Browsing by Author "Demir, Mustafa Muammer"

Now showing 1 - 20 of 35

Bir fosfor dönüşümlü beyaz led paketi
(Türk Patent ve Marka Kurumu, 2020) Demir, Mustafa Muammer; Güner, Tuğrul; Şentürk, Ufuk; Demir, Mustafa; Izmir Institute of Technology; Izmir Institute of Technology; Izmir Institute of Technology
Bu buluş bir fosfor dönüşümlü beyaz LED paketi ile, özellikle fosfor kullanım miktarını azaltmayı ve ışık rengi sıcaklığının ayarlanmasını sağlayan cam küre formunda optik elemanlar içeren bir fosfor dönüşümlü beyaz LED paketi ile ilgilidir.
Characterization and expansion behaviour of perlite
(Izmir Institute of Technology, 2016-07) Gül, Dilek; Demir, Mustafa; Şentürk, Ufuk; Demir, Mustafa Muammer
Perlite is a silica-based organic glass. When it is instantaneously heated, it shows expansion by releasing a certain amount of water. Approximately 70% of perlite reserves in the world are found in Turkey. The porous structure of expanded perlite leads to use this material in many industrial areas; therefore, this is highly advantageous from the economic standpoint. In this study, the expansion behaviours of perlite samples, which were taken from İzmir (Bergama Mine), Kütahya (Avdan Mine), and Erzincan (Mollatepe Mine) were both characterized and investigated in the laboratory. The instrumentation techniques, X-ray Diffraction (XRD), X-ray Fluorescence (XRF), Scanning Electron Microscopy (SEM), Thermogravimetric Analysis (TGA), and Optic Microscope were used to characterize the perlite. The determination of the amorphous and crystal structure of perlite, the chemical composition of perlite, the measurement of loss of mass, the morphological characterization of perlite, and the form of raw and expanded perlite samples were obtained by XRD, XRF, TGA, SEM and Optic Microscopy, respectively. Four different heat treatments as 600 ˚C, 800 ˚C , 900 ˚C and 1000 ˚C and four different sizes 400-500 μm, 315-400 μm, 200-315 μm and 160-200 μm were conducted in the laboratory and the their bulk volumes and densities were compared before and after heat treatment. The perlite sample from Bergama 01 showed the largest expansion in all samples. As results of the analyses, the amount of crystal and water release in the perlite which had an influence on expansion process were consistent with the literature. The analyses were conducted until 1000 ˚C due to forming sinter above softening temperature. Additionally, the expanded perlite samples in the industrial furnace were compared with the expanded perlite samples in the laboratory more porous structure was observed. As a conclusion, the perlite samples from the different regions of Turkey were characterized and analyzed in terms of expansion behaviour in this study.
Chitosan-plasmid DNA nanoparticles: Cytotoxic and cytostatic effects on human cell lines
(Izmir Institute of Technology, 2015-12) Bor, Gizem; Demir, Mustafa; Şanlı Mohamed, Gülşah; Demir, Mustafa Muammer
Although chitosan nanoparticles (CNs) became a promising tool for several biological and medical applications owing to their inherent biocompatibility and biodegrability, studies regarding their effects on cytotoxicity and cytostatic properties still remain insufficient. Therefore, in the present study, we decided to perform comprehensive analysis of the interactions between CNs – pKindling-Red-Mito (pDNA) and different cell line models derived from blood system and human solid tissues cancers. The resulting CNs-pDNA was investigated with regard to their physical-chemical properties, cellular uptake and transfection efficiency, cytotoxic and cytostatic properties. The nanoparticles showed high encapsulation efficiency and physical stability even after 2 days for various formulations. Moreover, high gene expression levels were observed already 96 h after transfection. CNs-pDNA treatment, despite the absence of oxidative stress induction, caused cell cycle arrest in G0/G1 phase and as consequence led to premature senescence, which turned out to be both, p21-dependent and p21-independent. Also, observed DNMT2 upregulation may suggest the activation of different pathways protecting from the resulting CNs-mediated stress. In conclusion, treatment of different cell lines with CNs-pDNA showed that their biocompatibility was limited and effects were cell type-dependent.
