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Öğe Analysis of the thermalization dynamics of two-layer thin films irradiated by femtosecond laser(Elsevier Gmbh, 2020) Tunc, K. M. Murat; Gunay, M. Erdem; Bayata, FatmaIn this work, ultrafast thermalization dynamics was examined for a variety of two layer-thin films (Au/Si, Au/Ni, Au/W, Au/Al and Au/Pb). Non-equilibrium energy transport under laser irradiation was formulated for the electron and lattice sub-systems of the thin films. A significant reduction in the temperature of the electron and the lattice of the gold surface was observed especially for Au/Si and Au/Ni thin films due to their large G values. Next, the effects of laser power intensity and laser heating duration on the temperature distributions were examined for Au/Ni two-layer thin film. It was found that, as the laser intensity increased, the maximum electron temperature increased dramatically; on the other hand, as the pulse heating duration increased, the electron temperature gradually decreased. It was then concluded that thermal damage threshold of the gold surface can be improved by depositing gold layer on a substrate material with high electron-phonon coupling factor. Hence the thermal failure of thin films used in optical components of ultrafast laser systems or micro/nano electro mechanical systems can be prevented.Öğe Çok Katmanlı Nanotübüler Termoelektrik Malzeme Tasarım, Üretim ve Karakterizasyonu(2020) Bayata, Fatma; Mısırlıoğlu, İbrahim BurçTermoelektrikler (TE), atık ısının dogrudan elektrik enerjisine çevrilmesinde kullanılan önemli bir enerji dönüsüm teknolojisidir. TE cihazlar TE modüllerlerden olusmaktadır ve TE modüller, içerdikleri TE malzemelere uygulanan sıcaklık gradyanı sayesinde elektrik üretmektedir. TE teknolojisinin yenilenebilir olması, yani sıcaklık gradyanı saglandıgı sürece elektrik üretebilir özellikte olması bu teknolojiyi önemli kılan bir özelliktir. TE teknolojisinde enerji verim oranının büyüklügü kullanılan TE malzemelerinin performansına baglı oldugu için yüksek TE davranısı gösteren malzemelerin gelistirilmesi durumunda önemli ölçüde enerji verimi saglamak mümkündür. Proje kapsamında çok katmanlı titanyum ince film kaplamalar alümina plakalar üzerine manyetik alanda sıçratma ve katodik ark fiziksek buhar biriktirme yöntemleri ile biriktirilmistir. Titanyum soy ve bazik metallerle alasımlandırılarak kaplamalarda kullanılmıstır. Proje kapsamında kullanılan alasım elementleri son derece pahalı oldugu için proje kapsamında katot ve hedef malzemeleri modifiye edilerek alasım elementlerinin yapıya girmesi saglanmıstır. Kaplamalar 3, 5 ve 7 kat seklinde uygulanmıstır. Alümina üzerinde ilk katman saf titanyum uygulanmıs, böylece daha iyi yüzeye tutunma saglanmıstır. Üzerine alasım katmanı ve tekrar saf titanyum kaplanmıstır. Bu sekilde katmanlar üretilmis ve kaplamalar karakterize edilmistir. Üretilen kaplamalar elektrokimyasal olarak oksitlenmis ve nanotübüler termoelektrik oksit katmanları üretilmistir. Yapısal olarak karakterize edilen katmanlar ardından termoelektrik açıdan karakterize edilmistir. Ilave edilen soy metallerin elektronik iletkenlikte önemli ölçüde artısa neden oldugu görülmüstür. Buna karsın Seebeck katsayısı degerlerinde ise bir düsüs gözlenmistir. Termal iletkenlikte ise ölçümün dogrudan yapılmasında güçlükle karsılasılmıstır ancak birbirleri ile uyumludur ve en iyi (en düsük) sonuç altın içeren numunede görülmüstür. Altın ilavesi kaplamaların termoelektrik performansını arttırmıstır. Projenin son asamasında üretilen malzemelerde çalısır bir modül üretimi gerçeklestirilmistir. Üretilen modül, ince film ve yıgın formundaki elektrotlardan olustugu için melez bir modül olarak tanımlanmıstır. Modül hem termoelektrik güç üretimi hem de Peltier sogutucu olarak çalıstırılmıstır. Sıcaklık farkı altında voltaj farkı saglanırken, voltaj farkı uygulandıgında iki yüzey arasındaki sıcaklık farkı kontrol edilebilmistir.