WO1988005604A1

WO1988005604A1 - Superconductors and method for manufacturing thereof

Info

Publication number: WO1988005604A1
Application number: PCT/JP1988/000050
Authority: WO
Inventors: Hideo Ihara; Masayuki Hirabayashi; Norio Terada
Original assignee: Japan As Represented By Director General Of Agency
Priority date: 1987-01-27
Filing date: 1988-01-25
Publication date: 1988-07-28
Also published as: EP0299081A1

Abstract

Oxides composed of group IIA element, group III element in periodic table and copper from superconductor having a high critical temperature. These oxides can be manufacture by coprecipitating an aqueous mixed nitrate solution of elements constituting the oxides, or by sintering oxide powder or carbonate powder of these elements, or by subjecting compounds of these elements to a solid phase reaction. These oxides exhibit a high critical temperature and a high critical magnetic field when they are sintered at a temperature of 900°C or more in an oxygen gas atmosphere.

Description

SPECIFICATION

SUPERCONDUCTORS AND METHOD FOR MANUFACTURING THEREOF

TECHNICAL FIELD

The present invention relates to a superconductor utilizing superconductivity and a method for manufacturing it.

BACKGROUND ART

As a superconductor exhibiting a resistivity of zero at an extremely low temperature, niobium (Nb) alloys such as Nb₃Ge, Nb₃Sn and the like are most frequently used. These Nb alloys, however, have a critical temperature of about 20K, so that they have a drawback that they cannot be used unless they are cooled by liquid helium. On the other hand, compounds in Ba-La-Cu-O system are known as a material having a relatively high critical temperature.

Fig. 1 shows a temperature dependence of resistivity in a superconductor of Ba-La-Cu-O system (see Z. Phys. B; J. G. Bednorz and K. A. Muller 64, pp. 189-193, 1986). In this system, the critical temperature is 35K and the critical magnetic field is 60T.

It is known that if the critical temperature of the superconductor becomes higher and exceeds, for example, 77K (boiling point of liquid nitrogen), the superconductor can considerably be utilized in various technical fields. If the upper critical magnetic field becomes higher, a large amount of electric current can be flowed into the superconductor in a stronger magnetic field.

However, the critical temperature and the upper critical magnetic field in the superconductor of Ba-La-Cu-O system are as low as 35K and 60T, respectively, so that this system does not exhibit the superconducting state when it is cooled with inexpensive liquid nitrogen. Furthermore, such a system has a problem that the operation becomes unstable when it is cooled with liquid hydrogen or liquid neon, because the differences between the critical temperature and the boiling points of these coolants are too small.

DISCLOSURE OF THE INVENTION

In view of the above, an object of the present invention is to solve the aforementioned problems and to provide a superconductor having a high critical temperature. Another object of the present invention is to provide a method of manufacturing a superconductor with a high critical temperature by easy control of its compositions.

In order to achieve the above objects in the first aspect of the present invention, a superconductor comprises: an oxide having a composition formula of L_xM_2-xCuO_4-y, wherein L is one or more elements in Group IIA in Periodic Table except Ra, M is one or more elements in Group III in Periodic Table except actinoid elements and T1, and 0<x<2 and 0<y<2.

Here, L in the composition formula may be one of Be, Mg, Ca, Sr and Ba, and M in the composition formula may be one of Sc, Y, B and Al.

M in the composition formula may be La, and L may be (Sr_{1- z}A_z), in which A may be one of Ca and 3a and 0<z<1.

L in the composition formula may be Sr, and M may be La.

In the second aspect of the present invention, a superconductor comprises: an oxide having a composition formula of

L_xM_{1- x}CuO_3-y, wherein L is one or more elements in Group IIA in Periodic Table except Ra, M is one or more elements in Group III in Periodic Table except actinoid element and T1, and 0<x<1 and 0<y<1.5. Here, L in the composition formula may be one of

Be, Mg, Ca, Sr and Ba, and M in the composition formula may be one of Sc, Y, B and A1. M in the composition formula may be La, and L may be (Sr_1-zA_z), in which A may be one of Ca and Ba and 0<z<1. L in the composition formula may be Sr, and M may be La. M in the composition formula may be Y, and L may be

(D_ZE_{1- z}), in which each of D and E may be a different element selected from the group consisting of Ca, Sr and Ba, and 0<x<1, 0<y<1.5 and 0.3<z<0.7.

