Fabrication of oxide superconductors by melt growth method

Superconductive oxide bodies such as wires, ribbons, rods, and other bulk bodies can be fabricated by a process that comprises melting precursor material, cooling at least of the melt such that a solid body of a desired shape results, and heat treating the solid body in an oxygen-containing atmosphere. The precursor material exemplarily is in the form of pressed superconductive oxide powder. The re-solidified superconductive material is relatively dense, typically textured, with relatively large grain size, and has improved properties, e.g., higher critical current density. An exemplary technique for melting of the precursor material is zone melting.

FIELD OF THE INVENTION 
This invention pertains to methods for producing superconductive bodies, 
and to apparatus and systems comprising a superconductive body produced by 
such a method. 
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 or 
intermetallic compounds (e.g., Nb.sub.3 Ge, probably the material with the 
highest transition temperature T.sub.c known prior to 1986). 
Recently, superconductivity was discovered in a new class of materials, 
namely, metal oxides. See, for instance, 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. 
The above 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.sub.c above 77K, the 
boiling temperature of liquid N.sub.2 (see, for instance, M. K. Wu et al, 
Physical Review Letters, Vol. 58, Mar. 2, 1987, page 908; and P. H. Hor et 
al, 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 discovery of composition and processing 
techniques that result in the formation of bulk samples of material that 
can be substantially single phase material and can have T.sub.c above 90K 
(see, for instance, R. J. Cava et al, Physical Review Letters, Vol. 
58(16), pp. 1676-1679), incorporated herein by reference. 
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 considered to be 
one of the most advantageous cryogenic refrigerants, and attainment of 
superconductivity at or above liquid nitrogen temperature was a 
long-sought goal which until very recently appeared almost unreachable. 
Although this goal has now been attained, there still exist barriers that 
have to be overcome before the new "ceramic" superconductors can be 
effectively utilized in technological applications. In particular, the 
ceramic high T.sub.c superconductive materials are relatively brittle. 
Development of techniques for fabricating the brittle compounds into 
bodies of desirable size and shape (e.g., wires or tape), and of 
techniques for improving the strength and/or other mechanical properties 
of ceramic superconductive bodies, is an urgent task for the technical 
community. Furthermore, techniques for increasing the critical current 
density J.sub.c of bodies formed from superconductive compounds are also 
of great significance. 
For a general overview of some potential applications of superconductors 
see, for instance, B. B. Schwartz and S. Foner, editors, Superconductor 
Applications: SQUIDS and MACHINES, Plenum Press 1977; and S. 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, 
as well as junction devices and detectors. It is expected that many of the 
above and other applications of superconductivity would materially benefit 
if high T.sub.c superconductive material could be used instead of the 
previously considered relatively low T.sub.c materials. 
The art has followed three approaches in producing ceramic superconductive 
compound bodies. One approach comprises providing the desired compound in 
powder form, producing a bulk body from the powder by any appropriate 
technique (e.g., cold or hot pressing in or through a die of desired size 
and shape, or forming a slurry and producing a tape therefrom by the 
doctor blade technique) and heat treating the resulting body. See U.S. 
patent application, Ser. No. 036,168, filed Apr. 6, 1987 for E. M. Gyorgy 
et al, titled "Apparatus Comprising a Ceramic Superconductive Body, and 
Method for Producing Such a Body now abandoned." The heat treatment 
invariably comprises treatment at a relatively high temperature that is 
intended to produce sintering of the powder particles, followed typically 
by optimization of the oxygen content of the material. The thus produced 
superconductive bodies typically are relatively porous (e.g., about 85% 
dense, depending on processing conditions). Furthermore, powder particles 
may not always be in intimate contact with their neighbors. The presence 
of voids and/or poor contact between particles is thought to be a possible 
reason for the relatively low strength and critical current of bodies 
produced from superconductive oxide powder by ceramic processing 
techniques. 
A recently filed U.S. patent application Ser. No. 046,825, filed May 5, 
1987 for S. Jin et al now abandoned) disclosed that some properties of 
superconductive compound bodies (e.g., their mechanical strength) can be 
improved by admixture of an appropriate metal powder (e.g., Ag) to the 
superconductive powder. 
The second approach typically comprises forming a "preform" by introducing 
a quantity of superconductive compound powder into a tubular normal metal 
body, reducing the cross section of the preform by, e.g., drawing through 
a die (or dies) or rolling, until the desired wire or ribbon is produced. 
The wire or ribbon is then typically wound into a coil or other desired 
shape, followed by a sintering treatment and, possibly, an oxygen 
content-optimizing treatment. Two recent U.S. patent applications Ser. No. 
036,160, filed Apr. 6, 1987 for S. Jin et al now abandoned, and Ser. No. 
046,825, filed May 5, 1987 for S. Jin et al now abandoned) disclose 
techniques for forming metal-clad high T.sub.c superconductive bodies. 
Such bodies typically are also relatively porous, and have the relatively 
low T.sub.c associated with high T.sub.c superconductors produced by 
ceramic processing techniques. 
The third approach to forming superconductive compound bodies comprises 
depositing a thin layer of the superconductive compound on an appropriate 
substrate. Deposition can be by any appropriate method, e.g., electron 
beam evaporation, sputtering, or molecular beam epitaxy. Another recently 
filed U.S. patent application Ser. No. 037,264, filed Apr. 10, 1987 for C. 
E. Rice now abandoned) discloses that thin superconductive films can be 
produced by forming a solution on a subtrate, and heat treating the thus 
formed thin layer. The high T.sub.c compound thin films known to the art 
are thought to be substantially 100% dense, and at least in isolated 
instances relatively high critical currents have been observed in such 
layers. 
