U.S. patent application number 11/375744 was filed with the patent office on 2010-09-23 for processes for growing carbon nanotubes using disordered carbon target.
Invention is credited to Gillian Althea Maria Reynolds, David Herbert Roach.
Application Number | 20100239490 11/375744 |
Document ID | / |
Family ID | 42737831 |
Filed Date | 2010-09-23 |
United States Patent
Application |
20100239490 |
Kind Code |
A1 |
Roach; David Herbert ; et
al. |
September 23, 2010 |
Processes for growing carbon nanotubes using disordered carbon
target
Abstract
Processes for producing single-wall carbon nanotubes without
catalysts are provided. The nanotubes are produced by vaporizing
silicon carbide and carbon.
Inventors: |
Roach; David Herbert;
(Hockessin, DE) ; Reynolds; Gillian Althea Maria;
(Wilmington, DE) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY;LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1122B, 4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Family ID: |
42737831 |
Appl. No.: |
11/375744 |
Filed: |
March 15, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60661975 |
Mar 15, 2005 |
|
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|
Current U.S.
Class: |
423/447.2 ;
423/447.1; 977/742; 977/750; 977/842; 977/844 |
Current CPC
Class: |
D01F 9/12 20130101 |
Class at
Publication: |
423/447.2 ;
423/447.1; 977/742; 977/750; 977/842; 977/844 |
International
Class: |
D01F 9/12 20060101
D01F009/12 |
Claims
1. A process comprising: a) providing a target comprising silicon
carbide and carbon; b) vaporizing the target in a catalyst-free
environment in an inert atmosphere at a pressure from about
10.sup.-3 Torr to 1000 Torr); and c) forming a product comprising
at least one single-wall carbon nanotube.
2. The process of claim 1, wherein the vaporization step is carried
out by laser ablation.
3. The process of claim 2, wherein the laser ablation is performed
at a temperature from about 100.degree. C. to about 1500.degree.
C.
4. The process of claim 2, wherein the laser ablation is performed
at a temperature from about 1000.degree. C. to about 1200.degree.
C.
5. The process of claim 1, wherein the pressure is about 500 Torr
or greater.
6. The process of claim 1, further comprising an annealing step
after the formation of the at least one single-wall carbon
nanotube.
7. The process of claim 1, wherein the vaporization is carried out
in the presence of an inert gas selected from argon, neon, helium,
nitrogen and mixtures thereof.
8. A single-wall carbon nanotube produced by the process of claim
1.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to processes for growing
single-wall carbon nanotubes in the absence of a catalyst.
BACKGROUND OF THE INVENTION
[0002] In the field of molecular nanoelectronics, few materials
show as much promise as nanotubes, and in particular carbon
nanotubes, which comprise hollow cylinders of graphite. Nanotubes
can be incorporated into electronic devices such as diodes and
transistors, depending on the nanotube's electrical
characteristics. Nanotubes are unique for their size, shape, and
physical properties. Structurally, a carbon-nanotube resembles a
hexagonal lattice of carbon rolled into a cylinder.
[0003] Besides exhibiting intriguing quantum behaviors at low
temperature, carbon nanotubes exhibit the following important
characteristics: a nanotube can be either metallic or semiconductor
depending on its chirality (i.e., conformational geometry).
Metallic nanotubes can carry extremely large current densities.
Semiconducting nanotubes can be electrically switched on and off as
field-effect transistors (FETs). The two types may be covalently
joined (sharing electrons). These characteristics point to
nanotubes as excellent materials for making nanometer-sized
semiconductor circuits.
[0004] Nanotubes can be formed as single-wall carbon nanotube
(SWNTs) or multi-wall carbon nanotube (MWNTs). SWNTs can be
produced, for example, by arc-discharge and laser ablation of a
carbon target (U.S. Pat. No. 6,183,714). Local growth of tubes on a
surface can also be obtained by chemical vapor deposition (CVD).
The growth of the nanotubes is made possible by the presence of
metallic particles, such as Co, Fe, and/or Ni, acting as catalyst.
The resultant carbon nanotubes comprise contaminants, e.g.,
catalyst particles. For most potential nanotube applications, the
use of clean nanotubes can be important, for example, where
nanotubes are incorporated as an active part of electric devices.
The presence of contaminating atoms and particles can alter the
electrical properties of the nanotubes. The metallic particles can
be removed; however the process of cleaning or purifying the
nanotubes can be complicated and can alter the quality of the
nanotubes.
