U.S. patent application number 10/474066 was filed with the patent office on 2004-08-05 for process and apparatus for the production of carbon nanotubes.
Invention is credited to Dai, Liming, Hammel, Ernst, Huang, Shaoming, Johansen, Oddvar, Mau, Albert, Tang, Xinhie.
Application Number | 20040149209 10/474066 |
Document ID | / |
Family ID | 3828217 |
Filed Date | 2004-08-05 |
United States Patent
Application |
20040149209 |
Kind Code |
A1 |
Dai, Liming ; et
al. |
August 5, 2004 |
Process and apparatus for the production of carbon nanotubes
Abstract
A process for preparing carbon nanotubes comprising locating a
substrate (1) capable of supporting carbon nanotube growth in a
localised heating zone within a reaction chamber (7), said
localised heating zone being provided by a heating element (2)
located within the reaction chamber (7), passing a gaseous
carbonaceous material into the reaction chamber (7) such that the
gaseous material passes over and contacts the substrate (1) in the
localised heating zone, whereby the gaseous material undergoes
pyrolysis under the influence of the heat to form carbon nanotubes
on the substrate (1). Embodiments of the process prepare multilayer
carbon nanotubes and hetero-structured multilayer carbon nanotube
films.
Inventors: |
Dai, Liming; (Hudson,
OH) ; Huang, Shaoming; (Glen Waverley, AU) ;
Johansen, Oddvar; (Somerville, AU) ; Mau, Albert;
(Glen Waverley, AU) ; Hammel, Ernst; (Vienna,
AT) ; Tang, Xinhie; (Vienna, AT) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 PENNSYLVANIA AVENUE, N.W.
SUITE 800
WASHINGTON
DC
20037
US
|
Family ID: |
3828217 |
Appl. No.: |
10/474066 |
Filed: |
March 26, 2004 |
PCT Filed: |
April 4, 2002 |
PCT NO: |
PCT/AU02/00437 |
Current U.S.
Class: |
118/715 ;
427/249.1; 428/408 |
Current CPC
Class: |
C23C 16/46 20130101;
C30B 25/02 20130101; B82Y 30/00 20130101; C01B 32/162 20170801;
B82Y 40/00 20130101; C01B 2202/34 20130101; D01F 9/127 20130101;
D01F 9/133 20130101; Y10T 428/30 20150115; C30B 29/02 20130101;
C30B 29/605 20130101; C01B 2202/08 20130101; C23C 16/26 20130101;
C23C 16/0281 20130101 |
Class at
Publication: |
118/715 ;
428/408; 427/249.1 |
International
Class: |
B32B 009/00; C23C
016/26; C23C 016/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 4, 2001 |
AU |
PR 4217 |
Claims
The claims defining the invention are as follows:
1. A process for preparing carbon nanotubes comprising: locating a
substrate capable of supporting carbon nanotube growth in a
localised heating zone within a reaction chamber, said localised
heating zone being provided by a heating element located within
said reaction chamber, passing a gaseous carbonaceous material into
said reaction chamber such that the gaseous material passes over
and contacts said substrate in the localised heating zone, whereby
said gaseous material undergoes pyrolysis under the influence of
said heat to form carbon nanotubes on said substrate.
2. A process according to claim 1 wherein the localised heating
zone has a temperature greater than 300.degree. C.
3. A process according to claim 2 wherein the localised heating
zone has a temperature between 400.degree. C. and 800.degree.
C.
4. A process according to claim 1 wherein the substrate is quartz
glass, mesoporous silica, nanoporous alumina, a ceramic plate,
glass, graphite or mica.
5. A process according to claim 4 wherein the substrate is
glass.
6. A process according to claim 1 wherein the gaseous carbonaceous
material is selected from an alkane, alkene, alkyne or aromatic
hydrocarbon.
7. A process according to claim 6 wherein the gaseous carbonaceous
material is selected from methane, ethylene, benzene or
acetylene.
8. A process according to claim 7 wherein the gaseous carbonaceous
material is acetylene.
9. A process according to claim 1 wherein pyrolysis of the
carbonaceous material occurs in the presence of a catalyst.
10. A process according to claim 9 wherein the catalyst is coated
on the substrate.
11. A process according to claim 9 or 10 wherein the catalyst
comprises a transition metal selected from Ni, Fe, Co, Al, Mn, Pd,
Cr or alloys thereof.
12. A process according to claim 11 wherein the catalyst comprises
Ni.
13. A process according to claim 1 wherein the pyrolysis conditions
are controlled to provide aligned carbon nanotubes.
14. A process according to claim 1 wherein the pyrolysis conditions
are controlled to provide non-aligned carbon nanotubes.
15. A process according to claim 1 wherein the pyrolysis conditions
are controlled to provide homogeneous carbon nanotube growth on the
substrate.
16. A process according to claim 1 wherein the pyrolysis conditions
are controlled to provide patterned carbon nanotube growth on the
substrate.
