SUPERCONDUCTIVE WIRE MATERIAL AND METHOD OF PRODUCING THE SAME

Abstract: A superconductive wire material used for superconductive magnets and a method of producing the same. Conventional superconductive wire materials cannot be used for the above application because of their low critical current densities (Jc). According to the invention, pinning centers consisting of a nonsuperconductive material are introduced into a superconductive material to greatly improve the density Jc of the superconductive wire utilizing the pinning effect and to solve the problems of long production time and high production cost inherent in the prior arts. The superconductive wire material having a greatly improved density Jc of the invention is suitable for superconductive magnets.

Description: TECHNICAL FIELD The present invention relates to a superconducting wire used for a superconducting electromagnet and methods of manufacturing a superconducting wire structure. BACKGROUND ART In recent years, superconducting wires made of various materials have remarkably been put to practical use. To advance the practical use of such wires, it is important to increase the amount of electric current, which can flow through the wires, that is, to increase the critical current density (Jc). One method of increasing Jc is increasing the pinning force for capturing magnetic flux in a superconducting wire within a magnetic field produced by a coil of the superconducting wire, which magnetic flux enters the superconducting wire. This method is called "pin stop", and a portion having the pin stop effect is termed "pinning center." In the prior art, an alloy superconducting wire is subjected to heat treatment at the time of final wire drawing, and a normal conducting deposited portion or strained portion, produced through the heat treatment, is employed as a pinning center. It is known that a pinning center in a compound superconducting wire is normally present at a grain boundary. The pinning center in the compound superconducting wire may be explained, for example, with reference to a manufacturing method illustrated in Figs. 5A to 5E. An Nb or Nb alloy ingot 41, as shown in Fig. 5A, is buried in a bronze (containing Sn) pipe 42, as shown in Fig. 5B. The resultant structure is processed into a hexagonal wire element 43 (see Fig. 5C). Subsequently, as is shown in Fig. 5D, a plurality of wire elements 43 are gathered and buried in a bronze (containing Sn) pipe 44. After the resultant structure is subjected to wire drawing, as illustrated in Fig. 5E, it is thermally treated to produce an intermetallic compound layer (Nb₃Sn layer), thus obtaining a superconducting wire. The intermetallic compound of the superconducting wire is a polycrystal. A cross-sectional structure of the polycrystal is shown, for example, in Fig. 5F. That is, there are crystal grains 45 of Nb₃Sn and grain boundaries 46. In order to enhance the Jc of the above compound superconducting wire, it is generally practiced to control parameters such as the size, density and contamination of the grain boundaries, thereby introducing strong pinning centers. Conventionally, these parameters are controlled by varying the temperature and time for heat treatment or adding a third substance, such as Ta, Ti, Hf or Ga in a Nb core or a matrix to make the grain boundaries dirty. Regarding an alloy superconducting wire, in order to make pinning centers function effectively to enhance the pin stop effect, it is important to appropriately determine the size and arrangement of the pinning centers. Where the pinning centers are introduced by heat treatment or by utilizing process strain, it is very difficult to suitably control the sizes of normal conducting deposited portions and strained portions and the distance therebetween. Further, when the wire is finally processed, heat treatment must be repeated several times. Consequently, manufacturing time increases and also the number of manufacturing steps increases. Where grain boundaries are employed as pinning centers in the compound superconducting wire, a pinning force Fp and an applied magnetic field B have the relationship: Fp ∝ (1-B/Bc) ² where Bc: critical magnetic field. As the intensity of the applied magnetic field approaches that of the critical magnetic field, the pinning force decreases suddenly and Jc decreases at high magnetic field, which is disadvantageous. GB-A-2038532 discloses a superconducting wire containing pinning centres of diameter 50 Angstroms or less. The present invention has been made in consideration of the above circumstances and its object is to provide a superconducting wire made of various superconducting substances and having enhanced pin stop effect and Jc. DISCLOSURE OF THE INVENTION The present invention provides a superconducting wire having an elongated filament comprising a plurality of elongated superconducting wire elements (3), said superconducting wire elements each being made of an alloy-based superconducting substance (8) and said