Enamel-coated superconducting wire material and method for using enamel-coated superconducting wire material

Abstract

Provided are: an enamel-coated superconducting wire material having an insulating layer of an enamel layer formed using a polyimide resin, wherein the insulating layer can be easily peeled off by chemical peeling; and a method for using the enamel-coated superconducting wire material.　Provided are: an enamel-coated superconducting wire material comprising a superconducting wire 2 and an insulating layer 3 covering the outer circumference of the superconducting wire 2, wherein the insulating layer 3 is an enamel layer formed using a polyimide resin, and the imidization degree of the insulating layer 3 is 45% to 92%; and a method for using the enamel-coated superconducting wire material.

Description

Enamel-coated superconducting wire and method of using the same

[0001] The present invention relates to an enamel-coated superconducting wire and a method for using the enamel-coated superconducting wire.

[0002] Conventionally, enamel-coated superconducting wires have been used, with an insulating layer made of enamel resin formed on the outermost periphery (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 and remove the insulating layer from the end of the wire that they wish to connect to other electrodes, etc., and then solder the wire to various devices, etc. In particular, high-temperature superconducting wires have a short standard length, so when used in large coils, etc., peeling and connecting operations must be performed frequently, making the productivity of the peeling process important.

[0003] Methods for stripping the insulating layer of enamel-coated superconducting wire include mechanical stripping, laser stripping, and chemical stripping. Mechanical stripping is a method in which a rotary blade or similar is pressed against the wire to mechanically strip it. However, mechanical stripping requires special equipment and a stripping blade that corresponds to the shape of the wire to avoid cutting through the conductor wire (superconducting wire), which results in poor productivity.

[0004] Laser delamination is a method of removing an insulating layer by irradiating it with laser light to melt the insulating layer or by generating and bursting bubbles between the wire and the insulating layer. However, laser delamination requires the preparation of a laser device, precise adjustment of the laser beam irradiation angle, and frequent part replacement and equipment adjustment, resulting in poor productivity. Furthermore, there is a concern that laser heat may damage the superconducting layer.

[0005] In contrast, chemical stripping is a method in which the insulating layer is immersed in a stripping solution to swell or dissolve it, thereby stripping it from the wire. Chemical stripping is simple, as it only requires immersion in a stripping solution, and is superior in productivity because it does not require the preparation of parts or adjustment of equipment, as compared to mechanical stripping and laser stripping.

[0006] JP 2012-59403 A

[0007] However, chemical stripping, which uses a chemical reaction to swell or dissolve the insulating layer, can require a long reaction time. Stripping an enamel layer made of polyimide resin, which has a high heat resistance, can be particularly time-consuming. For example, in the inventors' study, stripping a film thickness of about 25 μm could take more than three minutes, and depending on the degree of resin adhesion, it could take as long as 10 minutes. Therefore, under conditions requiring a long reaction time, chemical stripping may not necessarily be a highly productive method.

[0008] The present invention aims to provide an enamel-coated superconducting wire that has an insulating layer made of polyimide resin and that can be easily stripped by chemical stripping, and a method for using the enamel-coated superconducting wire. Another object of the present invention is to maintain flexibility, strength, and insulation properties even in the cryogenic environment in which the superconducting wire operates (e.g., the temperature of liquid helium or liquid nitrogen).

[0009] The above object can be achieved by the present invention, which is described below.

[0010] <1> An enamel-coated superconducting wire comprising 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 92% or less.

[0011] <2> The enamel-coated superconducting wire according to <1>, wherein the insulating layer has a 5% weight loss temperature of 408°C or lower as determined by thermogravimetry (TG).

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

[0013] <4> A method for using the enamel-coated superconducting wire according to <1>, comprising chemically stripping the insulating layer from an end of the enamel-coated superconducting wire when conducting electrical current to another member.

[0014] According to the present invention, it is possible to provide an enamel-coated superconducting wire that has an insulating layer made of a polyimide resin and that can be easily stripped by chemical stripping, and a method for using the enamel-coated superconducting wire. Furthermore, according to the present invention, it is possible to maintain flexibility, strength, and insulation properties even in a cryogenic environment (e.g., the temperature of liquid helium or liquid nitrogen) in which the superconducting wire operates.

