JP2009099573A - Mehod of manufacturing superconducting coil - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing a superconducting coil by which a non-circular coil having a small-curvature portion is formed in an intended shape. <P>SOLUTION: Provided is the method of manufacturing the superconducting coil which has a non-circular shape and has the curvature portion having curvature not more than the limit curvature of a superconducting wire rod, the method of manufacturing the superconducting coil includes the steps of: forming a body in a pancake coil shape by winding a (Bi, Pb)2223 precursor wire rod; and heat-treating the body in the pancake coil shape in a pressurized atmosphere, the total pressure in the pressurized atmosphere being ≥0.5 MPa at a temperature of ≥600°C in the step of the heat treatment. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method of manufacturing a superconducting coil in which a (Bi, Pb) 2223 superconducting wire is used as a conductor and has a shape (non-circular shape) different from a circle.

Oxidation mainly composed of (Bi, Pb) ₂ Sr ₂ Ca ₂ Cu ₃ O _{10 ± δ} (δ is a number of about 0.1: hereinafter referred to as (Bi, Pb) 2223) phase produced by a metal sheath method A superconducting wire is a useful wire that has a high critical temperature and exhibits a high critical current value even under relatively simple cooling such as liquid nitrogen temperature (for example, see Non-Patent Document 1).

  In addition, by using the (Bi, Pb) 2223 superconducting wire, it is possible to reduce energy loss far more than when using a conventional normal conductor, and a large current can flow at a low voltage. The (Bi, Pb) 2223 superconducting wire is tape-shaped to increase the critical current value. Further, superconducting application equipment such as a superconducting coil and a superconducting magnet using the tape-like (Bi, Pb) 2223 superconducting wire as a conductor is being developed at the same time.

  As a shape of the superconducting coil, a circular pancake-shaped coil is generally used. In this circular pancake coil, the wire is wound so that the curvature of the wire increases continuously and monotonously from the inside toward the outside. By the way, depending on the purpose of use, as a coil shape, not only a circular shape but also a racetrack shape, a pancake coil whose non-circular curvature is increased such as an elliptical shape or a substantially rectangular shape (other than a circular shape is called a non-circular coil) is selected. Some devices are built in.

  As a manufacturing method of the above superconducting coil using an oxide superconducting wire as a conductor, a heat and heat treatment is performed, and a desired superconducting phase is formed in the wire, and then the wire is wound into a desired shape. (See Patent Document 1) and the Wind and React method (see Patent Document 2) in which a superconducting wire is wound into a desired shape and then heat-treated to generate a target superconducting phase.

  When producing a superconducting coil having a relatively large diameter, a react and wind method is employed. On the other hand, when forming a superconducting coil having a small diameter, the wind and react method is used. In the latter, when the oxide superconductor in the superconducting wire is generated after the heat treatment, bending the wire with a small curvature destroys the superconducting portion made of ceramics and lowers the critical current value. Therefore, when forming a small-diameter coil, it is a manufacturing method in which a desired shape is formed in advance and the shape is baked and hardened.

JP 2003-318017 A JP-A-9-63881 SEI Technical Review, July 2006, No. 169, p103-108

  When forming a superconducting coil having a relatively large diameter can be accurately formed a shape intended even coils of any shape, including circular by React and wind method. Moreover, even if it is a small-diameter coil, if it is circular, it can form the circular coil which did not lose shape by the wind-and-react method. In the wind-and-react method, heat treatment is performed after the desired shape is obtained. By the way, in this heat treatment, the superconducting wire may expand in the thickness direction, and it may not be easy to complete the heat treatment while maintaining the original shape. If the curvature increases monotonously, and the tension applied to the wire and the pressing force in the coil radial direction are almost uniform, the wire is almost uniformly thermally expanded and is not easily deformed. Therefore, the wind-and-react method can be applied to the small-diameter circular coil.

  However, since the curvature does not increase monotonously in the non-circular coil described above, various stresses applied to the wire also differ depending on the location. Therefore, the rate of thermal expansion differs depending on the location, and it is difficult to perform heat treatment so as to maintain the intended shape.

  Then, this invention makes it a subject to provide the manufacturing method of the superconducting coil which forms the non-circular coil which has a small curvature part as the intended shape.

  The inventors of the present invention have investigated the properties of (Bi, Pb) 2223 superconducting wire and improved the heat treatment of the wind and react method, thereby completing the invention capable of solving the above-mentioned problems. The present invention will be described below.

