U.S. patent application number 10/984619 was filed with the patent office on 2006-02-09 for high tensile strength carbon nanotube film and process for making the same.
Invention is credited to Satish Kumar, Tao Liu, Sreekumar T. Veedu, Xiefei Zhang.
Application Number | 20060029537 10/984619 |
Document ID | / |
Family ID | 35757603 |
Filed Date | 2006-02-09 |
United States Patent
Application |
20060029537 |
Kind Code |
A1 |
Zhang; Xiefei ; et
al. |
February 9, 2006 |
High tensile strength carbon nanotube film and process for making
the same
Abstract
A conductive carbon nanotube film having high tensile strength
and initial tensile modulus comprises primarily oxidized
small-diameter carbon nanotubes wherein the diameter of the
small-diameter carbon nanotubes are at most about 3 nm. A method
for making the film comprises refluxing an aqueous mixture
comprising carbon nanotubes and an oxidizing agent to form a
refluxed nanotube dispersion; forming a carbon nanotube film from
the refluxed carbon nanotube dispersion; optionally removing nitric
acid or other oxidizing agent from the carbon nanotube film; drying
the carbon nanotube film; and heat-treating the carbon nanotube
film to form a heat-treated carbon nanotube film. The method can
also comprise sonicating the nanotubes prior to or after refluxing.
A heat-treated small-diameter carbon nanotube film can have a
tensile strength of over 70 MPa and an initial tensile modulus of
about 5 GPa.
Inventors: |
Zhang; Xiefei; (Atlanta,
GA) ; Veedu; Sreekumar T.; (Kannur, IN) ; Liu;
Tao; (Mt. Vernon, IN) ; Kumar; Satish;
(Lawrenceville, GA) |
Correspondence
Address: |
WILLIAMS, MORGAN & AMERSON, P.C.
10333 RICHMOND, SUITE 1100
HOUSTON
TX
77042
US
|
Family ID: |
35757603 |
Appl. No.: |
10/984619 |
Filed: |
November 9, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60523806 |
Nov 20, 2003 |
|
|
|
Current U.S.
Class: |
423/447.1 |
Current CPC
Class: |
B82Y 30/00 20130101;
D01F 11/12 20130101; D01F 11/123 20130101; D01F 11/122 20130101;
D01F 11/121 20130101 |
Class at
Publication: |
423/447.1 |
International
Class: |
D01F 9/12 20060101
D01F009/12 |
Goverment Interests
[0002] This invention was made in part with United States
Government support under Grant No. F49620-03-1-0124 awarded by the
Air Force Office of Scientific Research and under Grant No.
N00014-01-1-0657 awarded by the Office of Naval Research.
Government may have certain rights in the invention.
Claims
1. A method for making a conductive carbon nanotube film,
comprising: (a) refluxing an aqueous mixture comprising carbon
nanotubes and an oxidizing agent to form a refluxed nanotube
dispersion; (b) forming a carbon nanotube film from the refluxed
nanotube dispersion; (c) drying the carbon nanotube film; and (d)
heat-treating the carbon nanotube film to form a heat-treated
carbon nanotube film.
2. The method of claim 1, wherein the carbon nanotubes are
purified.
3. The method of claim 1, wherein the carbon nanotubes comprise
single-wall carbon nanotubes.
4. The method of claim 1, wherein the carbon nanotubes comprise
multi-wall carbon nanotubes, wherein the multi-wall carbon
nanotubes have diameters at most about 3 nm.
5. The method of claim 1, wherein the oxidizing agent is nitric
acid.
6. The method of claim 5, wherein the concentration of nitric acid
in the aqueous mixture is in the range of about 3 Molar and about
10 Molar.
7. The method of claim 5, wherein the concentration of nitric acid
in the aqueous mixture is in the range of about 3 Molar and about 6
Molar.
8. The method of claim 5, wherein the concentration of nitric acid
in the aqueous mixture is in the range of about 6 Molar and about
10 Molar.
9. The method of claim 1, wherein the oxidizing agent is selected
from the group consisting of ozone, potassium persulfate, a mixture
of nitric acid and sulfuric acid, a mixture of nitric acid and
hydrogen peroxide, steam, carbon dioxide, halogens,
halogen-containing compounds, and combinations thereof.
10. The method of claim 1, further comprising removing the
oxidizing agent from the carbon nanotube film.
11. The method of claim 10, wherein the removing is done by washing
with a solvent selected from the group consisting of acetone,
alcohol, water and a combination thereof.
12. The method of claim 1, wherein the forming is done by filtering
the carbon nanotubes.
13. The method of claim 1, wherein the forming is done on an
adsorbent or non-adsorbent surface.
14. The method of claim 1, wherein the drying is done in a
vacuum.
15. The method of claim 1, wherein the drying is done in an
atmosphere selected from the group consisting of a vacuum, nitrogen
and inert gas.
16. The method of claim 1, wherein the drying is done at a
temperature in the range of about 15.degree. C. and about
200.degree. C.
17. The method of claim 1, wherein the heat-treating is done in an
oxygen-containing atmosphere.
18. The method of claim 1, wherein the heat-treating is done in an
inert atmosphere.
19. The method of claim 1, wherein the heat-treating is done at a
temperature in the range of at least about 200.degree. C. and about
1000.degree. C.
20. The method of claim 1, wherein the heat-treated carbon nanotube
film has a tensile strength of at least about 15 MPa.
21. The method of claim 1, wherein the heat-treated carbon nanotube
film has a tensile strength of at least about 25 MPa.
22. The method of claim 1, wherein the heat-treated carbon nanotube
film has a tensile strength of at least about 50 MPa.
23. The method of claim 1, wherein the heat-treated carbon nanotube
film has a tensile strength of at least about 70 MPa.
24. The method of claim 1, wherein the carbon nanotube film
comprises crosslinked carbon nanotubes.
25. The method of claim 1, further comprising sonicating the carbon
nanotubes in water before refluxing.
26. The method of claim 1, further comprising sonicating the
mixture, the dispersion or both.
