JP6404556B2 - Oxide superconducting conductor and manufacturing method thereof - Google Patents

Description

  The present invention relates to an oxide superconductor and a method for manufacturing the same.

One of the electrical devices that can solve energy problems in recent years is a superconducting device such as a cable, a coil, a motor, and a magnet using an oxide superconductor as a low-loss conductive material. As superconductors used in these superconducting devices, for example, oxide superconductors such as RE-123 series (REBa ₂ Cu ₃ O _7-x : RE is a rare earth element including Y, Gd, etc.) are known. This oxide superconductor exhibits superconducting properties near the temperature of liquid nitrogen and can maintain a relatively high critical current density even in a strong magnetic field, and thus is expected as a practically promising conductive material.

  In order to use the above-described oxide superconductor in an electric device, it is common to process the oxide superconductor into a wire and use it as a power supply conductor or coil. Specifically, an oxide superconducting layer is formed on a metal substrate via an intermediate layer with good crystal orientation, and a protective layer and a stabilization layer are formed so as to cover the oxide superconducting layer. An oxide superconductor can be obtained.

As an oxide superconducting conductor of this type, for example, as described in Patent Document 1 below, an oxide superconducting layer is formed on a tape-like substrate via an intermediate layer, and on this oxide superconducting layer, A structure in which a highly conductive metal stabilizing layer is formed is generally known.
In addition, in order to improve the superconducting characteristics of the oxide superconducting layer, it is important to adjust the crystal orientation of the intermediate layer that is the base of the oxide superconducting layer. For example, as described in Patent Document 2 below, an intermediate layer having a good crystal orientation is formed on a substrate by an ion beam assist method, and a cap layer having a further good crystal orientation is formed on the intermediate layer. An oxide superconducting conductor is known in which an oxide superconducting layer is formed on the cap layer.

Japanese Patent Application Laid-Open No. 07-073758 JP 2004-071359 A

  Forming an intermediate layer with good crystal orientation by the ion beam assist method described above ensures a good crystal orientation as the intermediate layer and forms a cap layer with further good crystal orientation on the intermediate layer. It is based on the knowledge that if the oxide superconducting layer is formed on the top, the crystal orientation of the oxide superconducting layer can be improved. In addition, the ion beam assist method performs particle deposition while irradiating an ion beam from an oblique direction of the substrate during film formation, and the grain boundary tilt angle formed by the crystal axes of the crystal grains can be reduced. It is known as an effective technique for manufacturing a superconducting layer in which superconducting current flows easily. In order to further improve the critical current density in the oxide superconducting layer, it is considered necessary to further improve the grain boundary weak coupling.

However, when the oxide superconducting conductor is studied as a conductive material, there is a problem of further improving the superconducting characteristics such as critical current density. Therefore, the oxide superconducting layer and various intermediate layers serving as the underlying layer are more excellent. Research and development is underway to obtain superconducting properties.
In addition, since the RE-123 series oxide superconductor exhibits a specific critical temperature (Tc) of around 92K, it is cooled using a refrigerant such as liquid nitrogen. Accordingly, when a structure in which a plurality of layers are laminated on a substrate is employed, it is necessary to employ a layer structure that does not lower the critical temperature of the oxide superconducting layer.

  The present invention has been made in view of the above circumstances, and employs a structure in which an alkaline earth metal is diffused in the grain boundary of an oxide superconductor to reduce the critical current value without lowering the critical temperature. It is an object of the present invention to provide an oxide superconducting conductor that can be made higher and a method for manufacturing the same.

The oxide superconducting conductor of the present invention includes a base material, an alignment layer formed on the base material, a cap layer and an oxide superconducting layer, and a protective layer or a thin film layer formed on the oxide superconducting layer. A protective layer in contact with the oxide superconducting layer or the thin film layer containing an alkaline earth metal element, and an alkaline earth at the grain boundary of the oxide superconductor constituting the oxide superconducting layer. It is characterized by containing a similar metal element.
When alkaline earth metal elements are present at the grain boundaries of oxide superconductors, the carrier concentration at the grain boundaries is improved, and the occurrence of weak bonds at the grain boundaries is suppressed to improve the critical current value. Contribute to. Further, even if an alkaline earth metal element is present at the grain boundary of the oxide superconductor constituting the oxide superconductor layer, the critical temperature of the oxide superconductor layer does not decrease. An oxide superconductor having a high critical current value while having the same excellent critical temperature as an oxide superconductor not containing a metal element can be provided.

In the present invention, the alkaline earth metal element may be contained on the grain boundary side in the crystal grains of the oxide superconductor constituting the oxide superconducting layer.
If alkaline earth metal elements diffuse a lot inside the crystal grains of oxide superconductors, the crystal structure of oxide superconductors will collapse and cause a decrease in the critical current value of the oxide superconductor layer. It is preferable that the element is diffused in the grain boundary of the oxide superconductor and in the inner region of the crystal grain close to the grain boundary. The presence of an alkaline earth metal element in the region close to the grain boundary of the crystal grain of the oxide superconductor and the grain boundary can improve the critical current characteristic without lowering the critical temperature.

In the present invention, the alkaline earth metal element is preferably Ca or Sr.
If Ca and Sr are selected as the alkaline earth metal elements contained in the oxide superconducting layer, these elements can be diffused to the grain boundaries of the crystal grains of the oxide superconductor. The effect of increasing the critical current value while maintaining the temperature can be obtained.

