Note: Descriptions are shown in the official language in which they were submitted.
x ~
~ W094/00385 PCT/AU93/00300
TITLE
SUPERCONDUC ~ GOXIDESBYCOPRECP~A~ONATCONSTANTPH.
FIELD OF THE INVENTION
THIS INVENTION relates to superconductors and
particularly high t~ rature superconductors and to an
improved preparation of high temperature superconductors
and fabrication of articles cont~i ni ng high temperature
supercon~uctors.
BACKGROUND ART
High temperature superconductors were first
disclosed in 1987 and are characterised by having
superconductor transition temperatures (Tc) above the
temperature of liquid helium and many with a Tc above the
temperature of liquid nitrogen (77.3K). The three broad
families of high temperature superconductors (HTSC) are
as follows -
(i) the Y-Ba-Cu-O system
(ii) the Bi-Pb-Sr-Ca-Cu-O ~y~t- ; and
(iii) the Tl-Ba-Ca-Cu-O system.
The system (i) which is reported in Phys. Rev.
Lett. 58 405 (1987) is easy to synthesise and has a wide
composition range over which superconductivity may
appear. It has a relatively low Tc which is
approximately 9OK.
System (ii) does not contain rare earth
elements and has a high Tc which is approximately 108K.
However it has a narrow composition range which permits
superconductivity. Sy~Lc(ii) is reported in Japan J.
Appl. Phys 27 L 1041 (1988).
System (iii) which is reported in Nature 332
138 (1988) has a high Tc which is approximately 120K but
the toxicity of th~ um iS of conc~n.
Other high temperature conducting systems
include -
(a) The 214 ~y~ . ie. (La,Ba)2CuO~
(b) 124(YBa2Cu~08) and 247 (Y2Ba~Cu7O15)
W094/0038~ 7 ~ PCT/A~I93/00300 ~
(c) The Bi2SrzCuO6 (CaCuO2) n ( N50,1,2) system and
its variants with Pb stabilisation
(d) The T12Ba2CuO6(CaCuO2)n(n-O,1,2,3) system
(e) The (Nd,Ce)2CuO4 system and its variants.
The two main techn~ues to form HTSC are
gr~ n~ ~ ng the respective HTSC precursors together or
solution co-precipitation of the respective HTSC
precursors.
Gr~ n~ ~ ng HTSC precursors is a lengthy process
and re~uires several gri n~ ~ ng and sorting stages to
obtain a particle size distribution and mix which will be
suitable for formation of the HTSC. However, it is
difficult to obtain phase purity and repeatable results.
Phase purity is critical in the efficiency of the HTSCs
and with many HTSC families, even slight variations from
the correct ratio of the various components will result
in loss or degradation of HTSC properties.
Solution co-precipitation is a tp~-hn1que to
prepare precursor (for example of HTSC co-"~ounds)
compounds so that they are mixed in a very intimate
-nn~r, perhaps as closely as on a molecular level. In
solution co-precipitation, the various precursor
components are initially dissolved in an aqueous solution
(usually cont~n;~g an acidifying agent such as nitric
acid). The dissolved precuL~ are precipitated from
solution by addition of a precipitating agent. These
agents have included carbonates, acetates, formates,
hydloxides and oxalates. The resultant co-precipitate is
filtered usually by a centrifuge and subjected to a low
te p~rature calcination process to l _ve or ~P~-ompose
the anion to provide a mixture of the respective cation
oxides. Further heating at elevated temrPratures of
about 90OC results in transformation of the cation oxides
to the high temperature superco~ cting material.
To obtain HTSC ~ ounds having high efficiency
and a high transition temperature, it is critical to
ensure that the respective precursors are present in the
213~7~
~ W094/00385 ~ PCT/AU93/00300
correct ratio. The ratio must be extremely carefully
controlled to obtain the high phase purity required.
Grin~i~g has not generally been successful in
being able to obtain commercial amounts of consistently
pure products.
To date, solution co-precipitation of HTSC
precursors has also not been able to provide conclstent
phase purities on anything more than extremely small
amounts and therefore this te~hn~que has been unsuitable
in commercial applications which includes continuous or
large scale processing.
STATEMENT OF INVENTION
We have now developed a t~-hn~que which allows
co-precipitation to occur and wherein the co-precipitated
precursors may have a consistent phase purity to allow
production of HTSC ~ol~ounds having high efficiency. The
t~chn~que can be used to generate HTSC compounds on a
commercial scale.
The t~chn~que may be used to co-precipitate
precu x~x to all copper oxidè based ceramics, such as
semi~o~ otor, insulator and superconductor phAQeQ in
YBCO, BSSCO and th~ um based systems.
The t~chn~que involves the surprising and
~nexpected discovery that mi ni ~cing the variation in the
pH of the solution during the co-precipitation process
can result in the co-precipitate precursors having a high
and conQlstent phase purity. The co-precipitates may
also exhibit a narrow variation in particle size ranges.
Therefore, in one form, the invention resides
in a method for co-precipitating superconducting
precursor components, the method comprising the steps
of -
dissolving the superconducting precursor
components into solution,
A~ ing a precipitating agent to the aqueous
solution, and
maint~ n ~ ng the pH of solution substantially
W094/00385 ~ 3 8 ~ 7 0 PCT/AU93/00300 ~
constant during the co-precipitation process.
The precursor components may be those which
form copper, bismuth and ~hAll~um based HTSC compounds.
Suitably aqueous solutions are used. It is
found that the pH of the aqueous solution does not
significantly vary (ie. variations of greater than 0.1)
upon initial dilution of the solution. Therefore, while
not w~ sh~ ng to be bound by theory, there appears to be a
buffer effect present.
The co-precipitation process suitably is caused
to occur within the buffer range or just outside of the
buffer range. That is, it is preferred that the initial
aqueous solution is diluted until ~ust before the pH
begins to significantly vary and that the co-
precipitation process is carried out at this dilution
value. We believe that this maximises the buffer effect
during the precipitation process and therefore ~ n~ ~ ses
the pH variation during co-precipitation.
Suitably, the superconductor precursor
components are those which form high t~ ,?rature
superco~ ctors. The super~on~llctor precursor comron~nts
are preferably dissolved in an acidic aqueous solution.
The solution may be acidified by an organic or inorganic
acid and nitric acid is a typical acidifying agent. The
initial pH may vary dep~nA~ ng on the type of
superconducting components dissolved therein and various
other variants such as volume of water and acidifying
agent used but typically, the initial pH range is between
0.1 to 3 and suitably about 0.7 (for the yttrium based
family) and about 0.45 (for the bismuth based family).
Suitably, the precipitating agent is a
dicarboxylic acid or derivatives thereof. The
derivatives may include salts, esters anhydrides, amides
and the like. Ammonium oxalate is one suitable
derivative. A suitable dicarboxylic acid is oxalic acid.
