Alkaline-doped superconductors of the formula EQU X M.sub.2 Ca.sub.2 Cu.sub.3 O.sub.8+.alpha. are provided where X is selected from the group consisting of TI, Pb, Mo, Hg and mixtures thereof, M is selected from the group consisting of Ba, Sr and mixtures thereof, and a ranges from zero to about 0.2, and being doped with a dopant selected from the group consisting of Na and Li up to a level of up to about 12% molar ratio, based upon the amount of the element X taken as 100%. The superconductors of the invention exhibit extremely high T.sub.c onset and T.sub.cO values and have high J.sub.c properties as well. The superconductors can be fabricated at relatively low annealing temperatures (750.degree.-820.degree. C.) making them suitable for use as thin films with a variety of conventional substrates.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is concerned with alkaline (Na and Li) doped 1223 
superconductors which have record high T.sub.c onset values and which can 
be fabricated at low annealing temperatures permitting their use as thin 
films on a wide variety of substrates. More particularly, the invention 
pertains to alkaline-doped superconductors of the formula 
EQU X M.sub.2 Ca.sub.2 Cu.sub.3 O.sub.8+.alpha. 
where X is selected from the group consisting of TI, Pb, Mo, Hg and 
mixtures thereof, M is selected from the group consisting of Ba, Sr and 
mixtures thereof, and .alpha. ranges from zero to about 0.2, and being 
doped with a dopant selected from the group consisting of Na and Li up to 
a level of up to about 12%, based upon the amount of the element X taken 
as 100%. 
2. Description of the Prior Art 
Element doping in superconducting material systems has been explored since 
the discovery of high temperature superconducting oxides. In recent years, 
Hg-based cuprates (HgBa.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.2n+2+.varies., 
n=1,2,3,4) have been discovered having very high transit on temperatures 
on the order of 135K. Accordingly, the chemical doping effect of many 
different elements in these cuprates has been studied so as to explore the 
possibility of obtaining superconductivity at even higher temperatures. 
Thus, Hg has been replaced by Re, Mo, Pb and TI, and Ba has been replaced 
by Sr. However, with a few exceptions most of these replacements have 
resulted in the reduction of T.sub.c values. 
In all of these studies, the concept is generally to replace an existing 
element by another which has a similar atomic radius and the same valence 
so that a similar crystalline structure plus a slightly perturbed 
electronic structure can be achieved. 
Chemical doping using elements with dramatically different atomic radii and 
different valences from that of Hg and Ba, presents a significant 
departure from the prior art. For example, because the alkaline elements 
possess significant size and valence differences as compared to the heavy 
elements such as Hg or Ba, it has generally been believed that the 
alkaline elements cannot enter the lattice sites of Hg-based cuprates. 
It is also known that the formation of 1223 phase cuprates requires 
processing (annealing) temperatures above 870.degree. C. In the context of 
Hg-1223 superconductors, such high temperature annealing is also carried 
out in the presence of Hg vapor. Fabrication of Hg-1223 thin films thus is 
extremely difficult and film-substrate interface chemical reactions at 
these severe processing conditions are significant. Up until the present 
time, only SrTiO.sub.3 and LaAIO.sub.3 substrates have been used 
successfully with Hg-based cuprate thin films. It is believed that if 
Hg-based cuprates are to achieve commercial use as thin films on 
substrates, the severe processing conditions and consequent 
superconductor/substrate interface problems must be overcome. 
SUMMARY OF THE INVENTION 
The present invention overcomes the problems outlined above and provides 
improved alkaline-doped superconductors having very high T.sub.c onset and 
T.sub.cO critical temperatures which can be produced using a relatively 
low temperature annealing process making the superconductors suitable for 
use as thin films applied to substrates. More particularly, the invention 
relates to alkaline-doped superconductors of the formula 
EQU X M.sub.2 Ca.sub.2 Cu.sub.3 O.sub.8+.alpha. 
where X is selected from the group consisting of TI, Pb, Mo, Hg and 
mixtures thereof, M is selected from the group consisting of Ba, Sr and 
mixtures thereof, and .alpha. ranges from zero to about 0.2, and being 
doped with a dopant selected from the group consisting of Na and Li up to 
a level of up to about 12% molar ratio, based upon the amount of the above 
formula taken as 100%. Superconductors in accordance with the invention 
have T.sub.c onset values in excess of 140K, and further exhibit 
relatively high irreversibility lines, which is consistent with very high 
J.sub.c values. 
