Abstract:
This invention relates to an oxide superconducting wire comprising oxide superconducting filaments, a high-density ceramic layer uniformly surrounding each of the filaments, and a silver sheath that directly covers the ceramic layer. The ceramic layer becomes non-superconducting when the filaments are cooled to an operating temperature of oxide superconductors. The oxide superconductors can be isolated by the ceramic that acts as a highly resistive material or an insulator. A high normal resistance is achieved, and thereby AC loss is reduced remarkably.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS  
       [0001]    This application is a division of application Ser. No. 09/938,829, filed Aug. 27, 2001, now pending, and based on Japanese Patent Application No. 2000-258753, filed Aug. 29, 2000, by Naoki AYAI This application claims only subject matter disclosed in the parent application and therefore presents no new matter. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to an oxide superconducting wire and method of producing the same. More particularly, the invention relates to oxide superconducting wires and methods of producing the same that are intended for applications in such fields as electric power supply, transportation, high-energy physics, and medical instruments.  
           [0004]    2. Description of the Background Art  
           [0005]    In recent years, it has been reported that sintered oxide materials exhibit superconducting property at high critical temperatures, and practical applications of superconduction technology are being promoted by utilizing such superconductors. It has also been reported that an yttrium-based oxide brings about superconducting property at the temperature of 90 K and a bismuth-based oxide at the temperature of 110 K. These oxide superconductors are hopefully expected to serve practical use, because they exhibit superconducting property in liquid nitrogen, which can be available at relatively low cost.  
           [0006]    In order to use such a superconductor to carry AC current for electric power supply, for example, the superconductor is formed into a superconducting wire or strand, in which it is covered with a silver sheath that is coated with a high-resistance material having a metal coating thereon.  
           [0007]    Superconducting wires having such structures as described above are disclosed in International Publication No. WO96-28853, and Japanese Patent Application Laid-open Gazette Nos. 11-7846 (Patent Gazette No. 2992502), 10-50152, 10-247428, 3-15116, 11-312420, 3-15115, and 1-140520 (Patent Gazette No. 2877149).  
           [0008]    International Publication No. WO96-28853 discloses a structure of an oxide superconducting wire, wherein each of the oxide superconductors is individually surrounded by, for example, a silver sheath that is surrounded by a metal casing, which is oxidized to form a high-resistance layer between the silver and the casing.  
           [0009]    Japanese Patent Application Laid-open Gazette No. 11-7846 discloses a structure of an oxide high-temperature superconducting wire comprising superconducting ceramic filaments each of which is surrounded by a first layer of silver alloy that is covered with an oxygen-permeable non-superconducting ceramic layer that is surrounded by a second layer of silver alloy.  
           [0010]    Japanese Patent Application Laid-open Gazette No. 10-50152 discloses a structure of an oxide superconducting wire comprising oxide superconducting filaments each of which is surrounded by a silver sheath that is covered with a resistive alloy (a high-resistance material) that is oxidized to form an insulating oxide between the silver sheath and the high resistive alloy.  
           [0011]    Japanese Patent Application Laid-open Gazette No. 10-247428 discloses a structure of a multifilament oxide superconducting wire comprising a plurality of oxide superconducting strands that are individually covered with metal layers in which the resistivity of the outer metal layer is higher than that of the inner metal layer.  
           [0012]    Japanese Patent Application Laid-open Gazette No. 3-15116 discloses an oxide superconducting wire having anisotropy. The wire comprises a core and a superconducting layer thereon. The superconducting layer surrounds the core in such a configuration that a specific crystallographic axis of the layer is oriented toward the core. The superconducting layer is divided into a plurality of segments and provided in configurations elongated in the circumferential direction as compared with the radial directions. This publication also discloses another oxide superconducting wire, wherein a high-resistance layer consisting of a resistive material is placed between a plurality of superconducting layers.  
           [0013]    Japanese Patent Application Laid-open Gazette No. 11-312420 discloses an oxide superconducting wire comprising a plurality of oxide superconductors, each surrounded by a silver-based sheath that is surrounded by a high-resistance layer formed of a material including a heat-resistant ceramic, and further comprising an overall coating made of a material inert to the material of the high-resistance layer in a hot oxidizing atmosphere.  
           [0014]    Japanese Patent Application Laid-open Gazette No. 3-15115 discloses a metal-coated oxide superconducting wire in which an oxidation-resistant metal is used as a cladding.  
           [0015]    Japanese Patent Application Laid-open Gazette No. 1-140520 discloses a technology for subjecting a metal pipe, which is filled with a ceramic powder, first to plastic deformation to reduce its cross sectional area and thereafter to a heat treatment. This application also discloses a method of subjecting a metal pipe to hot plastic deformation so as to produce a small-diameter oxide superconducting wire having sufficient strength and toughness as well as high critical current density and high critical temperature.  
           [0016]    The aforementioned oxide superconducting wires, however, have the following disadvantages.  
           [0017]    A multifilament oxide superconducting wire incorporating silver sheaths is disadvantageous in that AC loss cannot be reduced sufficiently because silver as a sheath has low resistance. In the techniques disclosed in the aforementioned publications, superconducting filaments (covered by silver) are enclosed by barriers consisted of such as metal oxide having the insulating or resistive property for the purpose of the reduction of AC loss. However, it is difficult to make such barriers with a uniform density and thickness in limited spaces between the filaments. This subsequently makes it difficult to increase perpendicular resistance between the filaments to such a desirable level as to reduce AC loss. Furthermore, making the barrier layers thicker to realize sufficient resistance may, in turn, lead to decreasing the ratio of the superconducting filaments contained in the total oxide superconducting wire and subsequently making current density lower.  
           [0018]    Conventionally, barriers of a high-resistance material consisting of, for example, a metal oxide are formed by coating a slurry containing a metal oxide on the outer surface of the silver sheath covering the filaments. Upon coating the slurry, the barrier layers tended to have such a low density that they were prone to irregular deformation and were not formed uniformly.  
