JP2007302507A - Method for producing superconducting thin film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that the main burning which is a crystallization step takes a long time in a coating pyrolysis method which is one of the methods for producing a superconducting thin film. <P>SOLUTION: In the coating pyrolysis method, a coating film 3 containing the constituent element of a superconductor is formed on the upper layer of a substrate 1. Further, a surface side seed material layer 4 is formed on the surface side of the coating film 3 to perform temporary burning and main burning. During the main burning, crystallization is performed towards the surface from the substrate 1 side, and crystallization is also performed towards the substrate side from the surface side seed material layer 4. As a result, the time required for crystallization is shortened by performing crystallization from both directions. Furthermore, it is desirable to form a buffer layer 2 between the substrate 1 and the coating film 3. The buffer layer 2 prevents the chemical reaction of the substrate 1 with the coating film 3 and functions as a seed for crystallization. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a superconductor filter used for a base station for a mobile communication body, a superconductor substrate used for a conductor of a current limiter for electric power equipment, a superconducting wire for large current transport, and the like. The present invention relates to a method for manufacturing a conductive thin film.

Regarding superconducting materials, various applications have been developed since the discovery of the high-temperature oxide superconducting material in 1986. The high temperature oxide superconductors, conventional low-temperature superconducting material is, compared to the required liquid the He (4.2 K) for cooling, to handle because it can cooled with liquid N _{2 (77K)} Comparison And various uses are expected.
Applications of high-temperature oxide-based superconducting materials include bulk materials, wires, and thin films. Among these, thin-film materials are used for mobile communication base stations and radio station base stations. It is expected to be applied to a superconductor filter, a superconductor substrate used in a current limiter for electric power equipment, or a superconducting wire for transporting a large current.

  A critical current density Jc value is the most important index showing the characteristics of the high-temperature oxide superconducting thin film. By increasing the Jc value, a large current can be transported at a high temperature. In order to obtain a high Jc value, it is necessary to epitaxially grow a crystal of the high-temperature oxide superconductor on the support of the high-temperature oxide superconductor thin film in the c-axis direction.

A method for producing a high-temperature oxide-based superconducting thin film that is currently commercialized is the Thermal Co-evaporation method of THEVA, Germany (see Non-Patent Document 1). In this method, YBa ₂ Cu ₃ O _7-6 (YBCO) is used as a superconductor. The manufacturing procedure will be described. First, a substrate on which a YBCO superconducting thin film is formed is attached to a turntable. The substrate is heated to 700 ° C. by a Thermocoaxheater while rotating. Further, the inside of the chamber is kept at a high vacuum of 2 × 10 ⁻⁵ mbar. Then, Y, Ba, and Cu, which are constituent metal elements of the YBCO superconducting thin film, are respectively installed in the thermal boat, and the boat is heated by electric resistance. The amount of the constituent metal element that evaporates is measured by a quartz crystal monitor, and the amount of heat generated in the thermal boat is adjusted so that the intended thin film composition is obtained. Oxygen constituting the YBCO superconducting thin film is introduced from Pocket. Since the clearance between the Pocket and the rotating substrate is extremely small, the oxygen pressure inside the Pocket can be maintained at 250 times the pressure in the chamber. Within this chamber, the substrate periodically passes through the film formation region while rotating, and a monoatomic layer of the constituent metal elements is generated on the substrate surface. Next, the monoatomic layer is oxidized with oxygen pocket to form a YBCO superconducting thin film. In the next rotation, the YBCO monolayer overlaps the same thin film through the same process. By repeating the above process, YBCO grows epitaxially in the c-axis direction. The problems of this manufacturing method are that the mechanism of the apparatus is complicated and the manufacturing method requires a high vacuum, so that the manufacturing cost of the superconducting thin film is high and it is difficult to manufacture a large area thin film.

