High-temperature oxide superconductor

Abstract: Disclosed herein are high-temperature oxide super­ conductors of RBa₂Cu₄O₈ type, with Ba partly repaced by Sr or Ca, or with R and Ba partly replaced by Ca and Sr, respectively, as represented by the chemical composition formula of R(Ba 1-y Sr y )₂Cu₄O₈ or R(Ba 1-z Ca z )₂Cu₄O₈ or (R 1-x Ca x )(Ba 1-y Sr y )₂Cu₄O₈. They exhibit superconductiv­ ity at high temperatures. Especially, the last one exhib­ its superconductivity at a higher temperature than the former two. All of them can be made with a less amount of Ba as a deleterious substance, and the first two have improved sinterability. The best results are obtained when they are produced by the process involving the hot hydrostatic pressure treatment of the mixture of raw mate­ rials at 850-1100°C in an atmosphere composed of an inert gas and oxygen. The process permits a wider selection of Ba raw materials.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-temperature oxide superconductor which has a superconducting transition temperature (referred to as T c ) higher than the boiling point of liquid nitrogen and is less liable to the variation of T c due to the liberation of oxygen that takes place during fabrication at a high temperature. 2. Description of the Prior Art Among the known typical oxide superconductors having a T c higher than the boiling point of liquid nitrogen is RBa₂Cu₃O₇ of three-layer perovskite structure which has a T c of 90K. (R denotes Y or at least one element selected from the lanthanoid rare earth elements.) [Appl. Phys. Lett. Vol. 51 (1987) P57] Unfortunately, the above-mentioned oxide superconductor has the property that oxygen atoms as the constituent members are liable to get out of it when it is affected by head during fabrication. For this reason, the oxygen content in the superconductor varies depending on the heating conditions for fabrication. The variation of oxygen content brings about the orthorhombic-tetragonal phase transition, which in turn causes the T c to vary over a broad range from 0K to 90K. [Phys. Rev. B36 (1987) P5719] For example, RBa₂Cu₃O₇ powder becomes deteriorated in superconductivity due to the partial loss of oxygen atoms which occurs during the sintering heat treatment in the production of a superconducting wire by the silver-sheath wire drawing method. This method involves the steps of filling the oxide semiconductor powder in a silver pipe, working the silver pipe into a wire by cold drawing, and sintering the powder at 800-900°C. In contrast with the above-mentioned oxide superconductor, there is known another oxide superconductor of RBa₂Cu₄O₈ type, which has the double-layered Cu-O chains instead of the single-layered Cu-O chains in the RBa₂Cu₃O₇-type crystal structure. It has a stable chemical structure, retaining oxygen atoms until it is heated to about 850°C. Moreover, it has the T c in the neighborhood of 80K, which is higher than the boiling point of liquid nitrogen. Thus it is regarded as an important substance from the standpoint of practical use. The oxide superconductor of RBa₂Cu₄O₈ type can be produced by one of the following two methods which have been proposed so far. (1) A method involving the heat treatment of a calcined powder in a high-pressure atmosphere of pure oxygen. (E.g., at 930°C for 8 hours under an oxygen pressure of 10 MPa (100 atm)) [High-pressure oxygen method; T c = 81K, Nature 336 (1988) P660-662, or Phys. Rev. B 39 (1988) P7347-7350] (2) A method involving the heat treatment of a mixture of a calcined powder with a catalyst (such as sodium carbonate) in an oxygen stream for a long time. (E.g., at 800°C for 72 hours) [Normal-pressure method; T c = 77K, Nature 338 (1989) P328-330] Unfortunately, the superconductor compound of R-Ba-Ca-O type presents a problem associated with its production process because it contains a large portion of Ba and its synthesis needs a large amount of Ba compounds such as BaO, BaCO₃, and Ba(NO₃)₂, which are deleterious substances. (For example, the Ba content in YBa₂CU₄O₈ is 13.33 mol%.) Another disadvantage of this oxide superconductor is that it cannot be made with cheap BaCO₃ by the above-mentioned methods because BaCO₃ decomposes at 1000°C or above under normal pressure, whereas the reaction in the above-mentioned methods takes place at a comparatively low temperature. Therefore, it has to be made with an expensive compound, such as Ba(NO₃)₂, in place of BaCO₃. In addition, the present inventors' investigation revealed that an oxide of YBa₂Cu₄O₈ type is poor in sinterability and is liable to give a sintered body having a rather low density. The sintered body of low density has a low critical current density as a matter of course. OBJECT OF THE INVENTION It is an object of the present invention to provide an oxide superconductor which has a high density, is almost free of oxygen absorption and liberation at high temperatures, exhibits good head stability during fabrication of any kind, and can be made with a less amount of barium as a deleterious substance. It is another object of the present invention to provide an oxide superconductor which can be made with a Ba raw material in great variety. These and other objects of the invention will become more apparent in the detailed description and examples which follow. SUMMARY OF THE INVENTION The present invention provides an oxide superconductor of RBa₂Cu₄O₈ type which is represented by the chemical composition formula of R(Ba 1-y Sr y )₂Cu₄O₈ (0.001 ≦ y ≦ 0.6) or R(Ba 1-z Ca z )₂Cu₄O₈ (0.001 ≦ z ≦ 0.2), where R denotes Y or at least one element selected from the lanthanum series elements. This oxide superconductor is characterized by that Ba in RBa₂Cu₄O₈ is partly replaced with Sr or Ca. This leads to the reduction of Ba content and improved sinterability as compared with the conventional oxide superconductor of RBa₂Cu₄O₈ type. In the present invention, a part of R also may be substituted with Ca in an amount of lower than 30% when Ba is partially substituted with Sr. Such a superconductor is represented by the chemical composition formula of (R 1-x Ca x ) (Ba 1-y Sr y )₂Cu₄O₈ (0 ≦ x ≦ 0.3 and 0.001 ≦ y ≦ 0.6), where R denotes Y or at least one element selected from the lanthanide series elements. This oxide superconductor is characterized by that R and Ba in RBa₂Cu₄O₈ are partly replaced with Ca and Sr, respectively. This leads to the reduction of Ba content and permits a wide selection of raw materials. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic drawing illustrating the crystal structure of RBa₂Cu₄O₈. Fig. 2 is a schematic drawing illustrating the crystal structure of RBa₂Cu₃O₇. Fig. 3 is an X-ray diffraction pattern (powder method) of a sample in Example 1 which is represented by Y(Ba 1-y Sr y )₂Cu₄O₈ (where y = 0.3). Fig. 4 is a graph showing the resistance-temperature characteristics of a sample in Example 1 which is represented by Y(Ba 1-y Sr y )₂Cu₄O₈. Fig. 5 is a graph showing the results of thermogravimetry for a sample in Example 1 which is represented by Y(Ba 1-y Sr y )₂Cu₄O₈. Fig. 6 is an X-ray diffraction pattern (powder method) of a sample in Example 2 which is represented by Ho(Ba 1-y Sr y )₂Cu₄O₈ (where y = 0.3). Fig. 7 is a graph showing the resistance-temperature characteristics of a sample in Example 2 which is represented by Ho(Ba 1-y Sr y )₂Cu₄O₈. Fig. 8 is a graph showing the results of thermogravimetry for a sample in Example 2 which is represented by Ho(Ba 1-y Sr y )₂Cu₄O₈. Fig. 9 is an X-ray diffraction pattern (powder method) of a sample in Example 5 which is represented by Y(Ba 1-z Ca z )₂Cu₄O₈ (where z = 0.1). Fig. 10 is a graph showing the resistance-temperature characteristics of a sample in Example 5 which is represented by Y(Ba 1-z Ca z )₂Cu₄O₈. Fig. 11 is a graph showing the results of thermogravimetry for a sample in Example 5 which is reprsented by Y(Ba 1-z Ca z )₂Cu₄O₈ (where z = 0.1). Fig. 12 is an X-ray diffraction pattern (powder method) of a sample in Example 6 which is represented by Ho(Ba 1-z Ca z )₂Cu₄O₈ (where z = 0.1). Fig. 13 is a graph showing the resistance-temperature characteristics of a sample in Example 6 which is represented by Ho(Ba 1-z Ca z )₂Cu₄O₈. Fig. 14 is a graph showing the results of thermogravimetry for a sample in Example 6 which is represented by Ho(Ba 1-z Ca z )₂Cu₄O₈ (where z = 0.1). Fig. 15 is an X-ray diffraction pattern (powder method) of a sample in Example 9 which is represented by (R 1-x Ca x )(Ba 1-y Sr y )₂Cu₄O₈ (where x = 0.1 and y = 0.3). Fig. 16 is a graph showing the resistance-temperature characteristics of a sample in Example 9 which is represented by (R 1-x (Ca x )(Ba 1-y Sr y )₂Cu₄O₈ (where x = 0.1 and y = 0.3). Fig. 17 is a graph showing the a.c. susceptibility of a sample in Example 9 which is represented by (R 1-x Ca x )(Ba 1-y Sr y )₂Cu₄O₈ (where x = 0.1 and y = 0.3). Fig. 18 is a graph showing the results of thermogravimetry of a sample in Example 9 which is represented by (R 1-x Ca x )(Ba 1-y Sr y )₂Cu₄O₈ (where x = 0.1 and y = 0.3). Fig. 19 is an X-ray diffraction pattern (powder method) of a sample in Example 10 which is represented by (R 1-x Ca x )(Ba 1-y Sr y )₂Cu₄O₈ (where x = 0.1 and y = 0.3). Fig. 20 is a graph showing the resistance-temperature characteristics of a sample in Example 10 which is represented by (R 1-x Ca x )(Ba 1-y Sr y )₂Cu₄O₈ (where x = 0.1 and y = 0.3). Fig. 21 is a graph showing the a.c. susceptibility of a sample in Example 10 which is represented by (R 1-x Ca x )(Ba 1-y Sr y )₂Cu₄O₈ (where x = 0.1 and y = 0.3). Fig. 22 is a graph showing the results of thermogravimetry of a sample in Example 10 which is represented by (R 1-x Ca x )(Ba 1-y Sr y )₂Cu₄O₈ (where x = 0.1 and y = 0.3). DETAILED DESCRIPTION OF THE INVENTION The present inventors carried out their researches to improve the sinterability of the oxide superconductor of RBa₂Cu₄O₈ type. As the result, it was found that the object is achieved when Ba in the oxide superconductor is partly replaced by Sr or Ca. The thus obtained oxide superconductor contains a reduced amount of Ba as the chemical composition (after replacement) indicates. This is an important point in the present invention. The oxide superconductor pertaining to the present invention has the basic structure of RBa₂Cu₄O₈ which is schematically shown in Fig. 1, whereas the conventional oxide superconductor has the basic structure of RBa₂Cu₃O₇ which is schematically shown in Fig. 2. In Figs. 1 and 2, the reference numerals 1, 2, 3, and 4 denote R, Ba, Cu, and O arranged on the intersecting point of lines, respectively. The crystal structure of RBa₂Cu₄O₈ in Fig. 1 has the Cu-O chains in two layers, whereas that of RBa₂Cu₃O₇ in Fig. 2 has the Cu-O chains in one layer. The oxide superconductor pertaining to the present invention is represented by the chemical composition formula of R(Ba 1-y Sr y )₂Cu₄O₈ or R(Ba 1-z Ca z )₂Cu₄O₈, which is derived when Ba in RBa₂Cu₄O₈ is partly replaced by Sr or Ca. The oxide superconductor of the present invention represented by R(Ba 1-y Sr y )₂Cu₄O₈ or R(Ba 1-z Ca z )₂Cu₄O₈ has the T c higher than 80K and can be easily sintered into a sintered body having a porosity lower than 10%, whereas the oxide superconductor represented by RBa₂Cu₄O₈ (as the base material) has the T c of 80K and is difficult to sinter. As the result of their investigation into a more useful oxide superconductor, the present inventors have proposed an oxide of (R 1-x Ca x )Ba₂Cu₄O₈ type, which is a derivative of the oxide superconductor of RBa₂Cu₄O₈ type, with R partly replaced by Ca. (See Japanese Patent Application No. 176472/1989.) The oxide superconductor of (R 1-x Ca x )Ba₂Cu₄O₈ type has a T c which is much higher than the boiling point of liquid nitrogen on account of the partial replacement of R by Ca. In addition, it is stable at high tempeatures (even when heated at about 850°C) with no loss of oxygen. After their continued investigation, the present inventors found that better results can be obtained when the oxide superconductor of RBa₂Cu₄O₈ type has not only R partly replaced by Ca but also Ba partly replaced by Sr. The resulting oxide superconductor contains a smaller amount of Ba as a deleterious substance. Moreover, it has a higher T c than the oxide superconductor with Ba alone partly replaced by Sr. This finding is the basis on which the present invention was completed. The oxide superconductor pertaining to the present invention is represented by the chemical composition formula of (R 1-x Ca x )(Ba 1-y Sr y )₂Cu₄O₈, which is derived when R and Ba in RBa₂Cu₄O₈ (shown in Fig. 1) are partly replaced by Ca and Sr, respectively. The oxide superconductor of the present invention reprsented by (R 1-x Ca x )(Ba 1-y Sr y )₂Cu₄O₈ (where 0 < x ≦ 0.3 and 0.001 ≦ y ≦ 0.6) contains a reduced amount (down to 5.33 mol%) of Ba, whereas