METHOD OF MANUFACTURING SUPERCONDUCTOR

Abstract: According to this method of manufacturing a superconductor, powder materials of Y₂O₃, BaCO₃ and CuO are first prepared as raw materials and blended and mixed to satisfy composition of Y₁Ba₂Cu₃O x . Mixed powder thus obtained is compression-molded and thereafter sintered. A sintered body thus obtained shows the Meissner effect under the temperature of liquid nitrogen. This sintered body is pulverized into fine particles. A magnetic field is applied to the fine particles under the temperature of liquid nitrogen, thereby to select only superconductive particles. The selected superconductive particles are compression-molded with application of a magnetic field and then sintered, thereby to obtain a superconductor having high critical current density.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method of manufacturing a superconductor having anisotropic conducting axes, and more particularly, it relates to a method of manufacturing a superconductor, which can improve critical current density. Description of the Prior Art In the case of a superconductor consisting of an alloy or an intermetallic compound, its superconductivity is not influenced by azimuths of crystal axes. Therefore it is not necessary to consider such azimuths of crystal axes but the superconductor can be manufactured by various methods such as a powder sintering method, a melting method and a sputtering method, depending on its use. In the case of a ceramic superconductor which is mainly manufactured by the powder sintering method or the like and recently being watched as the one having a high critical temperature, however, its superconductivity is varied with azimuths of crystal axes of crystal grains forming the same, to have anisotropic conducting axes. Thus, it has been recognized that current density of a superconductor cannot be effectively improved unless the same is so manufactured as to orient its crystal axes in a constant direction. As hereinabove described, a ceramic superconductor of a polycrystalline substance manufactured by a conventional method such as the powder sintering method or the melting method is anisotropic with respect to superconductivity, if crystal axes thereof are not oriented in a constant direction. When a current is fed to such a ceramic superconductor of a polycrystalline substance, for example, loss is caused by anisotropy between crystals, to prevent flow of a large current. Therefore, such a superconductor is manufactured by performing heat treatment for monocrystallizing the same or setting a condition of growing a thin film so that crystal axes are oriented in a constant direction in case of forming a thin film, in order to improve current density of the superconductor. In such a method, however, labor is extremely required in order to manufacture conductor materials of high volume or in bulk. A superconductor manufactured by the melting method or the powder sintering method generally consists of a polycrystalline substance, while crystals thereof are randomly arranged. Therefore, heat treatment is repeated on the polycrystalline substance to orient its crystal axes in a constant direction or temperature treatment is performed with a temperature gradient, thereby to bring the entire polycrystalline substance into a completely superconducting state. However, the degree of superconductivity is varied with products in such a method, and hence much time and labor are required in order to manufacture uniform superconductors. General guidelines for techniques to optimize manufacturing are listed in Advanced Ceramic Materials-Ceramic Superconductors, vol. 2, no. 3B, 20 th July 1987, pages 586-592, ACerS, Westerville, OH, US; J.W. Ekin : "Transport critical current in bulk sintered Y₁Ba₂Cu₃O x and possibilities for its enhancement". A use of electromagnetic forces to align the grains of a ceramic superconductor material in the powder state is suggested among several possibilities. SUMMARY OF THE INVENTION Accordingly, the present invention has been proposed to solve the aforementioned problems, and an object thereof is to provide a simple method of manufacturing a superconductor suitable for mass production, which can obtain a superconductor having high critical current density with respect to a material being anisotropic in superconduction within superconductive materials. Provided according to the present invention is a method of manufacturing a superconductor having anisotropic conducting axes, which comprises: (a) a step of blending, mixing and solidifying raw materials for a superconductive material so that a solid body thus obtained is at least partially superconductive; (b) a step of pulverizing the solid body thus solidified into fine particles; (c) a step of applying a magnetic field to the fine particles under a temperature below the critical temperature, thereby to select superconductive particles; and (d) a step of compression-molding the superconductive particles thus selected with application of a magnetic field and sintering the same. In the manufacturing method according to the present invention, a solid body is first manufactured to be at least partially superconductive. This solid body consists of a polycrystalline substance having different azimuths of crystal axes. The solid body is pulverized into fine particles, which are then exposed to a magnetic field under a temperature below the critical temperature for selection processing. At this time, superconductive particles are subjected to strong repulsive force in the magnetic field because of the Meissner effect, which is one of basic properties of superconduction. Consequently, only the superconductive particles can be moved in the magnetic field, to be selected. The superconductive particles thus selected are subjected to compression-molding with application of a magnetic field to be sintered, so that the same are solidified with crystal axes thereof being oriented in a constant direction. A superconductor thus obtained in the form of an aggregate of crystal grains, having conducting axes oriented in a constant direction, is excellent in superconductivity and has particularly high critical current density. In this manufacturing method, the step of mixing and solidifying the raw materials so that the solid body thus obtained is at least partially superconductive may be performed by the powder sintering method of compression-molding powder obtained by stoichiometrically blending raw materials of elements and compounds by press-working, sintering and solidifying the same, or by the melting method of melting/mixing the raw materials and thereafter solidifying the same. If the solid body solidified in the aforementioned manner is pulverized into fine particles of up to about several µm in particle size, most of the particles are in the form of monocrystal grains regardless of crystal grain size. When the particles are subjected to compression-molding with application of a magnetic field after selection, crystal axes of most of the crystal grains are oriented. Thus, the solid body to be subjected to selection of superconductive particles is preferably pulverized into the finest possible particles of 1 to 10 µm in particle size. The present invention is directed to a superconductor of a ceramic compound such as Y-Sr-Cu-O, Ba-Y-Cu-O, La-Sr-Cu-O or La-Ba-Cu-O. However, the method according to the present invention is not restricted to manufacturing of such a ceramic superconductor, but also includes a superconductor having anisotropic conducting axes, which may be developed in the future. These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawing. BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is a process drawing schematically showing a method of manufacturing a superconductor according to the present invention along steps. DESCRIPTION OF THE PREFERRED EMBODIMENT A method of manufacturing a superconductor according to the present invention is carried out schematically along steps as shown in Fig. 1. Description is now made on Example of the present invention. First, powder materials of Y₂O₃, BaCO₃ and CuO were prepared as raw materials for a Y-Ba-Cu-O oxide superconductor. These powder materials were respectively weighed to satisfy composition of Y₁Ba₂Cu₃O x , and thereafter thoroughly mixed with each other. Then mixed powder thus obtained was press-worked under pressure of 1 ton/cm², to be shaped into a prescribed configuration through compression-molding. A molded substance thus obtained was sintered at a temperature of 920°C for 12 hours. A sintered body thus produced was cooled to the room temperature. Examination was made on superconductivity of this sintered body, which showed the Meissner effect at a temperature of 77 K. Then the sintered body was introduced into a ball mill, to be pulverized into fine particles of 3 to 10 µm in particle size. The fine particles were cooled to to the temperature of liquid nitrogen of 77 K, to be introduced on a circular magnet having magnetic flux density of about 0.2T (2000 Gs). Only particles repulsively moved by the Meissner effect were collected by a magnetic field provided around the magnet, while other particles were removed. In such case, the magnetic field to be applied to the fine particles preferably has magnetic flux density of 100 to 0.3T (3000 Gs). If the magnetic flux density is not more than 100 G, repulsive force of the particles caused by the Meissner effect is insufficient. On the other hand, if the magnetic flux density exceeds 0.3T (3000 Gs), the magnetic flux infiltrates into the particles, and the Meissner effect gradually decreases. Only the particles collected in the aforementioned manner were again compression-molded by press working, to be sintered at a temperature of 950°C for 12 hours. In this case, compression-molding was performed while applying a magnetic field having magnetic flux density of about 1T (10⁴ Gs) only in a direction identical to that of pressurization. The magnetic field to be thus applied preferably has magnetic flux density of 1-2, T (10⁴ to 2 x 10⁴ Gs). Crystal axes of respective crystal grains were well-oriented in a sintered body thus finally obtained, while its critical current density J C at a temperature of 77 K and magnetic flux density of 0 T (tesla) was 1200 A/cm², which value was higher than former critical current density J C of 800 A/cm² measured in advance to the aforementioned pulverizing step.

