with gold substrate upon post-annealing at 200°C leads to a recovery of the initial position of GNR-related bands with respect to the Fermi level, thus proving tunability of the induced n-doping. Contrary, changes in the electronic structure of 7-AGNRs induced by the deposition of Li are not reversible. It is demonstrated that via lithium doping 7-AGNRs can be transformed from a semiconductor into a metal state due to the partial filling of the conduction band. The band gap of Li-doped GNRs is reduced and the effective mass of the conduction band carriers is increased.
39
D'angelo, Anthony Joseph. "Investigation and Synthesis of Novel Graphene-Based Nanocomposites for Hydrogen Storage". Scholar Commons, 2012. http://scholarcommons.usf.edu/etd/4024.
Texto completoResumen
It is of great interest to develop and utilize a high surface area material with optimized hydrogen sorption properties. The need for a renewable energy source to replace automobile gasoline has become more critical in the past decade. Hydrogen is a viable fuel source for automobile usage; however, the question of how hydrogen will be safely and efficiently stored still remains. Critical factors for optimum hydrogen storage include ambient conditions and low activation temperature for adsorption and desorption phenomena. In order for optimum hydrogen adsorption to be achieved, the properties of (1) high surface area, (2) optimum hydrogen adsorption energy, and (3) Kubas interactions between metals and hydrogen molecules need to be considered. Fullerenes have recently become more popular with the discovery and mass production of graphene sheets derived from graphite. Graphene is a modified form of graphite that takes the form of sheets with less agglomeration than its respective graphitic form. This form has the potential for high surface area and storage capabilities. Storage of hydrogen at room temperature must be optimized by increasing the surface area and having an adsorption enthalpy between 15 - 20 KJ/mol. Graphene (G) sheets and graphene oxide (GO) sheets have been utilized as a matrix for hydrogen storage. These materials can also be cross-linked with organic spacers in order to form a porous framework of higher surface area. Metal decorating by calcium and platinum of the G/GO matrix has been used to enhance Kubas interactions, adsorption enthalpies, and spillover phenomenon. The use of a polymer matrix has also been implemented. Polyaniline is a novel superconducting polymer with unique electronic properties. Complexes of Polyaniline with graphene and graphene oxide have been investigated for hydrogen storage properties. Graphene and graphene oxide surface modification via metal decoration have been investigated in order to determine the most efficient synthesis and particle size on the G/GO matrix. Characterization by XRD, BET, adsorption enthalpy, PCT, TGA, FT-IR, and TEM/SEM (when applicable) were employed to optimize and compare the materials in the effort to develop a suitable storage material.
40
Vargas, Hernandez Jesus. "Structural and Morphological modification of TiO2 doped metal ions and investigation of photo-induced charge transfer processes". Thesis, Le Mans, 2017. http://www.theses.fr/2017LEMA1018/document.
