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Citation Count: 5
2-D digital filter realization without overflow oscillations
(Pergamon-elsevier Science Ltd, 2013) Singh, Vimal; Department of Mechatronics Engineering
A novel criterion for the elimination of overflow oscillations in 2-D state-space digital filters described by the Roesser model employing two's complement overflow arithmetic is presented. The criterion takes the form of linear matrix inequality (LMI) and, hence, is computationally tractable. The criterion is a generalization and improvement over an earlier criterion. An example shows the effectiveness of the new criterion. (C) 2012 Elsevier Ltd. All rights reserved.
Citation Count: 27
Acoustic Control of Flow over NACA 2415 Airfoil at Low Reynolds Numbers
(Asce-amer Soc Civil Engineers, 2016) Genc, M. Serdar; Acikel, H. Hakan; Akpolat, M. Tugrul; Ozkan, Gokhan; Karasu, Ilyas; Airframe and Powerplant Maintenance
In this study, the effects of acoustic excitation frequency on flow over an NACA 2415 airfoil were determined, and all of the experiments were done both with and without the presence of the acoustic excitation. The acoustic excitation was applied for a range of angles of attack (0 degrees-25 degrees) and Reynolds numbers of 50,000, 75,000, 100,000, 150,000, and 200,000. To examine the effects of acoustic excitation on the flow, force measurements, pressure measurements, hot-wire anemometry, smoke-wire flow-visualization, and particle image velocimetry techniques were employed. The results indicated that for stall and some limited poststall angles of attack of the acoustic excitation having a frequency in a certain range forced the separated shear layer to reattach to the surface of the airfoil. As the Reynolds number increased, the effective excitation frequency increased, but the range of Zaman number [St/(R-1/2)] was the same. With the acoustic excitation, the stall angle was delayed from 12 degrees to 16 degrees at R = 50,000, and there was a 30% and 50% increase on the maximum value of the lift coefficient and the ratio of the lift and drag forces, respectively. Moreover, the stall angle was delayed from 13 degrees to 17 degrees at R = 75,000, from 15 degrees to 18 degrees at R = 100,000, from 15 degrees to 17 degrees at R = 150,000. Furthermore, it was concluded that acoustic excitation shrunk laminar separation bubble, and an effect of the acoustic control on the separation bubble decreased as the Reynolds number increased. (C) 2016 American Society of Civil Engineers.
Citation Count: 1
Adaptive Stimulus Design for Dynamic Recurrent Neural Network Models
(Frontiers Media Sa, 2019) Doruk, R. Ozgur; Zhang, Kechen; Electrical-Electronics Engineering
We present an adaptive stimulus design method for efficiently estimating the parameters of a dynamic recurrent network model with interacting excitatory and inhibitory neuronal populations. Although stimuli that are optimized for model parameter estimation should, in theory, have advantages over nonadaptive random stimuli, in practice it remains unclear in what way and to what extent an optimal design of time-varying stimuli may actually improve parameter estimation for this common type of recurrent network models. Here we specified the time course of each stimulus by a Fourier series whose amplitudes and phases were determined by maximizing a utility function based on the Fisher information matrix. To facilitate the optimization process, we have derived differential equations that govern the time evolution of the gradients of the utility function with respect to the stimulus parameters. The network parameters were estimated by maximum likelihood from the spike train data generated by an inhomogeneous Poisson process from the continuous network state. The adaptive design process was repeated in a closed loop, alternating between optimal stimulus design and parameter estimation from the updated stimulus-response data. Our results confirmed that, compared with random stimuli, optimally designed stimuli elicited responses with significantly better likelihood values for parameter estimation. Furthermore, all individual parameters, including the time constants and the connection weights, were recovered more accurately by the optimal design method. We also examined how the errors of different parameter estimates were correlated, and proposed heuristic formulas to account for the correlation patterns by an approximate parameter-confounding theory. Our results suggest that although adaptive optimal stimulus design incurs considerable computational cost even for the simplest excitatory-inhibitory recurrent network model, it may potentially help save time in experiments by reducing the number of stimuli needed for network parameter estimation.
