Method for producing an Nb.SUB.3.Sn superconductor wire

Abstract

A method for the production of a superconducting wire (20) uses a monofilament (1) having a powder core (3) that contains at least Sn and Cu, an inner tube (2), made of Nb or an alloy containing Nb, that encloses the powder core (3), and an outer tube (4) in which the inner tube (2) is arranged. The outer side of the inner tube (2) is in contact with the inner side of the outer tube (4) and the outer tube (4) is produced from Nb or from an alloy containing Nb. The outer tube is disposed in a cladding tube. The superconducting current carrying capacity of the superconducting wire is thereby improved.

Claims

1. A method for producing a superconducting wire having Nb.sub.3SN superconductor from a monofilament and a precursor, the monofilament comprising: a powder core that contains at least Sn and Cu; an inner tube that encloses the powder core, the inner tube being made of Nb or of an alloy containing Nb; an outer tube in which the inner tube is disposed, wherein an outer side of the inner tube is in contact with an inner side of the outer tube, the outer tube being made of Nb or of an alloy containing Nb; and a cladding tube in which the outer tube is disposed, the inner tube having a round inner cross-section and a round outer cross-section and the outer tube having a round inner cross-section and a round outer-cross-section, wherein 4≤W.sub.outertube/W.sub.innertube≤50 with W.sub.outertube: wall thickness of the outer tube and W.sub.innertube: wall thickness of the inner tube, wherein the monofilament is produced by a method comprising the steps of: a) filling the powder core into the inner tube; b) drawing, following step a), the inner tube, thereby compacting the powder core; c) inserting the outer tube into the cladding tube, and d) inserting, following step b), the drawn, filled inner tube into the outer tube, wherein step c) is performed prior to or after step d), the precursor being made by a method comprising the steps of: e) drawing one or more of the monofilaments produced in step d) to produce monofilaments having a hexagonal outer cross-section; f) bundling a plurality of drawn monofilaments in a wire cladding tube; and g) extruding and/or drawing the wire cladding tube containing the bundled and drawn monofilaments, thereby obtaining the precursor of the superconducting wire, and the superconducting wire is made from the precursor using the following steps: h) mechanically deforming the precursor made in step g) or winding the precursor made in step g) to form a coil, and i) temperature-treating the deformed precursor or temperature-treating the deformed precursor at a maximum temperature of 700° C. or less, wherein Nb from the inner tube and the outer tube reacts with Sn from the powder core to form Nb.sub.3SN.

2. The method of claim 1, wherein the cladding tube has a hexagonal outer cross-section and a round inner cross-section.

3. The method of claim 1, wherein 1.2≤D.sup.outside.sub.outertube/D.sup.outside.sub.innertube≤2.0 with D.sup.outside.sub.outertube: outer diameter of the outer tube and D.sup.outside.sub.innertube: outer diameter of the inner tube.

4. The method of claim 1, wherein the powder core has a content of 2 weight % to 12 weight % of Cu, 3 to 9 weight % of Cu or the powder core contains elementary Cu powder.

5. The method of claim 1, wherein the powder core is compacted in the inner tube of has a density of at least 40% or of at least 50% of a theoretical density.

6. The method of claim 1, wherein the powder core contains NbSn.sub.2 and or a Nb.sub.6Sn.sub.5 and/or elementary Sn.

7. The method of claim 1, wherein the inner tube and/or the outer tube are produced from an alloy containing Nb and containing Ta and/or Ti, have a summed content of at least 0.5 weight % of Ta and/or Ti or have a summed content of maximally 10 weight % Ta and/or Ti.

8. The method of claim 1, wherein the inner tube and the outer tube are produced from different materials.

9. The method of claim 1, wherein the cladding tube is made of Cu.

10. The method of claim 1, wherein the cladding tube already has a hexagonal outer cross-section prior to step c).

11. The method of claim 1, wherein, in step e), the monofilaments already have a hexagonal outer cross-section of the cladding tube and a round outer cross-section of the outer tube prior to drawing, wherein the drawn monofilaments also have a hexagonal outer cross-section of the cladding tube and a round outer cross-section of the outer tube after drawing.

