The successful prediction of spacecraft motion is often heavily based upon assumptions used to simplify the problem without compromising solution accuracy. For many analyses, a primary assumption used is the decoupling of trajectory and attitude dynamics when calculating trajectories. In cases where spacecraft or objects have high area to mass ratios, non-conservative effects such as atmospheric drag and solar radiation pressure can greatly perturb spacecraft translational motion based on rotational state. A modular, six degree of freedom (6DOF) simulation with coupled orbit and attitude dynamics has been developed to model spacecraft and orbits of arbitrary geometries. First, the basis for the modular rotational and translational equations of motion are introduced. Next, formulations are provided for the gravity gradient torque, solar radiation pressure, aerodynamic, and non-spherical gravity potential sources of perturbations, and the Marshall Engineering Thermosphere atmospheric model used is described. A first test case is performed using the 6DOF simulation to simulate the deorbit of the spacecraft Lightsail 1, which flew in 2015. Next, predictive cases are demonstrated using the simulation for a theoretical sail-boom-rocket combination representative of a debris removal scenario, and for the Aerodynamic Deorbit Experiement, which will demonstrate a passively stable drag sail technology and characterize its effectiveness on orbit. All simulation cases have had aerodynamic perturbation formulations compared against high fidelity Direct Simulation Monte Carlo runs, and suggestions have been made for the future development of the simulation tool.

Information on the sensitivity of explosives is highly valuable, and the short time scales in which chemical reactions occur in explosives, along with the ability of microstructure to have significant effects on sensitivity, often make this information difficult and expensive to acquire and interpret. Significant changes in impact and shock sensitivity are expected as a result of inducing structural damage in an explosive sample, and thermally damaged HMX-based samples can incur a solid-solid phase transition from beta to delta with non-extreme thermal inputs. Changes in sensitivity due to this phase transition, as well as the simultaneously induced damage, and their relative influence on sensitivity, are of interest to determine experimentally. <div><br></div><div>Drop-weight impact tests are a commonly used measure of explosive impact sensitivity. Often, simply the L50 of a given material is reported and compared with that of other materials to give a sense of its impact sensitivity. The practice of reporting the impact sensitivity as a single number, the L50, is likely inadequate. It is important to additionally report a measure of the spread of the distribution of reaction probabilities in order to assess the hazard of reaction in situations that may induce a stimulus level well below the L50 of a material. Additionally, multiple distribution forms have been suggested previously for fitting of binary sensitivity data; these distributions typically deviate from each other most near the tails (low and high stimulus levels). The consequences of choosing one distribution form over another in the analysis of explosive drop-weight impact results is explored.<br></div><div><br></div><div>Changes in impact sensitivity due to phase change have received some previous exploration, though the phase change influence is generally conflated with the induced damage upon said phase transition; however, sensitivity changes in the shock regime due to beta to delta-phase change have received little attention. Work is shown which includes methods to isolate variables of HMX phase transition and damage typically incurred upon said phase transition.<br></div>

abstract: The availability of bulk gallium nitride (GaN) substrates has generated great interest in the development of vertical GaN-on-GaN power devices. The vertical devices made of GaN have not been able to reach their true potential due to material growth related issues. Power devices typically have patterned p-n, and p-i junctions in lateral, and vertical direction relative to the substrate. Identifying the variations from the intended layer design is crucial for failure analysis of the devices. A most commonly used dopant profiling technique, secondary ion mass spectroscopy (SIMS), does not have the spatial resolution to identify the dopant distribution in patterned devices. The possibility of quantitative dopant profiling at a sub-micron scale for GaN in a scanning electron microscope (SEM) is discussed. The total electron yield in an SEM is shown to be a function of dopant concentration which can potentially be used for quantitative dopant profiling. Etch-and-regrowth is a commonly employed strategy to generate the desired patterned p-n and p-i junctions. The devices involving etch-and-regrowth have poor performance characteristics like high leakage currents, and lower breakdown voltages. This is due to damage induced by the dry etching process, and the nature of the regrowth interface, which is important to understand in order to address the key issue of leakage currents in etched and regrown devices. Electron holography is used for electrostatic potential profiling across the regrowth interfaces to identify the charges introduced by the etching process. SIMS is used to identify the impurities introduced at the interfaces due to etch-and-regrowth process. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2019

