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WO2019176169A1 - Superconducting thin-film wire material and production method for superconducting thin-film wire material - Google Patents

Superconducting thin-film wire material and production method for superconducting thin-film wire material Download PDF

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WO2019176169A1
WO2019176169A1 PCT/JP2018/042125 JP2018042125W WO2019176169A1 WO 2019176169 A1 WO2019176169 A1 WO 2019176169A1 JP 2018042125 W JP2018042125 W JP 2018042125W WO 2019176169 A1 WO2019176169 A1 WO 2019176169A1
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mgb
thin film
superconducting
wire
temperature
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French (fr)
Japanese (ja)
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拓夢 岩中
楠　敏明
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株式会社日立製作所
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    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B35/00—Boron; Compounds thereof
    • C01B35/08—Compounds containing boron and nitrogen, phosphorus, oxygen, sulfur, selenium or tellurium
    • C01B35/10—Compounds containing boron and oxygen
    • C01B35/12—Borates
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G1/00—Methods of preparing compounds of metals not covered by subclasses C01B, C01C, C01D, or C01F, in general
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
    • H01B12/00—Superconductive or hyperconductive conductors, cables, or transmission lines
    • H01B12/02—Superconductive or hyperconductive conductors, cables, or transmission lines characterised by their form
    • H01B12/06—Films or wires on bases or cores
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
    • H01B13/00—Apparatus or processes specially adapted for manufacturing conductors or cables
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F6/00—Superconducting magnets; Superconducting coils
    • H01F6/06—Coils, e.g. winding, insulating, terminating or casing arrangements therefor
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/60—Superconducting electric elements or equipment; Power systems integrating superconducting elements or equipment

Definitions

  • the present invention relates to a superconducting wire using MgB 2 (magnesium diboride) and a method for producing the same.
  • a superconducting magnet system of 4.2 K operation for example, a nuclear magnetic resonance apparatus (NMR), a magnetic resonance imaging apparatus (MRI), a magnetic levitation railway (Maglev Railway)
  • NMR nuclear magnetic resonance apparatus
  • MRI magnetic resonance imaging apparatus
  • Magnlev Railway magnetic levitation railway
  • the superconducting wire for constructing the superconducting magnet is a long wire (for example, 1 km or more), can maintain a high current density even in the high magnetic field generated by the superconducting magnet itself, The wire is required to withstand. From this point of view, MgB 2 superconductor itself is a relatively new material and is still under development, so MgB 2 superconducting wire has various research and development in the production method of long wire, improvement of superconducting properties and mechanical properties. Has been done.
  • the powder-in-tube (PIT) method is a process that uses raw material powder (mixed powder of Mg (magnesium) powder and B (boron) powder or MgB 2 powder, and further mixed with a third element) as metal. This is a method in which after the tube is filled and drawn, heat treatment (usually 600 ° C. or higher) is performed to generate and sinter the superconducting phase.
  • the PIT method is advantageous for the production of a long wire, but the MgB 2 superconducting wire produced by the PIT method generally has a weak point in terms of superconducting properties.
  • a method for manufacturing a superconducting device such as a Josephson element
  • a vacuum process thin film process
  • the MgB 2 superconducting thin film produced by the vacuum process has the advantage of exhibiting a J c (critical current density) characteristic that is one order of magnitude higher in a 4.2 K magnetic field than the MgB 2 superconducting wire produced by the PIT method.
  • Jc critical current density
  • Patent Document 1 discloses a laminate in which a metal substrate, a three-fold symmetric MgO (111) layer formed on the metal substrate by an ion beam assist (IBAD) method, and an MgB two layer are laminated.
  • a superconducting conductor characterized by comprising a body is described. Further, it is disclosed that this superconducting conductor has an anti-diffusion layer interposed between the metal substrate and the MgO (111) layer.
  • MgB 2 is crystallized with good crystal orientation by forming MgB 2 layer on the three-fold symmetric MgO (111) layer in which MgO (111) is oriented in the normal direction of the base material. It is said that it can be laminated.
  • the superconducting conductor described in Patent Document 1 uses an element other than Mg and B for the MgO (111) layer for epitaxially growing the MgB 2 thin film and the diffusion preventing layer. Thereby, there is a problem that the manufacturing apparatus is complicated and the manufacturing process is increased (that is, the manufacturing cost is increased).
  • the object of the present invention is to achieve good superconducting properties (eg in a 20 K magnetic field) without complicating the manufacturing equipment and without increasing the manufacturing process (ie without significantly increasing the manufacturing cost).
  • An object of the present invention is to provide a superconducting thin film wire having a balance between a critical current density and excellent mechanical properties.
  • One aspect of the present invention is a superconducting thin film wire in which an MgB 2 thin film is formed on a metal substrate, wherein the MgB 2 thin film is a first MgB 2 thin film formed on the metal substrate. And a second MgB 2 thin film laminated thereon, and the B content of the first MgB 2 thin film is smaller than the B content of the second MgB 2 thin film.
  • the present invention can add the following improvements and changes to the superconducting thin film wire (I).
  • the average crystal grain size of the first MgB 2 thin film is smaller than the average crystal grain size of the second MgB 2 film.
  • a thin oxide layer is interposed between the first MgB 2 thin film and the second MgB 2 thin film.
  • Another aspect of the present invention is a method for producing an MgB 2 superconducting thin film wire, the step of forming a first MgB 2 thin film on a metal substrate at a first temperature, and the first superconducting thin film of a step of forming a second MgB 2 thin film at a second temperature on the MgB 2 thin film has a first temperature being lower than said second temperature
  • the manufacturing method of a wire is provided.
  • the present invention can be improved or changed as follows in the method (II) for producing a superconducting thin film wire.
  • the B content of the first MgB 2 thin film is smaller than the B content of the second MgB 2 thin film.
  • the average crystal grain size of the first MgB 2 thin film is smaller than the average crystal grain size of the second MgB 2 film.
  • the present invention it is possible to provide a superconducting thin film wire in which good superconducting characteristics and excellent mechanical characteristics are balanced in a well-balanced manner without complicating the manufacturing apparatus and increasing the manufacturing process.
  • FIG. 3A Is an electron microscopic image showing the microstructure of the second MgB 2 thin film on the surface of the superconducting thin film wire according to the present invention.
  • FIG. 4A It is a schematic diagram of FIG. 4A. It is a graph which shows an example of the relationship between the critical current density and the allowable tensile strain of the superconducting thin film wire according to the present invention.
  • the MgB 2 thin film is improved in crystallinity as it is formed at a higher temperature, and the superconducting properties such as critical temperature (T c ) and critical current density (J c ) are improved.
  • T c critical temperature
  • J c critical current density
  • the present inventors further investigated and studied the relationship between the film formation and properties of the MgB 2 thin film, and obtained the following knowledge.
  • the allowable tensile strain tends to decrease as the B content in the entire MgB 2 thin film increases.
  • the B content strongly depends on the deposition rate ratio of Mg and B and the film formation temperature, and tends to increase as the deposition rate ratio of B to Mg increases and as the film formation temperature increases.
  • the influence of the film formation temperature is considered to be because the amount of re-evaporation / re-evaporation rate of Mg atoms increases rapidly as the film formation temperature increases, making it difficult for Mg atoms to adhere / remain on the substrate.
  • Mg has high chemical reactivity, it becomes easier to generate a reaction phase (impurity phase) other than the MgB 2 phase by chemically reacting with a base material component (for example, Cu) as the film is formed at a higher temperature.
  • a reaction phase impurity phase
  • a base material component for example, Cu
  • MgB 2 thin films deposited at low temperatures have low crystallinity and low superconducting properties such as critical temperature and critical current density, but it has been found that the allowable tensile strain is higher than expected. This is because in addition to the crystal grain size of the MgB 2 thin film becoming smaller, the re-evaporation amount / re-evaporation rate of Mg atoms decreases as the film formation temperature decreases, and the B content in the entire MgB 2 thin film decreases. This is probably because
  • the film formation at a low temperature reduces the chemical reaction between the base material component and Mg, thereby lowering the probability that the formation / growth of MgB 2 crystals is hindered and unwanted impurity phases are mixed. .