The method for producing an oxide superconducting conductor of the present invention includes an intermediate layer forming step of forming an intermediate layer on a substrate, an oxide superconducting layer forming step of forming an oxide superconducting layer on the intermediate layer, and the oxide A protective layer forming step for forming a laminate by forming a protective layer on the superconducting layer, or a thin film protective layer forming step for obtaining a laminate by forming a thin film layer and a protective layer on the oxide superconducting layer; and And an oxygen annealing step of diffusing an alkaline earth metal element contained in the protective layer or the thin film layer to the oxide superconducting layer side by oxygen annealing treatment.
Oxygen is supplied to the crystal of the oxide superconducting layer through the protective layer by oxygen annealing treatment, and the critical current characteristics are improved by adjusting the crystal structure of the oxide superconducting layer. During this oxygen annealing treatment, alkaline earth metal elements are diffused from the protective layer or thin film layer to the oxide superconducting layer side to thereby disperse the alkaline earth metal elements at the grain boundaries of the oxide superconductor constituting the oxide superconducting layer. Can be diffused. The alkaline earth metal element diffuses into the grain boundary of the oxide superconductor crystal grain at a faster rate than the alkaline earth metal element diffuses inside the oxide superconductor crystal grain. Metal elements mainly diffuse into the grain boundaries of the oxide superconductor crystal grains. For this reason, the effect of improving the carrier concentration is mainly exhibited at the grain boundary of the oxide superconductor crystal grains, and the critical current characteristics are improved.

In the present invention, the alkaline earth metal element can be diffused into the grain boundary of the crystal grains of the oxide superconductor constituting the oxide superconducting layer by the oxygen annealing step.
In the present invention, the protective layer or the thin film layer is formed on the oxide superconducting layer by a film formation method, and the target used when the film formation method is performed can contain the alkaline earth metal element. .
If these layers are protective layers or thin film layers, an alkaline earth metal element is added to the target when these layers are formed by a film formation method, and the alkaline earth metal elements are formed simultaneously with the formation of the protective layer or thin film layer. A layer containing can be obtained. Then, an alkaline earth metal element can be diffused to the oxide superconducting layer side by utilizing thermal diffusion during oxygen annealing, and an oxide superconducting layer in which the alkaline earth metal element is diffused to the grain boundary can be obtained. it can.

  In the present invention, Ca or Sr can be used as the alkaline earth metal element, and by using these elements, the critical current characteristics can be improved without causing a decrease in the critical temperature.

  ADVANTAGE OF THE INVENTION According to this invention, a critical current density can be made high, without reducing a critical temperature, and the oxide superconducting conductor provided with the oxide superconducting layer which exhibits the outstanding superconducting characteristic can be provided.

1 shows a first embodiment of an oxide superconductor according to the present invention, FIG. 1 (A) is a partial cross-sectional perspective view, and FIG. 1 (B) is a crystal grain of an oxide superconductor constituting an oxide superconductor layer. FIG. It is a flowchart which shows an example of the process of manufacturing the oxide superconducting conductor of the embodiment. The fragmentary sectional perspective view which shows 2nd embodiment of the oxide superconductor based on this invention. It is a perspective view which shows an example of the superconducting coil comprised using the oxide superconducting conductor shown in FIG. The graph which shows an example of Ca distribution state of the grain-boundary part in the oxide superconducting layer of the oxide superconducting conductor manufactured in the Example. The graph which shows an example of the Sr distribution state of the grain-boundary part in the oxide superconducting layer of the oxide superconducting conductor manufactured in the Example.

Hereinafter, a first embodiment of an oxide superconductor and a method of manufacturing an oxide superconductor will be described in detail with reference to the drawings. The present invention is not limited to the embodiments described below. In addition, the drawings used in the following description may show the main parts in an enlarged manner for convenience in order to make the features of the present invention easier to understand, and the dimensional ratios of the respective components are the same as the actual ones. Not always.
As shown in FIG. 1 (A), the oxide superconducting conductor 1 of the first embodiment has an intermediate layer 5, an oxide superconducting layer 6, a protective layer 7 and metal stabilization on one surface (surface) of a base material 2. The layers 8 are laminated in this order, and the periphery thereof is covered with a covering layer 10 made of an insulating material. Note that the coating layer 10 is not an essential component, and may be omitted if insulation is not necessary.

  In the oxide superconducting conductor 1, the base material 2 is preferably in the form of a tape, a sheet or a thin plate in order to obtain a long flexible superconducting conductor. The material used for the substrate 2 is preferably a material having a relatively high mechanical strength, heat resistance, and a metal that can be easily processed into a wire, such as stainless steel and hastelloy. Examples thereof include various heat-resistant metal materials such as nickel alloys, or materials obtained by arranging ceramics on these various metal materials. Among them, Hastelloy (trade name, manufactured by Haynes, USA) is preferable as a commercial product. This kind of Hastelloy includes Hastelloy B, C, G, N, W, etc., which have different amounts of components such as molybdenum, chromium, iron, cobalt, etc., and any kind can be used here. Moreover, what is necessary is just to adjust the thickness of the base material 2 suitably according to the objective, and is 10-500 micrometers normally, Preferably it is 20-200 micrometers.

As the intermediate layer 5, a structure in which a base layer 3 </ b> A composed of a diffusion prevention layer or a bed layer, an alignment layer 3 </ b> B, and a cap layer 3 </ b> C are stacked in this order can be applied as an example.
When the base material 2 or another layer receives a thermal history as a result of heat treatment when forming another layer on the upper surface side of this layer, a part of the constituent elements of the base material 2 diffuses. And it has a function which suppresses mixing in the oxide superconducting layer 6 side as an impurity. A specific example of the diffusion preventing layer is not particularly limited as long as it can exhibit the above functions, but Al ₂ O ₃ , Si ₃ N ₄ , or GZO (Gd _A single layer structure or a multilayer structure composed of ₂ Zr ₂ O ₇ ) or the like is desirable.

The bed layer is used to suppress the reaction of constituent elements at the interface between the base material 2 and the oxide superconducting layer 6 and to improve the orientation of the layer provided above this layer. The specific structure of the bed layer is not particularly limited as long as it can exhibit the above functions, but Y ₂ O ₃ , CeO ₂ , La ₂ O ₃ , Dy ₂ O ₃ , Er ₂ O, which have high heat resistance. ₃ , a single layer structure or a multilayer structure composed of rare earth oxides such as Eu ₂ O ₃ and Ho ₂ O ₃ is desirable. Although both the diffusion prevention layer and the bed layer may be provided, or only one of them may be provided, in the embodiment of FIG. 1, these are collectively shown as one underlayer 3A.

