Superconducting wire

Abstract

Superconducting wire (10) comprising: a substrate (1) having a first main surface (1a) and a second main surface (1b) opposite the first main surface (1a), and a superconducting material layer (5) arranged on the first main surface (1a) of the substrate (1), wherein along at least a part of the superconducting wire (10) in a direction in which the superconducting wire (10) extends, the superconducting material layer (5) is arranged to cover a side surface of the substrate (1) in a width direction of the substrate (1) and to cover at least a part of the second main surface (1b), wherein a thickness of the superconducting material layer (5) located on the first main surface (1a) varies along the width direction, wherein a maximum thickness of the superconducting material layer (5) located on the second main surface (1b) is smaller than a maximum thickness of the superconducting material layer (5) located on the first main surface (1a).

Description

TECHNICAL FIELDThe present invention relates to a superconducting wire, and more particularly relates to a superconducting wire in which a superconducting material layer is formed on a substrate.PRIOR ARTIn recent years, development of a superconducting wire in which a superconducting material layer is formed on a metal substrate has been advanced. In particular, an oxide superconducting wire is of interest. The oxide superconducting wire includes a superconducting material layer of an oxide superconductor which is a high temperature superconductor having a transition temperature equal to or higher than the temperature of liquid nitrogen.Such an oxide superconducting wire is generally manufactured by forming an intermediate layer on an orientation-aligned metal substrate, forming an oxide superconducting material layer on the intermediate layer, and further forming a stabilizing layer of silver (Ag) or copper (Cu) (see, for example, JP 2013-12 406 A).US 2017 / 0 140 852 A1 discloses a superconducting wire comprising a substrate having a first main surface and a second main surface located opposite to the first main surface, and a superconducting material layer disposed on the first main surface of the substrate, wherein along at least a part of the superconducting wire in a direction in which the superconducting wire extends, the superconducting material layer is disposed to cover a side surface of the substrate in a width direction of the substrate and to cover at least a part of the second main surface,SUMMARY OF THE INVENTIONTechnical ProblemThe superconducting wire configured as described above has a multilayer structure in which a ceramic layer composed of the intermediate layer and the superconducting material layer is formed on the metal substrate. When such a superconducting wire is cooled to its critical temperature, a difference in thermal expansion coefficient between the metal substrate and the ceramic layer causes a stress from the metal substrate to act on the ceramic layer in the multilayer structure. However, the ceramic layer cannot yield to the mechanical stress. Therefore, the bonding strength at the interface between the metal substrate and the ceramic layer is lowered, resulting in the problem of local peeling of the edges of the ceramic layer. Due to this, breakage, deformation, or the like is likely to occur in a part of the superconducting material layer, resulting in deterioration of superconducting properties.An object of the present invention is to provide a superconducting wire having stable superconducting properties since local peeling of the superconducting material layer is suppressed.Solution of the ProblemA superconducting wire according to an aspect of the present invention includes: a substrate having a first main surface and a second main surface opposite to the first main surface; and a superconducting material layer disposed on the first main surface of the substrate. Along at least a part of the superconducting wire in a direction in which the superconducting wire extends, the superconducting material layer is disposed so as to cover a side surface of the substrate in a width direction of the substrate and to cover at least a part of the second main surface. A thickness of the superconducting material layer located on the first main surface varies along the width direction. A maximum thickness of the superconducting material layer located on the second main surface is smaller than a maximum thickness of the superconducting material layer located on the first main surface.Advantageous Effects of the InventionAccording to the above, in the superconducting wire in which the superconducting material layer is formed on the substrate, local peeling of the superconducting material layer can be suppressed. In this way, a superconducting wire having stable superconducting properties can be implemented.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 is a schematic cross-sectional view showing a configuration of a superconducting wire in a first embodiment. FIG. 2 is a schematic cross-sectional view showing a configuration of a multilayer stack in the first embodiment. FIG. 3 is a schematic cross-sectional view showing a configuration of a multilayer stack of a superconducting wire in a comparative example. FIG. 4 is a flow chart showing a method of manufacturing the superconducting wire in the first embodiment. FIG. 5 is a schematic cross-sectional view for illustrating the method of manufacturing the superconducting wire in the first embodiment. FIG. 6 is a schematic cross-sectional view for illustrating the method of manufacturing the superconducting wire in the first embodiment. FIG. 7 is a schematic cross-sectional view for illustrating the method of manufacturing the superconducting wire in the first embodiment. FIG. 8 is a schematic cross-sectional view for illustrating the method of manufacturing the superconducting wire in the first embodiment. FIG. 9 is a schematic cross-sectional view showing a configuration of a superconducting wire according to a first modification in the first embodiment. FIG. 10 is a schematic cross-sectional view showing a configuration of a superconducting wire according to a second modification in the first embodiment. FIG. 11 is a schematic cross-sectional view showing a configuration of a superconducting wire in a second embodiment. FIG. 12 is a schematic cross-sectional view showing a configuration of a superconducting wire in a third embodiment. FIG. 13 is a schematic cross-sectional view showing a configuration of a superconducting wire in a fourth embodiment. FIG. 14 is a flow chart showing a method of manufacturing the superconducting wire in the fourth embodiment. FIG. 15 is a schematic cross-sectional view showing a configuration of a superconducting wire in a fifth embodiment. FIG. 16 is a flow chart showing a method of manufacturing the superconducting wire in the fifth embodiment. FIG. 17 is a diagram schematically showing a configuration of a slot device used for a wire thinning step. FIG. 18 is a schematic cross-sectional view for illustrating the method of manufacturing the superconducting wire in the fifth embodiment. FIG. 19 is a schematic cross-sectional view showing a configuration of a superconducting wire according to a modification in the fifth embodiment.DETAILED DESCRIPTIONDESCRIPTION OF EMBODIMENTS OF THE INVENTIONFirst, aspects of the present invention will be described sequentially.(1) A superconducting wire 10 (see FIG. 1 ) according to an aspect of the present invention includes a substrate 1 and a superconducting material layer 5. the substrate 1 includes a first main surface 1 aand a second main surface 1 blocated opposite to the first main surface 1 a. The superconducting material layer 5 is disposed on the first main surface 1 aof the substrate 1. Along at least a part of the superconducting wire 10 in a direction in which the superconducting wire 10 extends, the superconducting material layer 5 is arranged to cover a side surface (at least one of a first side surface 1 cand a second side surface 1 d) of the substrate 1 in a width direction of the substrate 1 and to cover at least a part of the second main surface 1 b. A thickness of the superconducting material layer 5 located on the first main surface 1 avariates along the width direction. A maximum thickness T 2 of the superconducting material layer 5 located on the second main surface 1 bof the substrate 1 is smaller than a maximum thickness T 1 of the superconducting material layer 5 located on the first main surface 1 a.Superconducting material layer 5 is thus formed to cover first main surface 1 aof substrate 1 and also to cover side surface(s) of substrate 1 and at least a part of second main surface 1 b. Therefore, at the widthwise ends of the substrate 1, the bonding strength between the substrate 1 and the superconducting material layer 5 can be increased. Accordingly, occurrence of local peeling of the superconducting material layer 5 can be suppressed, and therefore deterioration of the superconducting properties of the superconducting wire 10 can be suppressed.Since the maximum thickness T 2 is smaller than the maximum thickness T 1, the strength of the superconducting material layer 5 located on the second main surface 1 bis lower than the strength of the superconducting material layer 5 located on the first main surface 1 a. Accordingly, the probability that the superconducting material layer 5 located on the second main surface 1 brupts due to a stress acting on the superconducting material layer 5 before the superconducting material layer 5 located on the first main surface 1 arupts increases. Consequently, the protection of the superconducting material layer 5 located on the first main surface 1 acan be given a higher priority than the protection of the superconducting material layer 5 located on the second main surface 1 bfrom breakage or deformation. Superconducting material layer 5 located on first main surface 1 ais a main portion of a path in which superconducting current flows. Since the protection of this portion is given higher priority, deterioration of the superconducting properties of the superconducting wire 10 can be suppressed effectively. As a result, a superconducting wire 10 having stable superconducting properties can be implemented.