JP7795368B2 - Oxide superconducting laminate, oxide superconducting wire and connection structure - Google Patents

Description

The present invention relates to an oxide superconducting laminate, an oxide superconducting wire, and a connection structure.

Patent Document 1 discloses an oxide superconducting wire having a structure in which an intermediate layer, a superconducting layer, and a protective layer are layered in this order on a substrate. The oxide superconducting wire can be used by connecting it to an object to be connected (such as another superconducting wire or an electrode) by soldering or the like.

JP 2014-89954 A

Oxide superconducting wire can sometimes have high connection resistance at the connection point with the object to be connected.

One aspect of the present invention aims to provide an oxide superconducting laminate, an oxide superconducting wire, and a connection structure that can reduce connection resistance at the connection.

One aspect of the present invention provides an oxide superconductor laminate comprising a substrate, a superconducting layer formed on the substrate and made of an oxide superconductor, and a protective layer formed on the superconducting layer in contact with the superconducting layer, wherein voids are formed inside the protective layer, and the number of voids formed in the protective layer that contact the interface with the superconducting layer is less than 12 per 1 μm of length of the interface in a cross section along the thickness direction of the protective layer.

It is preferable that the number of voids in contact with the interface is 0 per 1 μm of the length of the interface.

It is preferable that the number of voids in contact with the interface be three or more per 1 μm of the length of the interface.

It is preferable that the size of the voids in contact with the interface in the longitudinal direction of the interface be 11.0 nm to 70.2 nm.

It is preferable that a mixed region is formed between the superconducting layer and the protective layer, in which a partial region of the protective layer and a partial region of the superconducting layer are mixed.

In a cross-sectional view of the mixed region, the ratio (Li/L) of the interface length (Li) between the superconducting layer and the protective layer to the length (L) of the oxide superconducting wire in the width or longitudinal direction is preferably in the range of 0.03 to 1.56.

It is preferable that at least a portion of the interface between the superconducting layer and the protective layer in the mixed region intersects with a direction along the ab plane of the crystal of the oxide superconductor in the superconducting layer.

One aspect of the present invention provides an oxide superconducting wire in which a stabilizing layer is formed on the outer periphery of the oxide superconducting laminate.

One aspect of the present invention provides a connection structure having the oxide superconducting wire.

According to one aspect of the present invention, it is possible to provide an oxide superconducting laminate, an oxide superconducting wire, and a connection structure that can reduce connection resistance at the connection.

1 is a cross-sectional view of an oxide superconducting wire according to an embodiment. 2 is a schematic diagram showing a part of a cross section of an oxide superconducting layer and a protective layer. FIG. FIG. 2 is a diagram showing a part of a cross section of an oxide superconducting layer and a protective layer. 1 is a cross-sectional view showing a connection structure using an oxide superconducting wire according to an embodiment. 1A is an image of a cross section of an oxide superconducting layer and a protective layer of a comparative example, and FIG. 1B is an enlarged image of FIG.

The oxide superconducting laminate and oxide superconducting wire according to embodiments of the present invention will be described in detail below with reference to the drawings. For the sake of clarity, the drawings used in the description may show enlarged essential parts, and the dimensional proportions of the components may not necessarily be the same as in reality.

[Oxide superconducting laminates and oxide superconducting wires]
An oxide superconducting laminate and an oxide superconducting wire according to an embodiment will be described with reference to FIG.
Fig. 1 is a cross-sectional view showing an oxide superconductor laminate 5 and an oxide superconducting wire 10 according to an embodiment. Fig. 1 is a view showing a cross section perpendicular to the longitudinal direction of the oxide superconductor laminate 5 and the oxide superconducting wire 10.

