Method of forming superconductive articles by hydrostatic extrusion

A method for forming elongated articles including metallic oxide superconductor material by hydrostatic extrusion at temperatures less than about 800.degree. C., and even at temperatures less than about 450.degree. C. The method includes providing superconductive core material that is substantially free of carbon or organic additives and that has an equivalent density at least about 55% of full density, and enclosing the densified material in a metal container, to become a cladding, prior to extrusion. In a preferred embodiment, the cladding material is a dispersion hardened metal or metal alloy.

FIELD OF THE INVENTION 
The invention relates to the field of methods for the production of bodies 
comprising metallic oxide superconductor material. 
ART BACKGROUND 
Many potential applications have emerged for metallic oxide superconductors 
that exhibit superconductivity at relatively high temperatures, e.g., 
temperatures that can be maintained via liquid nitrogen cooling. For many 
such applications, e.g., applications involving the production, 
distribution, and utilization of electric power, it is desirable to 
provide superconductive bodies having elongated shapes, for example wires 
or rods comprising metallic oxide superconductor material. 
A number of investigators have attempted to use wire-drawing or extrusion 
techniques to produce rods or wires comprising metallic oxide 
superconductor material. For example, M. R. Notis, et al., "Fabrication 
and Characterization of Ceramic Superconducting Composite Wire," in 
Advances in Superconductivity, Proc. 1st Int. Symp. on Superconductivity 
(1989) pp. 371-375, and M-S Oh, et al., "Fabrication and Microstructure of 
Composite Metal-Clad Ceramic Superconducting Wire," J. Am. Ceram. Soc. 72 
(1989) pp. 2142-2147, have reported the use of wire drawing to achieve 
areal reduction ratios (also referred to as "extrusion ratios") up to 
about 1.4 for Ba.sub.2 YCu.sub.3 O.sub.7 cores surrounded by silver 
claddings or composite claddings of silver and stainless steel. These 
authors also suggested, without providing any guidance, that hydrostatic 
extrusion may be used as an alternative method to wire drawing. For many 
applications, however, it is desirable to achieve extrusion ratios greater 
than 1.4. Moreover, these authors observed displacement reactions 
occurring between the core material and the cladding material. Such 
reactions are undesirable because they may degrade the performance of the 
superconductive core. 
Other investigators have employed extrusion techniques. For example, S. K. 
Samanta, et al., "Manufacturing of High T.sub.c Superconducting Ceramic 
Wires by Hot Extrusion," Annals of the CIRP 37 No. 1 (1988) pp. 259-261, 
reported an extrusion ratio of 9 achieved by means of conventional (i.e., 
non-hydrostatic) extrusion of metal-clad powder at temperatures of 
825.degree. C. and 895.degree. C. R. N. Wright, et al., "Deformation 
Processing of High T.sub.c Superconducting Wire," in Processing and 
Applications of High T.sub.c Superconductors, W. E. Mayo, ed., The 
Metallurgical Society (1988) pp. 139-150, reported extrusion of metal-clad 
powder at even higher extrusion ratios of 15 at an extrusion temperature 
of 850.degree. C. However, extrusion at temperatures substantially greater 
than about 800.degree. C., and even at temperatures substantially greater 
than about 450.degree. C., is not generally desirable because, inter alia, 
at least some commonly used hydrostatic fluids for pressure distribution 
during hydrostatic extrusion are difficult or impossible to use at such 
temperatures. 
Investigators have also sought to achieve relatively high extrusion ratios 
at relatively low extrusion temperatures. P. J. McGinn, et al., "Texture 
Processing of Extruded YBa.sub.2 Cu.sub.3 O.sub.6+x Wires by Zone 
Melting," Physica C 165 (1990) pp. 480-484, and P. J. McGinn, et al., 
"Zone Melt Texturing of YBa.sub.2 Cu.sub.3 O.sub.6+x with Silver 
Additions," Physica C 167 (1990) pp. 343-347, reported cold extrusion of 
powdered Ba.sub.2 YCu.sub.3 O.sub.7 material mixed with organic solvent, 
binder, dispersant, and plasticizer. However, it is generally believed 
that the organic materials must be removed from the core in order to 
achieve a useful superconductive article. Thus, for example, R. N. Wright, 
et al., cited above, reported achieving extrusion ratios of 10, and even 
of 70, by cold extrusion of core material comprising 55 vol. % 
superconductor powder and 45 vol. % polyethylene spheres. It was observed 
that the extrudate was not electrically continuous, but could be made 
superconducting by extracting the polyethylene. However, carbon-containing 
residues are believed capable of degrading the superconductive properties 
of the core material. Moreover, removal of the polyethylene involves 
heating an unencapsulated core. As an unintended side effect, it is 
possible for the stoichiometry of the core material to be changed, for 
example by oxygen evolution. As a consequence, extrusion techniques that 
involve organic additives may be unacceptable for at least some 
applications. 