We also note that the co-precipitated precursor
oxalates used by our t~r-hnlque can be calcined to the
~ W094/00385 ~13 ~ ~ ~ O PCT/AU93/00300
,
precursor oxides at a fraction of the time currently used
to form precursor oxides by other processes. While again
not w~h1 ng to be bound by theory, we believe that the
improved times results from the high phase purity we
achieve by our terhnique, the intimate mixing, and
possibly the constant particle size obtained.
The advantage of being able to calcine the co-
precipitate precursors at a much quicker rate than
hitherto achieved, is that conventional high through
volume driers such as drop tube furnaces, fluidized bed
furnaces and inr.l ~ ned furnaces can be used to form the
precu~sor oxides in commercial quantities.
In another form, the invention resides in a
method for forming shaped articles or products which
contain HTSC material.
Since the discovery of high temperature super
conductors (HTSC) in 1987, there have been various
t~chniques developed to fabricate shaped articles
cont~n~g HTSC. These ~h~p~ articles can include
wires, tapes, coils, thick and thin ~ilms, coatings,
tubes, rods and other bodies. These æh~ppA articles find
applications in power transmission, levitation trains,
magne~ic shielding, microwave cavities, magnets and
energy storage devices. With a large number of existing
and potential uses of HTSC contA~n1ng articles, there is
a need to develop a process which allows manufacture of a
variety of shaped articles easily and which can be
readily adapted to any type of shape.
There are various known techn~ques which have
been used to provide ~h~p~ articles cont~ n; ng HTSC. In
one known t~rhn~que, Calcined 123 powder is placed into
suspension by using toluene as a solvent and other
organics as dispersants. It is n~r~c~ary to add
dispersants so as to deflo~r~l~te or disperse the slurry
because agglomeration of the slurry results in the poor
quality of formed products. This slurry can be cut into
shapes. However, the additives such as dispersants need
W094/00385 ~ 3 g 6 7 a PCT/A~193/00300
to be burned out in a very controlled manner from the
"green" product. This results in increased porosity and
lack of product density and can also result in the
formation of cracks, faults and defects. It is also
known to use thick~nl ng agents such as cellulose to
aqueous solutions in order to form a slurry.
Another known t~-hnique is disclosed in U.S.
Patent 5,026,683 and uses a powder in tube method. This
process ~onc~ts of f~ ng a hollow metal tube with an
HTSC powder and then rolling the tube to reduce the
overall wire diameter. This process is expensive and
requires the use of very sophisticated -chinery to give
multi-filament, circular cross-section wires. This
process is also not suitable for YBCO superconductors as
these compounds require heat treatment in oxygen which
does not readily flow through the metal walls.
Therefore, it is not possible to obtain high Jc with YBCO
powders by this process. There may also be physical
limitations on the length and the diameter of wires made
by this process.
Another techn1que is disclosed in U.S. Patent
4,975,416 which consists of heating an oxide composition
to above the melting ~- ,A~ature and then cooling the
melt through an orifice to form a wire. This wire is
then suitably heat treated to make it superconducting.
Although some sllccecs is obt~ne~ by this process for the
oxide materials, it is not possible to form wires in the
YBCO system by this process because of its ~ncongruent
melting behaviour. (A compound which melts ~ncongruently
results in more than one ~o,..~ound upon melting, none of
which is the original composition). Wires made by this
process are also very brittle and cannot be formed into
coils with any ease.
A further known t~chn~que of forming wire/coils
is by extrusion which forces a ceramic paste through an
orifice. This process can be adapted for HTSC ~o~ ounds.
In the process a powder of Calcined 123 compound is mixed
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~ W094/00385 . PCT/AU93/00300
with a binder, dispersant, plasticizer and a solvent.
The formed slurry is extruded through a die forming
flexible green wires. At this green or unfired state the
wires can be coiled or wrapped around a former. The
wires are then heat treated to burn out the additives and
to sinter the HTSC form.
Again, this t~chnique suffers from the
disadvantage of forming a non-homogenous mixture due to
the separate addition of binder, dispersant, plasticizer
and solvent.
Australian Patent 603001 discloses a method of
forming super conductive products using casting or screen
printing HTSC powders to form bulk Ch~pP~ or coatings.
However, in this method a calcined HTSC powder is used
which is already inherently brittle. Therefore, in order
to form a stable suspension, additives are necessary such
as deflocc~ ts and plasticizers and these must be in
relatively large amounts - up to 40% in order to provide
a sufficiently plastic/flowing mass.
We have now developed a te~hnique which can be
adapted to allow manufacture of a large number of
different ~h~pe~ products and articles cont~ining HTSC
materials and wherein the resultant formed article or
product can be substantially more dense than conventional
t~chniques. By optimizing parameters of slip casting
process we may remove substantially voids, cracks or
faults which can adversely affect the superconducting
nature of the article or product.
Our co-precipitation approach allows similar
downstream proc~ccing for all copper oxide based
ceramics. This downstream processing includes all shapes
such as ~hose prepared by slip casting, extrusion, thick
film and doctor blading.
Ceramic materials including HTSC compounds are
known to be inherently non-plastic. Therefore in order
to form shapes such as wires and crucibles, plasticity
providing agents such as binders, dispersants, lubricants
W094/00385 ~ 7 Pcr/Au93/0030b ~
and plasticizers must be added. The present invention
can produce a precursor mix with no additives, and which
is plastic. This plastic mix may be added to HTSC
_ -unds or their precursors, metals and other ceramics
thereby providing a mix which may be ch~pPA by
conventional t~hniques.
We can achieve this by using a co-precipitation
t~chn~ue to form the precursor HTSC precipitates and by
selecting a precipitating agent which confers plastic
properties to the precipitated product, and which can
also be easily removed, such as by heating, to provide
the resultant product.
In this manner, there is no requirement to
include separate additives to the precipitated product
such as dispersants, plasticizers and the like. This, in
turn, allows our precipitated product to be substantially
homogeneous while still having plastic properties
sufficient to allow the precipitated product to be Sh~peA
by extrusion, mol-lA~ng~ casting and the like. The
finally formed HTSC compound can also be more dense.
In another form, therefore, the invention
resides in a method for shaping HTSC cont~ nl ng materials
~III~L ising the steps of -
dissolving HTSC precursor components into solution,25 ~AA i ~g a pecipitant which will cause the precursor
components to precipitate, the precipitant being selected
to provide plastic properties to the co-precipitated
product, sub~ecting the resultant co-precipitated
product to a shaping step, and, L~- -ving the precipitant
from the ch~pPA product.
