DE2130380A1 - Process or device for the production of a composite superconductor - Google Patents

Description

PATENT LAWYERS DR.-PHIL G. NICKEL · DR.-ING. J. DORNER 2130380

8 MONKS 15

LANDWEHRSTR. 35 POST BOX! O4

TEL. (08 11) 555719

Munich, June 15, 1971 Attorney's files: 33 - Pat. 5

Cryomagnetics Corporation, 4955 Bannock Street, Denver, Colorado 8021.6, United States

Method and device for the production of a composite Superconductor.

The invention relates to methods and devices for manufacturing composite superconductors or for manufacturing of billets with a specific core configuration for the manufacture of composite superconductors. In particular, relates the invention focused on the production of superconductor wire of the highest quality.

A composite superconductor wire contains strands or threads made of superconductor material, which are embedded in a metal carrier, for example made of copper, and such a superconductor wire is often made from a billet by extrusion. Contains a certain type of superconductor wire for example up to a thousand threads made of a superconductor alloy, which in a certain arrangement in a common Metal supports, for example made of copper, are embedded. The said filaments should go from one end to the other of the wire or tape concerned and be continuous over their entire length through the carrier material.

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be separate. Such composite components are often made by a composite billet process are extruded and then by pressing, drawing or rolling to form the superconducting end product Wires, rods or strips can be further processed. Billets used in extrusion are generally of the shape Regular circular cylinder made of the carrier metal, for example made of copper, in which rods made of superconductor material are arranged in this way are that their longitudinal axes are parallel to the longitudinal axis of the carrier metal cylinder body. Generally one has Compression bars of this type are made by drilling holes in a copper bolt and then filling them with superconductor material.

Another way of making it was that in a number holes were drilled from disks of the carrier metal in a predetermined arrangement. The discs were then stacked on top of one another that their holes were aligned and could accommodate rods made of superconducting material. The arrangement of discs and bars was then made included to form a billet.

Yet another method for producing press ingots provides for a bundle of rods made of carrier metal and superconductor material to be assembled in a specific arrangement. This bundle of bars is then either included in the formation of a Preßbarrens or processed to a thinner bundle, which can be further processed ψ without the application of the extrusion process by pressing or drawing to the final product. Processes of this type are described in U.S. Patents 3,465,429 and 3,465,440.

However, in the above-mentioned methods of manufacturing composite superconductors, certain surface parts are adhered. For example the material and costs involved in drilling holes in a carrier metal body are very large. A covering and enclosing of the beam and bars is also expensive. Furthermore occurs

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especially in connection with the stacking disc-rod construction the disadvantage in appearance that the superconductor rods break when deformed or reduced to the final shape tend. Furthermore, in those cases in which the billets must necessarily be enclosed in a jacket, this sheathing can be removed after extrusion or the sheathing causes undesirable burr formation or serration on the finished wire.

The object of the invention is to be achieved, a method and a device for producing a composite To create superconductors, which or which works simply, cheaply and reliably and the production in particular made possible by superconductor wire, which has a significantly higher quality than can be achieved with known manufacturing processes is. This object also includes the manufacture of composite superconductor billets which are substantially free from any mistakes are.

The object is achieved in that a number of rods in a certain arrangement within a mold in a Melt of a carrier metal is set that the melt is brought to solidification in a controlled manner and that then from the Solidified melt of the carrier metal and the rod assembly formed body is removed from the mold and a press ingot to produce the composite superconductor.

The basic idea of the invention is therefore to produce a press billet by a uniform casting process. Previous suggestions for the casting of billets of this type have not proven themselves in practice, for which reason the generally accepted ones The assumption was that extremely expensive and complex casting equipment would be necessary in order to obtain a flawless casting of the desired Dimensions and the desired size to produce, what because of the strong shrinkage and others in the solidification normal carrier metals occurring processes was to be feared.

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The above manufacturing method according to the invention bears a certain resemblance to that in a completely different field, sometimes the manufacture of fuel assemblies processes used for nuclear reactors, such as in the publication by A.W. Haer and R.F. Dickerson, Volume 66 Transactions of American Foundrymen's Society, 1958, pp. 210 ff.