Still other investigators have attempted warm extrusion without organic 
additives. S. Samajdar, et al., "A Phenomenological Model On The 
Deformation Mechanism Of YBa.sub.2 Cu.sub.3 O.sub.7-x +Ag.sub.2 O 
Composite," J. Mat. Sci. Lett. 9 (1990) pp. 137-140, and S. K. Samanta, et 
al., "A Novel Processing Technique For Fabrication of Flexible YBa.sub.2 
Cu.sub.3 O.sub.7-x Wire," J. Appl. Phys. 66 (1989) pp. 4532-4534, have 
reported extrusion, at 450.degree. C., of Ba.sub.2 YCu.sub.3 O.sub.7 
powder containing 50-70 vol. % silver oxide (Ag.sub.2 O). An extrusion 
ratio of 9 was reportedly achieved. However, the presence of substantial 
quantities of non-superconductive material in the core may limit the 
critical current density of the (superconducting) core, and may even 
threaten the electrical continuity of the core. Thus it is desirable to 
have a core containing less than about 30 vol. % non-superconductive 
material. 
Thus, investigators have sought, hitherto without success, a method for 
extrusively forming, at temperatures below about 800.degree. C., and 
especially at temperatures below about 450.degree. C. (exclusive of 
adiabatic heating during extrusion), elongated bodies that comprise 
metallic oxide superconductor material that is substantially free of 
organic additives, and is diluted by less than about 30 vol. % 
non-superconductor material. Investigators have also sought, hitherto 
without success, an extrusive method for forming, at such relatively low 
temperatures, bodies that are substantially free of elemental carbon or 
organic additives, and that have experienced an extrusion ratio greater 
than about nine. 
SUMMARY OF THE INVENTION 
In a broad sense, the invention is a method for making a superconducting 
article comprising metallic oxide superconductor material by hydrostatic 
extrusion. Core material, which comprises the metallic oxide 
superconductor material, is provided in the form of at least one slug or 
pellet. Significantly, no elemental carbon or organic additive, e.g., 
organic binder or organic plasticizer, is added to the core material. 
Preferably, the core material includes less than 30 vol. % inorganic, 
non-superconductor material. The slug (or pellets) is enclosed within a 
hollow cylinder comprising a cladding material. (The assembled cylinder 
and slug are collectively referred to as the "billet".) Significantly, the 
equivalent density of the pellets or slug, just prior to assembly of the 
billet, should be at least about 55% of the full (i.e., fused) density of 
the metallic oxide superconductor. (By "equivalent density" is meant that 
portion of the density that is attributable to the metallic oxide 
superconductor. Where the core material contains metallic oxide 
superconductor having a volume fraction v.sub.ox and also contains one or 
more additives having densities d.sub.1, d.sub.2, etc. and respective 
volume fractions v.sub.1, v.sub. 2, etc., the equivalent density 
d.sub.equiv is determined from the actual density D according to the 
formula D=v.sub.ox d.sub.equiv +v.sub.1 d.sub.1 +v.sub.2 d.sub.2 + . . . ) 
The billet is heated to a temperature less than about 800.degree. C. and, 
preferably, less than about 450.degree. C., and hydrostatically compressed 
such that a composite extrudate, comprising core material surrounded by 
cladding material, is produced. 
In one embodiment of the invention, the core material may initially be 
provided in powder form. The initial core material powder may be formed 
into a slug or pellets by, e.g., isostatic compression followed by 
sintering. 
In a preferred embodiment, the cladding material comprises a dispersion 
strengthened metal or a dispersion strengthened metal alloy. 