Suitably, the precipitant is an organic
dicarboxylic acid or derivative thereof. A suitable
organic dicarboxylic acid is oxalic acid and the
derivatives may include salts, esters, anhydrides, and
am~des.
The resultant co-precipitated product may be
treated to increase or decrease its viscosity depending
~ 3~670
_ W094/00385 PCT/AU93/00300
on the particular type of shaping step reguired. Water
can be added or removed to ad~ust the viscosity and
therefore the plasticity of the precipitate.
By plasticity is meant that the precipitate or
slurry is of sufficient viscosity to allow it to be
moulded, extruded, cast, coated and the like. Various
additives may be added to the precipitated product.
These may include HTSC compounds/precursors, inert
fillers and the like. Various additives may also be
added to the solution prior to precipitation to allow
intimate mixing to occur.
The precipitant can be removed by a heating
step, sufficient to burn off or decompose the
precipitant. If the precipitant is an oxalate, the
heating step should be sufficient to reduce the oxalate
to form the various cation oxides.
If desired, the heating step may be contin~
or ad~usted to result in the shaped product having HTSC
properties. Suitably, the initial precipitation
t~hn~que is similar or identical to that disclosed above
with buffering of the pH of the medium during the co-
precipitation process.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a cnh~-~tic view of process scheme.
Figure 2 is a schematic flow sheet of Example l.
Figure 3 are pH values of Example l.
Figure 4 is an XRD plot of Y-123 powder.
Figure 5 shows XRD plots of HTSC materials formed at
different times from oxides.
Figure 6 is an XRD plot of HTSC formed directly from
oxalates.
Figure 7 is a susceptibility v temperature for wire
prepared by Example 4.
Figure 8 is an XRD plot of wire prepared by Example 4.
Figure 9 is a susceptibility v temperature for Y-123 wire
prepared by example 5.
Figure lO is a Jc v sintering temperature of Y-123 wire
W094/00385 2 ~ 3 8 ~ ~ ~ PCT/AU93/00300 ~
prepared by Example 5.
Figure 11 is a plot of resistivity v temperature of a Y-
123 wire prepared by Example 3.
Figure 12 is a susceptibility v t-mr~rature for the
samples prepared by Example 7.
Figure 13a is a trAns~sion electron micrograph of
Y8a2Cu3O7d powder YBCO calcined powder (500C).
Figure 13b is a transmission electron micrograph image
formed from calcined precursors.
Figure 14 is an XRD plot of BSSCO powder of Example 11.
Figures 15a and 15b are scAnn~ng electron mi~L~y~aphs of
YBa2Cu3O7d powders-
Figure 16 is a susceptibility v temperature for wire made
by Example 8.
Figure 17 is an XRD of Pb doped BSSCO powder of process 2
in Example 10.
Figure 18a is an XRD plot of Bi-2223 precursor powder of
process 1 of Example 10.
Figure 18b is an SEM micrograph of the powder of process
1 of Example 10.
Figure 18c is an SEM micrograph of the powder of Example
10 .
Figure 19 is an XRD plot of Y-211 powder made by Example
9.
Figures 20a and 20b are SEM photos of sample of process
2, F.XA~r1 e 10.
Figure 20c is a magnetic susceptibility v temperature of
sample of process 2, Example 10.
DETAILED DESCRIPTION
EXAMPLE 1:
This er- ,le illustrates the co-precipitation
process to manufacture pre-cursors and HTSC powders of
the compound Y1Ba2Cu3O,d(123). An overall cr-he~~tic
process is illustrated in figure 1.
78.0g of reagent grade yttrium oxide, Y2O3, and
500g of reagent grade copper nitrate trihydrate
CuN2O6.3H20, and 360.6g of barium nitrate, BaN2O6, are
2~ 38~70
W094/00385 ~ PCT/AU93/00300
11
charged into a 20 litre measuring plastic cont~i~er. To
the above mixture is added 10 litre of de-ionised water
and 240ml of co~c~ntrated nitric acid. The above mixture
is mechAn~cally mixed until it turns into a clear blue
solution. This mixing is accomplished in about 6 hours.
At the end of ~xi ng, a pH meter is used to continuously
monitor the pH of this solution. Initially, a pH of
about 0.7 is re~che~ for this solution. It is seen that
with addition of water the pH initially does not rise.
It is to be noted that the pH of water is about 7.0 and
such a water addition is expected in other systems to
increase the pH of a solution. However, only after about
4 litre of water addition, the pH shows a perceptible
increase. At about 6 litre of water addition, the pH of
the solution is about O.9. (The volume of the solution
is now about 17 litres). Such behavior indicates that
there is a buffer zone present in the nitrate solution.
This zone is controlled by addition or subtraction of
water. We observe that if sufficient water is added to
surpass this buffer zone then we obtain ro~ci~tency in
co-precipitation. The aqueous solution may be buffered
and then surpassed by substituting water addition with
addition of other solvents in the final stage. These
solvents may be those that do not cause i ~~iate
precipitation such as acetic acid. However, this
addition shows no observable im~lo~ement in precipitate
properties.
To another plastic ContA~ ne~ are weighed out
1008g of oxalic acid. 16 litre of de-io~sed water are
added to the oxalic acid and r?c~nically stirred. In
about 4 hours, the solution is clear. A pH meter is now
used cont~nllAlly to monitor the pH. At this stage the pH
is about 0.8. To this mixture of oxalic acid and water
is added 850 ml of ammonium llydluxide and allowed to mix
for about 15 minutes. The pH of the mixture is now about
3.45. It is also feasible to achieve a pH of 3.45 by
taking a mixture of oxalic acid and ammonium oxalate.
W094/00385 ~ I 3 ~ ~ 7 Q PCT/AU93/00300
12
The two solutions (i.e. nitrates and oxalic
acid) have the same volume; for example, in this case, 17
litre. The two contAin~s with their respective
solutions are ~onnerted to peristaltic pumps which drive
the two solutions at identical speeds into a reaction
tank. This reaction tank also acts as a convenient
medium for quality control measuL~ ?nts. Within the
reaction tank, and included with the pumping lines, the
mixed solutions rapidly form a blue precipitate which
consists of the respective cation-oxalate
co-precipitates. In this example, these co-precipitates
are yttrium-oxalate, barium-oxalate and copper oxalate as
well as mixed oxalates such as barium and copper and
oxalate. From the reaction tank, the mixture may then be
transferred to a centrifuge for filtration. Alternately,
the precipitated oxalate mix in the reaction tank may be
sufficiently dried and used directly for forming. A
~r,~ -tic flow sheet of this arrangement is shown in
Figure 2.