27. The method of claim 1, wherein the heat-treated carbon nanotube
film comprises primarily single-wall carbon nanotubes.
28. The method of claim 1, wherein the heat-treated carbon nanotube
film comprises primarily small-diameter carbon nanotubes having
diameters at most about 3 nm.
29. The method of claim 1, wherein the heat-treated carbon nanotube
film has a thickness in the range of about 0.1 micron and about
10,000 microns.
30. The method of claim 1, wherein the heat-treated carbon nanotube
film has a thickness in the range of about 1 micron and about 1,000
microns.
31. The method of claim 1, wherein the heat-treated carbon nanotube
film has a thickness in the range of about 1 micron and about 100
microns.
32. An film comprising primarily small-diameter carbon nanotubes,
wherein the nanotubes have been oxidized, and wherein the film has
a thickness in the range of about 0.1 micron and about 10,000
microns and tensile strength of at least about 15 MPa.
33. The film of claim 32, wherein the film has a tensile strength
of at least about 25 MPa.
34. The film of claim 32, wherein the film has a tensile strength
of at least about 50 MPa.
35. The film of claim 32, wherein the film has a tensile strength
of at least about 70 MPa.
36. The film of claim 32, wherein the film has a thickness in the
range of about 1 micron and about 1,000 microns.
37. The film of claim 32, wherein the film has a thickness in the
range of about 1 micron and about 100 microns.
38. A film consisting essentially of small-diameter carbon
nanotubes, wherein the small-diameter carbon nanotubes are
crosslinked.
39. The film of claim 38, wherein the film has a tensile strength
of at least about 25 MPa.
40. The film of claim 38, wherein the film has a tensile strength
of at least about 50 MPa.
41. The film of claim 38, wherein the film has a tensile strength
of at least about 70 MPa.
42. The film of claim 38, wherein the film has a thickness in the
range of about 0.1 micron and about 10,000 microns.
43. The film of claim 38, wherein the film has a thickness in the
range of about 1 micron and about 1,000 microns.
44. The film of claim 38, wherein the film has a thickness in the
range of about 1 micron and about 100 microns.
45. A conductive carbon nanotube film made by the process
comprising: (a) refluxing an aqueous mixture comprising carbon
nanotubes and an oxidizing agent to form a refluxed nanotube
dispersion; (b) forming a carbon nanotube film, (c) drying the
carbon nanotube film; and (d) heat-treating the carbon nanotube
film to form a heat-treated carbon nanotube film.
46. The film of claim 45, wherein the carbon nanotubes are
purified.
47. The film of claim 45, wherein the carbon nanotubes comprise
single-wall carbon nanotubes.
48. The film of claim 45, wherein the carbon nanotubes comprise
small-diameter carbon nanotubes, wherein the small-diameter carbon
nanotubes have diameters of at most about 3 nm.
49. The film of claim 45, wherein the oxidizing agent is nitric
acid.
50. The film of claim 45, wherein the oxidizing agent is removed
from the carbon nanotubes.
51. The film of claim 45, wherein the forming is done by
filtering.
52. The film of claim 45, wherein the heat-treating is done at a
temperature in the range of at least about 200.degree. C. and about
1000.degree. C.
53. The film of claim 45, wherein the heat-treated carbon nanotube
film has a thickness in the range of about 0.1 micron and about
10,000 microns.
54. The film of claim 45, wherein the heat-treated carbon nanotube
film has a thickness in the range of about 1 micron and about 1,000
microns
55. The film of claim 45, wherein the heat-treated carbon nanotube
film has a thickness in the range of about 1 micron and about 100
microns.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority from provisional United
Patent application Ser. No. 60/523,806, filed Nov. 19, 2003, which
application is incorporated herein by reference.
FIELD OF THE INVENTION
[0003] This invention relates generally to carbon nanotubes, and
more particularly to a high tensile strength film comprising carbon
nanotubes.
BACKGROUND OF THE INVENTION
[0004] Small-diameter carbon nanotubes having diameters between
about 0.5 and about 3 nanometers, commonly known as "buckytubes,"
have been the subject of intense research since their discovery due
to their unique properties, including high strength, stiffness,
thermal and electrical conductivity. The walls of small-diameter
carbon nanotubes are fullerenes consisting essentially of
sp.sup.2-hybridized carbon atoms typically arranged in hexagons and
pentagons. Some small-diameter carbon nanotubes have only one wall,
and others have more than one. Large-diameter multi-wall carbon
nanotubes (MWNT), having diameters in excess of about 4 nanometers,
are multiple nested carbon cylinders. Because large-diameter
multi-wall carbon nanotubes have substantially greater density of
defects in their side-walls, they are, consequently, mechanically
less strong and electrically less conductive than small-diameter
carbon nanotubes. Additionally, compared to the large-diameter
multi-wall carbon nanotubes, small-diameter carbon nanotubes have
considerably higher available surface area per gram of carbon.
[0005] The exceptional mechanical properties of carbon nanotubes
make them useful in composites requiring high tensile strength and
modulus, such as in structural reinforcement for sports equipment,
buildings, vehicles, ship hulls, aircraft, and artillery vehicle
and personal body armor. Besides incorporating nanotubes dispersed
in a matrix material, nanotubes in other forms, such as fibers and
films, are useful in the construction of laminates for composite
components of aircraft, automobiles and other structures. The
fabrication of high strength films comprising small-diameter
nanotubes for these and other applications remains a major
challenge.
SUMMARY OF THE INVENTION
[0006] In one embodiment, the present invention relates to a
conductive film comprising carbon nanotubes, wherein the film has
high tensile strength and a high initial tensile modulus, and
method for making the same.
