WO2003091158A1

WO2003091158A1 - Oxide superconductive thin-film, process for producing the same and superconducting fault current limiter

Info

Publication number: WO2003091158A1
Application number: PCT/JP2003/004933
Authority: WO
Inventors: Shuji Hahakura; Kazuya Ohmatsu
Original assignee: Sumitomo Electric Industries, Ltd.
Priority date: 2002-04-26
Filing date: 2003-04-17
Publication date: 2003-11-06
Also published as: AU2003235237A1; JP2005344125A

Abstract

A process for forming an oxide superconductive thin-film of high critical current density and large critical current characteristics having a thickness of more than 0.7 μm on a substrate including a sapphire single-crystal substrate by the use of laser deposition technique wherein a target is irradiated with laser beams and the matter scattered from the target thereby is deposited on the substrate; the oxide superconductive thin-film produced by the process; and a superconducting fault current limiter including the oxide superconductive thin-film.

Description

Specification

Oxide superconducting thin film, method for producing the same, and superconducting current limiter

The present invention relates to an oxide superconducting thin film, a method for producing the same, and a superconducting current limiter.In particular, a method for forming an oxide superconducting thin film on a sapphire substrate, an oxide superconducting thin film produced by the method, The present invention relates to a superconducting current limiter using the oxide superconducting thin film. Background art

For thin-film materials on single-crystal substrates, the development of practical applications such as superconducting current limiters for power applications, and superconducting microwave filters for electronic applications is expected. In particular, when a superconducting thin film formed on a sapphire (Α 1 ₂ Ο ₃ ) single crystal substrate is used for a current limiter, sapphire has excellent thermal conductivity, so heat is released to release heat from the superconducting thin film. There are benefits. In addition, sapphire single crystal is inexpensive compared to other single crystal substrates, and has the advantage that a substrate having a relatively large area can be manufactured.

As long as the superconducting thin film has the same critical current density, the thicker the film, the larger the current that can flow. Therefore, especially for power applications, a thick film is desired. For example, if a thick superconducting thin film capable of conducting a large current is used for the superconducting current limiting device, a compact superconducting current limiting device can be manufactured.

However, a superconducting thin film formed on a sapphire single-crystal substrate could only form a thin film having a thickness less than a conventional limit value. Specifically, the limit thickness of YBa2Cu3〇7-X (YBCO) formed on a sapphire is typically 0.25 to 0.3 / zm. In the following literature, 0.7 μπι is reported as the highest value.

References: R. Wordenweber et al., "Large-Area YBCO Films on Sapphire for Microwave Applications", IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 9, NO. 2, JUNE 1999, pp. 2486-2491 Previous reports have suggested that when the thickness of a superconducting thin film on a sapphire single crystal substrate exceeds a certain limit, cracks occur in the thin film. The number of cracks increases with time. A thin film having a thickness exceeding the limit value has cracks, so that the critical current density is almost zero force or excellent characteristics cannot be obtained. The reason why cracks occur in the superconducting thin film when the thickness exceeds the critical film thickness or the reason why excellent characteristics are not obtained is that (1) the difference in thermal shrinkage between the single crystal substrate and the superconducting thin film, and (2) the superconducting layer and its This is due to mismatch of lattice constant with the base (substrate or intermediate layer). In particular, when using the thermal co-evaporation method, which was considered to be the predominant method for superconducting thin films, the raw material that has been heated and vaporized is deposited on the substrate, so that dense crystals grow slowly. There is also. Disclosure of the invention

Therefore, one object of the present invention is to provide a thick oxide superconducting thin film having no cracks or excellent critical current density characteristics and a method for producing the same.

Another object of the present invention is to provide a high-thickness oxide superconducting thin film having no cracks or excellent critical current density characteristics, thereby exhibiting a high critical current value during normal operation, and in the case of a short circuit accident or the like. It is an object of the present invention to provide a superconducting current limiter exhibiting a high taench (normal conduction transition) resistance.

In the method for producing an oxide superconducting thin film of the present invention, the oxide superconducting thin film is formed on a substrate containing sapphire by a laser vapor deposition method in which a raw material is irradiated with laser light and a substance scattered from the raw material is deposited on the substrate. It is characterized by being formed with a film thickness exceeding 7 μπι.

