JP4822781B2 - Nb3Al superconducting coil connection method - Google Patents

Description

The present invention relates to a method for connecting Nb ₃ Al superconducting coils. Specifically, the present invention is _Nb 3 to enable connections with other superconducting coils, such as _Nb 3 Al superconductive coil together or _Nb 3 Al superconductive coil and NbTi superconducting coil and _Nb 3 Sn superconductive coils Al The present invention relates to a method for connecting a superconducting coil.

The NMR magnet is configured by connecting a plurality of superconducting coils in series. An NMR magnet is required to have high temporal stability of a magnetic field in order to improve resolution and sensitivity of NMR measurement. For example, in solution NMR having the strictest specifications regarding magnetic field stability, the magnetic field attenuation rate is required to be 0.1 ppm or less per hour. In order to realize this specification, it is necessary to set the connection resistance to 10 ⁻¹¹ ohm or less per connection part of the superconducting coil.

  Conventionally, a solder dipping method has been widely used for connection between superconducting coils. In the solder dipping method, the copper base material or bronze base material of the superconducting wire is heated and removed in a strong acid or solder bath to expose the superconducting core wire, and then a solder layer is formed on the surface of the superconducting core wire by solder dipping Then, the connecting portion is formed by filling the wire with solder (Patent Documents 1 and 2).

In the NMR magnet, an Nb ₃ Sn superconducting coil is used in the high magnetic field region, and an NbTi superconducting coil is used in the low magnetic field region. However, when realizing an ultra-high field NMR as 1GHzNMR, _Nb 3 since Sn superconducting properties in ultra high magnetic field superconducting coil is not always sufficient, _Nb 3 Al than having excellent superconducting properties in high magnetic field Use of conductive wire is desired.

In high magnetic field MRI for medical use, the diameter of the superconducting coil is large, and a large electromagnetic stress is applied to the superconducting coil. The Nb ₃ Sn superconducting coil has a problem that its superconducting characteristics deteriorate due to electromagnetic stress, and it is desired to use an Nb ₃ Al superconducting coil having excellent stress characteristics.
US Pat. No. 5,690,991 U.S. Pat. No. 4,744,506

However, since the Nb ₃ Al superconducting wire uses niobium as the base material, it is impossible to remove the metal base material and to expose the superconducting core wire, which are indispensable for the conventional solder dipping method, and to connect the Nb ₃ Al superconducting coil. Until now, Nb ₃ Al superconducting coils could not be applied to NMR and MRI.

A typical cross-sectional configuration of the Nb ₃ Al superconducting wire is shown in FIG. A large number of niobium and aluminum composite core wires 2 are embedded in the niobium base material 1, and the outer peripheral portion is clad with the stabilized copper 3. The niobium base material 1 is difficult to dissolve in chemicals such as acids and cannot be removed even in a solder bath. Therefore, it is impossible to remove the base material essential for the solder dipping method and to expose the superconducting core wire. Could not be formed.

The present invention has been made in view of such circumstances, and other superconducting coils such as Nb ₃ Al superconducting coils or Nb ₃ Al superconducting coils and NbTi superconducting coils and Nb ₃ Sn superconducting coils. It is an object of the present invention to provide a method for connecting an Nb ₃ Al superconducting coil that enables connection to the Nb ₃ Al.

The present invention solves the above-mentioned problems. First, after the surface of the Nb ₃ Al superconducting wire before heat treatment is coated with solder, heat treatment is performed together with the Nb ₃ Al superconducting coil. Nb 3 _Sn layer was formed on the outer periphery of the niobium base metal, followed after coating the outer circumference of the Nb 3 _Sn layer by soldering, it is characterized in that connected via the solder mating superconducting wire.

Second , the present invention is characterized in that a lead bismuth tin solder layer is formed on the outer periphery of the Nb ₃ Sn layer.

The present invention, in the third to remove the metal base material from NbTi superconducting wire connected to the Nb 3 _Al superconductive wire, the surface of the NbTi superconducting wire, solder coated surface of the Nb 3 _Al superconductive wire It is characterized by coating with the same solder.