According to the invention, a number of rods are put together in a predetermined mutual position to form a core arrangement, which then within a heated container from a molten carrier metal of predetermined purity and composition. The top of the container and its contents will be Maintained above the melting point of the carrier metal and at the same time a mold plate with the center of the bottom of the container brought into contact while the sides of the container remain in a hot environment. Frozen in this way the contents of the container or crucible from the bottom upwards and from the center outwards, so that the solidification front in the shape of an upward-pointing cone upwards wanders. Errors due to shrinkage, porosity and other causes are excluded in this way, so that produce a non-porous, uniform carrier metal body leaves. The cast billet can be used without costly and complicated zone melting systems, vibration casting devices, Establish centrifugal casting or the like. The inventive The process works extremely cheaply.

An important advantage of the invention is that the one core assembly superconductor ingots containing superconductor has a highly uniform support metal structure. Be on the other hand several rods made of carrier metal brought together in a sheath, so it causes considerable difficulties - to connect the rods Bi'fe, which are made of the same metal, to one another well. Some authors keep a full connection between the

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Carrier metal elements are not necessary, but has been tried With the method according to the invention it has been found that such a complete connection is extremely important. One reason that the process according to the invention results in a product leads to higher quality, it is precisely that the press ingot unites one has uniform metal support, which does not have any defective areas Having connection. For a better understanding of the invention, the known ones are mentioned below Procedure detailed.

It is a bundle of metal bars assembled in a certain way and superconductor rods are placed in a container or a jacket and reduced together, so that they connect ductile but superconductor rods axed almost instantly with the neighboring rods made of normal carrier metal. Each other but contacting carrier metal bars shift, slide and align when the tensions of common reduction take effect. They then transmit non-uniform voltages on the ductile superconductor threads, so that the resulting Threads in the assembled billet no longer have a uniform cross-section over their length. Are further the bars from the carrier metal are not completely cleaned, so they can be connected even less well and the The resulting thread arrangement becomes all the more irregular. Through this however, it limits the critical current density of the superconductor wire drawn from the assembled billet or otherwise produced. After the product produced by the method according to the invention is uniform Has superconductor threads, there are also the critical current densities significantly higher than in the case of corresponding composite superconductors which have been produced according to known processes are.

A known measure to improve the connection between the individual carrier metal elements of a rod in certain

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The composite billet arrangement consisted of compressing the billet at a relatively high temperature or reduce. Such high temperatures but have the effect that the surfaces of the carrier metal with the active Gases in the surrounding atmosphere react, reducing the connection properties of the carrier metal parts are greatly deteriorated and the quality of the product is lowered. When using this method, it has therefore been found necessary to pre-reduce the assembled structure in a vacuum or in a noble gas atmosphere or inert gas atmosphere. This will make this procedure though complex and expensive, which according to the invention, however, is just avoided shall be.

It is often advantageous to use a large length of a composite Use superconductor wire instead of two or more short pieces. Great lengths of such compound However, wires are more susceptible to the undesirable low critical current density because they have a greater probability is that a long wire has a thread flaw at some point along its length. The invention also offers the advantage of producing large lengths of assembled superconductor wires with very high critical current density values can.

A common measure is to twist assembled superconductors f in order to increase the useful critical current density when used in magnets. If composite superconductors are twisted in this way, however, the effect of connection errors on the carrier metal increases, so that the advantageous effect of the twisting is again reduced. It is therefore also an advantage of the invention that the wire produced can be twisted without the faultless condition of the thread arrangement being destroyed, whereby the improvement that can be achieved by twisting with respect to the critical current density is fully utilized.

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The invention also makes it possible to determine the resistance ratio in the case of composite superconductor wires easy, precise and economical to adjust. For this purpose, an alloy additive or a doping material is used Carrier metal melt added in such an amount that there is a certain resistance ratio in the superconductor in question adjusts.

After the cast part has solidified in the manner described above, the cast part provided with a core assembly is made removed from the crucible or container so that the rods can now be removed if necessary. In this case, the educated Durohbrbruch or holes are filled with superconductor material, so that you get a billet, which through Drawing or other suitable reduction processes to form a superconductor in the form of a wire, rod or strip can be further processed. The product produced in this way not only has superconductor operating properties, which are far superior to those of components which are manufactured according to known processes, but rather a corresponding one Superconductor wire is also free of burrs or jagged protrusions, such as those caused by outer sheaths or containers which are mentioned, for example, in US Pat. No. 3,4:65,430 in this context. The at achieved the application of the manufacturing process according to the invention, smooth surface enables easier processing and isolation.