As a result of intensive studies, the inventors of the present invention have formed a superconducting oxide thin film on a sapphire substrate by using a laser vapor deposition method instead of the conventional thermal co-evaporation method and appropriately setting various conditions during laser vapor deposition. As a result, it has been found that an oxide superconducting thin film having excellent critical current density characteristics can be formed with a thickness exceeding 0.7 μπι. This is because, in the case of the laser vapor deposition method, it is considered that a certain amount of particles are scattered as compared with the case of the thermal co-evaporation method. This is considered to be because distortion is easily alleviated. In other words, in laser deposition, By properly setting the conditions, the particles in a lump scatter and the oxide superconducting thin film is formed.Therefore, the difference in thermal shrinkage between the single crystal substrate and the superconducting thin film, and the difference between the superconducting layer and It is considered that thermal or mechanical distortion such as mismatch of lattice constant with the substrate or intermediate layer) can be reduced. As a result, a large current can be passed, and an oxide superconducting thin film suitable for power use can be manufactured. An excimer laser (ArF laser: wavelength 193 nm, KrF laser: wavelength 248 nm, XeCl laser: wavelength 308 nm) can be used as the laser.

In the above-described method for producing an oxide superconducting thin film, preferably, the repetition frequency (hereinafter, referred to as a laser frequency) of pulse irradiation of a laser beam applied to a raw material is divided into at least two steps to thereby obtain an oxide superconducting film. A thin film is formed.

Thereby, it becomes possible to manufacture an oxide superconducting thin film having a higher critical current density than when forming an oxide superconducting thin film at a single laser frequency. In the above method for producing an oxide superconducting thin film, preferably, the first-stage laser frequency is lower than the second-stage laser frequency.

By specifically controlling the laser frequency in this way, it becomes possible to manufacture an oxide superconducting thin film having a higher critical current density as described above. In the above method for producing an oxide superconducting thin film, the energy per pulse (hereinafter, referred to as laser power) is preferably at least 400 mJ.

By setting the laser power in this manner, an oxide superconducting thin film having a high critical current density can be obtained.

In the above method for producing an oxide superconducting thin film, the temperature of the substrate at the time of laser deposition is preferably at least 600 ° C. and less than 1200 ° C.

By setting the temperature of the substrate in this way, an oxide superconducting thin film having a high critical current density can be obtained.

In the above method for producing an oxide superconducting thin film, the gas pressure during laser deposition is from 1.33 Pa to 100 Pa, preferably from 1.33 Pa to 66.66. It is less than Pa.

By setting the gas pressure in this manner, an oxide superconducting thin film having a high critical current density can be obtained. In the above method for producing an oxide superconducting thin film, preferably, oxygen is contained in the atmosphere at the time of laser deposition.

By containing oxygen as described above, an oxide superconducting thin film having a high critical current density can be obtained.

The oxide superconducting thin film of the present invention is formed by the above manufacturing method, and has a thickness exceeding 0.7 μπι.

According to the oxide superconducting thin film of the present invention, it becomes possible to form a thick film exceeding 0.7 μπι having excellent critical current density characteristics and large critical current characteristics by a laser vapor deposition method. Since a thick oxide superconducting thin film can be realized as described above, a large current can be passed, and an oxide superconducting thin film particularly suitable for power use can be obtained.

In the oxide superconducting thin film described above, preferably, the critical current density under a self-magnetic field in liquid nitrogen is 1 × 10 ⁶ AZcm ² or more, and the critical current characteristics are excellent. As described above, since a thick oxide superconducting thin film having excellent critical current density characteristics can be formed by the laser deposition method, the critical current density is as high as 1 × 10 ⁶ A / cm ² or more, and It is possible to increase the critical current value.

In the above-mentioned oxide superconducting thin film, the critical current density under a self-magnetic field in liquid nitrogen is more than 7 OA / cm width per 1 cm width, and more than 280 A, cm width. This allows a large current to flow.

In the above oxide superconducting thin film, preferably, it has a RE123 structure, and the force RE is made of a material containing at least one of a rare earth element and a yttrium element.

