U.S. patent application number 09/781371 was filed with the patent office on 2002-02-14 for high performance (bi,pb)2sr2ca2cu2oy composites.
Invention is credited to Fleshler, Steven, Li, Qi, Michels, William J., Parrella, Ronald D., Riley, Gilbert N. JR., Teplitsky, Mark D..
Application Number | 20020019316 09/781371 |
Document ID | / |
Family ID | 24611837 |
Filed Date | 2002-02-14 |
United States Patent
Application |
20020019316 |
Kind Code |
A1 |
Li, Qi ; et al. |
February 14, 2002 |
High performance (BI,PB)2SR2CA2CU2Oy composites
Abstract
The present invention provides a (Bi,Pb)SCCO-2223 oxide
superconductor composite which exhibits improved critical current
density and critical current density retention in the presence of
magnetic fields. Retention of critical current density in 0.1 T
fields (77 K, .perp. ab plane) of greater than 35% is disclosed.
Significant improvements in oxide superconductor wire current
carrying capacity in a magnetic field are obtained by subjecting
the oxide superconductor composite to a post-processing heat
treatment which reduces the amount of lead in the (Bi,Pb)SCCO-2223
phase and forms a lead-rich non-superconducting phase. The heat
treatment is carried out under conditions which localize the
lead-rich phase at high energy sites in the composite.
Inventors: |
Li, Qi; (Waltham, MA)
; Michels, William J.; (Brookline, MA) ; Parrella,
Ronald D.; (Natick, MA) ; Riley, Gilbert N. JR.;
(Marlborough, MA) ; Teplitsky, Mark D.;
(Westborough, MA) ; Fleshler, Steven; (Brookline,
MA) |
Correspondence
Address: |
HALE AND DORR, LLP
60 STATE STREET
BOSTON
MA
02109
|
Family ID: |
24611837 |
Appl. No.: |
09/781371 |
Filed: |
February 12, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09781371 |
Feb 12, 2001 |
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09137733 |
Aug 21, 1998 |
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6188920 |
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09137733 |
Aug 21, 1998 |
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08651169 |
May 21, 1996 |
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5798318 |
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Current U.S.
Class: |
505/230 ;
505/231; 505/431; 505/501 |
Current CPC
Class: |
H01L 39/248 20130101;
Y10S 505/742 20130101; H01L 39/2419 20130101 |
Class at
Publication: |
505/230 ;
505/231; 505/431; 505/501 |
International
Class: |
H01B 012/00 |
Claims
What is claimed is:
1. A method of processing a (Bi,Pb)SCCO-2223 oxide superconductor
composite after oxide superconductor phase formation, comprising:
providing a (Bi,Pb)SCCO-2223 oxide superconductor composite, said
oxide superconductor composite comprising (Bi,Pb)SCCO-2223; heating
the oxide superconductor composite under conditions selected to
reduce the lead content of the (Bi,Pb)SCCO-2223 oxide
superconductor by about 5 percent to about 50 percent by weight and
to localize the exsolved lead in a secondary phase at high energy
sites of the composite.
2. A method of processing a (Bi,Pb)SCCO-2223 oxide superconductor
composite after oxide superconductor phase formation, comprising:
providing a (Bi,Pb)SCCO-2223 oxide superconductor composite, said
oxide superconductor composite comprising (Bi,Pb)SCCO-2223; heating
the oxide superconductor composite under oxidizing conditions, said
conditions sufficient to oxidize a portion of Pb.sup.2+present in
(Bi,Pb)SCCO-2223 into Pb.sup.4+ and to localize the Pb.sup.4+ in a
secondary phase at high energy sites of the composite.
3. A method for improving intergranular electrical properties of a
(Bi,M)SCCO-2223 oxide superconductor composite after oxide
superconductor phase formation, comprising: providing an oxide
precursor to (Bi,M)SCCO-2223 oxide superconductor composite, where
M is selected from the group consisting of Tl, Sb and Sn and is
present in an amount up to its solubility limit in the oxide
precursor; processing the composite so as to convert the oxide
precursor into (Bi,M)SCCO-2223; heating the oxide superconductor
composite under oxidizing conditions, said conditions sufficient to
oxidize a portion of M.sup.2+ present in (Bi,M)SCCO-2223 into
M.sup.4+ and to localize the M.sup.4+ in a secondary phase at high
energy sites of the composite
4. The method of claim 1, 2 or 3, wherein the high energy site
comprises one or more sites selected from the group consisting of
high angle c-axis tilt boundaries, pores, interfaces between the
superconducting and secondary phases and edge boundaries for the
superconducting phase.
5. The method of claim 1, 2 or 3, wherein the heating step is
carried out under oxidizing conditions.
6. The method of claim 1, 2 or 3, wherein the heat treatment is
effective to provide a composite which exhibits a critical current
retention at 0.1 T (77 K, .perp. ab plane) in the range of about
15% to about 50%.
7. The method of claim 1, 2 or 3, wherein the heat treatment is
effective to provide a composite which exhibits a critical current
retention at 0.1 T (77 K, .perp. ab plane) in the range of about
20% to about 40%.
8. The method of claim 1 or 2, wherein the (Bi,Pb)SCCO-2223 is
processed to impart intergranular connectivity of the oxide grains
before heat treatment of invention.
9. The method of claim 1, 2 or 3, wherein the heat treatment
comprises: heating the wire at a temperature in the range of about
500.degree. C. to about 800.degree. C. at an oxygen pressure of
about 0.03 atm to 100 atm O.sub.2 for a time sufficient to provide
a critical current retention at 0.1 T of at least 15% (77 K, .perp.
ab plane).
10. The method of claim 8, wherein the temperature is in the range
of 630.degree. C. to 790.degree. C. at an oxygen pressure of about
0.03 atm to 100 atm O.sub.2.
11. The method of claim 8, wherein the temperature is in the range
of 650.degree. C. to 750.degree. C. at an oxygen pressure of about
0.08 atm to 1.0 atm O.sub.2.
12. The method of claim 1 or 2, wherein the lead-rich secondary
phase comprises a hexagonal crystal structure characterized by an
X-ray diffraction pattern comprising the following peaks
(20(relative intensity)): 17.9(45), 32.3(100), 31.5(62), 44.8(42),
and 55.5(45).
13. The method of claim 1 or 2, wherein the (Bi,Pb)SCCO-2223
comprises lead in an amount in the range 3 wt % to about 8 wt %
before heat treatment.
14. The method of claim 1 or 2, wherein the (Bi,Pb)SCCO-2223
comprises lead in an amount in the range 4 wt % to about 6 wt %
before-heat treatment.
15. The method of claim 1 or 2, wherein the (Bi,Pb)SCCO-2223
comprises about 6.5 wt % lead.
16. The method of claim 3, wherein the (Bi,M)SCCO-2223 comprises M
in an amount of less than 10 wt %.
17. The method of claim 1 or 2, wherein the heat treatment is
carried out under conditions to reduce the lead content of
(Bi,Pb)SCCO-2223 in an amount in the range of about 15 wt % to
about 25 wt %.
18. The method of claim 1 or 2, wherein the heat treatment
comprises: heating the oxide superconductor under conditions which
are oxidizing to Pb.sup.+2 relative to a lead-rich phase stability
curve.
19. The method of claim 1 or 2, wherein a (Bi,Pb)SCCO-2223 is
obtained by heating in the range of 800.degree. C. to 850.degree.
C. for a first dwell time and heating in the range of 780.degree.
C. to 815.degree. C. for a second dwell time under an oxygen
partial pressure in the range of 0.01 to 1.0 atm.
20. The method of claim 1 or 2, wherein a (Bi,Pb)SCCO-2223 is
obtained by heating in the range of 825.degree. C. to 830.degree.