the conventional oxide superconductor represented by RBa₂Cu₄O₈ (where R = Y ) having a T c of 80K contains as much Ba as 13.33 mol% which is supplied from barium nitrate. The oxide superconductor of the present invention can be produced by the HIP treatment method (mentioned later), which permits Ba to be supplied from barium carbonate. Thus the present invention permits a wider selection of raw materials, which leads to the economical production of oxide superconductors having the superconducting transition temperature in the neighborhood of 80K. The results of thermogravimetry indicate that the oxide superconductors pertaining to the present invention remain stable without oxygen absorption or liberation. Therefore, they can be made into a stable, compactly sintered superconducting wire by the silver sheath method without any adverse effect on superconductivity int he final sintering step. The oxide superconductors pertaining to the present invention can be given a higher density than before by the high-temperature molding that employs a binder. This is because they are stable at high temperatures and hence can be freed from the binder by heating up to about 850°C. By contrast, the conventional oxide superconductor (RBa₂Cu₃O₇) cannot be freed from the binder by heating because it permits the liberation of oxygen atoms upon heating above 400°C. The higher density leads to the improvement of critical current density. In addition, the oxide superconductors pertaining to the present invention can be made into a thin film which is stabler to environments and has a stabler superconducting transition temperature than a thin film of the conventional oxide superconductor of RBa₂Cu₃O₇ type, which is easily deteriorated in the air at normal temperature on account of its great specific surface area. The oxide superconductor of the present invention can be produced by several methods including sputtering and coprecipitation. The most effective method is by the hot isostatic Pressure treatment (HIP treatment) of a powder mixture of R, Ca, Sr, Ba, Cu, and O (as raw materials) in an atmosphere composed of an inert gas and oxygen at 850-1100°C. Since the atmosphere for the HIP treatment is composed of an inert gas and oxygen, the total pressure of the atmosphere can be much higher so that the partial pressure of oxygen is equal to the pressure (say 20 MPa (200 atm)) required in the case where the atmosphere is pure oxygen. The atmosphere composed of an inert gas and oxygen in a molar ratio of 1:1 may have a total pressure of 40 MPa (400 atm), and with a molar ratio of 4:1, the total pressure can be raised to 100 MPa (1000 atm). It is considered that the atmosphere under high pressure increases the diffusion of Cu atoms and promotes the formation of the oxide superconductor of RBa₂Cu₄O₈ type. The HIP treatment is safer than the conventional process that employs high-pressure pure oxygen. The HIP treatment should be performed at 850°C or above so as to promote the formation of the oxide superconductor of the present invention, while suppressing the formation of the oxide of RBa₂Cu₃O₇ type. The HIP treatment at a temperature beyond the upper limit of 1100°C forms a mixed phase containing R₂Ba₄Cu₇O δ . EXAMPLES Example 1 Mixtures each having a chemical composition of Y(Ba 1-y Sr y )₂Cu₄O₈ (where y = 0, 0.01, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, or 0.7) were prepared from Y₂O₃, Ba(NO₃)₂, CuO, and Sr(NO₃)₂ (in powder form having a purity of 99.9%). The mixture was calcined at 850°C for 24 hours in oxygen. The calcined product was crushed and molded into a rectangular prism. The molded material underwent preliminary sintering at 800°C for 5 hours in oxygen. The sintered material underwent heat treatment in an atmosphere composed of 80% of argon and 20% of oxygen under a pressure of 100 MPa (1000 atm). The heat treatment consisted of heating up to 960°C at a rate of 200°C/hr and keeping this temperature for 6 hours, and further heating up to 1050°C at a rate of 200°C/hr and keeping this temperature for another 6 hours. The heat treatment was followed by cooling down to 300°C at a rate of 200°C/hr. The atmosphere was depressurized to 10⁵ Pa (1 atm) and the heat-treated material was discharged. The heat-treated material was crushed again and molded. The molded material was sintered at 800°C for 10 hours in oxygen. The thus obtained sintered body (sample) was tested for phase by X-ray diffractometry (powder method). It was found that the principal constituent of the sample has the crystal structure of YBa₂Cu₄O₈ type. The sample in which y = 0.3 gave an X-ray diffraction pattern (by powder method) as shown in Fig. 3. This pattern has the peaks attributable to the YBa₂Cu₄O₈ structure, which are indicated by indices. This sample is composed of a single superconducting phase. The samples obtained in this example have the phases as shown in Table 1. The samples in which y is 0 to 0.6 are composed of a single phase of Y(Ba 1-y Sr y )₂Cu₄O₈, whereas the sample in which y is 0.7 contains a second phase. The samples were tested for superconductivity by measuring their resistance. The results are shown in Table 1 and Fig. 4. In Table 1, T c on denotes the onset temperature for superconductivity transition from normal conductivity; T c R = 0 denotes the temperature at which the resistance decreases to zero; and ρ 300K denotes the resistivity at 300K. y Crystal phase T c on (K) T c R=0 (K) ρ 300K (× 10⁻³ Ω·cm) Porosity (%) Remarks 0.0 Superconducting single phase 80 75 7.2 39 Comparative Example 0.001 Ditto 81 76 5.1 22 Example 0.01 Ditto 82 77 3.6 15 Example 0.1 Ditto 82 78 1.2 9 Example 0.2 Ditto 82 77 1.6 7 Example 0.3 Ditto 82 76 1.5 5 Example 0.4 Ditto 82 76 1.3 8 Example 0.5 Ditto 81 76 0.9 13 Example 0.6 Ditto 81 76 2.1 19 Example 0.7 Superconducting phase plus impurity phase 81 71 8.4 24 Comparative Example It is noted from Table 1 and Fig. 4 that all the oxide superconductors Y(Ba 1-y Sr y )₂Cu₄O₈ pertaining to the present invention have the T c of 80K level, which is higher than the boiling point (77K) of liquid nitrogen. It is also noted that the samples decrease in resistance (at room temperature) in proportion to the Sr content (y). Those samples having a low resistance at room temperature are expected to have a high critical current density. The sample in which y is 0.5 has the lowest resistance at room temperature. The resistance