The present invention, in the fourth, the _Nb 3 Sn superconducting wire connected to the _Nb 3 Al superconductive wire, after forming the _Nb 3 Sn in the core by the heat treatment, removing the metal matrix, _Nb 3 Sn superconducting The surface of the core wire is coated with the same solder as the solder coated on the surface of the Nb ₃ Al superconducting wire.

According to the present invention, the Nb ₃ Al superconducting wire forming the Nb ₃ Al superconducting coil can be connected to another superconducting wire using solder, and the superconducting connection of the Nb ₃ Al superconducting coil is possible. It becomes. The Nb ₃ Al superconducting coil can be used as a superconducting coil for NMR or MRI. In particular, 1 GHz NMR can be realized by taking advantage of the high magnetic field characteristics of the Nb ₃ Al superconducting wire.

The Nb 3 _Al superconductive coil connection method of the present invention, the niobium periphery of the base material 1 shown in FIG. 1, to form a Nb 3 _Sn layer 4 as shown in FIG. 2, and, as shown in FIG. 3 In addition, a solder layer 6 excellent in superconducting characteristics such as lead bismuth tin solder is coated and connected. Specifically, the following operation is performed.

The Nb ₃ Al superconducting wire is drawn out from the Nb ₃ Al superconducting coil, the connecting part is coated with solder, and the connecting part is heated together with the Nb ₃ Al superconducting coil at 800 ° C. for 10 hours, and the composite core wire shown in FIG. 2, Nb ₃ Al is formed, and the stabilized copper 3 is changed to bronze 5 as shown in FIG. Since tin atoms in the bronze 5 diffuse during the heat treatment, the Nb ₃ Sn layer 4 is formed on the outer periphery of the niobium base material 1. Next, the connecting portion is immersed in a solder bath such as lead bismuth tin solder excellent in superconducting characteristics, and the bronze 5 is removed as shown in FIG. 3 to form a solder layer 6 on the surface of the Nb ₃ Sn layer 4. . The Nb ₃ Al superconducting wire thus treated is connected to the counterpart Nb ₃ Al superconducting wire, Nb ₃ Sn superconducting wire, and NbTi superconducting wire by a conventional solder dipping method.

The superconducting current passing through the Nb ₃ Al superconducting core wire flows in the superconducting state through the niobium base material 1 exhibiting superconducting characteristics, and flows to the superconducting wire connected via the Nb ₃ Sn layer 4 and the solder layer 6. Form a superconducting connection. When the connecting portion is magnetically shielded to 0.4 T or less, which is the critical magnetic field of niobium or lead bismuth tin solder, superconducting connection is favorably maintained under the magnetic field generated by the magnet.

1. Connection of Nb ₃ Al Superconducting Coil and NbTi Superconducting Coil First, the Nb ₃ Al superconducting wire is processed in the following three steps.

Process 1: The insulation coating is removed from the connection portion of the Nb ₃ Al superconducting wire before heat treatment drawn from the Nb ₃ Al superconducting coil, the outer periphery is covered with a lead tin solder having a low tin composition, and the connection portion is then coated with Nb. Heat treatment is performed at 800 ° C. for 10 hours together with ₃ Al superconducting coil By this heat treatment, Nb ₃ Al is formed on the composite core wire 2 shown in FIG. 1, and the stabilized copper 3 on the outer peripheral portion is changed to the bronze 5 as shown in FIG. Tin atoms in the bronze 5 are solid-phase diffused in the niobium base material 1, and an Nb ₃ Sn layer 4 is formed on the outer periphery of the niobium base material 1.

Process 2: The connecting portion is immersed in an inert gas such as Ar gas or a lead-free solder having a high tin composition melted in a vacuum, and the bronze 5 is diffused and removed in the solder bath, and around the Nb ₃ Sn layer. A lead-free solder layer is formed. The melting temperature of the lead-free solder having a high tin composition is set to a temperature within a range where the lower limit is about 200 ° C. at which the solder melts and the upper limit is 600 ° C. at which the characteristics of Nb ₃ Al change. For example, it can be 300 degreeC.

