U.S. patent application number 09/861248 was filed with the patent office on 2002-09-12 for processing of oxide superconductors.
Invention is credited to Carter, William L., Otto, Alexander, Riley, Gilbert N. JR..
Application Number | 20020128155 09/861248 |
Document ID | / |
Family ID | 26718574 |
Filed Date | 2002-09-12 |
United States Patent
Application |
20020128155 |
Kind Code |
A1 |
Otto, Alexander ; et
al. |
September 12, 2002 |
PROCESSING OF OXIDE SUPERCONDUCTORS
Abstract
A method for preparing a BSCCO-2223 oxide superconducting
article includes annealing an oxide superconductor article
comprised of BSCCO-2223 oxide superconductor at a temperature
selected from the range of about 500.degree.
C..ltoreq.T.ltoreq.787.degree. C. and an annealing atmosphere
having an oxygen pressure selected from within the region having a
lower bound defied by the equation,
P.sub.O2(lower).gtoreq.3.5.times.10.sup.10e- xp(-32,000/T+273) and
an upper bound defined by the equation,
P.sub.O2(upper).ltoreq.1.1.times.10.sup.12exp(-32,000/T+273). The
article is annealed for a time sufficient to provide at least a 10%
increase in critical current density as compared to the critical
current density of the pre-anneal oxide superconductor article. An
oxide superconductor having the formula
Bi.sub.2-yPb.sub.ySr.sub.2Ca.sub.2Cu.sub.3O.sub.10+x, where
0.ltoreq.x.ltoreq.1.5 and where 0.ltoreq.y.ltoreq.0.6 is obtained,
the oxide superconductor characterized by a critical transition
temperature of greater than 111.0 K, as determined by four point
probe method.
Inventors: |
Otto, Alexander;
(Chelmsford, MA) ; Riley, Gilbert N. JR.;
(Marlborough, MA) ; Carter, William L.;
(Chelmsford, MA) |
Correspondence
Address: |
HALE AND DORR, LLP
60 STATE STREET
BOSTON
MA
02109
|
Family ID: |
26718574 |
Appl. No.: |
09/861248 |
Filed: |
May 18, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09861248 |
May 18, 2001 |
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09451742 |
Nov 30, 1999 |
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09451742 |
Nov 30, 1999 |
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08779808 |
Jan 8, 1997 |
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08779808 |
Jan 8, 1997 |
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08198912 |
Feb 17, 1994 |
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08198912 |
Feb 17, 1994 |
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08041822 |
Apr 1, 1993 |
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Current U.S.
Class: |
505/100 |
Current CPC
Class: |
H01L 39/2464 20130101;
Y10S 505/782 20130101; Y10S 505/742 20130101; Y10S 505/785
20130101 |
Class at
Publication: |
505/100 |
International
Class: |
H01B 001/00 |
Claims
What is claimed is:
1. A method for preparing a BSCCO-2223 oxide superconducting
article, comprising the steps of: providing an oxide superconductor
article comprised of BSCCO-2223 oxide superconductor; and annealing
the superconducting article at a temperature selected from the
range of about 500.degree. C. .ltoreq.T.ltoreq.787.degree. C. and
an annealing atmosphere having an oxygen pressure selected from
within the region having a lower bound defined by the equation,
P.sub.O2(lower).gtoreq.3.5.- times.10.sup.10exp(-32,000/T+273) and
an upper bound defined by the equation,
P.sub.O2(upper).ltoreq.1.1.times.10.sup.12exp(-32,000/T+273), for a
time sufficient to provide at least a 10% increase in critical
current density as compared to the critical current density of the
pre-anneal oxide superconductor article.
2. A method for preparing a BSCCO-2223 oxide superconducting
article, comprising the steps of: providing an oxide superconductor
article comprised of BSCCO-2223 oxide superconductor; and annealing
the superconducting article at a temperature selected from the
range of about 500.degree. C. .ltoreq.T.ltoreq.760.degree. C. and
an annealing atmosphere having an oxygen pressure selected from
within the region having a lower bound defined by the equation,
P.sub.O2(lower).gtoreq.8.5.- times.10.sup.10exp(-32,000/T+273) and
an upper bound defined by the equation,
P.sub.O2(upper).ltoreq.2.62.times.10.sup.11exp(-32,000/T+273), for
a time sufficient to provide at least a 10% increase in critical
current density as compared to the critical current density of the
pre-anneal oxide superconductor article.
3. A method for preparing a BSCCO-2223 oxide superconductor
article, comprising the steps of: exposing an oxide superconductor
article to a heat treatment after deformation of the article, the
article comprised of at least BSCCO-2223, the heat treatment
comprising, (a) heating the article at a temperature substantially
in the range of 815-860.degree. C. for a period of time
substantially in the range of 0.1 to 300 hours at a P.sub.O2
substantially in the range of 0.001-1.0 atm; (b) subjecting the
article to a temperature substantially in the range of
790-845.degree. C. for a period of time substantially in the range
of 1 to 300 hours at a P.sub.O2 substantially in the range of
0.01-1.0 atm, with no deformation occurring after the heat
treatment; and annealing the superconducting article at a
temperature selected from the range of about 500.degree. C.
.ltoreq.T.ltoreq.787.degree. C. and an annealing atmosphere having
an oxygen pressure selected from within the region having a lower
bound defined by the equation, P.sub.O2(lower)
.gtoreq.3.5.times.10.sup.10exp(-- 32,000/T+273) and an upper bound
defined by the equation, P.sub.02(upper)
<1.1.times.10.sup.12exp(-32,000T+273).
4. The method of claim 3, wherein the step of annealing the
superconductor article comprises annealing at a temperature
selected from the range of about 500.degree. C.
.ltoreq.T.ltoreq.760.degree. C. and an annealing atmosphere having
an oxygen pressure selected from within the region having a lower
bound defined by the equation, P.sub.O2(lower).gtoreq.8.5.-
times.10.sup.10exp(-32,000/T+273) and an upper bound defined by the
equation, P.sub.O2(upper)
.ltoreq.2.62.times.10.sup.11exp(-32,000/T+273).
5. The method of claim 1 or 2, wherein the step of annealing
comprises annealing under temperature and oxygen pressure
conditions sufficient to provide at least a 30% increase in
critical current density as compared to the critical current
density of the pre-anneal oxide superconductor article.
6. The method of claim 1, 2 or 3, wherein the annealing atmosphere
is substantially at a pressure of one atmosphere and oxygen
pressure is obtained by controlling the oxygen concentration in the
annealing atmosphere.
7. The method of claim 3, wherein annealing atmosphere additionally
comprises an inert gas selected from the group consisting of argon,
nitrogen, helium and neon.
8. The method of claim 1, 2 or 3, wherein the step of annealing is
the final annealing to which the superconducting article is
subjected.
9. The method of claim 1, 2 or 3, wherein the anneal is carried out
at an oxygen pressure substantially in the range of
7.5.times.10.sup.-2 atm to 1.times.10.sup.-8 atm O.sub.2.
10. The method of claim 1, 2 or 3, wherein the anneal is carried
out at a the temperature in the range of 770 to 787.degree. C. and
an oxygen pressure in the range of 0.017 to 0.085 atm.
11. The method of claim 1, 2 or 3, wherein the anneal is carried
out at a temperature in the range of 750 to 770.degree. C. and an
oxygen pressure in the range of 0.0009 to 0.052 atm.
12. The method of claim 1, 2 or 3, wherein the anneal is carried
out at a temperature in the range of 730 to 750.degree. C. and an
oxygen pressure in the range of 0.005 to 0.029 atm.
13. The method of claim 1, 2 or 3, wherein the anneal is carried
out at a temperature in the range of 690 to 730.degree. C. and an
oxygen pressure in the range of 0.0001-0.015 atm.
14. The method of claim 1, 2 or 3, wherein the anneal is carried
out at a temperature in the range of 740 to 760.degree. C. and an
oxygen pressure in the range of 0.0016 to 0.009 atm.
15. The method of claim 1, 2 or 3, wherein the anneal is carried
out at a temperature in the range of 710 to 740.degree. C. and an
oxygen pressure in the range of 0.0006-0.005 atm.
16. The method of claim 1, 2 or 3, wherein the anneal is carried
out at a temperature in the range of 690 to 710.degree. C. and an
oxygen pressure in the range of 0.0003-0.002 atm.
17. The method of claim 1, 2 or 3, wherein the step of annealing
comprises annealing at progressively lower temperature and oxygen
pressure.
18. The method of claim 17, wherein the step of annealing comprises
continuously reducing temperature and/or oxygen pressure.
19. The method of claim 17, wherein the step of annealing comprises
stepwise reduction of temperature and/or oxygen pressure.
20. An oxide superconductor composite prepared according to any of
claims 1, 2 or 3.
21. An oxide superconductor article comprising an oxide
superconductor having the formula
Bi.sub.2-yPb.sub.ySr.sub.2Ca.sub.2Cu.sub.3O.sub.10+x, where
0.ltoreq.x.ltoreq.1.5 and where 0.ltoreq.y.ltoreq.0.6, the oxide
superconductor characterized by a critical transition temperature
of greater than 111.0 K as defined by zero resistance by a four
point linear probe method with zero resistance corresponding to a
resistivity of less than 10.sup.-8 .OMEGA.-cm.
22. The oxide superconductor article of claim 21, further
comprising silver.
23. The oxide superconductor article of claim 21, wherein the oxide
superconductor comprises a filamentary oxide superconductor phase
contained within a silver phase.