  • the present inventors have invented an MgB 2 superconducting thin film wire having a novel structure as shown in FIG. 2 in order to solve the above-mentioned new problems by comprehensively considering the above knowledge and considerations. .
  • FIG. 2 is a schematic diagram showing the structure of a cross section (cross section perpendicular to the surface of the metal substrate) of the superconducting thin film wire according to the present invention.
  • the schematic diagram of FIG. 2 is an image diagram of an image obtained by observing a cross section (a surface perpendicular to the surface of the metal substrate 2) of the manufactured MgB 2 superconducting thin film wire with a scanning electron microscope (SEM).
  • SEM scanning electron microscope
  • the method for producing a MgB 2 superconducting thin film wire basically includes a step of forming a first MgB 2 thin film 8a on a metal substrate 2 at a relatively low temperature, and subsequently the first step. It performs a process of relatively high temperature deposition forms a second MgB 2 thin film 8b on the MgB 2 thin film 8a.
  • the chemical reaction between the metal substrate 2 and Mg is prevented, and as a result, an unwanted impurity phase Can be prevented from being mixed or the crystal growth of the second MgB 2 thin film 8b can be inhibited.
  • a relatively low temperature for example, 200 ° C. or more and less than 290 ° C.
  • the second MgB 2 thin film 8b is formed at a relatively high temperature (eg, 290 ° C. or higher), the crystallinity of the MgB 2 thin film is improved, and superconducting properties such as critical temperature and critical current density are excellent. Can be demonstrated.
  • a relatively high temperature eg, 290 ° C. or higher
  • the first MgB 2 thin film 8a serves to alleviate the difference in allowable tensile strain between the metal substrate 2 and the second MgB 2 thin film 8b, which occurs when tensile stress is applied to the wire. And mechanical properties can be improved.
  • second MgB 2 thin film 8b improves the superconducting properties of the whole superconducting wire with the inhibitor for the formation of high crystallinity second MgB 2 thin film 8b is first MgB 2 thin film 8a prevent .
  • FIG. 1 is a schematic diagram showing an example of a vapor deposition apparatus for producing a MgB 2 superconducting thin film wire which is a superconducting thin film wire according to the present invention.
  • the vapor deposition apparatus generally includes a first MgB 2 thin film 8a on a metal substrate 2 at a first temperature (relatively low temperature, for example, 200 ° C. or more and less than 290 ° C.).
  • the first vacuum chamber 1a and the second vacuum chamber 1b are divided by a partition 10 and divided into a low temperature film formation region and a high temperature film formation region. Both are evacuated by a vacuum pump 7.
  • the metal substrate 2 is conveyed by the reel 3 from the first vacuum chamber 1a to the second vacuum chamber 1b.
  • the metal substrate 2 is fed to the film forming region in the first vacuum chamber 1a by the reel 3, and heated to a first temperature (relatively low temperature, for example, 200 ° C. or more and less than 290 ° C.) by the first heater 4a. The Then, Mg vapor and B vapor heated and evaporated from the first Mg vapor deposition source 5a and the first B vapor deposition source 6a, respectively, and the first MgB 2 thin film 8a is ejected onto the heated metal substrate 2. Are co-evaporated.
  • the metal base material 2 on which the first MgB 2 thin film 8a is formed is transported to the second vacuum chamber 1b by the reel 3, and is moved to a second temperature (relatively high temperature, for example, by the second heater 4b). 290 ° C or higher).
  • a second temperature relatively high temperature, for example, by the second heater 4b.
  • heated and evaporated Mg vapor and B vapor are ejected from the second Mg vapor deposition source 5b and formed on the first MgB 2 thin film 8a previously formed.
  • the MgB 2 thin film 8b is co-evaporated. That is, the film formation temperature of the first MgB 2 thin film 8a is lower than the film formation temperature of the second MgB 2 thin film 8b.
  • the first temperature (deposition temperature) for forming the first MgB 2 thin film 8a is set to 270 ° C.
  • a first MgB 2 thin film 8a having a thickness of 1 ⁇ m is formed on the metal substrate 2 as shown in FIG. 2, and a second MgB 2 thin film 8b having a thickness of 10 ⁇ m is formed thereon.
  • a film-formed MgB 2 thin film wire was obtained.
  • the microstructure of the obtained MgB 2 thin film wire (cross section perpendicular to the metal substrate surface) was observed using a scanning transmission electron microscope-energy dispersive X-ray analyzer (STEM-EDX). It was confirmed that a region having a high oxygen atom concentration (a thin layer considered to be an oxide) was interposed between the first MgB 2 thin film 8a and the second MgB 2 thin film 8b.
  • the reason for the formation of the oxide layer is that the growth of the MgB 2 thin film is temporarily interrupted between the step of forming the first MgB 2 thin film 8a and the step of forming the second MgB 2 thin film 8b.
  • Oxide particles in which the extreme surface of the first MgB 2 thin film 8a is oxidized by slightly remaining oxygen in the chamber and / or the excess Mg vapor floating in the vacuum chamber and slightly remaining oxygen are combined. It is conceivable that the first MgB 2 thin film 8a is deposited and deposited on the surface.
  • FIG. 3A is an electron microscope observation image showing the microstructure of the surface of the first MgB 2 thin film of the superconducting thin film wire according to the present invention.
  • the first MgB 2 2 is an image obtained by observing the surface of the thin film 8a with an SEM.
  • FIG. 3B is a schematic diagram of FIG. 3A.
  • 4A is an SEM observation image showing the microstructure of the surface of the second MgB 2 thin film of the superconducting thin film wire according to the present invention
  • FIG. 4B is a schematic diagram of FIG. 4A.
  • the average grain size of the MgB 2 crystal 9 of the first MgB 2 thin film 8a was about 100 nm, whereas that of the second MgB 2 thin film 8b
  • the average crystal grain size of MgB 2 crystal 9 was about 150 nm. This confirms that the average crystal grain size of the first MgB 2 thin film 8a is smaller than the average crystal grain size of the second MgB 2 thin film 8b.
  • the molar ratio between the amount of Mg vapor and the amount of B vapor during film formation of the MgB 2 thin film was controlled to be constant. From this, it is considered that the difference in the B content between the first MgB 2 thin film 8a and the second MgB 2 thin film 8b is caused by the difference in film formation temperature.
  • the first MgB 2 thin film 8a has a film formation temperature (here, 270 ° C.) lower than the film formation temperature of the second MgB 2 thin film 8b (here, 300 ° C.). It is considered that the B content of the MgB 2 thin film 8a was smaller than the B content of the second MgB 2 thin film 8b.
  • FIG. 5 is a graph showing an example of the relationship between the critical current density and the allowable tensile strain of the superconducting thin film wire according to the present invention. 5 shows, as a comparative sample, without forming the first MgB 2 thin film 8a on the metal substrate 2, MgB 2 thin film a second MgB 2 thin film 8b is directly deposited formed on the metal substrate 2 The wire data is also shown.
  • the MgB 2 superconducting thin film wire (Example) of the present invention has both improved J c value and allowable tensile strain compared to the comparative sample in which the first MgB 2 thin film 8a was not formed. is doing. Specifically, the examples of the present invention exhibit good J c characteristics (3800 A / mm 2 ) and excellent mechanical characteristics (allowable tensile strain 0.20%) in a temperature of 20 K and an external magnetic field of 5 T. It was confirmed.
  • the sample formed by forming both the first MgB 2 thin film 8a and the second MgB 2 thin film 8b at a high temperature (300 ° C.) is a comparative sample in FIG. It was confirmed separately that it showed the same characteristics as.
  • a sample in which the first MgB 2 thin film 8a and the second MgB 2 thin film 8b are both formed at a low temperature (270 ° C.) shows an allowable tensile strain equivalent to that of the example in FIG. It was separately confirmed that the c value was lower than that of the comparative sample.
