Enamel-coated superconducting wire and method of using the enamel-coated superconducting wire

Abstract

To provide an enamel-coated superconducting wire having an insulating layer of an enamel layer made of a polyimide resin and capable of easily peeling the insulating layer by chemical peeling. 【Solution means】An enamel-coated superconducting wire comprising a superconducting wire 2 and an insulating layer 3 covering the outer periphery of the superconducting wire 2, wherein the insulating layer 3 is an enamel layer made of a polyimide resin and the imidization rate of the insulating layer 3 is 92% or less.

Description

Technical Field

[0001] The present invention relates to an enamel-coated superconducting wire.

Background Art

[0002] Conventionally, an enamel-coated superconducting wire having an insulating layer formed of an enamel resin on the outermost periphery has been used (see Patent Document 1). The insulating layer formed on the outermost periphery prevents electrical contact with the outside and short circuits during winding. Users of enamel-coated superconducting wires peel off and remove the insulating layer at the end portion that they want to conduct with other electrodes or the like in the wire, and then solder and incorporate it into various devices. In particular, since high-temperature superconducting wires have a short fixed length, when used in large coils or the like, the peeling and connecting operations are required frequently, so the productivity of the peeling process becomes important.

[0003] Examples of methods for peeling the insulating layer of an enamel-coated superconducting wire include mechanical peeling, laser peeling, and chemical peeling. Mechanical peeling is a method in which a rotating blade or the like is pressed against the wire to mechanically peel it. However, in mechanical peeling, not only a special device must be prepared, but also a peeling blade corresponding to the shape must be prepared so as not to cut the conductor wire (superconducting wire), resulting in poor productivity.

[0004] Laser peeling is a method of removing the insulating layer by irradiating the insulating layer with laser light to melt the insulating layer or generate and rupture bubbles between the wire and the insulating layer. However, in laser peeling, not only a laser device must be prepared, but also the irradiation angle of the laser beam must be precisely adjusted, and frequent component replacement and device adjustment are required, resulting in poor productivity. In addition, in laser peeling, there is also a concern that the heat of the laser may damage the superconducting layer.

[0005] On the other hand, chemical stripping is a method of immersing in a stripping solution to swell or dissolve the insulating layer and strip it from the wire. Chemical stripping is simple as it only requires preparing the stripping solution and immersing the wire in it. Compared to mechanical stripping and laser stripping, it does not require the preparation of parts or adjustment of equipment, so it has excellent productivity.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, in chemical stripping where the insulating layer is swollen or dissolved by a chemical reaction, it may take time for the reaction. In particular, it tends to take time to strip an enamel layer made of a polyimide resin with a high heat-resistant temperature. For example, in the study by the inventors of the present invention, it took 3 minutes or more for stripping with a film thickness of about 25 μm, and in some cases, about 10 minutes depending on the degree of adhesion of the resin. Therefore, under conditions where the reaction takes time, chemical stripping may not necessarily be a method with excellent productivity.

[0008] An object of the present invention is to provide an enamel-coated superconducting wire that has an insulating layer of an enamel layer made of a polyimide resin and can easily strip the insulating layer by chemical stripping.

Means for Solving the Problems

[0009] The above object is achieved by the following present invention. That is, the aspects of the present invention are as follows.

[0010] <1> A superconducting wire and an insulating layer covering the outer periphery of the superconducting wire, wherein the insulating layer is an enamel layer made of a polyimide resin, An enamel-coated superconducting wire in which the imidization rate of the insulating layer is 92% or less.

[0011] <2> The enamel-coated superconducting wire according to <1>, wherein the 5% weight loss temperature by thermogravimetric measurement (TG) in the insulating layer is 408°C or less.

[0012] <3> The enamel-coated superconducting wire according to <1>, wherein the superconducting wire is in tape form, and an intermediate layer, a superconducting oxide layer, a protective layer, and a stabilizing layer are laminated in this order on a base material.

Advantages of the Invention

[0013] According to the present invention, it is possible to provide an enamel-coated superconducting wire having an insulating layer of an enamel layer made of a polyimide resin and capable of easily peeling the insulating layer by chemical peeling.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Modes for Carrying Out the Invention

[0015] Hereinafter, specific embodiments of the present invention will be described in detail with reference to the drawings. Note that the present invention is not limited to the following embodiments, and various modifications are possible without departing from the gist of the present invention.