In view of the fact that technologically significant superconductive wires, 
ribbons, and other bodies have to be able to carry relatively high current 
densities and to be able to withstand relatively large forces, fabrication 
methods that can result in high T.sub.c superconductive bodies having 
improved properties (including higher J.sub.c and, typically, greater 
strength and thermal conductivity) would be of considerable significance. 
This application discloses such a method. 
DEFINITIONS 
The Ba-cuprate system herein is the class of oxides of nominal general 
formula Ba.sub.2-x M.sub.1-y X.sub.x+y Cu.sub.3 O.sub.9-.delta., where M 
is one of Y, Eu, or La, and X is one or more optional element different 
from Ba and M and selected from the elements of atomic number 57-71, Sc, 
Ca, and Sr. Typically x+y is in the range 0-1 (with Ba and M being at 
least 50% unsubstituted), and typically 1.5&lt;.delta.&lt;2.5. In a particular 
preferred subclass of the Ba-cuprate system 0.ltoreq.y.ltoreq.0.1, with 
the original X being one or more of Ca, Sr, Lu and Sc. For further 
examples see D. W. Murphy et al, Physical Review Letters, Vol. 58(18), pp. 
1888-1890 (1987). 
A slightly different definition of the Ba-cuprate system that has also been 
used is based on the general formula Ba.sub.2-y (M.sub.1-x M.sub.x 
').sub.1+y Cu.sub.3 O.sub.9-.delta., where M and M' are chosen from Y, Eu, 
Nd, Sm, Gd, Dy, Ho, Er, Tm, Yb, Lu, La, Sc, Sr or combinations thereof, 
with typically 0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, and 1&lt;.delta.&lt;3. 
See, for instance, U.S. patent application Ser. No. 118,497, titled 
"Method of Producing Metal Oxide Material, and of Producing a 
Superconductive Body Comprising the Material", filed on Nov. 9, 1987 for 
S. Jin, M. Robbins, and R. C. Sherwood now abandoned. 
The La-cuprate system herein is the class of oxides of nominal general 
formula La.sub.2-x M.sub.x CuO.sub.4-.epsilon., where M is one or more 
divalent metals (e.g., Ba, Sr, Ca), and x.gtoreq.0.05, and 
0.ltoreq..epsilon..ltoreq.0.5. 
A "normal" metal herein is a metal that does not become superconductive at 
temperatures of technological interest, typically at temperatures of 2K 
and above. 
A body herein is "relatively dense" if at least a major part of the body 
has a density that is at least 90% of the theoretical density of the 
material in the part of the body. Preferably the density in the part of 
the body is greater than 95 or even 99% of the theoretical density. The 
theoretical density of Ba.sub.2 YCu.sub.3 O.sub.7 is about 6.4 g/cm.sup.3, 
and the density of sintered Ba.sub.2 YCu.sub.3 O.sub.7 bodies (heat 
treated to optimize the superconductive properties) typically is no more 
than about 5.5 g/m.sup.3 (about 85% of theoretical). 
SUMMARY OF THE INVENTION 
We have discovered that superconductive compound (e.g., Ba-cuprate and 
La-cuprate) bodies can be produced by a process that comprises melting a 
quantity of precursor material, and cooling at least a portion of the melt 
such that a solid body of a desired shape (e.g., a filament) results, with 
at least a substantial portion of the superconductive body having a 
density greater than about 90% of the theoretical density of the 
superconductive compound. The inventive method typically further comprises 
heat treating the solid body in an oxygen-containing atmosphere so as to 
impart the desired superconductive properties to the body. For instance, 
at least for an exemplary member of the Ba-cuprate system (nominal 
composition Ba.sub.2 YCu.sub.3 O.sub.7) an appropriate exemplary heat 
treatment comprises maintaining the body at a temperature in the 
approximate range 850.degree.-950.degree. C. in an oxygen-containing 
atmosphere for a period of time in the range 1-48 hours, followed by slow 
cooling. Although not a requirement, in many cases it will be desirable 
for the inventive method to be carried out such that the resulting body 
consists substantially of single phase material. 
The inventive technique represents a complete departure from prior art 
processing of bulk Ba-cuprate and La-cuprate material. These materials, 
which are ceramics, have in the past been processed by ceramic techniques. 
In general, ceramic processing techniques do involve high temperature 
treatment (e.g., sintering). However, standard high temperature processing 
of ceramic materials is carried out at temperatures below the melting 
point of the material, and melting of ceramic material is, to the best of 
our knowledge, not used in the production of any commercially significant 
ceramic. 
All the currently known high T.sub.c ceramic superconductive compounds are 
either La-cuprates or Ba-cuprates. Both these systems have complicated 
phase diagrams, with single phase-based superconductivity occurring only 
in relatively narrow compositional ranges. Due to these circumstances 
conventional theory suggests that solidification of material from a melt 
of composition corresponding to that of the superconductive phase will 
result in decomposition into a multiphase material that is not 
superconductive, or only partly superconductive at "high" temperatures 
(e.g., at or above 77K). 
Thus, not only is processing that involves melting of the ceramic material 
not within the normal repertoire of ceramicists but, based on the phase 
diagram of the prototypical Ba-cuprate YBa.sub.2 Cu.sub.3 
O.sub.9-.delta.), a man skilled in the art has good reason for not 
melt-processing the material. 
However, we have made the unexpected discovery that it is possible to 
produce superconductive material by a method that comprises cooling from 
the single phase liquid region (exemplarily sufficiently rapid cooling 
such that phase separation is substantially avoided or minimized), or that 
comprises cooling from the solid+liquid region of the phase diagram. 
