JP2013129853A - Rolled copper foil for superconducting film formation - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rolled copper foil which controls a shear band adjacent to the copper foil surface and has improved characteristics of a superconducting film formed on the surface itself.SOLUTION: The rolled copper foil for superconducting film formation which forms a film of a superconducting material on the surface itself is provided. In a situation before the following recrystallization heat treatment is performed, the total number of the shear band in the front and back surfaces, which reaches the copper foil surface across a line at a depth of 1/10 of the copper foil thickness in the thickness direction from the copper foil surface, as observed from a rolled parallel section is 0.1/μm or less, and crystal orientation, in which I/I≥50 (I is integrated intensity of diffraction peaks in the surface (200) obtained by X-ray diffraction on the rolling surface of the copper foil, and Iis integrated intensity of diffraction peaks in the surface (200) obtained by X-ray diffraction on fine copper powders), is exhibited by performing the recrystallization heat treatment at 700°C for 30 minutes.

Description

  The present invention relates to a rolled copper foil for forming a superconducting film in which a film of a superconducting material is directly or indirectly formed on its surface.

With the development of high-temperature superconducting materials, it has been studied to form a superconducting material on a substrate and process it into a wire or the like (Patent Document 1). Here, in order to obtain an excellent high-temperature superconducting wire, it is necessary to form a highly conductive superconducting film. In the technique described in Patent Document 1, a substrate (for example, Cu foil) in which metal atoms are biaxially oriented is used. The intermediate layer (for example, Ni film) is epitaxially grown on the substrate, and the superconducting film is epitaxially grown on the intermediate layer.
In addition, it is recommended to use a copper foil that is cold rolled at a high workability of 95% or more, subjected to orientation heat treatment at 200 ° C. or higher and below the melting point of copper, and has a cubic texture as the orientation substrate. Has been. Furthermore, a technique for clad bonding the orientation substrate to a support such as stainless steel has been developed (Patent Document 2).

JP 2006-127847 JP JP 2008-266686 A

However, the investigation of the surface properties of the copper foil for forming a superconducting film directly or indirectly on its surface is not yet sufficient, and the characteristics of the superconducting film (critical current density, etc.) are not sufficiently improved. There is a problem. For example, when a high degree of cold rolling is applied to give a cubic texture, a shear band develops near the surface of the copper foil. This shear band may degrade the properties of the superconducting film.
That is, the present invention has been made to solve the above problems, and provides a rolled copper foil for forming a superconducting film that improves the surface properties of the copper foil and improves the properties of the superconducting film formed on the surface. With the goal.

As a result of various studies, the present inventors have suppressed the shear band near the copper foil surface by adjusting the total workability in the final cold rolling and the workability for each pass in the final cold rolling, It has been found that the properties of the superconducting film formed on the surface are improved.
In order to achieve the above object, the rolled copper foil for forming a superconducting film of the present invention is a rolled copper foil for forming a superconducting material on its surface, and is subjected to the following recrystallization heat treatment. In the previous state, when viewed from the rolling parallel cross section, the total thickness of the front and back surfaces is 0.1 shear bands that reach the surface across the thickness of 1/10 of the copper foil thickness in the thickness direction from the copper foil surface. I / I ₀ ≧ 50 (I: (200) plane diffraction peak integrated intensity determined by X-ray diffraction of the rolled surface of the copper foil, by performing recrystallization heat treatment at 700 ° C. for 30 minutes, A crystal orientation expressed as I ₀ : (200) plane diffraction peak integrated intensity obtained by X-ray diffraction of fine powder copper appears.

It is preferable that the shear band is 0.05 or less per μm in total on the front and back surfaces.
The rolled copper foil for forming a superconducting film of the present invention is manufactured by repeating cold rolling and annealing after hot rolling an ingot, and finally performing final cold rolling, and the total workability of the final cold rolling is It is preferably 90.0 to 99.5% or less.
In the final cold rolling, there is a pass having a higher workability than the previous pass in the final 5 passes, and the maximum workability of any of the 5 passes is 40% or more, and in the final pass It is preferable that the degree of processing becomes the minimum in the five passes.