  The present invention is a method for manufacturing a superconducting coil in which a plurality of (Bi, Pb) 2223 superconducting wires are wound in a pancake shape, have a non-circular shape, and have a curvature portion equal to or less than the limit curvature of the superconducting wires. , (Bi, Pb) 2223 precursor wire is wound to form a pancake coil-shaped body, and the pancake coil-shaped body is heat treated in a pressurized atmosphere at 600 ° C. In the above temperature, the total pressure of the pressurized atmosphere is 0.5 MPa or more.

  In the present invention, in the step of heat treating the pancake coiled body, it is preferable to maintain the total pressure of the pressurized atmosphere at 0.5 MPa or more from the start to the end of the heat treatment.

  In the present invention, in the step of forming the pancake coil-shaped body, it is preferable that the sheet material containing a ceramic material and an organic binder and the (Bi, Pb) 2223 superconducting wire are wound together.

  According to the present invention, a superconducting coil having a small curvature portion and having a non-circular shape other than a circular shape can be manufactured in an intended shape.

(Embodiment)
FIG. 1 is a diagram showing a manufacturing process of a superconducting coil of the present invention. A specific process of the present invention will be described with reference to FIG.

First, raw material powders (Bi ₂ O ₃ , PbO, SrCO ₃ , CaCO ₃ , CuO) are mixed at a desired ratio, heat-treated and pulverized repeatedly to produce a precursor powder (step S1). The precursor powder is filled into a metal tube (step S2). This precursor powder is, for example, (Bi, Pb) ₂ Sr ₂ Ca ₁ Cu ₂ O _{8 ± δ} (δ is a number close to 0.1: hereinafter referred to as (Bi, Pb) 2212) phase or Bi ₂ Sr ₂ Ca. ₁ Cu ₂ O _{8 ± δ} (δ is a number close to 0.1: hereinafter referred to as Bi2212) phase, (Bi, Pb) 2223 phase and the like. As the metal tube, it is preferable to use silver or a silver alloy that hardly forms a compound with a precursor.

  Next, the wire is drawn to a desired diameter, and a single core wire coated with a metal such as silver with the precursor as a core material is produced (step S3). Next, many single core wires are bundled and fitted into a metal tube made of, for example, silver or the like (multi-core fitting: step S4). Thereby, the multi-core structure material which has many precursor powders as a core material is obtained.

  Next, the multi-core structural material is drawn to a desired diameter. Thereby, the precursor powder is embedded in a sheath portion made of silver or the like, and an isotropic multi-core bus bar having a circular or polygonal cross-sectional shape is obtained (step S5). Next, this isotropic multi-core bus is rolled (primary rolling: step S6). Thereby, a tape-shaped bus bar is obtained.

  Next, the tape-shaped bus bar is heat-treated (primary heat treatment: step S7). This heat treatment is performed at a temperature of about 800 ° C. to 850 ° C. in an atmosphere having an oxygen partial pressure of 1 to 20 kPa, for example. By this heat treatment, the precursor powder is almost transformed into the target (Bi, Pb) 2223 phase.

  Thereafter, the tape-shaped bus bar is rolled again (secondary rolling: step S8). In this way, voids generated by the primary heat treatment are removed by performing the secondary rolling. The state until after the secondary rolling is called a precursor wire. The secondary rolled tape-shaped precursor wire is wound together with a sheet material containing a ceramic material and an organic binder to obtain a desired shape, for example, a racetrack pancake coil (step S9).

  The wound precursor wire is put into a heat treatment furnace as it is and subjected to pressure heat treatment (step S10). The heat treatment conditions at this time are about 20 to 100 hours at a temperature of about 820 to 840 ° C. in an atmosphere having a total pressure of 0.5 MPa or more and an oxygen partial pressure of 1 to 20 kPa. This heat treatment causes transformation of the part that has not reacted in step S7 to the (Bi, Pb) 2223 phase, and (Bi, Pb) 2223 crystals, or (Bi, Pb) 2223 crystals and non-reacts. The superconducting phase is strongly bonded. After step 10, a superconducting wire having high characteristics is obtained, and a superconducting coil can be formed at the same time.

  Here, the small-diameter non-circular coil that is the subject of the present invention will be described. FIG. 2 is a top view of a non-circular pancake superconducting coil which is an object of the present invention. (A) a racetrack coil, (b) an elliptical coil, and (c) a substantially rectangular coil. These are merely examples, and any coil having a shape whose curvature does not increase monotonically is the subject of the present invention. (A) A racetrack coil will be described as an example. The coil has a large curvature portion 22 and a small curvature portion 21. In the racetrack shape, the curvature of the large curvature portion 22 is infinite. When this coil is composed of a single wire, the curvature applied to the wire is large → small → large → small →... From the inside and does not increase monotonously.

