17
Archived Theses
0
DOIs Assigned
0%
DOI Rate
Discipline
Birlikte buharlaştırma ve hibrit buhar çözeltisi sıralı yöntemi ile metal oksit delik (Hole) taşıma katmanlı düzlemsel perovskit güneş hücreleri
The recent success in achieving beyond 25% efficiency with perovskite solar cells (PSC) has called for further upgrading the fabrication techniques to meet the scalability requirements of the photovoltaic (PV) industry. Co-evaporation and a hybrid vapor-solution technique have been shown to produce uniform and efficient planar PSCs. Therefore, in this study, co-evaporation method was optimized studying the partial pressure of the organic precursor. Later electron transport layers in n-i-p and p-i-n structures were addressed achieving 13.0% and 16.1% efficiencies, respectively. Besides, mixed-halide perovskites were fabricated following a hybrid sequential method focusing on the deposition rate of PbI2 and a solution of methylammonium-halides to control the crystallization and morphology of the perovskite layer. This conferred efficiencies up to 19.8% in the case of MAPbI3-X-YBrXClY with 90 hours of operational stability. This is an important measure towards scalability due to the uniform deposition of the first inorganic layer by vacuum-deposition. As another step towards the scalability of perovskite PV, radio frequency (RF) magnetron sputtering was devised to deposit NiOX as a hole transport layer with wide bandgap, matched band structure with perovskite, and stability. The effect of Ar-partial pressure, deposition rate on the optoelectronic properties of the sputtered NiOX was investigated. The passivation of NiOX using organic (Poly-TPD) and inorganic (CuO) materials boosted the overall efficiency of the PSCs by 2.2% and 1.2%, respectively. Finally, Cu doping NiOX via co-sputtering enhanced the efficiency of the PSCs by 3%. This thesis provides a benchmark for applying scalable methods (from evaporation to sputtering) towards efficient PSCs.
Neuro-2a hücrelerinin femtosaniye lazer ile işlenmis silikon yüzeylerinin üstündeki davranışları
Cells are known to interact with their physical environment and respond to cues such as substrate topography. Knowledge of the cell responses to topography may give in- formation about cell behavior in health and disease, as well as be exploited in order to exert control on cells for various purposes. Cell responses to topography are depen- dent on cell type, substrate material and topographical features. In the present study, Neuro-2a cell line was used as a versatile and widely available neuronal cell model. The substrates consisted of polished or laser-structured silicon. Structuring was per- formed using an ultrafast infrared pulsed laser, which generated topographies such as laser induced periodic surface structures (LIPSS) and microcolumns. The substrates were characterized with scanning electron microscopy (SEM). Cells were grown on control substrates (glass or plastic), polished silicon and laser-structured silicon of different topographies for 3 hours or 24 hours to evaluate various cell behaviors. Ini- tial cell adhesion and initial motility, as well as cell adhesion and shape after 24 hours were studied in different substrates using fluorescence microscopy and SEM. Initial cell adhesion was found to be strongest on the microcolumn topography, allowing for selective cell patterning on microcolumn regions. After 24 hours, cell adhesion was found to be equal in all topographies. Moreover, cell motility was found to be fastest in polished silicon, and slowest in the microcolumn topography. On the other hand, cell area and perimeter was found to be larger on polished silicon and LIPSS, com- pared to microcolumn topography or glass. No difference was found in the average cell circularity for all substrates. Finally, cell preference for microcolumn stripes was found to be more prominent when LIPSS was found between the stripes, compared to when thee stripes were separated by flat regions. In conclusion, different cell be- haviors related to spreading, migration and adhesion were found to be dependent on the substrate topography. Topography on silicon may be promising to control these behaviors in the Neuro-2A cell line.
Mikro ölçekli güç üretimi için MEMS tabanlı mikrolitre hacimli mikrobiyal yakıt pili
Fuel cells can be a part of the solution to energy problem in the world. They can supply power in both macro and micro scales. Especially, MEMS based microscale microbial fuel cells (μMFC) may hold the answer to manufacture easy, cheap, fast, and mobile power sources and sensors. μMFCs are electrochemical devices converting chemical energy into electrical energy utilizing microorganisms as biocatalyst, instead of precious metal catalysts used in conventional fuel cells. They can be integrated to power, for example, lab-on-a-chip systems, or they can be used as stand-alone biosensors for sensing applications. This study focused on the development of a compact microbial fuel cell with microliter volume fabricated using silicon MEMS technology. The aim was to have high power density and low start-up time to be integrated as a power source for small devices. Several µMFC systems were operated under different conditions throughout the study. Effects of external load, anolyte type, operating conditions, and chemical modification of gold anode surfaces were compared in terms of start-up time and power densities using Shewanella oneidensis MR-1. Performances were evaluated using polarization curves, Electrochemical Impedance Spectroscopy, and Scanning Electron Microcopy. The results showed that µMFCs modified with cysteamine self-assembled monolayers resulted in more than a 50% reduction in start-up times due to better bacterial attachment on the anode surface. The volumetric power density (330 µW/cm3) was found to be similar in cysteamine-modified and bare gold µMFCs and was comparable to results reported in similar studies in the literature.
