Rare earth superconducting body and process for production thereof

Abstract: A rare earth superconducting composition includes grains of a rare earth superconducting oxide and a noble metal distributed in controlled concentrations in the rare earth superconducting composition. The noble metal may be silver. The noble metal may be present in a plurality of particles dispersed in each of the grains, and some of the particles have diameters up to 5 µm. A process for making a rare earth superconducting composition is disclosed. The method includes the steps of: (1) preparing a compact containing a rare earth superconducting oxide and a compound containing a noble metal wherein the noble metal is distributed in controlled concentrations in the compact; (2) heating the compact so that the compact becomes a semi-molten state; and (3) slowly cooling the compact so that grains of a rare earth superconducting oxide start growing from a portion of the compact in which a concentration of the noble metal is lowest. A seed crystal contacting with the compact may be used in the process. The seed crystal has a partial melting point higher than a partial melting point of the compact, and the seed crystal has a lattice constant similar to a lattice constant of the rare earth superconducting oxide.

Description: The present invention relates to a rare earth superconducting bodies and processes for making the same. Oxide superconductors belonging to the type II superconductor do not show phase transition from a superconducting phase to a normal conduction phase until they reach the second critical magnetic field even though the penetration of magnetic flux begins in the first critical field. Oxide superconductors differ from the type I superconductor wherein the penetration of magnetic flux begins at the critical magnetic field so that the phase undergoes a transition to normal conduction. Further, oxide superconductors have far higher critical temperatures than other superconductive materials such as alloys (e.g. Nb-Ti) and intermetallic compounds (e.g. Nb 3 Sn). Hence, active researches are under way on oxide superconductors in order to improve their critical magnetic fields and critical current density for practical application. Superconductivity is drawing attention because superconductors having superconductivity show complete diamagnetism, i.e. the Meissner effect. The Meissner effect is the most fundamental property possessed by superconductors; that is, any superconductor shows complete diamagnetism when an external magnetic field not larger than the critical magnetic field (the second critical magnetic field in the case of a type II superconductor) is applied thereto, because a supercurrent flows through the surface of the superconductor so as to cancel the external magnetic field and the magnetic flux are unable to enter the inside of the superconductor. Attempts have been made to apply a superconductor of diamagnetism, i.e. the Meissner effect to the development of material for magnetic shielding or material for magnetic levitation. Needless to say, the superconductor to be used in such a material is desired to have a supercurrent as large as possible. Type II superconductors have a property that they show no phase transfer to normal conduction until they reach respective second critical magnetic fields although the penetration of magnetic flux begins at the first critical magnetic fields. Hence, type II superconductors can have higher supercurrents by having a large number of pinning centers to preventing magnetic flux penetrating the superconductor from moving by a Lorenz force even when the penetration of magnetic flux begins in a magnetic field equal to or larger than the first critical magnetic fields. It is known that lattice defects act as pinning centers. Recently, it has been actively studied to obtain a superconductor of improved superconductivity by dispersing fine particles acting as pinning centers in an oxide superconductor. For example, Ogawa, Yoshida and Hirabayashi reported in ISTEC Journal, Vol. 4, No. 3, 1991, p. 3⊘, a process in which a platinum group element is added in a compact as a variation of a melting process. The process comprises heating a mixture of calcined powders of YBa 2 Cu 3 O 7-x and platinum powders to 1,1⊘⊘°C to temperatures higher than an incongruent temperature of YBa 2 Cu 3 O 7-x to put them in a semi-molten state where a solid phase of Y 2 BaCuO 5 and a liquid phase coexist, and slowing cooling the mixture to obtain a rare earth-based oxide superconductor containing platinum. According to the reference, fine particles of unreacted Y 2 BaCuO 5 are dispersed in grains of YBa 2 Cu 3 O 7-x and act as pinning centers, whereby the oxide superconductor shows a high critical current density even in a magnetic field. Also, the present applicant proposed in Japanese Patent Application No. 412529/199⊘ a rare earth-based oxide superconductor and a process for production thereof. The superconductor is obtained by adding a small amount of platinum or rhodium to disperse fine particles of RE 2 BaCuO 5 (RE is Y, Gd, Dy, Ho, Er or Yb) in grains of REBa 2 Cu 3 O 7-x and the fine particles act as pinning centers in the superconductor. Incidentally, when the volume percentage of a material acting as pinning centers is given, the material is desirably dispersed as finely and uniformly as possible. In producing a rare earth-based oxide superconductor by a melting process, there has hitherto been used a so-called temperature gradation process wherein solidification is allowed to proceed in one direction to obtain large grains of high orientation. In this temperature gradation process, a compact of rare earth-based oxide superconductor having uniform quality is molded, then melted, and slowed cooled. During the slowly cooling step the compact is subject to a temperature gradation of one direction to the superconductor and keeping the temperature gradation as each portion of the compact is cooled. Since the above superconductor as starting material is uniform and thus it has the same solidification point at any portion, crystallization starts from the lowest temperature portion and proceeds to higher temperature portions during the slowly cooling step. Alternatively, in S. Morita; S. Takebayashi; M. Tanaka; K. Kimura; K. Miyamoto; K. Sawano in "Advances in Superconductivity III; Proceedings of the 3rd International Symposium on Superconductivity (ISS '9⊘)" K. Kajimura; H. Hayakawa (Eds.), Spring-Verlag: Tokyo; 1991, pp. 733-736, a substitution process as a variation of melting process is reported to obtain a rare earth-based oxide superconductor of high orientation. This substitution process comprises preparing masses of rare earth-based superconducting oxides, each mass containing a different rare earth element and having a different melting point; laminating the masses in layers in such a way that their melting points change gradually to form a laminate, heating the laminate in a semi- molten state and slowly cooling the laminate. For example, a rare earth-based oxide superconductor of high orientation and high crystallinity is obtained by subjecting a laminate of a HoBa 2 CuO 7-x (melting point: 99⊘°C) layer, a YBa 2 Cu 3 O 7-x (melting point: 1,⊘⊘⊘°C) layer and a DyBa 2 Cu 3 O 7-x (melting point: 1,⊘1⊘ °C) layer to sintering and slowly cooling to give rise to crystallization. Also, a rare earth-based oxide superconductor of high crystal orientation is obtained by the above substitution process, using a laminate of II rare earth-based oxide superconductor layers all represented by Y y Yb 1-y Ba 2 Cu 3 O 7-x , in which layers y ranges from ⊘ to 1 at intervals of ⊘.1 and their melting points show a monotonous decrease. A so-called seed crystal process using a seed crystal is known, which is a variation of a melting process. In this seed crystal process, firstly a compact of a rare earth-based oxide superconductor is contacted with a seed crystal. The seed crystal is a rare earth-based oxide superconductor which has the same constitution as the compact except that the rare earth element in the seed crystal is different from the rare earth element of the compact so that the seed crystal has a higher partial melting point than the compact. Then both the compact and the seed crystal are heated to a temperature between the partial melting points of the compact and seed crystal to put only the compact having a lower partial melting point into a semi-molten state. Finally the compact and the seed crystal are slowly cooled to give rise to crystallization starting from the portion of the compact contacting with the seed crystal. The process gives a rare earth-based oxide superconductor of high crystal orientation [S. Morita; S. Takebayashi; M. Tanaka; K. Kimura; K. Miyamoto; K. Sawano in "Advances in Superconductivity III; Proceedings of the 3rd International Symposium on Superconductivity (ISS '9⊘)" K. Kajimura; H. Hayakawa (Eds.), Spring-Verlag: Tokyo; 1991, pp. 733-736]. In Japanese Patent Application No. 35⊘28/1992, the present applicant proposed a seed crystal process using a seed crystal essentially consisting of SrTiO 3 , MgO, LaAlO 3 , LaGaO 3 , NdGaO 3 , PrGaO 3 or the like because their lattice constants are similar to that of REBa 2 Cu 3 O 7-x . Since each of these compounds for a seed crystal has a melting point at least 1,5⊘⊘°C, and the melting point is far higher than the partially melting points of rare earth-based