Superconducting wire and its manufacturing method

Abstract

The present invention relates to an electrically conductive wire 4 formed as a flexible spiral with a constant or variable inclination, a constant or variable diameter, and a straight or curved extension, the electrically conductive wire comprising a superconductor. In this way, an electrically conductive wire is provided which comprises a superconductor and allows for a small bending radius, in particular less than 15 cm.

Description

[Technical Field]

[0001] The present invention relates to the field of conductive wires with superconductors, and in particular to conductive wires with superconductors for magnetic resonance imaging cryostats. [Background technology]

[0002] Superconducting magnets can be used in systems requiring strong magnetic fields, such as magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) spectroscopy. To achieve superconductivity, the magnet has one or more conductive coils formed from superconducting wire. To maintain the superconductivity of conventional superconductors, a cryogenic environment (cryostat) at temperatures close to absolute zero is required during operation. In the superconducting state, the conductive coil, called a superconducting coil, has virtually no electrical resistance and can therefore conduct very large currents and generate strong magnetic fields. In this regard, conventional metallic superconductors are well known, which typically operate below 77 K. Summary of the Invention [Problem to be solved by the invention]

[0003] High-temperature superconductors (HTS), operationally defined as materials that behave as superconductors at temperatures above 77 K, the boiling point of liquid nitrogen, one of the simplest cryogenic coolants, also exist. All materials currently known to perform at atmospheric pressure become superconducting at temperatures well below ambient temperature and therefore require cooling. The majority of high-temperature superconductors are ceramic materials. While ceramic superconductors are suitable for some practical applications, they still present many manufacturing challenges, and few have been successfully used in practical applications. Most ceramics are brittle, which makes the fabrication of wire from them problematic.

[0004] Magnesium diboride (MgB2) is a promising superconductor with a higher operating point than conventional Nb-Ti conductors. While MgB2 becomes superconducting below 39 K, which is much lower than the approximately 150 K of YBaCuO-HTC materials, it has the advantage of much higher cooling capacity at 39 K than the 9.7 K required to cool NbTi. This alleviates many of the thermal constraints that drive the cost of cryostats for magnetic resonance imaging (MRI) applications. However, as mentioned above, one drawback of conventional MgB2 wires is their brittleness. Bend radii below the order of 15 cm can cause cracking and loss of functionality, which is a significant limitation for wiring applications.

[0005] The methods for producing MgB2 wires are well known (see, for example, A. Ballarino and R. Flukiger 2017 J. Phys: Conf. Ser. 871 012098 or M.N. Kutukcu et al., “Composite Superconducting MgB2 Wires Made by Continuous Process,” in IEEE Transactions on Applied Superconductivity, vol. 28, no. 4, pp. 1-4, June 2018, Art. no. 6200704). The most important methods for producing MgB2 wires are ex situ and in situ. In addition, there is a magnesium diffusion technique. For example, in the ex situ method, pre-reacted MgB2 powder with a 1:2 molar ratio is inserted into a Nb tube, which is surrounded by a Cu.70Cu.30 tube, and drawn into a wire with a diameter of approximately 3.5 mm. Several pieces of wire are then inserted into nickel or Monel tubes, and OFHC Cu is added for stabilization. This composite is grooved and drawn into 2 mm diameter round wire, twisted before being rolled into tape (typically 3.6 x 0.65 mm²) or wire approximately 1 mm in diameter, and then subjected to a recrystallization heat treatment, typically at 965°C for 4 minutes. For in-situ MgB2 wire, the initial MgB2 mixture consists of Mg and B powders. This method has the advantage that the reaction to MgB2 occurs at a much lower temperature (approximately 650°C), resulting in less reaction layer in the metal sheath.

[0006] J. Breitschopf et al., 2020 IOP Conf. Ser:Mater. Sci. Eng. 756 012031, report the development of a super-cable-in-conduit (SuperCIC) designed for hybrid coil magnets. SuperCIC fully preserves the performance of the individual wires and can be formed into anything from flared-end windings for dipoles to layer-wound toroids and solenoids for tokamak hybrid windings. The SuperCIC winding structure is designed so that the Bi-2212, Nb3 Sn, and NbTi subwindings can be wound and heat-treated separately, then assembled and preloaded into the magnet.

[0007] The object of the present invention is to provide an electrically conductive wire having a superconductor, which allows for a small bending radius, in particular a bending radius of less than 15 cm, during wiring without cracking or loss of functionality. [Means for solving the problem]

[0008] According to the present invention, this object is addressed by the subject matter of the independent claims.

[0009] Preferred embodiments of the invention are set forth in the dependent claims.

[0010] Thus, in accordance with the present invention, there is provided an electrically conductive wire formed as a flexible helix having a constant or varying slope, having a constant or varying diameter, and having a straight or curved extension, the electrically conductive wire comprising a superconductor.