Yüksek performanslı nBn kızılötesi fotodedektörler için bariyer mühendisliği
Despite intensive studies, for high-performance applications, lowering dark current is still a challenging problem for pn-type infrared (IR) photodetectors. Over the last two decades, barrier-type IR detectors have been proposed as a solution for obtaining high-performance and high operation temperature conditions. However, the valence band discontinuity limits the material alternatives to which the barrier detector architecture can be applied. In this thesis work, it has been numerically shown that some material limitations in the barrier detector architecture can be eliminated using bandgap engineering techniques. Herein, simulations and analyses were performed by using Synopsys Sentaurus technology computer-aided design (TCAD) commercial device simulator via calculations of the current, continuity, and Poisson's equations with high precision. In this study, delta-doped layers, together with compositionally grading, were utilized to get InGaAs and HgCdTe nBn type IR barrier detector configurations. For the shortwave IR (SWIR) band InGaAs nBn detector, lattice-matched InAlAs and lattice-mismatched InGaAs were used for the barrier material. At least 40 and 20 times improvement, respectively, were calculated in the dark current level by suppressing the surface leakage and generation-recombination (G-R) current mechanisms without compromising any photo-response when compared to the conventional pn junction. This method was also applied successfully for obtaining an extended SWIR (eSWIR)/SWIR InGaAs dual-band nBn detector structure. In the case of HgCdTe material systems, strong suppression of G-R and trap assisted tunneling (TAT) currents were numerically demonstrated with the designed nBn structures in the SWIR, midwave IR (MWIR), and longwave IR (LWIR) bands, which could be useful for the alternative substrate HgCdTe technology. The HgCdTe dual-band nBn detector configuration was also examined in MWIR/LWIR bands again by using compositionally graded and delta-doped layers. Thanks to the flexibility of this method, the length and thickness of the barrier can be adjusted, while zero valence band offset is achieved at the same time for the compositionally bandgap adjustable materials.
Dalga önü modülasyonu kullanılarak lidar ve makine vizyonu için 3 boyutlu görüntüleme
Autonomous vehicles have proven to be very efficient in daily routine jobs and their impact will continue to increase given the recent developments in artificial intelligence, boosted by increased computation capacity. These vehicles are generally equipped with 2D imaging sensors and asked to accomplish tasks in a 3D world, which hamper their functionalities. In this study, we experimentally investigate and develop 3D imaging technologies. We first demonstrate colorful 3D imaging via time of flight measurements in order to map the target scene to a 3D point cloud data and secondly perform triangulation method. For time of flight measurements, we employ a digital micro-mirror device with a resolution of 1920x1080 and a range finder in order to scan the target. We scan the target with a resolution of 8x2, 16x2, and 8x4 using our first method. Second, we employ structured light method to scan the target in 3D using again the digital micro-mirror device in conjunction with a white light source. Using triangulation, we were able to scan the target with a resolution 1920x1080. Our results indicate that in terms of resolution triangulation method gives better results. We can foresee that triangulation and time of flight measurement methods are combined a robust and high-resolution 3D vision can be developed which will increase vision capabilities of autonomous vehicles that shape our future.
