JPH0791057B2 - Rare earth oxide superconductor - Google Patents

Description

TECHNICAL FIELD The present invention relates to a rare earth oxide superconductor having a novel structure and a method for producing the same.

[Prior Art] Conventionally, REBa ₂ Cu ₃ O _7-y (RE is Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er,
A superconductor (hereinafter also referred to as a rare earth-based superconductor) represented by a composition formula of one or more selected from the group consisting of Tm, Yb, and Lu, y is an oxygen deficiency amount, hereinafter referred to as 123 phase) is known. . As a method of manufacturing a bulk body of a rare earth-based superconductor, there is a method of synthesizing a crystal powder having the above composition, molding the powder, and sintering the powder. In addition, it is known to manufacture by a sol-gel method or a melt coagulation method.

The superconductors produced by these methods are all polycrystals, and the respective crystal grains are arranged in a disordered direction, and at the grain boundaries, a crystalline phase other than the 123 phase, an amorphous phase, and many In the case of, the pores are included. Since the direction in which the current easily flows in the crystal grains of the rare earth-based superconductor is determined, it is difficult for the current to flow in the grain boundaries between the crystal grains having different directions. Furthermore, since the grain boundary phase is not a superconductor, it acts as an insulating layer. For this reason, no conventional polycrystalline rare-earth superconductor having a high critical current density has been obtained.

The decrease in the critical current density due to such a grain boundary portion is
It is known that this phenomenon is more prominent in a magnetic field. As a field of application of the superconductor, it is mainly considered to use a wire material or a tape material as a coil to form a strong magnetic field. Therefore, in order to put the rare earth-based superconductor into practical use, the orientation of the crystal particles should be aligned.
It is considered necessary to produce a material having a high critical current density in a strong magnetic field by producing a structure in which grain boundaries are suppressed.

As a general method for producing a ceramic material having such a structure, a method of unidirectionally solidifying a melt under a temperature gradient is known. However, rare earth-based superconductors have RE ₂ BaCuO ₅ crystal (below 2
11 phase) and melts into a liquid phase. Therefore, when the melt having the same composition as the 123 phase is cooled, the 211 phase is precipitated first, and thus the 123 phase single crystal or oriented polycrystal cannot be obtained by the usual method.

The present inventor has previously produced a superconductor having a structure as shown in FIG. 2, and the superconductor having such a structure has a high critical current density and a decrease in the critical current density even when a magnetic field is applied. I suggested that. (JP-A-2-204322).

The structure of the superconductor shown in FIG. 2 has 123 phases as a matrix. The 123-phase is a crystal grown in a plate shape in a direction perpendicular to the c-axis and is a polycrystal as a whole, but the plate-shaped crystals are layered and aligned in the respective c-axis directions. The crystal is almost continuous as a single crystal even in the direction perpendicular to the c-axis, and the superconducting state is not easily broken even when a magnetic field is applied. Further, between the 123 phases, granular 211 phases having a particle diameter of several μm to several tens of μm are dispersed in an island shape. 2
The 11 phases are not oriented, and the crystals are not continuous. Therefore, this 211 phase does not interfere with the superconducting path.

[Problems to be Solved by the Invention] When a superconducting material is used in a strong magnetic field, it is required that the quantized magnetic flux penetrating the material be fixed to the pinning center. Fine precipitates, grain boundaries, and various defects can be considered as pinning centers. It is considered that in the rare earth-based superconductor as shown in FIG. 2, 211-phase finely dispersed fine particles can serve as the pinning center.

In order for such a fine precipitate to act as an effective pinning center, it is desired that the particle size be as small as 1 μm or less. However, when a rare earth superconductor such as that shown in FIG. 2 is manufactured by the unidirectional solidification method, if the grain size of the 211 phase is reduced by controlling the solidification conditions, the orientation of the 123 phase deteriorates. I will end up. as a result,
When only one kind of rare earth element is used, the particle size of the particles of the 211 phase is at least several μm or less, and it is unlikely that this acts as a good pinning center. It is considered that if the 211-phase precipitate can be finely dispersed, the critical current density will be further improved. The object of the present invention is 12
The purpose is to obtain a rare earth-based superconductor having a structure in which 211-phase particles are dispersed in a fine particle size so as to have a high pinning effect in a 3-phase matrix.

