Superconducting wire and superconducting coil

Abstract

To provide a superconducting wire capable of realizing a high residual resistance ratio (RRR) and a high strength, and also a superconducting coil using the superconducting wire.SOLUTION: A superconducting wire 10 includes a superconductor 11 and a superconductivity stabilization material 20. The superconductivity stabilization material 20 includes an inner copper material 21 and an outer copper material 22 including a copper material with a Cu content of 99.9 mass% or more. One or more kinds of elements selected from Ca, Sr, Ba, and a rare-earth element are added to the inner copper material 21, and Mg is added to the outer copper material 22. A length ratio A is 10% or less in a small-angle grain boundary and a subgrain boundary of the inner copper material 21. A ratio B / A is 2 or more with regard to the length ratio A in the small-angle grain boundary and the subgrain boundary of the inner copper material 21 and a length ratio B in a small-angle grain boundary and a subgrain boundary of the outer copper material 22.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a superconducting wire provided with a superconducting stabilizer, and a superconducting coil made of this superconducting wire. [Background technology]

[0002] The above-mentioned superconducting wires are used in fields such as MRI, NMR, particle accelerators, linear motor cars, and also in energy storage devices. This superconducting wire has a multi-core structure in which multiple strands made of superconductors such as Nb-Ti alloys and Nb3Sn are bundled together with a superconducting stabilizer in between. Tape-shaped superconducting wires made by laminating superconductors and superconducting stabilizers are also available. To further enhance stability and safety, superconducting wires equipped with a channel member made of pure copper are also available.

[0003] In the above-mentioned superconducting wire, if the superconducting state is broken in part of the superconductor, the resistance will increase significantly in that part, causing the temperature of the superconductor to rise, and the entire superconductor may rise above its critical temperature and transition to a normal conducting state. Therefore, in the superconducting wire, a superconducting stabilizer with relatively low resistance, such as copper, is arranged in contact with the superconductor, so that if the superconducting state is broken partially, the current flowing through the superconductor is temporarily diverted to the superconducting stabilizer, during which time the superconductor is cooled and restored to a superconducting state.

[0004] The above-mentioned superconducting stabilizer is required to have a sufficiently low resistance at cryogenic temperatures in order to efficiently divert the current. The residual resistance ratio (RRR) is widely used as an index of the electrical resistance of superconducting wires at cryogenic temperatures. This residual resistance ratio (RRR) is calculated by dividing the resistance ρ 293K and resistivity ρ at liquid helium temperature (4.2K) 4.2K Ratio ρ 293K / ρ 4.2K The higher this residual resistivity ratio (RRR), the better the performance of the material as a superconducting stabilizer.

[0005] Therefore, as a superconducting stabilizer (copper material) for forming a superconducting wire with a high residual resistance ratio (RRR), ultra-high purity copper (6NCu) with a purity of 99.9999 mass% or more, in which impurity elements have been reduced to the utmost limit, can be mentioned. Furthermore, Patent Document 1 proposes a superconducting stabilizer made of copper material to which one or more elements selected from Ca, Sr, Ba, and rare earth elements are added. In Patent Document 1, S, Se, and Te, which are elements that lower the residual resistance ratio (RRR) of superconducting wires, are fixed as compounds with one or more elements selected from Ca, Sr, Ba, and rare earth elements, thereby improving the residual resistance ratio (RRR) of superconducting wires. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 6642763 Summary of the Invention [Problem to be solved by the invention]

[0007] Here, the above-mentioned superconducting wire is wound into a coil to form a superconducting coil, and a direct current is passed through this superconducting coil to generate a magnetic field at the center, which is used as a superconducting magnet. Recently, there has been a demand for use in stronger magnetic fields than before. When used in a high-magnetic-field environment, superconducting wires can move and vibrate minutely, generating frictional heat and potentially destroying their superconducting state. For this reason, superconducting wires used in high-magnetic-field environments must be strong enough to prevent vibrations even in high-magnetic-field environments.

[0008] To achieve high strength, wire drawing is required as the final process. However, the wire drawing process creates a substantial processed structure, significantly increasing lattice defects such as dislocations, resulting in increased resistance and a decrease in the residual resistivity ratio (RRR) of the superconducting wire. Therefore, post-processing heat treatment is required to sufficiently reduce the lattice defects in the matrix as a recrystallized structure, thereby restoring the residual resistivity ratio (RRR) of the superconducting wire. However, if the heat treatment temperature is too high, the superconducting properties of the superconductor will deteriorate, so heat treatment at a lower temperature is required. In other words, a superconductor with a low recrystallization temperature is required. On the other hand, reducing the lattice defects in the matrix reduces strength.

[0009] Here, the superconducting stabilizer described in Patent Document 1 does not assume that the superconducting wire will be subjected to a thermomechanical treatment in an air atmosphere after being formed. Since elements such as Ca, Sr, Ba, and rare earth elements are easily oxidized, when they are subjected to thermal processing in the atmosphere, oxygen diffuses into the superconducting stabilizer, oxidizing elements such as Ca, Sr, Ba, and rare earth elements, which could prevent S, Se, and Te from being sufficiently fixed, and could prevent the residual resistance ratio (RRR) of the superconducting wire from being sufficiently improved.

