Note: Descriptions are shown in the official language in which they were submitted.
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FA~RICATIO~ OF SUPERCONDUCTI~G f~
~ETAL-OXIDE TEXTILES '~
This is a continuation-in-part of
5 ~pplication Serial Number 07/573,855 filed
~ugus~ 28, lg90.
T~chnica~ Field
This invention pertains to fibers, te~tiles
10 and shapes compos~d of superconductive metal o~ides
and to a process for their ~abrication.
Backaround
~uperconductivity, the ~irtual
15 disappearance of electrical resisti~ity9 was
initially discovered in mercury cooled to the
boiling temperature of liquid helium. This
discovery initiated the search for materials which
would be superconductive at higher temperatures. In
20 1987 came a significant advance. Superconductivity
was found at 95K in a material composed of several
phases containing yttrium, barium, copper and
o~ygen. ~he discovery was significant in that the
temperature at which superconductivity ~pp~are~ was
25 above the boiling temperature of liquid nitrogen,
which could then be used for the cooling ~edium.
The superconducting phase was found to correspond to
the crystalline, orthorhombic oxide YBa2Cu3O7. The
~uperconductive property was lost, however, upon
30 heating the orthorhombic phase under conditions
where o~ygen was depleted g;ving rise to a
tetragonal phase, the composition of which was close
to YBa~Cu3O6. The tr~nsition seemed to occur around
D-16390-1
~ ~ .
~ the composition YBa2Cu3O6 5. Hence the
superconductive property e~ists in compounds of the
formula YBa2Cu3O7_~ where ~ may vary ~rom 0 to 0.4,
the optimum being about ~.19.
Other high temperature ~uperconductors
which now have been identified include YBa~CuO7_~,
Ba~La5_~cu5Os(3-y)~ ~i2Sr~cu2O7~, Bi4~r3ca
and T12Ca~Ba2Cu3O~.
Superconductive metal oxide material can be
10 produced by traditional ceramic techniques of
grinding metal compounds in stoichiometric ratio to
bring the metal compounds into pro~imity.
Subsequent calcination allows the metal ions in
their respective crystalline compounds to diffuse
15 into the others. Repeated regrinding and
calcination under controlled conditions produces the
desired phase which has the superconductive property.
Most of the prospective appl~cations of
superconductors are based on the capability of
20 transmitting electric power loss free, and on the
production of powerful, compact magnets. Because
motors and generators are based on magnetism, there
is great potential for r~ducing their weights, sizes
and inefficiencies. Powerful magnets are ~onceived
25 to allow the suspension of objects such as a shaft
in a bearing and a train over a track.
The superconductive metal ozides, like
ceramics, are intrinsically brittle and their
fabrication into useful shapes, even basic wire,
30 presents many challenges. The most practiced method
to date for the formation of ~uperconduetive wires
has been the powder-in-tube technique. The
D-163~0-1
superconductive material in powder form is packed 2~9
into a silver, ~opper or stainless steel tube. The
tube is then swaged and drawn, or rolled, down to a
~mall diameter which can be further formed into a
5 useful configuration.
Lusk et al. in Supercon~. Sci. Technol. 1,
137 (1988) rep~rted ~n the fa~ricatlon of ~ cer~mic
superconducting wire by an e~trusion method.
Superconductor precursor material in powder form was
10 mi~ed with a binder such as epo~y resin, and the
mi~ture w~s e~truded into a wire form. The
e~trusion was heated in a nonreactive atmosphe,re to
remove the binder, and then sintered at high
temperature in air or o~ygen to develop strength an~
lS the superconductive phase. ~ra~ile wire with a
diameter of about 0.8 mm resulted from this method.
The preparation of superconductive fibers
by e~truding or spinning a polymer-metal precursor
was described by Chien et al. in Physical Review B,
20 3B, 1953 (1988). Metal ions in the desired atomic
ratios were complesed to a polymer. The polymer
solution was e~truded, dried and ~ound on a
mandrel. Heating in nitrogen pyrolyzed the polymer,
and subsequent heating in 02ygen converted the metal
25 intermediates to the superconductive o~ide. The
process produced fibers having diameters of 1 to 100
micrGns and grain sizes from 1 to 50 microns.
