U.S. patent application number 15/958375 was filed with the patent office on 2018-10-25 for carbon materials comprising carbon nanotubes and methods of making carbon nanotubes.
The applicant listed for this patent is CAMBRIDGE ENTERPRISE LIMITED. Invention is credited to Krzysztof Kazimierz Koziol, Agnieszka Ewa Lekawa-Raus, Rajyashree Sundaram, Alan Windle.
Application Number | 20180305211 15/958375 |
Document ID | / |
Family ID | 43401671 |
Filed Date | 2018-10-25 |
United States Patent
Application |
20180305211 |
Kind Code |
A1 |
Sundaram; Rajyashree ; et
al. |
October 25, 2018 |
Carbon Materials Comprising Carbon Nanotubes and Methods of Making
Carbon Nanotubes
Abstract
The present invention relates to carbon materials comprising
carbon nanotubes, powders comprising carbon nanotubes and methods
of making carbon nanotubes. In the methods of the present
invention, the size and/or formation of floating catalyst particles
is closely controlled. The resulting carbon nanotubes typically
exhibit armchair chirality and typically have metallic properties.
The carbon nanotubes produced by this method readily form bulk
materials, which typically have a conductivity of at least
0.7.times.10.sup.6 Sm.sup.-1 in at least one direction. The
invention has particular application to the manufacture of
components such as electrical conductors. Suitable electrical
conductors include wires (e.g. for electrical motors) and cables
(e.g. for transmitting electrical power).
Inventors: |
Sundaram; Rajyashree;
(Chitlapakam Chennai Tamil Nadu, IN) ; Koziol; Krzysztof
Kazimierz; (Cambridge, GB) ; Lekawa-Raus; Agnieszka
Ewa; (Cambridge, GB) ; Windle; Alan;
(Cambridge, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CAMBRIDGE ENTERPRISE LIMITED |
Cambridge |
|
GB |
|
|
Family ID: |
43401671 |
Appl. No.: |
15/958375 |
Filed: |
April 20, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13882721 |
Jul 30, 2013 |
9969619 |
|
|
PCT/GB11/01549 |
Nov 2, 2011 |
|
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15958375 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
Y10T 428/268 20150115;
D01F 9/133 20130101; B01J 23/745 20130101; B01J 37/086 20130101;
C01B 2202/30 20130101; C01B 32/162 20170801; B01J 35/006 20130101;
D01F 9/127 20130101; B82Y 30/00 20130101; D01F 9/1272 20130101;
C01B 2202/22 20130101; Y10T 428/2982 20150115; C01B 32/16 20170801;
Y10T 428/2927 20150115; B82Y 40/00 20130101; C01B 2202/36 20130101;
D01F 11/14 20130101 |
International
Class: |
C01B 32/162 20060101
C01B032/162; B01J 23/745 20060101 B01J023/745; B82Y 30/00 20060101
B82Y030/00; B01J 35/00 20060101 B01J035/00; D01F 9/127 20060101
D01F009/127; B01J 37/08 20060101 B01J037/08; B82Y 40/00 20060101
B82Y040/00; D01F 9/133 20060101 D01F009/133; C01B 32/16 20060101
C01B032/16; D01F 11/14 20060101 D01F011/14 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 2, 2010 |
GB |
1018498.4 |
Claims
1. A carbon fiber comprising at least 75% by weight of carbon
nanotubes, wherein at least 70% by number of the carbon nanotubes
have a diameter in the range from 1 nm to 2.5 nm, and wherein the
carbon fiber has a conductivity of at least
0.7.times.10.sup.6Sm.sup.-1 in at least one direction.
2-3. (canceled)
4. A carbon fiber according to claim 1 which has a length greater
than 0.5 m.
5. A carbon fiber according to claim 1, wherein the fiber has a
density of at least 0.1 g cm.sup.-3.
6. A carbon fiber according to claim 1, wherein the fiber has a
specific strength of at least 0.1 GPa SG.sup.-1 in at least one
direction.
7. A carbon fiber according to claim 1, wherein the fiber has a
specific stiffness of 30 GPa SG.sup.-1 or more.
8. A carbon material according to claim 1, having a plurality of
catalyst particles dispersed in the fiber, wherein the fiber
comprises 20 wt % or less of catalyst particles.
9. A carbon fiber according to claim 8 wherein at least 70% by
number of the catalyst particles have a diameter in the range from
0.5 nm to 4.5 nm.
10. A carbon fiber according to claim 1, comprising a plurality of
carbon nanotubes; a plurality of catalyst particles dispersed in
the fiber; and incidental impurities.
11. A carbon fiber according to claim 1, wherein at least 50% by
number of the carbon nanotubes are single-walled armchair carbon
nanotubes.
12. A carbon fiber according to claim 8 wherein the catalyst
particles comprise transition metal atoms.
13. A carbon fiber according to claim 8 wherein the catalyst
particles comprise sulphur atoms.
14. A carbon fiber according to claim 8 wherein the catalyst
particles comprise an inner core of transition metal atoms
surrounded by a shell or cage of sulphur atoms.
15. A carbon fiber according to claim 1 having a diameter equal to
or greater than 1 .mu.m and equal to or less than 10 cm.
16. A carbon fiber according to claim 1 wherein a high proportion
of the carbon nanotubes are metallic.
17. A carbon fiber according to claim 1 wherein a high proportion
of the carbon nanotubes have armchair chirality.
18. A carbon film comprising at least 75% by weight of carbon
nanotubes, wherein at least 70% by number of the carbon nanotubes
have a diameter in the range from 1 nm to 2.5 nm, and wherein the
carbon film has a conductivity of at least
0.7.times.10.sup.6Sm.sup.-1 in at least one direction.
19. A carbon material comprising at least 75% by weight of carbon
nanotubes, wherein at least 70% by number of the carbon nanotubes
have a diameter in the range from 1 nm to 2.5 nm, and wherein the
carbon material has a conductivity of at least 0.7.times.10.sup.6
Sm.sup.-1 in at least one direction, wherein a plurality of
catalyst particles are dispersed in the material, wherein the
material comprises 20 wt % or less of catalyst particles, wherein
the catalyst particles comprise sulphur atoms.
Description
BACKGROUND TO THE INVENTION
Field of the Invention
[0001] The present invention relates to carbon materials comprising
carbon nanotubes, powders comprising carbon nanotubes, and methods
of making carbon nanotubes. The present invention has particular,
but not exclusive, application to the manufacture of components
such as electrical conductors. Suitable electrical conductors
include wires (e.g. for electrical motors) and cables (e.g. for
transmitting electrical power).
Related Art
[0002] Carbon nanotubes are allotropes of carbon, which are tubular
and typically have a diameter in the nanometre range. The carbon
atoms of a carbon nanotube are each covalently bonded to three
other carbon atoms, to create a "hexagonal" lattice which forms a
wall of the tube. Accordingly, a carbon nanotube can be thought of
as a "rolled" graphene sheet. Single-walled carbon nanotubes have a
single layer of carbon atoms.
[0003] Double- and multi-walled carbon nanotubes have two or more
layers of carbon atoms, respectively.
[0004] The chirality of carbon nanotubes can vary, depending on the
orientation of the hexagonal lattice of the notional graphene sheet
with respect to the tube axis. Carbon nanotube chirality will be
well understood by a person skilled in the art. For example, carbon
nanotubes may have armchair chirality or zigzag chirality. Carbon
nanotubes with a chirality intermediate an armchair and a zigzag
chirality are generally referred to as chiral carbon nanotubes.
[0005] Without wishing to be bound by theory, it is believed that
all single-walled armchair carbon nanotubes are electrically
conductive, regardless of their diameter (i.e. it is believed that
all single-walled armchair carbon nanotubes are metallic). Zigzag
and chiral carbon nanotubes may be metallic or semiconducting.
[0006] (Nanotube chirality and properties, such as metallic and
semiconducting properties, are explained in detail in Reference 4,
which is incorporated herein by reference in its entirety.)
[0007] Production of bulk carbon nanotube materials is of
particular interest. Such carbon nanotube materials can have
particularly beneficial properties, such as relatively low density
and high strength.
[0008] WO2008/132467 describes densifying carbon nanotubes to
improve the efficiency of carbon nanotube packing, in order to
provide a fibre or film. For example, a density enhancement agent
such as divinyl benzene may be applied to the carbon nanotubes, in
order to improve the packing of the carbon nanotubes, which
provides a higher strength material. The fibres and films described
in WO2008/132467 may be at least one metre long.
[0009] Similarly, Koziol et al' have described the production of
carbon nanotube fibres with high specific strength and high
specific stiffness. This document describes the production of
carbon nanotubes by thermal chemical vapour deposition (CVD). In
the methods described, the resulting "aerogel" of carbon nanotubes
is drawn into a fibre, which is run through an acetone vapour
stream to enhance the densification. A winding rate of up to 20 m
min.sup.-1 is employed to draw the fibre.
[0010] Motta et al.sup.2 have described the effect of sulphur as a
promoter of carbon nanotube formation. They describe using
thiophene as a sulphur precursor in iron catalysed thermal CVD, to
produce nanotubes with a diameter of between 4 nm and 10 nm, which
were typically double-walled. The iron catalyst particles were
about 5 nm to 10 nm. The resulting aerogel was drawn into a fibre,
with a winding rate of 20 m min.sup.-1. Motta et al report a high
carbon nanotube growth rate of up to 0.1 to 1 mm sec.sup.-1.
