JPH02137762A - Oriented polycrystalline superconductor - Google Patents

Abstract

PURPOSE: To improve a current capacity by adding a specific ratio of rate earth elements anisotropic to YBa₂Cu₃O_7-y and LaBa₂Cu₃O_7-y and orienting the mixture by increasing alignment.

CONSTITUTION: Base material-forming powder (a) consisting of the respective oxides of Y and/or La, Ba and Cu or their precursors and having the ratios to yield the metal oxide meeting the compsn. of formula I, formula II (y is 0 to about 1) an their combination is prepd. An additive (b) mainly composed of the oxide of Ln (≥1 kinds among Nd, Eu, etc.), or its precursor mateal is prepd. and the mixture of both in which the additive (b) is about 1 to about 20 vol.% of the powder (a) is prepd. This mixture is brought into reaction at about 800°C to a temp. below the m.p. of the metal oxide described above, by which the product consisting of formula III, formula IV and their combination ((x) is about 0.01 to about 0.2) is obtd. This product is pulverized and is aligned by applying a magnetic field thereon. A compressed body is obtd. by compressing the aligned product. The compressed body is sintered at about 900°C to the temp. below the m.p. of the powder in an oxidation atmosphere to form a sintered compact having open porosity of 0 to about 20 vol.% and the sintered compact is then cooled in the oxidation atmosphere at speed at which a superconductor is formed.

COPYRIGHT: (C)1990,JPO

Description

DETAILED DESCRIPTION OF THE INVENTION This application is filed on September 6, 1988 and is assigned to the same assignee as the present invention.
K., W., Lay) Co-pending U.S. Patent Application No. 2402
It is related to No. 96. This patent application discloses a method for producing oriented polycrystalline superconductors.

The present invention provides Y B a2Cu3O7-y (Y -12
3) and (or) LaB a2Cu3O7-y
The present invention relates to a method of manufacturing an oriented polycrystalline superconductor by adding an anisotropic rare earth element to (La-123) to increase the degree of alignment.

In the industry, high temperature superconductors of composition 123 containing most of the lanthanide elements from Y or La to Lu have been produced. Note that the lanthanide elements (ie, Ce, Pr, and Tb), which produce relatively stable 4+ ions, are believed to form an exception. Much of the research to date has been conducted using the Y-123 compound. This material has been found to exhibit superconducting properties with a high degree of anisotropy. In particular, the critical current value in the (001) plane is much larger than the critical current value out of the plane.

By the way, it is expected that a polycrystalline block consisting of randomly arranged crystal grains does not exhibit a large critical current value because adjacent crystal grains are not aligned with each other. If this is true, applications requiring superconductors with large current capacities must use single crystals or polycrystals consisting of highly ordered grains. It turns out. Conventional ceramic forming techniques, consisting of forming a compact from a powder and then densifying the compact by sintering, are considered suitable as a method for producing bulk superconductors of composition 123. . Unfortunately, however, the crystal grains in the resulting high-density object exhibit only a random arrangement. Therefore, there is a need for a simple method for orienting grains.

One method that has been found to be effective for producing polycrystalline sintered bodies consisting of aligned grains is to take advantage of the anisotropy of the magnetic susceptibility of the materials described above. be. Crystals of the 123 compound have been shown to align in a magnetic field.

Y-123, Dy-123, N1423, S@-123
and Ho-123 are aligned such that the C-axis of the crystal is parallel to the magnetic field. However, Eu-123, Gd-12
3, Tom-123, Yb-123 and Er-123
are aligned so that the C-axis of the crystal is perpendicular to the magnetic field. The magnetic susceptibilities of Y-123 and La-,123 are derived from copper ions and conduction electrons. On the other hand, Ln
Lll-1 such that is within the range from P' to Ylt
Compound 23 exhibits a much larger magnetic susceptibility (and anisotropy in magnetic susceptibility) derived from the magnetic moment on the Ln ion. Note that Y and La ions are not magnetic ions.

