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Öğe The Annealing Effects in the Iron-Based Superconductor FeTe0.8Se0.2 Prepared by the Self-Flux Method(Springer, 2014) Guler, Nilay Kantarci; Ekicibil, Ahmet; Ozcelik, Bekir; Onar, Kubra; Yakinci, M. Eyyuphan; Okazaki, H.; Takeya, H.FeTe0.8Se0.2 single crystals as-cast and post-annealed were prepared by the self-flux method. We have investigated the structural properties of samples by using the XRD, scanning electron microscope (SEM), energy dispersive X-ray (EDX), and magnetic techniques. The SEM results clearly demonstrate that Te ions are quite well substituted for Se ions in the FeSe lattice for the samples. From the XRD and EDX spectra of the both samples, it has been concluded that the post-annealing causes no change in the tetragonal structure of FeTe0.8Se0.2. According to M-H measurements, the perfect diamagnetism has been observed only in low field at 5 and 10 K temperatures. The trend of the magnetization versus temperature curves, measured under a magnetic field of 10 Oe, also support our conclusion about diamagnetic contribution in FeTe0.8Se0.2 single crystal explored in this study. The as-cast and post-annealed samples show the onset of diamagnetism at temperatures, T-c.on(mag), 12.45 and 13.27 K, respectively. In addition, those curves indicate that the high field value and some impurities reveal ferromagnetic interactions.Öğe Effect of Tungsten (W) Substitution on the Physical Properties of Bi-(2223) Superconductors(Springer, 2014) Turk, Nihal; Gundogmus, Hakan; Akyol, Mustafa; Yakinci, Z. Deniz; Ekicibil, Ahmet; Ozcelik, BekirThe superconducting Bi1.6Pb0.4Sr2Ca2Cu3-x W (x) O10+y (x=0.00, 0.05, 0.10, 0.15) bulk samples were prepared by the solid-state reaction method. The effects of W substitution on the BSCCO system have been investigated by the electrical resistivity (rho-T), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), magnetic hysteresis and critical current density measurements. It has been found that the Bi-(2212) low-T (C) phase is formed for all the substitution levels, together with the Bi-(2223) high-T (C) phase. The results obtained from the XRD data show that the Bi-(2223) phase gradually transforms into the Bi-(2212) phase with increasing W substituting for Cu. In addition, from the magnetization measurements at the temperatures below the zero resistance temperatures of the samples, we have observed that a decreasing in magnitude of |M| with the increasing measurement temperature and W concentration. Therefore, the decreasing of |M| related to superconducting volume seems to imply an existence of flux-pinning centers in our samples.Öğe Effect of Vanadium-Titanium Co-doping on the BPSCCO Superconductor(Springer, 2011) Yazici, Duygu; Ozcelik, Bekir; Altin, Serdar; Yakinci, M. EyyuphanWe have produced the (BiPb)(2)V (x) Sr2Ca3Cu4-y Ti (y) O12+delta compounds for x=0.05 and y=0, 0.05, 0.10 and 0.20 by glass-ceramic method. The effects of vanadium adding and Ti doping on the structure have been investigated by electrical resistance, scanning electron micrographs (SEM), XRD patterns and magnetic hysteresis loop measurements. It has been found that the high-T (c) superconducting phase, (2223), is formed in the samples annealed at 845 degrees C for 185, with concentration x=y=0.05. However, with increasing Ti doping the (2223) phase transforms into the (2212) phase. We have observed no superconducting properties for x=0.05 and y=0.20 compound. It has completely transformed to semiconductor. In addition, the critical current densities (J (c)), calculated from the hysteresis loop measurements by using Bean's critical state model are obtained for the samples in the same doping range. Our data have indicated that J (c) decreases with increasing temperature and Ti concentration.Öğe Improvement of High Tc Phase Formation in BPSCCO Superconductor by Adding Vanadium and Substituting Titanium(Springer/Plenum Publishers, 2011) Yazici, Duygu; Ozcelik, Bekir; Yakinci, M. EyyuphanWe have produced the (BiPb)(2)V (x) Sr2Ca3Cu4-y Ti (y) O12+delta with x=0.1 and y=0.050,0.10,0.2 and 0.3 compounds by melt-quenching method. Structural and superconducting properties of the produced samples were investigated by Scanning Electron Microscopy, X-ray diffraction patterns, electrical resistance measurements and dc-magnetic hysteresis loop measurements. The pure high-T (c) phase (2223) is nearly found with Ti substitution for x=0.05 and 0.10. The onset critical temperature (T (c).