JP3159764B2 - Manufacturing method of rare earth superconductor - 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 manufacturing a rare earth oxide superconductor, and more particularly, to REBa ₂ Cu ₃ O _y (RE is Y, G
represents d, Ho, Er or Yb. The present invention relates to a method for obtaining an oxide superconductor by melting in the presence of a seed crystal.

[0002]

2. Description of the Related Art Oxide superconductors have been actively studied for practical use because of their high critical temperature. As the oxide superconductor, for example, JP-A-63-291814
In Japanese Patent Application Laid-Open Publication No. H11-163, a superconducting material composition containing rare earth elements of Y and Ce is proposed. When these oxide superconductors are obtained as a bulk material, a sintering method has conventionally been generally used. The oxide superconductor produced by the sintering method usually has a fine structure having a small particle size and a large number of grain boundaries inside. That is, in the oxide superconducting bulk body manufactured by the sintering method, individual superconducting particles are connected by a weak bond, and the critical current density of the oxide superconducting bulk body is
(Jc) is governed by this weak coupling, so a high Jc
Was not obtained.

On the other hand, it is known that a single-crystal superconductor does not have the above-described problem of grain boundaries and exhibits a high Jc even in a high magnetic field. Therefore, in order to obtain a bulk oxide superconducting body having a high Jc, attempts are being made to approximate a superconductor having a fine structure obtained by a sintering method to a single crystal structure.
Further, it has been proposed to introduce a so-called pinning center for dispersing fine-structure particles of a non-superconducting phase in the superconducting phase and fixing the penetrated magnetic flux lines.
tured growth method) has been proposed. The MTG method is generally used for oxide superconductors.
By slowly cooling the three phases (YBa ₂ Cu ₃ O _y , where Y is a rare earth element containing Y) from the decomposition melting temperature, a peritectic reaction occurs between the 211 phase (Y ₂ BaCuO ₅ ) and the liquid phase, and the crystal is formed. At the same time as growing, the peritectic reaction is incomplete so that a partially unreacted 211 phase exists inside the grown crystal. In the oxide superconductor obtained by this method, 211 phases remaining in the 123 phases act as pinning centers, and therefore exhibit high Jc even in a magnetic field.

However, the oxide superconductor obtained by the melting method has disadvantages such as a large grain size of the 211 phase, a nonuniform distribution thereof, and the presence of cracks along the crystal growth direction. As a method for improving the disadvantages of the MTG method, Japanese Patent Application Laid-Open No. 2-153803 discloses a QMG method (Qu
ench and Melt Growth method) has been proposed. This QM
The oxide superconductor obtained by the G method has crystal grown 1
It is disclosed that since the 211 phases existing inside the 23 phases are fine and homogeneously dispersed with a size of 20 μm or less, they exhibit an extremely strong pinning effect and exhibit excellent Jc in a high magnetic field. In addition, MPMG method (Melt Powder and Melt Growth
Law) has been proposed. The MPMG method is a method of improving the formability by pulverizing the melt-quenched solidified body in the QMG method, and it is proposed that the same effect can be obtained with a high Jc characteristic.

[0005] As for the oxide superconducting bulk material obtained by the above-mentioned proposed QMG method and MPMG method, those having a small size in the order of millimeters exhibit excellent superconducting characteristics and have an extremely high Jc, but have an extremely high Jc. When the size becomes large, it is not possible to obtain a material exhibiting the expected superconducting characteristics even if it is manufactured in the same manner. It is believed that this is due to the existence of grain boundaries and cracks generated at the grain boundaries. Therefore, as a method of bringing the YBaCuO superconducting compact obtained by the QMG method to a partial melting temperature as a method of bringing it closer to a single crystal, SmBa ₂ Cu ₃ O _x
A method of growing a single crystal on the surface of a molded article by recrystallizing as a seed crystal has been proposed (Japanese Journal of Applied Physics (JA)
PANESE JOURNAL OF APPLIED PHYSICS), Volume 30, N
o. 7A, L1157 to L1159 (July 1991)
reference).

