U.S. patent application number 12/755355 was filed with the patent office on 2010-10-14 for solar receiver utilizing carbon nanotube infused coatings.
This patent application is currently assigned to Lockheed Martin Corporation. Invention is credited to Daniel J. Adcock, Kristopher Heick, Harry C. Malecki, Tushar K. Shah.
Application Number | 20100258111 12/755355 |
Document ID | / |
Family ID | 42933343 |
Filed Date | 2010-10-14 |
United States Patent
Application |
20100258111 |
Kind Code |
A1 |
Shah; Tushar K. ; et
al. |
October 14, 2010 |
SOLAR RECEIVER UTILIZING CARBON NANOTUBE INFUSED COATINGS
Abstract
A solar receiver includes a heat absorbing element having an
outer surface and an inner surface opposite the outer surface and a
first coating including a carbon nanotube-infused fiber material in
surface engagement with and at least partially covering the outer
surface of the heat absorbing element. Solar radiation incident
onto the first coating is received, absorbed, and converted to heat
energy, and the heat energy is transferred from the first coating
to the heat absorbing element. A multilayer coating for a solar
receiver device includes a first coating that includes a
CNT-infused fiber material and an environmental coating disposed on
the first coating.
Inventors: |
Shah; Tushar K.; (Columbia,
MD) ; Malecki; Harry C.; (Abingdon, MD) ;
Adcock; Daniel J.; (Columbia, MD) ; Heick;
Kristopher; (Bel Air, MD) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY LLP
18191 VON KARMAN AVE., SUITE 500
IRVINE
CA
92612-7108
US
|
Assignee: |
Lockheed Martin Corporation
Bethesda
MD
|
Family ID: |
42933343 |
Appl. No.: |
12/755355 |
Filed: |
April 6, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61167386 |
Apr 7, 2009 |
|
|
|
Current U.S.
Class: |
126/635 ;
126/677 |
Current CPC
Class: |
C23C 28/42 20130101;
C23C 28/345 20130101; C23C 28/04 20130101; C23C 30/00 20130101;
C23C 28/3455 20130101; F24S 70/30 20180501; C23C 26/00 20130101;
C23C 28/322 20130101; C23C 28/34 20130101; F24S 20/20 20180501;
Y02E 10/44 20130101; Y02T 50/60 20130101; C23C 28/324 20130101;
F24S 10/95 20180501; F24S 70/225 20180501; C23C 28/00 20130101;
Y02E 10/40 20130101 |
Class at
Publication: |
126/635 ;
126/677 |
International
Class: |
F24J 2/32 20060101
F24J002/32; F24J 2/48 20060101 F24J002/48 |
Claims
1. A solar receiver comprising: a heat absorbing element having an
outer surface and an inner surface opposite the outer surface; and
a first coating comprising a carbon nanotube-infused fiber material
in surface engagement with and at least partially covering the
outer surface of said heat absorbing element; whereby solar
radiation incident onto said first coating is received, absorbed,
and converted to heat energy, and the heat energy is transferred
from said first coating to said heat absorbing element.
2. The solar receiver apparatus of claim 1, wherein said heat
absorbing element has a first end and a second end, wherein a heat
transfer fluid enters said heat absorbing element at said first end
and exits from said heat absorbing element at said second end.
3. The solar receiver of claim 1, wherein said heat absorbing
element comprises a heat pipe.
4. The solar receiver of claim 1, wherein said heat absorbing
element comprises a metal.
5. The solar receiver of claim 1, wherein said heat absorbing
element has grooves to sized accommodate said CNT-infused fiber
material.
6. The solar receiver of claim 1, wherein said CNT-infused fiber
material comprises a carbon nanotube-infused fiber tow.
7. The solar receiver of claim 1, further comprising an
environmental coating integrated within said first coating to form
a composite.
8. The solar receiver of claim 7, wherein said environmental
coating comprises a ceramic matrix material.
9. The solar receiver of claim 7 further comprising metal
particles.
10. The solar receiver of claim 1, further comprising an
environmental coating disposed on said first coating, wherein said
environmental coating comprises a low-emissivity coating.
11. The solar receiver of claim 1, further comprising an
environmental coating comprising a metal.
12. The solar receiver of claim 1, further comprising an
environmental coating comprising an anti-reflective material.
13. The solar receiver of claim 1, further comprising an annulus
surrounding said first coating and said heat absorbing element
creating a gap.
14. The solar receiver of claim 13, wherein the gap comprises
air.
15. The solar receiver of claim 13, wherein the gap is
evacuated.
16. The solar receiver apparatus of claim 1, wherein said apparatus
is configured to integrate with a power generation system.
17. A multilayer coating for a solar receiver device comprising: a
first coating comprising a CNT-infused fiber material; and an
environmental coating disposed on said first coating.
18. The coating of claim 17, wherein said first coating further
comprises a ceramic matrix.
19. The coating of claim 17, wherein said first coating further
comprises metal particles.
20. The coating of claim 17, wherein said environmental coating
comprises a metal film.
21. The coating of claim 17, wherein said environmental coating
comprises an anti-reflective coating.
22. The coating of claim 17, wherein said environmental coating
comprises a low emissivity coating.
Description
STATEMENT OF RELATED APPLICATIONS
[0001] This application claims priority under 35 U.S.C.119(e) to
U.S. Provisional Application 61/167,386 filed Apr. 7, 2009.
FIELD OF INVENTION
[0002] The present invention relates in general to a solar receiver
apparatus for receiving, absorbing, containing, and converting
received electromagnetic radiation into heat energy.
BACKGROUND
[0003] Solar thermal collectors have been developed to harness the
energy from solar radiation for various industrial processes, power
generation and water heating applications. Solar radiation incident
onto the earth's surface has an estimated power density of about 1
kW/m.sup.2 and wavelengths ranging from about 200 nanometers (nm)
for ultraviolet (UV) radiation to about 2500 nm for infrared (IR)
radiation. Solar thermal collectors generally include a reflector
to focus the solar radiation onto a thermal receiver. The thermal
receiver converts the photonic energy of the solar radiation into
thermal energy of a heat transfer fluid. Thermal receivers
generally include a thermal absorber which is a good absorber of
short-wave solar radiation, for example in the UV and visible
range. However, at least some thermal absorbers are also good
long-wave heat radiators in the infrared range, emitting heat via
IR radiation, when sufficiently excited by the absorption of
short-wave solar radiation. Although a high percentage of incident
solar radiation may be initially absorbed, thermal absorbers can
emit a high percentage as radiated heat, thereby lowering the
effective collection of the solar energy.
[0004] Several types of solar collectors have been developed,
including but not limited to flat plate solar collectors and
absorber tubes contained in evacuated glass tube housing. Absorber
surfaces can include a bare metal or a metal coated with a
selective absorber coating for absorbing radiation within the solar
radiation spectrum (i.e., about 200 nm to 2500 nm). Such solar
selective absorber coatings (having absorptivity, for example, in
the range of 0.92 to 0.96 and emissivity, for example, in the range
of 0.07 to 0.11) absorb practically all incident radiation but do
not generally emit heat at infra-red wavelengths. Examples of such
solar selective absorber coatings include very thin black metallic
oxide coating (e.g., on the order of about 0.5 to 1.0 microns) on a
highly reflective metal base, and galvanically applied selective
coatings such as black chrome, black nickel, and aluminum oxide
with nickel. Absorber tubes coated with solar selective coatings
are generally encased in glass tubes or evacuated glass tubes to
minimize the loss of heat to the ambient air via convection.
However, the evacuated glass tubes generally used in conjunction
with some of these coatings are costly to fabricate and prone to
damage when deployed. Additional components such as shrouds are
often employed to protect the vacuum seals from direct thermal
radiation, which results in losses in efficiency of about 2%
Alternative solar receivers having good absorbance and low
emissivity characteristics are, therefore, desirable. The present
invention satisfies this need and provides related advantages as
well.
SUMMARY OF THE INVENTION
[0005] In some aspects, embodiments disclosed herein relate to a
solar receiver that includes a heat absorbing element having an
outer surface and an inner surface opposite the outer surface; and
a first coating including a carbon nanotube-infused fiber material
in surface engagement with and at least partially covering the
outer surface of the heat absorbing element. Solar radiation
incident onto the first coating is received, absorbed, and
converted to heat energy, and the heat energy is transferred from
the first coating to the heat absorbing element.
[0006] In some aspects, embodiments disclosed herein relate to a
multilayer coating for a solar receiver device that includes a
first coating having a CNT-infused fiber material and an
environmental coating disposed on the first coating.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a profile view of an exemplary solar receiver
having a CNT-infused coating on the outer surface of a heat
absorbing element.
[0008] FIG. 2 is a profile of a solar receiver as shown in FIG. 1,
further including grooves on the outer surface of the heat absorber
element.
[0009] FIG. 3 is a profile view of a solar receiver as shown in
FIG. 1, further including an environmental coating over the
CNT-infused coating.
[0010] FIG. 4 is a profile view of a solar receiver as shown in
FIG. 3, further including grooves on the outer surface of the heat
absorber element, according to a fourth embodiment of the
invention;
[0011] FIG. 5 is a cross-sectional view of a ceramic low
emissivity, environmental coating integrated into a CNT-infused
coating and applied to an outer surface of a heat absorber element
of a solar receiver, according to an embodiment of the
invention;
[0012] FIG. 6 is a cross-sectional view of the ceramic low
emissivity, environmental integrated coating of FIG. 5, further
including an anti-reflective coating, according to an embodiment of
the invention;
[0013] FIG. 7 is a cross-sectional view of a metallic low
emissivity, environmental coating applied over the CNT-infused
coating, according to an embodiment of the invention;
[0014] FIG. 8 is a cross-sectional view of an anti-reflective
coating applied over the metallic low emissivity, environmental
integrated coating as shown in FIG. 7, according to an embodiment
of the invention;
[0015] FIG. 9 is a cross-sectional view of a layered cermet low
emissivity, environmental integrated coating applied over the
integrated coating as shown in FIG. 5, according to an embodiment
of the invention;
[0016] FIG. 10 is a cross-sectional view of the layered cermet low
emissivity, environmental integrated coating as shown in FIG. 9,
further including an anti-reflective coating, according to an
embodiment of the invention;
[0017] FIG. 11 is a cross-sectional view of an integrated cermet
low emissivity, environmental CNT-infused coating applied on an
outer surface of a heat absorber element of a solar receiver,
according to an embodiment of the invention;
[0018] FIG. 12 is a cross-sectional view of the integrated cermet
low emissivity, environmental CNT-infused coating as shown in FIG.
