Note: Descriptions are shown in the official language in which they were submitted.
-- 1 -
~34~394
APPARATUS AND SYSTEMS COMPRISING A CLAD SUPERCONDUCTIVE
OXIDE BODY, AND METHOD FOR PRODUCING SUCH BODY
Field of the Invention
This invention pertains to methods for producing clad superconductive wire-
like and
ribbon-like bodies and to apparatus and systems that comprise such bodies.
Background of the Invention
From the discovery of superconductivity in 1911 to the recent past,
essentially all
known superconducting materials were elemental metals (e.g., Hg, the first
known
superconductor) or metal alloys (e.g., Nb,Ge, probably the material with the
highest transition
temperature T~ known prior to 1986).
Recently, superconductivity was discovered in a new class of materials. See,
for
instance, B. Batlogg, Physica, Vol. 126, 275 (1984), which reviews the
properties of
superconductivity in barium bismuth lead oxide, and J.G. Bednorz and K.A.
Muller, Zeitschr.
f. Physik B - Condensed Matter, Vol. 64, 189 (1986), which reports
superconductivity in
lanthanum barium copper oxide.
Especially the latter report stimulated worldwide research activity, which
very quickly
resulted in further significant progress. The progress has resulted, inter
alia, to date in the
discovery that compositions in the Y-Ba-Cu-O system can have superconductive
transition
temperatures T~ above 77K, the boiling temperature of liquid N~ (M.K. Wu et
al, Ph~s. Rev.
Letters, Vol. 58, March 2, 1987, page 908; and F.H. Hor, ibid, page 911).
Furthermore, it has
resulted in the identification of the material phase that is responsible for
the observed high
temperature superconductivity, and in the disec>very of compositions and
processing techniques
that result in the formation of bulk samples of material that can be
substantially single phase
material and can have T~ above 90K (see the Canadian Patent Application Serial
No. 556,031
which was filed on January 7, 1988 in the names B.J. Batlogg, et al.)
._2_ ~ 341 39 4
The excitement in the scientific and technical community that was created by
the
recent advances in superconductivity is at least in part due to the
potentially immense
technological impact of the availability of materials that are superconducting
at temperatures
that do not require refrigeration with expensive liquid He. Liquid nitrogen is
generally
S considered to be a very convenient cryogenic refrigerant. Attainment of
superconductivity at
liquid nitrogen temperature was thus a long-sought goal which for a long time
appeared almost
unreachable.
Although this goal has now been attained, there still exists at least one
barrier that has
to be overcome before the new oxidic high T~ superconductive materials can be
utilized in
many technological applications. In particular, techniques for forming
superconductive bodies
of technologically significant shape have to be developed.
The superconductive oxide material is readily produced in powder form, and has
been
processed by ceramic techniques into various shapes such as pellets, discs,
and tori. Canadian
Patent Application Serial No. S61,S20 which was filed on March LS, 1988 in the
names E.M.
1S Gyorgy, et al, discloses techniques for making ceramic superconductive
bodies having at least
one relatively small dimension (S ~m - 1 mm). Such filamentary and sheet-like
bodies include
thin rods, filaments, tapes, and sheets, which can be incorporated into a
variety of apparatus
such as Bitter magnets, transmission lines, rotating machinery, maglev
vehicles, and fusion
devices.
Perhaps the economically most significant application of prior art metallic
superconductors (e.g., Nb3Sn) is in the form of magnet wires. Magnets
incorporating such
wires can be found in many scientific laboratories and, inter olio, are to be
used in the
proposed giant particle accelerator, the so-called "Superconducting
Supercollider". Prior art
superconductive wires universally have a composite structure, with one or more
2S superconductive filaments embedded in normal (i.e., non-superconductive)
metal, typically
copper. The normal metal serves several critical functions in such wires,
among them
provision of a by-pass electrical conduction path, provision of thermal
conductive means in the
event of local flux motion, and enhancement of the mechanical strength of the
wire.
~ 341394
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For an overview of some potential applications of superconductors
see, for instance, B. B. Schwartz and S. Foner, editors, Superconductor
Applications: S(~-UIDS and Machines, Plenum Press 1977; 5. Foner and
B. B. Schwartz, editors, Superconductor Material Science, Metallurgy,
Fabrications, and Applications, Plenum Press 1981. Among the applications are
power transmission lines, rotating machinery, and superconductive magnets for
e.g., fusion generators, MHD generators, particle accelerators, levitated
vehicles,
magnetic separation, and energy storage. The prior art has considered these
actual
and potential applications in terms of the prior art (non-oxidic)
superconductors.
