JP5400416B2 - Superconducting wire - Google Patents

Description

  The present invention relates to a superconducting wire in which a superconducting material layer made of an oxide superconducting material is formed on a substrate, and relates to a superconducting wire superior in characteristics (critical current density) than a conventional one.

  Superconducting materials that are expected to be applied to power equipment such as power transmission cables are classified according to their basic composition, but RE (rare earth) such as YBCO is considered promising from the viewpoint of practical application. ) There are superconducting materials. Since such a superconducting material itself is brittle and has poor toughness, it is used in the state of a superconducting material in which a superconducting material layer is formed on a linear or tape-like metal substrate.

  The characteristics of the superconducting wire depend on its material structure and are required to have a certain orientation structure. Therefore, in the production of a superconducting wire having a multilayer structure as described above, it is general that an oriented structure is developed on the surface of the substrate before the superconducting material layer is formed, and a superconducting material layer is formed thereon by epitaxial growth.

  Among such superconducting wires having a multi-layer structure, there is a structure using an IBAD substrate as a conventionally known configuration. In this method, a surface layer (IBAD layer) having an oriented structure is formed on an appropriate metal substrate such as Hastelloy by ion beam deposition or the like, and an appropriate intermediate layer and superconducting material layer are formed thereon. A superconducting wire using the IBAD substrate has a relatively good orientation of the superconducting material layer formed on the substrate, and is satisfactory in terms of characteristics. However, this type of superconducting wire has a problem in terms of production cost because the process for forming the IBAD layer is complicated.

  Therefore, a superconducting wire has been proposed in which the substrate itself is provided with orientation and a superconducting material layer or the like is formed thereon. This type of superconducting wire is intended to provide orientation on the substrate surface by improving the crystal structure by thermomechanical treatment, but an intermediate layer with a complicated manufacturing process such as an IBAD layer is unnecessary, and production cost reduction is expected. it can. As a superconducting wire using this oriented substrate, for example, Patent Document 1 describes one in which an oriented Ni alloy (Ni-W alloy) is applied as a substrate.

  Further, the inventors of the present application have found an oriented substrate made of pure copper as a substrate for a material by epitaxial growth such as a superconducting material (Patent Document 2). This copper substrate focuses on the ease of controlling the orientation of pure copper, and has a {100} <001> cubic texture in which the crystal axis deviation angle Δφ is Δφ ≦ 6 °. This substrate solves the problem of insufficient strength of pure copper containing no alloy element by surface activated bonding of a metal layer (support material) such as stainless steel.

  A superconducting wire to which the orientation substrate as described above is applied is excellent in terms of manufacturing efficiency and cost. However, these superconducting wires are considered to have insufficient characteristics or require further improvement. That is, since the nickel alloy substrate and the copper alloy substrate have inferior orientation, there can be room for improvement in the characteristics of the entire superconducting wire having a superconducting material layer formed on the substrate.

  On the other hand, with respect to a pure copper substrate, the orientation of the substrate itself is improved as compared with nickel alloys and copper alloys, but there is a limit to the improvement in orientation even with pure copper. Here, the orientation (orientation degree) of the orientation substrate is often evaluated by a deviation angle (Δφ) of a predetermined crystal plane of the constituent metal and its crystal axis with respect to the substrate. The orientation of a pure copper substrate is improved by adjusting the conditions of processing and heat treatment in the manufacturing process, but the limit is that the lower limit of Δφ is about 5 ° even by setting such conditions. Further, copper that has not been alloyed tends to oxidize, and a pure copper substrate has a problem in the film formability of the intermediate layer and the superconducting material layer due to surface oxidation.

JP 2007-115562 A JP 2008-266686A

  Accordingly, an object of the present invention is to provide a superconducting wire that can improve the orientation of the substrate as compared with the conventional superconducting wire to which the oriented substrate is applied, and exhibit high characteristics.

  As described above, even if the orientation of the pure copper substrate is relatively good, the improvement has a certain limit. The present inventors have examined the problem of improving the orientation of a pure copper substrate. When a predetermined metal thin film having a certain thickness or less is formed on the surface of the oriented metal, the orientation on the surface is the basis. We found a phenomenon that was improved over the metal layer. Further, it has also been found that by forming a metal thin film on a pure copper substrate, it is possible to improve the film forming properties of the subsequent intermediate layer and superconducting material layer, and to suppress peeling during film formation. Such a metal thin film is supposed to be an orientation improving layer for the base metal. By utilizing this, it is possible to improve the degree of orientation of the substrate surface and to improve the characteristics of the entire superconducting wire. I came up with the invention.

