JP2571936B2 - Superconducting material - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to an oxide superconducting (superconducting but here superconducting) material having a superlattice structure.

The present invention uses a thin-film type oxide superconducting material to be applied to a superconducting magnet or the like using a multi-layered oxide superconducting material by using stabilization, particularly stabilization related to crystal grain boundaries, and by using a higher critical temperature ( Tco).

“Conventional technology” Recently, oxide superconducting materials have been receiving attention. This is IB
Ba-La-Cu-O made at the M Zurich Institute
It originated in the development of oxide superconducting materials.

On the other hand, superconducting materials using metals such as Nb ₃ Ge have been well known. However, this superconductor has a very low Tco (temperature at which the electrical resistance becomes zero) of 23K, and must use liquid helium, which is not practical enough.

On the other hand, recently, oxide superconducting materials represented by YBa ₂ Cu ₃ O ₆ to ₈ are known. This is a material whose Tco is 90 to 100K, which is several tens of K higher than before.

"Conventional problems" However, when the properties of this oxide superconducting material, which has been attracting attention recently, are investigated, the case where Tco exists at 190 to 300K is found by accident, but it is extremely unstable. It is. The critical current density was only about 1 × 10 ^{2 to} 5 × 10 ³ A / cm ² , which was far from practical use.

As a cause, oxygen in the oxide superconducting material is easily degassed, and the crystal grains of the polycrystalline oxide superconducting material having a deformed perovskite structure are too large, and consequently the uneven distribution or localization of components on a micro surface. Structural deterioration has occurred. Therefore, although it is desired to allow current to flow in a direction parallel to the plane formed by the a-axis and the b-axis in this crystal, the crystal axis of the polycrystal is generally irregular (random). The current had to flow in the c-axis direction where the critical current density was small, and the crystal became the rate-limiting factor for improving the current density (the crystal that hindered the improvement most). It was also found that in the polycrystalline thin film, a barrier (barrier) was formed at the grain boundary of each polycrystalline grain, which hindered the flow of electricity.

"Means for Solving the Problem" The present invention is obtained by laminating superconducting thin films having different critical temperatures (Tco, ie, a temperature at which the electric resistance becomes zero) with a superlattice structure. The present invention provides a multilayer thin film structure of first and second oxide superconducting materials having different component elements or different stoichiometric ratios, and a third superconducting thin film in which each material is mixed or combined at the interface with a superlattice structure. Is provided. As shown in FIG. 2, an interface having a multilayer structure is positively formed as shown in FIG. 2 to prevent abnormal occurrence of crystal grain boundaries. The oxide superconducting material has at least two layers, preferably 5 to 500 layers. In FIG. 2A, a first oxide superconducting thin film (40-1), (40-2),... (40-n) is provided on a substrate (2), and a second oxide superconducting thin film (40-n) is sandwiched therebetween. Oxide superconducting thin film (41-
1), (41-2)... (41- (n-1)).
Then, each oxide superconducting thin film is thinned to 10Å to 1 μm, and these are sandwiched as a sandwich structure.
In addition, each of the thin film interfaces has a superlattice structure of 10 to 20
Third superconducting thin films (43-1), (43-
2),... (43-n) are provided to form a super lattice after annealing, so that two-dimensional conduction can be used more positively. In this manner, when forming the film, the film is formed in the c-axis direction, the ab plane as the film surface direction, and the third oxide superconducting thin film formed on the interface and the third oxide superconducting thin film. It has been found that high Tc and Tco can be obtained by using a synergistic effect with the first and second oxide superconducting thin films on the lower surface.

Further, by sandwiching the upper and lower surfaces of the first superconducting thin film with the second oxide superconducting thin film having an element different from that of the first oxide superconducting thin film or having a different mixing ratio (stoichiometric ratio), the crystal grain size is reduced. It has become possible to reduce the average in the plane direction to 10 to 10 μm, preferably 20 to 0.5 μm.

In the present invention, when forming a film of an oxide superconducting material, a sputtering method using a sputtering apparatus as shown in FIG. 1, an electron beam evaporation method, and other film forming methods are used. In particular, the apparatus shown in FIG. 1 has two sets or more of sputter systems (two systems of (1) and (1 ') are shown in FIG. 1) by one sputtering apparatus, and is continuously multi-layered. A film was formed. By rotating the drum (2) having the surface to be formed, when one region (22) forms the first superconducting thin film by sputtering, the other region (22 ') becomes the first oxide film. A second oxide superconducting thin film having a different element or a different mixing ratio from the superconducting thin film is formed. By rotating the cylindrical drum discontinuously or continuously, it has become possible to form each layer by laminating 5 to 500 layers.