Claims: A method of manufacturing a superconductor having anisotropic conducting axes, comprising: a step of blending, mixing and solidifying raw materials for said superconductor so that a solid body thus obtained at least partially shows superconductivity; a step of pulverizing said solid body thus solidified into fine particles; a step of applying a magnetic field to said fine particles under a temperature below the critical temperature for showing said superconductivity to select superconductive particles; and a step of compression-molding said selected superconductive particles with application of a magnetic field and sintering the same. A method of manufacturing a superconductor in accordance with claim 1, wherein said step of compression-molding and sintering said superconductive particles includes compression-molding of said superconductive particles with application of a magnetic field only in a direction identical to that of pressurization for said compression-molding. A method of manufacturing a superconductor in accordance with claim 1, wherein said step of mixing and solidifying said raw materials includes performing a powder sintering method. A method of manufacturing a superconductor in accordance with claim 1, wherein said step of mixing and solidifying said raw materials includes performing a melting method. A method of manufacturing a superconductor in accordance with claim 1, wherein said superconductor includes a ceramic compound. A method of manufacturing a superconductor in accordance with claim 5, wherein said ceramic compound includes a Y-Ba-Cu-O oxide.

Inventors: KAWASHIMA, MAUMI C/O OSAKA WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES LIMITED
Application year: 1988
Main classification: H01L39
IPC codes: H01B12, B03C1, C01G1, C04B35, H01B13, H01L39