POWDERS BASED ON NIOBIUM-TIN COMPOUNDS FOR PRODUCING SUPERCONDUCTIVE COMPONENTS

Abstract

A powder for the production of a superconducting component. The powder includes Nb.sub.xSn.sub.y, where 1≤x≤6 and 1≤y≤5, and three-dimensional agglomerates having a particle size D90 of less than 400 μm, as determined via a laser light scattering. The three-dimensional agglomerates have primary particles which have an average particle diameter of less than 15 μm, as determined via a scanning electron microscopy, and pores of which at least 90% have a diameter of from 0.1 to 20 μm, as determined via a mercury porosimetry.

Claims

1-18. (canceled)

19: A powder for the production of a superconducting component, the powder comprising: Nb.sub.xSn.sub.y, where 1≤x≤6 and 1≤y≤5; and three-dimensional agglomerates having a particle size D90 of less than 400 μm, as determined via a laser light scattering, the three-dimensional agglomerates comprising, primary particles having an average particle diameter of less than 15 μm, as determined via a scanning electron microscopy, and pores of which at least 90% have a diameter of from 0.1 to 20 μm, as determined via a mercury porosimetry.

20: The powder as recited in claim 19, wherein the powder comprises a proportion of Nb.sub.3Sn or Nb.sub.6Sn.sub.5 or NbSn.sub.2, respectively, of >90%, based on all crystallographic phases detected as determined via a Rietveld analysis of an X-ray powder diffraction pattern of the powder.

21: The powder as recited in claim 19, wherein the powder further comprises a specific surface area of from 0.5 to 5 m.sup.2/g as determined by a BET method.

22: The powder as recited in claim 19, wherein the powder further comprises: an oxygen content of less than 1.8 wt. % based on a total weight of the powder.

23: The powder as recited in claim 19, wherein the powder, after a milling of the three-dimensional agglomerates, has a particle size D99 of less than 15 μm as determined via a laser light scattering.

24: The powder as recited in claim 19, wherein the powder further comprises: a proportion of metallic tin of less than 3%, based on all crystallographic phases detected as determined via a Rietveld analysis of an X-ray diffraction pattern of the powder.

25: The powder as recited in claim 19, wherein the powder further comprises: powder particles, wherein, 95% of all of the powder particles have a Feret diameter of from 0.7 to 1 after atomization, the Feret diameter being a smallest diameter of a particle of the powder particles divided by a greatest diameter of the particle of the powder particles as determined via an evaluation of SEM images.

26: A process for producing the powder as recited in claim 19, the process comprising: reacting a niobium metal powder with a tin metal powder, wherein, the niobium metal powder is obtained via a reduction of niobium oxide using a gaseous reducing agent.

27: The process as recited in claim 26, wherein the niobium metal powder which is obtained via the reduction of niobium oxide using the gaseous reducing agent is obtained in situ.

28: The process as recited in claim 26, wherein the niobium metal powder comprises pores having a size of from 0.1 to 20 μm, as determined via a mercury porosimetry.

29: The process as recited in claim 26, wherein the niobium metal powder comprises three-dimensional agglomerates having a size with a D90 of less than 400 μm, as determined via a laser light scattering, and the three-dimensional agglomerates comprise primary particles having an average particle diameter of less than 10 μm, as determined via a scanning electron microscopy.

30: The process as recited in claim 26, wherein the niobium metal powder has a bimodal distribution of a pore size.

31: The process as recited in claim 26, wherein the niobium oxide is a compound selected from the group consisting of Nb.sub.2O.sub.5, NbO.sub.2, NbO, and mixtures thereof.

32: The process as recited in claim 26, wherein the gaseous reducing agent is magnesium vapor.

33: The process as recited in claim 26, wherein the niobium metal powder further comprises at least one dopant.

34: A method of using the powder as recited in claim 19 for producing a superconducting component, the method comprising: providing the powder as recited in claim 19; and using the powder to produce at least one of a superconducting wire and a superconducting coil as the superconducting component.