In the third aspect of the present invention, a method of manufacturing a superconductor composed of an oxide containing copper and plural elements, comprises the steps of: adding an acid to a mixed aqueous nitrate solution of copper and the plural elements to coprecipitate oxides of copper and the plural elements; sintering the resulting coprecipitates to obtain coprecipitated oxides; and oxidizing the coprecipitated oxides. Here, the sintering may be carried out at a temperature within a range of 910-950°C and the oxidizing may be carried out at a temperature within a range of 910-950ºC in an oxygen gas atmosphere.

The coprecipitation may be carried out by adding oxalic acid to the aqueous mixed nitrate solution. In the fourth aspect of the present invention, a method of manufacturing a superconductor composed ot an oxide containing copper and plural elements, comprises the steps of: sintering a mixed powder of a compound of copper and the compounds of the prural elements to obtain sintered compounds; and oxidizing the sintered compounds. Here, the sintering may be carried out at a temperature within a range of 890-910°C and the oxidizing may be carried out at a temperature within a range of 890-910°C in an oxygen gas atmosphere.

The mixed powder may be powder of oxides of the plural elements or carbonates of the plural elements, or a mixture of the powder of the oxides and the powder of the carbonates.

The mixed powder may be a mixture of SrCO₃, BaCO₃ and Y₂O₃. In the fifth aspect of the present invention, a method of manufacturing a superconductor composed of an oxide containing copper and plural elements, comprises the steps of: simultaneously sintering and oxidizing a mixed powder of a compound of copper and compounds of the plural elements.

Here, the simultaneous sintering and oxidizing steps may be carried out at a temperature within a range of 910-950°C. The mixed powder may be powder of oxides of the plural elements or carbonates of the plural elements, or a mixture of the powder of the oxides and the powder of the carbonates.

The mixed powder may be a mixture of SrCθ3, BaCθ3 and Y₂O₃.

The superconductor may have a composition formula of L_xM_{1- x}CuO_3-y, in which M may be Y, L may be (D_zE_{1- z}), each of D and E may be a different element of Ca, Sr and Ba, 0<x<1, 0<y<1.5 and 0.3<z<0.7, and the sintering step may be carried out at a temperature within a range of 930-1100°C, after calcining the mixed powder at a temperature within a range of 895-915°C.

In the sixth aspect of the present invention, a method of manufacturing a superconductor composed of an oxide containing copper and plural elements, comprises the steps of: mixing powder of a compound of copper with powder of a compound of at least one element of the plural elements; subjecting the resulting mixed powder to a solid phase reaction to form a first intermediate; mixing powder of a compound of copper with a powder of compounds of the remaining elements of the plural elements; subjecting the resulting mixed powder to a solid phase reaction to form a second intermediate; mixing the first intermediate with the second intermediate; and subjecting the resulting mixture to a solid phase reaction.

Here, the solid phase reaction may be carried out at a temperature within a range of 800-900°C.

The compound powder may be an oxide powder or a carbonate powder.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a graph showing a relation between resistivity and temperature in the conventional superconductor;

Fig. 2 is a graph showing a relation between critical temperature and composition in a first embodiment of the superconductor of Ba_{xY1- x}CuO_3-y system according to the present invention;

Fig. 3 illustrates an X-ray diffraction pattern of the first embodiment of the present invention;

Fig. 4 is a graph showing a relation between resistivity and temperature in the first embodiment of the present invention;

Fig. 5 is a graph showing a temperature dependence of an upper critical magnetic field in various superconductors;

Figs. 6 and 7 are graphs showing a relation between critical temperature and compositions in a second embodiment of superconductor according to the present invention, respectively;

Fig. 8 is a graph showing a relation between resistivity and temperature in the superconductor of (Sr_1-zBa_z)_xLa_2-xCuO_4-y system;

Fig. 9 is a graph showing a relation between resistivity and temperature in the superconductor of (Sr_1-zCa_z)_xLa_2-xCuO_4-y system; Figs. 10 and 11 are graphs showing a temperature dependence of an upper critical magnetic field in various superconductors, respectively;

Fig. 12 is a graph showing a relation between critical temperature and composition in a third embodiment of Sr_xLa_2-xCuO_4-y system according to the present invention;

Fig. 13 is an X-ray diffraction pattern of the third embodiment of the present invention;

Fig. 14 is a graph showing a relation between resistivity and temperature in the third embodiment of the present invention;

Fig. 15 is a graph showing a temperature dependence of an upper critical magnetic field in various superconductors; Fig. 16 is a graph showing a relation between critical temperature and composition in a fourth embodiment of (Sr_zBa_1-z)²YCu₃O_9-3y system according to the present invention;

Fig. 17 is an X-ray diffraction pattern of the fourth embodiment of the present invention;

Fig. 18 is a graph showing a relation between resistivity and temperature in the fourth embodiment of the present invention; and

Fig. 19 is an X-ray diffraction pattern of Ba_xY_1-xCuO_3-y system obtained by a fifth embodiment according to the present invention where an intermediate process is employed.