It is expected that many of the abave and other applications of
superconductivity
would materially benefit if high Tc superconductive wire could be used instead
of
the previously considered relatively low TG wire. We are disclosing herein
techniques for producing such wire, as well as other bodies such as tape.
Summary of the Invention
Disclosed is a method for producing elongate superconductive bodies
in which the superconductive material is a sintered oxide, typically a
cuprate, and
is surrounded by a cladding, typically a normal metal. Cuprates of interest
herein
typically are of nominal composition M3-mM'mCu34~,-S, with M being
preferably primarily Ba (substitution of all or some Ba by elements such as Ca
and Sr is contemplated), M' being preferably one or more of Y, La, Eu, Lu,
arid
Sc, _m being preferably about 1, S being typically in the range 1.5-2.5, and
the
divergence from the nominal formula amounts of M and M' being typically at
most 1096. Currently preferred cuprates have nominal composition Ba2YCu309-8
where $ is preferably about 2.1. This material will also be referred to as
(Ba, Y)
cuprate.
Such bodies are frequently referred to herein as "wires" or "tapes",
respectively. This usage is not intended to imply any limitation, e.g., with
regard
to cross section of the wire-like bodies (for instance, such bodies may
advantageously have noncircular cmss section and may also comprise a
multiplicity of coaxial superconductive bodies).
The inventive method comprises forming an intermediate body
comprising a cladding material surrounding a quantity of oxide powder, forming
an elongate body from the intermediate body by aneans of one or more cross
section-reducing operations (e.g., one or more passes through wire drawing
dies,
or through rolling apparatus), and heat treating the elongate body.
1 341 39 4
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Frequently the elongate body will be subjected to a shaping operation
prior to the heat treatment, such that the elongate body is put into a form
that
substantially corresponds to the shape in which the body is to be used. For
instance, the body may be wound helically on a mandrel into the shape of a
magnet coil.
The intermediate body typically comprises a quantity of oxide powder
surrounded by a diffusion barrier which in turn is surrounded by a normal
metal
jacket. Exemplarily, the normal metal jacket is a copper tube, the diffusion
barrier
comprises a thin-walled silver tube inside a thin-walled Ni tube, and the
oxide
powder is packed into the silver tube. If the normal metal jacket material is
inert
with respect to the oxide then a diffusion barrier may not be required. Ag is
such
an inert metal, at least with regard to (Bo, Y) cuprate.
The heat treatment of the elongate body is carried out such that
substantial sintering of the oxide powder occurs, and such that, after
completion of
the heat treatment, the chemical composition of the sintered powder is within
predetermined limits that are associated with the occurrence of
superconductivity
in the sintered oxide powder or an unclad sintered oxide body produced from
the
powder.
The oxides of concern herein are relatively unstable with regard to
their oxygen content (e.g., they can readily lose oxygen when heated to some
relatively high temperature) and are superconductive only within a relatively
narrow range of oxygen content. Therefore, the invention requires that
measures
be taken to insure that, upon completion of the heat treatment, the oxygen
content
of the sintered material is such that the material becomes supercanductive at
a
technologically significant temperature, typically > 77K. Among such measures
are hermetic sealing of the intermediate body at ambient or higher oxygen
partial
pressure, optionally together with placement of oxygen donor material (e.g.,
Ba02
or Ag0) inside the diffusion barrier, or introduction of oxygen into the space
inside the diffusion barrier directly through the powder material, or possibly
through a perforated tube placed axially inside the barrier or through
perforations
in the normal metal cladding.
The inventive method can be used to, inter alia, produce monofilament
or multifilament superconductive wire of a variety of cross sectional shapes,
or to
produce tape or ribbon containing one nr mare supercanductive elements. Many
systems as well as apparatus can advantageously Comprise wire or tape
according
1 341394
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to the invention. The availability of these superconductive bodies typically
will make possible
operation at a higher temperature than would 6e possible with prior art
superconductive wire.
