Note: Descriptions are shown in the official language in which they were submitted.
CR-8797
~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 ; ~
,' ~:
`." . ;: '
.,, .,, .~, .
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
12
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.
r!, . " ., , ~ "
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.
16
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
17
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
18
~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.