abstract: An airborne, tethered, multi-rotor wind turbine, effectively a rotorcraft kite, provides one platform for accessing the energy in high altitude winds. The craft is maintained at altitude by its rotors operating in autorotation, and its equilibrium attitude and dynamic performance are affected by the aerodynamic rotor forces, which in turn are affected by the orientation and motion of the craft. The aerodynamic performance of such rotors can vary significantly depending on orientation, influencing the efficiency of the system. This thesis analyzes the aerodynamic performance of an autorotating rotor through a range of angles of attack covering those experienced by a typical autogyro through that of a horizontal-axis wind turbine. To study the behavior of such rotors, an analytical model using the blade element theory coupled with momentum theory was developed. The model uses a rigid-rotor assumption and is nominally limited to cases of small induced inflow angle and constant induced velocity. The model allows for linear twist. In order to validate the model, several rotors -- off-the-shelf model-aircraft propellers -- were tested in a low speed wind tunnel. Custom built mounts allowed rotor angles of attack from 0 to 90 degrees in the test section, providing data for lift, drag, thrust, horizontal force, and angular velocity. Experimental results showed increasing thrust and angular velocity with rising pitch angles, whereas the in-plane horizontal force peaked and dropped after a certain value. The analytical results revealed a disagreement with the experimental trends, especially at high pitch angles. The discrepancy was attributed to the rotor operating in turbulent wake and vortex ring states at high pitch angles, where momentum theory has proven to be invalid. Also, aerodynamic design constants, which are not precisely known for the test propellers, have an underlying effect on the analytical model. The developments of the thesis suggest that a different analytical model may be needed for high rotor angles of attack. However, adding a term for resisting torque to the model gives analytical results that are similar to the experimental values. / Dissertation/Thesis / Masters Thesis Mechanical Engineering 2019

Orientador: Alexandre Mesquita / Resumo: Os materiais nanoestruturados vêm sendo extensivamente estudados, não somente pelas novas propriedades e suas possíveis aplicações tecnológicas, mas também pela busca de uma melhor compreensão dos aspectos físicos e químicos. Em relação a materiais semicondutores, estudos da estrutura em escala nanométrica tem recebido considerável interesse em razão do efeito de tamanho que exibem. Semicondutores nanocristalinos apresentam propriedades eletrônicas intermediárias entre aqueles de estrutura molecular e sólidos macrocristalinos e são objeto de intensa pesquisa, apresentando uma grande diversidade de aplicações quando na forma nanoestruturada. Dentre esses materiais, o silicato de zinco (Zn2SiO4) puro ou dopado tem recebido atenção em razão de notáveis propriedades fundamentais, versatilidade e potencial para diversas aplicações tecnológicas. Nesse contexto, essa dissertação teve como objetivo realizar a síntese e a caracterização de Zn2SiO4 puro e dopado com átomos de manganês e sua correlação com propriedades fotoluminescentes. Neste trabalho, foi utilizado o método químico de preparação conhecido por co-preciptação. Deste modo, a caracterização morfológica foi realizada através da técnica de microscopia eletrônica de varredura de alta resolução (FEG-MEV), a qual demonstra um aglomerado de grãos, tornando a determinação do tamanho do mesmo difícil. As propriedades estruturais das nanopartículas, bem como suas propriedades físicas e químicas, foram determinadas através das técn... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Nanostructured materials have been extensively studied, not only for new properties and their possible technological applications, but also for a better understanding of the physical and chemical aspects. With regard to semiconductor materials, nanoscale structure studies have received considerable interest because of the size effect they exhibit. Nanocrystalline semiconductors have intermediate electronic properties between those of molecular structure and macrocrystalline solids and are the subject of intense research, presenting a great diversity of applications when in nanostructured form. Among these materials, pure or doped zinc silicate (Zn2SiO4) has received attention because of its remarkable fundamental properties, versatility and potential for various technological applications. In this context, this dissertation aimed to synthesize and characterize pure and doped Zn2SiO4 with manganese atoms and its correlation with photoluminescent properties. In this work, the chemical preparation method known as co-precipitation was used. Thus, the morphological characterization was performed by the high resolution scanning electron microscopy (FEG-SEM) technique, which demonstrates a grain cluster, making the size determination difficult. The structural properties of nanoparticles, as well as their physical and chemical properties, were determined by X-ray diffraction, Raman spectroscopy and photoluminescence techniques, with which it is possible to observe the formation of na... (Complete abstract click electronic access below) / Mestre

