U.S. patent application number 10/729716 was filed with the patent office on 2004-12-30 for synthesis of coiled carbon nanotubes by microwave chemical vapor deposition.
Invention is credited to Mukhopakhyay, Kingsuk, Varadan, Vijay, Xie, Jining, Yadev, Jitendra.
Application Number | 20040265212 10/729716 |
Document ID | / |
Family ID | 32507816 |
Filed Date | 2004-12-30 |
United States Patent
Application |
20040265212 |
Kind Code |
A1 |
Varadan, Vijay ; et
al. |
December 30, 2004 |
Synthesis of coiled carbon nanotubes by microwave chemical vapor
deposition
Abstract
The present invention provides a coiled carbon nanotube and a
method for its manufacture. The coiled carbon nanotube comprises a
specific non-hexagonal/hexagonal carbon ring ratio, a specific
pitch, and a specific diameter. The invention employs a microwave
chemical vapor disposition system with novel processing conditions
and specialized catalysts to synthesize the coiled carbon
nanotubes.
Inventors: |
Varadan, Vijay; (State
College, PA) ; Xie, Jining; (State College, PA)
; Mukhopakhyay, Kingsuk; (Kanpur, IN) ; Yadev,
Jitendra; (Kanpur, IN) |
Correspondence
Address: |
Matthew T. Rogers, Esq.
McQuaide Blasko
811 University Drive
State College
PA
16801
US
|
Family ID: |
32507816 |
Appl. No.: |
10/729716 |
Filed: |
December 5, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60431888 |
Dec 6, 2002 |
|
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Current U.S.
Class: |
423/447.3 |
Current CPC
Class: |
D01F 9/12 20130101; B82Y
30/00 20130101 |
Class at
Publication: |
423/447.3 |
International
Class: |
D01F 009/12 |
Claims
Having thus described the invention, what is claimed is:
1. A coiled carbon nanotube having a non-hexagonal/hexagonal carbon
ring ratio in the range of 0.1:1 to 1:1.
2. The coiled carbon nanotube of claim 1 wherein the
non-hexagonal/hexagonal carbon ring ratio is 0.1:1.
3. The coiled carbon nanotube of claim 1 wherein the
non-hexagonal/hexagonal carbon ring ratio is 1:1.
4. The coiled carbon nanotube of claim 1 wherein the nanotube
comprises a substantially uniform distance between coils throughout
its length.
5. The coiled carbon nanotube of claim 1 wherein the nanotube
comprises a substantially uniform diameter throughout its
length.
6. The coiled carbon nanotube of claim 1 wherein the nanotube
comprises a substantially uniform distance between coils and
diameter throughout its length
7. The coiled carbon nanotube of claim 1 wherein the nanotube
comprises a diameter of less than 1000 nm.
8. The coiled carbon nanotube of claim 1 wherein the nanotube
comprises a diameter of less than 100 nm.
9. The coiled carbon nanotube of claim 1 wherein the nanotube
comprises a distance between coils of less than 1000 nm.
10. The coiled carbon nanotube of claim 1 wherein the nanotube
comprises a distance between coils of less than 200 nm.
11. The coiled carbon nanotube of claim 1 wherein the nanotube
comprises a diameter of less than 1000 nm and a distance between
coils of less than 1000 nm.
12. The coiled carbon nanotube of claim 1 wherein the nanotube
comprises a diameter of less than 100 nm and a distance between
coils of less than 200 nm.
13. A coiled carbon nanotube having a substantially uniform
diameter throughout its length.
14. The coiled carbon nanotube of claim 13 wherein the nanotube
comprises a diameter of less than 1000 nm.
15. The coiled carbon nanotube of claim 13 wherein the nanotube
comprises a diameter of less than 100 nm.
16. A coiled carbon nanotube wherein the nanotube comprises a
substantially uniform distance between coils throughout its
length.
17. The coiled carbon nanotube of claim 16 wherein the nanotube
comprises a distance between coils of less than 200 nm.