Briefly, the method of the present invention comprises: (a) Y and/or respective oxides of La, Ba and Cu or precursors thereof;
A metal oxide composition that matches the composition of one selected from the group consisting of u3O7-y, L a Ba2 Cu3O7-y, and combinations thereof (where y has a value within the range of 0 to about 1). (b) Ln oxide (however, Ln
are Nd, Em, Gd, Dy, Ho, Er, Tm, Yb
and a combination thereof) or a precursor thereof; (c) the oxide of Ln is approximately equal to the oxide of Y and/or La; (d) from about 800° C. to below the melting point of the metal oxide; By heating the mixture to a reaction temperature of , the precursor is decomposed to form the corresponding metal oxide before the reaction temperature is reached and the mixture is reacted, thereby producing Yl-.

(Ln)xBazcuiot-y, La,-x(L
n), Ba2Cu3o 7-y and combinations thereof, where X has a value within the range of about 0°01 to about 0.2, y has a value within the range of 0 to about 1, and (e) pulverizing said reaction product to obtain a sinterable powder; (f) pulverizing said reaction product to obtain a sinterable powder; substantially aligning said sinterable powder along its preferred axis of magnetization by applying an alignment magnetic field to said sinterable powder; and (g) compressing said sinterable powder thus aligned. (h) forming a compact body comprising said sinterable powder substantially aligned along its preferred axis of magnetization; By sintering the compressed body in an oxidizing atmosphere using a temperature of 0.
forming a sintered body with an open porosity in the range of ~20% (by volume) and then cooling said sintered body in an oxidizing atmosphere using a rate such that (i> produces a superconductor) It is characterized by consisting of various steps.

In carrying out the method of the invention, - firstly, yttrium oxide and (or > barium silanthane carbonate Pi) and copper oxide are used as matrix-forming powders. These matrix-forming powders are: , L
a B a2Cu3O7-y and combinations thereof, where y has a value within the range of 0 to about 1, but typically 0
1 selected from the group consisting of
used in proportions to provide a metal oxide composition that matches that of the individual.

Generally speaking, the additive is an oxide of Ln (however,
Ln is Nd, Em, Gd, Dy, Ho, Er, Tm, Y
b) and combinations thereof. Such additives are generally used in amounts ranging from about 1 to about 20% (by volume), and typically from about 2 to about 5% (by volume), based on yttrium oxide, lanthanum oxide, or combinations thereof. used in amounts of Note that the actual amount of additive used is determined experimentally.

If desired, particulate inorganic precursors of the oxide reactants, such as those described above, can also be used. Such precursors must be such that they completely decompose to produce the corresponding oxides and by-product gases and do not leave any contaminants in the reaction mass. Barium carbonate is a useful precursor to barium oxide. It is necessary that such precursors be used in amounts sufficient to produce the required amount of the corresponding oxide.

The oxide reactants or their precursors as mentioned above are
It must have a particle size that allows the reaction to occur. Generally, these powders are used within the commercially available particle size range, typically from less than 1 micron to about 100 microns. These powders also need to be free of hard agglomerates of large particle size (ie, particle size substantially greater than about 100 microns). This is because such aggregates may remain after the mixing operation and prevent sufficient contact between the reactants, thereby reducing the reaction rate.

A mixture is prepared by mixing the matrix forming powder and additives as described above. Preferably, such mixtures are homogeneous or substantially homogeneous in order to obtain a homogeneous or substantially homogeneous reaction product. A variety of conventional techniques can be used to prepare such mixtures, including ball milling.

A reaction product is obtained by reacting a mixture of such a base material forming powder and an additive.

Such reactions are generally conducted in an oxidizing atmosphere using reaction temperatures from about 800° C. to below the melting point of the metal oxide. Typically, the reaction temperature is about 850-1000°
C or within the range of about 900-950°C. Reaction times are determined experimentally.6 Generally, the reaction products are cooled to about room temperature in an oxidizing atmosphere. Generally speaking, the oxidizing atmosphere used in carrying out the reaction and cooling the reaction products will contain at least about 1% (by volume) oxygen, and the remainder of the atmosphere will not significantly affect the reaction products. Consists of gases that do not have any harmful effects.

Representative examples of such gases include nitrogen gas and noble gases such as argon and helium.

Note that the oxidizing atmosphere preferably consists of oxygen or air. Generally, such an oxidizing atmosphere need only be at approximately atmospheric pressure.

The above reaction product is Y+−x(LH)XB
a2Cu3O? -31,L &1-X(L 11)X
B a2C113O7-31 and combinations thereof, where X has a value within the range of about 0.01 to about 0.2;
y has a value within the range of 0 to about 1, and Ln is as defined above. In such reaction products, X is typically about 0,
02 to about 0.05, and y is typically 0
to about 0.7.

