A glass-ceramic superconductor is disclosed, having a nominal stoichiometric composition consisting essentially of the oxides of Bi, Ca, Sr, Cu and Zn in the stoichiometric formula range: Bi.sub.2 Ca.sub.x Sr.sub.z Zn.sub.n Cu.sub.w O.sub.y, wherein x ranges about 1-2, z ranges about 1-2, n ranges about 0.001-2, w ranges about 1-2, and y ranges about 6-11. These compositions exhibit greatly improved glass stability, yet can yield highly crystalline superconductors which may include large platelet crystals and which can exhibit very low room temperature resistivity.

BACKGROUND OF THE INVENTION 
The present invention is directed towards a superconducting glass-ceramic 
material and method for making the same. The composition of this 
glass-ceramic material is derived from the Bi-Ca-Sr-Cu-O family of 
superconductors. In particular, it has been discovered that when the Ca to 
Sr combination is maintained in certain proportions and, preferably, 
controlled additions of ZnO are provided, that relatively stable glasses 
crystallizable to glass-ceramics with good superconducting properties can 
be produced. 
Further, with appropriate heat treatment, a particularly desirable crystal 
morphology, also observed in Bi-Ca-Sr-Cu-O superconductors, can be 
developed in these stable glasses. This is a morphology comprising 
platelets or crystallites with an angularly juxtapositionally disposed 
orientation that exhibit very good superconductivity properties in 
combination with low room temperature resistivity. 
It is well known to those skilled in the superconductor art that this fast 
paced technology is in need of a composition and process whereby useful 
products can be manufactured. The ceramics of the first discovery, by 
Bednorz and Muller created excitement and happily spurred research, due to 
a significant raising of the critical temperature of superconductivity in 
their ceramic discovery. 
The yttrium-barium-copper oxide (1:2:3) superconductor of the Chu et al. 
discovery, while exhibiting superconductivity at higher temperatures, does 
not exhibit mechanical strength, useful crystal orientation, nor 
sufficient crystal positional proximity for useful products. The 1:2:3 
superconductivity discovery cannot be trivialized, however, since this 
discovery has inspired others to invent additional ceramic superconductive 
compositions. 
Recently, Maeda et al. in the Japanese Journal of Applied Physics 27, L209, 
(1988), disclosed the discovery of a new family of ceramic 
superconductors. This family is known as the Bi-Ca-Sr-Cu-O ceramic 
superconductors. The particular member of that family that excited 
interest is the BiCaSrCu.sub.2 O.sub.y, known as 1:1:1:2. The 1:1:1:2 
ceramic suffered similarly from the same poor properties exhibited by the 
initial 1:2:3 find. 
Since the Maeda et al. revelations, others have come forward with 
discoveries of cousins to the 1:1:1:2 material. Komatsu et al. in the 
Japanese Journal of Applied Physics, 27, 4, April 1988 discloses a 
glass-ceramic material with a composition from the Maeda family of 
superconductors. The glass-ceramics introduced by Komatsu are 3:2:2:4 and 
3:3:2:4 type Bi-Ca-Sr-Cu-O materials. Komatsu first makes a glass with the 
material, rapidly quenches the glass, then anneals the glass forming a 
thin layer of ceramic material on the surface. 
Similarly, Akamatsu et al. describe, in the Japanese Journal of Applied 
Physics, 27 (9) pages L1696--L1698 (September 1988), thick film 
superconducting compositions in the Bi-Ca-Sr-Cu-O composition system 
produced by the crystallization of mixed oxide melts on single-crystal MgO 
substrates. Nevertheless, prior art compositions of these known types tend 
to exhibit very poor glass stability and are difficult to form as glasses 
without uncontrolled crystallization. 
SUMMARY OF THE INVENTION 
The present invention includes compositions derived from the Bi-Ca-Sr-Cu-O 
family of superconductors. Additionally, the material is a glass-ceramic. 
