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Yeni tip insert uç ile Al6061 malzemeye bilyeli material yöntemi uygulaması ve yüzey özelliklerinin incelenmesi
Derin haddeleme, malzeme yüzeylerinin iyileştirilmesi için uygulanan ve yaygın olarak kullanılan bir yüzey düzeltme metodudur. Bu metot ile malzeme yüzeyindeki mekanik özellikler (sertleşme, aşınma direnci, yorulma) ve yüzey pürüzlülüğünde iyileşmeler meydana gelmektedir. Bu çalışmanın amacı, yeni tasarlanmış ve mevcut katerlere takılabilen bir ezici uç ile farklı tornalama işlemlerine sahip (boyuna, konik, radüslü) Al6061-T6 malzemenin derin haddelenmesi yoluyla kullanılabilirliğinin araştırılması ve deneysel olarak derin haddeleme sonuçlarının incelenmesidir. Çalışmada, WNMG insert uç modeli baz alınarak küresel uca sahip ezici tip uç imal edilerek, farklı parametreler kullanılmış ve Al6061-T6 malzemeye derin haddeleme işlemi uygulanmıştır. Derin haddeleme işlemleri CNC torna tezgâhında yapılmış ve işlem parametreleri olarak 143N, 330N, 495N haddeleme kuvveti, 0.04 mm/dev, 0.08 mm/dev ve 0.12 mm/dev ilerleme miktarı ile 400 dev/dak, 600 dev/dak ve 800 dev/dak devir sayısı belirlenmiştir. Tornalanan Al6061-T6 parçalardaki yüzey pürüzlülüğü incelenmiş, parametrelerin yüzey pürüzlülüğü üzerine etkileri analiz edilmiştir. Çalışma sonunda mevcut standart tornalama katerlerine uyumlu olarak tasarlanan küresel uçlu ezici ucun derin haddelemede kullanılabilir olduğu, farklı tornalama kontürlerine sahip torna parçalarının derin haddelenmesinde yüzey pürüzlülüğünü iyileştirdiği görülmüştür. Mikrosertlik sonucunda Ra üzerinde en ideal parametreler tespit edilmiştir. Optik mikroskopta yüzey görüntüleri alınmış ve ilerlemenin artışıyla yüzeyde bozulmaların meydana geldiği görülmüştür. SEM görüntülerinde ince taneli tabakaların varlığından söz edilebilir. EDX analizlerinde yüzey üzerinde en fazla değişimin %C oranında oluştuğu görülmüştür.
TIG ergitme yöntemi kullanılarak AISI 1040 orta karbonlu çelik yüzeyinin grafit takviyeli ferro ti ile kaplanabilirliğinin incelenmesi
Yüzey modifikasyonu olarak ifade edilen GTA ile kaplama yönteminde, arzu edilen bileşim ve oranlarda, yüzeyde ince bir tabaka oluşturmak için, esas malzemeye metalurjik olarak bağlanmış kaplama malzemesinin hızlı katılaştırılması işlemine dayanmaktadır. Gaz tungsten ark yöntemiyle yapılan kaplama teknolojisi yeni bir yöntem değildir. Fakat yapılan işlem bakımından, üretimin aynı şartlarda daha kalın bir tabaka elde etmenin ve istenilen bileşim ve oranlarda bir kaplama tabakası oluşturmanın çok daha kolay uygulanması ve ucuz olması bakımından benzer yöntemlere göre daha avantajlıdır. Bu tez çalışmasında, alt tabakası 1040 paslanmaz çelik yüzeyine farklı oranlarda grafit takviyeli TiC/Grafit tozu GTA kaynak yöntemi kullanılarak yüzey kaplama işlemi yapılmıştır. Kaplama işlemi sonrası numuneler kaplama kalınlıkları ve mikroyapı morfolojisi optik mikroskop ile incelenmiştir. Ayrıca numunelerin taramalı elektron mikroskobu (SEM), Enerji dağılımlı spektrometresi (EDS), X-Işını kırınımı (XRD) cihazları ile mikroyapı, elementel dağılım ve faz bileşenleri analiz edilmiştir. Ayrıca kaplama tabakasının mekanik özelliklerini belirlemek için mikrosertlik ve aşınma cihazları kullanılıp sertlik değerleri ölçülmüş olup aşınma sonuçları mikroyapı ile karakterize edilmiştir. SEM ve EDS analiz sonuçlarından daha düzgün ve homojen kaplamaların oluştuğu ve yoğun dendritik yapıların olduğu gözlenmiştir.Kaplama ve alt tabaka arasında iyi bir metalurjik bağın oluştuğu optik mikroyapı ve SEM sonuçlarından tespit edilmiştir. EDS sonuçlarından görüldüğü üzere kaplama tabakasından alt tabakaya doğru gidildikçe Ti, C elementinin azaldığı gözlemlenmiştir. C elementinin ise kaplama tabakası ile birlikte arayüzün alt kısımlarına doğru gidildikçe artışını devam ettirdiği bunun temel nedeni olarak da alt tabakada yer alan C elementinin yüzeye doğru çıkmasından kaynaklandığı düşünülmektedir. XRD analiz sonucunda tüm numunelerde baskın fazın TiC fazı ile birlikte iv Fe7C3, Fe3C ve martenzit fazlarının olduğu, düşük pik değerlerinde α-Fe ve δ-Fe fazlarının