Abstract:
The method of preparing an NB 3  Al superconducting wire comprises the steps of passing an Nb/Al composite wire consisting of an Nb metal or an Nb alloy and an Al metal or an Al alloy through a furnace for heating the same from the room temperature to a prescribed temperature, subsequently passing the same through the furnace for holding the same at the prescribed temperature, and subsequently passing the same through a cooling part for cooling the same from the prescribed temperature to the room temperature, and these steps are continuously carried out by continuously moving the wire. According to the present invention, it is possible to obtain an Nb 3  Al superconducting wire having homogeneous characteristics along its overall width with a high critical current density.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to methods of preparing an Nb 3  Al superconducting wire and an Nb 3  Al superconducting stranded wire, and more particularly, it relates to methods of preparing an Nb 3  Al superconducting wire and an Nb 3  Al superconducting stranded wire serving as high magnetic field superconducting materials for superconducting magnets which can be employed for nuclear fusion or the like. 
     2. Description of the Background Art 
     An Nb 3  Al superconducting material is expected particularly as a material for a nuclear fusion reactor which is exposed to high magnetic force in a high magnetic field, or a power storage superconducting material, due to a high critical current and excellent distortion resistance in a high magnetic field. However, it is difficult to work Nb 3  Al, which is an intermetallic compound, into a thin wire due to inferior workability, dissimilarly to an alloy superconducting material such as an Nb-Ti material. In order to obtain an Nb 3  Al superconducting wire, therefore, an Nb metal and an Al metal are generally composited with each other and then drawn into a thin wire, which in turn is finally heat treated. 
     Further, Nb 3  Al is stable only under a high temperature at least 1600° C. in a binary system of Nb-Al. In order to form Nb 3  Al at a lower temperature, therefore, it is necessary to reduce distance of Nb and Al diffusion pairs below several 10 nm by a jelly roll method or the like. 
     In an Nb/Al composite material which is prepared by compositing Nb and Al metals, however, the filament is ununiformly deformed when the material is worked into a thin wire, due to inferior workability. Therefore, an Nb 3  Al superconducting wire which is obtained by heat treating the composite material cannot attain a sufficiently high critical current density. 
     Further, a compound superconducting material such as Nb 3  Al is coiled before a heat treatment by the so-called wind-and-react method, since its superconductivity is extremely deteriorated by distortion such as bending. When the coil as formed is increased in size, however, it is difficult to carry out a uniform heat treatment as a whole. 
     In order to improve superconductivity, it is preferable to form Nb 3  Al which is close to a stoichiometric composition and provided with fine crystal grains by performing the heat treatment in a short time at the highest possible temperature, while maintaining this Nb 3  Al by rapidly cooling the same to the room temperature. However, it is difficult to heat treat and cool a large-sized coil in such a manner. 
     SUMMARY OF THE INVENTION 
     In order to solve the problems of the prior art, an object of the present invention is to provide methods of preparing an Nb 3  Al superconducting wire and an Nb 3  Al superconducting stranded wire having excellent superconductivity and uniform characteristics, which can be applied to manufacturing of large-sized conductors or coils. 
     According to an aspect of the present invention, a method of preparing an Nb 3  Al superconducting wire is provided. This method comprises the steps of passing an Nb/Al composite wire consisting of an Nb metal or an Nb alloy and an Al metal or an Al alloy through a furnace for heating the same from the room temperature to a prescribed temperature, further passing the heated Nb/Al composite wire through the furnace for holding the same at the prescribed temperature, and passing the Nb/Al composite wire, which is held at the prescribed temperature, further through a cooling part for cooling the same from the prescribed temperature to the room temperature, and the step of heating the Nb/Al composite wire from the room temperature to a prescribed temperature, the step of holding the same at the prescribed temperature and the step of passing the same through said cooling part are continuously carried out by continuously moving the Nb/Al composite wire. 
     According to the present invention, the step of heating the Nb/Al composite wire from the room temperature to a prescribed temperature and the step of holding the wire at the prescribed temperature are continuously carried out by continuously moving the wire thereby continuously heat treating the Nb/Al composite wire. Thus, it is possible to heat treat the Nb/Al composite wire at a higher temperature in a shorter time as compared with conventional batch treatment. Consequently, it is possible to homogeneously form Nb 3  Al which is close to a stoichiometric composition and provided with a high grain boundary density of Nb/Al composite for serving as a pinning center along the overall length of the wire. 