17. A process for preparing multilayer carbon nanotube materials
comprising: (a) synthesising a first layer of carbon nanotubes on a
substrate under a first set of pyrolysis conditions to provide a
nanotube coated substrate; (b) synthesising a second layer of
carbon nanotubes on the nanotube coated substrate under a second
set of pyrolysis conditions, wherein at least one of steps (a) and
(b) is performed using a process of claim 1.
18. A process according to claim 17 wherein step (b) is repeated at
least once.
19. A process according to claim 17 wherein the pyrolysis
conditions of step (a) are the same as the pyrolysis conditions of
step (b).
20. A process according to claim 17 wherein the pyrolysis
conditions of step (a) are different from the pyrolysis conditions
of step (b).
21. A process for the preparation of a hetero-structured multilayer
carbon nanotube film comprising: (a) synthesising a first layer of
carbon nanotubes on a substrate under a first set of pyrolysis
conditions to provide a nanotube coated substrate; (b) coating a
layer of pyrolysis resistant material onto the nanotube coated
substrate to provide a hetero-structured multilayer substrate; (c)
synthesising a second layer of carbon nanotubes on the
hetero-structured multilayer substrate under a second set of
pyrolysis conditions, wherein at least one of steps (a) and (c) is
performed using a process of claim 1.
22. A process according to claim 21 wherein steps (b) and (c) are
repeated at least once.
23. A process according to claim 21 wherein the pyrolysis
conditions of step (a) are the same as the pyrolysis conditions of
step (c).
24. A process according to claim 21 wherein the pyrolysis
conditions of step (a) are different from the pyrolysis conditions
of step (c).
25. A process according to claim 21 wherein the pyrolysis resistant
material is a metal, a semiconductor or a polymer.
26. A process according to claim 25 wherein the pyrolysis resistant
material is a metal.
27. A reactor for preparing carbon nanotubes comprising: a reaction
chamber, at least one support means located within said reaction
chamber capable of supporting a substrate, said substrate being
capable of supporting carbon nanotube growth, at least one heating
element located within said reaction chamber capable of providing
localised heating to said substrate within said reaction chamber,
means for passing a gaseous carbonaceous material into said
reaction chamber such that it passes over and contacts said
substrate.
28. A reactor according to claim 27 wherein the reaction chamber is
formed from metal, glass, plastic or a combination thereof.
29. A reactor according to claim 28 wherein the reaction chamber is
formed from glass or comprises at least one glass panel.
30. A reactor according to claim 27 wherein the heating element
comprises resistant wires, an induction field, microwave radiation
or infrared radiation.
31. A reactor according to claim 27 wherein the heating element is
located within the substrate support.
32. A reactor according to claim 31 wherein the heating element and
substrate support comprise a ceramic plate into which resistant
wires have been inserted.
33. A reactor according to claim 27 wherein the means for passing a
gaseous carbonaceous material into the reaction chamber is at least
one gas conduit.
34. A reactor according to claim 33 wherein the at least one gas
conduit is located above the substrate.
35. A reactor according to claim 33 wherein the at least one gas
conduit is located to allow the gaseous carbonaceous material to
flow across the surface of the substrate.
36. A reactor according to claim 27 comprising multiple support
means.
37. A reactor according to claim 27 comprising multiple heating
elements.
38. A reactor according to claim 27 further comprising a
pre-heating zone.
39. A reactor according to claim 38 wherein the pre-heating zone is
located in a separate chamber from the reaction chamber.
40. A reactor according to claim 27 further comprising a cooling
zone.
41. A reactor according to claim 40 wherein the cooling zone is
located in a separate chamber from the reaction chamber.
42. A reactor according to claim 27 further comprising a means of
transferring a substrate from a pre-heating zone to a support means
and/or from the support means to a cooling zone.
43. Carbon nanotubes prepared by the process of any one of claims 1
to 16.
44. Multilayer carbon nanotubes prepared by the process of any one
of claims 17 to 20.
45. Hetero-structured multilayer carbon nanotube films prepared by
the process of any one of claims 21 to 26.
Description
[0001] This invention relates to carbon nanotubes, in particular to
a process and apparatus for the preparation of carbon
nanotubes.
[0002] Carbon nanotubes usually have a diameter in the order of 0.4
nanometres to 100 nanometres and a length of up to about 1
centimetre. These elongated nanotubes consist of carbon hexagons
arranged in a concentric manner with both ends of the tubes
normally capped by pentagon-containing fullerene-like structures.
Carbon nanotubes may have a single wall or multiwall structure.
