No description available.

The aim of the project behind this report is to gain an understanding for the design criteria of launch pad flame deflectors used for launching rockets. This report presents an overview of the underlying phenomena, and then tackles the issue of estimating noise levels that come about during lift-off. Semi-empirical methods are used to accomplish this, and the same basic processes as those presented in a 1971 NASA report are used. The underlying assumptions and approximations used in this report are analysed, and a number of modifications are suggested. The results of these modifications are presented in comparison to the original model as well as data taken from the Ares I-X launch. It is found that the suggested methodology tends to overpredict where the SP-8072 method underpredicts, and that additional considerations regarding launch pad structure and water attenuation will need to be included for a more detailed understanding. Furthermore, the topic of acoustic reflectivity is investigated. It is demonstrated that this aspect is heavily dependent on the dimensions of the flame duct immediately downstream of the nozzle, and that further development is warranted. / Målet med detta projekt är att öka förståelsen för flamdeflektorer och vad som avgör utformningen av dessa. Rapporten presenterar en översikt av de fenomen och mekanismer som alstrar ljud, och fokuserar sedan på att beskriva hur dessa ljudnivåer uppskattas. Semi-empiriska metoder används, och de processer som presenteras i en NASA-rapport från 1971 tillämpas. De antaganden och approximationer som används i denna analyseras, och en rad ändringar föreslås. Resultaten från dessa ändringar presenteras tillsammans med den oförändrade modellen tillsammans med uppmätt data från uppskjutningen av Ares I-X. Den föreslagna modellen visar sig överuppskatta där den oförändrade tenderar att underuppskatta. Ytterligare aspekter gällande uppskjutningsplattformens geometri samt den dämpande effekten vatten har behöver iakttas för en mer detaljerad modell. Akustisk reflektivitet och dess ljudbidrag undersöks. Det visas att denna aspekt beror starkt på dimensionerna för avgaskanalen direkt nedströms från dysan. Det finns därmed anledning för vidareutveckling av denna modell.

The exploitation of the soft x-ray/extreme ultra-violet (EUV) region of the electromagnetic spectrum is possible mainly due to the development of multilayer (ML) mirrors. This region of the electromagnetic spectrum offers great opportunities in both science and technology. The shorter wavelength allows one to see smaller features in microscopy and write finer features in lithography. High reflectivity with moderate spectral bandwidth at normal/near-normal incidence can be achieved in soft x-ray/ EUV spectral range using these ML mirrors, where natural crystals with the required large periodicity are not available. These MLs are generally artificial Bragg’s reflectors, which consist of alternative high and low density materials with periodicity in the nanometer range. The main advantages of ML optics stem from the tunability of layer thickness, composition, lateral gradient, and the gradient along the normal to the substrate; these can be tailored according to the desired wavelength regime. They have the great advantage of being adaptable to figured surfaces, enabling their use as reflective optics in these spectral regions, for focusing and imaging applications. Broadband reflectivity and wavelength tunability are also possible by using MLs with normal and lateral gradient, respectively. However, fabrication of these ML mirrors requires the capability to deposit uniform, ultra-thin (a few angstroms-thick) films of different materials with thickness control on the atomic scale. Thus, one requires a proper understanding of substrate surfaces, individual layers, chemical reactivity at interfaces and, finally, of the ML structures required for particular applications. The performance of these MLs is limited by (the lack of) contrast in optical constants of the two materials, interfacial roughness, the chemical reactivity of two materials and, finally, errors in the thickness of individual layers. Soft x-ray/extreme ultra-violet ML mirrors have found a wide range of applications in synchrotron radiation beam lines, materials science, astronomy, x-ray microscopy, x-ray laser, x-ray lithography, polarizers, and plasma diagnostics. The Indus–1 synchrotron radiation (SR) source is an operational 450 MeV machine, which produces radiation up to soft x-rays. Indus-2 is a 2.5 GeV machine, which has been commissioned recently to produce hard x-rays (E > 25 keV). The combination of Indus-1 and Indus-2 will cover a broad energy spectrum from IR to hard x-rays. Therefore, there is a significant need and opportunity to study MLs of different pairs of materials, with different parameters such as periodicity and optimum thickness of individual layers. The goal of the present thesis is to fabricate MLs for soft x-ray optics and to study their physics for application as polarizers in the wavelength range from 67 Å to 160 Å on the Indus-1 synchrotron