Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
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Master Thesis Investigations on Nanoscale Wetting, Fluid Transport, and Droplet Evaporation at Nanostructured Surfaces by Molecular Dynamics Simulations(01. Izmir Institute of Technology, 2021) Şatıroğlu, Ezgi; Barışık, Murat; Özkol, ÜnverThere is a significant need to understand solid-liquid interactions at nanoscale to determine the fluid behavior in several revolutionary applications. Specifically, nanoscale surface wetting, nanoscale liquid transport, and nanoscale heat transfer are the most sought-after subjects in recent scientific and industrial applications. This thesis focuses on characterization and possible control of wetting, fluid flow, and heat transfer using nanoscale surface structures. First, wetting behavior on a nanostructured surface was studied to resolve contact angle hysteresis. The droplet was found stabilized at a metastable state with a contact angle significantly different from its equilibrium value due to contact line pinning from the surface asperities. The contact angle was found to increase linearly by increasing droplet size when the droplet is pinned. However, these pinning effects become negligible, and the contact angle reaches the equilibrium value of the corresponding surface when the surface structure size becomes negligible compared to droplet size. Second, fluid flow in nanostructured nanochannels was studied to determine the transport behavior. While the slip boundary condition on a smooth surface correlated with the wetting angle, transport in a nanostructured channel remained mostly independent from wetting condition of the corresponding surface structure. Lastly, droplet evaporation over nanopatterned surfaces was investigated. When the droplet temperature reached the Leidenfrost point, a sudden increase in the interface thermal resistance was observed, which significantly decreased the heat transfer to the droplet. Increasing the size of the surface structure pushed the Leidenfrost point to higher surface temperatures. Current results contribute to various disciplines in engineering and applied sciences.Master Thesis Production and Characterization of Emulsion Derived Porous Sioc+tio2 Submicron/Nanospheres(Izmir Institute of Technology, 2020) İçin, Öykü; Ahmetoğlu, Çekdar VakıfThe water resources are polluted because of the widespread use of dyes in the industry, resulting in a major ecological threat. Among the various water treatment techniques, adsorption and photocatalytic degradation methods are the most preferred owing to their easy applicability, low cost, and high efficiency. Silicon oxycarbide (SiOC), which is a type of polymer-derived ceramic, has the potential to be used in harsh environmental conditions thanks to its strong chemical stability and oxidation resistance, that being said it can also be used as a photocatalyst substrate. Titanium dioxide (TiO2) photocatalysts are extensively used for purification of contaminated waters. And also, TiO2 particles are synthesized with various material groups to investigate the adsorption and photocatalytic effect. In this thesis, initially, submicron/nano SiOC spheres were produced via an oil in water (o/w) emulsion technique by using parameters such as two types of preceramic polymer precursors (silicon oil and resin), mixing types (magnetically and ultrasonically), and different pyrolysis temperature (600-1200 oC). Upon the formation of submicron/nano SiOC spheres, selected samples were impregnated with a different molar of titanium oxide precursor solution (Titanium(IV) n-butoxide (TBT)) and calcined at 450 °C for 4 h. Various amounts of (0-5-10-20 wt.%) TiO2 containing submicron/nano SiOC spheres were produced and then characterized in depth by various techniques. Finally, the effects of pyrolysis temperatures and the amount of TiO2 were investigated in terms of adsorption and photocatalytic performance against aqueous cationic dye (methylene blue) (MB) solution. In the adsorption experiments, pure SiOC submicron/nanospheres (UM1200), pyrolyzed at 1200 oC, showed the best performance at the end of 24 h in the dark with 64% adsorption. In photocatalytic experiments, samples obtained by coating the SiOC substrate produced by pyrolysis at 600 oC with different amounts of TiO2 (UM600T5, UM600T10 and UM600T20) showed 79%, 80%, and 87% photodegradation efficiency.Master Thesis Quantum transport in nanostructured materials(Izmir Institute of Technology, 2017) Kurt, Gizem; Sevinçli, Haldun; Çakır, ÖzgürDue to the advances in the measurement and fabrication techniques at the nanoscale it is now possible to measure thermal transport across single molecule junctions[1], which makes it possible to consider nano-scale thermal devices. One of the building blocks for such thermal devices should be thermal switches. The aim of this study is to design a thermal switch, which is based on a single molecule junction and photoisomerism. We propose reversible photoisomerism as a key ingredient to build reversible thermal switches based on single molecule junctions. In this