increases in proportion to the value of y when y is greater than 0.6, and when y is 0.7, the resistance is higher than that when y is 0. The samples were tested for porosity by observing polished samples under an optical microscope. The results are shown in Table 1. It is noted that the porosity decreases in proportion to the value of y, reaching the minimum of about 5% when y is 0.3, and the porosity is not improved even when the value of y increases further. The results of X-ray diffractometry and the measurements of porosity as mentioned above suggest that the decrease of resistance (at room temperature) in proportion to the value of y is due to the formation of a solid solution by partial substitution of Sr for Ba in YBa₂Cu₄O₈. Consequently, according to the present invention, the value of y should be in the range of 0.001 to 0.6. Thermogravimetry performed on the samples in which the value of y is 0.1 and 0.01 gave the results as shown in Fig. 5(a). It is noted that the samples remain unchanged in weight over the range from normal temperature to 850°C and decrease in weight over the range from 850°C to 900°C. These results suggest that the samples remain stable up to a temperature as high as 850°C without absorption and liberation of oxygen. By contrast, the conventional superconductor YBa₂Cu₃O₇ loses a large amount of oxygen at 400°C to 800°C, as shown in Fig. 5(b). It will be understood from the foregoing that the superconductor YBa₂Cu₄O₈ as the base material is difficult to sinter and gives a sintered body having a porosity higher than 30%, whereas the samples in this example which are specified by the composition of Y(Ba 1-y Sr y )₂Cu₄O₈ (where 0.001 ≦ y ≦ 0.6) have the T c higher than 80K and give sintered bodies having a low porosity. In addition, these samples have a low electric resistance at room tempeature and remain stable up to 850°C without absorption and liberation of oxygen. Example 2 Mixtures each having a chemical composition of Ho(Ba 1-y Sr y )₂Cu₄O₈ (where y = 0, 0.01, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, or 0.7) were prepared from Ho₂O₃, Ba(NO₃)₂, CuO, and Sr(NO₃)₂ (in powder form having a purity of 99.9%). The mixture was calcined at 850°C for 24 hours in oxygen. The calcined product was crushed and molded into a rectangular prism. The molded material underwent preliminary sintering at 800°C for 5 hours in oxygen. The sintered material underwent heat treatment in an atmosphere composed of 80% of argon and 20% of oxygen under a pressure of 100 MPa (1000 atm). The heat treatment consisted of heating up to 950°C at a rate of 200°C/hr and keeping this temperature for 6 hours, and further heating up to 1050°C at a rate of 200°C/hr and keeping this temperature for another 6 hours. The heat treatment was followed by cooling down to 300°C at a rate of 200°C/hr. The atmosphere was depressurized to 10⁵ Pa (1 atm) and the heat-treated material was discharged. The heat-treated material was crushed again and molded. The molded material was sintered at 800°C for 20 hours in oxygen. The thus obtained sintered body (sample) was tested for phase by X-ray diffractometry (powder method). It was found that the principal constituent of the sample has the crystal structure of RBa₂Cu₄O₈ type. The sample in which y = 0.1 gave an X-ray diffraction pattern (by powder method) as shown in Fig. 6. This pattern has the peaks attributable to the RBa₂Cu₄O₈ structure, which are indicated by indices. This sample is composed of a single superconducting phase. The samples obtained in this example have the phases as shown in Table 2. The samples in which y is 0 to 0.6 are composed of a single phase of Ho(Ba 1-y Sr y )₂Cu₄O₈, whereas the sample in which y is 0.7 contains a second phase. The samples were tested for superconductivity by measuring their resistance. The results are shown in Table 2 and Fig. 7. y Crystal phase T c on (K) T c R=0 (K) ρ 300K (× 10⁻³ Ω·cm) Porosity (%) Remarks 0.0 Superconducting single phase 80 75 7.2 39 Comparative Example 0.001 Ditto 81 78 5.1 22 Example 0.01 Ditto 81 78 3.6 15 Example 0.1 Ditto 82 79 3.1 9 Example 0.2 Ditto 82 78 2.7 5 Example 0.3 Ditto 82 78 1.9 5 Example 0.4 Ditto 82 77 1.6 6 Example 0.5 Ditto 81 76 1.1 7 Example 0.6 Ditto 81 76 4.2 6 Example 0.7 Superconducting phase plus impurity phase 80 72 8.9 8 Comparative Example It is noted from Table 2 and Fig. 7 that all the oxide superconductors Ho(Ba 1-y Sr y )₂Cu₄O₈ pertaining to the present invention have the T c of 80K level, which is higher than the boiling point (77K) of liquid nitrogen. It is also noted that the samples decrease in resistance (at room temperature) in proportion to the Sr content (y). Those samples having a low resistance at room temperature are expected to have a high critical current density. The sample in which y is 0.5 has the lowest resistance at room temperature. The resistance increases in proportion to the value of y when y is greater than 0.6, and when y is 0.7, the resistance is higher than that when y is 0. The samples were tested for porosity by observing polished samples under an optical microscope. The results are shown in Table 2. It is noted that the porosity decreases in proportion to the value of y, reaching the minimum of about 5% when y is 0.3, and the porosity remains unchanged even when the value of y increases further. The results of X-ray diffractometry and the measurements of porosity as mentioned above suggest that the decrease of resistance (at room temperature) in proportion to the value of y is due to the formation of a solid solution by partial substitution of Sr for Ba in HoBa₂Cu₄O₈. Thermogravimetry performed on the samples in which the value of y is 0.1 and 0.01 gave the results as shown in Fig. 8(a). It is noted that the samples remain unchanged in weight over the range from normal temperature to 850°C and decrease in weight over the range from 850°C to 900°C. These results suggest that the samples remain stable up to a temperature as high as 850°C without absorption and liberation of oxygen. By contrast, the conventional superconductor HoBa₂Cu₃O₇ loses a large amount of oxygen at 400°C to 800°C, as shown in Fig. 8(b). It will be understood from the foregoing that the superconductor HoBa₂Cu₄O₈ as the base material has the T c of 80K and is difficult to sinter and gives a sintered body having a porosity higher than 30%, whereas the samples in this example which are specified by the