  Process 3: The connecting portion is immersed in an inert gas such as Ar gas or a lead bismuth tin solder bath melted in vacuum at about 250 ° C., and the lead-free solder layer is replaced with a lead bismuth tin solder layer excellent in superconducting critical magnetic field.

  Next, the following surface treatment is performed on the NbTi superconducting wire that is the connection partner.

  Process 4: Lead having a high tin composition melted in an inert gas such as Ar gas or vacuum after removing the insulation coating from the connection portion of the NbTi superconducting wire drawn from the NbTi superconducting coil and exposing the stabilized copper The copper base material is removed by dipping in free solder, and the periphery of the NbTi superconducting core wire is coated with lead-free solder. The melting temperature of the lead-free solder having a high tin composition is set to a temperature within a range where the lower limit is about 200 ° C. at which the solder melts and the upper limit is 500 ° C. at which the pinning characteristics of NbTi start to change. For example, it can be 300 degreeC. The method for removing the copper base material is not limited to immersion in a solder bath, but may be removal with a chemical such as nitric acid.

  Process 5: The connecting portion is immersed in an inert gas such as Ar gas or a lead bismuth tin solder bath melted in vacuum, and the lead-free solder layer is replaced with a lead bismuth tin solder layer. The melting temperature of the lead bismuth tin solder is a temperature within a range where the lower limit is about 200 ° C. at which the solder melts and the upper limit is 500 ° C. at which the pinning characteristics of NbTi start to change. For example, it can be set to 250 ° C to 300 ° C.

Then, as shown in FIG. 4, the Nb ₃ Al superconducting wire is drawn out from the Nb ₃ Al superconducting coil 7 and the NbTi superconducting wire 7 and the NbTi superconducting coil after the processing are finished. A superconducting connection 10 is formed by connecting the superconducting wires 7 and 8 with lead bismuth tin solder 9 by immersing them in a lead bismuth tin solder bath 9 which is arranged close by the method and melted in an inert gas such as Ar gas or vacuum. The melting temperature of the lead bismuth tin solder is a temperature within a range where the lower limit is about 200 ° C. at which the solder melts and the upper limit is 500 ° C. at which the pinning characteristics of NbTi start to change. For example, it can be 300 degreeC. The method of arranging the superconducting wires 7 and 8 is not limited to the switchback shown in FIG. Alternatively, the superconducting wires 7 and 8 may be flattened and the superconducting wires 7 and 8 may be abutted.

  In addition, the last connection process can be changed to the simultaneous process of the said process 3 and the process 5. FIG.

After connection, the superconducting current through the Nb 3 _Al superconductive wire is a niobium base metal, which is the second type superconducting material flow superconducting state, through an Nb 3 _Al layer and lead Bisumasusuzu solder layer NbTi than Flows through the conducting wire and forms a superconducting connection.

Sectional photographs of the Nb ₃ Al superconducting wire actually processed up to treatment 3 are shown in FIGS. An Nb ₃ Sn layer is formed on the outer periphery of the niobium base material, and a lead bismuth tin solder layer is formed around the Nb ₃ Sn layer. FIG. 6 shows the result of measuring the connection resistance with 4.2 Kelvin for the switch bag connecting portion of the Nb ₃ Al superconducting wire and the NbTi superconducting wire after the treatment. In the figure, the horizontal axis indicates the current at the connection portion, and the vertical axis indicates the voltage at the connection portion. A state where no voltage is generated is a superconducting state, and the superconducting state is maintained up to 0.4T. In particular, a sufficiently large superconducting current flows below 0.1T. Therefore, as shown in FIG. 7, the superconducting connection 14 is placed sufficiently far from the magnet 11 or the superconducting connection 14 is magnetically shielded to 0.4 T or less using a superconducting coil or a ferromagnetic body 15. The superconducting connection 14 can be maintained. As the superconducting coil used for the magnetic shield, a metal superconducting wire such as NbTi, Nb ₃ Sn or a high temperature superconducting wire wound can be used. The magnetic shield can also be a superconducting bulk.

