U.S. patent application number 11/415734 was filed with the patent office on 2007-05-24 for arrays of long carbon nanotubes for fiber spinning.
This patent application is currently assigned to The Regents of the University of California. Invention is credited to Paul Arendt, Raymond F. DePaula, James R. Groves, Qingwen Li, Yuntian T. Zhu.
Application Number | 20070116631 11/415734 |
Document ID | / |
Family ID | 38923724 |
Filed Date | 2007-05-24 |
United States Patent
Application |
20070116631 |
Kind Code |
A1 |
Li; Qingwen ; et
al. |
May 24, 2007 |
Arrays of long carbon nanotubes for fiber spinning
Abstract
An array of long carbon nanotubes (i.e. an array where the
average length of the nanotubes is greater than 0.5 millimeters) is
prepared by exposing a supported catalyst at elevated temperature
to a gas mixture of hydrocarbon, inert gas, and a relatively low
percentage of hydrogen. Addition of water vapor to the gas mixture
may result in an increase in the length of the nanotubes, an
increase the rate of growth, and a decrease in contamination of the
array by amorphous carbon. The temperature and growth time are also
chosen to minimize the amount of amorphous carbon that forms on the
array. Fibers spun from the array have a higher tensile strength
compared to known CNT fibers.
Inventors: |
Li; Qingwen; (Los Alamos,
NM) ; Zhu; Yuntian T.; (Los Alamos, NM) ;
Arendt; Paul; (Los Alamos, NM) ; DePaula; Raymond
F.; (Santa Fe, NM) ; Groves; James R.;
(Cupertino, CA) |
Correspondence
Address: |
LOS ALAMOS NATIONAL SECURITY, LLC
LOS ALAMOS NATIONAL LABORATORY
PPO. BOX 1663, LC/IP, MS A187
LOS ALAMOS
NM
87545
US
|
Assignee: |
The Regents of the University of
California
|
Family ID: |
38923724 |
Appl. No.: |
11/415734 |
Filed: |
May 1, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11051007 |
Feb 4, 2005 |
|
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11415734 |
May 1, 2006 |
|
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60620088 |
Oct 18, 2004 |
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Current U.S.
Class: |
423/447.3 ;
502/263; 977/843 |
Current CPC
Class: |
B82Y 30/00 20130101;
D01F 9/1273 20130101; C01B 2202/36 20130101; D01F 9/1277 20130101;
C01B 2202/08 20130101; C01B 2202/04 20130101; C01B 2202/34
20130101; D01F 9/127 20130101; B82Y 40/00 20130101; D01F 9/1271
20130101; C01B 32/162 20170801 |
Class at
Publication: |
423/447.3 ;
502/263; 977/843 |
International
Class: |
D01F 9/12 20060101
D01F009/12 |
Claims
1. A method for preparing an array of long nanotubes, comprising:
exposing a catalyst structure to a gaseous mixture for a chosen
amount of time within a chosen temperature range, wherein the
gaseous mixture comprises hydrocarbon, inert gas, and hydrogen,
wherein the percentage of hydrogen in the gaseous mixture is P, and
wherein P<20%, whereby an array of substantially aligned carbon
nanotubes forms on the catalyst, the carbon nanotubes of the array
having an average length of greater than about 0.5 millimeter.
2. The method of claim 1, wherein the catalyst structure comprises
a silicon substrate, a layer of silicon dioxide on the silicon
substrate, a layer of aluminum oxide deposited by ion beam assisted
deposition on the silicon dioxide layer, and a layer of metal on
the aluminum oxide layer.
3. The method of claim 1, wherein the layer of aluminum oxide is
amorphous.
4. The method of claim 1, wherein the layer of metal comprises
iron.
5. The method of claim 1, wherein the hydrocarbon in the gaseous
mixture is present in an amount in the range of from about 20
percent to about 80 percent.
6. The method of claim 1, wherein the hydrocarbon comprises
ethylene, acetylene, hexane, acetone, or mixtures thereof.
7. The method of claim 1, wherein the inert gas is present in an
amount in the range of from about 20 percent to about 80
percent.
8. The method of claim 1, wherein the inert gas comprises
argon.
9. The method of claim 1, wherein P.ltoreq.10%.
10. The method of claim 1, wherein P.ltoreq.6%.
11. The method of claim 1, wherein P.ltoreq.5%.
12. The method of claim 1, wherein P.ltoreq.4%.
13. The method of claim 1, wherein P.ltoreq.3%.
14. The method of claim 1, wherein the gaseous mixture further
comprises water vapor.
15. The method of claim 1, wherein the gaseous mixture is a flowing
gaseous mixture.
16. The method of claim 1, wherein the chosen amount of time and
the chosen temperature are selected to minimize the formation of
amorphous carbon on the array of substantially aligned carbon
nanotubes.
17. The method of claim 1, wherein the chosen amount of time is in
the range of from about 5 minutes to about 2 hours.
