CA2027898A1 - Superconductor and process for its preparation - Google Patents

Abstract

TITLE
NOVEL SUPERCONDUCTOR AND
PROCESS FOR ITS PREPARATION
ABSTRACT
A novel superconducting composition of the formula MBa2-xSrxCu4O8 and methods for its preparation are disclosed.

Description

~E
NOVEL SUPERCONDUCTOR AND
PROCESS FOR ITS PREPARATION
S B~CKGRO~J~ OF THF INyENTIoN
Fie~d_of~thc In~en~ion This invention relates to a novel superconducting orthorhombic phase having the formula MBa2_xSrxCu4Og, wherein x is from about 0.1 to about 1.2, and a process for preparing it.
Referen~es Bednorz and Muller, z. Phys. B64, 189 (1986), disclose a superconducting phase in the La-Ba-Cu-O ~ -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, Curren~_Science, 56, 47 (1987), Chu et al., Science, 235, 567 (1987), Chu et al., ~hYs. Rev. Lett., 58, 405 (1987), Cava et ~-al., Phys. Rev. Lett., 58, 408 (1987), Bednorz et al., EurQ~hys. Let~.~ 3, 379 (1987)]. The superconducting phase has been identified as the composition Lal_x(Ba,Sr,Ca)x4-y with the tetragonal K2NiF4-type -~ -structure and with x typically about 0.15 and y 25 indicating oxygen vacancies. ~ -~
Wu et al., Phy~. Rev. Tett., 58, 908 ~1987), disclose a superconducting phase in the Y-Ba-Cu-O
system with a superconducting transition temperature of about 90 K. The compounds investigated were ! '30 I prepared withinominal compositions (Yl-xBax)2CuO4-y and x = 0.4 by a solid-state reaction of appropriate amounts of Y2O3, BaCO3 and CuO in a manner similar to that described in Chu et al., Phys. Rev. Lett., 58, ~
405 ~1987). The superconducting phase was `-~ :-:' :

subsequently identified as YBa2Cu3O7_~ (1-2-3 type composition).
Hundreds of other papers have since disclosed similar solid state reaction processes for making YBa2Cu3O7_~ and the rare earth analogues. Other papers have disclosed various solution and precipitation methods for preparing the reactants to be heated at temperatures of 800-850C and above.
Hirano et al., Chemistry Letters, 665, (1988), disclose a process for producing Y-Ba-Cu-O
superconductors by the partial hydrolysis of a solution of 8a metal, Y(O-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 60C 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 60C and an amorphous precursor powder was obtained. The powder was calcined in flowing oxygen at temperatures between 800C and 950C
for up to 24 hours. The calcined powder was pressed and sintered in flowing oxygen at temperatures up to 920C and then annealed at temperatures between 450C
and 550C.
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 30 a process for making tetragonal MBa2Cu3Oy where y is ;
from about 6 to about 6.5, orthorhombic MBa2CU3x 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 650C to about 800C
and cooling appropriately to give tne desired product.
Wada et al., J~. J ApDl, Phys. 26, L706 (1987) disclose that the superconducting transition temperature of Y(Ba1_xSrx)2Cu3o7-~ decreases linearly from 94 K to 84 K as x increases from 0 to 0.4.
Saito et al., J~n. J A~l. Phys. 26, Suppl.
26-3, 1081 ~1987) disclose that the superconducting transition temperature of Y(Bal-xsrx)2cu3o7-~
decreases monotonically as x increases from 0 to 1. -There is no superconductivity for x = 1.
Baldha et al., Solid State Commun. 71, 839 (1989) disclose that the superconducting transition temperature, as determined by zero resistance, of;
YBa2_xCaxCu3O7_~ decreases from 90 K when x = 0 to 78 K when x = l. -~
Karpinski et al., Nat~l~ 336, 660 (1988), disclose a process for preparing in bulk YBa2Cu4Og 20 (1-2-4 type composition) at 400 bar (40 MPa) of 2 and 1040C. The transition temperature is 81 K.
Karpinski et al., J. Less-CommQn Met. 150, 129 (1989), further disclose that synthesis of bulk YBa2Cu4O~ is possible at pressures greater than 50 bar (5 MPa) Of 2 and at temperatures of approximately 1000C. They also disclose that bulk YBa2Cu3.sO7+x,;~
which can also be written as Y2Ba4Cu7Ols-y, with a Tc ~ 40 K can be synthesized at high oxygen pressures, i. e., about 1000-3000 bar (100-300 MPa) and at 30~ temperatures of about 1000-1200C. A mixture of Y2Ba4Cu7O1s_y with YBa2Cu3O7_~ appeared for samples sintered at T - 1050C under pressures of 200 bars ~20 MPa).
Morris et al., Phys. Rev. B 39, 7347 ~1989), 35 disclose the synthesis of YBa2cu4o8 and RBa2CU98r -~, ::