[0016] FIG. 1 is a perspective view showing a superconducting wire according to an exemplary embodiment of the present invention, and FIG. 2 is a cross-sectional view taken along the virtual plane P shown in FIG. 1.

[0017] As shown in FIG. 1(a), the superconducting wire 1 includes a tape-shaped superconducting wire 2 and an insulating layer 3. Among these, the superconducting wire 2 has a tape shape, and an intermediate layer, a superconducting oxide layer, a protective layer, and a stabilizing layer are laminated in this order on a base material. More specifically, as shown in FIG. 2, a tape-shaped metal substrate 21 serving as a base material, an intermediate layer 22 provided on one main surface of the metal substrate 21, a superconducting layer 23 which is a superconducting oxide layer provided on the surface of the intermediate layer 22, a protective layer 24 covering the periphery of the metal substrate 21, the intermediate layer 22, and the superconducting layer 23, and a stabilizing layer 25 covering the periphery of the protective layer 24 are preferably provided.

[0018] The metal substrate 21 constituting a part of the superconducting wire 2 is a tape-shaped metal substrate. The material of the metal substrate 21 is not particularly limited, and for example, a nickel alloy such as Hastelloy (registered trademark) can be used.

[0019] The intermediate layer 22 is provided on one main surface of the metal substrate 21. The main surface of the metal substrate 21 is a surface along the width direction of the metal substrate 21, and in FIG. 2, it is the upper and lower surfaces. The intermediate layer 22 is a layer for controlling the crystal orientation of the superconducting layer 23 laminated thereon. The intermediate layer 22 may have either a single-layer structure or a multilayer structure, and the material is not particularly limited. For example, metal oxides such as Gd2Zr2O7, MgO, ZrO2 - Y2O3 (YSZ), SrTiO3, LaMnO3, CeO2, Y2O3, Al2O3, Gd2O3, Zr2O3, Ho2O3, Nd2O3 can be used. The intermediate layer 22 may be a laminate composed of a plurality of layers such as four or five layers.

[0020] The superconducting layer 23 is provided on the surface of the intermediate layer 22. As the superconductor constituting the superconducting layer 23, it is preferably a RE-based superconductor (RE: rare earth element) that exhibits superconductivity at a temperature equal to or higher than the liquid nitrogen temperature. It is preferable to use so-called high-temperature superconductors such as GdBCO and SmBCO that use Sm or Gd as the rare earth element. In the superconducting wire 2, the metal substrate 21, the intermediate layer 22, and the superconducting layer 23 are laminated in this order to form a laminate.

[0021] The protective layer 24 covers the metal substrate 21 and the entire intermediate layer 22 and superconducting layer 23 provided on one side of the metal substrate 21. That is, the protective layer 24 covers the entire periphery of the laminate in which the metal substrate 21, the intermediate layer 22, and the superconducting layer 23 are laminated. The protective layer 24 is a metal layer formed by sputtering, vapor deposition, or the like, and is preferably composed of silver.

[0022] The stabilization layer 25 covers the periphery of the protective layer 24. That is, the stabilization layer 25 covers the entire outer periphery of the protective layer 24 that covers the entire periphery of the laminate in which the metal substrate 21, the intermediate layer 22, and the superconducting layer 23 are laminated. The stabilization layer 25 is a metal layer and is preferably composed of copper. The thickness of each of these layers may be appropriately set according to the purpose of use, specifications, desired performance, etc. of the superconducting wire 1.

[0023] The insulating layer 3 covers the outer periphery of the superconducting wire 2. That is, as shown in FIGS. 1 and 2, the insulating layer 3 covers the entire outer periphery of the protective layer 24 and the stabilization layer 25 that cover the entire periphery of the laminate in which the metal substrate 21, the intermediate layer 22, and the superconducting layer 23 are laminated. The insulating layer 3 can be formed by applying a coating liquid (hereinafter referred to as "polyimide precursor coating liquid") that becomes a precursor of the polyimide resin by a conventionally known method and heating and curing (imidizing). The thickness of the insulating layer 3 may be appropriately set according to the purpose of use, specifications, desired performance, etc. of the superconducting wire 1, but is approximately selected from the range of about 5 to 50 μm, and preferably from the range of about 10 to 25 μm.

