EP0366721A1 - Improved process for making 90 k superconductors - Google Patents

Abstract

Improved method of preparing a superconductive composition having the formula MBa2Cu3Ox, in which M is selected from the group consisting of Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu, x ranging from about 6.5 to about 7.0, said composition having a superconductive transition temperature of about 90 K. Said method essentially consists in heating a preliminary powder in an atmosphere containing oxygen, to a temperature ranging from about 875 ° C to about 950 ° C for a time sufficient to form MBa2Cu3Oy, where '' y '' ranges from about 6.0 to about 6.4, and to maintain the formulation MBa2Cu3Oy in an atmosphere containing oxygen while cooling for a sufficient time to obtain the desired product. Said preliminary powder is prepared by (a) mixing M2O3, Ba (OH) 2.8H2O and CuO powders in an atomic ratio of M: Ba: Cu of approximately 1: 2: 3, or (b) by making of an aqueous solution of M (NO3) 3, Ba (NO3) 2 and Cu (NO3) 2 in an atomic ratio of M: Ba: Cu of approximately 1: 2: 3. Then enough citric acid monohydrate is added to the solution obtained in order to transform the metals in question into their corresponding citrates, then the resulting solution is spray-dried in order to obtain the preliminary powder.

Description

TITLE IMPROVED PROCESS FOR MAKING 90 K SUPERCONDUCTORS

BACKGROUND OF, THE INVENTION Field of the Invention This invention relates to an improved process for making rare earth-barium-copper oxide superconductors with transition temperatures above 90 K.

Description of Related Art Bednorz and Muller, Z. Phys. B64, 189-193

(1986), disclose a superconducting phase in the La-Ba-Cu-O system with a superconducting transition temperature of about 35 K. Samples were prepared by a coprecipitation method from aqueous solutions of Ba-, La- and Cu-nitrate in their appropriate ratios. An aqueous solution of oxalic acid was msed as the precipitant.

Chu et al., Phys. Rev, Lett. 58, 405-407 (1987), report detection of an apparent superconducting transition with an on set temperature above 40. K under pressure in the LΑ-Ra-Cu -O compound system synthesized directly from a solid-state reaction of La₂O₃, CuO and BaCO₃ followed by a decomposition of the mixture in a reduced atmosphere. Chu et al., Science 235, 567-569 (1987) , disclose that a superconducting transition with an onset temperature of 52.5 K has been observed under hydrostatic pressure in compounds with nominal compositions given by (La_{0 .9}Ba_{0 .1}) ₂Cuo_4-y' where Y is undetermined. They state that the K ₂NiF₄ layer structure has been proposed to be responsible for the high-temperature superconductivity in the La-Ba-Cu-O system ( LBCO). They further state that, however, the small diamagnetic signal, in contrast to the presence of up to 100% K₂NiF₄ phase in their samples, raises a question about the exact location of superconductivity in LBCO.

Cava et al., Phys. Rev. Lett. 58, 408-410 (1987), disclose bulk superconductivity at 36 K in La_1.8Sr_{0 .2}CuO₄ prepared from appropriate mixtures of high purity La(OH)₃, SrCO₃ and CuO powders, heated for several days in air at 1000ºC in quartz crucibles. Rao et al., Current Science 56, 47-49 (1987), discuss superconducting properties of compositions which include La_1.8Sr_{0 .2}CuO₄, La_{1 .85}Ba_0.15CuO₄, La_1.8Sr_{0 .2}CuO₄,

(La_1-xpr_x)_2-ySr_yCuO₄, and (La_1.75Eu_0.25)Sr_0.2CuO₄. Bednorz et al., Europhys. Lett. 3, 379-384 (1987), report that susceptibility measurements support high-T_c superconductivity in the Ba-La-Cu-O system. in general, in the -La-Ba-Cu-O system, the superconducting phase has been identified as the composition La_1-x(Ba,Sr,Ca)_x CuO_4-y with the tetragonal K₂NiF₄-type structure and with x typically about 0.15 and y indicating oxygen vacancies. Wu et al., Phys. Rev. Lett. 58, 908-910

(1987), disclose a superconducting phase in the Y-Ba-Cu-O system with a superconducting transition temperature between 80 and 93 K . The compounds investigated were prepared with nominal composition (Y_1-xBa_x)₂CuO_4-y and x = 0.4 by a solid-state reaction of appropriate amounts of Y₂O_3' BaCO₃ and CuO in a manner similar to that described in Chu et al., Phys. Rev. Lett. 58, 405-407 (1987). Said reaction method comprises more specifically heating the oxides in a reduced oxygen atmosphere of 2x10 bars (2 Pa) at 900°C for 6 hours. The reacted mixture was pulverized and the heating step was repeated. The thoroughly reacted mixture was then pressed into 3/16 inch (0.5 cm) diameter cylinders for final sintering at 925°C for 24 hours in the same reduced oxygen atmosphere. The material prepared showed the existence of multiple phases.

