EP0494927A1

EP0494927A1 - Process for making supraconductors

Info

Publication number: EP0494927A1
Application number: EP90914838A
Authority: EP
Inventors: Harold Saul Horowitz; Stephan James Mclain
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1989-10-06
Filing date: 1990-09-25
Publication date: 1992-07-22
Also published as: CA2027019A1; AU6509890A; WO1991004946A1; KR920703453A; JPH05500651A

Abstract

Procédé de préparation du matériau supraconducteur MBa2Cu4O8, M représentant, entre autre, l'yttrium, consistant à chauffer une poudre de précurseur exempte de carbone contenant un mélange intime de composés de M, Ba et Cu, pour un rapport atomique de M:Ba:Cu de 1:2:4 dans un gaz contenant de l'oxygène, à une température comprise entre environ 700 °C et environ 840 °C.Process for preparing the superconducting material MBa2Cu4O8, M representing, among other things, yttrium, consisting in heating a carbon-free precursor powder containing an intimate mixture of compounds of M, Ba and Cu, for an atomic ratio of M: Ba: Cu of 1: 2: 4 in an oxygen-containing gas, at a temperature between about 700 ° C and about 840 ° C.

Description

PROCESS FOR MAKING SUPERCONDUCTORS

BACKGROUND OF THF. INVENTION FiffI of the Invent,ion

This invention relates to a process for making the superconducting orthorhombic phase having the formula MBa2Cu θ8. References Bednorz and Muller, Z. Phys . B64, 189 (1986), disclose a superconducting phase in the La-Ba-Cu-0 system with a superconducting transition temperature of about 35 K. The presence of this phase was subsequently confirmed by a number of investigators [see, for example, Rao and Ganguly, Current. Rπjenπe. 56, 47

(1987), Chu et al., S iencf.- 235, 567 (1987), Chu et al., Phys. Rev. Let .. 58, 405 (1987), Cava et al. , Phys . Rev. Lett . _r 58, 408 (1987), Bednorz et al. , Europhys . Let .. 3, 379 (1987) ] . The superconducting phase has been identified as the composition

Lai-χ (Ba,Sr,Ca) _xθ4-y with the tetragonal K2NiF4-type structure and with x typically about 0.15 and y indicating oxygen vacancies.

Wu et al., Phys. Rev. Let .. 58, 908 (1987), disclose a superconducting phase in the Y-Ba-Cu-0 system with a superconducting transition temperature of about 90 K. The compounds investigated were prepared with nominal compositions (Yι-χBa_x)2Cuθ4-y and x = 0.4 by a solid-state reaction of appropriate amounts of Y2O3, BaC03 and CuO in a manner similar to that described in Chu et al., Phvs . Rev. Let .. 58, 405 (1987) . This reaction method comprised 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' superconducting phase was subsequently identified as YBa₂Cu₃θ7_Λ.

Hundreds of other papers have since disclosed similar solid state reaction processes for making YBa2Cu3θ7_Δ. Other papers have disclosed various solution and precipitation methods for preparing the reactants to be heated at temperatures of 800-850°C and above.

Hirano et al., Chemistry Letters,. 665, (1988), disclose a process for producing Y-Ba-Cu-0 superconductors by the partial hydrolysis of a solution of Ba metal, Y(0-iPr)3 and Cu-acetylacetonate or

Cu-alkoxides in 2-methoxy or 2-ethoxy ethanol. The solution was stirred in dry nitrogen and heated at 60°C for 12 hours . The solution was then hydrolyzed by the slow addition of water diluted with solvent. Stirring and heating continued for several hours. Stirring continued while the solution was evaporated under vacuum at about 60°C and an amorphous precursor powder was obtained. The powder was calcined in flowing oxygen at temperatures between 800°C and 950°C for up to 24 hours. The calcined powder was pressed and sintered in flowing oxygen at temperatures up to 920°C and then annealed at temperatures between 450°C and 550°C.

