MAGNESIUMDIBORIDE POWDER-IN-TUBE WIRE

Abstract

A magnesiumdiboride (MgB.sub.2) powder-in-tube (PIT) wire has a cross-section showing —voids, —magnesiumdiboride, and —oxides, as measured by energy-dispersive X-ray spectroscopy. Oxides are located at the borders between the voids and the magnesiumdiboride. The MgB.sub.2 PIT wire has a higher degree of superconductivity.

Claims

1. A magnesiumdiboride powder-in-tube wire having a cross-section showing voids, magnesiumdiboride, and oxides, wherein oxides are located at the borders between the voids and the magnesiumdiboride.

2. The wire according to claim 1, said wire being an in situ magnesiumdiboride powder-in-tube wire.

3. The wire according to claim 1, wherein more than 60% of the present oxides are located at the borders between the voids and magnesiumdiboride.

4. The wire according to claim 1, wherein the oxides are magnesium oxides.

5. The wire according to claim 4, wherein the magnesium oxides are MgO.

6. The wire according to claim 1, wherein the amount of oxygen is less than 5 wt %, when measured over a cross-section voids not included.

7. A multi-wire comprising two or more wires according to claim 1.

8. A process of making a wire according to claim 1, said process comprising following steps: a) mixing magnesium powder and/or boron powder in a first carrier liquid to create a first slurry; b) washing said first slurry whereby magnesium oxides and/or boron oxides are washed away leaving a residue of magnesium powder and/or boron powder and remaining oxides; c) adding a second carrier liquid to said residue to create a second slurry; d) adding said second slurry to a preformed metallic sheath; e) closing said preformed metallic sheath to form a tube.

9. The process according to claim 8, wherein said first carrier liquid is equal to said second carrier liquid.

Description

BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS

[0034] FIG. 1 shows schematically first steps of making a single in situ magnesiumdiboride powder-in-tube wire.

[0035] FIG. 2 shows schematically following steps of making a multifilament wire.

[0036] FIG. 3a shows a scanned area of a prior art ex situ MgB.sub.2 wire, FIG. 3b shows energy dispersive X-ray mapping of oxygen of the prior art ex situ MgB.sub.2 wire in the scanned area of FIG. 3a.

[0037] FIG. 4a shows a scanned area of a prior art in situ MgB.sub.2 wire, FIG. 4b shows energy dispersive X-ray mapping of oxygen of the prior art in situ MgB.sub.2 wire in the scanned area of FIG. 4a.

[0038] FIG. 5a shows a scanned area of an invention in situ MgB.sub.2 wire, FIG. 5b shows energy dispersive X-ray mapping of oxygen of the invention in situ MgB.sub.2 wire in the scanned area of FIG. 5a.

MODE(S) FOR CARRYING OUT THE INVENTION

[0039] The first steps of making an in situ MgB.sub.2 PIT wire relate to the preparing of a slurry.

[0040] A first slurry is prepared by mixing B powder and/or Mg powder in a first carrier liquid. As mentioned before, there is no need to use small Mg powders. Mg powders with sizes ranging from 100 μm to 350 μm are perfectly suitable. The B powders may be of a nano to micro size. Dopant powders like SiC may be added. Examples of a suitable first carrier liquid are liquid hydrocarbon, ethyl alcohol, acetone, methyl acetate, and ethyl-acetate. Examples of a suitable second carrier liquid are alcohol and acetone.

[0041] This first slurry is then washed. The washing process removes not only the fluid substance but also a substantial part of the present oxides such as MgO and/or B.sub.2O.sub.3. A residue of B powders and Mg powders and possible dopant powders and a reduced amount of oxides remains.

[0042] After the washing step, a second liquid carrier is added to the residue to form a second slurry. This second liquid carrier can be different from the first liquid carrier, but is preferably the same.

[0043] Reference is now made to FIG. 1, which shows schematically the first steps of making a single in situ MgB.sub.2 PIT wire.

[0044] Step 100 is unwinding of a metallic or bi-metallic flat sheet 102. The sheet can be of Cu, Ni, Nb, Ti, Fe, stainless steel, Cu—Ni, Monel, Ag—Mg and Nb—Ti or of any combination of them. Preferably a bi-metallic sheet out of two metals is used or two sheets of different metals are used. One of the two metals is electrically more conducting than the other one. A preferred example is a combination of copper with steel.

[0045] Step 104 is the preforming of the metallic sheet 102, for example in a U-form by means of preforming elements 106. The U-form is suitable for receiving the second slurry with the various powders.

[0046] Step 108 is delivering the second slurry with the powders. This can be done in one step where all powders, B, Mg and any dopant powders are added via one nozzle 110 to the deformed sheet 102. Alternatively, the various powders in a separate second slurry can be added separately via nozzles 110, 112 and 114.

[0047] Step 116 is preconditioning the powders by means of a heater 118.

[0048] Step 120 is the closure of the metallic sheet 102 by means of preformers 122 and a welding operation to form a closed metallic tube.

[0049] Step 124 is the reduction of cross-section of the tube to form a single wire. This reduction can be done by means of rollers 126 or by means of a series of dies.

[0050] Step 128 is a quality control step.

[0051] The result of this first series of manufacturing steps 100, 104, 108, 116, 120, 124 and 128 is a single in PIT wire 130 with unreacted powders B and Mg and possible dopant powders inside a metallic sheet.

[0052] FIG. 2 illustrates the various steps for making a multifilament wire.

[0053] A single PIT wire 130 forms the main starting product for making a multifilament wire.

[0054] In step 200 various single PIT wires 130 are put next to each other and next to a copper or aluminium wire 202 ready to be unwound and twisted.

