Note: Descriptions are shown in the official language in which they were submitted.
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SUBSTRATES WITH IMPROVED OXIDATION RESISTANCE
Background of the Invention
The present invention relates to substrates for
superconductors, and more particularly to copper based
and copper-nickel based substrates, which have enhanced
oxidation.resistance, for the deposition of high
temperature superconducting copper oxide layers to form
superconducting coated conductors. Applicable
superconducting materials include YBCO (YBa2Cu30x, or
Yttrium-Barium-Copper-Oxide) and REBaZCu30X, in which the
Y component of YBCO has been partially or completely
replaced by rare earth (RE) elements. Other
superconducting phases of these yttrium and rare earth
superconductors, and other superconducting copper oxides
of the bismuth, thallum, and mercury families can also be
used.
YBCO (YBa2Cu30X) is an important superconducting
material for the development of superconducting tapes
that can be used in superconducting transmission cables,
superconducting current leads, superconducting magnets
for transformers, superconducting magnets for AC and DC
motor applications, and current limiters, as well as
other electrical conductors. These applications are based
on a basic property of a superconducting material: it
has no electrical resistance when cooled below its
transition temperature, and can carry an electric current
without power dissipation.
In the production of superconducting coated
conductors, thin substrate tapes (or foils) are typically
coated with a thin buffer layer, which in turn is coated
with a superconducting layer. A suitable heat treatment
is then performed to optimize the superconducting
properties of the superconducting layer. One of the
functions of the substrate is to impart mechanical
strength to the resulting superconducting tape. A second
function, which depends on the process type, is to act as
a template for a well-textured buffer layer. Compared to
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the substrate material, this buffer layer provides a much
better deposition surface for the superconductor layer in
terms of lattice match, texture, coefficient of thermal
expansion (CTE) and chemical compatibility. To obtain
S~good superconducting properties, the buffer layer is
preferably biaxially textured. In the particular case of
a cubic or tetragonal buffer layer material, the lattice
of the buffer layer is oriented such that the cube face
is parallel to the tape surface. In addition, the cube
edge in each crystallite is parallel to the cube edge in
all neighboring crystallites.
Some specialized techniques such as Ion Beam
Assisted Deposition (IBAD) or Inclined Substrate
Deposition (ISD) can deposit a biaxially textured buffer
layer on top of a random polycrystalline or even
amorphous substrate. In general, these deposition
techniques are very slow or are effective in only a
narrow region, and they are not suited for large scale
and economical manufacturing of superconducting coated
conductors. A more advantageous deposition method is the
epitaxial deposition of a biaxially textured buffer layer
(or YBCO superconducting layer) on top of a biaxially
textured metallic substrate. Examples of epitaxial
growth by vapor deposition, electroplating, or oxidation,
in which native oxide layers grow on parent metals, are
numerous and well known, as is the fact that many metals
can .form biaxial textures. Some of these biaxial
textures in metals are quite useful for deposition of
buffer layers and YBCO superconducting layers. For
example, a biaxial texture can be obtained in many
rolled, face-centered cubic (fcc) metals, which when
properly rolled and heat treated, result in a well-
developed, and very useful, texture. The cube faces are
parallel to the rolled surface and a cube edge typically
points in the same direction as the rolling direction.
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Such a texture is called a cube-on-cube texture, with a
crystallographic notation of (100)[001]. A second well
known cube texture is the Goss texture (100)[011].
Another well-known biaxial texture is the annealed brass
texture, often indicated by (113)[211]. These textures,
and many other biaxial textures, are also referred to as
sheet textures. In the following description of the
invention the (100)[001] texture will be referred to as
the ~~cube" texture.
One method to obtain a biaxially textured buffer
layer is to deposit the buffer layer epitaxially on a
biaxially textured substrate. In this method, the
substrate material needs to meet certain requirements.
The substrate must have a lattice constant and a
coefficient of thermal expansion (CTE) which are
compatible with the buffer layer material and also with
the superconducting layer. Ideally, the substrate will
yield a biaxial texture by simple thermo-mechanical
means. The substrates are preferably non-magnetic and
are electrically conductive at cryogenic temperatures,
that is, at temperatures between room temperature and
that of liquid helium, which is 4.2 degrees Kelvin. The
substrate must also be relatively strong at room
temperature, and oxidation resistant at elevated
temperatures. There are several metals, such as copper
or nickel, that can be biaxially textured by rolling a
selected copper or nickel stock, followed by a suitable
heat treatment. However, these pure metals have
significant drawbacks in that they are either
ferromagnetic (Ni) or are easy to oxidize (Cu). These
properties are detrimental to the superconducting
properties of an oxide layer and to the deposition of
buffer layers on the substrate.
Generally speaking, alloys are much more difficult
to biaxially texture than pure metals. It is known that
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some binary alloys (a single phase mixture of two metals)
can be made into a biaxially textured tape. One example
of a cube texture, which has been produced in an alloy,
is iron-nickel, but this alloy has proven to be
ferromagnetic. In addition, binary copper-nickel alloys
with small quantities of nickel have previously been
textured, however those working in the field believed
that the maximum Ni content in the Cu-Ni alloy should not
exceed 4.2 ~ nickel.
Non-ferromagnetic and oxidation resistant
biaxially textured alloys which are useful as substrate
materials for superconductors, have not been readily
available.
Summarv of the Invention
The present invention features biaxially textured
alloy articles having biaxially textured surfaces and
improved oxidation resistance. The alloys can have the
composition of Culoo-X-yNixEY, where the Ni content x can
vary, and the concentration y of the oxide former E can
vary between 0.1 and 25 atomic ~, with the balance, 100-
x-y, atomic % being copper. Preferably, x is from about
0 to about 45 atomic percent, more preferably from about
0 to about 48 atomic percent, and most preferably from
about 0 to about 50 atomic percent. When the magnetic
properties of the alloy are less important, x can be up
to 60 atomic percent.
The alloy articles of the present invention can be
for use as substrate materials for superconducting copper
oxide coatings, the combination forming a superconducting
copper oxide coated conductor. For use in YBCO or REBCO
coated conductors, for example, the oxidation resistance
of the substrate material needs to be as high as
possible. The alloys can be processed by thermomechanical
methods to form biaxially textured substrates. Oxide
formers, which form stable oxides, are included in the
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alloy to enhance its oxidation resistance during various
heat treatments. In most instances, the resulting
surface texture is a cube texture with no substantial
secondary texture. These alloy articles are non-
ferromagnetic and form good substrate materials for
subsequent epitaxial buffer layer and superconductor
layer deposition, for use in a variety of products.
The present invention provides for binary,
ternary, or quaternary alloy articles by the inclusion of
one or more additional oxide formers which can improve
the oxidation resistance of the alloy articles over
copper or copper-nickel alloys. This invention uses the
protective scale forming abilities of oxide former and
incorporates this capability into the substrate
production method. In certain aspects of the invention,
the properties of the alloy articles may be further
enhanced, without impairment of its biaxial surface
texture, by the inclusion of dispersed oxide particles,
which strengthen the alloy, or CTE-reducing materials
which tailor its CTE to its intended purpose.
There are several principles of importance in
selecting an oxide former and in some instances,
combinations of several oxide formers may be used. The
chosen oxide former must oxidize easily and form stable
oxides. In order to be effective, the oxides EO% (E
representing the oxide former) should be more stable,
both kinetically and thermodynamically, i.e. have higher
absolute energies of formation, than Cu or Ni oxides.
Examples of oxide formers with such stable oxides EOZ are
aluminum (Al), magnesium (Mg), titanium (Ti), zirconium
(Zr), hafnium (Hf), yttrium (Y), chromium (Cr), gallium
(Ga), germanium (Ge), beryllium (Be), silicon (Si) and
the rare earth elements lanthanum (La), cerium (Ce),
praseodymium (Pr), neodymium (Nd), Samarium (Sm),
europium (Eu), gadolinium (Gd), terbium, (Tb). dysprosium
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(Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium
(Yb), lutetium (Lu) and thorium (Th).
The oxide former must also be able to diffuse to
the surface of the substrate to form a protective scale,
a form of oxidation which is known as external oxidation.