[0010] The present invention has been made in consideration of the above-mentioned circumstances, and aims to provide a superconducting wire that can achieve a high residual resistance ratio (RRR) and high strength, and a superconducting coil using this superconducting wire. [Means for solving the problem]

[0011] In order to solve the above problems, a superconducting wire according to a first aspect of the present invention is a superconducting wire including a superconductor and a superconducting stabilizer, wherein the superconducting stabilizer has an inner copper material and an outer copper material made of a copper material having a Cu content of 99.9 mass% or more, wherein the inner copper material is doped with one or more elements selected from Ca, Sr, Ba, and rare earth elements, and the outer copper material is doped with Mg, and the misorientation of each crystal grain is analyzed by an EBSD method, and the length of the low-angle grain boundary and the subgrain boundary between adjacent measurement points where the misorientation between the adjacent measurement points is 2° or more and 15° or less is defined as L. LB The length of the high-angle grain boundary between adjacent measurement points where the misorientation between the measurement points exceeds 15° is defined as L. HB , the length ratio of the low-angle grain boundary and the subgrain boundary is L LB / (L LB +L HB ), the length ratio A of the low-angle grain boundaries and subgrain boundaries of the inner copper material is 10% or less, and the ratio B / A of the length ratio A of the low-angle grain boundaries and subgrain boundaries of the inner copper material to the length ratio B of the low-angle grain boundaries and subgrain boundaries of the outer copper material is 2 or more.

[0012] The superconducting wire of aspect 1 of the present invention comprises a superconductor and a superconducting stabilizer, and the superconducting stabilizer has an inner copper material and an outer copper material made of copper material with a Cu content of 99.9 mass% or more, and the inner copper material contains one or more elements selected from Ca, Sr, Ba, and rare earth elements, so that elements such as S, Se, and Te that reduce the residual resistivity ratio (RRR) can be fixed in the inner copper material by one or more elements selected from Ca, Sr, Ba, and rare earth elements. Furthermore, since Mg is added to the outer copper material, oxygen that would diffuse into the superconducting stabilizer during heat treatment in the air atmosphere can be trapped by Mg, and the diffusion of oxygen into the inner copper material can be suppressed, which prevents one or more elements selected from Ca, Sr, Ba, and rare earth elements contained in the inner copper material from being converted into oxides and consumed, thereby achieving a high residual resistivity ratio (RRR).

[0013] Furthermore, the length ratio A of the low-angle grain boundaries and subgrain boundaries in the inner copper material is 10% or less, and the ratio B / A of the length ratio A of the low-angle grain boundaries and subgrain boundaries in the inner copper material to the length ratio B of the low-angle grain boundaries and subgrain boundaries in the outer copper material is 2 or more. Therefore, in the inner copper material to which one or more elements selected from Ca, Sr, Ba, and rare earth elements are added, the density of dislocations introduced during processing is low, making it possible to increase the residual resistance ratio (RRR). In addition, in the outer copper material to which Mg is added and solid-solution strengthened, the density of dislocations introduced during processing is relatively high, making it possible to improve strength.

[0014] A superconducting wire according to a second aspect of the present invention is the superconducting wire according to the first aspect of the present invention, characterized in that the outer copper material contains Mg in a range of 10 ppm by mass to 130 ppm by mass, with the remainder being Cu and unavoidable impurities. According to the superconducting wire of aspect 2 of the present invention, the outer copper material contains Mg in the range of 10 mass ppm to 130 mass ppm, so that oxygen can be reliably trapped by Mg and workability can be ensured.

[0015] A superconducting wire according to a third aspect of the present invention is characterized in that, in the superconducting wire according to the first or second aspect of the present invention, the inner copper material contains one or more elements selected from Ca, Sr, Ba, and rare earth elements in a total amount ranging from 3 ppm by mass to 100 ppm by mass, with the remainder being Cu and unavoidable impurities. According to the superconducting wire of aspect 3 of the present invention, the inner copper material contains one or more elements selected from Ca, Sr, Ba, and rare earth elements in a total amount within the range of 3 mass ppm to 100 mass ppm. Therefore, S, Se, and Te can be sufficiently fixed by the one or more elements selected from Ca, Sr, Ba, and rare earth elements, and the residual resistivity ratio (RRR) can be sufficiently improved and processability can be ensured.

[0016] The superconducting wire of aspect 4 of the present invention is characterized in that, in the superconducting wire of any one of aspects 1 to 3 of the present invention, the GOS value C of the inner copper material is 0.55 or less, and the ratio D / C of the GOS value C of the inner copper material to the GOS value D of the outer copper material is 2.5 or more. According to the superconducting wire of aspect 4 of the present invention, the GOS value C of the inner copper material is 0.55 or less, and the ratio D / C of the GOS value C of the inner copper material to the GOS value D of the outer copper material is 2.5 or more. Therefore, the strength of the outer copper material, which has been solid-solution strengthened by the addition of Mg, is maintained sufficiently high, and the residual resistance ratio (RRR) of the superconducting wire can be further increased by the inner copper material to which one or more elements selected from Ca, Sr, Ba, and rare earth elements have been added.

[0017] A superconducting wire of a fifth aspect of the present invention is the superconducting wire of any one of the first to fourth aspects of the present invention, characterized in that in the inner copper material, a ratio X / Y of the total content of S, Se, and Te (Y ppm by mass) to the total content of Ca, Sr, Ba, and rare earth elements (X ppm by mass) is within a range of 0.5≦X / Y≦20. According to the superconducting wire of the fifth aspect of the present invention, in the inner copper material, the ratio X / Y of the total content of S, Se, and Te (Y ppm by mass) to the total content of Ca, Sr, Ba, and rare earth elements (X ppm by mass) is set within the range of 0.5≦X / Y≦20. Therefore, S, Se, and Te in the copper can be reliably fixed as compounds, and a decrease in the residual resistivity ratio (RRR) due to S, Se, and Te can be reliably suppressed.