C. for a first dwell time and heating in the range of 805.degree.
C. to 813.degree. C. for a second dwell time under an oxygen
partial pressure in the range of 0.01 to 1.0 atm.
21. The method of claim 18, further comprising heating in the range
of 780.degree. C. to 790.degree. C. for a third dwell time under an
oxygen partial pressure in the range of 0.01 to 1.0 atm.
22. The method of claim 1, 2 or 3, wherein the composite is in the
form of a silver sheathed wire.
23. The method of claim 21, wherein the composite is a
multifilamentary silver sheathed wire.
24. The method of claim 1 or 2, wherein the lead-rich secondary
phase is formed in a relative fraction in the range of about 0.002
to 0.5.
25. The method of claim 1 or 2, further comprising: providing
a(Bi,Pb)SCCO-2223 oxide superconductor composite comprising a noble
metal.
26. A method of preparing a (Bi,Pb)SCCO-2223 oxide superconductor
composite, comprising: modifying the lead content of a
(Bi,Pb)SCCO-2223 superconducting phase during processing of a
(Bi,Pb)SCCO-2223 oxide superconductor composite, such that the lead
content of the (Bi,Pb)SCCO-2223 superconducting phase is in the
range of 3% to 8% during formation of the (Bi,Pb)SCCO-2223 phase
and such that the lead content of the (Bi,Pb)SCCO-2223
superconducting phase is reduced up to 25% during post formation
processing of the oxide superconductor phase.
27. A (Bi,Pb)SCCO-2223 oxide superconductor composite wire,
comprising: a (Bi,Pb)SCCO-2223 oxide superconductor filament
substantially supported in a noble metal phase, wherein the
filament comprises a lead-rich secondary phase, the wire
characterized in that when tested over a current carrying distance
of 10 cm, the wire possess a J.sub.ret at 0.1 T in the range of
greater than 35% (77 K, .perp. ab plane).
28. A (Bi,Pb)SCCO-2223 oxide superconductor composite, comprising:
a (Bi,Pb)SCCO-2223 oxide superconductor phase supported in a noble
metal phase, the (Bi,Pb)SCCO-2223 oxide superconductor phase
comprising a lead-rich secondary phase localized at high energy
sites and a (Bi,Pb)SCCO-2223 phase.
29. A (Bi,Pb)SCCO-2223 oxide superconductor composite, comprising:
a (Bi,Pb)SCCO-2223 oxide superconductor phase supported in a noble
metal phase, the (Bi,Pb)SCCO-2223 oxide superconductor phase
comprising Bi:Pb:Sr:Ca:Cu in the nominal stoichiometry of
2.5(.+-.0.05):0.4(.+-.0.04- ):2.3(.+-.0.06):
2.3(.+-.0.04):3.0(.+-.0.15).
30. The composite of claim 28, further comprising: a lead-rich
secondary phase comprising Bi:Pb:Sr:Ca:Cu in the nominal
stoichiometrv of 0.9(.+-.0.09):1.1(.+-.0.21):1.6(.+-.0.06):
1.7(.+-.0.08):1.0(.+-.0.23).
31. The composite of claim 26 or 27, wherein the (Bi,Pb)SCCO-2223
is lead deficient.
32. The composite of claim 26 or 27, wherein (Bi,Pb)SCCO-2223
comprises lead from about 2 wt % to about 6.8 wt % lead.
33. The oxide superconductor of claim 30, wherein the
lead-deficient (Bi,Pb)SCCO-2223 phase, comprises lead about 4.75 to
about 5.5 percent by weight.
34. The oxide superconductor of claim 30, wherein the
lead-deficient (Bi,Pb)SCCO-2223 phase, comprises lead about 3.4 to
about 4.2 percent by weight.
35. The composite of claim 27, wherein the composite is in the form
of a wire and the oxide superconductor phase is in the form of a
filament.
36. A (Bi,M)SCCO-2223 oxide superconductor wire, comprising: a
(Bi,M)SCCO-2223 oxide superconductor filament supported in a noble
metal phase, wherein M is selected from the group consisting of Pb,
Tl, Sb, Sn, Te, Hg, Se, As and mixtures thereof, the wire
characterized in that when tested over a current carrying distance
of 10 cm, the wire possess a J.sub.ret at 0.1 T of greater than 35%
(77 K, .perp. ab plane).
37. The oxide superconductor composite of claim 27, wherein the
wire is characterized in that when tested over a current carrying
distance of 10 cm, the wire possess a J.sub.ret at 0.1 T in the
range of greater than 25% (77 K, .perp. ab plane).
38. The oxide superconductor wire of claim 27, the wire
characterized in that when tested over a current carrying distance
of 10 cm, the wire possess a J.sub.ret at 0.1 T of greater than
35%.
39. The oxide superconductor composite of claim 27, wherein the
wire is characterized in that when tested over a current carrying
distance of 10 cm, the wire possess a J.sub.ret at 0.1 T in the
range of about 35% to about 50% (77 K, .perp. ab plane).
40. The composite of claim 26, 27 or 33, further characterized in
that an increase in J.sub.ret of the composite does not produce a
proportional increase in J.sub.c (self field or zero field) of the
composite.
41. The oxide superconductor composite of claim 26 or 27, wherein
the lead rich secondary phase is present in a relative fraction in
the range of about 0.01 to about 0.5.
42. The oxide superconductor of claim 27, wherein the high energy
site comprises one or more sites selected from the group consisting
of high angle c-axis tilt boundaries, pores, interfaces between the
superconducting and secondary phases and surface boundaries for the
superconducting phase.
43. The oxide superconductor composite of claim 26 or 31, wherein
the wire comprises multiple filaments of (Bi.Pb)SCCO-2223 supported
by the noble metal phase.
44. The oxide superconductor composite of claim 26 or 27, wherein
the lead-rich secondary phase comprises a hexagonal crystal
structure characterized by an X-ray diffraction pattern comprising
the following peaks (20(relative intensity)): 17.9(45), 32.3(100),
31.5(62), 44.8(42), and 55.5(45).
45. The oxide superconductor composite of claim 26 or 27, wherein
the lead-rich secondary phase has a diffraction pattern
substantially that described in JCPDS card No. 44-0053.
Description
FIELD OF THE INVENTION
[0001] The invention relates to high performance oxide
superconductor composites exhibiting improved J.sub.c retention in
the presence of a magnetic field. The invention further relates to
a method for post-formation processing of an oxide superconductor
composite to improve electrical performance.
BACKGROUND OF THE INVENTION
[0002] In order to obtain high electrical performance in
(Bi,Pb).sub.2Sr.sub.2Ca.sub.2Cu.sub.3O.sub.y ((Bi,Pb)SCCO-2223)
high temperature superconducting composites, highlv phase pure
(Bi,Pb)SCCO-2223 with perfect texture and superior grain
connectivity is desired. "Texture" refers to the degree of
alignment of the oxide superconductor grains along the direction of
current flow. "Connectivity" refers to the positional relationship
of the oriented oxide superconductor grains, the nature of the
grains boundaries and the presence of phase impurities disrupting
intergrain connection.
[0003] Many parameters must be controlled and optimized during the
fabrication and thermomechanical processing of (Bi,Pb)SCCO-2223
tapes in order to obtain satisfactory electrical properties.
Electrical properties may be grouped into two categories:
intragranular electrical properties and intergranular electrical
properties. Intragranular electrical properties are those that are
effected by changes within individual oxide superconductor grains.
Critical transition temperatures (T.sub.c) is one electrical
property which is predominantly intragrain. Critical current
density (J.sub.c) and critical current retention (J.sub.c) also has
an intragrain component. Intergranular electrical properties are
those which relate to the transport of a supercurrent across oxide
superconducting grain boundaries and depend upon good intergrain
connectivity. Critical current density (J.sub.c) and critical
current retention in a magnetic field (J.sub.ret) have significant
intergrain character.