[0006]

However, the oxide superconductor obtained by the above-mentioned QMG method or MPMG method exhibits a stronger pinning effect than the oxide superconductor obtained by the melting method. Shows better Jc. In addition, in the case where a rapid solidification step is performed to finely disperse the 211 (RE ₂ BaCuO ₅ ) phase, a platinum crucible is essential for the melt-quick solidification step. However, when a platinum crucible is used in the melting step, a reaction between platinum and the rare-earth-based oxide superconductor may occur, and the superconducting characteristics may become non-uniform. There are also problems with methods and means for rapid quenching. Furthermore, the oxide superconducting compact obtained by the above-mentioned QMG method is subjected to a reheating treatment, and the SmB
The method of recrystallizing a ₂ Cu ₃ O _x as a seed crystal is described in Q
While having the above-mentioned problems of the MG method as it is, SmBa ₂ Cu ₃ O _x used for the seed crystal is decomposed at 1060 ° C., and therefore, depending on the RE component, it can be used only during the heat treatment process. It is complicated.

As described above, even in various proposed methods for obtaining an oxide superconductor having a uniform high Jc, the oxide superconductor has a large and uniform superconducting characteristic of 10 millimeters or more. It is still difficult to easily obtain a bulk oxide superconductor exhibiting high Jc. Therefore, industrially, there is a demand for a method for producing a large oxide superconductor having a superconducting property equivalent to a millimeter order and exhibiting a high Jc in a high magnetic field by a simple operation. The present inventors have conducted intensive studies on a method for easily manufacturing an oxide superconductor having excellent superconducting properties as a large bulk body. The present inventors have found a method for obtaining an oxide superconductor exhibiting excellent Jc in a high magnetic field by exhibiting an extremely strong pinning effect as in the case of the oxide superconductor described above, and completed the present invention.

[0008]

According to the present invention, RE-B
a-Cu-O-based oxide superconductor (RE is Y, Gd, H
represents o, Er or Yb. ) Is formed using a raw material powder containing RE, Ba, and Cu components, and SrTiO ₃ , MgO, LaAlO ₃ , La as seed crystals.
A decomposition melting step in which any one of GaO ₃ , NdGaO ₃ and PrGaO ₃ single crystals is arranged in contact with each other, and the formed body having the seed crystal arrangement is heated at a temperature not lower than the decomposition melting temperature of the oxide superconductor; A method for producing a rare earth-based oxide superconductor characterized by performing a treatment in a step and a heat treatment step is provided.

[0009]

According to the present invention, as described above, Y, Gd, Ho, Er
And a specific seed crystal is placed on a compact obtained from a raw material powder adjusted to constitute a REBa ₂ Cu ₃ O _y oxide superconductor containing at least one RE element component selected from Yb and Yb. Other than the above, REBa ₂ Cu ₃ O _y is melted, gradually cooled, and heat-treated in substantially the same manner as the conventional melting method.
An oxide superconductor can be obtained. Also obtained RE
Ba ₂ Cu ₃ O _y oxide superconductor fine RE ₂ Bac
contains uO ₅ phase, has few grain boundaries, and has RE
It has crystal orientation on the ab plane of the Ba ₂ Cu ₃ O _y -based oxide crystal structure, has excellent superconductivity, and exhibits high Jc.

In the present invention, in particular, by incorporating Ce component in the raw material powder, it is possible to RE ₂ BaCuO ₅ phase generated in REBa ₂ Cu ₃ in O _y particles are uniform and fine structure, By arranging a specific seed crystal in the decomposition melting, slow cooling, and heat treatment processes, REBa ₂
The grain structure of Cu ₃ O _y has few crystal grain boundaries, and the crystal can be grown with its surface oriented in the ab plane of the REBa ₂ Cu ₃ O _y crystal structure. In the conventional QMG method, when the raw material powder does not pass through a melting and quenching step, a RE ₂ BaCuO ₅ phase is generated at a decomposition melting temperature, and further, due to its aggregation and coarsening, a superconductor having a high Jc can be obtained. In contrast to the above, the RE-Ba-Cu-O-based oxide superconductor of the present invention has a low crystal grain boundary even in a bulk body, and has a high Jc Can be obtained.

