Superconducting composite

Abstract: Superconducting composite comprising a matrix made of superconducting sintered mass composed of perovskite type of quasi-­ perovskite type oxide and metal phase dispersed in the superconducting mass with a proportion of from 10 to 70 volume % with respect to said composite. The metal phase may consist of at least one of Cu, Ag, Au, Pt, Ni and Zn or their alloys. The superconducting sintered mass may be Ba-­ Y-Cu-O type compound oxide.

Description: Background of the Invention Field of the invention The present invention relates to a superconducting composite and a process for preparing the same; more particularly, it relates to a superconducting composite which can make good use of superconductivity which exhibits a higher critical temperature and a process for producing the same. Description of the related art FR-A-2.219.133 discloses a metal-ceramic composite and a method of manufacturing it, which composite comprises a sintered ceramic matrix whose pores contain a metallic material. Neither superconductor properties, nor the material proper to obtain such properties are disclosed. Under the superconducting condition, a perfect diamagnetism is observed and no difference in potential is observed for all that an electric current of a constant finite value is observed internally, and hence, a variety of applications of superconductivity have been proposed in the field of electric power transmission as a means for delivering electric power without loss. The superconductivity can be utilized in the field of power electric applications such as MHD power generation, power transmission, electric power reservation or the like; in the field of transportation for example magnetic levitation trains, or magnetically propelled ships; in the medical field such as high-energy beam radiation unit; in the field of science such as NMR or high-energy physics; high sensitive sensors or detectors for sensing very weak magnetic field microwave, radiant ray or the like, or in the field of fusion power generation. In addition to the abovementioned power electric applications, the superconducting materials can be used in the field of electronics, for example, as a Josephson device which is an indispensable switching device for realizing a high-speed computer which consumes very reduced power. However, their actual usage has been restricted because the phenomenon of superconductivity can be observed only at very low cryogenic temperatures. Among known superconducting materials, a group of materials having so-called A-15 structure show rather higher Tc (critical temperature of superconductivity) than others, but even the top record of Tc in the case of Nb₃Ge which showed the highest Tc could not exceed 23.2 K at most. This means that liquidized helium (boiling point of 4.2 K) is the only cryogen which can realize such very low temperature of Tc. However, helium is not only a limited costly resource but also requires a large-scaled system for liquefaction. Therefore, there had been a strong demand for other superconducting materials having higher Tc. But no material which exceeded the abovementioned Tc had been found for all studies for the past ten years. Possibility of existence of a new type of superconducting materials having much higher Tc was revealed by Bednorz and Müller who discovered a new oxide type superconductor in 1986 [Z. Phys. B64 (1986) 189]. This new oxide type superconducting material is [La, Ba]₂CuO₄ or [La, Sr]₂CuO₄ which are called K₂NiF₄-type oxides having a crystal structure which is similar to known perovskite type oxides. The K₂NiF₄-type oxides show such higher Tc as 30 to 50 K which are much higher than the known superconducting materials and hence it becomes possible to use liquidized hydrogen (b.p. = 20.4 K) or liquidized neon (b.p. = 27.3 K) as a cryogen which brings them to exhibit the superconductivity. It was also reported that C. W. Chu et al discovered in the United States of America another type of superconducting material having a critical temperature in the order of 90 K in February 1987, and hence possibility of existence of high-temperature superconductors has burst on the scene. However, the above mentioned new superconducting materials which were just born have been studied and developed only in the form of sintered bodies as a bulk produced from powders and hence are very fragile, so that careful handling is requested. In fact, they are apt to break or crack easily under mechanical stress, particularly, when they are shaped into a wire form. It is difficult to produce a sintered body composed of completely uniform polycrystal possessing higher superconductivity from particles having superconducting properties alone. Still more, the superconductivity is apt to be easily broken in stronger magnetic field or fluctuation and/or unhomogeneous distribution of temperature in the sintered body. The abovementioned oxide type superconducting materials possess rather higher specific resistance and lower heat-conductivity above the critical temperature. Therefore, if the phenomenon of superconductivity breaks locally, the sintered body generates locally Joule heat caused by the superconducting current and explosive evaporation of cryogen is induced when the heated portion of the sintered body contacts the cryogen. In order to avoid this trouble, in conventional metal type superconducting material, superconducting metal is shaped in the form of fine wires a plurality of which are embedded in electroconductive material which plays the role of a by-pass of electric current. The oxide type superconducting materials are, however, difficult to be shaped or deformed into such filaments, because they have not superior plasticity or processability in comparison with well-known metal type superconducting materials such as Ni-Ti alloy. EP-A-0 284 062, which is part of the art only so far as novelty is concerned, discloses a composite comprising a ceramic oxide superconductor matrix and non-superconductive material phase dispersed in the matrix, such as ferromagnetic or paramagnetic materials, Cu or Al which is inert to the matrix. This document does not disclose the surface of