U.S. patent application number 10/082848 was filed with the patent office on 2002-10-03 for fine uniform filament superconductors.
This patent application is currently assigned to American Superconductor Corporation. Invention is credited to Antaya, Peter D., Christopherson, Craig J., Craven, Christopher A., DeMoranville, Kenneth L., Garrant, Jennifer H., Hancock, Steven, Li, Qi, Riley, Gilbert N. JR., Roberts, Peter R., Seuntjens, Jeffrey M..
Application Number | 20020142918 10/082848 |
Document ID | / |
Family ID | 25415843 |
Filed Date | 2002-10-03 |
United States Patent
Application |
20020142918 |
Kind Code |
A1 |
Riley, Gilbert N. JR. ; et
al. |
October 3, 2002 |
Fine uniform filament superconductors
Abstract
A multifilamentary superconductor composite having a high fill
factor is formed from a plurality of stacked monofilament precursor
elements, each of which includes a low density superconductor
precursor monofilament. The precursor elements all have
substantially the same dimensions and characteristics, and are
stacked in a rectilinear configuration and consolidated to provide
a multifilamentary precursor composite. The composite is thereafter
thermomechanically processed to provide a superconductor composite
in which each monofilament is less than about 50 microns thick.
Inventors: |
Riley, Gilbert N. JR.;
(Marlborough, MA) ; Li, Qi; (Marlborough, MA)
; Roberts, Peter R.; (Groton, MA) ; Antaya, Peter
D.; (Sutton, MA) ; Seuntjens, Jeffrey M.;
(Singapore, SG) ; Hancock, Steven; (Worcester,
MA) ; DeMoranville, Kenneth L.; (Jefferson, MA)
; Christopherson, Craig J.; (Worcester, MA) ;
Garrant, Jennifer H.; (Natick, MA) ; Craven,
Christopher A.; (Bedford, MA) |
Correspondence
Address: |
HALE AND DORR, LLP
60 STATE STREET
BOSTON
MA
02109
|
Assignee: |
American Superconductor
Corporation
Westborough
MA
|
Family ID: |
25415843 |
Appl. No.: |
10/082848 |
Filed: |
February 26, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10082848 |
Feb 26, 2002 |
|
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08902421 |
Jul 29, 1997 |
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6370405 |
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Current U.S.
Class: |
505/231 ;
174/125.1; 428/702; 428/930; 505/236; 505/237; 505/704 |
Current CPC
Class: |
H01L 39/248 20130101;
Y10S 428/93 20130101; Y10S 505/704 20130101; Y10T 29/49014
20150115 |
Class at
Publication: |
505/231 ;
174/125.1; 428/702; 428/930; 505/236; 505/237; 505/704 |
International
Class: |
H01B 012/00 |
Claims
What is claimed is:
1. A mulitfilamentary superconductor including a plurality of
superconductor monofilaments each of which has in transverse
cross-section a width that is at least five (5) times the thickness
thereof, the thickness of each of said superconductor monofilaments
being less than about 50 microns, said superconductor having a Je
of not less than 12,000 A/cm.sup.2 at 77K, 0T; said
multifilamentary superconductor formed by processing a
multifilamentary high temperature superconductor precursor
composite comprising a plurality of precursor elements arranged in
horizontal or vertical alignment with each other, each of the
elements including an HTS (high temperature superconductor)
precursor monofilament, each of the monofilaments having a density
in the range of about 30% to about 70% theoretical density, the
superconductor precursor composite including fine grain metal
components surrounding each of the monofilamentary HTS precursors,
and said elements being consolidated.
2. The mulitfilamentary superconductor of claim 46 wherein said
thickness is not more than about 40 microns.
3. The mulitfilamentary superconductor of claim 46 wherein said
superconductor has a fill factor greater than about 30%.
4. The mulitfilamentary superconductor of claim 48 wherein said
fill factor is greater than about 40%.
5. The mulitfilamentary superconductor of claim 46 wherein the
thickness of each of said superconductor monofilaments is less than
about 10 microns.
6. The mulitfilamentary superconductor of claim 50 wherein the
thickness of each of said superconductor monofilaments is in the
range of about 2 to 7 microns.
7. The mulitfilamentary superconductor of claim 51 wherein the
thickness of each of said superconductor monofilaments is about 5
microns.
8. The mulitfilamentary superconductor of claim 46 wherein the
height of said superconductor, measured in the same direction as
the thicknesses of the superconductor monofilament, is less than
60% of a corresponding height of said multifilamentary
superconductor precursor composite.
9. The multifilamentary superconductor of claim 53 wherein the
height of said superconductor is in the range of 5% to 60% of said
corresponding height.
10. A multifilamentary superconductor including a plurality of
superconductor monofilaments each of which has in transverse
cross-section a width that is at least five (5) times the thickness
thereof, the thickness of each of said superconductor monofilaments
being less than about 50 microns, said superconductor having a Je
of not less than 12,000 A/cm.sup.2 at 77K, 0T; said
multifilamentary superconductor formed by processing a
multifilamentary high temperature superconductor precursor
composite comprising a plurality of precursor elements arranged in
horizontal or vertical alignment with each other, each of the
elements including an HTS (high temperature superconductor)
precursor monofilament surrounded by fine grain metal components,
each of the monofilaments having in transverse cross section a
width that is greater than the thickness thereof and the thickness
thereof being not more than about 50 microns.
11. The mulitfilamentary superconductor of claim 55 wherein said
thickness is not more than about 40 microns.
12. The mulitfilamentary superconductor of claim 55 wherein said
superconductor has a fill factor greater than about 30%.
13. The mulitfilamentary superconductor of claim 57 wherein said
fill factor is greater than about 40.
14. The mulitfilamentary superconductor of claim 55 wherein the
thickness of each of said superconductor monofilaments is less than
about 10 microns.
15. The mulitfilamentary superconductor of claim 59 wherein the
thickness of each of said superconductor monofilaments is in the
range of about 2 to 7 microns.
16. The mulitfilamentary superconductor of claim 60 wherein the
thickness of each of said superconductor monofilaments is about 5
microns.
17. The mulitfilamentary superconductor of claim 55 wherein the
height of said superconductor, measured in the same direction as
the thicknesses of the superconductor monofilament, is less than
60% of a corresponding height of said multifilamentary
superconductor precursor composite.
18. The multifilamentary superconductor of claim 62 wherein the
height of said superconductor is in the range of 5% to 60% of said
corresponding height.
19. A multifilamentary superconductor having a Je of not less than
12,000 A/cm.sup.2 at 77K, 0T; the multifilamentary superconductor
formed by processing a multifilamentary high temperature
superconductor precursor composite comprising a plurality of
precursor elements arranged in horizontal or vertical alignment
with each other, each of the elements including an HTS (high
temperature superconductor) precursor monofilament, each of the
monofilaments having a density in the range of about 30% to about
70% theoretical density, the superconductor precursor composite
including fine grain metal components surrounding each of the
monofilamentary HTS precursors, and said elements being
consolidated.
20. The mulitfilamentary superconductor of claim 64 wherein said
thickness is not more than about 40 microns.
21. The mulitfilamentary superconductor of claim 64 wherein said
superconductor has a fill factor greater than about 30%.
22. The mulitfilamentary superconductor of claim 66 wherein said
fill factor is greater than about 40%.
23. The mulitfilamentary superconductor of claim 64 wherein the
thickness of each of said superconductor monofilaments is less than
about 10 microns.
24. The mulitfilamentary superconductor of claim 68 wherein the
thickness of each of said superconductor monofilaments is in the
range of about 2 to 7 microns.
25. The mulitfilamentary superconductor of claim 69 wherein the
thickness of each of said superconductor monofilaments is about 5
microns.
26. The mulitfilamentary superconductor of claim 64 wherein the
height of said superconductor, measured in the same direction as
the thicknesses of the superconductor monofilament, is less than
60% of a corresponding height of said multifilamentary
superconductor precursor composite.
27. The multifilamentary superconductor of claim 71 wherein the
height of said superconductor is in the range of 5% to 60% of said
corresponding height.
28. A multifilamentary superconductor having a Je of not less than
12,000 A/cm.sup.2 at 77K, 0 T; the multifilamentary superconductor
formed by processing a multifilamentary high temperature
superconductor precursor composite comprising a plurality of
precursor elements arranged in horizontal or vertical alignment
with each other, each of the elements including an HTS (high
temperature superconductor) precursor monofilament surrounded by
fine grain metal components, each of the monofilaments having in
transverse cross section a width that is greater than the thickness
thereof and the thickness thereof being not more than about 50
microns.
29. The mulitfilamentary superconductor of claim 73 wherein said
thickness is not more than about 40 microns.
30. The mulitfilamentary superconductor of claim 73 wherein said
superconductor has a fill factor greater than about 30%.
31. The mulitfilamentary superconductor of claim 75 wherein said
fill factor is greater than about 40%.
