Monofilament for producing an Nb.SUB.3.Sn-containing superconductor wire, especially for internal oxidation

Abstract

A monofilament (100) for producing an Nb.sub.3Sn-containing superconductor wire (33) includes a powder core (1) with an Sn-containing powder, a reaction tube (3) composed of an Nb alloy that includes Nb and at least one further alloy component X. The powder core is disposed within the reaction tube. The monofilament also includes at least one source (4) for at least one partner component Pk. A respective source includes one or more source structures at a unitary radial position in the monofilament. The alloy component X and the partner component Pk form precipitates XPk on reaction annealing of the monofilament in which Sn from the powder core and Nb from the reaction tube react to produce Nb.sub.3Sn. The powder core is disposed in a moderation tube, which in turn is disposed within the reaction tube. This provides a monofilament for a powder-in-tube based Nb.sub.3Sn-containing superconductor wire with improved current carrying capacity.

Claims

1. A monofilament for producing an Nb.sub.3Sn-containing superconductor wire, comprising: a powder core comprising an Sn-containing powder, a reaction tube composed of an Nb alloy comprising Nb and at least one further alloy component X, wherein the powder core is disposed within the reaction tube, at least one source for at least one partner component Pk, wherein each source is disposed in a unitary radial position in the monofilament and comprises at least one source structure, wherein the alloy component X and the partner component Pk are selected to form precipitates XPk through reaction annealing of the monofilament, in which Sn from the powder core and Nb from the reaction tube react to produce Nb.sub.3Sn, and a moderation tube disposed within the reaction tube, wherein the powder core is disposed within the moderation tube, and wherein the reaction tube and the moderation tube have mutually differing chemical compositions.

2. The monofilament as claimed in claim 1, comprising at least two source structures, for the at least one partner component Pk, wherein the source structures are disposed at differing positions within the monofilament.

3. The monofilament as claimed in claim 1, wherein the moderation tube is fabricated at least partly from a material selected from the group: Cu, a Cu alloy, Ag, an Ag alloy, Sn, an Sn alloy, Nb, or an Nb alloy.

4. The monofilament as claimed in claim 1, wherein the at least one source for the at least one partner component Pk comprises a powder.

5. The monofilament as claimed in claim 1, wherein the at least one source for the at least one partner component Pk comprises a powder present in the powder core.

6. The monofilament as claimed in claim 5, wherein the source which comprises a powder present in the powder core comprises oxygen as the partner component Pk, and wherein the moderation tube is fabricated of Cu or a Cu alloy.

7. The monofilament as claimed in claim 5, wherein a wall thickness WM of the moderation tube is defined by: WM≤0.075*DP and/or WM≤0.2*WR where DP is the diameter of the powder core and WR is the wall thickness of the reaction tube.

8. The monofilament as claimed in claim 5, wherein the powder core comprises at least one of the following elements: Cu, Ag and Sn.

9. The monofilament as claimed in claim 1, wherein the at least one source for the at least one partner component Pk comprises an annular source, and wherein the moderation tube is disposed coaxially within the annular source.

10. The monofilament as claimed in claim 9, wherein the annular source comprises a powder in a circumferential powder layer.

11. The monofilament as claimed in claim 9, wherein the annular source comprises a powder in a powder filling of annularly disposed source tubules or source pockets.

12. The monofilament as claimed in claim 9, wherein the annular source comprises a metallic powder.

13. The monofilament as claimed in claim 9, wherein the annular source is adjacent to the reaction tube or to the moderation tube, and comprises a circumferential coating on the reaction tube or on the moderation tube.

14. The monofilament as claimed in claim 9, wherein the reaction tube is disposed coaxially within the annular source.

15. The monofilament as claimed in claim 14, further comprising: an ancillary reaction tube which comprises Nb, wherein the annular source is disposed coaxially within the ancillary reaction tube.

16. The monofilament as claimed in claim 9, wherein the annular source is disposed coaxially around the moderation tube and coaxially within the reaction tube.