<p>Climate change and the energy crisis are substantial challenges facing the human species, and they are projected to threaten life on our planet. For millions of years, the sun has been the main source of energy for life on Earth; this inspires ongoing research efforts focusing on a “sunlight to fuel” energy solution. Photosynthesis is nature’s tool to derive energy from the sun. Hence, scientists focus on the biochemistry of this phenomenon to employ photosynthesis in a man-made device. Such a device is able to convert solar energy to chemical energy through a light-driven cycle of the chemical reactions which produce hydrogen gas, later used as fuel. This process, often called “artificial photosynthesis,” needs efficient catalysts which can be incorporated into a molecular assembly and other microscopic structures or immobilized on an electrode surface. </p><p>Additionally, evolution, in the course of billions of years, chose manganese as an abundant and effective metal to facilitate the process of photosynthesis. These manganese atoms formed a cluster and an optimized ligand field to maximize efficiency. The photochemistry and photo-physics process behind photosynthesis is yet to be fully understood and implemented in a man-made apparatus with comparable efficiency and durability. </p><p>Photosynthesis requires a source of electrons. Water is an abundant molecule on earth that can provide the electrons needed for the photosynthesis. Although water is ubiquitous, it is one of the most stable molecules; hence, splitting it demands a well-designed system with strong oxidizing capability. Because a single atom of oxygen is highly reactive, there should be at least four oxidation states in the system to remove four electrons and release molecular oxygen: O2. The O-O bond formation is one of the most important steps in photosynthesis to fully understand. Lacking a thorough knowledge of this step prevents design and fabrication of robust and active water oxidizing catalysts. To fully understand O-O formation, one should perform a comprehensive study of each of the intermediates of the system. In other words, we need an understanding of the structure and electronic configuration of the system (natural or artificial) from the moment that a water molecule attaches to the catalyst (usually a metal core, central in the complex), until the moment that oxygen released as an O2 molecule. </p><p>There are multiple possible mechanisms to explain O-O formation. Two mechanisms that were extensively studied in this thesis are water nucleophilic attack and radical coupling. The prevailing view about oxygen formation in the catalysts that we study here explains the O-O bond formation by nucleophilic attack of a water molecule to a highly oxidized ruthenium (RuV=O) species. In this hypothesis, all polypyridine ligands that are coordinated to ruthenium remain neutral during the water oxidation process, while the formation of RuV=O (the key intermediate) would require a relatively high free energy (about 1.8 to 2 eV); use of computational (numerical) calculations determine this to be thermodynamically inaccessible. Furthermore, the failure of spectroscopic techniques to confirm the presence of RuV=O calls the validity of this model into question.)</p><p>Alternatively, radical coupling hypothesis considers another pathway to oxygen bond formation. Here, one of the nitrogen atoms coordinated to ruthenium in polypyridine plays a crucial role. We hypothesize that after formation of RuIV=O (which is spectroscopically observed), one nitrogen decoordinates from the metallic core (ruthenium) and oxidizes to form Ru-ON species. This N-oxide ligand can be further oxidized to form a ligand cation radical. It has been shown that [ligand-NO]+• can have almost no energy barrier for O-O bond formation via spin alignment. The study of the role of N-oxide is one of the main focuses of this work. Since this hypothesis does not require RuV=O nor water nucleophilic attack, it explains the process of water oxidation and opens further avenues for the design of future catalysts.</p><p>To confirm our hypothesis, I employed several spectroscopic methods and computational calculations. This new pathway predicts new intermediates exclusive to this model. Our objective is to prove their presence by in situ spectroscopy and test the possibility of formation of each intermediate computationally, to see if their formation is thermodynamically feasible. </p><div><br></div>