11, further including an antireflective coating, according to an
embodiment of the invention;
[0019] FIG. 13 is a cross-sectional view of a solar receiver with
an annulus, according to an embodiment of the invention;
[0020] FIG. 14 is a cross-sectional view of a solar receiver as
shown in FIG. 13, further including grooves as described in the
second embodiment shown in FIG. 2, according to an embodiment of
the invention.
[0021] FIG. 15 shows a process for producing CNT-infused carbon
fiber material in accordance with the illustrative embodiment of
the present invention.
[0022] FIG. 16 shows reflectivity data for a coating that includes
a CNT-infused fiber material.
[0023] FIG. 17 shows a scanning electron microscope (SEM) image of
the CNTs infused to a fiber material for use in a coating in a
solar receiver.
[0024] FIG. 18 shows an exemplary solar receiver.
DETAILED DESCRIPTION
[0025] The present invention is directed, in part, to a solar
receiver that incorporates a heat absorbing element having a first
coating that includes a carbon nanotube (CNT)-infused fiber
material which serves to absorb electromagnetic radiation in a wide
spectral range from ultraviolet (UV) at about 200 nm through
infrared (IR) at about 2500 nm. The CNTs of the CNT-infused fiber
material are good thermal conductors and serve as a conduit for
harvesting and converting light energy into heat. CNTs have some of
the highest thermal conductivities known for any material with some
indications as high as about 6,600 Wm.sup.-1 K.sup.-1 (Berber et
al. Phys. Rev. Lett. 84(20):4613-4616, (2000)).
[0026] Moreover, the fiber material itself of the first coating
provides a scaffold to organize the array of infused CNTs with
predictable alignments to optimize CNT orientation. CNTs can be
fabricated on fiber material substrates in controllably aligned
configurations in scalable quantities to provide access to large
surface area solar receiver panels. The control of CNT orientation,
which is difficult to achieve with "loose" CNT composites, can
enhance the light to heat conversion. Control of CNT alignment
combined with their high thermal conductivity allows heat to be
efficiently and directionally conducted along the CNT length to the
heat absorbing element and from the heating element to a heat
transfer fluid for use a variety of applications, including energy
generation.
[0027] The solar receivers of the present invention can be used in
numerous conventional solar heating collector configurations. For
example, the solar receivers can operate at relatively low
temperatures such as those that can be used in low-end heating
applications such as in a swimming pool heating system or
agricultural uses such as crop drying. The solar receivers of the
present invention can also be used in applications that employ high
temperatures, including temperatures that are used in energy
generation, such as steam generation, for example. The solar
receivers of the present invention can be configured in flat plate
designs as well as parabolic designs.
[0028] The coatings employed on solar receivers of the invention
can have absorptivity, for example, in the range from between about
0.92 to about 0.99. Moreover, the emissivity of the solar receiver
of the invention can be in a range from between about 0.01 to about
0.11. Coatings employed in the solar receivers of the invention can
absorb almost all incident radiation in a spectral band from the UV
through IR, while transfer to the heating element and subsequently
a heat transfer fluid, prevent thermal infra-red emission. It has
been indicated that with proper nanotube density, arrays of
vertically aligned single-walled CNTs can behave as nearly perfect
black body absorbers (Mizuno et al. Proc. Natl. Acad. Sci.
106:6044-6047 (2009)). One means to generate a black body absorber
is to suppress light reflection, which can be achieved when the
refractive index of the object is close to that of air. This
solution to minimize reflectance is evident from Fresnel's law:
R=(n-n.sub.0).sup.2/(n+n.sub.0).sup.2
where R is reflectance, n is the refractive index of the object,
and n.sub.0 is the refractive index of air. The CNT density on the
fiber material can be modulated in the continuous process described
herein below. By modulating CNT density, the CNT-infused fiber
material can be tuned to exhibit a refractive index, n, that
approximates that of air, n.sub.0.
[0029] In some embodiments, the coatings employed in the solar
receivers of the invention having CNT-infused fiber material can
behave as a black-body-like object and can exhibit high thermal
emissivity in the form of black body radiation. In some
embodiments, this loss of energy can be reduced or prevented by the
channeling of the thermal energy from the CNTs to the heating
absorbing element. The heating absorbing element, in turn, heats a
heat transfer fluid which can be used, for example, in power
generation. Reducing the emissivity of the system can also be
achieved by methods known in the art including, for example,
employing vacuum glass chambers about the heating element or
employing further coating materials, such as anti-reflective
coatings or the like.
[0030] In some embodiments, the coatings employed in the solar
receivers of the invention having CNT-infused fiber material can
behave as intrinsic solar selective materials that absorb nearly
all incident light, while have very low emissivity, obviating the
need for further coatings, instead efficiently transferring the
heat energy to the heat absorbing element and from the heat
absorbing element to the heat transfer fluid for use in a variety
of applications.
[0031] In some embodiments, a solar receiver includes a heat
absorbing element having an outer surface and an inner surface
opposite the outer surface. The receiver further includes a carbon
nanotube-infused ("CNT-infused") material in a first coating in
surface engagement with and at least partially covering the outer
surface of the heat absorbing element. CNT-infused fiber material
first coatings include, but are not limited to, a CNT-infused fiber
material and a CNT-infused fiber material in a matrix forming a
composite. The solar radiation incident on the CNT-infused fiber
material of the first coating is absorbed, contained, and converted
to heat energy. The converted heat energy is transferred from the
CNT-infused fiber material of the first coating on the outer
surface of the heat absorbing element to the inner surface of the
heat absorbing element and is then transferred from the inner
surface to a substance such as a heat transfer fluid.
[0032] In some embodiments, a solar receiver includes a heat
absorbing element having a plurality of grooves on the surface of
the heat absorbing element. In one embodiment, the grooves are on
the order of microns (pm) in size and depth. The grooves can be
arranged in a spiral configuration along the circumference of the
heat absorbing element to form a single groove extending from one
end of the heat absorbing element to the other on the outer
surface. Such a groove can accommodate, for example, a CNT-infused
fiber tow and can provide enhanced surface contact area between the
CNT-infused fiber material and the heat absorbing element. Without
being bound by theory, this increased surface area contact can
provide more efficient heat transfer to the outer surface of the
heat absorbing element. In a similar manner, an increased surface
area can be provided on the inner surface of the heat absorbing
element to increase the efficiency of heat transfer to the heat
transfer fluid.
[0033] In some embodiments, a solar receiver includes a low
emissivity, environmental coating covering or integrated into the
first coating having the CNT-infused fiber material. When
integrated into the first coating, it can function as a matrix
material to provide a first coating that is a composite structures.
The environmental coating allows for the transmission of
electromagnetic radiation (at least in ultra-violet to visual
range) incident on the outer surface of the environmental coating
onto the CNT-infused fiber material of the first coating for
absorption and conversion to heat energy. The environmental coating
has low emissivity characteristics so as to effectively reduce the
emission of heat energy by the CNT-infused coating back to the
external environment. The environmental coating can have a low
emissivity, particularly, in the infra-red spectrum, corresponding
to the spectrum at which the CNT-infused fiber material of the
first coating emits heat energy at the system operating
temperature.
[0034] In some embodiments, a solar receiver includes an annulus
surrounding the heat absorbing element at least partially covered
by the first coating having the CNT infused fiber material. In one
configuration, the annulus is radially spaced apart from the
CNT-infused coating. In an exemplary embodiment, the annulus can
include air pockets or air gaps disposed between the annulus and
the CNT-infused coating. In another embodiment, the annulus can be
evacuated and the gap held under vacuum. The annulus can be coated
with one or more of anti-reflective coatings and low emissivity
coatings applied to one or both of its outer and inner surfaces.
The annulus can further have infrared reflective coating applied to
its inner surface which faces the CNT-infused coating.
[0035] As used herein the term "fiber material" refers to any
material which has a fiber as its elementary structural component.
The term encompasses fibers, filaments, yarns, tows, tows, tapes,
woven and non-woven fabrics, plies, mats, and the like. Moreover,
the composition of the fiber material can be of any type including,
without limitation, glass, carbon, metal, ceramic, organic, or the
like.
[0036] As used herein the term "spoolable dimensions" refers to
fiber materials having at least one dimension that is not limited
in length, allowing for the material to be stored on a spool or
mandrel. Fiber materials of "spoolable dimensions" have at least
one dimension that indicates the use of either batch or continuous
processing for CNT infusion as described herein further below. One
exemplary fiber material that is a carbon fiber material of
spoolable dimensions is commercially available is exemplified by
AS4 12k carbon fiber tow with a tex value of 800 (1 tex=1 g/1,000
m) or 620 yard/lb (Grafil, Inc., Sacramento, Calif.). Commercial
carbon fiber tow, in particular, can be obtained in 5, 10, 20, 50,
and 100 lb. (for spools having high weight, usually a 3 k/12K tow)
spools, for example, although larger spools may require special
order. Processes of the invention operate readily with 5 to 20 lb.
spools, although larger spools are usable. Moreover, a pre-process
operation can be incorporated that divides very large spoolable
lengths, for example 100 lb. or more, into easy to handle
dimensions, such as two 50 lb spools.
[0037] As used herein, the term "carbon nanotube" (CNT, plural
CNTs) refers to any of a number of cylindrically-shaped allotropes
of carbon of the fullerene family including single-walled carbon
nanotubes (SWNTs), double-walled carbon nanotubes (DWNTs),
multi-walled carbon nanotubes (MWNTs). CNTs can be capped by a
fullerene-like structure or open-ended. CNTs include those that
encapsulate other materials.