Yüksek çözünürlüklü, değişken eşlenik yay sabitli titreşim tabanlı MEMS sıcaklık sensörü
This thesis presents the design, modelling, fabrication and characterization of a resonance based MEMS temperature sensor with improved sensitivity. The temperature sensor is composed of an electrostatically coupled double ended-tuning fork (DETF) MEMS resonator. The sensor utilizes the thermal expansion coefficient difference of the materials to detect the temperature change. The design consists of 2 resonator tines where each of them has two capacitive plates on each side. The capacitive plates facing each other on the inner side of the resonators are used for the electrostatic coupling, which is the crucial point of the study. The negative electrostatic coupling stiffness generated between these tines enables mode-ordering. By mode-ordering, the sensor can be operated closer to pull-in in order to achieve higher sensitivity for the out-of-phase mode. The outside capacitive plates are used for actuation and sensing, whose mechanisms are explained with the equations. The analytical model is presented with the thermo-electro-mechanical equations for the mode shapes and their corresponding natural frequencies. The model is verified by the Finite Element Analysis by comparing the resonance frequencies of the modes of interests. In FEM analysis, the effects of the electrostatic coupling are shown with the parametric sweep for the various proof mass voltage configurations with the thermal expansion physics node included. Having the thermal expansion, the effect of the temperature increase is shown in the modal analysis as the frequency shift in the mode of interests. The characterization tests are performed in a vacuum environment. The quality factor for the out-of-phase mode is about 25500 at a pressure of around 0.15mTorr. Resonance frequencies for the mode of interests of the resonator are close to the analytical model and the FEM simulation results. The effect of the electrostatic softening effect is investigated for the same, and the opposite sign proof mass voltage configurations. The frequency change for the opposite sign proof mass configuration from 6V to 20V is 3063Hz and 666Hz for out-of-phase and in-phase, respectively. For the same proof mass configuration, the resonance frequency shifts 579Hz. The out-of-phase mode and the in-phase mode frequency changes for the temperature increase from 25°C to 65°C are 1078Hz and 988 Hz for the VPM1=20V and VPM2=-20V. For the same proof mass configuration (VPM1=20V and VPM2=20V), the frequency change is 986Hz, still lower than the in-phase mode frequency of the opposite sign proof mass voltage configuration but very close as expected. The overall sensitivities are obtained using the maxima of the frequency response plots. For the opposite sign proof mass configuration, out-of-phase mode and in-phase mode temperature sensitivity are increased from 24.4Hz/K to 26.5Hz/K and 24Hz/K to 24.4Hz/K, respectively, with the voltage increase from 8V to 20V. This study shows that the temperature sensitivity can be increased by varying coupling stiffness by adjusting the proof mass voltages close to pull-in voltage and mode-ordering.
Ka bant radar uygulamaları için GaN / algan HEMT yapılarında alaşımlı ve alaşımsız ohmik kontakların karşılaştırılması
Gallium Nitride (GaN) based High Electron Mobility Transistor (HEMT) is the most powerful alternative for high-power and high-frequency applications due to their unique material properties, such as high breakdown field, high electron drift velocity, high operation temperatures, high radiation resistance, and so on. A low ohmic contact resistance (Rc) is critical to enhancing the device performance at high frequency and high output power. Various metal stacks with different metal types and individual metal layer thicknesses have been reported. Electron beam evaporated Ti/Al/Ni/Au metal stacks have been widely used for ohmic contacts for GaN-based HEMT devices. In general, Ti-based ohmic contacts require higher annealing temperatures above 800 °C. Annealing metal stacks at high temperatures (above 800 °C) lead to a deterioration in the metal surface morphology and metal edge acuity which makes it difficult to carry out further operations to form the gate region. High annealing temperatures may also result in thermal degradation of the epitaxial heterostructure. It is essential to decrease the transistor dimensions including the gate length (Lg) and the source-drain spacing to operate the device at Ka-band (26.5GHz-40GHz) and higher frequencies. To realize these potentials, the transistor should have a proper epitaxial design along with lower on-resistance and smoother ohmic contact morphology. Not only ohmic contact resistance but also ohmic contact morphology and metal edge acuities of the down-scaled HEMT are very crucial since it affects the next coming device processing steps including gate alignment. Low-temperature annealing (<600oC) of metal stacks with recess etching is an alternative way of forming ohmic contacts to down-scaled GaN-based HEMTs. Annealing at lower temperatures is good for keeping metal surface morphology reasonable but this method may result in higher contact resistances which is not good for HEMT devices. Implantation could be another way of forming ohmic contacts for down-scaled HEMTs. Implanted species need to be annealed at high temperature (>1200oC) which may results in deteriorated epitaxial quality since the growth temperature of epitaxial layers is generally in the range of 900oC-1200oC. Regrown of GaN-based materials gathering great interest for forming an ohmic contact to HEMT which does not require annealing of metals which results in much better metal surface morphology compared to alloyed