The alignment layer 3 </ b> B has a function of controlling the crystal orientation of the cap layer 3 </ b> C and the oxide superconducting layer 6 formed thereon and a function of suppressing the constituent elements of the base material 2 from diffusing into the oxide superconducting layer 6. And a function to alleviate a difference in physical characteristics such as a coefficient of thermal expansion and a lattice constant between the base material 2 and the oxide superconducting layer 6. The constituent material of the alignment layer 3B is not particularly limited as long as it can exhibit the above functions. When a metal oxide such as Gd ₂ Zr ₂ O ₇ , MgO, or ZrO ₂ —Y ₂ O ₃ (YSZ) is used, a crystal is formed in an ion beam assisted deposition method (hereinafter sometimes referred to as IBAD method). A layer with high orientation is obtained, and the crystal orientation of the cap layer 3C and the oxide superconducting layer 6 can be made better, which is particularly preferable.

The cap layer 3 </ b> C controls the crystal orientation of the oxide superconducting layer 6 more strongly than the orientation layer 3 </ b> B, diffuses elements constituting the oxide superconducting layer 6 to the intermediate layer 5 side, and stacks the oxide superconducting layer 6. It has a function of suppressing the reaction between the gas used sometimes and the intermediate layer 5. The constituent material of the cap layer 3C is not particularly limited as long as it can exhibit the above functions, but is CeO ₂ , Y ₂ O ₃ , Al ₂ O ₃ , Gd ₂ O ₃ , Zr ₂ O ₃ , Ho ₂ O _3. From the viewpoint of lattice matching with the oxide superconducting layer 6, metal oxides such as Nd ₂ O ₃ , Zr ₂ O ₃ , and LMnO ₃ are preferable. Among them, CeO ₂ or LMnO ₃ is particularly preferable because a layer having an orientation degree better than that of the orientation layer 3B can be obtained. Here, when CeO ₂ is used for the cap layer 3C, the cap layer 3C may include a Ce—M—O-based oxide in which a part of Ce is substituted with another metal atom or metal ion. The cap layer 3C may be a single layer or a plurality of layers.

The oxide superconducting layer 6 has a function of flowing current when in the superconducting state. The material used for the oxide superconducting layer 6 can be widely applied to an oxide superconductor having a generally known composition, such as a copper oxide such as a Y-based superconductor or a Bi-based superconductor. Examples include superconductors. Examples of the composition of the Y-based superconductor include REBa ₂ Cu ₃ O _7-x (RE is a rare earth element such as Y, La, Nd, Sm, Er, Gd, and x is oxygen deficiency). thereof _{include, Y123 (YBa 2 Cu 3 O} 7-x), include _{Gd123 (GdBa 2 Cu 3 O 7} -x). The composition of the Bi-based superconductor, for _{example, Bi 2 Sr 2 Ca n-} 1 Cu n O 4 + 2n + δ (n is the number of layers of CuO _2, [delta] represents an excess oxygen.) Include. The base material of this oxide superconductor is an insulator, but it becomes an oxide superconductor with a well-structured crystal structure by incorporating oxygen by the oxygen annealing treatment described in the manufacturing method described later, and has the property of exhibiting superconducting properties. Yes.
The oxide superconducting layer 6 of this embodiment contains an alkaline earth metal element diffused from a part of the alkaline earth metal element contained in the protective layer 7. Alkaline earth metal elements contained in the oxide superconducting layer 6 are mainly diffused in the grain boundaries of the oxide superconductor constituting the oxide superconducting layer 6. Further, a part of the alkaline earth metal element may be diffused slightly from the grain boundary of the oxide superconductor crystal grain to the inner side of the oxide superconductor crystal grain. The grain boundary side in the crystal grain means a range from the grain boundary until the additive concentration becomes constant.

  The oxide superconducting layer 6 of the present embodiment is a polycrystal formed by aggregating a plurality of crystal grains 6A in which oxide superconductor crystals are grown to a certain size as shown in FIG. 1B. In each crystal grain 6A, the a-axis or b-axis of the crystal axis is aligned, but the grain boundary tilt angle K indicated by the adjacent crystal grains 6A via the grain boundary R is excellent at about 1 ° to 5 °. It is a polycrystal formed in orientation. In order to show the crystal orientation corresponding to such an excellent single crystal, the orientation layer 3B is formed with a good crystal orientation by the IBAD method, and the cap layer 3C is laminated, This can be realized by forming the oxide superconducting layer 6 on the substrate. If the oxide superconducting layer 6 has such an excellent crystal orientation, when the superconducting conductor 1 is cooled below the critical temperature and energized, excellent critical current characteristics are exhibited.

The protective layer 7 becomes a current path that bypasses an overcurrent generated due to some accident when the oxide superconductor 1 is energized, and in order to make it easier for oxygen to be taken into the oxide superconductor layer 6, it transmits oxygen during heating. Has a function to facilitate. For this reason, it is preferable that the protective layer 7 is formed from Ag or a material containing at least Ag. The material for forming the protective layer 7 may be a mixture or alloy containing a noble metal such as Au or Pt, or a plurality of these may be used.
The protective layer 7 of this embodiment contains a specified amount of an alkaline earth metal element (one or more of Ca, Sr, Ba, and Ra). Examples of the content of the alkaline earth metal element contained in the protective layer 7 include a range of 1 mol% to 50 mol%. As content of alkaline-earth metal element, the range of 1-40 mol% is preferable, and the range of 1-30 mol% is more preferable.
As a means for containing the alkaline earth metal element in the protective layer 7, the alkaline earth metal element is previously contained in the target used when forming the protective layer 7 as described in detail in the manufacturing method described later. A method of introducing simultaneously with the membrane can be adopted. For this reason, the target of the alkaline earth metal element or the target of the alkaline earth metal element or the alloy target obtained by adding the alkaline earth metal element to the Ag or the Ag-based target constituting the protective layer 7 or the Ag-based target. It is preferable to contain a required amount of an alkaline earth metal element when forming the protective layer 7 using a composite target in which a compound target is combined.

