Superconducting joint between Nb3Sn tape and NbTi wire for use in superconducting magnets

Abstract: A method of forming a superconducting joint between the Nb 3 Sn layer (12) of a superconducting tape (10) and a superconducting NbTi (20) through use of a molten Pb-Bi bath, using tape (28) as a joint securing arrangement and a mold (32,34) around the joint.

Description: This invention relates to superconducting magnets, and in particular to the superconducting conductors used in such magnets. Superconducting multifilament niobium-titanium (NbTi) wires are widely used in large size superconducting magnets for magnetic resonance imaging (hereinafter MRI). Niobium-Tin (Nb 3 Sn) superconductor, generally in the form of tape rather than wire, is known to exhibit more desirable magnetic and operational characteristics than NbTi wire as regard important factors such as critical temperature, critical current and critical field. Nb 3 Sn has a critical temperature about 18K, and an upper critical field at zero K temperature, and about 28T at 4.2K temperature. With the same magnetic field and at 4.2 K temperature, the critical current density of Nb 3 Sn is generally a factor of 3 times higher than that of NbTi. It is thus clear that Nb 3 Sn is a superior material from a superconducting magnetic performance point of view for higher performance superconducting magnet construction, especially for high field superconducting magnets. However, the mechanical properties of Nb 3 Sn pose serious problems in the fabrications of suitable superconducting conductors and superconducting magnets. Nb 3 Sn is hard and brittle, and as a result when used commercially it is often fabricated utilizing thin tapes on which is deposited a thin layer of Nb 3 Sn by surface diffusion or by chemical vapor deposition procedures. In order to maximize performance, while also considering and balancing handling and cost differences in fabricating a superconducting magnet, there are advantages in using Nb 3 Sn tape conductors for winding superconducting magnets for use in the higher field regions of a MRI superconducting magnet, while using NbTi wires for the lower field regions of the same MRI superconductor magnet. However, the combination use of Nb 3 Sn tape conductor and NbTi wires in a single superconducting magnet presents the need and problem of fabricating superconducting joints between the Nb 3 Sn tape and NbTi wire. It is important that such superconducting joints provide enough current carrying capability and eliminate heating due to the high current flow through the joints, and in turn minimize cryogen boiloff resulting from such heating. In addition, in MRI applications superconducting switches are provided to keep a superconducting magnet in persistent operation. Such superconducting switches require high matrix resistance and good magnetic stability which is obtained with a thin layer of copper (about 0.001 inches thick, 1 inch = 2,54 cm) sandwiching the Nb 3 Sn tape conductor. The joining of such copper stabilized Nb 3 Sn tape and NbTi wire when also used in the same superconducting magnet requires a high performance superconducting joint in order to maintain a satisfactory drift rate in superconducting magnet persistent mode operation for the MRI equipment. It is an object of the present invention to provide improved superconducting joints between Nb 3 Sn tape and NbTi wire which are suitable for use in MRI applications. According to the present invention, there is provided a method of forming superconducting joints for use in superconducting magnets between an Nb 3 Sn superconductor and NbTi superconductor comprising; positioning said Nb 3 SN superconductor into contact with said NbTi superconductor to form a contact area; applying molten Pb-Bi to coat said contact area; and allowing said coating to cool. In accordance with one form of the present invention, a method of forming superconducting joints for use in superconducting magnets between Nb 3 Sn superconducting tape and NbTi superconducting wire is provided. When the Nb 3 Sn tape is sandwich between copper stabilizing layers, a copper layer and any copper encapsulation of the NbTi wire is removed by dipping the contact area into a molten tin bath maintained at approximately 350°C for a period in excess of 15 minutes. The Nb 3 Sn tape is then positioned into contact with the NbTi wire to form a contact area which may be maintained by wrapping a retaining tape around a small portion of the contact area applying molten Pb-Bi to coat the contact area and allowing the coating to cool. More particularly, the Pb-Bi is applied in a molten bath by dipping the contact area into the molten bath which is maintained at approximately 350°C with the dipping being for a period of 7 to 10 minutes. A detachable preheated mold is then temporarily fastened around the contact area prior to a second dipping. The mold is removed after the superconducting joint is formed in the bath and the joint is allowed to cool. The present invention provides a superconducting joint between Nb 3 Sn tape and NbTi wire which enables and facilitates their combined use in superconducting magnets, and which eliminates heat losses and minimizes drift rate in superconducting magnet operation. An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:- FIG. 1 illustrates the present invention. FIG. 2 is a block diagram chart useful in explaining the fabrication of the invention shown in FIG. 1 (1 inch = 2,54 cm). Referring to FIGS. 1 and 2, Nb 3 Sn superconducting tape 10 includes a layer of Nb 3 Sn 12 sandwiched between a pair of layers of stabilizing copper foil 14 and 16, the copper being in the order of about 0.001 inches thick. FIG. 1 is not drawn to scale with the thickness of layers 12, 14 and 16 being increased in order to better illustrate the invention. Superconducting NbTi wire 20 is positioned in contact with one side of superconducting tape 10 and Permeseal retaining tape 28 in the order of 12 mm wide is wrapped around the contact area at approximately 2 inches from end 21 of superconducting tape 10, and is also wrapped around a region approximately 6 inches from the end as indicated by arrow 24. Permeseal retaining tape 28 is utilized to retain superconducting tape 10 and superconducting wire 20 in position during subsequent formation of the superconducting joint in contact area 25 between end 21 of superconducting tape 10 and Permeseal retaining tape 28 at arrow 24. Permeseal retaining tape 28 at arrow 24 assists in preventing any sharp bending which could damage tape 10. With copper stabilizing layers 14 and 16 overlying Nb 3 Sn layer 12 of superconducting tape 10 and copper coated NbTi wire 20 as shown in FIG. 1, the copper is removed in contact area 25 before a superconducting joint is formed between the superconducting tape and wire to enable direct contact between the Nb 3 Sn and NbTi. This is accomplished by dipping or submerging 52 approximately one and one half inches of contact area 25 in a tin bath of 99.9% pure tin heated to 350°C for a period in excess of 15 minutes. Prior to the immersion, the top of the tin bath may be cleaned with a stainless steel spatula to removed any oxides or foreign materials. Once the contact area 25 for the joint has copper layer 14 etched off, the assembly or joint 8 is removed from the tin bath and immediately transferred 54 to a Pb-Bi bath up to the 1.5 inch mark close to arrow 22. The Pb-Bi bath is at 350°C and joint 8 remains in the bath for a period of approximately 7 to 10 minutes. Meanwhile, a mold or enclosing form 30 is set on top or adjacent to the bath to allow it to warm before molding the complete joint. At the end of the 7-10 minute period, the end joint 8 is lifted 56 out of the Pb-Bi bath to -allow the Pb-Bi coated ends of superconducting conductors 12 and 20 to solidify or harden. The ends of conductors 12 and 20 are then put into joint mold or enclosing form 30 consisting of upper half 34 and lower half 32 with a clamp or securing device 36, 38 on the outside of the mold for retaining the halves together when clamped 58 around contact area or joint 25. Mold 30 had an internal bore 41 slightly larger in diameter than Pb-Bi coated conductors 10 and 20 and extends approximately 2 inches from end 21, that is approximately up to arrow 22. The Pb-Bi coated conductors 12 and 24 along with their surrounding mold 30 are then submerged 60 into the Pb-Bi bath. Joint 8 including conductors 12 and 20 and mold 30 is shaken slightly to remove any air bubbles within the mold and to allow Pb-Bi to be formed within mold 30 and around superconductors 12 and 21 in contact area 25. Mold 30 and joint 8 are then carefully lifted out of the bath and transferred 62 to a heat sink stand with the joints being held straight in the mold until the Pb-Bi cools Upon cooling, mold 30 is removed and joint 8 is complete. Permeseal retaining tape 28 at arrow 26 may be removed, if desired, although this is not necessary. During the process care should be taken not to put sharp bends in the Nb 3 Sn because of potential damage to the superconducting material. Also care should be taken while clamping superconducting conductors 10 and 20 in heated mold 30 to prevent overtightening and deformation of tape 12 or joint 25. By way of summary, and with particular reference to FIG. 2, the initial step 50 is to prepare joint 25 by positioning and securing superconducting wire 20 and superconducting tape 10. If superconducting tape conductor 10 and/or superconducting wire 20 includes a copper stabilizer layer it is necessary to remove 52 the copper from the superconductors in contact area or joint 25 as indicated by arrow 51. This is accomplished in a tin bath for as much as more than approximately 15 minutes at approximately 350°C with one and one half inches of the end of the contact area being immersed. After the copper layer of encapsulation is removed joint 8 is removed from the tin bath and immediately transferred to a 350°C PbBi bath 54 for approximately 7-10 minutes after which joint 8 is removed 56 from the PbBi bath to allow the Pb-Bi coated conductors 10 and 20 to solidify. Conductors 10 and 20 are then clamped 58 in heated mold 30 which may be heated by placing it proximate to the heated PbBi bath. Mold 30 and conductors 10 and 20 are then dipped 60 approximately 6 inches into the same Pb-Bi bath. After mold 30 is filled with molten PbBi the mold and conductors 10 and 20 are transferred 62 to a heat sink to let the Pb-Bi around joint 25 cool. Mold 30 is then removed exposing Pb-Bi coated superconducting joint 25 suitable for use in MRI equipment through the joining of Nb 3 Sn superconducting tape 10 and NbTi superconducting wire 20. The suitability of joints 8 fabricated in accordance with the present invention for superconducting use were confirmed by testing which showed adequate and desirable magnetic characteristics.