24. An oxide superconductor article characterized by a critical
transition temperature of greater than 111.0 K as defined by zero
resistance by a four point linear probe method with zero resistance
corresponding to a resistivity of less than 10.sup.-8 .OMEGA.-cm
and an x-ray diffraction pattern having peaks at 17.4.degree.,
19.2.degree., 20.2.degree., 21.8.degree., 23.2.degree.,
23.9.degree., 26.2.degree., 27.8.degree., 29.degree., 29.7.degree.,
31.5.degree., 32.degree., 33.2.degree., 33.7.degree., 35.degree.,
35.6.degree., 38.degree., 38.8.degree., 41.6.degree., 43.8.degree.,
44.4.degree., 46.8.degree., 47.4.degree., 48.degree. and
49.degree..
25. An oxide superconductor article characterized by a critical
transition temperature of greater than 111.0 K as defined by zero
resistance by a four point linear probe method with zero resistance
corresponding to a resistivity of less than 10.sup.-8 .OMEGA.-cm
and an x-ray diffraction pattern having peaks at 17.4.degree.,
27.8.degree., 29.7.degree., 38.degree., 41.6.degree. and
46.8.degree..
Description
[0001] The application is a continuation-in-part application of
co-pending application U.S. Ser. No. 08/041,822 filed Apr. 1, 1993,
also entitled "Improved Processing for Oxide Superconductors".
FIELD OF THE INVENTION
[0002] The present invention relates to high-performance oxide
superconductors and oxide superconductor composites. The present
invention further relates to a method for healing defects
introduced into the oxide superconductor phase during processing
thereby improving superconducting properties. The present invention
also relates to the processing of high performance
bismuth-strontium-calcium-copper oxide superconductors and oxide
superconductor composites and a method for improving the critical
transition temperature (T.sub.c) and critical current density
(J.sub.c) of these oxide superconductors.
BACKGROUND OF THE INVENTION
[0003] Oxide superconductors of the rare earth-barium-copper-oxide
family (YBCO), bismuth(lead)-strontium-calcium-copper-oxide family
((Bi,Pb)SCCO) and thallium-barium-calcium-copper-oxide family
(TBCCO) form plate-like and highly anisotropic superconducting
oxide grains. Because of their plate-like morphology, the oxide
grains can be oriented by mechanical strain. Mechanical deformation
has been used to induce grain alignment of the oxide superconductor
c-axis perpendicular to the plane or direction of elongation. The
degree of alignment of the oxide superconductor grains is a major
factor in the high critical current densities (J.sub.c) obtained in
articles prepared from these materials.
[0004] Known processing methods for obtaining textured oxide
superconductor composite articles include an iterative process of
alternating heating and deformation steps. The heat treatment is
used to promote reaction-induced texture of the oxide
superconductor in which the anisotropic growth of the
superconducting grains is enhanced. Each deformation provides an
incremental improvement in the orientation of the oxide grains.
Additional heat treatment intermediate with or subsequent to
deformation is also required to form the correct oxide
superconductor phase, promote good grain interconnectivity and
achieve proper oxygenation.
[0005] Processing long lengths of oxide superconductor is
particularly difficult because deformation leads to microcracking
and other defects which may not be healed in the subsequent heat
treatment. Cracks that occur perpendicular to the direction of
current flow limit the performance of the superconductor. In order
to optimize the current carrying capability of the oxide
superconductor, it is necessary to heal any microcracks that are
formed during processing of the oxide superconductor or
superconducting composite.
[0006] Liquid phases in co-existence with solid oxide phases have
been used in processing of oxide superconductors. One type of
partial melting, known as peritectic decomposition, takes advantage
of liquid phases which form at peritectic points of the phase
diagram containing the oxide superconductor. During peritectic
decomposition, the oxide superconductor remains a solid until the
peritectic temperature is reached, at which point the oxide
superconductor decomposes into a liquid phase and a new solid
phase. The peritectic decompositions of
Bi.sub.2Sr.sub.2CaCu.sub.2O- .sub.8+x, (BSCCO-2212, where
0.ltoreq.x.ltoreq.1.5), into an alkaline earth oxide and a liquid
phase and of YBa.sub.2Cu.sub.3O.sub.7-.delta. (YBCO-123, where
0.ltoreq..delta..ltoreq.1.0) into Y.sub.2BaCuO.sub.5 and a liquid
phase are well known.
[0007] Peritectic decomposition of an oxide superconductor and the
reformation of the oxide superconductor from the liquid+solid phase
is the basis for melt textured growth of YBCO-123 and BSCCO-2212.
For example, Kase et al. in IEEE Trans. Mag. 27(2), 1254 (March
1991) report obtaining highly textured BSCCO-2212 by slowly cooling
through the peritectic point. This process necessarily involves
total decomposition of the desired 2212 phase into an alkaline
earth oxide and a liquid phase.
[0008] It is also recognized that an oxide superconductor itself
can co-exist with a liquid phase under suitable processing
conditions. This may occur by solid solution melting, eutectic
melting or by formation of non-equilibrium liquids.
[0009] Solid solution melting may arise in a phase system, in which
the oxide superconductor is a solid solution. As the temperature
(or some other controlling parameter) of the system increases (or
decreases), the oxide superconductor phase changes from a solid
oxide phase to a liquid plus oxide superconductor partial melt
(this happens at the solidus). A further increase in temperature
(or some other controlling parameter) affords the complete melting
of the oxide superconductor (this happens at the liquidus).
[0010] A phase diagram containing a eutectic point may provide an
oxide superconductor partial melt, known as eutectic melting, when
the overall composition is chosen so as to be slightly off
stoichiometry. As the temperature (or some other controlling
parameter) of the system increases (or decreases), the mixed phase
of oxide superconductor-plus-non-supercon- ducting oxide
(solid.sub.1+solid.sub.2) changes to a liquid-plus-oxide
superconductor partial melt (solid.sub.1+liquid).
[0011] Non-equilibrium liquids may also promote partial melting in
oxide superconductor systems. A non-equilibrium liquid is
established through the relatively rapid heating of a mixture of
oxides to a temperature above the eutectic melting point of local
stoichiometries present in the heterogeneous mixture of phases. As
the oxides form the desired oxide superconductor, the solid and
liquid phases can co-exist, if only temporarily.
[0012] Partial melting of
(Bi,Pb).sub.2Sr.sub.2Ca.sub.2Cu.sub.3O.sub.10+x ((Bi,Pb)SCCO-2223,
where 023 x.ltoreq.1.5) and (Bi).sub.2Sr.sub.2Ca.sub.1-
Cu.sub.2O.sub.10+x ((Bi)SCCO-2223, where 0.ltoreq.x.ltoreq.1.5),
collectively BSCCO-2223, at temperatures above 870.degree. C. in
air has been reported; see, for example, Kobayashi et al. Jap. J.
Appl. Phys. 28, L722-L744 (1989), Hatano et al. Ibid. 27(11), L2055
(Nov. 1988), Luo et al. Appl. Super. 1, 101-107, (1993), Aota et
al. Jap.J. Appl. Phys. 28, L2196-L2199 (1989) and Luo et al. J.
Appl. Phys. 72, 2385-2389 (1992). The exact mechanism of partial
melting of BSCCO-2223 has not been definitively established.
[0013] Guo et al. in Appl. Supercond. 1(1/2), 25 (January 1993)
have described a phase formation-decomposition-reformation (PFDR)
process, in which a pressed sample of (Bi,Pb)SCCO-2223 is heated
above its liquidus to decompose the 2223 phase, followed by a heat
treatment at a temperature below the solidus. The sample was
subsequently pressed again and reannealed. The final anneal of the
PFDR process includes a standard single step heat treatment in
which there is no melting.
[0014] The "high T.sub.c" oxide superconductor
Bi.sub.2-yPb.sub.ySr.sub.2C- a.sub.2Cu.sub.3O.sub.10+x, where
0.ltoreq.x.ltoreq.1.5 and 0.ltoreq.y.ltoreq.0.6 (BSCCO-2223 and
(Bi,Pb)SCCO-2223, hereinafter referred to as "BSCCO-2223 " to
indicate both lead-doped and undoped compositions), is particularly
desirable because of its high critical transition temperature
(T.sub.c.about.110 K) and high critical current (I.sub.c, J.sub.c).
The superconducting art constantly seeks to improve electrical
properties, such as, critical current density and critical
transition temperature.
[0015] Partial melting in the processing of oxide superconductors
has been used either to increase the yield of the BSCCO-2223 phase
or to improve the contiguity and texturing of the oxide
superconductor grains. The issue of healing defects, such as
microcracks, which develop during processing of the oxide
superconductor, has not been addressed. Further, the prior art has
not addressed the possibility of using a two-step process where the
oxide superconductor is stable in both steps for the healing of
cracks and defects.
[0016] Wang et al. ("Advances in Superconductivity",
Springer-Verlag, New York, Editors: Y. Bando and H. Yamauchi, pp.
291-294 (1993)) report an increase in T.sub.c by carrying out a
post-anneal step at 790.degree. C. at reduced total pressures. Wang
et al. observed T.sub.c by DC magnetization of 115 K and T.sub.c,
zero of 111 K by resistivity measurement. The technique used by
Wang et al. (vacuum encapsulation at 10.sup.-4 Torr of oxide
superconductor pellets, followed by annealing at 790.degree. C.)
does not permit determination of the oxygen pressure of the system.
The encapsulated pellets reach an equilibrium oxygen pressure
within the capsule by releasing oxygen. The pellet volume/capsule
volume plays an important role in determining the final equilibrium
oxygen pressure.
[0017] Critical transition temperatures (determined by
magnetization) as high as 117 K have been reported for multiphase
materials containing BSCCO-2223. Fisher et al. (Physica C 160, 466
(1990)) reported a T.sub.c of 115 K (determined by magnetization)
with the substitution of lead and antimony in the BSCCO-2223
system. A non-reproducible T.sub.c as high as 130 K was reported by
Hongbo et al. (Solid State Comm. 69, 867 (1989)).
[0018] While reports of high transition temperatures by
magnetization studies are of interest, they can sometimes be
misleading. The transition curves obtained by magnetization are
"soft", making extrapolation to zero resistivity highly subjective.