  ADVANTAGE OF THE INVENTION According to this invention, the rolled copper foil for superconducting film formation which suppresses the shear band of copper foil surface vicinity, improves the surface property of copper foil, and the characteristic of the superconducting film formed in the surface improves is obtained.

It is a figure which shows the superconducting material formed by forming the superconducting film on the surface of the orientation board for superconducting film formation formed by laminating | stacking the rolled copper foil for superconducting film formation on a support body, and the orientation plate for superconducting film formation. It is a figure which shows the method of measuring the number of shear bands. It is a figure which shows the SEM image of a structure | tissue when it sees from the cross section of a rolling parallel direction.

  Hereinafter, the rolled copper foil for superconducting film formation which concerns on embodiment of this invention is demonstrated. In the present invention, “%” means “% by mass” unless otherwise specified.

FIG. 1 shows a superconducting film forming orientation plate 10 formed by laminating a rolled copper foil 4 for forming a superconducting film according to an embodiment of the present invention on a support 2, and a surface of the superconducting film forming orientation plate 10 (superconducting film formation). A superconducting material 100 formed by forming a superconducting film 8 on the rolled copper foil 4 side) is shown.
The support 2 is for ensuring the strength of the superconducting film-forming alignment plate 10 and is preferably a nonmagnetic metal material (for example, stainless steel or nickel alloy).
The copper foil 4 is subjected to recrystallization (orientation) heat treatment (hereinafter also referred to as “heat treatment” as appropriate), and the cubic orientation of the copper foil 4 develops at that time. The temperature of this recrystallization (orientation) heat treatment is preferably 200 ° C. or higher and not higher than the melting point of pure copper. A heat treatment at a temperature lower than 200 ° C. may not provide a sufficiently oriented structure. The heat treatment is preferably 800 ° C. or lower. A more preferable heat treatment temperature is 300 to 700 ° C. The heat treatment time is preferably 1 to 30 minutes. When the heat treatment temperature is higher than 700 ° C. or the heat treatment time is longer than 30 minutes, the groove (groove) of the crystal grain boundary may become deep, and polishing for removing this may be required after the orientation treatment. . The heat treatment of the copper foil 4 may be performed before the copper foil 4 is laminated on the support 2 or after the copper foil 4 is laminated on the support 2.
As a method of laminating the rolled copper foil 4 on the support 2, both surfaces are cleaned by dry etching, then both are laminated with no pressure or pressure, and the surfaces are joined by atomic force on the surface. "Activated bonding" can be used (see Patent Document 2).

The superconducting substance constituting the superconducting film 8 is a substance that has an electric resistance of 0 when the substance is cooled below a specific temperature (critical temperature). In particular, from the viewpoint of practical use, a high-temperature superconducting material having a critical temperature equal to or higher than the boiling point (−196 ° C.) of liquid nitrogen is preferable. Examples of the high-temperature superconducting material include, but are not limited to, an yttrium-based superconductor (YBCO, Y123), a rare earth element-based oxide superconductor (R123), and a copper oxide high-temperature superconductor.
In the example of FIG. 1, a barrier layer 6 made of a Ni plating layer or the like is formed on the surface of the rolled copper foil 4 for forming a superconducting film. This is because when the superconducting film 8 is directly formed on the surface of the rolled copper foil 4 for forming a superconducting film, components (oxides, etc.) of the superconducting film 8 are diffused to the copper foil 4 side during the film formation to form copper oxide. This is because the copper foil 4 is easily oxidized by the high temperature during film formation. Therefore, it is preferable to form the barrier layer 6 on the surface of the rolled copper foil 4 for forming a superconducting film. As the barrier layer 6, nickel or a nickel alloy is preferably used.
In the example of FIG. 1, the rolled copper foil 4 for forming a superconducting film is formed on one side of the support 2, but the rolled copper foil 4 for forming a superconducting film may be formed on both sides of the supporting body 2. .

