Ph.D. Theses
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Browsing Ph.D. Theses by Author "Altıntaş, Sabri."
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Item Characterization of NiMnGa magnetic shape memory alloys(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2005., 2005.) Pirge, Gürsev.; Altıntaş, Sabri.Magnetic shape memory (MSM) alloys are a new class of actuator materials withhigh actuation frequency, energy density and strain and they can be used in themanufacturing of actuators, smart structures, sensors and transducers. NiMnGa alloys experience a reversible martensitic transformation, which is a temperature-dependent phasetransformation from a highly symmetric crystallographic structure (austenite) to lowsymmetry (martensite). These materials are ferromagnetic. Ferromagnetism is aphenomenon by which a material can exhibit a spontaneous magnetization, and is one of the strongest forms of magnetization. Ferromagnetic metal alloys whose constituents arenot ferromagnetic in their pure forms are called Heusler alloys, named after Friedrich Heusler. Applying a strong magnetic field to some of the Heusler alloys may inducereorientation of martensite variants with high magneto-crystalline anisotropy energy, which leads to a net shape change of the material. In this study, the effect of alloycomposition, cutting direction and heat treatment on the microstructure, local composition,and thermal and dilatometric properties of NiMnGa alloys were investigated.Characterization tests involved various crystals, with and without post-crystal growth heat treatment, by chemical analysis, differential scanning calorimetry (DSC), dilatometry,optical microscopy, scanning electron microscopy and radiography. Metallographic studies showed that as solidified, off-stoichiometric alloys had three distinct microstructural features-a Heusler phase, a Mn rich phase and a eutectic or eutectoid region. Various heattreatment procedures were applied to successfully remove the last phase and produce MSM effect. Heat treatment was also essential for the production of a distinct martensitetransformation in DSC and dilatometry traces and a martensitic transformation to occurover a narrow temperature range. Bulk and microanalysis showed that there are significant concentration variations in the boules grown by the Bridgman method, that lead to changesin phase transformation behavior which were observed by DSC. The presence of composition variations in the boules is a major issue because of its effect on the martensitetransformation temperature. For boules with composition variations, both transformed anduntransformed regions will exist over some temperature range, degrading the performanceof any actuator made from them. Clearly, further effort on the improvement of the crystalgrowth technique is needed to remove the composition gradient and variations and toobtain a fine dendritic structure, which would be much easier to homogenize. For thecurrent growth conditions, coarse cellular structures have been obtained which showsignificant solute segregation. An increase in the thermal gradient during the directionalsolidification process resulted in a finer cellular structure.Item Core-shell PVA / gelatin nanofibrous scaffolds using multinozzle aqueous electrospinning(Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2019., 2019.) Şengör, Mustafa.; Bedir, Hasan.; Altıntaş, Sabri.Biological scaffolds have been used in the reconstruction of the damaged tissues. They have similar morphology and structure to the host tissues. However, they can be produced using materials that can be harmful to humans and the environment. In this context, core-shell nano ber based sca olds, whose mechanical strengths are provided by PVA(poly vinyl alcohol) and recognition sites are provided by gelatin, were fabricated in a non-woven manner using multiple nozzles of electrospinning technique. Instead of widely used toxic, acidic or salt-based ionic solvents, deionized \water" was used as the only solvent for both polymers. Firstly, nano bers were produced from 8 % (w / w) gelatin and 8%(w / w) PVA solutions individually. Limits were determined for parameters such as voltage, feed rate, temperature and polymer concentrations. Although pure gelatin nano bers have diameters of less than 50 nm, they have beaded structure and have lower mechanical strengths. Smooth bers were obtained from 8% PVA. Fibers with PVA: gelatin core shell morphology were then produced at di erent feed rate ratios (FRR). Based on the ber diameter, the optimal FRR with a 15 kV voltage magnitude and 15 cm electrode distance was found to be 1: 1 with an average diameter of 280 nm. The ratio of 1: 3 and 1: 4 was seen as the formation of \beaded" bers and the pealing limit of gelatin over PVA, respectively. Mechanical and water resistance of the produced sca olds was further improved by cross-linking. Core - shell morphology was demonstrated by TEM, SEM, EDS analysis. The secondary structure of the gelatin from collagen and the e ects of the electrospinning were revealed by FTIR and DSC. Approximately 60% of all cross-linked sca olds were degraded in solution using lysozyme enzyme up to day 14.Item Fabrication and characterization of Al, AlMgSi, and AlSi foams(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2010., 2010.) Mahmutyazıcıoğlu, Nazım.; Altıntaş, Sabri.Manufacturing of metal foams by using powder compact melting (PCM) method, with the most important considerations like material and method selection, powder blending, cold compaction, hot compaction and hot extrusion and foaming is explained briefly. The effects of Al2O3 particle addition on the heat treatment, cell structure and mechanical properties of AlMgSi foams were studied. Alloy and composite foams were manufactured by powder compact melting (PCM) method. A pre-blended mixture of Al, Mg, Si and Cu representing the wrought AlMgSi alloy (6061) were mixed with heat treated TiH2 and Al2O3 ceramic particles, hot compacted and foamed at temperatures between 750 and 800 °C. The amount of <20μm sized Al2O3 particles in the composite