oxide superconductors. Thus, the temperature employed for the melting step has a large freedom and can be selected from a wide range. Any of the above-mentioned three melting processes for obtaining a rare earth-based oxide superconductor of high orientation has unresolved problems. First in the temperature gradation process, a special heating apparatus is needed. Moreover, since slowly cooling is made while keeping a temperature gradation of a compact, special temperature control of high technique is required. These are not preferable from an industrial standpoint. Moreover, because of said special temperature control, the shape of rare earth-based oxide superconductor produced is restricted to a simple shape. In the substitution process using various rare earth-based oxide superconductors of different rare earth elements or of the same rare earth element of different amount, there is utilized a phenomenon that these rare earth-based oxide superconductors have different partially melting points. Hence, the combinations of the rare earth elements usable together are restricted. Further, various powders containing two rare earth elements in various proportions must be prepared for production of a compact, requiring an additional step. In the seed crystal process, the seed crystal becomes a starting point of crystallization and controls crystal orientation only at the initial stage of crystallization. Hence, crystal growth tends to occur at random not only from the area of a rare earth-based oxide superconductor which is in contact of the seed crystal, but also from portions of the superconductor which is not in contact with the seed crystal. Thus, it has been difficult to allow crystal growth to occur only from the area of the superconductor which is in contact with the seed crystal and thereby obtain a large oxide superconductor of good orientation. SUMMARY OF THE INVENTION According to a first aspect of the invention, there is provided a rare earth superconductive body as set out in claim 1. The noble metal may be silver. Preferably the rare earth superconducting oxide is REBa 2 Cu 3 O 7-x where RE is at least one element selected from the group consisting of Y, Gd, Dy, Ho, Er and Yb and x ranges from ⊘ to 1. Bulk concentration of the noble metal may monotonously increase in one direction in the rare earth superconducting body. Preferably the noble metal is present in a plurality of particles, the particles dispersed in each of the grains. Preferably at least some of the particles have diameters up to 5 µm. According to a second aspect of the invention, there is provided a process for making a rare earth superconducting body as set out in claim 8. Bulk concentration of the compound may monotonously increase in a direction in the compact. Preferably the compound is at least one element selected from a group consisting of metallic silver and silver oxide. According to a third aspect of the invention, there is provided a process for making a rare earth superconducting body as set out in claim 12. Preferably the compound is distributed in controlled concentrations in the compact so that the compact surface contacted with the seed crystal has a lowest bulk concentration of the compound in the compact, and the bulk concentration of the compound in the compact may increase with distance from the surface. BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is a graph showing a relation between silver content in YBa 2 Cu 3 O 7-x and freezing point of the composition essentially consisting of YBa 2 Cu 3 O 7-x and silver. Figs. 2a-2h show cross sections of compacts of rare earth superconducting compositions before firing, and in each compact a compound containing a noble metal is distributed in controlled concentrations. Figs. 3a, 3b, and 3c show cross sections of explaining the vacuum melting step applicable to a compact of the rare earth superconducting composition of the present invention. Fig. 3a is before the vacuum melting, Fig. 3b is after the vacuum melting, and Fig. 3c is after polishing the compact subsequent to the vacuum melting step. Figs. 4a-4d show cross sections of conventional rare earth superconducting compositions or compacts thereof, wherein the concentration of noble metal or noble metal compound is zero or is uniform. DETAILED DESCRIPTION OF THE INVENTION In the noble metal gradation process of the present invention, a compact containing a rare earth superconducting oxide and a compound containing a noble metal is prepared, and the noble metal is distributed in controlled concentrations in the compact. The presence of a noble metal compound allows the compact of the rare earth superconducting composition to have a decreased freezing point, and the degree of freezing point decrease is in direct proportion, in a first-order approximation, to the content of the noble metal compound in a certain range of said content. For example, in Fig. 1, in a silver content range up to 5% by weight, the freezing point of YBa 2 Cu 3 O 7-x decreases from about 1,⊘1⊘° C to about 98⊘° C approximately in a direct proportion to the silver content; when the silver content reaches 1⊘ % or 12% by weight, the freezing point decreases to about 97⊘° C and levels off thereafter. Therefore, in the present invention, the maximum content of noble metal or noble metal compound in rare earth superconducting composition is preferably a point at which the freezing point of rare earth superconducting composition begins to level off. In the present specification a noble metal compound refers to a compound containing a noble metal, and thus the noble metal compound includes noble metal itself, such as silver. Preferably the compound containing a noble metal is metallic silver or silver oxide. In the noble metal gradation process of the present invention, the distribution of noble metal concentration in the compact of rare earth superconducting composition is controlled, whereby the distribution of freezing point in compact is controlled. This rare earth superconducting composition compact having a controlled distribution of freezing point is heated to a temperature higher than a partial melting point of the compact so as to put the compact into a semi-molten state and then slowly cooled. During the slow cooing step crystallization occurs first at the portion of the compact having the highest freezing point and proceeds to a direction of lower freezing point. The distribution of noble metal concentration in the compact is such that this crystallization smoothly proceeds from the portion of the compact having the highest freezing point to a direction of the portions of the compact having lower freezing points. In this way a rare earth superconducting composition of high orientation is obtained. In the seed crystal process of the present invention, a property of rare earth superconducting composition is utilized in which a rare earth superconducting composition gives a decreased freezing point, so that the partial melting point of the compact is lower than the melting point of the seed crystal. Thus, the crystal and the compact can be heated to a temperature between the partial melting points of the compact and the seed crystal to put only the compact in a semi-molten state. In the present process, a compact containing a rare earth superconducting oxide and a noble metal compound is arranged in contact with a seed crystal which is a single crystal of a rare earth superconducting composition having a higher melting point than said compact; the crystal and the compact are heated to a temperature between the melting points of the compact and the seed crystal to put only the compact in a semi-molten state; then, the compact is slowly cooled to give rise to crystallization starting from the portion of the compact contacting with the seed crystal; thereby, a rare earth superconducting composition of high crystal orientation is obtained. The seed crystal used in the present seed crystal process preferably has the same constitution as the constitution of the compact except that the seed crystal does not contain the noble metal compound. However, the constitution of the seed crystal is not restricted to that. When the seed crystal contains the rare earth superconducting oxide included in the compact, the seed crystal may also contain a noble metal compound as long as the content of the noble metal compound in the seed crystal is smaller than the content of the noble metal compound in the compact. Alternatively the seed crystal consisting of a rare earth superconducting oxide different from the rare earth superconducting oxide of the compact may be used , as long as the seed crystal has a higher partial melting point than that of the compact. In the present seed crystal process, the compact of a rare earth superconducting composition containing a noble metal compound may have a uniform constitution, but the noble metal compound contained in the compact may have a controlled concentration distribution because it makes easier the production of a rare earth superconducting composition of high crystal orientation. In the latter case, the seed crystal may be a single crystal of a rare earth superconducting composition having a partially melting point higher than the highest partially melting point of the compact, and the portion of the compact having the highest melting point is contacted with the seed crystal. A rare earth superconducting body of the present invention