Hor et al., Phys. Rev. Lett. 58, 911-912 (1987), disclose that pressure has only a slight effect on the superconducting transition, temperature of the Y-Ba-Cu-O superconductors described by Wu et al., supra.

Sun et al., Phys. Rev. Lett. 58, 1574-1576 (1987), disclose the results of a study of Y-Ba-Cu-O samples exhibiting superconductivity with transition temperatures in the 90 K range. The samples were prepared from mixtures of high-purity Y₂O₃ , BaCO₃ and CuO powders. The powders were premixed in methanol or water and subsequently heated to 100ºC to evaporate the solvent. Two thermal heat treaments were used. In the first, the samples were heated in Pt crucibles for 6 hours in air at 850°C and then for another 6 hours at 1000°C. After the first firing, the samples were a dark-green powder, and after the second firing, they became a very porous, black solid. In the second method, the powders were heated for 8-10 hours at 1000ºC, ground and then cold pressed to form disks of about 1 cm diameter and 0.2 cm thickness. The superconducting properties of samples prepared in these two ways were similar. X-ray diffraction examination of the samples revealed the existence of multiple phases.

Cava et al., Phys. Rev. Lett. 58, 1676-1679 (1987), have identified this superconducting Y-Ba-Cu-O phase to be orthorhombic, distorted, oxygen-deficient perovskite YBa₂Cu₃O_9-δ where δ is about 2.1, and have presented the X-ray diffraction powder pattern and lattice parameters for the phase. The single-phase YBa₂Cu₃O_9-δ was prepared in the following manner. BaCO₃, Y₂O₃ and CuO were mixed, ground and then heated at 950°C in air for 1 day. The material was then pressed into pellets, sintered in flowing O₂ for 16 hours and cooled to 200°C in O₂ before removal from the furnace. Additional overnight treatment in O₂ at 700°C was found to improve the observed properties. Takita et al., Jpn. J. Appl. Phys. 26,

L506-L507 (1987), disclose the preparation of several Y-Ba-Cu compositions with superconducting transitions around 90 K by a solid-state reaction method in which a mixture of Y₂O₃, CuO, and BaCO₃ was heated in an oxygen atmosphere at 950βC for more than 3 hours. The reacted mixture was pressed into 10 mm diameter disks for final sintering at 950° or 1000°C for about 3 hours in the same oxygen atmosphere.

Takabatake et al., Jpn. J. Appl. Phys. 26, L502-L503 (1987), disclose the preparation of samples of Ba_1-xY_xCuO_3-z (x = 0.1, 0.2, 0.25, 0.3, 0.4, 0.5,

0.6, 0.8 and 0.9) from the appropriate mixtures of BaCO₃, Y₂O₃ and CuO. The mixture was pressed into a disc and sintered at 900ºC for 15 hours in air. The sample with x = 0.4 exhibited the sharpest superconducting transition with an onset near 96 K.

Syono et al., Jpn. J. Appl. Phys. 26, L498-L501 (1987), disclose the preparation of samples of superconducting Y_0.4Ba_0.6CuO_2.22 with T_c higher than 88 K by firing mixtures of 4N Y₂O₃, 3N BaCO₃ and 3N CuO in the desired proportions. The mixtures were prefired at 1000°C for 5 hours. They were ground, pelletized and sintered at 900°C for 15 hours in air and cooled to room temperature in the furnace. They also disclose that almost equivalent results were also obtained by starting from concentrated nitrate solution of 4N Y₂O₃, GR grade Ba(NO₃)₂ and Cu(NO₃)₂.

Takayama-Muromachi et al., Jpn. J. Appl. Phys. 26, L476-L478 (1987), disclose the preparation of a series of samples to try to identify the superconducting phase in the Y-Ba-Cu-O system. Appropriate amounts of Y₂O₃, BaCO₃ and CuO were mixed in an agate mortar and then fired at 1173±2 K for 48-72 hours with intermediate grindings. X-ray diffraction powder patterns were obtained. The suggested composition of the superconducting compound is Y_1-xBa_xCuO_y where 0.6<x<0.7.