  • the first MgB 2 thin film 8a is formed at a relatively low temperature, and the second MgB 2 thin film 8b is formed thereon at a relatively high temperature, so that an unwanted impurity phase is formed.
  • There prevents or inhibits the production or crystal growth of the second MgB 2 thin film 8b, to improve the J c values of the whole MgB 2 thin wire, and the metal substrate 2 and the second MgB 2 It is confirmed that the difference in the allowable tensile strain from the thin film 8b is relaxed and the mechanical properties are improved.
  • the manufacturing method of the present invention forms the first MgB 2 thin film 8a and the second MgB 2 thin film 8b in succession without removing them from the vacuum chamber, so that it is as simple as the conventional manufacturing method. It has also been confirmed that it is possible to manufacture by a simple process and that the increase in equipment cost can be minimized (that is, the manufacturing cost is not significantly increased).

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Abstract

The purpose of the present invention is to provide a superconducting thin-film wire material that has well-balanced good critical current density in a 20-K magnetic field and superior mechanical characteristics, without involving complication of a production device and an increase in production processes. The superconducting thin-film wire material according to the present invention comprises a MgB2 thin film formed on a metal substrate, and is characterized in that the MgB2 thin film has a two-layer structure including a first MgB2 thin film formed on the metal substrate and a second MgB2 thin film layered on the first MgB2 thin film, and the content rate of B in the first MgB2 thin film is lower than that in the second MgB2 thin film.

Description

超電導薄膜線材及び超電導薄膜線材の製造方法Superconducting thin film wire and method for producing superconducting thin film wire
　本発明はMgB2（二硼化マグネシウム）を用いた超電導線材及びその製造方法に関する。 The present invention relates to a superconducting wire using MgB 2 (magnesium diboride) and a method for producing the same.
　MgB2超電導体は、金属系超電導体としては高い臨界温度（Tc＝39K）を有し、液体ヘリウムフリー（例えば10～20 K）で運転する超電導電磁石を実現しうる超電導材料として期待されている。また、MgB2超電導体を4.2 K運転の超電導マグネットシステム（例えば、核磁気共鳴装置（NMR）、磁気共鳴画像装置（MRI）、磁気浮上式鉄道（Maglev Railway））の超電導電磁石に適用すれば、温度マージン（臨界温度と運転温度との差）を従来よりも大きくできるので、クエンチが生じにくく、熱的安定性の高い超電導マグネットシステムが実現可能となる。 MgB 2 superconductor has high critical temperature (T c = 39K) as a metallic superconductor, and is expected as a superconducting material that can realize a superconducting electromagnet operating in liquid helium free (eg 10-20 K). Yes. In addition, if the MgB 2 superconductor is applied to a superconducting magnet system of 4.2 K operation (for example, a nuclear magnetic resonance apparatus (NMR), a magnetic resonance imaging apparatus (MRI), a magnetic levitation railway (Maglev Railway)), Since the temperature margin (difference between the critical temperature and the operating temperature) can be made larger than before, quenching hardly occurs and a superconducting magnet system with high thermal stability can be realized.
　超電導電磁石を構成するための超電導線材は、長尺線材（例えば1 km以上の長さ）であること、超電導電磁石自身が発生する高磁場中でも高い電流密度を維持できること、通電中に生じる電磁力に線材が耐えることが求められる。この観点において、MgB2超電導体自体が比較的新しい材料であり未だ開発途上であることから、MgB2超電導線材は、長尺線材の製造方法、超電導特性や機械的特性の向上において様々な研究開発が行われている。 The superconducting wire for constructing the superconducting magnet is a long wire (for example, 1 km or more), can maintain a high current density even in the high magnetic field generated by the superconducting magnet itself, The wire is required to withstand. From this point of view, MgB 2 superconductor itself is a relatively new material and is still under development, so MgB 2 superconducting wire has various research and development in the production method of long wire, improvement of superconducting properties and mechanical properties. Has been done.
　MgB2超電導線材の研究開発は、従来から、長尺線材を製造することを前提としてパウダー・イン・チューブ法で作製する超電導線材を対象とすることが多かった。パウダー・イン・チューブ（PIT）法とは、原料粉末（Mg（マグネシウム）粉末とB（硼素）粉末との混合粉末またはMgB2粉末、更にはそれらに第三元素を添加した混合粉末）を金属管に充填し、伸線加工した後に、超電導相を生成・焼結するための熱処理（通常600℃以上）を施す方法である。PIT法は長尺線材の製造に有利であるが、PIT法で作製したMgB2超電導線材は一般的に超電導特性の観点で弱点を有する。 Conventionally, research and development of MgB 2 superconducting wire has been focused on superconducting wire produced by the powder-in-tube method on the premise of producing a long wire. The powder-in-tube (PIT) method is a process that uses raw material powder (mixed powder of Mg (magnesium) powder and B (boron) powder or MgB 2 powder, and further mixed with a third element) as metal. This is a method in which after the tube is filled and drawn, heat treatment (usually 600 ° C. or higher) is performed to generate and sinter the superconducting phase. The PIT method is advantageous for the production of a long wire, but the MgB 2 superconducting wire produced by the PIT method generally has a weak point in terms of superconducting properties.
　一方、ジョセフソン素子などの超電導デバイスの製造方法として、真空プロセス（薄膜プロセス）を利用した方法がある。真空プロセスで作製したMgB2超電導薄膜は、PIT法で作製したMgB2超電導線材に比して、4.2 K磁場中で1桁以上高いJc（臨界電流密度）特性を示すという利点を有するが、従来は、真空プロセスであるが故に長尺線材の製造は困難という弱点を有していた。ところが、近年、酸化物超電導体において真空プロセスを利用した長尺線材の製造技術が進展したことにより、MgB2超電導体においても高い臨界電流密度（Jc）特性を有する薄膜線材が期待されるようになってきた。 On the other hand, as a method for manufacturing a superconducting device such as a Josephson element, there is a method using a vacuum process (thin film process). The MgB 2 superconducting thin film produced by the vacuum process has the advantage of exhibiting a J c (critical current density) characteristic that is one order of magnitude higher in a 4.2 K magnetic field than the MgB 2 superconducting wire produced by the PIT method. Conventionally, since it was a vacuum process, it had the weak point that manufacture of a long wire was difficult. However, in recent years, the progress in manufacturing technology for long wires using vacuum processes in oxide superconductors has led to expectations for thin-film wires with high critical current density ( Jc ) characteristics in MgB 2 superconductors. It has become.
　MgB2超電導薄膜の超電導特性を向上させるためには、MgB2結晶の結晶性の向上が有効である。結晶性の向上のためには、MgB2と格子定数や結晶構造が近い物質を基材上に結晶配向させて形成し、その上にMgB2膜をエピタキシャル成長させる方法などの種々の技術が報告されている。 In order to improve the superconducting properties of the MgB 2 superconducting thin film, it is effective to improve the crystallinity of the MgB 2 crystal. For improving the crystallinity of the MgB 2 and the lattice constant and the crystal structure is close material formed by crystal orientation on a substrate, various techniques such as a method of epitaxially growing a MgB 2 film is reported thereon ing.