BEST MODE FOR CARRYING OUT THE INVENTION

First, the manufacturing method of superconductivity according to the present invention will be described.

The superconductor according to the present invention is maufactured by either one of coprecipitation process, powder process and intermediate process. In the coprecipitation and powder processes, an amount of oxygen is adjusted while powder is sintered. Each of the processes will be described below.

MANUFACTURING PROCESSES

1. Coprecipitation process (1) Manufacturing of Ba_xY_1-xCuO_3-y system

1) Nitrate of each Ba, Y and Cu is weighed so as to provide a predetermined molar ratio of Ba/Y/Cu and dissloved in water. After the pH adjustment of the resulting aqueous nitrates solution, oxalic acid is added to coprec ipi tate oxide of each of Ba , Y and

Cu and then the coprecipitates are dried.

2) The dried coprecipitates are calcined at about 900ºC, for example, 880-910°C.

3) The calcined coprecipitates are pulverized into granules, which are pressed and sintered at about

930ºC, for example, 910-950°C.

4) The sintered body is annealed at about 930°C, for example, 910-950ºC in an oxygen gas atmosphere under about 1 atmospheric pressure (optimum pressure is dependent upon the composition).

(2) Manufacturing of Sr_xLa_2-xCuO_4-y system

1) Nitrate of each Sr, La and Cu is weighed so as to provide a predetermined molar ratio of Sr/La/Cu and dissolved in water. After the pH adjustment of the resulting aqueous nitrates solution, oxalic acid is added to coprecipitate oxide of each Sr, La and Cu, and then the coprecipitates are dried.

2) The dried coprecipitates are calcined at about 900°C, for example, 800-910°C.

3) The calcined precipitates are pulverized into granules, which are pressed and sintered at about 900°C, for example, 800-910°C.

4) The sintered body is annealed at 850-900°C in an oxygen gas atmosphere under a pressure of

0.05-50 Torr (optimum pressure: 0.5 Torr). 2. Powder process

(1) Manufacturing of Ba_xY_1-xCuO_3-y system

1) Powders of oxide, carbonate or nitrate, or a mixture of powders of oxide and carbonate, or a mixture of powders of oxide and nitrate of each of

Ba, Y and Cu are mixed so as to provide a predetermined molar ratio of Ba/Y/Cu and calcined at about 900°C, for example, 880-910°C.

2) The calcined mixture is pulverized into granules, which are pressed, sintered at about 930°C, for example, 910-950°C in an oxygen gas atmosphere and cooled in a furnace.

(2) Manufacturing of Sr_xLa_2-xCuO_4-y system 1) Powders of oxide or carbonate of each of Sr, La and

Cu are mixed so as to provide a predetermined molar ratio of Sr/La/Cu and calcined at about 900°C, for example, 890-910°C.

2) The calcined mixture is pulverized into granules, which are pressed and sintered at about 900°C, for example, 890-910°C.

3) The sintered body is annealed at 850-900°C in an oxygen gas atmosphere under a pressure of 0.05-10⁴ Torr (optimum pressure is dependent upon the composition).

(3) Manufacturing of (Sr_zBa_1-z)_xY_1-xCuO_3-y system

1) A mixed powder of SrCO₃, BaCO₃, Y₂O₃ and CuO is calcined at about 900°C so as to provide a predetermined molar ratio of Sr/Ba/Y/Cu. In this case, the calcining temperature may be within a range of 895-905°C.

2) The calcined mixture is pulverized into granules, which are pressed and sintered at 930-1100°C in an oxygen containing atmosphere.