Texto completoResumen
Le travail de thèse porte sur les méthodes de synthèse de nanostructures de dioxyde de titane et de leurs études physicochimiques afin de préciser les corrélations entre la morphologie, le dopage métallique, les caractéristiques structurales avec l'efficacité photocatalytique. Le grand intérêt pour les nanomatériaux TiO2 réside dans la mise au point de nouvelles sources d'énergie ou la conservation de l’environnement par des processus photocatalytiques. Cependant, la limitation principale de TiO2 est du au large gap électronique (eV ~3,2) du polymorphe Anatase. Ainsi, un des objectifs importants pour l'amélioration de l’efficacité des nanomatériaux TiO2 est d'augmenter leur photoactivité en décalantla création de paires d'électron-trou de l’UV à la gamme du visible. D'ailleurs, on a montré que l'utilisation de nanostructures 1D de TiO2 (nanotubes) a amélioré la collection de charges, en favorisant leur transport dans les structures 1D, qui par conséquent réduit au minimum la recombinaison et prolonge les durées de vie des électrons.La première partie de ce travail est dédiée à la synthèse des nanopoudres TiO2 dopées par des ions métalliques (Ag, Cu, Eu) préparés par sol-gel. Même avec différents éléments de dopage qui apparemment peuvent adopter le même état de valence (2+) (Cu2+, Ag2+, Eu2+), différents comportements ont été démontrés pour l'incorporation efficace de ces ions dans la structure de TiO2. L'anomalie entre les rayons ioniques des différents éléments utilisés module le rapport du dopage substitutionnel. Ceci est en effet réalisé pour Cu2+ mais dans moins d'ampleur pour Ag2+ tandis que les ions d'europium forment une ségrégation de phase Eu2Ti2O7. La dégradation de colorants de bleu de méthylène (MB) a étéaméliorée légèrement avec les échantillons dopés Ag. La raison a été attribuée aux clusters métalliques Ag qui ont été en effet mis en évidence à travers leur bande d’absorption plasmonique. La deuxième partie porte sur des couches minces de TiO2 dopés (Cu, Ag, et Eu) qui ont été élaborés par sol-gel et spin-coating et dipcoating. Les paramètres optimaux ont été obtenus pour réaliser les films cristallins mais présentant une organisation mésoporeuse qui dépend également du processus de dopage. Des études de Photocatalyse ont été également réalisées et l'efficacité des films ont été comparées en fonction des éléments dopants. La troisième partie de la thèse est liée à la modification morphologique des nanoparticules pour former des nanotubes à l'aide de la méthode hydrothermale sous pression contrôlée. Un plan d'expérience basé sur la méthode Taguchi a été utilisé pour la détermination des paramètres optimaux.Les nanotubes TiO2 augmentent la surface spécifique en comparaison avec les nanoparticules. La dégradation de bleu deméthylène par les nanotubes a montré une efficacité photocatalytique plus élevée qu’avec les nanopoudres TiO2 pures etdopés AgThe thesis work is focused on the synthesis methods of titanium dioxide nanostructures and their physico-chemical studies in order to point out the correlations between the morphology, metal doping, structural features with the photocatalytic efficiency. The great interest on TiO2 nanomaterials deals with new sources of energy or in the environment preservation through the photocatalytic properties. However, the main limitations is due to the wide band gap (~3.2 eV) of the anatase polymorph. Thus, a major objective for improvement of the performance of TiO2 nanomaterials is to increase theirphotoactivity by shifting the onset of the electron-hole pairs creation from UV to the visible range. Moreover, it was found that using onedimensional (1-D) TiO2 (nanotubes) improved the charge collection by 1D nanostructures which consequently minimizes the recombination and prolongate the electron lifetimes. The first part of this work is focused on the synthesis of TiO2 nanopowders doped with metallic ions (Ag, Cu, Eu) prepared by Solgel. Even with different doping elements which apparently can adopt the same valence state (2+) such as (Cu2+, Ag2+,Eu2+), different behaviors were demonstrated for the effective incorporation of these ions in the host structure of TiO2. The discrepancy between ionic radii of the different used elements modulates the ratio of the substitutional doping. This is indeed achieved for Cu2+ but in less extent for Ag2+ while Europium ions form segregated phase as Eu2Ti2O7. The experiments on the degradation of methylene blue (MB)dyes have shown slight improvement with Ag-doped samples. The reason was tentatively attributed to the Ag clusters which were indeed demonstrated through their plasmon optical band. The second part of the work concerns thin films of TiO2 doped (Cu, Ag, and Eu) which were elaborated by spin coating and dip coating. The optimal parameters were obtained to achieve crystalline films but presenting mesoporous organisation which also depends on the doping process. Photocatalysis investigations were also realized and the efficiency of the films compared as function of the doping elements.The third part of the thesis is related to the morphological modification from nanoparticles to nanotubes by using the hydrothermal method with controlled pressure. An experimental design based on Taguchi Method was employed for the determination of the optimal parameters. TiO2 nanotubes increase the surface area in comparison with TiO2nanoparticles. TiO2 nanotubes were tested for the methylene blue degradation and show a higher photocatalytic efficiency than TiO2 nanopowders and TIO2 doped with Ag
41
Xu, Ying. "Fabrication and Characterization of Photodiodes for Silicon Nanowire Applications and Backside Illumination". University of Dayton / OhioLINK, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=dayton1446313926.