After solidification of the melt a variety of treatments may be applied. 
For instance, the sample can be maintained in O.sub.2 at a temperature 
between the solidus and that of any solid state transformation (if such a 
transformation exists) for a relatively long period (e.g., 1-24 hrs) to 
facilitate homogenization and/or grain growth, followed by a slow cool 
(possibly with intermediate soaks) in O.sub.2 to room temperature. On the 
other hand, the solidified sample can be cooled slowly (typically in 
O.sub.2) to room temperature, with a later homogenization treatment in 
O.sub.2 at a temperature relatively close to but below the solidus 
(exemplarily 1-24 hours at 850.degree.-950.degree. C.). Intermediate 
treatment schedules are also possible. 
In a currently preferred embodiment the melt is cooled relatively rapidly 
(exemplarily within about 1-600 seconds) from the liquid region of the 
phase diagram to an intermediate temperature slightly below the 
solid+liquid region (exemplarily 10.degree.-100.degree. C. below the 
solidus and above any solid state phase transition temperature that may be 
present in the system), followed by a heat treatment that favors the 
growth of crystallites of the superconductive phase and avoids thermal 
shock and the consequent formation of microcracks. If the cooling from the 
melt is carried out too slowly (e.g., within more than about 10 minutes) 
an unacceptable amount of phase separation (typically more than about 15% 
by volume of non-superconductive phase) is likely to occur, and if the 
cooling is carried out too quickly (e.g., within less than about 1 
second), microcracks may form. The details obviously will depend, inter 
alia, on the amount and shape of the material that is to be solidified. 
The heat treatment of the solidified material exemplarily comprises a slow 
cool (e.g., furnace cool, 1-100 hours) in O.sub.2 to about room 
temperature, and optionally comprises a soak in O.sub.2 at the 
intermediate temperature, or some other elevated temperature. Exemplarily, 
a melt of composition YBa.sub.2 Cu.sub.3 O.sub.x (x.about.7) is maintained 
under O.sub.2 at about 1300.degree. C. for about 5 minutes, the melt is 
then rapidly cooled in O.sub.2 to about 950.degree. C., followed by a 
furnace cool in O.sub.2 to room temperature. The resulting material 
typically has spherulitic microstructure, with many grains being oblong, 
having a long axis of about 20-200 .mu.m. The material typically is 
substantially single phase, essentially 100% dense, and has T.sub.c of 
about 92K and J.sub.c of more than about 2000 A/cm.sup.2 (at 77K, with 
H=O). 
In a further exemplary embodiment the material is heated to a temperature 
in the solid+liquid region of the phase diagram, optionally maintained in 
that region under O.sub.2 for a period sufficient to result in 
establishment of approximate phase equilibrium (e.g., 5 second-5 hours), 
followed by a relatively slow cool (e.g., furnace cool 1-24 hours) in 
O.sub.2 to about room temperature (with an optional soak or soaks at one 
or more temperatures below the solidus not excluded). Exemplarily, a melt 
of composition YBa.sub.2 Cu.sub.3 O.sub.x is maintained at 1030.degree. C. 
for about 1 hour and furnace cooled. The resulting material typically has 
predominantly spherulitic microstructure, comprises, in addition to the 
superconducting phase, crystallites of nominal composition Y.sub.2 
BaCu.sub.3 O.sub.5 and also copper oxide and barium oxide. The material 
typically is essentially 100% dense, and has exemplarily T.sub.c of about 
92K and J.sub.c of about 1700 A/cm.sup.2 (at 77K, with H=O). 
As is apparent from the above cited properties, superconductive bulk 
samples produced according to the invention can have higher J.sub.c 
(including substantially greater J.sub.c in a magnetic field) than prior 
art bodies. Furthermore, bodies produced according to the invention 
typically also can have greater mechanical strength and greater thermal 
conductivity. 
The known superconductive oxides are relatively reactive and can be 
expected to interact with most common crucible materials. Consequently, it 
may be advantageous to use a crucible-free method of melting of the 
starting material, or to melt the starting material in a crucible that 
provides a particular constituent to a starting material that is initially 
deficient in that constituent. For instance, Y-poor Ba-Y-Cu oxide starting 
material can be melted in a Y.sub.2 O.sub.3 -lined crucible, with the 
starting composition, melt temperature, soak time, etc. chosen such that 
the solidified material has the desired 1:2:3 ratio of Y:Ba:Cu. 
The inventive method, or obvious variations thereof, can be used in 
conjunction with the known superconductive compounds, namely, the members 
of the Ba-cuprate system and of the La-cuprate system. There have been 
reports that, for instance, in some samples of nominal composition 
Ba.sub.2 Y.sub.1 Cu.sub.3 O.sub.6.9 indications of superconductivity were 
detected at temperatures above 100K. If these reports are correct, then it 
is likely that an unidentified phase of the material becomes 
superconductive at a temperature above 100K. We consider it likely that 
the inventive method, or an appropriate extension thereof, could be used 
to produce superconductive bodies that comprise the alleged high T.sub.c 
phase, should the phase exist. Furthermore, the inventive method, or an 
appropriate extension thereof may even likely be useful in connection with 
making superconductive bodies from non-cuprate superconductive compounds 
(e.g., nitrides, sulfides, hydrides, carbides, fluorides, and chlorides), 
should such non-cuprate superconductive compounds exist. However, in the 
remainder of this application we will generally only refer to cuprate 
superconductors, in particular, to the Ba-cuprate of nominal composition 
YBa.sub.2 Cu.sub.3 O.sub.7. This is for ease of exposition only, and does 
not imply a limitation of the inventive method to that system.