foams were 3, 5 and 10 per cent by volume. The effects of different heat treatments on the microhardness of the foams were investigated. Foams that were fully heat treated had the highest hardness values and they performed best with an increase in collapse strength up to 100 per cent over the untreated samples. It was found that the addition of Al2O3 did not affect the hardenability but the strength and the compression stiffness of the composite foams were increased with 3 and 5 vol. per cent Al2O3 addition. This was attributed to the improved cell structure and decreased drainage when the ceramic amount is not more than 5 per cent. The compression test results were interpreted in terms of the foam’s microstructure and correlations were made relating the unloading modulus and compression strength of the foams with the relative density. It was found that the foams were inhomogeneous and their mechanical properties were close to those expected from open cell foams. An important phenomenon in Al foam production is stabilization of the structure. Especially it is very difficult to obtain highly expanded foams made from pure Al powder. In this study, an improvement in the stabilization was achieved by Al2O3 and B4C ceramic powder addition to the Al-TiH2 mixtures. Compaction of the mixture was achieved by hot extrusion. Extruded dense semi-products (precursors) were foamed at 800 °C and the macro and microstructures of the foams were analyzed. It was found that both of the ceramics increased the number of cells and cell size homogeneity of Al foams. The stabilization seems to be enhanced with the presence of solid constituents but the mechanism acting could not be understood clearly. Most of the ceramic particles were partially wetted and segregation between the metal and gas interface was observed. The wetting of B4C particles by the aluminum matrix was relatively better and this resulted in smoother foam cell walls when compared with the cell walls of pure and Al2O3 added Al foams. Finally, a different matrix alloy, hypereutectic AlSi (AlSi14Cu2.5Mg0.6) was used for foam manufacturing by PCM method. The alloy has some advantages over the Mg, Cu and Zn added alloys like good wear resistance, high mechanical strength without aging heat treatment and dimensional stability. Foams manufactured were heat treated, in order to analyze the changes in the microstructure and the mechanical properties. The results showed that, heat treatment caused spheroidizing of eutectic Si phases and depending on the density, the compression strength of the foams could be increased up to 50 per cent. The deformation of the foam was similar to brittle foams and densification strain was higher than ductile foams which makes it a good candidate for energy absorption applications.Item Hydroxyapatite coating on Ti and Ti6A14V substrates by using electrophoretic deposition method(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2008., 2008.) Albayrak, Önder.; Altıntaş, Sabri.Electrophoretic deposition method was used to coat hydroxyapatite (HA) on titanium (Ti) and its alloy (Ti6Al4V). Main focus was elimination or decreasing the crack occurrence and increasing adhesion strength. For this purpose, chemically synthesized nano sized and naturally derived submicron sized HA powders were produced. First, these powders both in calcined and uncalcined states were deposited on Ti and Ti6Al4V substrates under different coating voltages to investigate the effect of calcination and coating voltage on coating quality. The cracks became less pronounced with decreasing applied voltage. Coatings obtained using calcined powders were seen to be free of cracks before and after sintering. Further studies were conducted to increase the adhesion strength by depositing nano-sized titanium dioxide (TiO2) powders using different coating voltages, as an inner layer between Ti6Al4V substrate and HA coating. Adhesion strength of the overall coating was found to increase with decreasing voltage used in TiO2 deposition. Furthermore, use of the TiO2 inner layer prevented HA decomposition. Finally, studies were conducted to create coating surface consisting of calcium phosphate (Ca-P) based bioceramics and TiO2. This surface was expected to combine the advantages of Ca-P (osseointegration) and TiO2 (photocatalytic bactericidal effect). TiO2 was allowed to grow from the titanium substrate to fill in the cracks in the Ca-P coating at sintering stage, and coalescence of Ca-P/TiO2 was observed.Item Production of antibacterial hydroxyapatite(Thesis (Ph.D.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2011., 2011.) İpekoğlu, Mehmet.; Altıntaş, Sabri.Hydroxyapatite was produced by rapid microwave synthesis method. Effects of microwave power and duration were examined on the powder morphology of the hydroxyapatite powder during the synthesis process and microwave aging was found to be an effective method to produce hydroxyapatite. Silver was added in different amounts to calcium deficient and stoichiometric hydroxyapatite during the synthesis process using microwave, the antibacterial activities of the powders produced were analyzed and silver addition was found to impart antibacterial properties to hydroxyapatite. Hydroxyapatite powders containing different amounts of silver were heat treated at temperatures between 900-1100 °C, the decomposition of the powders were analyzed using XRD and FTIR analysis, the increasing heat treatment temperature and increasing silver amount were found to negatively effect the decomposition of the powders. The hydroxyapatite powders produced which do and do not contain silver were used to produce coatings on titanium substrates, hydroxyapatite decomposition at high sintering temperatures was effectively inhibited by the use of titanium dioxide inner layer. The effects of coating voltage and coating duration of the titanium dioxide inner layer on the mechanical properties of the coatings were examined, use of lower voltage and shorter coating times for the titanium dioxide inner layer were found to increase the mechanical strength of the coated hydroxyapatite.