Citation Count: 20
Adopting automated whitelist approach for detecting phishing attacks
(Elsevier Advanced Technology, 2021) Azeez, Nureni Ayofe; Misra, Sanjay; Margaret, Ihotu Agbo; Fernandez-Sanz, Luis; Abdulhamid, Shafi'i Muhammad; Computer Engineering
Phishing is considered a great scourge in cyberspace. Presently, there are two major challenges known with the existing anti-phishing solutions. Low detection rate and lack of quick access time in a real-time environment. However, it has been established that blacklist solution methods offer quick and immediate access time but with a low detection rate. This research paper presents an automated white-list approach for detecting phishing attacks. The white-list is determined by carrying out a detailed analysis between the visual link and the actual link. The similarities of the known trusted site are calculated by juxtaposing the domain name with the contents of the whitelist and later match it with the IP address before a decision is made and further analyzing the actual link and the visual link by calculating the similarities of the known trusted site. The technique then takes a final decision on the extracted information from the hyperlink, which can also be obtained from the web address provided by the user. The experiments carried out provided a very high level of accuracy, specifically, when the dataset was relatively at the lowest level. Six different datasets were used to perform the experiments. The average accuracy obtained after the six experiments was 96.17% and the approach detects phishing sites with a 95.0% true positive rate. It was observed that the level of accuracy varies from one dataset to another. This result shows that the proposed method performs better than similar approaches benchmarked with. The efficiency of the approach was further established through its computation time, memory, bandwidth as well as other computational resources that were utilized with the minimum requirements when compared with other approaches. This solution has provided immense benefits over the existing solutions by reducing the memory requirements and computational complexity, among other benefits. It has also shown that the proposed method can provide more robust detection performances when compared to other techniques. (c) 2021 Elsevier Ltd. All rights reserved.
Advancing Mmwave Altimetry for Unmanned Aerial Systems: a Signal Processing Framework for Optimized Waveform Design
(Mdpi, 2024) Awan, Maaz Ali; Dalveren, Yaser; Kara, Ali; Derawi, Mohammad; Department of Electrical & Electronics Engineering
This research advances millimeter-wave (mmWave) altimetry for unmanned aerial systems (UASs) by optimizing performance metrics within the constraints of inexpensive automotive radars. Leveraging the software-defined architecture, this study encompasses the intricacies of frequency modulated continuous waveform (FMCW) design for three distinct stages of UAS flight: cruise, landing approach, and touchdown within a signal processing framework. Angle of arrival (AoA) estimation, traditionally employed in terrain mapping applications, is largely unexplored for UAS radar altimeters (RAs). Time-division multiplexing multiple input-multiple output (TDM-MIMO) is an efficient method for enhancing angular resolution without compromising the size, weight, and power (SWaP) characteristics. Accordingly, this work argues the potential of AoA estimation using TDM-MIMO to augment situational awareness in challenging landing scenarios. To this end, two corner cases comprising landing a small-sized drone on a platform in the middle of a water body are included. Likewise, for the touchdown stage, an improvised rendition of zoom fast Fourier transform (ZFFT) is investigated to achieve millimeter (mm)-level range accuracy. Aptly, it is proposed that a mm-level accurate RA may be exploited as a software redundancy for the critical weight-on-wheels (WoW) system in fixed-wing commercial UASs. Each stage is simulated as a radar scenario using the specifications of automotive radar operating in the 77-81 GHz band to optimize waveform design, setting the stage for field verification. This article addresses challenges arising from radial velocity due to UAS descent rates and terrain variation through theoretical and mathematical approaches for characterization and mandatory compensation. While constant false alarm rate (CFAR) algorithms have been reported for ground detection, a comparison of their variants within the scope UAS altimetry is limited. This study appraises popular CFAR variants to achieve optimized ground detection performance. The authors advocate for dedicated minimum operational performance standards (MOPS) for UAS RAs. Lastly, this body of work identifies potential challenges, proposes solutions, and outlines future research directions.
Citation Count: 4
Affective experiences of the parents of children with cancer: A qualitative study
(Springer, 2020) Demirtepe-Saygili, Dilek; Bozo, Ozlem; Department of Psychology
The present study aims to examine the affective experiences of the parents of children with cancer qualitatively. Semi-structured interviews were conducted with parents, whose children were being treated for different types of childhood cancers. Three affect themes, which were (a) negative affect (sadness, anxiety guilt, and anger), (b) emotion suppression, and (c) positive affect were revealed as a result of the transcribed and qualitatively analyzed interviews. Theoretical explanations were provided for each theme. Negative affect was the most predominant theme, yet positive affect was also present among parents, and emotion suppression was an important part of the affective experiences. The findings offered a descriptive picture about the emotions of parents with respect to caregiving a child with cancer. It was suggested that normalization and ventilation of the emotions can help the parents cope better with childhood cancers.
