Oxide superconducting wires, superconducting coils, and superconducting conductors

Abstract

This oxide superconductor wire material comprises: a tape-shaped metal substrate formed from a nickel alloy; an intermediate layer layered on the metal substrate; and an oxide superconductor layer layered on the intermediate layer. The average crystal grain size of the metal substrate is 3.08 μm or greater, but no greater than the metal substrate thickness.

Description

Technical Field

[0001] The present invention relates to an oxide superconducting wire, a superconducting coil, and a superconducting conductor. This application claims priority based on Japanese Patent Application No. 2022-172013 filed in Japan on October 27, 2022, the content of which is incorporated herein by reference.

Background Art

[0002] Patent Document 1 discloses an oxide superconducting wire including a tape-shaped metal substrate, an intermediate layer laminated on the metal substrate, and an oxide superconducting layer laminated on the intermediate layer.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Conventionally, when an excessive tensile stress is applied to an oxide superconducting wire in the longitudinal direction, there has been a problem that the characteristics of the oxide superconducting wire deteriorate. Generally, the oxide superconducting layer has higher rigidity than a metal substrate and is less likely to undergo elongation deformation. Therefore, when an excessive tensile stress is applied to the oxide superconducting wire along the longitudinal direction, the oxide superconducting layer may not withstand the elongation deformation of the metal substrate, and cracks may occur in the oxide superconducting layer. Since the resistance increases at the portion where cracks occur, there is a problem that the superconducting characteristics of the oxide superconducting layer deteriorate.

[0005] The present invention has been made in consideration of such circumstances, and an object thereof is to provide an oxide superconducting wire, a superconducting coil, and a superconducting conductor in which the superconducting characteristics are less likely to deteriorate even when a tensile stress is applied.

Means for Solving the Problems

[0006] To solve the above problems, the oxide superconducting wire according to embodiment 1 of the present invention comprises a tape-shaped metal substrate made of a nickel alloy, an intermediate layer laminated on the metal substrate, and an oxide superconducting layer laminated on the intermediate layer, wherein the average grain size of the metal substrate is 3.08 μm or more and is less than or equal to the thickness of the metal substrate.

[0007] According to embodiment 1 of the present invention, an oxide superconducting wire can be realized that has a high allowable strain amount in LN2, that is, the superconducting properties do not deteriorate easily even when tensile stress is applied.

[0008] Furthermore, in embodiment 2 of the present invention, in the oxide superconducting wire of embodiment 1, the standard deviation of the crystal grain size of the metal substrate is in the range of 2.32 to 14.66 μm.

[0009] According to aspect 2 of the present invention, an oxide superconducting wire with high tensile strength can be realized, which has a metal substrate in which recrystallization has not progressed very much.

[0010] Furthermore, in embodiment 3 of the present invention, in the oxide superconducting wire of embodiment 1 or embodiment 2, the average grain size is the average value of the grain size in a cross-section along the longitudinal and thickness directions of the metal substrate.

[0011] Furthermore, in embodiment 4 of the present invention, in any one of embodiments 1 to 3, the average grain size is the average value of the grain size measured by the reflection EBSD method. Furthermore, the superconducting coil according to embodiment 5 of the present invention is formed by winding one of the oxide superconducting wires from embodiment 1 to embodiment 4. Furthermore, in the superconducting conductor according to embodiment 6 of the present invention, multiple oxide superconducting wires from any one of embodiments 1 to 4 are assembled together. [Effects of the Invention]

[0012] According to the above-described embodiment of the present invention, it is possible to provide an oxide superconducting wire, a superconducting coil, and a superconducting conductor that do not easily deteriorate in superconducting properties even when tensile stress is applied. [Brief explanation of the drawing]

[0013] [Figure 1] This is a cross-sectional view showing an oxide superconducting wire according to an embodiment of the present invention. [Figure 2] This figure shows the relationship between the average grain size of the metal substrate and the allowable strain in LN2. [Figure 3] Figure 1 is a perspective view of a superconducting coil using oxide superconducting wire. [Figure 4A] This figure shows a superconducting conductor using oxide superconducting wires as shown in Figure 1. [Figure 4B] This figure shows other examples of superconducting conductors using oxide superconducting wires, as shown in Figure 1. [Figure 4C] This figure shows other examples of superconducting conductors using oxide superconducting wires, as shown in Figure 1. [Modes for carrying out the invention]