           [0019]    Furthermore, Japanese Patent Application Laid-open Gazette No. 63-239741 discloses a method of producing a superconducting wire. In the method, a slurry of an oxide-based superconductor and a slurry of a non-superconducting inorganic material are disposed such that the former is surrounded by the latter, and they are extruded through a die by either pressing or taking advantage of the weight of the slurries to produce a composite wire. After the composite wire is heated for drying and reduced its cross-section, the composite wire is covered with a metal tape, while being twisted at the same time.  
           [0020]    The superconducting wire produced by the aforementioned method, however, can be neither drawn nor rolled any more, as it is covered with a metal tape. This prevents the crystallic axis of the oxide superconductors from being oriented uniformly, lowers critical current, and consequently makes it difficult to apply the superconducting wire to a practical use.  
           [0021]    Further the conventional oxide superconductors were not capable of effectively releasing oxygen and other gases outside that were generated during a reaction process, as the superconductors were coated with silver sheaths. Consequently, as the reaction progressed to some extent, the internal pressure in the filaments increased to cause the crystal grains to be pushed apart, thereby creating interstices within the filaments. Japanese Patent Application Laid-open Gazette No.3-138820 discloses a method of producing an oxide superconductor that has a high critical current density. According to the method, the precursor of oxide superconductor is subjected to a second-stage plastic deformation to eliminate such interstices and increase its density, and then to a second-stage heat treatment. It is impossible, however, to completely bond the plural crystal grains of the oxide superconductor together, once they are formed as a result of the reaction, even if such subsequent second-stage plastic deformation and heat treatment are conducted. In heat treatment, excessively increased temperature for bonding the crystal grains may cause the once-formed oxide superconductor to melt and break up into different phases including a non-superconducting phase. A cross-section reduction ratio of plastic deformation and compressive stress applied to the oxide superconductor, if increased excessively, may create cracks of the crystal grains in the oxide superconductors and increase a portion where the bonding between the grains is not sufficient.  
           [0022]    As described so far, the conventional methods may lower the critical current density of an oxide superconductor depending on the conditions. Even if such conditions are carefully selected to realize the highest critical current density based on the conventional methods, filaments contained in an oxide superconducting wire are aggregation of crystal grains as small as several μm, in which non-superconducting phases remain dispersed. Some crystal grains in a superconducting phase are so weakly bonded that they may show resistance in small magnetic fields and electric currents, thus lowering the critical current density.  
           [0023]    Conventional oxide superconducting wires employed metal pipes, especially expensive silver pipes, considering their effective affinity with the oxide superconductors. For the production of multifilament superconducting wires based on conventional methods, a prolonged process is required, including steps of manufacturing both monofilament and multifilament wires. For the aforementioned reasons, overall production cost has soared and become a major impediment to wider applications of an oxide superconducting wire as an industrial product.  
         SUMMARY OF THE INVENTION  
         [0024]    It is an object of the present invention, by solving the aforementioned problems to provide an improved oxide superconducting wire capable of reducing AC loss, together with a method of producing the same.  
           [0025]    An oxide superconducting wire produced according to the present invention has a structure in which each of oxide superconductors is in contact and coated with a ceramic layer, and the ceramic layer is in contact and covered with a metal sheath. The ceramic layer becomes non-superconductive at the operating temperature of the oxide superconductors.  
           [0026]    The foregoing structure allows the oxide superconductors contained therein to be in contact with the ceramic layer. The ceramic layer functions as either a high-resistant barrier or an insulator that isolates the superconductors, making it possible to achieve a high perpendicular resistance therein. This results in a remarkable reduction of AC loss. Furthermore, the oxide superconductor can exhibit a high critical current density (Je) because it has the same volumetric ratio as in a conventional oxide superconducting wire having a silver sheath.  
           [0027]    The present invention facilitates the release of oxygen and other gases generated during a reaction of the production process because a ceramic enclosing the oxide superconductors is permeable to air to some extent. This enables the superconductive filaments contained therein to achieve a high density and allow crystal grains constituting the filaments to be bonded so strongly that a high critical current density can be realized.  
           [0028]    Moreover, this invention helps reduce the quantity of silver to be used, as the oxide superconductors are closely covered with ceramic layers. And as a result, the reduction of silver usage may also curtail overall production cost extremely.  
           [0029]    The ceramic layer interposed between the oxide superconductors and the metal sheath is also useful to prevent the two from causing atomic diffusion. Therefore, the ceramic layer permits the realization of a high critical current density as well as a wider selection of metals to be used for the metal sheath.  
           [0030]    It is preferable that the oxide superconductors are divided into plural segments, as this can make AC loss even smaller.  
           [0031]    Preferably, the oxide superconductors are formed spirally toward the central axis of the oxide superconducting wire. In this case, further reduction of AC loss can be achieved.  
           [0032]    Preferably, the ceramic layer consists of an oxide containing at least one kind selected from the group consisting of bismuth, lead, strontium, calcium, barium, titanium, niobium, molybdenum, tantalum, tungsten, vanadium, zirconium, copper, and silver. The oxide superconductor includes a bismuth-based superconductor. Bismuth-based oxide superconductors are useful for elevating a critical temperature of the superconductors and making the ceramic layer thereon easy to process. And the filaments, although being directly in contact with the ceramic layer, can be transformed to the oxide superconductors. This enables to form stable ceramic layer that is directly in contact with the oxide superconductors.  
           [0033]    Preferably, the ceramic layer contains an oxide including an alkali earth metal and copper. Such ceramic layer and the oxide superconductors thereunder have almost the same crystal structures, and this combination ensures excellent workability and a uniform sectional structure. As a result, it becomes possible to obtain a superconducting wire whose critical current density is higher and AC loss is smaller.  
           [0034]    Preferably, the metal pipes include at least one kind of metal selected from the group consisting of silver, copper, manganese, magnesium, antimony, iron, chromium, and nickel. As these metals are chemically stable, they react neither with the ceramic layer nor with the oxide superconductors, allowing the superconductors to surely demonstrate superconductivity. Furthermore, the use of manganese, magnesium, antimony, iron, chromium, and nickel can improve the mechanical strength of the oxide superconductors.  