Öğe Enhancement of high temperature thermoelectric performance of cobaltite based materials for automotive exhaust thermoelectric generators(Iop Publishing Ltd, 2022) Bayata, FatmaThermoelectric (TE) generators can directly convert exhaust waste heat into electricity in vehicles. However, the low conversion efficiency of TE generators is the main obstacle to their commercialization in automotive. Their efficiency mainly depends on the performance of the used materials which is quantified by the figure of merit (ZT value). In the present study, single- and co-doped calcium cobaltites (CCO) with rare-earth (Tb) and transition metals (Cu, Fe, Ni, Mn, Cr) were produced using sol-gel technique in order to improve their high temperature TE properties for heat recovery in exhaust manifold applications. By the combined effect of doping approach and the production technique used in this study, a remarkable decrease in the grain size of CCO was obtained, and thus its thermal conductivity dramatically decreased. Besides, thermopower values were improved significantly. The reduction in thermal conductivity and the increase in thermopower led to an enhancement in ZT value of CCO ceramics. Among all the co-doped samples, Tb-Cu co-doped CCO displayed the maximum ZT value of 0.116 at 873 K which is 2.5 times larger than that of pure CCO. The high thermal stability and the enhanced TE performance make Tb-Cu co-doped CCO material a potential candidate for heat recovery in automotive exhaust TE generators.Öğe İÇTEN YANMALI MOTORLARDA ÖSTENİTİK EGZOZ SUPAPLARININ YÜKSEK SICAKLIK AŞINMA MEKANİZMALARININ TESPİTİ(2021) Bayata, FatmaBu çalışmada 21-4N (%21 Cr, %9 Mn ve %3.88 Ni temel bileşenli alaşım) egzozsupap malzemesinin PL12 alaşımlı beyaz temper dökme demir baga malzemesikarşısında yüksek sıcaklıktaki aşınma davranışı incelenmiştir. Egzoz supap-bagaikilisinin aşınma testleri için özel olarak geliştirilmiş bir tribometre kullanılarak 25?, 350 ?, 550 ?’de ve 20 N yük altında testler yapılmıştır. Yapısal ve termodinamikanalizler ile 21-4N supap - PL12 baga ikilisinin ara yüzeyinde gerçekleşen yükseksıcaklık aşınma mekanizması literatürde ilk kez incelenmiştir. Aşınma sırasındauygulanan lokal basınç ve sıcaklık artışı ile spinel oksit yapılardan oluşan, aşınmayakarşı koruyucu bir tabakanın 21-4N yüzeyinde oluştuğu görülmüştür. Bu oluşantribofilm yüksek sıcaklıklarda 21-4N supap malzemesinin PL12 baga karşısındahacimsel aşınma kaybının azalmasına neden olmuştur. Ayrıca sıcaklığa bağlıoluşan/çözülen karbür ve karbonitrür fazları da termodinamik olarakhesaplanmıştır. Sıcaklık artışı ile pinin mukavemeti azalırken, 21-4N supapmalzemesinin 900°C’ye kadar kararlılık gösteren çökelti fazlarından dolayı mekaniközelliklerini koruduğu ve aşınma direncinin arttırdığı tespit edilmiştir.Öğe İçten yanmalı motorlarda östenitik egzoz supaplarının yüksek sıcaklık aşınma mekanizmalarının tespiti(Mühendislik Bilimleri ve Tasarım Dergisi, 2021-03-30) Bayata, FatmaÖZET: Bu çalışmada 21-4N (%21 Cr, %9 Mn ve %3.88 Ni temel bileşenli alaşım) egzoz supap malzemesinin PL12 alaşımlı beyaz temper dökme demir baga malzemesi karşısında yüksek sıcaklıktaki aşınma davranışı incelenmiştir. Egzoz supap-baga ikilisinin aşınma testleri için özel olarak geliştirilmiş bir tribometre kullanılarak 25 ?, 350 ?, 550 ?’de ve 20 N yük altında testler yapılmıştır. Yapısal ve termodinamik analizler ile 21-4N supap - PL12 baga ikilisinin ara yüzeyinde gerçekleşen yüksek sıcaklık aşınma mekanizması literatürde ilk kez incelenmiştir. Aşınma sırasında uygulanan lokal basınç ve sıcaklık artışı ile spinel oksit yapılardan oluşan, aşınmaya karşı koruyucu bir tabakanın 21-4N yüzeyinde oluştuğu görülmüştür. Bu oluşan tribofilm yüksek sıcaklıklarda 21-4N supap malzemesinin PL12 baga karşısında hacimsel aşınma kaybının azalmasına neden olmuştur. Ayrıca sıcaklığa bağlı oluşan/çözülen karbür ve karbonitrür fazları da termodinamik olarak hesaplanmıştır. Sıcaklık artışı ile pinin mukavemeti azalırken, 21-4N supap malzemesinin 900°C’ye kadar kararlılık gösteren çökelti fazlarından dolayı mekanik özelliklerini koruduğu ve aşınma direncinin arttırdığı tespit edilmiştir.