The reaction tank is placed on a magnetic
stirrer and constantly stirred. About 2000ml of the
reacted mix is maintA ~ n~A in the reaction tank by
controlling the flow rate out of the ~econA peristaltic
pump. Such a volume in the reaction tank increases the
homogeneity of the final co-precipitate and also serves
as another location for quality control measurements. A
pH meter is used to constantly read the pH of the reacted
mix as well as that of the filtrate. It is observed that
both pH values are constant during the course of the
reaction. pH of the reaction mix gives an indication of
the consistency of the co-precipi~ation process and
values are shown in Figure 3 for a number of runs. The
pH of the slurry is generally observed to be about 1.3,
with an average variation of +O.1.
The slurry at this stage can be used for at
least three different applications and may require at
least three different viscosity treatments. These are
~ W094/0038~ ~ 3 8 6 7 0 PCT/AUg3/00300
.
13
respectively:
(i) calcination to form HTSC powders,
(ii) extrusion to form wires/coils and
(iii) slip casting to form bulk bodies and/or to coat
thick films.
There are several synthesis procedures for
obtA~n~ n~ HTSC compound powders. The most commonly
practiced te~-hnique is by calcination of a stoichiometric
mix of oxides, carbonates and similar chemicals. Such a
calcination process can give phase pure YlBa2Cu30,d(123) in
a reproAllc~hle manner. However, achieving homogeneity
and adequate o~yyellation may be a problem for large
amounts of powders. In all cases, the time required for
complete conversion of raw materials (e.g. oxides,
carbonates etc.) to the HTSC compound is several hours.
In addition, intermittent grin~lng of the partially
calcined powders are often required during this lengthy
process.
The advantage of proc~Aing from a solution is
realised by several authors and there are various reports
of such processing. A spray drying process has been used
to dry a slurry of carbonate co-precipitated powders
which then required long calcination times with
intermittent slurrying to obtain YlBa2Cu307d(123) powder.
A spray dried nitrate solution of YlBazCu3O~d(123) cations
to obtain powders which were then calcined for long hours
to obtain YlBazCu30,d(123) powder has also been used.
Our co-precipitated powders can react to form
the 123 powder in a matter of a few minutes. This
process is valid for other co-precipitated HTSC powders
such as the bismuth-based HTSC ~ unds. Figure 4 is an
XRD plot of the Y-123 powder.
Example 2:
The coprecipitated oxalate as obt~n~ from the
centrifuge of Example 1 is dried in a vacuum oven at
100C. After about 12 hours, the precipitate is a dry
mass which can be easily ground in a mortar and pestle or
W094/00385 ~ 7 ~ PCT/A~93/00300
14
in a food proc~ceor to give a loose fluffy powder. This
powder may then be heat treated by two different methods.
The end products of both methods are pure YlBa2Cu3O7d(123)
powders with identical physical and chemical properties.
Method 1
(1) The powder is transferred to a refractory
beaker (eg alumina) and heat treated in a muffle furnace
to ~e ~ ? the oxalates to the oxides of the respective
cations. A temperature may be selected that is below the
first binary eutectic but above the ~P~- position
temperature of each oxalate. Thus, for example for the
batch prepared above, a temperature of 500C is selected.
The powder in the alumina beaker is transferred to the
muffle furnace and heat treated at a rate of 5C/min to
500C at which it is held for 15 hours. The powder is
then cooled down to room temperature at 5C/min. Further
experiments have shown that if the depth of the powder in
the refractory container is small, (eg 5mm), the time at
500C can be reduced to 5-10 minutes. Oxalate powders
thus calcined at 500C for various times were analysed by
x-ray diffraction. It is observed that in about 10
minutes, an oxalate co-precipitated powder as prepared by
our process can decompose to form oxides thereby giving
the same result as those obt~ine~ by other methods after
several hours of heat treatment. The rapid calcination
of oxalate powders may not be possible with precursors
formed by (i) improper co-precipitation methods or (ii)
other methods.
Figure 13a shows tr~nem~seion electron
mi~l~yLaph of the calcined powder and it is clearly seen
from the extent of lattice fringe images with individual
y~als that the particle size is less than 20 nm and
the various particles are intimately mixed. Figure 4
shows an XRD of Y-123 powder made from these calcined
powders and Figure 13b shows a transmission electron
micrograph of the YBa2Cu307 d (123) powder. The unit cell
dimensions of this micrograph indicate the HTSC
W094/0038~ ~ 3 ~ . - . PCT/AUg3/00300
1-- i '
superconducting phase is present.
Once the oxalates are ~eromposed to form the
oxides, they are heat treated at higher temperatures to
form the HTSC material. Figure 5 shows XRD patterns o
oxide powders as obt~ne~ above after heat treatment for
various times at 950C. It is observed that the ox~s
have reacted to form the HTSC material in about 5
minutes. This time is considerably reduced when compared
with conventional firing periods of several hours.
The advantage with the above two step firing is
that we may conveniently store powders which have been
heat treated at 500C. In comparison, the dried oxalate
powders tend to absorb water while the 123 HTSC compound
slowly degrades under humid conditions.
Method 2:
The dried oxalate powder as obt~; ne~ after
drying in a vacuum oven may be directly calcined to
produce the HTSC powder. In this example 80gm of oxalate
powder was placed in a shallow (~lcm height) alumina boat
and fired at 950C for various times in a muffle furnace.
Results obt~ne~ from this procedure are shown in Figure
6.
Thus, either Method 1 or Method 2 may be used
to convert the oxalates into the oxides; the high
reactivity of the precursor powders to firing and
subsequent transformation to HTSC material is due to (i)
the proper co-precipitation of oxalate precursors and
(ii) the fine, intimately-mixed nature of the
co-precipitated powders.
Since the reaction time for these
co-precipitated oxalate powders is in the order of a few
minutes, we are able to use a variety of industrial
equipment for calcinations. This equipment includes a
tl~nnel kiln, drop tube furnace, a fluidised bed furnace
and a horizontal zone furnace.
Further, due to the high reactivity of our
oxide mixture (or the powder as obt~ine~ after 500C
WO 94/0038~ r~ PClr/AU93/0030
16
calcination) we are able to produce pure YlBa2Cu307d(123)
powders but with a range of particle size distributions.
The mixture of oxides as obtA~ne~ after 500C calcination
is transferred to a refractory contA~n~r and charged
into a furnace. The sample may then be heat treated in
different ways to yield a variety of particle sizes in a
controlled manner. In each case, the material is HTSC
YlBa2Cu307 d Electron mi~-oylaphs of the various powders
thus obtA~ne~ are shown in Figures 15a and 15b and
illustrate a variety of particle sizes which can be
obt~n~ in a reproducible manner. Thus, for example, we
may use the following three firing schemes :
(a) Heat the furnace at 5C/minute to 800C and hold for
20h. The furnace is then cooled at 5C/min to 475C where
it is held for 5h. The furnace is then cooled to room
temperature at 5C/minute. The heat treatment of this
sample is carried out in an atmosphere of flowing oxygen.