Further, other effects, such as semiconductor to metallic
transitions, can mimic critical temperature transition behavior. It
is therefore desirable to rely on bulk resistivity measurements for
determining temperature at zero resistivity (T.sub.c, zero).
[0019] Idemoto et al. (Physica C 181, 171-178 (1991)) has
investigated the oxygen content and copper and bismuth valances of
BSCCO-2223 under a range of conditions, including temperatures in
the range of 500.degree. C. to 850.degree. C. and oxygen pressures
in the range of 0.005 to 0.20 atm. The samples were observed by
means of a microbalance under changing temperatures conditions at
constant oxygen pressures. Because the samples do not reach
equilibrium during the observation period, it is difficult to
determine the exact processing conditions experienced by the
samples. No investigation of the effect of reported conditions on
electrical properties is reported.
[0020] None of the previous research has indicated the desirability
of post-annealing the BSCCO-2223 phase at low temperatures and
oxygen pressures to enhance the electric transport properties of
the oxide superconductor, namely critical current.
[0021] It is the object of the present invention to provide a
method for improving superconducting performance of oxide
superconductors and superconducting composites by healing cracks
and defects formed during processing of oxide superconductors and
superconducting composites.
[0022] It is a further object of the invention to prepare oxide
superconducting articles having significantly less cracks and
defects than conventionally-processed articles.
[0023] A further object of the present invention is to provide a
process to increase the critical current density of BSCCO-2223 by a
method which also increases its critical transition temperature. It
is a further object of the present invention to provide a novel
high-T.sub.c BSCCO-2223 composition having a critical transition
temperature greater than 111.0 K.
[0024] A feature of the invention is a two-step heat treatment
after which no further deformation occurs which introduces a small
amount of a liquid phase co-existing with the oxide superconductor
phase, and then transforms the liquid back into the oxide
superconductor phase with no deformation occurring during or after
the heat treatment of the invention. A further feature of the
present invention is a low temperature, low oxygen pressure anneal
of the oxide superconductor.
[0025] An advantage of the invention is the production of highly
defect-free oxide superconductor and superconducting composites
which exhibit superior critical current densities. A further
advantage of the invention is a marked improvement in critical
transition temperature and critical current density as compared to
oxide superconductors and superconducting composites which are not
subjected to the method of the invention.
SUMMARY OF THE INVENTION
[0026] In one aspect of the present invention, an oxide
superconductor article containing a desired oxide superconductor
phase is exposed to a two-step heat treatment after deformation of
the article, which includes (a) heating the article at a
temperature sufficient to partially melt the article, such that a
liquid phase co-exists with the desired oxide superconductor phase;
and (b) cooling the article to a temperature sufficient to
transform the liquid phase into the desired oxide superconductor,
with no deformation occurring after the heat treatment of the
invention.
[0027] In another aspect of the invention, an oxide superconductor
article containing a desired oxide superconductor phase is exposed
to a two-step heat treatment after deformation of the article which
includes (a) forming a liquid phase in the oxide superconducting
article, such that the liquid phase co-exists with the desired
oxide superconductor solid phase; and then (b) transforming the
liquid phase into the desired oxide superconductor, with no
deformation occurring after the heat treatment of the
invention.
[0028] In preferred embodiments, the liquid phase wets surfaces of
defects contained within the oxide superconductor article. The
defects are healed upon transformation of the liquid to the desired
oxide superconductor. The partial melting of step (a) and the
transformation of step (b) are effected by selection of appropriate
thermodynamic state variables, for example, temperature, P.sub.O2,
P.sub.total and total composition. In principle, deformation may
occur during the heat treatment of the present invention up to
immediately prior to the completion of step (a), providing that the
liquid phase is available for a period of time sufficient to wet
defect surfaces.
[0029] By "two-step heat treatment" or "heat treatment of the
invention", as those terms are used herein, it is meant a heat
treatment for forming an oxide superconductor after which no
further deformation occurs. However, heat treatments for purposes
other than those stated herein, such as, for example, oxygenation
of the oxide superconductor, are possible. In all cases, not
further deformation occurs.
[0030] By "partial melt", as that term is used herein, it is meant
the oxide superconductor article is only partially melted, and that
the desired oxide superconductor is present during melting.
[0031] By "deformation" as that term is used herein, it is meant a
process which causes a change in the cross-sectional shape of the
article.
[0032] By "oxide superconductor precursor", as that term is used
herein, it is meant any material that can be converted to an oxide
superconductor upon application of a suitable heat treatment.
Suitable precursor materials include but are not limited to metal
salts, simple metal oxides, complex mixed metal oxides and
intermediate oxide superconductors to the desired oxide
superconductor.
[0033] By "desired oxide superconductor", as that term is used
herein, it is meant the oxide superconductor which it is desired to
ultimately prepare. An oxide superconductor is typically the
"desired" oxide superconductor because of superior electrical
properties, such as high T.sub.c and/or J.sub.c. The desired oxide
superconductor is typically a high T.sub.c member of a particular
oxide superconductor family, i.e., BSCCO-2223, YBCO-123, TBCCO-1212
and TBCCO-1223.
[0034] By "intermediate oxide superconductor", as that term is used
herein, it is meant an oxide superconductor which is capable of
being converted into a desired oxide superconductor. However, an
intermediate oxide superconductor may have desirable processing
properties, which warrants its formation initially before final
conversion into the desired oxide superconductor. The formation of
an "intermediate oxide superconductor" may be desired, particularly
during heat treatment/deformation iterations, where the
intermediate oxides are more amenable to texturing than the desired
oxide superconductor.
[0035] In yet another aspect of the present invention, a textured
oxide superconductor article is prepared by subjecting an article
containing an oxide superconductor precursor to at least one first
heat treatment/deformation iteration. The heat treatment is
effective to form a desired oxide superconductor. The resultant
oxide superconductor phase is textured upon application of the
first heat treatment/deformation iteration. The article is then
exposed to a two-step heat treatment in which (a) the article is
partially melted, such that a liquid phase co-exists with the
desired textured oxide superconductor phase; and (b) the liquid
phase is transformed into the desired oxide superconductor, with no
deformation occurring after the heat treatment of the
invention.
[0036] In yet another aspect of the present invention, a textured
oxide superconductor article is prepared by subjecting an article
containing an oxide superconductor precursor to at least one first
heat treatment/deformation iteration. The heat treatment is
effective to form an intermediate oxide superconductor. The
intermediate textured oxide superconductor phase is formed. The
article is then subjected to at least one second heat
treatment/deformation iteration. The heat treatment is effective to
form a desired oxide superconductor. The desired textured oxide
superconductor is formed. The article is then exposed to a two-step
heat treatment in which (a) the article is partially melted, such
that a liquid phase co-exists with the desired textured oxide
superconductor phase; and (b) the liquid phase is transformed into
the desired oxide superconductor, with no deformation occurring
after the heat treatment of the invention.
[0037] In preferred embodiments, the intermediate oxide
superconductor is BSCCO-2212 or (Bi,Pb)SCCO-2212 because it is
readily textured by the heat treatment/deformation iterations. The
intermediate oxide superconductor is then converted to a desired
oxide superconducting phase, typically BSCCO-2223 or
(Bi,Pb)SCCO-2223. The partial melting of step (a) may be carried
out at a temperature in the range of 820-835.degree. C. at 0.075
atm O.sub.2. The transformation of the liquid in step (b) may be
carried out at a temperature in the range of 790-820.degree. C. at
0.075 atm O.sub.2. In other preferred embodiments, the desired
oxide superconductor, may be YBCO-123,
Y.sub.2Ba.sub.4Cu.sub.7O.sub.14-.delta., (YBCO-247),
(Tl,Pb).sub.1Ba.sub.2Ca.sub.1Cu.sub.2O.sub.6.0.+-.y (TBCCO-1212) or
(Tl,Pb).sub.1Ba.sub.2Ca.sub.2Cu.sub.3O.sub.8.0.+-.y (TBCCO-1223),
where 0.ltoreq..delta..ltoreq.61.0 and y ranges up to 0.5. The
stated stoichiometries are all approximate and intentional or
unintentional variations in composition are contemplated within the
scope of the invention.
[0038] In other preferred embodiments, the liquid phase is formed
in the range of 0.1-30 vol %. In yet other preferred embodiments,
the heat treatment of the first and second heat
treatment/deformation iterations partially melts the oxide
superconductor article.
[0039] In yet another aspect of the invention, an oxide
superconductor article is exposed to a two-step heat treatment
after a deformation step, which includes (a) heating the article at
a temperature substantially in the range of 810-860.degree. C. for
a period of time substantially in the range of 0.1 to 300 hours at
a P.sub.O2 substantially in the range of 0.001-1.0 atm; and (b)
cooling the article to a temperature substantially in the range of
780-845.degree. C. for a period of time substantially in the range
of 1 to 300 hours at a P.sub.O2 substantially in the range of
0.001-1.0 atm, with no deformation occurring after the heat
treatment of the present invention.
[0040] In yet another aspect of the present invention, an oxide
superconductor article containing a desired oxide superconductor
phase is exposed to a two-step heat treatment after a deformation
step, which includes (a) subjecting the article to an oxygen
partial pressure sufficient to partially melt the oxide
superconducting article, such that a liquid phase co-exists with
the desired oxide superconductor; and (b) raising to an oxygen
partial pressure sufficient to transform the liquid phase into the
desired oxide superconductor.
[0041] Yet another aspect of the present invention provides for a
multifilamentary oxide superconductor composite containing a
plurality of oxide superconductor filaments contained within a
matrix material which has been subjected to the two-step heat
treatment of the invention.
[0042] In yet another aspect of the invention, a multifilamentary
oxide superconductor composite contains a plurality of oxide
superconductor filaments contained within a matrix material, the
composite having a J.sub.c of at least 14.times.10.sup.3 A/cm.sup.2
at 77K, self field, as measured over a length of at least 50 m.