18. The coiled carbon nanotube of claim 16 wherein the nanotube
comprises a distance between coils of less than 1000 nm.
19. A coiled carbon nanotube having a substantially uniform
diameter and a substantially uniform distance between coils
throughout its length.
20. The coiled carbon nanotube of claim 19 wherein the nanotube
comprises a diameter of less than 1000 nm and a distance between
coils of less than 1000 nm.
21. A method of manufacturing coiled carbon nanotubes, comprising:
placing a supported metal catalyst inside of a reaction chamber;
creating a microwave field inside said reaction chamber;
introducing a hydrocarbon source gas into said reaction chamber;
and reacting for a time and at a temperature sufficient to form
said coiled carbon nanotubes.
22. The method of claim 21, wherein an inert gas is introduced into
said reaction chamber.
23. The method of claim 21, wherein said source gas is
acetylene.
24. The method of claim 21, wherein said metal catalyst comprises a
metal selected from the group consisting of iron, nickel, cobalt,
and vanadium.
25. The method of claim 21, wherein said catalyst support is
selected from the group consisting of silica, zeolite, and
magnesium carbonate.
26. The method of claim 21, wherein said metal catalyst is iron and
said catalyst support is magnesium carbonate.
27. The method of claim 21, wherein said metal catalyst is iron and
said catalyst support is silica.
28. The method of claim 21, wherein said metal catalyst is nickel
and said catalyst support is zeolite.
29. The method of claim 21, further comprising the use of a stirrer
to make said microwave field uniform.
30. The method of claim 21, further comprising a stub tuner.
31. The method of claim 30, further comprising a port circulator
for controlling said stub tuner.
32. The method of claim 21, further comprising a circulating
chiller.
33. A method for manufacturing coiled carbon nanotubes, comprising:
placing a supported metal catalyst inside of a reaction chamber;
creating a microwave field inside said reaction chamber;
introducing a hydrocarbon source gas into said reaction chamber;
using a feedback system to control the temperature inside said
reaction chamber and the flow rate of said hydrocarbon source gas;
and reacting for a time and at a temperature sufficient to form
said coiled carbon nanotubes.
34. The method of claim 33, wherein an inert gas is introduced into
said reaction chamber.
35. The method of claim 33, wherein said source gas is
acetylene.
36. The method of claim 33, wherein said metal catalyst comprises a
metal selected from the group consisting of iron, nickel, cobalt,
and vanadium.
37. The method of claim 33, wherein said catalyst support is
selected from the group consisting of silica, zeolite, and
magnesium carbonate.
38. The method of claim 33, wherein said metal catalyst is iron and
said catalyst support is magnesium carbonate.
39. The method of claim 33, wherein said metal catalyst is iron and
said catalyst support is silica.
40. The method of claim 33, wherein said metal catalyst is nickel
and said catalyst support is zeolite.
41. The method of claim 33, further comprising the use of a stirrer
to make said microwave field uniform.
42. The method of claim 33, further comprising a stub tuner.
43. The method of claim 42, further comprising a port circulator
for controlling said stub tuner.
44. The method of claim 33, further comprising a circulating
chiller.
45. The method of claim 33, wherein said feedback system comprises:
a pyrometer; a switching power supply; a computer; a master flow
controller; and a mass flow controller.
46. A coiled carbon nanotube produced by the process of: placing a
supported metal catalyst inside of a reaction chamber; creating a
microwave field inside said reaction chamber; introducing a
hydrocarbon source gas into said reaction chamber; and reacting for
a time and at a temperature sufficient to form said coiled carbon
nanotubes.
47. The coiled carbon nanotube of claim 46, wherein argon is
introduced into said reaction chamber.
48. The coiled carbon nanotube of claim 46, wherein said source gas
is acetylene.
49. The coiled carbon nanotube of claim 46, wherein said metal
catalyst comprises a metal selected from the group consisting of
iron, nickel, cobalt, and vanadium.
50. The coiled carbon nanotube of claim 46, wherein said catalyst
support is selected from the group consisting of silica, zeolite,
and magnesium carbonate.
