JP2014003270A - Oxide superconducting thin film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the critical current characteristics while suppressing increase in the types of material.SOLUTION: The oxide superconducting thin film includes a substrate, a superconductive layer including a plurality of RE-based superconductor units formed on the substrate and composed to include a rare earth element, a CuO chain and a CuOplane, a CuO chain existing among the RE-based superconductor units around the interface of the superconductive layer and a layer adjoining the substrate side of the superconductive layer, out of a plurality of CuO chains, and is 1.2-2 times longer in the lamination direction than the length of a CuO chain determined by the lattice constant of the RE-based superconductor unit, and an edge dislocation adjoining the long CuO chain in the lamination direction.

Description

  The present invention relates to an oxide superconducting thin film.

  Conventionally, as a technique for putting an oxide superconducting material into practical use, there is a method in which a base material is prepared and an oxide superconductor film is formed on the base material to obtain an oxide superconducting thin film.

The oxide superconductor to be formed is represented by, for example, a composition formula of an RE-based superconductor (RE: rare earth element) that exhibits a superconducting phenomenon at a liquid nitrogen temperature (77 K) or higher, particularly REBa ₂ Cu ₃ O _7-δ. RE-based superconductors (hereinafter simply referred to as “(RE) BCO”) are often used.

  Such oxide superconducting thin films using RE-based superconductors are expected to be applied to superconducting current limiters, cables, and SMES (superconducting energy storage devices), and much attention is paid to RE-based superconductors and their manufacturing methods. Collecting.

However, one of the factors that impede the practical application of oxide superconducting thin films, including RE-based superconductors, is that it is not easy to improve critical current characteristics (hereinafter simply referred to as “Ic characteristics”). It is done.
This Ic characteristic is represented by a product of a critical current density characteristic (hereinafter simply referred to as “Jc characteristic”), a film thickness, and a width. The Jc characteristic depends on the state of the oxide superconductor (such as crystal orientation). The film thickness limit is determined by the thermal stress resulting from the difference in thermal expansion coefficient between the substrate and the oxide superconductor.

  Therefore, in Patent Document 1, an appropriate amount of vacancies is introduced into the (RE) BCO thin film, and the intermediate layer thin film different from the (RE) BCO thin film is not interposed between the (RE) BCO thin film and the single layer structure. By forming the film so as to have a multilayer structure including the point, the thermal stress caused by the difference in thermal expansion coefficient between the base material and the oxide superconductor is alleviated to increase the film thickness limit, thereby improving the Ic characteristic. Is disclosed.

JP 2008-140789 A

  However, in the configuration of Patent Document 1, since the superconducting layer has a structure including a (RE) BCO thin film and an intermediate layer thin film selected from RE 'different from the (RE) BCO thin film, the types of raw materials are increased. There is a risk of high costs.

  The present invention has been made in view of the above-described facts, and an object thereof is to provide an oxide superconducting thin film having good critical current characteristics while suppressing an increase in the types of raw materials.

The above-described problems of the present invention have been solved by the following means.
<1> a base material, a superconducting layer that is formed on the base material and includes a plurality of RE-based superconductor units including a rare earth element, a CuO chain, and a CuO ₂ surface; and a plurality of the CuO chains. Of these, the length of the CuO chain present in the RE-based superconductor unit around the interface between the superconducting layer and the layer adjacent to the base material side of the superconducting layer is determined by the lattice constant of the RE-based superconductor unit. An oxide superconducting thin film comprising a CuO chain that is 1.2 times to 2 times longer in the stacking direction and edge dislocations that are adjacent to the long CuO chain in the stacking direction.
The “layer adjacent to the base material side” includes not only the intermediate layer but also the base material itself.

<2> The oxide superconducting thin film according to <1>, having an intermediate layer containing another rare earth element capable of taking the valence of the rare earth element between the base material and the superconducting layer.

<3> The long CuO chain is present in the RE-based superconductor unit in the first layer or the second layer from the interface between the superconducting layer and the layer adjacent to the substrate side of the superconducting layer, <1> Or the oxide superconducting thin film as described in <2>.

<4> The oxide superconducting thin film according to <3>, wherein the first RE superconductor unit is laminated from the rare earth element.