[0015] FIG. 1 is a perspective view showing a superconducting wire according to an embodiment that is an exemplary aspect of the present invention. FIG. 2 is a cross-sectional view showing a superconducting wire according to an embodiment that is an exemplary aspect of the present invention, taken along the imaginary plane P shown in FIG. 1. FIG. 3 is a chemical reaction formula showing a reaction in which a polyimide resin is produced from a polyamic acid. FIG. 4 is a graph showing IR spectra of some measurement samples in Examples and Comparative Examples. FIG. 5 is a graph showing the relationship between the imidization rate and the peel time in a chemical strippability test in Examples and Comparative Examples. FIG. 6 is a graph showing the relationship between the 5% weight loss temperature and the peel time in a chemical strippability test in Examples and Comparative Examples.

[0016] Specific embodiments of the present invention will be described in detail below with reference to the drawings. Note that the present invention is not limited to the following embodiments, and various modifications are possible within the scope of the present invention.

[0017] [Enamel-coated superconducting wire] FIG. 1 is a perspective view showing an enamel-coated superconducting wire (hereinafter, sometimes simply referred to as "superconducting wire") according to an embodiment that is an exemplary aspect of the present invention, and FIG. 2 is a cross-sectional view taken along an imaginary plane P shown in FIG. 1.

[0018] As shown in Fig. 1(a), the superconducting wire 1 includes a tape-shaped superconducting wire 2 and an insulating layer 3. The superconducting wire 2 has a tape-like shape and is formed by laminating an intermediate layer, an oxide superconducting layer, a protective layer, and a stabilizing layer in this order on a substrate. More specifically, as shown in Fig. 2, the superconducting wire 1 preferably includes a tape-shaped metal substrate 21 serving as the substrate, an intermediate layer 22 provided on one main surface of the metal substrate 21, a superconducting layer 23 which is an oxide superconducting layer provided on the surface of the intermediate layer 22, a protective layer 24 which covers the metal substrate 21, the intermediate layer 22, and the superconducting layer 23, and a stabilizing layer 25 which covers the protective layer 24.

[0019] The metal substrate 21 that constitutes a part of the superconducting wire 2 is a tape-shaped metal substrate. The material of the metal substrate 21 is not particularly limited, but may be, for example, a nickel alloy such as Hastelloy (registered trademark).

[0020] The intermediate layer 22 is provided on one of the main surfaces of the metal substrate 21. The main surfaces of the metal substrate 21 are the surfaces along the width direction of the metal substrate 21, and are the upper and lower surfaces in FIG. 2. 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 multi-layer structure, and the material is not particularly limited, but may be, for example, Gd ２ Zr ２ O ７ , MgO, ZrO ２ -Y ２ O ３ (YSZ), SrTiO ３ , LaMnO ３ , CeO ２ , Y ２ O ３ , Al ２ O ３ , Gd ２ O ３ , Zr ２ O ３ , Ho ２ O ３ , Nd ２ O ３ The intermediate layer 22 may be a laminated body made up of a plurality of layers, such as four or five layers.

[0021] The superconducting layer 23 is provided on the surface of the intermediate layer 22. The superconductor constituting the superconducting layer 23 is preferably an RE-based superconductor (RE: rare earth element) that exhibits superconductivity at temperatures above liquid nitrogen temperature, and preferably a so-called high-temperature superconductor such as GdBCO or SmBCO that uses 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 stacked in this order to form a laminate.

[0022] Protective layer 24 covers the entire metal substrate 21, as well as intermediate layer 22 and superconducting layer 23 provided on one side of metal substrate 21. In other words, protective layer 24 covers the entire periphery of the laminate formed by laminating metal substrate 21, intermediate layer 22, and superconducting layer 23. Protective layer 24 is a metal layer formed by sputtering, vapor deposition, or the like, and is preferably made of silver.

[0023] 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, which covers the entire periphery of the laminate formed by laminating the metal substrate 21, the intermediate layer 22, and the superconducting layer 23. The stabilization layer 25 is a metal layer, and is preferably made of copper. The thickness of each of these layers may be set appropriately depending on the intended use and specifications of the superconducting wire 1, the desired performance, etc.