Following the nomenclature of the community, the compositions of the 
inventive glass-ceramic are such that Bi:Ca:Sr:Cu oxide phases of 2:1:2:2, 
2:2:2:3, 4:3:3:4, and 2:1:1:1 composition and combinations therebetween 
can be developed on suitable heat treatment. The material exhibits a 
variety of useful properties including good glass-forming characteristics, 
high mechanical strength, platelets or crystallites with highly defined 
geometries, and angular juxtapositional orientations conducive to 
conductivity. 
More specifically, the invention comprises superconducting glass-ceramic 
articles consisting essentially of the oxides of Bi, Ca, Sr, Cu and Zn, 
wherein oxides are present in the stoichiometric formula range: Bi.sub.2 
Ca.sub.x Sr.sub.z Zn.sub.n Cu.sub.w O.sub.y. In the stoichiometric 
formula, x ranges about 1-2, z ranges about 1-2, n ranges about 0.001-2, w 
ranges about 1-2, and y ranges about 6-11. 
In a further aspect, the invention comprises a method for making a 
superconducting glass-ceramic article offering significantly improved 
article forming capabilities. Broadly stated, that method includes the 
steps of, first, melting an oxide batch to provide a molten glass offering 
the desired improved glass stability characteristics. Such glasses will 
preferably consist essentially, in weight percent, of about 50-55% 
Bi.sub.2 O.sub.3, 5-12% CaO, 15-20% SrO, 8-20% CuO, and an amount of ZnO 
at least effective to stablilize the composition as a glass. 
For the purpose of enhancing glass stability, the ZnO content can be on the 
order of 0.01% by weight, or more preferably at least 0.1% ZnO by weight. 
For highest glass stability, ZnO concentrations of 0.5-12% by weight are 
particularly preferred. 
The molten glass thus provided is next formed into a glass shape of a 
selected configuration. The shape may be a thin coating, or it may be a 
glass shape of relatively substantial cross-section. Advantageously, the 
glass can in most cases be formed into a shape having at least one 
dimension in excess of 1 mm in thickness, yet will still be essentially 
free of crystalline precipitates or inclusions. 
After the glass shape has been provided, it is crystallized by heat 
treatment. This treatment will comprise heating the glass shape, 
preferably at a temperature in the range of about 800-890.degree. C. for a 
time sufficient to develop a superconducting Bi-Ca-Sr-Cu oxide crystal 
phase therein. 
Optional additives may be present in the glass-ceramic superconductors of 
the invention, provided they are maintained at levels below those causing 
either destabilization of the glass or loss of superconducting 
characteristics. Thus superconducting glass-ceramic articles such as above 
described may additionally contain one or more additives selected from the 
group consisting of alumina, silver, tin, lead, sodium, potassium and 
fluorine, but these additives will preferably not exceed, in total, about 
2% by weight of the glass-ceramic composition. 
The above-described compositions can produce a superconducting 
glass-ceramic wherein the mechanical properties of the material are 
enhanced, and wherein platelets or other crystallites exceeding 100 
microns in at least one dimension and ranging in size up to 1000 microns 
in one or more dimensions can be developed. Volume proportions of 
superconducting phases have ranged from as low as 1 to perhaps as high as 
60% by volume, as measured magnetically at 65.degree. K. However, crystal 
phase characteristics such as a large platey or ribbon-like habit, with 
interweaving of the crystals in certain cases, are considered to 
significantly improve properties such as room temperature conductivity. 