35: The method of using as recited in claim 34, wherein the superconducting component is produced via a powder-metallurgical process.

36: The method of using as recited in claim 35, wherein the powder-metallurgical process is a PIT process or an additive manufacturing process.

37: A method of using the powder as recited in claim 19 in an additive manufacturing process, the method comprising: providing the powder as recited in claim 19; and using the powder in the additive manufacturing process, wherein, the additive manufacturing process is selected, from at least one of a laser beam melting, and electron beam melting, and a laser cladding, or from a powder-metallurgical process.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present invention is described in greater detail below on the basis of embodiments and of the drawings in which:

[0020] FIG. 1 shows a scanning electron micrograph of a NbSn.sub.2 powder obtained per Example 1 of the present invention;

[0021] FIG. 2 shows a scanning electron micrograph of a Nb.sub.3Sn powder obtained per Example 2 of the present invention;

[0022] FIG. 3 shows a scanning electron micrograph of a Nb.sub.3Sn powder obtained per Example 3 of the present invention;

[0023] FIG. 4 shows a scanning electron micrograph of a conventional powder obtained per Comparative Example 1;

[0024] FIG. 4 shows a scanning electron micrograph of a conventional powder obtained per Comparative Example 2;

[0025] FIG. 6 shows a measurement of the pore size distribution of the niobium metal powder used for producing the powders according to the present invention, where the bimodal distribution of the pore sizes is readily apparent;

[0026] FIG. 7 shows a measurement of the pore size distribution of the powder according to the present invention as per Example 1 with a maximum at a pore size of about 3 μm; and

[0027] FIG. 8 shows a scanning electron micrograph of the niobium metal powder used for producing Comparative Examples 1 and 2.

DETAILED DESCRIPTION

[0028] The present invention thus firstly provides a powder for the production of superconducting components, the powder comprising Nb.sub.xSn.sub.y where 1≤x≤6 and 1≤y≤5, wherein the powder comprises three-dimensional agglomerates having a size having a D90 of less than 400 μm, for example, from 220 to 400 μm, determined by means of laser light scattering, the agglomerates being made up of primary particles which have an average particle diameter of less than 15 μm, for example, less than 8 μm, for example, from 0.1 μm to 5 μm, determined by means of scanning electron microscopy, and the agglomerate have pores of which 90% or more have a diameter of from 0.1 to 20 μm, for example, from 0.2 to 15 μm, for example, from 0.3 to 10 μm, determined by means of mercury porosimetry.

[0029] In an embodiment of the present invention, the agglomerates can, for example, have a particle size distribution having a D50 of from 100 to 300 μm, for example, from 150 to 220 μm, determined by means of laser light scattering.

[0030] The D90 or the D50 is the value which indicates the percentage of agglomerates in the powder which have a particle size of less than or equal to the size indicated.

[0031] In an embodiment, the Nb.sub.xSn.sub.y compound can, for example, be a compound selected from the group consisting of Nb.sub.3Sn, Nb.sub.6Sn.sub.5, NbSn.sub.2, and mixtures thereof.

[0032] It has surprisingly been found that the required diffusion paths within the niobium solid body in the powder of the present invention are significantly reduced due to the small primary particle size, and that the tin can very readily enter into the interior of the agglomerates during the reaction because of the particular pore structure, as a result of which the production process can be significantly accelerated. It has furthermore surprisingly been found that the temperatures at which the production of the powders is generally carried out, or the reaction times, could be significantly reduced. For example, NbSn.sub.2, which is stable only up to a temperature of 840° C., could thus be produced at a temperature of less than 800° C. in the context of the present invention in less than 6 hours and even in less than 4 hours. In the case of Nb.sub.3Sn, a time window of less than 6 hours was obtained at a temperature of 1050° C. and a time window of less than 8 hours was obtained at a temperature of 950° C.

