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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
31

Studies On The Development Of Magnetoelectric Ceramic Composites

Basaran, Yanki 01 June 2008 (has links) (PDF)
The aim of this thesis work was to develop magnetoelectric (ME) composites consisting of piezoelectric and magnetostrictive components. The piezoelectric constituent was selected as a PZT ceramic modified by strontium, bismuth and manganese. The magnetostrictive phase was nickel ferrite (NF) ceramic doped by cobalt, copper and manganese. The properties of component phases were optimized in order to enhance the ME effect in the composite. In the first part of the thesis, effects of sintering temperature on the dielectric and piezoelectric properties of PZT and on the electrical and magnetic properties of NF ceramics were investigated in the temperature range covered from 1150 to 1250 &deg / C. The best piezoelectric properties in PZT were attained at 1250 &deg / C. At this sintering temperature, values of piezoelectric strain coefficient, dielectric constant, and electromechanical coupling coefficient were 434 pC/N, 1320 and 0.48, respectively. NF ceramics showed poor densification / 80 %TD was attained at 1250 &deg / C. In order to obtain higher densities in ferrites, Bi2O3 was used as a sintering aid. Addition of Bi2O3 enhanced densification up to 97 %TD, and improved electrical and magnetic properties of ferrites. Highest DC-resistivity of 1.15*10^8 ohm-cm and highest magnetostriction of ~26 ppm were attained in NF ceramics doped with 1 wt% Bi2O3. In the second part of the thesis, ME composites were manufactured either as bulk composites or as laminated composites. The efficiency of different composite types was evaluated in terms of voltage output in response to the applied magnetic field. Higher outputs were observed in laminated composites.
32

Ab Initio Design Of Novel Magnesium Alloys For Hydrogen Storage

Kecik, Deniz 01 July 2008 (has links) (PDF)
A candidate hydrogen storing material should have high storage capacity and fast dehydrogenation kinetics. On this basis, magnesium hydride (MgH2) is an outstanding compound with 7.66 wt % storage capacity, despite its slow dehydriding kinetics and high desorption temperature. Therefore in this study, bulk and surface alloys of Mg with improved hydrogen desorption characteristics were investigated. In this respect, formation energies of alloyed bulk MgH2 as well as the adsorption energies on alloyed magnesium (Mg) and MgH2 surface structures were calculated by total energy pseudopotential methods. Furthermore, the effect of substitutionally placed dopants on the dissociation of hydrogen molecule (H2) at the surface of Mg was studied via Molecular Dynamics (MD). The results displayed that 31 out of 32 selected dopants contributed to the decrease in formation energy of MgH2 within a range of ~ 37 kJ/mol-H2 where only Sr did not exhibit any such effect. The most favorable elements in this respect came out to be / P, K, Tl, Si, Sn, Ag, Pb, Au, Na, v Mo, Ge and In. Afterwards, a systematical study within adsorption characteristics of hydrogen on alloyed Mg surfaces (via dynamic calculations) as well as calculations regarding adsorption energies of the impurity elements were performed. Accordingly, Mo and Ni yielded lower adsorption energies / -9.2626 and -5.2995 eV for substitutionally alloyed surfaces, respectively. MD simulations presented that Co is found to have a splitting effect on H2 in 50 fs, where the first hydrogen atom is immediately adsorbed on Mg substrate. Finally, charge density distributions were realized to verify the distinguished effects of most 3d and 4d transition metals in terms of their catalyzer effects.
33

An Evaluation Study On Instrumentation System Of Cindere Dam

Sezgin, Ozgun Ilke 01 August 2008 (has links) (PDF)
In this study, the first hardfill dam in Turkey, Cindere Dam, is evaluated regarding current and alternative measurement systems in detail. Cindere Dam is a notable project, especially with its height as an example of hardfill dam, not only in Turkey but also in the world. First, the current instrumentation system of Cindere Dam is evaluated as a whole with reference to the parameters measured, instruments and data acquisition system. Second, an alternative instrumentation system is developed by using additional parameter and instruments to the current instrumentation system. Furthermore, each instrumentation system equipped with manual and automatic data acquisition systems is considered. The comparison between the current and the alternative instrumentation systems are also carried out in terms of technical and economical feasibility aspects. Although, the current instrumentation system is found to be satisfactory, it is observed that the alternative system promotes the current system in various aspects without increasing the cost significantly.
34

A Comparative Study On Chemical Characterization Of Different Ink Ingredients Used In Ancient Ornamented Manuscripts