Circulating nucleic acid (CNA) separation from serum by electrospun membranes
(Izmir Institute of Technology, 2015-07) Işık, Tuğba; Demir, Mustafa; Demir, Mustafa Muammer; Horzum Polat, Nesrin
Early detection of diseases is a key factor that can be made provision and successfully treated. There is a wide array of methods to diagnose cancer like biopsy, endoscopy, magnetic resonance imaging (MRI) and blood tests. In blood, while some nucleic acids are found in intracellular fluids, circulating nucleic acids (CNAs) circulate freely in extracellular fluids. They are located at genomic regions and increased levels of CNAs imply a connection with cancer and tumor initiation. For further analysis, separation and concentration of CNAs have high potential in early cancer detection. A model system was constructed with bovine serum albumin (BSA) and single stranded Deoxyribonucleic acid (ss-DNA) for the investigation of membrane separation efficiency. The membranes made of poly styrene (PS) and poly(methylmethacrylate) (PMMA) were fabricated by electrospinning and they were placed in syringe columns. By using the absorption spectroscopy, the sorption efficiency of membranes was determined. The electrospun membranes are promising for BSA uptake over a wide pH range. Under the same circumstances, thanks to PS fibers, ss-DNA uptake is very with respect to BSA uptake. Our results revealed that PS membranes show a better affinity to BSA molecules by hydrophobic interactions. In the mixture of BSA and ss-DNA, ss- DNA cannot be held on the surface of the membrane and pass through with low sorption efficiency. By altering the membrane amount in syringe column and modifiying the surface of the membranes, the separation could be enhanced. The proposed technology promises fast and effective separation of CNAs from whole blood and body fluids.
A comparative study on the photocatalytic activity of dye-sensitized and non-sensitized graphene oxide-TİO2 composites under simulated and direct sunlight
(01. Izmir Institute of Technology, 2019-12) İlhan, Hatice; Demir, Mustafa; Varlıklı, Canan; Demir, Mustafa Muammer; 01. Izmir Institute of Technology
Amine modified graphene oxide (mGO) and TiO2 composite was synthesized by low temperature hydrothermal method. Characterization of the synthesized material was carried out by using X-ray diffraction, X-ray photoelectron spectroscopy, and BET analysis techniques. The films of mGO:TiO2 and formerly synthesized TiO2, N-TiO2, GO-TiO2 and GO:N-TiO2 were fabricated by doctor blade method and employed as photocatalysts for the photodegradation of Rhodamine-B (RhB) dye under simulated (Xe lamb) and direct sun-light. P25 was also used as reference photocatalyst for all of the synthesized ones. Photodegradation of RhB was monitored by UV-Vis spectroscopy. Among all the catalysts, GO:N-TiO2, the composite of GO and N-doped TiO2, presented the best photocatalytic activity and although the activity of mGO:TiO2 was better than the activities of P25 and TiO2, it presented lower degradation rate constant even than that of the N-TiO2. It is proposed that increased abundance of C-C bonds and decreased number of oxygenated functional groups on mGO:TiO2, in addition to the morphological difference between GO (sheet like) and mGO (dot like) has great influence on their photocatalytic activities. Among the GO containing photocatalysts including mGO:TiO2, specific surface area (SSA) and number of RhB molecules per film volume were the lowest and particle size was the highest for mGO:TiO2. Although the number of RhB molecules per film volume was higher in mGO:TiO2 than that of the N-TiO2, it is thought that approximately 2 folds higher SSA of N-TiO2 allowed better photocatalytic performance. Additionally, the films were sensitized with PTE dye to obtain effective catalysts in visible region and reusability of the films were also tested. Degradation rate constants of all fabricated films have increased under both of the irradiation media and no significant change in rate constants were detected after the reusability tests.
Determination of vitamin D by sensor technologies based on molecular imprinted polymers
(Izmir Institute of Technology, 2022-07) Ölçer, Yekta Arya; Demir, Mustafa; Eroğlu, Ahmet Emin; Demir, Mustafa Muammer
Vitamin D is an essential nutrient in the body; it plays important roles in human health. Both its lack and excess can have health risks. As a consequence, there is a great demand for development of simple and precise detection methods for vitamin D derivatives in different samples. Molecular imprinting polymers (MIPs) are artificial receptors that can recognize target molecules in solution. In this study, two different polymerization techniques were used to obtain MIP/NIP sorbents/films for the detection of vitamin D3. Firstly, molecular imprinted solid phase extraction (MISPE) method was proposed prior to HPLC-DAD analysis. Optimized parameters were as follows; sorbent amount of 5.0 mg for 5.0 mL of 1.0 mg/L vitamin D3 in 90:10 (v/v) ratio of H2O:MeOH solution, 5 hours sorption time and MeOH:HOAc ratio of 90:10 (v/v) as desorption solution. The accuracy of the method was verified with spike recovery test for PBS:MeOH in a ratio of 90:10 (v/v) and overall recovery was found as 85.1 (±4.3, n=3). In latter case, a quartz crystal microbalance (QCM) method was proposed for determination of vitamin D3. Electrochemical polymerization of poly(4-vinylpyridine) MIP/NIP films were achieved on gold working electrode by cyclic voltammetry (CV). Mass-transfer ability of the polymer films were analyzed by electrochemical impedance spectroscopy (EIS). The electrochemical QCM (eQCM) was used to develop thin polymer films on quartz crystals and vitamin D3 determination was achieved by QCM. In a preliminary test, as small a concentration as 0.0100 mg/L vitamin D was detected with the QCM method.