Exemplary of apparatus that advantageously comprises inventive wire or tape is
a superconductive
solenoid, and exemplary of such systems is a particle accelerator, a maglev
transportation system, a
fusion reactor with magnetic confinement, and a power transmission line.
Inventive bodies may
also be used as signal transmission lines in electronic apparatus.
Preferred embodiments become superconductive at a temperature T~ > 77K. An
example of a material with T~ > 77K is Ba,_~'Cu3U69. There have recently been
reported claims
that indications of superconductivity have been observed above 200K, at
temperatures as high as
240K, in some oxides (cuprates) of the type that is of concern herein. See,
for instance, New
York Times, Saturday, March 28, 1987, page 6, which reports on observations
made at Wayne
State University. See also J.T. Chen et al, "cJbservation of the Inverse ac
Josephson Effect at
240K", to be published. Similar claims have also been made by workers at
Berkeley University.
The inventive method for making elongate oxide supercond~rctive bodies
comprising a normal
metal cladding is broadly applicable to forming such bodies from oxide powder
and is, in
particular, applicable to forming such bodies from cuprate powders such as the
(La, Y) cuprate on
which the Wayne State and Berkeley experiments were done.
In accordance with one aspect of the invention there is provided a method of
producing an elongate superconductive body, CHARACTERIZED IN THAT the method
comprises
a) forming an intermediate body comprising a normal metal cladding sun~ounding
a quantity of
oxide powder and in contact therewith; b) forming an elongate body from the
intermediate body by
means of one or more cross-section-reducing operations; and c) heat treating
the elongate body
such that substantial sintering of the oxide powder occurs, with the oxide
powder being in contact
with an oxygen-containing atmosphere during at least a part of step c), with
the oxygen
2s concentration in the atmosphere such that the thus produced body manifests
superconductivity,
with T~ of 77K or above, wherein at least the portion of the cladding that is
in contact with the
oxide powder consists essentially of normal metal that is substantially inert
with respect to oxygen
and with respect to the oxide powder under the heat treating conditions.
In accordance with another aspect of the. invention there is provided an
article of
manufacture comprising an elongate superconductive body. wherein the
superconductive body
comprises a normal metal cladding material contactin'ly surrounding
substantially sintered oxide
powder, with at least a portion of the cladding that is in contact with the
oxide powder consisting
essentially of normal metal that is substantially inert with respect to oxygen
and with respect to the
oxide powder under heat treatment conditions used during manufacture of the
elongate body.
~ 341394
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Brief Descr~tion of the Drawings
FIGS. 1 and 2 show schematically in end view exemplary monofilament and
multifilament inventive wire, respectively;
FIG. 3 similarly depicts an exemplary inventive tape;
FIG. 4 schematically shows wire according to the invention shaped into a
helical coil;
FIG. 5 schematically depicts a superconductive magnet; and
FIG. 6 shows the resistance as a function of temperature of a clad
superconductive
oxide body according to the invention.
Detailed Description of Some Preferred Embodiments
For reasons similar to those given above for prior art superconductive wires,
wires and
tapes based on superconductive oxide also advantageously are composite bodies
that comprise
normal metal cladding that surrounds the superconductive oxide body or bodies.
A reason for
embedding the superconductive oxide body in a normal metal that is not present
in prior art
wires
1 ~4~ 39 4
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is the need to substantially eliminate interaction of the superconductive
material
with the environment. We have found, for instance, that the oxygen content of
some cuprate powders in air can decrease with time even at room temperature.
Such decrease can impair the superconduetive properties of the material.
Furthermore, the possibility of adverse reaction with water vapor, CC12, and
other
environmental gases exists. A still further reason is the need for mechanical
support of the generally relatively brittle sintered oxide body such that the
body
can withstand the Lorentz forces due to the interaction of the current through
the
body and the magnetic field created by the current.
FIG. 1 schematically depicts the end view of an exemplary wire ( 10)
according to the invention, in which 11 is the sintered oxide superconductive
filament, 12 is the normal metal (e.g., Cu) jacket, 13 and 14 are the two
layers of
a diffusion barrier, where 13 is a material (e.g., Ag, Au, Pd) that is
relatively inert
with respect to oxygen and the other constituents of the oxide, and 14 is a
material that substantially does not form an alloy with the material of 12 as
well
as with the material of 13. If 12 is Cu and 13 is Ag, then 14 can
advantageously
be Ni.
