Powders based on niobium tin compounds for the production of superconducting components

Abstract

Powder comprising Nb x Sn y with 1 ≤ x ≤ 6 and 1 ≤ y ≤ 5 for the production of superconducting components, characterized in that the powder has three-dimensional agglomerates with a particle size D90 of 220 to 400 µm, determined by laser diffraction, wherein the agglomerates are formed from primary particles having a mean particle diameter of 0.1 µm to 5 µm, determined by scanning electron microscopy, and the agglomerates have pores, of which 90% or more have a diameter of 0.1 to 20 µm, determined by mercury porosimetry.

Description

The present invention relates to powders based on niobium-tin compounds, in particular Nb x Sn y where 1≤x≤6 and 1≤y≤5, for the production of superconducting components, the powders being distinguished by a high porosity, to a process for the production thereof and to the use of such powders for the production of superconducting components.Superconductors refer to materials whose electrical resistance drops to zero when a specific temperature, the so-called transition temperature, is undershot. In the superconducting state, the interior of the material remains free from electric and magnetic fields and the current is transported losslessly. Superconductors are used, inter alia, for generating strong, constant magnetic fields or for producing low-loss transformers which, with the same power, have smaller dimensions and mass than conventional transformers and thus bring advantages above all in mobile operation.Superconductors can be classified into various categories such as metallic superconductors, ceramic superconductors and high temperature superconductors. At the latest since the discovery of the transition temperature of niobium tin (Nb 3 Sn) of 18.05 K, niobium and its alloys have come into focus as materials for the production of superconductors. Thus, superconducting cavity resonators made of niobium are used, for example, in particle accelerators (inter alia XFEL and FLASH at the DESY in Hamburg or CERN in Geneva).Superconducting wires which are used, inter alia, for the production of superconducting coils are of particular interest as superconducting components. For strong superconducting coils, as a rule, kilometre wires with conductor filaments / filaments only a few micrometers thick are required, which require complicated production processes.For the production of such wires, in particular based on niobium-tin alloys, recourse is made inter alia to the so-called bronze process, wherein a Cu-Sn alloy is used as starting material.Thus, EP 0 048 313 A1 describes superconducting wires based on bronze-Nb 3 Sn, which are applicable to high magnetic fields and which are characterized by a cubic phase in the bronze-Nb 3 Sn wire and which have stabilizing alloy components from the group LiBe Mg Sc Y U Ti Zr Hf V Ta Mo Re Fe Ru Ni Pd Zn Al Ga In TI Si Ge Sb in the weight percent range between 0.01 and 7, based on the Nb content, and / or between 0.05 and 10, based on the bronze content in the wire, which alloy components are based on bronze-Nb Sn and are characterized by a cubic phase in the bronze-Nb Sn wire.Alternatively, superconducting wires based on niobium-tin alloys can be produced by the so-called PIT (powder-in-tube) method, in which a powdered tin-containing starting compound is introduced into a niobium tube and then drawn out to form a wire. In a final step, a superconducting Nb 3 Sn interface is formed between the niobium-containing tube shell and the tin-containing powder filled therein by a heat treatment. With respect to the tin-containing starting compound, the phase composition, chemical purity and particle size, which must not be greater than the diameter of the finished filament, is decisive.DE 10 2008 048 614 A1 describes valve metal and valve metal oxide agglomerate powders which are suitable in particular for the production of solid electrolytic capacitors and which, after sintering to give highly porous sintered bodies, have a high skeletal density of above 88% of the theoretical.WO 00 / 67 936 A1 relates to a process for the production of metal powders from Ta and Nb, in which the corresponding metal oxides are contacted with a gaseous reducing agent.JP H08-92 668 A discloses a method for producing Nb 3 Sn superconductors, in which a Nb-Sn-based alloy powder having a higher content of Sn than that of Nb 3 Sn is mixed with a Nb powder, wherein the atomic ratio of Nb to Sn is 3:1. The mixed powder is then shaped and subjected to heat treatment.U.S. Pat. No. 3,325,888 A describes a method for producing a superconductor in