source. To accomplish this task, a UHV electron beam evaporation system has been developed indigenously for the fabrication of MLs. Three different ML systems viz., Mo/Si, Fe/B4C and Mo/Y have been fabricated, and their surfaces and interfaces were investigated thoroughly for the polarizer application. X-ray reflectivity (XRR) has been used extensively in the investigations of these MLs. This is because XRR is a highly sensitive non-destructive technique for the characterization of buried interfaces, and gives microscopic information (at atomic resolution) over a macroscopic length scale (a few microns). Numerical analysis of XRR data has been carried out using computer programs. Depth-graded x-ray photoelectron spectroscopy (XPS) has been used for compositional analysis at interfaces for some of the ML structures, as a technique complementary to XRR. The performance of some of these MLs has been tested in the soft x-ray region, using the Indus-1 synchrotron radiation (SR) source. Prior to studying the MLs, a detailed study of the surfaces and interfaces of thin films, bi-layers, and tri-layers was carried out using XRR and the glancing incidence fluorescence technique. The discontinuous-to-continuous transition and the mode of film growth, which are vital to the optimization of layer thickness (basically for the high-atomic number or high-Z layer) in the ML structures, were also investigated using in situ sheet resistance measurement method. Indus-1 is a soft x-ray SR source that covers atomic absorption edges of many low-Z materials. The present work demonstrates the possibilities of characterizing low-Z thin films and multilayers using soft x-ray resonant reflectivity. In one case, we have shown for first time that soft x-ray resonant reflectivity can be employed as a non-destructive technique for the determination of interlayer composition. In a second study using the Indus-1 SR source, we have shown, by observing the effect of the anomalous optical constant on reflectivity pattern when photon energy is tuned across the atomic absorption edge of the constituent low-Z element, that soft x-ray resonant reflectivity is an element-specific technique. This thesis is organized into 7 chapters. A brief summary of individual chapters is presented below. Chapter 1 gives a brief general introduction to x-ray ML optics. This is followed by a discussion of the importance of the soft x-ray region of electromagnetic radiation. The optical properties of x-rays are reviewed and optical constants are calculated for some of the important materials used for x-ray MLs. The refractive index in the x-ray region being less than unity (except absorption edges), the consequent limitation of conventional transmission lenses is discussed. The limitation of glancing angle incidence optics is presented, motivating the need for ML optics, which is discussed along with a theoretically calculated reflectivity profile. The procedure for materials for the MLs for application in different spectral regions is discussed, along with a survey of literature related to the present thesis. The importance of the quality of surfaces and interfaces on the performance of ML structures has been shown through simulations. The applications of soft x-ray MLs are discussed with emphasis on polarization. This is followed by a review of different modes of growth of thin films. Finally, the scope of the present work is highlighted. Chapter 2 provides brief descriptions of the experimental techniques used in the present investigations and of the numerical methods employed for quantitative data analysis. The XRR technique is discussed elaborately because it has been used extensively. Detailed calculations of x-ray reflectivity from single surfaces, thin films and bi-layers are presented, along with simulated values. The effect of critical angle and Brewster’s angle is also discussed. Data analysis methods for computing x-ray reflectivity from multilayer structures, based on dynamical and kinematical models, have been discussed. The effect of roughness on XRR has been discussed based on the recursion formalism of dynamical theory. Simulations of XRR and experimental XRR data fitting are carried out using computer programs. The XRR experimental set up is also outlined. A theoretical background is given for the electrical measurements on thin films. This is followed by a brief overview of x-ray photoelectron spectroscopy (XPS) and interpretation of spectra. Finally, the glancing incidence x-ray fluorescence (GIXRF) technique is outlined. Chapter 3 describes in detail the ultra-high vacuum electron beam evaporation system developed in house especially for the fabrication of thin films and x-ray multilayer optics. At the outset, a brief overview of different deposition techniques commonly used for the fabrication of x-ray optical elements is presented. Design, fabrication, and assembly of different accessories are discussed. The control of thickness and uniformity of the films deposited has been checked through the experiments, whose results are provided. The results obtained for ML test structures are presented to show the capability