[0014] Hereinafter, an oxide superconducting wire according to an embodiment of the present invention will be described with reference to the drawings. As shown in Figure 1, the oxide superconducting wire 10 according to this embodiment comprises a metal substrate 11, an intermediate layer 12, an oxide superconducting layer 13, a protective layer 14, and a stabilizing layer 16. Hereinafter, the metal substrate 11, the intermediate layer 12, the oxide superconducting layer 13, and the protective layer 14 may be collectively referred to as the "superconducting laminate 15".

[0015] Each of the metal substrate 11, intermediate layer 12, oxide superconducting layer 13, and protective layer 14 is formed in a tape shape. The metal substrate 11, intermediate layer 12, oxide superconducting layer 13, and protective layer 14 are stacked in this order in the thickness direction of the metal substrate 11 (the thickness direction of the oxide superconducting wire 10). The stabilizing layer 16 covers the outer periphery of the superconducting laminate 15. The oxide superconducting wire 10 is in the shape of a tape.

[0016] (Direction Definition) Here, in the present embodiment, an XYZ orthogonal coordinate system is set to describe the positional relationship of each component. The Z-axis direction (not shown) is the direction along the longitudinal direction of the oxide superconducting wire 10. The Y-axis direction is the direction orthogonal to the Z-axis direction and is the direction along the thickness direction of the oxide superconducting wire 10. The Y-axis direction is also the direction in which the layers 11 to 14 of the superconducting laminate 15 are laminated. The X-axis direction is the direction orthogonal to both the Z-axis direction and the Y-axis direction and is the direction along the width direction of the oxide superconducting wire 10. In this specification, the X-axis direction may be referred to as the width direction X, the Y-axis direction may be referred to as the thickness direction Y, and the Z-axis direction may be referred to as the longitudinal direction Z. Also, along the thickness direction Y, the direction from the metal substrate 11 toward the oxide superconducting layer 13 is referred to as the +Y direction or the upward direction. The direction opposite to the +Y direction is referred to as the -Y direction or the downward direction. One direction along the width direction X is referred to as the +X direction or the rightward direction. The direction opposite to the +X direction is referred to as the -X direction or the leftward direction.

[0017] As a specific example of the metal constituting the metal substrate 11, it is a nickel alloy typified by Hastelloy (registered trademark). The thickness of the metal substrate 11 may be appropriately adjusted according to the purpose, and for example, it is within the range of 10 to 1000 μm.

[0018] The intermediate layer 12 is laminated on the metal substrate 11 (the upper surface of the metal substrate 11). The configuration of the intermediate layer 12 is not limited to the example of FIG. 1. For example, the intermediate layer 12 may have a multilayer structure. In this case, the intermediate layer 12 may have, in order in the direction from the metal substrate 11 toward the oxide superconducting layer 13, a diffusion prevention layer, a bed layer, an orientation layer, a cap layer, and the like. These layers are not necessarily provided one by one, and there may be cases where some layers are omitted or cases where two or more layers of the same type are repeatedly laminated. The intermediate layer 12 may be a metal oxide. By forming the oxide superconducting layer 13 on the upper surface of the intermediate layer 12 having excellent orientation, an oxide superconducting layer 13 having excellent orientation can be easily obtained.

[0019] The oxide superconducting layer 13 is laminated on the intermediate layer 12 (the upper surface of the intermediate layer 12). The oxide superconducting layer 13 is composed of an oxide superconductor. As the oxide superconductor constituting the oxide superconducting layer 13, for example, an RE-Ba-Cu-O-based oxide superconductor (REBCO-based oxide superconductor) represented by the general formula RE1Ba2Cu3O y (RE123), etc. can be mentioned. Examples of the rare earth element RE include one or more of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. In the general formula of RE123, y is 7 - x (oxygen deficiency amount x: about 0 to 1). Also, the ratio of RE:Ba:Cu is not limited to 1:2:3, and non-stoichiometric ratios can also occur. The thickness of the oxide superconducting layer 13 is, for example, within the range of 0.5 to 5 μm. The oxide superconducting layer 13 can be formed by a PLD (pulsed laser ablation) film formation method or the like.