32. The mulitfilamentary superconductor of claim 73 wherein the
thickness of each of said superconductor monofilaments is less than
about 10 microns.
33. The mulitfilamentary superconductor of claim 77 wherein the
thickness of each of said superconductor monofilaments is in the
range of about 2 to 7 microns.
34. The mulitfilamentary superconductor of claim 78 wherein the
thickness of each of said superconductor monofilaments is about 5
microns.
35. The mulitfilamentary superconductor of claim 73 wherein the
height of said superconductor, measured in the same direction as
the thicknesses of the superconductor monofilament, is less than
60% of a corresponding height of said multifilamentary
superconductor precursor composite.
36. The multifilamentary superconductor of claim 80 wherein the
height of said superconductor is in the range of 5% to 60% of said
corresponding height.
37. A mulitfilamentary superconductor including a plurality of
superconductor monofilaments each of which has in transverse
cross-section a width that is at least five (5) times the thickness
thereof, the thickness of each of said superconductor monofilaments
being less than about 50 microns, said superconductor having a Je
of not less than 12,000 A/cm.sup.2 at 77K, 0T.
38. The mulitfilamentary superconductor of claim 82 wherein said
thickness is not more than about 40 microns.
39. The mulitfilamentary superconductor of claim 82 wherein said
superconductor has a fill factor greater than about 30%.
40. The mulitfilamentary superconductor of claim 84 wherein said
fill factor is greater than about 40%.
41. The mulitfilamentary superconductor of claim 82 wherein the
thickness of each of said superconductor monofilaments is less than
about 10 microns.
42. The mulitfilamentary superconductor of claim 86 wherein the
thickness of each of said superconductor monofilaments is in the
range of about 2 to 7 microns.
43. The mulitfilamentary superconductor of claim 82 wherein the
thickness of each of said superconductor monofilaments is about 5
microns.
44. The mulitfilamentary superconductor of claim 82 wherein the
height of said superconductor, measured in the same direction as
the thicknesses of the superconductor monofilament, is less than
60% of a corresponding height of said multifilamentary
superconductor precursor composite.
45. The multifilamentary superconductor of claim 89 wherein the
height of said superconductor is in the range of 5% to 60% of said
corresponding height.
Description
FIELD OF THE INVENTION
[0001] This invention relates to high temperature ceramic
superconductors. More particularly, it relates to multifilamentary
superconductor structures that include a multiplicity of thin and
uniform filaments; and to the manufacture of such structures from
near net shape precursors.
BACKGROUND OF THE INVENTION
[0002] Superconductors are materials having essentially zero
resistance to the flow of electrical current at temperatures below
a critical temperature, Tc. A variety of copper oxide ceramic
materials have been observed to exhibit superconductivity at
relatively high temperatures, i.e., above 77K. Since the discovery
of the first copper oxide based superconductor about ten years ago,
these superconducting ceramics have attracted wide interest, and
their physical and chemical properties have been widely studied and
described in many publications.
[0003] Composites of superconducting materials and metals are often
used to obtain better mechanical and electrical properties than
superconducting materials alone provide. These composites are
typically prepared in elongated wires, elements and cables by a
variety of known processes such as the well-known powder-in-tube
("PIT") process in which a metal container is filled with a
precursor powder and the filled container is then deformed and
thermomechanically processed to form filamentary composites having
the desired superconducting properties, and a variety of coated
conductor ("CC") processes in which a superconductor material or a
precursor thereof is deposited on a substrate which is then further
processed to form a composite including a superconducting filament.
However formed, a multiplicity of filaments may be bundled and/or
cabled, with additional deformation and thermomechanical processing
steps as needed, to provide multifilamentary composites.
[0004] To be commercially viable, high-temperature superconductor
(HTS) wire must have high performance (e.g., high critical current
density of the superconductor, Jc) and low cost. In the past,
considerable efforts have been directed to improving the Jc of
superconducting ceramics through densification and crystallographic
alignment or texture of the superconducting grains; more recently,
there has been increasing interest in and efforts to develop
manufacturing technologies through which long lengths of HTS wires
can be fabricated with higher, and commercially acceptable, price
to performance (as measured by $/kA m) ratios.
[0005] At this time, it is known in the HTS community that the
highest performing BSCCO (both 2212 and 2223) contain highly
aspected (an "aspected" element has, in transverse cross-section, a
width greater than its height) filaments with dimensions on the
order of 10.times.100 microns, and that composite Bi-2223
conductors fabricated using PIT techniques can achieve relatively
high Jc performance if asymmetric deformation resulting in an
aspected element is employed. For example, using asymmetric
deformation, a Jc value of 69,000 A/cm2 at 77K and self field has
been reported (Q. Li et al., Physica C, 217 (1993) 360); and it has
been predicted that the Jc performance of Bi-2223 conductors may be
improved drastically if the thickness of the superconducting layer
is decreased from the 30 micron level used by Li et al. to the
three micron level. A Jc value in excess of 100,000 A/cm2 (77K, 0
T) has been estimated for the Bi-2223 layer (about 1.5 micron
thick) that is immediately adjacent to the Ag in conventionally
fabricated elements. Other HTS wire types have shown short length
performance, e.g., coated conductors based on Y-123 which are
fabricated using thin film techniques using such equipment as
vacuum systems, lasers and ion guns.
[0006] It is difficult to achieve filament thicknesses in the range
of 3 microns using conventional PIT techniques in which
axisymmetric deformation is used to prepare a round
multifilamentary precursor that is subsequently rolled into a
highly aspected element, for two principal reasons. First, the
strain path for each filament is a function of its position within
the composite, and filaments in the edges of the final element will
be less textured and will have a lower performance level than those
in the central region of the element. Second, the pre-deformation
cross-section of each filament is typically circular, and it is
difficult to achieve a thin and wide filament by deforming an
initially round filament.
[0007] A variety of deformation processing procedures have been
proposed. Copending application Ser. No. 08/468,089, filed Jun. 6,
1995 entitled "Simplified Deformation-Sintering Process for Oxide
Superconducting Articles", and incorporated herein by reference in
its entirety, describes a method for preparing a highly textured
oxide superconductor article in a single, rather than a multiple
step, deformation-sinter process. In the procedure described a
precursor article, including a plurality of filaments extending
along the length of the article and comprising a precursor oxide
having a dominant amount of a tetragonal BSCCO 2212 phase and a
constraining member substantially surrounding each of the
filaments, is subjected to a heat treatment at an oxygen partial
pressure and temperature selected to convert a tetragonal BSCCO
2212 oxide into an orthorhombic BSCCO 2212 oxide. Thereafter, the
article is roll worked in a single high reduction draft in a range
of about 40% to 95% in thickness so that the filaments have a
constraining dimension is substantially equivalent to a longest
dimension of the oxide superconductor grains, and is then sintered
to obtain a BSCCO 2212 or 2203 oxide superconductor. Other
procedures are disclosed in copending application Ser. No.
08/651,688, filed Nov. 11, 1995 and entitled "Improved Breakdown
Process for Superconducting Ceramic Composite Conductors", which
application is also here incorporated by reference in its
entirety.
[0008] To be practical outside the laboratory, most electrical and
magnetic applications require flexible cabled lengths of conductor
manufacturable with high fill factors (i.e. a high volume percent
of superconductor in the composite multifilament structure) in
addition to high current-carrying capacity. Thus, in addition to
making individual filaments with high Jc, considerable effort also
has been directed to the manufacture of cables and the like which
include a multiplicity of HTS filaments. For example, copending
application Ser. No. 08/554,814, filed Nov. 11, 1995, entitled
"Cabled Conductors Containing Anisotropic Superconducting Compounds
and Method for Making Them," and also hereby incorporated by
reference in its entirety, discloses a cabled conductor comprising
a plurality of transposed strands each comprising one or more
preferably twisted filaments preferably surrounded or supported by
a matrix material and comprising textured anisotropic
superconducting compounds which have crystallographic grain
alignment that is substantially unidirectional and independent of
the rotational orientation of the strands and filaments in the
cabled conductor. The cabled conductor is made by forming a
plurality of suitable composite strands, forming a cabled
intermediate from the strands by transposing them about the
longitudinal axis of the conductor at a preselected strand lay
pitch, and, texturing the strands in one or more steps including at
least one step involving application of a texturing process with a
primary component directed orthogonal to the widest longitudinal
cross-section of the cabled intermediate, at least one such
orthogonal texturing step occurring subsequent to said strand
transposition step. In one embodiment, the filament cross-section,
filament twist pitch, and strand lay pitch are cooperatively
selected to provide a filament transposition area which is always
at least ten times the preferred direction area of a typical grain
of the desired anisotropic superconducting compound. For materials
requiring biaxial texture, the texturing step may include
application of a texturing process with a second primary component
in a predetermined direction in the plane of the widest
longitudinal cross-section of the conductor.