has a high grain orientation. This increases critical current density of the composition. The composition has grains and grain boundaries, and a supercurrent is generally interrupted by grain boundaries or cracks present inside the grains so that the loop radius of the supercurrent cannot be larger than the size of each grain. However, this is not the case with the body of the present invention. Adjacent grains of the body of the invention have substantially the same orientation so that the supercurrent can flow through the grains without interruption by the grain boundaries and can take a longer path, and the loop radius is not restricted to each grain size, and consequently a supercurrent has a larger loop radius and a higher critical current density. With respect to the Meissner effect, it is necessary for the practical application of a superconductor that the superconductor have a large supercurrent so that it can cancel an external magnetic field. The supercurrent flows through the surface of the superconductor in a circular loop. The larger the radius of the circle or loop, the larger the magnetic levitation force of the superconductor. Moreover, to have highly uniform orientation of grains is preferable for another reason. REBa 2 Cu 3 O 7-x oxides, which may be the main components of a rare earth superconducting composition of the invention, have a perovskite structure which has anisotropy, and the oxide accordingly show a superconductivity of two-dimensional anisotropy. That is, their critical current density is the largest on the ab face of its crystal lattice and only 1/3 thereof on the bc or ca face of its lattice. Therefore, with a rare earth superconducting body of the invention of uniform grain orientation, may have the supercurrent density much higher when the composition is used so that the crystal's c axis is in the same direction as a magnetic field applied to the composition, as compared with when the superconductor is used so that the crystal's c axis is in a direction perpendicular to the direction of the magnetic field. In both of the above present processes for obtaining a rare earth superconducting composition of high grain orientation, the noble metal compound is dispersed in a form of particles in the grains of rare earth superconducting composition in the melting step. Such dispersion of particles containing a noble metal in grains is effective for the prevention of growth of cracks, and the dispersion of a large number of small particles is particularly effective in such prevention. It is presumed that these particles act to make short and fine cracks before the cracks start growing. Moreover the portions of the superconductor where the particles are dispersed have higher mechanical strengths. Owing to the synergetic effect thereof, the supercurrent can flow without being interrupted by cracks and excellent superconductivity can be obtained even in a magnetic field. A source of cracks may be in a cooling step of a rare earth superconducting composition. A phase transition of a rare earth superconducting oxide from cubic system at higher temperatures to rhombic system at lower temperatures takes place at about 7⊘⊘ °C. This phase transition gives rise to a contraction in a direction of the c axis of the crystal lattice of the grains, which may cause cracks in grains and grain boundaries. Particularly when cracks are formed along the cleavage surfaces of crystals and thereby the cleavage of crystals is invited, the rare earth superconducting composition obtained gives a supercurrent having a significantly small loop radius and accordingly gives a lower magnetic field caused by the Meissner effect. In some cases, cracks are also formed in crystals by the thermal expansion, thermal impact etc. caused by the temperature change experienced during production or use of rare earth superconducting composition. Hereinafter dispersion of particles containing a noble metal in grains is explained in detail with emphasis when a noble metal is silver. A silver compound refers to a compound containing silver, and metallic silver and silver oxide are preferable. As explained above, a rare earth superconducting composition has grains of a rare earth superconducting oxide, and a plurality of particles are dispersed in each of the grains. The particles include a noble metal. Preferably the noble metal is silver. Some of the particles have diameters up to 5 µm. The inclusion of silver compound particles in grain has been known even in the conventional melting process, but their particle diameters have been at least 1⊘ µm and generally 2⊘-5⊘ µm. However, such fine dispersion of silver compound particles as seen in the present invention has been unknown. The present invention is described in more detail below. The rare earth superconducting composition of the present invention is represented by a formula REBa 2 Cu 3 O 7-x . In the formula, RE is at least one element selected from the group consisting of Y, Gd, Dy, Ho, Er and Yb, and x is a number which can be varied between ⊘ and 1 because the oxygen contents in raw material oxides of REBa 2 Cu 3 O 7-x have a non-stoichiometric relation with each other. x has a direct influence on the superconductivity of the composition. The present rare earth superconducting composition has a feature that the crystal structure is a multi-layered perovskite structure. An example of the main component of-the present rare earth superconducting composition is YBa 2 Cu 3 O 7-x . The rare earth element represented by RE is not restricted to one element and may be a mixture of two or more elements selected from the group consisting of Y, Gd, Dy, Ho, Er and Yb. This allows the rare earth superconducting composition to have a freedom for desired melting point. An example of such mixed use of rare earth elements is that RE is shown as Y y Yb 1-y where y ranges from ⊘ to 1. The raw materials for the REBa 2 Cu 3 O 7-x which is the main component of the present rare earth superconducting body are an oxide of RE, i.e. Y, Gd, Dy, Ho, Er or Yb; a carbonate, oxide or peroxide of Ba; and an oxide of Cu. Each raw material or a mixture thereof is used in the form of powder, calcined powder, frit powder or the like so as to produce, by firing, REBa 2 Cu 3 O 7-x and RE 2 BaCUO 5 . The kinds and proportions of the raw materials are not particularly restricted as long as the above two oxides are produced in desired proportions. The particle diameters of the raw material powders are not particularly restricted, but are generally in the range of 1-1⊘ µm. The noble metal refers to silver, gold or a platinum group metal. It is not restricted to only one of these metals and may be a mixture of two or more of these metals. The noble metal compound refers to a compound of such a mixture of two or more compounds of such noble metals. It is possible to use any combination of at least two members selected from the above-mentioned noble metals and noble metal compounds. In the present invention, the main purpose of addition of a noble metal compound to a rare earth superconducting composition compact is to lower the freezing point of said compact. Therefore, the kind of noble metal compound to be added is not restricted. The addition of silver or silver oxide is preferable in view of the cost. The addition of rhodium or platinum or an oxide thereof is preferable as well. The noble metal compound contained in the present rare earth superconducting body is not necessarily the same as those contained in the compact. That is, a possibility exists in which the noble metal compound contained in the compact reacts with some compound during the process for production of rare earth superconducting body, becoming a different compound. The possibility for the noble metal compound in the compact to react with other compound is low because any noble metal has low reactivity. The noble metal compound may be added to a mixture of powders of raw material oxides or calcined powders obtained by calcining said raw material oxides. The noble metal compound to be added, preferably has a powder form. Generally, a fine powder having particle diameters of about 2⊘ µm or less is preferred. When the particle diameters exceed about 2⊘ µm, the powder becomes agglomerates and does not decrease a freezing point of a superconducting oxide as much as expected for the added amount. Preferably the rare earth superconducting composition of the invention containing REBa 2 Cu 3 O 7-x as the main component may contain a platinum group metal (Pt or Rh) or a compound thereof in an amount of ⊘.⊘1-5% by weight as the platinum group metal. The presence of these metal in the composition prepared in the melt process like the processes of the invention, has an effect of generating fine particles of RE 2 BaCuO 5 uniformly dispersed in grains. These fine particles act as pinning centers and their presence is preferable. The Pt or Rh or compound thereof may be dispersed uniformly in the rare earth superconducting composition while the other noble metal compound is contained in a controlled concentration distribution. Alternatively, the Pt or Rh or compound thereof itself may be contained in a controlled concentration distribution. When the content of the above metal or metal compound in the rare earth superconducting body containing REBa 2 Cu 3 O 7-x as the main component is less than ⊘.