TITLE: Shaped superconductive materials.
European Patent Application EP0341030 A2

ABSTRACT:
Spinning or casting of superconductive oxide precursor from an aqueous
solution optionally containing a mixture of metal salts corresponding
to the superconductive YBa2Cu3Ox and a water soluble polymer results in
denser superconductive material upon firing.

INVENTORS:
Hsu, Che-hsiung

APPLICATION NUMBER: EP19890304420
PUBLICATION DATE: 11/08/1989
FILING DATE: 05/03/1989

ASSIGNEE: Pont DU. (US)
INTERNATIONAL CLASSES: C01G3/00; C01G1/00; C04B35/45; C04B35/622; D01F9/10; H01B13/00;
H01L39/24; (IPC1-7): H01L39/12; H01L39/24
EUROPEAN CLASSES: C04B35/45D2; C04B35/622F2H; H01L39/24J; H01L39/24J2B

DOMESTIC PATENT REFERENCES:
EP0308338 Method of making superconducting materials, and materials
 made thereby.
EP0316741 Method for preparation of high temperature superconductors.

OTHER REFERENCES:
JAPANESE JOURNAL OF APPLIED PHYSICS, vol. 26, no. 9, December 1987,
pages L1527-L1528, Tokyo, JP; T. GOTO et al.: "A method for producing
high-Tc Y-Ba-Cu-O superconducting filaments by suspension spinning
method"

CLAIMS:
1. A method for preparing superconductive fiber or film of improved
critical current density comprising forming a dispersion of a
superconductive oxide precursor in an aqueous medium containing a water
soluble polymer, extruding the dispersion into fiber or film through an
orifice, and drying and firing the fiber or film to convert them into
shaped super-conductive fiber or film, said superconductive oxide
precursor comprising a homogeneous mixture of BaCO3, CuO, and an
amorphous yttrium compound, in which the elements Y, Ba and Cu are
intimately mixed on a micron to sub-micron scale and are present in the
mixture in the ratio of 1:2:3 and which can be prepared by a process
comprising the steps of: (a) blending an aqueous mixture of yttrium
acetate, copper acetate, and a source of barium selected from barium
hydroxide and barium acetate; (b) removing excess solvent from the
mixture; and (c) calcining the product in air by heating while
increasing the temperature according to a predetermined timed sequence
from ambient to a temperature in the range of 450 DEG C to 750 DEG C
and maintaining that temperature for at least one hour.
2. A method according to claim 1 wherein the aqueous medium
additionally contains salts of yttrium, barium and copper, said metals
being present in the atomic proportions of 1:2:3, respectively.
3. Fiber or film prepared according to claims 1 or 2.
4. A method for preparing superconductive film of improved critical
current density comprising forming a dispersion of a superconductive
oxide precursor in an aqueous medium containing a water soluble
polymer, casting the dispersion onto a surface as a film, drying and
firing the film to convert it into a superconductive film, said
superconductive oxide precursor comprising a homogeneous mixture of
BaCO3, CuO, and an amorphous yttrium compound, in which the elements Y,
Ba and Cu are intimately mixed on a micron to sub-micron scale and are
present in the mixture in the ratio of 1:2:3 and which can be prepared
by a process comprising the steps of: (a) blending an aqueous mixture
of yttrium acetate, copper acetate, and a source of barium selected
from barium hydroxide and barium acetate; (b) removing excess solvent
from the mixture; and (c) calcining the product in air by heating while
increasing the temperature according to a predetermined timed sequence
from ambient to a temperature in the range of 450 DEG C to 750 DEG C
and maintaining that temperature for at least one hour.
5. Film prepared according to claim 4.

DESCRIPTION:
Shaped Superconductive Materials
This invention relates to shaped superconductive materials. Background
of the Invention
The use of a powder mixture for production of superconductive filaments
is taught in the Japanese Journal of Applied Physics, V.26 N.9
September 1987, pp. L1527-L1528, Goto et al. In this article is
described a process that involves mixing Y2O3, BaCO3 and CuO powders,
grinding and then heating them at 1223 DEG K for 5 hours. The product
is ground and suspended in an aqueous solution of poly(vinyl alcohol)
containing a dispersant. The suspension is then extruded as a filament
into an alkaline aqueous solution collected, neutralized, washed and
dried. The filament was then heated to remove volatile components.