[0011] A helix is ​​usually defined as a curve that winds with a constant slope around the mantle of a cylinder that defines a diameter. However, according to the present invention, such shapes are also understood to be helices that exhibit varying slopes and / or varying diameters along their respective extensions and / or straight or curved extensions. That is, the extensions of the helix need not be straight, like the axis of a cylinder. Rather, the present invention also allows for curved extensions of the helix that do not follow a straight line.

[0012] It is an essential aspect of the present invention that the bending of the spiral allows for smaller bending radii than would be permitted with a straight wire, since the stresses on the individual wire windings are reduced. The conductive wire according to the present invention is therefore very advantageous in routing wires, since the shape of the conductive wire in the form of a flexible spiral with a constant or varying inclination, with a constant or varying diameter, and with straight or curved extensions, significantly reduces the risk of cracking and loss of function of the wire. The present invention therefore makes use of the fact that the required bending moment is distributed over a longer wire length, or alternatively, that the bending moment is converted into a twist, which is also distributed over several loops.

[0013] Generally, different superconductors can be used for the conductive wire. However, according to a preferred embodiment of the present invention, the wire contains MgB2 as the superconductor. As mentioned above, MgB2 is a higher-temperature superconductor than conventional NbTi conductors. MgB2 becomes superconducting below 39 K, which is much lower than the approximately 150 K required for YBaCuO-HTC materials. However, MgB2 still has the advantage of being able to cool at 39 K, much higher than the 9.7 K required for NbTi. Therefore, many of the thermal constraints that affect the cost of MRI cryostats can be overcome in this manner. The drawback of conventional MgB2 wire, namely its brittleness and tendency to break, is overcome by the flexible spiral configuration. In this way, a bending radius of 15 cm or less can be easily achieved, which is a significant advantage when routing the wire. Various dimensions of the flexible spiral can be used. However, according to a preferred embodiment of the present invention, the spiral diameter is between 1.5 cm and 8 cm, preferably between 3.5 cm and 6.5 cm. It is further preferred that the conductive wire has a diameter between 0.4 mm and 1.6 mm, preferably between 0.8 mm and 1.2 mm. Furthermore, according to a preferred embodiment of the present invention, the conductive wire has a length such that, when adjacent windings are in contact with each other, the number of windings is between 6 and 12, preferably between 8 and 10. For example, this means that if the diameter of the flexible helix in its relaxed state is 5 cm and it has 10 windings, the length of the wire is about 157 cm.

[0014] The conductive wire may have a constant winding direction, meaning that the winding follows the shape of a circle with a constant inclination. However, according to a preferred embodiment of the present invention, the winding direction of the flexible spiral changes after each full turn. A full turn is defined as a winding in the range of 340° to 380°. In this regard, it is particularly preferred that the wire bends back 180° toward its previous path after each full turn. This may also be advantageous for magnetic field-sensitive applications, since winding sections running antiparallel to each other can compensate for each other.

[0015] While a single flexible helix will generally suffice for the present invention, a preferred embodiment provides two conductive wires formed as a double helix with alternating winding directions, which may be advantageous for magnetic field sensitive applications, as the winding sections running antiparallel to each other may compensate for each other.

[0016] The present invention is further directed to the use of an electrically conductive wire or wire assembly as described above for electrically connecting windings of a magnet in a magnetic resonance imaging cryostat, preferably the magnet being a superconducting magnet. As described above, such use is advantageous because routing of the wire is easier, as the risk of wire breakage is reduced. In this regard, the electrically conductive wire or wire assembly is preferably guided through bends of 90° or more or bends having a bend radius of 15 cm or less.

[0017] The present invention is also directed to an assembly for a magnetic resonance imaging device, comprising: a cryostat; a magnet having a plurality of magnetic field coils with superconducting windings disposed within the cryostat; and an electrically conductive wire formed as a flexible helix having a constant or varying slope, a constant or varying diameter, and a straight or curved extension, wherein the electrically conductive wire is a superconductor, or an electrically conductive wire assembly formed as a double helix with alternating winding directions of two electrically conductive wires, the electrically conductive wire or wire assembly being electrically connected to windings of adjacent magnetic field coils of the magnet. The present invention provides for bending the helix, which can reduce stress on the individual wire windings, thereby allowing for a smaller bending radius than would be permitted with straight wire. Thus, the conductive wire according to the present invention is highly advantageous when routing wire between adjacent field coils of high temperature superconductor based magnets, because the conductive wire is in the form of a flexible spiral with constant or varying slope, constant or varying diameter, and straight or curved extensions, thereby greatly reducing the risk of cracking of the wire and loss of function. Thus, the present invention takes advantage of the required bending moment being distributed over a longer wire length, or the bending moment being translated into a twist that is also distributed over multiple loops in the electrical connections between adjacent superconducting field windings, including strips of higher superconducting wire or superconducting field windings.

