JPH0479083B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to improvements in superconducting wires for persistent current switches used in superconducting magnets and the like. [Prior Art and Problems] Generally, when a wire for a switch transitions from superconductivity to normal conductivity, the higher the electrical resistance value of the wire, the better the switch characteristics. Therefore, no stabilizing material is used, and a high electrical resistance metal such as CuNi is used as the matrix. The switch wire is used to connect the starting and ending ends of the magnet coil to create a closed circuit when the superconducting magnet is turned off and switched to persistent current mode, and it reaches a predetermined magnetic field. Keep the switch in normal conductivity until
After the magnetic field arrives, switch from normal conductivity to superconductivity and turn off the power. A persistent current flows through the coil, which has become a closed circuit due to the switch wire becoming superconducting, and is able to maintain a magnetic field. The main uses are:
Examples of such magnets include NMR magnetic levitation magnets, which are used to disconnect the power supply to prevent heat intrusion and maintain a magnetic field for a long time. The characteristics of the switch must be stable superconductivity at the coil's rated current. On the other hand, during excitation, it must be in a normal conductive state due to heat. The switch must have a high normal conductivity electrical resistance and good SN response, and the wire must also have a high _IQ (quench current) and be stable. High electrical resistance during normal conduction (7-1 CuNi)
3CuNi is better). Here, "quench" means that when a current is passed through a superconducting coil to generate a magnetic field, heat is generated locally due to the movement of the conductor due to electromagnetic force, and the buds of the normal conductive region are generated. The normally conducting part propagates rapidly and with a current smaller than the critical current (I _C ),
This is a phenomenon in which a superconducting state suddenly transitions to a normal conducting state. The current value during this transition is called the quench current value ( _IQ value). Note that the critical current (I _C ) is defined as a current that produces an electrical resistance of 10 ^-12 Ωcm when a current is passed through a short sample (superconducting wire). Further, as superconducting wires for switches, wires made of thin wires made into a twisted wire structure, or wires molded into a rectangular shape are used. Furthermore, in order to improve the critical current density (Jc), which is a characteristic of superconducting wires, the strands are generally heat treated. However, the characteristics of the switch wire are determined not only by Jc but also by the quench current value, and the higher this value, the better the switch wire is recognized. However, in the case of the above-described stranded wire structure, the occupancy rate is low, so that even if the strands have excellent characteristics, for example, sufficient performance as a wire for a switch cannot be obtained. Incidentally, even if the wire is impregnated with epoxy resin or the like after winding in order to improve the occupancy rate, the magnet is likely to be quenched due to the movement of the wire, which is a disadvantage of winding with twisted wire. Furthermore, since the quench current depends on the surface area of the wire, when the wire is formed into a rectangular shape as described above, it is not possible to obtain a quench current value equivalent to that of a thin stranded wire. Furthermore, when heat treatment is performed, the quench current value becomes low and the characteristics are impaired because the heat treatment is performed in an attempt to increase the Jc of a wire material for which it is difficult to significantly improve the Jc. The present invention was made in order to solve the above-mentioned conventional problems, and aims to provide a superconducting wire for persistent current switch having an excellent quench current value. [Means for Solving the Problem] The present invention provides a stranded wire in which a plurality of strands of NbTi alloy filament embedded in a matrix made of a high electrical resistance metal are twisted together with an electrically insulated copper wire. This is a superconducting wire for a persistent current switch, which is characterized by being a rectangular shaped stranded wire. As the high electrical resistance metal, for example, CuNi or the like can be used. For example, the wire may be heated at 375°C to 450°C
