U.S. patent application number 10/115451 was filed with the patent office on 2003-05-15 for superconducting composite with high sheath resistivity.
Invention is credited to Christopherson, Craig J., Mason, Ralph L., Otto, Alexander, Roberts, Peter R..
Application Number | 20030091869 10/115451 |
Document ID | / |
Family ID | 22118618 |
Filed Date | 2003-05-15 |
United States Patent
Application |
20030091869 |
Kind Code |
A1 |
Otto, Alexander ; et
al. |
May 15, 2003 |
Superconducting composite with high sheath resistivity
Abstract
A superconducting article having a high bulk sheath resistivity,
and methods of manufacture of such an article. High-temperature
superconductor filaments are disposed in a ductile matrix
comprising a high silver content. The matrix is then coated with a
solute and heated to a temperature high enough to allow the solute
to diffuse into the matrix, but not high enough to allow
substantive degradation or poisoning of the superconductor. After
diffusion and cooling, the matrix comprises a silver alloy having a
higher bulk resistivity than the pure silver.
Inventors: |
Otto, Alexander;
(Chelmsford, MA) ; Mason, Ralph L.; (Ashland,
MA) ; Christopherson, Craig J.; (Worcester, MA)
; Roberts, Peter R.; (Groton, MA) |
Correspondence
Address: |
Elizabeth E, Nugent
Choate, Hall & Stewart
Exchange Place
53 State Street
Boston
MA
02109
US
|
Family ID: |
22118618 |
Appl. No.: |
10/115451 |
Filed: |
April 3, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10115451 |
Apr 3, 2002 |
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09782703 |
Feb 13, 2001 |
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09782703 |
Feb 13, 2001 |
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09240998 |
Feb 1, 1999 |
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6188921 |
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60074258 |
Feb 10, 1998 |
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Current U.S.
Class: |
428/697 ;
204/192.24; 205/51; 427/374.1; 427/376.2; 427/383.1; 427/428.06;
427/428.17; 427/428.2; 427/430.1; 427/62; 428/702; 505/236;
505/470 |
Current CPC
Class: |
Y10S 505/704 20130101;
H01L 39/248 20130101 |
Class at
Publication: |
428/697 ;
427/376.2; 427/383.1; 427/428; 427/430.1; 505/470; 428/702;
505/236 |
International
Class: |
H01L 039/00; B05D
005/12; B05D 003/02; B05D 001/28; B05D 001/18; C25D 001/00; H01L
039/24; B32B 019/00; B32B 009/00; C23C 002/00; B23K 001/00; B32B
001/00; H01F 006/00; H01B 012/00 |
Claims
What is claimed is:
1. An oxide superconductor article, comprising: at least one oxide
superconducting member in a silver-containing matrix, the matrix
having a bulk resistivity greater than 3 .mu..OMEGA.-cm at a
temperature of 77K, and a tensile fracture strain of greater than
0.5%, wherein no metallic constituent of the matrix has a boiling
point of less than 380.degree. C. at 1 atmosphere pressure.
2. The oxide superconductor article of claim 1, wherein the matrix
comprises a silver-rich solid solution of silver with at least one
other element selected from the group consisting of gallium, tin,
cadmium, zinc, indium, and antimony.
3. The oxide superconductor article of claim 1, wherein the matrix
comprises a silver-rich solid solution of silver with at least one
other element having a diffusivity in silver sufficient to allow
the element to diffuse into the silver in less than twenty hours at
a temperature of less than 550.degree. C.
4. The oxide superconductor article of claim 1, wherein the matrix
comprises a silver-rich solid solution of silver with at least one
other element having a diffusivity in silver of at least 10.sup.-12
cm.sup.2s at a temperature of less than 550.degree. C.
5. The oxide superconductor article of claim 1, wherein the oxide
superconducting member comprises BSCCO 2223 phase.
6. The oxide superconductor article of claim 1, wherein the oxide
superconducting member comprises BSCCO 2212 phase.
7. The oxide superconductor article of claim 1, wherein the oxide
superconducting member comprises a member of the YBCO family of
oxide superconductors.
8. The oxide superconductor article of claim 1, wherein the matrix
has a grain size of less than 50 .mu.m.
9. The oxide superconductor article of claim 1, wherein the matrix
has a grain size in the range of 0.1-15 .mu.m.
10. The oxide superconductor article of claim 1, wherein the matrix
comprises a silver-rich solid solution of silver and at least one
solute element, where the solute element comprises at least 2 atm %
of the bulk matrix composition.
11. The oxide superconductor article of claim 1, wherein the matrix
comprises a silver-rich solid solution of silver and at least one
solute element, where the solute element comprises at least 4 atm %
of the bulk matrix composition.
12. The oxide superconductor article of claim 1, wherein the matrix
comprises a silver-rich solid solution of silver and at least one
solute element, where the solute element comprises between 4 atm %
and 50 atm % of the bulk matrix composition.
13. The oxide superconductor article of claim 1, wherein the matrix
comprises a silver-rich solid solution of silver and at least one
solute element, where the solute element comprises between 4 atm %
and 18 atm % of the bulk matrix composition.
14. The oxide superconductor article of claim 1, wherein the bulk
matrix resistivity is greater than 5 .mu..OMEGA.-cm.
15. The oxide superconductor article of claim 1, wherein the bulk
matrix resistivity is in the range of 5-25 .mu..OMEGA.-cm.
16. The oxide superconductor article of claim 1, wherein the matrix
has a tensile fracture strain of greater than 1%.
17. The oxide superconductor article of claim 1, wherein the matrix
has a bend fracture strain of greater than 0.5%.
18. The oxide superconductor article of claim 1, wherein the matrix
has a bend fracture strain of greater than 1%.
19. The oxide superconductor article of claim 1, wherein the
article has an engineering critical current density of at least
3,000 A/cm.sup.2 at a temperature less than or equal to 90K.
20. The oxide superconductor article of claim 1, wherein the oxide
superconducting member has a critical temperature of at least
70K.
21. A method of preparing an oxide superconductor article having a
high resistivity sheath, the method comprising: after formation of
a final oxide superconductor phase in an article comprising an
oxide superconductor member in a silver-containing matrix, coating
the oxide superconductor article with a solute capable of forming a
silver-rich solid solution; heating the solute-coated composite
article to a temperature at which formation of a solute-silver
solid solution is favored, the temperature being below the boiling
point of the solute at one atmosphere pressure; maintaining the
composite at said temperature for a time sufficient to diffuse the
solute into the matrix and form the solute-silver solid solution;
and cooling the composite article to a temperature at which
substantially no further diffusion occurs, the cooling occurring at
a rate selected to kinetically substantially prevent the formation
of second phases in the matrix, wherein the matrix of the cooled
article is substantially free of second phases and has a
resistivity greater than the resistivity of the matrix before
coating.
22. The method of claim 21, wherein the heating temperature is
further selected such that formation of second phases is not
favored at the heating temperature.
23. The method of claim 21, wherein the solute element has a vapor
pressure of less than 0.1 atmospheres at the heating
temperature.
24. The method of claim 21, wherein the solute element is capable
of forming a second phase with silver which is thermodynamically
stable at room temperature; and cooling is sufficiently rapid that
said second phase is substantially kinetically prevented from
forming.
25. The method of claim 21, wherein said solute is further
characterized in that the solute has a diffusity in silver of at
least 10.sup.-12 cm.sup.2/s at a temperature less than 550.degree.
C.
26. The method of claim 21, wherein the composite is maintained at
the heating temperature for a period of no more than 20 hours.