The production of an analogous product is described in a subsequent
article by Goto et al. in the Japanese Journal of Applied Physics, V26,
N.12, December 1987, pp. L 1970-L 1972. This time the powder which has
been heated to 1223 DEG K is ground and suspended in an aqueous
solution containing Y, Ba and Cu nitrates as well as polyvinyl alcohol
and sodium dodecyl sulfate as a dispersant and then treated as before.
Summary of the Invention
A method for preparing superconductive fiber of improved critical
current density comprising forming a dispersion of a superconductive
oxide precursor in an aqueous medium of a water soluble polymer,
extruding the dispersion into fibers through an orifice, and drying and
firing the fibers to convert it into shaped superconductive fiber, said
superconductive oxide precursor comprising a homogeneous mixture of
BaCO3, CuO, and an amorphous yttrium compound, in which the elements Y,
Ba and Cu are intimately mixed on a micron to sub-micron scale and are
present in the mixture in the ratio of 1:2:3 and which is prepared by a
process comprising the steps of:
(a) blending an aqueous mixture of yttrium acetate, copper acetate, and
a source of barium selected from barium hydroxide and barium acetate;
(b) removing excess solvent from the mixture; and (c) calcining the
product in air by heating while increasing the temperature from ambient
to a temperature in the range of 450 DEG C to 750 DEG C and maintaining
that temperature for at least one hour.
Preferably, the aqueous medium containing the water-soluble polymer,
additionally contains salts of yttrium, barium and copper, said metals
being in the atomic proportions of 1:2:3. Films prepared from the
compositions are also contemplated. Detailed Description of the
Invention
In accordance with this invention, superconductive oxide precursor,
i.e., a precursor for a superconductive oxide, is combined with an
aqueous solution of a water-soluble polymer. The aqueous medium
advantageously contains in addition to the water-soluble polymer, salts
of yttrium, barium and copper, said metals being in the atomic
proportions of 1:2:3, respectively. The material is then extruded into
fiber, dried and fired to form superconductive material by techniques
known in the art. Alternatively, the material may be shaped into films,
then dried and fired as discussed above.
In the preparation of the spinning dispersion, it is desirable to first
prepare the metal salt solution by dissolving the metal salts in water
in the desired metal ratio. The soluble polymer and additional water
are then combined with the salt solution and heated until a spinnable
viscous solution is obtained. Finally, the particulate superconductive
oxide precursor is added stepwise with rigorous stirring or mixing
until a generally stable dispersion is obtained. Preferably the
particulate oxide precursor should be reduced to a particle size as
small as possible.
Examples of useful metal salts are the acetates and nitrates of
yttrium, barium and copper, said metals being in the atomic proportions
of 1:2:3 in the aqueous solution. They are employed in the aqueous
solution, preferably at a concentration in the range of 11 to 16% on a
wt. basis. The metal salts are believed to assist in producing a more
dense superconductive material.
The superconductive oxide precursor employed herein is a homogeneous
mixture of BaCO3 CuO, and an amorphous yttrium compound, in which the
elements Y, Ba and Cu are intimately mixed on a micron to sub-micron
scale and are present in the mixture in the ratio of 1:2:3. The
precursor is prepared by forming an aqueous mixture of yttrium acetate,
copper acetate, and a source of barium selected from barium hydroxide
and barium acetate in which the atomic ratio of Y:Ba:Cu is about 1:2:3,
followed by removing excess solvent from the mixture; and calcining the
product in air by heating while increasing the temperature according to
a predetermined timed sequence from ambient to a temperature in the
range of 450 DEG C to 750 DEG C and maintaining that temperature for at
least one hour, all as disclosed in said Connolly et al. application.
The superconductive oxide precursor may be employed in the dispersion
to be extruded or cast in an amount of from about 80 to 91 % by weight
of water-soluble polymer.