[0018] The superconducting wire of aspect 6 of the present invention is the superconducting wire of any one of aspects 1 to 5 of the present invention, characterized in that the total content of Fe, Ni, As, Ag, Sn, Sb, Pb, Bi, and P in the inner copper material is 100 mass ppm or less. According to the superconducting wire of aspect 6 of the present invention, the total content of Fe, Ni, As, Ag, Sn, Sb, Pb, Bi, and P in the inner copper material is 100 mass ppm or less, so that the decrease in the residual resistance ratio (RRR) due to these elements can be suppressed and the residual resistance ratio (RRR) can be further improved.

[0019] A superconducting wire according to a seventh aspect of the present invention is the superconducting wire according to any one of the first to sixth aspects of the present invention, characterized in that the tensile strength is 200 MPa or more. According to the superconducting wire of the seventh aspect of the present invention, the tensile strength is set to 200 MPa or more, and therefore the strength is excellent, and vibration when a high magnetic field is applied can be suppressed.

[0020] A superconducting wire according to an eighth aspect of the present invention is the superconducting wire according to any one of the first to seventh aspects of the present invention, characterized in that the residual resistance ratio (RRR) is 150 or more. According to the superconducting wire of the eighth aspect of the present invention, the residual resistance ratio (RRR) is set to 150 or more, so that when the superconducting state is broken, the current can be efficiently diverted to the superconducting stabilizer.

[0021] A superconducting coil according to a ninth aspect of the present invention is characterized by having a structure in which the superconducting wire according to any one of the first to eighth aspects of the present invention is wound. According to the superconducting coil of the ninth aspect of the present invention, the above-mentioned superconducting wire is wound, and therefore the residual resistance ratio (RRR) is sufficiently high and the strength is excellent, and when the superconducting state is broken, the current can be efficiently diverted to the superconducting stabilizer, and the coil can be used stably. [Effects of the Invention]

[0022] It is now possible to provide a superconducting wire that can achieve a high residual resistance ratio (RRR) and high strength, and a superconducting coil that uses this superconducting wire. [Brief explanation of the drawings]

[0023] [Figure 1]1 is a schematic cross-sectional view of a superconducting wire according to an embodiment of the present invention. [Figure 2] 1 is a flow diagram of a method for manufacturing a superconducting wire according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0024] A superconducting wire 10 according to one embodiment of the present invention will be described below. The superconducting wire 10 in this embodiment includes a superconductor 11 and a superconducting stabilizer 20 . The superconducting stabilizer 20 includes an inner copper material 21 disposed on the inner periphery side of the superconductor 11 and an outer copper material 22 disposed on the outer periphery side of the superconductor 11 .

[0025] In the superconducting wire 10 of this embodiment, the inner copper material 21 and the outer copper material 22 constituting the superconducting stabilizer 20 are made of copper material with a Cu content of 99.9 mass% or more, the inner copper material 21 is doped with one or more elements selected from Ca, Sr, Ba, and rare earth elements, and the outer copper material 22 is doped with Mg. That is, the inner copper material 21 and the outer copper material 22 are made of copper materials with different compositions.

[0026] Here, it is preferable that the inner copper material 21 contains one or more elements selected from Ca, Sr, Ba, and rare earth elements in a total amount within the range of 10 mass ppm to 130 mass ppm, with the remainder being Cu and unavoidable impurities. On the other hand, the outer copper material 22 preferably contains Mg in the range of 10 mass ppm to 130 mass ppm, with the remainder being Cu and unavoidable impurities.

[0027] In the inner copper material 21, it is preferable that the ratio X / Y of the total content of S, Se, and Te (Y ppm by mass) to the total content of Ca, Sr, Ba, and rare earth elements (X ppm by mass) is within the range of 0.5≦X / Y≦20. Furthermore, it is preferable that the total content of Fe, Ni, As, Ag, Sn, Sb, Pb, Bi, and P in the inner copper material 21 is 100 mass ppm or less.

[0028] In the superconducting wire 10 of this embodiment, the misorientation of each crystal grain is analyzed by the EBSD method, and the length of the small-angle grain boundary and the subgrain boundary between adjacent measurement points where the misorientation between the adjacent measurement points is 2° or more and 15° or less is defined as L. LB The length of the high-angle grain boundary between adjacent measurement points where the misorientation between the measurement points exceeds 15° is defined as L. HB , the length ratio of the low-angle grain boundary and the subgrain boundary is L LB / (L LB +L HB ), the length ratio A of the low-angle grain boundaries and subgrain boundaries of the inner copper material 21 is 10% or less, and the ratio B / A of the length ratio A of the low-angle grain boundaries and subgrain boundaries of the inner copper material to the length ratio B of the low-angle grain boundaries and subgrain boundaries of the outer copper material is 2 or more. It is also preferable that the GOS value C of the inner copper material 21 is 0.55 or less, and the ratio D / C of the LAM value C of the inner copper material 21 to the GOS value D of the outer copper material 22 is 2.5 or more.

[0029] Furthermore, the superconducting wire 10 of this embodiment preferably has a tensile strength of 200 MPa or more. Furthermore, in the superconducting wire 10 of this embodiment, the residual resistance ratio (RRR) is preferably 150 or more.

[0030] The reasons for specifying the component composition, crystalline structure, and various properties of the superconducting stabilizer 20 (the inner copper material 21 and the outer copper material 22) in the superconducting wire 10 of this embodiment as described above will be explained below.

[0031] (Composition of inner copper material 21) In the superconducting wire 10 of this embodiment, the inner copper material 21 arranged on the inner side of the superconductor 11 is composed of a copper material with a Cu content of 99.9 mass% or more to which one or more elements selected from Ca, Sr, Ba, and rare earth elements have been added. Among the unavoidable impurities contained in copper, S, Se, and Te are elements that significantly reduce the residual resistance ratio (RRR) when they dissolve in copper. Therefore, in order to improve the residual resistance ratio (RRR), it is necessary to eliminate the effects of S, Se, and Te.