27. The method of claim 21, wherein the solute is a metal selected
from the group consisting of gallium, tin, cadmium, zinc, indium,
and antimony.
28. The method of claim 21, wherein the solute is gallium.
29. The method of claim 28, wherein the article is heated to a
temperature between about 380.degree. C. and about 520.degree.
C.
30. The method of claim 28, wherein the concentration of gallium in
the solid solution is in the range of 2 to 18 atm %.
31. The method of claim 21, wherein the oxide superconductor member
comprises the desired superconductor phase.
32. The method of claim 21, wherein at least one of the cooling
step and the heating step is carried out at a rate greater than
1.degree. C./min.
33. The method of claim 21, wherein at least one of the cooling
step and the heating step is carried out at a rate greater than
10.degree. C./min.
34. The method of claim 21, wherein at least one of the cooling
step and the heating step is carried out at a rate greater than
20.degree. C./min.
35. The method of claim 21, wherein the rate of cooling is
sufficiently rapid to prevent dwell time in a temperature range
which favors the formation of a second phase sufficient to form a
significant amount of the second phase.
36. The method of claim 21, wherein the metal coating is applied to
the superconductor article by one of chemical vapor deposition,
physical vapor deposition, dip coating, roll coating, gravure roll
printing, doctor blading, stamping, sputtering, electrochemical
deposition, laser ablation and plasma spraying.
37. The method of claim 21, wherein the metal coating is applied to
the superconductor article by dip coating.
38. The method of claim 21, wherein the metal coating is applied to
the superconductor article by roll coating.
39. A method of preparing an oxide superconductor article having a
high resistivity sheath, the method comprising: after formation of
a final oxide superconductor phase in an article comprising an
oxide superconductor member in a silver-containing matrix, exposing
the oxide superconductor article to an environment at an elevated
temperature, the environment containing a solute element capable of
forming a silver-rich solid solution under conditions that favor
diffusion of the solute element into the silver-containing matrix,
the elevated temperature being less than the boiling temperature of
the solute element at one atmosphere pressure; maintaining the
oxide superconductor article in the environment for a time
sufficient to diffuse the solute into the matrix and form the
silver-rich solid solution; and cooling the composite article to a
temperature at which substantially no further diffusion occurs, the
cooling occurring at a rate selected to kinetically substantially
prevent the formation of second phases in the matrix, wherein the
matrix of the cooled article is substantially free of second phases
and has a resistivity greater than the resistivity of the matrix
before exposure to the elevated temperature environment.
40. The method of claim 39, wherein the solute element is capable
of forming a second phase with silver which is thermodynamically
stable at room temperature; and cooling is sufficiently rapid that
said second phase is substantially kinetically prevented from
forming.
41. The method of claim 39, wherein the environment containing the
solute element is held at a temperature less than 550.degree.
C.
42. The method of claim 39, wherein the cooling is carried out at a
rate of greater than 1.degree. C./min.
43. The method of claim 39, wherein the cooling is carried out at a
rate of greater than 10.degree. C./min.
44. The method of claim 39, wherein the cooling is carried out at a
rate of greater than 20.degree. C./min.
45. The method of claim 39, wherein the environment comprises a
liquid metal bath containing the solute element.
46. The method of claim 39, wherein the environment comprises a
vapor phase containing the solute element.
47. The method of claim 39, wherein the solute element is selected
from the group consisting of gallium, tin, cadmium, zinc, indium,
and antimony.
48. A method of preparing an oxide superconductor article having a
high resistivity sheath, the method comprising: after formation of
a final oxide superconductor phase in an article comprising an
oxide superconductor member in a silver-containing matrix, exposing
the oxide superconductor article to an environment at a selected
temperature and pressure under conditions that favor formation of a
silver-rich solid solution having a resistivity higher than the
resistivity of the silver-containing matrix, and do not favor the
formation of second phases, the environment containing a solute
element capable of forming the silver-rich solid solution, the
solute having a boiling temperature at the selected pressure
greater than the selected temperature; maintaining the oxide
superconductor article in the environment for a time sufficient to
form the silver-rich solid solution; and adjusting at a selected
rate an environmental parameter selected from the group consisting
of temperature, pressure, partial pressure of solute, and
combinations thereof, to produce an environment where substantial
diffusion of solute into the matrix occurs, the selected rate being
chosen such that the formation of second phases in the matrix is
substantially prevented.
49. A method of coating a superconducting composite with gallium,
comprising: immersing an electrode and a composite comprising a
metal and an oxide superconductor in a gallium-containing
electrolyte bath; and applying an electric potential difference to
the electrode and the composite to deposity gallium onto the
surface of the composite.
Description
[0001] This application is a continuation of U.S. patent
application Ser. No. 09/782,703, which is a divisional application
of U.S. patent application Ser. No. 09/240,998, filed Feb. 1, 1999,
now issued as U.S. Pat. No. 6,188,921, which claims benefit and
priority of U.S. Provisional Application No. 60/074,258, filed Feb.
10, 1998, both of which are incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention is related to oxide superconductor
composites having high sheath resistivity.
BACKGROUND OF THE INVENTION
[0003] Oxide superconducting wires and cables typically consist of
many filaments of superconducting material within a metal matrix
which separates the filaments from each other and from the local
environment. The matrix is typically a non-superconducting metal.
Silver and its alloys represent the matrix metals of choice because
the silver is ductile, chemically benign with respect to the oxide
superconductor material and relatively transparent to oxygen.
[0004] Recent advances in the development of oxide superconductors
have demonstrated their utility in applications such as power
transmission cables, fault current limiters, utility inductors,
motors and generators. For optimal performance, however, many of
these applications require matrix resistivities which are much
higher than that of pure silver at use (i.e., cryogenic)
temperatures. Pure silver has a resistivity at 80K of about 0.2-0.5
.mu..OMEGA.-cm, and this value decreases by an order of magnitude
as the temperature drops to 4K. As the term is used herein,
resistivity is defined as the bulk resistivity as determined by
measuring the current flow in a wire and applying the formula 1 = V
A I x
[0005] where .rho. represents resistivity, V represents voltage
measured over wire length x, A represents the cross-sectional area
of the wire, and I represents current.
[0006] There are many technical difficulties associated with the
manufacture of an oxide superconductor having a high resistivity
sheath. For example, processing steps associated with the formation
of the high resistivity sheath may not be compatible with the
processing of the oxide superconductor. In particular, under high
temperature conditions used to form oxide superconductor phases,
many of the sheath components likely to impart high resistivity to
the sheath react with and poison the oxide superconductor. In
addition, metals which are chemically compatible with the oxide
superconductor and the sheath metal typically are highly
electrically conductive.
[0007] One approach to increasing matrix resistivity consists of
the introduction of fine oxide particles into the metal matrix to
form a dispersed oxide/matrix metal alloy ("oxide-dispersion
strengthened" or ODS silver); however, this requires relatively
large volume fractions of the oxide phase in order to sufficiently
raise the bulk resistivity of the matrix. Such an approach is
limited because an increase in the oxide content of the matrix
metal increases its brittleness. Thus, only modest increases in
resistivity, e.g., 1-2 .mu..OMEGA.-cm, are possible while
maintaining a matrix with acceptable mechanical properties. In
order not to crack in ordinary coiling and winding operations, the
matrix should have a tensile fracture strain of at least 0.5%.
Fracture strains of higher than 1% are preferred for practical
handling of the superconducting composite. In addition, the oxide
precipitates used in ODS silver often interact detrimentally with
the oxide superconductor and tend to degrade the superconducting
properties of the composite.
