Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
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Master Thesis A Computational Study of Excitation Dynamics on Semiconductor Surfaces(Izmir Institute of Technology, 2019) Kaya, Birnur; Sevinçli, Haldun; Özçelik, Serdar; Sevinçli, Haldun; Özçelik, Serdar; 03.09. Department of Materials Science and Engineering; 04.01. Department of Chemistry; 03. Faculty of Engineering; 04. Faculty of Science; 01. Izmir Institute of TechnologyRecent experimental studies have shown that collodial quantum dots can be produced in large quantities and their optical properties can be tailored by controlling their composition, size and surface characteristics. Motivated by these studies, this thesis is devoted to the investigation of excitation dynamics on semiconductor surfaces, which are passivated with organic molecules. First, constructing a simplified model, excitation dynamics is investigated by computing time dependent occupations of frontier molecular orbitals for various scenarios regarding the values for the energy gap between the highest occupied and the lowest unoccupied molecular orbitals (HOMO and LUMO), as well as the coupling strengths. Second, the model is enhanced to address realistic systems. Passivation of ZnS surface with oleic acid (OA) is modeled using density functional theory based tight binding (DFTB) simulations. Extracting the Hamiltonian and overlap matrices, excitation dynamics is studied for Zn rich and S rich surfaces and different coverage ratios of surfaces. The excitation dynamics is compared and contrasted against the simplified model. Characteristic features are identified and typical decay rates are calculated for various molecular configurations. In addition to these, X-Ray diffraction spectra of quaternary ZnCdSSe nanoalloys have been investigated.Master Thesis Manufacturing of Fire Resistant Porous Ca-Silicate Ceramics(Izmir Institute of Technology, 2019) Oğur, Ezgi; Ahmetoğlu, Çekdar Vakıf; Adem, Umut; Ahmetoğlu, Çekdar Vakıf; Adem, Umut; 03.09. Department of Materials Science and Engineering; 03. Faculty of Engineering; 01. Izmir Institute of TechnologyCalcium silicate hydrates are a group of materials belonging to the calcium silicate family. Calcium silicate hydrates are crucial materials in the building industry, especially for thermal insulation and fire-resistant applications. Tobermorite and xonotlite, calcium silicate hydrates, are the most popular materials in the literature due to their structural properties. These material are synthesized by hydrothermal processes. In some publications, several pre-treatments are applied to the starting materials before hydrothermal synthesis. In this thesis, the effects of these processes on different starting materials and how does changing the parameters affect the process are examined. Besides, the main objective of the thesis is to produce the xonotlite phase using recycling materials as economically as possible. Calcium silicate hydrates powders containing approximately 61 wt. % xonotlite was produced with using lime and recycled glass by hydrothermal synthesis method. The obtained product has a mainly fibrous morphology due to the main phase is xonotlite. According to phase analysis, tobermorite, scawtite, and the trace of calcite phases are also present in the general structure. The thermogravimetric analysis demonstrated that approximately 20% loss is observed up to 800˚C (at about that temperature transformation of the xonotlite to the wollastonite phase is occur.). The mass change remained constant between 800˚C and 1200˚C. Calcium silicate powder (obtained by thermal treatments from CSH) was also thermally analyzed and consequently remained stable up to 1200˚C, (the loss was approximately <1%).Master Thesis Fabrication and Characterization of Ceramic Fibers From Preceramic Polymers(Izmir Institute of Technology, 2019) Özmen, Ecem; Ahmetoğlu, Çekdar Vakıf; Demir, Mustafa Muammer; Ahmetoğlu, Çekdar Vakıf; Demir, Mustafa Muammer; 03.09. Department of Materials Science and Engineering; 03. Faculty of Engineering; 01. Izmir Institute of TechnologyCeramic fibers which are classified as oxide and non-oxide fibers are preferred to use in applications which are carried out at high temperature since they have high strength, low thermal expansion, corrosion, and oxidation resistance. Non-oxide fibers are generally produced using preceramic polymers by the spinning method. The production of ceramic materials using preceramic polymers by spinning method is more advantageous than other methods since the production of complex materials could be achieved at lower temperatures. The preceramic polymer family is basically classified as polysiloxane, polysilazane and polycarbosilane. In this thesis, it was aimed to obtain ceramic fiber in the most economical way. In this context, a spinning device was designed and