Search

This paper describes two subjects, both aimed at the industrialization of Nb3Al conductors. One is the metastable solid-solution-strand restacking method, which is expected to be a low-cost Cu-stabilization method compared with the electroplating technique. In this paper, we mainly focus on recent efforts toward long-length production. The key point here is an improvement of the contact between the Cu tube and the restacked original strands. The second subject is the first attempt at “constant voltage” control in the continuous rapid-heating and quenching operation for 300-m-class precursor wire. With constant current control, changes in the diameter of the wire lead to variations in power per unit volume, so that the processing temperature changes. With constant voltage control, changes in diameter of the wire do not lead to changes in the power per unit volume, so that the processing temperature does not change and thus the uniformity of the longitudinal performance is improved. [ABSTRACT FROM AUTHOR]

Observations of fractured cross sections, which are frequently used to determine the grain size distribution in \ Nb3{\ {Sn}}, cannot be performed for \ Nb3{\ {Al}} since transgranular fracture is the dominant fracture mode. Instead, we have evaluated the grain size of \ Nb3{\ {Al}} using electron backscatter diffraction (EBSD). To check the validity of EBSD analysis for A15 grain structures, bronze-route \ Nb3{\ {Sn}} filaments were also examined by EBSD. The deformation-and-transformation of bcc supersaturated-solid solution \ Nb\ (Al)ss was found not only to homogenize and refine the grain size of the resultant \ Nb3{\ {Al}} but also to increase a ratio of high angle grain boundaries to low angle grain boundaries. The texture studies were made by pole figure works: the \ Nb\ (Al)ss retains a \langle 110\rangle fibrous texture as well as the filament barrier (Nb, Ta) does. The transformed \ Nb3{\ {Al}} also has the weak \langle 110\rangle fibrous texture, whereas \ Nb3{\ {Sn}} has almost never texture. A clear relationship between Jc and the inverse of grain size was not observed. [ABSTRACT FROM AUTHOR]

The critical current of a Nb3Al wire (RHQT process) with a rectangular cross section of 1.81\times 0.80\ mm^2 has been measured at 4.2 K under transverse compressive loads at fields between 15 T and 19 T. The same wire (HE2432) was previously used for winding an insert coil at NIMS (coil #B) that generated 4.5 T in a background field of 15 T at 4.2 K. In this work the load is applied perpendicularly to the narrow side of the conductor and the corresponding stresses reach 300 MPa. The critical current stays almost constant up to 150 MPa and shows a flat maximum around 75 MPa. At 300 MPa the critical current is reduced to 93%, 90% and 88% of its stress free value at 15 T, 17 T and 19 T, respectively. This indicates a rather small field dependence and significant degradation being seen only above 200 MPa. Depending on the field, unloading from 300 MPa yields to an almost complete recovery of Ic between 94% and 97% of its initial value, respectively. These observations are compared to the behavior of a Nb3Sn bronze wire. [ABSTRACT FROM AUTHOR]

Research and development of Nb3Al strands and cables for a high field accelerator magnet is ongoing under the framework of the CERN-KEK collaboration. In this program, new Ta-matrix Nb3Al strands were developed and their mechanical properties and superconducting properties were studied. The non-Cu Jc values of these strands were 750\sim 800\ \A/mm^2 at 15 T and 4.2 K. Using these strands, test fabrication of 27-strand Rutherford cable was carried out in collaboration with NIMS and Fermilab. The properties of the strands extracted from the cable were examined and it was found that there was no degradation of the superconducting properties of the strands. In this paper, we report the fabrication of the strands and the cable in brief and present some of the results obtained by studying their properties. [ABSTRACT FROM AUTHOR]

Sn-based sheets with Ta, B or Nb particle dispersion have been prepared from consolidated alloys. A small amount of Ti addition strengthened the bonding between dispersed particles and the Sn matrix, which improved the workability of the sheet. The sheet was laminated with a Nb sheet, and wound into a Jelly Roll (JR) composite. The composite was easily fabricated to a wire without intermediate annealing. Thick and uniform \ Nb3{{\ {Sn}}} layers with nearly stoichiometric Sn concentration were synthesized by the mutual diffusion between Nb and Sn-based alloy layers. An offset Tc of \sim18.1 K and an offset Bc2 (4.2 K) of \sim26.5 T have been obtained after the heat treatment. The non-Cu Jc of the wires increased with decreasing the wire diameter. A non-Cu Jc of \sim \ 150 A/mm^2 has been obtained at 4.2 K and 22 T. The Sn–Ta, Sn–B and Sn–Nb based sheet JR wires exhibited similar microstructures and superconducting performances. [ABSTRACT FROM AUTHOR]

Rapid heating, quenching and transformation (RHQT)-processed Nb3Al wires possess better strain tolerance than Nb3Sn wires. However, RHQT-processed Nb3Al wires so far developed show lower non-copper critical current density (Jc) than recently developed high-(Jc) Nb3Sn wires in the field range of 12-15 T. Therefore, the development of Nb3Al wires having enhancing Jc in this field range is a major concern for accelerator magnet designers. Among a number of fabrication parameters that might affect properties, we analysed the effect of thickness of Al and Nb foils in the jelly-roll-shaped filaments embedded in the precursor wires. Mechanical and superconducting properties of these wires with various Al-foil thicknesses in the filaments were investigated. [ABSTRACT FROM AUTHOR]