In the structure of the oxide superconducting conductor 1 of this embodiment, a metal stabilizing layer 8 is laminated on the protective layer 7. The use of the metal stabilization layer 8 varies depending on the use of the oxide superconducting conductor 1. For example, when used for a superconducting cable, a superconducting motor, or the like, it is used as a main part of a bypass that commutates an overcurrent generated when a quench occurs due to some accident and the oxide superconducting layer 6 transitions to a normal conducting state. At this time, the material used for the metal stabilization layer 8 is made of a relatively inexpensive material such as copper, a Cu-Zn alloy (brass), a copper alloy such as a Cu-Ni alloy, aluminum, an aluminum alloy, and stainless steel. It is preferable to use copper, and it is preferable to use copper because it has high conductivity and is inexpensive. Further, when the oxide superconducting conductor 1 is used for a superconducting fault current limiter, the stabilization layer is used to instantaneously suppress an overcurrent generated when a quench occurs and the state is changed to a normal conducting state. In the case of this application, examples of the material used for the metal stabilization layer 8 include high-resistance metals such as Ni-based alloys such as Ni-Cr.
The structure of the metal stabilizing layer 8 may be a layer structure like the other layers, and a laminated body from the base material 2 to the protective layer 7 is laminated from the front surface side of the protective layer 7 to the back surface side of the laminated body. It may be a metal stabilizing layer having a cross-sectional C-shaped structure in which a metal tape is bent so as to cover the width. Moreover, it is good also as a structure which comprised the metal stabilization layer 8 by metal plating, and covered the whole laminated body formed from the base material 2 to the protective layer 7 with the plating layer.

  Further, in the structure of the oxide superconducting conductor 1, the base material 2, the intermediate layer 5, the oxide superconducting layer 6, the protective layer 7, and the metal stabilizing layer 8 are laminated as in the structure of the present embodiment shown in FIG. Alternatively, a coating layer 10 made of a tape-like or sheet-like insulating material may be formed so as to cover the entire circumference of the laminated body. The coating layer 10 made of an insulating material has a function of reinforcing the oxide superconducting conductor 1 by providing insulation from the outside. Although the material used for the coating layer 10 will not be specifically limited if it is an insulating material, For example, insulating resin, such as a polyimide, is mentioned. Note that the covering layer 10 is not necessarily a necessary element, and may be omitted when it is not necessary to individually insulate the oxide superconducting conductor 1.

In the oxide superconducting layer 6 of this embodiment, since the alkaline earth metal element is diffused in the crystal grain boundary of the oxide superconductor, the critical current characteristics are excellent and the critical temperature is not lowered. Demonstrates superconducting properties.
If an alkaline earth metal element is present at the grain boundary R of the crystal grain 6A of the oxide superconductor constituting the oxide superconducting layer 6, the carrier concentration at the grain boundary portion is improved, and weak coupling hardly occurs at the grain boundary R. As a result, the obstacle when the superconducting current flows through the grain boundary R is reduced, and the critical current characteristic is improved.
Since alkaline earth metal elements such as Ca are divalent with respect to the oxide superconductor crystal located at the grain boundary R, it replaces the position of the Ba atom of the RE123 rare earth oxide superconductor and the valence changes. It can be estimated that the increase in the number of carriers of electrons improves the carrier concentration and facilitates the flow of the superconducting current.

Next, the manufacturing method of the oxide superconductor 1 according to the present invention will be described. Here, FIG. 2 is a flowchart showing the manufacturing method of the oxide superconducting conductor 1 according to the present invention in the order of steps.
First, a base layer 3A composed of a diffusion prevention layer and a bed layer is formed on the surface of the tape-shaped substrate 2 (base layer forming step: S1).
Since the diffusion preventing layer is not particularly limited in crystallinity, it can be formed by a film forming method such as a normal sputtering method. Moreover, what is necessary is just to normally set the film thickness of a diffusion prevention layer to 10-400 nm. The bed layer can be formed by a film forming method such as a normal sputtering method because crystallinity is not particularly limited as in the case of the diffusion preventing layer. The film thickness of the bed layer is usually 10 to 100 nm. Note that it is not always necessary to use both the diffusion prevention layer and the bed layer, and it is sufficient if the structure uses at least one of these layers. In some cases, both layers may be omitted. When both the diffusion preventing layer and the bed layer are omitted, the alignment layer 3B is formed directly on the surface of the substrate 2.

An alignment layer 3B is formed on the underlayer 3A (alignment layer forming step: S2).
A number of methods such as a vacuum deposition method, a laser deposition method, and an ion beam assisted deposition method (IBAD method) can be selected as a method for forming the alignment layer 3B, but the crystal orientation of the superconducting layer 6 and the cap layer 3C is higher. It is preferable to use the IBAD method because it can be controlled. Here, the IBAD method is a method of orienting crystal axes by irradiating an ion beam such as Ar with a predetermined incident angle with respect to a crystal film formation surface during film formation. In addition, the alignment layer 3B may have a single layer structure using one of the materials described above, or may have a multilayer structure using a plurality of materials.

Next, the cap layer 3C is formed on the alignment layer 3B (cap layer forming step: S3).
For example, CeO ₂ is used to form the cap layer 3C. In this case, a film can be formed by a pulsed laser deposition method (hereinafter sometimes referred to as a PLD method), a sputtering method, or the like, but it is preferable to use the PLD method from the viewpoint of obtaining a high film formation rate. The cap layer 3C preferably has a thickness of 40 nm or more in order to obtain sufficient orientation. By forming the cap layer 3 on the orientation layer 3B, the crystal orientation of the orientation layer 3B is maintained or better. In this embodiment, since the underlayer 3A, the alignment layer 3B, and the cap layer 3C are referred to as the intermediate layer 5, the underlayer forming step S1, the alignment layer forming step S2, and the cap layer forming step S3 are combined. This is referred to as a layer forming step S4. When the intermediate layer 5 is composed only of the alignment layer 3B, the alignment layer formation step S2 is an intermediate layer formation step, and when the intermediate layer 5 is composed of the alignment layer 3B and the cap layer 3C, the alignment layer formation step. S2 and cap layer forming step S3 are intermediate layer forming steps.