FIG. 2 similarly depicts schematically the end view of an exemplary
multifilament wire (20), in which each of three sintered oxide filaments 21 is
surrounded by a diffusion barrier 23, and is embedded in normal metal 22. A
further exemplary embodiment (not shown) comprises a superconductive oxide
filament surrounded by a dielectric layer which in turn is surrounded by a
tubular
superconductive oxide body, (possibly with diffusion barriers where needed)
with
this coaxial assembly being surrounded by normal metal cladding.
FIG. 3 schematically depicts an inventive tape 30, with 31 being the
sintered oxide body, 33 the diffusion barrier, and 32 the normal metal jacket.
Another exemplary inventive tape (not shown) contains a multiplicity of ribbon-
like superconduetive bodies embedded in a normal metal cladding.
A significant aspect of the invention is processing that results in a
normal metal-clad oxide superconductive body or bodies (filament(s),
ribbon(s))
having technologically significant superconductive properties. Among the most
important of these properties is the transition temperature Tc (herein
considered to
be the highest temperature at which the DC resistance is zero to within
measurerr~nt limits). Desirably TG > 77K. Furthermore, the transition
temperature of the composite clad body is desirably close to (preferably no
less
341394
-7-
than 9090 of) the transition temperature of a bulk ceramic body of essentially
the
same composition.
The processing is a multistep procedure that typically comprises some
or all of the steps below. The oxide starting material can be produced by a
known process that exemplarily comprises mixing metallic oxides, hydroxides,
carbonates, hydrates, oxalates or other reactive precursors with a lubricating
liquid
in the appropriate ratio to obtain the desired final composition, filtering
and drying
the slurry, fragmenting the dried cake, and calcining the fragments in an 02
containing atmosphere (exemplarily heating to 900°C, 2 hours at
temperature,
furnace cool). The calcined fragments are again milled, re-fragmented and
fired,
as needed to achieve homogeneity. The homogeneous material is then fragmented
to produce a powder of the desired mesh size.
The thus produced powder aptianally is heat treated (typically 300-
700oC, 10 minutes - 2 haurs, 02 partial pressure 0.1-10 afro, and/or is
optionally
mixed with oxygen donor powder (e.g., finely divided silver oxide such as Ag0)
or grain growth inhibitor (Ag powder). The powder is then packed into a
container that comprises the normal metal (e.g., Cu, Ag, maraging steel) outer
jacket and, optionally, one or more diffusion barriers (e.g., Ni and Ag). The
outer
jacket surface may be protected against oxidation by means of a layer of
appropriate material (e.g., Ag). The container is then closed (e.g., pinched
off or
welded), or loss of oxide powder prevented by other appropriate means, and
subjected to some appropriate cross section-reducing steps) such as drawing
through a series of dies, or rolling, swaging or extruding, either at room
temperature or at some other (typically elevated) temperature. The thus
produced
elongate composite body is then optionally subjected to a shaping operation
(e.g.,
wound on a mandrel into a rail shape).
The (shaped or unshaped) elongate body is then heat treated to result
in substantial sintering of the oxide powder. The currently preferred heat
treatment typically comprises heating the body to a temperature in the range
700-
950°C, maintaining it at that temperature until substantial sintering
has taken
place (exemplarily 0.1-100 haurs), relatively slaw cooling to a temperature in
the
range 300-700°C, and maintaining it at that temperature until the
desired oxygen
concentration is established in the sintered material (exemplarily 1-24
hours).
1 X41394
If the composite body comprises precipitation-hardenable normal
metal (e.g., managing steel) then the above heat treatment advantageously is
followed by a known precipitation hardening treatment.
The need to embed the oxide body in normal metal, together with the
tendency of the relevant oxides to lose oxygen a relatively high temperatures
(and
to take up oxygen at somewhat lower temperatures) requires novel processing
features. Among these features typically is the need to prevent contact of the
powder with material that can oxidize at temperatures encountered during
processing. The currently preferred technique for preventing such contact is
to
surround the oxide with a thin layer of an appropriate non-reactive material,
e.g.,
Ag or Au. Various materials such as Pd, Ru, Rh, Ir, Os, Pt, Ni, and stainless
steel
may also be useful under some circumstances. We refer to this layer as the
diffusion barrier.