[0004] In references too numerous to identify individually. the
effects of powder composition, mechanical deformation, and heat
treatment time, temperature and atmosphere on oxide superconductor
formation have been studied. Not surprisingly, these studies have
shown that heat treatment affects the rate of formation of the
superconductor phase, the quality of the superconductor phase and
the presence of secondary, non-superconducting phases. Thus, the
heat treatment used in the formation of the oxide superconductor
phase is important to the overall performance of the oxide
superconductor composite.
[0005] Post-formation heat treatments have been investigated as a
means for modifying the intragranular structure to boost
performance properties of the oxide superconductor phase.
Intragrain factors which affect electrical properties include the
presence or absence of defects in the superconductor phase, and the
phase purity of the superconductor phase and stoichiometric
modifications thereof which may improve or degrade superconducting
behavior. "Post-formation", as that term is used herein, means
processing of the oxide superconductor after formation of the
desired oxide superconductor phase from precursor oxide phases is
substantially complete.
[0006] Typical post-processing heat treatments include annealing to
alter the oxygen stoichiometry of the oxide superconductor phase,
such as described by E. Ozdas and T. Firat in "Oxygenation
Intercalation and Intergranular Coupling in the 110-K
Bi.sub.1.7Pb.sub.0.3Sr.sub.1.8Ca.sub.-
2Cu.sub.2.8O.sub.9.45+.delta. Superconductor" (Phys. Rev. B
48(13):9754-9762 (October, 1993)). Idemoto et al. in "Oxygen
Nonstoichiometry of 2223 Phase Bi--Pb--Sr--Ca--Cu--O System
Superconducting Oxide" (Physica C 181:171-178 (1991)). They
reported on the effect of heating (Bi,Pb)SCCO-2223 powders at
temperatures from 500.degree. C. to 800.degree. C. and oxygen
pressures of 0.2 to 10.sup.-3 atm. Idemoto et al. observed the
formation of secondary phase Ca.sub.2PbO.sub.4 and evaporation of
PbO, while Ozdas and Firat reported inhomogeneities forming at
oxide superconductor grain boundaries.
[0007] Um et al. (Jpn. J. Appl. Phys. 32: 3799-3803 (1993))
investigated the effect of a post-sintering anneal on
(Bi,Pb)SCCO-2223 powders. They observe that T.sub.c is affected by
the anneal temperature and oxygen pressure and found annealing at
temperatures below 700.degree. C. and at oxygen partial pressure of
0.01 atm to provide optimized T.sub.c. Um et al. noted that the
superconducting phase decomposes at temperatures higher than
700.degree. C. Wang et al. (Advances in Supercond. V (1992)) also
found that post-annealing under vacuum at 790.degree. C. improved
T.sub.c of (Bi,Pb)SCCO-2223 oxide superconductor powders.
[0008] These prior art references investigate the intragranular
electrical properties of oxide superconductor powders and the
authors are primarily interested in T.sub.c optimization. Oxide
powders have no intergranular boundaries because of the random
loose-packed nature of powder, and they provide no insight into the
optimization of electrical transport properties (J.sub.c,
J.sub.ret) the Bi,Pb)SCCO-2223 superconductor current carriers,
such as wires, tapes and the like.
[0009] Interestingly, the above-mentioned prior art noted the
decomposition of the superconducting oxide phase and formation of
secondary phases while optimizing intragranular electrical
properties. Conventional wisdom would suggest that microstructures
containing a non-superconducting secondary phase is undesirable
because these particles disrupt local alignment of the BSCCO-2223
grains and decrease superconducting volume fraction in the
composite. Thus, prior investigations have suggested that it is
highly desirable to reduce the amount of secondary phases to as low
a level as possible.
[0010] There has been little or no investigation of conditions
which optimize the interconnectivity of (Bi,Pb)SCCO-2223
superconductor grains in a silver sheathed wire or which
investigate its retention of critical current in the presence of a
magnetic field. In the case of silver sheathed high temperature
superconducting wires, good intergranular connectivity is critical
to performance, yet processing is complicated by the need to move
oxygen through the silver to the oxide superconductor. Observations
made for oxide superconductor powders, which are an open system
exposed directly to the furnace atmosphere and which systems do not
include silver/oxygen interfaces, may not apply to silver sheathed
tapes and the like.
[0011] The effects of cooling on the electrical properties of the
oxide superconductor composite has been investigated by Lay et al.
in "Post-Sintering Oxygen Pressure Effects on the Jc of
BPSCC--Silver Clad Tapes" (Mat. Res. Symp. Proc. 275:651-661
(October, 1992). Lay et al. reported cooling in air at 1 C/min
resulted in a J.sub.c (77K, 0 T) increase over tapes cooled at
3.degree. C./min. Lay et al. also noted that holding the
(Bi,Pb)SCCO-2223 samples at temperatures of 810.degree. C. or
780.degree. C. in reducing atmospheres improved J.sub.c.
[0012] While critical current (I.sub.c) and critical current
density (J.sub.c) in self-fields may be useful indications of the
quality of an oxide superconductor composite, an important
performance parameter for in-field operations of oxide
superconducting devices is their ability to retain their
superconducting transport properties in the presence of a magnetic
field. Many applications using oxide superconducting wires must be
accomplished in the presence of its own induced magnetic field or
in-applied field ranging form 0.01 T to 100 T. Superconducting
properties degrade dramatically in even relatively weak fields.
Oxide superconductors show their most dramatic loss in critical
current capacity perpendicular to the ab plane. Parallel to the ab
plane, capacity loss is only a few percent. For example, weaklv
linked yttrium-barium-copper oxide superconductor (YBCO) exhibits a
ten-fold drop in J.sub.c in magnetic field strengths of 0.01 T (B
.perp. oxide superconductor tape plane). Conventionally processed
BSCCO-2223 loses the majority of its critical current capacity in a
0.1 T field (77 K, .perp. tape plane). Even a few percent increase
in critical current retention would have a dramatic effect on wire
performance.
[0013] Thus, there remains a need to optimize the intergrain
connectivity of high temperature superconducting wires and tapes so
as to improve current carrying performance. Preferably, processing
of an oxide superconductor wire or tape would enhance the
intragranular properties of the conductor without detriment to the
intergranular transport properties of the conductor. Due to
secondary phase formation under conditions which optimize
intragranular electrical properties, it is desirable to process the
superconductor in a manner which minimizes the formation and/or
detrimental effect of secondary phases on intergrain
connectivity.
[0014] It is the object of the present invention to provide an
oxide superconductor article with improved critical current
retention and/or improved critical current density in the presence
of a magnetic field.
[0015] It is a further object of the present invention to provide a
method of treating the oxide superconductor composition to improve
critical current retention and/or critical current density.
[0016] It is yet a further object of the present invention to
increase flux pinning sites and/or intragranular coupling in a
(Bi,Pb)SCCO-2223 oxide superconductor composite.
[0017] It is yet a further object of the invention to improve grain
interconnectivity by reducing formation and/or the detrimental
effect of secondary phase formation.
[0018] It is yet a further object of the present invention to
provide a method form obtaining optimal intragranular properties of
an oxide superconductor wire or tape, while minimizing the
detrimental effects to intergranular transport properties.
[0019] These and other objection of the invention are accomplished
by the invention as set forth hereinbelow.
