JP3775304B2 - Magnesium diboride superconducting wire manufacturing method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a magnesium diboride superconducting wire used for a superconducting magnet for generating a high magnetic field or a conductive cooling magnet.
[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 superconducting powder as a starting material, the reacted MgB ₂ superconducting powder is filled in a metal pipe made of Ta, Ni, Fe, etc., and the outer side is coated with a Cu pipe and then drawn. A single hexagonal wire is assembled into a multi-core structure by drawing it into multiple pipes and then drawn. Finally, the multi-core wire is heat-treated in a range of 600 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 the method using a mixed powder of Mg and B as a starting material, the fine powder of Mg and B is mixed so that the molar ratio is about Mg: B = 1: 2, and then filled into a metal pipe and extruded. After processing, a single hexagonal wire is drawn to form a multi-core structure by incorporating it into a plurality of pipes. 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, if a different layer (insulating layer or normal conductive layer) such as an oxide film or MgB ₄ is present on the surface of MgB ₂ , a different phase precipitates at the grain boundary even after the final heat treatment. Even if the state is shown, the current is hindered at the grain boundary part, and the critical current is not improved.
[0007]
For the above reasons, there are many reports of bulk bodies of MgB ₂ superconductors, but in the case of multi-core structure wires, a high critical current is obtained with a magnetic field of 8 T or more of Nb—Ti and Nb ₃ Sn wire grades already in practical use. The performance shown is not obtained.
[0008]
In the method (2), the crystal structure of Mg is hexagonal, and the orientation in which the crystal is easily deformed is limited when plastic processing such as wire drawing or rolling is performed at room temperature. Further, after annealing (annealing) at 400 ° C. or higher like Cu or Al, it does not soften even when returned to room temperature, and it is difficult to perform plastic working at room temperature. As a result, breakage frequently occurs during drawing and drawing using a die, and it is very difficult to process a long wire. Although wire drawing is possible by processing using a swager or the like, the cross section tends to be non-uniform, and there is a difficulty in uniform processing. In order to enable wire drawing, the thickness of the metal pipe filled with powder is increased, the space factor in the wire cross section of the ductile metal matrix is increased, and the tensile strength of the wire is increased. Although drawing at room temperature becomes possible, the space factor of the superconducting portion is lowered, and the overall current density of the final multi-core wire is lowered. In addition, since the extrusion ratio and the area reduction processing rate at the time of wire drawing cannot be increased, the degree of processing becomes low, the number of wire drawing increases, and the number of processes increases.
[0009]
This invention is made | formed in view of this point, and it aims at providing the manufacturing method of the magnesium diboride superconducting wire which can produce the superconducting wire of a multi-core structure with a high critical current density.
[0010]
[Means for Solving the Problems]
In order to solve the above-mentioned problem, a method for manufacturing a magnesium diboride superconducting wire according to the present invention comprises a mixed powder of Mg and B, or a mixed powder obtained by mixing Mg, B, and other additive elements or compound powder, The billet having a structure filled in the pipe is hydrostatically extruded while being heated to a temperature of 250 ° C. or higher and 450 ° C. or lower.
[0011]
In addition, the wire rod obtained by the hydrostatic extrusion is drawn at a temperature of 250 ° C. or higher and 450 ° C. or lower.
[0012]
Here, the grounds for setting the preheating temperature during the hydrostatic extrusion or wire drawing to 250 ° C. or higher will be described. The shear stress of Mg sharply decreases from about 250 ° C., and the shear stress of all crystal orientations of hexagonal crystals is reduced to improve workability. Therefore, workability is remarkably improved by extruding or drawing and drawing in a state heated to 250 ° C. or higher, and a long wire can be produced. Therefore, it is good to extrude in the state of 250 degreeC or more, or to perform the wire drawing process by die drawing.
[0013]
Furthermore, the grounds for setting the preheating temperature during isostatic pressing or wire drawing to 450 ° C. or less will be described. As described in the above-mentioned basis, the workability of the wire is improved by heating to 250 ° C. or higher. However, if extrusion or wire drawing is performed in a state of being heated to 450 ° C. or higher, the work heat accompanying plastic deformation is spared. The heating temperature is added, and the wire temperature rises to 600 ° C. or higher during processing. Considering that the melting point of Mg is 650 ° C. and that the formation of MgB ₂ starts from around 600 ° C., when processed at 450 ° C. or higher, the hardness of the coated metal is extremely softened by melting Mg. There is a possibility that non-uniform deformation and disconnection occur due to a large difference between the two. In addition, hard and non-ductile MgB ₂ is generated as it is processed, and the long workability is drastically reduced. Therefore, the preheating temperature during processing is preferably set to 450 ° C. or less.
[0014]
In the magnesium diboride method of manufacturing a superconducting wire of the present invention, as the heat treatment for generating the MgB _2, 600 ° C. or higher, and characterized in that subjected to the following heat treatment 900 ° C..
