Phd Degree / Doktora
Permanent URI for this collectionhttps://hdl.handle.net/11147/2869
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Doctoral Thesis Production and Characterization of Ceramic Components Via Current Sintering Techniques(01. Izmir Institute of Technology, 2023) Karacasulu, Levent; Ahmetoğlu, Çekdar Vakıf; Adem, UmutThis dissertation aims to utilize contemporary advanced sintering techniques such as cold sintering, reactive hydrothermal liquid phase densification, fast firing, flash sintering, and ultrafast high-temperature sintering for sintering of various ceramic materials. The ceramics produced through these methods are compared with their traditional counterparts in terms of processing-structure-property relationships. In the first section, a brief overview of the advanced sintering techniques used is provided. Chapters 2-7 give a review study on low-temperature densification techniques, and the studies conducted using the cold sintering process and reactive hydrothermal liquid phase densification process, namely cold sintering techniques, which allow densification below 400 °C. Chapters 8&9 presents research related to ceramic materials produced via the fast-firing technique with rapid heating rates compared to conventional sintering, widely employed in the industry. Chapters 10-12 cover sintering studies conducted utilizing joule heating based sintering techniques allowing very fast heating rates such as flash sintering and ultrafast high-temperature sintering. Chapter 13 presents a comparison of current sintering techniques used in terms of applicability, equipment, materials, and so on. The pros and cons of such techniques were explained. In conclusion, there may be no guarantee that every ceramic material will yield successful results in all sintering processes. It is essential to recognize that each sintering process occurs within distinct sintering mechanisms. The selection of the appropriate advanced sintering method and conditions should be based on an assessment of the specific material's characteristics and the desired properties in the final product.Doctoral Thesis Production and Characterization of Porous Ceramics for High Temperature Applications(Izmir Institute of Technology, 2022) Semerci, Tuğçe; Ahmetoğlu, Çekdar Vakıf; Akdoğan, YaşarThis thesis focuses on the production and characterization of different porous polymer derived ceramic (PDC) components (foams, additively manufactured (AM) honeycombs, and aerogels) and demonstrates their potential for high temperature applications, including gas permeability (up to ~700 o C), molten metal filtration, and heat exchanger. The foams were produced via the replica technique and different pore sizes, ranging from 300 μm to 2 mm, silicon oxycarbide (SiOC) ceramic foams were able to be formed. The average total porosity of the foams was 96 vol% with a specific surface area (SSA) of ~80 m2 /g. AM-made honeycomb-like cellular structures with different cell sizes (578 μm, 1040 μm) were obtained via fused filament fabrication. Finally, SiOC aerogels were synthesized using siloxane resin, then dried at ambient pressure and room temperature. The produced SiOC aerogels showed a total porosity of around 80 vol% and an SSA reaching 250 m2 /g. Regarding the high temperature applications of porous PDC components, initially, the gas permeability of SiOC foams was tested, and the results showed stability up to 700 °C in the air without any loss of functionality, offering reusability even in aggressive environments. In the subsequent studies, filtration of molten aluminum alloy was tested using various porous components. PDC foams demonstrated better performance in comparison to the AM-made cellular structures and commercial SiC foams. Finally, heat exchange analysis was performed to evaluate the heat transfer of SiOC foams, and an increase in pressure drop was found to be directly proportional to the rate of increase in air velocity.Doctoral Thesis Processing Foam-Like Porous Glass Structure Using a Combined Process of Glass Powder Expansion in Aqueous Environment and Sintering Process(Izmir Institute of Technology, 2019) Zeren, Doğuş; Güden, Mustafa; Akdoğan, YaşarSoda-lime glass foams were formed by the controlled pore structure of inorganic particle-liquid suspensions at room temperature and then sintered at elevated temperatures between 650oC-800 oC. The slurries were prepared using the glass particles below 38 µm (fine), between 38 and 45 µm (medium) and between 45 and 56 µm (coarse) and with 50, 55, and 60 wt% solid content and 2, 3, and 4 wt% carboxymethyl cellulose (CMC) binder addition. The slurries were foamed using an Al-based foaming agent and a calcium hydroxide alkali activator with an amount of 1 wt%. An increase in CMC content and a decrease in particle size shifted the slurries from a Newtonian to a non-Newtonian behavior and slurry stabilization with the CMC addition. The extensively increased initial bubble pressure in high viscosity slurries resulted in higher linear expansion rate initially followed by a bursting of gas bubbles. The maximum foam linear expansion of the slurries increased with CMC addition until about ~5 Pa s and the expansions stayed almost constant over 400% expansion, while the slurries with the