Finely dispersed silver is optionally added to the metallic oxide 
superconductor material for the purpose, e.g., of improving the extrusion 
characteristics of the superconductor material. (As noted, the total 
amount of silver added is preferably less than 30 vol. % of the core 
material.) Thus in one aspect, the invention further involves a 
combination of steps for adding finely dispersed silver to the oxide 
superconductor material while it is still in the initial powdered form. 
Such steps include: mixing powdered oxide superconductor with powdered 
silver oxide, ball milling the mixture in an appropriate solvent, 
exemplarily methylene chloride, such that a slurry is formed, vacuum 
filtering the slurry such that a filtrate is retained, drying the filtrate 
such that dry cake is formed, and heating the dry cake for about two hours 
at about 400.degree. C.

DETAILED DESCRIPTION 
In a preferred embodiment, the core material consists of metallic oxide 
superconductor material (here referred to as "oxide superconductor") to 
which finely dispersed silver is optionally added. The total fraction of 
core material that is not metallic oxide superconductor material is 
preferably less than 30 vol. %. Moreover, the core material should be 
substantially free of elemental carbon or organic materials. It is 
believed that any metallic oxide superconductor material can be used. An 
exemplary, specific metallic oxide superconductor material is Ba.sub.2 
YCu.sub.3 O.sub.7. For illustrative purposes, the following process steps 
are described with reference to this particular oxide superconductor 
compound. 
With reference to FIG. 1, the oxide superconductor is initially provided 
(step A) in the form of a powder, having an average grain size of, e.g., 
10-100 .mu.m. Methods for producing an appropriate powder are well known 
in the art. An exemplary method is described in D. W. Johnson, et al., 
"Fabrication of Ceramic Articles from High T.sub.c Superconducting 
Oxides," Advanced Ceramic Materials, vol. 2, No. 3-B (1987) p. 364. At 
various stages of the processing to be described below, the oxide 
superconductor is characterized, inter alia, by its density (or equivalent 
density, discussed above). The density is expressed here as a relative 
density, i.e., as a percentage of a reference density corresponding to the 
density of fused oxide superconductor as measured by the method of 
Archimedes. The reference density (also referred to as "full density") of 
Ba.sub.2 YCu.sub.3 O.sub.7 was determined to be 6.2 g/cm.sup.2. (However, 
it should be noted that small variations are generally expected between 
density measurements made by different investigators, using different 
equipment, of the same fused oxide superconductor. As a consequence, 
absolute determinations of the reference density are expected to vary 
somewhat between laboratories.) 
Prior to extrusion, the core material is formed (step B) into partially 
densified pellets, or, preferably, a partially densified slug, by, e.g., 
isostatic compression (B1) followed, e.g., by sintering (B2). 
Densification by, e.g., sintering is necessary because if the equivalent 
density of the core material is less than about 55%, the walls of the 
canister will tend to collapse against the (relatively compressible) core 
material during extrusion, resulting in accordion-like corrugations in the 
cladding. After isostatic compression alone, the equivalent density of the 
pellets is typically about 30%-35%. Sintering typically raises this 
density to about 55%-65%. Although hot isostatic compression is preferred 
because a higher green density can be achieved thereby, cold isostatic 
compression is readily employed. After the pellets or slug are prepared, 
they are loaded (step D) into a bore hole drilled in a metal canister. The 
diameter of the pellets or slug is slightly smaller than the diameter of 
the bore hole, and is typically 0.3 inches (0.76 cm). The length of the 
canister, and concomitantly of the bore hole that is filled with core 
material, is typically 4 inches (10.16 cm) (depending on the size of the 
extrusion press). Thus, the slug, if used, is typically 4 inches (10.16 
cm) long. Pellets, if used, are typically made about 0.5 inches (1.27 cm) 
thick. The loaded canister is subsequently heated (step E) and extruded 
(step F). 
With reference to FIG. 2, it should be noted in this regard that if pellets 
are used, the interfaces between the stacked pellets in the canister may 
give rise to a spatial instability during extrusion. The effect of this 
instability is to create constrictions 10 in the extruded core 20 at 
irregular intervals. In some cases, the constrictions may also appear in 
the outer surface of the extruded jacket 30. As a consequence, the core 
(and possibly also the jacket) may assume a wavy, or sausage-like, 
appearance. For this reason, inter alia, the use of an integral slug is 
preferred. However, it has been discovered that the instability can be 
mitigated, and even eliminated, by adding finely divided silver (step C of 
FIG. 1) to the core material, as noted above and discussed in greater 
detail below. Moreover, the instability is dependent on the die angle. 