[0032] Here, one or more additive elements selected from Ca, Sr, Ba, and rare earth elements are highly reactive with S, Se, and Te, and therefore, by forming compounds with S, Se, and Te, it is possible to suppress the solid solution of S, Se, and Te in copper, thereby sufficiently improving the residual resistivity ratio (RRR) of the superconducting stabilizer 20 (inner copper material 21).

[0033] In addition, one or more additive elements selected from Ca, Sr, Ba, and rare earth elements are elements that are difficult to dissolve in copper, and even if they dissolve in copper, they have little effect on reducing the residual resistance ratio (RRR). Therefore, even if they are added in excess of the content of S, Se, and Te, the residual resistance ratio (RRR) of the superconducting stabilizer 20 (inner copper material 21) will not be significantly reduced.

[0034] Here, by setting the content of one or more elements selected from Ca, Sr, Ba, and rare earth elements to 3 ppm by mass or more in the inner copper material 21, the effect of fixing S, Se, and Te can be fully achieved. On the other hand, by setting the content of one or more elements selected from Ca, Sr, Ba, and rare earth elements to 100 ppm by mass or less, the generation of coarse precipitates containing these elements can be suppressed, and workability can be ensured. Therefore, in this embodiment, it is preferable that the inner copper material 21 contains one or more elements selected from Ca, Sr, Ba, and rare earth elements in a total amount within the range of 3 mass ppm to 100 mass ppm, with the remainder being Cu and inevitable impurities.

[0035] In order to more reliably fix S, Se, and Te, the lower limit of the content of one or more elements selected from Ca, Sr, Ba, and rare earth elements is preferably set to 4 ppm by mass or more, and more preferably set to 5 ppm by mass or more. On the other hand, in order to further ensure processability, the upper limit of the content of one or more elements selected from Ca, Sr, Ba, and rare earth elements is preferably set to 100 ppm by mass or less, more preferably set to 50 ppm by mass or less, and even more preferably set to 20 ppm by mass or less.

[0036] As described above, one or more elements selected from Ca, Sr, Ba, and rare earth elements form compounds with elements such as S, Se, and Te. When the ratio X / Y of the total content of S, Se, and Te (Y ppm by mass) to the total content of one or more elements selected from Ca, Sr, Ba, and rare earth elements (X ppm by mass) is 0.5 or greater, the content of one or more elements selected from Ca, Sr, Ba, and rare earth elements is sufficient, and elements such as S, Se, and Te can be sufficiently immobilized. On the other hand, when the ratio X / Y of the total content of S, Se, and Te (Y ppm by mass) to the total content of one or more elements selected from Ca, Sr, Ba, and rare earth elements (X ppm by mass) is 20 or less, the presence of a large amount of excess additive elements that do not react with S, Se, and Te is suppressed, thereby ensuring workability. In view of the above, in this embodiment, it is preferable that the ratio X / Y of the total content of S, Se, and Te (Y ppm by mass) to the total content of one or more elements selected from Ca, Sr, Ba, and rare earth elements (X ppm by mass) is within the range of 0.5 to 20.

[0037] In order to reliably fix elements such as S, Se, and Te as compounds, the ratio X / Y of the total content of S, Se, and Te (Y ppm by mass) to the total content of one or more elements selected from Ca, Sr, Ba, and rare earth elements (X ppm by mass) is preferably set to a lower limit of 0.75 or more, and more preferably set to a lower limit of 1.0 or more. In order to reliably suppress deterioration of workability, the ratio X / Y of the total content of S, Se, and Te (Y ppm by mass) to the total content of one or more elements selected from Ca, Sr, Ba, and rare earth elements (X ppm by mass) is preferably set to a lower limit of 17 or less, and more preferably set to a lower limit of 11 or less.

[0038] In addition, since specific impurity elements such as Fe, Ni, As, Ag, Sn, Sb, Pb, Bi, and P have the effect of lowering the residual resistance ratio (RRR), by specifying the total content of these elements, it is possible to reliably suppress the decrease in the residual resistance ratio (RRR) of the inner copper material 21. Therefore, in this embodiment, it is preferable that the total content of Fe, Ni, As, Ag, Sn, Sb, Pb, Bi, and P in the inner copper material 21 is 100 mass ppm or less. In addition, in order to more reliably suppress the decrease in the residual resistance ratio (RRR) of the superconducting wire 10, it is more preferable that the total content of Fe, Ni, As, Ag, Sn, Sb, Pb, Bi, and P in the inner copper material 21 be 50 mass ppm or less, and even more preferable that it be 40 mass ppm or less.

[0039] (Composition of outer copper material 22) In the superconducting wire 10 of this embodiment, the outer copper material 22 arranged on the outer periphery of the superconductor 11 is made of a copper material containing 99.9 mass % or more of Cu to which Mg has been added. Mg is an element that easily reacts with oxygen to generate Mg oxides. Therefore, by adding Mg to the outer copper material 22 arranged on the outer periphery of the superconductor 11, oxygen diffusing from the outside of the superconducting wire 10 can be trapped by the Mg in the outer copper material 22, and the diffusion of oxygen into the inner copper material 21 can be suppressed.