where R = Nd, Sm, Eu, Gd, Dy, Ho, Er and Tm.
YBa2Cu4Og was sintered in high pressure oxygen [pressure ~2) ~ 120 atm (12 MPa)] for 8 hours at 930C. Preparation of the rare earth compounds required different synthesis temperatures and pressures. They also report finding the additional phase Eu2BagCu70x and Gd2Ba4Cu7Ox with Tc ~ 40-50 ~
and report that this phase was prepared in Y, Dy, Ho and Er systems by varying the synthesis conditions.
Morris et al., ~hy~i~a C 159, 287 (1989) disclose the synthesis of YBa2cu4o8~ RBa2CU48, Y2Ba4cu7ol5-y and R2Ba4cu7ols-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 700C, 50 min to 600C, 100 min to 500C, 100 min to 400C, furnace cool). To maximize homogeneity, each sample was then reground, pressed, fired and cooled a second time under the same conditions. They disclose that YBa2Cu4Og can be prepared as described above at 930C
, and an oxygen pressure ~ 35 atm (3.5 Mpa). They also disclose that Y2Ba4Cu7Ols_y can be prepared as described above at 930C and an oxygen pressure ~ 15 atm (1.5 Mpa).
Cava et al., Nature 338, 328 (1989), disclose the synthesis of YBa2Cu4Og 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 750C 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 2- Best results are obtained when an intermediate mixing and grinding step is performed after the first few hours of reaction at 750C. This pre-reacted 5 powder is ground and then mixed with an approximately ~-equal volume of an alkali carbonate such as Na2CO3 or K2CO3. This mixture is ground, placed in a silver foil and heated at 800C in flowing 2 for 3 days.
YBa2Cu4Og is the majority phase obtained at heating temperatures from 700C to 82SC. 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 YBa2Cu4Og by a process in which Y2O3, BaCuO2 and ~ ~ -CuO are ground together, die-pressed into pellets and initially reacted at 900C. The YBa2Cu3O7_~ phase is formed at this point, with CuO, BaCuO2, and Y2BaCuOs ;~
present as impurities. After grinding and ~ -~
re-pressing, the pellets are sintered at temperatures between 790C and 830C in flowing oxygen, with good results at 815C. X-ray diffraction patterns show a substantial proportion of the YBa2Cu4Og phase after 1 day of sintering. Phase purity improved with repeated grinding and sintering. They also disclose the 25 preparation of Y2Ba4Cu7Ols-y in a similiar fashion ~ ~-with the primary difference being the reaction/
sintering temperature which in the preparation of Y2Ba4Cu7O1s-y is between 845C and 870C.
Stoichiometric quantities of Y2O3, Ba(NO3)2 and CuO
are mixed and pre-reacted to decompose the nitrate, then reacted/sintered in flowing oxygen over several days at 860C-870C , preferably with intermediate ---grinding. Pooke et al. also disclose that the reaction rate for both YBa2CugOg and Y2BagCu7O1s_y is --35 improved by the additions of very small quantities of ~ -~

