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.

Examples of background art can be found in <CIT> concerning superconducting articles having dual sided structures, <CIT> concerning manufacture of niobium3-tin superconducting wire, <CIT> concerning superconducting wire rod and method of producing the same, and <CIT> concerning a method for forming electrolytic copper plating filn on a surface of rare earth metal-based permanent magnet.

Oxide superconducting wire requires tensile strength as a mechanical property. A nickel alloy substrate is generally used for the substrate of the oxide superconducting wire, and copper (Cu) is generally used for the stabilization layer; however, the hardness of Cu is lower than that of the substrate. Therefore, the thicker the stabilization layer, the greater the proportion of the stabilization layer in the cross-sectional area of the oxide superconducting wire, and the lower the tensile strength of the oxide superconducting wire as a whole.

The present invention has been made in view of the above circumstances, and provides an oxide superconducting wire having excellent tensile strength.

In accordance with the independent claim <NUM> an oxide superconducting wire is provided which includes a superconducting laminate including an oxide superconducting layer on a substrate, and a stabilization layer which is a Cu plating layer covering the outer periphery of the superconducting laminate. A Vickers hardness of the Cu plating layer is in a range of <NUM> to <NUM> HV, and a tensile strength of the oxide superconducting wire (<NUM>) is <NUM> MPa or more.

Further in accordance with claim <NUM> there is provided the oxide superconducting wire, in which the superconducting laminate includes a protection layer being composed of Ag or an Ag alloy, the Cu plating layer is formed on the protection layer, an average crystal grain size of the Cu plating layer is in a range of <NUM> to <NUM>, and the average crystal grain size of the Cu plating layer is the average crystal grain size in a cross section parallel to a longitudinal direction of the oxide superconducting wire.

According to the invention, an average number of grain boundaries per <NUM> length of the Cu plating layer is <NUM> or more.

According to the above-described aspect of the present invention, since the Cu plating layer constituting the stabilization layer has a large Vickers hardness, it is possible to provide an oxide superconducting wire having excellent tensile strength.

<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> 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 in accordance with an object, 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>. The protection layer is composed of silver (Ag) or a silver 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> can be 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 thickness direction.

The stabilization layer <NUM> has functions such as bypassing the overcurrent and mechanically reinforcing the oxide superconducting layer <NUM> and the protection layer <NUM>. The stabilization layer <NUM> is a plating layer constituted by 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>, 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 equivalent.

In the Cu plating layer constituting the stabilizing layer <NUM>, the Vickers hardness of the Cu plating layer is in the range of <NUM> to <NUM> HV. Since the Vickers hardness of the Cu plating layer is large, the tensile strength of the oxide superconducting wire <NUM> can be improved. The Vickers hardness can be measured in accordance with, for example, JIS Z <NUM> (Vickers hardness test - Test method). The tensile strength of the oxide superconducting wire <NUM> is affected by the proportion of each layer to the cross-sectional area perpendicular to the longitudinal direction of the oxide superconducting wire <NUM>. Therefore, if the Vickers hardness of the Cu plating layer is large, the tensile strength of the oxide superconducting wire <NUM> can be improved even when the stabilizing layer16 is thickened. The relationship between the Vickers hardness of the substrate <NUM> and the Vickers hardness of the stabilizing layer <NUM> is not particularly limited; however, the former may be larger than the latter, the former may be smaller than the latter, and the former and the latter may be substantially equivalent.

In the Cu plating layer constituting the stabilization layer <NUM>, the average crystal grain size of the Cu plating layer is in the range of <NUM> to <NUM>. Since the average crystal grain size of the Cu plating layer is relatively small, the metal structure becomes dense and the Vickers hardness can be increased.

In addition, in the Cu plating layer constituting the stabilization layer <NUM>, the average number of grain boundaries per <NUM> length of the Cu plating layer is <NUM> or more. Since the average number of grain boundaries per unit length of the Cu plating layer is large, the metal structure becomes dense and the Vickers hardness can be increased. Examples of the unit length include a length of <NUM> in the longitudinal direction of the oxide superconducting wire <NUM>. The upper 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>, and <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 small or the average number of grain boundaries per unit length is large, so that the Vickers hardness of the stabilization layer <NUM> can be increased. As a result, even if the stabilization layer <NUM> is made thicker, it is possible to suppress a decrease in the tensile strength of the oxide superconducting wire <NUM> or to improve the tensile strength of the oxide superconducting wire <NUM>.

Although the present invention has been described above based on the preferred embodiments, the invention is defined in the appended independent claim <NUM>.

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. The diffusion prevention layer is constituted by 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> 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 composition of the plating solution was copper sulfate pentahydrate, sulfuric acid, hydrochloric acid, and additives, and the current density was changed for each sample in the range of <NUM> to <NUM> A/dm<NUM>.

Next, the tensile strength of the obtained oxide superconducting wire <NUM> was measured. Moreover, the Vickers hardness and the average crystal grain size of the Cu plating layer constituting the stabilization layer <NUM> were measured.

As the average crystal grain size of the Cu plating layer, <NUM> cross-sectional SEM photographs (<NUM> × <NUM> viewing range) 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>.

Generally, the tensile strength required for the oxide superconducting wire is <NUM> MPa or more. Therefore, as an evaluation result, a sample having a tensile strength of <NUM> MPa or more was determined to be a non-defective product (Good), and a sample having a tensile strength of less than <NUM> MPa was determined to be a defective product (Not Good). The result was shown that the larger the Vickers hardness of the Cu-plated layer constituting the stabilization layer, the higher the value of the tensile strength of the oxide superconducting wire.

In the sample No. <NUM>, the current density when forming the Cu plating layer was increased in order to increase the Vickers hardness. However, it became a so-called "plating burn" state, and it became impossible to observe the crystal grains and measure the tensile strength. Therefore, it was determined to be a defective product (NG) due to poor appearance. Therefore, in the sample of No. <NUM>, only the Vickers hardness was measured.

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>); substrate; and
a stabilization layer (<NUM>) which is a Cu plating layer covering an outer periphery of the superconducting laminate (<NUM>),
wherein the superconducting laminate (<NUM>) includes a protection layer (<NUM>) being composed of Ag or an Ag alloy,
wherein the Cu plating layer is formed on the protection layer (<NUM>), characterized in that,
a Vickers hardness of the Cu plating layer is in a range of <NUM> to <NUM> HV,
in that a tensile strength of the oxide superconducting wire (<NUM>) is <NUM> MPa or more,
wherein an average crystal grain size of the Cu plating layer is in a range of <NUM> to <NUM>,
wherein the average crystal grain size of the Cu plating layer is the average crystal grain size in a cross section parallel to a longitudinal direction of the oxide superconducting wire (<NUM>), and
wherein an average number of grain boundaries per <NUM> length of the Cu plating layer is <NUM> or more.