U.S. patent application number 11/282872 was filed with the patent office on 2006-10-12 for carbon nanotube with a graphitic outer layer: process and application.
This patent application is currently assigned to UNIVERSITY OF CENTRAL FLORIDA. Invention is credited to Lee Chow, Stephen Kleckley, Dan Zhou.
Application Number | 20060228288 11/282872 |
Document ID | / |
Family ID | 24104918 |
Filed Date | 2006-10-12 |
United States Patent
Application |
20060228288 |
Kind Code |
A1 |
Chow; Lee ; et al. |
October 12, 2006 |
Carbon nanotube with a graphitic outer layer: process and
application
Abstract
A method for manufacturing carbon nanotubes with an integrally
attached outer graphitic layer is disclosed. The graphitic layer
improves the ability to handle and manipulate the nanometer size
nanotube device in various applications, such as a probe tip in
scanning probe microscopes and optical microscopes, orbs an
electron emitting device. A thermal chemical vapor deposition
reactor is the preferred reaction vessel in which a transition
metal catalyst with an inert gas, hydrogen gas and a
carbon-containing gas mixture are heated at various temperatures in
a range between 500.degree. C. and 1000.degree. C. with gases and
temperatures being adjusted periodically during the reaction times
required to grow the nanotube core and subsequently grow the
desired outer graphitic layer.
Inventors: |
Chow; Lee; (Orlando, FL)
; Zhou; Dan; (Orlando, FL) ; Kleckley;
Stephen; (Orlando, FL) |
Correspondence
Address: |
LAW OFFICES OF BRIAN S STEINBERGER
101 BREVARD AVENUE
COCOA
FL
32922
US
|
Assignee: |
UNIVERSITY OF CENTRAL
FLORIDA
|
Family ID: |
24104918 |
Appl. No.: |
11/282872 |
Filed: |
November 18, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10424336 |
Apr 25, 2003 |
7011884 |
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11282872 |
Nov 18, 2005 |
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09528259 |
Mar 17, 2000 |
6582673 |
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10424336 |
Apr 25, 2003 |
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Current U.S.
Class: |
423/447.3 ;
977/843 |
Current CPC
Class: |
D01F 9/1271 20130101;
D01F 9/133 20130101; B82Y 40/00 20130101; Y10S 977/847 20130101;
C01B 32/162 20170801; Y10T 428/2918 20150115; Y10S 977/745
20130101; Y10T 428/2991 20150115; Y10S 977/86 20130101; Y10S
977/742 20130101; B82Y 30/00 20130101; Y10S 977/869 20130101; D01F
9/1272 20130101; D01F 9/127 20130101 |
Class at
Publication: |
423/447.3 ;
977/843 |
International
Class: |
D01F 9/12 20060101
D01F009/12 |
Claims
1-21. (canceled)
22. A system for manufacturing a nanotube device having a carbon
nanotube core integrally attached outer graphitic layer comprising:
means for preparing a transition metal catalyst on a selected
substrate; means for placing the transition metal catalyst and the
substrate of step in a reaction vessel to create a reaction
product; means for purging the reaction vessel by beating said
vessel to a temperature between approximately 500.degree. C. and
approximately 750.degree. C. while introducing a gaseous mixture of
hydrogen and an inert gas; means for growing a carbon nanotube core
by changing the gaseous mixture to a stream consisting of
approximately 10% carbon-containing gas and approximately 90% inert
gas, while increasing the temperature of the reaction vessel to a
range between approximately 750.degree. C. and approximately
900.degree. C.; means for growing an integrally attached graphitic
outer layer on the carbon nanotube core by changing the reaction
parameters to promote the distinct formation of carbon material
selected from the group consisting of amorphous carbon, graphitic
carbon and mixtures thereof; means for removing the carbon nanotube
with graphitic outer layer from the reaction vessel; and means for
mechanically breaking away a portion of the graphitic outer layer
to leave the carbon nanotube core partially exposed.
