METHOD FOR RESTORING SUPERCONDUCTIVITY OF AN MgB2 WIRE

Abstract

A method for restoring superconductivity of a wire having a core of reacted MgB.sub.2 comprises subjecting said wire to a two-phase heat treatment, wherein a first phase comprises heating in a range of 800? C. to 1000? C. during 20 minutes to 40 minutes and a second phase comprises heating in a range of 550? C. to 750? C. during 45 minutes to 75 minutes.

Claims

1. A method of restoring superconductivity of a wire having a core of reacted MgB.sub.2, said method comprising the following steps: subjecting said wire to a heat treatment of two phases, 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.

2. The method according to claim 1, wherein the wire contains multiple MgB.sub.2 cores.

3. The method according to claim 1, wherein said wire has a circular cross section.

4. The method according to claim 3, wherein said wire has a diameter between 0.2 mm and 1.5 mm.

5. The method according to claim 1, wherein said wire has a profiled or square or rectangular cross section.

6. The method according to claim 5, wherein said wire is flattened and has a width to thickness ratio ranging from 1.1 to 10, preferably from 1.1 to 5.0.

7. A method of restoring superconductivity of a cable comprising a plurality of wires having a core of reacted MgB.sub.2, said method comprising the following steps: subjecting said cable to a heat treatment of two phases, 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.

8. A method of restoring superconductivity in a joint of superconducting wires having a core of reacted superconducting MgB.sub.2, said method comprising the following steps: subjecting said joint to a heat treatment of two phases, 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.

Description

BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS

[0047] FIG. 1 is a schematic presentation of a preparation of a joint submitted to the heat treatment according to the invention.

[0048] FIG. 2 shows a joint submitted to a heat treatment according to the invention.

MODE(S) FOR CARRYING OUT THE INVENTION

[0049] 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.

[0050] The first superconducting wire 100 has a sheath 104 of low carbon steel and a core 106 of reacted MgB.sub.2.

[0051] 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.

[0052] 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.

[0053] The first end 116 and the second end 118 of the metal tubular connector 112 are first deburred.

[0054] 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.

[0055] Both the first and 116 and the second end 118 are polished followed by alcohol washing and vacuum drying.

[0056] 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.

[0057] After drilling, the tubular metal connector 112 is flattened over its whole length L.sub.tot.

[0058] 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.

[0059] 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 super-conducting wires 100 and 102 dipped in a diluted solution of HCl, followed by washing in alcohol and drying in vacuum.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] As mentioned, the heat treatment comprises two phases.

[0066] During the first phase the assembly is heated at 900? C. during 30 minutes. In this first phase, the following reaction takes place:

[00002]$2{\mathrm{MgB}}_2.fwdarw.\mathrm{Mg}+{\mathrm{MgB}}_4$

[0067] 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:

[00003]$\mathrm{Mg}+{\mathrm{MgB}}_4.fwdarw.2{\mathrm{MgB}}_2$

[0068] 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.

[0069] 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.

[0070] As already mentioned, and out of the context of joining two superconducting wires, the heat treatment can also be used to restore the superconductivity in MgB.sub.2 wires where superconductivity was lost. Two examples below show how superconductivity was recovered in damaged samples.

Example 1. Compression

[0071] Reference samples of superconducting wires having a superconducting core of reacted MgB.sub.2 and having a diameter of 0.75 mm were provided. The core of MgB.sub.2 had been sintered at 700? C. during 30 minutes. The critical current, I.sub.c, was measured at different temperatures. A linear regression was used to normalize the critical current values measured in different samples at different temperatures. Results obtained at respectively 25K, 20K and 15K are reported in table 1.

TABLE-US-00001 TABLE 1 Superconductivity test results obtained on different samples: Step 1 Step 2 Critical current Temp., time, Temp., time, Ic at Ic at Ic at Sample ? C. min ? C. min 25K, A 20K, A 15K, A REF 1 700 30 n/a n/a 139 252 365 REF 2 n/a n/a n/a n/a 38 76 114 COMP 1 900 30 650 30 68 90 113 COMP 2 900 10 650 60 75 145 214 INV 1 900 30 650 60 164 240 315 INV 2 900 40 650 60 179 267 365

[0072] REF 1 in Table 1 refers to the reference sample.