<5> The RE-based superconductor units sandwiching the long CuO chain in the stacking direction are deviated from 1/8 to 1/2 of the RE-based superconductor unit in the ab plane direction, <2> to <4 > The oxide superconducting thin film according to any one of the above.

<6> The oxide superconducting thin film according to any one of <2> to <5>, wherein at least the superconducting layer side layer of the intermediate layer is made of CeO ₂ or REMnO ₃ .

  ADVANTAGE OF THE INVENTION According to this invention, the oxide superconducting thin film which has a favorable critical current characteristic can be provided, suppressing the increase in the kind of raw material.

FIG. 1 is a view showing a laminated structure of oxide superconducting thin films according to an embodiment of the present invention. FIG. 2 is a diagram showing a crystal structure of an RE superconductor constituting the superconducting layer shown in FIG. FIG. 3 is a schematic configuration diagram of a superconducting fault current limiter according to an embodiment of the present invention. FIG. 4 is a diagram showing an ABF image of the interface between the intermediate layer (CeO ₂ layer) and the superconducting layer (YBCO layer) in the stacking direction P cross section of the oxide superconducting thin film of the thin film type superconducting element according to this example. is there. FIG. 5 is a cross section in the stacking direction P of the oxide superconducting thin film of the thin film type superconducting element according to the present embodiment, particularly where FIG. 4 at the interface between the intermediate layer (CeO ₂ layer) and the superconductive layer (YBCO layer) is different It is a figure which shows another different ABF image.

  Hereinafter, an oxide superconducting thin film according to an embodiment of the present invention will be specifically described with reference to the accompanying drawings. In the drawings, members (components) having the same or corresponding functions are denoted by the same reference numerals and description thereof is omitted as appropriate.

<Configuration of oxide superconducting thin film>
FIG. 1 is a diagram showing a laminated structure of an oxide superconducting thin film 1 according to an embodiment of the present invention.
As shown in FIG. 1, the oxide superconducting thin film 1 has a laminated structure in which an intermediate layer 12, a superconducting layer 13, and a stabilizing layer (protective layer) 14 are formed on a substrate 11 in order of the lamination direction P. .

The substrate 11 is a low magnetic metal substrate or ceramic substrate. The shape of the base material 11 is not particularly limited on the assumption that there is a main surface, and various shapes such as a plate material, a wire material, and a strip can be used. For example, when a tape-shaped substrate is used, the oxide superconducting thin film 1 can be applied as a superconducting wire.
As the metal substrate, for example, metals such as Cr, Cu, Ni, Ti, Mo, Nb, Ta, W, Mn, Fe, and Ag, which are excellent in strength and heat resistance, or alloys thereof can be used. Particularly preferred are stainless steel, Hastelloy (registered trademark), and other nickel-based alloys which are excellent in corrosion resistance and heat resistance. Various ceramics may be arranged on these various metal materials. As the ceramic substrate, for example, MgO, SrTiO ₃ , yttrium-stabilized zirconia, sapphire, or the like can be used.
Although the thickness of the base material 11 is not specifically limited, For example, it is 1 mm.

The intermediate layer 12 is a layer formed on one main surface of the base material 11 in order to realize high in-plane orientation in the superconducting layer 13 and adjacent to the base material 11 side of the superconducting layer 13. Or a multilayer film. The intermediate layer 12 is not particularly limited, but at least the outermost layer (the layer on the superconducting layer 13 side) is a material selected from, for example, CeO ₂ and REMnO ₃ , and is preferably CeO ₂ . The valence of Ce is usually tetravalent, but can be trivalent.
The film thickness of the intermediate layer 12 is not particularly limited, but is 20 nm, for example.

The superconducting layer 13 is formed on the intermediate layer 12 and contains an RE-based superconductor as a main component. The “main component” indicates that the content is the highest among the constituent components contained in the superconducting layer 13, and preferably indicates that it is more than 90%. Typical examples of RE superconductors include REBa ₂ Cu ₃ O _7-δ (RE-123), REBa ₂ Cu ₄ O ₈ (RE-124), RE ₂ Ba ₄ Cu ₇ O _15-δ ( RE-247). Each of them has a layered perovskite structure, but the structure existing inside is divided into a part where RE, Ba, and Cu have an oxygen and perovskite structure and a part where Cu and oxygen are bonded in a chain. The part of the perovskite structure has a CuO ₂ surface in the structure and is known as a part through which a superconducting current flows. The portion of the CuO chain includes a CuO single chain when CuO has only a single chain and a CuO heavy chain in which CuO is doubled. All of the CuO chains are called RE-123, those having a single chain and a heavy chain alternately are called RE-247, and those having all the heavy chains are called RE-124.