[0015]
The grounds for performing heat treatment at 600 ° C. or higher and 900 ° C. or lower as the MgB ₂ generation heat treatment will be described. At a heat treatment temperature of 600 ° C. or lower, the MgB ₂ production reaction does not proceed and the critical current does not improve. On the other hand, when heat treatment at 900 ° C. or higher is performed, the coarsening of the MgB ₂ crystal grains is promoted, and the precipitation of non-superconducting substances (MgB ₄ , Mg and B oxides) that obstruct the superconducting current becomes prominent at the grain boundaries. This is because the critical current is extremely reduced at the boundary portion, and the critical current of the entire line as viewed macroscopically is reduced.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0017]
(Embodiment)
FIG. 1 is a cross-sectional view showing a configuration of an MgB ₂ superconducting wire according to an embodiment of the present invention.
[0018]
In the MgB ₂ superconducting wire 10 shown in FIG. 1, a mixed powder 11 in which Mg and B powders are mixed at a molar ratio of 1: 2 is filled in a Ta pipe 12 and pressed, and the outside is covered with a Cu pipe 13. It is formed by doing.
[0019]
The MgB ₂ superconducting wire 10 is further plugged at the front end and the rear end of the pipe to form a single-core billet, and after extrusion, drawn and drawn to form a thin wire, and finally a hexagonal cross-section single wire (single wire) Core wire). A bundle of a plurality of hexagonal wires, which are single core wires, are roughened into a Cu pipe and plugged at the front and rear ends of the pipe to form a multi-core billet. By reacting Mg and B, the MgB ₂ multicore superconducting wire can be formed.
[0020]
Such MgB ₂ superconducting wire 10 and the actual manufacturing method of MgB ₂ multi-core superconducting wire using MgB ₂ superconducting wire 10 will be described with reference to the explanatory diagram of a manufacturing process of FIG.
[0021]
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.
[0022]
In step 202, the mixed powder 11 was filled into a Ta pipe 13 having an inner diameter of 21 mm, an outer diameter of 24 mm, and a wall thickness of 1.5 mm, and this was pressed in step 203 to a powder filling rate of 62%. Further, a Cu pipe 13 having an inner diameter of 24.1 mm, an outer diameter of 28 mm, and a thickness of about 2 mm was coated on the outside of the Ta pipe 13 filled with powder. Thereby, the MgB ₂ superconducting wire 10 was obtained. An Fe plug was set at the rear end of the Cu pipe 13 and a Cu plug was set at the front end to obtain a single-core billet having an outer diameter of 28 mm.
[0023]
In step 204, the single-core billet was extruded to an outer diameter of 10 mm by hot isostatic extrusion in a preheated state at 320 ° C.
[0024]
In step 205, the extruded material was set in a cylindrical heater, heated to 300 ° C. until just before drawing, and the die was drawn in a state preheated to 250 ° C.
[0025]
In Step 206, a hexagonal wire having a distance of 2.65 mm was obtained by drawing with a hexagonal die. 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.
[0026]
In step 208, 61 hexagonal wires 21 are selected. 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 this case, a Fe plug is set at the rear end of the pipe after incorporation, a Cu plug is set at the front end, and a multi-core billet having an outer diameter of 28 mm is preheated at 320 ° C. to obtain a wire having an outer diameter of 10 mm by hydrostatic extrusion. In step 211, the wire is further preheated to 300 ° C. and drawn. Then, in step 212, the extruded material is drawn to a wire diameter of 1 mm, and finally in step 213, in an Ar atmosphere. Then, heat treatment for generating MgB ₂ at 700 ° C. for 50 hours was performed. As a result, an MgB ₂ multicore superconducting wire was obtained. This is hereinafter referred to as the superconducting wire of the present embodiment.
[0027]
Next, in order to verify the improvement of the characteristics of the superconducting wire of this embodiment, an MgB ₂ multi-core superconducting wire was produced using the conventional MgB ₂ superconducting wire 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 a Ta pipe 12a having an inner diameter of 16 mm, an outer diameter of 22 mm, and a wall thickness of 3 mm in the step 202 shown in FIG. In step 203, this was pressed to a powder filling rate of 62%. Further, a Cu pipe 13a having an inner diameter of 22.1 mm, an outer diameter of 28 mm, and a thickness of about 3 mm was coated on the outside of the Ta pipe 12a filled with powder. As a result, a conventional MgB ₂ superconducting wire 30 was obtained. A Fe plug was set at the rear end of the Cu pipe 13a and a Cu plug was set at the front end to obtain a single-core billet having an outer diameter of 28 mm.
[0028]
In Step 204a, the single-core billet was extruded to an outer diameter of 15 mm by performing an isostatic extrusion process at room temperature.