23. The system of claim 22, wherein the growing means includes:
means for introducing to the reaction vessel a gaseous mixture
consisting of approximately 5% carbon-containing gas, approximately
5% hydrogen gas and approximately 90% inert gas while increasing
the temperature of the reaction vessel to a range between
approximately 900.degree. C. and approximately 1000.degree. C. to
promote the growth of an amorphous carbon material; means for
maintaining the conditions of the preparing means for a period of
time from approximately 30 minutes to approximately one hour; and,
thereafter; and means for annealing the reaction product by
introducing to the reaction vessel a gaseous mixture consisting of
approximately 10% carbon-containing gas and approximately 90% inert
gas while maintaining the temperature of the reaction vessel at
approximately 1000.degree. C. to promote the growth of a graphitic
carbon coating.
24. The system of claim 22, wherein the graphitic outer layer forms
on the carbon nanotube core at temperatures between approximately
900.degree. C. and approximately 1000.degree. C.
25. The system of claim 22, wherein the graphitic outer layer forms
on the carbon nanotube core at temperatures of approximately
1000.degree. C.
26. The system of claim 22, wherein the transition metal catalyst
is selected from the group consisting of cobalt, nickel, iron,
mixtures and alloys thereof.
27. The system of claim 22, wherein the transition metal catalyst
is a mixture of 50% iron and 50% nickel.
28. The system of claim 22, wherein the selected substrate for the
transition metal catalyst is silicon.
29. The system of claim 22, wherein the inert gas is selected from
a group consisting of argon, helium and nitrogen.
30. The system of claim 22, wherein the carbon-containing gas is
selected from the group consisting of methane, ethane, propane,
butane, ethylene, cyclohexane, carbon monoxide and carbon
dioxide.
31. The system of claim 22, wherein the carbon-containing gas is
methane.
32. A system for manufacturing a nanotube device having a carbon
nanotube core integrally attached outer graphitic layer comprising:
means for preparing a transition metal catalyst on a selected
substrate; means for placing the transition metal catalyst and the
substrate of step in a reaction vessel to create a reaction
product; means for purging the reaction vessel by heating said
vessel to a temperature between approximately 500.degree. C. and
approximately 750.degree. C. while introducing a gaseous mixture of
hydrogen and an inert gas; means for growing a carbon nanotube core
by changing the gaseous mixture to a stream consisting of
approximately 10% carbon-containing gas and approximately 90% inert
gas, while increasing the temperature of the reaction vessel to a
range between approximately 750.degree. C. and approximately
900.degree. C.; and means for growing an integrally attached
graphitic outer layer on the carbon nanotube core by changing the
reaction parameters to promote the distinct formation of carbon
material selected from the group consisting of amorphous carbon,
graphitic carbon and mixtures thereof, wherein a carbon nanotube
device can be formed therefrom.
33. A system for manufacturing a nanotube device, comprising: a
nanotube core grown inside of an outer layer; a mechanically
removable portion of the outer layer that is mechanically removable
to reveal and expose a tip portion of the nanotube core.
34. The system of claim 33, wherein the nanotube core is
carbon.
35. The system of claim 33, wherein the outer layer is graphitic
carbon.
36. The system of claim 34, wherein the outer layer is graphitic
carbon.
Description
[0001] The present invention relates to a novel method of
manufacturing carbon nanotubes, and in particular to the production
of a structure comprising a carbon nanotube with an outer graphitic
layer that can function as a handle for attaching and/or
manipulating the tip of the nanotube.
BACKGROUND AND PRIOR ART
[0002] Material scientists have been exploring the properties of
fullerenes which are geometric structures built of carbon atoms. In
1991, a new fullerene joined the buckyball, a cage-like structure
built of 60 carbon atoms. Scientists found that the buckyball
structure can be extended to form long slender tubes--carbon
nanotubes--single molecules comprised of rolled graphene
(graphite-like) sheets capped at each end. Thus, carbon nanotubes,
the newest fullerene structure, are effectively buckyballs played
out as long strands, so thin they can not be seen under an ordinary
microscope and certainly not with the naked eye. In fact, it is
suggested by Boris I. Yakobson and Richard E. Smalley in American
Scientist (July-August 1997), "Fullerene Nanotubes: C.sub.1,000,000
and Beyond," that when Roger Bacon used the electric arc in the
early 1960s to make "thick" carbon whiskers, the nanotube discovery
was a matter of looking more closely at the smallest products
hidden in the soot, but Bacon lacked the high-power microscope
required to see them.