[0007] In another embodiment, a method for making a conductive
carbon nanotube film comprising refluxing an aqueous mixture
comprising carbon nanotubes and an oxidizing agent to form a
refluxed nanotube dispersion; forming a carbon nanotube film from
the refluxed nanotube dispersion; drying the carbon nanotube film;
and heat-treating the carbon nanotube film to form a heat-treated
carbon nanotube film. In one embodiment, the method can further
comprise removing the oxidizing agent from the carbon nanotube
film. In another embodiment, the method can further comprise
sonicating the nanotubes prior to or after refluxing. In yet
another embodiment, the method can further comprise sonicating the
nanotubes prior to adding the oxidizing agent. In another
embodiment, the carbon nanotubes comprise single-wall
small-diameter carbon nanotubes. In another embodiment, the carbon
nanotubes comprise small-diameter carbon nanotubes wherein the
small-diameter carbon nanotubes have a diameter of at most about 3
nm and can have one or more walls. In another embodiment, the
forming of the carbon nanotube film is done by filtering.
[0008] In another embodiment, a film consists essentially of
small-diameter carbon nanotubes, wherein the small-diameter carbon
nanotubes are crosslinked.
[0009] In another embodiment, a heat-treated carbon nanotube film
consists essentially of small-diameter carbon nanotubes and has a
tensile strength of at least about 70 MPa (megapascals) and an
initial tensile modulus of up to about 5 GPa (gigaPascals).
[0010] In yet another embodiment, a carbon nanotube film comprises
primarily small-diameter carbon nanotubes, i.e. greater than about
50 wt % small-diameter carbon nanotubes, wherein the nanotubes have
been oxidized, and wherein the film has a thickness in the range of
about 0.1 micron and about 10,000 microns and has a tensile
strength of at least about 15 MPa. In other embodiments, a film
comprises primarily small-diameter carbon nanotubes and has a
tensile strength of at least about at 25 MPa, at least about 50 MPa
or at least about 70 MPa.
[0011] In another embodiment, a carbon nanotube film comprises
small-diameter carbon nanotubes and has a tensile strength of about
74 MPa, i.e., a tensile strength that is more than seven times
greater than that of a comparable film prepared without nitric acid
treatment of the nanotubes.
[0012] In yet another embodiment, a carbon nanotube film comprises
small-diameter carbon nanotubes and has an initial tensile modulus
of up to about 5 GPa, an initial tensile modulus which is over six
times greater than that of a comparable film prepared without
nitric acid treatment of the nanotubes.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 shows the dynamic mechanical behavior of a film
comprising small-diameter carbon nanotubes oxidized with 6 Molar
(M) nitric acid.
[0014] FIG. 2A shows a Scanning Electron Micrograph (SEM) of a
heat-treated film comprising small-diameter carbon nanotubes
processed with 3M nitric acid.
[0015] FIG. 2B shows a SEM of a heat-treated film comprising
small-diameter carbon nanotubes processed in 6M nitric acid.
[0016] FIG. 2C shows a SEM of a heat-treated film comprising
small-diameter carbon nanotubes processed in 10M nitric acid.
[0017] FIG. 2D shows a SEM of a heat-treated film comprising
small-diameter carbon nanotubes processed in 3M nitric acid,
wherein the film was further subjected to a second heat-treatment
at 900.degree. C. in nitrogen.
[0018] FIG. 2E shows a SEM of a heat-treated film comprising
small-diameter carbon nanotubes processed in 6M nitric acid,
wherein the film was further subjected to a second heat-treatment
at 900.degree. C. in nitrogen.
[0019] FIG. 2F shows a SEM of a heat-treated film comprising
small-diameter carbon nanotubes processed in 10M nitric acid,
wherein the film was further subjected to a second heat-treatment
at 900.degree. C. in nitrogen.
[0020] FIGS. 3A-D show Raman spectra of the Radial Breathing Mode
(RBM) peaks of films prepared by various embodiments of the present
invention.
[0021] FIG. 3A shows the RBM peaks of a Raman spectrum for a
heat-treated control film of small-diameter carbon nanotubes,
wherein the film was further subjected to a second heat-treatment
at 900.degree. C. in nitrogen.
[0022] FIG. 3B shows the RBM peaks of a Raman spectrum for a
heat-treated film comprising small-diameter carbon nanotubes which
were oxidized in 3M nitric acid, wherein the film was further
subjected to a second heat-treatment at 900.degree. C. in
nitrogen.
[0023] FIG. 3C shows the RBM peaks of a Raman spectrum for a
heat-treated film comprising small-diameter carbon nanotubes which
were oxidized in 6M nitric acid, wherein the film was further
subjected to a second heat-treatment at 900.degree. C. in
nitrogen.
[0024] FIG. 3D shows the RBM peaks of a Raman spectrum for a
heat-treated film comprising small-diameter carbon nanotubes which
were oxidized in 10M nitric acid, wherein the film was further
subjected to a second heat-treatment at 900.degree. C. in
nitrogen.
[0025] FIG. 4A shows a plot of the relative fraction of 1.1 g-nm
diameter carbon nanotubes as a function of nitric acid
concentration.
[0026] FIG. 4B shows a plot of the relative fraction of 1.11-nm
diameter carbon nanotubes as a function of nitric acid
concentration.
[0027] FIG. 4C shows a plot of the relative fraction of 1.07-nm
diameter carbon nanotubes as a function of nitric acid
concentration.
[0028] FIG. 4D shows a plot of the relative fraction of 1.02-nm
diameter carbon nanotubes as a function of nitric acid
concentration.
[0029] FIG. 4E shows a plot of the relative fraction of 0.89-nm
diameter carbon nanotubes as a function of nitric acid
concentration.
[0030] FIG. 4F shows a plot of the relative fraction of 0.88-nm
diameter carbon nanotubes as a function of nitric acid
concentration.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
[0031] In this application, the terms "dispersion" and "suspension"
are intended to have the same meaning and will be used
interchangeably. A dispersion or suspension of carbon nanotubes is
intended to be generally homogeneous, but may be temporary and
unstable over time. For example, the carbon nanotubes may be
dispersed in water during active sonication or mixing, but settle
out in time afterwards. The carbon nanotubes can be present in the
dispersion as individual nanotubes or nanotube bundles or
ropes.
