PROCESS FOR PREPARATION OF THE INTERMETALLIC COMPOUND Nb3Sn BY MELT METALLURGICAL PROCEDURE

Abstract

The invention relates to a process for preparation of the intermetallic compound Nb.sub.3Sn by melt metallurgical procedure. The process comprises the steps of pressing Nb particles and Sn particles to form a start electrode, whereby the pressed start electrode is remelted in a vacuum in an electric arc, whereby a first moulded body is obtained. Alternatively, the process comprises the steps of remelting an Nb start electrode in an electric arc in a vacuum, whereby Sn particles are being introduced into the molten Nb forming during the remelting, whereby a first moulded body is obtained. The molar ratio of Nb and Sn is selected appropriately such that the first moulded body obtained contains at least 50% by weight of the intermetallic compound Nb.sub.3Sn as the A15 phase as well as free Nb and/or Sn and inevitable impurities, if applicable.

Claims

1. A process for preparation of the intermetallic compound Nb.sub.3Sn by a melt metallurgical procedure, the process comprising: i. Pressing Nb particles and Sn particles to form a start electrode, whereby the pressed start electrode is remelted in a vacuum in an electric arc, whereby a first moulded body is obtained or ii. Re-melting an Nb start electrode in a vacuum in an electric arc, whereby Sn particles are introduced into the molten Nb that forms in the process during the reforming, whereby a first moulded body is obtained, whereby the molar ratio of Nb and Sn is selected appropriately such that the first moulded body obtained contains at least 50% by weight of the intermetallic compound Nb.sub.3Sn as A15 phase and, if applicable, free Nb and/or Sn and inevitable impurities.

2. The process of claim 1, further comprising: a. Mechanical disintegration of the first moulded body to form particles; b. Mixing of the particles obtained in step a. with Nb particles and/or Sn particles and pressing the mixture to form a second moulded body, whereby the fraction, in the second moulded body, of the particles obtained in step a. is at most 80% by weight; c. Remelting the second moulded body in an electric arc in a vacuum, whereby a third moulded body is obtained; whereby the molar ratio of Nb and Sn is selected appropriately such that the third moulded body obtained contains at least 70% by weight of the intermetallic compound Nb.sub.3Sn as the A15 phase and, if applicable, free Nb and/or Sn and inevitable impurities.

3. The process of claim 1, wherein, according to step ii., the particles are introduced successively into the forming molten Nb material.

4. The process of claim 2, wherein the particles obtained from the first moulded body comprise a mean cross-sectional area that is at most one fourth of the cross-sectional area of the second moulded body.

5. The process of claim 1, wherein the fraction of Sn particles in the total amount before the remelting to form the first moulded body is 30.0% by weight to 33.0% by weight.

6. The process of claim 1, wherein the fraction of Sn in the second moulded body before the remelting to form the third moulded body is 30.0% by weight to 33.0% by weight.

7. The process of claim 1, wherein the vacuum during the remelting comprises a partial pressure of no more than 500 mbar.

8. The process of claim 1, wherein the remelting in an electric arc in a vacuum is carried out in a protective gas that is inert with respect to Nb and Sn.

9. The process of claim 8, wherein a noble gas is used as the protective gas.

10. The process of claim 9, wherein helium is used as the protective gas.

11. The process of claim 1, wherein the molar ratio of Nb and Sn is selected appropriately such that the first moulded body and/or the third moulded body contains 1 to 5% by weight of free Sn, whereas the remainder of the moulded body essentially comprises Nb.sub.3Sn in the A15 phase.

12. The process of claim 1, wherein the first and/or third moulded body is/are taken up into a container, which is provided with an electrically insulating layer on its inside, during the remelting.

13. The process of claim 12, wherein the electrically insulating layer is a metal oxide layer.

14. An intermetallic niobium-tin compound, obtainable through a process according to claim 1.

15. An intermetallic niobium-tin compound according to claim 14, wherein the fraction of A15 phase is at least 95% by weight.

16. An intermetallic niobium-tin compound according to claim 14, wherein the fraction of Nb.sub.6Sn.sub.5 in the composition is no more than 1% by weight.

17. An intermetallic niobium-tin compound according to claim 14, wherein the fraction of NbSn.sub.2 in the composition is no more than 1% by weight.

