GB2223489A - Oriented polycrystal superconductor - Google Patents

Description

RD-18,2-1 A ORIENTED POLYCRYSTAL SUPERCONDUCTOR This invention relates to

the production of an oriented polycrystal superconductor by doping YBa 2 Cu 3 0 7-y (Y-123) and/or LaBa 2 CU3 0 7-y (La-123) with an anisotropic rare earth to increase alignability.

The art has made high temperature superconductors with the 123 composition containing Y or most of the lanthanide group from La to Lu. The exception seems to be the lanthanides which have fairly stable 4+ ions -- Ce, Pr, and Th. Most of the work to-date has been done with the Y-123 compound. It has been found that this material has quite anisotropic superconducting properties. In particular the critical current is much higher in the (001) plane than out of the plane. It may be that bulk polycrystalline samples with randomly oriented grains will not exhibit high critical currents due to the misorientation of adjacent grains. If this is the case, it means that any application which requires a superconductor with a high current carrying capacity will require either a single crystal or a polycrystal with a high degree of ordering of the grains. Traditional ceramic forming techniques based on forming a body from a powder and then densifying the body by sintering would be the preferred technique for making bulk superconductors of 123. Unfortunately this results in RD-18,271 random orientations of the grains in the dense body. A simple technique of orienting the grains is desired.

One method which has been demonstrated to result in align ed grains in a polycrystalline sintered body is to use the anisotropic magnetic susceptibility of the is materials. It has been shown that crystals of the 123 materials will align in a magnetic field. Y-123, Dy-123, Nd-123, Sm-123, and Ho-123 align with the c-axis of the crystal parallel to the magnetic field. Eu- 123, Gd-123,

Tm-123, Yb-123; and Er-123 align with the c-axis of the crystal perpendicular to the magnetic field. The magnetic susceptibility of the Yand La-123 compounds are due to the copper ions and the conduction electrons. The magnetic susceptibilities of the Ln-123 compounds where Ln ranges from Pr to Yb, on the other hand have much larger susceptibilities (and anisotropies in the susceptibilities) due to the magnetic moments on the Ln ions. Neither Y or La are magnetic ions.

Briefly stated, the present process comprises providing a combination of matrix-forming powders of an oxide or precursor therefor of Y and/or La, Ba and Cu wherein the metal oxide composition corresponds to the composition of a member selected from the group consisting of YBa 2 CU 3 0 7-y, LaBa 2 Cu 3 0 7-y and a combination thereof wherein y ranges from zero to about 1, providing an additive consisting essentially of an oxide or precursor therefor of Ln where Ln is selected from the group consisting of Nd, Eu, Gd, Dy, Ho, Er, Tm, Yb and a combination thereof and wherein y ranges from zero to about 1, forming a mixture of said matrix-forming powders and additive, said oxide of Ln ranging from about 1% to about 20% by volume of said oxide of Y and/or La, reacting said mixture at a temperature ranging from greater than about 800C to below the melting -2.RD-18,2'I point of said metal oxides to produce a reaction product selected from the group consisting of Y 1-x (Ln) X Ba 2 Cu 3 0 7-ye La 1-x (Ln) X Ba 2 Cu 3 0 7-y and a combination thereof where x ranges from about 0.01 to about 0.2 and where y ranges from zero to about 1, said precursor decomposing below said reaction temperature producing said oxide, comminuting said reaction product to produce a sinterable powder, applying an aligning magnetizing field to said sinterable powder to substantially align said sinterable powder substantially along its preferred axis of magnetization, forming the resulting aligned material into a compact wherein said sinterable powder is substantially aligned along its preferred axis of magnetization, sintering said compact in an oxidizing atmosphere at a temperature ranging from about 9000C to below the melting point of said sinterable powder producing a sintered body having an open porosity ranging from zero to about 20% by volume of said body, and cooling said body in an oxidizing atmosphere at a rate which produces a superconductive body. 20 Generally, in carrying out the present process, yttrium oxide and/or lanthanum oxide, barium carbonate and copper oxide are used as the matrix-forming powders. They are formulated to have a metal oxide composition which corresponds to the composition of a member selected from the group consisting of YBa 2 Cu 3 0 7-y, LaBa 2 Cu 3 0 7-y and a combination thereof wherein y ranges from zero to about 1, frequently from zero to about 0.7. The additive powder generally is an oxide of Ln where Ln is a rare earth selected from the group consisting of Nd, Eu, Gd, Dy, Ho, Er, Tm, Yb and a combination thereof. Generally, the oxide additive is used in an amount ranging from about 1% to about 20%, frequently from about 2% to about 5%, by volume of the volume of yttrium oxide, or RD-18,271 lanthanum oxide, or a combination thereof. The particular amount of additive is determined empirically.