[0008] In another approach, a metal may be alloyed with the sheath
metal prior to composite fabrication to raise the resistivity of
the sheath. While many metals may be readily alloyed and
incorporated into the metal sheath, this process requires that the
solute metal be present during high temperature processing of the
oxide superconductor. Unfortunately, known low-cost solutes which
significantly increase resistivity typically poison the
superconductor or themselves are subject to oxidation under these
processing conditions.
[0009] Shiga et al. in U.S. Pat. No. 5,296,456 disclose alloying a
variety of metals with the metal sheath covering the oxide
superconductor to obtain high conductivity (low resistivity) and
low conductivity (high resistivity) regions in the sheath. As is
discussed in greater detail below, most of the metals disclosed by
Shiga et al. are not very effective for increasing electrical
resistivity. Further, many metals which are highly effective in
raising the net resistivity of the matrix are not good candidates
for alloying with the metal sheath because they tend to readily
form second phases, e.g., intermetallic compounds, within the
matrix metal. Intermetallics tend to embrittle the matrix, and do
not raise net resistivity sufficiently.
[0010] Due to the limitations of prior art processes, a need
remains for sheathed oxide superconducting composites which combine
suitably high resistivity with good superconducting properties.
SUMMARY OF THE INVENTION
[0011] It is an object of the present invention to provide an oxide
superconductor composite with high sheath resistivity and, in
particular, high bulk sheath resistivity.
[0012] It is a further object of the invention to provide a method
of obtaining a high resistivity sheath without deleterious effect
on the electrical properties of the article.
[0013] It is yet a further object of the invention to provide a
process for making an oxide superconductor composite having a high
sheath resistivity, where the process is adaptable to continuous or
bulk processing of composites.
[0014] It is another object of the invention to identify materials
of suitable resistivity and compatibility with the oxide
superconductor for use in the high resistivity sheath of the
invention.
[0015] In one aspect, the invention comprises a composite oxide
superconductor article comprising one or more oxide superconducting
members in a silver-containing matrix. The matrix has a resistivity
of at least 3 .mu..OMEGA.-cm, preferably of greater than 5
.mu..OMEGA.-cm, and most preferably in the range of 5-25
.mu..OMEGA.-cm. The matrix does not comprise any metallic element
having a boiling point below 380.degree. C. at one atmosphere
pressure. The matrix further has a tensile fracture strain of at
least 0.5%, and preferably of at least 1%. The matrix may also have
a bend fracture strain of at least 0.5%, or preferably of at least
1%. The matrix may comprise a silver-rich solid solution with one
or more other elements, such as gallium, tin, cadmium, zinc,
indium, or antimony. The solute element may be selected so as to be
able to diffuse into a silver matrix in less than twenty hours at a
temperature of less than 550.degree. C., and/or to have a
diffusivity in silver at less than 550.degree. C. of at least
10.sup.-12 cm.sup.2/s. The solute element may represent at least 2
atm % of the matrix composition, preferably at least 4 atm %, more
preferably 4-50 atm %, and most preferably 4-18 atm %. The matrix
material may be fine grained, with a grain size of less than 50
.mu.m, or preferably in the range of 0.1-15 .mu.m. The oxide
superconducting member preferably may comprise BSCCO 2223 phase,
BSCCO 2212 phase, or a member of the YBCO family, and may have a
critical temperature of at least 70K. The engineering current
density of the superconductor article may be at least 3,000
A/cm.sup.2 at a temperature less than or equal to 90K (as measured
by a 1 .mu.V/cm criterion).
[0016] In another aspect, the invention comprises a method of
preparing an oxide superconductor. After formation of an article
comprising at least one oxide superconductor member in a
silver-containing matrix, the method comprises coating the article
with a solute capable of forming a silver-rich solid solution,
heating the coated article for a time sufficient to allow the
solute to diffuse into the matrix to form the solid solution, and
cooling the article to a temperature at which substantially no
further diffusion occurs. The heating step is carried out at a
temperature below the boiling temperature of the solute at one
atmosphere pressure, and preferably at a temperature at which the
vapor pressure of the solute is less than or equal to 0.1
atmospheres. The solute element may be chosen from a material
system which possesses a thermodynamically stable second phase with
silver, and cooled sufficiently rapidly to prevent formation of
this phase. The article may be coated by any of a number of
processes, including chemical vapor deposition, physical vapor
deposition, dip coating, roll coating, gravure roll printing,
doctor blading, stamping, sputtering, electrochemical deposition,
laser ablation, and plasma spraying. The solute may be a metal such
as gallium, tin, cadmium, zinc, indium, or antimony, and may be
selected to have a diffusivity in silver of at least 10.sup.-12
cm.sup.2/sec at a temperature below 550.degree. C. If the solute is
gallium, it may have a concentration in the range of 3-18 atm %.
The heating temperature may be selected so that the formation of
second phases is not favored, for example in the range of
380.degree. C.-520.degree. C. when the solute is gallium. The
heating and cooling steps may be accomplished sufficiently rapidly
that the formation of second phases rich in solute is suppressed.
The composite may be held at the heating temperature for a period
of less than or equal to 20 hours. The superconducting member of
the article may comprise the desired final superconducting
phase.
[0017] In yet another aspect, the invention comprises a method of
preparing an oxide superconductor article having a high resistivity
sheath. After formation of an article comprising at least one oxide
superconductor member in a silver-containing matrix, the method
comprises exposing the article to an environment containing one or
more solute elements capable of forming a silver-rich solid
solution, at an elevated temperature and other conditions which
favor diffusion of the solute into the matrix, and holding the
article in that environment for a time sufficient for such
diffusion to occur. The elevated temperature is less than the
boiling point of the solute element at one atmosphere pressure. The
method further comprises cooling the composite article to a
temperature at which substantially no further diffusion occurs. The
solute element may be chosen from a material system which possesses
a thermodynamically stable second phase with silver, and cooled
sufficiently rapidly to prevent formation of this phase. The
environment may be held at a temperature of less than 550.degree.
C., and cooling may be carried out at a rate of at least 1.degree.
C./min, or preferably of at least 10.degree. C./min, or more
preferably of at least 20.degree. C./min. The environment may
comprise a liquid metal bath or a vapor phase containing the solute
element. The solute element may be a metal such as gallium, tin,
cadmium, zinc, indium, or antimony.
[0018] In a further aspect, the invention comprises a method of
preparing an oxide superconductor having a high resistivity sheath.
After formation of a superconducting composite article comprising
an oxide superconducting member in a silver-containing sheath, the
method comprises exposing this article to an environment which
favors formation of a silver-rich solid solution with an increased
resistivity, and which does not favor the formation of second
phases. The environment comprises a solute element whose boiling
point is above the given temperature at the given pressure, and
which is capable of forming the silver-rich solid solution. The
article is maintained in this environment for a sufficient time to
form the solid solution, and then the environment is adjusted to a
condition in which diffusion of solute into the matrix is
substantially suppressed. The environment may be so changed by
changing the temperature, pressure, partial pressure of solute, or
combinations thereof. The rate of change of the environment is
selected so that formation of second phases in the matrix is
substantially prevented.
[0019] In still another aspect, the invention provides methods of
electroplating gallium onto the surface of a superconducting
composite, by applying a potential to the composite and an
electrode in a gallium-containing electrolyte.
[0020] By "silver-rich solid solution" as that term is used herein,
it is meant a composition having more than 50 atm % (atomic
percent) silver, preferably more than 75 atm % silver and most
preferably greater than 82 atm % silver.
