JP2914799B2 - Manufacturing method of oxide superconducting bulk material - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a RE-Ba-Cu-O-based oxide superconducting bulk material having high critical current density and high magnetic field characteristics.

[0002] Oxide superconductors are formed into various forms of bulk and used for superconducting bearings, superconducting magnets, magnetic shields and the like.

[0003]

2. Description of the Related Art A high critical temperature is a great advantage for superconducting applications. In particular, the discovery of RE-Ba-Cu-O and Bi-based and Tl-based oxide superconductors exceeding the temperature of liquid nitrogen has led to the reduction of cooling costs and the availability of nitrogen, which is more resource-rich than helium. This has the potential to greatly expand the field of superconductivity.

However, for superconductivity applications, the magnitude of the critical current at the operating temperature is more important than the critical temperature. In order to aim at application in liquid nitrogen, it is essential to improve the critical current density at this temperature. The critical current density is not a characteristic inherent to the material, but can be improved by changing the structure of the material. For example, by performing crystal orientation by a melting method, defects that hinder superconducting current can be removed, and a higher critical current density can be achieved as compared with a material manufactured by a sintering method or the like. As the melting method, QMG
Method (Nippon Steel), MPMG method (ISTEC) or MT
G method (AT & T) and the like. The QMG method includes a melting-rapid cooling step and a partial melting-gradual cooling step. The MPMG method is composed of a melt-quenching-pulverization-molding and a partial melting-slow cooling step. Further, the MTG method includes a molding-melting-gradual cooling step.

However, the critical current density itself is still lower than the practical level (T = 77 K, He = 10 ⁴ A at several T).
/ Cm ² ). This is because in order to obtain a high critical current density, it is necessary to not only remove defects that hinder the superconducting current but also to introduce pinning points for suppressing the movement of magnetic flux. When the pinning is simply performed by a melting method, a high critical current density cannot be obtained because such pinning has to rely on point defects or dislocations which are naturally introduced. In order to suppress the movement of this magnetic flux, RE-B
RE ₂ BaCuO which is a non-superconducting phase in a-Cu-O system
₅ phase (211 phase) is REBa ₂ Cu ₃ O which is a superconducting phase
Finely dispersed in the _7-x phase (123 phase).

[0006]

The above-mentioned QMG method and M
In the TG method, each component is uniformly dispersed before melting, but when melted, an RE oxide is generated, which is coarsened and segregates. The RE ₂ BaCuO ₅ phase is generated with this oxide as a nucleus. For this reason, it is difficult to uniformly disperse the fine RE ₂ BaCuO ₅ phase in the REBa ₂ Cu ₃ O _7-x phase. In the MTG method, R
The E ₂ BaCuO ₅ phase becomes coarse. This is considered to be due to coarsening due to a long partial melting treatment time. As a result, the pinning effect of the magnetic flux by the RE ₂ BaCuO ₅ phase is not sufficient, and cracks occur in the REBa ₂ Cu ₃ O _7-x phase, and a critical current density that can withstand practical use at liquid nitrogen temperature in a high magnetic field is obtained. There was a problem that can not be.

[0007] Therefore, the present invention relates to REBa ₂ Cu ₃ O
_An object of the present invention is to provide a method for producing a bulk oxide superconducting material in which a RE ₂ BaCuO ₅ phase is finely and uniformly dispersed in a _7-x phase and a high critical current density can be obtained.

[0008]

According to the present invention, there is provided a method of manufacturing a bulk oxide superconducting material, which comprises melting a mixed powder of a Ba oxide powder and a Cu oxide powder, quenching and pulverizing the mixed powder. Make powder. Then, RE ₂ O ₃ or RE ₂ O ₃ , RE ₂ Ba is added to the Ba—Cu composite oxide powder.
A mixed powder of CuO ₅ is added, kneaded and molded. And
Partially melt the molded body, and then heat-treated for crystal growth,
The oxygen content adjustment heat treatment is performed. Here, RE is Y, Sm, E
It refers to at least one element selected from the group consisting of u, Gd, Dy, Ho, Er, Tm, Yb, and Lu.