The commonly assigned application, "Process for Making Superconductors and Their Precursors", S. N. 372,726, filed June 28, 1989, a continuation-in-part of S. N. 214,702, filed July 1, 1988, discloses a process for making tetragonal MBa2C 3θy where y is from about 6 to about 6.5, orthorhombic MBa2Cu3θ_x where x is from about 6.5 to about 7, or mixtures thereof by forming an essentially carbon-free precursor powder of compounds of M, Ba and Cu with an atomic ratio of M:Ba:Cu of 1:2:3, heating said precursor powder in an inert gas such as nitrogen or argon at a temperature of about 650°C-to about 800°C and cooling appropriately to give the desired product.

Karpinski et al.. Nature 336, 660 (1988), disclose a process for preparing in bulk YBa2Cu4θ8 at 400 bar (40 MPa) of 02 and 1040°C. The transition temperature is 81 K. Karpinski et al., J. Less-Common Met. 150, 129 (1989) , further disclose that synthesis of bulk YBa2C 4θ8 is possible at pressures greater than 50 bar (5 MPa) of 02 and at temperatures of approximately 1000°C. They also disclose that bulk YBa2Cu3.5θ7+_x, which can also be written as Y2Ba4Cu7θi5-y, with a T_c ~ 40 K can be synthesized at high oxygen pressures, i.e., about 1000-3000 bar (100-300 MPa) and at temperatures of about 1000-1200°C. A mixture of 2Ba4Cu7θi5-y with YBa2Cu3θ7_Δ appeared for samples sintered at T » 1050°C under pressures of 200 bars (20 MPa) .

Morris et al., Phys. Rev. B 39, 7347 (1989), disclose the synthesis of YBa2Cu θ8 and RBa2Cu4θ8, where R = Nd, Sm, Eu, Gd, Dy, Ho, Er and Tm. Ba2C 4θ8 was sintered in high pressure oxygen [pressure (02) = 120 atm (12 MPa)] for 8 hours at 930°C. Preparation of the rare earth compounds required different synthesis temperatures and pressures. They also report finding the additional phase E 2Ba4Cu7θ_x and Gd2Ba4Cu7θ_x with Tc ~ 40-50 K and report that this phase was prepared in Y, Dy, Ho and Er systems by varying the synthesis conditions.

Morris et al., Physica C 159, 287 (1989) disclose the synthesis of YBa2C 4θ8, RBa2Cu4θ8, Y2Ba4Cu7θi5-y and R2Ba4Cu7θi5-y, where R = Nd, Sm, Eu, Gd, Dy, Ho, Er and Tm. Samples were prepared by the solid state reaction of Y2O3 or R2O3 with BaO and CuO. The fine powder ingredients are ground together and pressed into 6 mm tablets at 3500 kg/cm² (350 MPa) . The samples were individually wrapped in gold foil and calcined for 8 hours in an externally heated high pressure oxygen furnace. Calcining was followed by slow cooling to room temperature (50 min to 700°C, 50 min to 600°C, 100 min to 500°C, 100 min to 400°C, furnace cool) . To maximize homogeneity, each sample was then reground, pressed, fired and cooled a second time under the same conditions . They disclose that YBa2Cu4θ8 can be prepared as described above at 930°C and an oxygen pressure ~ 35 atm (3.5 Mpa) . They also disclose that Y2Ba4Cu7θi5-y can be prepared as described above at 930°C and an oxygen pressure ~ 15 atm (1.5 Mpa) .

Cava et al.. Nature 338, 328 (1989), disclose the synthesis of YBa2Cu4θ8 in a two-step process. In the first step nitrates of Y, Ba and Cu in the correct stoichiometric proportion are mixed and heated very slowly to 750°C in alumina crucibles and held at this temperature and allowed to react for 16-24 hours. All heating, soaking and cooling is carried out in flowing 02 • Best results are obtained when an intermediate mixing and grinding step is performed after the first few hours of reaction at 750°C. This pre-reacted powder is ground and then mixed with an approximately equal volume of an alkali carbonate such as a2C03 or K2CO3. This mixture is ground, placed in a silver foil and heated at 800°C in flowing 02 for 3 days. YBa2Cu4θ8 is the majority phase obtained at heating temperatures from 700°C to 825°C. The product is washed to remove excess alkali carbonate and then dried by gentle heating in air.

