(Hg or Pb)-Pr-Tl-Sr-Cu-O based superconductors

A high temperature superconducting system comprising M--R--Tl--Sr--Cu--O wherein: M is at least one compound selected from the group consisting of Hg, Pb, K, and Al; and R represents rare earth metals. In one embodiment, a composition forms a 93K superconducting phase having the composition: M--R--Tl--Sr--Cu--O wherein: M is selected from the group consisting of Hg and Al; and R is a rare earth metal. In another embodiment, the composition comprises M--R--Tl--Sr--Cu--O wherein: M is selected from the group of Pb and/or K; and R is a rare earth metal.

BACKGROUND OF THE INVENTION 
The present invention relates to high temperature superconducting systems 
and the processes for making same. 
A variety of high temperature superconducting systems have been developed. 
Such superconducting systems include: Y--Ba--Cu--O; Bi--Sr--Ca--Cu--O; 
Tl--Ba--Cu--O; and Tl--Ba--Ca--Cu--O. A number of such systems are set 
forth in pending patent applications of which one of the inventors of the 
present invention is a coinventor. 
For example, U.S. Pat. No. 4,962,083 discloses Tl--Ba--Ca--Cu--O 
superconductors and processes for making same. Additionally, that 
application discloses TlSrBaCuO superconductors and processes for making 
same. U.S. Pat. No. 4,994,432 discloses TlBaCuO superconductors and 
processes for making same. U.S. Pat. No. 5,036,044 discloses RTlSrCaCuO 
superconductors and process for making same, wherein R is a rare earth 
metal. U.S. Pat. No. 5,164,362 discloses TlSrCaCuO superconductors and 
processes for making same. 
Despite the existence of known superconducting systems, and the fact that 
the above-identified patent applications provide superconductors and 
methods for making same, new superconducting systems are desirable for 
several reasons. A new system could provide a basis for the discovery of 
higher-temperature superconductors. In turn, higher-temperature 
superconductors could provide low cost processing and manufacturing. 
SUMMARY OF THE INVENTION 
The present invention provides a composition having superconductive 
properties comprising M--R--Tl--Sr--Cu--O, wherein R represents rare earth 
metals and M is at least one compound selected from the group consisting 
of Hg, Pb, K, and Al. 
In an embodiment, the present invention provides a composition having 
superconductive properties at a temperature of approximately 93K 
comprising M--R--Tl--Sr--Cu--O wherein: 
R is selected from the rare earth metals; and 
M is selected from the group consisting of Hg and Al. 
In another embodiment, the present invention provides a composition having 
a Tc of at least approximately 93K to approximately 100K. The composition 
comprising M--R--Tl--Sr--Cu--O wherein: 
R is Pr; and 
M is at least one element selected from the group consisting of Pb and K. 
In an embodiment, the invention provides a material having superconductive 
properties having the nominal composition HgPr.sub.2 Tl.sub.2 Sr.sub.2 
Cu.sub.3 O.sub.12. 
In an embodiment, the invention provides a material having superconductive 
properties having the nominal composition HgPr.sub.2 Tl.sub.2 Sr.sub.2 
Cu.sub.3 O.sub.13. 
In an embodiment, the invention provides a material having superconductive 
properties having the nominal composition Pb.sub.0.5 Pr.sub.2 Tl.sub.2 
Sr.sub.3 Cu.sub.3 O.sub.13. 
In an embodiment, the invention provides a material having superconductive 
properties having the nominal composition KPb.sub.0.5 Pr.sub.2 Tl.sub.2 
Sr.sub.3 Cu.sub.3 O.sub.13. 
In a further embodiment, the present invention provides a method of 
preparing the high-temperature superconductors. The method includes the 
steps of: mixing together the components of the composition; and heating 
the mixture. 
In an embodiment, the mixture is heated at a temperature of approximately 
1000.degree. C. for about 5 minutes in flowing oxygen. 
In an embodiment, the mixture is pressed into a pellet prior to being 
heated. 
Additional features and advantages of the present invention are further 
described, and will be apparent from the detailed description of the 
presently preferred embodiments and from the drawings.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
The present invention provides new high-temperature superconductors and the 
processes for making them. To this end, the present invention provides a 
composition having super-conductive properties comprising the elements: 
EQU M--R--Tl--Sr--Cu--O 
wherein: 
M is at least one compound selected from the group consisting of Hg, Pb, K, 
and Al; and 
R is selected from the group consisting of rare earth metals. 
In an embodiment, R is Pr. In a further embodiment, R is Pr and M is Pb 
and/or K. 
The inventors of the present invention have found that particular elemental 
dopings with Hg, Al, Pb, and/or K into a Pr--Tl--Sr--Cu--O system results 
in a compound having a higher Tc. Specifically, Hg- or Al-doping produced 
a 93K superconducting phase, while Pb- or K-doping increased the 
temperature from 93K to 100K. 