17. The monofilament as claimed in claim 16, wherein the monofilament further comprises an ancillary moderation tube, wherein the annular source is disposed coaxially within the ancillary moderation tube, and wherein the ancillary moderation tube is disposed coaxially within the reaction tube.

18. The monofilament as claimed in claim 1, wherein the at least one source comprises a chemical compound comprising a partner component Pk of the at least one partner component Pk, wherein the at least one source comprises a chemical compound comprising a partner component Pk, wherein, on heat treatment at a first temperature T1, the chemical compound releases the partner component Pk at a first fraction and, on heat treatment at a second temperature T2>T1, the chemical component releases the partner component Pk at a second fraction.

19. The monofilament as claimed in claim 1, wherein the at least one source comprises at least two differing partner components Pk, which on reaction annealing form differing precipitates XPk with the at least one further alloy component X.

20. The monofilament as claimed in claim 1, wherein the at least one source comprises a first chemical compound and a second chemical compound, which each comprise a mutually same partner component Pk, and wherein, on heat treatment, the first chemical compound releases the partner component Pk at a lower temperature than does the second chemical compound.

21. The monofilament as claimed in claim 1, wherein the at least one partner component Pk comprises oxygen, and the at least one alloy component X comprises a metal which is less noble than Nb.

22. The monofilament as claimed in claim 21, wherein the source for the at least one partner component Pk comprises a powder comprising a metal oxide.

23. The monofilament as claimed in claim 21, wherein the at least one alloy component X comprises at least one of the elements selected from the group consisting of Mg, Al, V, Zr, Ti, Gd or Hf.

24. A monofilament for producing an Nb.sub.3Sn-containing superconductor wire, comprising: a powder core comprising an Sn-containing powder, a reaction tube composed of an Nb alloy comprising Nb and at least one further alloy component X, wherein the powder core is disposed radially within the reaction tube, at least one source for at least one partner component Pk, wherein each source is disposed in a unitary radial position in the monofilament and comprises at least one source structure, wherein the alloy component X and the partner component Pk are selected to form precipitates XPk through reaction annealing of the monofilament, in which Sn from the powder core and Nb from the reaction tube react to produce Nb.sub.3Sn, and wherein the precipitates XPk comprise nonoxidic precipitates, and a moderation tube disposed within the reaction tube and in which the powder core is disposed.

25. The monofilament as claimed in claim 1, wherein the precipitates comprise nonmetallic precipitates.

26. A method for producing a superconductor wire, comprising: subjecting a plurality of monofilaments as claimed in claim 1 to single-stage or multistage bundling and reshaping, to provide a precursor wire, bringing the precursor wire into a desired shape, and subjecting the shaped precursor wire to a reaction annealing, wherein precipitates XPk are formed from the at least one alloy component X and the at least one partner component Pk, and wherein Nb.sub.3Sn is formed from the Nb of the reaction tubes and the Sn of the powder cores.

27. A monofilament for producing an Nb.sub.3Sn-containing superconductor wire, comprising: a powder core comprising an Sn-containing powder, a reaction tube composed of an Nb alloy comprising Nb and at least one further alloy component X, wherein the powder core is disposed radially within the reaction tube, at least one source for at least one partner component Pk, wherein each source is disposed in a unitary radial position in the monofilament and comprises at least one source structure, wherein the alloy component X and the partner component Pk are selected to form precipitates XPk through reaction annealing of the monofilament, in which Sn from the powder core and Nb from the reaction tube react to produce Nb.sub.3Sn, wherein the precipitates XPk comprise nonoxidic and nonmetallic precipitates, and wherein the at least one partner component Pk comprises sulfur, and the at least one alloy component X comprises zinc, and a moderation tube disposed within the reaction tube and in which the powder core is disposed.

