JPH05279029A - Rare-earth oxide superconductor and its production - Google Patents

Abstract

PURPOSE:To improve superconductivity by mixing a rare-earth oxide superconductor and Ag or an Ag compd. in a specified weight ratio, heating, melting and then cooling the mixture to a solidifying point at a specified rate. CONSTITUTION:A Pt powder is added by 0.01-5wt.% to the raw powders of BaCO3 CuO, etc., and mixed and the mixture is calcined in a gaseous oxygen current and then crushed to obtain the calcined powder of BaCu2O, etc. Meanwhile, the powders of Y2O3, BaCO3, and CuO are mixed in a specified ratio, and further the Ag or Ag compd. grains having <=5mum diameter are admixed with the calcined powder by 5-12% of the Ag plus superconductor. This two kinds of mixed powders are mixed and compacted, and the compact is heated and melted at 1000-1200 deg.C in the atmosphere, then cooled to the solidifying point at a rate of >=100 deg.C/hr and then slowly cooled at a rate of 0.1-1 deg.C/hr to grow the crystal. The compact is then heat-treated at 700-400 deg.C in an oxygen atmosphere to provide a rare-earth oxide superconductor in which the vol.% of the Ag grains is larger at the end of the crystal grain than at the center.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rare earth oxide superconductor and a method for producing the same, and more specifically, it has a high critical current density obtained by dispersing silver particles or silver compound particles in crystal grains even under a high magnetic field. The present invention relates to a rare earth oxide superconductor and a method for producing the same.

[0002]

2. Description of the Related Art Oxide superconductors, unlike first-class superconductors in which magnetic flux penetrates when a critical magnetic field is reached and undergoes a phase transition to the normal conducting state, have a second critical value even if the penetration of magnetic flux begins in the first critical magnetic field. It belongs to a type II superconductor that does not undergo a phase transition to a normal conductor until it reaches a critical magnetic field. Moreover, Nb has superconducting properties
-Since the critical temperature is much higher than alloys such as Ti and intermetallic compounds such as Nb ₃ Sn, it is possible to improve the critical magnetic field and the critical current value for practical use. Active research is being conducted. In practical use, these oxide superconductors are often used in some kind of electromagnetic field. Therefore, a material that exhibits superconducting properties even in an environment in which a high magnetic field or high current flows is desired.

Therefore, in order to obtain a high superconducting current value even in a magnetic field, a large amount of so-called pin centers that can prevent the movement of magnetic flux lines due to Lorentz force even if the magnetic flux begins to enter the superconductor in the first critical magnetic field. Must exist
Lattice defects also act as pin centers, but recently, studies have been actively conducted to intentionally disperse fine particles that act as pin centers in oxide superconductors. For example, Ogawa, Yoshida, and Hirabayashi ISEC Journal Vol. 4 No. Three
1991: p30, in a rare earth oxide superconductor, a calcination powder of YBa ₂ Cu ₃ O _7-x and a platinum powder were added.
A platinum-added melting method has been announced in which the material is heated to 0 ° C. and brought into a semi-molten state in which a solid phase and a liquid phase of Y ₂ BaCuO ₅ coexist and then gradually cooled. Then, it has been reported that unreacted Y ₂ BaCuO ₅ fine particles are dispersed in the crystal grains, and these fine particles act as pin centers, so that the oxide superconductor exhibits a high critical current density even in a magnetic field.

Similarly, the applicant has found that trace amounts of platinum or R
RE ₂ BaCuO ₅ (RE
Fine particles of Y, Gd, Dy, Ho, Er, and Yb) are finely dispersed in the crystal grains of REBa ₂ Cu ₃ O _7-x , and these fine particles act as pin centers for rare-earth oxidation. Proposed a superconductor and a manufacturing method thereof (Japanese Patent Application No. 2-412529). If the volume ratio of the pin centers is constant, it is desirable that the pin centers be dispersed as finely and uniformly as possible.

One approach to increasing the magnetic field generated by the Meissner effect is to increase the loop radius of the superconducting current in the oxide superconductor. Since the magnetic field generated by the Meissner effect is due to the annular superconducting current flowing on the surface of the superconductor, it is known that the larger the loop radius of the superconducting current, the larger the magnetic field generated by the Meissner effect. Is. However, in reality, in the oxide superconductor, the superconducting current is often blocked by grain boundaries and cracks, and the superconducting current often flows in an annular shape around the crystal grains. Therefore, in order to increase the loop radius of the superconducting current, it is important to increase the grain size and reduce the cracks.