  In order to solve these problems, a coating pyrolysis method is currently being developed (see Patent Document 1). In this method, a coating solution is prepared by dissolving each metal compound in a solvent so that the composition of the constituent metal elements of the high-temperature oxide superconductor is obtained. Next, after apply | coating a coating solution on a board | substrate, it dries and forms the thin film (coating film) of a metal containing compound. Thereafter, the coating film is heated and baked to be converted into a superconducting thin film. In this case, the heating and firing conditions may be any conditions as long as the metal-containing compound forms a composite metal oxide, and is generally 500 to 1000 ° C. When the metal-containing compound is an organic compound, the same component is thermally decomposed or oxidized at 200 to 500 ° C., and the formation and crystallization of the composite metal oxide is performed at 500 to 1000 ° C. The baking time is about 1 to 72 hours. Moreover, the atmosphere of heat-firing is air, oxygen, nitrogen, argon, etc. Next, after heating and firing, the superconducting thin film can be obtained by slowly cooling the resulting composite metal oxide thin film together with the substrate to room temperature.

In addition, a coating pyrolysis method has been developed in which a buffer layer is interposed between the substrate and the superconducting thin film to epitaxially grow a superconductor crystal in the c-axis direction to obtain a high-performance superconducting thin film (Non-Continued) Patent Document 2).
This method will be described with reference to FIG. 2. A buffer layer 11 serving as a seed is formed on a substrate 10, and a coating film 12 is provided on the upper layer by the coating solution described above. By pre-baking the substrate 10, the coating film 12 is decomposed, and CO, CO ₂ and H ₂ O are generated and released, and the constituent metal elements remain to form the thin film 12a. By baking this substrate 10, the thin film 12 a changes to a composite metal oxide, and the composite metal oxide is crystallized using the buffer layer 11 as a seed to obtain a superconducting thin film 13.
JP-A 64-65003 Werner Prusseit et al., “Series production of large area YBa2Cu3O7-films for electronic-, microwave-, and electrical powerapplications”, ISS ', Japan, 1999,17.-19.10.99. T Manabe et al., “Two-dimensional large-size YBa2Cu3O7 films (30 × 10cm2) on CeO2-buffered sapphire by a coating pyrolysis process”, Superconductor Science and Technology 1, Japan, 7 (2004), P.354-357

  Since the conventional coating pyrolysis method is configured as described above, unlike the thermal co-evaporation method, the device mechanism is simple and the manufacturing method is performed under atmospheric pressure that does not require high vacuum. . Therefore, the manufacturing cost of the superconducting thin film is low, and the manufacturing of the large area thin film is easy. However, since this firing requires several hours and a long time, there is a problem that it is not an efficient manufacturing method.

  The present invention has been made to solve the problems of the conventional coating pyrolysis method as described above, and provides an efficient method for producing a superconducting thin film in which the time required for the main baking is shortened. It is aimed.

  That is, among the methods for producing a superconducting thin film of the present invention, the invention according to claim 1 is characterized in that a coating film is provided on an upper layer of a base material, and after that temporary firing is performed, then main firing is performed to superimpose the coating film. In the coating pyrolysis method for forming a conductive thin film, a surface-side seed material layer is provided on the surface side of the coating film provided on the upper layer of the base material before the preliminary baking, and the coating film is formed on the base layer during the main baking. A superconducting thin film is obtained by crystallization from the material side and the surface side seed material layer.

  The invention of the method for producing a superconducting thin film according to claim 2 is characterized in that, in the invention according to claim 1, a buffer layer made of a seed material on the substrate side is interposed between the substrate and the coating film. And

  According to a third aspect of the present invention, there is provided a method for producing a superconducting thin film, wherein the seed material of the surface-side seed material layer has the same composition as the superconducting thin film obtained by the main firing. It is characterized by that.

  The invention of the method for producing a superconducting thin film according to claim 4 is characterized in that, in the invention according to claim 2, the seed material of the surface side seed material layer and the base material side seed material are made of the same material. To do.