[0033] It has been found to be particularly advantageous to use phase-pure powders for the production of superconductors in order to avoid influencing the superconducting properties by any foreign phases. It has surprisingly been found that the powder of the present invention also displays a high purity which results, for example, in the powder having only a small proportion of phases of compounds other than the respective target compound. In an embodiment, the powder of the present invention is therefore characterized in that the compounds Nb.sub.3Sn or Nb.sub.6Sn.sub.5 or NbSn.sub.2 make up a proportion of in each case more than 90%, for example, more than 94%, for example, more than 97%, based on all detected crystallographic phases and determined by Rietveld analysis of an X-ray diffraction pattern of the powder of the present invention.

[0034] In an embodiment, the powder of the present invention can, for example, be characterized in that the proportion of metallic tin in the powder is less than 3%, for example, less than 0.5%, based on all detected crystallographic phases and determined by Rietveld analysis of an X-ray diffraction pattern of the powder of the present invention. In an embodiment, the powder of the present invention can, for example, be essentially free of metallic tin. “Essentially free” in this context means that the proportion of metallic tin is not detectable in an X-ray diffraction pattern.

[0035] The presence of oxygen can lead to a deterioration in the properties of the superconductor. It has here surprisingly been found that the powders according to the present invention do not have increased oxygen contents compared to the prior art despite the increased porosity. In an embodiment, the powder can, for example, have an oxygen content of less than 1.8% by weight, for example, from 0.35 to 1.5% by weight, in particular from 0.5 to 1.2% by weight, based on the total weight of the powder.

[0036] The powder of the present invention can, for example, have a specific surface area determined by the BET method of from 0.5 to 5 m.sup.2/g, for example, from 1 to 3 m.sup.2/g. The specific surface area determined by the BET method can be measured in accordance with ASTM D3663.

[0037] It has been found to be advantageous to use powders having a particularly fine particle size for the production of superconducting wires by the PIT method. For this reason, the powders used are usually milled before introduction into the tube. It has here surprisingly been found that the powders of the present invention display an improved milling behavior, as a result of which it is possible to realize smaller particle sizes or, as an alternative, it is possible to employ more gentle milling processes. In an embodiment, the powder of the present invention after milling of the agglomerates can, for example, have a particle size having a D99 of less than 15 μm, for example, less than 8 μm, for example, from 1 μm to 6 μm, determined by means of laser light scattering. In an embodiment, the powder of the present invention after milling of the agglomerates can, for example, have a D90 of less than 10 μm, for example, less than 7 μm and, for example, from less than 0.5 μm to 5 μm, in each case determined by means of laser light scattering.

[0038] For the production of superconducting components using additive manufacturing methods, for example, LBM (laser beam melting), EBM (electron beam melting) and/or LC (laser cladding), it has been found to be advantageous to use powders having a particularly spherical particle shape. It has here surprisingly been found that the powders of the present invention can be atomized very readily by known methods to give powders having spherical particles, for example, using the EIGA method (electrode induction-melting gas atomization). In an embodiment, at least 95% of all powder particles of the powder of the present invention therefore can, for example, have a Feret diameter of from 0.7 to 1, for example, from 0.8 to 1, after atomization, where the Feret diameter is, in the context of the present invention, defined as the smallest diameter divided by the largest diameter of a particle, which can be determined by evaluation of SEM images.

[0039] The powder of the present invention in particular displays a high purity. In an embodiment, the present invention therefore provides that the proportion of all metallic elements can, for example, be less than 500 ppm, for example, less than 300 ppm, for example, from 1 ppm to 150 ppm, in the powder, in each case based on the total weight of the powder. The metallic elements are especially ones which are introduced via milling processes. For the purposes of the present invention, ppm figures are in each case by mass.

