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Title:
SUPERCONDUCTIVE WIRES AND ASSOCIATED METHOD OF MANUFACTURE
Document Type and Number:
WIPO Patent Application WO/2014/135893
Kind Code:
A1
Abstract:
There is provided a superconductive wire comprising a central core surrounded by a sheath, wherein the central core comprises an innermost region (74) formed from a first set of constituents surrounded by an outer region (70) formed from a second set of constituents. The constituent(s) of the outer region are selected to act as a barrier between the sheath and the first set of constituents. A method of forming the wire to avoid voids in the central core is provided.

Inventors:
ATAMERT SERDAR (GB)
SCANDELLA JEAN-LOUIS (FR)
BERKAN RIZA (US)
GLOWACKI BARTLOMIEJ (GB)
Application Number:
PCT/GB2014/050681
Publication Date:
September 12, 2014
Filing Date:
March 07, 2014
Export Citation:
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Assignee:
EPOCH WIRES LTD (GB)
International Classes:
H01L39/24; H01L39/14
Domestic Patent References:
WO2008122802A12008-10-16
Foreign References:
EP1995797A22008-11-26
US20020198111A12002-12-26
Other References:
GOLDACKER W ET AL: "Mechanically Reinforced MgB2 Wires and Tapes with High Transport Currents", INTERNET CITATION, 29 April 2002 (2002-04-29), XP002367726, Retrieved from the Internet [retrieved on 20060213]
NAKANE T ET AL: "Fabrication of Cu-sheathed MgB2 wire with high Jc-B performance using a mixture of in situ and ex situ PIT techniques", PHYSICA C, NORTH-HOLLAND PUBLISHING, AMSTERDAM, NL, vol. 469, no. 15-20, 15 October 2009 (2009-10-15), pages 1531 - 1535, XP026447609, ISSN: 0921-4534, [retrieved on 20090530]
Attorney, Agent or Firm:
FORSYTH, Helen et al. (90-92 Regent Street, Cambridge Cambridgeshire CB2 1DP, GB)
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Claims:
Claims

1. A superconductive wire comprising a central core surrounded by a sheath, wherein the central core comprises an innermost region formed from a first set of constituents surrounded by an outer region formed from a second set of constituents.

2. A superconductive wire according to claim 1, wherein the constituent s) of the outer region are selected to act as a barrier between the sheath and the first set of constituents.

3. A superconductive wire according to claim 1 or claim 2, wherein the first set of constituents are in-situ constituents being one of Mg with B, or Nb with Ti or Nb with Zr, Nb with Al, or Nb with Sn.

4. A superconductive wire according to any of claims 1 to 3, wherein the second set of one or more constituents are one of Mg, B, MgB2, NbTi, Nb3Sn, NbZr or Nb3Al.

5. A superconductive wire according to claim 1, wherein the first set of constituents are in-situ constituents being Mg with B, and the second set of constituents are ex-situ MgB2.

6. A superconductive wire according to any of claims 1 to 5, wherein first set and/or second set of constituents contain doping additions such as nitride boroxides, silicides, carbon or carbon inorganics, metal oxides, metallic elements or organic compounds.

7. A superconductive wire according to any of the preceding claims, wherein the sheath is made from a single layer of electrically conductive material.

8. A superconductive wire according to any of claims 1 to 6, wherein the sheath is a bimetallic sheath.

9. A superconductive wire according to claim 7 or claim 8, wherein each sheath layer is made from one of Cu, or Ni, or Nb, or Ti, or Fe, or stainless steel, or Cu-Ni, or Cu-Be, or Monel, or Ag-Mg or Nb-Ti.

10. A superconductive wire according to claim 8, wherein an inner layer of the bimetallic sheath surrounding the outer region is formed from Cu.

11. A superconductive wire according to claim 8 or claim 10, wherein the outer layer of the bimetallic strip is formed from steel.

12. A superconductive wire according to any of the preceding claims, wherein, the bimetallic sheath is coated with an additional external layer.

13. A superconductive wire according to claim 1, wherein the first set of constituents are Mg with B, the second set of constituents are ex-situ MgB2, and the sheath is a bimetallic sheath having a steel outer layer and, copper inner layer with a third layer of copper coating applied to the steel outer layer.

14. A superconductive wire according to any of claims 1 to 3, 6 to 12, wherein the second set of constituents is a single element powder.

15. A superconductive wire according to any of the preceding claims when made according to any of the method claims 28 to 69.

16. A superconductive wire comprising a central core of superconductive material surrounded by a sheath, wherein the sheath comprises two layers of different metallic materials.

17. A superconductive wire according to claim 16, wherein the two layers have different electrical conductivities.

18. A superconductive wire according to claim 16 or claim 17, wherein each layer is made from one of Cu, Ni, Nb, Ti, Fe, stainless steel, steel, Cu-Ni, Cu-Be, Monel, Ag-Mg and Nb-Ti.

19. A superconductive wire according to any of claims 16 to 18, wherein the central core comprises an innermost region formed from a first set of constituents surrounded by an outer region formed from a second set of one or more constituents.

20. A superconductive wire according to any of claims 16 to 19, wherein the constituent(s) of the outer region are selected to act as a barrier between the sheath and the first set of constituents.

21. A superconductive wire according to any of claims 16 to 20, wherein the sheath is coated with an additional external layer.

22. A superconductive wire according to any of claims 14 to 18, wherein the first set of constituents are one of Mg with B, or Nb with Ti or Nb with Zr, Nb with Al or Nb with Sn.

23. A superconductive wire according to any of claims 16 to 22, wherein the second set of constituents are ex-situ constituents being one of MgB2, NbTi , Nb3Sn, NbZr or Nb3Al.

24. A superconductive wire according to any of claims 16 to 23, wherein the first and/or second set of constituents contain doping additions such as nitride boroxides, silicides, carbon or carbon inorganics, metal oxides, metallic elements or organic compounds.

25. A superconductive wire according to claim 19, wherein the first set of constituents are in-situ constituents Mg with B, and the second set of constituents are MgB2.