28. A monofilament for producing an Nb.sub.3Sn-containing superconductor wire, comprising: a powder core comprising an Sn-containing powder, a reaction tube composed of an Nb alloy comprising Nb and at least one further alloy component X, wherein the powder core is disposed radially within the reaction tube, at least one source for at least one partner component Pk, wherein each source is disposed in a unitary radial position in the monofilament and comprises at least one source structure, wherein the alloy component X and the partner component Pk are selected to form precipitates XPk through reaction annealing of the monofilament, in which Sn from the powder core and Nb from the reaction tube react to produce Nb.sub.3Sn, wherein the precipitates XPk comprise nonoxidic and nonmetallic precipitates, and, wherein the at least one partner component Pk comprises silicon, carbon or a halogen, and a moderation tube disposed within the reaction tube and in which the powder core is disposed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is shown schematically in the drawing and is explained in more detail through exemplary working examples.

(2) FIG. 1 shows an embodiment of a monofilament of the invention, with a source for the at least one partner component integrated into the powder core;

(3) FIG. 2 shows an embodiment of a monofilament of the invention, with an annular source around the moderation tube and with an ancillary moderation tube around the annular source;

(4) FIG. 3 shows an embodiment of a monofilament of the invention with an annular source around the reaction tube and with a diffusion barrier around the annular source;

(5) FIG. 4 shows an embodiment of a monofilament of the invention with a first source integrated into the powder core and with an annular second source around the moderation tube;

(6) FIG. 5 shows an embodiment of a monofilament of the invention with a first source integrated into the powder core and with an annular second source around the reaction tube;

(7) FIG. 6 shows an embodiment of a monofilament of the invention with an annular first source, which is disposed around the reaction tube, and with an annular second source, which is disposed around an ancillary reaction tube that surrounds the first annular source;

(8) FIG. 7 shows an embodiment of a monofilament of the invention with an annular source which is configured with recess-like source pockets on the inside of the reaction tube;

(9) FIG. 8 shows an embodiment of a monofilament of the invention with an annular source which is configured with closed source pockets in the interior of the reaction tube;

(10) FIG. 9 shows an embodiment of a monofilament of the invention with an annular source which is configured with source tubules which are disposed between the reaction tube and an ancillary reaction tube;

(11) FIG. 10 shows a precursor for an Nb.sub.3Sn superconductor wire, in which a plurality of monofilaments of the invention are bundled;

(12) FIG. 11 shows an oven arrangement for implementing a reaction annealing in accordance with the invention;

(13) FIG. 12 shows an SEM micrograph of the Nb.sub.3Sn phase of a superconductor wire fabricated in accordance with the invention, with visible precipitates on a fracture face.

DETAILED DESCRIPTION

(14) FIGS. 1 to 9 show embodiments of monofilaments 100 of the invention in a schematic cross section (that is, perpendicularly to the longitudinal direction of the monofilaments 100), which can be used in the context of the invention to fabricate an Nb.sub.3Sn-containing superconductor wire using the powder-in-tube approach.

(15) All embodiments comprise a central powder core 1, in which there is an Sn-containing powder or powder mixture, and also a moderation tube 2, in which the powder core 1 is disposed, and a reaction tube 3, which is fabricated from an alloy which comprises Nb (usually in a very large fraction, preferably at not less than 50 wt % or not less than 80 wt %) and further comprises at least one alloy component X (usually with a small fraction, such as with 0.3-3.0 wt %, for example). Reaction annealing is accompanied by formation of the Nb.sub.3Sn phase from the Sn of the powder core 1 and from the Nb of the reaction tube 3. Outwardly, the monofilament 100 is bounded in each case by a matrix 5 (also called shell tube), which is usually fabricated of Cu or a Cu alloy, and which in the embodiments shown has a hexagonal outer contour in each case, in order to facilitate bundling; alternatively, the matrix 5 may also be configured with a round outer cross section.

(16) In each embodiment, furthermore, the monofilament 100 comprises at least one source 4 for at least one partner component Pk. The reaction annealing is accompanied by the formation, from the partner component Pk and the alloy component X, of precipitates XPk which (according to the time at which they are formed) lead to grain refinement of the developing Nb.sub.3Sn phase or to the formation of local precipitates in the existing Nb.sub.3Sn phase, which act as pinning centers, and thereby improve the superconducting current carrying capacity of the completed superconductor wire.