  The invention of the method for producing a superconducting thin film according to claim 5 is the invention according to any one of claims 1 to 4, wherein the surface-side seed material layer is smaller than the size of gas molecules generated during the main firing in the surface direction. Large gaps are distributed.

  The superconducting thin film manufacturing method according to claim 6 is the invention according to any one of claims 1 to 5, wherein the surface-side seed material layer is obtained by applying a seed material powder to the surface of the coating film. It is characterized by being.

  According to the present invention, the coating film is provided on the upper layer of the substrate, and the surface-side seed material layer is provided on the surface side of the coating film. When the surface-side seed material layer is formed by coating, the solvent used in the coating is removed and pre-baking is performed. The removal of the solvent and the pre-baking may be performed by a series of heating processes or may be performed by another heating process.

In the pre-baking, organic salts of constituent metal elements of the coating film are decomposed and oxidized and gasified into CO, CO ₂ , H ₂ O and the like. The gas is released from the coating film and the surface-side seed material layer. For this reason, as for the surface side seed material layer, it is desirable for the clearance gap to be distributed in the surface direction. The gaps may be scattered, or the gaps may be connected. When the seed material for crystallization is provided on the surface of the coating film without any gap, it is difficult to remove the organic salt decomposition gas from the surface of the coating film. The gap can also be obtained by applying the surface side seed material as powder to the surface of the coating film at an appropriate density.

The seed material powder preferably has a circle-equivalent diameter of 0.001 to 1 μm in the surface direction when applied. This is because when the particle size is less than 0.001 μm, the regularity of the particles is disturbed, or the function as a nucleation site is deteriorated due to the influence of adsorbed gas such as oxygen and water molecules, and sufficient nucleation cannot be performed. This is because the thickness increases with this, the ratio of the thickness to the film thickness increases, and the unevenness of the surface also increases, leading to deterioration in characteristics. The surface coverage of the seed material powder is desirably 10 to 95%. If it is less than 10%, it is insufficient in quantity as a nucleation site. If it exceeds 95%, organic salts of constituent metal elements of the coating film are decomposed and oxidized to generate CO, CO _2. This is because gas such as H ₂ O is difficult to escape and may cause peeling of the film.

  During the main firing, which is the next step, crystallization proceeds from the substrate side toward the coating film surface side. By interposing the buffer layer between the substrate and the coating film, crystal growth in the c-axis direction is performed using the buffer layer as a seed. At the same time, crystallization proceeds from the surface-side seed material layer on the coating film surface side. Thereby, the time for crystallizing the whole coating film is significantly shortened compared with crystallizing only from the base material side.

  As described above, in the present invention, the provisional baking is performed in the coating pyrolysis method in which the coating film is provided on the upper layer of the substrate, and then the preliminary baking is performed and then the main baking is performed to make the coating film a superconductive thin film. Before, the surface side seed material layer is provided on the surface of the coating film provided on the upper layer of the base material, and the coating film is crystallized from the base material side and the surface side seed material layer, respectively, during the main baking. Since the superconducting thin film is obtained, crystallization can be completed at an early stage. Thereby, the time required for the main firing can be shortened and an efficient method for producing a superconducting thin film can be provided.

  When a buffer layer is interposed between the base material and the coating film, the crystal using the buffer layer as a seed is epitaxially grown almost in the c-axis direction. When forming the crystal, the crystal using the surface-side seed material layer as a seed becomes a polycrystal. The crystal grain boundaries of this polycrystal act as pinning points and can increase the critical current density of the resulting superconducting thin film. In addition, by applying a magnetic field to the polycrystalline body, it is possible to increase the c-axis orientation of the crystal and increase the critical current density.