Öğe Mechanical properties of vanadium-alloyed austempered ductile iron for crankshaft applications(Higher Education Press, 2023) Bayata, Fatma; Vatan, Sueleyman BatuhanThis study focused on the development of austempered ductile iron (ADI) with desirable combination of mechanical properties for crankshaft applications by the combined effect of vanadium (V) alloying and an optimized heat treatment process. The produced unalloyed GGG60, 0.15% V-alloyed GGG60 (V-15), and 0.30% V-alloyed GGG60 samples were subjected to austenitizing at 900 & DEG;C for 1 h and subsequent austempering processes at 250, 300, and 350 & DEG;C for 15, 30, 60, 90, and 180 min. As a result of these austempering processes, different bainitic structures were obtained, which led to the formation of diverse combinations of mechanical properties. The mechanical properties of the austempered samples were tested comprehensively, and the results were correlated with their microstructures and the stability of the retained austenite phases. From the microstructural observations, the V-alloyed samples exhibited a finer microstructure and a more acicular ferrite phase than unalloyed samples. The V addition delayed the coarsening of the acicular ferrite structures and considerably contributed to the improvement of the mechanical properties of GGG60. Moreover, the X-ray diffraction results revealed that the retained austenite volume and the carbon enrichment of austenite phases in ADI samples were remarkably affected by the addition of vanadium. The increase in volume fraction of retained austenite and its carbon content provided favorable ductility and toughness to V-15, as confirmed by the elongation and impact test results. Consequently, the dual-phase ausferrite microstructure of V-15 that was austempered at 300 & DEG;C for 60 min exhibited high strength with substantial ductility and toughness for crankshaft applications.Öğe Multi-objective optimum design of an alpha type Stirling engine using meta-models and co-simulation approach(Pergamon-Elsevier Science Ltd, 2021) Yildiz, Cengiz; Bayata, Fatma; Mugan, AtaAn alpha type Stirling engine was optimized using meta-models considering uninterrupted electric power supply concurrently with natural gas combi boilers at homes during electricity interruptions. To predict and optimize the power and efficiency of the designed Stirling engine, an artificial neural network (ANN) model was trained as a meta-model. The ANN modeling method was used in solving a multi-objective Pareto optimization problem under some constraints to determine the optimum engine parameters. The design parameters were swept volume, hot and cold cylinder temperatures, gas constant, charge pressure and engine operation speed. Feed forward and Levenberg-Marquardt back propagation algorithms were evaluated to determine the best resulting network architecture that was found as 6-12-8-1. Subsequently, the fraction of variance (R-f) value was calculated close to 1 and the absolute mean error percentage (AMEP) was calculated as 6.07%. Trained ANN model was used in solving the multi-objective optimization problem. Using the optimum design parameters, the meta model predicted the power as 73.3 W and efficiency as 32.2%. Co-simulation approach was followed to verify the optimization results, and the nominal power output and corresponding efficiency were calculated using the Schmidt theory and the calibrated 1-D model created by the GT-Suite software that yield respectively, 144.6 W and 85.8 W for the power and 35% and 35.1% for the cycle efficiency. Consequently, the use of an ANN model in solving the associated optimization problem proved itself as a fast, accurate enough and powerful method to find the optimum design parameters and predict the engine performance.