Hereinafter, the present invention will be described in more detail. RE-Ba-Cu-O-based oxide superconductor of the present invention,
RE has a multilayer perovskite structure containing at least one rare earth element selected from Y, Gd, Ho, Er, and Yb, and includes, for example, REBa ₂ Cu ₃ O such as YBa ₂ Cu ₃ O _{7.
y It is an} oxide superconductor. The rare earth element component represented by RE in the present invention is Y, Gd, Ho, Er or Y
b, and among the rare earth elements, Ce, Nd, Eu, Sm,
Sc, Tb, Dy, Tm, La, Lu and Pr are excluded because the effects of the present invention cannot be obtained.

The oxide superconductor of the present invention is formed into a predetermined shape using the following raw material powder,
_{(2) In} a decomposition melting step of heating at a temperature equal to or higher than the decomposition melting temperature of the Cu _{3 Oy} oxide superconductor, a slow cooling step, and a heat treatment step in an oxygen atmosphere, the seed crystal is obtained by placing the seed crystal in contact with the compact. Can be. In the present invention, the decomposition melting heat treatment other than disposing the seed crystal, the slow cooling, and the heat treatment under an oxygen atmosphere are the same as the known melting method, and are extremely simple operation steps. Thus, a RE-Ba-Cu-O-based superconductor exhibiting a desired high Jc can be obtained.

The seed crystal of the present invention is made of SrTiO ₃ , Mg
O, LaAlO ₃ , LaGaO ₃ , NdGaO ₃ and P
Any of single crystals of rGaO ₃ (hereinafter, simply referred to as single crystals of SrTiO ₃ or the like) can be used. These oxide single crystals are used as a crystal substrate in a study for forming a thin film of an oxide superconducting element having a high critical temperature (Journal of the Japan Society for Crystal Growth, Vol. 17, No. 2, No. 86).
Pp. 94 (1990)). The present invention provides a single crystal used for forming an oxide superconductor of these thin films by RE-
This was the first application to the production of a Ba-Cu-O-based superconducting bulk body, and the effects obtained by its application were not predictable. SrTi used as the above seed crystal
Single crystals such as O ₃ have a melting point of at least 1500 ° C. and are at or above the decomposition melting temperature of the rare-earth oxide constituting the RE-Ba—Cu—O-based superconductor of the present invention, and are preferably used. Can be. Further, a single crystal such as SrTiO ₃ which is commercially available for forming a thin film of an oxide superconducting element can be appropriately selected and used.

[0014] The REBa-Cu-O system RE for constituting the oxide superconductor, a raw material powder containing Ba and Cu components, REBa ₂ Cu ₃ O _y and RE ₂ BaCuO ₅ after firing
Is not particularly limited as long as it is blended so as to constitute. The raw material of each component is RE, which is a rare earth element, that is, an oxide of Y, Gd, Ho, Er, or Yb, a carbonate, oxide or peroxide of Ba, and C
An oxide mixed powder obtained by mixing and mixing the oxides of u as described above, a calcined powder of the oxide mixed powder, a frit powder of the oxide mixed powder, and the like can be used. Also, the particle size of the raw material powder is not particularly limited, but is generally 20 μm or less, particularly preferably 1 to 5 μm. Raw material powders having a diameter of more than 20 μm are not preferred because the composition becomes non-uniform at the time of decomposition and melting. 1 μm
As the following raw material powder, for example, it is preferable to use a powder or the like generated by a coprecipitation method, but as long as it is a fine powder of 1 μm or less, a powder obtained by another method can be used.

Further, it is preferable to add a Ce component to the raw material powder of the present invention. The Ce component is contained in the oxide superconductor that is the fired body to be obtained in an amount of 0.01 on an element basis.
It is preferable to add and convert the amount to be contained so as to contain ５5% by weight. If the added amount is out of the above range, RE ₂ BaCu
The effect of suppressing the grain growth of the O ₅ phase is poor. In particular, if it exceeds 5% by weight, the precipitation of foreign phases increases, which is not preferable. The Ce component is
It can be added to the raw material powder as a simple metal or a compound such as an oxide. Usually, a single powder is used, and it is preferable to add it in powder form according to the raw material powder, and more preferably, to add it in fine powder form having a particle size of about 20 μm or less. C
If the component e is a fine powder having a particle size of about 20 μm or less, the resulting oxide superconductor will have improved homogeneity without remaining as agglomerates, and excellent superconductivity with good uniformity and no variation will be obtained. be able to.