With crystal sizes ranging up to 1000 microns being attainable, these 
materials promise improved utility for the production of ribbon-like 
conductive media. Additionally, since the material is at first a glass, it 
may be drawn out similarly to a fiber, formed into shapes as a glass, 
coated on a substrate, and subsequently crystallized to form the ceramic 
portion of the glass-ceramic. The substantially improved glass forming and 
handling characteristics of the compositions of the invention are 
critically important where existing glass forming processes are to be 
utilized for superconducting component manufacture.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The process of producing the superconducting glass-ceramics of the 
invention can follow conventional glass melting practice. The material is 
batched from carbonates and/or oxides of bismuth, strontium, calcium, zinc 
and copper. The batched materials are melted at temperatures from 
1000.degree. to 1350.degree. C. and then cast or, if desired, rapidly 
quenched, producing an amorphous structure. This intermediate material is 
a glass. The glass is subsequently heated at about 800 to 890.degree. C., 
preferably between about 800-850.degree. C., in an oxygen rich environment 
or air, to form platelets or crystallites intimately interspersed with the 
glass, forming a glass-ceramic superconductor. 
An advantage provided by these relatively stable glasses is the gain in 
control over the grain size of the final crystallized material. This 
control permits the attainment of very fine crystal grains, which are 
useful to provide high critical fields for magnetic shielding in these 
materials. Alternatively, heat treatment variations can provide a large 
overlapping platelet crystal structure which imparts high room temperature 
conductivity to the superconducting material. 
In a generalized preferred procedure for synthesizing a superconducting 
glass-ceramic in accordance with the invention, quantities of batch 
materials such as Bi.sub.2 O.sub.3, CaO, SrCO.sub.3, ZnO and CuO are 
tumble-mixed together for times on the order of an hour. The batches are 
then placed in alumina crucibles and heated in a furnace in air to a 
temperature of approximately 1150.degree. C. for a time sufficient to 
achieve a homogeneous melt. 
The melts thus provided are then withdrawn from the furnace and poured onto 
a stainless steel table into glass patties approximately 
4.times.8.times.0.5 inches in size. If necessary, the melts can be pressed 
with a stainless steel block to provide rapid quenching and still further 
reduce any possibility that devitrification of the casting will occur. The 
resulting solids are typically black and exhibit glassy fracture. 
Samples of glasses produced as described are then placed in a tube or other 
furnace equipped for air and oxygen flow. Heat treatment is carried out 
while the furnace is supplied with a continuous flow of air or oxygen. Gas 
flow is generally continued during heat-up, high temperature dwell, and 
cool-down intervals. The furnace is preferably maintained at peak 
operating temperatures in the 800-850.degree. C. range, with these 
temperatures being maintained for times on the order of about 12 hours. 
Cooling of the samples is typically carried out at the furnace rate, i.e., 
over an interval which may be as long as 15 hours. 
One technique for controlling crystal morphology in crystallized 
superconductors made as above described relates to the positioning of the 
samples during heat treatment. For flat plate samples such as produced in 
accordance with the above casting procedure, fine-grained crystal 
development is favored by horizontal plate orientation in the furnace, 
while large platelet growth is favored by a vertical plate orientation. 
The reason for this difference in crystallization behavior is not fully 
understood, although it is presently hypothesized that vertical plate 
orientation creates an environment more akin to crystallization from a 
melt, and thus larger and more oriented crystals. 
The percent of superconductive phases present in the materials described 
herein is determined from magnetization measurements carried out at low 
temperatures. Magnetization vs. applied magnetic field measurements are 
made with a vibrating sample magnetometer, using fields up to 10,000 
Gauss. Cooling is provided by a flow cryostat. Data can be taken down to 
65K by pumping on the liquid nitrogen coolant. Samples for these tests 
consist of ground powders of superconducting glass-ceramic positioned in 
small nylon sample holders. The field is applied perpendicular to the long 
axis of the sample holders. 
Electrical resistivity measurements are made by the four probe method. Four 
gold strips are evaporated onto solid glass-ceramic samples and electrical 
contacts are made to these with silver epoxy. A flow cryostat, using 
liquid helium coolant, is used to lower the temperature of samples to 
about 10K. 