In addition, two-fold embodiments of the invention form the subject matter of the attached claims. In the following, the invention is illustrated by the description of preferred exemplary embodiments explained in more detail with reference to the accompanying drawings. Corresponding parts are in the various Drawing figures are provided with the same reference numbers. It it should be noted that the drawings are not to scale, but only the basic principles of the invention Let parts be recognized. It represent;

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Figure 1 is an end view of a core assembly for a press billet, as it can be produced by the method according to the invention,

FIG. 2 is a sectional view of that shown in FIG

Core arrangement corresponding to the sectional plane indicated by the line 2-2 in Figure i,

FIG. 3 is a schematic sectional drawing of part of a furnace and a crucible for carrying out a preferred form of the method according to the invention,

FIG. K shows a schematic illustration to explain the manner in which a carrier body of a press billet is controlled and made to solidify while maintaining a certain direction, and FIG

Figure 5 is a schematic illustration of the facilities

to control the intended solidification of the support body of the press billet according to Figure k.

In the core arrangement according to the drawings 1 and 2 is a HJLteplatte 10 attached both to a core rod holding rod 12 and to a lower perforated plate 14. The components mentioned are made of high-purity graphite.

The lower perforated plate Ik and a correspondingly designed upper perforated plate 16 have bores 18 which are designed so that they receive the ends of a number of quartz rods 20, in particular hollow quartz rods, which are sealed off at least at one end, approximately at 22 are to prevent the escape of air and the entry of copper during the egg diving process to be described.

The core assembly shown in Figure 1 is assembled by the core rod holding rod 12 made of graphite into the lower one

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Support plate 10 is screwed in, after which the lower perforated plate 14 is lifted over the core rod holding rod and naoh downwards is pushed, Hiernaoh, the upper perforated plate 16 is pushed over the holding rod 12 and brought to a certain point. Both the upper and the lower perforated plate are then in their position by means of small tungsten pins or graphite pins, as indicated at 26 and 28, fixed. A corresponding number of quartz tubes 20 is then inserted into the partial Assembled core assembly inserted by straightening it with the closed end 22 upwards (Figure 2 right lying) are guided through the holes IS of the upper perforated plate 16 so that they are finally in the respective associated holes the lower perforated plate 14 are enough. An upper support plate 24 is then placed over the support rod to complete the core assembly 12 pushed and, as already indicated above, secured by means of pins.

After the core assembly of Figures i and 2 in the described Way assembled, it is put in an oven for heating. At the same time, a shown in Figure 3 is Crucible 30 on a located in the furnace 34, approximately sleeve-like Bock 32 posed. The inner walls of the crucible 30 are here slightly conical, with a slope of about 2.1 mm to a length of 10 cm each, so that the inner diameter of the crucible at the lower end is smaller than at the upper end, the latter with an insulating plug 35 is completed. The plug 35 is made of graphite and formed so that it has the upper end of the crucible and can accommodate a tube 36, by means of which, if necessary, an inert gas into the interior of the crucible can be directed. The furnace has one or more primary heat supply channels, as indicated at 38, as well as one or several secondary heat supply channels, for example at 40. At the bottom of the furnace there is an opening 42 for receiving a chill block 44 which passes through the bracket 32 can be moved upwards so far that it rests against the bottom of the crucible 30. A temperature sensing element 46 occurs enters the furnace at the top of the crucible and extends

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along the side of the crucible to keep its temperature at different Record points along the crucible. Another temperature sensor element 48 is next to the Kokillenblook 44 down to the floor of the crucible upwards. Both temperature sensor elements are connected to a temperature indicator and temperature controller 50, which will nooh be mentioned in connection with FIG.

The temperature indicator and controller 50 provides output lines 52 and 5 ^ output signals for controlling a primary heater 56 and a secondary heater 58, which supply thermal energy to the furnace via the primary or secondary heat supply channels, as can be seen from FIG. The temperature indicator also provides and controller 50 on line 58 has an output signal to control or regulate the cooling water supply 60 for the mold, from where the cooling water via a line 62 to a cooling water space 64 of the mold block 44 of the furnace will. The heater and the cooling water supply can also can be set and regulated by hand.