[0038] As used herein "uniform in length" refers to length of CNTs
grown in a reactor. "Uniform length" means that the CNTs have
lengths with tolerances of plus or minus about 20% of the total CNT
length or less, for CNT lengths varying from between about 1 micron
to about 500 microns. At very short lengths, such as 1-4 microns,
this error may be in a range from between about plus or minus 20%
of the total CNT length up to about plus or minus 1 micron, that
is, somewhat more than about 20% of the total CNT length.
[0039] As used herein "uniform in distribution" refers to the
consistency of density of CNTs on a fiber material. "Uniform
distribution" means that the CNTs have a density on the fiber
material with tolerances of plus or minus about 10% coverage
defined as the percentage of the surface area of the fiber covered
by CNTs. This is equivalent to .+-.1500 CNTs/.mu.m.sup.2 for an 8
nm diameter CNT with 5 walls. Such a figure assumes the space
inside the CNTs as fillable.
[0040] As used herein, the term "infused" means bonded and
"infusion" means the process of bonding. Such bonding can involve
direct covalent bonding, ionic bonding, pi-pi, and/or van der Waals
force-mediated physisorption. For example, in some embodiments, the
CNTs can be directly bonded to the fiber material. Bonding can be
indirect, such as the CNT infusion to the fiber material via a
barrier coating and/or an intervening transition metal nanoparticle
disposed between the CNTs and fiber material. In the CNT-infused
fiber materials disclosed herein, the carbon nanotubes can be
"infused" to the fiber material directly or indirectly as described
above. The particular manner in which a CNT is "infused" to a
carbon fiber materials is referred to as a "bonding motif."
[0041] As used herein, the term "transition metal" refers to any
element or alloy of elements in the d-block of the periodic table.
The term "transition metal" also includes salt forms of the base
transition metal element such as oxides, carbides, nitrides, and
the like.
[0042] As used herein, the term "nanoparticle" or NP (plural NPs),
or grammatical equivalents thereof refers to particles sized
between about 0.1 to about 100 nanometers in equivalent spherical
diameter, although the NPs need not be spherical in shape.
Transition metal NPs, in particular, serve as catalysts for CNT
growth on the fiber materials.
[0043] As used herein, the term "matrix material" refers to a bulk
material than can serve to organize CNT-infused fiber materials in
particular orientations, including random orientation. The matrix
material can benefit from the presence of the CNT-infused carbon
fiber material by imparting some aspects of the physical and/or
chemical properties of the CNT-infused fiber material to the matrix
material. In some embodiments, the matrix material can act as the
environmental coating that helps retain the heat generated upon
absorption of solar radiation by the CNTs. In some embodiments, the
matrix material is a ceramic. In some embodiments, the matrix
material reflects infrared radiation back to the CNTs preventing
heat loss to the environment.
[0044] As used herein, the term "material residence time" refers to
the amount of time a discrete point along a fiber material of
spoolable dimensions is exposed to CNT growth conditions during the
CNT infusion processes described herein. This definition includes
the residence time when employing multiple CNT growth chambers.
[0045] As used herein, the term "linespeed" refers to the speed at
which a fiber material of spoolable dimensions can be fed through
the CNT infusion processes described herein, where linespeed is a
velocity determined by dividing CNT chamber(s) length by the
material residence time.
[0046] In some embodiments, the present invention provides a solar
receiver that includes a heat absorbing element having an outer
surface and an inner surface opposite the outer surface; and a
first coating that includes a carbon nanotube-infused fiber
material in surface engagement with and at least partially covering
the outer surface of the heat absorbing element, whereby solar
radiation incident onto the first coating is received, absorbed,
and converted to heat energy, and the heat energy is transferred
from the first coating to said heat absorbing element.
[0047] Solar receivers of the invention can operate low, medium,
and high temperature applications as known in the art. High
temperature receivers are used in numerous power generating
applications, for example, in driving a turbine with steam. High
temperature applications can be any application utilizing
temperatures greater than about 400.degree. C. Low temperature
applications include, for example, pool heating or crop drying.
Such temperatures can be about 10-100.degree. C. higher than
ambient temperatures. Any applications utilizing temperature
between about 100.degree. C. and 400.degree. C. are considered mid
temperature applications. Exemplary mid temperature application can
include, for example, a parabolic trough or concentrating solar
power plant.
[0048] The solar receiver apparatus has a heat absorbing element
having a first end and a second end and a heat transfer fluid that
enters the heat absorbing element at said first end and exits from
the heat absorbing element at the second end. The heat absorbing
element can have grooves on the inner and/or outer surface to
provide greater surface area contact with the first coating on the
outside and/or with the heat transfer fluid on the inside of the
heat absorbing element. The first and second ends of the heating
element can be used to transport the heat transfer fluid to and
from the receiver. The receiver itself is configured to integrate
into existing systems and can be incorporated in parabolic and flat
panel type receivers.
[0049] The heat absorbing element is generally a heat pipe made of
metal, although any conducting material can be used. Moreover, the
heat absorbing element need not be cylindrical like a pipe. The
heat absorbing element can be any shape and can be chosen for
improved surface area on the inner and outer surfaces. For example,
in some embodiments, the solar receiver heat absorbing element can
have grooves sized to accommodate the CNT-infused fiber material.
When the CNT-infused fiber material is a CNT-infused fiber tow, the
grooves can be helically disposed on the outer surface of the
heating element and the CNT-infused fiber tow wrapped inside the
groove and it contact with the wells of the groove. In some
embodiments, when a CNT-infused fiber tow is employed, the tow can
also be spread onto the heating element.
[0050] In some embodiments, the solar receiver of the invention has
a CNT-infused fiber material includes a carbon nanotube-infused
fiber tow that includes a material selected from carbon, metal,
glass, ceramic and the like.
[0051] In some embodiments, the solar receiver of the invention can
further include an environmental coating integrated within said
first coating to form a composite. Such materials forming an
environmental coating include, without limitation a ceramic matrix
material. In some embodiments, the composite formed with the matrix
material can further include metal particles. The metal particles
can be used to further increase conductive pathways to disperse the
heat collected by the CNT infused material. They can serve, for
example, as conduits for thermal heat transfer between neighboring
CNTs, while serving as a infra-red reflector.
[0052] In some embodiments, the solar receiver of the invention can
further include an environmental coating disposed on the first
coating, and this environmental coating can include a
low-emissivity coating. In such embodiments, the environmental
coating can also include the matrix type environmental coating
integrated within the CNT-infused fiber material. In some
embodiments the environmental coating includes a metal such as
copper.
[0053] Solar receivers of the invention can exhibit very low
emissivity. Any environmental coating can serve this purpose.
Additionally, in some embodiments, the solar receiver of the
invention further includes an environmental coating that includes
an anti-reflective material. This can be used to reflect infrared
heat radiated from the CNTs or the heat absorbing element back
towards the CNTs and heating element to prevent heat loss to the
environment.
[0054] In still further embodiments, the solar receiver of the
invention further includes an annulus surrounding the first coating
and the heat absorbing element creating a gap. This gap can include
air or the gap can be substantially evacuated.
[0055] The solar receivers of the invention are configured to
integrate with a power generation system. In this regard, the
overall design of the receiver can be nominally the same as those
known in the art.
[0056] In some embodiments, the present invention also provides a
multilayer coating for a solar receiver device that includes a
first coating having a CNT-infused fiber material; and an
environmental coating disposed on the first coating. The first
coating further can include a ceramic matrix and the first coating
can further include metal particles as described above and herein
below.
[0057] The multilayer coating of the invention can include
environmental coatings that include a metal film, an
anti-reflective coating, and/or a low emissivity coating as
described above and further described below.
[0058] It is to be understood that the figures and descriptions of
the present invention have been simplified to illustrate elements
that are relevant for a clear understanding of the present
invention, while eliminating, for purposes of clarity, many other
elements found in typical solar receivers and collectors. However,
because such elements are well known in the art, and because they
do not facilitate a better understanding of the present invention,
a discussion of such elements is not provided herein. The
disclosure herein is directed to all such variations and
modifications known to those skilled in the art.
[0059] Referring to FIG. 1, there is illustrated a profile view of
a solar receiver 100, according to the first embodiment of the
invention. Solar receiver 100 includes a heat absorbing element
110, and a CNT-infused coating 120 applied to at least a portion of
an outer surface 115 of heat absorbing element 110.
[0060] In one configuration, heat absorbing element 110 is a hollow
element adapted to receive a heat transfer substance, for example,
a heat transfer fluid therewithin. By way of non-limiting example
only, the heat transfer fluid may include water, anti-freeze
solution (e.g., water and glycol), air, various gases, oil, and
other high temperature (high heat capacity) fluids. In an exemplary
embodiment, heat absorbing element 110 is a metallic or alloy
absorber tube having a first end 112 and a second end 114. Heat
absorbing element 110 has an outer surface 115 and an inner surface
117 opposite to outer surface 115. By way of non-limiting examples
only, heat absorbing element 110 may be made of stainless steel,
carbon steel, or aluminum. One skilled in the art will appreciate
that other metals and alloys may also be used. The thickness of
heat absorbing element 110 and the material properties of heat
absorbing element 110 are selected to efficiently transfer heat
from outer surface 115 to inner surface 117 which heats a heat
transfer substance present in heat absorbing element 110 generally
in surface engagement with inner surface 117. In an exemplary
configuration, an absorber tube may have a length of about 3 meters
(m), a diameter of about 70 millimeters (mm), and a wall thickness
of about 2 mm. While heat absorber element 110 referred to herein
takes the form of a tube or tubular structure, it is understood
that heat absorber element 110 may be configured in various
geometric forms, including by way of example only, cylindrical,
conical, polygonal or other shapes and configurations.
[0061] In one configuration, heat absorbing element 110 is an open
system wherein a heat transfer substance such a heat transfer fluid
enters at the first end 112 at a first temperature and exits from
the second end 114 at a second temperature higher than the first
temperature. In another configuration, heat absorbing element 110
may be a closed system, such as a heat pipe, wherein the heat
transfer fluid is retained within heat absorbing element 110. In
the illustrated embodiment, heat absorbing element 110 has an outer
surface 115, which is generally uniform.