[0043] The present invention provides oxide superconductors which
exhibit marked improvement in critical current density (J.sub.c)
over samples processed in an otherwise similar manner, lacking only
the two-step heat treatment of the present invention.
[0044] In yet another aspect of the present invention, a BSCCO-2223
oxide superconducting article is prepared by providing an oxide
superconductor article including BSCCO-2223 oxide superconductor,
and annealing the superconducting article at a temperature selected
from the range of about 500.degree. C..ltoreq.T.ltoreq.787.degree.
C. and an annealing atmosphere having an oxygen pressure selected
from within the region having a lower bound defined by the
equation, P.sub.O2(lower).gtoreq.3.5.times.10.sup.10-
exp(-32,000/T+273) and an upper bound defined by the equation,
P.sub.O2(upper).ltoreq.1.1.times.10.sup.12exp(-32,000/T+273). The
sample is annealed for a time sufficient to provide at least a 10%
increase in critical current density as compared to the critical
current density of the pre-anneal oxide superconductor article.
[0045] In yet another aspect of the invention, a BSCCO-2223 oxide
superconducting article is prepared by providing an oxide
superconductor article including BSCCO-2223 oxide superconductor,
and annealing the superconducting article at a temperature selected
from the range of about 500.degree. C..ltoreq.T.ltoreq.760.degree.
C. and an annealing atmosphere having an oxygen pressure selected
from within the region having a lower bound defined by the
equation, P.sub.O2(lower).gtoreq.8.5.times.10.sup.10-
exp(-32,000/T+273) and an upper bound defined by the equation,
P.sub.O2(upper).ltoreq.2.62.times.10.sup.11exp(-32,000/T+273). The
sample is annealed for a time sufficient to provide at least a 10%
increase in critical current density as compared to the critical
current density of the pre-anneal oxide superconductor article.
[0046] In yet another aspect of the invention, a BSCCO-2223 oxide
superconductor article is prepared by exposing the article
including at least BSCCO-2223 to a heat treatment after deformation
of the article, including (a) heating the article at a temperature
substantially in the range of 815-860.degree. C. for a period of
time substantially in the range of 0.1 to 300 hours at a P.sub.O2
substantially in the range of 0.001-1.0 atm; and (b) subjecting the
article to a temperature substantially in the range of
790-845.degree. C. for a period of time substantially in the range
of 1 to 300 hours at a P.sub.O2 substantially in the range of
0.01-1.0 atm, with no deformation occurring after the heat
treatment. The superconducting article is then annealed at a
temperature selected from the range of about 500.degree.
C..ltoreq.T.ltoreq.787.degree. C. and an annealing atmosphere
having an oxygen pressure selected from within the region having a
lower bound defined by the equation,
P.sub.O2(lower).gtoreq.3.5.times.10.sup.10exp(-3- 2,000/T+273) and
an upper bound defined by the equation,
P.sub.O2(upper).ltoreq.1.1.times.10.sup.12exp(-32,000/T+273).
[0047] By "anneal of the present invention", it is meant a low
pressure, low temperature heat treatment under equilibrium
conditions during which no further formation of the desired oxide
superconducting phase occurs; however, the internal chemistry of
the oxide superconductor (i.e.; oxygen stoichiometry) and grain
growth of the existing oxide superconductor phase may be
affected.
[0048] In preferred embodiments, the annealing atmosphere is
substantially at a pressure of one atmosphere and oxygen pressure
is obtained by controlling the oxygen concentration in the
annealing atmosphere. The annealing atmosphere may additionally
contain an inert gas selected from the group consisting of argon,
nitrogen and helium. The anneal is carried out at an oxygen
pressure substantially in the range of 7.5.times.10.sup.-2 atm to
1.times.10.sup.-8 atm O.sub.2. The annealing of the invention is
preferably the final annealing to which the superconducting article
is subjected.
[0049] In other preferred embodiments, the anneal is carried out at
a the temperature in the range of 770 to 787.degree. C. and an
oxygen pressure in the range of 0.017 to 0.085 atm; The anneal is
carried out at a temperature in the range of 750 to 770.degree. C.
and an oxygen pressure in the range of 0.0009 to 0.052 atm; the
method of claim 1, 2 or 3, wherein the anneal is carried out at a
temperature in the range of 730 to 750.degree. C. and an oxygen
pressure in the range of 0.005 to 0.029 atm; the anneal is carried
out at a temperature in the range of 690 to 730.degree. C. and an
oxygen pressure in the range of 0.0001-0.015 atm; the anneal is
carried out at a temperature in the range of 740 to 760.degree. C.
and an oxygen pressure in the range of 0.0016 to 0.009 atm; the
anneal is carried out at a temperature in the range of 710 to
740.degree. C. and an oxygen pressure in the range of 0.0006-0.005
atm; and the anneal is carried out at a temperature in the range of
690 to 710.degree. C. and an oxygen pressure in the range of
0.0003-0.002 atm.
[0050] In yet other preferred embodiments, sample is annealed at
progressively lower temperature and oxygen pressure. This may be
accomplished by continuously reducing temperature and/or oxygen
pressure or by stepwise reduction of temperature and/or oxygen
pressure.
[0051] Yet another aspect of the invention includes a
superconductor having the formula
Bi.sub.2-yPb.sub.ySr.sub.2Ca.sub.2Cu.sub.3O.sub.10+x, where
0.ltoreq.x.ltoreq.1.5 and where 0.ltoreq.y.ltoreq.0.6, the oxide
superconductor characterized by a critical transition temperature
of greater than 111.0 K as defined by zero resistance by a four
point linear probe method with zero resistance corresponding to a
resistivity of less than 10.sup.-8 .OMEGA.-cm. An article
containing the oxide superconductor may additionally include
silver.
[0052] Yet another aspect of the invention includes an oxide
superconductor article characterized by a critical transition
temperature of greater than 111.0 K as defined by zero resistance
by a four point linear probe method with zero resistance
corresponding to a resistivity of less than 10.sup.-8 .OMEGA.-cm
and an x-ray diffraction pattern having peaks at 17.4.degree.,
19.2.degree., 20.2.degree., 21.8.degree., 23.2.degree.,
23.9.degree., 26.2.degree., 27.8.degree., 29.degree., 29.7.degree.,
31.5.degree., 32.degree., 33.2.degree., 33.7.degree., 35.degree.,
35.6.degree., 38.degree., 38.8.degree., 41.6.degree., 43.8.degree.,
44.4.degree., 46.8.degree., 47.4.degree., 48.degree. and
49.degree..
[0053] Yet another aspect of the present invention provides for a
multifilamentary oxide superconductor composite containing a
plurality of oxide superconductor filaments contained within a
matrix material which has been subjected to an anneal according to
the present invention.
[0054] The oxide superconductor prepared according to the method of
this invention exhibit superior electric transport properties and
enhanced T.sub.c.
BRIEF DESCRIPTION OF THE DRAWING
[0055] In the Drawing;
[0056] FIG. 1 is a processing profile of the two-step heat
treatment of the invention; and
[0057] FIG. 2 is an optical photomicrograph of (a) a pressed oxide
superconductor article without the two step heat treatment of the
invention, and (b) a pressed oxide superconductor article subjected
to the heat treatment of the invention;
[0058] FIG. 3 is a processing profile used to obtain an textured
oxide superconductor according to the method including the two-step
heat treatment of the invention;
[0059] FIG. 4 is a prior art Van't Hoff diagram for BSCCO-2223;
[0060] FIG. 5 is a phase stability limit diagram for BSCCO-2223
indicating the low pressure, low temperature anneal conditions of
the present invention;
[0061] FIG. 6 is a phase stability limit diagram for BSCCO-2223
indicating the low pressure, low temperature anneal conditions of
the present invention;
[0062] FIG. 7 is a plot of the critical transition temperature and
critical current v. anneal temperature for an article subjected to
the two-step heat treatment and low pressure, low temperature
anneal of the invention; and
[0063] FIG. 8 is an x-ray diffractogram of an article subjected to
the two-step heat treatment and low pressure, low temperature
anneal of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
I. Two-Step Heat Treatment
[0064] The present invention is a method for improving the critical
current density of oxide superconductor articles by healing
defects, such as micro- and macrocracks, incurred upon deformation.
The present invention calls for a final two-step treatment after
deformation of the oxide superconductor article, in which (a) a
liquid phase is formed such that the liquid phase co-exists with
the desired oxide superconductor; and (b) the liquid phase is then
transformed into the desired oxide superconductor without any
intermediate deformation. The methods of the invention can be used
to heal defects in any oxide superconductor or superconducting
composite as long as a liquid phase can co-exist with the desired
oxide superconductor phase. It is recognized, that complete
transformation of the liquid to the oxide superconductor occurs
under ideal conditions and that, under some circumstances, not all
of the liquid may be transformed into the desired oxide
superconductor.
[0065] The liquid phase is formed upon partial melting of the oxide
superconductor article. During partial melting of the article,
non-superconducting materials and intermediate oxide phases may be
present with the desired oxide superconductor phase. During the
partial melting step of the invention the desired oxide
superconductor, the non-superconducting materials, oxide
superconducting precursors, the desired oxide superconductor or a
mixture of these components may melt to form the liquid phase.
[0066] The above process, which required that liquid co-exist with
the desired oxide superconductor phase, is distinguished from those
which involve the peritectic decomposition of the oxide
superconductor, such as described by Guo et al. and Kase et al., in
which the desired oxide superconductor decomposes during the
melting process.
[0067] FIG. 1 shows a processing profile of the final heat
treatment of the invention. A dashed line 10 indicates a processing
point at which a liquid phase is formed for a given set of
processing conditions, e.g., T, P.sub.O2, P.sub.total and/or oxide
composition.