Pooke et al., preprint, disclose the preparation of YBa2Cu4θ8 by a process in which Y2O3, BaCuθ2 and CuO are ground together, die-pressed into pellets and initially reacted at 900°C. The YBa2Cu3θ7_Λ. phase is formed at this point, with CuO, BaCuθ2, and Y2BaCuθ5 present as impurities. After grinding and re-pressing, the pellets are sintered at temperatures between 790°C and 830°C in flowing oxygen, with good results at 815°C. X-ray diffraction patterns show a substantial proportion of the YBa2Cu4θ8 phase after 1 day of sintering. Phase purity improved with repeated grinding and sintering. They also disclose the preparation of Y2Ba Cu7θi5-y in a similiar fashion with the primary difference being the reaction/sintering temperature which in the preparation of Y2Ba Cu70i5-_y is between 845°C and 870°C. Stoichiometric quantities of Y2O3, Ba(N03)₂ and CuO are mixed and pre-reacted to decompose the nitrate, then reacted/sintered in flowing oxygen over several days at 860°C-870°C , preferably with intermediate grinding. Pooke et al. also disclose that the reaction rate for both YBa2Cu θ8 and Y2Ba4Cu7θi5-y is improved by the addition of very small quantities of an alkali nitrate to the precursor materials. They disclose that nearly single phase YBa2Cu θ8 can be prepared by mixing stoichiometric proportions of Y2O3, Ba(N03)2 and CuO with up to 0.2 molecular quantities of NaNθ3 or KNO3, pre-reacting as a loose powder for 30 minutes, then grinding, die-pressing pellets and reacting for at least 12 hours at 800°C in flowing oxygen. Phase purity is improved with repeated grinding and sintering. Improved crystallinity was observed if BaCuθ2 replaced Ba(N03)2 as a precursor. Complete substitution of some rare earths for Y is reported also to enhance the rate of formation of MBa2Cu₄θβ and Y2Ba4C 7θi5-y. The preparation of single phase ErBa2Cu4θ8 at 815°C without alkali is also disclosed. Miyatake et al.. Nature 341, 41 (1989) disclose the preparation of Yι-_xCa_xBa2Cu4θ8 by heating a mixture of Y2θ3, Ba(N03)2, CuO and CaC03 at 900°C in flowing-oxygen for 12 hours . The resulting powder was compacted at 100 MPa and then lightly sintered at 800°C in an oxygen atmosphere. The hot isostatic pressing (100 MPa) in a gas environment of argon with 20% oxygen was repeated twice. The first pressing was at 950°C for 6 hours. The second pressing was at 1050°C for 3 hours. The product was reported to be high quality polycrystalline material with no secondary phases .

It is highly desirable to form precursors that can be used to produce powders that have small size particles, i.e., generally sub-micron in size, that can be pressed into desired shapes, sintered and converted to superconducting MBa2Cu4θ8 and to produce such powders without the need to grind and reheat, and without the use of high oxygen pressures, catalysts or conventional solid state reaction techniques with their inherent slow kinetics.

Summary of the Invention

This invention provides a process for preparing a powder of the superconducting orthorhombic phase having the formula MBa2Cu4θ8, wherein M is selected from the group consisting of Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, T , Yb, and Lu, said process consisting essentially of

(a) preparing an essentially carbon-free precursor powder containing an intimate mixture of M, Ba and Cu compounds with an atomic ratio of M:Ba:Cu of 1:2:4;

(b) heating said precursor powder in an oxygen-containing atmosphere, preferably substantially pure oxygen at 1 atm (1 x 10^ Pa) total pressure, at a temperature from about 700°C to about 840°C, preferably from about 750°C to about 825°C, for a time sufficient. typically about 12 hours, to form a powder of MBa2Cu4θs; and

(c) cooling said MBa2Cu4θ8 powder in an' oxygen-containing atmosphere, e.g. air, but preferably substantially pure oxygen.

It should be noted that the atomic ratio of M:Ba:Cu of 1:2:4 may not be sacrosanct. Slight variation due to the presence of impurities or weighing errors may still provide superconductive materials which, however, may not be single phase.

