JPH0570137A - Oxide superconductor and its production - Google Patents

Abstract

PURPOSE:To provide an oxide superconductor of a Bi based 2212 phase exhibiting Tc of >=90K. CONSTITUTION:An oxide superconductor containing a different phase mainly containing magnesium and oxygen in a crystalline phase mainly containing bismuth, strontium, calcium, barium, copper and oxygen and having the ratio of Bi:Sr:Ca:Ba:Cu being 1:0.6 to 1.2:0.35 to 0.7:0.05 to 0.2:0.8 to 1.2, expressed in terms of atomic ratio and having the ratio of Bi, Sr, Ca, Ba and Cu being 3.5:0.05 to 2:3.95 to 6.65, expressed in terms of atomic ratio.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an oxide superconductor and a method for producing the same.

[0002]

2. Description of the Related Art As a conventional oxide superconductor, 198
The Bi-Sr-Ca-Cu-O system containing bismuth, strontium, calcium and copper as the main components, which was discovered by the Maeda Researchers of the Institute for Metals Technology in 1988 (hereinafter referred to as B
There is an oxide superconductor of the i-type), but this Bi-type oxide superconductor has a problem that a 2223 phase having a critical temperature at which electric resistance becomes zero (hereinafter referred to as Tc) of around 110 K is hard to be generated. was there. For this reason, it has been attempted to utilize the 2212 phase, which has a low Tc but a wide generation temperature range.

[0003]

However, since the Tc of the Bi-based oxide superconductor 2212 is around 80K, the difference from Tc is small and the superconducting property is unstable when liquid nitrogen is cooled (77K). May not be usable.

As a method of increasing the Tc of the 2212 phase, Japanese, Journal, Of, Applied, Physics (Japanese Journal of Applied Physics) Vo
l. No. 27, 9 (published in September 1988), L1626-L
1628 and Vol. No. 27, 12 (1988 1
February issue), pages L2327 to L2329, and Advances, Inns, and Superconductivity II (Advances).
in Superconductivity II), pp. 149-152, 50
A method has been reported in which a heat treatment is performed at a temperature of 0 to 880 ° C. and then rapidly cooled in liquid nitrogen or air.

Since this method includes a step of quenching, it has a drawback that it is difficult to produce a large-sized molded body although it is possible to produce a small-sized molded body.

[0006] The present invention is 90K without a quenching process.
Bi with higher Tc and mainly consisting of 2212 phase
A system oxide superconductor and a method for producing the same are provided.

[0007]

According to the present invention, a crystal phase containing bismuth, strontium, calcium, barium, copper and oxygen as a main component contains a hetero phase containing mainly magnesium and oxygen. Bi: Sr: Ca: The atomic ratio of Ba: Cu is 1: 0.6 to 1.2: 0.35 to 0.7:
0.05-0.2: 0.8-1.2, and Bi,
In order to obtain the oxide superconductor having the atomic ratio of the total Sr, Ca, Ba and Cu: Mg: O of 3.5: 0.05 to 2: 3.95 to 6.65, and the above composition. The present invention relates to a method for producing an oxide superconductor, in which a mixture or a crystal phase containing bismuth, strontium, calcium, barium, copper and oxygen as main components is mixed with mainly magnesium oxide or a compound which becomes magnesium oxide by firing, and then firing. ..

In the present invention, there are no particular restrictions on the raw materials containing bismuth, strontium, calcium, barium, copper and magnesium, which are the main constituents of the oxide superconductor, but for example oxides, carbonates, nitrates, oxalates. , Acetate, etc. are used alone or in combination of two or more.

Bismuth, strontium, calcium,
The compounding ratio of barium and copper is that bismuth is 1 in atomic ratio,
Strontium 0.6-1.2, calcium 0.3
5 to 0.7, barium 0.05 to 0.2 and copper 0.
The range is from 8 to 1.2, and if it deviates from this range, it is difficult to obtain a 2212-phase Bi-based oxide superconductor having a Tc of 90 K or more without a quenching step. In the above composition, barium forms a solid solution in the 2212 phase, and therefore, when added in the above range, it exhibits a Tc of 90 K or more.
A 12-phase Bi-based oxide superconductor is easily generated.

The atomic ratio of magnesium to oxygen is such that the total of Bi, Sr, Ca, Ba and Cu: Mg: O is in the range of 3.5: 0.05 to 2: 3.95 to 6.65. , Tc of 90K or more when out of this range 22
It is difficult to obtain a 12-phase Bi-based oxide superconductor. It should be noted that magnesium does not form a solid solution in the 2212 phase and always forms a different phase, and tends to be dispersed in the crystal phase as island-shaped fine particles. This heterogeneous phase may contain one or more elements of bismuth, strontium, calcium, barium and copper in addition to magnesium and oxygen, and is not necessarily a single phase.

