U.S. patent application number 13/408984 was filed with the patent office on 2013-03-21 for systems and methods for growth of nanostructures on substrates, including substrates comprising fibers.
This patent application is currently assigned to Massachusetts Institute of Technology. The applicant listed for this patent is Richard Li, Stephen A. Steiner, III, Brian L. Wardle. Invention is credited to Richard Li, Stephen A. Steiner, III, Brian L. Wardle.
Application Number | 20130072077 13/408984 |
Document ID | / |
Family ID | 47881082 |
Filed Date | 2013-03-21 |
United States Patent
Application |
20130072077 |
Kind Code |
A1 |
Steiner, III; Stephen A. ;
et al. |
March 21, 2013 |
SYSTEMS AND METHODS FOR GROWTH OF NANOSTRUCTURES ON SUBSTRATES,
INCLUDING SUBSTRATES COMPRISING FIBERS
Abstract
Systems and methods for the formation of nanostructures,
including carbon-based nanostructures, are generally described. In
certain embodiments, substrate configurations and associated
methods are described.
Inventors: |
Steiner, III; Stephen A.;
(Cambridge, MA) ; Wardle; Brian L.; (Lexington,
MA) ; Li; Richard; (West Windsor, NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Steiner, III; Stephen A.
Wardle; Brian L.
Li; Richard |
Cambridge
Lexington
West Windsor |
MA
MA
NJ |
US
US
US |
|
|
Assignee: |
Massachusetts Institute of
Technology
Cambridge
MA
|
Family ID: |
47881082 |
Appl. No.: |
13/408984 |
Filed: |
February 29, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61537538 |
Sep 21, 2011 |
|
|
|
Current U.S.
Class: |
442/59 ; 118/33;
427/171; 428/367; 428/368; 428/375; 428/411.1; 428/426; 428/457;
428/522; 428/688; 428/702; 977/700; 977/742; 977/762; 977/890 |
Current CPC
Class: |
B82Y 40/00 20130101;
D06M 15/263 20130101; C01B 32/18 20170801; Y10T 428/292 20150115;
D06M 11/45 20130101; Y10T 428/31935 20150401; Y10T 428/249924
20150401; Y10T 428/2918 20150115; C01B 32/15 20170801; Y10T 442/20
20150401; B82Y 30/00 20130101; D06M 11/79 20130101; C01B 32/16
20170801; C23C 16/22 20130101; C01B 2202/08 20130101; Y10T
428/31504 20150401; Y10T 428/31678 20150401; Y10T 428/2933
20150115; D01F 9/127 20130101; D06M 11/74 20130101; C23C 16/44
20130101; Y10S 977/843 20130101; D06M 2101/40 20130101; Y10S
977/742 20130101; D06M 15/233 20130101 |
Class at
Publication: |
442/59 ; 118/33;
427/171; 428/411.1; 428/688; 428/426; 428/457; 428/702; 428/522;
428/368; 428/375; 428/367; 977/890; 977/700; 977/742; 977/762 |
International
Class: |
B32B 9/00 20060101
B32B009/00; B05D 3/12 20060101 B05D003/12; B05D 7/00 20060101
B05D007/00; B32B 27/00 20060101 B32B027/00; B32B 15/00 20060101
B32B015/00; B32B 5/02 20060101 B32B005/02; D02G 3/36 20060101
D02G003/36; B05C 9/10 20060101 B05C009/10; B32B 17/00 20060101
B32B017/00 |
Claims
1. An article, comprising: a growth substrate; an intermediate
material non-covalently associated with the growth substrate; and a
nanopositor configured to promote the growth of carbon-based
nanostructures from carbon-based nanostructure precursors
associated with the intermediate material.
2. An article as in claim 1, wherein the nanopositor is in direct
contact with the intermediate material.
3. An article as in claim 2, wherein the nanopositor is covalently
bonded to the intermediate material.
4. An article as in claim 2, wherein the nanopositor is ionically
bonded to the intermediate material.
5. An article as in claim 1, wherein the growth substrate comprises
carbon, glass, and/or a polymer.
6. An article as in claim 1, wherein the nanopositor comprises an
elemental metal and/or a metal oxide.
7. An article as in claim 6, wherein the nanopositor comprises an
elemental metal.
8. An article as in claim 7, wherein the nanopositor comprises
elemental iron.
9. An article as in claim 1, wherein the nanopositor comprises
iron.
10. An article as in claim 1, wherein the intermediate material
comprises a polymer.
11. An article as in claim 1, wherein the intermediate material
comprises a functional group capable of participating in a pi-pi
interaction with the substrate.
12. An article as in claim 1, wherein the intermediate material
comprises an aromatic group.
13. An article as in claim 12, wherein the intermediate material
comprises a phenyl group.
14. An article as in claim 1, wherein the intermediate material
comprises poly(styrene-alt-[maleic acid]).
15. An article as in claim 1, wherein the intermediate material
covers at least a portion of an exposed surface of the
substrate.
16. An article as in claim 1, wherein the intermediate material is
present as a coating over the substrate.
17. An article as in claim 16, wherein the intermediate material is
present as a substantially conformal coating over the
substrate.
18. An article as in claim 17, wherein the intermediate material is
present as a surface layer over the substrate.
19. An article as in claim 18, wherein the intermediate material is
present as a monolayer over the substrate.
20. An article as in claim 16, wherein the intermediate material is
present as a non-conformal coating over the substrate.
21. An article as in claim 1, wherein the intermediate material
comprises an anion.
22. An article, comprising: an elongated carbon-based growth
substrate, wherein the substrate has a tensile strength of at least
about 1 GPa; and a plurality of substantially aligned carbon-based
nanostructures positioned over the elongated carbon-based growth
substrate.
23. An article as in claim 22, comprising a ceramic-containing
material positioned between the elongated carbon-based growth
substrate and the plurality of substantially aligned carbon-based
nanostructures.
24. An article as in claim 23, wherein the ceramic-containing
material comprises a metal oxide and/or a metalloid oxide.
25. An article as in claim 24, wherein the metal comprises
aluminum.
26-27. (canceled)
28. An article as in claim 1, wherein the carbon-based
nanostructures comprise carbon nanotubes.
29. An article as in claim 1, wherein the carbon-based
nanostructures comprise carbon nanofibers.
30. An article as in claim 1, wherein the growth substrate
comprises a fiber.
31. An article as in claim 30, wherein the fiber is part of a weave
of fibers.
32. An article as in claim 30, wherein the fiber is part of a
bundle of fibers.
33. An article as in claim 30, wherein the fiber is substantially
free of contact with other fibers.
34. An article as in claim 1, wherein the growth substrate
comprises a carbon fiber.
35. An article as in claim 1, wherein the nanopositor comprises a
catalyst.
36. A system for growing carbon-based nanostructures, comprising: a
growth substrate under a tensile force; and a nanopositor
positioned over the growth substrate; wherein the system is
configured to expose a carbon-based nanostructure precursor to the
nanopositor under conditions causing the formation of carbon-based
nanostructures on the nanopositor while the tensile force is
applied to the growth substrate.
37. A system as in claim 36, comprising an intermediate material
between the growth substrate and the nanopositor.
38-40. (canceled)
41. A system as in claim 36, wherein the nanopositor is in direct
contact with the growth substrate.
42-65. (canceled)
66. A system as in claim 36, wherein the magnitude of the tensile
force is such that it defines a stress that is greater than about
1% of the breaking strength of the substrate.
67. (canceled)
68. A method of growing carbon-based nanostructures, comprising:
applying a tensile force to a growth substrate over which a
nanopositor is positioned; and exposing a carbon-based
nanostructure precursor to the nanopositor under conditions causing
the formation of carbon-based nanostructures on the nanopositor
while the tensile force is applied to the growth substrate.
69-71. (canceled)
72. A method of growing carbon-based nanostructures, comprising:
exposing a carbon-based nanostructure precursor to a nanopositor
under conditions causing the formation of carbon-based
nanostructures on the nanopositor, wherein the nanopositor is
associated with an intermediate material that is non-covalently
associated with a growth substrate.
73-88. (canceled)
89. A method of growing carbon-based nanostructures, comprising:
exposing a ceramic-containing layer, positioned over an elongated
carbon-based growth substrate, to a carbon-based nanostructure
precursor under conditions causing the formation of substantially
aligned carbon-based nanostructures on the ceramic-containing
layer.
90-108. (canceled)
Description
RELATED APPLICATIONS
[0001] This application claims priority under 35 U.S.C.
.sctn.119(e) to U.S. Provisional Patent Application Ser. No.
61/537,538, filed Sep. 21, 2011, and entitled "Methods for Growth
of Nanostructures on Substrates Including Fibers," which is
incorporated herein by reference in its entirety for all
purposes.
TECHNICAL FIELD
[0002] Systems and methods for the formation of nanostructures,
including carbon-based nanostructures, are generally described. In
certain embodiments, substrate configurations and associated
methods are described.
BACKGROUND
[0003] The production of nanostructures (including carbon-based
nanostructures such as carbon nanotubes, carbon nanofibers, and the
like) may potentially serve as an important tool in the production
of emerging electronics and structural materials. Recent research
has focused on the production of, for example, carbon nanotubes
(CNTs) through chemical vapor deposition (CVD) and other
techniques. The selection of an appropriate substrate on which to
form the nanostructures is important when designing processes for
the production of carbon nanostructures. Many commonly used
substrate materials have one or more disadvantages associated with
them. For example, some substrate materials may react during growth
of the nanostructures, which can weaken the substrate and impede
nanostructure growth. Substrates configured to avoid reaction or
other degradation pathways would be desirable.
SUMMARY
[0004] Systems and methods for the formation of nanostructures,
including carbon-based nanostructures, are provided. The subject
matter of the present invention involves, in some cases,
interrelated products, alternative solutions to a particular
problem, and/or a plurality of different uses of one or more
systems and/or articles.
[0005] In one aspect, an article is provided. The article
comprises, in certain embodiments, a growth substrate; an
intermediate material non-covalently associated with the growth
substrate; and a nanopositor configured to promote the growth of
carbon-based nanostructures from carbon-based nanostructure
precursors associated with the intermediate material.
[0006] In certain embodiments, the article comprises an elongated
carbon-based growth substrate, wherein the substrate has a tensile
strength of at least about 1 GPa; and a plurality of substantially
aligned carbon-based nanostructures positioned over the elongated
carbon-based growth substrate.
[0007] In one aspect, a system for growing carbon-based
nanostructures is described. In certain embodiments, the system
comprises a growth substrate under a tensile force, and a
nanopositor positioned over the growth substrate, wherein the
system is configured to expose a carbon-based nanostructure
precursor to the nanopositor under conditions causing the formation
of carbon-based nanostructures on the nanopositor while the tensile
force is applied to the growth substrate.
[0008] In one aspect, a method of growing carbon-based
nanostructures is provided. The method comprises, in certain
embodiments applying a tensile force to a growth substrate over
which a nanopositor is positioned; and exposing a carbon-based
nanostructure precursor to the nanopositor under conditions causing
the formation of carbon-based nanostructures on the nanopositor
while the tensile force is applied to the growth substrate.
[0009] In some embodiments, the method comprises exposing a
carbon-based nanostructure precursor to a nanopositor under
conditions causing the formation of carbon-based nanostructures on
the nanopositor, wherein the nanopositor is associated with an
intermediate material that is non-covalently associated with a
growth substrate.
[0010] In certain embodiments, the method comprises exposing a
ceramic-containing layer, positioned over an elongated carbon-based
growth substrate, to a carbon-based nanostructure precursor under
conditions causing the formation of substantially aligned
carbon-based nanostructures on the ceramic-containing layer.
[0011] Other advantages and novel features of the present invention
will become apparent from the following detailed description of
various non-limiting embodiments of the invention when considered
in conjunction with the accompanying figures. In cases where the
present specification and a document incorporated by reference
include conflicting and/or inconsistent disclosure, the present
specification shall control.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] Non-limiting embodiments of the present invention will be
described by way of example with reference to the accompanying
figures, which are schematic and are not intended to be drawn to
scale. In the figures, each identical or nearly identical component
illustrated is typically represented by a single numeral. For
purposes of clarity, not every component is labeled in every
figure, nor is every component of each embodiment of the invention
shown where illustration is not necessary to allow those of
ordinary skill in the art to understand the invention. In the
figures:
[0013] FIGS. 1A-1C are schematic illustrations showing a system and
method by which nanostructures can be produced over a substrate and
an intermediate material, according to one set of embodiments;
[0014] FIGS. 2A-2B are, according to certain embodiments, schematic
illustrations of a system and method by which nanostructures can be
produced over a substrate and an intermediate material;
[0015] FIGS. 3A-3C are, according to some embodiments, schematic
illustrations of a system and method by which nanostructures can be
produced over a substrate, optionally with an intermediate
material, while a tensile force is applied to the substrate;
[0016] FIG. 4 is a schematic illustration of carbon nanotubes
positioned over a growth substrate, according to some
embodiments;
[0017] FIG. 5 includes SEM images of carbon nanotubes grown on
carbon fibers coated with alumina, according to one set of
embodiments;
[0018] FIG. 6 includes, according to some embodiments, SEM images
of sol-gel derived silica coatings on carbon fibers;
[0019] FIG. 7 includes SEM images of aligned carbon nanotubes grown
on alumina coated carbon fibers, according to certain
embodiments;
[0020] FIG. 8 shows SEM images of carbon nanotubes grown on carbon
fibers coated with alumina with and without h-PSMA undercoatings,
according to some embodiments;
[0021] FIG. 9 shows, according to some embodiments, FE-SEM images
and an Auger spectrum of a silica coating on a carbon fiber without
an h-PSMA underlayer;
[0022] FIG. 10 includes, according to certain embodiments, FE-SEM
images and an Auger spectrum of a silica coating on a carbon fiber
with an h-PSMA underlayer;
[0023] FIG. 11 shows SEM images of carbon nanotubes grown on carbon
fibers with a K-PSMA coating, according to some embodiments;
[0024] FIG. 12 illustrates Weibull distributions calculated from
single-fiber tensile tests of carbon fibers before and after heat
treatments, according to certain embodiments;
[0025] FIG. 13 illustrates, according to certain embodiments, the
losses in mean breaking strength of untreated carbon fibers as a
function of temperature and time spent at or above 480.degree.
C.;
[0026] FIG. 14 illustrates, according to some embodiments, the
decrease in mean tensile modulus of untreated carbon fibers as a
function of temperature and time spent at or above 480.degree.
C.;
[0027] FIG. 15 includes Weibull distributions calculated from
tensile tests of tensioned carbon fibers, according to some
embodiments; and
[0028] FIG. 16 illustrates, according to one set of embodiments;
Weibull distributions calculated from tensile tests of carbon
fibers coated with Fe.sup.3+-loaded K-PSMA coatings and
subsequently CVD processed.
DETAILED DESCRIPTION
[0029] Systems and methods for the formation of nanostructures,
including carbon-based nanostructures, are generally described. In
certain embodiments, substrate configurations and associated
methods are described. In some embodiments, articles, systems, and
methods for growing carbon-based nanostructures (including
elongated carbon-based nanostructures such as carbon nanotubes,
carbon nanofibers, and the like) on substrates such as carbon
fibers are provided. Some embodiments involve preparing a substrate
for growth of nanostructures on the substrate and/or on a
nanopositor proximate the substrate.
[0030] The ability to grow nanostructures, such as carbon-based
nanostructures, on a substrate while preserving the mechanical,
electrical, and/or thermal properties of the substrate is desirable
for a number of reasons. For example, the substrate on which
nanostructures are grown might be recycled for use in subsequent
growth processes. In other cases, the substrates themselves may be
incorporated into the finally-assembled structure, the properties
of which may be enhanced if the substrate is able to retain its
toughness, strength, electrical conductivity, thermal conductivity,
and/or other desirable properties. As one particular example,
nanostructures may be grown on a fiber (e.g., a carbon fiber) which
is subsequently incorporated into a composite material (e.g., by
weaving or otherwise assembling the fiber with other components to
form the composite). One architecture of particular interest
includes one or more carbon fibers circumferentially coated with an
array of aligned carbon nanotubes (CNTs), which can enable
through-thickness and inter-ply matrix reinforcement of carbon
fiber reinforced composites with multifunctional additional
benefits such as providing electrical and thermal conductivity
enhancement.