. : .:

an alkali nitrate to the precursor materials. They disclose that nearly single phase YBa2Cu4Og can be prepared by mixing stoichiometric proportions of Y2O3, Ba(NO3)2 and CuO with up to 0.2 molecular quantities of NaNO3 or KNO3, pre-reacting as a loose powder for 30 minutes, then grinding, die-pressing pellets and reacting for at least 12 hours at 800C in flowing oxygen. Phase purity is improved with repeated grinding and sintering. Improved crystallinity was observed if BaCuO2 replaced Ba(No3)2 as a precursor.
Complete substitution of some rare earths for Y is reported to also enhance the rate formation of MBa2Cu4Og and Y2Ba4Cu7Ols_y and the preparation of single phase ErBa2Cu4Og at 815C without alkali is disclosed.
Miyatake et al., Nature 341, 41 (1989) disclose the preparation of Y1-xCaxBa2Cu4o8~ with x = 0-0.1, by heating a mixture of Y2O3, Ba(NO3)2, CuO and CaC03 at 900C in flowing oxygen for 12 hours. The resulting powder was compacted at 100 MPa and then lightly sintered at 800C in an oxygen atmosphere. The hot isostatic pressing (100 Mæa) in a gas environment of argon with 20% oxygen was repeated twice. The first pressing was at 950C for 6 hours. The second 25 pressing was at 1050C for 3 hours. The product was ~-~
reported to be high quality polycrystalline material with no secondary phases. The superconducting transition temperature is about 80 K for x = 0 and `
about 90 K for x = 0.1.
It is desirable to have a superconductor of the 1-2-4 type with a superconducting transition temperature as high or higher than those of the 1-2-3 types since the 1-2-4 compositions do not exhibit the oxygen sensitivity found in the 1-2-3 compositions.

Summ~ry of th~ Tny~n~i~n This invention provides a novel superconducting orthorhombic phase having the formula MBa2-xsrxcu4o8r wherein M is selected from the group consisting of Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu, and x is ~rom about 0.1 to about 1.2. Preferably, x is from about 0.8 to about 1.2. Most preferably x is about 1.
These new compounds have superconducting transition temperatures higher than MBa2Cu4Og and comparable to those of YBa~Cu3O7_~.
This invention also provides a process for preparing a powder of the phase MBa2-xsrxcu4o8r wherein M is selected from the group consisting of Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu, and x is -from about 0.1 to about 1.2, said process consisting essentially of ~ a) preparing an essentially carbon-free mixed oxide precursor powder from an intimate mixture of M, Ba, Sr and Cu compounds with an atomic ratio of M:Ba:Sr:Cu of 1:2-x:x:4;
(b) heating said precursor powder in an oxygen-containing atmosphere, preferably substantially pure oxygen, at a pressure of about 100 bar to about 2 kbar tabout 10 MPa to about 200 MPa) and at a ~
25 temperature from about 850C to about 1050C, :~ :
preferably from about 925C to about 1050C, for a ~ -time sufficient, typically about 12 hours, to form a powder of MBa2_xSrxCu40g; and ~c) cooling said MBa2-xSrxCugOg powder in 30~ the oxygen-containing atmosphere of step (b) while ~
maintaining the pressure. ~-It should be noted that the atomic ratio of M:Ba:Sr:Cu of 1:2-x:x:4 may not be sacrosanct. Slight ~ ;
variation due to the presence of impurities or -35 weighing errors may still provide superconductive ; ~