ohmic contacts. Both Molecular Beam Epitaxy (MBE) and Metal-Organic Chemical Vapor Deposition (MOCVD) methods are used to grown non-alloyed GaN-based material for forming ohmic contacts. To this end, this work describes the development of MOCVD regrown InGaN non-alloyed ohmic contacts for GaN-based HEMT devices designated for Ka-band application. To do the best of our knowledge, we are the first group in the world to successfully implement MOCVD regrown of InGaN layers for forming non-alloyed ohmic contacts to GaN-based HEMT devices for Ka-band applications. Entire microfabrication processes were performed at Coatings and Thin Films Laboratory at Middle East Technical University (METU) and AB MicroNano Company. MOCVD regrown InGaN material which was grown on patterned wafer exhibited high doping concentration above 1020cm-3 which is crucial for forming ohmic contact and the corresponding surface morphology of metal contacts was excellent. HEMT with alloyed ohmic contacts were also fabricated to do a performance comparison. The highlights of this study include improved contacts resistance of non-alloyed regrown InGaN ohmic contacts down to 0.3 W.mm compared to HEMT with alloyed ohmic contacts. An almost 7% improvement was observed in both drain-source current (Ids), transconductance (gm), and small-signal performance. Large-signal measurements showed that the output power of the HEMT with non-alloyed regrown InGaN ohmic contact was 3.07 W/mm which was 9% higher compared to HEMT with alloyed ohmic contacts. In this work, Atomic Layer Deposition (ALD) of Aluminum doped Zinc Oxide (AZO) films were also characterized. Al composition of AZO films was varying in 2-4% with varying growth conditions. AZO films exhibited degenerate doping which corresponds to the sheet resistance of about 10-3 .cm. Having similar electronic properties with MOCVD of InGaN thin films, ALD of AZO could be an alternative for non-alloyed ohmic contacts to GaN-based HEMT devices. In this respect, an investigation of material properties of ALD of AZO films performed. Initial trials did not yield expected results due to not optimized recess etching and post-plasma cleaning conditions.
Mesa ve planar yapılı kısa dalgaboyu kızılötesi dedektörlerin karanlık akım optimizasyon tasarımları
This thesis aims to report novel designs to achieve lower dark current values for modified pin heterojunction InGaAs/InP SWIR photodiodes for both mesa and planar type production methodologies. Mesa type studies cover novel passivation methodology based on a fully depleted thin p-InP layer. Mesa structured detectors are targeted due to their competitive advantages for applications such as multicolor/hyperspectral imaging. Test detector pixels with different perimeter/area ratios are fabricated with and without etching the thin InP passivation layer between pixels in order to comparatively examine passivating behavior. I-V characteristics of the test detectors are measured at room temperature. Based on the results from differently sized pixel groups, bulk and surface dark current components are separated. Results show that thin InP layer decreases dark current by a factor of 3 while increasing photo current due to higher carrier collection efficiency Planar type studies with developing novel epilayer structures, junction depth optimization and successful suppression of surface and bulk generation and recombination (GR) dark current components resulted in diffusion current limited high performance planar type SWIR detectors. For 15 μm pixel pitch, we obtained12 fA pixel dark current at room temperature (~26°C) and 1.08 A/W responsivity (at 1.55 μm) that is corresponding 86% quantum efficiency (QE). Dark current modeling analysis is performed based on bias dependency of diffusion, GR and tunneling current components. Results show excellent match with experimental data. Utilizing the developed planar type detectors with the state-of-the-art performance, post process optimizations also completed which result high performance megapixel SWIR imaging sensors with record level operability.
Isıl işlemin eriyik filament ile imalat yöntemiyle üretilmiş PLA parçaların mekanik özelliklerine etkisinin incelenmesi
Additive manufacturing has become a disruptive technology for the production of load bearing components in a wide range of applications. Fused filament fabrication (FFF) is among the most effective and economical techniques for the printing of polymeric parts. There are numerous thermoplastic materials suitable for FFF. Among these, Polylactic acid (PLA) is a renewable, sustainable and cost-effective alternative. For better utilization of PLA parts produced by FFF, there is a need to understand the structure-property relationships in this system. This thesis investigated the effect of annealing on the mechanical properties of FFF produced parts made of polylactic acid (PLA). The as-printed specimens exhibit an amorphous structure, whereas annealing at elevated temperatures, ranging from 80°C to 140°C for 24 hours resulted in the formation of different crystalline phases. 100°C annealing for 24 hours provided the best results in terms of ductility and strength improvement; the tensile strength increased by 6.5 % and elongation at break increased by 33%. A further increase in the annealing temperature resulted in a decrease in the strength and ductility. In the second part of the thesis, the effect of annealing for different infill ratios were investigated. The 60% infill ratio case with 100 °C annealing showed the best normalized mechanical properties. The findings of the thesis show the importance of the microstructure on the mechanical properties and demonstrate the feasibility of annealing for the process and property optimization of FFF-produced parts.
