JP4058951B2 - Magnesium diboride superconducting wire precursor and magnesium diboride superconducting wire - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a magnesium diboride superconducting wire used for a superconducting magnet or a conductive cooling magnet for generating a high magnetic field.
[0002]
[Prior art]
MgB ₂ (magnesium diboride) is known to have a superconducting property, having a crystal structure in which Mg (magnesium) and B (boron) atoms are alternately stacked in layers. This MgB ₂ has an excellent superconducting transition temperature among those that are easy to use after being processed into metal or the like as a metal.
[0003]
Conventionally, as a method for manufacturing a MgB ₂ multi-core structure superconducting wire using MgB _2, for example, now described ▲ 1 ▼ and ▲ 2 ▼ production methods have been attempted.
[0004]
(1) In the method using MgB ₂ powder as the starting material powder, the reacted MgB ₂ superconducting powder is filled into a metal pipe such as Ta, Ni, Fe, etc., and the outer side is coated with a Cu pipe, followed by a drawing process. Hexagonal wire is used, and it is drawn into a multi-core structure by incorporating it into multiple pipes. Finally, the multi-core wire is heat-treated in the range of 800 to 900 ° C., and MgB ₂ powders are bonded to each other by a solid phase reaction to obtain a superconducting wire.
[0005]
(2) In a method of using a mixed powder of Mg and B as a starting material and filling a Ta pipe, after mixing Mg and B fine powder so that the molar ratio is about Mg: B = 1: 2, A non-reacting Ta pipe is filled, and the outer periphery thereof is covered with a Cu pipe and extruded, and then drawn into a single hexagonal wire, which is incorporated into a plurality of pipes to form a multi-core structure. This multi-core billet is again extruded and drawn, and finally heat treated to produce a MgB ₂ multi-core superconducting wire.
[0006]
[Problems to be solved by the invention]
However, the conventional method has the following problems. In the method (1), the starting material MgB ₂ powder has a high hardness (hard) and has no ductility, so that the workability is difficult. Even if it can be processed, a multi-core wire with a uniform cross section can be obtained. It was difficult. In addition, it is necessary to increase the tensile strength of the wire in order to prevent disconnection, and the MgB ₂ powder ratio (core ratio) of the core part to the total cross-sectional area of the wire cannot be increased. As a result, the non-superconductivity that is the coated metal part The space factor of the portion increased, and the critical current density of the wire rod overall tended to decrease.
[0007]
For these reasons, the cross-sectional structure of the wire material starting from MgB ₂ superconducting powder can only produce a single-core wire or a multi-core wire structure having a filament number as small as 10 or less and a filament diameter of 100 μm or more. This does not satisfy the structure of a practical superconducting wire having an ultrafine multi-core structure from the viewpoint of reducing AC loss of the superconducting wire and reducing magnetic instability (preventing flux jump).
[0008]
In addition, if a different phase (insulating phase or normal conducting phase) such as an oxide film or MgB ₄ is present on the surface of MgB ₂ , the different phase precipitates at the grain boundary portion even after the final heat treatment. Even when the superconducting state is shown, the current is hindered at the grain boundary portion, and the critical current may not be improved.
[0009]
In the method {circle around (2)}, compared to the method {circle around (1)}, by using a mixed powder of Mg and B as a starting material, workability is improved by the presence of Mg that is ductile to some extent. However, although Ta does not react with Mg, the section of the Ta portion is disturbed as the wire drawing is performed, and the Ta coating is broken, and Cu and Mg in the outer skin may be in direct contact with each other. In this case, Mg having a relatively low melting point of 650 ° C. diffuses to the Cu side during the final heat treatment for generating MgB _{2 to} form a Cu—Mg alloy, and the core portion has a stoichiometric composition (Mg: B = 1: 2). May deviate from B and become B-rich (boron-rich), which may deteriorate the superconducting characteristics. In particular, in the case of a multi-core wire, the Ta coating thickness is reduced to several tens of μm or less, so there is a high possibility that the coating will be partially broken due to disturbance, which is a factor in reducing the critical current of the multi-core wire. .
[0010]
The first cause of the disorder of the cross-section accompanying wire drawing is that the grain size of Ta is large, so there are few grain boundary parts, and the grain boundary sliding due to processing cannot be expected so much. The third is that the adhesion with Cu coated on the outer periphery is poor and the interface is easily isolated, and third, the difference in hardness with Cu is large, and there is a large difference in deformation resistance during wire drawing, etc. Presumed to be.
[0011]
The present invention has been made in view of the above points, and provides a magnesium diboride superconducting wire precursor and a magnesium diboride superconducting wire capable of producing a multiconductor superconducting wire having a high critical current density. For the purpose.
