Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2002-03-28
2004-04-06
Dunn, Tom (Department: 1725)
Stock material or miscellaneous articles
Composite
Of inorganic material
C505S237000, C505S238000
Reexamination Certificate
active
06716545
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to high temperature superconductors (HTS) and to their use. This invention is the result of a contract with the United States Department of Energy (Contract No. W-7405-ENG-36).
BACKGROUND OF THE INVENTION
One process in the production of coated conductors (superconductive tapes or films) has been referred to as a thick film process where the thickness of the superconductive layer is generally at least one micron in thickness. In the thick film process, it has been shown that YBCO thin films on single crystal substrates can achieve critical current density (J
c
) values of over 10
6
amperes per square centimeter (A/cm
2
) at 77 K. For non-epitaxial or amorphous substrates, the use of a suitable buffer layer to provide the necessary structural template has been previously developed.
One method of producing the coated conductor has been to deposit a textured film of a suitable buffer material onto a random, polycrystalline metal substrate. To obtain the best results, the textured film is deposited upon the metal substrate by ion-beam-assisted deposition. The preferred material for deposition by IBAD is magnesium oxide (MgO). IBAD MgO is an attractive process for making a textured film on a polycrystalline substrate because it is relatively fast and, hence, commercially feasible to scale into an industrial production process. The use of MgO has replaced the use of yttria-stabilized zirconia (YSZ). YSZ was the first material to be successfully deposited via IBAD to obtain a good textured film on which high quality YBa
2
Cu
3
O
y
(YBCO) was deposited with a high critical current density (J
c
) and high critical current (I
c
). The use of MgO rather than YSZ speeds up the process of forming a similar quality film by as much as 100 times. One reason that limited the move to the use of a MgO layer was difficulties with materials compatibility, i.e.; MgO tended to react with other materials in the various layers. For example, while IBAD YSZ layers were largely unreactive with the subsequently deposited superconducting films, MgO films have been found to react with a YBCO layer.
Thus, further improvements in the coated conductors have been desired. After extensive and careful investigation, improvements have been found to the preparation of superconducting films on polycrystalline substrates such as flexible polycrystalline metal substrates with an IBAD MgO layer.
SUMMARY OF THE INVENTION
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention provides a composite target material suitable for ablative deposition of a Cu-containing MgO layer upon a target substrate, said composite target material comprising: an intimate blend of MgO and CuO, said ratio of CuO:MgO being from about 0.01:100 to about 15:100 by weight.
In another embodiment, the present invention provides a composite target material suitable for ablative deposition of a Cu-containing (Sr
1−x−y
Ba
x
Ca
y
)TiO
3
layer upon a target substrate comprising: an intimate blend of CuO, TiO
2
, and one or more oxides selected from the group consisting of SrO, BaO and CaO, said ratio of CuO: SrO, BaO and CaO and TiO
2
being from about 0.01:100 to about 15:100 by mole percent.
In another embodiment, the present invention provides a superconducting structure comprising: a substrate, at least one intermediate layer upon said substrate, and a copper-containing superconducting layer upon at least one intermediate layer, wherein at least one intermediate layer is formed from a copper-containing composite target material whereby said copper within said intermediate layer does not adversely affect intermediate layer structure while reducing migration of copper from said copper-containing superconducting layer into said intermediate layer.
In another embodiment, the present invention provides a superconducting structure comprising: a substrate, at least one intermediate layer upon said substrate, and a superconducting layer of YBCO upon said at least one intermediate layer, wherein said superconducting layer includes excess copper as hereby loss of copper from said superconducting layer to said at least one intermediate layer does not lead to a copper deficient superconducting layer with diminished superconducting properties.
DETAILED DESCRIPTION
The present invention is concerned with high temperature superconducting composites. Specifically, the present invention is concerned with the addition of copper or excess copper to one or more of the layers of a superconducting composite structure to maintain high performance of a copper based superconducting layer. While not wishing to be bound by the present explanation, it is believed that through addition of copper or excess copper to one or more of the layers of a superconducting composite, the copper content of the copper based superconducting layer can be maintained at more optimal levels. Addition of excess copper to a copper based superconducting layer can allow for loss of some copper from the superconducting layer to other layers of the superconducting composite without significant degradation of the superconducting properties. Addition of copper to one of the various template and buffer layers found within superconducting composites, such layers typically formed from copper-free targets, can reduce the tendency for copper to migrate from the superconducting layer into these other template and buffer layers and avoid significant degradation of the superconducting properties.
A MgO layer in a superconducting composite does significantly react with a nearby YBCO layer. Both Cu
+
and Cu
2
+ ions are able to substitute into a MgO-rich solid solution. There is also an intermediate compound, Cu
2
MgO
3
that can form. The typical architecture of an IBAD MgO coated conductor includes a number of template and buffer layers. It has been found that copper from the final layer in these composite structures, the YBCO superconducting layer, can diffuse into or through the various buffer and template layers and reach the MgO layer whereat substitution can occur. Copper depletion in the YBCO layer has adverse effects on phase development and ultimate superconducting properties. One approach to this problem is to add excess copper to the system to make up for any deficiencies that may arise in the copper-based superconducting film. Copper can be added in at least three differing manners. First, copper can be added directly to the MgO layer. Second, copper can be added into intervening buffer layers, e.g., a strontium titanate buffer layer. Third, copper, as excess copper from the required stoichiometric copper content, can be added directly to the YBCO, i.e., the copper-based superconducting film, to compensate for any loss during processing. Additionally, combinations of these three manners can be employed.
In the present invention, the high temperature superconducting (HTS) material is generally YBCO, e.g., YBa
2
Cu
3
O
7−&dgr;
, Y
2
Ba
4
Cu
7
O
14+x
, or YBa
2
Cu
4
O
8
, although other minor variations of this basic superconducting material, such as use of other rare earth metals as a substitute for some or all of the yttrium as is well known, may also be used. Other superconducting materials such as bismuth and thallium based superconductor materials may also be employed. YBa
2
Cu
3
O
7−&dgr;
is preferred as the superconducting material.
In the present invention, the initial or base substrate can be, e.g., any polycrystalline material such as a metal or a ceramic such as polycrystalline aluminum oxide or polycrystalline zirconium oxide. Preferably, the substrate can be a polycrystalline metal such as nickel. Alloys including nickel such as various Hastalloy metals, Haynes metals and Inconel metals are also useful as the substrate. The metal substrate on which the superconducting material is eventually deposited should preferably allow for the resultant article to be flexible whereby superconduc
Arendt Paul N.
Ayala Alicia
Foltyn Stephen R.
Groves James R.
Holesinger Terry G.
Cooke Colleen P.
Cottrell Bruce H.
Dunn Tom
The Regents of the University of California
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