The particle size obt~ne~ by such a process is less than
a micron as shown in Figure 15a.
(b) Heat the furnace at 5C/minute to 900C and hold for
lh. The furnace is then cooled at 5C/minute to 475C and
held for 5h, after which it is cooled to room temperature
at 5C/minute. The heat treatment of this sample is
carried out in flowing oxygen. A typical particle size of
HTSC powder thus obt~in~ is lmicron, as shown in Figure
15b.
(c) The furnace is heated to 950C at 5C/minute and held
for 30 minutes. The sample is then cooled at 5C/minute
to 475C and held for 5h, after which it is cooled at
5C/minute to room ~ p~rature. The heat treatment of this
sample is carried out in flowing oxygen. A typical grain
size of HTSC powder obt~i~e~ by this method is about
2.5micron.
EXAMPLE 3:
This example illustrates that it is possible to
extrude wires from a plastic body/mass entirely
consisting of co-precipitated oxalates. About lOg of wet
-2 1 ~
~ W094/00385 ; PCT/AU93/00300
j .
17
oxalate slurry was transferred to a mortar and pestle.
The viscosity of the slurry was about that of bread
dough. It was further ground in the mortar and pestle to
break any poss$ble agglomeration and to increase
homogeneity. The mass was then extruded through a 0.6mm
die using about 400kPa applied pressure. Binders,
lubricants, or solvents were not used. Wires thus
obt~ne~ were dried for 2 days at room temperature.
After the wires were dry, they were placed in an alumina
boat and then transferred into a tube furnace. The tube
furnace was then heated from room temperature to 250C
where it was held for 3 hours, at the end of which it was
heated at 5C/minute to 960C. When the furnace r~Ache~
960C it was held for about 10 minutes and then shut down
and allowed to cool. The entire heat treatment was
performed in a flowing oxygen atmosphere. It is observed
that the wire held shape, although there was a
significant amount of porosity. However, the wire sample
showed a superconducting transition at 91K as observed by
a magnetic susceptibility measurement. If the wire is
heated directly to 960C without the 250C hold, it breaks
into pieces due to a fast release of water. Figure 11
shows a plot of resistivity v temperature of such a wire.
EXAMPLE 4:
This ~X~mrle shows that oxalates and 123 and/or
other HTSC compounds can be mixed to still form a plastic
mass which can then be formed into a shaped ceramic.
About lOg of calcined Y1Ba2Cu307d(123) powder is
mixed with about 5g of co-precipitated oxalate paste.
The oxalate paste had a viscosity about that of
toothpaste. The mix was ground in a mortar and pestle by
hand to increase homogeneity and break any possible
agglomeration. It was then extruded through a 0.6 mm die
using about 400kPa applied pressure. The wires thus
formed were dried at room temperature for about 2 days.
After adequately drying, the wires were placed on an
alumina boat and transferred into a tube furnace. The
W094/00385 ~ 3 8 ~ 7 ~ PCT/AU93/00300
18
furnace was then heat treated at 5C/minute to 250C where
lt was held for 3 hours, after which it was heated at
5C/minute to 970C. At 970C the furnace was held for lO
hours, after whlch it was cooled at 5C/minute to 450C
where it was held for 5 hours. The entire firing process
was carried out under flowlng oxyyell. Figure 7 shows a
supeL~o..d~cting transition in a sample of wire made by
this process, as evi~nce~ by a sharp drop to zero
resistance when cooled to below 9lK. Figure 8 shows an
XRD plot of a sample of wire made by this process. XRD
on the surface shows a high degree of orientation which
is illustrated by increased intensity of the 001
reflections. A typical critical current density, Jc, of
a sample made by this process is about 1O2A/cm2.
EXAMPLE 5:
This example illustrates that a more
conventional process of ~ ng hi n~er to the HTSC
compound may be used to fabricate ~h~pes such as wires.
However, due to the inherent fine particle size and
homogeneity of our powders, we may reduce the amount of
such binders to about 2 weight percent. In conventional
processes which require additions of binder, this
fraction is greater than 10 weight percent. As a
~-onæ~quence of such reduction in organic binder, less
material which is non-superconducting need be removed
from the sintered body and thus, we are able to obtain
dense wires with high current carrying capacity.
Wires have been extruded using the more
conventional route in which a binder is added. lOg of
YlBa2Cu3O7 d (123) powder as made by calcination of the
co-precipitated YBCO oxalate precursor was mixed with
0.2g of commercial HPMC (binder) and 50ml of water by
means of a magnetic stirrer in a glass beaker. In about
40 minutes, sufficient water had evaporated to leave a
residue that had viscosity sl ~1 ~r to toothpaste. This
viscous mass was then extruded and proc~se~ as in
Example 2 listed above. Magnetic susceptibility and
7 ~
W094/00385 ~ PCT/AU93/00300
19
phase purity results obtained from this sintered wire are
8~ r; 1 ~r to those for Example 2. The magnetic
susceptibility plot for this wire sample after sintering
a is shown in Figure 9. However, the current carrying
cAp~c~ty Jc as measured on a 0.6mm diameter wire over
lengths of several mi 11 ir?tres, is -lO A.cm . The
variation of critical current density, J~, measured by
transport, for wires prepared by this method and sintered
for various temperatures is shown in Figure lO. Repeat
measurements of Jc for different samples pror~cc~ by the
same methods are also shown in this figure.
EXAMPLE 6:
The co-precipitation process gives an oxalate
mixture that is inherently plastic and can stay in
suspension without the use of defloculants. In fact, the
oxalate powder used in this process, once dried, can be
re-susp~n~ in water without the addition of any organic
agent. Therefore, it h~r_ ~ S possible to use, for
forming, an aqueous mix of oxalates either by themselves
or with varying amounts of calcined HTSC powder and other
additives such as silver and/or superconducting
_ ,ounds. It further becomes possible to mix these
additives in solution, thereby obt~ n ~ ng intimate r~x~ ng
at an extremely fine-scale (e.g. sub-micrometer).
A slurry is prepared by addition of water to
oxalate co-precipitate powder in a glass beaker. The
slurry is continuously mixed by a magnetic stirrer and
water is added till the slurry pours evenly. A mold was
prepared from plaster of paris using a preformed crucible
or some other suitable shape. The slurry is poured into
the mold, and after about 5 ~eco~c~ drained out. In
this manner, a coating of the slurry mixture (e.g. YBCO
precursor oxalates) remained on the mold. After about 2
hours, the coating releases itself from the mould and can
be easily extracted and left to dry in open air. Such a
process of slip casting a ceramic slurry is known.
