Fabrication of high temperature superconductors

An improved method for producing high temperature superconductors comprising sintering ceramic superconductor material in a sealed confinement chamber made of non-reactive impervious material, thereby preventing loss of oxygen from the material during heating and eliminating the need for reoxygenation after sintering.

BACKGROUND OF THE INVENTION 
This invention relates to superconductors and more particularly to those 
that are referred to as "high temperature superconductors". 
High temperature superconductors are known in the art, illustrative of 
which are those described in co-pending U.S. patent application Ser. No. 
07/158,345 filed on Feb. 22, 1988 by Martin John Sablik. 
Among high temperature superconductors are several which are formed of a 
ceramic oxide, such as yttrium-barium-copper oxide. Although such high 
temperature ceramic oxide superconductors comprise a substantial step 
forward in the art, there have been problems in producing them. Typically, 
in the prior art, these high temperature superconductors have been 
fabricated by the so-called "shake" and "bake" method, according to which 
the metal oxide ingredients are ground together, compressed and then 
sintered in an oxygen atmosphere at elevated temperatures near the melting 
point. The elements react chemically to form the superconducting compound 
and fuse into a solid. However, it is known that oxygen is highly reactive 
in such a highly heated environment; and it is also known that in the 
absence of an oxygen rich atmosphere, some of the oxygen that is 
chemically bound to the other elements disassociates itself and migrates 
away, thus tending to destroy or degrade the superconducting properties of 
the material. 
SUMMARY OF THE INVENTION 
In practicing the principles of the invention, the superconducting material 
is fabricated within a hollow cylinder of essentially non-reacting, oxygen 
impermeable material which serves to block the loss of oxygen out of the 
superconducting material. This hollow cylinder which hereinafter is 
referred to as a confinement tube, is preferably made of silicon carbide 
with a binder constituent as hereinafter described. The silicon carbide 
confinement tube may contain ambient air or an oxygen rich atmosphere. 
OBJECTS AND FEATURES 
It is one general object of this invention to improve manufacture of 
high-temperature superconductors. 
It is another object of this invention to minimize the oxygen 
disassociation from ceramic oxide superconductors during the processing 
thereof. 
It is yet another object of the invention to process high-temperature 
ceramic oxide superconductors in an environment that is non-reactive and 
which facilitates bulk texturing of the super-conducting material. 
Accordingly, in accordance with one feature of the invention, through the 
efficacious use of silicon carbide, the material of the containment tube 
is non-reactive with the superconducting material ingredients, thereby 
facilitating processing at the aforementioned sintering temperature. 
In accordance with another feature of the invention, through the 
utilization of the aforementioned confinement tube, the superconducting 
material can be melt-textured while eliminating or reducing the need for 
subsequent oxygenation, and encouraging cystallites to grow in the 
melt-textured bulk along the principal axis of the confinement tube and in 
the direction of thermal gradient. 
It is another object of the invention to form thin or thick films on 
silicon carbide substrates because the substrates are non-reactive with 
the superconducting material. 
These and other objects and features of the invention will be apparent from 
the following detailed description, by way of examples of a preferred 
embodiment, with reference to the drawing.

DETAILED DESCRIPTION 
Now turning to the drawings, and more particularly to FIG. 1 thereof, it 
will be observed that there is disclosed in block diagram form a typical 
manufacturing process in accordance with heretofore known techniques. In 
that figure, material A, material B and/or other materials which are 
represented by the dashed line and the box labelled material "N" are 
ground and mixed together by introducing them to a suitable conventional 
grinder 13 where grinding and mixing are conducted. The powders are ground 
together, pressed into a pellet, and reacted at elevated temperature in 
the presence of flowing oxygen, and then after cooling in oxygen, ground 
again, pelletized again, and reacted again in sequence over and over until 
satisfactory powder of superconducting material is produced. This sequence 
is represented by return flow path 17. 
At the conclusion of the powder preparation, the powder is introduced into 
press 14 where the materials are compressed into a final pellet. 
Thereafter, the materials are heated in heater 15 where sintering of the 
pressed materials occurs, causing them to bind into a cohesive mass. 
Unfortunately, during the step of sintering the pressed materials as 
practiced in the prior art, a substantial amount of the oxygen contained 
in the superconductor materials is lost, and the materials cannot then 
become superconducting unless and until the oxygen is reintroduced. Such 
reintroduction is symbolized by oxygen restoration 16. However, typically 
such oxygen restoration involves many hours of baking the sintered 
material at relatively high temperatures in an oxygen atmosphere to effect 
the required restoration. Typically, the times and temperatures for such 
oxygen reintroduction are from eight hours to several days at temperatures 
of several hundred degrees Celsius. 