That is, certain samples were observed to exhibit instability when 
extruded at a die angle of 60.degree.. However, when identical samples 
were extruded at a die angle of 75.degree., the instability was less 
apparent, and at a die angle of 90.degree., it was undetectable, at least 
to the unaided eye. 
In an exemplary procedure for making pellets or slugs, core material is 
first provided, typically having a grain size ranging from about 10 .mu.m 
to about 100 .mu.m. (The grain size is not critical.) The core material is 
placed in a mold for isostatic compression and pressed into pellets at a 
pressure, typically, of 50 ksi (345 MPa). Isostatic compression may be at 
room temperature or at elevated temperatures that are readily apparent to 
practitioners in the art. The pellets or slugs are then sintered in an 
oxygen atmosphere in order to further densify them and to achieve optimal 
oxygen stoichiometry. As noted, further densification is desirable in 
order to avoid corrugating the cladding. The oxygen stoichiometry is 
important because at relatively high temperatures, e.g. above about 
450.degree. C. for Ba.sub.2 YCu.sub.3 O.sub.7, metallic oxide 
superconductors tend to evolve oxygen, and as a consequence the 
stoichiometric coefficient of oxygen remaining behind in the oxide falls 
below its nominal value. When this occurs, the superconductive electronic 
properties of the material may be impaired. The presence of an oxygen 
atmosphere during sintering tends to shift the thermodynamic equilibrium 
such that oxygen evolution is less favored. The pellets or slugs are 
sintered for about 24 hours at about 930.degree. C., and then cooled to 
room temperature at a constant rate for about 48 hours. As noted, such a 
sintering process typically increases the equivalent density of the core 
material to about 55%-65%. 
In a currently preferred process for preparing the oxide superconductor 
material and forming it into, e.g., slugs, finely divided silver is mixed 
into the core material. It has been discovered that the addition of silver 
mitigates or eliminates the spatial instability discussed above. Moreover, 
it has been discovered that the addition of silver may reduce the pressure 
required to achieve a given extrusion ratio. 
When silver is added in a conventional manner, it tends to segregate out, 
forming clumps or granules, during sintering of the slug. Thus, a 
currently preferred embodiment includes a sequence of steps that are 
effective for dispersing silver in the initial metallic oxide powder while 
avoiding such segregation and maintaining a uniform distribution of silver 
during sintering. 
The first steps are mixing (step C1 of FIG. 1) and milling (step C2), 
using, e.g., methylene chloride (also called dichloromethane, formula 
CH.sub.2 Cl.sub.2) as the milling solvent. A quantity of powdered oxide 
superconductor, e.g., 100 g of Ba.sub.2 YCu.sub.3 O.sub.7, is mixed with a 
quantity, e.g., 70 g, of silver oxide (Ag.sub.2 O) in a neoprene bottle 
having a total capacity of, e.g., 500 ml. Methylene chloride is added 
until the bottle is filled to 75% of capacity. Large zirconium oxide 
balls, for milling, are then added until the bottle is filled to capacity. 
The bottle is then agitated on a horizontal roller at 60 rpm for 4 hours. 
The resulting slurry is removed from the bottle and vacuum filtered (C3), 
and the resulting cake is baked for about 12 hours in a vacuum oven at 
60.degree. C. to dry it (C4). The dried cake is placed in a zirconia boat 
and heated in a tube furnace for two hours at 400.degree. C. (step C5). 
The resulting material is then isostatically compressed and sintered as 
described above. It has been found that this treatment produces a 
dispersion of silver particles averaging 1-2.5 .mu.m in size before 
sintering, and 3-5 .mu.m in size after sintering for 16 hours at 
925.degree. C., followed by furnace cooling in oxygen. It has also been 
found that the equivalent density of the material before sintering is 
about 30%-35%, and after sintering it is about 55%-65%. The corresponding 
extrudate has been found to have an equivalent density of about 92%-96%. 
