Browsing by Author "Erdem M."
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Scopus Ac Susceptibility Measurements and Mechanical Performance of Bulk MgB2(2015-07-24) Ozturk O.; Asikuzun E.; Kaya S.; Erdem M.; Safran S.; Kilic A.; Terzioglu C.The effects of Ar ambient pressure (vacuum and 0, 10, and 20 B) and annealing times (0.5 and 1 h) on microstructural, superconducting, and mechanical properties of bulk superconducting MgB2 are investigated. The samples are produced using the solid-state reaction method. X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements are performed for determination of the crystal structure and surface morphology of MgB2 samples, respectively. The superconducting properties are studied by AC magnetic susceptibility measurements. Microhardness analyses are made using the Vickers microhardness test for determination of mechanical properties of all samples. Increasing the Ar pressure decreases the lattice parameters and hence the average grain size. Increasing the annealing time results in larger lattice parameters and larger grain formation. The susceptibility measurements revealed a two-step transition which is reminiscent of granular superconductors. The intra-grain transition temperature is determined to be 38.4 K for all samples. The inter-grain transition temperature of 37.2 K is obtained for samples produced under Ar ambient pressure. The samples produced under Ar ambient pressure have better superconducting properties than the ones produced in vacuum. Increasing the annealing time under vacuum further decreases the superconducting properties probably due to Mg loss.Scopus Effect of annealing time on the structural, optical and electrical characteristics of DC sputtered ITO thin films(2014-10-01) Senol S.D.; Senol A.; Ozturk O.; Erdem M.Using an Indium tin oxide (ITO) ceramic target (In2O3:SnO2, 90:10 wt%), ITO thin films were deposited by conventional direct current magnetron sputtering technique onto glass substrates at room temperature. The obtained ITO films were annealed at 400 °C for different annealing times (1, 2, 5, 7, and 9 h). The effect of annealing time on their structural, optical and electrical properties was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microcopy (AFM), ultra violet–visible (UV–Vis) spectrometer, and temperature dependence Hall measurements. XRD data of obtained ITO films reveal that the films were polycrystalline with cubic structure and exhibit (222), (400) and (440) crystallographic planes of In2O3. AFM and Scanning Electron Microscopy SEM have been used to probe the surface roughness and the morphology of the films. The refractive index (n), thickness and porosity (%) of the films were evaluated from transmittance spectra obtained in the range 350–700 nm by UV–Vis. The optical band gap of ITO film was found to be varying from 3.35 to 3.47 eV with the annealing time. The annealing time dependence of resistivity, carrier concentration, carrier mobility, sheet resistance, and figure of merit values of the films at room temperature were discussed. The carrier concentration of the films increased from 1.21 × 1020 to 1.90 × 1020 cm−3, the Hall mobility increased from 11.38 to 18 cm2 V−1 s−1 and electrical resistivity decreased from 3.97 × 10−3 to 2.13 × 10−3 Ω cm with the increase of annealing time from 1 to 9 h. Additionally, the temperature dependence of the carrier concentration, and carrier mobility for the as-deposited and 400 °C annealed ITO films for 2 and 9 h were analysed in the temperature range of 80–350 K.Scopus Effect of Gd addition on the activation energies of Bi-2223 superconductor(2011-02-01) Erdem M.; Özturk O.; Yucel E.; Altintas S.P.; Varilci A.; Terziolu C.; Belenli I.We have investigated the effect of addition of Gd in Bi 1.8Pb0.35Sr1.9Ca2.1Cu 3GdxOy superconductor with x=0, 0.1, 0.2, 0.3, 0.4 and 0.5. The samples were prepared using the standard solid-state reaction method. The activation energies, irreversibility fields (Hirr), upper critical fields (Hc2) and coherence lengths at 0 K (ξ(0)) were calculated from the resistivity versus temperature (RT) curves under DC magnetic fields up to 7 T. The superconducting transition temperature, Tc, and activation energy, U0, were found to decrease with increase in Gd concentration and with increase in applied magnetic field. The offset transition temperature of the pure (Gd00) sample without applied magnetic field is 108 K, whereas for Gd05 sample, the offset transition temperature drops to 5 K with 7 T applied magnetic field. The activation energy of the Gd00 sample without applied magnetic field is 34,980 K, and for Gd05 sample with 7 T applied field it is 98 K. Hirr and Hc2 values also decrease with increase in Gd addition. The possible reasons for the observed degradation in microstructural and superconducting properties due to Gd addition were discussed. © 2010 Elsevier B.V. All rights reserved.Scopus Investigation of indentation size effect (ISE) and micro-mechanical properties of Lu added Bi2Sr2CaCu2Oy ceramic superconductors(2013-01-01) Ozturk O.; Erdem M.; Asikuzun E.; Yildiz O.; Yildirim G.; Varilci A.; Terzioglu C.In this study, we investigated the effect of the Lutetium (Lu) addition on microstructure and mechanical properties of the Bi-2212 superconductors annealed at 840 C for 50 h. The samples were prepared by the widely used conventional solid-state reaction method. For comparison, undoped sample was prepared in the same conditions. The prepared samples were characterized using X-ray powder diffraction (XRD), scanning electron microscope (SEM), and microhardness measurements (H v ). The volume fraction and lattice parameters were determined from the XRD measurements. The microstructure, surface morphology and orientation of the grains were investigated by SEM. In this study we have focused on microhardness measurements to investigate the mechanical properties. Vickers microhardness, load independent hardness, Young's modulus, fracture toughness and yield strength values were calculated separately for doped and undoped samples. Experimental results of hardness measurements were analyzed using the Meyer's law, proportional sample resistance (PSR)model, modified proportional sample resistance (MPRS) model, Elastic-Plastic deformation model (EPD), and Hays-Kendall (HK) approach. Finally, the Hays-Kendall (HK) approach was determined as the most successful model describing the mechanical properties of our samples. Moreover, lattice parameter c and volume fraction of Bi-2212 phase decreased with increasing Lu content. SEM measurements show that not only the surface morphology and grain connectivity were obtained to degrade but also the grain sizes of the samples were found to decrease with the increase of the Lu addition, as well. © 2012 Springer Science+Business Media, LLC.Scopus The Effect of Ar Ambient Pressure and Annealing Duration on the Microstructure, Superconducting Properties and Activation Energies of MgB2 Superconductors(2017-05-01) Ozturk O.; Asikuzun E.; Kaya S.; Koc N.S.; Erdem M.MgB2 samples are produced by ex situ reaction method under vacuum, and various (0, 10, 20 Bar) Ar pressure for 0.5 and 1 h. The effect of ambient pressure and annealing duration on the microstructure and electrical properties of bulk samples are investigated. XRD, SEM, and magnetoresistance measurements are made. The a and c lattice parameters and the grain size values are the highest for the samples produced under vacuum and their values decrease with increasing Ar pressure. Moreover, these values increase when the annealing duration increases from 0.5 to 1 h. The increasing pressure reduces the bond lengths between the atoms thus the grain sizes decrease. Smaller grain size promotes the connection between grains which results in an increase of the critical current density (Jc). SEM micrographs reveal that the produced samples have granular structure which is a characteristic feature of MgB2. The decrease of grain sizes and thus enhancement in grain connectivity with increasing pressure is also confirmed by SEM images. Magneto resistivity measurements show that Tc values of the samples produced under vacuum are the highest. Tc values decrease with increasing ambient pressure and applied magnetic field. The activation energies (U0) of the samples are calculated using Arrhenius plots due to thermally activated flux flow theory. Existence and increase of Ar pressure causes increase of activation energies. The samples produced with 0.5-h annealing have higher activation energies than the ones produced with 1-h annealing.Scopus The effect of pbse addition on the mechanical properties of Bi-2212 superconductors(2012-10-01) Cavdar S.; Deniz E.; Koralay H.; Ozturk O.; Erdem M.; Gunen A.The effects of PbSe addition and heat treatment on the structure and mechanical properties of Bi-2212 superconductors are analyzed. Glass ceramic method is used to produce PbSe added samples. The mechanical properties are derived using Vickers microhardness measurement results. Well known analysis methods like Meyer's law, PSR (Proportional Sample Resistance) Model, EDP (Elastic/ Plastic DeformationModel), Hays-Kendall approach and IIC (Indentation-Induced Cracking Model) are used to calculate load independent microhardness values of the samples. HK approach produces successful results for the samples A, E, F, G and H indicating ISE (Indentation Size Effect) behavior. The IIC model successfully explains RISE (Reverse Indentation Size Effect) behavior for the samples B, C and D. RISE behavior is observed only for the samples where the plastic deformation is dominant. ISE behavior is observed for the samples where both elastic and plastic deformation are produced. The results of XRD and SEM analysis show improvement with PbSe addition in the crystal structures and surface morphologies of superconducting samples. © Springer Science+Business Media, LLC 2012.Scopus The effect of Pr addition on superconducting and mechanical properties of Bi-2212 superconductors(2012-02-01) Ozturk O.; Asikuzun E.; Erdem M.; Yildirim G.; Yildiz O.; Terzioglu C.In this work, the effects of Pr doping on the superconducting, structural and mechanical properties of the samples are analyzed. Pr doped Bi-2212 superconductors are obtained using solid state reaction method. Dc resistivity measurements are made to investigate the superconducting properties, microhardness measurements are made to analyze the mechanical properties, XRD and SEM measurements are done for crystal structure determination and calculation of the lattice parameters. Using EDS measurements, the change in the elemental composition with doping is analyzed. The Vickers microhardness is calculated for undoped and doped samples. The experimental results of the microhardness measurements are analyzed using Kick's Law, PSR (proportional specimen resistance), modified PRS (MPSR) and Hays-Kendall (HK) approach. The microhardness values of the samples decrease with Pr addition. The results can be successfully explained by HK approach. © 2011 Springer Science+Business Media, LLC.