Patent Application
20090011941
The present invention relates to Nb3Sn superconducting wires by a powder process and precursors of the wires. In particular, the present invention relates to a Nb3Sn superconducting wire produced by a powder process, the wire being useful as a component of superconducting magnets used for generating high magnetic fields, and a precursor of the wire.
Among the fields in which superconducting wires are practically used, with respect to superconducting magnets used in high-resolution nuclear magnetic resonance (NMR) spectrometers, higher magnetic fields generated by magnets result in higher resolution. Accordingly, in recent years, there have been advances in the increase in magnetic field of superconducting magnets.
For example, Nb3Sn wires have been practically used as superconducting wires for use in superconducting magnets capable of generating high magnetic fields. Nb3Sn wires are mainly manufactured by the bronze process. In the bronze process, Nb-based cores are embedded in a Cu—Sn-based alloy (bronze) matrix and drawn into filaments (hereinafter referred to as “Nb-based filaments”). The filaments are bundled to make a filament bundle. The filament bundle is embedded in copper for stabilization (stabilizing copper) and subjected to drawing. The resulting bundle is subjected to heat treatment (diffusion heat treatment) at 600° C. to 800° C., thereby forming a Nb3Sn phase at each interface between a corresponding one of the Nb-based filaments and the matrix. However, the bronze process is disadvantageous in that the solid solubility of Sn in bronze has a limit (15.8% by mass or less), thereby resulting in a small amount of Nb3Sn phase. Furthermore, the crystallinity of Nb3Sn is degraded, thus resulting in poor properties in high magnetic fields.
In addition to the bronze process, an internal diffusion process is also known as a method for manufacturing a Nb3Sn superconducting wire. In this internal diffusion process, a Sn core is embedded in the middle of a Cu matrix. A plurality of Nb wires are arranged in the Cu matrix around the Sn core, subjected to diameter reduction, and heat treatment. Thus, Sn is diffused and allowed to react with Nb to form Nb3Sn (for example, Patent Document 1). This process has no limit of the Sn concentration, unlike the bronze process, which limits the Sn concentration due to the solid solubility limit. Accordingly, the Sn concentration can be set as high as possible to improve the superconducting properties of the resulting wire. However, the internal diffusion process has the following disadvantages: the Sn core is in direct contact with the Cu matrix, thereby easily forming a brittle Cu—Sn compound. Thus, annealing when being working is not applied. This results in a working limit, i.e., high deformation is difficult to perform.
Another known example of a method for producing a Nb3Sn superconducting wire is a powder process. For example, Patent Document 2 discloses subjecting Sn and at least one metal (alloy element) selected from the group consisting of Ti, Zr, Hf, V, and Ta to a melt-diffusion reaction at a high temperature to form an alloy or intermetallic compound (hereinafter, also referred to as a “Sn compound”), pulverizing the resulting Sn compound to form a Sn compound material powder, filling a sheath composed of Nb or a Nb-based alloy with the powder as a core (powder core described below), subjecting the sheath to diameter reduction, and heat treatment (diffusion heat treatment). Unlike the bronze process, in this process, there is no limitation of the amount of Sn. Furthermore, a Sn portion is not in direct contact with a Cu portion, making it possible to perform annealing when being working and thus high deformation. Moreover, it is possible to form a high-quality Nb3Sn layer. Thus, a superconducting wire can be obtained with excellent high-magnetic-field properties.
In the powder process, the powder core portion 2 is filled with a material powder containing at least Sn and subjected to diameter reduction, e.g., extrusion or wire drawing, to form a primary composite wire (precursor of a superconducting wire). The resulting wire is wound to form a coil. The coil is subjected to heat treatment to form a Nb3Sn superconducting phase at the interface between the sheath and the material powder.
To form a superconducting phase in such a Nb—Sn binary system, it is necessary to perform heat treatment at a high temperature of at least about 900° C. to about 1,000° C. At such a high temperature, a large heat treatment furnace is required. Furthermore, in the case where the wire is used as a high-magnetic-field superconducting magnet, the superconducting wire is closely wound to form a solenoid and then subjected to heat treatment. To prevent electrical short circuits, an insulator made of glass fibers is arranged on the periphery of the wire. However, the heat treatment at a high temperature disadvantageously causes embrittlement of the insulator made of glass fibers.
