U.S. patent application number 12/713147 was filed with the patent office on 2010-09-09 for method for the prevention of nanoparticle agglomeration at high temperatures.
This patent application is currently assigned to Lockheed Martin Corporation. Invention is credited to Jordan T. Ledford, Harry C. Malecki, Brandon K. Malet, Tushar K. SHAH.
Application Number | 20100227134 12/713147 |
Document ID | / |
Family ID | 42677114 |
Filed Date | 2010-09-09 |
United States Patent
Application |
20100227134 |
Kind Code |
A1 |
SHAH; Tushar K. ; et
al. |
September 9, 2010 |
METHOD FOR THE PREVENTION OF NANOPARTICLE AGGLOMERATION AT HIGH
TEMPERATURES
Abstract
A method includes: (a) conformally depositing a barrier coating,
provided in liquid form, on at least one surface of a substrate;
(b) embedding a plurality of nanoparticles in the barrier coating
to a selected depth; and (c) fully curing the barrier coating after
embedding the plurality of nanoparticles; the embedded plurality of
nanoparticles are in continuous contact with the cured barrier
coating. The order in which the barrier coating and nanoparticles
are deposited on the substrate can be switched or they can be
deposited simultaneously. An article includes a substrate having a
cured barrier coating conformally disposed on at least one surface
of the substrate and a plurality of nanoparticles embedded to a
selected depth in the barrier coating creating an embedded portion
of each of the plurality of nanoparticles. The embedded portion of
each of the plurality of nanoparticles in continuous contact with
the cured barrier coating.
Inventors: |
SHAH; Tushar K.; (Columbia,
MD) ; Malet; Brandon K.; (Baltimore, MD) ;
Ledford; Jordan T.; (Baltimore, MD) ; Malecki; Harry
C.; (Abingdon, MD) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY LLP
18191 VON KARMAN AVE., SUITE 500
IRVINE
CA
92612-7108
US
|
Assignee: |
Lockheed Martin Corporation
Bethesda
MD
|
Family ID: |
42677114 |
Appl. No.: |
12/713147 |
Filed: |
February 25, 2010 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61157096 |
Mar 3, 2009 |
|
|
|
61182153 |
May 29, 2009 |
|
|
|
Current U.S.
Class: |
428/213 ;
427/203; 427/372.2; 427/383.1; 427/397.7; 427/535; 428/323;
977/742 |
Current CPC
Class: |
Y10T 428/2495 20150115;
Y10T 428/25 20150115; C01B 32/162 20170801; B82Y 40/00 20130101;
D06B 19/00 20130101; B01J 35/0013 20130101; D06B 1/02 20130101;
D06M 11/74 20130101; C01B 32/16 20170801; B01J 37/0219 20130101;
B82Y 30/00 20130101; B01J 23/745 20130101; Y10S 977/842 20130101;
B01J 21/185 20130101; D06B 3/10 20130101 |
Class at
Publication: |
428/213 ;
427/372.2; 427/535; 427/383.1; 427/397.7; 427/203; 428/323;
977/742 |
International
Class: |
B32B 5/16 20060101
B32B005/16; B05D 3/10 20060101 B05D003/10; H05H 1/00 20060101
H05H001/00; B05D 1/36 20060101 B05D001/36; B32B 7/02 20060101
B32B007/02 |
Claims
1. A method comprising: (a) conformally depositing a barrier
coating on at least one surface of a substrate; said barrier
coating provided in liquid form; (b) embedding a plurality of
nanoparticles in said barrier coating to a selected depth creating
an embedded portion of each of said plurality of nanoparticles; and
(c) fully curing said barrier coating after embedding said
plurality of nanoparticles; said embedded portion of each of said
plurality of nanoparticles being in continuous contact with said
cured barrier coating.
2. The method of claim 1, wherein conformally depositing said
barrier coating and embedding said plurality of nanoparticles is
performed simultaneously.
3. The method of claim 1, wherein a thickness of said barrier
coating is about the same or less than the effective diameter of
said plurality of nanoparticles.
4. The method of claim 1, wherein a thickness of said barrier
coating is in a range from between about the same as the effective
diameter of said plurality of nanoparticles up to about 5,000%
greater than the effective diameter of said plurality of
nanoparticles.
5. The method of claim 1, wherein said substrate is treated with a
plasma prior to conformally depositing said barrier coating.
6. The method of claim 1, wherein said embedded plurality of
nanoparticles maintain an exposed surface when said plurality of
nanoparticles are in surface contact with said substrate.
7. The method of claim 1, wherein the step of depositing said
barrier coating is accomplished by a technique selected from dip
coating and spraying.
8. The method of claim 1, wherein said barrier coating comprises a
material selected from a siloxane, a silane, an alumina, a silicon
carbide ceramic, and a metal; said barrier coating being chosen for
its ability to adhere to said substrate.
9. The method of claim 1 further comprising partially curing said
barrier coating prior to embedding said plurality of
nanoparticles.
10. The method of claim 1, wherein said substrate is selected from
the group consisting of a metal, a ceramic, a silica wafer, a
fiber, a graphite sheet, and a high temperature plastic.
11. The method of claim 1 further comprising heating the
environment about said embedded plurality of nanoparticles, in the
presence of a feedstock material, to a temperature promoting growth
of a plurality of nanostructures from said feedstock material, said
embedded plurality of nanoparticles catalyzing said growth; wherein
the temperature is sufficient to cause agglomeration of said
plurality of nanoparticles in the absence of said barrier
coating.
12. The method of claim 11, wherein said nanoparticles comprise a
transition metal.
13. The method of claim 11, wherein said feedstock material is a
carbon source.
14. The method of claim 11, wherein said nanostructure is a carbon
nanotube.
15. The method of claim 1, wherein said nanoparticles comprise a
clay.
16. The method of claim 1, wherein said nanoparticles comprise
silica or alumina.
17. The method of claim 1, wherein said nanoparticles range in size
from between about 0.5 nm to about 500 nm.
18. A method comprising: (a) depositing a plurality of
nanoparticles on at least one surface of a substrate; (b)
conformally depositing a barrier coating over said substrate and at
least a portion of each of said plurality of nanoparticles,
creating an embedded portion of each of said plurality of
nanoparticles, said barrier coating provided in liquid form; and
(c) fully curing said barrier coating; wherein said plurality of
nanoparticles are in surface contact with said substrate and said
embedded portion of each of said plurality of nanoparticles is in
continuous contact with said cured barrier coating.
