Mechanical Engineering / Makina Mühendisliği
Permanent URI for this collectionhttps://hdl.handle.net/11147/4129
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Research Project Anot Destekli Katı Oksit Yakıt Hücrelerinde (koyh) Elektrolit Tabakası Yoğunluğunun Artırılması(2018) Sındıraç, Can; Büyükaksoy, Aligül; Akkurt, SedatGadolinyum katkılı Seryum (CGO), İtriya ile stabilize edilmiş zirkonya (YSZ) yerine, Orta Sıcaklık Katı Oksit Yakıt Hücrelerinde (OS-KOYH) elektrolit malzemesi olarak yaygın bir şekilde kullanılmaktadır. Ancak, CGO'nun tamamen gaz geçirmez bir tabaka olması için gereken % 95'in üzerinde göreceli yoğunluğa ulaşması 1400oC'nin üzerinde bir sinterleme sıcaklığını gerektirmektedir. Anot destekli KOYH tasarımlarında sıklıkla kullanılan birlikte sinterleme yönteminde anot ve elektrolit birlikte sinterlenir. Yüksek sinterleme sıcaklığı yanında tozların yüzey alanları, tane boyutları ve erime sıcaklığı farklılıkları birlikte-sinterleme (co-sintering) yöntemini zorlaştırır. Bu nedenle CGO'nun sinterleme sıcaklığının azaltılması gerekmektedir. Bu projede, CGO?nun 1400oC sıcaklığın altında sinterlenebilmesi için iki aşamalı bir yöntem önerilmektedir: Birinci aşamada düşük sıcaklıkta (800-1000oC) önceden sinterlenmiş gözenekli CGO gövdesine CGO ve Fe içeren çözeltilerin infiltre edilmesi; ikinci aşamada ise bu numunelerin 1000-1200oC aralığında sinterlenmesiyle yüksek yoğunlaşmaya ulaşılması hedeflenmiştir. Bu işlem sayesinde, sırasıyla CGO infiltre edilen numunelerde 1200oC?de %95,5 ve demir infiltre edilenlerde ise 1100oC?de %97,7 yoğunluğa başarıyla ulaşılmıştır. Böylece infiltre edilen çözeltiler bu gözenekleri homojen olarak başarıyla doldurmuşlardır. Bu durum SEM ve EDS analizleri ile de teyit edilmiştir. CGO infiltre edilen örnekler incelendiğinde, hem infiltre edilen çözeltinin konsantrasyonunun artması hem de infiltasyonun döngü sayısının artışının yoğunluğu arttırdığı gözlenmiştir. Projede amaçlanan 95% yoğunluğa CGO infiltre edildikten sonra 1200oC?de sinterlenen örnekte, kayda değer bir tane büyümesi olmadan ulaşılmıştır. Öte yandan, demir infiltrasyonu yoğunlaşmayı hızlandırmada daha verimli olup, %97,7 yoğunluğa 1100oC?de sinterlenme ile ulaşılmıştır. Bununla birlikte, sinterleme sıcaklığının 1200oC'ye yükseltilmesi, tane boyutu büyümesine ve yeni bir bileşiğin oluşmasına yol açmaktadır. Ayrıca, sonuçlar Fe ilavesi için optimum bir seviye olduğunu ve bu sınırdan daha fazla yükleme yapmanın göreceli yoğunluk üzerinde negatif bir etkiye neden olduğu gözlenmiştir. Son olarak, hem CGO hem de Fe ile infiltre edilen numunelerin iyonik iletkenlik değerleri, hemen hemen aynı yoğunluğa sahip 1400oC'de sinterlenmiş, infiltre edilmemiş CGO örneğiyle benzer sonuçlar göstermektedir. Bu da yeni yöntemin elektriksel özelliklerde beklenen değerleri sağladığını gösterilmiştir.Article Corrosion of Industrial Frit Furnace Refractories: a Postmortem Study(Anadolu Üniversitesi, 2016) Özcan, Selçuk; Akkurt, SedatMicrostructural and phase analyses of corroded frit furnace refractories forming the side walls and the bottom of an industrial frit furnace is reported in this study. Reflected light optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, and x-ray diffraction tools were used for the analyses. The microstructural analysis in combination with the saturation solubilities information in the phase diagrams was used to predict the corrosion behavior of the refractories. The frit and the refractory types were compared qualitatively for the dissolution potential and corrosion mechanisms. The dissolution of the refractory material was direct (congruent) for both the side wall refractories and bottom pavers. The first push exudation phenomenon was determined to be effective for the increase of porosity and pore dimensions which in turn caused accelerated wear rates when combined with corrosion. The corrosive potential of the transparent frit for corundum, mullite, and glassy phase in the refractories was determined to be excessive. The dissolution of these species in the molten transparent frit was predicted