The RE is a single rare earth element or a plurality of rare earth elements such as Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu, and among these, substitution with the Ba site hardly occurs. For the reasons described above, Y is preferable. Further, δ is an oxygen nonstoichiometric amount, for example, 0 or more and 1 or less, and is preferably closer to 0 from the viewpoint of a high superconducting transition temperature. The oxygen non-stoichiometric amount may be less than 0, that is, take a negative value when high-pressure oxygen annealing or the like is performed using an apparatus such as an autoclave.
The thickness of the superconducting layer 13 is not particularly limited, but is set to 200 nm, for example.

  The stabilization layer 14 is formed on the superconducting layer 13 and is made of, for example, silver. The film thickness of the stabilization layer 14 is not particularly limited, but is, for example, 200 nm.

  FIG. 2 is a diagram showing a crystal structure of the RE-based superconductor 20 constituting the superconducting layer 13 shown in FIG. In addition, the crystal structure shown in FIG. 2 shows a unit cell (unit cell) of RE-123 in the entire diagram, and the RE-based superconductor 20 of the present embodiment is not only composed of RE-123, The case where RE-123, RE-124, and RE-247 coexist and the crystal structure of the entire figure forms a part of the unit cell of the RE superconductor 20 is also included.

The RE-based superconductor 20, as shown in FIG. 2, in the unit cell, CuO ₂ and CuO ₂ plane 24 located along the c-axis both sides of the rare earth element (RE) 22, a rare-earth element 22 as a reference And a CuO single chain 26 located outside the surface 24 in the c-axis direction. In the present embodiment, the RE superconductor 20 of the unit cell is referred to as “RE superconductor unit 30”. In FIG. 2, the “RE-based superconductor unit 30” has the CuO ₂ surface as the lowermost layer of the unit, but the unit may be formed with the rare earth element 22 as the lowermost layer.
The superconducting layer 13 includes a plurality of the RE-based superconductor units 30 (in the stacking direction P and the width direction), and among the CuO single chains 26 present in the superconducting layer 13, Present in the RE-based superconductor unit 30 around the interface between the superconducting layer 13 and the intermediate layer 12 and 1.2 times or more in the stacking direction P than the length of the CuO single chain determined by the lattice constant of the RE-based superconductor unit 30 A CuO single chain that is two times or less long and edge dislocations that are adjacent to the long CuO single chain in the stacking direction P are provided. Here, the reason why the length of the long CuO single chain in the stacking direction P is 1.2 times or more the length of the CuO single chain determined by the lattice constant of the RE-based superconductor unit 30 is the a-axis / b-axis direction. This is because the interaction in the c-axis direction becomes smaller than the interaction of. Moreover, the reason for setting it as 2 times or less is for suppressing the deterioration of the orientation of the superconducting layer 13.
In the case of RE-124 and RE-247, the CuO chain that is long in the stacking direction may be a heavy chain instead of a single chain. In this case, the long heavy chain is 1.2 to 2 times longer in the stacking direction P than the length of the CuO heavy chain determined by the lattice constant of the RE-based superconductor unit 30.

