LEE- Nano Bilim ve Nano Mühendislik-Yüksek Lisans
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ÖgePEDOT:PSS-CB based interdigitated supercapacitors(Graduate School, 2023-01-31)The globe will meet the need for energy in many different ways as global energy consumption keeps rising. In order to enhance dependable and renewable energy and balance supply and demand, energy storage systems are now being expanded. Although significant progress has been achieved in the development of high-performance fuel cells and li-ion batteries, their applicability in many industries has been constrained by their poor power density and high maintenance requirements. Because they have qualities that conventional energy storage devices lack, supercapacitors have recently attracted a lot of attention. A key feature of energy storage devices is that they deliver high power density while also having a low charge-discharge rate. They offer high power density at the same time as a low charge-discharge rate, which is an important characteristic of energy storage devices. In addition to many renewable generation methods for energy production, the importance of storing this energy and using it later is obvious. Today, energy storage devices are used in automobiles, telephones, and all areas where energy is used. Frequently used storage devices can be supplied with Li-ion batteries, supercapacitors, and capacitors. The usage area of traditional capacitors has decreased rapidly from the past to the present. For this reason, studies between batteries and supercapacitors have been increasing rapidly in recent years and countries with high energy needs are investing in these areas. Batteries, especially li-ion batteries, are the most frequently encountered energy storage devices, from phones to automobiles. Although high energy capacity is its most important feature, different energy storage has been sought due to its limited lifetime, inability to withstand high cycles, lack of high power density and not being environmentally friendly after the end of its useful life. In response to this need, research on supercapacitors has brought to mind the idea of whether they can replace batteries. Supercapacitors are preferred due to the high number of cycles, high energy and power density, and flexible working areas. Besides the types of supercapacitors, there are also different configurations. Although sandwich-type supercapacitors are generally used, interdigitated supercapacitors have been used recently with the development of different production methods. Although conventional sandwich-type supercapacitors have relatively high capacitance, they have high ion transport resistance and low active surface area. Therefore, the power density is low. The production of interdigitated supercapacitors can be fast, wearable, flexible, and small in size. In on-chip applications, interdigitated supercapacitors can be applied to any surface in the form of a film. This study, it was tried to give general information about supercapacitors. It is aimed to operate devices that do not require high energy by connecting high-capacity comb supercapacitors in series. PEDOT:PSS polymer material was used as the electrode material due to its semiconductor and excellent electrical performance. PVA gel-based xxii material was preferred as the electrolyte. As a characterization method, supercapacitors were analyzed in detail using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) methods. To obtain interdigitated supercapacitors, patterns were created on the electrode surfaces by laser etching method. For this purpose, a laser with a 15 Watt power and a wavelength of 450 nm was used. To be able to process the patterns precisely, the laser was mounted on a plotter and the patterns were automatically processed by means of software. Since the conductivity of the PEDOT:PSS material used as the electrode material is not at a sufficient level, first of all, some materials were doped in order and the conductivity amount was aimed to reach the desired level. The doped materials are DMSO, ethylene glycol/methanol, and carbon black (CB), respectively. As a result of the added materials, PEDOT:PSS achieved the desired high conductivity. The glass surface with dimensions of 25mmx25mm was thoroughly cleaned with alcohol and water, and then the PEDOT:PSS-CB material, which was prepared beforehand, was applied to the surface by the drop-casting. Then, it was dried at 65 ⁰C for 2 hours and a thin film was obtained on the glass surface, and the coating process was completed. Then, a comb structure was formed on this surface by the laser etching method and the electrode was made ready for use. A copper current collector is affixed to the prepared electrode. Then, these current collectors are covered with insulating tape so that they do not come into contact with the electrolyte. Then, the prepared PVA-based gel electrolyte containing 6M KOH was poured onto the comb structures with the help of a syringe. The prepared energy cell is placed in a closed container so that the electrolyte does not dry out. In this way, both single and eight different samples were prepared. It is aimed to reach high voltage values by connecting the eight samples prepared in series. The operating range of the single cell is -1 V +1V and the operating range of the octa-series cell is determined as -6V +6V. As a result of the study, eight cells connected in series and a red LED bulb worked for 1 minute.