By using the oxide superconducting thin film having the RE123 structure capable of conducting a large current, an oxide superconducting thin film suitable for power use can be obtained.

Note that the "RE 1 2 3 Structure" herein, RE _X Te B a _y C u _z 〇 ₇ _ _d smell, 0. 7 x≤ l. 3, 1. 7≤ y≤ 2. 3, 2. It means that 7≤z≤3.3. RE in the “RE123 structure” means a material containing at least one of a rare earth element and a yttrium element.

The superconducting current limiter of the present invention is configured using the above-described oxide superconducting thin film. By forming a superconducting current limiter using a thick oxide superconducting thin film in this way, the superconducting current limiting device exhibits a high critical current value during normal operation and a high quenching resistance value during a short-circuit accident or the like. You can get a bowl.

In the above-described superconducting current limiter, preferably, the current is limited by transitioning the oxide superconducting thin film from the superconducting state to the normal conducting state.

As described above, the present invention can be applied to a so-called superconducting / normal conducting (SN) transition type superconducting current limiter. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagram for explaining a method for manufacturing an oxide superconducting thin film according to one embodiment of the present invention.

FIG. 2 is a view showing a process of forming an oxide superconducting thin film by dividing a laser frequency of a laser beam into two stages in a laser vapor deposition method.

FIG. 3 is a cross-sectional view schematically showing a configuration of the oxide superconducting thin film according to one embodiment of the present invention.

Figure 4 is a _{_{H o B a 2 C u 3}} O x (H o BCO) measured result indicating a critical current density relationship of the superconducting layer under the self-magnetic field in the gas pressure and liquid nitrogen during the laser deposition method . FIG. 5 is a diagram showing a configuration of a superconducting current limiter according to one embodiment of the present invention.

FIG. 6 is a plan view showing a coil made of a superconducting thin film used in the superconducting current limiter of FIG.

FIG. 7 is a cross-sectional view showing a coil formed of a superconducting thin film used in the superconducting current limiter of FIG. BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram illustrating a method for manufacturing an oxide superconducting thin film according to an embodiment of the present invention. Referring to FIG. 1, first, substrate 10 is placed on heater 2 in a state where substrate 10 is inclined at a predetermined angle with respect to target (raw material) 1. In this inclined state, a predetermined portion of the substrate 10 is covered with a mask (not shown), and a laser is applied to the target 1 by laser ablation. One light 3 is irradiated. As a result, the substance (plume) 4 scattered from the target 1 is deposited on the exposed surface of the substrate 10 to form an oxide superconducting thin film.

The laser frequency of the laser beam applied to the target 1 is preferably divided into two steps (steps S1 and S2) as shown in FIG. The laser frequency in the first laser irradiation (step S1) is preferably lower than the laser frequency in the second laser irradiation (step S2). In this laser irradiation, the laser power is preferably 40 OmJ or more, more preferably 600 mJ or more, and further preferably 800 mJ to 1,000 mJ.

In addition, the temperature of the substrate 10 during the laser deposition is preferably 600 ° C. or more and less than 1200 ° C., and more preferably 800 ° C. or more and less than 1200 ° C.

The gas pressure during laser deposition is from 1.33 Pa to 100 Pa, and preferably from 1.33 Pa to 66.6 Pa. Preferably contains oxygen.

Next, the oxide superconducting thin film formed by the above manufacturing method will be described. FIG. 3 is a cross-sectional view schematically showing a configuration of the oxide superconducting thin film according to one embodiment of the present invention. Referring to FIG. 3, oxide superconducting thin film 13 is formed on substrate 10. This substrate 10 has a sapphire single crystal substrate 11 and an intermediate layer 12 made of, for example, cerium oxide.

Oxide superconductor thin film 1 3, for example, summer than _{_{Ho B a 2 C u 3 O}} x, and has a thickness T of more than 0. 7 m. Further, the oxide superconducting thin film 13 preferably has a critical current density of 1 × 10 ⁶ A / cm ² or more under a self-magnetic field in liquid nitrogen, and has a critical current per 1 cm width. It is preferable that the width is 70 AZcm or more, and more than 28 OA / cm. The material of the oxide superconductor thin film 1 3 is not limited Ho B a ₂ C UsO, to, yo if a RE 1 2 3 Structure les. Further, RE in the RE123 structure is preferably at least one of a rare earth element and a yttrium element.