Texto completo
42
Ellis, Wade C. "Fundamental Studies and Applications of Ambient Plasma Ionization Sources for Mass Spectrometry". BYU ScholarsArchive, 2017. https://scholarsarchive.byu.edu/etd/6484.
Texto completoResumen
The field of ambient desorption/ionization mass spectrometry (ADI-MS) has existed for over a decade. ADI-MS is a technique that offers benefits including fast analysis time, simple ionization sources that are easily constructed, and little to no required sample preparation. The research presented here describes efforts to better understand plasma-based ADI-MS sources and to explore the use of hydrogen-doped plasma gases with these sources. The use of hydrogen-doped argon (H2/Ar) and hydrogen-doped helium (H2/He) as plasma gases for a dielectric barrier discharge (DBD) and an AC glow discharge is presented first. When using the DBD, the intensity of the signal obtained when analyzing organic molecules in positive ion mode was increased by factors up to 37 times. In negative ion mode, only H2/Ar was shown to enhance the signal of an organic analyte. The limits of detection for caffeine when using hydrogen-doped plasma gases were found to decrease by factors of 78 and 1.9 for H2/Ar and H2/He respectively. The same phenomenon was observed when using H2/Ar with the AC discharge, but no signal enhancement was observed when using hydrogen-doped helium with the AC discharge. Similarly, if the DBD was allowed to ground through a wire rather than through the air, no signal enhancement was observed for H2/He. Using H2/Ar with metal samples is presented second. By using the metal sample as the grounded electrode for the AC glow discharge, many different metals could be detected directly with a time-of-flight mass spectrometer (TOF-MS) in the form of atomic ions both on their own and in combination with water and ammonia from the discharge. Any refractory metals tested did not yield signal. In addition to direct analysis with a TOF-MS, the AC discharge was used as a sampling method for an inductively coupled plasma mass spectrometer (ICP-MS). When coupled with an ICP-MS, the AC glow discharge was found capable of sampling even refractory elements, though the power of the ICP was required for ionization and detection. Scanning electron microscope (SEM) images of a copper surface exposed to the plasma discharge showed signs of melting when using the H2/Ar. Finally, a computer simulation of the chemistry and flow dynamics of a DC glow discharge generated in helium is presented. The simulation explores many of the fundamental processes at work and how they depend on the composition of the plasma gas. The generation of important species in the plasma was found to depend more on the amount of N2 and H2O impurities in the plasma gas rather than on the humidity or air pressure.
43
Silva, Lucas Barboza Sarno da. "Otimização das propriedades de transporte em supercondutores de MgB2 com a adição de compostos de estrutura cristalina tipo AlB2 e fontes distintas de carbono". Universidade de São Paulo, 2013. http://www.teses.usp.br/teses/disponiveis/97/97135/tde-08102013-085005/.