26. A superconductive wire according to any of claims 14 to 22 or 24, wherein the second set of constituents is a single element powder.

27. A superconductive wire according to any of claims 14 to 22 made according to any of the method claims 28 to 69.

28. A method of manufacturing superconductive wires comprising of the following steps:

a. inserting a first strip of a first metallic material onto a continuously fed second strip of a second metallic material to create a bimetallic strip;

b. continuously forming the bimetallic strip into a "U" profile, with the first strip forming an inner face of the "U" profile;

c. placing at least one powder capable of forming a superconductive material into a channel formed by the "U" profile; and

d. sealing edges of the u profile together to produce a wire.

29. A method of manufacturing superconductive wires according to claim 28, wherein first and second strips have different electrical conductivities.

30. A method of manufacturing superconductive wires according to claim 28 or 29 wherein the first metallic material is one of Ni, or Co, or Nb, or Ti, or Fe, or alloys of these materials, or stainless steels, or Mg, or Cu bronze or Monel.

31. A method of manufacturing superconductive wires according to any of claims 28 to 30, wherein the second metallic material is one of Ni, or Co, or Nb, or Ti, or Fe, or alloys of these materials, or stainless steels, or steel, or Mg, or Cu bronze or Monel.

32. A method of manufacturing superconductive wires according to any of claims 28 to 31 wherein the method further comprises edge profiling on the continuously fed second strip.

33. A method of manufacturing superconductive wires according to any of claims 28 to 32, wherein the edges of the U-profile are sealed by welding in an inert atmosphere.

34. A method of manufacturing superconductive wires according to any of claims 28 to 33, wherein the sealed wire is further processed by rolling to smaller diameters.

35. A method of manufacturing superconductive wires according to any of claims 28 to 34, wherein the wire is heat treated.

36. A method of manufacturing superconductive wires according to any of claims 28 to 35, wherein at least two different powders are placed in the channel of the U- profile.

37. A method of manufacturing superconductive wires according to any of claims 28 to 36, wherein the powders comprise a first powder of being a single element powder or a powder of ex-situ constituents and a second powder of in-situ constituents.

38. A method of manufacturing superconductive wires according to claim 37, wherein the first powder is placed in the channel so as to surround the second powder.

39. A method of manufacturing superconductive wires according to any of claims 28 to 36 or 38, wherein at least three powders are placed in the channel, the first and third powders being a single element powder or a powder of ex-situ constituents and the second powder being in-situ constituents.

40. A method of manufacturing superconductive wires according to any of claims 32 to 39, wherein powders are placed in the "U" profile using multiple powder feeders.

41. A method of manufacturing superconductive wires according to claim 40, wherein different powders are introduced sequentially into the channel of the "U" profile.

42. A method of manufacturing superconductive wires according to any of claims 37 to 41, further comprising creating a groove in the first powder and placing the second powder in the groove.

43. A method of manufacturing superconductive wires according to any of claims 36 to 42, wherein the first powder is one of B, Mg, MgB2, NbTi, Nb3Sn, NbZr, Nb3Al.

44. A method of manufacturing superconductive wires according to any of claims 36 to43, wherein the second powder is one of Mg with B, or Nb with Ti or Nb with Zr, Nb with Al or Nb with Sn.

45. A method of manufacturing superconductive wires according to claim 37, wherein the first powder is MgB2 and the second powder is Mg and B.

46. A method of manufacturing superconductive wires according to any of claims 28 to 45, wherein the powders contain doping additions such as nitrides, borides, silicides, carbon or carbon inorganics, metal oxides, metallic elements or organic compounds.

47. A method of manufacturing superconductive wires according to any of claims 28 to 45, wherein the second strip is externally coated with a conductive layer.

48. A method of manufacturing superconductive wires according to any of claims 28 to 46, wherein electrically conductive material is wrapped around the wire to create external stabilisation.

49. A method of manufacturing superconductive wires according to any of claims 28 to 48, wherein the wire is drawn, extruded, swaged, rolled or deformed to smaller size by mechanical deformation to produce smaller diameter superconductive wire.

50. A method of manufacturing superconductive wires according to any of claims 28 to 49, wherein the wire is thermomechanically treated to react the power or powders and so create a superconductive core throughout the wire.

51. A method of manufacturing superconductive wires comprising of the following steps:

a. continuously feeding a strip of a metallic material;

b. continuously forming the strip into a "U" profile; c. placing at least two different powders capable of forming a superconductive material into a channel formed by the "U" profile; and

d. sealing edges of the "U" profile together to produce a wire.

52. A method of manufacturing superconductive wires according to claim 51, wherein the powders comprise a first powder being a single element powder or a powder of ex-situ constituents and a second powder of in-situ constituents.

53. A method of manufacturing superconductive wires according to claim 51 or claim 52, wherein the first powder is placed in the channel so as to surround second powder.

54. A method of manufacturing superconductive wires according to any of claims 51 to 53, wherein at least three powders are placed in the channel, the first and third powders being a single element powder or a powder of ex-situ constituents and the second powder being in-situ constituents.

55. A method of manufacturing superconductive wires according to any of claims 51 to 54, wherein powders are placed in the "U" profile using multiple powder feeders.

56. A method of manufacturing superconductive wires according to any of claims 51 to 55, wherein powders are introduced sequentially into the channel of the "U" profile.

57. A method of manufacturing superconductive wires according to any of claims 51 to 56, further comprising creating a groove in the first powder and placing the second powder in the groove.

58. A method of manufacturing superconductive wires according to any of claims 51 to 57, wherein the first powder is one of B, MgB2, NbTi, Nb3Sn, NbZr, Nb3Al.

59. A method of manufacturing superconductive wires according to any of claims 51 to 58, wherein the second powder is one of Mg with B, or Nb with Ti or Nb with Zr, Nb with Al, or Nb with Sn.

60. A method of manufacturing superconductive wires according to any of claims 51 to 53, wherein the first powder is MgB2 and the second powder is Mg and B.