[0021] By "second phase" as that term is used herein, it is meant a
phase comprising the solute element, other than the superconductor
phases, which is chemically distinct from the desired
resistivity-enhanced silver-rich phase. Common second phases are
"intermetallic compounds" or "intermetallics." Such compositions
are characterized by decreased fracture strains, as compared to the
corresponding solid solution. They may also provide a barrier to
further diffusion of the solute metal into the matrix metal.
[0022] Unless otherwise noted, "resistivity" refers to bulk
resistivity of the matrix which is determined across the many
grains of the matrix material along the wire axis.
[0023] By "reactive conditions" as that term is used herein, it is
meant conditions which are sufficient to create favorable kinetic
conditions for reaction of the oxide superconductor with the solute
element, or for significant degradation of the electrical
properties of the oxide superconductor.
[0024] By "engineering critical current density" as that term is
used herein, it is meant the total critical current of the
superconducting members of a superconducting composite, divided by
the cross-sectional area of the entire composite, including both
superconducting oxide filaments and silver-rich matrix. This
quantity is denoted by J.sub.e.
BRIEF DESCRIPTION OF THE DRAWING
[0025] The invention is described with reference to the several
figures of the drawing, in which,
[0026] FIG. 1 illustrates the preferred conditions for practice of
the invention on the silver-gallium phase diagram; and
[0027] FIGS. 2a and 2b illustrate two roll coaters which may be
used to coat a metal-sheathed superconductor with a liquid.
DETAILED DESCRIPTION
[0028] The composite of the present invention is well suited, for
example, for use in current limiters where the composite
demonstrates a high resistivity for currents beyond its intended
operating current level. By increasing the resistivity of the
sheath, the composite wire cannot carry significant excess current
beyond the critical current of the superconductor even when
increased voltages are applied. In normal superconducting
composites which comprise one or more oxide filaments encased in a
metal sheath, the sheath material and the superconductor constitute
a parallel circuit. When the current is increased to a level which
drives the electric current above the critical current carrying
capacity of the superconducting element, its resistivity is
abruptly increased to high levels (greater than 25 .mu..OMEGA.-cm).
Excess current may then flow through the sheath material. To limit
this excess current, it is desirable that the sheath have a high
resistivity.
[0029] In one aspect of the invention, an oxide superconductor
article is provided which possesses high bulk resistivity in the
matrix. The net bulk resistivity of the matrix is greater than 3
.mu..OMEGA.-cm, typically in the range of 5-25 .mu..OMEGA.-cm at
T.ltoreq.T.sub.c. This represents a significant increase over bulk
resistivities of previously reported oxide superconductor
composites.
[0030] The matrix resistivity is attained by formation of a solid
solution of the matrix metal with at least one solute element. The
solute desirably comprises one or more elements which result in a
large increase in the resistivity of silver when added in modest
quantities.
[0031] It is not necessary that the solute be distributed uniformly
in the matrix, as long as the bulk resistivity of the matrix is
increased sufficiently. For example, the solute may be localized to
the vicinity of the grain boundaries, and still provide a
substantial increase in resistivity, as long as no interconnected
pathway of low-resistivity material exists in the metal.
[0032] Suitable alloying elements are capable of imparting
resistivity to the matrix, such that the bulk resistivity of the
matrix increases. Elements with the greatest impact on alloy
resistance are generally most preferred; however, suitable elements
also demonstrate high diffusivity in silver and the ability to form
solid solutions with silver at low temperatures. In particular, it
is desirable that the solute have a melting point below 600.degree.
C. and preferably a melting point below 200.degree. C. Low melting
point metals may be easily applied (as is discussed hereinbelow) at
temperatures well below those which would affect the phase
composition of the oxide superconductor. However, it is also
desirable that the boiling point of the solute at one atmosphere
pressure be above the processing temperature, as the handling of
liquids is much simpler than the handling of vapors. By way of
example only, suitable solute metals which are anticipated to form
high resistivity, solid-solution alloys with silver under mild
conditions include tin (Sn), gallium (Ga), cadmium (Cd), zinc (Zn),
indium (In) and antimony (Sb) and alloys thereof. One or more
solute elements may be used.
[0033] As discussed above, Shiga et al. has disclosed a method of
producing a high-resistivity surface layer in an HTS/metal
composite, in order to reduce AC losses due to eddy currents, and
to reduce the thermal conductivity of the wire. Shiga et al. is not
concerned with reducing conductivity throughout the metal regions
of the wire, but only within a thin surface layer. High diffusivity
in silver is thus less important for the technique of Shiga et al.
than for the present invention, and a different set of processing
conditions and preferred metals is therefore recommended.
[0034] For the practice of the present invention, the matrix
preferably has a fine grained structure, particularly when the
primary diffusion path of the solute is via grain boundaries. The
finer the grain size of the matrix, the greater the interface area
available for the infusion of the resistivity-conferring solute
within the matrix. For the purposes of the present invention, fine
grained shall mean a grain size of less than 50 .mu.m and
preferably in the range of 0.1 to 15 .mu.m. Such grain sizes may be
achieved, for example, by the addition of a small quantity of a
boundary-pinning phase, such as fine oxide particles. The matrix
may be silver or a silver alloy. Silver is particularly well suited
as a matrix because of its inertness to oxidation and to the oxide
superconductor at typical formation temperatures for oxide
superconductors, and because of its formability. Alloying elements
which are added before formation of the desired superconducting
phase preferably share this inertness to oxidation and to the oxide
superconductor at formation temperatures.
[0035] Adding a solute having a significantly different atomic
radius from that of a pure metal usually has an effect of changing
the lattice parameter of the material, and thereby straining it.
When strain is introduced into one component of a composite system,
residual stresses and strains can arise. In the case of a
silver/superconductor composite, such stresses and strains may
introduce defects into the brittle superconductor phase upon
cooling to operating temperature (e.g., 77K). It has been found
that this effect is most pronounced for short lengths of wire. This
effect can be reduced or eliminated by mechanical stabilization of
the monofilament tapes. The mechanical stabilization process
involves attaching a tape on one or both sides to a stiff
mechanical support after the diffusion step. Attachment may be, for
example, by soldering or by bonding with epoxy or other known
adhesive. Suitable supports include stainless steel or fiberglass
plates.
[0036] The invention may be practiced with any desired oxide
superconductor or its precursors. By "desired oxide superconductor"
is meant the oxide superconductor intended for eventual use in the
finished article. Typically, the desired oxide superconductor is
selected for its superior electrical properties, such as high
critical temperature or critical current density. The desired oxide
superconductor is typically a member of a superconducting oxide
family which has demonstrated superior electrical properties, for
example, BSCCO 2223 or BSCCO 2212 in the BSCCO family. By
"precursor" is meant any material that can be converted to an oxide
superconductor upon application of a suitable heat treatment.