[0005] SWNTs have been identified as potential components of
electronic devices. The quality of nanotubes, e.g., their ability
to act as a semiconductor, can be affected by contaminants.
Therefore, a need exists for a method of catalyst-free growth of
single-wall carbon nanotubes.
[0006] U.S. Patent Application No. 2004/0035355 discloses a method
for growing single-wall nanotubes comprising providing a silicon
carbide semiconductor wafer comprising a silicon face and a carbon
face, and annealing the silicon carbide semiconductor wafer in a
vacuum at a temperature of at least about 1,350.degree. C. and a
pressure of 10.sup.-9 Torr thereby inducing formation of
single-wall carbon nanotubes on the silicon face. The disclosed
method utilizes relatively low pressures.
[0007] New and/or improved methods for making carbon nanotubes are
desired.
SUMMARY OF THE INVENTION
[0008] One aspect of this invention is a process comprising: [0009]
a) providing a target comprising silicon carbide and carbon; [0010]
b) vaporizing the target in a catalyst-free environment in an inert
atmosphere at a pressure from about 10.sup.-3 Torr to about 000
Torr; and [0011] c) forming a product comprising at least one
single-wall carbon nanotube.
[0012] Another aspect of the present invention is a single-wall
carbon nanotube produced by a process comprising: [0013] a)
providing a target comprising silicon carbide and carbon; [0014] b)
vaporizing the target in a catalyst-free environment in an inert
atmosphere at a pressure from about 10.sup.-3 Torr to about 000
Torr; and [0015] c) forming a product comprising at least one
single-wall carbon nanotube.
[0016] These and other aspects of the present invention will be
apparent to those skilled in the art, in view of the following
specification and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 shows a transmission electron micrograph of
agglomerates of single wall carbon nanotubes produced by one
embodiment of the present process.
DETAILED DESCRIPTION OF THE INVENTION
[0018] All documents cited herein are expressly incorporated herein
by reference in their entirety. Applicants also herein incorporate
by reference the co-owned and concurrently filed application
entitled "PROCESSES FOR GROWING CARBON NANOTUBES USING DISORDERED
CARBON SOURCE" (Attorney Docket # CL 2627).
[0019] When an amount, concentration, or other value or parameter
is given as either a range, preferred range, or a list of upper
preferable values and lower preferable values, this is to be
understood as specifically disclosing all ranges formed from any
pair of any upper range limit or preferred value and any lower
range limit or preferred value, regardless of whether ranges are
separately disclosed. Where a range of numerical values is recited
herein, unless otherwise stated, the range is intended to include
the endpoints thereof, and all integers and fractions within the
range. It is not intended that the scope of the invention be
limited to the specific values recited when defining a range.
[0020] The present invention provides a process for growing
single-wall carbon nanotubes (SWNTs) in the absence of a catalyst.
The process includes providing a target that is a mixture of
silicon carbide and carbon, which comprises 50 weight percent
silicon carbide or less, preferably about 45 weight percent or
less, more preferably about 42 weight percent or less. In some
embodiments, the amount of silicon carbide can be as low as 1
weight percent. Preferably, the amount of silicon carbide in the
mixture is at least about 1 weight percent. The target comprising
the mixture can be formed by, for example, forming a slurry of
silicon carbide and carbon powders in a volatile solvent, allowing
the solvent to evaporate, then compression molding the residual
solid. The compression molded silicon carbide/carbon article can be
optionally heated, preferably in an inert atmosphere, to
substantially remove traces of the solvent and harden the target.
Other methods for preparing such a target are known to those
skilled in the art.
[0021] Vaporization of the target can be carried out by laser
ablation or other suitable methods known to those skilled in the
art, such as, for example, rf induction heating and sputtering. The
vaporization can be carried out at temperatures between about
100.degree. C. and 1500.degree. C. and pressures of vacuum (e.g.,
about 10.sup.-3 Torr) to above atmospheric pressure.
[0022] In some preferred embodiments, the vaporization is carried
out at a temperature from about 1000.degree. C. to about
1200.degree. C. Preferably, the vaporization is carried out in the
presence of a non-oxidizing gas, such as argon, neon, helium,
nitrogen or mixtures thereof. By "non-oxidizing", as used herein,
is meant an atmosphere in which oxygen content is minimized.