61. A method of manufacturing superconductive wires according to any of claims 51 to 60, wherein the powders contain doping additions such as nitrides, borides, silicides, carbon or carbon inorganics, metal oxides, metallic elements or organic compounds.

62. A method of manufacturing superconductive wires according to any of claims 51 to 61, wherein the metallic material is one of Ni, or Co, or Nb, or Ti, or Fe, or alloys of these materials, or stainless steels, steel or Mg, or Cu bronze or Monel.

63. A method of manufacturing superconductive wires according to any of claims 51 to 62, wherein the edges of the U-profile are sealed by welding in an inert atmosphere.

64. A method of manufacturing superconductive wires according to any of claims 51 to 63, wherein the sealed wire is further processed by rolling to smaller diameters.

65. A method of manufacturing superconductive wires according to any of claims 51 to 64, wherein the wire is heat treated.

66. A method of manufacturing superconductive wires according to any of claims 51 to 66, wherein the strip is externally coated with a conductive layer.

67. A method of manufacturing superconductive wires according to any of claims 51 to 65, wherein electrically conductive material is wrapped around the wire to create external stabilisation.

68. A method of manufacturing superconductive wires according to any of claims 51 to 67, wherein the wire is drawn, extruded, swaged, rolled or deformed to smaller size by mechanical deformation to produce smaller diameter superconductive wire.

69. A method of manufacturing superconductive wires according to any of claims 51 to 68, wherein the wire is thermomechanically treated to react the power or powders and so create a superconductive core throughout the wire.

Description:
Title: Superconductive Wires and associated Method of Manufacture Field of the invention

This invention relates to superconductive wires and a method of manufacturing such wires.

Background to the invention

Superconductivity is a phenomenon occurring in certain materials at very low temperatures and is characterised by zero electrical resistance and the exclusion of an interior magnetic field. In general, the superconducting state is defined by three important parameters: critical temperature 7c, critical magnetic field He and the critical current density Jc.

Superconductors are often grouped into two classes according to their 7c; low temperature superconductors (LTS) and high temperature superconductors (HTS). They are also classified as type I and type II depending on their reaction to an external magnetic field. Type I superconductors completely expel any externally applied magnetic field, whereas type II superconductors allow the field to enter in the form of flux vortices.

The most commonly used LTS material is NbTi that is available in the market in the form of continuous solid wires. This material has a Jc value of 9.7K and is used extensively in the manufacture of MRI machines and other electronic applications. High critical current density Jc of >3000 A/mm 2 at 4 Tesla combined with an attractive cost makes NbTi a good superconductive material. However, NbTi is only superconductive at very low temperatures (9.8K) achieved using liquid helium. Alternative materials are required to replace NbTi due to finite stores of nonrenewable helium.

HTS materials can operate in liquid nitrogen (boiling point 77K). Their Jc value can be as high as 92K (YBa 2 Cu30 7 - 8 ). These materials are produced using highly complex methods such as deposition of biaxially textured thin films on textured buffer layers or substrates. Whilst having desirable properties, they are very expensive.

A recently discovered superconductive material Magnesium Diboride (MgB 2 ) is known to become superconductive at about 40K implying that MgB 2 can be cooled to an operational temperature by either indirect liquid hydrogen or readily available low cost closed-cycle refrigerators. In addition, MgB 2 consists of two simple elements magnesium (Mg) and boron (B), which are abundant in nature. MgB 2 can be used to replace NbTi at high magnetic field applications (i.e. MRI machines) and used instead of HTS materials at low field applications (i.e. power generation and transmission).

MgB 2 wires are commonly manufactured by a powder-in-tube (PIT) technique. In the PIT technique the precursor powder is packed in one or more metal tubes, these tubes are then drawn, swaged or drawn into wires or tapes, then this is followed by heat treatment in a vacuum, air or in an inert atmosphere. PIT wires can be classified into two main approaches depending upon the starting material. The first is the so- called in-situ route that uses the unreacted starting powders, Mg and B. The second is the ex-situ route that employs MgB 2 powder as the starting material. In-situ wires usually show higher current densities and offer more flexibility in introducing dopants and additives.

In the ex-situ PIT technique, a mixture of Mg and B are mixed and reacted at 900°C. A tube is filled using this reacted powder and then rolled into a smaller diameter. Cutting MgB 2 wires into identical lengths and packing them into another tube, produces multifilament wires. This tube is later rolled to a smaller diameter in order to produce a multifilament superconductive wire. The length of the tubes limits the total length of the wire, although up to 5km long single piece wires are possible.

US 6,687,975 is based on a well-established flux-cored welding wire manufacturing technique used in the welding industry in excess of 60 years to achieve a continuous tube forming and filling process. This technology can potentially produce unlimited lengths of wires. However, this technique produces wires with an overlapped sheath of strip wrapped around Mg+B or MgB 2 powder and needs to be inserted into another tube such as copper tube. The length of the final product is limited by the length of the copper tube although wire with lengths up to 4km are possible.

Despite good progress in introducing MgB 2 superconductive wires into industrial applications, neither of the above techniques is adequate to manufacture MgB 2 superconductive wires that are infinitely long and cost effective.

Yamaichi et al. submitted a patent in 1988 in which a continuous powder-in-tube method was proposed to overcome the difficulties discussed earlier. In their patent, a U-shaped cross-section of wire was filled in and welded in-situ and sintered to produce high temperature superconductive wires.

A similar technology was developed and successfully implemented in the manufacture of flux-cored welding wires.

The sheath material in PIT technology potentially could be any material, as long as it can be formed and reduced to a smaller diameter. However, the choice and design of sheath material should consider the influence of the sheath on providing mechanical support to the powder core, its electrical and thermal conductivity and its reaction with the powder core during heat treatments.

Many different sheath materials such as Cu, Ni, Nb, Ti, Fe, stainless steel (SS), Cu- Ni, Cu-Be, Monel, Ag-Mg and Nb-Ti have been chosen as sheath material to characterise the influence of sheath material on the overall performance of MgB 2 superconductive wires.

