Method for the manufacture of a superconductor containing an intermetallic compounds
BACKGROUND OF THE INVENTION 
     The present invention relates to a method for manufacturing a superconductor having a superconductive intermetallic compound of at least two elements. 
     In particular, the present invention relates to a method for manufacturing a superconductor having a superconductive intermetallic compound of at least two elements in which an intermediate conductor product is prepared from a first component containing one element of the compound and a second component which is an alloy consisting of a carrier metal and the remaining element or elements of the compound, and in which the intermediate conductor product is subjected to a heat treatment such that the desired intermetallic compound is formed by reaction of the element of the first component with the remaining element or elements of the second component. Such a manufacturing method for a superconductor is known from DE-OS No. 2 056 779. 
     Superconductive intermetallic compounds consisting of two components, each having one element of the desired intermetallic compound, such as Nb 3  Sn or V 3  Ga, which are of the A 3  B type and have A15 crystal structure, have very good superconductor properties and are distinguished particularly by a high critical flux density B c2  in a magnetic field, a high transition temperature T c  and a high critical current density I c . They are, therefore, particularly well suited as conductors for superconductor coils for producing strong magnetic fields. In addition, ternary compounds such as niobium-aluminum-germanium Nb 3  (A1 x  Ge.sub.(1-x)), are of special interest. Since these compounds are generally very brittle, however, it is very difficult to produce them in a form suitable, for example, for magnet coils. From the mentioned DE-OS No. 2 056 779, a method is known which makes possible the manufacture of superconductors with intermetallic compounds of two components in the form of long wires or ribbons. This method serves in particular for the manufacture of so-called multicore conductors with wires arranged in a normal-conducting matrix, for example, of Nb 3  Sn or V 3  Ga or with niobium or vanadium wires with surface layers of these compounds. Thus, a ductile element of the compound to be produced, in wire form, for example, a niobium or a vanadium wire, is surrounded by a jacket of ductile matrix material which contains a predetermined amount of the other elements in the form of an alloy, for example, of a tin or gallium bronze. A mulitiplicity of such wires also can be embedded in the matrix. The structure so obtained is then subjected to a cross section-reducing process and cut into a predetermined number of sections. These sections then are bunched together and again brought into an elongated form through cross section reduction. Through the cross section reductions, the diameter of the wire cores (consisting, for example, of niobium or vanadium) is reduced to a low value in the order of 10 μm or less, which is advantageous in view of the superconductor properties of the conductor. With this process step, a good metallurgical bond is obtained between the wire cores and the matrix material surrounding them without the occurrence of reactions which would embrittle the conductor. Thus, a not yet fully reacted intermediate product of the superconductor in the form of a long wire is obtained such as is subsequently required for the winding of coils. This intermediate product is finally subjected to an annealing treatment in vacuum or in an atmosphere of an inert gas such as argon, where element or elements of the superconductive compound to be formed, contained in the matrix, diffuse into the material of the wire cores, consisting of the other element of the compound, and thus react with the latter, forming a layer consisting of the desired superconductive compound. 
     It has been attempted repeatedly to increase the current carrying capacity of such multicore conductors by special alloying additives. Thus, small amounts of tantalum have been added to the core material niobium (IEEE Trans. Magnetics, MAG-14, No. 5, September 1978, pages 611 to 613). It is further known to add small amounts of gallium to a bronze matrix (J. Appl. Phys. 49(1), January 1978, pages 357 to 360). While an increase of the critical current density of the superconducting Nb 3  Sn layers can be achieved with these measure, particularly in magnetic fields with flux densities above 10 Tesla, the workability of one or the other conductor components is generally made more difficult by these additions, particularly due to alloy precipitation hardening. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the present invention to improve the known method for manufacturing a superconductor containing an intermetallic compound in such a manner that an increase of the critical current, and thus of the effective current density, of the superconductor in magnetic fields at a flux density above 10 Tesla is obtained without adversely affecting the workability of the conductor components. 
     According to a first embodiment of the present invention, this object is achieved by providing that the heat treatment performed in the known method is conducted in a hydrogen atmosphere. 
     The advantages of this process step are, in particular, that through the annealing in hydrogen instead of argon or vacuum, the effective current density of the superconductive intermetallic compound layers produced increases in magnetic fields with flux densities above 10 Tesla. This surprising effect is found if the diffusion anneal, i.e., the heating which is conducted after the separate components containing the elements of the desired intermetallic compound are brought together and subjected to cross-section reduction, takes place exclusively in hydrogen. If the doping with hydrogen is compared with the known doping by metallic additives, hydrogen doping can be carried out substantially more easily, since neither the core material nor the conductor matrix need be modified before processing. 