Development of a new antiscalant for minimization of stibnite scaling in geothermal binary plants
(01. Izmir Institute of Technology, 2021-03) Karaburun, Emre Mustafa; Demir, Mustafa; Demir, Mustafa Muammer; Baba, Alper; 01. Izmir Institute of Technology
Demand for renewable and sustainable energy resources has been increasing in recent years due to the adverse effects of fossil energy resources (gases, oil, coal ect,) on human health and nature. Since geology properties of our country is rich in geothermal energy resources, there is a remarkable increase in plant capacities every year. Geothermal energy is the energy obtained from the fluid, gas, and steam that have been stored in reservoirs by carrying the heat energy accumulated in the rocks in the depths of the earth or surfaced along the discontinuity zones.The increase in the use of geothermal energy and the increase in the capacity of the power plants has also revealed the problem of scaling. The main cause of deposition are the decrease in solubility of minerals by a decrease in pressure and temperature upon pumping the geothermal brine up to the ground. Calcium carbonate, calcium sulphate, metal silicates (Mg, Fe) are the most widely accepted types of scaling. In recent years, antimony and arsenic sulfide scaling have been encountered in volcanic and metamorphic regions. Between these two types of scaling, antimony sulfide is mostly seen in the heat-exchangers and preheaters where the temperature drops suddenly. In this thesis, the antimony sulfide scaling formed in the geothermal power plant was synthesized in an autoclave reactor under specified conditions. Water-soluble polymers nominee for being antiscalants such as Poly (vinyl sulfonic acid), Poly (acrylamide-co-vinyl sulfonic acid), Poly (acrylamide-co-vinyl phosphonic acid), Alginic Acid, Natural antiscalants, Polyacrylic acid, and Polyvinyl alcohol were employed. The results suggest that polymers containing vinyl sulfonic acid and acrylamide likely shows remarkable progress in increasing the concentration of ions in decantate, particularly at low dosages. (≈5 ppm)
Development of novel anticounterfeiting technologies using heavy metal free nanoparticles
(Izmir Institute of Technology, 2021-07) Taşcıoğlu, Didem; Demir, Mustafa; Özçelik, Serdar; Demir, Mustafa Muammer; Izmir Institute of Technology
Counterfeiting, the act of illegally copying a product, document or currency, is a growing problem and causes economic losses. Anticounterfeiting technology uses fluorescent inks that are invisible to the naked eye in daylight, but become visible under UV light. However, these inks have problems such as fading when exposed to sunlight or room light for a long time and disappear completely over time. This is due to the relevant inks are made using organic dyes that fade. The inks used in anticounterfeiting application preventing copying of secure documents such as banknotes, passports and ID cards must be health-friendly and chemically and optically stable for years. All of the existing security materials and equipments for ID cards, driver's licenses, passports, banknotes used in our country are imported. In this study, our aim is to create a new generation of security materials and codes to combat counterfeiters and to verify the generated security codes in a simple, efficient and fast way. In this study, it is aimed to produce nanoparticles, which do not contain heavy metals and show optical stability for a long time, emitting in visible region, on the basis of the security codes created. For this purpose, water and solvent-based nanoparticles synthesized which are non-toxic should have a long-term optical stability. The synthesized nanoparticles act like a pigment in security codes. The photoluminescence (emission color) of the security codes can be adjusted by size and chemical composition of nanoparticles. In this study, colloidally monodispersed and highly photoluminescent InP based nanoparticles were synthesized by the hot-injection approach under an inert atmosphere. In addition, a protective shell (ZnS, ZnSe) coating methods have been applied to provide optical stability to InP nanoparticles. Moreover, carbon-based nanoparticles with high optical stability and being dispersible in water were synthesized using the bottom-up method. Security codes that cannot be detected in daylight have been created on different subtrates (paper, polymer, glass, etc.) by using screen printing and inkjet printing methods, which are well known printing methods using the synthesized nanoparticles. In addition, the authenticity of the security codes was checked using a commercial fiber optic based spectrometer (Ocean Optics spectrometer) and a handy hand-held optical device called the Quantag sensor developed by Quantag Nanotechnologies. Thus, a verification method that can be distinguished by a simple detection device is proposed. The synthesized nanoparticles were furthermore dispersed in a polymer solution to create random