In preferred forms, the superconductors of the invention are of the formula 
Hg Br.sub.2 Ca.sub.2 Cu.sub.3 O.sub.8+.alpha., and the dopant level is 
from about 0.01-12%, most preferably about 10%. The most preferred dopant 
is sodium. 
The alkaline-doped superconductors of the invention can be formed as bulk 
superconductors or as thin films onto conventional substrates. In the 
latter case, the low annealing temperatures of the invention permit 
fabrication of thin films on a wide variety of substrates. The 
superconductors hereof are also of high purity. Generally speaking, they 
are at least about 90% pure 1223 phase, and more preferably at least about 
95% pure 1223 phase. 
The superconductors of the invention are prepared by first mixing together 
stoichiometric amounts (based upon the cations) of compounds of M, Ba, Cu 
and the dopant, generally the oxides and salts of these elements. The 
mixture is then sintered at a temperature of from about 
700.degree.-1000.degree. C. (more preferably from about 
800.degree.-950.degree. C.) in an oxygen-rich (i.e., at least about 80% 
oxygen) environment for a period of from about 10-30 hours (more 
preferably from about 20-30 hours) to form a sintered non-superconducting 
precursor. In the case of the most preferred superconductor, wherein the 
dopant is sodium and M is Ba, the precursor would have a nominal 
composition of Na.sub.y Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x where y ranges 
from about 0.01-0.12 and x is an unknown oxygen factor. 
In the next step, a stoichiometric compound of X is added to the sintered 
precursor with mixing to assure homogeneity. In the case of the most 
preferred superconductor, where X is Hg, the preferred compound is HgO. 
The mixture is then initially annealed at a temperature of from about 
750.degree.-820.degree. C. (more preferably from about 
760.degree.-810.degree. C.) for period of from about 1-15 hours to form a 
superconductor. 
In order to achieve the highest T.sub.c onset and T.sub.c values, the 
initially annealed superconductor is subjected to a secondary annealing 
step in a flowing oxygen atmosphere. Such secondary annealing is generally 
conducted at a temperature of from about 200.degree.-400.degree. C. (more 
preferably from about 275.degree.-350.degree. C.) for a period from about 
1-15 hours depending upon sample size. 
The impact of alkaline-doping in the preferred Hg-1223 superconductors is 
dramatic. Such superconductors have significant applicability in 
micro-electronic devices because of their record high T.sub.c values and 
relatively high irreversibility lines. Heretofore, fabrication of Hg-1223 
films have been extremely difficult owing to the high volatility of 
Hg-based compounds. Moreover, undoped Hg-1223 superconductors require 
annealing temperatures above 860.degree. C. under high Hg vapor pressures. 
These conditions cause severe superconducting film/substrate interface 
chemical diffusion problems even on the best substrate (SrTiO.sub.3). Many 
desirable substrates such as sapphire, garnet and the like are excluded 
when using undoped Hg-1223 superconductors because of these problems. 
However, the present discovery that annealing temperatures can be reduced 
without the need for high Hg vapor pressures will permit production of the 
desirable high T.sub.c film products using the heretofore unavailable 
substrates.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The following Example sets forth presently preferred alkaline-doped 
superconductors and methods of fabrication thereof. It is to be understood 
that these examples are provided by way of illustration only, and nothing 
therein should be taken as a limitation upon the overall scope of the 
invention. 
EXAMPLE 
In this example, a series of sodium-doped Hg-1223 bulk superconductors were 
fabricated. A two-step process was employed for making of the samples. 