     According to a second embodiment of the present invention, the same object is attained by providing that thermal post-treatment in a hydrogen atmosphere be performed subsequent to the heat treatment conducted for forming the superconductive compound. 
     The advantages achieved in accordance with this second embodiment are, in particular, that the effective current density of the so-produced layers of the superconductive intermetallic compound is increased, particularly in magnetic fields with flux densities above 10 Tesla. This surprising effect occurs if the superconductors are post-annealed for a short time in hydrogen after the customary anneal in, for example, argon. If the doping with hydrogen is compared with the known doping by metallic additives, the hydrogen doping can be carried out substantially more easily, since neither the core material nor the conductor matrix need be modified before processing. In addition, magnet coils which have already been wound from superconductors annealed in argon can be subjected to such a post-anneal in hydrogen in order to thereby increase the current-carrying capacity of their conductors. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Further explanation and details with respect to the methods according to the present invention are provided wit reference to the following embodiment examples and the tables. 
     The examples are based on a manufacturing method of wire samples of an Nb 3  Sn multi-filament conductor with 10,000 filaments and an overall diameter of 0.5 mm, the superconducting zones of which are formed by solid-state diffusion by means of the known bronze technique (See DE-OS No. 2 052 323). 
     EXAMPLE I 
     For comparison purposes, a corresponding intermediate conductor product was subjected to a generally known standard anneal at 700° C. for about 64 hours in an argon atmosphere of 0.5 bar. 
     EXAMPLE II 
     Instead of the anneal in an argon atmosphere of the intermediate conductor product as per Example I, two intermediate conductor products were annealed according to the present invention for about 64 hours at 700° C. in a hydrogen atmosphere of about 0.3 bar. 
     The critical current I c  and effective current density J eff  obtained in accordance with the Examples I and II above are given in the following table as a function of magnetic fields with flux densities acting thereon above 10 Tesla, where J eff  =I c  /F, F being the cross sectional area of the conductor. 
     The critical currents I c  are given in amps and the effective current densities J eff  in 10 -5  A/cm 2 . 
     
                       TABLE I                                                     
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            B [T]                                                         
            10    11        12      15                                    
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Example I: Standard Anneal 64h/700° C.                             
           (0.5 bar Ar)                                                   
         I.sub.c                                                          
              171     135       108    45                                 
         J.sub.eff                                                        
              0.87    0.69      0.55  0.23                                
Example II:                                                               
           64h/700° C. (0.3 bar H.sub.2)                           
Conductor 1:                                                              
           I.sub.c                                                        
                  164     135     111    57                               
           J.sub.eff                                                      
                  0.84    0.69    0.57  0.29                              
Conductor 2:                                                              
           I.sub.c                                                        
                  167     138     113   57.7                              
           J.sub.eff                                                      
                  0.85    0.70    0.58  0.29                              
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     As can be seen from the Table, the gain in effective current density attainable according to the invention is about 4 to 6% at flux densities of 12 Tesla and about 26% at 15 Tesla. 
     It was further found that the values of the critical current I c  and of the effective current density J eff  attainable according to the present invention are increased only at flux densities above 10 Tesla over the values for the conductors annealed in argon or in a vacuum. The conductors manufactured by the first method according to the invention are, therefore, most advantageously provided for superconducting devices such as magnet coils where they are subjected to magnetic fields with flux densities above 10 Tesla. 
     In the fabrication of the conductors according to the present invention, on which Example II was based, it was assumed that the heat treatment in te hydrogen atmosphere consists of a single treatment stage at a predetermined temperature. Optionally, several treatment stages at different temperatures also can be carried out in the hydrogen atmosphere. A pressure of about 0.3 bar was furthermore assumed. For raising the critical current I c  above 10 Tesla, however, lower pressures also are sufficient. In general, however, the pressure of the hydrogen atmosphere should be at least 10 -3  bar. 
     In general, a gain in effective current density over the superconductors treated in accordance with Example I can be obtained if a temperature of at least about 600° C., but preferably of about 700° C., is chosen; at the lower temperatures, the anneal time generally must be longer than at the high temperatures. 