Furthermore, in accordance with the prior art, where thin films of 
superconducting material are to be prepared, the aforementioned loss of 
oxygen has been sufficiently severe so as to destroy the superconducting 
characteristics of the film and thereby require reoxygenation as in the 
case of the bulk superconductors. For a thin film superconductor, the 
attendant increase in time of fabrication is particularly undesirable. 
Now turning to FIG. 2, it will be observed that it discloses a method in 
accordance with the invention hereof. There, superconducting powder is 
preformed as in the case of the prior art. However, in contrast with prior 
art, the powder is packed into a confinement tube 18 with much of the 
environmental atmosphere being squeezed out during packing. The packing is 
done in an environment of ambient air or an oxygen rich atmosphere. The 
confinement tube is then closed and sealed tightly before being heated to 
achieve sintering as indicated by symbol 19 (FIG. 2). Upon completion of 
sintering, the encapsulation tube may be opened and the material removed. 
As mentioned above, in consequence of the material having been enclosed 
within the confinement tube and since the material of the confinement tube 
is carefully selected to be non-reactive with the processed material and 
impermeable to oxygen, the processed material retains its superconductive 
properties and does not require reoxygenation as with the prior art. Other 
possible materials to be used in place of silicon carbide may be boron 
nitride, silicon nitride and tungsten carbide. 
Now turning to FIG. 3, the preferred form of the confinement tube is 
illustrated. As mentioned above, the walls 20 of the confinement tube 18 
are constructed of silicon carbide or any other material which has been 
found to be non-reactive with ceramic superconductors of the type herein 
described. In FIG. 3, the various members are shown in cross-section, and 
thus the walls 20 of confinement tube 18 are illustrated in section. 
Although the confinement tube preferred for practicing the invention is 
generally cylindrical, it will be evident to those skilled in the art that 
other geometric shapes could be employed. 
At the left end of the confinement tube as illustrated in FIG. 3, there is 
shown plug 21 which, after the tube has been filled with material 22, is 
inserted and sealed tightly. This plug may be made of any suitable 
material that will withstand the relatively high temperatures of sintering 
and without reacting with the superconductor material. The preferred 
material of such plug is also silicon carbide or other non-reactive 
material, such as suggested previously. 
After being filled and sealed, confinement tube 18 is passed into the 
interior opening 23 of furnace 24 where it is appropriately heated to 
chemically bind the material ingredients together and to sinter them as is 
heretofore described. In order to accomplish this, confinement tube 18 may 
be passed through the heating zone of the furnace 24 very slowly (e.g., 
progressive zone heating) or it may be placed stationary within the 
furnace and the entire tube heated uniformly. 
One additional advantage accruing from practicing the inventive principles 
hereof resides in the ease with which the material may be melt textured if 
desired by increasing the temperature of the furnace and/or by passing a 
heated zone along the tube. 
The principles of the invention will be further understood by reference to 
the following examples illustrating their use in preparing superconducting 
materials as described above. 
EXAMPLES 
YBa.sub.2 Cu.sub.3 O.sub.7-x was used as the superconductor powder in this 
example. The powder used was either obtained from Alrich Chemical Co. or 
was prepared in our laboratory according to the procedure that follows. 
Stoichiometric amounts of Y.sub.2 O.sub.3, BaCO.sub.3, and CuO were ground 
together in an agate crucible until the color and texture of the mixture 
was homogeneous. This grinding process took about 15 minutes. Next, the 
powder mixture was heated for 18 hours at 900.degree. C. in an oxygen 
atmosphere. The material was then quenched from 900.degree. C. to room 
temperature by removing it from the furnace. The cooled mixture was then 
reground in the crucible until a smooth, homogeneous, grayish powder was 
obtained. Pellets 1-inch thick in diameter and 1/8-inch thick were made in 
a press under pressure of about 5,000 pounds per square inch. These 
pellets were then heated in oxygen for six hours at 925.degree. C., 
quenched to room temperature, placed in a low temperature furnace with an 
oxygen atmosphere at 550.degree. C. for four hours and then slow-cooled to 
350.degree. C. at the rate of 1.degree. per minute. The pellets remained 
at 350.degree. C. for an additional twelve hours and then were slow-cooled 
to room temperature at about 1.degree. per minute. The pellets were then 
reground and new pellets made, which were reheated to 940.degree. C. for 
six hours in oxygen, and then were slow-cooled to room temperature at 
about 1.degree. per minute. The pellets were then again reground, new 
pellets made, and reheated to 940.degree. C. for six hours in oxygen and 
then again cooled to room temperature at about 1.degree. per minute. This 
grinding, repelletizing, heating and cooling were then repeated one 
additional time. It was found that reprocessing the pellets by grinding 
them, repelletizing, heating, and cooling resulted in an increase in 
density of the material. After the last firing at 940.degree. C., the 
pellets were reground once more to produce fine YBa.sub.2 Cu.sub.3 
O.sub.7-x powder. Care was then taken to protect the powder from moisture 
since it was found that the powder would take on water rapidly. 
