Note: Descriptions are shown in the official language in which they were submitted.
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Atty. Docket No.: 071226-0182
APPARATUS AND METHOD FOR THE PRODUCTION OF CARBON
NANOTUBES ON A CONTINUOUSLY MOVING SUBSTRATE
STATEMENT OF RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. Application No.
12/714,389 filed
February 26, 2010, which claims priority to U.S. Provisional Application No.
61/168,516
filed April 10, 2009 and to U.S. Provisional Application No. 61/295,624 filed
January 15,
2010, which are incorporated herein by reference in their entirety.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR
DEVELOPMENT
[0002] Not applicable.
FIELD OF THE INVENTION
[0003] The present invention relates in general to an apparatus and method for
the
production of carbon nanotubes on a continuously moving substrate.
BACKGROUND OF THE INVENTION
[0004] Current carbon nanotube (CNT) synthesis techniques can provide bulk
quantities
of "loose" CNTs for use in a variety of applications. These bulk CNTs can be
used as a
modifier or dopant in composite systems, for example. Such modified composites
typically
exhibit enhanced properties that represent a small fraction of the theoretical
improvements
expected by the presence of CNTs. The failure to realize the full potential of
CNT
enhancement is related, in part, to the inability to dope beyond low
percentages of CNTs (1-
4%) in the resulting composite along with an overall inability to effectively
disperse the
CNTs within the structure. This low loading, coupled with difficulties in CNT
alignment and
CNT-to-matrix interfacial properties figure in the observed marginal increases
in composite
properties, such as mechanical strength, compared to the theoretical strength
of CNTs.
Besides the physical limitation of bulk CNT incorporation, the price of CNTs
remain high
due to process inefficiencies and post processing required to purify the end
CNT product.
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[0005] One approach to overcome the above deficiencies, would be to develop
techniques
that grow CNTs directly on useful substrates, such as fibers, which can be
used to organize
the CNTs and provide a reinforcing materials in a composite. Attempts have
been made to
grow CNTs in a nearly continuous fashion, however, none have been successful
such that
they operate continuously, roll to roll without batch-wise processing. The
present invention
provides an apparatus and method that allows for continuous production of CNTs
on a
variety of substrates and provides related advantages as well.
[0006] Some processes attempt to grow CNTs directly on fiber substrates;
illustrative
thereof is the process disclosed in U.S. Pat. No. 7,338,684 to Curliss et al.
This patent
discloses a method for producing vapor-grown carbon-fiber-reinforced composite
materials.
According to the patent, a catalyst precursor such as a ferric nitrate
solution is applied as a
coating to fiber preform. The coated preform is then heated in air, typically
at a temperature
in the range of 300 C to 800 C, to decompose the precursor and yield an
oxidized catalyst
particle. Some of the examples disclose a heating time of 30 hrs. To reduce
the catalyst
particle to a metallic state, the preform is exposed to a flowing gas mixture
including
hydrogen. This is typically performed at a temperature of 400 C to 800 C for
a period of
time in the range of about 1 hour to about 12 hours.
[0007] Vapor grown carbon fiber is produced by contacting a gas phase
hydrocarbon gas
mixture with the preform at a temperature between about 500 C to 1200 C.
According to
the patent, the fibers grow on the composite preform resulting in a tangled
mass of carbon
fiber. The reaction time for growth varies between 15 minutes and 2 hours,
primarily as a
function of feed gas composition and temperature.
[0008] The processing times for the approach disclosed in 7,338,684 are too
long for
efficient processing. Furthermore, due to the extreme variation in the
processing time for
various steps, the process is unsuitable for implementation as a continuous
processing line for
the production of carbon nanotubes on a continuously moving substrate.
SUMMARY OF THE INVENTION
[0009] In some aspects, embodiments disclosed herein relate to an apparatus
capable of
linear and/or continuous CNT synthesis on spoolable length substrates. The
apparatus
includes at least one carbon nanotube growth zone having a substrate inlet
sized to allow a
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spoolable length substrate to pass therethrough, at least one heater in
thermal communication
with the carbon nanotube growth zone, and at least one feed gas inlet in fluid
communication
with the carbon nanotube growth zone, wherein the apparatus is open to an
atmospheric
environment during operation. The CNT growth is carried out at ambient or near
ambient
pressures. The apparatus is designed to be integrated into a system for the
continuous growth
of carbon nanotubes.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] Figure 1 shows a simplified perspective view of an apparatus for the
synthesis of
CNTs in a continuous process in accordance with an embodiment of the present
invention.