SUMMARY OF THE INVENTION
[0020] The present invention provides a (Bi,Pb)SCCO-2223 oxide
superconductor composite which exhibits improved critical current
density (J.sub.e or J.sub.c) and improved critical current
retention (J.sub.ret) the presence of magnetic fields. Retention of
critical current density in 0.1 T fields (77 K, .perp. to the tape
plane) of up to about 40% have been observed; and critical current
retention of greater than about 30% is typical. The improved
critical current retention is accompanied by the localization of a
lead-rich secondary phase at high energy sites within the
composite. The present invention recognizes that, contrary to the
commonly held belief that secondary non-superconducting phases are
detrimental to superconducting electrical properties, enhanced
critical current and critical current retention are obtained from a
composite containing a lead-rich non-superconducting secondary
phase.
[0021] In one aspect of the invention, a (Bi,Pb)SCCO-2223 oxide
superconductor composite wire is provided having a (Bi,Pb)SCCO-2223
oxide superconductor filament substantially supported in a noble
metal phase. The filament comprises a lead-rich secondary phase and
the wire possess a J.sub.ret at 0.1 T in the range of greater than
35% (77 K, .perp. ab plane) when tested over a current carrying
distance of 10 cm. The lead-rich secondary phase may be localized
at high energy sites. The (Bi,Pb)SCCO-2223 may be lead deficient.
In preferred embodiments, the (Bi,Pb)SCCO-2223 oxide superconductor
phase comprises Bi:Pb:Sr:Ca:Cu in the nominal stoichiometry of
2.5(.+-.0.05):0.4(.+-.0.04):2.3(.+-.0.06):
2.3(.+-.0.04):3.0(.+-.0.15). Unless otherwise noted, all references
are to atomic percent. The composite may additionally include a
lead-rich secondary phase comprising Bi:Pb:Sr:Ca:Cu in the nominal
stoichiometry of 0.9(.+-.0.09):1.1(.+-.0.21):1.6(.+-.0.06):
1.7(.+-.0.08):1.0(.+-.0.23).
[0022] The present invention further contemplates a (Bi,M)SCCO-2223
oxide superconductor wire including a (Bi,M)SCCO-2223 oxide
superconductor filament supported in a noble metal phase. M is may
include Pb, Ti, Sb, Sn, Te, Hg, Se, As and mixtures thereof. The
wire characterized in that when tested over a current carrying
distance of 10 cm, the wire possess a J.sub.ret at 0.1 T of greater
than 35% (77 K, .perp. ab plane).
[0023] Significant improvements in oxide superconductor wire
current carrying capacity in a magnetic field are obtained by
subjecting the oxide superconductor wire containing
(Bi,Pb)SCCO-2223 to a post-processing heat treatment which reduces
the lead content in the (Bi,Pb)SCCO-2223 phase by an amount in the
range of about 5 wt % to about 50 wt %, and typically to about 40
wt %, and to localize the exsolved lead in a lead-rich secondary
phase outside the superconducting grain colonies and/or at other
high energy sites in the composite. Reduction of lead in the
(Bi,Pb)SCCO-2223 phase improves intragranular electrical
properties. When the heat treatment is conducted under conditions
which localize secondary phases formed thereby at high energy
sites, the secondary phases do not significantly degrade the
intergranular transport properties of the composite.
[0024] The invention calls for the modification of the lead content
of the a (Bi,Pb)SCCO-2223 superconducting phase during processing
of a (Bi,Pb)SCCO-2223 oxide superconductor composite. The lead
content varies such that the lead content of the (Bi,Pb)SCCO-2223
superconducting phase is in the range of 3% to 8% during formation
of the (Bi,Pb)SCCO-2223 phase and such that the lead content of the
(Bi,Pb)SCCO-2223 superconducting phase is reduced up to 50% during
post formation processing of the oxide superconductor phase. This
results in the optimization of the product superconductor
electrical properties.
[0025] The method of the invention also contemplates heating the
oxide superconductor composite under oxidizing conditions, said
conditions sufficient to oxidize a portion of Pb.sup.2+ present in
(Bi,Pb)SCCO-2223 into Pb.sup.4+ and to localize the Pb.sup.4+ in a
secondary phase at high energy sites of the composite.
[0026] The term "wire" is used herein to mean any of a variety of
geometries having an elongated dimension suitable for carrying
current, such as but not limited to wires, tapes, strips and
rods.
[0027] By "fully formed (Bi,Pb)SCCO-2223", "desired
(Bi,Pb)SCCO-2223", and "final (Bi,Pb)SCCO-2223" as those terms are
used herein, it is meant an oxide superconductor phase in which
substantially all of the precursor oxide has been converted into
the desired (Bi,Pb)SCCO-2223 phase. There is no further processing
into a different oxide superconductor phase. The (Bi,Pb)SCCO-2223
may be obtained according to the methods described herein or
according to other prior art methods demonstrated to complete
conversion of the precursor oxides to (Bi,Pb)SCCO-2223.
[0028] "Critical current density retention", J.sub.ret as that term
is used herein means the ratio of the critical current density of
the composite in an applied field over the critical current density
of the composite in the absence of an applied field (self field or
zero field). The sample will generate its own self field, but that
field is expected to be at least an order of magnitude less than
the applied field.
[0029] High energy sites include high angle c-axis tilt boundaries,
pores, interfaces between the superconducting and secondary phases
and edge boundaries for the superconducting phase. Oxide grains
having a misorientation of greater than 10.degree. angular
deviation from perfect alignment of adjacent grains have a large
relative proportion of high energy sites.
[0030] The present invention is readily scalable and can be used to
process long lengths of oxide superconductor wire, in contrast to
techniques such as irradiation, which are expected to introduce
flux pinning and other site defects into the material.
BRIEF DESCRIPTION OF THE DRAWING
[0031] The invention is described with reference to the Drawing,
which is presented for the purpose of illustration only and is no
way intended to be limiting of the invention, and in which:
[0032] FIG. 1 are X-diffraction patterns of (a) a hexagonal
lead-rich secondary phase and (b) a (Bi,Pb)SCCO-2223 composite of
the invention including the heaxagonal lead-rich secondary
phase;
[0033] FIG. 2 is a plot of log P.sub.O2 (atm) vs. 1000/T (K)
showing a lead-rich phase reaction curve;
[0034] FIG. 3 is a temperature profile of the heat treatment of the
invention carried out (a) in a single step; (b) as a series of
steps; and (c) as a slow cooling step;
[0035] FIG. 4 is a heat treatment profile useful in preparing a
(Bi,Pb)SCCO-2223 oxide superconductor;
[0036] FIG. 5 is a plot of critical current density as a function
of temperature (500.degree. C. to 800.degree. C.) in the
post-formation heat treatment;
[0037] FIG. 6 heat treatment dwell time for oxide superconductor
wires heat treated at 724.degree. C. in 7.5% (0.075 atm)
O.sub.2;
[0038] FIG. 7 is a plot of critical current density as a function
of oxygen partial pressure (0.003-1.0 atm);
[0039] FIG. 8 is a bar graph illustrating critical current
retention at 0.1 T (77 K, .perp. tape plane) for a variety of
post-formation heat treatments;
[0040] FIG. 9 is a plot of critical current retention vs. field
strength for (Bi,Pb)SCCO-2223 wires post-formation heated under
various oxygen partial pressures;
[0041] FIG. 10 is a plot of critical current retention vs. field
strength for (Bi,Pb)SCCO-2223 wires post-formation heated under
various temperatures;
[0042] FIG. 11 is a plot of relative fraction lead-rich secondary
phase as a function of oxygen partial pressure;
[0043] FIG. 12 is a plot of relative fraction lead-rich secondary
phase as a function of temperature; and
[0044] FIG. 13 is a plot of J.sub.ret v. B demonstrating the effect
of heat treatment on critical current density retention above and
below the lead-rich phase stability line.