The oxide superconducting thin film 13 is formed directly on the sapphire single crystal substrate 11. May be.

Next, a superconducting current limiter using the above-described oxide superconducting thin film will be described.

FIG. 5 is a diagram showing a configuration of a superconducting current limiter according to an embodiment of the present invention. FIGS. It is a figure and a sectional view.

Referring to FIG. 5, the superconducting current limiter in the present embodiment has coils 28a and 28b that can be cooled by liquid nitrogen or the like. As the coils 28a and 28b, for example, flat superconducting coils are used as shown in FIGS. The superconducting coil 28 a is formed in a spiral shape on the surface of the insulating substrate 27, and the superconducting coil 28 b is formed in a spiral shape on the back surface of the insulating substrate 27. One end of each of the superconducting coils 28a and 28b is electrically connected to each other by a front and back circuit connecting portion 29. The other ends of the superconducting coils 28a and 28b are electrically connected to the terminals 26a and 26b, respectively.

Such a superconducting current limiter acts as a current limiter as follows.

Normally, current limiters using superconducting conductors are wound so that the impedance is minimized when viewed from the power system. Also, when the load current is flowing through the current limiter in the steady state, the power loss is very small because the superconducting state is maintained. On the other hand, if the short-circuit current becomes excessive at the time of the accident and the short-circuit current exceeds the tangent current of the superconducting wire used for the current limiter, the current limiter immediately changes from the superconducting state to the normal conduction state. The current limiter transitions from zero resistance to high resistance, the impedance increases instantaneously, and it becomes possible to immediately limit excessive short-circuit current. When the short-circuit fault recovers, the current limiter is cooled by the cooling medium and returns to the superconducting state, so that it operates as the original current system.

Hereinafter, examples of the present invention will be described.

(Example 1)

A HoBa ₂ Cu ₃ O _x superconducting layer was formed on a sapphire substrate with a cerium oxide intermediate layer (about 40 nm thick) deposited by laser deposition. In laser deposition, Ho B a ₂ C u ₃ 〇, on a sintered target was irradiated with Xe C l excimer laser (wavelength 3 08 nm), the laser energy to 90 Om J, repetition rate of the first step The frequency was set to 5 Hz, the laser of the second step was repeated at a frequency of ⁴ OHz (each film was formed simultaneously), the film was formed under the conditions of a substrate temperature of 9 ° C and an oxygen atmosphere of 13.33 Pa. Was. By varying the deposition time was deposited Ho B a ₂ Cu ₃ O _x greater conductivity layer having various thickness.

Further, on what was deposited cerium oxide intermediate layer on a sapphire substrate (thickness: about 40 nm) as a comparative example, it was formed _{_{H o B a 2 C u 3}} O x superconducting layer by thermal co-evaporation. Ho, Ba, and Cu metals were scattered by resistance heating using raw materials, placed on the opposite surface, and vapor-deposited on a substrate heated to 700 ° C.

We investigated the relationship between the critical current density and the thickness of each Ho B a ₂ Cu ₃ was produced by a laser deposition method and Netsutomo deposition of O _x film (Ho BCO film). The results are shown in Table 1. table 1

Critical current density and critical current value of HoBCO film produced by laser deposition method and thermal co-evaporation method * Relationship between * and film thickness (* Critical current value is value per film lcm width)

According to the results in Table 1, according to the thermal co-evaporation method, when the thickness of the Ho BCO film becomes 0.5 im or more, the critical current density becomes zero. It can be seen that the BCO film can maintain a high critical current value even at a thickness of 0.5 μπι or more, and can form a thick oxide superconducting thin film without causing cracks. Furthermore, in the case of a thick film exceeding 0.7 πι, the oxide superconducting thin film has a higher critical current density and a larger critical current value per 1 cm width of the film than the conventional thin film, and is thus an excellent oxide superconducting thin film. Understand.

Therefore, a superconducting thin film is formed on a sapphire substrate using laser evaporation. This indicates that an oxide superconducting thin film having a high critical current density and a large critical current can be formed with a thickness exceeding 0.7 / m.