[0031] Many growth substrates on which it is desirable to grow
nanostructures, such as carbon-based growth substrates, degrade
when subjected to certain growth conditions used to grow the
nanostructures. For example, growth of carbon nanotubes on carbon
fibers via chemical vapor deposition often leads to substantial
degradation of the tensile strength and/or stiffness of the carbon
fiber substrates, making the carbon fiber substrate less useful for
subsequent applications (either as a recycled growth substrate or
as a component of a composite). It has been discovered, within the
context of the present invention, that various substrate
configurations can be adopted that can reduce or eliminate the
degradation of the mechanical, electrical, and/or thermal
properties of substrates comprising carbon and/or other vulnerable
materials during the growth of nanostructures.
[0032] In one aspect, systems, articles, and methods are described
in which an intermediate material is located between a growth
substrate (e.g., a carbon fiber) and a nanopositor configured to
promote the growth of nanostructures. Traditional nanostructure
growth systems and methods load catalyst or catalyst precursor
directly onto the growth substrate surface using, for example,
incipient wetness techniques (e.g., dip-coating), in situ
deposition of nanoparticles, e-beam evaporation of catalyst metal
onto the substrate surface, and the like. Such approaches can be
disadvantageous for a variety of reasons. For example, incipient
wetness techniques generally require a wettable substrate surface
in order to attach catalyst to the substrate, which can necessitate
aggressive surface oxygenation through, for example, acid or
electrochemical etching. Such processing can, in turn, alter the
substrate surface morphology and reduce the substrate's ability to
transmit tensile load. Additionally, each of the aforementioned
traditional catalyst application approaches produces direct contact
between the catalyst and the growth substrate, providing a
configuration in which detrimental high-temperature interactions
(e.g., carbide formation and/or catalytic restructuring of the
substrate surface) can occur. Furthermore, such traditional
configurations often produce nanostructure (e.g., nanotube) arrays
that are unaligned; often, however, aligned or otherwise ordered
nanostructures are more advantageous, for example, for
nanoengineered composite architectures.
[0033] Positioning an intermediate material between the growth
substrate and the nanopositor can reduce or eliminate the
above-mentioned disadvantages. For example, the intermediate
material can be configured, in certain embodiments, to inhibit or
prevent interaction between the nanopositor and the growth
substrate. Inhibiting or preventing interaction between the
nanopositor and the growth substrate can reduce or eliminate damage
to the growth substrate, for example, due to undesirable
interactions between the nanopositor and the growth substrate. The
intermediate material can be configured, in some embodiments, to
inhibit or prevent interaction between the growth substrate and
environmental agents, including nanostructure precursors used
during nanostructure growth or other chemicals present in the
growth atmosphere. In some embodiments, the intermediate material
can be selected such that the wettability of the nanopositor
material is enhanced. In addition, positioning an intermediate
material between the nanopositor and the growth substrate can lead
to enhanced alignment or other ordering of the elongated
nanostructures grown from the nanopositor.
[0034] FIGS. 1A-1C are schematic diagrams illustrating the growth
of nanostructures 116 on a growth substrate 110 in which an
intermediate material 112 is employed. While an elongated, fiber
substrate 110 is illustrated in FIGS. 1A-1C, it should be
understood that the invention is not so limited, and, in other
embodiments, growth substrates with other shapes (e.g., plates,
spheres, etc.) can be employed.
[0035] In FIGS. 1A-1C, substrate 110 (e.g., a fiber such as a
carbon fiber) is provided. In some embodiments, an intermediate
material can be associated with the growth substrate. For example,
in the set of embodiments illustrated in FIGS. 1A-1C, intermediate
material 112 is positioned over growth substrate 110.
[0036] Intermediate material 112 and growth substrate 110 can be
associated in a variety of manners. In certain embodiments, the
growth substrate and the intermediate material can be
non-covalently associated with each other. The non-covalent
association between the intermediate material and the substrate can
comprise a variety of interactions. For example, non-covalent
association between the substrate and the intermediate material may
be present in the form of, for example, van der Waals forces, for
example, by physisorbing the intermediate material onto the
substrate surface. In certain embodiments, an ionic bond, a
hydrogen bond, a metal bond, or any other type of non-covalent bond
may be formed between growth substrate 110 and intermediate
material 112.
[0037] In some embodiments, the intermediate material may be
capable of participating in a pi-pi interaction with the growth
substrate. A pi-pi interaction (a.k.a., "pi-pi stacking") is a
phenomenon known to those of ordinary skill in the art, and
generally refers to a stacked arrangement of molecules adopted due
to interatomic interactions. Pi-pi interactions can occur, for
example, between two aromatic groups. One of ordinary skill in the
art can determine whether a material (e.g., an intermediate
material) is capable or participating in pi-pi interactions for a
particular growth substrate.
[0038] The use of intermediate materials that participate in
non-covalent interactions with the growth substrate can be
advantageous as such use can inhibit or prevent interactions
between the growth substrate and the intermediate material that
negatively impact the mechanical, electrical, and/or thermal
properties of the growth substrate. In addition, intermediate
materials that non-covalently interact with the growth substrate
can be selected such that they are removed relatively easily after
nanostructure growth, which can be desirable in certain
embodiments. Of course, the invention is not limited to
non-covalent interactions between the growth substrate and the
intermediate material, and in other embodiments (e.g., in some
embodiments in which intermediate material 112 comprises a ceramic,
as discussed in detail below), the intermediate material can be
covalently bonded to the growth substrate.
[0039] A variety of types of intermediate materials can be used in
association with the embodiments described herein. The intermediate
material can comprise, in some embodiments, a polymer. For example,
in some embodiments, the intermediate material can comprise
poly(styrene-alt-[maleic acid]) and/or poly(styrene-alt-[maleic
anhydride]). In some embodiments, the intermediate material can
comprise an ion (e.g., a cation or an anion), such as at least a
portion of a metal salt. Other examples of suitable polymers
include, but are not limited to, poly(diallyldimethylammonium
chloride), poly(styrene sulfonate), poly(allyl amine), and
poly(acrylic acid). In certain embodiments, the polymer comprises a
polyelectrolyte incorporating carboxylate, sulfonate, carbonate,
bicarbonate, amine, ammonium, phosphate, and/or phosphonate groups,
as described below.
[0040] In some embodiments, the intermediate material may comprise
an aromatic group. The aromatic group may, in some cases,
participate in one or more pi-pi interactions with the growth
substrate. Examples of suitable aromatic groups include, but are
not limited to, aryl groups (e.g., phenyl groups such as phenoxy
groups, benzyl groups, tolyl groups, o-xylyl groups, and the like),
and fused aromatic rings (e.g., napthalene, anthracene, pyrene, and
the like), among others.
[0041] Other suitable intermediate materials (e.g., for
non-covalent functionalization of the growth substrate) may include
polymers or small molecules comprising both a hydrophobic moiety
and a hydrophilic moiety in the same molecular structure.
[0042] In certain embodiments, the intermediate material provided
for non-covalent functionalization comprises a polyelectrolyte.
Suitable polyelectrolytes may include polymers comprising an
aromatic side group and a side group suitable for ion exchange. One
example of a suitable polyelectrolyte is the potassium salt of
poly(styrene-alt-[maleic acid]). Other suitable polyelectrolytes
(based on, for example, any of the polymers discussed above) may
similarly incorporate carboxylate, sulfonate, carbonate,
bicarbonate, amine, ammonium, phosphate, or phosphonate groups. In
some embodiments, an intermediate layer that binds to the substrate
is provided, and a second intermediate polyelectrolyte layer that
binds to that intermediate layer is provided. The use of such
materials can allow for advantageous methods of incorporating a
nanopositor into the growth system, as discussed in more detail
below.
[0043] The growth substrate and intermediate material can be
arranged relative to each other in a variety of suitable ways. In
some embodiments, the intermediate material can cover at least a
portion of an exposed surface of the growth substrate. In some
embodiments, the intermediate material can be present as a coating
over at least a portion of the substrate. The intermediate material
can form a uniform or a non-uniform coating over at least a portion
(and in some cases, all) of the substrate. In some embodiments, the
non-covalent functionalization process results in a substantially
conformal coating over the contour surfaces of the substrate (e.g.,
fiber). This coating may be present in a thickness of one
monolayer, several monolayers, or more. In certain embodiments, the
coating (continuous, or otherwise) may be present in a thickness of
from about 0.1 nm (e.g., which can be about the thickness of some
monolayers and/or bilayers) to about 100 micrometers, from about
0.1 nm to about 1000 nm, from about 0.1 nm to about 100 nm, from
about 1 nm to about 100 micrometers, from about 1 nm to about 1000
nm, or from 1 micrometers to about 100 micrometers. The coating
composition and thickness may be characterized by scanning electron
microscopy, transmission electron microscopy, Auger spectroscopy,
X-ray photoelectron microscopy, profilometry, and/or other methods.
In some embodiments, the intermediate material is present as a
surface layer over the substrate. The intermediate material can be
present, for example, as a monolayer, a bi-layer, several
monolayers, etc. over the substrate.
[0044] In the set of embodiments illustrated in FIGS. 1A-1C,
intermediate material 112 is present as a conformal coating over
growth substrate 110. Of course, it should be understood that, in
some embodiments, the intermediate material can be present as a
non-conformal coating, as a patterned layer, or in any other
suitable configuration, including those outlined above. In certain
embodiments, one or more intermediate materials can cover at least
about 50%, at least about 75%, at least about 90%, at least about
95%, at least about 99%, or substantially all of the growth
substrate (e.g., carbon fiber) over which the intermediate
material(s) are positioned.
[0045] In certain embodiments, a nanopositor material can be
associated with the intermediate material and/or the growth
substrate. In such embodiments, the nanopositor can be made of a
different material than the intermediate material(s). In FIG. 1B,
nanopositor material 114 can be positioned over intermediate
material 112. In some such embodiments, substrate 110, intermediate
material 112, and nanopositor 114 can be arranged such that
intermediate material 112 is positioned between substrate 110 and
nanopositor 114. That is to say, intermediate material 112 can be
positioned over growth substrate 110 and/or nanopositor 114 can be
positioned over intermediate material 112. In some embodiments, the
nanopositor is in contact with the intermediate material. For
example, the nanopositor can be, in some embodiments, covalently
bonded to the intermediate material or ionically bonded to the
intermediate material. In other embodiments, however, one or more
materials can be positioned between intermediate material 112 and
nanopositor 114.
[0046] The nanopositor can be added to the intermediate material
and/or the growth substrate using any suitable method. For example,
in certain embodiments, a nanopositor precursor (such as an
Fe.sup.3+ ion), a nanopositor nanoparticle (such as an Fe, Co, or
Ni nanoparticle) or other nanopositor (including a non-metallic
nanopositor (such as zirconia nanoparticles, polymer-coated
zirconia nanoparticles, or carbon-coated zirconia nanoparticles))
may be provided on a surface of a growth substrate and/or an
intermediate material. In some instances, the nanopositor
precursor, nanopositor nanoparticle, or other nanopositor (e.g.,
non-metallic nanopositor) is included as part of the intermediate
material used to non-covalently functionalize the carbon fiber
surface. In certain embodiments, the nanopositor precursor,
nanopositor nanoparticle, or other nanopositor (e.g., non-metallic
nanopositor) is added after the substrate (e.g., fiber) is
non-covalently functionalized. In some of these instances, the
nanopositor precursor, nanopositor nanoparticle, or other
nanopositor may participate in a chemical reaction with the
intermediate material used to non-covalently functionalize the
substrate surface. For example, a substrate over which a
polyelectrolyte (e.g., a potassium salt of poly(styrene-alt-[maleic
acid])) has been positioned may be dipped into an aqueous or
non-aqueous solution of ions (e.g., Fe.sup.3+ ions), resulting in
an ion exchange process between an ion in the intermediate material
(e.g., K.sup.+ for a potassium salt) and the ions in the solution
(e.g., Fe.sup.3+). Ions in solution suitable for exchange with ions
on the intermediate material (e.g., ions on a polyelectrolyte
intermediate material) include ions of cobalt, nickel, molybdenum,
zirconium, titanium, tantalum, silicon, aluminum, and/or other
metals and metalloids. Such ions can, in certain embodiments, serve
as nanopositors or precursors for nanopositors.
[0047] In certain embodiments, once the nanopositor has been
associated with the growth substrate and/or intermediate material,
a nanostructure precursor material can be used to grow
nanostructures. For example, in some embodiments, a carbon-based
nanostructure precursor can be exposed to the nanopositor under
conditions causing the formation of carbon-based nanostructures on
the nanopositor. In FIG. 1B, for example, a nanostructure precursor
material, may be delivered to growth substrate 110, intermediate
material 112, and/or nanopositor material 114. The nanostructure
precursor material may contact or permeate a surface of growth
substrate 110, a surface of intermediate material 112, and/or a
surface of nanopositor material 114. In certain embodiments, after
interaction between the nanostructure precursor material and
nanopositor material 114, nanostructures 116 can be formed from
nanopositor material 114, as illustrated in FIG. 1C.
[0048] Nanostructure precursor materials may be in any suitable
phase (e.g., solid, liquid, or gas) and include, for example,
hydrocarbons (e.g., methane, ethylene, acetylene, etc.), alcohols,
and the like. In the growth of carbon nanotubes, for example, the
nanostructure precursor material may comprise carbon, such that
carbon dissociates from the precursor molecule and may be
incorporated into the growing carbon nanotube, which is pushed
upward from the growth substrate in general direction 117 with
continued growth. Those of ordinary skill in the art would be able
to select the appropriate nanostructure precursor material for the
growth of a particular nanostructure. For example, carbon nanotubes
may be synthesized by reaction of a C.sub.2H.sub.4/H.sub.2 mixture
with a nanopositor. Other examples of nanostructure precursor
materials that may be used include, for example, methane, ethanol,
methyl formate, acetylene, and other alkynes. Examples of suitable
nanostructure fabrication techniques are discussed in more detail
in International Patent Application Serial No. PCT/US2007/011914,
filed May 18, 2007, entitled "Continuous Process for the Production
of Nanostructures Including Nanotubes," published as WO 2007/136755
on Nov. 29, 2007, which is incorporated herein by reference in its
entirety.
[0049] In certain embodiments, substrates prepared according to the
methods described herein may be processed by chemical vapor
deposition to cause growth of carbon-based nanostructures on the
substrate surface. A chemical vapor deposition process may include
providing thermal energy and providing a chemical atmosphere for a
length of time which may result in the formation of carbon-based
nanostructures. Substrates may be thermally processed in the
presence of hydrogen or another reducing agent to help render any
nanopositor precursor, or nanopositor on the substrate surface into
a state suitable for facilitating growth of carbon-based
nanostructures by chemical vapor deposition. In certain
embodiments, a substrate prepared according to the methods
described herein may be processed by chemical vapor deposition at a
temperature below this specific temperature (e.g., at a temperature
below about 600.degree. C., below about 550.degree. C., or below
500.degree. C.) and still result in growth of carbon-based
nanostructures. In some instances, a chemical vapor deposition
process employing carbon dioxide and an alkyne may be used. In some
of these instances, carbon dioxide and acetylene are present in a
molar ratio of approximately 1:1.
[0050] In certain embodiments, one or more properties of the growth
substrate (e.g., mechanical properties such as tensile strength,
stiffness, compressive strength, porosity, and the like; electrical
properties such as electronic conductivity; and/or thermal
properties such as thermal conductivity) can be preserved after the
one or more intermediate materials have been positioned (e.g.,
coated) over the growth substrate. In one set of embodiments, the
tensile strength of the substrate (e.g., carbon fiber(s)) can be
preserved after the one or more intermediate materials have been
positioned (e.g., coated) over the growth substrate. That is to
say, in certain embodiments, positioning the intermediate
material(s) over the growth substrate does not result in
substantial degradation of the tensile strength of the growth
substrate. In some embodiments, the tensile strength of the
substrate after positioning the intermediate material(s) over the
growth substrate is less than about 20% lower, less than about 10%
lower, less than about 5% lower, or less than about 1% lower than
the tensile strength of the substrate prior to positioning the
intermediate material(s) over the substrate. In some embodiments,
the stiffness, compressive strength, porosity, electronic
conductivity, and/or the thermal conductivity of the substrate
after positioning the intermediate material(s) over the growth
substrate is less than about 20% lower, less than about 10% lower,
less than about 5% lower, or less than about 1% lower than the
stiffness, compressive strength, porosity, electronic conductivity,
and/or the thermal conductivity, respectively, of the substrate
prior to positioning the intermediate material(s) over the
substrate.