It is preferred to have said precursor powder prepared by a solution route, for example, by drying a solution, a suspension or a precipitate of M, Ba and Cu carbon-free salts such as hyponitrites. It is especially preferred to have said precursor powder prepared by drying the oxides formed by the hydrolysis of M, Ba and Cu compounds dissolved in an organic solvent.

It is also preferred to have the oxygen-containing atmosphere be free of C02.

The process of the present invention provides an especially fine powder of MBa2Cu4θ8. Detailed Description of he Inventi n

This invention provides a relatively low temperature process for preparing a powder of MBa2Cu4θ8 wherein M is selected from the group consisting of Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu. This novel process makes it possible to produce a powder of superconducting MBa2Cu4θ8 with maximum process temperatures of about 840°C and a single heating step. The MB 2Cu4θβ powder can be produced by this process with primary particle size in the micron or sub- micron range. These powders should therefore be very useful for producing sintered shaped superconducting articles which are dense, since these small particles sinter better than larger size particles typical of powders made using conventional higher temperature solid state reactions or high temperature-high pressure ' techniques . The reactants used in the process of this invention must be of the type and form that will react at temperatures below about 840°C by this process to form MBa2Cu4θ8. It is necessary to avoid the use of BaC03 as a reactant in the process and to avoid the formation of BaC03 during the process since the presence of BaC03 necessitates reaction temperatures of at least about 900°C to insure substantially complete reaction, i.e., substantially complete decomposition of BaC03. While air can be used as the oxygen-containing atmosphere in the process of this invention, it is preferred to use an oxygen-containing atmosphere that is free of Cθ2 to avoid the formation of BaCθ3 during the process.

The process of this invention uses an essentially carbon-free precursor powder containing an intimate mixture of M, Ba and Cu compounds with an atomic ratio of M:Ba:Cu of 1:2:4. As used herein, essentially carbon-free means that there is less than 1 wt% carbon in the precursor powder. The precursor powder must be an intimately mixed fine-particle powder in order to facilitate the low temperature solid state reaction that it undergoes during the process. A solution route for the preparation of the precursor powder yields an intimately mixed fine-particle powder and solution- derived precursor powders are preferred. The precursor powder used in this invention can be prepared by drying a solution or suspension containing M, Ba and Cu compounds with an atomic ratio of M:Ba:Cu of 1:2:4.

One method for preparing the precursor powder is to form an aqueous nitrate solution of M, Ba and Cu with an atomic ratio of M:Ba:Cu of 1:2:4, mix said solution with an excess of a hyponitrite solution such as sodium hyponitrite or sodium peroxide to form a precipitate containing essentially all of the M, Ba and Cu present in the original nitrate solution, and collect and dry the precipitate.

A preferred method for preparing the precursor powder is to form a solution of M, Ba and Cu compounds with an atomic ratio of M:Ba:Cu of 1:2:4 in an organic solvent. Controlled hydrolysis results in the formation of oxides, or hydrous oxides, which when filtered, washed and dried serve as the precursor powder. Compounds suitable to form the solution must satisfy two criteria. They must be soluble in an organic solvent and they must react readily with water to produce metal oxide or metal hydroxide. The following list is not meant to be limiting, but some of the types of compounds which meet these criteria and representative examples are metal alkyls such as Cu(CH2SiMe3) and Y(CH2SiMe3)3, metal cyclopentadienides such as Y(C5Hs)3, Ba(C5Hs)2 and Ba(C5Me5)2, metal acetylides such as Cu[C≡CC(CH3)2θMe], metal aryls such as Cu(mesityl), metal alkoxides such Cu(OCMe3), Cu[OCH(CMe3)2], C (OCH2CH2OBU)2, Cu(OCH2CH2NEt2)2, Y5O(0CHMe2)13, Y(OCH2CH2OBU)3, Y(OCH2CH2NEt2)3, Ba(OCHMe2)2, Ba(OCH2CH2OBU)2 and Ba(OCH2CH2NEt2)2/ metal aryloxides such Y[0-2, , 6-C6H2 (CMe3)3]3, and metal amides such as Cu(NEt2), Cu(NBu2), Cu[N(SiMe3)2] and Y[N(SiMe3)2]3• The precursor powder is placed in an inert container or an inert tray, for example, on an alumina tray, and heated in an oxygen-containing atmosphere, preferably substantially pure oxygen, at a temperature from about 700°C to about 840°, preferably from about 750°C to about 825°C, for a time sufficient to form a powder of MBa2Cu4θ8- Twelve hours has proven to be sufficient time to form the MBa2Cu4θ8. but longer times can be used.