The oxide superconducting wire 10 includes an oxide superconducting laminate 5 and a stabilization layer 6 .
The oxide superconductor laminate 5 includes a metal substrate 1, an intermediate layer 2, an oxide superconductor layer 3, and a protective layer 4. The oxide superconductor laminate 5 has a structure in which the oxide superconductor layer 3 and the protective layer 4 are formed on the metal substrate 1 via the intermediate layer 2. That is, the oxide superconductor laminate 5 has a configuration in which the intermediate layer 2, the oxide superconductor layer 3, and the protective layer 4 are laminated in this order on one surface of the metal substrate 1. The oxide superconductor laminate 5 is an example of a "laminate."

The oxide superconductor laminate 5 is formed in a tape shape. The Y direction is the thickness direction of the oxide superconductor laminate 5, and is the direction in which the metal substrate 1, intermediate layer 2, oxide superconductor layer 3, and protective layer 4 are stacked. The X direction is the width direction of the oxide superconductor laminate 5, and is a direction perpendicular to the length and thickness directions of the oxide superconductor laminate 5.

Metal substrate 1 is made of metal. Specific examples of metals that make up metal substrate 1 include nickel alloys such as Hastelloy (registered trademark); stainless steel; and oriented Ni-W alloys, which are nickel alloys with textured structures. The thickness of metal substrate 1 can be adjusted appropriately depending on the purpose, and is, for example, in the range of 10 to 500 μm. One surface of metal substrate 1 (the surface on which intermediate layer 2 is formed) is referred to as the first main surface 1a, and the surface opposite first main surface 1a is referred to as the second main surface 1b. Metal substrate 1 is an example of a "substrate." First main surface 1a is an example of a "main surface."

The intermediate layer 2 is provided between the metal substrate 1 and the oxide superconducting layer 3. The intermediate layer 2 is formed on the first main surface 1a of the metal substrate 1. The intermediate layer 2 may have a multi-layer structure, and may have, for example, a diffusion prevention layer, a bed layer, an orientation layer, a cap layer, etc., in that order from the metal substrate 1 side to the oxide superconducting layer 3 side. These layers are not necessarily provided one by one; some layers may be omitted, or two or more layers of the same type may be repeatedly stacked. Note that the intermediate layer 2 is not an essential component of the oxide superconducting wire 10, and if the metal substrate 1 itself has orientation, the intermediate layer 2 may not be formed.

The diffusion prevention layer has the function of preventing some of the components of the metal substrate 1 from diffusing and mixing as impurities into the oxide superconducting layer 3. The diffusion prevention layer is made of, for example, Si ₃ N ₄ , Al ₂ O ₃ , GZO (Gd ₂ Zr ₂ O ₇ ), etc. The thickness of the diffusion prevention layer is, for example, 10 to 400 nm.

A bed layer may be formed on the diffusion prevention layer to reduce the reaction at the interface between the metal substrate 1 and the oxide superconducting layer 3 and to improve the orientation of the layer formed thereon. Examples of materials for the bed layer include Y ₂ O ₃ , Er ₂ O ₃ , CeO ₂ , Dy ₂ O ₃ , Eu ₂ O ₃ , Ho ₂ O ₃ , and La ₂ O _3. The thickness of the bed layer is, for example, 10 to 100 nm.

The alignment layer is formed from a biaxially oriented material to control the crystal orientation of the cap layer thereon. Examples of materials for the alignment layer include metal oxides such _{as Gd2Zr2O7} , _MgO , _ZrO2 - _Y2O3 ( _YSZ ), _SrTiO3 , _CeO2 , _Y2O3 , _Al2O3 , _Gd2O3 , _Zr2O3 , _{Ho2O3 , and Nd2O3 .} The alignment layer is preferably formed by _IBAD ( _{Ion -} Beam-Assisted Deposition).

The cap layer is formed on the surface of the orientation layer and is made of a material that allows crystal grains to self-orient in _the in-plane direction. Examples of materials for _the cap layer include _CeO2 , _Y2O3 , _Al2O3 , _Gd2O3 , _ZrO2 , _YSZ , _Ho2O3 , _Nd2O3 , and _{LaMnO3 .} The thickness of the cap layer is typically in the range of 50 to 5000 _nm .

