Flux pinning by precipitates in the Bi-Sr-Ca-Cu-O system

A fundamental pinning mechanism has been identified in the Bi-Sr-Ca-Cu-O system. The pinning strength has been greatly increased by the introduction of calcium- and copper-rich precipitates into the sample matrix. The calcium and copper are supersaturated in the system by complete melting, and the fine calcium and copper particles precipitated during subsequent crystallization anneal to obtain the superconducting phases. The intragrain critical current density has been increased from the order of 10.sup.5 A/cm.sup.2 to 10.sup.7 A/cm.sup.2 at 5 T.

BACKGROUND OF THE INVENTION 
This invention relates generally to enhanced current densities by the 
formation of precipitates in ceramic-oxide superconductors to improve flux 
pinning and more particularly to the improvement in current densities by 
the enrichment of certain components in the compositions to promote the 
formation of precipitates in the Bi-Sr-Ca-Cu-O system. 
High-T.sub.c superconductors in the Bi-Sr-Ca-Cu-O system have been reported 
to have low transport and magnetization critical current densities in 
polycrystalline form. The low transport property may be associated with 
the weak link effect resulting from the lattice misalignment and secondary 
phases at the grain boundaries, as in the case of YBa.sub.2 Ca.sub.3 
O.sub.7-x. The magnetization critical current density has been found to be 
strongly dependent on field and temperature and is much lower than that of 
YBa.sub.2 Cu.sub.3 O.sub.7-x. This difference has been attributed to the 
lack of pinning centers, such as twin planes, in the Bi-Sr-Ca-Cu-O 
superconductors prepared by conventional ceramic techniques. Previous 
studies have also indicated that thermally assisted flux creep is much 
more pronounced in the Bi-Sr-Ca-Cu-O system. Significant resistance is 
present even at temperatures well below T.sub.c in a magnetic field due to 
the flux creep effects. The pinning centers in YBa.sub.2 Cu.sub.3 
O.sub.7-x have been clearly identified as the twin boundaries. Therefore, 
it is of utmost importance to analyze the pinning mechanisms in this 
system so that additional pinning centers can then be introduced for the 
possible enhancement of the critical current density. 
Critical current densities in ceramic superconducting compositions relate 
to both intergrain current density involving transfer between grains and 
to intracurrent density involving transfer within grains. Flux pinning is 
particularly important for increased intergrain current densities. Flux 
pinning sites may vary in different compositions. 
OBJECTS AND SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to increase the 
number of magnetic flux-pinning centers in ceramic-oxide electrically 
conductive materials. 
Another object of the present invention is to promote the formation of 
precipitates in a Bi-Sr-Ca-Cu-O superconductor to increase the critical 
current density by increasing the number of flux pinning sites. 
A further object of the present invention is to form ceramic-oxide glass 
superconductors containing enriched concentrations of Ca and Cu with 
crystallization and precipitation induced by annealing for increasing the 
number of flux-pinning centers in the superconductor. 
This invention relates to the flux-pinning behavior identified in 
crystallized samples prepared by splat quenching in the Bi-Sr-Ca-Cu-O 
system. The magnetization critical current density has been greatly 
enhanced by introducing calcium-and copper-rich precipitates into the 
system. The results of theoretical fitting of the data show that the flux 
lines are pinned through different mechanisms in the samples containing 
different superconducting phases. Volume pinning has been identified in 
the samples with the majority of the 85K phase, an observation that is 
consistent with electron microscopy results. Two distinct pinning force 
peaks have been found in the multiphase samples, indicating two different 
pinning mechanisms. The increase of critical current density has been 
found to be directly related to the amount of precipitate introduced into 
the samples.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Several different types of ceramic oxides were first heated to temperatures 
above their respective melting points to form a liquid. Extremely dense 
glass samples with the nominal compositions of Bi.sub.2 Sr.sub.2 
CaCu.sub.2 O.sub.x (2212), Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x 
(2223), Bi.sub.2 Sr.sub.2 Ca.sub.3 Cu.sub.4 O.sub.x (2234), and Bi.sub.2 
Sr.sub.2 Ca.sub.4 Cu.sub.5 O.sub.x (2245), where x.congruent.10, were then 
made by a splat quenching method as described in D. Shi, M. Blank, M. 
Patel, D. G. Hinks, A. W. Mitchell, K. Vandervoort, and H. Claus, Physica 
C 156, 822 (1988); and D. Shi, M. Tang, K. Vandervoort, and H. Claus, 
accepted Phys. Rev. B, 1989. The as-quenched glass samples were 
subsequently annealed at 870.degree. C. in air for various times After 
annealing, the samples were slowly cooled to room temperature. The 
superconducting properties of the annealed products were analyzed using 
x-ray diffraction (XRD), electrical resistivity and magnetization 
shielding measurements, transmission electron microscopy (TEM), scanning 
electron microscopy (SEM), and an energy dispersive spectrum (EDS) 
technique. Improved superconducting properties were determined with high 
field magnetization measurements and flux pinning analysis of some of the 
crystallized samples. The magnetization data were taken on a commercial 
SQUID magnetometer at 10K up to 5 T. 