12. A method for producing a superconducting wire having Nb.sub.3SN superconductor from a monofilament and a precursor, the monofilament comprising: a powder core that contains at least Sn and Cu; an inner tube that encloses the powder core, the inner tube being made of Nb or of an alloy containing Nb; an outer tube in which the inner tube is disposed, wherein an outer side of the inner tube is in contact with an inner side of the outer tube, the outer tube being made of Nb or of an alloy containing Nb; and a cladding tube in which the outer tube is disposed, the inner tube having around inner cross-section and a round outer cross-section and the outer tube having a round inner cross-section and a round outer-cross-section, wherein 4≤W.sub.outertube/W.sub.innertube≤50 with W.sub.outertube: wall thickness of the outer tube and W.sub.innertube: wall thickness of the inner tube, wherein the monofilament is produced by a method comprising the steps of: a) filling the powder core into the inner tube; b) drawing, following step a), the inner tube, thereby compacting the powder core; c) inserting the outer tube into the cladding tube, and d) inserting, following step b), the drawn, filled inner tube into the outer tube, wherein step c) is performed prior to or after step d), the precursor being made by a method comprising the steps of: e) drawing one or more of the monofilaments produced in step d) to produce monofilaments having a hexagonal outer cross-section; f) bundling a plurality of drawn monofilaments in a wire cladding tube; and g) extruding and/or drawing the wire cladding tube containing the bundled and drawn monofilaments, thereby obtaining the precursor of the superconducting wire, and the superconducting wire is made from the precursor using the following steps: h) mechanically deforming the precursor made in step g) or winding the precursor made in step g) to form a coil, and i) temperature-treating the deformed precursor or temperature-treating the deformed precursor at a maximum temperature of 700° C. or less, wherein Nb from the inner tube and the outer tube reacts with Sn from the powder core to form Nb.sub.3Sn, wherein Nb from the inner tube and the outer tube reacts with Sn from the powder core to form Nb.sub.3Sn via a reaction front which advances substantially through an entire radial extent of the outer tube.

13. The method of claim 12, wherein temperature treatment is terminated in step i) before the reaction front has reached a boundary surface between the outer tube and the cladding tube.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 shows a schematic cross-sectional view of a first embodiment of an inventive monofilament with hexagonal cladding tube;

(2) FIG. 2 shows a schematic cross-sectional view of a second embodiment of an inventive monofilament with round cladding tube;

(3) FIG. 3 shows a schematic cross-sectional view of an embodiment of an inventive precursor of a superconducting wire;

(4) FIG. 4 shows a schematic diagram of the production of an inventive monofilament;

(5) FIG. 5 shows a schematic diagram of the production of an inventive superconducting wire; and

(6) FIG. 6 shows a schematic cross-sectional view of a section of an inventive superconducting wire in the area of a monofilament.

DESCRIPTION OF THE PREFERRED EMBODIMENT

(7) FIG. 1 schematically shows a first embodiment of an inventive monofilament 1 in cross-section perpendicular to its longitudinal direction.

(8) The monofilament 1 has an inner tube 2 which consists of niobium in the illustrated embodiment. A powder core 3 of a mixture of, in the present case, NbSn.sub.2 powder, Sn powder and Cu powder is disposed in the inner tube 2. The content of Cu in the mixture is approximately 5 weight % and the content of Sn in the mixture is typically at least 50 weight %.

(9) The inner tube 2 is arranged in an outer tube 4 which consists of NbTa7.5 in the present case. The outer side of the inner tube 2 thereby directly abuts the inner wall of the outer tube 4. The inner tube 2 and the outer tube 4 each have (circular) round inner and outer cross-sections.