DETAILED DESCRIPTION OF THE INVENTION
[0045] The critical current retention of BSCCO-2223 in a magnetic
field is relatively poor at high temperatures, e.g., 77 K. For
example, conventionally processed BSCCO-2223 loses the majority of
its critical current capacity in a 0.1 T field (77 K, .perp. tape
plane). Critical current retention may be improved in two ways. In
one method, defects can be introduced into the superconductor phase
that directly interact with magnetic flux vortexes and impede their
motion, so-called flux pinning. Defects may be particles giving
rise to point, line, plane or volume defects (zero, one, two or
three dimensional defects) or particles which create coherency
strain fields or differential coefficients of thermal expansion. In
another method, the crystal lattice of the oxide superconductor
itself is modified to improve the coupling of the carriers
responsible for superconducting behavior, so-called intrinsic
coupling. For example, carrier density may be modified by changing
the oxide superconductor stoichiometry. Note that both of these
mechanisms are intragranular.
[0046] In the case of high temperature superconducting wires, in
which multifilaments of oxide superconductor are sheathed in a
silver sheath, good intergranular connectivity is important to
maintain effective current carrying capacity along the wire length.
Good intergranular connectivity must be maintained, even as the
oxide superconductor wire is subjected to processes which enhance
intragranular properties.
[0047] The prior art discussed hereinabove enhances intragranular
properties (such as T.sub.c) with a resultant decomposition of the
oxide superconductor phase and formation of a secondary phase.
Thus, previous efforts to enhance intragranular superconducting
properties have indicated that intergranular connectivity is
substantially degraded as intragranular superconducting properties
are enhanced.
[0048] Thus, some prior art investigations into post-sintering
conditions have lead investigators to recommend regimes where
formation of secondary phases is minimized and intragranular
properties such as T.sub.c are optimized. This may, however, be a
processing regime which is not well suited to optimization of other
electrical properties, in particular, critical current retention.
For example, Um recommends post-sintering at temperatures of
500-700.degree. C. at partial oxygen pressures of 0.01 atm. As is
described herein, this processing regime may enhance T.sub.c and
avoid secondary phase formation, but it does not enhance critical
current retention.
[0049] The present invention has recognized for the first time that
processing conditions which optimize intragranular superconducting
properties such as T.sub.c or critical current retention in a
magnetic field (J.sub.ret) are different than those processing
conditions which optimize J.sub.c at self field or zero field, a
property having a dominant intergranular connectivity
characteristic. The method of the present invention recognizes the
need to balance these competing processes and provides a heat
treatment which maximizes the desired intragranular electrical
property, while minimizing the degradation of intergranular
connectivity. The method includes heating a (Bi,Pb)SCCO-2223 oxide
phase under conditions which modify the (Bi,Pb)SCCO-2223 phase to
optimize intragranular electrical properties and to localize any
secondary phase formed in post-processing heat treatments to
regions of the oxide superconductor composite where it is benign to
supercurrent flow, thereby optimizing intergranular
connectivity.
[0050] In the case of sheathed BSCCO-2223 wires, the following
issues need to be addressed when seeking to enhance intragranular
properties without cost to grain interconnectivity.
[0051] Processing conditions (T, PO.sub.O2, t) must be sufficient
to diffuse oxygen through the substantially dense ceramic filaments
and the metallic sheath. This is mainly a kinetic effect and
oxygenation can be accomplished, for example in a silver-based
alloy sheathed system, by use of temperatures greater than
500.degree. C., at times in excess of 1 hours and oxygen partial
pressures of greater than 0.01 atm.
[0052] Processing conditions must also be selected such that
secondary phase material, when formed, occupies a position in the
microstructure which is benign to supercurrent flow in the wire.
Secondary phases may arise in several possible situations. Oxygen
stoichiometry change may lead to a change in cationic states within
the oxide superconductor, resulting in material being exsolved
(expelled) from the superconductor phase. A likely candidate for
exsolution is lead (Pb). which may undergo an oxidation valance
change from Pb.sup.2+ to Pb.sup.1- during exsolution.
Alternatively, changes in the thermodynamic state may cause
decomposition of the oxide superconducting phase. Some types of
phase decomposition may result in enhanced flux-pinning. For
example, very small oxide secondary phases (10-5000 .ANG.) within
the superconductor oxide phase on the order of the coherence length
of the superconducting electron pairs can pin magnetic vortices. In
either case. it is desirable that secondary phases that do not
create vortices pinning occupy a position in the oxide
superconductor composite where they are substantially benign to
supercurrent flow.
[0053] Regardless of the driving force to the intragranular change
in the oxide superconducting phase, a secondary phase is formed. In
order to reap the benefits of the intragranular phase modification
of the oxide superconductor, the secondary phase desirably does not
disrupt the intergranular connectivity of the composite.
[0054] Applicants have discovered that by careful control of the
processing conditions by which the oxide superconductor phase
modification occurs, formation of the secondary phase may be
localized at high energy sites. Since supercurrent flow occurs
preferably at low energy sites, grain interconnectivity is not
disrupted. Localization of secondary phases at high energy sites
may be accomplished by balancing the energy of decomposition (of
the oxide superconductor phase to the secondary phase) and the rate
of entropy increase of a secondary phase at the various
microstructural sites.
[0055] Decomposition in a closed materials system, such as a
sheathed high temperature superconductor wire, has an associated
energy. The magnitude of this energy depends upon the specific
phases and the microstructure before and after decomposition. In
the present case, BSCCO-2223 has a small thermodynamic state
stability field, and is relatively difficult to form. As a result,
there is a strong driving force for the decomposing of BSCCO-2223
during the transition from conditions of BSCCO-2223 formation to
ambient conditions.
[0056] During "ramping" conditions (approach to formation
conditions or return to ambient conditions), the principles of
irreversible thermodynamics control rnicrostructural evolution.
This is in contrast to isothermodynamic state treatments in which
the principles of equilibrium thermodynamics (minimization of the
free energy of the system) control. A governing principle of
irreversible thermodynamics is that the time rate of entropic
increase is maximized. In the present case, one attempts to
simultaneously control equilibrium and irreversible thermodynamic
considerations in order to control microstructural evolution.
[0057] With respect to the microstructure, every structure within
the closed materials system has some associated free energy. For
example, the energy of a grain boundary increases as the number of
broken chemical bonds associated with it increases. Thus, a
high-angle grain boundary, where there is a greater degree of
PbO-PbO.sub.2"bond mismatch" between neighboring grains, is likely
to have a higher free energy than a low angle grain boundary. Other
examples of high energy sites includes (a) high angle c-axis tilt
boundaries, (b) pores, (c) interfaces between superconducting and
secondary phases, and (d) surface boundaries (boundaries
terminating perpendicular to the c-axis) for the superconducting
phase. Examples of low energy sites within the superconducting
composite include (a) within the superconducting grains, (b) c-axis
twist boundaries (tilt=0), (c) c-axis boundary with the silver
phase, (d) coincident site lattice boundaries and (e) twin
boundaries.
[0058] Because the energy associated with high energy sites is
high, there is a strong driving force to "grow" the decomposition
products at that point which decreases overall free energy of the
system. Thus, if time rate of entropic increase associated with the
decomposition of the oxide superconductor phase is small, then the
decomposition products will grow at high energy sites. However, if
the time rate of entropic increase is high then decomposition
products will form at low and high energy sites. Note that mass
transfer to high energy sites is more substantial than mass
transfer to low and high energy sites. A practical means to
maintain a low time rate of entropic increase (so that high energy
sites are favored) is to hold the article at a thermodynamic state
that is close to, but outside that of, the desired superconductor.
If the process state is far from the thermodynamic state of the
desired superconductor, irreversible thermodynamics governs
microstructural evolution.
