WO1997024192A1

WO1997024192A1 - Superconductor

Info

Publication number: WO1997024192A1
Application number: PCT/US1996/020845
Authority: WO
Inventors: Dingan Yu
Original assignee: Composite Materials Technology, Inc.
Priority date: 1995-12-29
Filing date: 1996-12-27
Publication date: 1997-07-10

Abstract

Ductile superconducting alloy NbTi sheets (1) are layered with transition metal Ta mesh (2) to produce a stack (3). The stack (3) is inserted into a copper can (4) along with a diffusion barrier (5). The resulting composite is then processed by a plurality of reduction and heat treatments to generate flux pinning sites within the superconducting alloy regions in order to obtain enhanced Jc and Hc2.

Description

SUPERCONDUCTOR NbTi alloy has long served as the backbone ofthe superconducting wire industry. Efforts to further develop this material have focused primarily on enhancement of J_c and/or H_c2 performance to meet growing demands for materials that perform adequately in high field (1 OT-12T) applications. Generally, it has been necessary to utilize Al 5-type superconductors in such applications. But Al 5 materials are far less durable and reliable than are NbTi alloy-based conductors. The goal of development work has thus been to retain the robust mechanical properties of NbTi alloy conductors while somehow obtaining the required high field performance. The present invention is aimed at meeting these demands. In order to obtain useful J_c performance in a superconductor, one requires defects within the superconductor that serve to inhibit the motion of magnetic flux lines. These defects, called "pinning sites", may be grain boundaries, voids, or any other discontinuity in the body ofthe superconductor. In the ease of NbTi, pinning sites are generally developed through a series of thermomechanical steps that result in the precipitation and elongation of α-Ti, A.W. West and D.C. Larbalestier, "Microstructural Changes Produced in a Multifilamentary Nb-Ti Composite by Cold Work and Heat Treatment", Metall. Trans. A., vol. 15A, pp. 843-852 (1984). D.C. Larbalestier and A.W. West, "New Perspectives on Flux Pinning in Niobium- Titanium Composite Superconductors", Acta Metallurgica, vol. 32, pp. 1871- 1881 (1984). When reduced to final size, α-Ti pinning sites serve as very effective flux pinners. Unfortunately, it is very difficult to precipitate more than 20%-25% by volume of α-Ti in the NbTi alloys normally used in industry (Nb47wt.%Ti, e.g.). This limits the potential J_c performance of conductors containing these alloys. It is possible to get higher α-Ti volumes in NbTi alloys with higher overall Ti contents, but such alloys tend to have severe ductility problems. In recent years, attempts have been made by various investigators to increase pinning volume and efficiency in NbTi alloys through the introduction of artificial pinning centers (APC), G.L. Dorofejev, E. Yu. Klimenko, S.V. Frolov, E.V. Nikulenkov, E.I. Plashkin, N.I. Salunin, and V. Y. Filkin, "Current-Carrying Capacity of Superconductors with Artificial Pinning Centers", Proc. Ofthe 9^,h International Conf. On Magnetic Technology, MT-9, Ed, by C. Marinucci and P. Weymuth, pp. 564-566 (1985), Zurich.; B.A. Zeitlin, M.S. Walker and L.R. Motowidlo, "Superconductors Having Controlled Laminar Pinning Centers, and Method of Manufacturing Same", U.S. Patent 4,803,310, 1989; J. Wong, et al., U.S. Patent No. 5,158,620, 5,160,550, 5,160,794, 5,174,830, 5,174,831, 5,223,348, and 5,230,748; K. Matsumoto, H. Takewaki, Y. Tanaka, O. Miura, K. Yamafuji, K. Funaki, M. Iwakuma, and T. Matsushita, "Enhanced J_c Properties in Superconducting NbTi Composites by Introducing Nb Artificial Pins with a Layered Structure", Appln. Phys. Lett., vol. 64 pp. 1 15-117 (1994); P.D. Jablonski and D.C. Larbalestier, U.S. Patent No. 5,226,947, 1993;. The basic idea behind NbTi APC conductors is that ductile pinning sites can be incorporated into the alloy by mechanical means, rather than relying upon precipitation. The primary advantage Is that the volume percentage and type ofthe artificial pins can be whatever one desires. Secondarily, since precipitation is not required for APC conductors, the numerous heat treatment steps necessary in conventional alloy processing can be eliminated, greatly streamlining the conductor fabrication process. An APC conductor would typically be fabricated by mechanically combining a NbTi alloy with a suitable pinning material, most