droplet and droplet/fiber patterns by electrospinning method. Thus, unique and inimitable security codes detectable under UV light were created which may be used in the fight against counterfeiting. To check the authenticity of the original security codes created; images collected with a simple smartphone microscope and a database was created in which the original patterns were recorded. The originality of the random patterns obtained was checked by comparing it with the patterns recorded in the database. In addition, the spectral information of the particle from the droplet/fiber pattern obtained was determined with a simple hand-held device (Ocean Optics optical spectrometer). Thus, by reading spectral information from the pattern, the spectral signature of the nanoparticles was determined and thus a second-step security was created. In this way, a two-stage anticounterfeiting technology that is impossible to imitate has been developed. As a conclusion, it is believed that the security codes developed in this study will pave the way for the commercialization of quantum labeling technology.
Development of whey protein isolate based nanocomposite food packaging film incorporated with chitosan and zein nanoparticles
(Izmir Institute of Technology, 2014) Oymacı, Pelin; Demir, Mustafa; Alsoy Altınkaya, Sacide; Demir, Mustafa Muammer
The purpose of this study was to investigate the effect of chitosan and zein nanoparticles addition on the barrier and mechanical properties of whey protein isolate (WPI) films as an alternative to conventional synthetic packaging materials. Chitosan nanoparticles (CSNP) were produced via ionic gelation method using sodium tripolyphosphate (TPP) and deacetylated chitosan. Zein nanoparticles (ZNP) were synthesized based on antisolvent procedure in the presence of sodium caseinate (SC) to enable dispersion in water. Both plain and nanoparticle added WPI films were prepared by solution casting method. Water vapor barrier and mechanical properties of films were measured and the improvements in these properties with nanoparticle addition was further investigated through surface wetting, morphological, viscoelastic and thermal properties of the films. Both nanoparticles significantly decreased the water vapor permeability (WVP) and improved the mechanical properties of the WPI film. The highest enhancement in barrier and mechanical properties of the WPI films were recorded with 20% (w/w of WPI) CSNP and 120% (w/w of WPI) ZNP addition which corresponded to the maximum nanoparticle loading levels. At these loadings, the average WVP of pure WPI films loaded with ZNP and CSNP decreased by 84% and 57%, and the average tensile strength increased by 304% and 161%, respectively. On the other hand, the nanoparticles did not change the elongation at break significantly. ZNP was found more effective than CSNP in improving barrier and mechanical properties of the WPI films due to its hydrophobic nature and better dispersion in the protein matrix which allowed much higher loadings compared with the maximum loading levels achieved with CSNP. CSNP addition imparted antibacterial activity to the WPI films.
Down-converting polymer composites and their white light applications
(Izmir Institute of Technology, 2018-12-05) Güner, Tuğrul; Demir, Mustafa; Demir, Mustafa Muammer
Down-converting materials, which are luminescent materials that can emit at lower energies than their excitation energies, have been employed in a broad range of application area including phosphor-converted white LEDs, display technologies, solar cells, etc. In general, they can be obtained in the form powder or in dispersion. Therefore, to use them in an application, various methods such as spin coating can be applied or they can be prepared in the form of ceramic plate or polymer composite. In this thesis, we fabricate polymeric composite of some of these down-converting materials to use them in white light applications. In this sense, we have prepared these down-converting materials together with suitable polymers to form polymer/down-converting material composite in order to obtain free-standing film. Among those, polydimethylsiloxane was used in most of our applications especially the ones that involve inorganic downconverting materials. Second, polystyrene was employed to produce electrospun fibers in the case of when organic down-converting materials were used. After obtaining these composites in the form of free-standing film, we have focused specifically their use as color conversion layers over blue or UV LED chip to produce white light. The last part of the thesis describes our future prospects. We think that halide perovskites, which have high quantum yield, low cost, ease of synthesis, and wavelength tunability, can be promising materials in order to be employed as color conversion layer in white light applications. The details of these material systems, and their recent use in phosphorconverted white LED applications were summarized.