Texto completoResumen
Em Janeiro de 2001, um supercondutor totalmente novo foi apresentado por Nagamatsu, o diboreto de magnésio (MgB2), com uma temperatura crítica, Tc, surpreendentemente alta de 39 K. Atualmente, o MgB2 é considerado o condutor de alto campo do futuro. É claramente aceito que os valores excepcionais de altos campos magnético crítico superior, Hc2, (Hc2 + (0) ? 40 T para Tc ? 35 - 40 K) mostram que o MgB2 é capaz de substituir o Nb3Sn (Hc2 (0) ? 30 T para Tc ? 18 K) como a escolha para aplicações de altos campos magnéticos. Neste trabalho foram preparadas pastilhas supercondutoras de MgB2 utilizando adições de diboretos metálicos de ZrB2, TaB2, VB2 e AlB2 e adições simultâneas de diboretos metálicos e fontes diversas de carbono, como carbeto de silício, grafite e nanotubos de carbono. O objetivo da adição desses novos elementos foi criar mecanismos para melhorar a capacidade de transporte do material, tanto pela dopagem substitucional como pela geração de defeitos na matriz supercondutora, atuando como eficientes centros de aprisionamento das linhas de fluxo magnético. Para isso foram utilizados dois diferentes métodos de preparação de amostras, insitu e ex-situ. O método de preparação in-situ seguiu padrões convencionais, como mistura em moinho de bola e tratamento térmico em fluxo de argônio. Para a preparação das amostras utilizando-se o método ex-situ foram utilizadas técnicas mais sofisticadas, como moagem de alta energia e tratamento térmico em altas pressões (Hot Isostatic Press, HIP). Em geral, as adições dos diboretos metálicos melhoraram a capacidade de transporte do material em baixos campos, as fontes de carbono aumentaram os valores de densidade de corrente crítica em altos campos magnéticos, enquanto que as combinações das duas adições melhoram a capacidade de transporte, para algumas amostras, em toda a faixa de campo magnético medida.In January 2001, a new superconductor was presented by Nagamatsu, the magnesium diboride (MgB2), with a critical temperature, Tc, extremely high of 39 K. MgB2 is considered the high field conductor of the future. The exceptional high values of upper critical magnetic field, Hc2, (Hc2 + (0) ? 40 T for Tc ? 35 - 40 K) show that the MgB2 is able to replace the Nb3Sn (Hc2 (0) ? 30 T for Tc ? 18 K) as the choice for applications in high magnetic fields. In this work, superconducting pellets of MgB2 were prepared with addition of other metal diborides of ZrB2, TaB2, VB2, and AlB2, and simultaneous additions of metal diborides and different carbon sources, such as silicon carbide, graphite and carbon nanotubes. The objective of these additions of new elements was to create mechanisms to improve the transport capacity of the material, by substitutional doping and by generation of defects in the superconducting matrix, acting as effective pinning centers of magnetic flux lines. Two different methods for sample preparation were used, the in-situ and the ex-situ method. The in-situ preparation method followed conventional standards, such as powder mixing in a ball mill and heat treatment in argon flow. The ex-situ preparation method used more sophisticated techniques, such as high energy ball milling and heat treatment under high pressures (Hot Isostatic Press, HIP). In general, the additions of metal diborides improved the transport capacity of the material at low fields, the carbon sources increased the critical current density at high magnetic fields, whereas the combination of these two additions improved the transport capacity, for some samples, in all range of applied magnetic field.
44
Ozouf, Guillaume. "Electrodes à base d’aérogels de SnO2, résistantes à la corrosion pour la réduction de l’oxygène dans les piles à combustible à membrane échangeuse de protons (PEMFC)". Thesis, Paris Sciences et Lettres (ComUE), 2017. http://www.theses.fr/2017PSLEM060/document.
Texto completoResumen
Afin d’augmenter la durabilité des PEMFC, des aérogels de dioxyde d'étain ont été étudiés pour remplacer le carbone comme support de catalyseur cathodique. SnO2 est un semi-conducteur de type n dont la conductivité électronique peut être améliorée en le dopant par des cations hypervalents tels que Nb5+, Ta5+ ou Sb5+. Pour être un support de catalyseur efficace, le matériau doit aussi posséder une surface spécifique élevée avec une morphologie mésoporeuse pour permettre à la fois la dispersion et l'activité du catalyseur (Pt). À cette fin, notre objectif était de développer des aérogels de SnO2 dopé. Dans cette étude, les aérogels ont été synthétisés par voie sol-gel en milieu acide à partir d’alcoxydes métalliques comme précurseurs. Nos matériaux présentent une morphologie aérée très intéressante avec une surface spécifique relativement élevée (80-90 m2/g). De plus, tous les aérogels SnO2 dopés au Sb ont présenté une amélioration très significative de la conductivité électronique pour atteindre une valeur d’environ 0,12 S/cm. Les nanoparticules de platine ont ensuite été déposées sur la surface de l'aérogel SnO2 dopé Sb en utilisant trois méthodes différentes. La