[0011] Figure 2 shows a simplified cross-sectional side view of an apparatus
for the
synthesis of CNTs in a continuous process in accordance with an illustrative
embodiment of
the present invention.
[0012] Figure 3 shows a cross-sectional side view of an embodiment of an
apparatus in
accordance with the present invention.
[0013] Figure 4 shows a cross-sectional side view of an embodiment of an
apparatus in
accordance with the present invention.
[0014] Figure 5 shows a top cross-sectional view of the apparatus of Figure 3
in
accordance with the present invention.
[0015] Figure 6 shows a top cross-sectional view of an embodiment of an
apparatus in
accordance with the present invention.
DETAILED DESCRIPTION
[0016] The present invention relates in general to an apparatus and method for
the
production of carbon nanotubes on a continuously moving substrate. 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. Carbon nanotubes exhibit impressive
physical
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properties. The strongest CNTs exhibit roughly eighty times the strength, six
times the
toughness (i.e., Young's Modulus), and one-sixth the density of high carbon
steel.
[0017] In accordance with some embodiments, apparatus 100 is used to grow,
produce,
deposit, or otherwise generate CNTs in situ directly onto or into moving
substrate 106 and
takes the form of an open ended, atmospheric, to slightly higher than
atmospheric pressure,
small cavity, chemical vapor deposition (CVD) CNT growth system. In accordance
with the
illustrative embodiment, CNTs are grown via CVD at atmospheric pressure and at
elevated
temperature (typically in the range of about 550 C to about 800 C) in a
multi-zone
apparatus 100. The fact that the synthesis occurs at atmospheric pressure is
one factor that
facilitates the incorporation of apparatus 100 into a continuous processing
line for CNT-on-
fiber synthesis. The fact that CNT growth occurs in seconds, as opposed to
minutes (or
longer) in the prior art, is another feature that enables using the apparatus
disclosed herein in
a continuous processing line. CNT-synthesis can be performed at a rate
sufficient to provide
a continuous process for functionalizing spoolable substrates. Numerous
apparatus
configurations facilitate such continuous synthesis.
[0018] Apparatus 100 includes at least one CNT growth zone 108 equipped with
growth
heaters 110 disposed between two quench or purge zones 114, 116. Any number of
growth
heaters can be included, (e.g., heaters 110a, 110b, 110c, 110d of Figure 4).
Apparatus 100
optionally includes pre-heater 132 that pre-heats feed gas 128 and feed gas
diffuser 136 to
evenly distribute feed gas 128.
[0019] In order to realize the potential enhancements afforded by CNT
introduction into
various materials and applications, an apparatus for applying CNTs directly to
substrate
surfaces is disclosed herein. CNTs applied directly on substrate surfaces,
particularly in the
case of silicon wafers or composite fiber materials, improves overall CNT
dispersion,
placement, and alignment in the completed structure. In the case of composite
materials, the
incorporation of CNTs on the fiber or fabric level improves CNT loading by
having the
CNTs preordered and placed in the composite structure, instead of having to
dope resins with
loose CNTs. To grow CNTs directly on a substrate in a continuous process
improves not
only these physical characteristics but also reduces overall CNT cost. By
having CNTs
grown directly on the final useful substrate surface, the auxiliary costs
involved with CNT
purification and doping/mixing/placement/dispersion are removed.
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[0020] Referring to Figure 1, apparatus 100 can include substrate inlet 118
sized to allow
spoolable length substrate 106 to continually pass therethrough, allowing for
the synthesis
and growth of CNTs directly on substrate 106. Apparatus 100 can be a multi-
zone apparatus
with seed or CNT growth zone 108 between a pre-process purge or first purge
zone 114 and
a post-process purge or second purge zone 116. Apparatus 100 can be open to
the
atmospheric environment during operation, with first end 120 and second end
124, such that
substrate 106 enters apparatus 100 through substrate inlet 118 in first end
120, passes through
first purge zone 114, CNT growth zone 108, second purge zone 116 and out
through
substrate outlet 122 (shown in Figure 2) in second end 124. In some
embodiments, the CNT
growth system can include additional zones that are specifically designed to
activate catalyst
particles via reduction reactions. In such embodiments, a catalyst activation
zone can be
placed between first purge zone 114 and CNT growth zone 108. Alternatively,
the catalyst
activation zone can be placed just before first purge zone 114, with its own
pre-purge zone.