(17) The different embodiments of monofilaments 100 differ primarily in the configuration of the one or the two or more sources 4, particularly their number, geometric disposition, and composition.

(18) In the case of the embodiment of FIG. 1, the powder core 1 serves simultaneously as the (sole) source 4 for the partner component Pk (indicated by double hatching of the powder core 1). The powder core 1 is formed here by a mixture of Sn powder and NbSn.sub.2 powder (as Sn source for the formation of Nb.sub.3Sn), and also Cu powder (as catalyst for the formation of Nb.sub.3Sn) and SnO.sub.2 powder (as a source for the partner component Pk, which here is oxygen).

(19) The moderation tube 2, here made of Cu, possesses a comparatively low wall thickness WM in comparison to the wall thickness WR of the reaction tube 3 and to the diameter DP of the powder core 1, here approximately with WM=0.15*WR and WM=0.055*DP. As a result it is possible to obtain sufficient diffusion of the oxygen through the moderation tube 2. It is noted that in this case the moderation tube 2 controls both the diffusion of the Sn and the diffusion of the partner component Pk; to balance out the diffusion of Sn and Pk, the powder core 1 may comprise suitable further additions, such as of Ag powder.

(20) The reaction tube 3 here is an NbZr1 alloy, in other words with zirconium as the alloy component X with a fraction of 1 wt %. If desired, the alloy of the reaction tube 3 may also comprise Ta or other alloy additions which are conducive to the formation of the Nb.sub.3Sn phase and/or to the superconducting properties thereof. The reaction annealing is accompanied here by the formation of ZrO.sub.2 precipitates in the Nb.sub.3Sn material.

(21) In the powder core 1 and/or in the source 4, besides SnO.sub.2, it is also possible, optionally, for a further oxygen-containing compound to be provided, Gd.sub.2O.sub.3 for example. As a result, during the heat treatment, oxygen is released twice at different times.

(22) Alternatively, rather than SnO.sub.2, the oxygen source 4 stockpiled in the powder core 1 may also be a Pk-containing chemical compound that breaks down in two stages, for instance a permanganate compound such as KMnO.sub.4. The permanganate may first release a part of its oxygen, with MnO.sub.2 forming as a first reaction product (first residual compound), among others. This MnO.sub.2 may then give off further oxygen on further supply of heat, forming elemental Mn as a second reaction product (second residual compound). This as well enables a temporally staggered, twofold release of oxygen.

(23) Through a temporally staggered, twofold release of the partner component Pk it is possible to make precipitates form both before Nb.sub.3Sn formation (for grain refinement of the Nb.sub.3Sn phase) and thereafter (for additional artificial pinning centers).

(24) In the embodiment of the monofilament 100 that is shown in FIG. 2, the powder core 1 serves only as a Sn source, but not as a source for a partner component (consequently the powder is shown with only single hatching). The powder core 1 is enclosed by the moderation tube 2, which here controls the diffusion only of Sn, but not of partner component Pk. The moderation tube 2 is here enclosed in turn by the annular (sole) source 4 for partner component Pk, which here is configured as a completely circumferential powder layer 6 comprising SnO.sub.2 powder (for the provision of partner component Pk, namely oxygen) and metallic Cu powder (for improving the deformation properties). Here, disposed between the powder layer 6 and the reaction tube 3, there is also an ancillary moderation tube 7, which jointly controls the diffusion of the partner component Pk and of the Sn into the reaction tube 3.

(25) In the case of the FIG. 3 embodiment of the monofilament 100, the annular (sole) source 4 for partner component Pk is configured in turn as a powder layer 6, but is disposed radially outside the reaction tube 3, with the powder layer 6 here directly enclosing the reaction tube 3. As a result, the partner component Pk has very rapid access to the reaction tube 3. In order to diffuse into the reaction tube 3 which comprises the alloy component X, the Sn from the powder core 1 has to cross the moderation tube 2. As a result, the diffusion of the Sn into the reaction tube 3 can be delayed, to allow precipitates of XPk to form in the reaction tube before the Nb.sub.3Sn phase is formed.

