Malzeme Bilimi ve Makine Mühendisliği Ana Bilim Dalı Tez Koleksiyonu

Permanent URI for this collectionhttps://hdl.handle.net/20.500.12573/422

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An accurate investigation of the mechanical response and damage model of aluminum 7068
(Abdullah Gül Üniversitesi, 2018) Karaveli, Kadir Kaan; AGÜ, Mühendislik Fakültesi, Makine Mühendisliği Bölümü; Karaveli, Kadir Kaan
The promising combination of high strength, high toughness, low density and corrosion resistivity have made aluminium (Al) alloys the material of choice in various applications, from buildings to aerospace, for decades. Especially, Al 7068 alloy is one of the recently developed materials used mostly in defence and automobile industries due to their exceptional mechanical properties. In this master thesis, the mechanical response and Johnson-Cook damage model of Al 7068-T651 alloy was investigated. Specifically, different Johnson-Cook damage parameters were determined for different application areas considering the maximum, minimum and average results. These damage parameters can be used for accurate Finite Element Analysis simulations. To determine these damage parameters tensile tests were conducted on notched and smooth specimen son both rolling direction and perpendicular to the rolling direction. The notch radius were selected as smooth, 0.4 mm, 0.8 mm and 2 mm to provide different stress triaxiality values and observe the mechanical response at these triaxiality values. Tensile tests were repeated seven times to obtain the accurate results. The final cross-sectional areas of fractured specimens were calculated through optical microscopy. The effects of stress triaxiality factor and rolling direction on the mechanical properties of Al 7068-T651 alloy were successfully investigated. All damage parameters were calculated via LevenbergMarquardt optimization method. Overall, three different Johnson-Cook damage parameters based on minimum, average and maximum equivalent strain values were calculated. These Johnson-Cook ii damage parameters can be utilized for the accurate damage simulations of different applications in Finite Element Analysis, which is a computational technique and is used to obtain approximate solution of several engineering problems
The boron-rich amorphous materials
(Abdullah Gül Üniversitesi, Fen Bilimleri Enstitüsü, 2023) KARACAOĞLAN, Ayşegül Özlem; AGÜ, Fen Bilimleri Enstitüsü, Malzeme Bilimi ve Makine Mühendisliği Ana Bilim Dalı
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.
Design, synthesis, and characterization of functional organic materials for optoelectronic applications / Optoelektronik uygulamalar için fonksiyonel organik malzemelerin dizaynı, sentezi ve karakterizasyonu
(Abdullah Gül Üniversitesi, 2018) ÖZDEMİR, MEHMET; AGÜ, Mühendislik Fakültesi, Malzeme Bilimi ve Nanoteknoloji Mühendisliği Bölümü; ÖZDEMİR, MEHMET
The development of ?-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 ?-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 ?-conjugated semiconductors with varied chemical structures for OTFT and OPV applications. In the first chapter, the general concepts and fundamentals of ?-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 ?-structure. The mechanism of OPVs’ working principle is also reviewed describing the electronic/structural effects of semiconductor ?-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, DDTIFDKT, 2EH-TIFDKT, and 2OD-TIFDKT that consist of highly ?-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 ?,?-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 ß-substituted counterparts. In the rational design of the new molecules, the repositioning of the insulating ß-substituents to molecular termini is found to significantly enhance the ?-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-4TBDY 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(2ODTBDY-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 BHJOPVs 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. Keywords: Organic Semiconductors, Solution-Processable Molecules and Polymers, Organic Thin-Film Transistors (OTFTs), Organic Photovoltaics (OPVs)