With reference to FIG. 3, after sintering, the slug 40 is enclosed within a 
canister 50. The canister is exemplarily formed by drilling and machining 
a solid metal cylinder. (The metal of the cylinder is here referred to as 
the "cladding metal.") The cladding metal should be relatively ductile, 
such that it can be extruded to form the cladding of the extrudate, at the 
desired extrusion ratios, without suffering fractures or tears. Moreover, 
the cladding metal should be capable of flowing relatively smoothly 
through the die during extrusion, without sticking to the inner die 
surface and stalling the extrusion press. Still further, the cladding 
metal should have a relatively high thermal conductivity in order to 
readily distribute heat generated by adiabatic heating during extrusion. 
In at least some cases, it may also be necessary to select a cladding metal 
that is chemically compatible with the superconductor material. That is, 
certain chemical elements, including copper and nickel, are capable of 
chemically reacting with metallic oxide superconductors with the result 
that the desirable superconductive electrical properties of the metallic 
oxide superconductors are degraded or eliminated. This may present a 
problem because the extrusion process often impairs the same desirable 
electrical properties, and the extrudate is typically annealed to restore 
those properties. If incompatible elements are present in the cladding, 
chemical reactions that poison the superconductor may take place during 
annealing. Thus, cladding materials are preferable that are relatively 
free of such incompatible elements. However, it should be noted in this 
regard that a cladding that contains incompatible elements is readily 
stripped away, prior to annealing, by conventional processes such as acid 
etching. (One appropriate acid solvent for this purpose is a 50% solution 
of nitric acid in water. It has been observed that this solvent can be 
used without a significant deleterious effect on the core material.) Thus, 
chemical incompatibility is less important when, for example, the extruded 
core is intended to be used without a cladding. 
It has further been observed that if the cladding material is too soft, it 
may not form an acceptable cladding. For example, some materials, such as 
pure silver, tend to flow away from the advancing front end of the 
extrudate during extrusion. As a result, the core, bare of any cladding, 
may advance a relatively short distance and then break. Moreover, this 
flow behavior tends to remove cladding metal from around the core material 
as it is being extruded. As a consequence, the cross sectional area of the 
core is reduced by a smaller factor than would be expected by simply 
comparing the cladding outer diameter before and after extrusion. In order 
to produce articles having an optimal extrusion ratio, and especially in 
order to produce articles having extrusion ratios greater than about 15, 
it is desirable to manufacture the canister from a hardened material. 
It has been found that dispersion hardened metals or dispersion hardened 
metal alloys are especially useful as cladding materials. (A dispersion 
hardened metal is one that is hardened by dispersing insoluble solid 
particles within the metal.) Dispersion hardened metals are generally 
better for this purpose than metals hardened by other processes, e.g., by 
forming solid solutions, because at elevated temperatures, they exhibit 
higher strain-hardening rates. This is an important property because 
extrusion is typically carried out at a temperature of 600.degree. F., 
(316.degree. C.) and adiabatic heating may raise the temperature during 
extrusion by as much as about 400.degree. F. (204.degree. C.). Moreover, 
it has been found that it is necessary to use a dispersion hardened 
cladding material in order to attain extrusion ratios greater than about 
30. 
A preferred cladding material is silver that is dispersion hardened with 
cadmium oxide at a concentration of, e.g., 10 mole %. This material is 
preferred because it is chemically compatible with many, if not all, 
currently known metallic oxide superconductors. 
Other exemplary cladding materials are silver that is dispersion hardened 
with nickel at a concentration of 10 mole %, and dispersion strengthened 
copper. Exemplary formulations of dispersion strengthened copper that are 
useful as cladding materials are the alumina-dispersed copper formulations 
going under the Copper Development Association designations C15715 (0.3 
Wt. % alumina) and C15760 (1.1 Wt. % alumina). These formulations are 
commercially available as GlidCop, a registered trademark of SCM Metal 
Products, Inc., of Cleveland, Ohio. 
As noted, the canister is exemplarily made by drilling and machining a 
solid metal cylinder. The outer diameter of the canister depends on the 
chamber size of the extrusion press to be used. On an exemplary press, the 
chamber has an inside diameter of 0.625 inches (1.59 cm), and a 
corresponding canister should have an outside diameter of about 0.620 
inches (1.57 cm). The corresponding bore hole diameter in the canister 
ranges, typically, from about 0.250 inches (0.635 cm) to about 0.435 
inches (1.10 cm). The slug or pellets fit within the bore hole with a 
clearance of no more than about 10 mils (0.254 mm) relative to the inner 
walls of the canister. 