Citation Count: 11
Ag nanostructures on a poly(3,4-ethylenedioxythiophene) film prepared with electrochemical route: A controllable roughened SERS substrate with high repeatability and stability
(Pergamon-elsevier Science Ltd, 2012) Dogan, Uzeyir; Kaya, Murat; Cihaner, Atilla; Volkan, Murvet; Chemical Engineering
A simple, reliable and reproducible one-step electrochemical method for the preparation of surface-enhanced Raman-active polymer-mediated silver nanoparticles (Ag NPs) on planar indium tin oxide (ITO) coated glass substrates was reported. Poly(3,4-ethylenedioxythiophene) (PEDOT) film was used as a support material for dispersing nanostructured silver nanostructures on the surface homogeneously, since 3,4-ethylenedioxythiophene (EDOT) monomer polymerizes regioregularly. The optical properties and morphologies of the silver substrates have been investigated by ultraviolet-visible (UV-vis) spectroscopy and field emission scanning electron microscopy (FE-SEM). The UV-vis and FE-SEM results revealed that the Ag nanostructures separately appeared on the PEDOT coated ITO after reduction. The effect of the thickness of PEDOT polymer film, reduction potential of silver, the concentration of silver ion solution and the amount of silver particle on the polymer film on the SERS response were studied as well as repeatability and temporal stability of prepared substrates. Brilliant cresyl blue (BCB) has been used as Raman probes to evaluate the properties of the new SERS substrates. Signals collected over multiple spots within the same substrate resulted in a relative standard deviation (RSD) of 9.34%, while an RSD of 11.05% was measured in signals collected from different substrates. The SERS-active substrates were robust and stable which lost only 5.71% of initial intensity after 1 month. (C) 2012 Elsevier Ltd. All rights reserved.
Citation Count: 11
Airport master planning in Turkey; planning and development problems and proposals
(Elsevier Sci Ltd, 2013) Saldiraner, Yildirim; Aviation Management
The purpose of this article is to draw attention to the problems experienced during the enlargement and development of existing airport facilities in Turkey, where air transport has been undergoing rapid growth over the past decade, and to provide recommendations to overcome these problems. In this study, civil air transport developments, bottlenecks, trends, and expectations worldwide and in Turkey are investigated. The earliest and latest master plan studies related to Ankara Esenboga, Antalya, Istanbul Ataturk and Sabiha Gokcen, and Izmir Adnan Menderes airports are examined and evaluated and proposals are made regarding the recurring planning problems. The analysis of earlier master plans, which were not brought into effect, indicated that some of the existing applications, such as the co-ordination, planning, and implementation procedures regarding the airport operators/state-regional planning authorities, need to be changed to clear the way for the development of airports. Otherwise, the latest master plans also will not be fully applicable and existing problems will continue to hinder airport development. (C) 2013 Elsevier Ltd. All rights reserved.
Citation Count: 7
Algorithm for adaptive learning process and improving learners' skills in Java programming language
(Wiley, 2018) Gavrilovic, Nebojsa; Arsic, Aleksandra; Domazet, Dragan; Mishra, Alok; Software Engineering
Adaptive approaches within distance learning systems enable adapting teaching process to the needs of each learner during the learning process. This paper presents an algorithm for creating an adaptive learning process that provides knowledge and skills improvement for learners in the Java programming language. Also, it presents the application of the tool that checks the learner's knowledge through solving practical tasks from the Java programming language. The adaptive learning process in this work leads the learner through teaching materials and practical tasks where the acquired knowledge is required to be applied. Also, the algorithm, based on the measurement of knowledge and time spent on a particular part of the learning process with detailed feedback and the demonstration of observed deficiencies, directs the learner to teaching materials that allow improving the demonstrated knowledge. Teaching materials are conceived as learning objects and, as such, allow for the application of adaptive approach. An analysis of the effectiveness of the algorithm and tool for practical knowledge testing from the Java programming language was done with a test group of learners who gave their opinions and grades.