[0009] Others, e.g., U.S. Pat. No. 5,508,254, have proposed forming
a multifilamentary structure by vertically stacking relatively
thick rolled tapes.
[0010] However, and despite all of the past and ongoing work in the
field, both cost and performance are still major constraints
limiting the widescale use of HTS wires in the marketplace. There
remain the needs to increase the Jc of HTS filaments, to provide
multi-filament composites of varying geometry having greater fill
factors and overall current-carrying capacity, and to accomplish
all of this at reduced cost.
SUMMARY OF THE INVENTION
[0011] The invention features a multi-filamentary superconductor
having a high fill factor (e.g., greater than 30% and preferably
greater than 35% to 40%) which is made in a semi-continuous
procedure from a number of superconductor precursor elements, each
of which has substantially the same overall geometry and which
include superconductor precursor monofilaments having the same
overall configuration. The superconductor precursor monofilaments
are provided on or in a metal component. Before rolling, the
precursor monofilaments have a low density (i.e., in the range of
25 to 70 percent, preferably 30-65 percent, and most preferably 40
to 60 percent, theoretical density); after rolling, the thickness
of the monofilaments is not more than about 50 microns (and
preferably not more than about 40 microns). The elements are
consolidated into a composite in which the spatial relationship of
the elements is such that all of the elements are symmetric
relative to each other and also both to the external shape of the
composite and to subsequent deformation. In the consolidated
precursor composite, metal components of the composite form a
bonded ladder structure with superconductor precursor monofilaments
in the space between adjacent "rungs". Both before and after
consolidation the configurations of the filaments of the different
precursor elements are substantially the same.
[0012] As used herein, "precursor"means any material that can be
converted to a desired anisotropic superconductor upon application
of a suitable heat treatment. If the desired anisotropic
superconductor is an oxide superconductor, for example, precursors
may include any combinations of elements, metal salts, oxides,
suboxides, oxide superconductors which are intermediate to the
desired oxide superconductor, or other compounds which, when
reacted in the presence of oxygen in the stability field of a
desired oxide superconductor, produces that superconductor.
Whatever the particular precursor, in the practice of the present
invention, the final aspect ratio of the composite, and of the
superconductor monofilaments in it, may be decoupled from the
aspect ratios of the individual precursor elements and
superconductor precursor filaments. "Consolidate", as used herein,
means to carry out operations that allow an assembly of elements to
behave as a unit, at least to the extent that there is no large
scale displacement of various elements of the assembly (e.g., the
precursor composite) during subsequent processing. Consolidation
may be accomplished by a number of procedures including heat
treatment (thermal processing), chemical adhesion, and drawing or
other deformation processes. The preferred procedure includes
sufficient thermal heating to accomplish an initial phase
transition of the superconductor precursor.
[0013] In one aspect of this invention, the composite precursor
includes a number of precursor elements, each of which includes at
least one HTS precursor monofilament, stacked in side by side
alignment to form a layer in which the tops and bottoms of the
elements and also the filaments in the elements are generally
aligned across the width of the composite. Metal (typically a noble
metal although other metals, with a buffer layer as required to
prevent interreaction with the superconductor components, may also
be used) is provided between each adjacent pair of precursor
filaments. In embodiments of this aspect, the composite may include
more than one layer of side-by-side precursor elements, in which
event the layers are stacked vertically in such a way that each HTS
precursor filament is either, in vertical alignment with or is
centered on the thin space between HTS precursor filaments in any
other layer. All the HTS precursor filaments in the different
precursor elements have essentially the same aspect ratio, width
and thickness. The stacked composite structure typically is
provided with a surrounding metal wrap or sheath and heated to
consolidate its various components.
[0014] In a second aspect of the present invention, each precursor
element includes metal having a layer of an HTS precursor deposited
on at least one face of the metal, and preferably on both of its
opposite faces, and in the composite metal overlies both an
otherwise exposed face and the side edges of each of the HTS
precursor layers.
[0015] In preferred practices of the invention, precursor
composites are thermomechanically processed in a 1DS or 2DS
procedure (as discussed hereinafter) in which the element is
reduced by about 40% to about 95% in thickness (with no subsequent
reduction in thickness in excess of about 5% prior to a sintering
step), and the thus-rolled precursor composite is sintered to
obtain a final superconductor composite structure having
effectively uniform filaments. In high performance structures, the
HTS filaments are typically less than about 10 microns, preferably
in the range of about 2-7 microns, and most preferably about 5
microns, thick.
[0016] Preferred precursor composites are made in PIT or coated
conductor processes in which a plurality of effectively identical
superconductor precursor elements having low density HTS precursor
monofilaments are (after drawing but before rolling for PIT
elements) stacked and surrounded with a supporting fine-grain
metal. In each precursor element, the HTS precursor filament is on
or in a fine grain metal. In PIT processes, the element is drawn to
smaller size in a procedure involving frequent anneals to maintain
the fine grain size and deformation characteristics of the metal.
By "fine-grained" is meant an average grain size that is typically
less than 300 micrometers, preferably less than 200 micrometers,
more preferably less than 100 micrometers, even more preferably
less than 50 micrometers, or, most preferably less than 20
micrometers. The maximum grain size is typically less than about
300 micrometers, preferably less than about 200 micrometers, more
preferably less than about 100 micrometers, and most preferably
less than about 50 micrometers. The stacked precursor elements and
support are then consolidated and thermomechanically processed. The
precursor elements may be made using a PIT process in which each
element is made from a metal tube filled with precursor powder to a
low density which then has been drawn to provide a structure not
more than about 600 microns in diameter and in which the
superconductor precursor monofilament is of low density and not
over about 300 microns thick. Such drawn elements may then be
rolled into high aspect ratio elements, which depending on the
procedure used may result in high density HTS precursor filaments.
The precursor elements also may be made using a deposition/coating
process in which each element comprises a fine-grain metal
substrate carrying a low density precursor superconductor layer. In
the latter event, a precursor superconductor layer is preferably
provided on both sides of the substrate, and the composite
precursor includes fine grain metal between adjacent precursor
elements.
DESCRIPTION OF THE DRAWINGS
[0017] The foregoing and other objects, features and advantages of
the invention will become more apparent from the following detailed
description of preferred embodiments thereof, taken together with
the attached drawings in which:
[0018] FIGS. 1 and 2 are cross-sections of composite superconductor
precursor structures according to the present invention; FIGS. 1a
and 2a are cross-sections of multifilamentary superconductor
structures made from, respective, the precursor structures of FIGS.
1 and 2.
[0019] FIG. 3 is a cross-section of a monofilament precursor
element made using a PIT procedure for use in the practice of the
present invention;
[0020] FIG. 4 is a schematic illustrating a procedure for forming
the composite of FIGS. 1 and 2;
[0021] FIG. 5 is a cross-section of a monofilament precursor
element made using a coated conductor technique for use in the
practice of the present invention;
[0022] FIG. 6 is a cross-section of a composite stack including a
multiplicity of the monofilament precursor elements of FIG. 5;
[0023] FIG. 7 is a cross-section of a second composite stack
including a multiplicity of the monofilament precursor elements of
FIG. 5;
[0024] FIG. 8 is a schematic illustrating a procedure for forming
the precursors of FIG. 6, and the composite structures of FIGS. 6
and 7;
[0025] FIGS. 9 is a cross section of another superconductor
precursor structure.
[0026] FIGS. 10 and 11 are graphs illustrating the relationship
between filament thickness and Je (engineering current density over
the cross-section of the entire superconductor including both HTS
and other structure).
[0027] FIG. 12 illustrates Je data.
[0028] FIG. 13 illustrates a tension take-up system.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0029] FIGS. 1 and 2 illustrate exemplar superconductor precursor
composite structures, designated 10 and 20, each of which includes
a plurality of HTS precursor monofilament elements, designated 11
through 15 and 21 through 25, respectively. Each element has an HTS
precursor monofilament, designated 11a through 15a and 21a through
25a, respectively; and in each of the composites all of the
elements and all of the monofilaments have substantially the same
size and cross-sectional configuration. It will also be noted that,
in each composite structure 10 and 20, there is a metal, e.g., a
noble metal such as Ag, layer 16, 26 between each adjacent pairs of
HTS precursor monofilaments. As discussed hereinafter, the Ag layer
16, 26 may, in the case of a element formed by a PIT process, be
provided by the sheath used in the process. In other circumstances,
e.g., when the HTS precursor filament is formed on a metal
substrate, some portions of the Ag between adjacent filaments may
be provided by separate spacers.
[0030] As used herein, the term "noble metal" means a metal which
is substantially non-reactive with respect to oxide superconductors
and precursors and to oxygen under the expected conditions
(temperature, pressure, atmosphere) of manufacture and use.
Preferred noble metals include silver, gold, platinum and
palladium. Silver (Ag) and its alloys, being lowest in cost of
these materials, are most preferred for large scale manufacturing.
