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Öğe Decrease of Ca3Co4O9+? thermal conductivity by Yb-doping(Elsevier Sci Ltd, 2015) KIirat, G.; Aksan, M. A.; Rasekh, Sh.; Madre, M. A.; Diez, J. C.; Sotelo, A.In this study, the effect of Yb-substitution on the structural, electrical and thermal transport properties of the Ca3Co4O9+delta system has been investigated in the low temperature region (between 10 and 300 K). The resistivity of samples increases with raising the Yb-concentration in the system. All the samples show a metal-semiconductor transition below 85 K. In the analysis based on Strongly Correlated Fermi Liquid Model, an increased bandwidth and a reduced electronic correlation are found. It has also been found that the energy gap, E-0, value also decreases with Yb-substitution. The samples show positive thermoelectric power, indicating that dominant charge carriers are holes in all the samples. The thermoelectric power value decreases with Yb-substitution. From Mott equation, it is determined that the Fermi energy and hole concentration decrease in the Yb-susbtituted samples, compared to the undoped ones. Thermal conductivity, kappa, is decreased in about 50% of the measured in undoped samples for the 0.01 and 0.03 Yb-doped samples at 300 K. Highest figure of merit, ZT, value is found to be 5.4 10(-3) at 300 K for the unsubstituted sample and the ZT value decreases by the substitution. (C) 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.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 The Effect of K Substitution on Magnetoresistivity and Activation Energy of Bi-2212 System(Springer, 2015) Ozcelik, B.; Yalaz, E.; Yakinci, M. E.; Sotelo, A.; Madre, M. A.In the present work, magnetoresistivity performance of polycrystalline Bi2Sr2Ca1-x K (x) Cu-2 O (8+y) superconductor with x=0.0,0.05,0.075, and 0.1 has been studied by change of flux pinning mechanism, using magnetoresistivity measurements. The samples were prepared using a polymer solution technique. The effects of K substitution for Ca on the irreversibility field, upper critical magnetic field, coherence length, and activation energies have been investigated. The magnetoresistivity of the samples has been measured under applied magnetic fields ranging from 0 to 9 T. Broadening of superconducting transition is observed under magnetic field, explained on the basis of thermally activated flux flow (TAFF) model. The upper critical magnetic field H (C2)(0) and the coherence length (zeta(0)) at T = 0 K were calculated using the resistivity data and from H (C2)(0) ,respectively. H (C2)(0) and xi(0) values have been calculated as 62, 72, 75, and 53 T and 23, 21, 20, and 25 for the 0.0, 0.05, 0.075, and 0.10 K doped samples, respectively. TAFF model has been studied in order to calculate the flux pinning energies. In particular, the flux pinning energies of Bi2Sr2Ca1-x K (x) Cu-2 O (8+y) where x = 0.075 are calculated as 0.0147 eV for 9 T and 0.29 eV for 0 T.Öğe Effect of Secondary Annealing Process on Critical Current Density in Highly Textured Bi-2212 Superconducting System(Springer, 2015) Aksan, M. A.; Madre, M. A.; Rasekh, Sh.; Constantinescu, G.; Torres, M. A.; Diez, J. C.; Sotelo, A.Bi-2212 samples prepared by a solid-state reaction technique have been grown from the melt using the laser floating zone method. After annealing the as-grown bars, the samples showed a good grain alignment and a high transport critical current density. Secondary annealing processes were performed on the annealed samples with the aim of producing Bi-2212 phase controlled decomposition. Hence, the Bi-2201 phase and the secondary phases, which act as effective pinning centers, were obtained with the secondary annealing process. After these thermal treatments, the transport critical current densities of samples significantly increased, when compared to the annealed ones. The maximum critical current density was achieved when the samples were subjected to secondary annealing at 680A degrees C for 168 h with an improvement of similar to 80%, compared to the annealed ones. Moreover, it was found that magnetization of the secondarily annealed samples was also increased. The magnetic critical current densities in these secondary annealed samples were about 3 times higher than the values obtained for the annealed ones. These results clearly indicate that the secondary annealing processes lead to the formation of effective pinning centers in the bulk material.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 Relationship Between Annealing Time and Magnetic Properties in Bi-2212 Textured Composites(Springer, 2013) Ozcelik, B.; Ozkurt, B.; Yakinci, M. E.; Sotelo, A.; Madre, M. A.In this study, we report the physical and magnetic properties of Bi2Sr2CaCu2O (x) textured materials prepared by a LFZ melting technique and annealed for different times (60, 72, 96 and 120 h). SEM images of the annealed samples for 96 and 120 hours indicate very good alignment with the longitudinal rod axis. In all cases, X-ray diffraction patterns show that the Bi-2212 phase is the major one. The magnetization measurements have been carried out as a function of the magnetic field up to 9 kOe. J (c) values of the samples were calculated by using the Bean model. The results indicate that the different annealing time has no significant effects on the T (c) values but, significant change on the critical current values of samples, J (c), has been obtained for sample annealed at 96 hours. We also found that the maximum critical density of J (c) is 5.5x10(5) A/cm(2) at 10 K for the 96 hours annealed sample.Öğe Relationship Between Growth Speed and Magnetic Properties in Bi-2212/Ag Textured Composites(Springer, 2013) Ozkurt, Berdan; Madre, M. A.; Sotelo, A.; Yakinci, M. Eyyuphan; Ozcelik, Bekir; Diez, J. C.In this paper, the measured magnetic properties of Bi2Sr2CaCu2O (x) /3 wt.% Ag textured composite materials prepared by a LFZ melting technique at different growth speeds (5, 15, 30, and 60 mm/h) are presented. X-ray diffraction patterns have shown that Bi-2212 phase is the major one in all cases. The magnetization measurements have been carried out as a function of the magnetic field up to 10 kOe. J (c) values, calculated using Bean's model, indicate that the growth speeds have a significant effect on J (c) . It has been found that the maximum critical current density, 4.42x10(5) A/cm(2) at 10 K, has been obtained for the 15 mm/h grown sample.Öğe Relationship Between Growth Speed, Microstructure, Mechanical and Electrical Properties in Bi-2212/Ag Textured Composites(Springer, 2012) Ozkurt, Berdan; Madre, M. A.; Sotelo, A.; Yakinci, M. Eyyuphan; Ozcelik, BekirTransport properties on Bi-2212 superconductors can be improved by grain orientation methods while mechanical ones can be tuned up by the addition of metallic silver. Both processes can be performed simultaneously using the Laser Floating Zone (LFZ) method. In this work, Bi2Sr2CaCu2O (x) /3 wt.%Ag textured composite materials were prepared by a LFZ melting technique at different growth speeds (5, 15, 30, and 60 mm/h). In all cases, the observed microstructure after annealing shows the Bi-2212 phase as the major one. Although growth speed has no effect on the measured T (c) values, a drastic change on the J (c) values has been found. The best results, both mechanical and transport properties, have been obtained for samples grown at 15 mm/h.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.