commonly Nb, and then reducing the composite to final size through a series of rebundling and drawing steps. Such processes have resulted in J_c performance at a low field (≤6T) that is superior to that obtainable with the conventional alloy approach. However, APC conductors of this type typically suffer from much inferior J_c performance at high field (≥6T), where J_c is at a premium. In 1989, Supercon, Inc. developed a unique APC process wherein pure Nb and pure Ti sheets are utilized to form the basic monofilament. J. Wong, et al., U.S. Patent No. 5,158,620, 5,160,550, 5,160,794, 5,174,830, 5,174,831, 5,223,348, and 5,230,748. During subsequent processing, a controlled amount of diffusion occurs, and a superconductor/normal structure is developed. The normal (non-superconductor or only weakly superconducting) regions serve as pinning sites in the final conductor. This process has been found to result in conductors displaying high filed J_c's comparable to those of conventional conductors. In the present invention, the methods of conventional NbTi alloy processing are combined with APC techniques to produce a NbTi superconductor displaying enhanced J_c and H_c2 properties. In the preferred embodiment, Nb47Ti alloy sheets are layered alternately with pure Ta mesh to form a monofilament. The monofilament is processed and restacked in accordance with standard industry practice. The restacked material is processed to final conductor size by conventional means, including precipitation heat treatment steps similar to those presently used in standard NbTi alloy processing. The dispersed Ta inclusions serve as flux pinning sites additional to those obtained through the heat treatment steps. Further, at final size, low temperature H_c2 enhancement is realized by virtue ofthe mechanical mixing ofthe NbTi and Ta. The invention can be extended to apply to other ductile superconducting alloys in combination with alternative transition metals without violating the spirit ofthe invention. Figure 1 is a schematic diagram illustrating the monofilament assembly process ofthe present invention. Figure 2 is a plot of critical current density versus applied magnetic field at 4.22K and 1.9K for a sample produced in accordance with the present invention as described in Example I. Figure 1 illustrates the monofilament assembly stage ofthe present invention. Ductile superconducting alloy sheets 1 are layered with transition metal mesh 2 to produce a structure of alternating alloy sheets and pure transition metal mesh 3. The sheets and mesh are cut to varying widths so that when stacked upon each other they form a cylindrical body of alloy sheets and Ta mesh. The stack 3 is inserted into a copper can 4 along with a diffusion barrier 5, which separates the can from the stack. In the preferred embodiment of the invention, the alloy sheets 1 are composed of Nb47wt.%Ti alloy, the mesh 2 is made of pure Ta, and the barrier 5 is made of Nb. Other NbTi alloys, higher or lower in Ti content, could be used in the practice ofthe invention. Alloys with transition element additions, such as NbTiTa, could also be used. The mesh layers could be composed of any appropriate transition element, not necessarily Ta. Possibilities include Nb, Ti, V, Hf, and Zr. The choice of alloy and mesh composition will depend upon the demands of the application. In another possible variation ofthe present invention, the alloy layers 1 and the mesh layers 2 are wound around a mandrel to form a jell-roll of alternating alloy mesh. Such a variation is described by Roberts et al. John A. Roberts and Peter R. Roberts, U.S. Patent No. 3,625,662, 1971 , in a similar context. After monofilament assembly, a nose and tail are welded into place and billet is processed to restack size in accordance with standard industry practice in the fabrication of NbTi monofilaments. The monofilament is then straightened and cut, and these filaments are then inserted into a copper can to form a secondary billet. The secondary billet is sealed as was the monofilament and is then processed to wire via well established NbTi conductor processing techniques, including appropriate precipitation heat treatment steps. In the preferred embodiment ofthe invention, none ofthe heating steps is sufficient to diffuse the Ta into the NbTi. The Ta serves as pinning material within the composite. The fact that it is arrayed as a mesh at the start means that the final composite contains a uniform