⊘1% by weight as metal, the metal or metal compound has no effect of generating fine particles of RE 2 BaCUO 5 dispersed in grains. When the content is more than 5% by weight, a large amount of a heterogeneous phase appears, which is not preferable. The Pt or Rh or compound thereof added to the powders of raw material oxides for rare earth superconducting composition, preferably has a powder form. Generally, fine powders having particle diameters of about 5 µm or less is preferred. When the particle diameters are larger than about 5 µm, the powder becomes agglomerates and does not disperse RE 2 BaCUO 5 particles much. (1) A calcined powder for the present rare earth superconducting composition or powder(s) of raw material oxides for the present rare earth superconducting composition, (2) a required amount of a noble metal compound powder and (3) other desired components are mixed uniformly to prepare a plurality of mixed powders of different noble metal compound contents. These mixed powders of different amounts of a noble metal compound are subjected to known molding such as press molding, injection molding, cast molding, isotropic press molding or the like so as to form a compact of desired shape in which the noble metal compound is contained in a controlled concentration distribution. In the above formation of said compact, two approaches are possible. In either approach, firstly a plurality of compacts are prepared, each of the compact has a desired shape and uniform constitution, and each compact has different noble metal compound concentration. Then in one approach the compacts are arranged so as to give a desired shape and are vacuum-melted or heated into a semi-molten state. In the other approach, said compacts are arranged as desired and then subjected to isotropic press molding or the like for bonding and integrating them, and the resulting one-piece mass is vacuum-melted or heated into a semi-molten state. There is another method for obtaining a compact by using press molding for bonding and integration. A first mixed powder is placed in a mold and pressed to obtain a first compact; without taking out the first compact, a second mixed powder is placed on the first compact in the mold and pressing is effected to obtain a laminate; without taking out the laminate, a third powder is placed thereon and pressing is effected to obtain a new laminate; the same procedure is repeated to obtain a final laminate, i.e. a compact. The compact obtained by either of the above two approaches can have any desired shape. The shape may be, as desired, a column; a prism; a parallelepiped; a rectangular parallelepiped; a cone; a pyramid; a spheroid; a sphere; a shape obtained by subjecting any of the above shapes to plastic deformation in any desired direction; a shape obtained by cutting any of the above shapes in any direction so as to give a flat or curved surface; and a shape obtained by making at least one hole in any of the above shapes, such as cylinder, doughnut or the like. In the case of a compact of complex shape, for example, a compact having a complex shape so as not to be able to take out from the mold used, it has been very difficult industrially by the conventional temperature gradation process to allow such a compact to have a controlled temperature distribution inside after it has been put in a semi-molten state. In the present noble metal gradation process and seed crystal process, however, even in a compact of complex shape, the concentration of noble metal compound in each portion of the compact can be controlled as desired. Therefore, uniform orientation of grains in sintered material is easier in the present noble metal gradation process and seed crystal process than in the conventional temperature gradation process. In the present noble metal gradation process, the concentration distribution of noble metal compound in compact of rare earth superconducting composition is preferably controlled so that in the gradually cooling step of the compact, only one single crystal or so-called pseudo-crystal may be formed. It is generally preferable that the compact be prepared so that the portion of highest freezing point is as small as possible. Crystallization can occur at any site of the compact whose temperature has become lower than the freezing point. Hence, if the portion of highest freezing point is too large, there is a high possibility that a plurality of crystals of different orientations be formed, making it difficult to obtain a superconducting composition of good orientation. When no seed crystal is used, the size of the portion of highest freezing point in compact is preferably about 1⊘ µm or less in diameter. In the present noble metal gradation process, use of the seed crystal process in combination is preferable because it enables production of a large superconducting composition of high orientation in desired direction. When a seed crystal is used, the size of the portion of highest freezing point in compact is preferably such that the distance between the circumference of said portion and the circumference of the contact area is not larger than about 1⊘ mm. In the present noble metal gradation process, the concentration distribution of noble metal compound in compact is controlled so as to give a desired distribution of freezing point. Therefore, the noble metal compound used need not be one compound and may be a plurality of compounds, and the desired freezing point distribution in compact can be achieved by controlling the concentration distributions of individual compounds. The use of a plurality of noble metal compounds provides a further advantage because it gives a higher freedom for the properties of superconducting composition obtained. Specific examples of the concentration distribution of noble metal compound in the compact for the present rare earth superconducting body are shown in Figs. 2a-2h. As mentioned above, each of the compact is prepared by arranging a plurality of compacts as desired. These compacts are subject to a normal press molding, a isotropic press molding or the like to be bonded together and integrated. Regardless of the method the compact is prepared, the concentration distribution of a noble metal compound in the compact is not restricted to the examples of Figs. 2a-2h. Concentrations of the compound may monotonously increase in a direction in the compact in any of the examples of Figs. 2a-2h. Figs. 2a - 2h show examples of the concentration distribution of noble metal in the compact used in the noble metal gradation process of the present invention. Figs. 2a to 2d are examples of the concentration distribution of noble metal (Figs. 2b and 2c) and noble metal oxide (Figs. 2a and 2d) in the compact used also in the seed crystal process. In Figs. 2b and 2d, on a compact of a rare earth superconducting composition containing a noble metal compound in a controlled concentration distribution is placed a seed crystal which is a single crystal of a rare earth superconducting composition having a melting point higher than the highest melting point of the compact. In Figs. 2a and 2c, the (1⊘⊘) surface 31s of a seed crystal 31 which is a thin sheet of a single crystal of MgO, is contacted with a compact in order to provide a starting point of crystallization and achieve controlled grain orientation. In Figs. 2a, 2c, 2d, 2e and 2f, various layers of rare earth superconducting compositions each containing a different concentration of a noble metal compound are arranged in a direction perpendicular to the axial direction of a column or prism formed by said arrangement of said layers; and in the column or prism, the concentration of the noble metal compound is on a monotonous increase or decrease in the axial direction. Meanwhile, in Figs. 2g and 2h, various layers of rare earth superconducting compositions each containing a different concentration of a noble metal compound are arranged in a direction parallel to the axial direction of a column formed by said arrangement of said layers; in the column, the concentration of the noble metal compound is on a monotonous increase or decrease in a direction perpendicular to the axial direction of the column. In Fig. 2h, when the column is seen from above, various layers are arranged in the form of concentric circles, and the concentration of the the noble metal compound is lowest in the innermost layer and is on a monotonous increase towards the circumference. Then, the compact of a rare earth superconducting composition containing a noble metal compound in a controlled concentration distribution is preferably subjected to a vacuum melting step, i.e. vacuum melting and subsequent cooling in the air. In this vacuum melting and subsequent cooling, considerable volume contraction and deformation takes place. For example, a column of Fig. 3a is applied vacuum melting. The center of the top portion is dented and the center of the side is slightly deformed after vacuum melting and cooling, and the overall shape becomes a hand drum shape as shown in Fig. 3b. Application of surface polishing thereto gives a material in which the concentration distribution of a noble metal compound is as shown in Fig. 3c. In such a concentration distribution, the direction of grain orientation can be controlled not only in a vertical direction but also in a horizontal direction. Meanwhile in the present seed crystal process, on a compact of a rare earth superconducting composition containing a noble metal compound is placed, at the center, a seed crystal, i.e. a