In order to increase resistance to cracking
W094/0038~ 8 ~ PCT/AU93/00300
during drying of the cast form, oxide powder or HTSC
l ~-unds may be added to the slurry. The oxide powder
is obt~ne~ by calc~n~ng oxalates at 500C. Both calcined
oxide powders and HTSC compounds behave similarly in this
type of oxalate slurry, and therefore, provide similar
products upon suitable firing.
EXAMPLE 7:
The example illustrates the use of a
co-precipitated oxalate slurry, made by dissolution of a
dried oxalate powder, with an addition of HTSC oxides for
the slip-casting of ch~p~ which show superconducting
properties and a critical current density >1O2A.cm
The following describes one specific example to
make a crucible of approximate ~i ?n~ions l.Ocm diameter,
l.Ocm height with l.Omm thick walls.
6.0 g of oxalate powder is added to 60ml of water in a
glass beaker and mech~ni cally stirred by magnetic stirrer
for 3 days. After this time, it is observed that the
mixture had h~: ~ quite viscous and poured like honey.
To this viscous mixture was added 12.0g of oxide powder.
This oxide powder had been prepared by calc~ n~ ng dry
oxalate powder to 500C for lOh in a muffle furnace. A
homogenous mix is obt~lne~ in about 5 minutes, and then
the slurry is poured into a pre-formed plaster of paris
mould. After about 30 s~con~c~ the slurry is poured out,
thereby leaving a coating about 1.5mm thick on the inside
of the mould. After about 2hours, the cast mixture of
oxalate and oxide releases from the mould and retains the
shape of the mould. The "green-body" in the shape of a
crucible is dried in air for 3 days. Once dried, it is
then fired to sinter the body, thereby forming the
supercon~-lcting .- , lund (e.g. Y1Ba2Cu3O7d (123)) in the
shape of the crucible. This sintered body also maintains
sufficient me~hAnical strength to withstand h~n~ g and
to contain liquids, such as liquid nitrogen.
A suitable firing scheme for these slip-cast
bodies may involve firing the "green-body" crucible at
~3~7~
W094/0038S PCT/A~193/00300
21
0.5C/minute to 500C, where it is held for 2h. This first
firing allows the water to escape very slowly, thereby
limiting the formation and propagation of cracks. After
the hold at 500C, the furnace is heated at 5C/minute to
960C and held for 2h, after which the furnace is cooled
to 475 C and held for 5h. The hold at 475C is essential
for converting a maximum ~lo~o~Lion of the tetragonal
phase (YBCO) to the superconducting orthorhombic phase.
The complete firing schF may be undertaken in a flowing
oxygen atmosphere, or, as also demonstrated, in air.
An electron micrograph illustrates the fine
grained morphology of the surface of slip cast bodies and
the high bulk density (>90~). An optimum selection of the
firing scheme may be used to texture (i.e. grain
orientation) or to improve individual grain morphology.
Figure 12 shows magnetic susceptibility measurements on
pieces of slip cast crucible fired at 960C (curve "a"),
930C (curve "b") and 910C (curve "c") for 10 hours in
flowing 2- It is observed that there is a superconducting
transition at 91K as is expected for YlBa2Cu34 d (123)
material. Critical current density measurements on a
piece of the crucible using a four-point probe yielded a
Jc value of ~200A.cm 2 at 77K and OT field for transport
of electrons over millimetre lengths.
EXAMPLE 8:
This example shows the use of a slurry to add
Y2BaCuOs (211), which is non-superconducting, in a defined
fashion - in terms of size, amount and shape - to use as
part of flux r~ nn~ ng experiments in which the ob~ective
is to surround small grains of 211 phase with 123
material.
About lOg of calcined YlBa2Cu307d (123) powder
is mixed with about 5g of co-precipitated Y2BaCuO5 (211),
oxalate paste. The oxalate paste has a v~ccos~ty about
that of toothpaste. The mix was ground in a mortar and
pestle, or by a me~-hAni~ed mixer under mild vacuum. These
latter i~g methods allow controlled mixing conditions
W094/0038~ 8 6 ~ O PCT/A~I93/00300 ~
22
as well as de-airing of the oxalate~HTSC mixture. This
mixture was then extruded through a 1.0 mm die using
about 400kPa applied pressure. The wires thus formed
were dried at room t~ rature for about 2 days. After
adequate drying, the wires were placed on an alumina boat
and transferred into a tube furnace. The furnace was
then heat treated at 5C/minute to 260C where it was held
for 3 hours, after which it was heated at 5C/minute to
990C. At 990C the furnace was held for 10 hours, after
which it was cooled at 5C/minute to 450C where it was
held for 5 hours. The entire firing process was carried
out under flowing oxygen. Figure 16 shows the results
from magnetic susceptibility measurements for this wire.
The result shows that the sample becs~-~ superconducting
at the transition temperature of ~9lK. The current
carrying c~r~c~ ty, J~, of this wire is -300A.cm~2. A
method to obtain higher performance of superconductivity,
particularly under an applied magnetic field, involves
dispersion of 211 phase into a 123 matrix. This potential
use of 211 phase to provide p~ nn~ ng centres has been
described in Supe~cond. Sci. Technol., 4, S49, 1991.
This example ~^~o~ctrates that an intimate mixture of 211
and 123 phases may be made at a sub-micron scale through
the use of an oxalate slurry formed by the
co-precipitation process.
EXAMPLE 9:
This example illustrates the application of the
process of co-precipitation to the manufacture of the
compound Y2Ba~Cul05. It further illustrates that our
process may be used to sl~cc~c~fully manufacture copper
oxide-based compounds other than the 123 stoichiometry.
Whilst the compound Y2Ba,Cu~05 is not known to be
superconducting, it is used for a variety of HTSC-related
applications, including those for doping and as a
substrate. The chemicals are taken in the molar
stoichiometry Y:Ba:Cu :: 2:1:1
45.16g yttrium oxide is added to a 1 litre
2~38~7a
W094/00385 ,~ PCT/AU93/00300
23
nApAr~ty flask. To the flask are also added lOOml
con~ntrated nitric acid and the volume is made to 1
litre by addition of de-ion~d water. The mixture is
- mechAn~Ally stirred till it becomes clear. A typical pH
value of the clear solution thus obtA~ned is 0.15.
52.47g of barium nitrate is added to a 1 litre
flask. To the flask are also added 49.29g of copper
nitrate and the volume is made to 1 litre by addition of
de-ionized water. The mixture is mec~An~cally stirred
till it becomes clear. A typical pH value of the clear
solution is 4.1.
The two solutions above are now mixed together
to obtain a clear solution. A typical pH value of this
clear solution is l.O. (Alternatively, we may add only
500 ml to the mixture of barium and copper nitrate. The
two nitrate solutions when mixed will have a typical pH
value of 0.7. If further water is added to a maximum of
500ml, the buffering effect may be seen).
