GB2523812A

GB2523812A - Superconducting jointing process

Info

Publication number: GB2523812A
Application number: GB1403981.2A
Authority: GB
Inventors: M Hamed Lakrimi; Adrian Mark Thomas; Mariusz Wozniak
Original assignee: Siemens PLC
Current assignee: Siemens PLC
Priority date: 2014-03-06
Filing date: 2014-03-06
Publication date: 2015-09-09
Also published as: GB201403981D0

Abstract

A lead free jointing process for superconductors comprising the steps of: chemically etching ends of superconducting wires to be jointed to remove any matrix material from end portions of each wire and to reveal superconducting filaments over a certain length at the end of each wire; twisting or plaiting the filaments together, and then winding the filaments around an inner NbTi or copper rod or tube; inserting the inner NbTi or copper rod or tube with the filaments inside an outer tube; placing the resultant assembly inside a pulsed magnetoforming machine; and causing the outer tube to collapse onto the inner rod or tube. The joint may further comprise MgB2 powder and be subjected to heat treatment. An additional superconducting jointing process is disclosed comprising cutting the ends of superconducting wires to expose elongate cross-sections of superconductive filaments; inserting the cut ends inside an outer tube; placing the resultant assembly inside a pulsed magnetoforming machine; and causing the outer tube to collapse onto the cut ends. A method for the production of a superconducting magnet for an MRI system, comprising a superconducting switch is also disclosed.

Description

SUPERCONDUCTING JOINTTNG PROCESS

The present invention provides methods for jointing superconducting components such as coils and switches, using powder and magnetic pulse forming. In the case of impregnated coils of superconducting wire and similar, the jointing process should preferably be carried out before impregnation.

Conventionally, Wood's metal is used for making persistent joints and it has been customary to first impregnate coils and switches with a thermosetting resin before wire jointing. The low melting point of the Wood's metal which is between 70 and 90°C is much below the gel temperature of Stycast for switch and resin for coil potting. The intention is to use magneto-pulse forming joints and thus be able to pot coils and switches after jointing. The advantages are that this technique is totally compliant with WEEE and Rol-iS and there are no residues other than the nitric acid.

Although the superconducting joints of prime interest are Nb3Sn, MgB2, and HTS, the method described here can also be implemented on NbTi.

The patent is about using the magneto pulse forming method as an alternative technique to for forming compact superconducting joints which do not contain lead (Pb) The invention accordingly provides methods and processes as recited in the appended claims. The characteristics, advantages and scope of the present invention may be more clearly understood by reference to the following description and the appended drawings, wherein: Fig. 1 shows an example superconducting wire which may be jointed according to the present invention; Fig. 2 shows an end of the wire of Fig. 1 cut at a shallow angle to expose superconducting filaments; Fig. 3 shows the wire of Fig. 2 from a different angle, such that the exposed filaments are visible; Fig. 4 shows an end of a wire such as shown in Fig. 1 where the matrix material has been removed to expose superconducting filaments; Fig. 5 shows two wires as shown in Figs. 2, 3 placed within a tube for jointing; and Figs. 6-S show example joints according to embodiments of the present invention.

According to one embodiment of the present invention, ends of superconducting wires to be jointed are first etched in nitric acid to remove any matrix material from end portions of each wire and to expose a corresponding length of filaments. The filaments are then twisted or plaited together and then wound around an inner NbTi or copper rod or tube which in turn is inserted inside an outer tube of NbTi or copper, for example. This assembly is then put inside a pulsed magneto-forming machine to collapse the outer tube onto the inner rod or tube. This method is relatively low risk and the joints produced are relatively robust.

According to certain embodiments of the present invention, a gap between the two tubes could be filled with a powder of a superconducting material such as NbTi, NbSn, or MgB,. The collapsing tube will compress the powder. Tn the case of Nb3Sn and MgB2, the joint will need to be heat treated. An MgB, superconductor may be formed in situ from Mg and B powders.