The present invention also provides methods for preparing high-temperature 
superconductors. Pursuant to the present invention, samples are prepared 
by mixing the components and heating the mixture in flowing oxygen. For 
example, compounds selected from the group consisting of HgO, Al.sub.2 
O.sub.3, PbO.sub.2, KO.sub.2, RE.sub.2 O.sub.3 (RE=rare earths), Tl.sub.2 
O.sub.3, SrO or Sr(NO.sub.3).sub.2, and CuO can be mixed to achieve the 
desired composition. 
In an embodiment of the procedure, the components are completely mixed, 
ground, and pressed into a pellet having a diameter of 7 mm and a 
thickness of 1-2 mm. The pellet is then heated in a tube furnace at a 
temperature of approximately 1000.degree. C. for about 5 minutes in 
flowing oxygen. The pellet can then be subjected to furnace-cooling or 
quenching. 
By way of example, and not limitation, examples of the superconducting 
composition and processes for making them are set forth below. For 
analyzing the resultant compositions created in the examples, resistance 
(ac, 27 Hz) was measured by a standard four-probe technique with silver 
paste contacts. All measurements were performed in a commercial APD 
refrigerator with computer control and processing. 
EXAMPLE 1 
A nominal Pr.sub.2 Tl.sub.2 Sr.sub.2 Cu.sub.3 O.sub.11 Sample (A) and a 
nominal HgPr.sub.2 Tl.sub.2 Sr.sub.2 Cu.sub.3 O.sub.12 Sample (B) were 
prepared according to the above method. The pellet was heated in a tube 
furnace at approximately 925.degree. C. 
FIG. 1 illustrates the resistance-temperature dependence for Sample A and 
Sample B. While Sample A had an onset temperature of 45K, Sample B 
exhibited a two-step transition at 88K and 43K, respectively. These 
results indicate that the addition of HgO facilitated the formation of a 
new superconducting phase with higher temperatures (approximately 90K). As 
set forth in Example 2, the superconducting behavior of the 
Hg--Pr--Tl--Sr--Cu--O samples was further enhanced by increasing the 
preparation temperature. 
EXAMPLE 2 
Two nominal HgPr.sub.2 Tl.sub.2 Sr.sub.3 Cu.sub.3 O.sub.13 Samples (C and 
D) were prepared at a higher temperature, by the method previously 
described, except that the temperature of the furnace was heated to 
approximately 1000.degree. C. Sample C was then furnace-cooled to 
700.degree. C. and remained at this temperature for 6 minutes. Sample D, 
on the other hand, was then furnace-cooled to room temperature. 
As illustrated in FIG. 2, Sample C exhibited a semi-metallic 
resistance-temperature behavior at the normal state. It had an onset 
temperature of 93K, and a zero-resistance temperature of 40K. Sample D had 
a similar onset temperature to Sample C, but reached zero-resistance at a 
much higher temperature (78K). Although not illustrated, Al-doping samples 
also exhibited a superconducting behavior similar to the Hg-doping 
samples. 
The results suggest to the inventors that: 1) either Hg or Al does not form 
a lattice in the superconducting phase, but only promotes the formation of 
the 93K superconducting phase; or 2) Hg or Al enters into the lattice but 
does not influence the conductivity temperature. 
EXAMPLE 3 
Pb-doping Pr--Tl--Sr--Cu--O samples exhibited different superconducting 
behavior as compared to the other doping elements. 
FIG. 3 illustrates resistance-temperature dependence for two Pb-doping 
samples (Pb1 and Pb2), consisting of a nominal composition of Pb.sub.0.5 
Pr.sub.2 Tl.sub.2 Sr.sub.3 Cu.sub.3 O.sub.13. The samples were prepared 
using the method previously described. 
As depicted in FIG. 3, both samples demonstrated a two-step superconducting 
transition at approximately 100K and 45K. The superconductivity at about 
100K in these Ca-free samples was reproducible. Further, this onset 
temperature of around 100K was higher than other doping elements. This 
100K superconducting transition was also observed in K-added Pb samples 
(K--Pb--R--Tl--Sr--Cu--O) as illustrated in FIG. 4. 
Compared with the Pr--Tl--Sr--Cu--O sample system, Pb (or Pb, K)-doping 
Pr--Tl--Sr--Cu--O samples do exhibit a higher superconducting temperature 
(about 100K). The inventors believe that these results indicate that Pb 
has entered the lattice structure of the superconducting phase, and has 
changed the superconducting behavior of the samples. Further, the Pb-doped 
Pr--Tl--Sr--Cu--O samples did not contain calcium as do other 
superconductors with conductivity temperatures at about 100K. Accordingly, 
the Pb--Pr--Tl--Sr--Cu--O system may be the first Ca-free superconducting 
system with reproducible temperatures of about 100K. 
The results also indicate that higher temperature superconductivity for Pb- 
and/or K-doping systems may be achieved by optimizing initial compositions 
and preparation conditions. Moreover, further elemental substitutions in 
these systems may lead to higher superconducting temperatures. 
It should be understood that various changes and modifications to the 
presently preferred embodiments described herein will be apparent to those 
skilled in the art. Such changes and modifications can be made without 
departing from the spirit and scope of the present invention and without 
diminishing its attendant advantages. It is therefore intended that such 
changes and modifications be covered by the appended claims.