JPH0791056B2 - Method for producing oxide superconductor having new structure - Google Patents

Description

The present invention relates to a method for producing an yttrium-based oxide superconductor having a novel structure.

[Conventional Technology] Conventionally, LnBa ₂ Cu ₃ O _7-y (Ln is Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er,
There is known a superconductor (hereinafter also referred to as yttrium-based superconductor) represented by a composition formula of at least one selected from the group consisting of Tm, Yb, and Lu, y is an oxygen deficiency amount, and hereinafter referred to as 123 phase. As a method of manufacturing the yttrium-based superconductor, there is a method of synthesizing a crystal powder having the above composition, and then molding and sintering the crystal powder. In addition, it is known to manufacture by a sol-gel method or a melt coagulation method.

[Problems to be Solved by the Invention] The superconductors manufactured by these methods are all polycrystals as shown in FIG. 3, and the respective crystal grains are arranged in a disordered direction and at the grain boundaries. , The structure contains grain boundary phases other than 123 phase. Fig. 3 shows the case where Ln is yttrium and the grain boundary phase has a crystalline phase other than 123 phase, an amorphous phase,
In many cases, pores are included. However, the yttrium-based superconductor has a drawback that current does not easily flow at grain boundaries between crystal grains having different directions because the direction in which the current easily flows is determined in the crystal grains.

Furthermore, since the grain boundary phase is not a superconductor, it acts as an insulating layer. Therefore, no conventional polycrystalline yttrium-based superconductor has a high critical current density.

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 yttrium-based superconductor into practical use, 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.

The yttrium-based superconductor decomposes and melts at a temperature of about 1000 ° C or higher, and separates into a Ln ₂ BaCuO ₅ crystal (hereinafter referred to as 211 phase) and 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.

On the other hand, Jin et al. Heat-melted a YBa ₂ Cu ₃ O _7-y crystal to a partially molten state in which a Y ₂ BaCuO ₅ phase and a Y-Ba-Cu-O-based liquid phase coexisted. Later, a method of solidifying under a slight temperature gradient was reported. (Physical Review B, Vol.37,7
850 (1988)) However, as shown in FIG.
Besides YBa ₂ Cu ₃ O _7-y phase, which contains a Y ₂ BaCuO ₅ phase and other grain boundary phase (CuO, BaCuO _2, the amorphous phase), YBa ₂ Cu ₃
Although the O _7-y phase was also oriented to some extent, the crystal grain phase was still in contact at an angle.

[Means for Solving the Problems] As a result of various investigations, the present inventor has conducted various studies for the purpose of obtaining an yttrium-based superconductor having a high critical current density and having a small decrease in the critical current density even when a magnetic field is applied. ,
It has been found that the above object can be achieved by making the superconductor a new structure. Thus, the present invention provides LnBa ₂ Cu ₃ O.
_A method for producing an oxide superconductor having a structure in which plate-like crystals represented by the composition formula of _7-y are layered and the crystals represented by the composition formula of Ln ₂ BaCuO ₅ are dispersed in islands therein. That is, from a partially molten state in which a solid phase of Ln ₂ BaCuO ₅ and a liquid phase of the Ln-Ba-Cu-O system coexist, a temperature gradient of 100 ° C / cm or more and a crystal growth of 2 mm / h or less. The present invention provides a method for producing an oxide superconductor that is cooled and crystallized by a unidirectional solidification method at a high speed.

The structure of the superconductor obtained by the manufacturing method of the present invention is 12
It uses three phases as a matrix. This 123 phase is c
It is a crystal that grows in a plate shape in a direction perpendicular to the axis and is a polycrystal as a whole, but the plate crystals are layered and aligned in the respective c-axis directions. Although the crystal is not completely continuous as a single crystal even in the direction perpendicular to the c-axis, even at the grain boundaries here, the planes of the c-axis are aligned, so that this grain boundary part is not affected by the application of a magnetic field. Superelectric state is hard to break.

In this superconductor, granular 211 phases are dispersed like islands between the above 123 phases. The 211 phase is not oriented and the crystals are not continuous. So this 211 phase is
It does not interfere with the superconducting path.

This superconductor is substantially free of crystalline or amorphous phases other than 123 phase and 211 phase. As long as it does not deteriorate the superconducting property, it may be possible to contain such a phase only partially.

In the present invention, in the above general formula, Ln is Y, La, Nd,
1 selected from the group consisting of Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu
More than a seed.

The production method of the present invention will be specifically described below. As described above, the yttrium-based superconductor decomposes and melts at a temperature of about 1000 ° C. or higher, and becomes a partially melted state in which the 211-phase solid phase and the Ln-Ba-Cu-O-based liquid phase coexist. It is necessary to make a temperature gradient from this state and solidify and crystallize in one direction under the following conditions. That is, a temperature gradient of 100 ° C / cm
As described above, it is necessary that the crystallization rate is 2 mm / h or less.
It is more preferable that the crystallization rate is 1 mm / h.