Next, the oxide superconducting layer 6 is formed on the cap layer 3C (oxide superconducting formation step: S5). The oxide superconducting layer 6 can be formed by sputtering, vacuum vapor deposition, laser vapor deposition, physical vapor deposition such as PLD or electron beam vapor deposition, chemical vapor deposition (CVD), or coating pyrolysis. A chemical vapor deposition method such as a method (MOD method) can be used. In general, the oxide superconductor used for the oxide superconducting layer 6 exhibits good critical current characteristics by adjusting the crystal orientation, so that the crystal c-axis is formed on the surface of the tape-like substrate 2 (synthetic material). It is necessary to form a film so that the a-axis or b-axis is oriented in the length direction of the tape-shaped substrate 2 while being oriented perpendicularly to the film surface.
In the above-described process, if the orientation layer 3B and the cap layer 3C are formed while being controlled to have good orientation, the crystal grains of the oxide superconductor formed on the cap layer 3C also have good orientation. Thus, the oxide superconducting layer 6 that is oriented with good properties and exhibits good superconducting properties can be obtained. The oxide superconducting layer 6 has a film thickness of about 0.5 to 5 μm and preferably a uniform thickness.

Subsequently, the protective layer 7 is formed on the oxide superconducting layer 6 (protective layer forming step: S6).
The protective layer 7 can be formed by a known film forming method, but can be formed by a sputtering method using a film forming apparatus such as a DC sputtering apparatus or an RF sputtering apparatus. Moreover, the film thickness of the protective layer 7 should just be normally 1-30 micrometers. A specified amount of an alkaline earth metal element is added to a target used for forming the protective layer 7. Here, the amount of the alkaline earth metal element added to the target may be in the range of 1 mol% to 50 mol%. The content of the alkaline earth metal element to be contained in the target is preferably 1 to 40 mol%, more preferably 1 to 30 mol%.

Next, an oxygen annealing process is performed on the laminated body in which up to the protective layer 7 is laminated on the base material 2 using a heating furnace (not shown) (oxygen annealing step: S7). Here, since the base material of the oxide superconducting layer 6 is an insulator, it does not show good superconducting characteristics as it is. For this reason, by supplying oxygen to the base material of the oxide superconducting layer 6 and adjusting the crystal structure thereof, the oxide superconducting layer 6 made of an oxide superconductor crystal with good superconducting characteristics is obtained.
Therefore, as a heating condition, the atmosphere in the heating furnace is preferably an atmosphere containing oxygen gas. The heating temperature during the oxygen annealing can be selected from the range of 300 to 500 ° C. and several hours to several tens hours.

By the heat treatment during the oxygen annealing, a part of the alkaline earth metal element contained in the protective layer 7 can be diffused to the oxide superconducting layer 6 side. The diffused alkaline earth metal element is present mainly by diffusing into the grain boundary of the crystal grains of the oxide superconductor constituting the oxide superconductor layer 6, and a part thereof is inside the crystal grains of the oxide superconductor. Also spread. In the oxide superconducting layer 6, the amount of the alkaline earth metal element diffusing to the grain boundary is larger than the amount of the alkaline earth metal element diffusing to the inside of the crystal grains of the oxide superconductor. In FIG. 6, many alkaline earth metal elements exist at the grain boundaries. This is because when elemental diffusion is performed on a polycrystalline body by thermal diffusion, the diffusion coefficient at the grain boundary is generally larger than the diffusion coefficient within the grain.
As described above, the oxide superconducting layer 6 excellent in superconducting characteristics in which the alkaline earth metal element is diffused in the crystal grain boundary of the oxide superconductor can be formed by the oxygen annealing treatment.

Next, although omitted in the process flow of FIG. 2, if the metal stabilization layer 8 is laminated on the protective layer 7 after the oxygen annealing treatment and covered with a coating layer 10 made of an insulating material as required, The oxide superconducting conductor 1 shown in FIG. In addition, when employ | adopting the structure which does not provide the metal stabilization layer 8 and the coating layer 10, these layers can be abbreviate | omitted.
The method for forming the metal stabilizing layer 8 is not particularly limited as long as it is a known method, but it is formed by a method of directly bonding a metal tape using solder, a method of forming a layer by a sputtering method or a vapor deposition method, or plating. It is preferable to select the method to do. The film thickness of the metal stabilizing layer 8 is not particularly limited, and can be adjusted as appropriate. However, considering the flexibility as the superconducting conductor 1, it is preferably 10 to 300 μm.
The method for forming the coating layer 10 made of an insulating material is not particularly limited, but the entire periphery of the laminate on which the metal stabilizing layer 8 is formed is covered with a tape-shaped, sheet-shaped, or thin-plate-shaped insulating coating layer. Can be formed.

FIG. 3 shows the oxide superconducting conductor according to the second embodiment of the present invention. The oxide superconducting conductor 11 according to the second embodiment comprises an intermediate layer 5, an oxide on one surface (surface) of the substrate 2. The superconducting layer 6, the thin film layer 70, the protective layer 7, and the metal stabilizing layer 8 are laminated in this order, and the periphery thereof is covered with a coating layer 10 made of an insulating material. Note that the coating layer 10 is not an essential component, and may be omitted if insulation is not necessary.
The oxide superconducting conductor 11 of the second embodiment is different from the oxide superconducting conductor 1 of the first embodiment in that a thin film layer 70 is interposed between the oxide superconducting layer 6 and the protective layer 7. Is a point. The oxide superconducting conductor 11 is characterized in that the thin film layer 70 is composed of a thin film containing an alkaline earth metal element (one or more of Ca, Sr, Ba, and Ra). In the thin film layer 70, the alkaline earth metal element is contained as a simple substance such as metallic calcium or an oxide such as CaO.
Further, similarly to the oxide superconducting layer 6 of the oxide superconducting conductor 1, an alkaline earth metal element is diffused in the oxide superconducting layer 6.