As previously noted, specific advantages of the superconducting 
glass-ceramic compositions of the invention include significantly improved 
glass melting and forming behavior. In many of these compositions, glass 
pieces well in excess of 1 cm in thickness can be formed by direct casting 
onto stainless steel without additional quenching. The products are 
completely homogeneous insofar as can be determined under electron 
microprobe analysis. This behavior is in marked contrast to that of 
ordinary Bi-Ca-Sr-Cu-O melts, which frequently require quenching and 
pressing to thicknesses on the order of 1 mm in order to provide glasses 
reasonably free from uncontrolled devitrification. 
The glass-ceramic superconductors of the invention, particularly when heat 
treated to provide a platelet crystal structure, can exhibit metallic 
conduction behavior characterized by exceptionally high room temperature 
conductivity. Typically, electrical resistivities in platelet-containing 
samples will not exceed about 10.sup.-4 ohm-cm at room temperature. When 
cooled to cryogenic temperatures, rapid decreases in sample resistivity 
associated with the superconducting transition are normally observed in 
the temperature regime of 75-85.degree. K. 
The invention is more fully illustrated by the following detailed examples, 
which are intended to be illustrative rather than limiting. 
EXAMPLE 1 
To provide a batch for a glass-ceramic superconductor, 544 grams of 
Bi.sub.2 O.sub.3, 69 grams of CaO, 50 grams of ZnO, 258 grams of 
SrCO.sub.3, 185 grams of CuO, and 15 grams of Al.sub.2 O.sub.3 were mixed 
and melted as a glass. This batch corresponds to a stoichiometric oxide 
composition of approximately: Bi.sub.2 Ca.sub.0.87 Sr.sub.1.46 Cu.sub.2.07 
Al.sub.0.39 Zn.sub.0.54 O.sub.8.78. The batch was melted at 1150.degree. 
C. in a platinum crucible in air, and the melt was then poured into a 
stainless steel mold. 
The cast glass plate thus provided was about 0.5 inch (1.72 cm) thick. The 
glass was converted to a glass-ceramic material by furnace heat treatment 
in accordance with the preferred procedure above described. The glass was 
initially heated to a temperature of about 850.degree. C., maintained at 
850.degree. C. for 12 hours, and then cooled at furnace rate to room 
temperature. 
After heat treatment, the sample had a flat, fine grained appearance. The 
inside was comprised of shiny crystallites, up to 1000 microns in size. 
Powder x-ray analysis showed a crystalline phase with a structure similar 
to that of Bi.sub.2 (CaSr).sub.3 Cu.sub.2 O.sub.8, unidentified lines, and 
glass. SEM indicated that the crystallites were comprised of flat, 
mica-like grains of up to several hundred microns in length and up to 1 
micron thick. Magnetization measurements at 67.degree. K. indicate 4 
volume percent of superconducting phases present in the sample. 
EXAMPLE 2 
A glass batch consisting of about 53.1 parts Bi.sub.2 O.sub.3, 6.4 parts 
CaO, 17.7 parts SrO, 18.1 parts CuO, and 4.6 parts ZnO is compounded for 
melting. This batch yields a melt for a glass having a stoichiometric 
composition of approximately Bi.sub.2 Ca.sub.1 Sr.sub.1.55 Cu.sub.2 
Zn.sub.0.5 O.sub.8 upon melting as described above in Example 1. 
The glass melt having the composition described is formed into a glass 
patty by casting onto a steel plate. The stability of the glass is 
sufficient to permit direct casting of the melt to a thickness of about 
0.25 inches without significant devitrification. Thus electron microprobe 
analysis demonstrates that the casting is substantially amorphous and 
homogeneous. 
Pieces of the amorphous glass patty produced as described were next heated 
at 850.degree. C. for 12 hours in oxygen. This treatment was sufficient to 
convert the glass pieces to highly crystalline glass-ceramics. 
Resistivity measurements were made on a typical glass-ceramic samples 
produced as described over a temperature range of about 70-290.degree. K. 
A superconducting transition was observed beginning at about 85.degree. 
K., although the point of zero resistance was not reached at 70K. 
