Process for heat treating superconductor wire

An oxide superconductor wire is prepared by preparing a length of precursor wire for processing into an oxide superconductor wire and coating the precursor wire with an isolating layer. The coated wire is wound onto a reel in a spiralling manner, such that each turn of the spiral is in alignment with the preceding turn of the spiral along an axis perpendicular to the axis of winding. The wound precursor wire is then heated to form the oxide superconductor. The removable isolating layer is prepared by coating the wire with a solution including a metal oxide and a porosity-inducing component, and heating the coated wire so as to induce porosity and control grain size of the metal oxide so as to render the coating removable. The coating should function to isolate the overlapping turns of the wound wire from neighboring wires, so that not diffusion bonding or adherence between the turns occurs. The coating should also be readily removable because the coating can interfere with subsequent processing of the oxide superconductor wire.

OBJECT OF THE INVENTION 
The invention relates generally to a system and process for manufacturing 
high transition temperature (T.sub.c) oxide superconductor wire. The 
invention more particularly relates to a system and apparatus for coating, 
winding and heat treating high T.sub.c oxide superconductor wire. 
BACKGROUND OF THE INVENTION 
Many applications of the high T.sub.c oxide superconductors requires 
forming the superconductor into a wire. Current processes include forming 
a precursor of the oxide superconductor or the oxide superconductor itself 
into a wire and heat treating the wire to obtain an oxide superconductor 
wire. Current developments in high T.sub.c oxide superconductor processing 
have resulted in the manufacture of increasingly longer lengths of oxide 
superconductor wire with acceptable current carrying capacity. 
The loading and unloading of oxide superconductor wire is an important step 
in the processing of the wire, in particular, to move the wire between 
wire deforming steps, such as pressing and rolling, and oxide 
superconductor phase-forming steps, such as sintering and annealing. The 
prior art wire deforming step is typically carried out by feeding the wire 
from a conventional spool through the deforming step and taking up the 
deformed wire onto a second conventional spool. Long lengths of wire can 
be efficiently wound onto the spool. However, the conventional spool is 
not readily adaptable for use in furnaces and results in inefficient use 
of furnace space. Further, the wire tends to sag and become distorted 
(wavy) because of the coiled form of the wire on the spool. Lastly, the 
multiple overlapping windings on the spool do not permit efficient 
oxidation and phase transformation of the oxide superconductor. 
A further disadvantage to winding the oxide superconductor wire prior to 
heat treatment is that overlapping contact between portions of the wire 
results in diffusion bonding of the wire to itself and the mandrel, 
thereby degrading superconducting properties and preventing the unspooling 
of the heat treated wire. Silver is commonly used as a protective cladding 
for the oxide superconductor, in particular because the cladding itself is 
electrically conductive and does not prevent oxygen diffusion to the oxide 
superconductor. However, even the silver cladding will diffusion bond to 
other portions of the silver-cladded wire which are in contact during heat 
treatment. 
U.S. Pat. No. 5,140,006 discloses a method and apparatus for coating a 
silver-cladded oxide superconductor wire with a diffusion-inhibiting 
material and taking up the coated wire onto a spool. Rare earth oxides are 
specifically disclosed as a desirable diffusion inhibiting material and no 
disclosure of the desirability of removing the material after treatment is 
disclosed. 
It is the object of the present invention to efficiently process 
high-T.sub.c superconducting oxide wire by increasing the simplicity and 
efficiency of the method used to load and unload wire during processing, 
by reducing adhesion of the wire to itself and by maximizing the use of 
furnace space during heat treatment. 
SUMMARY OF THE INVENTION 
In one aspect of the invention, an oxide superconducting wire is prepared 
by providing an oxide superconductor wire and applying an isolating layer 
to an external surface of the wire. The isolating layer includes an 
isolating material and a porosity-inducing component. The coated wire is 
heated so as to induce porosity into the isolating layer, thereby 
obtaining a removable porous isolating layer. The coated oxide 
superconductor wire is then further processed, as required. 
In another aspect of the invention, an oxide superconducting wire is 
prepared by providing an oxide superconductor wire and applying an 
isolating layer to an external surface of the wire. The isolating layer 
includes an isolating material and a porosity-inducing component. The 
coated wire is wound onto a reel in a spiralling manner, such that each 
turn of the spiral is in alignment with the preceding turn of the spiral 
along an axis perpendicular to the axis of winding. The coated oxide 
superconductor wire is then further processed, as required. 