Below, one Embodiment of this invention is described based on FIG.
The present invention is applicable to the production of any high-temperature oxide superconductor formed by a coating pyrolysis method. Superconductors example as the current commercialized study has been bismuth oxide is _{Bi 2 Sr 2 CaCu 20x (Bi} -2212). Bi ₂ Sr ₂ Ca ₂ Cu _30y (Bi-2223), yttrium-based oxide YBa ₂ Cu ₃ O _7-6 (YBCO) and the like can be mentioned, but the present invention is limited in the types of these superconductors. is not. In this embodiment, the case where YBCO is used as a superconductor will be described below.

  As the substrate, various materials and shapes can be used. For example, as a material, metals such as copper, titanium, lead, and stainless steel, metal oxides such as alumina, zirconia, and titania, ceramics such as silicon carbide, and the like are used. Further, the shape can be a curved surface or a flat surface, and for example, any shape such as a plate shape, a rod shape, a coil shape, a fiber shape, a nonwoven fabric shape, and a tubular shape is possible. Moreover, a porous thing may be sufficient.

In this embodiment, a sapphire (α-Al ₂ O ₃ ) single crystal (R plane) substrate is used as the base material. The reason for this is that sapphire, which has a high thermal conductivity and thermal shock resistance and can be increased in area, is considered optimal as a substrate material for a current limiting device superconducting thin film.
The sapphire substrate 1 functions as a support for the superconducting thin film. In addition, when using a superconducting thin film on a superconductor substrate as a conductor for a current limiter for power equipment, a large amount of heat generated in the superconductor during the operation of the current limiter is quickly transferred to the cooling medium. Restore state. Therefore, a sapphire substrate with high thermal conductivity is preferable.

  However, sapphire not only causes a chemical reaction with the YBCO superconductor, but also has a different crystal structure and a large lattice mismatch, so that it is difficult to epitaxially grow YBCO directly on the sapphire substrate 1 in the c-axis direction. Therefore, ceria having an intermediate lattice constant between YBCO and sapphire is formed on the sapphire substrate 1 as the buffer layer 2 to alleviate lattice mismatch. At the same time, it suppresses chemical reactions. In the present embodiment, the ceria buffer layer 2 is formed to 100 nm on the sapphire substrate by vacuum deposition. Since the buffer layer 2 has a flat surface at the nanometer level, YBCO can be epitaxially grown on the buffer layer 2 in the c-axis direction. In the present invention, it is possible to manufacture a superconducting thin film without providing a buffer layer depending on the selection of the substrate material. The buffer layer 2 can be formed by a reactive vacuum deposition method, a sputtering method, or the like. However, in the present invention, the method for forming the buffer layer is not particularly limited, and the thickness thereof can be appropriately changed.

  Next, an organic salt (acetylacetonate) of the constituent metal elements Y, Ba, and Cu of the YBCO superconducting thin film is dissolved in a solvent to form a coating solution, which is coated on the buffer layer 2. Note that the solute of the coating solution is not limited to acetylacetonate, and may be any compound that forms an oxide in the subsequent baking step. In general, any compound that thermally decomposes at 1000 ° C. or less, particularly 200 to 900 ° C. may be used. Examples thereof include alkoxides, organic acid salts, inorganic acid salts, metal halides, hydroxides, oxides and the like. Specifically, naphthenic acid, 2-ethylhexanoic acid, caprylic acid, stearic acid, lauric acid, acetic acid, propionic acid, oxalic acid, citric acid, lactic acid, phenol, catechol, benzoic acid, salicylic acid, nitric acid, carbonic acid, hydrochloric acid Examples thereof include metal salts of organic acids or inorganic acids such as, and metal alkoxides of alcohols such as ethanol, propanol, butanol, ethylene glycol, glycerin, and 2-penten-4-one-2-ol.