[0018] The present invention further provides a method for manufacturing a conductive wire, comprising: structuring i) pre-reacted MgB2 powder, or ii) Mg powder and B powder into a flexible spiral having a constant or varying inclination and a constant or varying diameter; heat treating the constructed material; In this regard, in case i), the heat treatment is preferably carried out at a temperature of 930°C to 1000°C, preferably 965°C, for a period of 2 to 6 minutes, preferably 4 minutes. Furthermore, in case ii), the heat treatment is preferably carried out at a temperature of 600°C to 750°C, preferably 650°C, for a period of 30 to 50 minutes, preferably 40 minutes.

[0019] Finally, the present invention relates to a method for manufacturing an electrically conductive wire, comprising the following method steps: a) placing a pure Mg rod in the center of a Ta or Nb tube; b) filling the region between the Mg rod and the Ta or Nb tube with B and C; c) repeating steps a) and b) for additional Mg rods and Ta or Nb tubes; d) bundling the filled rods; e) inserting the packed bundled rods into a Cu-Ni tube; f) cold deforming the filled Cu-Ni tube into the shape of a spiral with constant or varying taper, constant or varying diameter, and straight or curved extension; g) heat treating the constructed material; The present invention is directed to a method having the following.

[0020] From this viewpoint, C is preferably provided as a carbon layer on the surface of the B particles or as C14H12 powder. The heat treatment is preferably carried out at a temperature in the range of 630 to 650°C, preferably 640°C. In this way, Mg diffuses into the B particles and reacts to form a 10 to 30 μm thick MgB2 layer along the inner wall of the Ta tube.

[0021] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter, but such embodiments do not necessarily represent the full scope of the invention, and reference should therefore be made to the claims and this specification for interpreting the scope of the invention. [Brief explanation of the drawings]

[0022] [Figure 1] 1 is a diagram illustrating a schematic representation of a cryostat having two conductive wires each formed as a flexible spiral, in accordance with a preferred embodiment of the present invention; [Figure 2] FIG. 10 is a diagram illustrating a conductive wire assembly according to a preferred embodiment of the present invention, having two conductive wires, the two conductive wires being formed as double helices with alternating winding directions. [Figure 3] 1 is a schematic diagram of a conductive wire according to a preferred embodiment of the present invention, in which the winding direction of the spiral changes every 360° of the winding. [Figure 4] 1A-1D illustrate method steps of a method for manufacturing a conductive wire according to a first preferred embodiment of the present invention. [Figure 5] 5A-5C illustrate method steps of a method for manufacturing a conductive wire according to a second preferred embodiment of the present invention. [Figure 6] Schematic diagram showing a magnet (2) having multiple field coils with superconducting windings incorporating the helical conductive wire of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0023] FIG. 1 schematically illustrates an MRI cryostat 1 having two conductive wires 4 according to a preferred embodiment of the present invention. Both conductive wires 4 are formed as flexible helixes. Here, the term "flexible helix" does not refer solely to a conventional helix shape, defined as a curve wound around the mantle of a cylinder defining a diameter at a constant slope. Rather, a geometric shape exhibiting a varying slope and / or a varying diameter along each extension is also understood as a flexible helix. Furthermore, the extension of the helix need not be straight, like the axis of the cylinder. Rather, the present invention also allows for curved extensions of the helix that do not follow a straight line.

[0024] As shown in Figure 1, a conductive wire 4 is routed within an MRI cryostat 1 from two electrical terminals 3 to a superconducting magnet 2. This routing of the conductive wire 4 involves bending the wire 4, but because the conductive wire 4 is formed as a flexible spiral, the risk of breakage is minimal. Thus, in this preferred embodiment of the present invention, bending a spiral rather than a straight wire reduces stress on the individual wire windings, and has the advantage of allowing a smaller bending radius than would be possible with a straight wire. This means that the conductive wire 4 may be formed from a brittle material.

[0025] According to the preferred embodiment described herein, both conductive wires 4 contain MgB2 as the superconductor. As mentioned above, MgB2 has the advantage that it becomes superconducting below 39 K and has a much higher cooling capacity at 39 K than the 9.7 K required to cool NbTi. In this way, the disadvantage of conventional MgB2 wires, namely that they are brittle and therefore may easily crack, is overcome by the flexible spiral shape.

[0026] Here, the conductive wire has a spiral diameter of 5 cm, and the conductive wire 4 has a diameter of 1 mm. Also, as can be seen from Figure 1, each conductive wire 4 has 6 turns.