The oxide superconducting layer 3 is composed of an oxide superconductor. The oxide superconductor is not particularly limited, but examples include RE-Ba-Cu-O-based oxide superconductors (REBCO-based oxide superconductors) represented by the general formula REBa ₂ Cu ₃ O _x (RE123). Examples of rare earth elements include one or more of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Among these, one of Y, Gd, Eu, and Sm, or a combination of two or more of these elements, is preferred. Generally, X is 7-x (oxygen deficiency amount x: approximately 0 to 1). The thickness of the oxide superconducting layer 3 is, for example, approximately 0.5 to 5 μm. It is preferable that this thickness be uniform in the longitudinal direction. The oxide superconducting layer 3 is formed on a main surface 2 a of the intermediate layer 2 (the surface opposite to the metal substrate 1 side).
The oxide superconducting layer 3 is an example of a “superconducting layer.” The oxide superconducting layer 3 is provided on the first main surface 1 a of the metal substrate 1 via the intermediate layer 2 .

Protective layer 4 has functions such as bypassing overcurrents that occur in the event of an accident and suppressing chemical reactions that occur between oxide superconducting layer 3 and layers disposed above it. Examples of materials for protective layer 4 include silver (Ag), copper (Cu), gold (Au), gold-silver alloys, other silver alloys, copper alloys, and gold alloys. Protective layer 4 covers at least the main surface 3a of oxide superconducting layer 3 (the surface opposite to the intermediate layer 2). Protective layer 4 is in contact with main surface 3a of oxide superconducting layer 3. There are no particular limitations on the thickness of protective layer 4, but examples include approximately 1 to 100 μm.

5a is the first main surface of the oxide superconductor laminate 5 (main surface 4a of the protective layer 4). The first main surface 5a is the surface of the oxide superconductor laminate 5 on which the oxide superconductor layer 3 is formed. 5b is the side surface of the oxide superconductor laminate 5 (the side surface of the metal substrate 1, the side surface of the intermediate layer 2, the side surface of the oxide superconductor layer 3, and the side surface of the protective layer 4). 5c is the surface opposite the first main surface 5a and is the second main surface of the oxide superconductor laminate 5 (second main surface 1b of the metal substrate 1). The second main surface 5c is the surface of the oxide superconductor laminate 5 on which the metal substrate 1 is formed.

The stabilization layer 6 covers the first main surface 5a, side surfaces 5b, 5b, and second main surface 5c of the oxide superconductor laminate 5. The stabilization layer 6 is formed to surround the outer periphery of the oxide superconductor laminate 5. The stabilization layer 6 functions as a bypass section that diverts overcurrent that occurs when the oxide superconductor layer 3 transitions to a normal conducting state.

The stabilization layer 6 can be made of metals such as copper, copper alloys (e.g., Cu-Zn alloys, Cu-Ni alloys, etc.), aluminum, aluminum alloys, and silver. The thickness of the stabilization layer 6 is, for example, approximately 10 to 300 μm. The stabilization layer 6 can be formed by plating (e.g., electrolytic plating).

Figure 2 is a schematic diagram showing a portion of a cross section of the oxide superconducting layer 3 and protective layer 4 of the oxide superconducting laminate 5. Figure 2 shows a cross section along the thickness direction of the protective layer 4 (a cross section perpendicular to the longitudinal direction of the oxide superconducting wire 10). As shown in Figure 2, "7" denotes the interface between the oxide superconducting layer 3 and the protective layer 4. As shown in the schematic diagram of Figure 2, the interface 7 between the oxide superconducting layer 3 and the protective layer 4 can be observed using a transmission electron microscope (TEM). When observing the cross section of the oxide superconducting wire 10 with a TEM, it can be magnified to any desired magnification. For example, the cross section of the oxide superconducting layer 3 and the protective layer 4 shown in Figure 2 has a width of 1 μm in the depicted area, and the total length of the linear interface 7 is also 1 μm.