da oluştuğu tespit edilmiştir. Optik mikroskop yardımı ile ölçülen kaplama kalınlıklarıda ise 1144 j/mm enerji girdisine sahip N2 numunesinde kaplama kalınlığı 2270 μm iken, 899 j/mm enerji girdisine sahip N4 numunesinde ise 580 μm olarak ölçülmüştür. Elde edilen sonuçlar ile TIG yöntemi ile kaplama çalışmasında kaplama kalınlığında enerji girdisinin önemli bir faktör olduğu düşünülmektedir. Kaplama yüzeylerinden alınan mikrosertlik ölçüm sonuçları değerlendirildiğinde %5 C takviyeli numunenin 857 HV0.1 değeri ile en yüksek ortalama mikrosertlik değerine sahip olduğu gözlemlenmiştir. En düşük sertlik değeri ise. %1 C takviyeli numuneden ise 756 HV0.1 ortalama mikrosertlik değeri elde edilmiştir. Takviyesiz olarak ölçüm yapıldığında ise 715 HV0.1 ortalama mikrosertlik değeri elde edilmiştir. Sonuç olarak C miktarı arttıkça sertliğin de buna bağlı olarak arttığı gözlemlenmiştir. Yüksek değerlerin ana kaynağının ergime akabinde katılaşma sonrası kaplama bölgesinde oluşan karbür fazlarının etkisi olduğu düşünülmektedir. C ilavesiyle birlikte mikrosertliğin en yüksek %5 C kaplama numunesinden elde edilmiştir. Aşınma sonuçları değerlendirildiğinde C ilavesine bağlı olarak sürtünme katsayısının düştüğü ve aşınma direncinin arttığı belirlenmiştir. Elde edilen sonuçlara göre en düşük sürtünme katsayısının N6 nolu numunede 0.2 elde edilirken en düşük sürtünme katsayısının ise N1 nolu numunede 0.6 olarak ölçülmüştür. Bu sonuçlar ışığında aşınma ve milkrosertlik değeri düşük olan AISI 1040 çeliğinin TiC/C ile kaplanması sonucunda mikrosertlik değerlerinin yükseldiği ve aşınma direncinin arttığı tespit edilmiştir.
Karayolu ulaşımının Türkiye'de gelişim tarihi ve çevresel etkilerinin bilimsel çalışmalar ışığında araştırılması
İnsanoğlu, varlığından beri temel ihtiyaçlarını karşılamaları amacıyla bir yerden başka bir yere ulaşım amacıyla yaşam alanlarında sürekli hareket halinde olmuştur. Bu hareketliliği güven ve konfor içinde sağlayabilmek için coğrafik koşullara bağlı olarak farklı çalışma prensiplerine sahip ulaşım sistemleri ile gidermişlerdir. Dönemi teknolojisi bağlı olarak temel olarak karayolu, demiryolu, denizyolu, havayolu sistemlerini kullanarak ulaşım sağlanmıştır. Ancak, karayolu bunlar arasında en yaygın olarak kullanıla ulaşım sistemidir. Karayolu ulaşım ilk başlarda motorize olmayan araçlar ile yapılmakta iken günümüzde teknolojinin gelişmesi ile beraber üstün özelliklere sahip ve farklı enerji kaynakları kullanılan motor gücünden yararlanılan araçlar ile yapılmaktadır. Zira araç hareket kabiliyetini belirleyen motor gücünü sağlayan enerji kaynağı dünden günümüze fosil yakıtlar olmuştur. Gelişen araç teknolojisi ile beraber ulaşım hızlı, konforlu ve güven içerisinde yapılmaya başlanmış, sadece insan değil aynı zamanda yük taşımacılığı bölgesel ve küresel ölçekte gelişmiştir. Bu şekilde farklı kültürlere ve özeliklere sahip varlıkların etkileşimi ile hem bölgesel hem de küresel ölçekte çevresel tabanda değerlendirilebilecek sosyokültürel, ekonomik, biyo-çeşitlilik boyutunda çevresel yararlı ve/veya zararlı değişimler meydana gelmiştir. Bu tez çalışmasında, öncelikle karayolu ulaşımının (ülkemiz, Türkiye, odaklı) gelişme evreleri dönemsel tabanda araştırılmış ve sosyal-kültürel değişimi doğuşu ortaya konulmaya çalışılmıştır. Sonrasında karayolu ulaşımının çevresel etkileri sosyal, ekonomi, kültür ve biyo-çeşitlilik boyutuyla mevcut bilimsel çalışmalar ışığında incelenmiştir. Bu tez çalışması ile amaçlanan, dünden bugüne esasıyla ülkemiz, Türkiye, sınırları içerisinde karayolu ulaşım sisteminin gelişimini ortaya koymak, sonrasında küresel boyutta genişletilebilecek çevresel etkilerini, sosyal, ekonomi, kültür ve biyo-çeşitlilik bağlamında değerlendirerek, yararlar ve zararlar noktasında ilgili okuyucu kesimde bir bilinç oluşturmaktır. Nitekim mevcut çalışmalar, karayolu ulaşımı gelişiminin, Türkiye özelinde, oldukça zorlu bir süreçten geçtiği, çevreye kültürel, sosyal ve ekonomik bağlamda değerler kattığı ancak çevresel bağlamda ise küresel ısınmada etken unsur olma nedeniyle önemli ve geri dönülmez zararlar verdiği değerlendirilmiştir.