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materials of the compositions of this invention which, however, may not be single phase.
It is preferred to have said precursor powder prepared by a solution route, for example, by drying and decomposing to the oxides a solution, a suspension or a precipitate of M, Ba, Sr and Cu carbon-free salts such as nitrates or hyponitrites, and especially preferred to have said precursor powder prepared by drying the oxides formed by the hydrolysis of M, Ba, Sr and Cu compounds dissolved in an organic solvent.
Alternatively, a mechanically blended or milled mixture of the constituent oxides may be utilized as the precursor powder. This mixture can be prepared directly from the constituent oxides or by lS mechanically blending or milling M, Ba, Sr and Cu carbon-free salts such as nitrates and subsequently -~
heating the salts in an oxygen-containing atmosphere, preferably oxygen, which is preferably free of CO2.
The heating should be carried out at a temperature high enough to insure decomposition of all of the nitrates to the oxides. Temperatures of about 600C
to about 700C for a time of about 1 to about 2 hours should be adequate.
It is preferred to have the oxygen-containing -atmosphere, whenever used in the process of this invention, to be free of CO2.
~isf Descri~t;on of the Drawing The drawing consists of two figures. Figures 1 and 2 are plots of flux exclusion versus temperature for two YBaSrCu4Og samples.
Detailed Description_~f the Inv8~ion The novel superconductor of this invention, MBa2_xSrxCu4Og, wherein M is selected from the group consisting of Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu, and x is from about 0.1 to about 1.2, has an orthorhombic unit cell, the dimensions of which decrease from those of Msa2Cu4Og as x increases. The superconducting transition temperature Tc increases as x increases.
This invention also provides a process for preparing a powder of MBa2CU48-The reactants used in the process of this invention are preferably in the form of powders with small particle si~e in order to increase reaction kinetics. It is preferable to avoid the use of saCO3 or any other carbonate as a reactant in the process and to avoid the formation of saco3 during the process since the presence of BaCO3 necessitates reaction temperatures of at least about 850C to 900C at atmospheric pressure to insure substantially complete ; reaction, i. e., substantially complete decomposition of BaCO3, and even higher temperatures are re~uired at elevated pressure. While air can be used when an oxygen-containing atmosphere is required in the process of this invention, it is preferred to use an oxygen-containing atmosphere that is free of CO2 to -avoid the formation of BaCO3 during the process. If carbonates are used as reactants, formation of BaCO
is to be expected, and the mixture containing the -~
BaCO3 must be heated to at least about 850C to 900C
and held there for a sufficient time to decompose the BaCO3 and form a powder suitable for use as a precursor powder of the process of this invention. -~
The process of this invention uses an essentially carbon-free mixed oxide precursor powder prepared from an intimate mixture of M, Ba, Sr and Cu -~
compounds with an atomic ratio of M:Ba:Sr:Cu of 1:2-x:x:4. As used herein, essentially carbon-free means that there is less than 1 wt% carbon in the `~
35 precursor powder. The precursor powder should be an -, '`' ~