[0051] In some embodiments in which intermediate materials are
employed, the tensile strength of the growth substrate (e.g.,
carbon fiber(s)) can be preserved during growth of carbon-based
nanostructures. In some embodiments in which one or more
intermediate materials have been positioned over the growth
substrate, the tensile strength of the growth substrate after
growth of the nanostructures is less than about 20% lower, less
than about 10% lower, less than about 5% lower, or less than about
1% lower than the tensile strength of the substrate prior to growth
of the nanostructures.
[0052] As noted above, the use of intermediate materials in the
growth of nanostructures can provide a number of advantages. Such
advantages include, for example, the ability to load a nanopositor
precursor or nanopositor onto a substrate (e.g., carbon fiber)
without etching, oxygenating, or otherwise damaging the substrate
surface, which may result in a reduction in tensile strength,
tensile stiffness, thermal conductivity, and/or electrical
conductivity of the substrate. Other advantages include the ability
to leverage facile atmosphere-compatible solution-based processing.
Further advantages may include separation of the nanopositor
precursor or nanopositor and the substrate surface, for example, by
using the intermediate material as a sacrificial layer. Even
further advantages include the ability grow carbon-based
nanostructures on fiber fabrics (e.g., carbon fiber fabrics) such
as weaves.
[0053] In one set of embodiments, intermediate material 112
comprises a ceramic-containing material. In some embodiments, the
ceramic-containing material within intermediate material 112 can be
covalently bonded to growth substrate 110. The use of
ceramic-containing materials (e.g., metal oxides) can be
advantageous in certain embodiments in which it is desirable to
form aligned arrays of carbon-based nanostructures on carbon-based
substrates, such as carbon fibers. In certain embodiments, the
relatively smooth external surface provided by the
ceramic-containing material can enhance the degree to which
elongated nanostructures are aligned once produced at or near the
surface of the intermediate material. The use of ceramic-containing
materials as intermediate materials can also allow for relatively
easy separation of the nanostructures from the growth substrate on
which they are formed. For example, in certain embodiments,
nanostructures can be removed by etching or otherwise removing the
ceramic-containing material from between the growth substrate and
the nanostructures.
[0054] In some embodiments, nanostructures (e.g., carbon-based
nanostructures) can be formed on a ceramic-containing material
(e.g., in layer form) positioned over the growth substrate, with or
without an additional nanopositor present. FIGS. 2A-2B are
schematic diagrams of a system in which nanostructures 116 are
grown directly on intermediate material 112, which can comprise a
ceramic-containing material. In this set of embodiments,
ceramic-containing intermediate layer 112 has been exposed to a
carbon-based nanostructure precursor under conditions causing the
formation of substantially aligned carbon-based nanostructures on
the ceramic-containing layer. Of course, the invention is not
limited to such embodiments, and in other embodiments, a
nanopositor material 114 might also be added to the
ceramic-containing material to produce nanostructures.
[0055] A variety of ceramic-containing materials can be used in
intermediate materials, in association with the embodiments
described herein. For example, intermediate material 112 can
comprise a metal oxide, a metal nitride, a metal carbide, and/or a
metal boride (e.g., an oxide, nitride, carbide, and/or boride of
one or more of Al, Zr, Cr, Ba, Ca, Sr, Mg, Be, Na, K, Sc, Y, La,
Ti, Hf, V, Nb, Ta, Mo, W, Mn, Fe, Co, Ni, B, Ga, In, C, Sn, S,
and/or P). In certain embodiments, intermediate material 112 can
comprise a metalloid oxide, a metalloid nitride, a metalloid
carbide, and/or a metalloid boride, in addition to or in place of
the metal oxide, metal nitride, metal carbide, and/or metal boride.
For example, in certain embodiments, the intermediate material can
comprise an oxide, nitride, carbide, and/or boride of Si, Ge, As,
Sb, Te, and/or Po. In one particular set of embodiments, the
intermediate material comprises an aluminum oxide, a titanium
oxide, a magnesium oxide, and/or a silicon oxide.
[0056] Ceramic-containing materials can be formed over growth
substrates (such as carbon fibers or other growth substrates) using
a variety of methods. In certain embodiments, a ceramic-containing
material can be formed over a growth substrate via a sol-gel
technique. In some embodiments, a ceramic material can be formed
over a growth substrate via chemical vapor deposition of a suitable
precursor. Those of ordinary skill in the art, given the present
disclosure, would be capable of determining other methods by which
ceramic-containing material could be formed over a growth
substrate.
[0057] In some embodiments, the ceramic-containing material within
intermediate material 112 comprises an aluminum oxide. Aluminum
oxide can be formed on growth substrate 110 using any suitable
method. For example, in one set of embodiments, a substrate (e.g.,
comprising carbon fibers) is exposed to an aluminum alkoxide at
elevated temperatures. A coating (e.g., a substantially conformal
coating) of alumina may result over the contour surfaces of the
substrate, resulting in a surface layer of alumina. In another set
of embodiments, a sol-gel-based process is used to deposit a
coating (e.g., a substantially conformal coating) of alumina over a
surface of the substrate. Optionally, a nanopositor precursor or a
nanopositor may then be provided onto the surface layer of alumina.
The coated fiber then may be subsequently processed by chemical
vapor deposition resulting in the formation of an array of
carbon-based nanostructures which are substantially oriented (e.g.,
aligned or otherwise ordered) along their lengths relative to one
another.
[0058] In certain embodiments, the ceramic-containing layer can be
in direct contact with growth substrate 110. The invention is not
so limited, however, and in other embodiments, one or more
materials can be positioned between the growth substrate and the
ceramic-containing layer. For example, in certain embodiments, an
intermediate material that is non-covalently associated with growth
substrate 110 can be positioned between growth substrate 110 and
the ceramic-containing layer.
[0059] In another aspect, systems, articles, and methods are
described in which the growth substrate is under tension (e.g., due
to an applied tensile force) during nanostructure growth. It has
been unexpectedly discovered, within the context of the invention,
that applying a tensile force to a substrate during nanostructure
growth can lead to enhanced mechanical, thermal, and/or electrical
properties of the substrate after growth. For example, in certain
embodiments, when a tensile force is applied to a growth substrate
(such as a fiber) during nanostructure growth, the tensile strength
of the growth substrate after growth can be enhanced, relative to
the post-growth tensile strength that would be observed in the
absence of the tension during growth but under otherwise
essentially identical conditions.
[0060] Not wishing to be bound by any particular theory, it is
believed that the application of tension during nanostructure
growth can lead to favorable alignment of the molecules of the
growth substrate, reducing the degree to which the tensile strength
of the growth substrate is reduced during growth. During
manufacture of high-tensile-strength carbon fiber, the fiber is
often put under a high degree of tension. This can result in the
kinetic trapping of a highly oriented microstructure in the fiber.
This microstructure can become thermally activated above a
fiber-specific temperature, and the orientation and high degree of
overlap among grapheme domains can change, which can result in a
loss of tensile strength and stiffness in the fiber. For example,
in some embodiments in which carbon-based growth substrates are
employed, it is believed that, at relatively high growth
temperatures (e.g., temperatures above about 500.degree. C., or
higher), the component graphene (e.g., sheets, ribbons, etc.) can
become kinetically activated, which can allow these components to
shift. It is believed that, absent tension, the component graphene
unfurls and rearranges randomly, negatively affecting tensile
strength. On the other hand, it is believed that, when tension is
applied, alignment of the component graphene is actively maintained
in a direction that is favorable for the material properties of the
growth substrate.
[0061] FIGS. 3A-3C illustrate one set of embodiments in which a
tensile force is applied to a growth substrate during nanostructure
growth. In this set of embodiments nanopositor material 114 is
positioned over growth substrate 110 (e.g., a carbon fiber). In
some embodiments, the nanopositor can be in direct contact with the
growth substrate during the application of the tensile force and/or
during growth of the carbon-based nanostructures. For example, in
FIGS. 3B-3C, nanopositor material 114 is in direct contact with
growth substrate 110.
[0062] While FIGS. 3B-3C illustrate nanopositor material 114 in
direct contact with growth substrate 110, in other embodiments, one
or more materials (e.g., any of intermediate materials 112
described elsewhere) can be positioned between the growth substrate
110 and nanopositor material 114 during nanostructure growth while
tension is applied. In certain embodiments, the substrate (e.g., a
fiber) may have one or more substantially conformal coatings of
various substances over its contour surfaces, for example, forming
a surface layer. A nanopositor precursor or a nanopositor may be
present on the substrate (e.g., fiber) surface and/or on the
surface of the intermediate material (e.g., conformal coating).
[0063] As noted above, a tensile force can be applied to the growth
substrate, in certain embodiments. For example, in the set of
embodiments illustrated in FIGS. 3B-3C, a tensile force can be
applied to growth substrate 110 by applying forces having
components in the direction of arrow 310 and/or arrow 312. Tensile
forces can be applied to substrates using any methods known to
those of ordinary skill in the art. For example, in some
embodiments, the ends of the substrate can be affixed to a
stationary or movable mount using, for example, clamps, an
adhesive, and interlocking mechanism, or via any other suitable
mechanism. Subsequently, one or both of the clamped ends can be
moved away from the other, imparting a tensile stress on the
substrate.
[0064] The tensile force can be imparted to the growth substrate at
any suitable level and/or in any suitable direction. In certain
embodiments, the magnitude and/or the direction of the tensile
force can be selected such that the applied force enhances the
tensile strength of the growth substrate after growth of the
nanostructures (including exposure to a temperature within any of
the ranges described herein) relative to the tensile strength that
would be observed in the absence of the tensile force but under
otherwise essentially identical conditions. "Essentially identical
conditions," in this context, means conditions that are similar or
identical other than the application and/or magnitude of the
tensile force. For example, otherwise identical conditions may
mean, in certain embodiments, a growth substrate, intermediate
material, nanopositor, and nanostructure precursor that are
identical; nanostructure growth conditions (e.g., temperature,
atmosphere, pressure, growth time, etc.) that are identical; but
where the growth substrate is not constructed (e.g., by brackets or
other connections) to apply the anisotropic force on the subject
growth substrate. In certain embodiments, the magnitude and/or
direction of the applied tensile force can be selected such that
the post-growth tensile strength of the growth substrate is
enhanced by at least about 1%, at least about 5%, at least about
10%, or at least about 25% (and, in some cases, by less than about
50%), relative to the tensile strength of the growth substrate that
would be observed in the absence of the applied tensile force but
under otherwise essentially identical conditions.
[0065] In some embodiments, the magnitude of the tensile force can
be such that it defines an engineering strain in the growth
substrate in at least one direction of, for example, at least about
0.0001%, at least about 0.001%, at least about 0.01%, or at least
about 0.1% (and, in some embodiments, less than about 5%, less than
about 2%, or less than about 1%). One of ordinary skill in the art
would be capable of determining the amount of engineering strain
imparted to a given growth substrate by measuring the length (L) of
the growth substrate just prior to application of the tensile force
and during growth (i.e., at the temperature, pressure, and other
conditions at which growth occurs), measuring the length (l) of the
growth substrate after the application of the tensile force and
during growth, and calculating the engineering strain (e) as
follows:
e = l - L L .times. 100 % [ 1 ] ##EQU00001##
[0066] In some embodiments, the magnitude of the tensile force can
be such that it defines a stress of, for example, greater than
about 1 kPa, greater than about 10 kPa, greater than about 100 kPa,
greater than about 1 MPa, greater than about 25 MPa, greater than
about 75 MPa, greater than about 250 MPa, greater than about 500
MPa, greater than about 1 GPa, or greater than about 10 GPa.
[0067] The magnitude of the tensile force may be such that it
defines a stress of, for example, greater than about 1%, greater
than about 5%, or greater than about 10% of the breaking strength
of the substrate (e.g., fiber) (and/or, in some embodiments, less
than about 75%, less than about 60%, less than about 45%, less than
about 30%, less than about 20%, or less than about 15% of the
breaking strength of the substrate (e.g., fiber)). In this context,
the breaking strength of a substrate (e.g., fiber) is determined by
breaking a representative number of similar substrates (i.e.,
substrates originating from the same manufacturing lot as the
substrate being used) and number averaging these values, as
described in Example 8. For example, to determine the breaking
strength of a carbon fiber a priori, one of ordinary skill in the
art would determine the breaking strength of a representative
number of similar carbon fibers (e.g., carbon fibers sourced from
the same manufacturing lot) and taking the average of the measured
breaking strengths.
[0068] Suitable techniques for measuring the breaking strength of a
single fiber (e.g., a carbon or graphite fiber) can be found, for
example, in ASTM standard test D3379-75 ("Standard Test Method for
Tensile Strength and Young's Modulus for High-Modulus
Single-Filament Materials," 1989), which is incorporated herein by
reference in its entirety for all purposes. ASTM standard test
C1557-03 ("Standard Test Method for Tensile Strength and Young's
Modulus of Fibers," ASTM International, West Conshohocken, Pa.,
2003) could also be used and is expected to provide similar
results. Suitable techniques for measuring the breaking strength of
other substrates may be found, for example, in M. Madou,
"Fundamentals of Microfabrication," 2nd edition, CRC Press (2002),
which is incorporated herein by reference in its entirety.
[0069] In certain embodiments, the tensile force applied to the
growth substrate can include a component that is oriented in a
direction that is parallel to the external surface of the growth
substrate from which nanostructures are grown. In this context, the
"external surface of the growth substrate" refers to the geometric
surface of the growth substrate, which will be understood by those
of ordinary skill in the art to refer to the surface defining the
outer boundaries of the growth substrate, and does not include the
internal surface area (e.g., area within pores of a porous material
such as a foam, or surface area of those fibers of a mesh that are
contained within the mesh and do not define the outer boundary,
etc.).
[0070] For example, in the set of embodiments illustrated in FIG.
3B, a force applied in the direction of arrow 314 would include
component 310, which is parallel to the external surface 118 of
growth substrate 310 on which nanopositor material 114 is deposited
and from which nanostructures 116 are grown. In some embodiments,
the component of the applied force that is parallel to the external
surface of the growth substrate from which the nanostructures are
grown has a magnitude that lies within any of the ranges described
in the preceding paragraphs and/or defines a strain that lies
within any of the ranges within the preceding paragraphs.
[0071] In certain embodiments, the direction in which the tensile
force is applied is substantially parallel to the external surface
of the growth substrate from which the nanostructures are grown.
That is to say, in certain embodiments, the tensile force is
applied in a direction that defines a largest angle with the
external surface of the growth substrate that is less than about
5.degree., less than about 2.degree., less than about 1.degree., or
less than about 0.1.degree.. For example, in FIG. 3B, a tensile
force applied to growth substrate 110 in the direction of arrow 310
defines a largest angle with external surface 118 of growth
substrate 110 that is about 0.degree., and thus, a force applied in
the direction of arrow 310 would be substantially parallel to
external surface 118.
[0072] In some embodiments, while the tensile force is applied to
the growth substrate, a carbon-based nanostructure precursor can be
exposed to the nanopositor under conditions causing the formation
of carbon-based nanostructures on the nanopositor. In FIG. 3C, for
example, nanopositor 114 can be exposed to carbon-based
nanostructure precursor materials, resulting in the formation of
nanostructure 116. The nanostructures can be grown, for example,
using any of the techniques and procedures outlined above with
respect to FIGS. 1A-1C and FIGS. 2A-2B.
[0073] In one particular set of embodiments, carbon fibers are
tensioned at temperatures and/or other conditions suitable for
chemical vapor deposition growth of carbon-based nanostructures.
The tensioned carbon fiber may then be exposed to a chemical
atmosphere that results in the formation of carbon-based
nanostructures. In some instances, this chemical atmosphere may
comprise an inert gas, hydrogen, carbon dioxide, carbon monoxide,
an alkane, an alkene, an alkyne, water, an alcohol, ferrocene, or
an aromatic compound. In some instances, carbon-based
nanostructures result. In some of these instances, the substrate
(e.g., carbon fiber) exhibits a higher tensile strength than in a
similar process where tension is not applied.
