Yazar "Altin, E." seçeneğine göre listele
Listeleniyor 1 - 20 / 30
Sayfa Başına Sonuç
Sıralama seçenekleri
Öğe 3D like vortex behavior and the thermally activated flux flow mechanism in (Hg0.8Re0.2)Ba2Ca2Cu3Ox superconducting films(Springer, 2013) Yakinci, Z. D.; Altin, E.Approximately 1 mu m thick high quality epitaxial c-axis oriented (Hg0.8Re0.2)Ba2Ca2Cu3Ox superconducting films have been prepared on MgO (100) substrates using spraying technique and post-Hg-Vapor annealing. The effect of the heating temperature-time combinations and the filling factor of Hg (ff(Hg)) on the physical, electrical and magnetic properties of the thick films have been investigated. The XRD investigations showed that the a-b plane of HgRe-1223 phase align parallel to the substrate surface. The best T (c) and T (zero) were found to be 130.6 and 127.2 K, respectively. The superconducting transition of the films has been measured under applied magnetic field up to 6 T. The results obtained suggested that dissipative resistivity can be explained by thermally activated flux motion below critical temperature under applied magnetic field. The temperature and field dependences of the activation energy in the thermally activated flux flow region have also been investigated. The calculated values of m and alpha values were found to be 1.42-1.49 and 0.498-0.518 respectively and suggesting a 3D like behavior and the thermally activated flux flow mechanism for all films fabricated. Magnetic properties of the films up to 6 T have also been investigated. The calculated value of critical current density, J (c), was found to be 4.7 x 10(6) A/cm(2) at 10 K for the optimally treated films.Öğe Barrier lowering effect and dark current characteristics in asymmetric GaAs/AlGaAs multi quantum well structure(Springer Heidelberg, 2011) Altin, E.; Hostut, M.; Ergun, Y.In this study, we investigate dark current voltage characteristics of GaAs/AlGaAs staircase-like asymmetric multiquantum well structure at various temperatures experimentally. The activation energy is calculated by using Arrhenius plots at different voltages. It is found that the activation energy decreased with increasing electric field. This result is evaluated using a barrier lowering effect which is a combination of geometrical and Poole-Frenkel effects. Measured dark current density-voltage (J-V) characteristics compared with the Levine model, 3D carrier drift model and the emission capture model. The best agreement with the experimental results of dark current densities is obtained by the Levine model.Öğe Dark current and optical properties in asymmetric GaAs/AlGaAs staircase-like multiquantum well structure(Elsevier, 2013) Altin, E.; Hostut, M.; Ergun, Y.In this study, we investigate dark current voltage characteristics of GaAs/AlGaAs staircase-like asymmetric multiquantum well structure at various temperatures experimentally. Measured dark current density voltage (J(d)-V) characteristics are compared with the Levine Model. It is seen that the model fits well with the experimental dark current density. Ground state energy of electrons, heavy holes and light holes are calculated by Kronig Penney Model. Optical properties of sample are characterized by photoluminescence and photoconductivity measurements. The temperature-dependent photoluminescence (PL) spectra of the GaAs/GaAlAs QWIP show that the peaks corresponding interband transition from the ground heavy-hole subband to the ground electronic subband (E-hh1 - E-e1) are dominantly observed and the peak positions corresponding to the interband transitions of the PL spectrum are dependent on the temperature. Photoconductivity measurement is performed for different negative polarities at 37 K. (C) 2013 Elsevier B.V. All rights reserved.