[0040] For the production of superconducting components having acceptable properties, it is indispensable for the chemical purity of the powders used to be high and foreign materials to be introduced only in controlled form as dopants. Materials introduced unintentionally in the process, in particular metallic impurities and fluoride-containing compounds, should be minimized. One of the main sources of these impurities is contamination from the niobium and tin metal powders used. In an embodiment, the powder of the present invention can, for example, have a fluorine content of less than 15 ppm, for example, less than 5 ppm. In an embodiment, the powder of the present invention can, for example, have a total content of coincidental metallic impurities with the exception of tantalum of in total less than 0.8% by weight, for example, less than 0.5% by weight, for example, less than 0.25% by weight, in each case based on the total weight of the powder.

[0041] In an embodiment, the powder of the present invention can, for example, additionally contain dopants which, in contrast to the above-described materials introduced unintentionally in the process, are added in a defined way to the powder of the present invention, as is known from the prior art. It has surprisingly been here found that the dopants do not have to meet any particular requirements, but rather that it is possible to use the conventional dopants which are known to a person skilled in the art.

[0042] Some of the processes described in the prior art for producing superconducting wires based on Nb.sub.3Sn start out from a tantalum-tin alloy or from an intermetallic tin alloy based on tantalum and niobium as a precursor powder. This has the disadvantage, however, that residues of tantalum remain in the later core of the Nb.sub.3Sn wire filament, as a result of which, for example, production costs increase and the superconducting properties of the products are impaired. A further disadvantage which may be mentioned here is, in particular, the reduced thickness of the superconducting Nb.sub.3Sn boundary layer in the finished wire filament, with the expected adverse effects on the maximum current carrying capability. In an embodiment, the powder of the present invention can, for example, therefore be essentially free of tantalum and tantalum compounds. In an embodiment, the proportion of tantalum and compounds thereof in the powder of the present invention can, for example, be less than 1% by weight, for example, less than 0.5% by weight, for example, less than 0.1% by weight, in each case based on the total weight of the powder.

[0043] The powders of the present invention display, due to their high porosity combined with a small primary particle size, properties which are realized by means of a specific production process. The present invention therefore further provides a process for producing the powders of the present invention, which process allows precisely this combination of properties to be realized. The process of the present invention is characterized in that the powder of the present invention is produced by reacting a niobium metal powder with a tin metal powder, where the niobium metal powder is obtained by reduction of niobium oxide by means of a gaseous reducing agent. It has surprisingly been found that powders which, due to the advantageous combination of primary particle size, porosity, and chemical purity, are particularly suitable for the production of superconducting wires are thereby obtained. Suitable methods for reducing niobium oxide in the presence of a gaseous reducing agent are described, for example, in WO 00/67936.

[0044] In order to make the process efficient, it has been found to be advantageous to carry out the formation and reaction of the niobium metal powder in a combined process step. In an embodiment, the process of the present invention can, for example, be performed in which the niobium metal powder is obtained in situ. This can in particular be achieved by niobium oxide being reacted with the gaseous reducing agent in the presence of tin metal powder in a heat treatment.

[0045] It has surprisingly been found that this makes it possible to obtain structures which have the desired porosity. This is particularly unexpected because a person skilled in the art would not have expected porous NbSn products in the reduction of NbSn mixed oxides and hydroxides by means of magnesium.

[0046] The niobium oxide can, for example, be selected from the group consisting of Nb.sub.2O.sub.5, NbO.sub.2, NbO, and mixtures thereof.

[0047] It has surprisingly been found that a porosity of the powder which is advantageous for the production of superconducting wires can be achieved by suitable selection of the starting compounds. In an embodiment, the present invention provides that the niobium metal powder can, for example, comprise three-dimensional agglomerates having a size having a D90 of less than 400 μm, for example, from 200 to 350 μm, determined by means of laser light scattering, the agglomerates being made up of primary particles which have an average particle diameter of less than 10 μm, for example, less than 4 μm, for example, from 0.1 μm to 2 μm, determined by means of scanning electron microscopy, and the agglomerates

[0048] have pores with a size of from 0.1 to 20 μm, for example, from 0.2 to 15 μm, for example, from 0.2 to 5 μm, determined by means of mercury porosimetry.