Hosoya et al., Jpn. J. Appl. Phys. 26, L456-L457 (1987), disclose the preparation of various superconductor compositions in the L-Ba-Cu-O systems where L = Tm, Er, Ho, Dy, Eu and Lu. Mixtures of the proper amounts of the lanthanide oxide (99.9% pure), CuO and BaCO₃ were heated in air. The obtained powder specimens were reground, pressed into pellets and heated again. Hirabayashi et al., Jpn. J. Appl. Phys. 26,

L454-L455 (1987), disclose the preparation of superconductor samples of nominal composition

Y_1/3Ba_2/3CuO_3-x by coprecipitation from aqueous nitrate solution. Oxalic acid was used as the precipitant and insoluble Ba, Y and Cu compounds were formed at a constant pH of 6.8. The decomposition of the precipitate and the solid-state reaction were performed by firing in air at 900°C for 2 hours. The fired products were pulverized, cold-pressed into pellets and then sintered in air at 900°C for 5 hours. The authors found that the the sample was of nearly single phase having the formula Y₁Ba₂Cu₃O₇. The diffraction pattern was obtained and indexed as having tetragonal symmetry. Ek i no et al., Jpn. J. Appl. Phys. 26,

L452-L453 (1987), disclose the preparation of a superconductor sample with nominal composition Y_1.1Ba_0.9CuO_4-y. A Prescrihed amount of powders of Y₂O₃, BaCO₃ and CuO was mixed for about an hour, pressed under 6.4 ton/cm² (14 MPa) into pellet shape and sintered at 1000°C in air for 3 hours.

Akimitsu et al., Jpn. J. Appl. Phys. 26,

L449-L451 (1987), disclose the preparation of samples with nominal compositions represented by

(Y_1-xBa_x)₂CuO_4-y. The sp cecimens were prepcared by mixing the appropriate amounts of powders of Y₂O₃, BaCO₃ and CuO. The resulting mixture was pressed and heated in air at 1000°C for 3 hours. Some samples were annealed at appropriate temperatures in O₂ or CO₂ for several hours. The authors noted that there seemed to be a tendency that samples annealed in O₂ showed a superconducting transition with a higher onset temperature but a broader transition than non-annealed samples.

Semba et al., Jpn. J. Appl. Phys. 26, L429-L431 (1987), disclose the preparation of samples of Y_xBa_1-xCuO_4-d where x = 0.4 and x = 0.5 by the solid state reaction of BaCO₃, Y₂O₃ and CuO. The mixtures are heated to 950°C for several hours, pulverized, and then pressed into disk shape. This is followed by the final heat treatment at 1100ºC in one atmosphere 02 gas for 5 hours. The authors identified the phase that exhibited superconductivity above 90 K as one that was black with the atomic ratio of Y:Ba:Cu of 1:2:3. The diffraction pattern was obtained and indexed as having tetragonal symmetry.

Hatano et al., Jpn. J. Appl. Phys. 26, L374-L376 (1987), disclose the preparation of the superconductor compound Ba_{0 .7}Y_{0 .3}Cu₁O_x from the appropriate mixture of BaCO₃ (purity 99.9%), Y₂O₃ (99.99%) and CuO (99.9%). The mixture was calcined in an alumina boat heated at 1000°C for 10 hours in a flowing oxygen atmosphere. The color of the resulting well-sintered block was black. Hikami et al., Jpn. J. Appl. Phys. 26, L347-L348 (1987), disclose the preparation of a Ho-Ba-Cu oxide, exhibiting the onset of superconductivity at 93 K and the resistance vanishing below 76 K, by heating a mixture of powders Ho₂O₃, BaCO₃ and CuO with the composition Ho:Ba:Cu = 0.246:0.336:1 at 850ºC in air for two hours. The sample was then pressed into a rectangular shape and sintered at 800°C for one hour. The sample looked black, but a small part was green.

