JP2000143240A - Production of oxide superconductor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the critical current and magnetic field characteristics of superconductor in a wide temperature region by introducing a large number of effective pinning centers into the superconductor, in the production process for a Bi 2212 system superconductor. SOLUTION: This production process for a Bi 2212 system superconductor, comprises: mixing powdery raw materials together in prescribed ratios to obtain a mixture having such a mix ratios as to meet the formula Bi2-yPbySr2CaCu2Ox; heating the mixture to obtain the heated mixture in a partially molten state; then, gradually cooling the partially molten mixture to crystallize a highly- oriented Bi2-yPbySr2CaCu2Ox phase; annealing the resulting material consisting of the crystallized Bi2-yPbySr2CaCu2Ox phase at a temperature lower than the temperature required for effecting the partially molten state of the above mixture. Thus, the objective oxide superconductor in which a large number of grains having small grain size of a 451 phase (non-superconductor phase) are precipitated and distributed in the Bi2-yPbySr2CaCu2Ox phase, can be produced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present _{invention, Bi 2-y Pb y Sr} 2 CaCu 2
O _x The method for producing a (hereinafter, Bi2212 abbreviated) based superconductor.

[0002]

2. Description of the Related Art Hitherto, Bi-based superconductor wires have been produced by a heat melting method represented by a silver sheathing method and a coating method. In the silver sheath method, a raw material powder mixed at a predetermined ratio is filled in a silver tube or a silver alloy tube, subjected to a diameter reduction process or a rolling process, and then heat-treated. In the application method, a paste containing an oxide powder is continuously applied to a silver or silver alloy substrate by a method such as dip coating to form a film, and then heat treatment is performed. In any case, the heat treatment is performed by raising the temperature to around 890 ° C., which is the partial melting temperature of the Bi2212 phase, to bring the Bi2212 phase into a partially molten state, and then gradually cooling it to around 850 ° C., which is the crystallization temperature of the Bi2212 phase. Do. By performing such heat treatment, Bi
The 2212-based superconductor forms an extremely highly oriented structure such as a stack of flaky crystals.

[0003] In addition, for a bulk body or a film, there is also known a method of performing a similar heat treatment and gradual cooling after forming a predetermined molded body or applying a solution containing a raw material powder, such as a slurry or an organic acid salt, onto a substrate. Have been.

[0004] However, the Bi2212-based superconductor produced by such a heating and melting method has a high critical current density and a high magnetic field characteristic at 30 K or less, but when the temperature exceeds this temperature, the magnetic field characteristic rapidly decreases. . Therefore, there is a problem that it is difficult to use in an environment where a magnetic field is applied in a temperature range of liquid nitrogen temperature to 50 K which is expected to be most utilized. It is considered that the reason for the deterioration of the characteristics at a temperature of 30 K or more is that there is no effective pinning center in the Bi2212 phase.

On the other hand, a method of producing a Bi2212 superconductor by a powder method is also being studied. For example, "Physica
C 249 (1995) 241-246 ", the raw material powder was mixed at a predetermined ratio, calcined at 750 to 820 ° C, and then 850 ° C.
And a method of reheating at about 800 ° C. after the firing. The firing here (850 ° C.)
The firing in the powder method, that is, the solid-phase reaction is performed without partially melting the raw material powder.

The presence of pinning centers was confirmed in the Bi2212 superconductor obtained by this powder method, but the grain size was large, and the pinning centers were concentrated at the grain boundaries of the Bi2212 crystal grains. As a result, it was not a valid pinning center.

As described above, in the Bi2212-based superconductor, an effective pinning center cannot be introduced by any of the conventional manufacturing methods, and therefore, the critical current density and the magnetic field characteristics are limited. .

[0008]

SUMMARY OF THE INVENTION The present invention solves the above problems and introduces a number of effective pinning centers in a Bi2212 superconductor to improve the critical current density and magnetic field characteristics of the superconductor in a wide temperature range. The purpose is to let them. That is, an object of the present invention is to introduce an effective pinning center by distributing a large number of small-diameter pinning centers in a Bi2212-based superconductor structure, that is, by allowing them to exist in a finely dispersed state.