  That is, the present invention relates to a superconducting wire in which at least one intermediate layer and a superconducting material layer made of an oxide superconducting material are formed on a substrate made of a crystallographically oriented metal. 100} <001> a copper layer having a cubic texture, and an orientation improving layer made of nickel having a thickness of 1 to 5000 nm formed on the copper layer, and a crystal axis shift angle Δφ on the substrate surface Is a superconducting wire characterized by Δφ ≦ 5 °.

  Hereinafter, the present invention will be described in detail. The present invention is provided with a substrate mainly composed of oriented pure copper. This is because pure copper is the most excellent metal for adjusting the orientation as described above. The crystal orientation of the copper layer has a {100} <001> cubic texture because it has a face-centered cubic structure. Needless to say, even if the orientation improving layer described below is applied, it is preferable that the copper layer has good orientation. Preferably, the orientation of the copper layer is preferably such that the crystal axis deviation angle Δφ is 6 ° or less.

  The orientation improving layer formed on the copper layer is made of nickel. However, limiting to nickel is that the crystal structure and the lattice constant for the underlying copper are taken into account, and the effect of improving the orientation is particularly effective. It is because it is accepted. And the orientation improvement layer shall be 1-5000 nm as the thickness. This is because if the film thickness exceeds this range, the growth orientation of the intermediate layer and the superconducting material layer to be formed thereafter is shifted, and the effect of improving the orientation degree cannot be obtained at 1 nm or less. The film thickness of the orientation improving layer is more preferably 100 to 3000 nm.

  The orientation improving layer is preferably formed by a method using epitaxial growth such as plating. This is to maintain the orientation of the alignment substrate. And an orientation substrate provided with the orientation improvement layer demonstrated above improves (DELTA) phi in the surface, and will be 5 degrees or less.

  By the way, according to the present inventors, when the orientation improving layer is formed by epitaxial growth, extremely fine irregularities of the order of submicron may occur on the surface. This is because the epitaxial growth has a growth mechanism in which the metal crystal is oriented in a constant growth orientation and is laminated in a cube-on-cube relationship with the base, and unevenness is generated on the outermost surface according to the growth surface and the growth orientation. It depends. The irregularities are extremely fine, but if a superconducting material layer or the like is formed on the irregularities, minute distortion occurs, which may affect the characteristics.

  Therefore, in order to form a high-quality superconducting material layer or the like on the substrate, it is preferable that there is no fine unevenness as described above. Specifically, the surface roughness is preferably 20 nm or less. In addition, the surface roughness in this invention shows arithmetic mean roughness (Ra). Moreover, the preferable lower limit of surface roughness is 0.1 nm.

  In the present invention, from the viewpoint of the surface roughness of the substrate surface, it is preferable that the surface of the orientation improving layer contains palladium having a thickness equivalent to 30 nm or less. The reason why the surface roughness is improved when a small amount of palladium is added is not clear. However, according to the inventor's guess, the surface of the orientation improving layer formed by epitaxial growth has a high surface energy and tends to be stabilized by becoming smooth. When a different kind of metal called palladium is present on the surface of such an orientation improving layer, a catalytic action that promotes smoothing by forming a solid solution in the constituent metal (nickel) of the orientation improving layer occurs, which is on the submicron order. It is thought that the unevenness of the will disappear.

  Here, “equivalent to the film thickness”, which is the addition amount of palladium, is calculated from the surface area of the orientation improving layer, the abundance (weight) and density of palladium. The use of such an expression is because the amount of palladium present is extremely small, so it does not always form a complete uniform layer and coat the orientation improving layer, but is scattered on the surface of the orientation improving layer. Because there are things. The lower limit of this additional amount is 0.5 nm.

  Further, as described later, the smoothing of the orientation improving layer by the addition of palladium as described above is further promoted by heat treatment, but the substrate when the heat treatment is performed, the orientation improving layer ( In some cases, the underlying copper diffuses into (nickel) and the orientation improving layer forms a nickel-copper alloy. As described above, the treatment for improving the smoothness is to perform a heat treatment after adding palladium, and this heat treatment diffuses copper into the orientation improving layer. However, even if such alloying occurs, the orientation and smoothness of the substrate surface are not affected.

