METHOD AND APPARATUS FOR PRODUCING SUPERCONDUCTING Nb3-Al WIRE

Abstract: A very thin superconducting Nb₃-Al wire that exhibits a high current density in an intense magnetic field of the order of 20 teslas. For high degree of stability, the superconducting wire is coated with stabilizing material. A composite multi-conductor wire consisting of many Al or Al alloy members and an Nb matrix is cold-worked and is then immersed in a molten metal in an inert gas atmosphere so that the metal covers the composite wire. Thus, the stabilizing metal is formed to surround the superconducting wire.

Description: TECHNICAL FIELD This invention relates to a Nb 3 Al superconducting wire, and more particularly, to a process for making an electromagnetically stable Nb 3 Al superconducting wire which has a high current density at the high level of magnetic field such as 20 tesla and is coated with a stabilizing material, and an apparatus for making such a Nb 3 Al superconducting wire. BACKGROUND ART As disclosed in Applied Physics Letter 52(20), May 16, 1988, pp. 1724 - 1725, one of conventional Nb 3 Al superconducting wires has been made by cold drawing a composite consisting of a Nb matrix and a large number of Al-alloy cores into a wire having ultrafine multicores and then heat-treating the cold drawn wire at a temperature ranging from 750 to 950 °Cto form a Nb 3 Al multifilamentary wire ( refer to Composite Forming Process). Japanese Laid Open Patent Publication No. 29017/87 discloses a Nb 3 Al superconducting wire made by mixing Nb and Al powders, forming the mixture into a wire, irradiating an electron beam to the formed wire, and heat treating the irradiated wire at a temperature ranging from 500 to 1000 °C to provide a final Nb 3 Al superconducting wire ( refer to Electron Beam Irradiation Process ). The composite forming process described above has several disadvantages,i.e., a stabilizing metal-such as Cu or Al can not be compounded with a matrix; a stoichiometric composition of Nb 3 Al can not be obtained unless an ultrafine filament less than 1 µm in diameter is formed; the Nb 3 Al multifilamentary wire thus produced has a relatively low critical temperature and a critical magnetic field; and the critical current density falls steeply at a high level of magnetic field such as 20 Tesla. The electron beam irradiation process described above can not provide a Nb 3 Al superconducting wire having an electromagnetically stable multifilamentary wire structure. US-A-3710844 discloses a method and apparatus for making a stabilised Nb 3 Al superconductive wire, wherein a Nb 3 Al superconductive wire is dipped into a molten metal, thereby coating the wire with the metal. DISCLOSURE OF INVENTION Accordingly, it is an object of the present invention to provide a process for making an electromagnetically stable Nb 3 Al multifilamentary superconducting wire having high current density at a high level of magnetic field such as 20T, and covered with an electromagnetically stabilizing material, and an apparatus for making such an electromagnetically stable Nb 3 Al multifilamentary superconducting wire. According to the present invention, there is provided a process for making a Nb 3 Al multifilamentary superconducting wire comprising the steps of cold-drawing a composite consisting of an Al or Al alloy core material and a Nb matrix into a composite multicores wire , wherein the cold drawn composite multicore wire is dipped into a molten metal to heat treat the wire to form a stoichiometric composition of Nb 3 Al and to coat the molten metal around the composite multicore wire for forming an electromagnetically stabilizing metal coating on the periphery of the superconducting wire. Each of the Al or Al alloy core wires of the cold drawn composite multicore wire has a diameter or thickness ranging from about 0.1 to about 10 µm, and the molten metal consists of Cu , Cu alloy, Ag or Ag alloy. In the present invention, it is preferred to coat the periphery of the Nb 3 Al superconducting wire with an electromagnetically stabilizing metal by dipping a cold drawn composite multicore wire consisting of a large number of Al alloy core wires and a Nb matrix into a molten metal bath such as Cu or Cu alloy, Ag or Ag alloy heated to a temperature higher than 1200°C for coating the composite wire, rapidly cooling the coated composite wire, and then heat treating the cooled composite wire at a temperature ranging from about 