[0021] Apparatus 100 allows for the seamless transfer of substrate 106 into
and out of
CNT growth zone 108, obviating the need for batch runs. Spoolable length
substrate 106
effectively passes through an equilibrated growth system which has established
optimal
conditions for rapid CNT growth in real time as substrate 106 continually
moves through a
system that begins with spoolable length substrate 106 and winds the finished
product at the
end at CNT infusion on substrate 106. The ability to do this continuously and
efficiently,
while controlling parameters such as CNT length, density, and other
characteristics has not
been previously achieved.
[0022] In some embodiments, a continuous process for infusion of CNTs on
spoolable
substrates can achieve a linespeed between about 15 cm/min to about 1 m/min or
greater. In
this embodiment where CNT growth zone 108 is 100 cm long and operating at a
750 C
growth temperature, the process can be run with a linespeed of about 2 m/min
to about 11
m/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 30 cm/min to
about 2 m/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 15 cm/min to about
30 cm/min to
produce, for example, CNTs having a length between about 100 microns to about
200
microns. In some embodiments, a linespeed of up to at least 60 m/min can be
used for a
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continuous process for infusion. The CNT length is not tied only to linespeed
and growth
temperature, however, the flow rate of both the feed gas and inert purge gases
can also
influence CNT length. For example, a flow rate consisting of less than 1%
carbon feedstock
in inert gas at high linespeeds (2 m/min to 11 m/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 (2 m/min to 11 m/min) will result in
CNTs having
length between 5 microns to about 10 microns. Resulting growth rates for this
continuous
CNT growth system range depending on temperature, gases used, substrate
residence time,
and catalyst, however, growth rates on the range of 0.01-10 microns/second are
possible.
[0023] CNT growth zone 108 can be an open-air continuous operation, flow-
through
chamber. CNT growth zone 108 can be formed or otherwise bound by an enclosure
such as
stainless steel, titanium, carbon steel, INCONEL , INVAR , other high
temperature metals,
non-porous ceramics, or mixtures thereof, with additional features added to
improve
structural rigidity as well as reduce thermal warping due to repeated heat
cycling. CNT
growth zone 108 can be circular, rectangular, oval, or any number of polygonal
or other
geometrical variant cross-section based on the profile and size of substrate
passing
therethrough.
[0024] An internal volume of CNT growth zone 108 can be compared with a volume
of
substrate 106 having a length substantially equal to a length of CNT growth
zone 108. In
some embodiments, CNT growth zone 108 is designed to have an internal volume
of no more
than about 10000 times greater than the volume of substrate 106 disposed
within CNT
growth zone 108. In most embodiments, this number is greatly reduced to no
more than
about 4000 times. In other embodiments, this can be reduced to about 3000
times or less.
Similarly, cross sectional areas of CNT growth zone 108 can be limited to
about 10000,
4000, or 3000 times greater than a cross sectional area of substrate 106. In
some
embodiments, the volume of CNT growth zone 108 is less than or equal to about
10000% of
the volume of substrate 106 being fed therethrough. Without being bound by
theory,
reducing the size of CNT growth zone 108 ensures high probability interactions
between feed
gas 128 and substrates coated with catalyst particles. Larger volumes result
in excessive
unfavorable reactions as the treated substrate is only a small fraction of the
available volume.
CNT growth zone 108 can range from dimensions as small as millimeters wide to
as large as
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over 1600 mm wide. CNT growth zone 108 can have a rectangular cross-section
and a
volume of about 240 cm3 to as large as 8000 cm3. Temperature in CNT growth
zone 108 can
be controlled with imbedded thermocouples strategically placed on an interior
surface
thereof. Since CNT growth zone 108 is so small, the temperature of the
enclosure is nearly
the same temperature as the CNT growth zone 108 and gases inside. CNT growth
zone 108
can be maintained between about 550 C and about 900 C.
[0025] Referring now to Figure 2, Figure 3, and Figure 4, both purge zones
114, 116
provide the same function. As feed gas 128 (shown in Figure 2) from CNT growth
zone 108
exits apparatus 100, purge zones 114, 116 supply a continuous flow of purge
gas 130 (shown
in Figure 2) to buffer CNT growth zone 108 from the external environment. This
can
include optionally preheating purge zone 114 and/or cooling purge zone 116.
