110K Bi-Sr-Ca-Cu-O superconductor oxide and method for making same

A superconductor consisting of a sufficiently pure phase of the oxides of Bi, Sr, Ca, and Cu to exhibit a resistive zero near 110K resulting from the process of forming a mixture of Bi.sub.2 O.sub.3, SrCO.sub.3, CaCO.sub.3 and CuO into aparticulate compact wherein the atom ratios are Bi.sub.2, Sr.sub.1.2-2.2, Ca.sub.1.8-2.4, Cu.sub.3. Thereafter, heating the particulate compact rapidly in the presence of oxygen to an elevated temperature near the melting point of the oxides to form a sintered compact, and then maintaining the sintered compact at the elevated temperature for a prolonged period of time. The sintered compact is cooled and reground. Thereafter, the reground particulate material is compacted and heated in the presence of oxygen to an elevated temperature near the melting point of the oxide and maintained at the elevated temperature for a time sufficient to provide a sufficiently pure phase to exhibit a resistive zero near 110K.

BACKGROUND OF THE INVENTION 
Beginning with the discovery of superconductivity in the yttrium, barium, 
copper oxide, an enormous number of materials have been tested for 
superconductivity. Although the yttrium, barium, copper oxide material 
when it was first prepared became superconducting at elevated 
temperatures, upon examination of the material it was found that the 
actual superconducting phase was but a very minor amount of the material 
prepared in bulk. Apparently, the yttrium, barium, copper oxide system is 
superconducting because the superconducting phase is distributed 
throughout the lattice structure of the material and is capable of 
formulating current paths. This is not the case with the Bi, Sr, Ca, Cu 
oxide system. In the Bi, Sr, Ca, Cu oxide system, it has been found that 
the material when formulated is intercalated or interleaved. That is 
material having very low T.sub.c phase is interleaved with a 110 K T.sub.c 
phase. Because of the interleaving or intercalated nature of the Bi, Sr, 
Ca, Cu oxide system, it has been impossible to isolate the superconducting 
phase and more particularly it has been impossible to isolate the 110 K 
transition temperature material. Heretofore, the Bi, Sr, Ca, Cu oxide 
material has only shown zero resistance below 100K, typically at 85K due 
to the inability to segregate the 110K phase. 
Accordingly, it is a principal object of this invention to provide a 
material which has sufficient phase purity to exhibit zero electrical 
resistance at 110K. 
It is also an object of the invention to provide a method of producing bulk 
quantities of single phase superconducting material. 
Another object of this invention is to provide a superconductor consisting 
of a sufficiently pure phase of the oxides of Bi, Sr, Ca and Cu to exhibit 
a resistive zero near 110K. 
Still another object of the invention is to provide a superconductor 
consisting of a sufficiently pure phase of the oxides of Bi, Sr, Ca and Cu 
to exhibit a resistive zero near 110K resulting from the process of 
forming a mixture of Bi.sub.2 O.sub.3, SrCO.sub.3, CaCO.sub.3 and CuO into 
a particulate compact wherein the atom ratios are Bi.sub.2, 
Sr.sub.1.4-2.0, Ca.sub.1.8-2.4, Cu.sub.3, heating the particulate compact 
rapidly in the presence of oxygen to a first elevated temperature near the 
melting point of the oxides to form a sintered compact, maintaining said 
sintered compact at the first elevated temperature for a prolonged period 
of time, cooling the sintered compact and regrinding same to form a 
particulate material, heating the reground particulate material in the 
presence of oxygen to a second elevated temperature near the melting point 
of the oxide, and maintaining the reground particulate material at the 
second elevated temperature for a time sufficient to provide a 
sufficiently pure phase to exhibit a resistive zero near 110K. 
A still further object of the invention is to provide a method of making a 
single phase superconductor, comprising formulating a mixture of oxide 
particulates, rapidly heating the oxide particulates in the presence of 
oxygen to a first elevated temperature near the melting point thereof to 
sinter same, maintaining the sintered particulates at the elevated 
temperature for a prolonged period of time, regrinding and compacting the 
sintered particulates and reheating to a second elevated temperature in 
the presence of oxygen, maintaining the reground and compacted material at 
the second elevated temperature for a time sufficient to provide a 
substantially pure single phase having a resistive zero at a predetermined 
superconducting temperature. 