Electrochemical properties of titania based powders
(Izmir Institute of Technology, 2015-10) Türkay, Cem; Demir, Mustafa; Demir, Mustafa Muammer; Çiftçioğlu, Muhsin
Global warming arising from the greenhouse effect is globally accepted as the main problem which may threaten the life on the earth. Excess emission of carbon dioxide which leads to the more absorption of solar radiation in the atmosphere is the main reason for global warming. Carbon dioxide present in the atmosphere is balanced by natural photosynthesis; however this balance was disturbed by the increasing amount of carbon dioxide emissions after industrial revolution. Intense efforts was made by many scientists to find solutions to decrease the carbon dioxide level in the atmosphere and the pioneering studies were conducted in the early 1970s which founded the basic theory of artificial photosynthesis. The conceptual idea on conducting photosynthesis by technologically feasible processes was accepted by many scientists and the research on artificial photosynthesis accelerated in the last 10 years. The enhancement of the efficieny of artificial photosynthesis, by which alternative fuels such as methane, methanol may be produced, can be realized by doping titanium dioxide which is the most widely used photocatalyst in the literature. The determination of new electrochemical properties obtained by doping titanium dioxide is crucial since the oxidation/reduction reactions are controlled by the electrochemical structure of this material. Bandgap and band position energy levels which are important properties in photocatalysis can be determined and the efficiency of photoreduction under UV or visible light corresponding to these energy levels can be improved. Cyclic voltammetry (CV) can be used to determine the electrochemical properties of titanium dioxide and these properties can be improved by using the information obtained with this method. The effects of rare earth element doping on the electrochemical properties of titanium dioxide were investigated through out this Msc study. It was found that doping of titanium dioxide is significantly increased the electrochemical activity with rare earth elements. The increase in the doping amount of elements showed that articial photosynthesis activity of titanium dioxide may be enhanced by rare earth element doping.
Fabrication and characterization of ceramic fibers from preceramic polymers
(Izmir Institute of Technology, 2019-12) Özmen, Ecem; Demir, Mustafa; Ahmetoğlu, Çekdar Vakıf; Demir, Mustafa Muammer
Ceramic fibers which are classified as oxide and non-oxide fibers are preferred to use in applications which are carried out at high temperature since they have high strength, low thermal expansion, corrosion, and oxidation resistance. Non-oxide fibers are generally produced using preceramic polymers by the spinning method. The production of ceramic materials using preceramic polymers by spinning method is more advantageous than other methods since the production of complex materials could be achieved at lower temperatures. The preceramic polymer family is basically classified as polysiloxane, polysilazane and polycarbosilane. In this thesis, it was aimed to obtain ceramic fiber in the most economical way. In this context, a spinning device was designed and made. Additionally, polysiloxane which is the most economical preceramic polymer was used to produce ceramic fiber. Polysiloxanes were spun by melt spinning. Obtained fibers were cured by different methods. As a result of pyrolysis, 65-130 μm thickness SiOC fibers were achieved.
Fabrication of colloidal photonic crystals via Langmuir Blodgett technique and their integration of polymer matrix
(Izmir Institute of Technology, 2019-07) İnci, Ezgi; Demir, Mustafa; Demir, Mustafa Muammer; Varlıklı, Canan
Colloidal films have potential uses in various fields such as photonics, electronics, sensors, membrane filters, and surface devices owing to their unique optical properties. Photonic crystals composed of uniform diameter colloidal silica particles have been arranged in a periodic structure by taking inspiration from nature. The periodic structure of silica particles has physical interaction with light in a visible range. This special interaction is known as structural coloration. The close-packed monolayers and multilayers of colloidal silica particles in large area can be produced by using Langmuir Blodgett method. The integration of these photonic films with transparent polymer matrices having an elastomer feature provides for their use in optical sensor applications. In this thesis, we examined the fabrication of mechano-sensitive nanostructured films based on colloidal particles. Silica colloidal particles were synthesized at different sizes by using Stöber Process. Langmuir-Blodgett deposition was used to create three-layer of photonic crystal films with different particle diameters. For this purpose, various substrates were examined for the Langmuir Blodgett deposition process before starting the coating. The coated silica particles on the glass substrate were then embedded in an elastomeric transparent matrix. The generation of structural coloration after stretching was examined in manufactured elastomer films. In accordance with this purpose, various polymers such as acrylates and siloxanes with elastomer properties have been used. The structural characterization of these composite films and their optical properties were summarized in this thesis.
