Applied Superconductivity最新文献

The effects of Ag substitution for Bi, Sr and Cu on the c-axis lattice parameter and critical temperature (T_c) were determined for the Bi_2−xSr_2−yCaCu_2−zO_8+δ (BSCCO-2212) superconducting phase. Two distinct regimes of behavior were observed: fast cooling represented the regime where the uptake of excess oxygen was the controlling factor and slow cooling represented the regime where the cation content was the controlling factor on the structure and properties. Samples showed higher T_c values and longer c-axes with increased cooling rate. A linear increase in T_c with increase in c-axis length was observed for the faster-cooled samples. For the slow-cooled samples, increased c-axis length was observed when Ag was substituted for Bi and Sr, whereas substituting Ag for Cu caused no change in c-axis length. In addition, any deviation from the ideal 2212 stoichiometry was shown to reduce the T_c values of samples that were slowly cooled and fully oxygenated. Preliminary results obtained by high-resolution transmission electron microscopy coupled with electron energy loss spectroscopy indicated that relatively small amounts of Ag (≈1 at.%) can occupy Bi sites and a larger amount (≈10 at.%) of Ag can substitute for Cu.

Ba₂GdNbO₆ has been developed as a new substrate suitable for the BSCCO superconductor. Ba₂GdNbO₆ has a complex cubic perovskite (A₂BB′O₆) structure with a lattice constant a=8.587 Å. The DTA studies revealed that there is no phase transition occurring in Ba₂GdNbO₆ in the temperature range of 30–1300°C. The thermal expansion coefficient of Ba₂GdNbO₆ is found to be 7.913×10⁻⁶°C⁻¹. The dielectric constant and loss factor of Ba₂GdNbO₆ are in a range suitable for its use as a substrate for microwave applications. The Bi(2223) superconductor does not show any detectable chemical reaction with Ba₂GdNbO₆ even under extreme processing conditions. The thick films of Bi(2223) dip-coated on polycrystalline Ba₂GdNbO₆ substrate gave a T_C(0) of 109 K and a current density of approximately 4×10³ A/cm² at 77 K and zero magnetic field.

We have performed a detailed analysis of the micro-Raman spectra recorded around the non-superconducting R₂BaCuO₅ inclusions embedded in different melt grown RBa₂Cu₃O₇ (R=Y, Nd) samples. The Raman spectra were recorded every 0.4 or 0.5 μm along a line crossing one or more inclusions on two specific samples. Sample A, YBa₂Cu₃O₇ with 33 vol% of Y₂BaCuO₅ was prepared following a fast-melt-processing scheme. The cooling rate of the melt, crossing the peritectic temperature, was 180°C/h. We observed a downshift of the phonon modes of the superconducting phase around the inclusions. This softening can be related to the presence of tensile stresses around the inclusions. Sample B, NdBa₂Cu₃O₇ with 40% of Nd₄Ba₂Cu₂O₁₀ was prepared following an oxygen-controlled-melt-growth method. A cooling rate of 1°C/h was applied to produce this sample. In this case we found a hardening of the phonon modes of the superconducting phase around the inclusions towards the energy of the phonon modes obtained in single crystals. Again, this shift of the Raman peak position can be related to the variation of mechanical stress around the inclusions. The hardening can be caused by the relaxation of tensile stress in the superconducting matrix around the inclusions.

Because of the macroscopic form of Ag-sheathed BSCCO-2223 tape conductors, as well as internal structure of grain build up and their mutual orientation within the Ag sheath, their volt–ampere characteristics are not isotropic with respect to external magnetic field orientation. For the same absolute field values the behaviour of E(I) curves are a sensitive gauge of vector B inclination to tape axis and to the surface plane. As a result of the proximity effect and the weak links structure, the increasing external magnetic field reduces overall integral current capacity of the sample. The effect is pronounced at a perpendicular orientation of B but an analogic picture is observed for planar orientation with a corresponding shift of E(I) curves to higher current levels. For selected and gradually increasing levels of external magnetic field the difference in the vector B, parallel and perpendicular inclination with respect to tape plane on E(I), characteristics are presented. In this way the anisotropy manifestation at the different field levels is studied. The field vector remains perpendicular to tape axis. Also revealed is the influence of thermal cycling on E(I) through the thermomechanically induced cracking process.

