OXIDE SUPERCONDUCTOR WIRE, METHOD OF PRODUCING THE SAME AND ARTICLE PRODUCED THEREFROM

Abstract: An oxide superconductor wire obtained by giving compressive prestrain (3, 7, 11,) to an oxide superconductor layer (1) in the lengthwise direction thereof. Even when the oxide superconductor wire is bent, very little tensile force is exerted on the oxide superconductor layer (1) and the superconductivity is not deteriorated. The substrate (2) has a coefficient of thermal expansion greater than that of the oxide superconductor layer (1). The oxide superconductor wire is produced by heat-treating the oxide superconductor layer (1) and the substrate (2) followed by cooling, or by heat-treating the oxide superconductor layer (1) under the condition where the tensile strain (5, 8) is given to the oxide superconductor layer (1) and tensile strain (6) or bending stress (9, 10) is given to the substrate, followed by cooling, in order to impart a compressive force to the oxide superconductor layer (1). The superconductivity is hardly deteriorated even when the oxide superconductor wire is used coils (26, 31) or cables.

Description: TECHNICAL FIELD The present invention relates to a method of manufacturing oxide superconductive wires and to products using the oxide superconductive wires. More specifically, the present invention relates to an improvement for preventing, when an oxide superconductive wire is bent, degradation of superconductivity caused by the bending. BACKGROUND ART Metal superconductors, compound superconductors and oxide superconductors have been known and various applications thereof have been studied. A superconductor has its electrical resistance made zero when it is maintained at a temperature not higher than a critical temperature. Generation of high magnetic field, high density transfer of large current and so on have been tried utilizing this characteristic. Recently, attention is beginning to center on oxide superconductive materials, which have higher critical temperature at which the superconductive phenomenon occurs, e.g. as known from PATENT ABSTRACTS OF JAPAN, vol. 13, no. 99 (E-724), 8 March 1989 & JP-A-63 274 017. Such superconductive materials can be used for power transmission and distribution, electrical connection between various equipment and elements, AC coils and so on, when they are turned into longitudinal wire bodies. Various methods have been known to fabricate wires of oxide superconductive materials. In one method, powder of oxide superconductive material is filled in a metal pipe and the cross section thereof is reduced. In another, a layer of oxide superconductive material is formed on a longitudinal base. Gas phase thin film deposition such as vapor deposition, sputtering and CVD may be applied as a method of forming the oxide superconductive layer. Generally, oxide superconductive materials are weak on strain, especially tensile strain, and when a tensile strain is generated, for example, superconductivity such as critical temperature and current density is significantly degraded. When a longitudinal oxide superconductive wire is bent, a tensile strain is generated in some portion or other inevitably. In order to lengthen the oxide superconductor while suppressing generation of strain such as the tensile strain as much as possible, a method of forming a thin oxide superconductive layer on a fiber-type or film-type thin or narrow flexible base has been known. By this method, the wire can be bent to have smaller diameter with the same allowable strain. From DE-A-1 640 184 a "low" temperature superconductor is known with a multilayer superconductor arrangement symmetrically sandwiched between covering copper layers. The covering copper layer comprises tensile strain while the sandwiched superconducter multilayer structure comprises compressive strain. However, there is a limit in the above described method, and oxide superconductive wires which are stronger against strain have been desired for practical use. EP-A-0 286 135 discloses a method for producing a ceramic oxide superconductor, wherein a superconductor layer is formed on a elongate substrate such as a tape, for example a copper tape, and wherein the superconductor composite material comprising said substrate and said superconductor layer is wound on a take-up reel. The wound composite material is subjected to post-heating. Therefore, an object of the present invention is to provide a method of manufacturing oxide superconductive wires which are stronger against strains. A further object of the present invention is to provide products using the above described oxide superconductive wires. DISCLOSURE OF THE INVENTION These objects are solved by the present invention as defined in claims 1, 2 and 3. The inventors of the present