In the composition of the present invention, the amount of oxygen depends on the amount of copper and the oxidation state of copper. Although the calculated value is within the range shown above, it is impossible to measure the oxidation state in a strict and accurate manner. Therefore, the amount of oxygen is not described in the examples.

After adding a raw material containing magnesium to a mixture of bismuth, strontium, calcium, barium and copper, which is the main component of the crystal phase, an oxide superconductor is produced through a mixing, calcination, fine pulverization and firing steps. Although it is also possible, by a method of preparing a crystal phase containing bismuth, strontium, calcium, barium, copper and oxygen as main components in advance, adding a raw material containing magnesium to this and uniformly mixing them, followed by firing again. It is preferable to produce an oxide superconductor.

In the latter case, the method for synthesizing the crystal phase containing bismuth, strontium, calcium, barium, copper and oxygen as main components is not particularly limited, but for example, bismuth, strontium, calcium, barium and copper are prescribed. Mix the raw material powders in a ratio of 7
After calcination in the range of 50 ° C. to 850 ° C., crushing and molding this, firing at a temperature near the temperature at which the Bi-based 2212 phase decomposes to form a liquid phase, for example, in the range of 750 to 900 ° C. Is preferred.

The method of mixing the crystal phase containing bismuth, strontium, calcium, barium, copper and oxygen as a main component with magnesium oxide or a compound which is converted into magnesium oxide by firing is not particularly limited, but for example, synthesis is possible. It is preferable to use a ball coated with a synthetic resin in a resin ball mill, add a solvent selected appropriately, and wet mix, and it is more preferable if the average particle diameter of the mixed powder is pulverized to, for example, 3 μm or less.

After molding this mixed powder as required, B
The oxide superconductor is obtained by firing at a temperature near the temperature at which the i-based 2212 phase decomposes and melts, for example, in the range of 750 to 900 ° C. If necessary, crushing, molding and firing may be repeated.

The firing atmosphere contains 0.01 to 10% by volume of oxygen.
The firing is preferably performed in an inert gas (for example, nitrogen, argon, etc.) contained therein.

The firing time may be less than 5 to 10 hours, but it is preferably 10 to 100 hours in order to improve the homogeneity of the crystals. The cooling rate is not particularly limited, and for example, cooling may be performed at about 200 ° C./hour.

[0018]

【Example】

Examples 1 to 4 Bismuth trioxide, strontium carbonate, calcium carbonate, barium carbonate and cupric oxide (all of which are high in proportion to the composition of bismuth, strontium, calcium, barium and copper in atomic ratio shown in Table 1). Purity Chemical Laboratory Co., Ltd., purity 99.9%) was weighed and used as a starting material.

Next, the above starting materials were filled in a synthetic resin ball mill together with steel balls covered with synthetic resin and ethyl acetate, and wet mixed for 72 hours under the condition of 50 rpm. After drying, the product was placed in an alumina bowl and calcined at 800 ° C. for 10 hours in the air using an electric furnace, and then coarsely crushed in a mortar to obtain a calcined powder. Thereafter, the calcined powder was press-molded at a pressure of 147 MPa into a pellet having a diameter of 30 mm and a thickness of 2 mm, and then 85 in a nitrogen atmosphere containing 4.76% by volume of oxygen.
The mixture was first baked at 0 ° C. for 15 hours and then roughly crushed in a mortar to obtain a mother phase powder containing bismuth, strontium, calcium, barium, copper and oxygen.

Next, the mother phase powder and magnesium oxide (manufactured by Kojundo Chemical Laboratory Co., Ltd., purity 99.9%) were weighed so as to have the weight ratio shown in Table 2, and mixed under the same conditions as above. A composition for oxide superconductor was obtained. The addition amount of magnesium shown in Table 2 was obtained by analyzing the composition of the powder after mixing by ICP.

After that, the composition for an oxide superconductor was added to
After press molding into a pellet having a diameter of 30 mm and a thickness of 2 mm at a pressure of 7 MPa, a secondary calcination is performed at 865 ° C. for 100 hours in a nitrogen atmosphere containing 4.76% by volume of oxygen.
It was cooled to room temperature at a rate of 0 ° C. to obtain a Bi-based oxide superconductor. The measurement results of the characteristics will be described later.