Öğe Effect of Bi2O3 Addition on the Single-Crystal BiSrCaCuO Whisker Growth(Springer, 2011) Altin, S.; Aksan, M. A.; Altin, E.; Balci, Y.; Yakinci, M. E.Bi-2212 superconducting whiskers have been fabricated by annealing a melt-quenched precursor using suitable heat-treatment cycles for Bi3+x Sr2Ca2Cu3O8+delta where x=1 and 2. Approximately 1-3 mm long whiskers were fabricated for x=1 and x=2 and their physical, electrical and magnetic properties were investigated. Crystallization activation energies of the materials were calculated according to Kissinger and Augis-Bennet method using the DTA data. Microstructural changes from glass to heat-treated glass-ceramic materials at different temperatures were investigated with SEM, EDX and XRD analyses. It was observed that the structure of the whiskers has high c-axis oriented single-crystalline Bi-2212 phase. The whiskers fabricated for x=1 and 2 showed very smooth surface without cracks and dislocations. T (c) of the whiskers for x=1 and 2 were obtained to be 83.2 K and 79.8 K respectively. The change of magnetization with applied magnetic fields (M-H) was investigated at three different temperatures, 10, 20 and 40 K, and J (c mag) was determined using the Bean model.Öğe Electrochemical effects and magnetic properties of B substituted LiCoO2: Improving Li-battery performance(Elsevier Science Sa, 2016) Oz, E.; Altin, S.; Demirel, S.; Bayri, A.; Altin, E.; Baglayan, O.; Avci, S.LiCo(1-x)BxO(2) (x = 0, 0.125, 0.25, 0.375, 0.5, 0.75 and 1) samples were synthesized via solid state reactions. They were characterized by SEM, X-ray powder diffraction, FTIR, Raman, electrical and magnetic measurements. XRD data show impurity phases for x >= 0.25 samples due to boron content. Based on FTIR studies, the peak position of CoO6 shifts with increasing B content. The Raman spectra of LiCoO2 shows the presence of three broad bands, indicating the possible formation of spinel structure. The effective magnetic moments of the samples were calculated using Curie-Weiss law with temperature independent term. The M-H curves of x = 0.375 and 0.5 samples show very weak hysteresis loops. Effects of boron substitution on battery performance are also investigated. x = 0.125 and x >= 0.25 samples give promising results due to their enhanced reversibility of cycling compared to pristine LiCoO2. Capacity retentions for 100 cycles for x = 0, 0.125 and 0.25 samples are 22%, 81% and 83%, respectively. (C) 2015 Elsevier B.V. All rights reserved.Öğe Elucidating the optimized P2 type Na0.67Mn1-xCuxO2 cathode active material via operando XAS(Elsevier Science Sa, 2023) Altin, S.; Altundag, S.; Altin, E.; Vanpoucke, D. E. P.; Avci, S.; Ates, M. N.Here we report on the structural and electrochemical properties of P2-type Na0.67Mn1-xCuxO2 (where x = 0.20-0.50) via various techniques. X-ray diffraction (XRD) reveals a reduction of the unit cell volume upon substitution of Cu elucidated through detailed Rietveld analysis. The cyclic voltammetry (CV) behavior is also affected by the Cu substitution indicating new redox reactions stemming from Cu substitution. Galvanostatic cycling measurements at room temperature show that when x = 0.35 in a P2-type Na0.67Mn1-xCuxO2 cathode active material, the best electrochemical performance is obtained. The Na-ion diffusion rate is found to be strongly dependent upon the environmental temperature. Changes in the valence state and the local structures of P2-type Na0.67Mn1-xCuxO2 during the charge/discharge are in-vestigated through the operando X-ray absorption spectroscopy (XAS) technique. (c) 2022 Elsevier B.V. All rights reserved.Öğe Enhanced thermoelectric properties induced by chemical pressure in Ca3Co4O9(Elsevier Sci Ltd, 2014) Demirel, S.; Avci, S.; Altin, E.; Altin, S.; Yakinci, M. E.We report the investigation of boron substitution on structural, electrical, thermal, and thermoelectric properties of Ca-3_xBxCo4O9 (x=0, 0.5, 0.75, and 1) in the temperature range between 300 K and 5 K. X-ray diffraction studies show that the Ca3Co4O9 phase is successfully preserved as the majority phase in the x=0.5 sample despite the small size of boron ions. Electrical transport measurements confirm that B3+ substitution for Ca2+ causes an increase in resistivity due to the decrease in carrier concentration. x=0.5 sample is found to have a Seebeck coefficient of 181 mu V/K at room temperature which is similar to 1.5 times higher than that of the pure Ca3Co4O9. Our results indicate that the chemical pressure due to the large ionic radii difference between B3+ (0.27 angstrom) and Ca2+ (1 angstrom) enhances the thermoelectric properties as long as the unique crystal structure of Ca3Co4O9 is preserved. (c) 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.