     According to the present invention, further, the aforementioned heat treatment is continuously carried out by continuously moving the wire. Therefore, it is possible to cool the Nb/Al composite wire in a shorter time as compared with the conventional batch treatment. Consequently, it is possible to maintain high characteristics of Nb 3  Al which is formed in the heat treatment steps. 
     Preferably, the time for heating the wire from the room temperature to the prescribed temperature is within 1 minute. If this time exceeds 1 minute, it is difficult to form Nb 3  Al which is close to a stoichiometric composition and provided with a high grain boundary density. 
     Preferably, the time for cooling the wire from the prescribed temperature to the room temperature is also within 1 minute. If this time exceeds 1 minute, it is difficult to maintain high characteristics of Nb 3  Al as formed. 
     Preferably, the prescribed temperature is at least 800° C., and the time for holding the wire at the prescribed temperature is within 10 hours. In this case, it is possible to form Nb 3  Al which is close to a stoichiometric composition and provided with a high grain boundary density for serving as a pinning center. When the prescribed temperature is the highest possible within a range not melting a matrix and the wire is held at the prescribed temperature in a short time of several 10 seconds, it is possible to form Nb 3  Al having higher characteristics. 
     Preferably, the method further comprises a step of heating the Nb/Al composite wire, which is passed through the cooling part, at a temperature of not more than 800° C. for at least 10 hours. In this case, ununiform distortion which is applied among crystals of an Nb 3  Al compound phase is so relieved that it is possible to obtain an Nb 3  Al superconducting wire having homogeneous characteristics along its overall length with a high critical current density. 
     According to the present invention, it is possible to employ an Nb/Al composite wire which is composited with copper or a copper alloy for stabilization. 
     According to the present invention, further, it is possible to employ an Nb/Al composite wire which is prepared by a jelly roll method, while an Nb layer in the filament may have a thickness of about 500 nm. 
     In general, it is necessary to reduce thicknesses of an Nb layer and an Al layer in the filament to not more than several 10 nm, as hereinabove described. Therefore, it is necessary to apply severe working on the wire, leading to wire breaking. According to the present invention, on the other hand, the Nb and Al layers may have thicknesses of about 500 nm, whereby it is possible to prevent wire breaking. 
     According to another aspect of the present invention, a method of preparing an Nb 3  Al superconducting stranded wire is provided. The method comprises the steps of passing an Nb/Al composite stranded wire which is formed by stranding up a plurality of Nb/Al composite wires, each consisting of an Nb metal or an Nb alloy and an Al metal or an Al alloy, with each other through a furnace for heating the same from the room temperature to a prescribed temperature, further passing the heated Nb/Al composite stranded wire through the furnace for holding the same at the prescribed temperature, and passing the Nb/Al composite stranded wire, which is held at the prescribed temperature, further through a cooling part for cooling the same from the prescribed temperature to the room temperature, and the step of heating the Nb/Al composite stranded wire from the room temperature to a prescribed temperature, the step of holding the same at the prescribed temperature and the step of passing the same through said cooling part are continuously carried out by continuously moving the Nb/Al composite stranded wire. 
     Thus, it is also possible to form Nb 3  Al having high characteristics by continuously heat treating an Nb/Al composite stranded wire which is formed by stranding up Nb/Al composite wires by continuously moving the wire. 
     According to still another aspect of the present invention, a method of preparing an Nb 3  Al superconducting stranded wire is provided. This method comprises the steps of passing an Nb/Al composite wire consisting of an Nb metal or an Nb alloy and an Al metal or an Al alloy through a furnace for heating the same from the room temperature to a prescribed temperature, further passing the heated Nb/Al composite wire through the furnace for holding the same at the prescribed temperature, passing the Nb/Al composite wire, which is held at the prescribed temperature, further through a cooling part for cooling the same from the prescribed temperature to the room temperature, and collecting and stranding up a plurality of Nb/Al composite wires which are passed through the cooling part, and the step of heating the Nb/Al composite wire from the room temperature to a prescribed temperature, the step of holding the same at the prescribed temperature and the step of passing the same through said cooling part are continuously carried out by continuously moving the Nb/Al composite wire. 