Jin et al. in ~ppl. Phys. Lett. ~1, 943
(1987) described three different laboratory
30 fabrications of YBa~Cu307_~ wire by molten o~ide
processing. In the melt drawing technique, the
center of a bar of YBa2Cu30~_g material was fused
D-16390-1
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:`` ~: :
with a laboratory blow torch flame, and the two 2~ ., O~.
unmelted ends pulled apart leaving a 1.2-mm diame~er
filament between them. In the melt spinning
technique, one end of a bar of YBa2Cu307_x material
5 was heated and a molten droplet allowed to ~all on
the outside of a rotating mandrel producing a ribbon
1.5 mm wide and 0.3 mm thick. Still another
e~periment employed a sil~er wire as 3 substrate
onto which Y~a2Cu307_~ powder in a binder was
10 deposited. The composite was dried, producing a
0.75-mm diameter composite wire containing an
0.2~-mm diameter metal core. The wire was fur.ther
processed by rapidly moving it through a torch flame
and melting the outer portion. ~he wire formed in
15 each of these three methods required a homogenizing
heat tr~atment followed by an o~ygen heat treatment
to develop the superconductive phase. In
production, any of these three techniques would
require a high temperature melting furnace and
20 precise control of operating variables.
The processes described above were all
directed to the fabrication of a sin~le filament. A
process for producing metal o~ide fibers, te~tiles
and shapes was described by Hamling in U.S. Patent
25 3,3B5,915. By te~tiles is meant a variety of
te~ile forms including single filament, s~aple
fibers, continuous tow and yarns, woven fabrics,
batting and f~lts composed of fibers.
The Hamling process comprises initially
30 impregnating a preform of organic polymeric te~tile
material with one or more compo~nds of metals as
desired in the final product. The impregna~ed
~-163~
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material is heated under c~nSrolled conditions which
prevent ignition o~ the organic material, but
pyrolize the organic material to predominantly
carbon and remove the ~arbon as a ~arbon-containing
5 gas. The heating continu~s to o~idi~e the metal
compounds. ~t least part of the heating is
- performed in the presence of an o~idizing ga~.
product results which has substantially the same
shape as the preform, but only about 40 to 60% of
10 its original size. The metal oside in the product
typically is substantially micro-crystalline, or
amorphous, that is, its crystallites are so small
that they are barely discernible by conventional
2-ray diffraction. This is indicative of a
15 crystallite size on the order of 0.1 microns or
less, which Hamling preferred for masimum strength
in his product. The process, however, is described
as capable of preparing fibers with crystallite
sizes up to approximately 1 micron. With larger
20 crystallite sizes, a significant 19ss in strength
occurred. Mechanical properties of the product were
impaired when the crystallite size e~ceeded
appro~imately one-tenth the diameter of the fibers.
It is known that material capable of
25 superconductive behavior must be in a crystalline
state. Hence the process as described by U.S.
3,385,915 would not produce superconductive metal
oxide.
Fabrics composed of metal oxides are
30 described by Hamling in U.S. Patent 3,663,182. Such
~abrics are produced by the process described in
U.S. Patent 3,385,915, which has been summarized
.
D-16390-1
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- 6 - ~ 2~90~
above. Hence, the fabric has all the
characteristics of a product of that process, and
would not be e~pected to ha~e superconductiYe
properties.
It is an object of the present invention to
provide a process for producing superconducting
metal oxide fibers, te~tiles and shapes. It i~ ~lso
an object to produce these products with fle~ibility
and strength so as to allow their further ~haping.
It is a feature of this invention that the
starting material is organic polymeric material
which can be preformed into the final product shape.
It is an advantage of this process that
complicated and irregular product shapes can be
15 produced from ;ne~pensive oryanic material~ which
are readily preformed into the desired final shape.
The preforming is ine~pensive in that costly
machining is unnecessary. Another advantage is that
a high temperature melting furnace is not required.