In addition, there have been numerous studies to improve the superconductivity of MgB 2 wires by altering the wire architecture in terms of its powder composition and preparation, doping additions, filament size, number and geometry. There have been also many attempts to improve electrical and thermal stability by introducing internal, external and both internal and external additions of electrically conductive materials such as copper.

This invention results from in-depth investigation and understanding of the three most important factors influencing the superconductivity behaviour of MgB 2 superconductive wires. These are discussed in detail below.

Electrical and Thermal Stabilisation

In normal operating conditions the current flows through the superconductor. However, in the case of slight disturbance the temperature may climb above the Jc causing loss of superconductivity. In circumstances like this it is desirable that the current is transferred to another electrically conductive material such as copper. If the heat transfer is sufficient then the conductor cools down and the flow of current through superconductor continues. Similarly, conductors for low magnetic field applications (<4 Tesla) are required to be very cryostable in the low field region when the critical current density in the core of the wire is the highest. In particular, MgB 2 wires with quite inhomogeneous filaments and structure very often show thermally- driven jumps at higher currents.

When cryogen-free operation of an MgB 2 coil is considered, it is essential to have a good electrical and thermal conductor, such as copper in close contact with the superconducting material. Stabilizing the conductor by offering low-resistance paths around the hotspot can stop thermal avalanches. It is often a necessity that dissipated energy due to quenching is captured by intra-filament stabilisation within an overall wire geometry. This is because an effective external stabilisation relies on good thermal and electrical conductivity through the thickness of the sheath material. The current limitations at low or zero fields are believed to be caused by thermal heating affects and by limiting the use of single filaments. Changing to multifilamentary wire is therefore suggested to resolve the thermal affects. This solution in turn creates complexity in design and manufacturing MgB 2 superconductive wires and increases the total cost of manufacturing. Densification

It is well established that good electrically conductive materials such as copper, silver, aluminium and their alloys fail to achieve high densification of core powder. This is due to their relatively low strength, low work hardening rate and low work hardening capacity. It has been demonstrated that MgB 2 superconductive wires with copper sheath fails to achieve intra-grinding of powders and provides an almost negligible pressurizing affect leading to poor current density. In addition, these materials have a strong affinity to Magnesium and react in case of in- situ MgB 2 wire configuration resulting in low current carrying capability. It has been proposed that Nb and Ta can be used as barriers to eliminate the reaction between copper and Mg. However, these materials have smaller thermal expansion compared to MgB 2 , producing micro cracks during heat treatment cycles. Stainless steels are considered to be a good alternative. However, due to presence of Cr and nickel, stainless steels show a strong reaction with Mg and B in in-situ MgB 2 wires.

Steel sheath has been demonstrated to be a good alternative due to its high mechanical strength and good work hardening behaviour. High density wires have been achieved reaching 75% of theoretical density of MgB 2 by the rolling process. Reinforcement using steel sheath is believed to produce higher densification of the powder, reducing porosity and decreasing the occurrence of local hot spots, resulting in an improved stabilisation. Drawbacks of steel sheath are the interaction of iron with boron at high temperatures, requirement of stress relieving to reduce work hardening in order to continue the wire size reduction process and relatively poor electrical conductivity of steel.

Connectivity and Doping

The connectivity between the MgB 2 crystals is a prerequisite to obtain good superconductivity. The connectivity depends on many factors including powder, physical and chemical properties, powder mixing and filling techniques, wire manufacturing technologies, etc. It is common that chemical reactions may take place in the core of superconductive wires if there are reactive elements present. For instance, it is very common that a volatile product like Magnesium could react with copper or other sheath material to form Mg-rich compounds during heat treatments. From the binary phase diagrams of Mg-Cu, Cu-B and Mg-B, it can be concluded that the interaction between boron and copper at MgB 2 formation temperatures is negligible and the expected phases after heat treatment of an in-situ MgB 2 /Cu wire are the Mg-Cu and Mg-B phases. This is confirmed by published equilibrium data of the Mg-Cu-B ternary system, which reported no ternary intermetallics and no solid state solubility of Cu in MgB 2 . Starting from a mixture of Mg and B powders in the stoichiometric ratio for MgB 2 in a copper sheath, MgB 2 is not the only phase formed. This is because of the very rapid reactive diffusion of copper from the sheath into magnesium at temperatures lower than the MgB 2 formation temperature.

In addition to the chemical reactions taking place during heat treatments, the formation of voids can also create lack of connectivity. In particular Mg+B to MgB 2 transformation produces approximately 25% volume contraction. Unless this volume is filled in, the core powder would consist of a significant volume of voids resulting in poor superconductivity characteristics.

It is well established that in type II superconductors, a magnetic field can enter into superconductors in the form of vortices and could destroy superconductivity unless they are pinned. The pinning of these vortices is obtained by the addition of small size (often in the order of nano size) foreign particles called dopants.

Summary of the invention

In accordance with a first aspect of the invention, there is provided a superconductive wire comprising a central core surrounded by a sheath, wherein the central core comprises an innermost region formed from a first set of constituents surrounded by an outer region formed from a second set of one or more constituents, the constituent(s) forming the outer region acting as a barrier between the sheath and first set of constituents. This ensures that chemical properties of the sheath cannot affect the reaction of the first set of components when heat treated. The first and second sets of constituents will generally be different, either by chemical or physical structure, although there may be at least one chemical element in common to each.

Preferably the constituents forming the outer region are selected to act as a barrier, i.e. chemical barrier, or sacrificial layer between the sheath and constituents of the innermost region, the second set of constituents reacting with sheath material to prevent the first set of constituents reacting with the sheath material.

The first set of constituents may be in-situ constituents such as Mg with B, or Nb with Ti or Nb with Zr, Nb with Al, or Nb with Sn.

The second set may be ex-situ constituents such as MgB 2 , NbTi or other superconductive materials such as Nb 3 , Sn, NbZr or Nb 3 Al.

By in-situ constituents are meant components that are unreacted and so are indivual elements or compounds that will react with each other upon heat treatment, by ex-situ constituents is meant reacted components such as compounds which further react upon heat treatment.

