Patent Description:
A RE123-based oxide superconductor (REBa<NUM>CU<NUM>Oy, RE is a rare earth element) exhibits superconductivity at a temperature (approximately <NUM>) exceeding a liquid nitrogen temperature (<NUM>). This oxide superconductor has a feature that the critical current density in a magnetic field is higher than that of other high-temperature superconductors. Therefore, application to coils, power cables, and the like is expected. For example, Patent Document <NUM> discloses an oxide superconducting wire obtained by forming an oxide superconducting layer and an Ag stabilizing layer on a substrate and then forming a Cu stabilizing layer by electroplating.

It is necessary that an oxide superconducting wire has a tensile strength as a mechanical property. Therefore, it is necessary to ensure the tensile strength of the stabilizing layer formed by plating.

The present invention has been made in view of the above circumstances, and an object thereof is to provide an oxide superconducting wire having an excellent tensile strength of a stabilizing layer.

In order to achieve the aforementioned objects, there is provided an oxide superconducting wire according to the independent claim <NUM> including a superconductor laminate including an oxide superconducting layer on at least one surface of a base material, and a plating layer which is included in a stabilizing layer of the superconductor laminate and formed by plating, in which a surface roughness Ra of the plating layer is <NUM> or more and <NUM> or less, and an average crystal grain size of the plating layer is <NUM> or more and <NUM> or less.

Accordingly, it is possible to suppress cracks due to the surface roughness of the plating layer being too large and cracks due to the average crystal grain size of the plating layer being too large. Therefore, it is possible to provide the oxide superconducting wire having an excellent tensile strength of the stabilizing layer.

Further according to claim <NUM>, an average crystal grain size of the plating layer is less on a side close to an inner surface that is adjacent to the superconductor laminate than an average crystal grain size of the plating layer on a side close to an outer surface that is apart from the superconductor laminate.

Accordingly, since the average crystal grain size of the plating layer on a side apart from the superconductor laminate is relatively large, a ductility on the side closer to the outer surface is high and a workability such as machinability is improved. Therefore, it is possible to provide the oxide superconducting wire having more excellent practicality.

According to the present invention, it is possible to provide an oxide superconducting wire having an excellent tensile strength of the stabilizing layer.

Hereinafter, the present invention will be described based on preferred embodiments.

<FIG> is a cross-sectional view of an oxide superconducting wire according to an embodiment.

As shown in <FIG>, an oxide superconducting wire <NUM> according to the embodiment includes a plating layer <NUM> as a stabilizing layer which covers an outer periphery of a superconductor laminate <NUM>. A base material <NUM> is, for example, a tape-shaped metal base material having each of a first main surface <NUM> and a second main surface <NUM> on each of two sides in a thickness direction. <FIG> shows an example in which the oxide superconducting layer <NUM> is formed on the first main surface <NUM>, in a case where the oxide superconducting layer <NUM> is provided on one surface of the base material <NUM>. The superconductor laminate <NUM> may include an oxide superconducting layer <NUM> on at least one surface of the base material <NUM>.

Specific examples of a material constituting the base material <NUM> include metals such as a nickel alloy typified by Hastelloy (registered trademark), stainless steel, and epitaxial Ni-W alloy in which a texture is introduced into the nickel alloy. In a case where a textured base material in which crystals of metal are aligned and oriented is used as the base material <NUM>, the oxide superconducting layer <NUM> can be formed directly on the base material <NUM>. The thickness of the base material <NUM> may be appropriately adjusted according to the purpose, and is, for example, in a range of <NUM> to <NUM>.

Although not particularly shown, at least one intermediate layer may be laminated between the base material <NUM> and the oxide superconducting layer <NUM>. The intermediate layer may have a multi-layer structure, and may include a diffusion prevention layer, a bed layer, a textured layer, a cap layer, and the like in an order from a side close to the base material <NUM> to a side close to the oxide superconducting layer <NUM>, for example. These layers are not always provided one by one, and some layers may be omitted, or two or more layers of the same layer may be repeatedly laminated. In a case where the textured base material described above is used as the base material <NUM>, the intermediate layer may be omitted.