which Nb powder and tin powder are mixed, pelletized and sintered in a weight ratio of 70-80% Nb and 30-20% Sn. The sintered pellets are reduced to a powder and a mixture of pulverized Nb and Zn is added to this powder. This mixture is introduced into a ductile metal shell and subjected to a heat treatment to obtain the superconductor.T. Wong et al describes a PIT process and the preparation of the tin-containing starting compound using the example of the NbSn 2( T. Wong et al, "Ti and Ta Addition to Nb 3 Sn by the Powder in Tube Process", IEEE Transactions on Applied Conductoritivity, Vol. 11, No. 1 (2001), 3584-3587). A disadvantage of the process is that a satisfactory reaction of niobium with tin to form NbSn 2 requires a multistage process comprising millings and thermal treatments of up to 48 hours. Furthermore, the general teaching is that the oxygen content should be as low as possible.U.S. Pat. No. 7,459,030 B2 describes a production method for a superconducting Nb 3 Sn wire according to the PIT process, in which a tantalum tin alloy powder is used as the starting compound. For the production of these, K 2 NbF 7 and K 2 TaF 7 are used, which are reduced to the respective niobium and tantalum metal before the reaction with tin. However, the method described has the disadvantages of some limitations for the use of these niobium and tantalum metals. Thus, only those with a maximum content of oxygen of less than 3000 ppm and hydrogen of less than 100 ppm can be used. Exceeding the oxygen content results in a lower quality of the finished wire. At hydrogen values above 100 ppm, safety-related problems arise in the process procedure, since the hydrogen escapes during the temperature treatment. Furthermore, the process described has the disadvantages that the target compound contains a high content of unreacted tin and the finished wire core also contains tantalum-containing compounds, which can have a negative effect on the superconducting properties of the wire. In addition, the reduction of the starting compounds K 2 NbF 7 and K 2 TaF 7 gives sparingly soluble metal fluorides such as, for example, MgF 2 or CaF 2 which cannot be completely separated off. Moreover, all fluorine-containing compounds in the process chain are very toxic.M. Lopez et al describe the synthesis of nano-intermetallic Nb 3 Sn by mechanical alloying and low temperature annealing (M. Lopez et al, "Synthesis of nano-intermetallic Nb 3 Sn by mechanical alloying and annealing at low temperature", Journal of Alloys and Compounds 612 (2014), 215-220). However, the process described has the disadvantage that impurities are introduced into the product as a result of the grinding.A. Godeke et al. provide an overview of the conventional PIT processes for the production of niobium tin superconductors (A. Godeke et al., "State of the art powder-in-tube niobium-tin superconductors", Cyrogenics 48 (2008), 308-316).All methods known in the prior art for producing superconducting wires from Nb 3 Sn or niobium-tin precursor powders necessary for their production, such as NbSn 2, for example, from the elements have the disadvantage that, for complete reaction of the niobium with tin, the latter must pass into the interior of the niobium particles via solid-state diffusion. This process is extremely slow and cannot be accelerated even by introduction of energy owing to the decomposition of NbSn 2 at temperatures above 840 DEG C. In particular, the production of NbSn 2 is difficult in practice. The known exothermicity of the reaction usually leads to the decomposition temperature of NbSn 2 being exceeded by 840° C. under the usual process conditions of 800 to 1200° C.Furthermore, the niobium-tin powders known in the prior art can only be ground up with difficulty, which leads to the introduction of impurities from the steel- or hard metal-containing grinding media used, such as iron, cobalt, nickel, silicon, tungsten or carbon, for example, and to products of inferior quality in wire production.A further disadvantage of known processes for the preparation of the niobium-tin powders is the use of very toxic halogen-containing compounds such as, for example, K 2 NbF 7 or NbCl 5. These serve as precursors for a reduction with a suitable reducing agent to give the niobium metal, which is reacted in a further step with tin to give the niobium-tin target compound.Furthermore, the known processes for the preparation of the niobium-tin powders have the disadvantage that a significant proportion of oxygen is transferred to the target compounds by oxygen introduction of the elements niobium and tin. Therefore, for example, the process