méthode basée sur la réduction chimique par l’intermédiaire d’un polyol fournit le meilleur résultat en terme d'activité catalytique massique, mesurée en électrode à disque tournant (Is = 32 mA/mgPt). Cette valeur est, par ailleurs, encore plus élevée que celle de l'électrocatalyseur TEC10E40E (Is = 27 mA/mgPt). Les AMEs intégrant notre aérogel SnO2 dopé au Sb ont enfin montré une très bonne durabilité à des potentiels élevésIn order to tackle the problem of low durability, tin dioxide aerogels were studied to replace carbon black as a catalyst support in proton exchange membrane fuel cells (PEMFCs). SnO2 is a well-known n-type semi-conductor whose electronic conductivity can be improved by doping with hypervalent cations such as Nb5+, Ta5+ or Sb5+. In addition, as a catalyst support, this material has to develop a high specific surface area with adequate mesoporous morphology to allow both good dispersion and activity of the catalyst (Pt). To this end, our objective was to develop doped SnO2 aerogels. In this study, SnO2 based-aerogels were successfully synthesized following an acid-catalyzed sol–gel route starting with metal alkoxides as precursors. Our materials have shown a very interesting airy morphology with among other a reasonable specific surface area (80–90 m2/g). Moreover, all Sb-doped aerogels exhibited significant improvement in electronic conductivity and reach a value of around 0.12 S/cm. Platinum nanoparticles were then deposed on the Sb doped SnO2 aerogel surface using three different methods. The method based on chemical reduction using a polyol route provided the best result in term of mass catalytic activity measured by RDE (Is = 32 mA/mgPt). This value is even higher than that of the reference electrocatalyst TEC10E40E (Is = 27 mA/mgPt). Sb doped SnO2 aerogel based MEAs have exhibited a very good durability at high potentials
45
Davis, Taylor Matthew. "Feasibility and Impact of Liquid/Liquid-encased Dopants as Method of Composition Control in Laser Powder Bed Fusion". BYU ScholarsArchive, 2021. https://scholarsarchive.byu.edu/etd/9256.
Texto completoResumen
Additive manufacturing (AM) – and laser powder bed fusion (LPBF) specifically – constructs geometry that would not be possible using standard manufacturing techniques. This geometric versatility allows integration of multiple components into a single part. While this practice can reduce weight and part count, there are also serious drawbacks. One is that the LPBF process can only build parts with a single material. This limitation generally results in over-designing some areas of the part to compensate for the compromise in material choice. Over-designing can lead to decreased functional efficiency, increased weight, etc. in LPBF parts. Methods to control the material composition spatially throughout a build would allow designers to experience the full benefits of functionality integration. Spatial composition control has been performed successfully in other AM processes – like directed energy deposition and material jetting – however, these processes are limited compared to LPBF in terms of material properties and can have inferior spatial resolution. This capability applied to the LPBF process would extend manufacturing abilities beyond what any of these AM processes can currently produce. A novel concept for spatial composition control – currently under development at Brigham Young University – utilizes liquid or liquid-encased dopants to selectively alter the composition of the powder bed, which is then fused with the substrate to form a solid part. This work is focused on evaluating the feasibility and usefulness of this novel composition control process. To do this, the present work evaluates two deposition methods that could be used; explores and maps the laser parameter process space for zirconia-doped SS 316L; and investigates the incorporation of zirconia dopant into SS 316L melt pools. In evaluating deposition methods, inkjet printing is recommended to be implemented as it performs better than direct write material extrusion in every assessed category. For the process space, the range of input parameters over which balling occurred expanded dramatically with the addition of zirconia dopant and shifted with changes in dopant input quantities. This suggests the need for composition-dependent adjustments to processing parameters in order to obtain desired properties in fused parts. Substantial amounts of dopant material were confirmed to be incorporated into the laser-fused melt tracks. Individual inclusions of 100 $nm$ particles distributed throughout the melt pool in SEM images. Howewver, EDX data shows that the majority of the incorporated dopant material is located around the edges of the melt pools. Variations of dopant deposition, drying, and laser scanning parameters should be studied to improve the resulting dopant incorporation and dispersion in single-track line scans. Area scans and multi-layer builds should also be performed to evaluate their effect on dopant content and dispersion in the fused region.