(Example 2)

Cerium oxide intermediate layer on a sapphire substrate on what was deposited (thickness: about 40 nm), holmium-based superconducting thin film _{_{(Ho B a 2 Cu 3 O}} x film: Ho B CO) by laser deposition method was formed. At that time, the repetition rate of the laser was varied as a parameter. After forming the film at the repetition frequency of the first step for 10 minutes, the film was formed at the repetition frequency of the second step for 10 minutes. The atmosphere at the time of film formation was 13.33 Pa of oxygen gas, the substrate temperature was 900 ° C., and the laser power was 90 OmJ, and each was constant. In order to observe the characteristics of the superconducting layer, we measured the critical current density of the HoBCO superconducting layer in liquid nitrogen under a self-magnetic field. The results are shown in Table 2.

Table 2 Laser repetition frequency of first and second stages and critical current density of HoBCO superconducting layer in liquid nitrogen under self-magnetic field

From the results in Table 2, it can be seen that, at the repetition frequency of the laser, when the repetition frequency of the first step is smaller than the repetition frequency of the second step, the critical current density increases.

(Example 3)

Cerium oxide intermediate layer on a sapphire substrate on what was deposited (thickness: about 40 nm), laser vapor deposition method by Horumiumu system superconducting thin film _{_{(Ho B a 2 Cu 3 O}} x: Ho BC O) was formed. At that time, the laser energy was varied as a parameter. In addition, At the time of film formation, in the first step, film formation was performed at a repetition frequency of 5 Hz for 10 minutes, and then, in the second step, film formation was performed at a repetition frequency of 40 Hz for 10 minutes. The film formation atmosphere was an oxygen gas of 13.33 Pa and the substrate temperature was 900 ° C, and each was constant. To see the characteristics of the superconducting layer, the critical current density of the HoBCO superconducting layer in liquid nitrogen under a self-magnetic field was measured. The results are shown in Table 3.

Table 3

I ^^^^^^^ and HoBCO superconducting layer in liquid nitrogen

Critical density under self magnetic field

From the results in Table 3, it can be seen that when the laser energy is 40 OmJ or more, the critical current density increases.

(Example 4)

A holmium-based superconducting thin film (HoBa ₂ Cu ₃ O _x : HoBCO) was formed on a sapphire substrate with a cerium oxide intermediate layer (about 40 nm thick) deposited by laser deposition. At that time, the substrate temperature was changed as a parameter. During the film formation, in the first step, the film was formed at a repetition frequency of 5 Hz for 10 minutes, and then, in the second step, the film was formed at a repetition frequency of 40 Hz for 10 minutes. The film formation atmosphere was oxygen gas at 13.33 Pa and the laser power was 900 mJ, each of which was constant. In order to examine the characteristics of the superconducting layer, the critical current density of the HBCO superconducting layer in liquid nitrogen under a self-magnetic field was measured. The results are shown in Table 4.

Table 4

Substrate temperature and critical density of superconducting layer in liquid nitrogen under self-magnetic field

Substrate temperature (° C) 400 500 600 800 900 1000 1100 1200

HoBCO critical current density 0.1 0.3 1.6 3.0 4.0 3.5 3.1 0.3 degree (MA / cm ² ) The results in Table 4 show that when the substrate temperature is between 600 ° C and 1200 ° C, the critical current density increases.

(Example 5)

Cerium oxide intermediate layer on a sapphire substrate on what was deposited (thickness: about 40 nm), laser vapor deposition method by holmium-based superconducting thin film _{_{(Ho B a 2 Cu 3 O}} x: Ho BC O) was formed. At that time, the pressure of the film-forming oxygen gas was changed as a parameter. During the film formation, in the first step, the film was formed at a repetition frequency of 5 Hz for 10 minutes, and then, in the second step, the film was formed at a repetition frequency of 40 Hz for 10 minutes. The substrate temperature was 900 ° C and the laser power was 900 mJ, and each was kept constant. In order to examine the properties of the superconducting layer, the critical current density of the HoBCO superconducting layer in liquid nitrogen under a self-magnetic field was measured. The results are shown in Table 5 and FIG. Table 5

Gas pressure and critical current density of HoBCO superconducting layer in liquid nitrogen under self-magnetic field

From the results in Table 5 and Fig. 4, the critical current density increases when the gas pressure is 1.33 Pa or more and l OOP a or less, and the critical current density increases when the gas pressure is 1.33 or more and 66.66 Pa or less. It can be seen that the degree is even higher.