One or more voids are formed inside the protective layer 4. The voids may or may not be in contact with the interface 7. Gases such as air, oxygen, or organic compound gases may be present within the voids.

If there are voids in contact with the interface 7 inside the protective layer 4, the number of voids is less than 12 per 1 μm of the length of the interface 7. This prevents a decrease in the contact area between the oxide superconducting layer 3 and the protective layer 4. This reduces the interfacial resistance between the oxide superconducting layer 3 and the protective layer 4. If there are voids in contact with the interface 7, the oxide superconducting layer 3 and the protective layer 4 are not in electrical contact at the voids. Therefore, the fewer the number of voids in contact with the interface 7, the lower the interfacial resistance. The number of voids in contact with the interface 7 is preferably zero, or three to less than 12, per 1 μm of the length of the interface 7. If the number of voids in contact with the interface 7 is zero, or three to less than 12, per 1 μm of the length of the interface 7, the lower the interfacial resistance between the oxide superconducting layer 3 and the protective layer 4. If there are no voids in contact with the interface 7, the entire length of the interface 7 is in contact with the oxide superconducting layer 3 in Figure 2.

In Figure 2, the protective layer 4 takes one of the following forms (i) and (ii): (i) There are zero voids in contact with the interface 7 (i.e., the entire length of the interface 7 is in contact with the oxide superconducting layer 3). (ii) There are more than zero and fewer than 12 voids in contact with the interface 7 per 1 μm of the length of the interface 7.

The number of voids in contact with the interface 7 may be the average number of voids in multiple observation images (e.g., TEM images with a field of view of several micrometers square) of a cross section that includes the interface between the oxide superconducting layer 3 and the protective layer 4. The number of voids in contact with the interface 7 may be, for example, the average number of voids in three or more observation images. The number of voids in contact with the interface 7 may be, for example, the average number per micrometer of length of the interface 7 for an interface 7 that is 3 micrometers or longer.

In the example shown in Figure 2, voids V1 to V8 formed inside protective layer 4 are in contact with interface 7. Therefore, the number of voids in contact with interface 7 is 8 per 1 μm of length of interface 7. Note that voids V9 to V16 are not in contact with interface 7, and therefore are not included in the "number of voids in contact with interface 7."

The size of the voids in contact with the interface 7 (the dimension in the longitudinal direction of the interface 7) is not particularly limited. The size of the voids that can affect the interfacial resistance between the oxide superconducting layer 3 and the protective layer 4 is, for example, 0.1 nm or larger. The size of the voids is, for example, 100 nm or smaller. Examples of void sizes are 11.0 nm to 70.2 nm. The longitudinal direction of the interface 7 is perpendicular to the thickness direction of the protective layer 4 (the left-right direction in Figure 2).

In Figure 2, the shapes of the voids V1 to V8 are semicircular, but the shape of the voids in contact with the interface 7 is not particularly limited. The shape of the voids in the cross section of the protective layer 4 may be arched, circular, elliptical, or the like. The three-dimensional shape of the voids is, for example, hemispherical.

Fig. 3 is a diagram showing a part of a cross section of the oxide superconducting layer 3 and the protective layer 4 of the oxide superconducting laminate 5. In Fig. 3, the dashed line shown in the diagram indicates the interface 7 between the oxide superconducting layer 3 and the protective layer 4. In Fig. 3, the length of the dashed line shown in the diagram is the length of the interface 7.
3, a mixed region 8 may be formed between the oxide superconducting layer 3 and the protective layer 4. The mixed region 8 is a region where a partial region 3A of the oxide superconducting layer 3 and a partial region 4A of the protective layer 4 are mixed.

The partial region 3A may be a convex portion that protrudes from the oxide superconducting layer 3 into the protective layer 4, or may be an independent island. The partial region 4A may be a convex portion that protrudes from the protective layer 4 into the oxide superconducting layer 3, or may be an independent island.