(2) Superconducting wire 10 further includes an intermediate layer 3 disposed between first main surface 1 aof substrate 1 and superconducting material layer 5. Along at least a part of the superconducting wire 10 in the direction in which the superconducting wire 10 extends, the intermediate layer 3 is disposed so as to cover the side surface of the substrate 1 and to cover at least a part of the second main surface 1 b. A maximum thickness T 4 of the intermediate layer 3 located on the second main surface 1 bis smaller than a maximum thickness T 3 of the intermediate layer 3 located on the first main surface 1 a(see FIG. 8 ).Thus, at the widthwise ends of the substrate 1, the bonding strength between the substrate 1 and the intermediate layer 3 can be increased, and peeling of the intermediate layer 3 of the substrate 1 can therefore be suppressed. Accordingly, occurrence of peeling of the superconducting material layer 5 due to peeling of the intermediate layer 3 can be suppressed. Since the maximum thickness T 4 is smaller than the maximum thickness T 3, the strength of the intermediate layer 3 located on the second main surface 1 bis lower than the strength of the intermediate layer 3 located on the first main surface 1 a. This increases the possibility that the intermediate layer 3 located on the second main surface 1 bbreaks due to a mechanical stress acting on the intermediate layer 3 before the intermediate layer 3 located on the first main surface 1 breaks. Consequently, the protection of the intermediate layer 3 and the superconducting material layer 5 located on the first main surface 1 acan be given a higher priority than the protection of the intermediate layer 3 and the superconducting material layer 5 located on the second main surface 1 bfrom breakage, deformation, or the like.(3) The superconducting wire 10 further includes a protection layer 7 formed on the superconducting material layer 5. Along at least a part of the superconducting wire 10 in the direction in which the superconducting wire 10 extends, the protective layer 7 is disposed so as to cover the side surface of the substrate 1 and to cover at least a part of the second main surface 1 b. A maximum thickness T 6 of the protective layer 7 located on the second main surface 1 bis smaller than a maximum thickness T 5 of the protective layer 7 located on the first main surface 1 a.Thereby, the protective layer 7 can be formed so as to cover the superconducting material layer 5 covering the side surface(s) of the substrate 1 and covering at least a part of the second main surface 1 b. It is therefore possible to protect the superconducting material layer 5 and contribute to preventing the superconducting material layer 5 from being peeled off. Since the maximum thickness T 6 is smaller than the maximum thickness T 5, the superconducting material layer 5 and the protection layer 7 located on the second main surface 1 bhave a smaller total thickness and thereby have a lower strength than the superconducting material layer 5 and the protection layer 7 located on the first main surface 1 a. Accordingly, the possibility that the superconducting material layer 5 located on the second main surface 1 bbreaks before the superconducting material layer 5 located on the first main surface 1 ais broken is not suppressed.(4) With respect to the superconducting wire 10 (see FIG. 1 ), the thickness of the superconducting material layer 5 located on the first main surface 1 avariates along the width direction in a manner that the thickness of a central portion of the superconducting material layer 5 in the width direction is larger than the thickness of at least one end of the superconducting material layer 5 in the width direction. Since the occurrence of local peeling of the superconducting material layer 5 of such a superconducting wire 10 can also be suppressed, deterioration of the superconducting properties of the superconducting wire 10 can be suppressed. Consequently, stable superconducting properties can be obtained.(5) With respect to the superconducting wire 10 (see FIG. 11 ), the thickness of the superconducting material layer 5 located on the first main surface 1 avariates along the width direction in a manner that the thickness of at least one end of the superconducting material layer 5 in the width direction is greater than the thickness of a central portion of the superconducting material layer 5 in the width direction. Since the occurrence of local peeling of the superconducting material layer 5 of such a superconducting wire 10 can also be suppressed, deterioration of the superconducting properties of the superconducting wire 10 can be suppressed. Consequently, stable superconducting properties can be obtained.(6) With respect to the superconducting wire 10, the superconducting material layer 5 located on one end of the second main surface 1 bin the width direction is separated along at least a part of the superconducting wire 10 in the direction in which the superconducting wire 10 extends from the superconducting material layer 5 located on another end of the second main surface 1 bin the width direction. In other words, one end, in the width direction, of the superconducting material layer 5 is formed to extend from above the first side surface 1 cto above a part of the second main surface 1 b, and the other end, in the width direction, of the superconducting material layer 5 is formed to extend from above the second side surface 1 dto above a part of the second main surface 1 b. On the second main surface 1 b, the two ends of the superconducting material layer 5 are separated from each other. Since the occurrence of local peeling of the superconducting material layer 5 of such a superconducting wire 10 can also be suppressed, deterioration of the superconducting properties of the superconducting wire 10 can be suppressed. As a result, stable superconducting properties can be obtained.(7) With respect to the superconducting wire 10, the superconducting material layer 5 is directly or indirectly disposed on the first main surface 1 aof the substrate 1. The fact that the superconducting material layer 5 is arranged indirectly on the first main surface 1 ain the present text means that the intermediate layer 3 or one or more further layers are located between the first main surface 1 aand the superconducting material layer 5. In both the case where the superconducting material layer 5 is directly disposed on the first main surface 1 aand the case where the superconducting material layer 5 is indirectly disposed on the first main surface 1 a, the bonding strength between the substrate 1 and the superconducting material layer 5 can be increased, and therefore, occurrence of local peeling of the superconducting material layer 5 can be suppressed.(8) With respect to the superconducting wire 10 (see FIG. 15 ), the first main surface 1 aof the substrate 1 includes a curved portion. Thereby, the surface area of the first main surface 1 ais larger than that of the flat first main surface 1 aof the substrate 1, and therefore it is possible to further increase the bonding strength between the first main surface 1 aand the superconducting material layer 5. Accordingly, the effect of suppressing the occurrence of peeling of the superconducting material layer 5 can be enhanced.(9) With respect to the superconducting wire 10 (see FIG. 19 ), the curved portion is disposed at an end of the first main surface 1 aof the substrate 1 in the width direction of the substrate 1. Thereby, at the one or more ends of the first main surface 1 ain the width direction, the correspondence of the contraction of the superconducting material layer 5 with the contraction of the substrate 1 upon cooling can be improved. Accordingly, occurrence of peeling of the superconducting material layer 5 can be suppressed.(10) With respect to the superconducting wire 10, the superconducting material layer 5 is made of an oxide superconducting material. Since local peeling of the superconducting material layer can be suppressed in this manner, an oxide superconducting wire having stable superconducting properties can be implemented.DETAILS OF EMBODIMENTS OF THE INVENTIONHereinafter, embodiments of the present invention will be described with reference to the drawings. In the accompanying drawings, the same or corresponding parts are denoted by the same reference numerals, and the description thereof will not be repeated.First EmbodimentConfiguration of Superconducting WireFIG. 