The invention consists of certain novel features and a combination of parts 
hereinafter fully described, illustrated in the accompanying drawings, and 
particularly pointed out in the appended claims, it being understood that 
various changes in the details may be made without departing from the 
spirit, or sacrificing any of the advantages of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Since the first reports, in early 1988, of high-Tc superconductivity in the 
Bi-Sr-Ca-Cu-0 system, many investigators have reported compositions with 
superconducting transition temperatures in the range of 75-110 K. The 
exact compositions and lattice parameters corresponding to the different 
phases are still not well defined. Superconducting compounds with nominal 
composition Bi.sub.2 Sr.sub.2 Ca.sub.1 Cu.sub.2 Oz (2212) and transition 
temperatures in the range 85-95K have been prepared. 
The 110K phase had generally been assumed to have the nominal composition 
Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x (2223) although this high 
T.sub.c phase has been observed only in multiphase mixtures, and has never 
previously been isolated. Typically, the high T.sub.c phase appears as a 
minor constituent. Nevertheless, lattice parameters for the high T.sub.c 
phase have been reported. For example, lattice parameters are reported by 
Kijima et al. on material of nominal composition Bi.sub.1 Sr.sub.1 
Ca.sub.3 Cu.sub.4 O.sub.x (1134). This material is multiphase but shows a 
nearly complete resistive transition at about 107K. 
The discovery which constitutes this invention is the nearly complete 
isolation of the 110 K phase. Isolation of the phase depends sensitively 
on the Sr content and on heat treatment procedures. The 110K phase 
apparently forms as intercalated regions in the low T.sub.c matrix rather 
than a random phase mix. Consequently, a very large volume fraction of the 
high T.sub.c phase may exist without resulting in a resistive zero. We 
have obtained a sufficiently pure phase to observe a resistive zero near 
110K. 
A series of samples with compositions in the vicinity of the 2223 nominal 
material have been tested. Best results were obtained for materials with 
compositions near Bi.sub.4 Sr.sub.3 Ca.sub.4 Cu.sub.6 O.sub.x (4346), 
i.e., somewhat Sr deficient. 
The samples were prepared from mixtures of Bi.sub.2 O.sub.3, SrCO.sub.3, 
CaCO.sub.3, and CuO in appropriate ratios. Other salts, such as nitrates 
may be employed, but carbonates and oxides have been used to illustrate 
but not limit the invention. The mixture of starting materials was ground 
in a ball mill or by hand, compacted into a disk, heated rapidly 
(300.degree. C./h) to 860.degree. C. and held at 860.degree. C. for 20 
hours in air. The sintered compacts were reground, mixed thoroughly and 
recompacted for the final heat treatments. The presintered compacts were 
then heated to 815.degree. C. at a rate of 300 degrees per hour, and then 
to 870.degree. C. at 8.degree. C./hour. Samples were then soaked at 
870.degree. C. for an additional 120 hours in air before cooling at a rate 
of 40 degrees per hour to 400.degree. C. The samples were then held at 
400.degree. C. for 28 hours and were furnace cooled to room temperature. 
After treatment, samples are somewhat conical in shape (smaller at the 
top). Samples from successive heat treatments show somewhat variable 
superconducting properties. It is believed this is due to variations of 
heat treatment temperature (resulting from different sample positions in 
the furnace). One sample (labeled sample f below) was subjected to 
additional heat treatments consisting of 84 h at 850.degree. C. and 42 h 
at 840.degree. C. The last (840.degree. C.) treatment gave no significant 
change (from the 850.degree. C. treatment) in the resistivity. This sample 
was air quenched. 
In general, we have found rapid heating is necessary in the initial 
calcining step. 300.degree. C./hr is an acceptable heating rate and simply 
inserting the material into a preheated furnace may be preferred. Heating 
more slowly forms undesirable phases. The temperature to which the sample 
is heated should be no lower than 20.degree. C. below the melting point of 
the sample. If the calcination takes place at a lower temperature, vastly 
inferior results occur. Preferably, a temperature within 10.degree. C of 
the melting point is used. 
The calcining should continue for up to about 20 hours, but successful 
material has been prepared at 12 hours and to about 10 hours should be 
sufficient. After regrinding and reheating, the samples should be 
maintained at the elevated temperature for several days, with 3-5 days 
being sufficient. Cooling to an intermediate temperature of about 
400.degree. C. has been employed, but is not believed to be required. 
The superconducting properties have been determined from both resistivity 
and magnetization measurements. Resistivities were measured using a 
standard four-point probe with an alternating 190 Hz measuring current. 
