Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
2001-07-25
2003-10-14
Dunn, Tom (Department: 1775)
Stock material or miscellaneous articles
Composite
Of metal
C428S699000, C428S930000, C505S237000, C505S238000, C505S475000, C505S434000, C505S731000, C427S062000
Reexamination Certificate
active
06632539
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a polycrystalline thin film having a pyrochlore type crystalline structure with well-aligned crystal orientation and a method of producing the same, and an oxide superconductor element of excellent superconducting property comprising an oxide superconducting layer formed on the polycrystalline thin film which has a pyrochlore type crystalline structure with well-aligned crystal orientation, and a method of producing the same.
BACKGROUND ART
The oxide superconducting materials which have been discovered in recent years are good superconducting materials that have critical temperatures above the temperature of liquid nitrogen However, there remain various problems to be solved before the oxide superconducting materials can be used as practical superconductors. One of the problems is the low critical current densities of the oxide superconducting materials.
The problem that the critical current density of the oxide superconducting material is low stems mainly from the electrical anisotropy which is intrinsic to the crystal of the oxide superconducting material. It is known that electric conductivity of the oxide superconducting material is high in the a-axis and b-axis directions of the crystal, but is low in the c-axis direction. Thus, in order to use an oxide superconducting layer formed on a substrate as a superconductor element, it is necessary to form an oxide superconducting layer of good crystal orientation on the substrate and to align the a-axis or b-axis of the crystal of the oxide superconducting material to the intended direction of current flow, while aligning the c-axis of the oxide superconducting material to the other direction.
Accordingly, a practice has been employed such that an intermediate layer having good crystal orientation made of MgO, SrTiO
3
or the like is formed on a substrate such as a metal tape by means of a sputtering apparatus, and an oxide superconducting layer is formed on the intermediate layer. However, the oxide superconducting layer formed on an intermediate layer of this type by a sputtering apparatus has a critical current density (typically about 1000 to 10000 A/cm
2
) which is far lower than that of the oxide superconducting layer (typically several hundred thousands of A/cm
2
) which is formed on a single crystal substrate made of such a material. The cause of this problem is supposedly as follows.
FIG. 16
is a sectional view of an oxide superconductor element made by forming an intermediate layer
2
on a substrate
1
made of a polycrystalline material in the form of a metal tape or the like by means of a sputtering apparatus, and then forming an oxide superconducting layer
3
on the intermediate layer
2
by the sputtering apparatus. In the structure shown in
FIG. 16
, the oxide superconducting layer
3
is in a polycrystalline state in which a multitude of crystal grains
4
are bonded together in a random manner. These crystal grains
4
individually show the c-axis of crystal being oriented somewhat perpendicularly to the substrate surface, but the a-axis and b-axis are randomly oriented.
When the a-axis and b-axis are randomly oriented among the crystal grains of the oxide superconducting layer, degradation in the superconducting properties, particularly in the critical current density, would be caused due to quantum coupling of the superconducting state being lost in the grain boundaries in which crystal orientation is disturbed.
The cause of the oxide superconductor element turning into a polycrystalline state with the a-axis and b-axis randomly oriented is supposedly as follows: since the intermediate layer
2
formed below the oxide superconducting layer is polycrystalline where the a-axis and b-axis are randomly oriented, the oxide superconducting layer
3
would be grown in such a condition as to match the crystal structure of the intermediate layer
2
.
The present inventors found that an oxide superconductor element having a sufficient critical current density can be produced by forming an intermediate layer of YSZ (yttrium-stabilized zirconia) which has well-oriented a-axis and b-axis on a polycrystalline substrate by means of a special process, and forming an oxide superconducting layer on the intermediate layer. With respect to this technology, the present inventors have filed applications by way of Japanese Patent Application No. Hei 4-293464, Japanese Patent Application No. Hei 8-214806, Japanese Patent Application No. Hei 8-272606, and Japanese Patent Application No. Hei 8-272607.
The technology proposed in these patent applications makes it possible to, when a film is formed on a polycrystalline substrate using a target made of YSZ, selectively remove YSZ crystals of unfavorable crystal orientation by means of an ion beam-assisted process in which the film forming surface of the polycrystalline substrate is irradiated in an oblique direction at an incident angle from 50 to 60° with a beam of ions such as Ar
+
thereby to selectively deposit YSZ crystals of good crystal orientation, so that an intermediate layer of YSZ crystal having good crystal orientation is formed.
According to the technology proposed in the previous applications of the present inventors, a polycrystalline thin film of YSZ with the a-axis and b-axis being favorably oriented can be formed. Also it was verified that the oxide superconducting layer formed on the polycrystalline thin film has a sufficient critical current density, and the inventors of the present application commenced research to develop a technology for producing polycrystalline thin films having more favorable properties than other materials.
FIG. 17
is a sectional view showing an example of the oxide superconductor element which the inventor shave been using recently. The oxide superconductor element D of this example has a four-layer structure made by forming, with the ion beam-assisted technology described previously, an orientation control intermediate layer
6
made of YSZ or MgO on a substrate
5
in the form of metal tape, then forming a reaction stopper intermediate layer
7
made of Y
2
O
3
thereon and forming the oxide superconducting layer
8
thereon.
The reason for using the four-layer structure is that, in order to form an oxide superconducting layer having a composition of Y
1
Ba
2
Cu
3
O
7−X
, it is necessary to apply a heat treatment at a temperature in a range from 600 to 800° C. after forming the oxide superconducting layer which has the desired composition by sputtering or other film forming process, but diffusion of elements may proceed between the oxide superconducting layers that have the compositions of Y
1
Ba
2
Cu
3
O
7−x
and YSZ, due to the heat supplied during the heat treatment, while the diffusion may cause deterioration of the superconducting property and must be prevented. The YSZ crystal which constitutes the orientation control intermediate layer
6
has a cubic crystal structure, and the oxide superconducting layer having the composition of Y
1
Ba
2
Cu
3
O
7−x
has a crystal structure called perovskite. Both of these crystal structures belong to a class of face-centered cubic crystals and have similar crystal lattices, but there exists a difference of about 5% in the lattice size between the two structures. For example, distance between nearest atoms, namely the distance between an atom located at a corner of the cubic lattice and an atom located at the center of the face of the cubic lattice is 3.63 Å (0.363 nm) in the case of YSZ, 3.75 Å (0.375 nm) in the case of Y
2
O
3
, and is 3.81 Å (0.381 nm) in the case of the oxide superconducting layer having the composition of Y
1
Ba
2
Cu
3
O
7−X
. Thus Y
2
O
3
has an intermediate value between those of YSZ and Y
1
Ba
2
Cu
3
O
7−X
and is useful for bridging the difference in lattice size and can be advantageously used as a reaction stopper layer due to the similarity in the compositions.
With the four-layer structure shown in
FIG. 17
, however, the number of required layers is increased, whi
Iijima Yasuhiro
Kimura Mariko
Saitoh Takashi
Bell Boyd & Lloyd LLC
Cooke Colleen P.
Dunn Tom
Fujikura Ltd.
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