[0024] The size of the entire superconducting wire 1 may be appropriately set according to the purpose of use, standards, desired performance, etc. As for the width (in the direction of arrow X in FIGS. 1 and 2), it is selected from a range of approximately 1 to 20 mm, preferably from a range of approximately 2 to 12 mm. Also, as for the thickness of the entire superconducting wire 1 (in the direction of arrow Y in FIGS. 1 and 2), it is selected from a range of approximately 25 to 500 μm, preferably from a range of approximately 50 to 150 μm.

[0025] In this embodiment, the insulating layer 3 is an enamel layer made of polyimide resin, and the imidization rate of the insulating layer 3 is 92% or less. As shown in the examples described later, by suppressing the imidization rate to 92% or less, the time required for chemical peeling can be suppressed. The upper limit of the imidization rate of the insulating layer 3 is preferably 90% or less.

[0026] On the other hand, the lower limit of the imidization rate of the insulating layer 3 is not particularly limited, but if it is too low, film formation is likely to be insufficient, so it is preferably 45% or more, more preferably 60% or more, and even more preferably 70% or more.

[0027] The coating of polyimide resin (enamel layer) can be formed by imidizing (dehydrating and oxidizing) the polyamic acid, which is a precursor, by heating or the like. Polyamic acid dissolves in an organic solvent, but it becomes insoluble when it becomes polyimide after the reaction. Therefore, the polyamic acid is applied to the object to be coated in a state dissolved in an organic solvent (that is, a polyimide precursor coating solution), and then heat treatment (also referred to as "baking") is performed to remove the solvent and proceed with the imidization reaction, thereby forming a coating of polyimide resin.

[0028] FIG. 3 shows a chemical reaction formula for generating polyimide resin from polyamic acid. The number of benzene rings (symbol B) on the left and right sides of the chemical reaction formula does not change, but an imide structure (symbol I) is newly generated by imidization. The imidization rate can be determined by measuring the amount of generation of this imide structure by FT-IR (Fourier transform infrared spectroscopy).

[0029] In FT-IR measurement, a sample is irradiated with infrared light, and an IR spectrum is obtained from the amount of transmitted or reflected light. Since the IR spectrum shows a unique pattern depending on the molecular structure, the molecular structure of the sample can be analyzed using this. The spectra of the benzene ring with an unchanged number in polyamic acid and polyimide and the imide group inherent in the polyimide structure were obtained by FT-IR, and the ratio of the imidized polyimide structure obtained by heat treatment was determined from the intensity ratio, and this was defined as the imidization rate.

[0030] In the examples described later, an IR spectrum was obtained by FT-IR measurement of the insulating layer of the cured polyimide resin, and an example thereof is shown in FIG. 4. Referring to FIG. 4, the peak at a wavelength of 1494 [cm -1 (arrow B in FIG. 4) indicates the presence of a benzene ring, and the peak at 1712 [cm -1 (arrow I) indicates the presence of an imide structure. Since the number of benzene rings does not change before and after the reaction as described above, by calculating the ratio of the peak of the imide structure (arrow I in FIG. 4) based on this, the "imidization rate" in the present embodiment can be determined.

[0031] In order to suppress the imidization rate of the insulating layer 3 to 92% or less, the heating conditions for imidization when curing from polyamic acid to polyimide may be appropriately adjusted. Generally, the imidization rate can be lowered by lowering the heating temperature and / or shortening the heating time. Note that the required heating temperature and heating time vary depending on the resin type of the polyimide (polyamic acid) used, so these preferred conditions cannot be specifically shown.

[0032] In addition, in the present embodiment, as the insulating layer 3, it is preferable that the 5% weight loss temperature (hereinafter, may be simply referred to as "5% weight loss temperature") by thermogravimetric measurement (TG) is 408 °C or lower. As shown in the examples described later, by suppressing the 5% weight loss temperature to a lower level, the time required for chemical peeling can be suppressed. As the upper limit of the 5% weight loss temperature of the insulating layer 3 by thermogravimetric measurement (TG), it is more preferable that it is 400 °C or lower.

[0033] On the other hand, as the lower limit of the 5% weight loss temperature of the insulating layer 3, there is no particular limitation, but since heat resistance higher than that of the high-temperature superconducting wire is desired, it is preferably 200 °C or higher, and more preferably 215 °C or higher.

[0034] Thermogravimetric measurement (TG) is a method of thermally analyzing changes in the physical and chemical properties of a sample by measuring the weight of the sample when the temperature is raised at a constant rate, and it is standardized in Japanese Industrial Standard JIS K0129. The weight loss temperature is the temperature at which the weight ratio before and after heating obtained by thermogravimetric measurement (TG) reaches a set threshold value (5% in the present embodiment).