The thickness of the protective oxide scale is less than
one micrometer, and preferably less than 0.2 micrometer,
since and the substrates are quite thin, (25 to 50
micrometers). The protective oxide scale prevents or
slows down oxygen diffusion into the substrate during the
buffer layer and/or superconducting layer deposition
process. Additionally, if some oxygen diffuses into the
interior of the substrate, it should bind to the oxide
former and form small oxide particles, a process that is
known as internal oxidation. Both types of oxidation of
oxide formers are beneficial to the substrate, but in
accordance with the invention, a thin, highly adherent,
stable oxide scale with low oxygen permeability is
needed.
The selected oxide former should not induce a
random orientation in the substrate, an undesired type of
texture, or a mixture of textures in which the undesired
textures exceed about 15 %. Desirable textures include,
among others, the cube texture, the annealed brass
texture, and the Goss texture. Typically, the cube
texture is preferred. Most oxide formers which form
stable, non-spalling oxide scales, are known to preserve
the cube texture in pure Cu only when present in
relatively small quantities, typically less than 1.0 to 3
atomic %. The absolute maximum level depends on the
nature of oxide former. In accordance with the
invention, a similar effect is observed in for
homogeneous CuNi alloys.
In accordance with another embodiment of the
invention, those alloys with higher percentages of a
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second oxide former are found to be quite beneficial when
used as a core material in a substrate or alloy article,
in combination with a sheath consisting essentially of
Cu, Ni, or CuNi and 0 to 3 atomic % of a first oxide
S former. The core comprises 0 to 100 atomic % of a second
oxide former and nickel with the balance being copper.
Preferably, the alloy includes from about 0 to about 45
atomic percent nickel, more preferably from about 0 to
about 48 atomic percent nickel, and most preferably from
about 0 to about 50 atomic percent nickel. When the
magnetic properties of the alloy are less important, the
nickel content of the alloy can be up to 60 atomic
percent. In a preferred embodiment the second oxide
former is included in an amount of 3 to 25 atomic % and
in another preferred embodiment the second oxide former
is A1, Hf, Yb, Ce, Ti, Zr, or a mixture of these and
forms 100 atomic % of the core. Because of the low
concentration of the first oxide former in the sheath,
the surface of the substrate can be biaxially textured.
The core thus provides the added second oxide former,
which diffuses towards the external surface of the sheath
and dorms a protective oxide layer without affecting the
biaxial texture, preferably a cube texture, in the
surface of the substrate or in any deposited buffer layer
or superconducting layer. The core also acts as a site
for oxygen adsorption during the various heat treatments,
or acts to repair damaged oxide sheathing by providing
essential elements. The overall composition of the
substrate is 0.1 to 25 atomic % of the oxide former and
nickel, with the balance being copper. Preferably, the
alloy includes from about 0 to about 45 atomic percent
nickel, more preferably from about 0 to about 48 atomic
percent nickel, and most preferably from about 0 to about
50 atomic percent nickel. When the magnetic properties
of the alloy are less important, the nickel content of
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the alloy can be up to 60 atomic percent. The first and
second oxide formers may be the same or different.
In another aspect, the invention features an
article that includes an alloy containing copper, nickel
and at least about one atomic weight percent of an
additional metal selected from Mg, A1, Ti, Cr, Ga, Ge,
Zr, Hf, Y, Si, Pr, Eu, Gd, Tb, Dy, Ho, Lu, Th, Er, Tm,
Be, Ce, Nd, Sm, Yb, La or combinations thereof. The
article has a biaxially textured surface, such as a cube
textured surface. The article can be a superconductor
substrate. The article can include at least about two
atomic weight percent of the additional metal. The
article can include at most about four atomic weight
percent of the additional metal. The article can include
at least about two weight percent aluminum and/or at most
about four weight percent aluminum. The article can
include at least about 25 weight percent nickel.
Preferably, the article includes from about 0 to about 45
weight percent nickel, more preferably from about 0 to
about 48 weight percent nickel, and most preferably from
about 0 to about 50 weight percent nickel. When the
magnetic properties of the article are less important,
the nickel content of the article can be up to about 60
weight percent.
In another aspect, the invention features an
article that includes an alloy containing copper, nickel
and aluminum. The alloy can further include an
additional metal selected from Mg, A1, Ti, Cr, Ga, Ge,
Zr, Hf, Y, Si, Pr, Eu, Gd, Tb, Dy, Ho, Lu, Th, Er, Tm,
Be, Ce, Nd, Sm, Yb, La or combinations thereof. The
article has a native oxide exterior formed of alumina.
The surface of the article can be biaxially textured,
such as cubic textured. The article can be a
superconductor substrate.
In another aspect, the invention features an
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article including an alloy of copper and at least about
25 weight percent nickel. The article has a biaxially
textured surface, such as a cubic textured surface.
Preferably, the alloy includes from about 0 to about 45
atomic percent nickel, more preferably from about 0 to
about 48 atomic percent nickel, and most preferably from
about 0 to about 50 atomic percent nickel. When the
magnetic properties of the alloy are less important, the
nickel content of the alloy can be up to 60 atomic
percent. The alloy can also include at least about one
weight percent of Mg, A1, Ti, Cr, Ga, Ge, Zr, Hf, Y, Si,
Pr, Eu, Gd, Tb, Dy, Ho, Lu, Th, Er, Tm, Be, Ce, Nd, Sm,
Yb, La or a combination thereof.
The present invention also includes several
methods for producing substrates with biaxially textured
surfaces and superconducting composites and their
resulting products. Among these products and methods are
the following.
A preferred embodiment which provides a process
for forming an alloy or article with a biaxially textured
surface by melting, includes creating a mixture of 0.1 to
atomic % of an oxide former and nickel with the
balance being copper. Preferably, the alloy includes
from about 0 to about 45 atomic percent nickel, more
25 preferably from about 0 to about 48 atomic percent
nickel, and most preferably from about 0 to about 50
atomic percent nickel. When the magnetic properties of
the alloy are less important, the nickel content of the
alloy can be up to 60 atomic percent. A cube textured
surface may be obtained if less than about 1 atomic % to
3 atomic % of the oxide former is used. The mixture is
melted to form a liquid which is solidified to form an
alloy. Remelting is optionally performed to enhance
homogeneity. The solidified alloy is shaped and
homogenized by heat treatment. Deforming the alloy by
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mechanical techniques, followed by a recrystallization
procedure, produces a controlled fine grain size. Again
deforming the alloy by mechanical techniques and heat
treating the deformed alloy produces a biaxial texture in
S the alloy. The substrate can optionally be annealed in a
low oxygen partial pressure atmosphere to form an
epitaxial oxide layer.
Another preferred embodiment of the invention
features a method to produce substrates with biaxially
textured surfaces by a sheath and core approach. A can
of a sheath material consisting of Cu, Ni, CuNi and 0 to
3 atomic ~ of first oxide former is prepared. A core is
prepared having 0 to 100 atomic ~s, and preferably 3 to
100 atomic ~ of a second oxide former, and nickel with
the balance being copper. Preferably, the core includes
from about 0 to about 45 atomic percent nickel, more
preferably from about 0 to about 48 atomic percent
nickel, and most preferably from about 0 to about 50
atomic percent nickel. When the magnetic properties of
the core are less important, the nickel content of the
alloy can be up to 60 atomic percent. The method includes
placing the core in the can and co-reducing the
combination to form an article. Planar deformation of
the article is performed to a reduction in thickness
between 85 and 99.9, followed by heat treating the
deformed article to develop a biaxial texture on the
surface of the article. The substrate can optionally be
annealed in a low oxygen partial pressure atmosphere to
form an epitaxial oxide layer.
Variants of the sheath and core approach are based
on a powder metallurgy or rolled foil approach. This is
an alternative to melting the alloy, and offers
advantages in processing capabilities and cost. A
thinner can may be used in the powder metallurgy variant
increasing the overall concentration of the second oxide
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former. For powder metallurgy, a powder mixture of 0 to
100 atomic % of a second oxide former and nickel with the
balance being copper, is placed into a can or container
having a composition as described for the general sheath
and core method. Preferably, the powder mixture includes
from about 0 to about 45 atomic percent nickel, more
preferably from about 0 to about 48 atomic percent
nickel, and most preferably from about 0 to about 50
atomic percent nickel. When the magnetic properties of
the powder mixture are less important, the nickel content
of the alloy can be up to 60 atomic percent.
A rolled foil approach uses individual foils of
Cu, Ni, and a second oxide former or alloys thereof,
which are stacked together and rolled together to form a
bar, which is the used as a core material or a wrapping
for a central core. Cores made in this manner are placed
inside a can having a composition as described for the
sheath and core method. Co-reduction, planar
deformation, and heat treatment of the core and can are
similar to that described for the general sheath and core
process. Oxidation resistant substrates, with biaxially
textured and preferably cube textured surfaces, are
produced.