In the present embodiment, the copper base material is taken as an example of the NbTi superconducting wire connected to the Nb ₃ Al superconducting wire. However, the NbTi superconducting wire is made of, for example, a cupronickel base material used for a permanent current switch. It is good. The NbTi superconducting wire was drawn from the NbTi superconducting coil, but a NmTi superconducting wire of several meters was used, one end of this NbTi superconducting wire was connected to the Nb ₃ Al superconducting wire, and the other end was connected. It is also possible to connect to an Nb ₃ Sn superconducting coil, an NbTi superconducting coil, an NbTi permanent current switch, or the like by a solder dip method or the like. Furthermore, no particular restriction on the Nb 3 _Al superconductive wire to form a Nb 3 _Al superconductive coil, it is possible to cover all the Nb 3 _Al superconductive wire having a cross sectional structure shown in FIG. _Nb 3 Al superconducting wire according to rapid heating and quenching transformation method, _Nb 3 Al superconducting wire according to the low temperature diffusion method, various _Nb 3 Al superconductive wire, such as _Nb 3 Al superconductive wire improved due TRUQ method or DRHQ method However, the basic cross-sectional configuration is the same as that shown in FIG.
2. Performing three-step process in the processing 1 processing 3 Nb ₃ Al connection Nb ₃ Al superconducting coil of a superconducting coil and _Nb 3 Sn superconductive coils _Nb 3 Al superconductive wire drawn out for connection. The Nb 3 _Sn superconducting wire drawn out from the other side of the Nb 3 _Sn superconducting coil connecting, surface treatment of the following two steps.

Process 6: Nb 3 After removing the finished Nb 3 _Sn insulating coating from the connecting portion of the superconducting coil of _Sn of heat treatment, the connecting portion of the high lead-free solder of tin composition with an inert gas or in vacuum, such as Ar gas Immerse in a bath and remove the stabilized copper and bronze base material of the Nb ₃ Sn superconducting wire. The melting temperature of the lead-free solder having a high tin composition is set to a temperature within a range where the lower limit is about 200 ° C. at which the solder melts and the upper limit is 500 ° C. at which Nb ₃ Sn begins to be formed. For example, it can be 300 degreeC. After the Nb ₃ Sn superconducting core wire is exposed, a lead-free solder layer is formed around it. When removing the stabilized copper or bronze base material, a step of rough removal with a strong acid such as nitric acid can be added.

Process 7: The connecting portion is immersed in an inert gas such as Ar gas or a lead bismuth tin solder bath melted in vacuum, and the lead-free solder layer on the outer periphery of the connecting portion is replaced with a lead bismuth tin solder layer excellent in superconducting critical magnetic field. The melting temperature of the lead bismuth tin solder is set to a temperature within a range where the lower limit is about 200 ° C. at which the solder melts and the upper limit is 500 ° C. at which Nb ₃ Sn begins to be formed. For example, it can be 300 degreeC.

Nb ₃ Al superconducting wire and Nb ₃ Sn superconducting wire that have been processed are arranged in close proximity by a switchback method, etc., and immersed in an inert gas such as Ar gas or a lead bismuth tin solder bath melted in a vacuum. Connected via lead bismuth tin solder with superconducting critical magnetic field characteristics. The melting temperature of the lead bismuth tin solder is a temperature within a range where the lower limit is about 200 ° C. at which the solder melts and the upper limit is 500 ° C. at which Nb ₃ Sn begins to be formed. For example, it can be 250 degreeC. The arrangement method for connecting both superconducting wires is not limited to switchback, and may be cross arrangement or end face butt.

  In addition, the last connection process can be changed to the simultaneous process of the said process 3 and the process 7. FIG.