Öğe Role of aluminum doping on phase transformations in nanoporous titania anodic oxides(Elsevier Science Sa, 2015) Bayata, Fatma; Urgen, MustafaThe role of aluminium doping on anatase to rutile phase transformation of nanoporous titanium oxide films were investigated. For this purpose pure and aluminum doped metal films were deposited on alumina substrates by cathodic arc physical deposition. The nanoporous anodic oxides were prepared by porous anodizing of pure and aluminum doped titanium metallic films in an ethylene glycol + NH4F based electrolyte. Nanoporous amorphous structures with 60-80 nm diameter and 2-4 mm length were formed on the surfaces of alumina substrates. The amorphous undoped and Al-doped TiO2 anodic oxides were heat-treated at different temperatures in the range of 280-720 degrees C for the investigation of their crystallization behavior. The combined effects of nanoporous structure and Al doping on crystallization behavior of titania were investigated using X-ray diffraction (XRD) and micro Raman analysis. The results indicated that both Al ions incorporated into the TiO2 structure and the nanoporous structure retarded the rutile formation. It was also revealed that presence or absence of metallic film underneath the nanopores has a major contribution to anatase-rutile transformation. (C) 2015 Elsevier B.V. All rights reserved.Öğe The analyses of frictional losses and thermal stresses in a diesel engine piston coated with different thicknesses of thermal barrier films using co-simulation method(Sage Publications Ltd, 2023) Bayata, Fatma; Yildiz, CengizThis study comparatively presents the thermal and mechanical effects of different Thermal Barrier Coatings (TBCs) and their thicknesses on the performance of aluminum diesel engine piston by combining Finite Element Analyses (FEA) and Artificial Neural Network (ANN) methods. The piston structure of MWM TbRHS 518S indirect injection six-cylinder diesel engine was modeled. The clustered TBCs (NiCrAlY-Gd2Zr2O7, NiCrAlY-MgO-ZrO2, NiCrAl-Yttria Partially Stabilized Zirconia (YPSZ), and NiCrAlY-La2Zr2O7) were implemented to the related surface of aluminum alloy piston and then static, thermal, and transient structural FEA were conducted for each model. Based on both of the temperature and equivalent stress distributions, NiCrAlY-Gd2Zr2O7 coated model displayed the best performance. Additionally, the effects of top coating thicknesses of TBCs were investigated in the range of 0.1-1.0 mm with 0.1 mm increments in FEAs. The thermally effective top coating thickness was predicted as 0.95 mm for the selected TBC using ANN method. Then the effects of coating thickness on frictional performance were revealed by generating transient structural FE models and utilizing stribeck diagram. The uncoated and 0.95 mm NiCrAlY-Gd2Zr2O7 coated models were adjusted as transient and the related crank angle - dependent in-cylinder combustion pressure data was implemented. The friction force was reduced by at least 15% in NiCrAlY-Gd2Zr2O7 coated model.Öğe The effects of design parameters on mechanical failure of Ti-6Al-4V implants using finite element analysis(Pergamon-Elsevier Science Ltd, 2020) Bayata, Fatma; Yildiz, CengizThe failure of dental implant is frequently occurred as the result of mechanical complications under daily biting forces. The long-term stability and the success of a dental implant are highly depended upon developing an implant system having high fracture resistance under cyclic biting forces. The design of implant has a crucial role in improving the fracture resistance of implant by decreasing stress concentration around the implant system. In the present study, three-dimensional (3D) models of the internally connected Ti-6Al-4V implant systems were developed in different dimensions and the failure analysis of these implants was performed under real biting forces according to ISO 14801 using Finite Element Analysis (FEA). In these models, the design parameters (implant diameter, abutment diameter, taper angle of abutment, implant wall thickness, etc.) were optimized in order to prevent the failure of implants in long-term use. The implant model exhibiting the best biomechanical behavior was determined and the stress value was decreased to 170.34 MPa (von-Mises stress) in this implant under cyclic biting loads. Besides the high fracture resistance of this model, longer service life was predicted and relatively smoother stress transmission was achieved from the implant to the bone tissues under biting forces. The implant stability was also increased by optimizing implant tightening torques. Additionally, the mechanical behavior of this implant model was investigated under different biting conditions.