Minyatür soğutmasız kızılötesi mikrobolometre pikseli geliştirilmesi
This thesis reports the development of 12 µm pixel pitch single layer microbolometer structures for use in the 8-12 µm wavelength regions, following recent trends in the last decade in microbolometers. Various pixel structures are designed, simulated, fabricated, and characterized to obtain acceptable detector performance in these small pixels. The pixel structures are improved by reducing the pixel thermal conductance and by using planar type electrode structures where the active material is selected as VWOx (Vanadium-Tungsten Oxide). Predictive modeling with electro-thermal simulations has aided in attaining the best possible pixel structures. Besides that, absorptance is optimized using the cascaded transmission line (CTL) simulation method to improve the pixel absorption between 8-12 µm spectral region show that more than 85% level. Designed detector pixels are fabricated using a CMOS compatible process flow. Each fabrication step is optimized considering a possible array fabrication on CMOS wafers. Then, pixels are characterized electrically, thermally, and optically. These characterizations include temperature coefficient of resistance (TCR), noise, thermal conductance, absorption, responsivity, and thermal time constant measurements. The TCR of the active material, VWOx, is measured around -4 %/K with an optimum resistance value. The thermal conductance values of pixels are measured as low as 20 nW/K. Additionally, the corner frequency is measured 0.85 kHz at 10 μA bias current. Finally, a responsivity value of 37 kV/W at 473 µV is measured for the best-performed fabricated pixel with a low thermal time constant of less than 2 ms, suitable for applications that require high frame rates. Fabrication of 640x480 format FPAs is conducted in the framework of this thesis with the optimized detector structure. Among measured FPAs, the responsivity and operability are measured as high as 10 mV/K and >99%, respectively. Using these measurement results, the lowest NETD value of a 640x480 FPA measured minimum is 200 mK. The success of the developed detector structures is verified through the images obtained from the detectors that are monolithically built on the CMOS readout circuit. It should be noted that this thesis reports the first successfully fabricated 640x480 FPA array using a 12 µm pixel pitch single layer microbolometer in Turkey. The development in this small size FPA production may open a path to different applications to be developed and used in Turkey in the future.
Hücre zarı geçirgenliğinin sürekli bir termo-akışkan mikrokanallı sistemde farklı donmada koruyucu maddeler ile incelenmesi
Modeling cell membrane permeability in different solutions is a critical requirement in controlling the response of cells during preconcentration processes in biotechnological applications, such as drug delivery, fluorescence imaging, and cryopreservation . Current multi-step methods employed in loading cells with high concentrations of cryoprotectant agents (CPAs) prior to cryopreservation for long term storage affect cell viability as a result of extended exposure times associated with these methods. One of the objectives of this research to observe the response of different types of cells in a continous microfluidic system allowing for reduced exposure times to the CPAs and to model the transport of CPA through the cell membrane. In this study, a micro thermo-fluidic device designed for faster and continuous preconcentration of cells with CPAs is used to study cell membrane permeability. Cells are encapsulated in uniform aqueous droplets with a low CPA concentration, The concentration of the CPA in the droplet is increased along the microchannel by controlling the temperature, and thus the water solubility of the oil phase. Selective diffusion of water out of the droplet causes the droplet to shrink and get concentrated vi in CPAs. The response of the cell to the changes in the extracellular concentration is observed and analysed. The dynamic extracellular CPA concentration data is integrated in a permeability model to calculate the dynamic permeability of the cell membrane to a specific CPA. Two-phase flow conditions for droplet generation are optimized to determine the flow rate to achieve the desired droplet size. A linear relationship is found between flow rates and the size of the droplets, regardless of the size and geometry of the microfluidic device. The performance of encapsulation of cells in the droplets based on the flow rate is assessed. The undesired phenomena, such as the encapsulation of cells in multiples and the sedimentation inside the microchannels, are found to be associated with the low flow rates. Employing a continuous two-phase microfluidic system has proved valuable in observing the response of the cell to increasing CPA concentrations. Encapsulation in smaller droplets has yielded a higher increase in the intracellular CPA concentration. A conventional mathematical model for the permeability of the cell membrane is modified by using the dynamic extracellular concentrations obtained from the microfluidic system. Membrane permeability parameters are determined for MDA-MB231 cells using glycerol as the CPA, where concentration is increased from 1 M to 2.05 M in the aqueous droplets with heating to 40°C in the microfluidic channels.