made. Additionally, polysiloxane which is the most economical preceramic polymer was used to produce ceramic fiber. Polysiloxanes were spun by melt spinning. Obtained fibers were cured by different methods. As a result of pyrolysis, 65-130 μm thickness SiOC fibers were achieved.Master Thesis Investigation of Tribological Performance of B4c Reinforced Aluminium Matrix Composites(Izmir Institute of Technology, 2019) Serkir, Sevgi; Akdoğan, Yaşar; Kandemir, Sinan; Kandemir, Sinan; Akdoğan, Yaşar; 03.09. Department of Materials Science and Engineering; 03.10. Department of Mechanical Engineering; 03. Faculty of Engineering; 01. Izmir Institute of TechnologyAluminium, on account of its easy accessibility and superior metallic characteristics, has a wide variety of applications. Increasing demand on the use of aluminium in areas such as automobile, aviation and space industries which requires high performance has led to development of aluminium metal matrix composites. For this purpose, the ceramic reinforcing particles are mostly preferred to provide better mechanical and tribological properties than their conventional counterparts. In this study, aluminium metal matrix composite (AMC) reinforced with 5 wt.%, 10 wt.% and 15wt.% of B4C were fabricated using the powder metallurgy method. In order to obtain the optimum processing parameters necessary for efficient fabrication, several trials, at first place, were studied under different conditions by changing milling parameters such as milling time, milling medium, milling speed and process control agent, and sintering process parameters such as sintering time, sintering temperature. The production of composite powders was carried out using a planetary ball mill in a wet medium for 7 hours with 0.05 wt.% of stearic acid process control agent which helps to avoid contamination and cold welding of ductile Al particle. The milled powders were pressed at 314 MPa at RT and composite samples with a diameter of 30 mm and a height of 4 mm were obtained. The samples were sintered at 550, 575, 600 and 625ºC for one hour under argon atmosphere. The micro-structures of samples were analysed by scanning electron microscopy and the X-ray diffraction techniques. The wear behaviour of sintered composite samples with ball-on-disc dry wear tester and the mechanical behaviour of the samples with Vickers hardness test were investigated.Master Thesis Fabrication of Colloidal Photonic Crystals Via Langmuir Blodgett Technique and Their Integration of Polymer Matrix(Izmir Institute of Technology, 2019) İnci, Ezgi; Demir, Mustafa Muammer; Varlıklı, Canan; Demir, Mustafa Muammer; Varlıklı, Canan; 04.04. Department of Photonics; 03.09. Department of Materials Science and Engineering; 03. Faculty of Engineering; 04. Faculty of Science; 01. Izmir Institute of TechnologyColloidal films have potential uses in various fields such as photonics, electronics, sensors, membrane filters, and surface devices owing to their unique optical properties. Photonic crystals composed of uniform diameter colloidal silica particles have been arranged in a periodic structure by taking inspiration from nature. The periodic structure of silica particles has physical interaction with light in a visible range. This special interaction is known as structural coloration. The close-packed monolayers and multilayers of colloidal silica particles in large area can be produced by using Langmuir Blodgett method. The integration of these photonic films with transparent polymer matrices having an elastomer feature provides for their use in optical sensor applications. In this thesis, we examined the fabrication of mechano-sensitive nanostructured films based on colloidal particles. Silica colloidal particles were synthesized at different sizes by using Stöber Process. Langmuir-Blodgett deposition was used to create three-layer of photonic crystal films with different particle diameters. For this purpose, various substrates were examined for the Langmuir Blodgett deposition process before starting the coating. The coated silica particles on the glass substrate were then embedded in an elastomeric transparent matrix. The generation of structural coloration after stretching was examined in manufactured elastomer films. In accordance with this purpose, various polymers such as acrylates and siloxanes with elastomer properties have been used. The structural characterization of these composite films and their optical properties were summarized in this thesis.Master Thesis Obtaining and Characterization of Artificial Leather Using Different Types of Plasticizers(Izmir Institute of Technology, 2019) Akkuş Altındağ, İffet; Akdoğan, Yaşar; Adem, Umut; Akdoğan, Yaşar; Adem, Umut; 03.09. Department of Materials Science and Engineering; 03. Faculty of Engineering; 01. Izmir Institute of TechnologyArtificial leather is the material which has wide range of use in life from