Note: Descriptions are shown in the official language in which they were submitted.
WO 2010/118176 PCT/US2010/030300
SOLAR RECEIVER UTILIZING CARBON NANOTUBE INFUSED COATINGS
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 April 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/m2 and wavelengths ranging from about 200 nanometers (nn) 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
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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] Figure 1 is a profile view of an exemplary solar receiver having a CNT-
infused
coating on the outer surface of a heat absorbing element.
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[0008] Figure 2 is a profile of a solar receiver as shown in Figure 1, further
including
grooves on the outer surface of the heat absorber element.
[0009] Figure 3 is a profile view of a solar receiver as shown in Figure 1,
further
including an environmental coating over the CNT-infused coating.
[0010] Figure 4 is a profile view of a solar receiver as shown in Figure 3,
further
including grooves on the outer surface of the heat absorber element, according
to a fourth
embodiment of the invention;
[0011] Figure 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] Figure 6 is a cross-sectional view of the ceramic low emissivity,
environmental
integrated coating of Figure 5, further including an anti-reflective coating,
according to an
embodiment of the invention;
[0013] Figure 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] Figure 8 is a cross-sectional view of an anti-reflective coating
applied over the
metallic low emissivity, environmental integrated coating as shown in Figure
7, according to
an embodiment of the invention;
[0015] Figure 9 is a cross-sectional view of a layered cermet low emissivity,
environmental integrated coating applied over the integrated coating as shown
in Figure 5,
according to an embodiment of the invention;
[0016] Figure 10 is a cross-sectional view of the layered cermet low
emissivity,
environmental integrated coating as shown in Figure 9, further including an
anti-reflective
coating, according to an embodiment of the invention;
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[0017] Figure 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] Figure 12 is a cross-sectional view of the integrated cermet low
emissivity,
environmental CNT-infused coating as shown in Figure 11, further including an
anti-reflective coating, according to an embodiment of the invention;
[0019] Figure 13 is a cross-sectional view of a solar receiver with an
annulus, according
to an embodiment of the invention;
[0020] Figure 14 is a cross-sectional view of a solar receiver as shown in
Figure 13,
further including grooves as described in the second embodiment shown in
Figure 2,
according to an embodiment of the invention.
[0021] Figure 15 shows a process for producing CNT-infused carbon fiber
material in
accordance with the illustrative embodiment of the present invention.
[0022] Figure 16 shows reflectivity data for a coating that includes a CNT-
infused fiber
material.
[0023] Figure 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] Figure 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 rim 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
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conductivities known for any material with some indications as high as about
6,600 Wm-'K-1
(Berber et al. Phys. Rev. Lett. 84(20):4613-4616, (2000)).
[00261 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.
[00271 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.
[00281 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
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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-no)2/(n+no)2
where R is reflectance, n is the refractive index of the object, and no 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, no.
[00291 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.
[00301 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.
[00311 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
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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
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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/1,000m) or 620 yard/lb (Grafil, Inc.,
Sacramento, CA).
Commercial carbon fiber tow, in particular, can be obtained in 5, 10, 20, 50,
and 100 lb. (for
spools having high weight, usually a 3k/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 (S)VNTs), double-walled carbon nanotubes (D)VNTs),
multi-walled
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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/ m2 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
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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 radation
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
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generating applications, for example, in driving a turbine with steam. High
temperature
applications can be any application utilizing temperatures greater than about
400 C. Low
temperature applications include, for example, pool heating or crop drying.
Such
temperatures can be about 10-100 C higher than ambient temperatures. Any
applications
utilizing temperature between about 100 C and 400 C are considered mid
temperature
applications. Exemplary mid temperature application can include, for example,
a parabolic
trough or concentrating solar power plant.
[00481 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.
[00491 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.
[00501 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.
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[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
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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 Figure 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
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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 Figure 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. CNT-infused 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
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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 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 February 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 CNT-infused
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). CNT-,infused
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coating 120 may include carbon nanotubes having a length in the range of about
ten (10)
microns to about hundreds of microns.
[00671 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.
[00681 Referring now to Figure 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
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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 Figure 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
Figure 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 Figure 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 Figure 2), further including environmental coating 310 as described
for solar receiver
300 (of Figure 3).
[0072] Referring now to Figure 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
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at least solar radiation to permit the incident radiation to reach CNT infused
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 to 500 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 Figure 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 Figure 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
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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 to 500 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
(Mo02), Tungsten oxide (WO),titanium oxide (TiO), Titanium nitride (TiN) ,
Iron (Fe), and
Ferric oxide (Fe203).
[0075] Referring now to Figure 8, in another configuration of solar receiver
800, metallic
environmental coating 710 (of Figure 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 Figure 9, in another configuration of solar receiver 900,
the
integrated ceramic environmental coating 510 and CNT-infused coating 120 (of
Figure 5) is
further coated with a metal coating 710 (of Figure 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
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provides additional structural integrity to the underlying integrated ceramic
coating 510 and
CNT-infused coating 120.
[0077] Referring now to Figure 10, in another configuration of solar receiver
1000, the
integrated cermet coatings of Figure 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 Figure 11, in another configuration of solar receiver
1100, the
integrated ceramic environmental coating 510 and CNT-infused coating 120 (of
Figure 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 CNT infused coating
120 and
integrated ceramic coating 510 doped with metal particles 1110.
[0079] Referring to Figure 12, in another configuration 1200, the integrated
layer of
coatings of Figure 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 Figure 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 Figure 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
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WO 2010/118176 PCT/US2010/030300
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.
[00811 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.
[00821 In an exemplary embodiment, solar receiver 1300 may include air gaps or
air
pockets between annulus 13 10 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 CNT infused 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
Figures 5-12.
[00831 Referring now to Figure 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,
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WO 2010/118176 PCT/US2010/030300
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 Figure 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 Figures 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
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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] Figure 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.
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
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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.
[0095] 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.
[0096] As described further below and in conjunction with Figure 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.
[0097] With reference to the illustrative embodiment of Figure 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 C. Accordingly, operation 1504
involves
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heating the barrier-coated carbon fiber material to a temperature in the
aforementioned range
to support carbon nanotube synthesis.
[0098] 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.
[0099] 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.
[0100] 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
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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.
[01011 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.
[01021 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
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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, NH).
[0103] 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.
[0104] In some embodiments heating of the carbon fiber material can be at a
temperature
that is between about 500 C and 1000 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.
[0105] 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 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.
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[0106] 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 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.
[0107] 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.
[0108] 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.
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[0109] 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.
[0110] 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.
[0111] 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.
[0112] 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
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.
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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.
[01131 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 C to about 800 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.
[01141 CNT synthesis reactors in accordance with the various embodiments
include the
following features:
[01151 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.
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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
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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.
[01161 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.
[01171 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
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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.
[0118] 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 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.
[0119] 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
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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.
[0120] 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 3k 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.
[0121] 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
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adhesion to particular resins. Thus, in some embodiments, the present
invention provides
CNT-infused carbon fiber materials having functionalized CNTs.
[0122] 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.
[0123] 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.
[0124] 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 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 I 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
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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.
[0125] 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.
[0126] 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
[0127] This example shows the manufacture of CNT infused coating for use in a
solar
receiver and characterization of a model.
[0128] A CNT based coating can be manufactured by the following procedure:
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[0129] 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.
[0130] Figure 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 Figure 17.
[0131] 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
Figure 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.
[0132] 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.
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