SUPRALEITENDER OXIDDRAHT

Description

TECHNICAL FIELD

[0001] The present invention relates to an oxide superconducting wire.BACKGROUND ART

[0002] The RE123-based oxide superconductor (REBa 2 Cu 3 O y , RE is a rare earth element) shows superconductivity at a temperature (approximately 90K) exceeding the liquid nitrogen temperature (77K). Since such superconductors have a higher critical current density in a magnetic field than other high-temperature superconductors, they are expected to be applied to coils, power cables, and the like. For example, Patent Document 1 describes an oxide superconducting wire in which an oxide superconducting layer and an Ag stabilization layer are formed on a substrate, and then a Cu stabilization layer is formed by electroplating.PRIOR ARTPATENT DOCUMENT

[0003] [Patent Document 1] Japanese Patent Application, First Publication, No. 2007-80780 [Patent Document 2] JP 2016 149247 A [Patent Document 3] JP 2017 188339 A [Patent Document 4] US 5 654 098 A [Patent Document 5] JP 2012 252825 A DISCLOSURE OF INVENTIONPROBLEMS TO BE SOLVED BY THE INVENTION

[0004] The stabilization layer serves as a detour for the current flowing at that time when the superconducting state of the oxide superconductor becomes partially unstable. Therefore, it is required that the electrical resistance of the stabilization layer is low at a low temperature.

[0005] The present invention has been made in view of the above-described circumstances, and provides an oxide superconducting wire having a stabilization layer having a low electrical resistance at a low temperature.MEANS FOR SOLVING THE PROBLEMS

[0006] A first aspect of the present invention is defined in the independent claim 1.

[0007] A second aspect of the present invention is defined in the independent claim 2.

[0008] An additional optional aspects aspects of the present invention are defined in the dependent claims 3-7.EFFECTS OF THE INVENTION

[0009] According to the above-described aspect of the present invention, an average crystal grain size of the Cu plating layer constituting the stabilization layer is large, or the average number of grain boundaries per unit length is small, so that a stabilization layer having the low electrical resistance at a low temperature can be provided.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1 is a cross-sectional view of an oxide superconducting wire according to the embodiment.BEST MODE FOR CARRYING OUT THE INVENTION

[0011] Hereinafter, the present invention will be described with reference to the drawings based on the preferred embodiments.

[0012] As shown in FIG. 1, an oxide superconducting wire 10 according to the embodiment includes a superconducting laminate 15 and a stabilization layer 16 that covers the outer periphery of the superconducting laminate 15. The superconducting laminate 15 includes an oxide superconducting layer 13 disposed, either directly or indirectly, on a substrate 11. The superconducting laminate 15 may be a structure including, for example, the substrate 11, an intermediate layer 12, an oxide superconducting layer 13, and a protection layer 14.

[0013] The substrate 11 is, for example, a tape-shaped metal substrate including a first main surface 11a and a second main surface 11b on both sides in a thickness direction, respectively. Specific examples of the metal constituting the metal substrate include nickel alloys represented by Hastelloy (registered trademark), stainless steel, oriented Ni-W alloys in which a texture is introduced into the nickel alloy, and the like. When an oriented substrate in which the arrangement of metal crystals is aligned and oriented is used as the substrate 11, the oxide superconducting layer 13 can be directly formed on the substrate 11 without forming the intermediate layer 12. A side on which the oxide superconducting layer 13 is formed on the substrate 11 is referred to as a first main surface 11a, and the back surface opposite to the first main surface 11a is referred to as a second main surface 11b. The thickness of the substrate 11 may be adjusted as appropriate, and is, for example, in the range of 10 to 1000 µm.

[0014] The intermediate layer 12 may have a multi-layer structure, and may have a diffusion prevention layer, a bed layer, an orientation layer, a cap layer, and the like in the order from a side of the substrate 11 to a side of the oxide superconducting layer 13, for example. These layers are not always provided one by one, and a portion of the layers may be omitted, or two or more layers of the same type may be repeatedly laminated. The intermediate layer 12 may be a metal oxide. By film-forming the oxide superconducting layer 13 on the intermediate layer 12 having excellent orientation, it becomes easy to obtain the oxide superconducting layer 13 having excellent orientation.

[0015] The oxide superconducting layer 13 is composed of, for example, an oxide superconductor. Examples of the oxide superconductor include a RE-Ba-Cu-O-based oxide superconductor represented by the general formula REBa 2 Cu 3 O y (RE123) and the like. Examples of the rare earth element RE include one of or two 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). In addition, the ratio of RE: Ba: Cu is not limited to 1: 2: 3, and there may be an indefinite ratio. The thickness of the oxide superconducting layer 13 is, for example, approximately 0.5 to 5 µm.