[0074] Application of tensile forces to nanostructure growth
substrates can provide a number of advantages. For example, such
systems and methods can allow one to grow carbon-based
nanostructures (including carbon nanotubes) on substrates such as
carbon fibers at the temperatures suitable for chemical vapor
deposition (e.g., greater than 650.degree. C.), without subjecting
the substrate to substantial tensile strength and/or tensile
stiffness degradation (which is often observed in many traditional
growth processes that do not employ tension, at temperatures of,
for example, above about 500.degree. C.). Additionally, the
application of tensile forces to growth substrates can be easily
integrated into existing growth substrate manufacturing processes
(e.g., carbon fiber manufacturing processes), many of which apply
tension to substrates (e.g., fibers) during production of the
substrates.
[0075] In some embodiments, the application of a tensile force to
the growth substrate (e.g., carbon fiber) during nanostructure
growth can enhance the post-growth tensile strength of the growth
substrate. In certain cases, the tensile strength of the growth
substrate can be preserved even after carbon-based nanostructure
growth a relatively high temperatures (e.g., temperatures above
about 600.degree. C.). In some embodiments in which a tensile force
is applied during nanostructure growth, the tensile strength of the
substrate after growth of the nanostructures is less than about 20%
lower, less than about 10% lower, less than about 5% lower, or less
than about 1% lower than the tensile strength of the substrate
prior to growth of the nanostructures.
[0076] The systems and methods described herein can be used to
produce nanostructures over elongated, carbon-based substrates
(e.g., carbon fibers) while maintaining a relatively high tensile
strength of the elongated, carbon-based substrates. In certain
embodiments, the tensile strength of an elongated carbon-based
growth substrate can be at least about 1 GPa, at least about 2 GPa,
at least about 3 GPa or at least about 4 GPa (and, in some
instances, less than about 5 GPa) after nanostructures have been
grown over the growth substrate.
[0077] In certain embodiments, the articles, systems, and methods
described herein can be used to grow substantially aligned
nanostructures (i.e., such that the longitudinal axes of the
nanostructures are substantially aligned relative to each other).
For example, in FIGS. 1C, 2B, and 3C, nanostructures 116 are
substantially aligned with one another.
[0078] In certain embodiments, each nanostructure within a
plurality of nanostructures is positioned relative to an adjacent
nanostructure at a distance so as to together define an average
distance between adjacent nanostructures. In the set of embodiments
illustrated in FIGS. 1C, 2B, and 3C, the average distance between
adjacent nanostructures is roughly equal for each nanostructure. In
other embodiments, the distances between adjacent nanostructures
may vary.
[0079] The aligned nanostructures can be positioned, for example,
side-by-side relative to each other, as illustrated in FIGS. 1C,
2B, and 3C. In some embodiments, the nanostructures can be grown
such that the plurality of nanostructures extends a distance at
least 10 times greater than the average distance between adjacent
nanostructures in each of two orthogonal directions, each direction
perpendicular to the long axes of the nanostructures. In some
cases, the plurality of nanostructures extends, in two orthogonal
directions each perpendicular to the long axes of the
nanostructures, a distance at least 100 times greater, at least
1000 times greater, at least 10,000 times greater or longer than
the average distance between adjacent nanostructures.
[0080] In certain embodiments, the aligned nanostructures can be
non-parallel to the external surface of the growth substrate (e.g.,
surface 118 in FIG. 3B). In some embodiments, the aligned
nanostructures can be substantially perpendicular to the external
surface of the growth substrate at the point of contact. For a
non-planar growth substrate, the angle between the nanostructure
and the growth substrate surface is measured relative to a plane
tangent to the external surface of the growth substrate at the
point at which the longitudinal axis of the nanostructure
intersects the external surface of the growth substrate. FIG. 4 is
a cross-sectional schematic illustration of an elongated growth
substrate 110 on which nanostructures 116 have been formed. In FIG.
4, substrate 110 can correspond to a substantially cylindrical
substrate, such as a fiber. In the set of embodiments illustrated
in FIG. 4, the as-grown nanostructures 116 are arranged such that
the longitudinal axes of the nanostructures extend in a direction
normal to the external surface of the growth substrate 110 from
which they are grown.
[0081] A variety of growth substrates may be used in accordance
with the systems and methods described herein. Growth substrates
may comprise any material capable of supporting nanopositors and/or
nanostructures as described herein. The growth substrate may be
selected to be stable under the conditions described herein. In
some cases, the growth substrate comprises a substantially flat
surface. In some cases, the growth substrate comprises a
substantially nonplanar surface. For example, the growth substrate
may comprise a cylindrical surface.
[0082] In some embodiments, the growth substrate comprises a fiber,
such as a carbon fiber. In some embodiments, the growth substrate
can be elongated. For example, the ratio of the length of the
growth substrate (e.g., a fiber substrate) to the diameter or other
cross-sectional dimension of the growth substrate can be, in some
embodiments, at least about 2:1; at least about 3:1; at least about
5:1; at least about 10:1; at least about 50:1; at least about
100:1; at least about 500:1; at least about 1000:1; at least about
10,000:1; at least about 100,000:1; at least about 10.sup.6:1; at
least about 10.sup.7:1; at least about 10.sup.8:1; or at least
about 10.sup.9:1.
[0083] In certain embodiments, fibers (e.g., carbon fibers) used as
growth substrates can have relatively large cross-sectional
dimensions (e.g., relative to the nanostructures formed over the
fiber substrate). For example, in certain embodiments, a fiber
growth substrate can have a smallest cross-sectional dimension of
at least about 1 micrometer, at least about 5 micrometers, or at
least about 10 micrometers (and/or, in certain embodiments, less
than about 1 mm, less than about 100 micrometers, or less than
about 20 micrometers). Generally, the smallest cross-sectional
dimension is measured perpendicularly to the length of the fiber
and through the longitudinal axis of the fiber.
[0084] In some embodiments, the growth substrate can comprise a
fiber that is part of a bundle of fibers. For example, in certain
embodiments, the growth substrate can comprise a fiber that is part
of a tow, a weave, or a yarn of fibers. In certain embodiments, the
growth substrate can comprise a fiber that is substantially free of
contact with other fibers.
[0085] The growth substrate can comprise a variety of materials. In
some embodiments, the substrate can comprise carbon (e.g.,
amorphous carbon, carbon aerogel, carbon fiber, graphite, glassy
carbon, carbon/carbon composite, graphene, aggregated diamond
nanorods, nanodiamond, diamond, and the like). In some embodiments,
the growth substrate can comprise a polymer. In certain
embodiments, at least about 50 wt %, at least about 75 wt %, at
least about 90 wt %, at least about 95 wt %, or at least about 99
wt % of the substrate can be made of polymer). In some embodiments,
the growth substrate can comprise a glass. In certain embodiments,
at least about 50 wt %, at least about 75 wt %, at least about 90
wt %, at least about 95 wt %, or at least about 99 wt % of the
substrate can be made of glass). Substrates suitable for use in the
invention include high-temperature prepregs, high-temperature
polymer resins, inorganic materials such as metals, alloys,
intermetallics, metal oxides, metal nitrides, ceramics, and the
like. As used herein, the term "prepreg" refers to one or more
layers of thermoset or thermoplastic resin containing embedded
fibers, for example fibers of carbon, glass, silicon carbide, and
the like. In some cases, the substrate may be a fiber, tow of
fibers, a weave (e.g., a dry weave), and the like. The substrate
may further comprise a conducting material, such as conductive
fibers, weaves, or nanostructures. Examples of suitable polymers
that can be used in the growth substrate include, but are not
limited to, relatively high temperature fluoropolymers (e.g.,
Teflon.RTM.), polyetherether ketone (PEEK), and polyether ketone
(PEK), and the like.
[0086] In some embodiments, the substrate and the nanopositor can
be selected such that they would be reactive with each other if in
direct contact and exposed to the growth conditions under which the
carbon-based nanostructures are grown (e.g., temperatures of
greater than about 600.degree. C., for some embodiments).
[0087] In certain embodiments, carbon-based growth substrates such
as carbon fibers can be employed. In some embodiments, the
carbon-based growth substrate contains carbon in an amount of at
least about 75 wt %, at least about 90 wt %, at least about 95 wt
%, or at least about 99 wt %. That is to say, in some embodiments,
at least about 75 wt %, at least about 90 wt %, at least about 95
wt %, or at least about 99 wt % of the carbon-based growth
substrate is made of carbon.
[0088] As noted elsewhere, in one set of embodiments, the growth
substrate comprises one or more carbon fibers. Any suitable type of
carbon fiber can be employed including, for example,
aerospace-grade carbon fibers, auto/sport grade carbon fibers,
and/or microstructure carbon fibers. In certain embodiments,
intermediate modulus (IM) or "high modulus (HM) carbon fibers can
be employed. In some embodiments, poly(acrylonitrile)-derived
carbon fibers can be employed. Certain embodiments of the invention
are advantageous for use with carbon fibers that carry a large
degree of their tensile strengths in their outer skins (e.g.,
fibers in which at least about 50%, at least about 75%, or at least
about 90% of the tensile strength is imparted by the portion of the
fiber located a distance away from the outer skin of the fiber of
less than about 0.1 times or less than about 0.05 times the
cross-sectional diameter of the fiber), such as aerospace grade
intermediate modulus carbon fibers.
[0089] Carbon fibers, including poly(acrylonitrile)-derived carbon
fibers, can be particularly challenging substrates upon which to
grow nanostructures such as carbon nanotubes using traditional
methods for a number of reasons. For example, it is believed that
the majority of the tensile load carried by a carbon fiber is
transmitted in the outer skins of the fiber; as such, any
disruption to the surface can result in a disproportionate loss of
tensile properties. Many commonly employed nanostructure catalysts
(e.g., metals such as Fe, Ni, etc.) react with or dissolve carbon
at nanostructure growth temperatures (e.g., 700.degree. C. to
900.degree. C.), which can degrade the carbon fiber. In addition,
the carbon within carbon fibers can react with oxygen, water, and
hydrogen at temperatures above 400.degree. C., and many substances
commonly found in the environment (e.g., Na.sup.+ and K.sup.+ ions
from human skin) can catalyze microstructural transformations in
carbon at these temperatures. The outer surface of carbon fibers is
also generally highly graphitic and therefore presents a low
wettability with few active binding sites to which traditional
coatings can be applied; frequently acid or electrochemical etching
the surface of the carbon fiber is required to overcome the
wettability problem. In addition, aligned growth of nanostructures
such as carbon nanotubes is not natively facilitated on carbon
substrates and generally requires the presence of specific support
materials. As such, it has traditionally been challenging to adhere
catalyst materials for nanostructure growth to the fiber surface
without adding a chemical step that circumstantially results in
reduction of fiber tensile properties. In addition, even if the
catalyst material can be applied and adhered without such a
treatment using traditional methods (e.g., through in situ
deposition of floating catalyst particles during nanostructure
growth), damage to the carbon fiber is likely to result at the
temperatures at which nanostructures such as carbon nanotubes are
grown. The systems, articles, and methods described herein address
these issues and generally allow for more effective growth of
nanostructures such as carbon nanotubes on carbon fibers.
[0090] Of course, the invention is not limited to the use of growth
substrates comprising carbon, and other growth substrate materials
can be used. Other examples of suitable growth substrate materials
(e.g., materials that are vulnerable to degradation when
traditional methods are employed, but display improved post-growth
mechanical, thermal, and/or electrical properties when used in
association with the embodiments described herein) include, but are
not limited to, boron-based growth substrates (e.g., boron fibers),
glass- or quartz-based substrates (e.g., glass or quartz fibers),
ceramic substrates (e.g., ceramic fibers, including fibers
comprising aluminum oxide), and polymer substrates (e.g., polymer
fibers).
[0091] As used herein, the term "carbon-based nanostructure" refers
to articles having a fused network of aromatic rings, at least one
cross-sectional dimension of less than about 1 micrometer, and
comprising at least about 30% carbon by mass. In some embodiments,
the carbon-based nanostructures may comprise at least about 40%, at
least about 50%, at least about 60%, at least about 70%, at least
about 80%, at least about 90%, or at least about 95% of carbon by
mass, or more. The term "fused network" might not include, for
example, a biphenyl group, wherein two phenyl rings are joined by a
single bond and are not fused. Example of carbon-based
nanostructures include carbon nanotubes (e.g., single-walled carbon
nanotubes, double-walled carbon nanotubes, multi-walled carbon
nanotubes, etc.), carbon nanowires, carbon nanofibers, carbon
nanoshells, graphene, fullerenes, and the like.
[0092] In some embodiments, the carbon-based nanostructure can be
an elongated structure. In some embodiments, a carbon-based
nanostructure may have an aspect ratio of at least about 2:1; at
least about 3:1; at least about 5:1; at least about 10:1; at least
about 50:1; at least about 100:1; at least about 500:1; at least
about 1000:1; at least about 10,000:1; at least about 100,000:1; at
least about 10.sup.6:1; at least about 10.sup.7:1; at least about
10.sup.8:1; or at least about 10.sup.9:1.
[0093] In some embodiments, a carbon-based nanostructure may have a
least one cross-sectional dimension of less than about 500 nm, less
than about 250 nm, less than about 100 nm, less than about 75 nm,
less than about 50 nm, less than about 25 nm, less than about 10
nm, or, in some cases, less than about 1 nm. Carbon-based
nanostructures described herein may have, in some cases, a maximum
cross-sectional dimension of less than about 1 micrometer, less
than about 500 nm, less than about 250 nm, less than about 100 nm,
less than about 75 nm, less than about 50 nm, less than about 25
nm, less than about 10 nm, or, in some cases, less than about 1
nm.
[0094] In some embodiments, the carbon-based nanostructures
described herein may comprise carbon nanotubes. As used herein, the
term "carbon nanotube" is given its ordinary meaning in the art and
refers to a substantially cylindrical molecule or nanostructure
comprising a fused network of primarily six-membered rings (e.g.,
six-membered aromatic rings) comprising primarily carbon atoms. In
some cases, carbon nanotubes may resemble a sheet of graphite
formed into a seamless cylindrical structure. It should be
understood that the carbon nanotube may also comprise rings or
lattice structures other than six-membered rings. Typically, at
least one end of the carbon nanotube may be capped, i.e., with a
curved or nonplanar aromatic structure. Carbon nanotubes may have a
diameter of the order of nanometers and a length on the order of
millimeters, or, on the order of tenths of micrometers, resulting
in an aspect ratio greater than 100, 1000, 10,000, 100,000,
10.sup.6, 10.sup.7, 10.sup.8, 10.sup.9, or greater. Examples of
carbon nanotubes include single-walled carbon nanotubes (SWNTs),
double-walled carbon nanotubes (DWNTs), multi-walled carbon
nanotubes (MWNTs) (e.g., concentric carbon nanotubes), inorganic
derivatives thereof, organic derivatives thereof, and the like. In
some embodiments, the carbon nanotube is a single-walled carbon
nanotube. In some cases, the carbon nanotube is a multi-walled
carbon nanotube (e.g., a double-walled carbon nanotube). In some
cases, the carbon nanotube comprises a multi-walled or
single-walled carbon nanotube with an inner diameter wider than is
attainable from a traditional catalyst or other nanopositor
material. In some cases, the carbon nanotube may have a diameter
less than about 1 micrometer, less than about 500 nm, less than
about 250 nm, less than about 100 nm, less than about 50 nm, less
than about 25 nm, less than about 10 nm, or, in some cases, less
than about 1 nm.
[0095] As used herein, the term "nanopositor" refers to a material
that, when exposed to a set of conditions selected to cause
formation of nanostructures, either enables formation of
nanostructures that would otherwise not occur in the absence of the
nanopositor under essentially identical conditions, or increases
the rate of formation of nanostructures relative to the rate that
would be observed under essentially identical conditions but
without the nanopositor material. "Essentially identical
conditions," in this context, means conditions that are similar or
identical (e.g., pressure, temperature, composition and
concentration of species in the environment, etc.), other than the
presence of the nanopositor. In one set of embodiments, a
"nanopositor" is not consumed in a reaction involving the formation
of nanostructures which it enables or for which it increases the
rate, i.e., atoms or molecules that make up the nanopositor are
not, via reaction, incorporated into the nanostructure. In some
embodiments, a nanopositor can comprise a catalyst. In some
embodiments, a nanopositor can be configured to promote the growth
of carbon-based nanostructures from carbon-based nanostructure
precursors. For example, the nanopositor material can be selected
such that, when the nanopositor is exposed to a given set of
reaction conditions and/or nanostructure precursor material,
carbon-based nanostructures are produced.