THE DETERMINATION OF HYDROGEN EMBRITTLEMENT BEHAVIOR OF ARMOR STEELS BY EXPERIMENTAL METHODS AND THE OPTIMIZATION OF HYDROGEN BACK-DIFFUSION OPERATION
(Abdullah Gül Üniversitesi, Fen Bilimleri Enstitüsü, 2021) BAYRAM, Ferdi Caner; AGÜ, Fen Bilimleri Enstitüsü, Malzeme Bilimi ve Makine Mühendisliği Ana Bilim Dalı
Hydrogen embrittlement, also known as hydrogen attack or hydrogen-assisted cracking, is a process whereby metallic materials (i.e., high-strength steels, titanium alloys and aluminum alloys) become brittle or fractures due to the exposure, introduction and diffusion of hydrogen atoms through the microstructure of metals. It is a serious matter that drastically degrades the mechanical properties (e.g., ductility and toughness) of a wide range of different structural materials which include pipeline steels, armor steels, advanced high strength steels, etc. This thesis study aims to investigate the hydrogen embrittlement behavior of armor steels that conform to MIL-DTL-12560 Class 4a and MIL-DTL-46100 military specifications used by FNSS Defense Systems Inc. and to optimize the temperature and duration parameters of hydrogen back-diffusion operation to reduce the risk of hydrogen embrittlement. To characterize the embrittlement behavior of armor steels used by FNSS, various mechanical tests, including tensile tests, compression tests, high strain rate tests, hardness tests, CVN impact tests and ballistic tests, were carried out with as-received and hydrogen-uncharged specimens in order to unveil the adverse effects of hydrogen embrittlement on the mechanical properties of these steels. A cathodic hydrogen charging system was used to charge the specimens with hydrogen for mechanical tests. The effects of microstructure on the mechanical response of materials were also investigated for deeper understanding.
Development of hybrid membrane processes for energy and water recovery from municipal wastewaters
(Abdullah Gül Üniversitesi, Fen Bilimleri Enstitüsü, 2024) Özcan, Özlem; AGÜ, Fen Bilimleri Enstitüsü, Malzeme Bilimi ve Makine Mühendisliği Ana Bilim Dalı
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.
Development of rubber based materials with high performance, preventing hydrogen embrittlement and material modelling by introducing microstructure / Yüksek performanslı kauçuk esaslı malzemeler geliştirilmesi, hidrojen gevrekliğinin önüne geçilmesi ve mikroyapı tanıtılması ile malzeme modellemesi
(Abdullah Gül Üniversitesi, Fen Bilimleri Enstitüsü, 2021) Özge, DOĞAN; AGÜ, Fen Bilimleri Enstitüsü, Malzeme Bilimi ve Makine Mühendisliği Ana Bilim Dalı
Bu çalışmada, üç değişik nano malzemenin (Nano-Karbon Siyahı, Nano-ZnO ve Çok Duvarlı Karbon Nanotüp (ÇDKNT)) iki değişik kauçuk türü üzerinde (Kloropren Kauçuk (CR) ve AKrilonitril Bütadien Kauçuk (NBR)) etkileri deneysel olarak araştırılmıştır. Bu amaca ulaşmak için mekanik testler ve detaylı yaşlandırma testleri gerçekleştirilmiştir. Nano malzemelerin mekanik özellikler üzerinde hem olumlu hem de olumsuz etkilerinin olduğu tespit edilmiştir. En önemlisi, ÇDKNT ilavesi ile baskı set değerinin azaldığı gözlemlenmiştir. Böylece, ÇDKNT ilavesi ile daha yüksek sızdırmazlık kapasitesine ve daha uzun servis ömrüne sahip kauçuk ürünler elde edilebilecektir. Ayrıca, sert-krom elektrokaplama prosesi, buna karşı gelen hidrojen gevrekliği ve fırınlamanın hidrojen difüzyonu üzerindeki etkileri araştırılmıştır. Bu amaçla Ham 4340, Krom Kaplanmış 4340 ve Krom Kaplanmış & Fırınlanmış 4340 çeliği kullanılmıştır. Mikroyapı analizleri ve mekanik analizler sert-krom elektrokaplama prosesiyle malzeme içine hidrojen girdiğini ve elektrokaplama prosesinden sonra yapılan fırınlama işleminin hidrojenin ters difüzyonunu sağladığını göstermiştir. Ayrıca, hidrojenin benzer alaşım elementlerine ve kimyasal kompozisyona sahip α-Fe bazlı yapıların gerilme tepkisi Moleküler Dinamik (MD) simülasyonları kullanılarak simüle edilmiştir.