19. The method of claim 18, wherein a thickness of said barrier
coating is about the same or less than the effective diameter of
said plurality of nanoparticles.
20. The method of claim 18, wherein a thickness of said barrier
coating is in a range from between about the same as the effective
diameter of said plurality of nanoparticles up to about 100%
greater than the effective diameter of said plurality of
nanoparticles.
21. The method of claim 18, wherein said substrate is treated with
a plasma prior to depositing said plurality of nanoparticles.
22. The method of claim 18, wherein the step of depositing said
barrier coating is accomplished by a technique selected from dip
coating and spraying.
23. The method of claim 18, wherein said barrier coating comprises
a material selected from a siloxane, a silane, an alumina, a
silicon carbide ceramic, and a metal; said barrier coating being
chosen for its ability to adhere to said substrate.
24. The method of claim 18, wherein said substrate is a metal, a
ceramic, a silica wafer, a fiber, a graphite sheet, and a high
temperature plastics.
25. The method of claim 18 further comprising heating the
environment about said embedded plurality of nanoparticles, in the
presence of a feedstock material, to a temperature promoting
catalyzed growth of a plurality of nanostructures from said
feedstock material, said embedded plurality of nanoparticles
catalyzing said growth; wherein the temperature is sufficient to
cause agglomeration of said plurality of nanoparticles in the
absence of said barrier coating.
26. The method of claim 25, wherein said plurality of nanoparticles
comprise a transition metal.
27. The method of claim 25, wherein said feedstock material is a
carbon source.
28. The method of claim 25, wherein said nanostructure is a carbon
nanotube.
29. The method of claim 18, wherein said nanoparticles comprise a
clay.
30. The method of claim 18, wherein said nanoparticles comprise
silica or alumina.
31. The method of claim 18, wherein said nanoparticles range in
size from between about 0.5 nm to about 500 nm.
32. An article comprising: a substrate having a cured barrier
coating conformally disposed on at least one surface of said
substrate; and a plurality of nanoparticles embedded to a selected
depth in said barrier coating creating an embedded portion of each
of said plurality of nanoparticles, said embedded portion of each
of said plurality of nanoparticles in continuous contact with said
cured barrier coating.
33. The article of claim 32, wherein said embedded plurality of
nanoparticles are in surface contact with said substrate.
34. The article of claim 32, wherein said barrier coating comprises
a material selected from a siloxane, a silane, an alumina, a
silicon carbide ceramic, and a metal; said barrier coating being
chosen for its ability to adhere to said substrate.
35. The article of claim 32, wherein said substrate is selected
from the group consisting of a metal, a ceramic, a silica wafer, a
fiber, a graphite sheet, and a high temperature plastic
36. The article of claim 32, wherein said plurality of
nanoparticles comprise a transition metal.
37. The article of claim 32, wherein said plurality of
nanoparticles comprise a clay.
38. The article of claim 32, wherein said plurality of
nanoparticles comprise silica or alumina.
39. The article of claim 32, wherein said nanoparticles range in
size from between about 0.5 nm to about 500 nm.
40. The article of claim 32, wherein a thickness of said barrier
coating is about the same or less than the effective diameter of
said plurality of nanoparticles.
41. The article of claim 32, wherein a thickness of said barrier
coating is in a range from between about the same as the effective
diameter of said plurality of nanoparticles up to about 5000%
greater than the effective diameter of said plurality of
nanoparticles.
Description
STATEMENT OF RELATED APPLICATIONS
[0001] The present invention claims priority under 35 U.S.C.
.sctn.119(e) to provisional applications 61/157,096 filed Mar. 3,
2009, and 61/182,153 filed May 29, 2009 each of which is
incorporated by reference herein in their entirety.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not applicable.
FIELD OF THE INVENTION
[0003] The present invention relates to nanoparticles, more
specifically to nanoparticles used in high temperature
processes.
BACKGROUND OF THE INVENTION
[0004] Many nanoparticles are used in applications that involve gas
phase reactions which can expose the nanoparticles to elevated
temperatures. It has been observed that with decreasing particle
size, the melting point of the nanoparticle also decreases. The
reduced melting point coupled with surface diffusion
characteristics (driven by the large surface area to volume ratio)
of nanoparticles on various substrates can cause sintering and
particle agglomeration during high temperature processes. Sintering
occurs when the surfaces of nanoparticles approach temperatures
close to their melting point, resulting in an outer surface layer
change to a liquid phase, while the nanoparticle core remains
solid. The outer liquid surface of multiple particles may then mix
together forming solid agglomerates upon cooling. The sintering and
agglomeration of the nanoparticles result in reduced particle
specific surface areas and consequently diminished effectiveness in
various applications.
[0005] As an example, in the specific application of transition
metal nanoparticles as catalysts for carbon nanotube (CNT) growth,
nanoparticle sintering and agglomeration due to high temperature
exposure of more than 450.degree. C. can contribute to reduced
ability to nucleate CNTs.
[0006] It has been demonstrated that a ceramic coating
encapsulating individual nanoparticles can provide a thermal and
physical barrier to prevent agglomeration. This encapsulation,
while keeping the particles separated to prevent sintering and
agglomeration, can also prevent the nanoparticles from achieving
target temperatures for catalytic reactions and/or can present a
physical barrier to active catalytic surfaces of the nanoparticle.
Such discrete particles can also be mobile on a surface leading to
a lack of consistent distribution and density.
[0007] In the silicon wafer industry, it has been indicated that
nanoparticle mobility and subsequent sintering can be reduced by
providing, for example, an alumina layer on the silicon surface.
This alumina layer, however, channels the nanoparticles into a
pre-determined template. Moreover, the channels can be of
substantial depth and dimensions such that they impact the
subsequent reaction chemistry of the nanoparticle. For example,
when growing carbon nanotubes (CNTs), CNT morphology such as CNT
diameter, growth direction, and branching can all be affected by
the pre-constructed alumina template.