1 is a schematic cross-sectional view showing a configuration of a superconducting wire in a first embodiment. FIG. 1 shows a cross section in the direction transverse to the direction in which a superconducting wire 10 in the first embodiment extends. Therefore, the direction across the plane of the drawing is the longitudinal direction of the superconducting wire, and superconducting current in a superconducting material layer 5 is to flow in the direction across the plane of the drawing. Moreover, in the schematic cross-sectional views in FIG. 1 and the subsequent drawings, the difference between the size in the up-down direction (hereinafter also referred to as "thickness direction") and the size in the left-right direction (hereinafter also referred to as "width direction") of the rectangular cross-section is shown small for better visibility of the drawings. However, in reality, the size in the thickness direction of the cross section is sufficiently smaller than the size in the width direction of the cross section.As seen in FIG. 1, the superconducting wire 10 in the first embodiment has an elongated shape (ribbon shape) having a rectangular cross section, and the relatively larger areas of the wire extending in the longitudinal direction of the elongated shape are defined herein as major surfaces. Superconducting wire 10 includes substrate 1, intermediate layer 3, superconducting material layer 5, protective layer 7, and stabilizing layer 9.The substrate 1 has a first main surface 1 aand a second main surface 1 b. The second main surface 1 bis located opposite to the first main surface 1 a. The substrate 1 further has a first side surface 1 cand a second side surface 1 dopposing the first side surface 1 c. Preferably, the substrate 1 is made of, for example, a metal and has an elongated shape (band shape) having a rectangular cross section. When the superconducting wire is to be wound into a coil shape, the substrate 1 preferably extends over a long distance of about 2 km.More preferably, an orientation-aligned metal substrate is used as the substrate 1. The orientation-oriented metal substrate means a substrate in which crystal orientations are oriented in two axis directions in a plane of the substrate surface. For the orientation-oriented metal substrate, for example, an alloy of at least two metals selected from nickel (Ni), copper (Cu), chromium (Cr), manganese (Mn), cobalt (Co), iron (Fe), palladium (Pd), silver (Ag), and gold (Au) is preferably used. These metals and another metal or alloy may be superimposed. For example, an alloy such as SUS which is a high strength material may also be used. The material for substrate 1 is not limited to those mentioned above, and any material other than metal may be used, for example.The superconducting wire 10 has, for example, a width direction size of about 4 mm to 10 mm. In order to increase the density of current flowing in the superconducting wire 10, a smaller cross-sectional area of the substrate 1 is preferable. However, an excessively small thickness of the substrate 1 (in the top-down direction in FIG. 1 ) may result in deterioration of the strength of the substrate 1. Therefore, the thickness of the substrate 1 is preferably about 0.1 mm.The intermediate layer 3 is formed on the first main surface 1 aof the substrate 1. The superconducting material layer 5 is formed on the main surface (the upper main surface in FIG. 1 ) of the intermediate layer 3 opposite to the main surface thereof facing the substrate 1. The superconducting material layer 5 is disposed on the first main surface 1 aof the substrate 1 with the intermediate layer 3 between the superconducting material layer 5 and the substrate 1. The material forming the intermediate layer 3 is preferably, for example, yttria stabilized zirconia (YSZ), ceria (CeO 2), magnesia (MgO), yttria (Y 2 O 3) or strontium titanate (SrTiO 3). These materials have extremely low responsiveness to the superconducting material layer 5 and do not deteriorate the superconducting properties of the superconducting material layer 5 even at the boundary abutting the superconducting material layer 5. Particularly, in the case where a metal is used as a material constituting the substrate 1, the intermediate layer can satisfy the function of mitigating a difference in orientation orientation alignment between the superconducting material layer 5 and the substrate 1 having crystal orientation alignment in its surface, thereby preventing metal atoms from leaking from the substrate 1 in the superconducting material layer 5 while forming a superconducting material layer 5 at a high temperature. The material forming the intermediate layer 3 is not particularly limited to those mentioned above.The intermediate layer 3 can consist of several layers. In the case where the intermediate layer 3 is composed of multiple layers, the layers constituting the intermediate layer 3 may be formed of respective materials different from each other, or some of the layers constituting the intermediate layer 3 may be made of the same material.The superconducting material layer 5 is a thin film layer in the superconducting wire 10, and current flows superconductingly in this superconducting material layer 5. although the superconducting material is not particularly limited, for example, the superconducting material is preferably an SE-123-based oxide superconductor. "SE-123-based oxide superconductor" means a superconductor represented by SEBa 2 Cu 3 O y (y is 6 to 8, more preferably 6.8 to 7, and SE represents yttrium or a rare earth element such as Gd, Sm, Ho or the like). In order to improve the magnitude of the superconducting current flowing in the superconducting material layer 5, the superconducting material layer 5 preferably has a thickness of 0.5 μm to 10 μm.The protective layer 7 is formed on the main surface (the upper main surface in FIG. 1 ) of the superconducting material layer 5 opposite to the main surface thereof facing the intermediate layer 3. Preferably, the protective layer 7 is made of, for example, silver (Ag) or silver alloy, and has a thickness of not less than 0.1 μm and not more than 50 μm.The substrate 1, the intermediate layer 3, the superconducting material layer 5, and the protective layer 7 as described above form a multilayer stack 20. In the present embodiment, the stabilizing layer 9 is disposed so as to cover the outer periphery of the multilayer stack 20, namely, to cover substantially the entire outermost surface of the multilayer stack 20. Note that the "circumferential edge of the multilayer stack" in the present invention is not limited to the entire circumferential edge, and need only be a main surface of the multilayer stack. The stabilizing layer 9 is formed of a highly conductive metal foil or plating layer or the like.The stabilizing layer 9 functions together with the protective layer 7 as a bypass for commutating the current in the superconducting material layer 5 when the transition of the superconducting material layer 5 from the superconducting state to the normally conducting state takes place. The material forming the stabilizing layer 9 is, for example, preferably copper (Cu) or a copper alloy or the like. Although there are no particular restrictions on the thickness of the stabilizing layer 9, the thickness is preferably 10 μm to 500 μm in order to physically protect the protective layer 7 and the superconducting material layer 5.FIG. 2 is a schematic cross-sectional view showing a configuration of the multilayer stack 20 in the first embodiment. FIG. 2 is a cross section in the direction transverse to the direction in which the superconducting wire 10 in the first embodiment extends.In the multilayer stack 20 in the first embodiment, the superconducting material layer 5 is disposed so as to cover the side surface(s) of the substrate 1 in the width direction (left-right direction in FIG. 2 ) and cover at least a part of the second main surface 1 b.Specifically, in the multilayer stack 20 shown in FIG. 2, one end, in the width direction, of the superconducting material layer 5 is formed to extend from above the first side surface 1 cto above a part of the second main surface 1 b, and the other end, in the width direction, of the superconducting material layer 5 is formed to extend from above the second side surface 1 dto above a part of the second main surface 1 b. The two ends of the superconducting material layer 5 are separated from each other on the second main surface 1 b. In other words, the superconducting material layer 5 is disposed so as to completely cover the first main surface 1 aand the side surfaces 1 c, 1 dof the substrate 1 and partially cover the second main surface 1 b. This configuration enables an increase in bonding strength between the substrate 1 and the superconducting material layer 5 as compared to a conventional superconducting wire in which the superconducting material layer 5 covers only the first main surface 1 aof the substrate 1.In detail, when a superconducting wire in which a superconducting material layer which is a ceramic layer is formed on a metal substrate is cooled to its critical temperature, a stress is generated between the metal substrate and the superconducting material layer due to a difference in thermal expansion coefficients between the metal and the ceramic material. More specifically, each layer in the wire contracts when the superconducting wire is cooled. At this moment, because the superconducting material layer has a lower thermal expansion coefficient than the metal substrate, the superconducting material layer cannot contract to the same extent as the metal substrate and is accordingly subjected to a mechanical stress. Consequently, in the conventional superconducting wire, the superconducting material layer might be peeled off, particularly at the widthwise ends of the substrate.In a superconducting wire still having an intermediate layer between the substrate and the superconducting material layer, the intermediate layer, which is a ceramic layer, may peel off from one end, in the width direction, of the substrate, like the superconducting material layer described above. For example, peeling of the superconducting material layer or the intermediate layer is more likely to occur, which might lead to deterioration of the superconducting properties, in the superconducting material layer being broken or deformed.In the superconducting wire 10 according to the first embodiment, the superconducting material layer 5 extends from above the side surfaces 1 c, 1 dof the substrate 1 to over at least a part of the second main surface 1 b. Thereby, as compared with the conventional superconducting wire, the bonding area between