Ductile, composite superconductors having a continuous Re-Ba-Cu-O type or Bi-Sr-Ca-Cu-O type phases and a continuous noble metal phase and a process for their preparation are disclosed.

FIELD OF THE INVENTION 
This invention relates to superconducting oxides. Several publications are 
referenced in this application by Arabic numerals within parentheses in 
order to more fully described the state of the art to which this invention 
pertains. Full citations for these references are found at the end of the 
specification immediately preceding the claims. 
BACKGROUND OF THE INVENTION 
Since high-temperature superconductivity with onset at 30.degree. K. was 
discovered for a mixed structure in the La-Ba-Cu-O system (1), a great 
deal of effort has been devoted to the synthesis of a new oxide 
superconductor exhibiting a high critical temperature (T.sub.c). Thus, 
high critical temperatures of 92.degree. K. and 95.degree. K. were 
recently obtained in oxide compounds based on the Y-Ba-Cu-O (2) and 
Yb-Ba-Cu-O (3) systems, respectively. The crystal structure of these 
compounds has been identified as that of oxygen deficient perovskite, 
tetragonal K.sub.2 NiF.sub.4. 
Subsequently, non-rare earth type superconducting oxides containing bismuth 
were discovered (4, 5, 6) and found to have superconducting dual phases 
which are responsible for a high T.sub.c of 105.degree. K. and a low 
T.sub.c of 75.degree. K., respectively. Initially, the crystal structure 
of these phases was reportedly indexed as an orthorhomic structure (pseudo 
tetragonal structure) in which lattice parameters a, b are very similar, 
i.e., a.about.b (7, 8, 9, 10). However, a high resolution electron 
microscopy study on the Bismuth oxides showed that the orthorhombic cell 
has a modulated structure along the a-axis which is five times the subcell 
size b.sub.sc, i.e., b.about.5b.sub.sc and b=27 .ANG.(11, 12, 13). All the 
observed lattice parameters are approximately within the range of 
a.about.5.41+0.015 .ANG., b.about.5.4+015 .ANG.; c.about.30.8+0.1 .ANG.. 
The high T.sub.c associated with these oxides represents a potential for 
significant technological applications. High T.sub.c oxide superconductors 
have been prepared through many processes of blending, pressing and 
sintering and the shape is usually in bulk form. For many significant 
technological applications, it is desired to fabricate superconducting 
oxides in wire or filament form. However, processing and fabrication of 
superconducting oxides into desired forms such as wire and filament are 
accompanied by serious problems due to inherent material properties such 
as brittleness, anisotropy in current flow, weak link behavior caused by 
defect structures, etc. In an effort to alleviate these problems, 
diversified approaches to processing and fabrication have been explored. 
One such approach is the "precursor alloy-oxidation route" in which 
precursor alloys containing the constituent elements, with the exception 
of oxygen, of the superconducting oxide are prepared by rapid 
solidification (14, 15, 16) or mechanical alloying (17, 18) followed by 
oxidation at high temperatures for a prolonged period of time in order to 
form the superconducting orthorhmobic perovskite structure. While the 
precursor alloys are homogeneous and often show amorphous structures, the 
resulting superconducting oxides are still inherently brittle and 
therefore not suited for fabrication into wire or filament. 
In another approach, superconducting oxide powders are compacted into a 
metal tube, sealed under vacuum and then extruded into wires at high 
temperatures. However, the as-extruded wires are not superconducting 
because the powders have lost oxygen during the high temperature extrusion 
process. Therefore, oxygen must be replenished by annealing at 
intermediate temperatures so that the powders can regain superconductivity 
by absorption of oxygen which penetrates through the metal wall of the 
wire to the powders. However, the time required for oxygen to penetrate 
the metal wall of the wire to reach the powders is too long to be 
practical. Alternatively, the metal wall of the wire can be chemically 
etched to expose the powders and the wire is then immersed in an oxygen 
atmosphere. However, this alternative is too costly to be economically 
practical. 
In still another process, the addition of up to 50 weight percent of silver 
(19) or gold (20) to the precursor alloys in rare earth metal systems and 
the addition of up to 60 weight percent of silver in the bismuth system 
(21) have improved ductility in the resultant superconducting oxides. 