[0016] Artificial pins made of different materials may be introduced into the oxide superconducting layer 13 as artificial crystal defects. Examples of different materials used for introducing artificial pins into the oxide superconducting layer 13 can include at least one or more kinds of BaSnO 3 (BSO), BaZrO 3 (BZO), BaHfO 3 (BHO), BaTIO 3 (BTO), SnO 2 , TiO 2 , ZrO 2 , LaMnO 3 , ZnO, and the like.

[0017] The protection layer 14 has functions such as bypassing overcurrent and suppressing chemical reaction that occurs between the oxide superconducting layer 13 and the layer provided on the protection layer 14. Examples of the material constituting the protection layer 14 can include silver (Ag), copper (Cu), gold (Au), or alloys including one or more kinds of these (for example, Ag alloy, Cu alloy, Au alloy). The thickness of the protection layer 14 is preferably approximately 1 to 30 µm, and when the protection layer 14 is thinned, the thickness may be 10 µm or less, 5 µm or less, 2 µm or less, or the like. The protection layer 14 may also be formed on a side surface 15s of the superconducting laminate 15 or a second main surface 11b of the substrate 11. The thicknesses of the protection layers 14 formed on the different surfaces of the superconducting laminate 15 may be substantially the same or different. The protection layer 14 may be constituted by two or more kinds of metals or two or more metal layers. The protection layer 14 can be formed by a vapor deposition method, a sputtering method, or the like.

[0018] The stabilization layer 16 is formed over the entire periphery including the first main surface 15a, the second main surface 15b, and the side surface 15s of the superconducting laminated body 15. The first main surface 15a of the superconducting laminate 15 is, for example, the surface of the protection layer 14; however, is not limited thereto. The second main surface 15b of the superconducting laminate 15 is, for example, the second main surface 11b of the substrate 11; however, the present invention is not limited thereto, and for example, the second main surface 15b may be a surface on the protection layer 14 when the protection layer 14 is also formed on the second main surface 11b of the substrate 11. The side surfaces 15s of the superconducting laminate 15 are the respective surfaces on both sides in the width direction.

[0019] The stabilization layer 16 has functions such as bypassing the generated overcurrent and mechanically reinforcing the oxide superconducting layer 13 and the protection layer 14. The stabilization layer 16 is a plating layer constituted by a metal such as copper (Cu). The thickness of the stabilization layer 16 is not particularly limited; however, is preferably approximately 2 to 300 µm, and may be, for example, 200 µm or less, 100 µm or less, 50 µm or less, approximately 20 µm, approximately 10 µm, approximately 5 µm, or the like. The thicknesses of the stabilization layers 16 formed on the first main surface 15a, the second main surface 15b, and the side surfaces 15s of the superconducting laminated body 15 may be substantially equal to each other.

[0020] As an index for evaluating the electrical resistance or conductivity of the stabilization layer 16 at a low temperature, a residual resistance ratio (RRR) can be described. The residual resistance ratio is the ratio of the specific resistance (resistivity) at two predetermined temperatures, and is obtained by RRR = ρ high / ρ low as the ratio of the specific resistance ρ high at a high temperature and the specific resistance ρ low at a low temperature. As an example, the ratio of the specific resistance ρ 293K at 293K and the specific resistance ρ 15K at 15K is ρ 293K / ρ15K . It is shown that the larger the RRR, the higher the conductivity at a low temperature than at a high temperature (normal temperature). The Cu plating layer ρ 293K / ρ15K is preferably 100 or more, for example.

[0021] In accordance with a particular technical solution as defined in the independent claim 1, in the Cu plating layer constituting the stabilization layer 16, the average crystal grain size of the Cu plating layer is 2 µm or more. Since the average crystal grain size of the Cu plating layer is large, the electrical resistance at a low temperature can be lowered. The average crystal grain size of the Cu plating layer is more preferably 2.5 µm or more, further preferably 3 µm or more. The upper limit of the average crystal grain size of the Cu plating layer is not particularly limited; however, examples thereof include 4 µm, 5 µm, 6 µm, 7 µm, 8 µm, 9 µm, and 10 µm. In this particular technical solution, the average crystal grain size of the Cu plating layer is equal to or less than the thickness of the Cu plating layer.