fashion garment, upholstery to technical applications such as defense industries. PVC artificial leather is preferred material by manufacturers because of its low cost and modification can be done easily by using true plasticizers which are fitting for purpose. In this study, number of six different plasticizers which are non-phthalate plasticizers; trioctyl trimelliate (TOTM), tributyl trimelliate (TBTM), dioctyl terephthalate (DOTP), tributyl citrate (TBC), dioctyl adipate (DOA) and dioctyl succinate (Plast BIO) were used for obtaining artificial leather. To characterize the properties of different kinds of plasticizer in artificial leather, mechanical tests including tensile and tear strengths, elongation at break, cold flexibility and effects of migrability tests were applied. Materials were formulated at three degrees of plasticizers ratios. Also, chemical changes during plasticization were observed using FTIR spectroscopy with ATR accessory, according to types and used levels of plasticizers. In conclusion, plasticizers showed different properties, i.e. material with TOTM plasticized had maximum tear and tensile strengths, while DOA and TBC including materials showed maximum elongation under same load. Considering cold flexibility of six plasticizers, DOA, BIO and TBC have more resistance to cold were obtained, respectively. In addition, migration studies showed that plasticizers including TOTM and TBTM have the least migration properties.Master Thesis Effects of Manganese Promotion on Reactants and Intermediates of Fischer Tropsch Synthesis on a Model Cobalt Surface-A Density Functional Theory Investigation(Izmir Institute of Technology, 2019) Gençoğlu, Merve; Sevinçli, Haldun; Kızılkaya, Ali Can; Kızılkaya, Ali Can; Sevinçli, Haldun; 03.02. Department of Chemical Engineering; 03.09. Department of Materials Science and Engineering; 03. Faculty of Engineering; 01. Izmir Institute of TechnologyThe effects of manganese promotion on the adsorbates and specific elementary reactions of Fischer-Tropsch Synthesis (FTS) was investigated using periodic Density-Functional Theory (DFT) calculations on a close packed cobalt surface, Co(111). In particular the effects of MnO promotion on the adsorbates of CO, HCO, CH, CH2, C2H2, OH, H2O, C, O and on the reactions of direct CO dissociation, H-assisted CO dissociation and carbon hydrogenation were studied for MnO coverages of 0.25 ML and 0.11 ML. Mn was modeled in the chemical form of MnO. MnO was modeled as a singular monomer on the Co(111) surface, based on the findings from experimental studies. The results indicate that MnO promotion increases the adsorption energies of all adsorbates, except H and C2H2. In particular, CO and H2O adsorption energies increase significantly, which indicate that the selectivity increases to long chain hydrocarbons is mainly due to an increased surface coverage of CO with respect to H. The results also indicate that the relative effect of MnO on adsorption energies are strongly dependent on MnO coverage. MnO promotion is found to decrease the activation barriers for HCO and CH formation, while increasing the activation barriers for direct CO dissociation and HCO dissociation. The results point out that MnO does not promote the direct dissociation of CO and the activity increase due to Mn promotion is most probably due to a H or OH assisted CO dissociation pathway or another rate limiting step.Master Thesis A Novel Approach for Fabrication of Free-Standing Conductive Network: Pedot: Pss Based Bendable Chemo and Photoresistor(Izmir Institute of Technology, 2019) Mutlu, Mustafa Umut; Demir, Mustafa Muammer; Mutlu, Mustafa Umut; Yıldız, Ümit Hakan; Yıldız, Ümit Hakan; Demir, Mustafa Muammer; 03.09. Department of Materials Science and Engineering; 04.01. Department of Chemistry; 03. Faculty of Engineering; 04. Faculty of Science; 01. Izmir Institute of TechnologyElectrospinning is a simple and versatile technique for the fabrication of polymeric nanofibrous substrate with high surface to volume ratio. Besides high surface to volume ratio, their dimensional stability and flexibility make it a perfect candidate for conductive network for various sensor applications. Free-Standing conductive network can be fabricated by deposition of PEDOT:PSS or MWCNT through bendable nanofibrous substrate. As a simple example for sensor applications, the moving object has been sensed through the electrostatic interactions between fibers and object. The sensing range has been found to be 1-5 cm above the surface of fabric. By the controlled combination of conductive polymers and electrospun polymer nanofibers effective device miniaturization has been provided without loss of performance. The noncontact motion sensor platform has unique flexibility and light weight holding a potential for wearable sensor technology. For another application as a wearable electronics, the controlled