[0035] In order to suppress the 5% weight loss temperature of the insulating layer 3 to 408 °C or lower, the heating conditions for imidization when curing from polyamic acid to polyimide may be appropriately adjusted. Generally, the 5% weight loss temperature can be lowered by lowering the heating temperature and / or shortening the heating time. Note that since the required heating temperature and heating time vary depending on the resin type of the polyimide (polyamic acid) used, these preferable conditions cannot be specifically shown.

[0036] A sample of the insulating layer 3 for FT-IR measurement for obtaining the imidization rate and thermogravimetric measurement (TG) for obtaining the 5% weight loss temperature can be obtained by directly scraping off the insulating layer 3 coated on the outermost periphery of the target superconducting wire 1 with a spatula or the like.

[0037] The embodiments described above are merely examples of typical forms of the present invention, and the present invention is not limited to the above embodiments. For example, in the above embodiments, as the superconducting wire 2, an example has been described in which the entire periphery of a laminate formed by laminating a metal substrate 21, an intermediate layer 22, a superconducting layer 23, and a protective layer 24 is covered with a stabilizing layer 25. However, in the present invention, it is not limited to such a layer configuration. The stabilizing layer 25 does not have to cover the entire periphery of the laminate. For example, the superconducting wire may have a configuration in which an intermediate layer, a superconducting oxide layer, a protective layer, and a stabilizing layer are laminated in this order on a base material.

[0038] Also, not all of these layers are essential components. Layers other than the superconducting layer 23 may be appropriately omitted or may be divided into two or more layers. Further, in addition to or instead of these layers, other functional layers may be formed. Regardless of the layer configuration of the superconducting wire 2, as long as the outermost periphery thereof is covered with an insulating layer characteristic of the present invention, the wire corresponds to the enamel-coated superconducting wire of the present invention.

[0039] In addition, those skilled in the art can implement various modifications in accordance with conventionally known knowledge without departing from the gist of the present invention. As long as the modified form still has the configuration of the enamel-coated superconducting wire of the present invention, it is of course included in the scope of the present invention.

Examples

[0040] Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to the following examples.

[0041] (Preparation of superconducting wire) As the superconducting wire to be used in the tests of the examples and comparative examples, a superconducting wire 2 having the layer configuration shown in FIGS. 1 and 2 was prepared.

[0042] (Preparation of polyimide precursor coating solution) As the polyimide precursor coating solution, two types of coating solutions with different heat curing conditions (a "polyimide precursor coating solution A" with high heating conditions and a "polyimide precursor coating solution B" with low heating conditions) were prepared.

[0043] (Fabrication of Superconducting Wire) On the outer periphery of the prepared superconducting wire, polyimide precursor coating solution A or polyimide precursor coating solution B was applied by the dip coating method. The coating amount of the coating solution was adjusted so that the film thickness of the insulating layer after curing would be 25 μm.

[0044] For the superconducting wire coated with polyimide precursor coating solution A or polyimide precursor coating solution B, heat treatment was carried out by appropriately changing the conditions (baking conditions) within the range of heating temperature from 160 to 300 °C and heating time from 3 to 60 minutes, respectively. Then, Examples 1 to 4 and Comparative Examples 1 to 4 in which an enamel layer of polyimide resin A was formed by polyimide precursor coating solution A, and Examples 5 to 8 and Comparative Examples 5 to 17 in which an enamel layer of polyimide resin B was formed by polyimide precursor coating solution B were fabricated for each superconducting wire material.

[0045] (Sampling of Measurement Specimens) A part of the insulating layer formed on the surface of each of the obtained superconducting wire materials of Examples 1 to 8 and Comparative Examples 1 to 17 was scraped off with a spatula to collect each measurement specimen.

[0046] (Measurement of Imidization Rate) For each of the collected measurement specimens of Examples 1 to 8 and Comparative Examples 1 to 17, FT-IR measurement was performed using an FT-IR apparatus to obtain an IR spectrum. The imidization rate was calculated from the obtained IR spectrum by the method described above. The results are summarized and shown in Table 1 and Table 2 below. Note that, as a representative, only for Example 5, Comparative Example 6, Comparative Example 12, and Comparative Example 15, a graph showing the IR spectrum is shown in Figure 4 shown below.

