Note: Descriptions are shown in the official language in which they were submitted.
WO 2012/040368 CA 02809484 2013-02-25PCT/US2011/052616
METHODS FOR IN SITU DEPOSITION OF COATINGS AND ARTICLES
PRODUCED USING SAME
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of priority under 35 U.S.C. 119
from
United States Provisional Patent Application serial number 61/385,899, filed
September
23, 2010, which is incorporated herein by reference in its entirety.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR
DEVELOPMENT
[0002] Not applicable.
FIELD OF THE INVENTION
[0003] The present invention generally relates to coatings, and, more
specifically,
to methods for producing coatings.
BACKGROUND
[0004] Coatings are frequently used in a variety of applications to protect
the
materials beneath the coating from environmental exposure and/or to modify
other
physical properties of the materials beneath the coating. Physical properties
that can be
altered by a coating can include, but are not limited to, optical properties,
thermal
properties and mechanical properties. More specifically, thermal and
electromagnetic
absorption and emission properties of an article can be profoundly influenced
by the
presence of even a vanishingly thin layer of a coating deposited thereon.
[0005] A number of different techniques have been developed for depositing
thin
layer coatings. These techniques can include, for example, sputtering,
evaporative
deposition, pulsed laser desorption, electroplating, electroless plating,
chemical vapor
deposition, and the like. In the manufacture of articles, most of these
coating techniques
have to be conducted separately from other manufacturing steps used in the
fabrication of
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the article. Further, many of these deposition techniques can require
specialized
equipment that can increase the time and expense required for fabricating an
article
containing a coating.
[0006] In view of the foregoing, simple, low cost techniques for
producing a
coating, particularly during the fabrication of an article, would be of
substantial benefit in
the art. The present disclosure satisfies this need and provides related
advantages as well.
SUMMARY
[0007] In some embodiments, methods for depositing a coating on a
metal surface
are described herein. The methods include heating a metal surface to a
temperature not
greater than its melting point; while heating the metal surface, applying a
vacuum thereto;
and while heating the metal surface, releasing the vacuum and backfilling with
a first
purge gas. The first purge gas is reactive with the heated metal surface so as
to deposit at
least one layer of a coating thereon.
[0008] In some embodiments, methods for depositing a coating on a
solar receiver
are described herein. The methods include applying a vacuum to an annulus
having an
outer surface defined by a material that is at least partially transparent to
solar radiation
and an inner surface that is defined by a metal tube; heating the metal tube,
while
applying the vacuum thereto, to a temperature not greater than its melting
point; and,
while heating the metal tube, releasing the vacuum and backfilling with a
first purge gas,
where the first purge gas is reactive with the heated metal tube so as to
deposit at least
one layer of a coating thereon.
[0009] In some embodiments, solar receivers can be prepared by a
process that
includes applying a vacuum to an annulus having an outer surface defined by a
material
that is at least partially transparent to solar radiation and an inner surface
that is defined
by a metal tube; heating the metal tube, while applying the vacuum thereto, to
a
temperature not greater than its melting point; and, while heating the metal
tube,
releasing the vacuum and backfilling with a first purge gas, where the first
purge gas is
reactive with the heated metal tube so as to deposit at least one layer of a
coating thereon.
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[0010] The foregoing has outlined rather broadly the features of the
present
disclosure in order that the detailed description that follows can be better
understood.
Additional features and advantages of the disclosure will be described
hereinafter, which
form the subject of the claims.
BRIEF DESCRIPTION OF THE DRAWING
[0011] For a more complete understanding of the present disclosure,
and the
advantages thereof, reference is now made to the following description to be
taken in
conjunction with the accompanying drawing describing a specific embodiment of
the
disclosure, wherein:
[0012] FIGURE 1 shows a schematic of an illustrative solar receiver.
DETAILED DESCRIPTION
[0013] The present disclosure is directed, in part, to methods for
depositing
coatings on a metal surface. The present disclosure is also directed, in part,
to metal
surfaces having a coating deposited thereon, particularly solar receivers.
[0014] Although coatings are frequently used with great utility in a
wide variety
of applications, techniques for depositing coatings can considerably add to
the time and
expense required to fabricate articles containing a coating. The methods
described herein
can advantageously address these shortcomings in the art by providing simple
techniques
for preparing coatings on a metal surface. More particularly, in certain
cases, the coating
methods described herein can be used for the in situ deposition of a coating
on a metal
surface. That is, during the fabrication of an article, a coating can
advantageously be
applied to the article using simple modifications of at least some of the
operations that are
already being used for the fabrication of the article. For example, coatings
can be applied
to articles according to the present methods when a vacuum bakeout (e.g., a
hydrogen
bakeout) is used during fabrication of the article.
