Patent Description:
The RE123-based oxide superconductor (REBa<NUM>Cu<NUM>Oy, RE is a rare earth element) shows superconductivity at a temperature (approximately <NUM>) exceeding the liquid nitrogen temperature (<NUM>). Since such superconductors have a higher critical current density in a magnetic field than other high-temperature superconductors, they are expected to be applied to coils, power cables, and the like. For example, Patent Document <NUM> describes an oxide superconducting wire in which an oxide superconducting layer and an Ag stabilization layer are formed on a substrate, and then a Cu stabilization layer is formed by electroplating.

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

The present invention has been made in view of the above-described circumstances, and provides an oxide superconducting wire having a stabilization layer having a low electrical resistance at a low temperature.

A first aspect of the present invention is defined in the independent claim <NUM>.

A second aspect of the present invention is defined in the independent claim <NUM>.

An additional optional aspects aspects of the present invention are defined in the dependent claims <NUM>-<NUM>.

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

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

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

As shown in <FIG>, an oxide superconducting wire <NUM> according to the embodiment includes a superconducting laminate <NUM> and a stabilization layer <NUM> that covers the outer periphery of the superconducting laminate <NUM>. The superconducting laminate <NUM> includes an oxide superconducting layer <NUM> disposed, either directly or indirectly, on a substrate <NUM>. The superconducting laminate <NUM> may be a structure including, for example, the substrate <NUM>, an intermediate layer <NUM>, an oxide superconducting layer <NUM>, and a protection layer <NUM>.

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

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

The oxide superconducting layer <NUM> is composed of, for example, an oxide superconductor. Examples of the oxide superconductor include a RE-Ba-Cu-O-based oxide superconductor represented by the general formula REBa<NUM>Cu<NUM>Oy (RE123) and the like. Examples of the rare earth element RE include one of or two or more of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. In the general formula of RE123, y is <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 materials may be introduced into the oxide superconducting layer <NUM> as artificial crystal defects. Examples of different materials used for introducing artificial pins into the oxide superconducting layer <NUM> can include at least one or more kinds 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.

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

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

The stabilization layer <NUM> has functions such as bypassing the generated overcurrent and mechanically reinforcing the oxide superconducting layer <NUM> and the protection layer <NUM>. The stabilization layer <NUM> is a plating layer constituted by a metal such as copper (Cu). The thickness of the stabilization layer <NUM> is not particularly limited; however, is preferably approximately <NUM> to <NUM>, and may be, for example, <NUM> or less, <NUM> or less, <NUM> or less, approximately <NUM>, approximately <NUM>, approximately <NUM>, or the like. The thicknesses of the stabilization layers <NUM> formed on the first main surface 15a, the second main surface 15b, and the side surfaces <NUM> of the superconducting laminated body <NUM> may be substantially equal to each other.

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

In accordance with a particular technical solution as defined in the independent claim <NUM>, in the Cu plating layer constituting the stabilization layer <NUM>, the average crystal grain size of the Cu plating layer is <NUM> or more. Since the average crystal grain size of the Cu plating layer is large, the electrical resistance at a low temperature can be lowered. The average crystal grain size of the Cu plating layer is more preferably <NUM> or more, further preferably <NUM> or more. The upper limit of the average crystal grain size of the Cu plating layer is not particularly limited; however, examples thereof include <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>. In this particular technical solution, the average crystal grain size of the Cu plating layer is equal to or less than the thickness of the Cu plating layer.

In accordance with an alternative technical solution as defined in the independent claim <NUM>, in the Cu plating layer constituting the stabilization layer <NUM>, the average number of grain boundaries per <NUM> length of the Cu plating layer along a longitudinal direction of the oxide superconducting wire is <NUM> or less. Since the average number of grain boundaries per unit length of the Cu plating layer is small, the electrical resistance at a low temperature can be lowered. Examples of the unit length include a length of <NUM> in the longitudinal direction of the oxide superconducting wire <NUM>. The average number of grain boundaries per <NUM> length of the Cu plating layer is more preferably <NUM> or less, and even more preferably <NUM> or less. The lower limit of the average number of grain boundaries per <NUM> length of the Cu plating layer is not particularly limited; however, examples thereof include <NUM>, <NUM>, <NUM>, <NUM>, <NUM> grains, or the like.

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

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

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

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

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

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

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

The diffusion prevention layer of the intermediate layer <NUM> has a function of suppressing a portion of the components of the substrate <NUM> from diffusing and being mixed as impurities to the oxide superconducting layer <NUM> side. Examples of the diffusion prevention layer include 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>.

The bed layer of the intermediate layer <NUM> has functions such as reducing the reaction at the interface between the substrate <NUM> and the oxide superconducting layer <NUM> and improving the orientation of the layer formed on the substrate <NUM>. Examples of the material of the bed layer include 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. The thickness of the bed layer is, for example, <NUM> to <NUM>.

The orientation layer of the intermediate layer <NUM> is formed from a biaxially oriented substance to control the crystal orientation of the cap layer formed thereon. Examples of the material of the orientation layer include metal oxides such as Gd<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>, Nd<NUM>O<NUM>, and the like. The oriented layer is preferably formed by the IBAD method.

The cap layer of the intermediate layer <NUM> is formed on the surface of the orientation layer, and the crystal grains are oriented in an in-plane direction. Examples of the material of the cap layer include CeO<NUM>, Y2O<NUM>, Al2O<NUM>, Gd<NUM>O<NUM>, ZrO<NUM>, YSZ, Ho<NUM>O<NUM>, Nd<NUM>O<NUM>, LaMnO<NUM>, and the like. The thickness of the cap layer is, for example, <NUM> to <NUM>.

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

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

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

First, a superconducting laminate <NUM> having a predetermined width was prepared by the following procedure.

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

Next, a stabilization layer <NUM> having a thickness of <NUM> was formed by copper sulfate plating.

In the examples, the oxide superconducting wires <NUM> of sample numbers <NUM> to <NUM> were obtained under the conditions shown in Table <NUM>.

Next, with respect to the obtained oxide superconducting wire <NUM>, the residual resistivity ratio and the average crystal grain size of the Cu plating layer constituting the stabilization layer <NUM> were measured.

The residual resistivity ratio (RRR) of the Cu plating layer was calculated as ρ<NUM>/ρ<NUM> by the ratio of the specific resistance ρ<NUM> at <NUM> and the specific resistance ρ<NUM> at <NUM>.

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

The three line segments drawn in the longitudinal direction of the oxide superconducting wire <NUM> were located at a depth of approximately <NUM>, approximately <NUM>, and approximately <NUM> from the surface of the stabilization layer <NUM> having a thickness of <NUM> (i.e., positions of approximately <NUM>%, approximately <NUM>%, and approximately <NUM>%, respectively, with respect to the thickness of the stabilization layer <NUM>).

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

The measurement results described above are shown in Table <NUM>.

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

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