18. The method of claim 1, wherein the chosen amount of time is in
the range of from about 10 minutes to about one hour.
19. The method of claim 1, wherein the chosen amount of time is in
the range of from about 15 minutes to about 30 minutes.
20. The method of claim 1, wherein the chosen temperature range is
from about 700 degrees Celsius to about 800 degrees Celsius.
21. The method of claim 1, wherein the chosen temperature range is
from about 730 degrees Celsius to about 780 degrees Celsius.
22. The method of claim 1, wherein the temperature is about 750
degrees Celsius.
23. The method of claim 1, wherein the array of substantially
aligned carbon nanotubes comprises a length of from about 20 .mu.m
to about 4.5 mm.
24. An array of carbon nanotubes prepared by a method comprising
exposing a catalyst structure to a gaseous mixture for a chosen
amount of time within a chosen temperature range, wherein the
gaseous mixture comprises hydrocarbon, inert gas, and hydrogen,
wherein the percentage of hydrogen in the gaseous mixture is P,
wherein P<20%, whereby an array of substantially aligned carbon
nanotubes forms on the catalyst, the carbon nanotubes of the array
having an average length of greater than about 0.5 millimeters.
25. A densely packed array of substantially straight and aligned
carbon nanotubes comprising an average nanotube length of at least
2.5 millimeters.
26. The array of claim 25, wherein the average nanotube length is
at least 4 millimeters.
27. The array of claim 25, wherein the average nanotube length is
at least 4.5 millimeters.
28. A method for preparing a fiber, comprising: exposing a catalyst
structure to a gaseous mixture for a chosen amount of time at a
temperature in a chosen temperature range, wherein the gaseous
mixture comprises hydrocarbon, inert gas, and hydrogen, wherein the
percentage of hydrogen in the gaseous mixture is P, wherein
P<20%, whereby an array of substantially aligned carbon
nanotubes forms on the catalyst, the carbon nanotubes of the array
having an average length of greater than about 0.5 millimeters; and
spinning a fiber from the array.
29. A fiber prepared by a method comprising: exposing a catalyst
structure to a gaseous mixture for a chosen amount of time at a
temperature in a chosen temperature range, wherein the gaseous
mixture comprises hydrocarbon, inert gas, and hydrogen, wherein the
percentage of hydrogen in the gaseous mixture is P, wherein
P<20%, whereby an array of substantially aligned carbon
nanotubes forms on the catalyst, the carbon nanotubes of the array
having an average length of greater than about 0.5 millimeters; and
spinning a fiber from the array.
30. The fiber of claim 29, wherein said fiber has a tensile
strength S, wherein S.gtoreq.1 GPa.
31. A spun fiber of carbon nanotubes having a tensile strength T,
wherein S.gtoreq.1 GPa.
32. A ribbon prepared by a method comprising: exposing a catalyst
structure to a gaseous mixture for a chosen amount of time within a
chosen temperature range, wherein the gaseous mixture comprises
hydrocarbon, inert gas, and hydrogen, wherein the percentage of
hydrogen in the gaseous mixture is P, wherein P<20%, whereby an
array of substantially aligned carbon nanotubes forms on the
catalyst, the carbon nanotubes of the array having an average
length of greater than about 0.5 millimeters; and pulling a ribbon
from the array.
33. A composite structure, comprising: a silicon substrate; a layer
of silicon dioxide on the silicon substrate; a layer of aluminum
oxide having a thickness of from about 2 nanometers to about 20
nanometers deposited by ion beam assisted deposition on said layer
of silicon dioxide; and a layer of iron having a thickness of from
about 0.1 nanometers to about 5 nanometers on said layer of
aluminum oxide.
34. The structure of claim 33, wherein the aluminum oxide layer is
at least partially amorphous.
35. The structure of claim 33, wherein the aluminum oxide layer is
completely amorphous.
36. The structure of claim 33, wherein the aluminum oxide layer is
comprised of fine grains.
Description
RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 11/051,007 filed Feb. 4, 2005, and also claims
the benefit of U. S. Provisional Application Ser. No. 60/620,088
filed Oct. 18, 2004, both incorporated by reference herein.
STATEMENT REGARDING FEDERAL RIGHTS
[0002] This invention was made with government support under
Contract No. W-7405-ENG-36 awarded by the U.S. Department of
Energy. The government has certain rights in the invention.
FIELD OF THE INVENTION
[0003] The present invention relates generally to carbon nanotubes,
and more particularly to the preparation of a supported array of
long carbon nanotubes, to a catalyst for preparing the array, and
to fibers spun from the array.
BACKGROUND OF THE INVENTION
[0004] Individual carbon nanotubes (CNTs) are at least one order of
magnitude stronger than any other known material. CNTs with perfect
atomic structures have a theoretical strength of about 300 GPa [1].
In practice carbon nanotubes do not have perfect structures.
However, CNTs that have been prepared have a measured strength of
up to about 150 GPa, and the strength may improve upon annealing.
