Effect of Fe substitution for Cu on microstructure and magnetic properties of laser floating zone (LFZ) grown Bi-2212 rods

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.This work was supported by the Scientific and Technological Research Council of Turkey (TUBITAK) in the scope of 2214 International Doctoral Research Fellowship Program. A. Sotelo and M.A. Madre acknowledge MICINN-FEDER (Project MAT2008- 00429) and DGA (Consolidated research group T12) for financial support.Peer Reviewe

Grain alignment and its relationship with superconductivity and thermal transport of Ni-substituted Bi-2212 textured rods fabricated at two different growth rates

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 μVK−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, Tconset and Tcoffset, and activation energy, Uo, 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.This work was supported by the Scientific and Technological Research Council of Turkey (TUBITAK) under 2214 International Doctoral Research Fellowship Program (M. Ozabaci). A. Sotelo and M.A. Madre acknowledge DGA (Consolidated research group T12) for financial support. M. A. Madre acknowledges MINECO-FEDER (Project MAT2011-22719) for funding.Peer Reviewe

Thermal conductivity and thermoelectric power of potassium and sodium-substituted Bi-2212 superconductor prepared by PEI technique

In the present study, thermal conductivity (κ − T), thermoelectric power (S − T), and figure of merit (ZT) measurements from room temperature down to 20 K of polycrystalline Bi2Sr2Ca1−xNaxCu2O8+y and Bi2Sr2Ca1−xKxCu2O8+y superconductors prepared by a polymer solution technique using polyethyleneimine, PEI, have been carried out. The thermoelectric power, S(T), of samples have both negative (at high T) and positive values (at low T), and are found to increase with decreasing temperature, reaching their maximum values (peaks) around their Tc values and dropping rapidly to zero below Tc. Na and K substitutions also have a considerable effect on the thermal conductivity, κ. Namely, with increasing Na concentration the magnitude of κ is decreased, but for the K substitution, the magnitude of κ is drastically increased for x = 0.75.This work is supported by the Research Fund of Çukurova University, Adana, Turkey, under Grant Contract Nos FBE.2013YL45 and FEF2013BAP22. A. Sotelo and M. A. Madre wish to thank the Gobierno de Aragon and the Fondo Social Europeo (Research Group T12) and MINECO-FEDER (Project MAT2013-46505-C3-1-R) for the financial support.Peer Reviewe

Sintering effects in Na-substituted Bi-(2212) superconductor prepared by a polymer method

In this study, Na-substituted Bi2Sr2Ca0.9Na0.1 Cu2O superconductor samples were prepared by a polymer solution method. Three different sintering temperatures (850, 860, and 870 °C) were used to study the effect of Na substitution. The samples have been characterized using X-ray diffraction, scanning electron microscopy (SEM), DC electrical resistivity, and DC magnetic measurements. Magnetoresistivity measurements have shown a broadening of the superconducting transition under magnetic field which is explained on the basis of the thermally activated flux flow (TAFF) model. The calculated flux pinning energies of the samples varied from 0.17 to 0.02 eV by means of increasing magnetic field 0 to 9 T. The upper critical magnetic field Hc2(0) and the coherence length ( ζ(0)) at T = 0 K were calculated using the resistivity data. Hc2(0) and ξ(0) values have been calculated as 194, 144, and 139 T and 15.5, 15.2, and 13 Å at 850, 860, and 870 °C, respectively. TAFF model has shown Bi2Sr2Ca0.9Na0.1Cu2O8+y flux pinning energies are 0.015 eV at 9 T in all cases, while they were 0.165, 0153, and 0.149 eV at 0 T for samples sintered at 850, 860, and 870 °C, respectively.This work is supported by the Research Fund of Çukurova University, Adana, Turkey, under grant contracts no. FEF2013YL18.Peer Reviewe

Effect of Na substitution on superconducting properties of Bi-2212 ceramics prepared by Sinter-Forged process

Bulk textured Bi2Sr2Ca1-xNaxCu2Oy (x = 0.0, 0.05, 0.075 0.1, 0.15, and 0.20) ceramics have been prepared by a polymer solution method followed by sinter-forging process. The microstructure and superconducting properties of textured samples were affected by Na substitution. According to XRD data, all samples show that Bi-2212 phase is the major one, and main peaks corresponded to the (00l) diffractions confirming that the crystallographic c-axis is perpendicular to the surface. SEM micrographs showed that samples are composed of well-stacked and oriented grains. The best critical temperature, TC, was measured as 85.6 K for x = 0.075. The critical current density Jc, values decrease gradually with the increase of applied magnetic field. The maximum calculated Jc was 1.38 × 105 A/cm2 at 10 K for 0.075 Na sample which is about four times higher than the 0.34 × 105 A/cm2 value determined for the pure sample. The anisotropic properties of these textured materials have been investigated using magnetic measurements