[0068] In the oxide superconductors and superconducting composites
disclosed herein, processing conditions for obtaining the requisite
liquid and solid oxide phases are well established and the
relationship between temperature, oxygen partial pressure and total
pressure is reasonably well understood. For further information on
the phase diagrams for YBCO, BSCCO and the thallium-based systems,
the interested reader is directed to "Phase Diagrams for High
T.sub.c Superconductors", John D. Whitler and Robert S. Roth, Ed.;
American Ceramic Society, Westerville, Ohio.
[0069] Presence of a liquid phase can also be determined
experimentally by use of such conventional techniques as
differential thermal analysis (DTA). In DTA, exothermic and
endothermic reactions as a function of temperature can be
identified and attributed to various thermodynamic and chemical
processes. It is possible to identify endothermic processes
corresponding to partial melting; i.e. liquid phase formation.
[0070] It is desired that only a small amount of liquid be formed
during partial melting. The reason for this is that, at the time
that the final heat treatment is applied, the article substantially
is already textured. Complete or significant liquid formation at
this point would result in loss of texture. Volume percent of the
liquid phase is typically in the range of 0.1 to 30.
[0071] The oxide superconductor is deformed at a point 11 before
the final heat treatment, at which time defects such as microcracks
may be introduced into the article. Suitable deformation can
include swaging, extruding, drawing, pressing, hot and cold
isostatic pressing, rolling, and forging of wires, tapes and a
variety of shaped articles.
[0072] The method of the invention is particularly useful for oxide
superconductor articles in which deformation processing introduces
defects perpendicular to the direction of current flow. In rolling,
the deformation is largely plane strain, that is, extension in the
length of the article entirely compensates for the reduction in
thickness resulting in cracking in the transverse direction and
perpendicular to the current flow. FIG. 2a shows an optical
photomicrograph of a pressed oxide superconductor filament without
the final heat treatment of the invention. Many fine cracks are
visible and clearly disrupt the percolative pathway for current
flow. It is expected therefore, that rolled samples will benefit to
a greater extent from the final heat treatment of the
invention.
[0073] Other deformation processes, such as pressing, results in
reduction of the article thickness accommodated by lateral spread,
i.e., an increase in width. Cracks in this case form parallel to
the current flow. While defects of this type, can be healed by the
final heat treatment of the invention, the improvement to the
electrical properties may not be as marked.
[0074] Referring again to FIG. 1, the processing conditions are
adjusted to bring the article to point 12 where the article is
partially melted and a liquid phase co-exists with the desired
oxide superconductor phase. The article is held at point 12 for a
period of time during which the defect surfaces contained within
the oxide superconductor are wet by the newly-formed liquid. In the
case of BSCCO-2223, a temperature of 820-835.degree. C. at 0.075
atm O.sub.2 for 0.1-300 hours is sufficient. The processing
parameters are then adjusted to bring the oxide superconductor to
point 13 where the liquid phase is consumed and the desired oxide
superconductor phase is formed from the melt. In the case of
BSCCO-2223, a temperature of 820-790.degree. C. at 0.075 atm
O.sub.2 for 1 to 300 hours is sufficient.
[0075] The final heat treatment includes heating the article at a
temperature of substantially in the range of 815.degree. C. to
860.degree. C. for a period of time substantially in the range of
0.1 to 300 h at a P.sub.O2 substantially in the range of 0.001-1.0
atm. The processing temperature will vary dependent upon the oxygen
pressure. Additionally, variations in the chemical composition of
the article will also affect selection of temperature and pressure.
In particular, it has been noted that addition of silver to the
oxide composition lowers the temperature range for partial melting,
particularly at higher P.sub.O2 (0.1-1.0 atm).
[0076] Hence, the two-step heat treatment heals cracks and other
defects while effecting the final conversion of the oxide phases to
the desired oxide superconductor phase. The formation of BSCCO-2223
from BSCCO-2212 is kinetically enhanced by the presence of the
liquid phase, in part, due to the enhanced diffusivity of the oxide
superconductor constituents. The partial melting during the final
part of the process can perform two tasks. Firstly, the cracks
formed during the prior deformation steps are healed by rapid
growth of the oxide superconductor grains at the crack site.
Secondly, the conversion rate of 2212 to 2223 is greatly
accelerated, allowing the formation of a microscopically
crack-free, interconnected 2223 phase.
[0077] Various processing parameters can be controlled to obtain
the necessary partial melt and oxide reforming steps. For example,
P.sub.O2 can be held constant and temperature can be raised to
promote melting and formation of the liquid phase and lowered to
regenerate the desired oxide superconductor. Alternatively,
temperature can be held constant, and P.sub.O2 can be lowered to
promote the partial melting of the oxide superconductor article and
raised to reform the oxide superconductor.
[0078] The processing conditions can be changed rapidly from point
12 to point 13 of the process (fast ramp rate). Alternatively, the
oxide superconductor can be subjected to gradually changing
conditions (of temperature or pressure) between point 12 and point
13 of the process designated by the curve 14 in FIG. 1 (slow ramp
rate). In another alternative embodiment, there need be no "hold"
at 13. The processing conditions can be slowly ramped from the
processing conditions defined at point 12 to the processing
conditions defined for point 13. This process is illustrated by
curve 15 in FIG. 1.
[0079] The method of forming textured oxide superconducting
articles is described with reference to oxides of the BSCCO family;
however, this is in no way meant to limit the scope of the
invention. The present invention can be practiced with any oxide
superconductor system in which a liquid phase co-exists with an
oxide superconductor phase and which is amenable to
deformation-induced texture.
[0080] Texture may be induced by reaction conditions and/or
deformation. In reaction-induced texture, processing conditions are
chosen to kinetically favor the anisotropic growth of oxide
superconductor grains. Reaction-induced texture can occur in a
solid phase system or, preferably, in a solid-plus-liquid phase
system. The presence of a liquid phase enhances the kinetics of
anisotropic grain growth, probably through increased rates of
diffusion of the oxide components. In deformation-induced texture,
a strain is applied to the superconducting article to induce
alignment of the oxide grains in the plane or direction of
elongation. Deformation-induced texture requires aspected grains or
an anisotropic rheology in order to effect a preferential alignment
of the grains.
[0081] FIG. 3 shows a processing profile for a method of the
invention used to obtain highly textured oxide superconductor
articles. The general process for the formation of a textured oxide
superconductor may consist of three distinct steps.
[0082] In a first step, an oxide superconductor precursor is
subjected to one or more first heat treatment/deformation
iterations, denoted by step 20 and step 21, respectively, of FIG.
3. The oxide superconductor precursor can be any combination of
materials which will yield the desired oxide superconductor upon
reaction. In particular, it may be a metallic alloy containing the
metallic constituents of the desired oxide superconductor and
optionally containing silver. Alternatively, the constituent simple
metal oxides, mixed metal oxides, metal salts and even intermediate
oxide superconductors of the desired oxide superconductor may be
used as a precursor. The precursor may optionally be mixed with a
matrix metal, such as silver, and/or may be sheathed in a matrix
material in a powder-in-tube configuration.
[0083] The heat treatment 20 of the heat treatment/deformation
iteration serves two purposes in the process. Firstly, the anneal
is sufficient to form an oxide superconductor and results typically
in a mixture of superconducting and secondary phases. "Secondary
phases" include sub-oxide or non-superconducting oxide species
which require further processing to form an oxide superconductor
phase. BSCCO-2212 is often the intermediate oxide superconductor
because it is readily textured during mechanical deformation.
BSCCO-2223 is the typical desired oxide superconducting phase
because of its high critical temperature. Secondly, the heat
treatment promotes reaction-induced texture.
[0084] The deformation 21 of the article promotes
deformation-induced texture. One or more iterations can be
performed. FIG. 3 shows two first heat treatment/deformation
iterations, by way of example. If more than one iteration is
performed, both conversion to the superconducting phase and
development of texture can be done in incremental stages.
[0085] If the desired oxide superconductor is not formed in the
first heat treatment/deformation iterations, the second step of the
process may consist of one or more second heat
treatment/deformation iterations to form the desired oxide
superconductor and to further texture the oxide superconductor
phase. The article is heated in a step indicated by 22 whereby the
desired oxide superconductor is formed and reaction-induced texture
can occur. Secondary phases react with BSCCO-2212 to form the
desired oxide superconductor, BSCCO-2223. The article is deformed
in a subsequent step indicated by 23, whereby deformation-induced
texture can occur. If more than one iteration is used, only a
portion of the intermediate oxide superconductor, need be converted
into the desired oxide superconductor with each iteration.
Conditions known to form intermediate and desired oxide
superconductors are well known in the art. Suitable conditions are
described in Sandhage, et al. JOM, 21 (Mar. 1991), hereby
incorporated by reference.
[0086] Practically, the incremental improvement in alignment for
both anneal/deformation cycles will decrease markedly after several
iterations, however, there is no theoretical limit to the number of
iterations that can be used. The strain introduced in the
deformation step can range up to 99%. The strains applied in each
deformation/anneal iteration may be constant or they may be changed
for each subsequent iteration. It is particularly desirable in some
embodiments, to use decreasing strains with each subsequent
iteration.
[0087] It is also possible to adjust the processing conditions to
promote partial melting during the heating step 20 or 22 of the
heat treatment/deformation iterations, indicated by step 24, to
assist in grain growth and enhance reaction kinetics
(reaction-induced texture). Heating in the range of 820-835.degree.
C. in 0.075 atm O.sub.2 and 1 atm total pressure for 0.1 to 100
hours is typical for partial melting to occur.
[0088] The final part of the process consists of a two-step heat
treatment in which (a) the article is processed to form a liquid
phase which co-exists with the desired oxide superconductor
followed by (b) processing of the article under conditions that
favor the formation of the desired oxide superconductor phase.
Thus, step (b) of the two-step heat treatment is designed to
promote conversion of the non-"desired oxide superconductor into
the desired oxide superconductor. This process has been described
in greater detail, above.