[0040] Here, by setting the Mg content in the outer copper material 22 to 10 ppm by mass or more, it is possible to reliably trap oxygen and further suppress the diffusion of oxygen into the inner copper material 21. On the other hand, by setting the Mg content to 130 ppm by mass or less, it is possible to suppress the generation of coarse precipitates containing Mg, and it is possible to ensure workability. Therefore, in this embodiment, the outer copper material 22 preferably contains Mg in the range of 10 mass ppm to 130 mass ppm, with the remainder being Cu and inevitable impurities. In order to reliably trap oxygen, the lower limit of the Mg content is preferably 20 ppm by mass or more, and more preferably 30 ppm by mass or more, while in order to further ensure workability, the upper limit of the Mg content is preferably 100 ppm by mass or less, and more preferably 80 ppm by mass or less.

[0041] In addition, by specifying the total content of specific impurity elements such as Fe, Ni, As, Ag, Sn, Sb, Pb, Bi, and P in the outer copper material 22, it is possible to reliably suppress a decrease in the residual resistance ratio (RRR) of the inner copper material 21. In this embodiment, it is preferable that the total content of Fe, Ni, As, Ag, Sn, Sb, Pb, Bi, and P in the outer copper material 22 is 100 mass ppm or less. In addition, in order to more reliably suppress the decrease in the residual resistance ratio (RRR) of the outer copper material 22, it is more preferable that the total content of Fe, Ni, As, Ag, Sn, Sb, Pb, Bi, and P be 50 mass ppm or less, and even more preferable that it be 40 mass ppm or less.

[0042] (Length ratio of low-angle grain boundaries and subgrain boundaries) Among grain boundaries, low-angle grain boundaries and subgrain boundaries are regions with a high density of dislocations introduced during processing. Here, by setting the length ratio A of the low-angle grain boundaries and subgrain boundaries of the inner copper material 21 to 10% or less, the density of dislocations introduced during processing in the inner copper material 21 is reduced, the influence of dislocations can be suppressed, and the residual resistivity ratio (RRR) can be improved. On the other hand, by specifying the ratio B / A of the length ratio A of the low-angle grain boundaries and subgrain boundaries in the inner copper material to the length ratio B of the low-angle grain boundaries and subgrain boundaries in the outer copper material to be 2 or more, the density of dislocations introduced during processing in the outer copper material 22 increases while the residual resistivity ratio (RRR) of the superconducting wire 10 is high, and work hardening can improve the strength of the superconducting wire 10. If the ratio B / A of the length ratio A of the low-angle grain boundaries and subgrain boundaries in the inner copper material to the length ratio B of the low-angle grain boundaries and subgrain boundaries in the outer copper material is less than 2, the residual resistivity ratio (RRR) and tensile strength of the superconducting wire will decrease.

[0043] The length ratio A of the low-angle grain boundaries and subgrain boundaries in the inner copper material 21 is preferably 9% or less, and more preferably 8% or less. In addition, in this embodiment, the ratio B / A of the length ratio A of the low-angle grain boundaries and subgrain boundaries in the inner copper material 21 to the length ratio B of the low-angle grain boundaries and subgrain boundaries in the outer copper material 22 is preferably 2.5 or more, more preferably 3.0 or more, and even more preferably 4.0 or more.

[0044] (GOS value) The GOS (Grain Orientation Spread) value measured by EBSD is the average of the angular differences between all pixels within a crystal grain. Furthermore, the average GOS value is calculated by averaging the calculated GOS values across the measurement field. The average GOS value is calculated using the number of crystals, not the size of each crystal region. In other words, a large GOS value indicates localized strain within the crystal grain. Note that calculations are performed for pixels with an angular difference of 5° or less. By keeping the GOS value low, strain is uniformly distributed, resulting in uniform grain growth and effectively suppressing localized grain coarsening.

[0045] Here, when the GOS value C of the inner copper material 21 is 0.55 or less, the crystal grains in the inner copper material 21 grow uniformly, and the residual resistivity ratio (RRR) can be further improved. Furthermore, by specifying the ratio D / C between the GOS value C of the inner copper material 21 and the GOS value D of the outer copper material 22 to be 2.5 or more, a relatively large amount of strain exists in the outer copper material 22, making it possible to further improve the strength of the superconducting wire 10. The GOS value C of the inner copper material 21 is preferably 0.50 or less, and more preferably 0.45 or less. Furthermore, the ratio D / C of the GOS value C of the inner copper material 21 to the GOS value D of the outer copper material 22 is preferably 3.0 or more, and more preferably 3.5 or more.

[0046] (tensile strength) When the superconducting wire 10 is used in a high magnetic field environment, if the mechanical strength of the entire superconducting wire 10 is insufficient, the superconductor 11 and the superconducting stabilizer 20 (inner copper material 21, outer copper material 22) will vibrate slightly, which will generate frictional heat and may cause the superconducting state to be broken. For this reason, it is preferable that the tensile strength of the superconducting wire 10 of this embodiment is set to 200 MPa or more. In the superconducting wire 10 of this embodiment, the tensile strength is more preferably 220 MPa or more, and even more preferably 240 MPa or more.

[0047] (Residual resistance ratio (RRR)) In the superconducting stabilizer 20 (inner copper material 21, outer copper material 22), in order to have a low resistance value at cryogenic temperatures and to divert current well, the residual resistance ratio (RRR) of the superconducting wire 10 is required to be high. Here, if the residual resistance ratio (RRR) of the superconducting wire 10 is less than 150, the superconducting stabilizer 20 may not be able to effectively divert the current. The lower limit of the residual resistance ratio (RRR) of the superconducting wire 10 is preferably 200 or more, and more preferably 250 or more.

[0048] Next, an example of a method for manufacturing the superconducting wire 10 according to this embodiment will be described with reference to the flow chart shown in FIG.