Notwithstanding this improvement in ductility, only ribbons of limited 
length. A greater improvement in ductility is required to obtain longer 
ribbons and wires. However, in the rare earth metal system, it has been 
found that a composite formed by oxidation of a Eu-Ba-Cu precursor alloy 
containing 70 weight percent gold was not a superconductor because the 
oxide particles were too widely separated. In other words, the 
discontinuity of the oxide phase prevented superconductivity. 
Thus, a significant advance in the art would be achieved by providing a 
composite simultaneously possessing the ductility required for practical 
applications (e.g. wire or ribbon of substantial length) and a continuous 
superconducting oxide phase required for superconductivity. 
OBJECTS OF THE INVENTION 
It is an object of the invention, therefore, to provide a composite, 
superconducting material having continuous, noble metal and oxide phases, 
having improved ductility and having a transition temperature of at least 
77.degree. K. 
It is another object of the invention to provide a simple and economic 
process for preparing a superconducting composite having continuous noble 
metal and oxide phases, improved ductility and a transition temperature of 
at least 77.degree. K. 
These and other objects, features and advantages of the invention will 
become readily apparent from the ensuing description and the novel 
features will be particularly pointed out in the appended claims. 
SUMMARY OF THE INVENTION 
Ductile superconductors are desired for applications requiring ribbons or 
wires of substantial length and flexibility. In rare earth(Re)-Ba-Cu-O 
type superconducting oxides, some degree of ductility has been achieved by 
using a precursor alloy incorporating up to 50 atomic percent of a noble 
metal, rapidly solidifying the molten composite and then annealing in air 
to achieve the superconducting oxide. However, the resulting 
superconducting oxide composites still do not have the ductility required 
for wires or ribbons greater than one foot in length. While increasing the 
noble metal content of the precursor alloy achieves greater ductility, the 
resulting composite is not superconducting after conventional annealing 
procedures due to discontinuous superconducting oxide phases. 
In contrast, the invention makes possible the production of composite 
superconductors having a noble metal content of greater than 50 atomic 
percent which has the degree of ductility required to produce flexible 
ribbons or wires of substantial length, advantageously greater than one 
foot in length, and which are superconducting by virtue of a continuous 
superconducting oxide phase. In accordance with the invention, the 
continuous superconducting oxide phase, which imparts the 
superconductivity to the composite, is made possible by using an inert gas 
atmosphere during preliminary annealing of the rapidly solidified 
precursor composite alloy. It is believed that the annealing process in 
accordance with the invention causes the superconducting oxide phase to 
migrate to the outer limits of the composite material to form a continuous 
superconducting oxide phase which encapsulates a continuous noble metal 
core. The inner core of noble metal affords the improved ductility of the 
composite superconductors of the invention. 
Therefore, in one aspect, the invention is broadly directed to a process 
for preparing a ductile, composite superconductor which comprises the 
steps of forming a mixture having an overall composition comprising about 
1 to 16 atomic percent of a first constituent selected from the group 
consisting of rare earth metals, about 2 to 25 atomic percent of a second 
constituent selected from the group consisting of Group II substances, 
about 3 to 35 atomic percent of copper and greater than about 50 atomic 
percent of a fourth constituent selected from the group consisting of 
noble metals, heating the mixture to form a molten alloy, rapidly 
solidifying the molten alloy and annealing the solidified alloy in an 
inert atmosphere followed by an oxygen containing atmosphere. 
In another aspect, the invention is directed to a ductile, composite 
superconductor having a continuous noble metal phase, a continuous 
superconducting oxide phase and having an overall composition comprising 
about 0.8 to 3.3 atomic percent of a first constituent selected from the 
group consisting of rare earth metals, about 1.7 to 6.6 atomic percent of 
a second constituent selected from the group consisting of Group II 
substances, about 2.5 to 9.9 atomic percent of copper, about 5 to 30 
atomic percent oxygen and greater than about 50 atomic percent of a fifth 
constituent selected from the group consisting of noble metals. 