It is known that the addition of Cu to the material powder allows the reaction to proceed even at a heat treatment temperature of about 600° C. to about 750° C. Therefore, in the powder process, it is common to add an appropriate amount of a Cu powder to a material powder and then perform heat treatment for forming an intermetallic compound.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 49-114389
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 11-250749
As described above, it is known that the addition of Cu to the material powder reduces the heat treatment temperature to about 600° C. to about 750° C. In the case where the heat treatment is performed at a temperature within the range above, disadvantageously, the diffusion rate of Sn into Nb or a Nb-based alloy (hereinafter, also referred to collectively as a “Nb-based metal”) is very low. In the powder process including adding a Cu powder to a material powder, with respect to the reaction for forming Nb3Sn, the diffusion of Cu into the Nb-based metal and the diffusion of Sn into the Nb-based metal are both required in order that the reaction of Sn proceeds effectively. Thus, even when the powder process having no limit of the solid solubility of Sn is employed, the reaction is limited to diffusion rates of Sn and Cu. The advantage that a large amount of Sn is contained is not sufficiently taken of the process. In many cases, adequate reaction efficiency is not obtained because unreacted Sn is present in the powder core.
The present invention has been accomplished under the foregoing circumstances. It is an object of the present invention to provide a Nb3Sn superconducting wire having excellent superconducting properties, the wire being produced by a powder process, and a precursor thereof capable of increasing the efficiency of the formation reaction of Nb3Sn even in a relatively low practical temperature range of about 600° C. to about 750° C.
According to an aspect of the present invention, a precursor achieving the foregoing object of the present invention is a precursor of a Nb3Sn superconducting wire produced by a powder process including filling a sheath containing at least Nb with a material powder containing at least Sn, subjecting the resulting sheath filled with the powder to diameter reduction to form a wire, and subjecting the resulting wire to heat treatment to form a superconducting layer at the interface between the sheath and the powder, in which the material powder contains a Cu component, and the sheath has a structure in which a Nb or Nb-based-alloy portion is combined with a Cu or Cu-based-alloy portion.
With respect to the precursor of the present invention, preferred requirements are as follows: (A) the mass ratio of the Nb or Nb-based-alloy portion (hereinafter, also referred to, collectively, as a “Nb-based-metal portion”) to the Cu or Cu-based-alloy portion (hereinafter, also referred to, collectively, as a “Cu-based-metal portion”) in the sheath, i.e., Nb-based-metal portion:Cu-based-metal portion, is 50:1 to 5:1 (mass ratio); (B) the sheath is configured in such a manner that the material powder is not in contact with the Cu-based-metal portion before heat treatment; and (C) the precursor includes a Sn diffusion barrier layer on the outer periphery of the sheath, the Sn-diffusion-barrier layer being composed of Nb or Ta.
The material powder used in the present invention preferably has a Cu component content of 2 to 15 percent by mass with respect to the whole of the material powder. Another preferred embodiment of the material powder is a mixture of an alloy powder composed of Sn and at least one metal selected from the group consisting of Ti, Zr, Hf, V, and Ta, or an intermetallic-compound powder composed of Sn and at least one metal selected from the group consisting of Ti, Zr, Hf, V, and Ta, a Sn powder, and a Cu powder.
In the case where a superconducting wire is produced using the precursor of the present invention, a single-core wire including a Cu coating portion that covers the outer periphery of the sheath is subjected to wiredrawing to form a primary composite wire, and the resulting primary composite wire is subjected to heat treatment to form a single-core superconducting wire. Furthermore, a single-core wire including a Cu coating portion that covers the outer periphery of the sheath is subjected to wiredrawing to form a primary composite wire, a plurality of primary composite wires are inserted into a copper billet to form a multicore composite wire, and the multicore composite wire is subjected to wiredrawing and then heat treatment to form a multicore Nb3Sn superconducting wire.
In the present invention, the use of the sheath having the structure in which the Nb or Nb-based-alloy portion is combined with the Cu or Cu-based-alloy portion facilitates the formation reaction of a Nb3Sn phase. Furthermore, the Cu or Cu-based-alloy portion serves as a bypass for the diffusion of Sn, thereby increasing the diffusion rate of Sn. Thus, the amount of Sn remaining in the core is minimized even at a heat treatment temperature of about 600° C. to about 750° C., thereby uniformly forming the Nb3Sn superconducting phase with sufficient reaction efficiency. This results in a Nb3Sn superconducting wire having a high critical current density.
To achieve the foregoing object, the inventors have conducted intensive studies from various angles and have found that in the case where a Nb3Sn superconducting wire is produced by a powder process, the use of a sheath constituted by a combination of a Nb-based-metal portion and a Cu-based-metal portion achieves the object. This finding has resulted in the completion of the present invention.
The structure of the present invention will be described below with reference to the drawings.