51. The coiled carbon nanotube of claim 46, wherein said metal
catalyst is iron and said catalyst support magnesium carbonate.
52. The coiled carbon nanotube of claim 46, wherein said metal
catalyst is iron and said catalyst support is silica.
53. The coiled carbon nanotube of claim 46, wherein said metal
catalyst is nickel and said catalyst support is zeolite.
54. The coiled carbon nanotube of claim 46, further comprising the
use of a stirrer to make said microwave field uniform.
55. The coiled carbon nanotube of claim 46, further comprising a
stub tuner.
56. The coiled carbon nanotube of claim 55, further comprising a
port circulator for controling said stub tuner.
57. The coiled carbon nanotube of claim 46, further comprising a
circulating chiller.
58. The coiled carbon nanotube of claim 46, further comprising the
use of a feedback system to control the temperature inside said
reaction chamber and the flow rate of said hydrocarbon source
gas.
59. A coiled carbon nanotube produced by the process of claim 58,
wherein said feedback system comprises: a pyrometer; a switching
power supply; a computer; a master flow controller; and a mass flow
controller.
60. An article of manufacture produced by the process of: placing a
supported metal catalyst inside of a reaction chamber; creating a
microwave field inside said reaction chamber; introducing a
hydrocarbon source gas into said reaction chamber; and reacting for
a time and at a temperature sufficient to form said coiled carbon
nanotubes.
61. The article of manufacture of claim 60, wherein argon is
introduced into said reaction chamber.
62. The article of manufacture of claim 60, wherein said source gas
is acetylene.
63. The article of manufacture of claim 60, wherein said metal
catalyst comprises a metal selected from the group consisting of
iron, nickel, cobalt, and vanadium.
64. The article of manufacture of claim 60, wherein said catalyst
support is selected from the group consisting of silica, zeolite,
and magnesium carbonate.
65. The article of manufacture of claim 60, wherein said metal
catalyst is iron and said catalyst support is magnesium
carbonate.
66. The article of manufacture of claim 60, wherein said metal
catalyst is iron and said catalyst support is silica.
67. The article of manufacture of claim 60, wherein said metal
catalyst is nickel and said catalyst support is zeolite.
68. The article of manufacture of claim 60, further comprising the
use of a stirrer to make said microwave field uniform.
69. The article of manufacture of claim 60, further comprising a
stub tuner.
70. The article of manufacture of claim 69, further comprising a
port circulator for controlling said stub tuner.
71. The article of manufacture of claim 60, further comprising a
circulating chiller.
72. The article of manufacture of claim 60, further comprising the
use of a feedback system for controling the temperature inside said
reaction chamber and the flow rate of said hydrocarbon source
gas.
73. The article of manufacture of claim 72, wherein said feedback
system comprises: a pyrometer; a switching power supply; a
computer; a master flow controller; and a mass flow controller.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application No. 60/431,888 filed Dec. 6, 2002.
FIELD OF THE INVENTION
[0002] The present invention relates to the field of carbon
nanotubes. More particularly, the invention relates to
mass-produced coiled carbon nanotubes and to a method for their
synthesis using microwave chemical vapor deposition (CVD).
BACKGROUND OF THE INVENTION
[0003] Prior art methods of catalytic chemical vapor deposition
(CCVD) have been used to prepare carbon fibers/tubules with
different morphologies in different sizes. Among them, fibrous
carbon materials with coil morphology have especially attracted a
wide interest. This interest relates not only to academic research
interests but also to potential versatile commercial applications.
Motojima et al. first reported regular coiled carbon fibers in
micron size by CCVD with thiopene vapor as an impurity gas, named
carbon microcoils [Motojima S. et al., "Preperation of coiled
carbon fiders by pyrolysis of acetylene using a Ni catalyst and
sulfur or phosphorus compound impurity" Appl. Phys. Lett. 62
2322-3, 1993]. In this paper, the growth mechanism of carbon
microcoils was postulated as resulting from the anisotropic
properties of the catalyst for carbon deposition. Such carbon
microcoils found applications in EM absorbers, micro springs, etc.