The fabrication of plasmonic/photonic nanostructures in polymers: Mechanical sensor applications
(Izmir Institute of Technology, 2019-07) Topçu, Gökhan; Demir, Mustafa; Demir, Mustafa Muammer; Eanes, Mehtap
Functional polymer nanocomposites offer futuristic properties by the association of inorganic additive micro-/nanostructures into the polymers. With the growing knowledge of the physical fundamentals, stimuli-responsive polymeric composites enable detection of chemical, thermal, and mechanical changes by optical sensors and probes. Since the accurate real-time detection of the change in mechanical loading is vital for construction and industrial fields, the use of colorimetric pressure elements in a static body is important for the prediction of catastrophic failures. In this thesis, strain/pressure responsive colorimetric films were produced. A number of polymer nanocomposite-based mechanical sensors are presented. By transferring the optical activity (coherent reflection and plasmonic coupling) of the additives into various polymeric matrices having different mechanical features, the strain and pressure sensors are developed for practical applications. There are two approaches used for the fabrication of polymeric mechanical sensors: i) PDMS/SiO2 composites, ii) PAAm/Au NP composites. The coherent reflectivity of SiO2 colloidal particle arrays was used to develop strain sensors while controllable localized surface plasmon resonance of Au NPs was employed for pressure sensors. These optical systems were separately associated with viscoelastic and elastic polymeric systems, and sensor properties were discussed.
Fabrication of transparent polymer nanocomposites containing pmma-grafted CeO2 particles
(Izmir Institute of Technology, 2011) Parlak, Onur; Demir, Mustafa; Demir, Mustafa Muammer
The composite materials prepared by transparent polymer and nanosized particles possess promising future in optical design and applications since their controllable optical properties. In this study, transparent/translucent composite films based on polystyrene (PS) and poly(methyl methacrylate) (PMMA)-grafted CeO2 nanoparticles were prepared. CeO2 nanoparticles were precipitated from Ce(NO3)3·6H2O and urea in dimethyl formamide at 120°C. The surface of the nanoparticles was modified with a polymerizable surfactant, 3-methacyloxypropyltrimethoxy silane (MPS) in situ at 0°C. The size of the particles was fixed to 18 nm in diameter. The particles were dispersed into a mixture of MMA:toluene solution. The free radical solution polymerization was carried out in situ at 60°C using benzoyl peroxide (BPO) as initiator. A PMMA layer is formed around CeO2 nanoparticles. The thickness of the shell ranged from 9 to 84 nm was controlled by the amount of BPO using 6 and 0.5 wt %, respectively with respect to monomer. The layer thickness was found to be inversely proportional with the amount of initiator. The resulting PMMA-grafted CeO2 particles were blended with PS in tetrahydrofuran and the solution was spin-coat on a glass slide. CeO2 content in the composite films was fixed to 5.5 wt %. The transmission of the films was examined by UV-vis spectroscopy. The transmission of the PS composite prepared by neat CeO2 particles was 71 %. It was increased to 85 % when the composite prepared with PMMA-grafted CeO2 particles whose PMMA thickness is 9 nm. We believe that the achievement in transparency is most probably due to the reduction in refractive index mismatch between CeO2 particles and PS matrix using PMMA layer at interface.
Field emission electron source based on silicon carbide nanopillars
(Izmir Institute of Technology, 2017-07) Yeşilpınar, Damla; Demir, Mustafa; Çelebi, Cem; Demir, Mustafa Muammer
In this thesis work, I studied the fabrication and the field emission characteristics of SiC nanopillar based electron field emitters. The first objective of this thesis was to fabricate a large area nanopillar array on bulk 6H-SiC substrate. Accordingly, a nanosphere assisted technique was developed to create a conventional Cr/Ni hard mask to acquire desired etch mask pattern on the C-terminated face of 6H-SiC. The nanopillars were then fabricated by ICP-RIE. Two sets of nanopillars with different aspect ratios and geometries were fabricated for two different ICP-RIE durations. 1 min long etch resulted in nanopillar arrays with blunt tip apex and an aspect ratio of 3.4, where 2 min long etch produced nanopillar arrays with an aspect ratio of 4.9 and a sharp tip apex with an estimated radius of curvature of about 18 nm. As the second objective; the electron field emission characteristics of the produced nanopillars with two different aspect ratios and geometries were investigated and the obtained results were compared with each other. We found that the nanopillars with sharp tip apex produced field emission currents up to 240 μA/cm2 under 17.4 V/μm applied electric field, as the nanopillars with blunt tip apex produced an emission current of 70 μA/cm2. The threshold electric fields were found to be 9.1 V/μm and 7.2 V/μm for the nanopillars with blunt and sharp tip apex, respectively. Time dependent stability measurements yielded stable electron emission without any abrupt change in the respective current levels of both samples.