[0015] One example of an article containing a metal surface in which
a coating
can be deposited in situ during the article's fabrication is a solar receiver.
FIGURE 1
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shows a schematic of an illustrative solar receiver 100. Solar receiver 100
can be used as
the thermal energy collector in a parabolic trough solar receiver array, in
which solar
receiver 100 can absorb theunal energy from focused solar radiation, while
emitting as
little heat as possible back to the ambient environment. Illustrative solar
receiver 100
contains an inner metal tube 110, which has a heat transfer fluid (e.g., an
oil or like high
boiling fluid) flowing through its interior space in order to carry collected
heat away from
solar receiver 100. In order to maximize the amount of collected heat and
reduce thelinal
emission, inner metal tube 110 is typically surrounded by a vacuum. To this
end, solar
receiver 100 contains an outer surface 120 that is at least partially
transparent to solar
radiation (e.g., a glass), which defines armulus 115 together with inner metal
tube 110
and contains the vacuum. Vacuum can be applied to annulus 115 through port
130. Most
often, inner metal tube 110 is modified with a coating that increases its
thermal
absorptivity.
[0016] During the fabrication of typical solar receivers, inner metal
tube 110 is
heated using a heater (not shown), while applying vacuum to annulus 115 in
order to
achieve outgassing and desorption of species that would otherwise degrade the
applied
vacuum and potentially alter the thermal absorptivity. However, it has been
advantageously discovered according to the embodiments described herein that
if, instead
of sealing annulus 115 to maintain the vacuum therein, the vacuum is broken
and annulus
115 is backfilled with a purge gas while heating inner metal tube 110, an in
situ coating
can be applied to inner metal tube 110. Thereafter, fabrication of solar
receiver 100 can
be completed simply by re-applying vacuum to annulus 115, followed by sealing
to
maintain the vacuum. Therefore, the present methods offer the opportunity to
simply
modify the surface of inner metal tube 110 with a coating, using only simple
modifications of the operations already in place for fabrication of a solar
receiver.
[0017] As used herein, the term "vacuum" will refer to any pressure
that is less
than atmospheric pressure. Unless otherwise specified, the term vacuum should
not be
construed to constitute any particular magnitude of vacuum. In some
embodiments, a
suitable vacuum can be about 1 x 10-5 torr or lower. In other embodiments, a
suitable
vacuum can be about 1 x 10-6 torr or lower.
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[0018] As used herein, the term "coating" refers to a material on a
metal surface
that is at least a monolayer in thickness. Illustrative coatings that can be
applied to a
metal surface according to the methods described herein include, but are not
limited to,
metal oxide coatings, metal nitride coatings, metal carbide coatings, and
metal fluoride
coatings. Unless otherwise specified, the term coating should not be construed
to
constitute any particular type of coating or any particular number of layers
in the coating.
[0019] In some embodiments described herein, methods for depositing a
coating
on a metal surface can include heating a metal surface to a temperature not
greater than
its melting point; while heating the metal surface, applying a vacuum thereto;
and while
heating the metal surface, releasing the vacuum and backfilling with a first
purge gas that
is reactive with the heated metal surface so as to deposit at least one layer
of a coating
thereon. In some embodiments, the methods can further include depositing at
least one
layer of a coating thereon.
[0020] In general, any type of metal surface can be modified with a
coating
according to the embodiments described herein. In this regard, both pure
metals and
metal alloys can be used. In various embodiments, suitable metals can include,
without
limitation, titanium, copper, iron, aluminum, tungsten, and any combination
thereof.
Other suitable metals can be envisioned by one having ordinary skill in the
art. In some
embodiments, the metal surface can be polished to remove native oxides
therefrom, in
order to promote deposition of the coating. In some embodiments, the metal
surface can
be a metal tube such as, for example, in a solar receiver. In particular, in
the case of a
metal tube used in solar receiver, steels such as, for example, stainless
steel, carbon steel
or a combination thereof can be used.
[0021] Although certain embodiments of the present disclosure have
been
described in the context of a solar receiver, it is to be understood that any
type of article
having a metal surface can be modified with a coating according to the
embodiments
described herein. That is, the description herein regarding a solar receiver
should be
considered to be illustrative in nature and not limiting. More particularly,
it is to be
understood that any type of article having a metal surface can be modified
with a coating
according to the embodiments described herein by heating the metal surface and
either
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applying vacuum to the metal surface directly (e.g., via an annulus, cavity or
like opening
in the article) or indirectly (e.g., by placing the article or a portion
thereof in a heating
device such as a vacuum furnace, for example, that can be placed under vacuum
and
backfilled with a purge gas thereafter).