small piece of a single crystal of a rare earth superconducting composition having a higher melting point than said compact; then, they are heated, by a desired known method, to a temperature between the melting point of the seed crystal and the melting point of the compact to put only the compact in a semi-molten state. When the noble metal compound is contained in the compact in a controlled concentration distribution and as a result the compact has no single melting point, the melting point of the seed crystal must be higher than the highest melting point of the compact. In this case, the site of the compact on which the seed crystal is placed, is the center of the portion of the compact having the highest melting point. As the seed crystal, there may be also used single crystals of oxides such as SrTiO 3 , MgO, LaAlO 3 , LaGaO 3 , NdGaO 3 , PrGaO 3 and the like. In this method, the polished (1⊘⊘) surface of a single crystal of one of these oxides is contacted with a compact; they are heated to put only the compact in a semi-molten state; then, they are gradually cooled to a temperature below the freezing point of the compact to give rise to crystallization. Since the (1⊘⊘) surface of the oxide single crystal is close to the ab surface of the rare earth superconducting composition compact in lattice constant, it is possible to, by the above method, selectively grow the ab surface of the rare earth superconducting composition on the (1⊘⊘) surface of the oxide single crystal. Further, since the melting point of the oxide single crystal is 1,5⊘⊘°C or above and is far higher than the melting point of the compact, use of the seed crystal imposes no restriction on the temperature at which the compact is put in a semi-molten state. In this case, the area of the contact surface of the oxide single crystal as seed crystal is preferably larger than the contact area of the compact. The reason is as follows. The compact which is a rare earth superconducting composition, has a tetragonal system while the oxide single crystal has a cubic system and the (1⊘⊘), (⊘1⊘) and (⊘⊘1) surfaces thereof are equivalent to each other. Therefore, if there is used, as the seed crystal, an oxide single crystal having a contact area smaller than the area of the contact surface of a compact, the ab surface of a rare earth superconducting composition is formed on each of the (1⊘⊘), (⊘1⊘) and (⊘⊘1) surfaces of the oxide single crystal and crystallization proceeds in a plurality of directions. Consequently, there is obtained no superconducting oxide of uniform orientation. In the present noble metal gradation process, the temperature at which a compact of a rare earth superconducting composition is put into a semi-molten state may range from a lower temperature end at which the main component of the compact is melted and decomposed into a solid phase of RE 2 BaCuO 5 and a liquid phase to a higher temperature end at which a solid phase of RE 2 BaCuO 5 undergoes redecomposition into a solid phase of RE 2 O 3 and a liquid phase. The temperature to be set as above differs depending upon the kind of rare earth element (Y, Gd, Dy, Ho, Er or Yb) used and the kind and amount of noble metal compound used, and may be appropriately selected in consideration also of the size of compact obtained, the presence or absence of seed crystal and kind thereof, the heating condition, etc. Specifically, the temperature may be appropriately selected in the range of about 99⊘-1,15⊘°C, about 1,⊘2⊘-1,18⊘°C, about 1,⊘⊘⊘-1,16⊘ °C, about 98⊘-1,14⊘°C, about 96⊘-1,12⊘°C or about 89⊘-1,15⊘ °C when the rare earth element is Y, Gd, Dy, Ho, Er or Y, respectively. The compact is kept in a semi-molten state at a given temperature, such as above, for a given length of time for partially melting the compact. The time length is not particularly restricted and, similarly to the above heating temperature, can be appropriately selected depending upon the heating condition, etc., but is generally 2⊘ minutes to 2 hours. Then, the melt is rapidly cooled to about its freezing point. The cooling condition is important. That is, the cooling is conducted to the freezing point or a lower temperature by quenching at a rate of at least 1⊘⊘°C per hour. When the cooling rate is less than 1⊘⊘°C per hour, the silver compound particles in each grain are large and no fine particles of 5 µm or less are obtained. The end point of quenching is determined depending upon the freezing point of the compact because the freezing point of the compact is lower than the freezing point of REBa 2 Cu 3 O 7-x depending upon the kind of rare earth element substituted and/or the amount of silver compound added. When the end point of quenching is too high, the silver compound particles in each grain are large and, when the end point is too low, the grain size is small, which are both unpreferable. Subsequently, the compact is gradually cooled in the same manner as in an ordinary melting process. The temperature at which the gradually cooling is started, is about 5°C higher than the freezing point of the compact; the temperature at which the gradually cooling is stopped, is about 3⊘ °C lower than said freezing point. The rate of gradually cooling is preferably ⊘.1-1°C per hour. When the compact has a range of freezing points, the starting temperature of gradually cooling is about 5°C higher than the highest freezing point of the compact and the lowest temperature of gradually cooling is about 15 °C lower than the lowest freezing point of the compact, and the rate of gradually cooling is preferably ⊘.2-2°C per hour. When there is used, as the seed crystal, a single crystal of a rare earth superconducting composition having a melting point higher than that of the compact, gradually cooling is preferably started from the temperature at which the compact is put in a semi-molten state, because there occurs diffusion of the seed crystal into the compact in the vicinity of the portion of the compact contacting with the seed crystal and consequently said portion of the compact comes to have a higher freezing point. In the case of, for example, YBa 2 Cu 3 O 7-x containing ⊘-1⊘% by weight of silver, gradually cooling is preferably conducted from 1,⊘15°C to 94⊘°C at a rate of 1°C per hour. In the noble metal gradation process of the present invention, the rare earth superconducting composition compact of semi-molten state is then gradually cooled. In the gradually cooling, crystallization starts from the point of the compact having the highest freezing point and proceeds to the portion of lower freezing point, whereby a rare earth superconducting composition of high orientation is obtained. The rare earth superconducting composition compact is beforehand prepared so as to have a controlled freezing point distribution and give crystallization as mentioned above. Meanwhile, in the seed crystal process of the present invention, only the compact having a melting point lower than that of the seed crystal arranged in contact with the compact is put in a semi-molten state and then gradually cooled, whereby crystallization starts from the portion of the compact contacting with the seed crystal having a lattice constant similar to that of the compact and a rare earth superconducting composition of high orientation in a desired direction can be obtained. In the seed crystal process, the compact of a rare earth superconducting composition containing a noble metal compound may have a uniform constitution, but: preferably contains said noble metal or said noble metal compound in a controlled concentration distribution because such a concentration distribution of noble metal compound enables easy production of a large rare earth superconducting composition of high orientation in a desired direction. The rare earth superconducting composition after gradually cooling is heat-treated in an oxygen atmosphere at a given temperature in the same manner as in a known melting process, whereby the main component (REBa 2 Cu 3 O 7-x ) in said rare earth superconducting composition is allowed to absorb oxygen to adjust the value of x to a positive real number of ⊘.2 or less so that the resulting rare earth superconducting composition can exhibit superconductivity. The heat treatment in an oxygen atmosphere is ordinarily conducted at 6⊘⊘°C for 5-1⊘ hours, at 5⊘⊘°C for 1⊘-2⊘ hours and at 4⊘⊘°C for 2⊘-5⊘ hours in this order. Thus, in the noble metal gradation process of the present invention, a compact of a rare earth superconducting composition containing a noble metal compound in a controlled concentration distribution is prepared (this compact has a controlled distribution of freezing point); the compact is heated to a semi-molten state and then gradually cooled; during the gradually cooling, crystallization starts from the portion of the compact having the highest freezing point and proceeds to the portion of lower freezing point; thereby, a rare earth superconducting composition of high orientation can be obtained. In this case, by utilizing the property of a rare earth superconducting composition that its freezing point is lower when it contains a noble metal compound, the freezing point distribution in the compact is controlled beforehand at its preparation so that in the compact, crystallization can start from the portion of highest freezing point and proceed to the portion of lower freezing point. In the seed crystal process of the present invention, on a compact of