By "oxide superconducting wire", as that term is used herein, it is meant a 
wire at all stages of manufacture, but which can ultimately be processed 
into a superconductor wire. Therefore, precursor wires, which are 
converted into an oxide superconductor wire is deemed a superconductor 
wire for the purposes of the invention. Likewise, a wire which is only 
partially converted into the oxide superconductor, or which contains the 
oxide superconductor but requires further processing to optimize the 
electrical properties are likewise deemed an oxide superconductor wire for 
the purposes of the invention. 
By "in alignment" as that term is used herein, it is meant that each turn 
of the spiral is in alignment with and substantially completely 
overlapping with the previous turn of the spiral, such that the wound wire 
takes on a substantially two-dimensional appearance. The wound wire is 
similar in appearance to a wound cassette reel of audio tape. 
By "porosity-inducing component" as that term is used herein, it is meant a 
primarily carbon-containing material which is capable of combustion or 
thermal decomposition with very little or no residues. The space formerly 
occupied by the component, after such decomposition, provides the 
requisite porosity. 
The oxide superconducting wire prepared by the present invention possesses 
superior isolation of successive wire layers and the spool and is capable 
of easy removal of the layer for further wire processing.

DETAILED DESCRIPTION OF THE INVENTION 
The applicants have discovered that coating a superconductor wire with the 
isolating layer of the present invention results in superior isolation of 
successive wire layers and in easy removal of the layer for further wire 
processing. The ease of coating, superiority of its isolating properties 
and ease of removal provides several processing advantages. Further, the 
applicants have discovered that the oxide superconductor wire, when wound 
onto a reel-like support according to the method of the invention, can be 
processed more efficiently in a furnace environment, without detrimental 
affect to processing conditions or final superconducting properties. The 
present invention permits an oxide superconductor wire to be rapidly and 
efficiently loaded and unloaded onto a cassette reel for furnace 
processing during oxide superconductor phase-forming steps. The wire then 
may be transferred to conventional spools for wire-forming steps, if 
desired. In addition, cassette reels are easily interfaceable with other 
processing equipment, in particular automated processing equipment, which 
improves the efficiency of the manufacturing process, if desired. 
The applicants have found that use of a porous isolating layer effectively 
prevents bonding of the wire turns to each other, while being capable of 
easy removal. The porosity of the layer increases the ease with which the 
coating can be removed for the wire by reducing surface area contact with 
the wire and increasing coating brittleness. The isolating material may be 
any material which prevents sticking of overlapping layers of 
superconductor wire and which does not poison the superconducting wire and 
degrade superconducting properties. 
A variety of refractory metal oxides may be used in the practice of the 
invention. The applicants have found oxides of aluminum, calcium, 
tantalum, magnesium, zirconium and tungsten to be particularly effective. 
Selection of a particular refractory metal oxide is based on the ease of 
its removal and effectiveness in isolation of the wire turns. Magnesium 
oxide (MgO) is a particularly preferred metal oxide. It is preferred to 
use refractory metal oxides having a relatively large particle size and, 
in particular, a particle size in the range of 1 to 50 microns. Refractory 
metal oxide particles of small grain size promote adherence to the wire 
and make it difficult to remove. The adherence is of a mechanical, rather 
than a chemical, nature. 
The porosity of the layer improves the ease of its removal subsequent to 
the heat treatment. The ability to remove the isolating layer after heat 
treatment is important because the layer may impede further processing of 
the wire. For example, where the isolating layer is not of a uniform 
thickness, rolling operations result in uneven stress being applied along 
the wire length and/or width. Porosity may be obtained by including a 
porosity-inducing component in the isolating layer which is capable of 
thermal decomposition or combustion with little or no residue. In the 
subsequent heat treatment, the porosity-inducing component decomposes or 
combusts, leaving voids in the isolating layer and thereby introducing 
porosity into the layer. Suitable porosity-inducing components, include 
but are in no way limited to, cellulose, wood fiber, saw dust, graphite, 
paraffm, polypropylene and polyethylene. 
The isolating layer, including an isolating material and a 
porosity-inducing component, may be applied using an inert solvent as the 
carrier liquid. By inert solvent, it is meant herein that the isolating 
material and the porosity-inducing component are stable in the solvent 
with no adverse reactions between the solvent and the added materials. It 
is further required that the porosity-inducing material be insoluble in 
the carrier liquid, since a soluble material does not occupy a significant 
volume in the resulting layer. 