Preferred embodiments of the powder metallurgy and
rolled foil approaches provide a method to strengthen the
substrate material using internal oxidation. The method
includes creating rolled foils or a powder mixture with a
combined concentration of 3 to 50 atomic % of an oxide
former and Ni with the balance being copper. Preferably,
the alloy includes from about 0 to about 45 atomic
percent nickel, more preferably from about 0 to about 48
atomic percent nickel, and most preferably from about 0
to about 50 atomic percent nickel. When the magnetic
properties of the alloy are less important, the nickel
content of the alloy can be up to 60 atomic percent. The
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Ni and Cu powders or foils contain a combined
concentration of 0.2 to 1 weight g oxygen. The powder
mixture or foils are placed into a can of composition as
described for the sheath and core method and are co-
y reduced to form an article. Co-reduction, planar
deformation, and heat treatment of the core and can are
similar to that described for the general sheath and core
process. Hiaxially textured oxidation resistant
substrates, preferably of a cube texture, are produced.
A small amount of the oxide former absorbs the oxygen in
the Ni and Cu powders to form oxide particles which
strengthen the article. When any oxide former is
internally oxidized into small oxide particles, a major
strengthening effect is obtained. These small oxide
particles, (between 0.002 and 1 micrometer in size) are
extremely efficient in pinning the movement of
dislocations, even when their volume percentage is as
small as 0.2-0.5%. This internal oxidation method forms
high strength alloy articles with biaxially textured
surfaces in accordance with the invention, which are
typically also oxidation resistant.
In another aspect of the invention, a substrate
material has a controlled coefficient of thermal
expansion (CTE) which makes it more compatible with the
relatively low CTE of superconducting oxides. In a
preferred embodiment, substrates with a biaxially
textured surface, preferably a cube textured surface,
high oxidation resistance and controlled CTEs are
provided in accordance with previously described aspects
of the invention. The substrates are produced by
performing any of the above mentioned methods of the
invention (i.e. the melt process, the sheath and core
process, or the powder metallurgy, rolled foil, or the
internal oxidation variants of the sheath and core
process) or prior art substrate-forming processes, with
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the additional step of placing one or more rods of Nb,
Ta, Ti, NbTi, NiAl, Ni3Al,V, Cr, Zr, Pd, Sb, or mixtures
thereof in the core or billet. The rod preferably occupy
between 5 and 40 volume % of the billet. The billet or
S core is processed according to the selected method into
the final substrate and then heat treated. The substrate
can optionally be annealed in a low oxygen partial
pressure atmosphere to form an epitaxial oxide layer.
This produces a substrate with a embedded low CTE rod
which reduces the overall CTE of the substrate. In
preferred embodiments, the overall CTE of the substrate
has a value of about 10-15 x 10-6/~C.
A method to reduce the deleterious effect of
surface groove formation during the texturing heat
treatment is also provided according to the invention, as
well as a substrate having improved surface smoothness.
This method is utilized with prior art substrate-forming
processes, or with any of the processes described above,
by selecting and completing one of the processes to make
a substrate, including the texturing anneal step. This
method adds the additional steps of rolling the substrate
formed by the selected process using no more than 3 low
reduction passes and very smooth rolls, and then low
temperature stress annealing the substrate in a
protective atmosphere without recrystallization. This
produces a substrate with a 5 to 50 nanometer Ra surface
roughness.
In another aspect of the invention, a process for
forming a superconducting composite begins by forming a
substrate with biaxially textured and preferably cube
textured surfaces from an alloy of 0.1 to 25 atomic ~ of
an oxide former, nickel, and the balance being copper.
Preferably, the alloy includes from about 0 to about 45
atomic percent nickel, more preferably from about 0 to
about 48 atomic percent nickel, and most preferably from
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about 0 to about 50 atomic percent nickel. When the
magnetic properties of the alloy are less important, the
nickel content of the alloy can be up to 60 atomic
percent. The substrate can optionally be annealed in a
S low oxygen partial pressure atmosphere to form an
epitaxial oxide layer. The substrate is coated with a
buffer layer, which is coated with a superconducting
oxide.
A product, according to the invention, can be a
superconducting composite having a substrate with
biaxially textured, and preferably cube textured,
surfaces formed from an alloy of 0.1 to 25 atomic ~ of an
oxide former, nickel and the balance being copper.
Preferably, the alloy includes from about 0 to about 45
atomic percent nickel, more preferably from about 0 to
about 48 atomic percent nickel, and most preferably from
about 0 to about 50 atomic percent nickel. When the
magnetic properties of the alloy are less important, the
nickel content of the alloy can be up to 60 atomic
percent. The substrate can optionally be annealed in a
low oxygen partial pressure atmosphere to form an
epitaxial oxide layer. A buffer layer is coated on the
substrate and a superconducting oxide layer is coated on
the buffer layer.
Brief Description of the Drawin4s
These and other features of the invention will
become more readily apparent from the following detailed
description together with the accompanying drawings in
which:
Fig. 1 is a block diagram illustrating a process
of forming a biaxially textured alloy.
Fig. 2 is a block diagram illustrating a sheath
and core approach for forming a biaxially textured alloy.
Fig. 2A illustrates foil rolling.
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Fig. 2B illustrates a rolled foil as a wrap
material for a core.
Fig. 2C illustrates a rolled foil as a core for a
can.
Fig. 3 is a block diagram illustrating a powder
metallurgy variant of the sheath and core approach for
forming a biaxially textured alloy.
Fig. 4 is a block diagram illustrating an oxide
dispersion process for forming a biaxially textured
alloy.
Fig. 5 is a block diagram illustrating a process
for forming a biaxially textured alloy with a reduced
thermal expansion coefficient.
Fig. 6 is a block diagram illustrating a process
for forming a biaxially textured alloy with reduced
surface grooving.
Fig. 7 illustrates a partial cross-sectional view
of a substrate with a sheath and a powder metallurgy
core.
Fig. 8 illustrates a partial cross-sectional view
of a substrate with a sheath and a core.
Fig. 9 illustrates a partial cross-sectional view
of a superconductor composite formed with a biaxially
textured alloy substrate and textured buffer layer.
Fig. 9A and 9B illustrate partial cross-sectional
views of a superconductor composite with multiple buffer
layers.
Fig. 10 illustrates a partial cross sectional view
of a composite similar to the one illustrated in
conductor as in Fig. 9, in which the core includes a
material with a low CTE.
Fig. 11 illustrates a (111) pole figure of a cube
textured alloy made in accordance with the invention.
Description of the Embodiments
The present invention features an alloy with a
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biaxial texture which has the following composition:
Culoo-X-yNixEy.
where the Ni content x is preferably from about 0
to about 45 atomic %, more preferably from about 0 to
about 48 atomic %, and most preferably from about 0 to
about 50 atomic %. When the magnetic properties of the
composition are less important, the composition can
include up to about 60 atomic % nickel. The content of
oxide former can vary between 0.1 and 25 atomic %, with
the balance 100-x-y being atomic copper. For Cu-Ni
alloys, the enhanced Ni content achieves many of the
desirable features in the alloy, such as an increased
oxidation resistance, decreased CTE, and increased room
temperature and high temperature strength.
The third alloying element, an oxide former, which
is a stable oxide former, is preferably added to the
binary CuNi alloy to give a ternary alloy having enhanced
oxidation resistance. Alternatively, an oxide former E
can be added to Cu without adding Ni. Further, a
combination of various oxide formers E1, E2, E3 etc. can
be used so long as the total of their concentrations does
not exceed 25 atomic %. Suitable oxide formers which can
be used with the present invention include A1, Mg, Ti,
Cr, Ga, Ge, Zr, Hf, Be, Y, Si and the rare earth elements
La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu
and Th, and mixtures thereof, which form a protective
scale on the alloy.
When the magnetic properties of the composition
are relatively important, the upper limit of the atomic
percentage of nickel (e.g., 50 atomic %) can be
established to minimize the risk of developing
ferromagnetic properties in the alloy which are
detrimental to the superconducting properties of a
supported superconducting oxide layer. Further, by
remaining below this upper limit, a strong biaxial
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texture of the alloy can be maintained, which can allow
the alloy to be advantageously used as a substrate for
superconducting applications, even though small amounts
of oxide former E are added to the Ni-Cu mixture. With
S the appropriate buffer layer material, the Cu-Ni alloy
substrate. does not adversely affect the superconducting
properties of a supported layer E.