The closed end of the canister is desirably formed into a conical taper 60, 
exemplarily by machining. (Alternatively, the taper may be a separate 
piece that is soldered onto the end of the canister.) The taper may end 
bluntly, or it may optionally converge to a point. The full vertex angle 
.theta. of the cone is substantially the same as the extrusion angle 
(which is determined by the selection of the die to be used). The borehole 
preferably does not extend into the taper region. 
After the core material has been loaded into the bore hole, the open end of 
the canister is closed with an end cover 70. The end cover is typically 
made of the cladding material, and comprises at least one disk-shaped 
portion having an outer diameter equal to that of the canister. For 
example, a currently preferred end cover has a T-shaped cross section, 
such that a lower portion comprising a disk of relatively small diameter 
can be inserted in a plug-like manner into the open end of the canister, 
while an upper portion, comprising a disk of greater diameter, presses 
against, and is sealed to, the canister material. The cover is initially 
assembled on the canister by press fitting, and a final seal is made by 
compression in the extrusion press. Various alternative cover designs can 
readily be used, such as a disk of uniform outer diameter that is 
compression sealed to the canister, or a cover that is both T-shaped and 
threaded, and that is screwed into place. The fully assembled canister 80, 
including the slug and the cover, is here referred to as a "billet." 
With reference to FIG. 4, when the billet 80 is assembled within a typical 
extrusion press preparatory to extrusion, the closed end 90 of the 
canister is situated adjacent to a die 100 having an inward-facing bevel 
that is tapered at the so-called die angle. The bevel surrounds an 
aperture 110 through which the extrudate emerges during extrusion. 
The die angle has been found to affect the extrusion process. That is, die 
angles ranging from about 60.degree. to about 90.degree. may be usefully 
employed. However, as noted above, when the core material is in pellet 
form it is desirable to use a die angle of at least about 75.degree.. 
Moreover, it has been observed that when the core material is in the form 
of an integral slug, the highest extrusion ratios are achieved at a given 
pressure when the die angle is about 70.degree.-75.degree., and thus, in 
at least this case, a die angle of about 70.degree.-75.degree. is 
preferred. 
Devices for hydrostatic extrusion are well known in the art. An exemplary 
hydrostatic extrusion apparatus that is commercially available is the 
0.625-inch (1.59 cm) bore, 4-inch (10.16 cm) length, hydrostatic extrusion 
facility manufactured by Naples Research and Manufacturing Company of 
Naples, Fla. This apparatus is designed to reach a maximum hydrostatic 
pressure of 325,000 psi (2240 MPa), and it can be electrically heated to 
about 316.degree. C. by an electrical heating coil surrounding the 
chamber. The hydrostatic fluid used is a silicone oil sold by Amoco under 
the tradename Synthalube, and also sold by Naples Research and Development 
under the tradename JCT Extroil 111. The chamber has adequate heat 
resistance up to 316.degree. C.-371.degree. C. The die 100 is made of tool 
steel, and the ram 120 and die stem 130 are made of tungsten carbide. The 
use of such an extrusion press is well known to those skilled in the art, 
and is described only briefly here. 
Prior to insertion of the billet, a small quantity of the hydrostatic fluid 
140 is placed within the (preheated) chamber such that during extrusion, 
the billet will be substantially surrounded by the fluid and as a result 
will be subjected to hydrostatic or quasi-hydrostatic pressure. In order 
to leave room for the hydrostatic fluid, the diameter of the billet is 
smaller than the diameter of the chamber by at least about 5 mils (0.127 
mm), but not more than about 15-20 mils (0.381-0.508 mm). 
In a preferred sequence of steps, the slug (or, alternatively, a stack of 
pellets) is first assembled in the canister. The cover of the canister is 
then press fit or screwed over the open end of the canister. As noted, the 
loaded canister is referred to here as the billet. The billet and the 
empty chamber are separately preheated to the extrusion temperature. 
Hydrostatic fluid is placed within the chamber, as noted, above. The 
preheated billet is then placed within the chamber, and the die is placed 
over the billet. The ram is then advanced until the die engages the die 
stem and is properly aligned and set over the taper of the canister. A 
short period of time, typically about 30 seconds, is allowed for 
temperature equilibration before extrusion commences. Extrusion is 
commenced by advancing the ram of the extrusion press against the die 
stem. Typical strain rates are 0.08-0.2 inches/second (0.2-0.5 cm/second). 