Öğe Enhancement of battery performance of LiMn2O4: correlations between electrochemical and magnetic properties(Royal Soc Chemistry, 2016) Demirel, S.; Oz, E.; Altin, S.; Bayri, A.; Altin, E.; Avci, S.We report the results of a systematic investigation of structural, electrical transport, magnetic, and electrochemical properties of LiBxMn2-xO4 (where x = 0.0-1), synthesized via a one-step solid state reaction technique. We find that the parent compound (x = 0) has the non-stoichiometric Li1.05Mn2O4 phase and the B ions successfully incorporate the structure for x <= 0.5. The resistivity anomaly of the parent compound below 100 K is attributed to the antiferromagnetic correlations. The linear part of the chi(-1)-T data are used to calculate the effective magnetic moments and to determine the total number of Mn3+ and Mn4+ ions. Magnetization measurements show that Jahn-Teller active Mn3+ ions have high spin configuration. Average valance of Mn ions increases with increasing boron content. Electrochemical studies show improved capacity retention for x = 0.125 for 100 charge-discharge cycles. This improvement is attributed to subtle modifications in the structural and magnetic properties upon substitution.Öğe An enhancement ZT and spin state transition of Ca3Co4O9 with Pb doping(Elsevier Science Sa, 2015) Demirel, S.; Altin, E.; Oz, E.; Altin, S.; Bayri, A.We reported the structural, electrical, thermal and magnetic properties of Pb-doped Ca3Co4O9 in the range of 300-5 K. DTA analysis showed that the stability of the Ca3Co4O9 was increased with Pb doping. According to XRD analysis, it is found that Pb ions were successfully doped in the Ca3Co4O9 structure. The temperature of resistivity minima, T-min, increased by increasing the Pb doping level and it is seen that incommensurate spin density wave state becomes more stable with Pb doping. The enhancement of thermopower was explained that Pb doping in Ca sites caused a decrease of Co4+ fraction such that Co4+ ions transformed into Co3+ or Co2+. The room temperature ZT value of the polycrystalline sample reaches about 16 times larger value than that of the un-doped polycrystalline sample which is the promising candidate for high temperatures in the thin film applications. According to magnetic susceptibility measurement, the increase of effective magnetic moment by Pb concentration was explained by spin state transition of Co3+ from low spin to intermediate spin and high spin state together with some orbital angular momentum contribution which comes from T-5(2) term due to a decrease of the ligand field splitting energy. (C) 2014 Elsevier B. V. All rights reserved.Öğe Fabrication of Ca-Mn-Nb-O compounds and their structural, electrical, magnetic and thermoelectric properties(Iop Publishing Ltd, 2018) Oz, E.; Demirel, S.; Altin, S.; Altin, E.; Baglayan, O.; Bayri, A.; Avci, S.CaMn1-xNbxO3-delta (0 <= x <= 1) were synthesized by conventional solid state reaction method. The structural properties were determined by FTIR, Raman, XRD, XAS measurements. The FTIR and Raman modes change by increasing Nb content and the lattice volume increases by increasing Nb content. The solubility limit of Nb is determined as x <= 0.3 and impurity phases start to appear above this limit. The temperature dependence of the magnetization data shows an antiferromagnetic transition below 120 K for low Nb content. Increasing the Nb content causes a change in the magnetic phase from antiferromagnetic to paramagnetic. The oxygen deficiency in CaMnO3-delta may cause the formation of polaron effect which is destroyed by the Nb ions. The difference graph of XAS data for x = 0 and 0.5 show that the number of Mn3+ ions increases by increasing Nb content to maintain the charge neutrality.Öğe Fabrication of Cr doped Na0.67Fe0.5Mn0.5O2 compounds and investigation of their structural, electrical, magnetic and electrochemical properties(Springer, 2019) Altin, E.; Altundag, S.; Altin, S.; Bayri, A.The main purpose of the study is to investigate the battery properties behind the electrical and magnetic properties. The first part of the study is about the fabrication of Cr-doped Na0.67Fe0.5Mn0.5O2 and the structural properties of the powders, which were investigated by XRD, EXAFS, FTIR and Raman Spectroscopy. Temperature-dependent resistivity measurements showed that room temperature resistivity values increased with Cr-ion content and all samples show an exponential decrease with increasing temperature. The magnetic properties of the Cr-doped samples were investigated, and it was found that the Cr ions strongly affected the magnetic structure of the samples. The second part of the study is about battery performance using the samples as cathode materials. Hence, CR2032 coin cells were fabricated by using the Cr-doped samples and the CV measurements showed that a high level doping of Cr ions in the structure prevented Na diffusion in the cells. The obtained CV graphs were discussed from the viewpoint of lattice dynamics. The EIS and cycling properties showed that Cr-ions in Na0.67Fe0.5Mn0.5O2 decrease of the capacity fade of the cells.