However, in the manufacturing method such as the melting method of the rare earth oxide superconductor, cracks are inevitably generated in the crystal grains and grain boundaries of the superconductor. 1000-1200 in the melting method
Gradually cool the semi-molten oxide superconductor at ℃,
A phase transition from a high temperature tetragonal crystal to a low temperature orthorhombic crystal occurs around 700 ° C. Since this phase transition causes shrinkage in the c-axis direction, cracks are likely to occur in crystal grains and grain boundaries. In particular, when cracks occur along the cleavage plane of the crystal and the crystal is divided,
The loop radius of the superconducting current is significantly reduced, and the magnetic field generated by the Meissner effect is reduced. In addition, cracks may occur due to thermal expansion, thermal shock resistance, etc. associated with temperature changes during manufacture or use.

[0007]

For the purpose of reducing the generation of cracks at such crystal grains or grain boundaries, the grain size of the rare earth oxide superconductor is 20 to 50 .mu.m.
It has been known to include 10% by weight or more of silver particles or silver compound particles of m. Then, the silver particles or silver compound particles inside the crystal grains inhibit the growth of cracks in the vicinity thereof,
It is known that short and fine cracks are dispersed inside crystal grains. However, in the conventional method, the generation of cracks cannot be sufficiently prevented unless the addition amount of the silver compound is 50% by weight or more, and the addition of such a large amount of silver compound causes the volume of the oxide superconductor to increase. Since the rate is decreased, the critical current density naturally lowers.

Therefore, the inventor has conducted earnest research on a rare earth oxide superconductor in which the critical current density is not lowered by the addition of such a large amount of silver and the loop radius of the superconducting current is not lowered by the generation of cracks. The present invention has been completed.

[0009]

That is, according to the present invention,
There is provided a rare earth oxide superconductor characterized by containing silver particles or silver compound particles having a grain size of 5 μm or less in the crystal grains. Further, according to the present invention, the crystal particles contain silver particles or silver compound particles, and the particle diameter of the silver particles or the silver compound particles is smaller at the end portions of the crystal particles than at the central portion of the crystal particles. Or a rare earth oxide superconductor characterized by having a large volume fraction of those particles.

Further, according to the present invention, after forming a compact from a raw material powder or a calcined powder of a rare earth oxide superconductor, the compact is heated to melt and then cooled to be crystallized. In the method for producing an oxide superconductor, silver or a silver compound is contained such that the silver element contained in the molded body in the silver or silver compound is 5 to 12 wt% of the sum of the rare earth oxide superconductor and the silver or silver compound. And a cooling rate from the melting temperature of the formed body to the freezing point after heating and melting is 100 ° C. or more per hour, and a method for producing a rare earth oxide superconductor is provided.

[0011]

According to the present invention, silver, silver oxide or another silver compound is contained in the silver compound so that the silver element is 5 to 12% by weight of the sum of the rare earth oxide superconductor and the silver compound. Then, the powder of these silver compounds is added to the raw material powder or the calcined powder of the rare earth oxide superconductor to prepare a molded body, and the molded body is heated and melted, and then the melting temperature or the semi-molten state of the molded body is obtained. From the temperature to the freezing point at 100 ° C. or more per hour, and then by slowly cooling to crystallize the rare earth oxide superconductor, particles of these silver compounds having a particle diameter of 5 μm or less were dispersed inside the crystal particles. A rare earth oxide superconductor can be obtained. In the present specification, the term “silver compound” is a general term for compounds containing silver alone and containing silver in the composition formula.
In the microstructure of the rare earth oxide superconductor thus obtained, silver compound particles having a particle size of 3 to 5 μm are dispersed at the crystal grain end portions along the grain boundaries, and silver is present at the crystal grain central portion. The particle size of the compound particles was 10 to 20 μm.

Although the silver compound particles themselves in the crystal grains were known even by the conventional melting method, the particle size is at least 10 μm.
Since it was m or more and usually 20 to 50 μm, such dispersion of fine particles was not known. It was also unknown that the grain size of the silver compound particles was controlled in this way at the crystal grain edge portion and the crystal grain central portion.

It has been confirmed in the present invention that the dispersion of such fine silver compound particles in the crystal grains is effective in preventing the growth of cracks and the like, and in particular, the fine particles are distributed at the edges of the crystal grains. It turned out to be particularly preferable. These particles have an effect of forming a large number of short and fine cracks before the cracks grow long, and this effect is more effective for fine particles. Also, the local mechanical strength of the part where the particles are dispersed seems to be high. Due to these cooperative effects, the superconducting current can flow without being divided by cracks, and excellent superconducting characteristics can be obtained even under a magnetic field. In particular, the fine silver compound particles are dispersed at the crystal grain ends along the grain boundaries because cracks generated inside the crystal grains are prevented from growing to the grain boundaries. It is preferable because it can pass through the grain boundaries without being divided and easily flow to the adjacent crystal grains.