They can behave as a semiconductor or metal depending on their
diameter and helicity of the arrangement of graphitic rings in the
walls, and dissimilar carbon nanotubes may be joined together
allowing the formation of molecular wires with interesting
electrical, magnetic, nonlinear optical, thermal and mechanical
properties. These unusual properties have led to diverse potential
applications for carbon nanotubes in material science and
nanotechnology. Indeed, carbon nanotubes have been proposed as new
materials for electron field emitters in panel displays,
single-molecular transistors, scanning probe microscope tips, gas
and electrochemical energy storages, catalyst and proteins/DNA
supports, molecular-filtration membranes, and energy-absorbing
materials (see, for example: M. Dresselhaus, et al., Phys. World,
Jan. 33, 1998; P.M. Ajayan, and T. W. Ebbesen, Rep. Prog. Phys.,
60,1027, 1997; R. Dagani, C&E News, Jan. 11, 31, 1999). The
importance of carbon nanotechnology is evidenced by increasing
research and development funding (C & E News, May 1, 2000,
pp.41-47).
[0003] For most of the above applications, it is highly desirable
that the carbon nanotubes are aligned and/or formed into patterns
so that the properties of the individual nanotubes can be easily
assessed and they can be incorporated effectively into devices.
[0004] Carbon nanotubes have been synthesised using arc discharge
(S. Iijima, Nature, 354, 56-68, 1991; T. W. Ebbesen and P. M.
Aegean, Nature, 358, 220-222, 1992) and catalytic pyrolysis (see,
for example: M. End. et Aa J. Pays Chum. Solids, 54, 1841-1848,
1994; V. Ivanov, et Al Chum. Pays. Let. 223, 329-335, 1994) and
often exist in an randomly entangled state. Patterned and
non-patterned carbon nanotube films having the nanotubes aligned
perpendicularly with the substrate have been prepared by pyrolysis
of iron (II) phthalocyanine in a flow reactor comprising a quartz
glass tube heated by a dual furnace (J. Phys. Chem. B., 104, 2000,
1891). Ren et al., have synthesised large arrays of well-aligned
carbon nanotubes by radio-frequency sputter-coating of a thin
nickel layer onto a substrate, followed by plasma-enhanced hot
filament chemical vapour deposition of acetylene in the presence of
ammonia gas at approximately 666.degree. C. (Science, 282, 1998,
1105).
[0005] Carbon nanotubes may be prepared at a variety of
temperatures, although generally higher temperatures, for example,
600.degree. C. to 1100.degree. C., are required for the preparation
of aligned carbon nanotubes. For economic reasons it is preferable
to prepare carbon nanotubes at lower temperatures, for example,
between 300.degree. C. to 800.degree. C.
[0006] Carbon nanotubes can be prepared in flow reactors comprising
a glass tube surrounded by a dual furnace. This technique results
in the entire reactor, including the glass tube, being heated and
maintained at pyrolysis temperature. Furthermore, carbon is not
only deposited on the substrate, but also on the other hot surfaces
in the reactor, such as the inside of the glass tube. The carbon
deposits on the glass can obscure the view of the substrate, making
it difficult to visually monitor the growth of the nanotubes. The
positioning of the furnace also generally obscures the view of the
substrate and the growth of the nanotubes.
[0007] It is an object of the present invention to overcome or at
least alleviate one or more of the disadvantages of the prior
art.
[0008] According to a first aspect of the invention there is
provided a process for preparing carbon nanotubes comprising:
[0009] locating a substrate capable of supporting carbon nanotube
growth in a localised heating zone within a reaction chamber, said
localised heating zone being provided by a heating element located
within said reaction chamber,
[0010] passing a gaseous carbonaceous material into said reaction
chamber such that the gaseous material passes over and contacts
said substrate in the localised heating zone, whereby said gaseous
material undergoes pyrolysis under the influence of said heat to
form carbon nanotubes on said substrate.
[0011] According to a second aspect of the invention there is
provided a reactor for preparing carbon nanotubes comprising:
[0012] a reaction chamber,
[0013] at least one support means located within said reaction
chamber capable of supporting a substrate, said substrate being
capable of supporting carbon nanotube growth,
[0014] at least one heating element located within said reaction
chamber capable of providing localised heating to said substrate
within said reaction chamber,
[0015] means for passing a gaseous carbonaceous material into said
reaction chamber such that it passes over and contacts said
substrate.
[0016] According to the present invention the substrate is heated
by a heating element in a localised heating zone within a reaction
chamber, thereby avoiding the need to heat the entire reaction
chamber to pyrolysis temperatures. While the pyrolysis can be
achieved at any suitable temperature in the localised heating zone,
the process of the invention conveniently allows the preparation of
carbon nanotubes at temperatures as low as 300.degree. C. The
carbon nanotubes are grown on a substrate that is heated to the
required temperature by the heating element. In view of the lower
temperatures required and the fact that the heating is localised,
the present invention can provide substantial energy and cost
savings relative to conventional methods. Also, since the heating
is localised to the heating zone, the growth of carbon nanotubes at
sites within the reaction chamber other than on the substrate and
the production of amorphous carbon byproducts inside the reaction
chamber are minimised. This also leads to a cleaner reaction
chamber and purer carbon nanotube films being formed. If amorphous
carbon is deposited on other hot surfaces, for example, exposed
areas of the heating element, they are readily removed by heating
the heating element in air, causing the amorphous carbon to be
oxidised to CO.sub.2. The reaction chamber therefore may be easily
cleaned.
