Temel Bilimler Bölümü Bölümü Koleksiyonu

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https://hdl.handle.net/20.500.14034/38

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Investıgatıon of dıelectrıc propertıes of MBBA-5CB nematıc lıquıd crystal structures
(2022) Önsal, Gülnur
The dielectric anisotropy (????) is the response of the LC molecules to an applied electric field. In this study, the dielectric properties of 4-pentyl-4'-cyanobiphenyl (5CB) and N-(4-methoxybenzylidene)-4-butylaniline (MBBA) nematic liquid crystals with positive and negative dielectric anisotropy were investigated in detail. In addition, a structure with zero dielectric anisotropy was obtained by mixing MBBA and 5CB liquid crystals in certain rate (1:0.02). The behavior of the samples under 0V and 20V voltages was investigated in detail. Dielectric measurements were made with dielectric spectroscopy technique, which is a powerful method to investigate the molecular interaction mechanisms of liquid crystal materials, at a frequency of 1 kHz-10 MHz and a voltage of 0-20 V at room temperature. Some important parameters such as dielectric anisotropy (????), relaxation frequency (????), relaxation time (??) were determined for the samples. When electric field is applied to nematic liquid crystals, it is seen that, depending on the voltage, the relaxation frequency decreases, and the relaxation time increases.
Thermoluminescence characterization and kinetic parameters of Dy3+ activated Ca3Y2B4O12
(Elsevier, 2022) Hakami, J.; Sonsuz, M.; Kaynar, Ümit Hüseyin; Ayvacikli, M.; Oğlakçı, Mehmet; Topaksu, M.; Can, N.
In this study, thermoluminescence (TL) characteristics of Ca3Y2B4O12:xDy (0 < x < 0.07) phosphor samples were studied. The samples were exposed to beta irradiation in the dose range from 0.1 Gy to 100 Gy to investigate TL dose response. The concentration of Dy3+ in Ca3Y2B4O12 phosphor was optimized and found to be 1 mass % in terms of TL signal quality. The TL glow curve appears to be consisted of three peaks which were discernible at 72 degrees C, 280 C and 376 degrees C. The trapping parameters (E, b, and s) were calculated using initial rise (IR), and variable heating rate (VHR) techniques. The trapping parameters, order of kinetics, frequency factor, and figure of merit have been all determined by means of the Glow Curve Deconvolution (GCD) method (tgcd:An R package). Ca3Y2B4O12:Dy phosphor displays efficient thermoluminescence properties.
Thermoluminescence glow curve analysis and kinetic parameters of Eu doped Li2MoO4 ceramic phosphors
(Elsevier Sci Ltd, 2022) Souadi, G.; Oğlakçı, M.; Kaynar, Ümit Hüseyin; Correcher, V.; Benavente, J. F.; Bulcar, K.; Ayvacıklı, M.
LiMoO4: x Eu ceramic phosphors with x = 0.5, 1, 2, 3, 5, and 7 mol% were synthesized using a gel combustion method. X-ray diffraction (XRD) measurements confirmed a rhombohedral structure (space group R-3) of synthesized compounds. Following irradiation with 50 Gy beta dose, the sample doped with 5 mol% Eu exhibited the highest integrated thermoluminescence (TL) intensity. In order to evaluate dose-response, samples were irradiated with beta radiation for 10-1000 Gy. TL intensity with 1000 Gy dose without saturation yielded the highest integrated value. Different methods were employed to determine the number of peaks, the trap structure, and the kinetic parameters of the thermoluminescence glow curve of Eu doped Li2MoO4: the Hoogenstraaten method, the Booth-Bohun-Parfianovitch method, the initial rise method (IR), combined with the T-M-T-stop experiment, various heating rates (VHR), and glow curve fitting with two different software packages. Based on the glow curve deconvolution obtained using both software packages, the component TL glow peaks present in the complex glow curve are composed of well-isolated nine overlapping glow peaks. Two software packages have shown quite similar activation energies and frequency factors.
Thermoluminescence characteristics of a novel Li2MoO4 phosphor: Heating rate, dose response and kinetic parameters
(Pergamon-Elsevier Science Ltd, 2022) Souadi, G.; Kaynar, Ümit. Hüseyin; Oğlakçı, M.; Sonsuz, M.; Ayvacıklı, M.; Topaksu, M.; Canımoğlu, A.