It should be noted in this regard that adiabatic heating is more 
pronounced when higher strain rates are used. 
The extrusion temperature is typically about 316.degree. C., is preferably 
less than about 800.degree. C., and is still more preferably less than 
about 450.degree. C., as explained below. (These temperatures do not 
include the additional temperature increment due to adiabatic heating 
during extrusion.) It has been observed that heating even to such 
relatively low temperatures results in a dramatic, and quite surprising, 
reduction in the pressures required to achieve extrusion. 
It is desirable to avoid heating the billet, at least prior to extrusion, 
above the useful temperature range of the hydrostatic material. Such 
ranges are typically below about 500.degree. C., and in the case of 
Synthalube, referred to above, the manufacturer's recommended range is 
below about 350.degree. C. Furthermore, at sufficiently high temperatures, 
for example above about 450.degree. C. for Ba.sub.2 YCu.sub.3 O.sub.7, 
oxygen tends to be evolved from the metallic oxide. This may tend not only 
to impair the superconductive electrical properties of the core, but may 
also interfere with the extrusion process. Although such temperatures are 
typically not reached by electrical heating alone, such temperatures may 
be reached by a combination of electrical heating (via the heating coils) 
and adiabatic heating of the extrudate during extrusion. Thus, another 
reason to restrict electrical heating of the billet to temperatures below 
about 500.degree. C. (and preferably below about 450.degree. C.) is to 
avoid chemical decomposition of the core material at temperatures reached 
by the added-on effects of adiabatic heating. However, it should be noted 
in this regard that the core material is sealed within a canister that is 
substantially non-reactive toward oxygen at the process temperatures and 
pressures, and the core material is substantially free of readily 
oxidizable impurities, e.g., organic plasticizers or binders. As a result, 
most of the oxygen that may be evolved is confined within the canister. 
After the billet has been electrically heated to a predetermined, 
appropriate temperature, extrusion is initiated. For extrusion ratios in 
the range 10-100, pressures in the range 100-350 ksi (690-2410 MPa) are 
generally required. (FIG. 5 shows the relationship between pressure and 
extrusion ratio for an illustrative series of extrusions.) The maximum 
pressure that can be used is limited by the design of the extrusion press 
to be used. Extrusion ratios greater than 4, and, in fact, ranging up to 
100 and even more, are readily achieved by this method. 
After extrusion, the superconductor material of the core may be so flawed 
by shear strain that it is no longer superconductive. However, 
superconductivity is readily restored by annealing. By way of example, 
superconductivity in Ba.sub.2 YCu.sub.3 O.sub.7 is readily restored by 
heating for 2-5 hours at 925.degree. C., followed by cooling to room 
temperature at a rate of about 10.degree. C./hour. As noted above in 
connection with heating during extrusion, the core material is sealed 
within a cladding that is substantially non-reactive toward oxygen at the 
process temperatures and pressures, and the core material is substantially 
free of readily oxidizable impurities, e.g., organic plasticizers or 
binders. As a result, most of the oxygen that may be evolved during 
annealing after extrusion is confined within the core. 
It should be noted that if the cladding contains incompatible elements such 
as copper or nickel, the annealing conditions may also promote chemical 
poisoning of the superconductor. Thus, for example, poisoning of Ba.sub.2 
YCu.sub.3 O.sub.7 by copper may begin at temperatures as low as about 
700.degree. C., at which copper acquires significant diffusive mobility. 
Such poisoning is readily avoided by stripping away the cladding prior to 
annealing. 
EXAMPLE 
A slug of Ba.sub.2 YCu.sub.3 O.sub.7 containing 28 vol. % dispersed silver 
was prepared by the method described above. The slug was 0.325 inches 
(0.826 cm) in diameter. The slug was loaded into a canister of silver that 
was dispersion hardened with 10 mole % cadmium oxide. The canister was of 
one piece, including a pointed taper machined at the die angle of 
70.degree.. Using a die with a circular aperture and a pressure of about 
1890 MPa, an extrusion ratio of 100 was achieved. No obvious porosity was 
observed on subsequent metallographic examination of the extruded core 
material.