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Archived Theses
Thermoelectric properties of silicon doped germanium
Seebeck coeffcient is a property of material which shows the electrical potential difference created by the temperature difference. This electrical potential is useful in thermocouples and thermopower generation applications. Silicon doped germanium is a good material for thermopower generation applications. Moreover, in interface temperature measurement of bulk growth is possible with the thermoelectric property of Ge-Si alloy. Seebeck coeffcient measurements are made by the small ?T technique. In this technique, potential difference between two probes are measured and with changing the temperature of one probe potential difference change with respect to temperature can be calculated. Seebeck coeffcients of evaporation grade germanium, 5 atm% and 50 atm% silicon doped germanium has been measured. Moreover, electrical resistivity of 5-25 atm% silicon doped germanium at room temperature has been measured. Also, high temperature electrical resistivity measurements for 5 atm% and 50 atm% has been attempted.
A computational approach to the core of Eunicellins
The asymmetric Diels-Alder reaction has attracted much interest for decades. Most recently, interest in the use of catalysts in cycloadditions has gained momentum due to their capacity to provide complex stereoselectivity throughout natural product synthesis. In this study, the origin of diastereoselectivity of intramolecular Diels-Alder reaction with chiral imidazolidinones is investigated to construct the skeleton of eunicellin which is a marine metabolite and two of medium ring ethers found in some natural products. The first reaction of interest was the intramolecular Diels-Alder reaction of acyclic precursor, seven, eight and nine membered reactants with and without aldehyde substituent in order to better understand the origin of diastereoselectivity. The exo and endo selectivity of the cycloadditon reactions, and the rate of thermal reactions are discussed in detail. Secondly, an organocatalyst controlled intramolecular Diels-Alder reaction of seven membered ring to synthesize the tricyclic core is investigated. Lastly, the cycloaddition reactions of eight and nine membered model Diels-Alder precursors are further explored to rationalize the effects of medium ring constraints and to elucidate the difference in diastereoselectivity of the intramolecular Diels-Alder reaction with chiral imidazolidinones. In this work, the effect of MacMillan Catalyst on the mechanism and stereoselectivity of the reaction have been studied with DFT and discussed in detail.
A Novel system model implementation of network attacks and countermeasures
The Internet is an open and efficient TCP/IP protocol which is most widely usedin the world. As usage of the Internet and TCP/IP protocols increases, their lack ofbuilt-in security has become more and more problematic. Nowadays, network attacksare the most important and serious problem on the Internet. DoS attack is the oneof these network attacks which takes advantage of the lack of authenticity in the IPprotocol and stateless nature of the Internet.This thesis examines the TCP/IP architecture, general network structures andnetwork security mechanisms.Second, DDoS attacks and server system models are covered in this work. Newattack models are proposed and developed. These new models and also existing attackmodels are simulated. While generating dierent attacks, new proposed server model isalso designed and coded. This server is called \virtual server with a new perspective".Third, after generating the custom attacks against the custom server system,protection and warning mechanisms are required on the server side. In that case,custom intrusion detection and intrusion prevention system models are designed. Inaddition, numerous of simulation are completed.Finally, at the end of the research, many point of views about conutermeasuressystems are proposed, and their results are investigated.
Nutrient export coefficient modelling of the Melen watershed using estimated retention in the water body
The Melen Watershed provides fresh drinking water to most of Istanbul. According to The Ministry of Forest and Water Works of Turkey, regarding the pollution status, Melen River Basin should be primarily dealt with. Nutrient pollution such as nitrogen (N) and phosphorus (P) can cause water quality degradation in rivers, which are used as a source of drinking water. This situation raises the question of how the nutrients have moved depending on many factors such as land use and anthropogenic sources.Researchers developed several nutrient export coefficient models depending on aforementioned factors. For this purpose, statistical data includes a number of factors such as historical water quality data for the research field, land use, population and infrastructure were used. Nutrient export coefficients are estimates of the total load or mass of nitrogen (N) or phosphorus (P) exported from a watershed standardized to unit area and unit time (e.g., kg/km2/day). Almost no study has been conducted regarding this issue in Turkey.Within this study, nutrient export coefficients for the Melen Watershed were determined using the model that covers Frequentist and Bayesian approaches. River Retention Coefficient was determined and introduced into the model as an important variable. Frequently used hydrologic model, Soil & Water Assessment Tool (SWAT), was used as a tool for modeling of diffuse source pollution. As a brief recommendation, agricultural best management practices (BMP) were evaluated using SWAT model to determine the best management system to reduce nutrient loads in the Melen Watershed. This study is aimed to be an important precedent for other basins that are determined as in priority in terms of pollution by The Ministry of Forest and Water Works of Turkey.