[0089] The oxide superconductors which make up the oxide
superconductor articles of the present invention are brittle and
typically would not survive a mechanical deformation process, such
as rolling or pressing. For this reason, the oxide superconductors
of the present invention are typically processed as a composite
material including a malleable matrix material. In particular,
silver is preferred as the matrix material because of its cost,
nobility and malleability. The oxide superconductor composite may
be processed in any shape, however, the form of wires, tapes, rings
or coils are particularly preferred. The oxide superconductor may
be encased in a silver sheath, in a version of the powder-in-tube
technology. The oxide superconductor can take the form of multiple
filaments embedded within a silver matrix. For further information
on superconducting tapes and wires; see, Sandhage et al.
II. Low Pressure. Low Temperature Anneal
[0090] The present invention may also call for a low temperature,
low pressure annealing treatment after formation of a BSCCO-2223
oxide superconductor phase. The anneal is carried out in a low
temperature, low pressure region which is within the phase
stability region for BSCCO-2223. The phase stability region is that
range of processing conditions for which BSCCO-2223 is a
thermodynamically stable phase. Both oxygen pressure and
temperature are known to be important (but not exclusive)
processing parameters in determining the phase stability range of
BSCCO-2223. The applicants have surprisingly discovered that an
anneal of the BSCCO-2223 oxide superconductor according to the
invention provides significant improvements in electrical
properties, in particular, T.sub.c, I.sub.c, and J.sub.c.
Improvements of up to 10%, 20% 30% and even up to 50% in critical
current upon annealing according to the invention has been
observed. Bulk zero resistivity above 111.0 K after anneal also has
been observed.
[0091] The low temperature, low pressure anneal of the present
invention is particularly directed to
Bi.sub.2-yPb.sub.ySr.sub.2Ca.sub.2Cu.sub.3O.s- ub.10+x, where
0.ltoreq.x.ltoreq.1.5 and 0.ltoreq.y.ltoreq.0.6. As is well known
in the art, stated stoichiometries are all approximate. Intentional
and unintentional variations in composition are contemplated within
the scope of the invention. It is also well known in the art to
partially substitute one or more of the elements making up the
oxide superconductor. In a particular embodiment, lead is
substituted for between zero and thirty percent of the bismuth.
Such partial substitution of elements of the
Bi.sub.2Sr.sub.2Ca.sub.2Cu.sub.3O.sub.10+x oxide superconductor are
contemplated within the scope of the invention and are considered
represented within the notation "BSCCO-2223" used to describe the
composition.
[0092] The low pressure, low temperature anneal of the invention is
effective to improve T.sub.c and J.sub.c of any BSCCO-2223 article
regardless of the process used to form the oxide superconducting
phase. The BSCCO-2223 phase can be prepared from any conventional
method, including by way of example only, solid state reaction of
metal oxides, reaction of metal-organic precursors, oxidation of
metallic precursors and thin film deposition and reaction
processes. Additionally, the article can be textured by any
conventional method to induce orientation of the oxide
superconductor grains (texturing). By way of example only, the
article may be processed by zone-refined melt growth techniques and
mechanical deformation such as pressing and rolling, as described
hereinabove. The two-step heat treatment of the invention is
preferably used in conjunction with the low pressure, low
temperature anneal to obtain superior results.
[0093] Additional superconducting and non-superconducting phases
can be included in the article, so long as they do not interfere
with the superconducting properties of BSCCO-2223. By way of
example only, other superconducting phases may include BSCCO-2212.
By way of example only, non-superconducting phases may include
noble metals which add ductility and formability to the
superconducting article. Noble metals include silver and gold;
silver is a preferred noble metal. The article may be a
multifilamentary oxide superconductor in a silver matrix as
described hereinabove.
[0094] The anneal of the invention is most effective when carried
out as the final processing step in the preparation of the oxide
superconducting article. That is, the formation and texturing of
the BSCCO-2223 phase should be optimized before annealing the
article using the low pressure, low temperature anneal of the
invention. The invention recognizes that additional post-processing
thermal treatments may be required in the preparation of a usable
article (for example, insulation of the superconducting article).
Those steps are to be considered separate from the preparation of
the oxide superconductor, for which the annealing of the present
invention is preferably the final processing step.
[0095] FIG. 4 shows a prior art phase stability diagram for
BSCCO-2223 as a function of oxygen pressure and temperature, which
is taken from Rubin et al. (J. Appl. Phys. Let. 61(16), 1977
(1992)). The region 31 above a line 30 (indicated with closed and
open circles) defines the temperature and oxygen pressure
conditions at which the BSCCO-2223 phase is stable. The region 32
below line 30 defines the temperature and oxygen pressures at which
the BSCCO-2223 phase is unstable. The majority of the prior art
processing of BSCCO-2223 is carried out under conditions defined by
the smaller region 33 in the upper left hand corner of the diagram
bounded by the line 30 and lines 34.
[0096] The applicants have surprisingly discovered that the
critical current and critical transition temperature of the
BSCCO-2223 oxide superconductor can be improved by annealing the
BSCCO-2223 phase under conditions defined by a hatched region 40 in
FIG. 5. The annealing range lies above and roughly parallel to the
published stability boundary; however, there also exists an upper
boundary, above which superconducting properties will diminish.
This region is satisfied by the following equations (for T
(C.degree.) and P.sub.O2 (atm)):
500.degree. C..ltoreq.T.ltoreq.787.degree. C. (1);
P.sub.O2.gtoreq.3.5.times.10.sup.10exp(-32,000/(T+273)) is the
lower bound; and (2)
(3) P.sub.O2.ltoreq.1.1.times.10.sup.12exp(-32,000/(T+273)) is the
upper bound. (3)
[0097] The vertices of the parallelogram defining region 40 are
defined by the following coordinates [T(C.degree.), P.sub.O2(atm)]:
(787, 0.09); (787, 0.003); (500, 1.13.times.10.sup.-6); and (500,
3.7.times.10.sup.-8), which represent the corner bounds for
temperature and pressure conditions. The anneal is preferably
applied under conditions close to, but not exceeding, the upper
boundary of region 40. While approaching the upper bound, the
temperature is preferably as low as kinetically and practically
possible.
[0098] With reference to the phase diagram of FIG. 6, the
BSCCO-2223 phase is most beneficially annealed under conditions
defined by a hatched region 50. The annealing range lies parallel
and closely approaches the upper boundary of the annealing
conditions. This region is satisfied by the following equations
(for T (C.degree.) and P.sub.O2 (atm)):
500.degree. C..ltoreq.T.ltoreq.760.degree. C. (1);
P.sub.O2.gtoreq.8.5.times.10.sup.10exp(-32,000/(T+273)) is the
lower bound; and (2)
P.sub.O2.ltoreq.2.62.times.10.sup.11exp(-32,000/(T+273)) is the
upper bound. (3)
[0099] The vertices of the parallelogram defining region 50 are
defined by the following coordinates [T(C.degree.), P.sub.O2(atm)]:
(760, 0.0092); (760, 0.003); (500, 2.8.times.10.sup.-7); and (500,
9.0.times.10.sup.-8), which represent the corner bounds for
temperature and pressure conditions for region 50. The conditions
of regions 50 are particularly well-suited to improve critical
current and critical transition temperature of the oxide
superconductor.
[0100] The oxygen pressures of the invention are carefully
controlled by maintaining a total annealing atmosphere at
substantially one atmosphere. The annealing atmosphere includes an
inert gas such as argon or nitrogen. The inert gas is mixed with a
precisely controlled amount of oxygen to obtain the desired total
oxygen pressure. This gas is continuously introduced above the
oxide superconductor article during anneal. Hence, the annealing
atmosphere remains essentially constant, since any changes in
oxygen composition due to outgassing from the sample are mitigated
by the flow of annealing atmosphere over the sample.
[0101] The oxide superconductor is annealed for a time sufficient
to effect the improvements in electrical properties of the present
invention. The annealing time is a function of the anneal
temperature and the size of the superconducting article. Annealing
time increases as temperature decreases and hence poses a limit to
anneal temperatures which can be practically applied. At
660.degree. C., greater than 150 hours anneal time are required. If
the superconducting article is sufficiently thin, then the
annealing time decreases and the anneal may be practically applied
for temperatures as low as 500.degree. C. Anneal times typically
range from 5 to 300 hours.
[0102] In preferred embodiments, the temperature is in the range of
770-787.degree. C. and the oxygen pressure is in the range of
0.017-0.085 atm; or the temperature is in the range of
750-770.degree. C. and the oxygen pressure is in the range of
0.0009-0.052 atm; or the temperature is in the range of
730-750.degree. C. and the oxygen pressure is in the range of
0.005-0.029 atm; or the temperature is in the range of
690-730.degree. C. and the oxygen pressure is in the range of
0.0001-0.015 atm. In more preferred embodiments, the temperature is
in the range of 740-760.degree. C. and the oxygen pressure is in
the range of 0.0016-0.009 atm; or the temperature is in the range
of 710-740.degree. C. and the oxygen pressure is in the range of
0.0006-0.005 atm; or the temperature is in the range of
690-710.degree. C. and the oxygen pressure is in the range of
0.0003-0.002 atm.
[0103] The annealing step can take the form of a single anneal at
temperatures and oxygen pressures within the annealing regions
defined by region 40 and 50 in FIGS. 5 and 6, respectively.
Additional benefits to the electrical properties of the BSCCO-2223
article are observed when the annealing step includes application
of progressively lower temperatures and/or oxygen pressures. In one
preferred embodiment, the anneal may comprise two or more discrete
"bakes" at progressively lower temperatures and oxygen pressures.