[0032] In one embodiment of the present invention, a high tensile
strength and high tensile modulus conductive film consists
essentially of small-diameter carbon nanotubes oxidized with nitric
acid or other oxidizing agent.
[0033] The carbon nanotube material used to make the carbon
nanotube film can comprise small-diameter single-wall carbon
nanotubes, small-diameter multi-wall carbon nanotubes or a
combination thereof. The nanotubes can be made by any known process
for making carbon nanotubes, however, the composition of the
nanotube material can influence the conductivity, strength and
modulus of the nanotube film. For example, a nanotube material
having more small-diameter carbon nanotubes with respect to
large-diameter multi-wall carbon nanotubes and amorphous carbon,
would be expected to be more conductive and stronger than a
nanotube material with a lower concentration of small-diameter
carbon nanotubes in a mixture of large-diameter multi-wall carbon
nanotubes and amorphous carbon. In one embodiment of the invention,
the carbon nanotube material comprises primarily single-wall carbon
nanotubes (i.e. greater than 50 wt % of the carbon-containing
material). In another embodiment, the carbon nanotube material
comprises primarily small-diameter carbon nanotubes, in which the
diameter of the small-diameter multi-wall carbon nanotubes is at
most about 3 nm. Preferably, the carbon nanotube material comprises
about 50 wt % to about 100 wt % single-wall carbon nanotubes or
about 50 wt % to about 100 wt % small-diameter carbon nanotubes, in
which the diameter of the small-diameter single-wall and/or
multi-wall carbon nanotubes is at most about 3 nm, and in some
embodiments between about 0.5 and about 3 nanometers.
[0034] The present invention involves film made with both
small-diameter single-wall and/or multi-wall carbon nanotubes.
Because small-diameter carbon nanotubes have a strong affinity for
each other and are held strongly together by van der Waals forces,
dispersing small-diameter carbon nanotubes is much more difficult
than for large-diameter multi-wall carbon nanotubes. Therefore,
procedures that are effective for dispersing and processing
small-diameter carbon nanotubes are generally effective for
multi-wall carbon nanotubes, however, the reverse is generally not
the case, i.e., procedures that are suitable for multi-wall carbon
nanotubes are generally not effective for small-diameter carbon
nanotubes.
[0035] The carbon nanotubes can be made by any known means. The
carbon nanotubes can be used as synthesized or after purification.
Purification of the nanotube material can be done to remove
amorphous carbon, metallic impurities and non-nanotube carbon. For
certain applications, purification may be preferred and can be done
by any known means. Suitable procedures for purification of carbon
nanotubes are related in International Patent Publications "Process
for Purifying Single-Wall Carbon Nanotubes and Compositions
Thereof," WO 02/064,869 published Aug. 8, 2002, and "Gas Phase
Process for Purifying Single-Wall Carbon Nanotubes and Compositions
Thereof," WO 02/064,868 published Aug. 8, 2002, and included herein
in their entirety by reference.
[0036] In one embodiment, a method for making a conductive film
comprising carbon nanotubes wherein the film has high tensile
strength and high initial tensile modulus comprises refluxing an
aqueous mixture comprising carbon nanotubes and an oxidizing agent
to form a refluxed nanotube dispersion; forming a carbon nanotube
film from the refluxed carbon nanotube dispersion; drying the
carbon nanotube film; and heat-treating the carbon nanotube film to
form a heat-treated carbon nanotube film. The heat treatment can be
carried out in an oxidizing environment, which may comprise liquid
oxidizing agents, gaseous oxidizing agents or a combination
thereof. The method can further comprise sonicating the nanotubes
prior to or after refluxing. The method can further comprise
sonicating the carbon nanotube suspension.
[0037] The oxidizing agent can comprise any compound that can
oxidize carbon nanotubes. Oxidation can occur anywhere on the
carbon nanotube, but, typically, occurs at the end caps and/or at
defect sites of the nanotubes. Oxidizing agents that can oxidize
carbon nanotubes, include, but are not limited to, nitric acid,
ozone, potassium persulfate (K.sub.2S.sub.2O.sub.8), sulfuric acid,
sulfuric acid with hydrogen peroxide, oxygen, steam, carbon
dioxide, halogens, halogen-containing compounds, sulfuric acid with
nitric acid, and combinations thereof. For conciseness, nitric acid
will be used as an exemplary oxidizing agent, even though other
oxidizing agents could be used.
[0038] In another embodiment, the method comprises sonicating the
carbon nanotubes in water prior to adding the nitric acid. Other
mixing means can be used in addition to, or in lieu of, sonication
to disperse the nanotubes in water. Other suitable mixing means
include, but are not limited to, magnetic stirring, homogenization,
grinding, high-shear mixing, application of high pressures between
about 10 and about 100,000 pounds per square inch, and combinations
thereof. Heat can also be applied before or during or after the
dispersion process.
[0039] After sonication and/or suitable agitation of the nanotubes
in water, nitric acid is added to the carbon nanotube/water
mixture. Among other parameters, the concentration of the nitric
acid can affect the tensile strength of the film. Generally, higher
concentrations of nitric acid result in higher tensile strength
carbon nanotube films. The concentration of nitric acid in the
aqueous mixture is typically in the range of about 3 Molar and
about 10 Molar. The nitric acid concentration can be in the range
of about 3 Molar and about 6 Molar or in the range of about 6 is
Molar and about 10 Molar. The concentration of nitric acid and
other parameters can be adjusted to achieve the desired oxidation
of the nanotubes and tensile strength of the film.
[0040] After the desired amount of nitric acid is added to the
carbon nanotube-water mixture to obtain the desired nitric acid
concentration, the acidified mixture can be sonicated, or otherwise
agitated, again. As above, other suitable mixing means can be used
with, or in lieu of, sonication. Agitation can be done from hours
to days, such as from 12 hours to 3-4 days. A suitable amount of
agitation is that time needed to effectively disperse the
nanotubes.