The diffusion prevention layer has a function of suppressing some components of the base material <NUM> from diffusing and being mixed as impurities into the oxide superconducting layer <NUM>. The diffusion prevention layer is configured with, for example, Si<NUM>N<NUM>, Al<NUM>O<NUM>, GZO (Gd<NUM>Zr<NUM>O<NUM>), and the like. The thickness of the diffusion prevention layer is, for example, <NUM> to <NUM>.

A bed layer may be formed on the diffusion prevention layer in order to reduce a reaction at an interface between the base material <NUM> and the oxide superconducting layer <NUM> and improve an orientation of the layer formed thereon. As a material of the bed layer, Y<NUM>O<NUM>, Er<NUM>O<NUM>, CeO<NUM>, Dy<NUM>O<NUM>, Eu<NUM>O<NUM>, Ho<NUM>O<NUM>, La<NUM>O<NUM>, and the like are exemplary examples. The thickness of the bed layer is, for example, <NUM> to <NUM>.

The textured layer is formed of a biaxially textured material to control the crystal epitaxy of the cap layer thereon. The material of the textured layer, a metal oxide such as Gd<NUM>Zr<NUM>O<NUM>, MgO, ZrO<NUM>-Y<NUM>O<NUM> (YSZ), SrTiO<NUM>, CeO<NUM>, Y<NUM>O<NUM>, Al<NUM>O<NUM>, Gd<NUM>O<NUM>, Zr<NUM>O<NUM>, Ho<NUM>O<NUM>, and Nd<NUM>O<NUM>, and the like are exemplary examples. This textured layer is preferably formed by an Ion-Beam-Assisted Deposition (IBAD) method.

The cap layer is formed of a material that is formed on a surface of the textured layer described above and allows crystal grains to self-epitaxy in the in-plane direction. As the material of the cap layer, CeO<NUM>, Y<NUM>O<NUM>, Al<NUM>O<NUM>, Gd<NUM>O<NUM>, ZrO<NUM>, YSZ, Ho<NUM>O<NUM>, Nd<NUM>O<NUM>, LaMnO<NUM>, and the like are exemplary examples. The thickness of the cap layer is in a range of <NUM> to <NUM>.

The oxide superconducting layer <NUM> is configured with an oxide superconductor. Although, the oxide superconductor is not particularly limited, the oxide superconductor is, for example, a RE-Ba-Cu-O-based oxide superconductor represented by a general formula REBa<NUM>Cu<NUM>O<NUM>-x (RE123). As a rare earth element RE, one kind or two or more kinds of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu are exemplary examples. In the general formula of RE123, y is <NUM>-x (oxygen deficiency amount). In addition, the ratio of RE: Ba: Cu is not limited to <NUM>: <NUM>: <NUM>, and there may be an indefinite ratio. The thickness of the oxide superconducting layer <NUM> is, for example, approximately <NUM> to <NUM>.

Artificial pins made of different types of materials may be introduced into the oxide superconducting layer <NUM> as artificial crystal defects. As the different types of materials used for introducing the artificial pins into the oxide superconducting layer <NUM>, at least one or more of BaSnO<NUM> (BSO), BaZrO<NUM> (BZO), BaHfO<NUM> (BHO), BaTiO<NUM> (BTO), SnO<NUM>, TiO<NUM>, ZrO<NUM>, LaMnO<NUM>, ZnO, and the like are exemplary examples.

