Superconducting wire and method of manufacturing the same

Abstract: A superconducting wire comprises an oxide superconductor (31, 33) and a metal core member (32, 34) buried in the oxide. A starting material for forming the oxide superconductor is filled into a metal tube. The metal core member (32,34) is inserted into the tube longitudinally. The filled member is processed into a wire shape and the metal tube is then removed. The resultant wire is subjected to heat treatment to convert the wire into an oxide superconductor.

Description: The present invention relates to a superconducting wire using a compound superconductor and a method of manufacturing the same and, more particularly, to a superconducting wire suitable for an oxide superconductor and a method of manufacturing the same. Conventionally, A15, B1, Chevrel, and Laves metal-based compound superconductors are known. Oxide ceramic-based superconductors having a crystal structure such as perovskite and layered perovskite superconductors are also known. Of these superconductors, a layered perovskite oxide superconductor such as an La-Ba-Cu-O superconductor has a critical temperature of 30K or higher at which the superconductor does not exhibit superconducting characteristics. Of a oxygen deficient perovskite oxide superconductor, e.g., a Y-Ba-Cu-O superconductor, some have critical temperatures exceeding 90K. A variety of applications have been proposed for these oxide ceramic-based superconductors since they have high critical temperatures, and these superconductors are receiving a lot of attention. High-temperature superconductors are applied to, e.g., a superconducting magnet, a transmission line, and the like. These applications are wires, and a demand has arisen for manufacturing a wire using the above-mentioned oxide ceramic-based superconductor. However, since sintering is performed in the manufacture of the oxide superconductor, pellets can only be manufactured. Therefore, the oxide superconductor can hardly be applied to a superconducting magnet and a transmission line. It is an object of the present invention to provide a superconducting wire using a compound superconductor, especially, an oxide superconductor and a method of manufacturing the same. It is another object of the present invention to provide a superconducting wire which allows a current to effectively flow therethrough, and a method of manufacturing the same. It is still another object of the present invention to provide a superconducting coil using an oxide superconducting wire. According to a first aspect of the present invention there is provided a superconducting wire as claimed by claim 1. According to a second aspect of the present invention there is provided a method of manufacturing a superconducting wire as claimed by claim 9. In view of the conflicting European Patent application EP-A-0 304 116 (designated to the contracting states DE, FR, GB, IT, NL and SE), the invention is claimed by a set of claims for CH, and a different set of claims for DE, FR, GB, NL and SE. According to the present invention, since an oxide superconductor is integrated with a core member to form a wire, a superconducting wire using the oxide superconductor can be very easily obtained. Since the wire can be a continuous member, a current can be effectively flowed through the wire. The superconducting wire of the present invention can be wound to obtain a superconducting coil. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a cross sectional view showing a superconducting wire of a first arrangement according to an invention described and claimed in our co-pending European Patent Application No. 88302050.5 (EP-A-0 282 286); and Fig.2 is a sectional view showing a superconducting wire according to the invention. Fig.3 is a sectional view showing a superconducting wire according to an embodiment of the present invention. The present invention will be described in detail with reference to the accompanying drawings. Fig. 1 is a sectional view showing a superconducting wire according to a first configuration of the copending European application. In Fig. 1, reference numeral 1 denotes an oxide superconductor. Oxide superconductor 1 is covered by tubular sheath member 2. Oxide superconductor 1 may be a continuous oxide exhibiting superconducting characteristics, and is normally formed by a sintering powder. Oxide superconductor 1 may be formed of an oxide having a perovskite type structure in a broad sense such as a layered perovskite oxide having a composition represented by (La 1-x M x ) 2 CuO 4 ( x indicates atomic %, and falls within the range of 0 ≦ x < 0.1, and M is an element selected from the group consisting of: Ba, Sr, and Ca), or, a oxygen deficient perovskite oxide having a composition represented by ABa 2 Cu 3 O 7-δ (δ ≦ 0.2, and A is an element selected from the group consisting of: Y, Yb, Ho, Dy, Eu, Sm, Gd, Nd, La, Er, Tm, and Lu). The former has a critical temperature of about 30K, and the latter has a critical temperature of about 90K. In the latter, Ba may be partially substituted with Sr or Ca. The present invention will be described below. As shown in Fig. 2, 1 or more (5 in Fig. 2) metal core members 32 are buried in oxide superconductor 31 along the longitudinal direction of the wire. Oxide superconductor 31 has the same composition as in the first configuration. Each core member 32 is preferably a metal wire having high heat conductivity and electric conductivity, good machinability, and high mechanical resistance. These conditions need not be satisfied by one type of metal, and a plurality of types of metal wires may be bundled or twisted as the metal wire. As core member 32, Cu, Ag, Pu, Pt, Pd, Ti, or their alloys, stainless steel, or the like can be used. Note that as shown in Fig. 3, planar core member 34 may be buried in oxide superconductor 33. In this case, core member 34 can comprise an alloy plate of a plurality of types of thin metal plates. In order to manufacture the superconducting wire described above, the same oxide superconductor material powder as in the first or second configuration or a crystallized perovskite type oxide superconductor powder is inserted in a metal tube together with the