The low-field d.c. magnetization measurements were performed using a SQUID 
magnetometer. Magnetization measurements were taken (1) on samples that 
had been cooled in zero field (shielding) and (2) on samples cooled in 2 
Gauss field (Meissner effect). A 2 Gauss measuring field was used in both 
cases; in each case data were acquired on warming. Flux trapping (or 
remanence) was measured in zero field on warming after initially cooling 
the sample to 4K in a field of 2 Gauss. 
X-ray powder diffraction measurements on sintered compacts were made using 
a GE diffractometer. Measurements were made at room temperature using 
CuK.sub.a radiation. Since the sintered compacts were X-rayed without 
further powdering, sample texturing could influence relative intensities 
of the diffraction lines. 
Superconducting samples of the general formula Bi.sub.2 Sr.sub.x Ca.sub.y 
Cu.sub.3 O.sub.z where Sr content x is varied between 1.2 and 2.2 for 
three composition series with Ca contents specified by y=2, 2.2 and 2.4 
were prepared. Best results were obtained for a sample of composition 
x=1.6 and y=2.0. FIG. 1 shows the resistivity, FIG. 2 shows the 
magnetization (both Meissner effect and shielding measurements), and FIG. 
3 shows the X-ray powder diffraction pattern for this sample. The sample 
displays a resistive zero at T=110.+-.1K, in agreement with the onset of 
diamagnetism at 109K shown in FIG. 2. The Meissner effect shows a very 
sharp transition, being essentially complete by 100K. However, the 
shielding signal increases in magnitude to about 60K indicating some 
degree of inhomogeneity. At low temperatures the Meissner signal amounts 
to about 60% of the shielding signal increasing to 100% a few degrees 
below the superconductive transition temperature. The shielding signal, 
corrected for demagnetization effects, should in principle yield the 
volume fraction of the superconductive phase. However, due to the 
irregular shape and variation in packing density of the samples, only 
approximate values can be calculated for the superconductive volume 
fraction. For all samples tested, the superconductive volume fraction at 
4K lies between 50 and 100%, i.e., in all cases the samples are bulk 
superconductors. When cooled to 77K, the sample of FIGS. 1 and 2 (about 1 
gm) levitates approximately 3/8 inch above a small Nd-Fe-B magnet which 
confirms bulk superconductivity for this sample. Though not apparent in 
the Meissner data, the shielding data of FIG. 2 indicate that a small 
admix of the low T.sub.c phase (T.sub.c near 75K) remains in the sample. 
This phase can also be detected in the X-ray diffraction pattern of FIG. 
3. As we discuss subsequently, the two peaks with 2.theta.=27.6 and 31.1, 
appear to provide the best indicator of contamination by the low T.sub.c 
phase. 
FIG. 4 shows resistivity data for a series of samples prepared 
independently from the sample of FIGS. 1 to 3. The Ca concentration is 
fixed at y=2 with the Sr concentration being systematically decreased from 
x =2.2 (curve a) to x=1.4 (curve e). Curve f in FIG. 4 is for the same 
concentration as that of curve d (x =1.6) but after an additional heat 
treatment as explained above. Two distinct superconductive phases are 
clearly present--one with a T.sub.c near 110K, the other near 75K. As the 
Sr-concentration decreases the 110K phase becomes more and more dominant. 
Apparently, the fraction of high T.sub.c phase is very sensitive to the Sr 
content. At the 2223 composition (curve b), only a small drop in 
resistivity occurs near 110 K. These samples were processed together but 
subsequent to the sample of FIGS. 1-3. Because processing steps were 
somewhat different (the later samples were calcined at 820.degree. C. 
after heating at a rate of 70.degree. C./h) for the two heat treatments, 
final results for samples with the same nominal compositions are somewhat 
different. In particular, a larger residual of the low T.sub.c. phase 
persists for the later heat treatment. For sample f, the additional heat 
treatment (discussed above) sharpened the transition although it was 
shifted slightly to lower Tc. 
FIG. 5 displays the shielding curves for four of the samples of FIG. 4. The 
diamagnetic signal of all four curves has been normalized to -10 at 4K. 
The shapes of the curves are quite similar to the corresponding resistive 
transitions in FIG. 4. The ratio of the two phases as determined from the 
two steps in the magnetic transitions (FIG. 5) is remarkably close to the 
step ratio seen in the resistance. For example, the magnetic data for 
sample c indicate the presence of about 30% of the 110K phase. This large 
amount of the high T.sub.c phase should be more than enough to obtain a 
superconductive path. However, the resistance at 80K for this sample is 
still about 40% of its normal state value just above 110 K. This clearly 
indicates that the two phases are not randomly distributed but are highly 
correlated. The 110K phase is formed by intercalation of additional 
Ca-CuO.sub.2 layers into the low T.sub.c structure. The resulting 110K 
layers are thus well separated by the low T.sub.c phase inhibiting an 
electrical short. 
