Superconductor technology: apparatus – material – process – Processes of producing or treating high temperature... – Process of making wire – tape – cable – coil – or fiber
Reexamination Certificate
2001-03-02
2004-03-02
Dunn, Tom (Department: 1725)
Superconductor technology: apparatus, material, process
Processes of producing or treating high temperature...
Process of making wire, tape, cable, coil, or fiber
C505S125000, C505S126000, C505S780000, C505S781000, C505S450000, C505S452000, C505S470000
Reexamination Certificate
active
06699820
ABSTRACT:
TECHNICAL FIELD
The present invention relates to superconductors, particularly to the current carrying capabilities of superconductors. It is a proven fact that the critical currents of many superconductors, here polycrystalline superconductors, are limited by grain boundaries formed in them. The invention overcomes this limitation of present superconductors by using alterations for improving the current transport properties of the superconductors' grain boundaries. This is done in principle by doping these grain boundaries to dopant concentrations different from the concentrations of the grains, thus aiming at diminishing detrimental effects caused by space charge layers formed at the grain boundaries.
BACKGROUND OF THE INVENTION
Based on the new class of superconductors, henceforth referred to as high-T
c
superconductors, which were discovered by Bednorz and Müller and disclosed in their article “Possible High-T
c
Superconductivity in the Ba—La—Cu—O System”, Zeitschrift für Physik B, Condensed Matter, Vol. B64, 1986, pp.189-193, a variety of superconducting wires, cables and tapes have been developed for the transport of electrical current. A key parameter defining the performance and thus the economic benefit of these conductors is given by their so-called critical current density, which is the maximum density of the current these conductors can carry as so-called supercurrents in the superconducting state. The critical current density is a specific property for a given superconductor, and, for the practical use of a superconductor, one aims at maximizing this critical current density.
Chaudhari et al. have taught in their article “Direct Measurement of the Superconducting Properties of Single Grain Boundaries in YBa
2
Cu
3
O
7−&dgr;
”, Physical Review Letters, Vol. 60, 1988, pp.1653-1655, that the limiting factor for the critical current density of polycrystalline high-T
c
superconductors is the electronic behavior of the boundaries formed by the crystalline grains of these materials. It was shown that the critical current densities of these grain boundaries are smaller by one to two orders of magnitude than the critical current densities of the grains abutting the grain boundaries.
Further, Dimos et al. have taught in their publication “Superconducting Transport Properties of Grain Boundaries in YBa
2
Cu
3
O
7−&dgr;
Bicrystals”, Physical Review B, Vol. 41, 1990, pp. 4038-4049, that superconducting grains with a small misorientation (below typically 8° to 10°) behave as strongly coupled superconductors whereas larger misorientations (also called large-angle grain boundaries) are weakly coupled, showing Josephson junction-like properties. The teaching by Dimos et al. is the finding that the detrimental effect of the grain boundaries can be reduced by aligning the superconducting grains with respect to their crystalline main axes.
Following this proposal, wires and tapes of high-T
c
superconductors have been fabricated, the critical currents of which are enhanced by aligning the superconducting grains by a variety of means, such as rolling processes or ion beam assisted techniques. Although these technologies have lead to the fabrication of high-T
c
superconductors with current densities of the order of 100 000 A/cm
2
at temperatures of 4.2 K, it remains desirable to fabricate high-T
c
superconductors with still higher critical current densities or with processes which are less costly than the known ones.
Schmehl et al. in “Doping Induced Enhancement of the Critical Currents of Grain Boundaries in YBa
2
Cu
3
O
7−&dgr;
”, Europhysics Letters, Vol.47, 1999, pp. 110-115; Mannhart et al. in European Patent Application EP 1 006 594 A1, “Superconductor with Enhanced Current Density and Method for Making such a Superconductor”, and G. A. Daniels et al. in “Improved Strong Magnetic Field Performance of Low Angle Grain Boundaries of Calcium and Oxygen Overdoped YBa
2
Cu
3
O
x
” Appl. Phys. Lett., Vol.77, 2000, pp. 3251-3253, have shown that the critical current densities of the grain boundaries can be enhanced by doping of the superconductor to dopant concentrations different from concentrations that would provide the optimum superconducting properties of the grains of the superconductors. By this, large grain boundary critical current densities are achieved. Because the critical temperature of the grains is reduced by this doping (see FIG.