[0011] The carbon materials produced by the methods described in
these documents include a mixture of carbon nanotubes with a wide
distribution of diameters and with a wide distribution of
chiralities (including armchair, zigzag and intermediate
chiralities). Increasing the degree of control of carbon nanotube
formation would provide a greater control of the properties of the
resulting carbon materials produced, but although many researchers
have made efforts to provide such control, the present inventors
are not aware of any disclosure of significant recent progress in
this area.
SUMMARY OF THE INVENTION
[0012] The present inventors have devised the present invention in
order to address one or more of the above problems.
[0013] The present inventors have realised that by ensuring close
control of the size of catalyst particles in gas phase formation of
carbon nanotubes, for example in chemical vapour deposition (CVD),
it is possible to control the diameter of the carbon nanotubes
produced. The resulting carbon nanotubes typically exhibit armchair
chirality and typically have metallic properties. The carbon
nanotubes produced in this method readily form bulk materials, for
example by the densification methods described by Koziol et
al'.
[0014] Thus, the present inventors have for the first time
demonstrated that it is possible to produce an electrically
conductive carbon material in bulk form, which includes a narrow
size range of small diameter carbon nanotubes.
[0015] Accordingly, in a first preferred aspect, the present
invention provides a carbon material comprising carbon nanotubes,
wherein at least 70% by number of the carbon nanotubes have a
diameter in the range from 1 nm to 2.5 nm.
[0016] The term "material" here is intended to mean a substance
that has the form of a solid and has independent existence, in the
sense that it has no requirement to be supported by a substrate.
Thus, the material can be self-supporting (or, more generally,
capable of being self-supporting). However, the materials of the
invention may cooperate with other materials (such as substrates)
in order to provide the materials with additional
functionality.
[0017] The carbon material is preferably provided in the form of at
least one fibre. The fibre typically comprises a very large number
of carbon nanotubes.
[0018] The carbon material preferably has a conductivity of at
least 0.7.times.10.sup.6 Sm.sup.-1 in at least one direction (at
room temperature). Preferably, the carbon material comprises at
least 75% by weight of carbon nanotubes. The carbon material may
be, for example, a fibre or a film. It may have at least one
dimension greater than 0.5 m.
[0019] In a second preferred aspect, the present invention provides
a method of producing carbon nanotubes, the method comprising:
[0020] providing a plurality of floating catalyst particles,
wherein at least 70% by number of the catalyst particles have a
diameter less than or equal to 4.5 nm; and [0021] contacting the
floating catalyst particles with a gas phase carbon source to
produce carbon nanotubes.
[0022] The significance of contacting floating catalyst particles
with the gas phase carbon source is that, at least during the
formation of the carbon nanotubes, the catalyst particles are not
supported on a substrate but instead are held (e.g. suspended)
within a gas.
[0023] Preferably, the step of providing a plurality of catalyst
particles comprises initiating growth of catalyst particles and
subsequently arresting the growth of the catalyst particles using
an arresting agent. The steps of initiating growth of catalyst
particles and subsequently arresting their growth is preferably
carried out in the gas phase.
[0024] In another preferred aspect, the present invention provides
a method of producing carbon nanotubes, the method comprising:
[0025] providing a plurality of floating catalyst particles; and
[0026] contacting the floating catalyst particles with a gas phase
carbon source to produce carbon nanotubes
[0027] wherein the floating catalyst particles are provided by:
[0028] initiating growth of the catalyst particles by thermal
degradation of a catalyst source substance, the thermal degradation
of the catalyst source substance beginning at a first onset
temperature, and subsequently [0029] arresting the growth of the
catalyst particles using an arresting agent, the arresting agent
being provided to the catalyst particles by thermal degradation of
an arresting agent source substance, the thermal degradation of the
arresting agent source substance beginning at a second onset
temperature, wherein the second onset temperature is in the range
of temperatures from 10.degree. C. more than the first onset
temperature to 350.degree. C. more than the first onset
temperature.
[0030] Further preferred temperature ranges are set out below.
[0031] WO2010/014650 reports the preparation of metallic
single-wall carbon nanotubes. This document describes dispersing
Fe-containing catalyst particles on a substrate, then treating the
catalyst particles to obtain the desired catalyst particle size,
for example an average particle diameter ranging from 0.2 nm to 5
nm, or from about 0.9 nm to 1.4 nm. The catalyst particles, which
are immobilised on the substrate, are then contacted with a gaseous
carbon source, to produce carbon nanotubes which are
correspondingly immobilised on the substrate. This document reports
the production of predominantly metallic single wall nanotubes,
under certain reaction conditions. However, the formation of carbon
nanotubes on a substrate does not provide a route for the
production of a carbon nanotube material, such as an electrically
conductive material. A similar method is described by Harutyunyan
et al.sup.3.
[0032] Gas phase production of carbon nanotubes typically results
in a low density mass of carbon nanotubes. A typical density of
this mass of carbon nanotubes is less than 10.sup.-1 g cm.sup.-3,
or less than 10.sup.-2 g cm.sup.-3.
[0033] In the literature, such a mass of carbon nanotubes is
sometimes referred to as an "aerogel", although the use of this
term is not systematically applied. Such a mass of carbon nanotubes
can be densified to provide a carbon material, such as a fibre or
film. However, alternatively, the mass of carbon nanotubes may be
crushed, chopped, cut or otherwise processed to form a powder. It
will be understood that the powder may not exhibit significant
electrical conductivity.
[0034] Accordingly, in a third preferred aspect, the present
invention provides a carbon powder comprising carbon nanotubes,
wherein at least 70% by number of the carbon nanotubes have a
diameter in the range from 1 nm to 2.5 nm.
[0035] Preferably, the carbon powder is provided in an amount of at
least 10 g.
[0036] In a further preferred aspect, the present invention
provides a carbon material or a carbon nanotube powder obtained or
obtainable by the method of the second preferred aspect. It will be
understood that the carbon materials and the carbon powders
described herein may be obtained or obtainable by the methods of
making carbon nanotubes described herein.
[0037] In a further preferred aspect, the present invention
provides a current carrying component comprising a carbon material
according to the first preferred aspect.
[0038] In a further preferred aspect, the present invention
provides a current carrying component consisting of: carbon
nanotubes; optionally, remaining catalyst particles; and incidental
impurities, wherein at least 70% by number of the carbon nanotubes
have a diameter in the range from 1 nm to 2.5 nm.
[0039] The current carrying component preferably has a length of at
least 0.5 m, more preferably (for some embodiments) at least 1 m,
at least 10 m or at least 100 m. The current carrying component
may, for example, be provided in the form of an electrical cable,
an electrical interconnect or an electrical wire. The diameter of
the current carrying component is not particularly limited in the
present invention, but will typically be determined by the
application to which the component will be put, taking into account
the required current carrying capacity for that application. The
current carrying component is preferably to be used at or near
ambient temperature.
[0040] The current carrying component may be used in a range of
electrical applications. The current carrying component may be used
in a power transmission cable. The current carrying component may
be used in a lightning protection system. Alternatively, the
current carrying component may be used in general electrical wiring
applications, e.g. to replace conventional copper wiring. In a
preferred embodiment, the current carrying component may be used as
the current-carrying windings of an electromagnet, for example in a
solenoid or more preferably in an electric motor. The combination
of properties of the preferred current carrying components (high
current density, high strength, low density) are particularly well
suited to the manufacture of small size and/or low weight electric
motors.
[0041] In a further preferred aspect, the present invention
comprises a woven or non-woven fabric, comprising a carbon material
according to the first preferred aspect.
[0042] In a further preferred aspect, the present invention
provides a woven or non-woven fabric consisting of: carbon
nanotubes; optionally, remaining catalyst particles; and incidental
impurities, wherein at least 70% by number of the carbon nanotubes
have a diameter in the range from 1 nm to 2.5 nm.
[0043] The woven or non-woven fabric may comprise a plurality of
fibres, each fibre being formed of a large number of carbon
nanotubes.
[0044] The woven or non-woven fabric may be used in clothing. For
example, the clothing may include sensors, for monitoring the
condition of the wearer. The sensors may be arranged to transmit
information from the sensors to a remote receiver, providing for
remote monitoring of the condition of the wearer, for example
remote monitoring of the health of the wearer. For example, the
clothing may provide for remote monitoring of the temperature of
the wearer. The clothing may be worn, for example, by a patient or
by a soldier. It will be understood that the electrically
conductive fibres of the woven or non-woven fabric may form part of
the sensor and information transmission system.
[0045] Further preferred or optional features of the above aspects
will now be set out. Any aspect of the invention may be combined
with any other aspect, unless the context demands otherwise. Any of
the preferred or optional features of any aspect may be combined,
either singly or in combination, with any aspect of the invention,
unless the context demands otherwise. Where a series of end points
for a particular range is given, it is to be understood that any
one of those end points can be applied independently to the
invention.
[0046] The carbon material of the present invention is electrically
conductive. Preferably, it has a conductivity of at least
0.7.times.10.sup.6 S m.sup.-1 in at least one direction (at room
temperature). More preferably, it has a conductivity of at least
0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, or
2.0.times.10.sup.6S m.sup.-1 in at least one direction (at room
temperature). It is preferred that the carbon nanotubes dominate
the electrical properties of the material, thus providing the
material with its electrical conductivity.
[0047] Preferably, at least at room temperature, the carbon
material has a positive coefficient of resistivity with increasing
temperature.