If desired, a particulate inorganic precursor of the reactant oxides can be used. The precursor should decompose completely to form the oxide and by-product gas or gases leaving no contaminants in the reacted mass. Barium carbonate is a useful precursor for barium oxide. The precursor should be used in an amount sufficient to produce the respective oxide in the required amount.

The reactant oxides or precursors therefor should be of a size which allows the reaction to take place. Generally, these powders are used in the particle size range in which they are available commercially, which ordinarily ranges from submicron up to about 100 microns. The powders should be free of large, hard aggregates, i.e. significantly above 100 microns in size, which might survive the mixing process and prevent sufficient reactant contact for satisfactory reaction rates.

The matrix-forming powders and additive are admixed to form a mixture which preferably is uniform or substantially uniform in order to produce a reaction product which preferably is uniform or substantially uniform. Mixing of the powders can be carried out by a number of conventional techniques such as, for example, ball milling. The mixture of matrix-forming powders and additive is reacted to produce the present reaction product. The mixture is reacted in an oxidizing atmosphere generally at a temperature ranging from greater than about 8000C to below the melting point of the metal oxides. -Frequently, reaction 30 temperature ranges from about 850"C to 1000C gr from about 9000C to 9500C. Reaction time is determined empirically. Generally, the reaction product is cooled in an oxidizing atmosphere to about room temperature. Generally, the RD-12,2-11 oxidizing atmosphere, i.e. the atmosphere for carrying out the reaction as well as for cooling the reaction product, is comprised of at least about 1% by volume of oxygen and the remainder of the atmosphere is a gas which has no significant deleterious effect on the reaction product. Representative of such gases is nitrogen or a noble gas suc.-. as argon or helium. Preferably, the oxidizing atmosphere is comprised of oxygen or air. Generally, the oxidizing atmosphere is at about atmospheric pressure.

The reaction product is selected from the group consisting of Y 1-X (Ln) X Ba 2 Cu 3 0 7-y. La 1-x (Ln) X Ba 2 Cu 3 0 7-y and a combination thereof where x ranges from about 0.01 to about 0.2 and where y ranges from zero to about 1. Frequently, in the reaction product, x ranges from about 0.02 to about 0.05 and y rariges from 0 to about 0.7.

The reaction product is comminuted to produce the desired sinterable powder. Comminution can be carried out in a conventional manner such as, for example, by milling. Generally, the sinterable powder has an average particle size ranging from submicron to about 10 microns, frequently from about 0. 1 micron to about 5 microns, or from about 0.2 micron to about 4 microns. Average particle size can be determined by conventional techniques.

In carrying out the present process, an aligning magnetizing field is applied preferably at room temperature to the sinterable powder to align, or at least substantially align, it along its preferred axis of magnetization which is parallel or perpendicular to the "C" axis. Specifically, the sinterable powder will align along the preferred axis of magnetization of the rare earth of the additive. The aligning magnetizing field is applied to the sinterable powder before it is formed into a compact, and preferably, it is maintained while it is being formed into a compact.

RD-18---.--- Generally, the aligning magnetizing field ranges from about 1 kiloersted to about 100 kiloersteds and is determined empirically.

