JP4455902B2 - Oxide superconducting thin film and manufacturing method thereof - Google Patents

Description

  The present invention relates to an oxide-based superconducting thin film used for a superconducting wire or a superconducting filter, and a method for producing the same.

Oxide superconductors as represented by Y-Ba-Cu-O system, in order to show a high critical temperature T _C than the liquid nitrogen temperature (77K), its application to superconducting wire and the superconducting filter is expected ing. When such an oxide-based superconductor is applied to a superconducting wire or a superconducting filter, it is necessary to improve the critical current density _JC . Previous studies, the composition of the superconductor for improving the critical temperature T _C and the critical current density J _C has been controlled only by optimizing the deposition conditions for the superconductor.

It is known that a Y-Ba-Cu-O-based oxide superconductor generally has a composition of YBa ₂ Cu ₃ O _7-δ and includes a CuO ₂ structure that forms a superconductive surface in the ab plane. It has been. Here, in order to realize a high T _c , it is necessary to dope the optimum concentration of holes on Cu by reducing the value of δ, that is, by sufficiently introducing oxygen. However, oxygen is not sufficiently introduced at the temperature (about 900 ° C.) for forming the Y—Ba—Cu—O-based oxide superconductor. In the case of a bulk sample prepared under equilibrium conditions, oxygen has been introduced by slow cooling in an oxygen stream (see Non-Patent Document 1 and Non-Patent Document 2).

On the other hand, in a thin film of Y—Ba—Cu—O-based oxide superconductor (made under non-equilibrium conditions) It is considered that an inhomogeneous distribution of T _c and a compositional deviation occur, and further a decrease in J _c is observed. Therefore, it is very difficult to optimize T _c and J _c with good reproducibility only by the film forming conditions.

K. Kishio, J. Shimoyama, T. Hasegawa, K. Kitazawa and K. Fueki: Jpn. J. Appl. Phys. 26 (1987) L1228 J. Shimoyama, S. Horii, K. Otzschi and K. Kishio: Mat. Res. Soc. Symp. Proc. 689 (2002) E8.18

When an oxide-based superconducting thin film is applied to a superconducting wire or a superconducting filter, it is necessary to realize a high critical current density _Jc with high reproducibility. One factor that reduces the reproducibility of the T _c and J _c of the oxide superconductor thin film, oxygen nonstoichiometry of oxide superconductor are contemplated. Depending on the film forming conditions, without performing uniform introduction of oxygen into the film, T inhomogeneous distribution of _c and J _c occurs and results in a decrease in T _c and J _c of the film as a whole as a result Conceivable.

  Furthermore, if superconductor thin film is exposed to high temperature for a long time to introduce oxygen into superconductor thin film, the deterioration of superconductor over time will progress (especially superconductivity exposed to the atmosphere). This is remarkable in a thin film), and a method for introducing a necessary amount of oxygen in a short time is required.

Accordingly, an object of the present invention is to provide a superconducting thin film that realizes a high critical current density _Jc with high reproducibility. Another object of the present invention is to provide a superconducting thin film having a high critical current density with high reproducibility while promptly controlling the oxygen content of the superconductor without causing deterioration of the superconductor over time. It is to provide a method of manufacturing.

A superconducting thin film according to the first embodiment of the present invention comprises a substrate and an oxide superconducting phase formed on the substrate and having a composition of ErBa ₂ Cu ₃ O _7-δ (0 ≦ δ ≦ 1). A superconducting thin film including the superconducting layer, wherein the superconducting layer includes at least one hole. It is desirable that one or more holes are arranged per 1 cm 2. Moreover, the void | hole may penetrate the said superconductive layer. Desirably, the superconducting thin film of the present embodiment has a critical current density of 1 × 10 ⁵ A / cm ² or more under a magnetic field having an absolute temperature of 77K and 5T. The superconducting thin film of this embodiment can be used as a superconducting wire or a superconducting device.