The present invention will be described in more detail below. In the rare earth oxide superconductor according to the present invention, RE is a rare earth element, and REBa ₂ Cu ₃ O _7-x is a main component from at least one element in the group consisting of Y, Gd, Dy, Ho, Er and Yb. To do. Here, x represents a non-stoichiometric ratio of 0 or more and 1 or less because these compounds are non-stoichiometric, and this value directly affects the expression of superconducting properties. Further, the crystal structures of these compounds have common characteristics, and have a multi-layer perovskite structure. A specific example of the main component of the rare earth oxide superconductor according to the present invention is YBa ₂ Cu ₃ O _7-x . The rare earth element represented by RE is not necessarily limited to only one element, and any two or more elements selected from the group consisting of Y, Gd, Dy, Ho, Er, and Yb may be mixed, and a rare earth oxide. The freedom to choose a preferred melting point for the superconductor is increased. As such an example, there is a case where RE can be expressed as Y _y Yb _1-y (y is a real number of 0 or more and 1 or less).

The raw material powder of the rare earth oxide superconductor represented by the composition formula REBa ₂ Cu ₃ O _7-x is RE, that is, Y, G
d, Dy, Ho, Er or Yb oxide, Ba carbonate, oxide or peroxide, or copper, copper (II) oxide, copper (I) oxide, other copper compound, or any of these REBa ₂ Cu ₃ O _7-x and RE ₂ BaCuO after firing mixed powder, calcinated powder, or frit powder mixed with compounds
There is no particular limitation as long as it is compounded so as to form ₅ . The particle size of the raw material powder is also not particularly limited, but a particle size of 1 to 10 μm is generally used.

The silver or silver compound contained in the rare earth oxide superconductor according to the present invention may be added to and mixed with the above raw material powder, or may be added to the calcined body of the rare earth oxide superconductor. Good. Here, it is preferable to use silver oxide powder. Further, the silver compound added to the raw material powder of the rare earth oxide superconductor is preferably a powder in accordance with the raw material powder, and usually, a powder having a particle size of about 20 μm or less is preferable. If the particle size exceeds about 20 μm, it remains as aggregates and deteriorates the homogeneity, which causes variations in superconducting properties, which is not preferable.

In the present invention, the Rh or Pt element of the platinum group or the oxide thereof is represented by REBa ₂ Cu ₃ O _{7-x in} the composition formula of REBa ₂ Cu ₃ O _7-x. When it is contained in an amount of 5% by weight, the particles of RE ₂ BaCuO ₅ have the effect of being finely and uniformly dispersed in the crystal grains.
Then, these fine particles act as pin centers, which is preferable.

In this case, if the content of the above-mentioned elemental components in the rare earth oxide superconductor represented by the composition formula REBa ₂ Cu ₃ O _7-x is less than 0.01% by weight based on the element, the particle dispersion effect is obtained. In addition, if it exceeds 5% by weight, the amount of precipitation of different phases increases, which is not preferable. Further, the Rh or Pt element or these oxides added to the raw material powder of the rare earth oxide superconductor is preferably made into a powder according to the raw material powder, and usually, a fine powder having a particle size of about 5 μm or less. Are preferred. This is because if the particle size exceeds about 5 μm, the particles remain as agglomerates, which deteriorates the homogeneity. A decrease in homogeneity is not preferable because it causes variations in superconducting properties.

The rare earth oxide superconductor according to the present invention is
After adding silver or a silver compound to the raw material powder and molding the mixture into a predetermined shape using a mixed powder that is sufficiently mixed so that the silver component is uniformly dispersed in the raw material powder, the corresponding REBa ₂ Cu ₃
The O _7-x oxide superconductor is heated to a temperature at which it becomes a semi-molten state or a temperature at which it melts. Next, after cooling to the freezing point of the molded body at a cooling rate of 100 ° C. or higher per hour, the solidified point is gradually cooled from the freezing point to crystallize. Then, by performing heat treatment in an oxygen atmosphere, the oxygen composition ratio in the superconducting oxide is adjusted to exhibit superconducting properties, and the rare earth oxide superconductor according to the present invention can be obtained.

As a molding method, a well-known molding method such as a doctor blade method, a press molding method, or a casting molding method can be used to obtain a bulk body of a rare earth oxide superconductor. Further, it can also be obtained as a molded body in which a molded body layer is formed by spray coating, powder coating or the like on the substrate of metal, ceramics or the like with the above-mentioned mixed powder.

The shape of the molded product containing silver or a silver compound thus obtained is arbitrary, and if necessary, a cylinder, a polygonal prism, a parallelepiped, a rectangular parallelepiped, a cone, a polygonal pyramid, a spheroid, a sphere, etc. Not only contains the shape of, but also the shape obtained by plastically deforming these shapes in any direction, or the shape obtained by cutting these shapes with a flat surface or a curved surface in any direction, further, A shape in which one or more holes are drilled in any of the above shapes, for example, a cylinder or a donut shape is also included.