the non-superconductive material phase to be oxidized previously, and thus reduction of the sintered oxide may occur, with losses of superconductive property. EP-A-0 286 289 which is also part of the art only so far as novelty is concerned disclosed a method for preparing oxidemetal composite by oxidizing a mixture of metallic elements which will be changed to an oxide superconductor after oxidation, and noble metal. Also no disclosure of previous oxidation of the surface of the metallic phase is present. Therefore, an object of the present invention is to overcome the abovementioned problems and to provide a new superconducting composite which can be used under more stable conditions safely and a process for producing the same. Summary of the Invention The present invention provides a sintered superconducting composite comprising a superconducting sintered mass composed of composite oxide having a crystal structure of perovskite or quasi-perovskite and represented by the general formula : α w β x Cu y O z in which "α" represents one element selected from IIa group elements, "β" represents one element selected from IIIa group elements, each of "w", "x" and "y" represents a number which is not zero but is less than 1, and "z" represents a number which is selected from a range from 1 to 5, and a metal phase dispersed in said superconducting sintered mass at a proportion of from 10 to 70 % by volume in said composite, said metal phase being particles made of Cu, Ag, Au, Pt, Ni, Zn or alloys of these metals whose surface has been oxidized previously. Namely, the essence of the present invention resides in such a structure that electroconducting metal particles possessing higher heat conductivity and higher specific heat than a surrounding sintered mass or matrix composed of superconducting sintered oxide are dispersed in the latter. Therefore, even if the superconductivity breaks accidentally, electric current passes through the metal particles dispersed in the superconducting sintered oxide body so that excessive local heat generation caused by breakage of superconductivity becomes avoidable. Still more, the heat generated by Joule heat can be dispersed effectively because the metal phase possesses superior heat conductivity. Particularly, locally defined minute breakage of superconductivity may be annulled self-adjustably and hence the composite according to the present invention has a function for stabilizing the superconductivity. Furthermore, the composite according to the present invention shows improved mechanical strength and malleability, so that metal working for producing elongated articles such as rod, wire or a variety of parts of superconducting material can be facilitated and also the products possess improved mechanical strength. Namely, such disadvantages of sintered body as poor mechanical strength or fragility can be compensated with the metal phase which has higher resistance to plastic deformation and which is filled in pores in the sintered body. The performance as superconductivity of the abovementioned composite according to the present invention is not spoiled by the presence of metal particles dispersed in the superconducting sintered body because the continuity of the sintered body itself is not lost. In practice, the metal phase may consist of at least one of the metals Cu, Ag, Au, Pt, Ni and Zn or their alloys. Copper or copper alloys may be preferably selected for the reason of economy and stability of electric current, while Pt, Au, Ag which possess smaller values of free energy of oxide formation may be selected in order to prevent oxidation of the metal particles. The oxides which compose the superconducting sintered body are preferably compound oxides in which said element "α" is selected from a group consisting of Ba, Sr, Ca, Mg or Be and said element "β" is selected from a group consisting of Y, La, Sc, Ce, Gd, Ho, Er, Tm, Yb, Lu or the like. The volumetric percentage of the metal phase in the compact is preferably from 10 to 70 %. When the percentage of the metal phase is not higher than 10 %, the effect for stabilizing the superconducting condition cannot be expected because of inadequate heat-absorption capacity of the metal phase. To the contrary, if the percentage exceeds 70 %, higher current density of superconducting as a bulk cannot be obtained because the continuity of superconducting phase in the sintered body cannot be maintained. According to another preferred aspect of the present invention, platinum (Pt) is preferably used as the metal phase since the abovementioned sintered oxide is apt to be reduced when it is contacted with the metal and loses the superconducting property. Oxidizing the surface of the metal particles previously prevents the abovementioned reduction by the metal phase. For example, when the copper is used as metal particle of the metal phase, the copper particles are preferably surface-treated to form a surface layer of CuO or Cu₂O. The present invention also provides a process for producing the abovementioned superconducting composite, namely a process for producing a superconducting composite comprising a superconducting sintered mass (1) composed of composite oxide having a crystal structure of perovskite or quasi-perovskite and represented by the general formula : α w β x Cu y O z in which "α" represents one element selected from IIa group elements, "β" represents one element selected from IIIa group elements, each of "w", "x" and "y" represents a number which is not zero but is less than 1, and "z" represents a number which is selected from a range from 1 to 5, and a metal phase (2) dispersed in said superconducting sintered mass at a proportion of from 10 to 70 % by volume in said composite, said metal phase (2) being particles made of Cu, Ag, Au, Pt, Ni, Zn or alloys of these metals, surfaces of said metal phase (2) being oxidized, characterized by the steps of preparing a powder mixture comprising a powder of oxide or carbonate of said element "α", a powder of oxide or carbonate of said element "β" and a powder of oxide or carbonate of copper so that atom ratios of said elements "α":"β":Cu are adjusted to w:x:y, wherein each of "w", "x" and "y" represents a number which is not zero but is less than 1, sintering said powder mixture preliminarily, pulverizing the resulting preliminarily sintered mass, admixing the resulting pulverized powder with a metal powder of Cu, Ag, Au, Pt, Ni, Zn or alloys of these metals whose surface has been oxidized previously, compacting the resulting metal containing powder mixture into a shaped body, and subjecting the shaped body to the final sintering. It is supposed that the critical temperature of superconductivity can be improved when the size of crystal grains is reduced to increase the total amount of the grain boundaries. Therefore, it is preferable that the sintered mass which constitutes the skeleton of the superconducting composite has as fine a crystal structure as possible. In practice, the material powder to be sintered previously has preferably a particle size of less than 5 µm. When the particle size of the material powder exceeds 5 µm, the crystal grains in the finally sintered body cannot be reduced to desired dimension, more precisely the particle size of the sintered mass will have an average particle size of more than 6 µm, resulting in deterioration of superconductivity. To the contrary, larger crystal grains which exceed 10 µm reduce the total amount of the grain boundaries. The decrease of the grain boundaries is not preferable to improve the critical temperature, since it is supposed that the superconductors of the perovskite type oxide or quasi-perovskite type oxide exhibit their improved property particularly in the neighborhood of their grain boundaries. The sequence of preliminary sintering, pulverization and compacting to the shaped body is preferably repeated for several times in order to proceed to solid solution reaction of the material powder or the preliminary sintered mass. It is also preferable to pulverize the preliminary sintered powder into finer crystal grains. The sequence of preliminary sintering, pulverization and compacting to the shaped body is preferably repeated for at least three times. The temperature of the final sintering is a very critical factor to achieve higher critical temperature of superconductivity so that the temperature is controlled to satisfy such conditions that the sintering proceeds in solid phase reaction and that excessive crystal growth of perovskite type or quasi-perovskite type oxide does not occur. Therefore, it is preferable that the final sintering is carried out at a temperature selected in a range of from 800 to 1,000 °C. Namely, if the sintering temperature is not higher than 800 °C, the sintered product obtained shows very low mechanical strength. To the contrary, if the sintering temperature exceeds 1,000°C, a liquid phase is produced during the sintering operation or larger crystal grains are produced, resulting in lowering the Tc of the sintered body. Because of the same reason as above, the preliminary sintering is preferably carried out at a temperature ranging from 700 °C to 950 °C. When the temperature of the preliminary sintering is not higher than 700 °C, a satisfactory solid reaction does not proceed so that the desired crystal structure cannot be obtained, while, if the preliminary sintering is carried out at a temperature which exceeds 950 °C, the resulting preliminary sintered mass contains a solid solution phase or larger crystal grains, which makes it difficult to obtain finer crystal grains. Brief Description of the Drawings FIG.1 is an illustrative drawing of the superconducting composite according to the present invention, showing a structure thereof. FIG. 2 is also another illustrative view of another type of superconducting composite, showing a different structure thereof. The typical structures of the superconducting composite according to the present invention are illustrated in Fig. 1 and Fig. 2. In the first type shown in Fig. 1, a relatively small amount of the metal phase 2 is dispersed in a matrix consisting of superconducting sintered body 1. In this type, electric current can pass through the continuous sintered body 1 without breakage. This type of superconducting composite is realized when the volumetric percentage of the metal phase in the composite is in a range of from 10 to 20 %. In the second type shown in Fig. 2, the volumetric percentage of the metal phase is increased to 60 to 70 %, so that the amount of the metal phase 2 is larger than that of the sintered body. Even in this case, a superconducting property can be maintained through the matrix of the superconducting sintered body 1 which extends three-dimensionally. Now, embodiments of the present invention will be described with reference to illustrative Examples, but the scope of the present invention should not be limited thereto. Examples 1 to 6 At first, powders of BaCO₃, Y₂O₃, and CuO having a purity of more than 3N and an average particle size of 3 µm are mixed uniformly in such proportions that the value of "x" and "y"in the formula: (Ba 1-x Y x )Cu y O z becomes equal to x = 0.4 to 0.6 and y = 1.0 and 0.5 respectively. The powder mixture is sintered preliminarily at 920 °C for 12 hours in air to obtain a cake-like solid mass of preliminary sintered body. Then, the solid mass is pulverized in a mortar and then by means of alumina balls in a ball mill for 5 hours to prepare a powder having an average particle size of 5 µm. The same procedure as the abovementioned combination of preliminary sintering and pulverization is repeated for three times. A powder of electrolytic copper having a particle size of 325 mesh and a purity of more than 3N is admixed uniformly with the pulverized powders. Then, the resulting powder mixture is compacted in a rubber mold under a pressure of 0.5 ton/cm² statically to obtain a compact having a size of 30ø x 50 mm. Each sample having a thickness of 1.2 mm, a width of 5 mm and a length of 20 mm is cut from the compact and is sintered finally at 920 to 940 °C for 12 hours in O₂ to obtain a shaped and sintered body. The product thus obtained contains a copper metal phase of volumetric percentage which is shown in Table 1 in which the results of other examples using Gd and Yb in place of Y are also summarized.