Matsushita et al., Jpn. J. Appl. Phys. 26, L332-L333 (1987), disclose the preparation of

Ba_0.5Y_0.5Cu₁O_x by mixing appropriate amounts of BaCO₃ (purity 99.9%), Y₂O₃ (99.99%) and CuO ( 99. 9% ) . The mixture was calcined at 1000°C for 11 hours in a flowing oxygen atmosphere. The resultant mixture was then pulverized and cold-pressed into disks. The disks were sintered at 900βC for 4 hours in the same oxygen atmosphere. The calcined powder and disks were black. A superconducting onset temperature of 100 K was observed. Maeno et al., Jpn. J. Appl. Phys. 26,

L329-L331 (1987), disclose the preparation of various Y-Ba-Cu oxides by mixing powders of Y₂O₃, BaCO₃ and

CuO, all 99.99% pure, with a pestle and mortar. The powders were pressed at 100 kgf/cm² (98x10⁴ Pa) for 10-15 minutes to form pellets with a diameter of 12 mm. The pellets were black. The heat treatment was performed in two steps in air. First, the pellets were heated in a horizontal, tubular furnace at 800°C for 12 hours before the heater was turned off to cool the pellets in the furnace. The pellets were taken out of the furnace at about 200°C. About half the samples around the center of the furnace turned green in color, while others away from the center remained black. The strong correlation with location suggested to the authors that this reaction occurs critically at about 800ºC. The pellets were then heated at 1200°C for 3 hours and then allowed to cool. Pellets which turned light green during the first heat treatment became very hard solids whereas pellets which remained black in the first heat treatment slightly melted or melted down. Three of the samples exhibited an onset of superconductivity above 90 K.

Iguchi et al., Jpn. J. Appl. Phys. 26, L327-L328 (1987), disclose the preparation of superconducting Y_{0 .8}Ba_{1 .2}CuO_y by sintering a stoichiometrical mixture of Y₂O₃, BaCO₃ and CuO at 900°C and at 1000°C in air.

Hosoya et al., Jpn. J. Appl. Phys. 26, L325-L326 (1987), disclose the preparation of various superconducting specimens of the L-M-Cu-O systems where L = Yb, Lu, Y, La, Ho and Dy and M = Ba and a mixture of Ba and Sr by heating the mixtures of appropriate amounts of the oxides of the rare earth elements (99.9% pure), CuO, SrCO₃ and/or BaCO₃ in air at about 900ºC. Green powder was obtained. The powder samples were pressed to form pellets which were heated in air until the color became black. Takagi et al., Jpn. J. Appl. Phys. 26, L320-L321 (1987), disclose the preparation of various Y-Ba-Cu oxides by reacting mixtures containing the prescribed amounts of powders of Y₂O₃, BaCO₃ and CuO at 1000°C, remixing and heat-treating at 1100°C for a few to several hours. An onset temperature of superconductivity at 95 K or higher was observed for a specimen with the nominal composition of

(Y_0.9Ba_0.1)CuO_y.

Hikami et al., Jpn. J. Appl. Phys. 26,

L314-L315 (1987), disclose the preparation of compositions in the Y-Ba-Cu-O system by heating the powde rs of Y₂O₃ , BaCO₃ and CuO to 800°C or 900°C in air for 2-4 hours, pressing into pellets at 4 kbars (4x10⁵ Pa) and reheating to 800°C in air for 2 hours for sintering. The samples show an onset of superconductivity at 85 K and a vanishing resistance at 45 K. Bourne et al., Phys. Letters A 120, 494-496

(1987), disclose the preparation of Y-Ba-Cu-O samples of Y_2-xBa_xCuO₄ by pressing finely ground powders of Y₂O₃, BaCO₃ and CuO into pellets and sintering the pellets in an oxygen atmosphere at 1082°C. Superconductivity for samples having x equal to about 0.8 was reported.

Moodenbaugh et al., Phys. Rev. Lett. 58, 1885-1887 (1987), disclose superconductivity near 90 K in multiphase samples with nominal composition Lu_1.8Ba_{0 .2}CuO₄ prepared from dried Lu₂O₃, high-purity BaCP₃ (presumably BaCO₃), and fully oxidized CuO. These powders were ground together in an agate mortar and then fired overnight in air at 1000°C in Pt crucibles. This material was ground again, pelletized, and then fired at 1100°C in air for 4-12 hours in Pt crucibles. Additional samples fired solely at 1000°C and those fired at 1200°C show no signs of superconductivity.

Hor et al., Phys. Rev. Lett. 58, 1891-1894 (1987), disclose superconductivity in the 90 K range in ABa₂Cu₃O_6+x with A = La, Nd, Sm, Eu, Gd, Ho, Er, and Lu in addition to Y. The samples were synthesized by the solid-state reaction of appropriate amounts of sesquioxides of La, Nd, Sm, Eu, Gd, Ho, Er, and Lu, BaCO₃ and CuO in a manner similar to that described in Chu et al., Phys. Rev. Lett. 58, 405 (1987) and Chu et al., Science 235, 567 (1987).