[0022]

[Table 1]

[0023]

[Table 2]

Comparative Examples 1 and 2 Bismuth trioxide, strontium carbonate, calcium carbonate and cupric oxide (all manufactured by Kojundo Chemical Laboratory Co., Ltd.) so that the ratio of bismuth, strontium, calcium and copper is as shown in Table 3. , Purity 99.9%) was weighed and used as a starting material.

Next, the above starting materials are mixed, calcined, primary fired and coarsely pulverized in the same manner as in Examples 1 to 4, and wetted under the same conditions as in the above mixing step without adding magnesium. The composition was pulverized to obtain a composition for oxide superconductor.

Thereafter, the composition for an oxide superconductor was subjected to press molding and secondary firing under the same conditions as in Examples 1 to 4 to obtain a Bi-based oxide superconductor.

[0027]

[Table 3]

Comparative Examples 3 and 4 Bismuth trioxide, strontium carbonate, calcium carbonate, barium carbonate and cupric oxide so that the composition ratios of bismuth, strontium, calcium, barium and copper are shown in Table 4 in atomic ratio. (All manufactured by Kojundo Chemical Laboratory Co., Ltd., purity 99.9%) were weighed, and the same steps as those of Comparative Examples 1 and 2 were performed to obtain Bi-based oxide superconductors.

[0029]

[Table 4]

Next, fracture surfaces of the Bi-based oxide superconductors obtained in Examples 1 to 4 were observed with an X-ray microanalyzer (S680 manufactured by Hitachi, Ltd.). As a result, the secondary electron image of the Bi-based oxide superconductor obtained in Example 1 and the analysis position of the composition are shown in FIG. 1, and the analysis result of the composition is shown in Table 5. In FIG. 1, 1 to 8 indicate analysis positions.

[0031]

[Table 5]

As shown in FIG. 1 and Table 5, barium is solid-dissolved in the Bi-based oxide superconductor phase, but magnesium is not solid-dissolved in the Bi-based oxide superconductor phase. It can be seen that the particles become fine particles and are dispersed in the Bi-based oxide superconductor. Further, when the crystal phase was examined, it was confirmed to be the 2212 phase of the Bi type superconductor.

Next, the Tc of the Bi type oxide superconductors obtained in Examples 1 to 4 and Comparative Examples 1 to 4 was measured by the DC four-terminal method. The results are shown in Table 6.

[0034]

[Table 6]

As shown in Table 6, it is understood that the oxide superconductors according to the examples of the present invention have Tc of 90K or more. Further, B obtained in Examples 1 to 4 and Comparative Examples 1 to 4
The critical current density (hereinafter referred to as Jc) of the i-based oxide superconductor in a magnetic field of liquid nitrogen temperature was measured by a DC four-terminal method. The results are shown in Table 7. In Table 7, Jc ₀ is 77.
Jc at K zero field is shown.

[0036]

[Table 7]

As shown in Table 7, it is understood that the Bi-based superconductors according to the examples of the present invention show a small decrease in Jc in a magnetic field. From this result, Bi according to the embodiment of the present invention can be obtained.
In the oxide superconductor of the system, it is considered that fine particles of different phase mainly containing magnesium dispersed in the matrix act as pinning points of magnetic flux.

[0038]

According to the present invention, a 2212-phase Bi-based oxide superconductor exhibiting a Tc of 90 K or more can be obtained without going through a quenching step. Further, Jc in a magnetic field can be improved as a concomitant effect.

[Brief description of drawings]

FIG. 1 is a micrograph showing a crystal structure of a fracture surface of a Bi-based oxide superconductor according to an example of the present invention.

[Explanation of symbols]

 1 to 8 ... Analysis position

Claims (2)

[Claims] 1. A crystal phase containing bismuth, strontium, calcium, barium, copper and oxygen as main components,
Contains a heterogeneous phase containing mainly magnesium and oxygen, B
The ratio of i: Sr: Ca: Ba: Cu is 1: 0.
6-1.2: 0.35-0.7: 0.05-0.2:
0.8 to 1.2, and the total ratio of Bi, Sr, Ca, Ba and Cu: Mg: O is 3.5: 0.
05-2: 3.95-6.65 oxide superconductor. 2. A mixture or crystal phase containing bismuth, strontium, calcium, barium, copper and oxygen as main components so as to have the composition according to claim 1, and magnesium oxide or a compound which becomes magnesium oxide by firing. A method for producing an oxide superconductor, comprising: