Materials Science Forum Vols. 546-549

Abstract: GdBaCuO bulk superconductors from precursors of Gd123 and ultrafine Gd211 powder have been fabricated by melt-textured-growth method in air. The ultrafine Gd211 powder was synthesized by an improved Pechini method. It was found that the lower temperature of synthesizing ultrafine Gd211 powder, the higher peritectic reaction temperature of GdBaCuO bulk superconductors. Jc reaches 4.7×104 A/cm2 at 77 K and self-field for ultrafine Gd211 starting particles with 0.2μm in diameter, and higher compared to larger Gd211 starting particles fabricated by solid state reaction. However, a large secondary peak effect did not appeared at Jc-B curves. ,

Abstract: Superconducting single crystals of La1.85Sr0.15CuO4 have been grown at various temperatures without single crystal seeds by the traveling-solvent floating-zone method. In order to avoid the formation of bubbles during the crystal growth process, a flowing atmosphere of 2 atm oxygen or 1 atm air was applied in different temperature range. It was found that the crystal quality could be improved by raising the growth temperature in a certain range, and the orientation changed from (110) to (100) at higher temperature. X-ray diffraction results showed that the full-width at half-maximum of the best as-prepared crystal was 0.086°. The crystals grown in 2 atm oxygen showed a superconducting transition temperature (Tc) of 37.3 K, while the crystals grown in floating air showed a Tc of about 35 K and it could be improved to 36.5 K by annealing in flowing oxygen.

Abstract: The layer growth of the A15 (Nb3Sn) superconducting phase produced by the internal tin method on model monofilament wires was studied by neutron diffraction and electron microscopy techniques. Different ratios of the inner part of the wire (Sn/Cu) and the effect of the addition of 1% of zirconium to niobium were studied. The high temperature heat treatment plays a key role on the A15 formation and on the cost of the final wires produced. Different temperatures in the range 650-725°C and different annealing times in the range 8-200 hrs have led to the preparation of 72 different samples. The neutron diffraction study has evidenced the different phases formed during the heating treatment and the electron microscopy study has evidenced a power law for the growth rate of the A15 phase. In both studies the addition of zirconium in niobium is very effective on the growth rate of the A15 phase.

Abstract: In order to optimize the synthesis process of internal-Sn Nb3Sn wires, magnetization measurements have been performed on samples reacted at various temperatures between 650 and 725 °C and for time ranging from 8 to 200 h We compared samples of various origins, from NIN in Xi'an and Alstom in Belfort. These measurements give us access to critical and irreversibility temperatures. Although all Nb-Sn phases are superconductive, their critical temperatures are varying from 6 K (18at%Sn) to 18.3 K (25%atSn : stoichiometric Nb3Sn). Thus, Tc and T* measurements are related to the progress of tin diffusion and reaction in the wires. We correlate the results of magnetization with SEM (Scanning Electron Microscopy) and EDS (Energy Dispersive Spectroscopy) observations, which indicate directly the Sn content distribution across Nb filaments.

Abstract: The thick MgB2 films have been successfully grown on the Cu substrate by the technique of hybrid physical-chemical vapor deposition (HPCVD). The films are about 2 ~ 3 μm and quite dense. They possess the Tc (onset), as high as 37-38 K, and sharp transition ~ 0.8 K. X-ray diffraction (XRD) indicates their polycrystalline character. The upper critical field at T=0K, HC2(0), is extrapolated as 15.3T. The controlled growth of MgB2 film on Cu substrate opens a new route in the preparation of MgB2 tape materials.

Abstract: Bi-2223 tapes are usually subjected to tensile strain, bending strain and compressive stress in the process of fabrication and applications. In this study, we have measured the critical current of Bi-2223 tapes as a function of transverse compressive stress. The effects of the cross section size and processing defects on the stress characteristics of AgMn-alloy sheathed Bi-2223 tapes are compared. The experimental results showed that the onset transverse compressive stress for degradation of Ic was lower in the thinner tape with a thin Ag sheath due to the stress concentration on the irregular interface. The bubbled tapes were more sensitive to the transverse stress compared to the nonbubbled tapes.

Abstract: Development efforts at Oxford Superconducting Technology (OST) have recently been aimed at manufacturing long length round wires with improvement of transport properties for high field magnet applications. Recently, significant improvements in the JE and Jc performance have been achieved by optimizing the starting precursor composition, the deformation processes, and the heat treatments. The highest JE of 1580 A/mm2 (Jc of 6140 A/mm2) at 4.2 K, 0 T, JE of 420 A/mm2 at 4.2 K, 31 T and JE of 312 A/mm2 at 20 K, 2 T were obtained in 0.81 mm wire with the optimized condition. In addition, significant progress on braided insulation has been made for enabling a robust procedure for wind-and-react Bi-2212 solenoid coils. Ic and generated field have been measured in a series of such coils of increasing dimensions. In this paper the progress on the development of Bi-2212 round wires and coils will be reported.

Abstract: Quench propagation velocity is an important parameter to the stability and protection issues of superconducting magnet. In this paper, the finite element method (FEM) numerical simulation of quench propagation velocity has been performed for using the powerful analysis software COSMOS by establishing a suitable thermal analysis model of Bi-2223/Ag superconducting multifilamentary tape. The effects of quench energy and operating temperature on the quench propagation velocity have been studied. The analysis shows that the simulation result basically coincides with the experimental result.

Abstract: High magnetic field superconducting magnet technology has been developed in the recent years for all kinds of special applications in China. In the paper, the successful development of high magnetic field superconducting magnet technology is presented.