[0003] To comprehend the size of a single-wall carbon nanotube,
imagine holding in your hand a wand with a hollow core that is a
single molecule. Such a wand would be a few atoms in circumference.
In fact, nanotubes sufficient to span the 250,000 miles between the
earth to the moon could be loosely rolled into a ball the size of a
poppyseed. Together, the smallness-of the nanotubes and the
chemical properties of carbon atoms packed along their walls in a
honeycomb pattern are responsible for their fascinating and useful
qualities and, present significant production challenges.
[0004] Precursors to the development of carbon nanotubes are
reported in patents issued prior to 1991. U.S. Pat. No. 4,025,689
discloses a process requiring the use of inert gas and temperatures
up to 3000.degree. C. for preparing a light-weight, hollow carbon
body having outstanding properties as activated carbon. U.S. Pat.
No. 4,228,142 prepares a high density carbon coated silicon carbide
particle which can be substituted for commercial-grade diamonds by
reacting a fluorocarbon and silicon carbide in an inert gas
atmosphere at tempertures >800.degree. C. U.S. Pat. No.
4,855,091 discloses the formation of a fishbone-like graphite layer
along an axis of carbon filaments when a carbon containing gas is
heated to temperatures between 250.degree. C. and 800.degree. C. in
the presence of ferromagnetic metal particles. U.S. Pat. No.
5,165,909 discloses the growth of hollow carbon fibrils having
layers and a core made up of concentric rings, like a tree.
Metal-containing particles are used as catalysts in reactions with
inexpensive, readily available carbon containing raw materials;
high temperature graphitizing reactions are avoided. The fibrils
have a high surface area, high tensile strength and modulus
required in reinforcement applications. U.S. Pat. No. 5,747,161
discloses a method for forming a tubular shaped carbon filament
less than 30 nanometers in diameter using an arc discharge
process.
[0005] In summary, the pursuit of carbon structures is well
documented and the foundation is laid for the use of processes
employing inert gas atmospheres, metal catalysts having an affinity
for carbon such as iron, nickel, cobalt (Fe, Ni, Co, respectively)
and high temperatures to create the desired structure.
[0006] After the discovery of carbon nanotubes in 1991, scientific
efforts have been devoted to the production of carbon nanotubes in
higher yield; the production of carbon nanotubes with consistent
dimensions, e.g., diameter and length; processes which separate
nanotubes from other reaction products; processes which eliminate
the entanglement of tubes with each other and the development of
useful applications. For example, U.S. Pat. No. 5,346,683 discloses
uncapping and thinning carbon nanotubes to provide open
compartments for inserting chemicals. U.S. Pat. No. 5,456,986
discusses the production of magnetic nanoparticles and nanotubes
from graphite rods packed with magnetic metal oxides or rare earths
and subjected to carbon arc discharge. U.S. Pat. No. 5,543,378
discloses carbon nanostructures which encapsulate a palladium
crystallite, allowing the delivery of substances suitable for x-ray
diagnostic imaging in a safe, encapsulated form. Another method of
producing encapsulated nanoparticles, nanotubes and other closed
carbon structures is disclosed in U.S. Pat. No. 5,780,101 and its
divisional counterpart, U.S. Pat No. 5,965,267; a transition metal
catalyst is contacted with a gas mixture containing carbon monoxide
in a temperature range between 300.degree. C. and 1000.degree. C.
Methods for isolating and increasing the yield of carbon nanotubes
are disclosed in U.S. Pat No. 5,560,898 which discusses a physical
separation technique; U.S. Pat. No. 5,641,466 reveals a method for
purification of carbon nanotubes by the oxidation of co-existing,
but undesired carbon structures; and U.S. Pat. No. 5,698,175 claims
a chemical technique for separating nanotubes from carbon
by-products.
