KR960013848B1 - Method for connecting (nbti) super conductor wire - Google Patents

Abstract

a first step of preparing a single core composite rod consisting of Cu part(1) and NbTi part(2), and forming the rod to sleeve shape by forming a hole(5) in the NbTi part, the diameter of hole(5) being two times larger than that of super conductive wire(4); a second step of removing the outer insulation layer(7) of the super conductive wire(4) to about 30mm distance from the edge, and melting away copper part of exposed filament(8) in nitric acid solution; a third step of cleaning the sleeve(6) and the NbTi filament(8); and a fourth step of twisting the filament, inserting the twisted filament(8) into the hole(5) of the sleeve(6), and pressing the sleeve to the thickness of 1.5-2mm by small press. The method minimizes resistance loss in connecting part of the super conductive wires, improves mechanical properties and enhances stability of magnet in magnetic field and at very low temperature.

Description

Ultra-low resistance connection method of NbTi superconductor wire

1 is a process diagram of the present invention.

* Explanation of symbols for main parts of the drawings

1: Oxygen-free copper (Cu) part 2: NbTi part

3: Cu / NbTi single core composite rod 4: Superconducting wire

5: hole 6: slave

7: insulation layer 8: NbTi superconducting filament

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an extremely low resistance connection method of NbTi superconductor wires for joining superconducting wires of a winding part in the manufacture of high precision high uniformity superconducting magnets such as NMR and MRI (nuclear magnetic resonance tomography apparatus).

In general, the superconducting wire used to manufacture the superconducting magnet is limited in length due to the processing limitation of the billet when manufacturing the wire rod. In addition, since the length of the superconducting wire to be used in the case of a large superconducting magnet is limited, it is required to use the wires connected to each other when manufacturing the magnet.

Especially in magnets operating in permanent current mode like MRI magnets, the superconducting wire of permanent current switch and the magnet wire of main body should be connected at least two places. At this time, the magnetic field generated by the permanent current needs to have a very high time stability of the magnetic field. When the current attenuation occurs on the permanent current circuit, the magnetic field stability decreases. For commercially available superconducting MRI magnets, the magnetic field stability should be about 0.1 ppm / h. This magnetic field stability significantly affects the quality of the tomography. In addition, the current attenuation is most affected by the contact resistance of the superconducting wire connecting portion. In general, metal welding or soldering connection has a drawback in that a loss of resistance causes a large current attenuation.

Therefore, the lower the contact resistance, the higher the magnetic field stability. In the case of general soldering, the contact resistance is about 10 ^-9 to 10 ^-0 Ω, so a contact resistance of 10 ^-13 Ω or less is required for the magnetic field stability for tomography. do.

Looking at the connection method of superconducting wires described so far,

(1) A method of inserting a filament of superconducting wire into a copper sleeve and compressing it. (M.J. Leupold and Y.Iwasa: Cryogenics Apr. p. 125, 1976)

(2) A method of winding a filament portion of a superconducting wire to be connected with a copper foil and spot welding. (Mitsubishi Denki, Japan Patent No. 1626237)

(3) Method of winding spot filament of superconducting wire to be connected with NbTi foil and spot welding. (G.Luderer: Cryogenics Sep. p.518, 1974)

(4) Spot welding of filaments of superconducting wires (T.Tominaka et al .: IEEE on Mag.vol 27, p. 1846, 1991)

(5) Cold welding method (D. Cornish et al .: Proc. Of the 7th Symp. On Engineering Problem of Fusion Research, IEEE, p. 1267. 1977)

(6) Ultrasonic Connection (T.Fugioka et al .: Toshiba Review vol. 46, p. 389, 1991)

(7) Explosive welding method (a. Berruyer et al, Adv. In Cryogenics Engineering 15, p. 158, 1969)

Etc. are representative.

Among the above methods, the methods of (1), (2), (3), and (4) are frequently used to fabricate a highly uniform magnet.

The method of (1) is characterized by being relatively simpler than other methods, but since the filament is easily in contact with the copper part, the current flows from the superconducting part (filament) to the phase conducting part (copper) → the superconducting part (filament). There is a problem that a resistance loss occurs as a flow.

The method of (2) has a problem similar to that of the method of (1), and there is a problem in that the cryogenic stability of the connection part is inferior because the copper foil is used.

The method of (3) can solve the problem of (1), but there is also a problem in cryogenic stabilization because there is no copper part to play a stabilizing role.