The YBa.sub.2 Cu.sub.3 O.sub.7-x powder was used to pack commercial carbide 
containment tubes which were obtained commercially. In this example, tubes 
made of commercial grade silicon carbide with a clay binder were obtained 
from Bolt Technical Ceramics Company in Conroe, Texas. The silicon carbide 
tubes used were 1/4 inch internal diameter and 1/2 inch outer diameter. 
The foregoing silicon carbide tubes were closed at one end. YBa.sub.2 
Cu.sub.3 O.sub.7-x powder was added to the open end of the silicon carbide 
tubes and compacted as it was added. The silicon carbide tubes containing 
compacted powder were then heated in a horizontal position at 950.degree. 
C. for between 30 and 45 minutes and then slowly quenched to room 
temperature. The material was then removed from the silicon carbide tubes 
and was tested for superconducting properties by measuring its resistance 
while progressively lowering the temperature to the superconducting 
region. 
FIG. 4 illustrates test results. From an examination of FIG. 4, it will be 
evident that there is evidence of a superconducting transition. In this 
instance, superconducting transition temperatures for the cooling and 
warming curves appear to be different only because the sample was not in 
thermal equilibrium with the thermocouple during the cooling and warming 
process. It is clear that the sample, prepared in the confinement tube and 
quenched without additional reoxygenation, is a good superconductor. 
The clay binder in the silicon carbide confinement tube is useful to 
include because it is believed that the clay binder effectively seals 
porosity in the silicon carbide thereby effectively sealing it and 
preventing oxygen from escaping while the material therein is being 
sintered. 
A silicon carbide tube obtained from Lindberg Furnace Company was also used 
to produce superconducting material without reoxygenation, but in that 
case, the material produced was not as good, exhibiting a transition 
temperature step but not zero resistivity below the transition. The 
silicon carbide tube from Lindberg was somewhat porous and did not serve 
therefore as an effective barrier for preventing loss of oxygen from the 
superconducting material. 
In a separate experiment, YBa.sub.2 CU.sub.3 O.sub.7-x powder was compacted 
and sealed inside a silicon carbide tube with clay binder, as obtained 
from Bolt Technical Ceramics. Applying an oxy-hydrogen torch to the 
outside of the silicon carbide tube, the powder inside was heated to a 
temperature above that at which it begins to melt incongruently. Heating 
was applied to all sides of the silicon carbide tube and the torch 
elevation was very slowly decreased as time progressed. In this way, the 
hot zone descended slowly from the top to the bottom of the silicon 
carbide tube. In all, three zone passes of this type were accomplished 
over a 3 hour period. The experiment simulates the melt-texturing 
procedure of the prior art; but in this case, the superconducting material 
was inside a confinement tube instead of being exposed to the open air. 
FIG. 5 shows the result of melt-processing. The material exhibits an 
apparent superconducting transition but the temperature was not reduced to 
a point where full zero resistivity was obtained. With a more carefully 
controlled procedure, using for example, an rf heater, it is expected that 
even better superconducting material can be melt-processed by this 
confinement procedure without the need for oxygenation afterwards. 
It will now be evident that there has been described herein an improvement 
in the fabrication of high temperature superconductors which exhibits 
significant advantages over the corresponding prior art. Although the 
inventive concepts hereof have been illustrated by way of a preferred 
embodiment, it will be evident to those skilled in the art that 
adaptations and modifications may be employed without departing from the 
spirit or scope of the invention. Thus, for example, other forms of 
binders may be available to seal the porosity of the silicon carbide and 
thus render it efficacious for containment tubes. 
The terms and expressions employed herein have been used as terms of 
description and not of limitation, and there is no intent in the use 
thereof to exclude equivalents, but on the contrary, it is intended to 
include any and all equivalents, adaptations and modifications that can be 
employed without departing from the spirit and scope of the invention as 
described in the specification and claims herein.