Yapay burun: Optofluidic fotonik Bragg fiberleri ile uçucu organik bileşiklerin teşhisi
Artificial nose system, comprising of a bundle of photonic Bragg fibers used for identification of industrial toxic gases is reported. The system, otherwise known as optoelectronic nose, harvests the unique infrared spectrum of volatile organic compounds (VOCs), in conjunction to a fabricated multilayered photonic Bragg fibers that filters the incident spectrum of the infrared to a narrow transmission band. The sensing mechanism of the device comprises the measurement of infrared absorption of volatile analytes inside the hollow cores of optofluidic Bragg fibers. An array of six fibers is used, where each fiber target a different region of the midinfrared in the range of 2-14 µm with transmission bandwidths of about 1-3 µm. The quenching in the transmission of each fiber due to the presence of analyte molecules in the hollow core is measured separately and the cross response of the array allows the identification of virtually any (VOC). The device was used for the identification of seven industrial VOC vapors with high selectivity using a standard blackbody source and an infrared detector. The array response is registered as a unique six digit binary code for each analyte by assigning a threshold value to the fiber transmissions. The developed prototype is a comprehensive and versatile artificial nose that is applicable to a wide range of analytes.
Küçük ölçekli insansız hava aracı motor pistonlarının alüminyum matrisli kompozit ile üretimi ve karakterizasyonu
Conventional pistons are produced from the Al-Si alloy which has high-temperature resistance with the addition of 1 wt.% each of Cu, Mg, and Ni. These elements are added to increase the strength of the alloy. High Si content provides a lower expansion coefficient and higher wear resistance; however, it decreases the strength of the piston. The heat treatment of piston alloy is generally T5 and T6. This modification can be modal or compositional. That is to say, to increase the piston life, either the design of the piston should be changed, or the material of piston should be improved. Therefore, in this study, the material of the piston head was replaced with the metal matrix composite material. The characterizations of the new pistons and conventional ones were done. The results were collected and compared to their performances, physical, and chemical properties. Then, the results show that the composite pistons, especially produced by the Rheo-sintering technique, have superior mechanical properties such as higher wear resistance, higher fatigue life, and higher flexural strength.
Fotoelektrokimyasal suyu ayrıştırma tepkimesinin BiVO4-bazlı fotoanotlar üzerinde incelenmesi
Fotoelektrokimyasal (PEC) hücreler, suyu H2 ve O2'ye ayrıştırmak suretiyle güneş enerjisini kimyasal enerjiye çeviren cihazlardır. Güneş enerjili su ayrışması sırasında (foto) anot üzerinde oksijen evrimi reaksiyonu (OER) gerçekleşir ve (foto) katot üzerinde hidrojen açığa çıkarma reaksiyonu (HER) gerçekleşir. Oksijen evrimi reaksiyonundaki fotoanotların zayıf aktivitesi, PEC sistemlerinin yaygın kullanımında bir sınırlayıcı faktördür. Diğer bir faktör ise yük taşıyıcıların verimsiz kullanımıdır. Bu nedenle, daha işlevsel fotoanotlara için etkin foto emici materyallere ihtiyaç duyulmaktadır. OER'in verimliliğini, HER'in verimliliğinin seviyesine çekebilmek için oksijeni evrimleştirme yeteneğine sahip oksit yarı iletken ışık soğuran malzemeler sıklıkla kullanılır. Görünür ışık spektrumunda uygun ışık soğurma özelliklerine, iyi katalitik özelliklere, yeterli elektron / boşluk hareketliliği ve yük taşıyıcı ömrüne, yüksek kimyasal kararlılığa ve düşük maliyetli fotoanotlar PEC cihazlarının etkili kullanımı için gereklidir. Bir n-tipi yarı iletken olan bizmut vanadat (BiVO4), diğer metal oksit yarı iletken fotoanotlara göre nispeten dar bant boşluğu (Eg = 2.4 eV) ve uygun bant sınırları nedeniyle su ayırma reaksiyonlarında fotoanot olarak kullanımı için çok dikkat çekmiştir. OER potansiyelleri ile ilgili pozisyonlar. BiVO4 tabanlı fotoanotlardan elde edilen daha yüksek PEC aktiviteleri, morfoloji kontrolü, doping ve oksijen evrim kinetiklerini kolaylaştıran yardımcı katalizör ilavesi ile elde edilebilir. Bu çalışmada, BiVO4 fotoanode filmleri hidrotermal sentez yöntemi ile hazırlanmıştır. FTO kaplamalı cam substratlar üzerinde doğrudan büyütme ve üzerine toz birikimi arasında metodolojileri, doğrudan büyütme en iyi yaklaşım olarak bulunmuştur. Su oksidasyon kinetiklerini geliştirmek için tungsten (W) katkısı kullanıldı ve en iyi aktivite 2.0at% W-katkılı BiVO4 örneklerinden elde edildi. Daha fazla iyileştirme için, iridyum oksit (IrOx) yardımcı katalizörü, su ve ozon oksitleyicileri ile ALD yöntemi kullanılarak fotoanotlara yüklenmiştir. En iyi su oksidasyon aktivitesi, simüle edilmiş güneş ışığı aydınlatması altında 1.23 mA / cm2'de 1.23 V'de RHE'ye karşı oksijen akışı altında suyla oksitlenmiş IrOx yüklü (30 ALD döngüsü) BiVO4'ten ısıl işlemden sonra elde edilmiştir. Hidrotermal reaksiyon süresi ve hidrotermal sıcaklığın etkisi de araştırılmış ve BiVO4 fotoanotlanndan en iyi fotokatalitik performansları elde etmek için 180 ° C'de 2 saat reaksiyon süresine sahip olacak şekilde optimize edilmiştir. Kristalinite ve morfolojinin farklı sürfaktan kullanımı ile modifiye edilebileceği bulunmuştur. Morfoloji dağılımı ve kristallikte artan homojenliğin, BiVO4 fotoanotları için foto akımlarını iyileştirmediği gösterilmiştir.