composition of Ho(Ba 1-y Sr y )₂Cu₄O₈ (where 0.001 ≦ y ≦ 0.6) have the T c higher than 80K and give sintered bodies having a porosity lower than 22%. In addition, these samples have a low electric resistance at room temperature and remain stable up to 850°C without the absorption and liberation of oxygen. Example 3 Samples were prepared in the same manner as in Example 1 except that R in R(Ba 1-y Sr y )₂Cu₄O₈, was replaced by Nd, Sm, Eu, Gd, Dy, Er, Tm, Yb, or Lu, with the value of y fixed at 0.3. The samples were evaluated in the same manner as in Example 1. The results are shown in Table 3. R X-ray diffractometry T c on (K) T c R=0 (K) ρ 300K (× 10⁻³ Ω·cm) Porosity (%) Remarks Nd Superconducting single phase 83 78 0.89 8 Example Sm Ditto 82 78 0.82 7 Example Eu Ditto 82 76 0.76 7 Example Gd Ditto 82 76 0.72 6 Example Dy Ditto 81 76 0.68 5 Example Er Ditto 80 76 0.72 5 Example Tm Ditto 80 77 0.75 5 Example Yb Superconducting phase plus impurity phase 79 76 1.02 7 Example Lu Superconducting phase plus impurity phase 79 76 1.11 9 Example It is noted from Table 3 that the same effect is produced even when R is changed from Ho to any of Nd, Sm, Eu, Gd, Dy, Er, Tm, Yb, or Lu. Example 4 Samples were prepared in the same manner as in Example 1 except that R in R(Ba 1-y Sr y )₂Cu₄O₈ was replaced by Y and Ho in varied amounts, with the value of y fixed at 0.3. (In other words, the samples are represented by (Y₁- p Ho p )(Ba 0.7 Sr 0.3 )₂Cu₄O₈, with the value of p varied.) The samples were evaluated in the same manner as in Example 1. The results are shown in Table 4. ρ X-ray diffractometry T c on (K) T c R=0 (K) ρ 300K (× 10⁻³ Ω·cm) Porosity (%) Remarks 0.2 Superconducting single phase 83 76 1.4 5 Example 0.4 Ditto 83 78 1.2 5 Example 0.6 Ditto 82 78 1.0 4 Example 0.8 Ditto 82 78 1.7 5 Example It is noted from Table 4 that the same effect is produced even when R is replaced by Y and Ho in varied amounts. It is expected that the same effect will be produced even when R is replaced by Y and two or more elements selected from lanthanide rare earth elements. Example 5 Mixtures each having a chemical composition of Y(Ba 1-z Ca z )₂Cu₄O₈ (where z = 0, 0.01, 0.05, 0.1, 0.15, 0.2, or 0.25) were prepared from Y₂O₃, Ba(NO₃)₂, CuO, and CaCO₃ (in powder form having a purity of 99.9%). The mixture was calcined at 850°C for 24 hours in oxygen. The calcined product was crushed and molded into a rectangular prism. The molded material underwent preliminary sintering at 800°C for 5 hours in oxygen. The sintered material underwent heat treatment in an atmosphere composed of 80% of argon and 20% of oxygen under a pressure of 100 MPa (1000 atm). The heat treatment consisted of heating up to 900°C at a rate of 200°C/hr and keeping this temperature for 6 hours, and further heating up to 1020°C at a rate of 200°C/hr and keeping this temperature for another 6 hours. The heat treatment was followed by cooling down to 300°C at a rate of 200°C/hr. The atmosphere was depressurized to 10⁵ Pa (1 atm) and the heat-treated material was discharged. The heat-treated material was crushed again and molded. The molded material was sintered at 800°C in oxygen. The thus obtained sintered body (sample) was tested for phase by X-ray diffractometry (powder method). It was found that the principal constituent of the sample has the crystal structure of YBa₂Cu₄O₈ type. The sample in which z = 0.1 gave an X-ray diffraction pattern (by powder method) as shown in Fig. 9. This pattern has the peaks attributable to the YBa₂Cu₄O₈ structure, which are indicated by indices. This sample is composed of a single superconducting phase. The samples obtained in this example have the phases as shown in Table 1. The samples in which z is 0 to 0.2 are composed of a single phase of Y(Ba 1-z Ca z )₂Cu₄O₈, whereas the sample in which y is 0.25 contains a second phase. The samples were tested for superconductivity by measuring their resistance. The results are shown in Table 5 and Fig. 10. z X-ray diffractometry T c on (K) T c R=0 (K) ρ 300K (× 10⁻³ Ω·cm) Porosity (%) Remarks 0.0 Superconducting single phase 81 77 6.2 37 Comparative Example 0.01 Ditto 82 78 2.3 10 Example 0.05 Ditto 82 78 0.74 8 Example 0.1 Ditto 83 80 0.42 5 Example 0.15 Ditto 81 78 0.52 6 Example 0.2 Ditto 80 77 0.88 7 Example 0.25 Superconducting phase plus impurity phase 80 72 8.3 6 Comparative Example It is noted from Table 5 and Fig. 10 that all the oxide superconductors Y(Ba 1-z Ca z )₂Cu₄O₈ pertaining to the present invention have the T c of 80K level, which is higher than the boiling point (77K) of liquid nitrogen. It is also noted that the samples decrease in resistance (at room temperature) in proportion to the Ca content (z). Those samples having a low resistance at room temperature are expected to have a high critical current density. The sample in which z is 0.15 has the lowest resistance at room temperature. The resistance increases in proportion to the value of z when z is greater than 0.15, and when z is 0.25, the resistance is higher than that when z is 0. The samples were tested for porosity by observing polished samples under an optical microscope. The results are shown in Table 5. It is noted that the porosity decreases in proportion to the value of z, reaching the minimum of about 5% when z is 0.15, and the porosity remains unchanged even when the value of z increases further. The results of X-ray diffractometry and the measurements of porosity as mentioned above suggest that the decrease of resistance (at room temperature) in proportion to the value of z is due to the formation of a solid solution by partial substitution of Ca for Ba in YBa₂Cu₄O₈. Consequently, according to the present invention, the value of z should be in the range of 0.001 to 0.2. Thermogravimetry performed on the sample in which the value of z is 0.1 gave the results as shown in Fig. 11(a). It is noted that the samples remain unchanged in weight over the range from normal temperature to 850°C and decrease in weight over the range from 850°C to 900°C. These results suggest that the samples remain stable up to a temperature as high as 850°C without absorption and liberation of oxygen. By contrast, the conventional superconductor YBa₂Cu₃O₇ loses a large amount of oxygen at 400°C to 800°C, as shown in Fig. 5(b). It will be understood from the foregoing that the superconductor YBa₂Cu₄O₈ as the base material is difficult to sinter and gives a sintered