Polymer-dendron based micelles as drug delivery systems
Polymeric micelles are remarkable nominees for drug delivery systems due to their excellent biocompatibility also they provide high loading capacity to hydrophobic inner core and increase drug solubility.In water, micelles can be formed via self-assembly and this micelle formation is connected to the solubility difference of the hydrophilic and hydrophobic part. Hydrophobic parts form the inner core of the micelle and hydrophilic part which surrounds the core form the sphere of micelle.In our study, polymer-dendron conjugates are prepared via Huisgen type `click? reaction using polyester dendrons and PEG. Biocompatible PEG is used as the polymer to provide water solubility to hydrophobic drug molecules. The conjugation of Dendron to PEG followed by functionalization of the dendron periphery with hydrophobic moiety. This system provided the hydrophilic-hydrophobic balance necessary to form the polymeric micelles in aqueous media. Dendron surface was further decorated with drug molecules to yield the micellar drug delivery system.
WeFlow: We follow the flow
Web applications, such as social networking platforms and Web X.0 applications have transformed average users from consumers to producers of content. While this transition has been very successful with respect to content, the same can not be said for behavior generation. Non-savvy Web users have practically no ability to introduce any behavior on the Web. The computational powers offered by the Web are limited to those who are in the know or can afford to develop applications. Yet, there are many simple applications that average users could conceive and utilize if they were empowered to introduce behavior. This work aims to empower average web users with the ability to create simple web applications for purposeful communities. We suggest an environment with information and processes specific to a communities needs, as opposed to the ad hoc information sharing and coordination achieved via social networking platforms, in order to retain the long term value of the generated information. An interesting class of Web applications are human computation applications, where tasks are distributed among humans and computers based on their suitability for performing those tasks. Web based human computation applications seem appropriate for purposeful communities. This thesis presents a framework for the development of human computation applications for purposeful communities. In order to create such an environment, we propose a web based workflow framework. The WeFlow Framework, supports the specification, generation, and execution of community specific virtual environments. These environments are based on a workflow model, which are defined by the communities that use them. The environment is based on a workflow model, which consists of tasks, control and data flow among tasks, and people who perform those tasks. A WeFlow framework prototype is shown by examples and case studies.
Synthesis, HPLC and NMR studies on axially chiral pyridine compounds as potential bidentate N,N'-ligands
In this study, axially chiral enantiomeric and diastereomeric 2-arylimino-3-aryl-thiazolidine-4-ones have been synthesized and their stereostructures have been investigated. In these compounds, the rotation around N3-aryl bond is restricted resulting in axial chirality. Therefore, M and P enantiomers or RM, RP and SM, SP diastereomers exist. Enantiomeric and diastereomeric isomers of the compounds have been investigated by 1H-NMR and HPLC.In 3-(pyridin-2-yl)-2-(pyridin-2-ylimino)thiazolidine-4-one, it was found that the rotation around N3-aryl bond is too fast to make the enantiomeric isomer separation observable by enantioselective HPLC. On the other hand, the interconversion rate between the enantiomers of 3-(3-methylpyridin-2-yl)-2-(3-methylpyridin-2-ylimino)thiazolidine-4-one and the energy barrier for this compound have been determined by thermal racemization of the micropreparatively resolved enantiomers. Rotational barrier of the diastereomers of 5-methyl-3-(3-methylpyridin-2-yl)-2-(3-methylpyridin-2-ylimino) thiazolidine-4-one has also been determined by following the interconversion between the unequally populated diastereomers with time by HPLC. Also, 3-(3-methylpyridin-2-yl)-2-(3-methylpyridin-2-ylimino)thiazolidine-4-thione was synthesized by converting the C-4 carbonyl oxygen to a sulphur atom via the Lawesson Reagent.It was found that replacing one C-5 proton with a methyl group decreased the rotational barrier by 2.2 kJ/mol. On the other hand, replacing C-4 carbonyl oxygen with a sulphur atom increased the barrier by 18.3 kJ/mol.