A number of conventional procedures can be used to he form the sinterable powder into a compact. For example, sinterable powder can be extruded, injection moided, die-pressed, slip cast or tape cast to produce the compact of desired shape. Specifically, in one technique, an aligning magnetizing field can be applied to the sinterable powder, frequently in the form of a layer, in a press and the aligned material can be compressed, preferably in the magnetizing field, to produce the compact.

In another technique, the sinterable powder is dispersed in an organic liquid such as heptane to form a is slurry, the slurry is placed in a suitable firing container such as an alumina boat, an aligning magnetizing field is applied to the slurry-and maintained while the liquid evaporates away thereby forming the present aligned compact in the container which is then fired to produce the present sintered product.

In another technique, a slurry of the sinterable powder is slip cast in a conventional manner in a porous mold in an aligning magnetizing field thereby forming the present aligned compact.

In still another technique, a slurry of the sinterable powder is cast into tape in an aligning magnetizing field.

Lubricants, dispersants, binders or similar form-promoting materials useful in producing the compact can be admixed with the sinterable powder. Such materials are well known in the art and can be used in a conventional manner with the particular amount thereof being determined empirically. Generally, they are organic preferably of the -6 RD- 18, 271 type which evaporate or decompose on heating at relatively low temperatures, preferably below 500'C, leaving no residue or no significant residue. The form-promoting material should permit magnetic alignment of the sinterable powder and should have no significant deleterious effect in the present process.

The compact should have a density at least sufficient to produce the present sintered body. Preferably, it has a density of at least about 45% of its theoretical density to promote densification during sintering.

Sintering of the compact is carried out in an oxidizing atmosphere which generally is about atmospheric pressure. The oxidizing atmosphere should be at least sufficiently oxidizing to pr6duce a sintered body wherein the 0, i.e. oxygen, component has a value of at least about 6.0. Generally, the sintering, i.e. firing, atmosphere contains at least about 1% by volume of oxygen and the remainder of the atmosphere should be a gas which has no significant deleterious effect on the sintered product. Representative of such gases is nitrogen or a noble gas such as argon or helium. Most preferably, the sintering atmosphere is comprised of oxygen.

Sintering is carried out at a temperature ranging from about 900'C to below the melting temperature of the oxide constituents of the body. Generally, sintering temperature ranges from about 9001C to about 10000C, and typically it ranges from about 9500C to about 9750C. The particular sintering temperature is determined empirically and depends largely on particle size, density of the compact and final density desired in the sintered product. Generally, higher sintering temperatures produce sintered bodies of higher density and larger grain size.

RD-18,2-1 Sintering time can vary and is determined empirically. Longer sintering times generally produce sintered bodies with larger grains. Generally, sintering time ranges from about two hours to eight hours.

The sintered body is cooled in an oxidizing atmosphere generally at about atmospheric pressure at a rate which produces the present superconductive body. The cooling schedule can vary and is determined empirically. Generally, the cooling oxidizing atmosphere contains at least about 20% by volume of oxygen and the remaining gas should have no significant deleterious effect on the superconductive product. Preferably, the oxidizing atmosphere is air but more preferably it is oxygen.

Specifically, during the cooling procedure, generally at a temperature ranging from about 7000C to about 4000C, the sintered body should be cooled at a rate sufficient to produce the orthorhombic crystal structure in an amount at least sufficient to produce the superconductive body. Generally, in this temperature range of about 700'C to about 4000C additional oxygen is incorporated into the body. Sufficient oxygen should be incorporated in the body to permit formation of the required orthorhombic crystal structure. Cooling of the body from about 4000C can be at a more rapid rate, but not so fast as to fracture the body by thermal shock. The body is usually cooled to room temperature, i.e. from about 20C to about 300C. The present process has no significant effect on the amounts of the other, i.e. non-oxygen, components of the body. 30 The sintered body and the resulting. superconductive body have the same density or porosity. The body may have some closed porosity and generally has open porosity. Preferably, pores are small, preferably less than R.r,'- 18, 22 7 1 one micron, and sufficiently distributed in the body so that they have no significantly deleterious effect on mechanical properties. Porosity can be determined by standard metallographic techniques, such as, for example, optically 5 examining a polished cross section of the body.