[0062] Still referring to FIG. 1, CNT-infused coating 120 is
disposed on outer surface 115 of heat absorbing element 110.
CNT-infused coating 120, therefore, at least partially covers outer
surface 115 of heat absorbing element 110. CNT-infused coating 120
is wound under tension on outer surface 115 of heat absorbing
element 110 to establish and maintain an effective surface
engagement or contact with outer surface 115 of heat absorbing
element 110 while minimizing the gaps therebetween. CNTinfused
coating 120 receives incident electromagnetic radiation (typically
in the form of solar radiation) and converts the received radiation
into heat or thermal energy. The converted heat or thermal energy
is transferred to outer surface 115 of heat absorber element 110.
In an exemplary embodiment, outersurface 115 of heat absorbing
element 110 is substantially completely covered by CNT-infused
coating 120. In another embodiment, one or more pre-defined areas
of outer surface 115 may be left uncovered by CNT-infused coating
120.
[0063] In one configuration, CNT-infused coating 120 takes the form
of a glass rope or fiber infused with carbon nanotubes. Other
examples of CNT-infused coatings include carbon nanotube-infused
fibers and fabrics, such as carbon fibers infused with carbon
nanotubes, vapor growth carbon fibers, carbon nanofibers, and
graphene. In an exemplary embodiment, CNT-infused coating 120 may
have a thickness in the range of about 15 microns (pm) to about
1000 pm. CNT-infused coating 120 may optionally include a matrix of
a high temperature cement, resin or epoxy, doped with carbon
nanotubes or metal nanoparticles, to provide structural integrity
to CNT-infused coating 120.
[0064] In an exemplary embodiment, CNT-infused coating 120 may be
fabricated in the form of glass fibers using in situ carbon
nanotube growth techniques. For example, a glass fiber may be fed
through a growth chamber maintained at a given temperature of about
5000 to 750.degree. C. Carbon containing feed gas is then
introduced into the growth chamber, wherein carbon radicals
dissociate and initiate formation of carbon nanotubes on the glass
fiber, in presence of catalyst nanoparticles. One such technique is
described in the commonly owned Provisional U.S. Application No.
61/155,935, entitled "Low Temperature CNT Growth Using A
Gas-Pre-heat Method," and filed Feb. 27, 2009, which application is
incorporated by reference herein in its entirety. Other such
methods by which carbon nanotube infused fibers in the form of a
composite cover layer or thread or rope layer are to be generated
may be utilized to obtain CNT-infused coating 120.
[0065] As is known in the art, the electromagnetic radiation
absorptivity of a carbon nanotube-based structure is, in part, a
function of the carbon nanotube length as well as the nanotube
volume-filling fraction of the structure. The nanotube
volume-filling fraction represents the fraction of the structure's
total volume occupied by the nanotubes. In an exemplary embodiment,
the nanotube volume-filling fraction of CNTinfused coating 120 is
in the range of about 0.5% to about 25%. The average spacing
between the carbon nanotubes in CNT-infused coating 120 ranges from
about 2 nanometers (nm) to about 200 nm. The nanotube volume
filling of CNT-infused coating 120 may be tailored by selective
positioning of carbon nanotubes therein to control the range of
electromagnetic radiation that can be effectively absorbed by
CNT-infused coating 120. The gaps between the nanotubes in
CNT-infused coating 120 may be used to selectively capture and
absorb radiation having one or more given wavelengths.
[0066] The longer the carbon nanotube in the CNT-infused coating,
the higher the absorptivity of electromagnetic radiation (at least
in the visible light spectrum). CNTinfused coating 120 may include
carbon nanotubes having a length in the range of about ten (10)
microns to about hundreds of microns.
[0067] As is known in the art, thermal conductivity of a carbon
nanotube is dependent upon its structural configuration. In
particular, the carbon nanotube has a higher thermal conductivity
in the direction of its longitudinal axis as compared with that in
a direction perpendicular to its longitudinal axis. In one
configuration, CNT-infused coating 120 may, therefore, include
carbon nanotubes which are aligned generally perpendicular to outer
surface 115, carbon nanotubes which are aligned generally parallel
to outer surface 115 and carbon nanotubes which are aligned neither
parallel nor perpendicular to outer surface 115. Those carbon
nanotubes generally perpendicular to outer surface 115 effectively
conduct heat converted from the incident radiation to outer surface
115. Those carbon nanotubes not generally perpendicular to outer
surface 115 do not conduct any significant heat to outer surface
115 directly. However, those carbon nanotubes not generally
perpendicular to outer surface 115, form thermal paths to the
generally perpendicular carbon nanotubes within CNT-infused coating
120, thereby increasing overall heat transfer from CNT-infused
coating 120 to outer surface 115. Thus, the alignment of carbon
nanotubes in CNT-infused coating 120 may be tailored to maximize
the thermal conductivity of CNT-infused coating 120 to heat
absorbing element 110.
[0068] Referring now to FIG. 2, there is illustrated a solar
receiver 200, according to another embodiment of the invention.
Solar receiver 200 is generally similar to solar receiver 100.
However, receiver 200 has a heat absorbing element 110 having
grooves 215 formed on outer surface 115. In one configuration,
grooves 215 take the form of a spiral configuration extending along
the length of heat absorbing element 110. It will be appreciated by
one skilled in the art that machining a spiral groove is a simple
and well known process. In an exemplary embodiment, grooves 215 may
have a size ranging from about 50 pm to about 5000 pm. Grooves 215
effectively increase the surface area of outer surface 115 of heat
absorbing element 110 exposed to CNT-infused coating 120. The
increased surface area, in turn, increases the effectiveness of
heat transfer from CNT-infused coating 120 to outer surface 115 of
heat absorbing element 110. In an exemplary embodiment, grooves 215
are particularly effective when combined with a CNT-infused coating
consisting of CNT-infused fiber tows 120. Groove 215 may be sized
to maximize the contact area between interior surface of groove 215
and the outer surface of one or more individual fibers of
CNT-infused coating 120. In an exemplary embodiment, groove 215 may
be sized to have a size and depth approximately similar to a
CNT-infused fiber of CNT-infused coating 120, thereby accommodating
and seating the CNT-infused fiber of CNT-infused coating 120 in a
close fit within groove 215 and maximizing the surface contact
between groove 215 and CNT-infused coating 120. In other
embodiments, groove 215 may accommodate a plurality of CNT-infused
fibers of CNT-infused coating 120.
[0069] In one configuration, grooves 215 may take the form of a
single groove spirally defined on outer surface 115 and extending
continuously along the entire length of absorber element 110. In
another embodiment, grooves 215 may include a series of
discontinuous or segmented grooves defined on outer surface 115 of
heat absorber element 110. Such grooves 215 may be aligned
longitudinally with one another and sized to accommodate at least a
portion of one or more CNT-infused fibers wound about absorber
element 110.
[0070] Referring to FIG. 3, there is illustrated a solar receiver
300, according to another embodiment of the invention. Solar
receiver 300 is generally similar to solar receiver 100 (of FIG.
1). In one configuration, an environmental coating 310 may be
applied to the top surface of CNT-infused coating 120 to protect
CNT-infused coating 120 and to improve the reflective and emissive
characteristics of the combination of CNT-infused coating 120 and
environmental coating 310. Several embodiments of environmental
coating 310 are schematically depicted in FIGS. 5-12, and described
herein.
[0071] Referring now to FIG. 4, there is illustrated a solar
receiver 400, according to another embodiment of the invention.
Solar receiver 400 is generally similar to solar receiver 200 (of
FIG. 2), further including environmental coating 310 as described
for solar receiver 300 (of FIG. 3).
[0072] Referring now to FIG. 5, in one configuration of solar
receiver 500, there is shown a ceramic environmental coating 510
integrated with CNT-infused coating 120 for protecting CNT-infused
coating 120 from the environment and for reducing the emission of
thermal energy from CNT-infused coating 120. Environmental coating
510 is transparent to at least solar radiation to permit the
incident radiation to reach CNTinfused coating 120. Furthermore,
environmental coating 510 is reflective of thermal radiation,
including infra-red radiation, emitted by CNT-infused coating 120,
thereby reflecting thermal radiation back to CNT-infused coating
120 for reabsorption. Thus, environmental coating 510 has low
emissivity characteristics. In an exemplary embodiment,
environmental coating 510 may include a ceramic (dielectric) based
material applied as a liquid and converted to a glass through a
high temperature curing cycle. In another embodiment, environmental
coating 510 may be applied through a chemical vapor deposition
process, or through plasma sputtering. As such coating application
processes are known in the art, they are not described in further
detail for the sake of brevity. In one configuration, environmental
coating 510 is adapted to withstand high temperatures of
CNT-infused coating 120 and heat absorbing element 110, which may
reach as high as 400.degree. to 500.degree. C. In another
configuration, environmental coating 510 may be adapted to be
hydrophobic to protect CNT-infused coating 120 from environmental
moisture. In an exemplary embodiment, environmental coating 510 may
have a thickness in the range of about 50 nm to about 500 nm.
Examples of materials which may be used to form environmental
coating 510 include alumina, silicon dioxide, cesium dioxide, zinc
sulfide, aluminum nitride, and zirconium oxide.
[0073] Now referring to FIG. 6, in another configuration of solar
receiver 600, the integrated ceramic environmental coating 510 and
CNT-infused coating 120 is further coated with an anti-reflective
coating 615. The amount of incident radiation lost due to
reflectance by the integrated environmental coating 510 and
CNT-infused coating 120 may be reduced by disposing anti-reflective
coating 615 thereon. Anti-reflective coating 510, therefore,
effectively reduces the reflectance loss of underlying integrated
environmental coating 510 and CNT-infused coating 120 and increases
the amount of incident radiation absorbed by CNT-infused coating
120. Examples of such anti-reflective coatings include magnesium
fluoride, fluoropolymers and silica-based coatings. The use of such
anti-reflective coatings is known in the art and so will not be
described in further detail.