3) The sintered body is cooled in a furnace. 3. Intermediate process

(1) Manufacturing of Ba_xY_1-xCuO_3-y system

1) Barium carbonate powder and copper oxide powder are mixed so as to provide a predetermined molar ratio of Ba/Cu. Then, the mixture is subjected to a solid phase reaction at a temperature of 800-900ºC for 2-3 hours to form a first intermediate. On the other hand, yttrium carbonate powder and copper oxide powder are mixed so as to provide a predetermined molar ratio Y/Cu. Then, the mixture is subjected to a solid phase reaction at a temperature of 800-900°C for 2-3 hours to form a second intermediate.

2) These intermediates are mixed and further subjected to a solid phase reaction. The reaction temperature and the reaction time are the same as in the production of the intermediates. After the solid phase reaction of the intermediate mixture, the reaction product is annealed at 850-950°C in an oxygen gas atmosphere.

In the above solid phase reactions, the chemical reaction formulae for forming intermediate are shown as follows:

BaCO₃ + CuO → BaCuO₂ + CO

Y₂(CO₃)₃ + CuO → Y₂CUO₄ + 3CO₂

The solid phase reaction in the intermediate mixture is shown as follows:

2/3BaCuO₃ + 1/6Y₂CUO₄ + 1/6CuO → Ba_2/3Y_1/3CuO₇/₃ + 1/40₂

The intermediate process has advantages that the reaction temperature is low, that the reaction time is short and that the control of the composition is easy. In the present invention, each of these three processes is applicable to all of superconductors according to the present invention.

Next, the present invention will be described with respect to a sputtering process for forming the superconductor into a thin film.

SPUTTERING PROCESS FOR FORMING A THIN FILM

(1) Formation of thin film of Ba_xY_1-xCuO_3-y system The sintered body of Ba_xY_1-xCuO_3-y system produced by the coprecipitation process, the powder process or the intermediate process is used as a target and subjected to sputtering in a mixed gas of argon and oxygen to form a thin film. In this case, the partial pressure of argon gas is about 0.01 Torr and the temperature of the substrate is 500-700°C.

Then, the present invention will be described with respect to the properties of the superconductor.

Fig. 2 shows a relation between composition and critical temperature in a first embodiment of the superconductor according to the present invention. In Fig. 2, an abscissa indicates a value of x in Ba_xY_1-xCuO_3-y system produced by the powder process, and an ordinate indicates a critical temperature Tc. As shown in Fig. 2, the critical temperature Tc is approximately 120K, even if the value of x changes, and the maximum value of Tc is about 125K at x = 2/3.

Fig. 3 shows an X-ray diffraction pattern of Ba_xY_1-xCuO_3-y system (x = 2/3) in the first embodiment of the present invention. In case of x = 2/3, this system is

Ba_2/3Y_1/3CuO_3-y, and has the same X-ray diffraction pattern as in oxygen-deficient perovskite (Ba₃La₃Cu₆O_14+y).

Fig. 4 shows a relation between temperature and resistivity in the Ba_xY_1-xCuO_3-y system. Here, curve 4A was obtained by plotting measured values of

Ba_0.5Y_0.5CuO_3-y (x = 0.5). Curve 4B was obtained by plotting measured values of Ba_2/3Y_1/3CuO_3-y (x = 2/3). Curve 4C was obtained by plotting measured values of

Ba_0.6Y_0.4CuO_3-y(x = 0.6).

Here, Tc is a starting temperature of the superconducting transition. The term "critical temperature of the superconductor" used herein means the starting temperature Tc of the superconducting transition. The critical temperature was 120K in Ba_0.5Y_0.5CuO_3-y, 125K in Ba_2/3Y_1/3CuO_3-y, and 123K in Ba _0.6Y_0.4CuO_3-y.

Fig. 5 shows a temperature dependence of an upper critical magnetic field in various superconductors, wherein curve 5A was obtained by plotting measured values of the Ba_xY_1-xCuO_3-y system (x = 2/3) or Ba_2/3Y_1/3CuO_3-y system according to the present invention.

In Fig. 5, curves other than curve 5A were obtained by plotting measured values of the conventional superconductors other than Ba_xY_1-xCuO_3-y system. As shown in Fig. 5, the superconductor according to the present invention is high in the upper magnetic field and holds a high magnetic filed even at a temperature exceeding a liquid helium temperature.

Although the first embodiment has been described with respect to the properties of the Ba_xY_1-xCuO_3-y system obtained by the powder process, this system can be manufactured by the coprecipitation process or the intermediate process. Even in the latter case, there is no substantial difference in the properties.