It should be noted that in some circumstances the noble metal may
also be a stoichiometric excess of one of the metallic elements of
the desired superconducting ceramic, such as copper. It should also
be noted that, although noble metals such as Ag are preferred,
other deformable metals may be used, particularly as substrates on
which superconductor precursors are deposited. The use of metals
such as nickel, certain nickel alloys or stainless steel, often
with an oxide buffer layer, is known in the art. See, e.g., U.S.
Pat. No. 5,284,825.
[0031] It will be evident that the particular superconductor
ceramic of which the HTS monofilaments are precursors is not
critical. For example, superconducting ceramics of the oxide,
sulfide, selenide, telluride, nitride, boron carbide or
oxycarbonate types may be used. Superconducting oxides, e.g.,
members of the rare earth (RBCO) families of superconductors, the
bismuth (BSCCO) family of oxide superconductors, the thalliium
(TBSCCO) family of oxide superconductors, of the mercury (HBSCCO)
family of superconductors, are preferred. The bismuth and rare
earth families are most preferred. Thallinatioin, the addition of
doping materials, variations from ideal stoichiometric proportions
and other variations in the formulation of the desired
superconducting oxides may also be used in manners similar to those
now known to the art. At present, it appears that BSCCO materials
(either 2212 or 2223) and YBCO materials are the most viable
materials for use in HTS electrical systems, and the presently most
preferred materials are BSCCO 2223 and YBCO 123. It will be
appreciated that the compositions of the BSCCO, YBCO and other
materials are well-known in the art, as are procedures for making
of superconductor precursors employing these materials, and for
thermomechanically treating the precursors to produce the desired
superconductor from the precursor. See, e.g., the aforementioned
'089 and '814 applications, and the various patents and
publications referred to therein.
[0032] As shown in FIG. 1, the drawn (but not yet rolled) precursor
elements 11 through 15 of precursor composite 10 are generally
square in cross section and about 600 microns in overall thickness
and width. The superconductor precursor filaments 11a through 15a
in each element has a low density (e.g., less than about 60% of
theoretical density) and an overall thickness of about 300 microns
(e.g., the thickness of the monofilament is about half the overall
thickness of the entire precursor element. The five elements 11
through 15 are stacked side-by-side, with the tops and bottoms of
all of the elements lying in substantially the same respective
parallel planes and the HTS precursor filaments similarly aligned.
The principal axes of the filaments are parallel to those of the
composite, and to the principal displacements and loads of
subsequent deformations. Although the composite 10 of FIG. 1
includes five side-by-side generally square precursor elements; it
will be evident that different composites may include different
numbers of precursor elements similarly arranged side-by-side, and
that the precursors may be aspected, whether of high or low
density. Typically a composite having this side-by-side precursor
element geometry will include between about 5 and about 20
precursor elements across its width; as discussed hereinafter with
respect to FIG. 9, it will be apparent that composites constructed
in accord with the present invention may include a number of
vertically stacked layers, each of which includes a number of a
side-by-side elements as shown in FIG. 1.
[0033] By way of further example, the precursor elements 21 through
25 of precursor composite 20 of FIG. 2 have been rolled so that
they have a high aspect ratio (i.e., they have a width that it at
least twice and typically several times the thickness). Before
rolling, the dimensions and density of the filaments of elements 21
through 25 were substantially the same as those of elements 11
through 15. After rolling, each of elements 21 through 25 is not
more than about 100 microns thick, the monofilaments 21a through
25a within the respective elements are not more than about 50
microns thick, and the monofilaments have a relatively high
density. As shown in FIG. 2, elements 21 through 25 are stacked
vertically, with the opposite side edges of all of the elements
generally aligned with each other, and the tops and bottoms of the
elements, and of the precursor filaments 21a through 25a of the
elements, generally parallel. The composite 20 of FIG. 2 includes
five vertically stacked elements, but it will be evident that other
composites may have different numbers. Typically, a composite
having this vertically stacked, aligned edges, precursor element
geometry will include between 2 and 10 vertically stacked
layers.
[0034] No matter what particular stack arrangement may be used, the
dimensions of the precursor elements and the arrangement of the
elements in the stack is such that the aspect ratio (i.e.,
height:width) of the stack is not greater than one (1). The stacked
elements are consolidated and thereafter thermomechanically
processed (as discussed hereinafter and typically using procedures
known in the art) to product the final desired multifilamentary
superconductor, e.g., the multifilamentary superconductor 10' shown
in FIG. 1b that is produced from a precursor composite such as
composite 10, or the multifilamentary superconductor 20' shown in
FIG. 2b that is produced from a precursor composite such as
composite 20. With a too high aspect ratio it difficult to
accomplish further processing while maintaining the integrity of
and arrangement of elements in the composite. As shown in FIGS. 1
and 2, and also as discussed in more detail hereinafter, the
elements of the respective composite are typically surrounded by
some metal, e.g., a noble metal such as Ag, elements 16, 26, that
support and locate the elements relative to each other, at least
until such time as the elements have been consolidated.
[0035] Reference is now made to FIGS. 3 and 4 which illustrate
certain aspects of the manufacture of superconductor precursor
composites such as those of FIGS. 1 and 2 from monofilament
precursor elements made according to a PIT process.
[0036] As is well-known, in PIT processes a round wire by (a)
forming a powder of the superconductor precursor material, (b)
filling a metal container such as a tube, billet or grooved sheet
with the precursor powder, and (c) deformation processing the
filled container to provide a composite of reduced cross-section
that includes a filament of the superconductor precursor in a
surrounding metal matrix. In the instant invantion, the matrix must
be a fine-grained metal, and the deformation processing typically
includes a number of successive draws in which the diameter of the
wire is reduced to, e.g., about 0.3 to 0.6 mm with a fill factor
of, nominally, 47%. The drawn wire is then annealed (e.g., at 300
C. for about 30 minutes) one or more times in the course of drawing
to recrystallize the Ag sheath, and further softened (e.g., at
above 450 C. for over a half hour). The purpose of the anneals is
to soften the noble metal without creating excessive grain growth,
thereby to maintain the fine grains of the noble metal throughout
drawing (and any subsequent rolling) of the monofilament element.
Typically, the element is annealed sufficiently frequently that the
applied strain on the element does not exceed one (1) before
another anneal is performed. Anneals are typically done in the
range of 200 to 400 C. for between 15 and 60 minutes. A method for
fabricating silver or silver alloy articles and tube stock suitable
for the matrix, particularly thin-walled tubes and other articles
having small cross-sectional areas, is described in commonly owned
application Ser. No. 08/831,504 filed Mar. 31, 1997 by Jeffrey M.
Seuntjens and entitled "Silver and Silver Alloy Articles." This
application, which is here incorporated by reference in its
entirety, discloses the making of structures that are free from
defects, have a fine grain size, are amenable to uniform
deformation, and that can be used to make superconductor
monofilaments or multifilament articles.
[0037] In the practice of the present invention it is important to
insure that the superconductor product contains fine monofilaments.
This is accomplished, in part, by the just-mentioned provision of a
fine-grained metal matrix and annealing often enough to prevent
grain growth in the noble metal. Additionally, the precursor powder
is loaded into the metal tube at low density so that, after drawing
(which itself does not significantly increase the density of the
superconductor precursor although the diameter of the precursor
filament is significantly reduced) the monofilament superconductor
precursor will have a density that is in the range of about 25% to
70% (and most preferably about 40% to about 60%) of theoretical
density. Also, so that the filaments will be not more than about 50
microns thick after they are rolled (and rolling, which increases
the density and texturing of the filaments, may be accomplished
either before or after the elements are stacked to form a
multifilamentary superconductor precursor composite), the thickness
of the drawn precursor element typically is not more than about 600
microns and that of the precursor monofilament in the element is
not more than about 300 microns.
[0038] As previously indicated, the composite precursor stack of
PIT-made elements may, as discussed above in connection with FIG.
1, be made from elements that, because they have been drawn but not
rolled, have low density HTS precursor filaments. The composite may
also, as in the case of composite 20 of FIG. 2, be made from
elements that have been rolled into thin elements 30 such as shown
in. FIG. 3 after drawing (and thus typically have filaments with
80% or more theoretical density) e.g., using rolls between 2 and 5
cm in diameter. Rolling a wire having a low density filament core
reduces the thickness of the wire by between 70% and 85%, e.g., a
drawn round wire having a diameter of about 0.3. to 0.6 mm will be
formed into an aspected element having a nominal thickness of about
0.07 to 0.10 mm and a width of about 1.5 mm. As is evident in FIG.
3, the superconductor precursor filament 32 (which remains largely
in the form of a precursor powder) within the Ag sheath 34 of
element 30 is generally flat along most of its width, but of
reduced thickness adjacent its ends. Throughout most of the width
of the element, the filament 32 has a thickness (e.g., not more
than about 50 microns) that is less than two thirds, and preferably
not more than half, that of the overall element.