[0074] Referring to FIG. 7, in one configuration of solar receiver
700, a metallic environmental coating 710 is applied over
CNT-infused coating 120. In an exemplary embodiment, environmental
coating 710 may be a metal thin film that is transparent to at
least solar radiation to permit the incident radiation to reach
CNT-infused coating 120. Furthermore, environmental coating 710 has
low emissivity characteristics, by being reflective of thermal
radiation, including infra-red radiation, from CNT-infused coating
120 back to CNT-infused coating 120 for reabsorption. In an
exemplary embodiment, environmental coating 710 may include a metal
thin film material applied through a chemical vapor deposition
process, or through plasma sputtering or spray. In one
configuration, environmental coating 710 is adapted to withstand
high temperatures of CNT-infused coating 120 and heat absorbing
element 110, which may reach as high as 400.degree. to 500.degree.
C. In another configuration, environmental coating 710 may be
adapted to be hydrophobic. In an exemplary embodiment,
environmental coating 710 may have a thickness in the range of
about 1 nm to about 250 nm. Examples of materials which may be used
to form environmental coating 510 include, but not limited to,
Molybdenum (Mo), Silver (Ag), Copper (Cu), Nickel (Ni), Titanium
(Ti), Platinum (Pt), Tungsten (W), Chromium (Cr), Cobalt (Co), Gold
(Au), Cupric oxide (CuO), Cobalt oxide (Co304), Molybdenum dioxide
(MoO2), Tungsten oxide (WO), titanium oxide (TiO), Titanium nitride
(TiN), Iron (Fe), and Ferric oxide (Fe203).
[0075] Referring now to FIG. 8, in another configuration of solar
receiver 800, metallic environmental coating 710 (of FIG. 7) is
further coated with an anti-reflective coating 615. Examples of
such anti-reflective coatings include magnesium fluoride,
fluoropolymers and silica-based coatings.
[0076] Referring to FIG. 9, in another configuration of solar
receiver 900, the integrated ceramic environmental coating 510 and
CNT-infused coating 120 (of FIG. 5) is further coated with a metal
coating 710 (of FIG. 7), thereby forming a layered cermet coating
on heat absorbing element 110. The layered cermet coating includes
metallic coating 710 overlying the integrated ceramic coating 510
and CNT-infused coating 120. The combination of ceramic layer 510
and metallic layer 710 effectively increases the environmental
protection provided to CNT-infused coating 120 and effectively
reduces thermal radiation losses from underlying CNT-infused
coating 120 by reflecting thermal radiations back to CNT-infused
coating 120 for reabsorption. The layered cermet layer provides
additional structural integrity to the underlying integrated
ceramic coating 510 and CNT-infused coating 120.
[0077] Referring now to FIG. 10, in another configuration of solar
receiver 1000, the integrated cermet coatings of FIG. 9 are further
coated with an anti-reflective coating 615. Examples of such
anti-reflective coatings include magnesium fluoride, fluoropolymers
and silica-based coatings.
[0078] Referring now to FIG. 11, in another configuration of solar
receiver 1100, the integrated ceramic environmental coating 510 and
CNT-infused coating 120 (of FIG. 5) is doped with metal particles
1110. In one configuration, particles 1110 may include the metals
described for coating 710, and may be applied via colloidal
dispersions or selective plasma sputtering or sprays. Particle
sizes may be between several microns to several nanometers. This
configuration thus provides an integrated layer of CNTinfused
coating 120 and integrated ceramic coating 510 doped with metal
particles 1110.
[0079] Referring to FIG. 12, in another configuration 1200, the
integrated layer of coatings of FIG. 11 is further coated with an
anti-reflective coating 615. Examples of such anti-reflective
coatings include magnesium fluoride, fluoropolymers and
silica-based coatings.
[0080] Referring now to FIG. 13, there is illustrated a solar
receiver 1300, according to yet another embodiment of the
invention. Solar receiver 1300 is generally similar to solar
receiver 300 (of FIG. 3). Solar receiver 1300 additionally includes
an annulus 1310 surrounding heat absorbing element 110 coated with
CNT-infused coating 120. In an exemplary embodiment, annulus 1310
takes the form of a glass annulus. In other embodiments, annulus
1310 may be made of other materials such as quartz or other doped
glass materials which are transparent to incident electromagnetic
radiation, for example, solar radiation. In one configuration,
annulus 1310 may be coated with an anti-reflective coating on its
outer surface, inner surface, or both inner and outer surfaces to
maximize the amount of incident radiation transmitted through
annulus 1310. In an exemplary embodiment, anti-reflective coating
may include multiple thin film structures having alternating layers
of contrasting refractive index. Layer thicknesses may be chosen to
produce destructive interference in the beams reflected from the
interfaces, and constructive interference in the corresponding
transmitted beams. Examples of such anti-reflective coatings
include magnesium fluoride, fluoropolymers and silica-based
coatings.
[0081] In another configuration, annulus 1310 may be additionally
or alternatively coated with a low emissivity coating on the outer,
inner or both inner and outer surfaces to reduce radiation heat
loss from emission from annulus 1310. In an exemplary embodiment, a
low emissivity coating is a thin film metal or metallic oxide layer
deposited on annulus 1310. Non-limiting examples of such low
emissivity coatings include Molybdenum (Mo), Silver (Ag), Copper
(Cu), and Nickel (Ni) with thicknesses ranging between 500-50 nm.
In yet another configuration, annulus 1310 may be additionally or
alternatively coated with an infra-red reflective coating on its
inner, outer, or both inner and outer surfaces. As is known in the
art, heat may be from lost through infra-red radiation from heat
absorbing element 110 covered with CNT-infused coating 120. Annulus
1310 coated with infra-red reflective coating reflect such
infra-red radiation, emitted by CNT-infused coating 120, back to
heat absorbing element 110, where CNT-infused coating 120
re-absorbs such reflected IR radiation. Thus, effective heat loss
from infra-red radiation is reduced via reabsorption of the emitted
radiation. An example of such an infra-red reflective coating is a
cadmium stannate film.
[0082] In an exemplary embodiment, solar receiver 1300 may include
air gaps or air pockets between annulus 1310 and heat absorbing
element 110 at least partially covered with CNT-infused coating
120. In another embodiment, annulus 1310 may be evacuated to reduce
heat loss due to convection in the air present between CNTinfused
coating 120 and annulus 1310. In yet another exemplary embodiment,
solar receiver 1300 may further include one or more of the
environmental, low emissivity coatings described in relation to
FIGS. 5-12.
[0083] Referring now to FIG. 14, a solar receiver 1400 is
illustrated according to an embodiment of the invention. Solar
receiver 1400 is generally similar to solar receiver 400. Solar
receiver 400 additionally includes an annulus 1310 surrounding heat
absorbing element 110 at least partially covered with CNT-infused
coating 120. Annulus 1310 may be coated with one or more of
anti-reflective coating on its outer, inner or both outer and inner
surfaces, low emissivity coating on its outer, inner, or inner and
outer surfaces, infra-red radiation reflective coating on its
inner, outer, or inner and outer surfaces, as described above
herein with regard to the embodiments of FIG. 13. In yet another
exemplary embodiment, solar receiver 1400 may further include one
or more of the environmental, low emissivity coatings described in
relation to FIGS. 5-12.
[0084] Below is an exemplary process for generating a CNT infused
fiber material. This process is exemplified with carbon fiber
material, however, one skilled in the art will appreciate that the
operational parameters will be similar for other material types,
including glass, ceramic, and metal fiber materials as well.
[0085] In some embodiments the present invention provides a
continuous process for CNT infusion that includes (a) disposing a
carbon nanotube-forming catalyst on a surface of a fiber material
of spoolable dimensions; and (b) synthesizing carbon nanotubes
directly on the fiber material, thereby forming a carbon
nanotube-infused fiber material. For a 9 foot long system, the
linespeed of the process can range from between about 1.5 ft/min to
about 108 ft/min. The linespeeds achieved by the process described
herein allow the formation of commercially relevant quantities of
CNT-infused fiber materials with short production times. For
example, at 36 ft/min linespeed, the quantities of CNT-infused
fibers (over 5% infused CNTs on fiber by weight) can exceed over
100 pound or more of material produced per day in a system that is
designed to simultaneously process 5 separate tows (20 lb/tow).
Systems can be made to produce more tows at once or at faster
speeds by repeating growth zones. Moreover, some steps in the
fabrication of CNTs, as known in the art, have prohibitively slow
rates preventing a continuous mode of operation. For example, in a
typical process known in the art, a CNT-forming catalyst reduction
step can take 1-12 hours to perform. CNT growth itself can also be
time consuming, for example requiring tens of minutes for CNT
growth, precluding the rapid linespeeds realized in the present
invention. The process described herein overcomes such rate
limiting steps.
[0086] The CNT-infused fiber material-forming processes of the
invention can avoid CNT bundling that occurs when trying to apply
suspensions of pre-formed carbon nanotubes to fiber materials. That
is, because pre-formed CNTs are not fused to the carbon fiber
material, the CNTs tend to bundle and entangle. The result is a
poorly uniform distribution of CNTs that weakly adhere to the
carbon fiber material. However, processes of the present invention
can provide, if desired, a highly uniform entangled CNT mat on the
surface of the fiber material by reducing the growth density. The
CNTs grown at low density are infused in the fiber material first.
In such embodiments, the fibers do not grow dense enough to induce
vertical alignment, the result is entangled mats on the carbon
fiber material surfaces. By contrast, manual application of
pre-formed CNTs does not insure uniform distribution and density of
a CNT mat on the carbon fiber material.
[0087] FIG. 15 depicts a flow diagram of process 1500 for producing
CNT-infused carbon fiber material in accordance with an
illustrative embodiment of the present invention. Again, the use of
a carbon fiber material is merely exemplary.
[0088] Process 1500 includes at least the operations of:
[0089] 1501: Functionalizing the carbon fiber material.
[0090] 1502: Applying a barrier coating and a CNT-forming catalyst
to the functionalized carbon fiber material.