to start at temperatures between 1000-1340oC while the operating temperature was 1470oC. The decrease in the extent of corrosion by zirconia inclusion either in the refractories or in the molten glass compositions was qualitatively discussed.Article Citation - WoS: 54Citation - Scopus: 58Sintering and Microstructural Investigation of Gamma–alpha Alumina Powders(Elsevier Ltd., 2014) Yalamaç, Emre; Trapani, Antonio; Akkurt, SedatSintering behaviors of commercially available alumina powders were investigated using constant-heating rate dilatometric experiments. Each powder had different proportion of alpha/gamma alumina. Densification behaviors of powders were studied up to 1600 °C with three different heating rates of 1, 3.3 and 6.6 °C/min. Compacts of different gamma content alumina powders exhibited systematic anomalous second peaks in the densification rate curves at certain heating rates and temperatures. At 3.3 °C/min heating rate experiments, densification curves of 10% gamma phase alumina powder compacts reached a plateau after 1450 °C, and did not increase any further at higher temperatures. This phenomenon was double checked to understand powder behavior during sintering. 10% gamma phase alumina powder compacts showed the highest density for each heating rate. It reached 94% theoretical density with 1 °C/min heating rate. But 20% gamma phase alumina powder compacts had the finest grain size of about 1.40 ?m. Final density and porosity of compacts were also tested by image analysis and the results were coherent with Archimedes results. © 2014 Karabuk UniversityArticle Citation - WoS: 171Characteristics of Brick Used as Aggregate in Historic Brick-Lime Mortars and Plasters(Elsevier Ltd., 2006) Böke, Hasan; Akkurt, Sedat; İpekoğlu, Başak; Uğurlu, ElifMortars and plasters composed of a mixture of brick powder and lime have been used since ancient times due to their hydraulic properties. In this study, raw material compositions, basic physical, mineralogical, microstructural and hydraulic properties of some historic Ottoman Bath brick-lime mortars and plasters were determined by XRD, SEM-EDS, AFM, TGA and chemical analyses. The mineralogical and chemical compositions, microstructures, morphologies and pozzolanicities of the brick powders and fragments used as aggregates in the mortars and plasters were examined to find out the relationship between hydraulic properties of the mortars and the bricks. The characteristics of bricks used in the bath domes were also determined to investigate whether the brick aggregates used in mortar and plasters were prepared from these bricks. The results indicated that the mortars and plasters were hydraulic owing to the presence of crushed brick powders that have good pozzolanicity. The brick powders bad high pozzolanicity because they contained high amounts of calcium-poor clay minerals in their raw materials that were fired at low temperatures. On the other hand, bricks used in the domes had poor pozzolanicity with different mineralogical and chemical compositions from bricks used in mortars and plasters. Based on the results of the analysis, it was thought that the bricks manufactured with high amounts of clays were consciously chosen in the preparation of hydraulic mortars and plasters. (C) 2006 Elsevier Ltd. All rights reserved.Article Citation - WoS: 14Citation - Scopus: 15Electrochemical Performance of La0.6sr0.4co0.2fe0.8o3-Ce0.9gd0.1o2 Composite Sofc Cathodes Fabricated by Electrocatalyst And/Or Electrocatalyst-Ionic Conductor Infiltration(Springer, 2019) Sındıraç, Can; Büyükaksoy, Aligül; Akkurt, SedatInfiltration of electrocatalyst precursor solutions into previously sintered porous ionic conductor scaffolds has been used recently as an alternative method to the conventional co-sintering route to fabricate electrocatalyst-ionic conductor composites for solid oxide fuel cell (SOFC) cathode applications. However, the aqueous nitrate solutions generally used to perform the infiltration process results in electrocatalyst precipitates that are disconnected from each other, yielding