[0047] (Measurement of 5% Weight Loss Temperature) For each of the collected measurement specimens of Examples 1 to 8 and Comparative Examples 1 to 17, thermogravimetric measurement (TG) was performed using a thermogravimetric analyzer (TGA) to measure the 5% weight loss temperature. The results are summarized and shown in Table 1 and Table 2 below.

[0048] (Chemical Peelability Test) For each of the superconducting wires of Examples 1 to 8 and Comparative Examples 1 to 17, a region of about 30 mm from the end thereof was immersed in a stripping solution containing 30% by mass of potassium hydroxide (manufactured by Meiwa Chemical Industry Co., Ltd., "SOLCOAT #MLJ"), and the time until the insulating layer was stripped was measured. Whether the insulating layer was stripped or not was determined by taking out the superconducting wire every minute and visually observing it. When the insulating layer was not stripped, it was immersed again in the stripping solution and repeated until it was stripped. On the other hand, when it was stripped, the test was terminated at that stage, and the immersion time in the stripping solution was obtained in minutes and used as the result of the stripping time by the chemical peelability test. The results are summarized in Table 1 and Table 2 below.

[0049] (Evaluation Results of Examples and Comparative Examples) The results of Examples 1 to 4 and Comparative Examples 1 to 4 using the polyimide precursor coating solution A are shown in Table 1 below.

[0050] [Table 1]

[0051] The results of Examples 5 to 8 and Comparative Examples 5 to 17 using the polyimide precursor coating solution B are shown in Table 2 below.

[0052] [Table 2]

[0053] The relationship between the imidization rate shown in Table 1 and Table 2 above and the stripping time by the chemical peelability test is shown in the graph of Figure 5 The graph of Figure 5 plots the results of each example and comparative example as points with the imidization rate (%) on the horizontal axis and the stripping time (minutes) on the vertical axis. The broken line in the graph of Figure 5 is the line with an imidization rate of 92%.

[0054] Also, the relationship between the 5% weight loss temperature shown in Table 1 and Table 2 above and the stripping time by the chemical peelability test is shown in the graph of Figure 6 The graph of Figure6 The graph plots the results of each example and comparative example as points, with the horizontal axis being the 5% weight loss temperature (°C) and the vertical axis being the peeling time (minutes). Figure 6 The dashed line in the graph of Figure is the line with a 5% weight loss temperature of 408 °C.

[0055] In addition, Figure 5 and Figure 6 The graph also includes the results of superconducting wire materials not included in Examples 1 to 8 and Comparative Examples 1 to 17.

[0056] (Discussion of Results) As is clear from looking at the graph of Figure 5 , in each of polyimide resin A and polyimide resin B, when the imidization rate is 92% or less (the range enclosed by the dashed line in the graph), the peeling time by the chemical peelability test is suppressed to a short time, whereas when it exceeds 92% (the range enclosed by the dash-dotted line in the graph), it can be seen that the peeling time becomes long.

[0057] Also, as is clear from looking at the graph of Figure 6 , in each of polyimide resin A and polyimide resin B, when the 5% weight loss temperature is 408 °C or less (the range enclosed by the dashed line in the graph), the peeling time by the chemical peelability test is suppressed to a short time, whereas when it exceeds 408 °C (the range enclosed by the dash-dotted line in the graph), it can be seen that the peeling time becomes long.

Explanation of Symbols

[0058] 1: Superconducting wire material, 2: Superconducting wire, 3: Insulating layer, 21: Metal substrate (base material), 22: Intermediate layer, 23: Superconducting layer, 24: Protective layer, 25: Stabilizing layer

Claims

1. A superconducting wire and an insulating layer covering the outer periphery of the superconducting wire, wherein the insulating layer is an enamel layer made of a polyimide resin, and the imidization rate of the insulating layer is 45% or more and 92% or less, an enamel-coated superconducting wire.

2. The enamel-coated superconducting wire according to claim 1, wherein the 5% weight loss temperature by thermogravimetric measurement (TG) in the insulating layer is 408 °C or lower.

3. The enamel-coated superconducting wire according to claim 1, wherein the superconducting wire is in tape form, and an intermediate layer, a superconducting layer, a protective layer, and a stabilization layer are laminated in this order on a substrate.

4. A method of using the enamel-coated superconducting wire according to claim 1, wherein when conducting with other members, the insulating layer is peeled off from the end of the enamel-coated superconducting wire by chemical peeling.

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