[0041] After suitable sonication and/or agitation, the aqueous
nitric acid-carbon nanotube mixture is refluxed to further disperse
and oxidize the nanotubes. The reflux temperature is approximately
the boiling point of the aqueous nitric acid mixture which can be
generally in the range of about 100.degree. C. and about
130.degree. C. The refluxing time is dependent on the amount of
nanotube oxidation desired. Higher concentrations of nitric acid
will generally require less refluxing time. The refluxing time is
the time to effectively oxidize the nanotubes to the amount
desired. Typically, refluxing time can range from about 30 minutes
to about 5 hours, more typically about 30 minutes to about 2
hours.
[0042] After refluxing, the refluxed aqueous nitric acid-carbon
nanotube mixture can be sonicated, or otherwise suitably agitated,
again. After sonication or suitable agitation, if done, a carbon
nanotube film is formed. The nanotube film can be made any known
means. In one embodiment, the nanotube film is formed by filtering
the nanotubes from the mixture, such that the nanotubes are
retained on the filter, and, thereby forming a film. Filtering the
dispersion can be done through any suitable membrane and/or filter,
with a pore size small enough that the nanotubes collect on a
filter or membrane. "Filter", "membrane" and "membrane filter" will
be used interchangeably in this application. Examples of suitable
filter material are polytetrafluoroethylene and Whatman filter
paper. An example of a suitable filter pore size is about 1 micron.
Whatman filter paper #1 is an example of a suitable filter. A
suitable filter is any filter that takes out most of the nanotubes
from the dispersion. Other means of forming a nanotube film from
the nanotube dispersion can be used. For example, the nanotube
dispersion could be poured on an absorbent or non-absorbent
surface, and the solvent could be removed by any convenient means,
such as, but not limited to, evaporation, heating, application of a
vacuum, or combinations thereof.
[0043] At this point, the nanotubes can be peeled from the filter
or other surface and dried. However, the nanotubes can also be
washed to remove nitric acid from the nanotubes before drying.
Repeated washings can be done with water or any suitable solvent,
such as acetone, alcohols, such as methanol and ethanol, and
combinations thereof. Preferably, the washing fluid is a polar
compound with the ability to solubilize nitric acid and remove it
from the nanotubes. The washing, if done, is conveniently performed
when the nanotubes are on a filter, such that after washing the
nanotubes, the nanotube film can be peeled from the filter and
dried.
[0044] Drying the carbon nanotube film to remove moisture and any
solvent from the nanotubes can be done by any known drying means,
such as with heat, vacuum, ambient solvent evaporation, or
combinations thereof. The drying atmosphere can be a vacuum or
under nitrogen or inert gas, such as argon. The film can also be
air dried. The drying is done at a temperature in the range of
about room temperature, i.e., about 15-20.degree. C., up to about
200.degree. C. Typically, the drying temperature can be in the
range of room temperature to about 100.degree. C. The drying time
and temperature are dependent on various parameters, including, but
not limited to, the particular solvent used and the amount of water
or solvent to be removed. The drying time is also dependent on
temperature, ambient pressure and the thickness of the
small-diameter carbon nanotube film. The amount of drying time is
that amount of time required to remove most of the water or
solvent.
[0045] After drying, the film is heat-treated to form a
heat-treated carbon nanotube film. Heat-treating is generally done
at higher temperatures than those for drying the film. Typical
heat-treating temperatures are in the range of about 200.degree. C.
and about 1000.degree. C. Heat-treating can be done in a vacuum or
in an atmosphere comprising air, an oxidizing gas, nitrogen, an
inert gas, or combinations thereof. Typical heat-treating times are
dependent on the heat-treating temperature. Suitable heat-treating
conditions for a carbon nanotube film can comprise heating at
200.degree. C. for 2 to 3 hours in nitrogen or air. High
temperature heat-treatment conditions can comprise 900.degree. C.
in nitrogen for about 2 minutes.
[0046] Heat-treatment temperatures in nitrogen or inert
environments can typically be higher than in oxidative
environments. Typically, heat-treatment in nitrogen or inert gases
can be done at temperatures between about 200.degree. C. to about
1000.degree. C. Heat-treatment in air or oxidative atmospheres can
be done at temperatures up to about 1000.degree. C. for some mild
oxidizing agents such as carbon dioxide. In one embodiment, the
duration of the heat-treating is about 2 hours at 200.degree. C. in
air, nitrogen or an inert atmosphere. The thickness of the carbon
nanotube film can range from about 0.1 micron to about 10,000
microns. Typically, the thickness of the carbon nanotube film can
range from about 1 micron to about 1,000 microns, and, even more
typically, the thickness of the nanotube film can range from about
1 micron and about 100 microns.
[0047] The resulting heat-treated film consists essentially of
small-diameter single-wall carbon nanotubes and/or small-diameter
multi-wall carbon nanotubes that have been oxidized. The
heat-treated carbon nanotube film can further comprise crosslinked
carbon nanotubes. Typically, the film comprises primarily
small-diameter carbon nanotubes or primarily small-diameter
single-wall and/or small-diameter multi-wall carbon nanotubes.
[0048] In another embodiment, the heat-treated carbon nanotube film
comprises primarily oxidized small-diameter carbon nanotubes, e.g.
primarily oxidized small-diameter single-wall and/or small-diameter
multi-wall carbon nanotubes, and has a tensile strength of at least
about 70 MPa.
[0049] In another embodiment, the heat-treated small-diameter
carbon nanotube film comprises at least about 50 wt %
small-diameter carbon nanotubes and has a tensile strength of at
least about 70 MPa and high initial tensile modulus of about 5 GPa.
In another embodiment, the carbon nanotubes are purified.
[0050] In another embodiment, the heat-treated small-diameter
carbon nanotube film comprises primarily oxidized small-diameter
carbon nanotubes, e.g. primarily oxidized small-diameter
single-wall carbon nanotubes and/or oxidized small-diameter
multi-wall carbon nanotubes, and has a tensile strength of at least
about 15 MPa, at least about 25 MPa, or at least about 50 MPa.