[0017] The reaction chamber may be defined by one or more walls,
and may be of any size or shape suitable for accommodating one or
more heating elements. The wall(s) may be formed from any suitable
material, including metals, such as steel, aluminium, copper,
silver, platinum or alloys, glass, such as quartz glass, normal
glass or the like, plastic, polymethylmethacrylate (PMMA), Mylar,
polypropylene (PP), polyethylene (PE) or their composites, or a
combination thereof. The localised heating zone in the vicinity of
the heating element ensures that the temperature of the wall(s) of
the chamber remain lower than the temperature in the localised
heating zone where pyrolysis occurs. If the chamber is large
enough, walls of the chamber remote from the heating zone will
remain at ambient temperature. Preferably the reaction chamber is
formed from glass or at least includes one or more glass panels.
Preferably, the chamber is tubular and its walls are transparent or
partially transparent. Advantageously, transparency allows the
visual observation of carbon nanotube growth and therefore
undesirable growth can be terminated at any stage. Visual
observation also allows easier control of the length of the carbon
nanotubes by stopping the growth process at a desired time.
[0018] The support means may be any support means capable of
supporting a substrate within the reaction chamber and capable of
withstanding the pyrolysis temperatures used. For example, the
support means may be in the form of a solid block, plate, grate,
bracket, cradle, stretcher, scaffold or the like and may be made
from any suitable material, for example, metal or ceramic
materials. The support means may be any size or shape suitable to
support the substrate.
[0019] The heating element may be any suitable heating means
capable of heating a substrate and providing a localised heating
zone. For example, suitable heating means may include resistant
wires, induction field, microwave radiation or infrared radiation.
The localised heating zone can also be heated from a remote point
by, for example, a focussed infrared beam or laser beam. In a
preferred embodiment, the heating element also acts as the support
means for the substrate. In this embodiment, the heating element
preferably forms a flat surface upon which the substrate is
supported. An example of a suitable heating element which also acts
as a substrate support is a ceramic plate into which resistant
wires have been inserted. The heating element/substrate support may
be formed in any shape or size appropriate to support and heat the
substrate and heating zone. Preferably, the heating element allows
the substrate to be heated homogenously, i.e., the temperature
distribution of the heated substrate is homogenous. One means of
achieving homogenous temperature distribution is to place a
conducting material, for example, a copper sheet, between the
heating plate and the substrate allowing even temperature
distribution.
[0020] The reactor of the invention also includes a means for
passing gaseous carbonaceous material into the reaction chamber
such that it passes over and contacts the substrate. This means may
be provided by at least one gas conduit. The at least one gas
conduit is positioned to allow the flow of gaseous carbonaceous
material into the localised heating zone. In a preferred
embodiment, the inlet for the carbonaceous material is positioned
directly above the substrate so that the gaseous carbonaceous
material is supplied directly to the localised heating zone.
Alternatively, the gaseous carbonaceous material may be supplied
through an inlet at one end of the chamber and allowed to flow
across the substrate in the localised heating zone. Multiple gas
conduits may be used to supply gaseous carbonaceous material to a
large localised heating zone or multiple localised heating zones
located within the reaction chamber.
[0021] The at least one gas conduit may also be used as a gas inlet
for supplying other gases to the reaction chamber and as a gas
outlet to allow the exit of gases from the chamber. One gas conduit
may be used as both gas inlet and gas outlet. Alternatively,
multiple gas conduits may be used, each functioning as a gas inlet
or gas outlet.
[0022] If a single gas conduit is used, it may be attached to all
gas sources to be supplied to the chamber and a vacuum so the
chamber may be evacuated. However, the vacuum may not be applied to
the chamber at the same time as gas is supplied. The vacuum is not
necessary if inert gases (e.g. Ar) are used to flush the reaction
chamber.
[0023] A gas inlet may be used to supply reducing or inert
atmospheres, for example, H.sub.2 and/or nitrogen or argon, to the
chamber before pyrolysis and to supply the gaseous carbonaceous
material to be pyrolysed. These gases may be supplied through a
single inlet or through separate inlets.
[0024] A gas outlet may be used to allow the exit of the unused
gases and byproducts of the pyrolysis reaction. A gas outlet may be
attached to a vacuum pump to allow evacuation of the reactor before
the introduction of a reducing and/or inert atmosphere. The gas
outlet may also be attached to a device, such as a bubbler, to
allow a slight positive pressure of gas to be maintained in the
chamber during the deposition of carbon nanotubes.
[0025] In the process of the invention, the substrate may be any
substrate-capable of withstanding the pyrolysis conditions employed
and capable of supporting carbon nanotube growth. Examples of
suitable substrates include quartz glass, mesoporous silica,
nanoporous alumina, ceramic plates, glass, graphite and mica.