The alkaline earth metal element diffused in the oxide superconducting layer 6 of the present embodiment is composed of an alkaline earth metal element diffused from the thin film layer 70 during oxygen annealing. In the present embodiment, the alkaline earth metal element is not positively added to the protective layer 7, and the alkaline earth metal element contained in the thin film layer 70 is partially diffused into the protective layer 7 during the oxygen annealing. Contains alkaline earth metal elements.
Since the thin film layer 70 is laminated on the oxide superconducting layer 6, the thin film layer 70 is formed to a thickness that does not become a resistance when the oxide superconducting layer 6 is energized. In order to energize the oxide superconducting layer 6, a configuration in which an electrode terminal is bonded to the metal stabilizing layer 8 laminated thereon via the protective layer 7 and energized is generally used. When the oxide superconducting layer 6 is energized through the protective layer 7, it is necessary to set the film thickness so that the thin film layer 7 does not become an obstacle. Further, during the oxygen annealing treatment, oxygen in the oxygen annealing atmosphere permeates through the protective layer 7 and it is necessary to supply oxygen to the oxide superconducting layer 6 side, so that the thin film does not hinder this oxygen permeation. The film thickness of the layer 70 is also required.
Taking these into consideration, the thickness of the thin film layer 70 is preferably 1000 nm or less, and more preferably 100 nm or less. In addition, since the thin film layer 70 may diffuse the alkaline earth metal element, the thin film layer 70 does not have to be a completely uniform film, and may be a film in which crystal grains are dispersed in an island shape. In view of these circumstances, the thickness of the thin film layer 70 is preferably 2 nm or more and 1000 nm or less, and more preferably 2 nm or more and 100 nm or less.

In the oxide superconducting conductor 11 of the present embodiment, the point that the alkaline earth metal element is diffused in the oxide superconducting layer 6 is equivalent to the point that the alkaline earth metal element is mainly diffused in the grain boundary. Moreover, it is the same as that of the oxide superconductor 1 of the first embodiment in that the alkaline earth metal element is slightly diffused on the grain boundary side of the crystal grain of the oxide superconductor located at the grain boundary.
Also in the oxide superconducting conductor 11 of the present embodiment, since the alkaline earth metal element is diffused in the grain boundary R of the oxide superconducting layer 6, the same action as that of the oxide superconducting conductor 1 of the previous first embodiment. An effect can be obtained.

  In order to manufacture the oxide superconducting conductor 11 of the second embodiment, after performing the oxide superconducting layer forming step S5 in the step described with reference to FIG. 2, the thin film layer 70 is formed by a film forming means such as sputtering. This can be realized by performing the thin film layer forming step S8 formed by the above, followed by the protective layer forming step S6, followed by the oxygen annealing step S7. In the present embodiment, the thin film layer forming step S8 and the protective layer forming step S6 are collectively referred to as a thin film protective layer forming step S9.

Even when the alkaline earth metal element diffused from the thin film layer 70 during oxygen annealing is diffused in the grain boundary side or part of the oxide superconducting layer 6 into the grain on the grain boundary side, the critical current characteristics are excellent. The oxide superconducting conductor 11 that exhibits excellent superconducting properties without lowering the temperature can be obtained.
In the present embodiment, when the thin film layer 70 is formed by a film forming method such as sputtering, laser vapor deposition, or electron beam vapor deposition, the target thin film layer 70 is obtained by including an alkaline earth metal element in the target to be used. Can be formed. The target used here is preferably a target containing an alkaline earth metal element. For example, any of a single metal target of Ca or Sr, or an oxide target such as CaO or SrTiO ₃ may be used. Of course, even if these targets contain an alkaline earth metal element other than Ca and Sr, such as Ba and Ra, the object can be similarly achieved.

The superconducting coil 17 can be formed by winding the oxide superconducting conductors 1 and 11 configured as described above, for example, as shown in FIG. . The coil bodies 15 and 16 shown in FIG. 4 are configured so that the winding directions are right-handed and left-handed in opposite directions, but the winding direction of the superconducting conductors 1 and 11 is not limited to the example in this figure.
Further, a superconducting device such as a superconducting magnet or a superconducting motor can be manufactured by using the necessary number of superconducting coils 17 shown in FIG. Here, the superconducting device is not particularly limited as long as it has the oxide superconducting conductor 1, and examples thereof include a superconducting transformer, a superconducting current limiter, and a superconducting power storage device. Of course, the oxide superconducting conductors 1 and 11 according to the present embodiment can be applied to a superconducting device in which a plurality of the oxide superconducting conductors 1 are assembled into a cable structure without forming a coil.

Hereinafter, the content of the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples.
"Example 1"
Prepare a tape-shaped substrate of Hastelloy (trade name Hastelloy C-276, manufactured by Haynes, USA) having a width of 10 mm, a thickness of 0.1 mm, and a length of 2000 mm, and polishing until the surface roughness Ra is 4 to 5 nm. And then washed with alcohol and an organic solvent.
Next, a diffusion prevention layer, a bed layer, an alignment layer, a cap layer, an oxide superconducting layer, and a protective layer were laminated in this order on one surface of a tape-like substrate under the following formation conditions.
First, a 100 nm-thick diffusion prevention layer made of Al ₂ O ₃ is formed on a tape-like substrate by ion beam sputtering, and then Y ₂ is formed on the diffusion prevention layer by ion beam sputtering. A bed layer made of O ₃ and having a thickness of 20 nm was formed.
Next, an alignment layer having a thickness of 10 nm made of MgO was formed on the bed layer by IBAD.