[0027] 2 schematically illustrates an assembly having two conductive wires 4 according to another preferred embodiment of the present invention, where the two conductive wires 4 are formed as a double helix wire assembly 5 with alternating winding directions. Such a design is advantageous for electromagnetic field sensitive applications, as the currents in such alternating windings can compensate each other.

[0028] 3 shows a schematic representation of a conductive wire 4 according to another preferred embodiment of the invention, which is also advantageous for applications sensitive to electromagnetic fields, in which a helix 6 is provided, the winding direction of which changes every 360° of the winding.

[0029] 4 shows the method steps of a manufacturing method according to a first preferred embodiment of the present invention, which comprises the following method steps: S1) structuring a material comprising i) pre-reacted MgB2 powder, or ii) MgB2 powder and B powder, into a spiral having a constant or varying slope, a constant or varying diameter, and a straight or curved extension; S2) heat treating the constructed material; This means that this preferred embodiment allows the use of two alternative forms: pre-reacted MgB2 powder, or Mg and B powders. In both cases, these materials are configured in the form of a spiral with a constant or varying inclination, a constant or varying diameter, and a straight or curved extension. However, the heat treatment of step S2 is different in both cases: in case i), the heat treatment is carried out at a temperature of 965°C for 4 minutes, whereas in case ii), the heat treatment is carried out at a temperature of 650°C for 40 minutes.

[0030] 5 shows the method steps of a manufacturing method according to a second preferred embodiment of the present invention, which method comprises the following method steps: a) placing a pure Mg rod in the center of a Ta or Nb tube; b) filling the region between the Mg rod and the Ta or Nb tube with B and C; c) repeating steps a) and b) for additional Mg rods and Ta or Nb tubes; d) bundling the filled rods; e) inserting the packed bundled rods into a Cu-Ni tube; f) cold deforming the filled Cu-Ni tube into the shape of a spiral with constant or varying taper, constant or varying diameter, and straight or curved extension; g) heat treating the arranged material; It has.

[0031] Here, C is provided as a carbon layer on the surface of the B particles or as C14H12 powder. Further, heat treatment is carried out at 640°C. In this way, Mg diffuses into the B particles and reacts to form a 10-30 μm thick MgB2 layer along the inner wall of the Ta tube.

[0032] FIG. 6 shows a schematic diagram of a magnet (2) having multiple field coils with superconducting windings incorporating the helically shaped conductive wire of the present invention. Magnet (2) has several field coils (61), each having a winding of superconducting material, e.g., MgB2, in a matrix of a conventional resistive metal. Power supply connections (62-1, 62-2) are provided for connection to a power source external to the magnet and even to the cryostat (1). A helical electrical wire (63) is provided between one of the power supply connections (62-2) and the outer field coil (61). Additionally, helical interconnects (64) are provided between adjacent field coils (61) to facilitate wiring of mechanically fragile electrical connections between the field coils (61).

[0033] While the invention has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims are not to be construed as limiting the scope. Moreover, for the sake of clarity, not all elements in the drawings have been labeled with reference signs. [Explanation of symbols]

[0034] 1. MRI cryostat 2. Magnets 3 Electrical terminals 4 Superconducting wire 5 Double Helix Wire Assembly 6. A spiral with a changing winding direction

Claims

1. Use of a wire assembly having two conductive wires, wherein the two conductive wires are formed as a double helix having alternating winding directions for electrically connecting the windings of adjacent magnetic field coils, and each magnetic field coil has a conductive or superconducting winding in a magnetic configuration within a magnetic resonance imaging cryostat.

2. The use according to claim 1, wherein the conductive wire or the conductive wire assembly is guided through bends of 90° or more and / or bends having a bending radius of 15 cm or less.

3. A magnet configuration for a magnetic resonance imaging apparatus, Cryostat and, A magnet having a plurality of magnetic field coils having conductive or superconducting windings arranged within the cryostat, A wire assembly having two conductive wires The two conductive wires are formed as a double helix having alternating winding directions for electrically connecting the windings of adjacent magnetic field coils, each having conductive or superconducting windings for the magnet in a magnetic resonance imaging cryostat. Magnetic configuration.

4. The wire is a superconductor of MgB ２ An assembly for a magnetic resonance imaging apparatus according to claim 3, having the features described above.

5. The assembly for a magnetic resonance imaging apparatus according to claim 3 or 4, wherein the diameter of the helical body is 1.5 cm to 8 cm or 3.5 cm to 6.5 cm.

6. The assembly for a magnetic resonance imaging apparatus according to any one of claims 1 to 5, wherein the diameter of the wire is 0.4 to 1.6 mm or 0.8 mm to 1.2 mm.

7. The assembly for a magnetic resonance imaging apparatus according to any one of claims 1 to 6, wherein the wire has a length such that the number of windings is 6 to 12 or 8 to 10 when adjacent windings are in contact with each other.

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