poor electrode performance. In this work, polymeric electrocatalyst (La0.6Sr0.4Co0.2Fe0.8O3-LSCF) precursors that produce interconnected thin films upon heat treatment were used to infiltrate porous ionic conductor Ce0.9Gd0.1O2-delta (GDC) scaffolds to overcome these issues. In addition, for the first time in the literature, a mixture of LSCF and GDC polymeric precursors, which would yield LSCF-GDC nanocomposite coatings on the grains of the porous GDC scaffold were used as the infiltration solution. Thus, further enhancement of the electrocatalyst/ionic conductor interfacial area and achievement of improved electrode performance was aimed. As a result of the optimization studies, the lowest measured area specific polarization resistance (ASR(cathode)) values of 0.47 and 0.73 omega.cm(2) were obtained for polymeric LSCF+GDC and LSCF precursor infiltrations respectively at 700 degrees C, in air. In addition, LSCF+GDC infiltration yielded electrodes with much improved long-term stability in comparison to those obtained by LSCF infiltration. [GRAPHICS] .Article Citation - WoS: 25Citation - Scopus: 26Lowering the Sintering Temperature of Solid Oxide Fuel Cell Electrolytes by Infiltration(Elsevier Ltd., 2019) Sındıraç, Can; Çakırlar, Seda; Büyükaksoy, Aligül; Akkurt, SedatA dense electrolyte with a relative density of over 95% is vital to prevent gas leakage and thus the achievement of high open circuit voltage in solid oxide fuel cells (SOFCs). The densification process of ceria based electrolyte requires high temperatures heat treatment (i.e. 1400-1500 degrees C). Thus, the minimum co-sintering temperatures of the anode-electrode bilayers are fixed at these values, resulting in coarse anode microstructures and consequently poor performance. The main purpose of this study is to densify gadolinia doped ceria (GDC), a common SOFC electrolyte, at temperatures lower than 1400 degrees C. By this aim, an approach involving the infiltration of polymeric precursors into porous electrolyte scaffolds, a method commonly used for composite SOFC electrodes, is proposed. By infiltrating polymeric precursors of GDC into porous GDC scaffolds, a reduction in the sintering temperature by at least 200 degrees C is achieved with no additives that might affect the electrical properties. Energy dispersive x-ray spectroscopy line scan analyses performed on porous GDC scaffolds infiltrated by a marker solution (polymeric FeOx precursor in this case) reveals a homogeneous infiltrated phase distribution, demonstrating the effectiveness of polymeric precursors.Article Citation - WoS: 11Citation - Scopus: 12Electrical Properties of Gadolinia Doped Ceria Electrolytes Fabricated by Infiltration Aided Sintering(Elsevier Ltd., 2019) Sındıraç, Can; Büyükaksoy, Aligül; Akkurt, SedatCommon solid oxide fuel cell (SOFC) electrolyte materials (e.g., gadolinia doped ceria - GDC) demand temperatures exceeding 1400 degrees C for densification by conventional solid state sintering. It is very desirable to reduce the densification of the SOFC electroltytes to i) avoid microstructural coarsening of the composite anode layers, which are co-sintered with the electolyte layer in the anode supported SOFC fabrication scheme and ii) reduce energy consumption during SOFC manufacturing. We have recently demostrated a novel infiltration-aided sintering route to densify GDC ceramics at 1200 degrees C. In the present work, we present the electrical properties of GDC ceramics fabricated thusly. Comparison of high density (>= 95%) samples fabricated by conventional or infiltration-aided sintering reveal that at 700 degrees C, similar total electrical conductivities are obtained, while at 300 degrees C, specific grain boundary resistivity is smaller in the latter. Bulk (grain) conductivity is higher in porous GDC ceramics (relative density <= 90%) fabricated by infiltration-aided sintering than the conventionally sintered ones with similar porosities. Finally, open circuit voltage of 0.84 V at 700 degrees C, obtained under dilute hydrogen and stagnant air conditions suggests