           [0035]    An oxide superconducting wire according to another embodiment of the present invention has a structure in which oxide superconductors are individually surrounded by a ceramic layer produced by an extrusion method, and the ceramic layer, which becomes non-conductive at the operating temperature of the superconductors, is covered with a metal sheath. Extrusion can produce a ceramic layer of high density including no blowholes, and subsequently, AC loss can be reduced. The ceramic layer is inserted into the metal pipe that enables subsequent drawing and rolling. This also makes it possible to orient the direction of crystal grains contained in the oxide superconductors, so that it becomes easier to improve critical current density.  
           [0036]    Preferably, the oxide superconducting wire employs silver-based sheaths interposed between the oxide superconductor and the ceramic layer. Such silver-based sheaths facilitate processing of the oxide superconductors therein.  
           [0037]    Preferably, the oxide superconductors are in contact and coated with the ceramic layer.  
           [0038]    The present invention provides methods of producing an oxide superconducting wire as follows:  
           [0039]    Method I includes the steps of:  
           [0040]    a) Forming precursor powders of oxide superconductor into a rod  
           [0041]    b) Forming a ceramic powder into a layer on the surface of the rod to produce a ceramic-coated rod  
           [0042]    c) Inserting a plurality of the ceramic-coated rods into a metal pipe to produce a multifilament billet  
           [0043]    d) Subjecting the multifilament billet to plastic deformation to produce a multifilament wire  
           [0044]    e) Subjecting the multifilament wire to heat treatment to produce an oxide superconductor  
           [0045]    This production method having the foregoing steps permits the formation of a ceramic layer on the surface of a rod consisting of precursor powder of oxide superconductor. Unlike conventional techniques, this production method does not form silver coatings directly on the surface of the rods, and yet helps improve the perpendicular resistance of the oxide superconductors to make AC loss smaller.  
           [0046]    Method □ 
           [0047]    a) Forming ceramic powders into a ceramic pipe  
           [0048]    b) Filling precursor powders of oxide superconductors in the ceramic pipe to produce a ceramic-coated rod  
           [0049]    c) Inserting a plurality of the ceramic-coated rods into a metal pipe to produce a multifilament billet  
           [0050]    d) Subjecting the multifilament billet to plastic deformation to produce a multifilament wire  
           [0051]    e) Subjecting the multifilament wire to heat treatment to produce an oxide superconductor  
           [0052]    This production method having the foregoing steps permits the formation of a ceramic layer on the surface of a precursor of oxide superconductors. Unlike conventional techniques, this production method does not form silver coatings directly on the surface of the oxide superconductors, and yet helps improve the perpendicular resistance of the oxide superconductors to make AC loss smaller.  
           [0053]    Method □ 
           [0054]    a) Forming ceramic powders into a billet which has a plurality of holes extending in a longitudinal direction  
           [0055]    b) Filling precursor powders of oxide superconductors in the holes of the billet to produce ceramic-coated rods therein  
           [0056]    c) Inserting a plurality of the ceramic-coated rods into a metal pipe to produce a multifilament billet  
           [0057]    d) Subjecting the multifilament billet to plastic deformation to produce a multifilament wire  
           [0058]    e) Subjecting the multifilament wire to heat treatment to produce an oxide superconductor  
           [0059]    This production method having the foregoing steps permits the formation of a ceramic billet providing ceramic powders on the surface of a precursor of oxide superconductors. Unlike conventional techniques, this production method does not form silver layers directly on the surface of the precursor, and yet helps improve the perpendicular resistance of the oxide superconductors and subsequently reduce AC loss.  
           [0060]    Preferably, the precursor powders contain a part of the chemical composition of the oxide superconductors, while the ceramic powders contain the remaining part of the chemical composition of the oxide superconductors.  
           [0061]    Method □ 
           [0062]    a) Filling a precursor powders of oxide superconductor in a silver-based sheath to produce a coated rod  
           [0063]    b) Forming a ceramic powder layer on the surface of the coated rod by extrusion to produce a ceramic-coated rod  
           [0064]    c) Inserting a plurality of the ceramic-coated rods into a metal pipe to produce a multifilament billet  
           [0065]    d) Subjecting the multifilament billet to plastic deformation to produce a multifilament wire  
           [0066]    e) Subjecting the multifilament wire to heat treatment to produce an oxide superconductor  
           [0067]    This production method having the foregoing steps does not produce blowholes in a high-resistant ceramic layer, as the layer is formed from a ceramic powder through extrusion. As a result, the layer can achieve a high density, which is effective in reducing AC loss.  
           [0068]    Preferably, the production method includes an additional step of twisting the multifilament wire before subjecting said wire to heat treatment. This will reduce AC loss furthermore.  
           [0069]    Further preferably, the ceramic layer, the ceramic pipe, or the ceramic billet is produced through extrusion. In this case, the extrusion effectively prevents blowholes from producing in said layers, pipes, or ceramic billets, so that each of them can become to high density. This ensures the formation of a uniform sectional structure, which can subsequently reduce AC loss.  
           [0070]    The oxide superconducting wires produced by the aforementioned methods according to the present invention have a structure such that plural oxide superconducting filaments are inserted in a ceramic matrix that is encased in a metal sheath. As compared with the conventional techniques to produce silver-coated oxide superconducting wires by a powder-in-tube process, the production methods according to the present invention employ ceramic, instead of silver, as a coating of the filaments, which can reduce the required amount of silver roughly by 50% as compared with the conventional methods. Furthermore, the new methods simplify the manufacturing process of a monofilament wire, which can curtail overall production cost. Filaments are surrounded by ceramic matrix that is more permeable to gases than a conventionally used metal. Therefore, oxygen and other gases generated during reaction process can be diffused, and subsequently, the filaments having high density can be obtained. As a result, crystal grains contained in the oxide superconductors are so strongly bonded as to achieve a high critical current density.  
           [0071]    Furthermore, the ceramic matrix effectively serve as barriers that prevent diffusion between the metal pipe and filaments. It is possible, therefore, to employ such materials that could not have been used for the metal pipes, without decreasing critical current density.  
