S. Murase

Densification effect on the microstructure and critical current density in (Bi,Pb)2Sr2Ca2Cu3OxAg sheathed tape

The density of oxide core in (Bi,Pb)2Sr2Ca2Cu3OxAg sheathed tape was estimated from Vickers hardness and found to be increased to as high as 93% of the theoretical density with increasing the degree of cold working. During heat treatment, this densification before heat treatment promoted the formation of a strong linked microstructure which is characterized by dense microstructure, a high degree of orientation of oxide crystal and smooth interface between the Ag sheath and oxide core. Consequently, we obtained high values of critical current density: 66 000 A/cm2 at 0 T and 14 500 A/cm2 at 1 T for 77 K.

Multifilament niobium-tin conductors

A new Nb 3 Sn wire fabrication method has been developed, improving wire drawing workability and superconducting properties, such as stability and ac losses. A cross section of the single filament wire consists of a niobium tube with a copper sheathed tin rod inside and high conductivity copper tube outside. These constituents show scarcely any workhardening. Wires with 54 to 295 filaments were drawn down to 0.2 mm to 1.0 mm diameter. Heat-treatment conditions to obtain the highest critical current were clarified as a function of the tin content inside the niobium tube. The effect of bend strain in Nb 3 Sn on the critical current was also examined for samples with different wire diameters and Nb 3 Sn layer thickness. Losses were measured for twisted and non-twisted samples by means of magnetization experiments. Results were compared with calculated values. It was found that the effective resistivity between Nb 3 Sn filaments was one order of magnitude higher than that of pure copper. A coil was constructed using a 1 km long Nb 3 Sn composite having 258 filaments with 1 × 2 mm cross section. The maximum field obtained was 10.65T at 236A in the 6T backing field by NbTi solenoid.

Highly-strengthened alumina-copper alloy matrix (Nb,Ti)/sub 3/Sn conductor fabricated by using the tube process

In order to fabricate a large-bore, high-field magnet which achieves a low foil weight and volume, a high strength compound superconducting wire is required. The alumina-copper strengthened (Nb,Ti)/sub 3/Sn wire which contained 30 vol.% alumina-Cu alloy fabricated by using the tube process has been developed and tested for critical current density, mechanical properties, transverse compressive stress effects and related properties. As a result, it was found that the reinforced (Nb,Ti)/sub 3/Sn wire had high transverse compressive stress tolerance, for example only 3% decrease which was 1/3 of the Cu matrix wire in I/sub c//I/sub c0/ at 60 MPa and reversibility in I/sub c/ between 0 and 200 MPa. Using the newly developed reinforced-(Nb,Ti)/sub 3/Sn wire, it will be possible to fabricate a lightweight, large-bore, high-field and compact superconducting magnet in the near future.

Synthesis of Nb3Ga and Nb3Al superconducting composites by laser beam irradiation

Nb3Ga and Nb3Al superconductors were successfully fabricated by continuous CO2 laser beam irradiation of a moving material. Transition temperatures after the irradiation are 17.0 and 16.5 K, respectively. These temperatures are increased by ∼2 K by the subsequent annealings. Critical current density Jc (for the reacted area) of 3×104 A/cm2 at 15 T and 4.2 K is obtained for Nb3Al tape with small magnetic field dependence up to 23 T. The results indicate that the continuous laser beam irradiation is a very attractive method for fabricating advanced superconductors, such as Nb3Ga, Nb3Al, Nb3(Al,Ge), etc.

Fabrication of multifilament Nb3Sn conductors

Multifilament conductors have been fabricated using niobium tubes with a sheathed tin‐copper alloy inside and pure copper matrix outside. These composites have good workability, cold drawn without any intermediate heat treatment. Sample evaluation has included measurements of critical temperature, resistivity, and critical current density. The Nb3Sn formed by this method has much higher critical current density than the previous one with approximately the same tin concentration (12 at.%), but lies on the line extrapolated from the bronze method over the solid solubility limit.

Properties and performance of the multifilamentary Nb 3 Sn with Ti addition processed by the Nb tube method

MF Nb 3 Sn conductors made by titanium added niobium tube process have been developed for use in high fields. Composites, consisting of Nb-1. Owt.%Ti tube with a copper sheathed tin core inside and a high conductivity copper tube outside, were stacked together up to 264 filaments, then drawn to the final sizes without any intermediate annealings. As workability was further improved by the titanium addition, it was possible to decrease the Cu/SC ratio for the conductor down to 0.67 and to increase the tin concentration up to 30 wt.% in Cu-Sn inside the filament. An enhanced layer growth rate and a slightly increased grain size of Nb 3 Sn were observed in the titanium added conductors. compared with those made by use of pure Nb tube. The critical current density at high fields, which was more sensitive for heat-treatment condition, was superior to that of the other conventional processed conductors. Especially, the conductors with 30 % tin content have high critical current density for Nb 3 Sn layer and an excellent field dependence of critical current density without copper: 1550 A/mm2at 10 T, 600 A/mm2at 15 T and 350 A/mm2at 17 T. The critical current was also measured in the temperature range 1.88 - 4.2 K. A coil, aiming at 14 T, was fabricated using the conductor with .25 % tin to assure high level performance in practical situations.