[0020] Artificial crystal defects such as artificial pins made of different materials may be introduced into the oxide superconducting layer 13. Examples of the different materials used to introduce artificial pins into the oxide superconducting layer 13 include at least one or more materials such as BaSnO3 (BSO), BaZrO3 (BZO), BaHfO3 (BHO), BaTiO3 (BTO), SnO2, TiO2, ZrO2, LaMnO3, ZnO, etc.

[0021] The protective layer 14 is laminated on the oxide superconducting layer 13 (the upper surface of the oxide superconducting layer 13). The protective layer 14 has functions such as bypassing the overcurrent generated during an accident and suppressing the chemical reaction that occurs between the oxide superconducting layer 13 and the layer provided on the protective layer 14. Examples of the material of the protective layer 14 include silver (Ag), copper (Cu), gold (Au), an alloy of gold and silver, other silver alloys, copper alloys, gold alloys, etc. The thickness of the protective layer 14 is, for example, within the range of 1 to 30 μm. The protective layer 14 may be composed of two or more metals or two or more metal layers. The protective layer 14 can be formed by a vapor deposition method, a sputtering method, or the like.

[0022] The stabilization layer 16 is formed around the entire circumference of the superconducting laminate 15. In other words, the stabilization layer 16 covers the top surface, bottom surface, and a pair of sides of the superconducting laminate 15. In this embodiment, the "top surface of the superconducting laminate 15" corresponds to the top surface of the protective layer 14, the "bottom surface of the superconducting laminate 15" corresponds to the bottom surface of the metal substrate 11, and the "sides of the superconducting laminate 15" correspond to the sides of each layer 11 to 14. The stabilization layer 16 has functions such as bypassing overcurrents generated during an accident and mechanically reinforcing the oxide superconducting layer 13 and the protective layer 14. The stabilization layer 16 is made of, for example, a copper (Cu) plating layer. The thickness of the stabilization layer 16 is not particularly limited, but is, for example, in the range of 1 to 300 μm.

[0023] Generally, oxide superconducting layers have higher rigidity than metal substrates and are less susceptible to tensile deformation. Therefore, if excessive tensile stress is applied along the longitudinal direction to an oxide superconducting wire, the oxide superconducting layer may not be able to withstand the tensile deformation of the metal substrate, and cracks may occur in the oxide superconducting layer. Since the resistance increases in the cracked areas, there is a problem that the superconducting properties of the oxide superconducting layer deteriorate.

[0024] In response to this problem, the inventors of the present invention have conducted thorough research and found that by appropriately setting the average grain size of the metal substrate 11, the decrease in superconductivity when tensile stress is applied can be suppressed. The appropriate value for the average grain size of the metal substrate 11 will be explained below using specific examples. Note that the present invention is not limited to the following examples. [Examples]

[0025] Oxide superconducting wires of Comparative Examples 1-6 and Examples 1-18 were fabricated. These oxide superconducting wires have a metal substrate 11 with a thickness of 30 μm, 50 μm, or 75 μm, and the average grain size of the metal substrate 11 differs from that of each oxide superconducting wire. Table 1 shows the thickness and average grain size of the metal substrate 11 of the oxide superconducting wires 10. Among the multiple oxide superconducting wires 10 that were fabricated, conditions other than the thickness and average grain size of the metal substrate 11 are common as listed below. Note that the average grain size of the metal substrate 11 can be adjusted by adjusting the conditions when rolling the metal substrate 11 (temperature, grain size of the substrate before rolling, etc.) and the conditions when depositing the intermediate layer 12 and the oxide superconducting layer 13 (temperature, time, etc.). Material of metal substrate 11: Hastelloy Width of metal substrate 11: 4mm Material of oxide superconducting layer 13: EuBCO+BHO Thickness of oxide superconducting layer 13: 2 μm Method for forming oxide superconducting layer 13: PLD film formation Material of protective layer 14: Ag Thickness of protective layer 14: 2 μm Method for forming protective layer 14: Sputter deposition Material of stabilization layer 16: Cu Thickness of stabilization layer 16: 5 μm Method for forming the stabilizing layer 16: Plating film formation For the intermediate layer 12, a commonly used type was adopted.