Öğe The high temperature wear mechanisms of iron-nickel steel (NCF 3015) and nickel based superalloy (inconel 751) engine valves(Elsevier Science Sa, 2021) Bayata, Fatma; Alpas, Ahmet T.High temperature sliding wear tests were performed on samples taken from NCF 3015 austenitic steel (Fe-32% Ni-14% Cr based 0.8% C alloy with Al, Ti, Nb, Mo) and Inconel 751 superalloy (Ni-16%Cr alloy with Al, Ti, Nb) engine valves using the counterfaces made of PL12 chromium alloyed cast iron seat insert. The contact surfaces were characterized by SEM-EDS, Optical Profilometer, Raman, XRD. The wear mechanisms of the valve systems were explored using the results of these analyses that determined compositions of the tribolayers that formed on the contact surfaces. The Calphad based thermodynamic analyses that revealed stability of the precipitates in the alloys at the test temperatures. An increase of wear occurred between 25 degrees C and 350 degrees C, but for T > 350 degrees C, a continuous reduction in volumetric wear losses were observed with increasing temperature for both alloys. At all temperatures Inconel 751 showed higher wear performance than NCF 3015. It was found that the formation of (Fe1-x-yCrxNiy)(3)O-4 mixed spinel oxides controlled the high temperature wear behavior of these alloys and these structures support the formation of tribolayer under local pressure and high temperature during sliding. Besides the direct effect of oxide compositions within tribolayer, the formation of high amount of (Ti,Nb)C carbides, Ni-3(Ti,Nb,Al) and Ni3Nb precipitates play important role in transition from severe to mild wear regime above 350 degrees C.Öğe The Mechanical Behaviors of Various Dental Implant Materials under Fatigue(2018) Bayata, Fatma; Yıldız, CengizThe selection of materials has a considerable role on long-term stability of implants. The materials having high resistance to fatigue are required for dental implant applications since these implants are subjected to cyclic loads during chewing. This study evaluates the performance of different types of materials (A ISI 3161 stainless steel, alumina and its porous state, CoCr alloys, yttrium-stabilized zirconia (YSZ), zirconia-toughened alumina (ZTA), and cp Ti with the nanotubular TiO2 surface) by finite element analysis (FEA) under real cyclic biting loads and researches the optimum material for implant applications. For the analysis, the implant design generated by our group was utilized. The mechanical behavior and the life of the implant under biting loads were estimated based on the material and surface properties. According to the condition based on ISO 14801, the FEA results showed that the equivalent von Miscs stress values were in the range of 226.95 MPa and 239.05 MPa. The penetration analysis was also performed, and the calculated penetration of the models onto the bone structure ranged between 0.0037389 mm and 0.013626 mm. L-605 CoCr alloy-assigned implant model showed the least penetration, while cp Ti with the nanotubular TiO2 surface led to the most one. However, the difference was about 0.01 mm, and it may not be evaluated as a distinct difference. As the final numerical evaluation item, the fatigue life was executed, and the results were achieved in the range of 4 x 10(5) and 1 x 10(9) cycles. These results indicated that different materials showed good performance for each evaluation component, but considering the overall mechanical performance and the treatment process (implant adsorption) by means of surface properties, cp Ti with the nanotubular TiO2 surface material was evaluated as the suitable one, and it may also be implied that it displayed enough performance in the designed dental implant model.