The temperature at which the RE component is Y, G
d, Dy, Ho, Er, or Yb,
Y is about 1000 to 1200 ° C, Gd is about 1050
˜1250 ° C., Dy about 1000 to 1200 ° C., Ho about 1000 to 1150 ° C., Er about 950 to 1100.
C, Yb is a temperature in the range of about 900-1100 ℃,
Depending on each rare earth element, the heating conditions, the size of the molded body, and the like may be taken into consideration within the above temperature range, and the selection may be made appropriately. In addition, the heat treatment to bring this into a semi-molten state is performed by holding the temperature thus determined for a predetermined time. The holding time is not particularly limited and can be appropriately selected depending on the heating conditions and the like as in the above temperature range, and is usually 10 minutes to 2 hours.

Next, the molded product is cooled to below the freezing point, but the cooling conditions at this time are important. That is, it is rapidly cooled to below the freezing point at a cooling rate of 100 ° C. or higher per hour. If the cooling rate is less than 100 ° C. per hour, the silver compound particles become coarse and fine particles of 5 μm or less cannot be obtained. Also, the end point of the quenching temperature depends on the type of rare earth element to be replaced and / or the addition amount of silver compound.
_{2 The} degree to which the freezing point of Cu ₃ O _7-x decreases is different, so adjust the end point of the quenching temperature according to the freezing point. If the end point of the quenching temperature is too high, the silver compound particles become coarse, and if it is too low, the crystal grain size becomes small, which is not preferable.

After the temperature reaches the freezing point, it is preferable to gradually cool at 0.1 to 1 ° C. per hour until the temperature drops by about 30 to 100 ° C. For example, in a YBa ₂ Cu ₃ O _7-x oxide superconductor, it is preferable to gradually cool from 970 ° C. to 940 ° C. at 0.5 ° C. per hour. Next, the REBa ₂ Cu ₃ O _7-x oxide superconductor can be obtained by holding in an oxygen atmosphere, usually at 650 to 400 ° C. for about 10 to 50 hours and performing heat treatment.

In the method for producing a rare earth oxide superconductor according to the present invention, the silver content of the compact is 5% by weight as silver element.
It is preferably 12% by weight or less. When the silver content is within this range, the silver compound particles are finely dispersed in a large amount in a fine amount of 5 μm or less at the end portions of the crystal grains, while the volume fraction of the silver compound particles at the center portion of the crystal grains is smaller than that at the end portions. The particle size also increases to 20 μm. Therefore, cracks are less likely to spread at the crystal grain end portions, while cracks are more likely to occur at the central portion. As a result, when thermal stress or the like is applied, the stress is relaxed by first generating a crack in the center portion of the crystal grain, but the crack is hard to extend to the end portion of the crystal grain.

However, when the silver content is less than 5% by weight, the silver compound particles are dispersed in the crystal grains as in the case where the silver content is 5% by weight or more and 12% by weight or less. Since the volume occupied by the silver compound particles is small and the effect of buffering the stress is small, it is impossible to prevent the extension of cracks to the crystal edges. On the contrary, when the silver content is more than 12% by weight, the silver compound becomes large in size of 20 μm or more, and the buffering effect becomes small. Further, in this case, there is no difference in the particle size and volume fraction of the silver compound particles between the crystal grain end portion and the center portion. As a result, cracks extend from the center of the crystal grain to the end of the crystal grain unless the silver content is about 50% by weight or more.

The mechanism by which fine silver compound particles are obtained inside the crystal grains by the method for producing a rare earth oxide superconductor according to the present invention is not clear, but it is presumed that the reason is as follows. To be done. When the content of the silver compound in the molded body is 12% by weight or less as the silver element, the silver compound is dissolved in the melt at the same time when the molded body is melted, and the solubility is lowered in the process of cooling to the freezing point. It is considered that precipitation of the compound begins. Here, when the cooling rate to the freezing point is as high as 100 ° C. or more per hour,
Precipitation of silver compounds occurs all at once, and fine silver compound particles are thought to occur. On the other hand, the cooling rate is 10 per hour
When the temperature is lower than 0 ° C., it is considered that precipitation of silver compound gradually occurs and coarse silver compound particles are produced.

On the contrary, when the content of the silver compound in the molded product is a large amount of 12% by weight or more as a silver element, the silver compound cannot be completely dissolved when the molded product is melted, and a part of the silver compound is suitable for liquid. Exist in the process of cooling to the freezing point,
Regardless of the cooling rate, it is presumed that a suitable silver compound was deposited and coarse silver particles were generated.

It is also clear that the method for producing a rare earth oxide superconductor according to the present invention provides a mechanism in which the volume fraction of silver compound particles is larger at the ends than at the center of the crystal grains. However, it is presumed that it is due to the following reasons. The content of the silver compound in the molded body is 1 as the silver element.
Below 2% by weight, as the silver compound content increases,
Since the freezing point of the molded body is lowered, solidification of the molten liquid of the molded body starts from the portion where the content of silver compound is small, and the silver compound dissolved in the molten liquid is excluded from the portion that solidifies first. However, since the crystal grains grow, the volume fraction of the silver compound becomes small in the central portion of the crystal grains. Then, in the process of growing the crystal grains, the freezing point of the melt is lowered because the content of the silver compound is relatively increased, and as a result of these, silver is added to the ends of the crystal grains solidified at the final stage of the growth of the crystal grains. It is assumed that the particles are dense.

In the rare earth oxide superconductor according to the present invention in which the crystal grains contain silver particles or silver compound particles having a grain size of 5 μm or less, fine silver particles or silver compound particles prevent the growth of cracks inside the crystal grains. Therefore, the superconducting current can take a longer path without being interrupted by minute cracks inside the crystal grains, and the critical current density can be improved, so that the superconducting current has high superconducting properties.

Further, the crystal grains contain silver particles or silver compound grains, and the grain size of the silver grains or the silver compound grains is smaller at the ends of the crystal grains than at the center of the crystal grains, or the grains thereof. The rare earth oxide superconductor having a large volume fraction with respect to the crystal grains of the rare earth oxide superconductor of the present invention prevents cracks generated inside the crystal grains from growing to grain boundaries, and the superconducting current is Since the superconducting current can have a larger loop radius and the magnetic field generated by the Meissner effect is improved because it can easily flow to the adjacent crystal grains by overcoming the grain boundaries without being divided at the crystal grain ends, the high superconductivity can be improved. Has characteristics.

Further, in the method for producing a rare earth oxide superconductor according to the present invention, the raw material powder or the calcined powder of the rare earth oxide superconductor has the silver element contained in the silver compound as a rare earth oxide superconductor. The powder of the silver compound is added so as to be 5 to 12% by weight of the sum of the silver compound, and after melting, the mixture is rapidly cooled to a solidification temperature at a cooling rate of 100 ° C. or more per hour, and otherwise. In the same manner as in the conventional melting method, the rare earth oxide superconductor having the above-mentioned excellent superconducting properties can be obtained by one-step melting operation.

[0033]

EXAMPLES The present invention will be described in detail below with reference to examples. However, the present invention is not limited to the following examples.

Example 1-3 0.5 to 0.5% of BaCO ₃ powder having an average particle size of about 3 μm and CuO powder having an equimolar amount thereto.
4 wt% platinum powder was added. Next, after mixing these powders for 3 hours with a wet pot mill, spread the mixed powder on a high-purity alumina plate in an oxygen stream,
It was calcined at 000 ° C. for 10 hours to obtain a BaCuO ₂ calcined body. Next, this calcined body was ground for 15 hours in a rotary mill using zirconia boulders, and the average particle size was about 5 μm.
A calcined powder of aCuO ₂ was obtained.

Next, Y ₂ O ₃ , BaCuO ₂ , CuO
Of each of the powders, the molar ratio of these compounds is 0.9: 2.
4: 1.0 (at this molar ratio, the relative molar ratio of the whole Cu is 3.
4) was added and mixed. In Example 1,
Further, 5.5% by weight of silver oxide powder (5.0% by weight in terms of silver element) was added thereto. In Example 2, 7.7% by weight of silver oxide powder (7.0% by weight in terms of silver element) was further added thereto. In Example 3, here, 11% by weight of silver oxide powder (10% in terms of silver element) was added.
Wt%) was added.

The respective mixtures were then mixed in a dry pot mill for 64 hours. 120 g of each of the mixed powders was put into a cylindrical mold having an inner diameter of 50 mm, and the load was 0.1 ton / cm.
After press-molding at ² , it was subjected to isotropic pressure molding at 0.5 ton / cm ² to obtain a cylindrical molded body having a diameter of about 46 mm and a height of about 20 mm. This cylindrical molded body was placed on a high-purity alumina plate, and then the cylindrical molded body on the alumina plate was held at 1100 ° C. for 1 hour in an electric furnace in the air atmosphere to melt.

In the atmosphere as it is, in Example 1,
Cool from 1100 ℃ to 980 ℃ at 100 ℃ per hour, and from 980 ℃ to 920 ℃ at 1.0 ℃ per hour
Then, YBa ₂ Cu ₃ O _7-x was crystal-grown on an alumina plate to obtain a columnar oxide having a diameter of 40 mm and a height of 15 mm. In the air atmosphere as it is, in Example 2, cooling was performed from 1100 ° C. to 975 ° C. at 200 ° C. per hour, and 975 ° C.
To 920 ° C. at 1.0 ° C. per hour, YBa ₂ Cu ₃ O _7-x was crystal-grown on an alumina plate to obtain a columnar oxide having a diameter of 40 mm and a height of 15 mm. In the air atmosphere as it is, in Example 3, it was cooled from 1100 ° C. to 970 ° C. at 100 ° C. per hour, gradually cooled from 970 ° C. to 920 ° C. at 1.0 ° C. per hour, and then YBa ₂ on the alumina plate. Cu
₃ O _7-x was crystal-grown to obtain a columnar oxide having a diameter of 40 mm and a height of 15 mm. Then, each columnar oxide was further heat-treated at 700 ° C. to 400 ° C. for 50 hours in a furnace in an oxygen atmosphere to obtain a cylindrical YBa ₂ Cu ₃ O _7-x oxide superconductor.

The cross section of the obtained oxide superconductor was optically polished, and the progress of cracks at the central portion and the end portion of the crystal grains was observed with an optical microscope. In any of Examples 1-3, cracks were observed at the center of crystal grains, but no cracks were observed at the ends of crystal grains. These results are summarized in Table 1 together with the experimental conditions.

Further, the central portion and the end portion of the crystal grain are 300 μm.
By image analysis from a micrograph of a region of mx200 μm, 50% particle size of silver compound particles and YBa ₂ Cu ₃ O of the particles were obtained.
_The volume fraction of the _7-x oxide superconductor was measured. The 50% grain size of the silver compound at the edge of the crystal grain is 2 in Example 1.
μm, 3 μm in Example 2, and 5 μm in Example 3, which were one digit smaller than the 50% grain size of the silver compound in the center of the crystal grain. The results are summarized in Table 1.

A sample of about 100 mg was randomly cut out from the end of the cylindrical body of the obtained YBa ₂ Cu ₃ O _7-x oxide superconductor, and the temperature in the ab plane of the crystal of the oxide superconductor was 77.
The critical current density Jc (A / cm ² ) at K and a magnetic field of 1 T was measured by an AC magnetic susceptibility measuring method. 29500 A / c in Example 1
m ² , 30300 A / cm ² in Example ² , 2890 in Example 3
It was 0 A / cm ² . The results are summarized in Table 1.

Using a cylindrical YBa ₂ Cu ₃ O _7-x oxide superconductor having a diameter of 40 mm and a height of 15 mm as a sample, the repulsive force with the permanent magnet was measured. After immersing the oxide superconductor sample in liquid nitrogen to develop superconducting properties, the circular surface of this cylindrical sample has a cylindrical shape with a diameter of 20 mm and a height of 10 mm. Density is 3500
The circular one surface of the Sm-Co permanent magnet of G was pressed as the lower surface, and the repulsive force when the gap between the both surfaces was 0.1 mm was measured by the load cell. The average values measured for five samples were 15 kgf in Example 1, 20 kgf in Example 2, and 18 kgf in Example 3. This is called levitation force measurement.

(Comparative Examples 1 and 2) As in Examples 1-3,
After each powder of Y ₂ O ₃ , BaCuO ₂ , and CuO was added and mixed, 55% by weight of silver oxide powder (50% by weight in terms of silver element) was further added thereto. Then, in the same manner as in Example 1-3, the respective mixtures were mixed in a dry pot mill, and press molding and isotropic pressure molding were performed to obtain a diameter of about 4
A cylindrical molded body having a height of 6 mm and a height of about 20 mm was obtained. Example 1-
In the same manner as in No. 3, this cylindrical molded body was melted by holding it on a high-purity alumina plate at 1100 ° C. for 1 hour in the air atmosphere.

In the atmosphere as it is, in Comparative Example 1,
From 100 ° C. to 1000 ° C., cooling is performed at 100 ° C. per hour, and in Comparative Example 2, 1100 ° C. to 970 ° C.
After cooling at 100 ° C. per hour, a cylindrical YBa ₂ Cu ₃ O _7-x oxide superconductor was obtained in the same manner as in Examples 1-3 below.

The cross section of the obtained oxide superconductor was optically polished, and the progress of cracks at the center and the end of the crystal grains was observed with an optical microscope. In each of Comparative Examples 1 and 2, no crack was observed at the center of the crystal grain or at the end of the crystal grain. Further, in the image analysis of the micrographs of the central part and the end part of the crystal grain, the 50% particle size of the silver compound particle is almost the same value in the central part and the end part of the crystal grain, and is about 30 to 40 μm.
Met. Critical current density at temperature 77K and magnetic field 1T
Jc (A / cm ²⁾ is, 19800A / cm ² in Comparative Example 1, Comparative Example 2
Was 21000 A / cm ² . The floating force measurement was 8 kgf in both Comparative Examples 1 and 2.