     According to the first embodiment of the invention outlined in Example II, the heat treatment of the intermediate conductor products for forming the superconducting intermetallic compound was carried out in a hydrogen atmosphere in order to increase the critical current and the effective current density. According to a second embodiment of the invention, however, this objective also can be achieved if a thermal post-treatment in a hydrogen atmosphere is carried out only after the heat treatment for forming the superconductive compound. Appropriate process steps are outlined in Examples III to VII. 
     EXAMPLES III TO VI 
     Nb 3  Sn diffusion conductors prepared in accordance with Example I were subjected, according to the invention, additionally to a post-anneal in a hydrogen atmosphere of about 0.3 bar at 700° C. where, according to Example III, the anneal in the hydrogen atmosphere was carried out for about 64 hours; according to Example IV about 8 hours; according to Example V about 4 hours and according to Example VI about 2 hours. 
     The critical currents I c  and effective current densities J eff  obtained in accordance with the Examples III to VI above are given in the following Table II as a function of magnetic fields acting thereon with flux densities B above 10 Tesla; for comparison purposes, the corresponding values from Example I are likewise given. Here, J eff  =I c  F, F being the cross sectional area of the conductor. The critical currents I c  are given in amps and the effective current density J eff  in 10 -5  A/cm 2 . 
     
                       TABLE II                                                    
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Example     B [T]                                                         
No.         10      11         12     15T                                 
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I      Standard Anneal: 64h/700° C. (0.5 bar Ar)                   
I.sub.c :   171     135        108    45                                  
J.sub.eff : 0.87    0.69       0.55  0.23                                 
III    64h/700° C. (0.5 bar Ar) + 64h/700° C. (0.3 bar      
       H.sub.2)                                                           
I.sub.c :   170     140        115    60                                  
J.sub.eff : 0.87    0.71       0.59  0.31                                 
IV     64h/700° C. (0.5 bar Ar) + 8h/700° C. (0.3 bar       
       H.sub.2)                                                           
I.sub.c :   174     144        118    61                                  
J.sub.eff : 0.89    0.73       0.60  0.31                                 
V      64h/700° C. (0.5 bar Ar) + 4h/700° C. (0.3 bar       
       H.sub.2)                                                           
I.sub.c :   173     143        117    59                                  
J.sub.eff : 0.88    0.73       0.60  0.30                                 
VI     64h/700° C. (0.5 bar Ar) + 2h/700° C. (0.3 bar       
       H.sub.2)                                                           
I.sub.c :   174     144        118    59                                  
J.sub.eff : 0.89    0.73       0.60  0.30                                 
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     As can be seen from the Table above, the gain obtained according to this embodiment of the invention as to the effective current density is up to 9% at flux densities of 12 Tesla and up to 35% at 15 Tesla. 
     It was further found that the values of the critical current I c  and the effective current density J eff  obtainable according to the invention are generally increased over the values for the conductors annealed only in argon or in a vacuum only at flux densities above 10 Tesla. The conductors fabricated by the method according to the invention are, therefore, most advantageously provided for superconducting devices such as magnet coils, where they are subjected to magnetic fields with flux densities above 10 Tesla. 
     In the manufacture of the conductors according to the present invention, such as those on which Examples III to VI were based, it was assumed that the pressure of the hydrogen atmosphere is approximately 0.3 bar. However, lower pressures also may be provided, and, in general, it is sufficient for raising the critical current density I c  above 10 Tesla if the pressure of the hydrogen atmosphere is at least about 10 -3  bar. It was further assumed that the heat treatment in the hydrogen atmosphere consists of a single treatment stage at a predetermined temperature. Optionally, however, several treatment stages at different temperatures also can be carried out in the hydrogen atmosphere. The temperature dependance was investigated in accordance with the following example. 
     EXAMPLE VII 
     The intermediate conductor product of an Nb 3  Sn multi-filament conductor was first annealed in accordance with Example I for 64 hours at 700° C. in argon to form the superconductive intermetallic compound Nb 3  Sn. Subsequently, various post-anneals were carried out in a hydrogen atmosphere at 0.3 bar, and specifically, at temperatures between 300° C. and 800° C. for periods between 20 hours and 1 hour. It was found that a gain in effective current density over the superconductors treated in accordance with Example I can be obtained if a temperature above 350° and preferably of about 400° C. or about 700° C. is chosen, where at the lower temperatures, generally longer annealing times must be provided than at high temperatures. 
     According to the examples, the manufacture of superconducting multi-filament conductors with the intermetallic compound Nb 3  Sn was assumed. The method according to the invention is equally well suited, however, for the manufacture of superconductors of other known intermetallic compounds by means of solid-state diffusion, such as of V 3  Ga conductors.