Electricity: conductors and insulators – Conduits – cables or conductors – Superconductors
Reexamination Certificate
1995-06-07
2001-10-30
Paladini, Albert W. (Department: 2841)
Electricity: conductors and insulators
Conduits, cables or conductors
Superconductors
C505S230000, C505S704000, C505S776000, C505S780000, C505S813000
Reexamination Certificate
active
06310297
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to strongly-linked polycrystalline oxide superconductor and oxide superconductor composites. This invention further relates to a method for producing strongly-linked polycrystalline oxide superconductor articles.
BACKGROUND OF THE INVENTION
Polycrystalline, randomly oriented oxide superconductors exhibit critical current densities orders of magnitude lower than that of single crystal or highly oriented materials. The decrease in critical current density, as measured across grain boundaries in a randomly oriented material, is attributed to grain boundary impurities or grain boundary mismatch. The decrease in critical current density (J
c
) as measured across grain boundaries and, in particular, in the presence of a magnetic field is known as weakly-linked behavior.
Grain alignment or crystallographic texture has been shown to mitigate weakly-linked behavior of certain oxide superconductors and to permit “strongly-linked” behavior, allowing high current densities even in strong magnetic fields. Techniques used to produce grain alignment in oxide superconductors include melt texturing, magnetic alignment and mechanical deformation. Both melt texturing and magnetic alignment suffer from extremely slow processing speeds and the inability to process large amounts of material.
Melt texturing has been used to process oxide superconductors, in particular, the yttrium barium cuprate (YBCO) and bismuth strontium calcium cuprate (BSCCO) systems. Melt textured oxide superconductors have good grain alignment, high critical current density and strongly-linked grains. These qualities are obtained by forming large (millimeter scale) well-aligned grains. While such large grains are highly desirable for current transport, melt textured materials are brittle and can not be easily formed into complex shapes. A fine grained material (micron scale) would have the greater flexibility required for bulk applications such as current-carrying wires.
Mechanical deformation is widely used in the metal and ceramic arts to induce alignment by a purely mechanical means, for example, the alignment of fibers in a fiber-reinforced metal. In ceramic and ceramic composite materials, deformation processing is used to rotate, and thereby align, grains and particles having high aspect ratios. Grain rotation by mechanical means in brittle materials, such as oxide superconductors, is challenging because of the tendency for the oxide superconducting grains to fracture and pulverize upon deformation.
It is generally felt in the art that RE
1
Ba
2
Cu
4
O
8
(124) type oxide superconductors have inherently poor grain boundaries and are weakly-linked, where RE designates rare earth elements. By analogy, the RE
2
Ba
4
Cu
7
O
x
(247) type oxide superconductors which are structurally related to the 124-type compound might be also considered weakly-linked. For example, Shibutani et al investigated the critical current characteristics of wires prepared from [Y(Ca)]
1
Ba
2
Cu
4
O
8
by mechanical deformation and reported the wires to have poor critical current retention (<20% at approx. 0.1T). Shibutani et al. characterized the material as having incomplete grain boundary coupling and as being weakly-linked.
It is therefore an object of the present invention to provide RE
1
Ba
2
Cu
4
O
8
type and RE
2
Ba
4
Cu
7
O
1
type oxide superconductor material that is strongly-linked and that can significantly retain its critical current density in a magnetic field.
It is a further object of the present invention to provide RE
1
Ba2Cu
4
O
8
type and RE
2
Ba
4
CU
7
O
x
type oxide superconductor material having well aligned oxide superconductor grains.
It is yet a further object of the present invention to provide a method for producing strongly-linked oxide superconductor material on a large scale.
SUMMARY OF THE INVENTION
In one aspect of the invention, a strongly-linked oxide superconductor article includes fine, highly aligned oxide superconductor grains having at least a 25% critical current density retention in a field of 0.1 Tesla at 4.2 K as measured in a four point probe over a distance of at least 1 cm. The oxide superconductor is selected from the group consisting of 124-type and 247-type oxide superconductors. In preferred embodiments, the grains are less than 1000 &mgr;m along a longest dimension, more preferably less than 500 &mgr;m and most preferably less than 100 &mgr;m along a longest dimension.
By “critical current density retention”, as that term is used herein, it is meant the amount of critical current density retained at a given magnetic field strength as compared to the critical current density measured at zero Tesla (no magnetic field) at 4.2K. The value is reported as a percent.
By “124-type” oxide superconductor, as that term is used herein, it is meant an oxide superconductor in which the ratio of the component yttrium or rare earth element, barium and copper, is substantially 1:2:4. The 124-type oxide superconductor is further characterized as having the following arrangement of lattice planes shown schematically in
FIG. 1
a
: CuO
2
plane/Y/CuO
2
plane/BaO/CuO chains/CuO chains/BaO. Likewise, by “123-type” and “247-type” oxide superconductor, as those terms are used herein, it is meant an oxide superconductor in which the ratio of component yttrium or rare earth element, barium and copper is substantially 1:2:3 and 2:4:7, respectively. The 123-type oxide superconductor is further characterized as having the following arrangement of lattice planes shown schematically in
FIG. 1
b
: CuO
2
plane/Y/CuO
2
plane/BaO/CuO chains/BaO. The 247-type oxide superconductor is further characterized as having the following arrangement of lattice planes shown schematically in
FIG. 1
c
: CuO
2
plane/Y/CuO
2
plane/BaO/CuO chains/CuO chains/BaO/CuO
2
plane/Y/CuO
2
plane/BaO/CuO chains/BaO. Further, the constituent metallic elements can be substituted by non-rare earth elements so long as the overall composition maintains the relative 1:2:4, 1:2:3 or 2:4:7 ratio.
By “highly aligned oxide superconductor grains”, it is meant that grains are oriented such that the a-b planes are locally aligned parallel to the direction of current flow.
By “strongly-linked” behavior, as that term is used herein, it is meant that the tested oxide superconductor retains at least 25% of the critical current density at 0.1 Tesla as compared to the current density at 0 field. Testing of critical current density is done using a four point probe technique over a distance of 1 cm.
In preferred embodiments, the oxide superconductor is YBa
2
Cu
4
O
8
or [Y(Ca)]
1
Ba
2
Cu
4
O
8
. The polycrystalline oxide superconductor article may further include a noble metal selected from the group consisting of silver, gold and their alloys. The noble metal may be intimately mixed with or contactingly surrounding the oxide superconductor. The strongly-linked polycrystalline oxide superconductor article may be in the form of a wire, ribbon or sheet.
By “noble metal”, as that term is used herein, it is meant a metal that is chemically inert with respect to reaction with the oxide superconductor and with oxygen.
In another aspect of the invention, a strongly-linked polycrystalline oxide superconductor composite article includes at least one oxide superconductor filament contained within a noble metal. The oxide superconductor is selected from the group consisting of 124-type and 247-type oxide superconductors. The oxide superconductor possesses fine, highly aligned oxide superconductor grains having at least a 25% retention of critical current density in a 0.1 Tesla field. In preferred embodiments, the grains are less than 1000 &mgr;m along a longest dimension, more preferably less than 500 &mgr;m and most preferably less than 100 &mgr;m along a longest dimension.
In preferred embodiments, the oxide superconductor of the composite article is YBa
2
Cu
4
O
8
or [Y(Ca)]
1
Ba
2
Cu
4
O
8
. The noble metal is selected from the group consisting of silver, gold and its alloys. The noble metal c
Masur Lawrence J.
Podtburg Eric R.
American Superconductor Corp.
Choate Hall & Stewart
Nugent Elizabeth E.
Paladini Albert W.
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