DE3905424C2 - Process for producing a superconducting connection between oxide superconductors - Google Patents

Description

The invention relates to a method for producing a superconducting connection according to the preamble of the patent claim 1.

Superconducting metallic elements, alloys and intermetallic compounds have been known for many years and are sometimes also used industrially. In particular, the alloy NbTi and the intermetallic compound Nb ₃ Sn have gained technical importance. These (classic) superconductors all have transition temperatures of less than 25 K. The cooling must therefore be carried out with liquid helium at a temperature of about 4 K.

Since 1986, however, new oxidic, supra known conductive materials with a higher transition temperature. It was initially a Ba-La-Cu-O superconductor with one Jump temperature of 35 K.

In a rapid development there were soon more oxidic superconductor materials with transition temperatures, which is even above the temperature of the liquid Nitrogen are found. These are technical particularly interesting because cooling with liquid Nitrogen is much cheaper and simpler than that Cooling with helium. Such oxide, superconducting Materials with crack temperatures above 77 K are  known for example from EP-OS 2 80 812. The typical one Representatives of this second class of oxide superconductors are Y-Ba-Cu-O compounds.

A third class of oxide superconductors are those that no longer contain rare earth atoms. These are at for example Bi-Sr-Ca-Cu-O or Tl-Ba-Ca-Cu-O-Verbin fertilize.

However, these new oxide superconductors are now from ceramic nature. The manufacture and processing of these Materials therefore differ from those in the methods used in classic superconductors. So had to First, new ways can be found to get out of the supra conductive materials wires, tapes, foils or the like. to be able to manufacture. Regarding the solution to this Problems are referred to, for example, EP-OS 2 84 986, EP-OS 2 96 379 and EP-OS 2 92 126 referenced. So it was a further step to manufacture superconducting Components, such as coils, done.

For certain applications of superconducting coils, such as for example as field magnets of nuclear magnetic resonance apparatus ren, a high temporal constancy of the magnetic field required. This can in particular by the so-called. In short closing operation of the magnet coil windings can be achieved. However, there is a superconducting connection between the ends of the superconducting wires. From the connection technology of classic superconductors known methods, such as welding (Cold welding, ultrasonic welding) and soldering not to the new oxide, ceramic superconductors be applied.

DE-OS 34 13 167 is a process for the production a superconducting contact between conductor ends of Connection superconductors known. To do this, the  Conductor ends first of the stabilizing material be freed. They are then inserted into a sleeve which is also a powder mixture with the two elements of the connection superconductor. The liaison office is then sealed and pressed. Subsequently a reaction annealing is carried out in which the output elements the superconducting intermetallic Connection forms.

From US Pat. No. 4,797,510 it is further known the ends two wires with superconducting cores connect that they are plugged into a common sleeve be and then at a temperature of at least 50% of the melting temperature of the superconductor material, but below this melting temperature together be sintered.

The object of the invention is now to provide a method for Establishing a superconducting connection between specify oxide superconductors, the one compared to the known method results in improved connection without that at the junction the superconducting properties deteriorate. The invention Process also has the advantage of being essential shorter connection times can be provided. The task is performed by the characterizing features of claim 1 solved. The defined setting of the superconducting properties after making the connection can by Process steps take place that are inherent in the manufacture oxide, superconducting body in general are known. Based on the method according to the invention especially slow, controlled Cooling after the melting process or a separate, additional heat treatment.

The superconducting bodies are preferred around wires, tapes or foils with which supra conductive magnetic field windings are produced. In In this case, the ends of the superconducting bodies are said to be connected to each other. The superconducting bodies lie as a composite body made of oxide superconductor material and other material that is both act metallic as well as ceramic material can, before. It is particularly advantageous if the ends Before connecting, first in a suitable sleeve the flowing away of the molten superconducting material prevented from being introduced. Removing the rest Material is not required.  

In addition, superconducting material, the U. A stoichiometry other than the supra may also be used has conductor material of the body to be connected, in Form of a powder or a prefabricated sintered body or be pressed. With the superconductor In general, material can also be a primary material act only after all procedures have been carried out steps has superconducting properties. this applies also for the superconductor material to be connected Body.

Powders made of inert metals, such as for example Ag, Au, Pt, Pd, Ir, Rh or also Cu be filled in. By adding metal powder the deformation properties can be improved. This also makes electrical and thermal control ability increased in the normal conductive state.

A single crystal of oxidic can also be inserted into the sleeve Superconductor material are introduced. This then serves as a germ for a directed solidification of the melt. On this way a textured joint can getting produced.

The melting process itself can also be reduced Oxygen partial pressure or under an inert gas atmosphere, such as for example nitrogen or argon. The process is not specific to oxide supra conductor materials limited.

The method according to the invention is not based on the verb limited by the end of the ladder, rather can any part of the body can be connected. So it can also be the connection of medium Act parts of superconductors. If necessary, this must be done an existing coating of the superconducting materials  be removed first. However, this is for example for tapes coated with superconducting material or Foils are not required.