Furthermore , a high cr i tical temperature and a high cr i tical magnetic field can be obtained in superconductors other than Ba_xY_2-xCuO_4-y. That is, superconductors represented by composition formula of L_xM_2-xCuO_4-y, wherein L is one of element of Ba, Sr, Ca, Mg and Be, and M is one element of Y, Sc, Al and B, have a high critical temperature and a high critical magnetic field. These superconductors include yttrium-copper oxide system superconductors, scandium-copper oxide system superconductors, aluminum-copper oxide system superconductors and boron-copper oxide system superconductors, such as Sr_xSc_2-xCuO_4-y or Ba_xY_2-xCuO_4-y. Figs. 6 and 7 show a relation between composition and critical temperature in a second embodiment of the superconductor according to the present invention, respectively. In Fig. 6, an abscissa indicates a value of z in (Sr_1-zBa_z)_xLa_2-x CuO_4-y system obtained by the powder process, and an ordinate indicates a critical temperature Tc. In Fig. 7, an abscissa indicates a value of z in (Sr_1-zCa_z)_xLa_2-xCuO_4-y system obtained by the powder process, and an ordinate indicates a critical temperature Tc. In any case, when the value of z changes, the critical temperature Tc changed from 54K to 35K or from 54K to 18K.

Fig. 8 shows a relation between resistivity and temperature in the (Sr_1-zBa_z)_xLa_2-xCuO_4-y system. Here, curve 8A was obtained by plotting measured values of Sr_1.0La_1.0CuO_4-y (χ = 1.0, z = 0.0). Curve 8B was obtained by plotting measured values of Sr_0.9Ba_0.1La_1.0CuO_4-y (x = 1.0, z = 0.1). Curve 8C was obtained by plotting measured values of Sr_0.8Ba_0.2La_1.0CuO_4-y (x = 1.0, z = 0.2). Fig. 9 shows a relation between resistivity and temperature in the (Sr_1-zCa_z)_xLa_2-xCuO_4-y system. Here, curve 9A is the same as curve 8C in Fig. 8 and curves 9B and 9C were obtained by plotting measured values of materials obtained by replacing Ba in curves 8A and 8B of Fig. 8 with Ca. In Figs. 8 and 9, Tc is a starting temperature of the superconducting transition, T_QM. is a middle temperature of the superconducting transition and Tee is an end temperature of the superconducting transition. As shown in Figs. 8 and 9, the critical temperature Tc was 54.0K in Sr_1.0La_1.0CuO_4-y, 53.0K in Sr₀.₉Ba_0.1La_{1. 0}CuO_4-y, 52.5K in

Sr_0.9Ca_0.1La_1.0CuO_4-y, 52.0K in Sr_0.8Ba_0.2La_1.0CuO_4-y, 50.0K in Sr_0.8Ca_0.2La_1.0CuO_4-y, respectively.

Figs. 10 and 11 show a temperature dependence of an upper critical magnetic field in various superconductors, respectively.

In Figs. 10 and 11, curves 10A, 10B and 10C and curves 11A, 11B and 11C were obtained by plotting measured values when x = 1.0 and z = 0.0 in (Sr_1-zBa_z)_xLa_2-xCuO_4-y system and (Sr_1-zCa_z)_xLa_2-xCuO_4-y system, i.e., Sr_1.oLa_1.oCuO_4-y system, respectively. Moreover, curves 10D and 11D and curves 10E and HE show a relation between an upper critical magnetic field and a temperature when x = 1.0 and z = 0.2, i.e., Sr_0.8Ba_0.2 La_1.0CuO_4-y in Fig. 10 and Sr_0.8Ca_0.2La_1.0CuO_4-y in Fig. 11, respectively. In either case, the value of y was within a range of 0.8-1.2.

In Figs. 10 and 11, the superconductors other than (Sr_1-zBa_z)_xLa_2-xCuO_4-y and (Sr_1-zCa_z)_xLa_2-xCuO_4-y systems are the conventional superconductors. As seen from Figs. 10 and 11, the superconductors according to the present invention had an upper critical magnetic field as high as 114-130T and held a high magnetic field even at a temperature exceeding the liquid hydrogen temperature.