In another preferred embodiment, the anneal may comprise continuous
slow temperature reduction of the superconducting article while
oxygen pressure is either continuously or stepwise decreased. In
yet another preferred embodiment, the anneal may also comprise
continuous decrease of oxygen pressure while the temperature is
stepwise decreased. A particularly preferred embodiment includes
treating the oxide superconductor by a stepwise or continuous
reduction in temperature and oxygen pressure from 787.degree. C. at
0.075 atm oxygen to temperatures in the range of 730.degree. C. to
690.degree. C. at 0.003 to 0.0003 atm oxygen. A prior anneal at
787.degree. C. and 0.075 atm oxygen is preferably carried out. At
all times, however, the anneal must remain within the proscribed
regions of either 40 or 50.
[0104] The method of approaching the low temperature, low oxygen
pressure conditions of the anneal of the present invention is also
a factor in the process. In preferred embodiments, the decrease in
temperature and oxygen pressure are synchronized and gradual. An
oxide superconductor article which has been formed at higher
temperatures and oxygen pressures (as is typically the case) will
retain an internal oxygen potential at the elevate prior processing
levels until diffusion permits the internal oxygen content to
accommodate the external change. If the article is rapidly
subjected to low temperature, low P.sub.O2 conditions, the
BSCCO-2223 oxide superconductor may undergo irreversible
decomposition because the internal high oxygen potential of the
article results in internal reaction conditions that are outside
the upper boundary for oxygen pressure as defined by the present
invention. By slowly changing the temperature and oxygen pressure
conditions, the internal conditions are allowed to adapt to the
changing external conditions and remain within the anneal region of
the present invention.
[0105] It is not certain what is the source of the improved
critical current performance in the low pressure, low temperature
anneal of the present invention. It has been suggested that oxygen
stoichiometry is optimized by the long anneal times at low oxygen
pressure and temperature. Alternatively, is may be that grain
growth is optimized and the number of grain boundaries is reduced,
thereby facilitating unimpeded current flow. An interesting
possibility is the formation of a new oxide superconductor phase by
atomic diffusion into the solid state structure of BSCCO-2223.
[0106] The annealing of the present invention has been found to be
particularly effective when used in conjunction with the two-step
final heat treatment described above. The two-step heat treatment
for BSCCO-2223 includes heating the article at a temperature
substantially in the range of 815.degree. C. to 860.degree. C. for
a period of time substantially in the range of 0.1 to 300 h at a
P.sub.O2 substantially in the range of 0.001-1.0 atm and followed
by reducing the article to a temperature substantially in the range
of 790.degree. C. to 845.degree. C. for a period of time
substantially in the range of 1 to 300 hours at a P.sub.O2
substantially in the range of 0.01 to 1.0 atm. The reader is
referred to "I. Two-Step Heat Treatment" of the Description of the
Preferred Embodiment, above, for further discussion of the two-step
heat treatment.
III. Examples of the Preferred Embodiment
[0107] The examples below describe the two-step heat treatment and
the anneal step of the present invention. The anneal is used in
conjunction with the two-step heat treatment; however, it can be
used on BSCCO-2223 oxide superconductor prepared by any
conventional process. The J.sub.c obtained from samples subjected
to the two-step heat treatment show significant improvements over
prior art materials. Even further improvements in J.sub.c are
observed by subjecting an oxide superconductor article obtained
from the two-step heat treatment process to a low pressure, low
temperature anneal.
EXAMPLE 1
[0108] The following example compares the transport critical
current characteristics of a samples treated with the two-step heat
treatment of the present invention to those of conventionally
processed samples.
[0109] Precursor powders were prepared from the solid state
reaction of freeze-dried precursor of the appropriate metal
nitrates having the nominal composition of 1.7:0.3:1.9:2.0:3.1
(Bi:Pb:Sr:Ca:Cu). Bi.sub.2O.sub.3, CaCO.sub.3, SrCO.sub.3,
Pb.sub.3O.sub.4 and CuO powders could be equally used. After
thoroughly mixing the powders in the appropriate ratio, a multistep
treatment (typically, 3-4 steps) of calcination (800.degree.
C..+-.10.degree. C., for a total of 15 h) and intermediate grinding
was performed in order to remove residual carbon, homogenize the
material and to generate the low T.sub.c BSCCO-2212 oxide
superconductor phase. The powders were packed into silver sheaths
having an inner diameter of 0.625" (1.5875 cm) and a length of 5.5"
(13.97 cm) and a wall thickness of 0.150" (0.38 cm) to form a
billet.
[0110] The billets were extruded to a diameter of 1/4" (0.63 cm).
The billet diameter was narrowed with multiple die passes, with a
final pass drawn through a 0.070" (0.178 cm) hexagonally shaped die
into silver/oxide superconductor hexagonal wires. Nineteen of the
wires were bundled together and drawn through a 0.070" (0.178 cm)
round die to form a multifilamentary round wire. The round wire was
rolled to form a 0.009".times.0.100" (0.023 cm.times.0.24 cm)
multifilamentary tape.
[0111] A length of the composite tape was then subjected to a heat
treatment according to the invention. The composite tape was heated
in a furnace in a first heat treatment at 820.degree. C. in 0.075
atm O.sub.2 for 48 h. The first heat treatment formed significant
amounts of the desired oxide superconductor phase, BSCCO-2223. The
composite tape was then rolled to reduce thickness by 11% (0.009"
to 0.008"). Lastly, the rolled composite tape was subjected to a
two-step heat treatment, namely, heating from room temperature at a
rate of 1.degree. C./min to 820.degree. C. in 0.075 atm O.sub.2 and
holding for 54 h, cooling to 810.degree. C. in 0.075 atm O.sub.2
and holding for 30 h. The sample was furnace cooled to room
temperature in 1 atm P.sub.O2.
[0112] A comparable length of composite tape was subjected to a
conventional heat treatment. The composite tape was heated in a
furnace in a first heat treatment at 820.degree. C. in 0.075 atm
O.sub.2 for 48 h. The first heat treatment caused significant
amounts of the desired oxide superconductor phase, BSCCO-2223 to
form. The composite tape was then rolled to reduce thickness by 11%
(0.009" to 0.008"). The control samples were then subjected to a
second heat treatment at 810.degree. C. in 0.075 atm O.sub.2 for 84
h. This was a single step heat treatment in which no melting of the
sample occurs. The microstructure of the samples were evaluated
under an optical microscope. The samples prepared according to the
method of the invention had a higher density and far less cracks
than the control samples.
[0113] The critical currents of the samples using a criterion of 1
.mu.V/cm, 77 K and zero applied field were determined. A single
critical current was determined end-to-end over a long length of
tape (7-10 m). Critical current for a number of 10 cm lengths of
composite tapes were determined and an average value was
determined. Critical current (I.sub.c in A) is related to the
critical current density (J.sub.c in A/cm.sup.2) by the
relationship, J.sub.c.apprxeq.1,250.multidot.I.sub.c. The results
are reported in Table 1 and show that samples processed according
to the method of the invention exhibited a factor of at least two
improvement in critical transport properties.
1TABLE 1 A comparative study of the method of the invention with a
conventional process. sample no. length (m) I.sub.c (A) % .sigma.
J.sub.c (A/cm.sup.2) Example 1-1 10 6.05 -- 7563 Example 1-2 0.1
9.52 13 11,900 Control 1-1 7 2.23 -- 2788 Control 1-2 0.1 4.08 16
5100
EXAMPLE 2
[0114] This example demonstrates that silver alloys can be used in
place of silver for the silver billet without detrimental effect on
the electrical properties of the composite. A composite tape is
prepared as described in Example 1; however, a silver alloy
containing low levels of Mg and Ni was used to sheathe the oxide
superconductor. The tape was processed as in Example 1 according to
the method of the invention. The average I.sub.c (77K, 10 cm) was
7.68 A as compared to ca, 4.08 A for a conventional process.
EXAMPLE 3
[0115] This example compares samples which have been pressed or
rolled as the intermediate deformation.
[0116] Composite tapes were prepared as described in Example 1. A
statistically designed experiment was performed using the following
process parameters for the first anneal and final heat
treatments.
2 two-step heat treatment first heat treatment high temperature low
temperature T(.degree.C.) t(h) T(.degree.C.) t(h) T(.degree.C.)
t(h) - 820 12 - 820 12 - 810 36 0 827 24 0 827 24 0 815 54 + 835 48
+ 835 48 + 815 72
[0117] Up to sixty four statistically selected combinations of
reaction conditions were run both with and without the two-step
heat treatment of the invention. The intermediate deformation step
between the first heat treatment and two-step heat treatment
consisted of a pressing with 12% strain reduction.
[0118] Comparable statistically designed experiments were carried
out for rolled samples with and without the two-step heat treatment
of the invention. The intermediate deformation step between the
first and two-step heat treatment consisted of rolling with 12%
strain reduction. Critical currents (77 K, 0T) were measured across
1 cm. The results are reported in Table 2.
3TABLE 2 Comparison of pressed and rolled composite tapes.
deformation second heat sample no. treatment treatment n I.sub.c
(.sigma.) (A) J.sub.c (A/cm.sup.2) 3-1 press two step 60 10.63
(2.43) 13,288 3-2 press one step 6 9.45 (0.20) 11,812 3-3 roll two
step 60 11.71 (1.09) 14,638 3-4 roll one step 64 3.41 (0.77)
4,263
[0119] The results found in Table 2 show that both pressed and
rolled samples benefitted from the two-step heat treatment of the
invention. The benefit is greater for rolled samples because
rolling results in microcracking perpendicular to the direction of
current flow, which is the most deleterious to transport critical
current and the most responsive to the healing effect of the
present invention.
EXAMPLE 4
[0120] This example shows the effect of the number of heat
treatment/deformation iterations on critical transport properties
of the samples.