<div>Global warming is considered to result from excessive emission of CO₂ caused by human activity. The security of long term CO₂ capture and sequestration on the subsurface depends on the integrity of caprocks. Natural and engineered subsurface activities can generate fractures in caprocks that can lead to CO₂ leakage. Reactive fluids that flow through a fracture may seal a fracture through mineral precipitation or open a fracture through dissolution. It is extremely useful to CO₂ storage to understand the behavior of reactive fluids that generates mineral precipitation that can seal a fracture. Experiments on non-reactive and reactive fluid mixing were performed to explore gravity-driven chemical dynamics that control the mixing and spatial distribution of mineral precipitates. Fracture inclination, fracture apertures, fluid pumping rates, and density contrasts between fluids were studied for their effects on fluid mixing. From non-reactive fluid mixing experiments, a less dense fluid was found to be confined to a narrow path (runlet) by the denser fluid under the influence of gravity. Fracture inclination angle affected the shape of the less dense fluid runlet. As the angle of inclination decreased, the area of the less dense runlet increased. Improved mixing and a potentially larger area of precipitation formation will occur during reactive fluid mixing when the fracture plane is perpendicular to gravity. Fracture aperture affected the time evolution of the mixing of the fluids, while pumping rate affected fluid mixing by controlling the relative velocities between the two fluids. The fact that the spatial distribution of the two fluids, instead of the fracture roughness, dominated the fluid mixing sheds light on the potential behaviors of reactive fluids mixing in fractures. The location for the majority of precipitation formation and the transport of precipitates can be accordingly predicted from knowledge of the properties of the two reactive fluids and the orientation of the fracture.</div><div>From a small study on wave propagation across fractures with precipitates, simulation results showed that the impedance difference between the matrix material and the precipitate affects the transmitted signal amplitude. Both the aperture and fraction of aperture filled with precipitates affect signal amplitude.</div><div><br></div>

The detection of Earth-like exoplanets and the characterization of their atmospheres is a challenge one needs to solve to assess their habitability and the presence of life in the universe. If this challenge is still unresolved today, even in the era of giant telescopes, it is mainly because of the very high contrast between these exoplanets and their host star and also their proximity. To overcome both of these constraints, a new method combining high-dispersion spectroscopy and nulling interferometry has been imagined. The idea is to use the nulling interferometry to attenuate the star light emission and detect the inner rocky planets with a high angular resolution. The high-dispersion spectroscopy is increasing the exoplanet detectability significantly which enables to relax the star attenuation requirement for an Earth-like observation. Our simulation made for an exoplanet similar to the Earth orbiting Proxima Centauri is giving a condition for the star attenuation ∼10−4 to detect it. Given this condition, we are able to evaluate the unability of a photonic device at our disposal to achieve such performance without dealing with its limitations. If a future project manages to overcome these limitations, this device could be part of a precursor instrument at IPAG to demonstrate experimentally the performance of high-dispersion nulling interferometry. / Upptäckten av jordliknande exoplaneter och karakteriseringen av deras atmosfärer är en utmaning man behöver lösa för att bedöma deras beboelighet och närvaron av liv i universum. Om denna utmaning fortfarande inte är löst idag, även i jätteteleskopens tid, beror det främst på den mycket höga kontrasten mellan dessa exoplaneter och deras värdstjärna och också deras närhet. För att övervinna båda dessa begränsningar har en ny metod som kombinerar högdispersionsspektroskopi och nullingsinterferometri föreställts. Idéen är att använda nullingsinterferometrin för att minska stjärnljusemissionen och upptäcka de inre steniga planeterna med hög vinkelupplösning. Spektroskopin med hög dispersion ökar exoplanetens detekterbarhet betydligt vilket gör det möjligt att minska stjärndämpningsbehovet för en jordliknande observation. Vår simulering för en exoplanet som liknar jorden som kretsar omkring Proxima Centauri ger ett tillstånd för stjärndämpningen att ∼10−4 för att upptäcka den. Med tanke på detta villkor kan vi utvärdera oförmågan hos en fotonisk enhet till vårt förfogande för att uppnå sådan prestanda utan att hantera dess begränsningar. Om ett framtida projekt lyckas övervinna dessa begränsningar kan den här enheten vara en del av ett föregångarinstrument på IPAG för att experimentellt visa prestanda för högdispersionsnullningsinterferometri.