body having a porosity higher than 30%, whereas the samples in this example which are specified by the composition of Y(Ba 1-z Ca z )₂Cu₄O₈ (where 0.001 ≦ z ≦ 0.2) have the T c higher than 80K and give sintered bodies having a porosity lower than 10%. In addition, these samples have a low electric resistance at room temperature and remain stable up to 850°C without absorption and liberation of oxygen. Example 6 Mixtures each having a chemical composition of Ho(Ba 1-z Ca z )₂Cu₄O₈ (where z = 0, 0.01, 0.05, 0.1, 0.15, 0.2, or 0.25) were prepared from Ho₂O₃, Ba(NO₃)₂, CuO, and CaCO₃ (in powder form having a purity of 99.9%). The mixture was calcined at 850°C for 24 hours in oxygen. The calcined product was crushed and molded into a rectangular prism. The molded material underwent preliminary sintering at 800°C for 5 hours in oxygen. The sintered material underwent heat treatment in an atmosphere composed of 80% of argon and 20% of oxygen under a pressure of 100 MPa (1000 atm). The heat treatment consisted of heating up to 900°C at a rate of 200°C/hr and keeping this temperature for 6 hours, and further heating up to 1020°C at a rate of 200°C/hr and keeping this temperature for another 6 hours. The heat treatment was followed by cooling down to 300°C at a rate of 200°C/hr. The atmosphere was depressurized to 10⁵ Pa (1 atm) and the heat-treated material was discharged. The heat-treated material was crushed again and molded. The molded material was sintered at 800°C for 20 hours in oxygen. The thus obtained sintered body (sample) was tested for phase by X-ray diffractometry (powder method). It was found that the principal constituent of the sample has the crystal structure of RBa₂Cu₄O₈ type. The sample in which z = 0.1 gave an X-ray diffraction pattern (by powder method) as shown in Fig. 12. This pattern has the peaks attributable to the RBa₂Cu₄O₈ structure, which are indicated by indices. This sample is composed of a single superconducting phase. The samples obtained in this example have the phases as shown in Table 6. The samples in which z is 0 to 0.2 are composed of a single phase of Ho(Ba 1-z Ca z )₂Cu₄O₈, whereas the sample in which z is 0.25 contains a second phase. The samples were tested for superconductivity by measuring their resistance. The results are shown in Table 6 and Fig. 13. z X-ray diffractometry T c on (K) T c R=0 (K) ρ 300K (× 10⁻³ Ω·cm) Porosity (%) Remarks 0.0 Superconducting single phase 80 75 7.8 33 Comparative Example 0.01 Ditto 81 77 1.8 10 Example 0.05 Ditto 82 78 0.72 7 Example 0.1 Ditto 83 78 0.53 5 Example 0.15 Ditto 83 80 0.55 5 Example 0.20 Ditto 82 78 0.81 6 Example 0.25 Superconducting phase plus impurity phase 82 68 9.4 6 Comparative Example It is noted from Table 6 and Fig. 13 that all the oxide superconductors Ho(Ba 1-z Ca z )₂Cu₄O₈ pertaining to the present invention have the T c of 80K level, which is higher than the boiling point (77K) of liquid nitrogen. It is also noted that the samples decrease in resistance (at room tempeature) in proportion to the Ca content (z). Those samples having a low resistance at room temperature are expected to have a high critical current density. The sample in which z is 0.15 has the lowest resistance at room temperature. The resistance increases in proportion to the value of z when z is greater than 0.15, and when z is 0.25, the resistance is higher than that when z is 0. The samples were tested for porosity by observing polished samples under an optical microscope. The results are shown in Table 6. It is noted that the porosity decreases in proportion to the value of z, reaching the minimum of about 5% when z is 0.1, and the porosity remains unchanged even when the value of z increases further. The results of X-ray diffractometry and the measurements of porosity as mentioned above suggest that the decrease of resistance (at room temperature) in proportion to the value of z is due to the formation of a solid solution by partial substitution of Ca for Ba in HoBa₂Cu₄O₈. Thermogravimetry performed on the sample in which the value of z is 0.1 gave the results as shown in Fig. 14(a). It is noted that the sample remains unchanged in weight over the range from normal temperature to 850°C and decrease in weight over the range from 850°C to 900°C. These results suggest that the sample remains stable up to a temperature as high as 850°C without absorption and liberation of oxygen. By contrast, the conventional superconductor HoBa₂Cu₃O₇ loses a large amount of oxygen at 400°C to 800°C, as shown in Fig. 14(b). It will be understood from the foregoing that the superconductor HoBa₂Cu₄O₈ as the base material as the T c of 80K and is difficult to sinter and gives a sintered body having a porosity higher than 30%, whereas the samples in this example which are specified by the composition of Ho(Ba 1-z Ca z )₂Cu₄O₈ (where 0.001 ≦ z ≦ 0.2) have the T c higher than 80K and give sintered bodies having a porosity lower than 20%. In addition, these samples have a low electric resistance at room temperature and remain stable up to 850°C without absorption and liberation of oxygen. Example 7 Samples were prepared in the same manner as in Example 5 except that R in R(Ba 1-z Ca z )₂Cu₄O₈ was replaced by Nd, Sm, Eu, Gd, Dy, Er, Tm, Yb, or Lu, with the value of z fixed at 0.1. The samples were evaluated in the same manner as in Example 5. The results are shown in Table 7. It is noted from Table 7 that the same effect is produced even when R is changed from Ho to any of Nd, Sm, Eu, Gd, Dy, Er, Tm, Yb, or Lu. R X-ray diffractometry T c on (K) T c R=0 (K) ρ 300K (× 10⁻³ Ω·cm) Porosity (%) Remarks Nd Superconducting single phase 84 80 0.66 9 Example Sm Ditto 83 78 0.59 8 Example Eu Ditto 83 78 0.55 8 Example Gd Ditto 82 77 0.54 6 Example Dy Ditto 83 79 0.60 7 Example Er Ditto 82 78 0.52 8 Example Tm Ditto 82 77 0.56 5 Example Yb Superconducting phase plus impurity phase 81 77 0.72 7 Example Lu Superconducting phase plus impurity phase 81 76 0.96 8 Example Example 8 Samples were prepared in the same manner as in Example 5 except that R in R(Ba 1-z Ca z )₂Cu₄O₈ was replaced by Y and Ho in varied amounts, with the value of z fixed at 0.1. (In other words, the samples are represented by (Y 1-p Ho p )(Ba 0.9 Ca 0.1 )₂Cu₄O₈, with the value of p varied.) The samples were evaluated in the same manner as in Example 5. The results are shown in Table 8. It is noted from Table 8 that the same effect is