[0008] Methods that aid in the prevention of nanoparticle sintering
and agglomeration during high temperature processes while
maintaining catalytic activity, reducing particle mobility, and
providing consistent targeted nanoparticle densities would be
useful. In particular, providing a method for preventing sintering
and agglomeration under CNT growth conditions, while reducing or
preventing the effects of templating on CNT morphology is also
desirable. The present invention satisfies these needs and provides
related advantages as well.
SUMMARY OF THE INVENTION
[0009] In some aspects, embodiments disclosed herein relate to a
method that includes a method that includes: (a) conformally
depositing a barrier coating on at least one surface of a
substrate; the barrier coating provided in liquid form; (b)
embedding a plurality of nanoparticles in the barrier coating to a
selected depth creating an embedded portion of each of the
plurality of nanoparticles; and (c) fully curing the barrier
coating after embedding the plurality of nanoparticles; the
embedded portion of each of said plurality of nanoparticles being
in continuous contact with the cured barrier coating.
[0010] In some aspects, embodiments disclosed herein relate to a
method that includes: (a) depositing a plurality of nanoparticles
on at least one surface of a substrate; (b) conformally depositing
a barrier coating over the substrate and at least a portion of each
of the plurality of nanoparticles, creating an embedded portion of
each of the plurality of nanoparticles, the barrier coating
provided in liquid form; and (c) fully curing the barrier coating;
the plurality of nanoparticles are in surface contact with the
substrate and the embedded portion of each of the plurality of
nanoparticles is in continuous contact with the cured barrier
coating.
[0011] In some aspects, embodiments disclosed herein relate to an
article that includes a substrate having a cured barrier coating
conformally disposed on at least one surface of the substrate, and
a plurality of nanoparticles embedded to a selected depth in the
barrier coating creating an embedded portion of each of the
plurality of nanoparticles. The embedded portion of each of the
plurality of nanoparticles in continuous contact with the cured
barrier coating.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 shows nanoparticles embedded in a barrier coating
disposed conformally over a substrate.
[0013] FIG. 2 shows nanoparticles embedded in a barrier coating and
in surface contact with a substrate.
[0014] FIG. 3 shows exemplary growth of carbon nanotubes on the
embedded nanoparticles shown in FIG. 1.
[0015] FIG. 4 shows a system for producing a high temperature
ceramic fiber composite.
[0016] FIG. 5 shows a system for producing CNTs on carbon
fiber.
DETAILED DESCRIPTION
[0017] The present disclosure is directed, in part, to methods that
employ barrier coatings on a substrate to "lock" nanoparticles
distributed on a substrate surface in place to substantially reduce
nanoparticle sintering and agglomeration at high temperatures. The
barrier coatings employed in the methods disclosed herein are in
contact with the nanoparticles. In some embodiments, the barrier
coating does not fully encapsulate the nanoparticles, allowing the
nanoparticles to be exposed to desired reaction environments while
preventing nanoparticle sintering and agglomeration. In some
embodiments, the barrier coating does fully encapsulate the
nanoparticles. In such applications, the function of the
nanoparticle can be, for example, as a means of absorbing high
energy radiation. The heat associated with such absorption can be
sufficient to cause nanoparticle sintering in the absence of the
barrier coating. The barrier coating and nanoparticles can be
disposed on the substrate surface sequentially in any order or they
can be applied to the substrate simultaneously.
[0018] The barrier coatings employed in methods disclosed herein
can be provided as a sufficiently thin layer (equal to or less than
the effective nanoparticle diameter) that the barrier coating
itself does not influence the reactivity profile and/or course of
the reactions catalyzed or seeded by the nanoparticles. For
example, when using CNT growth catalysts embedded in nanochanneled
template materials for aligned CNT growth, the template dictates
the CNT dimensions, including width, and direction of CNT growth
(Li et al. App. Phys. Lett. 75(3):367-369 (1999)).
[0019] In some embodiments, the barrier coating can completely
embed the nanoparticles. In some embodiments, a barrier coating can
embed the nanoparticles while also allowing a degree of diffusion
through the barrier coating to allow access to the embedded
nanoparticles. Methods of the invention embed nanoparticles in the
barrier coating in a dense array without the restrictions of any
kind of pre-formed template. This can provide a greater
nanoparticle density, as well as a more uniform density of
nanoparticles. These benefits are realized by providing the barrier
coating in a liquid form which allows the barrier coating to
conform to the nanoparticle dimensions. This is particularly
beneficial in CNT synthesis applications because sintering is
prevented and CNT morphology is controlled by the nanoparticle
itself, rather than a pre-determined channel in which the CNT
resides.
[0020] The barrier coatings employed in methods disclosed herein
provide a means to prevent sintering and agglomeration of
nanoparticles under high mobility conditions by preventing
nanoparticle-to-nanoparticle interactions. The barrier coatings can
also prevent nanoparticle-to-substrate interactions by means of
physical separation and mechanical interlocking of the
nanoparticles in the barrier coating, as exemplified in FIG. 1. For
example, a metallic nanoparticle can form an alloy with a metal
substrate. The barrier coating can prevent such alloy formation.
Similarly, in the area of CNT growth, the barrier coating can
prevent nanoparticle-to-substrate interactions between a transition
metal catalyst and a carbon rich substrate. Such
nanoparticle-to-substrate interaction can poison the transition
metal nanoparticle catalyst by providing an excessive amount of
carbon as feedstock under CNT growth conditions. More generally,
the barrier coatings employed in methods disclosed herein
facilitate the use of nanoparticles with substrates that would
otherwise be incompatible in the absence of the barrier
coating.
[0021] In some embodiments, the embedded nanoparticles can be in
surface contact with the substrate as shown in FIG. 2 while still
avoiding or reducing nanoparticle-to-substrate interactions. For
example, the barrier coating can be used to minimize the contact
area between the substrate and the nanoparticles. In some
embodiments, even where there is still appreciable contact area
between the nanoparticles and the substrate, a sufficiently thick
barrier coating can provide a thermal barrier so that the
nanoparticle-substrate contact interface is at a sufficiently low
temperature to avoid any deleterious interactions. In some
embodiments, when the nanoparticle is in contact with a substrate
surface, a barrier coating thickness can be used that encapsulates
the nanoparticle while still allowing diffusion of reactive
materials through the barrier coating to allow nanoparticle
catalyzed reactions to take place. For example, in the case of CVD
CNT growth, carbon atoms from a CVD carbon feedstock can diffuse
through an appropriate barrier coating material. In such
embodiments, it can be desirable to have a barrier coating
thickness that is approximately the same or just slightly more than
the effective diameter of the nanoparticle catalysts.