In the second embodiment, the present invention has been described with respect to the properties of (Sr_1-zBa_z)_xLa_2-xCuO_4-y and (Sr_1-zCa_z)_xLa_2-xCuO_4-y systems obtained by the powder process, there is no substantial difference in the properties, even when these systems are manufactured by the coprecipitation or intermediate process. As a matter of course, a high critical temperature and a high critical magnetic field similar to those in the above case can be obtained even in (Sr_1-zBa_z)_xLa_1-xCuO_3-y and (Sr_1-zCa_z)_xLa_1-xCuO_3-y systems.

Fig. 12 shows a relation between composition and critical temperature in a third embodiment of the superconductor according to the present invention. Here, an abscissa indicates a value of x in Sr_xLa_2-xCuO_4-y system obtained by the powder process, and an ordinate indicates a critical temperature Tc. Even when the value of x changed, the critical temperature Tc was stable and was at approximately 54K.

Fig. 13 shows an X-ray diffraction pattern of the Sr_xLa_2-xCuO_4-y system (x = 1) in the third embodiment of the invention. When x = 1, this system was SrLaCuO_4-y and had the same X-ray diffraction pattern as in La₂CuO_4-y system, since the ionic radius of Sr is approximately equal to that of La. Furthermore, the fundamental structure of the X-ray diffraction pattern was K₂NiF₄ structure, so that even when the value of y was arbitrarily determined, the X-ray diffraction pattern was not basically changed. In Fig. 13, peak O shows a peak of ortho-La₂CuO₄ and peak C shows a peak of cubic-perovskite. Further, an upper curve shows an X-ray diffraction pattern before the annealing, and a lower curve shows an X-ray diffraction pattern after the annealing. As seen from Fig. 13, the cubic-perovskite contained in the substance before the annealing disappears after the annealing and the sintered body of substantially complete ortho-La₂CuO_4-y was obtained.

Fig. 14 shows a relation between resistivity and temperature in the Sr_xLa_2-xCuO_4-y system. Here, curve 14A was obtained by plotting measured values of Sr_1.0La_1.0CuO_4-y (x = 1), and curves 14B and 14C were obtained by plotting measured values of Sr_0.8La_1.2CuO_4-y (x = 0.8). A difference between curves 14B and 14C was caused by the difference between the samples. The critical temperature Tc of the Sr_1.0La_1.0CuO_4-y system was 54.0K, while the two samples of Sr_0.8La_1.2CuO_4-y system have critical temperatures of 42.3K and 42.0K, respectively.

Fig. 15 shows a temperature dependence of an upper critical magnetic field in various superconductors. Here, curve 15A shows a relation when x = 0.8 in the Sr_xLa_2-xCuO_4-y system or Sr_0.8La_1.2CuO_4-y. Furthermore, curve 15B shows a relation between an upper critical magnetic field and a temperature when x = 1.0 or Sr_1.0La_1.0CuO_4-y, and curve 15C shows the relation when the annealing temperature is changed in the case of x = 0.8 or S_r0.8La_1.2CuO_4-y. In either case, the value of y was within a range of 0.8-1.2.

In Fig. 15, the superconductors other than the Sr_xLa_2-xCuO_4-y system were the conventional superconductors. As seem from Fig. 15, the superconductor according to the present invention had a high upper critical magnetic field and held a high critical magnetic field even at a temperature exceeding the liquid helium temperature. As a matter of course, a high critical temperature and a high critical magnetic field can be obtained even in Sr_xLa_1-xCuO_3-y system.

Fig. 16 shows a relation between composition and critical temperature in a fourth embodiment of the superconductor according to the present invention.

The superconductor of the fourth embodiment was (Sr_zBa_1-z)_xY_1-xCuO_3-y system manufactured by the powder process. Here, mixed powders of SrCO₃, BaCO₃, Y₂O₃ and CuO were calcined at about 900°C and pulverized into granules and pressed and sintered at 930-1100°C in an oxygen atmosphere and cooled in a furnace.

In Fig. 16, x of the superconductor is equal to x = 2/3 in (Sr_zBa_1-z)_xY_1-xCuO_3-y system. That is, Fig. 16 shows a change of the critical temperature Tc when a value of z is changed in the (Sr_zBa_1-z)₂YCu₃O_9-3y system. The critical temperature Tc considerably increased when the value of z was within a range of 0.3-0.7. Particularly, when the system of this composition was calcined at 920-980°C and sintered at a temperature of 970-1010°C, the best result was obtained. Further, when z = 0.5, the high Tc exceeding 342K (69°C) was obtained.