[0048] The carbon material may have a current density of at least
15 A mm.sup.-2, more preferably at least 20, at least 25, at least
30, at least 35, at least 40, at least 50, at least 60 or at least
70 A mm.sup.-2. As used herein, the term "current density" refers
to the amount of current density which can be carried by the carbon
material without requiring force cooling to avoid runaway
heating.
[0049] The carbon material may preferably be a fibre or a film.
Where the carbon material is a fibre, the carbon nanotubes may have
their principal axes substantially aligned with the length
direction of the fibre. Similarly, where the carbon material is a
film, the principal axes of the carbon nanotubes may be
substantially aligned with each other and may lie substantially in
the plane of the film. The carbon material may comprise bundles of
carbon nanotubes, in which bundles the principal axes of the carbon
nanotubes may be substantially aligned with each other.
[0050] The carbon material may be a yarn, comprising bundles of
fibres (which fibres may comprise bundles of carbon nanotubes). It
will be understood that the yarn may consist of bundles of fibres,
optionally remaining catalyst particles, and incidental
impurities.
[0051] The carbon material preferably has at least one dimension
greater than 0.5 m. The carbon material may have at least one
dimension greater than 1 m, 2 m, 5 m, 10 m, 15 m or 20 m.
[0052] Where the carbon material is a fibre or a yarn, said at
least one dimension may be the length of the fibre. Where the
carbon material is a fibre, typically the fibre has a diameter in
the range from 1 .mu.m to 10 cm. More preferably, the fibre has a
diameter in the range from 1 .mu.m to 1 mm, or from 1 .mu.m to 100
.mu.m, or from 1 .mu.m to 50 .mu.m. A typical fibre diameter is 10
.mu.m.
[0053] Where the carbon material is a film, said at least one
direction may be the length of the film. The film may have a
thickness of at least 10 nm, for example at least 20 nm, at least
30 nm or at least 40 nm. The film may have a thickness of 1 mm or
less, more preferably 500 .mu.m or less, 250 .mu.m or less, 100
.mu.m or less, 1 .mu.m or less, or 100 nm or less. A typical
thickness is 50 nm. It will be understood that two or more films
may be placed on top of each other e.g. to provide a plurality of
overlying layers, which may together have a thickness greater than
those set out above.
[0054] A particular advantage of the carbon material of the present
invention is that it may provide a relatively high electrical
conductivity while having relatively low density, compared for
example with metals and alloys typically employed as electrical
current carrying components. Typically, the carbon material has a
density of 0.1 g cm.sup.-3 or more. It may have a density of at
least 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 or 0.8 g cm.sup.-3 or more.
Preferably, the carbon material has a density of 2.0 g cm.sup.-3 or
less, such as a density of 1.5, 1.4, 1.3, 1.2 or 1.1 g cm.sup.-3 or
less. In contrast, aluminium typically has a density of 2.7 g
cm.sup.-3, and copper typically has a density of 8.9 g
cm.sup.-3.
[0055] Where the carbon material is a fibre, its linear density may
instead be considered. For example, it may have a linear density
which is 1 g km.sup.-1 or less, for example 0.5, 0.4, 0.3, 0.2, 0.1
or 0.05 g km.sup.-1 or less. Such a low linear density may be
suitable for some specific applications. However, it is to be
understood that other specific applications (e.g. electrical power
cabling applications) will require a much higher linear
density.
[0056] Another advantage of the carbon materials of the present
invention is that they may provide a relatively high electrical
conductivity combined with a relatively high strength. The carbon
material preferably has a specific strength of at least 0.1 GPa
SG.sup.-1 in at least one direction, such as at least 0.2, 0.3,
0.4, 0.5, 0.6, 0.7 or 0.8 GPa SG.sup.-1. In contrast, aluminium
typically has a specific strength of 0.026 GPa SG.sup.-1, and
copper typically has a specific strength of 0.025 GPa
SG.sup.-1.
[0057] (As used herein, specific strength is the ultimate tensile
strength (UTS, measured in GPa) of the material concerned, divided
by its specific gravity (SG). Specific gravity is a dimensionless
value, obtained by dividing the density of the substance in
question by the density of a reference substance, in this case
water. The calculation of specific strength is explained in detail
Reference 1, which is incorporated herein by reference in its
entirety.)
[0058] Similarly, the specific stiffness of the carbon materials of
the present invention is relatively high. The carbon material
preferably has a specific stiffness of 30 GPa SG.sup.-1 or more,
more preferably 40 or 50 GPa SG.sup.-1 or more. In contrast,
aluminium typically has a specific stiffness of 26 GPa SG.sup.-1,
and copper typically has a specific stiffness of 13 GPa SG.sup.-1.
(Here, stiffness is the elastic modulus of the material, and
specific stiffness is determined by dividing this value by the
specific gravity of the material. The calculation of specific
stiffness is explained in detail Reference 1, which is incorporated
herein by reference in its entirety.)
[0059] Preferably, the carbon material or the carbon nanotube
powder comprises at least 75% by weight of carbon nanotubes. It may
comprise at least 80%, 85%, 90%, 95%, 96%, 97%, 98% or 99% by
weight of carbon nanotubes.
[0060] It will be understood that the carbon material of the
present invention, and the carbon nanotube powder of the present
invention, may comprise other components. For example, residual
catalyst particles employed in the synthesis of the carbon
nanotubes may remain in the carbon material. Accordingly, the
carbon material of the present invention, and the carbon nanotube
powder of the present invention, may comprise a plurality of
catalyst particles dispersed in the material. Preferably, the
material or powder comprises 20% by weight or less of catalyst
particles, for example 15%, 10%, 5%, 4%, 3%, 2% or 1% by weight or
less of catalyst particles.
[0061] The catalyst particles may have any of the features
described below in relation to the catalyst particles employed in
the methods of the present invention. For example, the catalyst
particles may comprise transition metal atoms, such as iron, cobalt
and/or nickel atoms. The catalyst particles may comprise sulphur
atoms. In a particularly preferred embodiment, the catalyst
particles may comprise an inner core of transition metal atoms
surrounded by a shell or cage of sulphur atoms.
[0062] At least 70% by number of the catalyst particles may
preferably have a diameter less than or equal to 4.5 nm, or less
than or equal to 3.5 nm. For example, at least 70% of the catalyst
particles may have a diameter in the range from 0.5 nm to 4.5 nm,
more preferably from 0.5 nm to 3.5 nm, or from 1.5 nm to 3.5 nm.
More preferably, at least 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98% or
99% by number of the catalyst particles have a diameter less than
or equal to 4.5 nm, or less than or equal to 3.5 nm. It will be
understood that at least 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98% or
99% by number of the catalyst particles may have a diameter in the
range from 0.5 nm to 4.5 nm, more preferably from 0.5 nm to 3.5 nm,
or from 1.5 nm to 3.5 nm. The number and size of catalyst particles
may be determined by transmission electron microscopy (TEM), as
described in more detail below with reference to the methods of the
present invention.
[0063] The carbon material may, for example, consist of a plurality
of carbon nanotubes, optionally a plurality of catalyst particles
dispersed in the material, and incidental impurities. Similarly,
the carbon nanotube powder may consist of a plurality of carbon
nanotubes, optionally a plurality of catalyst particles in the
powder, and incidental impurities.
[0064] As described above, the methods of the present invention can
provide a high degree of control of carbon nanotube diameter,
chirality and metallic properties. Typically, the methods result in
the production of a population of carbon nanotubes having a high
proportion of metallic carbon nanotubes. Typically, the methods
result in the production of a population of carbon nanotubes having
a high proportion of armchair carbon nanotubes.
[0065] Accordingly, the present inventors have made available for
the first time carbon materials comprising a high proportion of
metallic carbon nanotubes. Accordingly, it will be understood that
the carbon materials of the present invention, and the carbon
nanotube powders, typically comprise a high proportion of metallic
carbon nanotubes. For example, substantially all of the carbon
nanotubes may be metallic.
[0066] Similarly, the present inventors have made available for the
first time carbon materials comprising a high proportion of
armchair carbon nanotubes. Accordingly, it will be understood that
the carbon materials of the present invention, and the carbon
nanotube powders, typically comprise a high proportion of armchair
carbon nanotubes. For example, substantially all of the carbon
nanotubes may have armchair chirality.
[0067] The person skilled in the art will be familiar with methods
for probing the metallic or semiconducting properties of carbon
nanotubes. One suitable method for probing the metallic or
semiconducting properties of carbon nanotubes, for example in a
carbon material or carbon nanotube powder, employs Raman
spectroscopy.
[0068] One vibrational mode of carbon nanotubes is the radial
breathing mode. This radial breathing mode can be probed using
Raman spectroscopy. For a given wavelength of incident light, only
radial breathing modes of carbon nanotubes with certain diameters
will be resonant, and so only certain diameters of carbon nanotubes
will give rise to radial breathing mode (RBM) peaks in the Raman
spectrum. The wavenumber of a given RBM peak can be used to
determine the diameter of the carbon nanotube which gave rise to
that peak, using the equation d=239/.omega..sub.RBM, wherein d is
the nanotube diameter in nm, and .omega..sub.RBM is the wavenumber
of the radial breathing mode peak in cm.sup.-1.
[0069] Once the diameter of the carbon nanotube(s) giving rise to
the RBM peak has been determined, it is possible to determine
whether those carbon nanotube(s) are metallic or semiconducting.