Claims: A sintered superconducting composite comprising a superconducting sintered mass (1) composed of composite oxide having a crystal structure of perovskite or quasi-perovskite and represented by the general formula : α w β x Cu y O z in which "α" represents one element selected from IIa group elements, "β" represents one element selected from IIIa group elements, each of "w", "x" and "y" represents a number which is not zero but is less than 1, and "z" represents a number which is selected from a range from 1 to 5, and a metal phase (2) dispersed in said superconducting sintered mass at a proportion of from 10 to 70 % by volume in said composite, said metal phase (2) being particles made of Cu, Ag, Au, Pt, Ni, Zn or alloys of these metals whose surface has been oxidized previously. The sintered superconducting composite set forth in claim 1 wherein said metal phase is made of copper particles whose surface is oxidized to CuO or Cu₂O. A process for producing a superconducting composite comprising a superconducting sintered mass (1) composed of composite oxide having a crystal structure of perovskite or quasi-perovskite and represented by the general formula : α w β x Cu y O z in which "α" represents one element selected from IIa group elements, "β" represents one element selected from IIIa group elements, each of "w", "x" and "y" represents a number which is not zero but is less than 1, and "z" represents a number which is selected from a range from 1 to 5, and a metal phase (2) dispersed in said superconducting sintered mass at a proportion of from 10 to 70 % by volume in said composite, said metal phase (2) being particles made of Cu, Ag, Au, Pt, Ni, Zn or alloys of these metals, surfaces of said metal phase (2) being oxidized, characterized by the steps of preparing a powder mixture comprising a powder of oxide or carbonate of said element "α", a powder of oxide or carbonate of said element "β" and a powder of oxide or carbonate of copper so that atom ratios of said elements "α":"β":Cu are adjusted to w:x:y, wherein each of "w", "x" and "y" represents a number which is not zero but is less than 1, sintering said powder mixture preliminarily, pulverizing the resulting preliminarily sintered mass, admixing the resulting pulverized powder with a metal powder of Cu, Ag, Au, Pt, Ni, Zn or alloys of these metals whose surface has been oxidized previously, compacting the resulting metal containing powder mixture into a shaped body, and subjecting the shaped body to the final sintering. The process set forth in claim 3 wherein the final sintering stage is carried out at a temperature ranging from 800°C to 1,100°C. The process set forth in claim 3 or 4 wherein the preliminary sintering stage is carried out at a temperature ranging from 700°C to 900°C. The process set forth in any one of claims 3 to 5 wherein the steps of the preliminary sintering, pulverization and compacting of the pulverized powder are repeated for at least three times. The process set forth in any one of claims 3 to 6 wherein the preliminarily sintered mass is pulverized into a powder having an average particle size of less than 8 µm.

Inventors: JODAI TETSUJI ITAMI WORKS OF S, SIBATA KENICHIRO ITAMI WORKS O, YAMAGUCHI TAKESHI ITAMI WORKS, YAZU SHUJI ITAMI WORKS OF SUMI, JODAI, TETSUJI ITAMI WORKS OF SUMITOMO, SIBATA, KENICHIRO ITAMI WORKS OF SUMITOMO, YAMAGUCHI, TAKESHI ITAMI WORKS OF SUMITOMO, YAZU, SHUJI ITAMI WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES LIMITED
Application year: 1988
Main classification: H01L39
IPC codes: H01L39