The 2212 sample annealed at 870.degree. C. for 1 day contains mostly the 
85K phase. The 2223 sample annealed at 870.degree. C. for 1 day exhibited 
a single transition near 85K in both resistivity and magnetization 
shielding measurements. X-ray diffraction data also show that the sample 
contains mostly the 85K phase (it has the composition of Bi.sub.2 
Csr.sub.2 CaCu.sub.2 Ox) and a very small amount of Ca.sub.2 CuO.sub.3. 
The 110K superconducting phase (having the composition of Bi.sub.2 Sr.sub.2 
Ca.sub.2 Cu.sub.3 O.sub.x) starts to form as the calcium and copper 
concentration in the starting composition is increased to 2234 and 2245. A 
single resistive and magnetization transition was observed near 110K in 
the 2234 sample annealed at 870.degree. C. for 10 days and in the 2245 
sample annealed for 3 days. A large volume percent of the 110K phase and 
some 85K phase have been identified by x-ray diffraction in these samples. 
The samples of all compositions investigated which were annealed at 
870.degree. C. for 1 day were found to be mostly the 85K phase. 
Based on TEM and XRD data, it was found that in all the crystallized 
samples, there exists a certain amount of calcium-and copper-rich impurity 
phase Ca.sub.2 CuO.sub.3, which precipitated from the amorphous matrix 
during the crystallization process. The amount of precipitate was found to 
increase as the calcium and copper levels increase in the starting 
materials. Our TEM experimental results indicate that only a very small 
quantity of the precipitates was observed in the 2223 sample which was 
annealed at 870.degree. C. for 1 day and had mostly the 85K phase. A large 
amount of the calcium- and copper-rich phase formed in the 2234 and 2245 
samples. As shown in FIG. 1, which is a TEM photograph of Bi.sub.2 
-Sr.sub.2 -Ca.sub.4 Cu.sub.5 O.sub.x with the diffraction pattern taken 
from the (001) zone axis of the matrix shown in the inset, for a 2245 
sample annealed at 870.degree. C. for 3 days, the calcium- and copper-rich 
precipitates are finely dispersed rather evenly in the sample matrix, and 
their dimensions vary widely from less than 0.01 um to almost 0.10 um. The 
inset in FIG. 1 is an electron diffraction pattern taken from the (001) 
zone axis of the matrix area which is the 110K superconducting phase. 
Magnetization curves shown in FIGS. 2a-2f were taken at 10K up to 5 T for 
five crystallized samples with nominal compositions of 2223, 2234 and 
2245. As can be seen in these figures, the samples exhibit rather 
different hysteresis. The overall hysteresis width increases rapidly from 
FIG. 2a to FIG. 2f, indicating increased pinning strength. As can also be 
seen, the field required to reduce the hysteresis to less than 5% of the 
maximum value (Ho) increases from the 2212 composition of FIG. 2a to the 
2245 composition of FIG. 2f. For the 2212 and 2223 samples the H.sub.o 
value is less than 5 T, while respectable hysteresis still remains at the 
same field for the 2234 and the 2245 samples. 
The intra-grain critical current density, J.sub.c, has been estimated using 
the Bean model described in C. P. Bean, Rev. Mod. Phys. 36, 31 (1964) and 
A. Umezawa, G. W. Crabtree, J. Z. Liu, H. W. Weber, W. K. Kwok, L. H. 
Nunez, T. J. Moran, and C. H. Sowers, Phys. Rev. B 36 (13), 7151 (1988), 
based on magnetization measurements. The J.sub.c values are plotted 
against the applied field H, shown in FIG. 3. As can be seen in FIG. 3, 
the J.sub.c values of the 2212 and the 2223 samples are on the order of 
10.sup.6 A/cm.sup.2 in the low field range (&lt;1T) and the decrease to the 
order of 10.sup.5 A/cm.sup.2 near 5 T. The critical current density 
J.sub.c values of the 2234 and 2245 samples increase substantially to 
approximately 0.5.times.108 A/cm.sup.2 at near zero field and drop only 
slightly as the field increases to 5 T. It is to be noted that the J.sub.c 
(H) values increase with increasing calcium and copper content in the 
nominal composition, and the highest J.sub.c (H) has been obtained in the 
2245 sample annealed for 3 days at 870.degree. C. The amount of 
precipitates greatly increases as the calcium and copper content is 
increased in the nominal composition. Therefore, this increase in critical 
current density is a consequence of the enhanced pinning effect due to the 
increased precipitation. 
FIG. 4 shows the flux pinning force density, F.sub.p versus the reduced 
field h (=H/H.sub.o), for all the samples whose J.sub.c (H) values are 
shown in FIG. 3. H.sub.o corresponds to the value of the applied field at 
which the magnetic irreversibility disappears. It is also referred to as 
the "quasi de Almeida-Thouless line". For some samples, H.sub.o values 
could only be obtained by extrapolation since the available field cannot 
exceed 5 T. Thermally activated flux creep becomes much more pronounced in 
the Bi-Sr-Ca-Cu-O system at temperatures approaching the transition 
temperature; however, our experimental results are obtained at 10K, where 
the flux creep effects are not as significant. 