Citation Count: 49
An ambipolar low band gap material based on BODIPY and EDOT
(Elsevier Science Bv, 2009) Algi, Fatih; Cihaner, Atilla; Chemical Engineering
A novel donor-acceptor type conducting polymer based on BODIPY dye as acceptor and EDOT units as donor parts is synthesized electrochemically. The unique combination of BODIPY and EDOT units provides an ambipolar (n- and p-doping processes) low band gap material (4). This is the first example of p-n junction in an organic pi-conjugated material where BODIPY unit is incorporated directly in the main chain. Furthermore, the polymer film exhibits electrochromic behavior upon p-doping: a color change from light violet (neutral) to indigo (oxidized). (C) 2009 Elsevier B.V. All rights reserved.
Citation Count: 33
An ambipolar neutral state green polymeric electrochromic
(Elsevier, 2009) Algi, Fatih; Cihaner, Atilla; Chemical Engineering
The synthesis and properties of highly stable neutral state green polymeric electrochromic, P1, which is based on 5,8-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-2,3-di(1H-pyrrol-2-yl)quinoxaline were highlighted. P1 exhibits three well-defined absorption bands with a narrow band gap (1.17 eV). The n- and p-doping (ambipolar) processes suggest that P1 is one of the most promising neutral state green polymers and it can be used in supercapacitors as either cathode or anode electrode material. Apart from the stability and the robustness of the polymer film, P1 shows multi-electrochromic behavior; gray-purple in the reduced form and highly transmissive blue color in the oxidized state. (C) 2009 Elsevier B.V. All rights reserved.
Citation Count: 78
Amine grafted silica supported CrAuPd alloy nanoparticles: superb heterogeneous catalysts for the room temperature dehydrogenation of formic acid
(Royal Soc Chemistry, 2015) Yurderi, Mehmet; Bulut, Ahmet; Caner, Nurdan; Celebi, Metin; Kaya, Murat; Zahmakiran, Mehmet; Chemical Engineering
Herein we show that a previously unappreciated combination of CrAuPd alloy nanoparticles and amine-grafted silica support facilitates the liberation of CO-free H-2 from dehydrogenation of formic acid with record activity in the absence of any additives at room temperature. Furthermore, their excellent catalytic stability makes them isolable and reusable heterogeneous catalysts in the formic acid dehydrogenation.
Citation Count: 25
Amine-functionalized graphene nanosheet-supported PdAuNi alloy nanoparticles: efficient nanocatalyst for formic acid dehydrogenation
(Royal Society of Chemistry, 2018) Bulut,A.; Yurderi,M.; Kaya,M.; Aydemir,M.; Baysal,A.; Durap,F.; Zahmakiran,M.; Chemical Engineering
Formic acid (HCOOH), a major by-product of biomass processing with high energy density, stability and non-toxicity, has a great potential as a safe and a convenient liquid hydrogen (H2) storage material for combustion engines and fuel cell applications. However, high-purity hydrogen release from the catalytic decomposition of aqueous formic acid solution at desirable rates under mild conditions stands as a major challenge that needs to be solved for the practical use of formic acid in on-demand hydrogen generation systems. Described herein is a new nanocatalyst system comprised of 3-aminopropyltriethoxysilane-functionalized graphene nanosheet-supported PdAuNi alloy nanoparticles (PdAuNi/f-GNS), which can reproducibly be prepared by following double solvent method combined with liquid-phase chemical reduction, all at room temperature. PdAuNi/f-GNS selectively catalyzes the decomposition of aqueous formic acid through the dehydrogenation pathway (∼100% H2 selectivity), in the absence of any promoting additives (alkali formates, Brønsted bases, Lewis bases, etc.). PdAuNi/f-GNS nanocatalyst provides CO-free H2 generation with a turnover frequency of 1090 mol H2 mol metal−1 h−1 in the additive-free dehydrogenation of formic acid at almost complete conversion (≥92%) even at room temperature. The catalytic activity provided by PdAuNi/f-GNS nanocatalyst is higher than those obtained with the heterogeneous catalysts reported to date for the additive-free dehydrogenation of formic acid. Moreover, PdAuNi/f-GNS nanoparticles show high durability against sintering, clumping and leaching throughout the catalytic runs, so that the PdAuNi/f-GNS nanocatalyst retains almost its inherent catalytic activity and selectivity at the end of the 10th recycle. © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.