[0012]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, a magnesium diboride superconducting wire precursor of the present invention is a mixed powder of Mg and B, or a mixed powder obtained by mixing Mg, B, and other additive elements or compound powders, and an Nb pipe. Alternatively, in the case where the Nb-Ta alloy pipe is coated and the pipe is coated with a Cu (copper) pipe or a Cu alloy pipe, the Ta pipe is disposed inside the Nb or Nb-Ta alloy pipe ( It is characterized by having a (Mg + B) / Ta / Nb / Cu (or Cu alloy) or (Mg + B) / Ta / Nb-Ta / Cu (or Cu alloy) structure.
[0015]
That is, a mixed powder of Mg and B does not dissolve (react) with Mg and does not react with B only at a high temperature, or a composite pipe of Ta and Nb (Ta / Nb structure), or Ta and Nb- In order to fill the Ta composite structure pipe (Ta / Nb · Ta structure) and improve the wire drawing workability, the outer periphery of the Nb pipe or Nb-Ta alloy pipe is covered with a Cu pipe. According to this structure, the crystal grain of the Nb-Ta alloy added with several at% of Ta is refined, and cracks and disturbances during plastic working such as wire drawing or rolling can be prevented. Ta has a higher reaction temperature with B than Nb. When MgB ₂ heat treatment is performed at a high temperature of about 800 ° C. or higher, the reaction between the coated pipe and B is better when the contact portion with the mixed powder is Ta. Can be prevented. Also, a plurality of wires having a structure of (Mg + B) / Ta / Nb / Cu (or Cu alloy) or (Mg + B) / Ta / Nb-Ta / Cu (or Cu alloy) are aggregated. It is a feature.
[0016]
The space factor of the mixed powder portion relative to the total cross-sectional area of the wire (mixed powder portion cross-sectional area / single-core wire total cross-sectional area = core ratio) is 0.55 or less.
[0017]
The reason for setting it to 0.55 or less is to increase the core ratio in the single core wire preparation step (magnesium diboride superconducting wire preparation step), that is, the ratio of the mixed powder part area to the total cross sectional area of the single core wire is 0.55 or more. This is because if the height is increased, the possibility of disconnection during wire drawing increases due to the influence of the core portion having low ductility compared to metal-coated pipes such as Nb and Cu. In particular, when a large number of magnesium diboride superconducting wires are applied to a multicore wire whose coating thickness around the core is reduced to several tens of μm or less, even if the wire can be drawn, the coating of Nb, Ta, etc. is partially broken, This is because there is a very high possibility that the superconducting characteristics will deteriorate due to the reaction between Mg and Cu during the final MgB ₂ production heat treatment.
[0018]
Further, the ratio (t / r) between the coating thickness (t) of the wire other than the mixed powder portion and the radius of the mixed powder portion (= core radius: r) is 0.12 or more. .
[0019]
The reason for setting this to 0.12 or more is that if the thickness of the Ta / Nb or Ta / Nb · Ta-coated pipe is too thin, the outer Cu pipe is thick, and even if it can be drawn, the cross section is disturbed and Nb This is because the possibility that the superconducting characteristics deteriorate due to the reaction between Mg and Cu during the final heat treatment for generating MgB ₂ is greatly increased.
[0020]
In addition, the heat treatment of the magnesium diboride superconducting wire precursor that generates MgB ₂ is characterized in that heat treatment at 600 ° C. or higher and 900 ° C. or lower is performed.
[0021]
The reason why this heat treatment is set to 600 ° C. or more and 900 ° C. or less is that, at a heat treatment temperature of 600 ° C. or less, the formation reaction of MgB ₂ does not proceed and the critical current does not improve. On the other hand, when heat treatment at 900 ° C. or higher is performed, coarsening of MgB ₂ crystal grains is promoted, and precipitation of non-superconducting substances (MgB ₄ , Mg and B oxides) that inhibit superconducting current becomes prominent at the grain boundaries. This is because the critical current is extremely reduced at the grain boundary portion, so that the critical current of the entire line as viewed macroscopically is reduced.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0023]
(Embodiment)
FIG. 1 is a cross-sectional view showing a configuration of a MgB ₂ superconducting wire precursor according to an embodiment of the present invention.
[0024]
The MgB ₂ superconducting wire precursor 10 shown in FIG. 1 is a Ta (powder) 11 in which Mg and B are mixed at a molar ratio of about 1: 2 and arranged inside an Nb or Nb—Ta alloy pipe 12. It is formed by filling the composite pipe with the pipe 13 and pressing the powder, and coating the outside with the Cu pipe 14.
