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Öğe Crystallization activation energy and hole concentration properties of the Bi2Sr2Ca1-xCdxCu2O8+y glass-ceramic superconductor rods(Kluwer Academic/Plenum Publ, 2002) Özhanli, Z; Yakinci, ME; Balci, Y; Aksan, MAThe glass rods with the nominal compositions of Bi2Sr2Ca1-xCdxCu2O8+y, where x = 0.0, 0.4, and 0.8, were prepared by melt-casting method. XRD studies exhibited that the Cd substitution caused a transition from Bi-2212 phase to Bi-2201 phase. The multiphase structure was obtained for higher Cd concentration cases. It was determined that the activation energy for crystallization was decreased with the increase of the Cd concentration. When the Cd concentration was increased in the system the superconducting properties decreased. The hole concentration, p, was calculated using the Presland's method. The increase in the Cd concentration increased the hole concentration and the results showed that the Cd-substituted samples were highly in the overdoped region.Öğe Degradation of superconducting properties in MgB2 by Cu addition(Pergamon-Elsevier Science Ltd, 2006) Aksan, MA; Güldeste, A; Balci, Y; Yakinci, METhe (MgB2)(2-x)CUx (x=0-0.5) superconducting system was prepared by a solid-state reaction technique. Microstructural evolution and transport properties including resistivity versus temperature up to a magnetic field of 6 T, activation energy, thermoelectric power and Fermi energy, E-F, and the corresponding velocity, V-F, values of the samples prepared were also investigated. The XRD analysis showed a multiphase formation and no detectable solution of Cu in MgB2. Two different impurity phases, MgCu2 and CuB24, have been identified and their peak intensity increased when the Cu concentration increased. The temperature dependence of the resistivity of the samples showed a metallic behavior down to T-c. But, for the Cu concentrations above 0.3 the superconducting phase transition completely disappeared. The magnetic field strongly affects the electrical properties. For x = 0.0 samples, the transition is found to be sharp, Delta T similar to 1K, but it becomes broader with increasing magnetic field and Cu concentration. The calculated values of carrier concentration, n, of the samples are showed a sharp decrease with increasing Cu content. For x = 0.0 sample the n was calculated to be 12 X 10(21) cm(-3), but for the x = 0.5 sample it decreased to 1.3 X 10(21) cm(-3). We found that the activation energy, U(B), decreased sharply with increasing magnetic field. According to thermoelectric power and Fermi energy, EF, calculations the decrease of the carrier concentration by the additions of Cu into MgB2 gives a decrease in EF and this could be attributed to a shift of the Fermi level towards the top of the sigma-hole band. (c) 2005 Elsevier Ltd. All rights reserved.Öğe Degradation of superconducting properties in MgB2 by formation of the MgB4 phase(Springer/Plenum Publishers, 2002) Yakinci, ME; Balci, Y; Aksan, MA; Adigüzel, HI; Gencer, ASuperconducting MgB2 polycrystalline samples have been fabricated under two different conditions in order to determine the effect of MgB4 phase. A series of samples was placed in an alpha-alumina container closed with a cup and fired under high purity argon gas. The other series of samples was placed in an a-alumina boot without any lid and fired under similar conditions. For the first series of samples, we have found pure MgB2 phase formation and a narrow transition width at 0.4 K. For the second series of samples, significant amount of MgB4 phase were formed and the T-zero was decreased to 27 K. For both the group of samples magnetization hysteresis loops obtained at various temperature range and applied field up to 2 T. The best J(cmag) for the first series of samples was 1.9 x 10(5) x A/cm(2) at 10 K and 0 T, and for the second series of samples was 0.7 x 10(4) x A/cm(2) at 10 K and 0 T.Öğe Effect of substrates on Ic anisotropy in thick film Bi1.7Ga0.3Sr2Ca2Cu3O10+y glass-ceramic superconductors(Elsevier, 2004) Yakinci, ZD; Balci, Y; Yakinci, ME; Aksan, MABi1.7Ga0.3Sr2Ca2Cu3O10+y material was produced by the glass technique and the glass powders obtained then coated on to YSZ and Al2O3 substrates to fabricate 25-30 mu thick films using spray pyrolsis method. The films were then converted to glass-ceramic structure with suitable heat treatment cycles. The effects of substrates on the J(c) and the I-c anisotropy coefficients have been investigated and the results obtained were presented. (C) 2004 Elsevier B.V. All rights reserved.Öğe Enhanced Jc properties of MgB2 materials processed under a magnetic field(Elsevier, 2004) Aksan, MA; Yakinci, ME; Balci, YSuperconducting MgB2 samples with T-c in the range of 35.9-39.7 K were prepared under a magnetic field up to 1 T for 75 h. Crystallographic and transport properties of the samples were characterized by means of XRD, R-T, M-H, and J(c) analysis. According to XRD results, the intensity of the samples prepared in 1 T magnetic field increased more than 35% compared to standard MgB2 samples. The R-T results showed a slight change, while the transport J(c) and J(cmag) increased significantly. (C) 2004 Elsevier B.V. All rights reserved.