The preparation used for samples (a) through (e) did not yield as high a 
volume fraction of the 110K phase as the preparation used for the sample 
of FIGS. 1-3. This may be due to the difference in the calcine step. 
However, additional heat treatment can significantly increase the 110K 
volume fraction. For example, curve (f) in FIGS. 4 and 5 shows data for 
sample (d) after the additional treatment mentioned above. 
Additional magnetic measurements on a two phase sample (sample c of FIGS. 4 
and 5) are shown in FIG. 6. In addition to shielding data, FIG. 6 also 
shows Meissner and flux trapping data. (For comparison the flux trapping 
signal is inverted.) All three curves are quite similar in nature showing 
the same relative amounts of the two phases. As has been recognized in 
other high T.sub.c single phase materials, the sum of the magnitude of 
Meissner and flux trapping signals equals the shielding signal to within 
5% over the whole temperature range. Thus, all measurements presented here 
on two phase samples display features reflecting the special morphology of 
the two phase mixture. 
For the two Ca enriched y=2.2 and 2.4 series in the Bi.sub.2 Sr.sub.x 
Ca.sub.y Cu.sub.3 O.sub.z materials with 1.4.ltoreq.x.ltoreq.2.2, results 
very similar to those of FIGS. 4 and 5 were produced. The strongly 
mixed-phase behavior of curve (c) (FIGS. 4 and 5) is observed when Sr is 
dilute or deficient. The resistivity results for y=2.2 appear to be 
somewhat superior to those for y=2.0 and 2.4, with resistivity zeroes 
observed near 110K for the two Sr dilute (deficient) samples but no 
clearly discernable difference appears in magnetization measurements for 
samples with the same x (Sr content). 
FIG. 7 shows X-ray diffractometer traces for the sample series of FIGS. 4-5 
where y=2.0 and 1.4.ltoreq.x.ltoreq.2.2. These data show the evolution, 
with composition, from the low T.sub.c phase to the high T.sub.c phase. 
Index lines for the low T.sub.c phase (sample a, x =2.2) are shown in the 
upper trace; index lines for the high T.sub.c phase (sample e, x=1.4) are 
indicated on the bottom trace. (001) lines change dramatically in position 
for the two phases while lines dependent only on a.sub.o and b.sub.o 
change very little (since a.sub.o and b.sub.o are nearly equal; e.g., the 
200 line). Consistent with the results of FIGS. 4-5, lines (0010), (0012), 
and (0014) denoting the high T.sub.c phase become prominent for x.ltoreq. 
1.6. These lines appear to be broad, perhaps a consequence of the 
intercalated nature of this phase which might suggest that the 
crystallographic order of the high T.sub.c phase in the c-direction is 
imperfect. The systematic variation between the two phases illustrated in 
FIG. 7 shows that the lines (155) and (157) of the low T.sub.c phase 
provide sensitive indicators of the presence of the low T.sub.c phase in a 
mixture of the two phases. 
We have found that superconductivity may be obtained at 100K with 
compositions near 2223, but that phase purity is improved when the 
composition is Sr deficient and possibly somewhat Ca rich. This suggests 
that the high temperature phase tolerates a substantial Sr vacancy content 
and may imply that Ca substitutes on Sr sites. For a given oxygen 
stoichiometry, Sr site vacancies could cause the fraction of Cu.sup.3+ 
ions to be increased. 
By examining varied compositions of Bi-Sr-Ca-Cu-O in the vicinity of 
Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x (2223), we have succeeded in 
isolating the 110K superconducting phase. A resistance zero has been 
observed at 110K coincident with a diamagnetic onset. Because of the 
intercalated nature of the material, a very high volume fraction of the 
high T.sub.c phase is needed before interconnection and a resistance zero 
is observed. Results are very dependent on heat treatment conditions. For 
processing conditions cited above, the best superconducting properties and 
highest phase purity were obtained for samples with composition near x=1.5 
and y=2, i.e., Bi.sub.4 Sr.sub.3 Ca.sub.4 Cu.sub.6 O.sub.x (4346). 
While there has been disclosed what is considered to be the preferred 
embodiment of the present invention, it is understood that various changes 
in the details may be made without departing from the spirit, or 
sacrificing any of the advantages of the present invention.