[0024] 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 stabilizing layer 25, which overlap and cover the entire periphery of the laminate formed by laminating the metal substrate 21, intermediate layer 22, and superconducting layer 23. The insulating layer 3 can be formed by applying a coating liquid (hereinafter referred to as a "polyimide precursor coating liquid") that serves as a precursor to polyimide resin using a conventionally known method and then heat-curing (imidizing). The thickness of the insulating layer 3 can be appropriately set depending on the intended use, specifications, desired performance, etc. of the superconducting wire 1, but is selected from the range of approximately 5 to 50 μm, preferably from the range of approximately 10 to 25 μm. Polyimide has properties such as high heat resistance, cold resistance, electrical insulation, and chemical resistance, making it suitable for use at low temperatures.

[0025] The overall size of the superconducting wire 1 may be set appropriately depending on the intended use, specifications, desired performance, etc. The width (direction of arrow X in FIGS. 1 and 2) is selected from the range of approximately 1 to 20 mm, preferably from the range of approximately 2 to 12 mm. The overall thickness of the superconducting wire 1 (direction of arrow Y in FIGS. 1 and 2) is selected from the range of approximately 25 to 500 μm, preferably from the range of approximately 50 to 150 μm.

[0026] 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 will be shown in the examples described later, by keeping the imidization rate at 92% or less, the time required for chemical stripping can be reduced. The upper limit of the imidization rate of the insulating layer 3 is preferably 90% or less.

[0027] On the other hand, there is no particular restriction on the lower limit of the imidization rate of the insulating layer 3, but if it is too low, film formation tends to be insufficient, so it is preferably 45% or more, more preferably 60% or more, and even more preferably 70% or more. Furthermore, if the imidization rate of the insulating layer 3 is too low and the precursor content is high, the insulating layer is prone to deterioration, which becomes a cause of poor insulation, so this is suppressed by setting the imidization rate to 45% or more.

[0028] A polyimide resin coating (enamel layer) can be formed by imidizing (dehydrating and oxidizing) the precursor polyamic acid through heating or other means. Polyamic acid dissolves in organic solvents, but becomes insoluble after the reaction to form polyimide. Therefore, polyamic acid dissolved in an organic solvent (i.e., a polyimide precursor coating solution) can be applied to the substrate, followed by a heat treatment (also known as "baking") to remove the solvent and promote the imidization reaction, forming a polyimide resin coating.

[0029] Figure 3 shows the chemical reaction formula for producing polyimide resin from polyamic acid. The number of benzene rings (symbol B) remains the same on the left and right sides of the chemical reaction formula, but imide structures (symbol I) are newly generated through imidization. The imidization rate can be determined by measuring the amount of these imide structures generated using FT-IR (Fourier transform infrared spectroscopy).

[0030] In FT-IR measurements, infrared light is irradiated onto a sample, and an IR spectrum is obtained from the amount of transmitted or reflected light. Because the IR spectrum exhibits a unique pattern depending on the molecular structure, it can be used to analyze the molecular structure of a sample. FT-IR spectra of the benzene rings, the number of which remains the same in polyamic acid and polyimide, and the imide groups specific to the polyimide structure are obtained, and the proportion of imidized polyimide structures obtained by heat treatment is calculated from the intensity ratio, which is the imidization rate.

[0031] In the examples described later, the IR spectrum of the cured polyimide resin insulating layer was obtained by FT-IR measurement, and an example of this spectrum is shown in Figure 4. Referring to Figure 4, －１ ] (arrow B in Figure 4) indicates the presence of a benzene ring, and the peak at 1712 [cm －１ The peak of imide structure (part indicated by arrow I) indicates the presence of an imide structure. As described above, the number of benzene rings remains unchanged before and after the reaction, and therefore, the "imidization ratio" of the present embodiment can be determined by calculating the percentage of the imide structure peak (part indicated by arrow I in FIG. 4) based on this.

[0032] In order to keep the imidization rate of the insulating layer 3 at 92% or less, the heating conditions for imidization when curing polyamic acid to polyimide can be appropriately adjusted. In general, the imidization rate can be reduced by lowering the heating temperature and / or shortening the heating time. Note that the required heating temperature and heating time vary depending on the type of polyimide (polyamic acid) resin used, so it is not possible to specify preferred conditions for these.