For comparison, Kevlar fibers currently used in bullet-proof vests
have a strength of only about 3 GPa, and carbon fibers used for
making space shuttles and other aerospace structures have strengths
of only about 2-5 GPa [2].
[0005] Presently, CNT fibers can be drawn from CNT-polymer
solutions or directly from CNT arrays. However, due to practical
difficulties in dispersing, assembling and aligning carbon
nanotubes using a CNT-polymer route, a strategy based on direct
spinning of fibers from CNT arrays is more attractive.
[0006] The spinnability of CNT arrays depends greatly on the
quality of the arrays, including CNT alignment, density, purity,
length, and other factors. Due to difficulties in growing long CNT
arrays that are conducive to spinning, CNT yarns so far can only be
drawn from arrays of less than 300 .mu.m long, and have a tensile
strength of only around a few hundreds of MPa.
[0007] The preparation of suitable catalysts is important for
synthesizing arrays of long CNTs. Hata et al., for example,
recently reported the preparation of such a catalyst in
"Water-Assisted Highly Efficient Synthesis of Impurity-Free
Single-Walled Carbon Nanotubes," Science (2004) vol. 306, pp.
1362-1364, incorporated by reference herein. An array of long
nanotubes by a water assisted chemical vapor deposition (CVD)
procedure using ethylene as a carbon source was synthesized using a
catalyst prepared by sputtering a thin layer of iron on a buffer
layer of aluminum oxide. The aluminum oxide layer was previously
deposited on the silicon dioxide surface layer of a silicon
substrate. An abbreviation of this catalyst is
SiO.sub.2/Al.sub.2O.sub.3(10 nm)/Fe(1 nm), where the positions of
the layers in the abbreviation indicate that the aluminum oxide
layer is in between the silica layer and the iron layer. Using this
catalyst, other researchers have prepared arrays of multi-walled
CNTs with CNTs that are less than 2.2 mm in length. Arrays of long
multi-walled CNTs can also be obtained using a catalyst structure
having a buffering layer of MgO (instead of Al.sub.2O.sub.3).
[0008] A problem with current procedures for preparing CNT arrays
is a requirement of a large amount of hydrogen gas in the
precursor. Presently, it appears that a feed gas that includes
hydrogen in an amount greater than 20 percent and as high as 50
percent hydrogen is required for the growth of long CNT arrays.
Hydrogen is relatively expensive and can be dangerous when large
amounts are used in the laboratory and industrially. Importantly,
CNT arrays of the prior art are generally not good precursors for
fibers because they tend to be contaminated with amorphous
carbon.
[0009] There remains a need for long carbon nanotube fibers with
improved strength, and for better methods for preparing arrays of
carbon nanotubes that could be used as precursors for fibers of
carbon nanotubes.
SUMMARY OF THE INVENTION
[0010] In accordance with the purposes of the present invention, as
embodied and broadly described herein, the present invention
includes a method for preparing an array of long nanotubes. The
method involves exposing a catalyst structure to a gaseous mixture
for a chosen amount of time within a chosen temperature range,
wherein the gaseous mixture comprises hydrocarbon, inert gas, and
hydrogen, wherein the percentage of hydrogen in the gaseous mixture
is P, wherein P<20%, whereby an array of substantially aligned
carbon nanotubes forms on the catalyst, the carbon nanotubes of the
array having an average length of greater than about 0.5
millimeter.
[0011] The invention also includes an array of long carbon
nanotubes prepared by a method comprising exposing a catalyst
structure to a gaseous mixture for a chosen amount of time within a
chosen temperature range, wherein the gaseous mixture comprises
hydrocarbon, inert gas, and hydrogen, wherein the percentage of
hydrogen in the gaseous mixture is P, wherein P<20%, whereby an
array of substantially aligned carbon nanotubes forms on the
catalyst, the carbon nanotubes of the array having an average
length of greater than about 0.5 millimeters.
[0012] The invention also includes a densely packed array of
substantially vertically aligned long carbon nanotubes comprising
an average nanotube length of at least 2.5 millimeters.
[0013] The invention also includes a method for preparing a fiber.
The method involves exposing a catalyst structure to a gaseous
mixture for a chosen amount of time at a temperature in a chosen
temperature range, wherein the gaseous mixture comprises
hydrocarbon, inert gas, and hydrogen, wherein the percentage of
hydrogen in the gaseous mixture is P, wherein P<20%, whereby an
array of substantially aligned carbon nanotubes forms on the
catalyst, the carbon nanotubes of the array having an average
length of greater than about 0.5 millimeters; and spinning a fiber
from the array.
[0014] The invention also includes a fiber prepared by a method
comprising exposing a catalyst structure to a gaseous mixture for a
chosen amount of time at a temperature in a chosen temperature
range, wherein the gaseous mixture comprises hydrocarbon, inert
gas, and hydrogen, wherein the percentage of hydrogen in the
gaseous mixture is P, wherein P<20%, whereby an array of
substantially aligned carbon nanotubes forms on the catalyst, the
carbon nanotubes of the array having an average length of greater
than about 0.5 millimeters; and spinning a fiber from the
array.