Description

EXEMPLARY EMBODIMENTS

[0074] Exemplary embodiments of the invention shall be illustrated in the following on the basis of one schematic figure and two diagrams, though without limiting the scope of the invention. In the figures,

[0075] FIG. 1 shows a schematic depiction of a first alternative of the process according to the invention in the form of a flow diagram;

[0076] FIG. 2 shows a schematic depiction of a second alternative of the process according to the invention in the form of a flow diagram;

[0077] FIG. 3 shows an x-ray diffractometric analysis of the product according to the process based on a batch with a stoichiometric composition;

[0078] FIG. 4 shows an x-ray diffractometric analysis of the product according to the process based on a batch with an excess of tin.

[0079] FIG. 1 shows a schematic depiction of a first alternative of the process according to the invention in the form of a flow diagram. Initially, in a first step 101, Nb particles and Sn particles are pressed to form a start electrode. For example, Nb chips and tin granules (metal grains) can be used in this context. In a subsequent step 102, the start electrode is remelted in an electric arc in a vacuum as a consumable electrode, whereby a first moulded body is obtained as first product after the molten material has cooled. Said first moulded body already comprises Nb.sub.3Sn as the main phase (A15 phase). To improve the purity of the product even more, the first moulded body can be mechanically disintegrated to form particles in a further process step 103. The Nb.sub.3Sn particles thus obtained are subsequently mixed with Nb particles and Sn particles in a step 104 and are pressed to form a second moulded body. The Nb.sub.3Sn particles thus obtained are remelted again in an electric arc in a vacuum in a step 105, whereby a third moulded body made of Nb.sub.3Sn is obtained.

[0080] FIG. 2 shows a schematic depiction of a second alternative of the process according to the invention in the form of a flow diagram. In a first step 201, an ingot made of Nb is provided as consumable electrode. In a subsequent step 202, the electrode is remelted in an electric arc in a vacuum, whereby Sn particles are being trickled into the forming molten material during this remelting process. For example, Sn granules (metal grains) can be used for the trickling. The product of remelting, the first moulded body, already comprises Nb.sub.3Sn as the main phase (A15 phase). To further improve the purity of the product, the first moulded body can be mechanically disintegrated to form particles in a further process step 203. The Nb.sub.3Sn particles thus obtained are subsequently mixed with Nb particles and Sn particles in a step 204 and are pressed to form a second moulded body. The Nb.sub.3Sn particles thus obtained are remelted again in an electric arc in a vacuum in a step 205, whereby a third moulded body made of Nb.sub.3Sn is obtained.

Example 1

[0081] The production according to the first alternative of the process according to the invention through the use of a single remelting process is illustrated in the following based on a concrete exemplary embodiment.

[0082] Nb chips and Sn granules were used initially to produce a pressed electrode with dimensions of 3532480 mm in a rectangular die using a pressing force of 500 t. The weight of the electrode thus obtained was 3.74 kg. A total of 100 g of Nb.sub.3Sn particles were used as bottom protection in the vacuum-arc remelting process. The electrode was remelted in an electric arc furnace as a consumable electrode in a water-cooled copper crucible with an internal diameter of 80 mm. In order to minimise the evaporation of Sn, the partial pressure was 200 mbar. The copper crucible was insulated on the inside, in order to prevent lateral electric arc discharges. Argon was used as the partial pressure gas. There was a slight formation of plasma during the melting, during which the melting power was 70 kW (2.0 kA/35 V). The weight of the ingot produced by melting, i.e. of the first moulded body, was 3.79 kg. Accordingly, the total loss of Sn was 50 g.

[0083] In an improved batch, helium rather than argon was used as the partial pressure gas.

[0084] A pressed electrode made of Nb and Sn with a total weight of 3.77 kg was used in the batch. In this case, a first moulded body with a total weight of 3.80 kg was obtained. The Sn loss was 70 g. Due to the use of helium as the partial pressure gas, the formation of a plasma was prevented, i.e. no formation of plasma was observed during the remelting process.

Example 2

[0085] The production according to the second alternative of the process according to the invention through the use of a single remelting process is illustrated in the following based on a concrete exemplary embodiment.

[0086] A niobium rolled section rod with a width across flaps of 32 mm and a length of 480 mm was remelted as a consumable electrode in an electric arc furnace. The weight of the niobium electrode was 3.57 kg. A total of 100 g of Nb.sub.3Sn were used as bottom protection. To generate the intermetallic composition Nb.sub.3Sn during the melting, 1.64 kg tin granules (metal grains) were semi-continuously trickled into the molten material. After each 20 mm of travel of the electrode, 68.3 g of tin granules were added into the molten niobium using a rotary disk conveyor. The partial pressure during the melting and trickling process was 200 mbar, whereby helium was used as the partial pressure gas. The use of a partial pressure of 200 mbar and helium again served to minimise the evaporation of Sn and the formation of a plasma. In order to prevent lateral electric arc discharges, an inside-cooled copper crucible and a constantly rotating stirring field were used. The melting power was 70 kW (2.0 kA/35 V). The weight of the ingot produced by melting, i.e. of the first moulded body, was 5.14 kg. The Sn loss was 70 g.