As shown in Figure 3, when the mixed region 8 is formed, the interface 7 between the oxide superconducting layer 3 and the protective layer 4 is not flat. The interface 7 is formed by layers 3 and 4 intertwining vertically. The length of the interface 7 shown in Figure 3 (the length of the dashed line) is approximately 5.6 μm, calculated using the scale bar in the figure. The length of the interface 7 is longer than the width of the image in Figure 3 (approximately 3.6 μm). Therefore, when the mixed region 8 is formed, the contact area between the oxide superconducting layer 3 and the protective layer 4 is larger than in the case of a flat interface without the mixed region 8. Therefore, the interfacial resistance between the oxide superconducting layer 3 and the protective layer 4 is lower. It can be said that the contact area between the oxide superconducting layer 3 and the protective layer 4 is correlated with the length of the interface 7 obtained by cross-sectional observation of the oxide superconducting layer 3 and the protective layer 4. For example, when observing the cross section of the oxide superconducting layer 3 and the protective layer 4, if the interface 7 is observed to be linear and the interface 7 is observed to be curved in the thickness direction of the layers 3 and 4, the contact area is larger in the latter case (when the interface 7 is observed to be curved in the thickness direction).

Furthermore, since the crystals that make up the oxide superconductor have electrical anisotropy, the following can be speculated as the reason why the interface resistance is lowered when the mixed region 8 is formed.
The oxide superconductor contained in the oxide superconducting layer 3 is crystalline. The a-axis and b-axis of the crystal are aligned along the plane of the oxide superconducting layer 3. The c-axis of the crystal is aligned perpendicular to the plane of the oxide superconducting layer 3. The plane formed by the a-axis and b-axis (the ab-plane) coincides with the plane of the oxide superconducting layer 3 (the left-right direction in Figure 3). Oxide superconductors have the property that electricity flows easily along the ab-plane and does not flow easily along the c-axis. Therefore, if the interface 7 between the oxide superconducting layer 3 and the protective layer 4 is perpendicular to the ab-plane, it is thought that the interfacial resistance between the oxide superconducting layer 3 and the protective layer 4 will be low. If the interface 7 between the oxide superconducting layer 3 and the protective layer 4 is parallel to the ab-plane, it is thought that the interfacial resistance between the oxide superconducting layer 3 and the protective layer 4 will be high.
In the mixed region 8, the oxide superconducting layer 3 and the protective layer 4 are intertwined with each other, so that at least a part of the interface 7 between the oxide superconducting layer 3 and the protective layer 4 in the mixed region 8 intersects with the direction along the ab plane of the crystal of the oxide superconductor in the oxide superconducting layer 3. Therefore, the interface resistance can be reduced at this interface.
At least a part of the interface 7 between the oxide superconducting layer 3 and the protective layer 4 in the mixed region 8 may be perpendicular to the direction along the ab plane of the crystal of the oxide superconductor.

The interface between the oxide superconducting layer 3 and the protective layer 4 in the mixed region 8 is, for example, at least one of the interface between the partial region 3A and the protective layer 4, the interface between the partial region 4A and the oxide superconducting layer 3, and the interface between the partial region 3A and the partial region 4A.

[Method for producing oxide superconducting laminate and oxide superconducting wire]
Next, a description will be given of an example of a method for manufacturing the oxide superconducting laminate and the oxide superconducting wire 10. Note that the manufacturing method described below is just an example, and other manufacturing methods may also be adopted.

As shown in Figure 1, an intermediate layer 2 is formed on a metal substrate 1. The intermediate layer 2 can be formed, for example, using the well-known IBAD method.

Next, the oxide superconducting layer 3 is formed on the intermediate layer 2. The oxide superconducting layer 3 can be formed by a vapor deposition method such as a PLD method or an MOCVD method.
For example, the oxide superconducting layer 3 can be formed by a first step and a second step. In the first step, for example, film formation is performed by a PLD method using a target made of a REBCO-based material. In the second step, for example, film formation is performed by a PLD method using a target made of a REBCO-based material and Ag. The second step is a step of performing vapor deposition using a target containing an oxide superconductor and the material of the protective layer 4. The target used in the second step has an Ag content of 10 to 50 vol %.

