EP0009181B1 - Method for making an electrical contact between a normal conducting body and at least one superconductor - Google Patents

Description

The invention relates to a method for producing an electrical contact of a magnetic winding, in which a contact body consisting of normally conductive material is electrically conductively connected to the end piece of at least one superconductor made of an intermetallic compound, which is formed by in-situ annealing of a preliminary conductor product becomes.

Superconducting intermetallic compounds of type A ₃ B with an A15 crystal structure, such as Nb ₃ Sn or V ₃ Ga, have good superconducting properties and are distinguished by high critical values. Conductors with these materials are therefore particularly suitable for superconducting magnet coils for generating strong magnetic fields. In addition to the superconducting binary compounds mentioned, ternary compounds such as niobium-aluminum-Geπnanium Nb ₃ Al _0.8 Ge _{0.2 are also} of particular interest for conductors of such magnets.

However, these intermetallic compounds are generally very brittle, so that their preparation in a form suitable, for example, for magnetic coils is difficult. Special processes have therefore been developed with which such superconductors with an A15 crystal structure can be produced in the form of long wires or strips. In these methods, which in particular enable the production of so-called multi-core conductors, a first component, which is a wire-shaped ductile element of the intermetallic compound to be produced, is generally surrounded by a sheath which consists of a ductile carrier metal and an alloy containing the other elements of the compound consists. For example, a niobium or vanadium wire is surrounded by a sheath made of a copper-tin-bronze or a copper-gallium-bronze. A large number of such wires can also be embedded in a matrix made of the corresponding alloy. The structure obtained from these two components is then subjected to a cross-sectional machining. This gives a long wire-shaped structure, as is required for coils, without reactions that would embrittle the conductor. After the reduction in cross-section, the superconductor pre-product consisting of one or more wire cores and the surrounding matrix material is then subjected to an annealing treatment in such a way that the desired superconducting compound having an A15 crystal structure is formed by a reaction of the core material with the further element of the compound contained in the surrounding matrix becomes. The element contained in the matrix diffuses into the core material consisting of the other element of the connection (cf. DE-OS 20 44 660).

Superconducting magnetic coils made from such superconductors are generally produced by two different processes. In the first process, which is also referred to as the "react first-windthen process", the preliminary conductor product of the superconductor to be produced is wound onto a provisional winding body and then subjected to the annealing treatment required to form the desired superconducting compound. The superconductor thus produced is then unwound again from the provisional winding body and can be processed further. There is a general risk, particularly when winding magnetic coils, that the brittle intermetallic connections of the conductor are damaged due to impermissible deformations and their superconducting properties are correspondingly impaired.

These dangers do not exist in the second method for producing the superconducting connection from the intermediate conductor product. In this process, which is also referred to as the “wind-and-react technique”, the coil body of the magnet to be provided with the winding is first wound with the as yet unreacted conductor preliminary product of the superconductor and then the entire magnet thus wound is subjected to diffusion annealing. This annealing is also referred to as “in situ” annealing. This procedure avoids all difficulties in processing a brittle conductor material. It is also possible to manufacture coils with small inner diameters with relatively thick conductors. With this method, however, all the materials used to build the coil must withstand the high temperatures required for diffusion annealing, which can be 700 ° C. in the case of niobium tin, for example, for several hours to days.

With such in-situ annealed solenoids, the production of connection contacts is difficult. Because of the strong brittleness of the fully reacted A15 superconductors, it is practically no longer possible to bend the leads after in-situ annealing. Therefore, the ends from the pre-conductor of these lines can be arranged on specially shaped contact bodies made of normally conductive material, especially copper, before the in-situ annealing (Proc. Of 6th Int. Conf. On Magn. Techn. (MT-6) , Bratislava, CSSR, 29 Aug. - 2 sept. 1977, page 998). In the subsequent annealing, however, a reactive element of the superconducting compound to be produced, for example tin, can diffuse from the conductor matrix of the intermediate conductor product into the copper of the contact body. This leads to depletion of one component of the superconducting compound. The result is weaker superconductor zones and thus a reduced current carrying capacity of the superconductors. Other difficulties may arise sharp bends and kinks occur. In general, this does not play a role in the case of the unreacted intermediate conductor product, however, such spots can become critical after the annealing because of the mechanical stresses that are difficult to understand. Such stresses are caused in particular by materials of different shrinkage lying in the contact area.

