Phd Degree / Doktora

Permanent URI for this collectionhttps://hdl.handle.net/11147/2869

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Author: search.filters.author.Çiftçioğlu, MuhsinCOAR Access Rights: search.filters.coarAccessRights.open access

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Now showing 1 - 8 of 8
  • Doctoral Thesis
    Preparation and Characterization of Titania Based Powders and Suspensions for Photocatalytic Applications
    (Izmir Institute of Technology, 2015) Yurtsever, Hüsnü Arda; Çiftçioğlu, Muhsin
    Growing environmental concerns related to the extensive use of nonsustainable fossil fuels and increasing energy demand will force mankind, sooner or later, to tap into clean and sustainable sources of energy and artificial photosynthesis is being considered as a promising prospect for this concern. Photocatalysis is expected to make a great contribution to solutions for environmental problems and renewable energy generation. Titania based photocatalytic materials are one of the widely used materials in artificial photosynthesis research due to their unique chemical and optical properties. Recent research have shown that rare earth element (REE) doping improves the photocatalytic activity of TiO2 phases in the visible region. The effects of REE doping level and heat treatment temperature on the microstructure evolution, optical properties and the photocatalytic water splitting/CO2 photoreduction activities of mostly sol-gel derived titania powders were investigated in this thesis. The results of this thesis indicated that REE ions may enter the interstitial voids of TiO2 lattice or segregate on the grain boundaries of TiO2 as a separate phase and RE2Ti2O7 phases were formed for the REEs with relatively smaller ionic sizes. The experimental work conducted revealed the importance of surface area, crystallinity, light absorption, the presence of oxygen vacancies and structural defects in the efficiency of photocatalytic processes. Results have shown that REE doping improves the artificial photosynthesis activity of TiO2 at relatively low doping levels and 700oC heat treatment temperature as the production rate of H2 and CO/CH4 were significantly higher with REE doped TiO2 compared to pure TiO2.
  • Doctoral Thesis
    Rheological Behavior of Nanocrystalline / Submicron Ceramic Powder Dispersions
    (Izmir Institute of Technology, 2009) Çağlar Duvarcı, Özlem; Çiftçioğlu, Muhsin
    Several rheometric techniques were applied to submicron and nano ceramic powder dispersions systematically in this study. The rheological behavior of the dispersions was determined by steady shear and dynamic shear rheology. Dynamic shear rheological techniques are scarcely used for the characterization of ceramic powder dispersions contrary to polymers.The flow behaviors of the submicron and nano dispersions were found to be dependent on the solids content and fructose concentration. The submicron alumina, nano alumina, and nano titania dispersions in fructose solution showed shear thinning behavior and were fitted to the Herschel-Bulkley model.The dynamic shear rheology measurements showed that the solid part of the dispersions was dominant over the liquid part for both submicron and nano powder dispersions. The elastic modulus was higher than the viscous modulus in stress and frequency sweep measurements. The elastic moduli of the dispersions with solids content lower than 40 vol% were dependent on the angular frequency which indicated a gel-like behavior. However, the elastic moduli of the dispersions with 40 vol% solids were independent of angular frequency which indicated a solid like behavior. Further increase in fructose content had significant effects on both steady shear and dynamic shear rheological behavior of the dispersions regardless of particle size. The submicron and nano ceramic powder dispersions can be prepared by using fructose for the regulation of the rheological behavior of ceramic powder dispersions. The characterization of powder surfaces is essential for the effective adsorption of fructose.
  • Doctoral Thesis
    Gas Permeation Through Sol-Gel Derived Alumina and Silica Based Membranes
    (Izmir Institute of Technology, 2009) Topuz, Berna; Çiftçioğlu, Muhsin
    The scope of this thesis is to design defect-free microporous and mesoporous ceramic membranes having micro-engineered pore network that would contribute to the enhancement of pore control abilities as well as the thermal stability.In this study, mono-dispersed silica sols having well-defined silica spheres ranging in size from 5 to 700 nm were prepared through sol-gel methods and thin membrane layers were consolidated on either y-alumina support or unsupported form.The packing of 5 nm silica spheres resulted in micropores of 0.87 nm in 400 oC treatedmembranes with the porosity of 0.32 which are in well aggrement with the porosity level of random loose packing. Silica spheres with varying concentration and size were incorporated into polymeric network to complement the percolative structure of sphere packing with interpenetrated polymeric silica network in order to design well-defined thermally stable transport pathway. Low shrinkage value was obtained for sphere incorporated system providing the high thermal stability by affecting the thermally induced microcrack formation as well as the structural relaxation during consolidation.The resulting hybrid structure enabled the detailed transport properties that support to be able to control the pore structure but N2/CO2 separation properties are needed to be improved.Stable polymeric alumina sols having particle sizes smaller than 2 nm could be obtained when the hydrolysis conditions were accurately controlled. The mixture of prepared polymeric silica and alumina sols in mullite compositions (3:2) provided to the crystallization of mullite with homogeneously mixed stable oxide network upon heat treatment at 775 oC.
