Near net shape processing of continuous lengths of superconducting wire

A system and method for mechanically forming a ceramic superconductor product. A system for making the ceramic superconductor includes a metallic channel portion having a cross section for receiving a ceramic superconductor powder, a roll to mechanically reduce the channel cross section and included superconductor powder and a cap portion welded to the channel portion using a localized high energy source. The assembled bar is then mechanically reduced to form a tape or wire end product.

The present invention is concerned generally with an improved method of 
manufacturing ceramic superconductor tape. More particularly, the 
invention is concerned with a method of rolling a ceramic superconductor 
to continuously manufacture a clad ceramic superconductor tape. The 
rolling operation uses a closed metal channel shaped to hold the ceramic 
powder and the rolling proceeds by use of a roll head with a shape 
complementary to the metal channel shape. 
A class of ceramic materials have been discovered which have 
superconducting properties above the boiling point of liquid nitrogen. 
Ceramic superconductors can be produced by a number of conventional 
ceramic processing techniques, but effective commercial utilization of 
these ceramic superconductors will require substantial improvement of the 
product materials. For example, the ceramic superconductors would be very 
useful if they could be manufactured in large rolls of tape or wire. 
However, the extremely brittle mechanical properties make the manufacture 
of such products difficult. In addition such ceramics conduct electricity 
preferentially along selected crystallographic planes; thus, it is 
desirable to produce textured materials with the selected planes oriented 
optimally for best electrical performances. However, such textured 
materials are not easily produced using conventional ceramic forming 
techniques. 
Powder in tube techniques have been developed for the high temperature 
superconductors wherein a metallic nonsuperconducting tube is muzzle 
loaded with a superconducting ceramic powder. The filled tube is extruded 
or drawn or swaged to small cross-sections, and the reduced size rod is 
drawn or rolled into wire or tape. The reduced wire or tape is then 
sintered to densify the powder and establish the superconducting 
properties. Attempts have been made to enhance the crystallographic 
texture of the reduced wire or tape; but substantial limitations have been 
encountered, such as being limited to the manufacture of fixed lengths of 
wire or tape due to the batch nature of the process (loading powder into a 
fixed length tube or channel and processing that piece). In addition, 
defects (such as cracks) can be generated by these conventional thermal 
and mechanical processing techniques. 
It is therefore an object of the invention to provide an improved method of 
mechanically manufacturing a high temperature ceramic superconductor. 
It is another object of the invention to provide a novel method of 
continuously manufacturing a length of wire or tape of ceramic 
superconductor. 
It is also an object of the invention to provide an improved method of 
making a continuous length of ceramic superconductor using a rolling 
channel and matingly shaped roll to reduce the superconductor 
cross-section. 
It is a further object of the invention to provide a novel apparatus for 
rolling a ceramic superconductor by forming a channel having the same 
general shape as the final product which enables generaling a product with 
more uniform stress. 
It is yet another object of the invention to provide an improved apparatus 
for mechanically reducing the cross section of a ceramic superconductor 
using a rectangular shaped channel and complementary shaped roll. 
It is a further object of the invention to provide a novel apparatus for 
mechanically reducing a ceramic superconductor using a shaped, recessed 
channel and a shaped roll having a confined width to control the 
mechanical reduction operation. 
It is still a further object of the invention to provide a novel article of 
manufacture of highly textured ceramic superconductor wire or tape. 
These and other objects, features and advantages of the present invention 
will be readily apparent from the following description of the preferred 
embodiments thereof, taken in conjunction with the accompanying drawings 
described below wherein like elements have like numerals throughout the 
several views.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
A method and an apparatus characteristic of the invention can be understood 
by reference to the figures and particularly to FIGS. 1-5. In order to 
perform the method of the invention, a rolling apparatus 10 (see FIG. 2) 
is first used to form a compacted ceramic superconductor powder in 
preparation for rolling to a superconductor wire or tape product 12 shown 
in FIG. 6. The rolling apparatus 10 includes a channel portion 14 with a 
flat bottom 16 and substantially perpendicular side walls 18 (see FIG. 1). 
Such a geometry can be accomplished by forming a ductile metal stock 
material into the desire shape. The channel portion 14 should have 
sufficient depth and wall thicknesses to sustain several reduction steps 
before achieving the final dimensions. The ductile metal stock can be any 
one of a variety of elemental metals and/or metal alloys, provided the 
metal does not chemically react to detrimentally affect the desired 
superconducting properties. A most preferred metal is silver which is 
easily formed into the channel portion 14 and does not react with ceramic 
superconductors to affect superconductor electrical properties. The 
ductile metal stock preferably also has high electrical conductivity, 
formability and high oxygen solubility, as does silver. The high 
electrical conductivity property makes the channel portion 14 a better 
conductor around any potential gaps which might form in the 
superconducting path of the finished superconductor product 12. High 
solubility for oxygen allows the oxygen content of the ceramic 
superconductor to be changed in a known manner by elevated temperature 
heat treatments, preferably performed after fabrication of the 
superconductor product 12. 