The method of (4) has a problem that spot welding is not easy because the filament diameter of the superconducting wire is generally very small, and the methods of (2), (3) and (4) are weak in the mechanical strength of the connecting portion. It's a flaw.

The present invention has been invented to solve the above problems, and an object thereof is to provide an extremely low resistance connection method of a NbTi superconductor wire for improving the magnetic field stability and cryogenic stabilization of a superconducting magnet operating in a permanent current mode.

Hereinafter, with reference to the accompanying drawings will be described in detail a preferred embodiment of the present invention.

1st process

A superconducting wire (4) to prepare a Cu / NbTi single core composite rod 3 made of anoxic copper (Cu) portion 1 and NbTi (2), and to be connected to the NbTi portion 2 of the single core composite rod 3 (4). ) Drill a hole (5) corresponding to a diameter of about twice the diameter with a drill (not shown) to form a sleeve (6).

2nd process

NbTi superconducting filaments of the superconducting wire 4 of the superconducting wire 4 from which the insulating layer 7 is removed after removing the insulating layer 7 by a length of about 30 mm from both ends of the superconducting wire 4 to be connected to the hole 5 ( 8) Put 20mm length part into nitric acid solution (HNO ₃ ) to dissolve copper part.

20-40 ml of hydrogen fluoride (HF) was placed in a beaker with a solution for removing and washing the oxide film of the sleeve 6 and the NbTi superconducting filament (8), followed by mixing 20-40 ml of distilled water (H ₂ O), followed by nitric acid ( HNO ₃ ) Mix 20-40 ml to make a washing solution. In addition, when the concentration of each solution is 20ml or less, there is a disadvantage that the oxide film is not sufficiently removed, and in a solution of 40ml or more, the NbTi surface is eroded, so that a fitting occurs, so that the concentration of each solution is 20 to 40ml. Is most preferred.

3rd process

Put the slab 6 and the NbTi superconducting filament 8 into the washing solution, and when bubbles are generated, it is taken out after 2 or 3 seconds, immediately put into distilled water, shaken and immediately put into alcohol.

4th process

After washing the slab 6 and the NbTi superconducting filaments 8, twist the NbTi superconducting filaments 8 of the superconducting wire 4 into the holes 5 of the sleeve 6 and then compact Press by applying pressure so that the thickness of the slave 6 to 1.5 ~ 2mm using a press. In addition, the press work has the effect of sufficient mutual contact between the NbTi superconducting filaments 8 and destroying the remaining oxide film.

As described above, the present invention allows the superconducting current to flow only in the superconductor region which does not pass through the non-superconducting region portion when energizing, thereby minimizing the resistance loss of the superconducting wire connecting portion, increasing the mechanical strength and securing the stabilizer portion and the MRI magnet. It is possible to improve the magnetic field stability and cryogenic stability of the superconducting magnet in the same permanent current operation.

Claims (2)

A superconducting wire to prepare a Cu / NbTi single core composite rod 3 consisting of an oxygen-free copper (Cu) portion 1 and an NbTi portion 2, and to be connected to the NbTi portion 2 of the single core composite rod 3 ( 4) a first step of drilling a hole (5) corresponding to a diameter of about twice the diameter to form a sleeve (6); The superconducting wire 4 to be connected to the hole 5 is removed from the both ends of the insulating layer 7 by a length of about 30 mm, and then the NbTi superconducting filament of the superconducting wire 4 having the insulating layer 7 removed therefrom ( 8) the second step of melting the copper part by putting the nitric acid solution (HNO ₃ ) by 20 mm length; A third step of inserting the sleeve 6 and the NbTi superconducting filament 8 into the washing liquid and then removing the bubbles after 2 or 3 seconds into the distilled water immediately after putting them in the distilled water; After washing the sleeve 6 and the NbTi superconducting filament 8, twist the NbTi superconducting filament 8 of the superconducting wire 4 into the hole 5 of the sleeve 6 and then press the small press And a fourth step of pressing by pressing so that the thickness of the sleeve (6) becomes 1.5 to 2 mm. The cleaning solution for removing and washing the oxide film of the sleeve 6 and the NbTi superconducting filament 8 is 20 to 40 ml of hydrogen fluoride (HF) + 20 to 40 ml of distilled water (H ₂ O). ₃ ) Ultra low resistance connection method of NbTi superconductor wire, characterized in that consisting of 20 ~ 40ml.