[0039] The precursor elements (whether rolled or unrolled) are then
formed into a composite stack, such as the stacks shown in FIGS. 1
and 2. As previously discussed, FIG. 2 illustrates a composite
stack 40 of five precursor elements, designated 21a through 21e
(each of which is typically made in the same way as just-discussed
element 30), and the entire stack is wrapped with a thin (e.g.,
0.04 mm.times.1.5 mm) Ag element 26. The manner in which the stack
is formed (in a configuration similar to that shown in FIG. 1) is
schematically shown in FIG. 4. As there shown, lengths of the
monofilament elements 21a-21g are placed on spools, one for each
element in the composite stack 20. The spools typically are mounted
on a magnetic break stand to pay-off to a coplanar series of
idlers. A rectangular guide 44, also mounted on the stand, collates
the elements together into the stack 40. The stack formed at the
downstream side of guide 44 pays off to the center of a Turks Head
horizontal cabling line, generally designated 46. In the cabling
line, a spool of the Ag element 42 is placed on a magnetic
break-damped shaft on the rotating member of the cabling line. An
idler pulley 47 and guide die 48 guide the Ag element 26 into
side-by-side position in the stack, and the capstan 49 pulls the
stack axially at a fixed speed relative to the rotation of the
rotating member carrying the wrapping element 26 to define the
desired wrap lay pitch. Preferably, the pitch is selected so that
the wrapped element 42 does not overlap, and so that the gap
between adjacent turns is less than the element width. The Turks
Head wrapping line consolidates the stack with minimal plastic
deformation of the precursor elements. The wrapped stack is then
thermally bonded (e.g., at temperatures which, although above 500
C., are known not adversely to affect the precursor powder) to
sinter the Ag element 42 and Ag sheath 34 of the elements 30
together, thus resulting in a multifilament precursor composite,
e.g., composite 20 shown in FIG. 2 or composite 10 shown in FIG. 1,
ready for final heat treatment and deformation processing. As
previously discussed, it will be appreciated that the sheath 34 and
element 26 may be a metal other than silver. It will be noted that
the Ag wrap element 26 shown in FIG. 4 is slightly thicker (a
thickness of less than about 0.003 mm is usually preferred) than
typical; and the filaments in the particular elements 30 in the
stack 40 shown in FIG. 4 are also somewhat thicker than is
preferred for high performance.
[0040] It should also be noted that the fill factor of composite
stacks 10 and 20 is typically is more than 40%, and can be much as
50% or more. This is considerably higher than the typical 25-35%
fill factor range achieved for conventional HTS multifilament
conductors. Also, the over-wrap element 26 may be very thin, just
enough to hold the stack together until bonded in situ during heat
treatment. Unlike a standard multifilament sheath, the wrap need
not co-deform with the filaments during break-down and asymmetric
rolling. The increased fill factor does not negatively effect the
conductor stability and has benefits to increase Je and reduce
cost.
[0041] The above-described technique provides improved performance
simultaneously with reduced cost, providing significant savings in
terms of required capital equipment, labor, processing time, labor
and raw material yield; current estimates indicate an overall fifty
percent cost reduction in terms of labor and raw materials. Table
I, set forth below, compares the above-described procedure (at the
right of the Table), starting with monofilament wire and ending
with a multifilament precursor composite stack ready for final
thermomechanical processing, with a standard multifilament
processing procedure (at the left of the Table) The process set
forth in each column starts with monofilament at one or more mm
diameter cross section in coil form, and ends with a multifilament
conductor ready for heat treating. As is evident, the process of
the present invention requires far fewer steps.
1TABLE I Fine element PIT Standard multifilament PIT process
assembly process (Incoming monofilament coil) (Incoming
monofilament/ .dwnarw. fine metal coil) Monofilament hex .dwnarw.
Monofilament straighten and cut Multi-die fine wire draw
Monofilament clean with frequent anneals Multifilament can
preparation/cleaning Multifilament billet packing Roll to element
(optional Multifilament billet end cap/sealing at this time in the
process) Multifilament evacuation Multifilament hot isostatic
press/thermal bond Multifilament billet extrusion Multifilament
large rod breakdown Wire draw to diameter for start of rolling Roll
to element Respool (Multifilament composite at heat treatment)
Stack Roll (Multifilament composite at heat treatment)
[0042] Reference is now made to FIGS. 5-8 which illustrate other,
and preferred, precursor components and a procedure for forming
them.
[0043] According to the procedure of FIGS. 5-8, the precursor
elements, designated 50 in FIG. 5, are made by depositing a layer
52 of precursor powder on a fine-grained Ag or other noble metal
substrate 54, typically using either a continuous electrophoretic
coating technique such as that described in L. D. Woolf, et al.,
Appl. Phys. Let 58 (1991) 534, a slurry/dip coating technique such
as described by S. E. Dorris, N. Ashcom and N. Vasanthamohan in an
Argonne National Laboratory Dec. 1996 Superconductor Development
Quarterly Progress Report, or any of a number of other known
procedures for depositing thick films including physical film
forming methods such as sputtering or ion beam assisted deposition
(IBAD; see, e.g., U.S. Pat. No. 5,079,224) and chemical film
forming methods such as chemical vapor deposition (CVD; see, e.g.,
U.S. Pat. No. 5,231,074).
[0044] The particular technique used to form the precursor powder
layer 62 on the substrate 64 is not important. Further, and as
discussed previously, although BSCCO 2223 and YBCO 123 are
preferred, any of a large number of other ceramic superconductor
precursors, particularly members of the RBCO, BSCCO, TBSCCO and
HBSCCO families of oxide superconductor precursors, may be
employed.
[0045] As shown in FIG. 5, the Ag substrate 54 has a width (e.g.,
typically about 1500 microns) substantially equal to that of the
desired precursor element 50 but is quite thin. Typically it has an
aspect ratio (in transverse cross-section) of between 10:1 (e.g., a
thickness of about 150 microns) and 20:1 (e.g., a thickness of
about 75 microns). A precursor layer 52 is deposited on both sides,
and along the side edges, of the substrate 54. After the precursor
has been deposited, the edge portions of the coated element are
slit (e.g., along the dashed lines 56 shown in FIG. 6) and the
coated edges 58 are removed.
[0046] Although, depending on the particular deposition procedure
used, the precursor layer 52 may be somewhat thicker near the edges
of the element, the layer generally has essentially uniform
thickness both across and along the length of the Ag substrate 54.
The particular thickness chosen will depend on a variety of
factors, including the particular precursor employed, the manner in
which the precursor layer is formed and the resulting density of
the layer, the manner in which the precursor will be
thermomechanically processed, and the intended use of the end
product multifilamentary superconductor. By way of example,
although the thickness of the precursor layer 52 typically will be
in the range of about 10 to about 100 microns, it may range from as
thin as 5 to as thick as 1000 microns. In the preferred practice in
which the precursor layer comprises a BSCCO-2223 precursor
deposited at a density of about 40% of theoretical density, a
typical thickness is about 15 to 20 microns.
[0047] FIG. 6 illustrates a composite stack 60 of four elements,
designated 50a through 50d, positioned in a fine-grained metal,
e.g., Ag, trough 62 the top of which is covered with a fine-grained
metal, again typically Ag, foil 64. It will be noted that the four
elements provide a total of eight filament precursors. Within the
trough, the adjacent elements 50 are separated by a total of three
fine grain metal, e.g., Ag, spacer strips 66. The complete stack
structure is heated (as in the case of composite 20 previously
discussed to temperatures which will not adversely affect the
precursor) to bond the elements, intermediate strips, trough and
covering foil together. In a slightly modified procedure, an
ensemble of coated precursor-Ag strip pairs may inserted into an
appropriately dimensioned Ag tube, which is then heat treated to
bond the components together although the quality of the thermal
bond is less critical. In either procedure, the bonded
eight-filament composite precursor structure 670 is ready for final
heat treatment and deformation processing. It will be noted that
FIG. 6 is a broken view, and thus does not show the full width of
composite 60. As previously indicated, the full width of the
composite is at least as great as its full width, and typically is
considerably greater.
[0048] FIG. 7 illustrates a composite 70 of fifteen elements,
designated 50a through 50e, 50a' through 50e', and 50a" through
50e', arranged in three layers, designated 56, 56' and 56"
respectively. Each layer includes five elements placed in a
side-by-side configuration. The entire composite is wrapped with a
metal foil layer 72. It will be noted that each element provides
two precursor filaments, and that the fifteen elements thus provide
a composite having a total of thirty precursor filaments; by way of
comparison, a similar arrangement of elements formed by a PIT
procedure would, as should be evident from the prior discussions,
provide only half as many precursor filaments. In each layer of the
composite 70, a metal, e.g., Ag, spacer 74 is placed between the
adjacent sides of each pair of elements 50.