core member, and the two ends of the tube are closed by a metal member. Thereafter, following the same procedures as in the first configuration, the outer diameter of the metal tube is reduced to 1/10 or less and preferably, 1/20 or less an original outer diameter, thus forming a wire. Thereafter, the resultant wire may be subjected to rolling as needed. In the above description (see also copending application EP-A-0 282 286), the starting material is inserted in the form of the powder. However, the powder may be compression-molded to prepare a columnar molded body. The molded body may be inserted in the metal tube, and the resultant tube may be subjected to a diameter-reduction process. According to the present invention, the molded body may be manufactured while the core member is buried in the powder. Alternatively, when linear core members are employed, a molded body having a plurality of holes extending along the longitudinal direction is formed, and the core members may be inserted in the holes. When a planar core member is employed, a plurality of molded bodies having shapes divided by the core member are prepared, and after the core member is inserted in the metal tube, the molded bodies may be inserted therein. In these cases, the molding pressure is preferably set to fall within the range of 1 to 5 t/cm 2 (9.807 x 10 7 Nm -2 to 4.9 x 10 8 Nm -2 ). In this manner, when the diameter-reduction process is performed after the superconducting oxide is compression-molded, the packing density of the starting material can be increased, and generation of pores or cracks inside the superconducting wire can be minimized. The material of the metal tube is not particularly limited as long as it has good machinability. However, in view of machinability and economy, Cu is preferable. After the diameter-reduction process, the metal tube is removed from the resultant wire. This process is achieved such that the metal tube is brought into contact with an etching solution such as nitric acid to be etched. The wire without the metal tube is subjected to a heat treatment to obtain a superconducting wire. The heat treatment is preferably performed in an oxygen-containing atmosphere at 850 to 980°C for 8 to 80 hours. Thereafter, the temperature is gradually decreased from 600°C at a low cooling rate of 1°C/min, so that oxygen is introduced in oxygen deficiencies of the oxide superconductor, thus improving the superconducting characteristics. In order to achieve such oxygen introduction, the temperature may be maintained at 300 to 700°C for 3 to 50 hours. As the core members, since Ag, Au, Pt, Pd, or their alloys are not oxidized by a heat treatment, they are more preferable. In contrast to this, if a metal which is easily oxidized, such as Cu or a Cu alloy is employed, an oxygen deficient layer and the like are formed on the surface of the oxide superconductor, and the superconducting characteristics are degraded. However, since these layers do not expand beyond a given thickness, they do not cause disadvantages. In this manner, since core members are provided, the wire can be relatively easily formed into a coil. Example 1 An oxygen-deficient perovskite type superconductor powder was obtained following the following procedure: 2 mol% of a BaCO 3 powder, 0.5 mol% of a Y 2 O 3 powder, and 3 mol% of a CuO powder were sufficiently mixed. The mixture was calcined in air at 900°C for 48 hours to react with each other and ground. Thereafter, the powder starting material was reannealed in an oxygen atmosphere at 900°C for 24 hours to react with each other. After oxygen was introduced to oxygen vacancy, the reannealed body was reground using a ball mill and was classified, thus obtaining a oxygen deficient perovskite superconductor powder (composition ratio of YBa 2 Cu 3 O 7-δ ) having an average grain size of 2 µm and a ratio of diameter to thickness of 3 to 5. The resultant oxide superconductor powder was filled in a copper tube in which five silver wires having a diameter of 3 mm and a length of 100 mm were placed and which had an outer diameter of 20 mm, an inner diameter of 15 mm, and a length of 100 mm and had one end closed by a copper member. The other end of the tube was closed by a copper stop. Thereafter, the resultant wire was cold-worked into a wire having an outer diameter of 2 mm at room temperature by a swaging machine. The copper tube was removed by etching using HNO 3 . The resultant wire was sintered in an oxygen-containing atmosphere at 940°C for 24 hours, and the temperature was then gradually decreased from 600°C at a rate of 1°C/min, thereby obtaining an oxide superconducting wire having a section as shown in Fig. 2. The critical current density of the resultant oxide superconducting wire in an external magnetic field of 0 T at 77K was 5,000 A/cm 2 . When a tension of 200 kgf/cm 2 (1.9614 x 10 7 Nm 2 ) was applied, the critical current density in an external magnetic field of 0 T at 77K was 4,800 A/cm 2 . Thus, the superconducting characteristics was only slightly degraded. Example 2 Following the same procedures as in Example 2, a wire having an outer diameter of 1.8 mm was prepared, except that two thin silver plates having a width of 10 mm, a thickness of 3 mm, and a length of 100 mm, and a single thin silver plate having a width of 15 mm, a thickness of 3 mm, and a length of 100 mm were placed to be parallel to each other in place of the silver wires. Thereafter, the resultant wire was rolled by roller dies to obtain a tape-like member having a thickness of 1 mm and a width of 2.5 mm. Thereafter, the outer copper layer was removed by etching using HNO 3 . The resultant structure was sintered in an oxygen-containing atmosphere at 940°C for 24 hours, and the temperature was then gradually decreased from 600°C at a rate of 1°C/min, thereby obtaining an oxide superconducting wire. The critical current density of the resultant oxide superconducting wire in an external magnetic field of 0 T at 77K was 4,900 A/cm 2 . When a tension of 2,000 kgf/cm 2 (1.9614 x 10 8 Nm -2 ) was applied, the critical current density in an external magnetic field of 0 T at 77K was 4,750 A/cm 2 . Thus, the superconducting characteristics was only slightly degraded.