[0022] In accordance with an alternative technical solution as defined in the independent claim 2, in the Cu plating layer constituting the stabilization layer 16, the average number of grain boundaries per 100 µm length of the Cu plating layer along a longitudinal direction of the oxide superconducting wire is 50 or less. Since the average number of grain boundaries per unit length of the Cu plating layer is small, the electrical resistance at a low temperature can be lowered. Examples of the unit length include a length of 100 µm in the longitudinal direction of the oxide superconducting wire 10. The average number of grain boundaries per 100 µm length of the Cu plating layer is more preferably 40 or less, and even more preferably 35 or less. The lower limit of the average number of grain boundaries per 100 µm length of the Cu plating layer is not particularly limited; however, examples thereof include 10, 15, 20, 25, 30 grains, or the like.

[0023] The average crystal grain size of the Cu plating layer and the average number of grain boundaries per unit length of the Cu plating layer can be measured by, for example, a cross-sectional photograph of the Cu plating layer using a scanning electron microscope (SEM).

[0024] The copper plating layer constituting the stabilization layer 16 can be formed by, for example, electroplating. When the copper plating layer is formed by electroplating, a metal layer such as silver (Ag), copper (Cu), tin (Sn) may be formed in advance as a base layer by a vapor deposition method, a sputtering method, or the like. Examples of the plating bath used for electroplating the Cu plating layer can include a copper sulfate plating bath, a copper cyanide plating bath, and a copper pyrophosphate plating bath. As the copper sulfate plating solution, an aqueous solution including copper sulfate pentahydrate, sulfuric acid, additives, chlorine ions and the like is generally used.

[0025] At least a portion of the Cu plating layer can be formed by electroless plating. In such a case, a formaldehyde bath, a glyoxylic acid bath, a hypophosphate bath, a cobalt salt bath, and the like are used. A general formaldehyde bath uses a plating solution including a cupric salt, a reducing agent (formaldehyde, and the like), a complexing agent (Rossel salt, and the like), a pH adjuster (sodium hydroxide), and an additive (cyanide).

[0026] As a method of adjusting the average crystal grain size of the Cu plating layer or the average number of grain boundaries per unit length of the Cu plating layer, changing the conditions in the electroplating of Cu by at least one or more can be described. Specific conditions for electroplating can include, for example, the concentration of the plating solution, the type of plating bath, the current density, the degree of overvoltage, the temperature, the presence or absence of additives, the presence or absence of heat treatment after electroplating, or the like. For example, the higher the current density, the smaller the average crystal grain size of the Cu plating layer tends to be. In addition, by performing the heat treatment after electroplating, the average crystal grain size of the Cu plating layer becomes large. The additive for the plating bath is not particularly limited; however, examples thereof can include a complexing material, a pH adjuster, a leveler, and the like.

[0027] According to the oxide superconducting wire 10 of the present embodiment, in the Cu-plated layer constituting the stabilization layer 16, the crystal grain size is large or the average number of grain boundaries per unit length is small, electrical resistance due to the grain boundaries can be suppressed. The electrical resistance of a metal is divided into a portion having the large temperature dependence due to such as thermal vibration of a metal atom and a portion having the small temperature dependence (residual resistance) due to the imperfections of the metal crystal. The electrical resistance of the portion with the large temperature dependence decreases at a low temperature; however, the residual resistance remains as a finite value even at a low temperature. Therefore, by reducing the residual resistance, the RRR can be increased and the conductivity at a low temperature can be increased.

[0028] Although the present invention has been described above based on the preferred embodiments, the present invention is defined by the appended claims.

[0029] The film forming method of the intermediate layer 12 and the oxide superconducting layer 13 is not particularly limited as long as an appropriate film forming can be performed according to the composition of the metal oxide. Examples of the film forming method include a sputtering method, a dry film forming method such as a vapor deposition method, and a wet film forming method such as a sol-gel method. The vapor deposition methods include an electron beam deposition (IBAD) method, a pulsed laser deposition (PLD) method, and chemical vapor deposition (CVD) method.

[0030] The diffusion prevention layer of the intermediate layer 12 has a function of suppressing a portion of the components of the substrate 11 from diffusing and being mixed as impurities to the oxide superconducting layer 13 side. Examples of the diffusion prevention layer include Si 3 N 4 , Al 2 O 3 , GZO(Gd 2 Zr 2 O 7 ), and the like. The thickness of the diffusion prevention layer is, for example, 10 to 400 nm.

[0031] The bed layer of the intermediate layer 12 has functions such as reducing the reaction at the interface between the substrate 11 and the oxide superconducting layer 13 and improving the orientation of the layer formed on the substrate 11. Examples of the material of the bed layer include Y 2 O 3 , Er 2 O 3 , CeO 2 , Dy 2 O 3 , Eu 2 O 3 , Ho 2 O 3 , La 2 O 3 , and the like. The thickness of the bed layer is, for example, 10 to 100 nm.