2~
SuMMARy ~F T~E INVENTI~N
The invention is a process for producing
superconductive metal-o~ide textiles. It comprises
initially impregnating a preformed, organic,
25 polymeri~ te~tile material with one or more
compounds containing the desired metal elements in
substantially the atomic ratios as o~cur in the
superconductive material. The impregnated material
is dried and heated in a controlled atmosphere to a
3D temperature suffi~iently high to pyrolize and
o~idize the or~anic material and remove it as a
carbon-containing gas. The heating is carried out
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without igni~ion of the ~rganic and without melting
the metal o~ide. The material is then cooled at
least partly in an o~idizing atmosphere to further
o~i~ize and develop a crystalline supercon~uctive
5 metal osideO
ERIEF DE~CRTPTION C~HE DR~I~
Fig. 1 is the X-ray diffraction pattern
obtained on a specimen o~ YBa2Cu3O7_~ te~tile
1~ product by the process provided by this invention.
Fig. 2 is ~he ~-ray difEraction pattern
obtained on a specimen produced by grinding m~tal
powders providing metal elements in the atomic
ratios indicated in YBa2Cu3O7_~ and sintering at
15 950C in o~yQen.
; D~AILED_DESC~IP~IQ~
The starting material in this invention can
be any organic material capable of swelling and
20 ab~orbing a solvent and not melting on heating
during the subsequent proc~ssing. Any cellulosic
material can be employed including ray~n, saponified
cellulose acetate, cotton, wool and ramie. Usable
synthetics include acrylics, p~lyesters, vinyls ~nd
25 polyur~thanes. Rayon is a preferred material
beca~se of its physical uniformity, high
absorptivity and low impurity content.
The starting organic material is
impregnated with ~ompounds of metal elements to
30 provide metal elements in ~he organic material in
substantially the atomic ratios as are desired to
appear in the product. When thesP compounds are
D-16390-1
. . ~
salts highly soluble in water, the impregnation c ~ 9 ~
be carried out by immersing the organic material in
a concentrated aqueous solution of such ~alts in
proper ratio. Alternatively, the organic makerial
5 may be immersed ~eguentially in ~everal solutions,
each containing at least one of the desired
compounds, thereby accumulating the ~esired metal
content in the organic.
To obtain strength in the final product, it
10 is desirable to impregnate the starting materials
with metal compounds to the e~tent of at least 0.25
moles and preferably 1 to Z moles of the metal-
compounds per base mole of cellulose. Base mole as
used herein refers to the molecular weight of
15 glycosidic unit of the cellulose chain, namely a
molecular weight of 162. With non-cellulosic
materials, the degree of impregnation should be at
least 0.1 and preferably 0.5 to 1.0 gram-equivalent
of metal per gram of organic material.
Water is the preferred solvent for metal
compounds to impregnate cellulosic material. For
impregnating vinyl and polyurethane materials,
esters and ketones are suitable solvents. For
impregnating acrylic and polyester materials,
25 ~uitable ~olvents for the metal compounds include
aromatic alcohols and amines such as aniline,
nitro-phenol, meta-cresol and paraphenylphenol.
To increase the rat~ and e~tent of salt
impregnation in cellulosic starting material, it may
30 be preswelled by soaking in water prior to soaking
in salt solution. For acrylic and polyester
materials, aromatic alcohols are suitable swelling
D-16390-1
_ _
- agents. For vinyl and polyurethane materials, 2 0 9 ~ ~ ~ O
I ketones are useful.
¦ An alternate method of impregnating the
organic material is to use metal compounds which
5 hydrolyze or react with water ~o form a metal oxide
which is substantially insoluble in water. The
selected compounds may be dissolved i~ ~n oryanic
solvent immiscible with water, such as carbon
tetrachloride, ether, or benzene, to the e~tent of 5
10 to 50 grams of metal compound per 100 ml of
solvent. ~he starting organic material i~ prepared
by e~posing it to air having a relative humidity of
between 50 to 90 percent. It absorbs 5 to 30
percent by weight of water and swells. In this
15 swollen state, it is immersed in the prepared
solution. As the metal compounds ;n the solution
penetrate the swollen organic material, they react
- with the water, and the resultant o~ide prec~pitates
in the organic material structure. If the metal
20 compound is a gas or liquid, the swollen starting
organic material may be e~posed directly to the gas
or liquid to accomplish the precipitation of metal
oxide.