<Composition>
As the component composition of the rolled copper foil for forming a superconducting film, tough pitch copper (TPC) or JIS-H3100 (C1020) oxygen-free copper (OFC) standardized to JIS-H3100 (C1100) can be suitably used.
Further, the additive element is selected from the group consisting of Ag, Sn, In, Ti, Zn, Zr, Fe, P, Ni, Si, Te, Cr, Nb, and V as an additive element with respect to the above-described tough pitch copper or oxygen-free copper. You may contain 20-1500 mass ppm in total of 1 or more types. For example, 10 to 500 mass ppm of Sn and / or 10 to 500 mass ppm of Ag can be contained as an additive element with respect to the above-described tough pitch copper or oxygen-free copper.
When the total content of the above elements is less than 20 ppm by mass, the softening temperature is low, and the storability at normal temperature may be deteriorated.
The thickness of the rolled copper foil is preferably 100 μm or less, more preferably 50 μm or less, and more preferably 20 μm or less. Further, the lower limit of the thickness of the rolled copper foil is not particularly limited, but considering the manufacturability and the like, the thickness of the rolled copper foil is preferably 4 μm or more, more preferably 5 μm or more, and further preferably 6 μm or more.

<Crystal orientation of copper foil>
A copper foil used as a substrate for a superconductive film is required to develop a cubic orientation after recrystallization heat treatment. As an evaluation method of the cube orientation, the ratio (I ₀ ) of the strength (I) of the (200) plane obtained by X-ray diffraction of the rolled surface to the strength (I ₀ ) of the (200) plane obtained by X-ray diffraction of fine powder copper / I ₀ ) is measured. According to the study by the present inventors, for example, a copper foil manufactured by cold rolling with a high workability disclosed in Patent Document 2 and the like exhibits a high I / I ₀ value of 50 or more after the recrystallization heat treatment. When the I / I ₀ value is less than 50, the characteristics of the superconducting film are remarkably deteriorated. Therefore, the I / I ₀ value is defined to be 50 or more. The I / I ₀ value is preferably 60 or more, more preferably 80 or more.
On the other hand, the upper limit value of the I / I ₀ value is not restricted in terms of the characteristics of the superconducting film, and is generally better as it is higher. However, in the copper foil of the present invention manufactured in the process described later, The I ₀ value never exceeds 200.

As described above, the recrystallization heat treatment is performed in order to orient the copper foil in the cubic orientation in the manufacturing process of the orientation plate for forming a superconducting film, and the heat treatment temperature ranges from 200 ° C. to the melting point of pure copper. More preferably, it is 800 degrees C or less, More preferably, it is 300-700 degreeC. The heat treatment time is preferably 1 to 30 minutes. If the copper foil is sufficiently recrystallized, the effect on the I / I ₀ value is negligible even if the temperature or time of the heat treatment changes somewhat within the above range.

<Shear band>
Metallic materials cause slip deformation when rolled, but when deformed at a high workability, nonuniform deformation due to plastic instability occurs and shear bands occur. The shear band refers to a thin planar structure inclined at 30 to 60 degrees with respect to the surface of the rolled sheet (for example, “Iron and Steel” 70th year (1984) No. 15, p. 18). The shear band has a crystal orientation almost similar to that of the surrounding matrix, but has a dense cell structure and recrystallization nucleation is likely to occur. Therefore, in a material with a developed shear band, recrystallization occurs unevenly between the shear band and the matrix, and as a result, the development of the recrystallized texture is hindered.
In the state before the recrystallization heat treatment, the number of shear bands reaching the surface across the line of the depth of 1/10 of the copper foil thickness in the thickness direction from the copper foil surface in the thickness direction as seen from the rolled parallel cross section When the total value is adjusted to 0.1 / μm or less, the characteristics (for example, critical current density Jc) of the superconducting film formed on the copper foil surface are improved. When the number of shear bands is defined in this way, it is considered that recrystallization occurs uniformly by subsequent recrystallization heat treatment, and the superconducting film is easily epitaxially grown on the surface of the copper foil. As a result, the characteristics of the superconducting film are improved. Note that the number of shear bands decreases (or disappears) after the recrystallization heat treatment. The number of shear bands can be controlled by adjusting the conditions of final cold rolling as will be described later.
Here, even if the number of shear bands crossing the center of the copper foil thickness is small, the characteristics of the superconducting film deteriorate when the shear bands develop near the surface. Therefore, in the present invention, in order to evaluate the shear band in the vicinity of the copper foil surface, what reaches the surface across a line having a depth of 1/10 of the copper foil thickness in the thickness direction from the copper foil surface is regarded as a shear band. .