The MBa2Cu4θ8 powder is then cooled in an oxygen- containing atmosphere, preferably substantially pure oxygen. Unlike MBa2Cu3θ7_Δ (where 0 < Δ < 1) , MBa2Cu4θ8 does not exhibit oxygen non-stoichiometry within its temperature stability range; thus, the rate of cooling is not important and can be carried out as a slow cool (as slow a cooling rate as practical) or as a fast cool (as fast a cooling rate as practical) or any cooling rate in between these extremes. The nature of the final product is independent of the cooling program.

The MBa2Cu4θ8 product powder is typically composed of primary particles the majority of which are micron and sub-micron in size as determined by scanning electron microscopy.

The product powder can be stored for later use. However, it displays the same reactivity toward Cθ2 and H2θ as has been reported for the MBa2Cu3θ7_Δ phase. Hence, appropriate precautions must be taken.

The presence of superconductivity can be determined by the Meissner effect, i.e., the exclusion of magnetic flux by a sample when in the superconducting state. This effect can be measured by the method described in an article by E. Polturak and B. Fisher in Physical Review B. 36, 5586 (1987) . It is well known that particles with dimensions of the order of or less than the penetration depth do not exhibit flux exclusion. Particles of the powders of this invention are typically sub-micron. Estimates of the penetration depth of these materials are of the same order of magnitude , i.e., 0.1-1.0 μm, at 77 K. Therefore, the absence or weakening of the Meissner effect for these particles is to be expected. Because of the temperature dependence of the penetration depth, a depressed value of T_c might also be anticipated.

The superconducting compositions of this invention can be used to conduct current extremely efficiently or to provide a magnetic field for magnetic imaging for medical purposes. Thus, by cooling the composition in the form of a wire or bar to a temperature below the superconducting transition temperature, by exposing the material to liquid nitrogen or liquid helium in a manner well known to those skilled in this art, and initiating a flow of electrical current, one can obtain such flow without any electrical resistive losses. To provide exceptionally high magnetic fields with minimal power losses, the wire mentioned previously could be wound to form a coil which would be exposed to liquid helium or nitrogen before inducing any current into the coil. Magnetic fields provided by such coils can be used to levitate objects as large as railroad cars. These superconducting compositions are also useful in Josephson devices such as SQUIDS (superconducting quantum interference devices) and in instruments that are based on the Josephson effect such as high speed sampling circuits and voltage standards.

EXAMPT.ES OF THE INVENTION EXAMPLE 1

A YBa2Cu4θ8 precursor powder was prepared by dissolving Y5θ(OCHMe2) 13 (0.641 g) , Ba(OCHMe2)2 (1.332 g) and Cu(NBu2) (2.000 g) in 40 mL of tetrahydrofuran (THF) to give a homogeneous solution. Hydrolysis was carried out by dropwise addition of this solution to a solution of degassed water (2.58 g) in 40 mL of THF. The mixture was refluxed under an argon atmosphere for 16 hours, and filtered to give an orange solid. This orange solid was washed first with THF, then with pentane, and dried under high vacuum at 100°C to yield 1.96 g of orange precursor powder.