invention have found that the oxide superconductor is weak on tensile strain but relatively strong against compressive strain, and the inventors have attained the present invention based on this finding. The oxide superconductive wire has an oxide superconductive layer formed on a single longitudinal, flexible, tape-type base, wherein the compressive strain is remained in the oxide superconductive layer in the longitudinal direction to solve the above described technical problem. The base used in the present invention is, a tape-type base. Y-Ba-Cu-O, Bi-Sr-Ca-Cu-O, Bi-Pb-Sr-Ca-Cu-O, Tl-Ba-Ca-Cu-O, Tl-Pb-Ba-Ca-Cu-O and other materials are used as the oxide superconductive materials for forming the oxide superconductive layer in the present invention. Any of these materials exhibits superconductivity by heat treatment at 400 to 1000°C. In the present invention, the above mentioned step of heat treatment and a succeeding step of cooling can be advantageously used to provide preliminary strain of compression on the oxide superconductive layer. In the oxide superconductive wire having an oxide superconductive layer formed on a base by painting or gas phase deposition, which may have a stabilizing layer and a protective layer formed in addition to the oxide superconductive layer, the cross sectional area of the base is the largest among those of the oxide superconductive layer, the stabilizing layer and protective layer. Therefore, the strain in the oxide superconductive layer is applied by physical action derived from expansion or contraction of the base itself, such as thermal expansion of the base or contraction which occurs when the base is heated or cooled. In this manner, the preliminary compressive strain applied to the oxide superconductive layer is from the base, and such remaining strain is typically applied by the following methods. Fig. 1 shows a method helpful for understanding the invention. An oxide superconductive layer 1 formed on a base 2 is heat treated and then cooled. At this time, if there is a relation α 1 < α 2 between the thermal expansion coefficient α 1 of the oxide superconductive layer 1 and the thermal expansion coefficient α 2 of the base 2, the preliminary compressive strain is applied to the oxide superconductive layer 1 as shown by an arrow 3 during cooling after heat treatment. Materials of the base 2 satisfying the above mentioned relation α 1 < α 2 comprise zinc, aluminum, indium, silver, tin, lead, aluminum alloy, and copper alloy. Fig. 2 shows a method in accordance with the present invention. A tensile stress is applied as shown by arrows 4a and 4b to the oxide superconductive layer 1 as well as to the base 2. Consequently, a tensile strain is applied to the oxide superconductive layer 1 and the base 2 as shown by arrows 5 and 6. Heat treatment is carried out in this state. By the heat treatment, only the tensile strain in the oxide superconductive layer 1 is released. In order to enable release of the strain only in the superconductive layer, a material such as yttria stabilized zirconia (YSZ) or alumina must be used as the material of the base 2, whose tensile strain is not released under the heat treatment condition applied to the oxide superconductive layer. Then the preliminary compressive strain is applied to the oxide superconductive layer 1 as shown by an arrow 7 when it is cooled. In the method of the present invention described above, the preliminary compressive strain is applied from the base 2 to the oxide superconductive layer 1. Materials of the base 2 comprise zinc, aluminum, indium, silver, tin, lead, copper, and aluminum alloy. Further, the present invention provides a coil or cable using the above described oxide superconductive wires. In these products, the oxide superconductive wire having an oxide superconductive layer formed on one surface of a longitudinal flexible base is bent with the oxide superconductive layer positioned outside and a base positioned inside about the flexural center. Coils using the oxide superconductive wires and, cables having the oxide superconductive wire wound spirally on the surface of a longitudinal body are the claimed products of the above mentioned products. In the oxide superconductive wire employed in the present invention, the oxide superconductive layer included therein has a preliminary compressive strain in the longitudinal direction. When such an oxide superconductive wire is bent, the preliminary compressive strain is either released in accordance with the present invention, or further increased. When the compressive strain of the oxide superconductive layer is released, the factor affecting the superconductivity is reduced, so that degradation of the superconductivity generated when the oxide superconductive wire is