  Another feature of the coil targeted by the present invention resides in the small curvature portion 21. This small curvature portion 21 is smaller than the limit curvature of the superconducting wire constituting the coil. After heat treatment, if the wire is bent with a small curvature in the state where the oxide superconductor is generated in the superconducting wire, the superconducting portion made of ceramic is destroyed and the critical current value is lowered. The limit curvature is a diameter of curvature at which the critical current value of the superconducting wire becomes 95% or less of the straight shape when the straight superconducting wire is bent into a circle. For example, the limit curvature of a silver-coated (Bi, Pb) 2223 superconducting wire having a thickness of about 0.2 mm is about 50 mm. A coil having such a small curvature portion less than the limit curvature cannot be manufactured without a decrease in the critical current value of the superconducting wire by the conventional react-and-wind method.

  The present invention is a method for producing a pancake-like coil having a curvature that does not increase monotonously and having a portion that is equal to or less than the limit curvature as described above.

  The second feature of the present invention resides in the pressure heat treatment in step 10. Since the pressure heat treatment is performed by applying a high pressure from the outside, the bonding between the superconducting crystals can be strengthened. Therefore, there exists an effect | action that the performance of wire itself can be made high. Another effect is that the shape of the wound coil can be maintained after the heat treatment. In the silver-coated superconducting wire, a gas such as oxygen is released from the precursor powder during heat treatment. This gas remains inside the silver coating and thermally expands to expand the wire itself.

  In the circular coil, the tension applied to the wire and the pressing force in the coil radial direction are substantially uniform. Therefore, thermal expansion of the wire material occurs almost uniformly, it is difficult to lose its shape by heat treatment, and it is easy to maintain a circular shape. On the other hand, in a non-circular coil, various stresses applied to the wire also differ depending on the location. Therefore, the rate of thermal expansion varies from place to place. For example, in the racetrack coil shown in FIG. 2A, the large curvature portion 22 has a small pressing force applied to the wire in the coil radial direction, and the small curvature portion 21 has a large pressing force. When such a coil is heat-treated, the wire is likely to expand at the large curvature portion 22 and hardly expand at the small curvature portion 21. Therefore, even if the heat treatment is started with the shape of FIG. 2A, the shape after the heat treatment may be an ellipse of FIG. 2B.

  Further, not only the overall shape change but also the gap interval between the wires may change. This is due to thermal expansion and contraction during heating and cooling. The wire expands in the thickness direction by heating, and deforms in a direction in which the wire moves away from each other. On the other hand, the position of the wire does not follow the contraction in the thickness direction at the time of cooling, and it is cooled at the position as it is. Therefore, the gap between the wire rods becomes large and the coil becomes loose. In the small curvature portion where the pressing force in the radial direction is strong, the wire interval is not so wide, but in the large curvature portion where the pressing force is weak, the gap tends to be large. This phenomenon affects the case where an insulating material is sandwiched between wire materials to be described later.

The temperature range in which pressure is applied during the pressure heat treatment needs to be 600 ° C. or higher. Examples of gas components contained in the superconducting ceramic part include carbon (C) and oxygen (O). Carbon becomes CO ₂ from about 650 ° C., and oxygen is released from the superconducting ceramic part in the form of O ₂ from about 800 ° C. Therefore, if a pressure is applied in a temperature range of 600 ° C. or higher, gas emission caused by carbon and oxygen contained in these superconducting ceramic portions can be suppressed.

  The precursor powder removes the adsorbed moisture and fills the metal tube. If the precursor powder cannot be sufficiently removed, moisture exists in the metal tube. This moisture evaporates at 100 ° C. or more and causes the wire material to expand. Therefore, even if a precursor powder from which moisture has not been completely removed is used, expansion due to moisture can be suppressed by applying pressure from a temperature of 100 ° C. or lower (heat treatment start temperature). Therefore, it is preferable to apply pressure from the start of heat treatment at 100 ° C. or lower.

  When the temperature drops, the gas shrinks, and in principle, expansion does not occur.However, if for some reason the precursor wire before heat treatment has local holes such as pinholes in its silver coating, High-pressure atmospheric gas enters the filament. When the high pressure gas enters the filament and the outlet is blocked by the coupling between the superconducting crystals, the high pressure gas is confined in the filament. If the silver coating is soft when the temperature is lowered, that is, if the pressure is lowered under a high temperature condition, the trapped high-pressure gas may expand over the external pressure. Such a part is in a locally swollen state. Therefore, even if the high-pressure gas is confined in the filament, if the pressure is increased to a temperature at which the silver coating becomes sufficiently hard, that is, close to 100 ° C., expansion can be prevented by curing the silver coating. Therefore, it is preferable to maintain the pressurized state until the end of the heat treatment.

