Tape-formed oxide superconductor

Details Tape-formed oxide superconductor Tape-formed oxide superconductor

2001-10-12

2003-08-26

Dunn, Tom (Department: 1725)

Superconductor technology: apparatus, material, process

High temperature devices, systems, apparatus, com- ponents,...

Superconductor next to two or more nonsuperconductive layers

C505S238000, C505S239000, C428S699000, C428S701000, C428S702000

Reexamination Certificate

active

06610632

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a tape-form oxide superconductor and, more particularly, it relates to a tape-form oxide superconductor used for superconducting magnets, superconducting cables and electric power machines and instruments.
2. Description of the Related Art
Since the critical temperatures of oxide superconductors exceed the temperature of liquid nitrogen, they find application as superconducting magnets, superconducting cables and electric power machines and instruments.
In order to apply superconductors to the above fields, it is necessary to fabricate a wire having a high critical current density (Jc) and a long length. In order to obtain a long tape, it is necessary to form an oxide superconductor on a metal tape for mechanical strength and flexibility. Since oxide superconductors change their superconducting characteristics with changes in their crystalline alignment, it is necessary to improve in-plane alignment and it is therefore necessary to form an oxide superconductor on a tape-form, in-plane aligned substrate. In order to improve the critical current density, it is necessary that the c-axis of the oxide superconductor be aligned perpendicularly to the plane of the substrate and that its a-axis (or b-axis) have in-plane alignment almost parallel to the direction of current (hereinafter, referred to as “c-axis alignment” and “a-axis in-plane alignment”, respectively) so as to keep the quantum connectivity of a good superconducting state.
When an oxide superconducting layer is formed on a tape substrate by means of sputtering, pulse laser vapor deposition (PLD), vapor deposition or chemical vapor deposition using metalorganic salts (MOCVD), the crystal alignment of the substrate is usually as random polycrystals and, therefore, the oxide superconductor formed on this substrate is affected by the substrate and is unable to have a high degree of alignment.
Therefore, a method of using biaxially textured Ni (100) substrates as the tape-form substrates has been investigated. In this method, a nickel substrate is cold rolled and then heated in vacuo to make a highly aligned product called RABiTS (trade mark Rolling-Assisted Biaxially Textured-Substrates). There has also been reported a method wherein cerium is deposited on the biaxially textured nickel substrate in an atmosphere of inert gas at high temperature by means of electron beam evaporation. Hydrogen is present during this deposition whereby a thin epitaxial layer of CeO
2
is formed, then a thick film of YSZ (yttria-stabilized zirconia) is formed thereon at high temperature in vacuo by sputtering and the resulting product is used as a substrate.
In the above method, a layer of YBCO (as used herein “YBCO” refers to the Y—Ba—Cu—O type) is formed by pulse laser vapor deposition on the above-described substrate (refer, for example, to John Emathis, et al., Jpn. J. Appl. Phys., Vol. 37 (1988), pages L1379-1382).
The CeO
2
layer on the biaxially textured Ni substrate is aligned so as to depress reaction of the Ni substrate with YSZ and to prevent oxidation of the Ni substrate which forms NiO islands on the nickel, while the YSZ layer is aligned so as to depress reaction with the superconducting layer. In other words, the YSZ layer functions as a buffer layer for preventing the diffusion of Ni, whereby reduction in the superconducting characteristic is prevented and the matching with the superconducting layer is maintained. With the biaxially textured Ni substrate, when the YSZ is directly deposited, Ni in the substrate reacts with Zr in the YSZ at their interface whereby no epitaxial growth takes place. When YSZ is directly aligned on the Ni substrate, the oxidation of the Ni substrate occurs whereby no epitaxial growth takes place. Accordingly, the YSZ is superimposed on the CeO
2
layer which does not react with the Ni substrate, whereby diffusion of the element constituting the substrate into the superconducting layer is prevented. Since CeO
2
is easily broken, a thick film of YSZ is formed on the thin film of CeO
2
.
In the above method, YBCO is formed on YSZ having a good matching with YBCO but, since CeO
2
has a better crystallographic matching with YBCO than YSZ and further since CeO
2
is also better in terms of reactivity with an MOD solution, in one reported method a thin film of CeO
2
is further formed on the YSZ and a YBCO layer is formed thereon by MOD (metal organic deposition) to give a five-layered structure of biaxially textured Ni substrate/CeO
2
/YSZ/CeO
2
/YBCO (A. P. Malozemoff, et al., Eucas Conference, Sep. 14-17, 1999).
Metal organic deposition is a method where a metalorganic salt is applied and then thermally decomposed. Thus, a solution in which an organic compound having metal component is uniformly dissolved is applied on a substrate and heated for thermal decomposition whereupon a thin film is formed on the substrate. It does not require a vacuum and is able to achieve a high deposition rate at a low cost, whereby it is suitable for the manufacture of long tape oxide superconducting wires.
Since the MOD method uses a metalorganic salt as a starting material, it is also applicable to formation of an RE (123) superconductor, i.e. an RE
1+X
Ba
2−X
Cu
3
O
Y
superconductor (as used herein, RE means Y, Nd, Sm, Gd, Eu, Yb or Ho) and to formation of an intermediate layer such as CeO
2
. When organic salts are thermally decomposed, alkaline earth metal (such as Ba) carbonate is usually produced and, since a high-temperature thermal treatment of 800° C. or higher is necessary for the formation of an oxide superconductor by a solid state reaction of the carbonate, a method has been intensively used in recent years wherein an organic salt containing F (such as a TFA salt [trifluoroacetate]) is used as a starting material in a thermal treatment in a steam atmosphere, with control of steam partial pressure, whereupon an RE (123) superconductor is formed.
In that method where the TFA salt is used as a starting material, no nucleation results in a precursor and the RE (123) superconductor can be epitaxially grown on the substrate by the reaction of the steam with an amorphous precursor containing fluorine.
FIG. 8
shows a tape-form oxide superconductor
10
in a five-layered structure where CeO
2
, YSZ, CeO
2
and RE (123) superconducting layers are sequentially formed by the MOD method on the above-mentioned biaxially textured Ni substrate. In the drawing, there is shown a structure where a first intermediate layer
12
comprising CeO
2
, a second intermediate layer
13
comprising YSZ and a third intermediate layer
14
comprising CeO
2
are formed on the biaxially textured Ni substrate
11
and, on the third intermediate layer
14
, an RE (123) superconducting layer
15
is formed by an MOD method using a TFA salt.
In the above-mentioned tape-form oxide superconductor
10
with a five-layered structure, the CeO
2
first intermediate layer, the YSZ second intermediate layer, the CeO
2
third intermediate layer and the superconducting layer are epitaxially grown on a biaxially textured Ni substrate and, in addition, reaction among the elements constituting the biaxially textured Ni substrate and the superconducting layer is depressed whereby it is possible to prevent loss of the superconducting characteristic. In principle, the foregoing is an excellent method for the manufacture of a tape-formed superconductor.
However, with regard to the biaxially textured Ni substrate used in this method, it is necessary that a cold rolled Ni substrate be subjected to a thermal treatment in vacuo to give a high degree of orientation and there is the disadvantage that, during the recrystallization by such a thermal treatment, grain growth will take place to the extent of forming grains as big as 100 &mgr;m or more and adversely influence the superconducting layer to the extent that the expected Jc value is not obtained.
Thus, at the (surface) grain boundary in the biaxially textured Ni substrate, disorder in the crystal structu

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