[0049] Reference is now made to FIG. 8 which illustrates, somewhat
schematically, a complete process for manufacturing a
multifilamentary superconductor composite according to this aspect
of the present invention. In general, a precursor production
station, generally designated 80, is provided to make each of the
desired number of individual precursor elements 50; although only
one station 80 is shown, the total number of stations will of
course depend on the number of elements that are used to form the
complete composite. At each of the stations, a fine grain Ag
substrate strip 54 is passed through a slurry bath 82 to deposit
the precursor layer 52 on the substrate and form the element 50,
the element is then sintered by heater 83 and, after its edge
portions have been removed, a fine grain Ag spacer strip 76 is
placed in position, either on one or both of the top and bottom
surfaces of the element 50 in a vertical stack, or between adjacent
elements 50 in a side-by-side stack. The "sandwich" 84a formed by
the elements and spacers is then advanced to the composite
production station 86.
[0050] At the inlet to composite production station 86, element
sandwich 84a is juxtaposed with essentially identical element
sandwiches 84b, 84c, 84d and 84f (it will be recognized that when
the elements are vertically stacked, each element sandwich will
require only a single spacer strip, on the underside of the
element; when they are stacked horizontally side-by-side, a spacer
is needed only between adjacent elements) and passed through
grooved inlet rolls 86 which stack and align the element
sandwiches. The resulting multi-element structure is then placed in
a fine grain conduit trough 72, the trough 72 with the elements 50
and spacers 76 therein is covered with a fine grain foil 74, and
the entire structure is then consolidated by rolls 88 and heat
bonded by heater 89, thus producing the bonded multifilament
composite precursor structure 70 shown in FIG. 7, ready for final
heat treatment and deformation processing.
[0051] As schematically shown in FIG. 8, the precursor structure
60, 70 is thermomechanically processed, i.e., is subjected to the
desired number of deformation (e.g., by rolling to impart
deformation-induced texturing) and sintering (e.g., by heating to
impart reaction-induced texturing) steps, to develop the desired
density and degree of texture in the final multi-filament
superconductor, which is then wound onto a take-up roll 100. As
discussed in somewhat more detail hereinafter, the other precursor
composites discussed herein are is similarly thermomechanically
processed to produce, e.g., superconductor composites such as those
shown in FIG. 1 and 2.
[0052] Table II below compares, in somewhat more general terms than
Table I previously discussed in connection with the procedure for
making composite precursors using a PIT procedure, the above
described coated conductor procedure for forming a multifilamentary
superconductor composite with a conventional procedure for forming
a multifilamentary PIT structure. As will be evident, the coated
conductor composite precursor procedure, to an even greater degree
than procedures according to the present invention utilizing
PIT-based monofilament elements, provides significant savings.
2TABLE II FINE ELEMENT COATED STANDARD POWDER PRECURSOR IN TUBE
Prepare Slurry of HTS Prepare HTS Powder Deposit and sinter HTS on
Ag Pack HTS Powder in Ag Tube Substrate Draw Wire (more than 10
passes) Wrap HTS coated elements Restack Multiple Wires into Tube
Consolidate Draw Multifilamentary Wire to Smaller Diameter (more
than 10 passes) Roll Composite (1-5 Passes) Roll Composite (1-5
passes)
[0053] It should be particularly noted that the deposited/coated
conductor procedure shown in FIG. 8 permits a substantially
continuous manufacturing procedure, starting with rolls of the fine
grain Ag or other metal substrate used for the individual elements
64, and ending with a reacted composite multifilamentary
superconductor composite. Additionally, and largely because the
composite structure 70 formed using these deposited or coated
conductor techniques are not but drawn but are only rolled, the
high silver (or other metal) content required to support the draw
loads and forces employed in PIT procedures is not required. As a
result, the substrates 64 and conduit trough 72 can be quite thin,
and the total amount of silver silver or other metal used in the
product significantly reduced. This permits further increase in the
fill factor, which typically is about 30% and may be as high as 40%
or more.
[0054] No matter what type of precursor element is employed, it
will be apparent that the process of the present invention also
allows a manufacturer to choose a composite Ic by varying the
number of strands assembled in the composite. This offers great
flexibility to the manufacturer, since only one type of strand
(monofilament) needs to be inventoried to be responsive to a wide
variety of commercial conductor needs. It also allows for
composites with different Ic's to be fabricated from common strands
and similar strand deformation paths. By way of comparison, in
standard multifilament composite processes, the manufacture must
have several different multifilament billet configurations,
developed for different applications, and must maintain inventories
of various multifilament composites at intermediate or final
processing stages. To change the Jc in a conventional process, one
or more of the billet size, monofilament restack size, filament
count and multifilament design, or total multifilament strain, must
be varied. According to the preset invention, substantially all of
this may be accomplished simply by varying the manner in which
common strands are stacked in composites and thereafter deformed
and heat treated.
[0055] As an example, a conductor Jc requirement may be 140 A
minimum for the composite cable. Due to winding requirements in the
application, the maximum composite thickness may be 0.2 mm,
noninclusive of any lamination material. The optimal monofilament
strand dimension may be nominally 0.1 mm thick and 1.5 mm wide and
the reproducible, long-length Je level may be 12,000 A/cm.sup.2.
For these dimensions, this Je level equates to an Ic of 18 A/strand
in the composite. Therefore, a configuration of 4 strands wide by 2
strands thick yields an Ic of 144 A in a nominal 0.2.times.6 mm
cross-section, not including strengthening members.
[0056] In such a conductor, in which the precursor elements from
which the composite conductor is made are placed in a both
horizontally and vertically stacked configuration, the elements
typically will be stacked in the manner previously adverted to in
connection with the FIGS. 1 and 2, i.e., each horizontal row or
layer of elements will positioned so that the filaments of the
elements in the different rows are vertically aligned. It is also
possible to provide a configuration in which, similar to the
placement of bricks in a brick wall, the filaments in the elements
in the different rows are not all vertically aligned. A stack in
which the elements are arranged in such a "brick wall"
configuration provides greater stability and permits the stack to
have a higher aspect ratio that is usually otherwise desirable.
[0057] FIG. 9 illustrates such an alternative arrangement. The
composite 90 of FIG. 9 includes a total of fifteen (15) precursor
elements 92, each of which is substantially the same as an element
30 in FIG. 3, although it will be recognized that elements such as
those shown in FIGS. 1, 6 or 7 may be used also. The elements 92
are arranged in three vertically stacked layers, designated layers
a through c respectively, although it will be recognized that a
stack including more than three layers may also be used. As shown,
each layer includes five precursor elements 92 arranged in a
side-by-side configuration similar to that of the elements in the
composite 10 of FIG. 1. The respective elements 92a in layer a and
92c in layer c are vertically aligned with each other. The elements
92b in layer b are offset relative to elements 92a and 92c. That
is, the elements in the various layers of the precursor composite
are positioned so that the precursor HTS filaments in different
vertically spaced layers are either substantially vertically
aligned with each other (as are the filaments in layers a and c) or
the adjacent layers are positioned such that the center of the
relative thin space between adjacent filaments in a layer (i.e.,
the thin metal sheath or spacer between monofilaments) is placed in
vertical alignment (above or below as the case may be) with the
center of the filaments in the adjacent layers; e.g., the Ag
between adjacent filaments in layer b is aligned with the centers
of the filaments in layers a and c, and visa versa.
[0058] As discussed in aforementioned application Ser. No.
08/468,089, there are a number of known procedures for the
thermomechanical processing of superconductor precursors; these
typically involves repeated steps of deforming and sintering, at
varying pressures and temperatures depending on the particular
employed. These processes are often 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 and 3DS processes.
[0059] Although the particular thermomechanical processing
procedure is not critical to the practice of the present invention,
the preferred practice is to use a deformation/sintering iteration
process that uses as few iterations as possible, i.e., a 1DS
process such as described in the '089 application, or if that
cannot be done a 2DS process. In the preferred IDS practice, the
bonded multifilament composite precursor structure, e.g., a
composite of low-density monofilaments produced using PIT
procedures or a composite of low density monofilaments made using
coated conductor procedures, is roll worked in a high reduction
draft in the range of about 40% to 95% in thickness. There is no
further reduction in thickness in excess of about 5% after the high
reduction draft step and prior to a sintering step, and the
thus-rolled composite structure is then intered to obtain the final
oxide superconductor composite multifilamentary product. Similar
reductions in thickness are achieved using a 2DS practice. If the
composite is made of monofilament elements which have been rolled,
and thus have a high rather than a low density before they are
formed into a composite, subsequent thermomechanical processing of
the composite will result in a much smaller reduction in thickness.
However, and no matter what types of precursor elements are
employed, the present invention permits the processing of the
individual precursor elements to be decoupled from, and to be
accomplished in a manner that is largely independent of, later
processing of the consolidated multi-element precursor
composite.