[0121] Ten meter lengths of composite tape were prepared as
described for Example 1. The composite tapes were heated in a
furnace in a first heat treatment at 815.degree. C. in 0.075 atm
O.sub.2 for 48 h. The composite tapes were then rolled to reduce
thickness by 12%. The above heat treat and deform iteration was
carried out for x=2, x=3 and x=4 iterations on three samples,
respectively. Lastly, the deformed composite tapes were subjected
to a two-step heat treatment according to the invention, namely,
heating at 824.degree. C. in 0.075 atm O.sub.2 for 54 h, followed
by heating at 815.degree. C. in 0.075 atm O.sub.2 for 30 h. The
final thickness for each tape was 0.0080" (0.020 cm). The critical
transport measurements for the three samples are given in Table 3.
All samples exhibited critical currents higher than that of the
control sample (3 A). In this series of samples, the incremental
improvement to critical current was maximized at n=3; however,
dependent upon the particular experimental conditions, it may be
desirable to perform more or less iterations.
4TABLE 3 Effect of Iteration Number on Critical Transport sample
no. x I.sub.c (A) J.sub.c (A/cm.sup.2) 4-1 2 6.35 7938 4-2 3 7.83
9788 4-3 4 6.98 8725
EXAMPLE 5
[0122] The affect of precursor powder stoichiometry was
investigated. Composite tapes were prepared as described in Example
1, with the following exception. Powders of different stoichiometry
were used in the preparation the composite tapes.
5 Powder A: 1.8:0.4:2.0:2.2:3 Bi:Pb:Sr:Ca:Cu Powder B:
1.7:0.3:1.9:2.0:3.1 Bi:Pb:Sr:Ca:Cu
[0123] The composite tapes prepared from powders A and B were
subjected to the following heat treatment/deformation cycle:
6 (1) heat treatment: 815.degree. C., 0.075 atm O.sub.2, 48 h (2)
deformation: roll, 12% strain (3) heat treatment: 815.degree. C.,
0.075 atm O.sub.2, 48 h (4) deformation: roll, 12% strain (5) two
step heat treatment as described in Example 4
[0124] The final thickness of the tapes was 0.008" (0.020 cm).
Critical current measurements are reported in Table 4.
7TABLE 4 Effect of sample composition on critical current. 60 m
length 10 cm lengths sample no. powder I.sub.c (A) J.sub.c
(A/cm.sup.2) I.sub.c (.sigma.) (A) J.sub.c (A/cm.sup.2) 5-1 A 8.44
7,174 11.8 (1.3) 10,030 5-2 B 10.4 8,840 11.1 (1.2) 9,435
EXAMPLE 6
[0125] A composite tape was prepared according to the processed
described in Example 1. The composite tape having a length of 44 m
was subjected to an heat treatment/deformation iteration comprised
of heating at 815.degree. C. for 48 h under 0.075 atm O.sub.2 and
an 18% rolling deformation. The iteration was repeated three times.
The two-step heat treatment consisted of heating at 824.degree. C.
for 96 h (0.075 P.sub.O2), followed by heating at 815.degree. C.
for 30 h (0.075 P.sub.O2). The sample had a materials J.sub.C
measurement of greater than 17,000 A/cm.sup.2 (77 K, self
field).
EXAMPLE 7
[0126] This example illustrates the improved critical current for
articles prepared according to the low pressure, low temperature
anneal of the present invention.
[0127] A BSCCO-2223 oxide superconducting article is prepared using
the "metallic precursor" process (MP). Bi--Pb--Ca--Sr--Cu--Ag
alloys are made by mechanical alloying of the metallic elements in
stoichiometric proportions of 1.84:0.34:1.85:2.01:3.5
Bi:Pb:Sr:Ca:Cu with 64 wt % silver. The precursor metallic alloy is
fabricated into a multifilament metallic precursor-silver
composites tape as described in Example 1, above. The tape contains
361 precursor filaments in a silver matrix. The composite tape is
oxidized in oxygen at approx. 400.degree. C. for 150 to 400 hours
to convert the metallic precursor into simple and complex metal
oxides ("suboxides"). The suboxides are then reacted at 760 to
800.degree. C. (0.075 atm P.sub.O2, 1-20 hr) to form BSCCO-2212 and
remaining reactants necessary to form BSCCO-2223. Deformation by
rolling having a total strain in the range of 60 to 90% are used to
texture the BSCCO-2212 phase, as described in Example 1, above.
BSCCO-2212 is converted to BSCCO-2223 as described above and in
Example 1. Specifically, the composite tape is heated at
830.degree. C. for 40 hours in air, followed by roll deformation at
ambient to approx. 16% strain, followed by the two-step heat
treatment of heating at 830.degree. C. in 0.075 atm oxygen (balance
argon for a total pressure of one atmosphere) for 40 hours and
heating at 811.degree. C. for 120 hours.
[0128] Other samples may be prepared using the oxide powder in tube
method (OPIT), according to Example 1.
[0129] After formation of the BSCCO-2223 phase according to the
above methods (or other conventional methods), the temperature is
lowered at 4.degree. C./min to 787.degree. C. Oxygen pressure is
maintained at 0.075 atm and total pressure is one atmosphere
(balance of atmosphere is argon). The oxide superconductor
composite is heated at 787.degree. C. for 30 hours. The furnace
atmosphere is maintained at the desired composition by carefully
mixing argon with the proper amount of oxygen to attain an oxygen
concentration of 0.075 atm. The gas mixture flows through the
furnace and over the sample during the heat treatment.
[0130] Critical current is measure as described in Example 1 across
a 1 cm length. Critical current (I.sub.c) increased from a 4.5 A
pre-anneal value to a 5.5 A post-anneal value for a single sample.
(Critical current can be readily converted to critical current
density by division or the area, which is typically approximately a
factor of 0.00077 cm.
EXAMPLE 8
[0131] Example 8 shows the additional benefit to electrical
properties when an article is subjected to anneal at successively
lower temperatures and oxygen pressures.
[0132] An oxide superconductor composite is prepared according to
Example 7, including a low pressure, low temperature anneal at
787.degree. C. and 0.075 atm O.sub.2. The oxide superconductor
composite is then subjected to an additional low temperature, low
P.sub.O2 treatment. In a typical procedure, the sample is reduced
in temperature at 2.degree. C./min from 787.degree. C. to the
target lower temperature, which ranges as low as 724.degree. C. in
this example. The furnace atmosphere is maintained at the desired
composition by carefully mixing argon with the proper amount of
oxygen. The gas mixture is introduced into the furnace and passes
over the sample during the heat treatment. The gas composition is
adjusted with time such that oxygen pressure is approx. 0.01 atm at
the start of the period and 0.0035 atm as the target lower
temperature is attained. The conditions are then held for 45 hours,
followed by cool down at 10.degree. C./min to ambient
P.sub.O2=0.003).
[0133] FIG. 7(a) illustrates the improvement in critical current as
anneal temperature is decreased. The improvement is shown as the
percent increase of critical current density over the critical
current observed after the first anneal at 787.degree. C. (0.075
atm O.sub.2) shown in Example 7. It is understood that the sample
from Example 7 is already an improvement over a conventionally
processed material and even over materials processed according to
Examples 1-6. Increases of greater than 30% over the critical
current observed after the first anneal at 787.degree. C. (0.075
atm O.sub.2) shown in Example 7 were observed. The sample shows a
steady improvement in critical current density up to 738.degree. C.
(.DELTA.J.sub.c=32%), after which the improvement is somewhat
lessened. This may represent a kinetic effect of annealing at
reduced temperatures. It is expected that improved J.sub.c may be
obtained at even lower temperatures with longer anneal times. It is
further expected that further reduction in temperature and oxygen
pressure (under appropriate reaction times) will result in even
greater improvements in critical current density.
[0134] FIG. 7(b) illustrates the improvement in critical transition
temperature as anneal temperature is decreased. T.sub.c,zero as
high a 111.1 K were obtained for materials which were annealed at
745-752.degree. C. Critical transition temperature is measured by
the standard four probe method which involves placing two voltage
taps between two current taps, followed by passing a current
through the sample and recording the voltage with changing
temperature. The sample is considered to be superconducting with
effectively zero resistance when resistance is less than 10.sup.-8
.OMEGA.-cm. The data presented in FIG. 7(b) represents zero
resistance measurements.
[0135] X-ray diffraction analysis of a powdered sample obtained
according to the method of Example 8, exhibited a unique
diffraction pattern. In particular, the diffraction pattern (shown
in FIG. 8) contained peaks at 2.theta. values of 17.4.degree.,
19.2.degree., 20.2.degree., 21.8.degree., 23.2.degree.,
23.9.degree., 26.2.degree., 27.8.degree., 29.degree., 29.7.degree.,
31.5.degree., 32.degree., 33.2.degree., 33.7.degree., 35.degree.,
35.6.degree., 38.degree., 38.8.degree., 41.6.degree., 43.8.degree.,
44.4.degree., 46.8.degree., 47.4.degree., 48.degree. and
49.degree.. The peaks on the diffraction pattern of FIG. 8
represent the peaks typically associated with BSCCO-2223 and
additional unidentified peaks. The peaks most typically associated
with BSCCO-2223 are 19.2.degree., 20.2.degree., 21.8.degree.,
23.2.degree., 23.96.degree., 26.2.degree., 29.degree.,
31.5.degree., 32.degree., 33.2.degree., 33.7.degree., 35.degree.,
35.6.degree., 38.8.degree., 44.4.degree., 47.4.degree., 48.degree.
and 49.degree.. These new peaks suggest the formation of one or
more new oxide superconductor phases.
[0136] As can be seen by the above examples, the method of the
invention is highly versatile and can be successfully used with a
variety of deformation processes, oxide superconductor
compositions, silver alloy compositions and processing
conditions.
[0137] Other embodiments of the invention will be apparent to those
skilled in the art from a consideration of the specification or
practice of the invention disclosed herein. It is intended that the
specification and examples be considered as exemplary only, with
the true scope and spirit of the invention being indicated by the
following claims.
* * * * *