according to U.S. Pat. No. 7,459,030 B2 is restricted to the use of niobium and tantalum metal powders having an oxygen content of not more than 3000 ppm and tin having an oxygen content of not more than 2000 ppm.The object of the present invention is therefore to provide suitable starting compounds for the production of superconducting components, in particular superconducting wires, which make it possible to shorten the long solid phase diffusion step and have improved grinding behavior, and a process for the production thereof which additionally makes do without the reduction of halogen-containing precursors.It has surprisingly been found that this object is achieved by a powder which has agglomerates having a specific porosity.A first subject of the present invention is therefore a powder for the production of superconducting components comprising Nb x Sn y with 1≤x≤6 and 1≤y≤5, wherein the powder has three-dimensional agglomerates having a size with a D90 value of 220 to 400 μm, determined by laser diffraction, the agglomerates are formed from primary particles which have an average particle diameter of 0.1 μm to 5 μm, determined by scanning electron microscopy, and the agglomerates have pores, 90% or more of which have a diameter of 0.1 to 20 μm, determined by mercury porosimetry.In a preferred embodiment, the agglomerates have pores of which 90% or more have a diameter of 0.2 to 15 μm, preferably 0.3 to 10 μm, determined by mercury porosimetry.In a preferred embodiment of the present invention, the agglomerates have a particle size distribution having a D50 value of 100 to 300 μm, preferably 150 to 220 μm, determined by laser diffraction.The D90 value and the D50 value, respectively, is the value indicating the percentage of agglomerates in the powder having a particle size less than or equal to the indicated size.In a preferred embodiment, the Nb x Sn y- compound is a compound selected from the group consisting of Nb 3 Sn, Nb 6 Sn 5, NbSn 2 and mixtures thereof.It has surprisingly been found that the required diffusion paths within the niobium solids in the powder according to the invention are significantly reduced on account of the small primary particle size and, owing to the particular pore structure, the tin can pass very well into the interior of the agglomerates during the reaction, as a result of which the production process can be significantly accelerated.It was also surprisingly found that the temperatures at which the powders are generally prepared, or the reaction times, could be significantly reduced. For example, NbSn 2, which is stable only up to a temperature of 840° C., could be produced at a temperature of less than 800° C. in less than 6 hours and even in less than 4 hours in the context of the present invention. For Nb 3 Sn, a time window of less than 6 hours was obtained at a temperature of 1050° C. or a time window of less than 8 hours was obtained at a temperature of 950° C.For the production of superconductors, it has proven particularly advantageous to use phase-pure powders in order to avoid influencing of the superconducting properties by any foreign phases. It has surprisingly been found that the powder according to the invention is furthermore distinguished by a high purity level, which is evident inter alia in that it comprises only a small proportion of phases of compounds other than the respective target compound. In a preferred embodiment, the powder according to the invention is therefore characterized in that the compounds Nb 3 Sn or Nb 6 Sn 5 or NbSn 2 each make up a proportion of greater than 90%, preferably greater than 94%, particularly preferably greater than 97%, based on all crystallographic phases detected and determined by Rietveld analysis of an X-ray diffractogram of the powder according to the invention.In a further preferred embodiment, the powder according to the invention is characterized in that the proportion of metallic tin in the powder is less than 3%, preferably less than 0.5%, based on all crystallographic phases detected and determined by Rietveld analysis of an X-ray diffractogram of the powder according to the invention. In a particularly preferred embodiment, the powder according to the invention is substantially free of metallic tin. In this context, "substantially free" is to be understood as meaning that the proportion of metallic tin is not detectable in an X-ray diffractogram.The presence of oxygen can lead to an impairment of the properties of the superconductor. Here, it has surprisingly been found that the powders according to the invention have no oxygen contents increased compared to the prior art