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Article Citation - WoS: 2Citation - Scopus: 2Analysis of Optical Constants and Temperature-Dependent Absorption Edge of Gas0.75se0.25< Layered Crystals(Pergamon-elsevier Science Ltd, 2017) Isik, Mehmet; Gasanly, NizamiGaS0.75Se0.25 single crystals were optically characterized through transmission and reflection measurements in the wavelength range of 450-1000 nm. Derivative spectrophotometry analyses on temperature dependent transmittance spectra showed that band gap energies of the crystal increase from 239 eV (T=300 K) to 2.53 eV (T=10 K). Band gap at zero temperature, average phonon energy, electron phonon coupling parameter and rates of change of band gap energy with temperature were found from the temperature dependences of band gap energies under the light of different models reported in literature. Furthermore, the dispersion of room temperature refractive index was discussed in terms of single effective oscillator model. The refractive index dispersion parameters, namely oscillator and dispersion energies, zero-frequency refractive index, were determined as a result of analyses. (C) 2017 Elsevier Ltd. All rights reserved.Article Citation - WoS: 199Citation - Scopus: 195Existence and uniqueness of a common fixed point on partial metric spaces(Pergamon-elsevier Science Ltd, 2011) Abdeljawad, T.; Karapinar, E.; Tas, K.In this work, a general form of the weak phi-contraction is considered on partial metric spaces, to get a common fixed point. It is shown that self-mappings S, T on a complete partial metric space X have a common fixed point if it is a generalized weak phi-contraction. (C) 2011 Elsevier Ltd. All rights reserved.Article Citation - WoS: 11Citation - Scopus: 12A Low Band Gap Polymer Based on Selenophene and Benzobis (thiadiazole)(Pergamon-elsevier Science Ltd, 2017) Abdulrazzaq, Mohammed; Ozkut, Merve Icli; Gokce, Gurcan; Ertan, Salih; Tutuncu, Esra; Cihaner, AtillaA new derivative of benzobis(thiadiazole) based donor-acceptor-donor type monomers, namely 4,7-di (selenophen-2-yl)benzo[1,2-c;4,5-c']bis[1,2,5]thiadiazole (SeBTSe), was synthesized and its polymerization was carried out successfully via electrochemical polymerization in an electrolyte solution of 0.1 M tetrabutylammonium hexafluorophosphate dissolved in dichloromethane. The monomer SeBTSe is a deep red chromophore and it has four redox states: one oxidation, one neutral and two reduction states. The electrochemical behaviour of the corresponding polymer called PSeBTSe was studied by cyclic and differential pulse voltammetry. There is a good agreement between electrochemical (0.62-0.66 eV) and optical (0.63 eV) bandgaps of the polymer. Like the monomer, the ambipolar polymer has four redox states and electrochromic properties: gray beige at neutral state, smoky azurite at oxidized state, beige at first reduced state and dark beige at second reduced state. (C) 2017 Elsevier Ltd. All rights reserved.Article Citation - WoS: 29Citation - Scopus: 30The Temporal Communication Behaviors of Global Software Student Teams(Pergamon-elsevier Science Ltd, 2012) Swigger, Kathleen; Hoyt, Matthew; Serce, Fatma Cemile; Lopez, Victor; Alpaslan, Ferda NurThis paper examines the global software development process by using content analysis techniques, as described in an earlier study (Serce et al., 2011), to determine time-variant patterns of communication behaviors among student teams engaged in a global software development project. Data gathered from two software development projects involving students in the US, Panama, and Turkey were used to determine how globally distributed team behavior is temporally patterned in complex ways. A formal, quantitative methodology for time variant analysis of the transcripts of global software student teams based on content analysis is established. Results from the analysis suggest a positive correlation between a team's temporal communication patterns and project outcomes as well as a relationship between variations in communication behaviors and different phases of the software development cycle. The research also found that the temporal variations in communication behaviors between software phases were similar for the two projects. Such findings are intended to strengthen the case for developing new temporal measures for analyzing groups and teams. (C) 2011 Elsevier Ltd. All rights reserved.Article Citation - WoS: 2Citation - Scopus: 2Trap Distribution in Agin5s8< Single Crystals: Thermoluminescence Study(Pergamon-elsevier Science Ltd, 2018) Delice, S.; Işık, Mehmet; Isik, M.; Gasanly, N. M.; Işık, Mehmet; Department of Electrical & Electronics Engineering; Department of Electrical & Electronics EngineeringDistribution of shallow trap levels in AgIn5S8 crystals has been