This helps to
prevent unwanted mixing of feed gas 128 with the outside atmospheric
environment, which
could cause unintended oxidation and damage to substrate 106 (shown in Figure
3 and
Figure 4) or CNT material. Purge zones 114, 116 are insulated from CNT growth
zone 108
to prevent excessive heat loss or transfer from heated CNT growth zone 108. In
some
embodiments, one or more exhaust ports 142 (shown in Figure 2) are placed
between purge
zones 114, 116 and CNT growth zone 108. In such embodiments, gas does not mix
between
CNT growth zone 108 and purge zones 114, 116, but instead exhausts to the
atmospheric
environment through ports 142. This also prevents gas mixing which is
important in
situations where multiple CNT growth zones 108 (e.g., 108a, 108b, 108c, in
Figure 6) can
be used in series, attached, or otherwise utilized together to extend the
overall effective CNT
growth zone. Purge zones 114, 116 in this embodiment still provide a cool gas
purge to
ensure reduced temperatures as substrate 106 enters/exits CNT growth zone 108.
[0026] Feed gas 128 can enter CNT growth zone 108 of apparatus 100 via one or
more
feed gas inlets 112 (e.g., 112a and 112b of Figure 4). Feed gas 128 can pass
through feed
gas inlet manifold 134 (shown in Figure 5) and into CNT growth zone 108 via
feed gas
diffusers 136 (shown in Figure 5). Feed gas 128 can react with catalyst
particles present on
or in substrate 106 to create CNTs, with any leftover feed gas 128 passing
through exhaust
manifold 140 (shown in Figure 6) or otherwise exit CNT growth zone 108. Purge
gas 130
can be used to prevent the hot gases inside CNT growth zone 108 from mixing
with the
oxygen rich gas outside CNT growth zone 108 and creating local oxidizing
conditions that
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could adversely affect substrate 106 entering or exiting CNT growth zone 108.
Purge gas
130 can enter purge zones 114, 116 of apparatus 100 at purge gas inlets 126,
127 (shown in
Figure 2), allowing for a buffer between CNT growth zone 108 and the external
environment. Purge gas 130 can prevent ambient gasses from entering CNT growth
zone
108, and can either exit through substrate inlet 118 or substrate outlet 122
at respective ends
120, 124 of apparatus 100 as indicated in Figure 2, or purge gas 130 can exit
through
exhaust manifold 140 (shown in Figure 6).
[0027] Purge gas preheater 132 (shown in Figure 3) can preheat purge gas 130
prior to
introduction into first purge zone 114. CNT growth zone 108 can be further
heated by
heaters 110 (shown in Figure 3) contained within CNT growth zone 108. As
illustrated,
heaters 110 are on either side of substrate 106. However, heaters 110 can be
anywhere
within CNT growth zone 108, either placed along the length or in cases of wide
systems,
along the width of CNT growth zone 108, to ensure isothermal heating for well
controlled
CNT growth processes. Heaters 110 can heat CNT growth zone 108 and maintain an
operational temperature at a pre-set level. Heaters 110 can be controlled by a
controller (not
shown). Heaters 110 can be any suitable device capable of maintaining CNT
growth zone
108 at about the operating temperature. Alternatively, or additionally,
heaters 111 (shown in
Figure 5 and Figure 6) can preheat feed gas 128. Any of heaters 110, 111, 132
can be used
in conjunction with CNT growth zone 108, so long as the particular heater is
in thermal
communication with CNT growth zone 108. Heaters 110, 111, 132 can include long
coils of
gas line heated by a resistively heated element, and/or series of expanding
tubes to slow
down gas flow, which is then heated via resistive heaters (e.g., infrared
heaters). Regardless
of the method, gas can be heated from about room temperature to a temperature
suitable for
CNT growth, e.g. from about 25 C to about 900 C, or up to 1000 C or more.
In some
instances, heaters 110, 111, and/or 132 can provide heat such that the
temperature within
CNT growth zone 108 is about 550 C to about 850 C or up to about 1000 C.