46
POLO, ANNALISA. "TERNARY OXIDE SEMICONDUCTOR PHOTOANODES FOR SOLAR ENERGY CONVERSION". Doctoral thesis, Università degli Studi di Milano, 2021. http://hdl.handle.net/2434/827287.
Texto completoResumen
Solar energy conversion and storage into hydrogen is a valuable approach to capture the energy that is freely available from sunlight and to turn it into a clean fuel. Photoelectrochemical (PEC) water splitting through the dual-absorber tandem cell technology has emerged as a promising strategy to this aim. The work conducted in the frame of this PhD thesis aimed at playing a part in the development and optimization of efficient oxide-based semiconductor photoanodes for water oxidation, which is the kinetic bottleneck of the overall PEC water splitting process.
Photoanodes based on films of absorbing materials were successfully synthesized with a high optical transparency as important requirement for maximizing the solar energy conversion efficiency of the final tandem cell device. Subsequently, their intrinsic properties as single photoabsorber photoanodes were largely improved, on the basis of the results obtained through comprehensive PEC studies in parallel with thorough structural, morphological, and spectroscopic investigations.
The attention was focused on three different classes of promising ternary metal oxides, able to absorb a large portion of the solar spectrum, namely i) BiVO4 (bandgap Eg = 2.4 eV), known for its excellent solar light to hydrogen conversion efficiency, ii) the copper tungstate-based materials CuWO4 (Eg = 2.3 eV) and CuW(1-x)Mo(x)O4 (Eg = 2.0 eV), ideal to be employed as visible-light active alternative to WO3, and iii) ZnFe2O4 (Eg = 2.0 eV) belonging to the spinel ferrites class, possessing excellent photothermal and chemical stability.
Specifically, BiVO4 was studied either as a visible light sensitizer towards TiO2 or as a single photoanode material to focus on the identification and improvement of its intrinsically poor electron transport and interfacial transfer properties. In the first case, the TiO2/BiVO4 heterojunction system was proved to be effective in producing highly reductive electrons, suitable for overall water splitting, through TiO2 sensitization towards visible light. This, together with the counterintuitive mechanism at the basis of the observed impressive functionality, was effectively disclosed through combined PEC and photocatalytic reduction test studies. The multifaceted role of Mo6+ doping onto both the bulk and surface properties of BiVO4 films was also revealed through an in-depth PEC and impedance spectroscopy study. By improving either the bulk conductivity or the interfacial charge transfer of optimized Mo6+ doped BiVO4 photoanodes a conspicuous enhancement was attained of their photoactivity towards water oxidation with respect to the pure material.
The presence of intra-gap states in CuWO4, acting as electron traps and thus being responsible for a severe internal charge recombination, was verified by means of the first ultrafast transient absorption study performed with this material, in combination with both an electrochemical and a photochromic characterization. This issue, which strongly limits the PEC performance of CuWO4 photoanodes, was addressed by adopting a 50% Mo for W substitution resulting in CuW0.5Mo0.5O4 photoanodes, exhibiting not only a greatly extended visible light-induced photoactivity compared to the pure material, as a result of enhanced absorption, but also a considerably improved charge separation. All these factors contributed to the much better PEC performance attained with respect to CuWO4 electrodes. This study was finalized by the identification of a suitable hole scavenger species for copper tungstate-based materials, able to ensure enhanced photocurrent generation compared to pure water oxidation while minimizing dark currents.
Finally, in the frame of my seven months stage in Prof. Sivula’s group at the EPFL in Lausanne, a thorough study was performed on the impact that several parameters, such as the annealing temperature, the film thickness and the creation of oxygen vacancies through a reductive treatment in hydrogen atmosphere, have on the PEC performance of ZnFe2O4 photoanodes. The verified synergism between the higher crystallinity of the films subjected to a high-temperature annealing treatment and the hydrogenation efficiency, which proved effective in optimizing charge separation in the thicker photoactive layers, allowed one to maximize the performance of ZnFe2O4 electrodes for water oxidation. This study also shed light onto the strict correlation occurring between structural parameters, i.e. the film crystallinity and the spinel inversion degree, and the resulting PEC performance, which proved to be in turn controlled by the film morphology.