  The superconducting wire according to the present invention forms a superconducting material layer on a substrate provided with the orientation improving layer described above. Usually, an intermediate layer is formed between the substrate and the superconducting material layer. This intermediate layer functions as a buffer layer in consideration of the difference in lattice constant between the superconducting material (YBCO or the like) and the metal constituting the substrate, and a barrier for preventing the diffusion of metal elements contained in the substrate (copper). It has a function as a layer. The intermediate layer preferably has a three-layer structure of a seed layer, a barrier layer, and a cap layer. Each intermediate layer is made of any of oxide, carbide, and nitride, and preferably has a thickness of 10 to 1000 nm.

  Specific examples of the constituent material of the intermediate layer include oxides such as cerium oxide and zirconium oxide, composite oxides such as LaMnO, LaZrO, and GdZrO, and nitrides such as TiN. In the case of oxides and complex oxides, perovskite type, fluorite type oxides and complex oxides are preferable. Particularly preferably, the seed layer is made of a rare earth element oxide or a complex oxide containing a rare earth element, the barrier layer is made of an oxide containing zirconium oxide, and the cap layer is made of a rare earth element oxide or a rare earth element. It consists of a complex oxide containing elements.

Further, the oxide superconducting material constituting the superconducting material layer is preferably an RE-based superconducting material, in particular, an RE · Ba ₂ Cu ₃ O _x superconducting material (RE is one or more rare earth elements), specifically YBCO, SmBCO, GdBCO, Y _0.3 Gd _0.7 BCO, and the like. Further, the superconducting material layer may be made only of these superconducting materials, or may be an artificial pin added with an oxide different from these superconducting materials in order to improve superconducting characteristics. The thickness of the superconducting material layer is preferably 100 nm or more.

  The superconducting wire according to the present invention preferably has a reinforcing metal layer that supports the substrate bonded to the back surface of the substrate in order to ensure strength. The reinforcing metal layer is preferably made of any one of stainless steel and nickel alloy (Hastelloy alloy, Inconel alloy, Incoloy alloy, Monel alloy, etc.). Moreover, there is no limitation in particular about the thickness and shape of an orientation board | substrate, The shape according to a use, such as plate shape, foil shape, and tape shape, is applicable.

  Moreover, the superconducting wire according to the present invention may have a stabilizing layer on the surface of the superconducting material layer. The stabilizing layer is intended to stabilize the superconducting layer during use, and is made of a conductive metal. Preferably, silver or a layer in which a layer made of silver and a layer made of copper are stacked (in this case, a silver layer is formed on the superconducting material layer) is preferable. And it is preferable that the thickness shall be 10-1000 nm.

  The superconducting wire according to the present invention is manufactured by forming the intermediate layer and the superconducting material layer after manufacturing the above-described substrate. For the production of the substrate, a strip material (tape material) is obtained by cold working with a processing rate of 95% or more based on a plate material / wire material made of pure copper, and heat-treated (750 ° C.) in a non-oxidizing atmosphere { 100} <001> A highly oriented copper layer having a cubic texture can be obtained. Then, an orientation improving layer is formed on the copper layer. As described above, the orientation improving layer is preferably a thin film formation method based on epitaxial growth, such as PLD (pulse laser deposition), CVD (chemical vapor deposition), sputtering, vacuum deposition, ion plating, ion Although it can be manufactured by various thin film manufacturing processes such as a beam evaporation method, a spin coating method, MBE (molecular beam epitaxy method), and a plating method, the plating method is particularly preferable.

  Moreover, as a method of adding palladium for smoothing the surface of the orientation improving layer, the same method of the orientation improving layer can be applied, and a plating method is particularly preferable. In order to smooth the surface of the orientation improving layer, it is preferable to perform a heat treatment after adding palladium. This is for promptly exerting the catalytic action by the addition of palladium as described above. The heat treatment temperature is preferably 400 ° C. or higher and not higher than the melting point of palladium. This is because if the temperature is lower than 400 ° C., the movement of atoms for smoothing the surface becomes slow. The heat treatment time is preferably 10 minutes to 2 hours. This is because if the heat treatment time is less than 10 minutes, the movement of atoms for smoothing the surface becomes insufficient, and even if the heat treatment exceeds 2 hours, there is no difference in effect. The atmosphere of the heat treatment is not particularly limited as long as it is a non-oxidizing atmosphere.