(Example 6)

Cerium oxide intermediate layer on a sapphire substrate on what was deposited (thickness: about 40 nm), laser vapor deposition method by holmium-based superconducting thin film _{_{(Ho B a 2 Cu 3 O}} x: Ho BC O) was formed. At that time, the gas type of the film formation atmosphere was changed as a parameter. During the film formation, in the first step, the film was formed at a repetition frequency of 5 Hz for 10 minutes, and then, in the second step, the film was formed at a repetition frequency of 40 Hz for 10 minutes. The substrate temperature was 900 ° C, the laser power was 90 Om J, and the atmospheric gas pressure was 13.33 Pa. And each was kept constant. In order to see the characteristics of the superconducting layer, the critical current density of the HBCO superconducting layer in liquid nitrogen under a self-magnetic field was measured. Table 6 shows the results.

Table 6

Critical current density of gas species and HoBCO superconducting layer in liquid nitrogen under self-magnetic field

The results in Table 6 show that when the gas type is oxygen, the critical current density is high.

The embodiments and examples disclosed this time are to be considered in all respects as illustrative and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims. Industrial applicability

As described above, according to the method for producing an oxide superconducting thin film of the present invention, an oxide superconducting thin film is formed on a sapphire substrate by a laser deposition method under appropriate conditions, so that 0.7 μππι It is possible to form an oxide superconducting thin film having a high critical current density and a large critical current characteristic with a thick film exceeding the above. For this reason, it becomes possible to manufacture an oxide superconducting thin film suitable for power applications requiring a large current.

Claims

The scope of the claims

1. The oxide superconducting thin film having a thickness of more than 0.7 m is formed on the substrate containing sapphire by a laser vapor deposition method in which a material scattered from the raw material is deposited on the substrate by irradiating the raw material with laser light. A method for producing an oxide superconducting thin film, characterized by being formed.

2. The method for producing an oxide superconducting thin film according to claim 1, wherein the laser frequency of the laser light applied to the raw material is divided into at least two stages to form the oxide superconducting thin film.

3. The method for producing an oxide superconducting thin film according to claim 1, wherein the first stage laser frequency is lower than the second stage laser frequency.

4. The method for producing an oxide superconducting thin film according to claim 1, wherein the laser power is 40 OmJ or more.

5. The production of the oxide superconducting thin film according to any one of claims 1 to 4, wherein the temperature of the substrate at the time of the laser deposition is not less than 600 ° C and less than 1200 ° C. Method.

6. The method for producing an oxide superconducting thin film according to any one of claims 1 to 5, wherein a gas pressure at the time of the laser deposition is 1.33 Pa or more and 100 Pa or less.

7. The production of the oxide superconducting thin film according to any one of claims 1 to 5, wherein a gas pressure at the time of the laser deposition is 1.33 Pa or more and 66.66 Pa or less. Method.

8. The method for producing an oxide superconducting thin film according to any one of claims 1 to 7, wherein oxygen is contained in an atmosphere during the laser deposition.

9. An oxide superconducting thin film formed by the method for producing an oxide superconducting thin film according to any one of claims 1 to 8, and having a thickness exceeding 0.7 / zm.

10. The oxide superconducting thin film according to claim 10, wherein a critical current density under a self-magnetic field in liquid nitrogen is 1 × 10 ⁶ AZcm ² or more.

11. The oxide superconducting thin film according to claim 9, wherein a critical current density under a self-magnetic field in liquid nitrogen is not less than 70 AZcm width per 1 cm width.

12. The oxide superconducting thin film according to claim 9, wherein the oxide superconducting thin film has a RE 123 structure and is made of a material containing at least one of a rare earth element and a yttrium element.

1 3. A superconducting current limiter using the oxide superconducting thin film according to any one of claims 9 to 12.

14. The superconducting current limiter according to claim 13, wherein current is limited by causing the oxide superconducting thin film to transition from a superconducting state to a normal conducting state.