E. Yanmaz

Fabrication and properties of dense ex situ magnesium diboride bulk material synthesized using spark plasma sintering

High density ex situ magnesium diboride bulks were synthesized from commercial MgB2 powder using spark plasma sintering under a range of applied uniaxial pressures between 16 and 80 MPa. The microstructure was characterized using x-ray diffraction, scanning electron microscopy, polarized optical microscopy, Vickers hardness measurements, and density measurements using the Archimedes method. By combining these data with those for other bulk samples we have developed a correlation curve for the hardness and density for magnesium diboride for relative densities of 60–100%. The superconducting properties were determined using magnetization measurements. Comparison to samples of significantly higher porosity indicates a positive correlation between magnetization critical current density and bulk density for magnesium diboride bulks up to around 90% density. Above this level other microstructural processes such as grain growth begin to influence the critical current density, suggesting that full elimination of porosity is not necessary to obtain high critical current density. We conclude that the best superconducting properties are likely to be obtained with a combination of small grain size and minimal porosity.

Synthesis of Cobalt Sulfide/Sulfur Doped Carbon Nanocomposites with Efficient Catalytic Activity in the Oxygen Evolution Reaction.

Cobalt sulfide/sulfur doped carbon composites (Co9 S8 /S-C) were synthesized by calcining a rationally designed sulfur-containing cobalt coordination complex in an inert atmosphere. From the detailed transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analyses, the electrocatalytically active Co9 S8 nanoparticles were clearly obtained and combined with the thin sulfur doped carbon layers. Electrochemical data showed that Co9 S8 /S-C had a good activity and long-term stability in catalyzing oxygen evolution reaction in alkaline electrolyte, even better than the traditional RuO2 electrocatalyst. The excellent electrocatalytic activity of Co9 S8 /S-C was mainly attributed to the synergistic effect between the Co9 S8 catalyst which contributed to the oxygen evolution reaction and the sulfur doped carbon layer which facilitated the adsorption of reactants, prevented the Co9 S8 particles from aggregating and served as the electrically conductive binder between each component.

AC losses and irreversibility line of Bi(Pb)SrCa CuO high-tc superconductors

Abstract We performed measurements of χ′( T , H ac , f ) and χ′( T , H ac , f ) on a nominal composition of Bi 1.6 Pb 0.4 Sr 2 Ca 3 Cu

Light-assisted deposition of CdS thin films

The effects of white light illumination during the deposition of CdS thin films in a quasi-closed volume on the structural, photoelectrical and optical properties are investigated. The films were highly c-axis oriented with an increasing intensity of (002) reflection as the illumination increases. The room temperature resistivity values of the CdS films decreased in the range of 107-104???cm. The photosensitivity in the fundamental absorption region and the transparency in the transmission region considerably increased as the illumination increased. Under 100?mW?cm-2 insolation, the efficiencies of the CdS/CdTe solar cells based on CdS window materials which were deposited: (1) in the dark; and (2) under an illumination of 150?mW?cm-2; were found to be 1.8% and 7.3%, respectively.

Boron-functionalized graphene oxide-organic frameworks for highly efficient CO2 capture

The capture and storage of CO2 have been suggested as an effective strategy to reduce the global emissions of greenhouse gases. Hence, in recent years, many studies have been carried out to develop highly efficient materials for capturing CO2 . Until today, different types of porous materials, such as zeolites, porous carbons, N/B-doped porous carbons or metal-organic frameworks (MOFs), have been studied for CO2 capture. Herein, the CO2 capture performance of new hybrid materials, graphene-organic frameworks (GOFs) is described. The GOFs were synthesized under mild conditions through a solvothermal process using graphene oxide (GO) as a starting material and benzene 1,4-diboronic acid as an organic linker. Interestingly, the obtained GOF shows a high surface area (506 m2  g-1 ) which is around 11 times higher than that of GO (46 m2  g-1 ), indicating that the organic modification on the GO surface is an effective way of preparing a porous structure using GO. Our synthetic approach is quite simple, facile, and fast, compared with many other approaches reported previously. The synthesized GOF exhibits a very large CO2 capacity of 4.95 mmol g-1 at 298 K (1 bar), which is higher those of other porous materials or carbon-based materials, along with an excellent CO2 /N2 selectivity of 48.8.