thesis, the thermal conductances of molecular junctions built by azobenzene and its derivatives are computed using density functional theory based tight binding method combined with atomistic Green’s functions. These molecules show photoisomeric behaviour by switching their three-dimensional structure when exposed to radiation. We investigate the effects of different linker groups as well as the details of the reservoirs. Carbon nanotubes are used as reservoirs, while generic reservoirs are also investigated to illuminate the effects of the reservoir details. We show that thermal conductance can be altered by switching the molecule from trans to cis configuration. The effect is robust under the change of the linkers that bind the molecules to the reservoirs and under the change of the particular molecular species.Master Thesis Growth and Characterization of Carbon Nanotubes by Thernal Chemical Vapor Deposition Method(Izmir Institute of Technology, 2008) Aksak, Meral; Selamet, YusufThis thesis work is focused on producing carbon nanotubes (CNTs) by methane gas thermal chemical vapor deposition method on very thin Cobalt, Iron, and Nickel catalyst thin films deposited onto SiO2/Si substrates by DC magnetron sputtering. This thesis is also devoted to understanding some parameters affecting the growth of CNTs; such as catalyst material, temperature, and catalyst layer thickness effects In this study, CNT growth was performed on directly Si substrates, which was observed that the growth was too difficult and requiring very high temperatures. Hence, very thin catalyst films were deposited on SiO2/Si substrates, and the CNT growth was observed. The temperature effect was also examined. When the growth temperature was increased, the average diameters of the CNTs were decreased up to a critical temperature, but after this point the average diameter of CNTs were increased. This effect was studied systematically by utilizing Fe and Co catalyst thin films and with the help of Raman spectroscopy and Scanning Electron Microscopy results.Catalyst thickness effect was also examined. For this aim, Ni catalyst thin films with three different thicknesses; 0.7 nm, 1.4 nm, and 6 nm, were utilized. It was observed that CNTs were grown well on 0.7 and 1.4 nm thick Ni films, while there was a little growth on 6 nm thick Ni film. The roughness analysis of 0.7 nm and 1.4 nm thick Ni films were also done. Some of as-grown CNTs were also examined by X-ray diffraction method, and the results were compared one another.Master Thesis The Preparation Characterization and Sintering of Nanocrystalline Ceramics(01. Izmir Institute of Technology, 1999) Çağlar, Özlem; Çiftçioğlu, MuhsinNanocrystalline Titania was prepared by a chemical synthesis technique commonly known as sol-gel method. In the sol gel method, Titanium (IV) Isopropoxide was mixed with Isopropanol and Nitric Acid solution in predetermined ratios. A rapid hydrolysis reaction occurs between Titanium (IV) Isopropoxide and water in the Nitric Acid solution resulting in the formation of Titan oxide (Titania). The sols were clear sols and then gelled without any change in its clarity.Nanocrystalline Titania were tried to prepare by two different techniques in this work. The first technique involved the drying of the gel and subsequent sintering of the dried gel. A number of organic additives (oxalic acid, acetic acid, polyacrylic acid and stearic acid) were mixed into the sol before gelation in order to control drying (drying control chemical addives-DCCAs). Powders was prepared from sols and gels by several processes and a solid form was obtained by dry pressing and subsequently sintered in second technique. Oxalic acid was the most efficient DCCA among the others.The dried gels and powder compacts were sintered at 650, 700, 750, 800, and 850C. The sintering behaviors of them were examined. Relative densities of the dried gels were between 79-99% depending on the sintering temperature. The green body density of the pellets were varied between 41-52%. Their relative densities after sintering were varied between 55-83% depending on the sintering temperature. The pellets were pressed at different pressures to observe the pressure effect on the densification. Increase in pressure improve the densification behavior. The best route for the nanocrystalline powder preparation was the Route 4. This powder had smaller size of agglomerate most probably the agglomerates were broken during the ultrasonic radiation.The pore size analyses showed the pore structure of the gel. The pore size of the gels are about 35 nm. FTIR Spectra gave the crystal structure of the sols gels and powders. As a result, the sintering behavior of the dried gels is better than the powder compacts. The pellets can be densified to higher densities by appropriate forming technique. Although, the dried gels have significantly high densities, the shape and the weight of the gels can not be controlled.Master Thesis Development of Layered Silicate/Epoxy Nanocomposite(Izmir Institute of Technology, 2006) Kaya, Elçin Dilek; Tanoğlu, MetinLayered silicate/polymer nanocomposites are materials that display rather unique properties, even at low silicate content, by comparison with more conventional particulate-filled polymers. These nanocomposites exhibit improved mechanical, thermal, optical, gas permeability resistance and fire retardancy properties when compared with the pure polymer.In this study, layered silicate/polymer nanocomposites were prepared using Na+ cation containing montmorillonite (MMT) and epoxy resins. Silicate particles were treated with hexadecyltrimethylammonium chloride (HTAC) to obtain the complete homogenous dispersion of the nano plaques within the polymer matrix which forms the exfoliated microstructure. In this way, organophilic silicates (OMMT) were obtained.Modification of the silicate expands the silicate galleries (from 14 to 18 )that promote the formation of exfoliated composite structure. SEM results showed that nanocomposites with organically modified MMT exhibited better dispersion than those with MMT. It was found that the tensile and flexural modulus values are increased, whereas the fracture toughness is decreased with increasing silicate content. Thermal analysis results revealed that the glass transition temperature(Tg) of the neat epoxy resin (63.6oC) increases to 68.9 oC for the nanocomposites with 3 wt. % of OMMT. By incorporation of silicate particles, the dynamic mechanical properties of epoxy; including the storage and loss modulus and Tg are increased. Optical transmission values of the epoxy were affected by MMT and OMMT silicate incorporation. It was found that flame resistance at the polymer improved by the incorporation of MMT particles to the neat epoxy.Master Thesis Preparation and Physical Characterization of Clay/Epdm Nanocomposites(Izmir Institute of Technology, 2008) Karşal, Çiçek; Tanoğlu, MetinPolymer/clay nanocomposites have been extensively studied in recent years because they often exhibit improved properties different from their micro and macrocomposite counterparts. Addition of organophilic layered silicates to the polymer produces effective polymer nanocomposites by intercalation of macromolecules into the interlayer spaces. The performance of polymer/clay composites is not only related to the nature of the clay but also to the reinforcing mechanism of filler and the preparation conditions.In this study, the effects of mixing conditions and effect of aging on mechanical,physical and thermal properties of ethylene-propylene-diene rubber (EPDM)/Organo modified montmorillonite (OMMT) nanocomposites were studied at two different clay loadings 5 wt.% and 10 wt.%. EPDM/OMMT nanocomposites were prepared by melt blending method. The experimental results of X-ray diffraction (XRD) and scanning electron microscopy showed that the organically modified MMT existed in the form of an intercalated structure and that was exfoliated in EPDM matrix depending on the mixing conditions. XRD patterns showed that the interlayer distance of the organically modified clay was 30.9A, which was larger than those of the unmodified clay (14.6A).The mechanical evaluation of the nanocomposites was performed by tensile and tear testing. The mechanical tests showed that the properties of nanocomposites were significantly improved with addition of OMMT. The effects of the processing conditions were manifested in both the morphology and mechanical properties, which showed significant increase when optimized process conditions are applied. In addition, chemical test was performed on the nanocomposites to monitor the degradation of the mechanical properties. It was found that the reduction of the mechanical properties of nanocomposites after aging process is lower as compared to those of neat EPDM.Master Thesis Processing and Characterization of Nanocrystalline Materials(Izmir Institute of Technology, 2004) Genç, Gözde; Çiftçioğlu, MuhsinNanocrystalline materials with grain sizes under 100 nm have been counted as a very promising class of ceramic materials. The decrease in grain size imparts superior structural properties to the ceramic materials like increased fracture toughness, wear resistance and superplastic deformation at high temperatures. So, nanocrystalline ceramics can be used as structural materials in a wide range of uses covering advanced engineering applications such as aircraft and automotive technologies as well as electronics, computation and material sciences. Nanocrystalline alumina has been a popular research subject for the last decade because of the superior structural properties of alumina besides lower cost compared to other nanocrystalline materials like zirconia and titania. The raw materials are generally transitional aluminas or aluminum hydroxides such as boehmite and gibbsite all of which are widely used in industry.The main problem in nanocrystalline alumina production is to cope with agglomeration of fine powders or nano-particles in solutions due to interaction forces. Agglomeration cause nonhomogeneous microstructure with respectively larger pores which are very hard to eliminate during sintering. In this study a nanocrystalline commercial boehmite powder is used as the starting material and a homogeneous microstructure with respectively high density is aimed. Ultrasonic treatment is applied in order to reduce particle size and the stable dispersions of boehmite powder with a solid content of 30wt% was dried by spray drying. The effect of ultrasonic treatment, spray drying and compaction conditions were investigated and compared with bodies compacted from untreated powders. To see the effect of seeding on phase transformation behavior of boehmite, alpha alumina particles produced by combustion synthesis and broken into crystallites by ultrasonic treatment were used with a seed content less than 0.5%. Even that low amount of a-alumina reduced the transformation temperature by 100C but did not improve densification of alumina compacts. The homogeneity of microstructure obtained by ultrasonic treatment and granulation by spray drying yielded the highest relative densities of 65.5 % of the theoretical density. The treatment has also led more uniform microstructures compared to untreated and seeded samples during the a-phase transition which is the most important stage for densification.Master Thesis Layered Silicate / Polypropylene Nanocomposites(Izmir Institute of Technology, 2006) Işık, Kıvanç; Tanoğlu, MetinLayered silicate nanocomposites are new generation materials that have unique properties obtained by low particulate loadings. In this study, layered silicate/polypropylene nanocomposites were prepared by melt intercalation method.Homopolymer PP alone and maleic anhydride-grafted polypropylene (PPgMA) as a compatibilizer were used as the matrix. Clay (Na+ montmorillonite, MMT) particles were used with and without structural modification to obtain silicate nano-layers within the PP matrix. Structural modification of MMT using hexadecyltrimethyl ammonium chloride (HTAC) was applied to obtain organophilic silicates (OMMT). XRD results demonstrated that the dispersion of the modified silicate layers and compatibilized with PPgMA (OMMT/PPgMA) is better than those for incompatibilized compositions. The addition of silicate layers increased the crystallization temperature of PP as well as the thermal stability, but the melting temperature of the nanocomposites was decreased by the addition of silicate as compared with neat PP. The mechanical characterizations exhibited an increase of 62% on tensile modulus and 15% on tensile stress at break as compared to neat PP due to the improved dispersion of silicate layers within PP in 3 wt.% OMMT/PPgMA/PP nanocomposites. The effect of clay modification and PPgMA compatibilization on the light transmission of PP nanocomposites was characterized by optical transmission analysis. For the OMMT/PPgMA/PP nanocomposites, light transmission was improved as the dispersion was enhanced. The flammability results demonstrated that unmodified MMT and modified OMMT decreased the burning rate of PP nanocomposites. The organic modification of clay and compatibilization decreased the rate of flammability. A decrease of 26% on the burning rate of PP was recorded in 10%wt. OMMT/PPgMA/PP nanocomposites.Master Thesis Investigation of the Rheological Behaviour of Nanosuspensions(Izmir Institute of Technology, 2011) Çelen, Öykü; Çiftçioğlu, MuhsinThe rheology of concentrated calcium carbonate, titania, silica, alumina and zirconia suspensions of submicron/nanosized particles in non-aqueous media was investigated. Calcium carbonate and silica particles were synthesized by carbonation route and Stöber method respectively while titania, alumina and zirconia powders were obtained commercially. Suspensions of the ball milled natural calcium carbonate powders were also prepared. The dispersion of these powders especially at high solids loadings was observed to be difficult. The viscosity of the 45 vol% suspensions of precipitated CaCO3 particles increased about 21 times at a shear rate of 400 sec-1 relative to its initial value indicating shear-thickening behaviour. The onset of shear thickening was at 20, 30 and 40 vol% for three different commercial titania powder suspensions. The onset of shear thickening was at lower solids volume contents and the increase in viscosity with shear rate was far less dramatic for the titania suspensions with polydispersed particle size distributions. The critical shear rate was 187 sec-1 for silica suspensions at 47 vol% with a discontinuous jump in viscosity. Rheological data at higher shear rates was not obtained due to the torque limitations of the rheometer. The critical shear rate of the polydispersed alumina powder suspensions were higher than their relatively monodispersed counterparts. The highest solids loading achieved in alumina based suspensions was 62 vol%. The dynamic shear rheology measurements showed that the solid part of the silica suspension was becoming more dominant over the liquid part as the solid content was increased. The volume fraction dependence of the high shear viscosity was fitted to a modified Krieger-Dougherty model for titania and silica suspensions. The fit of the rheological data to the Power law and to a modified version of Cross model were analysed.