At this time, crystallization proceeds as follows. In the partially melted state, the unoriented 211 phase and the Ln-Ba-Cu-O-based melt coexist. From here, if cooling is carried out under a temperature gradient, 123 phases will precipitate from the 211 phase and the melt at the decomposition and melting temperature. Under the above conditions, the 123 phase has a structure in which the c-axes of the crystals are perpendicular to the temperature gradient and the plate-like crystals that are polycrystals but have the c-axes parallel to each other are layered. At this time, the 211 phase does not completely change to the 123 phase, and some of the 211 phase remains unreacted in the 123 phase lamellar structure, resulting in a structure in which granular crystals are dispersed in an island shape.

The 211 phase does not exhibit superconductivity, but each particle is independent, does not interfere with the current path, and does not adversely affect the superconducting properties. All of the melt is consumed to form the 123 phase, and the solid phase does not show any crystalline or amorphous phase other than the 123 and 211 phases.

In order to create the above-described partially molten state, it is preferable to heat the 123-phase sintered body to a temperature not lower than the decomposition melting temperature but not higher than the liquidus temperature. From this state, solidify in one direction to 123
A structure in which the phase and the 211 phase coexist is formed. In this case, it means that Ba and Cu are reduced from the original composition. This is mainly due to Ba and
This is probably because Cu is being emitted. In addition, it is considered that a part of it is volatilized. It is considered that this is also related to the fact that the crystal structure of this solidified material largely depends on the crystal growth rate. That is, the crystal growth rate is 2 mm /
If it is larger than h, diffusion of Cu and Ba components will not be in time, and as shown in FIG. A superconductor having such a structure cannot be obtained. If the crystal growth rate is 1 mm / h or less, it is more preferable in this sense as well.

[Example] Example 1 A calcined powder of an oxide having an atomic ratio of Y: Ba: Cu of 1: 2: 3 was prepared, and the powder was pressed by a die press to 70 mm x 40 mm x 2 mm.
And then baked in an oxygen stream at 930 ° C for 10 hours.
A sintered body of Ba ₂ Cu ₃ O _7-y was obtained. This sintered body was cut out to a width of 2 mm using a diamond cutter to obtain a square columnar sintered body having a square bottom surface of about 2 mm on each side and a height of a little less than 70 mm.

Next, the upper portion of this square-cast sintered body was gripped and hung in a vertical tubular resistance heating furnace having a temperature distribution as shown in Fig. 4, and the maximum temperature inside the furnace was maintained at 1090 ° C. While keeping it flowing, oxygen gas was made to flow from the bottom, and it was moved from the bottom to the top at a speed of 0.7 mm / h. At this time, even in the partially melted state, this sample did not lose its overall shape and no special support was required.

The resulting solidified product is further heated at 900 ° C in an oxygen atmosphere.
The mixture was heated up to 30 ° C./h and slowly cooled to absorb oxygen sufficiently.

This coagulated product was analyzed with an optical microscope, a scanning electron microscope, and an X-ray.
Observation with a line element analyzer revealed that plate-shaped YBa ₂ Cu ₃ O _7-y crystal grains as shown in Fig. 1 were layered, and granular Y ₂ BaCuO ₅ crystals were island-shaped. It was confirmed that the organization had an independently analyzed organization. Further, this solidified product was cut into a length of 0.90 mm × 0.15 mm × 10 mm, and the superconducting property was measured by the DC four-terminal method. The critical temperature showing zero resistance is 84K, and the critical current density in the magnetic field of 77K, 1T is 40K.
It was 00A / cm ² .

[Effects of the Invention] The superconductor of the present invention has a high orientation of the 123 phase and contains only the granular 211 phase dispersed in an island shape as the other phase, so that the critical current density is high and a magnetic field is applied. When
Decrease in critical current density is small.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing the structure of a superconductor according to an embodiment of the present invention. FIG. 2 is a schematic diagram showing the structure of the yttrium-based superconductor obtained by the conventional unidirectional solidification method.
FIG. 3 is a schematic diagram showing the structure of the yttrium-based superconductor of the sintered body. FIG. 4 is a diagram showing the temperature distribution of the electric furnace used in the examples of the present invention. In FIG. 4, the vertical axis represents the temperature, and the horizontal axis represents the position in the furnace by the distance from the portion showing the maximum temperature. + Is above the maximum temperature part, and-is below.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location H01L 39/12 ZAA C

Claims (1)

[Claims] 1. LnBa ₂ Cu ₃ O _7-y (Ln is Y, La, Nd, Sm, Eu, Gd, Dy,
One or more 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 the composition of Ln ₂ BaCuO ₅ A method for producing an oxide superconductor having a structure in which granular crystals represented by the formula are dispersed in an island shape, wherein a solid phase of Ln ₂ BaCuO ₅ and Ln-B
From the partially molten state in which an a-Cu-O liquid phase coexists,
A method for producing an oxide superconductor, which is cooled and crystallized by a unidirectional solidification method with a temperature gradient of 00 ° C / cm or more and a crystal growth rate of 2 mm / h or less.