Even if any of the structures shown in
To provide the effect, preferably, the ratio of the Nb-based-metal portion to the Cu-based-metal portion in the sheath is appropriately adjusted. From such a viewpoint, the Cu-based-metal portion is preferably arranged in such a manner that the ratio of the Nb-based-metal portion to the Cu-based-metal portion is 50:1 or more (ratio by mass). When the Cu-based-metal-portion content is very high, Cu acts as an impurity in Nb3Sn formed and is liable to cause a reduction in superconducting properties. Furthermore, the effective area of Nb3Sn is reduced; hence, the ratio should be 5:1 or less. Preferably, the Nb-based alloy constituting the sheath of the present invention has a Nb content of 90% by mass or more and contains alloy elements, such as Ta and Ti, in an amount of 10% by mass or less. The Cu-based alloy combined with the Nb or Nb-based alloy in the sheath has a Cu content of 90% by mass. It is possible to use the Cu-based alloy containing alloy elements, such as Pb, Fe, Zn, Al, Mn, and P, in an amount of 10% by mass or less as long as the processability of the superconducting wire is not impaired.
On the other hand, a material powder used in the powder process contains at least Sn. In a structure in which Sn is in direct contact with Cu, a brittle Cu—Sn compound may be formed by heat treatment to degrade wire-drawing processability. Thus, as shown in
After the outer periphery of the single-core wire shown in each of
In any process, a Cu coating layer is formed on the outer periphery of the sheath. In the case of the sheath including the Cu portion described above, the extremely rapid diffusion of Sn may result in the penetration of Sn into the Cu coating layer to contaminate the Cu coating layer. To eliminate such a disadvantage, as shown in
The material powder used in the present invention needs to contain at least Sn serving as a component constituting the Nb3Sn phase. To allow the formation reaction of Nb3Sn to efficiently proceed even when diffusion heat treatment is performed at a relatively low temperature (600° C. to 750° C.), the material powder needs to contain a Cu component. To provide the effect, the material powder preferably has a Cu-component content of 2% by mass or more. An excessively large Cu-component content may result in an increase in impurity content to reduce the superconducting properties in the same way as the Cu-based-metal portion in the sheath. Thus, the material powder should have a Cu-component content of 15% by mass or less.
With respect to the material powder, it is known that the incorporation of Sn and at least one metal (alloy element) selected from the group consisting of Ti, Zr, Hf, V, and Ta can result in a small amount of a solid solution in a reaction layer during the formation of Nb3Sn to improve superconducting properties. In the case where such a process is employed, powders of Sn, at least one metal selected from the group consisting of Ti, Zr, Hf, V, and Ta, and Cu are appropriately weighed and mixed. The resulting mixture is subjected to heat treatment, followed by pulverization. However, when the powder process is performed according to the procedure, a very hard Cu—Sn compound is simultaneously formed during the heat treatment. The presence of the Cu—Sn compound causes the abnormal deformation in the course of diameter reduction and, at worst, induces a break in the wire.
The inventors, therefore, have conducted intensive studies on a material powder eliminating the disadvantage and capable of forming a Nb3Sn superconducting wire with satisfactory superconducting properties and have conceived that during a melt-diffusion reaction, the total amount of Sn as a material is not allowed to react and that the minimum amount of Sn necessary to alloy the alloy element, such as Ti, Zr, Hf, V, or Ta, may be allowed to react. Furthermore, the inventors have conceived that with respect to Cu, Cu is not added during the melt-diffusion reaction and that Cu is added to the material powder after the reaction in order to effectively provide the effect of the addition of Cu on a reduction in heat treatment temperature. Then the inventors have found that a mixture of an alloy powder composed of Sn and at least one metal selected from the group consisting of Ti, Zr, Hf, V, and Ta, or an intermetallic compound powder composed of Sn and at least one metal selected from the group consisting of Ti, Zr, Hf, V, and Ta (hereinafter, referred to as a “Sn compound powder”), a Sn powder, and a Cu powder eliminates the foregoing disadvantage and provides satisfactory superconducting properties.
As the material powder used in the present invention, the above-described material powder may be used. In the material powder, the Cu powder is added after the Sn compound powder is prepared. Thus, the material powder provides a wire without the formation of a hard Sn—Cu compound during the formation reaction of the Sn compound (melt-diffusion reaction) and minimizes abnormal deformation and the occurrence of a break in the wire during the processing of the wire.