investigated by thermoluminescence (TL) measurements performed below room temperature (10-300 K). One broad TL peak centered at 33 K was observed as constant heating rate of 0.2 K/s was employed for measurement. The peak shape analysis showed that the TL curve could consist of several individual overlapping TL peaks or existence of quasi-continuous distributed traps. Therefore, TL experiments were repeated for different stopping temperatures (T-stop) between 10 and 34 K with constant heating rate of 0.2 K/s to separate the overlapping TL peaks. The E-t vs T-stop indicated that crystal has quasi-continuously distributed traps having activation energies increasing from 13 to 41 meV. Heating rate effect on trapped charge carriers was also investigated by carrying out the TL. experiments with various heating rates between 0.2 and 0.6 K/s for better comprehension of characteristics of existed trap levels. Analyses indicated that the trap levels exhibited the properties of anomalous heating rate behavior which means the TL intensity and area under the TL peak increase with increasing heating rate.Article Citation - WoS: 2Citation - Scopus: 3Effect of Phthalimide and Thieno[3,4-C]pyrrole Acceptors on P-Conjugated Donor-Acceptor Monomers: Experimental and Theoretical Investigations of Photophysical and Electrochemical Properties(Pergamon-elsevier Science Ltd, 2023) Cakal, Deniz; Arabaci, Elif Demir; Yildirim, Erol; Cihaner, Atilla; Onal, Ahmet M.In this study, two novel donor-acceptor-donor monomers were synthesized by coupling a phthalimide (PI) acceptor unit with thiophene (T) and ethylenedioxythiophene (EDOT (E)) donor units. These two novel PI-based monomers, namely 2-(2-ethylhexyl)-4,7-di(thiophen-2-yl)isoindoline-1,3-dione (T-PI) and 4,7-bis(2,3-dihydrothieno[3,4-b] [1,4]dioxin-5-yl)-2-(2-ethylhexyl)isoindoline-1,3-dione (E-PI), were fully characterized via UV-vis and fluorescence spectroscopy as well as electrochemical methods, and the results were directly compared with those of previously synthesized thieno[3,4-c]pyrrole-4,6-dione (TPD)-based analogues to report the results for four classes of monomers. In this regard, the ef-fects of different donor groups and structural modifications in the acceptor unit as changed from PI to TPD have been investigated to reveal the precise effect of the donor and acceptor on the optical and electrochemical properties of the monomers. All monomers exhibits an intramolecular charge transfer band in the low energy region due to strong donor and acceptor interactions. The oxidation and reduction potentials of the monomers are found to vary depending on the strengths of both donor and acceptor units. PI-based monomers show more positive oxidation and less negative reduction potentials than TPD-based analogues. Similarly, thiophene-based analogues reveal the same trend in their oxidation and reduction potentials as compared to EDOT-based analogues. Lastly, PI-based monomers could not undergo polymerization via electrochemical method, unlike TPD-based monomers, since the dihedral angle arising between donor and PI acceptor disturbs molecular planarity significantly, as demonstrated by first principle calculations.(C) 2023 Elsevier Ltd. All rights reserved.Article Citation - WoS: 6Citation - Scopus: 15Investigation of a Gas Turbine-Modular Helium Reactor Using Reactor Grade Plutonium With 232th and 238u(Pergamon-elsevier Science Ltd, 2016) Sahin, Sumer; Erol, Ozgur; Sahin, Haci MehmetUtilization of natural uranium (nat-U) and thorium as fertile fuels has been investigated by in a Gas Turbine - Modular Helium Reactor (GTMHR) using reactor grade plutonium as driver fuel. A neutronic analysis for the full core reactor was performed by using MCNP5 with ENDF/B-VI cross-section library. Different mixture ratios were tested in order to find the appropriate mixture ratio of fertile and fissile fuel particles that gives a comparable k(eff) value of the reference uranium fuel. Time dependent calculations were performed by using MONTEBURN2.0 with ORIGEN2.2 for each selected mixture. Different parameters (operation time, burnup value, fissile isotope change, etc.) were subject of performance comparison. The operation time and burnup values were close to each other with nat-U and thorium, namely 3205 days and 176 GWd/MTU for the former and 3175 days 181 GWd/MTU for the latter fertile fuel. In addition, the fissile isotope amount changed from initially 6940.1 kg-4579.2 kg at the end of its operation time for nat-U. These values were obtained for thorium as 6603.3 kg-4250.2 kg, respectively. (C) 2016 Elsevier Ltd. All rights reserved.Article Citation - WoS: 13Citation - Scopus: 15A Novel Data Encryption Method Using an Interlaced Chaotic Transform(Pergamon-elsevier Science Ltd, 2024) Gokcay, Erhan; Tora, HakanWe