In the present invention, when a superconducting material is sputtered, a halogen element or an additive element such as Si (silicon), Ge (germanium), Sn (tin), Pb (lead), In (indium), Sb (Antimony) is added to the target in advance. Or using these halide gases,
The materials sputtered from the target are made to fly into the atmosphere in which the gas is present, and are added to each other by reacting with each other. As a result, heat resistance and process resistance were imparted. According to the present invention, when a target is hit by a sputtering method to fly an oxide superconducting material and form it on a surface to be formed, a necessary amount of oxygen and a gas for an additive are simultaneously added to an inert gas which is a gas for hitting the target. It is enclosed.
SiF ₄ , Si ₂ F ₆ , GeF ₄ , SnCl ₄ , PbCl ₄ , InCl
A gas such as ₃ , SbCl ₃ , SnF ₄ , PbF ₄ , InF ₃ , SbF ₃ or the like is introduced, the gas is turned into plasma, and its elements, for example, silicon and fluorine are added to the oxide superconducting material on the surface to be formed.

Then, the whole is heat-treated, the interfaces of the multilayers are fused with each other, a superconducting thin film for forming a third super lattice in the interface region is formed, and the best characteristics having a higher Tco are obtained. is there.

As a component of the first oxide superconducting material, (A _1-x
Bx) yCuzOwXv, (x = 0.1 to 1, y = 2.0 to 4.0, preferably 2.
5 to 3.5, z = 1 to 4, preferably 1.5 to 3.5, w = 4 to 10, preferably 6 to 8, and v has a positive number of 3.0 or less. On the other hand, as a component of the second oxide superconducting material, (A '_1-x'
B'x ') y'Cuz'Ow'X'v', x '= 0.1 ~ 1.0, y' =
2.0-4.0, z '= 1.0-4.0, w' = 4.0-8.0, v 'has a positive number of 3.0 or less).

In the present invention, A, A ', B, B', X, X ', x, x', y,
Any of y ', z, z', v, and v 'may be different. A,
A ′ is Y (yttrium), Gd (gadolinium), Yb (ytterbium), Eu (europium), Tb (terbium), Dy (dysprosium), Ho (holmium), Er (erbium), Tm (thulium), Lu ( Lutetium), Sc (scandium) or other elements selected from one or more members of Group IIIa of the Periodic Table III. B and B 'are Ra (radium),
Ba (barium), Sr (strontium), Ca (calcium), Mg (magnesium), Be (beryllium) are selected from the Periodic Table II group a of the element. X and X 'are selected from elements of Group IVb of the periodic table consisting of silicon, germanium, tin, lead, indium and antimony. In addition, it is selected from elements of Group VIIb of the periodic table such as fluorine and chlorine. Carbon is unsuitable because it becomes a gas in the oxidation treatment.

As a specific example, (YBa ₂ ) Cu _3-4 O _6-8 Xv, (YBaSr)
Cu _3-4 O _6-8 X'v '(in this case, Ba and Sr are used as B, and x = 0.67, y = 3 is given to the sum), (YBaCa) Cu _3-4 O _6-8 Xv, (YBaSr _0.55 C
a _0.5 ) Cu _3-4 O _6-8 X'v '(in this case, B
a, Sr, and Ca were used, and x = 0.67, y = 3 was given as a sum thereof). A, A ′
Lanthanide elements other than the above-mentioned elements in the periodic table can be used.

The periodic table of elements in this specification is based on the Dictionary of Physical and Chemical Sciences (Iwanami Shoten published on April 1, 1963).

In the present invention, the growth of the first or second oxide superconducting thin film in the deformed perovskite structure in the c-axis direction is suppressed by the upper and lower second or first thin film surfaces,
The mutual reaction at the interface obtains characteristics that cannot be obtained in each case. Second, by using the reaction at the interface, it was found that the ab plane grows in the plane direction, and that the grain size and the barrier at the grain boundary (barrier) can be spontaneously reduced. It is intended to be used.

In the present invention, a holder for holding a substrate in the same apparatus as a sputtering apparatus is provided in the form of a disk (of the same quality), and a substrate having a surface to be formed is disposed on the upper surface thereof. Further, a multilayer film may be stacked using a plurality of targets located above the multilayer film.

In such a case, lamination of 5 to 500 layers on a plate-like substrate is possible by continuously rotating the disk.