Optoelektronik uygulamalar için fonksiyonel organik malzemelerin dizaynı, sentezi ve karakterizasyonu
The development of π-conjugated semiconducting small molecules and polymers as functional organic materials is an emerging and continuously growing research area in (opto)electronics. Semiconducting small molecules and polymers are envisioned as key components of high-performance organic thin-film transistors (OTFTs) and photovoltaics (OPVs) for next-generation (opto)electronic technologies such as plastic logic circuits, flexible displays, rollable solar panels, and electronic skins. The main motivations in continuously designing and synthesizing new π-frameworks in the past few decades do not only include improving charge-transport and device characteristics and realizing novel functions, but also better understanding and addressing molecular structure-(opto)electronic property-electrical performance relationships. This thesis studies and explores the rational design, synthesis, and characterization of novel π-conjugated semiconductors with varied chemical structures for OTFT and OPV applications. In the first chapter, the general concepts and fundamentals of π-conjugated semiconducting materials and organic electronic devices are discussed along with a deep literature review focusing on most up-to-date results in the field. OTFTs are classified and described based on the major charge carrier type as n-channel, p-channel, and ambipolar devices, which is closely related to the semiconductor's π-structure. The mechanism of OPVs' working principle is also reviewed describing the electronic/structural effects of semiconductor π-structure on these devices. The second chapter deals with the design, synthesis, and characterization of a series of new indeno[1,2-b]fluorene-6,12-dione-thiophene small molecules, DD-TIFDKT, 2EH-TIFDKT, and 2OD-TIFDKT that consist of highly π-conjugated donor–acceptor molecular architectures based on indeno[1,2-b]fluorene-6,12-dione acceptor unit and thiophene donor units. The semiconductor structures have low band gaps of 1.7–1.8 eV, and they are α,ω-end-functionalized with linear –n-C12H25 chains or swallow-tail 2-ethylhexyl-/2-octyldodecyl chains. The detailed study on the effects of alkyl chain size and orientation on the optoelectronic properties, intermolecular cohesive forces, thin-film microstructures, and charge transport performances of the new semiconductors, which revealed crucial structure–property–function relationships. The solution-processed OTFT devices of the current semiconductors, 2EH-TIFDKT and 2OD-TIFDKT, exhibit excellent ambipolar behavior with carrier mobilities of 0.04–0.12 cm2/V·s and 0.0003–0.02 cm2/V·s for electrons and holes, respectively, and Ion/Ioff ratios of 105 to 106, which indicates two–three orders of carrier mobility enhancement compared to those of solution-processed β-substituted counterparts. In the rational design of the new molecules, the repositioning of the insulating β-substituents to molecular termini is found to significantly enhance the π-core planarity while maintaining a good solubility, which improved the charge-transport characteristics. In the third chapter, three novel solution-processable BODIPY-based semiconducting materials (BDY-3T-BDY, BDY-4T-BDY, and BDY-PhAc-BDY) were synthesized. All these materials displayed n-channel OTFT device operation. BDY-4T-BDY based bottom-gate/top-contact devices exhibited extremely high Ion/Ioff ratios of >108 and electron mobilities of up to 0.01 cm2/V·s. Up to now, this result is one of the best charge-carrier mobilities among the known BODIPY based materials in the literature. BDY-3T-BDY showed electron mobilities of 2.7×10-4 cm2/V·s and Ion/Ioff ratio of 9.6×105 . BDY- PhAc –BDY displayed electron mobilities of 0.004 cm2/V·s and Ion/Ioff ratio of 105-106. The chemical structures, optical/electrochemical properties, and thin-film microstructures for these semiconductors were fully characterized by 1H/13C NMR, mass spectrometry, cyclic voltammetry, UV-Vis absorption spectroscopy, thermogravimetric, atomic force microscope (AFM) and X-ray diffraction (XRD) analysis. In the fourth chapter, boron containing polymers P(2OD-TBDY-T) and P(2OD-TBDY-TT) were synthesized and their optoelectronic properties in OTFT and OPV devices were investigated. P(2OD-TBDY-T) based bottom-gate/top-contact OTFT devices exhibited Ion/Ioff ratios of >108 and hole mobilities of 0.005 cm2/V·s. Inverted BHJ-OPVs employing (P(2OD-TBDY-T):PC71BM active layer exhibited excellent power conversion efficiencies (PCE) of 6.2% with a short-circuit current of 16.6 mA/cm2. These results showed that rationally designed BODIPY based donor copolymers could be used in high-performance OPVs. The findings presented in this thesis suggest that through computational modeling guided rational design and synthetic tailoring, physicochemical/optoelectronic properties and electron/hole transport characteristics of molecular and polymeric semiconductors can be significantly improved realizing new functions. We believe that our results will provide key structural/electronic information and additional motivation in the field to investigate and optimize structurally varied semiconductors for high-performance organic (opto)electronic applications.
