The present work aims to reveal the microstructural evolution and post-processing mechanical behavior of difficult-to-work alloys upon severe plastic deformation. Severe plastic deformation is applied using equal channel angular extrusion (ECAE) where billets are pressed through a 90o corner die achieving simple shear deformation. Three different materials are studied in this research, namely Ti-6Al-4V, Ti-6Al-4V reinforced with 10% TiC and AISI 316L stainless steel. Microstructure and mechanical properties of successfully extruded billets were reported using light microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), tension and compression experiments and microhardness measurements. The effects of extrusion conditions (temperature and processing route) on the microstructure and mechanical properties are investigated. The underlying mechanisms responsible for observed mechanical behaviors are explored. It is seen that ECAE shear deformation leads to refinement in α plates and elimination of prior β boundaries in Ti-6Al-4V. Decreasing extrusion temperature and increasing number of passes decreases α plate size and grain size. Refined α grain size leads to a significant increase in tensile and compressive flow stresses at room temperature. Texture produced by ECAE has a pronounced effect on mechanical properties. Specifically it leads to tension/compression asymmetry in flow strengths and strain hardening coefficients may be described by the activation of differing slip systems under tension and compression loading. ECAE of Ti-6Al-4V+10%TiC samples also improved mechanical properties due to α plate size refinement. Nevertheless, further extrusion passes should be carried out for tailoring reinforcement size and distribution providing optimum strength and ductility. ECAE deformation of AISI 316L stainless steel at high homologous temperatures (0.55 to 0.60 Tm) results in deformation twinning as an effective deformation mechanism which is attributed to the effect of the high stress levels on the partial dislocation separation. Deformation twinning gives rise to high stress levels during post-processing room temperature tension and compression experiments by providing additional barriers to dislocation motion and decreasing the mean free path of dislocations. The highest tensile flow stress observed in the sample processed at 700 oC following one pass route A was on the order of 1200 MPa which is very high for 316L stainless steel. The ultimate goal of this study is to produce stabilized end microstructures with improved mechanical properties and demonstrate the applicability of ECAE on difficult-to-work alloys.

NiTiHf alloys have attracted considerable attention as potential high temperature Shape Memory Alloy (SMA) but the instability in transformation temperatures and significant irrecoverable strain during thermal cycling under constant stress remains a major concern. The main reason for irrecoverable strain and change in transformation temperatures as a function of thermal cycling can be attributed to dislocation formation due to relatively large volume change during transformation from austenite to martensite. The formation of dislocations decreases the elastic stored energy, and during back transformation a reduced amount of strain is recovered. All these observations can be attributed to relatively soft lattice that cannot accommodate volume change by other means. We have used Equal Channel Angular Extrusion (ECAE), hot rolling and marforming to strengthen the 49.8Ni-42.2Ti-8Hf (in at. %) material and to introduce desired texture to overcome these problems in NiTiHf alloys. ECAE offers the advantage of preserving billet cross-section and the application of various routes, which give us the possibility to introduce various texture components and grain morphologies. ECAE was performed using a die of 90º tool angle and was performed at high temperatures from 500ºC up to 650ºC. All extrusions went well at these temperatures. Minor surface cracks were observed only in the material extruded at 500 °C, possibly due to the non-isothermal nature of the extrusion. It is believed that these surface cracks can be eliminated during isothermal extrusion at this temperature. This result of improved formability of NiTiHf alloy using ECAE is significant because an earlier review of the formability of NiTiHf using 50% rolling reduction concluded that the minimum temperature for rolling NiTi12%Hf alloy without cracks is 700°C. The strain level imposed during one 90° ECAE pass is equivalent to 69% rolling reduction. Subsequent to ECAE processing, a reduction in irrecoverable strain from 0.6% to 0.21% and an increase in transformation strain from 1.25% to 2.18% were observed at a load of 100 MPa as compared to the homogenized material. The present results show that the ECAE process permits the strengthening of the material by work hardening, grain size reduction, homogeneous distribution of fine precipitates, and the introduction of texture in the material. These four factors contribute in the increase of stability of the material. In this thesis I will be discussing the improvement of mechanical behavior and stability of the material achieved after various passes of ECAE.