(Comparative Examples 3 and 4) As in Examples 1-3,
After each powder of Y ₂ O ₃ , BaCuO ₂ and CuO was added and mixed, 3.3% by weight of silver oxide powder (3% by weight in terms of silver element) was further added thereto. Then, in the same manner as in Example 1-3, the respective mixtures were mixed in a dry pot mill, and press molding and isotropic pressure molding were performed to obtain a diameter of 4 mm.
A cylindrical molded body having a height of 6 mm and a height of 20 mm was obtained.

In the same manner as in Example 1-3, this cylindrical molded body was melted by holding it on a high-purity alumina plate in the atmosphere at 1100 ° C. for 1 hour.

In the atmosphere as it is, in Comparative Example 3,
Cooled from 1100 ° C. to 990 ° C. at 100 ° C. per hour, in Comparative Example 4, cooled from 1100 ° C. to 980 ° C. at 100 ° C. per hour, and in the same manner as in Examples 1-3 below, a cylindrical YBa was used. _{A 2} Cu ₃ O _{7- x} oxide superconductor was obtained.

The cross section of the obtained oxide superconductor was optically polished, and the progress of cracks at the center and the end of the crystal grains was observed with an optical microscope. In each of Comparative Examples 3 and 4, it was observed that the crack at the center of the crystal grain extended to the end of the crystal grain. Further, from the image analysis of the micrographs of the central portion and the end portion of the crystal grain, in any of Comparative Examples 3 and 4, the 50% particle diameter of the silver compound particle was 2 μm at the end portion of the crystal grain. It was about 20 μm at the center.

[0049] Temperature 77K, critical in the magnetic field 1T current density Jc (A / cm ²⁾ is, 29800A / cm ² in Comparative Example 3 showed a value satisfactory and 31700A / cm ² in Comparative Example 4. However, the levitation force measurement was 1 kgf in Comparative Example 3 and 2 kgf in Comparative Example 4, which was not a satisfactory value.

(Comparative Example 5) As in Example 1-3, Y ₂
After powders of O ₃ , BaCuO ₂ and CuO were added and mixed, 7.7% by weight of silver oxide powder (7% by weight in terms of silver element) was further added thereto. Then, Example 1
Similar to -3, mix each of these mixtures with a dry pot mill, press-mold and isostatically press-mold it to a diameter of about 46 m.
A cylindrical molded body having a height of about 20 mm was obtained.

In the same manner as in Example 1-3, this cylindrical molded body was melted by holding it on a high-purity alumina plate in the air atmosphere at 1100 ° C. for 1 hour.

In an air atmosphere as it is, it was cooled from 1100 ° C. to 975 ° C. at 20 ° C. per hour, and thereafter, in the same manner as in Example 1-3, a cylindrical YBa ₂ Cu ₃ O _7-x oxide superconducting material was obtained. Got the body

The cross section of the obtained oxide superconductor was optically polished, and the progress of cracks at the central portion and the end portion of the crystal grains was observed with an optical microscope. In each of Comparative Examples 3 and 4, a part of the crack at the center of the crystal grain extended to the end of the crystal grain. In addition, from the image analysis of the micrographs of the central part and the end part of the crystal grain, the 50% particle diameter of the silver compound particle was 14 μm at the end part of the crystal grain and 31 μm at the center part of the crystal grain. Critical current density Jc at temperature 77K and magnetic field 1T
(A / cm ² ) is 24400 A / cm ² , and the levitation force measurement is 5
It was kgf.

(Comparative Example 6-8) As in Example 1-3,
Powders of Y ₂ O ₃ , BaCuO ₂ , and CuO were added and mixed. In Example 6, 11% by weight of silver oxide powder (10% by weight in terms of silver element) was further added.
In Example 7, 16.5% by weight of silver oxide powder (15% by weight in terms of silver element) was further added. In Example 8, 22% by weight of silver oxide powder (20% by weight in terms of silver element) was further added.

Then, in the same manner as in Example 1-3, the respective mixtures were mixed in a dry pot mill, press-molded and isotropically pressure-molded to give a cylindrical molded body having a diameter of about 46 mm and a height of about 20 mm. Got Then, in the same manner as in Example 1-3, this cylindrical molded body was placed on a high-purity alumina plate in the atmosphere at 110
It was held at 0 ° C. for 1 hour to melt.

Comparative Examples 6 and 7 in the air atmosphere as they are
Then, from 1100 ° C to 1000 ° C, 10 per hour
Cooled at 0 ° C., and in Comparative Example 8, 1100 ° C. to 970 ° C.
And cooled at 100 ° C. per hour until the following Examples 1-3.
In the same manner as described above, a cylindrical YBa ₂ Cu ₃ O _7-x oxide superconductor was obtained.