           [0072]    In contrast with the conventional structure in which a layer of a high-resistant material is formed only around the silver matrix surrounding the filaments, the present invention allows a high-resistant layer to be thicker than the conventional one. Such thicker high-resistant layer ensures the isolation of the superconducting filaments and a high perpendicular resistance, subsequently resulting in a remarkable reduction of AC loss. The oxide superconducting wire based on the present invention is capable of maintaining the same volumetric ratio of the oxide superconductor as the conventional oxide superconducting wires with silver sheaths, and so higher current density can be obtained. The ceramic matrix can be made conductive by adding a suitable carrier to the composition thereof mainly consisting of insulating material. This makes it possible to adjust the resistivity to a level that is optimal in terms of both terminal connection and AC loss.  
           [0073]    In the manufacturing methods of the present invention, it is possible to allow an oxide superconducting wire to have a constitution such that a precursor powder of the filaments or a rod formed from the powder contains a portion of the chemical composition of oxide superconductors, while the ceramic matrix enclosing the filaments includes the remaining portion of the composition.  
           [0074]    For example, in the case of producing (Bi, Pb) 2  Sr 2  Ca 2  Cu 3 O X , the precursor powder of the filaments or the rod formed from the powder employs single-phase (Bi, Pb) 2  Sr 2  Ca Cu 2 O X  superconductors, while the ceramic matrix employs a single-phase (Ca, Sr) Cu O 2  compound. In this case, the structure prior to heat treatment at the last stage of producing the wire is such that a plurality of (Bi, Pb) 2  Sr 2  Ca Cu 2 O X  superconducting filaments are embedded in a (Ca, Sr) Cu O 2  matrix that is inserted in a metal pipe or sheath. Therefore, there are generated phase boundaries of (Bi, Pb) 2  Sr 2  Ca Cu 2 O X  and (Ca, Sr) Cu O 2  on the outer circumferences of filaments.  
           [0075]    At the early stage of heat treatment, the diffusion of these boundaries leads to the formation of (Bi, Pb) 2  Sr 2  Ca 2  Cu 3  O X  At this point, cores of the filaments remain to be (Bi, Pb) 2  Sr 2  Ca Cu 2  O X , around which thin phases of (Bi, Pb) 2  Sr 2  Ca 2  Cu 3 O X  are formed, and the thin phases are enclosed by the matrix of (Ca, Sr) Cu O 2 . As diffusion progresses, Ca and Cu are transferred from the matrix to the filament cores consisting of (Bi, Pb) 2  Sr 2  Ca Cu 2 O X , allowing (Bi, Pb) 2  Sr 2  Ca 2  Cu 3 O X  crystals to grow gradually until the entire filaments are occupied by large crystals of (Bi, Pb) 2  Sr 2  Ca 2  Cu 3 O X .  
           [0076]    In conventional methods, wherein a raw material powder, which is adjusted and preliminarily sintered to the finalized composition of (Bi, Pb) 2  Sr 2  Ca 2  Cu 3 O X , is used as a precursor of the filaments, a filament contains a number of different phases of non-superconductivity as well as numerous cores of superconducting phase. As described heretofore, according to progress in diffusion the ratio of (Bi, Pb) 2  Sr 2  Ca 2  Cu 3 O X  contained in the filaments increases while the different phases of non-superconductivity decrease. However, such different phases are not completely eliminated and there still remain some weakly bonded crystals of (Bi, Pb) 2  Sr 2  Ca 2  Cu 3 O X .  
           [0077]    In the method of producing an oxide superconducting wire based on the present invention, a ceramic layer, a ceramic pipe or a ceramic billet can be formed through extrusion, which enables these ceramic bodies to obtain uniform density and thickness. This method also allows the formation of high-density ceramic bodies using a small amount of binder. Even after thermal decomposition occurs in the binders, the ceramic layer finds only a slight decrease in its density. It is possible, therefore, to prevent the occurrence of irregular distortion in a cross-sectional shape at the subsequent plastic deformation process.  
           [0078]    Ceramic-coated rods may be subjected to a specified heat treatment to complete the thermal decomposition of binders before they are bundled to form a multifilament billet. This method allows the thermal decomposition of the binder to be completed uniformly in a short time because the heat treatment can be done in a state where there are no impediments to prevent gases from flowing in and out around ceramic layers. Since the thermal decomposition of binder caused prior to plastic deformation enables a multifilament billet to have a higher filling density, it is possible to obtain a more uniform sectional structure by a subsequent plastic deformation process. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0079]    [0079]FIG. 1 is a cross-sectional view of an oxide superconducting wire produced according to a first embodiment of the present invention.  
         [0080]    [0080]FIG. 2 is a perspective view of a rod formed from precursor powders of oxide superconductor at the first stage of manufacturing the oxide superconducting wire of FIG. 1.  
         [0081]    [0081]FIG. 3 is a perspective view of an equipment employed at the second stage of producing the oxide superconducting wire of FIG. 1.  
         [0082]    [0082]FIG. 4 schematically shows a cross section of the equipment of FIG. 3.  
         [0083]    [0083]FIG. 5 is a perspective view of a ceramic-coated rod produced at the second stage of manufacturing the oxide superconducting wire of FIG. 1.  
         [0084]    [0084]FIG. 6 is a perspective view of a multifilament billet produced at the third stage of manufacturing the oxide superconducting wire of FIG. 1.  
         [0085]    [0085]FIG. 7 is a perspective view of a multifilament round wire produced at the fourth stage of of manufacturing the oxide superconducting wire of FIG. 1.  
         [0086]    [0086]FIG. 8 is a cross-sectional view of an oxide superconducting wire produced according to a second embodiment of the present invention.  
         [0087]    [0087]FIG. 9 is a cross-sectional view of an equipment employed at the first stage of producing the oxide superconducting wire of FIG. 8.  
         [0088]    [0088]FIG. 10 is a perspective view of a ceramic pipe produced at the first stage of manufacturing the oxide superconducting wire of FIG. 8.  