[0051] In another embodiment, a film comprises at least about 80 wt
% oxidized single-wall carbon nanotubes and/or oxidized
small-diameter multi-wall carbon nanotubes, wherein the film has a
tensile strength of at least about 15 MPa, at least about 25 MPa,
at least about 50 MPa, or at least about 70 MPa.
[0052] In one embodiment of the present invention, a conductive
small-diameter carbon nanotube film is prepared with small-diameter
carbon nanotubes that have been oxidatively treated with nitric
acid, and has a substantially higher tensile strength and initial
tensile modulus than a film prepared with small-diameter carbon
nanotubes without nitric acid treatment. For example, in one
embodiment of the present invention, a carbon nanotube film
comprising small-diameter carbon nanotubes is prepared from
small-diameter carbon nanotubes treated in 10M nitric acid and has
a tensile strength and an initial tensile modulus of about 74 MPa
and about 5 GPa, respectively. These strength and modulus values
are about seven and six times greater, respectively, than those for
comparable film made with nanotubes without nitric acid
treatment.
[0053] In another embodiment, a film comprising small-diameter
carbon nanotubes that have been treated with nitric acid, has an
electrical conductivity value on the order of about 10.sup.4
Siemens/m, which is comparable to the electrical conductivity of
films prepared with nanotubes without nitric acid treatment and
also comparable to electrically conductive polymers, such as
polythiophene and polypyrrole.
[0054] In another embodiment, a film comprising oxidized
small-diameter carbon nanotubes has a stable mechanical performance
over a large temperature range. The stable mechanical performance
in an oxidative environment is expected to be up to about
400.degree. C., and up to about 1000.degree. C. in nitrogen or an
inert environment.
[0055] The following examples are included to demonstrate preferred
embodiments of the invention. It should be appreciated by those of
skill in the art that the techniques disclosed in the examples
which follow represent techniques discovered by the inventor to
function well in the practice of the invention, and thus can be
considered to constitute preferred modes for its practice. However,
those of skill in the art should, in light of the present
disclosure, appreciate that many changes can be made in the
specific embodiments which are disclosed and still obtain a like or
similar result without departing from the spirit and scope of the
invention.
EXAMPLE 1
[0056] This example demonstrates a method for preparing a high
strength film comprising predominantly single-wall carbon nanotubes
(SWNT). Purified, HIPCO.RTM. small-diameter carbon nanotubes,
comprising almost entirely single-wall carbon nanotubes, obtained
from Rice University and Carbon Nanotechnologies, Inc., were made
in a high temperature, high pressure, gas phase process through the
disproportionation of carbon monoxide to primarily single-wall
carbon nanotubes and CO.sub.2 using iron as the transition metal
catalyst. (HIPCO is a registered trademark of Carbon
Nanotechnologies, Incorporated, Houston, Tex.) 100 mg purified
small-diameter carbon nanotubes were dispersed in 100 mls distilled
water and sonicated for 2 hours using a Fisher Scientific bath
sonicator (frequency 43 KHz, power 150 Watts). Nitric acid was then
added to the dispersion to obtain a nitric acid concentration of
3M, 6M, or 10M. Each dispersion was then sonicated for 2 more
hours, then refluxed for two hours, and subsequently sonicated for
another 20 minutes. Each dispersion was filtered through a
polytetrafluoroethylene (PTFE) membrane filter (Gelman Laboratory,
1-.mu.m pore diameter) and repeatedly washed with distilled water.
The resulting carbon nanotube films comprising small-diameter
nanotubes were easily peeled off the PTFE membrane filter.
[0057] A control sample was made by sonicating an aqueous
dispersion of small-diameter carbon nanotubes without any nitric
acid for 4 hours, filtering the dispersion through a PTFE membrane
filter and peeling off a small-diameter carbon nanotube film from
the PTFE membrane.
[0058] Each film was dried at 70.degree. C. in a vacuum, and
heat-treated at 200.degree. C. for 2 hrs in air. The heat-treated
films made in this example are referred to the "as prepared" film
samples in further examples.
EXAMPLE 2
[0059] Tensile mechanical properties and dc conductivity were
measured on the small-diameter carbon nanotube films prepared in
Example 1. The film samples tested in the tensile tests were 1 mm
wide and 0.06 mm thick. The gauge length was 10 mm between the
clamps. The tensile tests were conducted on a Rheometrics Solids
Analyzer, RSA III, at a strain rate of 0.5% per second. The tensile
test data are summarized in Table 1.
[0060] The mechanical studies showed a tensile strength of 74 MPa
for the "as prepared" (heat-treated at 200.degree. C.)
small-diameter carbon nanotube film prepared with 10M nitric acid,
a greater than seven-fold increase in tensile strength over the
control film, which had a tensile strength of 10 MPa. The results
also showed an initial tensile modulus of 5.0 GPa for the
heat-treated small-diameter carbon nanotube film prepared with 10M
nitric acid, a greater than six-fold increase in initial tensile
modulus over the control film, which had an initial tensile modulus
of 0.8 MPa. Mechanical tests on films made with small-diameter
carbon nanotubes processed with 6M and 10M nitric acid also showed
that the 200.degree. C. heat-treatment increased the tensile
strength and tensile modulus over the films that were only dried.
The heat-treatment did not substantially change the
elongation-to-break or the electrical conductivity.
[0061] In-plane dc electrical conductivity of the films was
measured at room temperature by the four-probe method. Conductivity
is quantified in units of Siemens (S) per unit length, such as S/cm
or S/m. (Note: Siemen=mho=1/ohm.) Resistivity, the inverse of
conductivity, is quantified in units of ohm-length, such as
ohm-cm.