The present invention will be described in detail with reference to FIGS. In the figure, as an example, BaCO ₃ ,
The case where CuO, Y ₂ O ₃ is used is shown.

The mixing ratio of BaCO ₃ powder and CuO powder is as follows: Ba: Cu = (1.5-2.5) :( 2.5-3.
5) is preferable. Melt heating temperature of mixed powder is 110
0 ° C. or higher, and the heating and holding time is about 3 to 30 min. Melting the mixed powder of BaCO ₃ powder and CuO powder,
By rapid cooling, a solidified massive Ba-Cu composite oxide is obtained. The melting of the mixed powder causes Ba
CO ₃ in CO ₃ disappears. In order to avoid separation of Ba and Cu, the quenching rate is desirably -10 ° C / sec or more.

The particle diameter of the powder obtained by pulverizing the Ba—Cu composite oxide is preferably 0.2 mm or less.

[0012] Y ₂ O _{3 is} added to the Ba-Cu composite oxide powder.
Alternatively, a mixed powder of Y ₂ O ₃ and Y ₂ BaCuO ₅ is added, and then molded. This composition is Y: Ba: Cu = 1: (1 to 1
2): Desirably (1-3).

In the partial melting heat treatment of the compact, the compact is
Heat to 000 to 1350 ° C and hold for about 0.1 to 2 hours. If the heating temperature of the molded body is lower than 1000 ° C., partial melting occurs, but the amount is small and the critical current density is not improved. Conversely, if the heating temperature of the molded body exceeds 1350 ° C., a fine Y ₂ BaCuO ₅ phase cannot be obtained. In addition, the molded body is completely melted and cannot remain in the original form, and a bulk material having a required shape cannot be obtained. Then, 1050-1000 ° C
Relatively high cooling rate (eg, -50 to 1 ° C./mi)
Cool in n).

In the crystal growth heat treatment, the compact is heated to the temperature of 10 ° C.
Slowly cool from 50-1000 ° C to 960-940 ° C. The cooling rate is -200 ° C / hr or less. When the slow cooling rate exceeds -200 ° C / hr, YBa ₂ Cu ₃ O _7-x
Since the phase crystal grains do not grow sufficiently, the number of grain boundaries increases and the critical current density decreases. By such a heat treatment, since the superconducting phase contains a fine Y ₂ BaCuO ₅ phase, the structure is fine and the mechanical strength is improved.

In the oxygen content adjusting heat treatment, the compact is cooled in an oxygen atmosphere or air. Cooling rate is -10-1 ° C /
hr. In the oxygen amount adjusting heat treatment, YBa ₂ Cu ₃
Adjusting the amount of oxygen O _7-x in O _{7- x.}

[0016]

When the molded body is heated to 1000-1350 ° C., it becomes a semi-molten state, and RE ₂ O ₃ and a liquid phase (BaCu oxide)
Reacts to form fine needle-like RE ₂ B with RE ₂ O ₃ as the core.
The aCuO ₅ phase grows. This RE ₂ BaCuO ₅ phase is divided into fine phases. In the semi-molten state, fibrous R
Since the E ₂ BaCuO ₅ phase absorbs the liquid phase, the shape of the compact is almost maintained.

When the molded body in a semi-molten state is gradually cooled, RE ₂
REBa ₂ C by peritectic reaction between BaCuO ₅ phase and liquid phase
u ₃ O _7-x phase is precipitated. The nucleation is suppressed by further slow cooling from this state, and large crystal grains with few grain boundaries can be obtained by controlling the crystal growth.
The structure formed at this time contains a fine RE ₂ BaCuO ₅ phase in the REBa ₂ Cu ₃ O _7-x phase, and this RE ₂ B
The presence of the aCuO ₅ phase disperses the stress generated during the heat treatment including the crystal formation process, thereby reducing the grain boundaries and cracks, and forming an aggregate of crystal grains of several millimeters or more with uniform orientation.