[0060] Used with superconductor precursor composite in which the
monofilament superconductor precursors have a low density, such a
single high draft deformation greatly decreases the thickness of
the superconductor precursor in the composite 40, 70, e.g., to a
thickness of 1 to 3 microns, and in combination with the sintering
step also increases the density to as much as 95% theoretical
density. As shown in the graphs of FIGS. 10 and 11, discussed in
more detail below and which show the relationship between Jc and
filament thickness, the Jc of a superconductor filament is highly
dependent on filament thickness; all else being equal, a thinner
filament will have a far greater Jc than a thicker one. In large
measure because the present invention enables the production of
composite structures with very thin (e.g., less than 3 microns
thick) monofilaments, it is anticipated that BSCCO-2223 composite
structures made according to the present invention will achieve Jc
levels in excess of 100,000 A/cm2 (77K, 0 T) .
[0061] In the preferred practice of the invention, the
superconductor precursor composite is constrained during rolling by
positioning it between two elongated constraining membered in
contact therewith on opposite sides of the superconductor precursor
composite. The composite, together with the two constraining
members in contact therewith, is passed through rollers to form a
textured superconductor composite. The rolling assembly for
single-pass rolling of a precursor superconductor composite tape
includes first and second rolls and two constraining members. The
first roll is aligned to rotate about a first axis, and the second
groove roll is aligned to rotate about a second axis parallel to
the first axis. The rolls are spaced apart a selected distance to
form a passage therebetween. The two constraining members have
mechanical properties similar to that of the precursor
superconductor composite and are placed in contact with the
precursor superconductor wire on opposite sides of the precursor
superconductor when the precursor superconductor composite is
passed through the rollers to form the textured superconductor
composite. A fixture for feeding the precursor superconductor
composite and the constraining members between the first and second
rolls includes a slot dimensioned to receive the precursor
superconductor composite and the constraining members. The fixture
includes a tapered surface for guiding the precursor superconductor
composite and the constraining members between the first and second
rolls.
[0062] Constrained rolling permits deformation with large area
reduction while improving the filament quality as compared to other
deformation techniques. Constrained rolling particularly
facilitates the texturing of superconductor composites which are
sensitive to cracking and/or shearing during deformation, and
provides increased dimensional control over that of other
techniques. The procedure is described with more particularity in
commonly owned, co-pending applications Ser. Nos. 08/902,587 and
08/902,588, both filed by DeMoranville et al. on the same day as
this application and respectively entitled "Constrained Rolling and
Superconductor Wire Formed Thereby and " Constrained Rolling and
Textured Superconductor Wire Formed Thereby; both such applications
are hereby incorporated by reference in their entirety.
EXAMPLE I
[0063] To examine the relationship between thickness and Jc for
monofilaments made using a PIT procedure, monofilament BSSCO-2223
wire made with a fine grain silver tube manufactured in accordance
with the "Silver and Silver Alloy Articles" patent application
referred to above was drawn to nominally 0.6, 0.45 and 0.3 cm
diameter and annealed at 300 C. for 30 minutes at strain increments
less than 1.0 to recrystallize the Ag sheath. Rolling studies using
2 and 10 cm diameter rolls were performed to various thickness
reductions from about 70% to about 90%. FIG. 10 in which the legend
for the symbols defines the roll diameter/monofilament wire
diameter in inches, shows the result of the study. The Je results
on the 2 and 10 cm rolls correlate to the filament thickness,
despite the different widths caused by the different roll
diameters. Je as high as 14,500 A/cm2 were obtained.
EXAMPLE II
[0064] As part of a program directed to the development of cables,
the relationship between the final thickness of the superconductor
filaments in the cable and the Jc of the filaments was examined.
FIG. 11 is a graph of Je (77K, self field, 1 microvolt/cm) vs.
filament (both monofilament strand and multifilament strand)
thickness. As with Example I, Je was shown to be highly dependent
on, and greatly to increase with decreases in, filament
thickness.
EXAMPLE III
[0065] Using a non-optimized Bi-2223 powder, a coated element
precursor was fabricated using electrophoretic deposition. The
thickness of the Bi-2223 precursor was about 20 microns with good
dimensional uniformity across the width and along the length of the
fine grain Ag strip. The density of the precursor was estimated to
be .about.40% theoretical. Twenty 12" lengths of the coated
precursor were assembled and stacked them on top of each other,
separating each one with fine grain Ag strip. This multifilamentary
stack was then placed into a rectangular fine grain Ag trough,
covered with fine grain Ag strip and heated to diffusion bond the
elements of the composite to each other. The composite was then
rolled to achieve a high level of density and texture within the
ceramic filaments. The filaments had thicknesses in the 1 to 5
micron range. In a first set of heat treatments, a Jc value of
20,000 A/cm2 (77K, 0 T) was achieved.
EXAMPLE IV
[0066] The effects of densification on a composite including
approximately 15 micron thick HTS layers (density substantially
equal to 30% theoretical) on opposite sides of a silver alloy
substrate approximately 75 microns thick were calculated. If all
deformation of such a composite during subsequent thermomechanical
processing is densification, the thickness of the HTS layers will
be reduced to approximately 5 micron.
EXAMPLE V
[0067] An eighteen (18) filament ultrafine HTS filament composite,
having BSCCO-2223 superconductor filaments and fine grain AG
substrate and spacers, was made using a coated conductor process
generally as discussed above with reference to FIGS. 6-8. The
composite had a Jc of 20,000 A/cm2 and an Ic of 20 A (77K, 0
T).
EXAMPLE VI
[0068] Square monofilament elements including fine grain silver
were fabricated using drawing to 0.0239". To achieve finer filament
dimensions, the element was subsequently Turk's Headed to 0.018"
square. Eleven elements were aligned consistent with FIG. 1 and
consolidated using a thermal process. The consolidated precursor
was rolled in one pass using side constraining wires of 0.021"
resulting in thin HTS filaments (,50 microns). The Je of this
material was 6,700 A/cm2 (77, sf).
EXAMPLE VII
[0069] The same procedure was followed as in Example VI, except
drawing was used to fabricate 0.018" square elements. The Je was
7,500 A/cm2.
EXAMPLE VIII
[0070] The same procedure was followed as in Example VII as Example
VII except consolidation was carried out using a thermomechanical
process that included drawing and thermal treatments.
EXAMPLE IX
[0071] Rectangular monofilament elements were fabricated using
drawing, and the elements were aligned consistent with FIG. 2 and
consolidated using a thermomechanical process that included drawing
and thermal treatments. The consolidated precursor was rolled in
one pass resulting in thin HTS filaments (m10 microns). The Jc of
this material was 42,000 A/cm2 (77, SF).
EXAMPLE X
[0072] The same procedure was followed as in Example IX, except
consolidation was carried out using a mechanical process that
included drawing. Anneal temperatures were kept less than 300
C.
EXAMPLE XI
[0073] The same procedure was followed as in Example X, except
consolidation was carried out using a heat treatment at 785 C. in
0.075 atm 02 for 1 h.
EXAMPLE XII
[0074] A coated precursor was fabricated by dip coating a buffered
(YSZ) fine grained Ni substrate with YBCO. The coated precursor
element was calcined giving a coating density less than 70%. Two
precursor elements were stacked according to FIG. 2 and wrapped
with an Ag alloy. The precursor composite was consolidated using a
thermal process.
EXAMPLE XIII
[0075] Lengths of rolled monofilament tapes were placed on spools,
one for each strand in the stack. These spools were mounted on a
magnetic break stand to pay-off to a co-planer series of idlers. A
rectangular guide, also mounted on the stand, collated the tapes
together into stack. The stack of tape strands paid-off through the
die to the center of the horizontal cabling line, which was set up
to over-wrap the stack. A spool of think (,0.05 mm) fine Ag tape
was placed on a magnetic break-damped shaft on the rotating member.
An idler puller guided the Ag foil wrap onto the stack. The capstan
pulled the stack fixed ratio to the wrap rotation to define a fixed
wrap lay pitch.
[0076] The Ag wrap material was made by rolling annealed Ag wire
under high tension. The wrap material was typically 0.04
mm.times.1.5 mm cross section. Reconfiguration of the horizontal
cabling line to a wrapping line was straightforward.
[0077] The system was configured for 7 monofilament tapes and the
line ran without incident. The monofilament tape back-tension was
adjusted to be just high enough to fabricate a tight wrap
(.about.0.7N). The first run was not optimal in that the wrap was
0.05 mm thick and was placed with nominally a 50% gap between turns
of the wrap. Due to the higher than optimal wrap thickness, the
wrap remained oval shaped in the product. The wrap could be
consolidated by turk head with minimal plastic deformation on the
monofilament tapes. Short lengths of the stack cable were used in a
mini optimization study investigating the first heat treatment
(HT1) conditions and intermediate strain for a fixed final heat
treatment. FIG. 12, which is a 3-D plot, shows the optimization
variables: HT1 time (20 and 40 hr.), HT1 temperature (820.degree.
and 827.degree. C.), and intermediate rolling strain (10% and 20%)
make up the three axis. The corners of the cube contain the final
Je data for the appropriate condition.