　例えば、特許文献１には、金属基材と、その金属基材上にイオンビームアシスト（IBAD）法により形成された3回対称MgO（111）層と、MgB2層と、が積層された積層体よりなることを特徴とする超電導導体が記載されている。また、この超電導導体は、金属基材上とMgO（111）層との間に、拡散防止層が介在されてなることが開示されている。この超電導導体では、MgO（111）が基材法線方向に向いた3回対称MgO（111）層上に、MgB2層を形成することにより、MgB2を良好な結晶配向性で結晶化し、積層することができるとされている。3回対称MgO（111）面のMg面とMgB2（111）面のMg面は同じ三角格子であり、Mg原子間距離もほぼ等しいためにエピタキシャル成膜が可能となる。これにより、臨界電流密度が高く、超電導特性の良好な超電導導体を提供することが出来ることが記載されている。 For example, Patent Document 1 discloses a laminate in which a metal substrate, a three-fold symmetric MgO (111) layer formed on the metal substrate by an ion beam assist (IBAD) method, and an MgB two layer are laminated. A superconducting conductor characterized by comprising a body is described. Further, it is disclosed that this superconducting conductor has an anti-diffusion layer interposed between the metal substrate and the MgO (111) layer. In this superconducting conductor, MgB 2 is crystallized with good crystal orientation by forming MgB 2 layer on the three-fold symmetric MgO (111) layer in which MgO (111) is oriented in the normal direction of the base material. It is said that it can be laminated. Since the Mg plane of the three-fold symmetric MgO (111) plane and the Mg plane of the MgB 2 (111) plane are the same triangular lattice, and the distance between Mg atoms is substantially equal, epitaxial film formation is possible. Thus, it is described that a superconducting conductor having a high critical current density and excellent superconducting characteristics can be provided.
特開2010-287475号公報JP 2010-287475 A
　特許文献１に記載されている超電導導体は、MgB2薄膜をエピタキシャル成長させるMgO（111）層や、拡散防止層にMgとB以外の元素を用いている。これにより、製造装置の複雑化や製造プロセスの増加（すなわち、製造コストが増加する）という課題がある。 The superconducting conductor described in Patent Document 1 uses an element other than Mg and B for the MgO (111) layer for epitaxially growing the MgB 2 thin film and the diffusion preventing layer. Thereby, there is a problem that the manufacturing apparatus is complicated and the manufacturing process is increased (that is, the manufacturing cost is increased).
　したがって、本発明の目的は、製造装置を複雑化せず、かつ製造プロセスを増加せずに（すなわち、製造コストを大幅に増加せずに）、良好な超電導特性（例えば、20 K磁場中の臨界電流密度）と優れた機械的特性とをバランスよく両立させた超電導薄膜線材を提供することにある。 Therefore, the object of the present invention is to achieve good superconducting properties (eg in a 20 K magnetic field) without complicating the manufacturing equipment and without increasing the manufacturing process (ie without significantly increasing the manufacturing cost). An object of the present invention is to provide a superconducting thin film wire having a balance between a critical current density and excellent mechanical properties.
　（I）本発明の一態様は、金属基材上にMgB2薄膜が形成された超電導薄膜線材であって、前記MgB2薄膜は、前記金属基材上に形成された第１のMgB2薄膜と、その上に積層された第２のMgB2薄膜との２層構造を有し、前記第１のMgB2薄膜のB含有率が前記第２のMgB2薄膜のB含有率よりも小さことを特徴とする超電導薄膜線材、を提供するものである。 (I) One aspect of the present invention is a superconducting thin film wire in which an MgB 2 thin film is formed on a metal substrate, wherein the MgB 2 thin film is a first MgB 2 thin film formed on the metal substrate. And a second MgB 2 thin film laminated thereon, and the B content of the first MgB 2 thin film is smaller than the B content of the second MgB 2 thin film A superconducting thin film wire characterized by the above is provided.
　本発明は、上記の超電導薄膜線材（I）において、以下のような改良や変更を加えることができる。
（i）前記第１のMgB2薄膜の平均結晶粒径は、前記第２のMgB2薄膜の平均結晶粒径よりも小さい。
（ii）前記第１のMgB2薄膜と前記第２のMgB2薄膜との間に酸化物の薄層が介在している。 The present invention can add the following improvements and changes to the superconducting thin film wire (I).
(I) the average crystal grain size of the first MgB 2 thin film is smaller than the average crystal grain size of the second MgB 2 film.
(Ii) A thin oxide layer is interposed between the first MgB 2 thin film and the second MgB 2 thin film.
　（II）本発明の他の一態様は、MgB2超電導薄膜線材の製造方法であって、金属基材上に第１の温度で第１のMgB2薄膜を成膜する工程と、前記第１のMgB2薄膜の上に第２の温度で第２のMgB2薄膜を成膜する工程と、を有し、前記第１の温度が前記第２の温度よりも低いことを特徴とする超電導薄膜線材の製造方法、を提供するものである。 (II) Another aspect of the present invention is a method for producing an MgB 2 superconducting thin film wire, the step of forming a first MgB 2 thin film on a metal substrate at a first temperature, and the first superconducting thin film of a step of forming a second MgB 2 thin film at a second temperature on the MgB 2 thin film has a first temperature being lower than said second temperature The manufacturing method of a wire is provided.
　本発明は、上記の超電導薄膜線材の製造方法（II）において、以下のような改良や変更を加えることができる。
（iii）前記第１のMgB2薄膜のB含有率は、前記第２のMgB2薄膜のB含有率よりも小さい。
（iv）前記第１のMgB2薄膜の平均結晶粒径は、前記第２のMgB2薄膜の平均結晶粒径よりも小さい。 The present invention can be improved or changed as follows in the method (II) for producing a superconducting thin film wire.
(Iii) The B content of the first MgB 2 thin film is smaller than the B content of the second MgB 2 thin film.
(Iv) the average crystal grain size of the first MgB 2 thin film is smaller than the average crystal grain size of the second MgB 2 film.
　本発明によれば、製造装置を複雑化せず、かつ製造プロセスを増加せずに、良好な超電導特性と優れた機械的特性とをバランスよく両立させた超電導薄膜線材を提供することができる。 According to the present invention, it is possible to provide a superconducting thin film wire in which good superconducting characteristics and excellent mechanical characteristics are balanced in a well-balanced manner without complicating the manufacturing apparatus and increasing the manufacturing process.
本発明に係る超電導薄膜線材を作製する蒸着装置の一例を示す概略模式図である。It is a schematic diagram which shows an example of the vapor deposition apparatus which produces the superconducting thin film wire which concerns on this invention. 本発明に係る超電導薄膜線材の断面（金属基材表面に垂直な断面）の構造を示す模式図である。It is a schematic diagram which shows the structure of the cross section (cross section perpendicular | vertical to the metal base-material surface) of the superconducting thin film wire which concerns on this invention. 本発明に係る超電導薄膜線材の第１のMgB2薄膜の表面の微細組織を示す電子顕微鏡観察像である。Is an electron microscopic image showing the microstructure of a first MgB 2 thin film on the surface of the superconducting thin film wire according to the present invention. 図３Ａの模式図である。It is a schematic diagram of FIG. 3A. 本発明に係る超電導薄膜線材の第２のMgB2薄膜の表面の微細組織を示す電子顕微鏡観察像である。Is an electron microscopic image showing the microstructure of the second MgB 2 thin film on the surface of the superconducting thin film wire according to the present invention. 図４Ａの模式図である。It is a schematic diagram of FIG. 4A. 本発明に係る超電導薄膜線材の臨界電流密度と許容引張ひずみとの関係の一例を示すグラフである。It is a graph which shows an example of the relationship between the critical current density and the allowable tensile strain of the superconducting thin film wire according to the present invention.
　以下、本発明に係る実施形態について、図面を参照しながら説明する。 Hereinafter, embodiments according to the present invention will be described with reference to the drawings.
　MgB2薄膜は、高温で成膜するほど結晶性が向上し、臨界温度（Tc）や臨界電流密度（Jc）などの超電導特性が向上することが、知られている。本発明者等は、MgB2薄膜に関する様々な研究から、MgB2薄膜の結晶性を高めると、許容引張ひずみが低下してクラックが生じ易くなり、超電導線材としての機械的特性が低下するという新たな知見を今回得た。これは、MgB2薄膜を超電導線材として利用・実用化する上での新たな解決すべき課題と考えられた。 It is known that the MgB 2 thin film is improved in crystallinity as it is formed at a higher temperature, and the superconducting properties such as critical temperature (T c ) and critical current density (J c ) are improved. From various studies on the MgB 2 thin film, the present inventors have found that when the crystallinity of the MgB 2 thin film is increased, the allowable tensile strain is decreased and cracks are easily generated, and the mechanical properties as a superconducting wire are decreased. This time, I got a new knowledge. This was considered to be a new problem to be solved in using and commercializing MgB 2 thin film as a superconducting wire.