Apart from the water, the final component in the dispersion to be
extruded or cast is a water-soluble polymer, which holds the
particulate material in shape so that it may be ultimately fired to
form the superconductive article. Poly(vinyl alcohol) is the preferred
polymer but other water-soluble polymers such as poly(ethylene oxide)
are suitable. These polymers are used in amounts sufficient to
accomplish the temporary function of maintaining the shape of the
article since they are burned off in the firing step of forming the
superconductive article. Amounts of as little as 6 wt. % based on the
water present has been found useful. Generally, from 6 to 15 wt. % is
employed.
The compositions are extruded into fibers or films or if desired cast
into films which are then dried. The dried fibers or films are then
fired to form the superconductive products. Measurements Critical
Current Density(Jc)
The Jc at a given temperature is defined as the threshold current which
passed through a given cross-sectional area of a superconductor to
transform the material from the superconductive state to a normal
state. The measurement is done using a 4-probe method known in the art.
An amalgam is used as electrode material to reduce contact resistance
to <0.5 ohm. The two inner electrodes are 6 mm apart. While the fiber
is immersed in liquid nitrogen, current is applied to the two outer
electrodes. The voltage corresponding to the current applied through
the two outer electrodes is measured between the two inner electrodes
with an electrometer. As current is increased, at a certain point the
sample is driven to the normal state. When the sample shows a voltage
of 1 microvolt, the corresponding current is defined as threshold
current.
The threshold current normalized with the cross-section of the fiber
sample is defined as Jc.
The following examples are illustrative of the present invention and
are not to be construed as limiting: Example 1
A solution of cupric acetate monohydrate (16.4 g, Fluka >99% purity),
barium acetate (14.0 g Aldrich ACS reagent grade), yttrium nitrate
pentahydrate, (10.0 g Aldrich >99% purity), d-sorbitol (17.5 g) and
water (97.0 g) was first prepared at 50 DEG C. To 21.0 g of this
solution, 7.5 g polyvinylalcohol ("Elvanol" HV, Du Pont) with average
MW of 115,000 and a degree of polymerization of 1,900, and 30.0 g water
were added. The mixture was poured into a loosely capped plastic bottle
which was then placed in an oven set at 85 DEG C. Within 3 hours the
mixture became a viscous and spinnable solution. To the SIMILAR 80 DEG
C solution 30.0 g superconductive oxide precursor prepared as described
below, was added at several intervals. Between each addition, the
mixture was stirred vigorously with a spatula while kept at SIMILAR 80
DEG C.
The precursor was prepared as follows: (a) Copper acetate monohydrate
(230.0g) was dissolved in 1.6 liter of purified water at about 75 DEG C
forming a cloudy blue-green solution. To this was added a clear
solution of yttrium acetate hydrate -- 128.6 g dissolved in 0.6 liter
of purified water at 75 DEG C. A cloudy barium hydroxide solution
(containing some white precipitate after its preparation from heating
242.2 g of barium hydroxide octohydrate in 0.8 liter of purified water
at 75 DEG C) was added to the yttrium/copper mixture over a period of 5
minutes.
The resulting brownish mixture was maintained at 75 DEG C for 1 hour
after which the water was evaporated by "spray" drying using a Buchi
190 mini spray dryer with a 0.7 mm nozzle. (b) The dry powder from (a)
was calcined in air, i.e., in an oxygen-containing atmosphere,
according to the following predetermined timed sequence to 500 DEG C --
1 hour - 100 DEG C (time-set temperature), 1/2 hour - 230 DEG C, 1 hour
- 270 DEG C, 15 min - 320 DEG C, 1 hour - 400 DEG C, 1 hour - 480 DEG
C, 15 min - 500 DEG C - then held for 1 hour at 500 DEG C. The calcined
material is an intimate and homogeneous blend of stable BaCO3, CuO and
amorphous yttrium compounds. (c) Four batches treated as in (a) and (b)
were hammermilled together to form a large lot from which the precursor
powder for this Example was taken.