Preferably the substrate is ordinary glass. Preferably the surface
of the substrate upon which the carbon nanotubes are grown is
smooth.
[0026] The gaseous carbonaceous material may be any carbonaceous
compound or substance which may be gasified and which is capable of
forming carbon nanotubes when subjected to pyrolysis. Examples of
such compounds are alkanes, alkenes, alkynes and aromatic
hydrocarbons, for example, methane, ethylene, benzene or acetylene.
Preferably the carbonaceous material is acetylene.
[0027] Pyrolysis is performed in the presence of a catalyst. The
catalyst may be any compound, element or substance suitable for
catalysing the conversion of a carbonaceous material to carbon
nanotubes under the pyrolysis conditions. Preferably the catalyst
comprises a transition metal including Ni, Fe, Co, Al, Mn, Pd, Cr
or alloys thereof in any suitable oxidation state. Most preferably,
the catalyst comprises Ni. For example, the catalyst may be
prepared from polyvinylalcohol/Ni(NO.sub.3).- sub.2.6H.sub.2O (PVA
Ni.) Preferably, the surface of the substrate is coated with a
substance from which the catalyst is prepared. For example, a
spin-coated PVA Ni layer, subjected to oxidation at 500.degree. C.
for 30 minutes and reduction at 600.degree. C. for 30 minutes
provided a catalyst coating showing strong adhesion onto a glass
substrate, even when subjected to compressed air. Reduction of the
coating in the reactor is readily performed by supplying a mixture
of H.sub.2/Ar to provide the catalyst-coated substrate. The
substrate may then be maintained in an inert atmosphere, for
example, nitrogen or argon, to prevent the catalyst being
oxidised.
[0028] The pyrolysis conditions employed will depend on the nature
of the gaseous carbonaceous material, the catalyst used, and the
length and density of the carbon nanotubes required. It is possible
to vary the pyrolysis conditions, such as temperature, time,
catalyst, pressure or flow rate through the reactor to obtain
carbon nanotubes having different characteristics.
[0029] Pyrolysis may be performed at temperatures above 300.degree.
C. Preferably in the process of the invention temperatures in the
heating zone are between 400.degree. C. and 800.degree. C. The
selection of catalyst affects the temperature at which carbon
nanotubes may be formed. The carbon formed during pyrolysis is then
selectively deposited on the hot surface of the substrate in the
heating zone, forming carbon nanotubes. Temperatures below
400.degree. C. are demonstrated to be suitable for the nanotube
growth with the ratio of carbon nanotubes to carbon nanofibre,
their morphology and alignment depending on the conditions used.
Surprisingly, it was found using the process of the present
invention that well-aligned carbon nanotubes were easily formed
well below the softening point of normal glass plates (ca.
640.degree. C.). Within the temperature ranging from 400.degree. C.
and 800.degree. C., the nanotube deposition is completed within a
couple of seconds to 20 minutes. The formation of carbon nanotubes
is a typical transient reaction and its deposition rate can be
controlled by adjusting the pressure of the carbonaceous gas. With
a low feed of carbonaceous gas, the carbon nanotube growth is from
edge to centre of the substrate. In contrast, a homogenous coating
is seen over the substrate surface within a couple of seconds at a
high gas feed rate. Therefore, the deposition reaction can be
region-specifically controlled by controlling the feed of the
carbonaceous gas.
[0030] The carbon nanotubes produced by the process may be aligned
or non-aligned. Aligned or non-aligned carbon nanotubes may be
selected for by varying temperatures, type of catalyst used and the
density of the catalyst coating on the substrate. For example, a
low density of catalyst coating will favour non-aligned carbon
nanotube growth whereas a high density of catalyst coating will
favour aligned carbon nanotube growth.
[0031] The length of aligned carbon nanotubes may be varied over a
certain range (from a sub-micrometre to several tens of
micrometres) in a controllable fashion by changing the experimental
conditions such as the pyrolysis time and gas flow rate. The size
and shape of the aligned carbon nanotube film is, in principle,
limited only by the size and shape of the substrate.
[0032] According to one embodiment of the invention, the reaction
chamber may have a pre-heating zone, where the substrate may be
pre-heated to a predetermined temperature before entering the
localised heating zone where carbon nanotube deposition occurs. The
reaction chamber may also have a cooling zone where the substrate
is cooled after carbon nanotube deposition is complete. In one
embodiment of the invention, the reactor includes a means of moving
a substrate from the pre-heating zone to the localised heating zone
and from the localised heating zone to the cooling zone. For
example, the substrate may sit on a transporting belt.
[0033] The reaction chamber of the present invention may be adapted
to have multiple localised heating zones by having multiple heating
elements located within the chamber. Carbon nanotubes may then be
deposited on multiple substrates simultaneously or in separate
areas of a larger substrate. In a reactor containing multiple
localised heating zones, a gas inlet may be attached to a gas
distributor which allows the gaseous carbonaceous material to be
supplied to each of the multiple localised heating zones
simultaneously. Alternatively, the gaseous carbonaceous materials
may be supplied through an inlet at one end of the chamber and
allowed to flow across each substrate and through each localised
heating zone sequentially.
