Elongated superconductor structure with a high-Tc...

Details Elongated superconductor structure with a high-Tc... Elongated superconductor structure with a high-Tc...

2001-08-01

2003-07-22

Dunn, Tom (Department: 1725)

Stock material or miscellaneous articles

Composite

Of inorganic material

C428S702000, C505S230000, C505S237000, C505S238000, C505S434000, C505S470000, C427S062000, C427S419300

Reexamination Certificate

active

06596421

ABSTRACT:

BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to an elongated superconductor structure for carrying an electric current in a predetermined direction. The structure is intended to have the following parts: a biaxially textured mount composed of metallic material, an intermediate layer system which is deposited on the mount and has at least two intermediate layers composed of different oxide materials, and a superconducting layer which is deposited on the intermediate layer system and is composed of a high-T
c
superconductor material of the (RE)M
2
Cu
3
O
x
type. The RE component contains at least one rare earth metal (including yttrium) and the M component contains at least one alkaline-earth metal. The invention also relates to a method for producing such a superconductor structure. A corresponding superconductor structure and a method for producing it can be found in
Applied Superconductivity,
1996, Vol. 4, Nos. 10-11, pages 403 to 427.
Superconducting metal-oxide compounds having high critical temperatures Tc of above 77 K are known. They are therefore also referred to as high-T
c
superconductor materials or HTS materials. Their particular advantage is that they allow a liquid nitrogen (LN
2
) cooling technique. Metal-oxide compounds such as these include, in particular, cuprates based on specific material systems such as that of the (RE)-M—Cu—O type with the RE components containing at least one rare earth metal (including Y), and the M component containing at least one alkaline-earth metal. The main representative of this type is the material YBa
2
Cu
3
O
x
(referred to as “YBCO”).
Attempts are being made to deposit these known HTS materials on different mounts (substrates) for different application purposes. The aim is, in general, to produce a textured superconductor material with as high a phase purity as possible, in order to achieve a high current carrying capacity. The term texturing in this case means the alignment of the crystallite of a polycrystalline structure. In particular, elongated metallic mounts are intended for conductor applications. See, for example, U.S. Pat. No. 4,921,833 (corresponding European EP 0 292 959 A2) to Takano.
In the case of a corresponding superconductor structure for conductor applications, the HTS material is generally not deposited directly on a metallic mount strip used as a substrate; instead, this mount strip is first of all covered by at least one thin intermediate layer, which is also referred to as a buffer layer. This intermediate layer has a thickness in the order of magnitude of about 1 &mgr;m and is intended to prevent metal atoms from diffusing out of the mount material into the HTS material, in order to prevent the superconducting characteristics from being adversely affected as a result of this. At the same time, such an intermediate layer, which is used as a diffusion barrier, allows the surface to be smoothed and the adhesion of the HTS material to be improved. Appropriate intermediate layers are composed in particular of oxides of metals such as zirconium, cerium, yttrium, aluminum, strontium or magnesium, or of alloys with these metals, and are thus in general dielectrics.
In addition to the characteristic as a diffusion barrier, this at least one intermediate layer is, furthermore, intended to satisfy the requirements of allowing textured growth of the HTS material to be applied to it. In consequence, the intermediate layer must itself have a corresponding texture. The transfer of the crystallographic orientation during the growth of a layer on a substrate of a chemically different type is known by the term heteroepitaxy. In that case, the unit cells in the intermediate layer must have dimensions which are matched as well as possible to the lattice constants of the HTS material. Furthermore, it should have a thermal coefficient of expansion which at least approximately matches that of the HTS material in order in this way to avoid undesirable mechanical stresses during the temperature cycles, which are unavoidable for applications relating to superconducting technology and for layer preparation, and possible damage such as exfoliation resulting from this.
The choice of the “mount-intermediate layer” system is subject to similar requirements. In that case as well, good adhesion characteristics are desirable, and, at the same time, the desired heteroepitaxy between the intermediate layer and the HTS layer which is growing on it must not be adversely affected.
For the reasons mentioned above, the literature reference cited initially provides for a metal strip whose surface is biaxially textured by means of a rolling process and which is composed of copper or nickel to be used as the mount strip, on which an intermediate layer in the form of a CeO
2
layer (as the first buffer layer) and a thicker layer composed of ZrO
2
stabilized with Y (Zr(Y)O
2
as the second buffer layer) are deposited. This combination was chosen since CeO
2
can be deposited heteroepitaxially in a reducing atmosphere on textured nickel in order to avoid oxidation of the metal surface. However, the oxygen deficit which results in this case in the CeO
2
and the enlargement of the lattice constants associated with this mean that there is a tendency for the layer to form cracks. CeO
2
layers with a maximum thickness of only 100 nm are therefore used, in order to restrict the surface density at corresponding cracks. This is because thicker CeO
2
layers tend to form cracks more easily than thin layers. This makes it necessary to use a second layer composed of Zr(Y)O
2
in order to allow any cracks or other mechanical damage to be covered. This second layer must be made sufficiently thick, for example up to 1 &mgr;m, that it represents the actual diffusion barrier. A corresponding intermediate is layer system is described, inter alia, in U.S. Pat. Nos. 5,739,086 and in 5,741,377.
Such a conductor structure is thus subject to the requirement for deposition of crack-free intermediate layers, which are resistant to diffusion, on metal strips. Furthermore, a biaxially textured metal strip must be assumed in order to achieve the heteroepitaxy which is important for high critical current densities J
c
, in order in this way to allow the texture of the strip to be transferred to the superconducting material. The aim of this is to allow the production of conductors in the form of strips and having a long length that are coated with superconducting YBCO material or a corresponding HTS material.
High critical current densities J
c
in the YBCO layer of at least 1×10
5
A/cm
2
and with the YBCO material having a layer thickness of, for example, 0.8 &mgr;m are required for the applications which have been conceived of so far for loss-free transmission of high currents, for example in the form of wound strips in solenoid coils or transformers, which are then subject to high magnetic fields. This could be achieved only by aligning the crystallites of the YBCO layer in the same way in the crystalline a-b plane since then, as is known, small angle grain boundaries result in a high current carrying capacity. This is also referred to as a biaxial texture. This leads to the necessity for the biaxial structure which already exists in the mount to be transferred through the intermediate layers to the YBCO. What is referred to as the full width at half maximum (FWHM) in an x-ray “Phi scan” is used as a measure of the quality of the biaxial texture; this should not significantly exceed 10° for the intermediate layer system. Corresponding experiments are described in the
Journal of Materials Research,
Volume 7, No. 7, July 1992, pages 1641-51, in particular pages 1644-1645.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a high-temperature superconductor structure and a corresponding production method, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which specifies an intermediate layer system that satisfies the requirements ment

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