[0073] A damage was introduced by subjecting the reference sample REF 1 to a compression under a press of 4 tonnes during 10 minutes. REF 2 refers to the damaged sample and shows a drop in the critical current values, indicating a loss of superconductivity.

[0074] REF 2 was then subjected to different heat treatments to recover superconductivity.

[0075] COMP 1 and COMP 2 in Table 1 refer to comparison samples that were deformed as for REF2, then subjected to a two-phase heat treatment with durations outside the range of the present invention.

[0076] As an example, the second phase was too short for the comparative sample COMP 1 to recover superconductivity, as shown by the remaining low I.sub.c values.

[0077] As a second example, superconductivity was only partially recovered in the comparative sample COMP 2 due to a too short duration of the first phase of the recovery heat treatment, as shown by the increased I.sub.c values, yet too low compared to the reference sample REF 1.

[0078] Samples INV 1 and INV 2 were deformed as for REF 2, then subjected to the two-phase heat treatment of the invention, i.e. with a first phase at 900? C. during 30 minutes or 40 minutes, followed by a second phase at 650? C. for 60 minutes.

[0079] Both samples INV 1 and INV 2 have recovered superconductivity, as shown in table 1 with I.sub.c values close to the original values measured in sample REF 1.

Example 2. Bending

[0080] Another sample not reported in Table 1, sample INV 3, was obtained, starting from REF 1 and bending it over a tube of diameter of 19 mm diameter and straightening it.

[0081] Another sample, sample INV 4 was obtained starting from REF 1 and bending it over a tube of 19 mm diameter and straightening it, then pressing it uniaxially to obtain a flat wire with a width over thickness (w/t) ration of 2.

[0082] Samples INV 3 and INV 4 were subjected to the recovery heat treatment of the invention, i.e. samples were subjected to a heat treatment of two phases, a first phase of heating at 900? C. during 30 minutes and a second phase of heating at 650? C. during 60 minutes.

[0083] During the first phase the deformed wires were heated at 900? C. during 30 minutes. In this first phase, the following reaction takes place:

[00004]$2\mathrm{MgB}2.fwdarw.\mathrm{Mg}+\mathrm{MgB}4$

[0084] During the second phase immediately following the first phase, the deformed wires were kept at a temperature of 650? C. during 60 minutes. During this second phase, the following reaction takes place:

[00005]$\mathrm{Mg}+\mathrm{MgB}4.fwdarw.2\mathrm{MgB}2$

[0085] The critical current, I.sub.c was measured samples INV 3 and INV 4 after heat treatment, and the critical current was measured to be Ic=152 A at 25 K in both samples. This value was found to be close to the value measured in the reference sample REF 1 before deformation.

[0086] Additionally, a sample obtained in the same conditions as sample INV 3, i.e. by bending it over a tube of diameter of 19 mm diameter and straightening it, 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=0 A, meaning that superconductivity was not recovered.

[0087] Although experiments have determined reasonable temperature and time ranges, the claimed temperature and time ranges may be wider, when executed on other equipment, for other chemical compositions.

[0088] A cable comprising a plurality of wires having a core of reacted MgB.sub.2, wherein at least one wire is damaged, will recover superconductivity when subjected to the two phase heat treatment of the invention.

LIST OF REFERENCE NUMBERS

[0089] 100 first superconducting wire [0090] 102 second superconducting wire [0091] 104 sheath of first superconducting wire [0092] 106 reacted MgB.sub.2 inside first superconducting wire [0093] 108 sheath of second superconducting wire [0094] 110 reacted MgB.sub.2 inside second superconducting wire [0095] 112 tubular metal connector [0096] 114 unreacted Mg powder and B powder [0097] 116 first end of tubular metal connector [0098] 118 second end of tubular metal connector [0099] 120 pressing tool [0100] 122 first pressed end of tubular metal connector [0101] 124 second pressed end of tubular metal connector [0102] 126 central pressed part of tubular metal connector

LIST OF ABBREVIATIONS IN FIGURES

[0103] L.sub.1 length of insertion of first superconducting wire [0104] L.sub.2 length of insertion of second superconducting wire [0105] L.sub.tot length of the tubular metal connector [0106] L.sub.cr critical length [0107] L.sub.center length of pressure zone in central part of tubular metal connector