In this paper, we present the results of various magnetic measurements on an NdBa₂Cu₃O_7−δ (NdBCO) single crystal. Special features observed in the magnetization loops (MHLs) are discussed. From these loops, critical current densities, j_c as a function of the applied field, H_a, and volume pinning forces, F_p=j_c×B, are derived. The samples exhibit a strong fishtail or peak effect at temperatures above 30 K. In a small temperature window between 50 K and 74 K, even three peaks are seen in the j_c(T,H_a) curves. A good scaling of the j_c (T,H_a) curves is obtained, which allows the identification of the peaks. The scaled volume pinning forces (vs the reduced field, h=H_a/H_irr), in the temperature regime between 60 and 90 K give experimental evidence for a strong pinning at extended superconducting defects (interaction volume V_pin∼ξ²d). These defects are ascribed to spatial composition fluctuations found in light rare earth superconductors, providing a scatter of the transition temperature, T_c. Furthermore, the scaling analysis of F_p is extended towards low temperatures. It is shown that at 5 K another peak in F_p is found, which is due to pinning at twin boundaries. Using this scaling analysis of both j_c and F_p, the irreversibility line (IL) is determined in the entire temperature range 0–92 K. Magneto-optic (MO) imaging is employed to investigate the flux distributions of various NdBCO single crystals. These images confirm the important role of twin boundaries as pinning sites at low temperatures and low applied fields.

PACS numbers: 74.60 Ec, 74.60 Ge, 74.60 Jg

The pulsed-field magnetization (PFM) technique has been elaborated as the most efficient and effective tool in magnetizing the melt-processed high-T_c bulk superconductor in the temperature range 30–80 K. The dynamic motion of magnetic fluxes penetrating into the YBCO superconductor during the PFM operation has been analyzed and the flux propagation speed has been accurately evaluated. The viscous force is found to play a very crucial role in the PFM process. Detailed analysis of the flux motion in the presence of both viscous and pinning forces enabled us to find a very efficient method for magnetization by the PFM operation. The iteratively magnetizing pulsed field operation with reducing amplitudes abbreviated as ‘IMRA’ has been developed and proved to be very effective in magnetizing high-T_c superconductors at temperatures as low as 30 K. By making full use of these findings, we constructed a prototype quasi-permanent magnet system capable of producing magnetic field of 0.8 T in the outerspace at 7 mm above the surface of the magnetized YBCO superconductor.

The oxygen controlled melt growth (OCMG) process for filamentary Nd123 superconductors has been studied to enhance the J_c values. The filamentary Nd_1.18Ba_2.12Cu_3.09O_x (1) and Nd_1.25Ba_2.05Cu_3.10O_x (2) precursors were prepared by dry spinning through a homogeneous aqueous solution containing mixed acetates of Nd, Ba and Cu, poly(vinyl alcohol). The calcined filaments were partially melted in flowing 0.1% O₂+Ar. The T_c values of filament (1) was 94.5 K and 88.5 K for filament (2). By controlling the heating condition, maximum J_c values of 2.4×10⁴ A/cm² and 1.8×10⁴ A/cm² at 77 K and 0 T were attained for filaments (1) and (2), respectively. A window of the optimum heating conditions for filament (2) was narrow and the reproducibility of the high J_c value was poor.

We have studied the effects of Nd₄Ba₂Cu₂O₁₀ (Nd422) on the pinning characteristics of NdBa₂Cu₃O_7−δ (Nd123) crystals. Like Y₂BaCuO₅ particles in the Y–Ba–Cu–O system, the addition of Nd422 could enhance critical current density J_c at lower fields. The peak field (B_pk) and the J_c value at B_pk were almost independent of Nd422 contents although a slight decrease in J_c at B_pk was observed with increasing Nd422 contents. The normalized relaxation rate (S) increased for all the samples when the field exceeded B_pk. The addition of Nd422 was effective in preventing a rapid increase in S above B_pk. The exponent μ, defined in the collective creep model, was deduced by fitting the experimental data and was found to show strong field dependence. We found that μ changes from positive to negative at around B_pk, suggesting that the flux creep mechanism is different below and above B_pk.