In the next step of the process invention a ceramic superconductor powder 
20 (see FIG. 2) is packed into the channel portion 14 using the rolling 
apparatus 10. The ceramic superconductor powder 20 preferably takes the 
form of plate-like particles with superconducting properties 
preferentially oriented relative to the plate-like shape. For example, a 
superconductive plane can be oriented parallel to the plate-like shape, or 
perpendicular to the thickness dimension of the plate-like particle. The 
plate-like shape therefore can be used to establish a preferential 
crystallographic texture for the ceramic superconductor powder 20. This 
texture ultimately permits formation of the macroscopic superconductor 
product 12 having superconducting properties with optimized property 
values along the current carrying direction. In order to help establish 
this preferential texture for the powder 20, a stepped roll 22 is used to 
compress the powder 20 (see in FIGS. 7 and 8 the appearance of the powder 
20 before and after compaction, respectively). This stepped roll 22 in the 
rolling apparatus 10 causes compression which results in the 
superconducting planes of the powder 20 being oriented substantially along 
the axis of the channel portion 14. This initial rolling process also 
provides an initial densification of the powder 20, as well as establishes 
the appropriate level and thickness of the powder 20 in the channel 
portion 14. 
In a next step of the method the channel portion 14 is fitted with a cap 
portion 24 (see FIG. 4) preferably constructed of the same metal as the 
channel portion 14. The cap portion 24 is tightly fitted onto the open top 
of the channel portion 14 above the powder 20. The channel portion 14 is 
then welded to the cap portion 24, preferably by using a high energy 
source, such as, for example, an electron beam source 28, a TIG source and 
most preferably a laser 26. A completed bar 29 is then ready for rolling 
mill reduction. While other conventional means can be used to join the cap 
portion 24 to the channel portion 14, such as mechanically rolling of the 
seams, a high energy input process is preferred. Such a process prevents 
heat from the welding process from being conducted through the silver (or 
other appropriate metal stock) and causing melting of the superconducting 
ceramic particles. Such a melting event can result in formation of 
nonsuperconducting phases and/or misorientation of the superconducting 
planes relative to the preferred texture described hereinbefore. 
After completing the welding of the cap portion 24 to the channel portion 
14, the channel portion 14 of the bar 29 is reduced in thickness and 
increased in length by passage through use of a bar rolling apparatus 30 
(see FIG. 5). The rolling apparatus 30 preferably includes shafts 32 and 
33 rotating in opposite directions, and each of the shafts 32 and 33 
include integrally coupled roll disks 34-39 The roll disks 34 and 38 
define a controllable open space 40 through which the bar 29 passes and is 
rolled to a tape or wire of smaller dimension. The roll disks 34, 36, 37 
and 39 act to restrain the bar 29 from sideways deformation forcing 
lengthwise deformation and forming the elongated type product 12. This 
arrangement of the roll disks ensure that the rolling energy goes into 
lengthening, not widening, the dimension of the contained superconductor 
powder 20 which is undergoing mechanical reduction. 
During the rolling operation, the bar 29 is periodically annealed at a 
temperature high enough to permit recrystallization of the silver (or 
other metal stock), but low enough to avoid any substantial sintering of 
the enclosed superconducting ceramic. Such annealing acts to remove 
excessive cold work buildup, enabling the continued reduction of the cross 
section of the contained powder 20 being mechanically fabricated. When the 
thickness of the bar 29 has been sufficiently reduced to be within several 
reduction cycles of the final intended tape or wire cross section, the bar 
29 is preferably subjected to a sintering heat treatment to densify the 
superconducting powder 20. Preferably, after completion of the sintering 
operation and mechanical reduction to the final size, the product is 
annealed in an oxidizing atmosphere to optimize the electrical properties. 
The entire process is continuous to the extent a continuous arbitrary 
length of the bar 29 can be fed into the bar rolling apparatus 30 for 
continuous reduction to the final cross section desired. In assembly of 
the bar 29 the channel portion 14 can also be continuously filled with 
superconductor powder 20, the cap portion 24 continuously welded to the 
channel portion 14 and the previously described rolling operation carried 
out. 
In other forms of the invention, the channel portion 14 of the bar 29 can 
have a trapezoidal, hexagonal or other polygonal cross section susceptible 
to rolling using a complementary shaped roll and channel opening. Thus, 
the final product can be virtually any final cross sectional shape, such 
as octagonal, and can be obtained by controlling the geometry of the bar 
rolling apparatus 30. Generally, the final product, such as the sintered 
wire form of the superconductor product 12 shown in FIG. 6, preferably has 
the same cross section as the shape of the superconductor powder 20 within 
the channel portion 14 of the bar 29. Maintaining the same type of cross 
section for the end superconductor product 12 as the starting channel 
shape assists in minimizing excessive mechanical stresses which can 
accumulate during the reduction process. Accumulation of too much stress 
can cause cracking or formation of other undesirable mechanical defects. 