[0049] The use of niobium metal powders having a bimodal pore size distribution has been found to be particularly advantageous. In an embodiment, the present invention provides that the niobium metal powder can, for example, have a bimodal pore size distribution, for example, with a maximum in the range from 0.9 to 20 μm, for example, from 1 to 10 μm, and a maximum in the range from 0.1 to 0.9 μm, for example, in the range from 0.2 to 0.8 μm, determined by means of mercury porosimetry. Without wishing to be tied to a particular theory, it is assumed that the biomodal distribution of the pore sizes makes possible an improved intrusion of the tin through the pores into the interior of the agglomerates.

[0050] It has surprisingly been found that limiting the oxygen content of the niobium metal powders, as described in the prior art, is not necessary for the production of the powders of the present invention. It has nevertheless been found to be advantageous for the oxygen content not to be too high. The niobium metal powder used for the production of the powders of the present invention can therefore have, for example, an oxygen content of less than 2.2% by weight, for example, from 0.35 to 1.9% by weight, for example, from 0.4 to 1.6% by weight, in each case based on the total weight of the niobium metal powder. It is also not necessary to limit the hydrogen content of these niobium metal powders to less than 100 ppm. It is thus also possible to use niobium metal powders having a hydrogen content of greater than 100 ppm, even from 110 to 400 ppm, in the process of the present invention. In an embodiment, the present invention provides that the niobium metal powder used for the production of the powders of the present invention can, for example, have a hydrogen content of from 110 to 400 ppm, based on the total weight of the niobium metal powder.

[0051] To prevent the formation of undesirable and sometimes sparingly soluble Mg halides and Ca halides, for example, MgF.sub.2 and CaF.sub.2, a niobium metal powder having a very low fluorine and chlorine content can, for example, be used. For this reason, niobium metal powders produced by metallothermic vapor reduction of niobium oxides are preferred over niobium metal powders produced by reduction of fluorine-containing compounds, for example K.sub.2NbF.sub.7, or electrochemical reduction in a halide-containing salt melt. In an embodiment, the niobium metal powder used can, for example, contain less than 15 ppm of each of fluorine and chlorine, for example, in each case less than 10 ppm, for example, in each case less than 2 ppm, based on the total weight of the niobium metal powder.

[0052] To provide the necessary chemical purity of the powders of the present invention, use of niobium metal powders having a total of all metallic elements of less than 800 ppm, for example, less than 500 ppm can, for example, be used.

[0053] As gaseous reducing agent, alkaline earth metal compounds have been found to be particularly advantageous. In an embodiment, the gaseous reducing agent can, for example, be magnesium vapor. It has been found that a good wetting of the niobium oxide powder particles can thereby be achieved. Magnesium, in particular as a reducing agent, has the advantage that it can be removed simply and efficiently after the conversion into MgO, as a result of which unnecessary contamination of the products is avoided.

[0054] The desired target compounds can be produced by the process of the present invention at lower temperatures and in a shorter time than has to date been described in the prior art. In an embodiment, the process of the present invention provides that the reaction of the metallic niobium powder with the metallic tin can, for example, be carried out at a reaction temperature in the range from 300 to 1100° C., for example, from 600 to 1050° C., for example, from 650 to 790° C. The reaction time can, for example, be less than 8 hours, for example, from 0.5 to 7 hours, for example, from 1 to 4 hours.

[0055] As indicated above, dopants via which the properties of the powder can be adapted can be added to the powder. It has surprisingly been found to be advantageous for these dopants to be introduced right at the beginning of the process. For this reason, the present invention provides an embodiment in which the niobium oxide used for producing the niobium metal powder can, for example, additionally comprise dopants. There are no particular requirements which need be met in respect of the dopants used. It has rather surprisingly been found that all customary dopants known to a person skilled in the art can be used.

