K. Tagawa

Enhanced current capacity of jelly-roll processed and transformed Nb/sub 3/Al multifilamentary conductors

In order to enhance the current carrying capacity, we have developed an improved fabrication process where the wire diameter can be increased from 0.5 to 1.25 mm and the Nb-matrix ratio decreased from 1.5 to 0.52, without degrading the critical current density, J/sub c/, of Nb/sub 3/Al phase. The critical current for a monolithic conductor at 21 T and 4.2 K has now been enhanced to 166 A which used to be 15 A. The compacted-strand-cables were fabricated to investigate feasibility for large-scale application uses. We have found that stranding and flat-rolling the as-quenched Nb/Nb(Al)/sub ss/ composite cause no degradation in J/sub c/. Attempts were made to stabilize the resulting high current conductors.

Incorporation of stabilizer to rapid-quenched and transformed Nb/sub 3/Al multifilamentary superconductors

A Nb/sub 3/Al superconducting conductor based on Jelly-Rolled (JR) Nb/Al composites is fabricated by the rapid-heating and quenching of a wire of such composites with subsequent transformation-annealing to form Nb/sub 3/Al phase. Since the conductor is heated up to 1900/spl deg/C, far above the melting point of Cu, during fabrication, Cu stabilizer cannot directly be included as a basic composite constituent. In the present study, attempts have been made to incorporate the stabilizer to the Nb/sub 3/Al conductor. In a mechanical-cladding method, the as-quenched composite is wrapped longitudinally with a Cu tape, and groove-rolled for mechanical bonding. For internal stabilization, Jelly-Rolled Nb/Al filaments are isolated from the stabilizers by a Nb-filaments barrier and the formation of unwanted Nb-Al-Cu (Ag) compounds are prevented. Ag-stabilizer, little reactive with Nb, can dramatically decrease the Nb-barrier thickness required and allows the increase in the volume fraction of the stabilizer instead.

Stabilization and coil performance of RHQT-processed Nb/sub 3/Al conductors

A Nb/sub 3/Al superconductor based on jelly-rolled (JR) Nb/Al composites of high performance can be fabricated by rapid-heating and quenching of a wire of such composites with subsequent transformation-annealing to form the Nb/sub 3/Al phase (RHQT process). Since the conductor thereby is heated up to 2000/spl deg/C, far above the melting point of Cu, the Cu stabilizer needs to be externally incorporated by a mechanical-cladding method between quenching and annealing. Such an appropriate deformation promotes the transformation from a disordered bcc phase, and thereby enhances the critical current density J/sub c/. A solenoid coil was fabricated by a wind-and-react method: the Cu-clad conductor (1.61/sup w//spl times/0.71/sup t/ mm/sup 2/) with Al/sub 2/O/sub 3/ yarn insulation was wound into a coil (19.7/40.8/sup /spl phi///spl times/49.7/sup H/ mm), transformed at 800/spl deg/C, and impregnated with beeswax. The resultant coil, while carrying a current of 179 A at 2.1 K in a superconducting back-up field of 21.2 T, generated an additional 1.3 T. The total magnetic field of 22.5 T is the highest record ever for a metallic superconducting coil.

Strain tolerance in technical Nb3Al superconductors

We observed crack formation in transformation-processed Nb3Al wires at room temperature, the wire being bent with a small clamp fixture with a curvature. The polished cross-section parallel to the longitudinal axis was observed, using a high power optical microscope or a field-emission scanning electron microscope. The bend strain limit for microcrack formation is found, changing the radius of the curvature of the clamp. The bend strain limit was found to be around 0.3% for standard Nb3Al wires. This corresponds to the irreversible tensile strain limit of the Ic characteristics determined with a 0.1 µV cm−1 criterion. Reduction of the barrier thickness should be avoided to keep to the bend strain limit. A new configuration of the Nb3Al wire is demonstrated to improve the bend strain limit. The filament is divided into segments in the transverse cross-section. The wire is fabricated by a double-stacking method. The bend strain limit is enhanced to about 0.85% for the wire surface; the equivalent strain of the outermost filament location is about 0.66%. A simple react and wind test for this wire was performed, where the wire experienced 0.86% bend strain. The degradation of Jc was found to be very small.

Development of Nb/sub 3/Al superconducting wire for accelerator magnets

Nb/sub 3/Al superconductors have shown promising performance compared to Nb/sub 3/Sn conductors when processed by rapid heating/quenching process. Therefore we have started an R&D program of Nb/sub 3/Al conductors for future accelerator magnets. Several test wires of around 0.8 mm diameter, which have relatively small filament (/spl sim/50 micron diameter) and low matrix ratio (/spl sim/1.0), were fabricated, and the heat treatment and area reduction conditions after the rapid quenching process were studied. The highest noncopper J/sub c/ achieved during this study was 1734 A/mm/sup 2/ at 10 T and 4.2 K.