The object of the present invention is therefore to create a method for producing contacts for conductors from such brittle intermetallic superconducting connections, in which these difficulties do not occur or are of only minor importance.

This object is achieved according to the invention for a method of the type mentioned at the outset in that the corresponding end piece of the intermediate conductor product is applied to a molded body made of a heat-resistant material which does not react with the elements of the intermediate conductor product during the annealing and that after the annealing Molded body is exchanged for a contact body with a corresponding shape.

The advantages achieved with this method are in particular that at no point in the end piece of the preliminary conductor product can a reduction in the current carrying capacity of the superconductor due to the loss of a component of the superconducting connection to be formed, since the preliminary conductor product only in the contact area during the in-situ annealing comes into contact with materials that practically do not react with the elements of the intermediate conductor product. This method can also be used to produce contacts that are compact and have great mechanical stability.

According to a further development of the method, the end piece of the intermediate conductor product is advantageously placed in helically arranged grooves of a cylindrical shaped body, after the annealing the shaped body is unscrewed from the conductor spiral solidified during the annealing and formed from the superconductive end piece, and a cylindrical contact body with a predetermined outer diameter is inserted into the conductor spiral inserted. In this way, deformations of the superconductor can largely be avoided during the course of the method and the prerequisites for a good electrical connection between the contact piece and the conductor end piece can be created.

The end piece of the intermediate conductor product is expediently held on the molded body by means of a clamping device. In the case of in-situ annealing, the conductor intermediate product is prevented from slipping on the molded body. After the annealing, the molded body can then be removed relatively easily and the contact body can be introduced into the space enclosed by the conductor spiral.

An exemplary embodiment of the method according to the invention and its further developments characterized in the dependent claims are explained in more detail below with reference to the schematic drawing. Various process steps are indicated in FIGS. 1 to 3, while in FIGS. 4 and 5 details of a conductor lead through the coil flange of a magnet winding are illustrated.

With the method according to the invention, in particular mechanically sufficiently stable superconductors of the A15 type, for example monolithic conductors with a relatively large cross section or stranded conductors, can be contacted. For the production of magnets with such conductors, for example from the brittle intermetallic compound Nb ₃ Sn, it is assumed that a conductor preliminary product is used, as described, for example, in German Offenlegungsschrift 2044660. To form this intermediate conductor product, a niobium wire is first surrounded with a sheath made of copper-tin-bronze. A large number of such wires can also be embedded in a matrix made of bronze. This structure is then subjected to a cross-sectional machining. If necessary, individual intermediate annealing can be carried out. The result is a long wire that is sufficiently ductile as a preliminary conductor product. This wire-shaped intermediate conductor product is then applied to the winding body of a magnetic coil.

According to the method according to the invention, a provisional arrangement of the preliminary conductor product 2 is first provided on a hollow cylindrical winding device 3, which is shown schematically in FIG. 1 as a longitudinal section. This device is fastened on the upper flat side of a coil flange 4 of a winding support of a magnetic coil, which is not shown in the figure. The winding device 3 contains a hollow cylindrical base 5 made of a heat-resistant insulating material such as ceramic. Its predetermined outer diameter is denoted by d. On the upper flat side 6 of this base there is also a hollow cylindrical shaped body 7 with a comparatively somewhat larger outer diameter. The outer circumferential surface of the molded body is provided with a groove 8 which is so deep that it leads upwards in a helical shape, such that the base of the groove lies on a common imaginary cylinder surface with the diameter d of the ceramic base 5.

The materials used for the molded body 7 are advantageously insulating materials which do not react with the elements of the preliminary conductor product during an annealing treatment and which can be processed with conventional metal-cutting machines (for example from Rosenthal-Stemag, D 8560 Lauf: ERGAN). Corresponding materials can only change to the hard ceramic state after a special annealing (e.g. company Ore & Metal Comp. Ltd., Johannisburg, South Africa: Wonderstone or Rosenthal-Stemag, Lauf: STENAN). Metallic materials with a corresponding non-metallic temperature-resistant coating are also suitable.

The winding device 3 also contains an annular disk-shaped cover part 11 arranged on the upper flat side 10 of the shaped body 7. This cover part, which also has the outer diameter d and is made of metal, for example, is provided on its outer surface with at least one fastening device, for example a screwable clamping device 13 in order to be able to temporarily fix the preliminary conductor product to it. The entire device comprising the cover part 11, the shaped body 7 and the ceramic base 5 is detachably fastened to the coil flange 4 with the aid of a central screw 15 engaging the cover part 11.