Optoelektronik aygıtlar için yarıiletken kolloidal kuantum noktaları ve kuantum kuyularının sentezi ve karakterizasyonu
In the last few decades, semiconductor quantum dots (QDs) have become one of the important branches of the nanomaterials thanks to their size (2-10 nm) dependent optical properties and excitonic features, narrow emission bandwidth, excellent photo and thermal stabilities. All these properties have named QDs as exotic nanomaterials for optoelectronic applications such as light emitting diodes, solar cells and colloidal lasers. The main focus of this thesis study is to demonstrate high quality and stable colloidal nanocrystals synthesis and present their optoelectronic applications. To realize this purpose, high quality, monodisperse and pure color emitting CdSe/ZnS QDs have been synthesized and high-quality white light emitting diodes (w-LED) have been obtained by using free-standing flexible polymeric films of these QDs. The results obtained with these films have been presented both for display and lighting applications with white light parameters of NTSC color gamut of 122.5 (CIE-1931), CRI of 88.6, LER of 190 lm/Woptand CCT of 2763 K. In addition to CdSe based QDs, due to the environmental concern towards Cd-based nanomaterials, we focused our attention to Cd-free QDs as well. Evaluation and the performance of the QDs for various applications depend on their optical properties as QY, FWHM and tunable emission wavelength. To improve optical properties of the environmentally friendly QDs, various synthesis protocols and synthesis recipe have been used with different chemicals, precursor concentrations and structure. By the help of optimized precursor concentration and proposed structure of alloyed core/shell InPZnS/ZnS QDs, 78 % of QY and 45 nm of FWHM have been obtained by carefully designed synthesis recipe. The variation of the optical properties of the QDs have been characterized with steady state and time resolved photoluminescence (TRPL) analysis by monitoring the synthesis products at all stages of the synthesis. The lifetime of the alloyed core increased from 20.3 ns to 50.4 ns with shell coating by the suppression of the nonradiative decay components. For further improvements of Cd-free QDs synthesis, we have systematically studied the type and concentration of the Zn-precursor. Green emissive QDs have been synthesized with 87% of QY, having 54 nm of FWHM. Emission kinetics and Förster Resonance Energy Transfer (FRET) efficiency between donor and acceptor pairs of the green and red emitting QDs have been investigated by using steady state and TRPL analysis. Efficient green emissive Cd-free QDs have provided 70.3 % FRET efficiency by mixing with red emissive Cd-free QDs in their polymeric film structure. As an alternative matrix to polymer structures to incorporate the nanocrystals, salt macrocrystals have recently emerged as an efficient platform to keep QY and optical properties of the emitters in their solid forms. FRET efficiency, photo-stability and white-LED performance of Cd-free QDs embedded salt pellets have been investigated by varying the acceptor to donor ratio in the salt matrix. 65% of FRET efficiency, 84.7 of CRI with 324 lm/Wopt of high LER has been achieved from pellets form of Cd-free QDs. We have also focused on the synthesis of the two-dimensional colloidal quantum wells to efficiently use them in optical gain and laser application. However, low QY and stability of the core/shell nanoplatelets (NPLs) that produced with c-ALD methods limits their performance in an application. So, first we have started to improve their optical properties and stability both in solution and film form. Finally, near-unity emitting CdSe/ZnS core/shell NPLs have been successfully synthesized by using hot-injection shell growth approach. Synthesized NPLs with our new synthesis protocol exhibited outstanding photo, thermal-stability and optical gain performance with lasing thresholds as low as 7 µJ cm-2.