[0032] The orientation layer of the intermediate layer 12 is formed from a biaxially oriented substance to control the crystal orientation of the cap layer formed thereon. Examples of the material of the orientation layer include metal oxides such as Gd 2 Zr 2 O 7 , MgO, ZrO 2 -Y 2 O 3 (YSZ), SrTiO 3 , CeO 2 , Y 2 O 3 , Al 2 O 3 , Gd 2 O 3 , Zr 2 O 3 , Ho 2 O 3 , Nd 2 O 3 , and the like. The oriented layer is preferably formed by the IBAD method.

[0033] The cap layer of the intermediate layer 12 is formed on the surface of the orientation layer, and the crystal grains are oriented in an in-plane direction. Examples of the material of the cap layer include CeO 2 , Y2O 3 , Al2O 3 , Gd 2 O 3 , ZrO 2 , YSZ, Ho 2 O 3 , Nd 2 O 3 , LaMnO 3 , and the like. The thickness of the cap layer is, for example, 50 to 5000 nm.

[0034] In order to secure electrical insulation with respect to the periphery of the oxide superconducting wire, an insulating tape such as polyimide may be wrapped or a resin layer may be formed around the outer periphery of the oxide superconducting wire. An insulating coating layer such as an insulating tape or a resin layer is not always necessary, and an insulating coating layer may be appropriately provided depending on the use of the oxide superconducting wire, or a configuration without an insulating coating layer may be provided.

[0035] To manufacture a superconducting coil using an oxide superconducting wire, for example, the oxide superconducting wire is wound along the outer peripheral surface of the winding frame with the required number of layers to form a coil-shaped multi-layer wound coil, and the oxide superconducting wire can be fixed by impregnating a resin such as an epoxy resin so as to cover the wound oxide superconducting wire.EXAMPLES

[0036] Hereinafter, a method of manufacturing the oxide superconducting wire 10, not covered by claims, will be described with reference to specific examples. The following examples do not limit the present invention.

[0037] First, a superconducting laminate 15 having a predetermined width was prepared by the following procedure. (1) Polishing of the tape-shaped substrate 11 of Hastelloy (registered trademark) C-276. (2) Degreasing and cleaning of the substrate 11 with acetone. (3) Film formation of the Al 2 O 3 diffusion prevention layer by an ion beam sputtering method. (4) Film formation of the Y 2 O 3 bed layer by an ion beam sputtering method. (5) Film formation of the MgO oriented layer by an IBAD method. (6) Film formation of the CeO 2 cap layer by a PLD method. (7) Film formation of the GdBa 2 Cu 3 O 7 -x oxide superconducting layer 13 by a PLD method. (8) Film formation of the Ag protection layer 14 by a sputtering method from the surface direction of the oxide superconducting layer 13. (9) Oxygen anneal treatment of the superconducting laminate 15. (10) Thinning treatment of the superconducting laminate 15 by slitting with a width of 4 mm.

[0038] Next, a Cu base layer was formed on the superconducting laminate 15 by a sputtering method from the direction of the first main surface 15a and the direction of the second main surface 15b.

[0039] Next, a stabilization layer 16 having a thickness of 20 µm was formed by copper sulfate plating.

[0040] In the examples, the oxide superconducting wires 10 of sample numbers 1 to 5 were obtained under the conditions shown in Table 1. (Table 1)No.PLATING SOLUTIONHEAT TREATMENTCURRENT DENSITY1COPPER SULFATE PENTAHYDRATE, SULFURIC ACID, HYDROCHLORIC ACID, AND ADDITIVESNOT PERFORMED10A / dm 2< 2COPPER SULFATE PENTAHYDRATE, SULFURIC ACID, AND HYDROCHLORIC ACIDNOT PERFORMED10A / dm 2< 3COPPER SULFATE PENTAHYDRATE, SULFURIC ACID, HYDROCHLORIC ACID, AND ADDITIVESNOT PERFORMED10A / dm 2< 4COPPER SULFATE PENTAHYDRATE, SULFURIC ACID, HYDROCHLORIC ACID, AND ADDITIVESPERFORMED10A / dm 2< 5COPPER SULFATE PENTAHYDRATE, CHLORINE, AND LEVELERPERFORMED10A / dm 2<

[0041] Next, with respect to the obtained oxide superconducting wire 10, the residual resistivity ratio and the average crystal grain size of the Cu plating layer constituting the stabilization layer 16 were measured.

[0042] The residual resistivity ratio (RRR) of the Cu plating layer was calculated as ρ 293K / ρ 15K by the ratio of the specific resistance ρ 293K at 293K and the specific resistance ρ 15K at 15K.

[0043] As the average crystal grain size of the Cu plating layer, 45 cross-sectional SEM photographs (23 µm × 23 µm viewing range) parallel to the longitudinal direction of the oxide superconducting wire 10 were taken for each one sample, three line segments were drawn in the longitudinal direction of the oxide superconducting wire 10 for each one photograph, the number of crystal grains that are completely cut by the line segments according to the cutting method of JIS H 0501 (copper grain size test method) was counted, and the crystal grain size in µm unit obtained as an average value of the cutting length thereof was used as it was for the average crystal grain size of the Cu plating layer.

[0044] The three line segments drawn in the longitudinal direction of the oxide superconducting wire 10 were located at a depth of approximately 3.2 µm, approximately 10.0 µm, and approximately 16.8 µm from the surface of the stabilization layer 16 having a thickness of 20 µm (i.e., positions of approximately 16%, approximately 50%, and approximately 84%, respectively, with respect to the thickness of the stabilization layer 16).

[0045] The average number of grain boundaries (pieces) per 100 µm length of the Cu plating layer was calculated as a numerical value obtained by dividing the 100 µm length by the above-described average crystal grain size (µm).

[0046] The measurement results described above are shown in Table 2. (Table 2)No.AVERAGE NUMBER OF GRAIN BOUNDARIES PER 100- µm LENGTH [PIECES]AVERAGE CRYSTAL GRAIN SIZE [µm]RESIDUAL RESISTANCE RATIOEVALUATION RESULT11700.595NOT GOOD21300.7745NOT GOOD31200.8360NOT GOOD4303.30110GOOD5253. 95130GOOD

[0047] When a tape-shaped Cu foil, which has a proven suitable for a stabilization layer for oxide superconducting wires, is used as the stabilization layer, the residual resistivity ratio is approximately 100. Therefore, as an evaluation result, a sample having a residual resistivity ratio of 100 or more was determined to be a non-defective product (Good), and a sample having a residual resistivity ratio of less than 100 was determined to be a defective product (Not Good). It was shown that the larger the average crystal grain size of the Cu plating layer, the better the value of the residual resistivity ratio of the Cu plating layer.DESCRIPTION OF THE REFERENCE SYMBOLS

[0048] 10: Oxide superconducting wire, 11: Substrate, 11a: First main surface of the substrate, 11b: Second main surface of the substrate, 12: Intermediate layer, 13: Oxide superconducting layer, 14: Protection layer, 15: Superconducting laminate, 15a: First main surface of superconducting laminate, 15b: Second main surface of superconducting laminate, 15s: Side surface of superconducting laminate, 16: Stabilization layer.

Claims

1. An oxide superconducting wire (10) comprising: a superconducting laminate (15) comprising an oxide superconducting layer (13) disposed, either directly or indirectly, on a substrate (11); and a stabilization layer (16) which is a Cu plating layer covering the outer periphery of the superconducting laminate (15), characterized in that an average crystal grain size of the Cu plating layer is 2 µm or more and equal to or less than a thickness of the Cu plating layer.

2. An oxide superconducting wire (10) comprising: a superconducting laminate (15) comprising an oxide superconducting layer (13) disposed, either directly or indirectly, on a substrate (11); and a stabilization layer (16) which is a Cu plating layer covering the outer periphery of the superconducting laminate (15), characterized in that an average number of grain boundaries per 100 µm length of the Cu plating layer along a longitudinal direction of the oxide superconducting wire (10) is 50 or less.

3. The oxide superconducting wire (10) according to Claim 1 or 2, wherein an average crystal grain size of the Cu plating layer is 3.30 µm or more and equal to or less than a thickness of the Cu plating layer.

4. The oxide superconducting wire (10) according to Claim 1 or 2, wherein an average crystal grain size of the Cu plating layer is 3.30 µm or more and 3.95 µm or less.

5. The oxide superconducting wire (10) according to any one of Claims 1 to 4, wherein a residual resistance ratio of the stabilization layer (16), which is a ratio of a specific resistance at 293K to a specific resistance at 15K, is 110 or more and 130 or less.

6. The oxide superconducting wire (10) according to any one of Claims 1 to 5, wherein the superconducting laminate (15) further comprises a protection layer (14) including Ag, and wherein the stabilization layer (16) is formed on the protection layer (14).

7. The oxide superconducting wire (10) according to any one of Claims 1 to 6, wherein an average crystal grain size of the stabilization layer (16) is obtained from a cross-section of the oxide superconducting wire (10) parallel to the longitudinal direction of the oxide superconducting wire (10).