Glucose biosensor applicatiın of electrospun polyvinyl alacohol (PVA) fibers
(Izmir Institute of Technology, 2016-09) Berber, Emine; Demir, Mustafa; Demir, Mustafa Muammer; Yıldız, Ümit Hakan
Electrospinning is a simple and versatile technique for the fabrication of polymeric nanofibrous membranes with high surface to volume ratio. Besides the large surface area of the fibrous membranes, their dimensional stability and flexibility allows the immobilization of biomolecules on to the nanofiber surfaces. Therefore, electrospun nanofibers have been extensively used in enzyme electrodes. This thesis examines the glucose biosensor application of electrospun polyvinyl alcohol (PVA) nanofibers – carbon nanotube (CNT) nanocomposite membranes. By manipulating the structural design and the composition of the nanocomposite membranes, glucose sensing efficiency of the five different enzyme electrodes a) Glucose oxidase (GOx) immobilized PVA electrospun electrode, b) Glucose oxidase (GOx) immobilized PVA electrospun electrode containing multi-walled carbon nanotube (MWCNT), c) Glucose oxidase (GOx) immobilized PVA electrospun electrode containing Poly(diallyldimethylammonium chloride) (PDDA) functionalized multi-walled carbon nanotube (MWCNT) d) Glucose oxidase (GOx) immobilized PVA electrospun electrode containing Poly(diallyldimethylammonium chloride) (PDDA) functionalized single-walled carbon nanotube (SWCNT), e) Interfacially cross-linked PVA electrospun electrode containing Poly(diallyldimethylammonium chloride) (PDDA) functionalized multi-walled carbon nanotube (MWCNT) were comperatively studied. PVA electrospun nanofibers were fabricated by using electrospinning technique. Morphology and average diameter of the fibers were characterized by using Scanning Electron Microscopy (SEM). Average diameter for the neat PVA electrospun fibers were 115 nm. Carbon nanotubes were oxidatively functionalized by acid treatment and addition of functional groups after acid treatment was proved by using Raman Spectroscopy. Glucose sensing activities of the electrodes were amperometrically measured at an applied voltage -0.5 V (vs. Ag/AgCl) in 0.1M phosphate buffer solution (PBS pH 7). Glucose detection sensitivies of the electrodes were calculated as 19.6, 27.7, 67.5, 44.4, 4.0 μA mM-1cm-2 respectively.
Integration of red & blue TL materials to different polymer end-use
(Izmir Institute of Technology, 2016-07) İncel, Anıl; Demir, Mustafa; Demir, Mustafa Muammer
Triboluminescence (TL) is known as the emission of light upon the application of any mechanical force. In this master thesis, two organometallic-based TL crystals, which are EuD4TEA and Cu(NCS)(py)2(PPh3) were obtained and they were integrated in the transparent polymers: poly (methylmetacrylate) (PMMA), poly (styrene) (PS), poly (urethane) (PU) and polyvinylidene fluoride (PVDF) for different end-use. In the development of composites, two different processes were carried out: i) embedding (or blending) and ii) surface impregnation. The different end-use polymers were used as transparent polymer film, electrospun nanofibers, and nanobeads. TL performance of composites were investigated by using drop tower system which was specficically designed for this research. Atomic force microscopy (AFM), scanning electron microscopy (SEM) were used to characterize the topographic and morphologic properties of both polymers and composites. Additionally, fluorescence microscopy helped to understand the signal of emitted light by composites. Lastly, piezoelectric properties of composite materials were invetigated by oscilloscope. According to results, type of process, particle size of crystal, surface property and form of host material (polymer), the concentration of crystalline particles in composites were determined as the main parameters and the results were estimated with respect to these parameters. PU-based composite film and fiber show better stability towards mechnical stress rather than PMMA, PS, and PVDF due to the roughness surface of thin-film surface for film-based composites, smallest wickerwork formation of electrospun mats for fiber-based composite, and the chemical affinity of PU with TL crystals.