The desired sinterable powder is obtained by pulverizing the reaction product as described above. Conventional techniques can be used to effect such comminution, including milling. Such sinterable powders generally have a particle size of less than 1 micron to about 10
It has an average particle size in the range of up to microns, but typically has an average particle size in the range of about 0.1 to about 5 microns or about 0.2 to about 4 microns. Note that the average particle size can be measured by conventional techniques.

In carrying out the method of the invention, the sintering powder is preferably sintered at room temperature in order to align the sinterable powder at least substantially along its preferred magnetic axis (parallel or perpendicular to the c-axis). An alignment magnetic field is applied to the magnetic powder. Specifically, the sinterable powder will align along the preferential magnetization axis of the rare earth element in the additive. Although the alignment magnetic field is applied to the sinterable powder before forming the compacted body from the sinterable powder, it is preferable to continue applying it even during the formation of the compacted body. Generally, the alignment field will be in the range of about 17 to about 100 kiloersteds, although the actual strength will be determined experimentally.

A variety of conventional techniques can be used to form compacts from sinterable powders. For example, a compact of the desired shape can be formed from the sinterable powder by extrusion, injection molding, die compaction, slip casting or tape casting. In detail, according to one method, applying an alignment magnetic field to the sinterable powder (usually in layers) in a press and then compressing the aligned sinterable powder (preferably in the alignment magnetic field). A compressed body is formed.

According to another method, a slurry is prepared by dispersing a sinterable powder in an organic liquid such as heptane. This slurry is placed in a suitable firing vessel (eg, an alumina boat) and the liquid is evaporated while an alignment magnetic field is applied to form compacted bodies aligned within the vessel. Next, by firing this compressed body, the sintered body of the present invention will be obtained.

According to yet another method, an aligned compact is formed by placing a slurry of sinterable powder in a porous mold followed by conventional slip casting in an alignment magnetic field.

According to yet another method, a tape is cast from a slurry of sinterable powder in an alignment magnetic field.

The above-mentioned sinterable powder can also be mixed with forming aids such as lubricants, dispersants, binders, etc. that are useful in forming a compressed body. Such molding aids may be those known in the art and may be used in accordance with conventional methods, with the amount used being determined experimentally. They are generally organic materials but at relatively low temperatures (preferably 500°C
It is preferable to use a material that evaporates or decomposes and produces no or almost no residue when heated under the following conditions. Such shaping aids must permit magnetic alignment of the sinterable powder and must not have a significant adverse effect on the process of the invention.

The compressed body described above must have at least a density sufficient to produce the sintered body of the present invention. In order to facilitate high density during sintering, the compacted body preferably has a density equal to at least about 45% of the theoretical density.

Sintering of the compact is generally carried out in an oxidizing atmosphere at approximately atmospheric pressure. Such an oxidizing atmosphere must be at least sufficiently oxidizing to produce a sintered body in which the content of oxygen (0) in the chemical formula is at least about 6.0. Generally, the sintering (or firing) atmosphere should contain at least about 1% (by volume) oxygen, and the remainder of the atmosphere should be a gas that does not have a significant adverse effect on the sintered body. . Representative examples of such gases include nitrogen gas and noble gases such as argon and helium. Note that it is most preferable that the sintering atmosphere consists of oxygen.

Sintering is carried out at a temperature ranging from about 900°C to below the melting point of the oxide component of the compact.

Sintering temperatures generally range from about 900°C to about 1000°C, and typically from about 950°C to about 975°C. The actual sintering temperature is determined experimentally and is highly dependent on the particle size of the sinterable powder, the density of the compact, and the final density desired in the sintered body. Generally speaking, the higher the sintering temperature, the higher the density and grain size of the sintered body.

Sintering times can vary over a wide range, but the actual sintering time is determined experimentally. Generally speaking, the longer the sintering time, the greater the grain size of the sintered body. Generally, the sintering time will be in the range of about 2 to 8 hours.

The sintered body thus obtained is cooled in an oxidizing atmosphere, generally at about atmospheric pressure, using a rate that produces the superconductor of the invention. Cooling conditions can vary over a wide range, but actual cooling conditions are determined experimentally. Generally speaking, the oxidizing atmosphere for cooling should contain at least about 20% (by volume) oxygen, and the additional gas should not have a significant adverse effect on the resulting superconductor. The oxidizing atmosphere preferably consists of air, and more preferably of oxygen.