Without being bound by the following theory
25 regardin~ impregnation, organic polymeric materials
such as cellulose, ~hich are preferred starting
materials for this process, are composed of small
crystallites of polymer chains held together in a
matri~ of amorphous polymer. Upon immersion of the
30 organic polymer in a prepared solution of metal
salts, the amorphous regions absorb solution and
enlarge or swell. The swollen amorphous regions
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_ 10 --
then comprise 50 to 90 percent of the volume of th~ 0 ~ 0 1 4
swollen organic. When the swollen organic i5
removed from the impregnating solution and dried, as
by evaporation of the solvent, the metal compounds
5 remain in the amorphous regions. The amorphous
regions are so small, about 50 angstroms in size in
cellulosic material, that the metal compounds do not
crystallize.
After withdrawing the Starting organic
10 material rom the impregnating solution, it is
necessary to remove e~cess solution adhering to the
surface of the starting material before this
solution dries. Fibers bonded together by dried
salt are likely to be similarly bonded in the final
lS o~ide product and cause reduced strength and
increa~ed brittleness. EYcess solution can be
removed by ~lotting the impregnated material with
adsorbent paper or cloth using moderate pressure to
press out excess solution from the material.
20 Alternatively, washing, blowing with a gas stream,
vacuum filtration or centrifugation may be employed.
Next, the impregnated material is dried by
any convenient means such as a warm gas stream.
Rapid drying is desirable to avoid migration of salt
25 from the interior ts the surface of the impregnated
material. A drying time of one hour or less is
preferred. The drying ~an also be accomplished
during the first por~ion of the heating step which
is described following.
The ne~t principal step is to heat the
impregnated organic material under controlled
conditions to pyrolize the organic structure,
D-16390-1
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:
~ eliminate the carbon and convert the metal compound~ O n ~ ~ A
to thQ desired metal o~ide. Pyrolysis is defined as
~hemical change brought about by the action of
heat. Ignitisn and uncontrolled temperature ri~e of
S the organic material during the heating iG ko be
avoided. Otherwise the organic material may
disintegrate ~efore the metal compounds h~ve
intered together sufficiently to maintain the
structural inteyrity of the working material. Also
10 e~cessive crystallization and grain growth may occur
resulting in escessive loss of strength in the final
product.
Ignition and uncontrolled temperature rise
may be avoided ~y heating at a moderate, controlled
15 rate to a desired ma~imum temperature in an
atmosphere of not more than weakly vxidizing
capability. The ma~imum temperature will fall in
the range from 500C to 1000C, and will depend on
the particular superconducting material desired and
20 the treatment necessary to develop the appropriate
o~idation state. The heating may be performed by
suspending the impregnated material in a furnace
having walls which are raised in temp~rature at a
controlled rate. By radiation from the wall~ and
25 convection from the furnace atmosphere heat is
transferred to the impregnated material so that its
temperature approximates the furnac2 wall
temperature. Ignition and uncontrolled temperature
rise would be a temperature rise in the impregnated
30 material above the temperature of the furnace walls.
Heating rates of 60 C to 600 C per hour
have been suitable~ Heating rates at the low end of
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.
this range are preferred in heating to about 400~C, 2
during which interval most of the pyrolysis of the
organic will occur. 9y pyrolysi~ is meant chemical
change brought about by the action of heat and with
5 little osidatio~. By oxidation is meant ch~mical
change which involves combination with o~ygen. At
temperatures higher than 400C, heating rates at the
higher end of the range are preferred. A ~uitable
atmosphere of weakly o~idizing capability was found
10 to be carbon dioxide. Operative are atmospheres
containing nonreaetive gases with ~arbon dioxide,
preferably in e~cess of 50% carbon dio~ide, e.g.,
from about 55% to about 100% carbon dio~ide; and
preferably from about 70% to about 100% carbon
15 dio~ide.