The authors have been developing the Nb3Al conductors through transformation process, where the Nb3AL phase forms from the BCC (body-centered cubic) Nb(Al) supersaturated solid solution phase; the BCC phase is quenched by rapid-cooling from a temperature around 2000°C. It was reported several times that deformation to the quenched BCC Nb(AI) phase influences the superconducting properties, usually improving Jc. But, the results have not been discussed in detail so far from a metallurgical viewpoint. This work investigates the fundamental relationship between the BCC-deformation, transformation temperature, microstructure and superconducting properties. The transformation temperature is identified by DTA measurement, detecting the release of the free energy between the BCC and the Nb3Al phases. It was found that the transformation temperature has a tendency of monotonic decrease with the deformation. But, the transformation temperature does not seem to go down below a temperature of about 720°C even by strong deformation. The microstructures observed by FESEM suggests that the grain size is reduced almost proportionally with the deformation. In case of reduction in area of 99%, the grain size is reduced down to about 250 nm from 1.2 μm. The Jc increases with deformation, not showing a peak. The Jc of the wire (ME365) with reduction in area of 86% shows the best performance of the Nb3A1 wire ever reported. But, at least high-field Jc seems to saturate at a certain value. [ABSTRACT FROM AUTHOR]

To demonstrate the RHQT Nb3Al conductor to be really a candidate for a high-field magnet, we have fabricated 1 km-class Cu stabilized RHQT Nb3Al flat-wires, manufactured two insert real-sized coils #A and #B, and tested in a superconducting back-up field of 15 T at 4.2 K. Using the coil #A, a thinning of insulator, a winding technique, a new transformation annealing profile in a vacuum with a fast temperature-ramp above 500°C, decarbonization, an impregnation technique used for the Nb3Sn coil were preliminarily checked to be no matter. The coil #B that was wound with a 764 m piece could produce an additional 4.5 T (total 19.51) at 4.2 K in a clear bore of 30 mm without any training, when the coil current achieved the Ic of short samples that were simultaneously transformation annealed with the coil #B. [ABSTRACT FROM AUTHOR]

Recent studies of RHQ-processed Nb3Al wires for future accelerator magnets are presented and discussed. Test wires were prepared with different fabrication parameters, such as the Nb matrix ratio, RHQ current, area reduction ratio of the wire after an RHQ treatment and the 2nd heat treatment condition. Measurements of the critical current density (Jc) and n-value have been performed during the past several years. Recently, we performed the critical temperature (Tc) measurement, and the relationship between Jc and Tc was investigated by an RHQ treatment, a 2nd heat treatment, the Nb matrix ratio, and the area reduction effect. [ABSTRACT FROM AUTHOR]

Whereas Nb3Sn wires are commercially available and produced in long lengths for the fabrication of large high field magnets, Nb3Al is emerging as a candidate material for even higher fields, particularly for nuclear fusion devices. We are therefore investigating the basic superconducting parameters of both materials, such as the transition temperature, the critical current densities and the upper critical fields, as a function of gamma dose and fast neutron fluence. Whereas the transition temperature of these multifilamentary wires is found to decrease slightly, both Jc and the upper critical fields (as determined from Kramer plots) increase with neutron fluence, in agreement with previous work on A15 systems. The enhancement of the critical currents amounts to approximately 15%. Further irradiation work to higher fluences is under way, in order to investigate the limits of these materials for applications in a radiation environment. [ABSTRACT FROM AUTHOR]

We have developed a nearly-stoichiometric Nb/sub 3/Al multifilamentary superconductor by exploiting the transformation from bcc supersaturated-solid-solution Nb(Al)/sub ss/. This is a candidate conductor for 1 GHz NMR magnets. Flat-roll forming, an effective way to increase the packing factor of the coil and thereby the coil-current-density, has been carried out on the multifilamentary Nb(Al)/sub ss//Nb wire, and its effects are compared with that of the conventional Nb tube processed conductor. A Nb(Al)/sub ss//Nb wire, 0.74 mm in diameter, was successfully deformed into a tape conductor (the thinnest case: 0.15/sup t//spl times/2.2/sup w/ mm) without mechanical fracture. The flat-roll forming creates very little anisotropy in critical parameters and scarcely degrades the J/sub c/ properties. Excess flat-rolling causes a wide distribution of Nb(Al)/sub ss/ filament areas and thus reduces the n-index in the voltage-current characteristic. However, it is probably possible to retain the high value of the n-index of a wire, by appropriately designing the overall-aspect-ratio of a flattened conductor. [ABSTRACT FROM PUBLISHER]

Abstract: Magnesium diboride (MgB2) superconducting wires are interesting as alternative conductor of Nb-based superconducting wire applying for the advanced nuclear fusion reactor from viewpoint of decay time of induced radioactivity. However, J c property of MgB2 wire is insufficient for alternative material of Nb-based superconductor, and it is necessary to improve further J c property. In order to improve J c property, we tried to fabricate the several trace amounts of Cu addition MgB2/Fe-sheathed wires through PIT process using Mg2Cu as Cu addition source. T c values of Cu addition samples were higher than those of non-Cu addition samples and they were improved with increasing amount of Cu addition. On the other hand, core J c value were decreased with increasing amount of Cu addition and the maximum core J c values were estimated to be over 100A/mm2 at 4.2K under 8T for 1 and 3at% Cu addition sample. [Copyright &y& Elsevier]