[0059] It follows then that, in order to obtain a silver sheathed
BSCCO-2223 wire with enhanced critical current and critical current
retention, one processes the wire under conditions that are very
close to the phase boundary between BSCCO-2223 and the
decomposition phase, thereby minimizing the force driving secondary
phase formation at low and high energy sites as indicated in FIG.
13. In prior art compositions, secondary phase growth occurs
without selectivity at both low and high energy sites. In such
instances, the rate of entropic increase is high and secondary
phase growth is indiscriminate.
[0060] According to the invention, the BSCCO-2223 wire is heat
treated in a processing space in which the decomposition products
form at high energy sites. The decomposition reaction of interest
is one which achieves the desired enhancement of intragranular
properties. This processing space balances the irreversible and
equilibrium thermodynamics and has the additional benefit of
minimizing the absolute magnitude of decomposition. Thus, the heat
treatment of the invention simultaneously minimizes the formation
of the secondary phase and its detrimental effect to the electrical
properties of the composite.
[0061] In one embodiment of the invention, modifications of the
lead content in the oxide superconducting phase achieve the desired
results. Under oxidizing conditions, lead is exsolved (expelled)
from the oxide superconducting phase, presumably undergoing a
change in valence state from 2+ to 4+. The exsolved lead forms a
secondary phase which has a high lead content. The formation of the
lead-rich non-superconducting phase is associated with the
reduction of lead in the (Bi.Pb)SCCO-2223 oxide superconductor
phase. Lead loss in (Bi,Pb)SCCO-2223 may be in the range of about 5
wt % to about 50 wt % lead. Preferably lead loss is in the range of
about 15 wt % about 25 wt %. Lead loss is reported as a percentage
of the amount of lead originally present in the (Bi,Pb)SCCO-2223.
It is contemplated that other metal capable of +2 to +4 (or +1 to
+3) valance state changes that are soluble in the oxide
superconductor phase may be used according to the invention.
Suitable cations include, but are in to way limited to, Pb, Tl, Sb,
Te, Hg, Se, As and Sn.
[0062] The lead-rich secondary phase has a hexagonal crystal
structure. Although the crystal symmetry does not change, the
chemical composition varies with the temperature of formation. For
example, a lead-rich secondary phase formed at 724.degree. C. has
an elemental composition, Bi:Pb:Sr:Ca:Cu, of 1:1:2:2:1, whereas a
lead-rich phase formed at 784.degree. C. has an elemental
composition of 1:2:2:3:1. Both hexagonal lead-rich phases have the
same X-ray diffraction pattern, which is shown in FIG. 1a. The
diffraction pattern corresponds to that catalogues in the JCPDS
files as 44-0053. FIG. 1b is an X-ray diffraction pattern of a
(Bi,Pb)SCCO-2223 composite of the invention including the lead rich
secondary phase. Peaks marked with an asterisk are attributable to
the secondary lead-rich phase. The remaining peaks are attributable
to the BSCCO-2223 oxide superconductor. Flukiger et al. have
previously observed this phase and the interested reader is
directed to Physica C 235-240:505-506 (1994) for further
information, herein incorporated by reference.
[0063] The marked improvement of critical current retention in the
oxide superconductor wires of the present invention correlates to
the reduction of lead content in the oxide superconducting phase.
While not being bound to any particular theory of operation, it is
hypothesized that the change in oxygen activity of lead (Pb) leads
to a decrease in the lead content in the oxide superconductor. The
altered stoichiometry may introduce oxygen defect sites which are
effective flux pinning sites and/or change the intrinsic coupling.
Flux pinning sites are known to improve critical current
performance in a magnetic field.
[0064] The heat treatment of the invention should satisfy both the
kinetic and thermodynamic criteria set forth above, that is, the
heat treatment should support material transport throughout the
composite and should support oxidations of the divalent metal
dopant in the BSCCO-2223 phase while remaining close to the
stability line of the decomposition phase. A reasonable guideline
in determining the appropriate processing conditions is to process
under conditions of the unitary oxide stability phase, e.g.;
PbO--Pb.sub.3O.sub.4--PbO.sub.2 in the context of the appropriate
multicomponent oxide phase. FIG. 2 is a plot of log P.sub.O2 vs.
1000/K (K.sup.-1) on which a calculated stability curve 20 for the
lead-rich phase is shown. The region above the plot represents
conditions which are oxidizing for Pb.sup.2-, resulting in
formation of the lead-rich phase. The temperature range is bounded
one the low side by kinetic considerations and on the high side by
concerns for indiscriminate mass transfer. In one embodiment of the
invention, the heat treatment is conducted in a space 22 at
pressures above the lead-rich phase reaction curve 20 over a
temperature on the range of about 500.degree. C. to 800.degree. C.
In a preferred embodiment, heat treatment is conducted in a space
24 above the lead-rich phase reaction curve 20 over a temperature
in the range of about 790.degree. C. to 630.degree. C. and in a
most preferred embodiment. the heat treatment is conducted in a
space 26 at pressures above the lead-rich phase reaction curve 20
over a temperature on the range of about 650.degree. C. to about
750.degree. C. Upper limit on oxygen pressure is about 100 atm.
[0065] To summarize, heat treatment used according to the present
invention may be in the range of 800.degree. C. to about
500.degree. C. at an oxygen content of 0.03 to 100 atm. Preferably
the heat treatment is conducted at a temperature in the range of
about 790.degree. C. to about 630.degree. C. and most preferably at
a temperature in the range of about 650.degree. C. to about
750.degree. C. The oxygen pressure is preferably in the range of
0.075 atm to 1.0 atm O.sub.2. such that the pressure is above the
reaction curve 20 at all times.
[0066] The heat treatment may be carried out in a variety of ways,
as indicated in FIG. 3. The heat treatment may be a single "bake"
at a single temperature (FIG. 3a) or it may be a series of shorter
"bakes" as progressively lower temperatures (FIG. 3b).
Alternatively, the heat treatment may be accomplished by a very
slow ramp (cool down) through the temperature range of interest. so
that the total dwell time in the effective temperature is achieved
(FIG. 3c). Curves 30 and 32 bound the effective temperature range
for the heat treatment. Preferred dwell time is greater than 20
hour and preferably greater than 30 hours.
[0067] The composite is desirably subjected to preliminary
treatment in order to provide good intergranular connectivity prior
to the post-formation heat treatment of the present invention. Good
grain interconnectivity is accomplished by proper alignment of the
oxide grains and substantially complete conversion of the oxide
precursor materials into the BSCCO-2223 oxide superconductor.
Conventional methods are available in the prior art to accomplish
this. Suitable methods are described hereinbelow.
[0068] Any conventional method may be used to prepare the
(Bi,Pb)SCCO-2223 phase used in the present invention. A preferred
method for preparation of a (Bi,Pb)SCCO-2223 oxide superconductor
phase is a multistep heat treatment. Heat treatments at different
points in the process play a different role in the manufacture of
the (Bi,Pb)SCCO-2223 composite. After thermomechanical processing
of the precursor oxide (typically,(Bi,Pb)SCCO-2212) into a wire of
desired orientation and dimension (see below), a multistep heat
treatment is carried out to convert the precursor oxide into
(Bi,Pb)SCCO-2223. The first step of the heat treatment is conducted
at a relatively high temperature under conditions sufficient to
form a liquid phase to partially melt the oxide phase which heals
cracks induced in previous deformation processing and converts
(Bi,Pb)SCCO-2212 into (Bi,Pb)SCCO-2223. The second step of the heat
treatment at a slightly lower temperature converts any liquid at
the (Bi,Pb)SCCO-2223 grain boundaries formed in the previous heat
treatment into (Bi,Pb)SCCO-2223. An optional third step of the heat
treatment at an even lower temperature "cleans" the
(Bi,Pb)SCCO-2223 grain boundaries (reacts away undesirable phase
impurities) to obtain good intergranular connectivity and completes
conversion of the precursor to (Bi,Pb)SCCO-2223. A typical heat
profile is shown in FIG. 4, where T.sub.c=850-800.degree. C., and
preferably 830-825.degree. C. (40 h, 0.075 atm O.sub.2),
T.sub.2=815-780.degree. C., and preferably 813-805.degree. C. (40
h, 0.075 atm 02) and T.sub.3=790-780.degree. C., and preferably
787.degree. C. (30 h, 0.075 atm, O.sub.2). The interested reader is
directed to co-pending application U.S. Ser. No. 08/041,822 filed
Apr. 1, 1993, the contents of which are herein incorporated in its
entirey by reference.