A portion (0.77 g) of this precursor powder was spread in a thin layer in an alumina tray and heated at 20°C/min to 825°C in 1 atm (1 x 10⁵ Pa) of oxygen. The sample was held at 825°C in 1 atm (1 x 10⁵ Pa) of oxygen for 60 hours. The f rnace was then turned off and the sample allowed to cool in oxygen to room temperature, about 20°C. The resulting powder was black and the yield was 1.05 g. An X-ray diffraction powder pattern shows the powder to be YBa2Cu4θ8 and a trace of CuO. The X-ray diffraction powder pattern is in excellent agreement with previously refined and published powder patterns and shows an orthorhombic unit cell, space group Ammm and unit cell dimensions a = 3.871 A

(0.3871 nm) , b = 3.840 A (0.3840 nm) and c = 27.25 A (2.725 nm) .

Magnetic flux exclusion measurements confirmed superconductivity and showed the sample to have an onset of superconductivity T_c at about 83 K. Scanning electron microscopy showed that the size of the primary particles is in the range of about 0.5-1.0 μm.

EXAMPLE 2 A portion (0.32 g) of a hydrolyzed organometallic precursor prepared essentially as described in Example 1 was spread in a thin layer in an alumina tray and heated at 20°C/min to 750°C in 1 atm (1 x 10⁵ Pa) oxygen. The sample was held at 750°C in 1 atm (1 x 10⁵ Pa) oxygen for 60 hours. The furnace was then turned off and the sample allowed to cool in oxygen to room temperature, about 20 K. The resulting powder was black and the yield was 0.30 g. An X-ray diffraction powder pattern of the material shows it to be YBa2Cu4θs and a trace of

CuO along with one very weak unidentified peak at approximately 3.08 A. The phase purity of the YBa2Cu40s in this experiment as judged by X-ray diffraction results is comparable to that prepared in Example 1.

Magnetic flux exclusion measurements confirm^'very weak superconductivity with a Tc onset of about 90 K. The extremely weak signal is consistent with sub-micron particle size and the onset at 90 K suggests the presence of YBa2C 3θ7_^ in an amount too small to be detected by X-ray diffraction.

COMPARATIVE EXPERIMENT A Y2O3 (0.757 g) , Ba(N03)2 (3.504 g) and CuO

(2.133 g) were milled together for 30 min in a 2 oz. plastic jar with 50 g of Zrθ2 balls on a high intensity shaker mill. The CuO used was obtained from the decomposition of Cu(NO3)2-x^O at 300°C in oxygen for one hour. This milled mixture was placed in an alumina tray and fired at 825°C in 1 atm (1 x 10⁵ Pa) oxygen for 149 hours with one interruption for regrinding. The sample was allowed to slowly cool in oxygen to room temperature. X-ray diffraction of the powder showed it to be predominately YBa2Cu4θ8 but with second phases of CuO and Y2Ba2Cu0s. An unidentified peak at approximately 3.08 A is also present. Magnetic flux exclusion measurements confirmed superconductivity and showed the sample to have an onset of superconductivity T_c of about 85 K with another slight inflection in the flux exclusion curve at about 75 K. The multi-phase nature of the magnetic flux exclusion curve suggests contributions both from YBa2Cu4θ8 and a trace amount of YBa₂Cu3θ₇_Δ. The results of both the magnetic flux exclusion measurements and the X-ray diffraction powder pattern demonstrate that the phase purity of Ba2Cu4θs obtained by conventional solid state reaction techniques, even when fired for longer times and with an interruption for regrinding, is inferior to that obtained by the single firing decomposition of an organometallic-derived precursor as described in Example 1. The phase purity of the conventionally prepared sample as judged by X-ray diffraction results is also inferior to that obtained by a single lower temperature decomposition of an organometallic-derived precursor as described in Example 2.