  When manufacturing a superconducting material having a reinforcing metal layer, it is preferable to bond the reinforcing metal layer before forming the intermediate layer and the superconducting layer after manufacturing the substrate by the above-described steps. As a method for joining the reinforcing metal layer and the substrate, surface activated joining is preferably applied. Surface activated bonding is a method of bonding immediately after exposing the substrate (pure metal) by performing dry etching (surface activation) on the bonding surfaces of the members to be bonded, removing oxides and adsorbates on the bonding surfaces. It is. This method is a method of joining based on metal atomic force generated between atoms (molecules) having no impurities such as oxides on the surface. As a specific method of dry etching for surface activation, either ion beam etching such as argon or atomic beam etching, or plasma etching can be applied. This dry etching needs to be performed in a non-oxidizing atmosphere, and is particularly preferably performed under vacuum.

  The surface activated bonding enables bonding without applying pressure, and can be bonded only by superimposing materials to be bonded. However, pressurization may be performed for alignment of both materials or for stronger bonding. However, the applied pressure in this case is low enough to prevent deformation of the material, and is preferably 0.01 to 300 MPa. Further, surface activated bonding can be performed at room temperature. Therefore, it is not necessary to heat the processing atmosphere at the time of joining. It should be noted that a non-oxidizing atmosphere is also preferable during this bonding.

  Methods for producing each oxide to be an intermediate layer on a substrate include PLD method, CVD method, sputtering method, ion plating method, ion beam evaporation method, spin coating method, MBE, MOD method (metal organic acid salt) (Deposition method) can be applied. The same applies to the superconducting material layer. The stabilization layer can be formed by a film formation method such as sputtering or vapor deposition, or can be formed by bonding a foil-like copper layer with a brazing material after forming a silver layer by these methods.

  The superconducting wire according to the present invention described above applies a superconducting wire having a high characteristic by applying, as a substrate, a pure copper having orientation with a nickel orientation improving layer for further improving the orientation. To do. The present invention has higher performance than a superconducting wire using a conventional IBAD substrate, and is preferable from the viewpoint of manufacturing efficiency and cost.

  Hereinafter, the best embodiment of the present invention will be described.

(Manufacture and evaluation of substrates)
First embodiment : A tape-shaped copper plate having a plate thickness of 1000 μm was prepared, and this was set to a processing rate of 95% with a rolling roll, and cold-rolled at room temperature to obtain a tape material of 50 μm. After rolling, the copper plate was heat treated to orient the crystal structure, and a {100} <001> cubic texture was obtained. This heat treatment was performed by heating at a temperature of 750 ° C. for 2 hours in an atmosphere composed of 95% nitrogen gas and 5% hydrogen gas.

The copper plate subjected to the above orienting treatment was plated with nickel as an orientation improving layer. In the nickel plating, the substrate was subjected to acid degreasing and electrolytic degreasing, followed by electrolytic plating in a nickel plating bath (watt bath). The plating conditions were a temperature of 40 ° C., a current density of 1 A / dm ² , a plating time was adjusted, and nickel having a thickness of 500 nm was plated. In the case where nickel is formed by plating as the orientation improving layer, it is preferable to set conditions within a range of a current density of 1 to 5 A / dm ² and a bath temperature of 40 to 60 ° C.

Then, a tape-shaped stainless steel (SUS304) plate having a thickness of 100 μm that was rolled in advance as a reinforcing metal layer was joined to a copper substrate plated with nickel as the orientation improving layer. For joining the stainless steel plates, the surface of both the copper substrate and the stainless steel plate was activated with a high-speed atomic beam (argon) using a surface activated joining device, and both were joined with a rolling roll. The conditions during surface activated bonding are as follows.
・ Degree of vacuum: 10 ⁻⁵ Pa
(The inside of the vacuum chamber and etching chamber is in an argon gas atmosphere)
-Applied voltage: 2 kV
・ Etching time: 5 minutes ・ Pressure applied during bonding: 2 MPa

Second Embodiment : Here, in the first embodiment, a substrate having improved smoothness by adding palladium to the surface of the orientation improving layer was manufactured to manufacture a superconducting wire. An orientation improving layer (nickel) was plated on copper subjected to orientation treatment in the same manner as in the first embodiment, and palladium was further added by plating. For the plating, a commercially available palladium plating solution was used, and a bath temperature of 30 to 50 ° C. and a current density of 1 to 3 A / dm ² were adjusted to adjust the addition amount corresponding to the film thickness. Then, after the addition of palladium, the surface was smoothed by heat treatment at 700 ° C. for 1 hour in a non-oxidizing atmosphere. Thereafter, similar to the first embodiment, an intermediate layer and a superconducting material layer were formed.

