Abstract:
A high Tc superconducting ceramics material is produced by a method in which a mixture of chemicals in suitable amounts is compacted into a desired form. The compact mixture is then fired and, at the same time, a magnetic field is apilied to the compacted mixture in a predetermined direction. By virtue of the application of magnetic field during firing, the orderliness of molecular arrangement is enhanced and an elevated transition temperature Tc is obtained.

Description:
BACKGROUND OF THE INVENTION 
     This invention generally relates to the field of superconducting ceramics and particularly to manufacturing methods for making superconducting ceramics under a magnetic field. 
     It has long been known that metals such as mercury and lead, intermetallics such as NbNd, Nb 3 Ge and NbGa and ternary materials such as Nb 3 (Al 0.8 Ge 0.2 ) demonstrates superconductivity. However, the transition temperature of such long known conventional superconducting materials cannot exceed 25° K. 
     In more recent years, superconducting ceramics have attracted widespread interest. A new material was first reported by researchers at the Zurich laboratory of IBM Corp. as Ba—La—Cu—O-type high temperature superconducting oxides. Subsequently, La—Sr—Cu(II)-O-type superconducting oxides were also proposed. Another type of superconducting material that has been found is (YBa 2 )Cu 3 O 6-8 . By virtue of the fact that these superconducting ceramics form a quasi-molecular atomic unit in a crystalline structure whose unit cell is constructed with one layer in which electrons have essentially one-dimensional motion, whereas three dimensional electron condition occurs in the long known materials mentioned above, higher transition temperatures were achieved. 
     Much work has been undertaken by researchers in the field who have endeavoured to elevate Tco, the temperature at which resistance vanishes, above the levels previously obtained and preferably above the boiling point of nitrogen (77° K) or even higher. As described in our European Patent Application No.87309081.5, we have investigated superconducting ceramic materials having the stoichiometric formulae (A 1−x B x ) y Cu z O w , where A represents one or more elements of Group IIIa of the Periodic Table, e.g., the rare earth elements, and B represents one or more elements of Group IIa of the Periodic Table, e.g. the alkaline earth elements including beryillium and magnesium, and in the continuation of these investigations we have disclosed that the existence of voids and grain boundaries in superconducting ceramic materials makes it difficult to obtain an elevated Tco. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide superconducting ceramics having a higher transition temperature than hitherto and to manufacture superconducting ceramics substantially devoid of imperfection. 
     Whereas in previous attempts to find higher Tc superconducting materials attention has been focussed upon the composition or molar ratios of the constituent elements, in accordance with the present invention there is principally provided an improved process for the manufacture of superconducting ceramics in accordance with which, during the formation of superconducting structure, that is to say the chemical compounds which are mixed together to form the superconducting ceramics, a magnetic field is applied to thereto. The magnetic field is preferably applied in the direction normal to the (a,b) plane. The strength of the applied magnetic field is preferably not lower than 500 Gauss, e.g. 100,000 Gauss. By virtue of the magnetic field the ceramic mixture is given a special orientation in which the atomic arrangement is ordered and made more simple, e.g. on the (a,b) plane, so that few grain boundaries and imperfections exist in the final ceramic material. Accordingly, the higher transition temperature is obtained, and therefore the superconducting ceramics are endowed with a higher critical current. For example, the Tc onset is elevated to 50-107K and the critical current is elevated 10 5  to 10 6  A/cm 2  much higher than prior art has obtained on the order of 10 2  A/cm 2 . The fundamental concept of the present invention is based on the combination effect of the diamagnetism of superconductor and the current induced by the applied magnetic field. 
     Other feature of the invention are set forth with particularity in the appended claims and will become clear to those possessed of the relevant skills form consideration of the following description of exemplary embodiments. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Described hereinafter are a couple of examples illustrating the manufacture by a method according to the present invention of superconducting ceramics conforming to the stoichiometric formulae (A 1−x B x ) y Cu z O w , where A is one or more elements of Group IIIa of the Periodic Table, e.g., the rare earth elements, B is one or more elements of Group IIa of the Periodic Table, e.g., the alkaline earth metals including beryllium and magnesium, and x=0-1; y=2.0-4.0, preferably 2.5-3.5; z=1.0-4.0, preferably 1.5-3.5; and w=4.0-10.0, preferably 6.0-8.0. 
     EXAMPLE 1 
     Prescribed amounts of BaCO 3 , CuO and Y 2 O 3  (High Purity Chemical Industries Co. Ltd. 99.95% or higher) were used for preparing a superconducting ceramic material of the above formula where x=0.67, y=3, z=3 and w=6-9, i.e., in consistence with (YBa 2 )Cu 3 O 6-9 . The factor “w” was controlled by adjusting the sintering condition. 