The superconducting thin film manufacturing method according to the second embodiment of the present invention includes a superconducting layer including an oxide superconducting phase having a composition of ErBa ₂ Cu ₃ O _7-δ (0 ≦ δ ≦ 1) on a substrate. , A step of forming at least one hole in the superconducting layer, and a step of heat-treating the superconducting layer in an oxidizing atmosphere. The heat treatment step is desirably performed at a temperature in the range of 200 to 600 ° C. and for a time in the range of 1 minute to 45 minutes. The oxidizing atmosphere preferably contains an oxidizing gas having a partial pressure of 200 Torr (26.7 kPa) or more. It is desirable to arrange one or more holes per 1 cm 2, and it may penetrate through the superconducting layer. The manufacturing method of this embodiment can be used for manufacturing a superconducting wire or a superconducting device.

By adopting the embodiment as described above, even when an ErBa ₂ Cu ₃ O _7-δ oxide superconducting thin film is produced under non-equilibrium conditions, superconductivity can be achieved without causing deterioration over time in heating during formation. It is possible to provide an ErBa ₂ Cu ₃ O _7-δ oxide superconducting thin film with high reproducibility, which can quickly control the oxygen content of the body, has a uniform composition throughout, and has a high critical current density. Become. In particular, the obtained superconducting thin film exhibits excellent critical current characteristics even under application of a magnetic field. The superconducting thin film and the method for producing the same of the present invention can be used for superconducting wires and superconducting devices and the production thereof.

  An exemplary configuration of the superconducting thin film of the present invention is shown in FIG. The superconducting thin film of FIG. 1 has a structure in which a superconducting layer 2 is formed on a substrate 1, and the superconducting layer includes at least one hole 3.

As the substrate 1 of the present invention, perovskite crystals such as SrTiO ₃ and LaAlO ₃ ; rock salt crystals such as MgO and NiO; spinel crystals such as MgAl ₂ O ₄ ; fluorite crystals such as yttrium-stabilized zirconia and CeO ₂ A rare earth C-type crystal; an oxide such as a Pikuroa-type crystal; and a metal substrate (a Ni-based alloy substrate such as pure Ni, Ni-Cr and Ni-W, a Cu-based alloy substrate such as pure Cu and Cu-Ni, or Fe) -Fe-based alloy substrates such as Si and stainless steel) can be used. Alternatively, the substrate 1 may be the above-described oxide substrate, the above-described metal substrate, nitride substrate, or semiconductor substrate on which a buffer layer made of the above-described oxide or boride (such as MgB ₂ ) is formed. . More preferred substrates include MgO substrates, metal substrates, and metal substrates coated with oxides. In particular, it is advantageous to use a long metal substrate covered with an oxide when forming a superconducting wire. The material for forming the substrate 1 is not particularly limited, but desirably has a lattice constant close to that of the oxide-based superconductive material forming the superconductor layer.

The superconducting layer 2 of the present invention includes a superconducting phase formed from an oxide-based superconducting material represented by the chemical formula ErBa ₂ Cu ₃ O _7-δ (0 ≦ δ ≦ 1). Since the superconducting material forming the superconducting layer 2 is c-axis oriented (the c-axis is perpendicular to the substrate surface), the ab surface which is the superconducting surface is parallel to the substrate and parallel to the substrate. A large current can be passed in the direction.

  The superconducting layer 2 is formed by a method using non-equilibrium conditions such as physical vapor deposition such as laser vapor deposition or sputtering vapor deposition, or chemical vapor deposition such as organic chemical vapor deposition, preferably pulsed laser vapor deposition ( PLD) can be used.

When the superconducting layer 2 is laminated by the PLD method, a metal oxide (Er ₂ O ₃ , BaO, CuO) that forms the superconducting layer 2 is mixed and sintered at a desired ratio, and used as a target. The target is irradiated with a laser in an atmosphere containing oxygen. At this time, heating the substrate to a temperature of 700 to 790 ° C., preferably 750 to 780 ° C. is important in order to align the superconductor to be laminated in the c-axis orientation. As the laser for irradiating the target, any laser known in the art, such as an Ar—F excimer laser, can be used. In order to introduce oxygen into the superconductor material, the PLD is 0.1 Torr (13.3 Pa) or more, preferably 0.1 to 1.0 Torr (13.3 to 133 Pa), more preferably 0.2. It is desirable to carry out at an oxygen partial pressure of ˜0.65 Torr (26.7 to 86.7 Pa).