In yet another aspect, the invention is directed to a ductile, composite 
superconductor having a continuous noble metal phase and a continuous 
superconducting oxide phase, said superconductor having been prepared by a 
process comprising the steps of forming a mixture having an overall 
composition comprising about 1 to 16 atomic percent of a first constituent 
selected from the group consisting of rare earth metals, about 2 to 25 
atomic percent of a second constituent selected from the group consisting 
of Group II substances, about 3 to 35 atomic percent of copper and greater 
than about 50 atomic percent of a fourth constituent selected from the 
group consisting of noble metals, heating the mixture to form a molten 
alloy, rapidly solidifying the molten alloy and annealing the solidified 
alloy in an inert atmosphere followed by an oxygen containing atmosphere. 
In still another aspect, the invention is directed to an alloy having an 
overall composition comprising about 1 to 16 atomic percent of a first 
constituent selected from the group consisting of rare earth metals, about 
2 to 25 atomic percent of a second constituent selected from the group 
consisting of Group II substances, about 3 to 35 atomic percent of copper 
and greater than about 50 atomic percent of a fourth constituent selected 
from the group consisting of noble metals. 
In another aspect, the invention is directed to an alloy having been 
prepared by a process comprising the steps of forming a mixture having an 
overall composition comprising about 1 to 16 atomic percent of a first 
constituent selected from the group consisting of rare earth metals, about 
2 to 25 atomic percent of a second constituent selected from the group 
consisting of Group II substances, about 3 to 35 atomic percent of copper 
and greater than about 50 atomic percent of a fourth constituent selected 
from the group consisting of noble metals, heating the mixture to form a 
molten alloy and rapidly solidifying the molten alloy. 
In still another aspect, the invention is directed to a process for 
preparing a ductile, composite superconductor comprising the steps of 
forming a mixture having an overall composition of about 1 to 30 atomic 
percent of bismuth, about 2.0 to 35 atomic percent of a second constituent 
selected from the group consisting of Group II substances, about 1.5 to 30 
atomic percent of copper and greater than about 60 atomic percent of a 
fourth constituent selected from the group consisting of noble metals, 
heating the mixture to form a molten alloy, rapidly solidifying the molten 
alloy and annealing the solidified alloy in an oxygen containing 
atmosphere. 
In yet another aspect, the invention is directed to a ductile, composite 
superconductor having a continuous noble metal phase, a continuous 
superconducting oxide phase and having an overall composition comprising 
about 1 to 8 atomic percent of bismuth, about 1 to 14 atomic percent of a 
second constituent selected from the group consisting of Group II 
substances, about 1 to 8 atomic percent of copper, about 3 to 30 atomic 
percent oxygen and greater than about 60 atomic percent of a fifth 
constituent selected from the group consisting of noble metals. 
In another aspect, the invention is directed to a ductile, composite 
superconductor having a continuous noble metal phase and a continuous 
superconducting oxide phase, said superconductor having been prepared by a 
process comprising the steps of forming a mixture having an overall 
composition comprising about 1 to 30 atomic percent of bismuth, about 2 to 
35 atomic percent of a second constituent selected from the group 
consisting of Group II substances, about 1.5 to 30 atomic percent of 
copper and greater than about 60 atomic percent of a fourth constituent 
selected from the group consisting of noble metals, heating the mixture to 
form a molten alloy, rapidly solidifying the molten alloy and annealing 
the solidified alloy in an oxygen containing atmosphere. 
In another aspect, the invention is directed to an alloy having an overall 
composition comprising about 1 to 30 atomic percent of bismuth, about 2 to 
35 atomic percent of a second constituent selected from the group 
consisting of Group II substances, about 1.5 to 30 atomic percent of 
copper and greater than about 60 atomic percent of a fourth constituent 
selected from the group consisting of noble metals. 
In yet another aspect, the invention is directed to an alloy having been 
prepared by a process comprising the steps of forming a mixture having an 
overall composition of about 1 to 30 atomic percent of bismuth, about 2 to 
35 atomic percent of a second constituent selected from the group 
consisting of Group II substances, about 1.5 to 30 atomic percent of 
copper and greater than about 60 atomic percent of a fourth constituent 
selected from the group consisting of noble metals, heating the mixture to 
form a molten alloy and rapidly solidifying the molten alloy

DETAILED DESCRIPTION OF THE INVENTION 
The continuous superconducting oxide phase of the composite superconductors 
prepared in accordance with one embodiment of the invention can be any 
Re-Ba-Cu-O type superconducting oxide having a critical temperature 
(T.sub.c) of at least 30.degree. K and advantageously at least 77.degree. 