[0009]

SUMMARY OF THE INVENTION The present invention relates to a Bi2212 superconductor, in which raw powders are mixed at a predetermined ratio to obtain Bi2 _-yP.
a mixture having a mixing ratio of _{b y Sr 2 CaCu 2 O x} , and heating the mixture, the _{Bi 2-y Pb y Sr 2} CaCu 2 O x phase and partially melted state, and then gradually cooled so Bi _{2-y Pb y Sr 2 CaCu} 2 O x layer was crystallized, then characterized by annealing at a temperature lower than the temperature required for the said partially melted state.

[0010] partial melting together a part of Bi is substituted by Pb - after annealing treatment, by _{annealing, Bi 2-y Pb y Sr} 2 C
a Separate the Cu ₂ O _x phase into a stable phase and add Bi _2-y Pby _y Sr ₂ C
aCu during ₂ O _x, smaller particle diameters at a pinning center (Pb B
_{i) 4 (Sr Ca) 5} CuO x phase (hereinafter referred to as 451-phase) can be precipitated.

Hereinafter, the method for producing an oxide superconductor of the present invention will be described in detail.

In the production method of the present invention, first, as a raw material powder, a compound such as an oxide or a carbonate of a metal element constituting a superconducting material, for example, Bi ₂ O ₃ , Pb ₂ O ₃ , CaCO ₃ , SrCO ₃ ,
Mix CuO and the like at a predetermined ratio. The predetermined ratio, Bi in these raw material powders, Pb, Sr, Ca, Cu , the respective elements of O, a ratio such that the ratio of _{Bi 2-y Pb y Sr 2} CaCu 2 O x. In the present invention, by adding lead, a Bi2212-based superconductor in which a part of bismuth is replaced by lead can be obtained, and a 451 phase which is a phase-balanced and stable non-superconducting phase can be formed. The 451 phase becomes a pinning center by precipitating in the Bi2212 phase. The amount of lead to be replaced is 1 to 30% with respect to bismuth, preferably 1%.
0 to 20%. If the substitution amount is less than 1%, 451
No phase is formed and no effective pinning center is obtained.
On the other hand, if it is more than 30%, it is difficult to form a Bi2212 phase, so that a superconductor cannot be produced.

The mixture of the raw material powders thus mixed is formed into a desired shape prior to the heat treatment. As the molding method, various molding methods can be used depending on the desired shape. For example, in the case of a wire, a silver sheath method of filling a superconducting powder into a silver tube and a method of applying a superconducting paste on a silver or silver alloy substrate can be adopted. In the case of a bulk body, a pressure molding bulk method of filling and compressing a raw material powder in a rubber mold, and in the case of a film, a method of applying a paste or a metal salt on a silver single crystal or polycrystal substrate to form a film. And a method of forming a raw material powder on a substrate by sputtering or vapor deposition.

Next, the molded product of the mixture of the obtained raw material powders is heated to bring the Bi2212 phase into a partially molten state. By this heating, the mixture of the raw material powders is turned into a solid phase ((Sr, Ca) -Cu
-O phase and Bi-Sr-Ca-O phase) are mixed, that is, a partially melted state. The partial melting temperature of the Bi2212 phase is 860 to 90
The temperature is about 890 ° C when molded by a method of contacting with a silver tube or a silver plate at 0 ° C. In the present invention, 870 to 890
It is desirable to heat at a temperature of about 5 to 15 minutes. still,
The heat treatment can be performed by an electric furnace or zone melting.

Next, this is gradually cooled to crystallize the Bi2212 phase and grow the crystal. The crystallization temperature of the Bi2212 phase is about 850 ° C., but the final temperature may be appropriately set according to the degree of crystal growth and superconductivity.
The cooling rate of the slow cooling is 0.1 to 10 ° C./min, more preferably 0.1 to 5 ° C./min.

The compound which has been slowly cooled is substantially composed of a single layer of the Bi2212 phase, in which a part of bismuth in the Bi-O layer is replaced by lead. This Bi2212 phase is a metastable phase at a temperature of 800 ° C. or lower, but is separated into a stable phase by annealing after slow cooling.

The annealing is performed at a temperature lower than the temperature required for the partial melting state. That is, the superconductor must not be in a partially melted state again. Specifically, annealing is performed at 700 to 850 ° C, more preferably 700 to 800 ° C. By performing such annealing, 45
One layer is formed, and a large number of fine particles precipitate in the Bi2212 phase.