[0022] An additional use of the barrier coating can be to protect
sensitive substrates from high temperature and/or reactive
environments used in connection with reactions of the embedded
nanoparticles. For example, some carbon-based substrates may not be
stable under high reaction temperatures or when exposed to a
variety of reaction conditions, such as a strongly oxidative
environment.
[0023] The present invention is also directed, in part, to articles
that include a substrate having a barrier coating conformally
disposed on at least one surface of the substrate with a plurality
of nanoparticles embedded in the barrier coating. Such articles can
be used in further reactions to modify the substrate and hence
properties of the article. For example, CNTs can be grown on the
surface of the substrate, when employing transition metal
nanoparticles. Such CNTs can be useful in the manufacture of
organized CNT arrays for use in surface enhanced Raman applications
and microelectronic structures, in the preparation of reinforcing
materials in composites and other composite applications such as
EMI shielding, signature control, and lightning strike protection.
Articles of the invention can also include barrier coated
substrates with embedded nanoparticles in which the nanoparticles
serve as catalysts for other reactions where high temperatures are
employed, but in which the article remains unchanged. For example,
articles can include immobilized catalyst nanoparticles for
combustion reactions, as might be employed in a catalytic
converter.
[0024] As used herein, the term "conformally depositing," when used
in reference to the application of a barrier coating to a
substrate, refers to a process in which the barrier coating is
deposited on, and in surface contact with a substrate, regardless
of substrate geometry. Conformal deposition of a barrier coating on
a substrate to which nanoparticles have already been deposited does
not interfere with the exposure of at least a portion of the
nanoparticle surface when desired. In such embodiments, the barrier
coating can be formulated to fill the voids between nanoparticles
without completely encapsulating the nanoparticles. This can be
achieved by altering the concentration and/or viscosity of the
liquid form of the barrier coating.
[0025] As used herein, the term "barrier coating" refers to any
coating used to reduce or prevent undesirable
nanoparticle-to-nanoparticle interactions such as sintering and
agglomeration on a substrate surface. The term also includes
coatings used to reduce or prevent undesirable
nanoparticle-to-substrate interactions. "Barrier coatings" can be
further selected for adherence to particular substrates and/or to
protect a substrate from a reactive environment that is used in a
reaction in which a nanoparticle is used as a catalyst, seed
material, or reactant. Barrier coatings of the invention are
thermal insulators that can be applied to a substrate in liquid
form, such as gels, suspensions, dispersions, and the like. By
providing the barrier coating in a liquid form, it can be
subsequently partially or fully cured. The curing process generally
involves the application of heat. Exemplary barrier coatings
include, for example, spin-on glass or alumina.
[0026] As used herein, the term "agglomeration" refers to any
process in which nanoparticles disposed on a substrate are fused
together. Conditions for agglomeration can include heating to a
melting point of the entire nanoparticle or a portion of the
nanoparticle, such as its surface. In addition, agglomeration
refers to conditions that accelerate surface diffusion of the
nanoparticles on the substrate, which includes heating. With
respect to the latter conditions, the term "agglomeration" can be
used interchangeably with the term "sintering."
[0027] 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. Such
nanostructured materials encompass any geometry lacking a large
aspect ratio with respect to all dimensions.
[0028] As used herein, the term "effective diameter" refers to the
average nanoparticle diameter of approximately spherical
nanoparticles.
[0029] As used herein, the term "embedding," when used in reference
to nanoparticles in barrier coatings, refers to the process of
surrounding the nanoparticles with the liquid form of the barrier
coating to any depth, including in surface contact with a
substrate, and/or encapsulating the nanoparticle completely.
"Embedding" the nanoparticles of the invention in the barrier
coating and curing the barrier coating can mechanically lock the
particles in place preventing their migration and subsequent
agglomeration. "Embedding" the nanoparticles in the barrier coating
can include placing the particles in the barrier coating to a depth
that the nanoparticles are also in surface contact with the
substrate on which the barrier coating is deposited, while still
maintaining an exposed surface of the nanoparticle. Nanoparticles
can also be "embedded" in the barrier coating by applying the
barrier coating after placing nanoparticles on a substrate.
Nanoparticles can also be embedded in the barrier coating by
simultaneous application of the barrier coating and the
nanoparticles.
[0030] As used herein, the term "carbon nanotube" or "CNT" 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.
[0031] 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,
acetates, and the like.
[0032] In some embodiments, the present invention provides a method
that includes (a) conformally depositing a barrier coating on at
least one surface of a substrate; the barrier coating is provided
in liquid form; (b) embedding a plurality of nanoparticles in the
barrier coating to a selected depth creating an embedded portion of
each of the plurality of nanoparticles; and (c) fully curing the
barrier coating after embedding the plurality of nanoparticles. The
embedded portions of each of the plurality of nanoparticles are in
continuous contact with the cured barrier coating. The barrier
coating does not affect the arrangement of the plurality of
nanoparticles embedded therein. Thus, the barrier coating does not
behave as a template dictating the relative placement of the
nanoparticles. The result of this process is a barrier-coated
substrate with locked nanoparticles that can be used in a variety
of contexts depending on the exact choice of nanoparticle and
substrate employed, as further described below. In some
embodiments, the step of conformally depositing the barrier coating
and embedding the plurality of nanoparticles is simultaneous. Thus,
the barrier coating material can also be applied to the substrate
in situ with the nanoparticles via solutions that contain both the
barrier coating and nanoparticle material (`hybrid solutions`).
[0033] In some embodiments, the methods described herein control
particle dispersion on a variety of shaped objects. This includes
an efficient means of coating composite materials like fibers or
fabrics and irregular shaped materials. Moreover, methods of the
invention control and maintain a nanoparticle density on substrate
surfaces, even when exposed to conditions that might cause NP
diffusion and/or sintering.