K. These include the so-called "1, 2, 3" superconducting oxides such as 
La.sub.1 Sr.sub.2 Cu.sub.3 O.sub.7-x, Yb.sub.1 Ba.sub.2 Cu.sub.3 
O.sub.7-x, Eu.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, and Ho.sub.1 Ba.sub.2 
Cu.sub.3 O.sub.7-x. 
Thus, in accordance with the process of the invention, a "precursor" alloy 
is formed from a mixture having an overall composition comprising about 1 
to 16 atomic percent of a first constituent selected from the group 
consisting of rare earth metals, about 2 to 25 atomic percent of a second 
constituent selected from the group consisting of Group II substances, 
about 3 to 35 atomic percent of copper and greater than about 50 atomic 
percent of a fourth constituent selected from the group consisting of 
noble metals. 
Rare earth metals include elements in the Periodic Table with atomic 
numbers 57 through 71, advantageously lanthanum, ytterbium, europium, and 
holmium. The first constituent can be a single rare earth metal or a 
mixture of two or more rare earth metals. 
Group II substances comprising the second constituent include beryllium, 
magnesium, calcium, strontium, barium and radium. Calcium, strontium and 
barium are advantageously used. The second constituent can be a single 
Group II substance or a mixture of two or more Group II substances. For 
example, the first constituent can be a mixture of calcium and strontium, 
calcium and barium, calcium, strontium and barium or strontium and barium. 
However, the second constituent is very advantageously barium alone. 
The fourth constituent comprises greater than about 50 atomic percent of a 
noble metal. These are metals which are not readily oxidized, in 
particular, silver, gold, platinum and palladium. Silver, gold and 
platinum are advantageously used. The fourth constituent can be a single 
noble metal or a mixture of two or more noble metals. Advantageously, the 
fourth constituent is a single noble metal, for example, silver. The 
amount of noble metal is advantageously about 55 to 95 atomic percent, 
more advantageously about 60 to 95 atomic percent and most advantageously 
about 70 to 90 atomic percent. In a preferred embodiment, the amount of 
noble metal is about 90 atomic percent. 
In one embodiment, a precursor alloy comprises about 6.7 atomic percent 
ytterbium, about 13.3 atomic percent barium, about 19.9 atomic percent 
copper and about 60 atomic percent silver. In another embodiment, a 
precursor alloy comprises about 1.6 atomic percent ytterbium, about 3.4 
atomic percent barium, about 5 atomic percent copper and about 90 atomic 
percent silver. 
The mixture of component elements of the precursor alloy are then heated to 
form a molten alloy. Advantageously, the molten alloy is homogeneous. The 
temperature used to form the molten alloy is higher than the melting point 
of the element having the highest melting point of all the elements in the 
mixture. Induction melting can be used to form the molten alloy. The 
heating is advantageously carried out in an inert atmosphere such as 
argon, helium or nitrogen. The component elements of the alloy can be 
melted and formed into an ingot which is then remelted to insure 
homogeneity of the molten alloy. 
The molten alloy is then rapidly solidified. The term "rapidly solidified" 
as used herein means that the molten alloy is quenched at a rate such that 
it solidifies substantially instantaneously. Thus, the quench rate ranges 
from about 10.sup.3 degrees per second to about 10.sup.7 degrees per 
second, advantageously from about 10.sup.4 degrees per second to about 
10.sup.6 per second. A quench rate of 10.sup.5 degrees per second is most 
advantageously used. Rapid solidification provides for the uniform 
dispersion of the noble metal through the other component materials. Rapid 
solidification can be achieved by a number of processing techniques 
well-known in the art, such as, for example, splat-quenching and 
melt-spinning. Each of these techniques involves withdrawing a small 
amount of the molten alloy and then instantaneously quenching the small 
amount so that it rapidly solidifies. For the preparation of precursor 
alloys in the form of ribbon or wire, melt-spinning is advantageously 
used. Rapid solidification is conducted in an inert atmosphere. 