700 to about 950 °C. It is also preferred to increase the critical current density of the Nb 3 Al superconducting wire at the high magnetic field by cold drawing the large number of Al alloy core materials into fine wires within the range of inducing no proximity effect, heating the fine wires to a temperature higher than 1200°C, rapid cooling them, and thereafter heat treating them at a temperature ranging from about 700 to about 950 °C Further, it is preferable to assemble a number of composite multicore wires coated with an electromagnetically stabilizing metal to integrate or strand them for providing an electromagnetically stable Nb 3 Al superconducting material for a large current capacity. In the process of the present invention, a long Nb 3 Al superconducting wire with a high accuracy can be economically produced by dipping a long composite multicore wire consisting of a large number of Al alloy core materials and a Nb matrix into a molten metal bath kept at a predetermined temperature under an inert gas atmosphere of 0.1013 bar (0.1 or more atmospheric pressure). The preferred embodiment of the present invention presents an apparatus used in the process for making Nb 3 Al superconducting wire described above, wherein a spool for winding off a composite multicore wire, a spool for winding up the wound off composite multicore wire and guide rolls for guiding the composite multicore wire are provided within a sealing chamber having an inert gas supply means and a vacuum exhauster and wherein a molten metal bath and a cooling bath are successively arranged between said guide rolls. The present invention can intimately coat the periphery of a Nb 3 Al superconducting wire with an electromagnetically stabilizing metal by dipping a composite multicore wire consisting of a large number of Al core wires and a Nb matrix into a molten metal such as Cu , Cu alloy, Ag or Al alloy kept at a temperature higher than 1200 °C. This process can achieve the heat treatment required for forming a stoichiometric composition of Nb 3 Al and the coating of the electromagnetically stabilizing metal simultaneously. Although a part of the matrix Nb may be eluted into the molten Cu kept at 1200 °C or higher temperatures, the elution of Nb from the matrix can be prevented by preforming a solid solution of Nb with Cu. Ag can not form a solid solution with Nb at temperatures ranging from 1400 to 1700°C and thus the matrix Nb is not eluted into the molten Ag. On the other hand, Cu, Cu alloy, Ag or Ag alloy has a high vapor pressure at high temperatures and thus it is liable to evaporate. This can be inhibited by holding such a molten metal under an inert gas atmosphere of 0.1013 bar (0.1 or more atmospheric pressure). When the reaction of the molten metal with the Nb matrix is unavoidable, it is an effective method to precoat the surface of the Nb matrix with ceramics or metals other than Nb which have a high melting point and are unreactive to Nb and the molten metal. In any event, an excellent Nb 3 Al superconducting wire coated with the stabilizing metal can be obtained by coating the composite multicore wire with the stabilizing metal, rapid cooling the coated wire and thereafter heat treating the cooled wire. Even if such a coated stabilizing metal is Cu-Nb alloy, Nb is little occluded in Cu at temperatures of 700 to 950 °C and thus Nb is crystallized out of the Cu-Nb alloy layer and the coating layer results in Cu having a low electric resistivity thereby maintaining the function as an electromagnetically stabilizing material. When the composite multicore wire has richer Nb than that of stoichiometric composition, a Nb-Al supersaturated solid solution which is rich in Nb and has a body-centered cubic structure can be brought to room temperature by heating the solid solution to temperatures higher than 1200 °C and rapidly cooling it to room temperature. As this Nb-Al supersaturated solid solution has a relatively good workability, it can provide a Nb 3 Al superconducting wire having the high critical current density by heating the solid solution to temperatures ranging from 700 to 950 °Cafter plastic deformation thereof to allow the Nb 3 Al phase of fine crystals to precipitate from the matrix. Considering the proximity effect observed in conventional Nb 3 Al multifilamentary wire, the present invention can provide a Nb 3 Al multifilamentary wire having a high critical temperature at high critical