dispersion of Ta pinning sites. These pinning sites are in addition to whatever pinning is developed through α-Ti precipitation. The overall volume percentage of pinning sites is thus increased. J_c is enhanced accordingly. Another feature of the invention is that when the thickness and spacing of the Ta inclusions is reduced to a size on the order of the NbTi coherence length, the overall structure will display properties consistent with the homogeneous NbTiTa alloy, in accordance with the theory of Meingast et al. C. Meingast, P.J. Lee and D.C. Larbalestier, "Quantitative Description of a High J_c Nb-Ti Superconductor During Its Final Optimization Strain: I. Microstructure, T_cl, H_c2, and Resistivity", J. Appln. Phys. Vol. 66, pp. 5962- 5970 (1989). If the relative volumes of NbTi and Ta are chosen so that their average composition falls in the region where the conventional NbTiTa alloy would have enhanced low temperature (2K) H_c2 compared to conventional NbTi, the same will be true for the APC material at final size. Low temperature, high field performance is improved as a consequence. The present invention envisions a structure composed of a ductile superconducting alloy, such as NbTi, containing a dispersion of a transition metal, such as Ta. Such structures have been described in the literature in the context of APC, as cited previously. It must be pointed out, however, that this prior art does not disclose a process in which such a composite structure is subjected to heat treatment for the purposes of precipitation in the manner of the present invention. This is an essential feature ofthe invention, since the transition metal pinning sites are only intended to add to the pinning provided by conventional heat treatment. The composite structures described by McDonald et al. W. . McDonald, U.S. Patent No. 4,262,412, 1981, and Flukiger et al. R. Flukiger, W. Specking, M. Klemrn, and S. Gauss, "Composite Core Nb₃Sn Wires: Preparation and Characterization (Invited)", IEEE Trans. Mag..., vol. 25, no. 2, pp. 2192-2199, March, 1989, while similar in kind that described in the present invention, are aimed at the fabrication of Nb₃Sn conductors, not NbTi. Furthermore, as in the other cases, no precipitation heat treatments are employed in the process of these structures. EXAMPLE I In one embodiment ofthe invention, a combination of Nb47wt.%Ti alloy sheets and Ta mesh was used to form a composite as described hereafter. The described process resulted in "monocore" wire, i.e., there was no copper between the filaments ofthe secondary billet. For this reason, the barrier 5 of the monofilament was composed of NbTi, not pure Nb. For most practical applications, the secondary billet will be multifilamentary, and a Nb barrier will be preferred on the monofilament. This is a trivial modification that in no way impacts the validity ofthe invention as described. Nb47wt.%Ti alloy sheets having a thickness of 1.01 mm were combined in alternating fashion with Ta mesh having a thickness of 0.178 mm and 70% of open area. 31 pairs of NbTi sheet and Ta mesh were utilized to form a stack having a diameter of approximately 3.8 cm. The overall composition ofthe stack — i.e., if it had been a homogeneous alloy — was Nb40.1 wt.%Til4.6wt.%Ta, close to optimum for a conventional ternary alloy. The stack was inserted into a copper can having an external diameter of 5.72 cm and an internal diameter of 4.15 cm. The can was lined with a 0.35 mm thick Nb47et.%Ti sheet prior to stack insertion. A copper nose and tail were TIG (Tungsten Inert Gas) welded to the ends ofthe can and the billet was evacuated at a temperature of approximately 425 °C. The billet was then sealed. The monofilament billet was cold isostatically pressed at 446 Mpa and was then machined to an outer diameter of 5.08cm. After a 2 hour preheat at 650°C, the billet was extruded to 1.27 cm diameter. The extruded rod was cropped and then drawn at an areal reduction rate of 20% per die pass to a final diameter of 1.02 mm. This wire was straightened and cut into approximately 2,012 individual filaments. The copper was etched off of the filaments and the bare filaments were inserted into a copper can having a 5.51 cm outer diameter and a 4.13 cm inner diameter. The can was lined with a 0.39 mm thick Nb diffusion barrier prior to insertion ofthe filament bundle.. The assembled billet was TIG welded shut, evacuated, and sealed in the same manner as was the monofilament.