[0026] Table 1 summarizes the results of measuring the average grain size of the metal substrate 11, the standard deviation of the grain size of the metal substrate 11, and the allowable strain in LN2 (details below) for each of the oxide superconducting wires of Comparative Examples 1-6 and Examples 1-18. In Table 1, the tensile strength was judged as "pass" if the allowable strain in LN2 was 0.40% or more, and as "fail" if the allowable strain in LN2 was less than 0.40%. Figure 2 is a graph showing the relationship between the average grain size of the metal substrate 11 and the allowable strain in LN2.

[0027] [Table 1]

[0028] The LN2 allowable strain is a parameter that indicates the resistance of oxide superconducting wire to tensile stress. Specifically, the LN2 allowable strain is defined as the maximum strain at which the properties (superconductivity) of the oxide superconducting wire are maintained. Here, strain is a parameter that represents the elongation rate of the oxide superconducting wire, and is defined by the following equation, where L0 is the natural length of the oxide superconducting wire and L1 is the length of the oxide superconducting wire when it is stretched. Distortion amount [%]=100×(L1-L0) / L0 Whether or not the superconducting properties are maintained is determined by the ratio (Ic1 / Ic0) of the critical current value Ic0 when the oxide superconducting wire is at its natural length to the critical current value Ic1 when the oxide superconducting wire is stretched. If the value of Ic1 / Ic0 is 0.95 or higher, it is determined that the properties are maintained; if it is less than 0.95, it is determined that the properties are not maintained.

[0029] Furthermore, the average grain size of the metal substrate 11 in the fabricated oxide superconducting wire was calculated as the average value of the grain size measured by reflection EBSD (electron backscatter diffraction). The observation conditions for the sample in the EBSD method were as follows. Acceleration voltage: 15kV Irradiation current: 15nA Sample tilt angle: 70° Number of sample observation points: Any one point Spacing: 150nm / step The sample observation area was set to 45 × 45 μm for Comparative Examples 1 and 2 and Examples 1 to 12, which had a substrate thickness of 50 μm; to 25 × 25 μm for Comparative Examples 3 and 4 and Examples 13 to 15, which had a substrate thickness of 30 μm; and to 70 × 70 μm for Comparative Examples 5 and 6 and Examples 16 to 18, which had a substrate thickness of 75 μm. Furthermore, when measuring the grain size using the reflection EBSD method, the grain size of the grains obtained when the crystal orientation angle difference was 5° or more and the Σ3 twin boundary was used as the grain boundary was defined as the grain size. In addition, regions with low reliability in the crystal orientation assignment of the reflection EBSD pattern were excluded, and regions with a reliability parameter CI (Confidence Index) value of 0.1 or higher were adopted. Specifically, for measuring the grain size of the metal substrate 11, cross-sectional samples were prepared along the longitudinal and thickness directions of the metal substrate 11 by mechanical polishing and Ar ion milling, and these cross-sectional samples were observed and measured. A JEOL thermal field emission scanning electron microscope (TFE-SEM) JSM-6500F was used for observing the grain size. Furthermore, the area-average grain size, weighted by the area ratio of each observed grain to the entire field of view, was defined as the average grain size.

[0030] The reason for determining whether the tensile strength was "pass" or "fail" based on whether the allowable strain in LN2 was 0.40% or more is as follows: Generally, in superconducting coils and the like, oxide superconducting wires are subjected to tensile stress due to the electromagnetic force of the superconducting coil. Oxide superconducting wires are required to maintain their superconducting properties even when subjected to this tensile stress caused by electromagnetic force. The inventors of this application investigated oxide superconducting wires that showed deterioration in properties after being coiled and oxide superconducting wires that did not show deterioration in properties after being coiled, and found that the difference between those showing deterioration and those not showing deterioration lies at an allowable strain of 0.40% in LN2. Therefore, in this embodiment, the tensile strength is judged to be "acceptable" or "unacceptable" depending on whether the allowable strain in LN2 is 0.40% or more.