produced even when R is replaced by Y and Ho in varied amounts. It is expected that the same effect will be produced even when R is replaced by Y and two or more elements selected from lanthanide rare earth elements. p X-ray diffractometry T c on (K) T c R=0 (K) ρ 300K (× 10⁻³ Ω·cm) Porosity (%) Remarks 0.2 Superconducting single phase 83 80 0.48 5 Example 0.4 Ditto 84 79 0.50 4 Example 0.6 Ditto 83 79 0.50 4 Example 0.8 Ditto 83 78 0.53 4 Example Example 9 Mixtures each having a chemical composition of (R 1-x Ca x )(Ba 1-y Sr y )₂Cu₄O₈ (where x = 0, 0.001, 0.01, 0.1, 0.3, or 0.5, and y = 0, 0.001, 0.01, 0.1, 0.3, 0.5, 0.6, or 0.7) were prepared in an inert atmosphere from R₂O₃, Ba(NO₃)₂, CuO, and Sr(NO₃)₂ (in powder form having a purity higher than 99.9%). The mixture was calcined at 750°C for 10 hours and then at an arbitrary temperature from 750°C to 900°C for 10 hours in an oxygen stream. The calcined product was crushed and molded into a rectangular prism. The molded material underwent preliminary sintering at 800°C for 5 hours in an oxygen stream. The sintered material underwent heat treatment in an atmosphere composed of 80% of argon and 20% of oxygen under a pressure of 100 MPa (1000) atm. The heat treatment consisted of heating up to 1000°C at a rate of 200°C/hr and keeping this temperature for 10 hours. The heat treatment was followed by cooling down to 300°C at a rate of 200°C/hr. The atmosphere was depressurized to 10⁵ Pa (1 atm) and the heat-treated material was discharged. The heat-treated material was crushed again and molded. The molded material was sintered at 800°C for 20 hours in an oxygen stream. Incidentally, R in R₂O₃ (mentioned above) represents any one or a combination of Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, and Tm. Examples of the combination include Y : Er = 1 : 1, Y : Eu = 1 : 1, and Y : Ho = 1 : 1. The combinations of R's, the combination of x and y, and the starting materials are shown in Table 9(1). The thus obtained sintered body (sample) was tested for phase by X-ray diffractometry (powder method). It was found that the principal constituent of the sample has the crystal structure of YBa₂Cu₄O₈ type. The sample in which x = 0.1 and y = 0.3 gave an X-ray diffraction pattern (by powder method) as shown in Fig. 15. This pattern has the peaks attributable to the YBa₂Cu₄O₈ structure, which are indicated by indices. This sample is composed of a single superconducting phase. The samples in which x is 0 to 0.15 and y is 0 to 0.5 are composed of a single superconducting phase. The samples obtained in this example have the phases as shown in Table 9(2). The samples were tested for superconductivity by measuring their resistance. The results are shown in Table 9(2) and Fig. 16. It is noted from Table 9(2) and Fig. 16 that all the oxide superconductors (R 1-x Ca x )(Ba 1-y Sr y )₂Cu₄O₈ in this example have the T c of 80K level if the value of x (Ca content) is from 0 to 0.3 and the value of y (Sr content) is from 0 to 0.6. They also exhibit diamagnetism at temperatures above 80K as proved by the measurement of a.c. susceptibility. (See Fig. 17.) This suggests the emergence of superconductivity at temperatures above 80K and supports the results of the measurement of resistance. The T c is higher than the boiling point (77K) of liquid nitrogen. The samples were analyzed for Ba content. The results are shown in Table 9(2). It is noted that as the Sr content increases the Ba content decreases. This is advantageous to production. When the value of y (or the Sr content) is 0.6, the Ba content is about 5 mol%. With an excessive Sr content, the sample has a low T c . For example, the sample No. 26 in which y = 0.7 has the T c as low as 40K. Thermogravimetry performed on the sample in which x = 0.1 and y = 0.3 gave the results as shown in Fig. 18. It is noted that the sample remains unchanged in weight over the range from normal temperature to 850°C and decreases in weight over the range from 850°C to 900°C. These results suggest that the sample remains stable up to a temperature as high as 850°C without absorption and liberation of oxygen, as in the case of the conventional oxide superconductor of YBa₂Cu₄O₈ type. As explained above, the present invention provides oxide superconductors having the composition represented by (R 1-x Ca x )(Ba 1-y Sr y )₂Cu₄O₈ (where R denotes one of or any combination of Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, and Tm; x is 0.001 to 0.3; and y is 0.001 to 0.6). All of them have the T c higher than 80K. They remain stable up to about 850°C without absorption and liberation of oxygen. They can be produced with as small a Ba content as 5.33 mol%. This is a great advantage over the conventional oxide superconductor of YBa₂Cu₄O₈ type which contains as much Ba as 13.33 mol% and hence needs special are in its production. According to the present invention, it is possible to produce an improved oxide superconductor which contains a smaller amount of Ba (40% of the Ba content in the conventional one) and yet has the T c higher than 80K. Example 10 Mixtures each having a chemical composition of (R 1-x Ca x )(Ba 1-y Sr y )₂Cu₄O₈ (where x = 0, 0.001, 0.01, 0.1, 0.3, or 0.5, and y = 0, 0.001, 0.01, 0.1, 0.3, 0.5, 0.6, or 0.7) were prepared in an inert atmosphere from R₂O₃ (where R denotes Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, or Tm), CaCO₃, BaCO₃, SrCO₃, and CuO (in powder form having a purity higher than 99.9%). The mixture was calcined at an arbitrary temperature from 900°C to 950°C for 10 hours in an oxygen stream. The calcined product was crushed and molded into a rectangular prism. The molded material underwent preliminary sintering at 950°C for 5 hours in an oxygen stream. The sintered material underwent heat treatment in an atmosphere composed of 80% of argon and 20% of oxygen under a pressure of 100 MPa (1000 atm). The heat treatment consisted of heating up to 1000°C at a rate of 200°C/hr and keeping this temperature for 10 hours. The heat treatment was followed by cooling down to 300°C at a rate of 200°C/hr. The atmosphere was depressurized to 10⁵ Pa (1 atm) and the heat-treated material was discharged. The heat-treated material was crushed again and molded. The molded material was sintered at 800°C for 20 hours in an oxygen stream. The combinations of R's, the combinations of x and y, and the starting materials are shown in Table 10(1). The thus