1
), the enhancement of the grain boundary critical current density is not obtained at 77 K (see
FIG. 2
) but only in the temperature range below. The temperature of liquid nitrogen, 77 K, is a preferable temperature for the operation of superconducting cables. As shown in “Doping Induced Enhancement of the Critical Currents of Grain Boundaries in High-T
c
Superconductors” by Mannhart et al, Physica C, Vol. 341-348, 2000, pp. 1393-1396, doping of YBa
2
Cu
3
O
7−&dgr;
with Co achieves the opposite effect and enhances the sample's normal state resistance.
As described in the above-cited European Patent Application EP 1 006 594 A1, “Superconductor with Enhanced Current Density and Method for Making such a Superconductor”, it was recognized that a preferential doping of the grain boundaries is advantageous in many cases.
The present invention now aims to achieve large critical current densities in a wide temperature regime, in particular in the range of 77 K and above, by avoiding the degradation of the grains due to their non-optimal doping. This different and novel approach according to the invention is—in brief—the teaching that by use of dopant multilayers, the critical current densities of the grain boundaries in a superconductor, and therefore also of the polycrystalline conductors, can be enhanced by doping the grain boundaries to dopant concentrations different from the dopant concentrations of the superconducting grains. Ivanov et al. report in “Properties of Locally Doped Bicrystal Grain Boundary Junctions”, Physica B, Vol. 194-196, 1994, pp. 2187-2188, experiments in which they investigated the effects of Fe and Pt embedded into grain boundaries in YBa
2
Cu
3
O
7−&dgr;
films on the transport properties of these grain boundaries. In this work, the grain boundaries were also doped to concentrations different from the dopant concentrations of the grains. This was achieved by using specially prepared bicrystalline substrates. Prior to the growth of the high-T
c
superconductors, Fe and Pt dopants were embedded into the grain boundary of the substrate, which was a ZrO
2
bicrystal. During the growth of the superconducting film, the substrate acted as a source of the dopants. In these experiments, a strong reduction of the critical current density was observed, i.e. just the opposite of the aim of the present invention. The critical current reduction in the Ivanov et al. experiment presumably took place due to the unsuited doping configuration as well as due to an unsuited selection of the dopants.
SUMMARY OF THE INVENTION
A general objective of the present invention is to provide a solution for an increased use of high-T
c
and other superconducting materials by improving the current transport mechanisms within such materials, in particular for increasing the upper limit of the achievable supercurrent density within such materials.
A specific objective, as mentioned above, is to provide an approach by which the current carrying capabilities of superconductors, e.g. superconducting wires or tapes, can be significantly improved in a wide temperature range.
A further objective of the invention is to provide a technique for a simpler manufacturing process, resulting in cheaper mass production of polycrystalline superconductors with large critical current densities than with presently used techniques. At present, the fabrication of such superconductors requires cumbersome and costly processes to optimize the grain boundary geometries, such as improving their alignment or enhancing the effective grain boundary area.
A still further objective is to provide an approach for reducing the sensitivity of
Bielefeldt Hartmut Ulrich
Gotz Barbel Martha
Hammerl German
Hilgenkamp Johannes Wilhelmus Maria
Mannhart Jochen Dieter
Bachman & LaPointe P.C.
Cooke Colleen P.
Dunn Tom
LandOfFree
Method for making a superconductor with enhanced current... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for making a superconductor with enhanced current..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for making a superconductor with enhanced current... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3246041