[0034] In some embodiments, the present invention provides a method
that includes (a) conformally depositing a barrier coating on at
least one surface of a substrate and (b) embedding a plurality of
nanoparticles in the barrier coating, wherein the thickness of the
barrier coating is about the same or greater than the effective
diameter of the plurality of nanoparticles. In such embodiments,
the thickness of the barrier coating can be between about equal to
the effective diameter of the plurality of nanoparticles up to
about 5,000% greater than this effective diameter. Thus, the
thickness of the barrier coating can be 0.01% greater than this
diameter or 0.1%, or 0.5%, 1%, 5%, 10%, 20%, 30%, 40%, 50%, 60%,
70%, 80%, 90%, 100%, 150%, 200%, 250%, 300%, 350%, 400%, 500%,
1,000%, 1,500%, 2,000%, and so on up to about 5,000% greater than
the effective diameter of the plurality of nanoparticles, including
an value in between and fractions thereof.
[0035] In some embodiments, the nanoparticles are prevented from
agglomerating when subjected to heating, for example. In some
embodiments, a barrier coating that encapsulates the plurality of
nanoparticles can be useful in applications where reactant access
to the NPs is not employed. For example, in electromagnetic
interference (EMI) shielding applications, the barrier coatings can
be transparent to electromagnetic radiation, but the NPs can
effectively absorb the EM radiation. This absorption can cause the
NPs to heat; thus, the barrier coating can prevent sintering in
such instances. In some embodiments, the barrier coating can
encapsulate the plurality of NPs without denying access to the
particle when, for example, a porous barrier coating is employed.
In such embodiments, although the particle is technically
encapsulated, the porous nature of the barrier coating allows
access to reactive surfaces of the NP.
[0036] In some embodiments, the plurality of nanoparticles can be
embedded partially in the barrier coating providing a physical
boundary between the nanoparticle and the substrate, as shown in
FIG. 1. In other embodiments, the embedded nanoparticles can be in
surface contact with substrate, as shown in FIG. 2. In still
further embodiments, the embedded nanoparticles can be a mixture of
a first portion separated from the substrate and a second portion
and in surface contact with the substrate. In some applications it
can be beneficial to avoid direct surface contact between the
substrate and the nanoparticles. For example, with a metal
substrate and a metal nanoparticle, partial embedding of the
nanoparticle can help avoid formation of alloys when the
nanoparticle is exposed to high temperatures. Similarly, in the
case of CNT growth with transition metal nanoparticle catalysts, it
can be useful to separate the catalyst from a carbon rich substrate
that might react with the nanoparticle.
[0037] In some embodiments, the nanoparticles are completely
encapsulated in the barrier coating, but an exposed surface is
created through a number of subsequent processes. For example, when
fully curing the barrier coating some materials can form fissures
in the coating in the vicinity of nanoparticles which can provide
an interface between the nanoparticles and a reactive environment.
Other barrier coating materials can create the necessary access to
the nanoparticles through the formation of a porous cured
structure.
[0038] In some embodiments, fully encapsulated nanoparticles can be
treated with a plasma to roughen the surface of the barrier coating
and create exposed nanoparticle surfaces. Similarly, the barrier
coating with encapsulated nanoparticles can be treated with a wet
chemical etching agent for a period sufficient to expose a portion
of the surface of the nanoparticles.
[0039] In still further embodiments, fully encapsulated
nanoparticles can be treated under mechanical roughening conditions
to expose a portion of the surface of the nanoparticles. This can
be done through any physical abrasive method such as sand blasting,
laser ablation, ball milling, plasma etching, and the like.
[0040] Regardless of the degree with which the nanoparticles are
embedded in the barrier coating, the barrier coating can serve to
mechanically lock the nanoparticles in place to prevent their
agglomeration or sintering when subjected to heat. Without being
bound by theory, this is accomplished by restricting the movement
of the nanoparticles on the substrate surface reducing NP
diffusion. Thus, the nanoparticle-to-nanoparticle interaction is
substantially reduced or eliminated by the presence of the barrier
coating.
[0041] The barrier coating can also provide a thermal barrier for
use with low melting substrates. In this regard, the barrier
coating can minimize or reduce to zero the surface area contact
between the plurality of nanoparticles and the substrate to
mitigate the effects of the exposure of the substrate to
temperatures which the nanoparticles might be heated or, more
generally, to avoid exposure of the substrate to the reaction
environment to which the plurality of nanoparticles can be at least
partially exposed.
[0042] In some embodiments the thickness of the barrier coating is
generally chosen to be about equal to, less than, or slightly less
than the effective diameter of the plurality of nanoparticles so
that there remains an exposed nanoparticle surface for subsequent
exposure to a reaction environment. In other embodiments, the
thickness can also be more than the effective diameter of the
nanoparticles by employing any number of techniques described above
to create an exposed surface of the nanoparticles. In some
embodiments, the thickness of the barrier coating is between about
0.1 nm and about 100 nm. In some embodiments, the thickness can be
less than 10 nm, including 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7
nm, 8 nm, 9 nm, and any value in between. The exact choice of
barrier coating thickness can be chosen to approximately match or
be less than the effective diameter of the plurality of
nanoparticles. In some embodiments, the embedded plurality of
nanoparticles maintains an exposed surface even when the
nanoparticles are in surface contact with the substrate. In some
embodiments, the thickness of the barrier coating coats is such
that it covers about half the nanoparticle surface area. In some
embodiments, the thickness of the barrier coating covers about 10%
of the nanoparticle surface area, while in other embodiments, the
thickness of the barrier coating covers about 5%, 10%, 15%, 20%,
25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%,
90%, 95%, 98%, and 100% of the surface area of the nanoparticles,
including all values in between. In still other embodiments, the
barrier coating covers the nanoparticle when applied but a portion
of the nanoparticle is exposed upon further treatments or choice of
porous barrier coating.
[0043] In some embodiments, the methods of the invention can
include treating the substrate with a plasma prior to conformally
depositing the barrier coating. Treating the substrate in a plasma
process can serve a dual role of creating functional groups and
roughening the substrate surface, thereby increasing its effective
surface area, to improve the wetting properties of the substrate
and thus improve the conformal deposition of the barrier coating.
Substrate 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.
[0044] In some embodiments, the step of depositing the barrier
coating is accomplished by a technique selected from dip coating
and spraying. Thus, the barrier coating can be solution based and
applied via dip bath configuration, spray methods, or the like in
some embodiments. The exact choice of method can be dictated by a
number of factors, including, for example, the substrate geometry.