[0031] As shown in Figure 2 and Table 1, the larger the average grain size of the metal substrate 11, the larger the allowable strain in LN2. Examples 1-18, where the average grain size of the metal substrate 11 is 3.08 μm or larger, passed the tensile strength test, while Comparative Examples 1-6, where the average grain size of the metal substrate 11 is less than 3.08 μm, failed the tensile strength test. Furthermore, as shown in Table 1, Examples 1-18, which passed the tensile strength test, have a larger standard deviation of grain size in the metal substrate 11 compared to Comparative Examples 1-6, which failed the tensile strength test. These results will be discussed below.

[0032] Generally, in metallic solids, the smaller the average grain size, the smaller the overall strain of the metallic solid, and the larger the average grain size, the larger the overall strain of the metallic solid. However, the smaller the average grain size, the greater the variation in strain within the metallic solid, making it easier for areas to become uniquely heavily strained. Conversely, the larger the average grain size, the smaller the variation in strain within the metallic solid, making it less likely for areas to become uniquely heavily strained.

[0033] Similarly, in a metal substrate 11 with a small average grain size, when tensile stress is applied, the overall strain of the metal substrate 11 is small, but areas of particularly large strain are likely to occur. This is thought to be because, during the process of grain refinement, areas where solute elements adhere to mobile dislocations appear, resulting in areas with small strain and areas with large strain. It is then thought that cracks occur in the oxide superconducting layer 13 in these areas of large strain, leading to a decrease in superconductivity. On the other hand, in a metal substrate 11 with a large average grain size, when tensile stress is applied, the overall strain of the metal substrate 11 is large, but areas of particularly large strain are less likely to occur. Therefore, cracks are less likely to occur in the oxide superconducting layer 13, and superconductivity is ensured. Thus, it can be concluded that the larger the average grain size of the metal substrate 11, the greater the tensile strength (allowable strain in LN2). Based on the above considerations, it is expected that the allowable strain in LN2 will similarly increase even when the average grain size of the metal substrate 11 is greater than 16.20 μm (Example 15).

[0034] Furthermore, during the process of forming the intermediate layer 12 or the oxide superconducting layer 13, high heat is applied to the metal substrate 11. This heat causes recrystallization of the metal contained in the metal substrate 11. Generally, when rolled metal recrystallizes, the grain size becomes smaller, so from the viewpoint of increasing the allowable strain in LN2, it is desirable that recrystallization does not proceed. If recrystallization has almost completed, most of the crystal grains present in the metal substrate 11 will have a small diameter, so the standard deviation of the crystal grain size will be small. Conversely, if recrystallization has not progressed much, the metal substrate 11 will contain a mixture of crystal grains with small diameters and crystal grains with large diameters, so the standard deviation of the crystal grain size will be large. Therefore, a large standard deviation of the crystal grain size of the metal substrate 11 means that the recrystallization of the metal contained in the metal substrate 11 has not progressed much. In other words, in Comparative Examples 1 to 6, the standard deviation of the grain size of the metal substrate 11 is relatively small (less than 2.32 μm), suggesting that recrystallization has almost completed and the average grain size has decreased. On the other hand, in Examples 1 to 18, the standard deviation of the grain size of the metal substrate 11 is relatively large (2.32 μm or more), suggesting that recrystallization has not progressed much and the average grain size has increased.

[0035] Based on the above, this embodiment proposes an oxide superconducting wire 10 comprising a tape-shaped metal substrate 11 made of a nickel alloy, an intermediate layer 12 laminated on the metal substrate 11, and an oxide superconducting layer 13 laminated on the intermediate layer 12, wherein the average crystal grain size of the metal substrate 11 is 3.08 μm or more and is less than or equal to the thickness of the metal substrate. The upper limit of the average crystal grain size of the metal substrate 11 is based on the fact that the average crystal grain size of the metal substrate 11 will not exceed the thickness of the metal substrate 11.

[0036] This configuration makes it possible to realize an oxide superconducting wire with a high allowable strain in LN2, meaning that the superconducting properties do not deteriorate easily even when tensile stress is applied.

