JPH02204322A - Oxide superconductor having novel structure - Google Patents

Abstract

PURPOSE:To obtain the superconductor having a high critical current density which is not significantly decreased even when impressed with a magnetic field by forming the structure in which specified plate crystals are laminated and specified granular crystals are dispersed therein in the form of islands. CONSTITUTION:The oxide superconductor has a structure in which the plate crystals shown by the composition formula are laminated and the granular crystals shown by Ln2BaCuO5 are dispersed therein in the form of islands. In the formula, Ln is >=1 kind among Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu, and (y) is the oxygen deficiency amt. To produce the oxide superconductor, the material is decomposed and melted at >= about 1000 deg.C to form a partially molten state in which a 211 solid phase and an Ln-Ba-Cu-O-based liq. phase are coexistent. The product is unidirectionally solidified and crystallized at >=100 deg.C temp. gradient and <=2mm/hr crystallization rate, and the desired superconductor is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a yttrium-based oxide superconductor having a novel structure and a method for producing the same.

[Prior art] Conventionally, LnBagCusOy-y (Ln is Y, La
, Nd, Sm, Eu.

G d + D y , H r E r + Tta
A superconductor (hereinafter also referred to as a yttrium-based superconductor) represented by a composition formula of one or more selected from the group consisting of + Y b , Lu, and y is the amount of oxygen vacancies (hereinafter referred to as 123 phase) is known. As a method for producing an yttrium-based superconductor, there is a method in which a crystal powder having the above composition is synthesized, and then this is formed and sintered. In addition, it is known to manufacture by a sol-gel method or a melt solidification method.

[Problems to be Solved by the Invention] The superconductors produced by these methods are all polycrystalline, as shown in Figure 3, and each crystal grain is arranged in a disordered direction, and there are no grain boundaries. , the structure includes grain boundary phases other than the 123 phase. FIG. 3 shows a case where Ln is yttrium, and the grain boundary phase includes a crystalline phase other than the 123 phase, an amorphous phase, and pores in many cases. However,
Yttrium-based superconductors have a drawback in that, because the direction in which current flows easily within the crystal grains is determined, it is difficult for current to flow at grain boundaries between crystal grains that are oriented in different directions.

Furthermore, since the grain boundary phase is not a superconductor, it acts as an insulating layer. Therefore, conventional polycrystalline yttrium-based superconductors have not been able to exhibit high critical current density.

The decrease in critical current density caused by such grain boundaries is
It is known that this phenomenon is more noticeable in a magnetic field. The main field of application for superconductors is considered as electromagnets that create strong magnetic fields by processing wire or tape materials into coils. Therefore, in order to put yttrium-based superconductors into practical use, it is considered necessary to create a material that has a high critical current density in a strong magnetic field by creating a structure that suppresses one grain boundary.

The yttrium-based superconductor decomposes and melts at a temperature of about 1000° C. or higher and separates into Ln2BaCuO5 crystals (hereinafter referred to as 211 phase) and a liquid phase. Therefore, when a melt with the same composition as the 123 phase is cooled, the 211 phase will precipitate first, so
A 123-phase single crystal or an oriented polycrystalline body cannot be obtained using normal methods.

On the other hand, Jin et al. heated and melted YBalCuJt-y crystals to form Y2BaCuO5 phase and Y-Ba-Cu-
They report a method in which the material is made into a partially molten state in which a zero-based liquid phase coexists, and then solidified under a slight temperature gradient. (Ph
physical Review B, Vol, 37.78
50 (19 g + 1)) However, as shown in Figure 2, the obtained solidified product contains, in addition to the YBazCusOy-y phase, Y, BaCu0s phases, and other grain boundary phases (Cu
b, BaCu0a amorphous phase), and YB
Although the azCusOy-y phase is also oriented to some extent,
The crystal grain phases were still in contact at a certain angle.

[Means for Solving the Problem] The present inventor has conducted various studies with the aim of obtaining a yttrium-based superconductor that has a high critical current density and whose critical current density does not decrease even when a magnetic field is applied. ,
It has been found that the above object can be achieved by forming a superconductor into a new structure. Thus, the present invention provides LnBas
The present invention provides an oxide superconductor having a structure in which plate-like crystals represented by the compositional formula of CuzOt-y overlap in a layered manner, and crystals represented by the compositional formula of Ln2BaCuO5 are dispersed therein in a layered manner.

The structure of the superconductor of the present invention has 123 phases as a matrix. This 123 phase is a crystal that has grown in a plate shape in a direction perpendicular to the C axis, and although it is polycrystalline as a whole, these plate crystals are layered with their respective C axes aligned in the same direction. Even in the direction perpendicular to the C axis, the crystal is not completely continuous like a single crystal, but even at the grain boundaries here, C
Since the planes of the axes are aligned, the superconducting state of this grain boundary region is unlikely to be destroyed even when a magnetic field is applied.

In the superconductor of the present invention, granular 211 phases are dispersed in the form of islands between the 123 phases.

The 211 phase is not oriented, and each crystal is not continuous. Therefore, this 211 phase does not interfere with the superconducting path.

The superconductor of the present invention is substantially free of crystalline or amorphous phases other than the 123 phase and 211 phase, and only partially contains such phases as long as they do not deteriorate the superconducting properties. It's okay to be there.

In the present invention, in the above general formula, Ln is Y,
La, Nd, Sm, Eu, Gd, 'Dy, I
(One or more types selected from the group consisting of o, Er, Tea, Yb, and Lu.

For producing the yttrium-based superconductor of the present invention, for example, the following method can be adopted.

