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    Experimental and theoretical approaches on magnetoresistivity of Lu-Doped Y-123 superconducting ceramics
    (Springer, 2013) Türköz, Mustafa Burak; Nezir, Saffet; Varilci, Ahmet; Yıldırım, Gürcan; Akdogan, Mehmet; Terzioglu, Cabir
    This study discusses the change of the flux pinning mechanism, electrical and superconducting properties of Lu added YBa2Cu3O7-delta bulk superconducting ceramics prepared by the liquid ammonium nitrate and derivatives at 970 A degrees C for 20 h by means of magnetotransport measurements conducted in the magnetic filed range from 0 to 6 kG. The critical transition (both and) temperatures, residual resistivity (rho(0)), residual resistivity ratios (RRR), irreversibility fields (mu H-0(irr)), upper critical fields (mu (0) H (c2) ), penetration depths (lambda) and coherence lengths (xi) of the YBa2LuxCu3O7-delta materials are evaluated from the magnetoresistivity curves. The resistivity criteria of 10 and 90 % normal-state resistivity serve as the important parameters for the description of the irreversibility and upper critical fields, respectively. Moreover, rho (0) , mu (0) H (irr) (0) and mu (0) H (c2) (0) values of the bulk samples are theoretically calculated using the extrapolation method at absolute zero temperature (T = 0 K). Likewise, the xi and lambda values are inferred from mu (0) H (irr) (0) and mu (0) H (c2) (0) values obtained, respectively. At the same time, activation energies of the samples studied are determined from thermally activated flux creep (TAFC) model. The results obtained indicate that the pinning mechanism, electrical and superconducting properties of the samples enhance with the increment of the Lu addition up to level of 0.1 wt% beyond which these properties start to deteriorate systematically and in fact reach the local minimum points for the sample doped with 0.9 wt% Lu due to the degradation of pinning ability, density, crystallinity and connectivity between grains. Similarly, the presence of the magnetic field results in the reduction of these properties as a consequence of the decrement in the flux pinning in the samples prepared. Namely, the maximum of 94.6 K and of 92.5 K are observed for the sample doped with 0.1 wt% Lu whereas the minimum temperature values are obtained to be about 71.2 and 50.3 K for the sample doped with 0.9 wt% Lu. In fact, the value decreases to 20.5 K with the increment in the applied magnetic field up to 6 kG. Besides, the flux pinning energies of the samples decrease with ascending applied magnetic field. The pure sample has the flux pinning energy of 15,211 K at zero field while the maximum and minimum values are found to be about 16,722 K and 2,058 K for the samples added with 0.1 and 0.9 wt% Lu, respectively. The U (0) of 158 K at 6 kG applied magnetic field is obtained for the latter sample, showing that this sample exhibits much weaker flux pinning, lesser crystallinity and connectivity between grains compared to the other samples produced as a result of the stronger pair-breaking mechanism. The dissipation mechanism is also discussed by means of the magnetic field dependence of the activation energy.
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    Investigation of Lu effect on YBa2Cu3O7-delta superconducting compounds
    (Springer, 2013) Türköz, Mustafa Burak; Nezir, Saffet; Terzioğlu, Cabir; Varilci, Ahmet; Yildirim, Gürcan
    This study reports the effect of Lu addition on the microstructural and superconducting properties of YBa2LuxCu3O7-delta (Y123) superconducting samples with x = 0, 0.1, 0.3, 0.5 and 0.7 by means of X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), electron dispersive X-ray (EDX), electrical resistivity and transport critical current density (J(c)) measurements. The samples prepared by the liquid ammonium nitrate and derivatives are exposed to various annealing time (20, 40 and 60 h) and temperature (950, 960 and 970 A degrees C), and the best ambient for the sample fabrication is determined to be 970 A degrees C for 20 h. Zero resistivity transition temperatures (T-c), critical current densities (J(c)), variation of transition temperatures, hole-carrier concentration, grain size, lattice parameter, surface morphology, element distribution, crystallinity and resistivity (at room temperature) values of the bulk superconducting samples prepared at 970 A degrees C for 20 h are compared with each other. T-c and J(c) values of the samples are inferred from the dc resistivity and the critical current measurements, respectively. The results show that the T-c value of the pure sample is about 90.6 K while the sample doped with 0.1 wt% Lu has the maximum T-c value (92.5 K). However, beyond x = 0.1, the T-c value is observed to decrease toward to 83.5 K with increment in the Lu addition. Similarly, the J(c) values measured are found to reduce from 142 to 76 A/cm(2) with the addition. Moreover, XRD measurements show that both pure and Lu-doped samples exhibit the polycrystalline superconducting phase with the changing intensity of diffraction lines and contain Y123 and Y211 phase, confirming the incorporation of Lu atoms into the crystalline structure of the samples studied. At the same time, comparing of the XRD patterns of samples, the intensity ratio of the characteristic (110) and (013) peaks on the sample doped with 0.1 wt% Lu is more than that on the other samples prepared. Additionally, SEM images display that the sample doped with 0.1 wt% Lu obtains the best crystallinity, grain connectivity and largest grain size whereas the worst surface morphology is observed for the maximum doped sample (x = 0.7). Further, EDX results demonstrate that the Lu atoms doped are successfully introduced into the microstructure of the Y123 samples studied and the maximum Cu element level is observed for the sample doped with 0.1 wt%, explaining that why this sample obtains the best superconducting properties compared to others. According to all the results obtained, it is concluded that the 0.1 wt% Lu addition into the Y123 system improves the microstructural and superconducting properties of the samples studied.
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    Investigation of Sm → Ca substitution in Bi(Pb)SrCaCuO high temperature superconductor by low field AC magnetic susceptibility
    (Springer, 2007) Terzioglu, Cabir; Yegen, Dincer; Yilmazlar, Mustafa; Görür, Osman; Akdogan, Mustafa; Varilci, Ahmet
    We have investigated the effect of Sm substitution in Bi(Pb)SrCaCuO system by performing AC susceptibility (chi = chi' + i chi ''), XRD (X Ray Diffraction) and SEM (Scanning Electron Microscopy) measurements. The Sm -> Ca substitution (Bi(1.)6Pb(0.4)Sr(2)Ca(2-x)Sm(x)Cu(3)O(y)) was carried out by conventional solid-state reaction method. The susceptibility measurements were carried out at different values of the AC field amplitudes. The imaginary part of susceptibility is used to estimate the intergranular critical current density by means of the Bean's model. The intergranular critical current density (J(c)) of pure sample is found to be about 68 A/cm(2) at 95 K. The intergranular J(c) is seen to decrease with increasing Sm substitution. XRD pattern and SEM micrographs are given to provide information about Bi-2223 phase and grain size respectively.