The electricity generation using photovoltaic (PV) solar cells is the most viable and promising alternative to the fossil-fuel based technologies which are threatening world&rsquo / s climate. PV cells directly convert solar energy into electrical power through an absorption process that takes place in a solid state device which is commonly fabricated using semiconductors. These devices can be employed for many years with almost no degradation and maintenance. PV technologies have been diversified in different directions in recent years. Many technologies with different advantages have been developed. However, with more than %85 percent market share, Si wafer based solar cells have been the most widely used solar cell type. This is partly due to the fact that Si technology is well known from the microelectronic industry. This thesis is concerned with the production of single crystalline silicon solar cells and optimization of process parameters through the characterization of each processing step. Process steps of solar cell fabrications, namely, the light trapping by texturing, cleaning, solid state diffusion, lithography, annealing, anti reflective coating, edge isolation have all been studied with a systematic approach. Each sample set has been characterized by measuring I-V characteristics, quantum efficiencies and reflectance characteristics. The best efficiency that we reached during this study is 10.37% under AM1.5G illumination. This is below the efficiency values of the commercially available solar cells. The most apparent reason for the low efficiency value is the series resistance caused by the thin metal contacts. It is observed that the efficiency upon the reduction of series resistance effect is reduced. We have shown that the texturing and anti-reflective coating have a critically important effect for light management for better efficiency values. Finally we have investigated the fabrication of metal nanoparticles on the Si wafer for possible utilization of plasmonic oscillation in them for light trapping. The self assembly formation of gold nanoparticles on silicon surface has been successfully demonstrated. The optical properties of the nanoparticles have been studied / however, further and more detailed analysis is required.

Quantum well infrared photodetectors (QWIPs) have recently emerged as a potential alternative to the conventional detectors utilizing low bandgap semiconductors for infrared applications. There has been a considerable amount of experimental and theoretical work towards a better understanding of QWIP operation, whereas there is a lack of knowledge on the underlying physics. This work provides a better understanding of QWIP operation and underlying physics through particle simulations using the ensemble Monte Carlo method. The simulator incorporates Gamma, L, and X valleys of conduction band as well as the size quantization in the quantum wells. In the course of this work, the dependence of QWIP performance on different device parameters is investigated for the optimization of the QWIP structure. The simulations on AlGaAs/GaAs QWIPs with the typical Al mole fraction of 0.3 have shown that the L valley of the conduction band plays an important role in the electron capture. A detailed investigation of the important scattering mechanisms indicates that the capture of the electrons through the L valley quantum well (L-QW) affects the device performance significantly when Gamma and L valley separation is small. The characteristics of electron capture have been further investigated by repeating the simulations on QWIPs for quantum well widths of 36 and 44 &Aring / . The results suggest that the gain in the shorter well width device is considerably higher, which is attributed to the much longer lifetime of the photoexcited electrons as a result of lower capture probability (pc) in the device. The effects of the L-QW height on the QWIP characteristics have also been studied by artificially increasing this height from 63 to 95 meV in Al0.3Ga0.7As/GaAs QWIPs. The increase in the L valley (L-QW) height resulted in higher pc and lower gain due to high rate of capturing of these electrons when Gamma and L valley separation is small.

In this study, effect of fillet rolling process on fatigue performance of a diesel engine crankshaft was investigated. Crankshafts from two different materials, were studied / ductile cast iron EN-GJS 800-2 and micro-alloyed steel 38MnVS6. Resonance bending fatigue tests were conducted with crankshaft samples. Test plan according to staircase test methodology was used. Statistical analyses were carried out with the test data by Maximum Likelihood Estimation method in order to calculate the fatigue limits and construct the S-N curves based on Random Fatigue Limit (RFL) and Modified Basquin models. Fatigue limit calculations were also conducted by Dixon-Mood method and by Maximum Likelihood Estimation methodology for Normal and Weibull distributions. Fillet rolling process was simulated by computer based analysis in order to calculate the compressive residual stress profile at the fillet region to shed more light on the mechanisms and effect of fillet rolling. Fatigue performances of crankshafts from two types of materials were evaluated both at unrolled and fillet rolled states. Effect of fillet rolling load on fatigue performance was also evaluated with steel crankshafts. It was found that ductile cast iron showed better performance under bending fatigue tests than the steel crankshaft both at the fillet rolled and unrolled conditions. On the other hand, fillet rolling process was found to be more effective on steel crankshaft than ductile cast iron crankshaft in terms of fatigue performance improvement. It was also seen that fatigue limit increases with the fillet rolling load up to a limit where surface quality is deteriorated. Residual stress analysis showed that a higher magnitude of residual stress can develop on steel crankshaft fillet region whereas the effective depth of the residual stress is higher on ductile cast iron crankshaft with the same rolling condition. Residual stress analysis of steel crankshafts rolled at different rolling conditions show that, peak residual stress increase with the increasing rolling load is not significantly high and main effect of increased rolling load is the increased effective depth of residual stresses. The MLE methodology used in statistical analysis of the test data was found to be effective for life regression and fatigue strength distributions analysis. RFL model has provided better life regression analysis and fatigue limit calculations than Modified Basquin model. Dixon-Mood method was found to be overestimating the fatigue limit.