[0091] 1504: Heating the carbon fiber material to a temperature
that is sufficient for carbon nanotube synthesis.
[0092] 1506: Promoting CVD-mediated CNT growth on the
catalyst-laden carbon fiber.
[0093] In step 1501, the carbon fiber material is functionalized to
promote surface wetting of the fibers and to improve adhesion of
the barrier coating.
[0094] To infuse carbon nanotubes into a carbon fiber material, the
carbon nanotubes are synthesized on the carbon fiber material which
is conformally coated with a barrier coating.
[0095] In one embodiment, this is accomplished by first conformally
coating the carbon fiber material with a barrier coating and then
disposing nanotube-forming catalyst on the barrier coating, as per
operation 1502. In some embodiments, the barrier coating can be
partially cured prior to catalyst deposition. This can provide a
surface that is receptive to receiving the catalyst and allowing it
to embed in the barrier coating, including allowing surface contact
between the CNT forming catalyst and the carbon fiber material. In
such embodiments, the barrier coating can be fully cured after
embedding the catalyst. In some embodiments, the barrier coating is
conformally coated over the carbon fiber material simultaneously
with deposition of the CNT-form catalyst. Once the CNT-forming
catalyst and barrier coating are in place, the barrier coating can
be fully cured.
[0096] In some embodiments, the barrier coating can be fully cured
prior to catalyst deposition. In such embodiments, a fully cured
barrier-coated carbon fiber material can be treated with a plasma
to prepare the surface to accept the catalyst. For example, a
plasma treated carbon fiber material having a cured barrier coating
can provide a roughened surface in which the CNT-forming catalyst
can be deposited. The plasma process for "roughing" the surface of
the barrier thus facilitates catalyst deposition. The roughness is
typically on the scale of nanometers. In the plasma treatment
process craters or depressions are formed that are nanometers deep
and nanometers in diameter. Such surface modification can be
achieved using a plasma of any one or more of a variety of
different gases, including, without limitation, argon, helium,
oxygen, nitrogen, and hydrogen. In some embodiments, plasma
roughing can also be performed directly in the carbon fiber
material itself. This can facilitate adhesion of the barrier
coating to the carbon fiber material.
[0097] As described further below and in conjunction with FIG. 15,
the catalyst is prepared as a liquid solution that contains
CNT-forming catalyst that comprise transition metal nanoparticles.
The diameters of the synthesized nanotubes are related to the size
of the metal particles as described above. In some embodiments,
commercial dispersions of CNT-forming transition metal nanoparticle
catalyst are available and are used without dilution, in other
embodiments commercial dispersions of catalyst can be diluted.
Whether to dilute such solutions can depend on the desired density
and length of CNT to be grown as described above.
[0098] With reference to the illustrative embodiment of FIG. 15,
carbon nanotube synthesis is shown based on a chemical vapor
deposition (CVD) process and occurs at elevated temperatures. The
specific temperature is a function of catalyst choice, but will
typically be in a range of about 500 to 1000.degree. C.
Accordingly, operation 1504 involves heating the barrier-coated
carbon fiber material to a temperature in the aforementioned range
to support carbon nanotube synthesis.
[0099] In operation 1506, CVD-promoted nanotube growth on the
catalyst-laden carbon fiber material is then performed. The CVD
process can be promoted by, for example, a carbon-containing
feedstock gas such as acetylene, ethylene, and/or ethanol. The CNT
synthesis processes generally use an inert gas (nitrogen, argon,
helium) as a primary carrier gas. The carbon feedstock is provided
in a range from between about 0% to about 15% of the total mixture.
A substantially inert environment for CVD growth is prepared by
removal of moisture and oxygen from the growth chamber.
[0100] In the CNT synthesis process, CNTs grow at the sites of a
CNT-forming transition metal nanoparticle catalyst. The presence of
the strong plasma-creating electric field can be optionally
employed to affect nanotube growth. That is, the growth tends to
follow the direction of the electric field. By properly adjusting
the geometry of the plasma spray and electric field,
vertically-aligned CNTs (i.e., perpendicular to the carbon fiber
material) can be synthesized. Under certain conditions, even in the
absence of a plasma, closely-spaced nanotubes will maintain a
vertical growth direction resulting in a dense array of CNTs
resembling a carpet or forest. The presence of the barrier coating
can also influence the directionality of CNT growth.
[0101] The operation of disposing a catalyst on the carbon fiber
material can be accomplished by spraying or dip coating a solution
or by gas phase deposition via, for example, a plasma process. The
choice of techniques can be coordinated with the mode with which
the barrier coating is applied. Thus, in some embodiments, after
forming a solution of a catalyst in a solvent, catalyst can be
applied by spraying or dip coating the barrier coated carbon fiber
material with the solution, or combinations of spraying and dip
coating. Either technique, used alone or in combination, can be
employed once, twice, thrice, four times, up to any number of times
to provide a carbon fiber material that is sufficiently uniformly
coated with CNT-forming catalyst. When dip coating is employed, for
example, a carbon fiber material can be placed in a first dip bath
for a first residence time in the first dip bath. When employing a
second dip bath, the carbon fiber material can be placed in the
second dip bath for a second residence time. For example, carbon
fiber materials can be subjected to a solution of CNT-forming
catalyst for between about 3 seconds to about 90 seconds depending
on the dip configuration and linespeed. Employing spraying or dip
coating processes, a carbon fiber material with a surface density
of catalyst of less than about 5% surface coverage to as high as
about 80% coverage, in which the CNT-forming catalyst nanoparticles
are nearly monolayer. In some embodiments, the process of coating
the CNT-forming catalyst on the carbon fiber material should
produce no more than a monolayer. For example, CNT growth on a
stack of CNT-forming catalyst can erode the degree of infusion of
the CNT to the carbon fiber material. In other embodiments, the
transition metal catalyst can be deposited on the carbon fiber
material using evaporation techniques, electrolytic deposition
techniques, and other processes known to those skilled in the art,
such as addition of the transition metal catalyst to a plasma
feedstock gas as a metal organic, metal salt or other composition
promoting gas phase transport.
[0102] Because processes of the invention are designed to be
continuous, a spoolable carbon fiber material can be dip-coated in
a series of baths where dip coating baths are spatially separated.
In a continuous process in which nascent carbon fibers are being
generated de novo, dip bath or spraying of CNT-forming catalyst can
be the first step after applying and curing or partially curing a
barrier coating to the carbon fiber material. Application of the
barrier coating and a CNT-forming catalyst can be performed in lieu
of application of a sizing, for newly formed carbon fiber
materials. In other embodiments, the CNT-forming catalyst can be
applied to newly formed carbon fibers in the presence of other
sizing agents after barrier coating. Such simultaneous application
of CNT-forming catalyst and other sizing agents can still provide
the CNT-forming catalyst in surface contact with the barrier
coating of the carbon fiber material to insure CNT infusion.
[0103] The catalyst solution employed can be a transition metal
nanoparticle which can be any d-block transition metal as described
above. In addition, the nanoparticles can include alloys and
non-alloy mixtures of d-block metals in elemental form or in salt
form, and mixtures thereof. Such salt forms include, without
limitation, oxides, carbides, and nitrides. Non-limiting exemplary
transition metal NPs include Ni, Fe, Co, Mo, Cu, Pt, Au, and Ag and
salts thereof and mixtures thereof. In some embodiments, such
CNT-forming catalysts are disposed on the carbon fiber by applying
or infusing a CNT-forming catalyst directly to the carbon fiber
material simultaneously with barrier coating deposition. Many of
these transition metal catalysts are readily commercially available
from a variety of suppliers, including, for example, Ferrotec
Corporation (Bedford, N.H.).
[0104] Catalyst solutions used for applying the CNT-forming
catalyst to the carbon fiber material can be in any common solvent
that allows the CNT-forming catalyst to be uniformly dispersed
throughout. Such solvents can include, without limitation, water,
acetone, hexane, isopropyl alcohol, toluene, ethanol, methanol,
tetrahydrofuran (THF), cyclohexane or any other solvent with
controlled polarity to create an appropriate dispersion of the
CNT-forming catalyst nanoparticles. Concentrations of CNT-forming
catalyst can be in a range from about 1:1 to 1:10000 catalyst to
solvent. Such concentrations can be used when the barrier coating
and CNT-forming catalyst is applied simultaneously as well.
[0105] In some embodiments heating of the carbon fiber material can
be at a temperature that is between about 500.degree. C. and
1000.degree. C. to synthesize carbon nanotubes after deposition of
the CNT-forming catalyst. Heating at these temperatures can be
performed prior to or substantially simultaneously with
introduction of a carbon feedstock for CNT growth.
[0106] In some embodiments, the present invention provides a
process that includes removing sizing agents from a carbon fiber
material, applying a barrier coating conformally over the carbon
fiber material, applying a CNT-forming catalyst to the carbon fiber
material, heating the carbon fiber material to at least 500.degree.
C., and synthesizing carbon nanotubes on the carbon fiber material.
In some embodiments, operations of the CNT-infusion process include
removing sizing from a carbon fiber material, applying a barrier
coating to the carbon fiber material, applying a CNT-forming
catalyst to the carbon fiber, heating the fiber to CNT-synthesis
temperature and CVD-promoted CNT growth the catalyst-laden carbon
fiber material. Thus, where commercial carbon fiber materials are
employed, processes for constructing CNT-infused carbon fibers can
include a discrete step of removing sizing from the carbon fiber
material before disposing barrier coating and the catalyst on the
carbon fiber material.