Minimization of oxygen in the atmosphere during nanotube production
is desirable because oxygen can oxidize the carbon, thereby
reducing the production of the desired nanotubes. However, the
total absence of oxygen is not required. Thus, in one illustrative
embodiment, the target material is mixed, pressed and heated in an
inert atmosphere at 1150.degree. C. to harden the target before it
is placed into a laser ablation system, wherein the oxygen content
is minimized. Generally it is preferred that the atmosphere
comprise no more than about 100 ppm oxygen, preferably about 50 ppm
or less, more preferably about 25 ppm or less. Carbon nanotubes can
desirably be formed in the presence of a non-oxidizing gas, such as
argon, neon, helium, nitrogen or mixtures thereof. Commercially
available tanks of gases, such as 99.9% pure argon, are suitable
for the process of forming carbon nanotubes.
[0023] While the selection of the gas under which vaporization is
carried out is not critical, the nature of the gas can affect the
amount of nanotubes produced. While it is not intended that the
invention be bound by any particular theory, it is believed that
the thermal conductivity of the gas can affect the formation of
nanotubes. For example, the use of helium may result in the
formation of fewer nanotubes than would the use of nitrogen,
because the higher degree of cooling expected to occur with helium
can result in a cooler, and therefore less active, growth zone.
[0024] In one embodiment of this invention, a SWNT-containing
product produced using the processes disclosed herein can serve as
a target for one or more additional cycles of vaporization and
SWNT-formation.
[0025] The process can further comprise an annealing step.
Annealing does not require substantial additional processing, and
can be accomplished by allowing the newly formed nanotubes to
remain undisturbed and cool following ablation. The annealing can
be performed in an ultra-high vacuum (UHV) (e.g., at a pressure
less than about 10.sup.-9 Torr), or at higher pressures, even above
atmospheric pressure (760 torr). Generally, a pressure of about 500
torr is suitable.
[0026] It is generally desirable to grow the tubes at pressures of
at least 1 millitor, and preferably at 500 Torr or above. In some
preferred embodiments, the pressure is about 1000 Torr. It is
generally not desirable that the pressure be greater than about
1000 Torr. Although a reduction in pressure below about 500 Torr
has not been observed to undesirably affect the rate of growth of
nanotubes, pressures of about 500 Torr or greater are often
practical. In embodiments, the pressure is from about 300 Torr to
about 600 Torr. The nanotubes that are formed are predominately
SWNTs as shown in the Figure. The SWNTs can be very long and have a
good crystalline quality. "Good crystalline quality" means
substantially free of observable defects under transmission
electron microscopy.
[0027] All of the compositions and processes disclosed and claimed
herein can be made and executed without undue experimentation in
light of the present disclosure. While the compositions and
processes have been described in terms of preferred embodiments, it
will be apparent to those of skill in the art that variations can
be applied to the processes and methods and in the steps or in the
sequence of steps of the processes described herein without
departing from the concept, spirit, and scope of the invention. All
substitutions 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.
EXAMPLE
[0028] The present invention is further defined in the following
Example. It should be understood that this Example, while
indicating a preferred embodiment of the invention, is given by way
of illustration only. From the above discussion and this Example,
one skilled in the art can ascertain the preferred features of this
invention, and without departing from the spirit and scope thereof,
can make various changes and modifications to adapt it to various
uses and conditions. This Example shows the production of single
wall nanotubes by vaporizing silicon carbide with no added
catalyst.
[0029] A silicon carbide target was made by mixing 36.1 grams (g)
of silicon carbide powder (Third Millennium Technologies, Inc.,
Knoxville, Tenn.) with 51.5 g of Dylon.RTM.graphite cement (Dylon
Industries, Inc., Cleveland, Ohio) in 80 ml of methanol. The
methanol was allowed to evaporate overnight. The remaining solid
was broken into small pieces in a mortar and pestle and compression
molded at 130.degree. C. for 1 hour. The molded article was then
baked at 1150.degree. C. for 10 hours in flowing Ar, and then
inserted into a furnace at 1100.degree. C. The target was ablated
with Nd--Yag lasers running at 30 Hz with a pulse width of 10
nanoseconds. The pressure in the furnace was maintained at 500
torr. The target was rotated during ablation to achieve even
ablation, and 1.12 g of product were collected after 1 h of run
time. The micrographs in FIG. 1 show the presence of single wall
carbon nanotubes.
* * * * *