[0049] (Superconducting stabilizer preparation process S01) First, the inner copper material 21 and the outer copper material 22 that constitute the superconducting stabilizer 20 are prepared. Here, in this embodiment, the inner copper material 21 and the outer copper material 22 constituting the superconducting stabilizer 20 are manufactured by a manufacturing process including a melting and casting process, a hot processing process such as hot extrusion, a plastic processing process, a heat treatment process, and a cold plastic processing process. Note that a copper wire rod having the composition shown in this embodiment may be manufactured by a continuous casting and rolling method (for example, an SCR method) or the like, and the inner copper material 21 constituting the superconducting stabilizer 20 of this embodiment may be manufactured using this as a raw material. In this case, the production efficiency of the inner copper material 21 constituting the superconducting stabilizer 20 of this embodiment is improved, and it becomes possible to significantly reduce the manufacturing cost. The continuous casting and rolling method referred to here is a process in which a copper wire rod is manufactured using, for example, continuous casting and rolling equipment equipped with a belt-wheel type continuous casting machine and a continuous rolling device, and this copper wire rod is used as a raw material to manufacture a drawn copper wire.

[0050] (Assembly process S02) Next, the inner copper material 21 and outer copper material 22 prepared as described above and the superconductor 11 are assembled so as to have an arrangement as shown in the cross-sectional view of FIG.

[0051] (Hot extrusion process S03) Next, the assembly of the inner copper material 21, the outer copper material 22 and the superconductor 11 is subjected to hot extrusion. After holding the material in an air atmosphere at 500°C or higher and 1000°C or lower for 1 hour or longer, hot extrusion is carried out at 500°C or higher and 1000°C or lower.

[0052] (Wire drawing process S04) After the hot extrusion step S03, wire drawing is performed. The processing rate (area reduction rate) at this time is not particularly limited, but the total processing rate (total area reduction rate) of the wire drawing is preferably 10% or more. There are no particular limitations on the temperature conditions in the wire drawing step S04, but it is preferable to set the temperature within the range of -50°C to 150°C, which corresponds to cold or warm conditions.

[0053] (Heat treatment process S05) After the wire drawing process S04, a heat treatment is carried out in which the wire is held at a temperature of 300° C. to 450° C. for at least one hour. The wire drawing process S04 and the heat treatment process S05 are repeated several times to optimize the superconducting properties.

[0054] (Finishing wire drawing process S06) After the heat treatment step S05, finish wire drawing is carried out to obtain a predetermined wire diameter. The working rate (area reduction rate) at this time is not particularly limited, but the total working rate (total area reduction rate) of the wire drawing is preferably 10% or more. There are no particular limitations on the temperature conditions in the finish wire drawing step S06, but it is preferable to set the temperature within the range of -50°C to 150°C, which is cold or warm.

[0055] (Finishing heat treatment process S07) After the finish wire drawing process step S06, a heat treatment is carried out in an air atmosphere at a temperature of 150° C. to 300° C. for 1 minute or more. This finish heat treatment step S07 removes strain.

[0056] The superconducting wire 10 of this embodiment is manufactured through the above-described steps. The superconducting coil of this embodiment has a structure in which the superconducting wire 10 of this embodiment described above is wound around a bobbin or the like.

[0057] The superconducting wire 10 of this embodiment configured as described above includes a superconductor 11 and a superconducting stabilizer 20 as shown in FIG. 1. The superconducting stabilizer 20 has an inner copper material 21 and an outer copper material 22 made of copper material with a Cu content of 99.9 mass% or more. The inner copper material 21 contains one or more elements selected from Ca, Sr, Ba, and rare earth elements. Therefore, in the inner copper material 21, elements such as S, Se, and Te that reduce the residual resistivity ratio (RRR) can be fixed by one or more elements selected from Ca, Sr, Ba, and rare earth elements.

[0058] Furthermore, since Mg is added to the outer copper material 22, oxygen that diffuses from the outside into the inside of the superconducting wire 10 during heat treatment in the air atmosphere can be trapped by the Mg, and the diffusion of oxygen into the inner copper material 21 can be suppressed. This makes it possible to suppress the consumption of one or more elements selected from Ca, Sr, Ba, and rare earth elements contained in the inner copper material 21 as oxides, and reliably fix elements such as S, Se, and Te, thereby obtaining a high residual resistivity ratio (RRR).

[0059] Furthermore, since the length ratio A of the low-angle grain boundaries and subgrain boundaries of the inner copper material 21 is set to 10% or less, the density of dislocations introduced during processing in the inner copper material 21 to which one or more elements selected from Ca, Sr, Ba, and rare earth elements are added is low, and it is possible to increase the residual resistance ratio (RRR). On the other hand, since the ratio B / A of the length ratio A of the low-angle grain boundaries and subgrain boundaries in the inner copper material to the length ratio B of the low-angle grain boundaries and subgrain boundaries in the outer copper material is set to 2 or more, the density of dislocations introduced during processing in the outer copper material 22 that has been solid-solution strengthened by the addition of Mg is relatively high, making it possible to improve strength through work hardening.

[0060] In the superconducting wire 10 of this embodiment, when the outer copper material 22 contains Mg in the range of 10 mass ppm to 130 mass ppm, with the remainder being Cu and unavoidable impurities, the Mg can reliably trap oxygen diffusing from the outside, and processability can be ensured.

[0061] In the superconducting wire 10 of this embodiment, when the inner copper material 21 contains one or more elements selected from Ca, Sr, Ba, and rare earth elements in a total amount within the range of 3 mass ppm to 100 mass ppm, with the remainder being Cu and unavoidable impurities, S, Se, and Te can be sufficiently fixed by the one or more elements selected from Ca, Sr, Ba, and rare earth elements, and the residual resistivity ratio (RRR) can be sufficiently improved while ensuring workability.