Öğe Growth mechanism and magnetic and electrochemical properties of Na0.44MnO2 nanorods as cathode material for Na-ion batteries(Elsevier Science Inc, 2015) Demirel, S.; Oz, E.; Altin, E.; Altin, S.; Bayri, A.; Kaya, P.; Turan, S.Nanorods of Na0.44MnO2 are a promising cathode material for Na-ion batteries due to their large surface area and single crystalline structure. We report the growth mechanism of Na(0.44)Mno(2) nanorods via solid state synthesis and their physical properties. The structure and the morphology of the Na0.44MnO2 nanorods are investigated by X-ray diffraction (XRD), scanning and tunneling electron microscopy (SEM and TEM), and energy-dispersive X-ray (EDX) techniques. The growth mechanism of the rods is investigated and the effects of vapor pressure and partial melting of Na-rich regions are discussed. The magnetic measurements show an antiferromagnetic phase transition at 25 K and the Pelf is determined as 3.41 and 3.24 mu(B) from the chi-T curve and theoretical calculation, respectively. The electronic configuration and spin state of Mn3+ and Mn4+ are discussed in detail. The electrochemical properties of the cell fabricated using the nanorods are investigated and the peaks in the voltammogram are attributed to the diffusion of Na ions from different sites. Na intercalation process is explained by one and two Margules and van Laar models. (C) 2015 Elsevier Inc. All rights reserved.Öğe Hysteresis loops of MgB2 + Co composite tapes(Springer, 2013) Altin, E.; Gokhfeld, D. M.; Komogortsev, S. V.; Altin, S.; Yakinci, M. E.The (MgB2)(1-x) Co (x) composite tapes fabricated by the ex situ PIT method were investigated. The structural and magnetic properties were characterized. Only MgB2 and Co phases were observed in the composites while x a parts per thousand currency sign 0.2. A superconducting transition temperature was stable for these samples. Enhancement of the critical current density is found for x = 0.1.Öğe Improved battery performance of silicon modified Na0.67Fe0.5Mn0.5O2 and its structural and electrochemical properties: An investigation of infrared thermal imaging(Elsevier, 2021) Altin, S.; Altundag, E.; Altin, E.; Altundag, S.Here, we present a systematically Si-doping strategy to improve the structural stability and battery performances of P2-type Na0.67Mn0.5Fe0.5O2 samples. Through Si4+ doping intended to settlement of the interstitial region in the crystal structure and investigate the effect on the electrical conductivity of the Na0.67Mn0.5Fe0.5O2. The structural properties were examined by x-ray absorption near-edge structure (XANES), x-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) measurements. The XANES spectra showed that the valence of both Manganese and iron ions was unchanged by Si. According to charge/discharge cycling measurements for constant current at room temperature, the lowest capacity fade was obtained for Na0.67Mn0.5Fe0.48Si0.02O2. The cycling measurements were also investigated at 50 degrees C, and it was observed that the electrochemical properties were changed with temperature. It was developed an experimental setup for the measurements of the internal temperature of the battery cells during the charging/discharging process of CR2032 coin cells using an infrared thermal camera. The ohmic heat was calculated from chronoamperometry measurements using a developed setup, and the heat generation was explained by a quadratic equation in the system.