[0015] The invention also includes a spun fiber of carbon nanotubes
having a tensile strength S, wherein S.gtoreq.1 GPa.
[0016] The invention also includes a ribbon prepared by a method
comprising exposing a catalyst structure to a gaseous mixture for a
chosen amount of time within a chosen temperature range, wherein
the gaseous mixture comprises hydrocarbon, inert gas, and hydrogen,
wherein the percentage of hydrogen in the gaseous mixture is P,
wherein P<20%, whereby an array of substantially aligned carbon
nanotubes forms on the catalyst, the carbon nanotubes of the array
having an average length of greater than about 0.5 millimeters; and
pulling a ribbon from the array.
[0017] The invention also includes a composite structure that
comprises a silicon substrate, a layer of silicon dioxide on the
silicon substrate, a layer of aluminum oxide having a thickness of
from about 2 nanometers to about 20 nanometers deposited by ion
beam assisted deposition on the layer of silicon dioxide, and a
layer of iron having a thickness of from about 0.1 nanometers to
about 5 nanometers on the layer of aluminum oxide.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The accompanying drawings, which are incorporated in and
form a part of the specification, illustrate the embodiments of the
present invention and, together with the description, serve to
explain the principles of the invention. In the drawings:
[0019] FIG. 1a top shows a CNT array of the present invention next
to a small stack of dime coins. FIG. 1a bottom shows a magnified
scanning electron microscopy (SEM) image of the CNT array. FIG. 1b
shows a transmission electron microscope (TEM) image of CNTs in the
array.
[0020] FIG. 2 shows a graph that summarizes growth of a CNT array
with the passage of time.
[0021] FIG. 3 provides a graph of the growth rate of a CNT array
versus temperature with water (top graph) and without water (bottom
graph).
[0022] FIG. 4 shows a graphical representation of the effect of
temperature on the growth rate of CNT arrays.
[0023] FIG. 5a-b shows images of CNT arrays grown by a pulsed
injection of ethylene at time intervals of 1, 2, 3, 4, and 5
minutes, each interval separated by 1 minute.
[0024] FIG. 6 shows a schematic diagram relating to spinning a
fiber from an array of long CNTs according to the present
invention.
[0025] FIG. 7 shows a stress-strain curve of a fiber spun from a
CNT array of the invention. According to the curve, the fiber
demonstrates a non-brittle behavior.
[0026] FIG. 8 shows a scanning electron microscopy (SEM) image of a
ribbon of the invention.
DETAILED DESCRIPTION
[0027] Briefly, the invention is concerned with the preparation of
an array of long (greater than 0.5 millimeters), substantially
straight and aligned carbon nanotubes (CNTs). The invention is also
concerned with a catalyst useful for preparing the array, and with
fibers that are spun from the array. A very efficient, relatively
long-lived catalyst useful for preparing a high purity,
well-aligned array of multi-walled, long CNTs was prepared by
depositing a thin film of iron (Fe) on a layer of aluminum oxide
(Al.sub.2O.sub.3). An array of long CNTs was prepared by exposing
the catalyst to a gaseous mixture of hydrocarbon, inert gas, and a
relatively low percentage of hydrogen at an elevated temperature.
The addition of water vapor to the gaseous mixture results in an
increase in the final length of the CNTs in the array.
[0028] Arrays of long CNTs of this invention have been prepared
with lengths of about 3 millimeters (mm), 3.5 mm, 4 mm, and 4.5 mm.
A growth rate of about 120 .mu.m/min was achieved, which is nearly
ten times faster than the reported growth rates of CNT arrays that
were grown from SiO.sub.2 substrates [5]. These arrays of long CNTs
(i.e. arrays where the CNTs have an average length greater than 0.5
millimeters) are preferred for spinning because they lead to fibers
that can be used for carrying heavier loads than fibers prepared
from arrays of shorter CNTs. Thus, fibers spun from these arrays of
the invention have a tensile strength greater than that of known
CNT fibers. These fibers may also have a higher electrical
conductivity than fibers prepared from shorter arrays.
[0029] The catalyst used for synthesizing CNT arrays according to
the invention was prepared using a silicon support having a thin
surface layer of silicon dioxide (SiO.sub.2). First, a layer of
aluminum oxide (Al.sub.2O.sub.3), typically a layer having a
thickness of about 10 nm, was deposited on the silicon dioxide,
preferably by ion beam assisted deposition (IBAD). Although the
layer of aluminum oxide could also be prepared using other
techniques, such as but not limited to electron beam evaporation,
the IBAD technique can deposit an Al.sub.2O.sub.3 layer that is
fully, or at least partially amorphous. After the IBAD deposition,
a thin layer (of about 1-3 nm in thickness) of iron was magnetron
sputter deposited onto the aluminum oxide layer. It is believed
that the aluminum oxide layer in some way improves catalytic
activity and extends the lifetime of the catalyst. For the
description that follows, unless specially mentioned, the catalyst
used for preparing CNT arrays has a 1 nm thick Fe layer, a 10 nm
thick aluminum oxide layer, and 1 micrometer (.mu.m) thick silicon
dioxide layer on a silicon substrate, wherein the aluminum oxide
layer is deposited on the silicon dioxide layer by ion beam
assisted deposition (IBAD), and the Fe layer is magnetron sputter
deposited on the aluminum oxide layer.