the substrate 1 and the superconducting material layer 5 can be increased, and the bonding strength between the substrate 1 and the superconducting material layer 5 can be therefore enhanced. Thus, when the superconducting wire 10 is cooled, the correspondence of the contraction of the superconducting material layer 5 with the contraction of the substrate 1 is improved. Peeling of the superconducting material layer 5 of the substrate 1 can therefore be suppressed. As a result, the superconducting material layer 5 can be prevented from being broken or deformed, and deterioration of the superconducting properties of the superconducting wire 10 can therefore be suppressed.In the multilayer stack 20 shown in FIG. 2, the intermediate layer 3 is disposed so as to completely cover the first main surface 1 aand the side surfaces 1 c, 1 dof the substrate 1 and partially cover the second main surface 1 b. As a result, as well as the bonding strength between the substrate 1 and the superconducting material layer 5, the bonding strength between the substrate 1 and the intermediate layer 3 can be increased. Peeling of the intermediate layer 3 of the substrate 1 can therefore be suppressed. Accordingly, occurrence of peeling of the superconducting material layer 5 due to peeling of the intermediate layer 3 can be suppressed. Preferably, the superconducting material layer 5 covers the ends, in the width direction, of the intermediate layer 3 as shown in FIG. 2. Accordingly, the effect of suppressing peeling of the intermediate layer 3 can be enhanced.In the multilayer stack 20 shown in FIG. 2, the protective layer 7 is disposed so as to completely cover the first main surface 1 aand the side surfaces 1 c, 1 dof the substrate 1 and partially cover the second main surface 1 b. Thereby, the protective layer 7 can be formed to cover the superconducting material layer 5 covering the side surfaces 1 c, 1 dand the second main surface 1 bof the substrate 1. The protective layer 7 thus protects the superconducting material layer 5 and can contribute to preventing the superconducting material layer 5 from being peeled off. Preferably, the protective layer 7 covers the ends, in the width direction, of the superconducting material layer 5 as shown in FIG. 2. Accordingly, the effect of suppressing peeling of the superconducting material layer 5 can be enhanced.As long as the superconducting material layer 5, the intermediate layer 3, and the protective layer 7 in the superconducting wire 10 in the first embodiment cover at least a part of the second main surface 1 balong at least a part of the superconducting wire 10 in the direction in which the superconducting wire 10 extends (longitudinal direction), the bonding strength between the substrate 1 and the superconducting material layer 5 and the intermediate layer 3 can be enhanced.In the multilayer stack 20 shown in FIG. 2, the intermediate layer 3, the superconducting material layer 5, and the protective layer 7 located on the first main surface 1 aeach have a cross-sectional shape protruding at its central portion in the width direction. Specifically, the intermediate layer 3, the superconducting material layer 5, and the protective layer 7 each have an upper surface in an outward curved convex shape. Accordingly, the intermediate layer 3, the superconducting material layer 5, and the protective layer 7 located on the first main surface 1 aeach have a thickness that varies along the width direction. In the example in FIG. 2, the apex of this curve is located substantially at the center, in the width direction, of the upper side. The intermediate layer 3, the superconducting material layer 5 and the protective layer 7 therefore each have a thickness greater in the widthwise central portion thereof than the thickness at the widthwise end thereof.In the multilayer stack 20, a maximum thickness T 2 of the superconducting material layer 5 located on the second main surface 1 bis smaller than a maximum thickness T 1 of the superconducting material layer 5 located on the first main surface 1 a(T 2<T 1).Since T2 is smaller than T1, the strength of the superconducting material layer 5 located on the second main surface 1b is lower than the strength of the superconducting material layer 5 located on the first main surface 1a. Thus, when the superconducting material layer 5 is subjected to a stress applied to the substrate 1, there is a high possibility that the superconducting material layer 5 located on the second main surface 1 bbreaks before the superconducting material layer 5 located on the first main surface 1 abreaks. As a result, the protection of the superconducting material layer 5 located on the first main surface 1a from the stress applied to the superconducting material layer 5 is given a higher priority. Since the superconducting material layer 5 located on the first main surface 1 ais a main portion of the path on which the superconducting current flows, higher priority can be given to protection of this portion to effectively suppress deterioration of the superconducting properties.If T2 is much smaller than T1, there may be a possibility that sufficient bonding strength cannot be maintained between the second main surface 1b and the superconducting material layer 5. From this viewpoint, the ratio of T2to T1(T2 / T1) is preferably 0.1% or more and 95% or less. When this ratio is 95% or less, it is ensured that the strength of the superconducting material layer 5 located on the second main surface 1 bis lower than the strength of the superconducting material layer 5 located on the first main surface 1 a, and therefore the above-described effect can be sufficiently realized. In contrast, when this ratio is less than 0.1%, sufficient bonding strength cannot be maintained between the substrate 1 and the superconducting material layer 5 located on the second main surface 1 b, and there arises a possibility that the above-described effect cannot be sufficiently realized.Further, the superconducting wire 10 in the first embodiment can exhibit favorable effects over a comparative example shown in FIG. 3. FIG. 3 is a schematic cross-sectional view showing a configuration of a multilayer stack of a superconducting wire in a comparative example. FIG. 3 shows a cross section of the superconducting wire in the comparative example in the direction transverse to the direction in which the superconducting wire extends.As seen in FIG. 3, a multilayer stack 200 in the comparative example basically has a similar configuration to the multilayer stack 20 shown in FIG. 2, however, the multilayer stack 200 is different from the multilayer stack 20 shown in FIG. 2 in that the former includes an intermediate layer 3, a superconducting material layer 5, and a protective layer 7 that are disposed so as to cover a first main surface 1 aof the substrate 1 and partially cover side surfaces 1 c, 1 dof the substrate 1. In other words, with respect to the multilayer stack 200, the superconducting material layer 5 and the intermediate layer 3 do not extend over the second main surface 1 b. Therefore, the multilayer stack 20 shown in FIG. 2 has both a larger bonding area between the substrate 1 and the superconducting material layer 5 and a larger bonding area between the substrate 1 and the intermediate layer 3 as compared with the multilayer stack 200.The substrate 1 contracts upon cooling not only in the width direction but also in the thickness direction. In the multilayer stack 20 shown in FIG. 2, the superconducting material layer 5 and the intermediate layer 3 extend to above the second main surface 1 b, and therefore, the correspondence is high not only in the contraction in the width direction but also in the contraction in the thickness direction. In contrast, with respect to the multilayer stack 200, although the compliance with the contraction in the width direction of the substrate 1 can be realized by the superconducting material layer 5 and the intermediate layer 3 located on side surfaces 1 c, 1 d, the compliance with the contraction in the thickness direction of the substrate 1 is low.Further, the widthwise end portions of the superconducting material layer 5 each have a U-shape in a cross section of the multilayer stack 20 shown in FIG. 2. Therefore, the widthwise end portions of the superconducting material layer 5 are substantially in a state of catching the second main surface 1 b. In this state, the end portions serve as hooks for securing the superconducting material layer 5 to the substrate 1 in view of the structure. As a result, a structure having a greater resistance to the stress of the substrate 1 than the comparative example can be implemented.For the reasons set forth above, the superconducting wire 10 in the first embodiment can provide a stronger effect of suppressing peeling of the superconducting material layer 5 and / or the intermediate layer 3 from the substrate 1 than the superconducting wire in the comparative example.In the first embodiment, the intermediate layer 3 and the superconducting material layer 5 covering the side surface(s) of the substrate 1 and covering at least a part of the second main surface 1 bmay be respectively formed to cover both the first side surface 1 cand the second side surface 1 d, as shown in FIGS. 1 and 2, or may be formed to cover only one of the first side surface 1 cand the second side surface 1 d. In other words, the intermediate layer 3 and the superconducting material layer 5 may be disposed to cover at least one of the first side surface 1 cand the second side surface 1 dand cover at least a part of the second main surface 1 b. Both configurations allow the bonding strength between the substrate 1, the intermediate layer 3, and the superconducting material layer 5 to be increased, as compared with the conventional superconducting wire and the comparative example (FIG. 3 ).Method for Producing Superconducting WireNext, a method as shown in FIGS. 4 to 8 for manufacturing the superconducting wire in the first embodiment will be described.FIG. 4 is a flow chart showing a method of manufacturing the superconducting wire in the first embodiment. Hereinafter, the present embodiment will be described in conjunction with a method for manufacturing a superconducting wire 10 using a substrate 1 subjected to wire thinning to a width of 4 mm as an example.As seen in FIG. 4, a substrate manufacturing step (S 10) is first performed. Specifically, as seen in FIG. 5, a substrate 1 is manufactured which is made of an orientation-aligned metal substrate and has a tape shape with a desired width (width: 4 mm, for example). The substrate 1 has a first main surface 1 aand a second main surface 1 bdisposed opposite to the first main surface 1 a, and a first side surface 1 cand a second side surface 1 ddisposed opposite to the first side surface 1 c. The thickness of the substrate 1 can be appropriately adjusted to achieve any purpose, and may be usually in a range of 10 μm to 500 μm. For example, the thickness of the substrate 1 is about 100 μm.Next, an interlayer forming step (S 20 in FIG. 4 ) for forming an interlayer 3 on the substrate 1 is performed. Specifically, as seen in FIG. 6, the intermediate layer 3 is formed on the first main surface 1 aof the substrate 1. As the method for forming the intermediate layer 3, any method may be used. For example, a physical vapor deposition method such as a pulse laser deposition (PLD) method may be used.Next, a superconducting material layer forming step (S 30 in FIG. 4 ) for forming a superconducting material layer 5 on the intermediate layer 3 is performed. Specifically, as seen in FIG. 7, the superconducting material layer 5 made of an SE-123-based oxide superconductor is formed on the main surface (the upper main surface in FIG. 7 ) of the intermediate layer 3 opposite to the main surface thereof facing the substrate 1. As the method for forming the superconducting material layer 5, any method may be used. For example, a vapor phase method, a liquid phase method, or a combination thereof may be used to form the layer. Examples of the vapor phase method are laser evaporation methods, sputtering methods, electron beam evaporation methods and the like. This step can be carried out by at least one of a laser deposition method, a sputtering method, an electron beam method, and an organometallic deposition method to form the superconducting material layer 5 whose surface has excellent crystal orientation alignment and surface smoothness.Next, a protective layer forming step (S 40 in FIG. 4 ) for forming a protective layer 7 on the superconducting material layer 5 is performed. Specifically, as seen in FIG. 8, the protective layer 7 made of silver (Ag) or silver alloy is formed on the main surface (the upper main surface in FIG. 8 ) of the superconducting material layer 5 opposite to the main surface thereof facing the intermediate layer 3 by a physical vapor deposition method such as sputtering, an electroplating method, or the like. The protective layer 7 may be formed to protect the surface of the superconducting material layer 5. Thereafter, oxygen annealing, namely, heating in an oxygen environment (an oxygen introduction step), is carried out to introduce oxygen into the superconducting material layer 5. Through the above-described steps, a multilayer stack 20 having a width direction size of about 30 mm is formed.Next, a stabilizing layer forming step (S 50 in FIG. 4 ) for forming a stabilizing layer 9 on the circumferential edge of the multilayer stack 20 is performed. Specifically, the stabilizing layer 9 made of copper (Cu) or copper alloy is formed by the well-known plating to cover the outer periphery of the multilayer stack 20 to cover substantially the entire outermost surface of the multilayer stack 20. The method of forming the stabilizing layer 9 may be bonding copper foil instead of plating. Through the above-described steps, the superconducting wire 10 in the first embodiment shown in FIG. 1 is produced.In the multilayer stack 20 shown in FIG. 8, a maximum thickness T 4 of the intermediate layer 3 located on the second main surface 1 bis preferably smaller than a maximum thickness T 3 of the intermediate layer 3 located on the first main surface 1 a(T 4<T 3).Since T 4 is less than T 3, the strength of the intermediate layer 3 located on the second main surface 1 bis lower than the strength of the intermediate layer 3 located on the first main surface 1 a. Thus, when the intermediate layer 3 is subjected to a stress applied from the substrate 1, the intermediate layer 3 located on the second main surface 1 bis likely to crack before the intermediate layer 3 located on the first main surface 1 ais broken. Accordingly, the protection of the intermediate layer 3 located on the first main surface 1 ais given a higher priority from the stress acting on the intermediate layer 3. Consequently, higher priority can be given to protection of the superconducting material layer 5 located on the first main surface 1 a, for example, from breakage or deformation.In the multilayer stack 20, a maximum thickness T 6 of the protective layer 7 located on the second main surface 1 bis preferably smaller than a maximum thickness T 5 of the protective layer 7 located on the first main surface 1 a(T 6<T 5).Since T 6 is smaller than T 5, the intermediate layer 3, the superconducting material layer 5, and the protection layer 7 located on the second main surface 1 bhave a smaller total thickness and therefore have a lower strength than the intermediate layer 3, the superconducting material layer 5, and the protection layer 7 located on the first main surface 1 a. Accordingly, the probability that the superconducting material layer 5 located on the second main surface 1 bbreaks before the superconducting material layer 5 located on the first main surface 1 ais broken is not suppressed.Modification of the First EmbodimentWith reference to FIGS. 9 and 10, modifications of the superconducting wire in the first embodiment will be described.FIG. 9 is a schematic cross-sectional view showing a configuration of a superconducting wire 10A according to a first modification in the first embodiment. FIG. 9 shows a cross section in the direction transverse to the direction in which the superconducting wire 10A extends.As seen in FIG. 9, the superconducting wire 10A of the first modification basically has a similar structure to the superconducting wire 10 shown in FIG. 1, except that the shape of a stabilizing layer 9 of the superconducting wire 10A is different from that of the superconducting wire 10.In the superconducting wire 10A, the thickness of the stabilizing layer 9 disposed at widthwise ends of the first main surface 1 aof the substrate 1 is larger than the thickness of the stabilizing layer 9 located above a widthwise central portion of the first main surface 1 a. The thickness of the stabilizing layer 9 disposed at widthwise ends of the second main surface 1 bof the substrate 1 is also larger than the thickness of the stabilizing layer 9 located above a widthwise central portion of the second main surface 1 b.In the superconducting wire 10A, the intermediate layer 3 and the superconducting material layer 5 are also formed to extend from above the side surfaces 1 c, 1 dof the substrate 1 to a part of the second main surface 1 b. The maximum thickness T 2 of the superconducting material layer 5 located on the second main surface 1 bis smaller than the maximum thickness T 1 of the superconducting material layer 5 located on the first main surface 1 a. Further, the respective maximum thicknesses of the intermediate layer 3 and the protective layer 7 located on the second main surface 1 bare smaller than the respective maximum thicknesses of the intermediate layer 3 and the protective layer 7 located on the first main surface 1 a. The superconducting wire 10A can therefore realize similar effects to the case of the superconducting wire 10 shown in FIG. 1.A method for manufacturing a superconducting wire 10A has basically similar structural elements to the method for manufacturing the superconducting wire in the first embodiment described above with reference to FIGS. 4 to 8, except that the conditions for forming the stabilizing layer in the stabilizing layer forming step (S 50 in FIG. 4 ) of the former method are different from those of the first embodiment. For example, when the electroplating method is used to form the stabilizing layer 9 to cover the outer periphery of the multilayer stack 20 with the stabilizing layer 9, the current is likely to concentrate at the corners of the multilayer stack 20. Thereby, the stabilizing layer 9 shown in FIG. 9 can be formed. Thus, the superconducting wire 10A is obtained.FIG. 10 is a schematic cross-sectional view showing a configuration of a superconducting wire 10B according to a second modification in the first embodiment. FIG. 10 is a cross section in the direction transverse to the direction in which the superconducting wire 10B extends.As seen in FIG. 10, the superconducting wire 10B of the second modification basically has a similar structure to the superconducting wire 10 shown in FIG. 1, except that the structure of the multilayer stack 20 is different from that of the multilayer stack 20 shown in FIG. 2.In the superconducting wire 10B, the protective layer 7 located on the second main surface 1 bis disposed so as to completely cover the second main surface 1 b. In the superconducting wire 10B, the intermediate layer 3 and the superconducting material layer 5 are also formed to extend from above the side surfaces 1 c, 1 dof the substrate 1 to a part of the second main surface 1 b. The maximum thickness T 2 of the superconducting