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    AC losses and determination of some superconducting parameters of Ag2O added Bi2223 superconductors
    (ELSEVIER SCIENCE BV, 2006) Duzgun, I; Karaca, I; Ozturk, A; Celebi, S
    The effects of Ag2O addition on electromagnetic properties in a Bi2223 superconductor have been investigated by means of AC susceptibility measurements and theoretical analysis. Intergranular critical current density was found to be decreased with increased amount of Ag2O addition (0, 15, 30 wt.%). The effective volume fractions of the grains were estimated as a function of Ag2O amount. The field and temperature dependence of the intergranular critical current density for each sample has been accounted by the function J(cm) (T) = (J(0)/B-n) (1 - T/T-cm)(P), where alpha(0) denotes the pinning strength parameter at T = 0. The field exponent n, temperature exponent p, and maximum intergranular loss peaks, which are determined from the comparison of the experimental and calculated curves, increased with increasing amount of Ag2O addition. Experimental isothermal hysteresis losses as a function of field amplitude are also seen to adequately be reproduced by model calculation using the same fitting parameters as for AC susceptibility chi(n)(T) data. (c) 2006 Elsevier B.V. All rights reserved.
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    Effect of Ag2O addition on the intergranular properties of the superconducting Bi-(Pb)-Sr-Ca-Cu-O system
    (IOP PUBLISHING LTD, 2003) Karaca, I; Celebi, S; Varilci, A; Malik, AI
    The effect of Ag2O addition on the Bi-(Pb)-Sr-Ca-Cu-O system has been investigated in terms of ac susceptibility, phase evolution, critical current density and critical temperature. It was found that as the amount of Ag2O addition increases, the intergranular critical current density decreases in our samples (Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06O10) fabricated by ammonium nitrate technique. The analysis for comparison is based on the suppression degree of the diamagnetic behaviour with respect to fields, rapid or slow shift of the summit in chi'(T) to lower temperature with increasing field amplitude and the sharpness of the transition of chi'(T) for intergranular component for the same field amplitude. We also qualitatively discuss experimental results in the framework of the critical state model. The room temperature XRD diagram indicates the presence of large amount of high-T-c (2223) phase. The percentage of Bi-2223 phase in the phase mixture was estimated from the intensities of high-T-c (2223) and low-T-c (2212) phase peaks as 78% for the pure BSCCO sample. Among the Ag2O-added BSCCO samples studied, the one in which 5 wt%Ag2O was added shows the highest rate of Bi-2223 formation as 92%. The SEM analysis reveals some morphological changes induced by silver addition.
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    Effect of growth rate and composition on the primary spacing, the dendrite tip radius and mushy zone depth in the directionally solidified succinonitrile-Salol alloys
    (ELSEVIER SCIENCE BV, 2003) Cadirli, E; Karaca, I; Kaya, H; Marasli, N
    The succinonitrile (SCN)-Salol alloys for four different concentrations Salol were unidirectionally solidified for five different growth rates in a constant temperature gradient. The microstructure parameters, viz., the primary dendrite arm spacing, lambda(1), dendrite tip radius, R and mushy zone depth, d, were measured. The dependence of the microstructure parameters on the solidification parameters for SCN-Salol alloys were determined by linear regression analysis. The results are compared with theoretical models and published data. (C) 2003 Elsevier Science B.V. All rights reserved.
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    Fabrication and characterization of Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06O10 superconductors prepared by a wet technique
    (ELSEVIER SCIENCE SA, 1998) Celebi, S; Karaca, I; Ozturk, A; Nezir, S
    We have carried out measurements of resistivity on Bi1.84Pb0.34Sr1.91Ca2.03Cu3.06O10 granular samples prepared by the liquid ammonium nitrate method. The standard four-probe technique was used to measure the resistivity and the critical current of the specimen. We estimated the irreversibility line from the broadening of the resistive-superconducting transition in external DC magnetic fields up to 447 mT for the sample fabricated. A non-linear Arrhenius plot of resistive tail has been found. The power law relation B-R(irr)=B-0(1-T/T-c)(3/2) gives a good fit to OUT data Obtained from zero-resistivity temperature in selected magnetic fields. We have also characterized the prepared samples in terms of XRD, SEM and critical currents. (C) 1998 Elsevier Science S.A.
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    Frequency dependence of the intergranular AC loss peak in a high-T-c Bi-(Pb)-Sr-Ca-Cu-O bulk superconductor
    (ELSEVIER SCIENCE BV, 1998) Celebi, S; Karaca, I; Aksu, E; Gencer, A
    We have performed AC susceptibility measurements as a function of temperature; frequency and AC field amplitude on a rectangular bar shaped high-temperature superconductor with nominal composition of Bi1.8.4Pb0.34Sr1.91Ca2.03Cu3.06O10 prepared by the liquid ammonium nitrate method. We have observed that as the frequency (AC field amplitude) increases, the intra- and intergranular AC loss peaks move higher (lower) temperature, which indicates that AC losses are due to both hysteretic bulk pinning losses and viscous (flux flow) losses. M-H curves exhibit a hysteresis which is similar to that of hysteretic superconductors. Activation energy at the grain boundary is extracted from the loss peak at some selected AC fields. Experimental results are qualitatively discussed in the framework of the critical state model (CSM). (C) 1998 Elsevier Science B.V. All rights reserved.
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    Measurement of mass attenuation coefficients for YBaCuO superconductor at different energies
    (IOP PUBLISHING LTD, 2005) Cevik, U; Baltas, H; Celik, S; Karaca, I; Kopya, AI
    The mass attenuation coefficients for Y2O3, BaCO3, CuO compounds, and solid-state forms of YBa2Cu3O7 superconductor were determined at energies of 57.5. 59.5. 65.2, 74.8, 77.1, 87.3, 94.6, 98.4, 122, and 136 keV. The samples were irradiated using a Am-241 point source emitting 59.5 keV photon energies and a Co-57 point source emitting 122 and 136 keV photon energies. The other energies were obtained using secondary targets such as Ta, Bi2O3, and (CH3COO)(2)UO(2)2H(2)O. The gamma- and x-rays were counted by a Si(Li) detector with a resolution of 160 eV at 5.9 keV. Samples were selected on the basis of known composition and mass densities were measured using a densitometer. The experimental results obtained in this study are compared with theoretical values.
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    Production of BiPbSrCaCuO thin films on MgO and Ag/MgO substrates by electron beam deposition techniques
    (WILEY-V C H VERLAG GMBH, 2002) Varilci, A; Altunbas, M; Gorur, O; Karaca, I; Celebi, S
    Superconducting BiPbSrCaCuO thin films were prepared on MgO(001) and Ag/MgO substrates using an electron beam (e-beam) evaporation technique. The effects of annealing temperature and Ag diffusion on the crystalline structure and some superconducting properties, respectively, were investigated by X-ray diffraction, atomic force microscopy, and by measurements of the critical temperature and the critical current density. It was shown that an annealing of both types of films at 845 or 860 degreesC resulted in the formation of mixed Bi-2223 and Bi-2212 phases with a high degree of preferential orientation with the c-axis perpendicular to the substrates. The slight increase of the critical temperature from 103 K to 105 K, the enhancement of the critical current density from 2 x 10(3) to 6 x 10(4) A/cm(2), and the improved surface smoothness are-due to a possible silver doping from the substrate.