The ability of a propulsion system is very crucial for the capability of a missile or a rocket system. Unlike liquid propellant rocket motors, the only control mechanism of the thrust value is the propellant geometry in solid propellant rocket motors. When the operation of solid propellant rocket motor has started, it cannot be stopped anymore. For this main reason the advance of dual pulse motor technology has started. The aim of this study is to investigate the geometrical effects of design parameters on the flow behavior of a dual pulse solid propellant rocket motor by using commercial Computational Fluid Dynamics (CFD) methods. For the CFD analysis, a generic dual pulse rocket motor model is constituted. Within this model, initially four different geometry alternatives of Pulse Separation Device (PSD) are analyzed. To begin PSD analyses, mesh sensitivity analyses are performed on one PSD geometry alternative. By defined grid size, the analyses of PSD geometry alternatives are performed. Computed results were compared in terms of flow behavior (flow streamlines, velocity distribution, turbulent kinetic energy&hellip / etc.) with each other. With the selected PSD geometry alternative the effects of L/D ratio (Length/Diameter ratio) of first pulse chamber, Achamb/APSD ratio (Chamber area/PSD opening area) and APSD/Ath ratio (PSD opening area/Throat area) on the flow behavior is investigated. Flow analyses are performed by simulating the unsteady flow of second pulse operation. With the performed analyses, it is aimed to identify generic geometric definitions for a dual pulse rocket motor.

Optical Wireless Communication systems use lasers offering larger bandwidth, which facilitates higher data rates, comparing with radio communication systems. However, its performance is limited by atmospheric conditions, and is a function of wavelength. The objective of this study is the determination of the wavelength interval(s) at which the atmospheric transmittance is relatively high and has relatively low dependence on variations in temperature, relative humidity, wind speed and atmospheric pressure under the conditions such as path altitude of 10 meters, path geometry of horizontal to the Earth&rsquo / s surface and clean (includes no fog, rain or snow etc.) over sea surface atmosphere. The path length is taken to be 15 km. Alanya was assignated as geographical region and the required information about the atmospheric constituents and meteorological parameters was collected. Then, the variations in atmospheric transmittance due to the periodically measured meteorological parameters were calculated (for summer and winter seasons). Finally, individually calculated effects of these parameters on atmospheric transmittance are assembled in order to determine the desired wavelength interval(s). As a result, the most suitable wavelength interval was determined to be about between 3.99 &micro / m and 4.02 &micro / m. In addition, dependencies of atmospheric pressure, temperature, relative humidity, and wind speed on atmospheric transmittance have been established for both winter and summer seasons. Atmospheric transmittance is found to be inversely proportional to temperature, relative humidity and wind speed. The effect of pressure is relatively small comparing with other parameters.

The collision of impulsive gravitational waves, electromagnetic plane waves with collinear polarization and, especially, plane fronted parallel waves (pp waves) are considered. The solution of axisymmetric pp waves is reviewed and the structures of the resulting space-times are investigated with the help of curvature invariants.