Thus, in the RE-based superconductor unit existing around the interface between the superconducting layer 13 and the intermediate layer 12, there is a CuO single chain extending in the stacking direction P, so that a long CuO single chain in the stacking direction P is present. It is possible to prevent the RE-based superconductor unit 30 existing on the opposite side of the intermediate layer 12 from being affected by the structure of the intermediate layer 12, and to relax the lattice strain of the intermediate layer 12 and the superconducting layer 13. it can. As a result, a structure in which cracks due to heat shrinkage are difficult to occur is achieved, and a thick film can be formed. Further, the lattice constant of CeO ₂ is smaller than that of YBCO, and since the strain is reduced when the number of lattices of YBCO is smaller than the number of lattices of CeO ₂ , the blade adjacent to the long CuO single chain in the stacking direction P. If there is a dislocation, the number of lattices of YBCO in the superconducting layer 13 is reduced with respect to the number of lattices of, for example, CeO _{2 in} the intermediate layer 12, and the lattice mismatch between the intermediate layer 12 and the superconducting layer 13 is alleviated. As a result, a structure in which cracks due to heat shrinkage are difficult to occur is achieved, and a thick film can be formed.
As described above, if the film thickness can be increased, the Ic characteristic represented by the product of the Jc characteristic, the film thickness, and the width can be improved. Moreover, since it is not necessary to configure the superconducting layer as a (RE) BCO thin film and a thin film selected from RE 'different from the (RE) BCO thin film as in Patent Document 1, the kind of raw material for forming the superconducting layer is not required. The oxide superconducting thin film 1 having good Ic characteristics can be provided without increasing the value.
The above-mentioned “length of the CuO single chain determined by the lattice constant of the RE-based superconductor unit 30” is the lattice constant of the superconductor with respect to the lattice constant of the RE-based superconductor unit 30 determined by X-ray diffraction measurement. And the CuO single chain length ratio.
Further, the “periphery of the interface” means that it is within five RE-based superconductor units 30 from the interface between the superconducting layer 13 and the intermediate layer 12, and the entire superconducting layer 13 from the lower layer of the superconducting layer 13. From the viewpoint of alleviating lattice mismatch, it is preferably present in the RE superconductor unit 30 in the first or second layer from the interface between the superconducting layer 13 and the intermediate layer 12. The “blade dislocation adjacent to the long CuO single chain in the stacking direction P” means the blade existing in the RE superconductor unit 30 in the first layer to the third layer with respect to the long CuO single chain. This means a dislocation. The edge dislocation is preferably present in the RE superconductor unit 30 in the first layer, but can also be present in the RE superconductor unit 30 up to the third layer due to the unevenness of the intermediate layer 12. is there.

Furthermore, the RE in the intermediate layer (CeO ₂ ) and the RE-based superconductor unit 30 has good bonding properties, and when stacked, the RE in the RE-based superconductor unit 30 around the interface between the superconducting layer 13 and the intermediate layer 12 is stacked. The strain energy due to the mismatch of the lattice of CeO ₂ and YBCO is concentrated on the CuO single chain, and the CuO single chain is stretched to relax the strain. Therefore, it is easy to form a long CuO single chain. Therefore, the first RE superconductor unit 30 is preferably laminated so that the rare earth element 22 is the lowest layer. In order to realize this, it is necessary to promote solid solution at the interface by combining the Ce atoms in the intermediate layer and the valence of the rare earth element 22 of the RE-based superconductor unit 30. For example, the superconducting layer 13 and the intermediate layer 12 are fired in an inert atmosphere so that the valence of Ce of other rare earth elements, for example, CeO ₂ , particularly on the surface of the intermediate layer 12 is changed from the tetravalent to the trivalent side. In addition, by reducing the valence, the rare earth element 22 (assumed to be trivalent) of the first RE superconductor unit 30 in the superconducting layer 13 is strongly against other rare earth element sites on the surface of the intermediate layer 12. It is thought to combine.
Further, the oxygen ions on the CuO ₂ surface 24 of the first RE superconductor unit 30 are in the half unit cell position 28 of the RE superconductor unit 30 (see FIG. 2, specifically, closest to the CuO ₂ surface). It is preferable that it is located 1/8 or more and 1/2 or less in the ab surface direction 32 with respect to the intermediate position between Cu and Cu adjacent to each other. When oxygen ions are thus biased in the ab plane direction 32, the oxide superconducting thin film 1 having good Ic characteristics can be obtained. This good Ic characteristic is considered to be because the portion that is biased to the position of the oxygen ions on the CuO ₂ surface 24 of the RE-based superconductor unit 30 is a pinning center.
Further, from the viewpoint of weakening the bonding between lattices and facilitating the introduction of edge dislocations, RE-based superconductor units 30 sandwiching a long CuO chain in the stacking direction P are connected to each other in the ab plane direction 32 by the RE-based superconductor units 30. It is preferable that the deviation is 1/8 or more and 1/2 or less.