Amorf malzemelerin modellenmesi ve incelenmesi
The aim of this PhD dissertation is to investigate the behavior of different MOFs under hydrostatic and uniaxial stresses by using ab-inito molecular dynamics simulations (AIMD). The results obtained from computations are reported in three main chapters. In the first part, ab initio simulations within a generalized gradient approximation (GGA) were carried out to investigate the response of MOF-5 to high pressure. Similar to the previous experimental findings, a pressure-induced amorphization (PIA) was observed at 2 GPa through the simulations. The phase transformation was an irreversible first order transition and accompanied by a volume collapse around 68%. Remarkably, the transition arose from local distortions and contrary to previous suggestions, did not involve any bond breaking or formation. Additionally, a drastic band gap closure was perceived for the amorphous state. For the second part of this project, AIMD simulations were performed to probe the high-pressure behavior of ZIF-8 over wide pressure-range. Under compression, the enormous distortions in the ZnN4 tetrahedral units led to a crystal-to-amorphous phase transition at around 3 GPa. During the amorphization process, the Zn-N coordination was retained. No other phase change but a possible fracture of the system was proposed above 10 GPa. When the applied pressure was released just before the amorphization, the rotations of imidazolate linkers (swing effect) caused an isostructural crystal-to-crystal phase transition. In the tensile regime, no phase transition was perceived up to −2.75 GPa at which point the structural failure was observed. In the last part of this research project, the phase transitions of ZIF polymorphs (ZIF-1 to ZIF-3) under pressure were comprehensively simulated. ZIF-1 showed some consecutive crystal-crystal and crystal-amorphous phase transitions between -2 GPa (tension) and 10 GPa (compression). On the other hand, ZIF-2 and ZIF-3 presented similar pressure-volume relation in both tension and compression regions. In compression region, a rapid crystal-amorphous at relatively lower compression regime and most likely an amorphous-amorphous transition were explored whereas the structural failure was observed at around -3 GPa for all ZIFs.
Yeni [1]benzothieno[3,2-b][1]benzothiophene (BTBT)-temelli moleküler yarıiletkenler ve organik alan etkili transistör uygulamaları
DAcTTs have provided an excellent π-framework for the development of high mobility p-type molecular semiconductors in the past decade. However, n-type DAcTTs are rare and their electron transporting characteristics remain largely unexplored. In the second chapter of this thesis, the first example of an n-type BTBT-based semiconductor, D(PhFCO)-BTBT, has been realized via a two-step transition metal-free process without using chromatographic purification. The corresponding TC/BG-OFET devices demonstrated μe (max) = ~0.6 cm2/Vs and Ion/Ioff ratio = 107-108. The large band-gap BTBT π-core is a promising candidate for high mobility n-type organic semiconductors and, combination of very large intrinsic charge transport capabilities and optical transparency, may open a new perspective for next-generation (opto)electronics. In the third chapter of this thesis, a series of BTBT-based small molecules, D(C7CO)-BTBT, C7CO-BTBT-CC(CN)2C7, and D(C7CC(CN)2)-BTBT, have been developed in "S-F-BTBT-F-S (F/S: functional group/substituent)" molecular architecture. Combining with D(PhFCO)-BTBT, a molecular library with systematically varied chemical structures has been studied herein for the first time for low LUMO DAcTTs, and key relationships have been elucidated. The molecular engineering perspectives presented in this thesis may give unique insights into the design of novel electron transporting thienoacenes for unconventional optoelectronics.
Yüksek performanslı organik transistörler ve güneş pilleri uygulamaları için yeni yarı iletken malzemeler
In the first chapter, we review the historical and recent advances in the design and implementation of indenofluorene (IF)-based semiconductors in organic transistor and solar cell devices. In the second chapter, a series of n-type ambient-stable and solution- processable TIFDMT-based semiconducting molecules, β,β'-C8-TIFDMT, β,β'-C12- TIFDMT, and β,β'-C16-TIFDMT are reported. By utilizing alkyl chain engineering in TIFDMT-based molecules and semiconductor-dielectric interface engineering through PS-brush treatment onto the dielectric surface in their OFET devices, we optimize the semiconductors' morphologies and thin-film molecular packing motifs to attain high- performance OFETs. The PS-brush treated OFETs demonstrate high device performance with μe = 0.9 cm2/V.s and Ion/Ioff ratio = 107-108. In the third chapter, we demonstrate the design, synthesis, and characterizations of two novel meso-π-extended/-deficient BODIPY building blocks (2OD-T2BDY and 2OD-TTzBDY), a library of low band gap (Eg = 1.30-1.35 eV) donor-acceptor copolymers based on these building blocks, and the utilization of the D-A copolymers as donor materials in the bulk heterojunction organic photovoltaics. Power conversion efficiencies of up to 4.4% with a short-circuit current of 12.07 mA cm-2 are achieved. The findings of this thesis on molecular engineering and optoelectronic properties are unique and may provide critical insights into the future development of high performance materials for unconventional optoelectronics.