By closed porosity, it is meant herein closed pores or voids in the sintered body, i.e. pores not open to the surface of the body and therefore not in contact with the ambient atmosphere. Generally, closed porosity ranges from 0 to about 10%, preferably it is less than about 5%, or less than about 1% by volume of the body.

/0 By open porosity, it is meant herein pores or voids which are open to the surface of the sintered body, thereby making the interior surfaces accessible to the is ambient atmosphere.

The sintered body should have sufficient surface area to permit production of the superconductive body and this is determined empirically. Specifically, the sintered body, during cooling thereof in an oxidizing atmosphere, should have at least sufficient surface area for contact with oxygen to allow production of the superconductive body. Generally, a portion of the surface area of the sintered body is provided by its open porosity. For a very thin body, open porosity may not be needed. Generally, the present superconductive body has an open porosity ranging from 0 to about 20%, frequently from about 2% to about 20%, or from about 5% to about 15%, by volume of the body.

Typically, open porosity ranges from about 7% to about 10% by volume of the body.

Generally, the present superconductive sintered body is comprised of grains which are disc-like irregular plates or polygons, i.e. the edge of the plate is irregular.

Grain size in the longest direction is generally at least -g- RD - 18, 2_ - 1 about 1 micron and can range widely depending largely on the size of the sinterable powder and sintering conditions. For example, grain size in the longest direction can range from about 1 micron to about 100 microns. Frequently, grain size in the longest direction ranges from about 1 micron up to about 5 microns, or from about 2 microns up to about 5 microns, or from about 2 microns to about 4 microns.

The orientation of the grains in the superconductive body depends largely on the orientation of the sintering powder in the process. Generally, when the sintering powder aligns with its c-axis parallel to the orienting magnetic field, substantially all of the plates in the resulting superconductive body are stacked together. Generally, when the powder aligns with its c-axis - perpendicular to the orientihg magnetic field, the plates in the resulting superconductive body can be oriented in any direction, i.e. they can be arranged in 360 degrees, except that the c-direction is in a common plane perpendicular to the orienting magnetic field.

In the prior art, the Y-123 and La-123 sintered bodies were comprised of non-aligned grains and were restricted to a grain size generally of about 2 to 3 microns since a larger grain size would cause the material to crack on heating. In contrast to the prior art, the present superconductive body has a significant anisotropic thermal expansion since its grains are aligned. As a result, there is no restriction on grain size. Generally, grain boundaries are weak links and reduce critical current. Since the grains of the present superconductive body can. be relatively large if desired, it can have fewer.grain boundaries thereby enhancing critical current.

The present superconductive body has a composition selected from the group consisting of Y 1-X (Ln) X Ba 2 Cu 3 0 7-y11 r La 1-x (Ln) x Ba 2 Cu 3 0 7-y and a combination thereof where x ranges from about 0.01 to about 0.2 and where y ranges from zero to about 0.3. Frequently, x ranges from about 0.02 to about 0.05. Preferably, y ranges from zero to about 0.2. The particular values of x and y are determined empirically and depend largely on the particular superconductive body desired.

The present superconductive body contains the orthorhombic crystal structure in an amount at least sufficient to give the desired superconductivity. Generally, the presence of the orthorhombic phase can be determined by x-ray diffraction analysis, transmission electron microscopy, or polarized light microscopy. The -superconductive body is polycrystalline.

is Superconductivity nf the present body can be determined by conventional techniques. For example, it can be demonstrated by magnetic flux exclusion, the Meissner effect. Generally, the present superconductive body has a zero resistance transition temperature, i.e. a temperature below which there is no electrical resistance, greater than about 77K, preferably at least about 85K and most preferably higher than about 90K.

The present superconductive body is useful as a conductor for magnets, motors, generators, and power transmission lines.