Example 3

[0087] In the following, the further processing of the moulded body produced according to the first alternative (Example 1) or second alternative (Example 2) of the process according to the invention through the use of a second remelting process is illustrated in more detail.

[0088] The moulded body obtained according to either one of the aforementioned batches was extremely brittle and was therefore easy to break mechanically. Accordingly, a jaw crusher was used to produce particles with a maximum dimension of 5 mm from the first moulded body. For homogenisation, the particles that could be pressed to form a second moulded body were mixed by hand in a metal tub.

[0089] Subsequently a pressed electrode, i.e. the second moulded body, with dimensions of D35480 mm (D=variable thickness) was produced with a pressing force of 500 t to produce highly pure Nb.sub.3Sn in the A15 phase by melting. The weight of the pressed electrode was 5000 g.

[0090] Depending on whether or not a stoichiometric or over-stoichiometric (Sn excess) composition was used, the pressed electrode or the second moulded body had the following composition:

TABLE-US-00001 Fraction of particles from Fraction Fraction first moulded of Nb of Sn Ratio of body chips granules Batch no. Nb:Sn (wt. %) (wt. %) (wt. %) 1 stoichiometric 50 35 15 2 Sn excess 50 33 17

[0091] The pressed electrodes or second moulded body according to batch no. 1 or 2 were produced by melting in an electric arc furnace as the consumable electrode. The partial pressure during the melting process was 200 mbar, which, as before, served to minimise the evaporation of Sn. Helium was used as the partial pressure gas in order to prevent the formation of plasma, as before. An inside-insulated copper crucible was used in order to prevent lateral electric arc discharge. Moreover, a constantly rotating stirring field was used to further minimise lateral electric arc discharges. The melting power was 70 kW (2.0 kA/35 V).

[0092] The weight of the ingot produced by melting, i.e. of the third moulded body, according to batch 1 was 4.91 kg. The weight loss of tin was 90 g.

[0093] The weight of the ingot produced by melting, i.e. of the third moulded body, according to batch 2 was 4.93 kg. The weight loss of tin was 87 g.

[0094] Random fragments of the moulded body obtained were used for preparation of the subsequent analyses. One fragment was machined such to be planar in one place for x-ray diffractometric analysis. The fragment selected for chemical analysis was mechanically disintegrated.

[0095] FIG. 3 shows an x-ray diffraction analysis (X-ray diffractionXRD) of the ingot or third moulded body according to batch 1. The vertical lines along the x-axis show reference signals, i.e. the expected positions of the signals for Nb.sub.3Sn in the A15 phase (solid lines), NbSn.sub.2 (dotted lines) or free Sn (dashed lines). The signal intensity of the individual component of the sample is expressed by the peaks at the corresponding reference positions. It is evident in this context that the sample according to batch 1 essentially contains peaks in the area of the solid reference lines, i.e. peaks to be associated with Nb.sub.3Sn in the A15 phase. The peaks for NbSn.sub.2 and free Sn are very weak and/or hardly detectable in the noise of the individual signals.

[0096] FIG. 4 shows an x-ray diffraction analysis (X-ray diffractionXRD) of the ingot or third moulded body according to batch 2, in which an excess of Sn was used. As before, the signals and/or peaks of Nb.sub.3Sn in the A15 phase predominate, whereas the peaks to be assigned to pure Sn and NbSn.sub.2 are rather weak.

[0097] Accordingly, the x-ray diffractometric analyses demonstrate that the process according to the invention is well-suited for preparation of pure Nb.sub.3Sn in A15 phase.

[0098] The following table shows the composition of the products obtained according to Example 1 and Example 3 according to a chemical analysis. For Example 3, the two batches according to batch 1 and batch 2 are shown.

TABLE-US-00002 Example no. Nb (wt. %) Sn (wt. %) Example 1 71.1 28.8 Example 3, batch 70.6 29.3 1 (stoichiometric) Example 3, batch 69.0 30.9 2 (Sn excess)

[0099] Ideally, pure Nb.sub.3Sn has a content of 70.1% by weight and an Sn content of 29.9% by weight.