present a novel data encryption approach that utilizes a cascaded chaotic map application. The chaotic map used in both permutation and diffusion is Arnold's Cat Map (ACM), where the transformation is periodic and the encrypted data can be recovered. The original format of ACM is a two-dimensional mapping, and therefore it is suitable to randomize the pixel locations in an image. Since the values of pixels stay intact during the transformation, the process cannot encrypt an image, and known-text attacks can be used to get back the transformation matrix. The proposed approach uses ACM to shuffle the positions and values of two-dimensional data in an interlaced and nested process. This combination extends the period of the transformation, which is significantly longer than the period of the initial transformation. Furthermore, the nested process's possible combinations vastly expand the key space. At the same time, the interlaced pixel and value transformation makes the encryption highly resistant to any known-text attacks. The encrypted data passes all random-data tests proposed by the National Institute of Standards and Technology. Any type of data, including ASCII text, can be encrypted so long as it can be rearranged into a two-dimensional format.Review Citation - WoS: 71Citation - Scopus: 87Vision 2023: Assessing the Feasibility of Electricity and Biogas Production From Municipal Solid Waste in Turkey(Pergamon-elsevier Science Ltd, 2013) Melikoglu, MehmetTurkey imports most of its energy. However, according to the recently avowed Vision 2023 agenda the country also plans to produce 30% of its electricity demand from renewable energy sources by 2023. Meanwhile, each year around 25 million tonnes of municipal solid waste (MSW) is generated nationwide. Not only MSW pollutes the environment handling, processing and storage requires precious labour and capital. In that context, a synergistic solution can be created between MSW management and energy supply. In this study, economics and environmental impacts of electricity generation from MSW via (i) direct combustion and (ii) biogas harnessing in 81 cities of Turkey is analysed in detail for a period between 2012 and 2023. Firstly, it is estimated that nationwide 8500 GWh of electricity could have been generated by direct combustion of MSW in 2012. This is predicted to rise 9700 GWh in 2023. It is calculated that 3100 million m(3) of methane would be emitted from the landfills of Turkey in 2012. If no action taken this would rise to 3600 million m(3) in 2023. Furthermore, it is estimated that by capturing 25% of this methane via landfill bioreactors 2900 GWh or 0.5% of Turkey's annual electricity demand could be supplied in 2023. Simulations also showed that by realizing apposite landfill investments by 2023 annual energy savings worth 200-900 million (sic) could be generated from MSW. Consequently, this could lead to greenhouse gas emission savings up to 11.0 million tonnes of CO2 per annum. (C) 2012 Elsevier Ltd. All rights reserved.Conference Object Citation - WoS: 75Citation - Scopus: 92Modeling and Simulation of a Hybrid Photovoltaic (pv) Module-Electrolyzer Fuel Cell System for Micro-Cogeneration Applications(Pergamon-elsevier Science Ltd, 2015) Ozgirgin, Ekin; Devrim, Yilser; Albostan, AyhanThe rising cost of energy and power, depreciation of natural resources like fossil fuels and the global warming issues have all led the need for developing advanced clean energy systems. Hydrogen, which is clean energy carrier, can be produced by using solar electric energy from photovoltaic (PV) modules for the water electrolysis without emitting carbon dioxide. Modeling of PV module-electrolyzer hydrogen system is important for their planning and control strategies in many applications. In this respect, high-efficiency cogeneration systems for producing both heat and electricity coupled with clean energy sources such as PVs and fuel cells are gaining more attention, due to their advantages in terms of increasing efficiency and power quality, reducing harmful emissions and flexibility of operation. This study describes the analysis of the PV module-fuel cell hybrid system for house-hold micro co-generation applications. The system consists of PV modules, batteries, proton exchange membrane type water electrolyzer and proton exchange membrane fuel cell (PEMFC). The excess heat of PEMFC was used to supply hot water and/or heating energy of the house. Electrical energy was stored in the batteries. The analysis of the PV-electrolyzer-PEMFC system can be further used for designing co-generation systems for various application optimizing the PV module, electrolyzer and PEMFC sizes. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. 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