R & D for the next generation superconducting magnets of high-energy physics accelerators, MHD generators, SMES, nuclear fusion reactors, and higher resolution NMRs needs advanced superconductors. The conductors of Nb3Al are the possible candidates because of the higher magnetic field properties of critical current density, the excellent critical current density at lower- and medium-field ranges, and the higher strength against mechanical stress. The R & D of jelly-roll processed Nb3Al has given already well-qualified conductors. Newly proposed rapid heat, the rapid quench process for Nb3Al, is also attractive from an economic aspect because of the superconductive characteristics of conductors. However, further developments are needed for the choice of stabilizer metal, cross-section configuration design, insulation materials, reinforcement and impregnant, n-value, joint technology between the two conductor; thus a prompt start of R & D is being requested.
Nb3Al with stoichiometric composition shows Tc above 19K and Hc2 (4.2K) above 30T, much higher than in commercially available Nb-Ti wire and Nb3Sn wire. Therefore the developments of fabrication processes for Nb3Al conductors were performed early on with zeal, but ended without success because stoichiometric Nb3Al is unstable at any temperatures except about 2, 000°C. Moreover, the fabrication speed of Nb3Al through solid-state diffusion reaction is very slow. To overcome these difficulties, a recent new process, RHQT (rapid-heat, quench, and transformation), was developed for fabricating Nb3Al multifilamentary conductors with near-stoichiometric composition and fine-grained crystals. Through this process we have succeeded in the fabrication of Nb3Al multifilamentary wires showing 2 to 5 times larger Jc at 4.2K than those of commercialized superconductors in any field, and their Tc of about 17.6K and Hc2 (4.2K) of about 26T are similar to those of the (Nb, Ti)3Sn wire. Furthermore, excellent tolerance to mechanical strain is also obtained for the Nb3Al conductor. I believe the new Nb3Al conductor should soon be commercialized and take the place of Nb3Sn conductor. In this paper I would like to describe the historical progress in the Nb3Al conductor fabrication process up to the birth of the RHQT process.
Nb3Al multifilamentary conductors have been fabricated via a newly developed process where the “precursor” of supersaturated bcc-solid-solution Nb(Al)ss/Nb is first formed by quenching the Jelly-roll (or rod-in-tube) Nb/Al composites from the high-temperature region and subsequently transformed to Nb3Al of nearly stoichiometric composition with fine grains. Although the precursor contains many spherical defects (probably the Kirkendall void), it is flexible and ductile even at room temperatures, enough to be Cu stabilized, cabled, flat-roll formed, and wound to a coil before being aged and transformed. The transformed Nb3Al shows high-field critical current densities much larger than those for conventionally prepared Nb3Al conductors, where the Nb3Al phase is known to be off-stoichiometric. The degradation of critical current densities with -0.7% intrinsic strain is about 20% at 12T, comparable with those for conventional JR-Nb3Al conductors of high strain tolerance. An attempt is made to further improve the high field performance by adding Ge to Al.
Nb3Al superconducting wires fabricated by the RHQT (rapid-heat, quench, and transformation) process show excellent superconducting properties in high magnetic fields. To prove the uniformity of the Nb3Al superconducting monolith wire in a longitudinal direction, we made a small solenoid coil by using a Nb3Al monolith wire 0.5mm in diameter, 137m long, and 169A/mm2 in Jc (21T, 4.2K). The quench current of the coil was almost equal to the Ic of a short sample for this coil. As a next step, we tried to develop a Nb3Al superconducting wire with a large current capacity. We successfully produced a Nb3Al monolith wire with a diameter of 1.25mm and a Nb-matrix ratio of 0.52, which showed the Ic (21T, 4.2K) of 166A and the Jc (21T, 4.2K) of 207A/mm2. We also attempted to make a compacted strand cable as an alternative. The Nb3Al compacted strand cable showed a large current capacity without degradation, compared with the Ic summation of the original strand wires. These results indicate that the RHQT-processed Nb3Al superconducting wire has great potential for large-scaled and high-field applications.
Nb3Al superconductors have a strain tolerance superior to Nb3Sn superconductors. Nb3Al superconductors have been expected to be useful for high-field applications, such as high-energy physics (particle accelerators) and fusion reactors. In this paper, Nb3Al wire processing technologies are summarized, and the properties of Nb3Al strands made by the jelly-roll process are described. The jelly-roll process was suitable to develop copper-stabilized multifilamentary composites. For the high-energy physics application, a Rutherford-type cable conductor was developed. For the fusion reactor application, a chrome-plated strand with a critical current density performance of at least 600A/mm2 at 4.2K and 12T was developed, and 230km strands, the world's first mass-produced Nb3Al wire, were fabricated for the Insert Coil of ITER/EDA.