Öğe Improved performance of the NaFePO4/Hardcarbon sodium-ion full cell(Pergamon-Elsevier Science Ltd, 2023) Altundag, S.; Altin, S.; Yasar, S.; Altin, E.Chronoamperometry and galvanostatic procedures were applied in the LiFePO4-based ion exchange approach for the production of NaFePO4 cathodes. The materials obtained by various processes were employed as the cathode electrode in Na-ion half cells, and their structural and electrochemical properties were investigated. According to XRD patterns for both processes, it is seen that the Na-ions were successfully replaced with Li-ions. In addition of this, XPS and EDX-dot mapping analyses were supports the ion exchange procedure. The electrochemical properties of the cathode were investigated using CR2032 cells, and the diffusion coefficient and diffusion activation energy were calculated using the CV graphs at various scan rates and environmental temperatures. It was found that the Na-diffusion rate in the cell is faster at higher temperatures than at lower temperatures. The galvanostatic cycling tests were performed for C/10 and 1C rates at room temperature, 10 degrees C, and 50 degrees C and determined that the first capacities for C/10 and 1C at room temperature were 140mAh/g and 60mAh/g, respectively, with good stability. The full cells of NaFePO4/hard carbon were fabricated by pre-sodiation tech-nique and the battery performance tests revealed that the cells have a 74 mAh/g for C/10-rate with a voltage window of 2-4 V.Öğe Improving of the battery performance of Dy-substituted LiCoO2 and investigating the mechanism of the cells(Springer, 2019) Altin, S.; Altundag, S.; Altin, E.; Bayri, A.In this study, we successfully fabricated LiCo1-xDyxO2 (where x = 0.0-0.5) samples and investigated the structural and electrochemical properties. The Dy-substituted LiCoO2 samples were characterized by X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), Fourier-transform infrared (FTIR), and Raman measurements before and after cycling. The lattice volume and effective magnetic moment were increased by the substitution of the Dy ions in the structure. The capacity fading mechanism of Dy-substituted LiCoO2 via ex situ X-ray diffraction, XAS, Raman and FTIR spectroscopy were investigated. According to the electrochemical performance of the batteries, the x = 0.04 electrode had better cycling properties up to 400 cycles, which are better than that of the pure LiCoO2. We suggested that the critical number of Dy in LiCoO2 facilitates the Li-diffusion by increasing lattice volume. According to the battery performance temperature dependence analysis from 10 to 50 degrees C, the electrolyte just below degradation temperature shows better cycling since the ions are more mobile in this case.Öğe An investigation of the improvement in energy storage performance of Na2/3Mn1/2Fe1/2O2by systematic Al-substitution(Springer, 2020) Altin, S.; Altundag, S.; Altin, E.; Harfouche, M.; Bayri, A.We successfully fabricated Na2/3Mn1/2Fe1/2-xAlxO2, wherex = 0, 0.01, horizontal ellipsis 0.10, by a modified solid-state reaction technique. The structural properties of the Al-substituted samples were investigated by x-ray diffraction (XRD), scanning electron microscope (SEM), Fourier transform infrared spectroscopy and x-ray absorption fine structure (XAFS) measurements. It was found that there were no impurity phases in the XRD patterns of the samples and they fit the P63/mmc symmetry. The Al substitution in Na(2/3)Mn(1/2)Fe(1/2)O(2)causes a decrease in the a-lattice parameter, but the c-parameter starts to increase after a certain substitution value of Al. We suggest that a certain proportion of Al in the samples triggers the change of the spin configuration of the Fe ions, and it may cause an increase in the lattice parameters. The size of the grains was found to be less than 0.9 mu m, from SEM images for all samples. The valence states of the substituted samples as well as the local structure around Fe and Mn were investigated by means of XAFS measurements. The highest capacity for the first cycle was obtained as 134.3 mAh/g forx = 0.07, and the best capacity fade was found to be 0.23 forx = 0.08 substitution. So, the highest performance of the Al-substituted cells was found when 0.08 >= x >= 0.06. The environmental temperature effects on the battery cells were determined at 10 oC, room temperature and 50 oC, and it was found that the temperature plays a crucial role in the Na-ion batteries.