SUMMARY OF THE INVENTION This invention provides an improved process for preparing superconducting compositions having the formula MBa₂Cu₃O_x wherein M is selected from the group consisting of Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu; x is from about 6.5 to about 7.0; said composition having a superconducting transition temperature of about 90 K; said process ponsiεting essentially of heating a precursor powder in an oxygen-containing atmosphere at a temperature from about 875°C to about 950ºC for a time sufficient to form MBa₂Cu₃O_y, where y is from about 6.0 to about 6.4; and maintaining the MBa₂Cu₃O_y in an oxygen-containing atmosphere while cooling for a time sufficient to obtain the desired product; said precursor powder being prepared by (a) forming a mixture of Ba(OH)₂.8H₂O, M₂O₃ and CuO powders with the atomic ratio of M:Ba:Cu being about 1:2:3; or (b) forming an aqueous solution of M(NO₃)₃, Ba(NO₃)₂ and Cu(NO₃)₂ in an atomic ratio of M:Ba:Cu of about 1:2:3, adding to the resulting solution sufficient citric acid monohydrate to convert the metals present to their corresponding citrates, and spray drying the resulting solution to obtain the precursor powder. The precursor powder prepared in (a) can be pressed into a desired shape prior to heating. The invention also provides the shaped article prepared by the process of the invention. DETAILED DESCRIPTION OF THE INVENTION

The process of the invention provides an improved process for preparing superconducting compositions having,, the formula MBa₂Cu₃O_x. M is selected from the group consisting of Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu, but is preferably Y.

The parameter x is from about 6.5 to about 7.0, but is preferably from about 6.8 to about 7.0.

In the process of the invention a precursor powder is prepared for later heating. In one embodiment of the invention the precursor powder is prepared by mixing M₂O₃, Ba(OH)₂·8H₂O and CuO powders in an atomic ratio of M:Ba:Cu of about 1:2:3. The powders are mixed well in a mixing device or by hand using a mortar and pestle to obtain an intimate mixture of reactants. The use of Ba(OH)2·8H₂O as the source of Ba results in the preparation of a uniform, practically single-phase, superconducting MBa₂Cu₃O_x composition.

In another and preferred embodiment of the process of the invention the appropriate precursor powder is prepared by forming an aqueous solution of M(NO₃)₃, Ba(NO₃)₂ and Cu(NO₃)₂ in an atomic ratio of M:Ba:Cu of about 1:2:3. The aqueous solution of nitrates can be prepared by starting with the appropriate nitrate salts. Alternatively, the aqueous solution of nitrates can be prepared by reacting Ba(OH)₂·8H₂O or BaCO₃, M₂O₃ and CuO powders with sufficient concentrated nitric acid to convert the metals present to metal nitrates. Εxcess concentrated nitric acid can be used to speed the reaction. The amount of concentrated nitric acid used is typically between one and two times the amount needed to convert all the metals present to metal nitrates. If nitric acid conversion is used, the resulting mixture is diluted with water until a clear solution is obtained. As used here, "clear solution" means one containing no undissolved solids.

To the solution of nitrates is added an amount of citric acid monohydrate at least sufficient to convert all metals present to metal citrates. Typically, the amount of citric acid monohydrate used is between one and two times the amount needed to convert all the metals present to metal citrates. The acidic nitrate solution prevents precipitation of the citrates. The resulting citrate/nitrate solution is then spray dried using conventional spray-drying techniques and equipment to obtain the precursor powder. Spray drying the citrate/nitrate solution provides a well mixed precursor and results in the preparation of a uniform, practically single-phase, superconducting MBa₂Cu₃O_x product after fhe heating and cooling steps. The X-ray diffraction pattern of the superconducting product prepared by spray-drying to obtain the precursor powder and then heating and cooling as described herein has significantly less impurity than does the superconducting product prepared by nixing the oxides and Ba(OH)₂·8H₂O and heating and cooling as described herein.

Preferably, the starting materials used in the process of the invention are of high purity, e.g. 99.99% by weight for CuO and 99.9% by weight for M₂O₃. Less pure starting materials can be used; however, the product may then contain an amount of another phase material comparable to the amount ofimpurity in the starting materials. It is particularly important to avoid the presence of impurities containing iron and other transition, but non-rare earth, metals in the reactants.