         [0089]    [0089]FIG. 11 is a perspective view of a rod produced at the second stage of manufacturing the oxide superconducting wire of FIG. 8.  
         [0090]    [0090]FIG. 12 is a perspective view of a ceramic-coated rod produced at the third stage of manufacturing the oxide superconducting wire of FIG. 8.  
         [0091]    [0091]FIG. 13 is a perspective view of a multifilament billet produced at the fourth stage of manufacturing the oxide superconducting wire of FIG. 8.  
         [0092]    [0092]FIG. 14 is a perspective view of a multifilament round wire produced at a fifth stage of manufacturing the oxide superconducting wire of FIG. 8.  
         [0093]    [0093]FIG. 15 is a cross-sectional view of an oxide superconducting wire produced according to a third embodiment of the present invention.  
         [0094]    [0094]FIG. 16 is a cross-sectional view of an equipment employed at the first stage of producing the oxide superconducting wire of FIG. 15.  
         [0095]    [0095]FIG. 17 is a perspective view of a ceramic billet produced at the first stage of manufacturing the oxide superconducting wire of FIG. 15.  
         [0096]    [0096]FIG. 18 is a perspective view of a rod produced at the second stage of manufacturing the oxide superconducting wire of FIG. 15.  
         [0097]    [0097]FIG. 19 is a perspective view of a multifilament billet produced at the third and fourth stage of manufacturing the oxide superconducting wire of FIG. 15.  
         [0098]    [0098]FIG. 20 is a perspective view of a multifilament round wire produced at the fifth stage of manufacturing the oxide superconducting wire of FIG. 15.  
         [0099]    [0099]FIG. 21 is a cross-sectional view of an oxide superconducting wire produced according to a fourth embodiment of the present invention.  
         [0100]    [0100]FIG. 22 is a perspective view of a silver-coated rod produced at the second stage of manufacturing the oxide superconducting wire of FIG. 21.  
         [0101]    [0101]FIG. 23 is a perspective view of a ceramic/silver-coated rod produced at the third stage of manufacturing the oxide superconducting wire of FIG. 21.  
         [0102]    [0102]FIG. 24 is a perspective view of a multifilament billet produced at the fourth stage of the manufacturing the oxide superconducting wire of FIG. 21.  
         [0103]    [0103]FIG. 25 is a perspective view of a multifilament round wire produced at the fifth stage of manufacturing the oxide superconducting wire of FIG. 21.  
         [0104]    [0104]FIG. 26 is a cross-sectional view of an oxide superconducting wire produced according to the comparative embodiment 1.  
         [0105]    [0105]FIG. 27 is a perspective view of a silver-coated rod produced at the first stage of manufacturing the oxide superconducting wire of FIG. 26.  
         [0106]    [0106]FIG. 28 is a perspective view of a multifilament billet produced at the second stage of manufacturing the oxide superconducting wire of FIG. 26.  
         [0107]    [0107]FIG. 29 is a perspective view of a multifilament round wire produced at the third stage of manufacturing the oxide superconducting wire of FIG. 26.  
         [0108]    [0108]FIG. 30 is a cross-sectional view of an oxide superconducting wire produced according to the comparative embodiment 2.  
         [0109]    [0109]FIG. 31 is a perspective view of a silver-coated rod produced at the first stage of manufacturing the oxide superconducting wire of FIG. 30.  
         [0110]    [0110]FIG. 32 is a perspective view of a ceramic/silver-coated rod produced at the second stage of manufacturing the oxide superconducting wire of FIG. 30.  
         [0111]    [0111]FIG. 33 is a perspective view of a multifilament billet produced at the third stage of manufacturing the oxide superconducting wire of FIG. 30.  
         [0112]    [0112]FIG. 34 is a perspective view of a multifilament round wire produced at the fourth stage of manufacturing the oxide superconducting wire of FIG. 30. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0113]    Embodiment 1  
         [0114]    [0114]FIG. 1 shows a cross section of an oxide superconductor wire produced in accordance with the first embodiment of the present invention. The oxide superconducting wire  29  comprises oxide superconducting filaments  27 , a ceramic layer  22 , and a silver sheath  24  as a metal pipe. Each of the superconducting filaments  27  is in contact and coated with the ceramic layer  22 . The ceramic layer  22  containing a metal oxide becomes non-conductive at an operating temperature of the filaments  27 . The ceramic layer  22  is in contact and coated with the silver sheath  24 . The oxide superconducting wire  29 , which is formed into a tape, extends from an end whose sectional view is illustrated in FIG. 1 toward the other end not illustrated herein. The filaments  27 , the ceramic layer  22 , and the silver sheath  24  have flat structures elongated laterally. The filaments  27  consist of, for example, (Bi, Pb) 2  Sr 2  Ca 2  Cu 3 O x . The oxide superconducting wire  29  contains a plurality of the filaments  27 . The filaments  27  each enclosed by the ceramic layer  22  are arranged so as to extend spirally around the central axis of the oxide superconducting wire  29 .  
         [0115]    Next, an explanation will be given about the manufacturing method of the oxide superconducting wire shown in FIG. 1. First, a precursor powder of the filaments is formed into a rod  21  shown in FIG. 2.  
         [0116]    As illustrated in FIGS. 3 and 4, the rod  21  is inserted in an extruder  301   a . The extruder  301   a  is provided with a base body  302 , a hopper  311  fitted thereon, an exhaust pipe  304 , a crosshead mandrel  305 , and a screw  303 . The hopper  311  has an upward opening where the precursor powders are fed in a direction indicated with an arrow  311   a . The exhaust pipe  304  is arranged to penetrate the base body  302  and release gases to the direction  304   a , that are generated as a result of compressing ceramic powders  310 . The base body  302  has a hollow structure wherein the screw  303  is installed. The ceramic powder  310  is filled in the base body  302 , surrounding the screw  303 . When the screw  303  rotates, the ceramic powders  310  move toward the direction shown with an arrow. The screw  303  can be replaced with a plunger.  