[0078] An automated dancer-cassette take-up system was built to
provide for take-up with uniform and low tension. As shown
schematically in FIG. 13, the system consists of a moving pulley
131 on linear bearings connected to dead-weight, 132 and a
motorized drive. The motor drive is controlled by limit switches
133, 134 at the tope and bottom of the pulley travel. The cassette
take-up eliminates hard-way bends in the tape due to spool traverse
and has a large hub diameter to minimize bend strain. The unit is
mounted on a cart for potential use with any rolling mill.
[0079] The respooling process and the pay-off path on the stacking
line were also modified to reduce bend strain. The Ag wrap
thickness was reduced to 0.03 mm to reduce the need for
back-tension during the stacking process to obtain a tight
wrap.
[0080] A multistrand superconductor was made with these
improvements. The rolling mill take-up system worked well at loans
of 0.98N on the tapes (200 g mass load on the moving pulley) at
speeds over 5 m/min. It is thought that the machine is capable of
0.24N tension at high speeds. Monofilament samples taken from the
take-up cassette did not show transverse cracking. The Je levels of
these sample achieved 5000 A/cm.sup.2 at 77K in self field.
[0081] It will be recognized that the structures of the
superconductor precursor monofilaments and of the superconductor
precursor composites are important to the practice of the present
invention. For the final superconductor product to have the desired
high performance, each of its superconductor filaments should be of
essentially fine and uniform dimensions, preferably substantially
rectangular, and it is also highly desirable that the dimensions
and characteristics of different filaments in the multi-filamentary
product themselves be essentially the same. According to the
present invention, this is accomplished by providing precursor
elements that have substantially the same fine dimensions and
characteristics, and then assembling the elements into rectangular
(including square) in transverse cross-section composite stacks.
The configuration of the stack relative to the deformation forces
insures that during subsequent thermomechanically processing, when
the stack is passed between a pair of rolls or the like that apply
force generally vertically (i.e., perpendicular to the width and
parallel to the height) of the composite stack, each filament in
the stack will be subjected to substantially similar deformation
processing thus providing a multifilamentary superconductor in
which the all of the filaments have the desired, and substantially
the same, superconducting properties. In addition, the principal
axes of the precursor filaments and the stacks should be parallel
to those of the precursor and product. As previously indicated,
superconductor precursor monofilaments are arranged in the
composite stacks with the filaments aligned and the tops and
bottoms of the filaments generally parallel; and a metal layer is
provided between each pair of filaments and, typically, surrounding
the exterior of the entire composite stack. The metal layer may be
provided by, e.g., an Ag sheath 34, an Ag substrate 64, Ag wrap 42,
trough 70, foil 74 and/or a spacer strip.
[0082] As previously discussed, the precursor elements may be
stacked in a single vertical (as shown in FIGS. 2 and 6), or a
single horizontal (as shown in FIG. 1) row. A number of vertical
stacks also may be placed side by side, e.g., to produce an
arrangement in which the filaments are relatively located as shown
in FIG. 7, or a number of horizontal side-by-stacks may be placed
one on top of the other to provide structures such as those shown
in FIGS. 7 and 9. In each event, it will be noted that the
filaments of the composite are located so that, when pressure is
applied in a generally vertical direction during later
thermomechanical processing, each filament will be subjected to
substantially the same forces and deformation. It will also be
noted, as discussed above, that the width of each composite is no
less than the composite height to insure stability during
subsequent processing.
[0083] No matter what particular precursor is employed, the
following criteria are important for high performance:
[0084] 1. Appropriate Dimensions--The filament dimensions should be
generally uniform throughout the composite. This requires the
filaments in the superconductor precursor elements similarly to
have generally uniform dimensions (both within any particular
element and from one element to another) and further HTS filaments
in the final product to have a high aspect ratio. The particular
dimensions of a particular precursor will, of course, depend on
such things as overall structure of the precursor composite and the
particular nDS procedure used to thermomechanically process the
precursor composite. Typically, an unrolled precursor element will
be between 25 and 1250 microns thick, preferably between 30 and 600
microns thick, and, in most preferred embodiments, between 30 and
250 microns thick. The width of the precursor composite may vary
widely. Although it is always desirable that the composite width be
at least as great as the height, it may also be many times wider,
e.g., the overall width may be as much as 25000 microns and in many
circumstances will be 2500 microns or more. As discussed above, the
thickness of the metal, typically Ag, layers in the precursor
elements and precursor composites is less critical; typically
thicker layers are needed in elements used in PIT procedures for
reasons previously discussed.
[0085] 2. Density--Before rolling, the density of the precursor
filaments should be low, in the range of about 25% to about 70%,
preferably in the range of 30% to 65% and, particularly when the
precursor filaments of the precursor elements are made using a
deposition/coating procedure in the range of 35% to 60%. Providing
a low density precursor filament with a low degree of texturing
optimizes characteristics of the superconductor filaments produced
during subsequent thermomechanical processing.
[0086] 3. Thickness--The precursor monofilaments should be thin.
After rolling in a PIT process, a precursor filament should not be
more than about 50 microns thick. Deposited superconductor
precursors may be significantly thinner. In particular, for high
performance, each superconductor filament in the final product
should not be more than about 10 microns thick (and preferably 2-7
microns thick), although as discussed above thicker filaments
(e.g., not over 50 and preferably not more than 40 microns thick)
may be used in appropriate circumstances)). The width of the final
superconductor structure will typically be such greater than the
thickness of any HTS filament.
[0087] 4. Dimensional Uniformity--Both the precursor filaments and
precursor elements should be substantially rectangular and
generally uniform along the length of the precursor, as well as
generally uniform across the width of the filament and element. In
particular, the principal axes of the filaments and the precursor
stacks should be parallel to those of the precursor and
product.
[0088] 5. Fill Factor--Precursor elements, particularly those made
using a deposition/coating process rather than with a power-in-tube
technique, typically include a high percentage of superconductor
precursor filament relative to the amount of noble metal. This
leads to a higher fill factor in both the composite precursor and
in the final superconductor composite. The achievable fill factor
is further increased because the composite precursor requires less
noble metal, typically Ag, fill than a conventional
multifilamentary cable and, to an even greater degree, in the case
of deposition/coated precursor elements, because the amount of
noble metal required for structural support during drawing is
greatly decreased.
[0089] 6. The metal component, e.g., the sheath, spacer and/or
foils, should be a fine grained deformable metal at least through
the stacking step. As discussed above, the metal should be annealed
frequently during drawing under conditions selected to maintain the
fine grain size. Fine grain metals are required for fabrication of
high performance superconducting elements because they allow the
formation of the fine filament sizes associated with high Jc. In
PIT processes, they allow the fabrication of fine, uniform tubes
that encase the precursor powder. In coated conductor processes,
fine grain metals are needed to make uniform and fine dimension
substrates for improved fill factors.
[0090] The manner in which the precursor composite is constructed
using the monofilament precursor elements is also important.
Important criteria include:
[0091] 1. Forming and arranging the elements relative to each other
so that every monofilament in the multifilament composite can be
subjected to substantially the same pressure/deformation conditions
during later thermomechanical processing.
[0092] 2. Insuring that a fine grain metal, e.g., a noble metal
such as silver, layer is provided between adjacent HTS filaments.
The thickness of this layer is controlled so that it is not
significantly greater than required to provide sufficient
structural integrity during processing.
[0093] 3. Insuring that the configurations of both the individual
elements and that of the composite provide a high fill factor. This
is a function of both the relative metal/HTS material in the
individual elements, and the extent to which any additional metal
is used in forming the composite. The precursor elements of the
present invention, particularly those made using a
deposition/coating process rather than with a power-in-tube
technique, typically include a high percentage of superconductor
precursor filament relative to the amount of metal. This leads to a
higher fill factor in both the composite precursor and in the final
superconductor composite. The achievable fill factor is further
increased because the composite precursor requires less metal,
typically Ag, fill than a conventional multifilamentary cable and,
to an even greater degree, in the case of deposition/coated
precursor elements, because the amount of metal required for
structural support during drawing is greatly decreased.
[0094] It also is often desirable that the precursor composite,
often after appropriate treatment after the composite is formed but
before thermomechanical processing, have a relatively low volatile
content so that blisters and other defects do not form during heat
treating.
[0095] It will be apparent to those skilled in the art that the
methods and advantages of the present invention are capable of
being used in multifilamentary superconducting articles having a
variety of configurations and compositions, including both
superconductor precursors and superconducting ceramics now known
and preferred and those that will be hereafter discovered and
developed. The invention is not intended to be limited by any of
the particular description and examples set forth above, which are
set forth in the specification for purposes of illustration only,
and other structures, methods and embodiments will be within the
scope of the claims.
* * * * *