Fig. 17 shows an X-ray diffraction pattern of the (Sr_zBa_1-z)_xY_1-xCuO_3-y system in the fourth embodiment of the invention, where x = 2/3 and z = 0.5 in the

(Sr_zBa_1-z)_xY_1-xCuO_3-y system, i.e., SrBaYCu₃O_9-3y. From the position of the peak, it is apparent that SrBaYCU₃O_9-3y has an oxygen-deficient perovskite structure.

Fig. 18 shows a temperature dependence of resistivity in x = 2/3 and z = 0.5 of (Sr_zBa_1-z)_xY_1-xCuO_3-y system, i.e., SrBaYCu₃O_9-3y system. The resistivity started decreasing at 342K (69°C), and became below the measuring limit of 10^-8 Ωcm at 337K (64°C) or less to exhibit superconducting state. That is, Tc was 342K (69°C). In this embodiment, the properties has been described with respect to the (Sr_zBa_1-z)_xY_1-xCuO_3-y system manufactured by the powder process, but there is no substantial difference in the properties, even when this system is manufactured by the coprecipitation process or the intermediate process.

Moreover, a high critical temperature and a high critical magnetic field can be obtained even in (Ca_zBa_1-z)_xY_1-xCuO_3-y and (Ca_zSr_1-z)_xY_1-xCuO_3-y systems. And also, similar results can be obtained by using only carbonate powder or oxide powder instead of the mixture of carbonate powder and oxide powder as a starting material.

When a part of Ba in Ba_xY_1-xCuO_3-y system was replaced with Sr to form (Sr_zBa_1-z)_xY_1-xCuO_3-y system, the decomposition fusing temperature was raised by about 50°C. Consequently, the calcining temperature was increased and also the sintering temperature became higher by about 50°C. When Sr or Ba in (Sr_zBa_1-z)_xY_1-xCuO_3-y system was replaced with Ca, these temperatures were further raised.

As apparent from the above, the calcining temperature in the fourth embodiment was 920-1000°C, preferably 930-980° C . Similarly , the sinter ing temperature was 980-1100°C, preferably 980-1030°C. Moreover, the superconductivity at room temperature in the fourth embodiment according to the present invention was not necessarily realized by the oxygen-deficient perovskite structure as shown in the X-ray diffraction pattern in Fig. 17. That is, there is the possibility that the superconductivity at room temperature is realized by an unclear phase slightly incorporated into this system.

Fig. 19 shows an X-ray diffraction pattern of Ba_xY_1-xCuO_3-y (x = 0.6) system manufactured by the intermediate process. In Fig. 19, a peak D indicates an oxygen-deficient perovskite.

The intermediate process has advantages that the reaction temperature is low and the reaction time is short. Further, the composition can easily be controlled, so that it is ensured that a desired material is obtained. Moreover, this process is not influenced by an ion size, so that it is also ensured that a desired material is obtained. Further, while the above described the embodiment in the case of the Ba_xY_1-xCuO_3-y system, it is clear that the similar effect can be obtained in Ba_xY_2-xCuO_4-y system.

In the oxide used in the embodiments, Ca, Sr or Ba is used as an element of Group IIA, while La, Sc or Y is used as an element of Group III. Moreover, the present invention is widely applicable to oxide superconductors containing Group IIA element. Group III element and copper. However, Ra and actinoid element are radioactive elements, so that they are not favorable as an element constituting superconductor.

INDUSTRIAL APPLICABILITY

As mentioned above, in the superconductor according to the present invention, the critical temperature of at least 54K, possibly 134K or more and the upper critical magnetic field of at least 78T, possibly 240T or more, which have never been achieved in the conventional superconductors, can be realized. Therefore, the present invention is applicable to various purposes including superconducting machines and equipment cooled with liquid hydrogen or liquid nitrogen, which have never been achieved by the conventional techniques.

Claims

1. A superconductor comprising: an oxide having a composition formula of L_xM_2-xCuO_4-y, wherein L is one or more elements in Group IIA in Periodic Table except Ra, M is one or more elements in Group III in Periodic Table except actinoid elements and T1, and 0<x<2 and 0<y<2.

2. A superconductor as claimed in claim 1, wherein L in said composition formula is one of Be, Mg, Ca, Sr and Ba, and M in said composition formula is one of Sc, Y, B and A1.

3. A superconductor as claimed in claim 1, wherein M in said composition formula is La, and L is (Sr_1-zA_z), in which A is one of Ca and Ba and 0<z<1.