Under some circumstances, inventive wire (or tape) need not comprise
a diffusion barrier. For instance, if the normal metal jacket consists of
metal that
is substantially inert with respect to oxygen and does not "poison" the oxide
then
no diffusion barrier is required. We have discovered that, at least for (Bo,
Y)
cuprate, Ag is such a normal metal.
Novel processing features are also occasioned by the need to maintain
the oxygen content of the sintered powder within a relatively narrow range.
Sintering of the oxide particles is frequently carried out at temperatures
above
about 700°C. We have observtd that at such temperatures under ordinary
pressure the oxides of interest herein frequently lose oxygen. Thus, the
inventive
method typically comprises features designed to prevent the loss of the freed
oxygen from the powder-containing space. Exemplarily this is accomplished by
hermetic sealing of the elongate body prior to the heat treatment, possibly in
a
high 02-pressure environment, by connecting a high pressure 02 reservoir to
the
ends of the elongate body during heat treatment, or by carrying out the heat
treatment in relatively high pressure (e.g., 2-20 afro) oxygen. In the latter
case
oxidation of the normal metal surface has to be prevented. Thus, either the
normal metal jacket consists of relatively inert material (e.g., Ag), or the
surface
of the jacket is coated with a relatively inert material (e.g., Ag, Au, or
Pd).
Instead of, or in addition to, measures designed to prevent loss of 02
from the space occupied by the oxide power, the inventive method may also
comprise measures designed to introduce OZ into that space. Exemplarily this
can
1 34~ 39 4
be done by forcing a flow of 02 through the space, ar, preferably, by
introducing
an oxygen donor material (e.g., BaO2 or Ag0 powder) into the space. Such
donor material releases oxygen during heat treatment, with the released oxygen
then txing available for incorporation into the superconductive oxide. It will
be
appreciated by those skilled in the art that a material, in order to be useful
as an
oxygen donor must not be a poison of the superconductive oxide, i.e., react
with
the oxide in a manner that substantially impairs its superconductive
properties.
In some cases it may be possible to place a perforated tube into the
powder space of the intermediate body, such that, after carrying out the size-
reducing operation, a perforated channel exists through the powder. Through
this
channel 02 can then easily be supplied. In some cases, it may be advantageous
to
perforate, at appropriate intervals, the normal metal that surrounds the oxide
powder, and to contact the perforated elongate body with oxygen during the
heat
treatment.
As indicated above, the supercanductive oxides frequently give up
oxygen when heated to a relatively high temperature (e.g., about 900oC) in
ambient air or in moderate (e.g., 1 atmosphere) 02 pressure. On the other
hand,
these oxides frequently take up oxygen upon cooling to intermediate
temperatures.
This thermodynamic property of the oxide may form the basis of a novel heat
treatment in which the oxygen partial pressure over the oxide powder is
adjusted
such as to maintain optimum oxygen stoichiometry during the heat treatment.
Although the details of the treatment depend frequently on the composition of
the
powder as well as the temperature, it can be said that in the novel variable
O2-
pressure heat treatment the (relatively high temperature) sintering step
typically is
carried out at a relatively high (e.g., 1.5-20 arm) 02 partial pressure, and
the
subsequent (relatively low temperature) step is carried out at a relatively
low (e.g.,
1-5 atm) 02 partial pressure, with a slow cool from the high to the low
temperature being currently preferred.
As will be appreciated by those skilled in the art, the temperature at
which a particular heat treatment step is carried out depends inter olio on
the
length of the treatment step. Thus, the sintering can be carried out at a
relatively
low temperature (e.g., 700°C) if the sintering time is relatively long
(c.g., > 24
hours). Typically the sintering temperature is in the range 600-1100°C,
and the
time from 0.1 to 1000 hours. The length and temperature of the sintering step
typically also depend on the size of the oxide particles, with smaller
particles
~ ~4~ 39 4
to
making possible shorter time andlor lower temperature, due to the increased
thermodynamic driving force for the sintering process. It is thus currently
considered advantageous to use relative small particle size powders (e.g., < 5
p,m,
preferably < 2 ~tm, more preferably < 0.5 ~.m, average diameter) in the
practice
of the invention.