The isolating material and the porosity-inducing component are added to an 
inert solvent. The resultant mixture can be a solution (of the isolating 
material only), dispersion, slurry or a suspension; however, due to the 
low solubility of the isolating material and the porosity-inducing 
component, the mixture is most typically a suspension or slurry. The 
solvent is preferably volatile, so that evaporation of the solvent and 
subsequent adherence of the layer to the wire occurs rapidly. The solvent 
is preferably a low molecular weight liquid, such as ethanol, acetone, 
hexane or water. 
The weight percent of added solids (isolating material+porosity-inducing 
component) in the mixture may be in the range of 5 to 25 wt %. An 
exemplary mixture is prepared by adding approximately 100 g powder into 1 
liter of ethanol (approx. 10 wt %). The solids range from 5 wt% to 99 wt % 
isolating material, the balance porosity-inducing component. The greater 
the level of isolating material in the solids, the more effective the 
anti-bonding effect of the layer; however, this improvement is obtained at 
the cost of the removability of the layer. A preferred process uses 50 g 
MgO, 50 g cellulose in 1 liter of ethanol; however, ratios of 10% MgO/90% 
cellulose have been successfully used to provide a highly releasable 
layer. 
It may be desirable to vary the relative proportions of the isolating 
material and porosity-inducing component in the isolating layer during 
processing of the oxide superconductor wire, where the layer is repeatedly 
applied and removed. For example, in early processing steps, where greater 
problems with sticking of the wire turns is observed, it is desirable to 
apply an isolating layer of higher isolating material content, i.e., 
greater than 50% MgO. In later processing steps, where bonding of the wire 
turns is less marked, it is desirable to apply an isolating layer with 
higher levels of porosity-inducing component to facilitate easy removal of 
the layer. The appearance of the wire is also greatly improved. 
The mixture may be applied to the wire in any conventional manner, 
including but not limited to, spray coating and dip coating and the like. 
Spray coating includes generating droplets of the carrier liquid 
containing the isolating material and porosity-inducing component therein 
and impinging the droplets on the surface. Dip coating includes passing 
the wire through a bath which contains a mixture of the isolating material 
and porosity-inducing component therein. In other embodiments of the 
invention, the isolating material and the porosity-inducing component may 
be introduced in two separate application steps (i.e., spray coating of 
first one, and then the other, component onto the layer). In such 
instances, the porosity-inducing component should be applied closest to 
the wire in order to ensure easy release. 
The method is described with reference to an apparatus which is suitable 
for practicing the invention. Coating and loading a wire onto a reel may 
be accomplished from a second reel, which is useful when the method is to 
be integrated with automated reel-based technology. As shown in FIGS. 1(a) 
and (b), an apparatus 30 includes a base 31 which supports a receiving 
reel 32 using mounting means 33 which is capable of rotation. The 
receiving reel 32 is driven by a motor 34, which causes the reel to 
rotate. A wire 35 is first provided on a feeding reel 36 which may be 
rotatingly mounted on the base 31 by mounting means 37. Upon rotation of 
the feeding reel 36, wire 35 is unwound. Alignment of the receiving and 
feeding wheels 32 and 36, respectively, is preset to prevent twisting and 
distortion of the wire. The wire is loaded onto the receiving reel 32 as a 
spiral with each turn of the spiral aligned with the previous turn, so 
that the turns are aligned and substantially completely overlapping along 
an axis perpendicular to the axis of rotation. A control panel 39 controls 
the speed of the driving motor 34. 
In preferred embodiments, an isolating layer is applied to the wire before 
it is taken up onto the reel 32. The isolating layer prevents overlapping 
layers from sticking to each other after heat treatment. To this end, the 
apparatus 30 includes a receptacle 38 for holding a mixture which 
comprises an isolating material and a pore-inducing component in an inert 
solvent. The receptacle 38 may contain a guide pulley (not shown) for 
directing the wire down into the mixture contained in the receptacle and 
back out of the receptacle towards the reel 32. The guide pulley 
additionally serves as a means of stirring the mixture during a coating 
operation. Some means of agitation is required to maintain dispersion of 
the suspended insoluble particles. Alternative means of agitation include 
use of a circulating pump, mechanical stirring or convection means. 
In operation, one end of the wire 35 is attached to the receiving reel 32 
and the motor 34 is activated. The wire 35 is unwound from feeding reel 36 
and drawn around the pulley and into the receptacle 38. The wire contacts 
the mixture contained in the receptacle, thereby depositing a layer of 
isolating material on the wire. Before the wire is wound on the reel, the 
solvent dries to leave an isolating layer on the wire. Drying means (not 
shown) can be used to increase drying rate, if desired. Drying means 
include fans or heater or the like suitably between the receptacle 38 and 
reel 32. 