Specifically regarding the cooling process, the sintered body is generally sintered in a temperature range of about 700°C to about 400°C at a rate that produces at least a sufficient amount of orthorhombic crystal structure to form a superconductor. Needs to be cooled. Generally, additional oxygen is introduced into the sintered body in this temperature range of about 700 to about 400 degrees Celsius. In this case, it is necessary to introduce enough oxygen into the sintered body to produce the required orthorhombic crystal structure.

From about 400°C the sintered body may be cooled at a faster rate, but the rate must not be so fast that thermal shock causes the sintered body to fracture. Typically, the sintered body is cooled to room temperature (i.e.
to a temperature within the range of about 20 to about 3O<0>C.

This cooling process does not significantly affect the appearance of other components of the sintered body (ie, components other than oxygen).

The sintered body and the superconductor obtained therefrom have the same density or porosity. Although the sintered body may have some closed pores, it generally has open pores. The porosity of the sintered body is defined by the porosity, which is preferably small (preferably less than 1 micron) and sufficiently dispersed in the sintered body so as not to significantly adversely affect its mechanical properties. It can be measured by standard metallographic techniques, such as optically examining a Kenkyo section of the body.

The term "closed pores" as used herein means pores or voids that are not open on the surface of the sintered body and therefore not in contact with the surrounding atmosphere. Generally, the closed porosity is 0 for the entire sintered body.
to about 10% (by volume), preferably about 5%
% (by volume), and preferably less than about 1% (by volume).

The term "open pores" as used herein means pores or voids that are open on the surface of the sintered body, so that the inner surface can come into contact with the surrounding atmosphere.

The sintered body must have sufficient surface area to produce a superconductor, but the actual surface area is determined experimentally. Specifically, upon cooling in an oxidizing atmosphere, the sintered body must have at least enough surface area to form a superconductor upon contact with oxygen; generally speaking, sintering Part of the body's surface area is provided by its open pores. Note that in the case of an extremely thin sintered body, open pores may not be necessary. Generally, the superconductors of the present invention have a capacitance of 0 to about 20
% open porosity, preferably from about 2 to about 20 (
% by volume and an open porosity of - layer preferably about 5
It has an open porosity of ~15% (by volume). Note that the open porosity is typically within a range of about 7 to about 10% (by volume).

Generally speaking, the superconductor of the present invention consists of plate-shaped crystal grains having a disk or polygonal shape, and the outer circumference of such plate-shaped crystal grains is irregular.

The grain size in the longitudinal direction is at least about 1 micron, but the actual value can vary widely depending on the grain size of the sinterable powder and the sintering conditions. For example, grain size in the longitudinal direction can range from about 1 micron to about 100 microns. Typically, the grain size in the long axis direction is approximately 1 to
about 5 microns, about 2 to about 5 microns, or about 2 to about 4
It is typically in the micron range.

The orientation of the grains in the superconductor is highly dependent on the orientation of the sinterable powder during the implementation of the method of the invention. Generally speaking, if the C-axis of the sinterable powder is aligned so that it is parallel to the alignment magnetic field, almost all of the plate-like crystal grains in the resulting superconductor will be in a state where they are stacked on top of each other. . On the other hand,
If the C-axis of the sinterable powder is aligned perpendicular to the alignment magnetic field, the plate-like crystal grains in the resulting superconductor will have a C axis in a common plane perpendicular to the alignment magnetic field. have an axis, but are otherwise aligned in any direction (i.e., over a range of 360°).

Previously, Y-123 and La-123 sintered bodies were composed of unaligned grains and their grain size was generally limited to about 2-3 microns.6 The reason for this was that large grain sizes were This is because the sintered body having the above-mentioned structure cracks when heated. Unlike conventional superconductors, the superconductor of the present invention exhibits significant anisotropic thermal expansion due to the aligned crystal grains. As a result, there are no restrictions on grain size. Furthermore, in terms of -a, grain boundaries are weak bonding sites and reduce the critical current value. However, since the grains in the superconductor of the present invention can be relatively large if desired, it is possible to reduce grain boundaries and increase critical current values.