Further, the above-mentioned edge transition is preferably as many as the lattice constant ratio in the superconducting layer 13 from the viewpoint of relaxing the lattice mismatch. For example, when the intermediate layer 12 is made of CeO ₂ and the superconducting layer 13 is made of YBCO, the Ce interval of CeO ₂ is 3.83Å (lattice constant is 5.41Å), and the b-axis lattice constant of YBCO. Is 3.89 mm (a-axis is 3.81 mm), so CeO ₂ : YBCO ₂ = 65: 64 is preferable. That is, it is preferable that about one dislocation is seen per 70 lattices.
Further, the long CuO single chain described above has been described as being 1.2 times or longer, but it is 1.8 times longer than the length of the CuO single chain determined by the lattice constant of the RE-based superconductor unit 30. It is preferable that the length is longer in the direction P. This is because the interaction between atoms decreases rapidly when the atoms are separated from each other. In other words, the strength of the interaction is determined by the square of the distance. It is considered to be. Accordingly, the longer the CuO single chain, the smaller the interaction between the long CuO single chain and the lattices above and below it. It is considered that the interaction between the long CuO single chain and the respective upper and lower lattices is weakened, so that it is less affected by the lattice constant of the lower layer and the edge transition is likely to occur. However, if it exceeds twice, the interaction is excessively reduced and the orientation of the superconducting layer 13 is deteriorated.
Regarding the relationship between the length of the CuO single chain and the dislocation, since the RE superconductor 20 is an ionic crystal, if there is a defect such as an edge dislocation, a repulsive force will work and it will be partially unstable. However, if a long CuO single chain as in this embodiment is present, even if a lattice defect occurs in that portion, the repulsive force is smaller than in a normal state, and as a result, the defect is likely to occur. It is done.

In order to realize the above-described configuration, for example, the intermediate layer 12 is annealed in air at 800 ° C. or higher, or the superconducting layer 13 is formed in an inert atmosphere at a temperature range of 700 ° C. or higher and 900 ° C. or lower. What is necessary is just to adjust a manufacturing method suitably, such as annealing. Further, from the viewpoint of adjusting the length of the CuO chain, it tends to be longer when the temperature of the substrate in forming the intermediate layer 12 is lowered and longer when the oxygen partial pressure is lowered. Furthermore, when the temperature of the main firing at the time of forming the superconducting layer 13 is lowered, it becomes longer, and when the oxygen partial pressure in the inert atmosphere is lowered, it tends to become longer.
Further, the position of oxygen ions can be specified by using an ABF-STEM method (angle controlled annular bright field method) using a transmission electron microscope (TEM). An ABF image obtained by the ABF-STEM method can detect light elements such as oxygen by photographing electrons scattered at a small scattering angle with an annular detector.

<Modification>
Although the present invention has been described in detail with respect to specific embodiments, the present invention is not limited to such embodiments, and various other embodiments are possible within the scope of the present invention. It will be apparent to those skilled in the art, and for example, the plurality of embodiments described above can be implemented in combination as appropriate. Further, the following modifications may be combined as appropriate.

For example, the intermediate layer 12 and the stabilization layer 14 can be omitted as appropriate. Moreover, although the superconducting layer 13 of the oxide superconducting thin film 1 has been described as being a single layer, it may be a plurality of layers. Further, as the RE-based superconductor 20, for example, it cited _{REBa 2 Cu 3 O 7-δ} , may be doped with Co to Ca and Cu sites in the Ba site.

  In this embodiment, the oxide superconducting thin film 1 has been described. However, the oxide superconducting thin film 1 can be applied to various other devices. For example, it can be applied to devices such as a superconducting current limiting device, SMES (Superconducting Magnetic Energy Storage), a superconducting transformer, an NMR (nuclear magnetic resonance) analyzer, a single crystal pulling device, a linear motor car, and a magnetic separation device.

<Superconducting fault current limiter>
Next, the configuration of the superconducting fault current limiter when the oxide superconducting thin film 1 is applied to the superconducting fault current limiter will be described as an example.

  FIG. 3 is a schematic configuration diagram of the superconducting fault current limiter 40 according to the embodiment of the present invention.

  The superconducting fault current limiter 40 according to the embodiment of the present invention uses a superconducting-normal state transitions (S / N) of a superconductor, and normally has zero resistance and an overcurrent exceeding a critical current flows. This is a device with high resistance and a function to suppress overcurrent.