DETAILED DESCRIPTION OF SOME EMBODIMENTS 
Tentative ternary phase diagrams of the Y.sub.2 O.sub.3 -BaO-CuO system 
have recently been reported. See, for instance, K. G. Frase et al, "Phase 
Compatibilities in the System Y.sub.2 O.sub.3 -BaO-CuO at 950.degree. C.," 
submitted Apr. 7, 1987 to Communications of the American Ceramic Society. 
FIG. 1 shows a tentative partial schematic phase diagram, derived from the 
above-referred to ternary phase diagram, that illustrates some aspects of 
the invention. As will be understood by those skilled in the art, the 
phase diagram is a schematic cross section of the ternary diagram along 
the tie-line of non-superconducting BaY.sub.2 CuO.sub.5 and 
superconducting Ba.sub.2 YCu.sub.3 O.sub.7. It will also be appreciated 
that further research may require modification of the phase diagram. 
Compositions in fields 10, 11 and 12 of FIG. 1 are mixed solid phases 
(consisting of BaY.sub.2 CuO.sub.5 /Ba.sub.2 YCu.sub.3 O.sub.7 and 
Ba.sub.2 YCu.sub.3 O.sub.7 /BaCuO.sub.2 /CuO, respectively. Compositions 
in field 13 are liquid+solid (the solid being BaY.sub.2 CuO.sub.5). 
Finally, compositions in field 14 are single phase liquid. As is well 
known, the presently known high T.sub.c superconductive phase in the 
Ba-Y-Cu-O system has nominal composition Ba.sub.2 YCu.sub.3 O.sub.7. (The 
optimal oxygen content is not necessarily equal to 7 but may differ 
slightly therefrom, e.g., 6.9. We intend to include such minor departures 
within the above nominal composition.) 
Heating a sample of material of composition Ba.sub.2 YCu.sub.3 O.sub.7 
above the solidus line 17 (about 980.degree. C. in air or about 
1010.degree. C. in oxygen) results in (partial or complete) melting. For 
instance, if the sample is heated to a temperature in field 13 of FIG. 1 
(e.g., corresponding to point 15) then partial melting occurs, with the 
equilibrium compositions of the liquid and solid portions determined by 
the familiar lever rule. On the other hand, the equilibrium phase in filed 
14 of FIG. 1 is a uniform liquid. However, relatively slow heating through 
field 13 produces enrichment of the solid phase with Y, with attendant 
undesirable increase in the melting temperature. Consequently, in at least 
some of the embodiments of the invention it is considered advantageous to 
raise the temperature of the starting material relatively rapidly at least 
through field 13 to a temperature in field 14 (e.g., point 16). 
Exemplarily, the temperature is raised such that a sample of Ba-cuprate 
spends less than about 5 minutes (preferably less than 2 or even 0.5 
minutes) in field 13 (or an equivalent solid-liquid region of an 
applicable phase diagram). In some cases it may be advantageous to permit 
establishment of thermal equilibrium by soaking the sample at a 
temperature close to but below the solidus, followed by rapid heating to 
the liquid region. 
After being maintained in region 14 for a relatively non-critical period 
(typically long enough to ensure homogeneity of the melt and short enough 
to avoid undesired uptake [or possibly loss] of material by [from] the 
melt) at least a portion of the melt is rapidly cooled through field 13 
(or an equivalent solid+liquid region of an applicable phase diagram) to a 
temperature (e.g., 18) below the solidus but above the 
orthorhombic/tetragonal transition temperature of YBa.sub.2 Cu.sub.3 
O.sub.x at about 700.degree. C. The cooling rate should be such that phase 
separation is substantially avoided or minimized. Exemplarily, the cooling 
rate through field 13 for a sample of Ba-cuprate is typically greater than 
about 100.degree. C./min, and preferably greater than 200.degree. C./min. 
In many cases, it will be desirable to alter the shape of all or part of 
the molten material prior to its solidification, e.g., by drawing a fiber 
or a ribbon from the melt, by casting into a mold, or by extrusion through 
a die. The details of the shaping step as well as of the rapid cooling 
step depend, inter alia, on the desired shape of the superconductive body. 
For instance, a fiber or ribbon may be drawn or extruded by techniques of 
the type employed in the manufacture of optical fibers or of rapidly 
quenched glassy metals. The casting of bulk bodies will typically require 
cooling of the mold or die. The shape of the solidified body need not 
necessarily be the final shape of the superconductor, and at least in some 
cases it may be advantageous to carry out a separate solid state shaping 
step, e.g., hot forming at a temperature close to the solidus. 
Although currently not preferred, it may at times be advantageous to carry 
out the shaping of the superconductive material under conditions such that 
the material is in the solid+liquid region (field 13 of FIG. 1). In 
particular, the conditions can be chosen such that the solid+liquid 
mixture has an appropriate viscosity that allows easier shaping of the 
material. As will be readily appreciated, the processing time in the 
solid+liquid two phase region advantageously is kept to a minimum 
(exemplarily less than 10 minutes, preferably less than 1 minute) to 
reduce compositional separation. As discussed above, subsequent heat 
treatment should produce substantial homogenization. 
Subsequent to the shaping and rapid solidification the body is typically 
heat treated so as to obtain material of the appropriate composition 
(e.g., oxygen content) and crystal structure and, possibly, to homogenize 
the material. Exemplarily, bodies made from the currently preferred 
Ba-cuprate (nominally composition Ba.sub.2 YCu.sub.3 O.sub.6.9) can 
optionally be heat treated in oxygen (or an oxygen-containing atmosphere 
such as air) at a temperature in the range 850.degree.-950.degree. C. for 
a period in range 1-48 hours, typically followed by relatively slow 
cooling (e.g., average rate&lt;100.degree. C./hour) in oxygen (or an 
oxygen-containing atmosphere to a temperature below about 300.degree. C., 
so as to avoid formation of microcracks. Maintaining the sample at an 
intermediate temperature for some period (e.g., 10 minutes to 15 hours) 
may at times be advantageous, e.g., to facilitate attainment of the 
optimal oxygen content. 