a rare earth superconducting composition containing a noble metal compound is placed, as a seed crystal, a single crystal of a rare earth superconducting composition having a melting point higher than that of the compact; then, the seed crystal and the compact are heated to a temperature between their melting points to bring only the compact into a semi-molten state; thereafter, gradually cooling is conducted to give rise to crystallization starting from the portion of the compact contacting with the seed crystal; thereby, a rare earth superconducting composition of high orientation in a desired direction can be obtained. This seed crystal process has a feature that there are used, as a compact, a rare earth superconducting composition containing a noble metal compound and accordingly having a freezing point lower than that of a seed crystal used together and, as said seed crystal, a single crystal of a rare earth superconducting composition having a melting point higher than that of the compact. In the seed crystal process, the compact of a rare earth superconducting composition containing a noble metal compound may have a uniform constitution, but preferably contains said noble metal or said noble metal compound in a controlled concentration distribution because such a concentration distribution of noble metal compound enables easy production of a rare earth superconducting composition of high orientation in a desired direction, which has a large size. The present invention is hereinafter described in more detail by way of Examples. However, the present invention is not restricted to the following Examples. Example 11 To a mixture of Y 2 O 3 powders, BaO 2 powders and CuO powders in a molar ratio of ⊘.9:2.4:3.4. was added ⊘.5% by weight of platinum powders. They were placed in a dry type pot mill and mixed for 6 hours. The mixed powder was spread on a silver plate which was placed in an atmosphere consisting of oxygen and nitrogen (1:4 in volume ratio), and calcined at 7⊘⊘°C for 1⊘ hours. The calcined material was ground in a rotary mill using zirconia balls, for 15 hours to obtain calcined powders of YBa 2 Cu 3 O 7-x having an average particle diameter of about 3 µm. The calcined powders of YBa 2 Cu 3 O 7-x were mixed with 1, 2, 3, 4, 5, 6, 7, 8 and 9% by weight of silver oxide powders having an average particle diameter of about 2 µm to obtain 9 kinds of mixtures different only in silver oxide concentrations. For convenience, the calcined powder of YBa 2 Cu 3 O 7-x itself containing no silver oxide powder is herein called also as a mixture; therefore, 1⊘ kinds of mixtures of different silver oxide concentrations were obtained. Each mixture was mixed for 64 hours in a dry type pot mill to obtain 1⊘ kinds of mixed powders of different silver oxide concentrations each having an average particle diameter of about 3 µm. This method for preparation of raw material mixed powders using no BaCuO 2 is herein called as method 1 for raw material preparation. 15 g of the mixed powders containing no silver oxide were placed in a cylindrical mold of 5⊘ mm in internal diameter and subjected to press molding under a load of 2 tons to prepare a disc-shaped compact of 5⊘ mm in diameter and about 2.7 mm in thickness. On the compact in the mold was further placed 15 g of the mixed powder containing 1% by weight of silver oxide, and the same press molding was applied to prepare a disc-shaped laminate consisting of two layers each of which extends in a horizontal direction. On the laminate was placed 15 g of the mixed powder containing 2% by weight of silver oxide, and the same press molding was applied to prepare a new disc-shaped laminate consisting of three layers each extending in a horizontal direction. This procedure was repeated to obtain a columnar laminate consisting of 1⊘ layers each extending in a horizontal direction, wherein the silver oxide concentration increased from ⊘% by weight to 9% by weight upwards (see Fig. 2a). The columnar laminate was subjected to isotropic press molding under a load of 5⊘⊘ kgf/cm 2 at room temperature to obtain a laminated columnar compact of 46 mm in diameter and 25 mm in height where 1 kgf is equivalent to 9.8⊘665 N. The laminated columnar compact 11 was placed on a square MgO single crystal plate 31 of 5⊘ mm x 5⊘ mm x ⊘.5 mm (thickness) having a polished planar surface 31s of the (1⊘⊘) surface of the crystal lattice so that the layer of the compact containing no silver oxide contacted with the polished surface 31s of the single crystal plate, as shown in Fig. 2a. They were placed in an electric furnace in an air and the laminated compact 11 was partially melted at 1,1⊘⊘° C for 1 hour, then it was cooled from 1,1⊘⊘° C to 1,⊘15° C at a rate of 1⊘⊘° C per hour. Then, the compact was slowly cooled from 1,⊘15°C to 95⊘° C at a rate of ⊘.1-1.⊘ °C per hour to subject the compact to crystallization on the single crystal plate. Thereafter, the atmosphere in the furnace was changed to an oxygen atmosphere and a heat treatment was conducted at 65⊘-4⊘⊘°C for 5⊘ hours to obtain a columnar rare earth superconducting composition (a so-called pseudo single crystal) of about 4⊘ mm in diameter and about 15 mm in height. The columnar rare earth superconducting composition was visually observed for its entire surface, which showed neither grain boundary nor crack. The columnar rare earth superconducting composition of about 4⊘ mm in diameter and about 15 mm in height was also measured for repulsive force against a permanent magnet. A columnar Sm-Co permanent magnet of 2⊘ mm in diameter and 1⊘ mm in height having a surface magnetic flux density of 2,5⊘⊘ G was arranged with one of the circular surfaces directed downward. The columnar rare earth superconducting composition was dipped in liquid nitrogen and allowed to express superconductivity, and immediately one of the circular surfaces of the columnar composition was pressed onto the upper circular surface of the permanent magnet. When the gap between the two circular surfaces of the composition and the permanent magnet was ⊘.1 mm, a repulsive force was measured by a load cell. Five samples were measured for respective repulsive forces, giving an average of 1⊘.5 kgf. This measurement is called as measurement of levitation force. Further, the columnar rare earth superconducting composition was cut in various arbitrary directions and the sections were observed using a polarizing microscope having a magnification of 5⊘. Neither grain boundary nor crack was seen in these sections. Next, thin plates were cut off from the columnar rare earth superconducting composition and measured for grain orientation using an X-ray diffractometer. The c axis of each crystal grain was oriented in a direction of the columnar axis. Thus, since neither grain boundary nor crack was seen in the arbitrarily selected sections and uniform grain orientation was seen in the cut off thin plates, the rare earth superconducting composition is judged to be a single crystal. The test conditions used above and the test results obtained are shown in Tables 1 and 2. Example 12 To a mixture of BaCO 3 powders of about 3 µm in particle diameter and CuO powders in equal moles was added ⊘.54% by weight of a platinum powder. They were mixed in a wet type pot mill for 3 hours. The mixed powder was spread on a high-purity alumina plate which was placed in an oxygen stream, and calcined at 1,⊘⊘⊘°C for 1⊘ hours to obtain a calcined material of BaCuO 2 . The calcined material was ground in a rotary mill using zirconia balls, for 15 hours to obtain a calcined powder of BaCu 2 O having an average particle diameter of about 5 µm. Y 2 O 3 powders, the BaCuO 2 powders obtained above and a CuO powders were taken so as to give a molar ratio of ⊘.9:2.4:1.⊘. Thereto was added 3, 4, 5, 6, 7 or 8% by weight of silver powders having an average particle diameter of about 1 µm to prepare 6 kinds of mixtures different only in silver concentration. Each mixture was mixed in a dry type pot mill for 64 hours to prepare 6 kinds of mixed powders of different silver concentrations. This method for preparation of raw material mixed powders via BaCuO 2 is herein called as method 2 for raw material preparation. 