MgB2 superconducting wire joint

Abstract

The superconducting wire joint includes at least two superconducting wires (100, 102), each having a sheath (104, 108) and a core (106, 110) of reacted superconducting MgB2. At least one first superconducting wire (100) has a first flat end and at least one second superconducting wire (102) has a second flat end. The joint further includes a tubular metal connector (112) having a center portion filled with MgB2 material (114). A first flat end of a first superconducting wire (100) is inserted into one side of the connector until it contacts the MgB2 material, a second flat end of a second superconducting wire (102) is inserted into the other side of the connector until it contacts the MgB2 material, and the connector is pressed on both sides (116, 118) to secure the superconducting wires, and the center of the connector is pressed to compress the MgB2 material.

Description

[Technical Field]

[0001] The present invention relates to a superconducting wire joint having magnesium diboride (MgB2) as a superconducting core and a method for making such a joint. [Background technology]

[0002] Magnesium diboride is an inorganic compound with the chemical formula MgB2. The superconductivity of magnesium diboride was discovered in 2001. MgB2 superconducts at 39 K (-234 °C), the highest temperature of any conventional superconductor. This allows it to function as a superconductor without the need for liquid helium.

[0003] To maintain superconductivity, the electrical resistance present at the junction where the two ends of the superconducting wire are joined must also be very low - no easy task given the extremely brittle nature of superconducting materials.

[0004] The prior art discloses various alternatives for realizing superconducting junctions.

[0005] US Patent Application Publication No. A1-2009 / 0105079 discloses a superconducting connection between two ends of a superconductor using a sheath or bushing into which MgB2 as a superconducting contact material is inserted. The superconductor is a multi-wire superconductor. The connection shows the area inside the sheath or bushing where the superconducting wires overlap, i.e., a cross section shows the presence of superconducting wires from the two ends of the superconductor.

[0006] Korean Patent Application Publication No. A-10-2020-0103369 discloses a superconducting joint between two ends of a single wire-type superconductor. The single wire-type superconductor is a circular wire having an MgB2 superconducting core surrounded by a stabilizing layer and a metal sheath. To achieve this joint, the ends of the superconducting wire are flattened to a large extent to increase the bonding area. The width-to-thickness ratio is in the range of 3 to 100. The stabilizing layer and metal sheath are removed on one side of the flattened end so that the superconducting core is open at one end. The open ends of the superconducting core are then brought into contact with each other. The contact ends are introduced into a bonding container, where sintered powders of Mg and B or MgB2 are introduced and pressed. Summary of the Invention [Problem to be solved by the invention]

[0007] It is an object of the present invention to provide an alternative superconducting joint.

[0008] Another object of the present invention is to simplify the superconducting joint.

[0009] A particular object of the present invention is to make it possible to combine superconducting wire with already reacted MgB2. [Means for solving the problem]

[0010] According to a first aspect of the present invention, a superconducting wire joint is provided. The joint includes at least two superconducting wires, each having a sheath and an MgB2 superconducting core within the sheath. At least one first superconducting wire has a first flat end, and at least one second superconducting wire has a second flat end. The joint further includes a tubular metal connector. The connector has a central portion filled with magnesium, boron, or MgB2 material. The first flat end of the first superconducting wire is inserted into one side of the connector until it contacts the magnesium, boron, or MgB2 material. The second flat end of the second superconducting wire is inserted into the other side of the connector until it also contacts the magnesium, boron, or MgB2 material. The connector is pressed from both sides to secure the first and second superconducting wires. The central portion of the connector is pressed to compress the magnesium, boron, or MgB2 material.

[0011] Compared with the prior art, the joint of the present invention does not require overlapping of the superconducting wires, and in addition, it does not require removing the sheath of the superconducting wires.

[0012] The first and second flat ends have a width-to-thickness ratio to eliminate voids and provide a stable compressed wire that no longer deforms during pressing of the connector. Preferably, the flat ends have a width-to-thickness ratio in the range of 1.1 to 10.0, preferably 1.1 to 5.0, such as 1.25 to 2.5, for example 1.50 to 2.0.

[0013] The first and second superconducting wires have a core of reacted superconducting MgB2, and therefore also have interstices. This is because the volume of the reacted MgB2 is approximately 25% smaller than the original Mg and B powders. The ends of the superconducting wires are pressed to such an extent that the number and volume of interstices are reduced. Subsequent pressing of these already pressed ends of the superconducting wires no longer creates large cracks that would impede current flow.

[0014] The tubular metal connector preferably comprises low carbon steel.

[0015] The tubular metal connector may also comprise titanium.

[0016] The tubular metal connector may have a titanium barrier on the radially inner side and low carbon steel on the radially outer side.

[0017] The tubular metal connector may preferably be made solely of low carbon steel.