It is desirable to use a material that has been heat treated for ~8 days and then subjected to 10 to 30% cold working. The reason for limiting the heat treatment temperature is that if the heat treatment temperature is lower than 375°C, the effect of the heat treatment will not be fully achieved, whereas if it exceeds 450°C, the quench current value ( _IQ value) may decrease. The electrically insulated copper wire is provided with an electrically insulating coating on the copper wire that is twisted together with the superconducting strands, so that when the superconducting copper strands transition to a normal copper state (when the switch is turned off), the superconductor Stabilization in the sense that it prevents bypass from the copper wire to the copper wire, and dissipates heat generated when the superconducting wire locally transitions to a normal conductive state through the electrically insulated copper wire. It plays a part in its role as a material. As such an electrically insulated copper wire, for example, a cotton-wound copper wire, a formal copper wire, etc. can be used. For example, cotton-wound copper wire, formal copper wire, etc. can be used. [Function] According to the present invention, a plurality of wires each having a large number of NbTi alloy filaments embedded in a matrix made of a high electrical resistance metal are twisted together with electrically insulated copper wires acting as a stabilizing material. Therefore, even if the wire locally transitions from a superconducting state to a normal conducting state due to some external influence, the generated heat can be dissipated through the electrically insulated copper wire, and the wire can quickly return to a superconducting state. becomes possible. Furthermore, the quench current value ( _IQ value) can be improved by making the wire thinner and reducing the equivalent cross-sectional area. In the present invention, by forming the twisted strands into rectangular shaped strands, it is possible to improve the wire area ratio and also improve the density as a winding wire. Therefore, even when the superconducting state transitions to the normal conducting state, it is possible to quickly return to the superconducting state, and the wire area factor can be improved as a rectangular shaped stranded wire, and the density as a winding wire can also be improved. By improving _{the IQ} value, it is possible to obtain a superconducting wire for a persistent current switch having a high IQ value. In addition, if the wire is heat-treated at 375°C to 450°C for 4 to 8 days and then subjected to 10 to 30% cold working, it can be used for persistent current switches with even higher _IQ values. Superconducting wire can be obtained. [Examples] Examples of the present invention will be described in detail below. Example First, the CuNi ratio is 1.6 and the number of filaments is 4000.
After heat treating the CuNi-NbTi wire at 420°C for 6 days, cold working was performed by 20% to produce a wire with a diameter of 0.76 mm. Next, the five strands of wire and the two cotton-wrapped copper wires were twisted together by forming, and then processed into a rectangular shape, resulting in 4000 filaments with a cross section of 1.2 x 2.5 mm.
A superconducting wire made of rectangular molded stranded wire with 5 cores was manufactured. Reference example Seven strands similar to those in the example were twisted together by forming process, and then processed into a rectangular shape to produce a superconducting wire made of 7 rectangular shaped stranded wires with a cross section of 1.2 x 2.5 mm and 4000 filaments x 7. Manufactured. Comparative example Rectangular superconducting wire with a cross section of 1.3 x 2.6 mm (CuNi ratio 1.6, number of filaments 28000 CuNi−
NbTi single wire) was heat treated at 420°C for 6 days in the same manner as in the example, and then subjected to 20% cold working to produce a superconducting wire. The obtained superconducting wires of this example, reference example, and comparative example were wound into a cylindrical coil and placed in a magnetic field with a magnetic flux density of 1.5 Tesla (T) generated by a superconducting magnet. The current value (quench current value; _IQ value) at the time of transition from the superconducting state to the normal conducting state was measured by gradually increasing the applied current.

[Table] As is clear from Table 1, the superconducting wire of this example, which is made by twisting bare wires and cotton-wound copper wires and forming a rectangular stranded wire, is not only the superconducting wire of the comparative example, but also the plain wire. It can be seen that the _IQ value (per strand) is higher than that of the reference superconducting wire, which is a rectangular shaped stranded wire made by twisting only the wires together, and is more stable as a wire for switches. [Effects of the Invention] As detailed above, according to the present invention, it is possible to provide a superconducting wire for a persistent current switch in which the quench current value is significantly improved.

Claims (1)

[Claims] 1. A stranded wire made by twisting together a plurality of strands of NbTi alloy filaments embedded in a matrix made of a high electrical resistance metal together with an electrically insulated copper wire is called a rectangular shaped stranded wire. A superconducting wire for persistent current switches. 2. Claim 1, characterized in that the wire is heat treated at 375°C to 450°C for 4 to 8 days and then subjected to 10 to 30% cold working. superconducting wire for persistent current switches.