Although not particularly shown, a protective layer may be provided on the oxide superconducting layer <NUM> (between the oxide superconducting layer <NUM> and the plating layer <NUM>). The protective layer has functions of bypassing an overcurrent generated in a case of an accident, and suppressing a chemical reaction occurring between the oxide superconducting layer <NUM> and the plating layer <NUM>. As the material of the protective layer, silver (Ag), copper (Cu), gold (Au), an alloy of gold and silver, other silver alloys, copper alloys, and gold alloys are exemplary examples. The protective layer covers at least the surface of the oxide superconducting layer <NUM>. The thickness of the protective layer is, for example, approximately <NUM> to <NUM>. In a case where the protective layer is thinned, the thickness thereof may be <NUM> or less. The protective layer can be formed by a vapor deposition method, a sputtering method, or the like.

The plating layer <NUM> is formed as a stabilizing layer on an outer periphery of the superconductor laminate <NUM>. The stabilizing layer has functions of bypassing the overcurrent generated in a case of an accident, and mechanically reinforcing the oxide superconducting layer <NUM>. The plating layer <NUM> may be formed over the entire periphery of the superconductor laminate <NUM>. The thickness of the plating layer <NUM> is not particularly limited, and is preferably approximately <NUM> to <NUM>, and may be, for example, <NUM> or less, <NUM> or less, <NUM> or less, and <NUM> or the like. In a case where the plating layers <NUM> are formed on each of the surfaces of the superconductor laminate <NUM>, the thicknesses of the respective plating layers <NUM> may be substantially the same or different.

The plating layer <NUM> can be configured with a metal such as copper (Cu), silver (Ag), or gold (Au). The plating layer <NUM> can be formed by, for example, electroplating. In a case where the Cu plating layer is formed by electroplating, as a plating bath, a copper sulfate plating bath, a copper cyanide plating bath, a copper pyrophosphate plating bath, and the like are exemplary examples. As the copper sulfate plating solution, an aqueous solution containing copper sulfate pentahydrate, sulfuric acid, additives, and chlorine ions, and the like are generally used.

At least a part of the plating layer <NUM> can also be formed by electroless plating. In this case, a formaldehyde bath, a glyoxylic acid bath, a hypophosphorous acid bath, a cobalt salt bath, or the like is used. A general formaldehyde bath uses a plating solution containing a cupric salt, a reducing agent (formaldehyde or the like), a complexing agent (Rossel salt or the like), a pH adjuster (sodium hydroxide), and an additive (cyanide).

The plating layer <NUM> has an outer surface <NUM> on a surface opposite to the side in contact with the superconductor laminate <NUM>. That is, the plating layer <NUM> has the surface (outer peripheral surface) <NUM> and an inner surface <NUM> in contact with the superconductor laminate <NUM>. In the oxide superconducting wire <NUM> of the present embodiment, a surface roughness Ra of the plating layer <NUM> is in a range of <NUM> to <NUM>, and the average crystal grain size of the plating layer <NUM> is in a range of <NUM> to <NUM>. Here, the surface roughness Ra of the plating layer <NUM> may be the surface roughness of the surface (outer peripheral surface) <NUM>.

Accordingly, it is possible to suppress cracks due to the surface roughness of the plating layer <NUM> being too large and cracks due to the average crystal grain size of the plating layer <NUM> being too large. Therefore, it is possible to provide the oxide superconducting wire having an excellent tensile strength of the stabilizing layer.

The surface roughness Ra of the plating layer <NUM> can be measured, for example, as an arithmetic mean roughness Ra specified in JIS B <NUM> using a stylus type surface roughness measuring machine for the outer surface <NUM> of the plating layer <NUM>. As a method of adjusting the surface roughness Ra of the plating layer <NUM>, for example, the current density in electroplating may be changed. In a case where the surface roughness of the plating layer <NUM> is too large, stress is concentrated on the uneven portion of the surface, and cracks are likely to occur even with a relatively low stress.