Precursors may include any combination of elements, metal salts,
oxides, suboxides, oxide superconductors which are intermediate to
the desired oxide superconductor, or other compounds which, when
reacted in the stability field of a desired oxide superconductor,
produces that superconductor. For example, there may be included
elements, salts, or oxides of copper, yttrium, and barium for the
YBCO family of oxide superconductors; elements or oxides of copper,
bismuth, strontium, and calcium, and optionally lead, for the BSCCO
family of oxide superconductors; elements, salts, or oxides of
copper, thallium, calcium and barium or strontium, and optionally,
bismuth and lead, for the thallium (TBSCCO) family of oxide
superconductors; elements, salts, or oxides of copper, mercury,
calcium, barium or strontium, and optionally, bismuth and lead, for
the mercury (HBSCCO) family of oxide superconductors. The YBCO
family of oxide superconductors is considered to include all oxide
superconductors of the type comprising barium, copper, and a rare
earth selected from the group consisting of yttrium, lanthanum,
neodymium, samarium, europium, gadolinium, terbium, dysprosium,
holmium, erbium, thulium, ytterbium, and lutetium. By "oxide
superconductor intermediate to the desired oxide superconductor" is
meant any oxide superconductor which is capable of being converted
to the desired oxide superconductor. The formation of an
intermediate may be desired in order to take advantage of desirable
processing properties, for example, a micaceous structure amenable
to texturing, which may not be equally possessed by the desired
superconducting oxide. Precursors are included in amounts
sufficient to form an oxide superconductor. In some embodiments,
the precursor powders may be provided in substantially
stoichiometric proportion. In others, there may be a stoichiometric
excess or deficiency of any precursor to accommodate the processing
conditions used to form the desired superconducting composite. For
this purpose, excess or deficiency of a particular precursor is
defined by comparison to the ideal cation stoichiometry of the
desired oxide superconductor. Thalliation, the addition of doping
materials, including but not limited to the optional materials
identified above, variations in proportions and such other
variations in the precursors of the desired superconducting oxides
as are well known in the art, are also within the scope and spirit
of the invention.
[0037] The three-layer, high-Tc phase of a member of the BSCCO
family of superconductors (BSCCO 2223), such as
Bi.sub.2Sr.sub.2Ca.sub.2Cu.sub.3O.s- ub.x or
[Bi.sub.1-yPb.sub.y].sub.2Sr.sub.2Ca.sub.2Cu.sub.3O.sub.x
(0<y<0.5), is the desired superconducting oxide most
preferred for the operation of the present invention. Composites
including BSCCO 2223 have demonstrated the potential for superior
mechanical and electrical performance at long lengths when
adequately textured. Bi.sub.2Sr.sub.2Ca.sub.1Cu.sub.2O.sub.x and
[Bi.sub.1-yPb.sub.y].sub.2Sr.- sub.2Ca.sub.1Cu.sub.2O.sub.x (BSCCO
2212) are also preferred materials for the practice of the
invention.
[0038] A typical method of manufacture of the high resistivity
matrix composites of the invention involves application of a layer
of the solute composition to the composite after it has been
processed into a fully sintered oxide superconductor composite,
followed by diffusion of the solute into the matrix to form a solid
solution. The solute preferably diffuses into the matrix along
grain boundaries, which promotes the facile penetration of the
solute into the matrix bulk, but solutes whose primary transport
mechanism is bulk diffusion are also suitable for the practice of
the invention, as long as acceptable transport rates can be
achieved at a temperature which does not adversely affect the
properties of the composite.
[0039] The extent to which the solute favors grain boundary
diffusion is determined, in part, by the chemical system, i.e., the
composition of the matrix and the diffusing elements, and the
processing conditions. While both grain boundary and bulk diffusion
rates increase at higher temperatures, grain boundary diffusion
typically is predominant at lower temperatures. The interested
reader may find more information on diffusion pathways and typical
high- and low-temperature behavior in many materials science texts,
including Reed-Hill, et al, Physical Metallurgy Principles,
PWS-KENT Publishing, 1992 (chapter 12).
[0040] Many solute elements will degrade a superconducting oxide
when present under reactive conditions. It is therefore desirable
that such conditions be avoided when the solute element is present
in the matrix. This is difficult when the alloyed matrix is formed
before the oxide superconductor, since the conditions under which
the oxide superconductor is formed are necessarily reactive
conditions. Therefore, the high resistivity component preferably is
introduced at low temperatures and at a point in the manufacture of
the superconducting composite when no further treatments under
reactive conditions will occur.
[0041] In summary, it is desirable to introduce the high
resistivity component into the matrix at a point after the oxide
superconductor has been formed, noted herein as a "post-processing
step." By "post-processing step" as that term is used herein, it is
meant a procedure which is carried out after processing to obtain
the desired oxide superconductor is completed. The oxide
superconductor, thus processed, possesses all the electrical
properties required for its intended use, for example, appropriate
composition, oxide grain morphology and alignment, critical
transition temperature, and current carrying ability. The
post-processing step(s) desirably is carried out at low
temperatures in order to prevent degradation of the superconducting
properties of the composite. The post-processing step(s) may be
carried out at a temperature in the range of ambient to 600.degree.
C., and is preferably carried out at a temperature below
550.degree. C., but high enough to ensure rapid diffusion of the
solute in the matrix, in order to prevent degradation of the oxide
superconductor.
[0042] The alloying process may be accomplished by heating the
oxide superconductor composite in the presence of a solute. The
solute may be present as a vapor or liquid in contact with the
composite. When the solute is present in the form of a vapor or a
liquid having a high vapor pressure, it will generally be found
necessary to hold the composite in a pressure vessel or similar
sealed environment in order to avoid loss of solute. This
restriction imposes severe constraints on the processing of large
volumes of material, and may render continuous processing
impractical or even impossible.
[0043] The alloying process may be accomplished by depositing the
solute composition onto an exterior surface of the oxide
superconductor composite and heating, whereby the solute diffuses
into the matrix. The solute may be deposited by any conventional
method, including but not limited to chemical vapor deposition,
physical vapor deposition, dip coating, roll coating, gravure roll
printing, stamping, sputtering, electrochemical deposition, doctor
blading a powder onto the composite external surface, application
of a slurry paste onto the external surface of the composite
followed by a bake out heat treatment, laser ablation and plasma
spraying. Descriptions of these and other coating techniques can be
found in standard coating references, including Cohen, et al.,
Modern Coating and Drying Technology, VCH Publ., 1992, and
Schweizer, et al. (Eds.), Liquid Film Coating, Chapman-Hall, 1997.
Some preferred methods of applying the solute metal are roll
coating and meniscus/gravity controlled dip coating, further
described below. Because grain boundary diffusion is readily
accomplished at lower temperatures than bulk diffusion for grain
boundary-favoring elements, the process may be conducted at low
temperatures when these elements are used as solute metals, thereby
minimizing the possibility that the superconducting properties will
be degraded.
[0044] It is desirable to accomplish the diffusion of the solute
into the matrix while minimizing the formation of brittle
intermetallic phases in the matrix, since these phases will tend to
substantially reduce the fracture strain of the superconducting
composite if present in significant quantities. For some solute
metals, this can be accomplished by coating the silver matrix with
a quantity of the solute metal at a low temperature, and then
heating the superconducting composite to a selected diffusion
temperature. The composite is held at this temperature for a time
sufficient to diffuse the solute metal into the silver matrix, and
then the composite is cooled back to a low temperature. For an
understanding of the mechanics of this process, we examine the
thermodynamics and kinetics of the particular case of a
silver-gallium alloy.
[0045] A portion of the silver-gallium phase diagram is given as
FIG. 1. It will be seen that two intermetallic phases exist in the
silver-rich regions of the diagram: a high-temperature .beta.
phase, and a low-temperature .beta.' phase. Shaded region 10 shows
a preferred range of temperatures and compositions for which a
single-phase solid solution of gallium in silver is favored.
[0046] When pure gallium and pure silver are placed in contact and
heated slowly, the gallium melts at 30.degree. C., and begins to
diffuse into the silver primarily along grain boundaries. The
region of the silver near the interface becomes gallium-rich, and
intermetallic .beta.' forms because gallium diffusion into the bulk
is relatively slow. When the same metals are placed in contact and
heated rapidly, transport of gallium into the silver is rapid, and
if the amount of gallium is small relative to the amount of silver
(e.g., less than about 18 atm %), all of the gallium can diffuse
into the silver and be held stably in solid solution. If the metal
is then slowly cooled, .beta.' will nucleate and grow when line 12
is crossed on the phase diagram. If the metal is rapidly cooled
below a temperature at which gallium is mobile, this nucleation can
be suppressed, and the metal will remain substantially composed of
a solid solution of gallium in silver. Even if some .beta.' phase
is nucleated, its growth will be sufficiently slow at temperatures
below about 150.degree. C. that the volume fraction of
intermetallic phase will remain small, and the intermetallic
precipitates will be well-dispersed.