despite the increased porosity. Therefore, an embodiment is preferred in which the powder has an oxygen content of less than 1.8% by weight, preferably 0.35 to 1.5% by weight, in particular 0.5 to 1.2% by weight, based on the total weight of the powder.The powder according to the invention preferably has a BET specific surface area of from 0.5 to 5 m 2 / g, preferably from 1 to 3 m 2 / g. The BET specific surface area can be determined here according to ASTM D3663.For the production of superconducting wires according to the PIT method it has proven advantageous to use powders with a particularly fine particle size. Therefore, the powders used are usually ground up before being introduced into the tube. Here, it has surprisingly been found that the powders according to the invention exhibit improved grinding behavior, as a result of which smaller particle sizes can be realized or alternatively more gentler grinding methods can be used. In a preferred embodiment, the powder according to the invention, after milling of the agglomerates, has a particle size with a D99 value of less than 15 μm, preferably less than 8 μm, particularly preferably from 1 μm to 6 μm, determined by means of laser diffraction. In a further preferred embodiment, the powder according to the invention, after milling of the agglomerates, has a D90 value of less than 10 μm, preferably less than 7 μm and particularly preferably less than 0.5 μm to 5 μm, in each case determined by means of laser diffraction.For the production of superconducting components with additive manufacturing methods, such as LBM (laser beam melting), EBM (electron beam melting) and / or LC (laser cladding), it has proven advantageous to use powders with a particular spherical particle shape. Here, it has surprisingly been shown that the powders according to the invention can be very easily atomized by known methods to give powders having spherical particles, for example by the EIGA method (electrode induction-melting gas atomization). In a preferred embodiment, at least 95% of all powder particles of the powder according to the invention therefore have a Feret diameter after atomization of from 0.7 to 1, particularly preferably from 0.8 to 1, wherein the Feret diameter is defined within the scope of the present invention as the smallest diameter divided by the largest diameter of a particle, determinable by evaluation of SEM images.The powder according to the invention is distinguished in particular by its high degree of purity. Therefore, an embodiment is preferred in which the proportion in the powder of all metallic elements is less than 500 ppm, preferably less than 300 ppm, particularly preferably from 1 ppm to 150 ppm, in each case based on the total weight of the powder. In particular, the metallic elements are those which are introduced via grinding processes. In the context of the present invention, ppm data are in each case based on proportions by mass.For the production of superconducting devices having acceptable properties, it is essential that the chemical purity of the powders used is high and impurities are introduced only in a controlled form as dopants. Substances unintentionally introduced into the process, in particular metallic impurities and fluoride-containing compounds, must be minimized. One of the main sources of these impurities is contamination from the niobium and tin metal powders used. In a preferred embodiment, the powder according to the invention has a fluorine content of less than 15 ppm, preferably less than 5 ppm. In a further preferred embodiment, the powder according to the invention has a content of random metallic impurities with the exception of tantalum of in total less than 0.8% by weight, preferably less than 0.5% by weight, particularly preferably less than 0.25% by weight, in each case based on the total weight of the powder.In a preferred embodiment, the powder according to the invention further contains dopants which, in contrast to the described substances unintentionally introduced in the process, are added to the powder according to the invention in a defined manner, as is known from the prior art. It has surprisingly been found that no particular requirements are to be placed on the dopants, but rather the common dopants known to the skilled person can be used.Some of the methods described in the prior art for producing superconducting wires based on Nb 3 Sn proceed from a tantalum tin alloy or from an intermetallic tin alloy based on tantalum and niobium as precursor powder. However, this has the disadvantage that residues of tantalum remain in the later core of the Nb 3 Sn wire filament, as a result of which, inter alia, the production costs increase and the superconducting properties of the products are impaired. A further disadvantage here is, in particular, the reduced thickness of the superconducting Nb 3 Sn boundary layer in the finished wire filament with the expected negative effects on the maximum current carrying capacity. In a preferred embodiment, the powder according to the invention is therefore substantially free of tantalum and tantalum compounds. In a particularly preferred embodiment, the proportion of tantalum and its compounds in the powder according to the invention is less than 1% by weight, preferably less than 0.5% by weight, particularly preferably less than 0.1% by weight, in each case based on the total weight of the powder.The powders according to the invention are distinguished by their high porosity at a small primary particle size, properties which are realized by means of a special production process. The present invention therefore also provides a process for the preparation of the powders according to the invention which makes it possible to realize precisely this combination of properties. The method according to the invention is distinguished in that the niobium metal powder is reacted with tin metal powder at a temperature of 300 to 1100° C., wherein the niobium metal powder is obtained by reduction of niobium oxide with a vaporous reducing agent. It has surprisingly been found that powders are obtained in this way which are suitable in particular for the production of superconducting wires by the advantageous combination of primary particle size, porosity and chemical purity. Suitable processes for reducing niobium oxide in the presence of a vaporous reducing agent are described, for example, in WO 00 / 67 936 A1.In order to make the process procedure efficient, it has proven advantageous to carry out the recovery and reaction of the niobium metal powder in a combined process step. Therefore, an embodiment of the method according to the invention is preferred in which the niobium metal powder is obtained in situ. This can be achieved in particular by reacting niobium oxide with the vaporous reducing agent in the presence of tin metal powder in a temperature treatment.It has surprisingly been found that structures having the desired porosity are accessible in this way. This is unexpected in particular since the skilled worker would not expect porous NbSn products in the reduction of NbSn mixed oxides and hydroxides with magnesium.The niobium oxide is preferably one selected from the group consisting of Nb 2 O 5, NbO 2, NbO, and mixtures thereof.It has surprisingly been found that a porosity of the powder which is advantageous for the production of superconducting wires can be achieved by a suitable selection of the starting compounds. Therefore, an embodiment is preferred in which the niobium metal powder has three-dimensional agglomerates having a size with a D90 value of 200 to 350 μm, determined by laser diffraction, the agglomerates are formed from primary particles which have an average particle diameter of 0.1 μm to 2 μm, determined by scanning electron microscopy, and the agglomerates have pores having a size of 0.1 to 20 μm, preferably 0.2 to 15 μm, particularly preferably 0.2 to 5 μm, determined by mercury porosimetry.The use of niobium metal powders having a bimodal pore size distribution has proven to be particularly positive. Therefore, an embodiment is preferred in which the niobium metal powder has a bimodal pore size distribution, preferably with a maximum in the range from 0.9 to 20 μm, particularly preferably 1 to 10 μm, and a maximum in the range from 0.1 to 0.9 μm, preferably in the range from 0.2 to 0.8 μm, determined by means of mercury porosimetry. Without being bound by any particular theory, it is believed that the bimodal distribution of the pore sizes enables improved penetration of the tin through the pores into the interior of the agglomerates.It has surprisingly been found that for the production of the powders according to the invention, a limitation of the oxygen content of the niobium metal powders, as described in the prior art, is not necessary. Nevertheless, it has proven advantageous if the oxygen content is not too high. Therefore, in a preferred embodiment, the niobium metal powder used for the production of the powders according to the invention has an oxygen content of less than 2.2% by weight, preferably of 0.35 to 1.9% by weight, particularly preferably of 0.4 to 1.6% by weight, based in each case on the total weight of the niobium metal powder. Furthermore, no limitation of