Thus, in accordance with one embodiment of the invention, the molten alloy 
is expressed from an orifice, usually in the bottom of a crucible, to form 
a molten stream of the alloy which is then immediately impinged onto a 
rotating chill surface which is moving at such a rate with respect to the 
expressed molten stream that the stream rapidly solidifies to form an 
alloy ribbon. Alloy ribbons produced in this manner have a thickness of 
about 2 to 100 .mu.m and can be of unlimited length. Ribbons are 
advantageously prepared from homogeneous alloy buttons using a single 
roller melt spinner. The process is conducted in an inert atmosphere 
(e.g., argon, helium or nitrogen) in which the alloy button is melted in a 
crucible and the molten alloy is then expressed through an orifice in the 
crucible onto a rotating chill disk, e.g., a copper chill disk, with a 
circumferential speed of about 8 to 14 m/sec. The molten alloy is 
expressed at a pressure of about 5 to 20 psi onto the rotating disk at an 
angle of about 10.degree. to 80.degree.. One skilled in the art will 
readily appreciate that a variety of melt-spin processing parameters can 
be used depending on the size of the ribbon desired. Melt spun precusor 
alloy ribbons have a bend strain ductility of about 0.25 to 0.4. 
If it is desired to form the precursor alloy into a wire, the molten alloy 
can be expressed from an orifice into a rotating chill drum which is 
moving at such a rate with respect to the expressed molten stream of alloy 
such that the stream rapidly solidifies to form a wire of about 100 to 300 
.mu.m in diameter and of unlimited length. The chill drum contains a 
cooling fluid such as water or a suitable hydrocarbon mixture. 
The preformed, precursor alloy is then annealed in an inert atmosphere 
followed by an oxygen containing atmosphere. Annealing is carried out at a 
temperature and for a period of time sufficient to form a superconductor 
having a continuous noble metal phase and a continuous superconducting 
oxide phase. The resulting ductile, composite superconductor has a T.sub.c 
of at least about 30.degree. K. and advantageously at least about 
77.degree. K. The inert atmosphere can be any gas which does not react 
with the alloy. Suitable gases include argon, helium and nitrogen. Argon 
is advantageously used. The chamber in which annealing takes place can be 
evacuated before annealing begins. Thus, the inert atmosphere can be 
maintained within the annealing chamber at pressures ranging from about 
10.sup.-5 atmospheres to about 10 atmospheres, advantageously from about 
0.5 atmospheres to about 2 atmospheres. More advantageously, the inert 
atmosphere maintained in the chamber is at about 1.1 atmospheres to about 
1.5 atmospheres. 
In one embodiment, the alloy is heated in the inert atmosphere over a 
period of about ten minutes to two hours to a temperature between about 
300.degree. C. and 1050.degree. C. Once the temperature is reached, an 
oxygen containing atmosphere is introduced into the annealing chamber such 
that the total oxygen partial pressure is about 0.1 to 2 atmospheres, 
advantageously 0.1 to 1 atmosphere. A total oxygen partial pressure of 0.1 
to 0.2 atmosphere is very advantageous. The oxygen containing atmosphere 
can be ambient air or pure oxygen. The oxygen containing atmosphere is 
maintained throughout the remainder of the annealing procedure. Upon 
introduction of the oxygen containing atmosphere into the annealing 
chamber, the temperature in the chamber is maintained for a period of 
about 1 to 50 hours, then cooled over a period of about 1 to 5 hours to a 
second temperature between about 500.degree. C. and 700.degree. C., 
maintained at the second temperature for a period of about 5 to 50 hours, 
cooled over a period of about 1 to 5 hours to a third temperature between 
about 300.degree. C. and 500.degree. C., maintained at the third 
temperature for a period of about 5 to 24 hours and finally cooled over a 
period of about 1 to 5 hours to room temperature. 
In accordance with another embodiment of the invention, the continuous 
superconducting oxide phase of the composite superconductor can be any 
Bi-Sr-Ca-Cu-O type superconducting oxide having a T.sub.c of at least 
30.degree. K and advantageously at least 77.degree. K. These include 
superconducting oxides such as Bi.sub.1 Sr.sub.1 Ca.sub.1 Cu.sub.2 
O.sub.6-x, Bi.sub.4 Sr.sub.3 Ca.sub.3 Cu.sub.4 O.sub.15-x and Bi.sub.4 
Sr.sub.3 Ca.sub.3 Cu.sub.6 O.sub.17-x. 