[0030] The CNT arrays of the invention were prepared by chemical
vapor deposition (CVD). A supported catalyst prepared as described
above was placed in a quartz tube furnace. The diameter of the
furnace used will depend on the size of the catalyst used. For the
sizes of catalyst described in certain embodiments of this
invention, a furnace with a diameter of about one inch was used.
After placing the catalyst in the furnace, the furnace was heated
to an elevated temperature and a nonflammable gaseous mixture of
argon and about 6 percent hydrogen (the mixture is known in the art
as "forming gas") and a source of carbon (ethylene, for example)
were sent through the tube furnace. Sometimes, water vapor was
included in the gaseous mixture by passing a small amount of Ar gas
through a water bubbler.
[0031] Arrays of CNTs are prepared more safely using this invention
than by other methods because the invention may employ a hydrogen
concentration of about 6 percent or less, which is a lower
concentration of hydrogen than what is used in current methods. The
invention also employs a wider range of hydrocarbons (alkanes such
as but not limited to methane and ethane; alkenes such as but not
limited to ethylene; alkynes such as but not limited to acetylene;
gaseous sources; liquid sources such as but not limited to hexane;
functionalized hydrocarbons such as but not limited to alcohols and
ketones (acetone, for example), and the like), a wider range of
concentrations (from about 20 percent to about 80 percent of the
hydrocarbon, and from about 20 percent to about 80 percent of an
inert gas such as argon, helium, or an inert gas mixture) than
known methods. For the purposes of this invention, hydrocarbons
also include functionalized hydrocarbons (acetone, ethanol, and the
like). The percentage of a particular hydrocarbon used may depend
to some extent on the hydrocarbon employed.
[0032] The invention also employs a wider temperature range for the
synthesis of an array of long nanotubes. A typical growth
temperature is in the range of from about 700 degrees Celsius to
about 800 degrees Celsius. A preferable growth temperature is in
the range of from about 730 degrees to about 780 degrees.
[0033] Hydrogen is present in the feed in an amount less than 20
percent. More preferably, the hydrocarbon is present in an amount
less than or equal to about 10 percent. More preferably, the
hydrocarbon is present in an amount less than or equal to about 6
percent, which the amount present in forming gas. Other amounts of
the hydrocarbon include about 5 percent or less, about 4 percent or
less, and about 3 percent or less.
[0034] In an embodiment synthesis using forming gas, an array of
CNTs having an average length of about 1 mm was prepared on a
catalyst by placing the catalyst in a quartz furnace, adjusting the
furnace temperature to a temperature of about 750 degrees Celsius,
and sending a mixture of Ar and 6% H.sub.2 gas at a flow rate of
about 100 sccm and ethylene at a flow rate of about 100 sccm
through the furnace for about 15 minutes.
[0035] In another embodiment synthesis, a catalyst was placed in a
quartz furnace, the furnace temperature was adjusted to a
temperature of about 750 degrees Celsius, and a mixture of Ar and
6% H.sub.2 gas (flow rate of 100 sccm) and ethylene (flow rate of
100 sccm) and were sent through the tube for a period of about 2
hours, The result was an array of vertically aligned (with the
catalyst structure on a horizontal surface) carbon nanotubes having
an average nanotube length of 3.2 millimeters (mm). The top portion
of FIG. 1a shows an image of the array. For a size comparison, the
array was placed next to a small stack of coins (dimes). A closer
inspection of the array by scanning electron microscopy (SEM, see
the bottom portion of FIG. 1a) shows that the CNTs are densely
packed, substantially straight, and well aligned in a direction
substantially perpendicular to the surface of the corresponding
substrate. A greatly magnified image of individual CNTs of the
array using transmission electron microscopy (TEM) (see FIG. 1b)
shows that the CNTs of the array are multi-walled, with an average
diameter of about 10 nm.
[0036] FIG. 2 shows a graph of the growth of the CNT array. The
growth temperature is about 750 degrees Celsius. According to FIG.
2, over the first 20 minutes or so, the array grows at a
substantially even growth rate of about 60 .mu.m/min. Afterwards,
the growth rate decreases gradually. After about 90 minutes, growth
almost ceases, when the length of the CNTs of the array are about 3
mm.