47
Chicot, Gauthier. "Effet de champs dans le diamant dopé au bore". Phd thesis, Université de Grenoble, 2013. http://tel.archives-ouvertes.fr/tel-01062250.
Texto completoResumen
Alors que la demande en électronique haute puissance et haute fréquence ne fait qu'augmenter, les semi-conducteurs classiques montrent leurs limites. Des approches basées soit sur des nouvelles architectures ou sur des matériaux à large bande interdite devraient permettre de les dépasser. Le diamant, avec ses propriétés exceptionnelles, semble être le semi-conducteur ultime pour répondre à ces attentes. Néanmoins, il souffre aussi de certaines limitations, en particulier d'une forte énergie d'ionisation du dopant de type p (bore) qui se traduit par une faible concentration de porteurs libres à la température ambiante. Des solutions innovantes s'appuyant sur un gaz 2D et /ou l'effet de champ ont été imaginées pour résoudre ce problème. Ce travail est axé sur deux de ces solutions : i) le diamant delta dopé au bore qui consiste en une couche fortement dopée entre deux couches intrinsèques, afin d'obtenir une conduction combinant une grande mobilité avec une grande concentration de porteurs et ii) le transistor à effet de champ métal oxide semiconducteur( MOSFET ), où l'état " on " et l'état " off " du canal sont obtenus grâce au contrôle électrostatique de la courbure de bandes à l' interface de diamant/oxyde. Pour ces deux structures, beaucoup de défis technologiques doivent être surmontés avant de pouvoir fabriquer un transistor. La dépendance en température de la densité surfacique de trous et de la mobilité de plusieurs couche de diamant delta dopées au bore a été étudiée expérimentalement et théoriquement sur une large gamme de température (6 K
48
Burnham, Shawn David. "Improved understanding and control of Mg-doped GaN by plasma assisted molecular beam epitaxy". Diss., Available online, Georgia Institute of Technology, 2007, 2007. http://etd.gatech.edu/theses/available/etd-06122007-133821/.
Texto completoResumen
Thesis (Ph. D.)--Electrical and Computer Engineering, Georgia Institute of Technology, 2008.Doolittle, W. Alan, Committee Chair ; Ferguson, Ian T., Committee Member ; Cressler, John D., Committee Member ; Dorsey, John F., Committee Member ; Carter, W. Brent, Committee Member.
49
BONDARENKO, VLADIMIR. "The Synthesis and Study of TiO2/Aluminosilicate Composites as Components of Building Finishing Materials for Improvement of the Indoor Air Quality". Doctoral thesis, Università Politecnica delle Marche, 2017. http://hdl.handle.net/11566/251220.