.: - 1 0 intimately mixed fine-particle powder in order to facilitate the 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, Sr and Cu compounds with an atomic ratio of M:Ba:Sr:Cu of 1:2-x:x:4. One method for preparing such precursor powder is to form a nitrate solution of M, Ba, Sr and Cu, for example, by simply ~ -mixing aqueous solutions of the four component nitrates. This solution can be dried directly by heating, by spray-drying, or by spray-freezing ! followed by freeze-drying. Alternatively, precipitation can be achieved and a suspension formed by increasing the pH of the solution. The suspension can then be dried as indicated above. Spray-drying or spray-freezing followed by freeze-drying are preferred since they provide a more intimately mixed powder.
After drying, this mixed nitrate powder must be heated at about 600C to about 700C for about 1 to about 2 hours in an oxygen containing atmosphere, preferably oxygen, to decompose the nitrates and obtain a mixed oxide precursor powder.
Another method for preparing such precursor powder is to form an aqueous nitrate solution of M, Ba, Sr and Cu with an atomic ratio of M:Ba:Sr:Cu of 1:2-x:x:9, 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, Sr and Cu present in the original nitrate solution, and collect and dry the precipitate. After drying, this mixed hyponitrite powder must be heated at about 600C to about 700C
for about 1 to about 2 hours in an oxygen containing atmosphere, preferably oxygen, to decompose the hyponitrites and obtain a mixed oxide precursor powder.
A preferred method for preparing the precursor powder is to form a solution of M, Ba, Sr and Cu compounds with an atomic ratio of M:sa:Sr:Cu of 1:2-x:x:4 in an organic solvent. Controlled hydrolysis results in the formation of oxides, or hydrous oxides, which are filtered, washed and dried.
This hydrolysis product may contain water, either in the form of hydroxides or as chemically bound water, and should be heated prior to reaction at elevated -temperature and pressure. Heating at about 100C to about 600C, preferably at about 500C, for about 0.5 to about 3 hours, preferably about 1 hour, is sufficient. This heating should be conducted in a CO2-free, oxygen-containing atmosphere, preferably 20 oxygen. 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(C5H5)3, Ba(C5H5)2, ~-Ba(CsMes)2, Sr~CsHs)2 and Sr(CsMes)2, metal acetylides 30~ such as Cu [C3CC (CH3)2OMe]! metal aryls such as Cu(mesityl), metal alkoxides such Cu(OCMe3), ~ -~
Cu[OCH(CMe3~2], Cu(ocH2cH2oBu)2~ Cu(ocH2cH2NEt2)2 MsO(OCHMe2) 13, M(ocH2cH2oBu) 3~ M(ocH2cH2NEt2)3~ ,~
Ba(OCHMe2)2, Ba(ocH2cH2oBu)2~ Ba(ocH2cH2NEt2)2~ - -Sr(OCHMe2)2, Sr(ocH2cH2oBu)2 and Sr(ocH2cH2NEt2)2 metal aryloxides such Y[O-2,4,6-C6H2(CMe3)3]3, and metal amides such as Cu(NEt2), CU(NBu2)~ Cu[N(SiMe3)2]
and Y[N(SiMe3)2]~. The hydrolysis product may contain water, either in the form of hydroxides or as chemically bound water, and should be heated prior to reaction at elevated temperature and pressure.
Precursor powders can also be prepared by solid state methods. For example, nitrate salts, hyponitrite salts or mixtures thereof of the constitutent cations can be mechanically blended or milled and then decomposed, as described above, to a mixture of oxides. Alternatively, the constituent oxides or a combination of oxides and nitrate or hyponitrite salts can be mechanically mixed. If the salts are used in combination with the oxides, a . decomposition step as described above must be employed. Suitable oxides include M2O3, BaO, BaO2, -~
SrO, SrO2, CuO and Cu2O.
After synthesis the mixed oxide precursor, made by any of the above routes, is susceptible to reaction with moisture and CO2 and should be protected accordingly.
The mixed oxide precursor powder is heated at about 850C to about 1050C at a pressure of about -100 bar to about 2 kbar (about 10 MPa to about 200 MPa) for a period of time sufficient to obtain the desired M~a2_xSrxCu4Og phase. These conditions should be sustained for a period of 2-12 hours and then the sample cooled while the pressure is maintained. The 30 lpressure that the precursor powder experiences should be that of an oxygen-containing atmosphere, preferably oxygen, and preferably free of CO2. One method for accomplishing this is to use a CO2-free, oxygen containing atmosphere, preferably oxygen, as the pressurizing medium and to have this pressurizing medium in direct contact with the precursor powder.
Such an experimental arrangement will provide sufficient oxidizing potential to yield the desired phase, Msa2-xsrxcu4og~ with an average copper oxidation state > 2.0 even though the precursor powder has an average copper oxidation state S 2Ø In an alternate method, a different pressurizing gas such as air, nitrogen or argon can be used if the material is sealed in a pressure transmitting envelope along with its own oxygen producing source. For example, a precursor powder comprised of a mixture of CuO, Y203 and the peroxides of Ba and Sr can be loaded into a gold tube that is welded shut at both ends. At elevated temperature, decomposition of the peroxides yields sufficient gaseous oxygen so that the desired high oxidation state product may be obtained. When a precursor that does not employ peroxides is used, another oxygen-yielding compound can be used as an oxygen source. For example, KC103 can be sealed in the gold tube along with the mixed oxide precursor.
At elevated temperature, decomposition of the KC10 yields the required gaseous oxygen lea-~ing molten KCl as the decomposition product. Typically KC103 is placed within a second smaller gold tube that is --25 welded shut at one end, but crimped closed at the ~
other end in order to allow the escape of oxygen but ~ ~
contain the molten KCl. The smaller tube is sealed ~ -along with the precursor powder in the larger, pressure-transmitting gold tube. Using this 30 arrangement oxygen, air or inert gas such as nitrogen or argon can be used as the pressurizing medium. Air and especially oxygen are still preferred, however, since oxygen has a tendency to diffuse through gold at elevated temperature. Typcially, 0.5 - 1.0 gm of -35 KC103 per gram of mixed oxide precursor is adequate.