This is done using a plot called a Kataura plot, such as the plot
shown in FIG. 1. The diameter of the carbon nanotube is read from
the x-axis, and the excitation energy used to generate the RBM peak
in question is read from the y-axis. For a given diameter and
excitation energy, the plot indicates whether the carbon
nanotube(s) giving rise to the RBM peak are metallic or
semiconducting.
[0070] For example, a typical method of probing the metallic or
semiconducting properties of carbon nanotubes in a carbon material
or powder comprises the steps of: [0071] (i) taking a first Raman
spectrum using an incident wavelength of 633 nm; [0072] (ii)
identifying each peak falling in the range from 120 cm.sup.-1 to
350 cm.sup.-1 (RBM peaks); [0073] (iii) determining the position
(.omega..sub.RBM) of each RBM peak using Lorentzian fit; [0074]
(iv) determining the nanotube diameter associated with each RBM
peak, using the equation
[0074] d=239/.omega..sub.RBM [0075] wherein d is the nanotube
diameter in nm, and .omega..sub.RBM is the frequency of the radial
breathing mode peak in cm.sup.-1; [0076] (v) comparing this
diameter with the Katura plot shown in FIG. 1, using an excitation
energy of 1.96+/-0.1 eV (which corresponds to the 633 nm incident
light) to determine whether each RBM peak corresponds to metallic
or semiconducting carbon nanotubes; [0077] (vi) taking a second
Raman spectrum using an incident wavelength of 514 nm,
corresponding to an excitation energy of 2.41+/-0.1 eV, and
repeating steps (ii) to (v) for this second Raman spectrum.
[0078] In FIG. 1, the filled circles indicate semiconducting
nanotubes, and the open circles indicate metallic nanotubes.
[0079] When the above method is carried out on the carbon material
or carbon nanotube powder of the present invention, preferably at
least one of the first and second Raman spectra includes at least
one RBM peak which corresponds to metallic carbon nanotubes.
Preferably, neither of the first and second Raman spectra includes
any RBM peak which corresponds to semiconducting carbon
nanotubes.
[0080] In the above method for probing the metallic or
semiconducting properties of carbon nanotubes, the sample probed
may be a bulk carbon material. In that case, preferably the method
is carried out a plurality of times, on different regions of the
sample. For example, preferably at least 10 regions are probed.
Preferably, at least 70%, 80% or 90% of the probed regions meet one
or more of the conditions recited above. In the case of a fibre,
the incident light may be aligned with the fibre axis. The regions
probed may be equally spaced, e.g. at a spacing distance of 1 cm
along the fibre axis.
[0081] Alternatively, the carbon nanotubes of the carbon material
may be dispersed before the method for probing the metallic or
semiconducting properties of carbon nanotubes is carried out. In
this case, the method may be carried out on a single sample of
dispersed carbon nanotubes. Alternatively, the method may be
repeated for one or more samples of dispersed carbon nanotubes, for
example 10 samples. Preferably, at least 70%, 80% or 90% of the
probed samples meet one or more of the conditions recited
above.
[0082] In the case of a carbon nanotube powder, the method of
probing the metallic or semiconducting properties of carbon
nanotubes can be carried out on one or more, e.g. 10 samples.
Preferably, at least 70%, 80% or 90% of the probed samples meet one
or more of the conditions recited above.
[0083] A suitable laser source for the 633 nm incident light is
He/Ne. A suitable laser source for the 514 nm incident light is Ar
ion. The range of wavenumbers scanned for each spectrum may be at
least 100 cm.sup.-1 to 400 cm.sup.-1, e.g. 50 cm.sup.-1 to 3300
cm.sup.-1. A suitable spectroscope is the Renishaw Ramanscope 1000
system, available from Renishaw (www.renishaw.com). A suitable
laser spot size is 1 .mu.m.sup.2. A suitable acquisition time is 10
s.
[0084] The Raman spectroscopy methods described above are
particularly suited to probing single-walled carbon nanotubes. It
is preferable that the carbon material, or the carbon nanotube
powder, comprises single-walled carbon nanotubes. Preferably, at
least 50% by number of the carbon nanotubes are single-walled
carbon nanotubes. More preferably, at least 60%, 70%, 80%, 90%,
95%, 96%, 97%, 98% or 99% by number of the carbon nanotubes are
single-walled carbon nanotubes.
[0085] Methods of probing the properties of carbon nanotubes using
Raman spectroscopy are described in Reference 4 and Reference 5,
which are each incorporated herein by reference in their
entirety.
[0086] As explained above, the carbon materials of the present
invention, and the carbon nanotube powders, typically comprise a
high proportion of armchair carbon nanotubes. For example,
substantially all of the carbon nanotubes may have armchair
chirality.
[0087] The chirality of a single carbon nanotube, of a bundle of
carbon nanotubes or, more generally, of a population of carbon
nanotubes (e.g. in a carbon material or carbon nanotube powder) can
be probed using electron diffraction. As the skilled person
understands, TEM analysis allows the production of electron
diffraction patterns by suitable operation of the microscope. The
electron beam is directed through the carbon nanotube (or bundle of
carbon nanotubes) in a direction perpendicular to the principal
axis of the carbon nanotube. The resultant electron diffraction
pattern indicates the chirality of the carbon nanotube.
[0088] For both zigzag and armchair single-walled nanotubes, a
hexagonal pattern of six diffraction spots is generated. However,
the orientation of these spots with respect to the principal axis
of the nanotube is different for zigzag and for armchair nanotubes.
For armchair nanotubes, three of the six spots are positioned to
one side of the principal axis of the nanotube, and three are
positioned to the other side of the principal axis. In contrast,
for zigzag carbon nanotubes, two of the six spots are positioned to
one side of the principal axis, two are positioned to the other
side of the principal axis, and two spots are aligned with the
principal axis.
[0089] This is illustrated in FIG. 2. FIG. 2A shows a schematic
representation of the diffraction pattern generated by an armchair
carbon nanotube, and FIG. 2B shows a schematic representation of
the diffraction pattern generated by a zigzag carbon nanotube. The
"x" points indicate the position of the diffraction spots. The
solid vertical lines illustrate the principal axis direction of the
carbon nanotube.
[0090] Where a population (e.g. a bundle) of carbon nanotubes
having the same chirality is probed, similar results are observed.
Where a mixture of different chiralities is present, the
diffraction spots generated by carbon nanotubes of different
chiralities together form a circular pattern of diffraction spots.
In some cases, these spots may merge to form a continuous circular
diffraction pattern. Similarly, in multi-walled carbon nanotubes,
typically a circular diffraction pattern is generated, indicating a
mixture of chiralities.
[0091] In all cases, the spots tend to be slightly elongated
streaks, elongated in a direction substantially perpendicular to
the principal axis of the nanotube, in the plane of the diffraction
pattern.
[0092] Preferably, at least 50% by number of the carbon nanotubes
are single-walled armchair carbon nanotubes. More preferably, at
least 60%, 70%, 80%, 90%, 95%, 96%, 97%, 98% or 99% by number of
the carbon nanotubes are single-walled armchair carbon
nanotubes.
[0093] The percentage by number of single-walled carbon nanotubes
having armchair chirality can be determined using electron
diffraction, as described above. 50% by number of the carbon
nanotubes are considered to be armchair single-walled carbon
nanotubes wherein when at least 10 separate regions of the material
are probed by electron diffraction (e.g. 10 different carbon
nanotubes (not in the same bundle), or 10 different bundles of
carbon nanotubes) using a beam perpendicular to the principal axis
of the carbon nanotube, at least 50% of the probed nanotubes or
bundles give rise to an armchair diffraction pattern. An armchair
diffraction pattern is considered to be a hexagonal pattern of six
diffraction spots, where three of the six spots are positioned to
one side of the principal axis of the carbon nanotube, and three
are positioned, substantially mirror symmetrically, to the other
side of the principal axis of the carbon nanotube.
[0094] More preferably, an armchair diffraction pattern may be
considered to be a hexagonal pattern of six diffraction spots,
where the spots are located at positions 30.degree., 90.degree.,
150.degree., 210.degree., 270.degree. and 330.degree.+/-5.degree.
(or +/-4.degree., 3.degree., 2.degree. or 1.degree.) with respect
to the principal axis of the carbon nanotube. Here, the positional
angle is measured between the principal axis of the carbon nanotube
and a line extending from the centre of the beam (central spot) to
the centre of the diffraction spot.
[0095] Where a bundle of nanotubes is probed, the nanotubes should
preferably have their principal axes aligned. Conveniently, where
the carbon material is a fibre, the fibre axis may be taken to be
the principal axis of the carbon nanotubes, although this can of
course be easily confirmed during the TEM analysis.
[0096] (It will be understood that the principal axis of a carbon
nanotube extends along the elongation direction of the carbon
nanotube.)
[0097] Suitable armchair chiralities for the carbon nanotubes of
the material and powder of the present invention include the
following. The chiralities are given in terms of n and m (n, m) and
are listed alongside their diameter:
TABLE-US-00001 (3, 3) 0.41 .+-. 0.02 nm (4, 4) 0.54 .+-. 0.02 nm
(5, 5) 0.68 .+-. 0.02 nm (6, 6) 0.81 .+-. 0.02 nm (7, 7) 0.95 .+-.
0.02 nm (8, 8) 1.10 .+-. 0.02 nm (9, 9) 1.22 .+-. 0.02 nm (10, 10)
1.36 .+-. 0.02 nm (11, 11) 1.49 .+-. 0.02 nm (12, 12) 1.63 .+-.