[0062] The electrical conductivity results for the small-diameter
carbon nanotube films are also given in Table 1 below. The results
show that the in-plane dc electrical conductivity for the "as
prepared" small-diameter carbon nanotube films made with nitric
acid treatment is on the same order of magnitude as the control
film made without nitric acid treatment. The "as prepared" film
made with nanotubes treated in 10 M nitric acid had a conductivity
of 1.2.times.10.sup.4 Siemens/m versus 3.times.10.sup.4 Siemens/m
for the control film. Although not meant to be held by theory, the
somewhat lower conductivity for the carbon nanotube films made with
nitric acid-treated nanotubes could be attributed to acid-induced
creation of defects in the small-diameter carbon nanotube structure
and the presence of functional groups at the defect sites. The
presence of functional groups on the acid-treated carbon nanotubes
provides for enhanced inter-tube interaction, as well as for the
possibility of crosslinking. The somewhat lower conductivity could
be also due to reduced catalytic impurity, as well as a possible
reduction in metallic nanotube population. TABLE-US-00001 TABLE 1
Mechanical and electrical properties of various carbon nanotube
films Initial Strain-to- Tensile Tensile Failure Small-Diameter
Carbon Strength Modulus Elongation Conductiv- Nanotube Films (MPa)
(GPa) (%) ity (S/m) Control 10 .+-. 2 0.8 .+-. 0.1 5.6 .+-. 0.3 3.0
.times. 10.sup.4 Prepared in 3 M HNO.sub.3 16 .+-. 1 1.4 .+-. 0.1
1.4 .+-. 0.2 2.3 .times. 10.sup.4 Prepared Before 200.degree. 56
1.5 4.8 3.1 .times. 10.sup.4 in 6 M C. heat treat- HNO.sub.3 ment
After 200.degree. 71 .+-. 5 2.9 .+-. 0.2 3.4 .+-. 0.2 2.4 .times.
10.sup.4 C. heat treat- ment Prepared Before 200.degree. 68 4.5 3.0
1.3 .times. 10.sup.4 in 10 M C. heat treat- HNO.sub.3 ment After
200.degree. 74 .+-. 2 5.0 .+-. 0.2 3.0 .+-. 0.1 1.2 .times.
10.sup.4 C. heat treat- ment
EXAMPLE 3
[0063] Dynamic mechanical analysis (DMA) as a function of
temperature was done on the "as prepared" (200.degree. C.
heat-treated) small-diameter carbon nanotube films, prepared in
Example 1, at a frequency of 10 Hz and at 0.1% dynamic strain using
a RSA III Rheometrics Solids Analyzer. During the dynamic test, the
static force adjusted automatically to 40% larger than dynamic
force.
[0064] The results show a fairly constant E' storage modulus for
the "as prepared" small-diameter carbon nanotube films throughout
the temperature range of about 30.degree. C. to about 210.degree.
C., with a slight increase in storage modulus above 150.degree. C.
The Tan .delta. values were very low (i.e., about 0.02), indicating
that the films are fairly elastic throughout the 30.degree. C. to
about 210.degree. C. temperature range. A plot of the dynamic
mechanical behavior for the "as prepared" film made with
small-diameter carbon nanotubes processed in 6 M nitric acid is
shown in FIG. 1.
EXAMPLE 4
[0065] The effects of heat treating on the films of small-diameter
carbon nanotubes were studied by Scanning Electron Microscopy
(SEM). The "as prepared" films made with small-diameter carbon
nanotubes treated in nitric acid were compared to the same films
after an additional high temperature heat treatment. "High
temperature heat-treated" films were prepared from the
"as-prepared" films by heating the "as-prepared" films in nitrogen
in a TGA to 900.degree. C. at a rate of 20.degree. C./minute and
then holding for 2 minutes at 900.degree. C.
[0066] Scanning electron micrographs of the "as-prepared"
heat-treated films are shown in FIGS. 2A, 2B and 2C, which
correspond to films prepared from small-diameter carbon nanotubes
treated in 3M, 6M and 10M nitric acid, respectively. Scanning
electron micrographs of the "high temperature heat treated" films
are shown in FIGS. 2D, 2E and 2F, made by heat-treating the "as
prepared" films shown in FIGS. 2A, 2B and 2C, prepared with
small-diameter carbon nanotubes processed in 3M, 6M and 10M nitric
acid, respectively, at 900.degree. C. as described above.
[0067] FIGS. 2A and 2D are SEMs showing nanotube ropes on the
surface of the "as prepared" heat-treated and "high temperature
heat-treated" films, respectively, in which the nanotubes were
processed in 3M nitric acid. Ropes are not seen on the film
surfaces of the "as prepared" heat-treated films made with
nanotubes processed in 6 M nitric acid (See FIG. 2B) and 10 M
nitric acid (FIG. 2C). However, after the "as prepared" films were
heat-treated at 900.degree. C., the nanotube ropes can be clearly
seen on both the surfaces of the "high temperature heat-treated"
films. FIG. 2E shows a SEM of the "high temperature heat-treated"
film made from the "as prepared" film, shown in FIG. 2B, which was
made from nanotubes processed in 6M nitric acid. FIG. 2F shows a
SEM of the "high temperature heat-treated" film made from the "as
prepared" film, shown in FIG. 2C, which was made from nanotubes
processed in 10M nitric acid. Although not meant to be held by
theory, the material surrounding the nanotube ropes in films made
with 6M and 10M-nitric acid treated nanotubes is attributed, at
least in part, to amorphous carbon resulting from acid
treatment-induced nanotube decomposition. The 900.degree. C.
heat-treatment of the films made with 6M nitric acid- and 10M
nitric acid-treated nanotubes appears to have removed the
continuous phase attributed to amorphous carbon from the surface of
the films so that the nanotube bundles can be clearly observed.
These observations support the theory that the films prepared from
nitric acid-treated small-diameter carbon nanotubes have a
composite structure comprising nanotube ropes embedded in a
continuous matrix phase attributed to amorphous carbon and
polycyclic aromatic material, which was expected to have formed, at
least in part, from the decomposition of small-diameter carbon
nanotubes. The appearance of an amorphous phase is consistent with
the disappearance of small diameter nanotubes as the concentration
of nitric acid treatment was increased.