  The inventor has experimentally found that if the pressure during the heat treatment is 0.5 MPa or more, the expansion of the gas released from the superconducting filament portion and adsorbed moisture is suppressed.

  When used as a coil after heat treatment, insulation between wires (between turns) is necessary. As one method, in step S9 in FIG. 1, a tape-shaped precursor wire, a sheet material containing a ceramic material and an organic binder are wound together and heat-treated.

The organic binder disappears in the components of the sheet material after the heat treatment, and the ceramic material remains between the wire materials in the coil, and acts as an insulating material between the wire materials. As the ceramic material, a material having low reactivity with the superconducting material or the covering material is preferable. Examples thereof include magnesia, zirconia, alumina, silicon oxide, CuO, SrCO ₃ , ZrSrO _{4 and the} like. These may be used in combination.

  The organic binder is mainly intended to keep the ceramic material in the form of a sheet, and any material may be used as long as it is a material that disappears during the wire heat treatment. Examples include cellulosic and natural rubber materials.

  Since the ceramic material is difficult to react with the superconducting wire, it is not in close contact with the superconducting wire. If the gap between the wires becomes large after heat treatment, the ceramic material may fall off the coil when the coil is taken out of the furnace. If it does so, the insulation between wires will fall. Thus, as described above, it is important to perform the heat treatment so that the gap between the wires does not increase.

  Hereinafter, based on an Example, this invention is demonstrated further more concretely.

Raw material powder (Bi ₂ O ₃ , PbO, SrCO ₃ , CaCO ₃ , CuO) is in a ratio of Bi: Pb: Sr: Ca: Cu = 1.8: 0.3: 2.0: 2.0: 3.0 Are mixed in the atmosphere and subjected to pulverization in the air at 700 ° C. for 8 hours, pulverization, 800 ° C. for 10 hours, pulverization, and 840 ° C. for 4 hours to obtain a precursor powder. In addition, by injecting a nitric acid aqueous solution in which five types of raw material powders are dissolved into a heated furnace, the water in the particles of the metal nitrate aqueous solution evaporates, the thermal decomposition of the nitrate, and the reaction and synthesis of metal oxides. Precursor powder can also be produced by a spray pyrolysis method that instantly raises. The precursor powder thus produced is a powder mainly composed of (Bi, Pb) 2212 phase or Bi 2212 phase.

  The precursor powder produced as described above is filled in a silver pipe having an outer diameter of 25 mm and an inner diameter of 22 mm, and drawn to a diameter of 2.4 mm to produce a single core wire. The single core wires are bundled into 55, inserted into a silver pipe having an outer diameter of 25 mm and an inner diameter of 22 mm, and drawn to a diameter of 1.5 mm to obtain a multi-core (55 core) wire. This multi-core wire is rolled and processed into a tape-like wire having a thickness of 0.25 mm. The obtained tape-shaped wire is subjected to primary heat treatment at 820 ° C. to 840 ° C. and atmospheric pressure for 30 hours to 50 hours in an oxygen atmosphere of 8 kPa. The tape-shaped wire after the primary heat treatment is secondarily rolled to a thickness of 0.23 mm.

(Comparative example: coil production by the react and wind method)
A part of the tape-shaped wire rod after the secondary rolling is subjected to a secondary heat treatment at 820 ° C. to 840 ° C. for 100 hours in an oxygen partial pressure of 8 kPa and a total pressure of 0.5 MPa (react). The superconducting wire immediately after the secondary heat treatment is straight and has a critical current value of 210A. A racetrack coil shown in FIG. 3 is produced using this wire. FIG. 3 is a top view of a coil manufactured as a comparative example and an example.

  The straight part (L) is 30 cm, the inner diameter (b) of the semicircular part is 3 cm, and the outer diameter (B) is 5 cm. Further, the distance (A) between the straight line centers is 5 cm like B. A superconducting coil having this shape is formed by winding (winding) the wire after the secondary heat treatment and a tape-like sheet material having a thickness of 0.1 mm in which alumina fibers are held with a cellulose-based binder. Thereafter, no heat treatment is performed.