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While the inner tube is generally effective at containing the KCl, any KCl that does come into contact with the product can easily be removed by washing the product with water for 15 minutes and then drying.
The MBa2_xSrxCu40g product powder can be stored for later use. However, it displays the same reactivity toward CO2 and H2O as has been reported for the MBa2Cu307_~ and MBa2Cu4Og phases. 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 . Fisher in Physical Review B, 36, 5586 (1987).
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 in this field, 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 tsuperconducting quantum intexference devices) and in instruments that are based on the Josephson effect such as high speed sampling circuits and voltage standards.
EX~M~LES OF THE INVENTION
~a~æL~
A YBaSrCu4Og precursor powder was prepared by dissolving YsO(OCHMe2)13 [0.769 g], Ba(OCHMe2)2 [0.799 g~, Sr(OCHMe2)2 [0.644 g], and Cu(NBu2) [2.400 g] in 40 ml of THF to give a homogeneous solution. Hydrolysis was carried out by dropwise addition of this solution to a solution of degassed ;
water [3.10 g] in 40 ml of THF. The mixture was refluxed under an argon atmosphere for 16 h, and -~
filtered to give an orange solid. The solid was washed first with THF, then with pentane, and dried ; under high vacuum at 100C to yield 2.205 g of orange precursor powder. ~-A portion (-1 g) of this precursor powder was placed in a 1/2" ~1.3 cm) diameter gold tube sealed by flame welding at one end. This open gold tube was then placed in a tube furnace and heated at 500C in oxygen for one hour to drive off any water. The furnace was shut off and allowed to cool. After reaching room temperature the open gold tube containing the mixed oxide precursor was quickly removed from the tube furnace, and a smaller gold tube, welded shut at one end and crimped shut at the other, containing 1 g of KCl03, was placed inside the larger gold tube. The larger gold tube was then 30 I welded shut at its other end and placed in a high-pressure furnace. The sealed gold tube was externally pressurized with nitrogen at ~750 bar ~75 MPa) and the temperature of the bar was raised to 950C. The ~
pressure was adjusted to 1 kbar (100 MPa) and the tube ~ -was maintained at 950C and l kbar for 12 hours.