For irregular shaped substrates, it can be useful to employ dip
methods that avoid the use of directionally applied barrier
coatings, such as in spray applications. For substrates in which a
single side should be coated, such as a wafer substrate, it can be
useful to apply the barrier coating with spray or related
techniques (nebulizers, for example) to assure coating on only one
side. Other factors to consider in applying the barrier coating can
depend on the barrier coating material itself including, for
example, the ability to form solutions or homogenous suspensions
for dip or spray coating.
[0045] When applying the barrier coating via dip or spray methods,
for example, the thickness of the barrier coating can be controlled
by use of diluents. Diluents can include any solvent compatible
with both the substrate and nanoparticle materials. For dip
coating, in particular, the thickness of the barrier coating can be
a function of concentration of the barrier coating material and the
residence time in the dip bath. The residence time can also aid in
providing uniformity of the coating. Uniformity can also be insured
by employing multiple dip baths.
[0046] The barrier coating includes a material selected from a
siloxane, a silane, an alumina, a silicon carbide ceramic, a metal,
and mixtures thereof. In some embodiments, the choice of barrier
coating can be chosen for its ability to adhere to the substrate.
There are many types of barrier coating materials including, for
example, those that are siloxane-based, silane-based,
alumina-based, silicon carbide-based ceramics, and metallic based.
Alumina based materials include, for example, alumoxane, alumina
nanoparticles, and alumina coating solutions, including, for
example, alumina-based coatings available from Zircar Ceramics,
such as Alumina Rigidizer/Hardener Type AL-R/H. In some
embodiments, glass coatings such as spin on glass, glass
nanoparticles, or siloxane-based solutions, such as methyl siloxane
in isopropyl alcohol, can be used as barrier coating materials.
Metallic based barrier coatings useful in the invention include,
for example, molybdenum, aluminum, silver, gold, and platinum.
Silicon carbide based ceramics include, for example, SMP-10,
RD-212a, Polyaramic RD-684a and Polyaramic RD-688a available from
Starfire.
[0047] Barrier coatings can also act as multifunctional coatings
tailored to specific applications. A specific type of barrier
coating can be selected to both prevent sintering as well as
promote adhesion to the substrate. For composite applications, a
barrier coating can selected to prevent sintering as well as bond
well to the composite matrix material. In still further
embodiments, the barrier coating material can be selected for
adhesion both to the substrate as well a composite matrix material.
In yet further embodiments, more than one barrier coating can be
employed. A first barrier coating can be selected for its ability
to adhere to the substrate surface. A second barrier coating can be
selected for its ability to adhere, for example, to a composite
matrix material such as a resin, ceramic, metal, or the like.
[0048] In some embodiments, methods of the invention include
partially curing the barrier coating prior to embedding said
plurality of nanoparticles. Partial curing of the barrier coating
can provide a "sticky" surface to embed the nanoparticles while
preventing movement of the applied nanoparticles to minimize
particle-to-particle interaction. Partial curing can also be caused
by the method used to apply the nanoparticles to the barrier
coating. In such a case, the partial curing step and embedding step
are performed simultaneously. Partial curing temperatures are
generally below the normal cure temperature, and can include
temperature that are between about 50 to about 75% of the normal
cure temperature and for residence times on the order of
seconds.
[0049] In some embodiments, methods of the present invention
further include heating the environment about the embedded
plurality of nanoparticles, in the presence of a feedstock
material, to a temperature promoting growth of a plurality of
nanostructures from the feedstock material. In some embodiments,
the embedded plurality of nanoparticles can catalyze the growth of
the nanostructures. In some embodiments, the nanoparticles act as a
seed for growth of the nanostructure, without behaving as a true
catalyst. In still further embodiments, the nanoparticles catalyze
a reaction which does not alter the substrate, barrier coating, or
the nanoparticles. Thus, the nanoparticle can catalyze a gas phase
reaction in which the products remain in the gas phase, for
example. In some embodiments, the temperature of a given reaction
is sufficient to cause agglomeration of the plurality of
nanoparticles in the absence of the barrier coating. Thus, the
barrier coating provides an effective means for preventing
sintering.
[0050] In some embodiments, the nanoparticles include a transition
metal. The catalyst transition metal nanoparticle 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, such as acetates
and chlorides, and mixtures thereof. In some embodiments, the
transition metal is used as a CNT forming catalyst. Many of these
transition metal catalysts are readily commercially available from
a variety of suppliers, including, for example, Ferrotec
Corporation (Bedford, N.H.).
[0051] In some embodiments, the feedstock material is a carbon
source, which when used in conjunction with the aforementioned
transition metals, allows for the synthesis of nanostructures such
as carbon nanotubes (CNTs). These CNTs can be single-walled,
double-walled, or other multi-walled CNTs. One skilled in the art
will recognize the relationship between nanoparticle size and the
type of CNTs that can be grown. For example, single-walled CNTs are
normally accessible with nanoparticle catalysts less than about 1
nm. CNT growth conditions are typically between about 500 to about
1,000.degree. C., a temperature at which sintering is observable
and can impact successful CNT growth.
[0052] Many substrate types, such as carbon and stainless steel,
are not normally amenable to CNT growth of high yields when only a
catalyst nanoparticle is applied to the surface due to high levels
of sintering. Barrier coatings are useful, however, for high-yield
CNT growth, even on these challenging substrates.
[0053] On the surface of a substrate, a catalyst nanoparticle's
ability to nucleate CNT growth can depend on the presence of
sufficient barrier coating material at that location of the
substrate surface to substantially reduce or prevent sintering. CNT
growth can be performed when the catalyst nanoparticles are applied
to the substrate prior to the barrier coating ('reverse order').
The benefit of a `reverse order` process is that the barrier
coating keeps the catalyst locked onto the substrate, and therefore
allows for anchoring of the CNTs to the substrate surface. Without
being bound by theory, when barrier coating is applied prior to
catalyst coating the CNT nanoparticle catalyst tends to follow the
leading edge of CNT synthesis, that is, tip-growth results. The
`reverse order` coatings can promote base-growth.