Effect of Ce substitution on the magnetoresistivity and flux pinning energy of the Bi2Sr2Ca1-x CexCu2O8+δ superconductors

In this study, the effect of Ce doping on the properties of Bi 2Sr2Ca1-x Ce x Cu2O 8+δ ceramic superconductors, with x=0.0, 0.01, 0.05, 0.1, and 0.25, has been investigated. Samples' precursors were prepared using the conventional solid state method and subsequently textured using the Laser Floating Zone technique. The magnetoresistance measurements were studied under various applied magnetic fields. The activation energies, irreversibility fields (H irr ), upper critical fields (H c2) and coherence lengths at 0 K (ξ(0)) were calculated from the resistivity versus temperature (ρ-T) curves, under DC magnetic fields up to 5 T. The thermally activated flux flow model has been applied in order to calculate the flux pinning energies. The results indicated that H c2(0) varied from 416.19 to 115 T and the flux pinning energies varied from 1.46 to 0.042 eV at 0 T. © 2013 Springer Science+Business Media New York.This work is supported by the Research Fund of Çukurova University, Adana, Turkey, under grant contracts No: FEF2011YL27 and FEF2013BAP11. A. Sotelo and M.A. Madre wish to thank the Gobierno de Aragón (Research Group T12), for financial support. M.A. Madre also acknowledges the MINECO-FEDER (Project MAT2011-22719) for funding.Peer Reviewe

Improvement of thermoelectric performances of Bi2Sr2Co1.8Ox textured materials by Pb addition using a polymer solution method

El pdf del artículo es la versión pre-print.Bi1.6Pb0.4Sr2Co1.8Ox ceramics have been prepared through a polymer solution method using polyethyleneimine. From these powders, bulk textured materials have been prepared using a melt growth technique. Microstructure has been observed by scanning electronic microscopy (SEM) which has shown that samples are mainly composed by the thermoelectric phase, with small amounts of secondary phases. Electrical resistivity measurements showed very small values nearly constant with temperature, whilst thermopower increases from room temperature to 650 °C. Power factor at 50 °C is about 0.15 mW/K2·m and 0.30 at 650 °C, which makes these ceramics good candidates for power generation applications. © 2012 Elsevier B.V. All rights reserved.This research has been supported by the Spanish Ministry of Science and Innovation-FEDER (Project MAT2008-00429) and by the Gobierno de Aragón (Consolidated Research Groups T12 and T87). Sh. Rasekh also acknowledges a JAE-PreDoc 2010 grant from CSIC.Peer Reviewe

Thermoelectric ceramic materials, and the quest to improve their performance

El pdf del artículo es la versión post-print.Peer Reviewe

Effect of Na substitution and Ag addition on the superconducting properties of Bi-2212 textured materials

The effect of Ag addition on the structural and superconducting properties of Bi2Sr2Ca0.925Na0.075Cu2Oy + x wt% Ag materials with x = 0, 1, 3, and 5 prepared by sol–gel method, followed by directional growth using the laser floating zone (LFZ) technique has been investigated. Powder XRD measurements demonstrate that all samples are composed by nearly single Bi-2212 phase. SEM micrographs and EDX results confirms that Bi-2212 is the major phase with minor amounts of a Bi-free one (Sr, Ca)CuO2 and metallic Ag. According to the magnetic results, Ag does not modify the superconducting critical temperature, Tc, but significantly increases the magnetic hysteresis loop area. The magnetic critical current density is maximum in the 3 wt% Ag containing samples, due to the larger grain clusters found in these samples, reaching 2.1 × 105 A/cm2 at 10 K. Moreover, Ag addition also improves the rods mechanical properties, determined by three point bending tests. The highest bending stress is also reached in the 3 wt% Ag samples

Effect of Yb substitution in Bi-2212 ceramics prepared by laser floating zone technique

In the present work, the influence of Yb substitution for Bi on the structural, electrical and magnetic properties of textured Bi-2212 ceramics has been investigated. X-ray diffraction studies indicated that the major peaks correspond to the Bi-2212 phase. The SEM images clearly show that there is an increase in the number of secondary phases with raising Yb content while all samples maintain similar grain morphology. Magnetic analysis of all samples has been done by magnetic hysteresis measurements, indicating that an enhancement in the M-H loops is obtained by a low Yb doping. The critical current density of samples was calculated from the M-H curves by using Bean’s critical state model, indicating that sample with 0.05 Yb substitution possesses the highest Jc values.The authors wish to thank the Gobierno de Aragón and Fondo Social Europeo (Grupos de Investigacion Consolidados T12 and T87) and MINECO-FEDER (MAT2013-46505-C3-1-R) for financial support.Peer Reviewe