[0022] In various embodiments, methods for depositing a coating on
a metal
surface of a solar receiver can include applying a vacuum to an annulus having
an outer
surface defined by a material that is at least partially transparent to solar
radiation and an
inner surface that is defined by a metal tube; heating the metal tube, while
applying the
vacuum thereto, to a temperature not greater than its melting point; and while
heating the
metal tube, releasing the vacuum and backfilling with a first purge gas that
is reactive
with the heated metal tube so as to deposit at least one layer of a coating
thereon.
[0023] In the case of a solar receiver, various materials can be
used to form the
outer surface of the annulus. In general, the material forming the outer
surface of the
annulus needs to be at least partially transparent to solar radiation so as to
allow the solar
radiation to impinge upon the inner surface defined by the metal tube.
Further, the
material forming the outer surface of the annulus typically needs to have at
least some
degree of resistance to deformation under heating, since considerable heat can
be
generated when solar energy is focused upon the solar receiver. In some
embodiments, a
suitable material that is at least partially transparent to solar radiation
can be a glass. In
some embodiments, the glass can further include an anti-reflective coating
adapted to
minimize reflection therefrom so as to maximize the amount of solar radiation
transmitted to the metal tube.
[0024] After depositing the coating on the metal tube of the solar
receiver
according to the methods described herein, fabrication of the solar receiver
can be simply
completed by continuing with standard fabrication operations. To this end, in
some
embodiments, after depositing the coating and while heating the metal tube,
the methods
can further include re-applying a vacuum to the annulus. In some embodiments,
the
methods can further include sealing the annulus so as to maintain the vacuum
therein and
complete the fabrication of the solar receiver. In alternative embodiments, at
least one
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additional layer of coating on the metal tube can be deposited by repeating
the methods
described herein.
[0025] In some embodiments, at least one additional layer of
coating can be
deposited by repeating the operations of the methods described herein. In some
embodiments of the present methods, after depositing the coating and while
heating the
metal surface, a vacuum can be re-applied thereto. In some embodiments of the
present
methods, while heating the metal surface, at least one additional layer of the
coating can
be deposited by releasing the vacuum and backfilling with a second purge gas.
[0026] In some embodiments, the first purge gas and the second
purge gas can be
the same. That is, in some embodiments, the coating can contain multiple
layers in which
all the layers are the same. In other embodiments, the first purge gas and the
second
purge gas can be different. That is, in some embodiments, the coating can
contain
multiple layers in which at least some of the layers are different. By
repeating the
operations of the present methods, a coating having any number of layers can
be
deposited, for example, 1 to about 100 layers, or 1 to about 20 layers, or 1
to about 10
layers, or 1 to about 5 layers, or 1 layer, or 2 layers, or 3 layers, or 4
layers, or 5 layers, or
6 layers, or 7 layers, or 8 layers, or 9 layers, or 10 layers. In embodiments
in which
multiple layers are present, the various layers of the coating can impart
different
properties to the metal surface. For example, a first layer can enhance the
metal surface's
electromagnetic absorptive properties, and a second layer of a different
substance can
reduce its thermal emission properties.
[0027] Various types of coatings can be formed on metal surfaces
according to
the embodiments described herein. In some embodiments, the coatings can
include, for
example, at least one of an oxide coating, a nitride coating, a carbide
coating, or a
fluoride coating. The choice of the type of coating formed on the metal
surface can be a
matter of the intended use thereof, which will be evident to one having
ordinary skill in
the art. For example, an oxide coating can be formed by reacting the metal
surface with
an oxygen-containing purge gas under heating conditions. A nitride coating can
be
formed by reacting the metal surface with a nitrogen-containing purge gas,
particularly
molecular nitrogen, under heating conditions. A carbide coating can be formed
by
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reacting the metal surface with a carbon-containing purge gas, including but
not limited
to organic compounds, under heating conditions. A fluoride coating can be
formed by
reacting the metal surface with a fluorine-containing purge gas, particularly
hydrogen
fluoride, under heating conditions. Other types of coatings can be easily
envisioned by
one having ordinary skill in the art.
[0028] Various purge gases and combinations thereof can be used in
the
embodiments described herein. It is to be recognized that the purge gases
suitable for use
in the present embodiments can be either substances that are gases at room
temperature
and pressure or liquids or solids that have a high vapor pressure and are
readily
volatilized to form a vapor phase. Illustrative purge gases that can be
suitable for use in
the present embodiments include, for example, air, water vapor, oxygen, carbon
dioxide,
carbon monoxide, nitrogen, fluorine, chlorine, bromine, iodine, hydrogen
fluoride,
hydrogen chloride, hydrogen bromide, hydrogen iodide, boron trifluoride, boron
trichloride, boron tribromide, silicon tetrafluoride, sulfur hexafluoride,
sulfur
tetrafluoride, phosphorus trifluoride, phosphorus pentafluoride, nitrogen
trifluoride,
nitrous oxide, nitric oxide, nitrogen dioxide, dinitrogen tetroxide, diimide,
hydrogen,
gaseous organic compounds, and any combination thereof.