To carry out a preferred embodiment of the method according to the invention, a copper bar indicated by dashed lines at 65 in FIG. 3 is inserted into the crucible 30, this copper bar consisting of oxygen-free knife ^{5 of} high purity and high conductivity in order to exclude this source of impurities in the melt. This is particularly important if, according to the method according to the invention, superconductor wires are to be produced which have a high resistance ratio. This is to be understood as the ratio of the resistance values at room temperature on the one hand and in the superconducting state at around 4.2 ° K, for example. An example of desired resistance ratios are values from about 150 to 200, but this ratio value drops rapidly when impurities are present in the copper. the end

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For this reason, the use of a graphite crucible is advantageous, since the graphite does not combine with the copper, but the crucible can also be made of another material which has the property of not contaminating the copper. Other carrier materials can also be used instead of copper. If a superconductor with a low resistance ratio is to be produced on purpose, the copper or another carrier material can be arbitrarily alloyed or doped. For example, an addition of only 10% of nickel causes the value of the resistance ratio of the arrangement to drop to about 5: 1. It has been found that the method according to the invention is particularly well suited to setting the resistance ratio of assembled superconductor wires precisely and at low cost. With the aid of the method according to the invention, with respect to a given combination of a carrier metal and a superconductor metal, the resistance ratio of the relevant composite superconductor wire can be set with an accuracy which previously could not be achieved with previously available composite superconductor wires.

After the copper ingot has melted and overheated, the copper melt fills the crucible to a level which is indicated in Figure 3 by the line 66. Now the preheated core assembly is slowly lowered into the melt, see above that it is covered by molten copper. The primary heater is then switched off and the melt is regulated in a certain way Direction brought to solidification, which will be discussed in more detail below.

The temperature indicator and controller 50 has the effect that cooling water is fed from a cooling water source 60 via the line 62 to the cooling water space 6k of the mold block kk . At the same time, heat energy is transferred from the heater 58 via the secondary heat supply channels kO to the upper part of the crucible, so that the upper part of the melt is above the melting point of the

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Carrier metal is held, which in the case of oxygen-free, high-purity, highly conductive copper is 1083 ° C. Of the The crucible remains in the furnace so that its walls, although they are no longer heated, remain in a hot environment. In this way, the melt of the carrier metal gradually solidifies from the bottom to the top and from the inside to the outside below Formation of a conical solidification front, so that casting defects, as they generally occur with uncontrolled solidification, can be avoided. For example, occur when controlled Solidification no shrinkage voids or voids in the middle of the billets on how they can develop when the melt solidifies from the outside in.

It has been shown that by simultaneously regulating the cooling of the lower part of the crucible and by heating the upper part of the crucible during the solidification, a pyramidal or conical solidification front is created. As can be seen from Figure k , the melt 68 solidifies first from the bottom in the middle and then at the outer edges in such a way that a pyramid or a cone moves upwards, the one above on the already solidified part of the one below lying carrier metal sits. For example, the dashed line 70 can symbolize the solidification s front at a first point in time, the dashed line 72 then shows the solidification front at a subsequent point in time and the dashed line Ik finally allows the advanced solidification front to become one again

ψ ' recognize later point in time. It is precisely on this cone-like solidification front during the progressive solidification of the melt that the freedom from undesired cavities or common casting defects in castings according to the invention is based,

After the casting has cooled sufficiently, it is removed from the crucible together with the core arrangement cast therein. Those parts of the casting which contain the perforated plates and support plates are then separated and the outer surfaces

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Before the Gußstüokes is possibly to certain dimensions turned off. It should be noted here, however, that it is an advantage of the invention The process is that only a small proportion of the grit material originally used in the carrier material falls off. With the above-mentioned turning and separating the Looh plates and the support plates, for example, only about 5 $ of the carrier metal is disposed of unused, whereby this waste can be reused due to its high purity. Here It should be noted that when drilling holes either in a one-piece body or in individual disks the The purity of the waste material can only be maintained with difficulty so that it can be reused for manufacture of superconductor carrier bodies is not indicated.

