JOINT OF MgB2 SUPERCONDUCTING WIRES

Abstract

A joint of superconducting wires having at least two superconducting wires, each with a sheath and with a core of reacted superconducting MgB.sub.2. At least one first superconducting wire has a first flattened end and at least one second superconducting wire has a second flattened end. The joint further has a tubular metal connector having a centre filled with MgB.sub.2 material. The first flattened end of the first superconducting wire is inserted at one side of the connector until it is in contact with the MgB.sub.2 material, the second flattened end of the second superconducting wire is inserted at the other side of the connector until it is in contact with the MgB.sub.2 material, the connector is pressed at both sides to fix the superconducting wires, and the centre of the connector is pressed to compact the MgB.sub.2 material.

Claims

1. A joint of superconducting wires, said joint comprising at least two superconducting wires each with a sheath and with a core of reacted superconducting MgB.sub.2, at least one first superconducting wire having a first flattened end and at least one second superconducting wire having a second flattened end, said joint further comprising a tubular metal connector, said connector having a centre being filled with magnesium or boron or MgB.sub.2 material, said first flattened end of said at least one first superconducting wire being inserted at one side of said connector until it is in contact with said magnesium or boron or MgB.sub.2 material, said second flattened end of said at least one second superconducting wire being inserted at the other side of said connector until it is in contact with said magnesium or boron or MgB.sub.2 material, said connector being pressed at both sides to fix said at least one first superconducting wire and said at least one second superconducting wire, said centre of said connector being pressed to compact said magnesium or boron or MgB.sub.2 material.

2. The joint according to claim 1, wherein said first flattened end and said second flattened end have a width to thickness ratio ranging from 1.1 to 10, preferably from 1.1 to 5.0.

3. The joint according to claim 1, wherein said tubular metal connector comprises low carbon steel.

4. The joint according to claim 1, wherein said tubular metal connector comprises titanium.

5. (canceled)

6. A method of joining at least two superconducting wires having a superconducting core of reacted MgB.sub.2, said method comprising the following steps: a. providing at least one first superconducting wire and at least one second superconducting wire, said at least one first superconducting wire having a first end and said at least one second superconducting wire having a second end; b. flattening said first end of said first superconducting wire and flattening said second end of said second superconducting wire; c. providing a tubular metal connector having a length Ltot and having a centre; d. filling said centre of said connector with unreacted Mg powder and B powder; e. flattening said connector in case said connector has an original circular cross-section; f. inserting said first end of said least one first superconductor at one side in said connector over a length L1 until contact with said Mg powder and B powder; g. inserting said second end of said at least one second superconductor at the other side in said connector over a length L2 until contact with said Mg powder and B powder; h. pressing said connector at both sides to fix said at least one first superconductor wire and said at least one second superconductor wire in said connector; i. pressing said centre of said connector over a length Lcenter to compact said Mg and B powder, wherein said lengths Ltot, L1, L2 and Lcenter matching following equation: $\mathrm{Ltot}=L1+L2+2?\mathrm{Lcr}+\mathrm{Lcenter}$ where Lcr is a critical distance between, on the one hand, the first flattened end and the second flattened end, and, on the other hand, the pressed length Lcenter of the centre of said connector needed to avoid that the superconducting wires are pressed together with the pressing of said centre.

7. The method according to claim 6, wherein said method comprises the following step: subjecting said connector and said first flattened end and second flattened end to a heat treatment to create superconductivity in the centre and to restore superconductivity in said first flattened end and second flattened end.

8. The method according to claim 7, wherein said heat treatment comprises a first phase of heating in a range of 800? C. to 1000? C. during 20 minutes to 40 minutes and a second phase of heating in a range of 550? C. to 750? C. during 45 minutes to 75 minutes.

9. The method according to claim 6, wherein flattening said first end of said at least one first superconducting wire and flattening said second end of said at least one second superconducting wire are done to a degree where the width to thickness ratio of said first end and of said second end range from 1.25 to 2.5, preferably from 1.50 to 2.0.

10. The method according to claim 6, wherein said critical distance Lcr ranges from 0.6 mm to 5.0 mm.

11. The joint according to claim 1, wherein said tubular metal connector comprises low carbon steel, and wherein said tubular metal connector comprises titanium.

12. The joint according to claim 11, wherein said tubular metal connector has a titanium barrier inside and a low carbon steel around said titanium barrier.

Description

BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS

[0035] FIG. 1 is a schematic presentation of a preparation of a joint according to the invention.

[0036] FIG. 2 shows a joint according to the invention.

MODE(S) FOR CARRYING OUT THE INVENTION

[0037] Referring to FIG. 1 the subsequent steps or joining a first superconducting wire 100 and a second superconducting wire 102 will be described. The numerical values are given by way of non-limiting example.

