Y. Iijima

Nb3Al Multifilamentary Wires Continuously Fabricated by Rapid-Quenching

The Nb tube process has recently been developed at NRIM(Japan) for fabricating Nb3Al multifilamentary superconductors,1–3 which are characterized by the heat treatment at low temperature ( 1700°C), many attempts were made to form Nb3A1 by heat treating Nb/AI composite at high temperatures.4–6 However, rapid grain growth decreased the density of pinning center (grain boundary) in Nb3A1 and thereby degraded Jc in particular at low magnetic fields. To overcome this problem, Nb3A1 conductors fabricated through a rapid-quenching process have been studied. In this process the A15 Nb3A1 phase with fine grain structure can be precipitated from the supersaturated Nb-Al bcc phase by aging at ~800°C. However, no attempt was made to fabricate a practical-structure conductor such as a long multifilamentary wire through continuous rapid-quenching and subsequent annealing (post annealing) process.

Microstructures of rapidly-heated/quenched and transformed Nb/sub 3/Al multifilamentary superconducting wires

Microstructures of rapidly-heated/quenched and transformed Nb/sub 3/Al multifilamentary wires were studied by using transmission electron microscopy. Nb/Al composite wires are fabricated by a jelly-roll process. The Nb/Al composite filaments changed into an Nb-Al supersaturated bcc solid solution with rapid-heating/quenching. The Nb-Al bcc phase consists of many crystal grains with diameters of 2-4 /spl mu/m, surrounded with large-angle grain boundaries. Some spherical voids, about 0.1 micron in diameter, were also observed at the intra- and intergrains. All grain boundaries of the Nb-Al bcc phases are simple flat planes. Then, the Nb-Al bcc phases were transformed into A15 phases (grain size: 0.5-2.0 /spl mu/m in diameter) with additional annealing. Secondary phases were not observed in the A15 filaments. Grain boundaries of the A15 phases show zig-zag shape unlike those of the Nb-Al bcc phases, and every grain of A15 phases is an aggregation of sub-grains of 80-150 nm in diameter. Sub-grain boundaries are small-angle ones. Moreover we found that many stacking faults formed in the A15 sub-grains in parallel with spaces of 10-20 nm. These numerous plane defects seem to be the main pinning centers in the rapidly-heated/quenched and transformed Nb/sub 3/Al wires.

Improvements in current-carrying capacities of Nb 3 Sn composites in high fields through titanium addition to the matrix

Single-core and multifilamentary Nb 3 Sn composites with titanium addition to the matrix have been fabricated. The electron-probe microanalysis indicates that the titanium is more rapidly incorporated into the Nb 3 Sn layer from the matrix than from the core. The titanium addition of less than 1.5 at.% to the matrix does not deteriorate the workability of the Nb 3 Sn composites. The titanium addition to the matrix remarkably increases the growth rate and J c in high fields of the Nb 3 Sn layer. The optimum amount of titanium addition to the matrix to produce the highest overall J c at 16 T was found to be about 0.8 at.% for the 160-core Nb/Cu-7at.%Sn-Ti composite wires. The simultaneous titanium addition to the core and to the matrix produces further improvement in J c in high magnetic fields. An overall J c of about 2.7×104A/cm2at 16 T is obtained for the 370-core Nb-1.5at.%Ti/Cu-8at.%Sn-0.5at.%Ti composite wire reacted at 700°C for 200 h.

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.

Nb/sub 3/Al conductor fabricated by DRHQ (Double Rapidly-Heating/Quenching) process

For Nb/sub 3/Al compound, extremely high temperatures are required to obtain the stoichiometric A15 phase, so that the rapidly-heating/quenching process has been developed. In the process the Nb/Al composite filaments change into Nb-Al supersaturated bcc solid solution filaments with rapid-heating/quenching. The Nb-Al solid solutions are transformed into A15 phases with post-annealing at 800/spl deg/C, resulting in a 2-5 times higher J/sub c/(4.2 K) than those of commercial (Nb,Ti)/sub 3/Sn wires. However, the best values of T/sub c/=17.8 K and H/sub c2/=26 T are rather lower than those of stoichiometric Nb/sub 3/Al, which is due to its off-stoichiometry. Therefore, we tried a 2nd rapid-heating/quenching to the 1st rapidly-heated/quenched wires having Nb-Al solid solution filaments. The phase transformation was performed with the 2nd rapid-heating/quenching, which is a very-short high-temperature (about 1900/spl deg/C) heat treatment to obtain near-stoichiometric Nb/sub 3/Al. Without any additional elements, the Nb/sub 3/Al compounds synthesized by the DRHQ (Double Rapid Heating/Quenching) process show T/sub c/ and H/sub c2/(4.2 K) of about 18.4 K and 30 T, respectively. The most interesting result is its excellent J,(4.2 K)-B performance, e.g., 2nd A/mm/sup 2/ at 25 T, 270 A/mm/sup 2/ at 24 T and 330 A/mm/sup 2/ at 23 T.