As will be appreciated, the details of the heat treatment will depend, 
inter alia, on the shape of the body. For instance, resolidified bodies 
having one or two relatively small dimensions (e.g., ribbons and fiber, 
respectively) may not require as extended post-solidification heat 
treatment as bulk bodies, since the diffusion distances (e.g., for 
O.sub.2) are much smaller in the former cases than in the latter. Thus, it 
may be possible to carry out the heat treatment of fiber and/or ribbon as 
a continuous in-line process right after formation of the fiber or ribbon. 
FIGS. 5 and 7 show a photomicrograph and a scanning electron fractograph of 
prior art sintered ceramic high T.sub.c superconductive material (nominal 
composition YBa.sub.2 Cu.sub.3 O.sub.6.9, having T.sub.c of about 92K and 
J.sub.c of about 700 A/cm.sup.2 at 77K and H=0). As can be seen from the 
Figures, the prior art material is granular (average grain size 
substantially less than 10 .mu.m) and porous, essentially without texture. 
FIGS. 6 and 8 show a corresponding micrograph and fractograph, 
respectively, of superconductive material produced according to the 
invention. The nominal composition of the material of FIGS. 6 and 8 is the 
same as that of the sintered material of FIGS. 5 and 7. The inventive 
material also has T.sub.c of about 92K but has J.sub.c of about 3000 
A/cm.sup.2 at 77K and H=0. The material according to the invention was 
produced by a process that comprises rapidly (within less than about 5 
seconds) cooling the melt from about 1300.degree. C. to about 950.degree. 
C., followed by a furnace cool to room temperature. As can be seen from 
FIGS. 6 and 8, the inventive process results in essentially 100% dense 
material that is strongly textured, with a substantial portion being 
relatively large elongate crystallites (e.g., having a long dimension that 
is typically greater than about 10 .mu.m, with aspect ratio typically 10:1 
or greater, and with the long axis tending to lie in the basal plane of 
the orthorhombic superconductor), and that is predominantly single phase. 
The texture of the exemplary material is spherulitic. However, by 
providing for directional cooling macroscopically oriented growth can be 
obtained. Oriented growth (including spherulitic growth) typically results 
in a structure in which neighboring crystallites have similar 
orientations, with relatively low angle boundaries between adjacent 
crystallites. This is considered to be of significance in this layered 
material which has anisotropic superconductivity and thermal expansion, 
and may be an aspect that contributes to the improved J.sub.c of material 
produced according to the invention, as compared to prior art sintered 
material. 
A further embodiment of the inventive method is exemplified by FIGS. 9, 10 
and 11, which show respectively a low and two high magnification 
photomicrographs of high T.sub.c superconductive material produced 
according to the invention. The material was produced by maintaining a 
pressed and sintered sample of the starting material (powder of 
composition YBa.sub.2 Cu.sub.3 O.sub.x) in the solid+liquid region (13 of 
FIG. 1) of the phase diagram (1 hour at 1030.degree. C.), followed by a 
furnace cool to room temperature, all under O.sub.2. The resulting 
material had T.sub.c of about 92K, and J.sub.c of about 1700 A/cm.sup.2 at 
77K and H=0. 
In FIGS. 10 and 11 the needle-shaped crystallites are superconductive 
YBa.sub.2 Cu.sub.3 O.sub.7, the rounded crystallites are 
non-superconducting Y.sub.2 BaCuO.sub.5, and the bright phase is 
non-superconducting CuO(+BaO). The material thus is clearly not single 
phase (although more than 80% by volume typically is superconductive 
material), but the superconductive phase typically forms a continuous 
network. As FIG. 9 shows, the non-superconductive "bright" phase is 
non-homogeneously distributed, being concentrated mainly in thin regions 
between relatively large regions that are relatively free of the bright 
phase. FIG. 10 depicts a region that includes a "boundary" between two of 
the referred to large regions of FIG. 9, and FIG. 11 depicts a portion of 
the interior of one of the large regions. 
FIG. 12 gives exemplary data of critical current density as a function of 
applied magnetic field, for samples of nominal composition YBa.sub.2 
Cu.sub.3 O.sub.7 produced by a variety of techniques. Line 120 represents 
bulk samples produced according to the invention. Prior art bulk samples 
(i.e., sintered) typically fall into region 121. The improvement in 
J.sub.c, including the slower decrease of J.sub.c with increasing magnetic 
field, is apparent from a comparison of 120 and 121. Line 122 pertains to 
single crystal samples. Line 123 pertains to thin film results. Further 
thin films have been found to have J.sub.c (H=0) in the region indicated 
by bracket 124. 