The molding method in the present invention is not particularly limited. For example, using a known molding method such as a doctor blade method, a press molding method, a slurry casting molding method,
A desired oxide superconductor can be obtained as a bulk molded body of the order of 10 mm or more. Alternatively, a molded article can be obtained by forming a molded article layer on a substrate of a metal, ceramics, or the like by spray coating, powder coating, or the like using the mixed powder.

In the present invention, the formed bulk body obtained by the above-mentioned forming method is then treated in a step after decomposition and melting by disposing the above-mentioned single crystal such as SrTiO ₃ as a seed crystal on the surface of the formed body. I do. The seed crystal is arranged in such a manner that the surface of the shaped bulk body on which the seed crystal is arranged is REBa ₂ Cu ₃ O
The crystal is arranged so as to grow in accordance with the ab plane of the crystal.
For example, in a firing furnace for performing a decomposition melting step, a slow cooling step, and a heat treatment step, when a molded bulk body is placed on a support base on which a body to be fired is placed, that is, on a setter, as shown in FIG. Near the center of the upper body surface, SrT
A single crystal such as iO ₃ is placed as a seed crystal, or
As shown in FIG. 2, the formed bulk body may be placed on a single crystal plate of SrTiO ₃ or the like.

The temperature above the decomposition melting temperature of the RE-Ba-Cu-O-based superconductor in the present invention varies depending on the type of the RE element component contained, but is generally from 1050 to 1200.
° C and may be appropriately selected depending on the heating conditions, the size of the molded body, and the like. The rate of temperature rise to a temperature equal to or higher than the decomposition melting temperature is not particularly limited. Especially large RE-Ba-
When obtaining a fired product of a Cu-O-based superconductor, the temperature may be raised usually at a rate of about 10 to 300 ° C / hour using a large electric furnace or the like. Furthermore, the decomposition melting heat treatment can be performed by holding at a temperature corresponding to the RE element component in the above temperature range for a predetermined time. In this case, the holding time is not particularly limited, and can be appropriately selected according to the heating conditions and the like in the same manner as in the above temperature range. Usually, the treatment can be carried out for 10 minutes to 3 hours.

[0019] After the heat treatment step of the incongruent melting is passed through the conventional melt method as well as annealing step, under oxygen atmosphere, through a heat treatment step of heat-treating and held at a predetermined temperature REBa ₂ C
A u ₃ O _y oxide superconductor can be obtained. In this case, the slow cooling step is a step of growing REBa ₂ Cu ₃ O _y crystals, and it is usually sufficient to lower the temperature to the temperature of the next heat treatment step at a predetermined cooling rate or less. Preferably, the cooling can be performed at a cooling rate of 5 ° C./hour or less and 0.5 ° C./hour or more. In addition, the heat treatment step after the slow cooling step is a step for sufficiently introducing a predetermined amount of oxygen into the oxide superconducting crystal phase, and is usually performed under an oxygen atmosphere at 500 ° C. or lower and for as long as possible. . Generally, at 300-500 ° C,
It is preferable to hold for about 10 to 100 hours.

According to the present invention, as described above, a seed crystal is brought into contact with a molded bulk body molded from a predetermined raw material powder in a decomposition melting heat treatment step, a slow cooling step, and a heat treatment step substantially similar to the conventional melting method. By doing so, the RE-Ba-Cu-O-based superconducting crystal phase on the surface of the bulk body can be crystal-grown in accordance with the ab plane. Therefore, the surface of the RE-Ba-Cu-O-based superconducting bulk material obtained in the present invention is
Since the ab plane of the Ba ₂ Cu ₃ O crystal is oriented in a layered manner and has excellent superconducting properties and high Jc, for example, when used as a magnetic shield, it can exhibit high magnetic shielding ability.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to embodiments. However, the present invention is not limited by the following examples. Example 1 and Comparative Example 1 Y ₂ O ₃ , BaCO ₃ , and CuO powder were mixed with Y, Ba, and Cu.
Was adjusted to be 1.50: 2.25: 3.25, and mixed with a dry mixer for 1 hour. The obtained raw material powder was formed into a pellet having a thickness of 15 mm and a diameter of 20 mmφ under a pressure of 1 ton / cm ² by press molding. A 0.5 mm-thick, 3 mm square MgO single crystal (100) plate was placed on one surface of the two pellet compacts obtained as described above as shown in FIG. (Example 1), but the MgO single crystal plate was not placed on another pellet compact (Comparative Example 1). The pellet compacts, both pellets on which the MgO single crystal plate was placed and pellets without, were heated on a dense alumina setter in an electric furnace to 1100 ° C. in a nitrogen atmosphere for 10 hours. Dry air is introduced into the furnace and the atmosphere is 1100 ° C.
For 20 minutes. Next, it was gradually cooled from 1000 ° C. to 920 ° C. in 80 hours. Thereafter, oxygen was introduced into the furnace, and the atmosphere was changed to a 100% oxygen atmosphere at 600 ° C.
After cooling at 20 ° C / hour to
Heat treatment was carried out over a period of time, and then the mixture was left in a furnace to 100 ° C. or lower to obtain a sintered body.