[0056] The powder of the present invention is particularly suitable for the production of superconducting components. The present invention therefore further provides for the use of the powder of the present invention for producing superconducting components, in particular for producing superconducting wires. The superconducting component can, for example, be produced by powder-metallurgical processes or additive manufacturing processes or PIT processes. The superconducting wires can, for example, be produced via PIT processes.

[0057] The present invention further provides for the use of the powder of the present invention in additive manufacturing processes. The additive manufacturing processes can, for example, be LBM (laser beam melting), EBM (electron beam melting) and/or LC (laser cladding).

[0058] The present invention will be illustrated with the aid of the following examples which should, however, not in any way be construed as restricting the inventive concept.

Examples

[0059] Niobium metal powder was obtained in a manner analogous to the production process described in WO 00/67936 A1 by reaction of NbO.sub.2 with magnesium vapor. The niobium metal powder obtained had an oxygen content of 8500 ppm, a hydrogen content of 230 ppm, a fluorine content of 2 ppm, total metallic impurities of 310 ppm, and an agglomerate size having a D50 of 205 μm and a D90 of 290 μm. The average size of the primary particles was 0.6 μm, and the pore size distribution of the agglomerates was bimodal with maxima at 0.5 and 3 μm (see FIG. 6). This niobium metal powder was subsequently reacted with tin metal powder having an oxygen content of 6800 ppm under various conditions, and the products obtained were analyzed.

[0060] In Comparative Examples 1 and 2, use was made of powders for which conventional pore-free niobium metal powder having an oxygen content of 2900 ppm and a hydrogen content of 10 ppm was used. These comparative powders had a D90 of 95 μm (see FIG. 8).

[0061] For Comparative Examples 3 and 4, a niobium metal powder was produced by reduction of K.sub.2NbF.sub.7 with liquid sodium in a salt melt composed of KCl/KF at 850° C. and subsequently removing all salts by washing. The niobium metal powder produced in this way contained irregularly shaped particles without pores having a D90 of 29 μm which were not sintered together to form agglomerates. The niobium metal powder had an oxygen content of 2.4% by weight, a hydrogen content of 150 ppm, a fluorine content of 2500 ppm, and metallic impurities of 10 200 ppm in total, with the main impurities being nickel and iron.

[0062] In all Comparative Examples, the niobium metal powders were subsequently reacted with tin metal powder having an oxygen content of 6800 ppm under various conditions and the products obtained were analyzed.

[0063] The tin metal powder used had a particle size of less than 150 μm in all Examples.

[0064] The results are summarized in Table 1, with the pore sizes and particle sizes reported each relating to the agglomerates. The particle size was in each case determined by means of laser light scattering (MasterSizer S, dispersion in water and Daxad® 11, 5 minutes ultrasonic treatment). The size of the pores was determined by means of mercury porosimetry (Micromeritics; AutoPore IV) and relates to the position of the maximum of the pore size distribution. The oxygen content was determined by means of carrier gas hot extraction (Leco TCH600). Trace analysis of the metallic impurities was carried out by means of ICP-OES using the following analytical instruments PQ 9000 (Analytik Jena) or Ultima 2 (Horiba). All figures reported in % by weight are based on the total weight of the powder.

[0065] X-ray diffraction was carried out on pulverulent samples using an instrument from Malvern-PANalytical (X'Pert-MPD with semiconductor detector, X-ray tubes Cu LFF with 40 KV/40 mA, Ni filter).

[0066] The powder of Example 3 was analyzed after washing with H.sub.2SO.sub.4 and drying.

[0067] The powders of the Comparative Examples did not form agglomerates and displayed an inhomogeneous phase composition in the analyses. The powders of Comparative Examples 3 and 4 are furthermore characterized by very high fluorine contents of from 1300 to 1800 ppm and metallic impurities of from 6500 to 9000 ppm in total, and are therefore not suitable for producing superconducting components, in particular wires.