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ÖgeDevelopment of SAW sensors coated with metal organic framework and borophene for detection of Covid-19(Graduate School, 2023-06-05)The rapid and accurate detection of COVID-19 biomarkers is critical for the early diagnosis and effective treatment of the disease. In this study, surface acoustic wave (SAW) biosensors coated with metal-organic framework (MOF) and borophene layers were developed to detect COVID-19 biomarker gases, including isopropyl alcohol, n-butyraldehyde, acetone, and ethyl butyrate. In addition, the gas measurement system includes ethanol and n-hexane, which are already present in human exhaled air. The objective of this study is to investigate the response of different MOFs and borophene to these biomarker gases and to determine the optimal sensing material for COVID-19 biomarker detection. Seven different materials are coated onto SAW sensors using the drop-casting method as sensing layer. These materials include borophene, MIL-101 (Fe), MIL-101 (Fe)-Borophene, MIL-125, MIL-125-Borophene, MOF-5, and MOF-5-Borophene. They are selected based on their distinct metal components and their potential for detecting COVID-19 biomarker gases. When choosing MOFs, it is preferred to use MOFs that have same organic ligand but a different metal in their center. It is aimed at comparing metal effects and other results related to metal effects. Sensing molecules are characterized using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and Brunauer-Emmett-Teller (BET) analysis. SEM is employed for analyzing the thickness and roughness of coating films on SAW sensors. To examine the sensor's frequency change, a gas measurement system in TÜBİTAK MAM is employed. This system is for the preparation of gas mixtures at different concentrations and humidity levels, which are then sent to the surface of the sensors at room temperature with a constant flow rate. In addition to measuring the change in resonance frequency of the transducer device, the electrical conductivity of the surface is simultaneously measured under related gases to determine the influence of electrical conductivity on sensor performance. Results are processed using MATLAB to read, compare, and perform principal component analysis (PCA). The results of the gas sensing experiments show that the MOFs have a better response to the COVID-19 biomarker without borophene. MOFs also exhibited good selectivity for the target gases. This suggests that these sensors could be used for selective detection of COVID-19 biomarkers in a mixed environment. Keywords:surface acoustic wave(SAW) sensors, metal organic frameworks (MOFs), volatile organic compounds(VOCs), biomarkers, COVID-19
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ÖgeFabrication and characterization of novel membranes for battery separator applications(Graduate School, 2023-05-26)With the prospering industry of electrical mobility, the need for high-performance batteries and energy sources is on the rise. The demand on efficient and safe batteries has exponentially increased. In general, batteries consist of three main critical components; anode, cathode and a separator. While anode and cathode are the active parts in charge and power generation, battery separators are the non-active insulative part. They are a porous structure that allows the passage of ions back and forth while they are made of inherently electrically insulative materials that prohibits the contact of the two poles preventing any short circuit possibility. Battery separators can be made of several materials and substances and carry several key properties. They can be made of any porous insulative material with defined pore size and porosity for their intended application. Along their porous structure, they should carry high chemical stability, strong integrity, and stability under elevating temperatures. Since there are various reactions occurring inside a battery which may lead to an increase in the cell temperature, several approaches are used to fabricate battery separators with high thermal stability and flame retardancy. Among these approaches, coating the separator with inherently flame-retardant materials is a common method. Sodium alginate, as a natural polysaccharide, that shows significant flame retardancy performance when crosslinked with calcium. Cross-linked calcium alginate is reported to exhibit a limiting oxygen index (LOI) of 34. Several polymers are used to fabricate the battery separator. Ultra-high molecular weight polyethylene (UHMWPE) is widely used in battery separator applications due to its high mechanical strength and chemical stability. Film casting is a process widely used to fabricate battery separators. In the wet process film casting, UHMWPE is melt-mixed with a low molecular weight