After forming the alignment layer, a cap layer having a thickness of 300 nm made of CeO ₂ was formed on the alignment layer by a PLD method using a CeO ₂ target.
The film formation temperature by the PLD method can be set in the range of 700 to 900 ° C. In this example, the film was formed at 800 ° C.
Next, an oxide superconducting layer having a composition of GdBa ₂ Cu ₃ O _7-x having a thickness of 2 μm is sequentially formed on the cap layer of each laminate sample by the PLD method, and then an Ag target containing Ca is used. A protective layer made of Ag and having a thickness of 5 μm was formed on the oxide superconducting layer by DC sputtering.
In order to compare the influence of the amount of Ca contained in the protective layer, a plurality of targets are prepared by appropriately changing the content of Ca contained in the Ag target between 1 mol% and 50 mol% as shown in Table 1 below. Then, a plurality of laminate samples having different amounts of Ca contained in the protective layer were prepared using different targets.
Next, the laminated body on which the layers up to the protective layer were formed was subjected to oxygen annealing in a heating furnace filled with oxygen at 500 ° C. for 10 hours. Then, it removed from the heating furnace and obtained the tape-shaped oxide superconductor sample. Each oxide Ca addition amount of superconductor samples (amount added for capping layer) and the critical current (Ic / Ic _0; the Ic value of Ca oxide superconductor having an oxide superconducting layer without additives was Ic ₀ In this case, the relative value of Ic and the critical temperature (Tc: K) of each obtained oxide superconducting conductor sample were measured. The Ic value was measured at 77K by a four-terminal method in which a terminal was attached to the outer surface of the protective layer of each superconducting conductor sample. Also, a pass / fail column is provided in Table 1, and samples with an Ic / Ic ₀ value of 1.2 or more are marked with ◎, samples with a value of 1.0 to less than 1.2 are marked with ○, and samples with a value less than 1.0 are marked with △ It showed in.

As shown in the results shown in Table 1, in the oxide superconducting conductor sample in which Ca contained in the target for forming the protective layer is diffused and contained in the oxide superconducting layer side, all of the samples have a high critical current. The value is shown.
The Ca content in Table 1 is the Ca content relative to the target for forming the protective layer, but assuming that all the elements have migrated from the target to the protective layer side, the Ca contained in the protective layer and the oxide superconducting layer. It can be considered that the total amount of is almost equal to the amount of Ca contained in the target. For this reason, as a Ca amount (alkaline earth metal element amount) contained in both the protective layer and the oxide superconducting layer, the result shown in Table 1 is good if it is in the range of 1 mol% or more and less than 50 mol%. Thus, an oxide superconductor having both a critical current value and an excellent critical temperature can be produced. Moreover, as a range of Ca amount, from the result shown in Table 1, the range of 1 mol% or more and 40 mol% or less is more preferable, and the range of 1 mol% or more and 30 mol% or less is most preferable.
For comparison, at the time of preparing a sample of No. 4 Ca addition amount 5 mol% shown in Table 1, 5 mol% of Ca was contained in the target for generating the oxide superconducting layer to form an oxide superconducting layer. A formed oxide superconducting conductor sample of a comparative example was prepared. The oxide superconducting conductor sample had an Ic / Ic ₀ value of 0.8 and a critical temperature Tc of 90K. According to this sample, it is considered that Ca invaded into the crystal of the oxide superconducting layer when the oxide superconducting layer was formed, thereby affecting the generation of the oxide superconducting layer crystal itself. For this reason, the pass / fail of this comparative sample corresponds to Δ.

"Example 2"
In addition to the manufacturing process substantially equivalent to that of Example 1, a process for forming a thin film layer containing an alkaline earth metal element was added, and an oxide superconductor was manufactured without adding an alkaline earth metal element to the protective layer. Similarly to Example 1, a diffusion prevention layer, a bed layer, an alignment layer, a cap layer, and an oxide superconducting layer are formed on one surface of a tape-like substrate, and a thin film layer (thickness 50 nm) and a protective layer are formed thereon. (Thickness 5 μm) were laminated in this order.
The difference from the manufacturing process of Example 1 is that instead of using an Ag protective layer forming target containing Ca, an Ag protective layer forming target containing no Ca is used, and this protective layer forming target is used to form Ag. Before forming the protective layer, a thin film layer containing Sr was formed using a SrO target containing Sr. The difference from the first embodiment is that Sr is diffused from this thin film layer during oxygen annealing. By using this thin film layer, an oxide superconducting conductor provided with an oxide superconducting layer in which Sr was diffused into the crystal grain boundary was manufactured. Other manufacturing conditions were the same as in Example 1, and a plurality of oxide superconducting conductor samples were obtained.

Sr addition amount (addition amount with respect to the thin film layer) and critical current value (Ic / Ic ₀ ; Ic value of the oxide superconducting conductor provided with no Ca-added oxide superconducting layer of each oxide superconducting conductor sample shown in Table 2 The relative value of Ic and the critical temperature (Tc: K) of each oxide superconducting conductor sample in the case of Ic ₀ were measured. Moreover, when forming the thin film layer of Example 2, the thickness of the thin film layer varied from 2 nm to 5000 nm as shown in Table 3 below, with the film thickness of the thin film layer being 0 nm (example without the thin film layer). A plurality of samples (Sr addition amount of 5 mol%) having a thickness of 1 mm were prepared, and the (Ic / Ic ₀ ) value of each sample was measured. The results are shown in Table 3. The Ic value was measured at 77K by a four-terminal method in which a terminal was attached to the outer surface of the protective layer of each superconducting conductor sample.

As shown in the results shown in Table 2, in the oxide superconducting conductor samples containing Sr diffused to the oxide superconducting layer side, all the samples showed high critical current values.
The Sr content in Table 2 is the Sr content relative to the target. Assuming that all elements have migrated from the target to the thin film layer side, the total amount of Sr contained in the thin film layer and the oxide superconducting layer is the target. It can be regarded as being equivalent to the amount of Sr contained in. For this reason, as the Sr amount (alkaline earth metal element amount) contained in both the thin film layer and the oxide superconducting layer, from the results shown in Table 2, it is good if it is in the range of 1 mol% or more and 40 mol% or less. Thus, an oxide superconductor having both a critical current value and an excellent critical temperature can be produced. Further, as the range of the amount of Sr, it is considered that the range of 1 mol% or more and 30 mol% or less is more preferable from Table 2.