The deposition rate of the material in the PLD method can be appropriately set by adjusting, for example, the pulse frequency of the laser of the PLD device, the atmospheric pressure during film formation, and the like.
By changing the deposition rate in the PLD method in the second step, it is possible to adjust the number of voids contained in the protective layer 4 or in the interface between the superconducting layer 3 and the protective layer 4. For example, if the deposition rate is increased, the number of voids contained in the protective layer 4 or in the interface between the superconducting layer 3 and the protective layer 4 tends to decrease. Conversely, if the deposition rate is decreased, the number of voids contained in the protective layer 4 or in the interface between the superconducting layer 3 and the protective layer 4 tends to increase. The size of the voids is roughly the same regardless of the deposition rate.

Next, a protective layer 4 is formed on the oxide superconducting layer 3. The protective layer 4 can be formed by a method such as sputtering. This results in an oxide superconducting laminate 5. By forming this protective layer 4, the mixed region 8 described above is formed.

Next, oxygen annealing is performed. Specifically, the oxide superconducting laminate 5 is heated to, for example, 300 to 1000°C in an oxygen atmosphere (in the presence of an oxygen-containing gas). The oxygen-containing gas is, for example, oxygen gas or air. This oxygen annealing may be performed after the protective layer 4 is formed, or may be performed before the protective layer 4 is formed (after the oxide superconducting layer 3 is formed).
Through the above steps, the oxide superconductor laminate 5 shown in FIG. 1 is obtained.

Next, after obtaining the oxide superconductor laminate 5, a stabilization layer 6 is formed on the outer periphery of the oxide superconductor laminate 5. The stabilization layer 6 can be formed by plating or the like.
Through the above steps, the oxide superconducting wire 10 shown in FIG. 1 is obtained.

[Connection structure]
FIG. 4 is a cross-sectional view showing a connection structure 20 using the oxide superconducting wire 10. As shown in FIG.
4, the connection structure 20 is configured by connecting two oxide superconducting wires 10. The stabilization layer 6 of one oxide superconducting wire 10 and the stabilization layer 6 of the other oxide superconducting wire 10 are electrically connected via a solder layer 9.
The connection structure 20 makes it possible to manufacture a long oxide superconducting wire with reduced connection resistance.

The connection structure 20 shown in FIG. 4 has a structure in which the stabilization layer 6 of one oxide superconducting wire 10 and the stabilization layer 6 of the other oxide superconducting wire 10 are connected via a solder layer 9, but the protective layer 4 of one oxide superconducting wire 10 and the protective layer 4 of the other oxide superconducting wire 10 may also be connected via a solder layer 9. Alternatively, the protective layer 4 of one oxide superconducting wire 10 and the stabilization layer 6 of the other oxide superconducting wire 10 may also be connected via a solder layer 9. It is also possible to partially remove the stabilization layer 6 of one oxide superconducting wire 10 to expose the protective layer 4, and then connect the protective layer 4 or stabilization layer 6 of the other oxide superconducting wire 10 to the exposed protective layer 4 via a solder layer 9.

In the connection structure 20 shown in Figure 4, two oxide superconducting wires 10 are connected to each other, but the connection structure may also have a structure including two oxide superconducting wires and an oxide superconducting wire that serves as an intermediate connector that connects them. Even if the configuration of this embodiment is adopted only for the intermediate connector, the connection resistance can be reduced.

Oxide superconducting wire 10 can be electrically connected to objects to be connected, such as electrodes, in addition to other superconducting wires. Even in this case, the connection resistance can be reduced at the connection point between oxide superconducting wire 10 and the object to be connected.