Öğe Fabrication and properties of (Hg0.8Re0.2)Ba2Ca2Cu3Ox superconducting thick films(Iop Publishing Ltd, 2005) Yakinci, ME; Aksan, MA; Balci, YApproximately 50 mu m thick epitaxial c-axis orientated (Hg0.8Re0.2)Ba2Ca2Cu3Ox superconducting., films have been prepared on MgO(100) substrates using a spraying process and post-Hg-vapour annealing. The effects of the heating temperature-time combinations and the filling factor of Hg (ff(Hg),) on the physical, electrical and magnetic properties of the thick films have been investigated. The interesting observation was the formation of a similar to 1.5 mu m thick interfacial layer between the film and the substrates. This interfacial layer was found to be protective against excessive diffusion and dissolution of the Mg (2+) ions from the substrate or counter-diffusion of Ba, Ca and Cu ions from the films. The XRD investigations showed that the a-b plane of the HgRe- 1223 phase aligns parallel to the substrate surface. The best T-c and T-zero were found to be 13 1.9 and 129.3 K, respectively. Magnetic properties up to 5 T have been investigated. The calculated value of critical current density, J(c) was found to be 4.82 x 10(5) A cm(-2) at 4.5 K. The results obtained suggested that fabrication of HgRe- 1223 thick film superconductors using a spraying process with 130 K transition is possible and they are very promising particularly for electronic applications.Öğe In-situ single chamber arc sputtering process for YCd0.3Ba2Cu3O7-? superconducting thin films(Kluwer Academic/Plenum Publ, 1999) Yakinci, ME; Balci, Y; Aksan, MA; Aydogdu, Y; Ates, HSuperconducting YCd0.3Ba2Cu3O7-delta thin films have been deposited in-situ onto single crystal MgO substrates using a DC arc-sputtering process. The depositions were carried out in a single chamber deposition system equipped with two target holders. The films deposited at the optimum condition exhibited strong (001) orientation with a high peak: intensity. The best electrical properties were achieved to be 90 K for T-e, 81K for T-zero and the transport critical current density J(c) = 675 A/cm(2) at 77K and 2.3x10(3) A/cm(2) at 4.2 K for the sample deposited at the optimum conditions.Öğe Study on the physical and electrical properties of Bi2-?Zn?Sr2Ca2Cu3O10+y glass-ceramic superconductors(Kluwer Academic/Plenum Publ, 2002) Aksan, MA; Yakinci, ME; Balci, YWe have fabricated a series of glass-ceramic (Bi2-deltaZndelta) Sr2Ca2Cu3O10+y, where delta = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0, and investigated the effect of Zn ions on the glass formation, crystallization, thermal, electrical, and on the magnetic properties of the BSCCO-2223 superconductor system. The structural symmetry was found to be tetragonal in all the substitution levels. The best electrical performance was obtained from the 6 = 0 sample, the T-c and T-zero was obtained at 110 K and 107 K, respectively. The J(c) values of the samples were determined using the magnetization hysteresis and Bean's model. The crystallization kinetics were investigated using nonisothermal models of Augis-Bennett. The calculated activation energy, E-a, of the system was found to be in the range of 258-336 kJ/mol.Öğe Synthesis and characterization of Er-substituted Bi-2223 H-Tc glass-ceramic superconductors(Elsevier Science Sa, 2004) Aksan, MA; Yakinci, METhe Bi2Sr2Ca2Cu3-xErxO10+delta (x = 0.5 and 1.0) H-T-c system was prepared by glass-ceramic technique to investigate the microstructural formation. thermal and transport properties. We have found that Er+3 ions have a solid solubility limit in the Bi-2223 system and this limit was passed at x = 1.0. Below this limit glass samples were prepared and converted to the glass-ceramic form easily, but after the solid solubility limit glass formation was diminished and crystallized samples were formed after melt quenching. The substitution had considerable effect on the electrical properties comparing with the unsubstituted Bi-2223 material. The T-c values of the x = 0.5 and 1.0 sample were found to be 71 +/- 0.1 K. The thermoelectric power data were analyzed in view of two band model with linear T-term and Xin's two band model. Thermoelectric power was obtained to be positive for the both samples, indicating that the hole type conductivity is dominant in the materials prepared. The magnitude of thermal conductivity, kappa, was also influenced by the Er concentration. When the Er concentration was increased the magnitude of kappa was suppressed. It strongly indicates that the impurities play a significant role in limiting the heat transport due to a strong increase of the electron-phonon-impurity scattering rate. (C) 2004 Elsevier B.V. All rights reserved.