M. E. Yakinci

Synthesis and characterization of glass-ceramic Bi2-xGaxSr2Ca2Cu3O10+y superconductors

We have melt quenched a series of Ga substituted Bi2-xGaxSr2Ca2Cu3O10+y samples with the compositions with x = 0.2, 0.4, 0.6, 0.8 and 1.0. Microstructure, thermal properties including crystallization activation energy, magnetic and superconducting properties were investigated. We have found that Ga3+ ions have a solid solubility limit in the BSCCO system and this limit was passed at x = 0.6. Below this limit HTc BSCCO n = 3 phase was obtained, but after the solid solubility limit samples yield multiphase superconducting properties. Extended heat treatment at 840?C displayed no suppression of the solid solubility limit. The crystallization kinetic studies were investigated using non-isothermal kinetic models of Kissinger and Augis-Bennet. Higher crystallization activation energies, Ea, were obtained compared to unsubstituted BSCCO materials, glassification improved and glass handling time increased. The substitution had a slight effect on Jc, the flux pinning in the material was improved and better Jc values were obtained below the solid solubility limit.

Crystallization Activation Energy and Hole Concentration Properties of the Bi2Sr2Ca1 − xCdxCu2O8 + y Glass-Ceramic Superconductor Rods

The glass rods with the nominal compositions of Bi2Sr2Ca1 − xCdxCu2O8 + y, where x = 0.0, 0.4, and 0.8, were prepared by melt-casting method. XRD studies exhibited that the Cd substitution caused a transition from Bi-2212 phase to Bi-2201 phase. The multiphase structure was obtained for higher Cd concentration cases. It was determined that the activation energy for crystallization was decreased with the increase of the Cd concentration. When the Cd concentration was increased in the system the superconducting properties decreased. The hole concentration, p, was calculated using the Preslands method. The increase in the Cd concentration increased the hole concentration and the results showed that the Cd-substituted samples were highly in the overdoped region.

Degradation of Superconducting Properties in MgB2 by Formation of the MgB4 Phase

Superconducting MgB2 polycrystalline samples have been fabricated under two different conditions in order to determine the effect of MgB4 phase. A series of samples was placed in an α-alumina container closed with a cup and fired under high purity argon gas. The other series of samples was placed in an α-alumina boot without any lid and fired under similar conditions. For the first series of samples, we have found pure MgB2 phase formation and a narrow transition width at 0.4 K. For the second series of samples, significant amount of MgB4 phase were formed and the Tzero was decreased to 27 K. For both the group of samples magnetization hysteresis loops obtained at various temperature range and applied field up to 2 T. The best Jcmag for the first series of samples was 1.9 × 105 A/cm2 at 10 K and 0 T, and for the second series of samples was 0.7 × 104 A/cm2 at 10 K and 0 T.

Synthesis and characterization of novel polyacrylate-clay sol-gel materials

Organo-functional silanes which were able to form chemical bonds with kaolinite and could also have an affinity to the materials of concern here, were studied by the sol-gel process. Polymethacrylate with trialkoxy silyl functional groups were prepared, hydrolysed and co-condensed with kaolinite. The progress of the hydrolysis, which proceeded very slowly, was followed by Karl-Fischer titration. Thermal behavior was investigated by differential thermal analysis. The extent of the reaction leading to network formation was qualitatively followed by Fourier transform-infrared spectroscopy and X-ray diffraction. Free-radical polymerization was carried out ultrasonically in the presence of a catalyst. Trimethoxy silane end-capped silane was found to be covalently bonded to kaolinite. The copolymers, with various amounts of kaolinite, were then hydrolysed and co-condensed in the presence of a catalyst to yield sol-gel materials which have a controllable combination of properties of both the polymer and kaolinite.

Study on the Physical and Electrical Properties of Bi2 − δZnδSr2Ca2Cu3O10 + y Glass-Ceramic Superconductors

We have fabricated a series of glass-ceramic (Bi2 − δZnδ) Sr2Ca2Cu3O10 + y, where δ = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0, and investigated the effect of Zn ions on the glass formation, crystallization, thermal, electrical, and on the magnetic properties of the BSCCO-2223 superconductor system. The structural symmetry was found to be tetragonal in all the substitution levels. The best electrical performance was obtained from the δ = 0 sample, the Tc and Tzero was obtained at 110 K and 107 K, respectively. The Jc values of the samples were determined using the magnetization hysteresis and Beans model. The crystallization kinetics were investigated using nonisothermal models of Augis–Bennett. The calculated activation energy, Ea, of the system was found to be in the range of 258–336 kJ/mol.