The number of horizontal wells is increasing rapidly in all over the world with the growth of new technological developments. During horizontal well drilling, much more complex problems occur when compared with vertical well drilling, such as decrease in load transfer to the bit, tubular failure, tubular fatigue and tubular lock-up. This makes selection of appropriate tubular and making the right drill string design more important. As the total compression load on the horizontal section increases, the behavior of the tubular changes from straight to sinusoidal buckling, and if the total compression load continues to increase the behavior of the tubular changes to helical buckling. Determination of critical buckling loads with finite element method (FEM) in horizontal wells is the main objective of this study. Initially, a computer program (ANSYS) that uses FEM is employed to simulate different tubular and well conditions. Four different pipe sizes, four different wellbore sizes and three different torque values are used to model the cases. Critical buckling load values corresponding to significant variables are collected from these simulated cases. The results are classified into different buckling modes according to the applied weight on bit values and the main properties of the simulated model, such as modulus of elasticity, moment of inertia of tubular cross section, weight per unit length of tubular and radial clearance between the wellbore and the tubular. Then, the boundary equations between the buckling modes are obtained. The equations developed in this thesis by simulating the cases for the specific tubular sizes are used to make a comparison between the critical buckling load values from the models in the literature and this work. It is observed that the results of this work fit with literature models as the tubular size increases. The influence of torque on critical buckling load values is investigated. It is observed that torque has a slight effect on critical buckling load values. Also the applicability of ANSYS for buckling problems was revealed by comparing the ANSYS results with the literature models&amp / #8217 / results and the experimental study in the literature.

This thesis, for the first time in the literature, presents the effect of quadrature error compensation on the performance of a fully decoupled MEMS gyroscope and provides experimental data on the sources of quadrature error. Dedicated quadrature error cancellation electrodes operating with only differential DC potentials are designed. Gyroscopes with intentionally placed imperfections are fabricated with SOG based SOI process which provides higher yield and uniformity compared to SOG process. Tests show that the fully closed loop system with quadrature cancellation operates as expected. Gyroscope performance is improved up to 7.8 times for bias instability, 10 times for angle random walk (ARW) and 800 times for output offset with quadrature cancellation. The actual improvement is higher since some sensors cannot be operated without quadrature cancellation and they are not included in improvement calculations. The best obtained performance is bias instability of 0.39

This study focuses on the evaluation of seismic safety of masonry buildings in Turkey by using fragility curves. Fragility curves for masonry buildings are generated by two behavior modes for load bearing walls: in-plane and out-of-plane. By considering the previous research and site investigations, four major parameters have been used in order to classify masonry buildings with in-plane behavior mode. These are number of stories, strength of load-bearing wall material, regularity in plan and the arrangement of walls (required length, openings in walls, etc.). In addition to these four parameters, floor type is also taken into account for the generation of fragility curves by considering out-of-plane behavior mode. During generation of fragility curves, a force-based approach has been used. In this study there exist two limit states, or in other words three damage states, in terms of base shear strength for in-plane behavior mode and flexural strength for out-of-plane behavior mode. To assess the seismic vulnerability of unreinforced masonry buildings in Turkey, generated fragility curves in terms of in-plane behavior, which is verified by damage statistics obtained during the 1995 Dinar earthquake, and out-of-plane behavior, which is verified by damage statistics obtained during the 2010 Elazig earthquake, is combined. Throughout the analysis, ground motion uncertainty, material variability and modeling uncertainty have also been considered. In the final part of the study, a single-valued parameter, called as &lsquo / vulnerability score&rdquo / , has been proposed in order to compare the seismic safety of unreinforced masonry buildings in Fatih sub province of Istanbul and to assess the influence of out-of-plane behavior together with the in-plane behavior of these existing masonry buildings.

During severe floods, bed material around bridge piers and abutments are scoured by the flow and as a result, bridges are subject to damages. These damages are mostly unrepairable and can result in loss of lives and property. In this thesis study, abutment scour under clear-water condition was investigated and collars were tested as scour countermeasures around the abutments. The experimental study was carried out in a rectangular channel with an almost uniform cohesionless bed material of d50=1.50 mm for a test period of 3-hours. The channel was 28.5 m long and 1.5 m wide. The erodible bed material was placed into the test section that was 5.8 m long and 0.48 m deep. For this thesis study, 60 experiments were carried out with and without various collars placed at different elevations around the abutments. The scour formation around the abutments with collars was observed and scour reduction efficiencies of the collars were studied. Experiment results were compared with the previous studies of Dogan (2008) and Kayat&uuml / rk (2005) in terms of sediment size, abutment shape and flow depth, and the effects of these factors on collar&#039 / s scour reduction efficiency were studied. Based on the results of the experimental studies, it was observed that scour depths decreased as the collar width increased and the collar placed deeper into the sediment bed for a given abutment length. When the present study and the previous studies were compared, it was observed that sediment size and flow depth had no significant effect on the scour reduction performances of the collars.