The upper surfaces of the obtained two sintered pellets were visually observed. In the sintered compact on which the MgO single crystal plate was mounted, large single grains without grain boundaries presumed to have grown from the MgO single crystal mounting portion were observed, and the surface was YBa ₂ C.
It was consistent with the ab plane of the u ₃ O crystal. On the other hand, in the sintered pellets on which the MgO single crystal plate was not placed, crystals that randomly grew in 5 to 6 directions were observed. In addition, the magnetic levitation force was measured as superconductivity for each of the obtained sintered compact pellets. The magnetic levitation force was measured as magnetic repulsion (g / cm ² ) using the apparatus shown in FIG. In FIG. 3, after sintering the sintered pellet 3 in liquid nitrogen 5 in a container and attaching the rare earth magnet 4 (surface magnetic field 4000 Gauss) to the rod 6, the crosshead 7 is lowered at a speed of 10 mm / min. The magnetic repulsion when the distance between the sintered pellet 3 and the rare earth magnet 4 was 0.1 mm was measured. As a result, the sintered compact pellet of Example 1 was 370 g / cm.
² , the sintered compact pellet of Comparative Example 1 was 120 g / cm ²
Met. The levitation force of the YBa ₂ Cu ₃ O sintered body obtained by mounting the MgO single crystal plate by the method of the present invention is about three times that in the case where the MgO single crystal is not mounted, and has excellent superconducting properties. You can see that.

Example 2 and Comparative Example 2 Y ₂ O ₃ , BaCO ₃ , and CuO powder were mixed with Y, Ba and Cu.
Was 15 mm in thickness and 2 mm in diameter in the same manner as in Example 1 except that the atomic ratio was 1.8: 2.4: 3.4.
A 0 mmφ pellet was obtained. One of the two obtained pellet compacts is made of Mg having a thickness of 0.5 mm and a square of 20 mm square.
As shown in FIG. 2 (Example 2), it was placed on an O single crystal (100) plate (Comparative Example 1) without using another MgO single crystal plate for the other pellet molded body. The pellets placed on the MgO single crystal plate and the pellets not loaded were prepared in Example 1.
Each sintered body was obtained in the same manner as described above.

In the same manner as in Example 1, the upper surfaces of the two sintered pellets obtained were visually observed. In the sintered compact pellets mounted on the MgO single crystal plate, large single grains without grain boundaries presumed to have grown from the MgO single crystal mounting portion were observed, and the surface was ab of YBa ₂ Cu ₃ O crystal. Face was consistent. On the other hand, in the sintered body pellets not placed on the MgO single crystal plate, crystals that grew randomly in 5 to 6 directions were observed. The magnetic levitation force of each of the obtained sintered compact pellets was measured in the same manner as in Example 1. as a result,
The sintered body pellet of Example 2 was 400 g / cm ² , and the sintered body pellet of Comparative Example 2 was 150 g / cm ² . The superconducting property of the YBa ₂ Cu ₃ O sintered body obtained by mounting on the MgO single crystal plate by the method of the present invention is 2.7 times that of the case where the YBa ₂ Cu ₃ O sintered body is not mounted on the MgO single crystal. It can be seen that