[0018] According to a second aspect of the present invention, there is provided a method of joining at least two superconducting wires having a superconducting core of reacted MgB2, the method comprising: a) providing at least one first superconducting wire and at least one second superconducting wire, the first superconducting wire having a first end and the second superconducting wire having a second end; b) flattening a first end of the first superconducting wire and flattening a second end of the second superconducting wire; c) Length L tot providing a tubular metal connector having a central portion; d) filling the center of the connector with unreacted Mg powder and B powder; e) flattening the connector if it has an original circular cross section; f) inserting a first end of a first superconductor into one side of the connector over a length L1 until it contacts the Mg powder and the B powder; g) inserting a second end of the second superconductor into the other side of the connector over a length L2 until it contacts the Mg powder and the B powder; h) pressing the connector on both sides to secure the first superconducting wire and the second superconducting wire within the connector; i) Length L center Pressing the center of the connector over a length L to compress the Mg and B powders. tot , L1, L2 and L center is the following formula: L tot=L1+L2+2×L cr +L center where L cr is the critical distance between the first and second flat ends on the one hand and the pressing length Lcenter of the center of the connector on the other hand, which is necessary to avoid the superconducting wire being pressed together with the pressing of the center. Includes.

[0019] The tubular metal connector and the flat ends are subjected to a heat treatment to bring about superconductivity in the center of the tubular metal connector and to restore superconductivity in the flat ends.

[0020] This heat treatment preferably has two phases: 1) A first phase of heating in the range of 800°C to 1000°C for 20 to 40 minutes, immediately followed by 2) A second phase of heating in the range of 550°C to 750°C for 45 to 75 minutes. Includes.

[0021] Preferably, the ends of the superconducting wire are flattened to the extent that the width to thickness ratio of these ends is in the range of 1.10 to 10.0, preferably 1.10 to 5.0, most preferably 1.50 to 2.5, for example 1.50 to 2.0.

[0022] Critical distance L cr The preferred value of is in the range of 0.6 mm to 5.0 mm, preferably 0.8 mm to 2.0 mm, for example 0.8 mm to 1.2 mm. [Brief explanation of the drawings]

[0023] [Figure 1] 1 is a schematic diagram of the preparation of a joint according to the present invention; [Figure 2] 1 shows a coupling according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0024] The next step, namely bonding the first superconducting wire 100 and the second superconducting wire 102, will now be described with reference to Figure 1. Numerical values ​​are given as non-limiting examples.

[0025] The first superconducting wire 100 has a sheath 104 of low carbon steel and a core 106 of reacted MgB2.

[0026] The second superconducting wire 102 has a sheath 108 with a barrier layer of titanium and an outer layer of low carbon steel, and a core 110 of reacted MgB2.

[0027] A tubular metal connector 112 is provided, which includes unreacted boron powder and magnesium powder. The boron powder is preferably nanoboron powder, and the magnesium powder is preferably spherical magnesium powder. By way of example, the outer diameter of the tubular metal connector 112 is 5.6 mm to 6.0 mm, and the tubular metal connector 112 is approximately 22 mm long.

[0028] The first end 116 and the second end 118 of the tubular metal connector 112 are first deburred.

[0029] The first end 116 of the tubular metal connector 112 and the second end 118 of the tubular metal connector are then immersed in a dilute HCl solution for a few seconds and then dried under vacuum.

[0030] Both the first end 116 and the second end 118 are polished, then alcohol washed and vacuum dried.

[0031] A 1.25 mm hole is drilled at both the first end 116 and the second end 118 to fit the dimensions of the first and second superconducting wires 100 and 102. The length of the hole is L1 on the first end 116 side and L2 on the second end 118 side. L1 and L2 may both be approximately 6.5 mm.

[0032] After drilling, the tubular metal connector 112 has a total length L totis flattened over the

[0033] The first superconducting wire 100 and the second superconducting wire are flattened over a length that is longer than the drilling lengths L1 and L2.

[0034] The flattened ends of the first and second superconducting wires 100 and 102 are then bevel-ground to increase the surface area. After this grinding operation, they are again alcohol-washed and vacuum-dried. The flattened ends of the first and second superconducting wires 100 and 102 are then immersed in a dilute solution of HCl, followed by alcohol-washing and vacuum-drying.

[0035] The flat ends of the first and second superconducting wires 100 and 102 are inserted into the tubular metal connector 112 until they contact the boron and magnesium powders in the center.

[0036] Pressure is applied to both ends 116 and 118 of tubular metal connector 112 to secure superconducting wires 100 and 102 and seal ends 116 and 118 .

[0037] The central portion of the tubular metal connector 112 is pressed by a hydraulic press 120 to a length L center It is pressed over a length L center is, for example, 6.35 mm before flattening.

[0038] Critical length L of 1.0 mm cr remains between the flattened central portion of the tubular metal connector 112 and the drilled holes at both ends.

[0039] Finally, a heat treatment is applied to the assembly of the first and second superconducting wires 100, 102 and the tubular metal connector 112 with Mg and B powders 114.

[0040] As mentioned, the heat treatment involves two phases.

[0041] During the first phase, the assembly is heated to 900° C. for 30 minutes. In this first phase, the following reactions occur: 2MgB2 → Mg + MgB4.