As shown in FIG. 4, the pinning force density, Fp, is quite different in 
the six samples. The 2212 and 2223 samples annealed for one day have 
mostly the 85 k phase and the lowest values of the flux-pinning force 
density, because they have the least amount of precipitates. The 2234 and 
2245 samples annealed for one day contain mostly the 85K phase and have 
progressively larger values of the flux-pinning force density as the 
amount of calcium- and copper-rich precipitation increases. All four of 
these samples exhibit a maximum in the flux-pinning force density at 
h=0.33, which indicates volume normal pinning via the magnetic 
interaction. The remaining two samples are different in that they contain 
significant amounts of the 110K superconducting phase. Specifically, the 
2234 sample annealed 10 days exhibits two peaks, one at h=0.33 which 
corresponds to volume normal pinning via the magnetic interaction, and the 
other at h=0.60 which corresponds to surface .DELTA.K pinning via the core 
interaction. The first peak may relate to the pinning by precipitates as 
observed in the four samples annealed for one day. The second peak may 
indicate that the boundaries between the two superconducting phases are 
acting as pinning centers. The 2245 sample annealed for 3 days exhibits a 
peak at h=0.33, but the available field extends only to approximately 
h=0.40. Therefore, the second peak in the flux-pinning force density curve 
for this sample cannot be observed, although by analogy with the 2234 
sample annealed ten days and containing two superconducting phases, it is 
expected that a second peak exists. This sample exhibits the largest 
absolute values of flux pinning, a fact that is consistent with our TEM 
experiments showing that this sample contains the largest amount of 
calcium- and copper-rich precipitates. 
Theoretical curves of F.sub.p versus h have the form of h.sup.1/2 (1-h) for 
all the samples containing mostly the 85K phase, as shown in FIGS. 5a-5d. 
As pointed out by Dew-Hughes in Phil. Mag. 30(8), 293 (1974), the pinning 
function h.sup.1/2 (1-h) corresponds to normal volume pinning via the 
magnetic interaction. This correspondence is consistent with the TEM 
results that the calcium- and copper-rich precipitates are finely 
dispersed in the sample matrix and their sizes are, in all dimensions, 
greater than the inter-flux line spacing d [=1.07 (.PHI..sub.o /B).sup.1/2 
] (the d value is 218 .ANG. at 5 T). Although the precipitates are widely 
distributed in size, only those with the appropriate dimensions greater 
than the penetration depth, .lambda., can be responsible for the normal 
volume pinning via the magnetic interaction. 
In conclusion, a fundamental pinning mechanism in the crystallized 
Bi-Sr-Ca-Cu-O system has been identified, namely, normal volume pinning 
via the magnetic interaction by calcium-and copper-rich precipitates. This 
identification is based on substantial electron microscopy evidence and 
theoretical data fitting. By introducing large amounts of precipitates as 
pinning centers in the system, the magnetization critical current density 
J.sub.c (H) is increased by a factor of 30. In addition, for the 
multiphase sample (2234 annealed 10 days) the existence of two peaks in 
the flux pinning force density has been detected. These peaks indicate 
that there are two types of pinning centers, namely, the calcium- and 
copper-rich precipitates and the boundaries between the two 
superconducting phases. The rapid solidification technique is a unique 
method for the introduction of precipitates as pinning centers into this 
system by which the intragrain critical current density can be greatly 
enhanced in accordance with the present invention. 
There has thus been shown a process for producing and resulting 
ceramic-oxide superconductors with improved flux pinning giving rise to 
increased current densities by the enrichment of certain components in the 
ceramic-oxide superconducting compositions to promote the formation of 
precipitates in the Bi-Sr-Ca-Cu-O system. The process is also 
characterized by the formation of glass containing enriched Ca and Cu 
concentrations with crystallization and precipitation induced by 
annealing. The formation of the precipitates is promoted by enrichment of 
the composition with certain components and particularly calcium and 
copper. In the process, enriched compositions identified by formulas 
Bi.sub.2 -Sr.sub.2 -Ca.sub.3 Cu.sub.4 O.sub.x (2234) and Bi.sub.2 
-Sr.sub.2 -Ca.sub.4 Cu.sub.5 O.sub.x (2245) are converted to molten glass 
and then quickly quenched. Annealing is subsequently carried out at 
approximately 870.degree. C. to form crystals within the grains with a 
large number of small precipitates distributed throughout the grains. The 
precipitates vary in size from 0.01 to 0.1 microns. 
While particular embodiments of the present invention have been shown and 
described, it will be obvious to those skilled in the art that changes and 
modifications may be made without departing from the invention in its 
broader aspects. Therefore, the aim in the appended claims is to cover all 
such changes and modifications as fall within the true spirit and scope of 
the invention. The matter set forth in the foregoing description and 
accompanying drawings is offered by way of illustration only and not as a 
limitation. The actual scope of the invention is intended to be defined in 
the following claims when viewed in their proper perspective based on the 
prior art.