Texto completoResumen
Lo studio oggetto della tesi presenta il metodo di preparazione di materiali compositi per
deposizione chimica di TiO2 su matrici di alluminosilicati per deposizione da una soluzione
di Titanio solfato. Materie prime relativamente economiche e facilmente disponibili sono
state usate per la preparazione: i frammenti del processo di trattamento meccanico
dell'alluminio per l'ottenimento della soluzione di titanio solfato, argille e cenere volante
come matrice. L'analisi dei riferimenti di letteratura consente di assumere che questo metodo
produca materiali compositi con una larga superficie specifica grazie all'attivazione
termoacida e ad una elevata attività fotocatalitica spendibile in applicazioni indoor. Queste
proprietà sono confermate dai dati sperimentali ottenuti sui campioni di materiale composito
ottenuti, sui quali sono stati valutati: la composizione chimica, la morfologia superficiale con
SEM, la dimensione media delle particelle in microscopia ottica, la proprietà di adsorbimento
del vapore d'acqua, la capacità di adsorbimento del benzene, la struttura dei pori, l'analisi ai
raggi X, l'attività fotocatalitica con metodo di rimozione del MEK con due diversi tipi di box
di prova e sorgenti di luce. La stabilità nel tempo delle proprietà dei compositi sviluppati è
stata valutata dopo tre anni dalla loro preparazione. Sulla base dei risultati ottenuti sono state
discusse le indicazioni per l'ottenimento di Compositi stabili. La relativa semplicità della
tecnologia utilizzata ha consentito di descrivere l'organizzazione della produzione industriale
del materiale composito e un metodo di possibile promozione industriale. Sono stati
esaminati anche compositi contenenti dopanti non metallici come metodo per fornire attività
fotocatalitica nel campo della luce visibile. L'uso di cenere volante come matrice del
composito è stato studiato come metodo di valorizzazione di un rifiuto industriale. Infine i
materiali da costruzione preparati con i compositi a base di TiO2 sono stati testati con il
metodo della rimozione del MEK.This study offers the method for the synthesis of composite material by chemical deposition of TiO2 on aluminosilicate matrix from titanium sulfate solution (precursor). Relatively cheap and available raw materials were used during the preparation: the waste of mechanical processing of titanium (titanium shavings) for obtaining the precursor, montmorillonite, kaolinite, and fly ash as supports. The analysis of the literature sources allows assuming that such method produces the composite with a large specific surface area because of thermoacid activation and high photoactivity suitable for indoor conditions. These properties are confirmed by the experimental data for the obtained samples: Chemical contents; Morphology of surface and average particles size by SEM/optical microscopy; Adsorptive properties and parameters of porous structure, water vapor and benzene vapor adsorption capacity; X-rays analysis of phases; Photoactivity of the materials was estimated by the method of MEK removal in reactors of two different types (16 L & 0,45 L) with two types of UV-irradiation source (ULTRAVITALUX E27 lamp and DFL-5013UVC-380). The stability of the properties of the obtained composites after 3 years of storage was examine. Based on the results obtained, the recommendations for obtaining stable composites were discussed. The relative simplicity of technology allows organizing industrial production of the materials and possible upgrades of the method. Non-metal co-doping of the composites was examined as a method for providing visible light activity of the materials. The use of fly ash as matrix for composites was studied as possible way of valorization of industrial by-product. The building materials prepared with the obtained TiO2–containing component were tested by the method of methyl ethyl ketone removal.
50
Tewg, Jun-Yen. "Zirconium-doped tantalum oxide high-k gate dielectric films". Diss., Texas A&M University, 2004. http://hdl.handle.net/1969.1/1346.
Texto completoResumen
A new high-k dielectric material, i.e., zirconium-doped tantalum oxide (Zr-doped TaOx), in the form of a sputter-deposited thin film with a thickness range of 5-100 nm, has been studied. Important applications of this new dielectric material include the gate dielectric layer for the next generation metal-oxide-semiconductor field effect transistor (MOSFET). Due to the aggressive device scaling in ultra-large-scale integrated circuitry (ULSI), the ultra-thin conventional gate oxide (SiO2) is unacceptable for many practical reasons. By replacing the SiO2 layer with a high dielectric constant material (high-k), many of the problems can be solved. In this study, a novel high-k dielectric thin film, i.e., TaOx doped with Zr, was deposited and studied. The films electrical, chemical, and structural properties were investigated experimentally. The Zr dopant concentration and the thermal treatment condition were studied with respect to gas composition, pressure, temperature, and annealing time. Interface layer formation and properties were studied with or without an inserted thin tantalum nitride (TaNx) layer. The gate electrode material influence on the dielectric properties was also investigated. Four types of gate materials, i.e., aluminum (Al), molybdenum (Mo), molybdenum nitride (MoN), and tungsten nitride (WN), were used in this study. The films were analyzed with ESCA, XRD, SIMS, and TEM. Films were made into MOS capacitors and characterized using I-V and C-V curves. Many promising results were obtained using this kind of high-k film. It is potentially applicable to future MOS devices.