[0034] The reactor of the present invention may be adapted to allow
continuous carbon nanotube deposition on multiple substrates by
having a means of moving a substrate from a pre-heating zone to the
localised heating zone before carbon nanotube deposition and a
means of moving a substrate to a cooling zone after carbon nanotube
deposition is complete. Continuous carbon nanotube production may
be achieved by pre-heating a substrate or substrates in the
pre-heating zone, moving the substrate or substrates to the
localised heating zone, synthesising carbon nanotubes on the
substrate or substrates, moving the substrate or substrates into
the cooling zone, removing the cooled substrate or substrates
having a carbon nanotube film deposited on it/them from the
reactor, and supplying new substrate or substrates to the
pre-heating zone. Generally only one step, e.g. pre-heating, carbon
deposition or cooling, is performed at a certain part of the
reactor at any one time. The process may be continuously repeated.
The reduction of a substance from which the catalyst is prepared
may also be performed in the reactor by supplying a mixture of
Ar/H.sub.2 to the chamber to provide the catalyst coated
substrate.
[0035] In the continuous process, the substrate may be located in a
first chamber and then transported to a second chamber having a
pre-heating zone and/or localised heating zone and/or cooling zone.
Advantageously, the reduction of a substance from which the
catalyst is prepared may be performed in the first chamber to
provide a catalyst coated substrate. The substrate may then be
transported to the pre-heating zone or localised heating zone in
the second chamber when the atmosphere in the first chamber has
been flushed with inert gas. In a preferred embodiment, the two
chambers are connected and the catalyst coated substrate may pass
from the first chamber to the pre-heating zone or localised heating
zone in the second chamber through a connecting door.
[0036] In a reactor adapted for use in the continuous process of
the invention, the localised heating zone and the cooling zone may
be in the same chamber. Alternatively, the cooling zone may be in a
separate chamber from the localised heating zone. If the localised
heating zone and cooling zone are in separate chambers, the two
chambers may be connected such that the substrate in the localised
heating zone may be transferred to the cooling zone via a
connecting door after carbon deposition has occurred.
[0037] According to the present invention it is possible to prepare
multilayer carbon nanotube materials by synthesising a first layer
of carbon nanotubes on a substrate under a first set of pyrolysis
conditions, and then synthesising a second layer of carbon
nanotubes on the nanotube coated substrate under a second set of
pyrolysis conditions.
[0038] This process may be repeated until the desired number of
carbon nanotube layers are present. Each layer of carbon nanotubes
may be deposited using the same or different pyrolysis conditions.
After preparation of the multilayer structure the carbon nanotube
film may be removed from the substrate using appropriate
conditions.
[0039] It is also possible according to the present invention to
prepare hetero-structured multilayer carbon nanotube films by
interposing layers of carbon nanotubes between layers of pyrolysis
resistant materials, the carbon nanotubes being generated in
accordance with the process of the present invention.
[0040] The term "hetero-structured" as used herein refers to a
multilayer structure which includes one or more carbon nanotube
layers together with layers of other materials
[0041] The pyrolysis resistant material may be a metal, preferably
Au, Pt, Ni, Cu, a semiconductor, TiO.sub.2, MgO, Al.sub.2O.sub.3,
ZnO, SnO.sub.2, Ga.sub.2O.sub.3, In.sub.2O.sub.3, CdO or a polymer
or any other pyrolysis resistant material that is capable of
supporting the nanotube growth.
[0042] The pyrolysis resistant material may be applied to the
carbon nanotube coated substrate by any suitable means. Preferably
metals are applied by sputter-coating, polymers are applied by
spin-casting and semiconductors by sputter-coating or physical
deposition.
[0043] As is evident from the above description, the process and
apparatus of the invention allow the preparation of a large variety
of carbon nanotube films and structures. It is also possible to
provide patterned layers using appropriate masking and etching
techniques. At lower temperatures it may be possible to use the
reactor of the invention to prepare carbon nanofibres or mixtures
of carbon nanotubes and nanofibres.
[0044] The materials produced by the present invention may be used
in the construction of devices for practical applications in many
fields including electron emitters, field-emission transistors,
electrodes for photovoltaic cells and light emitting diodes,
optoelectronic elements, bismuth actuators, chemical and biological
sensors, gas and energy storages, molecular filtration membranes
and energy-absorbing materials.
[0045] The invention can be more fully understood from the
following detailed description of FIG. 1 and the examples. It
should be understood that the examples and Figures described are
only for illustration purposes, which does not intend to constitute
a limitation on the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0046] FIG. 1AA is a diagrammatic side-view representation of a
pyrolysis flow reactor of the invention having a gas inlet at one
end of the chamber, and a gas outlet at the other.