EXAMPLE 5
[0068] Raman studies were done on the "as prepared" and "high
temperature heat-treated" films comprising with small-diameter
carbon nanotubes processed in various concentrations of nitric
acid, as prepared in Example 1. "High temperature heat-treated"
films were prepared by heating the "as-prepared" films in nitrogen
in a thermogravimetric analyzer (TGA) at rate of 20.degree.
C./minute to 900.degree. C. and holding at 900.degree. C. for two
minutes.
[0069] Raman spectra were collected using a Holoprobe Research 785
Raman Microscope made by Kaiser Optical System, Inc. with an
incident laser wavelength of 785 nm. Diameter determinations were
made using the Raman radial breathing modes (RBM) peaks in the
range of about of 150 to 350 cm.sup.-1 that correlate with carbon
nanotube diameters. The nanotube diameters were calculated based on
the RBM band peak position of the control film using the empirical
equation .omega..sub.RBM=238/d.sup.0.93. Metallic carbon nanotube
configurations cannot be observed using a 785-nm laser wavelength,
thus the present Raman observations and analyses are limited to
semiconducting tubes that are observable with the 785-nm incident
wavelength.
[0070] No RBM peaks in the Raman spectra were observed for "as
produced" films.
[0071] Raman RBM peak spectra for the 900.degree. C. "high
temperature heat-treated" films are shown in FIGS. 3A-D. FIG. 3A
shows a dominant RBM peak at about 267 cm.sup.-1 for the control
film, while the dominant RBM peak shifts to lower frequency for
samples made with increasing nitric acid concentrations, consistent
with the destruction of smaller diameter tubes and survival of
larger diameter tubes. The stress-induced curvature around the
circumference of the nanotubes makes small-diameter nanotubes more
reactive and liable to chemical attack than larger diameter
nanotubes.
[0072] The changes in the diameter distribution of the nanotubes
resulting from the oxidative nitric acid treatment were quantified
by resolving the spectra in FIGS. 3A-D, using a Lorentz line shape
fitting routine. Using a 785-nm incident laser wavelength, six
different diameters in the range of 0.88-1.19 nm were identified in
these films. The RBM peaks for carbon nanotube films made with
nitric acid-treated nanotubes were also up-shifted by 1-3 cm.sup.-1
compared to the corresponding peaks for the control film processed
without nitric acid.
[0073] The relative fraction of a particular diameter of nanotube
was determined by taking the ratio of the area of the corresponding
peak to the sum of the area of all the RBM peaks. FIGS. 4A, 4B, 4C,
4D, 4E and 4F show plots of the relative fractions of
small-diameter carbon nanotubes having diameters of 1.19 nm, 1.11
nm, 1.07 nm, 1.02 nm, 0.89 nm and 0.88 nm, respectively, in "high
temperature heat-treated" small-diameter carbon nanotubes films as
a function of nitric acid concentration. FIGS. 4E and 4F show that
the relative fractions of 0.88-nm and 0.89-nm small-diameter
nanotubes, respectively, significantly decreased with increasing
nitric acid concentration. That the percentage of these
small-diameter nanotubes decreased from 70% in the control film to
less than 20% in samples processed in 10 M HNO.sub.3, supports the
theory of selective degradation of the small-diameter carbon
nanotubes by nitric acid. The destruction of small-diameter
nanotubes, corresponds to an increase of the relative fraction of
the 1.19-nm and 1.11-nm diameter small-diameter carbon nanotubes,
the amount of which increased, generally monotonically, with
increasing nitric acid concentration.
[0074] The chemical reactivity of small-diameter carbon nanotubes
with respect to their diameter is further demonstrated by the
relative population changes of 1.02 and 1.07-nm diameter nanotubes.
The relative fraction of 1.07-nm diameter nanotubes was fairly
constant when processed in 6 M and 10 M HNO.sub.3, however, the
1.02-nm diameter fraction decreased when processed in 6 M nitric
acid and further decreased when processed in 10 M nitric acid. This
suggests that the 1.07-nm diameter nanotubes have higher resistance
to oxidative degradation than the 1.02-nm diameter nanotubes.
Although the diameter difference between these two tubes is only
5%, there is a measurable difference between their oxidative
resistances. In FIGS. 4A-F, the circle symbol in the plots
represents the relative fraction of small-diameter carbon nanotubes
in a film sample where the nanotubes were processed in 6 M nitric
acid and the film was subjected to a more severe heat-treatment of
900.degree. C. for 30 minutes and 700.degree. C. for 4.5 hrs in
N.sub.2. The similarity in the relative populations between the
high temperature treatment (squares) and the more severe heat
treatment (circles) suggests that little or no additional nanotube
degradation occurred by subjecting the nanotubes to the more severe
heat treatment. These results are consistent with the higher
propensity for the smaller diameter carbon nanotubes to be attacked
and damaged first by the nitric acid.
EXAMPLE 6
[0075] The "as-prepared" films made with small-diameter carbon
nanotubes processed in 3M, 6M and 10M nitric acid as described in
Example 1 were analyzed by thermogravimetric analysis (TGA) using a
TA Instruments TGA2950. The samples were heated to 900.degree. C.
at 10.degree. C./minute in nitrogen. Ash residues were 80, 65, 57,
and 52 wt % for the small-diameter nanotube control, and the
nanotubes processed in 3M, 6M, and 10M nitric acid, respectively.
The results are consistent with the production of more amorphous
carbon with increasing nitric acid concentration.
[0076] All of the compositions and methods disclosed and claimed
herein can be made and executed without undue experimentation in
light of the present disclosure. While the compositions and methods
of this invention have been described in terms of preferred
embodiments, it will be apparent to those of skill in the art that
variations may be applied to the compositions and methods and in
the steps or in the sequence of steps of the method described
herein without departing from the concept, spirit and scope of the
invention. More specifically, it will be apparent that certain
agents which are chemically related may be substituted for the
agents described herein while the same or similar results would be
achieved. All such similar substitutes and modifications apparent
to those skilled in the art are deemed to be within the spirit,
scope and concept of the invention as defined by the appended
claims.
* * * * *