A portion of the dispersion was placed in a 10 ml plastic syringe for
extrusion through the nozzle. The extruded filaments were hung on a
horizontal rod. The extrudate was left to dry in air before being
placed in alumina crucibles for air-firing in a Fisher #495 Ashing
Furnace at the following cycle: 1 DEG C/minute from RT to 160 DEG C, 3
hours at 160 DEG C, 2 DEG C/minute from 160 DEG C to 915 DEG C, 1 hour
at 915 DEG C, cooled down in air at the natural cooling rate of the
furnace. Filaments of the fired extrudate show a sharp onset transition
at 90 DEG K in excluding magnetic flux, indicating superconductivity.
Electrical resistivity of the filaments determined by a 4-probe
technique is 2.7 x 10<-><3> ohm-cm at room temperature (RT). A
precipitous drop in electrical resistance occurs at 90 DEG K and
reaches a point where resistance cannot be detected at 83 DEG K.
The superconductivity transition is extremely sharp, indicating that
the filaments are highly crystalline and pure. The assessment is
supported by X-ray powder diffraction data. The characteristic 2 theta
peaks at 32.8 DEG and 33.2 DEG are extremely sharp, indicating the
material is highly crystalline. No other crystalline species were
detected. The filaments have a density of 4.8 g/cc, equivalent to 75%
of the theoretical density. Example 2
To 31.0 g of the metal salt solution prepared in Example 1, 7.5 g
"Elvanol" HV and 22.0 g water were added. The mixture was converted to
a solution according to the procedure described in Example 1. To the
solution was added 30.0 g of a superconductive oxide precursor prepared
as in Example 1 above. A dispersion, extrudate and filaments were
prepared in the same manner as described in Example 1. A powder
prepared from the air-fired filaments shows the same sharp 2 theta
peaks. No other crystalline species was detected. Electrical
resistivity of the filaments is 1.2 x 10<-><3> ohm-cm at RT. They
undergo a precipitous drop in electrical resistance at 90 DEG K and
reach a point where resistance cannot be detected at 83 DEG K. The
sharp superconductivity transition confirms that the materials are pure
and highly crystalline. Example 3
The dispersion prepared as in Example 1 was cast on microscope slides
for making thick films. A portion of the dried films were air-fired
according to the cycle described in Example 1. The air-fired films show
an onset transition of Meissner effect at 90 DEG K. Density of the
films is 5.4 g/cc., equivalent to 84% of the theoretical density.
Example 4
A metal salt solution identical to the one described in Example 1 was
prepared. To 33.9 g of the solution was added 12.0 g "Elvanol" HV and
48.1 g water. The mixture was converted to a viscous and spinnable
solution according to the procedure described in Example 1. To the 80
DEG C solution, 48 g of super-conductive oxide precursor was added. The
procedure described in Example 1 for preparation of a dispersion was
followed. The dispersion was transferred to a twin-cell mixer as
described in U.S. 3,767,756. The twin cells were kept at 80 DEG C in a
water bath while the dispersion was pumped back and forth for 55
minutes to ensure thorough mixing of the dispersion between the cells.
The water bath was then removed and one of the twin cells was connected
to a 20-mil spinneret. A 20-mesh screen was used for filtration.
Filaments were extruded and collected on sheets for drying.
The dried filaments were air-fired as described in Example 1. Average
size of the filaments was 0.3 mm in diameter. The filaments showed a
sharp onset transition at 90 DEG K in excluding magnetic flux,
indicating that they are superconductive. Electrical resistivity of the
filaments at RT was 1.0 x 10<-><3> ohm-cm. They undergo a precipitous
drop in resistance at 90 DEG K and reach a point where resistance
cannot be detected at 80 DEG K. Critical current density (Jc) of the
filaments at 77 DEG K is 64 A/cm<2>.
Scanning electric micrographs (SEM) of a fractured cross-section of the
filaments show a non-grainy morphology whereas the polished surfaces
show distinct randomly oriented needle-like crystals. X-ray diffraction
patterns of a powder prepared from the filaments show the sharp 2 theta
peaks as described in Example 1. Example 5
A solution of cupric acetate monohydrate (4.1 g), barium acetate (3.5
g), yttrium nitrate pentahydrate (2.5 g), d-sorbitol (4.4 g) and water
(24.3 g) was first prepared at 50 DEG C. To 31.0 g of the solution,
12.0 g "Elvanol" HV and 49.3 water were added. The mixture was
converted to a solution according to the procedure described in Example
1. To the solution 108 g of superconductive oxide precursor particulate
was added. This precursor and the dispersion thereof were prepared in a
manner similar to that of Example 1. The procedures for mixing (mixing
time is 75 minutes), extrusion and air-firing are described in Example
4. The filaments from the air-fired extrudate show an onset transition
at 90 DEG K in excluding magnetic flux. The electrical resistivity of
the filaments at RT is 1.7 x 10<-><3> ohm-cm.