Yeni yüzey yapısı ve geniş bant aralıklı taşıyıcı seçici katmanlarıyla silikon heteroeklem güneş hücresi performansının iyileştirilmesi
The photovoltaic (PV) industry is dominated by silicon-based solar cells owing to the abundance of silicon and its full-fledged technology. The main road for the PV industry points out to enhance the conversion efficiency of solar cells while decreasing production costs, which is crucial for improving renewable energy market share. The silicon heterojunction solar cells (SHJ) are receiving attention on this road map due to their higher conversion efficiencies, simple process flow, and low-temperature fabrication sequence. In order to further enhance the SHJ device performance, both electrical and optical properties should be improved simultaneously. In this Ph.D. thesis work, various aspects of SHJ solar cells, such as surface texturing, surface passivation, and material choices, were addressed. Firstly, surface texturing was studied to search for new approaches to reduce optical losses. Even though the well-established surface texturing method generating random pyramids on the surface reduces the reflection successfully, there is still room for improvement. A new and novel silicon surface texturing method based on copper-assisted chemical etching was developed for efficient light management on the surface. With this technique, tetragonal-star shaped inverted pyramids were formed, resulting in extremely low reflectance values. Secondly, the surface passivation of silicon was studied using different process conditions and material systems. The SHJ solar cell performance was significantly improved by understanding the chemical passivation kinetics and improving the surface passivation quality. Thirdly, wide band gap materials were integrated into the SHJ solar cell structure to decrease parasitic absorption losses. Furthermore, the free-carrier absorption losses were reduced significantly by tuning TCO's electrical and optical properties. Based on the theoretical and experimental explanations, the novel method for light trapping and integration of wide band gap materials to SHJ solar cell structure were shown to offer promising alternatives to existing technologies for future applications. With these new material systems and process improvements, we have achieved high-efficiency values of up to 21.2% in the SHJ solar cells fabricated at ODTÜ-GÜNAM
Katalitik konvertörlerden Pd, Pt ve Rh metallerinin geri kazanılması, fulleren çekirdekli yolk shell tipi nanopartikül sentezi, hidrojen depolama kapasitelerinin ölçülmesi
Paladyum, platin ve rodyum gibi hidrojen depolama teknolojilerinde kullanılan metaller, maliyetli ve sınırlı kaynaklardır. Bu metaller, otomotiv sektöründe egzoz gazlarını daha az zararlı bileşiklere dönüştüren katalitik konvertörlerde yaygın olarak kullanılmaktadır. Ancak zamanla işlevselliğini kaybeden bu parçalar değiştirilmekte ve atık haline gelmektedir. Kullanılan araca bağlı olarak bir katalitik konvertör 1 ila 5,5 gram arasında Pd, Pt ve Rh içermekte; bu değerli metaller ülkemizde geri kazanılamadığından yurt dışına düşük fiyatlarla satılmaktadır. Tez kapsamında, atık katalitik konvertörlerden PGM'lerin geri kazanımı için etkili bir hidrometalurjik yöntem başarıyla uygulanmış ve özellikle Platin (Pt) için %94,3'lük yüksek bir liç verimine ulaşılmıştır. Geri kazanılan metaller, ileri sentez aşamalarında kullanılmak üzere oto-indirgenebilir komplekslere dönüştürülmüştür. Bu kompleksler kullanılarak, fulleren (C₆₀) çekirdek ve metal bir kabuktan oluşan, tek ve iki tabakalı yolk-shell tipi nanomalzemeler sentezlenmiştir. Sentezlenen nanoyapıların morfolojik, yapısal ve kimyasal özellikleri XRD, SEM-EDX, HR-TEM ve BET gibi ileri karakterizasyon teknikleri ile detaylı olarak incelenmiştir. Sentezlenen malzemelerin hidrojen depolama özellikleri TPD/TPR cihazı ile analiz edilmiştir. Analizler sonucunda, tek tabakalı C₆₀@Pt yapısının yaklaşık 100-200°C aralığında en yüksek hidrojen desorpsiyon sinyalini vererek test edilen malzemeler arasında en üstün performansı sergilediği tespit edilmiştir. En düşük performans ise C₆₀@Pd numunesinde gözlemlenmiştir. Bu çalışma, atıkların değerlendirilmesiyle yüksek katma değerli nanomalzemelerin üretilebileceğini ve bu malzemelerin temiz enerji teknolojilerinden olan hidrojen depolamada etkin bir şekilde kullanılabileceğini ortaya koymaktadır.