[0054] In some embodiments, the feedstock can be a carbon source
mixed with other gases as might be found, for example, in a
combustion process. In such embodiments, embedded transition metal
nanoparticles, such as platinum, palladium, rhodium, cerium,
manganese, iron, nickel, or copper can be used to modulate the
oxidation of the carbon source. The favorable surface area to
volume of a nanoparticle can improve the catalytic performance in
such combustion processes. This type of reaction can find
application, for example, in catalytic converters. It can also be
useful in various industrial petroleum processes such as in
refining and in downhole operations to catalyze the cracking of
heavy hydrocarbons for enhanced oil recovery, thus maximizing
formation productivity.
[0055] In some embodiments, other uses of transition metal
nanoparticles include the manufacture of high density magnetic
recording media that employ FePt nanoparticles. One skilled in the
art will recognize that sintering of FePt nanoparticles is
problematic when attempting to induce phase the change to obtain
the useful face-centered tetragonal FePt structure. This phase
change is generally conducted by heating at about 550.degree. C.
and is accompanied by sintering. The barrier coatings disclosed
herein are useful in preventing this sintering.
[0056] In some embodiments, a transition metal nanoparticle can be
used in desulfurization processes. For example, nickel and
molybdenum catalysts have been used in the desulfurization of
bitumen. In such processes, expensive supports such as uranium
oxide have been employed to prevent sintering during recycle of the
catalyst. Methods of the present invention employing a barrier
coating can be employed to prevent such sintering, while avoiding
the use of expensive support materials.
[0057] In some embodiments, a transition metal nanoparticle can be
used in syngas production processes. It has been determined that
sintering of CeO.sub.2 in Rh--CeO.sub.2 catalysts limits the use of
this catalyst system. The barrier coating employed in methods
disclosed herein can be used to prevent this sintering and enhance
the biomass to syngas transformation, for example.
[0058] In some embodiments, the nanoparticles can include other
metal containing materials such as ceramics, for example, oxides,
carbides, borides, of zinc, titanium, aluminum, and the like. Other
materials that do not contain transition metals such as clays,
silica, silicates, aluminosilicates and the like can also be
used.
[0059] Any of the aforementioned nanoparticles can range in size
from between about 0.1 nm to about 100 nm. In some embodiments, the
size of the nanoparticles can be in a range from between about 1 to
about 75 nm, and between about 10 to 50 nm in other embodiments. In
some embodiments, the size of the nanoparticles is in a range from
between about 0.1 to about 1 nm. In other embodiments, the size of
the nanoparticles is in a range from between about 2 to about 10
nm. In still further embodiments, the size of the nanoparticles is
in a range from between about 10 to about 20 nm, from between about
20 to about 30 nm, from between about 30 to about 40 nm, from
between about 40 to about 50 nm, from between about 50 to about 60
nm, from between about 60 to about 70 nm, from between about 70 to
about 80 nm, from between about 80 to about 90 nm, and from between
about 90 to about 100 nm, including all values in between. The
choice of size can depend on the application. In catalytic
processes, as described above, it can be desirable to utilize
smaller particles to benefit from the larger surface area to
volume. More generally, at the nanoparticle scale, one skilled in
the art will recognize the quantized nature of the properties of
the nanoparticles and that an appropriate size can be determined
through theoretical considerations and calculations. For example, a
particular particle size can be designed to absorb specific
wavelengths of radiation.
[0060] The rate of sintering of a metallic nanoparticles can vary
depending on the substrate on which it is disposed. However, by
employing the barrier coatings in methods of the present invention,
any substrate type can be used. For example, the substrate can
include a metal, a ceramic, a silica wafer, a fiber, a graphite
sheet, high temperature plastics, such as polyimides, PEEK, PEI and
the like.
[0061] In some embodiments, the present invention provides a method
that includes: (a) depositing a plurality of nanoparticles on at
least one surface of a substrate; (b) conformally depositing a
barrier coating over the substrate and at least a portion of each
of the plurality of nanoparticles, creating an embedded portion of
each of the plurality of nanoparticles; the barrier coating is
provided in liquid form; and (c) fully curing the barrier coating.
The plurality of nanoparticles are in surface contact with the
substrate in such embodiments, and the embedded portion of each of
the plurality of nanoparticles is in continuous contact with the
cured barrier coating. This is described above as "reverse order"
process and is shown graphically in FIG. 2. In this configuration,
the barrier coating can also prevent the agglomeration of the
plurality of nanoparticles when exposed to heat, or other processes
that might cause sintering. As described above, the thickness of
the barrier coating can be about the same or slightly less than the
effective diameter of the plurality of nanoparticles allowing the
plurality of nanoparticles to maintain an exposed portion of their
surface. Alternatively the thickness of the barrier coating can be
greater than effective diameter of the plurality of nanoparticles.
In some embodiments, the methods described above for post barrier
coating handling can be used when the barrier coating encapsulates
the nanoparticles completely.
[0062] When employing the "reverse order" process, the substrate
can be treated with a plasma prior to depositing the plurality of
nanoparticles. This can provide the exposed substrate surface with
good wetting characteristics as described above. Similarly, the
step of depositing the barrier coating can be accomplished by a
technique selected from dip coating and spraying as described
above. Moreover, any of the above applications, conditions and
general considerations apply equally to the "reverse order" methods
of the invention.
[0063] The methods of the invention can be used to produce an
article that includes a substrate having a barrier coating
conformally disposed on at least one surface of the substrate and a
plurality of nanoparticles embedded in the barrier coating. The
barrier coating function can be to prevent the agglomeration of the
plurality of nanoparticles when subjected to heat or other chemical
and/or physical processes.
[0064] The thickness of the barrier coating in articles of the
invention can be about the same or slightly less than the effective
diameter of said plurality of nanoparticles allowing said plurality
of nanoparticles to maintain an exposed portion of their surface
when said nanoparticles are, optionally, in surface contact with
the substrate. In particular embodiments, the embedded plurality of
nanoparticles are in surface contact with the substrate. Articles
of the invention can include a substrate that is a metal, ceramic,
silica wafer, fiber, graphite sheet, and high temperature plastic,
as describe above.
[0065] Any of the nanoparticle types and sizes described above can
be used in connection with the articles of the invention. In some
embodiments, articles of the invention include, composite materials
having a matrix material and carbon nanotubes infused to a fiber.
In combustion and related catalyst applications articles of the
invention include a) catalytic converters, b) catalyst reaction
beds used in refining, syngas production, desulfurization and the
like, c) downhole tools used in oil recovery, and d) high density
storage media.