  It is desirable that the superconducting layer 2 has a film thickness sufficient to pass a desired amount of current. The superconductive layer 2 is preferably 300 nm or more, more preferably 300 nm to 3.0 μm, and most preferably 0.5 to 2.0 μm. However, a larger film thickness may be used in a specific application where a larger amount of current needs to be realized.

When the superconducting layer 2 is produced under non-equilibrium conditions as described above, oxygen having a desired composition ratio may not be introduced into the superconducting layer 2 as it is formed. In addition, there is a deviation in the composition (particularly the oxygen composition ratio) in the film, which may cause an inhomogeneous distribution of T _c and J _c . In order to achieve a homogeneous T _c and J _c distribution throughout the superconducting layer 2, one or more pores 3 are provided in the superconducting layer 2, and the heat treatment under an oxidizing atmosphere can be performed quickly. carry out.

  The air holes 3 are oxygen inlets for introducing oxygen more uniformly in a shorter time during the heat treatment described later. By providing the holes 3, oxygen enters from the side walls of the holes 3 in addition to the surface of the superconducting layer 2 during the heat treatment described later, and can diffuse into the superconducting layer 2. One or more of the holes 3 provided may not all penetrate the superconducting layer 2, but most of them penetrate the superconducting layer 2 in order to perform more efficient oxygen diffusion. It is preferable that all of them penetrate the superconducting layer 2. The air holes 3 may have an arbitrary shape such as a circular shape, a polygonal column shape, a truncated cone shape, and a truncated pyramid shape. When the circular holes 3 are used, it is desirable that the upper surface of the superconductive layer 2 has a diameter of 50 μm or less, preferably 30 μm or less, more preferably 10 μm or less. Even when holes having other shapes are used, they have the diameter of the opening corresponding to this (for example, the diameter of the circle inscribed in the opening on the upper surface of the superconducting layer 2 is in the above range). Is desirable).

  When a large area or long superconducting layer 2 is used, one or more per cm square, preferably one or more per mm square, more preferably, in order to perform uniform oxygen diffusion throughout the superconducting layer 2 Is preferably provided with a plurality of holes 3 at a density of 1 or more per 500 μm square. The plurality of holes 3 are not necessarily arranged at regular intervals, but are preferably arranged as evenly as possible in order to ensure the uniformity of the superconducting layer 2. When the long superconducting layer 2 is used as a wire, it is desirable to align a plurality of holes 3 in the current direction.

  The hole 3 is formed by any machining method known in the art using an etching method such as ion beam etching, electron beam etching, sputter etching, or laser ablation, or a microfabrication drilling device such as a microneedle or a microdrill. Can be formed.

After disposing one or more holes 3 in the superconducting layer 2, heat treatment is performed in an oxidizing atmosphere to diffuse oxygen into the superconducting layer 2. The oxidizing atmosphere includes an oxidizing gas such as O ₂ and O ₃ , and preferably includes O ₂ . The oxidizing atmosphere may contain an inert gas such as N ₂ , He, or Ar in addition to the oxidizing gas. The oxidizing gas in the oxidizing atmosphere has a partial pressure of 200 Torr (26.7 kPa) or more, preferably 300 Torr (40 kPa) or more, more preferably 760 Torr (101.3 kPa) or more. By having the above partial pressure, it is possible to introduce a sufficient amount of oxygen while shortening the heat treatment time and suppressing the deterioration of the superconducting layer 2 over time.