viscosity above 50 Pa s could not be foamed. The most effective factor on the maximum expansion was found the solid content followed by CMC content and the least effective factor was determined the particle size. Partial bonding of glass particles and excessive shrinkage of glass particles due to the melting of foam green bodies were seen at 650 and 800 oC sintering temperatures. Prepared foam glasses showed lower compressive strengths and thermal conductivities than the glass foams reported in the literature. Finally, foaming at room temperature with this technique was found to be more advantageous than conventional glass foam production techniques due to ease of pore formation controlling at room temperature.Doctoral Thesis Effect of Colloidal Interactions in the Forming of Lead Magnesium Niobate Ceramics(Izmir Institute of Technology, 2007) Deliormanlı, Aylin Müyesser; Polat, MehmetElectrokinetic properties of lead magnesium niobate (PMN) powders in liquid medium are investigated in this thesis. Isoelectric point of aqueous PMN suspensions was determined as a function of solids concentration. Dissolution character of both Pb+2 and Mg+2 cations in PMN structure were examined as a function of pH. Poly(acrylic acid)-poly(ethylene) oxide PAA/PEO comb polymers were tested as the novel dispersant in this system and their effect on the stability and flow behavior of aqueous PMN suspensions was investigated. Stability and flow behavior were mainly studied by rheological measurements and sedimentation experiments as a function of pH and polyelectrolyte concentration. Adsorption of comb polymers onto PMN surface and their effect on the powder dissolution were also investigated. Results indicated that the PAA/PEO comb polymers impart stability to PMN suspensions over a wide pH range and ionic strength where pure PAA fails. In the course of this study PMN powders were consolidated using two different wet shaping methods. First, PMN films were produced using aqueous tape casting method which is widely used in the manufacture of multilayer ceramic capacitors. Secondly, three-dimensional PMN lattices were produced using robocasting method. In the former case, the goal was to provide alternative recipes for the aqueous tape casting process to be utilized in industrial applications since the current technology is based on the organic solvent based techniques. Results showed that it was possible to produce high quality PMN films with thicknesses in the range of 10 to 250 .m using aqueous based tape casting process. In the latter case, the results showed that robocasting is a suitable technique for the preparation of three dimensional PMN ceramics. Based on these results overall conclusion and the contribution of this study can be summarized as follows: The work carried out gives new insights into the manufacturing of PMN based ceramics using aqueous based techniques. Such improvements may bring benefits in the manufacturing of smaller electronic components employed in the personal computers and mobile phones and other consumer products in the near future.Doctoral Thesis Gas Permeation Through Sol-Gel Derived Alumina and Silica Based Membranes(Izmir Institute of Technology, 2009) Topuz, Berna; Çiftçioğlu, MuhsinThe 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 The Use of Paper Processing Residues in the Development of Ceramics With Improved Thermal Insulation Properties(Izmir Institute of Technology, 2010) Sütçü, Mücahit; Akkurt, SedatRecycled paper processing residues, those are industrial wastes, are utilized in the manufacture of porous and lightweight ceramics with improved thermal insulation properties for structural brick and refractory firebrick applications. These residues that contained micro-sized calcium carbonate (<5 .m) and cellulose fibers (<20 .m of diameter), were successfully used as an additive to earthenware brick to create porous structure during firing. A solid porous brick and a vertically perforated porous brick had 0.4 W/mK and 0.158 W/mK of thermal conductivity, respectively. This means substantial potential energy savings in houses and other buildings. When the paper residue was added in extreme amounts to the brick clay, it was found that anorthite (CaO.Al2O3.2SiO2) formed in the brick at the high-end of the firing temperatures of 1000-1100 C. This inspired the second part of the thesis which dealt with the production of porous, lower density insulating firebrick. Addition of the paper residue in excessive amounts up to 30% into the clay systems and firing at higher temperatures (1100.1400 C) formed a crystalline anorthite phase. Highly porous anorthite lightweight ceramics from the mixtures with up to 30% sawdust addition (used as additional pore-former) was successfully produced. Thermal conductivities of the samples decreased from 0.25 W/mK (1.12 g/cm3) to 0.13 W/mK (0.64 g/cm3) with increasing sawdust addition. Samples were stable at high temperatures up to 1100 C, and their cold strength was sufficiently high. Two separate porous lightweight ceramics were developed for insulation in buildings to reduce heat losses, and for insulation in high temperature applications.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, MuhsinThe 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.