[0047] FIG. 1AB is a diagrammatic side view representation of a
pyrolysis flow reactor of the invention having a gas inlet
positioned above the localised heating zone and gas outlets
positioned on the respective ends of the reaction chamber.
[0048] FIG. 1AC is a diagrammatic end view representation of a
pyrolysis reactor of the invention having a gas inlet positioned
above the localised heating zone and a gas outlet positioned
below.
[0049] FIG. 1BA is a diagrammatic side view representation of a
pyrolysis reactor of the invention having multiple localised
heating zones.
[0050] FIG. 1BB is a diagrammatic top view representation of a
heating element supporting multiple substrates.
[0051] FIG. 1CA is a diagrammatic side view representation of a
pyrolysis reactor of the invention having a pre-heating zone, a
localised heating zone and a cooling zone and wherein inlet for
supplying the gaseous carbonaceous material is positioned above the
localised heating zone.
[0052] FIG. 1CB is a diagrammatic top plan view of a heating
element having a pre-heating zone, a localised heating zone and a
cooling zone.
[0053] FIG. 1D is a diagrammatic side plan view of a reactor of the
invention in which a substrate may be moved from the pre-heating
zone to the localised heating zone and finally to the cooling zone
before removal from the reactor.
[0054] FIG. 1EA is a series of diagrammatic side plan views of a
heating element designs suitable for use in the reactor of the
invention.
[0055] FIG. 1EB is a diagrammatic top plan view of a heating
element suitable for use in the reactor of the invention.
[0056] FIG. 1AA is a diagrammatic representation of a pyrolysis
reactor of the invention in which a substrate (1) has been
positioned on the heating element (2). The heating element (2) and
substrate (1) are located within a reaction chamber (7) defined by
chamber walls (3). A sheet of conducting material (4) is positioned
between the substrate (1) and the heating element (2) in order to
provide homogenous heating of the substrate (1). A gas inlet (5) is
positioned at one end of the reaction chamber and a gas outlet (6)
is positioned at the opposing end of the reaction chamber such that
any gas introduced into the reaction chamber (7) flows from one end
of the chamber to the other, flowing over the substrate (1).
[0057] FIG. 1AB is a diagrammatic representation of a pyrolysis
reactor of the invention similar to that shown in FIG. 1AA but
having a gas inlet (5) positioned above the heating element (2)
upon which the substrate (1) is supported. The gas is suppled
directly into the localised heating zone (8). Gas outlets (6) are
located at each end of the chamber.
[0058] FIG. 1AC is a diagrammatic end view representation of a
pyrolysis reactor of the invention similar to that shown in FIG.
1AA or 1AB but having a gas outlet (6) located below the localised
heating zone (8).
[0059] In operation, the reactor shown in FIGS. 1AA to 1AC is
flushed with an inert gas such as N.sub.2 or Ar, or is evacuated by
means of a vacuum pump connected to the gas outlet (6). A mixture
of H.sub.2 and Ar is introduced into the reaction chamber (7) to
ensure the catalyst is in a reduced state. The heating element (2)
is heated to the required temperature which heats the substrate (1)
to the required temperature. When the catalyst has been reduced, a
flow of Ar gas is maintained to ensure an inert atmosphere in the
chamber. A gaseous carbonaceous material, such as ethylene, is
introduced into the reaction chamber (7) through the gas inlet (5).
Pyrolysis of the carbonaceous material occurs in the localised
heating zone (8) and carbon is deposited on the hot surface of the
substrate (1) to produce a carbon nanotube layer. The substrate (1)
is then allowed to cool.
[0060] FIG. 1BA is a diagrammatic side view representation of a
pyrolysis reactor of the invention in which multiple substrates (1)
are positioned on a heating element (2). The heating element (2)
and the substrates are located within a reaction chamber (7)
defined by chamber walls (3). The gas inlet (5) is connected to a
gas distributor (9) that allows the carbon-containing material to
be simultaneously introduced into multiple localised heating zones
(8). The gas outlet (6) is positioned at one end of chamber (7) and
a connection to a vacuum pump (10) is positioned at the opposing
end of chamber (7).
[0061] FIG. 1BB is a diagrammatic top plan view representation of
the heating element (2) of the reactor shown in FIG. 1BA. Multiple
substrates (1) are positioned on the heating element (2).
[0062] In operation, the reactor shown in FIG. 1BA is evacuated by
a vacuum pump connected to outlet (10) and a mixture of H.sub.2 and
Ar is introduced into the reaction chamber (7). The heating element
(2) is heated to the required temperature which heats the multiple
substrates (1) to the required temperature. When the catalyst has
been reduced, a positive pressure of inert atmosphere (Ar) is
maintained in the reaction chamber (7). A gaseous carbonaceous
material is introduced into multiple localised heating zones (8)
from the gas inlet (5) by means of a gas distributor (9). Pyrolysis
of the carbonaceous material occurs in the multiple localised
heating zones (8) and carbon is deposited on the hot surfaces of
the multiple substrates (1) to produce a carbon nanotube layer on
each of the multiple substrates (1). The substrates are then
allowed to cool. Multiple substrates are simultaneously coated with
a layer of carbon nanotube in this reactor.