Claims: A method of forming superconducting joints for use in superconducting magnets between an Nb 3 Sn superconductor (10) and NbTi superconductor (20) comprising; positioning said Nb 3 Sn superconductor into contact with said NbTi superconductor (50) to form a contact area; applying molten Pb-Bi to coat said contact area (54); and allowing said coating to cool (56). The method of claim 1 wherein said Pb-Bi is applied in a molten Pb-Bi bath by dipping said contact area into said molten bath. The method of claim 2 wherein said bath is maintained at a temperature of approximately 350°C and said dipping is for a period of 7-10 minutes. The method of claim 3 wherein a mold (32,34) is positioned around said tape and said wire in the region of said contact area. The method of claim 4 wherein said mold is heated and molten Pb-Bi is applied to said contact area within said mold. The method of claim 4 wherein the application of Pb-Bi to said contact area within said mold is accomplished by a second dipping (60) of said contact area plus said heated mold into said bath of molten Pb-Bi. The method of claim 6 wherein said mold is clamped (58) around said contact area prior to said second dipping. The method of claim 7 wherein said Nb 3 Sn tape superconductor is Nb 3 Sn sandwiched between copper stabilizing layers (14,16) and said NbTi superconductor is a wire encapsulated in a copper stabilizer and the copper in the region of said contact area is removed prior to said second dipping. The method of claim 8 wherein said copper is removed (52) by a different dipping of said contact area into a molten tin bath. The method of claim 9 wherein said tin bath temperature is maintained at approximately 350°C and said different dipping is for a period of time in excess of 15 minutes.

Inventors: MCCLAM O'NEIL T, WARD GEER, XU BU-XIN, MCCLAM, O'NEIL T., WARD, GEER, XU, BU-XIN
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1996
Main classification: H01L39
IPC codes: H01L39, H01R43