Superconductor technology: apparatus – material – process – High temperature devices – systems – apparatus – com- ponents,... – Superconductor next to layer containing nonsuperconducting...
Reexamination Certificate
2001-06-11
2004-04-06
Dunn, Tom (Department: 1725)
Superconductor technology: apparatus, material, process
High temperature devices, systems, apparatus, com- ponents,...
Superconductor next to layer containing nonsuperconducting...
C505S237000, C427S062000, C427S596000, C427S126300
Reexamination Certificate
active
06716796
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a polycrystalline thin film having a crystalline structure of a type C rare earth oxide with a well-aligned crystal orientation and a method of producing the same, and an oxide superconductor element of excellent superconducting properties having a polycrystalline thin film which has a crystalline structure of a type C rare earth oxide with a well-aligned crystal orientation and an oxide superconducting layer and a method of producing the same.
BACKGROUND ART
Oxide superconducting materials which have been discovered in recent years are good superconducting materials which have critical temperatures above the liquid nitrogen temperature. However, there remain various problems to be solved before oxide superconducting materials can be used as practical superconductors. One of the problems is the low critical current densities of 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 crystals of the oxide superconducting material. It is known that the electric conductivity of oxide superconducting materials 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 with a good crystal orientation on a substrate and to align the a-axis or b-axis of the crystal of the oxide superconducting material with the intended direction of current flow, while aligning the c-axis of the oxide superconducting material with the other direction.
Accordingly, such a practice has been employed wherein an intermediate layer having a good crystal orientation and 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 this type of intermediate layer by a sputtering apparatus has a critical current density (typically about 1,000 to 10,000 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 assumed to be as follows.
FIG. 14
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 by forming an oxide superconducting layer
3
on the intermediate layer
2
by the sputtering apparatus. In the structure shown in
FIG. 14
, 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 each crystal being oriented perpendicular to the substrate surface, but the a-axis and b-axis are randomly oriented.
When the a-axes and b-axes 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 since quantum coupling of the superconducting state is lost in the grain boundaries in which the crystal orientation is disturbed.
The cause of the oxide superconductor element turning into a polycrystalline state with the a-axes and b-axes randomly oriented is assumed to be as follows: since the intermediate layer
2
formed below the oxide superconductor element is polycrystalline in which the a-axes and b-axes are randomly oriented, the oxide superconducting layer
3
would be grown in such a condition so as to match the crystal structure of the intermediate layer
2
.
The present inventors have 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 a well-oriented a-axis and b-axis, on a polycrystalline substrate by means of a special process, and by forming an oxide superconducting layer on the intermediate layer. With respect to this technology, the present inventors have filed applications by way of Japanese Unexamined Patent Application, First Publication No. Hei 4-293464, Japanese Patent Application, First Publication No. Hei 8-214806, Japanese Unexamined Patent Application, First Publication No. Hei
8-272606
, and Japanese Unexamined Patent Application, First Publication No. Hei 8-272607.
The technology proposed in these patent applications makes it possible, when a film is formed on a polycrystalline substrate using a target made of YSZ, to selectively remove YSZ crystals of an 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 with a beam of ions, such as Ar
+
, thereby selectively depositing YSZ crystals of a good crystal orientation, so that an intermediate layer of YSZ crystals having a 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-axes and b-axes being favorably oriented can be made. It was also verified that the oxide superconducting material formed on the polycrystalline thin film has a sufficient critical current density, and the present inventors began research into developing technology of producing better polycrystalline thin films from other materials.
FIG. 15
is a sectional view showing an example of the oxide superconductor element which the inventors have been using recently. An oxide superconductor element D of this example has a four-layer structure made by forming, with the technology described previously, an orientation control intermediate layer
6
of YSZ or MgO on a substrate
5
in the form of a metal tape, then forming a reaction stopper intermediate layer
7
made of Y
2
O
3
thereon, and forming an oxide superconducting layer
8
thereon.
The reason for using the four-layer structure is that, in order to make 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 of several hundred degrees centigrade after forming the oxide superconducting layer having the desired composition by sputtering or another film forming process, but diffusion of the elements may proceed between the oxide superconducting layers having the compositions of YSZ and Y
1
Ba
2
Cu
3
O
7−x
, due to the heat supplied during the heat treatment; the diffusion may cause deterioration of the superconducting properties and must be prevented. The YSZ crystals which constitute the orientation control intermediate layer
6
have a cubic crystal structure, and the oxide superconducting layer having a 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, the distance between the 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) for YSZ, 3.75 Å (0.375 nm) for Y
2
O
3
, and 3.81 Å (0.381 nm) for an 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 of the compositions.
With the four-layer structure shown in
FIG. 15
, however, the number o
Iijima Yasuhiro
Kimura Mariko
Saito Takashi
Bell Boyd & Lloyd LLC
Cooke Colleen P.
Dunn Tom
Fujikura Ltd
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