[0025]
The MgB ₂ superconducting wire precursor 10 is further plugged at the front and rear ends of the pipe to form a single-core billet, drawn and drawn after extrusion, and thinned, and finally a single wire having a hexagonal cross section with a hexagonal die. (Single core wire). A plurality of hexagonal wires, which are single wires, are bundled into a Cu or Cu-Ni pipe or the like, and the front and rear ends of the pipe are plugged into a multi-core billet. After extrusion, the wire is drawn and drawn into a thin wire. An MgB ₂ multicore superconducting wire can be formed by reacting Mg and B by heat treatment.
[0026]
Such MgB ₂ superconducting wire precursor 10 and the actual manufacturing method of MgB ₂ multi-core superconducting wire using MgB ₂ superconducting wire precursor 10 will be described with reference to the explanatory diagram of a manufacturing process of FIG.
[0027]
First, in step 201, by mixing an amorphous B powder having an average particle diameter of 0.1 μm and an Mg powder having an average particle diameter of 20 μm so that the molar ratio is Mg: B = 1: 2, A mixed powder 11 of Mg and B was obtained.
[0028]
In step 202, the mixed powder 11 was filled into a Ta pipe 13 having an inner diameter of 18 mm and an outer diameter of 19 mm, and this was pressed in step 203 to a powder filling rate of 62%. Further, the outside of the Ta pipe 13 filled with powder is coated with an Nb-1 at% Ta pipe (hereinafter referred to as an Nb-Ta alloy pipe) 12 having an inner diameter of 19.1 mm and an outer diameter of 22 mm, and the outer diameter is 22.1 mm. The Cu pipe 14 having an outer diameter of 28 mm was coated. Thereby, the MgB ₂ superconducting wire precursor 10 was obtained.
[0029]
In step 204, a Fe plug was set at the rear end of the pipe and a Cu plug was set at the front end to form a single-core billet, which was extruded to an outer diameter of 15 mm by hydrostatic extrusion.
[0030]
In step 205, the extruded material was drawn, and in step 206, the extruded material was drawn with a hexagonal die to form a hexagonal wire having a near distance of 2.65 mm. In step 207, the hexagonal wire was straightened by straightening, and then cut into a length of 150 mm. A schematic perspective view of the cut hexagonal line is denoted by reference numeral 21.
[0031]
In step 208, 61 hexagonal wires 21 are selected, and in step 209, the 61 hexagonal wires 21 are incorporated into a Cu pipe 22 having an inner diameter of 25.5 mm and an outer diameter of 28 mm (see schematic perspective view). In Fig. 1, a Fe plug was set at the rear end of the pipe after incorporation and a Cu plug was set at the front end, and a multi-core billet having an outer diameter of 28 mm was extruded to an outer diameter of 15 mm by hydrostatic extrusion.
[0032]
In step 211, the extruded material was drawn to a wire diameter of 1 mm, and finally, in step 212, MgB ₂ generation heat treatment was performed in an Ar atmosphere at 700 ° C. for 50 hours. This gave MgB ₂ multi-core superconducting wire using MgB ₂ superconducting wire precursor 10. This is hereinafter referred to as the superconducting wire of the present embodiment.
[0033]
Next, in order to verify the improvement in the characteristics of the superconducting wire of this embodiment, a MgB ₂ multicore superconducting wire was produced using the conventional MgB ₂ superconducting wire precursor 30 shown in FIG. In this production method, the same mixed powder 11 as in the superconducting wire production method of the present embodiment is filled in the Ta pipe 13a having an inner diameter of 18 mm and an outer diameter of 22 mm in the step 202, and this is pressed in the step 203 to obtain a powder. The filling rate was 62%. Further, a Cu pipe 14 having an inner diameter of 22.1 mm and an outer diameter of 28 mm was coated on the outside of the Ta pipe 13a filled with powder. Thus, a conventional MgB ₂ superconducting wire precursor 30 was obtained. Hereinafter, the MgB ₂ multi-core superconducting wire using the conventional MgB ₂ superconducting wire precursor 30 was obtained by processing in the same manner as the above steps 204 to 212. This is hereinafter referred to as a conventional superconducting wire.
[0034]
Next, the result of comparing both the superconducting wire of the present embodiment and the conventional superconducting wire will be described. As a result of observing a cross section of a multifilamentary wire having a wire diameter of 1 mm, the filament shape of the conventional superconducting wire having a Ta / Cu structure was uneven, and the size of the 61 filaments was also highly varied. In addition, the calculated thickness of the Ta barrier covering the periphery of the filament is about 12 μm, and the thickness distribution is not uniform and the barrier is partially broken. Due to this breakage, the Cu and Mg + B powder are in direct contact with each other. Observed.
[0035]
On the other hand, the filament shape of the superconducting wire of this embodiment having a Ta / Nb-Ta / Cu structure is greatly improved in uniformity and smaller in size variation than the conventional superconducting wire, and a Ta / Nb / Ta barrier. There was no part torn.
