Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
2000-07-14
2004-05-04
Dunn, Tom (Department: 1725)
Stock material or miscellaneous articles
Composite
Of metal
C428S697000, C428S699000, C428S701000, C428S702000, C428S930000, C505S236000, C505S237000, C505S238000, C505S239000
Reexamination Certificate
active
06730410
ABSTRACT:
TECHNICAL FIELD
The present invention generally relates to a method for controlling the surface character of bi-axially textured alloy substrates. The invention more particularly relates to the use of intermediate epitaxial films on textured metal substrates. More specifically, the present invention relates to the use of an epitaxial film deposited on a biaxially textured alloy substrate, the epitaxial film serving to stabilize the substrate surface against oxidation for subsequent deposition of an epitaxial film.
BACKGROUND OF THE INVENTION
Some high temperature superconductors require well-aligned crystallites with low angle grain boundaries in order to yield high critical currents at relatively high temperatures. For example, thin films consisting of polycrystalline yttrium-barium-copper-oxide superconductors (YBCO) can yield a critical current density exceeding 10
6
A/cm
2
at 77K, self field, when a substantial portion of the local grain boundary misorientations in the film are well below 10°, thereby mitigating the well known weak link behavior in current transport across boundaries between misoriented regions. This requirement for achieving high critical current densities in polycrystalline films can also be expressed as the YBCO film having a bi-axial texture in which, for example, the c-axis of each crystallite is substantially perpendicular to the film surface and the collective a-axes of all crystallites align in substantially the same direction in the plane of the film surface. To obtain an YBCO thin film with a good (for example, <<10° for a single crystal substrate) bi-axial texture, it can be deposited in an epitaxial manner on an oxide single crystal such as LaAlO
3
. This is not a commercially viable process for many applications since single crystal substrates cannot be economically produced in very long lengths or large areas. It is therefore more appropriate for industrial purposes to consider the epitaxial deposition of YBCO on a bi-axially textured buffer layer (often an oxide layer), which in turn has been deposited on a flexible metal substrate tape.
The flexible metal substrate can be used to provide a necessary template for texture and mechanical stability during handling and use in applications. Face centered cubic (fcc) metals and some alloys based on fcc metals are especially useful for substrate material, as they can be bi-axially textured using well known rolling deformation and annealing processes. A well-known texture in these metals and alloys is the so called “cube texture”, in which the c-axis of the substrate crystallites is substantially perpendicular to the substrate surface, and the a-axes align primarily along the tape direction. The cube texture can often be made with very low full-width at half-maximum (FWHM) values obtained from X-ray pole figures, an indication of collective alignment of both c- and a-axes of all crystallites. Under controlled rolling and annealing processes, these deformation textured metal tapes possess texture approaching that of single crystals. In practical application of the process, the FWHM texture is less than 10 degrees and more typically less than 8 degrees.
Nickel (Ni) is one fcc metal that can be made into thin substrates with a well-defined cube texture using the rolling and annealing process. Prior work has shown that oxide buffer layers can be deposited on a biaxially textured nickel surface using conditions under which nickel oxide is not stable, but the buffer layer (for example, CeO
2
or Y
2
O
3
) is stable, allowing the oxide to inherit the texture of the underlying nickel substrate (that is epitaxy). A limitation to use of Ni, however, is its ferromagnetic character that may preclude its use in superconducting tape for alternating current (AC) applications such as power transmission cables, motors, and transformers. In addition, Ni is mechanically weak following the typical annealing heat treatment used to form the cube texture.
For these reasons, Ni alloys and other alloys have been developed to make strong, non-magnetic bi-axially textured substrates. These alloys often have alloying elements such as chromium (Cr) or aluminum (Al) that have a tendency to form stable oxides under very low oxygen partial pressure (P
O2
). The growth of epitaxial layers on metal or alloy substrates is commonly carried out under P
O2
of less than 10
−17
Torr and 650° C., where the constituent elements such as Cr or Al will form surface oxides. The presence of these surface oxides can inhibit the growth of the epitaxial layers. This can be explained in part due to the fact that the surface oxides are typically randomly oriented on the textured alloy surface and can therefore interfere with a high quality epitaxial buffer layer deposition.
Texturing in HTS films has been demonstrated by epitaxial growth of the superconductor on appropriate templates. As used herein, “epitaxial” means that the crystallographic orientation of the superconducting film is derived from and directly related to the crystallographic orientation of the underlying template. Early work used single crystal oxide substrates as the HTS growth template. For many practical applications, the substrate must be flexible. A well-oriented template can be achieved by means of ion beam assisted deposition (IBAD) of oxide buffer layers on random polycrystalline metal substrates. High quality epitaxial superconducting films have been grown on such tapes. In alternative and preferred embodiments, texturing in substrates can be induced using the deformation texturing (“DeTex”) process (as set forth herein) and epitaxial deposition of buffer layer(s) and subsequent functional layer(s) can be accomplished as also described herein.
One major issue therefore relating to the epitaxial deposition of oxide buffer layers on a bi-axially textured substrate, whether a metal or an alloy, is the control of the oxygen partial pressure, or P
O2
. This is true for any buffer deposition technique, whether it is Physical Vapor Deposition (PVD) by pulsed lasers, sputtering, electron beam, or thermal evaporation, or by a non-vacuum process such as Metal-Organic-Deposition (MOD). The objective is to avoid formation of native oxide films on the surface of the substrate, thereby allowing the deposited buffer layer to nucleate and grow with the appropriate biaxial texture from the substrate surface. Some metals such as silver (Ag) have a natural ability to allow for growth using a great variety in P
O2
conditions, but suffer from other disadvantages such as being difficult to texture, having a large coefficient of thermal expansion (CTE), high price, low mechanical strength and the like. Others such as copper (Cu) will easily oxidize. In some metals (for example, Ni), the P
O2
can be carefully controlled at the deposition temperature to provide sufficient oxygen to stabilize the buffer layer but insufficient to oxidize the Ni due to basic thermodynamic considerations. Methods to control P
O2
below the thermodynamic stability limit for NiO formation have been developed. For this reason, Ni has been established as a good deposition surface for epitaxial oxide layers such as oxides of the rare earth metals (yttria, ytterbia, ceria and the like). As discussed above, however, a pure Ni substrate suffers from other deficiencies that preclude its use in various applications.
In the context of surface control of substrates, it would therefore be desirable to provide controlled methods and articles for producing a surface on nonmagnetic, high strength biaxially textured substrates upon which an epitaxial layer can be deposited, thereby overcoming the shortcomings associated with the prior art.
SUMMARY OF THE INVENTION
A method has been established to mitigate stable oxide formation until a first epitaxial (for example, buffer) layer has been produced on the biaxially textured alloy surface.
It is therefore an object of this invention to provide an improved process for producing biaxially textured epitaxial (for example, buffer) layers on biaxially textured metal or alloy
Annavarapu Suresh
Arendt Paul N.
Foltyn Stephen R.
Fritzemeier Leslie G.
Hans Thieme Cornelis Leo
Cooke Colleen P.
Dunn Tom
Electronic Power Research Institute, Incorporated
Fish & Richardson P.C.
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