of system in carrying out fabrication of high quality x-ray ML structures. Finally, the versatility of evaporation system incorporating in situ characterization facilities such as -situ electrical measurements for different substrate temperatures is illustrated. Chapter 4 presents a study of the growth of ultra-thin Mo films at different substrate temperatures using in situ sheet resistance measurements. First, a theoretical background is given on the different stages of island growth and on factors affecting thin film growth, followed by a discussion of the possible electrical conduction phenomena in continuous and discontinuous metal films. The nature of thin film growth and a detailed microscopic picture at different growth stages are derived from a modeling of sheet resistance data obtained in situ. The various conduction mechanisms have been identified in different stages of growth. In the island growth stage, the isotropic and anisotropic growth of Mo islands is identified from the model. In the insulator-metal transition region, experimentally determined values of critical exponent of conductivity agrees well with theoretically predicted values for a two-dimensional (2D) percolating system, revealing that Mo films on float glass substrate is predominantly a 2D structure. The minimum thickness for which Mo films becomes continuous is obtained as 1.8 nm and 2.2 nm for Mo deposited at substrate temperatures 300 K and 100 K, respectively. An amorphous-to- crystalline transition is also observed, and discussed. Chapter 5 covers the detailed study of the surfaces and interfaces studies in three different ML structures viz., Mo/Si, Fe/B4C and Mo/Y, meant for the polarizer application in the wavelength range of 67 Å to 160 Å. Multilayers with varying periodicity, varying number of layer pairs, and different ratios of high-Z layer thickness to the period, were fabricated using the electron beam system. Initially, a brief overview of the design aspects of ML structures is given, along with the theoretically calculated reflectivity at Brewster’s angle from the best material combinations. In Mo/Si MLs, the interlayer formed at the interfaces due to interdiffusion of the two elements is asymmetric in thickness, i.e., Mo-on-Si interlayer is thicker than the Si-on-Mo interlayer. To take account of these interlayers in XRR data fitting, a four layer model is considered. The effect of interlayers on reflectivity pattern was studied using simulations, and differences with respect to roughness are also discussed. The mechanism of formation of asymmetric interlayers is also discussed. The interlayer composition has determined using depth-graded XPS. The results reveal the formation of the MoSi2 composition at both the interfaces. The experimental results agree well with theoretical calculations based on solid-state amorphization reaction, which is a result of large heat of mixing. The effective heat of formation model reveals the formation of MoSi2 as the first phase. The soft x-ray reflectivity performance of the Mo/Si ML structure at Brewster’s angle is tested using Indus-1 synchrotron radiation (SR). Using XRR and GIXFR, a study of the surfaces and interfaces of bilayers of B4C-on-Fe and Fe-on- B4C, and tri-layers of Fe-B4C-Fe was carried out, with a systematic variation of Fe and B4C layer thicknesses. A sharp interface was observed in Fe-on-B4C, whereas a low density (w.r.t. Fe) interlayer is observed at the B4C-on-Fe interface. The interlayer properties fluctuates w.r.t. the bottom Fe layer thickness and is independent of the top B4C layer thickness. The nature of fluctuations has been discussed in detail. A study of the surfaces and interfaces of Fe/B4C MLs is described. Finally, a study of the surfaces and interfaces of bilayers, tri-layers, and MLs of the Mo/Y system is discussed in detail. Chapter 6 describes the application of soft x-ray resonant reflectivity for the characterization of low-Z thin films and interfaces in multilayer structures. Initially, a discussion of the energy dependence of atomic scattering factors and hence of optical constants is provided with simulations, with emphasis on the atomic absorption edge. Then, a brief overview of synchrotron radiation, with particular emphasis on the parameters of the Indus-1 synchrotron source is given. The possibilities of determining the composition of the buried interlayer with sub-nanometer scale sensitivity using soft x-ray resonant reflectivity are discussed. The methodology has been applied to study the Mo/Si interface both by simulations and by experiments on the Indus-1 SR, by tuning the photon energy to the Si L-absorption edge. Finally, direct evidence of elemental specificity of soft x-ray resonant reflectivity through the observation of the effect of anomalous optical constants on the reflectivity pattern is discussed. We demonstrate the method through simulations and experiments on the B4C material in B4C thin films and Fe/ B4C bi-layers, using Indus-1 SR tuned to the boron Kedge. Chapter 7 summarizes the main findings of the present work, and provides an outlook for further investigations in the field.