The superconducting current limiting device 40 includes a sealed container 42 that is sealed by closing a container body 42A with a lid 42B.
A refrigerator 44 is connected to the container main body 42 </ b> A, and liquid nitrogen is introduced into the sealed container 42 from the refrigerator 44. The lid 42B is connected to a current introduction / extraction portion 46 that introduces and flows out current from the outside to the inside of the sealed container 42. The current introduction / extraction unit 46 is configured by a three-phase AC circuit, and specifically includes three current introduction units 46A and three current outflow units 46B corresponding thereto.

Each of the current introduction portion 46A and the current outflow portion 46B includes a conducting wire 48 that penetrates the lid 42B and extends in the vertical direction, and a cylindrical body 50 that covers the conducting wire 48.
One end of the conducting wire 48 of the current introduction portion 46A exposed to the outside is connected to one end of the corresponding conducting wire 48 of the current outflow portion 46B exposed to the outside via an external resistor 52 as a shunt resistor.

An element housing container 54 is supported at the end of each cylinder 50 inside the container body 42A.
The element storage container 54 is built in the sealed container 42 and cooled to the inside by liquid nitrogen filled in the sealed container 42.

The element housing container 54 incorporates a current limiting unit 56 including a plurality of thin film type superconducting elements 60 formed by attaching electrodes to the oxide superconducting thin film 1. In the embodiment of the present invention, specifically, the current limiting unit 56 is configured by three sets in which the thin film superconducting elements 60 are arranged in four rows and two columns.
This current limiting unit 56 is supported by the other end inside the conducting wire 48 of the current introducing portion 46A, the other end inside the conducting wire 48 of the current outflow portion 46B, and the support column 58, and is a three-phase alternating current. The other end inside the conducting wire 48 of the current introducing portion 46A and the other end inside the conducting wire 48 of the current outflow portion 46B are electrically connected via the thin film superconducting element 60 so as to constitute a circuit. It is connected to the.

  Here, in order to put the superconducting fault current limiter into practical use, it is required to flow a current as large as possible with zero resistance. For this purpose, it is necessary to improve the critical current characteristics of the oxide superconducting thin film. In this embodiment, since the oxide superconducting thin film 1 showing good critical current characteristics is applied as the thin film type superconducting element 60 of the superconducting current limiter 40, a larger current can be flowed with zero resistance, and practical application is possible. It becomes.

Hereinafter, the oxide superconducting thin film according to the present invention will be described with reference to examples, but the present invention is not limited to these examples.
In this example, a thin film superconducting element used for a superconducting fault current limiter was fabricated as an oxide superconducting thin film.

<Production of thin film superconducting element>
In the production of the thin film superconducting element of this example, first, a sapphire substrate having the R plane of the sapphire single crystal as the main surface was prepared. Next, the sapphire substrate was pre-annealed at 1000 ° C. and cut out in the r-plane direction. Then, the cut sapphire substrate was annealed at 1000 ° C. Next, plasma is generated in 3 × 10 ⁻² Pa oxygen, and CeO ₂ is deposited by EB (electron beam) to about 10 nm to 40 nm with the sapphire substrate heated to 700 ° C. or more to form an intermediate layer. did.
Next, the sapphire substrate on which the intermediate layer was formed was annealed at 800 ° C., and surface treatment (planarization / valence control) was performed. As a result, the crystallinity of CeO ₂ increases, and as will be described later, when YBCO serving as a superconducting layer grows, the strain of CeO ₂ becomes smaller than that in the conventional case, and lattice relaxation can easily occur in YBCO.

Next, a solution of an organic complex of yttrium, barium, and copper was applied with a spin coater, and pre-baked in air at 500 ° C. Then, main baking was performed at 800 ° C. in an inert atmosphere, that is, in an inert gas stream having an oxygen partial pressure of about 100 ppm, and the oxygen atmosphere was switched to the middle. By carrying out the main firing in an inert atmosphere, the growth direction of the crystals forming YBCO was determined, and the lattice relaxation of YBCO could be caused at an early stage, and a YBCO thin film with good characteristics was obtained.
A superconducting current limiting element was produced by depositing a gold-silver alloy on the obtained superconducting thin film by sputtering and attaching electrodes. This superconducting current limiting element becomes a superconducting state by cooling to liquid nitrogen temperature. However, when a current of a certain level or more flows, it becomes a normal conducting state and can perform current limiting.

