Method of producing oxide superconducting wire

Abstract: A method of producing an oxide superconductor of Tl-Ba-Ca-Cu from a raw material comprises steps of performing first heat treatment on the raw material, performing plastic deformation on the material subjected to the first heat treatment, and performing second heat treatment on the material subjected to the plastic deformation. According to the present invention, denseness and orientation property are so improved that a superconductor having high critical current density can be produced.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method of producing a Tl-Ba-Ca-Cu oxide superconducting. wire Description of the Background Art A specific material exhibits diamagnetism under a superconducting phenomenon such that no potential difference is appeared although a finite stationary current flows in its interior. This superconducting phenomenon is applied to an extremely wide range of fields such as that of electric power fields including MHD power generation, power transmission and magnetic energy storage and that of transportation including a magnetic levitation train and an electromagnetically thrust ship. Further, a high-sensitive sensor for a magnetic field, a high frequency, radiation or the like using the superconducting phenomenon is applied to the field of measurement, and also superconductors are used in the field of including nuclear magnetic resonance (NMR), π-meson remedy and a high energy physical experimental apparatus, while the superconducting phenomenon is also expected in the field of electronics, which is represented by the Josephson device, as a technique which can not only reduce power consumption but implement an element of extremely high-speed operation. Superconductivity was until recently observed only under a very low temperature. Even Nb₃Ge, which has been referred to as that having the highest critical temperature T C of superconductivity within conventional superconducting materials, has an extremely low critical temperature of 23.2 K, which has been regarded as the limit critical temperature of superconduction for a long period of time. Therefore, a superconducting material has been generally cooled to a temperature below the critical temperature T C with liquid helium which boils at 4.2 K, in order to realize a superconducting phenomenon. However, such employment of liquid helium leads to technical and economic burdens due to cooling equipment including Refrigeration equipment, to hinder implementation of the superconducting technique. However, it has been recently reported that a composite oxide sintered body can show superconductivity at a high critical temperature, and development of the superconducting technique is abruptly being prompted with a superconductor whose critical temperature is not very low. It has been reported that a Y-Ba-Cu-O oxide superconductor superconducts at 90 K and a Tl-Ba-Ca-Cu oxide superconductor superconducts at 125 K respectively. Liquid nitrogen is relatively easily obtainable at a low cost, and in fact, development of the superconducting technique has been greatly advanced with discovery of a superconducting material which operates at the temperature of liquid nitrogen. However, in practical application to a superconducting magnet, a wiring member for a device, a power cable or the like, for example, it is necessary to attain not only a high critical temperature but current density exceeding 1000 A/cm², the current density of copper used with liquid nitrogen. EP-A-0 281 474 discloses a process for manufacturing a wire comprising the preparation of a mass of oxide having superconductivity from powder material, covering the mass of oxide with a metal pipe and then reducing the diameter of the metal pipe containing the mass of oxide into a wire form by plastic deformation, preferably by swaging. EP-A-0 302 791 and EP-A-0 308 326 have a publication date between the two priority dates of the present patent application. These applications disclose processes for an elongated sintered article or superconductor, respectively. Both comprise the steps of filling a metal pipe with powdered material, carrying out plastic deformation on the metal pipe, and then subjecting the material powder in deformed metal pipe to sintering operation. Summary of the invention An object of the present invention is to provide a method of producing a superconductor which shows superconductivity under a temperature of at least 100 K, by far exceeding the temperature of liquid nitrogen, and which can increase critical current density. The present invention provides a method of producing an oxide superconducting wire of Tl-Ba-Ca-Cu from a raw material, which method comprises steps of performing first plastic deformation on said raw material, performing first heat treatment on said raw material being subjected to said first plastic deformation, performing second plastic deformation on said material being subjected to said first heat treatment, and performing second heat treatment on said material being subjected to said second plastic deformation, wherein said first heat treatment, said first and second plastic deformation and said second heat treatment are performed in a state enclosed by a metal sheath. According to the inventive producing method, the 2nd plastic deformation step is inserted between the first heat treatment and the second heat treatment to increase denseness of the oxide superconductor as well as to improve its orientation property, thereby to extremely improve current density. An exemplary Tl-Ba-Ca-Cu oxide superconducting wire produced according to the present invention is composed of: Tl k Ba ℓ Ca m Cu n and oxygen where k, ℓ, m and n represent numbers satisfying 1 ≦ k, ℓ, m ≦ 3 and n ≧ 2. In the case of the Tl-Ba-Ca-Cu oxide superconductor having the above composition, the first heat treatment and the second heat treatment are preferably carried out within a temperature range of 800 to 1000°C. An element of the Tl-Ba-Ca-Cu oxide superconductor produced according to the present invention may be partially replaced by another element. For example, Tl may be partially replaced by Pb. A superconductor thus obtained by partially replacing Tl by Pb exhibits high current density. An example of such a superconductor is composed of: (Tl 1-x ,Pb x ) y Ba₂Ca n-1 Cu n and oxygen where x, y and n represent numbers satisfying 0 ≦ x < 1, 0.5 < y < 3 and n ≧ 2. With respect to the above composition, the first heat treatment and the second heat treatment are preferably carried out within a temperature range of 700 to 900°C. In the above composition, further, x preferably satisfies 0 ≦ x ≦ 0.5. In addition, it is preferable that x is around 0.1, n is around 4 and y is around 2 in the above composition, in consideration of blending. A material having such composition is subjected to heat treatment in the air atmosphere or in an oxygen atmosphere, whereby the Tl component volatilizes so that y is around 1. Powder of this material is charged in a metal sheath to provide a wire having excellent characteristics. In the inventive producing method, the first heat treatment, the 1st and 2nd plastic deformation and the second heat treatment are performed in a state enclosed by a metal sheath. In this case, the metal sheath is preferably formed of gold, gold alloy, silver or silver alloy. In the inventive producing method, the 2nd plastic deformation is preferably deformation with a reduction in area of at least 10 % and/or a load of at least 1000 kg/cm² (1 ton/cm²). Further, the second plastic deformation is preferably rolling, pressing and/or wire drawing. According to the present invention, the second plastic deformation is performed after the first heat treatment and then the second heat treatment is performed. In advance of the first heat treatment the raw material is previously subjected to a first plastic deformation. Higher current density can be attained through such a first plastic deformation performed in advance of the first heat treatment. According to the inventive producing method, the second plastic deformation step is inserted between the first heat treatment and the second heat treatment, thereby to increase denseness of the superconductor as well as to improve the orientation property thereof. Thus, a superconductor having higher current density as compared to the prior art can be produced. These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Example I Commercially available Tl₂O₃ powder, BaCO₃ powder, CaCO₃ powder and CuO powder were mixed in the ratios 2:2:2:3. This mixture was pulverized, mixed, calcined at 800°C for eight hours, subjected to heat treatment at 860°C for 16 hours, and charged in a gold pipe of 4 mm in inner diameter and 6 mm in outer diameter, to be drawn into a wire of 1 mm in diameter. Samples of such a wire were subjected to first heat treatment at 860°C for 12 hours in the atmosphere. Thereafter the samples were rolled with drafts of various reductions in sectional area shown in Table 1, and then subjected to second heat treatment at 860°C for 12 hours. Table 1 shows critical temperatures and critical current density values of these samples. The critical current density values were measured under a temperature of 77.3 K. For reference, a sample No. I-8 was prepared by charging a raw material in a gold pipe similarly to the above, drawing the same into a wire of 1 mm in diameter and performing heat treatment on the wire only once. Table 1 also shows the critical temperature and the critical current density of this sample. No. Reduction in Sectional Area (%) Critical Temperature (K) Current Density (A/cm²) I-1 7 118 750 I-2 13 118 1240 I-3 24 118 2050 I-4 30 117 2500 I-5 37 117 2800 I-6 49 117 4560 I-7 73 117 4520 I-8 -- 118 400 As clearly understood from Table 1, the inventive wires of sample Nos. I-1 to I-7 are superior in critical current density to the sample No. I-8 according to the prior art. In particular, those worked with reductions in sectional area of at least 10 %, more preferably at least 20 %, exhibit particularly excellent critical current density values. Example II Description is now made on Example II, in which Tl is partially replaced by Pb. Commercially available Tl₂O₃ powder, PbO powder, BaO powder, CaO powder and CuO powder were mixed in the ratios 1.8:0.2:2:3:4. This mixture was pulverized, mixed, subjected to heat treatment at 870°C for 12 hours, thereafter charged in a silver pipe of 4 mm in inner diameter and 6 mm in outer diameter to be drawn into a wire of 2 mm in diameter, and then rolled into a tape of 0.15 mm in thickness. Samples of such a tape-type wire were subjected to first heat treatment at 780°C for one hour in the atmosphere. Thereafter the samples were rolled with drafts of various reductions in sectional area shown in Table 2, and then subjected to second heat treatment at 780°C for four hours. Table 2 shows critical temperatures of these samples and critical current density values measured at 77.3 K. For reference, a sample No. II-8 was prepared by performing heat treatment only once on a tape-type wire in accordance with a conventional method. Table 2 also shows the critical temperature and the critical current density of this sample. No. Reduction in Sectional Area (%) Critical Temperature (K) Current Density (A/cm²) II-1 5 115 840 II-2 14 115 1980 II-3 23 115 3730 II-4 29 115 4000 II-5 32 115 7020 II-6 51 115 9800 II-7 72 115 9500 II-8 -- 115 700 As clearly understood from Table 2, the inventive wires of sample Nos. II-1 to II-7 exhibit higher current density values as compared with the sample No. II-8 according to the prior art. In particular, the wires worked with reductions in area of at least 10 %, more preferably at least 20 %, exhibit excellent critical current density values. Example III Description is now made on the case of performing plastic deformation on Example III, in which Tl is partially replaced by Pb, with application of a load. Commercially available Tl₂O₃ powder, PbO powder, BaO powder, CaO powder and CuO powder were mixed in the ratios 1.8:0.2:2:3:4. This mixture was pulverized, mixed, subjected to heat treatment at 870°C for 12 hours and charged in a silver pipe of 4 mm in inner diameter and 6 mm in outer diameter to be drawn into a wire of 2 mm in diameter, and further rolled into a tape of 0.15 mm in thickness. Samples of such a tape-type wire were subjected to first heat treatment at 780°C for one hour in the atmosphere. Then the samples were pressed under various loads shown in Table 3, and subjected to second heat treatment at 780°C for four hours. Table 3 shows critical temperatures of these samples and critical current density values measured at 77.3 K. For reference, a sample No. III-8 was prepared by performing heat treatment only once on a tape-type wire in accordance with a conventional method. Table 3 also shows the critical temperature and the critical current density of this sample. No. Load (ton/cm²) Critical Temperature (K) Current Density (A/cm²) III-1 0.5 115 920 III-2 1 115 1360 III-3 5 115 1910 III-4 10 115 4500 III-5 20 115 7800 III-6 50 115 9930 III-7 100 115 9880 III-8 - 115 700 As clearly understood from Table 3, the inventive wires of sample Nos. III-1 to III-7 exhibit higher current density values as compared with the sample No. III-8 according to the prior art. In particular, the wires worked with loads of at least 1 ton/cm², more preferably at least 10 ton/cm², exhibit excellent critical current density values.

Claims: A method of producing an oxide superconducting wire of Tl-Ba-Ca-Cu from a raw material, said method comprising the following steps: performing first plastic deformation on said raw material, performing first heat treatment on said raw material being subjected to said first plastic deformation, performing second plastic deformation on said material being subjected to said first heat treatment and performing second heat treatment on said material being subjected to said second plastic deformation, wherein said first heat treatment, said first and second plastic deformation and said second heat treatment are performed in a state enclosed by a metal sheath. A method of producing an oxide superconducting wire in accordance with claim 1, wherein said oxide superconducting wire of Tl-Ba-Ca-Cu is composed of: Tl k Ba l Ca m Cu n and oxygen where k, l, m and n represent numbers satisfying 1 ≦ k, l, m ≦ 3 and n ≧ 2. A method of producing an oxide superconducting wire in accordance with claim 1 or 2, wherein said first heat treatment and said second heat treatment are carried out within a temperature range of 800 to 1000°C. A method of producing an oxide superconducting wire in accordance with any of claims 1-3, wherein said oxide superconducting wire of Tl-Ba-Ca-Cu is a superconductor in which Tl is partially replaced by Pb. A method of producing an oxide superconducting wire in accordance with any of claims 1-4, wherein said oxide superconducting wire of Tl-Ba-Ca-Cu is composed of: (Tl 1-x ,Pb x ) y Ba₂Ca n-1 Cu n and oxygen where x, y and n represent numbers satisfying 0 ≦ x < 1, 0.5 < y < 3 and n ≧ 2. A method of producing an oxide superconducting wire in accordance with any of claims 4 or 5, wherein said first heat treatment and said second heat treatment are carried out within a temperature range of 700 to 900°C. A method of producing an oxide superconducting wire in accordance with claim 5 or 6, wherein said x satisfies 0 ≦ x ≦ 0.5. A method of producing an oxide superconducting wire in accordance with any of claims 1-7, wherein said second plastic deformation is deformation with a reduction in sectional area of at least 10 %. A method of producing an oxide superconducting wire in accordance with any of claims 1-8, wherein said second plastic deformation is working with a load of at least 1000 kg/cm² (1 ton/cm²). A method of producing an oxide superconducting wire in accordance with any of claims 1-9, wherein said second plastic deformation is rolling, pressing and/or wire drawing. A method of producing an oxide superconducting wire in accordance with any of claims 1-10, wherein said metal sheath is formed of gold, gold alloy, silver or silver alloy.

Inventors: HIKATA TAKESHI C O OSAKA WORKS, SATO KENICHI C O OSAKA WORKS O, TAKEI HIROMI C O OSAKA WORKS O, TORII YASUKO C O OSAKA WORKS O, HIKATA, TAKESHI C/O OSAKA WORKS OF SUMITOMO, SATO, KENICHI C/O OSAKA WORKS OF SUMITOMO, TAKEI, HIROMI C/O OSAKA WORKS OF SUMITOMO, TORII, YASUKO C/O OSAKA WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1989
Main classification: C04B35
IPC codes: H01L39, C04B35