To a 2 litre cont~e~ is added 126.07g of
oxalic acid and the volume is made to 2 litre by addition
of de-ionized water. The mixture is mechAn~cally mixed
till it h~ -~ clear. A typical pH of this solution is
3.5. To the oy~llc acid solution is added 150ml of
ammonium hydroxide. The pH is now typically at 3.9.
Alternatively a mixture of ammonium oxalate and oxalic
acid may be used to achieve the same pH.
Using the on-line mixing and centrifuge
assembly as described in example 1, the nitrate solution
is mixed with the oxalic acid solution, thereby obt~ n~ ng
co-precipitation of yttrium, barium, copper oxalates and
mixed cation oxalates. A typical pH of the reacted mix
is 1.3.
The slurry thus obtA~ned is dried in a vacuum
oven at the boiling point of water. The dry mass is
extracted in about 12 h and ground by a food processor.
The loose powder may then be further ground by a mortar
and pestle.
W094/00385 ~ 7 ~ PCT/AU93/00300
24
The ground oxalate powder is transferred to a
refractory cont~ine~ and charged into a muffle furnace.
The furnace is heated at 5C/minute to 500C and held for
lOh. The furnace is then cooled to room temperature. The
powder, now black is hand ground in mortar and pestle,
and charged into the furnace as before. The furnace is
heated to 980C and held for 12 h. It is then cooled at
5C/minute to room t- ,?~ature. The green powder obt~ne~
is characterized by x-ray diffraction and electron
mi~ o~ y to be pure YzBalCul05, as shown in Figure 19.
Similar to Example 1, we may heat treat the
500C calcined powder at different temperatures and times
to obtain a variety of particle size distributions.
EXAMPLE 10:
This example illustrates the process of
co-precipitation as applied to the manufacture of
bismuth- and lead-based HTSC precursor and compounds of
the formula (BiPb)2Ca2Sr2Cu3010.The chemicals are weighed in
the molar stoichiometry (Bi:Pb):Sr:Ca:Cu ::
(1.65:0.35):2:2:3.
100.05g of reagent grade bismuth nitrate
Bi(N03)2.5H20 is charged into a 1 litre flask. To this
are added 25ml co~c~ntrated nitric acid and the volume is
made up to 300ml by ~ing de-l on 1 7ed water. The mixture
is merh~n~cally stirred till it h~~ s clear. The pH of
the clear solution is typically 0.1.
In a 1 litre beaker are charged reagent grades
of 14.5g lead nitrate Pb(N03)2, 52.91g strontium nitrate
Sr(N03)2.4H20, 59.04g calcium nitrate Ca(N03)2 and 90.6g
copper nitrate Cu(N03)2.3H20. To these powders are added
300ml of de-io~ d water and mer-h~n~cally stirred till
the solution is clear. The pH of the clear solution is
typically about 2.4.
The two solutions noted above are mixed
together and mech~n~c~lly stirred for 10 minutes. The pH
is now continuously monitored. At this stage the mixed
solutions have a pH of 0.15. As the volume of the
2~8~73
W094/00385 PCT/AU93/00300
solution is made to 1 litre, the pH of the mix rises to
0.45. Just as in Example 1, addition of water initially
does not change the pH perceptibly. However, with further
addition, a buffer effect is observed at a pH of about
0.4.
To a 6 litre flask are charged l90.0g reagent
grade ammonium oxalate (NH~COO)2H2O and 4 litres of
de-ioni 7~ water. The mixture is me-ch~nically stirred
till it becomes clear. A typical pH value of this clear
solution is 6.6.
The solution of ammonium oxalate and the
solution of nitrates are then mixed together using the
peristaltic pumps and filtered using the centrifuge as
described in Example 1. A typical pH of the precipitated
mixture is 1.6, with an average variation of less than
+0.2.
The precipitate is dried in a vacuum drying
oven at the boiling point of water and in about 12 hours
the precipitate is dry. It is then ground by a food
pro~cco~ followed by mortar and pestle to obtain a fine
light blue coloured powder.
The powder is then calcined to convert the
oxalates to a fine mixture of oxides. This may be done
in a muffle furnace where the powder, in a refractory
cont~ine~, is heated at 5C/minute to 500C for lOh, and
cooled to room temperature at 5C/minute. High resolution
transmission electron mi~lGylaphs of this calcined powder
show that the powder consists of intimately mixed oxide
grains of size less than 20nm. X-ray microanalysis shows
that these grains are typically oxides of copper,
bismuth, calcium, strontium or lead, depenAi~g on the
copper-oxide family under consideration.
The powder obt~1ne~ above is then converted to
HTSC precursors or HTSC materials as described in Process
1 and Process 2.
Process 1 : This process of heat treatment
gives a precursor which is ready to be converted into the
W094/0038~ 2 ~ 7 Q PCT/AU93/00300
26
three layer HTSC superconductor (Bi,Pb)2Ca2Sr2Cu3Ol0. The
powder obt~ne~ after calr~n~ng at 500C is transferred
into a muffle furnace. The furnace is then heat treated
at 5C/minute to 800C and held at this t~ -rature for 2h.
It is then cooled to room temperature at 5C/minute. The
powder is ground up by hand and then placed into the
muffle furnace. The muffle furnace is heat treated at
5C/minute to 840C and held for lOh. The furnace is then
allowed to cool down to room L Arature and the powder
taken out. The powder is re-ground and then charged into
the furnace and heated again at 5C/minute to 840C where
it is held for lOh. The muffle is then cooled to room
temperature at 5C/minute. The powder thus obt~;ne~ is a
suitable precursor for making the compound
(Bi,Pb)2Ca2Sr2Cu30l0. Figure 18 illustrates, by XRD (Figure
18a) and SEM micrographs (Figures 18b and 18c), the
development of various ph~s~ during the conversion of
the initial oxalate to the final precursor. Our process
allows the synth~s~c of this precursor in various
particle sizes. These various particle sizes achieveable,
gives greater fl~y~h~llty in processing of the powders,
particularly in wire manufacture by the process known as
"powder-in-tube".
Process 2 : This process converts the precursor
obt~;neA in Process 1 to the superconAl~ctor phase
(BiPb)2Ca2Sr2Cu3Ol0. The precursor powder obt~nQ~ in
Process 1 is pressed into a disk, for example, less than
2cm diameter and about 0.5cm thick. The disk is placed on
a refractory setter such as MgO and transferred into the
muffle furnace. The muffle is heated at 5C/minute to 860C
and held for 15h. The sample is then cooled to room
temperature at 5C/minute. Figures 17, 20A-C illustrates,
by XRD, SEM and magnetic susceptibility data, the
development of the three layer HTSC compound
(Bi,Pb)2CazSr2Cu30l0 via this process. Note that a high
phase purity (about 99.9%) is obt~;n~ by this process
and the superconducting transition has an onset
2 1 ~
_ W094/00385 ~ PCT/AU93/00300
27
temperature of 108K.
