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

Improvement of the intergranular pinning energy in the Na-doped Bi-2212 superconductors

In the present study, magnetoresistivity performance of polycrystalline Bi2Sr2Ca1−xNaxCu2O8+y superconductor with x = 0.0, 0.05, 0.075, 0.10, 0.15, and 0.20 has been studied by change of flux pinning mechanism. Samples have been prepared using a polymer solution technique with polyethyleneimine. The effects of Na substitution for Ca on the activation energies, irreversibility field, upper critical magnetic field and coherence length have been studied. The magnetoresistance of samples has been measured at applied magnetic fields between 0 and 9 T. Broadening of superconducting transition has been observed under magnetic field, explained on the basis of Thermally Activated Flux Flow (TAFF) model. The upper critical magnetic field HC2(0) and the coherence length (ζ(0)) at T = 0 K were calculated using the resistivity data and HC2(0), respectively. HC2(0) and ξ(0) values have been calculated as 186.4, 195.5, 321.0, 296.0, 292.5, 280.9 T, and 13.29, 12.98, 10.13, 10.55, 10.61, 10.69 Å for the 0.0, 0.05, 0.075, 0.10, 0.15, and 0.20 Na-doped samples, respectively. TAFF model has been studied in order to calculate the flux pinning energies. In particular, the flux pinning energies of Bi2Sr2Ca1−xNaxCu2O8+y where x = 0.075 determined to be 0.019 eV for 9 T and 0.239 eV for 0 T.This work is supported by Research Fund of Cukurova University, Adana, Turkey, under grant contracts no: FEF 2013YL18 and FEF2013BAP22.Peer Reviewe

Drastic microstructural modification of Bi2Ca2Co2Oy ceramics by Na doping and laser texturing

Bi2Ca2-xNaxCo2Oy materials with x = 0 = x = 0.125, have been prepared either by sintering through the classical ceramic method, or textured using the laser floating zone technique. XRD results have shown that Bi2Ca2Co2Oy phase is the major one, independently of the Na content, in both kind of samples. SEM studies have shown a drastic microstructural modification between sintered and laser-textured materials. Na-doping increases density and grain sizes in sintered materials, while it enhances grain alignment in laser-grown materials. Moreover, it decreases secondary phase content in all cases. Electrical resistivity is also drastically reduced through texturing, when compared to the sintered samples, and Na-doping further decreases it. The lowest values determined in the laser-textured samples (26 mO cm at 650 °C) are around 40% lower than the best in sintered materials. On the other hand, S has been only slightly affected for Na-doping or processing method for all samples. Consequently, the highest power factor at 650 °C (0.18 mW/K2 m) has been obtained in laser-grown 0.075Na-doped samples, which is around 20 and 70% higher than the measured in undoped laser-textured samples, and sintered ones, respectively. Se han preparado materiales de composición Bi2Ca2-xNaxCo2Oy, con x = 0 ≤ x ≤ 0,125, por el método de estado sólido, además de texturarlos mediante la técnica de zona flotante inducida por láser. Los resultados de XRD han mostrado que la fase Bi2Ca2Co2Oy aparece como la mayoriraria, independientemente del contenido de Na, en ambos tipos de muestras. Los estudios SEM han encontrado una drástica modificación microestructural entre materiales sinterizados y texturados. El dopado con Na aumenta la densidad y el tamaño de grano en materiales sinterizados, mientras que mejora la orientación de los granos en materiales texturados. Además, disminuye el contenido de fases secundarias en todos los casos. La resistividad eléctrica disminuye drásticamente con el texturado, en comparación con las muestras sinterizadas, mientras que el dopado con Na la disminuye aún más. Los valores más bajos determinados en muestras texturadas (26 mΩ cm a 650 °C) son alrededor del 40% menor que los mejores datos publicados en materiales sinterizados. Por otro lado, S solo se ha visto afectado levemente, tanto por el dopado como por el método de procesado. En consecuencia, el máximo factor de potencia a 650 °C (0,18 mW/K2 m) se ha obtenido en muestras dopadas con 0,075 Na y texturadas, que es alrededor de un 20 a un 70% superior al medido en muestras texturadas sin dopar y sinterizadas, respectivamente

Detail investigation of thermoelectric performance and magnetic properties of Cs-doped Bi2Sr2Co2Oy ceramic materials

Bi2Sr2-xCsxCo2Oy materials with 0 = x = 0.15, have been fabricated via the classical ceramic technique. XRD results have indicated that undoped and Cs-substituted samples are composed of Bi2Sr2Co2Oy phase as the major one. Microstructural studies have demonstrated the formation of a liquid phase, which allows a drastic grain growth. This factor is responsible for a drastic improvement of relative density, reaching about 95% of the theoretical one for 0.125 Cs content. On the other hand, electrical resistivity has been reduced up to 14 mO cm at 650 °C for 0.125 Cs content, around 40% lower than the obtained in undoped samples. As a consequence, Seebeck coefficient has been decreased due to the raise in charge carrier concentration. The highest power factor at 650 °C (0.21 mW/K2 m) has been found for 0.125 Cs substituted sample, about 40% larger than the obtained in undoped samples, and very similar to the notified in single crystals (0.26 mW/K2 m). Magnetisation with respect to temperature results have demonstrated that measured samples have a paramagnetic property above 50 K, except 0.10 Cs. Magnetic hysteresis curves have shown that the slopes and the magnitudes have increased with decreasing temperature