[0037] In another embodiment synthesis, a catalyst was placed in a
quartz furnace, the furnace temperature was adjusted to a
temperature of about 750 degrees Celsius, and a flow of ethylene
(flow rate of 100 sccm) and a mixture of Ar and 6% H.sub.2 gas
(flow rate of 100 sccm) and water vapor were sent through the 15
quartz tube for a period of about 2 hours. The addition of water
vapor to the gaseous mixture did not appear to have any substantial
effect during the first 20 or so minutes of growth. However, the
final length of CNTs of the array was about 4.5 mm; the length of
the CNTs of the array increased by about 50 percent when water
vapor was included in the gaseous feed. The water added to the
gaseous feed seems to play a role in maintaining the activity (i.e.
the growth rate) and the lifetime of the catalyst.
[0038] In another embodiment synthesis, a growth rate of 46
.mu.m/min for an array of CNTs was observed when catalyst was
exposed to a combination of 100 sccm ethylene and 100 sccm forming
gas (Ar+6% hydrogen) at a temperature of 730 degrees Celsius. In
another embodiment synthesis of a CNT array, the growth rate
increased to about 72 .mu.m/min when the temperature was increased
to about 750 degrees Celsius. When water (about 1000 ppm) was added
to the feed by bubbling carrier gas through water at 8 sccm, the
average growth rate did not substantially change in a temperature
range of from about 730 degrees Celsius to about 750 degrees
Celsius. However, when the synthesis was repeated in the absence
and in the presence of water at a higher temperature of about 780
degrees Celsius, the growth rate with water was nearly twice the
growth rate without water. FIG. 3 provides a graph of the growth
rate versus temperature with water (top graph) and without water
(bottom graph).
[0039] Raman characterization reveals that CNT growth with water
leads to a lower D peak compared to a run at the same temperature
but in the absence of water, indicating that less amorphous carbon
was formed with water than without water. Further evidence to
support this conclusion comes from an observation of the appearance
of the arrays (with and without water) after about two hours of
growth. In the absence of water, the top of the CNT array appeared
brown after 2 hours, even at a temperature of 750 degrees Celsius
when the CNTs of the array about 3.2 mm in length. By contrast,
when water was added, the top of the array was still black after 2
hours when the CNTs were even longer, about 4.5 mm in length. From
these observations, it is believed that the addition of water slows
down the formation of amorphous carbon that contaminates the array
and deactivates the catalyst.
[0040] FIG. 4 shows a graphical representation of the effect of
temperature on the growth rate of CNT arrays. According to FIG. 4,
the highest growth rate achieved using an embodiment catalyst was
approximately 120 .mu./min at a temperature of about 780 degrees
Celsius. At this rate, a 3 mm long CNT array may be grown in about
40 minutes. A longer growth time at this temperature, even with the
addition of water to the gaseous feed, does not result in a longer
array.
[0041] Under the synthesis conditions employed using the invention,
CNT arrays appear to prefer to grow in a base growth mode (as
opposed to a tip growth mode), with or without water added to the
gaseous feed. FIG. 5a-b shows images of CNT arrays grown by a
pulsed injection of ethylene at time intervals of 1, 2, 3, 4, and 5
minutes, each interval separated by 1 minute. Growth marks appear
on the side of the array. The growth marks may be used as a simple
and indirect indication of the growth mode. For a tip growth mode,
the gap between two adjacent growth marks would gradually increase
with increasing distance from the substrate. For a base growth
mode, the opposite result would be seen. In this case, the largest
gap between marks is found at the bottom of the array, which
suggests a base growth mechanism. In addition, the size of each gap
appears to be proportional to the growth time, indicating again
that the growth evolves with time uniformly.
[0042] An aspect of the invention is concerned with controlling the
thickness of the individual CNTs of an array of the invention. The
CNT diameter, and the number of walls of the CNTs, may be adjusted
by adjusting the thickness of the Fe catalyst layer. Although long
CNT arrays may be synthesized using a catalyst structure having an
Fe catalyst layer thickness in the range of 0.3-2 nm, the CNT
diameter and the number of walls of the CNTs decrease when the
thickness of the deposited Fe film decreases. Using a Fe film
having a thickness of about 0.3 nm, for example, the majority of
CNTs are double-walled and the average CNT diameter is about 6
nanometers (nm). This suggests that the microstructure of the CNTs
can be tuned by adjusting the thickness of the catalyst (iron, for
example) film.
[0043] It is currently believed that the longer it takes to grow an
array of carbon nanotubes, the more amorphous carbon would be
deposited on the CNT array. The presence of amorphous carbon on the
array is detrimental for spinning fibers, and arrays with a
substantial amount of amorphous carbon are unsuitable for spinning.