In a particularly preferred embodiment, the in-situ constituents will be Mg with B, and the ex-situ constituents will be MgB 2 .

The first and second sets of constituents may contain doping additions such as nitride boroxides, silicides, carbon or carbon inorganics, metal oxides, metallic elements or organic compounds.

The sheath may be made from a single layer of electrically conductive material, and may be made from Cu, Ni, Nb, Ti, Fe, stainless steel, Cu-Ni, Cu-Be, Monel, Ag-Mg and Nb-Ti. Particularly preferred for a single layer is copper.

Preferably the sheath is a bimetallic sheath with an inner layer surrounding the outer region formed from a good electrical conductor, such as copper. Preferably the outer layer of the bimetallic strip is formed from a material with high mechanical strength and good work hardening behavior, and is thus preferably steel.

If desired, the sheath may be coated with an additional external layer, such as copper, preferably using the electroless process.

A particularly preferred configuration of wire is one having in-situ constituents of Mg with B, ex-situ constituents of MgB 2 , a bimetallic sheath with a stainless steel outer layer, a copper inner layer and external copper coating, so as to give a superconductive core surrounded by a triple layer sheath.

In accordance with a second aspect, the invention lies in a superconductive wire comprising a central core of superconductive material surrounded by a sheath, wherein the sheath comprises two layers of different metallic materials, so being a bimetallic sheath.

The two layers preferably have different electrical conductivities.

Each layer of the sheath may be made from one of Cu, Ni, Nb, Ti, Fe, stainless steel, Cu-Ni, Cu-Be, Monel, Ag-Mg and Nb-Ti. Preferably an inner layer of the sheath surrounding the central core is formed from a good electrical conductor, such as copper. The outer layer of the bimetallic strip is preferably formed from a material with high mechanical strength and good work hardening behaviour, and is thus preferably steel or stainless steel, nickel, nickel alloys or monel.

The central core may comprise an innermost region formed from a first set of constituents surrounded by an outer region formed from a second set of one or more constituents, as discussed above, and if necessary the constituents may be doped, as set out above.

If desired, the bimetallic sheath may be coated with an additional external layer, such as copper, preferably using the electroless process. In accordance with another aspect of the invention, there is provided a method of manufacturing superconductive wires comprising of the following steps:

a. inserting a first strip of a first metallic material onto a continuously fed second strip of a second metallic material to create a bimetallic strip;

b. continuously forming the bimetallic strip into a "U" profile, with the first strip forming an inner face of the "U" profile;

c. placing at least one powder capable of forming a superconductive material into a channel formed by the "U" profile; and

d. sealing edges of the U profile together to produce a wire.

The first and second strips may have different electrical conductivities, and typically the first metallic material will be one of Ni, or Co, or Nb, or Ti, or Fe, or alloys of these materials, or stainless steels, or Mg, or Cu bronze or Monel. If the first strip is selected to provide internal stabilisation then an electrically conductive material such as Cu, Al, or Al alloys is required. Such materials tend to be soft and require mechanical support. The second metallic material providing mechanical support may be one of steel, stainless steels, nickel or nickel alloys or Monel.

The method may further comprise edge profiling the continuously fed second strip before the first strip is inserted.

The edges of the U-profile may be sealed by welding in an inert atmosphere. This may be by laser beam, HF welding, or other welding techniques such as plasma, electron-beam, TIG, MIG, or Cold-Metal- Arc welding techniques.

Preferably the sealed wire obtained at the end of the initial process is further processed by rolling to smaller diameters, and may be heat treated.

Preferably at least two different powders are placed in the channel of the "U" profile prior to sealing. The powders may comprise a first powder of ex-situ constituents and a second powder of in-situ, or unreacted, constituents, with these constituents as discussed above. If necessary the constituents may be doped.

The first powder is preferably placed in the channel so as to surround second powder.

If desired, at least three powders may be placed in the channel, the first and third powders being ex-situ constituents and the second powder being in-situ, or unreacted, constituents.

For ease of placement, the powders may be placed in the "U" profile using multiple powder feeders, preferably being introduced sequentially into the channel.

The second strip may be externally coated with a conductive layer, such as Cu, to create a triple layer sheath. The external conductive Copper plating ensures external stabilisation. By having a triple clad sheath, electrically conductive/mechanically strong/electrically conductive materials are used to produce both internally and externally stabilised wire.

As an alternative, an electrically conductive material such as copper strip may be wrapped around the wire to create external stabilisation.

Typically the wire may be drawn, extruded, swaged, rolled or deformed to smaller size by mechanical deformation to produce smaller diameter superconductive wire.

The method preferably includes heat treatment to react the powder or powders and so create a superconductive core throughout the wire.

In accordance with a further aspect of the invention, there is provided a method of manufacturing superconductive wires comprising of the following steps:

a. continuously feeding a strip of a metallic material;

b. continuously forming the strip into a "U" profile; c. placing at least two different powders capable of forming a superconductive material into a channel formed by the "U" profile; and

d. sealing edges of the "U" profile together to produce a wire.

Preferably the powders comprise a first powder of a first set of constituents and a second powder of a second set of one or more constituents, the constituents as discussed above and if necessary doped with dopant.

The first powder may be placed in the channel so as to surround second powder.

If desired, at least three powders may be placed in the channel, the first and third powders being ex-situ constituents and the second powder being in-situ, or unreacted, constituents.

The powders may be placed in the "U" profile using multiple powder feeders, and preferably will be introduced sequentially into the channel.

A groove may be created in the first powder and the second powder placed in the groove.

The strip material may be one of Ni, or Co, or Nb, or Ti, or Fe, or alloys of these materials, or stainless steels, or Mg, or Cu bronze or Monel.

As above, the edges of the U-profile may be sealed by welding in an inert atmosphere, and if desired the sealed wire further processed by rolling to smaller diameters. Heat treatment may also be performed on the wire.

The invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 shows a schematic representation of a superconductive wire production line;

Figure 2 shows details of wire formation;

Figure 3 shows schematic representations of different embodiments of MgB 2 superconductive wire profiles in accordance with the invention; and

Figure 4 shows the morphology of sectional views of a superconductive wire in accordance with the invention.

Description

It should be noted that the present invention is not limited to the examples and sketches described herein, and various combinations and modifications can be implemented without changing the purpose of the invention.

Figure 1 is a schematic illustration of a seamless superconductive wire production line 10 in accordance with the invention. Steel strip 12 is delivered continuously from strip dispensers 14 to a strip accumulator 16. Two strip dispensers are shown enabling the preparation of the next coil of strip whilst decoiling from the other dispenser. Strip 12 passes from the decoiler into a strip accumulator 16. Accumulator 16 stores strip enabling the manufacturing mill to continue running whilst joining the coils of strip on the strip decoilers. This enables the production of infinite lengths of superconductive wires. Edges of strip 12 are profiled by edge profiling unit 18 and cleaned in a cleaning unit 20. Following the edge profiling and cleaning processes, sheath material in the form of electrically conductive strip 21, typically copper strip, is introduced onto the continuously fed steel strip 12 in chamber 22 that consists of a series of guide and forming rolls and so creates a bimetallic strip 26. A series of forming rolls 28 are used to convert strip 26 to a "U" profile cross-section, so producing a U-shaped channel 30, see Figure 2. The dimensions of sheath material could change from 6mm to 100mm wide and thickness can range from 0.5mm to 6mm. It is possible to produce seam welded tubes with larger size bimetallic sheath material but the most economical size range is likely to be 10 to 5mm wide and 0.8 to 2.5mm thick.

The channel 30 formed by the cavity of the "U" profile is filled with powders stored in hoppers 34, which are contained in an inert gas chamber 36. Each powder hopper 34 is linked to a separate powder feeder and depending on the configuration of wire required, powders are fed to the "U" profile in three different stages 40, see Figure 2. The resulting filled tube 42 is closed by closing rollers 44, welded using in- line welding equipment 46, rolled to smaller diameters using steady and small reductions by rollers 48 and finally stored in take-up units 50.

The detailed strip forming, powder filling and welding stages are shown in Figure 2. Strip 12 is profiled, edges are prepared 32 and the strip surface coated with continuously fed electrically conductive strip 21 to produce bimetallic strip 26. Bimetallic strip 26 is profiled to a "U" shape 30 by forming rolls 28 and filled in with powders 52 using multiple feeding technology 40. Three separate powder feeders 52, 54, 56 are used to add powder in stages. The core is filled in using a multiple- powder-feeding system using three powders stored in powder hoppers in a protective chamber 36. The speed of the powder feeders are independently controlled and synchronised with the strip speed to maintain the required powder strip ratio. A first feeder 52 is used to feed ex-situ MgB 2 powder into channel 30. After filling approximately 1/3 of the total cavity, an elongate U-shaped profile tamping device 58 having a complementary U-shaped profile to channel 30 but of reduced lateral extent is inserted into the ex-situ powder to create a depression 60 in the centre and push the ex-situ powder to the sides of the cavity. This cavity is next partially filled in with in- situ Mg+B powder using the second hopper and second feeder 54. The amount of in- situ powder is approximately 30% of the total fill. The remaining ex-situ MgB 2 powder is used to fill in the remaining part of the cavity above the in-situ powder using the third hopper and third drive system.

In the production of MgB 2 superconductive wires doping compounds could be nitrides, borides, silicides, carbon or carbon inorganics, metal oxides, metallic elements or organic compounds. These compounds are uniformly mixed with superconductive powder and are then fed into the core of the "U" profile sheath. They could be mixed either with in-situ or ex-situ powders.

The filled profile is later converted to an "O" shape 62 by closing rollers 44 and finally seam welded in an inert Argon atmosphere in an in-situ welding unit 46 to form a perfectly closed seam 64. The seam welding process uses a laser technology. The laser beam is perfectly focused onto the seam using a seam follower, and the intensity of the laser beam is adjusted making sure that a good penetration is achieved and no detrimental damage is introduced to the core powder. MgB 2 superconductive wires can also be manufactured using a High-Frequency-Seam- Welding technique as well as other welding techniques such as electron-beam-plasma-TIG-welding.

Powders in this technique for continuous forming of superconductive wire could be either a mixture of Mg and B, known as in-situ powder, or already reacted MgB 2 powder, known as ex-situ powder or a combination. It could also be any other superconductive powder such as NbTi or any other high temperature superconductive materials available in the form of powders. Particle size distribution of superconductive powders could vary from few nanometres to tens of Microns depending on the size of the final superconductive wire.

It has been found previously that undesirable chemical reactions occur between in-situ MgB 2 powder and a copper sheath, and the multicore-powder-feeding system avoids this by the core of the wire consisting of Mg+B mixture in the middle surrounded by ex-situ MgB 2 powder. This design creates a diffusion barrier between the copper and Mg+B and minimizes the risk of Mg+Cu intermetallics forming.

The additional benefit of this multi-core structure is to achieve greater Jc values. It was believed that Mg in the core of the composite structure would diffuse towards the sheath during heat treatment and not only could combine with B to form MgB 2 but also it creates a bond between MgB 2 ex-situ crystals to obtain better connectivity. To fully materialize the benefit of Mg diffusion, the Mg level is increased beyond the Mg:B stoichiometric ratio so that some excess Mg is available. Typically the core contains 10-20% excessive Mg powder beyond the stoichiometric need for MgB 2 formation.

As the constituents within the core react during heat treatment, volume contraction occurs in the core due to MgB 2 formation. The would create voids affecting the wire properties but by using the multi-step thermo-mechanical treatment discussed above gradual transformation of the central core to MgB 2 is achieved while voids generated as a result of phase deformation are filled in by the rolling process, so ensuring a void free core. This multi-step thermo-mechanical process also allows gradual Mg diffusion from the core to the ex-situ MgB 2 creating good connectivity as well as flux pinning. Relatively low temperatures (600-750°C) are used for thermo-mechanical treatment and a short period of time (2-10 seconds) combined with mechanical deformation to optimise the process.