[0007] The references reveal that prior art methods for producing
carbon nanotubes give undesirably low yields. Carbon nanotubes with
significant variations in structure and size are usually produced
and often include carbon materials of different shapes which may be
carbon nanoparticles and amorphous carbon. Carbon nanotubes are
further classified into one with a single hexagonal mesh tube
called a single-walled nanotube (abbreviated as "SWNT"), and one
comprising a tube of a plurality of layers of hexagonal meshes
called a multiwalled nanotube (abbreviated as "MWNT").
[0008] The type of carbon nanotube structure available is
determined to some extent by the method of synthesis, catalysts and
other conditions. Research continues in an effort to produce carbon
nanotubes of a consistent, predictable structure.
[0009] The present invention contributes a more consistent,
predictable method for manufacturing a particular configuration of
carbon nanotubes. Novel process conditions and reactants are
disclosed. The present invention also provides a solution to
problems associated with handling and manipulating the "small" wand
which is only visible with high-power electron microscopes, or
other costly visual aids. Through the process of the present
invention, a "graphitic outer layer" defined as carbon material
comprising one or more distinct structures, is intentionally formed
during carbon nanotube production and becomes an integral part of
the carbon nanotube device. The carbon material can be either a
soft amorphous carbon, a hard graphitic carbon, or a combination
thereof. If the soft amorphous carbon is formed prior to the
formation of the harder, more resilient graphitic carbon, the
amorphous carbon serves as a cushion between the carbon nanotube
and the harder graphitic carbon. The carbon material, either singly
or collectively, is called the "graphitic outer layer" and creates
bulk such that the submicroscopic nanotube can be handled easily
and efficiently.
[0010] Computer simulations and laboratory experiments show that
carbon nanotubes have extraordinary resilience, strength and
various unusual electronic and mechanical properties; for instance,
they can be formed into very strong ropes and can be used as probes
because of a very large Young's modulus, even greater than diamond.
They also exhibit electrical conductivity in a quantized fashion
that has led to experiments with tiny nanowires and nanoscale
transistors.
SUMMARY OF THE INVENTION
[0011] The first objective of the present invention is to provide a
method for producing a new configuration for multi-walled carbon
nanotubes that enhances utility. The same procedure can be used to
produce a similar configuration for single-walled carbon nanotubes
with appropriate modifications to process conditions.
[0012] The second objective of the present invention is to provide
a method for handling this new carbon nanotube configuration on the
tip of a scanning probe microscope.
[0013] The third objective of this invention is to demonstrate the
use of the new carbon nanotube configuration as an excellent
electron source.
[0014] The fourth objective of this invention is to provide a
graphitic carbon handle on a carbon nanotube to enable manipulation
of the nanotube probe tip using the power of a conventional optical
microscope.
[0015] The preferred embodiment for the production of carbon
nanotubes in the present invention results in nanotubes having a
diameter between about 1 nanometer (nm) and about 100 nm with an
integrally attached outer layer of graphitic material approximately
1 micrometer (.mu.m) to approximately 10 .mu.m in diameter. The
much larger diameter of the graphitic outer layer becomes a handle
so that the nanotube probe can be manipulated under a conventional
optical microscope. The handle also provides a means for attachment
and greatly enhances the utility of carbon nanotubes in a variety
of scanning probe microscopes, electron microscopes and on a
substrate as an electron emitter for flat panel displays.
[0016] Further objectives and advantages of this invention will be
apparent from the following detailed description of a presently
preferred embodiment which is illustrated in the accompanying
drawings.
BRIEF DESCRIPTION OF THE FIGURES
[0017] FIG. 1 is a diagram of a carbon nanotube with a graphitic
outer layer--amorphous carbon coated with graphitic carbon.
[0018] FIG. 2 is a scanning electron micrograph of a nanotube with
a graphitic outer layer.
[0019] FIG. 3A is a carbon nanotube probe mounted on a conventional
atomic force microscope tip.
[0020] FIG. 3B is a carbon nanotube probe mounted on the end of an
optical fiber.
[0021] FIG. 4 is a diagram of a thermal chemical vapor deposition
reactor for the production of a nanotube with a graphitic outer
layer.
[0022] FIG. 5 is a chart of process conditions used to produce a
nanotube with various graphitic outer layers.