Ba containing RE at a high temperature exceeding 1350 ° C.
Since the Cu composition is not melted, RE ₂
Since there is no aggregation or coarsening of O ₃ , the RE ₂ BaCuO ₅ phase becomes fine. Further, R in the REBa ₂ Cu ₃ O _7-x phase
The amount of the E ₂ BaCuO ₅ phase mainly depends on RE ₂ O ₃ and RE ₂ B
It is adjusted by the addition amount of the aCuO ₅ phase.

If the RE ₂ BaCuO ₅ phase is fine, the number of points at which the magnetic flux penetrating through the superconductor interlinks with the RE ₂ BaCuO ₅ phase increases, and the pinning effect is enhanced. Also, cracking is reduced. As a result, the critical current density improves.

[0020]

【Example】

[Example 1] A powder obtained by mixing BaCO ₃ powder and CuO powder in a molar ratio of 2: 3 was mixed in a platinum crucible for 1 hour.
After heating and melting at 400 ° C. for 3 minutes, it was quenched on a copper hearth (the cooling rate until solidification was about −10 ° C./sec). The obtained molded body was pulverized to a powder of 100 mesh or less, and a mixed powder of Y ₂ O ₃ and Y ₂ BaCuO ₅ was added thereto.
The mixing ratio of Y: Ba: Cu is 1: 2:
A mixed powder having a ratio of 3, 1.2: 2: 3 and 1.5: 2: 3 was filled in a mold, molded by a hydraulic press, and further subjected to CIP to increase the packing density to a diameter of 3 cm and a thickness. 1.5cm
Was molded.

The above pellets are heated to 1100 ° C.
Hold for min and partially melt the molded body. From this state, it was cooled to 1000 ° C. at −200 ° C./hr. 100
From around 0 ° C., the superconductor YBa ₂ Cu ₃ O _7-x crystal starts to grow. Crystal growth stops at about 960 to 940 ° C. In the temperature range during this period, the generation of crystal nuclei is suppressed in order to grow the crystal largely, and the crystal is gradually cooled (−
(1 ° C./hr). In addition, this temperature is -1 until normal temperature.
Cooled at 00 ° C / hr. Thereafter, heat treatment for adjusting the amount of oxygen is performed by reheating to a temperature of 600 ° C.
After holding for hr, it was cooled at -10 ° C / hr to 200 ° C.

FIG. 3 shows the microstructure of the Y-based oxide superconductor obtained by this heat treatment. FIG.
2: 3, FIG. 3 (b) shows a microstructure with a mixing ratio of 1.2: 2: 3. Both microstructures are YBa ₂ Cu ₃ O
_{In the 7-x} crystal, Y ₂ BaCuO ₅ crystals were finely dispersed to an average of 2 μm or less, and further fine cracks were hardly observed, and a good quality oxide superconductor bulk material was obtained.

The critical temperature at which the crystals of any composition ratio enter the superconducting state was 92 K as shown in FIG.

At 77K, the critical current density was converted from the magnetization hysteresis curve measured using a sample vibrating magnetometer.
When the mixture ratio of Y: Ba: Cu is 1: 2: 3, 2.3 × 10 ⁴ A / cm ² (see FIG. 5), 1.5: 2: 3
At a mixing ratio of 3.7 × 10 ⁴ A / cm ² , indicating an extremely high critical current density.