Mert, Esra 01 September 2008 (has links) (PDF)
Manuscripts have an essential importance as being irreplaceable parts of our cultural heritage. It has long been known that these works have been suffered from serious damages due to the corrosive effects of the inks. In this study, the influence of inks prepared according to historical recipes on the degradation of model paper has been investigated. Totally, sixteen ink solutions were prepared and applied on Whatman No. 41 filter papers. The mostly examined ink is the iron-gall-ink which is known as very corrosive. Changing the ratios of the ingredients in the ink, it is aimed to understand the degradation mechanism of the cellulose caused by the iron-gall-ink. Influence of iron to gallic acid ratio, copper to iron ratio and the effect of saffron on ink corrosion were examined. Also the influence of colored inks (red and green) / prepared according to the historical recipes on cellulose degradation was studied. In order to determine the changes in the cellulose in time, dry heat accelerated ageing was performed on the paper samples. UV-Vis spectroscopy, FTIR-ATR and Color spectrophotometry were used to investigate the changes in the chemical composition of the cellulose after accelerated ageing.
35

Effect Of Surface Roughness In Microchannels On Heat Transfer

Turgay, Metin Bilgehan 01 December 2008 (has links) (PDF)
In this study, effect of surface roughness on convective heat transfer and fluid flow in two dimensional parallel plate microchannels is analyzed by numerically. For this purpose, single-phase, developing, laminar fluid flow at steady state and in the slip flow regime is considered. The continuity, momentum, and energy equations for Newtonian fluids are solved numerically for constant wall temperature boundary condition. Slip velocity and temperature jump at wall boundaries are imposed to observe the rarefaction effect. Effect of axial conduction inside the fluid and viscous dissipation also considered separately. Roughness elements on the surfaces are simulated by triangular geometrical obstructions. Then, the effect of these roughness elements on the velocity field and Nusselt number are compared to the results obtained from the analyses of flows in microchannels with smooth surfaces. It is found that increasing surface roughness reduces the heat transfer at continuum conditions. However in slip flow regime, increase in Nusselt number with increasing roughness height is observed. Moreover, this increase is found to be more obvious at low rarefied flows. It is also found that presence of axial conduction and viscous dissipation has increasing effect on heat transfer in smooth and rough channels.
36

Reliability-based Analysis Of Time-dependent Scouring At Bridge Abutments

Kerpicci Kara, Sibel 01 February 2009 (has links) (PDF)
Deterministic scour prediction equations for bridge abutments do not involve uncertainties coming from scouring parameters and they only consider effects of hydraulic parameters. However, in order to safely design bridge abutments, treatment of these uncertainties and evaluation of possible risks are required. Two artificial neural network (ANN) models are constructed to describe scouring phenomenon using the parameters of two different equations. The equation to be used in the reliability analysis is then determined according to ANN modeling results. To conduct reliability analysis, Monte Carlo simulation technique is used in which different distributions and coefficients of variations are used for random variables to examine their effects on reliability. It is observed that probability distributions of governing variables have no impact on reliability. However, coefficients of variations of these variables influence reliability.
37

Low Velocity Impact Analysis Of A Composite Mini Unmanned Air Vehicle During Belly Landing

Yuksel, Serhan 01 June 2009 (has links) (PDF)
Mini unmanned Air Vehicles (UAV) have high significance among other UAV&#039 / s, in different categories, due to their ease of production, flexibility of maintenance, decrease in weight due to the elimination of landing gear system and simplicity of use. They are usually built to meet &#039 / hand launching&#039 / and &#039 / belly landing&#039 / criteria in order to have easy flight and easy landing features. Due to the hand take-off and belly landing features there is no need to have a runway and this feature is a very significant advantage in operational use. In an operation, belly landing mini UAV&#039 / s may encounter tough landing areas like gravel, concrete or hard soil. Such landing areas may create landing loads which are impulsive in character. The effect of the landing loads on the airframe of the mini unmanned air vehicle must be completely understood and the mini UAV be designed accordingly in order not to damage the mini UAV during belly landing. Typical impact speeds during belly landing is relatively low (&lt / 10 m/s) and in general belly landing phenomenon can be treated as low velocity impact. The purpose of this study is to analyze the impact loads on the composite substructures of a mini UAV due to the belly landing. &#039 / G&uuml / vent&uuml / rk&#039 / Mini UAV which is designed and built in METU Aerospace Engineering Department, is used as the analysis platform. This study is limited to the calculation of stresses and deformation that is caused by the low velocity impact forces encountered during belly landing. The main purpose of this work is to help the designer in making design decisions for a mini UAV that is tolerable to low velocity impact loads. Initial part of the thesis includes analytical treatment of low velocity impact phenomenon. In the simplified analytical approach the loading is assumed as quasistatic and comparisons of such a simplified method of analysis is made with explicit finite element solutions on isotropic and composite plate structures to investigate the applicability of simplified analytical method of analysis. Belly landing analyses of the mini UAV are done by MSC.Dytran, which is an explicit finite element solver. Model building and post processing are done via MSC.Patran. Stress and deformation response of the mini UAV is investigated by performing low velocity impact analysis using sub-structure built-up approach.
38

Characterization Of Zeolite Membranes By Gas Permeation

Soydas, Belma 01 June 2009 (has links) (PDF)
Zeolite membranes are attractive materials to separate gas and liquid mixtures. MFI is a widely studied zeolite type due to its ease of preparation and comparable pore size with the molecular size of many substances. In this study MFI type membranes were synthesized over porous &amp / #945 / -Al2O3 supports and characterized with XRD, SEM and gas permeation measurements. In the first part of this study the effect of soda concentration of the synthesis solution on the membrane morphology and crystal orientation was investigated. The synthesis was carried out from solutions with a molar composition of (0- 6.5)Na2O:25SiO2:6.9TPABr:1136H2O at 150oC. At soda concentrations between 0.45 and 1.8 the membrane layers with (h0h)/c-directed orientation were obtained. At lower and higher soda concentrations membrane layer formed from randomly oriented crystals. The (h0h)/c-oriented membranes showed H2/n-C4H10 ideal selectivities of 478 and 36 at 25&deg / C and 150&deg / C, respectively.In the second part, MFI membranes were synthesized from mixtures with different concentrations of template molecules. Tetrapropylammonium hydroxide, tetrapropylammonium bromide or mixture of both types were used as template. The nucleation period, the size of MFI crystals, membrane thickness decreased as the tetrapropylammonium hydroxide concentration increased. Besides conversion of SiO2 in the synthesis solution to MFI passed through a maximum with increasing concentration of tetrapropylammonium hydroxide in the synthesis solution. When tetrapropylammonium bromide was used as template thicker membranes were obtained. In the third part MFI type membranes with a thickness of 1.5-2 &amp / #956 / m were synthesized by mid-synthesis addition of silica to the synthesis medium. The membranes synthesized with and without mid-synthesis addition of silica have n-C4H10/i-C4H10 ideal selectivities of 47 and 8 at 100oC, respectively. The change of composition during the synthesis increases the crystal growth rate and the size of the crystals forming the membrane, thus better quality membranes can be obtained by mid-synthesis addition of silica to the synthesis medium. In the last part of this study, thin MFI type zeolite membranes were synthesized in a recirculating flow system at 95&deg / C on the inner side of the tubular &amp / #945 / - alumina supports. A membrane synthesized by two consecutive synthesis steps had a separation selectivity of 38 and 86 for equimolar mixtures of n- C4H10/CH4 and n-C4H10/N2 at 25oC, respectively. The membrane selectively permeated large n-C4H10 over small CH4 and N2, suggesting that the separation is essentially adsorption-based and the membrane has few nonselective intercrystalline pores.
39

Analysis Of Local Sectoral Policies Via System Dynamics Approach: The Case Of Alanya Tourism Sector

Dumbar, Burcu 01 June 2009 (has links) (PDF)
The purpose of this work is indicating System Dynamics Approach to be an appropriate tool for analysis of policies suggested for local sectors. Local sectors are very important for stability of the country&rsquo / s economy as well as welfare of the local community. Therefore, feasible and effective policies should be created and implemented in order to contribute to the local sectoral development. But before any policy implementation, policy analysis is required on to evaluate whether effectiveness and feasibility would be ensured in alternative policies. In this study, firstly the issues of Local Sectoral Dynamics and Policies are examined and specifically, the situation in Alanya Tourism Sector is studied. Afterwards, in accordance with policy analysis steps, problems of Alanya Tourism Sector are detailed and alternative policies that would aid in solution of the problem are idenfined. For evaluation of alternative policies, Dynamics of Alanya Tourism Sector are modeled by System Dynamics approach and the &lsquo / Formal Model&rsquo / is implemented in Stella 9.0.1. The model is simulated for all policy alternatives and the policy outcomes of each alternative are forecasted. Finally the performance of each policy are evaluated using the previously established criteria and combined policies having more superior outcomes than the present ones are created.
40

Road Extraction From High-resolution Satellite Images

Ozkaya, Meral 01 June 2009 (has links) (PDF)
Roads are significant objects of an infrastructure and the extraction of roads from aerial and satellite images are important for different applications such as automated map generation and change detection. Roads are also important to detect other structures such as buildings and urban areas. In this thesis, the road extraction approach is based on Active Contour Models for 1- meter resolution gray level images. Active Contour Models contains Snake Approach. During applications, the road structure was separated as salient-roads, non-salient roads and crossings and extraction of these is provided by using Ribbon Snake and Ziplock Snake methods. These methods are derived from traditional snake model. Finally, various experimental results were presented. Ribbon and Ziplock Snake methods were compared for both salient and non-salient roads. Also these methods were used to extract roads in an image. While Ribbon snake is described for extraction of salient roads in an image, Ziplock snake is applied for extraction of non-salient roads. Beside these, some constant variables in literature were redefined and expressed in a formula as depending on snake approach and a new approach for extraction of crossroads were described and tried.

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