If desired, the quartz rods can now be removed from the casting what can happen through a dissolution process or a leaching, in which the quartz through the action of molten sodium hydroxide is dissolved. The hot ingot is then quenched in the water to ensure that about undissolved quartz is also removed and in order to keep the oxidation of the ingot surface as low as possible. the Quartz rods can also be obtained by dissolving them in hydrofluoric acid solution or removed mechanically. Thereafter, the carrier cast body by brushing, etching and rinsing in suitable Liquids cleaned, resulting in clean, active surfaces. After that, the holes in the beam are made with rods superconducting elements or alloys, for example made of niobium or a superconducting alloy. Be like that for example, satisfactory results with a niobium Titanium alloy, which contains up to $ 70 titanium (Titan with 30 percent by weight niobium). In a preferred embodiment A composite billet are rods made of titanium with 45 percent by weight of niobium in a carrier made of oxygen pure, high-purity, highly conductive copper embedded. In any case The ingot is subsequently assembled by extrusion, pressing, rolling, drawing or in some other way

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processed in such a way that ultimately composite, Superconducting material in the form of wires, rods, ribbons, strips or the like results.

The Faohmann recognizes from the above description of the method according to the invention and the device for producing a uniform carrier that the superconductor wire thus produced has great uniformity and is high in quality. This is because such a superconductor wire is not only more uniform, but also definitely has fewer places with a thread defect than you will find on wires are manufactured using well-known processes. The corresponding wire can therefore be drawn out to significantly greater lengths without a reduction in the useful critical current density occurring. For example, comparisons were made between a composite superconductor wire produced by conventional methods and a composite Superconductor wire of the same diameter and the same composition employed, which, however, naoh the one described above Process has been established. It showed that the greatest length of a composite superconductor wire is longer Quality with a Durohmesser of 1.26 mm, which could be produced using the known process, only about 120Om was, while the corresponding length of the same wire, which was manufactured according to the invention, was 12,000 m, although this value could have been increased without difficulty.

In addition, the surface d ·· wire · »produced free of serrations or ridges as ti" occur upon application of known methods, since the composite Pr "ßbarr" n _t he present invention is produced "in accordance with, is not" divided egmentweise and therefore It is unnecessary to arrange the bar in a container or casing before drawing. There are therefore no unwanted preprints or parts of the wire on the "finished" wire, so that the wire can be insulated. Becomes a superconductor ^

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Wire, which is produced in the manner according to the invention, used in constructions such as a superconductor magnet, this way, a solenoid magnet can be excited much more easily, because it can be isolated better without short-circuits and because of that the flawlessness of the superconductor threads higher flux densities can be achieved.

Within the scope of the invention, there are still a large number of options available to the person skilled in the art vm the possibility of modifying certain forms and details, but which of the idea underlying the invention to be included. For example, it should be noted that the above-mentioned central core rod support rod is made of graphite that the quartz tubes can also be replaced by a surrounding holding sleeve or by strips running in the longitudinal direction can also be drilled out and that sioh the invention Process can also be applied to continuous casting or continuous casting. Instead of the above-mentioned materials other materials can be used. For example, rods made of stainless steel or other metals can be used as core elements coated with aluminum dioxide, titanium dioxide or other heat-resistant compounds, wherein these components can be used instead of the quartz rods mentioned above. If you use conical rods, leave them sioh remove these mechanically «

There is also the possibility of changing the shape of the mold crucible, the core elements and the press ingot within the scope of the invention change. For example, square or rectangular or hexagonal pressing bars can be produced and in cross section the achievement of an optimal packing factor or filling factor can make it appear expedient to use core bars with hexagonal, triangular or other geometric cross-section to be used.

The quartz rods can also be replaced by superconductor rods, the surfaces of which are coated with niobium, molybdenum, tungsten or the like in such a way that there is no connection with the

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Carrier metal results and / or that no compounds are formed
which are harmful in the production or in the achievement of a high useful critical current density of the superconducting product.

The invention thus also includes the insertion of core rods made of superconductor material directly into the core assembly for manufacture of a finished billet as a casting instead of a billet carrier body provided with cores, the following still has to be filled with superconductor material in order to finally form the finished composite billet. For example titanium-niobium core rods can be cast directly into an aluminum carrier. Instead of dipping the core assembly in the melt of the carrier metal can also be provided to fix the core assembly in the crucible and the melt of the carrier metal to be poured in from the outside. It should be noted here again that other carrier metals such as lead or tin are also used can be.

