Papers by Keyword: Superconductivity

Abstract: The influence of reactivity of different sheath metals such as Ag, Fe, Cu, Ni and SS on the superconducting properties of NdFeAsO_0.6F_0.4 wires has been investigated. The observed structural and superconducting properties of the metal sheathed wires conclude that among the five candidates Ag and Fe are technically compatible to be used as sheath materials in terms of reactivity. However, the best superconducting properties such as T_C and transport J_C were exhibited by Ag sheathed NdFeAsO_0.6F_0.4 wires.

Abstract: We report superconductivity, which is achieved in Sr_1-xNa_xFe₂As₂ at T_c = 36.5 K for x = 0.47. Single crystals of Na-substituted SrFe₂As₂ up to 2 × 2 mm² were grown from NaF flux. The superconducting transition temperature with a narrow width less than 2 K, confirmed by both magnetic and resistivity measurements, indicates the high quality of our samples. The upper critical fields (0) = 87 T and (0) = 217.5 T are extremely high. The anisotropy ratio γ ~ 2.5 is close to that of other hole-or electron-doped 122 phases but lower than γ ~ 4.3 – 5 of phases from the 1111 series and much lower than γ ~ 7 – 10 of superconducting cuprates. Magnetic relaxation measurements, which are consistent with the collective pinning model, reveal that the flux-creep rate in Na substituted 122 SrFe₂As₂ is smaller than in 1111 type iron pnictides.

Abstract: Technetium-99 is a key radioisotope from a nuclear waste perspective because of its long half-life (t_1/2= 2.13x10⁵ years) and its abundance in used nuclear fuel. As such, it is targeted in separation strategies such as UREX+, for isolation and encapsulation in solid waste forms for final disposal in a nuclear repository. We report here results regarding the incorporation of Tc-99 into ternary oxides of different structure types: pyrochlore (Nd₂Tc₂O₇), perovskite (SrTcO₃), and layered perovskite (Sr₂TcO₄). The objective was to determine synthesis conditions of these ceramic waste forms to immobilize Tc-99 as Tc (IV) and to harvest crystallographic, thermophysical and hydrodynamic data. The fabricated ceramic technetates exhibit good crystallinity and lattice parameters and atomic coordinates could be refined with high accuracies. Thermophysical properties of the oxides, such as the critical temperature (T_c) for superconductivity, were analyzed using AC magnetic susceptibility measurements. In our efforts to compare hydrodynamic properties of the ceramic waste forms with those to Tc-bearing borosilicate glass, we applied the standard test method ASTM C1220-10. Hereby matrix corrosion and Tc-leaching of Tc-99 containing LAWE4 type borosilicate glass and ceramic specimens were compared by static leaching experiments.

Abstract: We investigated the lattice dynamics and electron-phonon coupling (EPC) of superhard material OsB₂ by first-principles linear response calculations. The calculated EPC parameters for the optical phonon modes at Г indicate that the heavy Os atoms play the most important role in deciding the superconducting behavior, and there are sizeable contributions from lighter B atoms to EPC. Our calculated EPC constant is 0.42, and the estimated superconducting transition temperature T_c is 2.1 K using the Coulomb pseudopotential μ^*=0.125, in excellent agreement with the experimental ones.

Abstract: The amorphousization of Zr₆₅Nb₃₅ alloy was performed. The Zr-Nb based alloys contained Al and Co elements were fabricated by arc-melting and melt-spinning methods. The superconducting property of the Zr_(65-x)Nb_35-xAl_x (x = 0~15 at%) and Zr_(65-x)Nb₂₀Al₁₅Co_x alloys (x = 3~10 at%) was investigated by magnetic susceptibility measurements. The Zr_(65-x)Nb₂₀Al₁₅Co_x metallic glasses (x = 6~10 at%) with superconducting nanocrystalline particles dispersed in an amorphous matrix exhibited a superconductivity below about 3.5 K. The addition of Co element led drastically to the amorphousization of the superconducting Zr₆₅Nb₂₀Al₁₅ alloy.

Abstract: The effect of Pr-substitution at the Eu-site on the superconductivity and transport properties in Eu_1-xPr_xBaSrCu₃O_7-d (x = 0.0, 0.1, 0.2, 0.4, 0.5) has been investigated. T_{c onset} for x = 0.0, 0.1, 0.2, 0.4, 0.5 are 86 K, 75 K, 65 K, 38 K, 27 K, respectively. The critical temperature (T_c) decreased linearly with increase in Pr concentration x indicating a magnetic pair-breaking mechanism. The transport critical current density (J_c) decreased with increasing x. Analysis of the structure of the samples via X-ray powder diffraction (XRD) technique showed transformation from orthorhombic to pseudo-tetragonal structure at x = 0.2. Scanning electron microscope (SEM) showed no variation in the microstructure.

Abstract: Recently, an interest in a hybrid system combining only the merits of the conventional wheel-rail system and Maglev propulsion system is growing as an alternative to high-speed maglev train. This hybrid-type system is based on wheel-rail method, but it enables to overcome the speed limitation by adhesion because it is operated by a non-contact method using a linear motor as a propulsion system and reduce the overall construction costs by its compatibility with the conventional railway systems. Therefore, a comparative analysis on electromagnetic characteristics according to the structural combinations on the stator-mover of Linear Synchronous Motor (LSM) for Very High Speed Train (VHST) maintaining the conventional wheel-rail method is conducted, and the structure of coreless superconducting LSM suitable for 600 km/h VHST is finally proposed in this paper.

Abstract: The electronic and the superconducting properties of Pd were studied in the framework of density functional perturbation theory. We explored the superconducting transition temperature for bulk Pd and predicted possible superconductivity at ambient and high pressures. It is found that of Pd is 0.0356 K at ambient pressure and it decreases with pressure.

Abstract: We present the fabrication, structural, and transport properties of MgB2-based ceramic composites with magnetic nanospheres fabricated by spark plasma sintering. The nanospheres are either carbon–encapsulated iron or iron oxide. The former nanospheres have been prepared by laser pyrolysis whereas the iron oxide was obtained by the pyrolysis of the polysiloxane-based copolymers embedded into MgB2 matrix during the sintering process. The structural data show the shrinkage of the a-axis lattice constant as a result of the partial carbon substitution for boron. However, the transport data suggest that carbon diffusion is limited to the outer layer of the MgB2 grains in both cases.

Abstract: The effect of ferromagnetic layer inhomogeneity on the superconducting resistive transition in Superconductor/Ferromagnet/Superconductor (S/F/S) trilayers is studied. The critical temperature, T_c, and the resistive transition shape versus the F layer thickness, d_F, in Nb/Cu_0.41Ni_0.59/Nb and Nb/Pd_0.81Ni_0.19/Nb trilayers were analyzed. It is shown that the T_c(d_F) dependence is sensitive to the Ni concentration variation along the F layer for thickness of d_F corresponding to the π-superconducting state and to the 0-π crossover thickness.