Superconductive wires produced in this way are heat-treated using in-line heat treatment (In-line-baking) facilities in a protective atmosphere. The purpose of this in-line heat treatment could be either making sure that the fusion and chemical reactions are complete or softening the sheath material allowing further reduction to take place. During the heat treatment of superconductive MgB 2 wires there could be harmful chemical reactions forming insulating layers which inhibit the current transfer. These layers can result from iron reacting with boron or copper reacting with magnesium. These reactions may also include other elements which are added as doping additions.

In order to avoid the forming of these resistive layers at the interface between the sheath material and the powder we use multiple powder feeders to sequentially feed different powders and provide a defined powder structure in the core before heat treatment. A powder layer within the core and adjacent to the sheath material surrounds an innermost material components to create barrier and minimise reactions between the sheath material and the innermost material components.

For instance, if the sheath material is iron then the barrier powder could be magnesium. The powder mix within the centre could be Mg+B. During the heat treatment, Mg and B reacts together and form MgB 2 . The protective layer of magnesium prevents boron to react with iron sheath and prevents the formation of iron boride at the interface.

Alternatively, if the sheath material is copper then boron is added as a protective layer. In this case, Mg reacts with boron and forms MgB 2 while the formation of copper magnesium compound at the interface is prevented due to presence of boron. Alternatively the barrier powder can be ex-situ powder of common constituents to in- situ powder situated at the core centre.

After the superconductive wire is rolled and heat treated to a diameter close to finish size, it is copper coated by the electroless process in Copper Sulphate solution after surface cleaning. This process provides a thin layer of copper coating 86, see Figure 3(d), that gives protection against corrosion and also provides a layer of electrically conductive surface suitable for further copper coating.

The bobbins of finished wire are placed into a motorised decoiler. The decoiler is motorised to maintain constant tension on the wire at high production speeds. The wire passes through a twin die wet drawing unit, which reduced the wire down to a desirable diameter. The drawing process cleans the wire and also heats it up improving the coppering of the wire. The wire then passes through the coppering tank to give an even copper coating to the wire. After coppering, the wire passes through a rinsing stabilisation tank to remove the acid from the wire. Finally the wire passes through a second twin-die-drawing-unit to apply lubrication and corrosion protection to the wire and reduce it down to the final size. Great care is taken to prevent the acid from the coppering tank entering the stabilisation tank and hence reducing the consumption of acid and stabilisation components. The finished copper coated wire is then coiled onto standard bobbins and is ready for spooling.

The finished wires can be externally stabilised by wrapping copper strip around, or inserting the wire into a copper or any other electrically conductive strip, folding the strip to a "U" shape and welding the seam in-situ using the process described earlier.

The wire can be manufactured in a variety of different configurations as shown in Figure 3. A first profile 68 of an MgB 2 superconductive wire is shown in Figure 3(a) where the wire is manufactured with a bimetallic sheath 26 containing an in-situ mixture 70 of MgB 2 with or without doping additions. A second profile 72 is a representation of MgB 2 superconductive wires manufactured using in-situ 70 and ex- situ 74 MgB 2 with or without doping additions. This configuration is sealed with welded single layer sheath material 76 such as Cu, Ni, Nb, Ti, Fe, stainless steel, Cu- Ni, Cu-Be, Monel, Ag-Mg or Nb-Ti. When filling channel 30 with powder, first feeder 52 feeds ex-situ MgB 2 powder into channel 30 , then tamper device 58 forms a groove 60 which is filled with in-situ Mg+B powder by second feeder 54. Lastly third feeder 56 feeds more ex-situ MgB 2 powder into channel 30 so that the Mg+B powder is surrounded by Mg powder.

The two profiles 80, 82 shown in Figures 3(c) and (d) respectively represent multicore wire architecture with in-situ Mg+B surrounded by ex-situ MgB 2 as in Figure 3(b) but with a surrounding bimetallic strip 26. Profile 80 is internally stabilized. Profile 82 has an external coating 86 of copper as well to be both internally and externally stabilized. The powder used for both profiles 80, 82 can be with or without any doping additions.

Figure 4 illustrates the morphology of a wire profile as shown in Figure 3(b) with the ex-situ MgB 2 surrounding in-situ Mg+B. In this example the multicore wire was produced using only single layer copper sheath and MgB 2 and Mg+B powders with a particle size in the order of a few microns. The copper strip forming the sheath had dimensions of 35mm wide and 1.5mm thick. The strip was formed to a "U" profile by forming rolls and the powders were fed in three stages as explained above. A seam was welded using laser-seam welding and the wire was rolled to an overall diameter of 1.2mm as shown in Figure 4(a).

The wire was heat treated at 750°C for 15 minutes. Scanning electron microscopy shows microstructures before and after the heat treatment including the distribution of Mg, see Figure 4(b). It was revealed that excessive Mg diffused from the core of the wire to a cross sectional area homogenously. There was no sign of a Mg-rich compound at the copper sheath/ex-situ interface indicating that MgB 2 ex-situ powder acted like a barrier.

According to this invention, MgB 2 superconductive wires using the powder-in-tube process could be manufactured using a continuous automated seam welding process based on the use of bimetallic clad strip and multicore powder feeding system. The manufacturing system shown in Figure 1 produces internally stabilised multicore MgB 2 superconductive wires based on automated seamless and infinitely long PIT type superconductive wires. The bimetallic strip consisting of copper and steel achieves a combination of high densification and good internal stabilisation. The powder addition to the core was delivered using an innovative multiple belt system. Where ex-situ MgB 2 and in-situ Mg+B are used, the final product has a mixture of distinct ex-situ and in-situ powders. The ex-situ powder is located between the internal copper surface and the central in-situ powder and minimises the risk of unwanted diffusion driven chemical reactions. Magnesium rich central in-situ powder turns into MgB 2 and the excessive Mg penetrates to ex-situ MgB 2 and creates greater connectivity. The thermo-mechanical in-line treatment ensures the powders densify during phase transformation and the crystallization processes.