Subsequently, Pan et al. reported the synthesis of carbon tubule
nanocoils using iron-coated indium tin oxide as a catalyst [Pan L.
et al., "Growth and density control of carbon tubule nanocoils
using catalyst of iron compounds" J. Mater. Res. 17 145-8, 2001].
Recently, Varadan et al. synthesized carbon nanocoil fibers by
using Ni particles [V. K. Varadan, J. Xie, "Synthesis of carbon
nanocoils by microwave CVD" Smart Materials and Structures, 11
728-34, 2002]. Such nanocoiled carbon fibers/tubules have been
shown to be good candidates for commercial applications, especially
field emission display technology [Pan L. et al., "Field emission
properties of carbon tubule nanocoils" Jpn. J. Appl. Phys. 40
L235-7, 2001]. Note that all of the coiled carbon fibers/tubules
mentioned above are prepared using amorphous carbon materials.
[0004] In contrast to the conventional method of producing carbon
nanocoils, the present invention relates to coiled carbon
nanotubes. Since the discovery of carbon nanotubes by Iijima,
coiled carbon nanotubes have become objects of widespread interest.
The primary difference between coiled carbon nanotubes and carbon
nanocoils lies in the crystalline graphitic structures of the
nanotubes. Also, the diameter of coiled carbon nanotubes (<100
nm) is much smaller than the diameter of carbon nanocoils. The coil
morphology, together with as well as the extraordinary properties
of nanotubes, make coiled carbon nanotubes a promising material for
hydrogen storage, field emission, EM absorber and nanotechnology
applications in general.
[0005] Nanotubes prepared from CCVD methods tend to be produced in
straight or randomly curled morphologies. For example, accidentally
coiled carbon nanotubes were reported in extremely low yield by
Hernadi et al. ["Fe-catalyzed carbon nanotube formation" Carbon 34
1249-57, 1996]. Also, Amelinckx et al. reported a formation
mechanism for catalytically grown helix-shaped graphite nanotubes
["A formation mechanism for catalytically grown helix-shaped
graphite nanotubes" Science 265 635-9, 1994]. According to their
results, the coil morphology of carbon nanotubes is due to the
mismatch between the extrusion velocity and the rate of carbon
deposition.
[0006] Thus far the synthesis of coiled CNT has been reported as a
byproduct of regular CNT synthesis. It would be more accurate to
say that coiled CNTs have been found under microscope by accident
because there was no control for the synthesis of coiled CNT. To
date, neither specific process conditions nor special catalyst
compositions have been identified for the effective and consistent
synthesis of coiled CNTs. Despite the early indications of the
usefulness of coiled CNTs, there continues to be a strong need for
the effective synthesis of regular coiled carbon nanotubes.
[0007] CNTs, along with coiled carbon nanotubes, are the most
promising materials anticipated to impact future nanoscience and
nanotechnology. Their unique structural and electronic properties
have generated great interest for use in a broad range of
nanodevices. A significant amount of work has been done in the past
decade to reveal the unique structural, electrical, mechanical,
electromechanical and chemical properties of carbon nanotubes and
to explore the key applications of this novel material. Most of
these applications will require efficient fabrication methods
capable of producing pure CNTs, including coiled carbon nanotubes,
to meet device requirements. Another advantage of coiled carbon
nanotubes is that they are capable of forming in situ
semiconductor-metallic or semiconductor-insulator junctions which
one can utilize for the fabrication of nanodevices. Coiled carbon
nanotubes also have an greater surface area than CNTS which
increases their functionalization. Accordingly it is an object of
the present invention to provide effective and efficient methods of
fabricating CNTs, especially coiled carbon nanotubes, to enable the
commercialization of applications using such.