In certain embodiments, the alloy can include at
least about one weight percent of the oxide former and
still have a biaxially textured surface. In these
embodiments, the article can include at least about two
weight percent of the oxide former and/or at most about
four weight percent of the oxide former.
In other embodiments, the invention features an
alloy, or an article formed from the alloy, that includes
copper and nickel, and has a native oxide exterior formed
of alumina (i.e., the alumina exterior is grown by, for
example, exposing the alloy to oxidizing conditions,
rather than forming the alumina exterior by depositing
alumina). In these embodiments, the alloy can have a
biaxially textured surface, such as a cubic surface.
As described above, many metals and alloys with
various structures can be biaxially textured, and a
number of these have been of interest for use as
substrate materials. A particular case are metals or
alloys with the face centered cubic (fcc) structure,
which have been processed to display a biaxial cube
texture, often indicated by the crystallographic notation
(100)[001]. These can typically be formed by sheet or
tape rolling of a suitable metal or alloy, followed by an
appropriate heat treatment. These textured alloys are
particularly useful as substrate materials for
superconducting composites, and such metals or alloys
with a cube texture have crystallites in which the cube
faces are parallel to a tape surface, while one cube edge
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of the parallel cube faces points in the rolling
direction.
The effectiveness of the substrate is determined
by the quality of the biaxial texture. For example, in a
S.metal or alloy with a cube texture, the orientation of
the cube oriented grains should be within a few degrees
of the ideal orientation. Grains with an orientation
which deviates substantially from the cube texture should
be small in number, preferably below 15 volume %. The
quality of these textures is revealed in their X-ray
diffraction pole figures, which show the collective
orientations of all crystallites in the irradiated area.
Random orientations show no concentration of directions.
Quality textures, on the other hand, show sharp peaks in
the pole figures. The sharpness of these peaks is
expressed as a Full Width Half Maximum (FWHM) value and
is indicative of texture quality. The lower the FWHM
value, the sharper~the peak, and hence, the better the
texture. For polycrystalline materials, a FWHM value of
well below 10° is considered good. Higher FWHM values can
be acceptable for substrate purposes, in particular if
the reduced texture is offset by advantages in other
areas, such as improved oxidation resistance, or better
chemical or CTE compatibility. In general, alloy
substrates result in FWHM values that are often higher
than the FWHM values of the similarly treated pure metal
constituents, but the alloying does lead to advantages as
mentioned before, such as a non-magnetic substrate,
improved oxidation resistance, improved CTE etc. For
cube textured alloys of the present invention, FWHM
values in the range of about 6° to about 14° may be
obtained.
An example where a higher percentage of oxide
former leads to a different, i.e., not cube, but still
useful biaxial, texture is the annealed brass texture,
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often indicated by the notation (113)[211], and sometimes
by (227)[734]. Both notations indicate the same biaxial
texture in which the (113) or the nearly identical (227)
plane is parallel to the rolling surface. Instead of a
square cube face, the unit cell is now a square with
distorted right angles. This distortion is about 5-6°,
which is sufficiently low to make this texture useful for
substrate purposes. The sides of this distorted square
are 5-6~ longer, which is advantageous for Cu, Ni or CuNi
alloys where the actual lattice constant is about 10$
short of the ideal.
In accordance with a preferred application of the
substrates of the invention, superconducting composites
are formed using the above described alloy articles with
biaxially textured surfaces as substrates and by coating
at least one surface of each substrate with a
superconducting oxide. Preferably, a protective oxide
scale is formed on the substrate prior to or during the
coating process. The coating can include, for example, a
superconducting oxide such as yttrium-barium-copper-oxide
(YBCO) or a rare earth barium copper oxide (REBCO) or
mixtures of the two classes, wherein the YBCO yttrium is
partially or completely replaced by rare earth elements
such as lanthanum, cerium, praseodymium, neodymium,
samarium, europium, gadolinium, terbium, dysprosium,
holmium, erbium, thulium, ytterbium, lutetium and
thorium. Other possible superconductor oxides include the
mercury, bismuth, and thallium families. The coating of
the superconducting oxide is preferably approximately 0.2
to 20 micrometers thick, more preferably 1-20 micrometers
thick, and is applied by deposition techniques such as
electroplating, non-vacuum solution deposition, chemical
vapor deposition, physical vapor deposition techniques
such as sputtering, laser ablation, thermal evaporation,
electron beam evaporation, metallorganic or sol-gel
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solution precursor methods.
A preferred precursor approach uses a
metallorganic triflouroacetate precursor solution. With
this approach, high temperature superconductor films are
spun or dip coated onto substrates and then reacted to
form the superconducting YBCO phase. The as-coated
precursor includes an oxy-fluoride film containing BaF2.
Heat treatment in a controlled atmosphere, such as that
disclosed in U.S. Patent No. 5,231,074 issued to Cima, et
al., fully incorporated herein by reference, decomposes
the BaFz phase and thereby crystallizes the film. This
allows the nucleation and growth of an epitaxial YBCO
film. Superconductor oxide films characterized by highly
textured morphologies and fully dense, homogenous
microstructures are capable of sustaining critical
current densities in excess of 104 A/cm2 at 77 degrees
Kelvin when prepared on non-lattice matched substrates,
and or critical current densities in excess of 106 A/cm2
at 77 degrees Kelvin when prepared on lattice matched
substrates.
The surface characteristics of the substrate for
receiving the superconducting oxide coating can be
improved by depositing a buffer layer (or multiple buffer
layers) in an epitaxial manner onto substrate. Any of
the deposition processes listed above for the
superconducting oxide can be used. Other methods are
also available as is well known in the field.
Alternatively, a buffer layer or part of a buffer layer
can be grown epitaxially from the alloy articles of the
present invention as a native oxide. Irrespective of how
the buffer layer is created, the buffer layer preferably
has a thickness of approximately 0.05 to 10 micrometers,
more preferably 0.2 to 0.8 micrometers. It can include a
single metal or oxide layer. The buffer layer can also
be a multiple layered structure. Preferably, according
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to the invention, the resulting superconductive structure
includes a biaxially textured substrate, on which a
biaxially textured buffer layer is deposited, using an
epitaxial deposition process, and onto which a biaxially
textured superconducting layer is deposited, again using
an epitaxial deposition process. A protective oxide
scale grown from the substrate in accordance with one
aspect of the invention may form all or part of a buffer
layer.
The buffer layer and superconducting layer can be
on one side or both sides of the substrate, and can
partially or entirely surround the substrate. The buffer
layer can be a metal layer or an oxide layer or
combinations of metal layers and/or oxide layers. Each
layer must provide the texture, chemical compatibility,
lattice constant, and proper CTE match for the underlying
substrate. For example, the buffer layer can be a noble
metal or noble metal alloy, or an oxide with a cubic
structure such as Ce02, Ybz03, or yttria-stabilized
zirconia ("YSZ"), or any combination of oxides and/or
metals. Importantly, when the buffer layer or multiple
buffer layers are deposited in an epitaxial process, the
biaxial texture of the substrate is transferred onto each
successive layer, and finally to the top layer which is
the YBCO or superconducting layer. A metal cap layer
can be provided on top of the superconducting layer.
The metals useful for the buffer layer and cap
layer are preferably noble metals or noble metal alloys.
By "noble metal" is meant a metal which is
thermodynamically stable under the reaction conditions
employed relative to the desired superconducting ceramic,
and/or which does not react with the superconducting
ceramic or its precursors under the conditions of
manufacture of the composite. The noble metal can be a
metal different from the metallic matrix elements of the
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desired superconducting ceramic. The noble metal can be
silver or a silver/gold alloy, but it can also be a
stoichiometric excess of one of the metallic elements of
the desired superconducting ceramic, such as yttrium.
5~ Silver (Ag) and silver alloys are the most preferred
noble metals. Other noble metals which can be used are
platinum, gold, palladium, rhodium, iridium, ruthenium,
rhenium, rhenium or alloys thereof. Suitable oxides such
as MgO, cubic A1203, yttria, YSZ, or rare earth oxides
such as Ce02, Yb203 etc . or mixtures of these are
typically stable oxides with a cubic structure.