material layer 5 located on the second main surface 1 bis smaller than the maximum thickness T 1 of the superconducting material layer 5 located on the first main surface 1 a. Further, the respective maximum thicknesses of the intermediate layer 3 and the protective layer 7 located on the second main surface 1 bare smaller than the respective maximum thicknesses of the intermediate layer 3 and the protective layer 7 located on the first main surface 1 a. Therefore, the superconducting wire 10B can realize similar effects to the case of the superconducting wire 10 shown in FIG. 1.A method for manufacturing a superconducting wire 10B has basically similar structural elements to the method for manufacturing the superconducting wire in the first embodiment described above with reference to FIGS. 4 to 8, except that the conditions for forming the protective layer in the protective layer forming step (S 40 in FIGS. 4, 8 ) of the former method are different from those of the first embodiment. For example, when the electroplating method is used to form the protective layer 7 on the main surface of the superconducting material layer 5 opposite to the main surface thereof facing the intermediate layer 3, the second main surface 1 bmay be completely plated to form the protective layer 7 shown in FIG. 10. Thus, the superconducting wire 10B is obtained.Also in the modifications shown in FIGS. 9 and 10, the bonding strength between the substrate 1 and the superconducting material layer 5 can be increased as long as the superconducting material layer 5 covers a part of the second main surface 1 balong at least a part of the superconducting wire in the longitudinal direction. The intermediate layer 3 may cover a part of the second main surface 1 b, and the protective layer 7 may cover at least a part of the second main surface 1 balong at least a part of the superconducting wire in the longitudinal direction.Second EmbodimentFIG. 11 is a schematic cross-sectional view showing a configuration of a superconducting wire 10C in a second embodiment. FIG. 11 shows a cross section in the direction transverse to the direction in which the superconducting wire 10C extends.As seen in FIG. 11, the superconducting wire 10C in the second embodiment has basically a similar structure to the superconducting wire 10 shown in FIG. 1, except that the structure of the multilayer stack 20 is different from that of the multilayer stack 20 shown in FIG. 2.In the superconducting wire 10C, the intermediate layer 3, the superconducting material layer 5, and the protective layer 7 located on the first main surface 1 aprotrude respectively at the opposite ends in the width direction, and accordingly have a cross section whose central portion retracts toward the substrate 1 in the width direction. In other words, the upper surface of the intermediate layer 3, the superconducting material layer 5, and the protective layer 7 each has a concave shape curved toward the substrate 1. Thus, the thickness of the intermediate layer 3, the superconducting material layer 5, and the protective layer 7 located on the first main surface 1 avaries along the width direction. In the example in FIG. 11, the thickness at the ends, in the width direction, of the intermediate layer 3, the superconducting material layer 5, and the protective layer 7 is respectively larger than the thickness at the central portion in the width direction.In the superconducting wire 10C, the maximum thickness T 2 of the superconducting material layer 5 located on the second main surface 1 bis also smaller than the maximum thickness T 1 of the superconducting material layer 5 located on the first main surface 1 a. The respective maximum thicknesses of the intermediate layer 3 and the protective layer 7 located on the second main surface 1 bare smaller than the respective maximum thicknesses of the intermediate layer 3 and the protective layer 7 located on the first main surface 1 a. The superconducting wire 10C can therefore realize similar effects to the case of the superconducting wire 10 shown in FIG. 1.A method for manufacturing a superconducting wire 10C has basically similar structural elements to the method for manufacturing the superconducting wire in the first embodiment described above with reference to FIGS. 4 to 8, except that the conditions for forming the layers in the interlayer forming step (S 20 in FIGS. 4, 6 ), the superconducting material layer forming step (S 30 in FIGS. 4, 7 ), and the protective layer forming step (S 40 in FIGS. 4, 8 ) are different from those in the first embodiment.Third EmbodimentFIG. 12 is a schematic cross-sectional view showing a configuration of a superconducting wire 10D in a third embodiment. FIG. 12 shows a cross section in the direction transverse to the direction in which the superconducting wire 10D extends.As seen in FIG. 12, the superconducting wire 10D of the third embodiment basically has a similar structure to the superconducting wire 10 shown in FIG. 1, except that the structure of the multilayer stack 20 is different from that of the multilayer stack 20 shown in FIG. 2.In the superconducting wire 10D, the intermediate layer 3, the superconducting material layer 5, and the protection layer 7 located on the second main surface 1 bare each disposed so as to completely cover the second main surface 1 b. In the superconducting wire 10D, the maximum thickness T 2 of the superconducting material layer 5 located on the second main surface 1 bis also smaller than the maximum thickness T 1 of the superconducting material layer 5 located on the first main surface 1 a. The respective maximum thicknesses of the intermediate layer 3 and the protective layer 7 located on the second main surface 1 bare also smaller than the respective maximum thicknesses of the intermediate layer 3 and the protective layer 7 located on the first main surface 1 a. The superconducting wire 10D can therefore realize similar effects to the case of the superconducting wire 10 shown in FIG. 1.As long as the superconducting material layer 5 completely covers the second main surface 1 balong at least a part of the superconducting wire 10D in the longitudinal direction, the bonding strength between the substrate 1 and the superconducting material layer 5 can be increased. The intermediate layer 3 may completely cover the second main surface 1 b, and the protective layer 7 may completely cover the second main surface 1 balong at least a part of the superconducting wire 10D in the longitudinal direction.A method for manufacturing a superconducting wire 10D has basically similar structural elements to the method for manufacturing the superconducting wire in the first embodiment described above with reference to FIGS. 4 to 8, except that the conditions for forming layers in the interlayer forming step (S 20 in FIGS. 4, 6 ), the superconducting material layer forming step (S 30 in FIGS. 4, 7 ), and the protective layer forming step (S 40 in FIGS. 4, 8 ) are different from those in the first embodiment.Fourth EmbodimentFIG. 13 is a schematic cross-sectional view showing a configuration of a multilayer stack in a superconducting wire 10E in a fourth embodiment. FIG. 13 shows a cross section in the direction transverse to the direction in which the superconducting wire 10E extends.As seen in FIG. 13, the superconducting wire 10E in the fourth embodiment has basically a similar structure to the superconducting wire 10 shown in FIG. 1, except that the structure of the multilayer stack 20 is different from that of the multilayer stack 20 shown in FIG. 2.In the superconducting wire 10E, the intermediate layer 3, the superconducting material layer 5, and the protective layer 7 are arranged so as to completely cover the first main surface 1 aand the first side surface 1 cof the substrate 1 and partially cover the second main surface 1 b. In contrast, the second side surface 1 dof the substrate 1 is not covered by the intermediate layer 3, the superconducting material layer 5, and the protection layer 7.In the superconducting wire 10E, the intermediate layer 3, the superconducting material layer 5, and the protection layer 7 located on the first main surface 1 aprotrude each at one end in the width direction. In other words, respective upper surfaces of the intermediate layer 3, the superconducting material layer 5, and the protective layer 7 each have an outwardly curved convex shape. Therefore, the intermediate layer 3, the superconducting material layer 5, and the protective layer 7 located on the first main surface 1 aeach have a thickness that varies along the width direction. In the example shown in FIG. 13, the intermediate layer 3, the superconducting material layer 5, and the protective layer 7 each have a thickness at one end in the width direction that is larger than their thickness at the other end in the width direction.In the superconducting wire 10E, the maximum thickness T 2 of the superconducting material layer 5 located on the second main surface 1 bis also smaller than the maximum thickness T 1 of the superconducting material layer 5 located on the first main surface 1 a. The respective maximum thicknesses of the intermediate layer 3 and the protective layer 7 located on the second main surface 1 bare also smaller than the respective maximum thicknesses of the intermediate layer 3 and the protective layer 7 located on the first main surface 1 a. The superconducting wire 10E can therefore realize similar effects to the case of the superconducting wire 10 shown in FIG. 1.FIG. 14 is a flow chart showing a method of manufacturing the superconducting wire in the fourth embodiment. As shown in FIG. 14, the method of manufacturing the superconducting wire in the fourth embodiment basically has similar structural elements to the method of manufacturing the superconducting wire in the first embodiment described