The precursor powder is then heated in an oxygen-containing atmosphere at a temperature from about 875°C to about 950°C, preferably from about 900ºC to about 950°C, for a time sufficient to form MBa₂Cu₃O_y, where y is from about 6.0 to about 6.4. It has been determined by TGA data that when the precursor powder is heated to 900ºC, y is from about 6.0 to about 6.4. Alternatively, prior to heating when the precursor powder is made by mixing M₂O₃, Ba(OH)₂·8H₂O and CuO powders, the precursor powder can be pressed into a disk, bar or other desired shape using conventional techniques. For heating, the precursor powder is placed in a non-reactive container such as an alumina or gold crucible. The oxygen-containing atmosphere can be air or oxygen gas, but is preferably oxygen.

The container with the precursor powder is placed in a furnace and brought to a temperature of from about 875°C to about 950 °C . It is the total time tha t the precursor powder is at temperatures in this range that is important. For the Ba(OH)₂·8H₂O precursor powder heating rates of 10ºC per minute to 50°C per minute can be used to raise the temperature of the furnace containing the sample from ambient temperature to the final heating temperature of from about 875°C to about 950ºC. When the final heating temperature is 900ºC, 1/2 hour is sufficient time to maintain the sample at 900°C to produce, after cooling, practically single-phase superconducting MBa₂Cu₃O_x. When the final heating temperature is 940°C, 2 minutes is sufficient time to maintain the sample at 940°C to produce, after cooling, practically single-phase superconducting MBa₂Cu₃O_x. Alternatively, the container can be placed directly into an oven already heated to the final heating temperature. Longer heating times can be used.

For heating the spray-dried precursor, if slower heating rates are used, the minimum time for which the sample must be maintained at a final temperature of from about 875ºC to about 950ºC is shorter. If faster heating rates are used, the minimum time for which the sample must be maintained at a final temperature of from about 875°C to about950°C is longer. For example, when a heating rate of50°C per minute is used to raise the temperature ofthe furnace containing the sample from ambienttemperature to a final heating temperature of 900°C,1/2 hour is sufficient time to maintain the sample at900°C to produce, after cooling, practically single-phase superconducting MBa₂Cu₃O_x. Longer heating times can be used. After cooling as described herein, the MBa₂Cu₃O_x product can be pressed into a desired shape and sintered to provide a shaped article.

At the end of the heating time, the furnace is turned off, and the resulting material is allowed to cool in the oxygen-containing atmosphere for a time sufficient to obtain the desired product. Preferably, the material is cooled to below about 350ºC (a time interval of about 1-1.5 hours) before the sample container is removed from the furnace. During the cooling step, the oxygen content of the material increases to give the desired MBa₂Cu₃O_x product. The additional oxygen which enters into the crystalline lattice of the material during this cooling step to form the desired product does so by diffusion. The rate at which oxygen enters the lattice is determined by a complex function of time, temperature, oxygen content of the atmosphere, sample form, etc. Consequently, there are numerous combinations of these conditions that will result in the desired product. For example, the rate of oxygen uptake by the material at 500ºC in air is rapid, and the desired product can be obtained in less than an hour under these conditions when the sample is in the form of a loosely packed, fine particle powder. However, if the sample is in the form of larger particles, densely packed powders or shaped articles, the times required^to obtain the desired product at 500°C in air will increase. Well sintered, shaped articles will take longer to form the desired product than will more porous ones, and for larger, well sintered, shaped articles many hours may be required.

A convenient procedure for obtaining the desired product when the material is in the form of a powder or a small shaped object is to turn off the furnace in which the heating was conducted and to allow the material to cool in the furnace to a temperature approaching ambient temperature (about 22°C) which typically requires a few hours. In the examples, cooling in the furnace to below about 350°C was found to be sufficient. Increasing the partial pressure of oxygen in the atmosphere surrounding the sample during cooling increases the rate at which oxygen enters the lattice. If, in a particular experiment, the material is cooled in such a manner that the MBa₂Cu₃O_x product is not obtained, the material can be heated to an intermediate temperature, such as 500°C, between ambient temperature and the final temperature used in the heating step and held at this temperature for a sufficient time to obtain the desired product. If the MBa₂Cu₃O_x product is. pressed into a desired shape and sintered at about 900°C to absaut 650ºC the above cooling considerations would then apply to the resulting shaped article. The product powder formed by the process of the invention is practically single-phase and has orthorhombic symmetry as determined by X-ray diffraction measurements.