         [0117]    The rod  21  is inserted into the crosshead mandrel  305  from the direction indicated with an arrow  21   a . At the same time, the screw  303  rotates. The ceramic powders  310  being compressed by the screw  303 , are made to move along and cover the rod  21 . Taking these steps, ceramic-coated rod  23  as illustrated in FIG. 5 is formed, consisting of the rod  21  and ceramic layer  22 .  
         [0118]    [0118]FIG. 6 shows a production state in which a plurality of ceramic-coated rods  23  are inserted into a silver sheath  24  so as to produce a multifilament billet  25 . The multifilament billet  25  is then subjected to heat treatment to cause thermal decomposition of binders contained in the ceramic layer  22 .  
         [0119]    After the thermal decomposition of the binders, the multifilament billet  25  is subjected to plastic deformation for drawing into a multifilament round wire  26  as shown in FIG. 7.  
         [0120]    The multifilament round wire  26  shown in FIG. 7 is then formed into a structure having a rectangular cross section and subjected to heat treatment to transform an oxide superconductor in each filament. This produces an oxide superconducting wire  29  having a structure in which a plurality of oxide superconducting filaments  27  are embedded in the ceramic layer  22  that is surrounded by a silver sheath  24  as shown in FIG. 1.  
         [0121]    Embodiment 2  
         [0122]    [0122]FIG. 8 is a cross section of an oxide superconducting wire produced in accordance with the second embodiment of the present invention. The oxide superconducting wire  39  comprises oxide superconducting filaments  37 , a ceramic layer  31 , and a silver sheath  34  as a metal pipe. The filaments  37  are each in contact and coated with the ceramic layer  31 , which become non-conductive when the filaments  37  are cooled to an operating temperature. The ceramic layer  31  is in contact and coated with the silver sheath  34 . The oxide superconducting wire  39  is a tape shape extending from the end whose cross section is shown in FIG. 8 toward the other direction not illustrated herein. The filaments  37 , the ceramic layer  31  and the silver sheath  34  are formed into tape-shaped structures having a flat cross-section elongated laterally. The filaments  37  consist of, for example, (Bi, Pb) 2  Sr 2  Ca 2  Cu 3 O x . The oxide superconducting wire  39  includes a plurality of the filaments  37  which are each surrounded by the ceramic layer  31 . A plurality of the filaments  37  extend spirally around the central axis of the oxide superconducting wire  39 .  
         [0123]    FIGS.  9  to  14  show a process of producing the oxide superconducting wire of FIG. 8. First, an extruder  301   b  is prepared as shown in FIG. 9. Similar to the extruder  301   a  of FIGS. 3 and 4, the extruder  301   b  is also provided with a hopper  311 , a base body  302 , an exhaust pipe  304 , and a screw  303 . A mandrel  307  is installed on one end of the base body  302 , which contains a cylindrical rod  307   a  in its central part. The base body  302  is filled with ceramic powders  310 . The rotating screw  303  allows the ceramic powders  310  to be extruded from the mandrel  307 , and then a pipe-shaped ceramic layers  31  is formed extending toward the direction indicated with an arrow  31   a  in FIG. 10.  
         [0124]    According to the same method as Embodiment 1, a rod  32  shown in FIG. 11 is formed from a precursor powder to be transformed to an oxide superconducting filament.  
         [0125]    [0125]FIG. 12 shows a ceramic-coated rod  33  having a structure in which the rod  32  is inserted into the pipe-shaped ceramic layer  31 .  
         [0126]    As shown in FIG. 13, a plurality of the ceramic-coated rods  33  are made into a bundle, which is then inserted into a silver-based sheath  34  to form a multifilament billet  35 . The multifilament billet  35  is subjected to heat treatment to cause thermal decomposition of binders contained in the ceramic layers  31 .  
         [0127]    [0127]FIG. 14 illustrates a multifilament round wire  36  formed by drawing the multifilament billet  35  after the thermal decomposition of the binders.  
         [0128]    The multifilament round wire  36  illustrated in FIG. 14 is formed into a shape having a rectangular cross section, and is subjected to a heat treatment so as to convert each filament into an oxide superconductor. This produces an oxide superconducting wire  39  having a structure shown in FIG. 8 in which a plurality of the filaments consisting of oxide superconductors are embedded in the ceramic layer  31  that is encased in the silver sheath  34 .  
         [0129]    Embodiment 3  
         [0130]    [0130]FIG. 15 is a cross section of an oxide superconducting wire produced in accordance with the third embodiment of the present invention. The oxide superconducting wire  49  comprises oxide superconducting filaments  47 , a ceramic layer  48 , and a silver sheath  44  as a metal pipe. The filaments  47  are each in contact and coated with the ceramic layer  48 , which become non-conductive when the filaments  47  are cooled to an operating temperature. The ceramic layer  48  is in contact and coated with the silver sheath  44 . The oxide superconducting wire  49  is a tape shape extending from the end whose cross section is shown in FIG. 15 toward the other direction not illustrated herein. The filaments  47 , the ceramic layer  48  and the silver sheath  44  are formed into tape-shaped structures having a flat cross-section elongated laterally. The filaments  47  consist of, for example, (Bi, Pb) 2  Sr 2  Ca 2  Cu 3  O x . The oxide superconducting wire  49  includes a plurality of the filaments  47  which are each surrounded by the ceramic layer  48 . These filaments  47  are formed into a shape spirally extending around the central axis of the oxide superconducting wire  49 .  
         [0131]    [0131]FIGS. 16 through 20 illustrate a method of producing the oxide superconducting wire of FIG. 15 by Embodiment 3 of the present invention. First, an extruder  301   c  is prepared as shown in FIG. 16. Similar to the other extruder employed in Embodiment 1, this extruder  301   c  is provided with a base body  302 , a hopper  311 , an exhaust pipe  304 , and a screw  303 . A mandrel  308  installed on one end of the base body  302  contains a plurality of cylindrical rods  308   a . The rotating screw  303  allows ceramic powders  310  to be extruded through the mandrel  308  and form into a honeycomb-structured ceramic billet  41 . The honeycomb-structured ceramic billet  41  contains a plurality of holes as indicated in FIG. 17. The holes  41   a  extend in the same direction as does the ceramic billet  41 .  