[0062] In the superconducting wire 10 of this embodiment, when the GOS value C of the inner copper material 21 is 0.55 or less and the ratio D / C of the GOS value C of the inner copper material 21 to the GOS value D of the outer copper material 22 is 2.5 or more, the strength of the outer copper material 22, which has been solid-solution strengthened by the addition of Mg, is maintained sufficiently high, while the residual resistance ratio (RRR) of the superconducting wire 10 can be further increased by the inner copper material 21 to which one or more elements selected from Ca, Sr, Ba, and rare earth elements have been added.

[0063] In the superconducting wire 10 of this embodiment, when the ratio X / Y of the total content of S, Se, and Te (Y ppm by mass) to the total content of Ca, Sr, Ba, and rare earth elements (X ppm by mass) in the inner copper material 21 is set within the range of 0.5≦X / Y≦20, S, Se, and Te in the copper can be reliably fixed as compounds, and a decrease in the residual resistivity ratio (RRR) due to S, Se, and Te can be reliably suppressed.

[0064] In the superconducting wire 10 of this embodiment, when the total content of Fe, Ni, As, Ag, Sn, Sb, Pb, Bi, and P in the inner copper material 21 is 100 mass ppm or less, the decrease in the residual resistance ratio (RRR) due to these elements can be suppressed, and the residual resistance ratio (RRR) can be further improved.

[0065] In the superconducting wire 10 of this embodiment, when the tensile strength is 200 MPa or more, the strength of the superconducting wire 10 is excellent, and vibrations when a high magnetic field is applied can be suppressed.

[0066] In the superconducting wire 10 of this embodiment, if the residual resistance ratio (RRR) is 150 or more, when the superconducting state is broken, the current can be efficiently diverted to the superconducting stabilizer.

[0067] The superconducting coil of this embodiment has a structure in which the superconducting wire of this embodiment described above is wound, and therefore has a sufficiently high residual resistance ratio (RRR) and excellent strength. When the superconducting state is broken, the current can be efficiently diverted to the superconducting stabilizer, and the coil can be used stably.

[0068] The above describes the superconducting wire and superconducting coil according to the embodiment of the present invention, but the present invention is not limited to this and can be modified as appropriate within the scope of the technical concept of the invention. In the above embodiment, an example of a method for manufacturing a superconducting wire is described, but the method for manufacturing a superconducting wire is not limited to the one described in the embodiment, and an existing manufacturing method may be appropriately selected for manufacturing the superconducting wire. [Example]

[0069] The results of confirmation experiments conducted to confirm the effects of the present invention will be described below.

[0070] As the inner copper material, billets (diameter 160 mm, length 1000 mm) and wire rods (diameter 8 mm to 1 mm, length 1000 mm) with the compositions listed in Tables 1 to 3 were prepared using various master alloys made from copper raw materials with a Cu purity of 99.99 mass% or more, pure metal with a purity of 2N (purity 99 mass%) or more, and copper raw materials with a Cu purity of 99.99 mass% or more. As the outer copper material, billets with the compositions listed in Tables 1 to 3 were prepared using copper raw material with a Cu purity of 99.99 mass% or more and pure Mg with a purity of 2N (purity of 99 mass%) or more, and hot-extruded pipes (outer diameter 300 mm, inner diameter 260 mm, length 1000 mm) were produced.

[0071] Next, the inner copper billet and NbTi alloy superconductor were placed inside the outer copper tube, and assembled as shown in Figure 1, with the gaps filled with the inner copper wire. This assembly was hot extruded at 800°C in an air atmosphere, then subjected to wire drawing and heat treatment at 300°C for 1 hour. Next, finish wire drawing and finish heat treatment were carried out for 1 hour under the conditions shown in Tables 4 to 6 to obtain a superconducting wire with a diameter of 1.5 mm.

[0072] The superconducting stabilizers (inner and outer copper materials) constituting the obtained superconducting wires of the present invention and comparative examples were measured for composition, length ratio of low-angle grain boundaries and subgrain boundaries, GOS value, tensile strength, and residual resistance ratio (RRR) of the superconducting wire as follows. The measurement results are shown in Tables 4 to 6. The composition, length ratio of low-angle grain boundaries and subgrain boundaries, and GOS value were measured at the target locations on a cross section of a superconducting wire with a diameter of 1.5 mm. The tensile strength and residual resistivity ratio (RRR) were measured using the superconducting wire.

[0073] (composition) Mg was measured by inductively coupled plasma atomic emission spectrometry, and other elements were measured using a glow discharge mass spectrometer (GD-MS).

[0074] (Length ratio of low-angle grain boundaries and subgrain boundaries) The length ratios of low-angle grain boundaries and subgrain boundaries were calculated using an EBSD measurement device and OIM analysis software as follows: After mechanical polishing using waterproof abrasive paper and diamond abrasive grains, a final polishing was performed using a colloidal silica solution to prepare a measurement sample. Then, using an EBSD measurement device (FEI Quanta FEG 450, EDAX / TSL (now AMETEK) OIM Data Collection) and analysis software (EDAX / TSL (now AMETEK) OIM Data Analysis ver. 7.3.1), the electron beam acceleration voltage was 15 kV, and multiple fields of view were used to measure the total area of 10,000 μm, so that a total of 1,000 or more crystal grains were included. 2 The measurement results were analyzed using the data analysis software OIM to obtain the CI value for each measurement point in the measurement area above. Except for measurement points where the CI value was 0.1 or less, the misorientation of each crystal grain was analyzed using the data analysis software OIM. The boundaries between measurement points where the misorientation between adjacent measurement points was 2° or more and less than 15° were defined as low-angle grain boundaries and subgrain boundaries, and their lengths were defined as L LB The boundary between adjacent measurement points where the misorientation between the measurement points is 15° or more is defined as a high-angle grain boundary, and its length is defined as L HB The ratio of the length of low-angle grain boundaries and subgrain boundaries to the total grain boundaries, L LB / (L LB +L HB ) was sought.