The effect of Ag-doping on the magnetic behaviour of Bi-(Pb)-Sr-Ca-Cu-O ceramic superconductors

Abstract AC susceptibility χ ′( T , H ac ) and χ ″( T , H ac ) measurements have been carried out on rectangular bar shaped high-temperature superconductors with nominal composition of Bi 1.6 Pb 0.4 Sr 2 Ca 3 Cu 4− x Ag x O 12 ( x =0–1.0) prepared by the solid state reaction method. The effect of Ag-doping on the Bi–(Pb)–Sr–Ca–Cu–O system has been investigated in terms of an AC susceptibility study. It was found that as the amount of Ag doping increases, the critical current density decreases. We also qualitatively discuss experimental results in the framework of the critical state model (CSM). Comparison of the observations and the results of the CSM suggests the Bean like dependence of the intergrain (matrix) depinning critical current density j cm on the magnetic flux density B in the low field range studied. The temperature dependence of the pinning strength parameter α ( T ) obtained from AC dissipation is in good agreement with the relation α = α 0 (1− T / T c ) 2.37 for undoped Bi–(Pb)–Sr–Ca–Cu–O.

Ag-doped 120 K YBa2Cu3O7- delta superconductors prepared by the flame-quench-melt-growth (FQMG) method

The effect of Ag-doping on the microstructure and physical properties of YBa2Cu3O7- delta superconductors prepared by a flame-quench-melt-growth (FQMG) process, was investigated. Pellets of YBa2Cu3O7- delta containing 0-30 weight percentage (wt.%) of Ag were quenched by LPG-O2 flame melting and then crushed to obtain powder which was compacted and then subjected to a melt-growth process. As a result of this process, large grained, textured YBCO material was produced. It was found that YBCO samples with 10 wt.% Ag doped showed a transition temperature of 120 K and further increasing the Ag content caused a decrease of transition temperature. Using this modified quench-melt-growth technique, the reaction between crucibles and material can be completely avoided during the preparation of the precursor materials.

Thermally activated flux dissipation in Ag-added YBa2Cu3O7-x (123) prepared by flame-quench-melt-growth (FQMG) method

The resistive transitions of large grained and textured YBa2Cu3O7−x polycrystals fabricated by the flame-quench-melt-growth (FQMG) process were studied in various selected magnetic fields. It was found that the appropriate amount of silver addition enhances the formation of the superconducting phase and slightly increases the transition temperature. The magnetic behaviour of YBCO with silver and without silver were determined by measurements of the resistivity (ρ) in an external magnetic field. It was found that samples with silver showed a lower transition temperature in a magnetic field. The activation energy, Uo, for thermally activated flux flow (TAFF) was estimated for all samples from the slope of the Arrhenius plot of ln ρ versus T−1. It was found that the activation energy of the samples decreases with increasing silver content and magnetic field.

The effects of Ag addition and magnetic field on melt-processed YBa2Cu3Ox superconductors

Abstract Superconducting YBa 2 Cu 3 O x materials were produced by the FQMG method and their microstructures were defined by XRD analysis and optical microscopy. The optimum growth temperature and time were estimated by XRD and resistivity measurements to be 1050°C, which is in the range of the peritectic temperature of the YBCO system, and over 10 h, respectively. The effects of increasing Ag addition and applied magnetic field on the microstructure and physical properties of this system were investigated. It was found that the critical temperature ( T c,zero ) of specimens slightly increased on increasing the Ag content up to 20 wt.% and the transition curves broadened on increasing the magnetic field. Additionally, the effect of oxygen annealing on the Ag-added specimens was examined and the curves of resistive transitions became sharper and T c,zero significantly increased.

Effect of Er doping on the superconducting properties of porous MgB 2

MgB2 bulk sample with porous structure was produced by using the in-situ solid-state reaction method under argon (Ar) atmosphere of 10 bar. Elemental Er in powder form was doped into MgB2 with different compositions (Mg 1−xErx)B2, where x=0.00, 0.03 and 0.05, in order to investigate the effect of rare-earth (RE) element Er on the structural and electromagnetic properties of porous MgB2. The Er-doped samples result in small grain size structure compared to the undoped one. The lattice constants a and c of the doped samples, determined from X-ray diffraction (XRD) analysis, increase with the increasing Er content, and consequently the superconducting transition temperature (Tconset) of MgB2, determined from resistivity measurements, is slightly suppressed. Also, the upper critical field (Bc2), the irreversibility field (Birr) and the critical current density (Jc) values are significantly enhanced in the doped samples. For the best sample (x=0.03), at 15 K under a magnetic field of 4 T, the Jc value reaches 2.4 ×104 A cm−2, which is higher than that of the porous sample by an order of 103, and the Birr value at 20 K reaches 9.7 T. These results imply that the RE element Er fills the pores, enhances the density and the grain connectivity. Hence, the superconducting properties of the porous MgB2 sample improve by Er doping.