As FIG. 12 demonstrates, there exists a very large difference in J.sub.c 
between polycrystalline bulk samples (especially prior art material, i.e., 
material with randomly oriented small grains) and single crystals and thin 
films. It is widely believed that this difference is due, at least in 
significant part, to grain boundary resistance effects. Such effects could 
be caused, inter alia, by the presence of inhomogeneities or impurities, 
mechanical defects (e.g., stress concentration or microcracks), altered 
stoichiometry (e.g., oxygen content), structural deviation, or crystal 
orientation change at grain boundaries. The presence of voids may also 
adversely affect the transport properties. These and/or possibly other 
effects are likely to result in low J.sub.c regions at grain boundaries 
separating high J.sub.c grains. Thus a treatment that eliminates or 
reduces these low J.sub.c regions or perhaps increases the current 
carrying capacity of these regions could result in increased J.sub.c of a 
bulk sample. We believe that the improved J.sub.c observed in samples 
produced according to the invention is due to such an effect. In 
particular, the larger grain size and, significantly, the high degree of 
texture (with the attendant reduction in average orientation change at 
grain boundaries) are thought to at least contribute to the observed 
improvement. Other, so far unidentified factors, may of course also be 
contributing to the improvement. 
The superconductive bodies produced according to the invention may be 
coated (e.g., for purpose of electrical or thermal stabilization, or 
mechanical or environmental protection) with a suitable material, e.g., a 
normal metal such as Ag, Cu, Zn, In, Cd, Al, Sn, etc. The coating can be 
applied by any suitable process, e.g., by evaporation, or by dipping in 
the molten metal. The bodies may also be coated with insulating material 
(e.g., polymers or some oxides), either alone or on top of a metal 
coating. FIG. 2 schematically depicts in cross section an exemplary coated 
wire according to the invention, wherein 21 is the melted and resolidified 
high T.sub.c ceramic superconductive material, and 22 is cladding 
material, exemplarily a metal cladding. 
It is also envisaged to form a "wire" by a process substantially as 
described in the above referred to U.S. patent application Ser. No. 
036,160, now U.S. Pat. Nos. 4,952,554 and 046,825, now abandoned; 
incorporated herein by reference), except that the cladding material is a 
relatively ductile high melting point metal such as stainless steel Nb or 
Ta (including a cladding consisting of ductile high melting point first 
metal matrix with second metal particles embedded therein, the second 
metal also having a high melting point and having substantially different 
chemical behavior than the first metal, such that the second metal 
particles can be removed by selective etching), with typically a barrier 
layer on the inside of the cladding. After drawing the wire from a preform 
and, typically, shaping the wire such that it is at least approximately in 
its final form, the wire is heated (either all of it simultaneously or 
consecutively, e.g., by passing a hot zone along the length of wire) such 
that the oxide powder core of the wire melts, followed by resolidification 
and heat treatment. 
In one exemplary embodiment the cladding is hermetically sealed such that 
all the O.sub.2 that was given off by the oxide during melting and high 
temperature treatment is still available to re-oxidize the material at 
lower temperatures. In another exemplary embodiment O.sub.2 is pumped into 
the wire from the wire ends through the voids that resulted from the 
densification of the oxide material upon melting and resolidification. And 
in still another exemplary embodiment the wire is made porous by selective 
etching of the second metal particles, and O.sub.2 is supplied to the 
oxide through the porous cladding during the heat treatment. 
The invention can be practiced in a number of ways. For instance, the 
material that is to be melted (the "charge") can be a sintered body or 
compressed powder pellet of superconductive oxide produced by a 
conventional technique, or the charge can be a mixture (in appropriate 
ratios) of the starting materials for oxide production (e.g., BaCO.sub.3, 
Y.sub.2 O.sub.3, and CuO powder, or nitrate, acetates, oxalates, etc., 
containing the desired metals). The composition of the charge can be such 
that the relevant metals (e.g., Y, Ba, and Cu) are present in the same 
ratio as they are found in the relative superconductive phase (e.g., 
1:2:3), or the melt can be deficient in one (or more) of the metals, with 
the deficient metal(s) to be augmented in the melt. For instance, 
Y-deficient starting materials can be melted in a Y.sub.2 O.sub.3 -lined 
crucible, with Y being augmented from the crucible liner. Such a procedure 
typically will reduce the likelihood of poisoning of the melt. 
Furthermore, material once melted according to the invention can be 
re-melted, for instance, to facilitate further shaping or to eliminate 
trapped bubbles and/or other inhomogeneities. In some instances it may be 
advantageous to heat the charge slowly to a temperature below the liquidus 
(or to maintain the charge at that temperature for some period), followed 
by rapid heating into (and preferably through) the solid +liquid region. 
Such treatment may reduce oxygen evolution in the liquid material. 
The charge can be melted in a crucible, contact-free (e.g., by torch, RF, 
electrical heater, or laser melting the end or middle of a suspended rod 
of the starting material), or such that the molten charge is in contact 
only with other charge material (e.g., by forming a molten puddle in a 
quantity of the starting material). Furthermore, a core of a first 
material (e.g., silver wire) can be coated with superconductive material 
(e.g., Ba.sub.2 YCu.sub.3 O.sub.7 powder mixed with a binder, and possibly 
also with a metal powder such as Ag powder; the presence of the latter can 
improve the adhesion of the ceramic to the metal core. See the above 
referred to U.S. patent application Ser. No. 046,825 now abandoned). The 
coated core is then moved through a heating zone such that the binder is 
driven off and at least the outer portion of the superconductive material 
coating is melted and rapidly resolidified, without substantial melting of 
the first material core. This can be accomplished by a variety of 
techniques, such as by means of circumferentially arranged lasers or 
electron beams, by means of a well controlled ring burner, or by microwave 
heating. FIG. 3 schematically depicts, in cross-sectional view, an 
exemplary body of the above described type, wherein 31 is the core (e.g., 
silver wire), 32 is the ceramic superconductive shell, with material 
outside of line 33 being resolidified (relatively dense) material, and 
material inside of 33 being less dense material, for instance, being in 
the sintered state. Layer 34 is a coating (exemplarily a polymer coating). 