The work presented in this thesis was broadly focussed on the investigation of the magnetic behaviour of different superconducting materials in the form of bulk (singe crystals and pellets) and thin films (nanomagnetic devices like superconducting spin valves etc). Neutrons and muons were extensively used to probe the structural and magnetic behaviour of these systems at the nanoscale along with bulk characterisation techniques like high-sensitive magnetic property measurements, scanning probe microscopy and magneto-transport measurements etc. The nanoscale interplay of Superconductivity and Ferromagnetism was studied in the thin film structures using a combination of Polarised Neutron Reflectivity (PNR) and Low Energy Muon Spin Rotation (LE-µSR) techniques while bulk Muon Spin Rotation (µSR) technique was used for microscopic magnetic investigation in the bulk materials. In the Fe/Pb heterostructure, evidence of the Proximity Effect was observed in the form of an enhancement of the superconducting penetration depth (λs) with an increase in the ferromagnetic layer thickness (dF) in both the bilayered and the trilayered structures. The existence of an Inverted Magnetic Region was also detected at the Ferromagnet-Superconductor (F/S) interface in the normal state possibly originating from the induced spin polarisation within the Pb layer in the presence of the neighbouring Fe layer(s). The spatial size (height and width) of the Inverted Magnetic Region did not change much while cooling the sample below the superconducting transition temperature(Tc)and it also stayed unaffected by an increase in the Fe layer thickness and by a change of the applied magnetic field. In the superconducting spin valve structure containing Permalloy (Py) as ferromagnetic layer and Nb as the superconducting layer, LE-µSR measurements revealed the evidence of the decay of magnetic flux density (as a function of thickness) within the Nb layer symmetrically from the Py/Nb interfaces towards the centre of the Nb layer in the normal state. The thickness dependent magnetisation decay occurred over two characteristic length scales in the normal state that stayed of similar values in the superconducting state also. In the superconducting state, an additional contribution towards the magnetisation was found in the vicinity of the Py/Nb interfaces possibly originating from the spin polarisation of the singlet Cooper pairs in these areas. The nanoscale magnetic investigation on a highly engineered F/S/F structure (where each of the F blocks made of multiple Co/Pd layers with magnetic moments aligned perpendicular to the plane of these layers and neighbouring magnetic blocks separated by Ru layers giving rise to antiferromagnetic alignment) using LE-µSR showed an antisymmetric thickness dependent magnetic flux density profile with two characteristic length scales. In the superconducting state, the magnetic flux density profile got modified within the superconducting Nb₆₇Ti₃₃ layer near the F/S interfaces in a way similar to that of observed in the case of Py/Nb system, most likely because of the spin polarisation of the superconducting electron pairs. The vortex magnetic phase diagram of Bi₂Sr₂Ca₂Cu₃O10-δ was studied using the Muon Spin Rotation (µSR) technique to explore the effects of vortex lattice melting and rearrangements for vortex transitions and crossover as a function of magnetic field and temperatures. At low magnetic fields, the flux vortices undergo a first order melting transition from a vortex lattice to a vortex liquid state with increasing temperature while another transition also occurred with increasing field at fixed temperature to a vortex glass phase at the lowest temperatures. Evidence of a frozen liquid phase was found in the intermediate field region at low temperature in the form of a lagoon in the superconducting vortex state which is in agreement with earlier observations made in BiSCCO-2212. The magnetic behaviour of the unconventional superconductor Sr₂RuO₄ was investigated using µSR to find the evidence of normal state magnetism and the nature of the vortex state. In the normal state, a weak hysteretic magnetic signal was detected over a wide temperature and field range believed to be supporting the evidence of a chiral order parameter. The nature of the vortex lattice structure was obtained in different parts of the magnetic phase diagram and the evidence of magnetic field driven transition in the lattice structure was detected from a Triangular→Square structure while the vortex lattice stayed Triangular over the entire temperature region below Tc at low fields with a disappearance of pinning at higher temperatures.

