AU2012201062A1 - Superconducting composite wire made from magnesium diboride - Google Patents

Abstract

A superconducting composite wire with superconducting phase of magnesium diboride comprises: a core of conductive metal (1); a plurality of filaments in which 5 each filament (3) comprises a core of magnesium diboride (5), placed around said conductive metal core (1); - an outer metallic sheath (4) for containment and mechanical reinforcement, surrounding the said plurality of filaments; and - at least one layer (2, 2a, 2b) of metal chemically compatible with magnesium diboride and capable of acting as an obstacle to the diffusion of the conductive metal of said 10 conductive metal core (1) towards the said filaments (3), up to 980*C where the said at least one layer is applied a) as a coating (2) of the said conductive metal core and/or b) as a coating (2a) of the said filaments (3), and/or c) as a coating (2b) of the said magnesium diboride core (5) of the said filaments (3).

Description

AUSTRALIA Patents Act COMPLETE SPECIFICATION (ORIGINAL) Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority Related Art: Name of Applicant: Columbus Superconductors S.r.l. Actual Inventor(s): Andrea Malagoli, Giovanni Grasso, Antonio Sergio Siri Address for Service and Correspondence: PHILLIPS ORMONDE FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne 3000 AUSTRALIA Invention Title: SUPERCONDUCTING COMPOSITE WIRE MADE FROM MAGNESIUM DIBORIDE Our Ref : 935835 POF Code: 1249/480748 The following statement is a full description of this invention, including the best method of performing it known to applicant(s): - 1- 1A Superconducting composite wire made from magnesium diboride The present application is a divisional application from Australian Patent Application No.2004321817, the entire disclosure of which is incorporated herein by reference. 5 The present invention relates to a superconducting composite wire or strip, comprising superconducting magnesium diboride material. A reference herein to a patent document or other matter which is given as prior art is not to be 10 taken as an admission that that document or matter was, in Australia, known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims. Throughout the description of this specification the word "comprise" and variations of that 15 word, such as "comprises" and "comprising", are not intended to exclude other additives or components or integers. The use of superconducting materials in various industrial applications requires many precautions and protective measures. One of the most significant problems concerns the 20 appropriate protection of a winding made with superconducting wire, in the eventuality in which the superconductivity phenomenon is interrupted for any reason, even if only momentarily. Such an eventuality may arise, for example, if the coolant which keeps the superconducting 25 wire constantly below its critical superconductive transition temperature is lost, even if this loss affects only a small portion of the winding. This is because superconducting materials generally have significant electrical resistance when heated to above the critical temperature: this temperature generally varies from a few Kelvin to approximately 150 0 K, depending on the material. 30 If this happens, it is essential to have an immediately available alternative electrical path, as nearly as possible parallel to the superconducting path, for the purpose of protecting the superconducting path from the passage of the electric current, which would otherwise seriously damage it. 35 This aim is usually achieved by connecting a closely specified portion of metallic material with low electrical resistance, such as copper, in parallel with the superconducting wire. In 2 practice, this result can be obtained by jointly winding two wires, namely a superconducting wire and a metal wire, when the winding is formed. However, this method can cause difficulties and complications in the winding process for any 5 device. A greatly preferable solution is that of incorporating a low-resistance metallic material into the superconducting wire itself. However, in order to make this possible, there must be complete chemical and mechanical compatibility between the metallic material chosen as the stabilizer, the superconducting compound itself and the rest of the material making up the wire. 10 It is extremely difficult to find a solution to these problems in the case of superconducting wires made from magnesium diboride MgB2. This is because magnesium diboride shows considerable chemical incompatibility with virtually all the more conductive metallic elements which might be used as stabilizers, such as copper, silver or aluminium. These elements tend 15 to decompose the MgB2 because of their considerable affinity for magnesium. Therefore, it would be desirable to overcome or at least alleviate some or all of the aforementioned problems. 20 According to an embodiment of the present invention, there is provided superconducting composite wire with superconductive phase of magnesium diboride, comprising a plurality of filaments of magnesium diboride within an outer metallic sheath, further comprising: - a core of conductive metal selected from copper and silver; - said plurality of filaments, in which each filament comprises a core of magnesium 25 diboride, is placed around the conductive metal core; - said outer metallic sheath for containment and mechanical reinforcement, surrounds the said plurality of filaments; and - at least one layer of metal chemically compatible with magnesium diboride and capable of acting as an obstacle to the diffusion of the conductive metal of said conductive 30 metal core towards the said filaments at a temperature of at least 700*C, where the said layer is applied a) as a coating of the conductive metal core and/or b) as a coating of the said filaments, and/or as a coating of the said magnesium diboride core of said filaments. In some embodiments, said at least one layer of metal chemically compatible with 35 magnesium diboride is positioned as a coating of the conductive metal core.