Power to the furnace was then shut off and the furnace allowed to cool to room temperature, and, finally, the external pressure was released. The tube was opened to reveal a uniform, black powder. Approximately 0.67 g of this powder was recovered. X-ray diffraction indicated that this powder was comprised of a YBa2Cu4Og-type phase, but with smaller unit cell dimensions, and a trace of CuO. The unit cell dimensions for the orthorhombic YBaSrCu4Og were:
a = 3.798 A (0.3798 nm), b = 3.855 A (0.3855 nm), c = 26.984 A (2.6984 nm), and V = 395.1 A3 (0.3951 nm3).
Magnetic flux exclusion measurements confirmed superconductivity and showed the sample to have an onset of superconductivity Tc at about 87 K as shown by the plot in Figure 1.
~X~MPLE 2 YBaSrCu~Og was produced from a mixture of the binary oxides wherein the atomic ratio of Y:Ba:Sr:Cu was 1:1:1:3. Quantities of the binary oxides Y2O3 (0.353 g), BaO2 (0.529 g), SrO2 (0.374 g), and CuO (0.745 g) were weighed out and ground together employing a wrist ~;
action mechanical grinding apparatus until a fine grey -powder, homogeneous in appearance, was obtained. The 25 ground powder was then placed in the bottom of a 3/8th ~ `
inch (1 cm) diameter gold tube. The gold tube was flattened to exclude air and to allow for the expansion -~
of oxygen gas resulting from the decomposition of the `
barium and strontium peroxides. The flattened tube was sealed by flame welding and a pressure of - 750 bar (75 MPa) was applied to the tube and its contents with nitrogen gas. The furnace temperature was raised to 950C, the pressure was adjusted to 1 kbar (100 MPa) and the furnace was held at that temperature and pressure - -for 12 hours. Po~er to the furnace was then shut off and the furnace allowed to cool to room temperature. -When the furnace reached room temperature, the external pressure was released. The tube was opened to reveal a -uniform black powder. The powder consisted of mainly YBaSrCu9O8 as determined by powder X-ray diffraction.
As expected, the difference in stoichiometry between ~ -product and starting materials result in the presence of - ~
extra unindexed peaks. However, there is no evidence in -the X-ray diffraction pattern for the presence of YBa2Cu3O7_~ or Sr-doped YBa2Cu3O7_~. Least squares refinement of the X-ray diffraction data for the YBaSrCu4Og phase gave the following unit cell parameters: a = 3.794 A (0.3794 nm), b = 3.846 A
(0.3846 nm), c = 27.08 A t2.708 nm) and V = 395.2 A3 15 (0.3952 nm3). These lattice parameters are significantly different from the lattice parameters of YBa2Cu4Og given in Comparative Experiment A and clearly demonstrate a new phase.
Magnetic flux exclusion measurements confirmed 20 superconductivity and showed the sample to have a -sharp onset of superconductivity Tc at about 91 K as shown by the plot in Figure 2. -This example demonstrates that under the synthesis conditions used, YBa2Cu3O7_a and Sr-doped YBa2Cu3O7_a are unstable with respect to YBaSrCu9Og.
Thus any increase in Tc obtained with quantities of -~
reactants corresponding to the stoichiometry of MBa2_xSrxCu4Og reacted according to the conditions of the instant process cannot be attributed to a -30 1 1-2-3-type phase, but rather is the result of the formation of the novel MBa2_xSrxCu4Og.
COMPARATIVE EXPERIM~ A
YBa2Cu4Og was produced from a mixture of the binary oxides wherein the atomic ratio of Y:Ba:Cu was 1:2:4. Quantities of the binary oxides Y2O3 ~1.467 g), BaO2 (4.400 g), and CuO (4.133 g) were weighed out and ground together employing a wrist action mechanical grinding apparatus until a fine grey powder, homogeneous in appearance was obtained. The ground powder was then placed in the bottom of a 3/8th inch (1 cm) diameter gold tube. The gold tube was flattened to exclude air and to allow for the expansion of oxygen gas resulting from the decomposition of the barium peroxide. The flattened tube was sealed by flame welding and a pressure of 750 bar (75 MPa) was applied to the tube and its contents. The furnace temperature was raised to 950C, the pressure was adjusted to 1 kbar (100 MPa) and the furnace was held at that temperature and 15 pressure for 12 hours. Power to the furnace was then ~-shut off and the furnace allowed to cool to room temperature. When the furnace reached room temperature, the external pressure was released. The tube was opened to reveal a uniform black powder. The powder consisted of mainly YBa2Cu40g as determined by powder X-ray diffraction. A small amount of unreacted Y2O3 was also detected. Least squares refinement of ,the x-ray diffraction data gave the following unit cell parameters: a = 3.841 A (0.3841 nm), b = 3.871 A
(0.3871 nm), c = 27.24 A (2.724 nm), and V = 405.1 A3 -~
(0.4051 nm3). The sample exhibited a sharp Tc onset -~
of - 77 K as measured by flux exclusion.
This example demonstrates that under the ;-synthesis conditions used the 1-2-4 phase YBa2CugOa is 30 ! formed and has a Tc of ~77 K in contrast to the higher Tc found in Examples 1 and 2 for YBaSrCu4Og.

Claims (29)

1. A superconducting composition having the formula MBa2-xSrxCu4O8, wherein M is selected from the group consisting of Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu, and x is from about 0.1 to about 1.2.