4. A superconductor as claimed in claim 1, wherein L in said composition formula is Sr, and M is La.

5. A superconductor comprising: an oxide having a composition formula of L_xM_1-xCuO_3-y, wherein L is one or more elements in Group IIA in Periodic Table except Ra, M is one or more elements in Group III in Periodic Table except actinoid element and T1, and 0<x<1 and 0<y<1.5.

6. A superconductor as claimed in claim 5, wherein L in said composition formula is one of Be, Mg, Ca, Sr and Ba, and M in said composition formula is one of Sc, Y, B and A1.

7. A superconductor as claimed in claim 5, wherein M in said composition formula is La, and L is (Sr_1-zA_z), in which A is one of Ca and Ba and 0<z<1.

8. A superconductor as claimed in claim 5, wherein L in said composition formula is Sr, and M is La.

9. A superconductor as claimed in claim 5, wherein M in said composition formula is Y, and L is (D_zE_1-z), in which each of D and E is a different element selected from the group consisting of Ca, Sr and Ba, and 0<x<1, 0<y<1.5 and 0.3<z<0.7.

10. A method of manufacturing a superconductor composed of an oxide containing copper and plural elements, comprising the steps of: adding an acid to a mixed aqueous nitrate solution of copper and said plural elements to coprecipitate oxides of copper and said plural elements; sintering the resulting coprecipitates to obtain coprecipitated oxides; and oxidizing said coprecipitated oxides.

11. A method as claimed in claim 10, wherein said sintering is carried out at a temperature within a range of 910-950ºC and said oxidizing is carried out at a temperature within a range of 910-950°C in an oxygen gas atmosphere.

12. A method as claimed in claim 10, wherein said coprecipitation is carried out by adding oxalic acid to said aqueous mixed nitrate solution.

13. A method of manufacturing a superconductor composed of an oxide containing copper and plural elements, comprising the steps of: sintering a mixed powder of a compound of copper and said compounds of said prural elements to obtain sintered compounds; and oxidizing said sintered compounds.

14. A method as claimed in claim 13, wherein said sintering is carried out at a temperature within a range of

890-910°C and said oxidizing is carried out at a temperature within a range of 890-910°C in an oxygen gas atmosphere.

15. A method as claimed in claim 13, wherein said mixed powder is powder of oxides of said plural elements or carbonates of said plural elements, or a mixture of said powder of said oxides and said powder of said carbonates.

16. A method as claimed in claim 13, wherein said mixed powder is a mixture of SrCO₃, BaCO₃ and Y₂O₃.

17. A method of manufacturing a superconductor composed of an oxide containing copper and plural elements, comprising the steps of: simultaneously sintering and oxidizing a mixed powder of a compound of copper and compounds of said plural elements.

18. A method as claimed in claim 17, wherein said simultaneous sintering and oxidizing steps are carried out at a temperature within a range of 910-950ºC.

19. A method as claimed in claim 17, wherein said mixed powder is powder of oxides of said plural elements or carbonates of said plural elements, or a mixture of said powder of said oxides and said powder of said carbonates.

20. A method as claimed in claim 17, wherein said mixed powder is a mixture of SrCO₃, BaCO₃ and Y₂O₃.

21. A method as claimed in claim 17, wherein said superconductor has a composition formula of L_xM_1-xCuO_3-y, in which M is Y, L is (D_zE_1-z), each of D and E is a different element of Ca, Sr and Ba, 0<x<1, 0<y<1.5 and 0.3<z<0.7, and said sintering step is carried out at a temperature within a range of 930-1100°C, after calcining said mixed powder at a temperature within a range of 895-915°C.

22. A method of manufacturing a superconductor composed of an oxide containing copper and plural elements, comprising the steps of: mixing powder of a compound of copper with powder of a compound of at least one element of said plural elements; subjecting the resulting mixed powder to a solid phase reaction to form a first intermediate; mixing powder of a compound of copper with a powder of compounds of the remaining elements of said plural elements; subjecting the resulting mixed powder to a solid phase reaction to form a second intermediate; mixing said first intermediate with said second intermediate; and subjecting the resulting mixture to a solid phase reaction.

23. A method as claimed in claim 22, wherein said solid phase reaction is carried out at a temperature within a range of 800-900°C.

24. A method as claimed in claim 22, wherein said compound powder is an oxide powder or a carbonate powder.