EXAMPLE 11:
This example illustrates the process of
co-precipitation as applied to the manufacture of
bismuth- and lead-based HTSC precursor and compounds of
the formula Bi2Sr2CalCu2O8 (or 2212). Reagent grade
chemicals are weighed according to the molar
stoichiometry : Bi:Sr:Ca:Cu :: 2:2:1:2.
lOOg bismuth nitrate is added to a 1 litre
flask. To this flask is added 50ml nitric acid and the
mixture shaken to dissolve most of the blsmuth nitrate.
De-ionized water is added to make the volume to 1 litre
and the mixture is mechAnically stirred till it becomes
clear. A typical pH value of the clear solution is about
0.15.
43.64g strontium nitrate, 24. 33g calcium
nitrate, 49. 83g copper nitrate are added to a 1 litre
flask. De-ionized water is added to make the solution to
1 litre and mechanically stirred till the solution
becomes clear. A typical pH value of the clear solution
is 3.4.
The two nitrate solutions are mixed and
mechAnically stirred for lO minutes. A typical pH of the
clear solution is 0.4.
142.11g ammonium oxalate is added to a 4 litre
contA~ner and the volume made up to 4 litres by ~ ng
de-io~ 7~ water. The mixture is mechAnically stirred
till it becomes clear. A typical pH of the clear
solution is 6.7.
Using the on-line mixing ~y~ and the
centrifuge, the nitrate solution and the oxalate
solutions are mixed to form the co-precipitate and the
slurry collected in the centrifuge. A typical pH of the
reactant mix is 1.2, with an average variation of + O.1.
The precipitated oxalates thus obtA~ ne~ are
dried for 12h in a vacuum oven at the boiling point of
water. The dried mass is ground in a food processor
W094/00385 ~ 7 ~ PCT/AU93/00300
28
followed by fine gri n~ ng by hand in a mortar and pestle.
The blue ground powder is then transferred to a
refractory contAine~ and charged into a muffle furnace
for heat treatment. The furnace is heated to 500C at
5C/minute and held for lOh. It is then cooled to room
temperature at 5C/minute to room temperature. The powder,
now black, is ground by mortar and pestle and re-charged
as before into the furnace. The furnace is heated to 800C
at 5C/minute and held for 2h. It is then cooled ~o room
temperature at 5C/minute. The powder is L2 _ved,
re-ground and then re-charged into the furnace. The
furnace is heated to 840C at 5C/minute and held for lOh.
The sample is then cooled to room temperature at
5C/minute. The powder is ~ ved, re-ground, and
recharged into the furnace as before. The furnace is
heated at 5C/minute to 840C and held as before for lOh.
It is then cooled to room temperature at 5C/minute. X-ray
diffraction data in Figure 14 illustrate that the powder
consists of the HTSC phase Bi2Sr2Ca1Cu208.
At such stage of the production of the 2212
phase, various modifications to the average particle size
distribution can be made by a~lo~Liate modifications to
the heat treatment.
EXAMPLE 12:
Due to the inherent plasticity present in our
co-precipitated powder it becomes possible to add
cationic mixtures (or compounds) cont~n~ng barium,
yttrium, copper, or silver (as may be required for the
YBC0 system) or strontium, bisumth, calcium, lead or
silver (as may be required for the BSSC0 system). The
addition may be made to dried co-precipitated powder
which may contain one, few or all of the cations required
in the desired final compound. The process may be
facilitated through water medium to permit a simple
drying operation.
The already dried co-precipitates are ground up
and transferred to water where they are stirred by a
2~867~
W094/0038~ PCT/AU93/00300
29
mixer. This ~ix~ ng makes a finely dispersed suspension.
In a separate contAiner is prepared a solution of the new
or additional cation. This solution is then mixed to the
co-precipitate solution, which is then stirred and
evaporated to dryness. Because the co-precipitated
powders have the ability to form a fine suspension with
water, it allows remixing of dried powders with fresh
powders/precipitates/solution.
Example: To add barium cation to a co-
precipitated mixture which has the molar ratio Y:Ba:Cu::1:1.5:3.0 such that the final stoichiometry is 1:2:3.
The starting co-precipitate is made by the
process described earlier in the text and cs~c;~ts of
yttrium, barium, and copper oxalates. It is determined
by I.C.P. (Inductively coupled plasma atomic emission
spectroscopy) results that the cationic ratio of Y:Ba:Cu
is 1:1.5:3Ø (Such a composition finds application in
bulk HTSC shapes as it yields a fraction of the
insulating compound Y-211 which is expected to act as
flux p~nn~ ng centres). To 800 g of the dried co-
precipitate powder is added 500 ml of water and the
mixture is stirred vigorously. In another cont~;n~r
79.2g of barium nitrate is weighed out and dissolved in 1
litre of water. The barium nitrate solution is added to
the co-precipitate mix and stirred vigorously for 30
minutes in a blender. The blended mix is transferred to
a hot plate where it is continuously stirred till almost
dry. At this stage lt is transferred to a vacuum oven
where the mixture is allowed to dry at the boiling point
of water. Transmission electron microscopy on the dried
powder indicated that the powder consists of well mixed
cations on a n~no~eter level similar to powders obt~;ned
earlier. Similarly TEM on this powder after calcinating
at 500C (to eliminate oxalate anion) shows that the
cations are well mixed on a n~ ter level.
The new mix prepared may be treated in a ~nner
similar to that described earlier in the text for co-
W094/00385 ~ 7 ~ PCT/AU93/00300
precipitated powders. Thus for example, we may fire itin the same -nne~ to form superconducting Y-123. XRD
and I.C.P. results confirm that the cation stoichiometry
of the resulting powder is 1:2:3.
EXAMPLE 13:
Using the calcined co-precipitated oxalate
powders, sputtering targets were fabricated. Calcined
powders were ground and pressed into discs and then
sintered at between 920C and 980C where the
superconducting phase is stable. Since dense targets are
required for thin film fabrication via a well-known
sputtering process, the density of those targets were
measured. It was found that a 500C calcined powder
(mixture of oxides, no YBa2Cu3O7 d phase) gave 40-60~
higher density than high temperature (>800~C, mainly
YBa2Cu3O7d) calcined powders after sintering the final
products. This difference in density can be attributed
to the intermediate phase/s which assist the
densification during transformation from 500C calcined
oxide ph~s~ to a superconducting phase.
Various other changes and modifications can be
made to the emho~i~ents without departing from the spirit
and scope of the invention as claimed.