[Operation] As described above, conventionally, a single type of oxide superconducting thin film mixed more homogeneously is used as a thin film. However, on the other hand, in the present invention, two or more oxide superconducting thin films are laminated to form a multilayer of 5 to 500 layers, and a second
By providing the superconducting thin film in the superlattice structure, it was possible to prevent the local superconducting state from being destroyed due to current flow to a higher current density. As a result, the interatomic distance, crystal growth, and characteristic deterioration can be prevented, and the superconducting state of the surface can be effectively used stably for a long period of time.

It is important that the superconducting properties be higher in Tco and that the device can be used with a higher current density.
In the present invention, it can be increased to 1.3 × 10 ⁵ A / cm ² or more, which is 5 to 30 times as large as that in the case of making a bulk tablet, and the tablet can be stored.

FIG. 2 (B) shows a vertical cross-sectional view obtained by processing the multilayer film. Unnecessary portions are removed from the multilayer film by photoetching, laser scribing, or the like, and the remaining multilayer film is provided in a band shape, thereby forming a ribbon-shaped superconducting wire (band) as shown in FIG. 2 (B). It has become possible. By providing an insulating film again thereon and forming a multilayer film thereon again in a strip shape, it became possible to make a superconducting multiple coil.

 The present invention will be described below with reference to the drawings.

Example 1 FIG. 1 shows an outline of a sputtering apparatus for producing a superconducting material of the present invention.

In FIG. 1, first and second sputtering systems (1) and (1 '), reaction chambers (4) and (4'), doping systems (10) and (10 '), and an exhaust system (30) are shown. Have.

The doping system consists of argon (5), (5 '), oxygen (6), (6'), and a gas for the additive (here Si
Use F ₄ , GeF ₄ , Si ₂ F ₆ , InCl ₃ , SnCl ₄ or SbCl ₃ )
(7) and (7 ') are introduced. The exhaust system (30) comprises a mechanical booster pump or turbo molecular pump (8), (8 ') pressure regulating valves (9), (9'), and a rotary pump (11). The film forming surface of the holder (2) is heated from the inside by a halogen heater (3). The drum (2) has a cylindrical shape.

Since the multilayer film can be continuously formed by the rotation of the drum, the multilayer film can be laminated without being re-adhered to a different sputtering apparatus. However, although the reliability is very poor, the layers may be stacked by different sputtering apparatuses.

The substrate on the side on which the first oxide superconducting material is being formed, that is, the holder, is, for example, at room temperature to 700 ° C. Also the first
The second oxide superconducting thin film having a different element or a different ratio from the oxide superconducting thin film of Example 1 was also kept at room temperature to 700 ° C., for example, both at 650 ° C. The distance between the target (1), (1 ') and the surface on which the holder (2) is formed is 2 to 15 cm.

The first sputtering system (1) was used for forming the first oxide superconducting thin film. Its target (12) is (A _1-x B
x) yCuzOwXv, x = 0.1 ~ 1.0, y = 2.0 ~ 4.0, z = 1.0 ~ 4.0, w
= 4.0 to 8.0, v is provided by pressing a starting material for forming an oxide superconducting material represented by a positive number of 3.0 or less. The back side of this so-called target (12) comprises a packing plate (13), a magnet (14), a cooling water inlet (15), a cooling water outlet (16), and a shield plate (17). These are electrically separated from the main body of the sputtering apparatus by a Teflon insulator (18).

Further, a second oxide superconducting thin film having a different element or a different mixing ratio from the first oxide superconducting thin film is formed using the second sputtering system (1 '). Target (1
2 ') contains (A'1 _- X'B'x') y'Cuz'Ow'X '
v ', x' = 0.1 ~ 1.0, y '= 2.0 ~ 4.0, z' = 1.0 ~ 4.0, w '
= 4.0 to 8.0, v 'is a positive number of 3.0 or less, and a starting material for forming an oxide superconducting material is similarly provided. Others are the same as the first sputtering system (1).

Then, a high negative voltage is applied to the current introduction terminal (20) of each of the targets (12) (12 ') to perform sputtering.

When a DC (direct current) sputtering method is performed, the target is negatively applied, and the substrate is set at the ground potential.

When the AC (AC) sputtering method is performed, the substrate is electrically used as floating.

Experimental Example 1 A first oxide superconducting material was formed as a first sputtering system (1), and a second oxide superconducting material was formed as a second sputtering system (2). YBa ₂ as target (12)
Cu _3.6 O _6-8 was used. Further, YSr ₂ Cu _3.8 O ₆ to ₈ was used as the target (12 ′). The distance between each target and the substrate was 10 cm. Argon pressure is 4 × 10
^-1 Pa, oxygen amount 5 × 10 ⁻³ Pa, and SiF ₄ × 10 ⁻⁴ Pa. The output of DC sputtering was 1.5 W and 1.5 KW, respectively. Then, a film is formed on the portion where the drum (2) exists in the regions (22) and (22 '). Therefore, the film thickness can be controlled by changing the rotation speed of the drum. And the formed thin film is
(YBa ₂ Cu ₃ O _6-8 ) -mixed or compound- (YSr ₂ Cu ₃ O
_6-8 ) -mixed or compound- (YBa ₂ Cu ₃ O _6-8 )...