the hydrogen content of these niobium metal powders to less than 100 ppm is required either. Thus, niobium metal powders having a hydrogen content of greater than 100 ppm, even of 110 to 400 ppm, can also be used in the process according to the invention. Therefore, an embodiment is preferred in which the niobium metal powder used for the production of the powders according to the invention has a hydrogen content of 110 to 400 ppm, based on the total weight of the niobium metal powder.In order to prevent the formation of undesirable and partly sparingly soluble Mg and Ca halides, such as, for example, MgF 2 and CaF 2, a niobium metal powder having the lowest possible fluorine and chlorine content is preferred. Therefore, niobium metal powders produced by metallothermal vapor reduction of niobium oxides are preferably produced in comparison with niobium metal powders produced by reduction of fluorine-containing compounds, such as K 2 NbF 7, or electrochemical reduction in a halide-containing salt melt. In a preferred embodiment, the niobium metal powder used comprises in each case less than 15 ppm of fluorine and chlorine, preferably in each case less than 10 ppm, particularly preferably in each case less than 2 ppm, based on the total weight of the niobium metal powder.To ensure the necessary chemical purity of the powders according to the invention, a preferred embodiment is the use of niobium metal powder with a sum over all metallic elements of less than 800 ppm, preferably of less than 500 ppm.Alkaline earth metal compounds have proven to be particularly advantageous as vaporous reducing agents. In a preferred embodiment, the vaporous reducing agent is magnesium vapor. It has been found that good wetting of the niobium oxide powder particles can be achieved in this way. In particular, magnesium as a reducing agent has the advantage that it can be removed easily and efficiently after the conversion to MgO, whereby unnecessary contamination of the products is avoided.The process according to the invention makes it possible to prepare the desired target compounds at lower temperatures and in a shorter time than has been known hitherto in the prior art. Therefore, the reaction of the metallic niobium powder with the metallic tin takes place at a reaction temperature in the range from 300 to 1100° C., preferably 600 to 1050° C., particularly preferably 650 to 790° C. The reaction time is preferably less than 8 hours, preferably 0.5 to 7 hours, particularly preferably 1 to 4 hours.As already described, dopants can be added to the powder, whereby the properties of the powder can be adapted. It has surprisingly been found to be advantageous if these dopants are already introduced at the very beginning of the method. Therefore, an embodiment is preferred in which the niobium oxide used for producing the niobium metal powder further comprises dopants. No particular requirements need be placed with regard to the dopants used. Rather, it has surprisingly been found that all customary dopants known to the person skilled in the art can be used.The powder according to the invention is suitable in particular for the production of superconducting components. Therefore, a further subject of the present invention is the use of the powder according to the invention for the production of superconducting components, in particular for the production of superconducting wires. The superconducting component is preferably produced by powder metallurgical processes or additive manufacturing processes or PIT processes. The superconducting wires are preferably produced by means of PIT methods.The present invention further provides for the use of the powder according to the invention in additive manufacturing processes. The additive manufacturing methods can be, for example, LBM (laser beam melting), EBM (electron beam melting) and / or LC (laser cladding).The present invention is illustrated in more detail by the following examples, these being by no means to be understood as limiting the concept of the invention.Examples:Niobium metal powder was obtained by reacting NbO 2 with magnesium vapor in a manner analogous to the production process described in WO 00 / 67936A1. The niobium metal powder obtained had an oxygen content of 8500 ppm, a hydrogen content of 230 ppm, a fluorine content of 2 ppm, metallic impurities of 310 ppm in total and an agglomerate size having a D50 value of 205 μm and a D90 value 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 then reacted with tin metal powder having an oxygen content of 6800 ppm under various conditions and the resulting products were analyzed.Comparative