  The reason why the characteristics of the superconducting film (for example, the critical current density Jc) is improved by reducing the shear band to 0.1 pieces / μm or less is not clear, but for example, when forming YBCO as the superconducting film, The critical current density is improved when nucleation of c-axis oriented grains in the YBCO film is promoted, and the critical current density is not improved when a large number of a-axis oriented grains are generated. Therefore, the superconducting film can be identified by the surface properties of the copper foil. It is thought that the number of crystal grains oriented in the orientation increases.

<Identification of shear band>
In the shear band, a structure formed by intensive shear deformation on the rolling surface and a surface inclined by 30 to 60 degrees due to plastic instability due to strong processing appears on the observation surface. Therefore, the shear band is observed as a discontinuous surface of the rolled structure. Since the crystal orientation of the shear band portion is not different from the parent phase, the shear band cannot be defined by crystal orientation measurement. On the other hand, since the shear band extends in the depth direction, it can be specified by observing the cross section of the material. Therefore, when the cross section in the rolling parallel direction of the copper foil after the final rolling is observed, a discontinuous portion of the rolled structure inclined by 30 to 60 degrees is defined as a shear band. Specifically, an image of the above-mentioned cross-section microscope (metal microscope, scanning electron microscope (SEM), scanning ion microscope (SIM), etc.) is obtained, and a line inclined by 30 to 60 degrees is subjected to image analysis or visual observation. It can be determined as a shear band. The cross-section processing of the copper foil is preferably performed by FIB or CP, but a method such as mechanical polishing may be used.
FIG. 3 shows an SEM image of the structure as viewed from the cross section in the rolling parallel direction. In this figure, a line connecting two arrows represented by symbol Sh is a shear band. The white arrow is a shear band that does not reach the line C.

<Measurement of shear band>
As shown in FIG. 2, the shear band is measured by polishing the cross-section R in the rolling parallel direction RD of the copper foil before the recrystallization (orientation) heat treatment so that the width W in the RD direction is 200 μm or more and the thickness of the copper foil. An observation visual field V having a height t is determined, and an image of a scanning electron microscope (SEM) is obtained. And the number of shear bands obtained by dividing the number of shear bands Sh reaching the copper foil surface across the line C having a depth of 1/10 of the copper foil thickness in the thickness direction from the copper foil surface by the visual field width W (Books / μm). Further, since the lines C can be drawn from the front and back surfaces of the copper foil, the number of shear bands is the sum of the values measured for the front and back surfaces of the copper foil.
The significant shear band Sh is a line whose one end reaches the copper foil surface and the other end intersects the line C, and other shear bands (not reaching the copper foil surface or intersecting the line C). The shear band is not counted as a shear band in the present invention because it has a small effect on the recrystallization texture development and the properties of the superconducting film.

Next, an example of the manufacturing method of the rolled copper foil for superconducting film formation of this invention is demonstrated. First, an ingot made of copper, necessary alloy elements, and inevitable impurities is hot-rolled, and then cold-rolling and annealing are repeated, and finally, it is finished to a predetermined thickness by final cold-rolling.
In the final cold rolling, the material is finished to a predetermined thickness by repeatedly passing (passing) the material through a rolling mill. In the production of the rolled copper foil of the present invention, at least 5 passes are repeated. In this final cold rolling, the shear band can be adjusted to 0.1 or less per μm by adjusting the total workability and the workability of each pass as follows. Here, the total workability R is a sheet thickness reduction rate in the final cold rolling, and R = (T ₀ −T) / T ₀ (T ₀ : thickness before final cold rolling, T: final cold rolling) Later thickness). Further, the processing degree r per pass is a sheet thickness reduction rate when the rolling roll passes once, and r = (t ₀ -t) / t ₀ (t ₀ : thickness before passing the rolling roll, t: thickness after passing through the rolling roll).