The targets (12) and (12 ') are 80 cm in the vertical direction (depth direction) in the drawing of FIG. 1, and the vertical direction in FIG.
cm. The rotating drum (2) is cylindrical (diameter 30)
cm, length 1 m).
To form a thin film. These were further rotated in the direction of the arrow. Then, an oxide superconducting thin film was formed in each of the first and second sputtering systems. When this rotation is performed once, each film is formed one by one, and n layers (n =
2 to 500), n layers of each film are formed.

Further, this was annealed in oxygen at 950 ° C. for 5 hours and then slowly cooled, and at the interface, thin films having different Tcos of YBaSrCu ₃ O _{6 to 8} (43-1), (43-2),.
n), and this Tco has 80
It was presumed to have about 150K compared to ~ 90K, and current could also flow through this surface in two dimensions.

 FIG. 2A shows such a multilayer film.

In the drawing, a first oxide superconducting thin film (40-1) on a drum (2), a second oxide superconducting thin film (41-1) having a different element or a different mixing ratio from the first superconducting thin film (40-1), As a third superconducting thin film obtained by mixing or combining thin films of the above, a thin film having a superlattice structure having a high Tco (43
-1), (43-2),..., (43-n).

Further, for the laminate (42), a predetermined width (5
By patterning in a band shape in (0), band-shaped multilayer leads (lines) (44-1)... (44-n) can be formed. The width (50) of the strip-shaped lead may be determined according to the current value in actual use.

Under such conditions, for example, n = 50 and the thickness of the first oxide superconducting material was set to 200 °. The thickness of the second oxide superconducting thin film having a different element or a different mixing ratio from that of the first oxide superconducting thin film was set to 200 °. 950 in oxygen
Annealed at ℃ for 3 hours and then gradually cooled. In particular, the temperature was kept at 500 to 600 ° C., for example, 570 ° C., for 8 hours, and the crystal structure in the coating was altered. Then, a third superconducting thin film (thickness: 10 to 200 mm) having a higher Tco was obtained by mixing or combining the respective oxide superconducting materials at these interfaces.
The Tco could be made at 115K. The critical current density was 9.5 × 10 ³ A / cm ² (measured at 77K).

That is, in the present invention, the first and second oxide superconducting materials are microscopically diffused and reacted at the interface between each other by the thermal annealing. The reaction products can then preserve the most stable state of each other's components. Therefore, it is possible to mainly flow the superconducting current in this interface region (a region where a thickness is generated by reaction and diffusion). Additives are added where crystal grain boundaries are supposed to be formed, and the crystal grains are connected more closely to make them structurally stable, and thus Tco (temperature at which resistance becomes zero) critical current density Is estimated to have been greatly improved. When thermal annealing was not performed, superconductivity was not observed at all, and reproducibility was poor.

"Experimental example 2" YSrCaCu _3.6 O was used as a target in the sputtering system.
_6-8 X _2-1 were used. YbBa ₂ Cu _3.8 O ₆ to ₈ was used as a target of the sputtering system 2. The substrate temperatures were both room temperature. Argon was adjusted to 4 × 10 ^-1 Pa and oxygen was introduced. Further, GeF ₄ was added as an additive for 1 × from (7) in FIG.
10 ^-5 Pa was introduced. The resulting multi-layer coating (100 μm in thickness of each oxide superconducting layer, several hundred layers, total thickness of 1 μm) was thermally annealed at 750 ° C. for 5 hours in the air, and then gradually cooled.
Thus, a thin film having a superlattice structure was obtained. As a result, the superconducting material formed on the surface to be formed is 8.3 × 10 ⁴ A / cm ² (77K measurement)
I got 105K was obtained as Tco.

"Experimental example 3" YbBaSr _0.5 Ca _0.5 Cu _3.8 O as the first target
₆ to ₈ were used. The thickness of each layer was 70 ° and n = 500. Others are the same as the first embodiment. 103K as Tco,
A critical current density of 14.0 × 10 ⁴ A / cm ² (77 K) was obtained. In other words, the layers to be laminated are thinner, and the higher the number of layers, the higher the two-dimensional conduction can be promoted and the superlattice structure is more actively used.
It is assumed that it has a Tco and can have a large current density.