Example 2 Next, the same heat treatment as in Example 1 was performed by changing the raw material powder and the composition ratio thereof, and
An a ₂ Cu ₃ O _7-x bulk superconductor was prepared. Where R
Since E changes the temperature at which REBa ₂ Cu ₃ O _7-x is generated, the temperature at which slow cooling is started is changed accordingly. The slow cooling rate is -1 ° C / hr as in Example 1. The sample thus produced was subjected to a structure observation and a magnetization measurement at 77K. As a result, in each case, the RE in the generated REBa ₂ Cu ₃ O _{7-x
The 2} BaCuO ₅ phase was found to be finely dispersed to an average of 2 μm or less. It was also confirmed that the critical current density estimated from the magnetization measurement had a high value of 1 × 10 ⁴ A / cm ² or more under the conditions of 77 K and 1 T. Table 1 shows the composition of the implemented system, the annealing temperature range for growing REBa ₂ Cu ₃ O _{7 -x} , and the critical current density at 77 K and 1 T. Also, as an example, GdBa manufactured by the above-described method is used.
FIG. 6 shows the magnetization curve of the ₂ Cu ₃ O _7-x bulk superconductor and the magnetic field dependence of the critical current density estimated from the curve. The high critical current density as shown above is due to the elimination of weak bonds by the melting method and the RE ₂ Ba in the REBa ₂ Cu ₃ O _7-x phase.
This is realized by fine dispersion of CuO ₅ phase.

[0026]

[Table 1]

[0027]

According to the present invention, REBa ₂ Cu ₃ O
A superconducting bulk material in which a fine RE ₂ BaCuO ₅ phase is uniformly distributed in the _7-x phase can be obtained. In the obtained REBa ₂ Cu ₃ O _7-x phase, one particle size is as large as several millimeters, and there are few grain boundaries and cracks. As a result, fine RE
₂ BaCuO ₅ phase improves the pinning effect of magnetic flux,
Even in a high magnetic field, a higher critical current density can be obtained as compared with the conventional method.

[Brief description of the drawings]

FIG. 1 is a flowchart showing the steps of the present invention.

FIG. 2 is a diagram showing the steps of the present invention.

FIG. 3 is a photograph showing a crystal structure of an RE-based oxide superconductor.

FIG. 4 is a graph showing a critical temperature of the RE-based oxide superconductor of the present invention.

FIG. 5 is a magnetization hysteresis curve of the RE-based oxide superconductor of the present invention.

FIG. 6 is a graph showing a magnetization hysteresis curve of a Gd-based oxide superconductor and a relationship between a magnetic field and a critical current density.

Continued on the front page (72) Inventor Misao Hashimoto 1618 Ida, Nakahara-ku, Kawasaki-shi, Kanagawa Prefecture New Nippon Steel Corporation Advanced Technology Research Laboratory (72) Inventor Masamoto Tanaka 1618 Ida, Nakahara-ku, Kawasaki-shi, Kanagawa New Japan (72) Inventor Mitsuru Morita 1618 Ida, Nakahara-ku, Kawasaki City, Kanagawa Prefecture New Nippon Steel Corporation Advanced Technology Research Laboratory (72) 1618 Inventor Seiki Takebayashi 1618 Ida, Nakahara-ku, Kawasaki City, Kanagawa Prefecture Nippon Steel Corporation Advanced Technology Research Laboratories (56) References JP-A-2-204322 (JP, A) JP-A-4-119968 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , (DB name) C01G 3/00 C01G 1/00 H01B 13/00 H01L 39/12 H01B 12/00

Claims (1)

(57) [Claims] 1. A mixed powder of a Ba oxide powder and a Cu oxide powder is melted, quenched, and pulverized to produce a Ba—Cu composite oxide powder.
₂ O ₃ (where RE is Y, Sm, Eu, Gd, Dy, H
1 selected from the group consisting of o, Er, Tm, Yb, and Lu
Or more elements) or RE ₂ O ₃ , RE ₂ BaC
A method for producing a bulk oxide superconducting material, comprising adding a kneaded uO ₅ powder, kneading and shaping, subjecting the formed body to a partial melting heat treatment, a subsequent crystal growth heat treatment, and an oxygen content adjustment heat treatment.