  • Doctoral Thesis
    Preparation of Hydroxyapatite/Silk Protein Thin Film Implant Surfaces, Investigation of Their Microstructural Properties and Model Protein Interactions
    (Izmir Institute of Technology, 2009) Özcan, Selçuk; Çiftçioğlu, Muhsin
    Biocompatible hydroxyapatite (HAp) coatings of load bearing metallic in vivo hard tissue implants act as local scaffolds for enhanced osteoconduction, providing fast bone apposition and cementless fixation. In this study, in an attempt to exploit the potential of hydroxyapatite as a carrier of bone morphogenetic proteins for post operative accelerated healing, and implant durability, the tailored microstructural properties, and protein adsorption capabilities of thin film hydroxyapatite implant surfaces were investigated.A novel particulate sol method was used to fabricate HAp thin films on bioinert glass, and Ti6Al4V substrates by dip and spin coating. The microstructural characterization of the thin films was carried out by SEM/EDX, AFM, XRD, and FTIR, and their surface roughness, Vickers hardness and adhesion strength were determined. The effects of silk fibroin and sericin thin film layers on the HAp film microstructure, and model protein (bovine serum albumin, BSA) adsorption behavior (by the size exclusion HPLC method) were investigated. The minimum threshold solid content of the suspensions was determined as 15% by weight for a continuous HAp film structure. The silk sericin and fibroin intermediate layers drastically improved homogeneity of the HAp layer. The BSA adsorption of the glass/sericin/commercial-HAp film was 2.6 ug/cm2, more than twice of the glass/commercial-HAp, and glass/sericin/dry-milled-HAp films, evidencing the effectiveness of surface micro/nano topographical structure as well as chemical structure. The XRD patterns of spin coated commercial-HAp films on Ti6Al4V pointed out to a particular crystal orientation which increased the positive degree of cooperativity between HAp and proteins during adsorption or deposition.
  • Doctoral Thesis
    Synthesis and Characterization of Calcium Phosphate Cement Based Macroporous Scaffolds
    (Izmir Institute of Technology, 2012) Şahin, Erdem; Çiftçioğlu, Muhsin
    The goal of this thesis is to synthesize unique, clinically relevant macroporous calcium phosphate cement blocks to be utilized both in vivo and in vitro tissue engineering applications. Calcium phosphate cements which essentially consist of hydroxyapatite or brushite are constantly improved to overcome their inherent shortcomings such as low strength, low functional porosity, and low resorption. Recent literature on the topic points to monetite forming cements as an alternative phase. A novel method to utilize monetite that is finer and stronger with respect to brushite in load bearing scaffold applications is introduced in the results section of this thesis as a contribution to ever growing literature on this scope. In the preliminary study on the conversion extent of apatite forming cement, ionic strength of the setting liquid was determined as the prime effective factor on monetite conversion extent. Subsequently brushite forming β-tricalcium phosphate – monocalcium phosphate monohydrate cement system was modified by NaCl and citric acid so that brushite formation was selectively inhibited. Singular and synergistic monetite promoting effects of NaCl and citric acid were determined by monitoring the kinetics of cement setting in excess setting liquid. Spectrometric studies revealed the difference in brushite and monetite crystal surface site density which enabled selective inhibition of brushite and promotion of monetite by the synergistic effect of NaCl anc citric acid. Proposed phase control mechanism enables tailoring the composition of biphasic cements comprising of a predetermined monetite content and brushite or hydroxyapatite. In the final stage of the thesis, size distributed NaCl particles were introduced into the cement paste containing optimum amount of citric acid to enable complete monetite formation. Resultant macroporous monetite blocks were characterized in terms of microporosity, macroporosity, density, morphology, strength, phase composition, and surface area. Interconnectivity of the cement was optimized based on the correlation of porogen size distribution and morphological data.
  • Doctoral Thesis
    Enhancement of Tribological Properties of Mineral Oil by Addition of Sorbitan Monostearate and Borate
    (Izmir Institute of Technology, 2010) Atakul Savrık, Sevdiye; Çiftçioğlu, Muhsin
    The development of modern automobile and engine industries requires lubricants that can withstand high temperatures and pressures. Recent advances made in the chemistry provide the use of inorganic particles as lubricant additives. Therefore inorganic boron-based additives have been the focus of much attention, as they posses a good combination of properties, such as wear resistance, friction-reducing ability. In this study, the state of art in the field of inorganic particle, zinc borate synthesis and its employment in tribology were investigated. The synthesis of zinc borate was achieved not just by precipitation, but also production methods such as inverse emulsions. The products were characterized by SEM, FTIR, TGA, DSC, EDX. In lubrication part, the friction reducing and antiwear ability of the particles as an additive in the mineral oil was focused. Sorbitan monostearate was used to cover the surfaces of inorganic particles in order to provide better dispersion of additives in the oil. Friction and wear behavior of the lubricants were measured by four-ball wear test machine. The effects of dispersing agent, zinc borate type as well as surfactant concentration on the tribological properties of the lubricants were investigated. Sorbitan monostearate not only outperformed as a dispersing agent of inorganic particles, but also it proved to be an efficient antiwear agent. The lowest wear scar diameter was obtained by the lubricant containing zinc borate synthesized via coordination precipitation method. The addition of this sample with the surfactant in the oil reduced the wear scar diameter from 1.402 mm to 0.550 mm.
  • Doctoral Thesis
    Preparation and Characterization of Resorbable Calcium Phosphate Based Bioceramics
    (Izmir Institute of Technology, 2010) Çetin, Ali Emrah; Çiftçioğlu, Muhsin; Çiftçioğlu, Muhsin
    The main objective of this study was to determine the behavior of hydroxyapatite powders in solutions containing major ions found in human blood plasma. Research efforts however were focused on two different fields, chemical analysis and dissolution testing of hydroxyapatite. Arsenazo III-calcium analysis method was selected and modified for the low level calcium analysis whereas malachite green phosphate assay was used for phosphate determinations. Arsenazo III reagents with different dye concentrations and pH values were tested for the analysis of low levels of calcium. The effects of ions at their blood plasma concentration levels on the modified calcium analysis method were further evaluated. Effects of these ions on the accuracy of phosphate analysis by malachite green assay were also investigated. Dissolution behavior of synthesized and commercial hydroxyapatite was investigated by immersing pellets in several solutions. Calcium and phosphate release kinetics in ultrapure water, sodium chloride and sodium bicarbonate solutions were investigated. Commercial hydroxyapatite pellets were also immersed separately in magnesium chloride, and sodium chloride/bicarbonate solutions. Calcium concentrations in solutions were found to decrease in the following order: sodium chloride > ultrapure water > bicarbonate. Phosphate concentrations in solutions were found to follow an order of sodium bicarbonate > sodium chloride > ultrapure water. Bicarbonate-phosphate ion exchange was observed in bicarbonate ion containing solutions. Magnesium-calcium ion exchange and adsorption of phosphates on hydroxyapatite surfaces were found to be effective in magnesium ion containing solution.
  • Doctoral Thesis
    Investigation of Effects of Microstructural and Surface Properties of Ultrafiltration/ Nanofiltration Ceramic Membranes on Their Performance
    (Izmir Institute of Technology, 2009) Erdem, İlker; Çiftçioğlu, Muhsin
    The ceramic membranes with their superior chemical, thermal, mechanical and microbiological properties and long service lives are gaining importance in pressure driven filtration processes. The diverse requirements of different applications enforce preparation of tailor-made ceramic membranes with specific characteristics. This dissertation focused on the preparation and characterization and filtration performance of asymmetric multilayer ceramic membranes. Support is the layer responsible for mechanical stability while top layer is mainly responsible for separation and intermediate layer is balancing the microstructural difference between these two layers. The permeability of alumina support could be increased over 100 L / m2 h by 15% starch addition. The intermediate layer was prepared by coating fine alumina or zirconia powders and / or colloidal sols with thicknesses between 0.2-70 .. The top layer was formed from pure or mix of zirconia or titania polymeric sols with average particle sizes in the range of 3 . 50 nm with a thickness smaller than 1.. The physicochemical properties of these mixed oxides were modified by changing the composition and calcination temperature enabling preparation of top layers with varying Donnan exclusion capacities. The membranes prepared could reject sugar, PEG 1000 and PEG 4000 up to 10, 60 and 19%, respectively, that which can be increased via further optimization of parameters in coating / heat treatment processes.