[0069] The (Bi,Pb)SCCO-2223 phase is substantially single phase
2223; however, 100% conversion may not always be obtained. Small
amounts of starting materials and/or other non-superconducting
phases may be present. They should not be present at levels greater
than 10 vol %, and preferably less than 5 vol %.
[0070] The composite is desirably subjected to preliminary
thermomechanical treatment in order to orient or texture the
precursor (Bi,Pb)SCCO-2212 oxide grains before their conversion to
(Bi,Pb)SCCO-2223. Known processing methods for texturing
superconducting oxide composites include combination of heat
treatments and deformation processing (thermomechanical
processing). BSCCO-2212 superconducting oxide grains can be
oriented along the direction of an applied strain, a phenomenon
known as deformation-induced texturing (DIT). Deformation
techniques, such as pressing and rolling, have been used to induce
grain alignment of the oxide superconductor c-axis perpendicular to
the plane or direction of elongation. Heat treatment under
conditions which at least partially melt and regrow the BSCCO-2212
superconducting phase also may promote texturing by enhancing the
anisotropic growth of the superconducting grains, a phenomenon
known as reaction-induced texturing (RIT).
[0071] Typically, density and degree of texture are developed in
the composite by repeated steps of deformation (to impart
deformation-induced texturing) and sintering (to impart
reaction-induced texturing). The steps of deforming and sintering
may be carried out several times. The process may be designated by
the term "nDS", in which "D" refers to the deformation step, "S"
refers to the sintering or heating step and "n" refers to the
number of times the repetitive process of deformation and sintering
are carried out. Typical prior art processes are 2DS or 3DS
processes. See, Sandhage et al. (JOM 21 (March, 1991)), herein
incorporated by reference. A 1DS process is described in co-pending
application, U.S. Ser. No. 08/468,089 filed Jun. 6, 1995 and
entitled "Simplified Deformation-Sintering Process for Oxide
Superconductinc Articles", which is herein incorporated by
reference. The nDS process may be used to orient the precursor
oxide phase before its conversion into the (Bi,Pb)SCCO-2223 oxide
superconductor wire.
[0072] 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. The malleable
material is preferably a noble metal which is inert to oxidation
and chemical reaction under conditions used in the formation and
post-formation processing of the composite. Suitable nobel metals
include palladium, platinum, gold, silver and mixtures thereof. 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 formation of superconducting tapes and
wires, see Sandhage et al.
[0073] The advantages of the post-formation heat treatment of the
present invention is demonstrated in the following examples, which
are present for the purpose of illustration only and which are in
no means intended to be limiting of the invention. Note that
J.sub.c values are critical current density normalized to reflect
the different superconducting content of the wires. J.sub.c is the
critical current carried by an oxide superconductor filament. In
the instance of a multifilamentary oxide superconductor wire,
J.sub.c is a value obtainable by division of the total current of
the oxide superconductor multifilamentary wire by the oxide
superconductor cross-sectional area. J.sub.e is the critical
current over the entire cross-sectional area of the
multifilamentary wire, a value obtainable by division of the total
current by the cross- sectional area of the wire. Comparison of
J.sub.e among wires having different fill-factors is not
meaningful, however, wires having the same fill factor may be
readily compared.
[0074] A multifilamentary oxide superconductor tape (85 filament
count) is prepared from (Bi,Pb)SCCO-2212 powders having the overall
composition of Bi(1.74):Pb(0.34):Sr(1.9):Ca(2.0):Cu(3.03) as
follows. Precursor powders were prepared from the solid state
reaction of freeze-dried precursor of the appropriate metal
nitrates having the stated stoichiometry. 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 mLxing 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 to homogenize the material
and to generate the BSCCO-2212 oxide superconductor phase. The
powders were packed into silver sheaths to form a billet. The
billets were drawn and narrowed with multiple die passes, with a
final pass drawn through a hexagonally shaped die into silver/oxide
superconductor hexagonal wires. Eighty-five (85) wires were bundled
together and drawn through a round die to form a multifilamentary
round wire.
[0075] The round multifilamentary tape is heated at 760.degree. C.
for 2 hours in 0.001 atm O.sub.2 and rolled to the desired
thickness in a single draft process (from about 35.4 mil to about 6
mil). Heat treatment at 827.degree. C. (0.075 atm O.sub.2) for 40 h
and at 808.degree. C. (0.075 atm O.sub.2) for 40 h are used to
convert the (Bi,Pb)SCCO-2212 phase into (Bi,Pb)SCCO -2223.
[0076] The effect of the temperature, oxygen content and dwell time
of the post-formation heat treatment on superconducting properties,
microstructure and composition were investigated. A four-point
probe was used to measure critical current, with a voltage
criterion of 1 .mu.V/cm for the determination of J.sub.e.
[0077] The temperature of the post-formation heat treatment was
systematically varied while atmosphere and dwell time was held
constant (0.075 atm O.sub.2, 30 h). FIG. 5 is a plot of J.sub.e
performance as a function of temperature (500.degree. C. to
800.degree. C.). All measured J.sub.e represented an improvement
over pre-treatment performance. Optimal J.sub.e performance (ca.
11,600 A/cm.sup.2) was measured at a temperature in the range of
700-724.degree. C. Improvements in J.sub.e represent improvements
in both intragrain and intergrain characteristics of the oxide
superconductor.
[0078] For a different set of samples, FIG. 6 is a plot of J.sub.e
as a function of dwell time for oxide superconductor wires
post-formation heat treated at 724.degree. C. in 0.075 atm (7.5%)
O.sub.2. Significant improvement in J.sub.e with dwell time is
observed, with diminishing incremental improvement as dwell time
increases above 6 h.
[0079] The oxygen partial pressure of the post-formation heat
treatment also was systematically varied while temperature and
dwell time was held constant (724.degree. C., 30 h). FIG. 7 is a
plot of J.sub.e as a function of oxygen partial pressure (0.003-1.0
atm). Balance of gas is inert gas, such as nitrogen or argon.
Optimal J.sub.e performance (ca. 11,500 A/cm.sup.2) was measured at
an oxygen partial pressure in the range of 0.075 atm oxygen.
Interestingly, optimal T.sub.c was obtained at lower oxygen partial
pressures and optimal J.sub.ret was obtained at 1.0 atm oxygen.
See, FIG. 7. This is a dramatic illustration of how optimization
for particular intergranular and intragranular properties occurs in
different processing regimes.
[0080] In conclusion, it is apparent that temperature, oxygen
partial pressure and dwell time may be varied to optimize the
absolute J.sub.e performance of the (Bi,Pb)SCCO-2223 wire. A
preferred post-formation heat treatment for optimal J.sub.e is at a
temperature in the range of about 700.degree. C. to 730.degree. C.;
at an oxygen partial pressure of about 0.075 atm O.sub.2; and at a
dwell time of at least about 20 hr, and preferably at least about
30 h. Other preferred conditions are within the scope of the
invention. For example, at higher oxygen pressures, the preferred
temperature should decrease and the dwell time is expected to
increase.