Faisant confiance à leur intuition les urbanistes s’inscrivent souvent dans un flou théorique qui les rend susceptibles de succomber à l’idéologie dominante et aux modes du moment. La première partie de ce mémoire vise à démontrer que cette posture par rapport à la théorie entraine un manque de réflexivité qui constitue un obstacle majeur à l’exercice d’une pratique urbanistique véritablement professionnelle, éthique et respectueuse du territoire. En effet, selon Bourdieu, un corpus théorique autonome permettrait d’offrir à l’urbaniste un cadre de réflexivité lui permettant de prendre du recul sur le quotidien et d’échapper à l’influence indue de l’idéologie sur sa pratique. Ce mémoire vise à comprendre la configuration actuelle du champ des théories de l’urbanisme en Occident (compris selon la théorie des champs de Bourdieu) et à proposer un modèle synthèse spécifiquement destiné à servir d’outil de réflexivité à l’urbaniste. Pour y parvenir, il a été privilégié de faire une relecture herméneutique de modèles théoriques existants, mais considérés comme incomplets, des théories de l’urbanisme. Dans ce domaine, deux Écoles de pensée ont été définies (École française et École anglo-saxonne). Ensuite, une typologie emblématique de chaque École a été choisie pour être revue à la lumière de l’objectif fixé. Ainsi, les typologies de Françoise Choay (École française) et de Philip Allmendinger (École anglo-saxonne) ont été analysées et mises en commun au moyen de la construction d’une historiographie commune des théories de l’urbanisme. Finalement, un modèle théorique original (la typologie intégrée) a été formulé. Celui-ci définit quatre grands courants théoriques : le rationalisme instrumental, le rationalisme intersubjectif, le mouvement pragmatico-postmoderne et le mouvement patrimonial. Il apparaît alors que le rationalisme occupe une position hégémonique dans le champ des théories de l’urbanisme depuis sa fondation et qu’elle est assumée par le rationalisme intersubjectif depuis le milieu des années 1990. / Relying on their intuition, urbanists are often in a blurred theoric frame which make them more likely to go into the dominant ideology and the current main trends. The first part of this master thesis intend to illustrate that this attitude toward theory is leading to a lack of reflexivity which is an major obstacle to the exercice of a trully professional, ethical and respectful toward territory urbanistic practice. According to Pierre Bourdieu, an autonomous theoric corpus would give the urbanist a frame for reflexivity in order to take step back from his daily exercice of urban planning and to escape the concealed influence of ideology. This thesis intend to understand the actual outline of the field of urbanism theories (understood following the theory of fields of Bourdieu) in the western countries and to propose a synthetic theoric frame specifically dedicated to be use as a reflexivity tool by the urbanist. In order to achieve this goal, an hermeutical reading of the existing, but incomplete, urbanism theories models has been selected. Regarding this matter, two School of toughts has been outlined : the French and the Anglo-saxon Schools. After, a typology of urbanism theories representing each school has been selected to be re-examine. Thus, Françoise Choay’s (for the French School) and Philip Allemndinger’s (for the Anglo-saxon School) typologies has been analysed and put togheter by the construction of a common historiography of the urbanism theories. Finally, a brand new theoritical model (the integrated typology) has been proposed. It propounds that the field of urbanism theories has four great theoritical movements : the instrumental rationalism, the intersubjective rationalism, the pragmatic-postmodern movement and the heritage based movement. In conclusion, rationalism appears as the hegemonic trend in the field of urbanism theories since its foundation and that this dominant position is now overtake by the intersubjective rationalism movement since the mid-1990s.

Dans de nombreux cas, le développement de surfaces aux propriétés adhésives spécifiques fait appel à l’utilisation « d’interfaces décorées ». Ces interfaces sont composées d’un substrat solide sur lequel des chaînes de polymère sont plus ou moins bien ancrées. Ces chaînes se couplent mécaniquement au matériau environnant et contrôlent la transmission des contraintes de friction et d'adhésion aux interfaces. Ce couplage dépend en particulier de la pénétration des chaînes de surface dans la matrice et de leur dynamique. Dans cette thèse, les systèmes que nous avons étudiés sont constitués d’une couche de chaînes de polymère dont une extrémité est liée de manière covalente à un substrat solide. Ces brosses de polymère, constituent un système modèle pour des interfaces décorées. Notre objectif a été d’étudier la conformation et la dynamique de ces chaînes greffées lorsque ces dernières sont soumises à différents types de sollicitations afin de comprendre les mécanismes moléculaires régissant les propriétés d’adhésion et de friction de ce type d’interface. Dans un premier volet, nous avons étudié par réflectivité de neutrons la cinétique de cicatrisation d'une interface composée initialement de chaînes greffées recroquevillées sur un substrat et en contact avec un fondu. Lorsque le système est amené à une température supérieure à la température de transition vitreuse, les chaînes de polymère retrouvent une mobilité non nulle permettant ainsi la pénétration des chaînes greffées dans le fondu de polymère. La réflectivité de neutrons nous a permis d'une part de sonder à l'échelle moléculaire la cinétique de cicatrisation de ce type d’interface