The superconductor of the present invention has Y1-X(Ln), B
a2Cu3O7□, L al-x(Ln)XB a2C
u, o 7-y and combinations thereof, where X has a value within the range of about 0.01 to about 0.2, and y has a value of 0 to about 0
．． 3 and Ln is as defined above. Note that X typically has a value within the range of about 0.02 to about 0.05. Further, y preferably has a value within the range of 0 to about 0.2. The actual values of X and y are determined experimentally and are highly dependent on the desired superconductor.

The superconductor of the present invention contains at least a sufficient amount of orthorhombic crystal structure to impart desired superconductivity.

Generally, the presence of an orthorhombic crystal phase can be confirmed by X-ray diffraction analysis, transmission electron microscopy or polarized light microscopy. Note that the superconductor of the present invention is polycrystalline.

The superconductivity of the superconductor of the present invention can be confirmed by conventional techniques. For example, it can be demonstrated by the magnetic flux repulsion effect (ie Meissner effect). Generally speaking, the superconductor of the present invention is about 77
It has a higher zero resistance transition temperature (ie, the temperature at which electrical resistance ceases to exist). It should be noted that the superconductors of the present invention preferably have a zero resistance transition temperature higher than about 85, and most preferably have a zero resistance transition temperature higher than about 90.

The superconductors of the present invention are useful as conductors for magnets, motors, generators, and power transmission lines.

Examples are shown below to explain the present invention in more detail.

Example 1 1a06g Y2O3,7,65g(')E r20B
, 47, 72 g of CuO and 7a94 g of BaCO
A powder mixture consisting of three components was prepared. This powder mixture was heated at about 950°C for about 20 minutes in air at atmospheric pressure.
The reaction was allowed to take place over a period of time.

The reaction product was then milled in a ball mill for 4 hours, thereby separating the individual particles. As calculated from the results of one-point BET surface area measurement of the sinterable powder thus obtained, the average grain size in terms of sphere, which is an index of relative grain size, was about 0.97 microns.

X-ray diffraction analysis showed that the sinterable powder consisted of a single phase.

Based on the initial mixing ratio, this sinterable powder has Y o,
It was found to consist of aE rQ, 2B a2Cu3o 7-y. Also, based on the results of another study, the content of oxygen (0) was found to be in the range of about 6-7.

A slurry was prepared by mixing approximately 100 g of sinterable powder with hebutane at room temperature. In addition, several drops of an organic substance sold under the trade name Sarkosyl-0 were added as a dispersant.

After the slurry thus obtained was charged into an alumina boat, a magnetic field for alignment of about 40 kOersted was applied at room temperature in air. The alignment magnetic field was continuously applied for about 16 hours, during which time the liquid medium did not evaporate.
The alumina boat containing the compressed body thus obtained was placed in an alumina tube furnace.

The compacts were sintered at approximately 950° C. for 10 hours in a stream of oxygen gas at nearly atmospheric pressure.

Next, the sintered body was cooled to room temperature in a furnace in a flow of oxygen gas under near atmospheric pressure. To be more specific, about 2
The sintered body was cooled to room temperature at a rate of 0°C/hour. Note that the oxygen flow rate was about 1 cubic foot at 7:00.

A sample was obtained by cutting the superconductor thus obtained.

X-ray diffraction analysis of the sample revealed that only the 12B compound phase was present.

Again, as a result of X-ray diffraction analysis, it was found that the crystal grains were aligned such that their C axes were perpendicular to the direction of the alignment magnetic field. Such alignment direction is Er-123
This is the expected alignment direction for the crystal.

Examination of the polished samples by polarized light microscopy revealed the expected twinning in the superconductor phase within the grains. The maximum grain size ranged from about 2 microns to 50 microns.

Note that many of the grains had a maximum dimension of about 10-25 microns. The grain thickness was smaller, often around 5 microns. In the 12B compound,
An anisotropic grain shape in which the C-axis direction is thinner than the other two directions is common.

The sample was found to exhibit superconductivity at 77 as determined by the magnetic flux exclusion effect (ie Meissner effect).

Based on the results of other studies, the superconductors thus obtained are Y o, soE ro, 20B a2Cu3O?
-y, where y has a value of about 0.2 or less, and was found to have an open porosity of about 10% (by volume).

Such superconductors are believed to be useful in magnets, motors, generators, and other applications requiring high current, low loss conductors.