     Thus, high characteristics of Nb 3  Al are also maintained when Nb/Al composite wires are continuously heat treated by continuously moving the wire and thereafter stranded. 
     Preferably, the method further comprises a step of heating the stranded wire which is formed after the heat treatment at a temperature of not more than 800° C. for at least 10 hours. In this case, ununiform distortion which is applied among crystals of an Nb 3  Al compound phase is so relieved that it is possible to obtain an Nb 3  Al superconducting stranded wire having homogeneous characteristics among strands along its overall length with a high critical current density. 
     According to a further aspect of the present invention, an apparatus for preparing an Nb 3  Al superconducting wire from an Nb/Al composite wire consisting of an Nb metal or an Nb alloy and an Al metal or an Al alloy is provided. This apparatus comprises supply/carrier means for supplying and carrying the composite wire, temperature increase means which is provided on a carrier path for heating the composite wire from the room temperature to a prescribed temperature while carrying the same, soaking/holding means which is provided on the carrier path adjacently to the temperature increase means for soaking/holding the composite wire as heated at the prescribed temperature, and cooling means which is provided on the carrier path adjacently to the soaking/holding means for cooling the composite wire as soaked/held from the prescribed temperature to the room temperature. 
    
    
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view showing the structure of a hexagonal segment formed by a jelly roll method; 
     FIG. 2 is a sectional view schematically showing the structure of a Cu/Nb/Al composite multifilamentary wire; and 
     FIG. 3 is a sectional view showing an exemplary apparatus for preparing an Nb 3  Al superconducting wire according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     EXAMPLE 1 
     An Nb sheet and an Al sheet were lap-wound on a copper bar, and the winding as obtained was inserted in a copper pipe and thereafter drawn to have a hexagonal section. 
     FIG. 1 is a sectional view showing the structure of the hexagonal segment prepared in the aforementioned manner. 
     Referring to FIG. 1, this hexagonal segment is formed by a copper matrix 1 which is arranged on its center, an Nb sheet 2 and an Al sheet 3 which are alternately lap-wound on the matrix 1, and a copper matrix 4 which is formed around the same. The copper matrices 1 and 4 serve as stabilizing materials. In boundaries between the winding which is formed by alternately lap-winding the Nb and Al sheets 2 and 3 and the copper matrices 1 and 4 which are arranged on the center and the outer periphery of the winding, only the Nb sheet 2 is wound to define diffusion barrier layers 5 and 6 respectively. 
     520 such hexagonal segments were collected and inserted in a copper pipe and subjected to a wire drawing step with extrusion and drawing, a twisting step and a forming step, thereby preparing a Cu/Nb/Al composite multifilamentary wire having a section of 1.29 mmφ. 
     FIG. 2 is a sectional view schematically showing the structure of the Cu/Nb/Al composite multifilamentary wire obtained in the aforementioned manner. 
     Referring to FIG. 2, this composite multifilamentary wire is formed by a stabilizing material 7 consisting of copper and a number of filaments 8 which are embedded therein. A stabilizing material 9 consisting of copper is arranged in the center of each filament 8. 
     The composite multifilamentary wire obtained in the aforementioned manner was heat treated according to the present invention, to prepare an Nb 3  Al superconducting multifilamentary wire. This heat treatment step is now described in detail with reference to FIG. 3. 
     FIG. 3 is a sectional view showing an exemplary apparatus for preparing an Nb 3  Al superconducting wire according to the present invention. 
     Referring to FIG. 3, this apparatus is formed by a continuous heat treatment furnace (tubular electric furnace) 12 comprising a temperature increase zone 10 and a soaking zone 11, a cooling vessel 13, a supply reel 15 for feeding a composite multifilamentary wire 14 into the continuous heat treatment furnace 12, and a take-up reel 17 for taking up an Nb 3  Al superconducting wire 16 which is obtained by passing the composite multifilamentary wire 14 through the cooling vessel 13. 
     The Cu/Nb/Al composite multifilamentary wire 14 obtained in the aforementioned manner was passed through the temperature increase zone 10 by the supply reel 15. The composite multifilamentary wire 14 was passed through the temperature increase zone 10 for 30 seconds, to be heated from the room temperature up to 900° C. Then, the composite multifilamentary wire 14 was passed through the soaking zone 11 of 900° C. for 1 minute, to be held at the temperature of 900° C. for 1 minute. During this heating, the tubular electric furnace 12 comprising the temperature increase zone 10 and the soaking zone 11 was brought into a nitrogen atmosphere, for passing the composite multifilamentary wire 14 therethrough. Then, the heated wire 14 was passed through the cooling vessel 13 for 30 seconds, to be cooled to the room temperature, and thereafter taken up on the take-up reel 17. 