bent can be prevented. Even if the compressive strain is further increased, the oxide superconductive layer is relatively strong against compressive strain compared with the tensile strain, and therefore the superconductivity is not very much degraded. Therefore, the oxide superconductive wire can be bent without much degrading the superconductivity when it is coiled, either in the direction releasing the compressive remaining strain or in a direction further increasing the compressive strain. Therefore, treatment of the oxide superconductive wire can be facilitated. As described above, oxide superconductive wires strong against bending can be provided, which are readily used for various magnets, coils, cables and the like. The oxide superconductive wire manufactured in accordance with the present invention can be coiled after heat treatment. Conversely, if heat treatment is to be done after coiling, it is necessary that a bobbin or an insulating layer must be formed of a material which can withstand heat treatment. For the oxide superconductive wire bobbins formed of aluminum alloy, FRP or the like may be used, and common enamel insulation is available. In the product using the above described oxide superconductive wire in accordance with the present invention, an oxide superconductive wire having an oxide superconductive layer formed on one surface of a flexible base with a preliminary compressive strain applied in the longitudinal direction of the oxide superconductive layer is employed. Such an oxide superconductive wire is bent with the oxide superconductive layer positioned outside and the base positioned inside about the flexural center. Therefore, a tensile stress, if any, is applied to the oxide superconductive layer. However, such a tensile stress is merely to release the compressive strain which has been applied to the oxide superconductive layer, and therefore influence of the tensile strain, which is undesirable to the oxide superconductive layer, can be prevented more or less, in accordance with the present invention. Therefore, degradation of superconductivity of the oxide superconductive layer included in the oxide superconductive wire can be prevented. Consequently, in products such as coils and cables using the oxide superconductive wires in the above described manner, the superconductivity of the oxide superconductive wires can be fully utilized. For example, in a compact magnet for investigation, the oxide superconductive wire must be bent to have a radius of curvature as small as about 2 to 3 cm, for example. The oxide superconductive wire can be applied to such usage without problem. In addition, according to the present invention, a cable formed by winding an oxide superconductive wire spirally around a surface of a longitudinal body such as a pipe can be provided. In the case of such a cable, the longitudinal body such as the pipe can give rigid structure, and therefore the oxide superconductive wire is not necessarily be very rigid. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 to 2 show methods of applying preliminary compressive strain to an oxide superconductive layer, respectively; Fig. 3 is a front view showing a bobbin 19 on which an oxide superconductive wire 21 is wound in Example 1 of the present invention; Fig. 4 is an enlarged cross sectional view showing the oxide superconductive wire 21 wound around a core 20 of the bobbin 19 shown in Fig. 3. Fig. 5 is a front view showing a portion of a coil 26 provided by Embodiment 1 of the present invention; Fig. 6 is a cross sectional view showing in enlargement a portion of an oxide superconductive wire 25 included in the coil 26 shown in Fig. 5; Example 1 not being an embodiment of the invention A film of Y-Ba-Cu-O superconductive material was formed to be 1µm in thickness by laser deposition on a base (5 mm in width, 0.1 mm in thickness) of silver with platinum deposited to be 0.1 µm thereon as a diffusion preventing layer. The conditions of film formation were as follows. Target composition: Y 1 Ba 2 Cu 3 O x Film forming temperature: 750°C Gas pressure: 0.1 Torr Gas: O 2 Laser wave length: 193 nm Energy density: 1 J/cm 2 After the film is formed in this manner, the provided oxide superconductive wire was heat treated for 10 minutes at 900°C in a heat treatment chamber, with the speed of movement of the base being 4 cm/hour. Then, in a winding chamber, the oxide superconductive wire 21 was wound for 5 turns on a core 20 of a bobbin 19 as shown in Figs. 3 and 4. At this time, the oxide superconductive wire 21 was wound with the oxide superconductive layer 23 positioned outside and the base 24 positioned inside about the center of the core 20, that is, the flexural center 22, as shown in Fig. 4. The diameter of the core 20 was 40 