Öğe Synthesis and characterization of glass-ceramic Bi2-xGaxSr2Ca2Cu3O10+y superconductors(Iop Publishing Ltd, 2000) Aksan, MA; Yakinci, ME; Balci, YWe have melt quenched a series of Ga substituted Bi2-xGaxSr2Ca2Cu3O10+y samples with the compositions with x = 0.2, 0.4, 0.6, 0.8 and 1.0. Microstructure, thermal properties including crystallization activation energy, magnetic and superconducting properties were investigated. We have found that Ga3+ ions have a solid solubility limit in the BSCCO system and this limit was passed at x = 0.6. Below this limit HTc BSCCO n = 3 phase was obtained, but after the solid solubility limit samples yield multiphase superconducting properties. Extended heat treatment at 840 degrees C displayed no suppression of the solid solubility Limit. The crystallization kinetic studies were investigated using non-isothermal kinetic models of Kissinger and Augis-Bennet. Higher crystallization activation energies, E-a, were obtained compared to unsubstituted BSCCO materials, glassification improved and glass handling time increased. The substitution had a slight effect on J(c), the flux pinning in the material was improved and better J(c) values were obtained below the solid solubility limit.Öğe Thermal analysis study of Bi2Sr2Ca2Cu3-xErxO10+? glass-ceramic system(Springer, 2005) Aksan, MA; Yakinci, ME; Balci, YWe have fabricated glasses in the Bi-2223 HTc superconductor system with Bi2Sr2Ca2Cu3-xErxO10+delta nominal composition, where x=0.5 and 1.0, by the glass-ceramic technique. Using an analysis developed for non-isothermal crystallization studies, information on some aspects of crystallization temperature and thermal properties has been obtained. The crystallization studies were made using DTA with several uniform rates. The calculations of crystallization activation energies, E-a, and the Avrami parameters, n, were made based on the non-isothermal kinetic theory of Kissinger and the Ozawa's equations. The DTA data of the samples showed that the first crystallization temperature, T-x1, increases and the second crystallization temperature, T-x2, decreases by increasing the Er concentration. This suggests that the Er substitution had significant effect on the glassification of the BSCCO material due to change on the surface nucleation and increased ionic activities at high temperature region. The activation energy for crystallization, E., of the samples was also showed an increase at high Er concentration case. However, the Avrami parameter, n, decreased from 2.5 to 1.7 for x=0.5 and 1.0 samples, respectively. This suggests that the growth mechanism is diffusion-controlled and three-dimensional parabolic growth takes place near the first crystallization temperature. The oxidization rates and the activation barrier for oxygen out-diffusion process, E, was calculated using the TG data. It was found that the total mass gain in the x=0.5 sample is comparably smaller than that of the x=1.0 sample. This shows that the oxygen absorption of the x=1.0 sample is faster than the x=0.5 sample, leading to increase in the oxidization rate in the x=1.0 material.Öğe Thermal conductivity properties of Bi2-?Tl?Sr2Ca2Cu3O10+z glass-ceramic HTc superconductor rods(Kluwer Academic/Plenum Publ, 1999) Aksan, MA; Yakinci, ME; Balci, Y; Ates, HThe Glass rods with nominal composition of (Bi2-alphaTlalpha)Sr(2)Ca(2)Cu3O(10+z) where alpha = 0.15, 0.25 and 0.35 have been prepared by melt casting technique. Preferably c-axis oriented grains were grown after the sintering process. The thermal conductivity kappa(T) measurements were performed between 30K and 290K and calculations were mad: using Wiedemann-Franz law. The kappa(T) values of the samples varied between 3.8 to 8.3 mW/cm.K and showed strong dependence to the heating temperatures and compositions. The best T-c and T-zero were obtained to be 122 K and 115 K respectively.Öğe Thermal conductivity properties of glass-ceramic (Bi2-?-?Ga?Tl?)Sr2Ca2Cu3O10+x high-Tc superconductors(Kluwer Academic/Plenum Publ, 1999) Balci, Y; Yakinci, ME; Aksan, MA; Özdes, A; Ates, HGlass-ceramic BSCCO superconductors with nominal (Bi2-delta-gammaGadeltaTlgamma)Sr2Ca2Cu3O10+x compositions, where delta=0.20, 0.40, 0.60 and gamma=0.10, 0.30, 0.40, have been prepared by the melt quenching method. The thermal conductivity, kappa(T) measurements were performed between 30K and 285K and results obtained found to be between 3.1 and 9.9 mW/cm.K depending an the substitution levels and temperature. Results obtained showed that the phonon-defect scattering dominates above the T-c for all samples but below the T-c a strong coupling mechanism occurs. The best electrical performance was obtained to be 113K and 107K for T-c and T-zero, respectively.Öğe Thermoelectric power and thermal conductivity properties in superconducting (MgB2)2-xCrx materials(Elsevier, 2004) Yakinci, ME; Balci, Y; Aksan, MAExperimental results of thermoelectric power, S(T), and thermal conductivity, kappa(T), in (MgB2)(2-x)Cr-x, were studied. The both electronic and lattice contributions Of K(T) of the samples were investigated between 40 and 300 K. The total K(T) was found in the range of 80-260 mW/cm K and the S(T) measurements showed a variation between 5.9 and 14.3 muV/K at 300 K depending on the Cr substitution. The all S(T) data obtained were found to be positive. (C) 2004 Elsevier B.V. All rights reserved.