Critical currents and flux creep in orientated BiPbSrCaCuO 110 K phase made by the powder in tube method

Abstract Critical currents, magnetic relaxation, magnetic irreversibility and microstructure have been studied for a (Bi,Pb)2Sr2Ca2Cu3Ox phase tape conductor fabricated by the powder in tube method. This conductor, with transport critical current density, Jct (at 77 K and O T), of a few thousand amperes per square centimetre and ab plane orientation to the rolling flat surface, showed pinning potential energies, Uo, of 66 meV for the field perpendicular to the ab plane and 181 meV for the field parallel to the ab plane; these values were much larger than those reported for the Bi2Sr2CaCu2Ox phase single crystal. Moreover, the field dependence of the magnetic irreversibility temperature was smaller for this tape conductor than that for Bi based crystals. These results imply an improvement in the pinning ability, due to the severe cold working peculiar to this process. Transmission electron microscopy (TEM) showed some defects, such as dislocations and twin-like structures, which may act as pinning centres.

Studies on superconducting Nb3Sn formed from high‐tin‐concentration Cu‐Sn alloy

Multifilament conductors have been fabricated using niobium tubes with a sheathed tin‐copper alloy inside and pure copper matrix outside. These composites have good workability, cold drawn without any intermediate heat treatment. Reactions and their effect upon superconducting properties have been checked for temperatures in the 625–800 °C range and various inner filament sizes ranging from 18 to 1154 μm. The Nb3Sn layer thickness grows as t0.8 in most of the reaction time, which cannot be explained by an ordinary diffusion process. The pinning force is found to be inversely proportional to the square root of the grain size measured through a fractographic technique in the 0.1–0.3‐μm range. Also, the activation energy of the Cu/Sn‐Cu diffusion couple is obtained as 47 kcal/mol from the reaction‐temperature–time relation. These results have been discussed and compared with others.

Nb 3 Al and Nb 3 (Al,Ge) composite tapes fabricated by CO 2 laser beam irradiation

Nb 3 Al and Nb 3 (Al,Ge) tapes with excellent superconducting properties were fabricated by the laser beam irradiation method. Nb-25 at %Al and Nb-20 at % Al-5 at % Ge tapes were prepared by the powder metallurgy process. Continuous CO 2 laser beam irradiation was carried out on the tape surface in an argon gas atmosphere at 1-13m/min, velocities. Beam power was 0.3-4kW and beam diameter at the tape surface was 0.5-3mm. As the power density was high and irradiation time was short, the tape could be heated and cooled much faster than a tape that was heat treated by the conventional method. The results were fine grain structure and large J c values at high magnetic fields. T c for an as-irradiated tape was 16-18K. This T c increased by ∼2K by the subsequent annealings at 700°C for 100 hours. The maximum T c (onset) values obtained were 18.6K for the Nb 3 Al tape and 20.1K for the Nb 3 (Al,Ge) tape, which indicated that stoichiometric compounds could be formed by laser beam irradiation. Annealed tapes, after the irradiation, showed excellent J c values, especially at high magnetic fields. J c values over 2×10 4 A/cm2at 23 Tesla were obtained for both Nb 3 Al and Nb 3 (Al,Ge) tapes. These values are much larger than those obtained by conventional heat treatment. J c values of as-irradiated tapes at high fields were smaller than those for annealed tapes, due to the lower T c and, hence, lower H c2 . The large J c values, as well as easy scale-up procedure, indicate that the laser irradiation method is promising for the fabrication of advanced superconductors, e.g. Nb 3 Al and Nb 3 (Al,Ge) capable of generating fields over 20 Tesla.

Enhanced high-field current carrying capacities and pinning behavior of NbTi-based superconducting alloys

High‐field critical current densities Jc and pinning behavior are discussed for Nb‐63a/oTi, Nb‐61.7a/oTi‐3a/oHf, and Nb‐64.7a/oTi‐7.2a/oTa superconducting alloys. Jc properties for 8–12 T in these alloy superconductors under superfluid helium environments can be even superior to those in Nb3Sn at 4.2 K, when they are heavily cold‐worked after final heat treatments. A temperature scaling law of the flux pinning force Fp is found to hold for these alloys in the form of FP=K [Bc2(T)]nbp(1−b)q, where Bc2 is the upper critical field and b≡B/Bc2. The peak reduced field, bm, in the Fp−b curve depends upon how the alloys have been processed. The origins of n, p, and q are discussed in terms of the processing condition.