Lithium molybdate (Li2MoO4) phosphor was synthesized by a gel combustion method and its thermoluminescence properties were studied with the irradiation of beta. Various Heating Rate (VHR), Initial Rise (IR), and Computerized Glow Curve Deconvolution (CGCD) methods were used to determine the kinetic parameters (activation energy E (eV), frequency factor s (s(-1)), and kinetic order b) of the visible glow peaks. According to the kinetic study, the TL glow curve is made up of seven separate peaks with activation energies of 1.05, 0.76, 0.40, 0.60, 0.78, 1.81 and 1.25 eV and these peaks follow general-order kinetics. The results clearly showed that undoped Li2MoO4 has a potential to be considered in dosimetric applications where high doses have to be monitored as in the case of clinical dosimetry.
Structural and temperature-dependent luminescence of Terbium doped YAl3(BO3)4 phosphor synthesized by the combustion method
(Elsevier Sci Ltd, 2022) Hakami, Jabir Wali; Kaynar, Ümit Hüseyin; Ayvacıklı, Mehmet; Çoban, Mustafa Burak; Garcia-Guinea, J.; Townsend, Peter David; Oğlakçı, Mehmet
A series of Y1-xAl3(BO3)4:x Tb3+ (x = 0.5 to 7 wt%) phosphors synthesized by a gel combustion method have been systemically investigated by X-ray diffraction (XRD), Fourier transform infrared (FTIR), energy dispersive spectroscopy (EDS), and photoluminescence (PL) as a function of temperature from 300 K to 10 K and 300 K-550 K. An XRD analysis confirms that the phosphors crystallized, and its crystal structure was analysed. The synthesized phosphor matches the XRD pattern provided in the ICSD File No 96-152-6006. The FTIR analysis indicates that nitrates and organic matter have been completely removed and the BO3 groups are present. The broad PL band peaked at 420 nm with a shoulder circa 460 nm of YAl3(BO3)4 is associated with hydrous components which attached to the sample in environmental conditions after synthesis. The PL spectra of YAl3(BO3):Tb3+ phosphors exhibit a bright and narrow green main emission peak at 543 nm corresponding to the 5D4 -> 7F5 transition under 359 nm excitation. The PL intensity increases with increasing Tb3+ ion concentration up to 5 wt %, followed by evidence for concentration quenching. There is a possibility that higher concentration quenching could be from confinement effects of localised resonant energy transfer. PL data revealed that acti-vation energies for thermal quenching at 485 nm and 543 nm were found to be 0.659 and 0.092 eV, and 0.585 and 0.087 eV, respectively.
Set star-menger and set strongly star-menger spaces
(Walter De Gruyter Gmbh, 2022) Kocinac, Ljubisa D. R.; Konca, Şükran; Singh, Sumit
Motivated by the Arhangel'skii s-Lindelof cardinal function definition, Kocinac and Konca defined and studied set covering properties and set star covering properties. In this paper, we present results on the star covering properties called set star-Menger and set strongly star-Menger. We investigate the relationship among set star-Menger, set strongly star-Menger and other related properties and study the topological properties of set star-Menger and set strongly star-Menger properties.
Modeling and optimization for adsorption of thorium (IV) ions using nano Gd:ZnO: application of response surface methodology (RSM) and artificial neural network (ANN)
(Taylor & Francis Inc, 2022) Kaynar, Ümit Hüseyin
The waste problem created by nuclear materials both in nuclear reactors and after their medical and industrial use is evaluated differently from other wastes because they can harm human and environmental health. In this study, it is aimed to study the adsorption properties of Gd ions doped nano ZnO (Gd/nano-ZnO) material synthesized by microwave assisted ignition method for the adsorption of Thorium (IV) from aqueous medium. We tested how pH (3-8), temperature (20-60 degrees C), Th (IV) concentration (25-125 mg/L) and adsorbent amount (0.005-0.08 g) affect adsorption efficiency. The best possible combinations of these parameters were examined by Response Surface Methodology (RSM) and Artificial Neural Network (ANN). R-2 values for RSM and ANN were 0.9970 and 0.9666, respectively. According to the models, the experimental adsorption capacity under the optimum conditions determined for the RSM and ANN model was found to be 192.62 mg/g and 218.47 mg/g, respectively.
