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Öğe Effect of Fe Substitution for Cu on Microstructure and Magnetic Properties of Laser Floating Zone (LFZ) Grown Bi-2212 Rods(Springer, 2013) Ozabaci, M.; Sotelo, A.; Madre, M. A.; Yakinci, M. E.In this paper, the laser floating zone (LFZ) technique has been used to fabricate Bi2Sr2Ca1Cu2-x Fe (x) O (x=0,0.01,0.03,0.05,0.1) superconducting fibers. The effects of Fe substitution on grain alignment and superconducting properties of annealed fibers were studied using SEM/EDX microanalysis in addition to phase analysis and magnetic measurements realized through XRD, M-T, and M-H loops. In the same growth conditions, higher Fe contents lead to a more compact microstructure with lower porosity but also caused the formation of poor superconducting phases followed by the decrease of T (c) and J (cmag). No evidence of enhanced pinning capability was found in the magnetic measurements. Annealed rods indicated a weakly ferromagnetic-like behavior at relatively high doping levels.Öğe Effect of Substrates on Superconducting Properties of MgB2 Thick Films Fabricated by Spraying Method(Springer, 2011) Ozabaci, M.; Aksan, M. A.; Yakinci, M. E.MgB2 thick films have been prepared on Al2O3, MgO and SrTiO3 ceramic substrates using the spray method with two different heat-treatment cycles, 925 A degrees C with 10 min (Group A) and 610 A degrees C with 24 h (Group B). The structural/microstructural (XRD, SEM) and transport (R-T, MR-T) properties of the films prepared were investigated. XRD analysis showed that the films in Groups A and B consisted mainly of MgB2, but the peaks originating from the substrates were also observed in films fabricated on the Al2O3 substrate. From scanning electron microscopy, it was seen that the surface of MgB2 thick films prepared is not sufficiently homogeneous. Some cracks and heaps with different sizes were observed. The best electrical results were obtained for films in Group A on Al2O3 substrate. The T (c) value for films in Group A was found to be 36.1 K, 36 K and 35 K, for Al2O3, MgO and SrTiO3 substrates, respectively. No superconducting state was reached for films in Group B on the SrTiO3 substrate. In all the films which showed a superconducting state, the magnetic field strongly affected T (c) . A tail in the resistance curves was observed with increasing magnetic field for films in Group A on the MgO and SrTiO3 substrates. At magnetic fields above 1 T for films in Group A on the SrTiO3 substrate and above 2 T for films in Group A on the MgO substrate, the zero-resistance temperature, T (0), was not obtained.Öğe Enhancement of Magnetic and Transport Properties of Superconducting Fe1-xMnxSe0.5Te0.5 Single Crystals(Springer, 2019) Ozabaci, M.; Yakinci, K.; Yakinci, M. E.In this article, we provide the synthesis and characterization of manganese-substituted FeSe0.5Te0.5 single crystals in the nominal composition range of Fe1-xMnxSe0.5Te0.5 (x = 0.01, 0.05, 0.1) via the self-flux method. It is observed that Mn-substitution improved the bulk superconducting properties of FeSe0.5Te0.5 considerably by increasing the superconducting transition temperatures determined from both resistive and magnetic transitions. Our results reveal that x = 0.05 Mn-substituted sample yields the most appropriate superconducting performance, considering all the samples employed. The corresponding value is computed to be 4.7 x 10(4) A/cm(2) for x = 0.05 sample at 8 K under 0.2 kOe, and it decreases to the values 2.54 x 10(4) A/cm(2) and 1.34 x 10(4) A/cm(2) for the x = 0.01 and x = 0.1 samples, respectively.Öğe Grain alignment and its relationship with superconductivity and thermal transport of Ni-substituted Bi-2212 textured rods fabricated at two different growth rates(Springer, 2015) Ozabaci, M.; Kizilaslan, O.; Madre, M. A.; Yakinci, M. E.; Sotelo, A.The microstructure, grain alignment, superconducting and thermal transport properties of Ni-substituted Bi-2212 rods grown at two different speeds (15 and 30 mm h(-1)) through the Laser Floating Zone method, have been evaluated. Significant variations in grain size, grain alignment, electrical and thermoelectric power properties have been observed for the rods depending on the growth and substitution rates. The highest aligned structure was obtained on unsubstituted rods grown at 15 mm h(-1). Both increased substitution and growth rates degraded the grain alignment. The presence of Ni-based secondary phases showed that Ni is not totally incorporated into the crystal structure, which, in turn, caused a decrease on the average grain size of the rods. With increasing Ni concentration, peak values of thermoelectric power of the rods, which lie between 3.8 and 6.4 mu VK-1, monotonically decreased while thermal conductivity values did not show any systematic change. The activation energy of flux motion, Uo, was calculated from the field dependent resistivity-temperature curves in a range of 0-8 T. Superconducting transition temperatures, T-c(onset) and T-c(offset), and activation energy, U-o, were found to decrease with increasing Ni contents and applied magnetic field. It has been estimated from the magnetic field dependence of activation energy of the samples that plastic creep of the collective vortices is dominant in the rods.