[0096] Any suitable nanopositor can be used in association with the
embodiments described herein. In some embodiments, the nanopositor
can comprise an elemental metal and/or a metal oxide. The
nanopositor can comprise, in some embodiments iron, such as
elemental iron. In some embodiments, the nanopositor can include a
metal in a zero-oxidation state (e.g., during growth of the
nanostructures). Exemplary zero oxidation state metals include, but
are not limited to, iron, cobalt, nickel, platinum, gold, copper,
rhenium, tin, tantalum, aluminum, palladium, rhodium, silver,
tungsten, molybdenum, zirconium, or any other suitable metal. In
some embodiments, the nanopositor includes metal or metalloid atoms
in a non-zero oxidation state (e.g., during growth of the
carbon-based nanostructures). In some instances, the nanopositor
and/or nanopositor support may comprise metal oxides or metal
chalcogenides (e.g., metal sulfides, metal selenides, metal
tellurides, etc.). In some embodiments, the nanopositor or
nanopositor support may comprise metalloid oxides or metalloid
chalcogenides (e.g., metalloid sulfides, metalloid selenides,
metalloid tellurides, etc.). In some cases, the nanopositor or
nanopositor support may comprise a metal and/or metalloid carbide,
nitride, phosphide, silicide, or combination of these. Examples of
metal atoms in a non-zero oxidation state which may be particularly
suitable, in some embodiments, for use in nanopositors or
nanopositor supports include, but are not limited to, oxide and
chalcogenide forms of zirconium, hafnium, tantalum, niobium,
yttrium, lanthanum, molybdenum, lanthanide metals, titanium,
aluminum, rhenium, and calcium, among others. Examples of metalloid
atoms in a non-zero oxidation state which may be particularly
suitable, in some embodiments, for use in nanopositors or
nanopositor supports include, but are not limited to, silicon and
germanium among others. Specific examples of suitable nanopositors
include, but are not limited to, zirconia, doped zirconia, titania,
doped titania (e.g., Sn-doped titania), MoO.sub.3/ZrO.sub.2 blends,
FeS, and Si.sub.3N.sub.4.
[0097] In some instances, the nanopositor may comprise
nanoparticles. Generally, the term "nanoparticle" is used to refer
to any particle having a maximum cross-sectional dimension of less
than about 1 micrometer. In some embodiments, a nanopositor
nanoparticle may have a maximum cross-sectional dimension of less
than about 500 nm, less than about 250 nm, less than about 100 nm,
less than about 10 nm, less than about 5 nm, less than about 3 nm,
less than about 2 nm, less than about 1 nm, between about 0.3 and
about 10 nm, between about 10 nm and about 100 nm, or between about
100 nm and about 1 micrometer. A plurality of nanopositor
nanoparticles may, in some cases, have an average maximum
cross-sectional dimension of less than about 1 micrometer, less
than about 100 nm, less than about 10 nm, less than about 5 nm,
less than about 3 nm, less than about 2 nm, less than about 1 nm,
between about 0.3 and about 10 nm, between about 10 nm and about
100 nm, or between about 100 nm and about 1 micrometer. As used
herein, the "maximum cross-sectional dimension" refers to the
largest distance between two opposed boundaries of an individual
structure that may be measured. The "average maximum
cross-sectional dimension" of a plurality of structures refers to
the number average.
[0098] In some cases, the nanopositor may comprise one or more
nanoscale features. As used herein, a "nanoscale feature" refers to
a feature, such as a protrusion, groove or indentation, particle,
or other measurable geometric feature on an article that has at
least one cross-sectional dimension of less than about 1
micrometer. In some cases, the nanoscale feature may have at least
one cross-sectional dimension of less than about 500 nm, less than
about 250 nm, less than about 100 nm, less than about 10 nm, less
than about 5 nm, less than about 3 nm, less than about 2 nm, less
than about 1 nm, between about 0.3 and about 10 nm, between about
10 nm and about 100 nm, or between about 100 nm and about 1
micrometer. Not wishing to be bound by any theory, the nanoscale
feature may increase the rate at which a reaction, nucleation step,
or other process involved in the formation of a nanostructure
occurs. Nanoscale features can be formed, for example, by
roughening the surface of a nanopositor.
[0099] Additional suitable nanopositor materials and configurations
are described, for example, in U.S. Patent Application Publication
No. 2011/0027162, filed Jul. 30, 2010, and published Feb. 3, 2011,
entitled "Systems and Methods Related to the Formation of
Carbon-Based Nanostructures," to Steiner, III et al, which is
incorporated herein by reference in its entirety for all
purposes.
[0100] It should be understood that when a portion (e.g., layer,
structure, region) is "on", "adjacent", "above", "over",
"overlying", or "supported by" another portion, it can be directly
on the portion, or an intervening portion (e.g., layer, structure,
region) also may be present. Similarly, when a portion is "below"
or "underneath" another portion, it can be directly below the
portion, or an intervening portion (e.g., layer, structure, region)
also may be present. A portion that is "directly on", "immediately
adjacent", "in contact with", or "directly supported by" another
portion means that no intervening portion is present. It should
also be understood that when a portion is referred to as being
"on", "above", "adjacent", "over", "overlying", "in contact with",
"below", or "supported by" another portion, it may cover the entire
portion or a part of the portion.
[0101] As noted above, in some embodiments, a nanostructure
precursor (e.g., a carbon-based nanostructure precursor) can be
exposed to a nanopositor under conditions causing the formation of
nanostructures (e.g., carbon-based nanostructures), for example on
the nanopositor. Exposure to such conditions may comprise, for
example, exposure to a particular temperature, pH, solvent,
chemical reagent, type of atmosphere (e.g., nitrogen, argon,
oxygen, etc.), electromagnetic radiation, or the like. In some
cases, the conditions may be selected to facilitate nucleation,
growth, stabilization, removal, and/or other processing of
nanostructures. In some cases, the conditions may be selected to
facilitate reactivation, removal, and/or replacement of the
nanopositor. In some cases, the conditions may be selected to
maintain the activity of the nanopositor. Some embodiments may
comprise a set of conditions comprising exposure to a source of
external energy. The source of energy may comprise electromagnetic
radiation, electrical energy, sound energy, thermal energy, or
chemical energy. For example, the conditions can comprise exposure
to heat or electromagnetic radiation, resistive heating, exposure
to a laser, or exposure to infrared light. In some embodiments, the
set of conditions comprises exposure to a particular temperature,
pressure, chemical species, and/or nanostructure precursor
material. For example, in some cases, exposure to a set of
conditions comprises exposure to substantially atmospheric pressure
(i.e., about 1 atm or 760 torr). In some cases, exposure to a set
of conditions comprises exposure to a pressure of less than about 1
atm (e.g., less than about 100 torr, less than about 10 torr, less
than about 1 torr, less than about 0.1 torr, less than about 0.01
torr, or lower). In some cases, the use of high pressure may be
advantageous. For example, in some embodiments, the conditions
comprise exposure to a pressure of at least about 2 atm, at least
about 5 atm, at least about 10 atm, at least about 25 atm, or at
least about 50 atm. In some instances, the set of conditions
comprises exposure to a temperature below about 600.degree. C.,
below about 500.degree. C., or below about 400.degree. C. (and, in
some cases, above about 300.degree. C.).
[0102] In some embodiments, exposure to growth conditions comprises
performing chemical vapor deposition (CVD) of nanostructures on the
nanopositor. In some embodiments, the chemical vapor deposition
process may comprise a plasma chemical vapor deposition process.
Chemical vapor deposition is a process known to those of ordinary
skill in the art, and is explained, for example, in Dresselhaus M
S, Dresselhaus G., and Avouris, P. eds. "Carbon Nanotubes:
Synthesis, Structure, Properties, and Applications" (2001)
Springer, which is incorporated herein by reference in its
entirety.
[0103] The following patents and patent applications are
incorporated herein by reference in their entireties for all
purposes: International Patent Application Serial No.
PCT/US2007/011914, filed May 18, 2007, entitled "Continuous Process
for the Production of Nanostructures Including Nanotubes,"
published as WO 2007/136755 on Nov. 29, 2007; U.S. patent
application Ser. No. 12/227,516, filed Nov. 19, 2008, entitled
"Continuous Process for the Production of Nanostructures Including
Nanotubes," published as US 2009/0311166 on Dec. 17, 2009;
International Patent Application Serial No. PCT/US07/11913, filed
May 18, 2007, entitled "Nanostructure-reinforced Composite Articles
and Methods," published as WO 2008/054541 on May 8, 2008;
International Patent Application Serial No. PCT/US2008/009996,
filed Aug. 22, 2008, entitled "Nanostructure-reinforced Composite
Articles and Methods," published as WO 2009/029218 on Mar. 5, 2009;
U.S. patent application Ser. No. 11/895,621, filed Aug. 24, 2007,
entitled "Nanostructure-Reinforced Composite Articles and Methods,"
published as US 2008/0075954 on Mar. 27, 2008; U.S. Pat. No.
7,537,825, issued on May 26, 2009, entitled "Nano-Engineered
Material Architectures: Ultra-Tough Hybrid Nanocomposite System";
U.S. patent application Ser. No. 11/895,621, filed Aug. 24, 2007,
entitled "Nanostructure-Reinforced Composite Articles," published
as U.S. Patent Application Publication No. 2008/0075954 on Mar. 27,
2008; U.S. Provisional Patent Application 61/114,967, filed Nov.
14, 2008, entitled "Controlled-Orientation Films and Nanocomposites
Including Nanotubes or Other Nanostructures"; U.S. patent
application Ser. No. 12/618,203, filed Nov. 13, 2009, entitled
"Controlled-Orientation Films and Nanocomposites Including
Nanotubes or Other Nanostructures," published as U.S. Patent
Application Publication No. 2010/0196695 on Aug. 5, 2010; U.S.
patent application Ser. No. 12/630,289, filed Dec. 3, 2009,
entitled "Multifunctional Composites Based on Coated
Nanostructures," published as U.S. Patent Application Publication
No. 2010/0255303 on Oct. 7, 2010; U.S. patent application Ser. No.
12/847,905, filed Jul. 30, 2010, entitled "Systems and Methods
Related to the Formation of Carbon-Based Nanostructures"; U.S.
Provisional Patent Application No. 61/264,506, filed Nov. 25, 2009,
and entitled "Systems and Methods for Enhancing Growth of
Carbon-Based Nanostructures"; U.S. patent application Ser. No.
12/953,287, filed Nov. 23, 2010, entitled "Systems and Methods for
Enhancing Growth of Carbon-Based Nanostructures"; and U.S.
Provisional Patent Application Ser. No. 61/537,538, filed Sep. 21,
2011, and entitled "Methods for Growth of Nanostructures on
Substrates Including Fibers." The articles, systems, and methods
described herein may be combined with those described in any of the
patents and/or patent applications noted above. All patents and
patent applications mentioned herein are incorporated herein by
reference in their entirety for all purposes.
[0104] The following examples are intended to illustrate certain
embodiments of the present invention, but do not exemplify the full
scope of the invention.
Example 1
[0105] This example describes the sol-gel deposition of alumina on
carbon fiber substrates and subsequent growth of carbon nanotubes.
The carbon fibers used in these examples were taken from unsized
(i.e., never-sized) intermediate-modulus (high-tensile-strength)
carbon fiber tows obtained from industrial supplier TohoTenax
("fiber HTR-40") and a standard AS4 fiber ("fiber AS4"). The
unsized fibers described in these examples are not normally
commercially available, but rather, were spooled from their
respective production lines prior to the surface oxygenation and
sizing steps typically performed in their manufacture.
[0106] The sol-gel deposition of alumina via
propylene-oxide-assisted gelation was investigated. In this
process, 2.96 g of aluminum chloride hexahydrate
(AlCl.sub.3.6H.sub.2O, Sigma-Aldrich product number 237078, 99%)
was dissolved in a mixture of 20.0 g (20.0 mL) deionized water
(analytical reagent grade deionized water, Ricca Chemical Company
product number 9150-1) and 20.0 g (25.4 mL) 2-propanol
(isopropanol, Mallinckdrodt Chemicals product number 3032-16, ACS
grade). The mixture was stirred until the salt had fully dissolved.
Next, 7.86 g (9.5 mL) propylene oxide (C.sub.3H.sub.5O,
Sigma-Aldrich product number 110205, 99%, or 82320, .gtoreq.99.5%)
was added slowly into the solution via syringe with stirring. The
solution was then stirred another 5 min and allowed to solify. Gel
time was about 4 h.
[0107] Alternatively, a solution of 10.0 g (10.0 mL) deionized
water and 7.89 g (10.0 mL) absolute ethanol (anhydrous absolute
ethanol, C.sub.2H.sub.5OH, Pharmco-Aaper product number 111000200,
ACS/USP grade) could be used. In this case the gel time was reduced
to about 1 hour and 40 min. Gel time could be further adjusted for
either of these processes by increasing the amount of solvent used.
However, doing so also resulted in an increase in porosity.
[0108] The sol-gel deposition of alumina via
epichlorohydrin-assisted gelation was also investigated. A solution
of 15.78 g (20.0 mL) ethanol and 4.52 g (4.52 mL) H.sub.2O was
prepared. 2.96 g AlCl.sub.3.6H.sub.2O was then added to the
solution, with stirring. With this solution placed on a balance,
12.52 g (10.58 mL) epichlorohydrin (C.sub.3H.sub.50Cl,
Sigma-Aldrich product number 240699, .gtoreq.99%) was added slowly
by a syringe with an 18-gage needle (at a rate of a few drops per
second). The solution was then stirred for another 5 min. Gel time
was approximately 120 min. The density of the gel could be lowered
by adding more ethanol. At least the amount of water specified here
was required to dissolve all of the AlCl.sub.3. However if water
was added until the solution was greater than 30-40% water by
volume, the epichlorohydrin separated into a second phase and no
gelation occurred. Similarly, when using barely enough ethanol to
get the solution into a single phase, the solution often
spontaneously became hazy and epichlorohydrin separated out due to
evaporation of the ethanol.
[0109] Unsized (i.e., never-sized), never-surface-treated carbon
fiber tow (TohoTenax product number HTR40 N00 24k 1550tex) was used
for substrates. As noted above, this product is not generally
available commercially. The equivalent commercial version, HTA40
F22 24k 1550tex, is surface-treated and sized. Carbon fiber tows
(about 10 cm long) were cut and taped at one end with masking tape
(3M 2600) for ease of handling. Tows were soaked in freshly-mixed
alumina sol and removed at fixed time intervals: for 45-min gel
times, samples were removed at 15 min, 30 min, and right at
gelation; for 4-h gel times, samples were removed at 1 h, 2 h, and
right before gelation; for 120-min gel times, samples were removed
at 30 min, 1 h, and right at gelation. Next, a screw-top Erlenmeyer
flask containing just enough 2-propanol or ethanol (the same
solvent used in the sol-gel process) was used to provide a pool
about 1-cm deep at the bottom. Samples were then taped to the top
of the screw-top cap for the flask and screwed into place. This
provided a solvent-saturated atmosphere to allow sol coating on the
fibers to gel without drying out. Once the gel point had been
reached, samples were optionally soaked under 2-propoanol or
ethanol, respectively. This ensured excess water and leftover
reactive materials were removed from the pore network to minimize
syneresis of the gel coating. Finally, the alumina-gel-coated
fibers were optionally hung and allowed to dry in air.
[0110] Alumina-coated fibers were then dip-coated with 0.050 M
Fe(NO.sub.3).sub.3.9H.sub.2O in 2 propanol (Fe.sup.3+/IPA). Some
alumina-coated fibers were dip-coated before the coating was dry,
others were dip-coated after it was dry. After they were dip-coated
with catalyst solution, fibers were hung and allowed to air dry.
Once the tow was dry and ready for further processing, the taped
end was cut off.
[0111] To determine the efficacy of the alumina coating for growing
CNTs, the tow was CVD processed for CNT growth. CVD growth of CNTs
was performed in a fused quartz tube (54-mm outer diameter, 50-mm
inner diameter, 137-cm length) heated by a three-zone split-hinge
tube furnace (Lindberg/Blue M model HT55667C, 30-cm heated zone
lengths). In a typical process, specimens were placed in a fused
quartz process tube at the center of the third zone. The tube was
then flushed with a flow of 2070 sccm He for 10 min to displace
oxygen from the tube. Next, He was turned off and a flow of 1040
sccm H.sub.2 gas (Airgas, ultrahigh purity grade, >99.999%) was
introduced. The sample was then heated to 720.degree.