[0055] Step 204 is the twisting process where the various PIT wires 130 are twisted around the copper or aluminium wire 202 to form a twisted structure.

[0056] In step 208 elongated insulation material 210 is wrapped or braided around the twisted structure to form a consolidated multifilament 212.

[0057] Thereafter, preferably in line with the twisting and wrapping or braiding process, the consolidated multifilament 212 is subjected to a heat treatment in order to react the Mg with the B and to form MgB.sub.2.

[0058] In a final step 214 the multifilament 212 is impregnated with resin 216.

[0059] Energy-dispersive X-ray spectroscopy has been applied on several samples of two prior art MgB.sub.2 PIT superconductor wires and on several samples of an invention MgB.sub.2 PIT superconductor wire.

[0060] All samples were cut by a plasma of argon ions with a cross-section polisher of the type JEOL (JSM 09010). After making a clean cross-section, the samples were put on a sample holder for analysis and stored in a vacuum chamber until analysis could start. This was done to avoid oxidation by air.

[0061] Several analyses were carried out in a JEOL 7200F equipped with an Oxford X-max EDX detector from Oxford Instruments with a 80 mm.sup.2 window. The software used is Oxford Aztec version 3.3.

[0062] All samples were measured on process time 5 with a fixed number of counts/spectrum of 500000. All elements other than B, O or Mg were removed from the spectrum by eliminating them in the Oxford Aztec software.

[0063] The acceleration voltage used in the method was 5 kV or 15 kV.

[0064] The results are in weight percentage (wt %). The measured values have an accuracy of 0.1 wt %.

[0065] Prior Art Wire 1: Ex Situ MgB.sub.2 PIT Wire

[0066] FIG. 3a shows the scanned area of an ex situ MgB.sub.2 PIT wire. FIG. 3b shows the energy dispersive X-ray mapping of oxygen of the prior art ex situ MgB.sub.2 wire in the scanned area of FIG. 3a. The white spots in FIG. 3b correspond to oxygen.

[0067] The oxygen is distributed homogeneously over the cross-section area and is thus largely present in the MgB.sub.2 matrix.

[0068] The voids or cavities are widely spread over the whole cross-section.

[0069] Measurement of pure material without including a certain number of cavities was not possible.

[0070] Nine different samples were measured with different magnification (2000×, or 10000×) and with different acceleration voltage (5 kV or 15 kV). These nine samples show following ranges: [0071] B: 51.0 wt %-52.2 wt % [0072] O: 6.8 wt %-8.7 wt % [0073] Mg: 40.5 wt %-42.2 wt %

[0074] Prior Art Wire 2: In Situ MgB.sub.2 PIT Wire

[0075] FIG. 4a shows a scanned area of a prior art in situ MgB.sub.2 wire. FIG. 4b shows energy dispersive X-ray mapping of oxygen of the prior art in situ MgB.sub.2 wire in the scanned area of FIG. 4a.

[0076] The white spots in FIG. 4b correspond to oxygen. The oxygen is distributed in oxygen right areas and oxygen poor areas, but the oxygen concentration cannot be linked to the cavities or the voids.

[0077] In contrast to prior art wire 1, it was possible here to do measurements with inclusion and exclusion of cavities.

[0078] Including the cavities and using an acceleration voltage of 15 kV and magnification of either 2000× or 10000×, following ranges were found over four samples: [0079] B: 40.9 wt %-50.3 wt % [0080] O: 13.8 wt %-23.4 wt % [0081] Mg: 29.4 wt %-38.3 wt % These wide ranges confirm the inhomogeneous distribution of oxygen.

[0082] Excluding the cavities and using an acceleration voltage of either 5 kV or 15 kV, following ranges were found: [0083] B: 29.9 wt %-57.3 wt % [0084] O: 1.6 wt %-34.1 wt % [0085] Mg: 33.8 wt %-42.6 wt %

[0086] Again, these wide ranges confirm the inhomogeneous distribution of oxygen.

[0087] Invention In Situ MgB.sub.2 PIT Wire

[0088] FIG. 5a shows a scanned area of an invention in situ MgB.sub.2 wire. FIG. 5b shows an energy dispersive X-ray mapping of oxygen of the invention in situ MgB.sub.2 wire in the scanned area of FIG. 5a. The white spots in FIG. 5b correspond to oxygen. The oxygen is concentrated at the border between the MgB.sub.2 matrix and the cavities.

[0089] Including the cavities and using an acceleration voltage of 5 kV or 15 kV and magnification of either 2000×, following ranges were found over four samples: [0090] B: 52.8 wt %-54.4 wt % [0091] O: 6.2 wt %-8.4 wt % [0092] Mg: 37.2 wt %-40.2 wt %

[0093] In comparison with prior art in situ MgB.sub.2 PIT wire, these ranges are more narrow.

[0094] Excluding the cavities and using an acceleration voltage of 5 kV or 15 kV and varying magnification, following ranges were found: [0095] B: 56.0 wt %-58.1 wt % [0096] O: 0.9 wt %-2.3 wt % [0097] Mg: 41.0 wt %-42.3 wt %

[0098] Here the ranges are also quite narrow. The very low amount of oxygen (always below 2.5 wt %) is remarkable and explains the advantages of the present invention.

[0099] A MgB.sub.2 PIT wire according to the invention can be used in superconductors. The superconductor is preferably used in a superconducting magnet of a magnetic resonance imaging apparatus. A superconductor according to the invention may also be applied in magnetic levitating vehicles, superconducting electromagnetic propulsion ships, nuclear fusion reactors, superconducting generators, accelerators, electron microscopes, energy storing apparatus, and power cables.