　そこで、本発明者等は、MgB2薄膜の成膜と性状との関係を更に調査・研究し、次のような知見を得た。 Therefore, the present inventors further investigated and studied the relationship between the film formation and properties of the MgB 2 thin film, and obtained the following knowledge.
　MgB2薄膜は、Bが非常に硬く脆い物質のため、MgB2薄膜全体に占めるB含有率が大きくなるほど許容引張ひずみが低下する傾向がある。B含有率は、MgおよびBの蒸着レート比と成膜温度とに強く依存し、Mgに対するBの蒸着レート比が大きくなるほど、また成膜温度が高くなるほど大きくなる傾向がある。成膜温度の影響は、成膜温度が高くなるほどMg原子の再蒸発量／再蒸発率が急激に増大し、Mg原子が基材上に付着／残存しにくくなるためと考えられる。 In the MgB 2 thin film, since B is a very hard and brittle substance, the allowable tensile strain tends to decrease as the B content in the entire MgB 2 thin film increases. The B content strongly depends on the deposition rate ratio of Mg and B and the film formation temperature, and tends to increase as the deposition rate ratio of B to Mg increases and as the film formation temperature increases. The influence of the film formation temperature is considered to be because the amount of re-evaporation / re-evaporation rate of Mg atoms increases rapidly as the film formation temperature increases, making it difficult for Mg atoms to adhere / remain on the substrate.
　さらに、Mgは、化学反応性が高いことから、高温で成膜するほど基材成分（例えばCuなど）と化学反応して、MgB2相以外の反応相（不純物相）を生成し易くなる。その結果、MgB2結晶の生成・成長が阻害されたり、望まない不純物相が混入したりすることで、超電導特性が低下することも分かってきた。 Furthermore, since Mg has high chemical reactivity, it becomes easier to generate a reaction phase (impurity phase) other than the MgB 2 phase by chemically reacting with a base material component (for example, Cu) as the film is formed at a higher temperature. As a result, it has also been found that the superconducting properties deteriorate due to the inhibition of the formation and growth of MgB 2 crystals and the mixing of unwanted impurity phases.
　一方、低温で成膜したMgB2薄膜は、結晶性が低く、臨界温度や臨界電流密度などの超電導特性は低いが、許容引張ひずみは想像以上に高いことが分かってきた。これは、MgB2薄膜の結晶粒径が小さくなることに加えて、成膜温度が低くなるほどMg原子の再蒸発量／再蒸発率が低下し、MgB2薄膜全体に占めるB含有率が低下するためと考えられる。 On the other hand, MgB 2 thin films deposited at low temperatures have low crystallinity and low superconducting properties such as critical temperature and critical current density, but it has been found that the allowable tensile strain is higher than expected. This is because in addition to the crystal grain size of the MgB 2 thin film becoming smaller, the re-evaporation amount / re-evaporation rate of Mg atoms decreases as the film formation temperature decreases, and the B content in the entire MgB 2 thin film decreases. This is probably because
　さらに、低温で成膜した方が基材成分とMgとの化学反応が減少することで、MgB2結晶の生成・成長が阻害されたり望まない不純物相が混入したりする確率が低くなると考えられる。 Furthermore, it is considered that the film formation at a low temperature reduces the chemical reaction between the base material component and Mg, thereby lowering the probability that the formation / growth of MgB 2 crystals is hindered and unwanted impurity phases are mixed. .
　本発明者等は、上記の知見・考察を総合的に勘案し、前述のような新たな課題を解決するために、図２に示すような新規な構造を有するMgB2超電導薄膜線材を発明した。 The present inventors have invented an MgB 2 superconducting thin film wire having a novel structure as shown in FIG. 2 in order to solve the above-mentioned new problems by comprehensively considering the above knowledge and considerations. .
　図２は、本発明に係る超電導薄膜線材の断面（金属基材表面に垂直な断面）の構造を示す模式図である。図２の模式図は、作製したMgB2超電導薄膜線材の積層構造断面（金属基材2の表面に垂直な面）を走査型電子顕微鏡（SEM）で観察した像のイメージ図である。図２に示すように、本発明のMgB2超電導薄膜線材は、金属基材2の上に第１のMgB2薄膜8a（例えば、厚さ1μm）が形成され、さらにその上に第２のMgB2薄膜8b（例えば、厚さ10μm）が形成されている。 FIG. 2 is a schematic diagram showing the structure of a cross section (cross section perpendicular to the surface of the metal substrate) of the superconducting thin film wire according to the present invention. The schematic diagram of FIG. 2 is an image diagram of an image obtained by observing a cross section (a surface perpendicular to the surface of the metal substrate 2) of the manufactured MgB 2 superconducting thin film wire with a scanning electron microscope (SEM). As shown in FIG. 2, in the MgB 2 superconducting thin film wire of the present invention, the first MgB 2 thin film 8a (for example, 1 μm in thickness) is formed on the metal substrate 2, and the second MgB is further formed thereon. Two thin films 8b (for example, a thickness of 10 μm) are formed.
　本発明のMgB2超電導薄膜線材の製造方法は、基本的に、金属基材2の上に第１のMgB2薄膜8aを比較的低温で成膜形成する工程を行い、引き続いて、第１のMgB2薄膜8aの上に第２のMgB2薄膜8bを比較的高温で成膜形成する工程を行うものである。 The method for producing a MgB 2 superconducting thin film wire according to the present invention basically includes a step of forming a first MgB 2 thin film 8a on a metal substrate 2 at a relatively low temperature, and subsequently the first step. It performs a process of relatively high temperature deposition forms a second MgB 2 thin film 8b on the MgB 2 thin film 8a.
　第１のMgB2薄膜8aを比較的低温（例えば、200℃以上290℃未満）で成膜形成することによって、金属基材2とMgとの化学反応を防止し、その結果、望まない不純物相が混入したり第２のMgB2薄膜8bの結晶成長を阻害したりすることを防止することができる。同時に、比較的低温で成膜形成することにより、第１のMgB2薄膜8aのB含有率と平均結晶粒径とが小さくなる。 By forming the first MgB 2 thin film 8a at a relatively low temperature (for example, 200 ° C. or more and less than 290 ° C.), the chemical reaction between the metal substrate 2 and Mg is prevented, and as a result, an unwanted impurity phase Can be prevented from being mixed or the crystal growth of the second MgB 2 thin film 8b can be inhibited. At the same time, by forming the film at a relatively low temperature, the B content and the average crystal grain size of the first MgB 2 thin film 8a are reduced.
　一方、第２のMgB2薄膜8bを比較的高温（例えば、290℃以上）で成膜形成することによって、MgB2薄膜の結晶性が向上し、臨界温度や臨界電流密度などで優れた超電導特性を発揮することができる。 On the other hand, by forming the second MgB 2 thin film 8b at a relatively high temperature (eg, 290 ° C. or higher), the crystallinity of the MgB 2 thin film is improved, and superconducting properties such as critical temperature and critical current density are excellent. Can be demonstrated.