C.-730.degree. C. under H.sub.2 gas over the course of about 8 min
to reduce iron oxide nanoparticles on the specimen to
catalytically-active iron. The sample remained at these conditions
for an additional 7 min to further reduce remaining iron oxide
nanoparticles. A flow of 316 sccm ethylene (Airgas, ultrahigh
purity grade, >99.999%) was then added for 5 min to facilitate
CNT growth. Lastly, H.sub.2 and C.sub.2H.sub.4 were turned off and
the sample was allowed to cool to room temperature under a flow of
2070 sccm He.
[0112] A high yield of bundles of aligned CNTs was observed over
the surface of the fibers, as illustrated in FIG. 5. Generally,
carbon nanotube growth was observed where alumina was present on
the carbon fibers. While SEM imaging of the alumina coatings showed
that chunks of alumina were clinging to the fibers (believed to be
due to the alumina losing a substantial amount of volume on
drying), it is believed that conformal coatings of alumina could be
achieved by using multiple depositions to build up a contiguous
film (e.g., by employing multiple coatings of sol, followed by
low-temperature annealing). Alternatively, the alumina coating can
be improved by depositing an intermediate polymeric layer between
the alumina and the carbon fiber, as described below in Example
4.
Example 2
[0113] This example describes a sol-gel process for producing
silica coatings on carbon fibers. A rapid gelation process was
developed and evaluated as a means for coating carbon fibers with a
porous gel coating in less than one minute. The resulting silica
coating was then investigated for its ability to adhere to carbon
fibers and promote aligned CNT growth.
[0114] A solution containing 5.17 g Tetramethoxysilane (TMOS,
Sigma-Aldrich product number 218472, 98%) and 3.97 g anhydrous
absolute ethanol (Pharmco-Aaper product number 111000200,
200-proof) and a second solution containing 2.00 g NaOH.sub.(aq)
stock solution (19.1 mg/mL), 2.00 g NaF.sub.(aq) stock solution (1
mg/mL), and 2.36 g ethanol were prepared. The salt solution was
then added to the alkoxide solution and swirled vigorously. Gel
time was about 17 sec. Fibers were coated by quickly dipcoating
within this time period or alternatively by pouring the gelling sol
over a tow placed on aluminum foil.
[0115] Following application of the gel coating, the gel-coated tow
was soaked in ethanol for 1-4 h to remove unreacted materials from
the pore network. The tow was then air-dried, dip-coated with
Fe.sup.3+/IPA, and CVD processed for CNT growth as described in
Example 1.
[0116] FIG. 6 shows SEM images of carbon fibers coated with
sol-gel-derived silica before (left) and after (right) CNT growth.
The inset of the right image in FIG. 6 illustrates the silica/CNT
coverage over the carbon fiber substrate. While cracking in the
silica coating was observed, it is believed that more conformal
coatings could be achieved by repeating the coating process
multiple times. Alternatively, the silica coating can be improved
by depositing an intermediate polymeric layer between the silica
and the carbon fiber, as described below in Example 4.
Example 3
[0117] This example describes the chemical vapor deposition (CVD)
of alumina on carbon fiber substrates using a solid-phase aluminum
triisopropoxide (ATI) precursor in a hot-wall reactor.
[0118] Aluminum triisopropoxide (ATI, .gtoreq.98%, Sigma-Aldrich
product number 220418) served as the alumina precursor and was used
as received. An alumina ceramic crucible (Sigma-Aldrich part number
Z561738) was used to contain the ATI. Between 0.5-1.0 g of new ATI
was placed in the crucible prior to each deposition. CVD of alumina
was performed in a fused quartz tube (54-mm outer diameter, 50-mm
inner diameter, 137-cm length) heated by a three-zone split-hinge
tube furnace (Lindberg/Blue M model HT55667C, 30-cm heated zone
lengths). Positions of objects in the quartz tube are referred
herein by the zone number in which the object was placed (wherein
"Zone 1" is the zone on the side of the reactor in which gases
enter, "Zone 2" is the center zone, and "Zone 3" is the zone on the
side in which effluent exits) followed by the position in the zone
in which it was placed ("Left" for the side of the zone towards gas
entry, "Center" for the center, and "Right" for the side of the
zone towards effluent exit). Samples placed at positions straddling
two zones are denoted by "Zone X-Zone Y Boundary," where X and Y
are the respective zone numbers associated with the boundary.
[0119] The crucible containing ATI was placed in the center of the
first zone ("Zone 1 Center") in order to facilitate control over
the precursor temperature. Helium (Airgas, ultrahigh purity grade,
>99.999%) was used as a carrier gas and was delivered via a
computer-controlled mass flow controller (Unit model 1100A, 10 SLM
capacity). Although many alumina CVD processes are conducted at
reduced pressure, depositions in this work were performed at
atmospheric pressure to improve potential compatibility for
eventual integration with continuous CNT growth on carbon fiber
substrates. Target substrates were unsized carbon fiber tow
(TohoTenax, product number HTA40 N00 12K, formerly 12 KHTA-7CNS01)
cut to 6.5-cm lengths tied into knots at either end, and desized
carbon fiber weave (Tenax-J G40-800 24K EP03) cut to sizes of
either 4 cm.times.18 cm or 4 cm.times.6 cm. Weaves were received
sized and desized by baking in air at 200.degree. C. for 30 min
prior to deposition. Successful desizing was indicated by the
evolution of smoke from the weave during the first 5 min of the
heat treatment. Successful deposition of alumina on tows and weaves
was determined by the presence of a colored thin film observable by
optical microscopy at 50.times. magnification or below and
confirmed by SEM.
[0120] A low precursor set point of 150.degree. C.-230.degree. C.
(slightly above the melting point of ATI) and a substrate set point
of 700.degree. C.-780.degree. C. were used. Additionally, three
substrates (unsized tows) were positioned across the length of the
second zone to assess spatial-dependence of the deposition. Due to
proximity heating from the second zone, the zone containing the
precursor was observed to be much hotter than its set point
(ramping from 370.degree. C. to 495.degree. C. over the course of
the 10-min soak phase of the deposition). Despite this, successful
deposition of alumina was observed over most of the length of the
first tow (positioned at Zone 2 Left) and was visible under a
microscope and to the eye as a glossy rainbow-colored glaze. The
coatings appeared well-adhered and, as observed by SEM, did not
appear to crack or substantially flake off on handling of the tow.
The tows positioned at Zone 2 Center and Zone 2 Right, however, did
not show the presence of an alumina film, revealing the presence of
a limited region (up to about 12 cm downwind of the crucible) in
which deposition can be performed. The absence of white or glossy
deposits on the quartz tube in Zone 2 suggests that formation of
the observed deposit was surface-reaction-limited.
[0121] The successfully-coated specimen and an uncoated control
specimen were then dip-coated with Fe.sup.3+/IPA and CVD processed
for CNT growth. First, a catalyst solution of 0.050 M
Fe(NO.sub.3).sub.3.9H.sub.2O in 2-propanol (IPA) was prepared and
aged with stirring for 1-2 h. Specimens were then dipped into this
solution (Fe.sup.3+/IPA) for about 5 min and then hung to air dry.
Using the same CVD equipment used for alumina deposition described
above, specimens were placed into a dedicated quartz process tube
at Zone 3 Center. The tube was then flushed with a flow of 2070
sccm He for 10 min to displace oxygen from the tube. Next, He was
turned off and a flow of 1040 sccm H.sub.2 gas (Airgas, ultrahigh
purity grade, >99.999%) was introduced. The sample was then
heated to 650.degree. C. under H.sub.2 gas over the course of about
8 min to reduce iron oxide nanoparticles on the specimen to
catalytically-active iron. The sample remained at these conditions
for an additional 7 min to further reduce remaining iron oxide
nanoparticles. A flow of 316 sccm ethylene (Airgas, ultrahigh
purity grade, >99.999%) was then added for 5 min to facilitate
CNT growth. Lastly, H.sub.2 and C.sub.2H.sub.4 were turned off and
the sample was allowed to cool to room temperature under a flow of
2070 sccm He.
[0122] Following CNT growth processing the alumina-coated sample
was noticeably fuzzy and matte black in color, while the control
sample remained smooth and glossy black. SEM imaging of the
CNT-processed samples, shown in FIG. 7, revealed extremely
high-yield growth of aligned CNTs covering the alumina-coated tow
over nearly the entire length of the tow including interior fibers.
The top left, top right, and lower left images in FIG. 7 are SEM
images of aligned CNTs grown on the alumina-coated carbon fibers.
The lower right image in FIG. 7 shows the alumina-wrapped carbon
fibers after the mechanical delamination of the CNTs.
Example 4
[0123] In this example, methods of non-covalently functionalizing
carbon fibers with amphiphilic polymers derived from
poly(styrene-alt-[maleic anhydride]) (PSMA) is described. The use
of h-PSMA as a non-covalent functionalizing polymer and its
potassium salt (K-PSMA) as a polyelectrolyte for ion exchange were
investigated as methods for improving adhesion of sol-gel-derived
alumina coatings and solution-deposited iron catalyst precursors to
carbon fibers. These methods were then investigated for efficacy in
facilitating CNT growth.
[0124] Aqueous solutions with varying concentrations of h-PSMA were
prepared by dissolving 1.4 g, 4.2 g, or 7.0 g (corresponding to
concentrations of 0.5, 1.5, and 2.5 wt %, respectively) of PSMA
(Sigma-Aldrich, 99%, Mw=350,000) in 25 mL of acetone with gentle
heating. The PSMA solution was then added to 300 mL of 0.3 M NaOH
in deionized water with stirring and allowed to react for 3 h,
after which it was acidified with 0.1 M HNO.sub.3 to a pH of 8. The
acetone in the solution was then removed with a rotary
evaporator.
[0125] Unsized (i.e., never-sized), never-surface-treated carbon
fiber tow (TohoTenax product number HTR40 N00 24k 1550tex) was used
for substrates. As noted elsewhere, the equivalent commercial
version of the fibers that were used as substrates is HTA40 F22 24k
1550tex, which is surface-treated and sized. h-PSMA was coated over
the fibers by dip-coating a tow in aqueous h-PSMA solution for
about 5 min and subsequently allowing the tow to dry in air or
blow-drying with cool air (which took about 9 min). Upon removal of
the tow from h-PSMA solution, the tow became noticeably stiff and
hard to peel apart. To improve coating of fibers in the inner tow,
the tow could be dabbed up and down in the h-PSMA solution (as is
done to clean a watercolor paintbrush) over the 5-min period.
[0126] Auger spectroscopy revealed extensive coverage of a
well-adhered conformal h-PSMA coating, disrupted only where fibers
had clung together during dip-coating of the h-PSMA due to
capillary action.
[0127] In this coating process, PSMA was hydrolyzed to produce
carboxylate derivatives. Not wishing to be bound by any particular
theory, it is believed that phenyl groups extending off of the
polymer backbone undergo pi-pi stacking (.pi.-.pi. stacking) with
similarly-structured aromatic rings on the surface of the carbon
fiber, enabling the carbon fiber surface to be coated by the
polymer. Polar carboxylate moieties, also extending off of the
polymer backbone, are believed to be then repelled by the carbon
fiber surface and provide a polar surface functionality over the
fiber. Upon heating under inert atmosphere, it is believed that
this polymer results in a carbon coating over the fiber that can
potentially protect the underlying fiber from chemical interactions
with barrier and catalyst materials.
Example 5
[0128] In this example, h-PSMA-coated fibers were coated with
sol-gel-derived alumina or silica coatings to determine whether the
h-PSMA coating could improve the degree to which carbon fibers are
coated with alumina and/or silica. The efficacy of h-PSMA for
improving adhesion of sol-gel-derived coatings was evaluated using
FE-SEM spatial elemental analysis by Auger spectroscopy.
[0129] As discussed in Example 1, sol-gel deposition of alumina can
enable high-yield, aligned CNT growth, but, if too few coating
steps are employed, can also exhibit poor adhesion and can undergo
substantial volume contraction during both the gel aging phase (up
to 50%) and upon evaporative drying (up to 90% of the aged gel)
thereby leaving substantial areas of bare fiber. FIG. 8 includes
SEM images of (left) initially-uncoated carbon fibers covered with
alumina deposits produced via the techniques described in Example
1, followed by a dip-coating with Fe.sup.3+/IPA and subsequent CNT
growth by CVD and (right) h-PSMA-coated carbon fibers produced
according to the methods in Example 4, subsequently covered with
alumina via the methods described in Example 1. Although some
cracking was observed over the h-PSMA-coated fibers, the coverage
was noticeably improved. It is believed that subsequent coating
steps could produce a conformal coating of alumina over the carbon
fiber substrates.
[0130] The efficacy of h-PSMA for improving adhesion of silica
sol-gel-derived coatings, described in Example 2, was also
evaluated. As described in Example 2, silica coatings were
deposited from an alkoxide and often resulted in a high degree of
mass loss upon evaporative drying. Accordingly, additional
experiments were performed in which h-PSMA was formed over the
carbon fiber substrates and, subsequently, silica coatings were
deposited. Without the h-PSMA coating, unevenly-distributed silica
deposits were present over carbon fibers, as shown in FIG. 9.
Spatial elemental analysis by Auger spectroscopy (shown on the
bottom of FIG. 9) verified that lighter patches on the fiber
surface, present as both sporadic thin films and in thick chunks,
contained Si and O, whereas the surrounding dark regions only
contained C, indicating these dark regions were the uncoated carbon
fiber surface. On carbon fibers pre-coated with h-PSMA and
subsequently coated with silica, however, a well-adhered coating
with somewhat uneven thickness was observed, as shown in FIG. 10.
Auger spectroscopy was performed showing the presence of Si and O
over the fibers both in thicker deposits and over areas that at
first glance seem to be uncoated fiber surfaces (but were actually
just thin coatings). The Auger spectra are illustrated on the right
side of FIG. 10. These results indicated that pre-deposition of
h-PSMA could also improve the adhesion of alkoxide-derived gel
coatings, even those that undergo substantial mass loss upon
evaporative drying due to their high pore volumes.
Example 6
[0131] In this example, the development and optimization of a
polyelectrolyte comprising poly(styrene-alt-[dipotassium maleate])
(K-PSMA) is described, including its utility for enabling
high-yield, CVD growth of CNTs on carbon fiber tows at temperatures
below 500.degree. C.
[0132] A solution of 1.5 wt % h-PSMA was prepared as described in
Example 4. The h-PSMA polyacid was then neutralized by stirring
solid K.sub.2CO.sub.3 (Sigma-Aldrich part number 209619,
.gtoreq.99.0%) into the solution. At first, the K.sub.2CO.sub.3
simply dissolved in the solution, but upon further addition,
evolution of a CO.sub.2 fizz resulted. K.sub.2CO.sub.3 was added
until CO.sub.2 fizz no longer resulted upon further addition. This
point corresponded to a solution pH of about 11 (i.e., about 0.79 g
K.sub.2CO.sub.3/10.00 g 1.5 wt % h-PSMA solution).
[0133] Unsized (i.e., never-sized), never-surface-treated carbon
fiber tow (TohoTenax product number HTR40 N00 24k 1550tex) was used
for substrates. Carbon fiber tows (about 10 cm long) were cut and
taped at one end with masking tape (3M 2600) for ease of handling.
K-PSMA was coated over the fibers by dip-coating a tow in aqueous
K-PSMA solution for about 5 min and subsequently allowing the tow
to dry in air or blow-drying with cool air (which took about 9
min). Upon removal of the tow from K-PSMA solution the tow became
noticeably stiff and hard to peel apart, but less so than when
coating with h-PSMA. To improve coating of fibers in the inner tow,
the tow could be dabbed up and down in the h-PSMA solution (as is
done to clean a watercolor paintbrush) over the 5-min period.
[0134] K-PSMA-coated fibers were then dip-coated with iron catalyst
precursor solutions. 0.050 M Fe(NO.sub.3).sub.3.9H.sub.2O in
2-propanol (Fe.sup.3+/IPA) was used. In subsets of these
formulations, the Fe.sup.3+/IPA solution was aged for 30 min, 60
min, and 90 min prior to dip-coating. Some K-PSMA-coated fibers
were dip-coated before the K-PSMA deposit was dry, others were
dip-coated after it was dry. Alternatively, 0.1 M aqueous
Fe(NO.sub.3).sub.3 was used to dip-coat catalyst precursor. The
results outlined below were made by dipcoating with the
Fe.sup.3+/IPA solution aged for 60 min and dipcoating after the
K-PSMA coating was dry. After dip-coating with catalyst, the
masking tape at the end of the tow was cut off. At this point, the
tow was sufficiently stiff from and held together by its K-PSMA
coating that handling of the fibers was possible without tape.
