How to restore superconductivity in MgB2 wires

Abstract

A method of restoring superconductivity in a wire having a reacted MgB2 core includes subjecting the wire to a two-phase heat treatment, a first phase including heating in the range of 800°C to 1000°C for 20 minutes to 40 minutes, and a second phase including heating in the range of 550°C to 750°C for 45 minutes to 75 minutes.

Description

[Technical field]

[0001] The present invention relates to a method for restoring superconductivity in MgB2 wire by subjecting the wire to a two-phase heat treatment. [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 operating temperature of any conventional superconductor. This allows it to function as a superconductor without the need for the use of 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 achieving superconducting junctions.

[0005] US Patent Application Publication No. A1-2009 / 0105079 discloses a superconducting connection of 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 connection of two ends of a single wire-type superconductor. The single wire-type superconductor is a circular wire having a MgB2 superconducting core surrounded by a stabilizing layer and a metal sheath. To realize this connection, the ends of the superconducting wire are flattened to a significant extent to increase the bonding area. The width-to-thickness ratio is in the range of 3 to 100. The stabilizing layer and the metal sheath are removed on one side of the flattened end so that the superconducting core is open at one end. The open sides 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 [Means for solving the problem]

[0007] The object of the present invention is to provide a method for restoring superconductivity in a wire having a reacted MgB2 core, the method comprising the following steps: Wire the two phases: 1) a first phase of heating in the range of 800°C to 1000°C for 20 to 40 minutes; and 2) A second phase of heating in the range of 550°C to 750°C for 45 to 75 minutes. A step of subjecting the resultant to a heat treatment.

[0008] The above heat treatment is suitable for restoring superconductivity to a single wire having an MgB2 core that has lost superconductivity due to breakage or other events.

[0009] The above heat treatment is also suitable for restoring superconductivity in several single wires (cable structure) or several single wires with MgB2 cores (multifilament wires).

[0010] The above heat treatment is - the bonding process, - Damage from handling (excessive tension / compression of the wire during processing or winding), - Accidental / unexpected damage (shock to the wire or wire assembly, heat...) It is effective in repairing general defects that occur in the MgB2 phase due to

[0011] The method for restoring superconductivity can be applied to wires having a bent, flattened or twisted core of reacted MgB2.

[0012] The above heat treatment to restore superconductivity is independent of the shape of the superconducting wire: said superconducting wire can have a round or shaped cross section.

[0013] The method may also be applied to restore superconductivity in a cable containing multiple wires with an MgB2 core, where superconductivity has been lost in one or more wires due to a break or other event.

[0014] This method can also be applied to restore superconductivity in superconducting wire bonds.

[0015] As an illustration of the application of the method for restoring superconductivity in wires with reacted MgB2 cores, a superconducting wire joint is provided. The joint and method described below are not part of the present invention.

[0016] The joint includes at least two superconducting wires, each having a sheath and a MgB2 superconducting core inside 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 center portion filled with magnesium or 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 or 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 or boron or MgB2 material. The connector is pressed from both sides to secure the first and second superconducting wires. The center portion of the connector is pressed to compress the magnesium or boron or MgB2 material.

[0017] At this joint, the superconducting wires do not need to overlap, and the sheath of the superconducting wires does not need to be removed.

[0018] 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.

[0019] The first and second superconducting wires have a core of reacted superconducting MgB2 and therefore also have gaps 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 gaps are reduced. Subsequent pressing of these already pressed ends of the superconducting wires no longer creates large cracks that impede the flow of current.

[0020] The tubular metal connector preferably comprises low carbon steel.

[0021] The tubular metal connector may also include titanium.

[0022] The tubular metal connector may have a titanium barrier on the radially inner side and a low carbon steel on the radially outer side.

[0023] The tubular metal connectors are preferably made exclusively from low carbon steel.

[0024] A method for joining at least two superconducting wires having a superconducting core of reacted MgB2 is provided, the method comprising the steps of: 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 originally had a 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 and B powders; g) inserting a second end of a second superconductor into the other side of the connector over a length L2 until it contacts the Mg and B powders; 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 In the formula, L cr is the critical distance between the first flat end and the second flat end, on the one hand, and the pressing length Lcenter of the central part of the connector, on the other hand, which is necessary to avoid the superconducting wire being pressed together with the pressing of the central part; Includes.