Oth~r weakly ~idizing atmospheres may be
employed such as nitrous o~ide, nitrogen dio~ide or
sulfur trio~ide, mi~tures of two or more thereof and
mi~tures of two or more thereof with nonreactive
20 gases. In an atmosphere containing nitrous o~ide,
concentrations in e~cess of 10% nitrous o~ide are
operative, e.g., from about 20~ to about 100%
nitrous o~ide.
Alternativel~ usable is a nonreactive gas
25 rontaining a ~mall percentage of o~ygen, e.g.,
nitrogen, argon, or helium containing several
percent of ogygen. The oxygen content appropriate
will depend somewhat on the heating rate employed,
weaker o~idizing atmospheres in general allowing
30 somewhat faster heating rates. O~ygen contents up
to 2% by ~olume are operative, from about 0.05 to
about 1 % are preferred and from 0.05 to about 0.5 %
D-16390-1
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are most preferred. Above the critical content of 2 0 9 014
2% o~ygen, ignition and disintegration of the
impregnated mat~rial was found to occur.
An atmosphere which is totally
5 non-o~idizing is not suitable during the heatin~
step because carbon is then ~pparently entrapped in
the metal o~ide, is not sufficiently removed, Dnd is
deleterious to the formation of the superconducting
phase.
During the heating step, if ignition is
avoided, consolidation of the metal compounds occurs
which is evident as shrinkage of the starting
preform material. Typically, the longest dimension
of the startîng material shrinks 40 to ~0%. In
15 ~eneral, the shrinkage is inversely proportional to
the degree of impregnation of metal compounds
achieved in the starting material.
Particularly in the case of string or
tape-like starting materials, it has been desirable
20 to apply a light tension to the starting material
during the heating step. This tension ser~es to
r~duce wrinkling or warpage of the material.
Upon reaching the selected ma~;mum
temperature in the weakly o~idizing atmosphere,
25 these conditions are maintained until the reaction
activity approaches completion as evident by the
r~duction in evolution of gases from the starting
organic. At this stage the pyrolysis o~ the
starting material is substantially complete, and
30 oxidation of the carbon has at least begun.
The ne~t step is to convert to and maintain
a mildly o~idizing atmosphere while appro~imately
D-16390-1
- 14 -
maintaining temperature~ until the o~idation and
removal of the carbon as a gas has approached 2 0 n
completion at these conditions A mildly o~idizing
atmosphere can be conveniently pro~ided as a slowly
5 flowing nonreactive gas such as nitrogen, argon, or
helium containing from 0.5 to 5~ of o~ygen by
volume. The approach of reaction completion can be
determined by observing the decrease end leveling
off of the carbon dio~ide content in the effluent to
10 a small value, such as 0.5%, which has been observed
to occur in from 0.5 to 2 hours.
At this poin~, the atmosphere is converted
to at least a moderately o~idizing atmosphere, which
will further gasify the remaining carbon and at
15 least partially form the desired o~idation state in
the working material. Such atmospheres will contain
at least 20% o~ygen and preferably ~ubstantially
oxygen. In some instances, ozone with its greater
ozidizing power may be advantageous. These
20 conditions are maintained at least for 0.5 hours and
preferably for 5 hours.
Cooling may be performed in the latter
atmosphere, and preferably in an atmosphere of
substantially o~ygen. Cooling while the working
25 material is still at high temperature must be
performed in an atmosphere which will add o~ygen to
the working material, i.e., form or develop the
~uperconductive o~ide, and not remove o~ygen from
the working material, i.e., maintain the oxygen
30 content. The cooling rate may be in the range of 60
C to 600 C per hour. During the cooling, the
material is preferably in th~ established o~idizing
D-16390-1
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atmosphere for a time of 0.5 to ~ hours while in the 2~90
temperature range of 700 to 400C. Such treatment
;s favorable for the development of the o~idation
~tate whi~h is superconducting.