Fig. 9 illustrates a partial cross-sectional view
of a superconducting composite structure 900 according to
the invention. The composite 900 in which the substrate
901 has a buffer layer 902 with a superconducting oxide
layer 903 coated on at least one side thereof. Figure 9A
illustrates a partial cross-sectional view of a
superconducting composite structure where the buffer
layer includes two layers 904 and 905. Layers 904 and
905 can be metal or oxide layers or any combination of
layers. A superconducting layer 903 is then deposited on
layer 905. Alternatively, as shown in Figure 9B, the
buffer layer can include three or more layers, in which
the substrate 901 is coated with a metal or oxide layer
906 which in turn is coated with additional metal or
oxide layers 907 and 908 before deposition of the
superconducting layer 903.
Biaxially textured alloys of the present invention
may be formed by several methods. These methods produce
an alloy of Cu or CuNi to which one or more oxide formers
(such as A1, Mg, Ti, Cr, Ga, Ge, Zr, Hf, Be, Y, Si, La,
Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Th)
are added. The substrates and alloy articles
according to the invention can be manufactured using a
number of different process, each having its own
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advantages. For example, the melting and sheath and core
methods for creating alloy articles of the present
invention are described with reference to Figures 1 and
2. Each of these methods is useful for forming
substrates and can be used, with some variations in other
processes, described in reference to Figures 3-6 and the
examples which follow.
Referring to Figure 1, a block diagram illustrates
a melt process 100 for forming a alloy article with a
biaxially, and preferably cube textured surface. The
alloy articles of the present invention are created by
first selecting and weighing the constituent metals in a
mixture an appropriate amount of nickel, 0.1 to 25 atomic
~ of an oxide former E (such as A1, Mg, Ti etc.), with
the balance being copper (Step 101). Preferably, the
alloy includes from about 0 to about 45 atomic % nickel,
more preferably from about 0 to about 48 atomic % nickel,
and most preferably from about 0 to about 50 atomic
nickel. When the magnetic properties of the alloy are
less important, the alloy can include up to about 60
atomic ~ nickel. When a cube texture is desired the
amounts of oxide former should not exceed 1-3 atomic
depending on which oxide former is chosen. This mixture
is then melted (Step 102) by various processes known in
the art, such as arc melting, induction melting, melting
in an electrical resistance furnace, or furnace heated by
gas or coal. Melting temperatures range from 900°C to
1250°C. A certain level of homogenization is achieved
during the melt process due to convection, mechanical
stirring, or stirring induced by the melting techniques
such as in an induction melter. The melting can
optionally be preferred in air, or under a protective
atmosphere such as nitrogen, argon, helium, high vacuum
etc. Melting can be repeated a few times to further
increase homogenization (Step 103). The melt is then
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cooled within the furnace and the solidified melt is
shaped, preferably into a bar. The bar is reduced in
diameter, by a factor of 1.3 to 5, by rolling, swaging,
drawing or extrusion, and is then heat treated to further
homogenize the alloy (Step 104). A further mechanical
reduction in diameter, by similar mechanical techniques
follows, to a size where the planar deformation process
will commence (Step 105). Before or at this stage, a
heat treatment can be applied to recrystallize the alloy
and obtain a fine grain size of approximately 5 to 70
micrometers, preferably greater than 10 micrometers and
less than 40 micrometers (also Step 105). Alternatively,
other methods can be utilized to achieve a fine grain
size, such as the rapid solidification of the alloy after
melting. The alloy article is now deformed in an axially
symmetric manner, such as, by extruding, swaging, drawing
or rod rolling to a smaller size, which can be round,
square, or rectangular (Step 106). Alternatively, the
melt can be cast and rolled directly into a plate shape.
The plate can be further homogenized with a suitable heat
treatment, rolled to a thinner size, and recrystallized
to induce a desired fine grain size.
The fine grained alloy article is then deformed
further by various planar rolling methods known in the
art (Step 107), to reduce the thickness of the stock by
at least 85~ but not more than 99.9. A
recrystallization anneal (Step 108) in a protective,
e.g., high vacuum, low oxygen or reducing atmosphere, at
temperatures exceeding 250°C but not more than 95~ of the
melting temperature, and preferably between 400 and
1200°C, produces the desired biaxial texture (100)[001].
The article is positioned to provide oxidation resistance
during subsequent uses, such as during deposition of
superconductor or buffer layers. Alternatively, the
article may be annealed (Step 109) to form a protective
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epitzxial oxide layer.
Rolling processes suitable for use with methods of
the present invention as shown in Figs 1 and 2, utilize
the following parameters. Rolling is typically performed
at room temperature, with a rolling speed of between
O.lOm/minute and 100m/minute. The reduction schedule
typically follows a constant strain per pass, with
reduction steps being set at between 5 and 40% per pass.
The resulting tape can be lubricated during rolling, or
rolled without any lubricant. Bidirectional rolling is
preferred. The tapes can be rolled with large diameter
rolls (3.5 to 8" or larger in diameter) or with small
diameter rolls (0.75" to 2" in diameter) which are
preferably backed up by larger rolls, in a so-called
four-high arrangement. An alternative to the four-high
arrangement is the cluster rolling mill. A planetary
rolling mill can be used as well.
Referring to Figure 2, a block diagram illustrates
a process 200 for forming a biaxially textured alloy with
improved oxidation resistance, which uses a sheath and
core approach. A sheath is biaxially textured, which,
for example, can be a cube texture, while the core
provides a high concentration of oxide former needed to
provide the oxidation resistance during the subsequent
buffer layer and superconductor deposition processes.
For the sheath and core approach, a thick walled can
(Step 201) is made of CuNi or Ni stock, or alternatively,
of Cu, CuNi or Ni, with small (0.1 to 3 atomic %)
additions of an oxide former E. The thickness of the
wall is between about 5% and about 90% of the can outside
radius. A core is made to fit inside the can using a
melt process or one of the variations described below.
(Step 202). The core includes nickel, alloyed with 0 to
100 atomic % of an oxide former E (such as A1, Mg, Ti,
etc.), (preferably 3 to 100 atomic % of the oxide former
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E), and the balance copper.
In one variation, known as the "rolled foil" or
"jelly roll" variation, as shown in Fig. 2A, individual
foils 220a-220b of Cu, Ni, an oxide former or alloys
5- thereof 220c, can be stacked together and rolled into a
bar 222, a so called "jelly roll", which can be used as a
core material or a wrapping for a central core. Aluminum
is a particularly useful oxide former in making rolled
foils, due to its deformability. In Fig. 2B, the rolled
foil bar 222 is illustrated inside an outer layer of can
226 and is a wrap material for a core 228. In Fig. 2C,
the rolled foil 222 is illustrated inside a can 226
process and is the core for the can.
Referring to Figure 3, a block diagram illustrates
a process 300 for forming an alloy substrate with a
biaxially textured surface and improved oxidation
resistance, which uses a Powder Metallurgy variant of the
sheath and core approach. This is one of the preferred
embodiments of the general sheath and core method
illustrated in Figure 2. A sheath is worked into the
desired biaxial texture while a powder metallurgy core
provides the high concentration of oxide former needed to
provide the oxidation resistance during buffer layer and
superconductor layer deposition. For this approach 300, a
thick walled can (step 301) is made of Cu, CuNi or Ni
stock, or alternatively, of Cu, CuNi or Ni with small
(0.1 to 3 atomic %) additions of a first oxide former E
as generally described in Step 301. The thickness of the
wall is between about 5% and 20% of the can outside
diameter. The can is filled with a mixture of elemental
powders (step 302) or alternatively, pre-alloyed powders
including 0 to 100 atomic % of one or more second oxide
formers, nickel and the balance copper. Preferably, the
alloy includes from about 0 to about 45 atomic % nickel,
more preferably from about 0 to about 48 atomic % nickel,
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and most preferably from about 0 to about 50 atomic %
nickel. When the magnetic properties of the alloy are
less important, the alloy can include up to about 60
atomic % nickel. The oxide formers may be the same or
S different. The powder mixture is poured into the can at
tap-density (Step 302), or is compacted into the can
using a press with a compacting ram.
Each elemental or alloy powder should have the
ability to deform well when consolidated into a powder
mixture. The powders are then deformed to high areal
reductions in order to form the substrate. Many
elemental and alloyed fcc powders have been found to be
well suited. Some hexagonal powders, such as Mg, are
more difficult to deform and are easier to incorporate in
1S the as-alloyed fcc solid solution, such as Cu-2 atomic %
Mg. The same is true for an element such as, for
example, Ga which is difficult to deform, but readily
melts at ambient temperature processing. An alloy such
as Cu-5 atomic % Ga has been found to deform very well up
to high areal reductions; an atomized Cu-5 atomic % Ga
powder has been found to be the ideal way to incorporate
this element in the core of the substrate material.