[0047] Hatched region 10 on the phase diagram shows the region
which has been found to be preferred for the practice of the
invention, namely, a temperature range of from 380 to 550.degree.
C., and a composition range of from 2 to 18 atm % gallium. BSCCO
2223 phase has been shown to be substantially unaffected by
temperatures in this range for the hold time required for gallium
to diffuse evenly through a 0.02-0.2 mm silver matrix. It is
expected that suitable processing conditions exist for other
low-melting point metals which have such a silver-rich solid
solution phase field, such as tin, cadmium, zinc, indium, and
antimony. These metals are therefore also suitable for the practice
of the invention.
[0048] One method of applying a reproducible amount of liquid metal
such as gallium to the surface of a silver sheath is roll coating.
Various roll coating techniques exist; in general, they comprise
applying a controlled amount of liquid to a roller, which in turn
applies the liquid to the surface of the sheath. The amount of
liquid applied to the roller may be controlled, for example, by the
use of an applicator roll, which is partially immersed in a liquid
coating bath, and which is held at a controlled small distance from
the main roll, so that a well-defined quantity of liquid is applied
to the main roll. Other methods of controlling the amount of liquid
applied include fountain application, thinning with a gas knife, or
application with a doctor blade. These and other roll coating
techniques are known in the art and are discussed in more detail in
Gravure: Process and Technology, distributed by the Gravure
Association of America (1991).
[0049] Another method of applying a reproducible amount of liquid
metal such as gallium to the surface of a silver sheath is a
meniscus/gravity controlled dip coating. In this method, a
composite superconductor wire or tape is immersed in a liquid metal
bath, and is pulled upward out of the bath at a constant rate. This
process deposits an even coating of liquid metal on the wire, whose
thickness is a function of the surface energy and density of the
liquid metal, and of the pulling speed of the wire. It has been
found that for producing even and reproducible coatings of gallium
on silver, it is desirable to clean the silver sheath to give it a
uniform surface energy. One process which has been found suitable
is a two-step process involving ultrasonically cleaning the sheath
first in a bath of CITRANOX.TM., an anionic liquid detergent, and
then in a distilled water bath.
[0050] Another method of applying a reproducible amount of gallium
or another liquid metal to the surface of a silver sheath is wave
coating followed by thinning with a "gas knife." In this technique,
the superconducting composite wire or tape is passed through a
chamber where it is inundated by a liquid metal "wave," and thus
receives a heavy coating of molten metal. Excess metal is then
pared away from the surfaces of the tape by jets of inert gas from
upper and lower nozzles, leaving a thin and even layer. The gas
exiting the nozzles may be any desired composition; preferred
compositions for gas knife paring of a wave coating of gallium are
Cu-gettered argon, nitrogen, helium, or carbon dioxide. The
thickness of the final coating is a function of (at least) the
composition and application temperature of the liquid metal, the
temperature, composition, and velocity of the gas jet, and the
speed of initial coating of the wire or tape. In one preferred
embodiment, the wire or tape may be cooled to solidify the liquid
metal before spooling at the apparatus exit. Alternatively, the
wire or tape may be passed directly into a furnace for quick
heating to diffuse the liquid metal into the matrix of the
superconducting composite.
[0051] Yet another method for deposition of gallium or other metals
onto the sheath surface is electroplating. A method of brush
plating gallium is described in U.S. Pat. No. 4,521,328. We have
also devised an immersion method of gallium plating using gallium
dissolved to a concentration of 20 g/l to 130 g/l in a caustic
solution such as sodium hydroxide or potassium hydroxide. Possible
gallium sources include Ga.sub.2O.sub.3, Ga(OH).sub.3, GaCl.sub.3,
Ga.sub.2O, Ga.sub.2(SO.sub.4).sub.3, and Ga(NO.sub.3).sub.3. A DC
plating cell is run with a current density on the order of 1
A/cm.sup.2, using the HTS composite as the cathode and any material
which does not poison the cell as an anode. Anodes made from
platinum, nickel, tungsten, gallium, and stainless steel have been
successfully tested. This plating method is further described in
Example 5.
[0052] For some metals, such as gallium, it may facilitate further
handling to subject a liquid coated tape (coated by any of the
above methods) to a relatively low-temperature anneal which allows
the metal to form an intermetallic phase with the silver at the
surface of the tape.
[0053] In another embodiment of the invention, the coating and
diffusion steps may be combined, by placing the superconducting
composite in a solute-rich environment at the desired diffusion
temperature, e.g., a gallium or gallium/silver bath at 450.degree.
C.
[0054] Long lengths of coated superconducting composite tape can be
annealed in a batch process by co-winding the coated tape with a
stainless steel tape on a reel to form a pancake coil. The pancake
coil can then be laid flat in the furnace and diffusion heat
treated. The stainless steel and the treated composite can be
easily unwound and separated after heat treating, and the stainless
steel tape can be reused or disposed of. Heat treatment may also be
carried out by winding the coated tape on a mandrel (e.g., an
alumina mandrel). The tape may be wound in a single layer, or
parting agents may be used to treat multiple layers.
[0055] For some applications such as those using multiple lengths
of high resistivity conductor, it may be important that the
variation in sheath resistivity for the multiple lengths be
minimized. The invention includes process control steps whereby
this may be achieved. In the technique, composite superconducting
tapes are coated with a slight excess of gallium, and sheath
resistivity is directly measured during the diffusion heat
treatment. When the resistivity reaches a selected value, the
furnace is turned off to halt the heat treatment. While some
additional diffusion will probably occur during cooling, this
system significantly reduces batch-to-batch variation in
resistivity.
EXAMPLES
Example 1
Ga--Ag Alloy Matrix
[0056] Pure silver-sheathed monofilamentary BSCCO 2223 composite
tapes were fabricated by the standard oxide powder in tube process.
After the final sintering step, the critical wire current was about
16 A at 77K (1 .mu.V/cm criterion). Ten 15 cm lengths were then
coated with Ga manually by dipping latex gloved fingers into molten
Ga at about 40.degree. C., followed by repeated forward and reverse
swabbing onto the Ag tape surface of the Ga pool on the glove
fingers while slightly compressing the tape between those fingers.
The central region (about 3 inches long) was cut from each tape and
weighed. The known oxide fill factor and the external tape
dimensions allowed calculation of the weight percent Ga deposited
relative to the silver sheath weight. The average weight gain was
11.23%. Samples were subjected to 2 and 4 hour bakes at 480.degree.
C. in Cu-gettered argon, followed by I.sub.c and resistivity
measurements. The results are in Table 1 below. It will be seen
that while both the 2 and 4 hour bakes increased the resistivity of
the matrix, the 4 hour bake also resulted in excessive degradation
of the superconducting wires, as can be seen by the low values of
I.sub.c final/I.sub.c initial for these wires.
1TABLE 1 Resistivity at I.sub.c final / Temperature (C.) Time (hrs)
77K (.mu..OMEGA.-cm) I.sub.c (A) I.sub.c initial 480 2 6 13.6 0.85
13.9 0.87 480 4 6.2 2.3 0.14 10.3 0.64
Example 2
Resistivity of Pure Silver Tapes
[0057] Pure silver tapes were also subjected to the coating method
described in Example 1 to assess the resistivity potential of
Ga--Ag alloys and the coat and diffuse process. These samples were
heat treated for 25 hours at 480.degree. C. in Cu-gettered argon.