Extending through the coil flange 4 is an oblique feed-through slot 17, only indicated in the figure, which runs in such a way that the end of the conductor pre-product 2 which leads out of the winding space of the coil is wound helically around the outer surface of the hollow-cylindrical ceramic base 5 without a kink and then bumpless can be inserted into the helical groove 8 of the molded body 7. Since the preliminary conductor product 2 is not fixed on the ceramic base 5, a continuous transition from the slope of the lead-through slot 17 in the flange 4 to the slope of the groove 8 of the molded body 7 can be ensured on this part of the device 3. The end piece of the intermediate conductor product lying essentially in the groove 8 of the shaped body 7 and designated by 19 is placed a little further around the ring-shaped cover part 11 and then held in place by means of the clamping device 13.

This is followed by the reaction annealing of the preformed conductor by the winding, in which the intermetallic compound is formed, for example the niobium of the wire cores is converted with the tin from the bronze by diffusion to Nb ₃ Sn.

After the reaction annealing, the clamping device 13 is released and the molded body 7, together with the cover part 11, is carefully turned upwards. There then remains a conductor spiral 20, shown schematically in a side view in FIG. 2, of now superconductive, relatively rigid material, the free end piece of which projects beyond the ceramic base 5 and is designated by 21.

According to the longitudinal section shown schematically in FIG. 3, a hollow cylindrical contact body 23 made of electrically highly conductive material such as e.g. Copper with an outer diameter corresponding to the inner diameter d of the conductor spiral 20 formed from the superconductive end piece 21 according to FIG. 2 is inserted into the conductor spiral from above and fastened on the ceramic base 5. For this purpose, the disc-shaped bottom part 24 of the contact body, which rests on the upper flat side 6 of the ceramic base 5, is screwed onto the upper side of the coil flange 4 by means of a fastening screw 26. In order to ensure insulation of the screw and thus of the coil flange from the contact body 23, its bottom part 24, which is provided with a bore 27, is held by the screw via an annular insulation disk 28. The contact body 23 can expediently have a tinned outer surface 29, so that the bronze matrix material of the superconducting end piece 21 can be easily soldered to it and a good electrically conductive, large-area connection between these parts is achieved.

At least one normal conductor 32, which is only indicated in the figure, can be electrically conductively connected to a head piece 30 of the contact body 23, which protrudes upward beyond the conductor spiral from the end piece 21. In order to enable a large-area connection, for example by soldering, between these parts, the head piece 30 can have a square cross section, for example.

The conductor parts led out of the coil flange 4, in particular the parts lying on the ceramic base 5, are expediently fixed with a suitable fixing agent 34, e.g. a filled epoxy resin that hardens at room temperature or a ceramic putty to prevent conductor movement.

In the exemplary embodiment according to FIGS. 1 to 3, it was assumed that a large-area electrical contact is established between a normal conductor and a superconductor using the method explained, in order to enable current supply or current discharge for a superconducting magnet coil. However, the method according to the invention is also suitable for connecting superconducting parts of an in-situ annealed coil to one another. Individual superconducting conductor pieces of such a coil can namely be contacted with one another practically neither before nor after the annealing of the coil within the winding space of the coil. It is therefore necessary to lead them out of the changing room and connect them together outside. In this case, the end pieces of two conductors to be connected are passed through the coil flange and placed parallel to one another or lying one on top of the other in the grooves of the shaped body shown in FIG. 1. The remaining method steps differ from the method explained with reference to FIGS. 1 to 3 only in that instead of a single conductor end there are now two conductor ends and there is no need to solder a normal conductor to the head part of the contact body. After the annealing, the two superconductive end pieces are soldered to one another and to the contact body.

In order to be able to apply one or more conductor preliminary products to the winding device 3 according to FIG. 1 without kinks, the bushings for the conductor preliminary products have to pass through the coils flange 4 can be designed accordingly. 4 schematically illustrates the top view of such a coil flange. It is assumed that an intermediate contact between the conductor ends of two concentrically enclosing partial windings A and B of a magnetic winding is to be produced by the method according to the invention. Therefore, two feed-through slots 40 and 41 are required, which have a curved shape, in order to enable a continuous, kink-free transfer of the individual conductor ends to the outer jacket of the winding device 3. These slots can be made, for example, by milling, but guidance and support of the conductor is only possible in the radial direction. Due to the desired curved shape of the slots and the resulting possibility of almost all-round support and guidance of the conductor, the slots are advantageously spark-eroded. The electrodes required for this process can be obliquely milled pipe segments with a wall thickness corresponding to the slot width and can consist, for example, of copper or graphite.