Effect of Bi deficiency on grain alignment of BiPb-2223 thin film fabricated using rf sputtering process and on critical current density properties

Abstract In the present study, thin films prepared as a function of the Bi concentration in the BiPbSrCaCuO system were synthesised. Thin films were fabricated using radio frequency sputtering method. Crystal structure of the films fabricated was determined from X-ray diffraction measurements. The crystal orientation was analysed by X-ray pole figure and in-plane alignment. Both X-ray diffraction and pole figure analysis revealed that crystallinity in the films decreased significantly with decreasing Bi concentration in the system. A systematic decrease in the superconducting transition temperature and hole concentration per CuO plane was obtained with decreasing Bi concentration. The Jc values of the films were calculated using Bean formula. It exhibits a significant dependence on the magnetic field and temperature. It was found that Jc decreased sharply with increasing applied magnetic field. The highest Jc value was found to be 1·06×106 A cm−2 at 10 K, which corresponded to the best flux pinning among the films fabricated.

Fabrication and properties of (Hg0.8Re0.2)Ba2Ca2Cu3Ox superconducting thick films

Approximately 50??m thick epitaxial c-axis orientated (Hg0.8Re0.2)Ba2Ca2Cu3Ox superconducting films have been prepared on MgO(100) substrates using a spraying process and post-Hg-vapour annealing. The effects of the heating temperature?time combinations and the filling factor of Hg (ffHg) on the physical, electrical and magnetic properties of the thick films have been investigated. The interesting observation was the formation of a thick interfacial layer between the film and the substrates. This interfacial layer was found to be protective against excessive diffusion and dissolution of the Mg2+ ions from the substrate or counter-diffusion of Ba, Ca and Cu ions from the films. The XRD investigations showed that the a?b plane of the HgRe-1223 phase aligns parallel to the substrate surface. The best Tc and Tzero were found to be 131.9 and 129.3?K, respectively. Magnetic properties up to 5?T have been investigated. The calculated value of critical current density, Jc, was found to be 4.82 ? 105?A?cm?2 at 4.5?K. The results obtained suggested that fabrication of HgRe-1223 thick film superconductors using a spraying process with 130?K transition is possible and they are very promising particularly for electronic applications.

Charging and discharging of particles on the low-potential electrode under bipolar corona conditions

In this work, the process of charging and discharging of aerosol particles on the surface of a low-potential electrode and their behaviour depending on the charge that they have are investigated under bipolar corona conditions. A general expression to characterize the kinetics of charging of these particles is developed so that this expression yields many of the special equations that have been reported in the literature and are valid under certain conditions. It is found that the intensity of the corona discharging field affects both the amount of stable charge on the particle and its charging time; in general, the time constant for discharging and the coefficient of bipolarity determine the amount of stable charge. The obtained results indicate that, under suitable conditions, use of a bipolar corona field can be an alternative technique for cleaning and refining impure aerosol mixtures for desired purposes.

Adjustable tunneling barrier in bi-based high-T c cross-whisker junctions

We present a study of cross-whisker junctions with electronically modified tunneling barriers. Cross-whisker junctions were successfully prepared by annealing two crossed Bi-based whisker bars at temperatures of around 840 °C. In addition to the artificially produced junction at the interface, intrinsic Josephson junctions in the respective bars of the cross were observed. The artificial junction exhibited reproducible and almost ideal junction characteristics. The tunneling barrier/interface properties were controlled by carrier injection in c-axis direction. Using this process, we were able to reduce the tunneling resistance from 408 Ω to 30 Ω. At the same time, the critical current did rise by a factor of 4. Subsequently, the critical current was doubled while the tunneling resistance stayed constant. We interpret this observation in terms of the counterplay between transparency of the barrier and the carrier concentration of the electrodes. In this sense, we can consider the current injection procedure as an electronic gluing process of the two cross-junction whisker bars.

Effects of SnO on growth and physical properties of BSCCO whiskers

Abstract Superconductor whiskers doped with SnO have been fabricated by annealing a melt quenched (Bi2Sn1)-223 precursor using suitable heat treatment cycles. Approximately 5 μm thick, 90 μm wide and 5 mm long whiskers were fabricated, and their physical, electrical and magnetic properties were investigated. Crystallisation activation energies of glass phase fabricated were calculated to be 390 kJ mol–1 using Kissinger method based on the differential thermal analysis data. The T c value of the whiskers was found to be 94 K from M–T measurement. The magnetisation of whiskers before superconducting transition increased with decreasing temperature, and after superconducting transition, the magnetisation of whiskers decreased, from positive to negative, due to the diamagnetic nature of superconductivity. The change on magnetisation dependence of applied magnetic fields (M–H) showed that whiskers have paramagnetic–diamagnetic multiphase structure.