COMPARATIVE EXPE IMENT B

Y2O3 (0.757 g), Ba(Nθ3)2 (3.381 g) , NaNθ3 (0.058 g) and CuO (2.167 g) were milled together for 30 min in a 2 oz. plastic jar with 50 g of Zrθ2 balls on a high intensity shaker mill. The CuO used was obtained from the decomposition of Cu(NO3)2-x^O at 300°C in oxygen for one hour. This milled mixture was placed in an alumina tray and fired at 825^CC in 1 atm (1 x 10⁵ Pa) oxygen for 60 hours with one interruption for regrinding. The sample was allowed to slowly cool in oxygen to room temperature. X-ray diffraction of the powder showed it to be predominately YBa2 u4θ8 but with second phases of CuO and Y2Ba2Cuθ5. An unidentified peak at approximately 3.08 A is also present. The phase purity of the YBa2Cu 0s in this experiment is improved relative to that prepared in Comparative Experiment A but still inferior to that prepared in Examples 1 and 2. Magnetic flux exclusion measurements confirmed superconductivity and showed the sample to have a T_c of about 87 K and another slight inflection in the flux exclusion curve suggests contributions both from YBa2C 4θs and a trace amount of YBa2Cu3θ7_Δ- These results demonstrate that the phase purity of YBa2C 4θ8 obtained by conventional solid state reaction techniques, even when aided by the presence of an alkali nitrate catalyst (carried out in a manner described in a pre-print by D. M. Pooke et al., Dept. of Scientific and Industrial Research, Lower Hutt, New Zealand) , is inferior to that obtained by the single firing decomposition of an organometallic-derived precursor as described in Example 1. The phase purity of the - conventionally prepared sample, aided by the presence of alkali nitrate, as judged by X-ray diffraction results is also inferior to that obtained by a single lower temperature decomposition of an organometallic-derived precursor as described in Example 2.

Claims

1. A process for preparing a powder of the' superconducting orthorhombic phase having the formula MBa2Cu4θ8, wherein M is selected from Y, Nd, Sm, Eu, Gd,

Dy, Ho, Er, Tm, Yb and Lu, said process consisting essentially of

(a) preparing an essentially carbon-free

' precursor containing an intimate mixture of M, Ba and Cu compounds with an atomic ratio of M:Ba:Cu of 1:2:4;

(b) heating said precursor powder in an oxygen-containing atmosphere at a temperature from about 700°C to about 840°C for a time sufficient to form a powder of MBa2Cu4θs; and (c) cooling said MBa2Cu4θ8 powder in an oxygen-containing atmosphere.

2. The process of Claim 1 wherein said oxygen- containing atmosphere is substantially free of Cθ2.

3. The process of Claim 2 wherein said oxygen- containing atmosphere is substantially pure oxygen.

4. The process of Claim 3 wherein said precursor powder is heated at a temperature from about 750°C to about 825°C.

5. The process of Claim 4 wherein the total pressure of said oxygen is 1 atm (1 x 10^ Pa) .

The process of Claim 5 wherein M is Y.

7. The process of Claim 1 wherein said precursor powder is prepared by forming an aqueous solution of M, Ba and Cu nitrates, wherein the atomic ratio of M:Ba:Cu is 1:2:4; mixing said solution with an amount of sodium hyponitrite or sodium peroxide effective to precipitate substantially all of the M, Ba and Cu in said solution; and isolating the resulting precipitate.

8. The process of Claim 7 wherein said oxygen- containing atmosphere is substantially free of Cθ2•

9. The process of Claim 8 wherein said oxygen- containing atmosphere is substantially pure oxygen.

10. The process of Claim 9 wherein the precursor powder is heated at a temperature from about 750°C to about 825°C.

11. The process of Claim 10 wherein the total pressure of said oxygen is 1 atm (1 x 10^ Pa) .

12. The process of Claim 11 wherein M is Y.

13. The process of Claim 1 wherein said precursor powder is prepared by forming a solution of M, Ba and Cu compounds wherein the atomic ratio of M:Ba:Cu is 1:2:4 in an organic solvent, said compounds of M, Ba and Cu being soluble in said solvent and reactive with water to produce metal oxides or metal hydroxides; contacting the resulting solution with water to form said oxides or hydroxides; and filtering, washing and drying said oxides or hydroxides.

14. The process of Claim 13 wherein said oxygen- containing atmosphere is substantially free of Cθ2.

15. The process of Claim 14 wherein said oxygen- containing atmosphere is substantially pure oxygen.

16. The process of Claim 15 wherein the precursor powder is heated at a temperature from about 750°C to about 825°C.

17. The process of Claim 16 wherein the total pressure of said oxygen is 1 atm (1 x 10~> Pa) .

18. The process of Claim 17 wherein M is Y.