The bimetallic sheath is produced in-line, desirably from steel and copper with a copper strip introduced onto a continuously fed steel strip to achieve a sandwich structure.

This wire arrangement allows high powder densification to be achieved due to the outer sheath layer being steel and also a good internally stabilised core due to the copper layer. This technique has the additional advantage of being able to select different size steel and copper strips to manufacture different thickness bimetallic wires, to suit different applications. It is also possible to use commercially available bi-metallic strip to achieve the same result. It is also possible to use clad strip with electrically conductive material on both sides of the sheath to manufacture both internally and externally stabilised MgB 2 superconductive wires.

In order to achieve similar performance, different electrically conductive alloys such as bronze and other copper alloys can be chosen as an internal liner instead of copper.

Particular aspects and features of the invention are as follows:

Aspect 1 :

1) A method of manufacturing multicore internally stabilised MgB 2 superconductive wires comprising of the following steps; a. Inserting a copper strip onto a continuously fed steel strip to create a bimetallic strip

b. Forming a continuously fed bimetallic strip into a "U" profile by a series of forming rolls

c. Preparing edge profiling on the edges of continuously fed strip

d. Filling in the concave of the "U" profile with ex-situ/in-situ/ex- situ powders using multiple powder feeders to produce multi-core

e. Closing the "U" profile and welding the seam

f. Rolling the seam welded wire to smaller diameters

Features of aspect 1 are as follows:

2) A method of manufacturing MgB 2 superconductive wire based on aspect 1 above, wherein the sheath material is, Ni, Co, Nb, Ti, Fe, or alloys of these base materials, stainless steels (SS) or any other metallic alloy which can be shaped into the "U" profile by mechanical forming and its purpose being a mechanical support to the MgB 2 core powder.

3) A method of manufacturing MgB 2 superconductive wire based on aspect 1, wherein instead of copper, other electrically conductive strip materials such as Mg, Cu bronze or Monel are adopted.

4) A method of manufacturing MgB 2 superconductive wire based on aspect 1, wherein the powders contain doping additions such as nitrides, borides, silicides, carbon or carbon inorganics, metal oxides, metallic elements or organic compounds.

5) A method of manufacturing MgB 2 superconductive wire based on aspect 1, wherein a bimetallic clad strip is used.

6) A method of manufacturing MgB 2 superconductive wire according to aspect 1 and any of the associated dependent features, wherein the metallic strip contains high strength metallic products such as iron and or steel outside and electrically conductive materials such as copper inside to create stabilisation.

7) A method of manufacturing MgB 2 superconductive wire according to features 5 or 6, wherein the bimetallic products may be Ni, Co, Nb, Ti, SS or any other metallic alloy and Monel, Copper alloys or any other electrically high conductive materials inside for stabilisation.

8) The method of manufacturing MgB 2 superconductive wire according to aspect 1, wherein a triple clad sheath is used containing electrically conductive/mechanically strong/electrically conductive materials to produce both internally and externally stabilised wire.

9) A method of manufacturing MgB 2 superconductive wire according to aspect 1, wherein powder core includes only in-situ Mg+B or ex- situ MgB 2 powders and/or doping additions listed in claim 4.

10) A method of manufacturing MgB 2 superconductive wire according to aspect 1 and/or feature 9, wherein a clad bimetallic or triple-metallic clad strip are used.

11) A method of manufacturing MgB 2 superconductive wire according to aspect 1 or any of the above features, wherein the edges of the strip are welded using laser beam or HF welding.

12) A method of manufacturing MgB 2 superconductive wire according to aspect 1 or any of the above features, wherein the edges of the strip are welded using other welding techniques such as plasma, electron-beam, TIG, MIG, Cold-Metal-Arc welding techniques.

13) A method of manufacturing MgB 2 superconductive wire according to aspect 1 or any of the above features, wherein an electrically conductive Copper plating is applied onto the external surface of the wire to create external stabilisation. 14) A method of manufacturing MgB 2 superconductive wire according to aspect 1 or any of the above features, wherein other electrically conductive materials are used.

15) A method of manufacturing MgB 2 superconductive wire according to aspect 1 or any of the above features by plating onto the external surface of the wire to create external stabilisation.

16) A method of manufacturing MgB 2 superconductive wire according to features 13 and 14, wherein electrically conductive material is wrapped around the wire to create external stabilisation.

17) A method of manufacturing MgB 2 superconductive wire according to aspect

1, wherein electrically conductive material is wrapped around the wire to create external stabilisation.

18) A method of manufacturing MgB 2 superconductive wire according to aspect 1 or any of the above features, wherein the seam welded continuous tube is drawn, extruded, swaged, rolled or deformed to smaller size by mechanical deformation to produce smaller diameter superconductive wire.

19) A method of manufacturing MgB 2 superconductive wire according to aspect 1 or any of the above features, wherein the wire is thermomechanically treated to generate reacted in-situ powder in the centre and fused ex-situ powder outside to create better continuity for improved superconductivity.

A second aspect is as follows:

2. A MgB 2 superconductive seamless wire containing multicore powders ex- situ/in-situ/ex-situ produced by bimetallic or triple- metallic sheath. Preferred features of aspect 2 are as follows:

1) A MgB 2 superconductive seamed wire containing multicore powders ex- situ/in-situ/ex-situ produced by bimetallic or triple- metallic sheath.

2) A MgB 2 superconductive seamed or seamless wire containing multicore powders ex-situ/in-situ/ex-situ produced by bimetallic sheath and wrapped by an electrically conductive sheath.

3) A MgB 2 superconductive seamless wire according to aspect 2 or any of features 1 to 2, wherein the powders consist of doping additions.

4) A MgB 2 superconductive seamless wire containing only in-situ or ex-situ powders with or without doping additions produced by bi- metallic or triple-metallic sheath.

5) A method of manufacturing superconductive wire according to aspect 2, wherein the powder is any other superconductive powder i.e. NbTi or high temperature superconductive materials.




 
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