Superconductor technology: apparatus – material – process – Processes of producing or treating high temperature... – Process of making wire – tape – cable – coil – or fiber
Reexamination Certificate
2000-11-16
2003-10-07
Abraham, Fetsum (Department: 2826)
Superconductor technology: apparatus, material, process
Processes of producing or treating high temperature...
Process of making wire, tape, cable, coil, or fiber
C505S320000, C505S501000, C505S430000, C505S432000, C505S431000
Reexamination Certificate
active
06630426
ABSTRACT:
FIELD OF INVENTION
The present invention relates generally to the field of superconductors. More particularly, the present invention is directed to a method of increasing the critical temperature of a superconducting film by inducing residual strains into the film. The present invention is also directed to a superconducting structure made in accordance with the foregoing method.
BACKGROUND OF THE INVENTION
High critical temperature superconducting (HTS) films have important applications in, e.g., microwave, electronic and optical devices. Presently, however, useful applications for HTS films are limited by the relatively low critical temperatures that have been achieved by conventional HTS film growth. Devices comprising conventional HTS films must be cooled to the films' low operating temperatures using, e.g., a Stirling cycle refrigerator or a Gifford-McMahon-type cryocooler. Currently, typical refrigerators of this type have mean times between failure (MTBFs) of about 4,000 hours, the most reliable models having MTFBs of 150,000 hours, or about 17.5 years. Drawbacks of these refrigerators are that they are expensive and are much less reliable than desired.
Increasing the critical temperature, T
c
, of HTS films to 160 K or above would allow them to be cooled inexpensively and reliably. As a result, the use of HTS films could become more widespread. Cooling to 160 K could be provided by highly reliable solid state thermoelectric coolers that operate based on the Seebeck effect. MTFBs of 200,000 to 300,000 hours (34 years) are commonplace among such thermoelectric coolers. Furthermore, these devices are inexpensive, widely used and readily available. Another benefit that would be realized from being able to use thermoelectric coolers is that the power required to cool an HTS device having a T
c
greater than 160 K would be relatively small compared to a conventionally cooled HTS device. Therefore, operating costs of thermoelectrically cooled HTS devices would be lower.
As reported in the CRC Handbook of Chemistry and Physics, 80
th
Edition, pp. 12-91 to 12-92, CRC Press LLC (1999), the present record for the highest T
c
of an HTS film is 133 K, which is held by a mercury-based copper oxide superconducting compound. Critical temperatures of other HTS materials may also be found in the foregoing reference. In Gao et al., “Superconductivity up to 164 K in HgBaCaCuO under quasi-hydrostatic pressures,” Physical Review B, Vol. 50, pp. 4260-63, The American Physical Society (1994), it was reported that the T
c
of the subject compound can be increased to 164 K by applying 300,000 atmospheres of hydrostatic pressure to it. This hydrostatic pressure effect has also been observed in other superconducting compounds, including the lanthanum, bismuth and thalium compounds, and to a lesser degree in yttrium compounds. While these increases are of scientific interest, they are not suitable for practical applications.
Recently, J. Loquet et al. reported in “Doubling the critical temperature of LaSrCuO using epitaxial strain,” Nature, Vol. 394, 1998, pp. 453-56, that greater increases in T
c
can be achieved by inducing compressive strain into the a-b plane of the HTS film by growing the HTS film pseudomorphically on a substrate having a lattice constant smaller that the lattice constant of the bulk HTS material, rather than by applying hydrostatic pressure. To achieve the relatively large increases in T
c
that are desirable for allowing the use of less expensive and more reliable cooling apparatus, a relatively large lattice mismatch between the HTS film and the substrate is required.
However, such a large lattice mismatch causes problems in growing a high quality, low defect HTS film. For example, a large lattice mismatch increases the energy of formation required for the lattice structure of the HTS film to conform to the lattice structure of the substrate. This increase in energy increases the likelihood that defects, such as dislocations, will occur in the HTS film during its growth. Such defects can degrade the superconducting properties of the HTS film and decrease the strain in the film. Therefore, it is desirable to grow an HTS film at the lowest energy of formation as possible to achieve the highest quality film.
Conventional pseudomorphic epitaxy, however, cannot achieve the desired low energy of formation and thus places severe limitations on the thickness of an HTS film having its T
c
increased via a lattice mismatch between the substrate and HTS film. As the magnitude of the lattice mismatch increases, the maximum thickness to which a high quality film can be grown decreases. The lattice mismatch induces strain into the HTS film within only about the first few hundred angstroms of thickness adjacent the substrate. Beyond these first few hundred angstroms, the strain in the HTS film caused by the lattice mismatch is significantly diminished due to dislocations. These limitations on HTS film thickness may not be compatible with a desired application. For example, in some applications, such as microwave filters and current fault limiters, among others, it is desired that the thickness of the HTS film be on the order of 1000 Å or more. It is, therefore, desirable to induce the T
c
-raising strains into an HTS film after it has been grown at the lowest energy of formation possible so that the problems of pseudomorphic epitaxy are avoided.
Belenky et al. have reported, in “Effect of stress along the ab plane on the J
c
and T
c
of YBa2Cu3O7 thin films,” Physical Review B, Vol. 44, No. 10, pp. 10, 117-120, The American Physical Society (1991), that the T
c
of a thin YBa
2
Cu
3
O
7
film grown on a substrate to form a composite structure can be changed from the unstrained T
c
by bending the composite structure to induce a stress into the HTS film. For their experiment, one end of the composite structure was clamped into a fixed support such that a portion of the composite structure was cantilevered from the fixed support. Then, external forces were alternatingly applied to the composite structure adjacent the free end of the cantilevered portion in a direction normal to the HTS film to alternatingly induce compression and tension into the a-b plane of the HTS film. Belenky et al. found that inducing compression along the a-b plane leads to an increase in T
c
above the unstrained T
c
and, conversely, that inducing tension along the a-b plane leads to a decrease in T
c
below the unstrained T
c
. While these results are of experimental interest, the temporary strains induced into the HTS film are of no value to practical HTS film devices.
In order to facilitate an understanding of the present invention, following is a presentation of orientation conventions, terminology, equations and empirical data used in the present specification and/or claims appended hereto. It is noted that Equations {1}-{8} appearing below are generally valid only for relatively small magnitudes of strain, i.e., where the relationship between strain and T
c
is generally linear. These equations may not adequately describe the relationship between strain and T
c
for larger magnitudes of strain, wherein the relationship between strain and T
c
may be non-linear. Thus, Equations {1}-{8} are presented only to illustrate the general concepts embodied in the various aspects of the present invention.
FIG. 1A
shows the relative orientation of the a, b and c directions/axes and a-b plane with respect to a unit
20
of simple cubic crystal lattice structure, and
FIG. 1B
shows the relative orientation of the a, b and c directional axes and a-b plane of an HTS film
22
grown on a substrate
24
.
FIGS. 2A and 2B
illustrate, respectively, HTS film
22
epitaxially grown commensurate with respect to substrate
24
and, in the alternative, epitaxially grown pseudomorphic with respect to substrate
24
. HTS film
22
in
FIG. 2A
is denoted a commensurate film, characterized in that the b-direction lattice constant K
s
b
of substrate
32
is equal to the bulk b-di
Ference Thomas G.
Puzey Kenneth A.
Abraham Fetsum
Downs Rachlin & Martin PLLC
TeraComm Research Inc.
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