Bor esaslı nano yapıların modellenmesi ve incelenmesi
Polyhedral boron clusters and their applications have been subject to research in many fields such as medicine, materials science, catalytic applications, energy studies, etc. These molecules owe their popularity to their exceptional 3D stable structures, as well as their various sought-after properties in many applications. This doctoral thesis was prepared within the focus of a computational investigation of different polyhedral borane and carborane clusters by using DFT methods. The results of our studies were reported in two main chapters (Chapters 3 and 4). In the first part (Chapter 3), theoretical evaluation of relative stabilities and electronic structure for [BnXn]2− clusters were provided. The structural and electronic characteristics of [BnXn]2− clusters were examined by comparison with the [B12X12]2− counterparts with a focus on the substituent effects (X = H, F, Cl, Br, CN, BO, OH, NH2). The effects of the substituents were discussed in relation to their mesomeric (±M) and inductive (±I) effects. The results showed that the icosahedral barrier can be reduced through substitution by destabilizing the [B12X12]2−cluster with symmetry-reducing ligands or ligands with +M effects rather than stabilizing the larger clusters. In the second part (Chapter 4), the investigation of the photophysical properties of carborane-containing luminescent systems was presented. The o-CB-Anth system is known to exhibit a dual-emission property by radiating in the visible region from two low energy conformations with local excited (LE) and hybridized local and charge transfer (HLCT) characters, however, it shows a very low emission quantum yield in solution state similar to many other CB-luminescent systems. In this section, the excited-state potential energy surface (PES) of o-CB-Anth and o-CB-Pent were investigated in detail and the effect of a low-lying CT on the low quantum yield was discussed.
Bor zengini amorf malzemeler
In the scope of this thesis, boron-rich amorphous materials having different boron concentrations (B1-xNx, B1-xOx and B1-xSix) were created as a result of rapid cooling of their liquid state with the help of an ab initio molecular dynamics technique. Their structural, electrical and mechanical properties were exposed in detail. In all boron rich materials, the coordination number of B was found to increase steadily with increasing B content. Similarly N and Si atoms also attained high coordinated motifs with increasing B content. However, the coordination number of O atoms remained null for all compositions. Chemical segregations and hence phase separations were witnessed in most amorphous configurations. The materials with high boron ratios, as expected, consisted of B12 icosahedrons. In addition, the formation of nano-sized B7, B10, B14 and B16 clusters was observed in some boron-rich compounds. Each computer-generated material exhibited a semiconducting character. The mechanical properties (Bulk, Young and Shear moduli) were perceived to increase with increasing B content. Some amorphous compositions were proposed to be hard materials on the basis of their Vickers hardness estimation.
Amorf bor malzemelerin simülasyonu
Boron-based materials and their technological applications have great interests in many scientific and technological areas from materials science to medicine. This doctorate thesis was prepared for the purpose of investigating the atomic structure, electrical and mechanical properties of different boron based amorphous materials by using an ab-initio molecular dynamics technique. The results obtained via a computational method were presented in three main chapters. In the Chapter 3, the influence of hydrogenation on the atomic structure and the electronic properties of amorphous boron nitride (ɑ-BN) was examined. The structural evaluation of ɑ-BN and the hydrogenated (ɑ-BN:H) models revealed that their short-range order was mainly similar to each other. Hydrogenation suppressed the formation of twofold coordinated chain-like structures and tetragonal-like rings and leaded to more sp2 and even sp3 bonding. Furthermore, hydrogenation was found to have an insignificant impact on the electronic structure of ɑ-BN. Secondly, in the Chapter 4, an amorphous boron carbide (a-B4C) model was generated. The pentagonal pyramid-like motifs were found to be the main building units of B atoms in a-B4C and some of which yielded the development of B12 icosahedra. On the other hand, the fourfold-coordinated units were the leading configurations for C atoms. a-B4C was a semiconducting material and categorized as a hard material. In the Chapter 5, amorphous boron carbides (BxC1-x, 0.50x0.95) were systematically created. With increasing B/C ratio, more closed packed materials having pentagonal pyramid motifs form. All models were semiconducting materials. Some amorphous compositions were proposed to be hard materials. Keywords: Amorphous, Hydrogenation, Boron Nitride, Boron Carbide, Ab-initio molecular dynamics technique
Evsel atıksulardan enerji ve su geri kazanımında hibrit membran proseslerin geliştirilmesi
This thesis study aims to develop a hybrid innovative membrane-based process that maximizes circular benefit with the recovery of energy, nutrients, and water from municipal wastewater (MWW). This process was designed to be a sustainable alternative to the widely used advanced biological wastewater treatment plants (WWTP). For this purpose, the wastewater samples from the pre-sedimentation tank effluent of the Kayseri WWTP were used in laboratory-scale membrane-based process applications. In the first stage of the study, pre-concentration studies were performed to concentrate the organic matter and nutrients in the wastewater using the chemically enhanced primary sedimentation+direct ceramic microfiltration (CEPS+DCMF) process. Wastewater concentrated up to 8 times in the CEPS+DCMF process was fed to the anaerobic fluidized bed ceramic membrane bioreactor (AnFCMBR), which is the second stage of the study. The performance of the reverse osmosis (RO) process was evaluated for nutrient recovery performance in permeates of AnFCMBR and CEPS+DCMF processes. Chemical precipitation was performed on RO concentrate samples to recover struvite. With the innovative membrane-based hybrid wastewater treatment process, a net energy recovery potential of 0.126 kWh/m3 was attained by operating the AnFCMBR process at 6 hours hydraulic retention time, while an energy requirement of 0.08 kWh/m3 was attained and thus, an energy-positive process for treating MWW has been developed.