combination of conductive network and light-matter interaction provides opportunities to fabricate photo-resistor exhibits broad band response 400 to 1600 nm that holding promises for ultra-thin sensors used in telecommunication. As a final example, we report the effect of gold and iron oxide nanoparticles on the selectivity and sensitivity of MWCNT or PEDOT:PSS based chemiresistor responsive to VOCs. The interplay between conductive layer by gold and iron oxide nanoparticles resulted a significant conductivity improvement that affecting selectivity which is governed by the interaction between electron-donating VOCs and NP doped conductive layer due to variation in charge carrier densities in conductive layer lattice.Master Thesis Production and Characterization of Porous Ceramics for Aircraft Arresting Systems(Izmir Institute of Technology, 2019) Çapraz, Furkan; Ahmetoğlu, Çekdar Vakıf; Adem, Umut; Ahmetoğlu, Çekdar Vakıf; Adem, Umut; 03.09. Department of Materials Science and Engineering; 03. Faculty of Engineering; 01. Izmir Institute of TechnologyDespite developments in aviation, accidents still occur and lead to loss of lives. Technical or environmental factors may cause overrun accidents. In the case of aircraft that fail to stop along the runway distance during take-off or landing, such accidents are called overrun. Aircraft stopping systems (AAS) are passive safety systems used to prevent to take damage of passengers and aircraft during overrun. In the literature, foamed concrete was generally used as the AAS material. However, some studies have also been carried out on other materials that may be used as the AAS material. In present thesis, the aim was to produce porous calcium silicates instead of foamed concrete in AAS. The slip casting method was used to produce porous calcium silicate monoliths. The main components used in the manufacturing of porous wollastonite were xonotlite (as a matrix material), polymethyl methacrylate (as a sacrificial pore former, PMMA), polyvinyl alcohol (as a binder, PVA), Dolapix CE 64 (as a dispersing agent) and distilled water. Three different xonotlite ratios (8, 9 and 10% by volume) have been studied with four different xonotlite/ PMMA ratios (1, 0.82, 0.54 and 0.33 by volume). Also, specimens without PMMA were produced for each xonotlite ratio. The suspensions were stirred for 3 h at 750 rpm and then casted into the gypsum mold. The samples which were dried at room temperature (RT) were heat treated at 1000 oC. After production step microstructural, structural and mechanical analysis o the porous calcium silicates were carried out. Cold Crushing Strength (CCS) tests were performed out for determining mechanical properties. CCS values of the porous monolith parts ranged between 1376 kPa and 53.2 kPa depending on xonotlite/ PMMA ratios. In addition, the porosity values ranged from 93.38 to 87.82%. The resuts showed that some of these porous calcium silicate monoliths are suitable to use instead of foamed concrete as the AAS material.Master Thesis Determination of the Electrocalorinc Properties of the Ferroelectric (1-x)batio3-bi(li1/3ti2 (0?x?0.2) System(Izmir Institute of Technology, 2018) Demirtay, Tuğçe; Adem, Umut; Çiftçioğlu, Muhsin; Adem, Umut; Çiftçioğlu, Muhsin; 03.02. Department of Chemical Engineering; 03.09. Department of Materials Science and Engineering; 03. Faculty of Engineering; 01. Izmir Institute of TechnologyThe aim of this study is to develop electrocaloric (EC) materials for alternative eco-friendly cooling technologies. Considering the dielectric and piezoelectric properties of the (1-x)BaTiO3- xBi(Li1/3Ti2/3)O3 (0≤x≤0.2), this system was selected to study electrocaloric properties due to the presence of morphotropic phase boundary (MBP). The crystal structure of the samples that were synthesized by convential solid state reaction technique were determined by the X-Ray Diffraction experiments. The electrical properties of the ceramics were characterized by temperature-dependent dielectric measurements and the phase transition temperatures of the samples were identified. Temperature-dependent electrical polarization curves were measured for all compositions and electrocaloric temperature change values were calculated with the help of Maxwell equations. Using the dielectric measurements, a phase diagram was obtained. Strain-electric field measurements were done to complement dielectric and polarization measurements. A new antiferroelectric phase was discovered at high temperatures in the samples. As Bi(Li1/3Ti2/3)O3 content is increased, phase transition from the ferroelectric to antiferroelectric phase decreases. Since in the antiferroelectric phase the net polarization is very small, a large polarization change therefore large electrocaloric temperature change is obtained. Highest electrocaloric temperature change (ΔT=0.66 K under 22 kV/cm) is obtained for x=0.03 sample.