The total degree of work in the final cold rolling is preferably 99.5% or less, more preferably 99.0% or less, and even more preferably 98.0% or less. By reducing the total degree of processing, the development of shear bands can be suppressed. The total workability in the final rolling is preferably 90% or more. When the total workability is less than 90%, the I / I ₀ value of the (200) plane after recrystallization heat treatment may be less than 50. is there. The total degree of processing is more preferably 93% or more, and still more preferably 95% or more.
In addition to the above-mentioned upper limit of total workability, the workability of each pass of final cold rolling is the last of the last 5 passes (meaning 5 passes from the 4th pass before the final pass to the final pass). There is a path with a higher degree of machining than the first path, the maximum degree of machining of any of the five paths except the final path is 40% or more, and the degree of machining in the final path is the smallest of the five paths By setting so as to be, the shear band can be 0.1 or less per μm.
In the final 5 passes, there is a pass with a higher processing degree than the previous pass, and the maximum processing degree of any pass excluding the final pass is set to 40% or more, so that the copper foil is uniformly deformed in the thickness direction Thus, local deformation can be suppressed and the development of the shear band can be prevented. In addition, since a shear deformation structure develops on the material surface due to the friction between the material surface and the work roll during rolling, the formation of a shearing layer on the material surface is suppressed by rolling the final pass at a low workability. be able to. However, if a high degree of processing is applied when the material is thick, it does not deform uniformly in the material thickness direction, and a shear band is likely to be formed near the surface of the material, so the maximum processing degree is 2-4 passes before the final pass. Is preferred.

Ingots were cast by adding the elements shown in Table 1 to tough pitch copper (TPC) standardized to JIS-H3100 (alloy number C1100) or JIS-H3100 (alloy number C1020) oxygen-free copper (OFC). The produced ingot is hot-rolled at a temperature of 800 ° C. or more to a thickness of 10 mm, and after chamfering the oxide scale on the surface, after cold rolling and annealing are repeated, the thickness is further 0.006 to 0.1 mm in the final cold rolling. (See Table 1).
The final cold rolling was performed in 10 to 15 passes, and the total degree of work of the final cold rolling was set to the values shown in Table 1. In addition, the degree of processing in the final 5 passes of the final cold rolling (from 4 passes before the final pass to the final pass) was set to the values shown in Table 1.

Various characteristics of each copper foil sample thus obtained were evaluated.
(1) I / I ₀ value of (200) plane Simulating recrystallization (orientation) heat treatment, the sample was heated at 700 ° C. for 30 minutes, and then determined by X-ray diffraction of the rolled surface (200) plane strength The integral value (I) of was obtained. This value is divided by the integral value (I ₀ ) of the (200) plane strength of finely powdered copper (manufactured by Kanto Chemical Co., Inc., 325 mech,> 99.5% copper powder), and I / I ₀ The value was calculated. RINT2500 (manufactured by Rigaku Corporation) was used as the measurement apparatus, and Co was used as the X-ray source.

(2) Number of shear bands (frequency)
As shown in FIG. 2, the cross section R of the rolling parallel RD of the sample before the recrystallization (orientation) heat treatment is polished (mechanical polishing or CP (cross section polisher method) to obtain a width W = 200 μm or more in the RD direction, and copper An observation field of view V having a height t of the foil was determined, and an image of a scanning electron microscope (SEM) was obtained, and the number of shear bands Sh reaching the copper foil surface across the line C in FIG. The number divided by the visual field width W was visually counted as the number of shear bands (lines / μm).
In addition, draw lines C from the front and back surfaces of the copper foil, and measure the number of shear bands on the front and back surfaces of the copper foil, respectively, {(number of shear bands on the front surface) + (number of shear bands on the back surface)} / field of view. The number of shear bands was determined from the width W.

(3) Characteristics of superconducting film (critical current density Jc)
As a recrystallization (orientation) heat treatment, each obtained copper foil was heated at 700 ° C. for 30 minutes in an atmosphere consisting of 95% nitrogen and 5% hydrogen. Next, the heated copper foil and the support (SUS316 stainless steel, thickness 0.1 mm) were placed in a predetermined vacuum apparatus, and each joint surface was cleaned by argon ion beam etching. Thereafter, the copper foil and the support were laminated and pressed in a vacuum apparatus to obtain a superconducting film forming alignment plate.