  The solvent for the coating solution is not particularly limited as long as it can dissolve the above metal-containing compound, and the following various types can be used alone or in the form of a mixture. Examples of such solvents include hydrocarbons such as hexane, octane, benzene, toluene, and tetralin, alcohols such as methanol, ethanol, propanol, butanol, and amyl alcohol, ketones such as acetone, methyl ethyl ketone, and acetylacetone, and diesters. Ether cheeks such as butyl ether, aldehydes such as acetaldehyde and benzaldehyde, formic acid, acetic acid, propionic acid, butyric acid, caprylic acid, lauric acid, stearic acid, naphthenic acid, linoleic acid, oleic acid, oxalic acid, citric acid, lactic acid, phenol , Organic acids such as p-toluic acid, esters such as butyl butyrate, amines such as dimethylamine and aniline, amides such as N-methylacetamide and formamide, sulfur-containing compounds such as dimethyl sulfoxide, pyridine Heterocyclic substances such as furfural and the like. In addition, nitric acid aqueous solution, ammonia aqueous solution, water and the like can be mentioned. In this example, butanol, butyric acid, and furfural were used.

  The molar ratio of each metal element in the coating solution is Y: Ba: Cu = 1: 2: 3. Moreover, application | coating of an application | coating solution can be performed by normal methods, such as spin coating, screen printing, a dipping method, a spray method, a brush coating method, for example. In the present example, spin coating was performed. Next, the solvent is removed by heating and drying in air to form a coating film 3. The method for removing the solvent is not limited to heating / drying, and may be a combination of reduced pressure / drying or heating / drying, for example. Further, the solvent does not need to be completely removed and may remain somewhat.

Thereafter, a crystallization seed material solution is further applied to the surface of the coating film 3. The seed material is a fine powder of YBCO superconductor. The fine powder has a size of about 1 micron to submicron, and one particle is almost in a state of a single crystal. The fine powder can be obtained by mechanically pulverizing seed material crystals. A suitable seed material powder desirably has a circle-equivalent diameter of 0.001 to 1 μm in the surface direction when applied. Further, the seed material solution is preferably applied in a range of 10 to 95% in terms of the seed material surface coverage.
Next, the solvent is removed from the seed material coating by heating at an appropriate temperature, and the seed material is uniformly supported on the surface of the coating film to form the surface-side seed material layer 4. In the surface-side seed material layer 4, the seed material powder is arranged at an appropriate density, so that the gaps 4a are distributed at the same interval.

Next, the temperature of the substrate 1 is raised to, for example, 500 [° C.] at a predetermined speed as temporary baking, and is held in the atmosphere for a predetermined time. In the pre-baking, the intramolecular bonds of organic salts (acetylacetonato) of the constituent metal elements Y, Ba and Cu of the YBCO superconducting thin film are broken by thermal decomposition, and C and H are each oxidized by oxygen in the atmosphere to be CO, The gas is converted into CO ₂ and H ₂ O. These gases are removed into the atmosphere from the gap 4a of the fine powder seed material of the surface-side seed material layer 4 supported on the coating film 3. The seed material is applied. If the surface of the film 3 is covered without any gap, it is difficult to remove the gas, and pre-baking may not be performed, and as a result of the pre-baking, the coating film 3 has left the elements constituting the superconducting thin film. The thin film 3a is formed.

Thereafter, the substrate 1 is gradually cooled to room temperature and fired, for example, in the vicinity of 750 [° C.] under the flow of a mixed gas of an inert gas Ar and oxygen. In this step, it is desirable to precisely control the oxygen partial pressure and switch from 10 Pa to 10 ⁵ Pa in two stages. By performing the main baking, oxygen is taken into the thin film 3a and converted into a composite metal oxide, and a YBCO superconductor crystal is epitaxially grown on the ceria buffer layer 2 in the c-axis direction. In this case, crystallization proceeds in the direction of the surface of the thin film 3a using the buffer layer 2 on the substrate 1 as a seed. Furthermore, in the present invention, the seed material of the surface-side seed material layer 4 supported on the surface of the thin film 3a is used as a seed, and crystallization proceeds from the surface side toward the buffer layer 2 to obtain the YBCO superconducting thin film 5. As described above, crystallization proceeds with the buffer layer 2 on the substrate 1 as a seed at the time of main firing, and at the same time, crystallization proceeds so as to face the surface-side seed material layer 4 on the surface of the thin film 3a. Crystallization is completed early. Further, on the surface side of the superconducting thin film 5, a polycrystalline body 6a crystallized using the seed material of the seed material layer 4 as a seed is generated, and the crystal grain boundary has an effect of performing a pinning action together with the seed material 4a. .