[0042] During the second phase, which immediately follows the first phase, the assembly is held at a temperature of 650° C. for 60 minutes. During this second phase, the following reactions occur: Mg+MgB4→2MgB2.

[0043] This double phase heat treatment not only brings about superconductivity in the center of the tubular metallic connector 112, but also restores superconductivity in the flat ends of the first and second superconducting wires 100 and 102.

[0044] Figure 2 provides a schematic representation of the realized joint. More specifically, Figure 2 shows the pressed ends 122 and 124 of the tubular metal connector 112 and the flattened central portion 126. Typical dimensions of the finished joint, i.e., the joint after the flattening operation, are: overall length L tot 23.8mm, length of flattened central part L center The insertion length of the first superconducting wire is 7.9 mm, and the insertion length of the second superconducting wire is 8.7 mm. [Explanation of symbols]

[0045] 100 First superconducting wire 102 Second superconducting wire 104 Sheath of the first superconducting wire 106 Reacted MgB2 inside the first superconducting wire 108 Sheath of the second superconducting wire 110 Reacted MgB2 inside the second superconducting wire 112 Tubular metal connector 114 Unreacted Mg powder and B powder 116 first end of tubular metal connector 118 second end of tubular metal connector 120 Press Tool 122 Pressed first end of tubular metal connector 124 Pressed second end of tubular metal connector 126 Pressed center section of tubular metal connector

[0046] List of abbreviations in the figure L1: Insertion length of the first superconducting wire L2: Insertion length of the second superconducting wire L tot Tubular metal connector length L cr critical length L center Pressure zone length in the center of the tubular metal connector

Claims

1. A superconducting wire joint, Sheath and reacted superconducting MgB ２ and at least two superconducting wires each having a core of at least one first superconducting wire has a first flattened end and at least one second superconducting wire has a second flattened end, the first flattened end and the second flattened end being superconducting; the coupling further comprises a tubular metal connector; The connector is made of superconducting MgB ２ a central portion containing the material; The first flat end of the at least one first superconducting wire is formed by the superconducting MgB ２ a connector inserted into one side of the connector so as to contact the material; The second flat end of the at least one second superconducting wire is formed by the superconducting MgB ２ a second connector disposed on the other side of the connector so as to contact the material; the connector is pressed on both sides to secure the at least one first superconducting wire and the at least one second superconducting wire; The central portion of the connector is pressed to form the superconducting MgB ２ A joint that compresses the material.

2. The joint of claim 1, wherein the first flat end and the second flat end have a width to thickness ratio in the range of 1.1 to 10, preferably 1.1 to 5.

0.

3. The coupling of claim 1 , wherein the connector comprises low carbon steel.

4. The coupling of claim 1 , wherein the connector comprises titanium.

5. 5. The coupling of claim 3 or 4, wherein the connector has an internal titanium barrier and low carbon steel surrounding the titanium barrier.

6. Reacted MgB ２ 1. A method for joining at least two superconducting wires having a superconducting core of a. providing at least one first superconducting wire and at least one second superconducting wire, the at least one first superconducting wire having a first end and the at least one second superconducting wire having a second end; b. flattening the first end of the first superconducting wire to form a first flat end and flattening the second end of the second superconducting wire to form a second flat end; c. providing a tubular metal connector having a length Ltot and having a central portion; d. filling the center of the connector with unreacted Mg powder and B powder; e. flattening the connector if it has an original circular cross section; f. inserting the first end of the at least one first superconducting wire into one side of the connector over a length L1 until it contacts the Mg powder and the B powder; g. inserting the second end of the at least one second superconducting wire into the other side of the connector over a length L2 until it contacts the Mg powder and the B powder; h) pressing the connector on both sides to secure the at least one first superconducting wire and the at least one second superconducting wire within the connector; i. Pressing the center portion of the connector over a length Lcenter to compress the Mg powder and B powder, wherein the lengths Ltot, L1, L2, and Lcenter are determined by the following formula: Ltot=L1+L2+2×Lcr+Lcenter where L is a critical distance between the first and second flat ends, on the one hand, and the length L of the center of the connector, on the other hand, that is necessary to avoid the superconducting wire being pressed together with the press of the center; and j) subjecting the connector and the first and second flat ends to a heat treatment to induce superconductivity in the central portion and restore superconductivity in the first and second flat ends, the heat treatment comprising a first phase of heating in the range of 800°C to 1000°C for 20 to 40 minutes and a second phase of heating in the range of 550°C to 750°C for 45 to 75 minutes; A method comprising:

7. 7. The method of claim 6, wherein flattening the first end of the at least one first superconducting wire and flattening the second end of the at least one second superconducting wire is performed to an extent that the width-to-thickness ratio of the first end and the second end is in the range of 1.25 to 2.5, preferably 1.50 to 2.

0.

8. The method of claim 6, wherein the critical distance Lcr is in the range of 0.6 mm to 5.0 mm.

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