[0033] In this embodiment, the insulating layer 3 preferably has a 5% weight loss temperature (hereinafter sometimes simply referred to as "5% weight loss temperature") of 408°C or lower as determined by thermogravimetry (TG). As will be shown in the examples described later, by keeping the 5% weight loss temperature low, the time required for chemical peeling can be reduced. It is more preferable that the upper limit of the 5% weight loss temperature (TG) of the insulating layer 3 is 400°C or lower.

[0034] On the other hand, there is no particular restriction on the lower limit of the 5% weight loss temperature of the insulating layer 3, but since heat resistance equal to or greater than that of the high-temperature superconducting wire is desired, it is preferably 200°C or higher, and more preferably 215°C or higher.

[0035] Thermogravimetry (TG) is a method for thermally analyzing changes in the physical and chemical properties of a sample by measuring the weight of the sample as it is heated at a constant rate, and is standardized in Japanese Industrial Standards JIS K 0129. The weight loss temperature is the temperature at which the weight ratio before and after heating, obtained by thermogravimetry (TG), reaches a set threshold value (5% in this embodiment).

[0036] In order to keep the 5% weight loss temperature of the insulating layer 3 at 408°C or less, the heating conditions for imidization when curing polyamic acid to polyimide can be appropriately adjusted. In general, the 5% weight loss temperature 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 type of polyimide (polyamic acid) resin used, and therefore it is not possible to specify preferred conditions for these.

[0037] A sample of the insulating layer 3 to be subjected to FT-IR measurement to determine the imidization rate or thermogravimetry (TG) to determine the 5% weight loss temperature can be obtained by directly scraping off the insulating layer 3 covering the outermost periphery of the target superconducting wire 1 with a spatula or the like.

[0038] [Method of using enamel-coated superconducting wire] The method of using enamel-coated superconducting wire according to an embodiment of the present invention is a method of using enamel-coated superconducting wire according to the embodiment described above, in which the insulating layer is stripped from the end of the enamel-coated superconducting wire by chemical stripping when conducting electrical current to another component.

[0039] The above-described embodiments merely show typical examples of the present invention, and the present invention is not limited to the above-described embodiments. For example, in the above-described embodiments, the superconducting wire 2 is described as an example in which the stabilization layer 25 covers the entire periphery of a laminate formed by stacking a metal substrate 21, an intermediate layer 22, a superconducting layer 23, and a protective layer 24. However, the present invention is not limited to this layer configuration. The stabilization layer 25 does not have to cover the entire periphery of the laminate. For example, the superconducting wire may be configured by stacking an intermediate layer, an oxide superconducting layer, a protective layer, and a stabilization layer in this order on a substrate.

[0040] Furthermore, not all of these layers are necessarily essential components, and layers other than the superconducting layer 23 may be omitted as appropriate or may be divided into two or more layers. Furthermore, other functional layers may be formed in addition to or in place of these layers. Regardless of the layer configuration of the superconducting wire 2, as long as its outermost periphery is coated with an insulating layer characteristic of the present invention, the wire corresponds to the enamel-coated superconducting wire of the present invention.

[0041] 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 such modifications still provide the configuration of the enamel-coated superconducting wire of the present invention, they are of course included in the scope of the present invention.

[0042] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0043] (Preparation of Superconducting Wire) A superconducting wire 2 having the layer structure shown in Figs. 1 and 2 was prepared as a superconducting wire to be subjected to the tests of the examples and comparative examples.

[0044] (Preparation of Polyimide Precursor Coating Liquid) Two types of polyimide precursor coating liquids with different heat curing conditions ("Polyimide Precursor Coating Liquid A" with high heating conditions and "Polyimide Precursor Coating Liquid B" with low heating conditions) were prepared.

[0045] (Preparation of Superconducting Wire) The outer periphery of the prepared superconducting wire was coated by dip coating with either the polyimide precursor coating liquid A or the polyimide precursor coating liquid B. The amount of the coating liquid was adjusted so that the thickness of the insulating layer after curing would be 25 μm.

[0046] The superconducting wires coated with polyimide precursor coating liquid A or polyimide precursor coating liquid B were subjected to heat treatment under appropriately changed conditions (baking conditions), with the heating temperature in the range of 160 to 300°C and the heating time in the range of 3 to 60 minutes. Then, superconducting wires of Examples 1 to 4 and Comparative Examples 1 to 4 in which an enamel layer of polyimide resin A was formed using polyimide precursor coating liquid A, and superconducting wires of Examples 5 to 8 and Comparative Examples 5 to 17 in which an enamel layer of polyimide resin B was formed using polyimide precursor coating liquid B were fabricated.

