Abstract:
A laminated ceramic superconductor which comprises at least two layers of ceramic superconductor and a stabilizing metal layer interposed between said ceramic superconductor layers, which has improved flexibility and increased critical current density.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a laminated superconductor. More particularly, it relates to a laminated superconductor comprising at least two layers of ceramic superconductor and a stabilizing metal layer interposed between said ceramic superconductor layers, which can be used as a flexible wire for connecting electrical or electric devices or as a conductive wire of a superconducting magnet. 
     As superconductive materials, metallic, ceramic and organic compound types are known. Among them, the ceramic superconductor such as one having a layer perovskite (K 2  NiF 4 ) crystal structure has been found to have a higher critical temperature and is recently very actively studied and developed. For example, an oxide type ceramic superconductor such as [LaSr] 2  CuO 4  and [LaBa] 2  CuO 4  has a critical temperature of higher then 30K. 
     The ceramic superconductor is hard and brittle, and only a very thin film of ceramic superconductor has flexibility. For wiring, the ceramic superconductor should be as thin as about 10 μm. However, such thin film ceramic superconductor cannot have a large critical current density. 
     In addition, the ceramic superconductor is very brittle and its processing is not so easy as metal superconductors. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide a laminated ceramic superconductor with improved flexibility. 
     Another object of the present invention is to provide a laminated ceramic superconductor having good processability. 
     Further object of the present invention is to provide a laminated ceramic superconductor having an increased critical current density. 
     These and other objects are accomplished by a laminated ceramic superconductor which comprises at least two layers of ceramic superconductor and a stabilizing metal layer interposed between said ceramic superconductor layers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view of a conventional laminated ceramic superconductor, 
     FIG. 2 is a cross sectional view of one embodiment of the laminated ceramic superconductor according to the present invention, 
     FIG. 3 is a perspective view of another embodiment of the laminated ceramic superconductor according to the present invention, and 
     FIG. 4 is a cross sectional view of a further embodiment of the laminated ceramic superconductor according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As a ceramic superconductor, any of known ceramic superconductors may be used. Particularly, those having a layer perovskite crystal structure are preferred. Preferred examples of the ceramic superconductors are those having a composition formula of 
     
         (M.sup.1.sub.1-x M.sup.2.sub.x).sub.2 M.sup.3 M.sup.4.sub.4-y (I) 
    
     wherein M 1  is an element selected from the lanthanum series elements, M 2  is an element selected form the elements of the IM, IIA and IIIB groups, M 3  is an element selected from the group consisting of elements of IB, IIB and IIIA groups and transition elements, M 4  is an element selected from the group consisting of oxygen, boron, carbon, nitrogen, fluorine and sulfur, x is a number of larger than 0 (zero) and smaller than 1 (one), and y is a number of not smaller than 0 (zero) and smaller than 4. Typical examples of the ceramic superconductor of the formula (I) are (La 1-x  Sr x ) 2  CuO 4 , (La 1-x  Ba x ) 2  CuO 4 , (Y 1-x  Sr x ) 2  CuO 4  and (Y 1-x  Ba x ) 2  CuO 4 . 
     The layer of ceramic superconductor can be formed by ion beam spattering, multi-element vacuum deposition, plasma CVD, plasma spray coating, and a sol-gel method By these methods, the layer of ceramic superconductor is formed on the stabilizing metal layer without strain. 
     The thickness of ceramic superconductor layer is from 0.1 to 100 μm, preferably from 1 to 50 μm. 
     The stabilizing layer is formed by a conductive metal such as copper, aluminum, silver, gold and the like. The stabilizing metal layer may be supplied in the form of a thin foil or film or formed by any one of the methods which are used for forming the layer of ceramic superconductor 
     The thickness of the stabilizing metal layer is from 10 to 1,000 μm, preferably from 50 to 800 μm. 
     PREFERRED EMBODIMENTS OF THE INVENTION 
     Example 1 
     FIG. 2 shows a cross section of one preferred embodiment of a laminated ceramic superconductor in the sheet form according to the present invention, which comprises a copper tape 1 as a center stabilizing layer, two ceramic superconductor layers 2,2 which sandwich the copper tape 1, and two outer stabilizing copper layers 3,3 formed on the outer surfaces of the respective ceramic superconductor layers 2,2. 
     As the ceramic superconductor, La 1 .6 Sr 0 .4 CuO 4  was deposited on both surfaces of the copper tape 1 by sputtering under following conditions: 
     
         ______________________________________Source materials:          La.sub.1.8 Sr.sub.0.2 Cu.sub.1.5 O.sub.xApparatus:     RF magnetron sputtering apparatusConditions:Substrate temperature:          550° C.Pressure:      1 × 10.sup.-2 Torr          O.sub.2 /(O.sub.2 + Ar) = 10%Time:          17 hours______________________________________ 
    
     The thickness of each ceramic superconductor layer was 0.01 mm. 
     On an outer surface of each ceramic superconductor layer, a stabilizing copper layer having a thickness of 0.02 mm was formed by sputtering under following conditions; 
     
         ______________________________________Apparatus:    DC magnetron sputtering apparatusConditions:Temperature:  Room temperaturePressure:     2 × 10.sup.-2 Torr.DC powder:    100 WattsTime:         25 minutes______________________________________ 
    
     The produced laminated ceramic superconductor exhibited superconductivity stably at about 36K. 
     Example 2 
     FIG. 3 shows a perspective view of another preferred embodiment of a sheet form laminated ceramic superconductor according to the present invention, which comprises three ceramic superconductor layers 11a, 11b,11c each of which is made of a ceramic superconductor having a composition of YBa 2  Cu 3  O 6 .8 and has a thickness of 10 μm and three stabilizing metal layers 12a,12b,12c each of which is made of copper and has a thickness of 100 μm. 
     The laminated ceramic superconductor of FIG. 3 exhibited critical current of 30 A at 77K. 
     COMPARATIVE EXAMPLE 
     For comparison, a laminated ceramic superconductor having a structure shown in FIG. 1 was produced. Said two layer ceramic superconductor consisted of a ceramic superconductor layer 13 which was made of a ceramic superconductor having a composition of YBa 2  Cu 3  O 6 .8 and had a thickness of 30 μm and a stabilizing copper layer 14 which had a thickness of 100 μm. The critical current of the comparative laminated ceramic superconductor was 10 A at 77K. 
     Both the laminated ceramic superconductors of Example 2 and that of Comparative Example could be bent to a radius of curvature of 1 cm without decrease of current capacity. 
     In addition to the above preferred structures, the laminated ceramic superconductor of the present invention may be in the form of a wire as shown in FIG. 4, which comprises a core conductive metal wire 4 made of for example, aluminum, copper and the like, a ceramic superconductor layer 5, a stabilizing aluminum layer 6, a second super conductor layer 7, a stabilizing layer 8, a third ceramic superconductor layer 9 and a stabilizing layer 10.