The oxide powders used in the practice of the invention
advantageously have stoichiometric composition. By this we mean herein that
they have a composition that is associated with high temperature
superconductivity
in bulk ceramic bodies produced from such powders. We have discovered that in
at least some cases the partitioning (e.g., ball milling) process that is used
to
produce particles of the desired mesh size may, in addition to straining the
material, result in a change in composition. Far this reason, it may in some
circumstances be desirable to subject the properly sized powder to a
relatively low
temperature (e.g., 300-700°C) oxygen anneal (exemplarily 0.1-10 atm of
02) for
about 10 minutes - 2 hours prior to compacting the powder into the
intermediate
body.
As indicated above, in many cases it is desirable to shape the elongate
clad body that is produced from the intermediate body by some appropriate
known
cross section-reducing process prior to heat treating the elongate body. The
heat
treatment typically results in sintering of the oxide powder and therefore
typically
reduces the formability of the superconductive element. On the other hand, the
heat treatment frequently results in softening of the normal metal components)
of
the elongate body. In order to produce inventive bodies having 'both
formability
prior to sintering and good mechanical strength, it is frequently desirable to
use
precipitation hardenable normal metal as cladding material. As is well known,
such alloys (e.g., managing steel, or Cu-Ni-Sn spinodal alloy) can be hardened
by
means of a relatively low temperature treatment after wire drawing and
shaping.
Such treatment typically does not affect the superconductive properties of the
sintered oxide element(s), If applied to a properly shaped superconductive
wire or
tape (e.g., a helical coil) it can result in an article that can be readily
handled and
further processed.
Example I: Powder (approximately 2.5 ltam average particle size) of
nominal composition Ba2YCu306_g was produced by a known process and
subjected to a 600°C, 15 minutes anneal in 1 atm of 02. A bulk body
produced
from the thus prepared powder has TG(R~) of about 93 K. A silver tube (0.250
34~ 39 4
-11-
inches outside diameter, 0.030 inches wall thickness, was filled with the
powder
and the ends of the tube sealed. The thus produced preform was drawn down to
0.060 inches diameter in 15 passes at room temperature. The resulting wire was
wound into a coil on a 1.5 inch diameter mandrel. At this stage the coil does
not
exhibit superconductivity. The coil was then heat treated as follows: heated
to
900°C, maintained at 900°C for 8 hours; furnace cooled to
600°C, maintained at
600°C for 4 hours; furnace cooled to about 350°C. All of this
treatment was
carried out in about 1 atm of flowing 02. The coil was then removed from the
furnace, and a portion tested by standard DC-resistance measurement. The
results
of this measurement are given in FIG. 6. As can be seen, the wire was fully
superconducting at 91 K. The sintered oxide was removed from a portion of the
coil, powdered and analyzed by a standard powder X-ray technique. The spectrum
of the material was essentially the same as that of a sintered bulk sample of
composition Ba2YCu306.~.
Example II: A coil is prepared substantially as described in
Example I, except that a 3/16 inch OD, 1/8 inch ID copper tube is used. A
diffusion barrier was formed by first wrapping 0.002 inch thick Ni foil into
the
tube, followed by similarly wrapping 0.002 inch thick Au foil into the tube.
The
thus formed normal metal jacket is then filled with powder. A portion of the
wire
is measured. The superconductive properties are substantially the same as
those
of the wire of Example I.
Example III: A coil was produced substantially as described in
Example I, except that a 3/16 inch OD, 1/8 inch ID copper tube was used
instead
of the silver tube. No superconductivity above ?'7 K was observed in the coil.
Example IV: A coil was prepared substantially as described in
Example I. After winding of the coil but prior to heat treating the coil, the
Ag
clad is perforated at intervals of about 1 inch. The heat treated coil was
superconductive, with Tc(R=0) about 91 K.
Example V: A coil was prepared substantially as described in
Example II, except that 0.002 inch Pt foil was used instead of the Au foil. A
portion of the wire was measured and showed a broad transition to a low
resistance state, terminating at about 30 K.
Example VI: A tape is prepared substantially as described in
Example I, except that the sealed, powder-filled Ag tube is elongated by
rolling to
0.010 inch thickness in a standard mlling mill.
1 X41394
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Example YII: A coil is prepared substantially as described in
Example I except that Ag0 powder (about I.3 ~,m average particle size) is
mixed
with the cuprate powder (20% b.w. Ag0). The coil has a superconductive
transition substantially as shown in FIG. 6.