The secondary billet was hot isostatically pressed at 103 Mpa and 650°C for 4 hours. It was then machined to 5.08 cm diameter. After a 2 hour preheat at 650°C, the billet was extruded to 1.59 cm diameter. The rod was then drawn to wire of 0.14 mm - 0.20 mm diameter at a rate of 20% areal reduction per die pass. A total of 4 precipitation heat treatments were applied in the course of reduction in order to promote α-Ti precipitation. The heat treatment schedule was chosen based on the literature (well known in the art) describing heat treatment of Nb47et.%Ti alloys. Since the composite was not simply Nb47Ti alloy, it was not anticipated that this schedule would produce optimum results, but only indicate the potential of the ternary composite. The schedule was as shown in the following table:

Wire Heat Treatment Heat Treatment Diameter (mm) Temperature (C°) Time (hours)

8.84 405 40 5.99 405 20 3.02 405 20 1.70 405 70 Samples resulting from this heat treatment schedule were tested for non-copper J_c at 4.2K in applied magnetic fields up to 9T using a standard four-point probe technique. The best of these samples had a wire diameter of 0.149 mm. At this size, the thickness ofthe stacked NbTi alloy layers and the Ta inclusions project to be approximately 60 nm and 10 nm, respectively. This sample was further tested for J_c at 1.9K in applied fields up to 9T. The J_c results are listed in the following table:

Testing Applied Non-Copper Critical Temperature (K) Field (T) Current Density

(A/mm²)

5 2648

4.22 6 2096 7 1554

1.90 7 3301 8 2760 9 2209

Figure 2 shows a plot ofthe above J_c data as a function of field. The straight lines through the data points are parallel. They are displaced in the applied field dimension by 3.20T, as indicated in the figure. This means that the conductor shows a 4.2K - 1.9K operating temperature/field advantage of 3.20T for a given J_c. (J_c = 2500 A/mm² at the point chosen in the figure.) This is very close to the estimated value of 3.4T for a 15 wt.%Ta ternary alloy fabricated conventially, H. Liu, E. Gregory, N.D. Rizzo. J.D. McCambridge, X.S. Ling, and D.E. Prober, "Experimental Results on Nb25wt.%Ta45wt.%Ti Superconducting Wire", IEEE Trans. On Appl. Supercond.. vol. 3, No. 1, pp. 1350-1353, March, 1993. The overall layer composition described in the above example can be varied by changing the relative thickness' ofthe NbTi foil and the Ta mesh, and/or by altering the Ti content within the NbTi alloy. The heat treatment schedule described in the above example can, of course, also be varied. Either or both types of modification may be desirable in order to improve J_c or H_c2 performance.

EXAMPLE II

The process of Example I was modified to include increased cold- work and more aggressive heat treatment for the purpose of improving J_c performance. In this embodiment ofthe invention, approximately 750 ofthe copper-free monofilamentary rods described in Example I were used in the assembly of a secondary billet having an outer diameter of 3.81 cm. The billet was TIG welded shut, evacuated at a temperature of approximately 425°C, and sealed. It was then extruded to 1.12 cm in diameter after a two-hour preheat at 593°C. The extrusion temperature here was reduced as compared to that employed in the process of Example I in order that more cold-work might be retained in the extruded rod. The pre-extrusion HIP at 650°C was eliminated for the same reason. It is well known in the art that increased cold-work is beneficial to α-Ti precipitation and, ultimately, to J_c performance.

The extruded rod was thermomechanically processed into fine wire in accordance with the following precipitation heat treatment schedule:

Wire Heat Treatment Heat Treatment Diameter (mm) Temperature (°C) Time (hours)

6.22 420 89 2.87 420 87 1.32 420 80 Samples resulting from this thermomechanical processing were tested for J_c at 4.2 K in applied fields up to 9T. A standard four-point probe technique was utilized in the testing. The non-copper J_c's measured for a sample having a diameter of 0.019 mm are shown in the table below. It will be noted that the 5T and 7T current densities are significantly higher than those measured for the best sample produced by the process of Example 1 (shown in the applicable table therein).