[Means for Solving the Problem] The present inventor has found that the above object can be achieved by incorporating two or more kinds of rare earth elements as elements constituting a superconductor and producing the superconductor by a unidirectional solidification method. .

Thus, the present invention provides REBa ₂ Cu ₃ O _7-y (RE is Y, La, Nd, Sm, Eu,
Two or more kinds selected from the group consisting of Gd, Dy, Ho, Er, Tm, Yb, and Lu, where y is the oxygen deficiency amount) and plate-like crystals represented by the composition formula are overlapped in a layered manner. _{2 The} granular crystals represented by the composition formula of BaCuO ₅ are dispersed like islands, and
The present invention provides a rare earth oxide superconductor having a structure in which a weight percentage is 1 μm or less.

Since two or more kinds of rare earth elements are used in the present invention, the 211-phase crystal particles are unlikely to grow large even under the condition of unidirectional solidification in which the 123-phase plate-shaped crystals are likely to grow in the same direction. The reason for this is not clear, but for each rare earth element, the composition ratio in the molten state is 21
There is a difference between what is contained in one phase and what is contained in the liquid phase, and it is thought that due to this effect, a structure in which 211-phase particles are finely dispersed is formed compared to the case where only one kind of rare earth element is used. . In particular, when an element with a large ionic radius such as Sm or Eu is combined with an element with a relatively small ionic radius such as Y, Sm or Eu is preferentially taken into the 211 phase and the 211 phase crystal grains grow large. Hard to do. In order to fully bring out the effect of this rare earth mixture, two or more REs are used.
Of the RE, the second largest element content is 1m
It is preferably ol% or more. This content is 10 mol%
Is more preferable.

The method for producing the rare earth oxide superconductor of the present invention is preferably a solidification method from a melt. In particular, RE
_{2 From the} partially melted state in which the solid phase of BaCuO ₅ and the liquid phase of the RE-Ba-Cu-O system coexist, when cooled and crystallized, a solidified product of the structure in which the 211 phase is homogeneously dispersed is obtained. Therefore, it is preferable.

For solidification, a method of unidirectionally solidifying with a temperature gradient is preferably used. The unidirectional solidification conditions depend on the type of rare earth element selected for the melting temperature, but in both cases it is preferable to control the temperature gradient to 50 ° C./cm or more and the crystal growth rate to 5 mm / h or less.

[Example] Regarding two kinds of rare earth elements RE1 and RE2 shown in Table 1, RE1: R
E2: Ba: Cu atomic ratio of a: b: 17: 24 oxide calcination powder is made, and the powder is 70mm × 40mm by a die press.
It was molded into a 2 mm diameter, baked in an oxygen stream at 930 ° C. for 10 hours, cooled and cut out with a diamond cutter to obtain a 70 mm × 4 mm × 2 mm prismatic sintered body.

Next, one end of this prismatic sintered body was fixed, and the maximum temperature part was 1080 ° C under an oxygen stream, and the vertical temperature was 50 ° C / c each.
A solidified product was obtained by a zone melting method in which an electric furnace having a temperature distribution with a gradient of m was used to move at a speed of 2 mm / h. The solidified product obtained as a result was further heated to 700 ° C. in an oxygen atmosphere, gradually cooled at 15 ° C./h, and kept at 450 ° C. for 40 hours to obtain a rare earth oxide superconductor.

Observation of this oxide superconductor using a scanning electron microscope and an X-ray elemental analyzer revealed that, as shown in FIG. 1, plate-like 123-phase crystal grains were superposed in layers, and the granular 211 It was confirmed that the crystal grains of the phase had a structure in which they were dispersed in an island shape. The crystal grain size of the 211 phase was large, and 50% by volume or more of fine particles of 1 μm or less were observed at several μm. A combination of a rare earth element with a large ionic radius and a rare earth element with a small ionic radius precipitates 21
The grain size of one phase tends to be small. For example, in the case of a system containing Y and Sm, most of 211 phase crystals had a grain size of 1 μm or less. At this time, Sm was contained in a larger concentration in the 211 phase than in the 123 phase of the matrix.