         [0132]    Secondly, a rod  42  shown in FIG. 18 is formed from precursor powders to be transformed to an oxide superconducting filament, following the same method employed in Embodiment 1.  
         [0133]    As illustrated in FIG. 19, a plurality of the rods  42  are inserted into the holes  41   a  to produce ceramic-coated rods  43 . The ceramic-coated rods  43  are inserted into the silver-based sheath  44  functioning as a metal pipe to produce a multifilament billet  45 . The multifilament billet  45  is subjected to heat treatment to cause the thermal decomposition of binders contained in the ceramic billet  41 .  
         [0134]    As shown in FIG. 20, the multifilament billet  45  is drawn into a multifilament round wire  46  with plastic deformation, after the thermal decomposition of the binders takes place.  
         [0135]    The multifilament round wire  46  indicated in FIG. 20 is formed into a shape having a rectangular cross section and then subjected to heat treatment to transform oxide superconductors inside the filaments. As a result, the oxide superconducting wire  49 , in which a plurality of oxide superconducting filaments  47  shown in FIG. 15 are embedded in the ceramic layer  48  encased in a silver sheath, is produced.  
         [0136]    Embodiment 4  
         [0137]    [0137]FIG. 21 is a cross section of an oxide superconducting wire produced in accordance with the fourth embodiment of the present invention. The oxide superconducting wire  120  comprises oxide superconducting filaments  117 , silver sheaths  110 , a ceramic layer  112 , and a silver sheath  114  as a metal pipe. The filaments  117  are coated with the ceramic layer  112  produced through an extrusion process. The ceramic layer  112  becomes non-conductive at the operating temperature of the filament  117 . The ceramic layer  112  is directly covered with the silver sheath  114 . The silver sheaths  110  are each interposed between the respective filaments  117  and the ceramic layer  112 . The oxide superconducting wire  120  is formed into a tape shape, extending from one end whose cross section is shown in FIG. 21 to the other end not illustrated herein. The filaments  117 , the silver sheaths  110 , the ceramic layer  112 , and the silver sheath  114  are formed into flat and tape-shaped structures, extending laterally. The filaments  117  consist of, for example, (Bi, Pb) 2  Sr 2  Ca 2  Cu 3 O x . The oxide superconducting wire  120  comprises a plurality of the filaments  117  each enclosed by the respective silver sheaths  110 , which are in turn circumferentially coated with the ceramic layer  112 . A plurality of the filaments  117  extend spirally around the central axis of the oxide superconducting wire  120 .  
         [0138]    FIGS.  22  to  25  illustrate a method of producing the oxide superconducting wire of FIG. 21. First, a rod  100  is formed from precursor powders of oxide superconducting filaments. The rod  100  is inserted into a silver sheath  110  to create a silver-coated rod  101  as illustrated in FIG. 22.  
         [0139]    As illustrated in FIG. 23, the ceramic layer  112  is formed on the silver sheath  110  through an extrusion process as in Embodiment 1 to produce a ceramic/silver-coated rod  113 .  
         [0140]    As illustrated by FIG. 24, the ceramic/silver-coated rods  113  are inserted in a silver sheath  114  to create a multifilament billet  115 . The multifilament billet  115  is subjected to heat treatment to cause thermal decomposition of binders contained in the ceramic.  
         [0141]    As indicated in FIG. 25, the multifilament billet  115 , after thermal decomposition, is drawn into a multifilament round wire  116  with plastic deromation.  
         [0142]    The oxide superconducting wire  120 , in which a plurality of oxide superconducting filaments  117  are embedded in the ceramic layer  112  enveloped by the silver sheath  114  as shown in FIG. 21, is produced by forming the multifilament round wire  116  of FIG. 25 into a shape having a rectangular cross section and then subjecting it to a heat treatment to convert the filaments into oxide superconductors.  
       EXAMPLES  
       [0143]    Described below are conventional techniques for producing an oxide superconducting wire.  
       Comparative Embodiment 1  
       [0144]    [0144]FIG. 26 is a cross section of an oxide superconducting wire produced in accordance with the first comparative embodiment. The oxide superconducting wire  109  comprises oxide superconducting filaments  105 , a silver matrix  107 , and a silver sheath  102 . The oxide superconducting filaments  105  are surrounded by the silver matrix  107 , which is in turn encased in the silver-based sheath  102 . A plurality of the filaments  105  are formed such that they extend spirally in the direction that the oxide superconducting wire  109  extends.  
         [0145]    FIGS.  27  to  29  illustrate a method of producing the oxide superconducting wire of FIG. 26. As illustrated in FIG. 27, precursor powders for oxide superconducting filaments are formed into a rod  100 . The rod  100  is enclosed by a silver-based sheath  110  and then drawn into a silver-alloy coated rod  101 .  
         [0146]    As illustrated in FIG. 28, the silver-alloy coated rods  101  are inserted in a silver sheath  102  to form a multifilament billet  103 .  
         [0147]    As shown in FIG. 29, a multifilament round wire  104  is obtained by drawing the multifilament billet  103  with plastic deformation.  
         [0148]    The multifilament round wire  104  of FIG. 29 is formed into a shape having a rectangular cross section and subjected to heat treatment to transform oxide superconductors in the filaments. As a result, the oxide superconducting wire  109  illustrated in FIG. 26, in which a plurality of the oxide superconducting filaments  105  are surrounded by the silver matrix  107  and further circumferentially coated with the silver sheath  102 , is produced.  
       Comparative Embodiment 2  
       [0149]    [0149]FIG. 30 is a cross section of an oxide superconducting wire manufactured in accordance with the second comparative embodiment. The oxide superconducting wire  220  produced in the second comparative embodiment differs from Embodiment 4 in the step of producing ceramic layer  212 . While the ceramic layer  112  is formed by an extrusion method in Embodiment 4, the ceramic layer  212  is produced by a painting method in the second comparative embodiment.  