As described above, coating and loading the wire onto reel 32 may be 
accomplished from a cassette reel, which is useful when the method is to 
be integrated with automated processing. Alternatively, coating and 
loading the wire may be accomplished from spool to reel. To do so, the 
method and apparatus are modified, as shown in FIG. 2, where like elements 
are similarly numbered. A spool 40 is positioned adjacent to the feeding 
reel 36. The wire 35 is fed from the spool 40 and drawn over feeding reel 
36. The wire is then processed as described above for a reel to reel 
transfer. The spool 40 should be positioned a distance from reel 36 so 
that the wire 35 is not significantly bent or distorted as it is fed out 
from the spool. 
It is also within the scope of the invention to apply the isolating layer 
by feeding out the oxide superconducting wire from a conventional spool, 
through a receptacle containing a mixture of the isolating material and 
porosity-inducing component and onto a second conventional spool. The 
manner of the process is readily understood with reference to FIGS. 1 and 
2, above. Further, when the coating is not removed, it can function as a 
desirable insulating layer. It is also within the scope of the invention 
to spray coat the insolating layer onto the wire during the wire loading 
and unloading process. 
It is further contemplated that the wire may take on any geometry, 
including but not limited to, wires having a circular, avoid, ellipsoidal, 
rectangular, square and polygonal cross-section. The wire may have a width 
to thickness ratio in the range of 1 to 1000. 
The reel which is used in the above method may be made from a material 
capable of withstanding high furnace temperatures, such as nickel alloys 
and stainless steel. With reference to FIG. 3, the reel includes a central 
mandrel 42 and two opposing end plates 44 and 46, respectively. The 
mandrel 42 may be of substantially the same width as the wire. The end 
plates preferably contain apertures 48 to allow the passage of oxygen 
and/or other processing gases. Alternatively, the reel is made from 
components capable of assembly and disassembly. In this case, the end 
plates 44 and 46 are removed from the mandrel 48. The mandrel is a 
compressible three-piece mandrel which permits it to be removed from the 
spiral core. The pancake-like wire spiral can be placed on a flat ceramic 
sheet and heat treated without the reel. It may be preferred to use a 
retaining ring around the outer circumference of the spiral wire in order 
to retain its shape during heat treatment. When reel is remove prior to 
heat treatment, it is not required to be made out of special high 
temperature materials. The reel can be made of any material without regard 
to high temperature stability. Aluminum is acceptable. 
Subsequent to winding the wire onto the reel, the wire is ready for heat 
treatment. Such heat treatment includes forming the oxide superconductor 
phase or optimizing the superconducting properties of the wire. The heat 
treatment may be carried on the reel, or one or more plates of the reel 
may be removed prior to heat treatment. During heat treatment, the 
isolating layer prevents bonding of the neighboring layers. The isolating 
layer of the invention permits complete overlap of the wire turns without 
sticking, which is not possible in the prior art. 
The isolating layer may be easily removed by passing it under a stream of 
water, or against an abrasive surface. Ultrasonification will also remove 
the layer. Other conventional means of removing coatings are within the 
scope of the present invention. 
The method of the present invention can be used to process any oxide 
superconductor wire. By way of example, and in no way limiting, the oxide 
superconductor may include rare earth barium cuprates, bismuth strontium 
calcium cuprates and thallium barium calcium cuprates. The oxide 
superconductor wires are typically processed with silver or other 
malleable, inert metal to impart desirable mechanical properties to the 
wire, which is otherwise too brittle to function as a wire. 
The wire used in the invention may be a precursor to an oxide 
superconductor, such as the metallic alloys disclosed in U.S. Pat. No. 
4,826,808 to Yurek et al., herein incorporated by reference, which is then 
oxidized to form the desired oxide superconductor. Alternatively, the wire 
may include an oxide superconductor which is subjected to further heat 
treatment to optimize the formation and properties of the oxide 
superconductor. The wire preferably contains a malleable, but inert, metal 
for improved formability and mechanical flexibility. The preferred metal 
is silver. Oxide superconductor wires suitable for use in the method of 
the present invention are described in "Critical Issues OPIT Processing of 
High-T.sub.c BSCCO Superconductors" by Sandhage et al. (JOM 43(3), 21-25 
(1991)), herein incorporated by reference. 
Other embodiments of the invention will be apparent to those skilled in the 
art from a consideration of this specification or practice of the 
invention disclosed herein. It is intended that the specification and 
examples be considered as exemplary only, with the true scope and spirit 
of the invention being indicated by the following claims.