superconducting wire elements each including a pinning center made of at least one wire of a non-superconducting substance (9) buried in said alloy-based superconducting substance and extending in the longitudinal direction of said filament, characterised in that said at least one wire of non-superconducting substance has a diameter greater than or equal to 7 nm and less than 500 nm. It is necessary that the diameter of the wire of non-superconducting substance be greater than or equal to 7 nm and less than 500 nm. In addition, it is desirable that the ratio of the non-superconducting substance in the filament be 40% or less. When the diameter of the wire of non-superconducting substance is less than 7 nm, this diameter is too small as a pinning center, and the pin stop effect lowers. Thus, the enhancement of Jc cannot be expected. On the other hand, when the diameter of the wire exceeds 500 nm, the matching with the lattice of magnetic flux is not good. When the ratio of the non-superconducting substance in the filament exceeds 40%, the superconducting electron density in the filament decreases, and the enhancement of Jc cannot be expected. A superconducting wire structure can be manufactured using a superconducting wire of the invention by a method comprising the steps of: burying said at least one wire of a non-superconducting substance in the longitudinal direction of each of said superconducting wire elements; gathering a plurality of said superconducting wire elements; drawing the gathered superconducting wire elements into a filament; and gathering a plurality of said filaments and reducing the diameter of the resultant structure to obtain the superconducting wire structure. Also, a superconducting wire structure can be manufactured using a superconducting wire of the invention by a method comprising the steps of: burying a plurality of said wires of a non-superconducting substance in the longitudinal direction of said superconducting wire elements; reducing the diameter of the resultant structure to obtain a filament; and gathering a plurality of said filaments and reducing the diameter of the resultant body to obtain the superconducting wire structure. In these manufacturing methods, it is desirable that the diameter of the wire of non-superconducting substance be 7 to 500 nm. When the diameter of the wire of non-superconducting substance is less than 7 nm, this diameter is too small as a pinning center, and it is difficult to pin-stop the lattice of magnetic flux. Thus, the enhancement of Jc cannot be expected. When the diameter of the wire exceeds 500 nm, the matching with the lattice of magnetic flux deteriorates and the enhancement of Jc cannot be expected. According to this method, it is possible to freely control, at the design stage, the size, type, distance and position of the pinning center of non-superconducting substance introduced into the filament, in accordance with the use of the wire, e.g. the intensity of magnetic field. Thus, the matching with the lattice of magnetic flux can be enhanced and the elemental pinning force can be effectively exhibited and increased, whereby Jc can be greatly improved. Besides, since no heat treatment is performed, the manufacturing time and cost can be reduced. In the above alloy-based superconducting wire, it is desirable that the alloy which forms the alloy-based superconducting wire element be a known superconducting substance, e.g. NbTi alloy. It is desirable that the non-superconducting substance for forming the pinning center be Cu or a Cu alloy; Cu or a Cu alloy coated with Nb or Ta; Cu or a Cu alloy mixed with 0.5 to 5% by weight of magnetic substance; Cu or a Cu alloy coated with Nb or Ta and mixed with 0.1 to 5% by weight of magnetic substance; Ag or Au or an alloy of Ag or Au; or Ag or Au or an alloy of Ag or Au, mixed with 0.5 to 5% by weight of magnetic substance. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A to 1D illustrate a method of manufacturing a superconducting wire according to Example 1. Figs. 5A to 5F illustrate a conventional method of manufacturing a compound superconducting wire. BEST MODES OF THE INVENTION Example The outer periphery of a Cu rod having a diameter of 3 mm was coated with a Nb sheet. The Cu rod was inserted into a through hole formed in a central part of a NbTi alloy wire having a diameter of 35 mm. A plurality of NbTi alloy wires were inserted into a Cu billet having a diameter of 453 mm. Thereafter, the Cu billet was hot-excluded at 700 °C and drawn (the billet was removed during drawing), thus obtaining a superconducting wire element 1 of NbTi alloy having a hexagonal final cross section and a distance of 1.83 mm between parallel sides. In this case, the Cu rod serves as a pinning center, as will be described below. As is shown in Fig. 1A, 253 superconducting wire elements 1 were gathered and inserted into a Cu-10% Ni alloy billet 2 having its outer peripheral surface coated with Nb and having a diameter of 453 mm. The resultant body was