Design and analysis of polysilicon and single crystal silicon gyroscopes have been carried out. Variations in suspension design have been explored. Designs that utilize in-plane and out-of-plane sensing are studied. Damping plays an important role in determining the sense response. Reduction in damping directly affects sensor performance. The various damping mechanisms that are prevalent in gyroscopes are studied. Perforations on the proof mass are observed to significantly reduce the damping in the device when operated in air. The effects of perforation geometry and density have been analyzed. The analysis results show that there is a two orders of magnitude reduction in damping of thick gyroscope structures with optimized perforation design. Equivalent circuit lumped parameter models have been developed to analyze gyroscope performance. The simulation results of these models have been compared with results obtained from SABER, a MEMS specific system level design tool from Coventorware. The lumped parameter models are observed to produce faster simulation results with an accuracy comparable to that of Coventorware Three gyroscopes specific to the PolyMUMPS fabrication process have been designed and their performance analyzed. Two of the designs sense motion out-of-plane and the other senses motion in-plane. Results of the simulation show that for a given damping, the gyro design with in-plane modes gives a resolution of 4º/s. The out-of-plane gyroscopes have two variations in suspension. The hammock suspension resolves a rate of 25º/s in a 200 Hz bandwidth while the design with folded beam suspension resolves a rate of 2º/s in a 12 Hz bandwidth. A single crystal silicon in-plane gyroscope has been designed with vertical electrodes to sense Coriolis motion. This design gives an order of magnitude higher Capacitance change for a given rotation in comparison to conventional comb-finger design. The effects of process induced residual stress on the characteristic frequencies of the polysilicon gyroscopes are also studied. The in-plane gyroscope is found to be robust to stress variations. Analysis results indicate that the tuning fork gyroscope with the hammock suspension is the most susceptible to compressive residual stress, with a significant drop in sensitivity at high stress values.

We discuss two ways of measuring angular diameter distances in the Universe: (i) Baryon Acoustic Oscillation (BAO) , and (ii) strong gravitational lensing. For (i), we study the effects of survey geometry and selection functions on the 2-point correlation function of Lyman- alpha emitters in 1.9 < z < 3.5 for Hobby-Eberly Telescope Dark Energy Experiment (HETDEX). We develop a method to extract the BAO scale (hence a volume-averaged angular diameter distance D_V, which is a combination of the angular diameter distance and the Hubble expansion rate, i.e., [cz〖(1+z)〗^2 〖D_A〗^2 H^(-1) ]^(1/3)) from a spherically averaged 1-d correlation function. We quantify the statistical errors on such measurements. By using log-normal realizations of the HETDEX dataset, we show that we can determine DV from HETDEX at 2% accuracy using the 2-point correlation function. This study is complementary to the on-going effort to characterize the power spectrum using HETDEX. For (ii), a previous study (Para ficz and Hjorth 2009) looked at the case of a spherical lens following a singular isothermal distribution of matter and an isotropic velocity distribution, and found that combining measurements of the Einstein ring radius with the time delay of a strong lens system directly leads to a measurement of the angular diameter distance, D_A. Since this is a very new method, it requires more careful investigations of various real-world eff ects such as a realistic matter density pro file, an anisotropic velocity distribution, and external convergence. In more realistic lens confi gurations we find that the velocity dispersion is the dominant source of the uncertainty ; in order for this method to achieve competitive precision on measurements of DA, we need to constrain the velocity dispersion, down to the percent level. On the other hand, external convergence and velocity dispersion anisotropy have negligible e ect on our result. However, we also claim that the dominant source of the uncertainty depends largely on the image con figuration of the system, which leads us to the conclusion that studying the angular dependence of the lens mass distribution is a necessary component. / text

This dissertation presents a detailed investigation on the development of a quasi Yagi antenna with end fire radiation at 2.4 GHz (ISM band) over a metal reflector. Realization of an end fire radiator on top of a metal ground is very difficult because the reflected waves and image currents from the ground degrade the frequency bandwidth and steer the radiation pattern in the broadside direction. This destructive interference is reduced in this research through two quasi Yagi-ground configurations. The first arrangement utilizes a substrate of suitable thickness (7.5mm-0.19 ?g) to displace the ground away from the antenna. The second design implements a high impedance surface (HIS) as ground plane for the antenna. The preferred HIS is the Jerusalem Cross Frequency Selective Surface (JC-FSS) because of its compact size, numerous parameters for tuning and frequency stability in the operating band for a large angular spectrum of TE and TM polarized incident waves. In this work a new parameter is added to the model used for the derivation of the JC-FSS which accounts for the substrate of the antenna lying on top of the FSS, this addition allows for a smaller cell grid. The results for the quasi Yagi antenna over the 7.5mm grounded slab presented an operational bandwidth of 190 MHz with 40°; of beam steering in the end fire direction while the quasi Yagi over the JC-FSS offered 260 MHz of functional bandwidth and 54° of beam steering towards the end fire direction. In addition, the quasi Yagi design over the JC-FSS decreases the combined profile (antenna/backing structure) by 33% in relation to the 7.5mm grounded slab. This dissertation combines for the first time a quasi Yagi antenna with a JC-FSS to achieve end fire radiation in the presence of a ground metal.

ITC/USA 2013 Conference Proceedings / The Forty-Ninth Annual International Telemetering Conference and Technical Exhibition / October 21-24, 2013 / Bally's Hotel & Convention Center, Las Vegas, NV / We address the problem of detection in orbital angular momentum (OAM). The focus of our analysis will be on the power efficient Q-ary Pulse Position Modulation (Q - PPM). Free space optical signals sent through wireless channels are degraded by atmospheric turbulence. In this paper a novel detection approach based on a factor graph representation of OAM Q-PPM signalling is presented to equalize for the crosstalk among orbital angular momentum vortices. It will be shown that our proposed detection algorithm significantly outperforms the separate detection scenario in terms of error rate performance.