[0047] (Collection of Measurement Samples) A ​​portion of the insulating layer formed on the surface of each of the obtained superconducting wires of Examples 1 to 8 and Comparative Examples 1 to 17 was scraped off with a spatula to collect the respective measurement samples.

[0048] (Measurement of Imidization Ratio) FT-IR measurement was performed using an FT-IR device to obtain IR spectra for each of the collected measurement samples of Examples 1 to 8 and Comparative Examples 1 to 17. The imidization ratio was calculated from the obtained IR spectra using the method described above. The results are summarized in Tables 1 and 2 below. Note that, as a representative example, graphs showing the IR spectra of Example 5, Comparative Example 6, Comparative Example 12, and Comparative Example 15 only are shown in FIG. 4.

[0049] (Measurement of 5% Weight Loss Temperature) Thermogravimetric measurement (TG) was performed using a thermogravimetric analyzer (TGA) to measure the 5% weight loss temperature for each of the collected measurement samples of Examples 1 to 8 and Comparative Examples 1 to 17. The results are summarized in Tables 1 and 2 below.

[0050] (Chemical Strippability Test) For each of the superconducting wires of Examples 1 to 8 and Comparative Examples 1 to 17, a region approximately 30 mm from the end was immersed in a stripping solution containing 30 mass% potassium hydroxide ("SOLCOAT #MLJ" manufactured by Meiwa Chemical Industry Co., Ltd.), and the time until the insulating layer peeled off was measured. Whether the insulating layer had peeled off was determined by removing the superconducting wire every minute and visually inspecting it. If the insulating layer had not peeled off, the wire was immersed in the stripping solution again, and this was repeated until the insulating layer peeled off. On the other hand, if the insulating layer had peeled off, the test was terminated at that stage, and the immersion time in the stripping solution was calculated in minutes, which was used as the result of the chemical strippability test. The results are summarized in Tables 1 and 2 below.

[0051] (Evaluation Results of Examples and Comparative Examples) The results of Examples 1 to 4 and Comparative Examples 1 to 4 using Polyimide Precursor Coating Liquid A are shown in Table 1 below.

[0052]

[0053] The results of Examples 5 to 8 and Comparative Examples 5 to 17 using Polyimide Precursor Coating Solution B are shown in Table 2 below.

[0054]

[0055] The relationship between the imidization rate shown in Tables 1 and 2 and the peel time in the chemical peelability test is shown in the graph of Figure 5. In the graph of Figure 5, the horizontal axis represents the imidization rate (%) and the vertical axis represents the peel time (minutes), and the results of each Example and Comparative Example are plotted as points. The dashed line in the graph of Figure 5 indicates a 92% imidization rate.

[0056] The relationship between the 5% weight loss temperature and the peel time in the chemical peelability test shown in Tables 1 and 2 above is shown in the graph of Figure 6. In the graph of Figure 6, the horizontal axis represents the 5% weight loss temperature (°C) and the vertical axis represents the peel time (minutes), and the results of each Example and Comparative Example are plotted as points. The dashed line in the graph of Figure 6 indicates the 5% weight loss temperature of 408°C.

[0057] The graphs of FIGS. 5 and 6 also include results for superconducting wires that are not included in Examples 1 to 8 and Comparative Examples 1 to 17.

[0058] (Discussion of Results) As is clear from the graph in FIG. 5, for each of polyimide resin A and polyimide resin B, the peel time in the chemical peelability test was kept short when the imidization rate was 92% or less (the range enclosed by the dashed line in the graph), whereas the peel time became long when the imidization rate exceeded 92% (the range enclosed by the dashed line in the graph).

[0059] Furthermore, as is clear from the graph in FIG. 6, for each of polyimide resin A and polyimide resin B, the peel time in the chemical peelability test was kept short when the 5% weight loss temperature was 408°C or lower (the range enclosed by the dashed line in the graph), whereas the peel time became long when the temperature exceeded 408°C (the range enclosed by the dashed line in the graph).

[0060] 1: Superconducting wire, 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. An enamel-coated superconducting wire comprising 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 polyimide resin, and the imidization rate of the insulating layer is 45% or more and 92% or less.

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

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

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

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