0.02 nm (13, 13) 1.76 .+-. 0.02 nm (14, 14) 1.90 .+-. 0.02 nm (15,
15) 2.03 .+-. 0.02 nm (16, 16) 2.17 .+-. 0.02 nm (17, 17) 2.31 .+-.
0.02 nm (18, 18) 2.44 .+-. 0.02 nm (19, 19) 2.58 .+-. 0.02 nm (20,
20) 2.71 .+-. 0.02 nm (21, 21) 2.85 .+-. 0.02 nm (22, 22) 2.98 .+-.
0.02 nm
[0098] The (n, m) notation employed here will be familiar to the
person skilled in the art.
[0099] In the carbon materials and carbon nanotube powder of the
present invention, preferably at least 70% by number of the carbon
nanotubes have a diameter in the range from 1 nm to 2.5 nm, more
preferably from 1 nm to 2 nm. More preferably, at least 75%, 80%,
85%, 90%, 95%, 96%, 97%, 98% or 99% by number of the carbon
nanotubes have a diameter in the range from 1 nm to 2.5 nm, more
preferably from 1 nm to 2 nm. The diameter of carbon nanotubes in a
material can be determined by TEM. The size distribution of the
carbon nanotubes can be determined by counting. For example, TEM
may be carried out on 2, 5, 10 or more samples of the carbon
material or carbon nanotube powder to determine the size
distribution.
[0100] As set out above, in at least one aspect, the present
invention provides a method of producing carbon nanotubes, the
method comprising: [0101] providing a plurality of floating
catalyst particles, wherein at least 70% by number of the catalyst
particles have a diameter less than or equal to 4.5 nm; and [0102]
contacting the floating catalyst particles with a gas phase carbon
source to produce carbon nanotubes.
[0103] The method typically yields a mass of carbon nanotubes,
having a density of 10.sup.-2 g cm.sup.-3 or less, for example
10.sup.-3 g cm.sup.-3 or less. Typically, the mass of carbon
nanotubes has a density of 10.sup.-6 g cm.sup.-3 or more. In the
literature, this mass of carbon nanotubes is sometimes referred to
as an aerogel.
[0104] At least 70% by number of the catalyst particles have a
diameter which is less than or equal to 4.5 nm, or less than or
equal to 3.5 nm. For example, at least 70% by number of the
catalyst particles have a diameter in the range from 0.5 nm to 4.5
nm, more preferably from 0.5 nm to 3.5 nm or from 1.5 nm to 3.5 nm.
More preferably, at least 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98% or
99% by number of the catalyst particles have a diameter less than
or equal to 4.5 nm or less than or equal to 3.5 nm. It will be
understood that at least 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98% or
99% by number of the catalyst particles may have a diameter in the
range from 0.5 nm to 4.5 nm, more preferably from 0.5 nm to 3.5 nm
or from 1.5 nm to 3.5 nm.
[0105] The diameter of the catalyst particles can be determined by
TEM. The diameter of a catalyst particle is taken to be the largest
linear dimension of that particle visible on the TEM image.
Typically, the product of the process for forming the carbon
nanotubes includes not only the carbon nanotubes but also residual
catalyst particles, typically randomly dispersed amongst the
population of carbon nanotubes. Accordingly, the TEM can be carried
out on a sample of carbon nanotubes produced by the process, and
thus the size distribution of catalyst particles determined.
Alternatively, catalyst particles can be isolated during the
process, after their formation but before carbon nanotube
production. TEM can be carried out on the isolated catalyst
particles to determine their diameter.
[0106] The present inventors have realised that the size of the
catalyst particles can be conveniently controlled by initiating
growth of catalyst particles and subsequently arresting the growth
of the catalyst particles using an arresting agent. This may be
carried out in the gas phase.
[0107] The growth of catalyst particles may be initiated by
degradation of a catalyst source substance (e.g. a catalyst source
compound or element), and/or the arresting agent may be supplied by
degradation of an arresting agent source substance (e.g. an
arresting agent source compound or element). Typically, this may be
performed by: [0108] subjecting a mixture of catalyst source
substance and arresting agent source substance to catalyst source
substance degradation conditions; [0109] and subsequently
subjecting said mixture to arresting agent source substance
degradation conditions.
[0110] It will be understood that the term "degradation" as used
herein includes chemical breakdown of a compound to release e.g.
its component atoms or a simpler compound. It will be understood
that the term "degradation" as used herein also includes physical
change in a substance which results in the release of a catalyst
component such as a transition metal atom (to allow catalyst
particle growth), or release of arresting agent (to arrest catalyst
particle growth). For example, the physical change could be
vaporisation or sublimation. The term "degradation conditions"
should be interpreted accordingly.
[0111] Typically, degradation of the catalyst source substance
(e.g. compound) may be by thermal degradation. Typically, the
degradation of arresting agent source substance (e.g. compound) may
be by thermal degradation. This is desirable in embodiments where
the carbon nanotubes are produced by thermal chemical vapour
deposition (thermal CVD). However, it is noted here that other
manufacturing conditions may be used, e.g. plasma CVD.
[0112] Thermal degradation of the catalyst source substance may
begin at a first onset temperature, and thermal degradation of the
arresting agent source substance may begin at a second onset
temperature. Preferably, the second onset temperature is greater
than the first onset temperature. Preferably, the second onset
temperature is not more than 350.degree. C. greater than the first
onset temperature, for example not more than 300.degree. C.,
250.degree. C., 200.degree. C., 150.degree. C., 100.degree. C.,
90.degree. C., 80.degree. C., 70.degree. C., 60.degree. C. or
50.degree. C. greater than the first onset temperature. Preferably,
the second onset temperature is at least 10.degree. C. greater than
the first onset temperature, for example at least 20.degree. C.,
30.degree. C., 40.degree. C., 50.degree. C., 60.degree. C.,
70.degree. C., 80.degree. C., 90.degree. C. or 100.degree. C.
greater than the first onset temperature.
[0113] The first onset temperature may be at least 200.degree. C.,
or at least 300.degree. C. The first onset temperature may be
700.degree. C. or less, more preferably 600.degree. C. or less,
500.degree. C. or less, or 400.degree. C. or less. The second onset
temperature may be at least 300.degree. C., at least 400.degree.
C., at least 450.degree. C., or at least 500.degree. C. The second
onset temperature may be 800.degree. C. or less, or 700.degree. C.
or less, or 650.degree. C. or less, or 600.degree. C. or less, or
550.degree. C. or less, or 500.degree. C. or less, or 450.degree.
C. or less.
[0114] The carbon nanotubes may be produced at a carbon nanotube
formation temperature. This is typically higher than the second
onset temperature. Preferably, the carbon nanotube formation
temperature is at least 900.degree. C., for example at least
950.degree. C., 1000.degree. C., 1050.degree. C., 1100.degree. C.
or 1150.degree. C.
[0115] Preferably, the steps of: [0116] initiating growth of
catalyst particles; [0117] subsequently arresting the growth of the
catalyst particles using an arresting agent; and [0118] contacting
the catalyst particles with a carbon source to produce carbon
nanotubes
[0119] are all carried out in the same reaction chamber.
[0120] Each (e.g. all) of these steps may be carried out in the gas
phase.
[0121] The arresting agent source substance, the catalyst source
substance and the carbon source may pass through the reaction
chamber in a flow direction. For example, the arresting agent
source substance, the catalyst source substance and the carbon
source may pass through the chamber in the gas phase, e.g. in said
flow direction. The arresting agent source substance, the catalyst
source substance and the carbon source may be carried through the
chamber as part of a gas stream. The gas stream may include an
inert gas, such as a noble gas, for example helium or argon. The
gas stream may include a reductive gas, for example hydrogen.
[0122] The conditions inside the reactor may vary along the flow
direction. For example, the temperature in the reaction chamber may
vary along the flow direction. The temperature may increase from
the first onset temperature to the second onset temperature along
the flow direction. The temperature may then change (e.g. increase)
to the carbon nanotube formation temperature. For example, the
reaction chamber may be a furnace.
[0123] Preferably, the catalyst particles comprise transition metal
atoms. For example, the catalyst particles may comprise iron,
cobalt and/or nickel atoms, preferably iron atoms. Accordingly, it
may be preferred that the catalyst source substance (e.g. compound)
comprises at least one transition metal atom, for example at least
one iron atom, at least one nickel atom and/or at least one cobalt
atom. Preferably, the catalyst source substance (e.g. compound)
comprises at least one iron atom. These atoms may be released on
degradation of the catalyst source substance (e.g. compound).
[0124] (As used herein, the word "atom" is understood to include
ions of the relevant atoms. For example, the catalyst particles
and/or the catalyst source substance (e.g. compound) may include
one or more transition metal ions.)
[0125] For example, the catalyst source substance may be a
transition metal complex, for example a transition metal complex
including one, or preferably two, cyclopentadienyl ligands.
Alternatively or additionally, the transition metal complex may
include other ligands, such as one or more carbonyl ligands. The
transition metal complex may include only hydrocarbon ligands.
[0126] For example, the catalyst particle source substance may be
ferrocene. Other suitable catalyst particle source substances
include other metalocenes, such as nickelocene and cobaltocene.
Metal carbonyl compounds are also suitable, for example cobalt
carbonyl (e.g. dicobalt octacarbonyl), nickel carbonyl (e.g. nickel
tetracarbonyl) and iron carbonyl (e.g. iron pentacarbonyl). It will
be understood that transition metal complexes having a mixture of
cyclopentadienyl ligands and carbonyl ligands may also be
suitable.