diluent (also called porogen) and casted through a film die before its conveyed into stretching and extraction steps to obtain final porous membrane. Another type of common separator is the nanofiber-based. They can be fabricated via several methods including centrifugal spinning, where a rotor is ejecting the polymeric solution into fibers while rotating at very high speeds. The aim of this thesis is to fabricate lithium and sodium ion battery separators with different fabrication methods and enhance their thermal performance by coating them with calcium alginate. Film casting and centrifugal spinning processes were used to obtain two different membrane structures from two different polymers. In the film casting process, UHMWPE was mixed with paraffin oil (PO) then melted and extruded through a twin-screw extruder (TSE). 30% UHMWPE, 70% PO, and 1% antioxidant mixture was prepared. The temperature of the 6 heating zones and die was held at 130, 140, 150, 160, 170, 180, and 180℃ respectively. The screw rotating speed was held constant at 35rpm. The melt blend was cast through a stripe die with a thickness of 3 mm. The obtained sheets were hot pressed at 180℃ and 8 tons of load for 8 minutes then cooled down to room temperature. The final film thickness obtained was in the range of 200-400μm. Optimization experiments were conducted on the samples to study the effect of different stretching ratios, the importance of extraction step order, the effect of the constrained and nonconstrained uniaxial stretching, and the effect of heating distance between the sample and the stretching machine. It was found that the extraction of the oil after stretching led to a smaller and more uniform pore size in comparison to bigger pores formed when the oil was extracted before stretching. Heating of the samples in the stretching machine was done with the open system using thermal irradiators. Distances of 13.5, 16.5, and 19.5cm were studied. Heaters caused the film to melt fastly and close the pores formed at close distances while at 19.5cm the heating wasn't enough to initiate any pores. The films are then stretched with a custom-made uniaxial stretching machine, where pore formation is initiated. Two stretching ratios were applied; 2×1.5 and 4×1.5. Higher stretching ratios showed the closure of pores. The optimum parameters were found to be a 1.5×2 constrained stretching ratio with a 16.5 cm heating distance and 110℃ heating temperature. After that three Samples (S1, S2, and S3) were fabricated and hot pressed into three thicknesses of 280, 200, and 280μm. These samples are then stretched and annealed to obtain samples of three final thicknesses of 40, 60, and 80μm. Finally, these samples are immersed in n-hexane to extract the oil and obtain the desired porous structure and the final membrane. These membranes are then put in a coin cell configuration and tested for sodium ion (Na-ion) battery separator performance. The ionic conductivities of S1, S2, and S3 samples are 0.09, 0.48, and 0.04mS/cm2 respectively. The S2 sample showed better performance in comparison to other samples due to its higher porosity and uniform pore distribution. Nanofibrous membrane on the other hand was fabricated from thermoplastic polyurethane (TPU) by centrifugal spinning process. TPU was dissolved in dimethylformamide (DMF)/acetone mixture and spun into nanofibers. Optimization of the process was done by altering the polymer concentration, solvent ratios, needle diameter (gauge), and rotating speeds. The optimum nanofibers were obtained at 10% TPU dissolved in a 2:1 DMF/acetone ratio with 30G needle diameter and 13,000 rpm rotating speed. Obtained nanofibers were treated with sodium alginate which was then crosslinked with calcium chloride (CaCl2). After Calcium Alginate (Ca-alg, CA) treatment, the samples were hot pressed at 120℃ and 8 tonnes of load for 2 minutes. Treatment with CA enhances the thermal performance of the separators. The obtained coated nanofibrous membranes were then put into coin cell configuration with 1M LiPF6 and 1M NaClO4 electrolytes and its performance was measured. These membranes were tested for sodium ion (Na-ion) and lithium-ion (Li-ion) batteries. The nanofibrous membranes exhibited significantly better performance when compared to the commercial Celgard 2500 polypropylene (PP) separator. The ionic conductivities of the neat TPU, pressed TPU, 0.5 TPU, and 2 TPU were 0.23, 0.67, 0.17, 0.16, and 0.05 mS/cm2 respectively. The coated samples exhibited significantly larger ionic conductivity numbers and lower resistance values in comparison to the commercial PP separator. In full cell configuration, where anode and cathode are used, the samples exhibited the same trend with neat TPU performing better than other samples. The ionic conductivities of the full-cell samples