Experiments 1 and 2 used powders using conventional pore-free niobium metal powder having an oxygen content of 2900 ppm and a hydrogen content of 10 ppm. These comparative powders had a D90 value of 95 μm (see FIG. 8 ).For Comparative Experiments 3 and 4, a niobium metal powder was prepared by reducing K 2 NbF 7 with liquid sodium in a salt melt of KCl / KF at 850° C. and then removing all salts by washing. The niobium metal powder produced in this way contained irregularly shaped particles without pores having a D90 value of 29 μm, which were not sintered to form agglomerates. The niobium metal powder had an oxygen content of 2.4 wt%, a hydrogen content of 150 ppm, a fluorine content of 2500 ppm and metallic impurities of 10200 ppm in total, the main impurities being nickel and iron.In all the comparative experiments, the niobium metal powders were then reacted with tin metal powder having an oxygen content of 6800 ppm under various conditions and the products obtained were analyzed.The tin metal powder used in all experiments had a particle size of less than 150 μm.The results are summarized in Table 1, with the details of pore size and particle size relating in each case to the agglomerates. The particle size was determined in each case by means of laser diffraction (Master Sizer S, dispersion in water and Daxad11, ultrasonic treatment for 5 min). The size of the pores was determined by means of mercury porosimetry (Micromeritics company; AutoPore IV) and relates to the position of the maximum of the pore size distribution. The oxygen content was determined by hot carrier gas 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). Any figures in % by weight are based in each case on the total weight of the powder.X-ray diffraction is carried out on pulverulent samples using a Malvern-PANalytic instrument (X'Pert-MPD with semiconductor detector, X-ray tube Cu LFF with 40 KV / 40 mA, Ni filter).The powder of Example 3 was analyzed after washing with H 2 SO 4 and drying.The powders of the comparative examples did not form agglomerates and showed an inhomogeneous phase composition in the analyses. Furthermore, the powders of Comparative Experiments 3 and 4 are characterized by very high fluorine contents of 1300 to 1800 ppm and metallic impurities of in total 6500 to 9000 ppm and are therefore unsuitable for the production of superconducting components, in particular of wires.The powder from Example 1 was then milled in an oxygen-free atmosphere to achieve particle size distributions having a D90 value of 2.6 μm and a D99 value of 3.7 μm. The oxygen content after milling was 1.14 wt % and the difference of all added metallic impurities before and after the milling process was 140 ppm.In the context of the present invention, it has surprisingly been found that the increased porosity of the powders according to the invention is expected not to lead to an increase in the oxygen content, even if the powders are subjected to a further grinding step.Furthermore, the examples according to the invention show that no unreacted tin could be detected in the target compounds.FIGS. 1 to 3 show SEM images of the powders according to the invention, FIG. 1 showing the NbSn 2, FIG. 2 showing the Nb 3 Sn obtained according to Example 2 and FIG. 3 showing the Nb 3 Sn obtained according to Example 3. It can be clearly seen from all the photographs that the powders according to the invention have a porosity which was not observed in conventional powders, as can be seen in FIGS. 4 and 5 which show SEM photographs of comparative examples 1 and 2.FIG. 6 shows a measurement of the pore size distribution of the niobium metal powder used to produce the powders according to the invention, the bimodal distribution of the pore sizes being clearly evident.FIG. 7 shows a measurement of the pore size distribution of the powder according to the invention according to Example 1 with a maximum at a pore size of about 3 μm.FIG. 8 is an SEM image of the niobium metal powder used for producing Comparative Examples 1 and 2. Table 1: Table 1:See -1Nb+2Sn 790° C. / 2hNb, NbSn 2, Nb 3 Sn, NbONb: 51% NbSn 2: 24% Nb 3 Sn: 19% NbO: 6%--2677980,31,29See -23 Nb+Sn 1050 °C / 6hNb 3 Sn, NbO, Nb, NbSn 2Nb 3 Sn: 92% NbO: 3% Nb: 4% NbSn 2:1 %--3965850,251,42See -3Nb+2Sn 790° C. / 2hNbNb2, Nb, Nb3Nb, NbONbNb2: 69% Nb: 10% Nb3Nb: 15% NbO: 6%--1934490,292,9See -43 Nb+Sn 1050 °C / 6hNb3 Nb, NbO, NbNb3Nb: 90% NbO: 8% Nb: 2%--2839690,213,2Ex. 1Nb+2Sn 790° C. / 2hNbSn 2, NbONbNb 2: 97% NbO: 3%3<51502773441,71,2Ex. 23 Nb+Sn 1050 °C / 6hNb 3 Sn, NbONb 3 Sn: 96% NbO: 4%3<81562673601,41,3Ex. 33 NbO 2 + 6 Mg (g) + Sn 1050° C. / 6 hNb 3 Sn, NbSn 2, NbONb 3 Sn: 93% NbSn 2: 3% NbO: 4%1,5<21222342994,21,59