[0127] It will be understood that the catalyst source substance is
preferably a substance which begins thermal degradation at the
first onset temperature, for example as set out above. Preferably,
the catalyst source substance begins thermal degradation at the
first onset temperature under the conditions employed in the method
of the invention, for example under reductive conditions.
[0128] Preferably, the arresting agent is sulphur. Preferably, the
arresting agent source substance (e.g. compound) comprises at least
one sulphur atom. The sulphur atom may be released on degradation
of the arresting agent source substance. The arresting agent source
substance may be a compound comprising at least one sulphur atom
and at least one carbon atom covalently bonded to the sulphur atom.
In this case, at least one carbon-sulphur covalent bond may be
broken on degradation of the arresting agent source substance, in
order to release the sulphur atom.
[0129] A typical arresting agent source substance is carbon
disulphide (CDS).
[0130] Alternatively, the arresting agent source substance may be a
compound comprising at least one sulphur atom and at least one
hydrogen atom covalently bonded to the sulphur atom. In this case,
at least one hydrogen-sulphur covalent bond may be broken on
degradation of the arresting agent source substance, in order to
release the sulphur atom. Accordingly, it will be understood that a
further suitable arresting agent is hydrogen sulphide
(H.sub.2S).
[0131] A further suitable arresting agent source substance is
elemental sulphur. For example, solid sulphur may be supplied to
the reaction chamber. Sulphur atoms, ions, radicals or molecules
may be released within the reaction chamber, for example by
vaporisation or sublimation of the elemental sulphur (e.g. solid
sulphur). It will be understood that this release of sulphur atoms,
ions, radicals or molecules, e.g. by sublimation or vaporisation,
can be considered to be arresting agent source substance
degradation (e.g. thermal degradation). Alternatively, gaseous
sulphur may be supplied to the reaction chamber.
[0132] It will be understood that the arresting agent source
substance (e.g. compound) is preferably a substance which begins
thermal degradation at the second onset temperature, for example as
set out above. Preferably, the arresting agent source substance
(e.g. compound) begins thermal degradation at the second onset
temperature under the conditions employed in the method of the
invention, for example under reductive conditions.
[0133] It will be understood that the catalyst particles may
comprise transition metal atoms, such as iron, cobalt and/or nickel
atoms. The catalyst particles may comprise sulphur atoms. The
catalyst particles may consist of transition metal atoms such as
iron atoms, sulphur atoms, and incidental impurities. In a
particularly preferred embodiment, the catalyst particles may
comprise an inner core of transition metal atoms surrounded by a
shell or cage of sulphur atoms.
[0134] The degradation of catalyst source substance and/or the
degradation of arresting agent source substance may be carried out
under reductive conditions, for example in the presence of
hydrogen.
[0135] The amount of catalyst source substance and arresting agent
source substance supplied may provide a molar ratio of transition
metal atoms to sulphur atoms of 50:1 or less, more preferably 40:1,
30:1, 20:1, 15:1 or 10:1 or less. Preferably, the molar ratio of
transition metal atoms to sulphur atoms is at least 2:1, more
preferably at least 3:1, at least 4:1 or at least 5:1. A typical
molar ratio of transition metal atoms to sulphur atoms is 6:1.
[0136] A typical molar ratio of carbon atoms to transition metal
atoms is 8:1. Preferably, the ratio of carbon atoms to transition
metal atoms is at least 2:1, at least 3:1, at least 4:1, or at
least 5:1. Preferably, the ratio of carbon atoms to transition
metal atoms is 50:1 or less, or 40:1, 30:1, 20:1, 15:1 or 10:1 or
less.
[0137] The carbon source is not particularly limited. For example,
the carbon source may be a C.sub.1-C.sub.20 hydrocarbon, e.g. a
C.sub.1-C.sub.10 or C.sub.1-C.sub.5 hydrocarbon, including alkanes,
alkenes and alkynes. For example, ethane and acetylene are
suitable. Alternatively, the carbon source may be a
C.sub.1-C.sub.20 alcohol, e.g. a C.sub.1-C.sub.10 or
C.sub.1-C.sub.5 alcohol, such as a monohydroxy alcohol. Typical
carbon sources include methane and ethanol. It will be understood
that mixtures of carbon sources may be employed.
[0138] It will be understood that the carbon nanotubes may be
produced by chemical vapour deposition, for example thermal
chemical vapour deposition or plasma chemical vapour
deposition.
[0139] The method may further comprise densifying the carbon
nanotubes to produce a carbon material. For example, the
densification may comprise drawing the carbon nanotubes to form the
carbon material. Alternatively or additionally, the densification
may include supplying a densification agent to the carbon
nanotubes. Suitable densification agents include acetone and
divinyl benzene.
[0140] Suitable densification methods are described in
WO2008/132467 and in Reference 1, which are each incorporated
herein by reference in their entirety.
[0141] The method may further comprise the step of forming a fibre
or film of carbon nanotubes. For example, the method may further
comprise the step of drawing the carbon nanotubes into a fibre. The
method may further comprise the step of forming a yarn from such
fibres.
[0142] The method may further comprise the step of forming a carbon
nanotube powder, for example by crushing, chopping, cutting or
otherwise processing the carbon nanotubes produced.
[0143] It will be understood that the carbon material produced by
the method, e.g. the fibre, film or yarn, or the carbon nanotube
powder produced by the method, may advantageously have one or more
of the optional and preferred features described above with
reference to the carbon materials and carbon nanotube powders of
the present invention.
[0144] Similarly, it will be understood that the method of the
present invention preferably produces a population of carbon
nanotubes having the diameter, chirality and metallic properties
described above with respect to the carbon materials and carbon
nanotube powders.
[0145] The method may further comprise removing some or all of the
residual catalyst particles from the carbon nanotubes.
[0146] Preferably the method is performed substantially
continuously for at least 10 minutes, for example for at least 30
minutes, at least 1 hour or at least 5 hours.
BRIEF DESCRIPTION OF THE DRAWINGS
[0147] Preferred embodiments of the invention will now be
described, with reference to the accompanying drawings in
which:
[0148] FIG. 1 shows a Kataura plot.
[0149] FIGS. 2A and 2B show a schematic illustration of typical
diffraction patterns obtained for (2A) armchair carbon nanotubes,
and (2B) zigzag carbon nanotubes.
[0150] FIGS. 3A, 3B, and 3C illustrate an embodiment of the method
of the present invention, showing (3A) a schematic illustration of
the apparatus used, (3B) a typical temperature gradient within the
reaction chamber, and (3C) a schematic flow chart illustration of
the method.
[0151] FIGS. 4A, 4B, and 4C show SEM images of carbon materials
produced in the examples.
[0152] FIGS. 5A, 5B, 5C, and 5D show typical Raman spectra of
carbon materials produced in the examples.
[0153] FIGS. 6A, 6B, and 6C show in (6A) and (6B) extracts from a
typical Raman spectrum of a carbon material produced in the
examples, and (6C) an annotated Kataura plot.
[0154] FIGS. 7A, 7B, 7C, and 7D show TEM images of carbon materials
produced in the examples.
[0155] FIGS. 8A and 8B show the carbon nanotube diameter
distribution of carbon materials produced in the examples,
determined using TEM.
[0156] FIGS. 9A and 9B show in (9A) a HREM image of catalyst
particles withdrawn from the reactor in the examples, and (9B) the
diameter distribution of these particles.
[0157] FIGS. 10A and 10B show the thermal degradation temperatures
of reactants used in the examples, and relates them to the
temperature profile in the reactor.
[0158] FIGS. 11A and 11B show in (11A) an example electron
diffraction pattern obtained from a bundle of carbon nanotubes
having armchair chirality, and (11B) a marked up version of the
pattern of FIG. 11A with oval marks indicating the location of the
diffraction spots.
[0159] FIGS. 12A and 12B show in (12A) an example electron
diffraction pattern obtained from a bundle of carbon nanotubes
having armchair chirality, and (12B) a marked up version of the
pattern of FIG. 12A with oval marks indicating the location of the
diffraction spots.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0160] A preferred embodiment of the method of the present
invention will now be described. A schematic flow chart
illustration of the method is shown in FIG. 3C, and a schematic
illustration of the apparatus used is shown in FIG. 3A.
[0161] A gaseous mixture of carbon source (e.g. methane), catalyst
source substance (e.g. ferrocene) and arresting agent source
substance (e.g. carbon disulphide) is fed into a furnace, carried
in a stream of gas (e.g. hydrogen and/or helium). The gas mixture
flows through the furnace in a flow direction.
[0162] The temperature increases along the flow direction, so that
the mixture is first subjected to a first onset temperature, at
which temperature the catalyst source substance degrades to
initiate growth of catalyst particles. For example, iron atoms may
be released, to form catalyst particles comprising iron. Further
along the flow direction, the mixture is subjected to a second
onset temperature, at which temperature the arresting agent source
substance degrades. The arresting agent is thus released, and acts
to arrest the growth of the catalyst nanoparticles. The mixture is
then subjected to a carbon nanotube formation temperature, and
carbon nanotubes are produced.
[0163] As illustrated in FIG. 3A, the resulting carbon nanotubes
may be densified by supplying a densification agent (e.g. acetone).
The carbon nanotubes may be drawn into a fibre. A typical winding
rate is from 10 m s.sup.-1 to 20 m s.sup.-1. It will be understood
that much higher winding rates may be employed.