The average crystal grain size of the plating layer <NUM> can be measured, for example, by using a cross-sectional image of the plating layer <NUM> using a field emission (FE) scanning electron microscope (SEM). In a case of measuring the average crystal grain size by the cutting method, for example, the number of crystal grains that are completely cut by a linear line segment in which a distance from the side where the plating layer <NUM> is in contact with the superconductor laminate <NUM> is substantially constant is counted. And a crystal grain size in µm units, which is obtained as the average value of the cutting lengths thereof, can be used.

As a method for adjusting the average crystal grain size of the plating layer <NUM>, at least one or more of conditions for electroplating may be changed. As specific conditions for electroplating, for example, the concentration of a plating solution, the type of a plating bath, the current density, the degree of an overvoltage, the temperature, presence or absence of an additive, presence or absence of a heat treatment after electroplating, and the like are exemplary examples. Although, the additive for the plating bath is not particularly limited, a complexing material, a pH adjuster, a leveler, and the like are exemplary examples. In a case where the average crystal grain size of the plating layer <NUM> is too large, the ductility of the metal constituting the plating layer <NUM> is high, but a proof stress (strength) decreases and cracks are likely to occur.

As defined in claim <NUM>, the average crystal grain size of the plating layer <NUM> is less on a side close to the inner surface <NUM> that is adjacent to the superconductor laminate <NUM> than the average crystal grain size of the plating layer <NUM> on a side close to the outer surface <NUM> that is apart from the superconductor laminate <NUM>. In addition, in the plating layer <NUM>, the average crystal grain size in a first region near the inner surface <NUM> may be smaller than the average crystal grain size in a second region, which is apart from the inner surface <NUM> than the first region. Since the average crystal grain size of the plating layer <NUM> on a side apart from the superconductor laminate <NUM> is relatively large, the ductility on a side close to the outer surface is high and a workability such as machinability is improved. Therefore, it is possible to provide the oxide superconducting wire <NUM> having more excellent practicality.

In a case of measuring the average crystal grain size of the plating layer <NUM> on the side close to the inner surface <NUM> that is adjacent to the superconductor laminate <NUM>, the average crystal grain size measured on a line segment drawn at a predetermined position that is approximately <NUM>% to <NUM>% (for example, <NUM>%) of the thickness of the plating layer <NUM>, from the side of the plating layer <NUM> in contact with the superconductor laminate <NUM> may be defined as a representative value. That is, the first region may be a region in which the distance from the inner surface <NUM> is approximately <NUM>% to <NUM>% (for example, <NUM>%) of the thickness of the plating layer <NUM>.

In addition, in a case of measuring the average crystal grain size of the plating layer <NUM> on the side close to the outer surface <NUM> that is apart from the superconductor laminate <NUM>, the average crystal grain size measured on a line segment drawn at a predetermined position that is approximately <NUM>% to <NUM>% (for example, <NUM>%) of the thickness of the plating layer <NUM>, from the side of the plating layer <NUM> in contact with the superconductor laminate <NUM> may be defined as a representative value. That is, the second region may be a region in which a distance from the inner surface <NUM> is approximately <NUM>% to <NUM>% (for example, <NUM>%) of the thickness of the plating layer <NUM>.

The average crystal grain size representing the entire plating layer <NUM> may be obtained by an arithmetic mean of the following "first to third average crystal grain sizes". (<NUM>) The "first average crystal grain size" is the average crystal grain size measured on a line segment drawn at a predetermined position that is approximately <NUM>% to <NUM>% (for example, <NUM>%) of the thickness of the plating layer <NUM>, from the side of the plating layer <NUM> in contact with the superconductor laminate <NUM>. (<NUM>) The "second average crystal grain size" is the average crystal grain size measured on a line segment drawn at a predetermined position that is approximately <NUM>% to <NUM>% (for example, <NUM>%) of the thickness of the plating layer <NUM>, from the side of the plating layer <NUM> in contact with the superconductor laminate <NUM>. (<NUM>) The "third average crystal grain size" is the average crystal grain size measured on a line segment drawn at a predetermined position that is approximately <NUM>% to <NUM>% (for example, <NUM>%) of the thickness of the plating layer <NUM>, from the side of the plating layer <NUM> in contact with the superconductor laminate <NUM>. These first to third average crystal grain sizes are measured on a line segment parallel to a surface on a side where the plating layer <NUM> is in contact with the superconductor laminate <NUM>.