magnetic field by making the spacing between Al alloy core wires small in the range which the proximity effect does not occur, and combining the high temperature heat treatment, rapid cooling and relatively low heat treatment described above. In a case of the Nb 3 Al multifilamentary wire, the critical diameter of the core wires which the proximity effect is initiated is 0.1 µm or less. The object of the present invention can be achieved by making the diameter of the core wires 0.1 µm or more. It is also required to make the core wire of 10 µm or less diameter for providing an electromagnetically stable Nb 3 Al multifilamentary wire. The present invention can provide an electromagnetically stable Nb 3 Al superconducting wire for a large current capacity by assembling a number of Al alloy multicore wires/Nb matrix composite wire coated with an electromagnetically stabilizing metal to integrate or strand them. This is the construction of Nb 3 Al superconducting wire provided for the first time by coating the periphery of the Nb 3 Al core wires/Nb matrix composite wire with an electromagnetically stabilizing metal. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing steps of making a Nb 3 Al multifilamentary wire according to the present invention; Fig. 2 is a block diagram showing steps of making a Nb 3 Al multifilamentary wire according to the conventional method; Fig. 3 shows magnetic field-critical current density curves for Nb 3 Al multifilamentary wires made by present invention or conventional process at 4.2 K; Fig. 4 is a schematic cross-sectional view of stranded seven Nb 3 Al superconducting wires; Fig. 5 is an current-voltage curve of Nb 3 Al multifilamentary wire of the present invention; and Fig. 6 is a schematic diagram of an apparatus for making Nb 3 Al superconducting wires of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION Referring now to the drawings, preferred embodiments of the present invention will be described, compared with conventional methods. Fig. 1 is a block diagram showing steps of making a Nb 3 Al multifilamentary wire according to the present invention. Fig. 2 is a block diagram showing steps of making a Nb 3 Al multifilamentary wire according to the conventional method. An Al core material is combined with Nb pipe to drawing the composite into a fine wire. The process for making the Nb 3 Al multifilamentary wire comprises the steps of combining an Al core material with a Nb pipe, drawing the composite into a composite single-core fine wire, recombining a large number of the drawn composite single-core fine wires with a Nb pipe, and drawing the composite into a composite multicore fine wire. These steps are used both in the present invention and the conventional processes. In the conventional process, a large number of the composite multicore wires are further combined with a Nb pipe and drawing the composite into a composite multicore fine wire, and such steps are repeated until an Al alloy core wire having a diameter of 0.1 p m can be obtained. However, even if the diameter of the Al core wire is several µ m, the present invention can provide a sufficient electromagnetic property to the final product. This is because a phase of Nb 3 Al stable at high temperatures can be brought to room temperature by producing a supersaturated solid solution of stoichiometric Nb 3 Al or rich in Nb than that value and rapid-cooling the solid solution. Further, when the rapid cooled composite multicore wire is a solid solution richer in Nb than that of Nb 3 Al, further drawing of the multicore wire to a fine wire, or assembling a number of the composite fine wires to integrate or strand them can be carried out. Finally, an electromagnetically stable Nb 3 Al multifilamentary superconducting wire coated with an electromagnetically stabilizing metal and having a high current density at a high magnetic field can be obtained by heating the composite multicore wire of the final dimension to a temperature ranging from about 700 to 950 °C. and carrying out rearrangement and fining of Nb 3 Al crystals. Example 1 A composite single-core wire was fabricated by inserting an Al-5 at.