above with reference to FIGS. 4 to 8. However, the former method is different from the first embodiment in that the former method includes a wire thinning step.As seen in FIG. 14, a substrate manufacturing step (S 10) is first performed. Specifically, a substrate 1 is manufactured which is made of an orientation-aligned metal substrate and has a wide band shape. The width of the substrate 1 at this time may be, for example, a width (for example, 8 mm) that is about twice as large as the width (for example, 4 mm) of the substrate 1 in the superconducting wire 10E.Next, on this wide substrate 1, an interlayer forming step (S 20), a superconducting material layer forming step (S 30), and a protective layer forming step (S 40) are performed in this order. The interlayer forming step, the superconductive material layer forming step, and the protective layer forming step are performed in a similar manner to the corresponding steps in the first embodiment. These steps are thus performed to form a wide multilayer stack 20.Next, a wire thinning step (S 60) of cutting a wide multilayer stack 20 into those each having a predetermined width (for example, 4 mm) is performed. In particular, rotary blades are used to perform mechanical slitting to mechanically cut the multilayer stack of 8 mm width to those having a width of 4 mm.In the wire thinning step (S 60), a multilayer stack 20 having a width direction size of about 8 mm, for example, is cut in half in the width direction to produce two multilayer stacks 20 each having a width of 4 mm. The multilayer stack 20 shown in FIG. 13 is one of these multilayer stacks. Respective cut surfaces of two multilayer stacks 20 exposed by this cutting may each form one end surface in the width direction. In the multilayer stack 20 shown in FIG. 13, the second side surface 1 dof the substrate 1 is exposed and is not covered by any of the intermediate layer 3, the superconducting material layer 5, and the protective layer 7. The other multilayer stack 20 (not shown) facing the cut surface of the multilayer stack 20 shown in FIG. 13 has an exposed first side surface 1 cof the substrate 1. more specifically, in the multilayer stack 20 not shown, the intermediate layer 3, the superconducting material layer 5, and the protective layer 7 completely cover the first main surface 1 aand the second side surface 1 dof the substrate 1 and partially cover the second main surface 1 b. In contrast, the first side surface 1 cis not covered by the intermediate layer 3, the superconducting material layer 5, and the protective layer 7, and is therefore exposed.In the wire thinning step (S 60), laser processing may be performed to cut the multilayer stack into thin wires. The multilayer stack 20 shown in FIG. 13 can also be obtained by adjusting the conditions for the laser processing.Next, a stabilizing layer forming step (S 50) of forming a stabilizing layer 9 on the peripheral edge of the multilayer stack 20 subjected to the wire thinning is performed. The stabilizing layer forming step is carried out in a similar manner to the first embodiment. The above-described steps are carried out to thereby produce the superconducting wire 10E shown in FIG. 13.Fifth EmbodimentFIG. 15 is a schematic cross-sectional view showing a configuration of a multilayer stack of a superconducting wire 10F in a fifth embodiment. FIG. 15 is a cross section in the direction transverse to the direction in which the superconducting wire 10F extends.As seen in FIG. 15, the superconducting wire 10F in the fifth embodiment has basically a similar structure to the superconducting wire 10 shown in FIG. 1, except that the structure of the multilayer stack 20 is different from that of the multilayer stack 20 shown in FIG. 2.In the superconducting wire 10F, the first main surface 1 aof the substrate 1 has an outwardly curved convex shape. The apex of this curve is located substantially at the center of the first main surface 1 ain the width direction. The ends of the curve are located at the ends of the first main surface 1 ain the width direction. The intermediate layer 3, the superconducting material layer 5, and the protection layer 7 are formed along the first main surface 1 a. Therefore, the upper surface (the upper surface of the protective layer 7) of the multilayer stack 20 also has an outwardly curved convex shape. The intermediate layer 3, the superconducting material layer 5, and the protective layer 7 each have a thickness that varies along the width direction. In the multilayer stack 20 shown in FIG. 15, the maximum thickness T 2 of the superconducting material layer 5 located on the second main surface 1 bis also smaller than the maximum thickness T 1 of the superconducting material layer 5 located on the first main surface 1 a.With respect to the superconducting wire 10F, the first main surface 1 ais curved. Therefore, the surface area of the first main surface 1 acan be increased compared to the substrate 1 having the flat first main surface 1 a. The intermediate layer 3 and the superconducting material layer 5 are formed so as to completely cover the first main surface 1 ahaving the curved portion, and therefore, the bonding area between the substrate 1 and the intermediate layer 3 and the bonding area between the substrate 1 and the superconducting material layer 5 can be increased. Accordingly, the bonding strength between the substrate 1 and the intermediate layer 3 and the bonding strength between the substrate 1 and the superconducting material layer 5 can be further increased.The curved surface portion may be the entire first main surface 1 aas shown in FIG. 15, or may be a part of the first main surface 1 a. The curved surface portion may have an outward curved convex shape or may be curved toward the second main surface 1 b(concave shape).FIG. 16 is a flow chart showing a method of manufacturing the superconducting wire in the fifth embodiment. As shown in FIG. 16, the method of manufacturing the superconducting wire in the fifth embodiment basically has similar structural elements to the method of manufacturing the superconducting wire in the first embodiment described above with reference to FIGS. 4 to 8. However, the former method is different from the first embodiment in that the former method includes a wire thinning step.As seen in FIG. 16, a substrate manufacturing step (S 10) is first performed. Specifically, a substrate 1 is prepared which is made of an orientation-aligned metal substrate and has a wide band shape (about 30 mm width).Next, a wire thinning step (S 70) of cutting the substrate 1 having a width of 30 mm to those each having a predetermined width (for example, 4 mm) is performed. Specifically, as shown in FIG. 17, rotary blades are used to perform mechanical slitting to mechanically cut the substrate 1 having a width of 30 mm to those each having a width of 4 mm.FIG. 17 schematically shows a configuration of a slot device used for the wire thinning step. On the right side in FIG. 17, the configuration of the substrate 1 subjected to slitting by the slitting jig 30 is shown.As seen in FIG. 17, the slitter 30 includes a plurality of rotary blades 31 and a plurality of spacers 32. the slitter 30 includes, for example, a total of seven rotary blades 31. three rotary blades 31 each having a width of about 4 mm are disposed on an upper rotary shaft of the slitter 30. Between rotary blades 31 that are adjacent to each other in the direction of the rotation axis, the spacer 32 is disposed. On a lower rotary shaft of the slitter 30, there are also arranged four rotary blades 31 each having a width of about 4 mm. The width of the rotary blades 31 disposed on the upper rotary shaft and the lower rotary shaft may be set to any width.As shown in FIG. 17, the substrate 1 that is slit with rotary blades 31 that contact the second main surface 1 bhas a cross-sectional shape in which a central portion of the first main surface 1 aprotrudes in the width direction (the first main surface 1 ahas a convex shape) because slit conditions such as the clearance between adjacent rotary blades 31 and the vertical overlap of rotary blades 31 have been adjusted accordingly. In this way, a substrate 1 having a cross-sectional shape as shown in FIG. 18 can be obtained. In contrast, the substrate 1 that is slit with rotary blades 31 contacting the first main surface 1 ahas a cross-sectional shape in which a central portion of the second main surface 1 bprotrudes in the width direction (the second main surface 1 bhas a convex shape) because slit conditions such as the clearance between adjacent rotary blades 31 and the vertical overlap of rotary blades 31 have been adjusted accordingly.As described above, the mechanical slitting cuts the substrate 1 by shearing by means of the upper rotary blades 31 and the opposite lower rotary blades 31. the resulting thin wires (substrate 1) each have curved edges depending on the direction in which the rotary blades 31 are put on (the direction of slitting). Specifically, with respect to thin wires (substrate 1 e) formed by slitting from the first main surface 1 aside with the upper rotary blades 31, the edges of the substrate 1 are curved toward the first main surface 1 a. In contrast, with respect to thin wires (substrate 1 f) formed by slitting from the second main surface 1 bside with the lower rotary blades 31, the edges of the substrate 1 are curved toward the second main surface 1 b, and accordingly, the first main surface 1 ahas a convex shape.With respect to the mechanical slitting shown in Fig. 17, the rotary blades 31 applied to the first main surface 1a have an identical width with the rotary blades 31 applied to the second main surface 1b. However, the rotary blades 31 applied to the second main surface 1 bmay have a predetermined width (for example, 4 mm), and the rotary blades 31 applied to the first main surface 1 amay have a narrower width. In this way, the number of thinned wires (substrate 1 having the first main surface 1 ain a convex shape as shown in FIG. 18 ) obtained by slitting the second main surface 1 bside can be increased.Next, on the substrate 1 shown in FIG. 18, an interlayer forming step (S 20), a superconducting material layer forming step (S 30), and a protective layer forming step (S 40) are performed in this order. The interlayer forming step, the superconductive material layer forming step, and the protective layer forming step are each performed in a similar manner to the first embodiment. These steps are performed to thereby form the multilayer stack 20 shown in FIG. 15.Next, a stabilizing layer forming step (S 50) of forming the stabilizing layer 9 on the circumferential edge of the multilayer stack 20 is performed. The stabilizing layer forming step is carried out in a similar manner to the first embodiment. These steps are carried out to thereby produce the superconducting wire 10F shown in FIG. 15.Modification of Fifth EmbodimentFIG. 