Temperature
controls (not shown) can provide monitoring and/or adjustment of temperature
within CNT
growth zone 108. Measurement can be made at points (e.g., probe 160 of Figure
9) on plates
or other structures defining CNT growth zone 108. Because the height of CNT
growth zone
108 is relatively small, the temperature gradient between the plates can be
very small, and
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thus, measurement of temperature of the plates can accurately reflect the
temperature within
CNT growth zone 108.
[0028] Because substrate 106 has a small thermal mass, as compared with CNT
growth
zone 108, substrate 106 can assume the temperature of CNT growth zone 108
almost
immediately. Thus, preheat can be left off to allow room temperature gas to
enter the growth
zone for heating by heaters 110. In some embodiments, only purge gas is
preheated. Other
feed gas can be added to purge gas after purge gas preheater 132. This can be
done to reduce
long term sooting and clogging conditions that can occur in purge gas
preheater 132 over
long times of operations. Preheated purge gas can then enter feed gas inlet
manifold 134.
[0029] This embodiment does not occur only with purge gas preheater 132, but
with all
preheaters. Feed gas 128 can be added after the preheating process to ensure
that the
preheater is not clogged with soot over long time operation of the system.
[0030] Feed gas inlet manifold 134 provides a cavity for further gas mixing as
well as a
means for dispersing and distributing gas to all gas insertion points in CNT
growth zone 108.
These points of insertion are built into one or more feed gas diffusers 136,
e.g. gas diffuser
plates with a series of patterned holes. As illustrated in Figure 4, gas
diffusers 136 are used
in series to create "gas stagnation" regions therebetween, where the larger
volumes slow
down the gas flow rate. Gas diffusers 136 are used to speed up gas between
each zone to
create a "billowing effect" as gas accelerates to the new stagnation region.
These
strategically placed holes ensure a consistent pressure and gas flow
distribution. Feed gas
enters CNT growth zone 108, where heaters 110 can apply an even temperature
generation
source.
[0031] A gas diffusing element of the growth system can also exist as an
embodiment
where instead of a diffuser plate, feed gas inlet manifold 134 is packed with
a high
temperature porous material such as alumina or silica ceramic or sintered
metal foams to
diffuse and spread the gas which can be introduced through a diffuser plate
that has a simple
slot which runs along the width of the chamber.
[0032] Referring now to Figure 5, in one exemplary embodiment, substrate 106
enters
first purge zone 114, where purge gas 130, which has been preheated by purge
gas preheater
132 warms substrate 106 while simultaneously preventing ambient air from
entering CNT
growth zone 108. Substrate 106 then passes through substrate inlet 118 in
first end 120 of
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CNT growth zone 108. As illustrated in Figure 5 and Figure 6, substrate 106
enters CNT
growth zone 108, is heated by heaters 110 (shown in Figure 6) and exposed to
feed gas 128
(shown in Figure 2). Before entering CNT growth zone 108, feed gas 128 can
move from
any of heaters 111, through any of feed gas inlets 112, through feed gas inlet
manifold 134,
and through feed gas diffusers 136. Feed gas 128 and/or purge gas 130 can exit
first purge
zone 114 and/or CNT growth zone 108 via exhaust ports 142 and/or exhaust
manifold 140,
maintaining atmospheric or slightly above atmospheric pressure. Substrate 106
can continue
through additional CNT growth zones 108 as desired until sufficient CNT growth
has
occurred. As illustrated in Figure 5, substrate 106 passes through substrate
outlet 122 in
second end 124 of CNT growth zone 108 and into second purge zone 116.
Alternatively,
first purge zone 114 and second purge zone 116 can be the same zone and
substrate 106 can
turn around within apparatus 100 and pass out of CNT growth zone 108 via
substrate inlet
118. In either event, substrate passes into a purge zone and out of apparatus
100. Purge
zones 114 and 116 can each have purge gas introduced through purge gas inlet
126 and 127
(shown in Figure 2), such that purge gas 130 therein acts as a buffer and
prevents feed gas
128 from contacting ambient air. Purge gas 130 may be relatively cool, such
that purge
zones 114 and 116 optionally act as a cooling zone, e.g., by helping reduce
the temperature
of the process gases before they exit the system. Likewise, Purge zones 114
and 116 can
each have exhaust ports 142 (shown in Figure 2) and/or exhaust manifolds 140
(shown in
Figure 6) to accomplish appropriate buffering. Access plate 138 (shown in
Figure 5) can
provide access to CNT growth zone 108, for cleaning and other maintenance.