et d'autre part de la quantifier. L'influence des paramètres moléculaires sur cette cinétique de cicatrisation a pu être observée, ce qui nous a permis de proposer un modèle en loi d'échelle permettant d'apporter une interprétation physique au phénomène étudié. La deuxième partie de ce travail de thèse a consisté en l'élaboration d'un dispositif expérimental permettant de cisailler un système brosse/fondu au-dessus de la température de la température transition vitreuse et de geler la conformation des chaînes greffées dans leur configuration cisaillée. L'inversion des spectres de réflectivité neutrons associés a permis de mettre en évidence l'influence du cisaillement sur le degré d'interpénétration entre la brosse et le fondu qui régit la transmission des contraintes de friction sur ce type d'interface. De plus, nous avons pu mesurer la cinétique de relaxation de chaînes greffées, cisaillées au préalable, et la comparer aux expériences d’interdigitation simple. Cette comparaison a permis de divulguer l’importance du type de sollicitation sur la cinétique de relaxation d’une interface brosse / fondu.Nous avons également observé que la cinétique de relaxation et la conformation de chaînes greffées peuvent être altérées lorsque ces dernières sont confinées dans un film d'épaisseur comparable au rayon de giration des chaînes. Une étude systématique par réflectivité de neutrons a permis de mettre en évidence une accélération de la cinétique de relaxation du système en dessous d'une épaisseur critique qui pourrait être interprétée en termes de déplacement de la température de transition vitreuse. Dans un deuxième temps, nous avons étudié le glissement de solutions de polymères sur une surface greffée. La fraction volumique en chaînes libres dans la solution est un paramètre supplémentaire aux trois cas précédents qui contrôle ici le degré d'interpénétration entre chaînes libres et chaînes greffées. Une première approche théorique a permis de dissocier différents régimes de glissements à la paroi en fonction de fraction volumique. Nous avons entrepris une première série d'expériences de vélocimétrie laser après photolyse afin de mesurer le glissement à la paroi de solutions de polymère et de confronter les résultats expérimentaux avec notre approche théorique. / In many cases, the development of surfaces with specific adhesive properties involves the use of "decorated interfaces." These interfaces consist of a solid substrate on which polymer chains are more or less well anchored. These chains are mechanically coupled to the surrounding material and control the transmission of friction and adhesion stresses at the interfaces. This coupling depends particularly on the penetration of the surface chains within the matrix and on their own dynamics. In this thesis, the systems we investigated are composed of a layer of polymer chains whose end is covalently linked to a solid substrate. These, so called, polymer brushes, provide a model system for decorated interfaces. Our objective was to study the conformation and dynamics of these grafted chains when they are subjected to different types of stress in order to understand the molecular mechanisms governing the adhesion and friction properties of this type of interface.In the first part, we investigated the healing kinetics of an interface composed initially of grafted chains collapsed on a substrate and in contact with a molten by using neutron reflectivity. When the system is brought above the glass transition temperature, the polymer chains mobility is high enough to allow the penetration of the grafted chains within the polymer melt. Neutrons reflectivity allowed us to probe at the molecular scale and to quantify the healing kinetics of this type of interface. The influence of molecular parameters on this healing kinetics was observed, which allowed us to propose a scaling law model to give a physical interpretation to the phenomenon studied.The second part of this thesis consisted in the development of an experimental setup which is able to shear a brush / melt interface above the glass transition temperature and to freeze the conformation of chains grafted in their sheared conformation. The inversion of the associated neutron reflectivity spectra made it possible to demonstrate the influence of shear on the degree of interpenetration between the brush and the melt which governs the transmission of stresses. In addition, we measured the kinetics of relaxation of grafted chains previously sheared and we compared it to the interdigitation experiments. This comparison highlighted the influence of the kind of solicitation on the relaxation kinetics of a brush/melt interface.We also observed that the relaxation kinetics and the conformation of the grafted chains may be altered when they are confined in a film which thickness is comparable to the radius of gyration of the chains. A systematic study using neutron reflectivity was conducted and highlighted an acceleration of the relaxation kinetics of the system below a critical thickness which could be interpreted in terms of a shift in the glass transition temperature.Secondly, we studied the slip of polymer solutions onto a grafted surface. The volume fraction of free chains in solution is an additional parameter which controls the degree of interpenetration between free chains and grafted chains. A first theoretical approach showed that different slip regimes can occur as a function of volume fraction. We have undertaken a first series of experiments using laser velocimetry after photobleaching to measure the surface velocity of flowing polymer solutions and to compare the experimental results to our theoretical approach.