Other oxide formers, like Y, are also difficult to
deform, and require deformation at elevated temperatures
2S if an elemental incorporation is desired. The powder
approach has more flexibility in choice of composition
because the powder mixture can, in principle, have a very
wide compositional range without adversely affecting the
ability to mechanically deform the mixture. The
advantage of the powder metallurgy approach is the
reduced work hardening rate when using elemental powder
mixtures for a core compared to the melt processed core
approach. The compositional range of the powder core is
larger than with the melt processed core approach, with 3
3S to 50 atomic % preferred.
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Cores formed by a melt process, by a powder
metallurgy process, or by the rolled foil process of
Figs. 2A-2C, are placed inside the can and the assembly
is evacuated, sealed, and extruded, swaged, drawn, or
rolled to a smaller cross sectional bar or tape (Step
203). This is processed further to a desired starting
size to enable for the planar rolling to commence (Step
204). The resulting bar, wire, tape, sheet or foil is
deformed in a planar manner such as rolling (Step 205),
to a reduction in thickness of between 85% and 99.9%. A
partial cross sectional view of the substrate 700 in this
stage is shown in Fig. 7, with a powder metallurgy core
702 inside of a can 701. Example seven discusses the
details of a process that uses a copper can and a Cu+37
atomic % A1 PM core. In Fig. 8 a partial cross-sectional
view of a substrate 800, in this stage of the process,
shows a core, such as a melt process core, 802 inside a
sheath 801. A heat treatment (Step 205) follows in order
to develop biaxial texture on the surface of the sheath,
and to induce homogenization in the substrate.
Temperatures can range from 250°C to as high as 95% of the
melting temperature of the substrate. The oxide former
will diffuse towards the surface of the substrate, but
reach the surface after the biaxial texture has been
developed on its surface. The enrichment of the surface
layer with oxide formers will therefore not adversely
affect the quality of the established cube texture. Upon
diffusion, the oxide former is positioned to provide
oxidation resistance during the subsequent buffer layers
and superconductor deposition processes. Alternatively,
the textured substrate can be annealed (Step 207) in a
gas flow with a low oxygen partial pressure (typically
between 0.01 and 5 vol% oxygen) to form an epitaxial
oxide layer which is part of the buffer layer, or can
serve as the buffer layer needed for the later
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superconductor deposition process.
When using a copper sheath, a recrystallization
step at approximately 300°C remains possible before
commencing the rolling, to refine the Cu grain size to 5
to 50 micrometers. The refined grain size is beneficial
to obtain a cube texture in the rolled and heat treated
tapes.
With small amounts of first oxide former (less
than 1-3 atomic ~) in the sheath, a same or different
oxide former can be added in large concentrations
(typically 3 to 25 atomic ~) to the core. Sheaths
without oxide formers may also be used. Pure elemental
cores are also possible for certain oxide formers, such
as A1, Yb, or Hf, Ce, Ti, Zr, or mixtures of these
because of their deformation ability. A high quality
biaxial, and in preferred embodiments, cube texture is
obtained on the surface of the alloy article, where it is
needed for the subsequent epitaxial deposition of buffer
layers. The core supplies the oxide former, which
diffuses from the core to the surface of the substrate
after the texturing is completed, to form the protective
oxide scale. This diffusion does not adversely affect
the biaxial texture in the surface of the substrate.
Some oxide formers, such as A1, form epitaxial cubic
oxide layers at the surface, which can be successfully
incorporated into the buffer layer, or even form the sole
buffer layer of the superconducting composite.
Referring to Figure 4, a block diagram illustrates
a process 400 for forming an alloy article with a
biaxially textured surface and improved oxidation
resistance, and which uses a variation on the powder
metallurgy embodiment or the rolled foil embodiment of
the sheath and core process. When selecting the starting
powders or foils (Step 401), a Ni and/or Cu powder or
foil is chosen that contains 0.2 to 1 weight ~ oxygen.
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Although Fig. 4 shows 0 to 60 atomic % nickel, other
appropriate ranges of nickel can be used (e.g., 0 to 45
atomic %, 0 to 48 atomic % or 0 to 50 atomic %). Oxygen
is often a common contaminant in commercial powders,
S~especially Ni powders which often have an oxygen content
of around 0.6 weight %. The presence of oxygen can be
exploited by using it for the internal oxidation of some
of the oxide formers. Additional powders or foils, such
as an oxide former which is easily deformable, or a pre-
alloyed Cu powder or foil, is selected for a total
concentration, with the oxygen-containing powders or
foils, of 3 to 50 atomic % oxide former, and the balance
copper (Step 402). The composite is to be processed with
the oxygen-containing starting powders or foils. For
example, a Cu can is packed with a powder mixture that
includes 60 atomic % Cu-25 atomic % Ni-15 atomic % A1,
all in elemental powder form. The Ni powder contains 0.6
weight % oxygen, and the oxygen in the Cu and A1 powder
is negligible. The processing is similar to the approach
illustrated in Figure 200, except that intermediate
anneals are not recommended to avoid premature hardening
of the substrate material (Step 403). During the final
heat treatment (Step 404) at temperatures which can range
from 250°C to as high as 95% of the melting temperature of
the substrate, the oxygen reacts to binds with a portion
of the oxide former to form an oxide dispersion
strengthened alloy. Thus, in the example, a small
percentage of the A1 is used to bind the oxygen in the Ni
powder into A1203 to strengthen the substrate. Any
remaining A1 which is available enhances the oxidation
resistance of the substrate. These oxide particles
generally occupy 0.2 to 2 vol % of the core material.
For this type of strengthening, also known as oxide
dispersion strengthening, the result provides a
sufficiently large volume percentage of oxide particles
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to significantly enhance both the room temperature and
high temperature strength of the substrate. Both types
of strength enhancement are important; room temperature
handling of the substrate, high temperature handling
during the various deposition processes, and then room
temperature handling of the final coated conductor in
subsequent cabling or winding operations.
Referring to Figure 5, a block diagram illustrates
a process 500 for forming an alloy with a biaxially
textured surface and an improved CTE matches among the
substrate, the buffer layer, and the superconductor
layer. The CTE of YBCO, for example, depends strongly on
its crystallographic orientation. In the a-axis
direction, the CTE of YBCO at room temperature is
approximately 11 x 10'6 /°C, in the b-axis approximately 8
x 10-6/°C , and in the c-axis direction, approximately 18
x 10-6/°C. Most thin film deposition techniques are
geared towards depositing a film with the c-axis
perpendicular to the film surface, so the CTE in this
direction is of less importance. That means that
substrates need to be matched to the much lower CTE
values of YBCO in the a and b direction. Ni has a CTE of
13.5 x 10-6/°C and Cu of 17 x 10-6/°C. These metals, being
cubic, have the same CTEs for the a, b, and c axes which
are isotropic. This means that both metals place a
compressive strain on the YBCO layer when the sample is
cooled from the reaction or deposition temperature (which
can range from 650°C to 850°C, depending on the deposition
process) to cryogenic temperatures. In the IBAD process,
which uses a nickel-chromium alloy substrate with a CTE
that is comparable to that of Ni, the compressive strain
is about 0.5~. When using elements with a higher CTE
such as Cu (17x10-6/°C) or Ag (19x10-6/°C) , the compressive
strain exceeds 0.5~ by a considerable margin, and the
risk of spalling and crack formation in the ceramic layer
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becomes unacceptable. Whenever a considerable mismatch
exists between the CTE of the primary substrate material
and either the superconducting layer or the buffer layer,
the high CTE, of the substrate can be reduced by
incorporating into the alloy another element with a much
lower CTE, such as Nb, Mo, Ta, V, Cr, Zr, Pd, Sb, NbTi,
an intermetallic such as NiAl or Ni3Al, or mixtures
thereof, but these materials do not typically alloy or
texture as desired for high temperature superconductor
applications. In accordance with the invention, the CTE-
reducing material is preferably included as a rod
embedded in the alloy. In one embodiment multiple CTE-
reducing rods may be used. Nb and NbTi are preferred
elements because they are quite ductile, and can be
deformed in a Cu matrix. The effect is roughly
proportional to the volume of the Nb or NbTi; but at
elevated temperatures, when the Cu or CuNi begins
yielding at very low strains, the influence of the work
hardened Nb is even stronger as Nb does not recrystallize
at temperatures below 1100°C. In other words, only a
small amount of Nb {CTE: 7.5x10-6/°C) is needed in the
substrate to make it an effective CTE reducing agent.