It will be appreciated that 32 could in principle be completely 
resolidified superconductive material. The invention can also be practiced 
by forming a layer of precursor material (e.g., a YBa.sub.2 Cu.sub.3 
O.sub.7 layer produced by a prior art technique) on a substrate (e.g., 
ZrO.sub.2), melting all or a portion of the precursor material, rapidly 
cooling the melted material below the solidus, followed by an appropriate 
heat treatment. The melting can be carried out by any appropriate means, 
including laser melting or by means of a heat lamp. 
Another exemplary embodiment of the invention comprises casting 
superconductive slit lamellae of the type used in Bitter magnets, 
assembling a stack of lamellae, with appropriate insulator material 
between neighboring lamellae and with the lamellae arranged that any given 
lamella overlaps with its neighboring lamella or lamellae (thereby forming 
a continuous spiral), melting at least the overlap regions or otherwise 
establishing superconductive contact between the overlap regions, and heat 
treating the thus produced Bitter magnet as disclosed herein. 
A toroidal superconductive magnet can also be formed by casting of the 
molten oxide into a tubular mold. 
The inventive method typically can be used to fabricate high T.sub.c 
superconductive bodies that are relatively dense throughout substantially 
all (typically&gt;95%) of the resolidified portion of the body. Bodies 
according to the invention typically have substantially greater fracture 
toughness than less dense identically shaped bodies of the same 
composition, fabricated by a prior art technique such as sintering of 
pressed powder. The latter frequently has densities in the range 70-85% of 
the theoretical density, depending on the heat treatment. The improvement 
in fracture toughness typically is at least about 50%. Inventive bodies 
typically also have critical currents that are substantially larger 
(exemplarily at least about 20% larger) than those of identically shaped 
prior art superconductive bodies of the same composition. The transition 
temperature T.sub.c of bodies according to the invention can be identical 
to that obtainable in prior art bodies of the same composition. Some 
treatment conditions may lead to a slightly broadened transition, and 
materials produced according to the invention and having a somewhat 
broadened superconductive transition (but still reaching R=0 at or above 
77K) are also contemplated. 
Bodies according to the invention can be advantageously employed in a 
variety of apparatus and systems, e.g., those discussed in the previously 
referred to books by S. Foner and B. B. Schwartz. Exemplary such of 
applications is the superconductive solenoid schematically depicted in 
FIG. 4, wherein 41 is a clad superconductive wire according to the 
invention, and 42 is a tubular body that supports 41. 
EXAMPLE I 
Powder of nominal composition Ba.sub.2 YCu.sub.3 O.sub.7 and approximately 
5 .mu.m average diameter was prepared in a conventional manner (see, for 
instance, R. J. Cava et al, op. cit.), and pressed into a 
3.1.times.3.1.times.31 mm pellet. The pellet was heated such that its 
central portion melted. Melting and drawing apart of the solid end 
portions resulted in formation of an elongated neck-down region whose 
minimum diameter was about 1.25 mm. After rapid solidification of the 
molten portion the sample was heat treated in oxygen (16 hours at 
900.degree. C.; furnace cooled to 600.degree. C.; 2 hours at 600.degree. 
C.; furnace cooled to 250.degree. C.). The solidified portion of the 
sample was found to be superconductive, with transition onset temperature 
of about 98K, and completion (R=0) at 92K. The normalized magnetic 
susceptibility transition behavior of the resolidified material was 
essentially the same as that of an unmelted comparison sample of identical 
composition. In particular, the superconductive transition in both samples 
was substantially complete at 90K. 
EXAMPLE II 
Ba-cuprate powder as described in Example I was mixed with 17% by volume of 
Ag powder (1.3 .mu.m average diameter). To the mixture was added 50% b.v. 
of a commercial acrylic binder (Cladan No. 73140 obtained from Cladan, 
Inc., San Marcos, Calif.) dissolved in 1,1,1 trichlorethane. A 0.25 mm 
diameter Ag wire was dipped into the thus produced slurry. The coated wire 
was heat treated for 16 hours at 900.degree. C. in O.sub.2, followed by 
furnace cooling to room temperature. The resulting wire preform had a 
diameter of about 0.75 mm, a superconductive transition temperature (R=0) 
of about 92K, and a critical current density of about 100 A/cm.sup.2 at 
77K. The preform wire was then heated so that at least the outer portion 
of the ceramic coating was melted rapidly while substantially maintaining 
the geometry, followed by rapid cooling and resolidification. The wire was 
then heat treated substantially as described in Example I. The thus 
produced wire had a substantially 100% dense outer ceramic layer that 
adhered well to Ag core, T.sub.c (R=0) of 91K, and a critical current 
density of about 400 A/cm.sup.2. 
EXAMPLE III 
A ceramic-coated wire was produced substantially as described in Example 
II. The wire was then dipped into molten In, resulting in formation of a 
0.125 mm thick In coating. The wire was superconducting, with T.sub.c of 
about 92K. 
EXAMPLE IV 
A sintered pellet of Ba.sub.2 YCu.sub.3 O.sub.7 powder, produced in the 
conventional manner, was rapidly heated above the liquidus, and a drop 
(approximately 6 mm diameter) of the melt caused to fall on a '.degree. 
inclined steel plate. At the moment of impact the drop was flattened into 
ribbon shape by dropping a steel block onto it. The contacting surface of 
the steel block carried a grooved pattern (1.25 mm pitch, 0.375 mm depth). 
The resulting patterned ribbon was about 0.75 mm thick. The ribbon was 
heat treated for 16 hours at 900.degree. C., furnace cooled to 600.degree. 