[0025] Preferably, the flattening of the ends of the superconducting wire is carried out 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.

[0026] A preferred value of the critical distance Lcr 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.

[0027] The tubular metallic connector and the flat end are subjected to a heat treatment to bring about superconductivity in the tubular metallic connector and to restore superconductivity at the flat end. A second object of the present invention is to provide a heat treatment for restoring superconductivity at the joint of the superconducting wire.

[0028] 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, followed immediately by 2) A second phase of heating in the range of 550°C to 750°C for 45 to 75 minutes. Includes. [Brief description of the drawings]

[0029] [Figure 1] FIG. 2 is a schematic representation of the preparation of a joint to be subjected to heat treatment according to the present invention. [Diagram 2] 3 shows a joint subjected to a heat treatment according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] The subsequent step, namely bonding the first superconducting wire 100 and the second superconducting wire 102, is described with reference to Figure 1. Numerical values ​​are given as a non-limiting example.

[0031] The first superconducting wire 100 has a sheath 104 of low carbon steel and a core 106 of reacted MgB2.

[0032] 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.

[0033] A tubular metal connector 112 is provided, the tubular metal connector 112 including unreacted boron powder and magnesium powder. The boron powder is preferably nano boron powder, and the magnesium powder is preferably spherical magnesium powder. By way of example, the outer diameter of the tubular metal connector 112 is between 5.6 mm and 6.0 mm, and the tubular metal connector 112 is approximately 22 mm long.

[0034] The first and second ends 116, 118 of the tubular metal connector 112 are first deburred.

[0035] 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.

[0036] Both the first end 116 and the second end 118 are polished, then alcohol washed and vacuum dried.

[0037] 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 at the first end 116 side and L2 at the second end 118 side. L1 and L2 can both be about 6.5 mm.

[0038] After drilling, the tubular metal connector 112 has a total length L totis flattened over

[0039] The first superconducting wire 100 and the second superconducting wire are flattened over a length that is greater than the drilling lengths L1 and L2.

[0040] The so flattened ends of the first and second superconducting wires 100 and 102 are ground at an angle to increase the surface area. This grinding operation is followed again by alcohol rinsing and vacuum drying. 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 rinsing and vacuum drying.

[0041] The flat ends of the first and second superconducting wires 100 and 102 are inserted into the tubular metal connector 112 until it contacts the boron and magnesium powders in the center.

[0042] Pressure is applied to both ends 116 and 118 of the tubular metal connector 112 to secure the superconducting wires 100 and 102 and seal the ends 116 and 118 .

[0043] The central portion of the tubular metal connector 112 is pressed to a length L center The pressing is performed over a length L center is, for example, 6.35 mm before flattening.

[0044] 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.

[0045] 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 powders 114.

[0046] As stated, the heat treatment involves two phases.

[0047] 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.

[0048] During the second phase, which follows immediately after 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.

[0049] This dual phase heat treatment not only provides 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 .

[0050] Figure 2 provides a schematic diagram 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.

[0051] Besides the situation of joining two superconducting wires, as already mentioned, heat treatment can also be used to restore superconductivity in MgB2 wires where superconductivity has been lost. The following two examples show how superconductivity was restored in damaged samples. EXAMPLES

[0052] Example 1. Compression A reference sample of a superconducting wire having a reacted MgB2 superconducting core and a diameter of 0.75 mm was provided. The MgB2 core was sintered at 700 °C for 30 min. The critical current I c was measured at different temperatures. Linear regression was used to normalize the critical current values ​​measured on different samples at different temperatures. The results obtained at 25 K, 20 K and 15 K, respectively, are reported in Table 1.

[0053] [Table 1]

[0054] Reference 1 in Table 1 refers to the reference sample.

[0055] Damage was induced by compressing the reference sample, Ref. 1, under a pressure of 4 tons for 10 minutes. Reference 2 refers to the damaged sample and shows a drop in the critical current value, indicating that superconductivity was lost.

[0056] Reference 2 was then subjected to different heat treatments to restore superconductivity.

[0057] Comp. 1 and Comp. 2 in Table 1 refer to comparative samples that failed in the same manner as Reference 2 and were then subjected to a two-phase heat treatment using a duration outside the scope of the present invention.

