Superconductor structure with glass substrate and...

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Reexamination Certificate

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C428S702000, C428S699000, C428S689000, C505S238000, C505S239000

Reexamination Certificate

active

06309767

ABSTRACT:

BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a superconductor structure having a substrate, at least one buffer layer which has been deposited on the substrate, and a layer of a metal oxide superconductor material with a high critical temperature which has been deposited on the buffer layer, the substrate including a glass material which is sufficiently temperature-resistant to be able to withstand the maximum temperature during production of the buffer layer and the superconductor layer. The invention also relates to a current limiter device and a process for producing a corresponding superconductor structure. A structure and a corresponding production process of this general type can be found in “Physica C”, Vol. 267, 1996, pages 355 to 360.
Superconductive metal oxide compounds with high critical temperatures T
c
of over 77 K are known, and these compounds are therefore also known as high T
c
superconductor materials or HTS materials and, in particular, enable an LN
2
(liquid nitrogen) cooling technique to be used. Such metal oxide compounds include, in particular, cuprates of special material systems, for example of the Y—Ba—Cu—O or Bi—Sr—Ca—Cu—O types, in which the Bi component can be partially substituted by Pb. There can be a plurality of superconductive high T
c
phases within individual material systems, which phases differ through the number of copper-oxygen lattice planes or layers within the crystalline unit cell and have different critical temperatures.
It is desired for these known HTS materials to be deposited on different substrates for different applications, the aim generally being to form pure-phase superconductor material as far as possible. For example, metallic substrates are provided in particular for conductor applications. Furthermore, DE 195 20 205 A describes the general use of substrates made from glass material as supports for conductor tracks made from HTS material in current limiter devices. In order to allow textured growth of the HTS material, it is also known for a suitable buffer layer to be applied to that surface of the substrate which is to be coated with the HTS material.
The production of a biaxially oriented thin film made from the HTS material YBa
2
Cu
3
O
7−x
on various glass substrates is described in the literature reference from “Physica C” which was mentioned in the introduction. Materials with a coefficient of expansion &agr; of at most 4.6×10
−6
° C.
−1
were used for the glass substrate used in that document. These materials are so-called “hard glass”, since glass materials with &agr; values of below 6×10
−6
/K are generally referred to in this way, while glass materials with a &agr; value above this are referred to as “soft glasses” (see H. G. Pfaender: “Schott-Glaslexikon”, 1984, page 30). Moreover, the substrates of the known structure had a very small surface area to be coated, which was covered with oriented, Y-stabilized ZrO
2
. However, it has emerged that with the known structure it is only possible to achieve critical current densities J
c
of the order of magnitude of 10
4
A/cm
2
(in the zero field). Current densities of this level are regarded as being too low for many applications.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a superconductor structure with a glass substrate and a high-temperature superconductor deposited thereon, a current limiter device having the superconductor structure and a process for producing the structure, that overcome the above-mentioned disadvantages of the prior art devices and processes of this general type, in such a way that comparatively higher critical current densities can be achieved and industrial manufacture using commercially available, large-area, cost-effective glass materials is made possible.
With the foregoing and other objects in view there is provided, according to the invention, a superconductor structure, comprising a substrate, at least one buffer layer deposited on the substrate, and a layer of a metal oxide superconductor material with a high critical temperature deposited on the buffer layer, wherein the substrate is formed of a glass material which is sufficiently temperature-resistant to be able to withstand the maximum temperature during the production of the buffer layer and the superconductor layer, has a coefficient of thermal expansion which is greater than 6×10
−6
K
−1
, and has a transformation temperature of over 550° C.
The invention is based on the recognition that the (linear) thermal expansion coefficient of the glass material, which is to be regarded as “soft glass”, together in combination with the transformation temperature, which is of importance with regard to the maximum temperature required for the deposition or formation of the superconductor material, is the decisive variable with a view to obtaining a high critical current density J
c
. If a glass material having the claimed levels of expansion coefficient and transformation temperature is selected, it is advantageously possible to at least substantially avoid cracking in the HTS material, a phenomenon which was observed in the known superconductor structure described in the above-mentioned literature reference “Physica C”, since the claimed value of the coefficient of expansion is at least substantially matched to that of the HTS material, which is of the order of magnitude of 10×10
−6
K
−1
. A glass material with a coefficient of expansion of over 7×10
−6
K
−1
is therefore to be considered particularly advantageous. The measurement range for the above-mentioned values of the coefficients of expansion usually extends from 20° C. to 300° C.
Surprisingly, it has been found that commercial glass materials intended for applications in other specialist fields, such as for example in the field of safety glasses, with a sufficiently large surface area are available and, despite having a sufficiently high transformation temperature of over 550° C., preferably of at least 580° C., have the required high coefficient of expansion. In fact, the materials properties of commercially available glass materials with high coefficients of expansion mean that these materials generally do not have the required resistance to high temperatures such as those which are needed for the production of HTS materials. These problems arise, for example, in the case of known commercially available soda-lime glass materials, since the transformation temperatures of these materials are generally below 550° C.
Glass materials according to the invention which are suitable as the substrate material are relatively inexpensive, so that they can be used in particular for large-area substrates with a coatable area of at last 10 cm
2
, preferably over 100 cm
2
, as are to be provided in particular for current limiter devices. Specifically, in such devices a total surface area of HTS material of over 2 m
2
is required for a power of, for example, approximately 10 MVA which is to be limited.
Advantageously, a flat glass which has been produced in particular through the use of a drawing process is provided with a view to obtaining an inexpensive, large-area glass material.
If a thermally smoothed flat glass is used, the conditions for the deposition of a high-quality buffer layer which promotes the formation of a superconductor layer with a high critical current density are particularly good.
Advantageously, the glass materials selected may be special aluminosilicate glasses, since they are best able to satisfy the required combination of glass properties. Such a glass material preferably contains approximately 50 to 70% by weight SiO
2
and approximately 10 to 30% by weight Al
2
O
3
, it being possible for the above-mentioned values to deviate by ±5%.
Furthermore, the glass material used advantageously contains cerium, in particular in the form of a cerium oxide addition. This is because such an addition enables the transformation temperature of the glass mat

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