(10) The outer tube 4 is, in turn, arranged in a cladding tube 5 which consists of elementary copper in the present case. The cladding tube 5 has a round inner cross-section in the illustrated embodiment such that the outer tube 4 abuts the cladding tube 5 over the whole surface. The outer cross-section of the cladding tube 5 is hexagonal (“hexagonal tube”), thereby increasing the area portion of the finished superconducting wire that can be utilized for Nb.sub.3Sn.

(11) The main source of Nb for the reaction heat treatment is the outer tube 4. The inner tube 2 enables good compaction of the powder core 3 in a previous drawing step (see also FIG. 4 in this context) due to its comparatively thin wall. By way of example, the ratio VD between the outer diameter D.sup.outside.sub.outertube of the outer tube 4 and the outer diameter D.sup.outside.sub.innertube of the inner tube 2 is approximately 1.65 in the embodiment shown. The ratio VW between the wall thickness W.sub.outertube of the outer tube 4 and the wall thickness W.sub.innertube of the inner tube 2 is moreover approximately 4.2. The inner tube 2, the outer tube 4 and the cladding tube 5 are arranged concentrically. No solder is required for the monofilament 1.

(12) FIG. 2 schematically shows a second embodiment of an inventive monofilament 1 in cross-section perpendicular to its longitudinal extension.

(13) The second embodiment of the monofilament 1 largely resembles the embodiment of FIG. 1, in particular, with respect to the inner tube 2, the powder core 3 and the outer tube 4.

(14) However, the cladding tube 5 here is provided with a (circular) round outer cross-section. For this reason, the monofilament 1 is easy to manufacture. The cladding tube 5 is also produced of elementary copper and has a (circular) round inner cross-section.

(15) FIG. 3 shows a cross-section perpendicular to its longitudinal extension of an embodiment of an inventive precursor 10 of a superconducting wire.

(16) A plurality of drawn monofilaments 11 (in the present case seven as an example), which are each produced from one monofilament by means of a filament drawing process, are bundled in a wire cladding tube 12 (“casing tube”) in the precursor 10, and are subsequently subjected to an extrusion and/or wire drawing process in order to reduce the cross-section.

(17) The wire cladding tube 12 is preferably produced of elementary copper. Cavities at the inner edge of the wire cladding tube 12 are prevented or filled by means of filling profiles 13 which are preferably produced of elementary copper.

(18) In the present case, the monofilaments had a hexagonal outer cross-section (see FIG. 1) already prior to the filament drawing process such that the monofilaments were only radially compressed during the filament drawing process. The outer tubes 4 in the drawn monofilaments 11 correspondingly still have a round outer cross-section. The round outer cross-section of each outer tube 4 is maintained even after the extrusion and/or wire drawing process of the precursor 10. For this reason, starting from the powder core 3, the reaction front of Nb.sub.3Sn can uniformly and concentrically approach the outer edge of the outer tube 4 during reaction annealing. There are no particularly thin points where Sn could prematurely break through into the cladding tube 5. Nor are there particularly thick points where residual Nb not utilized for the reaction to Nb.sub.3Sn protrudes.

(19) FIG. 4 shows a schematic diagram of the respective cross-sections of the specified components for producing an inventive monofilament 1.

(20) Within the scope of the production variant illustrated here, the inner tube 2 is filled with the powder core 3 and subjected to a drawing step a). The cross-section of the comparatively thin-walled inner tube 2 is thereby reduced and the powder core 3 is compacted. The non-filled outer tube 4 is furthermore inserted into the cladding tube 5 (in the present case of hexagonal outer cross-section) in one step b). The steps a) and b) can thereby be performed in arbitrary order or also simultaneously. In step c), the drawn and filled inner tube 2 is subsequently introduced into the outer tube 4 which is already arranged in the cladding tube 5.

(21) Alternatively, the drawn and filled inner tube may also be initially inserted into the outer tube and the outer tube can subsequently be inserted into the cladding tube (not separately shown).

(22) FIG. 5 schematically illustrates the production process of a superconducting wire 20 from monofilaments 1 as produced e.g. in accordance with FIG. 4.