[0164] A typical temperature gradient within the furnace is
illustrated in FIG. 3B.
Examples
[0165] Continuous production of carbon nanotube fibres (schematic
as shown in FIG. 3A) was carried out in a vertical ceramic reactor
(d=80 mm, I=2 m), with a temperature profile as shown in FIG. 3B.
The feedstock contained a carbon precursor (methane) and vapours of
a catalyst source substance (ferrocene) and a sulphur source
substance, carried by helium. The feedstock was introduced in to
the reactor through a steel injector tube (d=12 mm, I=90 mm).
[0166] On thermolysis of the feedstock components in a reductive
atmosphere of hydrogen followed by synthesis of nanotubes, a plume
composed of entangled nanotubes was obtained which was continuously
drawn at 20 m min.sup.-1 and densified with an acetone spray into a
fibre. The fibre had a typical diameter of 10 .mu.m.
[0167] The effect of two different sulphur precursors, thiophene
and carbon disulphide (CDS), on the morphology of the carbon
nanotubes constituting the fibres produced was investigated. The
input concentrations of the various precursors were optimised
experimentally to provide continuous spinning of the fibre. The
elemental ratios used are presented in Table 1.
TABLE-US-00002 TABLE 1 Input precursor concentrations and the
elemental ratios Precursors mol min.sup.-1 Elemental ratio
(.times.10.sup.-5) (.times.10.sup.-3) Carbon Catalyst Promoter Fe/S
Fe/C Case 1 170 0.21 C.sub.4H.sub.4S 2.5 80 8 Case 2 170 0.21
CS.sub.2 18 6 8
[0168] (The higher input concentration of sulphur where CDS is
used, compared to where thiophene is used, reflects the fact that
smaller catalyst particles are formed where CDS is used (see
below); the surface area to volume ratio of these catalyst
particles is higher. Additionally, the CDS becomes available at an
earlier stage of catalyst particle growth (again, see below), at
which stage there is a higher number density of forming catalyst
particles.)
[0169] The analyses of the fibre microstructure and the
constituting nantotubes were carried out by electron microscopy
(FEI Tecnai F20-G2 FEGTEM, JEOL 2000FX and JEOL 6340 FEG), Raman
Spectroscopy using a Renishaw Ramanscope 1000 system (incident
light of wavelength 633 nm and 514 nm; acquisition time=10 s; laser
spot size=1 .mu.m). The mechanical properties of the fibres were
investigated with tensile tests using Textechno Favimat, a
dedicated fibre testing equipment which employs a load cell with a
force and displacement measurement range of 0-2 N
(resolution=0.0001 cN) and 0-100 mm (resolution=0.1 micrometer)
respectively. Testing was carried out at a standard gauge length of
20 mm and a test-speed of 2 mm min.sup.-1 to acquire the specific
strength and specific stiffness (expressed in N Tex.sup.-1, these
values are numerically equivalent to GPa SG.sup.-1) of the
fibres.
[0170] Results
[0171] Fibre Composition, Microstructure and Nanostructure
[0172] SEM Analysis
[0173] Typical SEM images of the condensed and the internal
structure of the uncondensed fibres from both the CDS and thiophene
runs are presented in FIG. 4. FIG. 4A shows a typical condensed
fibre, FIGS. 4B and 4C shows the internal structure of the fibre
prior to acetone densification, where CDS is used as the sulphur
precursor (B), and thiophene is used as the sulphur precursor (C).
The nanotubes shown are orientated in the fibre direction. It can
also be seen that the CDS fibre shows minimal presence of
extraneous materials (which are generally by-products of most CVD
processes) in comparison to the thiophene fibre.
[0174] Raman Spectroscopy
[0175] Raman spectra were acquired on the fibre samples with the
polarisation of the incident light parallel to the fibre axis. At
least 10 spectra were collected along the length of the fibre per
fibre sample. The fibre samples were 1 cm in length, and
acquisitions were spaced at equal intervals along the length. At
least five samples obtained from each sulphur precursor were
examined. The typical spectra for fibres obtained using CDS and
thiophene are presented in FIG. 5. FIG. 5A shows a typical spectrum
for a fibre obtained using CDS, and FIG. 5B shows a typical
spectrum for a fibre obtained using thiophene. FIG. 5C shows the M
and iTOLA regions of a typical Raman spectrum for a fibre obtained
using CDS, and FIG. 5D shows the IFM region of a typical Raman
spectrum for a fibre obtained using CDS.
[0176] The positions of the peaks are in the spectra of FIGS. 5A
and 5B and the D/G ratios (indicative of fibre purity and
crystallinity of the nanotubes) are presented in Table 2.
TABLE-US-00003 TABLE 2 The list of positions of the major peaks in
the Raman spectra Position cm.sup.-1 Fibre RBM D G G'
I.sub.D/I.sub.G CDS 194.5 .+-. 3.8 1320.9 .+-. 1.1 1589.7 .+-. 0.3
2627.2 .+-. 1.9 0.010 .+-. 0.003 Thiophene Absent 1331.3 .+-. 1.5
1583.8 .+-. 1.8 2656.4 .+-. 2.5 0.3 .+-. 0.04
[0177] The low D/G ratios shown by the fibres obtained using CDS in
comparison to those obtained using thiophene are in agreement with
the SEM results. They suggest minimal presence of extraneous
materials and low density of defects in the nanotubes produced
using CDS. The distinctive intense low frequency ring breathing
modes (RBMs) occurring in the spectra from the CDS fibres indicate
the presence of single-walled carbon nanotubes. In addition, the
upshifted G peak (to 1590 cm.sup.-1), the downshifted D peak (1320
cm.sup.-1), the presence of M (1750 cm.sup.-1), i-TOLA (1950
cm.sup.-1) and intermediate frequency vibration modes (IFM modes
600-1200 cm.sup.-1) confirm that the fibres obtained with CDS as
the sulphur precursor are composed of mainly single-walled carbon
nanotubes. All these vibrational features are completely absent in
fibres obtained with thiophene as the sulphur precursor and the G
band and D band position occurring at 1582 cm.sup.-1 and 1331
cm.sup.-1 are suggestive of the presence of carbon nanotubes with
more than one wall.
[0178] G Band and RBM Analysis of CDS Fibres
[0179] Further analysis of the G band (FIG. 6A) reveals an internal
structure and in addition to the G+ feature at 1590 cm.sup.-1
(Lorentzian fit), the G- band occurs as a broad feature at 1552
cm.sup.-1 fit with the Breit-Wigner-Fanoline shape which indicates
the predominant presence of metallic nanotubes. The Fano line shape
is given by:
I ( .omega. ) = I 0 [ 1 + ( .omega. - .omega. BWF ) / q .GAMMA. ] 2
1 + [ ( .omega. - .omega. BWF ) / .GAMMA. ] 2 ##EQU00001##
[0180] where I.sub.0, .omega..sub.0, .GAMMA. and q are intensity,
normalised frequency, broadening parameter and line shape parameter
respectively.
[0181] (FIG. 6A shows the internal structure of the G band, with
the Lorentzian G+ and the G- exhibiting the Fano lineshape (see the
above equation) with fit parameters I.sub.0, .omega..sub.0, .GAMMA.
and q=2256, 1556, 49.5 and -0.20 respectively.)
[0182] The position of the radial breathing modes (RBMs) can be
utilised to obtain the diameters of the nanotubes, as described
above. It was observed that all the RBM frequencies noticed in the
CDS fibre occur around 200 cm.sup.-1 at the excitation wavelength
of 633 nm (FIG. 6B) corresponding to the diameter range of
1.2.+-.0.2 nm (d=239/.omega..sub.RBM). This can be mapped to the
Kataura plot, which is a theoretical model that relates the
diameter of the nanotubes to the optical transition energies.
Nanotubes of the same diameter can be either metallic or
semiconducting, and the difference in the behaviour is shown in the
differences in their optical transition energies. From the Kataura
plot (FIG. 1 and FIG. 6C) it can be inferred that nanotubes in the
diameter range of 1.1-1.4 nm with optical transition energies in
the range of 1.96.+-.0.1 eV are metallic, while those with
transition energies in the range of 2.41.+-.0.1 eV are
semiconducting (the energy range of 0.1 eV takes in to account any
transition energy shifts caused due to environmental effects such
as nanotube bundling).
[0183] Only those tubes with optical transition energies that are
in resonance with the excitation energy (in the case of Raman
spectroscopy, the incident laser light) will yield an RBM. While
intense RBMs could be obtained with incident light of 633 nm
(E.sub.excitation=1.96 eV), no resonance, and hence no RBMs, was
observed when an incident light of 514 nm (E.sub.excitation=2.41
eV) was used (FIGS. 4B and 4C). This further confirms that the
single-walled carbon nanotubes that constitute the CDS fibre are
metallic.
[0184] (FIG. 6B shows the representative RBM region of a typical
Raman spectrum for a fibre obtained using CDS. It has a peak at 195
cm.sup.1 with .lamda..sub.excitation=633 nm and the absence of the
RBM peak with .lamda..sub.excitation=514 nm. FIG. 6C shows the
metallic and semiconducting window in the Kataura plot (non-filled
circles=metallic nanotubes, filled circles=semiconducting
nanotubes) are marked red and green respectively on the original
colour version of this drawing for nanotubes in the diameter range
of 1.1 to 1.4 nm in correlation to the excitation energies used to
acquire the Raman spectra (green region=2.41.+-.0.1 eV, 514 nm; red
region=1.96.+-.0.1 eV, 633 nm).)