hot-extruded at 700 °C and the diameter thereof was reduced, thus obtaining a secondary wire element (filament) 3 having a hexagonal final cross section and a distance of 3.15 mm between parallel sides (see Fig. 1B). The secondary wire element 3 is a composite of a number of superconducting wire elements 1. Then, as is shown in Fig. 1B, 151 filaments 3 were gathered and inserted into a Cu billet 4 having a diameter of 453 mm. The resultant body was hot-extruded at 600 °C and the diameter thereof was reduced, thus obtaining a tertiary wire element 5 having a hexagonal final cross section and a distance of 2.6 mm between parallel sides (see Fig. 1C). The tertiary wire element 5 is a composite of a number of secondary wire elements 3. The 241 tertiary wire elements 5 were gathered and buried in a CU billet 6 having a diameter of 453 mm, as shown in Fig. 1C. The resultant structure was hot-extruded at 600 °C and the diameter thereof was reduced, thus obtaining a superconducting wire 7 having a diameter of 10 mm or less. If the cross section of each superconducting wire element 1 constituting the superconducting wire 7, it is found that a Cu pinning center 9 coated with a Nb sheet is present at the center of a hexagonal NbTi alloy 8 (see Fig. 1D). For example, when the superconducting wire 7 had the diameter of 0.2 mm, the diameter of the filament 3 was 0.507 µm, the diameter of the pinning center 9 was 3.7 nm, and the distance between adjacent pinning centers 9 was 30 nm. Current densities were measured with respect to the above-stated superconducting wire having Cu pinning centers, a superconducting wire manufactured in like manner with pinning centers of Ag-30 wt% Pd alloy, and a superconducting wire of a NbTi alloy in which pinning centers were introduced by a conventional heat treatment method. The results are shown in Table 1. As is seen from Table 1, any of the superconducting wires according to the present Example has a Jc remarkably higher than that of conventional superconducting wire No. 12. The wires Nos. 2, 3, 7 and 8 have higher Jc than conventional wire No. 11. These results seem to have stemmed from the improved matching between the magnetic lattice and the size, position, etc. of the pinning center and also the enhanced pin stop effect.

Claims: A superconducting wire having an elongated filament comprising a plurality of elongated superconducting wire elements (3), said superconducting wire elements each being made of an alloy-based superconducting substance (8) and said superconducting wire elements each including a pinning center made of at least one wire of a non-superconducting substance (9) buried in said alloy-based superconducting substance and extending in the longitudinal direction of said filament, characterised in that said at least one wire of non-superconducting substance has a diameter greater than or equal to 7 nm and less than 500 nm. A method of manufacturing a superconducting wire structure using the superconducting wire according to claim 1, said method comprising the steps of: burying said at least one wire of a non-superconducting substance in the longitudinal direction of each of said superconducting wire elements; gathering a plurality of said superconducting wire elements; drawing the gathered superconducting wire elements into a filament; and gathering a plurality of said filaments and reducing the diameter of the resultant structure to obtain the superconducting wire structure. A method of manufacturing a superconducting wire structure using the superconducting wire according to claim 1, said method comprising the steps of: burying a plurality of said wires of a non-superconducting substance in the longitudinal direction of said superconducting wire elements; reducing the diameter of the resultant structure to obtain a filament; and gathering a plurality of said filaments and reducing the diameter of the resultant body to obtain the superconducting wire structure. The superconducting wire according to claim 1, wherein said non-superconducting substance is Cu or a Cu alloy. The superconducting wire according to claim 4, wherein 0.1 to 5% by weight of magnetic substance is added to the non-superconducting substance. The superconducting wire according to claim 4, wherein said non-superconducting substance is coated with Nb or Ta. The superconducting wire according to claim 6, wherein 0.1 to 5% by weight of magnetic substance is added to the non-superconducting substance. The superconducting wire according to claim 1, wherein said non-superconducting substance is Ag or Au or an alloy of Ag or Au. The superconducting wire according to claim 8, wherein 0.1 to 5% by weight of magnetic substance is added to the non-superconducting substance.

Inventors: MATSUMOTO KANAME, MIURA OHSUKE, SHIGA SHOJI, TANAKA YASUZO, MATSUMOTO, KANAME, MIURA, OHSUKE, SHIGA, SHOJI, TANAKA, YASUZO
Applicants: FURUKAWA ELECTRIC CO LTD, THE FURUKAWA ELECTRIC CO., LTD.
Application year: 1990
Main classification: H01B12
IPC codes: H01L39