SUMMARY OF THE INVENTION
[0008] The invention is a method for synthesizing coiled carbon
nanotubes using a microwave CVD system with inventive processing
conditions and specialized catalyst(s). Preferred conditions
include the use of acetylene as a hydrocarbon source gas in a
microwave CVD system without using an impurity gas and using an
iron supported on magnesium carbonate as a catalyst. The invention
also includes the resulting coiled carbon nanotubes.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 shows a schematic diagram of a conventional thermal
filament CVD system.
[0010] FIG. 2 shows a schematic diagram of a microwave CVD system
used in the present invention.
[0011] FIG. 3 shows an illustration of a coiled carbon
nanotube.
[0012] FIG. 4 shows a flow diagram for the flow control
systems.
[0013] FIG. 5 shows a SEM micrograph of coiled nanotube synthesized
using a microwave CVD system in accordance with the present
invention.
[0014] FIG. 6 shows a TEM image of coiled nanotubes obtained from a
microwave CVD system in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0015] Referring now to FIG. 1, an apparatus designed to perform
this method is designated in its entirety by the reference numeral
8. The method generally involves placing a catalyst 3 inside of a
reaction chamber 1 and then heating up the reaction chamber 1. In
one embodiment of the method, a quartz reaction tube 2 is used to
transport the catalyst in and out of the reaction chamber 1. The
temperature is monitored by a thermocouple 7. When the temperature
inside has reached a certain level (usually 700.degree. C.) a
hydrocarbon source gas 4 (such as acetylene) is pumped into the
reaction chamber 1 through a intake valve. The hydrocarbon source
gas 4 is then broken down into its elements which interact with the
catalyst 3 resulting in the growth of carbon nanotubes. The exhaust
gas 6 is removed from the reaction chamber 1. In one embodiment of
this method, argon 5 is pumped into the reaction chamber 1 for
purging.
[0016] FIG. 2 shows a microwave CVD system 9 according to a
preferred embodiment of the present invention. Until now, this
method has involved the use of a furnace to heat the reaction
chamber 33. The present invention uses a magnetron 10 in place of a
furnace. Although a magnetron 10 capable of producing 750 W is
preferred, any commercially-available magnetron may be used. The
magnetron 10 creates a microwave field inside the reaction chamber
33.
[0017] In the preferred method of the invention, a known amount of
the catalyst and catalyst support 15 are dispersed onto the
substrate 34. The substrate 34 is then loaded into the reaction
chamber 33. In this embodiment, the substrate 34 is loaded in and
out of the reaction chamber 33 in a quartz container 19. The
magnetron 10 is then switched on to heat the substrate 34 to the
reaction temperature. In this embodiment, the reaction temperature
is set to 700.degree. C. During heating, an inert gas 22, at an
optimized flow rate, can be used for purging, although the use of
an inert gas 22 is not required for the present invention. When the
reaction temperature is reached, a hydrocarbon source gas 21 is
introduced into the reaction chamber at an optimal flow rate. The
gases are pumped into the reaction chamber 33 through the gas inlet
17 and blown onto the substrate 34 using a quartz gas distributor
16. Exhaust gas leaves the chamber through the gas outlet 18. In
this embodiment the reaction is set to 30 minutes. After the
reaction, the resulting product is scratched from the substrate.
Previous experiments by Varadan, V. et al. ["Synthesis of carbon
nanocoils by microwave CVD" Smart. Mater. Struct. 11 (2002)
728-734] did not involve the production of carbon nanocoils, and
utilized different processing materials and conditions than
described in the present invention.
[0018] The advantage of using a microwave field is that it is
uniform throughout the reaction chamber 33. This uniform field
allows for a higher quantity of coiled carbon nanotubes with
consistent properties to be produced during each reaction. In
addition, a microwave field can be instantly turned off whereas a
reaction chamber heated by a furnace must be allowed to cool to
room temperature before any nanotubes can be extracted. This
dramatically reduces the lag time between production rounds. In the
preferred embodiment of the invention, a commercially available
stub tuner 12 is used prevent any reflected power from flowing into
the magnetron 10. In one example, the inventors manually adjusted
the stub tuner 12. In another example, a commercially available
three-port circulator 13 was used to automatically adjust the stub
tuner 12. In addition, the invention may comprise a circulating
chiller 14 which cools the magnetron 10 and therefore extends its
life. The invention may further comprise a stirrer 27, which
assists in making the microwave field uniform. The stirrer 27 is
driven by a motor 28.