The process of this invention proovides a single heating-step, method for preparing a superconducting MBa₂Cu₃O_x composition that does not require a special atmosphere during the heating step, subsequent grinding, reheating or annealing, extended heating times or refining of the product to separate the desired superconducting MBa₂Cu₃O_x composition from other phases. The best mode contemplated for carrying out the invention is described in Example 5.

As used herein the phrase "consisting essentially of" means that additional steps can be added to the process of the invention so long as such steps do not materially alter the basic and novel characteristics of the invention. Superconductivity can be confirmed by observing flux exclusion, i. e., the Meissner effect.

The invention is further illustrated by the following examples in which temperatures are in degrees Celsius unless otherwise indicated. The chemicals (with purity indicated) used in the following the examples are Ba(OH)₂·8H₂O - (48.6% BaO) obtained from Kali-Cheraie, CuO - (99.99%) obtained from Johnson and Matthey or Puratronic or (>99%) obtained from Fluka, Y₂O₃ - (99.99%) obtained from Research Chemicals. High purity chemicals were used to demonstrate that the process of the invention can result in single-phase or practically single-phase MBa₂Cu₃O_x.

EXAMPLE 1

Ba(OH)₂·8H₂O (2.644 g ), 1.000 g of CuO, and 0.474 g of Y₂O₃ were ground together in an agate mortar for 15 minutes, and the resulting mixed powder was pressed into disks approximately 1 cm in diameter and 0.2 cm in thickness. One disk was placed in an alumina (Al₂O₃) container and heated in air in a furnace from ambient temperature to a final heating temperature of 900º at a rate of about 50° per minute. The temperature was maintained at 900º for 30 minutes. The furnace was then turned off and allowed to cool to below 350° (an elapsed time of about 1-1.5 hours) after which the sample was removed. The fired disk was black. An X-ray diffraction powder pattern was obtained on the crushed disk. The X-ray diffraction pattern indicated that the product was YBa₂Cu,O_x with orthorhombic symmetry and contained a very small amount of Y₂BaCuO₅ as an impurity. The material exhibited the Meissner effect at about 90 K, thereby indicating a superconducting transition of about 90 K.

EXAMPLE 2 A disk prepared substantially as described in Example 1 was placed in an alumina container and heated in flowing oxygen by inserting the sample directly into a tube furnace already at a temperature of 900°. The temperature was maintained at 900° for 30 minutes. The furnace was then turned off and allowed to cool to about 350° (an elapsed time of about 1-1.5 hours) after which the sample was removed. The resulting fired disk was black. An X-ray diffraction powder pattern was obtained on the crushed disk. The pattern showed that the product was YBa₂Cu₃O_x with orthorhombic symmetry and contained a very small amount of Y₂BaCuO₅ as an impurity. The material exhibited the Meissner effect at about 90 K, thereby indicating a superconducting transition of about 90 K.

EXAMPLE 3 A disk prepared substantially as described in Example 1 was placed in an alumina container and heated in air in a furnace from ambient temperature to a final heating temperature of 940° at a rate of about 50° per minute. The temperature was maintained at 940° for 2 minutes. The furnace was then turned off and allowed to cool to about 350ºC (an elapsed time of about 1-1.5 hours) after which the sample was removed. The resulting fired disk was black. An X-ray diffraction powder pattern was obtained on the crushed disk. The pattern showed that the product was YBa₂Cu₃O_x with orthorhombic symmetry and contained trace amounts of impurity. The material exhibited the Meissner effect at about 90 K, thereby indicating a superconducting transition of about 90 K. EXAMPLE 4 Ba(OH)₂·8H₂O (7.929 g ), 3.000 g of CuO, and 1.419 g of Y₂O₃ were placed in a 250 ml beaker and 15 ml of concentrated nitric acid were added. When all of the CuO was dissolved, a mixture consisting of a blue solution and undiεsolved white material was obtained. The mixture was diluted with H₂O until all of the solid material was dissolved. The volume of the final solution was about 200 ml. Citric acid monohydrate (15 g) was added to the blue solution, and the resulting mixture was stirred for a minute to give a clear blue solution again. This solution was spray dried to give a blue powder. Spray drying was performed by using a Buchi No. 190 mini spray dryer operated with N₂ as the atomizing gas. An inlet temperature of 190º and an outlet temperature of 95°-115º were employed. The chamber atmosphere, was air. A portion of the resulting spray-dried material was placed in an alumina container and heated in air in a furnace from ambient temperature to a .final heating temperature of 900° at a rate of about 50° per minute. The temperature was maintained at 900° for 30 minutes. The furnace was then turned off and allowed to cool to about 350° (an elapsed time of about 1-1.5 hours) after which the sample was removed. The fired powder was black. An X-ray diffraction powder pattern was obtained and showed the product to be orthorhombic YBa₂Cu₃O_x with a trace of a second phase detected. The material exhibited the Meissner effect at about 90 K, thereby indicating a superconducting transition at about 90 K.