They undergo a precipitous drop in resistance at 92 DEG K and reach a
point where resistance cannot be detected at 84 DEG K. The Jc at 77 DEG
K is 130 A/cm<2>.
The filaments have the same morphology and X-ray diffraction pattern as
those described in Example 4. Example 6
A mixture of 12.0 g "Elvanol" HV and 69.3 g water was poured into a
loosely capped plastic bottle. The bottle was then placed in an oven
set at 85 DEG C. Within 2 hours the mixture became a homogeneous
solution. To the solution 117 g of superconductive oxide precursor
employed in Example 5 was added. The procedures for dispersion
preparation, mixing, extrusion and air-firing described in Example 5
were followed. Average size of the filaments from the air-fired
extrudates is SIMILAR 0.3 mm in diameter. They show an onset transition
at 90 DEG K in excluding magnetic flux. They undergo a precipitous drop
in electrical resistance at 90 DEG K and reach a point where resistance
cannot be detected at 82 DEG K. Jc at 77 DEG K was 48 A/cm<2>. The
filaments have the same morphology and X-ray diffraction pattern as
those of Examples 4 and 5. Example 7
A mixture of 1 g poly(ethylene oxide) having an MW of 5,000,000 from
Aldrich and 25 g water was placed in a loosely capped bottle. After the
polymer powder was thoroughly wetted, the bottle was placed in an oven
set at 80 DEG C. The tacky solution was cooled down to RT before 14 g
of superconductive oxide precursor similar to that employed in Example
5 was added to it at several intervals. At each addition, the
dispersion was vigorously stirred. The dispersion was drawn into fibers
by a spatula, which were dried on sheets. The dried fibers were
air-fired according to the following cycle: RT to 150 DEG C at 0.5 DEG
C/min., 150 DEG C for 30 minutes, 150 DEG C to 680 DEG C at 1 DEG
C/min., 680 DEG C to 800 DEG C at 2 DEG C/min., 800 DEG C for 5
minutes, 800 DEG C to 915 DEG C at 3 DEG C/min., 915 DEG C for 1 hr.,
and cooled down to RT at the oven's natural cooling rate.
X-ray diffraction patterns show that powder prepared by grinding the
filament is orthorhombic, the correct phase for superconductivity. The
air-fired fibers show super-conductivity transition in electrical
resistance at temperatures above 77 DEG K. They show an onset
transition at 90 DEG K in excluding magnetic flux. Example 8
A portion of the extruded fiber prepared in Example 6 was cut into
small pieces ( SIMILAR 8 cm long). These pieces were placed in alumina
boats which were put in a quartz container for firing in a Fisher #497
furnace. The container was fed with oxygen during the entire firing
cycle at a rate of 1.0 SCFH (standard cubic feet per hour). The firing
cycle was as follows: room temperature (RT) to 160 DEG C at 1 DEG
C/min, 1 hour at 160 DEG C, 2 DEG C/min to 850 DEG C, 30 minutes at 850
DEG C, 2 DEG C/min to 935 DEG C, 1 hour at 935 DEG C, cooled down to
600 DEG C at 5 DEG C/min, 1 hour at 600 DEG C, 5 DEG C/min to 300 DEG
C, one minute at 300 DEG C, 5 DEG C/min to 35 DEG C. The average
diameter of the oxygen-fired pieces was SIMILAR 0.3 mm. When tested,
they showed a sharp Meissner transition.
As a test sample was cooled down from RT to 20 DEG K, it started to
exclude magnetic flux at 95 DEG K and almost completely excluded flux
at 85 DEG K. Critical current density of the fired filaments ranged
from 861 to 1080 amps/cm<2> at 77 DEG K. The measurement was done with
silver instead of amalgam electrodes.