The use of the above described polygonal shapes for the channel portion 14 
advantageously results in the final product (the superconductor product 
12) containing a higher fraction of superconducting material than achieved 
by any conventional process (see, for example FIG. 6 and Example I). Such 
an improvement enhances electrical performance and reduces manufacturing 
costs. Further, by establishing a preferred texture for the superconductor 
powder 20 before welding the cap portion 24 to the channel portion 14 (see 
FIGS. 8 and 9), the resulting sintered wire form of the superconductor 
product 12 exhibits an improved texture giving rise to substantially 
improved superconductor properties. 
A further advantage of the invention is processing the ceramic 
superconductor during the steps when the channel portion 14 has a top open 
(the cap portion 24 is not in position). This mode of processing gives 
rise to the ability to intervene easily during the processing sequence. 
Conventional techniques use a metal overlayer which interferes with any 
intervention steps. For example, in Tl--Ba--Ca--Cu--O superconductors, the 
best current carrying capacity is achieved by reacting a Tl containing 
vapor with a layer of Ba--Ca--Cu--O. While conventional sealed tube 
processing methods would not permit this type of vapor processing, the 
instant invention allows such processing before sealing the cap portion 24 
to the channel portion 14. 
The following nonlimiting examples illustrate several particular methods of 
manufacturing superconductor wire in accordance with invention. 
EXAMPLE I 
In a most preferred embodiment the ceramic superconductor product is made 
by the following procedure: 
1. Form the channel portion using stepped rolls: Ag 0.010" thick, 0.040" 
tall, 0.125" wide. 
2. Form the cap portion using shaped rolls: 0.010" thick with 0.005" edges. 
3. Pack the channel portion with BiSCC0 2212 powder. Packing procedure 
involves: 
a. Pack powder into channel portion and press with spatula; 
b. Roll into place with stepped rolling apparatus; 
c. Check depth with comparator; 
d. Load powder again; 
e. Roll with stepped rolling apparatus; 
f. Check depth with comparator; 
g. Load powder again; 
h. Roll with stepped rolling apparatus; 
i. Three such operations were sufficient to pack the channel portion to a 
depth of 0.025". 
4. Weld cap portion to channel portion using conventional laser welding 
methods. 
5. Rolling schedule: 
a. Anneal welded part at 600.degree. F. for 10 minutes. 
b. Reduce using stepped rolls to a projected value of about 5% reduction 
per pass and annealing after 20% reduction: 
Pass 1 to 0.037"; 
Pass 2 to 0.036"; width 0.1245"; 
Pass 3 to 0.034"; 
Pass 4 to 0.032"; width 0.0124"; 
c. Anneal at 600.degree. F. for 7 minutes: 
Pass 5 to 0.029"; 
Pass 6 to 0.027"; width 0.1243"; 
Pass 7 to 0.025"; 
Pass 8 to 0.024" and a 1.5" section was archived. 
d. Anneal at 600.degree. F. for 7 minutes: 
Pass 9 to 0.020"; 
Pass 10 to 0.0195"; 
Pass 11 to 0.0175"; 
Pass 12 to 0.017" and a 2" section was archived. 
e. Anneal to 600.degree. F. for 7 minutes: 
Pass 13 to 0.015"; 
Pass 14 to 0.0135"; 
Pass 15 to 0.0125" and a 2" section was archived. 
f. Anneal at 600.degree. F. for 7 minutes: 
Pass 16 to 0.011"; 
Pass 17 to 0.0095"; 
Pass 18 to 0.0085". 
6. Heat Treatment Schedule in ambient: 
a. Heat from room temperature to 830.degree. C. at 2.degree. C. per minute; 
b. Heat from 830.degree. C. to 885.degree. C. at 1.degree. C. per minute; 
c. Hold at 885.degree. C. for 30 minutes; 
d. Cool from 885.degree. C. to 862.degree. C. at 0.167.degree. C. per 
minute; 
e. Hold at 862.degree. C. for 180 minutes; 
f. Cool to room temperature at 2.degree. C. per minute. 
7. Final rolling of the product with approximately 25% reduction was taken 
for a range of wire thicknesses from 0.024" to 0.085", and the most 
preferred size was a 0.011" wire rolled to 0.008"; 
8. Final heat treatment: 
a. Heat to 840.degree. C. at 3.degree. C. per minute; 
b. Hold at 840.degree. C. for 100 hours; 
c. Cool to room temperature at 1.degree. C. per minute. 
9. Test at 4.2K. The ceramic superconductor wire product with a thickness 
of 0.008" had the best measurable critical current density of 9770 
amps/cm.sup.2. 
While preferred embodiments of the invention have been shown and described, 
it will be clear to those skilled in the art that various changes and 
modifications can be made without departing from the invention in its 
broader aspects as set forth in the claims provided hereinafter.