[0063] FIG. 1CA is a diagrammatic side view representation of a
pyrolysis reactor of the invention in which a substrate (1) is
positioned on the heating element (2). The heating element (2) and
the substrate (1) is located within the reaction chamber (7)
defined by chamber walls (3). The heating element (2) is divided
into zones having different temperatures, a pre-heating zone (11),
a localised heating zone (8) and a cooling zone (12). The gas inlet
(5) is positioned above the localised heating zone (8) and
distributes the gaseous carbonaceous material evenly over the
substrate (1) when it is in the localised heating zone (8). The gas
outlet (6) is positioned at one end of the chamber (7) and a
connection to a vacuum pump (10) is positioned at the opposing end
of the chamber (7).
[0064] FIG. 1CB is a diagrammatic top plan view representation of a
heating element (2) of the reactor shown in FIG. 1CA. A substrate
(1) is initially positioned in the pre-heating zone (11), then the
substrate is moved to the localised heating zone and finally the
substrate is moved to the cooling zone (12).
[0065] In operation, a substrate (1) is pre-heated to a
predetermined temperature in the pre-heating zone (11). The
substrate (1) is then moved to the localised heating zone (8) and
is heated to pyrolysis temperature. A gaseous carbonaceous material
is introduced into the localised heating zone (8) through gas
inlet. The gaseous carbonaceous material is pyrolysed and carbon
nanotubes are deposited on the surface of the substrate (1). The
substrate (1), upon which carbon nanotubes have been deposited, is
moved to the cooling zone (12) and allowed to cool before being
removed from the reactor. The reactor may also include a mechanism
for transferring the cooled substrate (1) having a film of carbon
nanotubes out of the reactor and a mechanism for positioning a new
substrate in the pre-heating zone. The reactor may also include a
mechanism which allows transfer of a substrate from one zone to the
next within the chamber. (7) The process may be performed in a
continuous manner, for example, the substrate may sit on a
transporting belt. Such a device is also illustrated in FIG.
1D.
[0066] FIGS. 1EA to 1ED show a series of heating element
(2)/conducting material (4)/substrate (1) configurations which may
be useful in the reactor of the invention. In FIG. 1EA, the sheet
of conducting material (4) and the substrate (1) are embedded in
the heating element (2). In FIG. 1EB, the sheet of conducting
material (4) is embedded in the heating element (2) and a substrate
(1) which is smaller than the heating element (2) is placed in the
heating element (2) on the conducting metal sheet (4). In FIG. 1EC,
the surface of the heating element (2) is flat and a substrate (1)
having a smaller surface than the heating element (2) is placed on
a sheet of conducting material (4) also having a smaller surface
than the heating element (2). The sheet of conducting material (4)
and the substrate (1) protrude above the surface of the heating
element (2). In FIG. 1ED, the heating element (2) is coated with a
conducting material (4) across its entire surface and a substrate
(1) is positioned on top of the conducting material coating. The
configuration in FIG. 1ED is the most suitable for use in a reactor
where continuous carbon nanotube deposition occurs, for example,
the reactor shown in FIG. 1C or FIG. 1D.
[0067] FIG. 1EE is a diagrammatic top plan view of a preferred
heating element (2) having a substrate (1) positioned on it. The
heating element (2) has a sheet of conducting material (4) placed
on it to provide homogenous temperature distribution and a
resistant wire (13) is supported within the heating element to heat
the heating element.
EXAMPLES
Example 1
Preparation of Non-Aligned Carbon Nanotubes
[0068] A glass substrate was spin-coated with a PVA Ni layer (100
mM Ni(NO.sub.3).sub.2.6H.sub.2O and 3 wt % PVA) to provide a
catalyst for deposition of carbon nanotubes. The coated substrate
was oxidised at 500.degree. C. for 30 minutes and reduced at
600.degree. C. for 30 minutes. After such treatment, the catalyst
coating was strongly adhered to the substrate. The catalyst-coated
substrate was placed in the reactor as shown in FIG. 1A and the
atmosphere in the reaction chamber was replaced with H.sub.2/Ar to
ensure the catalyst was in a reduced state. The substrate was
heated at 650.degree. C. on the heating element and acetylene/Ar
gas (V:V=1:3), introduced at a total flow rate of 60 ml/min for a
time of 3 minutes, was pyrolysed resulting in the deposition of
non-aligned carbon nanotubes on the catalyst-coated substrate.
Example 2
Preparation of Aligned Carbon Nanotubes
[0069] Aligned carbon nanotubes were prepared by the same method as
applied in Example 1, but at a lower temperature (440.degree. C.)
was used. The resulting carbon nanotubes align almost normal to the
substrates surface and are densely packed with a fairly uniform
tubular length of ca. 1 .mu.m.
* * * * *