Hereinabove, the present invention has been described based on preferred embodiments, but the present invention is defined solely by the appended claims.

In order to ensure electrical insulation from the periphery of the oxide superconducting wire, an insulating tape such as polyimide may be wound around the outer periphery of the oxide superconducting wire, or a resin layer may be formed thereon. An insulating coating layer such as the insulating tape or the resin layer is not essential component, and an insulating coating layer may be appropriately provided depending on the use of the oxide superconducting wire, or a configuration without the insulating coating layer may be provided.

In order to manufacture a superconducting coil using the oxide superconducting wire, for example, the oxide superconducting wire is wound along an outer peripheral surface of a winding frame in the required number of layers to form a coil-shaped multilayered coil, and then a resin such as an epoxy resin is impregnated to cover the wound oxide superconducting wire to fix the oxide superconducting wire.

Hereinafter, the present invention will be specifically described with reference to Examples.

For each sample, a type of plating solution was selected from three types of a plating solution A (copper sulfate + chlorine), a plating solution B (copper sulfate + chlorine + leveler), and a plating solution C (copper sulfate pentahydrate + sulfuric acid + hydrochloric acid + additive), a current density was further specified, and an oxide superconducting wire including a Cu plating layer was manufactured.

The surface roughness Ra of the plating layer was measured regarding the surface of the plating layer in accordance with JIS B <NUM>.

The average crystal grain size at the "<NUM> position" of the plating layer was obtained by averaging values of <NUM> average crystal grain sizes Y [µm] calculated on a line segment drawn at the position of <NUM> in the thickness direction from the position in which the plating layer is in contact with the superconductor laminate.

The average crystal grain size at the "entire" plating layer was obtained by averaging values of <NUM> average crystal grain sizes Y [µm] in total calculated on each of line segments drawn at the positions of <NUM>, <NUM>, and <NUM> in the thickness direction from the position in which the plating layer is in contact with the superconductor laminate.

After applying a tensile force (stress) of <NUM> MPa in the longitudinal direction of the oxide superconducting wire, the surface of the plating layer was visually observed to confirm the presence or absence of cracks. In Table <NUM>, in a case where there was no crack, it was evaluated as "OK", and in a case where there was a crack, it was evaluated as "NG". In addition, in <FIG>, a sample without cracks is indicated as "O", and a sample with cracks is indicated by "x".

Table <NUM> shows the types of plating solutions, the surface roughness Ra, the average crystal grain size, and results of the tensile test. In addition, <FIG> shows a graph showing the presence or absence of cracks, with a value of surface roughness Ra [µm] as a horizontal axis and a value of average crystal grain size [µm] as a vertical axis.

Claim 1:
An oxide superconducting wire (<NUM>) comprising:
a superconductor laminate (<NUM>) including an oxide superconducting layer (<NUM>), directly or indirectly, on at least one surface of a base material (<NUM>); and
a plating layer (<NUM>) which is included in a stabilizing layer of the superconductor laminate (<NUM>) and formed by plating,
wherein a surface roughness Ra of the plating layer (<NUM>) is <NUM> or more and <NUM> or less, the oxide superconducting wire (<NUM>) characterized in that;
an average crystal grain size of the plating layer (<NUM>) is <NUM> or more and <NUM> or less, and
an average crystal grain size of the plating layer (<NUM>) is less on a side close to an inner surface (<NUM>) that is adjacent to the superconductor laminate (<NUM>) than an average crystal grain size of the plating layer (<NUM>) on a side close to an outer surface (<NUM>) that is apart from the superconductor laminate (<NUM>).