The heat treatment is performed by heating the laminate of the substrate 1 and the superconducting layer 2 to 250 to 600 ° C., preferably 300 to 600 ° C., more preferably 350 to 550 ° C. The heat treatment is preferably performed for 1 minute to 45 minutes, preferably 3 minutes to 30 minutes, more preferably 5 minutes to 20 minutes. By setting the treatment temperature and the treatment time as described above, it is possible to suppress the deterioration of the superconducting layer 2 due to heating with time, and at the same time, oxygen is quickly diffused into the superconducting layer 2 and has a desired composition ratio. The conductor ErBa ₂ Cu ₃ O _7-δ can be obtained.

  The heat treatment includes resistance heating by a resistor provided on the substrate support member, radiation heating by irradiating the superconducting layer 2 with an infrared lamp or the like, induction heating by applying an AC electric field to the substrate support member or the substrate, etc. It can be carried out using any heating means known in the art.

Example 1
A superconducting thin film in which an ErBa ₂ Cu ₃ O _7-δ oxide superconducting laminate was laminated on a SrTiO ₃ substrate was produced by the PLD method.

First, an excimer laser was irradiated to an ErBa ₂ Cu ₃ O _7-δ target, and a superconducting layer 2 having a thickness of 620 nm was laminated on a 1 cm square SrTiO ₃ substrate 1. At this time, the substrate temperature was set to 760 ° C., and the oxygen partial pressure in the vacuum chamber was set to 0.4 Torr (53 Pa).

When the obtained superconducting layer was analyzed by the X-ray diffraction method, it was confirmed that the c-axis of the ErBa ₂ Cu ₃ O _7-δ crystal was perpendicular to the film formation surface of the substrate. Furthermore, when the rocking curve was measured by the X-ray diffraction method, the half width of the peak was 0.2 degrees, and it was revealed that the obtained superconductor layer has high crystallinity. The resulting superconducting layer 2 exhibited a T _c of 90.9K.

After processing the obtained superconducting layer 2 to a sample width of 100 μm, it was cooled to an absolute temperature of 77 K and its critical current density was measured. When no magnetic field was applied (self-magnetic field condition), 1.2 × 10 ⁵ A / When a magnetic field of cm ² and 5 T was applied, a critical current density of less than 1 × 10 ⁴ A / cm ² was exhibited. The change in the critical current density of the superconducting layer before the heat treatment with respect to the strength of the applied magnetic field is shown in FIG. 2 (non-heated).

Next, with respect to another sample in which the superconducting layer 2 is laminated under the same conditions as described above, a plurality of diameters completely penetrating the superconducting layer 2 at a rate of one per 300 μm square using a microneedle. Holes 3 of 10 μm to 30 μm were formed. The laminated body in which the holes 3 were formed was placed in an image furnace having an oxygen partial pressure of 760 Torr (101.3 kPa) and heated to 450 ° C. for 15 minutes to obtain a superconducting thin film. The resulting superconducting thin films showed _{T c} of 90.5K.

After processing the heat-treated superconducting thin film to a sample width of 100 μm, it was cooled to an absolute temperature of 77 K and its critical current density was measured. When no magnetic field was applied, 1.9 × 10 ⁶ A / cm ² , 5T The critical current density of 1.2 × 10 ⁵ A / cm ² was exhibited when the magnetic field was applied. FIG. 2 shows the change in critical current density of the superconducting thin film obtained in this example with respect to the strength of the applied magnetic field (heating at 450 ° C.).

The critical current density obtained in the superconducting thin film after the heat treatment was heated both when no magnetic field was applied and when the magnetic field was applied, although _Tc showed almost the same value regardless of the presence or absence of the heat treatment. It became much larger than the value before processing. This indicates that the superconducting layer immediately after film formation under non-equilibrium conditions has a good T _c because a region exhibiting superconductivity is formed at least partially, but the whole has a uniform composition. It believed showed low J _c for free. On the other hand, in the sample obtained by the method of the present invention in which the heat treatment after providing the holes was performed, the oxygen composition ratio was made uniform by the heat treatment, and good _Tc and _Jc were exhibited. Conceivable.

(Example 2)
Example 1 was repeated to form a superconducting thin film, except that the heat treatment was carried out by heating to 425 ° C. for 15 minutes.