[0058] As an example, the comparative sample, Comparative 1, has a low I c As shown by the value, the time was too short to restore superconductivity.

[0059] As a second example, in the comparative sample Comparative 2, the superconductivity was only partially restored due to the duration of the first phase of the recovery heat treatment being too short, which is due to the fact that I c This is indicated by the increased values, which are still too low compared to the reference sample, Ref1.

[0060] Samples Inventive 1 and Inventive 2 were broken in the same way as Reference 2 and then subjected to a two-phase heat treatment according to the invention, i.e. a first phase at 900°C for 30 or 40 minutes, followed by a second phase at 650°C for 60 minutes.

[0061] Both samples of the present invention 1 and the present invention 2 recovered superconductivity, which is shown in Table 1 as I cThe values ​​are shown to be close to the original values ​​measured in sample reference 1.

[0062] Example 2. Curvature Starting with Reference 1, we bent it through a 19 mm diameter tube and then straightened it out to obtain Sample Inventive 3, another sample not reported in Table 1.

[0063] Starting with Reference 1, we bent it through a tube with a diameter of 19 mm, straightened it out, and then uniaxially pressed it to obtain a flat wire with a width / thickness (w / t) ratio of 2 to obtain another sample, Sample Inventive 4.

[0064] Samples Inventive 3 and Inventive 4 were subjected to the recuperative heat treatment of the invention, i.e. the samples were subjected to a heat treatment in two phases: a first phase of heating at 900° C. for 30 minutes and a second phase of heating at 650° C. for 60 minutes.

[0065] During the first phase, the deformed wire was heated to 900° C. for 30 minutes. During this first phase, the following reactions take place: 2MgB2 → Mg + MgB4.

[0066] During a second phase, immediately following the first phase, the deformed wire was maintained at a temperature of 650° C. for a period of 60 minutes. During this second phase, the following reactions take place: Mg+MgB4→2MgB2.

[0067] After heat treatment, the critical current Ic was measured for samples Inventive 3 and Inventive 4. The critical current was measured to be Ic=152 A at 25 K for both samples. This value was found to be close to the value measured for the reference sample, Ref. 1, before deformation.

[0068] Furthermore, a sample obtained under the same conditions as in sample invention 3, i.e. a sample obtained by bending and then straightening a tube with a diameter of 19 mm, was heated at 900°C for 4 hours, followed by heating at 850°C for 41 hours. The critical current measured after this heat treatment was Ic=0A, which meant that the superconductivity was not restored.

[0069] Experimentation has determined reasonable temperature and time ranges, but when performed with other equipment and other chemical compositions, the claimed temperature and time ranges may be broader.

[0070] A cable containing multiple wires with a reacted MgB2 core, in which at least one wire has been broken, regains superconductivity when subjected to the two-phase heat treatment of the present invention. [Explanation of symbols]

[0071] 100 The 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

[0072] List of abbreviations in the figure L1 Insertion length of the first superconducting wire L2 Insertion length of the second superconducting wire L totTubular metal connector length L cr critical length L center Pressure zone length in the center of the tubular metal connector

Claims

1. Reacted MgB ２ 1. A method of restoring superconductivity in a wire having a core of the wire comprising the steps of subjecting the wire to a two-phase heat treatment, 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.

2. The wire is made of a plurality of MgB ２ The method of claim 1 including a wick.

3. The method of claim 1 or 2, wherein the wire has a circular cross-section.

4. The method of claim 3, wherein the wire has a diameter of between 0.2 mm and 1.5 mm.

5. The method of claim 1 or 2, wherein the wire has a contoured, square or rectangular cross-section.

6. The method of claim 5, wherein the wire is flattened and has a width to thickness ratio in the range of 1.1 to 10, preferably 1.1 to 5.

0.

7. Reacted MgB ２ 1. A method of restoring superconductivity in a cable including a plurality of wires having a core of 1000° C., the method comprising the steps of subjecting the cable to a two-phase heat treatment, 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.

8. Reacted superconducting MgB ２ 1. A method of restoring superconductivity in a joint in a superconducting wire having a core of 0.1 mm or more, the method comprising the steps of subjecting the joint to a two-phase heat treatment, 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.

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