A benefit of arrays prepared according to the present invention is
that 25 such arrays include only a minimal amount of amorphous
carbon. This is likely due to the speed at which arrays may be
grown using the invention. CNT arrays can grow much faster
(typically 10 times faster) than CNT arrays grown from SiO.sub.2
substrates. Using a growth temperature of from about 720 degrees
Celsius to about 790 degrees Celsius, a growth time from about 5
minutes to about 20 minutes, and a catalyst prepared as described
above, long CNT arrays with lengths ranging from 700 .mu.m to 1.5
mm were synthesized (using ethylene and forming gas). These arrays
are useful for spinning fibers. Furthermore, the longer CNT arrays
synthesized here will enhance the strength of fibers spun from the
arrays.
[0044] Certain important aspects of the present invention that have
been described in some detail thus far have been concerned with
preparing catalysts and using them to synthesize arrays of CNTs.
Another aspect of the invention is concerned with using the arrays
to make strong fibers of CNTs. CNT fibers can be continuously spun
out of suitable CNT arrays like a thread spun from a silk cocoon.
However, not all arrays are conducive to spinning. Currently, it is
believed that arrays with good alignment, high purity and therefore
strong inter-tube interaction are favorable for spinning. We have
tried to spin fibers from various CNT arrays with different
diameters, different lengths and grown from different substrates.
Most of the spinnable arrays prepared according to the prior art
are shorter than 300 micrometers. We have found that CNT arrays
with long CNTs (arrays with CNTs longer than 500 micrometers)
having good alignment are preferred for spinning fibers. CNT arrays
grown at a temperature of about 750 degrees Celsius for 10 to 15
minutes without water, for example, are excellent for spinning
fibers. A longer growth time (greater than about 15 minutes)
sometimes results in the formation of amorphous carbon and more
rigid arrays. However, there appears to be a compromise among the
array length, array purity, and array rigidity. Long, spinnable CNT
arrays may be obtained at higher temperatures (780 degrees, for
example) when water is added to the gaseous feed and when the
growth is for a period of less than about 15 minutes.
[0045] FIG. 6 shows a schematic diagram relating to spinning a
fiber from an array of long CNTs according to the present
invention. As FIG. 6 shows, the fiber spins at a rate of .omega.
while being pulled at a speed of v. The spinning parameters .omega.
and v likely have an effect on the microstructural characteristics
(e.g. the fiber diameter, the helix angle of individual CNTs in the
fiber, and the like) of the resulting composite fiber. The spinning
parameters can be adjusted to optimize the fiber structure for
highest strength.
[0046] A spinning shaft with an end configured for nanotubes to
stick on (a hooked end, an end with adhesive, and the like) may be
used for preparing a fiber from the CNT array. When this end of the
spinning shaft makes contact with nanotubes from the supported
array, the nanotubes begin to twist around the shaft. Many
thousands of nanotubes are likely twisted together at the
beginning. A fiber begins to grow as the array moves relative to
the spinning shaft, and additional nanotubes from the array can
twist around the growing fiber to extend the length of the
fiber.
[0047] The as-spun fiber can be stretched to improve alignment of
the nanotubes.
[0048] An advantage of spinning the fiber from the supported array
is that the long nanotubes from the array are generally aligned
relative to one another before they are spun into a fiber. The
spinning process spirally aligns the nanotubes, and this spirally
aligned arrangement provides the CNT (or CNT/polymer composite)
fiber with high strength. CNT, or CNT/composite fibers of this
invention have a rope like structure that is made strong by
twisting the carbon nanotubes together and around each other.
[0049] CNT fibers spun from a CNT array of the invention display an
enhanced mechanical strength. Fiber samples were spun from 1 mm
long CNT array. Each has a diameter in the range of from about 2
.mu.m to about 3 .mu.m. Tensile tests were performed with these
fibers. For a tensile test, a fiber of about 1 cm in length was
glued onto a hard paper with a fixed 8 mm long oval-shaped cavity
in the center and then mounted in a commercial tensile test
machine. A typical stress-strain curve for a fiber of the invention
is shown in FIG. 7. According to the curve of FIG. 7, the fiber
demonstrates a tensile strength of greater than 3.0 GPa. These
fibers are much stronger than a fiber spun from a 300 .mu.m CNT
array. The enhancement of mechanical strength observed on fibers
from the longer arrays may be because longer CNTs have a better
interlocking ability during tensile testing and can therefore a
carry higher load than fibers spun from shorter tubes. It should
also be mentioned that the resistivity of as-made CNT fibers is
1.68.times.10.sup.-3.OMEGA.cm; thus, fibers prepared according to
this invention have a higher conductivity than any spun MWNT fibers
previously reported.
[0050] The nanotubes of the array may be coated with a polymer
solution before they are spun into fibers or during the spinning
process. The spinning process spirally aligns the polymer-coated
nanotubes, and when the nanotubes are carbon nanotubes, the
resulting fiber has a high volume fraction (60 percent of
nanotubes, and higher), and the twisting improves mechanical
interlocking between nanotubes.
[0051] CNT fibers spun from the array can also be
infiltrated/coated with a polymer matrix to form composite
fibers.