According to the schematic longitudinal section shown in FIG. 5, two slanted electrodes 43 and 44 are required to produce a slot, which electrodes are driven into the flange from the top side of the flange 4 or the bottom side by spark erosion. If several bushings are to be produced in one sink flange, then all electrodes to be driven in from the top of the flange and all corresponding electrodes to be driven in from the bottom of the flange are expediently mounted on a common carrier plate. By spark erosion with these two electrode arrangements, all lead-through slots can then be machined in only two work steps. Post-processing of the slots, e.g. Rounding off sharp edges can largely be dispensed with.

In the exemplary embodiment, a conductor or intermediate conductor product with a circular cross section was assumed. With the method according to the invention, however, profile conductors with a rectangular cross-section can be contacted just as well, which have sufficient mechanical inherent stability and are generally relatively difficult to bend over their narrow sides. Such profile conductors are also particularly suitable for an intermediate contact, since they can be easily and extensively soldered to one another and to the contact body.

Furthermore, it was assumed in the exemplary embodiment that the superconductor to be electrically conductively connected to the contact body consists of the intermetallic compound Nb ₃ Sn, which is formed by the so-called bronze technique by in-situ annealing of the conductor. With the method according to the invention, however, conductors made of other brittle superconducting materials can also be contacted with normal conductors or with one another, the conductor precursors of which are sufficiently ductile for safe winding of the provisional molding.

Claims (12)

1. A process for the production of an electrical contact of a magnet winding, in which a contact body (23) made of normally conductive material is connected, in electrically conductive manner to the terminal piece (19) of at least one superconductor which consists of an intermetallic compound formed by an in-situ annealing of a conductor preform (2), characterised in that, prior to the annealing step, the corresponding terminal piece (19) of the conductor preform (2) is applied to a shaped body (7) which consists of a heat-resistant material which does not react with the elements of the conductor preform (2) during the annealing step, and that, after the annealing step, the shaped body (7) is changed for a contact body (23) of corresponding shape.

2. A production process as claimed in Claim 1, characterised in that the terminal piece (19) of the conductor preform (2) is inserted in grooves (8) of a cylindrical shaped body (7) and the superconductive terminal piece (21) is arranged on the curved surface (29) of a cylindrical contact body (23).

3. A production process as claimed in Claim 2, characterised in that the terminal piece (19) of the conductor preform (2) is inserted into helically arranged grooves (8) of the shaped body (7), that after the annealing step, the shaped body (7) is withdrawn from the conductor helix (20) formed by the superconductive terminal piece (2), which has hardened during the annealing step, and a contact body (23) which has a predetermined outer diameter (d) and a smooth curved surface (29) is inserted into the conductor helix (20).

4. A production process as claimed in one of Claims 1 to 3, characterised in that the superconductive terminal piece (21) is soldered or welded to the contact body (23).

5. A production process as claimed in one of Claims 1 to 4, characterised in that the terminal piece (19) of the conductor preform (2) is held on the shaped body (7) by means of a clamping device (13) during the annealing step.

6. A production process as claimed in one of Claims 1 to 5, characterised in that the shaped body (7) or the contact body (23) is set up on a hollow, cylindrical carrier body (5) having a corresponding outer diameter (d).

7. A production process as claimed in Claim 6, characterised in that there is provided a carrier body (5) which consists of a heat-resistant insulating material which does not react with the elements of the conductor preform (2) during the annealing step.

8. A production process as claimed in Claim 6 or Claim 7, characterised in that the part of the annealed superconductor which is arranged on the carrier body (5) is fixed thereto.

9. A production process as claimed in one of Claims 1 to 8, characterised in that there is provided a shaped body (7) consisting of an insulating material.

10. A production process as claimed in one of Claims 1 to 9, characterised in that there is provided a shaped body (7) consisting of a material which hardens on heat treatment.

11. A production process as claimed in one of Claims 1 to 10, characterised in that the shaped body (7) and the contact body (23) are arranged on the outside of the flange (4) of a winding carrier for the magnet winding.

12. A production process as claimed in Claim 11, characterised in that a conductor lead- in slot (17 ; 40 ; 41) is formed through the flange (4) by spark erosion.

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