3 In some embodiments, each of said magnesium diboride filaments comprises a magnesium diboride core and a metallic sheath surrounding the magnesium diboride core, and in which the said at least one layer of metal compatible with magnesium diboride is applied as an outer coating of the said sheath. 5 In some embodiments, each of said magnesium diboride filaments comprises a magnesium diboride core and a metallic sheath surrounding the magnesium diboride core, and in which the said at least one layer of metal compatible with magnesium diboride is applied as a coating of the said magnesium diboride core inside the said sheath. 10 In some embodiments, said core of conductive metal has a coating layer of metal, chemically compatible with magnesium diboride, surrounding the said conductive metal core and capable of acting as an obstacle to the diffusion of said conductive metal of said conductive metal core; 15 In some embodiments, said plurality of magnesium diboride filaments placed around the said coated conductive core is provided with a coating or sheath of metal chemically compatible with magnesium diboride. 20 In some embodiments, said coating layer of metal, chemically compatible with magnesium diboride, consists of a metal chosen from the group consisting of niobium, tantalum, iron, nickel, tungsten, molybdenum, chromium and their alloys. In some embodiments, said coating layer consists of a tube, or a rolled sheet, or is produced 25 by electrochemical deposition or by evaporation. In some embodiments, said magnesium diboride filaments consist of single-filament wires, each wire comprising a superconducting magnesium diboride core and a sheath of metal chemically compatible with magnesium diboride, wherein said sheath surrounds each 30 individual magnesium diboride core of each wire. In some embodiments, said sheath of the single-filament wires consists of a metal chosen from the group consisting of niobium, tantalum, iron, nickel, tungsten, molybdenum, chromium and their alloys. 35 In some embodiments, said layer of metal chemically compatible with magnesium diboride is chosen from niobium, tantalum, iron and their alloys, while the said metallic sheath 3a surrounding the magnesium diboride core consists of a metal chosen from nickel, tungsten, molybdenum, chromium and their alloys. In some embodiments, said outer containing and reinforcing sheath consists of a metal 5 chosen from the group consisting of niobium, tantalum, iron, nickel, tungsten, molybdenum, chromium and their alloys. In some embodiments, said superconducting composite wire has a circular or flattened cross section. 10 Further advantages and characteristics of the invention will be made clear by the following detailed description, which refers to the attached drawings provided by way of example and without restrictive intent, in which: - Fig. 1 is a cross-sectional view of a superconducting wire according to the invention; - 15 Fig. 2 is a cross-sectional view of a superconducting wire having a structure similar to that of Fig. 1, made in the form of a flat wire or strip; - Figs. 3 and 4 are photographs of cross sections of superconducting wires, made according to the structure of Figs. 1 and 2; and - Figs. 5 and 6 are cross-sectional views of a superconducting wire in an alternative 20 embodiment. In Figs. 1 and 2, the number 1 indicates a central core of conductive metallic material . This conductive metallic material is preferably copper or silver, since these are the most conductive elements in electrical terms and can also withstand, without melting, the heat 25 treatment to which the conductor is subjected, at temperatures ranging from 600 0 C to approximately 10000C. Preferably, copper known as OFHC (Oxygen Free High Conductivity) copper is used, since this has the highest possible electrical conductivity at low temperatures. The central core 1 is provided with an outer coating 2 of metallic material chemically 30 compatible with magnesium diboride, to act as a barrier or impediment to the diffusion of the conductive metal towards the said superconducting 4 phase. This barrier can be made, . for example, from niobium, tantalum, iron, nickel, tungsten, molybdenum, chromium or alloys of these, and can have a sufficient minimum thickness to impede or slow down the diffusion of the internal metallic material. This barrier can be introduced as a thin tube or rolled sheet fitted around the high-conductivity core.. Alternatively, the material forming the barrier can be deposited electrochemically or by evaporation around the high conductivity core. A plurality of magnesium diboride filaments 3 are positioned to surround the barrier coating 2. These filaments preferably consist of single-filament wires, each comprising a superconducting core 5 of magnesium diboride and an outer metallic sheath 6, chemically compatible with magnesium diboride. As shown in greater detail in the examples of Figs. 5 and 6 described below, each single-filament wire 3 can optionally comprise a barrier coating 2a or 2b outside the sheath 6, or inside the sheath 6, in other words in direct~ contact with the core 5. The materials used for the sheath can be, for example, niobium, tantalum, iron,. nickel, tungsten, molybdenum, chromium, or alloys of these. The single-filament wire is preferably made by the powder-in tube method, by the mechanical deformation of a metallic tube which has been filled with powder consisting of MgB 2 or- a mixture of its constituents (essentially boron and magnesium powders). An external sheath 4 surrounds the single-filament wires 3. The material of the external sheath can be any material 5 having the function of containing the wire and forming the mechanical support of the wire. The materials forming the sheath can preferably be chosen from niobium, tantalum, iron, nickel, tungsten, molybdenum, chromium, or alloys of these. The composite assembled in this way is machined by mechanical deformation in order to produce a long conductor having a circular (Figs. 1 and 3) or flat (Figs. 2 and 4) section. The methods used for machining the composite can include extrusion, rolling, hammering and drawing. The quantity of wire produced will depend exclusively on the size of the initial assembly and the final size of the conductor which is to be manufactured. Typical dimensions for a superconducting wire with internal stabilization range from diameters of 0.2 mm to 2 mm. Similarly, it is possible to produce superconducting strips having thicknesses from 0.2 mm to 2 mm and widths from 1 mm to 5 mm. By contrast with the prior art, this type of structure of the magnesium diboride superconducting wire enables the conductor to be heat-treated at high temperatures (above 700 0 C) without contamination of the superconducting phase due to the presence of the metallic element. In a specific embodiment, shown in Figs. - 3 and 4, superconducting wires having the previously described structure were made, these wires having the cross section of either a round wire or a superconducting strip, with a central core of pure copper, a pure iron diffusion barrier, magnesium diboride superconducting filaments with pure nickel sheaths and a pure nickel outer sheath. Figs. 5 and 6 show alternative embodiments, which fall within the scope of the invention.