Citation Count: 32
Analysis of forward and reverse biased current-voltage characteristics of Al/Al₂O₃/n-Si Schottky diode with atomic layer deposited Al₂O₃ thin film interlayer
(Springer, 2019) Gullu, H. H.; Yildiz, D. E.; Department of Electrical & Electronics Engineering
The dark current-voltage (I-V) characteristics of Al/Al2O3/n-Si Schottky diode are investigated in a wide temperature range of 260-360 K. The diode shows four orders of magnitude rectification. In forward and reverse bias regions, the temperature-dependent I-Vcharacteristics are detailed in terms of diode parameters and dominant conduction mechanisms. Due to the existence of Al2O3 film layer and series resistance in the diode structure, current flow under the forward bias is observed in a deviation from pure exponential characteristics. The diode parameters are estimated from thermionic emission model with non-unity ideality factor, and this non-ideal behavior is resulted in the ideality factors greater than two. In addition to these values, zero-bias barrier height is found to be strongly temperature dependent, and this variation indicates a presence of inhomogeneties in the barrier according to Gaussian distribution (GD) approximation. This fact is investigated plotting characteristic plot of this model and by extracting mean barrier height with its standard deviation. In order to complete the work on the forward I-V region, the carrier transport characteristics of the diode are explained on the basis of thermionic emission mechanism with a GD of the harrier heights. In accordance with this approximation, the conventional Richardson plot exhibits non-linearity behavior and modified current relation based on GD model is used to calculate mean barrier height and Richardson constant. In addition, the values of parasitic resistances are determined using Ohm's law as a function of temperature for all bias voltage spectra. In the reverse bias region, Poole-Frenkel effect is found to be dominant on the conduction associated with the barrier lowering, and barrier height in the emission process from the trapped states, and high-frequency dielectric constant of Al2O3 film layer is calculated.
Citation Count: 0
Analysis of glow curve of GaS_0.5Se_0.5 single crystals
(Elsevier Science Bv, 2015) Isik, Mehmet; Delice, Serdar; Gasanly, Nizami; Department of Electrical & Electronics Engineering
Characterization of shallow trapping centers in GaS0.5Se0.5 crystals grown by a Bridgman method was carried out in the present work using thermoluminescence (TL) measurements performed in the low temperature range of 10-300 K. The activation energies of the trapping centers were obtained under the light of results of various analysis methods. The presence of three trapping centers located at 6, 30 and 72 meV was revealed. The analysis of the experimental glow curve gave reasonable results under the model that assumes slow retrapping which states the order of kinetics as b=1. Heating rate dependence of the observed TL peaks was studied for the rates between 0.4 and 1.0 K/s. Distribution of the traps was also investigated using an experimental technique based on the thermal cleaning of centers giving emission at lower temperatures. The distributed levels with activation energies increasing from 6 to 136 meV were revealed by increasing the stopping temperature from 10 to 52 K. (C) 2015 Elsevier B.V. All rights reserved.
Citation Count: 10
Analysis of non-isothermal warm deep drawing of dual-phase DP600 steel
(Springer France, 2019) Pepelnjak, T.; Kayhan, E.; Kaftanoglu, B.; Airframe and Powerplant Maintenance; Manufacturing Engineering
Improving the formability of the material is a key issue in the deep drawing process. Heating the material above its recrystallization temperature drastically increases formability, but in the case of dual phase (DP) steels, it results in a loss of their mechanical properties. To improve the drawing ratio, only the heating of the flange region in the warm temperature range up to 573K was studied on DP600 sheet steel by numerical simulation. A thermo-elastic-plastic finite element method (FEM) analysis of deep drawing at several drawing ratios was performed and compared with experimental results. During the experiments, the flange area of the blank was heated by induction heating, and the central part over the punch was cooled with spray water. Experimental results showed that limiting drawing ratio could be increased by 25.58%. The microstructure of the DP 600 steel was analyzed before and after the warm forming process. No significant changes were observed, and the high strength properties of the DP 600 steel remained intact. There was good agreement between numerical and experimental results.