Applied Non-Copper Critical

Field (T) Current Density (A/mm²)

5 2923

7 1769

8 1071

The NbTi/Ta composite may be formed by inserting Ta rods into a NbTi alloy matrix, inserting this assembly into a copper can, and then performing a series of drawing and rebundling steps so as to reduce the NbTi alloy and Ta thicknesses to a size suitable for the application of a heat treatment schedule like that described in Example I. As will be obvious to anyone knowledgeable in the art, there are a number of other ways in which the present invention may be practiced. These center on the manner in which the NbTi alloy and Ta are combined. They are briefly described in the following non-limiting examples: EXAMPLE III The NbTi/Ta composite may be formed as described in Example II, except that NbTi alloy rods are inserted into a Ta matrix. EXAMPLE IV The NbTi/Ta composite may be formed as described in Example II, except that the starting combination consists of NbTi alloy foil wrapped around a Ta rod. EXAMPLE V The NbTi/Ta composite may be formed as described in Example II, except that the starting combination consists of Ta foil wrapped around a NbTi alloy rod. EXAMPLE VI The NbTi/Ta a composite may be formed by mixing chopped Ta wire with chopped NbTi wire, inserting this amalgam into a copper can, and then processing as described in Example I. EXAMPLE VII The NbTi/Ta composite may be formed by mixing Ta powder with NbTi alloy powder, inserting this amalgam into a copper can, and then processing as described in Example I.

Claims

CLAIMS I claim: 1. The process of forming a ductile superconductor having a J_c in excess of 1000 A/mm² at 1.9K. and an applied field of 1 OT comprising the steps of combining a body of NbTi alloy with a plurality of bodies of transition metal selected from Ta, Nb, Ti, V, Hf, and Zr, and alloys thereof, coreducing said NbTi alloy and said transition metal bodies in multiple reduction steps sufficiently that said transition metal is sufficiently small to serve as pinning sites for said superconducter, said NbTi alloy being subjected to a plurality of α Ti precipitation heat treatment steps between reductions steps to provide a uniform disperion of α Ti pinning sites in the final product. 2. A ductile superconductor having a J_c in excess of 1000 A/mm² at 1.9K and an applied field of 10T comprising a NbTi alloy of about 47 w/o weight % Ti, said alloy containing both precipitated α Ti pinning sites and dispersed Ta inclusions also serving as pinning sites. 3. The process of forming a ductile superconductor having a J_c in excess of 1000 A/mm² at 1.9K and an applied field of 10T comprising the steps of containing a body of NbTi alloy with a plurality of bodies of Ta, coreducing said NbTi alloy and said Ta in multiple reduction steps sufficiently that said Ta is sufficiently small to serve as pinning sites for said superconductor, said NbTi alloy being subjected to a plurality of α Ti precipitation heat treatment steps between reductions steps to provide a uniform dispersion of α Ti pinning sites in the final product. 4. The process of claim 1 wherein the NbTi alloy is in the form of a sheet. 5. The process of claim 1 wherein the transition metal is in the form of a sheet or an expanded wire mesh. 6. The process of claim 1 wherein said α Ti precipitation heat treatment is at a temperature below the temperature at which Ta diffuses into said NbTi alloy. 7. The process of claim 1 temperature on the order of 400°C for a time of at least about 20 hours. 8. The process of claim 1 wherein the transition metal body is reduced to a size on the order ofthe superconducting coherence length. 9. The process of claim 1 wherein the amount ofthe starting NbTi alloy and the Ta additions are chosen so that the final average composition are about 33-41 wt % Ti and 15-25 wt % Ta, Nb balance. 10. The process of forming a ductile superconductor having a J_c in excess of 1000 A/mm² at 1.9K and an applied field of 10T comprising the steps of containing a body of NbTi alloy with a plurality of bodies of transition metal, coreducing said NbTi alloy and said transition metal in multiple reduction steps sufficiently that said transition metal is sufficiently small to serve as pinning sites for said superconductor, said NbTi alloy being subjected to a plurality of α Ti precipitation heat treatment steps between reductions steps to provide a uniform dispersion of α Ti pinning sites in the final product said transition metal being selected Ta, Nb, Ti, V, Hf and Zr, and alloys thereof.