[0023] FIG. 6 is a flow diagram of process steps for production of
nanotubes in the preferred embodiment.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0024] Before explaining the disclosed embodiment of the present
invention in detail it is to be understood that the invention is
not limited in its application to the details of the particular
arrangement shown since the invention is capable of other
embodiments. Also, the terminology used herein is for the purpose
of description and not of limitation.
[0025] The present invention produces a new carbon nanotube device
as shown in FIG. 1. The carbon nanotube (11) is produced in an
early phase of the manufacturing process and with adjustments in
reaction conditions, the graphitic outer layer (12) having a
diameter from approximately 1 micrometer (.mu.m) to approximately
10 micrometers grows on the outside of the nanotube. What is
surprising and not expected is that by judiciously selecting
reaction conditions and reactants, carbon nanotubes are produced
with one or more distinctive carbon or graphitic structures
integrally attached. For example, after the growth of the nanotube
(11) at specific process parameters, the gaseous mixture in the
reactor is changed to include hydrogen gas, and the reaction
temperature is increased to approximately 900.degree. C. to support
the growth of an amorphous carbon layer (12a) which is soft and
easily broken away from the nanotube core. A subsequent change in
the gaseous mixture to eliminate the hydrogen gas while maintaining
the reaction temperature at approximately 1000.degree. C., causes a
graphitic carbon layer (12b) to grow. The graphitic carbon layer
(12b) is harder than the amorphous carbon layer (12a); therefore,
the graphitic carbon layer (12b) is preferred as a handle for the
nanotube core. It is also possible to grow the nanotube core (11)
and change process conditions so that the amorphous carbon layer
(12a) is omitted and the harder graphitic carbon layer (12b) grows
as the only outer layer on the nanotube core. If the softer
amorphous carbon layer is preferred, the process conditions can be
adjusted or stopped after the amorphorus layer is formed, so that
the harder graphitic carbon layer does not form.
[0026] FIG. 2 shows the scanning electron micrograph of the new
carbon nanotube device after a portion of the graphitic material
(22) has been mechanically removed from the nanotube core (21).
[0027] The outer graphitic layer serves several purposes. For
example, it provides a mechanical coupling between the nanotube and
the probe tip when used as a tip for scanning probe microscopes
(SPM). SPM include a variety of proximity probe microscopes, for
example: scanning tunneling microscope (STM) atomic force
microscope (ATM), magnetic force microscope (MFM), scanning
capacitance microscope (SCM) and the like.
[0028] A conventional atomic force microscope (AFM) tip arrangement
is shown in FIG. 3A. Prior to the present invention, the pyramid
tip of a standard AFM was typically made out of silicon nitride
(Si.sub.3 N.sub.4) and can be used in contact mode with atomic
resolution. However, the silicon nitride pyramid-tip scanning probe
microscope can not meet the requirements of the semiconductor
industry where a deep narrow trench of approximately 200 nm width
and 500 nm depth is to be scanned. The current state of the art
technique reaches it limit with the production of an etched optical
fiber tip with a diameter of about 200 nm. As the components in the
semiconductor industry become smaller and smaller, it is necessary
to use the product of the present invention to meet the metrology
requirements. The graphitic material, acting as a handle, provides
a means for attaching the nanotube to the tip of the AFM.
[0029] In FIG. 3A, the nanometer size nanotube (30) is mounted to a
conventional cantilever by using a laser or focus ion beam to drill
a hole (31) approximately 1 micrometer to approximately 10
micrometers (on) in diameter in the conventional cantilever pyramid
tip (32). The nanotube with graphitic outer layer (33) is inserted
in the hole. The nanotube (30) and the graphitic outer layer (33)
are collectively sometimes called a "nanoprobe." A small amount of
adhesive (34) is applied to firmly fix the nanoprobe in the pyramid
tip (32) of a conventional AFM. Other means for affixing the
nanoprobe to the scanning probe tip may be used and are understood
to be within the scope of the present invention.