were 1.53, 0.19, 0.05, and 0.03 mS/cm2 respectively. The drop in the ionic conductivities of the samples can be referred to as the effect of coating the samples which leads to pore closure. This led to the domination of the diffusion phenomena in the samples which was demonstrated by the straight line in the Nyquist plot. In terms of thermal performance, significant enhancement was achieved. On the other hand, when these samples were examined for Li-ion separator tests, the ionic conductivities of Neat TPU, 0.5 TPU, 2 TPU, and PP separators were 0.10, 0.21, 0.09, and 0.07mS/cm2 respectively. The 0.5 TPU sample showed the best performance and proved that calcium alginate can enhance the chemical performance of the separator. The coated samples showed 0% shrinkage in comparison to 63% and 100% shrinkage of the neat TPU and commercial samples. The samples also showed significant fire retardation when exposed to flame source. A comparison between the condition of the samples after 1 and 10 seconds was recorded. Neat and pressed samples melted directly within the first seconds, while the treated 0.5 TPU and 2 TPU samples kept their integrity. 0.5 TPU sample showed marks of burning and its color started turning into brownish while the 2 TPU sample kept its initial state even after 10 seconds. The obtained results showed the high potential calcium alginate carries in battery applications. Studies to enhance the ionic conductivities of the calcium alginate can be conducted and better integrating methods can be proposed.
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ÖgeComposite carbon nanofiber anodes for na ion batteries(Graduate School, 2022-06-02)Nanofibers are one of the most important nanomaterials and have many applications in diverse fields such as biomedical engineering, drug delivery, filtration, sensors, energy storage etc. due to their unique properties like high specific surface area, small diameters, uniform interconnected 3D porous structure and low weight. There are different types of techniques to produce nanofibers like self-assembly, drawing, melt-blowing, phase separation, electrospinning and centrifugal spinning. Due to special features of centrifugal spinning such as high production yield and safe production, it is one of the most promising methods to prepare nanofibers. In this technique, nanofibers are made by applying centrifugal force on polymer solution or melt by using a high-speed rotating spinneret which lead to form nanofibers on collectors. In centrifugal spinning method, the morphology of nanofibers is dependent on diverse processing parameters such as viscosity, surface tension, molecular structure, molecular weight, solution concentration, solvent structure, additive; and operational conditions such as rotational speed, feeding rate, nozzle diameter, and nozzle-collector distance. Energy and environment head the list of top global problems facing society in the twenty-first century. Nanotechnology is responding to these challenges by designing and fabricating functional nanofibers optimized for energy and environmental applications. Nanofiber materials have been extensively studied as constituent parts of energy conversion and storage devices. Lithium ion batteries (LIBs) are rechargeable batteries and have been widely used in different applications like portable electronic devices and electric vehicles due to their high energy density, however limited lithium sources lead to find another option. Production of sodium ion batteries (SIBs) are gaining great attention due to low-cost and high abundance of sodium resources. Rechargeable batteries consist of an anode, a cathode, an electrolyte and a separator. Conventional materials used as anode suffer from large volume expansion, leakage, pulverization and low conductivity whereas carbon nanofibers have been widely used in electrochemical energy storage devices because of their excellent conductivity, extremely large surface area, high porosity, mechanical flexibility and structural stability that will improve capacity and cycling performance when they are used as electrodes in sodium-ion batteries. The performance of these materials is greatly influenced by the material type; structure; mechanical, chemical, thermal stability and physical properties. For example, if the surface area and porosity increased, the permeation of the electrolyte increases, so the electrode can provide higher sodium storage capability and the electrode will have shorter transport length for sodium ions. Heteroatom doping is one of the best ways to increase the surface area and porosity of nanofibers so enhanced sodium ion insertion and desertion at high rates can be achieved. Until now, composite carbon nanofiber using various materials such as tin, iron, antimony and molybdenum have been widely