<TEM evaluation>
The obtained thin film type superconducting element is processed, and a plurality of ABF (annular bright-field) images of the cross section in the stacking direction P of the oxide superconducting thin film in the thin film type superconducting element are processed using a TEM (transmission electron microscope). Observed.
FIG. 4 is a diagram showing an ABF image of the interface between the intermediate layer (CeO ₂ layer) and the superconducting layer (YBCO layer) in the stacking direction P cross section of the oxide superconducting thin film of the thin film type superconducting element according to this example. is there.

As shown in FIG. 4, the CuO single chain at the center of the stacking direction P in the figure is a CuO single chain of the RE-based superconductor unit (one unit cell of YBCO) in the first layer in the stacking direction P from the intermediate layer side. Was confirmed to be longer in the stacking direction than ordinary CuO single chains.
Specifically, the length of the CuO single chain when determined by the lattice constant of the RE superconductor unit is usually 4.3 mm, whereas the length of the CuO single chain of the RE superconductor unit in this example is It was confirmed that the film extends long in the stacking direction P to 7.9 mm (almost double). In this embodiment, the “stacking direction” can be regarded as equivalent to the “c-axis direction” of the RE-based superconductor unit.
Thus, if the CuO single chain extending in the stacking direction P is long in the superconducting layer, the lattice strain between the intermediate layer and the superconducting layer is alleviated, cracks due to thermal contraction are less likely to occur, and a thicker film can be formed. It becomes.

The first layer of YBCO grown on the intermediate layer (CeO ₂ layer) starts from the Y layer. That is, it was confirmed that the RE-based superconductor unit of the first layer is laminated from the rare earth element Y. This is believed to strengthen the binding force with CeO ₂ .
Furthermore, the oxygen ions on the CuO ₂ surface of the RE superconductor unit of the first layer (the oxygen ions on the CuO ₂ surface under the long CuO single chain in the figure) are relative to the half unit cell position of the RE superconductor unit. In addition, it was confirmed that the position was deviated in the range of 1/8 to 1/2 in the ab plane direction.
Furthermore, it was confirmed that RE-based superconductor units sandwiching a long CuO single chain in the stacking direction are shifted from one-eighth to one-half of RE-based superconductor units in the ab plane direction.

FIG. 5 is a cross section in the stacking direction P of the oxide superconducting thin film of the thin film type superconducting element according to the present embodiment, particularly where FIG. 4 at the interface between the intermediate layer (CeO ₂ layer) and the superconductive layer (YBCO layer) is different It is a figure which shows another different ABF image.

As shown in FIG. 5, at the interface between the intermediate layer (CeO ₂ layer) and the superconducting layer (YBCO layer), the number of white arrows (the number of upper YBCO lattices compared to the number of lower CeO ₂ lattices) Edge dislocations were confirmed in the discontinuous part).
If there are edge dislocations in this way, the number of lattices of YBCO in the superconducting layer with respect to CeO _{2 in} the intermediate layer is reduced, and the lattice mismatch between the intermediate layer and the superconducting layer is alleviated. As a result, a structure in which cracks due to heat shrinkage are difficult to occur is achieved, and a thick film can be formed.

By adjusting the length of the CuO single chain by changing the firing atmosphere (inert atmosphere) and firing temperature of the superconducting layer 13 and the intermediate layer 12 among the manufacturing methods of the above-described examples, as shown in Table 1, Thin film superconducting elements including RE-based superconductor units having different CuO single chain lengths were formed. Specifically, the main baking temperature before switching to the oxygen atmosphere at the time of forming the superconducting layer 13 (YBCO thin film), the oxygen partial pressure in the inert atmosphere, and the EB deposition at the time of forming the intermediate layer 12 (CeO ₂ ) The heating temperature and oxygen partial pressure of the sapphire substrate were adjusted as shown in Table 1. At this time, the length of the CuO single chain when determined by the lattice constant to be compared was set to 4.3 mm.