[0107] The step of synthesizing carbon nanotubes can include
numerous techniques for forming carbon nanotubes, including those
disclosed in co-pending U.S. Patent Application No. US 2004/0245088
which is incorporated herein by reference. The CNTs grown on fibers
of the present invention can be accomplished by techniques known in
the art including, without limitation, micro-cavity, thermal or
plasma-enhanced CVD techniques, laser ablation, arc discharge, and
high pressure carbon monoxide (HiPCO). During CVD, in particular, a
barrier coated carbon fiber material with CNT-forming catalyst
disposed thereon, can be used directly. In some embodiments, any
conventional sizing agents can be removed prior CNT synthesis. In
some embodiments, acetylene gas is ionized to create a jet of cold
carbon plasma for CNT synthesis. The plasma is directed toward the
catalyst-bearing carbon fiber material. Thus, in some embodiments
synthesizing CNTs on a carbon fiber material includes (a) forming a
carbon plasma; and (b) directing the carbon plasma onto the
catalyst disposed on the carbon fiber material. The diameters of
the CNTs that are grown are dictated by the size of the CNT-forming
catalyst as described above. In some embodiments, the sized fiber
substrate is heated to between about 550 to about 800.degree. C. to
facilitate CNT synthesis. To initiate the growth of CNTs, two gases
are bled into the reactor: a process gas such as argon, helium, or
nitrogen, and a carbon-containing gas, such as acetylene, ethylene,
ethanol or methane. CNTs grow at the sites of the CNT-forming
catalyst.
[0108] In some embodiments, the CVD growth is plasma-enhanced. A
plasma can be generated by providing an electric field during the
growth process. CNTs grown under these conditions can follow the
direction of the electric field. Thus, by adjusting the geometry of
the reactor vertically aligned carbon nanotubes can be grown
radially about a cylindrical fiber. In some embodiments, a plasma
is not required for radial growth about the fiber. For carbon fiber
materials that have distinct sides such as tapes, mats, fabrics,
plies, and the like, catalyst can be disposed on one or both sides
and correspondingly, CNTs can be grown on one or both sides as
well.
[0109] As described above, CNT-synthesis is performed at a rate
sufficient to provide a continuous process for functionalizing
spoolable carbon fiber materials. Numerous apparatus configurations
faciliate such continuous synthesis as exemplified below.
[0110] In some embodiments, CNT-infused carbon fiber materials can
be constructed in an "all plasma" process. An all plasma process
can being with roughing the carbon fiber material with a plasma as
described above to improve fiber surface wetting characteristics
and provide a more conformal barrier coating, as well as improve
coating adhesion via mechanical interlocking and chemical adhesion
through the use of functionalization of the carbon fiber material
by using specific reactive gas species, such as oxygen, nitrogen,
hydrogen in argon or helium based plasmas.
[0111] Barrier coated carbon fiber materials pass through numerous
further plasma-mediated steps to form the final CNT-infused
product. In some embodiments, the all plasma process can include a
second surface modification after the barrier coating is cured.
This is a plasma process for "roughing" the surface of the barrier
coating on the carbon fiber material to facilitate catalyst
deposition. As described above, surface modification can be
achieved using a plasma of any one or more of a variety of
different gases, including, without limitation, argon, helium,
oxygen, ammonia, hydrogen, and nitrogen.
[0112] After surface modification, the barrier coated carbon fiber
material proceeds to catalyst application. This is a plasma process
for depositing the CNT-forming catalyst on the fibers. The
CNT-forming catalyst is typically a transition metal as described
above. The transition metal catalyst can be added to a plasma
feedstock gas as a precursor in the form of a ferrofluid, a metal
organic, metal salt or other composition for promoting gas phase
transport. The catalyst can be applied at room temperature in the
ambient environment with neither vacuum nor an inert atmosphere
being required. In some embodiments, the carbon fiber material is
cooled prior to catalyst application.
[0113] Continuing the all-plasma process, carbon nanotube synthesis
occurs in a CNT-growth reactor. This can be achieved through the
use of plasma-enhanced chemical vapor deposition, wherein carbon
plasma is sprayed onto the catalyst-laden fibers. Since carbon
nanotube growth occurs at elevated temperatures (typically in a
range of about 500 to 1000.degree. C. depending on the catalyst),
the catalyst-laden fibers can be heated prior to exposing to the
carbon plasma. For the infusion process, the carbon fiber material
can be optionally heated until it softens. After heating, the
carbon fiber material is ready to receive the carbon plasma. The
carbon plasma is generated, for example, by passing a carbon
containing gas such as acetylene, ethylene, ethanol, and the like,
through an electric field that is capable of ionizing the gas. This
cold carbon plasma is directed, via spray nozzles, to the carbon
fiber material. The carbon fiber material can be in close proximity
to the spray nozzles, such as within about 1 centimeter of the
spray nozzles, to receive the plasma. In some embodiments, heaters
are disposed above the carbon fiber material at the plasma sprayers
to maintain the elevated temperature of the carbon fiber
material.
[0114] Another configuration for continuous carbon nanotube
synthesis involves a special rectangular reactor for the synthesis
and growth of carbon nanotubes directly on carbon fiber materials.
The reactor can be designed for use in a continuous in-line process
for producing carbon-nanotube bearing fibers. In some embodiments,
CNTs are grown via a chemical vapor deposition ("CVD") process at
atmospheric pressure and at elevated temperature in the range of
about 550.degree. C. to about 800.degree. C. in a multi-zone
reactor. The fact that the synthesis occurs at atmospheric pressure
is one factor that facilitates the incorporation of the reactor
into a continuous processing line for CNT-on-fiber synthesis.
Another advantage consistent with in-line continuous processing
using such a zone reactor is that CNT growth occurs in a seconds,
as opposed to minutes (or longer) as in other procedures and
apparatus configurations typical in the art.
[0115] CNT synthesis reactors in accordance with the various
embodiments include the following features:
[0116] Rectangular Configured Synthesis Reactors: The cross section
of a typical CNT synthesis reactor known in the art is circular.
There are a number of reasons for this including, for example,
historical reasons (cylindrical reactors are often used in
laboratories) and convenience (flow dynamics are easy to model in
cylindrical reactors, heater systems readily accept circular tubes
(quartz, etc.), and ease of manufacturing. Departing from the
cylindrical convention, the present invention provides a CNT
synthesis reactor having a rectangular cross section. The reasons
for the departure are as follows: 1. Since many carbon fiber
materials that can be processed by the reactor are relatively
planar such as flat tape or sheet-like in form, a circular cross
section is an inefficient use of the reactor volume. This
inefficiency results in several drawbacks for cylindrical CNT
synthesis reactors including, for example, a) maintaining a
sufficient system purge; increased reactor volume requires
increased gas flow rates to maintain the same level of gas purge.
This results in a system that is inefficient for high volume
production of CNTs in an open environment; b) increased carbon
feedstock gas flow; the relative increase in inert gas flow, as per
a) above, requires increased carbon feedstock gas flows. Consider
that the volume of a 12K carbon fiber tow is 2000 times less than
the total volume of a synthesis reactor having a rectangular cross
section. In an equivalent growth cylindrical reactor (i.e., a
cylindrical reactor that has a width that accommodates the same
planarized carbon fiber material as the rectangular cross-section
reactor), the volume of the carbon fiber material is 17,500 times
less than the volume of the chamber. Although gas deposition
processes, such as CVD, are typically governed by pressure and
temperature alone, volume has a significant impact on the
efficiency of deposition. With a rectangular reactor there is a
still excess volume. This excess volume facilitates unwanted
reactions; yet a cylindrical reactor has about eight times that
volume. Due to this greater opportunity for competing reactions to
occur, the desired reactions effectively occur more slowly in a
cylindrical reactor chamber. Such a slow down in CNT growth, is
problematic for the development of a continuous process. One
benefit of a rectangular reactor configuration is that the reactor
volume can be decreased by using a small height for the rectangular
chamber to make this volume ratio better and reactions more
efficient. In some embodiments of the present invention, the total
volume of a rectangular synthesis reactor is no more than about
3000 times greater than the total volume of a carbon fiber material
being passed through the synthesis reactor. In some further
embodiments, the total volume of the rectangular synthesis reactor
is no more than about 4000 times greater than the total volume of
the carbon fiber material being passed through the synthesis
reactor. In some still further embodiments, the total volume of the
rectangular synthesis reactor is less than about 10,000 times
greater than the total volume of the carbon fiber material being
passed through the synthesis reactor. Additionally, it is notable
that when using a cylindrical reactor, more carbon feedstock gas is
required to provide the same flow percent as compared to reactors
having a rectangular cross section. It should be appreciated that
in some other embodiments, the synthesis reactor has a cross
section that is described by polygonal forms that are not
rectangular, but are relatively similar thereto and provide a
similar reduction in reactor volume relative to a reactor having a
circular cross section; c) problematic temperature distribution;
when a relatively small-diameter reactor is used, the temperature
gradient from the center of the chamber to the walls thereof is
minimal. But with increased size, such as would be used for
commercial-scale production, the temperature gradient increases.
Such temperature gradients result in product quality variations
across a carbon fiber material substrate (i.e., product quality
varies as a function of radial position). This problem is
substantially avoided when using a reactor having a rectangular
cross section. In particular, when a planar substrate is used,
reactor height can be maintained constant as the size of the
substrate scales upward. Temperature gradients between the top and
bottom of the reactor are essentially negligible and, as a
consequence, thermal issues and the product-quality variations that
result are avoided. 2. Gas introduction: Because tubular furnaces
are normally employed in the art, typical CNT synthesis reactors
introduce gas at one end and draw it through the reactor to the
other end. In some embodiments disclosed herein, gas can be
introduced at the center of the reactor or within a target growth
zone, symmetrically, either through the sides or through the top
and bottom plates of the reactor. This improves the overall CNT
growth rate because the incoming feedstock gas is continuously
replenishing at the hottest portion of the system, which is where
CNT growth is most active. This constant gas replenishment is an
important aspect to the increased growth rate exhibited by the
rectangular CNT reactors.
[0117] Zoning. Chambers that provide a relatively cool purge zone
depend from both ends of the rectangular synthesis reactor.
Applicants have determined that if hot gas were to mix with the
external environment (i.e., outside of the reactor), there would be
an increase in degradation of the carbon fiber material. The cool
purge zones provide a buffer between the internal system and
external environments. Typical CNT synthesis reactor configurations
known in the art typically require that the substrate is carefully
(and slowly) cooled. The cool purge zone at the exit of the present
rectangular CNT growth reactor achieves the cooling in a short
period of time, as required for the continuous in-line
processing.