(26) In this embodiment, shown by way of example, there is also a diffusion barrier 8, made of Ta, for instance, on the inside of the matrix 5, and this barrier 8 makes it possible to prevent contamination of the matrix 5 by diffusion of elements from the interior (especially of Sn) and hence to prevent a reduction in the residual resistance ratio (RRR) of the matrix 5.

(27) In the FIG. 4 embodiment of a monofilament 100 of the invention there are two sources 4 for partner component Pk.

(28) On the one hand, the powder core 1 here serves simultaneously as a source 4 for a partner component Pk, with the powder core comprising a corresponding Pk-containing chemical compound—here, SnO.sub.2 powder for oxygen as partner component Pk. On the other hand, a further source 4 for a partner component Pk is here disposed around the moderation tube 2, and within the reaction tube 3 which comprises the alloy component X. This further source 4 is configured with a circumferential powder layer 6, which here likewise has SnO.sub.2 powder for oxygen as partner component Pk.

(29) The partner component Pk from the powder layer 6 reaches the reaction tube 3, during the reaction annealing, before the partner component Pk from the powder core 1. As a result it is possible to introduce the partner component Pk both before Nb.sub.3Sn formation and after Nb.sub.3Sn formation into the reaction tube 3 (or into the structurers arising therefrom).

(30) In the embodiment shown, the chemical compounds in both sources 4 are the same, providing the same partner component Pk. Alternatively, it is also possible to provide different chemical compounds in the various sources 4, for providing the partner component Pk, or else for providing different partner components Pk (in order to form precipitates XPk which in that case are also different). In particular, provision may be made for the formation both of oxidic precipitates and of nonoxidic precipitates in the course of a reaction annealing.

(31) In the embodiment of FIG. 5 there are likewise two sources 4 for the monofilament 100 therein.

(32) One source 4 is again formed by the powder core 1, which here is admixed with SnO.sub.2 powder. Also provided is an annular source 4 which in this case is configured as a powder layer 6 comprising SnO.sub.2 powder and which here directly encloses the reaction tube 3. The powder layer 6 is in turn enclosed, radially on the outside, by an ancillary reaction tube 9, which in this case has the same composition as the reaction tube 3, and in particular likewise comprises Nb and the alloy component X.

(33) With this construction, partner component Pk is able very rapidly to pass from the powder layer 6 both into the reaction tube 3 and into the ancillary reaction tube 9, in order to form precipitates XPk, in particular before the formation of the Nb.sub.3Sn phase, in order to bring about a fine microstructure for this phase. The partner component Pk from the powder core 1 arrives in large parts after the Sn from the powder core 1 in the reaction tube 3 and in the ancillary reaction tube 9, to then form punctiform precipitates as artificial pinning centers. The time of the inward diffusion of the Sn and of Pk from the powder core 1 can be adjusted suitably via the moderation tube 2, particularly the wall thickness thereof, with optional support from additions in the powder core 1.

(34) The FIG. 6 embodiment of the monofilament 100 likewise envisions two sources 4, which here are configured both as annular sources 4.

(35) One of the sources 4 is again configured as a powder layer 6 comprising SnO.sub.2 powder, and sits directly radially externally on the reaction tube 3, so that partner component Pk can penetrate rapidly from the powder layer 6 into the reaction tube 3 and form precipitates XPk with alloy component X present therein, in this case zirconium, especially before the Nb.sub.3Sn phase comes about. The other source 4 for partner component Pk is configured here as a coating 10 on the outside of the ancillary reaction tube 9. The ancillary reaction tube 9 in turn sits externally directly on the powder layer 6. Partner component Pk from the coating 10 reaches the reaction tube 3 later than the partner component Pk of the powder layer 6.

(36) By appropriately adjusting the wall thickness of the ancillary reaction tube 9 and the wall thickness of the moderation tube 2, it is possible to make Sn from the powder core 1 reach the reaction tube 3 before the partner component Pk from the coating 10 reaches the reaction tube 3.