The objectives of the present project were to set up, optimise and characterise a digitalholographic microscopy (DHM) laboratory set-up designed for the study of eyetissue and to implement and optimise digital data processing and noise reductionroutines. This work is part of a collaborative project aiming to provide quantitativemethods for the in vitro and in vivo characterisation of human corneal transparency.The laboratory set-up is based on a commercial laboratory microscope with zoomfunction (a “macroscope”). In continuation of previous work, we completed and optimised,and extended a software for holographic signal processing and numericalpropagation of the wavefront.To characterise the set-up and quantify its performances for standard operationand in its DHM configuration, we compare the magnification and resolution to theoreticalvalues for a given set of parameters. We determined the magnification factorand the rotation angle between the object and camera planes. With a laser wavelengthof 532 nm, a x1 objective and a zoom setting of x2.9 (which corresponds to aplane sample wavefront), we measured a magnification of 1.68. With the same parameters,we measure a holographic resolution of about 11 m. The wavefront phasecould be determined with a precision of a fraction of the wavelength.We subsequently performed analysis of the relative contribution of coherent noiseand implemented and evaluated several noise reduction routines. While the impactof coherent noise remained visible in the amplitude image, interferometric precisionwas obtained for the phase of the wavefront and the set-up was considered qualifiedfor its intended use for corneal characterisation.A first test measurement was performed on primate cornea.Subsequent work will address the further quantitative characterisation of the setupfor the full set of parameters (objectives, zoom positions, wavelengths), test measurementson samples with known transmission and light scattering properties (e.g.solutions of PMMA beads) and the comparison of the results with the predictions ofa theoretical model, and measurements on animal and human tissue.

A Kitaev quantum spin liquid is a phase of matter predicted to host excitations that can be used to preform fault-tolerant quantum computation. Though the theoretical prediction of such a state is on firm footing, its realisation in real materials has proven to be elusive. Recent developments have suggested honeycomb materials consisting of 3d transition metal ions as possible candidates. The focus of this thesis is the magnetic properties of one such material, K2Ni2–xCoxTeO6. It is part of a family of layered two dimensional materials consisting of honeycomb structured transition metal layers sandwiched between layers of alkali ions. A characterisation of the magnetic properties of K2Ni2–xCoxTeO6 has been carried out with the techniques of muon spin rotation/relaxation/resonance and bulk magnetisation as a function of the chemical composition. Further investigations of the detailed atomic structure and spin order using neutron scattering was also initiated. The results of such characterisations are presented and discussed in this thesis. / En Kitaev kvantspinvätska är en fas av materia som har förespåtts kunna husera exciterade tillstånd som kan användas for att konstruera en kvantdator. Även om de teoretiska rönen är väl underbyggda, har ett förverkligande av en sådan fas i verkliga material varit svår att åstadkomma. Nya rön har pekat ut bikakematerial bestående av 3d övergångsmetaller som potentiella kandidater. Därav fokuserar denna avhandling på ett sådant material, K2Ni2–xCoxTeO6. Det är en del av en familj av liknande material bestående av tvådimensionella lager av bikakeformade övergångsmetaller mellan lager av alkaliska joner. En karaktärisering av de magnetiska egenskaperna av K2Ni2–xCoxTeO6 har utförts genom att analysera data från myon spin rotation/dämpning/resonans samt magnetiserings mätningar som funktion av materialets kemiska samansättning. Ytterligare mätningar av den atomära strukturen och spinordning påbörjades också med hjälp av neutronspridningstekniker. I denna avhandling presenteras och diskuteras resultaten av dessa karaktäriseringar.