Rehabilitation of damaged coasts usind beach nourishment and a case study in Izmit Bay
Part of Izmit Bay had been filled with excavation waste after the 1999 Earthquake causing beaches around the Bay to lose their natural form and usability. The main purpose of this study is to investigate the existing conditions of a typical fill area and find a feasible solution to reinstate a natural coast. After theoretical and experimental investigations, beach nourishment is proposed as a solution to rehabilitate the coasts of İzmit Bay by transforming them into recreational beaches. The geology, sediment properties, morphological structure and wave regime of the selected pilot site, Derince, is determined by field measurements, laboratory experiments and computer analysis. Along the coastal profile, sediment diameter differs from place to place, and especially in deeper zones, the rate of fine material (silt and clay) increases significantly. Bathymetric measurements indicate that the coastal profile is not in equilibrium. A physical model is established to examine the interaction between the native mixed soil and the granular nourishment sand. Laboratory experiments showed that there is no mixing between the native fine material and nourishment sand. Even after extreme storms, no silt and clay have been found on the nourished nearshore profile. Therefore, it is concluded that the predefined coasts can be rehabilitated using beach nourishment. Using the site specific parameters, alternative beach nourishment designs are developed including calculation of the required sand volumes and the project cost.
Active noise control in a duct with flow
In this thesis, active noise control in a duct with flow is investigated. A one dimensional acoustic duct model, in which fluid medium inside the duct has a mean flow velocity, is presented. The acoustic duct model is solved in Laplace domain and infinite dimensional system transfer functions are obtained. For controller designs, appropriate microphone and noise canceling source locations inside the duct are determined. In numerical studies, ideal boundary condition case (open end) and general boundary condition case (frequency dependent impedance end) are investigated. For these boundary conditions, low order finite dimensional transfer function approximations of actual system transfer functions are obtained. It is found that, in a selected frequency range, approximations represent actual system in a satisfactory way. By using approximated system transfer functions, low order optimal H2 and H? controllers are synthesized via linear matrix inequalities method found in linear time invariant finite dimensional control theory. Closed loop frequency response and time domain simulations show that the controllers successfully suppress unwanted sound which propagates along the duct.
A comparison between students' mental models of atomic structure and visualizations in textbooks for the concept of atom
This study was designed for two major goals which are to describe students? mental models about atom concept from 6th to 11th grade and to compare students? mental models of atomic model with visual representations of atom in textbooks. in order to analyze the comparison of students? mental models from their drawings and textbook visuals Chi-square analyzes were used. Descriptive statistics were also used to describe students? mental models and textbook visual representations. The date was collected as qualitatively (Mental Model Atom Test) with 4 open ended questions including also drawings and quantitatively (Evaluation Rubric for Atomic Model Representations). The study was implemented in 2 elementary schools and 3 high schools with 180 students. in addition, 525 visuals from the Ministry of Education textbooks were evaluated in the study. The results indicated that Bohr?s Atomic Model were seen as the most frequent model in both the student? drawings and textbook atom visuals. The consistency with textbook visuals and students? drawings were not established statistically because of the 6th grade students and 10th grade students? drawings. The results obtained by qualitatively can show that motion was not seen frequently in students? drawings similarly textbooks visuals. Finally, the results stated that students are very stable to draw their previous model even though they can encounter with the new atom model.