(onset)) of the samples increases up to 111 K with doping up to x=0.20. In addition, considerable large values of the critical current densities (J (c)), calculated from the hysteresis loop measurements by using Bean's critical state model are obtained for the samples in the same doping range. Our data have indicated that J (c) decreases with increasing temperature and Ti concentration.Öğe Physical and magnetic properties of Sm0.2Gd0.8Ni4B compound(Trans Tech Publications Ltd, 2012) Ozcelik, Bekir; Kantarci, Nilay; Nane, Onur; Yakinci, M. EyyuphanPhysical properties of the Sm0.2Gd0.8Ni4B compound have been investigated by means of the X-ray powder diffraction, DC and AC-susceptibility techniques. The compound studied crystallizes in CeCo4B type structure with P6/mmm space group. The unit-cell parameters a and c are determined as 5.01 and 6.95 angstrom, respectively, and the unit-cell volume V is calculated as 151.08 angstrom(3). DC and AC magnetic measurements present the visible magnetic phase transition from paramagnetic to ferromagnetic, around definite transition temperature. The magnetic phase transition temperature of the compound is obtained from DC magnetization, AC-susceptibility and the well known Kouvel-Fisher method as 36.6, 35.7 and 35.2 K, respectively. The saturation magnetization (Ms) and the coercive fields (Hc) of the compound are found to be 3.7 mu(B)/f.u and 277 Oe, respectively, by using the hysteresis loops at 9.5 K. We have also investigated the non-linear AC-susceptibility of the compound, around its ferromagnetic transition temperature, as a function of temperature, frequency and amplitude of the AC-driving field. In order to explain the measured experimental results, we have used the theory developed for ferromagnetic, based upon the mean field model. The measurements exhibit both frequency and amplitude dependencies. Observed dependencies are compared with the existing theories of linear and nonlinear susceptibilities with reference to short- and long-range interactions. In Kouvel-Fisher method, one plots 1/chi(1)*(d chi(-1)/dT) against T, obtaining a straight line. The slope of this line gives the critical exponent gamma, and it intersects the T axis at Tc. In order to obtain d chi(-1)/dT and the best straight line, we used a two-point numerical differentiation program and linear regression method, respectively. The critical exponent gamma of the sample is calculated to be 2.78 +/- 0.05. The value of the critical exponent beta, which is characteristic of static phase transition to a ferromagnetic state, is estimated as 2.41 +/- 0.3 from the slope of the line obtained the plot of the absolute third-harmonic values versus the reduced temperature on a log log scale.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 Structure and physical properties of iron-selenide KxFe2-ySe2(Elsevier Science Sa, 2015) Guler, Nilay Kantarci; Ozcelik, Bekir; Ekicibil, Ahmet; Onar, Kubra; Yakinci, M. Eyyuphan; Okazaki, H.; Takeya, H.We have synthesized the superconducting KxFe2-ySe2 (x = 0.8) crystal by using the one step technique. SEM images demonstrate terrace-like structures on the surface and the sample contains two phases. XRD pattern gives the (00l) peaks. The main peaks are indexed to the I4/m space group and the lattice parameter c is calculated to be 13.94076 angstrom. ZFC magnetization measurement exhibits a very sharp transition which indicates the presence of superconductivity in the crystal. The superconducting transition temperature, T-c(mag), obtained from magnetization measurement is estimated to be 31 K. The critical current value, Jc(0), deduced from the M-H loops for sample is approximately 9 x 10(4) A/cm(2) at 5 K. From magnetoresistivity measurement the T-c, T-c(onset) and T-c(offset) temperatures at the absence of the external magnetic field are found as 32.2, 33.1 K and 31.5 K, respectively. The calculated flux pinning or activation energy U is around 0.46 eV for 0 T. (C) 2015 Elsevier B.V. All rights reserved.