This technigue can be applied to joint all sorts of joints, joining superconducting wires of a same, or different, superconducting materials: NbTi, Nb3Sn, MgB,, etc. MgB2 may be preferred as the heat treatment is only for 20 to 30mm at around 60000.

In the production of a typical superconducting magnet for an MRI system, separate superconducting coils must be electrically joined in series, and one or more superconducting switch electrically connected with the series connection of coils. In an example method, the superconducting switch would be assembled with joints according to the present invention before being impregnated with resin. The superconducting switch(es) will then be tested. In a parallel process, the superconducting coils would be jointed together. The series connection of superconducting coils could be fully jointed, and then impregnated with a resin, such as a STYCAST® resin.

The invention is related to the use of magnet-pulse forming joints to joint switches and coils before their impregnation.

The production of the joints using the magneto-pulse forming process may considerably simplify the switoh manufaoturing and magnet build processes. A whole superconducting magnet assembly, including the switch(es) may be jointed before potting. The technique of the present invention offers the potential to reduce a length of wire stored inside the joint cups, reduce the length of lead runs, and as the joints are compact then a relatively large number of joints may be provided in a magnet. One could embed the fastening of che coil wires and diagnostic wires in a manner that they could be impregnated, thus reducing time taken in assembly as compared to methods which conventionally reguire the use of a large number of tie-wraps. This technique suit for A2 bonded coils -that is to say, superconducting coils bondod onto a support structuro by thcir radially outor surfaco.

The superconducting wires and the joints may be coated with an electrically insulating layer, such as FOPIVIVAR® resin. The resulting joints may thereby be provided with complete electrical isolation.

The technique has the potential of removing the requirement for a board for mounting joints in a superconducting magnet. The joints could be encased inside a groove on a coil support structure, between coils.

The invention advantageously provides magnet-pulse forming joints to joint switches and coils before their impregnation.

No lead (Pb) need be used in forming joints according to the present invention. All joints may be formed from other superconductor powders which do not contain Pb.

The techniques of the present invention enable jointing of differing or similar types of superconducting wires and/or tapes, such as NbTi to MgH, or Nb-Sn or HIS, Nb1Sn to MgB, or HIS and MgH2 to HIS. The present invention also enables jointing of conductors which include pnictide-based superconductors.

According to a method of a further embodiment of the present invention, superconducting wires to be joined are cut at a shallow angle to reveal the filaments and also increase their contact surface area (Figs. 1-3) . Alternatively, superconducting filaments could be exposed by etching (i.e. with iron (TTI) chloride) away the matrix of the conductor (Fig. 4) . These are then placed inside a tube (Fig. 5) which is filled with a powder (Fig. 6) of a superconducting material.

After firing a magnetic pulse, the tube is crushed and thus compressing the powder. The method is resembles isostatic pressing.

Depending on the type of powder used, a heat treatment step may be required after joint formation, for the superconducting alloy to form.

This can be achieved by means of a furnace or induction heating.

The present invention accordingly provides a magneto pulse forming technique to carry out persistent superconducting jointing using powdcrs. Thc odvontagos providcd by thc tcchniquo of tho proscnt invention include: the absence of lead (Pb) in the finished joint; preparation for the jointing reguires little labour time and does not involve the use of dangerous acids although for some superconductors, the etching of the wires in the iron (III) chloride water solution for 1mm may be beneficial.

the finished joints are small and so are believed to be less prone to flux jumping than conventional superconducting joints which employ lead bismuth PbBi alloy in a jointing cup; the technique is very fast, requiring about one minute to line up the joint inside the machine, fire the pulse, and retrieve the joint; there is little scope for operator error; and the technique is compatible with conduction cooled joints.