[0068] The powder of Example 1 was subsequently milled in an oxygen-free atmosphere, giving particle size distributions having a D90 of 2.6 μm and a D99 of 3.7 μm. The oxygen content after milling was 1.14% by weight and the difference between all total metallic impurities before and after the milling process was 140 ppm.

[0069] In the context of the present invention, it was surprisingly found that the increased porosity of the powders of the present invention does not lead, contrary to expectations, to an increase in the oxygen content, even when the powders were subjected to a further milling step.

[0070] The examples according to the present invention furthermore show that no unreacted tin could be detected in the target compounds.

[0071] FIGS. 1 to 3 show scanning electron micrographs of the powders according to the present invention, with FIG. 1 showing the NbSn.sub.2 obtained as per Example 1, FIG. 2 showing the Nb.sub.3Sn obtained as per Example 2, and FIG. 3 showing the Nb.sub.3Sn obtained as per Example 3. It can clearly be seen on all micrographs that the powders according to the present invention have a porosity which was not observed in the case of conventional powders, as can be seen in FIGS. 4 and 5, which show scanning electron micrographs of Comparative Examples 1 and 2.

[0072] FIG. 6 shows a measurement of the pore size distribution of the niobium metal powder used for producing the powders according to the present invention, with the bimodal distribution of the pore sizes being readily visible.

[0073] FIG. 7 shows a measurement of the pore size distribution of the powder according to the present invention as per Example 1 with a maximum at a pore size of about 3 μm.

[0074] FIG. 8 shows a scanning electron micrograph of the niobium metal powder used for producing Comparative Examples 1 and 2.

[0075] The present invention is not limited to embodiments described herein; reference should be had to the appended claims.

TABLE-US-00001 TABLE 1 Phase Primary Particle Particle Particle BET O Composition Pore Particle Size Size Size Surface content X-ray from Rietveld Size Size D50 D90 D99 rea [% by Example Production Diffraction Analysis [μm] [μm] [μm] [μm] [μm] [m.sup.2/g] weight] Comparative Nb + 2 Sn Nb, NbSn.sub.2, Nb: 51% — — 26 77 98 0.3 1.29 Example 1 790° C./2 h Nb.sub.3Sn, NbSn.sub.2: 24% NbO Nb.sub.3Sn: 19% NbO: 6% Comparative 3 Nb + Sn Nb.sub.3Sn, Nb.sub.3Sn: 92% — — 39 65 85 0.25 1.42 Example 2 1050° C./6 h NbO, Nb, NbO: 3% NbSn.sub.2 Nb: 4% NbSn.sub.2: 1% Comparative Nb + 2 Sn NbSn.sub.2, Nb, NbSn.sub.2: 69% — — 19 34 49 0.29 2.9 Example 3 790° C./2 h Nb.sub.3Sn, Nb: 10% NbO Nb.sub.3Sn: 15% NbO: 6% Comparative 3 Nb + Sn Nb.sub.3Sn, Nb.sub.3Sn: 90% — — 28 39 69 0.21 3.2 Example 4 1050° C./6 h NbO, Nb NbO: 8% Nb: 2% Example 1 Nb + 2 Sn NbSn.sub.2, NbSn.sub.2: 97% 3 <5 150 277 344 1.7 1.2 790° C./2 h NbO NbO: 3% Example 2 3 Nb + Sn Nb.sub.3Sn, Nb.sub.3Sn: 96% 3 <8 156 267 360 1.4 1.3 1050° C./6 h NbO NbO: 4% Example 3 3 NbO.sub.2 + Nb.sub.3Sn, Nb.sub.3Sn: 93% 1.5 <2 122 234 299 4.2 1.59 6Mg.sub.(g) + Sn NbSn.sub.2 NbSn.sub.2: 3% 1050° C./6 h NbO NbO: 4%