Tomoyuki Yanagiya

Properties of Ag-Sheathed Bi-Pb-Sr-Ca-Cu-O Superconducting Tapes Prepared by the Intermediate Pressing Process

The influence of an intermediate pressing process on the superconducting properties of Ag-sheathed Bi-Pb-Sr-Ca-Cu-O superconducting tapes has been investigated. The critical current density (Jc) was improved by the effect of the uniaxial pressing and by a sintering process after the initial sintering. The maximum transport Jc at 77.3 K under zero magnetic field was indicated as 2.36×104 A/cm2, and that at 4.2 K under zero magnetic field was indicated as 1.5×105 A/cm2. The magnetic field dependence on the Jc at 4.2 K was summarized as follows: 5×104 A/cm2 (I⊥H, C-axis⊥H, 10 T) and 3×104 A/cm2 (I⊥H, C-axis//H, 10 T).

Fabrication and Superconducting Properties of AgCu Alloy-Sheathed BiSrCaCuO Oxide Tapes

Ag-Cu alloy-sheathed BiSrCaCuO tapes have been successfully fabricated. Critical temperature higher than 100 K (Tc-onset) was obtained for the Ag-13 at%Cu alloy-sheathed tape of the (Bi, Pb)2Sr2Ca2Cu3Ox phase. Tcs of ~80 K were obtained for the Ag-Cu alloy tapes of the Bi2Sr2Ca1Cu2Ox phase with Cu content less than ~25 at%. Critical current density reached 3.5×104 A/cm2 at 4.2 K and 12 T for the Ag-10 at%Cu alloy-sheathed Bi2Sr2Ca1Cu2Ox-phase tapes. Mechanical strength of the sintered tapes could be increased, as well as improvement in workability of the wires and tapes.

Critical current densities of AgCu-sheathed Bi-2212 multifilamentary round wires in long length

Abstract Ag–Cu alloy sheathed Bi-2212 round wires have been fabricated by the powder-in-tube method. Short samples show critical current ( I c ) values of more than 150 A at 4.2 K and 10 T, while I c for a small coil with total length of 10 m reduces to about 30% of the values obtained for short samples. In order to clarify the factors to deteriorate I c for coils, superconducting properties and the microstructures have been examined. The difference in transport I c between short samples and coils can be attributed not to the intrinsic properties of Bi-2212 grains but to the extrinsic limiting factors, such as voids that are formed by the released gases inside the wires with long length during heat treatment.

In Situ Observations of the Growth Process of C-Axis-Oriented Bi2Sr2Ca1Cu2Ox Superconducting Phase

The growth process of the highly oriented microstructure of Bi2Sr2Ca1Cu2Ox(2212) crystalline phase has been studied by a method of in situ high-temperature microscopy. The melting state of calcined powders and growth of 2212 crystals on an Ag foil substrate have been directly observed by an optical microscope. The 2212 calcined powders are resolved into a mixed phase at 890°C, which consists of liquid, Bi-free crystalline and Cu-free crystalline phase. The composition of the liquid phase is closer to Bi2Sr2Cu1Ox than to Bi2Sr2Ca1Cu2Ox of the starting material. The plate-shaped Bi2Sr2Ca1Cu2Ox crystals are grown with the a-b planes parallel to the free surface of molten samples.

High- J c Reproducibility of Ag-Sheathed Bi-2212 Tapes

In the fabrication of Ag-sheathed Bi-2212 superconducting tapes, J c reproducibility is obstructed by (1) swelling of the Ag sheath, (2) the rough interface of the sheath and core, (3) cracks in the oxide core, (4) inhomogeneities in composition, particle size, and particle hardness of the raw powder. The swelling is caused by the expansion of dissociated 02 gas within the weak, pure-Ag sheath during the high-temperature melting process; it can be eliminated by using powder calcined in low, partial O2 pressure. Complex, misoriented Bi-2212 layer structures often grow from rough interfaces, which are due to improper deformation processes. Cracks in the oxide core, easily introduced by mishandling, can be significantly reduced to achieve a mechanically stronger AgCu-alloy-sheathed tape. Improvement in high-J c reproducibility and the effects of Cu addition into the sheath on increasing J c and mechanical strength are presented.

Superconducting Properties of Bi-2212 Multifilamentary Round Wires With Double Layered Core Filaments

Multifilamentary Ag-sheathed Bi-2212 round-shaped wires with double layered core-filaments and AgCu alloy sheathed Bi-2212 round wires were fabricated by the powder in tube technique. Composites consisting of Ag outer sheath and the filaments with number of 7, 37 and 61 and a composite of AgCu alloy outer sheath having 37 and 61 filaments were drawn into wires of 1mm in diameter. The wires were heat treated by a conventional partial-melting process. High oxide core ratio of ∼50% to wire-cross-sectional area could realize high overall critical current densities (Jc) of 280A/mm2 at 4.2K, 10T in Ag sheathed wires with 61-filaments and Jc of 310A/mm2 in AgCu alloy sheathed wires with 37-filaments.

Bi-2212 Ag-Sheathed Round Cross Section Wires

Bi-2212 Ag-sheathed round cross sectional wires which are suited as winding materials for realizing high homogeneity superconducting magnets have been successfully fabricated by the powder-in-tube technique. High overall critical current density, Jc, up to 2.8x104 A/cm4 at 4.2 K, 10 T was achieved in the multifilamentary wire consisting of 37 double tubed core filaments. Highly deformed Bi-2212 microstructures which were formed along a small curvature Ag sheath surfaces of the core filaments were observed, which might be responsible for the high Jc properties. Effects of wire bending before the heat treatment on the Jc property have been examined with respect to a solenoid winding. It was confirmed that the decrease in Ic in a 40 mm bore solenoid compared with that of short sample is estimated to remain less than about 15%.

AgCu alloy sheathed Bi-system oxide superconducting wires

Abstract AgCu alloy sheathed Bi(Pb)SrCaCuO oxide wires have been fabricated with respect to improvement in high- J c reproducibility and mechanical reinforcement. High homogeneity in microscopic composition of oxide core and improved flatness between the core and sheath interface have been focused as important factors for realizing high- J c reproducibility. Complete elimination of Ag sheath swelling during partial melting for Bi-2212 tapes has also been examined. Remarkable increase in J c values for Bi-2223 samples prepared using the simultaneous addition of Cu and a small amount of Ti or Au has been obtained, probably due to an increased flux pinning.