[0019] The reaction chamber 33 can be made from any number of
materials without departing from the scope of the present
invention. For instance, in one embodiment the reaction chamber is
constructed out of aluminum. In a further embodiment, the reaction
chamber is made of steel. In the preferred embodiment, the reaction
chamber is a cylinder. The inventors used two reaction chambers
manufactured by HVS Technologies. The smaller reaction chamber had
dimensions of 14" in length and 5.75" in diameter. The larger
reaction chamber had dimensions of 70" in length and 35" in
diameter.
[0020] Although the catalyst can be made from various materials
without departing from the scope of the present invention, a
preferred catalyst is iron. Iron is preferred because it produces
the highest yield of coiled carbon nanotubes. Alternatively, other
transition metal catalysts can be used; including combinations of
transition metals (e.g., bimetallic catalysts). It is important to
note here that the indium-tin-iron catalyst disclosed in U.S. Pat.
No. 6,583,085 to Nakayama et al. is not preferred for this
invention. The presence of tin in the catalyst would cause the
catalyst to spark when placed in the microwave field. In addition,
the indium-tin-iron catalyst would not be preferred does not easily
absorb the microwaves.
[0021] The specific support used in the method of the present
invention is critical. The support must contain pores giving rise
to the growth of coiled carbon nanotubes according to the invention
as opposed to other formations, such as fibers. The supports must
also be able to easily absorb microwaves. Some non-limiting
examples include silica, zeolite, and magnesium carbonate
(preferred). Preferred pore sizes lie in the range of 0.1 to 10 nm
with a surface area of 250-300 m.times.m/g. The following are three
examples of catalyst supports and catalysts with which they were
combined (being just three examples of "supported metal
catalyst").
EXAMPLES
[0022] Catalyst and Catalyst Support #1
[0023] Iron nitrate and magnesium carbonate were weighed 1:1 weight
ratio. Iron nitrate was dissolved in water and the resulting
solution was added to magnesium carbonate, followed by continuous
stirring to obtain a semi-solid mixture. The semi-solid mixture was
kept inside an overnight at 500.degree. C. After allowing the
mixture to cool to room temperature, the resulting brown color
solid was powdered. While the pole size of the magnesium carbonated
varied somewhat throughout its surface, a majority were 10 nm in
diameter.
[0024] Catalyst and Catalyst Support #2
[0025] Iron nitrate and silica were weighed 1:1 weight ratio. Iron
nitrate was dissolved in water and the resulting solution was added
to silica, followed by continuous stirring to obtain a semi-solid
mixture. The semi-solid mixture was kept inside an oven at
120.degree. C. overnight. After allowing the mixture to cool to
room temperature, the resulting brown color solid was powdered. As
a porous substance, the pore sizes for silica varied throughout its
length.
[0026] Catalyst and Catalyst Support #3
[0027] The inventors used "hydrothermal processing" to manufacture
zeolite (although commercial grade zeolite may be used). The
hydrothermal processing method is described in Cundy, C. et al.
["The Hydrothermal Synthesis of Zeolites: History and Development
from the Earliest Days to the Present Time" Chem. Rev. 2003, 103,
663-701] Nickel acetate was dissolved in water and a proper amount
of zeolite was added into the solution with a Ni percentage in
zeolite of 14.5 wt %. The gel solution was stirred and kept in an
oven at 120.degree. C. overnight. After drying, the solid was
crushed into a fine powder. While the pore size for zeolite varied
throughout its surface, the majority were 1 nm in diameter.