6 In these figures, elements corresponding to those of Figs. 1 and 2 are indicated by the same reference numbers. In particular, the invention allows for the possibility that the diffusion barrier 2 is not necessarily placed around the central conductive core 1, but can also - or alternatively be placed around each filament 3, either as a 'coating of the containing sheath 6, or in direct contact with the superconducting core 5 of the magnesium diboride filaments. Thus all possible combinations, in twos, of the solutions shown in Figs. 1, 5 and 6 are allowed for, as is the use of all three solutions together. In the example of Fig. 5, the barrier coating, indicated by 2a, coats the containing sheath 6 of each filament 3. In the example of Fig. 6, the barrier coating, indicated by 2b, is in direct contact with the superconducting core 5 of each filament within the sheath 6. The preceding description is applicable to the materials forming the barrier coating 2a and 2b, to the materials forming the outer metallic sheath 6 and to their application. In the solutions of Figs. 5 and 6, the barrier coating 2a and 2b is preferably a metal or metal alloy chosen from the previously mentioned group, but different from the material forming the sheath 6. Thus it is preferable if the barrier coating 2a, 2b is chosen from niobium, tantalum, iron and their alloys, while the material forming the sheath 6 consists of or comprises a metal chosen from niobium, tantalum, iron, nickel, tungsten, molybdenum, chromium or their alloys, this material being different from the material forming the coating 2a, 2b. The scope of the invention also includes the case in which, 7 in the superconducting wire, some of the filaments 3 are made in accordance with the solution of Fig. 5 and other filaments are made in accordance with the solution of Fig. 6, optionally with the presence of a coating layer 2 on the conductive core 1. In all cases, the composite materials thus produced have undergone heat treatments at up to 980 0 C without decomposition of the magnesium diboride due to the presence of copper.