The invention is further illustrated by the following example:

EXAMPLE 1: A powder mixture comprised of 18.06 grams of Y 2 0 3' 30 7.65 grams of Er 2 0 3' 47.72 grams of CuO, and 78.94 grams of BaCO 3 was reacted in air at about atmospheric pressure for about 20 hours at about 9500C.

RD-18,2'71 The reaction product was then ball milled four hours to separate it into particles. From single point BE], surface area measurement of the resulting sinterable powder, a spherical equivalent average crystal diameter of about 0.97 microns was calculated which is an indication of its relative size.

X-ray diffraction indicated the material to be single phase.

From the initial mix, it was known that the powder was comprised of Y. a Er. 2 Ba 2 Cu 3 0 7-y and from other work it was known that 0 falls in the range of from about 6 to 7.

About 100 grams of the sinterable powder were admixed with heptane at room temperature to form a slurry. A few drops of an organic material sold under the trademark Sarkosyl-O was added as a dispersing agent.

The resulting slurry was placed in an alumina boat and an aligning magnetizing field of about 40 kiloersteds was applied to the slurry at room temperature in air. The magnetizing field was maintained for about 16 hours during which time the liquid had evaporated away. The alumina boat supporting the resulting compact was placed in an alumina tube furnace.

The compact was sintered in flowing oxygen at about atmospheric pressure at a temperature of about 9500C for 10 hours. The sintered body was then cooled in the furnace in flowing oxygen at about atmospheric pressure to room temperature. Specifically, it was cooled at a rate of about 200C per hour to room temperature. The oxygen flow rate was about I cubic foot per hour. 30 The resulting superconductive sintered body was cut into sample pieces. X-ray diffraction of a sample showed only the presence of a 123 phase.

RD- 18, 2 71 X-ray diffraction analysis of the sample showed the grains to he aligned with their c-axis perpendicular to the direction of the magnetic field during fabrication. This is the expected direction for alignment of Er- 123 crystals.

Examination of polished samples by polarized light microscopy showed the twins within the grain expected for the superconductivity phase. Grains varying from about 2 microns up to 50 microns in the longest dimension were seen. Many of the grains were about 10 to 25 microns in longest dimension. Grain thicknesses were smaller with many about 5 microns. Anisotropic grain shapes with the c-direction thinner than the other 2 directions are common in -123 materials.

The sample showed tuperconductivity at 77K as demonstrated by magnetic flux exclusion, the Meissner effect From other work, it was known that the superconductive sintered body was comprised of Y. so Er. 20 Ba 2 CU 3 0 7-y1f where y is about 0.2 or less and that it had an open 20 porosity of about 10% by volume.

This superconductive body would be useful as a conductor for a magnet, motor, generator, or any other application where a high current, low loss conductor is desired.

RD-18,27

Claims (23)

1. A process for producing a superconductive sintered body which comprises providing a combination of matrix-forming powders of an oxide or precursor therefor of Y and/or La, Ba and Cu wherein the metal oxide composition corresponds to the composition of a member selected from the group consisting of YBa 2 Cu 3 0 7-y, LaBa 2 Cu 3 0 7-y and a combination thereof wherein y ranges from zero to about 1, providing an additive consisting essentially of an oxide or precursor therefor of Ln where Ln is selected from the group consisting of Nd, Eu, Gd, Dy, Ho, Er, Tm, Yb and a combination theeof and wherein y ranges from zero to about 1, forming a mixture of said' matrix- forming powders and additive, said oxide of Ln ranging from about 1% to about 20% by volume of said oxide of Y and/or La, reacting said mixture at a temperature ranging from greater than about SOOOC to below the melting point of said metal oxides to produce a reaction product selected from the group consisting of Y 1-X (Ln) x Ba 2 CU 3 0 7-y, La I- x (Ln) x Ba 2 Cu 3 0 7-y and a combination thereof where x ranges from about 0.01 to about 0.2 and where y ranges from zero to about 1, said precursor decomposing below said reaction temperature producing said oxide, comminuting said reaction product to produce a sinterable powder, applying an aligning magnetizing field to said sinterable powder to substantially align said sinterable powder substantially along its preferred axis of magnetization, forming the resulting aligned material into a compact wherein said sinterable powder is substantially aligned along its preferred axis of magnetization, sintering said compact in an oxidizing atmosphere at a temperature ranging from about 900C to below the melting point of said sinterable powder producing a sintered body having an open porosity ranging from zero to about 20% by volume of said body, and cooling said body in an oxidizing atmosphere at a rate which produces a 35 superconductive body.