The resistivity data presented in Table 2 illustrates that the
Ga--Ag solid solution formed by the coat and diffuse method can
attain resistivities of greater than 25 .mu..OMEGA.-cm.
2 TABLE 2 Resistivity at 77K wt % Ga (.mu..OMEGA.-cm) 8.1 12.8 10.2
22.4 10.7 28.4 13.8 31.3
Example 3
Meniscus/Gravity Controlled Dip Coating
[0058] Pure silver-sheathed mono filamentary Bi-2223 composite tape
was fabricated by the standard oxide powder in tube process to a
cross-sectional dimension of 0.011 cm.times.0.221 cm. The tape
exhibited an average critical current (I.sub.c at 77 K, self field,
1 .mu.V/cm criterion) of 23.4 A with a 2.04 A standard deviation.
The corresponding engineering current density (J.sub.e) was 9.68
kA/cm.sup.2. Two long segments of this tape were then dip coated at
two different travel speeds, .about.25 cm/min and .about.33.5
cm/min. The coating method consisted of pulling the tape upward
through a slot in a silicone rubber seal into a molten Ga bath held
at 50.degree. C., with the tape traveling about 4" through the
molten Ga, exiting upward out of the Ga, dragging Ga into a
relatively uniform coating on its surface. The thickness of this
coating was dependent on the travel speed, with the faster speed
producing a thicker Ga layer. The tape was then chilled, and
wrapped onto a mandrel followed by a "drying" anneal consisting of
4 hrs at 80.degree. C., a process which formed Ga--Ag intermetallic
at the tape surface from the molten Ga, thereby allowing much
easier subsequent in-process handling. The tape was then cut into
test samples (each about 4" long), which were used to investigate
the effects of time at Ga diffusion temperature on I.sub.c and
average resistivity. The addition of the Ga to the sheath
immediately reduced J.sub.e to a value of 8.25 kA/cm.sup.2 for the
25 cm/min coating speed, by increasing the amount of
non-superconducting material in the wire.
[0059] For the diffusion experiments, five samples cut from the
long length were placed and sealed into the ambient temperature end
region of a preheated furnace. The atmosphere of the furnace was
either purged with Cu-gettered Argon or air, and samples were
thrust in the 480.degree. C. preheated hot zone of the furnace
(heating to temperature in less then 2 minutes). Samples were held
in the hot zone for a time in the range of 30-120 minutes and
pulled out, thereby rapidly quenching their temperature back to
ambient. The samples were then tested for their 77K, self-field
I.sub.c levels using a standard four point probe method with two
sets of 1 cm-spaced voltage taps. Sheath resistivity was determined
by measuring voltage between 1 cm spaced taps while a fixed small
current was passed through each composite as it was cooled down to
the transition temperature of a superconductor (.about.110K),
followed by extrapolation of the typically straightline voltage
temperature dependence to 77K. From the voltage v. temperature
data, resistivity is given by Equation 1, where V is measured
voltage, A is the cross-sectional area of the sheath assuming the
superconductor above Tc has a high resistivity compared to the
matrix, I is the applied fixed current, and x is the spacing
between the voltage taps.
[0060] Data is summarized in Table 3. The diffusion conditions
listed produced retained I.sub.c levels above 50%, with
corresponding J.sub.e levels above 4 kA/cm.sup.2 at 77 K.
Resistivities ranged from 5.1 to 13 .mu..OMEGA.-cm--all exceeding
the minimum 3 .mu..OMEGA.-cm level required in many current
limiting applications.
3TABLE 3 weight Coating gain retained sheath speed (% rel to Ga
diffusion I.sub.c J.sub.e resistivity (cm/min) Ag) treatment (%)
(kA/cm.sup.2) (.mu..OMEGA.-cm) 25 10.8 .5 hr 480.degree. C., 92
7.43 5.1 Ar 25 10.8 1 hr 480.degree. C., 78 6.24 7.9 Ar 25 10.8 2
hr 480.degree. C., 64 5.22 11.6 air 33.5 15 .5 hr 480.degree. C.,
63 4.66 8.33 air 33.5 15 1 hr 480.degree. C., 54 4.02 13.0 air
Example 4
Roll Coating
[0061] A desired amount of gallium or another liquid metal can be
applied to the exterior of a metal-sheathed superconductor via a
roll coating apparatus as shown in FIGS. 2a and 2b.
[0062] The roll coater of FIG. 2a comprises a feed roll 20, a
backup roll 22, a sump 24, and a doctor blade 26. The exterior
surface of the feed roll 20 is preferably of a material which is
not degraded by the molten metal, and may for example comprise an
elastomer or other polymer, ceramic, or metal surface. Most
preferably, the exterior of the roll 20 is an elastomer with a
surface profile which aids in the transport of gallium. Desirable
surface profiles include but are not limited to textured finishes
produced by grinding and quadrangle or pyramid cells similar to
those used in gravure printing.
[0063] Feed roll 20 is mounted horizontally and partially submerged
in liquid gallium 28 (or another liquid metal) which is contained
in the sump 24. When feed roll 20 is rotated as shown in FIG. 2a,
gallium will be transported on its surface. The doctor blade 26,
which is mounted peripherally to the feed roll 20, will act to
level and regulate the delivery of gallium. Subsequently, the
metal-sheathed superconductor 30 will be brought by action of the
backup roll 22 into contact with the gallium layer on the feed roll
20 and some or all of the gallium will be transferred to the
surface of the superconductor sheath 30. The sheathed
superconductor can move in the same or opposite direction as the
feed roll surface. The former is referred to as "forward" roll
coating, and the latter as "reverse" roll coating.
[0064] The second side of the sheathed superconductor 30 can be
coated by inverting the superconductor and then passing it through
the same or another roll coating apparatus.
[0065] In an alternative design, the doctor blade 26 of FIG. 2a
would be replaced by a metering roll (not shown) which would both
level and regulate the delivery of gallium. The gallium would be
regulated not only by the gap between the feed roll 20 and the
metering roll, but also by the direction and speed of rotation of
the metering roll. The use of a metering roll may provide several
advantages including the reduced wear of the feed roll surface.
[0066] In another alternative design, the feed roll 20 would not be
partially submerged in molten gallium 28 contained in the sump 24
of FIG. 2a. Instead, the gallium would be supplied to the feed roll
surface through a nozzle or slit (not shown) mounted adjacent to
the feed roll 20. This would reduce foaming and other undesirable
effects such as oxidation, which can occur due to introduction of
gases into the gallium. In this design, any excess gallium would be
collected in the sump 24 and returned to the gallium supply.
[0067] The roll coater of FIG. 2b comprises a feed roll 32, a
transfer or offset roll 34, a backup roll 22, a sump 24, and a
doctor blade 26. The feed roll 32 functions in substantially the
same manner as the feed roll 20 of FIG. 2a described above. Again,
it may be partially submerged in the molten gallium 28, or gallium
can be applied to its surface from a nozzle (not shown). Either a
metering roll (not shown) or a doctor blade 26 may be used as
described above to regulate the delivery of gallium.
[0068] Some or all of the gallium transported on the surface of the
feed roll 32 is transferred to the offset or transfer roll 34. The
details of this transfer are controlled at least by the direction
and speed of rotation of each of the two rolls and by the spacing
between them.