[0033] In some embodiments, multiple substrates 106 (e.g., 106a, 106b, 106c in
Figure
4) can pass through apparatus 100 at any given time. Likewise, any number of
heaters can be
used either inside or outside a particular CNT growth zone 108.
[0034] Some of potential advantages of the apparatus and method of the present
teachings
can include, without limitation: improved cross-sectional area; improved
zoning; improved
materials; and combined catalyst reduction and CNT synthesis.
[0035] Since most of the material processed is relatively planar (e.g., flat
tape or sheet-
like form), the conventional circular cross-section is an inefficient use of
volume. Such
circular cross-section can create difficulties with maintaining a sufficient
system purge,
because an increased volume requires increased purge gas flow rates to
maintain the same
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level of gas purge. Thus, the conventional circular cross-section is
inefficient for high
volume production of CNTs in an open environment. Further such circular cross-
section can
create a need for increased feed gas flow. The relative increase in purge gas
flow requires
increased feed gas flows. For example, the volume of a 12K fiber is 2000 times
less than the
total volume of exemplary CNT growth zone 108 having a rectangular cross-
section. In an
equivalent growth cylindrical chamber (e.g., a cylindrical chamber having a
width that
accommodates the same planarized fiber as the rectangular cross-section CNT
growth zone
108), the volume of the fiber is 17,500 times less than the volume of CNT
growth zone 108.
Although gas deposition processes (e.g., CVD, etc.) are typically governed by
pressure and
temperature alone, volume has a significant impact on the efficiency of
deposition. With
illustrative rectangular CNT growth zone 108 there is quite a bit of excess
volume-volume
in which unwanted reactions occur (e.g., gasses reacting with themselves or
with chamber
walls); and a cylindrical chamber 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 chamber, which is problematic for the development of a
continuous
process. Additionally, it is notable that when using a cylindrical chamber,
more feed gas is
required to provide the same flow percent as in the illustrative CNT growth
zones having a
rectangular cross-section. Another problem with the conventional circular
cross-section is
temperature distribution. When a relatively small-diameter chamber 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 required for commercial-scale production, the
temperature gradient
increases. Such temperature gradients result in product quality variations
across a substrate
(i.e., product quality varies as a function of radial position). This problem
is substantially
avoided when using CNT growth zone 108 having a cross-section more closely
matched to
corresponding substrate 106 (e.g., rectangular). In particular, when a planar
substrate is used,
CNT growth zone 108 can have a height maintained constant as the size of
substrate 106
scales upward. Temperature gradients between the top and bottom of CNT growth
zone 108
are essentially negligible and, consequently, thermal issues and the product-
quality variations
that result are avoided.
[0036] The conventional circular cross-sectional chamber also requires feed
gas
introduction. Because quartz tube furnaces are used, conventional CNT
synthesis chambers
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introduce feed gas at one end and draw it through the chamber to the other
end. In the
illustrative embodiment disclosed herein, feed gas can be introduced at the
ends of, the center
of, or within CNT growth zone 108 (e.g., symmetrically, either through the
sides or through
the top and bottom plates of CNT growth zone 108). This can improve the
overall CNT
growth rate because the incoming feed gas in continuously replenishing at the
hottest portion
of the system, which is where CNT growth is most active. This constant feed
gas
replenishment can be an important aspect to the increased growth rate
exhibited by CNT
growth zone(s) 108 in accordance with the present teachings.
[0037] When hot feed gas mixes with the external environment, degradation of
the
substrate material (e.g., fiber) would increase. Conventional CNT synthesis
processes
typically require that the substrate is carefully (and slowly) cooled. Purge
zones 114, 116 on
either or both ends of CNT growth zone 108 disclosed herein provide a
temperature buffer
between the internal system and external environments. Purge zone 116 achieves
the cooling
in a short period of time, as may be required for the continuous processing
line. Purge zones
114, 116 can also provide a buffer to prevent oxidation at the interface
between the internal
system and external environments.
[0038] The use of metal (e.g., stainless steel, INVAR , INCONEL , etc.) in
accordance
with the illustrative embodiment is uncommon and, in fact, counterintuitive.