obtained sintered body (sample) was tested for phase by X-ray diffractometry (powder method). It was found that the principal constituent of the sample has the crystal structure of YBa₂Cu₄O₈ type. The sample in which x = 0.1 and y = 0.3 gave an X-ray diffraction pattern (by powder method) as shown in Fig. 19. This pattern has the peaks attributable to the YBa₂Cu₄O₈ structure, which are indicated by indices. This sample is composed of a single superconducting phase. The samples in which x is 0 to 0.15 and y is 0 to 0.5 are composed of a single superconducting phase. The samples obtained in this example have the phases as shown in Table 10(2). The samples were tested for superconductivity by measuring their resistance. The results are shown in Table 10(2) and Fig. 20. It is noted from Table 10(2) and Fig. 20 that all the oxide superconductors (R 1-x Ca x )(Ba 1-y Sr y )₂Cu₄O₈ in this example have the T c of 80K level if the value of x (Ca content) is from 0 to 0.3 and the value of y (Sr content) is from 0 to 0.6. They also exhibit diamagnetism at temperatures above 80K as proved by the measurement of a.c. susceptibility. (See Fig. 21.) This suggests the emergence of superconductivity at temperatures above 80K and supports the results of the measurement of resistance. The T c is higher than the boiling point (77K) of liquid nitrogen. The samples were analyzed for Ba content. The results are shown in Table 10(2). It is noted that the Ba content decreases as the Sr content increases. This is advantageous to production. When the value of y (or the Sr content) is 0.6, the Ba content is about 5 mol%. With an excessive Sr content, the sample has a low T c . For example, the sample No. 26 in which y = 0.7 has the T c as low as 40K. Thermogravimetry performed on the sample in which x = 0.1 and y = 0.3 gave the results as shown in Fig. 22. It is noted that the sample remains unchanged in weight over the range from normal temperature to 850°C and decreases in weight over the range from 850°C to 900°C. These results suggest that the sample remains stable up to a temperature as high as 850°C without absorption and liberation of oxygen, as in the case of the conventional oxide superconductor of YBa₂Cu₄O₈ type. As explained above, the present invention provides oxide superconductors having the composition represented by (R 1-x Ca x )(Ba 1-y Sr y )₂Cu₄O₈ (where R denotes one of or any combination of Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, and Tm; x is 0.001 to 0.3; and y is 0.001 to 0.6). All of them have the T c higher than 80K. They remain stable up to about 850°C without absorption and liberation of oxygen. They can be produced with as small a Ba content as 5.33 mol%. They can also be produced with cheap carbonates. This is a great advantage over the conventional oxide superconductor of YBa₂Cu₄O₈ type which contains as much Ba as 13.33 mol% (whose source is an expensive Ba nitrate) and hence needs special care in its production. According to the present invention, it is possible to produce, from a wider selection of materials, an improved oxide superconductor which contains a less amount of Ba (40% of the Ba content in the conventional one) and yet has the T c higher than 80K. Although the invention has been described in its preferred form with a certain degree of particularity, it is to be understood that the embodiments are illustrative only and not restrictive and the invention may be embodied in several forms. The oxide superconductor of the invention will find use as wiring and magnetic shield for low-temperature electronic apparatus. Disclosed herein are high-temperature oxide superconductors of RBa₂Cu₄O₈ type, with Ba partly replaced by Sr or Ca, or with R and Ba partly replaced by Ca and Sr, respectively, as represented by the chemical composition formula of R(Ba 1-y Sr y )₂Cu₄O₈ or R(Ba 1-z Ca z )₂Cu₄O₈ or (R 1-x Ca x )(Ba 1-y Sr y )₂Cu₄O₈. They exhibit superconductivity at high temperatures. Especially, the last one exhibits superconductivity at a higher temperature than the former two. All of them can be made with a less amount of Ba as a deleterious substance, and the first two have improved sinterability. The best results are obtained when they are produced by the process involving the hot isostatic pressure treatment of the mixture of raw materials at 850-1100°C in an atmosphere composed of an inert gas and oxygen. The process permits a wider selection of Ba raw materials.

Claims: A high-temperature oxide superconductor represented by the chemical composition formula of R(Ba 1-z Ca z )₂Cu₄O₈ (where R denotes Y or at least one element selected from the lanthanide series elements, and z is in the range of 0.001 ≦ z ≦ 0.2). A high-temperature oxide superconductor represented by the chemical composition formula of (R 1-x Ca x )(Ba 1-y Sr y )₂Cu₄O₈ (where R denotes Y or at least one element selected from the lanthanide series elements, and x is in the range of 0 ≦ x ≦ 0.3 and y is in the range of 0.001 ≦ y ≦ 0.6). A high-temperature oxide superconductor according to Claim 1 or Claim 2 which is produced by the process involving the hot isostatic pressure treatment of sintered material derived from the mixture of raw materials at 850-1100°C in an atmosphere composed of an inert gas and oxygen.

Inventors: KORIYAMA SHIN-ICHI C O SUPERCO, KOSHIZUKA NAOKI C O SUPERCONDU, MIYATAKE TAKAYUKI C O SUPERCON, SAKURAI TAKASHI C O SUPERCONDU, SUZUKI NOBUO C O SUPERCONDUCTI, TANAKA SHOJI C O SUPERCONDUCTI, WADA TAKAHIRO C O SUPERCONDUCT, YAMAUCHI HISAO C O SUPERCONDUC, KORIYAMA, SHIN-ICHI, C/O SUPERCONDUCTIVITY RES., KOSHIZUKA, NAOKI, C/O SUPERCONDUCTIVITY RES. LAB, MIYATAKE, TAKAYUKI, C/O SUPERCONDUCTIVITY RES. LAB, SAKURAI, TAKASHI, C/O SUPERCONDUCTIVITY RES. LAB., SUZUKI, NOBUO, C/O SUPERCONDUCTIVITY RES. LAB., TANAKA, SHOJI, C/O SUPERCONDUCTIVITY RES. LAB., WADA, TAKAHIRO, C/O SUPERCONDUCTIVITY RES. LAB., YAMAUCHI, HISAO, C/O SUPERCONDUCTIVITY RES. LAB
Applicants: INT SUPERCONDUCTIVITY TECH, KOBE STEEL LTD, KYOCERA CORP, MATSUSHITA ELECTRIC IND CO LTD, MITSUBISHI METAL CORP, TOKYO ELECTRIC POWER CO, INTERNATIONAL SUPERCONDUCTIVITY TECHNOLOGY CENTER, KABUSHIKI KAISHA KOBE SEIKO SHO ALSO KNOWN AS KOBE STEEL LTD., KYOCERA CORPORATION, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD., MITSUBISHI METAL CORPORATION, THE TOKYO ELECTRIC POWER CO., INC.
Application year: 1990
Main classification: C01G3
IPC codes: H01L39, C04B35, C01G3