Typically the rod of CTE reducing material occupies 5 to
40 volts of the billet, with 10-20~ being preferred. In
accordance with the invention, an oxide former, such as
A1 or Mg, is included in the alloy that surrounds the
CTE-reducing rod to provide oxidation protection for the
rod during the buffer layer and superconductor layer
deposition processes. This approach to reduce the
overall CTE of the substrate can be used in any of the
substrate-forming processes discussed above (Step 501) or
in the prior art processes for forming superconducting
substrates.
In a preferred embodiment of the invention, one or
more rods of a CTE-reducing material are placed in one or
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more bores in the billet for process 100, or in the core
of the composite billet for processes 200, 300 or 400
(Step 502). The billet is processed into the final
substrate according to any of processes 100-400 (Step
S 503) with a standard texturing heat treatment. The final
substrate includes one or more rods of CTE-reducing
material which reduce the overall CTE of the substrate,
to preferably about 10-15x10-6/°C, the exact value
depending on the composition and the volume % of the
rods. However, because the rods are located inside the
substrate they do not impair any biaxial texture which is
developed on the surface of the substrate by the process
of the invention. An illustration of a partial cross
section of a substrate produced by this process is shown
in Fig. 10. In this figure, the center includes a rod of
CTE reducing material 1004, such as Nb, surrounded by the
substrate materia1.1001. A buffer layer 1002 completely
surrounds the substrate material 1001 and has a
superconducting layer 1003 on at least one side. In one
embodiment, the rod can be coated with a thin layer, such
as gold, which can prevent a reaction between the rod and
an alloy in the core.
Referring to Figure 6, a block diagram illustrates
a process 600 for forming a biaxially textured alloy with
improved surface smoothness it may be used as a final
step to smooth the substrate before commencing the buffer
layer deposition or coating superconductor. The surface
smoothness of the substrate is desirable aspect in the
deposition of a smooth, exclusively c-axis oriented
superconducting film (that is, with the c-axis normal to
the substrate surface), and has been shown to be
essential for YBCO films. If the surface roughness
exceeds 3-7 nm Ra the current carrying capability of the
film is strongly reduced. Rolling of substrate materials
as described heretofore can produce a very smooth
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surface, well within the 10 nm roughness range. The heat
treatment to bring out the texture however, nearly always
results in groove formation at the surface, located at
the grain boundaries. The grooving is conceivably caused
S by surface tension, which is reduced by a curved grain at
the surface, or by a diffusion of vacancies towards this
region in the surface. The grooving, often referred to
as "thermal grooving" results in a surface roughness that
can often exceed 100 nm Ra. Methods to remove these
grooves, such as mechanical or electro-polishing of the
substrate, remove substrate material as well, and can
lead to a loss in dimensional control.
In accordance with the aspect of the invention, a
low reduction rolling pass, following a recrystallization
heat treatment, restores the original surface smoothness,
while a low temperature stress anneal, at temperatures
below the recrystallization anneal, restores the high
quality biaxial texture to the surface of the substrate.
Any of the five processes 100-500 or a prior art
substrate forming process can be selected to make a
substrate with reduced surface grooving. The selected
process is first entirely completed, including the
texturing anneal (if any) (Step 601). The substrate is
subsequently rolled once or twice (Step 602) using a
reduction per pass of 5-30$, with rolls having an
extremely fine finish, such as tungsten carbide with a
2S-50 nm Ra surface roughness, or chromium-plated steel
rolls with a 5 nm Ra surface roughness. The substrate is
then given a low temperature stress anneal (Step 603), in
a protective environment which does not lead to a
recrystallization. A temperature range of 200-400°C is
typical. The resulting substrate has a very smooth
surface with a 5 to 50 nm Ra surface roughness and a well
developed, undisturbed, and well-preserved biaxial
texture.
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The following examples illustrate several
particularly preferred processes and structures according
to the invention.
Example 1
Copper metal of sufficient purity such as
Electrolytic Tough Pitch or Oxygen Free High Conductivity
Cu, Ni metal with a purity of more than 99%, A1 metal
with a purity of more than 98%, and Hf and Ti metals with
a purity of more than 98% are weighed to obtain a Cu-16
atomic % Ni-0.5 atomic % A1-0.05 atomic % Hf-0.05 atomic
% Ti mixture. The metals are in the form of powder,
chip, pellet, chunk, or rod, and enclosing can. The
weighed Cu, Ni, A1, Hf and Ti are put in a suitable
refractory crucible such as (but not limited to) alumina
or zirconia, and are melted together. Fox a clean melt,
an induction melter can be used, in which the melting is
done in vacuum or in a protective atmosphere, but melting
in air, and/or melting using other heater types such as
arc melting or the use of resistance furnaces are
possible. The alloy is remelted two or three times to
ensure additional compositional homogeneity. The melting
temperature is 1105°C. The cast is cleaned, and deformed
by rolling, swaging or extrusion to a smaller diameter
with sufficient size to allow subsequent deformation
processing. At this size, it is again homogenized by
holding the alloy at elevated temperatures for a few
hours to a few days, depending on temperature. Effective
temperatures should exceed 700°C. A preferable
combination is 12 hrs at 1000°C. The alloy bar is then
deformed by rod rolling, swaging, wire drawing or
extrusion to a smaller size, which is typically round or
rectangular in cross section, but can be oval or square
as well. All of these different cross sections have
been demonstrated to be equally effective for further
processing. The thinnest dimension typically varies
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between lmm and lOmm. The alloy wire, rod, tape or strip
is then rolled to a thin tape or foil. The reduction in
thickness is larger than 80% and can be as high as 99.9%.
One example is the extrusion of a homogenized 30.5mm or
15.7mm diameter bar to a 3.8 mm x 2mm tape. The tape is
rolled to 37 micrometers, a reduction in thickness of
98.1%. Another example is the swaging of a bar to a
diameter of 6.2mm and subsequent rolling to a thickness
of 250 microns, a reduction in thickness by rolling of
96.0%. The rolling is performed with a conventional wire
flattening mill. A wide variety of rolling conditions
have been used successfully. For example, we have rolled
the CuNi based substrate materials at 5%, 10%, 20% and
40% deformation per pass, using various lubrication
schemes, and at speeds as low as 0.1 meter per minute or
as fast as 100 meters per minute. In general, the lower
reductions per pass and lower processing speeds result in
somewhat improved textures.
The texturing anneal can be performed using a wide
range of temperatures, ranging from 250°C to close to the
melting temperature of the alloy (around 1105°C). The
higher temperatures require a shorter time and lead to
slightly better textures, but can increase surface
irregularities at the grain boundaries. This effect,
also known as thermal grooving, leads to depressions in
the surface at the grain boundaries due to surface
tension effects, and is undesired for high quality buffer
layers and superconducting layers. Lower temperature
anneals have a much lower rate of thermal grooving, but
also a less well developed texture. The temperature
range of 850-1000°C, for a period of 1 to 24 hrs, and
using a vacuum or protective atmosphere to avoid
oxidation of the substrate, are preferred conditions.
This process results in a substrate with a cube texture
and no substantial secondary textures, a FWHM value of 7-
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90.
The resulting thermal grooving is eliminated with
the following processing step. The texture annealed tape
is rolled once using very smooth rolls, typically with a
surface roughness of about 5 nm Ra, to a reduction of 5%
to 20%, with 10% being preferred. The substrate is then
stress annealed at low temperatures, 300°C being preferred
for the CuNi alloys, under protective atmosphere such as
a vacuum. This procedure does not adversely affect the
texture quality, or may improve it. It greatly enhances
the surface smoothness of the substrate material,
improving it to better than 5 nm Ra. The substrate is
then ready for the next step in the superconductor
manufacturing process, typically the application of a
buffer layer.
Fxample 2
Electrolytic Tough Pitch copper, nickel with a
purity grater than 99% aluminum with a purity greater
than 98% and hafnium and titanium with a purity greater
than 98% are weighed to obtain mixture containing 26.5
atomic % nickel, 0.5 atomic % aluminum, 0.05 atomic %
titanium, and 0.05 atomic % hafnium with the balance
copper. The weighed metals are placed in an alumina
crucible. To insure a clean melt the charge is vacuum
induction melted at 1250° Celsius and a vacuum of 50
millitorr, and cooled to room temperature. The alloy is
melted two more times to insure material homogeneity.