Improvement of the dielectric and electro-optic properties of phthalocyanine- and quantum dot-doped nematic liquid crystals under UV illumination
(Springer, 2022) Önsal, Gülnur
In this study, dielectric and electro-optic properties of 4-pentyl-4 '-cyanobiphenyl (5CB) nematic liquid crystal (LC) and different concentrations of nickel (II) phthalocyanine (Ni(II)Pc) and cadmium selenide sulfide/zinc sulfide (CdSeS/ZnS) quantum dot (QD)-doped LC have been investigated in dark and ultraviolet (UV) illumination. Some important dielectric parameters such as the real and imaginary part of the complex dielectric constant (epsilon ' and epsilon ''), relaxation frequency (f(R)), and relaxation time (tau) have been determined. The output of the study shows that Ni(II)Pc and CdSeS/ZnS contribution and UV illumination improved the dielectric properties. Optical band gap (E-gap) values of all samples have been obtained using the UV spectroscopy technique. The E-gap value decreased after dispersion of Ni(II)Pc and CdSeS/ZnS in the pure LC. The results may be beneficial for the design of new optoelectronic devices.
Comparison of experimental measurements and machine learning predictions of dielectric constant of liquid crystals
(Indian Acad Sciences, 2022) Taşer, Pelin Yıldırım; Önsal, Gülnur; Uğurlu, Onur
In this study, we investigated the dielectric properties of the phthalocyanine (Pc)-doped nematic liquid crystal (NLC) composite structures. 4-Pentyl-4 & PRIME;-cyanobiphenyl (5CB) NLC was dispersed with 1 and 3% wt/wt Pc to investigate the doping concentration effect. Dielectric measurements of the samples were carried out using the dielectric spectroscopy method. Moreover, the real and imaginary components of the dielectric constant values were estimated based on the input parameters (frequency, voltage value and dispersion rate) using two different traditional regression algorithms (k-Nearest Neighbor and Decision Tree Regression) and five different ensemble-based regression algorithms (Extreme Gradient Boosting, Random Forest, Extra Tree Regression, Voting and Bagging using k-Nearest Neighbor as a base learner). According to the obtained results, the Extra Tree Regression algorithm had the best prediction performance on real and imaginary components of the dielectric constant values. Moreover, it is seen from the obtained results that the ensemble-based regression algorithms are more successful than the traditional ones.
Comparison of feedforward perceptron network with LSTM for solar cell radiation prediction
(Mdpi, 2022) Özdemir, Tuğba; Taher, Fatma; Ayinde, Babajide O.; Zurada, Jacek M.; Tüzün Özmen, Özge
Intermittency of electrical power in developing countries, as well as some European countries such as Turkey, can be eluded by taking advantage of solar energy. Correct prediction of solar radiation constitutes a very important step to take advantage of PV solar panels. We propose an experimental study to predict the amount of solar radiation using a classical artificial neural network (ANN) and deep learning methods. PV panel and solar radiation data were collected at Duzce University in Turkey. Moreover, we included meteorological data collected from the Meteorological Ministry of Turkey in Duzce. Data were collected on a daily basis with a 5-min interval. Data were cleaned and preprocessed to train long-short-term memory (LSTM) and ANN models to predict the solar radiation amount of one day ahead. Models were evaluated using coefficient of determination (R-2), mean square error (MSE), root mean squared error (RMSE), mean absolute error (MAE), and mean biased error (MBE). LSTM outperformed ANN with R-2, MSE, RMSE, MAE, and MBE of 0.93, 0.008, 0.089, 0.17, and 0.09, respectively. Moreover, we compared our results with two similar studies in the literature. The proposed study paves the way for utilizing renewable energy by leveraging the usage of PV panels.