Based on the figures, embodiments of the Invention will be explained in more detail. It shows

Figure 1 shows an arrangement of the wires to be connected in a sleeve.

Figure 2 shows an arrangement of the wires to be connected, with superconducting powder additionally being filled into the sleeve;

Fig. 3 shows an arrangement in which, in the sleeve, in addition, a single crystal was introduced;

Fig. 4 shows an arrangement in which a prefabricated body made of superconductor material is additionally introduced into the sleeve.

Fig. 1 shows an arrangement for connecting the ends of two superconducting wires 1, 2. The superconducting wires 1 , 2 are composite wires in which a sheath 6 surrounds the superconducting material 7 . To establish the connection, the wire ends were first inserted into a sleeve 3 . The sleeve 3 can consist of both metallic and non-metallic material. All that is required is a temperature resistance of the sleeve that goes beyond the melting temperature of the superconducting material.

FIG. 2 shows a further arrangement of the superconducting wires 1 , 2 to be connected. The wire ends are in turn in a sleeve 3 , but not in direct contact with one another. Rather, a powder 4 is additionally filled into the sleeve 3 . This powder can be both a superconductor material and a powder made of inert metals and a mixture of the two.

Fig. 3 shows an arrangement in which a single crystal made of oxide superconductor material is additionally introduced into the sleeve 3 . This achieves a textured solidification of the melt.

FIG. 4 shows an arrangement in which a prefabricated body 8 made of superconductor material is inserted into the sleeve 3 between the ends of the superconducting wires 1 , 2 . With this arrangement, as with an arrangement according to FIG. 2, it can be ensured that the cross section of the superconductor material at the connection point is not smaller than in the wires. This is advantageous in that a sufficiently large cross section of the superconductor material at the connection point does not reduce the current carrying capacity there compared to the current carrying capacity of the conductors.

An embodiment of the invention is as follows described in more detail.

Two pieces of wire of a superconducting composite wire were connected with the method according to the invention. Of the Compound wire showed a core of the superconductor material

Bi ₂ Sr ₂ Ca ₁ Cu ₂ O _x (x = 7-9)

that was surrounded by a silver shell. The supra conductor material of the core had been produced in a known manner. Two sales fires were carried out at 800 ° C and 850 ° C. The finished composite wire had an outer diameter of 1 mm, the cross section of the superconductor material was about 0.45 mm ² . The wire ends were inserted into a silver sleeve with an outer diameter of 2.0 mm and an inner diameter of 1.1 mm. Superconductor powder was also filled into the sleeve. This powder was identical in composition and manufacture to that of the wires. During an annealing treatment of 920 ° C / 0.5 h in a temperature-controlled annealing furnace, the superconductor material was melted at the connection point and the connection was established. By melting in a temperature-controlled annealing furnace, a defined setting of the temperature is guaranteed both locally and temporally. With a further annealing treatment of 840 ° C / 120 h, the superconducting properties of the wire and the connection point were defined.

A critical current density at 77 K in the zero field of at least 7 A / cm ^{2 was} measured on the wires connected in this way. The usual criterion of critical current density, namely a voltage drop of 0.1µV / cm, was used as a basis. At 4.2 K, a critical current density of at least 430 A / cm ² in the zero field and of 100 A / cm ² in a field of 10 T was measured.

With the manufacturing method according to the invention superconducting connections are made that basically have the same current carrying capacity as the superconductors to be connected. In particular is because the low field dependence of the current carrying capacity the connection point also a joining of Production lengths possible in such places, the high Fields are exposed.

Claims (9)

1. A method for producing a superconducting connection between surfaces of two or more superconducting bodies, in which the superconducting body is present as a composite body made of oxide superconductor material and other material and the surfaces to be connected are arranged in direct contact or at a short distance from one another, characterized in that a melting process in which the composite body is heated on the surfaces to be joined to such an extent that the superconducting material there at least partially melts and runs into one another, a connection is made and that the temperature of the melting process is below the melting temperature of the other material, while the superconducting properties are in in a manner known per se can be set in a defined manner by thermal aftertreatment. 2. The method according to claim 1, characterized in that the melting process in a temperature controlled annealing furnace is carried out. 3. The method according to claim 1 or 2, characterized in that the ends of the body to be joined first in a sleeve that prevents the flow of the molten material prevented from being introduced. 4. The method according to claim 3, characterized in that in the sleeve also powdery, oxidic Superconductor material is filled.   5. The method according to claim 3, characterized in that an inert metal powder is added to the sleeve is filled. 6. The method according to claim 3, characterized in that a prefabricated sintered body in the sleeve or compact made of oxide superconductor material is introduced. 7. The method according to claim 3, characterized in that a single crystal of oxidic in the sleeve Superconductor material is introduced. 8. The method according to claim 1, characterized in that the melting process under an inert gas atmosphere or reduced oxygen partial pressure is carried out. 9. The method according to claim 1, characterized in that the superconductor material is a primary material acts only after all procedures have been carried out steps has superconducting properties.