[0135] The Fe.sup.3+/K-PSMA-coated fibers were then CVD processed
in a hot-wall configuration for CNT growth. Samples were placed in
a dedicated fused quartz process tube (25-mm outer
diameter.times.22-mm inner diameter.times.30-cm length) and heated
in an electric clam-shell tube furnace (Lindberg/Blue M MiniMite).
Samples were positioned at 75% along the length of the heated zone.
First, a flow of 750 sccm Ar (Airgas, ultrahigh purity grade,
>99.999%) was introduced into the reactor for 2 min to displace
oxygen from the process tube. Next, a flow of 400 sccm H.sub.2
(Airgas, ultrahigh purity grade, >99.999%) was added and the Ar
flow lowered to 200 sccm Ar. The samples were then heated to a set
point temperature of 480.degree. C. under H.sub.2/Ar flow to reduce
and coarsen iron ions adsorbed over the coated fiber surfaces to
iron nanoparticles. Once at this temperature, a flow 17 sccm
CO.sub.2 (Airgas, ultrahigh purity grade, >99.999%) and 167 sccm
10% C.sub.2H.sub.2 in Ar (Airgas, ultrahigh purity grade,
>99.999%, acetone-free) were introduced and the H.sub.2 and Ar
deactivated. The samples were soaked under these conditions for 15
min after which a flow of 750 sccm Ar was introduced and the
CO.sub.2 and C.sub.2H.sub.2/Ar mixture were deactivated. The
furnace was then opened and the samples were allowed to cool to
ambient conditions. Between CNT growths, the quartz process tube
was baked in air at 750.degree. C. for about 20 min to remove
deposited organics. In one variation of this process, samples were
not treated with H.sub.2 on ramp-up to the set point
temperature.
[0136] FIG. 11 shows SEM images showing the growth of CNTs on the
K-PSMA coated fibers. The top left image shows a representative
fiber covered with unaligned CNTs. The top right image shows an
overview of fuzzy fibers in a tow. The bottom left image shows the
detail of a single fuzzy fiber. The bottom right image shows
sparser growth that was observed in a few fibers.
[0137] Short (1-3 .mu.m), unaligned CNTs were observed in a
significant quantity (about 6-8 CNTs/.mu.m.sup.2) over the majority
of fibers in the tow (where the fibers successfully engaged the
coating solutions). As a control, an unsized tow without K-PSMA was
coated with iron catalyst precursor by baking on Fe.sup.3+/IPA
solution and processed with the same CVD procedure. No CNTs were
observed on this tow; instead, platelets of iron presenting an
observable grain structure were found delaminating from the fibers.
This example demonstrates that CNT growth on carbon fibers can be
achieved at 480.degree. C. without etching or chemically changing
the carbon fiber surface and using only solution-based methods to
apply coatings and catalyst precursor.
Example 7
[0138] In this example, carbon fibers were heated to relatively
high temperatures in the absence of applied tension, and their
post-heating tensile strengths were measured. The carbon fibers
used in this example were primarily unsized, never-surface-treated
TohoTenax HTR40-24k. Unsized AS4 fibers were also used in a few
studies, as noted. The cross-sectional diameters of the fibers were
generally about 7 micrometers. Unsized fiber was chosen in order to
eliminate possible fiber damage associated with desizing procedures
such as thermally decomposing the sizing under inert atmosphere or
interaction of the fiber with unremoved sizing.
[0139] Prior to use, the carbon fibers were rinsed with acetone and
evaporatively dried in air. Single fibers were carefully extracted
from the 24k tow using Q-Tips.RTM., which gently latched onto the
fibers and pulled them out from the other fibers. Care was taken
not to stretch fibers when pulling them out to avoid pretensioning
of the fiber. It should be noted that the force required to break a
single carbon fiber was about 40-180 mN, or about 0.01-0.04
lb.sub.f, corresponding to a stretch of only about 150-300
.mu.m.
[0140] After pre-processing, thermal process was performed, in
which the carbon fibers were heated to a range of temperatures. In
the thermal processing step, unstrained carbon fibers were heated
to various temperatures in the absence of CVD gases C.sub.2H.sub.4
and H.sub.2. Table 1 includes a summary of single-fiber tensile
test data for carbon fibers thermally processed in He atmosphere as
a function of temperature and time.
TABLE-US-00001 TABLE 1 Summary of single-fiber tensile test data
for carbon fibers thermally processed in He atmosphere as a
function of temperature and time. Weibull Processed No. of .sigma./
S( .sigma.)/ Modulus / S( )/ Sample Description as Samples GPa
(=.beta.) GPa .alpha. GPa GPa Temperature Study, TohoTenax HTR-40
Unsized HTR-40 Tow 19 4.49 0.75 5.96 223 7.88 Unsized HTR-40, Tow
19 4.37 0.83 5.27 213 12.9 480.degree. C. in He, 10 min at set
point Unsized HTR-40, Tow 18 2.50 0.34 7.42 211 11.9 580.degree. C.
in He, 10 min at set point Unsized HTR-40, Tow 20 1.47 0.28 5.30
194 13.1 730.degree. C. in He, 10 min at set point Temperature
Study, AS4 Unsized AS4 Tow 19 3.77 0.46 8.11 237 10.2 Unsized AS4,
Tow 17 3.55 0.66 5.34 239 9.99 480.degree. C. in He, 18 min
.gtoreq. 480.degree. C. Unsized AS4, Tow 20 3.74 0.69 5.40 234 13.3
580.degree. C. in He, 18 min .gtoreq. 480.degree. C. Unsized AS4,
Tow 18 1.09 0.20 5.41 215 10.2 730.degree. C. in He, 18 min
.gtoreq. 480.degree. C. Temperature Study vs. Time Study Unsized
HTR-40, Tow 20 4.49 0.94 4.76 217 7.54 480.degree. C. in He, 18 min
.gtoreq. 480.degree. C. Unsized HTR-40, Tow 20 3.56 0.36 9.97 212
8.96 580.degree. C. in He, 18 min .gtoreq. 480.degree. C. Unsized
HTR-40, Tow 20 3.09 0.52 5.94 205 14.0 730.degree. C. in He, 18 min
.gtoreq. 480.degree. C. Unsized HTR-40, Tow 20 4.20 0.95 4.44 212
12.6 480.degree. C. in He, 36 min .gtoreq. 480.degree. C. Unsized
HTR-40, Tow 19 2.61 0.43 6.11 200 13.3 580.degree. C. in He, 36 min
.gtoreq. 480.degree. C. Unsized HTR-40, Tow 18 1.94 0.63 3.07 192
12.2 730.degree. C. in He, 36 min .gtoreq. 480.degree. C.
[0141] After thermal processing was performed, single-fiber tensile
tests were performed in a universal testing machine (Nano-UTM, MTS
Nano Instruments) according to the ASTM standard test D3379-75
("Standard Test Method for Tensile Strength and Young's Modulus for
High-Modulus Single-Filament Materials," 1989), which is
incorporated herein by reference in its entirety for all purposes.
A baseline data set for as-received fibers was established each day
that tensile tests were run to normalize variations arising from
machine alignment and variations in materials properties along the
tow spool. As an extra validation step, conclusions regarding a
sample type were drawn based on differentials run against their
respective baseline dataset.
[0142] To prepare the fiber samples for testing, testing tabs were
laser-cut from tagboard to dimensions specified by ASTM standard
test D3379-75. Fibers were mounted on tagboard frames with epoxy. A
gage length of 25 mm was used for all samples. First, a straight
line was drawn down the center of the tagboard testing frame. A
length of carbon fiber tow was cut and laid out on a smooth surface
such as clean white copy paper. With gloved fingers, the ends of
the tow were spread slightly apart. A fiber grabber tool was used
to gently stick onto single carbon fibers in the bundle and pull
them away from the tow as described above. The fiber grabber was
used to draw a fiber over the line drawn on the testing frame. One
end of the fiber was then taped down in this position on the
testing frame. A second fiber grabber was then used to pull the
fiber taught at other end. This end of the fiber was then taped
down. Finally, each end of the fiber was glued in placed with dabs
of 5-min epoxy (Devcon, product number 14250) applied by Q-Tip.
[0143] Tensile testing was performed as follows. Fiber specimens
were first mounted in the testing machine. Proper vertical
alignment of the tagboard frame was important to ensure accurate
and repeatable tensile measurements. Tabs were gripped on the
tagboard 25 mm away from the fiber-epoxy joints. Once mounted in
the machine, the sides of the tagboard frame were separated into
top and bottom halves by snipping twice on each side as to remove a
section of tagboard--this ensured the top and bottom halves did not
catch on each other during pulling and introduce a spike or offset
in the force-displacement curve measured for the fiber from
incorrect autotaring by the instrument software. Fibers were pulled
at a strain rate of 400 .mu.strain/s which ensured no test took
longer than 60 s to complete. A nominal fiber diameter of 7 .mu.m
was used for stress calculations, which was verified by SEM to be a
consistent and reasonable value. Fibers typically broke at a load
of about 40-180 mN and an extension of about 150-300 .mu.m. While
ideally fiber pulls would only be considered if the test resulted
in a break in the middle of the test specimen, the extremely small
diameter of the fibers and large amount of elastic energy stored at
break almost always resulted in specimens springing off of the
frame, making it exceedingly difficult to assess where fiber
failure occurred. All fiber break test results are included in the
presented data as a result and it is expected that some of the
lower values of strength in particular are a result of fiber
breakage near the fiber-tab interface. Elastic modulus was
calculated by linear regression fit of all data points in the
linear region of the stress-strain curve for a given test such that
an R.sup.2 value of 0.99 or better was obtained (data points from
movement of the testing frame and non-linear behavior at the fiber
break point were excluded). The elastic modulus is represented as E
while S( E) represents the standard deviation of the elastic
modulus. In some datasets, force-displacement curves exhibited a
constant offset due to mistaring of the load cell (arising from
momentary collision of the tagboard frame top and bottom);
accordingly, this offset was subtracted from these datasets in
postprocessing.
[0144] Carbon fiber strength is inherently limited by flaws in the
fibers and is characterized according to ASTM standard test
D3379-75 using a Weibull distribution. Weibull distributions are
described, for example, in Harlow, E. G. and Phoenix, L., Journal
of Composite Materials, 1978, 12, pp. 195-214, which is
incorporated herein by reference in its entirety for all purposes.
The Weibull distribution represents the probability of failure for
a fiber at a particular tensile load (i.e., its tensile strength).
The probability of failure is given by:
p ( x ) = .alpha. .beta. ( x .beta. ) .alpha. - 1 - ( x .beta. )
.alpha. x .gtoreq. 0 0 x < 0 [ 2 ] ##EQU00002##
where x is the parameter of interest (here tensile strength),
.beta. is the location parameter, and .alpha. is a scale factor (or
"Weibull modulus"). It is assumed that fibers have random flaws
distributed along them which result in fiber failure according to a
weakest-link-in-chain model, and that these flaws follow
Weibull-Poisson statistics. Accordingly, the location parameter can
be approximated by the mean tensile strength:
.beta..apprxeq. x= .sigma. [3]
and the Weibull modulus, .alpha., can be approximated as the ratio
of the mean tensile strength to standard deviation in tensile
strength, S( .sigma.)
.alpha. .apprxeq. x _ S = .sigma. _ S ( .sigma. _ ) . [ 4 ]
##EQU00003##
[0145] These approximations were employed in the data reported in
this example. According to ASTM standard test D3379-75, a minimum
of 15 successful tests were required for calculation of a valid
Weibull distribution. As such, each condition surveyed involved
preparation and testing of at least 15 samples with the exception
of a few preliminary diagnostic tests. It is noted that the Weibull
distributions presented here may be wider than the true
distribution as they include all measurements regardless of fiber
break point. Testing did not allow for identification of break
location and it is anticipated that some fibers broke away from the
center of the testing frame (e.g., at the fiber-epoxy interface).
The asymmetric bias of the distributions towards lower tensile
strength values with increasing number of samples tested supports
this hypothesis, as more breaks at the mounting points would be
included in the dataset. Weibull parameters derived from tensile
tests of single alumina fibers used for CNT growth derived
exclusively considering fibers with breaks in the middle of test
specimens result in a comparatively narrower Weibull
distribution.
[0146] Table 1 (above) includes a summary of the various thermal
processing experiments that were performed, and the resulting
tensile strength of the fibers. In one set of experiments, unsized,
uncoated HTR-40 fibers were heated to 480.degree. C., 580.degree.
C., and 730.degree. C. such that the time the fibers were at or
above 480.degree. C. was equal to 18 min. FIG. 12 shows Weibull
distributions calculated from tensile tests from samples processed
under these conditions. No loss in tensile strength was observed
for fibers thermally processed in He at 480.degree. C. However,
substantial strength loss was observed in samples heated at
580.degree. C., and additional strength loss over this was observed
in samples heated at 730.degree. C.
[0147] Extending the length of the heat treatment had a further
detrimental effect at 580.degree. C. and 730.degree. C., but not
noticeably at 480.degree. C. This result suggested that the level
of damage was not a function of temperature alone. Both strength
(FIG. 13) and stiffness (FIG. 14) were further compromised with
longer treatment time at these temperatures. Based on these
results, it appeared that a thermally-activated process was
responsible for the observed degradation in tensile properties, and
primarily occurred above 480.degree. C.
[0148] To better characterize the nature of the hypothesized
thermally-activated process, thermogravimetric analysis (TGA) was
performed with HTR-40 fibers in He atmosphere (3 mg chopped fiber
per run). TGA of fibers at a constant heating rate of 100.degree.
C. min.sup.-1 (comparable to what is encountered during CNT growth
in the CVD furnace) revealed a sudden change in mass-loss rate
beginning at 550.degree. C. Similarly, TGA over the course of 60
min at constant temperatures of 400.degree. C., 500.degree. C., and
650.degree. C. revealed that time-dependent mass loss only occurred
at 650.degree. C.
[0149] To verify the observed results were not peculiar to the
specific TohoTenax carbon fibers that were used, single-fiber
tension tests were performed with heat-treated AS4 fibers as well.
Breaking strength and tensile stiffness for unsized AS4 as received
and after heat treatment in He at 480.degree. C., 580.degree. C.,
and 730.degree. C. showed similar temperature-dependent declines in
these properties, although less severe below 600.degree. C. than
HTR-40. Thus the observed responses of the HTR-40 carbon fibers
were not isolated to this specific product, but rather, were
generally relevant to poly(acrylonitrile)-derived carbon fibers
(i.e., PAN-derived carbon fibers) such as HTR-40 and AS4.
[0150] Drawing on these results, it was further hypothesized that
this mass loss may be correlated with a loss of HCN, the major
by-product generated in the pyrolysis of
poly(acrylonitrile)-derived carbon fibers such as HTR-40 and AS4. A
surface analysis using Auger spectroscopy was performed to detect
changes in the nitrogen-to-carbon ratio present on the carbon fiber
surface. It was theorized that, if HCN was being volatilized, N
must be present in the carbon fiber, and if N was coming off, a
decrease in the concentration of N should be measurable. The C:N
ratio of unsized HTR-40 fibers as received and after thermal
processing in He at 480.degree. C., 580.degree. C., and 730.degree.
C. was measured by Auger spectroscopy. A measurable,
temperature-dependent loss of N was clearly observed. Notably, the
largest drop in N content occurred between the as-received fibers
and the fibers heat-treated at 480.degree. C., even though strength
and (generally) stiffness were preserved at this temperature.