[0029] In some embodiments, the purge gas can further include a
diluent gas that
is not reactive with the metal surface. In some embodiments, the diluent gas
can be a
noble gas such as, for example, helium, argon, neon, krypton or xenon. When
present,
the diluent gas can be included in an amount ranging between about 0.1% and
about
99.9% of the gas mixture, including all subranges thereof. In some
embodiments, the
diluent gas can be present in an amount ranging between about 1% and about 90%
of the
gas mixture, or between about 5% and about 50% of the gas mixture, or between
about
10% and about 70% of the gas mixture. Without being bound by theory or
mechanism, it
is believed that by adjusting the quantity of purge gas that is present to
react with the
metal surface, by increasing or decreasing the amount of diluent gas, the
thickness of the
coating on the metal surface can be controlled.
[0030] The thickness of the coating on the metal surface can vary
over a
considerable range. In some embodiments, each layer of the coating on the
metal surface
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can range in thickness between about 1 nm and about 1 pm, including all
subranges in
between. In some embodiments, a thickness of each layer of the coating can
range
between about 1 nm and about 250 nmõ or between about 1 nm and about 100 rim,
or
between about 5 nm and about 50 nm, or between about 5 nm and about 100 nm, or
between about 10 nm and about 50 nm. That is, the coating can be
nanostructured in at
least some embodiments.
[00311 Suitable temperatures for practicing the present embodiments can
likewise
vary over a wide range. As one of ordinary skill in the art will recognize,
the ultimate
temperature range over which the present embodiments can be practiced will
depend
mainly upon the melting point of the chosen metal surface. Except for low
melting point
metals (e.g., metals having melting points of less than about 800 C, such as
aluminum),
suitable temperatures for practicing the present embodiments can vary over a
temperature
range of about 200 C to about 1000 C, including all subranges in between. In
some
embodiments, a suitable temperature can range between about 400 C and about
800 C.
In other embodiments, a suitable temperature can range between about 300 C and
about
600 C. In still other embodiments, a suitable temperature can range between
about
400 C and about 600 C. In some embodiments, a suitable temperature can be at
last
about 400 C. In other embodiments, a suitable temperature can be at least
about 500 C,
or at least about 600 C, or at least about 700 C, or at least about 800 C, or
at least about
900 C, or at least about 1000 C. It is to be further recognized that factors
other than the
melting point of the metal surface can also dictate the chosen temperature at
which the
present embodiments are practiced. For example, certain purge gases may become
flammable, explosive, or otherwise unstable if the temperature is excessively
high. Thus,
the temperature at which a particular coating is prepared according to the
present
embodiments will be a matter of routine experimental design that is within the
capabilities of one having ordinary skill in the art.
[0032] In some embodiments, the purge gas can be subjected to a pre-
heating
operation before being backfilled into a vacuum about a metal surface.
Possible reasons
one would desire to pre-heat the purge gas can include, but are not limited
to, to address
cooling of the purge gas due to adiabatic expansion that occurs upon
backfilling a
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vacuum space. Cooling of the purge gas can potentially impact its reaction
with a heated
metal surface.
[0033] Although the invention has been described with reference to the
disclosed
embodiments, one having ordinary skill in the art will readily appreciate that
these
embodiments are only illustrative of the invention. It should be understood
that various
modifications can be made without departing from the spirit of the invention.
The
particular embodiments disclosed above are illustrative only, as the present
invention
may be modified and practiced in different but equivalent manners apparent to
those
skilled in the art having the benefit of the teachings herein. Furthermore, no
limitations
are intended to the details of construction or design herein shown, other than
as described
in the claims below. It is therefore evident that the particular illustrative
embodiments
disclosed above may be altered, combined, or modified and all such variations
are
considered within the scope and spirit of the present invention. While
compositions and
methods are described in terms of "comprising," "containing," or "including"
various
components or steps, the compositions and methods can also "consist
essentially of' or
"consist of' the various components and operations. All numbers and ranges
disclosed
above can vary by some amount. Whenever a numerical range with a lower limit
and an
upper limit is disclosed, any number and any subrange falling within the
broader range is
specifically disclosed. Also, the terms in the claims have their plain,
ordinary meaning
unless otherwise explicitly and clearly defined by the patentee. If there is
any conflict in
the usages of a word or term in this specification and one or more patent or
other
documents that may be incorporated herein by reference, the definitions that
are
consistent with this specification should be adopted.
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