Also in the case of the connection between the Nb ₃ Al superconducting coil and the Nb ₃ Sn superconducting coil, as shown in FIG. 7, the superconducting connection 14 is placed in a magnetic field of 0.4 T or less sufficiently far from the magnet 11, The superconducting connection 14 can be maintained by magnetically shielding to 0.4 T or less using the conductive coil or the ferromagnetic material 15. As the superconducting coil, a coil in which a metal superconducting wire such as NbTi or Nb ₃ Sn is wound, or a coil in which an oxide high-temperature superconducting wire is wound can be employed. The magnetic shield can also be a superconducting bulk.
3. It performs processing of the processing 1 processing 3 Nb ₃ Al superconductive coil and _Nb 3 Al 2 pieces of _Nb 3 Al superconducting wire from the connection Nb ₃ Al superconductive coil drawn out for connection of the superconducting coil.

Two Nb ₃ Al superconducting wires that have been processed are arranged in close proximity by a switchback method, immersed in a lead bismuth tin solder bath melted in Ar gas or vacuum, and lead bismuth tin solder having high superconducting critical magnetic field characteristics Connect through. The melting temperature of the lead bismuth tin solder is a temperature within a range where the lower limit is about 200 ° C. at which the solder melts and the upper limit is 500 ° C. at which Nb ₃ Al formation starts. For example, it can be 250 degreeC. The method of arranging both superconducting wires for connection is not limited to switchback, and may be crossed arrangement or end face bonding.

  As shown in FIG. 7, the superconducting connection to be formed is placed at a location where a leakage magnetic field of 0.4 T or less is sufficiently far from the magnet 11, or is magnetized to a magnetic field of 0.4 T or less by a superconducting coil or a ferromagnetic material 15. Can be shielded and maintained.

The cross-sectional configuration of nb 3 _Al superconductive wire is a diagram schematically showing. Nb 3 _Sn layer is formed on the outer periphery of the niobium base metal, a diagram showing a state in which the stabilizing copper is changed into bronze. Nb is a diagram solder layer on an outer peripheral showed a state in which the formation of the 3 _Sn layer. It is the figure which showed the state which made the connection part adjoin by a switchback system and was immersed in the solder bath. (A) and (b) are cross-sectional photographs each showing a Nb ₃ Al superconducting wire before connection. Nb 3 _Al superconductive wire and NbTi voltage at the connection when connecting the superconducting wires - is a graph showing the current characteristics. It is the figure shown about arrangement | positioning etc. of the superconducting connection to form.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Niobium base material 2 Niobium and aluminum composite core wire 3 Stabilized copper 4 Nb ₃ Sn layer 5 Bronze 6 Solder layer 7 Nb ₃ Al superconducting wire 8 NbTi superconducting wire 9 Lead bismuth tin solder bath 10 Superconducting connection 11 Magnet 14 Super Conductive connection 15 Superconducting coils and ferromagnets

Claims (4)

After the surface of the Nb ₃ Al superconducting wire before heat treatment is coated with solder, heat treatment is performed together with the Nb ₃ Al superconducting coil, and simultaneously with the heat treatment , an Nb ₃ Sn layer is formed on the outer periphery of the niobium base material, and then Nb ₃ A method of connecting an Nb ₃ Al superconducting coil, wherein the outer periphery of the Sn layer is coated with solder and then connected to the superconducting wire on the other side via solder. Nb ₃ Sn layer _Nb 3 Al connection of the superconducting coil according to claim 1, wherein the outer circumference to form a lead Bisumasusuzu solder layer. Nb 3 _Al superconducting wire and the metal matrix is removed from the NbTi superconducting wire which connects, NbTi surface of the superconducting wire, Nb 3 _Al claim 1 same solder coated with solder and coated the surface of the superconducting wire or 2 _Nb 3 Al connection of the superconducting coil according to. For _Nb 3 Sn superconducting wire connected to the Nb ₃ Al superconductive wire, after forming the _Nb 3 Sn in the core by the heat treatment, removing the metal matrix, the surface of the _Nb 3 Sn superconducting wire, _Nb 3 Al Nb 3 _Al connection of the superconducting coil according to claim 1 or 2 coated with the same solder as solder coated surface of the superconducting wire.