Claims

Powder comprising Nb x Sn y where 1 ≤ x ≤ 6 and 1 ≤ y ≤ 5 for the production of superconducting components, characterized in that the powder has three-dimensional agglomerates having a particle size D90 of 220 to 400 μm, determined by laser diffraction, the agglomerates being formed from primary particles which have an average particle diameter of 0.1 μm to 5 μm, determined by scanning electron microscopy, and the agglomerates having pores of which 90% or more have a diameter of 0.1 to 20 μm, determined by mercury porosimetry.Powder according to Claim 1, characterized in that the proportion of Nb 3 Sn or Nb 6 Sn 5 or NbSn 2 in the powder in each case makes up more than 90%, preferably more than 94%, particularly preferably more than 97%, based on all crystallographic phases detected and based on a Rietveld analysis of an X-ray powder diffractogram of the powder.Powder according to one or more of the preceding claims, characterized in that the powder has a BET specific surface area of 0.5 to 5 m 2 / g, preferably 1 to 3 m 2 / g.Powder according to one or more of the preceding claims, characterized in that the powder has an oxygen content of less than 1.8% by weight, preferably 0.35 to 1.5% by weight, particularly preferably 0.5 to 1.2% by weight, based on the total weight of the powder.Powder according to one or more of the preceding claims, characterized in that the powder, after milling of the agglomerates, has a particle size D99 of less than 15 μm, preferably less than 8 μm, particularly preferably from 1 μm to 6 μm, determined by means of laser diffraction.Powder according to one or more of the preceding claims, characterized in that the powder has a proportion of metallic tin of less than 3%, preferably less than 0.5%, based on all crystallographic phases detected and determined by Rietveld analysis of an X-ray diffractogram of the powder according to the invention.Powder according to one or more of the preceding claims, characterized in that 95% of all powder particles of the powder according to the invention after atomization have a Feret diameter of 0.7 to 1, particularly preferably of 0.8 to 1, wherein Feret diameter is defined as the smallest diameter divided by the largest diameter of a particle, determinable by evaluation of SEM images.Process for producing a powder according to one or more of Claims 1 to 7, characterized in that niobium metal powder is reacted with tin metal powder at a temperature of 300 to 1100°C, the niobium metal powder being obtained by reducing niobium oxide with a vaporous reducing agent.The method according to claim 8, characterized in that the niobium metal powder is obtained in situ.Method according to one or more of claims 8 and 9, characterized in that the niobium metal powder has pores in a size of 0.1 to 20 μm, preferably 0.2 to 15 μm, determined by means of mercury porosimetry.Method according to one or more of Claims 8 to 10, characterized in that the niobium metal powder has three-dimensional agglomerates having a size with a D90 value of less than 400 μm, preferably of 200 to 350 μm, determined by means of laser diffraction, and the agglomerates are formed from primary particles which have an average particle diameter of less than 10 μm, preferably of less than 4 μm, particularly preferably of 0.1 μm to 2 μm, determined by means of scanning electron microscopyProcess according to one or more of Claims 8 to 11, characterized in that the niobium metal powder has a bimodal pore size distribution.The method according to one or more of claims 8 to 12, characterized in that the niobium oxide is a compound selected from the group consisting of Nb 2 O 5, NbO 2, NbO and mixtures thereof.Method according to one or more of Claims 8 to 13, characterized in that the vaporous reducing agent is magnesium vapour.Method according to one or more of claims 8 to 14, characterized in that the niobium metal further comprises dopants.Use of a powder according to one or more of Claims 1 to 7 for producing superconducting components, in particular for producing superconducting wires and coils.Use according to Claim 16, characterized in that the superconducting component is produced by powder-metallurgical processes, in particular PIT processes, or additive manufacturing processes.Use of a powder according to one or more of Claims 1 to 7 in additive manufacturing processes, in particular LBM (laser beam melting), EBM (electron beam melting) and / or LC (laser cladding), or powder metallurgical processes, in particular PIT processes.

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