Nanopartikül katkılı polimerik sensörlerin üretimi ve uçucu organik bileşenlere karşı hassasiyetinin belirlenmesi
Uçucu organik bileşikler (UOB), çevre ve insan sağlığı açısından kritik riskler oluşturan önemli kirleticilerdir. Metanol, etanol, aseton, toluen, ksilen, klorometan, hidrazin ve dimetilformamid gibi bileşikler, atmosferde düşük seviyelerde bulunsalar dahi akut ve kronik maruziyetlerle ciddi sağlık sorunlarına yol açabilmektedir. Bu nedenle, söz konusu bileşiklerin güvenli sınırlar içinde tutulması ve sürekli olarak izlenmesi büyük önem taşımaktadır. Çevresel izleme çalışmalarında kullanılan gaz sensörleri, yüksek hassasiyetleri ve seçicilikleri nedeniyle öne çıkmaktadır. Özellikle polimer esaslı ince filmler, yüksek yüzey alanı/hacim oranları, düşük maliyetleri ve esneklikleri sayesinde sensör tasarımlarında yaygın bir kullanım alanı bulmuştur. Çözücü döküm yöntemi, bu filmlerin üretiminde basitliği ve homojen yapılar elde edilebilmesi sebebiyle tercih edilmektedir. Ancak etkin bir gaz algılama performansı için filmlerin iletken ya da yarı iletken özelliklerle desteklenmesi gerekmektedir. Bu noktada, nanomalzeme katkıları önemli bir rol oynamaktadır. Metal oksit nanopartiküller, gaz algılama kapasitesini artıran başlıca katkı malzemeleri arasında yer almaktadır. Bu tez kapsamında, polivinil alkol (PVA) ve karboksimetilselüloz (CMC) polimerlerinden çözücü döküm yöntemi ile elde edilen ince filmler, gümüş, titanyum dioksit ve demir nanopartiküller ile fonksiyonelleştirilmiştir. Üretilen nanokompozit filmler farklı kombinasyonlarda hazırlanarak yapısal ve morfolojik karakterizasyonları gerçekleştirilmiş, ardından seçilen UOB'lere karşı sensör performansları test edilmiştir. Elde edilen bulgular, nanopartikül katkılı polimer ince filmlerin uçucu organik bileşiklerin tespitinde yüksek potansiyele sahip olduğunu ortaya koymaktadır.
Atık taş yünleri ile kompozit poliüretan köpük geliştirilmesi
Bu tez çalışmasında, sandviç panel uygulamalarında yaygın olarak kullanılan poliüretan esaslı rijit köpük sistemlerine atık taş yünü ilavesinin mekanik ve termal özellikler üzerindeki etkileri deneysel olarak incelenmiştir. Çalışmanın amacı, endüstriyel süreçler sırasında ortaya çıkan atık taş yünlerinin poliüretan köpük sistemlerinde değerlendirilerek çevresel sürdürülebilirliğin artırılması ve alternatif kompozit çekirdek malzemelerin geliştirilmesine katkı sağlamasıdır. Bu kapsamda, referans poliüretan köpük sistemi ile karşılaştırmalı olarak farklı oranlarda (%0.5, %1, %2, %4 ve %6) atık taş yünü içeren kompozit poliüretan köpük numuneleri hazırlanmıştır. Numuneler üzerinde ilgili standartlara uygun olarak basma dayanımı, çekme dayanımı ve ısıl iletkenlik katsayısı deneyleri gerçekleştirilmiştir. Ayrıca, taş yünü katkısının köpüklerin hücresel yapısı üzerindeki etkilerini değerlendirmek amacıyla optik mikroskop kullanılarak mikroyapısal incelemeler yapılmıştır. Elde edilen deneysel sonuçlar, atık taş yünü katkı oranına bağlı olarak poliüretan köpüklerin mekanik ve termal performansında belirgin değişimler meydana geldiğini göstermiştir. Düşük ve orta seviyedeki katkı oranlarında (%0.5 - %2), köpüklerin hücresel yapısının büyük ölçüde korunduğu ve mekanik dayanımların referans numuneye kıyasla dengeli değerler sergilediği belirlenmiştir. Buna karşılık, yüksek katkı oranlarında (%4 ve %6) hücresel yapıda bozulmalar meydana gelmiş, bu durum mekanik dayanımların azalmasına ve ısıl iletkenlik katsayısının artmasına neden olmuştur. Genel olarak çalışma sonuçları, atık taş yünü katkısının belirli bir oran aralığına kadar poliüretan köpük sistemlerinde uygulanabilir olduğunu ve özellikle %1 - %2 aralığındaki katkı oranlarının mekanik dayanım ile ısıl yalıtım performansı arasında optimum bir denge sağladığını ortaya koymaktadır. Bu bulgular, atık taş yünlerinin sandviç panel üretiminde alternatif kompozit çekirdek malzemesi olarak değerlendirilme potansiyeline sahip olduğunu göstermektedir.