(37) The ancillary reaction tube 9 here has the same composition as the reaction tube 3, hence also comprising niobium and zirconium, and so the volume of the ancillary reaction tube 9 as well can be utilized for the formation of an Nb.sub.3Sn phase. If desired, instead of the ancillary reaction tube 9, it is also possible to employ a tube made of a material (Cu, for instance) with which there is no notable contribution to the generation of Nb.sub.3Sn phase volumes (for instance, because little or no niobium is present, or because the tube is simply of relatively thin configuration), but with which a desired hindering of the diffusion of the partner component Pk from the coating 10 radially inward to the reaction tube 3 is achieved. In this case this tube is termed an auxiliary moderation tube 11.

(38) Materials particularly suitable for the coating 10 are those which are plastically deformable or exhibit low friction during deformation, since these materials do not produce any defects on further processing (for instance, in the event of a forming operation which reduces the cross section). The coating for example may consist of graphitic carbon as partner component Pk, which together with Fe (iron) as alloy component X in the reaction tube 3 is able to produce an iron carbide precipitate. In the powder layer 6 it is also possible to utilize materials which are not plastically deformable, as for instance oxidic powder particles of SnO.sub.2, since the powder form imparts a certain flowability. Oxygen from the oxidic powder particles may form, with Zr as alloy component X, for example, ZrO.sub.2 precipitates in the reaction tube; in this example, then, the reaction tube 3 (besides Nb) also comprises Fe and Zr as two alloy components X.

(39) FIG. 7 shows an embodiment of a monofilament 100 in which source pockets 12 are configured on the radial inside of the reaction tube 3. The source pockets 12 here are recesses on the reaction tube 3 that have a powder filling 13. The powder filling 13 comprises a powder which comprises the (at least one) partner component Pk, in this case SnO.sub.2 powder for the oxygen partner component, and here, furthermore, a metallic powder, Cu powder for instance, to improve the formability. The source pockets 12 are all disposed at a unitary (identical) radial position in the monofilament 100, and constitute a plurality of source structures which in their entirety form an annular source 4 for the partner component Pk in the monofilament 100. Remaining between the source pockets 12 in the circumferential direction are radially continuous structural regions 16, here configured by the reaction tube 3 and in contact with the moderation tube 2, which improve the processing properties of the monofilament 100.

(40) The FIG. 8 embodiment of a monofilament 100 likewise shows an annular source 4 formed of a plurality of source pockets 12 which are disposed at a unitary radial position in the monofilament 100 and which again have a powder filling 13 which here comprises SnO.sub.2 powder. In the embodiment shown, the source pockets 12 are configured as recesses, closed on all sides, within the reaction tube 3. Again there remain radially continuous structural regions 16 between the source pockets 12 in the reaction tube 3.

(41) In the embodiment of a monofilament 100 that is shown in FIG. 9, there is an annular source 4 which is formed of a multiplicity of source tubules 14 as source structures, which are installed in a radial gap 15 between the internally bordering reaction tube 3 and the externally bordering ancillary reaction tube 9. The source tubules 14 are formed here with a round cross section and have a powder filling 13 which comprises a powder comprising the at least one partner component Pk. It is noted that after a customary forming operation which reduces cross section, the voids between the source tubules 14 in the gap 15 are filled up (not shown) with material from the bordering reaction tube 3 and from the bordering annularly reaction tube 9.

(42) FIG. 10 shows a precursor 20 for an Nb.sub.3Sn-containing superconductor wire for the invention. The precursor 20 comprises a plurality of monofilaments 100 of the invention (“bundling”), the monofilaments 100 here being disposed annularly. The precursor 20 here, moreover, is fabricated of copper.

(43) Disposed typically in the precursor 20 are a plurality of monofilaments 100 with a hexagonal outer cross section and optionally (as shown here) a plurality of stabilizing elements 101 with the same hexagonal outer cross section, usually composed of Cu or a Cu alloy, which are placed lying against one another in hexagonal close packing in an outer tube.