[0069] The metal-sheathed superconductor 30 to be coated is brought
into contact with the gallium layer on the transfer or offset roll
34 and some or all of the gallium will be transferred to the
surface of the superconductor sheath 30. Either forward or reverse
roll coating configurations may be used. After coating on one side,
the sheath can be inverted and coated on the other side by the same
or another roll coater.
Example 5
Electroplating
[0070] In preparation for electroplating, an electrolyte was made
by dissolving 67 grams of Ga.sub.2O.sub.3 into 1 liter of 5 molar
NaOH at approximately 70.degree. C. The electrolyte was allowed to
cool to room temperature prior to plating. The plating equipment
consisted of a DC power supply, the electrolyte, and a solid
gallium electrode. The material to be plated was a composite tape
having dimensions of 0.01164 cm thick, 0.3745 cm wide, 40 cm long,
and a 40% fill factor. For the plating process, the composite tape
was made the cathode and the gallium electrode was made the anode.
A cathodic current density of 0.6 A/cm.sup.2was applied for 30
seconds. The resulting tape thickness was 0.01336 cm providing for
a 8.6 .mu.m gallium plating thickness. The plated tape was cut into
four 10 cm lengths and heat treated for 1 hour at 100.degree. C. in
air then 4 hours at 450.degree. C. in air. Electrical measurements
done at room temperature yielded longitudinal sheath resistivities
of 5.84 .mu..OMEGA.-cm. Critical current testing done at 77 K
yielded an average IC of 37.52 A (self field, 1 .mu.V/cm).
Identical critical current tests done on control samples
(non-plated and diffusion processed) yielded an average IC of 35.01
A, indicating that the post-processing did not degrade the
superconducting filaments.
Example 6
Mechanical Testing of Alloyed Matrix
[0071] Pure silver strips nominally 2.45 mm.times.0.095 mm in
cross-section were coated with Ga at ambient temperature via dip
coating. The samples were weighed before and after coating to
determine the quantity of Ga. The samples were then heated to
480.degree. C. and held at temperature for different lengths of
time in Cu-gettered argon. Finally, the strips were quenched to
ambient temperature. The strips were then tested for tensile
strength and ductility, using an Instron mechanical testing
machine. Results are presented in Table 4.
4TABLE 4 tensile elongation diffusion yield strength strength to
fracture wt % Ga time (hrs) (MPa) (MPa) (%) 6.8 1 142 267 14.5 6.8
2 108 239 20.3 6.8 4 102 216 16 11 4 123 231 23.2 15 4 143 292
11.1
[0072] These results clearly show that the alloys were quite
ductile, with all samples fracturing above 10% strain. Their yield
strengths were quite high, and their tensile strengths even
higher.
Example 7
Mechanical Testing of Composite Tapes
[0073] Fully processed monofilament BSCCO 2223 tape samples were
coated with Ga using a dip method to 6.2 and 12.3 wt % with respect
to the silver. The tapes were nominally 1.73 mm wide and 0.191 mm
thick, with 45 vol % BSCCO 2223. After coating, the samples were
heated to 480.degree. C. in Cu-gettered argon, held at temperature
for 4 hours, and quenched back to ambient temperature. Samples were
then wound onto a 2.55 mm diameter rod (tape surface tensile strain
of about 7.5%) and examined for surface damage.
[0074] Neither macroscopic nor microscopic observation showed any
signs of surface cracking. This strain level is more than 10-fold
greater than the strain tolerance of HTS oxides such as BSCCO 2223.
Although the tests were completed at ambient temperature, the
results are expected to apply equally well at cryogenic
temperatures.
Example 8
Meniscus/Gravity Controlled Dip Coating with Further Post-Diffusion
Processing
[0075] Pure silver-sheathed monofilament Bi-2223 composite tape was
fabricated by the standard oxide powder in tube process to a
cross-sectional dimension of 0.115 mm.times.1.85 mm. The tape
exhibited an average critical current (I.sub.c at 77 K, self field,
1 .mu.V/cm criterion) of 18.6 A. The corresponding engineering
current density (J.sub.e) was 8.74 kA/cm.sup.2. A long segment of
this tape was then dip coated at a speed of approximately .about.25
cm/min. As in Example 3, the coating method consisted of pulling
the tape upward through a slot in a silicone rubber seal into a
molten Ga bath held at 50.degree. C., with the tape traveling about
4" through the molten Ga, exiting upward out of the Ga, dragging Ga
into a relatively uniform coating on its surface. The tape was then
chilled, and wrapped onto a mandrel followed by a "drying" anneal
consisting of 2 hrs at 90.degree. C., a process which formed Ga--Ag
intermetallic at the tape surface from the molten Ga, thereby
allowing much easier subsequent in-process handling. The tape was
then cut into test samples (each about 4" long). The addition of
the Ga to the sheath immediately reduced J.sub.e to a value of 7.39
kA/cm.sup.2 by increasing the amount of non-superconducting
material in the wire.
[0076] Twelve samples cut from the long length were placed and
sealed into the ambient temperature end region of a preheated
furnace. The atmosphere of the furnace was purged with either
Cu-gettered Argon or air, and samples were thrust in the
450.degree. C. preheated hot zone of the furnace (heating to
temperature in less then 2 minutes). Samples were held in the hot
zone for 120 or 240 minutes and pulled out, thereby quenching their
temperature back to ambient.
[0077] After diffusion treatments, the gallium-treated silver
sheathed monofilament BSCCO 2223 composite tapes have an oxide
layer on their surfaces which gives the tapes a dull matte finish.
This presence of this oxide layer may interfere with soldering of
these conductors either during subsequent processing or during end
product fabrication. The twelve test samples were divided into
three lots to investigate the effects of oxide removal method on
IC. The first lot of four sample tapes were abraded using 600 grit
silicon carbide grinding paper to remove the oxide and to create a
fresh specular silver alloy surface. The second lot of four sample
tapes were chemically etched for approximately three minutes in a
solution of 50 parts ammonium hydroxide (NH.sub.4OH), 10 parts
hydrogen peroxide (H.sub.2O.sub.2), and 50 parts water. This
etching removed the oxide and created a specular silver alloy
surface. The third lot of four samples was maintained as a control
group.
[0078] The samples were then tested for their 77K, self-field
I.sub.c levels using a standard four point probe method with two
sets of 1 cm-spaced voltage taps. Sheath resistivity was determined
by measuring voltage between 1 cm spaced taps while a fixed small
current was passed through each composite as it was cooled down to
the transition temperature of the superconductor (.about.110K),
followed by extrapolation of the typically straightline voltage
temperature dependence to 77K.
[0079] Data is summarized in Table 5. The additional process steps
of chemical etching or abrasion after diffusion heat-treatments
resulted in significantly higher values of I.sub.c retention (76
and 80% compared to 46%). The resistivities ranged from 9.8 to 9.9
.mu..OMEGA.-cm--all exceeding the minimum 3 .mu..OMEGA.-cm level
required in many current limiting applications.
5TABLE 5 Wt % Gallium Drying and Average Sheath (relative to
Diffusion Oxide removal Retained J.sub.e resistivity Ag) Conditions
method I.sub.c (%) (kA/cm.sub.2) (.mu..OMEGA.-cm) 11 2 hours at
90.degree. C. none 46 3.16 9.9 and 2 hours at 450.degree. C. 11 2
hours at 90.degree. C. chemical 76 5.79 9.8 and 2 hours at etching
450.degree. C. 11 2 hours at 90.degree. C. abrasion 80 6.61 not and
4 hours at measured 450.degree. C.
[0080] Other embodiments of the invention will be apparent to those
skilled in the art from a consideration of the specification or
practice of the invention as 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.
* * * * *