Öğe Growth Speed and Substitution Effects on Alignment and Thermal Transport Properties of Bi-2212 Textured Superconductors(Springer, 2015) Ozabaci, M.; Rasekh, Sh.; Kizilaslan, O.; Madre, M. A.; Sotelo, A.; Yakinci, M. E.Fe-substituted superconducting thin BiSrCaCuO rods with nominal compositions of Bi2Sr2Ca1Cu2-x Fe (x) O8+delta (x = 0, 0.01, 0.03, 0.05, and 0.1) were fabricated using the laser floating zone technique at two different growth speeds, 15 mm h(-1) and 30 mm h(-1). The influences of growth speed and Fe substitution on the grain alignment in the rods were evaluated by means of x-ray pole figure studies. The obtained results showed that both applied growth speed and Fe substitution play a crucial role on grain alignment, which is strongly connected with the current-carrying capacity of the rods. It was found that the rods grown at 15 mm h(-1) (G15) have stronger orientation than the rods grown at 30 mm h(-1) (G30). However, in contrast to the G15 rods, an increased substitution rate improved the orientation of the G30 rods. Another important observation is that the increase on the substitution caused a decrease on the grain size of all the rods. The decrease of critical temperature values of the rods upon substitution was ascribed to both grain size effect and formation of a nonsuperconducting Fe-rich phase detected in scanning electron microscope/energy-dispersive x-ray analyses. The thermal conductivity values of the G15 and G30 rods were found to be in the range of 0.9-1.9 and 1.1-1.18 W m(-1) K-1 at 150 K, respectively. The higher values of figure of merit (ZT), at all temperature ranges, were obtained from the highest substituted rods (x = 0.1) for both of the applied growth speeds. In addition, it was observed that the ZT of G30 rods are up to three times higher than that of G15 ones.Öğe Preparation and characterization of CaO-Al2O3-SiO2 (CAS) glass-ceramics(Elsevier, 2016) Ozabaci, M.; Aksan, M. A.; Kirat, G.; Kizilaslan, O.; Yakinci, M. E.A glass-ceramic CaO-Al2O3-SiO2 (CAS) system with CaAl2Si2O8 (Anorthite) composition was prepared at two different heat treatment temperatures following the melt-quenching in an arc-melter system. Structural and magnetic properties of the samples were characterized using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and magnetic measurements (M-H, M-T). A clear phase transformation, hexagonal to triclinic, was obtained when heat treatment temperature increased from 930 degrees C to the 1100 degrees C Two different impurity phases were obtained for the sample heat treated at 930 degrees C, but then these phases dissolved in the main matrix when temperature increased to 1100 degrees C and single phase CaAl2Si2O8 was obtained. All the samples showed diamagnetic behavior between room temperature and similar to 150 K, but then ferri- or ferromagnetic phase transformation was obtained. This transformation is found to be independent from the heating temperature and believed to be associated with the trapping of free electrons at crystal imperfections and cation ordering between Al and Si in the main matrix. (C) 2016 Elsevier B.V. All rights reserved.Öğe Structural and Superconducting Properties of Magnetically Doped Bi-2212 Textured Rods Grown by Laser Floating Zone (LFZ) Technique(Springer, 2013) Yakinci, M. E.; Madre, M. A.; Ozabaci, M.; Sotelo, A.In this work, we have applied the laser floating zone technique, a well-known method to produce highly aligned crystals and dense morphology, with improved transport properties, to obtain Bi2Sr2Ca1Cu2-x Ni (x) O (x=0, 0.01, 0.03, 0.05 and 0.1) fibers of about 1.8 mm diameter x12 cm length. Microstructural and magnetic properties of textured materials grown in the optimum conditions were analyzed through scanning electron microscope (SEM), energy dispersive X-ray analysis (EDX), powder XRD patterns, M-T and M-H loops. XRD analysis showed that addition of Ni at x=0.1 level caused the formation of small amounts of Bi-2201 phase. Thermal treatment has produced the agglomeration of Ni particles up to a few mu m, detected by dot mapping EDX. Both T (c-onset) and J (cmag) values of the annealed rods decreased with the increasing Ni substitution rate, determined from the M-T and M-H loops, respectively.