The Sn compound powder described above is prepared by subjecting an alloy element, such as Ti, Zr, Hf, V, or Ta, and Sn to the melt-diffusion reaction. The mixing ratio of the alloy element to Sn is not particularly limited. The mixing ratio of the alloy element to Sn, i.e., alloy element:Sn, is preferably about 4:1 to about 1:2 (atomic ratio) from the viewpoint of achieving good superconducting properties.
The material powder is prepared by forming the Sn compound, pulverizing the compound to form the Sn compound powder, adding the Sn powder and the Cu powder to the resulting Sn compound powder, and mixing the resulting mixture. With respect to the mixing ratio in the material powder, the Sn powder is in the range of 15 to 90 parts by mass, and the Cu powder is in the range of 1 to 20 parts by mass relative to 100 parts by mass of the Sn compound powder. However, the Cu content is preferably in the range of 2 to 15 percent by mass and more preferably 2 to 5 percent by mass on the basis of the foregoing purport.
In the case of using the material powder, when the mixing ratio of the Sn powder is less than 15 parts by mass, it is difficult to provide the effect of the addition of Sn on improvement in superconducting property. When the mixing ratio of the Sn powder exceeds 90 parts by mass, the amount of the alloy element in the material powder is relatively reduced, causing elution of Sn due to heat generated during extrusion. When the mixing ratio of the Cu powder is less than 1 part by mass, the effect of the addition of Cu on a reduction in heat treatment temperature (temperature in diffusion heat treatment) is not provided. When the mixing ratio of the Cu powder exceeds 20 parts by mass, a large amount of a hard Cu—Sn compound is formed in the core during annealing, thereby degrading the processability of the wire and causing frequent breaks in the wire.
In the case of filling the sheath with the material powder, uniaxial pressing is usually employed. Instead of such a process, compacting treatment with isotropic pressure, e.g., cold isostatic pressing (CIP), is performed to increase the filling rate of the material powder and is preferred in order to perform uniform processing. For example, in the structure shown in
While the present invention will be described in more specific by examples, the following examples are not limited to the present invention. Design changes in accordance with the purport described above and below are included within the technical range of the present invention. For example, in these examples described below, single-core superconducting wires are exemplified. Alternatively, the present invention is also applicable to a superconducting wire including a multicore composite wire in which a plurality of single cores are arranged in a Cu matrix.
Ta and Sn powders were weighed with an electronic balance in an Ar gas atmosphere in such a manner that Ta:Sn=6:5 (atomic ratio). The powders were mixed in a V-blender for 30 minutes. The resulting powder mixture was subjected to heat treatment at 950° C. for 10 hours in vacuum to form a Ta—Sn compound.
The resulting Ta—Sn compound was roughly crushed and then pulverized for 1 hour in an Ar atmosphere with an automatic mortar to form the Ta—Sn compound powder having a particle diameter of 75 μm or less. To the Ta—Sn compound powder, 25% by mass of a Sn powder and 5% by mass of a Cu powder were added. The mixture was mixed to form a material powder (Sn-based powder).
On the other hand, members (A) to (E) described below were stacked in sequence to form a composite sheath (see
(A) Pipe, composed of a Nb alloy containing 7.5% by mass Ta, having an outer diameter of 35 mm and an inner diameter of 30 mm
(B) Cu pipe having an outer diameter of 37 mm and an inner diameter of 35 mm
(C) Pipe, composed of a Nb alloy containing 7.5% by mass Ta, having an outer diameter of 42 mm and an inner diameter of 37 mm
(D) Cu pipe having an outer diameter of 44 mm and an inner diameter of 42 mm
(E) Pipe, composed of a Nb alloy containing 7.5% by mass Ta, having an outer diameter of 55 mm and an inner diameter of 44 mm
The extrusion billet having the structure described above was extruded with a hydrostatic extruder and then formed into a wire having a diameter of 1.0 mm by wiredrawing with dies. In this billet, the mass ratio of Nb—Ta to Cu, i.e., Nb—Ta:Cu, was 5.7:1.
The wire was subjected to heat treatment at 700° C. for 100 hours in vacuum in order to form Nb3Sn. After the heat treatment, the critical current (Ic) of the resulting wire was measured while an external magnetic field from a superconducting magnet was applied to the wire. The critical current density of a non-copper area (nonCu-Jc) was evaluated by dividing the Ic by the non-copper area of the cross section of the wire. The critical current density (nonCu-Jc) was determined to be 470 A/mm2 at 4.2 K in a magnetic field of 18 T. The reaction rate of the sheath (rate obtained by dividing the cross-sectional area of the Nb3Sn layer by the total cross-sectional area of the sheath) after the reaction was measured and found to be 67%, which was a high reaction rate.