Evaluation was made as follows with respect to the interlayer bonding force of the CeO ₂ layer and the YBCO layer and the orientation of the superconducting layer of the obtained thin film superconducting element.

Interlayer bonding strength, CeO ₂ layer and evaluation CeO ₂ layer of the interlayer bonding force of the YBCO layer and the YBCO layer was evaluated by Maderungu energy. Specifically, the Madelung energy of atoms on the CuO ₂ surface was determined from the interatomic distance read from the TEM, and the energy from the CuO chain side and the energy from the RE ion side were evaluated. Of the obtained values, evaluation was performed according to the following criteria.
A: The energy on the RE ion side is more than twice the energy on the CuO chain side. ○: The energy on the RE ion side is 1 to 2 times the energy on the CuO chain side. X: The energy on the RE ion side is the energy on the CuO chain side. 1x or less

-Orientation of superconducting layer The orientation of the superconducting layer was evaluated by measuring the half-value width of the 006 peak of YBCO by XRD measurement, and among the obtained values, the following criteria were evaluated.
◎: 0.2 degrees or less ○: 0.2 degrees to 0.5 degrees ×: 0.5 degrees or more

-Performance of superconducting layer The performance of the superconducting layer was measured by the critical current density and evaluated according to the following criteria.
◎: 3MA / cm ² or more ○: 2MA / cm ² ultra ~3MA / cm ² less than ×: 2MA / cm ² or less

As can be seen from Table 1, when the length in the stacking direction P of the long CuO single chain is less than 1.2 times the length of the CuO single chain determined by the lattice constant of the RE-based superconductor unit, the CeO ₂ layer and the YBCO It was found that the interlayer bonding strength of the layers was strong and the performance of the superconducting layer was low. It was also found that the orientation of the superconducting layer was low when the length of the long CuO single chain in the stacking direction P was more than twice the length of the CuO single chain determined by the lattice constant of the RE-based superconductor unit.
On the other hand, if the length of the long CuO single chain in the stacking direction P is 1.2 to 2 times the length of the CuO single chain determined by the lattice constant of the RE superconductor unit, the CeO ₂ layer and the YBCO layer It was found that the performance of the superconducting layer was high because of its weak interlaminar bonding strength and high orientation of the superconducting layer.
Also, if the length of the long CuO single chain in the stacking direction P is 1.2 to 1.3 times the length of the CuO single chain determined by the lattice constant of the RE-based superconductor unit, the performance of the superconducting layer will be It turned out to be higher.

DESCRIPTION OF SYMBOLS 1 Oxide superconducting thin film 11 Base material 12 Intermediate layer 13 Superconducting layer 20 RE system superconductor 22 Rare earth element 24 CuO ₂ face 26 CuO single chain (CuO chain)
28 Half unit cell position 30 RE-based superconductor unit 32 ab plane direction P stacking direction

Claims (6)

A substrate;
A superconducting layer containing a plurality of RE-based superconductor units formed on the base material and including a rare earth element, a CuO chain, and a CuO ₂ surface;
Among the plurality of CuO chains, they exist in the RE-based superconductor unit around the interface between the superconducting layer and the layer adjacent to the substrate side of the superconducting layer, and depend on the lattice constant of the RE-based superconductor unit. A CuO chain that is 1.2 to 2 times longer than the determined CuO chain length,
Edge dislocations present adjacent to the long CuO chain in the stacking direction;
An oxide superconducting thin film. Between the base material and the superconducting layer, having an intermediate layer containing another rare earth element capable of taking the valence of the rare earth element,
The oxide superconducting thin film according to claim 1. The long CuO chain is present in the RE-based superconductor unit of the first layer or the second layer from the interface between the superconducting layer and the layer adjacent to the substrate side of the superconducting layer.
The oxide superconducting thin film according to claim 1 or 2. The first RE superconductor unit is laminated from the rare earth element.
The oxide superconducting thin film according to claim 3. The RE-based superconductor units sandwiching the long CuO chain in the stacking direction are displaced from each other by 1/8 to 1/2 of the RE-based superconductor unit in the ab plane direction.
The oxide superconducting thin film according to any one of claims 2 to 4. At least the layer on the superconducting layer side of the intermediate layer is made of CeO ₂ or REMnO ₃ .
The oxide superconducting thin film according to any one of claims 2 to 5.