Meyan kökü ve meyan kökü nanokristalin selülozekstraktıyla kitosan filmlerinde antimikrobiyalve uv bariyer özelliklerinin geliştirilmesi
Bu çalışmada, kitosan (KS) temelli biyobozunur filmlerin işlevsel özelliklerini geliştirmek amacıyla meyan kökü sulu ekstraktı (MKS) ve meyan kökünden elde edilen nanokristalin selüloz (NKS) birlikte kullanılmıştır. Amaç, çevre dostu, antimikrobiyal ve UV bariyer özellikleri yüksek biyokompozit bir film geliştirmektir. Kalınlık değerleri incelendiğinde, MKS ve NKS'nin tek başına etkisinin sınırlı, ancak birlikte kullanıldıklarında film kalınlığında anlamlı artış sağladığı belirlenmiştir (p<0,05). Mekanik testler, NKS'nin çekme dayanımını artırdığını, MKS'nin ise esnekliği bir miktar koruduğunu göstermiştir. En yüksek dayanım KS-MKS-NKS filminde (37,9 MPa) elde edilmiştir. Şişme ve çözünürlük testleri, NKS'nin hidrofilik yüzey grupları nedeniyle su tutma eğilimini artırdığını, MKS'nin ise ağ yoğunluğunu artırarak şişmeyi sınırladığını ortaya koymuştur. Su buharı geçirgenliği, her iki katkı maddesiyle birlikte anlamlı biçimde azalmıştır (p<0,05). UV-Vis analizleri, özellikle MKS katkılı filmlerin güçlü UV bloklama kapasitesine sahip olduğunu göstermiştir. Antioksidan testlerinde, KS-MKS-NKS filmi en yüksek DPPH inhibisyon oranını göstermiş (%71,6 ± 2,6), bu durum meyan kökünün fenolik bileşenlerinin NKS ile sinerjik etkisini doğrulamıştır. Antimikrobiyal etkinlik gözlenmemiştir. MKS ve NKS'nin birlikte kullanımı kitosan filmlerinin mekanik, bariyer ve antioksidan özelliklerini belirgin biçimde iyileştirmiş; bu kombinasyonun sürdürülebilir, aktif gıda ambalajı üretiminde umut verici bir yaklaşım sunduğu belirlenmiştir.
Kenevirden (Cannabis sativa) üretilen nanoselüloz yapılarına antibiyotik bağlanması ve bunların antibakteriyel özelliklerinin incelenmesi
Bu yüksek lisans tez çalışmasında, kenevirden (Cannabis sativa) nanoselüloz üretimi ve siprofloksasin ile birlikte antibakteriyel etkisinin incelenmesi araştırılmıştır. Selüloz, biyolojik olarak parçalanabilirlik ve biyouyumluluk gibi önemli özelliklere sahip bir malzemedir. Bu özellikler, selülozu kompozitlerde doğal bir takviye malzemesi olarak kullanılmaya uygun hale getirmektedir. Bu çalışmada, öncelikle kenevir lifinden yaklaşık 4.5 μm çapında saf selüloz elde edilmiş ve ardından bu selüloz, asit hidrolizi ve karbonizasyon işlemleriyle 34.7 nm çapında nanoselüloz (CNC) haline getirilmiştir. Siprofloksasin (CIP) antibiyotiği, hem selüloz hem de CNC'ye yüklenerek, bu malzemelerin antibakteriyel özelliklerinin artırılması hedeflenmiştir. Optimum yükleme koşulları belirlenmiş ve CIP'nin hem selüloz hem de CNC yüzeyine başarılı bir şekilde bağlandığı gözlemlenmiştir. Elde edilen kompozitlerin (C+CIP ve CNC+CIP) antibakteriyel ve antibiyofilm aktiviteleri, farklı bakteri türlerine karşı değerlendirilmiştir. Sonuçlar, CNC'nin, özellikle CIP ile kombinasyon halinde, daha yüksek antibakteriyel ve antibiyofilm aktiviteye sahip olduğunu göstermiştir. Bu bulgular, kenevir lifinden elde edilen selüloz ve CNC'nin, antibiyotik direnci sorunuyla mücadelede potansiyel bir çözüm olabileceğini ve farmasötik ve tıbbi alanlardaki yeni nesil antibiyotik taşıyıcı sistemlerinin geliştirilmesi için de ilham kaynağı olabilir.