that GDC ceramics densified by infiltration-aided sintering are suitable for use as SOFC electrolytes.Article Citation - WoS: 15Citation - Scopus: 16Fabrication of Lscf and Lscf-Gdc Nanocomposite Thin Films Using Polymeric Precursors(Springer, 2020) Sındıraç, Can; Ahsen, Ali; Öztürk, Osman; Akkurt, Sedat; Birss, Viola, I; Büyükaksoy, AligülLa1-xSrxCoyFe1-yO3 (LSCF) and LSCF-gadolinia-doped ceria (LSCF-GDC) composites are used as solid oxide fuel cell (SOFC) cathodes. In the present study, to maximize the LSCF/gas and LSCF/GDC interfacial area and thus enhance the performance, we fabricated both single-phase LSCF and composite LSCF-GDC thin-film electrodes using a facile and cost-effective polymeric precursor technique. This method involves molecular level mixing of cations in solution form and results in average particle sizes of ca. 72 nm and 60 nm upon annealing at 700 degrees C, respectively. For LSCF, electrochemical impedance spectroscopy measurements indicate very low electrode polarization resistances of ca. 0.6 omega cm(2) per electrode at 600 degrees C. However, the addition of GDC results in poorer electrochemical activity but better microstructural and electrochemical stability, all at 600 degrees C. Surface analysis revealed that Fe surface segregation occurs in the single-phase LSCF, while predominantly Co segregation is observed at the LSCF-GDC composite electrode surface.Article Citation - WoS: 9Citation - Scopus: 10Microstructural Investigation of the Effect of Electrospraying Parameters on Lscf Films(Elsevier Ltd., 2020) Sındıraç, Can; Akkurt, SedatIntermediate temperature solid oxide fuel cells (IT-SOFC) require an effectively functioning cathode layer whose performance depends largely on their microstructures. Improved electrochemical performance of the cathode layer can be possible by tailoring the microstructure to ensure that both the oxygen reduction reaction (ORR) occurs fast along the triple-phase boundaries (TPB) and the diffusion pathway is short enough for fast ion diffusion through the cathode layer. Electro spray deposition (ESD) method is a low-cost deposition method which allows the optimization of microstructure by changing the spraying parameters. In this study, gadolinium doped ceria (GDC) electrolyte layer is deposited with La1-xSrxCo1-yFeyO3-? (LSCF) derived from polymeric precursor salts, symmetrically. As a solvent couple, 2-butoxyethanol and ethylene glycol are used instead of the conventional solvent couples frequently employed in the literature. The use of the new type of solvents in the precursor solution leads to promising results on modifying the microstructure of the deposited layer. The effect of electrospraying parameters on the cell performance was also studied. Promising results were obtained as measured by impedance spectroscopy when this new solvent couple was employed. © 2020 Hydrogen Energy Publications LLCArticle Citation - WoS: 9Citation - Scopus: 10Formation of La1-Xsrxco1 Cathode Materials From Precursor Salts by Heating in Contact With Cgo Electrolyte(Elsevier Ltd., 2016) Sındıraç, Can; Akkurt, SedatThe purpose of this study is to determine the solid state reactions leading to the formation of La0.6Sr0.4Co0.8Fe0.2O3 and La0.6Sr0.4Co0.2Fe0.8O3 which are widely used as cathode material in solid oxide fuel cells (SOFC) from precursor salts. Interactions between the cathode and the electrolyte layers are also investigated while the cathode layer formed upon heating in contact with the surface of cerium-gadolinium oxide (CGO) electrolyte substrates. Almost all combinations of precursor salt mixtures were tested to see if all solid state reactions are completed and what phases eventually formed. Most of the transformation was complete after 1050 °C heat treatment to yield different mixed oxides. The cathode layer was usually in porous form but was found to spread well over the substrate. Uneven diffusion of La, Sr, Co or Fe into the substrate influenced the stoichiometry of the resulting cathode layer in varying degrees. Fe was found to diffuse into the substrate.