Claims (13)

1. Superconducting composite wire with superconductive phase of magnesium diboride, comprising a plurality of filaments of magnesium diboride within an outer metallic sheath, further comprising: - a core of conductive metal selected from copper and silver; - said plurality of filaments, in which each filament comprises a core of magnesium diboride, is placed around the conductive metal core; - said outer metallic sheath for containment and mechanical reinforcement, surrounds the said plurality of filaments; and - at least one layer of metal chemically compatible with magnesium diboride and capable of acting as an obstacle to the diffusion of the conductive metal of said conductive metal core towards the said filaments at a temperature of at least 700*C, where the said layer is applied a) as a coating of the conductive metal core and/or b) as a coating of the said filaments, and/or as a coating of the said magnesium diboride core of said filaments.

2. Superconducting composite wire according to Claim 1, wherein said at least one layer of metal chemically compatible with magnesium diboride is positioned as a coating of the conductive metal core.

3. Superconducting composite wire according to Claims 1 or 2, wherein each of the said magnesium diboride filaments comprises a magnesium diboride core and a metallic sheath surrounding the magnesium diboride core, and in which the said at least one layer of metal compatible with magnesium diboride is applied as an outer coating of the said sheath.

4. Superconducting composite wire according to Claims 1 or 2, wherein each of the said magnesium diboride filaments comprises a magnesium diboride core and a metallic sheath surrounding the magnesium diboride core, and in which the said at least one layer of metal compatible with magnesium diboride is applied as a coating of the said magnesium diboride core inside the said sheath.

5. Superconducting composite wire according to Claims 1 or 2, wherein: - said core of conductive metal has a coating layer of metal, chemically compatible with magnesium diboride, surrounding the said conductive metal core and capable of acting as an obstacle to the diffusion of said conductive metal of said conductive metal core; 9 - said plurality of magnesium diboride filaments placed around the said coated conductive core is provided with a coating or sheath of metal chemically compatible with magnesium diboride.

6. Superconducting composite wire according to Claim 5, wherein said coating layer of metal, chemically compatible with magnesium diboride, consists of a metal chosen from the group consisting of niobium, tantalum, iron, nickel, tungsten, molybdenum, chromium and their alloys.

7. Superconducting composite wire according to Claim 6, wherein said coating layer consists of a tube, or a rolled sheet, or is produced by electrochemical deposition or by evaporation.

8. Superconducting composite wire according to any one of the preceding claims, wherein said magnesium diboride filaments consist of single-filament wires, each wire comprising a superconducting magnesium diboride core and a sheath of metal chemically compatible with magnesium diboride, wherein said sheath surrounds each individual magnesium diboride core of each wire.

9. Superconducting composite wire according to Claim 8, wherein said sheath of the single-filament wires consists of a metal chosen from the group consisting of niobium, tantalum, iron, nickel, tungsten, molybdenum, chromium and their alloys.

10. Superconducting composite wire according to any one of Claims 3 and 4, wherein said layer of metal chemically compatible with magnesium diboride is chosen from niobium, tantalum, iron and their alloys, while the said metallic sheath surrounding the magnesium diboride core consists of a metal chosen from nickel, tungsten, molybdenum, chromium and their alloys.

11. Superconducting composite wire according to any one of the preceding claims, wherein said outer containing and reinforcing sheath consists of a metal chosen from the group consisting of niobium, tantalum, iron, nickel, tungsten, molybdenum, chromium and their alloys.

12. Superconducting composite wire according to any one of the preceding claims, with a circular or flattened cross-section. 10

13. Superconducting composite wire according to any one of the embodiments substantially as herein described with reference to the accompanying drawings.