A distributed depth first search based algorithm for edge connectivity estimation
(IEEE, 2020) Uğurlu, Onur; Akram, Vahid Khalilpour; Eliiyi, Deniz Türsel
The edge connectivity of a network is the minimum number of edges whose removal disconnect the network. The edge connectivity determines the minimum number of edge-disjoint paths between all nodes. Hence finding the edge connectivity can reveal useful information about reliability, alternative paths and bottlenecks. In this paper, we propose a cost-effective distributed algorithm that finds a lower bound for the edge connectivity of a network via finding at most c depth-first-search trees, where c is the edge connectivity. The proposed algorithm is asynchronous and does not need any synchronization between the nodes. In the proposed algorithm, the root node starts a distributed depth-first-search algorithm, and the nodes select next node in the tree based on their available edges to maximize the total number of established trees. The simulation results show that the proposed algorithm finds the edge connectivity with an average of 48% accuracy ratio.
Whale optimization and sine-cosine optimization algorithms with cellular topology for parameter identification of chaotic systems and Schottky barrier diode models
(Springer, 2021) Turgut, Mert Sinan; Şağban, Hüseyin Muzaffer; Turgut, Oğuz Emrah; Özmen, Özge Tüzün
This research study aims to enhance the optimization accuracy of the two recently emerged metaheuristics of whale and sine-cosine optimizers by means of the balanced improvements in intensification and diversification phases of the algorithms provided by cellular automata (CA). Stagnation at the early phases of the iterations, which leads to entrapment in local optimum points in the search space, is one of the inherent drawbacks of the metaheuristic algorithms. As a favorable solution alternative to this problem, different types of cellular topologies are implemented into these two algorithms with a view to ameliorating their search mechanisms. Exploitation of the fertile areas in the search domain is maintained by the interaction between the topological neighbors, whereas the improved exploration is resulted from the smooth diffusion of the available population information among the structured neighbors. Numerical experiments have been carried out to assess the optimization performance of the proposed cellular-based algorithms. Optimization benchmark problems comprised of unimodal and multimodal test functions have been applied and numerical results have been compared with those found by some of the state-of-the-art literature optimizers including particle swarm optimization, differential evolution, artificial cooperative search and differential search. Cellular variants have been outperformed by the base algorithms for multimodal benchmark problems of Levy and Penalized1 functions. Then, the proposed cellular algorithms have been applied to two different parameter identification cases in order to test their efficiencies on real-world optimization problems. Extensive performance evaluations on different parameter optimization cases reveal that incorporating the CA concepts on these algorithms not only improves the optimization accuracy but also provides considerable robustness to acquired solutions.
Synthesis and enhanced photoluminescence of the BaSiF6:Dy3+ phosphors by Li+ doping via combustion method
(Elsevier, 2022) Souadi, G. O.; Kaynar, Ümit H.; Ayvacıklı, M.; Canımoğlu, A.; Can, N.
Undoped BaSiF6, Dy3+ doped BaSiF6, and Dy3+, Li+ co-doped BaSiF6 phosphors were synthesized through a gelcombustion method. The prepared samples were characterized by powder x-ray diffraction (XRD), Fourier transform infrared (FTIR), energy dispersive x-ray spectroscopy (EDS), and photoluminescence (PL) techniques. The XRD data revealed that both the Dy3+ doped and Li+ co-doped BaSiF6 phosphors exhibited a single-phase structure belonging to the space group R (3m) over bar which matched well with the standard JCPDS files (No. 002-6613). FTIR spectra showed absorption bands at 3417 cm -1 , 1640 cm(-1), and 1620 cm(-1) corresponding to water molecules. EDS analysis confirmed the chemical composition of the prepared samples. The PL emission spectra of BaSiF6:Dy3+ by different co-doping concentrations of Li+ exhibited prominent emission peaks at 490 nm, 572 nm, 672 nm and 758 nm. The incorporation of Li+ is beneficial for enhancing the photoluminescence intensity. The optimum Li+ amount was 8% for BaSiF6:Dy3+ and then started to decrease. The enhancement could be due to the occurrence of oxygen vacancies due to the incorporation of Li+ ions. The x = 0.301 and y = 0.361 coordinates of this phosphor with varying Li+ dopant concentration determined by the Commission Internationale de l'Eclairage (CIE - 1931) were in the white range. The present work demonstrates how a simple and effective method can be used to prepare novel nanophosphors for applications in the field of visible light emitting devices with enhanced white emission.