An example method according to an embodiment of the invention may proceed as follows: 1. Take two or more wires (Fig. 1) or tapes (even a mixture) and cut them at a shallow angle (Fig. 2) to expose elongated cross sections of superconducting filaments (Fig. 3) 2. Use where possible a weak solution of ferric chloride to clean the exposed surfaces of the filaments and remove any swarf which may be deposited on the cut face. As an alternative and in the case where the use of water is not recommended (e.g. MgB2) , the cut faces can be polished clean.

3. Alternatively, a matrix material, typically copper, may be removed by a chemical etching process to reveal superconducting filaments over a certain length at the end of each wire or tape (Fig. 4).

4. Insert the cut or etched ends of the wires in a highly conductive tube (Fig. 5) . The tube may be of Cu, Al, and other such materials, or alloys of these. The wires and/or tapes may be lined up side by side, as shown; or facing each other; or on top of one another. The wires and/or tapes do not need to be touching each other. Tn another embodiment, the wires and tapes could be inserted from different ends of the tube. The tube could be lined with a thin layer of superconducting material, such as to enhance the shielding properties of the joints and also to serve to carry current.

5. The tube is then at least partially filled with an appropriate powder. Although NbTi, NbSn and pnictide powders could be used, MgE2 powder may be preferred because of its short time heat treatment which is compatible with NbTi commonly used in superconducting wires.

6. A magnetic pulse is then fired to compress the tube onto the power and the ends of the wires. A compressed joint structure results (Fig. 6) to compress the powders.

7. The joint is then subjected to an appropriate heat treatment as necessary to form a superconducting joint.

Fig. 7 illustrates an example of the tube made in several parts, which may facilitate assembly.

Fig. B shows an embodiment which comprises more than two wires, jointed together; and wires which enter from opposite ends of the tube.

Claims

ChAINS 1. A superconducting jointing process comprising the steps of: -chemically etching ends of superoonduoting wires to be jointed to remove any matrix material from end portions of eaoh wire and to reveal superconducting filaments over a certain length at the end of each wire; -twisting or plaiting the filaments together, and then winding the filajiteriLs around an inner NbTI or copper rod ur bube; -inserting the inner NbTi or copper rod or tube with the filaments inside an outer tube; -placing the resultant asserrbly inside a pulsed magneto-forming machine; -performing pulsed magneto-forming to cause the outer tube to collapse onto the inner rod or tube.
2. A method according to claim 1 wherein a gap is provided between the outer tube and the inner tube.

O
3. A method according to claim 2 wherein the gap is filled with a powder of a superconducting material such that the collapsing tube will compress the powder. (4
4. A superconducting jointing process comprising the steps of: -cutting ends of superconducting wires to expose elongated cross sections of superconducting filaments at the end of each wire; -inserting the cut ends of the superconducting wires inside an outer tube; -placing the resultant asserrbly inside a pulsed magneto-forming machine; -performing pulsed magneto-forming to cause the outer tube to collapse onto the cut ends of the superconducting wires.
5. A method according to claim 3 or claim 4 wherein the joint is heat treated.
6. A method according to claim 5 wherein the powder comprises Mg and B powders; and an MgB2 superconductor is formed in situ therefrom.
7. A method according to any of claims 5-6 wherein the heat treatment is performed for 20 to 30mm at 60000.
S. A method according to any preceding claim wherein the wires are inserted from different ends of the outer tube.
9. A method according to any preceding claim wherein che outer tube is lined with a layer of superconducting material.
10. A method according to any preceding claims wherein the superconducting wires and the joints are coated with an electrically insulating layer.
11. A method for production of a superconducting magnet for an MRI system, wherein: -separate superconducting coils are electrically joined in series; and -at least one superconducting switch is electrically connected with the series connection of coils, 0 -the resultant assembly is then impregnated with resin, r wherein the superconducting switch is assembled with joints provided by a method according to any preceding claim. (4
12. A method according to claim 11 wherein the joints are encased inside a groove on a coil support structure, between coils.