[Effects of the oxide superconducting laminate and oxide superconducting wire according to the embodiment]
The oxide superconductor laminate 5 and the oxide superconducting wire 10 have a small number of voids in contact with the interface 7 in the protective layer 4, so that the contact area between the oxide superconducting layer 3 and the protective layer 4 can be increased. This reduces the interfacial resistance between the oxide superconducting layer 3 and the protective layer 4. Therefore, when the oxide superconducting laminate or the oxide superconducting wire 10 is connected to an object to be connected (another superconducting wire, an electrode, etc.), the electrical resistance at the connection can be reduced.
In the oxide superconducting laminate and the oxide superconducting wire 10, the electrical resistance at the joints is low, which reduces Joule heat generated at the joints. Since the interface resistance between the oxide superconducting layer 3 and the protective layer 4 is low, a bypass current easily flows from the oxide superconducting layer 3 to the protective layer 4 during a quench.

The present invention has been described above based on preferred embodiments, but the present invention is not limited to the above-described embodiments, and various modifications are possible without departing from the spirit of the present invention. For example, the structure of the oxide superconductor laminate is not limited to the structure shown in Figure 1. The oxide superconductor laminate may include layers other than the metal substrate, intermediate layer, oxide superconductor layer, and protective layer.

The present invention will be specifically described below with reference to examples.
Example 1
A sample of the oxide superconducting wire 10 shown in FIG. 1 was produced as follows.
An intermediate layer 2 was formed on one surface (first main surface 1a) of a tape-shaped metal substrate 1 made of Hastelloy (registered trademark) by the IBAD method or the like.

An oxide superconducting layer 3 was formed on the intermediate layer 2 through a first step and a second step. In the first step, film formation was performed by a PLD method using a target formed of a REBCO-based material ( _EuBa2Cu3Oy ). In the second step, film formation was performed by a PLD method using a target formed of _{a REBCO} -based material ( _EuBa2Cu3Oy ) and Ag. In the second step _, film formation was performed at a deposition rate of 0.1 _Å /sec.

A protective layer 4 made of Ag was formed by sputtering on the oxide superconducting layer 3. In this way, an oxide superconducting laminate 5 was obtained.
The oxide superconducting laminate 5 was subjected to oxygen annealing by heating it to 500° C. in an oxygen atmosphere.
A stabilizing layer 6 was formed by copper plating on the outer periphery of the oxide superconducting laminate 5. In this way, an oxide superconducting wire 10 was obtained.

A cross section of the protective layer 4 along the thickness direction was observed with a TEM, and the presence or absence of voids in contact with the interface 7 was confirmed from the TEM image of the cross section. If voids were present, the number of voids per 1 μm in length of the interface 7 and the size of the voids (the size of the interface 7 in the longitudinal direction) were investigated. In Example 1, the number of voids per 1 μm in length of the interface 7 was 7. The size of the voids ranged from 11.0 nm to 70.2 nm. In addition, to quantitatively evaluate the size (size) of the contact area between the oxide superconducting layer 3 and the protective layer 4, the ratio (Li/L) of the interface length (Li) between the oxide superconducting layer 3 and the protective layer 4 to the length ( _L ) in the width direction or longitudinal direction of the oxide superconducting wire 10 was investigated. It is considered that the larger the value of the ratio _Li /L, the larger the contact area between the oxide superconducting layer 3 and the protective layer 4. The interface length _Li was measured by importing a TEM image of the cross section of the oxide superconducting wire 10 into the image processing software "ImageJ" and measuring the length of the interface 7 shown in the TEM image using the drawing tool and length measurement tool of this image processing software. The value of _Li /L can be a substitute index for knowing the size (dimension) of the contact area between the oxide superconducting layer 3 and the protective layer 4. In Example 1, the value of the ratio Li/L was 1.03.

Next, to evaluate the connection resistivity, two oxide superconductor laminates 5 were prepared, and the protective layers 4 of both were electrically connected via a solder layer 9, and the connection resistivity at 77 K and 4 K was determined. The results are shown in Table 1. The connection resistivity is determined by dividing the connection resistance obtained by measurement by the connection area. The connection resistance can be measured using the four-terminal resistance measurement method. Another method (attenuation method) involves constructing a closed circuit with a known inductance L and the desired circuit resistance R, and measuring the change in the decaying current over time to determine the connection resistance. For the example and comparative example samples shown in Table 1, the connection resistivity at 77 K was measured using the four-terminal resistance measurement method. The connection resistivity at 4 K was measured using the attenuation method.