2. The composition of Claim 1 wherein x is from about 0.8 to about 1.2.

3. The composition of Claim 2 wherein x is about 1.

4. The composition of Claim 1 wherein M is Y.

5. The composition of Claim 4 wherein x is from about 0.8 to about 1.2.

6. The composition of Claim 5 wherein x is about 1.

7. A process for preparing a powder of the composition MBa2-xSrxCu4O8, wherein M is selected from the group consisting of Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu, and x is from about 0.1 to about 1.2, said process consisting essentially of (a) preparing an essentially carbon-free mixed oxide precursor powder from an intimate mixture of M, Ba, Sr and Cu compounds with an atomic ratio of M:Ba:Sr:Cu of 1:2-x:x:4;
(b) heating said precursor powder in an oxygen-containing atmosphere at a pressure of about 100 bar to about 2 kbar (about 10 MPa to about 200 MPa) and at a temperature from about 850°C to about 1050°C for a time sufficient to form a powder of MBa2-xSrxCu4O8; and (c) cooling said MBa2-xSrxCu4O8 powder in the oxygen-containing atmosphere of step (b) while maintaining the pressure.

8. The process of Claim 7 wherein said oxygen-containing atmosphere is substantially free of CO2.

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

10. The process of Claim 9 wherein the heating temperature is from about 925°C to about 1050°C.

11. The process of Claim 10 wherein x is from about 0.8 to about 1.2.

12. The process of Claim 11 wherein x is about 1.

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

14. The process of Claim 7 wherein said precursor powder is prepared by forming an aqueous solution of M, Ba, Sr and Cu nitrates, wherein the atomic ratio of M:Ba:Sr:Cu is 1:2-x:x:4; drying said solution to obtain a mixed nitrate powder; and heating said mixed nitrate powder at about 600°C to about 700°C for about 1 to 2 hours in an oxygen-containing atmosphere.

15. The process of Claim 14 wherein said oxygen-containing atmosphere is substantially free of CO2.

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

17. The process of Claim 7 wherein said precursor powder is prepared by forming an aqueous solution of M, Ba, Sr and Cu nitrates, wherein the atomic ratio of M:Ba:Sr:Cu is 1:2-x:x:4; forming a suspension by increasing the pH of the solution; drying said suspension to obtain a mixed nitrate powder; and heating said mixed nitrate powder at about 600°C to about 700°C
for about 1 to 2 hours in an oxygen-containing atmosphere.

18. The process of Claim 17 wherein said oxygen-containing atmosphere is substantially free of CO2.

19. The process of Claim 18 wherein said oxygen-containing atmosphere is substantially pure oxygen.

20. The process of Claim 7 wherein said precursor powder is prepared by forming an aqueous solution of M, Ba, Sr and Cu nitrates, wherein the atomic ratio of M:Ba:Sr:Cu is 1:2-x:x:4; mixing said solution with an amount of sodium hyponitrite or sodium peroxide effective to precipitate substantially all of the M, Ba, Sr and Cu in said solution; isolating the resulting precipitate; and heating said precipitate at about 600°C
to about 700°C for about 1 to 2 hours in an oxygen-containing atmosphere.

21. The process of Claim 20 wherein said oxygen-containing atmosphere is substantially free of CO2.

22. The process of Claim 21 wherein said oxygen-containing atmosphere is substantially pure oxygen.

23. The process of Claim 7 wherein said precursor powder is prepared by forming a solution of M, Ba, Sr and Cu compounds with an atomic ratio of M:Ba:Sr:Cu is 1:2-x:x:9 in an organic solvent, said compounds of M, Ba, Sr and Cu being soluble in said solvent and reacting readily with water to produce metal oxides or metal hydroxides; contacting the resulting solution with water to form said oxides or hydroxides; filtering, washing and drying said oxides or hydroxides; and heating said oxides or hydroxides at about 100°C to about 600°C for about 0.5 to about 3 hours.

24. The process of Claim 23 wherein said oxygen-containing atmosphere is substantially free of CO2.

25. The process of Claim 24 wherein said oxygen-containing atmosphere is substantially pure oxygen.

26. The process of Claim 7 wherein said precursor powder is prepared by forming an intimate mixture of oxides of M, Ba, Sr and Cu with an atomic ratio of M:Ba:Sr:Cu is 1:2-x:x:4.

27. The process of Claim 26 wherein said oxygen-containing atmosphere is substantially free of CO2.

28. The process of Claim 27 wherein said oxygen-containing atmosphere is substantially pure oxygen.

29. The process of Claim 26 wherein the oxides are M2O3, BaO2, SrO2, and Cu2O.