     After being mixed in a ball mill, the high purity chemicals were pressed in a capsule at 30 Kg/cm and formed into a cylindrical shaped tablet of 5 mm diameter and 15 mm height. The tablet was heated (fired) and oxidized at 500-1200° C., e.g., 700° C. for 8 hours in an oxdizing atmosphere such as in air, this step being called pre-firing hereinafter. 
     The tablet was subjected to a magnetic field during the pre-firing. The magnetic field is induced by an electromagnet whose North pole was positioned proximately to one end of the tablet and whose South pole was positioned proximately to the opposite end to subject the cylinderial tablet to a magnetic field in the axial direction. The strength of the applied magnetic field was 500 Gauss. The application of a stronger magnetic field is preferred if available. 
     Next, the tablet was finely ground in a ball mill so that average grain diameter was 200-0.03 microns, e.g., 10 microns or smaller. Then, the powder was pressed in a capsule at 50 Kg/cm to form a cylindrical tablet again. 
     The tablets were then fired, hereinafter called main-firing, for 10-50 hours at 500-1200° C., for example 15 hours at 900° C. in an oxidizing atmosphere such as in air. Then, the tablets were heated and reduced in an O 2  atmosphere with a 5-20 atom % Ar at 600-1200° C. for 3-30 hours, for example at 800° C. for 20 hours. 
     The tablets were subjected to a magnetic field during the main-firing and reduction in the same manner after the pre-firing for 10 unites or longer. The relationship between the temperature and the resistance was investigated, and as a result the highest Tc onset temperature and Tco were measured to be 101° K and 94° K respectively. The critical current densities were obtained to be as larger as 3×10 5  A/cm 2 . 
     For reference, the manufacturing process was repeated again making no use of magnetic field. As a result, the Tc onset was 93° K and the Tc was 73° K. 
     EXAMPLE 2 
     The procedure of Example 1 was repeated but with 50% of Y substituted by Yb and with an alternating magnetic field of 50 Hz. The Yb was introduced as an oxide. As a result, Tc onset was measured to be 109° K and Tco was 93° K. 
     For reference, this manufacturing process was repeated again making no use of magnetic field. As a result, the Tc onset was 94 °K and the Tc was 81° K. Accordingly, Tc onset and Tc were elevated by 5-12° K respectively. 
     The foregoing Examples are merely illustrative of the invention, and do not include all of the combinations of elements and process variations which may be used to produce superconducting ceramic material in accordance with the invention, other combinations also being effective to provide improved superconducting materials. 
     Superconducting ceramics for use in accordance with the present invention also may be prepared in consistence with the stoichiometric formulae (A 1−x B x ) y Cu z O w , where A is one or more elements of Group Vb of the Periodic Table such as Bi, Sb and As, B is one or more elements of Group IIa of the Periodic Table, e.g., the alkaline earth metals including beryllium and magnesium, and x=0.3-1; y=2.0-4.0, preferably 2.5-3.5; z=1.0-4.0, preferably 1.5-3.5; and w=4.0-10.0, preferably 6.0-8.0, e.g. BiCaSrCu 3 O 6-8 . 
     While for several embodiments have been specifically described, it is to be appreciated that the present invention is not limited to the particular examples described and that modifications and variations can be made without departure from the scope of the invention as defined by the appended claims. For example, The tablet may be pressed in a die at 50 Kg/cm 2  during the main-firing. The constituent atoms are movable more easily at a higher temperature, so that the tablet becomes dense by this press. Hereinabove, superconducting ceramics are prepared in the form of tablets. However, after pre-firing or main-firing, the tablet may be ground to be powder, so that the ground material can be dispersed into solvent. The solution (or suspension) is applied to the surface of an object such as a substrate to form a thin film thereon. After being dried, the film on the surface is fired under a magnetic field in an oxidizing atmosphere to provide a supercoducting thin film. Furthermore, it also effective to apply a magnetic field to a thin film which is being formed by sputtering or spraying. Still further, superconducting films can be formed by ion plating or plasma melting spray method, in which magnetic field is applied thereto during film formation. Anyway, during formation of superconducting ceramics, the well-ordered structure is enhanced if the material which is just being formed into a film is heated, e.g. at 100-900° C. 
     In this description, the term “superconducting” is used to direct the property of material that superconductivity arise when the temperature is below the Tc. Also, Group IIIa is directed to a group consisting of transition elements as shown in “Physics and Chemistry Dictionary” published by Iwanami, Japan. In this connection, Group IIIa consists of typical elements in “The Penguin Dictionary of Science” published by Penguin Books.