Citation Count: 2
Analysis of optical constants and temperature-dependent absorption edge of GaS_0.75Se_0.25 layered crystals
(Pergamon-elsevier Science Ltd, 2017) Isik, Mehmet; Gasanly, Nizami; Department of Electrical & Electronics Engineering
GaS0.75Se0.25 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.
Citation Count: 0
Analysis of Space Efficiency in High-Rise Timber Residential Towers
(Mdpi, 2024) Ilgin, Hueseyin Emre; Aslantamer, Ozlem Nur; Interior Architecture and Environmental Design
High-rise timber residential towers (>= eight-stories) represent a burgeoning and auspicious sector, predominantly due to their capability to provide significant ecological and financial advantages throughout their lifecycle. Like numerous other building types, spatial optimization in high-rise timber residential structures stands as a pivotal design factor essential for project viability. Presently, there exists no comprehensive investigation on space efficiency in such towers. This study analyzed data from 51 case studies to enhance understanding of the design considerations influencing space efficiency in high-rise timber residential towers. Key findings included (1) the average space efficiency within the examined cases was recorded at 83%, exhibiting variances ranging from 70% to 93% across different cases, (2) the average percentage of core area to gross floor area (GFA) was calculated at 10%, demonstrating fluctuations within the range of 4% to 21% across diverse scenarios, and (3) no notable distinction was observed in the effect of various core planning strategies on spatial efficiency when properly designed, and similar conclusions were drawn regarding building forms and structural materials. This research will aid in formulating design guidelines tailored for various stakeholders such as architectural designers involved in high-rise residential timber building developments.
Citation Count: 9
Analysis of temperature-dependent forward and leakage conduction mechanisms in organic thin film heterojunction diode with fluorine-based PCBM blend
(Springer, 2020) Yildiz, D. E.; Gullu, H. H.; Toppare, L.; Cirpan, A.; Department of Electrical & Electronics Engineering
The forward and reversed biased current-voltage behaviors of the organic diode were detailed in a wide range of temperatures. In this diode, a donor-acceptor-conjugated copolymer system was constructed with poly((9,9-dioctylfluorene)-2,7-diyl-(2-dodecyl-benzo[1,2,3]triazole)) as a partner of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Two-order of magnitude rectification ratio was achieved, and the temperature-dependent values of saturation current, zero-bias barrier height, and ideality factor were extracted according to the thermionic emission model. The temperature responses of these diode parameters showed an existence of inhomogeneity in the barrier height formation. As a result, the observed non-ideal behavior was explained by Gaussian distribution of barrier height where low-barrier regions are effective in the forward biased conduction mechanism at low temperatures. Together with this analysis, series resistances were evaluated using Cheung's functions and also density of interface states were investigated. On the other hand, reverse biased current flow was found under the dominant effect of Poole-Frenkel effects associated with these interfacial traps. The reverse current conduction mechanism was detailed by calculating characteristic field-lowering coefficients and barrier height values in the emission process from the trapped state in the range of temperatures of interest.
Citation Count: 2
Analysis of temperature-dependent transmittance spectra of Zn_0.5In_0.5Se (ZIS) thin films
(Springer, 2019) Isik, M.; Gullu, H. H.; Delice, S.; Gasanly, N. M.; Parlak, M.; Department of Electrical & Electronics Engineering
Temperature-dependent transmission experiments of ZnInSe thin films deposited by thermal evaporation method were performed in the spectral range of 550-950nm and in temperature range of 10-300K. Transmission spectra shifted towards higher wavelengths (lower energies) with increasing temperature. Transmission data were analyzed using Tauc relation and derivative spectroscopy. Analysis with Tauc relation was resulted in three different energy levels for the room temperature band gap values of material as 1.594, 1.735 and 1.830eV. The spectrum of first wavelength derivative of transmittance exhibited two maxima positions at 1.632 and 1.814eV and one minima around 1.741eV. The determined energies from both methods were in good agreement with each other. The presence of three band gap energy levels were associated to valence band splitting due to crystal-field and spin-orbit splitting. Temperature dependence of the band gap energies were also analyzed using Varshni relation and gap energy value at absolute zero and the rate of change of gap energy with temperature were determined.

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