  About the board | substrate in the above 1st, 2nd embodiment, the surface roughness and orientation degree ((DELTA) (phi)) of the orientation improvement layer surface were measured. In the measurement, the surface roughness and Δφ were measured by performing AFM (atomic force microscope) observation and X-ray diffraction analysis (φ scan) on the substrate surface before forming the intermediate layer and the superconducting material layer. Moreover, the value about the pure copper substrate before formation of an orientation improvement layer was also measured as a comparison. Table 1 shows the results.

  From the table, it can be seen that by plating nickel as the orientation improving layer, the orientation of the substrate surface is improved and Δφ can be less than 5 °. Moreover, when it sees about 2nd Embodiment which added trace amount palladium to the orientation improvement layer and heat-processed, it turns out that surface roughness is reduced and smoothing is made | formed.

(Manufacturing and evaluation of superconducting wire)
An intermediate layer and a superconducting material layer were formed on the oriented substrate manufactured by the above steps to obtain a tape-shaped superconducting wire. Table 2 shows the configuration of the superconducting wire manufactured in this embodiment. These intermediate layer and superconducting material layer were formed by the PLD method.

  And in order to confirm the effect of the orientation improvement layer and smoothing in each embodiment, the characteristic (critical current density) of each superconducting wire was evaluated. The results are shown in Table 3. In this evaluation, measurements were also made on a superconducting wire (conventional example) in which a substrate without an orientation improving layer was used (comparative example) and a similar intermediate layer and superconducting material layer were formed on an IBAD substrate.

  From Table 3, it can be seen that the superconducting wire to which the pure copper substrate plated with nickel is applied as the orientation improving layer has a critical current density equal to or higher than that to which the IBAD substrate is applied and has good characteristics. In particular, the superconducting wire according to the second embodiment in which the orientation improving layer is further subjected to the smoothness improving treatment exhibits a critical current density that is twice or more that of the conventional example.

  In addition, about the pure copper board | substrate which does not form the orientation improvement layer used as a comparative example, peeling of the intermediate | middle layer occurred after intermediate | middle layer formation, and the process to a superconducting wire itself was impossible. Thereby, the orientation improving layer not only improves the degree of orientation of the substrate, but also has a function of stably forming the intermediate layer and the superconducting material layer to be formed thereafter, and applies the pure copper substrate to the superconducting wire. It was confirmed that it was essential.

  As described above, the superconducting wire according to the present invention has higher performance than a conventional superconducting wire using an IBAD substrate, and is preferable from the viewpoint of manufacturing efficiency and cost. The present invention broadens the applicability of superconducting materials to various power devices such as power transmission cables.

Claims (7)

In a superconducting wire in which at least one intermediate layer and a superconducting material layer made of an oxide superconducting material are formed on a substrate made of a crystal-oriented metal,
The substrate made of the crystal orientation metal comprises a copper layer having a {100} <001> cubic texture, and an orientation improving layer made of nickel having a thickness of 1 to 5000 nm formed on the copper layer,
The orientation improving layer includes palladium on its surface in a film thickness equivalent to 30 nm or less, and the surface roughness (Ra) of the substrate surface is 20 nm or less,
A superconducting wire having a crystal axis shift angle Δφ on the substrate surface of Δφ ≦ 5 °. 2. The intermediate layer according to claim 1, wherein the intermediate layer has a three-layer structure of a seed layer, a barrier layer, and a cap layer, and each intermediate layer is made of any of oxide, carbide, and nitride, and has a thickness of 10 to 1000 nm . Superconducting wire. The seed layer is made of a rare earth element oxide or a complex oxide containing rare earth elements, the barrier layer is made of an oxide containing zirconium oxide, and the cap layer is made of a rare earth element oxide or a complex oxide containing rare earth elements. The superconducting wire according to claim 2, which is made of a material. The superconducting wire layer according to any one of claims 1 to 3 , wherein the superconducting material layer is an RE-based superconducting material. The superconducting wire according to any one of claims 1 to 4 , wherein a reinforcing metal layer that supports the substrate is bonded to a back surface of the substrate. The superconducting wire according to any one of claims 1 to 5 , further comprising a stabilizing layer made of a conductive metal on a surface of the superconducting material layer. The superconducting wire according to claim 6 , wherein the stabilization layer is a laminate of silver or a layer made of silver and a layer made of copper.