[0066] 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 examples are
intended to illustrate but not limit the present invention.
Example I
[0067] This example shows how a barrier layer can be used in a
ceramic fiber composite structure to prevent sintering of iron
nanoparticles applied to the ceramic fiber surface for enhanced
signature control characteristics.
[0068] FIG. 4 depicts system 400 for producing a high temperature
ceramic fiber composite with enhanced signature control
characteristics in accordance with the illustrative embodiment of
the present invention. System 400 includes a ceramic fiber 402,
barrier coating solution bath 404, nanoparticle solution bath 406,
coating curing system 408, filament winding system 410, and a resin
infusion system 412, interrelated as shown.
[0069] The ceramic fiber 402 used is a Silicon Carbide Sylramic.TM.
fiber tow (1600 denier10 micron diameter) (COI Ceramics, Inc).
[0070] A barrier coating 404, consisting of the Starfire SMP-10,
RD-212a solution is applied to the ceramic fiber 402 via a dip
process. A diluted solution of 1 part SMP-10 and 10 parts isopropyl
alcohol is used in the dip process to apply a 2-4 nm thick
coating.
[0071] The nanoparticle solution 406 used is GTP 9700
(NanoChemonics), an iron oxide nanoparticle mixed in a toluene
solution. The nanoparticle solution is used to apply a uniform
distribution of iron oxide nanoparticles on the surface of the
barrier coating 404. Solutions containing less than 10% iron oxide
by weight is used to create nanoparticle coatings with 20-40 nm
spaced nanoparticles.
[0072] The coating curing system 408 consists of a set of heaters
used to cure the combine barrier and nanoparticle coating 409. The
coated fiber is exposed to a temperature of 200 C for 2 hours along
with a platinum-based catalyst to aid in the curing process.
[0073] The cured coating locks the nanoparticles into position, and
the coated fiber is wound into a component using the filament
winding system 410.
[0074] The filament wound component 411 is then infused with a
bismaleimide matrix using the resin infusion system 412.
[0075] The final cured high temperature ceramic fiber composite
structure 413 is able to withstand brief high temperature exposure
as high as 600 C while maintaining signature control
characteristics which are imparted due to the dispersed iron oxide
nanoparticle coating. This nanoparticle coating will not sinter as
a result of its interaction with the cured barrier coating.
Example II
[0076] This example shows how carbon nanotubes (CNTs) can be grown
on the surface of a carbon fiber using a barrier coating to prevent
sintering of the iron nanoparticle catalyst.
[0077] FIG. 5 depicts system 500 for producing CNTs on carbon fiber
(34-700 12 k unsized carbon fiber tow with a tex value of
800--Grafil Inc., Sacramento, Calif.) in accordance with the
illustrative embodiment of the present invention. System 500
includes a carbon fiber material payout and tensioner station 505,
plasma treatment station 515, barrier coating application station
520, air dry station 525, catalyst application station 530, solvent
flash-off station 535, CNT-growth station 540, and carbon fiber
material uptake bobbin 550, interrelated as shown.
[0078] Payout and tension station 505 includes payout bobbin 506
and tensioner 507. The payout bobbin delivers an unsized carbon
fiber material 560 to the process; the fiber is tensioned via
tensioner 507. For this example, the carbon fiber is processed at a
linespeed of 2 ft/min.
[0079] Unsized fiber 560 is delivered to plasma treatment station
515. For this example, atmospheric plasma treatment is utilized in
a `downstream` manner from a distance of 1 mm from the spread
carbon fiber material. The gaseous feedstock is comprised of 100%
helium.
[0080] Plasma enhanced fiber 565 is delivered to barrier coating
station 520. In this illustrative example, a siloxane-based barrier
coating solution is employed in a dip coating configuration. The
solution is `Accuglass T-11 Spin-On Glass` (Honeywell International
Inc., Morristown, N.J.) diluted in isopropyl alcohol by a dilution
rate of 40 to 1 by volume. The resulting barrier coating thickness
on the carbon fiber material is approximately 40 nm. The barrier
coating can be applied at room temperature in the ambient
environment.
[0081] Barrier coated carbon fiber 590 is delivered to air dry
station 525 for partial curing of the nanoscale barrier coating.
The air dry station sends a stream of heated air across the entire
carbon fiber spread. Temperatures employed can be in the range of
100.degree. C. to about 500.degree. C.
[0082] After air drying, barrier coated carbon fiber 590 is
delivered to catalyst application station 530. In this example, an
iron oxide-based CNT forming catalyst solution is employed in a dip
coating configuration. The solution is `EFH-1` (Ferrotec
Corporation, Bedford, N.H.) diluted in hexane by a dilution rate of
200 to 1 by volume. A monolayer of catalyst coating is achieved on
the carbon fiber material. `EFH-1` prior to dilution has a
nanoparticle concentration ranging from 3-15% by volume. The iron
oxide nanoparticles are of composition Fe.sub.2O.sub.3 and
Fe.sub.3O.sub.4 and are approximately 8 nm in diameter.
[0083] Catalyst-laden carbon fiber material 595 is delivered to
solvent flash-off station 535. The solvent flash-off station sends
a stream of air across the entire carbon fiber spread. In this
example, room temperature air can be employed in order to flash-off
all hexane left on the catalyst-laden carbon fiber material.
[0084] After solvent flash-off, catalyst-laden fiber 595 is finally
advanced to CNT-growth station 540. In this example, a rectangular
reactor with a 12 inch growth zone is used to employ CVD growth at
atmospheric pressure. 98.0% of the total gas flow is inert gas
(Nitrogen) and the other 2.0% is the carbon feedstock (acetylene).
The growth zone is held at 750.degree. C. For the rectangular
reactor mentioned above, 750.degree. C. is a relatively high growth
temperature. The addition of the barrier coating prevents sintering
of the catalyst nanoparticle at CNT growth temperatures, allowing
for effective high density CNT growth on the surface of the carbon
fiber.
[0085] CNT coated fiber 597 is wound about uptake fiber bobbin 550
for storage. CNT coated fiber 597 is loaded with CNTs approximately
50 .mu.m in length and is then ready for use in composite
materials.
[0086] 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.
[0087] 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 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.
* * * * *