(44) Following introduction of the monofilaments 100, the precursor 20 is subjected to a forming operation which reduces cross section (optionally a multiple bundling and cross-sectionally reducing forming operation is also employed), to give a precursor wire which is subsequently wound into a desired form, such as the form of a coil, for instance.

(45) FIG. 11 shows the precursor wire 20, wound into a coil 31 on a spool 30, arranged in an oven 32.

(46) In the oven 32, the precursor wire 20 is subjected to reaction annealing, typically at a temperature between 400° C. and 800° C.; this reaction annealing may last several days. During this procedure, superconducting Nb.sub.3Sn forms from the Sn of the powder core and from the Nb of the reaction tubes, and precipitates XPk form from the at least one partner component Pk of the at least one source and from the at least one alloy component X of the reaction tube.

(47) After the reaction annealing, an Nb.sub.3Sn-containing superconductor wire 33 has formed from the precursor wire 22, and the Nb.sub.3Sn phase of this wire 33 also comprises the precipitates XPk. The superconductor wire 33 exhibits a particularly high superconducting current carrying capacity.

(48) FIG. 12 shows an SEM micrograph of the Nb.sub.3Sn phase of a superconductor wire fabricated in accordance with the invention. The associated monofilament contained SnO.sub.2 powder as oxygen source in the powder core, the powder core having been enclosed in a copper moderation tube. The surrounding reaction tube consisted of an NbZr1 alloy. The reaction annealing involved a treatment time of 300 h in a multistage temperature program with a maximum temperature of 640° C. The wire diameter was 1 mm, and the diameter of a monofilament in the superconductor wire was 40 μm. Clearly apparent are ZrO.sub.2 precipitates (light-colored dots) on the Nb.sub.3Sn grains, which may act as artificial pinning centers. The Nb.sub.3Sn grains typically have diameters of around 30-80 nm. In a control experiment in which the powder core contained no SnO.sub.2, no precipitates and a relatively coarse microstructure (not shown) were observed.

(49) In summary, the invention relates to a monofilament for a powder-in-tube precursor wire for an Nb.sub.3Sn-containing superconductor wire, with an Sn-containing powder core disposed radially within an Nb-containing reaction tube, where the monofilament is set up for the formation of precipitates XPk in the region of the Nb.sub.3Sn phase during the reaction annealing. The monofilament comprises at least one source for at least one partner component Pk, and the reaction tube is admixed with at least one alloy component X, allowing the precipitates XPk to be formed in the region of the reaction tube during the reaction annealing. The powder core is in a moderation tube, in order for the delivery of the Sn to the reaction tube and hence the formation of Nb.sub.3Sn to be controlled (in particular, delayed) and, overall, to be temporally harmonized, together with the construction of the monofilament, with the formation of the precipitates, more particularly so that precipitates are formed at least before and preferably also after formation of the Nb.sub.3Sn phase. The configuration of the monofilament (and the implementation of the reaction annealing) is preferably such that during the reaction annealing, partner component Pk reaches the reaction tube in at least two temporally successive waves—more particularly, where one wave reaches the reaction tube primarily before the formation of the Nb.sub.3Sn phase, and one wave reaches the reaction tube primarily after the formation of the Nb.sub.3Sn phase. For this purpose, the monofilament may in particular have at least two separate sources for partner component Pk at different radial positions, or may have at least two different chemical compounds in the at least one source, each delivering partner component Pk at different temperatures, or may have, in the at least one source, a chemical compound which delivers partner component Pk in two stages at different temperatures, in other words by way of an intermediate stage.

LIST OF REFERENCE SYMBOLS

(50) 1 powder core 2 moderation tube 3 reaction tube 4 source 5 matrix 6 powder layer 7 ancillary moderation tube 8 diffusion barrier 9 ancillary reaction tube 10 coating 11 auxiliary moderation tube 12 source pockets 13 powder filling 14 source tubules 15 gap 16 radially continuous structural region 20 precursor 22 precursor wire 30 spool 31 coil 32 oven 33 superconductor wire 100 monofilament 101 stabilizing element DP diameter of powder core WM wall thickness of moderation tube WR wall thickness of reaction tube