　すなわち、金属基材2上へのMgB2薄膜の成膜形成を上記のように２工程に分けて行うことにより、第１のMgB2薄膜8aで金属基材2との化学反応が抑制されると共に許容引張ひずみが高まり、第２のMgB2薄膜8bで優れた超電導特性が発揮される。その結果、超電導線材全体としては、線材への引張応力時に生じる、金属基材2と第２のMgB2薄膜8bとの間の許容引張ひずみの差異を第１のMgB2薄膜8aが緩和する役目を果たし、機械的特性を向上させることができる。また、結晶性が高い第２のMgB2薄膜8bの形成に対する阻害因子を第１のMgB2薄膜8aが防ぐと共に第２のMgB2薄膜8bが超電導線材全体としての超電導特性を向上させることができる。 That is, by forming the MgB 2 thin film on the metal substrate 2 in two steps as described above, the chemical reaction with the metal substrate 2 is suppressed by the first MgB 2 thin film 8a. At the same time, the allowable tensile strain increases, and excellent superconducting properties are exhibited by the second MgB 2 thin film 8b. As a result, as a whole superconducting wire, the first MgB 2 thin film 8a serves to alleviate the difference in allowable tensile strain between the metal substrate 2 and the second MgB 2 thin film 8b, which occurs when tensile stress is applied to the wire. And mechanical properties can be improved. Further, it is possible to second MgB 2 thin film 8b improves the superconducting properties of the whole superconducting wire with the inhibitor for the formation of high crystallinity second MgB 2 thin film 8b is first MgB 2 thin film 8a prevent .
　図１は、本発明に係る超電導薄膜線材であるMgB2超電導薄膜線材を作製する蒸着装置の一例を示す概略模式図である。図１に示したように、当該蒸着装置は、概略的には、第１の温度（比較的低温、例えば200℃以上290℃未満）で第１のMgB2薄膜8aを金属基材2上に成膜する第１の真空チャンバ1aと、第２の温度（比較的高温、例えば290℃以上）で第２のMgB2薄膜8bを第１のMgB2膜8a上に成膜する第２の真空チャンバ1bと、金属基材2を搬送するリール3と、第１の温度で金属基材2を加熱する第１のヒーター4aと、その際にMgB2薄膜を共蒸着する第１のMg蒸着源5aおよび第１のB蒸着源6aと、第２の温度で金属基材2を加熱する第２のヒーター4bと、その際にMgB2薄膜を共蒸着する第２のMg蒸着源5bおよび第２のB蒸着源6bと、真空ポンプ7とを有する。 FIG. 1 is a schematic diagram showing an example of a vapor deposition apparatus for producing a MgB 2 superconducting thin film wire which is a superconducting thin film wire according to the present invention. As shown in FIG. 1, the vapor deposition apparatus generally includes a first MgB 2 thin film 8a on a metal substrate 2 at a first temperature (relatively low temperature, for example, 200 ° C. or more and less than 290 ° C.). A first vacuum chamber 1a for forming a film, and a second vacuum for forming a second MgB 2 thin film 8b on the first MgB 2 film 8a at a second temperature (relatively high temperature, for example, 290 ° C. or higher). A chamber 1b, a reel 3 for transporting the metal substrate 2, a first heater 4a for heating the metal substrate 2 at a first temperature, and a first Mg deposition source for co-depositing an MgB 2 thin film at that time 5a and the first B deposition source 6a, the second heater 4b for heating the metal substrate 2 at the second temperature, and the second Mg deposition source 5b and the second for co-evaporating the MgB 2 thin film. B deposition source 6b and a vacuum pump 7.
　第１の真空チャンバ1aと第２の真空チャンバ1bとは、仕切り10によって区切られ、低温成膜領域と高温成膜領域とに分けられている。両者は真空ポンプ7によって真空排気される。金属基材2は、リール3によって第１の真空チャンバ1aから第２の真空チャンバ1bへ搬送される。 The first vacuum chamber 1a and the second vacuum chamber 1b are divided by a partition 10 and divided into a low temperature film formation region and a high temperature film formation region. Both are evacuated by a vacuum pump 7. The metal substrate 2 is conveyed by the reel 3 from the first vacuum chamber 1a to the second vacuum chamber 1b.
　金属基材2は、リール3によって第１の真空チャンバ1a内の成膜領域に送り出され、第１のヒーター4aによって第１の温度（比較的低温、例えば200℃以上290℃未満）に加熱される。そして、第１のMg蒸着源5aと第１のB蒸着源6aとから、それぞれ加熱蒸発したMg蒸気とB蒸気とが噴出し、加熱された金属基材2上に第１のMgB2薄膜8aが共蒸着される。 The metal substrate 2 is fed to the film forming region in the first vacuum chamber 1a by the reel 3, and heated to a first temperature (relatively low temperature, for example, 200 ° C. or more and less than 290 ° C.) by the first heater 4a. The Then, Mg vapor and B vapor heated and evaporated from the first Mg vapor deposition source 5a and the first B vapor deposition source 6a, respectively, and the first MgB 2 thin film 8a is ejected onto the heated metal substrate 2. Are co-evaporated.
　その後、第１のMgB2薄膜8aが成膜形成された金属基材2は、リール3によって第２の真空チャンバ1bへ搬送され、第２のヒーター4bによって第２の温度（比較的高温、例えば290℃以上）に加熱される。第２のMg蒸着源5bと第２のB蒸着源6bとから、それぞれ加熱蒸発したMg蒸気とB蒸気とが噴出し、先に成膜形成された第１のMgB2薄膜8a上に第２のMgB2薄膜8bが共蒸着される。すなわち、第１のMgB2薄膜8aの成膜温度は、第２のMgB2薄膜8bの成膜温度よりも低くなっている。 Thereafter, the metal base material 2 on which the first MgB 2 thin film 8a is formed is transported to the second vacuum chamber 1b by the reel 3, and is moved to a second temperature (relatively high temperature, for example, by the second heater 4b). 290 ° C or higher). From the second Mg vapor deposition source 5b and the second B vapor deposition source 6b, heated and evaporated Mg vapor and B vapor are ejected from the second Mg vapor deposition source 5b and formed on the first MgB 2 thin film 8a previously formed. The MgB 2 thin film 8b is co-evaporated. That is, the film formation temperature of the first MgB 2 thin film 8a is lower than the film formation temperature of the second MgB 2 thin film 8b.
　本実施例では、第１のMgB2薄膜8aを成膜する第１の温度（成膜温度）を270℃とし、第２のMgB2薄膜8bを成膜する第２の温度（成膜温度）を300℃とし、加熱蒸発するMg蒸気量とB蒸気量とのモル比が「Mg：B＝10：1」となるように各Mg蒸着源と各B蒸着源とを制御した。その結果、図２に示したような、金属基材2上に厚さ1μmの第１のMgB2薄膜8aが成膜形成され、その上に厚さ10μmの第２のMgB2薄膜8bが成膜形成されたMgB2薄膜線材を得た。 In this embodiment, the first temperature (deposition temperature) for forming the first MgB 2 thin film 8a is set to 270 ° C., and the second temperature (deposition temperature) for forming the second MgB 2 thin film 8b. Was set to 300 ° C., and each Mg vapor deposition source and each B vapor deposition source were controlled so that the molar ratio of the amount of Mg vapor to be evaporated by heating and the amount of B vapor was “Mg: B = 10: 1”. As a result, a first MgB 2 thin film 8a having a thickness of 1 μm is formed on the metal substrate 2 as shown in FIG. 2, and a second MgB 2 thin film 8b having a thickness of 10 μm is formed thereon. A film-formed MgB 2 thin film wire was obtained.
　得られたMgB2薄膜線材の断面（金属基材表面に垂直な断面）の微細組織を、走査型透過電子顕微鏡－エネルギー分散型Ｘ線分析装置（STEM-EDX）を用いて観察したところ、第１のMgB2薄膜8aと第２のMgB2薄膜8bとの間に酸素原子濃度の高い領域（酸化物と思われる薄い層）が介在していることが確認された。酸化物層形成の要因としては、第１のMgB2薄膜8aを成膜する工程と第２のMgB2薄膜8bを成膜する工程との間でMgB2薄膜の成長が一旦途切れる際に、真空チャンバ中にわずかに残存する酸素によって第１のMgB2薄膜8aの極表面が酸化した、および／または真空チャンバ中に浮遊する余剰のMg蒸気とわずかに残存する酸素とが化合した酸化物粒子が第１のMgB2薄膜8aの表面に付着堆積した等が考えられる。 The microstructure of the obtained MgB 2 thin film wire (cross section perpendicular to the metal substrate surface) was observed using a scanning transmission electron microscope-energy dispersive X-ray analyzer (STEM-EDX). It was confirmed that a region having a high oxygen atom concentration (a thin layer considered to be an oxide) was interposed between the first MgB 2 thin film 8a and the second MgB 2 thin film 8b. The reason for the formation of the oxide layer is that the growth of the MgB 2 thin film is temporarily interrupted between the step of forming the first MgB 2 thin film 8a and the step of forming the second MgB 2 thin film 8b. Oxide particles in which the extreme surface of the first MgB 2 thin film 8a is oxidized by slightly remaining oxygen in the chamber and / or the excess Mg vapor floating in the vacuum chamber and slightly remaining oxygen are combined. It is conceivable that the first MgB 2 thin film 8a is deposited and deposited on the surface.