Example 2
Oxide superconducting wire 10 was produced in the same manner as in Example 1, except that in the second step of forming oxide superconducting layer 3, the deposition rate was set to 0.2 Å/sec. The number of voids per 1 μm length of interface 7 was 3. The void sizes were 20.1 nm to 65.4 nm. The value of the ratio Li/L was 1.12. The results are shown in Table 1.

Example 3
Oxide superconducting wire 10 was produced in the same manner as in Example 1, except that in the second step of forming oxide superconducting layer 3, the deposition rate was set to 0.4 Å/sec. The number of voids in contact with interface 7 was zero. The value of the ratio Li/L was 1.56. The results are shown in Table 1.

(Comparative Example)
An oxide superconducting wire 10 was produced in the same manner as in Example 1, except that the oxide superconducting layer 3 was formed only in the first step.
TEM images of the cross section are shown in Figures 6(A) and 6(B). Figure 6(B) is an enlarged image of Figure 6(A). As shown in Figure 6(B), numerous voids V were confirmed in the protective layer 4. The number of voids in contact with the interface 7 was 12 per 1 µm of the length of the interface 7. The value of the ratio Li/L was 1.03. The results are shown in Table 1.

As shown in Table 1, in Examples 1 to 3, no voids were observed in contact with the interface 7. In Examples 1 to 3, the connection resistivity at 77K and 4K was kept low.
In contrast to this, in the comparative example, voids were observed in contact with the interface 7. In the comparative example, the connection resistivity at 77K and 4K was high.

DESCRIPTION OF SYMBOLS 1... Metal substrate (substrate), 3... Oxide superconducting layer (superconducting layer), 3A... Partial region, 4... Protective layer, 4A... Partial region, 5... Oxide superconducting laminate, 7... Interface, 8... Mixed region, 10... Oxide superconducting wire, 20... Connection structure, V, V1 to V16... Void.

Claims (9)

A substrate;
a superconducting layer formed on the substrate and made of an oxide superconductor;
a protective layer provided on the superconducting layer in contact with the superconducting layer,
A void is formed inside the protective layer,
Among the voids formed in the protective layer, the number of the voids in contact with the interface with the superconducting layer is less than 12 per 1 μm of length of the interface in a cross section along the thickness direction of the protective layer.
Oxide superconducting laminate. The number of the voids in contact with the interface is 0 per 1 μm of the length of the interface.
The oxide superconducting laminate according to claim 1. the number of the voids in contact with the interface is 3 or more per 1 μm of the length of the interface;
The oxide superconducting laminate according to claim 1. The size of the void in contact with the interface in the length direction of the interface is 11.0 nm to 70.2 nm.
The oxide superconducting laminate according to claim 3 . a mixed region in which a partial region of the protective layer and a partial region of the superconducting layer are mixed is formed between the superconducting layer and the protective layer;
The oxide superconducting laminate according to any one of claims 1 to 4. In a cross-sectional view of the mixed region, a ratio (Li/L) of the interface length (Li) between the superconducting layer and the protective layer to the length (L) of the oxide superconducting wire in the width direction or longitudinal direction is in the range of 1.03 to 1.56.
The oxide superconducting laminate according to claim 5 . at least a part of the interface between the superconducting layer and the protective layer in the mixed region intersects with a direction along the a-b plane of the crystal of the oxide superconductor of the superconducting layer;
7. The oxide superconducting laminate according to claim 5 or 6. A stabilization layer is formed on the outer periphery of the oxide superconducting laminate according to any one of claims 1 to 7.
Oxide superconducting wire. The oxide superconducting wire according to claim 8,
Connection structure.