First a precursor of nominal composition Na.sub.y Ba.sub.2 Ca.sub.2 
Cu.sub.3 O.sub.x was made from a mixture of NaCl, Ba(NO.sub.3).sub.2, CaO 
and CuO powders with molar ratio of Na:Ba:Ca:Cu equals y:2:2:3. y was 
chosen to be 0.05, 0.1, 0.15 and 0.2, based upon the ultimate amount of Hg 
to be used taken as 1. The powders were thoroughly ground, mixed and then 
fired at 900.degree. C. for 1000 min. in a tube furnace in the presence of 
flowing oxygen. The tube furnace was heated from atmospheric to 
900.degree. C. at a rate of about 1.degree. C./min. The pressure within 
tube furnace during the 1000 min. sintering step was maintained at 
approximately 1 atmosphere. At the end of the sintering period, the 
heating of the tube furnace was stopped and was cooled at a rate of about 
5.degree. C./min., with constant oxygen flow through the tube furnace, 
until ambient temperature was reached. 
The sintered precursor was then reground and mixed with HgO powder in a 
cation ratio of (Hg.sub.1-y Na.sub.y):Ba:Ca:Cu equals 1:2:2:3 and the 
mixture was then manually pressed into a 12 mm diameter pellet using a 
laboratory press. In order to reduce the possible detrimental effect of 
moisture and CO.sub.2 in the air, the mixing and grinding steps were 
carried out in a plastic bag filled with pure Ar gas. 
The pressed pellet was next encapsulated into a precleaned and evacuated 
quartz tube (I.D. =0.5 in with a length of 1.5-2 in.) which was enclosed 
in a steel cylinder for protection. The ends of the steel tube were closed 
to form a sample assembly. The sample assembly was then placed in a tube 
furnace and was subjected to a rapid heating rate of 50.degree. C./min. in 
order to bring the sample temperature from room temperature to a desired 
initial annealing temperature ranging from ,750.degree.-870.degree. C. The 
rapid heating rate was adopted so as to reduce the formation of 
HgCaO.sub.2 in the sample. Once the desired annealing temperature was 
reached, the temperature was maintained for 10 hrs. Thereafter, the 
furnace was set for a cool-down rate of 2.5.degree. C./min. for cooling of 
the sample to ambient temperature. 
At this point, certain of the samples were tested for T.sub.c values 
(resistivity versus temperature curves), magnetization as a function of 
temperature and applied magnetic field, and by X-ray crystallography. This 
was to provide a comparison between the samples at this stage in the 
preparation and the final samples after oxygen annealing. 
The Hg-annealed samples Were then placed in the quartz tube for secondary 
oxygen annealing in the tube furnace. The oxygen annealing step involved 
heating the sample in the quartz tube to approximately 350.degree. C. at a 
rate of 10.degree. C./min., followed by maintenance of the oxygen 
annealing temperature for 10 hrs. During this procedure oxygen was passed 
through the quartz tube and the pressure within the tube was maintained at 
about 1 atmosphere. Thereafter, the furnace was cooled at a rate of about 
10.degree. C./min., with continued oxygen flow until ambient temperature 
was reached. The completed bulk superconductor was then removed from the 
quartz tube. 
The finished samples were then tested for T.sub.c and magnetization values, 
and also subjected to additional X-ray crystallography examinations. FIGS. 
1 and 2 illustrate the temperature dependence of resistivity in the 
Hg-1223 samples with y=0.05 and 0.1, respectively, both before and after 
oxygen annealing (Hg annealing temperature of 810.degree. C.). These 
graphs demonstrate that both samples experienced a superconducting 
transition at above 100K before oxygen annealing (about 109K for y=0.05 
and near 120K for y =0.1). The zero resistance is achieved at 105K for 
y=0.05 and at 115K for y=0.1. 
After oxygen annealing, the y=0.05 samples exhibited two transitions, one 
at 137K and the other at 125K, and a zero resistance temperature of 123K. 
The y=0.1 samples exhibited only a single transition at around 143K and a 
zero resistance temperature at about 131K. Additional y=0.1 samples gave 
an onset T.sub.c in the range of 141-147K and a zero resistance T.sub.c of 
131.varies.136K. 