Ta and Sn powders were weighed with an electronic balance in an Ar gas atmosphere in such a manner that Ta:Sn=6:5 (atomic ratio). The powders were mixed in a V-blender for 30 minutes. The resulting powder mixture was subjected to heat treatment at 950° C. for 10 hours in vacuum to form a Ta—Sn compound.
The resulting Ta—Sn compound was roughly crushed and then pulverized for 1 hour in an Ar atmosphere with an automatic mortar to form the Ta—Sn compound powder having a particle diameter of 75 μm or less. To the Ta—Sn compound powder, 25% by mass of a Sn powder and 5% by mass of a Cu powder were added. The mixture was mixed to form a material powder (Sn-based powder).
The resulting material powder was placed in a rubber mold and subjected to CIP at 200 MPa for 15 minutes to form a columnar formed article having an outer diameter of 32 mm and a length of 181 mm.
The resulting formed article was mechanically processed into a columnar formed article having an outer diameter of 30 mm and a length of 180 mm. A sheet (Nb—Ta sheet), composed of Nb containing 7.5% by mass Ta, having a thickness of 0.1 mm was wound around the periphery of the formed article in such a manner that the number of turns was 30. A Cu sheet having a thickness of 0.03 mm was inserted. The Nb—Ta sheet was wound together with the Cu sheet in such a manner that the number of turns of the two-sheet winding was 10. Then only the Nb—Ta sheet was wound in such a manner that the number of turns was 80, thereby producing a composite member. In this case, Nb—Ta:Cu (mass ratio) was 48:1.
The composite member was combined with an oxygen-free copper pipe having an outer diameter of 65 mm and an inner diameter of 55 mm, thereby forming an extrusion billet. The extrusion billet having the structure described above was extruded with a hydrostatic extruder and then formed into a wire having a diameter of 1.0 mm by wiredrawing with dies.
The wire was subjected to heat treatment at 700° C. for 100 hours in vacuum in order to form Nb3Sn. After the heat treatment, the critical current (Ic) of the resulting wire was measured while an external magnetic field from a superconducting magnet was applied to the wire. The critical current density of a non-copper area (nonCu-Jc) was evaluated by dividing the Ic by the non-copper area of the cross section of the wire. The critical current density (nonCu-Jc) was determined to be 490 A/mm2 at 4.2 K in a magnetic field of 18 T. The reaction rate of the sheath (rate obtained by dividing the cross-sectional area of the Nb3Sn layer by the total cross-sectional area of the sheath) after the reaction was measured and found to be 70%, which was a high reaction rate.
Ta and Sn powders were weighed with an electronic balance in an Ar gas atmosphere in such a manner that Ta:Sn=6:5 (atomic ratio). The powders were mixed in a V-blender for 30 minutes. The resulting powder mixture was subjected to heat treatment at 950° C. for 10 hours in vacuum to form a Ta—Sn compound.
The resulting Ta—Sn compound was roughly crushed and then pulverized for 1 hour in an Ar atmosphere with an automatic mortar to form the Ta—Sn compound powder having a particle diameter of 75 μm or less. To the Ta—Sn compound powder, 25% by mass of a Sn powder and 5% by mass of a Cu powder were added. The mixture was mixed to form a material powder (Sn-based powder).
The resulting material powder was filled into a sheath, composed of Nb containing 7.5% by mass Ta, having an outer diameter of 55 mm and an inner diameter of 30 mm. The sheath was combined with an oxygen-free copper pipe having an outer diameter of 65 mm and an inner diameter of 55 mm arranged on the outer periphery of the sheath, thereby forming an extrusion billet. The extrusion billet having the structure described above was extruded with a hydrostatic extruder and then formed into a wire having a diameter of 1.0 mm by wiredrawing with dies.
The wire was subjected to heat treatment at 700° C. for 100 hours in vacuum in order to form Nb3Sn. After the heat treatment, the critical current (Ic) of the resulting wire was measured while an external magnetic field from a superconducting magnet was applied to the wire. The critical current density of a non-copper area (nonCu-Jc) was evaluated by dividing the Ic by the non-copper area of the cross section of the wire. The critical current density (nonCu-Jc) was determined to be 310 A/mm2 at 4.2 K in a magnetic field of 18 T. The reaction rate of the sheath (rate obtained by dividing the cross-sectional area of the Nb3Sn layer by the total cross-sectional area of the sheath) after the reaction was measured and found to be 38%, which was a low reaction rate.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-061299 | Mar 2006 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2007/054356 | 3/6/2007 | WO | 00 | 8/22/2008 |