Thus, in accordance with the process of the invention, a "precursor" alloy 
is formed from a mixture having an overall composition comprising about 1 
to 30 atomic percent of bismuth, about 2 to 35 atomic percent of a second 
constituent selected from the group consisting of Group II substances, 
about 1.5 to 30 atomic percent of copper and greater than about 60 atomic 
percent of a fourth constituent selected from the group consisting of 
noble metals. 
The Group II substances and noble metals are as described above. The amount 
of noble metal is advantageously about 65 to 95 atomic percent, more 
advantageously about 70 to 90 atomic percent. In a preferred embodiment, 
the amount of noble metal is about 90 atomic percent. 
In an embodiment, a precursor alloy comprises about 2.8 atomic percent 
bismuth, about 2.1 atomic percent strontium, about 2.1 atomic percent 
calcium, about 2.8 atomic percent copper and about 90 atomic percent 
silver. 
The mixture of component elements of the precursor alloy are then processed 
in substantially the same manner as that described with reference to the 
rare earth-based alloys described above to afford ductile, composite 
superconductors having a T.sub.c of at least 30.degree. K., advantageously 
at least 70.degree. K. In general, annealing is carried out at a 
temperature and for a period of time sufficient to form a superconductor 
having a continuous noble metal phase and a continuous superconducting 
oxide phase. In one embodiment, an alloy ribbon is heated in air at a rate 
of 10.degree. C. per minute to a temperature of 850.degree. C., held at 
that temperature for six hours and then cooled to room temperature at a 
rate of 5.degree. C. per minute. 
A better understanding of the present invention and of its many advantages 
will be had by referring to the following examples, given by way of 
illustration. 
Example 1 
A. Preparation of (Bi.sub.4 Sr.sub.3 Ca.sub.3 Cu.sub.4).sub.10 Ag.sub.90 
Precusor Alloy Ribbon 
An alloy button was prepared by mixing 0.2873 g Bi, 0.0892 g Sr, 0.0408 g 
Ca, 0.0863 g Cu and 4.5 g Ag. The mixture was then melted in an arc 
furnace under argon atmosphere at a temperature of about 1100.degree. C. 
Upon cooling, the resulting alloy button was turned over and remelted to 
ensure homogeneity. 
The alloy button was then broken into a few pieces and charged into the 
quartz crucible of a melt-spinner apparatus. The quartz crucible had an 
internal diameter of 14 mm and an orifice of 0.7 mm in diameter. The alloy 
was induction melted under an atmosphere of argon at about 0.33 to 0.5 
atmospheres. When the melt temperature reached about 1100.degree. C., the 
quartz crucible was pressurized with argon and the melt expressed at a 
pressure of 5-8 psi through the orifice onto a spinning disk with a 
circumferential speed of 8 to 10 m/sec. The resulting ribbon, shown in 
FIG. 1, had a thickenss of 40-70 um and a width of 2-3 mm. 
B. Annealing of the Precusor Ribbon 
The precusor alloy ribbon was placed in a furnace and annealed in air by 
heating at a rate of 10.degree. C. per minute to a temperature of about 
850.degree. C. This temperature was maintained for six hours. The alloy 
was then allowed to room temperature at a rate of 5.degree. C. per minute. 
The X-ray diffraction pattern, shown in FIG. 2, indicates the presence of 
the superconducting orthorhombic crystal structure in the oxide phase. 
EXAMPLE 2 
A precursor alloy ribbon (Yb.sub.1 Ba.sub.2 Cu.sub.3).sub.40 Ag.sub.60 was 
prepared in the manner described in Example 1A. The ribbon was then placed 
in a furnace and heated in an argon atmosphere to a temperature of about 
860.degree. C. When this temperature was reached, oxygen was pumped into 
the furnace and the temperature of 860.degree. C. was maintained for about 
20 hours. The alloy was then allowed to cool over a period of about 5 
hours to 550.degree. C., held at that temperature for about 11.5 hours, 
further allowed to cool over a period of about 5 hours to 300.degree. C., 
held at that temperature for about 16.5 hours and then allowed to cool to 
room temperature. FIG. 3 is an optical micrograph of a section of the 
annealed alloy showing the separated silver (A) and oxide (B) phases. It 
is believed that the oxide phase is superconducting. 
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