Browsing by Author "Soylu N."

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Breaking point of the harmony between Gd diffused Bi-2223 slabs with diffusion annealing temperature
(2013-01-01) Aydın H.; Babanli A.; Altintas S.; Asikuzun E.; Soylu N.; Ozturk O.; Dogruer M.; Terzioglu C.; Yildirim G.
This comprehensive study reports the role of annealing temperature on the microstructural, superconducting and mechanical characteristics of the Gd diffused Bi-2223 superconducting ceramics produced by the conventional solid-state reaction route at 840 C for the annealing duration of 48 h. For the material characterization, the standard experimental methods such as dc resistivity (ρ-T), transport critical current density, X-ray powder diffraction, scanning electron microscopy and Vickers microhardness measurements are performed systematically. All the results obtained show that all the measured characteristic properties, being in charge of the applications in the industry, engineering and technology, improve until a certain diffusion annealing temperature of 800 C beyond which they tend to degrade considerably. The increase in the properties is mostly related to the transition from the inherent overdoped state of the pure Bi-2223 material to optimum doped state with the diffusion annealing temperature, confirming the penetration of the sufficient Gd nanoparticles into the crystal structure. On the other hand, the suppression in the superconducting properties stems from the appearance of the porosity, defects, disorder and localization problem in the polycrystalline Bi-2223 superconducting matrix. This is attributed to the decrement of the average crystallite size and mobile hole concentration in the Cu-O 2 layers and especially the retrogression of the crystallinity in the system. As for the mechanical characteristics, Vickers microhardness measurements exerted in the applied indentation test load range of 0.245-2.940 N indicate that the Gd diffused bulk superconducting samples exhibit the typical indentation size effect behavior. With the enhancement in the annealing temperature up to 800 C, the significant increase in the elastic modulus, yield strength and fracture toughness is one of the most striking points in the paper. The long and short of it is that the excess diffusion annealing temperature damages the fundamental characteristics of the Bi-2223 system. © 2013 Springer Science+Business Media New York.
Evaluation of microstructural and mechanical properties of ag-diffused bulk MgB2 superconductors
(2014-01-01) Yilmazlar M.; Terzioglu C.; Dogruer M.; Karaboga F.; Soylu N.; Zalaoglu Y.; Yildirim G.; Ozturk O.
Electrical, microstructural, and mechanical properties of undiffused and Ag-diffused bulk MgB2 superconductors are systematically studied using dc resistivity, scanning electron microscopy (SEM), and Vickers microhardness (H V ) measurements. The resistivity (at room temperature), critical (onset and offset) temperature, variation of transition temperature, hole-carrier concentration, surface morphology, Vickers microhardness, elastic modulus, and yield strength values of the samples are obtained and compared with each other. One can see that all superconducting parameters given above depend on the Ag diffusion on MgB2 system. The obtained results illustrate that the room temperature resistivity reduces with the increment of diffusion annealing temperature because of the hole filling when the onset (Tconset) and offset (Tcoffset) critical temperatures determined from the resistivity curves are obtained to enhance from 38.4 to 39.7 K and from 36.9 to 38.8 K, respectively. Further, SEM studies carried out for the microstructural characterization demonstrate that the surface morphology and grain connectivity also improve with the increase of the diffusion annealing temperature. In fact, the best surface morphology is observed for the Ag-diffused bulk MgB2 superconductor exposed to 850 C annealing temperature. Besides, it is obtained that the load-dependent microhardness values reduce nonlinearly as the applied load increases until 2 N, beyond which the curves shift to the saturation region, presenting that all the samples exhibit the indentation size effect (ISE) behavior. Further, the elastic modulus and yield strength values observed decrease with the enhancement of the applied load. © 2013 Springer Science+Business Media New York.
Investigation of microstructural, Vickers microhardness and superconducting properties of YBa2Cu3-xGdxO 7-δ (0 ≤ x ≤ 0.150) superconducting ceramics via experimental and theoretical approaches
(2013-04-01) Dogruer M.; Yildirim G.; Ozturk O.; Varilci A.; Soylu N.; Gorur O.; Terzioglu C.
This study manifests the change of pinning mechanism, electrical, structural, physical, mechanical and superconducting properties of YBa 2Cu3-xGdxO7-δ superconductors samples prepared by the conventional solid-state reaction method (x = 0, 0.025, 0.050, 0.100 and 0.150) by use of dc resistivity, X-ray analysis (XRD), scanning electron microscopy (SEM) and Vickers microhardness measurements. Zero resistivity transition temperatures (T coffset ) of the samples are deduced from the dc resistivity measurements. Additionally, the lattice parameters are determined from XRD measurements when the microstructure, surface morphology and microhardness of the samples studied are examined by SEM and mechanical measurements, respectively. The results obtained demonstrate that T coffset values of the samples decrease slowly with the increase in the Gd content. The maximum T coffset (92.0 K) is obtained for the pure sample prepared at 940 C for 20 h in air atmosphere while the minimum value of 83.3 K is found for the sample doped with 0.150 Gd content. Moreover, it is obtained that J c values reduce from 132 to 34 A/cm2 with the enhancement of the Gd level in the crystalline structure. Further, the peak intensities belonging to Y123 (major) phase are obtained to decrease whereas the peak intensities of the minor phases such as BaCuO2 and Y211 are found to enhance systematically with the increment in the Gd content in the system, illustrating that partial substitution of Cu2+ ions by Gd3+ ions are carried out successfully. Moreover, SEM images display that the undoped sample obtains the best crystallinity and connectivity between superconducting grains and largest grain size whereas the worst surface morphology is observed for the maximum doped sample (x = 0.150). At the same time, Vickers microhardness, elastic modulus, load independent hardness, yield strength, fracture toughness and brittleness index values, playing important roles on the mechanical properties, are computed for all the samples. The experimental results of the microhardness measurements are examined using the Meyer's law, PSR (proportional specimen resistance), modified PRS, Elastic-Plastic deformation model (EPD) and Hays-Kendall (HK) approach. The microhardness values obtained increase with the enhancement of the Gd content in the samples. Besides, it is noted that the Hays-Kendall approach is the most successful model explaining the mechanical properties of the samples studied in this work. © 2012 Springer Science+Business Media New York.
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.