Superconducting properties of Nb3Al multifilamentary wire

Nb3Al multifilamentary wire has been fabricated by a newly developed process, in which a composite consisting of a niobium matrix and a large number of Al‐Mg alloy cores is cold drawn into a wire with ultrafine Al‐Mg alloy filaments (about 0.1 μm), and then heat treated to form Nb3Al filaments by the diffusion reaction between the Al‐Mg alloy cores and the Nb matrix. The wire shows excellent high‐field superconducting properties comparable to those of the practical Nb3Sn multifilamentary wire.

Effects of additive elements on continuous ultra-fine Nb/sub 3/Al MF superconductor

Nb/sub 3/Al multifilamentary (MF) wire can be fabricated by the composite process using various Al-based alloy cores and pure Nb matrix. Additive elements of Mg, Ag, Cu and Zn harden the Al core preferentially and then improve the workability of the Nb/Al composite, permitting it to be cold-drawn into a wire with ultrafine Al-based alloy filaments (filament number: 1.8 million, diameter: about 0.1 mu m). Reacted wires at 700-900 degrees C show T/sub c/ of 15-16 K, mu /sub 0/H/sub c2/ (4.2 K) of 21-22 T and J/sub c/ (4.2 K, 10 T) of 1-1.5*10/sup 9/ A/m/sup 2/. A two-stage reaction consisting of a reaction above 950 degrees C and a subsequent reaction around 700 degrees C is carried out to improve critical values; T/sub c/ and mu /sub 0/H/sub c2/ are increased to 17.4 K and 25.4 T, respectively. Excellent superconducting properties involving good strain tolerance ( epsilon /sub irrev/=1.3%) indicate that this Nb/sub 3/Al is very promising for practical high-field superconducting cables. >

Manufacture and superconductivity of tantalum matrix RHQT processed Nb/sub 3/Al superconductors

The replacement of Nb matrix with Ta for rapid-heating, quenching and transformation annealing (RHQT) processed Nb/sub 3/Al conductor seems to be advantageous for facilitating the RHQ operation due to a higher mechanical strength at elevated temperatures and less reactivity with the molten Ga in the quench bath and suppressing flux jumps in low magnetic fields. We have fabricated three grades of the Ta matrix precursor, in which the volume fraction of Ta matrix is different, so as to examine the drawability of such Ta matrix precursors. Also examined are the effects of using the Ta matrix on the RHQ condition, T/sub c/ values after quenching and subsequent annealing, respectively, J/sub c/ (core), overall residual resistivity, and magnetization curves. We have succeeded in drawing all of Ta matrix precursors and confirmed the favorable existence of a plateau region in the I/sub RHQ/-T/sub c/ relationship even for the Ta matrix RHQT Nb/sub 3/Al conductors. The resulting superconducting properties were comparative to those of Nb matrix conductors. The replacement of Nb matrix with Ta was very effective in suppressing flux jumps even at 4.2 K.

Effect of flat-roll forming on critical current density characteristics and microstructure of Nb/sub 3/Al multifilamentary conductors

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.

Pinning mechanism in a continuous ultrafine Nb3Al multifilamentary superconductor

For Nb3Al multifilamentary superconductors fabricated by a newly developed composite process using continuous ultrafine Al‐based alloy cores and pure Nb matrix, Jc properties have been investigated in detail in regard to the size and morphology of the Al core. A significant enhancement in pinning force caused by reducing the Al core size and an anisotropy in Jc observed in a rolled tape have indicated that the Nb3Al‐matrix (or core) interface acts as a dominant pinning center of fluxoids.