　図３Ａは、本発明に係る超電導薄膜線材の第１のMgB2薄膜の表面の微細組織を示す電子顕微鏡観察像である。具体的には、第１のMgB2薄膜を成膜する工程が終了した段階で取り出した試料（すなわち、第２のMgB2薄膜を成膜する工程を行っていない試料）において、第１のMgB2薄膜8aの表面をSEMで観察した像である。図３Ｂは、図３Ａの模式図である。図４Ａは、本発明に係る超電導薄膜線材の第２のMgB2薄膜の表面の微細組織を示すSEM観察像であり、図４Ｂは、図４Ａの模式図である。 FIG. 3A is an electron microscope observation image showing the microstructure of the surface of the first MgB 2 thin film of the superconducting thin film wire according to the present invention. Specifically, in the sample taken out after the step of forming the first MgB 2 thin film (that is, the sample not subjected to the step of forming the second MgB 2 thin film), the first MgB 2 2 is an image obtained by observing the surface of the thin film 8a with an SEM. FIG. 3B is a schematic diagram of FIG. 3A. 4A is an SEM observation image showing the microstructure of the surface of the second MgB 2 thin film of the superconducting thin film wire according to the present invention, and FIG. 4B is a schematic diagram of FIG. 4A.
　図３Ａ，Ｂおよび図４Ａ，Ｂに示すように、第１のMgB2薄膜8aのMgB2結晶9の平均結晶粒径が約100 nmであったのに対し、第２のMgB2薄膜8bのMgB2結晶9の平均結晶粒径は約150 nmであった。このことから、第１のMgB2薄膜8aの平均結晶粒径は、第２のMgB2薄膜8bの平均結晶粒径よりも小さいことが確認される。 As shown in FIGS. 3A and 3B and FIGS. 4A and 4B, the average grain size of the MgB 2 crystal 9 of the first MgB 2 thin film 8a was about 100 nm, whereas that of the second MgB 2 thin film 8b The average crystal grain size of MgB 2 crystal 9 was about 150 nm. This confirms that the average crystal grain size of the first MgB 2 thin film 8a is smaller than the average crystal grain size of the second MgB 2 thin film 8b.
　また、別の実験として、金属基材2上に第１のMgB2薄膜8aのみを成膜形成した試料と、金属基材2上に第２のMgB2薄膜8bのみを成膜形成した試料とを用意し、誘導結合プラズマ－発光分光分析法（ICP-AES）により、第１のMgB2薄膜8aおよび第２のMgB2薄膜8bそれぞれのB含有率を測定した。その結果、第１のMgB2薄膜8aのB含有率が56.5原子％であり、第２のMgB2薄膜8bのB含有率が66.6原子％であった。 As another experiment, a sample in which only the first MgB 2 thin film 8a was formed on the metal substrate 2 and a sample in which only the second MgB 2 thin film 8b was formed on the metal substrate 2 Were prepared, and the B content of each of the first MgB 2 thin film 8a and the second MgB 2 thin film 8b was measured by inductively coupled plasma-emission spectroscopy (ICP-AES). As a result, the B content of the first MgB 2 thin film 8a was 56.5 atomic%, and the B content of the second MgB 2 thin film 8b was 66.6 atomic%.
　前述したように、MgB2薄膜の成膜形成の際のMg蒸気量とB蒸気量とのモル比は一定となるように制御した。このことから、第１のMgB2薄膜8aと第２のMgB2薄膜8bとのB含有率の差異は、成膜温度の差異に起因するものと考えられる。言い換えると、第１のMgB2薄膜8aの成膜温度（ここでは270℃）が第２のMgB2薄膜8bの成膜温度（ここでは300℃）よりも低かったことに起因して、第１のMgB2薄膜8aのB含有率が第２のMgB2薄膜8bのB含有率よりも小さくなったと考えられる。 As described above, the molar ratio between the amount of Mg vapor and the amount of B vapor during film formation of the MgB 2 thin film was controlled to be constant. From this, it is considered that the difference in the B content between the first MgB 2 thin film 8a and the second MgB 2 thin film 8b is caused by the difference in film formation temperature. In other words, the first MgB 2 thin film 8a has a film formation temperature (here, 270 ° C.) lower than the film formation temperature of the second MgB 2 thin film 8b (here, 300 ° C.). It is considered that the B content of the MgB 2 thin film 8a was smaller than the B content of the second MgB 2 thin film 8b.
　次に、超電導薄膜線材の臨界電流密度と許容引張ひずみとの関係を調査する実験を行った。作製したMgB2薄膜線材に所定量の引張ひずみを加え、温度20 K、外部磁場5 Tの環境で臨界電流密度（Jc）の測定を行った。引張ひずみを加えなかったMgB2薄膜線材（基準試料）のJc値から5％低下する引張ひずみを許容引張ひずみとした。 Next, an experiment was conducted to investigate the relationship between the critical current density of the superconducting thin film wire and the allowable tensile strain. A predetermined amount of tensile strain was applied to the prepared MgB 2 thin film wire, and the critical current density (J c ) was measured in an environment of a temperature of 20 K and an external magnetic field of 5 T. An allowable tensile strain was defined as a tensile strain that was reduced by 5% from the Jc value of the MgB 2 thin film wire (reference sample) to which no tensile strain was applied.
　図５は、本発明に係る超電導薄膜線材の臨界電流密度と許容引張ひずみとの関係の一例を示すグラフである。図５には、比較試料として、金属基材2上に第１のMgB2薄膜8aを形成せず、金属基材2上に第２のMgB2薄膜8bを直接成膜形成させたMgB2薄膜線材のデータも併せて示した。 FIG. 5 is a graph showing an example of the relationship between the critical current density and the allowable tensile strain of the superconducting thin film wire according to the present invention. 5 shows, as a comparative sample, without forming the first MgB 2 thin film 8a on the metal substrate 2, MgB 2 thin film a second MgB 2 thin film 8b is directly deposited formed on the metal substrate 2 The wire data is also shown.
　図５に示したように、本発明のMgB2超電導薄膜線材（実施例）は、第１のMgB2薄膜8aを形成させなかった比較試料よりも、Jc値と許容引張ひずみとが共に向上している。具体的には、本発明の実施例は、温度20 K、外部磁場5 T中における良好なJc特性（3800 A/mm2）と、優れた機械的特性（許容引張ひずみ0.20％）を示すことが確認された。 As shown in FIG. 5, the MgB 2 superconducting thin film wire (Example) of the present invention has both improved J c value and allowable tensile strain compared to the comparative sample in which the first MgB 2 thin film 8a was not formed. is doing. Specifically, the examples of the present invention exhibit good J c characteristics (3800 A / mm 2 ) and excellent mechanical characteristics (allowable tensile strain 0.20%) in a temperature of 20 K and an external magnetic field of 5 T. It was confirmed.