Öğe Jc enhancement and flux pinning of Se substituted YBCO compound(Springer, 2013) Yakinci, Z. D.; Gokhfeld, D. M.; Altin, E.; Kurt, F.; Altin, S.; Demirel, S.; Aksan, M. A.Y-2/3 Se1/3Ba2Cu3Ox compound was fabricated by using solid state fabrication technique. Optimum heat treatments conditions for Y0.77Se0.33Ba2Cu3Ox were investigated. It was determined that the XRD results of these samples were similar to Y-123 phase with some impurities. Magnetization dependence of applied magnetic fields was measured in the range of 0-9 T at 10-50 K. The symmetric and asymmetric M-H loops were obtained for the samples. Magnetization loops obtained from measurements were successfully described by the extended Valkov-Khrustalev model. The temperature and applied magnetic field dependencies of magnetization of sample were estimated and critical current density of samples was calculated by Bean model and pinning force of samples was calculated by using Lorentz force. It is found from critical current density values that Se additions were acted as a pinning center which increased critical current density.Öğe Li1+xFePO4 (x=0-0.5) production from Fe3+ sources by glass-ceramic technique with different carbon sources and investigation of structural, thermal and electrochemical performance(Elsevier, 2022) Altin, E.; Altundag, S.; Gultek, E.; Altin, S.Li1+xFePO4 (x=0 - 0.5) samples were successfully fabricated by the glass-ceramics technique with quenching from high temperatures. Thermal properties were analyzed by DTA, TG and the glass transition temperature, crystallization activation energy and Avrami parameters were calculated as 3.99 indicating that three dimensions of growth and one representing a constant nucleation rate. The XRD analysis shows that there are two-phase of LiFePO4 and Li3Fe2P3O12 which compete with each other according to increasing heat treatment temperatures. Glucose and ascorbic acid were used as the carbon source for increasing the conductivity of the powders during to crystallization process with different heating and cooling rates and it was found that the heating/cooling rates are effective for battery performance. The battery cells exhibit similar cycling voltammetry (CV) data with the undoped LiFePO4 battery cells. The charging/discharging cycles measurements for C/5- rate and C/20- rate for different environmental temperatures were made and it was found that the best capacity values for different carbon sources were obtained as 165.8 mAh/g and 169.9 mAh/g for using ascorbic acid and glucose addition, respectively. The environmental effect on the battery performance was investigated and the Li-diffusion rate depending on the temperature was calculated and it was found that the diffusion rate obtained from CV curves for 50 degrees C, room temperature, and 10 degrees C and the diffusion activation energy were found as 2.93 and 8.03 meV for glucose and ascorbic acid respectively.Öğe LiNi0.8Co0.15Ti0.05O2: synthesis by solid state reaction and investigation of structural and electrochemical properties with enhanced battery performance(Springer, 2020) Bayri, A.; Gocer, E.; Altin, E.; Altundag, S.; Oz, E.; Harfouche, M.; Altin, S.Solid state synthesis is an essential technique for large-scale production of electrode active materials in battery industry. However, solid state synthesis of LiNi0.8Co0.15Al0.05O2(NCA), which is a well-known commercial cathode material for Li-ion batteries, provides electrochemically inactive compound. Here, we report the solid state synthesis of Li(x)Ni(0.8)Co(0.15)Ti(0.05)O(2)wherex = 1.03, 1.06, and 1.09, which is a modified version of conventional NCA. Our thorough studies consist of characterization of compounds by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and magnetization measurements. The results point out the significant effects of Li content on structural and magnetic properties of the samples. Battery performance tests show that Li(1.06)Ni(0.8)Co(0.15)Ti(0.05)O(2)exhibits better cycling properties than conventional NCA. X-ray absorption spectroscopy (XAS) technique is utilized to determine structural modifications upon cycling of this compound via ex-situ analysis. We conclude that substitution of Ti ions in Li(1.06)Ni(0.8)Co(0.15)Ti(0.05)O(2)improves the cycling capability of the cells by reducing the formation of NiO insulating layer which hinders the redox reactions. The capacity value ofx = 1.06 sample increases up to 150 mAh g(-1)at C/3 rate during cycling and the capacity fade is negative for the first 10 cycles. Possible mechanism for the negative capacity fade is also discussed.