Claims: A superconducting wire comprising: an oxide superconductor (31, 33); and a plurality of metal core members (32, 34) buried in said oxide superconductor. A superconducting wire according to claim 1, characterized in that said oxide superconductor (31, 33) has a perovskite type crystal structure. A superconducting wire according to claim 2, characterized in that said oxide superconductor (31, 33) contains Cu and at least one element selected from the group consisting of: Ba, Sr, Ca, Y, Yb, Ho, Dy, Eu, Sm, Gd, Nd, La, Er, Tm, and Lu. A superconducting wire according to claim 3, characterized in that said oxide superconductor (31, 33) contains a composition represented by (La 1-x M x ) 2 CuO 4 ( x indicates atomic %, 0 ≦ x < 0.1, and M is an element selected from the group consisting of Ba, Sr, and Ca). A superconducting wire according to claim 3, characterized in that said oxide superconductor (31, 33) contains a composition represented by ABa 2 Cu 3 O 7-δ (δ ≦ 0.2, and A represents an element selected from the group consisting of: Y, Yb, Ho, Dy, Eu, Sm, Gd, Nd, La, Er, Tm, and Lu). A superconducting wire according to claim 1, characterized in that said core member (32, 34) contains a material selected from the group consisting of: Ag, Au, Pt, Pd, Cu, Ti, and their alloys. A superconducting wire according to claim 6, characterized in that said core member (32) has a linear shape. A superconducting wire according to claim 6, characterized in that said core member (34) has a plate-like shape. A method of manufacturing a superconducting wire, comprising the steps of: filling a starting material for forming an oxide superconductor, in a metal tube to form a filled member; inserting a plurality of metal core members (32, 34) in said metal tube along a longitudinal direction of said metal tube; processing said filled member into a wire shape; removing said metal tube from said filled member; and performing a heat treatment on the resultant wire to convert the wire into an oxide superconductor (31, 33) which is conductive enough to effectively transmit a current therethrough. A method according to claim 9, characterized in that said oxide superconductor (31, 33) has a perovskite type crystal structure. A method according to claim 10, characterized in that said oxide superconductor (31, 33) contains Cu and at least one element selected from the group consisting of: Ba, Sr, Ca, Y, Yb, Ho, Dy, Eu, Sm, Gd, Nd, La, Er, Tm, and Lu. A method according to claim 11, characterized in that said oxide superconductor (31, 33) contains a composition represented by (La 1-x M x ) 2 CuO 4 ( x indicates atomic %, 0 ≦ x < 0.1, and M is an element selected from the group consisting of: Ba, Sr, and Ca). A method according to claim 11, characterized in that said oxide superconductor (31, 33) contains a composition represented by ABa 2 Cu 3 O 7-δ (δ ≦ 0.2, and A represents an element selected from the group consisting of: Y, Yb, Ho, Dy, Eu, Sm, Gd, Nd, La, Er, Tm, and Lu). A method according to claim 9 characterized in that said metal core member (32, 34) contains a material selected from the group consisting of: Ag, Au, Pt, Pd, Cu, Ti, and their alloys. A method according to claim 9, characterized in that said starting material of the oxide superconductor is compression-molded, and is inserted in said metal tube in the form of a molded body. A method according to claim 15 , characterized in that said molded body is molded at a pressure of 1 to 5 t/cm 2 . A method according to claim 15, characterized in that said core member (32, 34) is buried in said molded body upon the molding of said starting material. A method according to claim 15, characterized in that a hole is formed in said molded body along a longitudinal direction of the wire, and said core member (32) is inserted in the hole. A method according to claim 9, characterized in that said metal tube is removed by an etching process. A superconducting coil formed by winding a superconducting wire according to any one of claims 1 to 6.

Inventors: FUKUSHIMA NOBURU, KOIZUMI MISAO, MURASE SATORU, NAKAYAMA SHIGEO, NIU HIROMI, YAMADA YUTAKA, YOSHINO HISASHI, FUKUSHIMA, NOBURU, KOIZUMI, MISAO, MURASE, SATORU, NAKAYAMA, SHIGEO, NIU, HIROMI, YAMADA, YUTAKA, YOSHINO, HISASHI, Fukushima, Noburu, c/o Patent Division, Koizumi, Misao, c/o Patent Division, Murase, Satoru, c/o Patent Division, Nakayama, Shigeo, c/o Patent Division, Niu, Hiromi, c/o Patent Division, Yamada, Yutaka, c/o Patent Division, Yoshino, Hisashi, c/o Patent Division
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1988
Main classification: H01L39
IPC codes: H01L39, C04B35