[0037] Furthermore, the standard deviation of the crystal grain size of the metal substrate 11 is within the range of 2.32 to 14.66 μm. This configuration makes it possible to realize an oxide superconducting wire with high tensile strength, having a metal substrate 11 that has not undergone much recrystallization.

[0038] The technical scope of the present invention is not limited to the embodiments described above, and various modifications can be made without departing from the spirit of the invention.

[0039] For example, the oxide superconducting wire 10 does not need to have a protective layer 14 or a stabilizing layer 16.

[0040] Alternatively, as shown in Figure 3, a pancake-type multilayer coil (superconducting coil 100) may be formed by winding a tape-shaped oxide superconducting wire 10 many times in the thickness direction and stacking them. For example, the superconducting coil 100 comprises a laminate in which oxide superconducting wires 10 and metal tapes are alternately stacked, and an impregnating resin layer. The impregnating resin layer is impregnated into the laminate and covers the outer surface of the laminate. Examples of resins constituting the impregnating resin layer include epoxy resin and phenolic resin. The superconducting coil 100 can be manufactured, for example, by coiling an oxide superconducting wire 10 coated with a resin (such as epoxy resin) and a metal tape together, and then curing the resin by heating or the like. Alternatively, as a method for manufacturing the superconducting coil 100, the oxide superconducting wire 10 and the metal tape may be coiled together, the coil may be impregnated with the resin under reduced pressure, and then the resin may be cured by heating or the like. Such a superconducting coil 100 can be used in superconducting magnets, superconducting motors, and the like.

[0041] Furthermore, as shown in Figures 4A to 4C, superconducting conductors 101, 102, and 103 may be formed by bundling multiple tape-shaped oxide superconducting wires 10 together. While it is possible to carry a current of several tens to several hundreds of amperes through a single oxide superconducting wire 10, it is possible to carry a larger current by bundling multiple oxide superconducting wires 10 to form superconducting conductors 101, 102, and 103. Moreover, superconducting conductors 101, 102, and 103 can be easily wound into wires.

[0042] As an example of a superconducting conductor, as shown in Figure 4A, a spiral-type superconducting conductor 101 is provided, in which N tape-shaped oxide superconducting wires 10-1 to 10-n are spirally wound around the outer circumference of a core material C, and a coating J is further provided on the outer circumference. Another example of a superconducting conductor is a laminated superconducting conductor 102, as shown in Figure 4B, which is formed by laminating multiple oxide superconducting wires 10 and covering the outer circumference of the laminate with a stabilizing material S. Another example of a superconducting conductor is the ROEBEL-type superconducting conductor 103, shown in Figure 4C, which is made by twisting together multiple oxide superconducting wires 10 bundled at a binding portion B into a meandering pattern.

[0043] Furthermore, without departing from the spirit of the present invention, the components in the above-described embodiments may be replaced with well-known components as appropriate, and the above-described embodiments and modifications may be combined as appropriate. [Explanation of Symbols]

[0044] 10…Oxide superconducting wire 11…Metal substrate 12…Interlayer 13…Oxide superconducting layer 100…Superconducting coil 101, 102, 103…Superconducting conductors

Claims

1. A tape-shaped metal substrate made of nickel alloy, An intermediate layer laminated on the aforementioned metal substrate, The intermediate layer comprises an oxide superconducting layer laminated on the intermediate layer, The average grain size of the metal substrate is 3.08 μm or more, and is less than or equal to the thickness of the metal substrate. The standard deviation of the crystal grain size of the metal substrate is within the range of 2.32 to 14.66 μm. Oxide superconducting wire.

2. The average grain size is the average value of the grain size in a cross-section along the longitudinal and thickness directions of the metal substrate. The oxide superconducting wire according to claim 1.

3. The average grain size is the average value of the grain sizes measured by the reflectance EBSD method. The oxide superconducting wire according to claim 1.

4. A superconducting coil formed by winding the oxide superconducting wire described in any one of claims 1 to 3.

5. A superconducting conductor formed by assembling a plurality of oxide superconducting wires according to any one of claims 1 to 3.

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