(Example: Winding and Reacting Coil Manufacturing)
The tape-shaped wire after the secondary rolling is wound with a tape-shaped sheet material in the same manner as in the comparative example so as to have the shape shown in FIG. The coiled body was subjected to a secondary heat treatment at 820 ° C. to 840 ° C. for 100 hours (react) from the start to the end of the heat treatment so as to have an oxygen partial pressure of 8 kPa and a total pressure of 0.5 MPa. Example 1).

(Example 2-6 and Comparative Example 2-6: Production of Coil by Wind and React Method)
After forming a coil shape in the same manner as in Example 1, a superconducting coil serving as a plurality of Examples and Comparative Examples is manufactured by performing secondary heat treatment under different conditions. Examples and comparative example numbers and secondary heat treatment conditions are shown in Table 1.

  The critical current value (Ic) of the produced superconducting coil is measured by the 4-terminal method in liquid nitrogen. Let Ic be the current value at which a voltage of 1 μV / cm is generated. The results are shown in Table 1. Table 1 also shows the distance (A) between the straight line centers after the secondary heat treatment.

  The superconducting coil of Comparative Example 1 manufactured by the react-and-wind method has an Ic (A) of 100 A, which is significantly lower than the Ic (210 A) before forming the coil. This is because the wire was bent to a limit curvature (5 cm in the case of the main wire) or less after formation of the superconducting phase (secondary heat treatment). It can be seen from this example that a superconducting coil having a diameter equal to or less than the limit curvature of the superconducting wire used is difficult to manufacture by the react and wind method without degrading the performance of the original superconducting wire. However, since the heat treatment is not performed after the coiled body is formed, the wire does not expand. Therefore, the shape maintains the wound shape (distance (A) between the straight line centers remains 5 cm).

  On the other hand, the superconducting coil of Example 1 manufactured according to the present invention has an Ic of 197A. This is energization in the form of a coil, and considering the fact that a magnetic field is generated, it can be said that the original performance of the used wire is almost realized.

  As a measure of the change in the coil shape, the distance (A) between the straight line centers is 5.1 cm. This indicates that there is no expansion of the wire and the shape before the heat treatment can be maintained. In Comparative Example 2 in which the secondary heat treatment is not the pressure heat treatment, Ic is as low as 152 A, and the distance (A) between the straight line centers is 6.1 cm, which is deformed into an ellipse. None of these results from the expansion of the wire because no pressure was applied. In such a coil, since the interval between the wire members in the straight portion is wide, the insulating ceramic material is likely to fall off.

  Comparing Comparative Examples 3, 4 and Examples 1, 4, 5, and 6, it can be seen that the coil heat-treated at a total pressure of 0.5 MPa or higher has a high Ic and a small change in coil shape. This is because the expansion of the wire can be suppressed by heat treatment at a total pressure of 0.5 MPa or more.

  Comparing Comparative Examples 5 and 6 and Examples 1, 2, and 3, when the pressurization start and end temperatures are 600 ° C. or lower, in other words, pressurization is performed to cover a temperature region of 600 ° C. or higher. It can be seen that a good superconducting coil with high Ic and small deformation can be obtained. That is, pressurization may be performed so as to cover a temperature region where carbon dioxide and oxygen are generated from the superconducting ceramic portion. In consideration of moisture, it is preferable to pressurize from room temperature.

  It should be understood that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

It is a figure which shows the manufacturing process of the superconducting coil of this invention. It is a top view of the non-circular pancake-shaped superconducting coil which is the subject of the present invention. (A) a racetrack coil, (b) an elliptical coil, (c) a substantially rectangular coil. It is a top view of the coil produced as a comparative example and an Example.

Explanation of symbols

21 Small curvature part 22 Large curvature part

Claims (3)

(Bi, Pb) 2223 superconducting wire is a method for producing a superconducting coil, wherein a plurality of superconducting wires are wound in a pancake shape, have a non-circular shape, and have a curvature portion equal to or less than a limit curvature of the superconducting wire,
(Bi, Pb) comprising a step of winding a 2223 precursor wire to form a pancake coil-shaped body, and a step of heat-treating the pancake coil-shaped body in a pressurized atmosphere,
A method of manufacturing a superconducting coil, wherein the total pressure of the pressurized atmosphere is 0.5 MPa or higher at a temperature of 600 ° C. or higher in the heat treatment step.   2. The method of manufacturing a superconducting coil according to claim 1, wherein in the step of heat-treating the pancake coil-like body, the total pressure of the pressurized atmosphere is maintained at 0.5 MPa or more from the start to the end of the heat treatment.   3. The step of forming the pancake coil-like body, wherein the sheet material containing a ceramic material and an organic binder and the (Bi, Pb) 2223 precursor wire are wound together. Manufacturing method of superconducting coil.

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