　なお、図５中には記載していないが、第１のMgB2薄膜8aと第２のMgB2薄膜8bとを共に高温（300℃）で成膜形成した試料は、図５中の比較試料と同等の特性を示すことを別途確認した。また、第１のMgB2薄膜8aと第2のMgB2薄膜8bとを共に低温（270℃）で成膜形成した試料は、図５中の実施例と同等の許容引張ひずみを示すが、Jc値が比較試料よりも低下することを別途確認した。 Although not shown in FIG. 5, the sample formed by forming both the first MgB 2 thin film 8a and the second MgB 2 thin film 8b at a high temperature (300 ° C.) is a comparative sample in FIG. It was confirmed separately that it showed the same characteristics as. A sample in which the first MgB 2 thin film 8a and the second MgB 2 thin film 8b are both formed at a low temperature (270 ° C.) shows an allowable tensile strain equivalent to that of the example in FIG. It was separately confirmed that the c value was lower than that of the comparative sample.
　このような実験結果から、第１のMgB2薄膜8aを比較的低温で成膜形成し、その上に第２のMgB2薄膜8bを比較的高温で成膜形成することにより、望まない不純物相が生成したり第２のMgB2薄膜8bの結晶成長を阻害したりすることを防止し、MgB2薄膜線材全体としてのJc値を向上させること、および金属基材2と第２のMgB2薄膜8bとの間の許容引張ひずみの差異が緩和されて、機械的特性を向上させることが確認される。 From these experimental results, the first MgB 2 thin film 8a is formed at a relatively low temperature, and the second MgB 2 thin film 8b is formed thereon at a relatively high temperature, so that an unwanted impurity phase is formed. There prevents or inhibits the production or crystal growth of the second MgB 2 thin film 8b, to improve the J c values of the whole MgB 2 thin wire, and the metal substrate 2 and the second MgB 2 It is confirmed that the difference in the allowable tensile strain from the thin film 8b is relaxed and the mechanical properties are improved.
　また、本発明の製造方法は、真空チャンバから取り出すことなく第１のMgB2薄膜8aと第２のMgB2薄膜8bとを連続して成膜形成することから、従来の製造方法と同様の簡易なプロセスで製造することが可能であり、かつ装置コストの上昇を最小限に抑えることができる（すなわち、製造コストを大幅に増加させない）ことも確認された。 Further, the manufacturing method of the present invention forms the first MgB 2 thin film 8a and the second MgB 2 thin film 8b in succession without removing them from the vacuum chamber, so that it is as simple as the conventional manufacturing method. It has also been confirmed that it is possible to manufacture by a simple process and that the increase in equipment cost can be minimized (that is, the manufacturing cost is not significantly increased).
　上述した実施形態は、本発明の理解を助けるために説明したものであり、本発明は、記載した具体的な構成のみに限定されるものではない。例えば、実施形態の構成の一部を当業者の技術常識の構成に置き換えたり、実施形態の構成に当業者の技術常識の構成を加えたりすることが可能である。すなわち、本発明は、本明細書の実施形態の構成の一部について、発明の技術的思想を逸脱しない範囲で、他の構成に置換や、他の構成の追加をすることが可能である。 The above-described embodiments are described for helping understanding of the present invention, and the present invention is not limited only to the specific configurations described. For example, it is possible to replace a part of the configuration of the embodiment with the configuration of common technical knowledge of those skilled in the art, or to add the configuration of common technical knowledge of those skilled in the art to the configuration of the embodiment. That is, in the present invention, a part of the configuration of the embodiment of the present specification can be replaced with another configuration or added with another configuration without departing from the technical idea of the present invention.
　1a…第１の真空チャンバ、1b…第２の真空チャンバ、2…金属基材、3…リール、4a…第１のヒーター、4b…第２のヒーター、5a…第１のMg蒸着源、5b…第２のMg蒸着源、6a…第１のB蒸着源、6b…第２のB蒸着源、7…真空ポンプ、8a…第1のMgB2薄膜、8b…第2のMgB2薄膜、9…MgB2結晶、10…仕切り。 DESCRIPTION OF SYMBOLS 1a ... 1st vacuum chamber, 1b ... 2nd vacuum chamber, 2 ... Metal base material, 3 ... Reel, 4a ... 1st heater, 4b ... 2nd heater, 5a ... 1st Mg vapor deposition source, 5b ... second Mg deposition source, 6a ... first B deposition source, 6b ... second B evaporation source 7 ... vacuum pump, 8a ... first MgB 2 thin film, 8b ... second MgB 2 thin film, 9 ... MgB 2 crystals, 10 ... dividers.

Claims (6)

  1. 　金属基材と、
    前記金属基材の上に形成された第１のMgB2薄膜と、
    前記第１のMgB2薄膜の上に形成された第２のMgB2薄膜と、を有し、
    前記第１のMgB2薄膜のB含有率が前記第２のMgB2薄膜のB含有率よりも小さいことを特徴とする超電導薄膜線材。     A metal substrate;
    A first MgB 2 thin film formed on the metal substrate;
    And a second of MgB 2 thin film formed on the first MgB 2 thin film,
    A superconducting thin film wire, wherein the B content of the first MgB 2 thin film is smaller than the B content of the second MgB 2 thin film.
  2. 　請求項１に記載の超電導薄膜線材において、
    前記第１のMgB2薄膜の平均結晶粒径が前記第２のMgB2薄膜の平均結晶粒径よりも小さいことを特徴とする超電導薄膜線材。     The superconducting thin film wire according to claim 1,
    Superconducting thin film wire, wherein the average crystal grain size of the first MgB 2 thin film is smaller than the average crystal grain size of the second MgB 2 film.
  3. 　請求項１または請求項２に記載の超電導薄膜線材において、
    前記第１のMgB2薄膜と前記第２のMgB2薄膜との間に酸化物の薄層が介在していることを特徴とする超電導薄膜基材。     In the superconducting thin film wire according to claim 1 or 2,
    A superconducting thin film substrate, characterized in that a thin oxide layer is interposed between the first MgB 2 thin film and the second MgB 2 thin film.
  4. 　金属基材上に第１の温度で第１のMgB2薄膜を成膜する工程と、
    前記第１のMgB2薄膜の上に第２の温度で第２のMgB2薄膜を成膜する工程と、を有し、
    前記第１の温度が前記第２の温度よりも低いことを特徴とする超電導薄膜線材の製造方法。     Depositing a first MgB 2 thin film on a metal substrate at a first temperature;
    And a step of forming a second MgB 2 thin film at a second temperature on the first MgB 2 thin film,
    The method for producing a superconducting thin film wire, wherein the first temperature is lower than the second temperature.
  5. 　請求項４に記載の超電導薄膜線材の製造方法において、
    前記第１のMgB2薄膜のB含有率が前記第２のMgB2薄膜のB含有率よりも小さいことを特徴とする超電導薄膜線材の製造方法。     In the manufacturing method of the superconducting thin film wire according to claim 4,
    A method for producing a superconducting thin film wire, wherein the B content of the first MgB 2 thin film is smaller than the B content of the second MgB 2 thin film.
  6. 　請求項４または請求項５に記載の超電導薄膜線材の製造方法において、
    前記第１のMgB2薄膜の平均結晶粒径が前記第２のMgB2薄膜の平均結晶粒径よりも小さいことを特徴とする超電導薄膜線材の製造方法。     In the manufacturing method of the superconducting thin film wire according to claim 4 or 5,
    The method of manufacturing a superconducting thin film wire having an average crystal grain size of the first MgB 2 thin film is equal to or smaller than the average crystal grain size of the second MgB 2 film.
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* Cited by examiner, † Cited by third party
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JP2003323822A (en) * 2002-05-02 2003-11-14 Sumitomo Electric Ind Ltd Thin film superconducting wire and method of manufacturing the same
WO2016084513A1 (en) * 2014-11-28 2016-06-02 株式会社日立製作所 Magnesium diboroide superconducting thin-film wire material and method for producing same

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2003323822A (en) * 2002-05-02 2003-11-14 Sumitomo Electric Ind Ltd Thin film superconducting wire and method of manufacturing the same
WO2016084513A1 (en) * 2014-11-28 2016-06-02 株式会社日立製作所 Magnesium diboroide superconducting thin-film wire material and method for producing same

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