Further, from the results shown in Table 3, when the thin film layer is interposed between the oxide superconducting layer and the protective layer, the value of Ic / Ic ₀ is deteriorated if the thin film layer is thicker than necessary. did. This can be presumed that when the thin film layer is too thick, the thin film layer itself becomes a resistance layer, which impedes energization when measuring the Ic value. Therefore, even if a thin film layer for diffusing an alkaline earth metal element is provided, the film thickness is preferably 1000 nm or less, and a film thickness in the range of 2 nm to 100 nm is more preferable.

  Next, for the sample No. 3 shown in Table 1, the distribution of Ca diffused in the crystal grain boundary of the oxide superconductor was examined. A part of the sample is cut out, the protective layer is removed by etching, the oxide superconducting layer is exposed, one grain boundary of the oxide superconducting layer is specified, and the X-ray photoelectron spectroscopy (XPS) method is used. Elemental analysis was performed over a distance of 3 μm from the center of the crystal grain boundary to the crystal grain side present on both sides of the grain boundary. The result is shown in FIG. The vertical axis in FIG. 5 indicates the intensity of X-rays, but is proportional to the relative abundance ratio of the Ca element. Therefore, a point with a large value on the vertical axis indicates a position where the Ca concentration is high. That is, the existence width of Ca in the inspected sample is about 1 μm, and Ca diffused in the oxide superconductor layer is almost present at the grain boundary of the oxide superconductor, and only a part thereof is the oxide superconductor. It was found that it diffused to the inner side of the crystal grains (side closer to the grain boundary).

  Next, for the sample No. 3 shown in Table 2, the distribution of Sr diffused in the grain boundaries was examined. A part of the sample is cut out, the protective layer is removed by etching, one of the grain boundaries of the oxide superconducting layer is specified, and the grain boundary is determined by X-ray photoelectron spectroscopy (XPS). Elemental analysis was performed over a distance of 3 μm from the center to the crystal grain side present on both sides of the grain boundary. The result is shown in FIG. The vertical axis in FIG. 6 indicates the intensity of X-rays, but is proportional to the relative abundance ratio of the Sr element. Therefore, a point with a large value on the vertical axis indicates a position where the Sr concentration is high. In other words, the existence width of Sr in the inspected sample is about 1 μm, and Sr diffused in the oxide superconductor layer is almost present at the grain boundary of the oxide superconductor, and only a part of the Sr is present in the oxide superconductor. It was found that it diffused to the inner side of the crystal grains (side closer to the grain boundary).

  As described above, the oxide superconducting layer of this example has a structure in which Ca or Sr, which is an alkaline earth metal element, is mainly diffused in the crystal grain boundary of the oxide superconductor constituting the oxide superconductor. As shown in Tables 1 and 2, it has been found that an excellent critical current value can be exhibited while having an excellent critical temperature.

  DESCRIPTION OF SYMBOLS 1 ... Oxide superconducting conductor, 2 ... Base material, 3A ... Underlayer, 3B ... Orientation layer, 3C ... Cap layer, 5 ... Intermediate layer, 6 ... Oxide superconducting layer, 6A ... Crystal grain, R ... Grain boundary, K DESCRIPTION OF SYMBOLS: Grain boundary inclination angle, 7 ... Protective layer, 8 ... Metal stabilization layer, 10 ... Covering layer, 15, 16 ... Coil body, 17 ... Superconducting coil, 70 ... Thin film layer.

Claims (6)

A substrate, an intermediate layer and an oxide superconducting layer formed on the substrate, and a protective layer formed on the oxide superconducting layer;
The oxide superconducting layer is made of a RE-123 oxide superconductor,
The protective layer is formed of Ag or a material containing at least Ag, has a function of permeating oxygen during heating, the protective layer is to contact the oxide superconducting layer,
The Ca or Sr is contained in the protective layer as an alkaline earth metal element, the oxide oxide superconductor grains with the alkaline earth is contained in the protective layer to the boundary metal elements constituting the superconducting layer And the amount of the alkaline earth metal element contained in the grain boundary rather than the amount of the alkaline earth metal element contained on the inner side of the crystal grains of the oxide superconductor in the oxide superconducting layer. Oxide superconducting conductor, characterized in that there are more .   2. The oxide superconductor according to claim 1, wherein the alkaline earth metal element is contained on the grain boundary side inside the crystal grains of the oxide superconductor constituting the oxide superconductor layer. The alkaline earth metal element is Ca , and the protective layer and oxide superconductivity without Ca of the critical current value Ic of the oxide superconducting conductor provided with the protective layer and the oxide superconducting layer containing Ca are contained. 3. The oxide superconductor according to claim 1, wherein a relative value Ic / Ic ₀ with respect to the critical current value Ic ₀ of the oxide superconductor having a layer is in a range of 1.13 to 1.35. . An intermediate layer forming step of forming an intermediate layer on a substrate,
An oxide superconducting layer forming step of forming an oxide superconducting layer formed of an oxide superconductor of RE-123-based on the intermediate layer,
A protective layer formed of Ag or a material containing at least Ag on the oxide superconducting layer and containing Ca or Sr as an alkaline earth metal element is formed so that the protective layer is in contact with the oxide superconducting layer. a protective layer formation step of obtaining a laminate,
The alkaline earth metal element contained in the protective layer is diffused to the oxide superconducting layer side while supplying oxygen to the oxide superconducting layer side through the protective layer by performing oxygen annealing treatment on the laminate. The alkaline earth metal element contained in the protective layer is contained in grain boundaries of crystal grains of the oxide superconductor constituting the oxide superconductor layer, and the oxide superconductor layer includes the oxide includes an oxygen annealing step you have more right amount of the alkaline earth metal elements than the amount of the alkaline earth metal element diffuses into side grain superconductor diffused into the grain boundary is large, the A method for producing an oxide superconducting conductor. And forming a film-forming method the protective layer on the oxide superconducting layer, according to claim 4, characterized in that the inclusion of the alkaline earth metal element target used when carrying out the film formation method Manufacturing method for oxide superconducting conductors. 6. The method for producing an oxide superconducting conductor according to claim 5 , wherein the alkaline earth metal element is Ca and the content of Ca contained in the target is in the range of 1 to 40 mol% .

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