The cross section of the obtained oxide superconductor was optically polished, and the progress of cracks at the center and the end of the crystal grains was observed with an optical microscope. In each of Comparative Examples 6-8, it was observed that a part of the crack at the center of the crystal grain extended to the end of the crystal grain. Further, from the image analysis of the micrographs of the central part and the end part of the crystal grain, the 50% particle size of the silver compound particles in Comparative Examples 6 and 7 was about 20 μm at the end part of the crystal grain.
And in Comparative Example 8, it was 35 μm.

In any of Comparative Examples 5-7, the critical current density Jc (A / cm ² ) at a temperature of 77 K and a magnetic field of 1 T was about 2000.
It was 0 A / cm ² , and the levitation force measurement was 3 kgf. None of the values was satisfactory.

The 50% particle size of the silver compound particles at the edges of the crystal grains in Example 1-3 is 2 to 5 μm, and Example 1
The levitation force of -3 was significantly superior to the levitation force of any of Comparative Examples 1-8. In Comparative Examples 3 and 4 as well, the 50% particle size of the silver compound particles at the edges of the crystal grains takes a small value of 2 μm, but since the volume fraction of the silver compound particles is small, the central portion of the crystal grains to the edges of the crystal grains are small. Since it was not possible to prevent the cracks from growing, it seems that the superconducting properties were not satisfactory.

Comparative Examples 1, 2 and 6-8 show that if the amount of silver compound added is too large, the grain size of the silver compound particles at the edges of the crystal grains increases and the superconducting properties deteriorate. ..
Comparing Example 2 with Comparative Example 5, even if the addition amount of the silver compound is 12% by weight or less in terms of silver element, if the cooling rate during firing is slow, the silver compound at the end of the crystal grain is It can be seen that the particle size of the particles becomes large.

From these results, it can be seen that the growth of cracks inside the crystal grains can be prevented and the superconducting characteristics are remarkably improved by reducing the grain size of the silver compound particles at the ends of the crystal grains.

[0062]

[Table 1]

[0063]

INDUSTRIAL APPLICABILITY The rare earth oxide superconductor according to the present invention in which the crystal grains contain silver particles or silver compound particles having a particle size of 5 μm or less, the fine silver particles or silver compound particles grow cracks inside the crystal grains. It is possible to improve the superconducting property by preventing the above. Further, the crystal grains contain silver particles or silver compound particles, and the particle size of the silver particles or silver compound particles is smaller at the ends of the crystal grains than at the center of the crystal grains, or the volume of the particles is The rare earth-based oxide superconductor according to the present invention having a high rate prevents cracks generated inside the crystal grains from growing to grain boundaries, and thus improves superconducting properties.

Further, in the method for producing a rare earth oxide superconductor according to the present invention, the raw material powder or the calcined powder of the rare earth oxide superconductor has the silver element contained in the silver compound as a rare earth oxide superconductor. The powder of the silver compound is added so as to be 5 to 12% by weight of the sum of the silver compound, and is cooled at a cooling rate of 100 ° C. or more per hour from the melting temperature of the molded body to the freezing point. Can obtain a rare earth oxide superconductor having the above-mentioned excellent superconducting properties in the same manner as the conventional melting method. As described above, since the rare earth oxide superconductor according to the present invention has improved superconducting properties, it is one step toward practical application of the rare earth oxide superconductor, and the present invention is industrial. Can contribute to development.

Claims (5)

[Claims] 1. A rare earth oxide superconductor characterized in that the crystal grains contain silver particles or silver compound particles having a particle size of 5 μm or less. 2. The crystal grains contain silver particles or silver compound particles, and the particle size of the silver particles or the silver compound particles is smaller at the ends of the crystal grains than at the center of the crystal grains. A rare earth oxide superconductor characterized by the following. 3. The crystal grains contain silver particles or silver compound particles, and the volume fraction of the silver particles or the silver compound particles is closer to the end portions of the crystal grains than to the central portions of the crystal grains. A rare earth oxide superconductor characterized by being large. 4. RE is Y, Gd, Dy, Ho, Er and Y.
The main component of the rare earth oxide superconductor is represented by a composition formula of REBa ₂ Cu ₃ O _{7-x in} which at least one element in the group consisting of b is included and x is 0 or more and 1 or less. The rare earth oxide superconductor according to claim 1, 2 or 3. 5. A rare earth oxide superconductor prepared by preparing a molded body from a raw material powder or a calcined powder of a rare earth oxide superconductor, heating the molded body to melt, and then cooling to crystallize the molded body. In the production method, the silver or the silver compound is contained in the molded body such that the silver element contained in the silver or the silver compound is 5 to 12% by weight of the sum of the rare earth oxide superconductor and the silver or the silver compound. A method for producing a rare earth oxide superconductor, wherein the cooling rate from the melting temperature of the molded body to the freezing point after heating and melting is 100 ° C. or more per hour.