         [0150]    FIGS.  31  to  34  illustrate a method of producing the oxide superconducting wire  220  of FIG. 30. First, precursor powders for oxide superconducting filaments are formed into a rod  100 . The rod  100  is inserted in a silver sheath  110  to create a silver-coated rod  101 .  
         [0151]    A mixture of ceramics and binders is painted to the surface of the silver sheath  110  to form the ceramic layer  212  thereon, whereby a ceramic/silver-coated rod  113  of FIG. 32 is produced. The volumetric ratio of the binder contained in the mixture is specified as 50%.  
         [0152]    As indicated in FIG. 33, a plurality of the ceramic/silver-coated rods  113  are bundled together and inserted in a silver-based sheath  114  to produce a multifilament billet  115 . The multifilament billet  115  is subjected to heat treatment to cause thermal decomposition of the binders contained in the ceramic layers  212 .  
         [0153]    As illustrated in FIG. 34, the multifilament billet  115 , where thermal decomposition of the binders has completed, is then drawn into a multifilament round wire  116  with plastic deformation.  
         [0154]    The multifilament round wire  116  of FIG. 34 is formed into a wire having a rectangular cross section and subjected to heat treatment to transform oxide superconductors in the filaments. This produces the oxide superconducting wire  220  in which a plurality of the oxide superconducting filaments  117  are each encased in the silver sheath  110 , and further circumferentially coated with the ceramic layer  212  that is enclosed by the silver sheath  114 .  
         [0155]    Preparation  
         [0156]    (1) Samples A to F  
         [0157]    First, samples were prepared as shown in the Table.  
                                                                             TABLE                                               Twist           Samples   Embodiments   Filaments   Layers   Sheaths   1c (77K, OT)   pitch   Normalized AC loss                                A   Comparative   Precursor → (Bi, Pb)   Silver   Silver-based   50A   15 mm   1           Embodiment 1   based 2223       B   Comparative       Silver/(Ca,   Silver   44A       0.86           Embodiment 2       Sr)—Cu—O       C   Embodiment 4           Silver-based   48A       0.53       D   Embodiment 1       (Ca, Sr)—Cu—O       43A       0.45       E   Embodiment 2               51A       0.55       F   Embodiment 3               43A       0.43       G   Embodiment 1   (Bi, Pb)           92A       0.47               based 2212               powders → (Bi, Pb)               based 2223                  
 
         [0158]    As for the preparation of Samples A to F, the oxides or carbonates of Bi, Pb, Sr, Ca, and Cu were mixed according to the composition ratio of (Bi, Pb) 2  Sr 2  Ca 2  Cu 3 O x  superconductors and were repeatedly subjected to heat treatment and pulverization to produce preliminarily sintered powders as their precursors. The superconducting wires were manufactured from the precursors according to the methods of Embodiments 1 to 4 and Comparative Embodiments 1 to 2. In the individual production methods, the wires were twisted prior to the last stage of plastic deformation, that is, before the wires were formed into a structure having a rectangular cross section. The oxide superconducting wires were composed of 61 filaments, and formed to a tape-like shape of 3.5 mm width and 0.23 mm thickness.  
         [0159]    (2) Sample G  
         [0160]    As for Sample G, the oxides or carbonates of Bi, Pb, Sr, Ca, and Cu were mixed according to the composition ratio of (Bi, Pb) 2  Sr 2  Ca 2  Cu 3 O x  superconductors, and were repeatedly subjected to heat treatment and pulverization to produce a powder of the precursor. Ceramic layers employed in Sample G were formed specifically from a (Ca, Sr)—Cu oxide. The precursor and the ceramic layers were used to produce an oxide superconducting wire with the method specified in Embodiment 1. Similar to the aforementioned cases of Samples A to F, the wire produced from Sample G was twisted. The oxide superconducting wire thus obtained was required to contain 61 filaments and have a shape of a tape whose width and thickness were specified respectively as 3.5 mm and 0.23 mm. During the heat treatment conducted on the wire to form said wire into a final shape, a (Ca, Sr)-Cu oxide matrix and the precursor react with each other to produce (Bi, Pb) 2  Sr 2  Ca 2  Cu 3 O x  superconductors.  
         [0161]    Results  
         [0162]    The individual critical current (Ic) of Samples A to G was measured in liquid nitrogen whose temperature was set at 77 K. The individual AC loss of Samples A to G was also measured in liquid nitrogen of 77 K by the magnetization method, wherein an external magnetic field (the magnetic field: 0.1T, frequency: 50 Hertz) was applied individually on the wires in a direction perpendicular to the longitudinal direction thereof and parallel to the flat surfaces thereof. The AC loss of each sample was estimated according to the following formula based on the length and critical current of the respective wire.  
         Wi=(AC loss of Sample i)/[critical current value of Sample  1  length of Sample i] 
         [0163]    The unit of Wi is W/A·m.  
         [0164]    The AC loss values thus calculated for the respective samples were divided by the AC loss value of Sample A according to the following formula, and the values thus obtained were defined as the normalized AC losses.  
         [0165]    Normalized AC loss &lt;Wi&gt;=Wi/WA  
         [0166]    Results of the normalized AC loss are indicated in the Table.  
         [0167]    As can be seen from the Table, Samples A and B of the comparative embodiments exhibited relatively large AC losses, while the AC losses of Samples C to G of the present invention were extremely small. This proves that an oxide superconducting wire having a relatively low AC loss can be produced according to the present invention.  
         [0168]    Aside from the representative implementations herein described, other varied embodiments are also conceivable with regard to the present invention. For example, not only a Bi2223-based oxide superconductor but also a Bi2212-based oxide superconductor can be used.  
         [0169]    Preferably, ceramic powders for enclosing such superconductors include bismuth, lead, strontium, calcium and copper. Most preferably, the ceramic powders contain oxidized calcium, strontium and copper.  
         [0170]    The embodiments herein disclosed are for the purpose of examples in all aspects and should not be considered as excluding other embodiments. The scope of the present invention should be defined by the scope of claims, instead of the foregoing description, and should include any changes made within the scope of equivalency thereof.