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  • Küçük Resim
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    Effect of Mn Addition on Structural and Superconducting Properties of (Bi, Pb)-2223 Superconducting Ceramics
    (Springer, 2012) Yildirim, G.; Bal, S.; Yucel, E.; Dogruer, M.; Akdogan, M.; Varilci, A.; Terzioglu, C.
    This study deals with the effect of Mn addition on the structural and superconducting properties of Bi1.8Pb0.4Sr2Mn (x) Ca2.2Cu3.0O (y) ceramics with x=0,0.03,0.06,0.15,0.3 and 0.6 by means of X-ray analysis (XRD), scanning electron microscopy (SEM), electron dispersive X-ray (EDX), resistivity, and transport critical current density (J (c)) measurements. Zero-resistivity transition temperatures (T (c)) of the samples produced via the standard solid-state reaction method are estimated from the dc resistivity measurements. Moreover, the phase fraction and lattice parameters are determined from XRD measurements while the microstructure, surface morphology and element composition analyses of the samples are investigated by SEM and EDX measurements, respectively. It is found that T (c) values are obtained to decrease from 109 K to 85 K; likewise, J (c) values are observed to reduce from 3200 A/cm(2) to 125 A/cm(2) with increasing Mn addition. According to the refinement of cell parameters done by considering the structural modulation, the Mn addition is confirmed by both an increase of the lattice parameter a and a decrease of the cell parameter c of the samples in comparison with that of the pure sample (Mn0). SEM measurements show that not only the surface morphology and grain connectivity are seen to degrade but the grain sizes of the samples are found to decrease with the increase of the Mn addition as well. The EDX results reveal that the elements used for the preparation of samples distribute homogeneously and the Mn atoms enter into the crystal structure by replacing Sr and Cu atoms. The possible reasons for the obtained degradation in microstructural and superconducting properties are also interpreted.
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    Öğe
    Fabrication and Transport Critical Current Densities of MgB2/Fe/Cu Multifilament Tapes without Any Intermediate Annealing
    (Polish Acad Sciences Inst Physics, 2012) Yucel, E.; Terzioglu, C.; Varilci, A.; Gencer, A.; Belenli, I.
    We have fabricated superconducting 6 and 7 filaments MgB2/Fe/Cu tapes by ex situ powder-in-tube method using Cu-sheath without any intermediate annealing. Properties of two different multicore MgB2/Fe/Cu tapes annealed at 900 degrees C for 2 h in high purity argon gas atmosphere were compared. The samples were characterized using scanning electron microscope, X-ray diffraction, electron dispersive spectroscopy, optical microscopy, critical transition temperature, transport critical current density, and magnetic measurements. Transport critical current densities of the 6 and 7 filaments tapes were found to be 450 A/cm(2) and 190 A/cm(2) at 20 K, respectively. From X-ray diffraction measurements, lattice parameters a and c were determined. From dc resistivity measurements, the connectivity between grains was investigated by using Rowell's connectivity analysis.
  • Küçük Resim
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    Influence of diffusion-annealing temperature on physical and mechanical properties of Cu-diffused bulk MgB2 superconductor
    (Springer, 2013) Dogruer, M.; Zalaoglu, Y.; Gorur, O.; Ozturk, O.; Yildirim, G.; Varilci, A.; Yucel, E.
    This study reports not only the effect of Cu diffusion on physical and mechanical properties of bulk MgB2 superconductors with the aid of Vickers microhardness (H-v) measurements but also the diffusion coefficient and the activation energy of copper (Cu) in the MgB2 system using the resistivity measurements for the first time. Cu diffusion is examined over the different annealing temperature such as 650, 700, 750, 800 and 850 A degrees C via the successive removal of thin layers and resistivity measurement of the sample. Further, Vickers microhardness, elastic modulus, yield strength, fracture toughness and brittleness index values of the samples studied are evaluated from microhardness measurements. It is found that all the results obtained depend strongly on the diffusion annealing temperature and applied load. The microhardness values increase with ascending the annealing temperature up to 850 A degrees C owing to the increment in the strength of the bonds between grains but decreasing with the enhancement in the applied load due to Indentation Size Effect behaviour of the bulk samples. Moreover, the diffusion coefficient is observed to enhance from 2.84 x 10(-8) to 3.22 x 10(-7) cm(2) s(-1) with the increase of the diffusion-annealing temperature, confirming that the Cu diffusion is more dominant at higher temperatures compared to lower ones. Besides, temperature dependence of the Cu diffusion coefficient is described by the Arrhenius relation D = 2.66 x 10(-3) exp(-1.09 +/- A 0.05 eV/k(B)T) and the related activation energy of the Cu ions in the MgB2 system is obtained to be about 1.09 eV. Based on the relatively low value of activation energy, the migration of the Cu ions primarily proceeds through defects such as pore surfaces and grain boundaries in the polycrystalline structure, resulting in the improvement of the physical and mechanical properties of the bulk MgB2 samples.
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    Vickers hardness measurements and some physical properties of Pr2O3 doped Bi-2212 superconductors
    (Springer, 2012) Asikuzun, E.; Ozturk, O.; Cetinkara, H. A.; Yildirim, G.; Varilci, A.; Yilmazlar, M.; Terzioglu, C.
    This study deals with the effect of Pr2O3 addition on the structural, superconducting and mechanical properties of Bi-2212 superconductor by means of X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), electron dispersive X-ray (EDX), Vickers microhardness and resistivity measurements. The samples studied are prepared using the standard solid-state reaction method. Onset transition temperatures (T (c) (onset) ) of the samples are estimated from the dc resistivity measurements. Furthermore, the phase ratio and lattice parameters a and c are determined from XRD patterns when the microstructure, surface morphology and element composition analyses of the samples are investigated by SEM and EDX measurements, respectively. Additionally, vickers microhardness, elastic modulus, yield strength and fracture toughness values of the samples are deduced from microhardness measurements. It is found that T (c) (onset) values of the samples increase from 87 to 97 K with the Pr2O3 addition. According to the refinement of cell parameters done by considering the structural modulation, the doping is confirmed by both an increase of the lattice parameter a and a decrease of the cell parameter c of the samples in comparison with that of the pure sample. As for SEM measurements, it is obtained that the surface morphology and grain connectivity degrade with the increase of the Pr2O3 addition. Moreover, EDX images show that the elements used for the preparation of samples distribute homogeneously and the Pr atoms enter into the crystal structure by replacing Sr atoms. To sum up, the Pr2O3 addition is found to suppress the mechanical, microstructural and superconducting properties of the Bi-2212 superconductor.