The melt is allowed to cool slowly, under vacuum, to
minimize voids due to shrinkage. The cast billet is 33
mm in diameter by 75 mm long. The billet is machined to
31.8 mm diameter to improve surface finish. The machined
billet is swaged to 16.8 mm diameter. After swaging the
billet is homogenized at 950° Celsius for 24 hours in a
protective argon 5% hydrogen reducing atmosphere. After
homogenization the billet is machined to 15.6 mm and
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hydrostatically extruded to a tape with a 2 mm by 3.8 mm
cross-section. The tape is then rolled with a constant
reduction of 0.127 mm per pass to 0.051 mm final
thickness, the reduction of the final pass being adjusted
S~as required to achieve the desired thickness. The
rolling is done on a four high wire flattening mill with
25 mm diameter work rolls and a speed of 3m per minute.
The finished tape is then annealed at 850° Celsius for 4
hours in a protective argon 5~ hydrogen reducing
atmosphere. This process produces a tape having a cube
texture d surface with a FWHM of 12°, and no substantial
secondary texture.
Exaarple 3
Electrolytic Tough Pitch copper, nickel with a
purity greater than 99~ and aluminum with a purity of
greater than 98% are weighed to obtain a mixture
containing 37 atomic ~ nickel, 0.5 atomic ~ aluminum,
with the balance copper. The weighed metals are placed
in an alumina crucible. To insure a clean melt the
charge is vacuum induction melted at 1280° Celsius and a
vacuum of 50 millitorr, and cooled to room temperature.
The alloy is melted two more times to insure material
homogeneity. The melt is allowed to cool slowly, under
vacuum, to minimize voids due to shrinkage. The cast
billet is 33 mm in diameter by 75 mm long. The billet is
machined to 31.8 mm diameter to improve surface finish.
The machined billet is swaged to 16.8 mm diameter. After
swaging the billet is homogenized at 1000° Celsius for 24
hours in a protective argon 5~ hydrogen reducing
atmosphere. After homogenization the billet is machined
to 15.6 mm and hydrostatically extruded to a tape with a
1.52 mm by 3.8 mm cross-section. The tape is then rolled
with a constant reduction of 0.127 mm per pass to 0.061
mm final thickness, the reduction of the final pass being
adjusted to achieve the desired thickness. The rolling
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is done on a four high wire flattening mill with 25 mm
diameter work rolls and a speed of 3m per minute. The
finished tape is then annealed at 850° Celsius for 4
hours in a argon 5~ hydrogen atmosphere. This process
produces a tape and a cube textured surface with a FWHM
of 14°, and no substantial secondary texture. Figure 11
shows the (111) pole figure for this material.
Example 4
An alloy comprising Cu-1.2 atomic ~ A1 is made
according to example 1. The alloy is made into a 16 mm
round bar, and is drilled along the axis to create a bore
in order to accommodate a 9.5 mm diameter Nb rod. This
CuAl+Nb composite billet is extruded to a 3.2 mm diameter
round exthudate, and subsequently drawn and rolled to
achieve a 97~ reduction in thickness. An anneal at 850°C
yields a biaxially textured substrate. The Nb core does
not interfere with the surface texture of the substrate.
The CTE for this composite material is measured to be
13.4x10-6/°C at room temperature. In the extruded
material, the volume ~ of the Nb in the composite is
determined to be 37.6 volume %. This percentage yields a
calculated average CTE of 13.4x10'6/°C using the Rule of
Mixtures, confirming the measured value. The Rule of
Mixtures predicts that the CTE of a composite material is
the average of the CTE of its components (which are
17.0x10-6/°C for CuAl and 7.5x10'6/°C for Nb) , taking into
account their relative volume percentages. This
demonstrates that the CTE of a substrate can be carefully
adjusted to provide an improved CTE match with the buffer
layer and superconducting layer.
Example 5
Electrolytic Tough Pitch copper with a purity
greater than 99~ and aluminum with a purity of greater
than 98~ are weighed to obtain a mixture containing 9
atomic ~ aluminum, with the balance copper. The weighed
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metals are placed in an alumina crucible. To insure a
clean melt the charge is vacuum induction melted at 1100°
Celsius and a vacuum of 50 millitorr, and cooled to room
temperature. The alloy is melted to more times to insure
material homogeneity. The melt is allowed to cool
slowly, under vacuum, to minimize voids due to shrinkage.
The cast billet is 33 mm in diameter by 75 mm long. The
billet is machined to 31.8 mm diameter to improve surface
finish. The machined billet is swaged to 16.8 mm
diameter. After swaging the billet is homogenized at
950° Celsius for 24 hours in a protective argon 5%
hydrogen reducing atmosphere. After homogenization the
billet is machined to 15.6 mm and hydrostatically
extruded to a tape with a 1.52 mm by 3.8 mm cross-
section. The tape is then rolled with a constant
reduction of 0.127 mm per pass to 0.061 mm final
thickness. the reduction of the final pass being
adjusted to achieve the desired thickness. The rolling
is done on a four high wire flattening mill with 25 mm
diameter work rolls and a speed of 3m per minute. The
finished tape is then annealed at 850° Celsius for 4
hours in a protective argon 5%hydrogen reducing
atmosphere. The finished substrate is heat treated at
830° Celsius using an oxidizing environment selected to
be typical of the environment utilized during one YBCO
deposition process, which is argon 1 vol oxygen gas,
followed by a 100% oxygen anneal at 400° Celsius. The
thin 40 micrometer thick substrate retains a biaxial
surface texture and is protected from the oxidizing
environment by the formation of a continuous native oxide
film.
Example 6
Electrolytic Tough Pitch copper with a purity
greater than 99% aluminum with a purity of greater than
98% are weighed to obtain a mixture containing 5 atomic %
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aluminum, with the balance copper. The weighed metals
are placed in an alumina crucible. To insure a clean
melt the charge is vacuum induction melted at 1080°
Celsius and a vacuum of 50 millitorr, and cooled to room
temperature. The allow is melted two more times to
insure material homogeneity. The melt is allowed to cool
slowly, under vacuum, to minimize voids due to shrinkage.
The cast billet is 33 mm in diameter by 75 mm long. The
billet is machined to 31.8 mm diameter to improve surface
finish. The machined billet is swaged to 16.8 mm
diameter. After swaging the billet is homogenized at
950° Celsius for 24 hours in a argon 5% hydrogen
atmosphere. After homogenization the billet is machined
to 15.6 mm and hydrostatically extruded to a tape with a
1.52 mm by 3.8 mm cross-section. The tape is then rolled
with a constant reduction of 0.127 mm per pass to 0.061
mm final thickness, the reduction of the final pass being
adjusted to achieve the desired thickness. The rolling
is done on a four high wire flattening mill with 25 mm
diameter work rolls and a speed of 3m per minute. The
finished tape is then annealed at 850° Celsius for 4
hours in an argon 5% hydrogen atmosphere. The finished
substrate is heat treated at 830° Celsius using an
oxidizing environment selected to be typical of the
environment utilized during one YBCO deposition process,
which is argon 1 vol% oxygen gas, followed by a 100%
oxygen anneal at 400° Celsius. The thin 40 micrometer
thick substrate retains a biaxial surface texture and is
protected from the oxidizing environment by the formation
of a continuous native oxide film.
Example 7
A Cu-14.4 atomic % A1 alloy is made using a
powder metallurgy sheath and core approach. A copper
powder made from electrolytic tough pitch copper, with a
particle size of 250 micrometers, and an aluminum powder
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made by gas atomization, with a purity of 99%, and a
particle size of 220 micrometers, are mixed in a ratio of
63 atomic % Cu and 37 atomic ~ A1. The well-mixed Cu+A1
powder is compacted into an oxygen free high conductivity
S.copper billet which has an external diameter of 30.5 mm
and an internal diameter of 21.5 mm. The billet is
evacuated and extruded to a 9 mm bar. The bar is drawn
through round and rectangular drawing dies to a final
dimension of 2.4 mm x 3.6 mm. This rectangular product
is subsequently rolled to a tape of 65 microns thick
(97.3 % reduction). This tape is two-step annealed at
600°C and 800°C under protective atmosphere. This yields
a Cu 14.4 atomic % A1 substrate with a cube textured
surface, which has excellent oxidation resistance.
