JP3115915B2 - Method for producing rare earth oxide 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 producing a rare earth oxide superconductor, and more particularly, to a RE having excellent superconducting properties in which an Ag component is uniformly dispersed in a superconducting crystal phase.
The present invention relates to a method for producing a Ba-Cu-O-based oxide superconductor (RE represents a rare earth element).

[0002]

2. Description of the Related Art Oxide superconductors have been actively studied for practical use because of their high critical temperature. In recent years, various proposals have been made to obtain an oxide superconductor having a high critical current density (Jc) in a bulk material. For example, there is a melting method in which fine structure particles of a non-superconducting phase are dispersed in a superconducting phase, and a so-called pinning center is introduced to fix a penetrating magnetic flux line.
ured Growth method) has been proposed, but the oxide superconductor obtained by this method is Y ₂ BaCuO ₅ (211 phase).
Are disadvantageous in that the grain size is large, their distribution is not uniform, and cracks exist along the crystal growth direction.

As a method for improving the above-mentioned disadvantages of the MTG method, Japanese Patent Application Laid-Open No. 2-153803 discloses a method in which
A QMG method (Quench and Melt Growth method) in which fine particles are dispersed in Ba ₂ Cu ₃ O ₇ (123 phase) particles has been proposed. The superconductor obtained by this method exhibits an extremely strong pinning effect and exhibits excellent Jc in a high magnetic field, but has disadvantages such as complicated preparation operations. Further, as an improvement of the above-mentioned QMG method, there has been proposed a method of adding superconducting properties and mechanical properties by adding Ag powder.

[0004]

In most of the conventional methods such as the above-mentioned QMG method, the growth of the crystal phase constituting the rare earth-based superconductor starts at about 1000 ° C. in the air atmosphere, so that the melt heating step is continued. Under the same atmosphere, about 950-1
Slow cooling was started in the range of 000 ° C. to grow a superconductor crystal. However, according to the inventors, as described above,
In the method of increasing the superconducting properties by adding Ag powder, when the cooling rate is lowered in the above-mentioned crystal growth temperature range and the cooling is performed slowly to promote the growth of the superconducting crystal phase, the Ag component aggregates and coarsens. The phenomenon of solidification in the outer periphery of the crystal phase was found. Therefore, it was found that despite the addition of the Ag component for the purpose of improving the superconductivity and mechanical properties, the dispersibility of the Ag component was inferior and the superconductivity was not excellent. The present inventors have intensively studied a method of uniformly dispersing the Ag component in the superconducting crystal phase while elucidating the above-mentioned phenomenon, and as a result, completed the present invention.

[0005]

According to the present invention, RE-B
An a-Cu-O-based oxide superconductor (RE represents a rare-earth element) is formed by molding a raw material powder comprising RE, Ba and Cu components and an Ag component. Heat treatment at a temperature not lower than the decomposition melting temperature of the body, and then gradually cooling from a temperature below the freezing point of the Ag component in an atmosphere having an oxygen partial pressure of 0.001 to 0.05 atm to grow the oxide superconducting crystal phase A method for producing a rare earth-based oxide superconductor is provided.

[0006]

The present invention is constructed as described above, whereas the conventional method continuously cools down in the atmosphere of the melting and heating step, whereas the oxygen partial pressure in the atmosphere at the time of slow cooling is 0.001 to 0. By controlling the pressure to 05 atm, the growth temperature range of the rare-earth superconducting crystal can be reduced from the normal 950 to 1000 ° C to the freezing point of Ag component of 960 ° C or lower. Thereby, since the superconducting crystal can be grown with the slow cooling start temperature being equal to or lower than the freezing point of the Ag component, the Ag component is solidified in a uniformly dispersed state in the molded body in which the Ag component is uniformly dispersed before the slow cooling is started, Thereafter, it can be assumed that the superconducting crystal grows. Therefore, the obtained rare earth superconductor is
The Ag component is uniformly dispersed and fixed in the superconducting crystal phase, and has a high Jc and excellent superconducting characteristics.
In contrast, in the conventional method, the growth of the rare-earth superconducting crystal phase occurs before the solidification of the Ag component, and the Ag component dispersed in the molten state gradually aggregates on the outer periphery of the crystal phase.
It is presumed that the dispersibility of the Ag component deteriorated and high Jc could not be exhibited.