The superconducting thin film after the heat treatment was cooled to an absolute temperature of 77 K, and its critical current density was measured. When no magnetic field was applied, 2.5 × 10 ⁶ A / cm ² , and when a magnetic field of 5 T was applied, 1.0 × A critical current density of 10 ⁵ A / cm ² was exhibited. The change in critical current density of the superconducting thin film obtained in this example with respect to the strength of the applied magnetic field is shown in FIG. 2 (at 425 ° C. heating).

  The obtained critical current density was three times that before heat treatment when no magnetic field was applied, and about twice that before heat treatment when a magnetic field of 5T was applied. This effect is considered to be due to the uniform introduction of oxygen by the heat treatment after providing the holes.

(Example 3)
Example 1 was repeated to form a superconducting thin film, except that the heat treatment was carried out by heating to 450 ° C. for 60 minutes.

The superconducting thin film after the heat treatment was cooled to an absolute temperature of 77 K, and its critical current density was measured. As a result, it was 5 × 10 ⁵ A / cm ² when no magnetic field was applied, and 1 × 10 ⁴ A / when a magnetic field of 5 T was applied. A critical current density of cm ² was shown.

  The obtained critical current density increased approximately twice before the heat treatment when no magnetic field was applied, but showed the same value before and after the heat treatment when a 5T magnetic field was applied. This result is considered to be because although the oxygen composition ratio was made uniform over the entire superconducting layer by the heat treatment, the superconducting layer was deteriorated with time by extending the heating time.

It is sectional drawing which shows the exemplary structure of the superconducting thin film of this invention. It is a graph which shows the change of the critical current density of the superconducting layer with respect to the applied magnetic field at the time of heating at 450 degreeC and 425 degreeC when not heating.

Explanation of symbols

1 Substrate 2 Superconducting layer 3 Hole

Claims (14)

A superconducting thin film comprising a substrate and a superconducting layer formed on the substrate and including an oxide superconducting phase having a composition of ErBa ₂ Cu ₃ O _7-δ (0 ≦ δ ≦ 1), superconducting layer viewed contains at least one pore, wherein the at least one cavity is superconducting thin film characterized in that through said superconducting layer.   2. The superconducting thin film according to claim 1, wherein at least one of the holes is arranged per 1 cm 2. 3. The superconducting thin film according to claim 1 , having a critical current density of 1 × 10 ⁵ A / cm ² or more under a magnetic field having an absolute temperature of 77 K and 5 T. 4. A superconducting wire comprising the superconducting thin film according to any one of claims 1 to 3 . A superconducting device comprising the superconducting thin film according to any one of claims 1 to 3 . Forming a superconducting layer including an oxide superconducting phase having a composition of ErBa ₂ Cu ₃ O _7-δ on a substrate;
Forming at least one hole in the superconducting layer;
And a step of heat-treating the superconducting layer in an oxidizing atmosphere.   The method of manufacturing a superconducting thin film according to claim 6, wherein the heat treatment step is performed at a temperature within a range of 250 to 600 ° C. The method of manufacturing a superconducting thin film according to claim 6 or 7 , wherein the oxidizing atmosphere includes an oxidizing gas having a partial pressure of 200 Torr (26.7 kPa) or more. The method of manufacturing a superconducting thin film according to any one of claims 6 to 8 , wherein the heat treatment step is performed for a time within a range of 1 minute to 45 minutes. The method for producing a superconducting thin film according to any one of claims 6 to 9 , wherein a PLD method is used in the step of forming the superconducting layer. The method for producing a superconducting thin film according to any one of claims 6 to 10 , wherein one or more of the at least one hole is arranged per 1 cm 2. 12. The method of manufacturing a superconducting thin film according to claim 6, wherein the at least one hole penetrates the superconducting layer. A method for producing a superconducting wire, comprising using the production method according to claim 6 . A method for manufacturing a superconducting device, wherein the method according to any one of claims 6 to 12 is used.

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