The room temperature resistivity of a typical superconducting glass-ceramic 
sample produced in accordance with this Example is about 10.sup.-4 ohm-cm. 
This is an order of magnitude lower than typical prior art Bi-Ca-Sr-Cu-O 
glass-ceramics, and also lower than that of conventional ceramic 
superconductors of this composition. 
Electron microprobe analyses indicate the presence, in the glass-ceramic 
samples produced as above described, of a combination of superconducting 
phases. Thus both Bi.sub.2 Ca.sub.1.5 Sr.sub.1.5 Cu.sub.2 O.sub.8 ("4334", 
Tc=85K) and Bi.sub.2 Ca.sub.1 Sr.sub.1 Cu.sub.1 O.sub.6 ("2111", Tc=35K) 
phases appear to be present. Magnetization tests at 67.degree. K. show 
about 8 volume % of superconducting phases present in the samples. 
Glass samples of the above described composition which are heated to 
850.degree. C. for 12 hours in air yield a crystalline material that again 
comprises a mixture of "4334" and "2111" phases. These are magnetically 
similar to the earlier described glass-ceramics, but have room temperature 
resistivities on the order of 10.sup.-3 ohm-cm. 
Table I below summarizes additional data on superconducting glass-ceramics 
provided in accordance with the invention. Included in Table I for each of 
the compositions evaluated are an oxide composition, in weight percent 
oxide as batched for each of the glass melts, and information relating to 
the heat treatments used to develop superconducting phases in the 
glass-ceramic products. 
Heat treatment data includes a report of heat treatment atmosphere (HT 
Atm.), indicated whether the treatment was carried out in air, in oxygen 
(Oxy), or in a mixture of air and oxygen (O/A). Also reported are the peak 
heat treatment temperatures (HT Temp) in .degree.C. and times at peak 
temperature (HT Time) in hours. The orientation of each sample during heat 
treatment (HT Orien.), whether aligned parallel with (horizontal) or 
perpendicular to (vert.) the bottom of the heat treating furnace, is also 
given. 
Finally, the volume percent of superconducting phases present int he 
samples (SC Phase) are recorded, as determined by magnetization 
measurements at 67.degree. K. As would be expected, variations in the 
volume percent of superconducting phases present can result from heat 
treatment variables as well as variations in sample composition. 
TABLE I 
______________________________________ 
Superconducting Glass-Ceramics 
______________________________________ 
Oxides Example No. 
(wt %) 3 4 5 6 7 8 
______________________________________ 
Bi.sub.2 O.sub.3 
53.1 53.1 53.1 52.1 52.1 53.6 
CaO 6.4 6.4 6.4 6.6 6.6 9.7 
SrO 17.7 17.7 17.7 17.3 17.3 17.9 
CuO 18.1 18.1 18.1 17.7 17.7 18.3 
ZnO 4.6 4.6 4.6 4.8 4.8 0.5 
Al.sub.2 O.sub.3 
-- -- -- 1.4 1.4 -- 
HT Temp. 850 850 850 850 850 830 
(.degree.C.) 
HT Atm. Air Air Oxy Air Oxy Oxy 
HT Time 12 12 12 12 12 12 
(hours) 
HT Orien. 
Hor. Hor. Vert. Vert. Vert. Hor. 
SC phase 8 1.5 6.5 1 1 5 
(% vol.) 
______________________________________ 
Oxide 9 10 11 12 13 14 
______________________________________ 
Bi.sub.2 O.sub.3 
53.6 53.6 53.9 53.9 53.8 53.8 
CaO 9.7 9.7 9.7 9.7 9.7 9.7 
SrO 17.9 17.9 17.9 17.9 18.0 17.9 
CuO 18.3 18.3 18.0 13.8 13.8 9.2 
ZnO 0.5 0.5 0.47 4.7 4.7 9.4 
Al.sub.2 O.sub.3 
-- -- .sup..about. 2 
.sup..about. 2 
-- -- 
HT Temp. 830 890 860 860 845 845 
(.degree.C.) 