Following i~ an example of the preparation
of a superconducting tape of Y8a2Cu3O7_~ pursuant to
the process of this invention. A solution was
10 prepared by dissolving 4 grams of Y(NO3)3O6H2O, 6
grams of ~a(NO3)~ and B.4 grams of Cu(NO3)3-3H2O
into 100 cc of distilled water at 60C. This
con~entration was near the highest achievable,
inasmuch as Ba(NO3)2, the least soluble of the three
15 salts, would precipitate at lower temperatures. A
rayon tape 1.25 inch wide, 0.018 inch thick,
approximately 12 in~hes long, with individual fiber~
0.001 inch in diameter was soaked for 4 hours in
this solution. E~cess solution was pressed out by
20 rolling the tape between sheets of absorbing paper.
About 10% by weight of salts in the tape was
achieved on a dry basis.
The imbibed tape under slight tension of 50
grams was heated in a furnace at a rate between
25 60 C to 600 C~ per hour to 850C in a lowing
atmosphere of carbo~ dio~ide ~as.
It was found that to form the desired
YBa2Cu3O7_~ compound, the precursor tape had to be
heated to at least 750C and pre~erably to 050C.
30 Above 950C, fracture of the precursor tape commonly
occurred, apparently because of partial melting of
the metal o~ides. At temperatures above 950~C, a
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molten peritectic region exists with ~ompositions ~ h
that e~tend close to the desired YBa2Cu3O7_x
compound.
~perimentation with nitrogen during the
5 heating step was also conducted, but did not produce
the desired ~uperconducting product. Pyroly~ig of
the starting organic material in a totally
nono~idizing atmosphere apparently caused entrapment
of carbon in the resulting metal o~ide which was
10 detrimentsl to the formation of the ~uperconducting
oxide. Pyrolysis in air caused the impregnated tape
~o ignite and disintegrate apparently because the
rayon fibers lost their structural integrity before
the metal o~ides had sintered together
15 sufficiently. Carbon dioxide gas for the initial
heating phase, where pyrolysis occurs, proved to be
an acceptable atmosphere which avoided ignition yet
allowed the o~idation and elimination of carbon.
Vpon reaching 850C in khe carbon dio~ide
20 atmosphere, these conditions were maintained for 1.5
hours. The atmosphere was then changed to nitrogen
~ontaining 1% o~ygen and maintained for 18 hours.
By this time, the carbon dio~ide content in the
e~iting gas had decreased to less than 0.5~
25 indicati~g that the pyrolysis and carbon elimination
reactions had approached completion. The atmosphere
was then changed to 100% oxygen and maintained for 1
to 2 hours. Then tbe tape was slowly cooled to
450C at a rate of about 60 C per hour in oxygen.
0 Below 450C the tape was cooled rapidly to room
temperature while the o~ygen flow was maintained.
The product tape typically was 0.4 inches
wide and 4 inches long with individual fibers
D-16390-1
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appro~imately 0.0004 inches in diameter. By
chemical analysis, the tape composition was 16.67
atom % Y, 33.33 % Ba and 50.18% Cu which compared
favorably with the ideal composition of 16.767 ~ Y,
5 33.67 ~a and ~0.00% Cu. Observed under a
microscope, ~he tape had a morphology characteristic
of the starting rayon tape.
~ -ray diffraction of the product tape
displayed well-defined peaks characteristic of
10 superconducting orthorhombic YBa2Cu3O7~ as shown in
Fig. 1. However, the relative magnitudes of the
peaks differed from the relative magnitudes of
orthorhombic YBa2Cu3O7_~ prepared by the traditional
ceramic route of grinding and sinterin~ of solid
15 starting materials as shown in Fig. 2. This
indicates that a degree of orientation of crystal
axes had occurred in the crystallites comprising the
product tape compared to the random orientation
occurring in the ground and sintered material.
Although the invention has been described
with a degr~e of particularity, the present
disclosure has been made only by way of example, and
numerous chanyes In the details and arrangement of
various steps in the process may be resorted to
25 without departing from ths spirit and scope of the
invention as hereinafter claimed.
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