[0036]
The characteristics (Jc-B characteristics) between the critical current density Jc (A / mm ² ) and the external magnetic field B (T) in the liquid helium of each of the superconducting wire of this embodiment and the conventional superconducting wire are shown in FIG. Shown in
[0037]
Jc at 5T, relative 480A / mm ² of conventional superconducting wire 42, forms the superconducting wire 41 of the present embodiment was about 2.5 times the 1220A / mm ^2. The superconducting wire 41 of the present embodiment exhibited a Jc of about twice or more in all magnetic field regions as compared with the conventional superconducting wire 42. The reason for this is considered that the superconducting wire 41 of the present embodiment was superior in filament uniformity and barrier soundness in the wire cross section.
[0038]
Therefore, according to the present superconducting wire, a high critical current density can be obtained.
[0039]
If a superconducting magnet is formed by winding such a superconducting wire of this embodiment, this superconducting magnet can stably generate a high magnetic field. Further, since the superconducting wire of the present embodiment has a high critical temperature of 39K, the cooling temperature is increased to about 10K as compared with a conductive cooling magnet (cooling to about 5K) using a conventional metal superconducting magnet. As a result, in the refrigerator using the superconducting magnet, the load can be significantly reduced.
[0040]
【The invention's effect】
As described above, according to the present invention, the mixed powder of Mg and B is filled in a Ta pipe that does not dissolve in Mg (does not react) and reacts only with B at a high temperature. In order to improve wire drawing workability, the outer periphery of the Ta pipe was covered with an Nb pipe or an Nb-Ta alloy pipe, and the outer periphery of the Nb pipe or Nb-Ta alloy pipe was further covered with a Cu pipe. In this structure, Nb is a good bonding between the Cu compared to Ta, the difference in deformation resistance during working is small and also, Nb-Ta alloy containing several a t% of Ta crystal grain fine Therefore, it is possible to prevent cracking and turbulence during plastic working such as wire drawing or rolling, so that a high critical current density can be obtained. Therefore, a multiconductor superconducting wire having a high critical current density can be produced .
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing a configuration of a MgB ₂ superconducting wire precursor according to an embodiment of the present invention.
FIG. 2 is an explanatory diagram of a production process of an MgB ₂ multiconductor superconducting wire using the MgB ₂ superconducting wire precursor and the MgB ₂ superconducting wire precursor according to the embodiment.
FIG. 3 is a cross-sectional view showing a configuration of a conventional MgB ₂ superconducting wire precursor .
FIG. 4 shows a critical current density Jc (A / mm ² ) and an external magnetic field B (in a liquid helium of an MgB ₂ multicore superconducting wire using the MgB ₂ superconducting wire precursor of both the present embodiment and the conventional example. It is a figure which shows the characteristic (Jc-B characteristic) with T).
[Explanation of symbols]
10 MgB ₂ superconducting wire precursor of this embodiment 11 Mixed powder of Mg and B 12 Nb-Ta alloy pipe 13, 13a Ta pipe 14, 22 Cu pipe 21 Hexagonal wire 30 Conventional MgB ₂ superconducting wire precursor 41 This implementation Multiconducting superconducting wire of the form 42 Conventional multiconducting superconducting wire

Claims (5)

A mixed powder of Mg (magnesium) and B (boron), or mixed powder of Mg, B, and other additive element or compound powder is coated with an Nb (niobium) pipe or an Nb-Ta (tantalum) alloy pipe. In the case where this pipe is coated with a Cu (copper) pipe or a Cu alloy pipe, by placing a Ta pipe inside the Nb or Nb-Ta alloy pipe, (Mg + B) / Ta / Nb / Cu ( Or a Cu alloy), or a (Mg + B) / Ta / Nb-Ta / Cu (or Cu alloy) structure, a magnesium diboride superconducting wire precursor . A plurality of wires having a structure of (Mg + B) / Ta / Nb / Cu (or Cu alloy) or (Mg + B) / Ta / Nb-Ta / Cu (or Cu alloy) are collected. The magnesium diboride superconducting wire precursor according to claim 1. The space factor of the mixed powder part with respect to the total cross-sectional area of the wire (mixed powder part cross-sectional area / single-core wire total cross-sectional area = core ratio) is 0.55 or less. Magnesium diboride superconducting wire precursor . The ratio (t / r) between the coating thickness (t) of the wire other than the mixed powder portion and the radius of the mixed powder portion (= core radius: r) is 0.12 or more. The magnesium diboride superconducting wire precursor according to any one of 1 to 3. The magnesium diboride superconducting wire precursor according to any one of claims 1 to 4 is heat-treated at 600 ° C to 900 ° C to produce MgB ₂ (magnesium diboride). Magnesium boride superconducting wire.

Images