Solid phase epitaxial thickening of boron and phosphorus doped polycrystalline silicon thin films formed by aluminium induced crystallization technique on glass substrate
(Elsevier Science Sa, 2019) Özmen, Özge Tüzün; Karaman, Mehmet; Sedani, Salar Habibpur; Şağban, Hüseyin Muzaffer; Turan, Raşit
Aluminium induced crystallization (AIC) technique can be used to form the high-quality and large-grained polycrystalline silicon (poly-Si) thin films, which are with the thickness of similar to 200 nm and used as a seed layer, on silicon nitride coated glass substrate. Thanks to aluminium metal in AIC process, the natural doping of AIC thin films is p(+) type (similar to 2 x 10(18) cm(-3)). On the other hand, recombination of carriers can be controlled by partial doping through the defects that may have advantages to improve the thin film quality by the overdoping induced passivation. In this study, boron (B) and phosphorus (P) doped AIC seed layers were thicken to similar to 2 mu m by solid phase epitaxy (SPE) technique at 800 degrees C for 3 h under nitrogen flow in a tube furnace. During the crystallization annealing, exodiffusion of dopants was formed through the SPE film from the AIC seed layer. Optical microscope and electron back scattering diffraction technique (EBSD) were used to analyse the structural quality of the Si films. The poly-Si layer with an average grain size value of similar to 32 mu m was formed by AIC + SPE technique for P doped samples while EBSD analysis gave no results for B doped samples due to the quite deterioration on the surface of the films. AIC + SPE films were analyzed in terms of structural properties by using micro-Raman Spectroscopy and X-ray diffraction systems. The results showed that the crystallinity of compressive stress formed AIC + SPE films reached up to 98.55%. Additionally, the Raman analysis pointed out that no temperature-induced stress were generated in the AIC + SPE films while compressive stress was induced by increasing the annealing duration for doped AIC film. For all samples, the preferred orientation was < 100 >, and the crystallite size up to 44.4 nm was formed by phosphorus doping of AIC films. The doping efficiency was determined by time-of-flight secondary ion mass spectroscopy for doped samples. A graded n(+)n doping profile was obtained by exo-diffusion of phosphorus from the overdoped seed layer during the epitaxial thickening while boron doping of SPE film has failed with exo-diffusion of boron from AIC seed layer into SPE film. Finally, high-quality n(+)n type poly-Si films were fabricated on glass substrate by using AIC + SPE technique.
Enhancing the blue luminescence behaviour of the Li co -doped novel phosphor ZnB 2 O 4: Tm 3+
(Elsevier Science Sa, 2020) Kücük, N.; Kaynar, Ümit H.; Akça, S.; Alajlani, Y.; Yin, L.; Wang, Y.; Can, N.
[No Abstract Available]
Set-Menger and related properties
(Elsevier, 2020) Kocinac, Ljubisa D. R.; Konca, Şükran
In this work, we consider new types of covering properties called set-Menger, set-Rothberger, set-Hurewicz and set-Gerlits-Nagy. We investigate the relationships between set-Mengerness and some other Menger-type covering properties (such as Mengerness, weak Mengerness, quasi-Mengerness) and study topological properties of set-Menger and related spaces. In particular, we give a game-theoretic characterization of set-Menger spaces. (C) 2019 Elsevier B.V. All rights reserved.
Variations of some star selection properties
(Amer Inst Physics, 2021) Kocinac, Ljubisa D. R.; Konca, Şükran; Singh, Sumit
In this paper we introduce some new types of star covering properties which we call set (strongly) star Menger, set (strongly) star Rothberger, set (strongly) star Hurewicz properties. We consider also weaker versions of these properties, discuss their relationships with some other selective covering properties, and study topological properties of introduced classes of spaces.
Influence of laser excitation power on temperature-dependent luminescence behaviour of Ce- and Tb-incorporated BaMgAl10O17 phosphors
(Pergamon-Elsevier Science Ltd, 2020) Kaynar, Ümit H.; Kaynar, S. Cam; Ayvacıklı, M.; Karabulut, Y.; Souadi, G. O.; Can, N.
