Thin film Hg-based superconductors, thermoelectric materials...

Superconductor technology: apparatus – material – process – Processes of producing or treating high temperature... – Coating

Reexamination Certificate

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C505S501000

Reexamination Certificate

active

06395685

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is broadly concerned with improved methods for forming superconducting materials and thermoelectric materials comprising highly volatile elements. In one embodiment, the methods are useful for the production of highly epitaxial Hg-containing film superconductors exhibiting very high J
c
values, along with novel films of this character. In another embodiment, the methods are useful for the production of thermoelectric materials which have very low thermal conductivities. In either embodiment, a process is carried out whereby precursor structures including a nonvolatile element are subjected to energy in the presence of a vapor comprising a volatile element so as to cause the nonvolatile element to be replaced by the volatile element without substantial alteration of the crystalline structure of the precursor.
2. Description of the Prior Art
Hg-based superconductors have a record high superconducting transition temperature (T
c
~135K). Since a higher T
c
promises higher device operation temperature and a better stability at a given temperature, it is very important to develop viable technologies for fabrication of high-quality Hg-based superconducting films. Prior technologies generally involve two steps: deposition of amorphous rare-earth cuprate precursor films with or without Hg, followed by Hg-vapor annealing at temperatures above 800° C. under a controlled Hg-vapor pressure in order to form the superconducting phases.
All high T
c
superconductors have layered structures and their physical properties are anisotropic parallel or perpendicular to the layers. The alignment of grains during the growth is crucial for the quality of the films. Since most applications of the superconductor require the capability of carrying high current, epitaxial growth of grains is essential.
Since Hg-based compounds are volatile, the control of the growth conditions is very difficult using conventional techniques. Though c-axis-oriented superconducting Hg-1212 and Hg-1223 films have been obtained with their T
c
up to 124K and 130K, respectively, high-quality epitaxial growth has not been achieved. In other words, the superconducting grains are connected more or less randomly in the plane of the substrates. This lack of epitaxy is reflected in the poor x-ray pole figures and high X
min
values of prior Hg-1212 and Hg-1223 films, which are on the order of 100%.
A direct effect of this substantial non-epitaxy is that the J
c
values of the films are much lower than expected in view of the intragrain J
c
values and the high irreversibility line of the Hg-based superconductors. When current passes grain boundaries, it is significantly reduced owing to the fact that the grains are not aligned epitaxially. The other effect of this non-epitaxy is a relatively rough film surface which hinders many potential applications of the Hg-based superconducting thin films, e.g., for use in microelectronics.
The volatility of Hg presents a particular problem in the fabrication of micro-bridges in microelectronic devices. Because Hg-based superconductors are so delicate and tend to react with etching chemicals and water, fabrication of the Hg-containing micro-bridges directly from Hg-containing films using regular photolithography processes generally results in degradation of the samples. This, in turn, leads to unreliable connections in the circuit.
There is accordingly a real and unsatisfied need in the art for an improved method of fabricating Hg-containing superconductors to yield films having a high degree of epitaxy and correspondingly high T
c
and J
c
values.
The properties of thermoelectric materials are demonstrated by calculating the figure of merit ZT=TS
2
/&kgr;&rgr;, where T is the operating temperature of the material, S is the Seebeck coefficient, &rgr; is the resistivity, and K is the thermal conductivity of the material. Current state-of-the-art Peltier refrigerators use semiconducting Bi
2
Te
3
—Sb
2
Te
3
alloys (with a ZT of <1) that only produce moderate amounts of cooling and are inefficient compared to compressor-based refrigerators. As a result, thermoelectric refrigerators are generally used in applications where reliability and convenience are more important than economy.
Many attempts have been made to design improved thermoelectric materials having a higher ZT. Theoretically, a solid that is simultaneously a poor conductor of heat and a good conductor of electricity (the “electron crystal and phonon glass”) could have a ZT as high as 2-4, making it ideal for thermoelectric applications.
It has been proposed that synthesizing semiconducting compounds in which one of the atoms or molecules is weakly bound in an oversized atomic cage would cause the weakly bound atom to undergo local anharmonic vibrations, somewhat independent of the other atoms in the crystal, forming what is known as a “rattler.” These localized rattlers can, in some cases, dramatically lower the phonon thermal conductivity (&kgr;
lattice
) to values comparable to that of a glass of the same composition. The theoretical lower limit of &kgr;
lattice
is designated &kgr;
min
and corresponds to the thermal conductivity of an amorphous solid in which the mean-free path of the heat-carrying phonons approaches the order of the phonon wavelength. In an electrically conducting solid, heat is transported by both the charge carriers (electrons or holes) and phonons (lattice); hence, &kgr;=&kgr;
electron
+&kgr;
lattice
. Therefore, a significantly enhanced ZT is expected from a minimized &kgr;
lattice
.
One class of materials that satisfies many of the requirements of an electron-crystal and phonon-glass solid is the filled skutterudites. Filled skutterudites have the general formula RM
4
X
12
, where typically: X is P, As, or Sb; M is Fe, Ru, or Os; and R is La, Ce, Pr, Nd, or Eu. Skutterudites are body-centered, cubic crystals with 34 atoms in the conventional unit cell and a space group of Im3. This structure consists of square, planar rings of four pnicogen atoms (i.e., X from the general formula) with rings of four oriented along either the (100), (010), or (001) crystallographic directions. The metal atoms M form a simple cubic sublattice, and the rattler atoms R are positioned in the two remaining holes in the unit cell. The structure of filled skutterudites differs from basic skutterudites in that basic skutterudites do not contain a rattler atom R. It has previously been shown that the &kgr;
lattice
of filled skutterudite antimonides is nearly an order of magnitude lower than that of the basic skutterudites due to the presence of the rattling atoms R.
Filled skutterudites are difficult to synthesize in pure form by conventional arc melting processes. Moreover, the observed low &kgr;
lattice
of these filled skutterudite antimonides is still significantly higher than the predicted &kgr;
min
. There are a few possible sources for this problem, one of which is the presence of impurity phases such as RX
2
or MX
2
. Another possible source may be that the filled skutterudites generated thus far are not actually similar to phonon glasses. Thus, the R elements used likely do not have sufficiently large anharmonic deflection to allow the skutterudites to approach the phonon glass limit.
SUMMARY OF THE INVENTION
The present invention overcomes the problems outlined above and provides new methods for the production of desirable high T
c
and J
c
(both magnetic and transport) value Hg-containing film superconductors having significantly improved epitaxial characteristics. As is conventional in the art, transport J
c
refers to the measure of current density through a film when a current is directly applied to the film whereas magnetic J
c
refers to the measure of current density through a film that is induced by the application of a magnetic field to the film. The present invention also provides methods for the production of thermoelectric materials having high ZT's and low thermal conductivities. As is conventional in th

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