(23) A monofilament 1 is transformed into a drawn monofilament 21 by drawing in step a′) (“filament drawing process”). The cross-sectional surface area is thereby reduced. If the monofilament 1 already has a hexagonal outer cross-section (as illustrated in FIG. 5), drawing merely effects radial compression. This is preferred since in this case, a round outer cross-section of the outer tube 4 can be easily obtained after drawing. If the monofilament 1 has a non-hexagonal outer cross-section (e.g. a round outer cross-section) a hexagonal outer cross-section is also impressed during drawing according to a′).

(24) A plurality of drawn monofilaments 21 are then bundled in a wire cladding tube 12 in step b′). The number of drawn monofilaments is thereby basically arbitrary. Seven drawn monofilaments 21 are bundled in the illustrated variant. In the bundled configuration, the monofilaments in the core area can be replaced by hexagonal Cu elements.

(25) Extrusion and/or drawing is subsequently performed in step c′) (“wire drawing process”) which is again accompanied by a reduction in cross-section, thereby obtaining a precursor 10 of a superconductor. This precursor 10 already has the cross-sectional shape and cross-sectional size of the subsequent superconducting wire but can still be plastically deformed.

(26) For finishing the superconducting wire, the precursor 10 must be shaped in step d′) so as to have the shape required for the superconducting wire as determined by the desired application. In the illustrated variant, the application concerns a magnet coil 23. The precursor 10 is correspondingly wound onto a carrier 22.

(27) Temperature treatment (“reaction annealing”) of the formed precursor is subsequently carried out in step e′). Towards this end, the magnet coil 23 is put into a furnace 24 that is heated to a temperature of maximally 700° C. Sn from the powder cores reacts with Nb of the inner and outer tubes in the monofilaments of the precursor to Nb.sub.3Sn at these temperatures. Temperature treatment is terminated before the reaction front reaches the outer edge of the outer tubes. The formed precursor has been transformed into a superconducting wire 20 by means of the temperature treatment, the Nb.sub.3Sn filaments of which can carry an electrical current (with corresponding cooling e.g. with liquid helium) practically without ohmic losses. The superconducting wire 20 should not be deformed again after temperature treatment in order to prevent breaking of the enclosed brittle Nb.sub.3Sn filaments.

(28) FIG. 6 is a schematic cross-section illustrating a section of the superconducting wire 20 in the area of a temperature-treated monofilament 61.

(29) A reaction front 62 has radially advanced from the inside to the outside in a temperature-treated monofilament 61 and has generated a relatively homogeneous fine-grained area 63 of Nb.sub.3Sn. The reaction front 62, however, has not completely crossed the outer tube 4 but has left a circumferential border 64 of non-reacted material of the outer tube 4 (in the present case of NbTa7.5, i.e. Nb with 7.5 weight % Ta). The border 64 has an approximately uniform thickness S over its entire circumference. The thickness S is adjusted by the temperature treatment program to be just sufficiently large in order to reliably prevent breakthrough of Sn into the matrix 65 (formed from previous cladding tubes) of copper, thereby maintaining the electrical conductivity of the matrix 65 at a high level. Due to the round outer cross-section of the outer tube 4, a large portion of the cross-sectional area of the superconducting wire 20 can react to Nb.sub.3Sn. In particular, there are no remaining useless bulges of material of the outer tube 4 (as would be generated at the edges of an outer tube having an outer hexagonal cross-section). Accordingly, Nb from the inner tube and the outer tube reacts with Sn from the powder core to form Nb.sub.3Sn via a reaction front which advances substantially through an entire radial extent of the outer tube.

(30) The round outer cross-section of the outer tube 4 can already be obtained prior to drawing of the monofilaments (see step a′) in FIG. 5) through a hexagonal outer cross-section of the cladding tubes.

(31) A residual core 66 resulting from the powder core with a reduced amount of Sn generally remains in the temperature-treated monofilament 61.

(32) The temperature-treated monofilament 61 is substantially free of gaps and cavities

LITERATURE

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