[0052] The spinning approach has several advantages over a drawing
approach. One advantage relates to the relative ease a spinning
process provides for preparing fibers compared to a drawing
process.
[0053] Another advantage of the spinning approach versus the
drawing approach relates to the helical orientation of the
nanotubes that results from spinning the nanotubes and twisting
them around each other. This helical orientation contributes to
improving load transfer because the twisted nanotubes can squeeze
radially against each other when the composite fiber is under load,
which increases the bonding strength and consequently load-transfer
efficiency. Untwisted carbon nanotubes/polymer composite fibers
prepared by drawing are not strong fibers, presumably because the
nanotube-polymer interface is slippery, making it difficult to
transfer load onto the nanotubes.
[0054] Another advantage of spinning process of this invention is
that the twisting squeezes out excess polymer so that individual
CNTs can be closely spaced together. This close spacing increases
the CNT volume fraction of the composite fiber.
[0055] Another advantage of the invention relates to using a
substantially aligned array of carbon nanotubes to prepare a
composite fiber. The alignment of the nanotubes prior to spinning
guarantees alignment in the spun composite fiber.
[0056] Composite fibers prepared using nanotube arrays of the
invention may be used for a variety of applications. These fibers
could be used to prepare superior laminates, woven textiles, and
other structural fiber composite articles. Fiber composites of this
invention could be used to prepare strong and light armor for
aircraft, missiles, space stations, space shuttles, and other high
strength articles. The reduced weight would allow aircraft and
projectiles to fly faster and for longer distances. These features
are also important for spacecraft for future space missions (to the
moon and to Mars, for example), where high strength and lightweight
features of the composite fibers are very important.
[0057] Another advantage of this invention becomes apparent when
metallic carbon nanotubes are used to prepare the composite fiber.
Usually a fraction of carbon nanotubes in an array is metallic.
Metallic carbon nanotubes have been shown to be highly electrically
conductive [6]. Thus, composite fibers of this invention prepared
using precursor carbon nanotubes would not only be very strong but
also highly electrically conductive.
[0058] Composite fibers of this invention are prepared using a
substantially parallel, aligned carbon nanotube array of the type
illustrated in FIG. 1. Arrays like these can be used after they are
prepared, or they can be coated with a dilute solution of polymer
by, for example, immersing the nanotube array in a polymer
solution, and then ultrasonically vibrating the immersed array to
promote wetting. Polymer solutions that have been used in the past
to prepare carbon nanotube-polymer composites could be used with
this invention and include, but are not limited to, polystyrene
dissolved in toluene [8], low viscosity liquid epoxy [6],
poly(methyl methacrylate) (PMMA) dissolved in PMF [9], polyvinyl
alcohol (PVA) in water [10], and poly(vinyl pyrrolidone) (PVP) in
water [10].
[0059] For the case involving polymer-coated nanotubes, after
spinning and stretching, solvent is evaporated and the polymer is
cured at an appropriate temperature. Detailed treatment parameters
depend on the specific polymer and solvent that are used during the
preparation. A vacuum oven may be used for solvent removal and
curing.
[0060] The cured composite fiber of the invention can be evaluated
in tension to obtain the strength, the dependency of the strength
on the length (i.e. size effect), the Young's modulus, the
ductility, and other properties. The fracture surface of the
composite fiber may be examined using Scanning Electron Microscopy
(SEM) to investigate the failure mode in order to evaluate the
strength of the CNT/polymer interface. Transmission electron
microscopy (TEM) may be used to examine individual CNT arrangements
in the composite fiber and the CNT/matrix interface.
[0061] The invention is also concerned with the preparation of CNT
ribbon from an array of the invention. FIG. 8 shows an SEM image of
CNT ribbon prepared from an array of the invention.
[0062] In summary, arrays of long carbon nanotubes are prepared on
a supported catalyst using a feed of hydrocarbon, inert gas, and a
relatively small amount of hydrogen. Water may be added to the
feed. The arrays are relatively rigid, of a high-purity, and have
very good CNT alignment. A balance between length and quality of
CNTs may be achieved to produce an array with a length of from
about 500 .mu.m to about 1.5 mm. Fibers spun from such long CNT
arrays exhibit a mechanical strength and electrical conductivity
greater than for known CNT fibers.
[0063] The foregoing description of the invention has been
presented for purposes of illustration and description and is not
intended to be exhaustive or to limit the invention to the precise
form disclosed, and obviously many modifications and variations are
possible in light of the above teaching.
[0064] The embodiments were chosen and described in order to best
explain the principles of the invention and its practical
application to thereby enable others skilled in the art to best
utilize the invention in various embodiments and with various
modifications as are suited to the particular use contemplated. It
is intended that the scope of the invention be defined by the
claims appended hereto.
REFERENCES
The following references are incorporated by reference herein.
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[0066] 2. Demczyk et al, Mater. Sci. Eng. A334, (2002) pp. 173-178.
[0067] 3. Concise Encyclopedia of Composite Materials, edited by A.
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