Relationship Between BCC-Deformation, Transformation Temperature and Microstructure in

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 2000degC. It was reported several times that deformation to the quenched BCC Nb(Al) phase influences the superconducting properties, usually improving . 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 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 degC 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 1.2mum . The 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 wire ever reported. But, at least high-field Jc seems to saturate at a certain value.

{\rm Nb}_{3}{\rm Al}

Recent progress in the internally stabilized rapid-heating, quenching and transformation (RHQT) processed Nb3Al superconductors is described and their various features are compared to those of the externally stabilized Cu clad conductor. Based on the favourable feature that Ag is almost non-reactive to Nb during joule heating, we have succeeded in thinning a diffusion barrier of Nb to less than 5 µm in the finished cross section by adopting an Ag rod jacketed with Nb as a stabilizer module when stacking the subelements. The Ag/non-Ag ratio could be increased to as much as 0.345 by additionally replacing the Nb-centre rod of a jelly roll (JR) with Ag. The superconducting properties of the critical current density, the residual resistance ratio, the magnetoresistance and the n-index in the voltage–current characteristic were studied for both as-quenched and subsequently deformed conductors, finally transformed at 800 °C for 10 h. A loading test of a small coil wound with a 30 m length of internally stabilized RHQT JR conductor has shown the uniformity of the internally stabilized conductor used.

Wires

A Jelly-Roll processed Nb3Al superconductor with excellent superconducting properties has been fabricated by the process of rapid heating, quenching and transformation (RHQT). In order to fabricate a long length of wire with homogeneous superconducting properties, the optimization of the RHQT conditions is particularly important because RHQT conditions determine the final microstructure and superconducting properties. In this paper, we have studied the variation of microstructure and superconducting properties with RHQ and transformation annealing conditions, to determine the optimum processing condition. In the study of the RHQ treatment, we found that the RHQ conditions are divided into four regions according to the joule heating current (IRHQ). In the optimum region, the wire is quenched from the temperature in which a bcc solid solution extends to more than 25 at%Al, and the scatter of critical current density (Jc) after transformation annealing is almost negligible. In the study of the deformation and transformation heat treatment, we have found that the deformation of the supersaturated solid solution improves the resultant superconducting properties, and prevents degradation of superconducting properties that occurs when the temperature ramp-up rate is slow.

Development of internally stabilized RHQT Nb3Al superconductors

The surface of RHQT-processed Nb/sub 3/Al wires with pure Nb matrix are covered by a strong stable Nb oxide surface layer. In order to obtain good mechanical, electrical and thermal bonding between the Cu stabilizer and RHQT-processed Nb/sub 3/Al wires, it is required to destroy the Nb oxide layer on the surface of the wire. We tried to fabricate a thin Cu layer on the surface of the RHQT-processed Nb/sub 3/Al wires through the Cu ion-plating technique. Before electroplating of thick Cu stabilizers, Cu was ion-plated to a thickness of about 1 /spl mu/m. The Cu ion-plated wire showed no folded projections, cracks, or exfoliation of the Cu stabilizer even when the wire was bent through 180 degrees, showing that Cu and the wire were tightly bonded mechanically. This tight bonding between Cu and the wires should be due to the removal of stable Nb oxide layers from the surface of the wire. The V-I characteristic of wires that were ion-plated and then applied with an appropriate amount of Cu by an electroplating showed no quenching, and the wires were able to carry current up to the normal critical current transition. The measured values of recovery current were almost equivalent to the calculated values, showing that the ion-plated Cu/Nb interface had a very tight bond, allowing good electrical and thermal conductivity. In addition, a reel to reel Cu-ion plating apparatus for the long length wires is demonstrated, and Cu ion plating has been successfully carried out on 100 m long RHQT-processed Nb/sub 3/Al wire at present.

Effect of rapid heating, quenching and transformation conditions on the superconducting properties and microstructure of Jelly-Roll processed Nb3Al superconductors

We have produced so far about a 400 m length of Cu stabilized RHQT (rapid-heating, quenching and transformation) Nb3Al flat-wire using a small-sized multi-billet (10 kg). Although our goal is to make a 2.5 km unit length of Cu cladding flat-wire for high-field NMR uses, an attempt was made to fabricate a 1 km class unit-length of conductor as a milestone of commercialization. A long-length of precursor wire over 2.5 km was prepared by using a large-sized multi-billet (50 kg). A 1.3 km length RHQ operation by using a newly installed large-scale RHQ apparatus was performed and then a km-class length Cu cladded flat-wire was fabricated to evaluate the uniformity of resultant superconducting characteristics of wire