The amount of precipitation of the 451 layer changes depending on the amount of replaced lead and the annealing time. As the annealing time elapses, the precipitation amount of the 451 phase increases, but annealing for 150 hours or more undesirably decomposes the Bi2212 phase and lowers the superconductivity. If the time is less than 30 hours, formation of the 451 phase becomes difficult. Therefore, the annealing time is preferably 30 hours to 150 hours.

The Bi2212 superconductor thus obtained exhibits excellent magnetic field characteristics even in an environment of 20 to 50K.
It can be used for various purposes. For example, when formed into a wire, it becomes a superconducting magnet and a cable conductor, when it is formed into a pressurized bulk body, it becomes a current lead, an element for a current limiter, a magnetic shield, and when formed into a superconducting film, it becomes a It can be used for elements, magnetic shields, antennas, filters and the like.

[0020]

Embodiments of the present invention will be described below. Example 1 Bi ₂ O ₃ , Pb ₂ O ₃ , SrCO ₃ , CaCO ₃ and Ca
O was mixed at different mixing ratios to obtain a mixture. Compounding ratio, the value of y _{Bi 2-y Pb y Sr 2} CaCu 2 O x was varied so that the values shown in Table 1. The mixture was filled in a silver tube, subjected to diameter reduction and rolling, and then placed in an electric furnace and heated to a maximum temperature of 885 ° C., thereby bringing the Bi2212 phase into a partially molten state. This is then cooled at a cooling rate of 8 ° C./min.
It was cooled to ° C. After that, this is put in air at 800 ° C. for 5 minutes.
Annealed for 0 hours. Table 1 shows the measurement results of the 451 phase fraction and the critical current density Jc at 40 K and 3 Tesla of the obtained superconductor.

[0021]

[Table 1] As can be seen from this result, by using a raw material in which a part of bismuth was replaced with lead and annealing under predetermined conditions,
An extremely excellent critical current density was obtained even in the temperature range of 40K. In particular, when the lead content was 0.2, a high critical current density was exhibited.

Example 2 Bi ₂ O ₃ , Pb ₂ O ₃ , SrCO ₃ , CaCO ₃ and C 2
Prepare a mixture of aO and Bi _1.8 Pb _0.2 Sr ₂ CaCu ₂ O _x so as to have a compounding ratio, fill this into a silver tube, perform diameter reduction processing and rolling processing, and then put it in an electric furnace. Then, the temperature was raised to a maximum temperature of 885 ° C., and the Bi2212 phase was partially melted. Then, it was gradually cooled to 830 ° C. at a cooling rate of 8 ° C./min. Thereafter, it was annealed at 800 ° C. in air with various annealing times. Table 2 shows the relationship between the annealing time of the obtained superconductor and the phase fraction of the 451 phase.

[0023]

[Table 2] From this result, it was found that 451 phase was favorably generated by annealing. In particular, when the annealing time is set to 30 hours or more, a high 451 fraction can be obtained.

[0024]

As described above, according to the method for manufacturing an oxide superconductor of the present invention, an oxide superconductor in which a large number of 451 phases (non-superconducting phases) having a small grain size are distributed in Bi2212 phase is obtained. Obtainable. That is, in the Bi2212 superconductor, a number of effective pinning centers could be introduced, and thereby the critical current and magnetic field characteristics of the superconductor could be improved in a wide temperature range.

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Claims (4)

[Claims] 1. Bi _-y Pb _y is prepared by mixing raw material powders at a predetermined ratio.
A mixture having a mixing ratio of Sr ₂ CaCu ₂ O _x, and partially melted state by heating the mixture, and then gradually cooled this crystallized the _{Bi 2-y Pb y Sr 2} CaCu 2 O x phase A method for producing an oxide superconductor, further comprising annealing at a temperature lower than a temperature required for the partial melting state. 2. A mixed raw material powder at a predetermined ratio Bi _2-y Pb _y
A mixture having a mixing ratio of Sr ₂ CaCu ₂ O _x, and partially melted state by heating the mixture at eight hundred sixty to nine hundred ° C., and then gradually cooled _{so, Bi 2-y Pb y Sr} 2 CaCu 2 O x A method for producing an oxide superconductor, which comprises crystallizing a phase and further annealing at 700 to 850 ° C. 3. The method according to claim 1, wherein the annealing is performed for 30 hours or more. 4. The content _{y of} Pb in Bi _2-y Pby _y Sr ₂ CaCu ₂ O _x
Is in the range of 0.02 to 0.6, the method for producing an oxide superconductor according to any one of claims 1 to 3, wherein