Effect of K substitution on structural, electrical and magnetic properties of Bi-2212 system

In this work, the effect of K in the Bi2Sr2Ca1−xKxCu2O8+y superconductor with x = 0.0, 0.05, 0.075, and 0.1, has been investigated. The samples were prepared by a polymer solution technique using polyethyleneimine, PEI. The effects of K substitution have been investigated by electrical resistivity (ρ−T), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy, and magnetic characterizations. SEM and XRD results have shown that the Bi-(2212) phase is the major one independently of the K concentration. Moreover, the microstructure of samples is improved with K-concentration up to x = 0.075. From electrical resistivity measurements we have found that Tc is slightly higher than 91 K for K-concentrations up to 0.075, and then it decreases for higher doping for about 0.5 K. Moreover, this trend is maintained in the magnetic measurements in which the hysteresis loops are increased until 0.075 K-concentrations. The maximum calculated Jc, using Bean’s model, has been found at around 4.5 106A/cm2 at 10 K and ~1,000 Oe for the 0.05 K doped samples.This work is supported by Research Fund of Cukurova University, Adana, Turkey, under Grant Contracts No: FEF 2013YL17 and FEF2013BAP22.Peer Reviewe

Improvement of superconducting properties in Na-doped BSCCO superconductor

In this study, the effect of Na substitution on the structural, electrical and magnetic properties of Bi2Sr2Ca1−xNaxCu2O8+y superconductor with x = 0.0, 0.05, 0.075, 0.1, 0.15, and 0.2 has been investigated. The effects of partial replacement of Na for Ca have been investigated by electrical resistivity (ρ–T), scanning electron microscopy (SEM), X-ray diffraction (XRD), and magnetic characterizations. XRD results have shown that the Bi-(2212) phase is the major one independently of the Na-concentration. SEM results indicate that the microstructure of samples is improved with increasing of Na-contents. From the electrical resistivity measurements it has been found that Tc is slightly higher than 92 K for Na-concentrations up to 0.20. Moreover, this trend is maintained in the magnetic measurements where the hysteresis loops are increased with increasing Na-concentrations. The maximum calculated Jc, using Bean’s model, is around 10 × 106 A/cm2 at 10 K and ~1,000 Oe for the 0.075 Na doped samples.This work is supported by Research Fund of Cukurova University, Adana, Turkey, under Grant Contracts No: FEF2013BAP22.Peer Reviewe

Improvement of the intergranular pinning energy in the Na-doped Bi-2212 superconductors

In the present study, magnetoresistivity performance of polycrystalline Bi2Sr2Ca1-xNaxCu2O8+y superconductor with x = 0.0, 0.05, 0.075, 0.10, 0.15, and 0.20 has been studied by change of flux pinning mechanism. Samples have been prepared using a polymer solution technique with polyethyleneimine. The effects of Na substitution for Ca on the activation energies, irreversibility field, upper critical magnetic field and coherence length have been studied. The magnetoresistance of samples has been measured at applied magnetic fields between 0 and 9 T. Broadening of superconducting transition has been observed under magnetic field, explained on the basis of Thermally Activated Flux Flow (TAFF) model. The upper critical magnetic field HC2(0) and the coherence length (?(0)) at T = 0 K were calculated using the resistivity data and HC2(0), respectively. HC2(0) and ?(0) values have been calculated as 186.4, 195.5, 321.0, 296.0, 292.5, 280.9 T, and 13.29, 12.98, 10.13, 10.55, 10.61, 10.69 Å for the 0.0, 0.05, 0.075, 0.10, 0.15, and 0.20 Na-doped samples, respectively. TAFF model has been studied in order to calculate the flux pinning energies. In particular, the flux pinning energies of Bi2Sr2Ca1-xNaxCu2O8+y where x = 0.075 determined to be 0.019 eV for 9 T and 0.239 eV for 0 T. © 2015, Springer Science+Business Media New York

Effect of Cooling Rate on Structure, Composition, and Superconducting Properties of FeTe0.6Se0.4 Prepared by Self-Flux Technique

We have reported the synthesis and characterization of FeTe0.6Se0.4 prepared by self-flux technique with two different cooling rates, namely 0.8 and 4.5°C/h. The effect of cooling rate on the samples has been characterized by using scanning electron microscopy (SEM) together with energy dispersive X-ray (EDX) spectrometer, X-ray diffraction (XRD), magnetization and magnetic hysteresis techniques. Four strong peaks were observed in the powder XRD patterns of both samples corresponding to the reflected intensities from the (001), (002), (003), and (004) planes of the tetragonal structure having space group P4/nmm. However, sample 2 has extra peaks corresponding to various non-superconducting binary phases of FeSe and FeTe. It can be concluded that the multiphase behavior of system increases with increasing cooling rate. The critical current value at 5 K, Jc(0), deduced from the M - H loops is approximately 5.6 × 104 A/cm2 for sample 1 and 7.5 × 104 A/cm2 for sample 2. Increasing in the critical current value can be attributed to non-superconducting binary phases acting as effective pinning centers in the system. © 2016, Springer Science+Business Media New York