HT Atm. Oxy Oxy Oxy Oxy Oxy Oxy 
HT Time 12 12 8 8 12 12 
(hours) 
HT Orien. 
Vert. Vert. Hor. Hor. Hor. Hor. 
SC phase 8 9 &lt;1 &lt;1 5 3 
(% vol.) 
______________________________________ 
Based on testing such as hereinabove described, a region of good glass 
formability in the Bi-Ca-Sr-Cu-Zn oxide system has been identified which 
offers an excellent combination of forming and superconducting properties. 
That region is presently considered to comprise glass and glass-ceramic 
compositions consisting essentially, in weight percent on an oxide basis, 
of about 50-55% Bi.sub.2 O.sub.3, 5-12% CaO, 15-20% SrO, 8-20% CuO, and 
0.5-12% ZnO. Optional additions of oxide, halide, and metallic 
constituents selected from the group consisting of Al.sub.2 O.sub.3, PbO, 
Na.sub.2 O, Li.sub.2 O, SnO.sub.2, K.sub.2 O, fluoride and silver, in 
amounts totalling not more than about 2% by weight, may be included if 
desired. 
Chemical analyses of samples provided in accordance with the invention in 
some cases show the presence of several impurities, such as carbon, 
silicon, iron, and magnesium. However, it is believed that these 
impurities have little effect on the superconductivity of the examples at 
the low levels observed. 
Referring again to the drawings, FIG. 1 shows angularly juxtaposed 
platelets in superconducting glass-ceramics wherein large crystals can be 
grown. The scale is 100 microns. Within this view, crystallites of up to 
1000 microns are apparent. In expanded views crystallites of 1000 microns 
were noted. Generally, larger crystals indicate a higher aspect ratio. 
The grain sizes noted have been found to depend upon the heating 
temperature as well as composition. The lower the temperature the smaller 
the grain size. FIG. 2 shows that the same platelets can be disposed 
parallel to one another with a consistent form. The scale is 100 microns. 
FIG. 3 shows an overall view of the composite superconducting material 
wherein the scale is again 100 microns. Parallel or interwoven platelets 
may be important in the production of viable superconducting structures 
since the disposition of the platelets will provide a means for 
communicating, one from the other. It is noted that the platelets are 
interwoven. Interwoven platelets add vertical and lateral mechanical 
strength. The orientation of the platelets and/or crystallites may be 
effected by the orientation of the sample when heat treated. The aggregate 
crystal structure of 1 millimeter has been noted. By aggregate crystal 
structure is meant, a unified oriented crystallite or platelet 
aggregation. 
FIG. 4 shows a plot of resistance in ohms versus Temperature in Kelvins for 
a typical superconducting Bi-Sr-Ca-Cu-O glass-ceramic. The onset of bulk 
superconductivity in the sample is manifested by the rapid decrease in 
resistivity commencing at about 85.degree. K. 
FIG. 5 is a plot of a magnetization measurement. The dotted line shows 
where the measured magnetization deviates from linearity indicating the 
first critical field of greater than 2000 Gauss. 
In summary, then, ZnO has been found to be a surprisingly effective 
additive, in the Bi-Ca-Sr-Cu-O oxide composition system, for the purpose 
of improving the stability or resistance of the oxide melt to 
crystallization on cooling. Additions of only 0.1% ZnO by weight to the 
composition are found to suppress crystallization of cast molten material, 
and additions of as little as 0.01% by weight are considered to be useful 
and effective for the purpose of improving melt behavior and control of 
crystal development. Hence, while ZnO concentrations of 0.5-20% by weight 
yield the highest improvements in glass stability, additions of 0.01%, or 
preferably at least 0.1%, will be found to yield significant improvements 
in glass quality as determined by visual observation, X-ray diffraction 
and/or electron microscopic examination of glass articles produced 
directly from the molten oxides.