Superconductor technology: apparatus – material – process – Processes of producing or treating high temperature... – Producing josephson junction – per se
Reexamination Certificate
1999-07-30
2002-01-08
Cuneo, Kamand (Department: 2841)
Superconductor technology: apparatus, material, process
Processes of producing or treating high temperature...
Producing josephson junction, per se
C174S125100, C505S235000, C505S238000, C029S599000
Reexamination Certificate
active
06337307
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an oxide superconducting conductor allowing a large amount of superconducting critical current to pass therethrough incurring a small power loss, an oxide superconducting element wire for use in fabricating this conductor, and methods for fabricating the conductor and the element wire.
2. Description of the Related Art
In a metallic superconducting conductor fabrication it has been widely practiced to fabricate a superconducting conductor by bundling superconducting element wires. When bundling the element wires, it is required to increase the critical current density of the element wire and to pass currents evenly through each of the element wires in order to attain a large current flow in a superconducting state, or a current flow incurring a small loss. To decrease the AC power loss in a metallic superconducting conductor the use of a great number of fine element wires in a strand is practiced.
Further, in obtaining a superconducting element wire, efforts are being made to obtain a finer superconducting element wire having excellent flexibility and providing a greater critical current (Ic). In the case of a metallic superconducting element wire, a method to draw a bundle of coated wires is practiced.
In the oxide superconducting fabrication a similar method in practiced in metallic superconducting fabrication is also practiced in fabricating a Bi type oxide superconducting element wire, in which Bi type element wire has a Bi type superconducting core with a metallic sheath.
Recently, in fabricating a R
X
Ba
Y
Cu
Z
O
W
-based oxide superconducting element wire, in which the said wire has a carbon fiber or a metallic fiber as a core and superconducting film as a conductor, was attempted.
Such arts are described for example in Japanese Patent Laid-open No. Hei 1-93006, J. G. Wang et al., “Y—Ba—Cu—O superconducting fibers and wires by spray pyrolysis on carbon fibers”, J. Appl. Phys. Vol. 67 No. 4, pp. 2160-2162, or L. D. Woolf et al., “Continuous fabrication of high-temperature superconductor coated metal fiber and multifilamentary wire”, Appl. Phys. Lett. Vol. 58 No. 5, pp. 534-536.
Crystals of oxide superconductors generally have the orthorhombic crystal structure. The orthogonal axes of the orthorhombic lattice are called a-axis, b-axis, and c-axis in ascending order of the lengths of the axes. Of these axes, since the a-axis and b-axis have nearly the same length, they are frequently not discriminated from each other and are called the a-axis or the ab-axis. Since the superconducting current flows in the CuO plane perpendicular to the c-axis, a great superconducting current flow takes place in the direction perpendicular to the c-axis. On the other hand, the superconducting critical current density (Jc) is low in the direction parallel to the c-axis. Accordingly, in an oxide superconducting element wire aimed at obtaining a large current flow, it becomes necessary to arrange the c-axis of the crystal perpendicular to the direction of the current flow.
In a tape-formed or plate-formed substrate, a thin film whose c-axis is oriented in the perpendicular direction to the surface of a base material (such a film may hereinafter be called a c-axis oriented film) was formed by suitably selecting the forming conditions, such as depositing temperature and the like, for depositing the oxide superconducting thin film.
Another factor impeding the flow of a superconducting current, especially in the c-axis oriented film is a grain boundary caused by the crystals having different orientations in the a- and b-axes. This tendency is particularly remarkable in the case of a R
X
Ba
Y
Cu
Z
O
W
-type oxide superconductor. Hence in order to obtain a large Jc value, it is essential to eliminate such grain boundaries and therefore to form a thin film in which the ab-axes are aligned.
A method tried first to form an intermediate layer having a crystal alignment by an ion beam assist deposition method or an inclined substrate method (ISD method) and then to form a superconducting thin film with an crystal alignment on the intermediate layer by a pulse laser deposition method (PLD method) or a metal organic chemical vapor deposition method (MOCVD method) on a plate formed substrate for reducing the number of the grain boundary. Another method was tried which uses a thin film formed on a long and narrow single crystalline base material by such a method as a liquid phase epitaxial method, so that a crystal orientation is provided for the oxide superconducting thin film.
As to the ion beam assist deposition method, reference is made to the disclosure, for example, in Y. Iijima et al., “In-plane aligned YBa
2
Cu
3
O
7−x
thin films deposited on polycrystalline metallic substrates”, Appl. Phys. Let. Vol. 60 No. 6, 1992, pp. 769-771, and, as to the inclined substrate method and the liquid phase epitaxial method, reference is made to the disclosure, for example, in Y. Yamada et al., “liquid Phase Epitaxy of YBCO Single-Crystalline Oxide Fibers for Power Application”, Advances in Superconductivity IX Vol. 2, 1996, pp. 653-655.
(1) Of the above mentioned related art examples, that using a fiber as the core was not succeeded to align the crystal orientation of the superconducting layer and, hence, there were problems such that (a) the critical current density was restricted to a low value. And if applying the thick superconducting layer for passing enough electric current, then (b) the mechanical characteristics such as flexibility tend to deteriorate because of the superconducting layer thickness.
These problems are described in detail below.
(a) In order to form an oxide superconducting thin film with excellent superconducting characteristics it is required to align the crystal orientation of the oxide superconducting thin film. To align the said orientation, it is required either to irradiate the substrate with an ion beam directed thereto at a specific angle or to deposit a material on the substrate in a specific angular direction.
However, with a fine and round base material such as a fiber, it was thought to be impossible to make constant the irradiating angle of the ion beam or the deposited angle of the material with respect to the surface of the base material, and therefore it was thought to be impossible to obtain an oxide superconducting thin film with good crystal alignment. Hence, the critical current density was restricted to a low value. One example of the attempt was reported to provide a thin superconducting layer by a pulse laser deposition method with a carbon fiber as a core (A. Al-Sharif et al., “Attempts to prepare Bi-based superconductor on a carbon fiber substrate”, J. Appl. Phys. Vol. 67 No. 9, pp. 5023-5025).
(b) As to the thickness of the superconducting layer, a superconducting current is known that flows only very near the surface of the superconductor. The pertinent thickness is considered to be around 1 &mgr;m, against which, the superconducting layer in the above described related art example was as thick as several tens of &mgr;m.
The thick superconducting layer has no contribution to the current flow amounts, and further, by the strain produced by bending at the circumferential portion of the layer, where the current flow is large, and results in deterioration of the superconducting characteristic of the wire element.
(2) As to the shape of an element wire in a tape form or plate form, the conductor design is greatly restricted and it is difficult to calculate the inductance accurately. This is described in detail below.
A tape-formed and a plate-formed element wire having a high Jc value obtained in the above-described related art, however, the direction to be bent is restricted and prevents processing in the desired conductor shape. Especially when the obtained superconducting element wires are stranded for making conductor, the manner of the stranding is greatly restricted. For example, when the wires are twisted together, the pitch in the strand must be made somewhat larger. Thi
Konishi Masaya
Nakahara Tuneo
Sato Ken'ichi
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