[0118] Non-contact, hot-walled, metallic reactor. In some
embodiments, a hot-walled reactor is made of metal is employed, in
particular stainless steel. This may appear counterintuitive
because metal, and stainless steel in particular, is more
susceptible to carbon deposition (i.e., soot and by-product
formation). Thus, most CNT reactor configurations use quartz
reactors because there is less carbon deposited, quartz is easier
to clean, and quartz facilitates sample observation. However,
Applicants have observed that the increased soot and carbon
deposition on stainless steel results in more consistent, faster,
more efficient, and more stable CNT growth. Without being bound by
theory it has been indicated that, in conjunction with atmospheric
operation, the CVD process occurring in the reactor is diffusion
limited. That is, the catalyst is "overfed;" too much carbon is
available in the reactor system due to its relatively higher
partial pressure (than if the reactor was operating under partial
vacuum). As a consequence, in an open system--especially a clean
one--too much carbon can adhere to catalyst particles, compromising
their ability to synthesize CNTs. In some embodiments, the
rectangular reactor is intentionally run when the reactor is
"dirty," that is with soot deposited on the metallic reactor walls.
Once carbon deposits to a monolayer on the walls of the reactor,
carbon will readily deposit over itself. Since some of the
available carbon is "withdrawn" due to this mechanism, the
remaining carbon feedstock, in the form of radicals, react with the
catalyst at a rate that does not poison the catalyst. Existing
systems run "cleanly" which, if they were open for continuous
processing, would produced a much lower yield of CNTs at reduced
growth rates.
[0119] Although it is generally beneficial to perform CNT synthesis
"dirty" as described above, certain portions of the apparatus, such
as gas manifolds and inlets, can nonetheless negatively impact the
CNT growth process when soot created blockages. In order to combat
this problem, such areas of the CNT growth reaction chamber can be
protected with soot inhibiting coatings such as silica, alumina, or
MgO. In practice, these portions of the apparatus can be dip-coated
in these soot inhibiting coatings. Metals such as INVAR.RTM. can be
used with these coatings as INVAR has a similar CTE (coefficient of
thermal expansion) ensuring proper adhesion of the coating at
higher temperatures, preventing the soot from significantly
building up in critical zones.
[0120] Combined Catalyst Reduction and CNT Synthesis. In the CNT
synthesis reactor disclosed herein, both catalyst reduction and CNT
growth occur within the reactor. This is significant because the
reduction step cannot be accomplished timely enough for use in a
continuous process if performed as a discrete operation. In a
typical process known in the art, a reduction step typically takes
1-12 hours to perform. Both operations occur in a reactor in
accordance with the present invention due, at least in part, to the
fact that carbon feedstock gas is introduced at the center of the
reactor, not the end as would be typical in the art using
cylindrical reactors. The reduction process occurs as the fibers
enter the heated zone; by this point, the gas has had time to react
with the walls and cool off prior to reacting with the catalyst and
causing the oxidation reduction (via hydrogen radical
interactions). It is this transition region where the reduction
occurs. At the hottest isothermal zone in the system, the CNT
growth occurs, with the greatest growth rate occurring proximal to
the gas inlets near the center of the reactor.
[0121] In some embodiments, when loosely affiliated carbon fiber
materials, such as carbon tow are employed, the continuous process
can include steps that spreads out the strands and/or filaments of
the tow. Thus, as a tow is unspooled it can be spread using a
vacuum-based fiber spreading system, for example. When employing
sized carbon fibers, which can be relatively stiff, additional
heating can be employed in order to "soften" the tow to facilitate
fiber spreading. The spread fibers which comprise individual
filaments can be spread apart sufficiently to expose an entire
surface area of the filaments, thus allowing the tow to more
efficiently react in subsequent process steps. Such spreading can
approach between about 4 inches to about 6 inches across for a 3 k
tow. The spread carbon tow can pass through a surface treatment
step that is composed of a plasma system as described above. After
a barrier coating is applied and roughened, spread fibers then can
pass through a CNT-forming catalyst dip bath. The result is fibers
of the carbon tow that have catalyst particles distributed radially
on their surface. The catalyzed-laden fibers of the tow then enter
an appropriate CNT growth chamber, such as the rectangular chamber
described above, where a flow through atmospheric pressure CVD or
PE-CVD process is used to synthesize the CNTs at rates as high as
several microns per second. The fibers of the tow, now with
radially aligned CNTs, exit the CNT growth reactor.
[0122] In some embodiments, CNT-infused carbon fiber materials can
pass through yet another treatment process that, in some
embodiments is a plasma process used to functionalize the CNTs.
Additional functionalization of CNTs can be used to promote their
adhesion to particular resins. Thus, in some embodiments, the
present invention provides CNT-infused carbon fiber materials
having functionalized CNTs.
[0123] As part of the continuous processing of spoolable carbon
fiber materials, the a CNT-infused carbon fiber material can
further pass through a sizing dip bath to apply any additional
sizing agents which can be beneficial in a final product. Finally
if wet winding is desired, the CNT-infused carbon fiber materials
can be passed through a resin bath and wound on a mandrel or spool.
The resulting carbon fiber material/resin combination locks the
CNTs on the carbon fiber material allowing for easier handling and
composite fabrication. In some embodiments, CNT infusion is used to
provide improved filament winding. Thus, CNTs formed on carbon
fibers such as carbon tow, are passed through a resin bath to
produce resin-impregnated, CNT-infused carbon tow. After resin
impregnation, the carbon tow can be positioned on the surface of a
rotating mandrel by a delivery head. The tow can then be wound onto
the mandrel in a precise geometric pattern in known fashion.
[0124] The winding process described above provides pipes, tubes,
or other forms as are characteristically produced via a male mold.
But the forms made from the winding process disclosed herein differ
from those produced via conventional filament winding processes.
Specifically, in the process disclosed herein, the forms are made
from composite materials that include CNT-infused tow. Such forms
will therefore benefit from enhanced strength and the like, as
provided by the CNT-infused tow.
[0125] In some embodiments, a continuous process for infusion of
CNTs on spoolable carbon fiber materials can achieve a linespeed
between about 0.5 ft/min to about 36 ft/min. In this embodiment
where the CNT growth chamber is 3 feet long and operating at a
750.degree. C. growth temperature, the process can be run with a
linespeed of about 6 ft/min to about 36 ft/min to produce, for
example, CNTs having a length between about 1 micron to about 10
microns. The process can also be run with a linespeed of about 1
ft/min to about 6 ft/min to produce, for example, CNTs having a
length between about 10 microns to about 100 microns. The process
can be run with a linespeed of about 0.5 ft/min to about 1 ft/min
to produce, for example, CNTs having a length between about 100
microns to about 200 microns. The CNT length is not tied only to
linespeed and growth temperature, however, the flow rate of both
the carbon feedstock and the inert carrier gases can also influence
CNT length. For example, a flow rate consisting of less than 1%
carbon feedstock in inert gas at high linespeeds (6 ft/min to 36
ft/min) will result in CNTs having a length between 1 micron to
about 5 microns. A flow rate consisting of more than 1% carbon
feedstock in inert gas at high linespeeds (6 ft/min to 36 ft/min)
will result in CNTs having length between 5 microns to about 10
microns.
[0126] In some embodiments, more than one carbon material can be
run simultaneously through the process. For example, multiple tapes
tows, filaments, strand and the like can be run through the process
in parallel. Thus, any number of pre-fabricated spools of carbon
fiber material can be run in parallel through the process and
re-spooled at the end of the process. The number of spooled carbon
fiber materials that can be run in parallel can include one, two,
three, four, five, six, up to any number that can be accommodated
by the width of the CNT-growth reaction chamber. Moreover, when
multiple carbon fiber materials are run through the process, the
number of collection spools can be less than the number of spools
at the start of the process. In such embodiments, carbon strands,
tows, or the like can be sent through a further process of
combining such carbon fiber materials into higher ordered carbon
fiber materials such as woven fabrics or the like. The continuous
process can also incorporate a post processing chopper that
facilitates the formation CNT-infused chopped fiber mats, for
example.
[0127] It is understood that modifications which do not
substantially affect the activity of the various embodiments of
this invention are also included within the definition of the
invention provided herein. Accordingly, the following example is
intended to illustrate but not limit the present invention.
Example I
[0128] This example shows the manufacture of CNT infused coating
for use in a solar receiver and characterization of a model.
[0129] A CNT based coating can be manufactured by the following
procedure:
[0130] CNTs are infused to a carbon fiber tow (carbon fiber being
exemplary) in a reel-to-reel system as outlined above. The CNT
infused fiber tow is then wrapped over a heating element.
Additional reflective layers are added as needed. A coating made by
this procedure is expected to exhibit characteristics of being a
solar selective coating. The exact characteristics of a coating
employing CNT-infused fibers will depend on CNT length and
density.
[0131] FIG. 16 shows the reflectivity data for a model of this
CNT-infused fiber coating, namely Buckypaper, with an overlay of a
theoretical ideal coating indicated as a dashed line. The
CNT-infused fiber wrapped around a heating element has an
arrangement of CNTs similar to Buckypaper. The arrangement of CNTs
in Buckypaper are shown in the SEM image of FIG. 17.
[0132] The coating having CNT-infused fiber can be formed onto to
the outer surface of a heat absorber element for incorporation into
a solar receiver, such as the one exemplified in FIG. 18. This
solar receiver includes an annulus surrounding the heat absorbing
element coated with CNT-infused coating. The annulus can be
borosilicate glass with an anti-reflective coating on its outer
surface, inner surface, or both inner and outer surfaces to
maximize the amount of incident radiation transmitted through
annulus. The annulus can be evacuated to a pressure (less than or
equal to 0.0001 Torr) to minimize heat loss due to convection in
the air present between CNT-infused coating and annulus.
[0133] While the foregoing invention has been described with
reference to the above-described embodiment, various modifications
and changes can be made without departing from the spirit of the
invention. Accordingly, all such modifications and changes are
considered to be within the scope of the appended claims.
* * * * *