investigated as the anode materials in SIBs, since they possess high theoretical capacity, environmental benignity, safety, and low cost. In the first part of the study; it was aimed to fabricate carbon nanofibers as anodes for sodium ion batteries via centrifugal spinning. PAN was used as carbon fiber precursor because of its high carbon yield, high thermal, chemical and mechanical properties. Also, PS was blended with PAN in order to increase the porosity of carbon nanofibers. Moreover, graphene with a two-dimensional honeycomb structure was used to improve the electrochemical performance of the LIBs and SIBs, due to its high theoretical specific capacity, large specific surface area, and good electronic conductivity. In addition, the effect of Molybdenum disulfide (MoS2) on performance and electrochemical capacity of LIBs and SIBs was investigated. PAN/PS/graphene polymer blend solution was prepared in DMF and centrifugally spun in order to obtain nanofibers. Nanofibers were fabricated at the rotational speed of 4000 rpm, feeding rate of 60 ml/h, with 0.5 mm nozzle diameter and 20 cm collector distance. After obtaining nanofibers, blend nanofibers were stabilized in air atmosphere at 280 ℃ for 2.5 h with a heating rate of 5 ℃/min and then carbonized in nitrogen atmosphere at 800 ℃ for 2 h with a heating rate of 2 ℃/min. Furthermore, MoS2 decorated graphene-containing porous carbon nanofibers were fabricated via hydrothermal synthesis. The morphology and average fiber diameter distribution were analyzed with SEM. Porous structure of the carbon nanofibers was observed via TEM images. XRD and Raman spectroscopy were used for structural characterization. Porous structure enhanced the electrochemical performance of electrodes. Furthermore, MoS2 decorated graphene included porous CNF improved the electrochemical capacity up to 860 mAh/g in Li-ion cells and 455 mAh/g in Na-ion cells with excellent cycling performance. In the second part of the study; it was aimed to fabricate carbon nanofibers by using water soluble polymers such as PVA and PVP. Considering environmental concerns, it is vital to fabricate carbon nanofibers from environmentally friendly materials. PVA/PVP nanofibers were fabricated via fast and safe centrifugal spinning. The effect of PVP content on the morphology and thermal properties of PVA/PVP blend nanofibers were studied by using SEM and DSC studies. Moreover, the effect of carbonization conditions including stabilization time, stabilization temperature, carbonization time, carbonization temperature on morphology and carbon yield was investigated.
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ÖgeSynthesis and characterization of graphene oxide with enhanced mechanical properties(Graduate School, 2022-11-16)With the ever-increasing multiple threats and conflicts, and with the insufficiency of the actual protection systems made of polymeric fibers such as Kevlar, Dyneema, and Zylon against improvised explosive devices (IEDs) and lethal ammunition, the development of personal protection systems with heightened mechanical proprieties has received great interest in this decade, and this interest will continue to increase as the market of personal protective armor systems is predicted to rise to $5.3 billion by 2024. As it can offer solutions and evolutions for today's world, nanotechnology holds -undoubtedly- the opportunity to provide the breakthroughs that defense technology so desperately needs. The emergence of nanomaterials with exotic proprieties makes them an excellent choice for ballistic armor materials, among the most ideal ones is graphene: graphene is already known as the world's strongest material with a theoretical modulus of more than TPa. Moreover, graphene has a low density, which is a very interesting propriety for body armor application since it provides better mobility for the soldier due to its lightweight attribute and fatigue reduction. According to recent studies, graphene has also an intrinsic ability to absorb sudden impacts and dissipate their high energy. what is essential at this point is to effectively exfoliate the raw material: graphite into large quantities of high-quality graphene. Graphene oxide (GO), the oxidative derivative of pristine graphene can be produced via a solution-based chemical exfoliation method which is a top-down process susceptible to economical large-scale production. The oxygen groups of GO can increase also its interaction with different functional groups of polymeric fibers such as Kevlar fibers, or with the polymer constituting the "brick" part of the nacre-like protective system. GO can be further converted to graphene through chemical or thermal reduction, which makes the potential of fabricating graphene-based body armors very high soon. However, GO quality remains the determinant