Hereinafter, the present invention will be described in more detail. RE-Ba-Cu-O-based oxide superconductor of the present invention,
RE is Sc, Y, La, Ce, Pr, Nd, Sm, Eu,
Multilayer perovskite structure containing one or more rare earth elements of Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu (excluding compositions containing only one of Sc, Ce, Pr or Tb) For example, (YCe) ₁ B
It is an oxide superconductor such as a ₂ Cu ₃ O ₇ .

[0008] The raw material powder containing RE, Ba and Cu components for constituting the RE-Ba-Cu-O-based oxide superconductor is RE, that is, an oxide of a rare earth element, a carbonate, an oxide of Ba or an oxide of Ba. An oxide mixed powder of each component of the oxide superconductor mixed with an oxide and an oxide of Cu, a calcined powder of the oxide mixed powder, a frit powder of the oxide mixed powder, and the like are fired, and then subjected to REBa ₂ Cu ₃ O. There is no particular limitation as long as _y and RE ₂ BaCuO ₅ are blended. In addition, the raw material powder of the present invention contains the above RE-Ba-
It is preferable that the Ag component powder is further added to and contained in the mixed powder of the components constituting the Cu-O-based oxide superconductor, and the whole is used in a uniform mixed state. The addition amount of the Ag component
There is no particular limitation. Usually, it can be added and contained in the range of 1 to 20% by weight. Ag to be added
The form of the components is not particularly limited. For example, Ag
It may be added as a simple powder or a silver oxide (Ag ₂ O) powder, or may be added using an aqueous solution of a water-soluble silver compound, for example, silver nitrate.

Although the particle size of the raw material powder is not particularly limited, it is generally 20 μm or less, especially 1 to 5 μm.
Those having a small m are preferable. 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. As the raw material powder having a size of 1 μm or less, for example, it is preferable to use a powder generated by a coprecipitation method.
As long as it is a fine powder of μm or less, a powder obtained by another method can be used. Further, in the present invention, the raw material powder comprising the respective components constituting the rare earth oxide superconductor and the Ag component further has a fine dispersibility of 211 phases and a
In order to promote the crystal growth of the 23 phases, known additives can be added and used. For example, noble metal elements such as Pt, Pd, and Au, rare earth elements other than the rare earth elements constituting the superconductor, potassium, and the like.

The oxide superconductor of the present invention is formed into a predetermined shape using the above-mentioned raw material powder. As the molding method, a known molding method such as a doctor blade method, a press molding method, and a slurry casting method can be used to obtain a bulk oxide superconductor. 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, a compact obtained by molding the raw material powder containing the components constituting the rare earth oxide superconductor, the Ag component, and, if necessary, the additive, as described above, into a predetermined shape, Heat treatment to a temperature higher than the decomposition melting temperature of the corresponding RE-Ba-Cu-O-based oxide superconductor,
The superconductor can be obtained by slow cooling in a predetermined oxygen-containing atmosphere and heat treatment in an oxygen-containing atmosphere similarly to the conventional method.

In the present invention, the decomposition melting temperature of the oxide superconductor during the heat treatment varies depending on the type and combination of the RE element components contained, but generally ranges from 1050 to 12 hours.
The temperature is 00 ° C. and may be appropriately selected depending on the heating conditions, the size of the molded body, and the like. In the present invention, the melting heat treatment including the temperature rise to a temperature equal to or higher than the decomposition melting temperature and the subsequent heat treatment and the like can be usually performed according to the schedule shown in Table 1.

[0013]

[Table 1]

The present invention can be carried out according to the heat treatment schedule shown in Table 1. In the conventional method, the slow cooling step is carried out at 950 to 1000 ° C. in an air atmosphere. No. The slow cooling step 4 was performed using an oxygen partial pressure of 0.0
Under an atmosphere of 01 to 0.05 atm, the freezing point of the Ag component is 9
The difference is that the heat treatment is performed at a temperature of 60 ° C. or less, that is, a temperature range of 900 to 960 ° C., and other heat treatment schedules are not significantly different from the conventional method. However, as is clear from the examples and comparative examples described below, in the present invention, the conditions of the above-described slow cooling step are extremely important, and have a great effect on the superconducting properties of the rare earth-based oxide superconductor obtained thereby. Will have an effect. In the slow cooling step of the present invention, if the oxygen partial pressure is less than 0.001 atm, it is not preferable because sufficient oxygen is not supplied to generate the 123 phase. Further, at an oxygen partial pressure exceeding 0.05 atm, the growth of the rare-earth superconducting crystal starts before the solidification of the Ag component,
Slow cooling at a temperature of 960 ° C. or less prevents sufficient crystal growth, making it impossible to obtain a rare-earth oxide superconductor having excellent superconducting properties.