  In the above embodiment, a series of steps for forming a coating film, forming a surface-side seed material layer, pre-baking, and forming a superconducting thin film by main baking has been described. It is also possible to increase the thickness of the conductive thin film. In this case, the surface-side seed material layer may be formed every time the superconducting thin film is formed, and only on the coating film when the final superconducting thin film is formed. You may make it provide in.

  As mentioned above, although this invention was demonstrated based on the said embodiment, this invention is not limited to the content of the said description, Of course, it can change in the range which does not deviate from this invention.

Hereinafter, an embodiment of the present invention will be described in comparison with a comparative example.
A YBCO superconducting thin film was manufactured based on the conditions and steps described in the above embodiment. The powder seed material used for the surface-side seed material layer was adjusted to an equivalent circle diameter of 0.1 μm and applied to the coating film surface with a surface coverage of 50%. The thickness of the superconducting thin film obtained as a result of the production was 0.25 μm, and the holding time required for the main firing was 1 hour. Moreover, the critical current density of the obtained superconducting thin film was 1.2 MA / cm ² .

(Comparative example)
As a comparative example, the coating pyrolysis method was performed in the same manner as in the above embodiment except that the surface-side seed material layer was not provided.
That is, the same substrate, buffer layer, and coating film as those in the above embodiment were used. Then, temporary baking was performed by the same method as the Example. Next, the main firing was performed in the same manner as in the embodiment, but crystallization proceeded only in the direction of the coating film surface using the buffer layer on the substrate as a seed. It took a long time to complete the crystallization, and the holding time required for the main baking was longer than that of the example and was 2 hours. Moreover, the film thickness of the obtained superconducting thin film was 0.23 μm. Furthermore, the critical current density was 0.8 MA / cm ² , which was lower than that of the example.

  The reason for the difference in critical current density is estimated as follows. That is, the crystal using the buffer layer as a seed is epitaxially grown substantially in the c-axis direction, but the crystal using the surface-side seed material as a seed is a polycrystal. Therefore, it is considered that the grain boundary acts as a pinning point and the critical current density is increased.

It is process drawing which shows the manufacturing method of one Embodiment of this invention. It is process drawing which shows the conventional manufacturing method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Sapphire substrate 2 Buffer layer 3 Coating film 4 Surface side seed material layer 5 YBCO superconducting thin film

Claims (6)

  In the coating pyrolysis method in which a coating film is provided on the upper layer of the base material and then calcined and then subjected to main firing to make the coating film a superconducting thin film, it is provided on the base material upper layer before the temporary firing. A surface-side seed material layer is provided on the coating film surface side, and the superconducting thin film is obtained by crystallizing the coating film from the base material side and the surface-side seed material layer, respectively, during the main baking. A method for producing a superconducting thin film.   The method for producing a superconducting thin film according to claim 1, wherein a buffer layer made of a base material-side seed material is interposed between the base material and the coating film.   The seed material of the said surface side seed material layer consists of the same composition as the superconducting thin film obtained by this baking, The manufacturing method of the superconducting thin film of Claim 1 or 2 characterized by the above-mentioned.   The method for producing a superconducting thin film according to claim 2, wherein the seed material of the surface-side seed material layer and the base material-side seed material are made of the same material.   The method for producing a superconducting thin film according to any one of claims 1 to 4, wherein in the surface-side seed material layer, gaps larger than the size of gas molecules generated during main firing are distributed in the plane direction. .   The method for producing a superconducting thin film according to any one of claims 1 to 5, wherein the surface-side seed material layer is obtained by applying seed material powder to the surface of the coating film.

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