FIGS. 1-2 also show that the resistivity of both samples have metallic 
linear temperature dependencies before oxygen annealing. .rho. 
(300K)/.rho. (150K) is about 3.29 for y=0.05 and about 2.09 for y=0.1. The 
former is similar to the reported value for undoped Hg-1223. After oxygen 
annealing however, this ratio becomes 2.14 for y=0.05 and 1.08 for y=0.1. 
Moreover, the resistivity in both cases increases after oxygen annealing. 
These results differ from literature reported observations of undoped 
Hg-1223 phase superconductors where .rho. versus T curves become steeper 
and resistivity values decrease after oxygen annealing. 
FIGS. 3-4 illustrate the resistance versus temperature curves for 
sodium-doped Hg-1223 samples (y=0.1) sintered at different Hg annealing 
temperatures, namely 830.degree. C. (FIG. 3) and 870.degree. C. (FIG. 4), 
both before and after oxygen annealing at 350.degree. C. This data and 
that of FIG. 2 established that the normalstate resistivity increases 
after oxygen annealing. The slopes of the R versus T curves increase with 
annealing temperature T.sub.a. For example, for T.sub.a =830.degree. C., 
.rho. (300K)/p (150K) is about 2.91 and 1.88 before and after oxygen 
annealing, whereas for T.sub.a =870.degree. C., this ratio is about 3.3 
and 2.05 before and after oxygen annealing. At T.sub.a =810.degree. C., 
only one phase is observed to be superconducting above 130K for y=0.1 as 
shown in FIG. 2. When T.sub.a is increased, the second phase is formed 
which has a lower T.sub.c (around 125K) as indicated by the knee in the R 
versus T curves (after oxygen annealing) in FIGS. 3 and 4. This results is 
unexpected, however, because oxygen annealing at low temperatures would 
not normally be expected to give phase decomposition. 
The chemical compositions of the samples were analyzed using powder X-ray 
diffraction on a Philip PW 1830 diffractometer with a step size of 
0.050.degree. and a scan speed of 0.05.degree./sec. As shown in FIGS. 5-6, 
the dominant phase for both y=0.05 (FIG. 5) and y=0.1 (FIG. 6) have a 
structure similar to that of the Hg-1223 phase. The lattice constants are 
calculated from the XRD patterns as a=3.856 .ANG. and c=15.805 .ANG. for 
y=0.05 and a=3.847 .ANG. and c=15.816 .ANG. for y=0.1. These values are 
comparable with the published lattice constants of a=3.845 .ANG. and 
c=15.797 .ANG. for pure Hg-1223 phase. In the case of y=0.05, two known 
impurity phases were observed: Hg-1212 and CaHgO.sub.2 as labeled by open 
circles. The starred peaks represent unknown impurities. The content of 
the Hg-1212 impurity phase in the y=0.05 samples is estimated to be less 
than 30% using the (013) peaks from both Hg-1212 and Hg-1223 phases. In 
the case of the y=0.1 samples, no Hg-1212 peaks are visible while 
CaHgO.sub.2 is still the major impurity (which is possibly caused by the 
detrimental effect of air during sample preparation). 
The magnetization (M) of the y=0.1 sample of FIG. 2 was measured in a 3 Oe 
applied magnetic field in both field-cool and zero-field-cool modes, and 
these results are shown in FIG. 7. A transition temperature at around 135K 
was observed which is slightly higher than that seen in the transport 
measurement. Assuming a complete Meissner effect it low temperature, the 
superconducting phase volume portion was estimated to be less than 70% 
from the zero-field-cool curve and higher than 42% from the field-cool 
curve. 
The y=0.15 and 0.2 samples exhibited no superconducting properties, and in 
fact were insulators. This indicated a critical upper limit in the amount 
of sodium doping which could be accommodated and still obtain high T.sub.c 
superconductors. 