[0028] The substrate 34 is made of silicon carbide. Although the
hydrocarbon source gas 21 can be any gas containing carbon, in the
preferred embodiment the hydrocarbon source gas is acetylene. The
inventors found that the optimal flow rate for acetylene is 30 sccm
for the smaller reaction chamber (14".times.5.75") and 600 sccm for
the larger one (70".times.35"). Other non-limiting examples include
methane, ethane and propane.
[0029] When an inert gas 22 is also used as described herein,
helium is preferred, although any inert gas can be used (such as
argon). The inventors found the optimal flow rate for helium is 190
sccm for the smaller reaction chamber and 3500 sccm for the larger
one. It is important to mention here that for the synthesis of
coiled carbon nanotubes by a conventional CCVD method, the presence
of an impurity gas (e.g. thiophene) is necessary, while in the
microwave CVD method, no impurity gas is required.
[0030] To optimize the processing conditions, the temperature of
the reaction chamber 33 and the gas flow rates can be monitored by
a computer 26. FIG. 4 is a software flow chart for the flow control
systems. The temperature of the reaction chamber 33 is monitored by
a pyrometer 23, which in the preferred embodiment is an optical
pyrometer 25. The temperature readings taken by the pyrometer 23
are transmitted to a computer 26. The computer 26 then compares the
temperature of the reaction chamber 33 with the set temperature for
processing (preferably 700.degree. C.). The computer 26 then
controls the switching power supply 11 which in turn controls the
magnetron 10. If the reaction chamber temperature is too low, the
computer 26 will tell the switching power supply 11 to turn on the
magnetron 10 and increase the temperature. If the reaction chamber
temperature is too low, the computer 26 will tell the switching
power supply 11 to turn off the magnetron 10. The computer 26 also
communicates with the master flow controller 20 which controls the
mass flow controllers 24. The mass flow controllers 24 control the
flow rates of the inert 22 and hydrocarbon source gas 21.
[0031] An illustration of a coiled carbon nanotube is shown in FIG.
3. The distance between the coils is substantially uniform
throughout the length of each coiled carbon nanotube. In addition,
the diameter of each coiled carbon nanotube will also be
substantially uniform. The coiled carbon nanotube 29 is composed of
carbon rings in the shapes of pentagons 32, hexagons 30, and
heptagons 31. Depositing the various shapes in specific locations
along the surface of the carbon nanotube causes the carbon nanotube
to assume a coiled shape. Depending upon the distance between the
coils, the non-hexagonal/hexagonal ratio of the carbon rings ranges
from 0.1:1 to 1:1. A non-hexagonal/hexagonal ratio of 0.1 produces
a "loose" coil with a large pitch. A non-hexagonal/hexagonal ratio
of 1:1 produces a "tight" coil with a small pitch. The exact
morphology of coiled carbon nanotubes will depend on the
catalyst/catalyst support that is used and the conditions of the
microwave CVD.
[0032] A scanning electron microscope (SEM 3000N manufactured by
Hitachi) was used to investigate the morphology of the coiled
carbon nanotubes. Due to the conducting property of carbon
nanotubes, no gold coating is necessary for SEM operation. A
transmission electron microscope (TEM 420T manufactured by Philips)
was used to study the nanostructure of the coiled carbon nanotubes.
TEM samples were prepared by ultrasonic vibration of a small amount
of material in acetone followed by dropping on a TEM grid (Lacey
carbon film on 300 mesh copper grid, Electron Microscopy
Science).
[0033] In the SEM micrograph shown in FIG. 5, the coiled morphology
for the microwave CVD samples is clearly revealed. From the TEM
micrograph shown in FIG. 6, the hollow structure as well as coiled
morphology for the microwave CVD samples was confirmed.
[0034] Under optimized conditions, the inventors have achieved
.about.90% yield of coiled carbon nanotubes. While the invention
has been particularly shown and described with reference to
preferred embodiments thereof, it will be understood by those
skilled in the art that various alterations in form and detail may
be made therein without departing from the spirit and scope of the
invention. In particular, the particular metal catalysts, supports,
source gas, and flow rates used can vary significantly and still be
within the optimization scope of the present invention.
* * * * *