[0030] FIG. 3B shows a nanoprobe (35) mounted on the end of an
optical fiber (36). In this embodiment, a hole (37) that is 1 .mu.m
to approximately 5 .mu.m in diameter is drilled at the end of an
optical fiber (36). The depth of the hole can be from approximately
10 microns to approximately 1000 microns. A small amount of
adhesive is applied to the surface interface between the nanoprobe
and the hole drilled in the optical fiber.
[0031] Another purpose served by the graphitic outer layer is to
provide a means for the carbon nanotube to be manipulated under an
optical microscope. In other applications, such as an electron
emitter for the field emission electron microscope or for arranging
an array of nanotubes on a silicon substrate used in flat panel
displays, the manipulation and arrangement of the nanotubes are
facilitated by the graphitic outer layer.
[0032] The method for manufacturing the unique carbon nanotube
device of the present, invention will now be described in detail.
The configuration shown in FIG. 4 is generally known as a thermal
chemical vapor deposition (CVD) reactor which is the preferred
device for production. FIG. 5 charts the nanotube device
manufacturing process parameters. FIG. 6 is a flow chart of the
manufacturing method used in the Example which illustrates the
invention.
[0033] FIG. 4 is a diagram of the quartz tube reactor comprising a
furnace (41) a quartz tube (42) with an inlet means for gases (43)
and an internal position for the catalytic metal substrate (44).
Quartz tube reactors are commercially available and known to those
skilled in the art. The reactor used in the present invention was
supplied by Lingberg-Blue M, 304 Park Street, Watertown, Wis.
53004. Reaction conditions are shown in FIG. 5.
[0034] Preparation for the manufacture of the carbon nanotube
device begins with the deposition of transition metal or transition
metal alloy particles on a silicon substrate in 1-100 Torr of an
inert gas atmosphere. Substrates can be used that do not react with
the carbon-containing raw material gas at high temperatures or do
not contain the catalysts selected for the present process. The
transition metal catalyst (pure element or alloy) can be selected
from the group consisting of cobalt, nickel or iron (Co, Ni or Fe,
respectively), or mixtures thereof which can be deposited as
nanoparticles on silicon. The presence of a catalyst also allows
the use of a lower reaction temperature.
[0035] Thus, the reaction temperatures may vary with choice and
amount of catalysts employed. Temperature ranges for various
reactions in the present invention are disclosed in FIG. 5. Purging
of the reaction vessel occurs between approximately 500.degree. C.
and approximately 750.degree. C. Nanotube growth occurs between
approximately 750.degree. C. and approximately 900.degree. C.
Amorphous carbon forms a soft outer coating of the nanotube at
temperatures between approximately 900.degree. C. and approximately
1000.degree. C. Graphitic carbon forms a hard outer coating on
either the nanotube or the amorphous carbon at a temperature of
approximately 1000.degree. C.
[0036] The inert gas atmosphere is effective when the growth rate
is too high, or when alleviating toxicity or explosivity of the raw
material gas. The inert gases useful in the present invention
include such gases as argon, helium and nitrogen; argon being
preferred. As one skilled in the art would understand, process
conditions can vary based on the selection of gases. The silicon
substrate with catalyst is placed inside the quartz tube reactor.
Then a mixture of hydrogen gas and argon gas is passed through the
reactor while the tube is being heated to 750.degree. C. using a
resistivity tube furnace. The ratio of hydrogen gas to argon gas is
1:1. The temperature of the furnace is measured with a thermocouple
probe. This phase of the reaction is shown in FIG. 5 as Step 1. In
Step 1, catalyst reduction and purging of impurities in the vessel
are accomplished.
[0037] In Step 2 of FIG. 5, carbon nanotube growth occurs when the
gases fed to the reaction vessel are changed to a mixture of a
carbon-containing raw material gas and an inert gas while the
temperature is increased to 900.degree. C. The ratio of raw
material gas to inert gas is 1:10. Theoretically, any carbon
containing gas can be used as a raw material for the present
process. Preferably the carbon-containing raw material gas is a
hydrocarbon such as methane, ethane, propane, butane, hexane,
cyclohexane or an oxide such as carbon monoxide. To reiterate,
process conditions may require adjustment with changes in the raw
materials used. The ratio of raw material gas to inert gas, should
preferably be within a range of from approximately 1:20 to
approximately 20:1. The better results are from experiments with a
ratio of 1:20 to approximately 1:10. At lower ratios, the nanotubes
are thinner and in a preferred structural arrangement. A carbon
nanotube is generated in a period of time within a range of from 30
minutes to 1 hour after the introduction of the raw material
gas.