[0015]

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. Examples 1 to 5 and Comparative Examples 1 to 4 Y ₂ O ₃ , BaCO ₃ , and CuO powder were prepared by mixing the atomic ratio Y: Ba:
Formulated so that Cu = 1.50: 2.3: 3.2,
Calcination was performed at 800 ° C. for 10 hours in the air. 4% by weight of Ag powder and 0.1% by weight of Pt powder in the obtained calcined powder
The resulting mixture was ground in a rotary mill using zirconia cobblestone in ethanol for 15 hours. The average particle size of the obtained powder is 4μ.
m. 1 ton /
Pressed under pressure of ² cm2, thickness 20mm, diameter 40
Nine pellets of mmφ were obtained.

Each of the obtained pellets was heated up to 1100 ° C. at a rate of 5 ° C./min in a nitrogen atmosphere on a dense alumina setter in an electric furnace. Thereafter, dry air was introduced into the electric furnace to change the atmospheric gas from nitrogen to air, and the temperature was changed to 1100 ° C.
For 30 minutes. Next, the atmospheric oxygen partial pressure was adjusted while the temperature was lowered at a rate of 2 ° C./min to each of the slow cooling start temperatures shown in Table 2, and the predetermined oxygen partial pressure shown in Table 2 was obtained. After reaching the slow cooling start temperature and the oxygen partial pressure, respectively, the temperature was lowered from each slow cooling start temperature to a temperature lower by 50 ° C. at an extremely slow cooling rate of 1 ° C./hour to crystallize the superconducting phase. Thereafter, the temperature was decreased to 500 ° C. at a rate of 1 ° C./min, oxygen was introduced into the furnace, and the atmosphere was changed to a pure oxygen atmosphere at 500 ° C. for 2 hours.
After the heat treatment for 4 hours, each sintered pellet was obtained by furnace cooling.

The cross section of each of the obtained sintered pellets is polished,
The dispersion state of the Ag component was observed using an electron beam microanalyzer (EPMA). The results are shown in Table 2. A 1 mm cube sample was cut out from each sintered pellet, and magnetic hysteresis was measured using a vibrating magnetometer. The critical current density at liquid nitrogen temperature and magnetic field 1T (tesla) was measured.
Jc was calculated. The results are shown in Table 2.

[0018]

[Table 2]

As is clear from the above examples, the rare-earth oxide superconductor obtained by the method of the present invention has a good dispersion state of the Ag component, a high Jc, and has excellent superconducting properties. I understand. On the other hand, in Comparative Examples deviating from the method of the present invention, a superconductor having favorable superconducting properties cannot be obtained. That is, in Comparative Examples 3 and 4, when the oxygen partial pressure was 0.20 atm or higher and higher than 0.05 of the present invention, when the cooling was started from 960 ° C., the dispersion state of the Ag component was good, but the superconductivity was high. Crystal growth of the phase is hardly observed, and Jc is remarkably reduced. Further, in Comparative Example 2, even when the oxygen partial pressure was set to 0.05 atm of the present invention, the slow cooling was started at 9 atm.
At 80 ° C., the crystal growth of the superconducting phase is good,
Ag component aggregation was observed and Jc was low. In Comparative Example 1, when the oxygen partial pressure is 0.0005 atm, which is lower than 0.001 atm of the present invention, crystal growth of the superconducting phase is not observed and Jc is low even if the cooling is started from 950 ° C.

[0020]

According to the method for producing a rare-earth oxide superconductor of the present invention, it is not necessary to employ a complicated operation step as in the conventional melting method, and each of the RE-Ba-Cu-O-based oxide superconductors is required. By using a raw material powder composed of a component and an Ag component, after a heat treatment for melting, a predetermined slow cooling step is performed, thereby having a high Jc and obtaining a rare-earth oxide superconductor having excellent superconducting properties. .

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C01G 1/00-3/00 C04B 35/00

Claims (1)

(57) [Claims] 1. A molding obtained by molding a raw material powder comprising RE, Ba and Cu components and an Ag component constituting a RE-Ba-Cu-O-based oxide superconductor (RE represents a rare earth element). The body is subjected to a heat treatment at a temperature not lower than the decomposition melting temperature of the oxide superconductor, and then gradually cooled in an atmosphere having an oxygen partial pressure of 0.001 to 0.05 atm from a temperature not higher than the freezing point of the Ag component. A method for producing a rare earth oxide superconductor, comprising growing a superconductor crystal phase.