[0029]
In step 205a, the extruded material is drawn and drawn at room temperature. In step 206, the extruded material is drawn with a hexagonal die to form a hexagonal wire having an opposite side distance of 2.65 mm. In step 207, the hexagonal wire is corrected and straightened. After making it into a shape, it was cut into a length of 150 mm. In step 208, 61 of the cut hexagonal wires 21 are selected, and in step 209, the 61 hexagonal wires 21 are incorporated into the Cu pipe 22 having an inner diameter of 25.5 mm and an outer diameter of 28 mm. Further, in step 210a, A Fe plug is set at the rear end of the pipe after the assembly, and a Cu plug is set at the front end. A multi-core billet having an outer diameter of 28 mm is formed into a wire having an outer diameter of 15 mm by hydrostatic extrusion, and the wire is further processed in step 211a. After drawing at room temperature, the extruded material was drawn to a wire diameter of 1 mm in step 212, and finally, in step 213, MgB ₂ generation heat treatment was performed at 700 ° C. for 50 hours in an Ar atmosphere. . As a result, an MgB ₂ multicore superconducting wire was obtained. This is hereinafter referred to as a conventional superconducting wire.
[0030]
The characteristics (Jc-B) of the critical current density Jc (A / mm ² ) and the external magnetic field B (T) per total cross-sectional area of the wire in the liquid helium of each of the superconducting wire and the conventional superconducting wire in this embodiment. The characteristics are shown in FIG.
[0031]
The Jc at 5T was 1220 A / mm ² , which was approximately twice that of the superconducting wire 41 of the present embodiment compared to 570 A / mm ² of the conventional superconducting wire 42. The reason why the Jc of the superconducting wire 41 of the present embodiment is improved is that the occupation ratio of the MgB ₂ superconducting filament in the cross section of the wire is improved by about 1.7 times that of the conventional superconducting wire, and the workability is good due to heat drawing. This is because the non-uniform deformation of the filament is reduced and a uniform superconducting filament is formed in the wire length direction. 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.
[0032]
Therefore, according to the present superconducting wire, a high critical current density can be obtained.
[0033]
Such a superconducting wire of the present embodiment can produce a long wire with the same processing equipment as a conventional Nb—Ti wire or Nb ₃ Sn wire.
[0034]
In addition, if a superconducting magnet is formed by winding the superconducting wire of the present 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.
[0035]
That is, by producing a magnet using the superconducting wire of the present embodiment, it is possible to provide a superconducting magnet that can be stably operated without quenching due to a high critical temperature and an ultrafine multi-core structure.
[0036]
【The invention's effect】
As described above, according to the present invention, in manufacturing a magnesium diboride superconducting wire, a mixed powder of Mg and B, or a mixed powder obtained by mixing Mg, B and other additive element or compound powder, A billet having a structure filled in a metal pipe is subjected to isostatic extrusion while being heated to a temperature of 250 ° C. or higher and 450 ° C. or lower. The wire obtained by the hydrostatic extrusion is drawn at a temperature of 250 ° C. or higher and 450 ° C. or lower. In these processes, since the long workability is good, the occupation ratio of the MgB ₂ superconducting filament in the cross section of the wire is improved, the non-uniform deformation of the filament is reduced, and a uniform superconducting filament is formed in the length direction of the wire. 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 an MgB ₂ superconducting wire according to an embodiment of the present invention.
[Figure 2] MgB ₂ superconducting wire according to the above embodiment and is an explanatory view of a manufacturing process of MgB ₂ multi-core superconducting wire using MgB ₂ superconducting wire.
FIG. 3 is a cross-sectional view showing a configuration of a conventional MgB ₂ superconducting wire.
FIG. 4 shows a critical current density Jc (A / mm ² ) and an external magnetic field B (T) in liquid helium of a MgB ₂ multicore superconducting wire using both the present embodiment and the conventional MgB ₂ superconducting wire. It is a figure which shows the characteristic (Jc-B characteristic).
[Explanation of symbols]
10 MgB ₂ superconducting wire 11 of this embodiment Mg and B mixed powder 12, 12a Ta pipe 13, 13a Cu pipe 21 Hexagonal wire 30 Conventional MgB ₂ superconducting wire 41 Jc of multi-core superconducting wire of this embodiment -B characteristic curve 42 Jc-B characteristic curve of a conventional multi-core superconducting wire

Claims (4)

A billet having a structure in which a mixed powder of mixed powder of Mg (magnesium) and B (boron) or mixed powder of Mg, B and other additive element or compound is filled in a metal pipe is 250 ° C or higher and 450 ° C. A method for producing a magnesium diboride superconducting wire, which is subjected to an isostatic extrusion while being heated to the following temperature. The method for producing a magnesium diboride superconducting wire according to claim 1, wherein the wire obtained by the hydrostatic extrusion is drawn while being heated to a temperature of 250 ° C or higher and 450 ° C or lower. Obtained by drawing a billet with a structure in which a mixed powder of Mg and B, or mixed powder of Mg, B and other additive element or compound powder is filled in a metal pipe, after hydrostatic extrusion. The method is a method of drawing a billet of a multi-core structure after hydrostatic pressure extruding a billet having a structure in which a plurality of wires are assembled, each of the hydrostatic pressure extruding and wire drawing being performed at 250 ° C. or more, A process for producing a magnesium diboride superconducting wire characterized by processing in a state heated to a temperature of 450 ° C. or lower. 4. The method for producing a magnesium diboride superconducting wire according to claim 1, wherein a heat treatment for generating MgB ₂ (magnesium diboride) is performed at a temperature of 600 ° C. to 900 ° C. 5.

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