The irreversibility line (H.sub.irr) of a high T.sub.c superconductor 
separates the irreversible region from the reversible region in the H/T 
phase space. This line sets the upper H/T limit for most applications of 
high T.sub.c superconductors while the exponent n is found depending an 
the anisotropy of the material. For a less anisotropic system such as YBCO 
n.about.1.5 and for a highly anisotropic system such as Bi-Tl-based 
systems, n.about.5.5. The higher the exponent n, the lower the 
irreversibility line, which limits practical applications. For Hg-1212 and 
Hg-122 systems, the exponent n is found to be .about.2.5 which suggests a 
superior property for various devices, such as in large current 
applications. 
With Na doping of y=0.1, the exponent n in the irreversibility line can be 
further increased to n .about.1.66 as shown in FIG. 8. This is close to 
the highest reported irreversibility line (on YBCO superconductors) to 
date. This high irreversibility line plus the record high T.sub.c in 
Na-doped Hg-1223 phase establishes the unique nature of the present 
superconductors. 
Low level Na doping in Hg-1123 phase renders the superconductivity of the 
material more sensitive to the oxygen content. Before oxygen annealing, 
the onset of superconducting transition (T.sub.c onset) is near 125K and 
zero resistance temperature (T.sub.cO) is near 107.5K (shown at zero 
annealing temperature) for the y=0.1 Na-doped superconductor (FIG. 9). 
200.degree. C. oxygen annealing for 10 hours raised the T.sub.c onset to 
146K and T.sub.c to 121 K. As shown in FIG. 9, a variety of oxygen 
annealing temperatures can influence the T.sub.c onset and T.sub.cO 
values, with the highest such values being found at around 
275.degree.-350.degree. C. oxygen annealing temperature. 
The mechanism of the sodium doping effect in Hg-1223 phase is not yet 
clear. Two possibilities might be considered: formation of new 
superconducting phase, and sodium-assisted growth of Hg-1223 phase. In the 
former case, sodium should enter the crystal lattice, while in the latter 
it may only stay as an interstitial ingredient. Since the atomic, size of 
the Na is much smaller than that of others in Hg-1223, lattice contraction 
might be induced if sodium entered the lattice. This conflicts with the 
XRD data which shows almost identical lattice constants, for both cases of 
y=0.05 and y=0.1, to that of the undoped Hg1223 phase. Furthermore, no Na 
can be identified in the study of high resolution transmission electron 
microscopy (HRTEM) and energy dispersive X-ray spectroscopy (EDS). The 
HRTEM/EDS analysis identifies structures and compositions similar to 
Hg-1223, Hg-1212, CaHgO.sub.2 and other Ba-Cu compounds which consists 
with the XRD result. Even though the crystalline structure of the 
sodium-doped Hg-1223 looks the same as that of the undoped Hg-1223, 
differences in the electrical transport properties have been noticed 
between them. First of all, the slope of the normal-state R versus T curve 
decreases with the content of sodium and an insulating behavior is 
resulted as y is more than 0.1. Second, this slope decreases after low 
temperature oxygen annealing while T.sub.c and resistivity increase. If, 
as for the undoped Hg-1223 phase, more oxygen is brought into the Hg-O 
layer during this oxygen annealing, this may indicate that sodium stays as 
an interstitial near the Hg-O layer. On one hand, the presence of these 
interstitials may affect the local distribution of the oxygen in the Hg-O 
layer which could account for the different slopes in the R versus T 
curves of the sodium-doped Hg-1223. On the other hand, as the amount of 
the oxygen in the Hg-O varies during the oxygen annealing, the 
distribution of the sodium may also be modified since interstitials 
presumably have fairly low binding energy which could be comparable to the 
oxygen annealing temperature of 350.degree. C. This could then explain 
while T.sub.c is increased with oxygen being annealed in the resistivity 
also increases caused by competition of increase of charge carrier density 
due to oxygen optimization and redistribution of sodium as scattering 
center for the charge carrier or formulation of Na.sub.2 O. Finally, it 
should also be noted that these sodium interstitials promotes formation of 
Hg-1223 phase at much lower temperatures than that required for the 
undoped Hg-1223 phase.