     The wire treated in the aforementioned manner was further heat treated in a vacuum electric furnace at 750° C. for 50 hours, thereby obtaining an Nb 3  Al superconducting wire. 
     EXAMPLE 2 
     The Cu/Nb/Al composite multifilamentary wire described in Example 1 was passed through the tubular electric furnace 12 under a nitrogen atmosphere and the cooling vessel 13 similarly to Example 1, except heating conditions. The composite multifilamentary wire was passed through the temperature increase zone 10 for 30 seconds to be heated up to 1050° C. in 30 seconds, held at this temperature in the soaking zone 11 for 1 minute, and thereafter passed through the cooling vessel 13, to be cooled to the room temperature in 30 seconds. 
     The wire treated in the aforementioned manner was further heat treated in a vacuum electric furnace at 750° C. for 50 hours similarly to Example 1, to obtain an Nb 3  Al superconducting wire. 
     EXAMPLE 3 
     The Cu/Nb/Al composite multifilamentary wire described in Example 1 was passed through the tubular electric furnace 12 under a nitrogen atmosphere and the cooling vessel 13 similarly to Example 1, except heating conditions. The composited multifilamentary wire was passed through the temperature increase zone 10 for 1 minute to be heated up to 1050° C. in 1 minute, held at this temperature in the soaking zone 11 for 1 minute, and thereafter passed through the cooling vessel 13, to be cooled to the room temperature in 30 seconds. 
     The wire treated in the aforementioned manner was further heat treated in a vacuum electric furnace at 750° C. for 50 hours similarly to Example 1, to obtain an Nb 3  Al superconducting wire. 
     EXAMPLE 4 
     The Cu/Nb/Al composite multifilamentary wire described in Example 1 was passed through the tubular electric furnace 12 under a nitrogen atmosphere and the cooling vessel 13 similarly to Example 1, except heating conditions. The composite multifilamentary wire was passed through the temperature increase zone 10 for 2 minutes to be heated up to 1050° C. in 2 minutes, held at this temperature in the soaking zone 11 for 1 minute, and thereafter passed through the cooling vessel 13, to be cooled to the room temperature in 30 seconds. 
     The wire treated in the aforementioned manner was further heat treated in a vacuum electric furnace at 750° C. for 50 hours similarly to Example 1, to obtain an Nb 3  Al superconducting wire. 
     EXAMPLE 5 
     The Cu/Nb/Al composite multifilamentary wire described in Example 1 was passed through the tubular electric furnace 12 under a nitrogen atmosphere and the cooling vessel 13 similarly to Example 1, except heating conditions. The composite multifilamentary wire was passed through the temperature increase zone 10 for 30 seconds to be heated up to 1050° C. in 30 seconds, held at this temperature in the soaking zone 11 for 2 minutes, and thereafter passed through the cooling vessel 13, to be cooled to the room temperature in 30 seconds. 
     The wire treated in the aforementioned manner was further heat treated in a vacuum electric furnace at 750° C. for 50 hours similarly to Example 1, to obtain an Nb 3  Al superconducting wire. 
     EXAMPLE 6 
     The Cu/Nb/Al composite multifilamentary wire described in Example 1 was passed through the tubular electric furnace 12 under a nitrogen atmosphere and the cooling vessel 13 similarly to Example 1, except heating conditions. The composite multifilamentary wire was passed through the temperature increase zone 10 for 30 seconds to be heated up to 1050° C. in 30 seconds, held at this temperature in the soaking zone 11 for 5 minutes, and thereafter passed through the cooling vessel 13, to be cooled to the room temperature in 30 seconds. 
     The wire treated in the aforementioned manner was further heat treated in a vacuum electric furnace at 750° C. for 50 hours similarly to Example 1, to obtain an Nb 3  Al superconducting wire. 