Investigation on emission features of TTBC aggregates in PVA fiber mats by electrospinning
(Izmir Institute of Technology, 2010) Özen, Bengisu; Demir, Mustafa; Demir, Mustafa Muammer
1,1’,3,3’-tetraethyl-5,5’,6,6’ -tetrachlorobenzimidazolocarbocyanine (TTBC) is a frequently used cyanine dye that undergoes two different types of molecular aggregate (J and H-type). Dye molecules, in general, come into aggregation in ionic solutions and solid surfaces without control over the type and orientation of the resulting aggregate. In this research, we focused on electrospinning of aqueous poly(vinyl alcohol) (PVA)/TTBC solutions and investigated whether the aggregate formation could be controlled by solution and instrumental parameters of this process. Initially, TTBC was molecularly dispersed in aqueous PVA solution with a weight fraction of 0.001- 0.65 % and the precursor solution was subjected to electrospinning under electrical field ranging from 0.95-1.81 kV/cm. A stationary horizontal electrospinning set-up was used including two parallel-positioned metal strips as counter electrode. Both randomlydeposited and uniaxially aligned fibers were achieved. For the reason of comparison, reference films were prepared by spin-coating and film casting. Photoluminescence and polarized FTIR spectroscopy techniques were employed to examine spectral properties of the fibers. While H- and J-type aggregates coexist within spin-coated films and only J-aggregates exist within cast films, only H-type aggregates were observed within the fibers regardless of their alignment. A strong polarized emission was obtained from the unaxially aligned fibers due to the orientation of H-aggregates along the fibers. Consequently, electrospinning was found to be an alternative method to bring individually dispersed dye molecules into oriented H-type aggregates within submicron diameter fibers. Similar experimentation was also applied to TTBC/PS and Pyrene/Polystyrene(PS) systems to investigate aggregation behavior of dye molecules. TTBC exhibited similar behavior in PS/Dimethylformamide (DMF) system observed in PVA/H2O system. However, electrospinning has no remarkable influence on aggregation of pyrene in excimers. It slightly disassemble excimer structure.
Kinetics of silica polymerization at various conditions
(01. Izmir Institute of Technology, 2022-03) Hasköylü Toker, Öykü Çağ; Demir, Mustafa; Demir, Mustafa Muammer; Baba, Alper
Silica is the most abundant element on Earth because the Earth's crust is composed mainly of metal silicates. The source of this silica is mainly volcanic rocks, which come to the surface through tectonic activity and are the primary source of heat for geothermal activity. The silica concentration in a geothermal fluid is higher than the solubility limit of natural waters, so scaling of (metal) silicates is often observed in geothermal operations. This situation has become critical for geothermal power plants. Since silicates have an insulating structure, they lead to a reduction in energy efficiency during fluid transport. The formation of silica-rich deposits should be understood to minimize the negative effects of the scaling. Briefly, silicic acid molecules in the reservoir system are condensed, and the monomeric silicic acid molecules bind to each other via covalent bonds. In the course of this reaction, dimers, tetramers and short oligomers are formed, and eventually a large polymeric silica network is formed. In the presence of metals, both the kinetics of polymerization and the structure of the network are inevitably affected. In this study, the presence of kinetic parameters (different salts such as FeCl3, MgCl2, AlCl3 and NaCl), the reaction process, the rate and the activation energy of silica polymerization at different temperatures between 25 and 90 °C were investigated. The yellow silicomolybdate method was used to determine the concentration of monomeric silica. The order of the polymerization reaction was given as 3. The polymerization occurs in the initial phase, in the first 40 minutes, where the activation energy was about 29.52 ± 2.28 kJ/mol and the rate constant was of the order of 4x10-8 mol-2∙L2∙s-1. The results also confirmed that pH has a stronger effect on the kinetics of silica polymerization than temperature. The neutral solution decreases rapidly, while the acidic solution has an induction phase in the first hour of polymerization. Different temperatures did not affect the polymerization rate as much as pH. At 25°C the experiment showed the fastest polymerization, but at 90°C the low concentration changed from the beginning. During all these experiments, no scaling of amorphous silica was observed, only the polymerization of silica.