[0081] The ability for the (Bi,Pb)SCCO-2223 wire to retain critical
current in an applied magnetic field was also investigated. A
(Bi,Pb)SCCO-2223 wire subjected to the heat treatment of the
invention demonstrates a retention of up to about 40% and
preferably about 25% to about 35% of current carrying capacity at
0.1 T (77 K, .perp. tape surface). Field strengths of this
magnitude are of interest because they are comparable to the
applied field in certain applications. FIG. 8 is a bar graph
illustrating J.sub.ret for a variety of post-formation heat
treatments. All samples retained at least 25% of initial critical
current density. J.sub.ret showed a maximum at heat treatments of
724.degree. C./1 atm O.sub.2/30 h. These J.sub.ret performances
represent a significant improvement over performances reported in
the prior art.
[0082] FIGS. 9 and 10 are plots of J.sub.c retention vs. field
strength for (Bi,Pb)SCCO-2223 wires heated under different oxygen
partial pressures and temperatures, respectively, which demonstrate
that critical current flow remains in fields up to 0.5 T.
[0083] Note that samples processed for maximum J.sub.e do not
necessarily also exhibit optimal critical current retention. Table
1 shows the Td.sub.c, J.sub.e, and J.sub.ret values for samples
held at constant pressure (0.075 atm, 30 h) at varying temperatures
(Ex. 1-5) and for samples held at constant temperature (724.degree.
C., 30 h) at varying oxygen pressures (Ex. 6-8). J.sub.e values can
only be compared within a samples series, as fill factor changes.
Note that optimal T.sub.c, optimal J.sub.c and optimal J.sub.ret
result at different processing conditions. This supports the
earlier observation that factors maximizing the two properties need
not be identical.
1TABLE 1 T.sub.c, J.sub.e, and J.sub.ret values for variously
heat-treated samples. parameter T.sub.onset .DELTA.T.sub.c
J.sub.e.degree. No. constant variable (K) (K) (amp/cm.sup.2)
J.sub.ret (%) comments 1 0.075 atm O.sub.2, 500.degree. C. 107 6.5
6500 26 30 h 2 0.075 atm O.sub.2, 700.degree. C. 108 4.0 10,400 29
30 h 3 0.075 atm O.sub.2, 724.degree. C. 108 3.3 11,600 32 30 h 4
0.075 atm O.sub.2, 750.degree. C. 109 4.0 9600 29 optimal 30 h
T.sub.c.sup.+ 5 0.075 atm O.sub.2, 800.degree. C. 109 8.0 7500 30
30 h 6 724.degree. C., 30 h 0.03 atm 108 4.0 8000 25 7 724.degree.
C., 30 h 0.075 atm 108 3.0 11,600 32 optimal J.sub.c 8 724.degree.
C., 30 h 1.0 atm 105.5 5.0 5100 37 optimal J.sub.ret .degree.
J.sub.e values may only be compared within the same sample series.
.sup.+note that both sample nos. 4 and 5 have comparable
T.sub.onset; however, sample no. 4 has a smaller .DELTA.T.
[0084] Formation of a new lead-rich non-superconducting phase is
observed during the post-formation heat treatment of the invention
and the amount of this phase increases with dwell time. This phase
was not observed during (Bi,Pb)SCCO-2223 formation heat treatments.
The appearance of the lead-rich secondary phase and the increased
formation with increased dwell time correlates well with the
observed improvements in J.sub.c and J.sub.ret in the
post-formation heat treatment. FIG. 11 is a plot of the relative
fraction of the lead-rich secondary phase in the final
(Bi,Pb)SCCO-2223 wire as a function of oxygen partial pressure
(724.degree. C., 30 h). The relative fraction of the lead-rich
phase increases with increased oxygen partial pressure. This
correlates well with conditions producing the maximum critical
current retention. Note that no lead rich secondary phase appears
to have formed at 0.003 atm oxygen, the processing condition which
optimized T.sub.c. Thus, the appearance of this phase positively
affects the performance of the post-formation heat treatment
samples. FIG. 12 is a plot of the relative fraction of the lead
rich phase as a function of temperature. Curve 110 is for samples
at 0.075 atm O.sub.2. Significant lead-rich secondary phase
formation is observed for temperatures in the range of
724-775.degree. C. Results from FIG. 11 (at 724.degree. C.) are
included in this plot and suggest that at higher P.sub.O2, a
greater temperature range may provide significant amounts of the
lead-rich phase.
[0085] The effect of heat treatment on critical current density
retention above and below the lead-rich phase stability line is
demonstrated in FIG. 13. Curve 120 represents the J.sub.ret for
samples processed under conditions above the lead-rich phase
stability line. Open circle data point represent samples heat
treated at 724.degree. C. at 0.075 atm for 30 hours. Closed diamond
data points represent samples heat treated at 724.degree. C. at 1.0
atm for 30 hours. Curve 122 represents the J.sub.ret for samples
processed at 724.degree. C. at 0.003 atm for 30 hours--conditions
above the lead-rich phase stability line. Performance represented
by curve 122 is significantly compromised.
[0086] The lead-rich secondary phase formation and the effect of
its formation on (Bi,Pb)SCCO-2223 were investigated by scanning
electron microscopy (SEM) and energy dispersive spectrometry (EDS),
which permitted determination of the elemental composition of both
phases. The results are reported in Table 2. The relative
stoichiometry of the (Bi,Pb)SCCO-2223 phase prior to the
post-formation heat treatment is consistent with a nominal 2:2:2:3
stoichiometry. However, after heat treatment, the level of lead in
the superconducting phase has been reduced significantly and a
secondary phase rich in lead and poor in copper is formed. The
relative fraction of the lead-rich secondary phase increases with
dwell time and appears decorating the perimeter of the BSCCO-2223,
grains. Further, the lead-rich phase appear to concentrate at the
BSCCO-2223 high-energy sites.
2TABLE 2 (Bi,Pb)SCCO-2223 and lead-rich phase compositions (at. %).
last heat compound treatment Bi Pb Sr Ca Cu (Bi,Pb)SCCO-
808.degree. C. 19.8 .+-. 0.4 4.9 .+-. 0.1 20.9 .+-. 0.5 23.6 .+-.
0.3 30.8 .+-. 0.6 2223 (40 h) post-formation 724.degree. C. 21.1
.+-. 0.5 3.8 .+-. 0.4 22.5 .+-. 0.6 22.9 .+-. 0.4 29.7 .+-. 1.4
heat treated (30 h) (Bi,Pb)SCCO- 2223 lead-rich phase 724.degree.
C. 14.2 .+-. 1.5 17.7 .+-. 3.3 25 .+-. 0.9 27.1 .+-. 1.3 16 .+-.
3.6 (30 h)
[0087] Although not intending to be limited to a single
interpretation, one possible explanation for the observed
appearance of the lead-rich phase under conditions which also
improve critical current retention is that the starting composition
is overdoped with lead and that the extra lead decomposes into the
lead-rich secondary phase. An alternative explanation is that the
lead-rich phase is the product of oxygen content modification in
the (Bi,Pb)SCCO-2223 lattice. In other words, oxygen defects are
introduced into the 2223 lattice for high performance and the doped
oxygen defects change the valance of the Pb in the 2223 lattice and
consequently the lead-rich phase is forced to decompose out from
the 2223 phase. Further, the increased flux pinning is derived from
the introduction of oxygen defect Therefore, the post-formation
heat treatment results in both oxygen defect formation and
lead-rich phase formation, which influences both the intergranular
and intragranular properties of the superconductor.
[0088] Other embodiments of the invention will be apparent to the
skilled in the art from a consideration of this 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.
* * * * *