The objective of this research is to demonstrate the impact of interconnect process variations, line-edge roughness and size effects on interconnect effective resistivity and ultimately chip performance. The investigation is accomplished through five tasks. In Task I, a new closed-form effective resistivity model, which is a function of line-edge roughness (LER), surface specularity and grain boundary reflectivity, is derived. In Task II, a critical path model is enhanced by including interconnect parasitics using the model in Task I. This enhancement also involves an extensive survey of foundry process data to shed light on the device resistance estimation used in the critical path model in Task II. Task III develops a Monte Carlo (MC) simulation framework called the Fast Interconnect Statistical Simulator (FISS). Using the latest International Technology Roadmap for Semiconductors (ITRS) projections, the FISS projects the impact of interconnect process variations and size effects onto high performance microprocessor units (HP-MPUs). Task IV fabricates metallic interconnect test structures with sub-100nm line-widths. The fifth task statistically calibrates the model from Task I using resistivity data measured from the test structures in Task IV.

In this thesis, we explore the four aspects of fiber Bragg grating sensors: mathematical modeling of Fiber Bragg Grating response/spectral characteristics, fabrication using phase mask, application and interrogation. Applications of fiber Bragg gratings, also known as in-fiber gratings, with emphasis on their sensing capabilities, interrogation of an array of sensors and their performance in structural health monitoring scenario are documented. First, we study the process of photosensitivity phenomenon in glasses, in particular GeO2:SiO2 glasses. For mathematical modeling we consider the 1-D refractive index profile along the propagation axis of an optical fiber drawn from the preform of such glasses. These 1-D index structures exhibit a bandgap for propagation along the fiber axis. We show how the bandgap is dependent on the two structural parameters: index periodicity and effective refractive index. The mathematical model provides the characteristics of three sensor parameters -resonance wavelength also known as the Bragg wavelength (λB ), filter bandwidth (ΔλB ), and reflectivity (R). We show that the evolution of the index structure in germanosilicate glasses is dependent on the inscription parameters such as exposure time, intensity of the laser used for inscribing, the interference pattern, and coherence of the laser system. In particular, a phase mask is used as the diffffacting element to generate the required interference pattern, that is exposed on the photosensitive fiber. We present a mathematical model of the electromagnetic diffraction pattern behind the phase mask and study the effect of the limited coherence of the writing laser on the interference pattern produced by the diffracting beams from the mask. Next, we demostrate the sensing capabilities of the fiber Bragg gratings for measuring strain, temperature and magnetic fields. We report linearity of 99.7% and sensitivity of 10.35pm/◦C for the grating temperature sensor. An array of gratings assigned with non-overlapping spectral windows is inscribed in a single fiber and applied for distributed sensing of structural health monitoring of an aircraft’s composite air-brake panel. The performance of these sensors is compared with the industry standard resistance foil gauges. We report good agreement between the two gauges (FBG and RSG). In some applications it is more desirable to know the spectral content, rather than the magnitude of perturbation. Fiber Bragg gratings sensors can be used to track events that occur in a very small span of time and contain high frequencies. Such applications demand very high speed wavelength demodulation methods. We present two interrogation techniques: wavelength-shift time-stamping (WSTS) and reflectivity division multiplexing (RDM). WSTS interrogation method employs the multiple threshold-crossing technique to quantize the sensor grating fluctuations and in the process produces the time stamps at every level-cross. The time-stamps are assembled and with the a priori knowledge of the threshold levels, the strain signal is reconstructed. The RDM methodology is an extension of the WSTS model to address multiple sensors. We show that by assigning unique reflectivities to each of the sensors in an array, the time-stamps from each of the sensors can be tagged. The time-stamps are collected by virtue of their corresponding pulse heights, and assembled to reconstruct the strain signal of each of the array sensor. We demonstrate that the two interrogation techniques are self-referencing systems, i.e., the speed at which the signals are reconstructed is instantaneous or as fast as the signal itself.

No description available.

No description available.