Table 1 shows the superconducting properties of the obtained superconductor. A sample cut into a size of 1 mm × 0.15 mm × 10 mm was used for these measurements. The critical temperature (Tc) is the temperature at which zero resistance is measured by the DC four-terminal method. The critical current density (Jc) was also measured by the DC four-terminal method at an external magnetic field of 5 Tesla at liquid nitrogen temperature (77K).

[Comparative Example] Regarding the rare earth elements RE shown in Table 2, the atomic ratio of RE: Ba: Cu is
Make a calcinated powder of oxide such that it becomes 13:17:24, mold the powder to 70 mm × 40 mm × 2 mm with a mold press, bake at 930 ° C for 10 hours in an oxygen stream, and after cooling, diamond It was cut out using a cutter to obtain a 70 mm × 4 mm × 2 mm prismatic sintered body.

Next, with respect to this prismatic sintered body, a solidified product was obtained by the zone melting method similar to the example. This solidified product is further heated to 700 ° C in an oxygen atmosphere, gradually cooled at 15 ° C / h, and then cooled at 450 ° C to 40 ° C.
After holding for a while, a rare earth oxide superconductor was obtained.

Observation of this oxide superconductor using a scanning electron microscope and an X-ray elemental analyzer revealed that plate-like 123-phase crystal grains were layered, with granular 211-phase crystal grains forming islands. It was confirmed to have a dispersed structure similar to that shown in FIG. The crystal grain size of the 211 phase was large, but no more than 1 μm fine particles were observed at about several tens μm.

Table 2 shows the superconducting properties of these superconductors obtained in the same manner as in Example 1.

[Advantages of the Invention] In the superconductor of the present invention, the 123 phase of the matrix has a high orientation, and as the other phases, very fine granular 211 phases are dispersed in an island shape, which results in good pinning of magnetic flux. Since it acts as a center, it has a high critical current density even in a strong magnetic field.

INDUSTRIAL APPLICABILITY The production method of the present invention is useful for producing a rare earth-based superconductor in which 123 phases are oriented.

[Brief description of drawings]

FIG. 1 is a schematic diagram of an image obtained by observing a cross-sectional structure of a superconductor of a system containing two or more kinds of rare earth elements of the present invention with a scanning electron microscope. FIG. 2 is a schematic diagram of an image obtained by observing a cross-sectional structure of a superconductor of a system containing one kind of rare earth element with a scanning electron microscope.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication location H01L 39/12 ZAA C (56) References Japanese Patent Publication 4-40289 (JP, B2) International Publication 90 / 13517 (WO, A) Appl. Phys. Lett. , Vol. 52, No. 24, P. 2074-2076 (1988) Phys. Rev. B, Vol. 37, No. 13, P.I. 7850-7853 (1988)

Claims (4)

[Claims] 1. REBa ₂ Cu ₃ O _7-y (RE is Y, La, Nd, Sm, Eu, Gd, Dy,
Two or more kinds selected from the group consisting of Ho, Er, Tm, Yb, and Lu, and y is an oxygen deficiency amount) plate-like crystals represented by the composition formula are layered, and RE ₂ BaCuO ₅ A rare earth oxide superconductor having a structure in which granular crystals represented by a composition formula are dispersed in an island shape, and 1% by volume or more of the granular crystals have a grain size of 1 μm or less. 2. The rare earth oxide superconductor according to claim 1, wherein the content of the element having the second largest amount in the selected two or more REs is 1 mol% or more based on the total RE. 3. The crystallization is performed by cooling from a partially molten state in which a solid phase of RE ₂ BaCuO ₅ and a liquid phase of RE—Ba—Cu—O coexist. Manufacturing method of rare earth oxide superconductor. 4. Cooling crystallization is carried out with a temperature gradient of 50 ° C./cm or more, 5
4. A unidirectional solidification method with a crystal growth rate of mm / h or less.
For producing a rare earth oxide superconductor of.