mm. By the time of the above described step of winding, the heat-treated oxide superconductive wire 21 was cooled, and since the thermal expansion coefficient of the base 24 was larger than that of the oxide superconductive layer 23, a preliminary compressive strain was applied in the longitudinal direction of the oxide superconductive layer 23. The oxide superconductive wire 21 wound around the bobbin 19 was dipped in liquid nitrogen and the critical current was measured, which was 5.5 A. When the oxide superconductive wire provided by Example 1 was cut in an appropriate length without winding, and the critical current in liquid nitrogen of the linear wire was measured, which was 6.0 A. Reference Example 1 The oxide superconductive wire 21 was wound around the core 20 of the bobbin 19 under the same condition as in Example 1 except that the oxide superconductive layer 23 was positioned inside. The critical current of the oxide superconductive wire 21 wound around the bobbin 19 was measured under the same condition, which was 4.75 A. Embodiment 1 An oxide superconductive wire was provided by using the same base and the same film forming conditions as in Example 1. By using the oxide superconductive wire, a superconductive coil was fabricated as will be described below. Referring to Fig. 5, an oxide superconductive wire 25 was wound for 5 layers to provide a coil 26, starting from a circumference distant from the center 27 of the coil 26 by the radius of 20 mm. At this time, the oxide superconductive wire was wound with the oxide superconductive layer 28 positioned outside and the base 29 positioned inside. The coil 26 provided in this manner was dipped in liquid nitrogen, and the critical current was measured, which was 25 A. Reference example 2 A coil was fabricated under the same condition as in Embodiment 1 except that the wire was wound with the oxide superconductive layer 28 positioned inside. The critical current was measured under the same condition, which value was 8.8 A. INDUSTRIAL APPLICABILITY As described above, the oxide superconductive wire can be advantageously applied to products such as superconductive magnets for investigation, magnets for nuclear magnetic resonance diagnostic apparatus, superconductive cables, superconductive generators, superconductive transformers, superconductive coils for linear motor cars, superconductive coils for electromagnetically propelled ships.

Claims: A method of manufacturing an oxide superconductive wire comprising the steps of preparing a single, longitudinal, flexible, tape-type base (2; 12; 24; 29; 34) and forming an oxide superconductive layer (1; 13; 23; 28; 33) on said single base through at least processes of heat treatment and succeeding cooling, with a compressive strain in said oxide superconductive layer of said wire in the longitudinal direction, wherein said compressive strain is applied to said superconductive layer by heat treating said oxide superconductive layer and said base with tensile strain applied to the oxide superconductive layer and the base in a non-bent state to release stress existing in the oxide superconductive layer and by cooling the oxide superconductive layer and said base, based on physical action provided by said base. A coil for generating a magnetic field, comprising an oxide superconductive wire (21; 25; 30) having an oxide superconductive layer (1; 13; 23; 28) formed on a single, longitudinal, flexible, tape-type base, wherein said oxide superconductive layer has a compressive strain in the longitudinal direction of said wire in said coil, and wherein said oxide superconductive wire is bent with said oxide superconductive layer positioned outside and said base positioned inside about a flexural center. A cable comprising an oxide superconductive wire (21; 25; 30) having an oxide superconductive layer (1; 13; 23; 28) formed on a single, longitudinal, flexible, tape-type base, wherein said oxide superconductive layer has a compressive strain in the longitudinal direction of said wire in said cable, and wherein said oxide superconductive wire is wound spirally on a surface of a longitudinal body with said oxide superconductive layer positioned outside and said base positioned inside about a flexural center.

Inventors: HAYASHI N, HITOTSUYANAGI H, OKUDA S, TAKANO S, HAYASHI, N., HITOTSUYANAGI, H., OKUDA, S., TAKANO, S., HAYASHI, N. OSAKA WORKS SUMITOMO ELEC. IND. LTD., HITOTSUYANAGI, H. OSAKA WORKS S. ELEC. IND. LTD., OKUDA, S. OSAKA WORKS SUMITOMO ELEC. IND. LTD., TAKANO, S. OSAKA WORKS SUMITOMO ELEC. IND. LTD.
Applicants: KANSAI ELECTRIC POWER CO, SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD., THE KANSAI ELECTRIC POWER CO., INC.
Application year: 1990
Main classification: H01B12
IPC codes: H01B12, H01B13, H01L39, H01F6