Browsing by Author "Coskunyurek M."

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Change of formation velocity of Bi-2212 superconducting phase with annealing ambient
(2013-11-01) Ozturk O.; Yildirim G.; Asikuzun E.; Coskunyurek M.; Yilmazlar M.; Kilic A.
This exhaustive study enables the researchers to recognize the role of the annealing conditions (temperature and time) on the microstructural, mechanical, electrical and superconducting properties of the Bi-2212 superconducting material with the aid of ρ-T, X-ray diffraction, scanning electron microscopy and Vickers microhardness (Hv) measurements. For this aim, the superconducting samples are elaborated by standard solid-state reaction route at different annealing temperature and different annealing duration. The results show that the annealing temperature of 840 C and the annealing duration of 72 h are the best for the formation velocity of Bi-2212 superconducting phase. In this study we have focused on microhardness measurements to investigate the mechanical properties. Vickers microhardness, Young's modulus, fracture toughness and yield strength values are calculated separately for all samples. Experimental results of hardness measurements are analyzed using the some models. Finally, the Hays-Kendall model is determined as the most successful model describing the mechanical properties of our samples. © 2013 Springer Science+Business Media New York.
Physical properties and diffusion-coefficient calculation of iron diffused BI-2223 system
(2012-10-01) Ozturk O.; Asikuzun E.; Kaya S.; Coskunyurek M.; Yildirim G.; Yilmazlar M.; Terzioglu C.
This study includes two parts: (I) investigation of the effect of different annealing time (10 h, 30 h, and 60 h) on physical, superconducting, and microstructural properties of Fe-diffused Bi-2223 superconductor ceramics prepared by the conventional solid-state reaction method with the aid of the X-ray diffraction (XRD), scanning electron microscopy (SEM), dc resistivity (φ-T ) and transport critical current density (Jc) measurements, and (II) determination of the diffusion coefficient and the activation energy of iron in the Bi-2223 system. In the former part, the zero-resistivity transition temperature (Tc), phase purity, volume fraction, hole-carrier concentration, lattice parameters, surface morphology, texturing, crystallinity, grain connectivity, grain size, and room temperature resistivity values of the bulk samples are found and compared with each other. The results obtained show that both the zero resistivity transition temperature (Tc) and transport critical current density (Jc) regularly enhance with the increment in the diffusion-annealing time. The maximum Tc of 107 ± 0.2 K and Jc of 50.0 Acm-2 are observed for the sample annealed at 830 °C for 60 h. As for the XRD investigations, according to the refinement of cell parameters done by considering the structural modulation, the enhance-ment in the diffusion-annealing is confirmed by both a decrease of the cell parameter a and an increase of the lattice parameter c of the samples, meaning that the greatest Bi-2223 phase fraction belongs to the sample annealed at 830 °C for 60 h. Moreover, SEM images display that the sample has the best crystallinity, grain connectivity, and largest grain size. Based on the results, the superconducting and microstructural properties improve with the increase in the diffusion-annealing time. In the latter part, Fe diffusion in the Bi-2223 system is examined in a range of 500-830 °C by the variation of the lattice parameters evaluated from the XRD patterns. The temperature dependence of the Fe diffusion coefficient is described by the Arrhenius relation D = 4.27 × 10-5 exp(-1.27 ± 0.10) eV/kBT, and the related activation energy of the iron in the Bi-2223 system is found to be about 1.27 eV. The relatively low value of activation energy obtained illustrates that the migration of the Fe ions primarily proceeds through defects such as pore surfaces and grain boundaries in the polycrystalline structure, leading to the improvement of the microstructural and superconducting properties of the samples, supported by the results of part I. All in all, the aim of the present study is not only to analyze the role of diffusion-annealing time on superconducting and microstructural properties of Fe-diffused Bi-2223 superconductors, but also to find the diffusion coefficient and activation energy of Fe in the Bi-2223 system. © Springer Science+Business Media, LLC 2012.
The effect of Nd2O3 addition on superconducting and structural properties and activation energy calculation of Bi-2212 superconducting system
(2014-01-01) Ozturk O.; Asikuzun E.; Coskunyurek M.; Soylu N.; Hancerliogullari A.; Varilci A.; Terzioglu C.
The effect of Nd2O3 addition on the microstructural and the superconducting properties of Bi-2212 superconductor ceramics, prepared by solid state reaction method, was analyzed by performing X-ray diffraction (XRD), scanning electronic microscope (SEM), energy dispersive spectroscopy (EDS) and dc Resistivity (ρ-T) measurements. The magnetoresistivity of the samples was measured for different values of the applied magnetic field strengths (0-7 T). Also, the activation energies were calculated using the Arrhenius equation. According to these results, the Tcoffset value of the undoped sample was decreased from 79 to 42 K with the growth of magnetic field. In the same way, the activation energy (U o) values were significantly diminished by the increasing of magnetic field. A similar situation was observed in other doped samples. Activation energy for 0.05 % Nd2O3 doped sample under 7 T magnetic field was 550 J/mol the least. In addition, lattice parameter c, calculated by analysis of XRD data, was decreased with doping while lattice parameter a was increased. SEM analysis shows that particles were shrinking with the addition. When compared with other elements for EDS analyses, it was analyzed an important decrease in the percentage of Sr with the increasing of Nd contribution. © Springer Science+Business Media New York 2013.