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Claims (25)

Claims 1. Process for the production of a composite superconductor, characterized in that a number of rods in a certain arrangement within a mold in a melt of a Carrier metal is set so that the melt is brought to solidification in a controlled manner and that then from the solidified Melt of the carrier metal and the rod assembly formed body is removed from the mold and a press ingot for production of the composite superconductor forms. 2. The method according to claim i, characterized in that the press billet is processed to reduce the cross section. 3. The method according to claim 1 or 2, characterized in that the composite superconductor a subsequent processing in particular by means of drawing into a finished superconductor product. 4 «Method according to claim 2 or 3, characterized in that that the press ingot is processed into a thin, elongated superconductor. 5. The method according to any one of claims 1 to 4, characterized in that the melt of the carrier metal from the lower end of the billet to be produced is brought upward to solidify in such a way that the solidification front migrates upward in the form of an upwardly pointing cone. - 17 - 109853/1262 6. The method naoh one of claims 1 to 5, characterized in that that the controlled solidification of the melt of the carrier metal is carried out in such a way that the melt of is cooled down while the mold is in a hot environment at a temperature above the melting point of the carrier metal is held and on the mold in the area of the upper end of the melt as long as a temperature above the melting point of the carrier metal is maintained until the upper end of the melt has solidified. 7. The method according to any one of claims 1 to 6, characterized in that that the rods are arranged around a central longitudinal axis in a frame and that the rod arrangement in one the mold-forming crucible is surrounded by the melt of the carrier metal. 8. The method according to any one of claims 1 to 7 »characterized in that that the rod assembly is removed from the carrier metal after its solidification, so that holes are formed in the billet will. 9. The method naoh claim 8, characterized in that the formation of a composite, superconducting billet Löoher can be filled with superconductor material. 10. The method according to any one of claims 1 to 7, characterized in that the rods are made of superconductor material. 11. The method naoh one of claims 1 to 9, characterized in that that the rods are made of quartz, 12. The method according to claim 11, characterized in that the quartz rods are removed from the carrier metal by introducing the casting into a bath which contains or consists of substances of the type of molten »sodium hydroxide or hydrofluoric acid solution. - 18 -109853/1262 13. The method according to claim 11 or 12, characterized in that quartz tubes are used as rods, which at least are closed at one end so that an escape of air and an entry of molten salt when the rods are introduced into them be avoided, I ² I. Method according to one of claims 1 to 9, characterized in that the rods are made of metal and are coated with a heat-resistant compound. 15. The method according to claim 10, characterized in that the rods contain superconductor metal which is surface-coated with a metal is selected from the group consisting of niobium, molybdenum and tungsten. 16. The method according to any one of claims 1 to 15, characterized in that that the Sohmelze an alloy material in certain Amount corresponding to a predetermined resistance ratio of the resulting composite superconductor is added. 17. A method for producing a press ingot for the formation of composite superconductors according to the method of any one of claims 1 to 16. 18. The method according to any one of claims 1 to 17, characterized in, that the carrier metal is melted in the mold or the crucible before the rod arrangement is introduced. 19. Device for carrying out the method according to one of claims 1 to 18, characterized by a frame (iO, 12, Ik, 16, 22) for holding a rod arrangement (20) in a certain position around a longitudinal axis, further by a device for Receipt of the melt of the carrier metal (66) and of the frame, in such a way that the base melt surrounds the rod arrangement and by means (50, 56, 58, 6o) for regulating the temperature of the melt in such a way that the melt at the lower end of the mold - 19 109853/1262 (30) is cooled while the upper end of the melt remains above the melting point, thereby solidifying from the bottom progresses above. 20. Device according to claim 19, characterized by a mold block (44), which is arranged so near the lower end of the mold (30) that the heat dissipated from the mold block causes the melt to solidify from bottom to top. 21. Device according to claim 19 or 20, characterized by a closure piece (35) to close the upper end of the mold (30) and through a feed line (36) for an inert gas into the Shape to the space between the melt (66) and the plug. 22. Device according to one of claims 19 to 21, characterized by heating devices (56, 38) for heating the mold (30) for the purpose of melting the carrier metal, 23. Device according to claim 22, characterized by further heating devices (58, 40) for heating the upper part of the Form (30) during the solidification of the melt of the carrier metal. 24. Device according to one of claims 19 or 21 to 23 and / or claim 20, characterized by devices for supply of coolant to the mold block (44) in such a way, ψ ' that the heat dissipated through the mold block causes the solidification of the melt to proceed from bottom to top. 25. Device according to one of claims 19 to 24, characterized in that that the frame contains two spaced apart retaining plates (14, 16), the means (lö) for holding of the ends of the rods in the predetermined arrangement. - 20 - 109853/1262 Blank page