C., maintained for 2 hours at 600.degree. C., then furnace cooled to 
200.degree. C., all in O.sub.2. The resulting ribbon had T.sub.c (R=0) of 
about 92K, and J.sub.c of about 200 A/cm.sup.2 at 77K. It was substantialy 
100% dense, appeared to be essentially single phase, and its fracture 
toughness is at least 50% greater than that of a sintered test body of 
identical shape and composition. 
EXAMPLE V 
A rod is produced from Ba.sub.2 YCu.sub.3 O.sub.7 powder by pressing in a 
conventional manner. The rod is heated such that a puddle of molten 
material is formed on the upper end of the rod, the melt is contacted with 
a silver-coated stainless steel bait rod wire, and the bait rod is 
withdrawn at a rate such that a 0.125 mm diameter fiber of the ceramic 
compound is continuously formed. The thus formed substantially 100% dense 
ceramic fiber is wound on a 1 m diameter spool with Ag-coated surface, and 
heat treated on the spool substantially as described in Example I. The 
heat treated fiber is coated by drawing it through molten Cd. The coated 
fiber has T.sub.c of about 93K, and is wound on a tubular mandrel (having 
1 m outer diameter), thereby producing a superconducting solenoid, 
EXAMPLE VI 
A substantially 100% dense ribbon (0.25.times.1.25 mm cross section) of 
nominal composition Ba.sub.2 YCu.sub.3 O.sub.7 is formed in continuous 
manner by melt spinning, i.e., by causing a continuous stream of the 
molten material to fall onto a spinning ceramic-coated wheel maintained at 
about 400.degree. C. After heat treatment substantially as described in 
Example I the ribbon is substantially single phase material and has 
T.sub.c of about 92K. 
EXAMPLE VII 
Powder as described in Example I was pressed into a 2.times.2.times.30 mm 
pellet, the single phase pellet heated to 950.degree. C., followed by 
rapid (about 200.degree. C./min) heating to about 1300.degree. C., and 
held at that temperature for about 2 minutes. The resulting single phase 
liquid was then rapidly cooled (about 200.degree. C./min) to about 
950.degree. C., held at that temperature for 20 minutes, followed by 
furnace cooling to room temperature. All of the heat treatment was carried 
out in O.sub.2 at ambient pressure. The sample was then given a 
homogenization and oxygen adjustment treatment as follows: heated to 
920.degree. C., soaked for 16 hours, cooled to 600.degree. C. at 
100.degree. C./hour, cooled to below 300.degree. C. at 20.degree. C./hour, 
all in O.sub.2 at ambient pressure. The sample had T.sub.c (R=0) of 93K, 
and J.sub.c (77K) at H=0, 50, 100, 200, and 10,000 gauss of 3100, 2300, 
1320, 570, and 130 A/cm.sup.2, respectively. 
A prior art sample of identical composition and geometry (sintered at 
920.degree. C. for 16 hours, furnace cooled to room temperature, all in 
O.sub.2) had T.sub.c (R=0) of 92K, and J.sub.c (77K) at 0, 50, 200, and 
10,000 gauss of 570, 130, 20, and 3 A/cm.sup.2, respectively. 
EXAMPLE VIII 
A pellet as in Example VII was heated to 1030.degree. C., maintained at 
that temperature for 2 hours, and furnace cooled to room temperature, all 
in O.sub.2. Subsequently, the sample was heated to 920.degree. C., 
maintained at that temperature for 16 hours, followed by a furnace cool to 
room temperature, all in O.sub.2. The material had T.sub.c (R=0) of 93K, 
and J.sub.c (77K) at 0, 50, 200, and 10,000 gauss of 1700, 1210, 310, and 
100 A/cm.sup.2, respectively. 
EXAMPLE IX 
A sample was prepared substantially as in Example VII, except that the 
pellet was maintained at 1030.degree. C. for only 20 minutes, followed by 
a rapid cool to 950.degree. C., followed by a furnace cool to room 
temperature. The sample had T.sub.c (R=0) of 91K and J.sub.c (77K) at 0, 
200, and 10,000 gauss of 1600, 380, and 120 A/cm.sup.2, respectively. 
EXAMPLE X 
A sample was prepared substantially as described in Example VII, except 
that the pellet was held at 1300.degree. C. for 5 minutes, then 
transferred rapidly (about 0.5 seconds) into a 980.degree. C. region of 
the furnace, and one end of the pellet exposed to a blast of O.sub.2, 
leading to rapid cooling with directional solidification. The sample had 
T.sub.c (R=0) of 92K and J.sub.c (77K, H=0) of 7400 A/cm.sup.2. 
EXAMPLE XI 
A 0.075 mm thick film of nominal composition YBa.sub.2 Cu.sub.3 O.sub.7 is 
formed on a ZrO.sub.2 substrate by coating a surface of the substrate with 
a slurry (containing about 30% by volume binder as in Example II, the 
remainder being YBa.sub.2 Cu.sub.3 O.sub.x powder), heating (50.degree. 
C./hour) the coated substrate to 900.degree. C. to remove the binder, then 
raising the temperature to 950.degree. C., maintaining it at that 
temperature for 2 hours, then rapidly (about 500.degree. C./min) heating 
the precursor material by means of a heat lamp to 1300.degree. C., 
maintaining this temperature for 10 seconds, then rapidly (200.degree. 
C./min) cooling the melted precursor material to 950.degree. C. The 
substrate with the solidified layer thereon is maintained at 950.degree. 
C. for 1 hour, then is slowly (30.degree. C./hour) cooled to room 
temperature. All of the heat treatment is carried out in O.sub.2 at 
ambient pressure. The thus produced superconductive film has T.sub.c of 
92K, and J.sub.c (H=0) in excess of 1000 A/cm.sup.2.