factor and the big challenge to overcome in order to integrate GO into body armor systems. The main objective of this thesis is to enhance GO quality and to make its chemical synthesis more suitable for large-scale production. By using expanded graphite as a starting material instead of natural graphite flakes, we promoted the synthesis of GO with large sheets (average of ~ 37 μm) and low defects degree thanks to the effective oxidant diffusion into graphene galleries after enlarging the interlayer spacing. The expanded graphite was obtained easily by treating graphite with cooled piranha solution without any washing or drying steps, and without involving any heat treatment nor requiring advanced equipment, unlike the traditional methods which require harsh conditions and result in a high cost and severe environmental pollution. An expansion volume of 430 ml/g was achieved under room temperature with mass ratios of +100 mesh graphite to sulfuric acid of 1:100 and hydrogen peroxide to sulfuric acid of 1:10. Thanks to this expansion, the oxidation temperature could be reduced from 50 °C to 35 °C and the oxidation time could be reduced to half. XRD, XPS, and NMR have shown that GO synthesized via this route that we called the "enhanced method" —reported to the best of our knowledge for the first time— has a high oxidation degree, while UV, XPS, and Raman have manifested the retain of more aromatic rings i.e. low defects compared to Tour group's method. Furthermore, after using the industrially suitable doctor-blade technique and hydroiodic acid (HI) reduction, rGO film obtained through this method has achieved a tensile strength of 190 MPa, a toughness of 5.7 MJ m-3 which is promising for the mass production of expanded graphite (EG) and GO due to the method simplicity, cost-effectiveness, and low environmental impact. The enhanced synthesis method was further used but with four different graphite sizes to study their effect on the volume expansion and GO properties in the scope of producing large graphene oxide from initially large graphite flakes. Other enhancements were done like reducing the acid quantity to reduce the total cost and make the synthesis more environmentally friendly, operating it at room temperature (20 °C), and minimizing the oxidant quantity to restrict any over-oxidation which can lead to more defects hardly removed through reduction. The strength and toughness were found to increase with increasing the starting graphite material size, except GO+100 mesh which was unexpectedly inferior to GO200 mesh. In this study, GO50 mesh exhibited the highest failure strength and toughness at 232 MPa and 11.3 MJ m-3 respectively, but despite that its starting material has a much larger size than that of GO200 mesh, the difference between their tensile curves was not that pronounced. This research work concludes that the starting graphite size can play an important role, but larger graphite flakes' size does not always lead to better GO despite that this trend is ordinarily correct. XPS shows that impurities such as organosulfate, and carboxylic groups located on the sheets' edges can reduce the properties of the final GO even if it is obtained from large graphite flakes. Raman and morphology studies reveal that as larger flakes need higher oxidant quantity, harsher oxidation may exist to overcome the diffusion-controlled oxidation pathways until achieving the flakes centers, which cuts off the sheets into smaller ones and creates more cracks and defects. Thus, there exists a balance between the large building blocks needed and the defects induced. In this study, evidence of how using +50 mesh graphite -with the enhancements made to the already enhanced method- can improve the resulting GO mechanical properties. However, to confirm the graphite size effect on GO properties, it would be better if a graphite size larger than +50 mesh can be used and the final GO size as well as its mechanical properties before and after reduction can be determined. It is important to make other characterizations for the graphite flakes and the resulting GO such as elemental analysis, XRD, and AFM. More than the tensile tests, advanced characterization can be made to GO and rGO free-standing films such as nanoindentation test, split Hopkinson bar, and gas gun measuring system for testing high strain rate behavior, and for determining failure stress and absorbed specific energy under dynamic conditions. For the immediate use of GO in the current body armor systems, Kevlar fibers can be coated with GO synthesized and enhanced through this thesis, then GO can be reduced to further ameliorate its mechanical properties. Our proposed enhanced method susceptible to cost-effective mass-scale production of GO makes the fabrication of such a body armor attainable in the near future.