[0151] To gain insights about microstructural changes throughout
the bulk of the fiber (the interior core), X-ray diffraction (XRD)
was performed on unsized HTR-40 fibers before and after thermal
processing in He at 730.degree. C. The as received fibers displayed
a superposition of two phases of graphite--one with a very small
domain size (tens of nm, the broad peaks at 44.degree. 2-.theta.
and 53.degree. 2-.theta.) and one with larger domain sizes
(hundreds of nm, the sharp peaks at 44.degree. 2-.theta. and
53.degree. 2-.theta. and possibly an accentuated feature at
25.7.degree. 2-.theta.). The disappearance of the phase associated
with the larger graphite domain sizes upon heat treatment in He at
730.degree. C., in combination with the chemical changes on the
carbon fiber surface observed by Auger spectroscopy, suggested that
a microstructural rearrangement within the carbon fiber occurred
beginning at 550.degree. C., wherein the minor strength-bearing
phase (the highly-oriented surface shell of the fiber)
restructured, leaving a weaker phase (the less-oriented interior
core of the fiber) as the primary load-bearing continuity.
[0152] Not wishing to be bound by any particular theory, it is
believed that there exists inherent mechanochemical coupling of
fiber strength with chemical aspects of the fiber's
microstructure.
Example 8
[0153] This example describes experiments in which tension was
applied to carbon fibers while the fibers are heated to relatively
high temperatures, and the resulting effect on the tensile strength
of the fibers. The carbon fibers used in this example were the same
as those described in Example 7. In addition, the carbon fibers
tested in this example underwent the same pre-processing steps as
those described in Example 7.
[0154] The role of tension during thermochemical processing of
carbon fibers was evaluated using a tensioning frame and
screw-clamp graphite weights. To prevent chemical contamination,
the frame and all fastening parts were machined exclusively out of
polycrystalline graphite. The clamp-on weights were used to impart
up to 0.5 GPa of tension into single carbon fibers. A set of
tungsten-core, graphite-shell weights was also produced and enabled
application of tension up to 3.2 GPa.
[0155] To mount the fibers onto the tensioning frame, a length of
carbon fiber tow (about 30 cm long) was cut and laid out on a clean
sheet of copy paper. A "fiber grabber" tool was fashioned to
extract fibers from the tow. The fiber grabber tool was made by
rolling a piece of masking tape (3M 2600) onto the end of a thin
wooden dowel (e.g., a toothpick) to make an approximately
0.5.times.1.0 cm "flag" of tape hanging off the end of the dowel
with the sticky side exposed. This tool was used to gently stick
onto single carbon fibers in the bundle and pull them away from the
tow through gentle application of torque rather than tension. This
was done to minimize stretching or "pretensioning" of fibers during
extraction from the tow. Next, with fiber attached to the fiber
grabber, the fiber was drawn out and laid across two
Parafilm-covered fiberglass blocks (about 5 cm.times.2.5
cm.times.4.3 mm) separated by a distance of about 18-20 cm, to
which the fiber could cling electrostatically. White paper could
optionally be placed between the translucent Parafilm and off-white
fiberglass to improve contrast of fibers placed atop the blocks.
Another, thinner fiberglass block (about 12 cm.times.5 cm.times.2.9
mm) without Parafilm was placed nearby for the next step. With a
gloved finger, the end of the fiber attached to the fiber grabber
tool was pressed down onto its Parafilm-covered fiberglass block
and the fiber grabber twisted away and detached. With fiber strung
between, the two Parafilm-covered fiberglass blocks were then
picked up and carefully placed down and over the larger, thinner
fiberglass plate such that the larger plate sat between the two
Parafilm-covered blocks lengthwise (12-cm length) with about 3 cm
of space between it and each Parafilm-covered block. Once set down,
the Parafilm-covered blocks were gently pulled apart to tension the
fiber, noting that the fiber was capable of slipping on the
Parafilm which self-corrects for and reduces concerns for
overtensioning.
[0156] Next, graphite clamp weights (or heavier
tungsten-core/graphite-shell clamp weights) were readied for
attachment to the fibers. The clamp weights were assemblies
comprising two small blocks with dimensions of 9.5 mm.times.9.5
mm.times.17.8 mm joined at one end with a graphite screw to form a
sandwich structure. The screw could be tightened or loosened to
open a gap between the two blocks, which sit parallel to each other
and perpendicular to the screw thread. Two such weight assemblies,
tightened such that an approximately 1 mm gap was left between the
two weight blocks, were placed on either side of the long
fiberglass plate and pushed against the plate with gap running
parallel to and under the fiber, in preparation for the fiber to be
laid down into them. The two Parafilm-coated blocks were then
carefully picked up and the taught fiber laid into the gaps of the
two clamp weight assemblies. Once in place, the screws on the
weight assemblies were screwed finger-tight resulting in clamping
of the weights onto the fiber.
[0157] Once the clamp weights were attached, the fibers were loaded
onto the tension frame. The tension frame was placed on top of an
adjustable-height lab jack to which a piece of clean white copy
paper had been taped. The lab jack was then placed onto a cardboard
tray which served as a semi-rigid carrying structure that helped to
dampen vibration from walking when moving the fiber-loaded frame
(as described below). Two fiberglass plate "guide rails" were also
taped down onto the paper to restrict sliding of the tension frame
to one dimension. A 6.3-mm-thick prop block was then placed to the
left of the tension frame. Pinching both of the clamp weights to
pick up the fiber, the fiber-weight assembly was moved over the
tension frame. Next, one clamp weight was propped up vertically
(screw-side-up) on the prop block on the left side of the tension
frame while the other weight was placed on its long edge atop the
right side of the tension frame in such a way as to not twist the
fiber. Finally, the right weight was carefully pulled and rotated
over the right edge of the tension frame and then gently released.
The prop block on the left was then carefully removed and the two
weights were left dangling off either edge of the frame, holding
the fiber in tension over the frame. The fiber loading process was
repeated up to three more times, loading the frame with up to four
fibers total. Once loaded onto the frames, neither the fiber nor
the weights were touched until after processing.
[0158] Determining the maximum level of tension that could be
reliably applied to single carbon fibers for thermal processing was
not straightforward. Carbon fibers of a given length exhibit
polydispersity in their tensile strengths. Accordingly, the maximum
applicable tension for a given length of fiber varies from specimen
to specimen and is not knowable in advance of tensile testing. The
maximum applicable tension is, at a minimum, less than the fiber's
breaking strength by some increment, and in practice, may be even
less due to limitations arising from gripping, dynamic loading, and
variations in material quality.
[0159] Assuming T.sub.app represents the tension applied to a
single fiber, one can set the maximum tension that can be
practicably applied to a fiber, T.sub.max, equal to the mean
breaking strength measured for fibers of the same type as
determined by single-fiber tensile tests. The tension applied to a
fiber can then be expressed as a proportion of the approximate
maximum applicable tension with the dimensionless ratio
R.sub.T=T.sub.app/T.sub.max. In this example, single fiber loadings
of R.sub.T=0.12, R.sub.T=0.45, and R.sub.T=0.75 were investigated.
At tensions of R.sub.T=0.75, less than 1 in 10 fibers survived
thermal processing. Accordingly, this level of tension was not
studied in depth.
[0160] Once the fibers had been loaded onto the tension frame, and
the tensile force was applied, the fibers were then transferred to
the tube furnace for thermal processing. To accomplish this, the
cardboard tray under the lab jack was carefully and slowly picked
up and the lab jack (with loaded tensioning frame on top) was moved
and placed in proximity to the tube furnace using extreme care so
as to not impart dynamic loading onto the fibers. With a process
tube inside the furnace, the lab jack was placed next to the
furnace and raised to the height of the process tube so that the
tension frame could be smoothly pushed into position with a push
rod.
[0161] Thermal processing was then performed by heating the fiber
samples. Table 2 includes a summary of the tests that were
performed.
TABLE-US-00002 TABLE 2 Summary of single-fiber tensile test data
for carbon fibers thermally processed in He atmosphere under
tension as single fibers and control samples processed untensioned
as tows. Weibull Processed No. of .sigma./ S( .sigma.)/ Modulus /
S( )/ Sample Description as Samples GPa (=.beta.) GPa .alpha. GPa
GPa Low Tension Study Unsized HTR-40, Tow on 20 3.91 1.07 3.66 214
17.6 untensioned, 730.degree. C. in He, Tension 18 min .gtoreq.
480.degree. C. Frame Unsized HTR-40, Single 21 4.46 0.72 6.23 215
10.7 tensioned (R.sub.T = 0.12), Fibers on 730.degree. C. in He, 18
min .gtoreq. 480.degree. C. Tension Frame Moderate Tension Study
Unsized HTR-40 Tow 19 3.46 0.79 4.40 212 12.5 Unsized HTR-40,
Single 24 3.10 0.71 4.36 202 11.6 tensioned (R.sub.T = 0.45 Fibers
on 730.degree. C. in He, 18 min .gtoreq. 480.degree. C. Tension
Frame
[0162] Following thermal processing, the frame was pulled out of
the furnace by one of its horns and placed back onto the lab jack
and the fibers were removed by grabbing both weights of each fiber
simultaneously, slacking the fiber, and resting it on a surface
with the weights. Some fibers were found to break spontaneously
during thermal processing or from bumping or vibration during
loading, observable by the presence of a dropped weight next to the
frame. Such fibers were no longer considered valid for tensile
tests. Each fiber on the frame could be cut in half to afford two
tensile test specimens.
[0163] Tensile testing was performed according to ASTM standard
test D3379-75, as described in Example 7. Test results are
summarized in Table 2. FIG. 15 shows Weibull distributions
calculated from tension tests performed on unsized HTR-40 fibers as
received and after heat treatment at 730.degree. C. (at or above
480.degree. C. for 18 min) with R.sub.T=0.12 and R.sub.T=0.45. The
application of a low level of tension (R.sub.T=0.12) was found not
only to preserve carbon fiber tensile stiffness and strength upon
thermal processing at 730.degree. C. but possibly also to enhance
tensile strength. Interestingly, the higher level of tension
(R.sub.T=0.45) resulted in the loss of about 10% of the tensile
strength (although this was still a 20-25% improvement in strength
retention compared with untensioned thermal processing of fibers.
These experiments demonstrated that the application of a low level
of tension is a viable strategy for preserving fiber strength and
stiffness during CNT growth on carbon fibers.
Example 8
[0164] This example describes the mechanical testing of
K-PMSA-coated carbon fibers on which carbon nanotubes have been
grown. CO.sub.2/C.sub.2H.sub.2 chemical vapor deposition was used
to grow carbon nanotubes on HTR-40 carbon fibers coated with
Fe.sup.3+-loaded K-PSMA at 480.degree. C., as described in Example
6. The tested carbon fibers were coated with K-PSMA (from h-PSMA
initially acidified with NaOH to a pH of 8 then neutralized with
K.sub.2CO.sub.3 to a pH of 11) and dip-coated with 0.050 M
Fe.sup.3+/IPA solution aged for 60 min. The fibers were then
processed in a 1'' CVD furnace first under 400 sccm hydrogen on
ramp-up to 480.degree. C. to reduce the Fe.sup.3+ to iron
nanoparticles, and then 17 sccm CO.sub.2/167 sccm 10%
C.sub.2H.sub.2 in Ar for 15 min to grow CNTs by oxidative
dehydrogenation.
[0165] Tension tests were performed using the devices and methods
described in Example 7. FIG. 16 includes Weibull distributions
calculated from tensile tests of carbon fibers coated with
Fe.sup.3+-loaded K-PSMA coatings and subsequently CVD processed.
Table 3 summarizes tension tests performed on unsized HTR-40 and
CVD-processed Fe.sup.3+/K-PSMA-coated HTR-40 fibers. As illustrated
in Table 3, the carbon fibers unexpectedly exhibited no substantial
changes in breaking strength or tensile modulus after carbon
nanotube growth. This example demonstrates that CNT growth can be
achieved on high-performance carbon fibers without resulting in
degradation of fiber tensile properties.
TABLE-US-00003 TABLE 3 Summary of single-fiber tensile test data
for carbon fibers coated with Fe.sup.3+-loaded K-PSMA CVD processed
for CNT growth with CO.sub.2 /C.sub.2H.sub.2, and as-received
control samples. Weibull Processed No. of .sigma./ S( .sigma.)/
Modulus / S( )/ Sample Description as Samples GPa (=.beta.) GPa
.alpha. GPa GPa Unsized HTR-40* Tow 19 3.46 0.79 4.40 212 12.5
K-PSMA/Fe.sup.3+ on Tow 20 3.88 0.85 4.55 213 13.2 HTR-40,
CO.sub.2/C.sub.2H.sub.2 CVD at 480.degree. C. *Repeat of listing
from Table 2; served as control for moderate tension study and CNT
growth study.
[0166] While several embodiments of the present invention have been
described and illustrated herein, those of ordinary skill in the
art will readily envision a variety of other means and/or
structures for performing the functions and/or obtaining the
results and/or one or more of the advantages described herein, and
each of such variations and/or modifications is deemed to be within
the scope of the present invention. More generally, those skilled
in the art will readily appreciate that all parameters, dimensions,
materials, and configurations described herein are meant to be
exemplary and that the actual parameters, dimensions, materials,
and/or configurations will depend upon the specific application or
applications for which the teachings of the present invention
is/are used. Those skilled in the art will recognize, or be able to
ascertain using no more than routine experimentation, many
equivalents to the specific embodiments of the invention described
herein. It is, therefore, to be understood that the foregoing
embodiments are presented by way of example only and that, within
the scope of the appended claims and equivalents thereto, the
invention may be practiced otherwise than as specifically described
and claimed. The present invention is directed to each individual
feature, system, article, material, and/or method described herein.
In addition, any combination of two or more such features, systems,
articles, materials, and/or methods, if such features, systems,
articles, materials, and/or methods are not mutually inconsistent,
is included within the scope of the present invention.
[0167] The indefinite articles "a" and "an," as used herein in the
specification and in the claims, unless clearly indicated to the
contrary, should be understood to mean "at least one."
[0168] The phrase "and/or," as used herein in the specification and
in the claims, should be understood to mean "either or both" of the
elements so conjoined, i.e., elements that are conjunctively
present in some cases and disjunctively present in other cases.
Other elements may optionally be present other than the elements
specifically identified by the "and/or" clause, whether related or
unrelated to those elements specifically identified unless clearly
indicated to the contrary. Thus, as a non-limiting example, a
reference to "A and/or B," when used in conjunction with open-ended
language such as "comprising" can refer, in one embodiment, to A
without B (optionally including elements other than B); in another
embodiment, to B without A (optionally including elements other
than A); in yet another embodiment, to both A and B (optionally
including other elements); etc.
[0169] As used herein in the specification and in the claims, "or"
should be understood to have the same meaning as "and/or" as
defined above. For example, when separating items in a list, "or"
or "and/or" shall be interpreted as being inclusive, i.e., the
inclusion of at least one, but also including more than one, of a
number or list of elements, and, optionally, additional unlisted
items. Only terms clearly indicated to the contrary, such as "only
one of" or "exactly one of," or, when used in the claims,
"consisting of," will refer to the inclusion of exactly one element
of a number or list of elements. In general, the term "or" as used
herein shall only be interpreted as indicating exclusive
alternatives (i.e. "one or the other but not both") when preceded
by terms of exclusivity, such as "either," "one of," "only one of,"
or "exactly one of." "Consisting essentially of," when used in the
claims, shall have its ordinary meaning as used in the field of
patent law.
[0170] As used herein in the specification and in the claims, the
phrase "at least one," in reference to a list of one or more
elements, should be understood to mean at least one element
selected from any one or more of the elements in the list of
elements, but not necessarily including at least one of each and
every element specifically listed within the list of elements and
not excluding any combinations of elements in the list of elements.
This definition also allows that elements may optionally be present
other than the elements specifically identified within the list of
elements to which the phrase "at least one" refers, whether related
or unrelated to those elements specifically identified. Thus, as a
non-limiting example, "at least one of A and B" (or, equivalently,
"at least one of A or B," or, equivalently "at least one of A
and/or B") can refer, in one embodiment, to at least one,
optionally including more than one, A, with no B present (and
optionally including elements other than B); in another embodiment,
to at least one, optionally including more than one, B, with no A
present (and optionally including elements other than A); in yet
another embodiment, to at least one, optionally including more than
one, A, and at least one, optionally including more than one, B
(and optionally including other elements); etc.
[0171] In the claims, as well as in the specification above, all
transitional phrases such as "comprising," "including," "carrying,"
"having," "containing," "involving," "holding," and the like are to
be understood to be open-ended, i.e., to mean including but not
limited to. Only the transitional phrases "consisting of" and
"consisting essentially of" shall be closed or semi-closed
transitional phrases, respectively, as set forth in the United
States Patent Office Manual of Patent Examining Procedures, Section
2111.03.
* * * * *