Jouvence of FLUOR (JouFLU) is a major overhaul of the FLUOR (Fiber Linked Unit for Optical Recombination) beam combiner built by the Laboratoire d’études spatiales et d’instrumentation en astrophysique (LESIA) and installed at the CHARA Array. These upgrades improve the precision, observing efficiency, throughput, and integration of FLUOR with the CHARA Array as well as introduce new modes of operation to this high-precision instrument for interferometry. Such high precision observations with FLUOR have provided the first unambiguous detections of hot dust around main sequence stars, showing an unexpectedly dense population of (sub)micrometer dust grains close to their sublimation temperature, 1400 K. Competing models exist to explain the persistence of this dust; some of which suggest that dust production is a punctuated and chaotic process fueled by asteroid collisions and comet infall that would show variability on timescales of a few years. By re-observing stars from the exozodiacal disks survey we have searched for variations in the detected disks. We have found evidence that for some stars the amount of circumstellar flux from these previously detected exozodiacal disks, or exozodis, has varied. The flux from some exozodis has increased, for some the flux has decreased, and for a few the amount has remained constant. These results are intriguing and will be no doubt useful for future modeling of this phenomenon. Furthermore, long-term monitoring is suggested for some of these objects to confirm detections and determine the rate of variation.

The scattering of H2 from Si(100) has been studied using pulsed molecular beam techniques and quantum state-specific detection methods. These studies can be used to test theoretical calculations and give insight into new theories of molecule-surface interactions, a fundamental study in a diverse field of science and technology. In this work, time-of-flight (TOF) spectra of the elastic scattering of H2(v=1. J=1) and H2(v=0, J=1) from clean Si were recorded over a wide range of surface temperatures. Two data processing strategies were developed to extract rich kinematic information from the scattering experiments, e.g., mean translational energy exchange, absolutely survival probability, and angular and speed distribution of the scattered molecules. No such set of quantitative results has been reported before for this system. Compared with close packed metal surfaces, these scattering experiments from a covalently bonded semiconductor surface showed a completely distinct dynamics, e.g. the finding of energy gain instead of loss from the substrate, much broader angular distribution and some counterintuitive surface temperature effects. From the studies of molecules/surface scattering experiments, the thermal excitation on Si(100) surface which depends on surface temperature can substantially alter the adsorption barrier and its distribution, and therefore changes the kinematics of scattered molecules. As a result, even the most basic understanding of the dynamics has to include phonon excitation and deexcitation of the silicon substrate. / text

In this thesis a kinetic theory for spin-1/2 particles is given a brief overview, focusing on the derivation of an evolution equation for the quasiprobability distribution function used in the theory to describe certain types of quantum plasma. The current theory is expanded upon by exploring conservation laws. A local conservation law for momentum is derived using two different expressions for electromagnetic momentum, given by Abraham and Minkowski respectively. There has been some controversy over which of these expressions should be used; in the case considered here the expression given by Minkowski seems to be more suitable. Based on the conservation law for momentum, a conservation law for angular momentum is also derived.

Magistro darbo autorius: Andrius Samuolis Magistro darbo pavadinimas: Judesio kiekio momento ryšio schemos grafo transformacija Vadovas: doc. dr. A. Deveikis Darbas pristatytas: Vytauto Didžiojo Universitetas, Informatikos fakultetas, Kaunas, 2011 birželis. Puslapių skaičius: 46 Lentelių skaičius: 2 Paveikslų skaičius: 43 Priedų skaičius: 3 Sudaromas supaprastintas judesio kiekio momento grafinis metodas skirtas transformacijos matricų vaizdavimui ir supaprastinimui. Siūlomas metodas apibendrina Vilniaus teorinės fizikos mokyklos sukurto grafinio metodo praktinio taikymo patirtį ir apima tik daţniausiai taikomas jo taisykles. Transformacijos matrica supaprastinama suvedant jos pavidalą į 6j ir 9j koeficientus. Metodo taisyklės yra pakankamai paprastos ir gali būti naudojamos atitinkamų kompiuterinių įrankių kūrimui. / Author of Master Thesis: Andrius Samuolis Full title of Master Thesis: Transformation of the angular momentum coupling graph Supervisor: doc. dr. A. Deveikis Presented at: Vytautas Magnus University, Faculty of Informatics, Kaunas, June 2011. Number of pages: 46 Number of tables: 2 Number of pictures: 43 Number of appendices: 3 Establishes a simplified graphical angular momentum method, which is intended to represent and simplified transformation matrix. Suggest method summarizes graphic method practical experience and includes only the most common form of the rules, which were created by Vilnius school of Theoretical Physics. The transformation matrix is simplified when it’s shape is reduced to 6j and 9j coefficients. Principle rules are simple and can be used in the development of computer tools.