% Mg alloy rod of 5.0 mm in diameter into a Nb pipe having the inside diameter of 7.0 mm and the outside diameter of 11.0 mm, and cold drawing the resulting composite into a wire having the outside diameter of 1.0 mm at room temperature. The composite single-core wire of 1.0 mm in diameter was then cut into short pieces and the bundled 331 short cut single-core wires were packed into a Nb pipe of 21.5 mm I.D. and 25.5 mm O.D. and was cold drawn into a wire having the 0.D. of 0.22 mm at room temperature to provide a composite multicore wire. At this time, each of the cold drawn Al-5 at.% Mg core wires has a diameter of about 5 µm. Then, this composite multicore wire was dipped into a molten silver bath which was heated to a temperature of 1700 °C to melt the silver under an argon gas atmosphere of 1.013 bar (1 atmospheric pressure) for ten minutes for covering the composite multicore wire with the silver coating having a thickness of about 50µm, and the composite multicore wire covered with silver was cooled to room temperature. Thereafter, it was heated under vacuum at 850 °C for 50 hrs. to obtained a Nb 3 Al multifilamentary superconducting wire covered with the silver coating. Next, samples were cut from this Nb 3 Al multifilamentary superconducting wire and their critical temperature T c and electric resistance of the matrix were measured at a temperature just above the critical temperature by changing temperatures using resistive method. These samples showed a T c of 17.9 K at the midpoint and a matrix resistivity of 1.6 x 10 -8 ohm cm. Conventional Nb 3 Al multifilamentary wire has a T c of 15.6 K and a matrix resistivity of 5.0 x 10 -7 ohm cm and thus the present multifilamentary wire has 2.3 K for T c and 1/30 for matrix resistivity superior to those of the conventional one, respectively. Their critical currents I c were also measured by changing the electric field at 4.2 K and the results are shown in Fig. 3. The critical current I c is defined as the current at a point of generating an electric field of 1 µV/cm and the critical current density is defined as I c /S, where S is the total cross-sectional area except that of a stabilizing material. Fig. 3 shows magnetic field as critical current density curves, and curve 1 is for the Nb 3 Al multifilamentary wire made according to this example 1 and curve 4 is for that made by conventional composite forming process. It is obvious from Fig. 3 that the present method can provide a Nb 3 Al multifilamentary wire having the critical current density higher than that of conventional method in the entire range of magnetic fields higher than 10 T and in particular, in the range greater than 15 T, for example, it has a critical current density of about 100 times that of the conventional one. Example 2 Similarly to Example 2, a composite single-core wire and a composite multicore wire were fabricated and finally a composite multicore wire consisting of 109,561 Al-5 at.% Mg cores each having a 1 µm in diameter and a Nb matrix and having the O.D. of 1 mm was produced. Then this composite multicore wire was dipped into a Cu-10 at.% Nb bath which was heated to 1400°C to melt it under an argon gas atmosphere of 133.3 mbar (100 torr) for ten minutes, to cover the composite multicore wire with the Cu-Nb alloy coating of 1 mm thickness, and then the covered wire was rapidly cooled to room temperature. Thereafter, the cooled wire was heated at 850° C for 50 hrs. under vacuum atmosphere to obtain a copper-coated Nb 3 Al ultrafine multicore superconducting wire. The critical temperature and magnetic field-critical current density characteristics of the samples, and electric resistance of the matrix of the samples were measured in a similar manner as Example 1. The samples showed a critical temperature of 17.3 K which is slightly lower than that of Example 1, and a matrix resistivity of 1.1 x 10 -8 ohm cm which is superior to that of Example 1. The magnetic field-critical current density curve of this sample is shown in Fig.3. The critical current density of this sample shown by curve 2 is remarkably improved, compared with that of the sample made by conventional method and shown by curve 4, but it is slightly lowered than that of Example 1 in the range of magnetic field higher than 17 T. This cause seems that the sample of Example 2 was heat treated at such a lower temperature as 1400 °C, compared with 1700 °C of Example 1. However, this Nb 3 Al multifilamentary superconducting wire has a high current density which can withstand practical use at magnetic field lower than 20 T and has a stable electromagnetic properties. Example 3 In order to produce a composite multicore wire of Nb-Al supersaturated solid solution richer than Nb 3 Al in Nb, a composite single-core wire having a cross-sectional ratio of an Al-5 at.