15 g of each of the mixed powders was placed in a cylindrical mold of 5⊘ mm in inside diameter and subjected to press molding under a load of 2⊘⊘⊘ kgf to prepare 6 kinds of disc-shaped material of different silver concentrations. Each of these disc-shaped materials was subjected to isotropic press molding at 5⊘⊘ kgf/cm 2 at room temperature to obtain 6 kinds of disc-shaped compacts of 46 mm in diameter and 2.5 mm in height having different silver concentrations. As shown in Fig. 3a, these compacts were laminated on a high-purity alumina plate so that the compact 12 containing 8% by weight of silver contacted with the alumina plate 35 and the silver concentration in compact became gradually lower upwards. The above-obtained laminated compact 12 on the alumina plate 35 was placed in an electric furnace whose inside was kept in vacuum, and was heated to 1,⊘1⊘°C for partially melting. Then, the furnace inside was changed to an air atmosphere, and the laminated compact was kept at 1,1⊘⊘°C for 1 hour to put in a molten state and then cooled to room temperature. This is called as vacuum melting. As shown in Fig. 3b, the laminated compact 12 consisting of 6 layers of different silver concentrations was integrated; at the same time, the center of the upper surface of the column 12, i.e., the laminated compact, was dented and the center of the side of the column was slightly deformed; and the overall shape became a hand drum shape. The upper surface and side of the column were dented or deformed in their center, and the column were polished to obtain a column with no deformation. The section of the column after polishing was observed by electron probe microanalysis (EPMA). As shown in Fig. 3c, the upper surface had a silver concentration distribution of concentric circles, that is, the silver concentration was lowest at the center and higher towards the circumference. The column 12 after polishing was placed on a high-purity alumina plate 35 so that the surface of the column containing 8% by weight of silver contacted with the alumina plate. A single crystal 32 of YBa 2 Cu 3 O 7-x of about 4 mm x 5 mm containing no silver compound was placed on the center of the upper surface of the column so that the cleaved ab surface 32s of the single crystal 32 contacted with the column (Fig. 2b). Then, they were placed in an electric furnace in an air atmosphere and kept at 1,⊘⊘⊘°C for 1 hour to partially melt the compact. In the furnace and the atmosphere, the compact was gradually cooled from 1,⊘⊘⊘°C to 95⊘°C at a rate of 1.⊘°C to give rise to crystallization of a rare earth superconducting composition on the alumina plate. Thereafter, the atmosphere in the furnace was changed to an oxygen atmosphere and subjected to a heat treatment at 65⊘-4⊘⊘ °C for 5⊘ hours to obtain a columnar rare earth superconducting composition. The columnar rare earth superconducting composition was visually observed for its entire surface, which showed neither grain boundary nor crack. The columnar rare earth superconducting composition of about 4⊘ mm in diameter and about 15 mm in height was also measured for repulsive force against a permanent magnet in the same manner as in Example 11. Five samples were measured for respective repulsive forces, giving an average of 11.2 kgf. Further, the columnar rare earth superconducting composition was cut in various arbitrary directions and the sections were observed using a polarizing microscope having a magnification of 5⊘. Neither grain boundary nor crack was seen in these sections. Next, thin plates were cut off from the columnar rare earth superconducting composition and measured for grain orientation using an X-ray diffractometer. The c axis of each crystal grain was oriented in a direction of the columnar axis. Thus, since neither grain boundary nor crack was seen in the arbitrarily selected sections and uniform grain orientation was seen in the cut off thin plates, the rare earth superconducting composition is judged to be a single crystal. The test conditions used above and the test results obtained are shown in Tables 1 and 2. Examples 13 and 15-18 In each of Examples 13 and 15-18, a YBa 2 Cu 3 O 7-x columnar compact 1⊘ for rare earth superconducting composition of 4⊘ mm in diameter and 15 mm in height was produced by a noble metal gradation process. In Examples 13, 16 and 18, the concentration of metallic silver was controlled and, in Examples 15 and 17, the concentration distribution of silver oxide was controlled. In the columns of Examples 13, 15 and 16, as shown in Figs. 2c, 2e and 2f, layers each containing a different concentration of silver or silver oxide and extending in a horizontal direction were laminated in a direction of the columnar axial direction so that the concentration of silver or silver oxide was on a monotonous increase or decrease in the columnar axial direction. In Example 13, there was produced a compact 13 consisting of 1⊘ laminated layers each containing a different concentration ranging from ⊘ to 9% by weight of silver and extending in a horizontal direction (Fig. 2c); the (1⊘⊘) surface 31S of a thin plate of a MgO single crystal 31 was contacted with the upper surface of the compact to provide a starting point of crystallization of a rare earth superconducting composition; and the grain orientation of the rare earth superconducting composition was controlled by a noble metal gradation process. In Example 15, there was produced a compact 15 consisting of 7 laminated layers each containing a different concentration, ranging from ⊘ to 6% by weight, of silver oxide and extending in a horizontal direction (Fig. 2e). In Example. 16, there was produced a compact 16 consisting of 6 laminated layers each containing a different concentration, ranging from 5 to 1⊘% by weight of silver and extending in a horizontal direction (Fig. 2f). In the columns 17, 18 of Example 17 (Fig. 2g) and Example 18 (Fig. 2h), layers each containing a different concentration of silver oxide or silver and extending in a direction of the columnar axis were laminated so that the concentration of silver oxide or silver changed in a direction perpendicular to the columnar axial direction. In Example 17, the concentration of silver oxide was on a monotonous increase from 5 to 1⊘% by weight in a direction perpendicular to the columnar axial direction (Fig. 2g). In Example 18, the concentration of silver was distributed in coaxial cylinders, that is, the silver concentration was 4% by weight at the cylindrical axis portion and was on a monotonous increase towards the cylinder circumference from 4 to 7% by weight at intervals of 1% by weight (Fig. 2h). The test conditions used and the test results obtained are shown in Tables 1 and 2. Example 14 In Example 14, there was produced, by a seed crystal process, a columnar YBa 2 Cu 3 O 7-x superconducting composition of 4⊘ mm in diameter and 15 mm in height which had the same shape as in Examples 11-13 and 15-18. There was produced, by the method 2 for raw material preparation and without using the vacuum melting step, a compact 14 consisting of 1⊘ laminated layers each containing a different concentration, ranging from ⊘ to 9% by weight at intervals of 1% by weight, of silver oxide and extending in a horizontal direction. Incidentally, the concentration distribution of silver oxide in this compact was similar to that of silver in Example 13 using a noble metal gradation process. However a shape of the compact 14 of Example 14 is little different from compact 13 of Example 13. A single crystal 33 of SmBa 2 Cu 3 O 7-x of about 4 mm x 5 mm containing no silver compound was placed on the center of the upper surface of the column so that the cleaved surface 33s of the single crystal 33 contacted with the column (Fig. 2d). The test conditions used and the test results obtained are shown in Tables 1 and 2. Example 19 In Example 19, a large columnar YBa 2 Cu 3 O 7-x superconducting composition of 1⊘⊘ mm in diameter and 2⊘ mm in height was produced. In contrast the size of those produced in Examples 11-18 was 4⊘ mm in diameter and 15 mm in height. In Example 19, an YBa 2 Cu 3 O 7-x superconducting composition containing silver in a concentration distribution ranging from 3 to 8% by weight was produced by a seed crystal process in the same conditions as in Example 12 except that the raw material powders were prepared in the method 1 for raw material preparation in place of the method 2 for raw material preparation used in Example 12. The test conditions used and the test results obtained are shown in Tables 1 and 2. Comparative Example 11 Y 2 O 3 powders, BaO 2 powders and CuO powders were taken so as to give a molar ratio of ⊘.9:2.4:3.4. Thereto was added ⊘.5% by weight of platinum powders to prepare a mixed powder. In the same manner as above except that part of Y 2 O 3 was substituted with Yb 2 O 3 , that is, 5 to 5⊘ mole % of Y 2 O 3 was substituted with Yb 2 O 3 at intervals of 5 mole %, there were further prepared 1⊘ more mixed powders having different degrees of substitution of Y with Yb. Each of the thus prepared 11 mixed powders was mixed in a dry type pot mill for 6 hours, after which it was spread on a silver plate in a mixed gas stream consisting of oxygen and nitrogen (1:4 by volume ratio) and calcined at 7⊘⊘° C for 1⊘ hours to obtain 11 calcined materials of Y 1-a Yb a Ba 2 Cu 3 O 7-x (a=b/2⊘; b is an integer of ⊘-1⊘). Each calcined material was ground in a dry type pot mill for 64 hours to obtain 11 calcined powders of Y 1-a Yb a Ba 2 Cu 3 O 7-x (a=b/2⊘; b is an integer of ⊘-1⊘) each having a different degree of substitution of Y with Yb and having an average particle diameter of about 3 µm. These calcined powders contained neither silver nor silver oxide. 15 g of each calcined powder was placed in a cylindrical mold of 5⊘ mm in inside diameter and subjected to press molding under a load of 2 tons to prepare 11 disc-shaped materials each having a different degree of substitution of Y with Yb. Each of the disc-shaped materials was subjected to isotropic press molding at 5⊘⊘ kgf/cm 2 at room temperature to prepare 11 disc-shaped compacts each of 46 mm in diameter and 2.5 mm in height having a different degree of substitution of Y with Yb. As shown in Fig. 4a, the disc-shaped compacts 21 were laminated on a high-purity alumina plate so that the compact of 5⊘ mole % substitution of Y with Yb contacted with the alumina plate 15 and the degree of substitution of Y with Yb decreased gradually upwards. A SmBa 2 Cu 3 O 7-x single crystal 33 of about 4 mm x 5 mm containing no silver was placed on the uppermost compact layer (YBa 2 Cu 3 O 7-x containing no Yb) so that the cleaved ab surface 33s of the single crystal contacted with the uppermost compact layer. The disc-shaped laminated compact on the alumina plate 35 was kept at 1,⊘3⊘°C for 1 hour in an electric furnace in an air atmosphere to put the compact in a semi-molten state for integration and, in the furnace in the atmosphere, was gradually cooled from 1,⊘3⊘°C to 93⊘°C at a rate of 1.