Example 3 and Comparative Example 3 Y ₂ O ₃ , BaCO ₃ , and CuO powder were mixed with Y, Ba, and Cu.
Was adjusted to be 1.5: 2.25: 3.25, and 0.5% by weight of CeO ₂ powder was further added and mixed with the mixture. Mixing was performed using a ball mill for 10 hours using ethanol as a solvent. The obtained mixed powder was calcined at 950 ° C. for 24 hours in an oxygen stream, and then pulverized by a dry pulverizer to obtain a raw material powder. The obtained raw material powder was press-formed to a thickness of 20 mm at a pressure of 0.5 ton / cm ^2.
To form a pellet having a diameter of 30 mmφ. As in Example 1, two of the obtained pellet molded bodies were formed on one upper surface with a thickness of 0.5 mm and a square of 3 mm square as shown in FIG.
gO was placed on the surface (Example 3), and the other pellet compact was left as it was (Comparative Example 3) in a nitrogen atmosphere at 1150 g.
The temperature was raised to 10 ° C. in 10 hours, and after the temperature was raised, dry air was introduced into the electric furnace, and maintained at 1150 ° C. for 30 minutes as an air atmosphere to decompose and melt. Then, from 1000 ° C to 920 ° C, 6
It cooled slowly in 0 hours. Thereafter, oxygen is introduced into the furnace, the atmosphere is changed to an atmosphere of 100% oxygen, cooled to 500 ° C. at 20 ° C./hour, heat-treated from 500 to 400 ° C. for 50 hours, and then left in the furnace to 100 ° C. or less for sintering I got a body.

The magnetic levitation force of each sintered pellet was measured in the same manner as in Example 1. As a result, the sintered body pellet of Example 3 was 410 g / cm ² , and the sintered body pellet of Comparative Example 3 was 220 g / cm ² . YBa ₂ Cu obtained by mounting on an MgO single crystal plate by the method of the present invention
Levitation force ₃ O sintered body, it is found to have a 1.9-fold and excellent superconducting characteristics when not placing a MgO single crystal.

[0027]

According to the present invention, a seed crystal is formed in a molded bulk body formed from a predetermined raw material powder in a one-stage melting operation of a decomposition melting heat treatment step, a slow cooling step, and a heat treatment step substantially similar to the conventional melting method. Are placed in contact with each other, so that the RE-Ba-Cu-O-based superconducting crystal phase on the surface of the bulk body can be crystal-grown in conformity with the ab plane, and the RE-Ba-Cu-O-based A superconductor can be obtained. Further, in the RE-Ba-Cu-O-based superconductor obtained in the present invention, particularly, the RE component is mixed by mixing a Ce component.
The RE ₂ BaCuO ₅ phase is uniformly dispersed in the Ba ₂ Cu ₃ O _y phase, and the superconducting bulk body is entirely homogeneous, so that excellent superconducting characteristics can be obtained and high Jc is obtained.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing one embodiment of the arrangement of seed crystals on a compact according to the present invention.

FIG. 2 is an explanatory view showing another embodiment of the arrangement of seed crystals on a compact according to the present invention.

FIG. 3 is an explanatory view of an apparatus for measuring a magnetic levitation force of superconductivity according to the present invention.

[Explanation of symbols]

 Reference Signs List 1 molded body 2 seed crystal 3 sintered body 4 rare earth magnet 5 liquid nitrogen 6 rod 7 crosshead

Claims (2)

(57) [Claims] 1. A raw material powder containing RE, Ba and Cu components constituting a RE—Ba—Cu—O-based oxide superconductor (RE represents Y, Gd, Ho, Er or Yb). SrTiO ₃ , Mg as seed crystals
O, LaAlO ₃ , LaGaO ₃ , NdGaO ₃ and P
Any one of the rGaO ₃ single crystals is arranged in contact with each other, and the formed body having the seed crystal arrangement is subjected to a heat treatment at a temperature not lower than the decomposition melting temperature of the oxide superconductor, a slow cooling step, and a heat treatment step. A method for producing a rare earth oxide superconductor. 2. An elemental element or a compound of Ce in said raw material powder such that said RE-Ba-Cu-O-based oxide superconductor contains 0.01 to 5.0% by weight of a Ce component on an element basis. The method for producing a rare earth oxide superconductor according to claim 1, wherein