19 is a schematic cross-sectional view showing a configuration of a superconducting wire 10G according to a modification in the fifth embodiment. FIG. 19 shows a cross section in the direction transverse to the direction in which the superconducting wire 10G extends.As seen in FIG. 19, the superconducting wire 10G according to the modification basically has a similar structure to the superconducting wire 10 shown in FIG. 1, except that the structure of the multilayer stack 20 is different from that of the multilayer stack 20 shown in FIG. 2.With respect to the superconducting wire 10G, the first main surface 1 aof the substrate 1 has curved portions at the widthwise ends of the substrate 1. The intermediate layer 3, the superconducting material layer 5, and the protective layer 7 each have a thickness that varies along the width direction. With respect to the superconducting wire 10G, the maximum thickness T 2 of the superconducting material layer 5 located on the second main surface 1 bis also smaller than the maximum thickness T 1 of the superconducting material layer 5 located on the first main surface 1 a. The respective maximum thicknesses of the intermediate layer 3 and the protective layer 7 located on the second main surface 1 bare smaller than the respective maximum thicknesses of the intermediate layer 3 and the protective layer 7 located on the first main surface 1 a.Since the curved portions are located at the widthwise ends of the substrate 1, the correspondence of the contraction of the intermediate layer 3 and the superconducting material layer 5 with the contraction of the substrate 1 at the widthwise ends of the first main surface 1 acan be improved. Accordingly, the intermediate layer 3 and the superconducting material layer 5 can be prevented from peeling off the substrate 1.Next, a method of manufacturing a superconducting wire 10G shown in FIG. 19 will be described. Basically, the superconducting wire 10G can be obtained by executing the steps (S 10 to S 70) shown in FIG. 16. In the above-described wire thinning step (S 70), the wide substrate 1 may be cut into those each having a predetermined width by laser processing, and the first main surface 1 aof the resultant substrate 1 that has been cut may be subjected to a process of forming curved portions at the ends of the first main surface 1 ain the width direction of the substrate 1.Regarding the configuration in which the superconducting material layer in the first to fifth embodiments is disposed so as to cover the side surface(s) of the substrate and cover at least a part of the second main surface, the configuration in which the intermediate layer, the superconducting material layer, and the protective layer respectively cover the side surface(s) of the substrate and at least a part of the second main surface has been described above. However, the present invention is not limited to this configuration, but further includes a configuration in which the intermediate layer and the superconducting material layer cover the side surface(s) of the substrate and cover at least a part of the second main surface, a configuration in which only the superconducting material layer covers the side surface(s) of the substrate and covers at least a part of the second main surface, and a configuration in which the superconducting material layer and the protection layer cover the side surface(s) of the substrate and cover at least a part of the second main surface. Among these configurations, the configuration in which the intermediate layer and the superconducting material layer cover the side surface(s) of the substrate and cover at least a part of the second main surface is preferable because the orientation orientation of the superconducting material layer can be improved not only in the first main surface but also in the side surface(s) of the substrate and the second main surface of the substrate, and because the peeling of the intermediate layer can be prevented.Although the configuration in which the stabilizing layer is formed to cover the outer periphery of the multilayer stack is illustrated above in connection with the first to fifth embodiments, the stabilizing layer may be disposed on at least the top surface of the multilayer stack. In this case, after the stabilizing layer is formed on the protective layer, an insulating coating layer may be formed to cover the outer periphery of the superconducting wire to protect the superconducting wire.

Claims

A superconducting wire (10) comprising: a substrate (1) having a first main surface (1a) and a second main surface (1b) located opposite to the first main surface (1a); and a superconducting material layer (5) disposed on the first main surface (1a) of the substrate (1), wherein along at least a part of the superconducting wire (10) in a direction in which the superconducting wire (10) extends, the superconducting material layer (5) is disposed to cover a side surface of the substrate (1) in a width direction of the substrate (1) and to cover at least a part of the second main surface (1b), wherein a thickness of the superconducting material layer (5) located on the first main surface (1a) varies along the width direction, wherein a maximum thickness of the superconducting material layer (5), which is located on the second main surface (1b), is smaller than a maximum thickness of the superconducting material layer (5) located on the first main surface (1a).The superconducting wire (10) according to claim 1, further comprising an intermediate layer (3) disposed between the first main surface (1a) of the substrate (1) and the superconducting material layer (5), wherein along at least a part of the superconducting wire (10) in the direction in which the superconducting wire (10) extends, the intermediate layer (3) is disposed so as to cover the side surface of the substrate (1) and cover at least a part of the second main surface (1b), and a maximum thickness of the intermediate layer (3) located on the second main surface (1b) is smaller than a maximum thickness of the intermediate layer (3) located on the first main surface (1a).The superconducting wire (10) according to claim 1 or 2, further comprising a protective layer (7) formed on the superconducting material layer (5), wherein along at least a part of the superconducting wire (10) in the direction in which the superconducting wire (10) extends, the protective layer (7) is disposed so as to cover the side surface of the substrate (1) and to cover at least a part of the second main surface (1b), and a maximum thickness of the protective layer (7) located on the second main surface (1b) is smaller than a maximum thickness of the protective layer (7) located on the first main surface (1a).The superconducting wire (10) according to any one of claims 1 to 3, wherein the thickness of the superconducting material layer (5) located on the first main surface (1a) varies along the width direction in a manner that the thickness of a central portion of the superconducting material layer (5) in the width direction is greater than the thickness of at least one end of the superconducting material layer (5) in the width direction.The superconducting wire (10) according to any one of claims 1 to 3, wherein the thickness of the superconducting material layer (5) located on the first main surface (1a) varies along the width direction in a manner that the thickness of at least one end of the superconducting material layer (5) in the width direction is greater than the thickness of a central portion of the superconducting material layer (5) in the width direction.The superconducting wire (10) according to any one of claims 1 to 5, wherein along at least a part of the superconducting wire (10) in the direction in which the superconducting wire (10) extends, the superconducting material layer (5) located on one end of the second main surface (1b) in the width direction is separated from the superconducting material layer (5) located on another end of the second main surface (1b) in the width direction.The superconducting wire (10) according to any one of claims 1 to 6, wherein the superconducting material layer (5) is directly or indirectly disposed on the first main surface (1a) of the substrate (1).The superconducting wire (10) according to any one of claims 1 to 7, wherein the first main surface (1a) of the substrate (1) includes a curved portion.The superconducting wire (10) according to claim 8, wherein the curved portion is disposed at an end of the first main surface (1a) in the width direction.The superconducting wire (10) according to any one of claims 1 to 9, wherein the superconducting material layer (5) is made of an oxide superconducting material.

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