Metal, and
stainless steel in particular, is more susceptible to carbon deposition (i.e.,
soot and by-product
formation). Quartz, on the other hand, is easier to clean, with fewer
deposits. Quartz also
facilitates sample observation. However, the increased soot and carbon
deposition on
stainless steel can result in more consistent, faster, more efficient, and
more stable CNT
growth. It is believed that, in conjunction with atmospheric operation, the
CVD process
occurring in CNT growth zone 108 is diffusion limited. That is, the catalyst
is "overfed;" too
much carbon is available in the system due to its relatively higher partial
pressure (than if
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 accordance with the illustrative embodiment, the inventors
thus
intentionally run the apparatus "dirty." Once carbon deposits to a monolayer
on the walls of
CNT growth zone 108, carbon will readily deposit over itself. Since some of
the available
carbon is "withdrawn" due to this mechanism, the remaining carbon radicals
react with the
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catalyst at a more acceptable rate - 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. While soot formation is important
to the
process, it is desirably contolled, especially in places of interest where
consistant orifice sizes
are important (e.g., gas inlets, manifolds, diffusers). In these areas, soot
inhibiting coatings
(e.g., MgO, Silica, Alumina) can prevent unwanted sooting.
[0039] Using apparatus 100 allows for both a catalyst reduction and CNT growth
to occur
within CNT growth zone 108. This is significant because the reduction step
cannot be
accomplished timely enough for use in a continuous process if performed as a
discrete
operation. Conventionally, the reduction step typically takes 1-12 hours to
perform. Both
operations occur in CNT growth zone 108 in accordance with the present
invention due, at
least in part, to the fact that, in some embodiments, feed gas is introduced
the center of CNT
growth zone 108, not the end. 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. The reduction process can
be affected by a
variety of factors including, but not limited to, the temperature, the
catalyst composition, feed
gas composition, and the component flow rates. For example, the feed gas
composition may
determine the amount of hydrogen available upon dissociation to reduce the
catalyst. In
another embodiment, hydrogen (e.g., H2) can be added of the feed gas to
increase the amount
of hydrogen available for reduction of the catalyst. At the hottest isothermal
zone in the
system, the CNT growth occurs, with the greatest growth rate occurring
proximal to the feed
gas inlets near the center of the CNT growth zone.
[0040] The illustrative embodiments can be used with any type of substrate.
The term
"substrate" is intended to include any material upon which CNTs can be
synthesized and can
include, but is not limited to, a carbon fiber, a graphite fiber, a cellulosic
fiber, a glass fiber, a
metal fiber (e.g., steel, aluminum, etc.), a metallic fiber, a ceramic fiber,
a metallic-ceramic
fiber, an aramid fiber, or any substrate comprising a combination thereof The
substrate can
include fibers or filaments arranged, for example, in a fiber tow (typically
having about 1000
to about 12000 fibers) as well as planar substrates such as fabrics, tapes, or
other fiber
broadgoods, and materials upon which CNTs can be synthesized.
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[0041] In some embodiments, the apparatus of the present invention results in
the
production of carbon-nanotube infused fiber. As used herein, the term
"infused" means
chemically or physically 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 substrate. Additionally, it is believed that some degree of
mechanical
interlocking occurs as well. Bonding can be indirect, such as the CNT infusion
to the
substrate via a barrier coating and/or an intervening transition metal
nanoparticle disposed
between the CNTs and substrate. In the CNT-infused substrates disclosed
herein, the carbon
nanotubes can be "infused" to the substrate directly or indirectly as
described above. The
particular manner in which a CNT is "infused" to a substrate is referred to as
a "bonding
motif."
[0042] CNTs useful for infusion to substrates include single-walled CNTs,
double-walled
CNTs, multi-walled CNTs, and mixtures thereof. The exact CNTs to be used
depends on the
application of the CNT-infused substrate. CNTs can be used for thermal and/or
electrical
conductivity applications, or as insulators. In some embodiments, the infused
carbon
nanotubes are single-wall nanotubes. In some embodiments, the infused carbon
nanotubes
are multi-wall nanotubes. In some embodiments, the infused carbon nanotubes
are a
combination of single-wall and multi-wall nanotubes. There are some
differences in the
characteristic properties of single-wall and multi-wall nanotubes that, for
some end uses of
the fiber, dictate the synthesis of one or the other type of nanotube. For
example, single-
walled nanotubes can be semi-conducting or metallic, while multi-walled
nanotubes are
metallic.