[0075] (GOS value) Using an EBSD measurement device (FEI Quanta FEG 450, EDAX / TSL (now AMETEK) OIM Data Collection) and analysis software (EDAX / TSL (now AMETEK) OIM Data Analysis ver. 8.6), the electron beam acceleration voltage was 15 kV, and the measurement interval was 1 μm, with a step size of 1 mm. 2 The observation surface of the sample was measured using the EBSD method in the above measurement area. Except for measurement points where the CI value was 0.1 or less, the misorientation of each crystal grain was analyzed using the data analysis software OIM. Boundaries between adjacent pixels where the misorientation between pixels was 5° or more were considered to be grain boundaries and analyzed, and the GOS value of all crystal grains was calculated. The total value was divided by the number of crystal grains to obtain the average GOS value.

[0076] (tensile strength) The test was conducted in accordance with JIS Z 2241:2011 "Method of tensile testing for metallic materials."

[0077] (Residual resistance ratio (RRR)) Electrical resistivity (ρ 293K ) and electrical resistivity (ρ 4.2K ) and RRR = ρ 293K / ρ 4.2K was calculated.

[0078] [Table 1]

[0079] [Table 2]

[0080] [Table 3]

[0081] [Table 4]

[0082] [Table 5]

[0083] [Table 6]

[0084] In Comparative Example 1, the outer copper material did not contain Mg, and the inner copper material did not contain Ca, Sr, or rare earth elements (RE), and the residual resistance ratio (RRR) of the superconducting wire was low at 98. In Comparative Example 2, the outer copper material did not contain Mg, and the residual resistance ratio (RRR) of the superconducting wire was low at 123. This is presumably because Ca in the inner copper material reacted with oxygen. In Comparative Example 3, the inner copper material did not contain Ca, Sr, or rare earth elements (RE), and the residual resistance ratio (RRR) of the superconducting wire was as low as 43. In Comparative Example 4, the ratio B / A of the length ratio A of the low-angle grain boundaries and subgrain boundaries in the inner copper material to the length ratio B of the low-angle grain boundaries and subgrain boundaries in the outer copper material was low, and the tensile strength was low at 185 MPa. In Comparative Example 5, the length ratio A of the small-angle grain boundaries and subgrain boundaries of the inner copper material was high, and the residual resistance ratio (RRR) of the superconducting wire was low at 69.

[0085] In contrast, the present invention provides a superconducting wire with a tensile strength of 215 MPa or more and a residual resistance ratio (RRR) of 151 or more, which allows realization of a high residual resistance ratio (RRR) and high strength. [Explanation of symbols]

[0086] 10 Superconducting wire 11 Superconductors 20 Superconducting stabilizer 21 Inner copper material 22 Outer copper material

Claims

1. A superconducting wire comprising a superconductor and a superconducting stabilizer, The superconducting stabilizer has an inner copper material and an outer copper material each made of a copper material having a Cu content of 99.9 mass% or more, The inner copper material is doped with one or more elements selected from Ca, Sr, Ba, and rare earth elements, and the outer copper material is doped with Mg, The misorientation of each crystal grain was analyzed by the EBSD method, and the length of the low-angle grain boundary and subgrain boundary between adjacent measurement points where the misorientation between the adjacent measurement points is 2° or more and 15° or less was defined as L. ＬＢ The length of the high-angle grain boundary between adjacent measurement points where the misorientation between the measurement points exceeds 15° is defined as L ＨＢ , the length ratio of the low-angle grain boundary and the subgrain boundary is L ＬＢ / (L ＬＢ +L ＨＢ ) and A superconducting wire characterized in that the length ratio A of the low-angle grain boundaries and subgrain boundaries of the inner copper material is 10% or less, and the ratio B / A of the length ratio A of the low-angle grain boundaries and subgrain boundaries of the inner copper material to the length ratio B of the low-angle grain boundaries and subgrain boundaries of the outer copper material is 2 or more.

2. 2. The superconducting wire according to claim 1, wherein the outer copper material contains Mg in a range of 10 ppm by mass to 130 ppm by mass, with the remainder being Cu and unavoidable impurities.

3. 2. The superconducting wire according to claim 1, wherein the inner copper material contains one or more elements selected from Ca, Sr, Ba, and rare earth elements in a total amount within a range of 3 ppm by mass to 100 ppm by mass, with the remainder being Cu and unavoidable impurities.

4. 2. The superconducting wire according to claim 1, characterized in that the GOS value C of the inner copper material is 0.55 or less, and the ratio D / C of the GOS value C of the inner copper material to the GOS value D of the outer copper material is 2.5 or more.

5. 2. The superconducting wire according to claim 1, wherein in the inner copper material, a ratio X / Y of a total content of S, Se, and Te (Y ppm by mass) to a total content of Ca, Sr, Ba, and rare earth elements (X ppm by mass) is within a range of 0.5≦X / Y≦20.

6. 2. The superconducting wire according to claim 1, wherein the total content of Fe, Ni, As, Ag, Sn, Sb, Pb, Bi, and P in the inner copper material is 100 mass ppm or less.

7. 2. The superconducting wire according to claim 1, wherein the superconducting wire has a tensile strength of 200 MPa or more.

8. 2. The superconducting wire according to claim 1, characterized in that the residual resistivity ratio (RRR) is 150 or more.

9. A superconducting coil having a structure in which the superconducting wire according to any one of claims 1 to 8 is wound.

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