[Effect] As shown in the present invention, a high critical current density is obtained by arranging crystal axes and homogenizing the crystal grain size by using the prepared multi-layered oxide superconducting material having different elements and different component ratios. I was able to. In addition, a part of each of them is replaced by IIa group element and IIIa group element by slow heat treatment, lowering the barrier at grain boundaries, improving Tco, improving critical current density, and improving heat resistance. Was effective.

In addition, halogen elements are filled on these interfaces and on the micro vacancies. This halogen element, for example, fluorine has the effect of improving stability at the same time by using SiF ₄ and GeF ₄ as additive gas.

In the method of the present invention, the substrate having the surface to be formed in FIG. 1 is provided on a drum so as to be detachable. It is effective to form a multi-layer oxide superconducting thin film on the upper surface of this substrate as shown in FIG.

In the present invention, each oxide superconducting thin film may be a multilayer film of three or more types. In that case, one sputtering system may be provided in FIG. 1 and three sputtering systems may be provided at a distance of 120 ° from each other.

Especially when the thickness of each layer is very thin, 10-200 °, they can have a substantially superlattice structure after annealing. Therefore, two-dimensional electric conduction can be performed. That is, it can be seen that by matching this two-dimensional plane direction with the ab plane in the deformed perovskite structure where the current flows most easily, a high current density due to the quantum interaction on that plane can be expected.

In the present invention, when a superlattice can be formed only by two layers to be laminated, the superlattice is simpler and thus industrialized easily. However, it is necessary to epitaxially grow at an interface to eliminate lattice irregularity.

In the present invention, a polycrystal is mainly shown. However, although a single crystal superlattice structure is more difficult to manufacture, a higher critical current density can be expected.

The direction of gravity in FIG. 1 may be a direction perpendicular to the drawing or a vertical direction in the drawing. These positions may be considered from the viewpoint of convenience such as a method of taking the thin film in and out.

[Brief description of the drawings]

 FIG. 1 shows a sputtering apparatus used in the present invention. FIG. 2 shows a multilayer superconducting material made according to the present invention.

Claims (4)

(57) [Claims] 1. A superlattice structure in which first and second oxide superconducting thin films having different critical temperatures are stacked, wherein the first and second oxide superconducting thin films are ab planes as a plane direction of the film. And the first oxide superconducting thin film has (A _1-x
Bx) yCuzOwXv, x = 0.1-1.0, y = 2.0-4.0, z = 1.0-4.0,
w = 4.0 to 8.0, v is a positive number equal to or less than 3.0, and the second oxide superconducting thin film is (A ′ _{1−X ′} B′x ′) y′Cuz′Ow ′
X'v ', x' = 0.1 ~ 1.0, y '= 2.0 ~ 4.0, z' = 1.0-4.
0, w '= 4.0 to 8.0, v' is a positive number of 3.0 or less, A, A 'is an element selected from one or more kinds of group IIIa of the periodic table, and B, B' is an element X is an element selected from one or more members of the periodic table group IIa, and X and X 'are Si (silicon), Ge (germanium), Sn (tin), Pb (lead), In (indium) or Sb A superconducting material comprising one or more elements selected from (antimony) and a halogen element. 2. The superconducting material according to claim 1, wherein the laminate has a band shape. 3. The superconducting material according to claim 1, wherein the oxide superconducting material thin film having a superlattice structure has a thickness of 10 ° to 200 °. 4. A multi-layer structure comprising a first oxide superconducting thin film and a second oxide superconducting thin film having a different element or a different stoichiometric ratio from the first oxide superconducting thin film. A superlattice structure in which a mixture of the first and second superconducting thin films or a third oxide superconducting thin film of a compound is laminated at an interface; The first oxide superconducting thin film has an ab plane as a plane direction.
_1-x Bx) yCuzOwXv, x = 0.1 ~ 1.0, y = 2.0 ~ 4.0, z = 1.0 ~
4.0, w = 4.0 to 8.0, v is a positive number of 3.0 or less, and the second oxide superconducting thin film is (A ′ _{1−X ′} B′x ′) y′Cuz′O
w'X'v ', x' = 0.1 ~ 1.0, y '= 2.0 ~ 4.0, z' = 1.0 ~
4.0, w '= 4.0 to 8.0, v' is a positive number less than or equal to 3.0, A, A 'is an element selected from one or more of Group IIIa of the periodic table, and B, B' is an element Periodic Table II An element selected from one or more members of Group a, where X and X 'are Si (silicon), Ge (germanium), Sn (tin), Pb (lead), and In (indium).
Alternatively, a superconducting material containing one or more elements selected from Sb (antimony) and a halogen element.