[0043] As is clear from the foregoing, two key distinctions between
conventional
chambers and illustrative apparatus and method are: catalyst reduction time
and CNT
synthesis time. In the illustrative methods, these operations take seconds,
rather than several
minutes to hours as per conventional systems. The inability of conventional
chambers to
control catalyst-particle chemistry and geometry results in processes that
include multiple
time-consuming sub operations that can only be performed in batchwise fashion.
[0044] In a variation of the illustrative embodiment, the continuous
processing line for
CNT growth is used to provide an improved filament winding process. In this
variation,
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CNTs are formed on substrates (e.g., graphite tow, glass roving, etc.) using
the system and
process described above, and are then passed through a resin bath to produce
resin-
impregnated, CNT-infused substrate. After resin impregnation, the substrate is
positioned on
the surface of a rotating mandrel by a delivery head. The substate then winds
onto the
mandrel in a precise geometric pattern in known fashion. These additional sub
operations
can be performed in continuous fashion, extending the basic continuous
process.
[0045] The filament winding process described above provides pipes, tubes, or
other
forms as are characteristically produced via a male mold. But the forms made
from the
filament 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 substrates. Such forms will
therefore
benefit from enhanced strength, etc., as provided by the CNT-infused
substrates.
[0046] As used herein the term "spoolable dimensions" refers to substrates
having at least
one dimension that is not limited in length, allowing for the material to be
stored on a spool
or mandrel. Substrates of "spoolable dimensions" have at least one dimension
that indicates
the use of either batch or continuous processing for CNT infusion as described
herein. One
substrate of spoolable dimensions that 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.
[0047] As used herein, the term "feed gas" refers to any carbon compound gas
(e.g.,
acetylene), solid, or liquid that can be volatilized, nebulized, atomized, or
otherwise fluidized
and is capable of dissociating or cracking at high temperatures into at least
some free carbon
radicals and which, in the presence of a catalyst, can form CNTs on the
substrate. In some
embodiments, feed gas can comprise acetylene, ethylene, methanol, methane,
propane,
benzene, natural gas, or any combination thereof. The term "feed gas" also
includes an inert
gas, e.g., nitrogen, and can also contain an auxiliary gas such as hydrogen
used to aid in soot
CA 02801186 2012-11-29
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inhibition and catalyst reduction The feed gas can therefore consist of a
mixture of 95%
nitrogen, 3% hydrogen, and 2% acetylene by volume and can be fed into the
system via the
various methods described hereinabove.
[0048] As used herein, the term "purge gas" refers to any gas capable of
displacing
another gas. Purge gas can optionally be cooler than corresponding feed gas.
In some
embodiments, purge gas can include an inert gas such as nitrogen, argon, or
helium.
[0049] 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 substrates.
[0050] As used herein, the term "material residence time" refers to the amount
of time a
discrete point along a substrate 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 zones.
[0051] As used herein, the term "linespeed" refers to the speed at which a
substrate of
spoolable dimensions can be fed through the CNT infusion processes described
herein, where
linespeed is a velocity determined by dividing CNT growth zone(s) length by
the material
residence time.
[0052] It is to be understood that the above-described embodiments are merely
illustrative
of the present invention and that many variations of the above-described
embodiments can be
devised by those skilled in the art without departing from the scope of the
invention. For
example, in this Specification, numerous specific details are provided in
order to provide a
thorough description and understanding of the illustrative embodiments of the
present
invention. Those skilled in the art will recognize, however, that the
invention can be
practiced without one or more of those details, or with other processes ,
materials,
components, etc.
[0053] Furthermore, in some instances, well-known structures, materials, or
operations
are not shown or described in detail to avoid obscuring aspects of the
illustrative
embodiments. It is understood that the various embodiments shown in the
Figures are
illustrative, and are not necessarily drawn to scale. Reference throughout the
specification to
"one embodiment" or "an embodiment" or "some embodiments" means that a
particular
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feature, structure, material, or characteristic described in connection with
the embodiment(s)
is included in at least one embodiment of the present invention, but not
necessarily all
embodiments. Consequently, the appearances of the phrase "in one embodiment,"
"in an
embodiment," or "in some embodiments" in various places throughout the
Specification are
not necessarily all referring to the same embodiment. Furthermore, the
particular features,
structures, materials, or characteristics can be combined in any suitable
manner in one or
more embodiments. It is therefore intended that such variations be included
within the scope
of the following claims and their equivalents.
17