A superconducting film comprising a YBCO film is formed by electroplating a 2 μm Ni plating layer as a barrier layer on the copper foil surface of this orientation plate for forming a superconducting film, and by TFA-MOD (Metal Organic Deposition using Tri Fluoro Acetates) method on the barrier layer. Formed. Then, the critical current density Jc was measured with a voltage reference of 1 μV / cm by a direct current four-terminal method in a self-magnetic field at 77K.
In addition, a case in which Jc is more than ^{100000A / cm 2 ◎, 10000A /} cm 2 Beyond ○ the case of 100000 A / cm ² or less, 100 more than the ^{A / cm 2 10000 A / cm} 2 or less of the case △ The case of 100 A / cm ² or less was expressed as x.

As is clear from Tables 1 and 2, in each example, the shear band reaching the surface across the line C as viewed from the rolling parallel section before the recrystallization (orientation) heat treatment was 0.1 / μm. The degree of orientation I / I ₀ of the cube texture after the recrystallization (orientation) heat treatment was 50 or more, and the characteristics (critical current density) of the superconducting film were improved. In particular, before recrystallization (orientation) heat treatment, Examples 1 to 3, 5, 7 to 9, 11, 12, 15 to 17 have a shear band of 0.05 / μm or less across the line C and reaching the surface. In this case, the characteristics (critical current density) of the superconducting film were further improved as compared with other examples. From this, it is more preferable that the number of shear bands crossing the line C and reaching the surface is 0.05 / μm or less.

On the other hand, in the final cold rolling, Comparative Example 1 in which the degree of processing in any of the final five passes was less than 40%, Comparative Examples 2, 3, in which the degree of processing in the final pass was not minimized in the five passes. In the final 5 passes, there is no pass having a higher processing degree than the previous pass (that is, the processing degree monotonously decreases toward the final pass) of Comparative Example 4 and Comparative Example 6 in which the total processing degree exceeds 99.5% In the case of Comparative Example 5 in which the degree of work in the final pass is not minimum in 5 passes and the total degree of work exceeds 99.5%, a shear band crossing line C appears before the recrystallization (orientation) heat treatment. The total value on the back surface exceeded 0.1 / μm. In these comparative examples, although the I / I ₀ of the (200) plane of the cubic texture after the recrystallization heat treatment was 50 or more, the characteristics (critical current density) of the superconducting film were inferior. This is presumably because recrystallization occurred unevenly during the recrystallization heat treatment, and the superconducting film became difficult to epitaxially grow on the copper foil surface.

Further, in Comparative Example 7 in which the total degree of work of the final cold rolling was less than 90.0%, I / I ₀ was less than 50, and the characteristics (critical current density) of the superconducting film were inferior.
Table 1 also shows the total value of the front and back surfaces of the shear band that reaches the surface across the center line in the thickness direction from the copper foil surface. It can be seen that the characteristics (critical current density) cannot be evaluated.

t Thickness C of copper foil Line Sh with 1/10 depth of copper foil thickness from the copper foil surface in the thickness direction

Claims (4)

A rolled copper foil for forming a superconducting material on its surface, which is formed from a copper foil in the thickness direction from the surface of the rolled copper foil as viewed from the rolled parallel section in the state before the following recrystallization heat treatment. The shear band reaching the surface across a line having a depth of 1/10 of the thickness is not more than 0.1 / μm in total on the front and back surfaces, and is subjected to a recrystallization heat treatment at 700 ° C. for 30 minutes. I ₀ ≧ 50 (I: diffraction peak integrated intensity of (200) plane determined by X-ray diffraction of rolled surface of copper foil, I ₀ : diffraction peak integral of (200) plane determined by X-ray diffraction of fine powder copper A rolled copper foil for forming a superconducting film that exhibits a crystal orientation of (strength).   2. The rolled copper foil for forming a superconducting film according to claim 1, wherein the shear band is 0.05 or less per μm in total on the front and back surfaces.   The ingot is manufactured by hot rolling, then cold rolling and annealing are repeated, and finally the final cold rolling is performed, and the total degree of work of the final cold rolling is 90.0 to 99.5%. The rolled copper foil for superconducting film formation as described in 2.   In the final cold rolling, there is a pass having a higher workability than the previous pass in the final 5 passes, and the maximum workability of any of the 5 passes is 40% or more, and in the final pass The rolled copper foil for forming a superconducting film according to claim 3, wherein the degree of processing is minimum in the five passes.