[0038] The first superconducting wire 100 has a sheath 104 of low carbon steel and a core 106 of reacted MgB.sub.2.

[0039] 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 MgB.sub.2.

[0040] A tubular metal connector 112 is provided and contains unreacted boron powder and magnesium powder. The boron powder is preferably a nano boron powder and the magnesium powder is preferably a spherical magnesium powder. By way of example, the external diameter of the tubular metal connector 112 is 5.6 mm to 6.0 mm and the metal tubular connector 112 is about 22 mm long.

[0041] The first end 116 and the second end 118 of the metal tubular connector 112 are first deburred.

[0042] Then, the first end 116 of the tubular metal connector 112 and the second end 118 of the tubular metal connector are immersed in a diluted HCl solution for a couple of seconds and thereafter dried under vacuum.

[0043] Both the first and 116 and the second end 118 are polished followed by alcohol washing and vacuum drying.

[0044] A hole of 1.25 mm is drilled in both the first end 116 and the second end 118 to match the dimensions of the first and the second superconducting wires 100 and 102. The length of the hole is L.sub.1 at the sided of the first end 116 and L.sub.2 at the side of the second end 118. Both L.sub.1 and L.sub.2 can be about 6.5 mm.

[0045] After drilling, the tubular metal connector 112 is flattened over its whole length L.sub.tot.

[0046] The first superconducting wire 100 and the second superconducting wire are flattened over a length that is greater than the drilling lengths L.sub.1 and L.sub.2.

[0047] The thus flattened ends of the first and second superconducting wire 100 and 102 are ground with an angle in order to increase the surface area. This grinding operation is again followed by alcohol washing and vacuum drying. Thereafter, the flattened ends of the first and second superconducting wires 100 and 102 dipped in a diluted solution of HCl, followed by washing in alcohol and drying in vacuum.

[0048] The flattened ends of the first and second superconducting wires 100 and 102 are inserted in the tubular metal connector 112 until they have contact with the boron and magnesium powder in the centre.

[0049] Pressure is exercised at both ends 116 and 118 of the tubular metal connector 112 in order to fix the superconducting wires 100 and 102 and to seal the ends 116 and 118.

[0050] The centre part of the tubular metal connector 112 is pressed over a length L.sub.center by means of a hydraulic press 120. The length L.sub.center before flattening is for example 6.35 mm.

[0051] A critical length L.sub.cr of 1.0 mm remains at both sides between the flattened centre part of the tubular metal connector 112 and the drilled holes.

[0052] 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 the Mg and B powder 114.

[0053] As mentioned, the heat treatment comprises two phases.

[0054] During the first phase the assembly is heated at 900? C. during 30 minutes. In this first phase, the following reaction takes place:

2MgB.sub.2.fwdarw.Mg+MgB.sub.4

[0055] During the second phase immediately following the first phase, the assembly is kept at a temperature of 650? C. during 60 minutes. During this second phase, the following reaction takes place:

Mg+MgB.sub.4.fwdarw.2MgB.sub.2

[0056] This double phase heat treatment not only create superconductivity in the centre of the tubular metal connector 112 but also restores the superconductivity in the flattened ends of the first and second superconducting wires 100 and 102.

[0057] FIG. 2 gives a schematical representation of a realized joint. More particularly, FIG. 2 shows the pressed ends 122 and 124 of the tubular metal connector 112 and the flattened central part 126. Typical dimensions in the finalized joint, i.e. after the flattening operations, are a total length L.sub.tot of 23.8 mm, a length L.sub.center of the flattened central part of 7.25 mm, a length of insertion of first superconducting wire of 7.9 mm and a length of insertion of second superconducting wire of 8.7 mm.

LIST OF REFERENCE NUMBERS

[0058] 100 first superconducting wire [0059] 102 second superconducting wire [0060] 104 sheath of first superconducting wire [0061] 106 reacted MgB.sub.2 inside first superconducting wire [0062] 108 sheath of second superconducting wire [0063] 110 reacted MgB.sub.2 inside second superconducting wire [0064] 112 tubular metal connector [0065] 114 unreacted Mg powder and B powder [0066] 116 first end of tubular metal connector [0067] 118 second end of tubular metal connector [0068] 120 pressing tool [0069] 122 first pressed end of tubular metal connector [0070] 124 second pressed end of tubular metal connector [0071] 126 central pressed part of tubular metal connector

LIST OF ABBREVIATIONS IN FIGURES

[0072] L.sub.1 length of insertion of first superconducting wire [0073] L.sub.2 length of insertion of second superconducting wire [0074] L.sub.tot length of the tubular metal connector [0075] L.sub.cr critical length [0076] L.sub.center length of pressure zone in central part of tubular metal connector