[0038] Step 3 in FIG. 5 shows a change in ratio of raw material gas
to inert gas, with the addition of hydrogen gas, so that there are
equal amounts of hydrogen and carbon-containing raw material gas
(methane). During this step the temperature of the vessel is
increased to approximately 1000.degree. C. An amorphous carbon
outer layer grows on the carbon nanotube core in a period of time
that ranges of from approximately 30 minutes to approximately 45
minutes or one hour after the gaseous mixture is adjusted to a
lower composition of raw material gas.
[0039] Step 4 in FIG. 5 shows the third and final adjustment of
gases being fed into the reactor tube. Hydrogen gas is turned off
and the ratio of carbon-containing raw material gas to inert gas is
again in a preferred ratio of 1:10. The temperature of the reactor
is maintained at approximately 1000.degree. C. for a period of time
from approximately 30 minutes to approximately 45 minutes.
Graphitic carbon layer growth and annealing of the carbon nanotube
device occurs in the final phase of the reaction.
[0040] For carbon nanotube production and growth of a graphitic
outer layer, the pressure within the reactor is one atmosphere. The
gas flow rate is 100 to 200 cubic centimeters per minute.
[0041] The present invention is shown in further detail in FIG. 6
and described in the following example.
EXAMPLE
[0042] 50 milligrams (mg) of FeNi nanoparticles, comprising a 1:1
ratio of Fe and Ni, are deposited on a silicon substrate having a
surface area of approximately 300 cm.sup.2 in 50-100 Torr argon
atmosphere. The FeNi transition metal catalyst on silicon substrate
is removed from the evaporator and placed in a thermal catalytic
vapor deposition (CVD) system consisting of a quartz tube inserted
into a furnace.
[0043] The reactor vessel is heated to approximately 725.degree. C.
with a flow of 50% argon and 50% hydrogen for 30 to 60 minutes.
This purges the vessel of undesirable impurities and reduces
oxides. Next, a gas flow comprising 10% methane (CH.sub.4) and 90%
argon (Ar), is introduced to the reactor at a temperature between
approximately 725.degree. C. and 900.degree. C. for a period of
thirty minutes to forty-five minutes for nanotube growth. Then the
gaseous mixture being fed to the reactor vessel is changed to 5%
methane, 5% hydrogen (H.sub.2) and 90% argon while the reactor is
heated to temperatures between 900.degree. C. and 1000.degree. C.
and maintained at this temperature range for a period of
approximately thirty minutes to approximately forty-five minutes.
During this period, an amorphous carbon coating forms on the carbon
nanotube core.
[0044] Hydrogen gas is turned off and graphitic carbon outer layer
growth continues with gas flow comprising 10% CH.sub.4 and 90% Ar
and the maintenance of the reactor at a temperature of
approximately 1000.degree. C. for an additional thirty minutes to
sixty minutes. During this period an outer graphitic layer forms on
the amorphous carbon covering the nanotube core. The reaction
product is removed from the reactor vessel, allowed to cool then
examined under an optical microscope. The outer graphitic layer is
broken away by a physical means to reveal the strong, resilient
nanotube core which is highly resistant to breaking.
[0045] In another embodiment, when referring to FIG. 5, the process
could omit step 4 and produce a carbon nanotube with a soft
amorphous carbon outer layer. Or, if step 3 is omitted, the outer
layer would be the harder graphitic carbon. Either singly or
collectively, the carbonaceous material intentionally formed on the
outside of the nanotube core is called the graphitic outer
layer.
[0046] While the invention has been described, disclosed,
illustrated and shown in various terms of certain embodiments or
modifications which it has presumed in practice, the scope of the
invention is not intended to be, nor should it be deemed to be,
limited thereby and such other modifications or embodiments as may
be suggested by the teachings herein are particularly reserved
especially as they fall within the breadth and scope of the claims
here appended.
* * * * *