Optoelektronik uygulamalar için özgün fonksiyonel organik malzemeler geliştirilmesi
In the first chapter, we review the historical and recent advances in the design and proposal of organic semiconductors and their (opto)electronic applications. In the second chapter of this thesis, we discovered the nanostructured film construction and Raman signal enhancement capabilities of a π-electron deficient low-LUMO BTBT molecule, 1,10-(benzo[b]benzo[4,5]thieno[2,3-d]thiophene-2,7-diyl)bis(octan-1-one) (D(C7CO)-BTBT), which includes 2,7-dicarbonyl functionalization along with n-heptyl (-n-C7H15) substituents. This molecule is arranged on the gram-scale in ambient via simplistic Friedel-Crafts acylation and precipitation/solvent washing without demanding any high-cost transition-metal catalyst and monotonous chromatographic/sublimation-based purification. In the third chapter of this thesis, we demonstrated the Hansen solubility approach to study the solubility behavior of an ambient-stable n-type semiconductor, 2,2'-(2,8-bis(3-dodecylthiophen-2-yl)indeno[1,2-b]fluorene-6,12-diylidene)dimalononitrile (β,β'-C12-TIFDMT), and to analyze potential green solvents for thin-film processing. In the fourth chapter of this thesis, present a unique molecular engineering on the BTBT π-system by employing mono-(aryl)carbonyl functionalization with one hexyl (n-C6H13) substituent, and demonstrate the design, synthesis, and characterization of a new asymmetric BTBT semiconductor, m-C6PhCO-BTBT. The new molecule was produced in gram-scale through a two-step transition-metal-free synthesis, and the detailed structural, physicochemical, and (opto)electronic characterizations were performed.
Yapay zeka destekli shot peening prosesinin optimizasyonu ve ikincil proseslerin SLM ile üretilen AlSi10Mg alaşımının hidrojen kırılganlık direnci ve mekanik performansı üzerindeki etkilerinin incelenmesi
This thesis investigates the optimization of shot peening processes and the mitigation of hydrogen embrittlement in AlSi10Mg alloys produced via Selective Laser Melting (SLM). Chapter one reviews process optimization techniques (e.g., Taguchi, Box-Behnken), additive manufacturing (AM) challenges like residual stress and porosity, and introduces hydrogen embrittlement mechanisms and testing methods. Chapters two and three focus on optimizing shot peening intensity using AI-based approaches validated by Almen tests and analyze real-world aviation failures, such as Bell 412EP and Piper PA-32R, to highlight hydrogen embrittlement's impact on component degradation. Chapters four and five explore the effects of strain rates and post-processing treatments, including shot peening and heat treatment, on mechanical performance, demonstrating significant improvements in fatigue resistance. Advanced strategies for mitigating hydrogen embrittlement are also proposed. The thesis concludes by emphasizing the societal benefits of enhanced material reliability and sustainability, suggesting future research into AI-assisted methods and real-time monitoring systems in manufacturing.
Bileşenlerin özel uyumluluğuna sahip düşük karbonlu yüksek performanslı katkılı portland çimentoları
This thesis aims to develop low-carbon, high-performance blended cements by holistically investigating the multidimensional interactions of Portland cement (PC), supplementary cementitious materials (SCMs), and superplasticizers (SP). A comprehensive dataset consisting of isothermal calorimetry, thermal analysis, and compressive strength data was generated, and different PC types, replacement rates, SCM fineness, water/binder (w/b) ratio, SP type, and dosage were systematically varied. These data were analyzed with regression-based machine learning models to obtain high-accuracy predictions. While linear models performed well for outputs focused on physical parameters such as early-age compressive strength, nonlinear models (especially Gaussian process regression and artificial neural networks) were found to perform significantly better (R2 ≈ 0.90–0.97) and with lower error for complex and high-variance outputs such as hydration kinetics, CH content, and late-age compressive strength. The Multi-Criteria Compatibility Index, developed by normalizing and weighting different criteria, was applied to a synthetic dataset consisting of 1602 mixtures. The results showed that highly compatible systems had low PC-high SCM content, low w/b ratios, and high SP dosages. Furthermore, SCM with low to medium CaO and high SiO2+Al2O3 content increased compatibility. In conclusion, machine learning methods modeled the multivariable interactions in superplasticized blended cements with high accuracy, and the developed compatibility index emerged as an innovative and flexible tool for the preliminary design of low-carbon, high-performance mixtures. This approach contributes to an environmentally, economically, and socially sustainable cement industry.