2. The process according to claim 1, wherein y ranges from zero to about 0.7.

3. The process according to claim 1, wherein said oxide of Ln ranges from about 2% to about 5% by volume of said oxide of Y and/or La.

4. The process according to claim 1, wherein said sinterable powder has an ave-rage particle size ranging from submicron to about 10 microns.

5. The process according to claim 1, wherein said mixture is reacted in air.

6. The process according to claim 1, wherein said reaction temperature ranges from about 85CC to 10OCC.

7. The process according to claim 1, wherein said aligning magnetizing field is also applied to said sinterable powder during said forming into a compact.

8. The process according to claim 1, wherein said sintering is carried out in oxygen.

9. The process according to claim 1, wherein said sintering temperature ranges from about 900C to about 10000C.

-Is- RD- 18, 2 71 -1

10. The process according to claim 1, wherein said cooling is carried out in oxygen.

_

11. The process according to claim 1, wherein said Ln is Er.

12. A superconductive polycrystalline body comprised of composition selected from the group consisting of Y 1-X (Ln) X Ba 2 CU 3 0 7-y. La 1x (Ln) X Ba 2 Cu 3 0 7-y and a combination thereof where x ranges from about.01 to about 0.2, where y ranges from zero to about 0.3, and where Ln is selected from the group consisting of Nd, Eu, Gd, Dy, Ho, Er, Tm, Yb and a combination thereof, said body having a grain.size in the longest direction of up to about 5 microns, said body having anopen porosity ranging up to 10 about 20% by volume of said body.

13. The superconductive body according to claim 12, wherein x ranges from about 0.02 to about 0.05.

14. The superconductive body according to claim 12, wherein y ranges from zero to about 0.2.

15. The superconductive body according to claim 12, wherein said body has an open porosity ranging from about 5% to about 15% by volume.

16. The superconductive body according to claim 12, wherein said grain size in the longest direction ranges from about 2 microns up to about 5 microns.

RD-18,271

17. The superconductive body according to claim 12, having a zero resistance transition temperature greater than about 77K.

18. The superconductive body according to claim 12, wherein Ln is Er.

19. A superconductive polycrystalline body consisting essentially of Y 1X (Ln) X Ba 2 Cu 3 0 7-y where x rances from about 0.01 to about 0.05, where y ranges from zero to about 0.2, and where Ln is selected from the group consisting of Nd, Eu, Gd, Dy, I.1o, Er, Tm, Yb and a combination thereof, said body being comprised of plates with irregular edges wherein said plates range in the longest direction up to about 5 microns, said body having an open porosity ranging up to about 20% by volume of said body.

20. The superconductive body according to claim 19, wherein Ln is Er.

21. A Process for producing a superconductive sintered body as claimed in claim 1 substantially as herein before described in Example 1.

22. A superconductive sintered body when produced by a process as claimed in any one of claims 1 to 11 and 21.

23. A superconductive body as claimed in claim 12 substantially as hereinbefore described in Example 1.

Publishe 1990 a, The Patent O;5ice.Siate House. 66 71 High H.-lborr.. Londor.WC1R4TP Further copies maybe obtairedfrL,,-TPePaientOt5ce ales Branch. S Mar:; Cray. Orpingtcr,. Kent BRC 3RD- Printed by MjItiplex techniques I.d, St Mai-j Cray- Kent. Con 1 E-