⊘°C per hour to give rise to crystallization of a superconducting composition on the alumina plate. Then, the atmosphere in the furnace was changed to an oxygen atmosphere and a heat treatment was conducted at 65⊘-4⊘⊘°C for 5⊘ hours to obtain a columnar rare earth superconducting composition. The columnar rare earth superconducting composition was visually observed for its entire surface, which showed neither grain boundary nor crack. The columnar rare earth superconducting composition of about 4⊘ mm in diameter and about 15 mm in height was also measured for repulsive force against a permanent magnet in the same manner as in Example 11. Five samples were measured for respective repulsive forces, giving an average of 1⊘.8 kgf. Further, the columnar rare earth superconducting composition was cut in various arbitrary directions and the sections were observed using a polarizing microscope having a magnification of 5⊘. Thin plates were cut off from the columnar rare earth superconducting composition and measured for grain orientation using a transmission electron microscope. The c axis of each crystal grain was oriented in a direction of the columnar axis. The test conditions used and the test results obtained are shown in Tables 1 and 2. Comparative Examples 12-14 In each of Comparative Examples 12 and 13, an YBa 2 Cu 3 O 7-x superconducting composition was produced using a seed crystal process alone (no noble metal gradation process was used). In Comparative Example 14, an YBa 2 Cu 3 O 7-x superconducting composition was produced without using any of a noble metal gradation process and a seed crystal process. In Comparative Examples 12-14, the compacts had uniform constitutions without gradiaton of a noble metal. In Comparative Example 12 (Fig. 4b), a mixed powder of YBa 2 Cu 3 O 7-x containing no silver was prepared by the method 2 for raw material preparation. In Comparative Example 13 (Fig. 4c), a mixed powder of YBa 2 Cu 3 O 7-x containing 5% by weight of silver was prepared by the method 2 for raw material preparation. In Comparative Example 14 (Fig. 4d), a mixed powder of YBa 2 Cu 3 O 7-x containing 1⊘% by weight of silver was prepared by the method 2 for raw material preparation. 15⊘ g of each of the mixed powders was placed in a cylindrical mold of 5⊘ mm in inside diameter and subjected to press molding under a load of 2 tons to prepare 3 columnar compacts. Each compact was subjected to isotropic press molding at 5⊘⊘ kgf/cm 2 at room temperature to prepare 3 columnar compacts of 46 mm in diameter and 25 mm in height. In Comparative Example 12, a SmBa 2 Cu 3 O 7-x single crystal 33 of about 4 mm x 5 mm containing no silver was placed on the upper surface of the column 22 layer (YBa 2 Cu 3 O 7-x containing no Yb) so that the cleaved ab surface 33s of the single crystal contacted with the uppermost compact layer. The column 22 was placed on an aluminum plate 35. In Comparative Example 13, the column 23 was placed on a polished planar surface 31t of the (1⊘⊘) surface of a square MgO single crystal plate 31 of 5⊘ mm x 5⊘ mm x ⊘.5 mm (thickness), as shown in Fig. 4c. Then, 3 columnar rare earth superconducting compositions were produced under the conditions shown in Tables 1 and 2. The columnar rare earth superconducting compositions were visually observed for the entire surfaces. In any of the compositions, grain boundaries and cracks were observed. The columnar rare earth superconducting compositions each of about 4⊘ mm in diameter and about 15 mm in height were also measured for repulsive force against a permanent magnet in the same manner as in Example 11. Five samples were measured for respective repulsive forces and the average of 5 measurements was 4.2 kgf in Comparative Example 12, 6.1 kgf in Comparative Example 13 and 3.3 kgf in Comparative Example 14. Further, each of the columnar rare earth superconducting compositions was cut in various arbitrary directions and the sections were observed using a polarizing microscope having a magnification of 5⊘. Thin plates were cut off from each of the columnar rare earth superconducting compositions and measured for grain orientation using a transmission electron microscope. In each columnar rare earth superconducting composition, the c axis of each crystal grain was oriented in a direction of'the columnar axis. The test conditions used and the test results obtained are shown in Tables 1 and 2. The YBa 2 Cu 3 O 7-x superconducting compositions obtained in Examples 11-19, as compared with those obtained in Comparative Examples 12-14, have larger crystal grains and higher grain orientation owing to the adoption of a noble metal gradation process during their production to allow each of the respective compacts to have a controlled freezing point distribution. Further, the YBa 2 Cu 3 O 7-x superconducting compositions obtained in Examples 11-19 are far superior in superconductivity represented by levitation force to the YBa 2 Cu 3 O 7-x superconducting compositions obtained in Comparative Examples 12-14.

Claims: A rare earth superconductive body comprising a rare earth superconducting oxide and having a noble metal distributed in said rare earth superconductive body, characterized in that the noble metal bulk concentration is different at at least two spatial regions in said rare earth superconductive body. A rare earth superconductive body according to claim 1, wherein said noble metal is silver. A rare earth superconductive body according to claim 2 wherein the amount of said silver is in the range 5 to 12% by weight. A rare earth superconductive body according to claim 1, 2 or 3 wherein said rare earth superconducting oxide is REBa 2 CU 3 O 7-x where RE is at least one element selected from the group consisting of Y, Gd, Dy, Ho, Er and Yb and x is in the range 0 to 1. A rare earth superconductive body according to any one of claims 1 to 4 wherein the bulk concentration of said noble metal monotonously increases in one direction in said rare earth superconductive body. A rare earth superconductive body according to any one of claims 1 to 3 wherein said rare earth superconducting oxide is present as one or more grains and said noble metal is present as a plurality of particles dispersed in each of said grains. A rare earth superconductive body according to claim 5 or 6 wherein said rare earth superconducting oxide is present as a plurality of grains with adjacent grains thereof having substantially the same orientation. A process for making a rare earth superconductive body comprising a rare earth superconducting oxide and a noble metal distributed in said rare earth superconductive body which comprises the steps of: (1) preparing a compact containing the rare earth superconducting oxide or a precursor therefor and a compound containing a noble metal distributed in said compact such that the bulk concentration of said noble metal is different at at least two spatial regions in said compact; (2) heating said compact so that said compact becomes in a semi-molten state; and (3) slowly cooling said compact so that grains of the rare earth superconducting oxide start solidifying from a portion of said compact in which the bulk concentration of said noble metal is lowest. A process according to claim 8 wherein said bulk concentration of said compound monotonously increases in a direction in said compact. A process according to claim 8 or claim 9 wherein said compound is at least one of metallic silver and silver oxide. A method according to any one of claims 8 to 10 wherein said compact has a plurality of layered regions each having a uniform concentration of noble metal compound while the concentration of noble metal compound is different in adjacent said layered regions. A process for making a rare earth superconductive body, which comprises the steps of: (1) contacting a surface of a seed crystal with a surface of a compact containing a rare earth superconducting oxide or a precursor therefor and a compound containing a noble metal distributed in said compact, said seed crystal having a partial melting point higher than the partial melting point of said compact, and said seed crystal having a lattice constant similar to the lattice constant of said rare earth superconducting oxide; (2) heating said compact and said seed crystal so that said compact becomes in a semi-molten state while said seed crystal remains intact; and (3) slowly cooling said compact and said seed crystal so that grains of said rare earth superconducting oxide start solidifying in said compact from said compact surface contacted with said seed crystal. A process according to claim 12 wherein said compound is distributed in said compact so that said compact surface contacted with said seed crystal has a lowest bulk concentration of said compound in said compact, and the bulk concentration of said compound in said compact increases with distance from said surface.

Inventors: HAYASE TOORU, MATSUDA KAZUYUKI, SHIMIZU HIDEKI, TANI MAKOTO, HAYASE,TOORU, MATSUDA, KAZUYUKI, SHIMIZU, HIDEKI, TANI, MAKOTO
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1993
Main classification: H01L39
IPC codes: H01L39, C04B35