ENCAPSULATION OF OMEGA-3 FATTY ACIDS INTO STARCH NANOPARTICLE STABILIZED PICKERING EMULSIONS
(Abdullah Gül Üniversitesi, Fen Bilimleri Enstitüsü, 2021) KORKUT, Ayşe; AGÜ, Fen Bilimleri Enstitüsü, Malzeme Bilimi ve Makine Mühendisliği Ana Bilim Dalı
The main purpose of the thesis is to produce starch nanoparticles to be used as an emulsion stabilizer. In the first part of the thesis, starch nanoparticles were produced via acid hydrolysis and the starch nanoparticles were characterized in terms of morphological properties and size, crystallinity and structural properties. Pickering emulsions were prepared in two different oil fractions (Φ0.6 and Φ0.8) with different oils (sunflower and corn oil). To determine the starch nanoparticle which provides the best emulsion stability, emulsions were prepared with addition of 2% (mg starch/g emulsion) starch nanoparticles. Emulsions were stored for 30 days at room conditions and phase separation was visually examined. The most stable emulsion was prepared with corn oil at Φ0.6 oil fraction when the starch nanoparticle (%2) produced with a 1:3 starch:H2SO4 ratio and 3 days hydrolysis (1:3 (3)) was used as stabilizer. In the second part of the thesis, omega-3 fatty acids were encapsulated in Pickering emulsions. Flaxseed oil was selected as the omega-3 source. The emulsions were prepared using flaxseed oil at a Φ0.2 oil fraction with the addition of 3% starch nanoparticles (1: 3 (3)). The emulsions were stored for 15 days at 25±1°C. Changes in the emulsions during storage were examined in terms of physical stability, peroxide number, pH, particle size, and zeta potential. Pickering emulsions stabilized with starch nanoparticles to encapsulate omega-3 fatty acids made flaxseed oil more resistant to primary oxidation.
FUNCTIONALIZED LOW LUMO [1]BENZOTHIENO[3,2-B][1]BENZOTHIOPHENE (BTBT)-BASED MOLECULAR SEMICONDUCTORS FOR ORGANIC FIELD EFFECT TRANSISTORS
(Abdullah Gül Üniversitesi, Fen Bilimleri Enstitüsü, 2021) Özdemir, Resul; AGÜ, Fen Bilimleri Enstitüsü, Malzeme Bilimi ve Makine Mühendisliği Ana Bilim Dalı
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.
Modelling and investigation of boron-based nanostructures
(Abdullah Gül Üniversitesi, Fen Bilimleri Enstitüsü, 2022) Tahaoğlu, Duygu; AGÜ, Fen Bilimleri Enstitüsü, Malzeme Bilimi ve Makine Mühendisliği Ana Bilim Dalı
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.
Modelling and investigations of amorphous materials
(Abdullah Gül Üniversitesi, 2019) ERKARTAL, MUSTAFA; AGÜ, Mühendislik Fakültesi, Malzeme Bilimi ve Nanoteknoloji Mühendisliği Bölümü; ERKARTAL, MUSTAFA
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.
Novel semiconducting materials for high-performance organic transistors and solar cells
(Abdullah Gül Üniversitesi, Fen Bilimleri Enstitüsü, 2022) Can, Ayşe; AGÜ, Fen Bilimleri Enstitüsü, Malzeme Bilimi ve Makine Mühendisliği Ana Bilim Dalı
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.
The simulations of amorphous boron materials
(Abdullah Gül Üniversitesi, Fen Bilimleri Enstitüsü, 2023) YILDIZ, Tevhide Ayça; AGÜ, Fen Bilimleri Enstitüsü, Malzeme Bilimi ve Makine Mühendisliği Ana Bilim Dalı
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, 0.50x0.95) 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
Synthesis and characterization of semiconductor colloidal quantum dots and quantum wells for optoelectronic devices /
(Abdullah Gül Üniversitesi, 2018) ALTINTAS, YEMLİHA; AGÜ, Fen Bilimleri Enstitüsü, Malzeme Bilimi ve Makine Mühendisliği Ana Bilim Dalı; ALTINTAS, YEMLİHA
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 (wLED) 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 Cdbased 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 ii 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 Znprecursor. 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 Cdfree 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 hotinjection 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 .

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