As mentioned above, yttrium-based superconductors have a temperature of about 1000°C.
It decomposes and melts at a temperature above, forming the 211 phase and Ln-Ba.
-Cu-0 system liquid phase coexists in a partially melted state. From this state, a superconductor having the structure of the present invention can be obtained by solidifying and crystallizing in one direction under the following conditions with a temperature gradient. That is, temperature gradient position 0
Conditions are required such that the crystallization rate is 0° C./c+a or higher and the crystallization rate is 2 m/h or lower, more preferably l ram/h or lower.

At this time, crystallization proceeds as follows. 211 phase that is not oriented in a partially melted state, and Ln-Ba-C
A u-0 type melt coexists.

From here, when cooling is performed under a temperature gradient, phase 123 is precipitated from phase 211 and the melt at the decomposition melting temperature. Under the above conditions, the 123 phase has a structure in which the C-axis of the crystal is perpendicular to the temperature gradient and, although it is a polycrystal, plate-shaped crystals whose C-axes are parallel to each other are stacked in layers. At this time, the 211 phase does not completely change to the 123 phase, and a portion remains unreacted in the layered structure of the 123 phase, resulting in a structure in which granular crystals are dispersed in the form of islands.

Although the 211 phase does not exhibit superconductivity, each particle is independent and does not interfere with the current path and does not have much of a negative effect on superconducting properties. All of the melt is consumed in the formation of the 123 phase.

No crystalline phase or amorphous phase other than the 123 phase and 211 phase appears in the solidified product.

In order to create the above-mentioned partially molten state, it is preferable to heat the 123-phase sintered body to a temperature above the decomposition melting temperature and below the liquidus temperature.If it is solidified in one direction from this state, 1
A structure in which 23 phase and 211 phase coexist is formed. In this case, Ba and Cu are reduced from the original composition. This is considered to be because Ba and Cu are mainly discharged toward the front in the crystal growth direction through the melt part.

It is also believed that some of it has evaporated. this is,
This seems to be related to the fact that the crystal structure of this solidified material largely depends on the crystal growth rate. That is, when the crystal growth rate is higher than 2 mm/h, Cu
-The diffusion of Ba growth cannot be completed in time, and as shown in Fig. 2, the CuO or BaCuO phase inevitably precipitates at the grain boundaries of the 123 phase, resulting in crystals that cannot be obtained in the superconductor having the structure of the present invention. If the growth rate is 1 mm/h,
In this sense, it is even more preferable.

[Example] Example 1 A calcined powder of an oxide with an atomic ratio of Y: Ba: Cu of 1:2:3 was prepared, and the powder was molded into a size of 70 mm x 40 wm x 2 mm using a mold press.
YBaiC was baked for 10 hours in an oxygen stream at 0°C.
A sintered body of usOt-y was obtained. This sintered body was cut into a width of 2a+m using a diamond cutter to obtain a prismatic sintered body with a square bottom surface of about 2 mm on a side and a height of just under 70 mm.

Next, the upper part of this colored sintered body was grasped and suspended in a vertical tubular resistance heating furnace having a temperature distribution as shown in Fig. 4, and the maximum temperature in the furnace was maintained at 1090°C. It was moved from bottom to top at a speed of 0.7 wa+/h while oxygen gas was flowing from below. At this time, even in a partially melted state, this sample did not lose its overall shape, and no special support was required.

The resulting solidified product was further heated for 900 min in an oxygen atmosphere.
The mixture was heated to 0.degree. C. and slowly cooled at 30.degree. C./h to sufficiently absorb oxygen.

This coagulated material was examined using an optical microscope, a scanning electron microscope, and an X-ray microscope.
When observed using a line elemental analyzer, it was found that the plate-shaped YBazCusOy-y crystal grains overlapped in a layered manner as shown in Figure 1, and the granular Y2BaCuO5 crystals were analyzed independently in the form of islands within the layered structure. It was confirmed that it has.

In addition, this solidified material was 0.90ma+X O, 15mmX
It was cut into a length of 10a+a+, and its superconducting properties were measured using the DC four-probe method. The critical temperature showing zero resistance is 84K, and the critical current density in a magnetic field of 77K is 400K.
It was 0A/cm'.

[Effect of the invention 1] The superconductor of the present invention has a high degree of orientation of the 123 phase, and contains only the granular 211 phase dispersed in island shapes as other phases, so it has a high critical current density and is easy to apply a magnetic field to. Even when this happens, there is little decrease in critical current density.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing the structure of a superconductor according to an example of the present invention. FIG. 2 is a schematic diagram showing the structure of a yttrium-based superconductor obtained by a conventional unidirectional solidification method. FIG. 3 is a schematic diagram showing the structure of a sintered yttrium-based superconductor. FIG. 4 is a diagram showing the temperature distribution of the electric furnace used in the embodiment of the present invention. In Figure 4. The vertical axis represents the temperature, and the horizontal axis represents the position within the furnace as a distance from the highest temperature section. + indicates above the highest temperature section, - indicates below. Figure 'L'

Claims (3)

[Claims] (1) LnBa_2Cu_3O_7_-_y (Ln is Y
, La, Nd, Sm, Eu, Gd, Dy, Ho, Er,
One or more selected from the group consisting of Tm, Yb, Lu, y
An oxide superconductor having a structure in which plate-like crystals represented by the composition formula Ln_2BaCuO_5 are layered and granular crystals represented by the composition formula Ln_2BaCuO_5 are dispersed in islands. (2) Phases of Ln_2BaCuO_5 and Ln-Ba-
2. The method for producing an oxide superconductor according to claim 1, wherein the oxide superconductor is crystallized by cooling from a partially molten state in which a Cu--O liquid phase coexists. (3) Cooling crystallization has a temperature gradient of 100°C/cm or more,
The manufacturing method according to claim 2, which is a unidirectional solidification method with a crystal growth rate of 2 mm/h or less.