     EXAMPLE 7 
     The Cu/Nb/Al composite multifilamentary wire described in Example 1 was passed through the tubular electric furnace 12 under a nitrogen atmosphere and the cooling vessel 13 similarly to Example 1, except heating conditions. The composite multifilamentary wire was passed through the temperature increase zone 10 for 30 seconds to be heated up to 1050° C. in 30 seconds, held at this temperature in the soaking zone 11 for 10 minutes, and thereafter passed through the cooling vessel 13, to be cooled to the room temperature in 30 seconds. 
     The wire treated in the aforementioned manner was further heat treated in a vacuum electric furnace at 750° C. for 50 hours similarly to Example 1, to obtain an Nb 3  Al superconducting wire. 
     EXAMPLE 8 
     The Cu/Nb/Al composite multifilamentary wire described in Example 1 was passed through the tubular electric furnace 12 under a nitrogen atmosphere and the cooling vessel 13 similarly to Example 1, except heating conditions. The composite multifilamentary wire was passed through the temperature increase zone 10 for 30 seconds to be heated up to 1050° C. in 30 seconds, held at this temperature in the soaking zone 11 for 20 minutes, and thereafter passed through the cooling vessel 13, to be cooled to the room temperature in 30 seconds. 
     The wire treated in the aforementioned manner was further heat treated in a vacuum electric furnace at 750° C. for 50 hours similarly to Example 1, to obtain an Nb 3  Al superconducting wire. 
     EXAMPLE 9 
     The Cu/Nb/Al composite multifilamentary wire described in Example 1 was passed through the tubular electric furnace 12 under a nitrogen atmosphere and the cooling vessel 13 similarly to Example 1, except heating conditions. The composite multifilamentary wire was passed through the temperature increase zone 10 for 30 seconds to be heated up to 1050° C. in 30 seconds, held at this temperature in the soaking zone 11 for 1 minute, and thereafter passed through the cooling vessel 13, to be cooled to the room temperature in 1 minute. 
     The wire treated in the aforementioned manner was further heat treated in a vacuum electric furnace at 750° C. for 50 hours similarly to Example 1, to obtain an Nb 3  Al superconducting wire. 
     EXAMPLE 10 
     The Cu/Nb/Al composite multifilamentary wire described in Example 1 was passed through the tubular electric furnace 12 under a nitrogen atmosphere and the cooling vessel 13 similarly to Example 1, except heating conditions. The composite multifilamentary wire was passed through the temperature increase zone 10 for 30 seconds to be heated up to 1050° C. in 30 seconds, held at this temperature in the soaking zone 11 for 1 minute, and thereafter passed through the cooling vessel 13, to be cooled to the room temperature in 2 minutes. 
     The wire treated in the aforementioned manner was further heat treated in a vacuum electric furnace at 750° C. for 50 hours similarly to Example 1, to obtain an Nb 3  Al superconducting wire. 
     EXAMPLE 11 
     The Cu/Nb/Al composite multifilamentary wire described in Example 1 was passed through the tubular electric furnace 12 under a nitrogen atmosphere and the cooling vessel 13 similarly to Example 1, under the same heating conditions. The composite multifilamentary wire was passed through the temperature increase zone 10 to be heated up to 900° C. in 30 seconds, held at this temperature in the soaking zone 11 for 1 minute, and thereafter passed through the cooling vessel 13, to be cooled to the room temperature in 30 seconds. Thereafter three such wires were stranded up at a strand pitch of 30 mm. 
     The stranded wire obtained in the aforementioned manner was further heat treated in a vacuum electric furnace at 750° C. for 50 hours similarly to Example 1, to obtain an Nb 3  Al superconducting stranded wire. 
     EXAMPLE 12 
     The Cu/Nb/Al composite multifilamentary wire described in Example 1 was passed through the tubular electric furnace 12 under a nitrogen atmosphere and the cooling vessel 13 similarly to Example 1, under the same heating conditions as those in Example 2. The composite multifilamentary wire was passed through the temperature increase zone 10 to be heated up to 1050° C. in 30 seconds, held at this temperature in the soaking zone 11 for 1 minute, and thereafter passed through the cooling vessel 13, to be cooled to the room temperature in 30 seconds. Thereafter three such wires were stranded up at a strand pitch of 30 mm. 
     The stranded wire obtained in the aforementioned manner was further heat treated in a vacuum electric furnace at 750° C. for 50 hours similarly to Example 1, to obtain an Nb 3  Al superconducting stranded wire. 