EXAMPLE 5 A portion of the spray-dried material prepared in Example 4 was placed in an alumina container and subjected to heat and cooling treatments similar to those described in Example 4 except that heating was conducted in flowing oxygen. The results were practically identical to those found in Example 4. The fired powder was black. An X-ray diffraction powder pattern was obtained and showed the product to be orthorhombic YBa₂Cu₃O_x with a trace of a second phase detected. The material exhibited the Meissner effect at about 90 K, thereby indicating a superconducting transition at about 90 K.

Claims

CLAIMS The Invention Being Claimed Is:

1. An improved process for preparing a superconducting composition having the formula MBa₂Cu₃O_x wherein M is selected from the group consisting of

Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu; x is from about 6.5 to about 7.0; said composition having a superconducting transition temperature of about 90 K; said process consisting essentially of heating a precursor powder in an oxygen-containing atmosphere at a temperature from about 875ºC to about 950°C for a time sufficient to form MBa₂Cu₃O_y, where y is from about 6.0 to about 6.4; and maintaining the MBa₂Cu,O_y in an oxygen-containing atmosphere while cooling for, a time sufficient to obtain the desired product; said precursor powder being prepared by

(a) mixing M₂O₃, Ba(OH)₂·8H₂O and CuO powders in an atomic ratio of M:Ba:Cu of about 1:2:3, or

(b) forming an aqueous solution of M(NO₃)₃, Ba(NO₃)₂ and Cu(NO₃)₂ in an atomic ratio of M:Ba:Cu of about 1:2:3, adding to the resulting solution sufficient citric acid monohydrate to convert the metals present to their corresponding citrates, and spray drying the resulting solution to obtain the precursor powder.

2. A process according to Claim 1 wherein the precursor powder is prepared by mixing M₂O₃,

Ba(OH)₂-8H₂O and CuO powders.

3. A process according to Claim 2 wherein the precursor powder is pressed into a desired shape prior to heating.

4. A process according to Claim 2 wherein the precursor mixture is heated at a temperature from about 900°C to about 950°C.

5. A process according to Claim 3 wherein the precursor mixture is heated at a temperature from about 900°C to 950°C.

6. A process according to Claim 4 wherein x is from about 6.8 to about 7.0.

7. A process according to Claim 5 wherein x is from about 6.8 to about 7.0.

8. A process according to Claim 6 wherein M is Y.

9. A process according to Claim 7 wherein M is Y.

10. A shaped article prepared according to the process of Claim 3.

11. A shaped article prepared according to the process of Claim 5.

12. A shaped article prepared according to the process of Claim 7.

13. A shaped article prepared according to the process of Claim 9.

14. A process according to Claim 1 wherein the precursor powder is prepared by forming an aqueous solution of M(NO₃)₃, Ba(NO,)₂, and Cu(NO₃)₂ in an atomic ratio of M:Ba:Cu of about 1:2:3, adding to the resulting solution sufficient citric acid monohydrate to convert the metals present to their corresponding citrates, and spray drying the resulting solution.

15. A process according to Claim 14 wherein the precursor powder is heated at a temperature from about 900°C to about 950°C.

16. A process according to Claim 15 wherein x is from about 6.8 to about 7.0.

17. A process according to Claim 16 wherein M is Y.

18. A process according to Claim 14 wherein, the MBa₂Cu₃O_x powder is pressed into a desired shape and sintered.

19. A process according to Claim 15 wherein the MBa₂Cu₃O_x powder is pressed into a desired shape and sintered.

20. A process according to Claim 16 wherein the MBa₂Cu₃O_x powder is pressed into a desired shape and sintered.

21. A process according to Claim 17 wherein the MBa₂Cu₃O_x powder is pressed into a desired shape and sintered.

22. A shaped article prepared according to the process of Claim 18.

23. A shaped article prepared according to the process of Claim 19.

24. A shaped-article prepared according to the process of Claim 20.

25. A shaped article prepared according to the process of Claim 21.