% Mg core material to a Nb matrix different from that of Example 1 was prepared. That is, first a composite single-core wire was produced by inserting an Al-5 at.% Mg alloy rod having a 5.5 mm diameter into a Nb pipe having the inside diameter of 11.0 mm and the outside diameter of 6.0 mm, and cold drawing the composite into a wire having the outside diameter of 1.0 mm at room temperature. Next, similarly to the process of Example 1, the bundled 331 composite single-core wires were packed into a Nb pipe and the composite was cold drawn into a composite multicore wire of a 0.2 mm diameter and comprising the multicore wires each having a 4 µm diameter. Then, this cold drawn composite multicore wire was dipped into a molten silver bath heated to 1700°C for 10 minutes under an argon gas atmosphere of 1 atmospheric pressure for covering the composite multicore wire with a silver coating having about 50 µm thickness and the covered wire was rapidly cooled to room temperature. Finally, the silver coated composite multicore wire having the outside diameter of 0.3 mm was cold drawn into a wire having the outside diameter of 0.2 mm at room temperature. This cold drawing or plastic deformation could be carried out because the core material of the composite multicore wire became Nb-Al supersaturated solid solution instead of A-15 type intermetallic compound. Thereafter, the silver coated composite multicore wire was heated at 850 °C for 10 hrs. under vacuum for precipitating Nb 3 Al intermetallic compound from the Nb-Al supersaturated solid solution to obtain a silver coated Nb 3 Al multifilamentary superconducting wire. Similarly to Examples 1 and 2, the critical temperature and magnetic field-critical current density characteristics of the samples, and the electric resistance of the matrix were measured and the results similar to Example 1 were obtained, for example, the critical temperature is 17.8 K and the resistivity of the matrix is 1.6 x 10 -8 ohm cm. However, as shown in Fig. 3, the magnetic field-critical current density characteristics in this example are different from those of Examples 1 and 2, that is, they are lower than those of Examples 1 and 2, are higher than those of conventional process at high magnetic fields greater than 14 T, and the drop rate in the critical current density is less than those of Examples 1 and 2 even at a magnetic field in the vicinity of 20 T. Such differences in the magnetic field-critical current density characteristics are attributable to the amountof Nb 3 Al compound, deviation from its stoichiometric composition, crystal grain size of Nb 3 Al, etc. In any event, an electromagnetically stable Nb 3 Al multifilamentary superconducting wire coated with an electromagnetically stabilizing metal and having a high current density enough to withstand the practical use at a high magnetic field could be obtained. Example 4 A composite multicore wire of which periphery is coated with a 50 p m thick Ag and which has a Nb-Al supersaturated solid solution was produced by a method similar to that of Example 3. At this time, the core material of the wire consisted of 109,561 wires and each of which had a diameter of 1 µ m. Then, such seven wires were stranded and the stranded wires were passed through a flat-type rolls and a flat-type die to form a flat-type stranded wire having a 5.0 mm width x 2.4 mm thick. Then, the stranded wire was heated at 850 °C for 10 hrs. under vacuum to precipitate a Nb 3 Al intermetallic compound from the Nb-Al supersaturated solid solution, thereby producing a Nb 3 Al multifilamentary superconducting wire. Fig. 4 shows a schematic cross-sectional structure of such a superconducting wire. The Nb 3 Al multifilamentary superconducting wire 5 consists of seven wires and each of which consists of a large number of Nb 3 Al core materials 7, a Nb matrix 8, and a silver coating 6 applied on the periphery of the wire 5. Next, samples were prepared from this superconducting wire and the critical current characteristics at an outer magnetic field of 20 T in liquid He were measured. The result is shown in Fig. 5. Defining a current value at the generation of a resistance of 1.1 x 10 -11 ohm cm as a critical current value, it was 240 amperes. As shown in Fig. 5, the present Nb 3 Al multifilamentary superconducting wire has an electromagnetical stability and large current capacity at the high magnetic field due to coating the wire with a stabilizing material, compared with the conventional wire. Example 5 Fig. 6 shows main components of an apparatus for making an Nb 3 Al superconducting wire according to the present invention. A long composite multicore wire 15 consisting of a large number of Al alloy core wires and a Nb matrix and wound around a spool 14 is passed through a stabilizing metal molten bath 16 and a cooling bath 17, and is wound around a winding spool 18. A molten metal coating apparatus 11 has a vacuum exhauster 12 and an inert gas cylinder 13 to control the pressure in the atmosphere 19 for the molten metal. The temperature for heating the molten metal bath 16 is selected depending on the structural stability of a Nb 3 Al compound and the wettability of the molten stabilizing metal with the Nb 3 Al compound and it requires at least 1200°C or more temperature, and the pressure of the atmosphere 19 is selected depending on a kind of the stabilizing metal and a heating temperature and it requires an inert gas atmosphere of 0.1013 bar (0.1 or more atmospheric pressure). A Ga-In eutectic alloy bath or a variety of salt baths are used for the cooling bath 17 so as to rapidly cool the superconducting wire from 1200 °C or more temperature to a temperature ranging from 700 to 800 °C. INDUSTRIAL APPLICABILITY As described above, the present invention can provide a Nb 3 Al superconducting wire having an ultrafine multicore structure coated with an electromagnetically stabilizing material and a high current density at a high level of magnetic field such as 20 T and thus has the following effectiveness. As this superconducting wire has a high current density at a high level of magnetic field such as 20 T which has been impossible to be realized by using only a superconducting magnet can be economically, produced using a smaller type of apparatus. In addition, the present superconducting wire has a multifilamentary structure coated with an electromagnetically stabilizing material, so that a high level of magnetic field such as 20 T can be stably generated in a short time without transferring to the normal conduction, thereby enabling to provide an economical apparatus with a reduced liquid helium consumption.

Claims: A process for making a Nb 3 Al superconductive wire (5), comprising the steps of cold-drawing an Al or Al-alloy core material and a Nb matrix (8) into a composite multi-core wire; and dipping the composite multi-core wire into a molten metal, thereby heat treating the wire to form a stoichiometric composition of Nb 3 Al (7) and coating the wire with the metal (6) simultaneously. A process according to claim 1, wherein a diameter or thickness of the core material is 0.1 to 10 µm. A process according to claim 1 or claim 2, wherein the wire is rapidly cooled after being dipped into the molten metal at more than 1200°C, and additionally heat-treated at 700 to 950°C. A process according to any preceding claim, wherein the metal is copper, copper-alloy, silver or silver-alloy. A process according to any preceding claim, wherein the molten metal is kept at a constant temperature under an inert gas at at least 0.1013 bar (10% of atmospheric pressure). A process according to any preceding claim, which additionally comprises combining a plurality of the coated wires into an integral or stranded product. Apparatus suitable for use in a process for making a Nb 3 Al superconductive wire as defined in any of claims 1 to 5, which comprises respective spools (18,14) for taking off and winding on the wire, guide rolls for guiding the wire in a closed chamber (11) which includes inert gas supply means (13) and vacuum exhaust means (12), and a bath for molten metal (16) and a cooling bath (17) arranged in series between the guide rolls.

Inventors: INOUE KIYOSHI, KAMATA KUNIHISA HITACHI CABLE, TADA NAOHUMI, WADAYAMA YOSHIHIDE, INOUE, KIYOSHI, KAMATA, KUNIHISA, TADA, NAOHUMI, WADAYAMA, YOSHIHIDE, INOUE, KIYOSHI, TSUKUBA BRANCH OFFICE OF NATIONAL, KAMATA, KUNIHISA, HITACHI CABLE LTD., TADA, NAOHUMI, HITACHI LABORATORY OF HITACHI,LTD., WADAYAMA, YOSHIHIDE, HITACHI LABORATORY OF
Applicants: HITACHI CABLE, HITACHI LTD, JAPAN AS REPRESENTED BY NATION, HITACHI CABLE, LTD., HITACHI, LTD., JAPAN AS REPRESENTED BY NATIONAL RESEARCH INSITUTE FOR METALS
Application year: 1991
Main classification: H01B13
IPC codes: C23C2, H01B13, H01B12, C22F1, B21F19, H01L39, C22C27