     EXAMPLE 13 
     Three Cu/Nb/Al composite multifilamentary wires described in Example 1 were stranded up at a strand pitch of 30 mm, to prepare a composite stranded wire. This composite stranded wire was passed through the tubular electric furnace 12 under a nitrogen atmosphere similarly to Example 1, under the same heating conditions. Namely, the stranded wire was passed through the temperature increase zone 10 to be heated up to 900° C. in 30 seconds, held at this temperature in the soaking zone 11 for 1 minute, and thereafter passed through the cooling vessel 13, to be cooled to the room temperature in 30 seconds. 
     The stranded wire treated in the aforementioned manner was further heat treated in a vacuum electric furnace at 750° C. for 50 hours similarly to Example 1, to obtain an Nb 3  Al superconducting stranded wire. 
     EXAMPLE 14 
     Three Cu/Nb/Al composite multifilamentary wires described in Example 1 were stranded up at a strand pitch of 30 mm, to prepare a composite stranded wire. This composite stranded wire was passed through the tubular electric furnace 12 under a nitrogen atmosphere similarly to Example 1, under the same heating conditions as those in Example 2. Namely, the stranded wire was passed through the temperature increase zone 10 to be heated up to 1050° C. in 30 seconds, held at this temperature in the soaking zone 11 for 1 minute, and thereafter passed through the cooling vessel 13, to be cooled to the room temperature in 30 seconds. 
     The stranded wire treated in the aforementioned manner was further heat treated in a vacuum electric furnace at 750° C. for 50 hours similarly to Example 1, to obtain an Nb 3  Al superconducting stranded wire. 
     Comparative Example 
     Three Cu/Nb/Al composite multifilamentary wires described in Example 1 were stranded up at a strand pitch of 30 mm, to prepare a composite stranded wire. This composite stranded wire was heat treated in a vacuum electric furnace at 800° C. for 10 hours, to obtain an Nb 3  Al superconducting stranded wire. 
     Evaluation 
     The Nb 3  Al superconducting wires and the Nb 3  Al superconducting stranded wires according to Examples 1 to 14 and comparative example were subjected to measurement of critical current densities of non-copper portions at 4.2 K and 12 T and upper critical magnetic field at 4.2 K. Table 1 shows the results. 
     
                       TABLE 1______________________________________      Critical current                    Upper Critical      Density       Magnetic Field (4.2K)______________________________________Example 1  680 A/mm.sup.2                    21.5 TExample 2  850 A/mm.sup.2                    22 TExample 3  780 A/mm.sup.2                    21.8 TExample 4  740 A/mm.sup.2                    21.6 TExample 5  790 A/mm.sup.2                    22 TExample 6  680 A/mm.sup.2                    22 TExample 7  530 A/mm.sup.2                    21.8 TExample 8  320 A/mm.sup.2                    21.5 TExample 9  820 A/mm.sup.2                    21.9 TExample 10 780 A/mm.sup.2                    21.7 TExample 11 550 A/mm.sup.2                    21.5 TExample 12 710 A/mm.sup.2                    22 TExample 13 650 A/mm.sup.2                    21.5 TExample 14 820 A/mm.sup.2                    22 TComparative      490 A/mm.sup.2                    20 TExample______________________________________ 
    
     It is clearly understood from Table 1 that the Nb 3  Al superconducting wires and the Nb 3  Al superconducting stranded wires of Examples 1 to 14 according to the present invention have higher critical current densities as compared with the Nb 3  Al superconducting stranded wire of comparative example. 
     While the above description has been made with reference to only a stranded wire, the present invention is also applicable to manufacturing of a coil. Namely, it is possible to manufacture an Nb 3  Al superconducting coil having a high critical current density by coiling an Nb 3  Al superconducting wire which is prepared according to the present invention and thereafter heat treating the same at a low temperature for a long time. 
     According to the present invention, as hereinabove described, it is possible to prepare an Nb 3  Al superconducting wire and an Nb 3  Al superconducting stranded wire having uniform characteristics along overall lengths of the wire and the strands respectively with high critical current densities. Thus, the present invention is also applicable to manufacturing of a large-sized conductor or coil. 
     According to the present invention, further, no wire breaking is caused since it is not necessary to apply severe working on the wire. Thus, the present invention is effective for elongation of the wire, with improvement in productivity. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.