Claims (20)

[Claims] 1. (a) consisting of Y and/or respective oxides of La, Ba and Cu or their precursors, and YBa_2Cu_3O_7_-_y, LaBa_2C
u_3O_7_-_y and combinations thereof (however,
y has a value within the range of 0 to about 1), the matrix forming powders are prepared in proportions to provide a metal oxide composition that matches the composition of one member selected from the group consisting of: b)L
oxide of n (however, Ln is Nd, Eu, Gd, Dy,
(c) the oxide of Ln is an element selected from the group consisting of Ho, Er, Tm, Yb, and combinations thereof) or a precursor thereof; ) about 1 to about 20 of La oxide
(d) at a reaction temperature within the range of about 800° C. to below the melting point of the metal oxide; By heating the mixture, until the reaction temperature is reached, the precursor is decomposed to form the corresponding metal oxide before the mixture is reacted, thereby
_1_-_x(Ln)_xBa_2Cu_3O_7_-
_y, La_1_-_x(Ln)_xBa_2Cu_3
O_7_-_y and combinations thereof, where x has a value in the range of about 0.01 to about 0.2, and y has a value in the range of 0 to about 1
and Ln is as defined above); (e) pulverizing said reaction product to make it sinterable; (f) substantially aligning the sinterable powder along its preferred axis of magnetization by applying an alignment magnetic field to the sinterable powder; (g) thus aligning the sinterable powder; (h) compressing the sinterable powder from about 900°C to form a compact body comprising the sinterable powder substantially aligned along its preferred axis of magnetization; sintering the compact in an oxidizing atmosphere using a temperature in the range below the melting point of the powder to produce a sintered body having an open porosity in the range of 0 to about 20% (by volume). and then (i) cooling said sintered body in an oxidizing atmosphere using a rate such as to produce a superconductor. 2. The method of claim 1, wherein y has a value within the range of 0 to about 0.7. 3. 2. The oxide of Ln is within the range of about 2% to about 5% (by volume) of the oxide of Y and/or La.
Method described. 4. The method of claim 1, wherein the sinterable powder has an average particle size within the range of less than 1 micron to about 10 microns. 5. The method of claim 1, wherein the mixture is reacted in air. 6. 2. The method of claim 1, wherein said reaction temperature is within the range of about 850-1000<0>C. 7. 2. The method of claim 1, wherein the alignment magnetic field is applied to the sinterable powder also during the step of forming a compact from the sinterable powder. 8. Claim 1, wherein the sintering step is carried out in oxygen.
Method described. 9. 2. The method of claim 1, wherein said sintering temperature is within the range of about 900 to about 1000<0>C. 10. 2. The method of claim 1, wherein said cooling step is carried out in oxygen. 11. 2. The method of claim 1, wherein Ln is Er. 12. Y_1_-_x(Ln)_xBa_2Cu_3O
_7_-_y, La_1_-_x(Ln)_xBa_2
Cu_3O_7_-_y and combinations thereof, where x has a value within the range of about 0.01 to about 0.2, y has a value within the range of 0 to about 0.3, and Ln Nd,
a crystal of up to about 5 microns in the long axis direction A polycrystalline superconductor characterized in that it has a grain size and an open porosity of up to about 20% (by volume). 13. 13. The superconductor of claim 12, wherein x has a value within the range of about 0.02 to about 0.05. 14. Claim 1 wherein y has a value within the range of 0 to about 0.2.
2. The superconductor according to 2. 15. 13. The superconductor of claim 12 having an open porosity of about 5 to about 15% (by volume). 16. 13. The superconductor of claim 12 having a grain size of about 2 to about 5 microns in the longitudinal direction. 17. 13. The superconductor of claim 12 having a zero resistance transition temperature greater than about 77K. 18. 13. The superconductor according to claim 12, wherein Ln is Er. 19. Y_1_-_x(Ln)_xBa_2Cu_3O
_7_-_y (where x has a value within the range of about 0.01 to about 0.05, y has a value within the range of 0 to about 0.2, and Ln is Nd, Em, Gd ,Dy,Ho,Er,
It has a composition consisting mainly of elements selected from the group consisting of Tm, Yb and combinations thereof), and is composed of irregular plate-like grains with an irregular outer circumference and a grain size of up to about 5 microns in the long axis direction. and an open porosity of up to about 20% (by volume). 20. The superconductor according to claim 19, wherein Ln is Er.