BaMgAl10O17 (BAM) is a highly suitable host lattice for various rare earth ions with excellent luminescence properties in different spectral regions, including a strong photoluminescence (PL) emission from the visible spectral region. A new Ce- and Tb-incorporated BaMgAl10O17 phase was synthesized successfully using a wet combustion method and it was studied as a function of the temperature and laser excitation power. We further characterize the obtained phosphors with X-ray diffraction at room temperature. Different fuel/oxidant (f/o) ratios were introduced to investigate the optimum synthesis conditions for the BAM phosphors and optimum ratio was found out to be 8. The photoluminescence (PL) spectra were collected under the excitation light generated by a Nd:YLF pulse laser at 349 nm as the temperature was increased from 10 K to 300 K. A strong green emission of Tb3+ was observed in the green region of the spectrum due to the D-5(4)-> F-7(J) transition. We also observed a wide emission band from the Ce3+ ion in the wavelength range of 350-650 nm. The luminescence intensities of all phosphors exhibited different patterns with an increase in the temperature. We also evaluated how the PL spectrum of the rare earth-activated BAM host matrix shifts under various laser excitation powers. The PL intensity of Ce-activated BAM significantly shifted (similar to 30 A) to the blue region of the spectrum with an increase in the laser excitation power, however we did observed no shift forTb(3+) activated BAM. The present findings suggest that Tb-incorporated BaMgAl10O17 can be effective as a green phosphor candidate material with a wide range of applications.
Eu3+ and Dy3+ doped La2MoO6 and La2Mo2O9 phosphors: Synthesis and luminescence properties
(Pergamon-Elsevier Science Ltd, 2020) Kaynar, Ümit H.; Kaynar, Sermin Çam; Alajlani, Yahya; Ayvacıklı, Mehmet; Karalı, Elçin Ekdal; Karabulut, Yüksel; Can, Nurdoğan
We report a detailed structural analysis and properties of the photoluminescence (PL) and thermoluminescence (TL) spectra of Eu3+ and Dy3+ incorporated into novel La2MoO6 and La2Mo2O9 phosphors synthesized successfully through gel combustion synthesis. The formation of a tetragonal phase and a cubic structure were verified for La2MoO6 and La2Mo2O9 phosphors via X-ray diffraction (XRD) studies. Dy doped samples exhibited blue and green emissions at 480 nm (F-4(9/2) -> H-6(15/2)) and 572 nm (F-4(9/2) -> H-6(13/2)), and also Eu doped samples showed a sharp emission peaks at 612 and 619 nm (D-5(0) -> F-7(2)) upon 349 nm pulse laser excitation. Peak shape (PS) technique was utilised to determine activation energy, frequency factor and order of kinetics associated with the main glow curves in undoped and Eu and Dy doped samples after X-ray irradiation. The present findings suggest that Eu and Dy incorporated La2MoO6 and La2Mo2O9 phosphors are highly auspicious candidates for applications in solid-state lighting.
PCDTBT:PCBM tabanlı organik Schottky diyotlarının ve Heteroeklem güneş hücrelerinin optoelektronik karakterizasyonu
(2019) Gökşen, Kadir; Kurtay, Merve Zayim; Özmen, Özge Tüzün; Şağban, Hüseyin Muzaffer; Köysal, Oğuz
Bu çalışmada Gümüş/n-tipi Silisyum/poly[N-9?-heptadecanyl-2,7–carbazole–alt-5,5-(4?,7?-di–2–thienyl-2?,1?,3?-benzothiadiazole:[6,6]-phenyl-C61-butyric acid methyl ester/Altın (Ag/n-Si/PCDTBT:PC61BM/Au) organik metalpolimer yarıiletken Schottky bariyer diyotları, polimer arayüz olarak %20 PCDTBT ve %80 PCBM karışımı kullanılaraküretilmiştir. Üretilen diyotların 240 - 350 K sıcaklık aralığında akım-voltaj ölçümleri yapılarak, bu sıcaklıklardaki diyotidealite faktörü değerlerinin 1,80 ve 2,26 aralığında değiştiği gözlemlenmiştir. Çalışmanın ikinci aşamasında, İndiyumKalay Oksit/PCDTBT:PCBM/Gümüş (ITO/PCDTBT:PCBM/Ag) organik güneş hücresi %20:%80 PCDTBT:PCBMhacimsel oranına sahip olacak şekilde üretilmiş ve güç dönüştürme verimi % 0,85 olarak bulunmuştur.

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