[0185] TEM and Electron Diffraction
[0186] Analysis by transmission electron microscopy indicates that
the bundles that constitute the fibres, from both CDS and
thiophene, are typically in the diameter range of 30-60 nm (FIGS.
7A and 7B respectively). From HREM analysis, the CDS fibres are
composed of SWCNTs and those obtained with thiophene as sulphur
precursor are composed of collapsed DWCNTs (FIGS. 7C and 7D
respectively), confirming the findings from Raman spectroscopic
analysis.
[0187] The diameters and diameter distribution of the nanotubes are
presented in Table 3 and FIGS. 8A and 8B. It can be seen that the
diameters obtained from TEM analysis of the CDS fibres are in close
agreement with those obtained from Raman spectroscopy (bulk
characterisation).
TABLE-US-00004 TABLE 3 Average diameters of the single-walled
carbon nanotubes obtained using CDS, and collapsed double-walled
carbon nanotubes obtained using thiophene, from TEM and RBM.
Diameter.sub.TEM Diameter.sub.RBM Fibre (nm) (nm) CDS: Metallic
SWCNT 1.4 .+-. 0.3 1.2 .+-. 0.2 Thiophene: DWCNT 7.6 .+-. 2.3
N/A
[0188] The diameter distributions, determined using TEM are
presented in FIGS. 8A and 8B, which shows (8A) the diameter
distribution of carbon nanotubes obtained using CDS, and (8B) the
diameter distribution of carbon nanotubes obtained using
thiophene.
[0189] Electron diffraction was carried out on fibre bundles (e.g.
those represented in FIGS. 7A and B). The electron pattern from
fibres obtained using CDS showed a pattern of clear spots,
positioned to indicate armchair (n,n) tubes, with a chiral angle of
30.degree.. In correlation with the diameter measurements, this
suggests that the tubes are (10,10) tubes. Armchair tubes are
metallic and hence, these results are in agreement with the
characterisation by Raman spectroscopy. The electron diffraction
patterns from the fibres obtained from thiophene are composed of
continuous rings corresponding to (10-10) and (11-20) reflections,
which shows that the nanotubes have a continuous distribution of
helicities (i.e. there is a mixture of different chiralities).
[0190] An example electron diffraction pattern is shown in FIG.
11A, for a bundle of carbon nanotubes having armchair chirality.
Only half of the hexagonal pattern of spots is shown, the remaining
spots are obscured by a shade. The arrow indicates the principal
axis of the carbon nanotubes. FIG. 11B shows the same image, which
has been marked up to show the location of the diffraction spots.
The position of the three visible diffraction spots of the
hexagonal pattern is indicated with white oval marker points. A
similar example electron diffraction pattern is shown in FIG. 12A.
FIG. 12B shows the same image as FIG. 12A, marked up to highlight
the location of the diffraction spots.
[0191] Catalyst Particles
[0192] The catalyst particles formed when CDS is used were
examined. The particles were frozen and withdrawn from the zone of
the reactor where they form (in the temperature range 400-600C. A
HREM image of the withdrawn catalyst particles is shown in FIG. 9A.
The diameter distribution of these particles, determined using HREM
is shown in FIG. 9B. This figure shows that the catalyst particles
have a narrow size distribution.
[0193] The average diameter values of the `frozen` catalyst
particles is 2.5.+-.0.8 nm and the ratio of the average diameter of
the catalyst particles to that of the single-walled carbon
nanotubes is about 1.8, which is in close agreement with that
reported in the literature.
[0194] Fibre Properties
[0195] Mechanical and Electrical Properties
[0196] The mechanical properties of the metallic single-walled
carbon nanotube fibre (obtained using CDS) and the double-walled
carbon nanotube fibre (obtained using thiophene) are presented in
Table 4. The fibres composed of collapsed DWCNT fibres are expected
to be superior mechanically, due to the large contact area between
the nanotubes held by van der Waals forces within the bundles,
which is evinced in the tensile strength and stiffness values.
TABLE-US-00005 TABLE 4 Fibre characteristics and mechanical
attributes properties of the metallic SWCNT and DWCNT fibres along
with those of copper wire of electrical wiring grade. Fibre
characteristics Mechanical properties Linear Sp. Sp. Diameter
density Strength Stiffness Material (.mu.m) (g Km.sup.-1) (GPa
SG.sup.-1) (GPa SG.sup.-1) Metallic SWCNT 10-15 0.04 0.5 10 fibre
DWCNT fibre 10-15 0.04 1 20 Copper AWG 10 1820 2.3 .times. 10.sup.4
0.03 14
[0197] Table 5 below illustrates typical properties of materials,
including a non-optimised fibre within the scope of the present
invention (final row).
TABLE-US-00006 Volumetric Linear Specific Current Specific Specific
Conductivity density density conductivity density Strength
Stiffness Material S/m .times. 10.sup.6 g/m.sup.3 .times. 10.sup.6
g/km S/m/g/m.sup.3 (A/mm.sup.2) GPa/SG GPa/SG Copper 58 8.9 -- 6.5
2-10 0.025 13 (electrolytic) Aluminium 38 2.7 -- 14.1 4 0.026 26
Steel/Iron 10 7.9 -- 1.3 -- 0.038 27 Carbon fibre 0.06 1.8 -- 0.03
-- 1.96 128 T300 TORAY High 0.14 1.9 -- 0.07 -- 2.06 309
performance carbonfibre (M60J) TORAY CNT yarn 0.1-0.7 0.8-1.2
0.02-0.1 0.1-0.7 30 0.8-1.2 60-140 "non (depending on metallic"
winding rate) CNT yarn 0.7-3 (so far) 0.8-1.2 0.02-0.1 0.7-3 80
0.8-1 60-140 "metallic" (depending on winding rate)
[0198] In this table, specific conductivity parameter which takes
into account both electrical conductivity and the density of a
conductor. In Table 5 below, the values for specific conductivity
were estimated, from experimentally obtained values for
conductance, length and linear density, using the equation
below:
.sigma. l = G * L LD 10 3 ##EQU00002##
[0199] wherein G is conductance (in Siemens), L is length (in
metres), and LD is linear density (in tex, or g km.sup.-1), and 6'
is specific conductivity in:
S m - 1 g cm - 3 ##EQU00003##
[0200] It will be understood that both the electrical conductivity
and the density of a conductor are important in many engineering
applications, for example in overhead power lines.
[0201] In the above examples, without wishing to be bound by
theory, it is believed that the thermal degradation of ferrocene in
hydrogen atmosphere begins at about 673K to yield iron atoms (d=0.3
nm) which subsequently grow into nanoparticles (which act as
catalysts for the nucleation and growth of nanotubes). These
nanoparticles are believed to pass through the reactor in the flow
direction, along the temperature profile. The thermal degradation
of the sulphur precursor (CDS or thiophene) in the reaction
feedstock leads to the interaction of the iron nanoparticles with
sulphur. We call this sulphudisation. The addition of sulphur, a
recognised promoter in carbon nanotube growth, allows the
production of long carbon nanotubes (typically mm). It is believed
that this can enhance the mechanical integrity of the mass of
carbon nanotubes produced. This can facilitate the production of
carbon materials, such as fibres and films, from the carbon
nanotubes.
[0202] The above examples probe the effect of different sulphur
precursors, with varied thermal degradation behaviour. This alters
when the sulphur becomes available to the growing iron
nanoparticles. Sulphur is believed to act as an arresting agent,
stopping or slowing the nanoparticle growth. This is believed to
affect the structure of the carbon nanotubes formed.
[0203] The thermal stability of CDS is lower than thiophene,
especially in a reductive hydrogen atmosphere. The adjacent double
bonds in CDS are expected to readily undergo hydrogenation followed
by elimination of sulphur in the form of H.sub.2S. This compound
readily sulphudises the iron nanoparticles. Thiophene on the other
hand is resistive to hydrogenolysis owing to its stability as an
aromatic compound. Where CDS is used, the temperature at which
sulphur becomes available to the catalyst particles is lower than
the temperature at which sulphur becomes available to the catalyst
particles when thiophene is used.
[0204] As shown in FIG. 10A, the thermal degradation temperatures
of ferrocene and CDS are close to each other. Therefore, it is
believed that the catalyst particles are sulphudised in the early
stages of their growth. This is believed to result in catalyst
particles wherein at least 70% by number of the catalyst particles
have a diameter in the range from 0.5 nm to 4.5 nm. It is believed
that the small catalyst particles tend to provide single-walled
carbon nanotubes.
[0205] In contrast, there is a much larger difference between the
thermal degradation temperatures of thiophene and ferrocene, as
shown in FIG. 10B. Therefore, the nanoparticles grow for much
longer before they encounter sulphur obtained by degradation of
thiophene. Before this, the iron particles grow to 8-10 nm in
diameter. These larger nanoparticles tend to produce larger
diameter carbon nanotubes, which tend to be double-walled and
collapsed.
[0206] The experiments were repeated with ethanol as the carbon
source (ethanol decomposition yields a carbon supply at much lower
temperatures (about 873K) than methane). In the case of carbon
nanotube fibres obtained from ethanol, CDS lead to the formation of
metallic single-walled carbon nanotubes, while thiophene yielded
collapsed double-walled carbon nanotubes. In addition, the effect
of the presence of helium (recently reported to play a role in the
formation of metallic nanotubes; Reference 3) on the formation of
metallic nanotubes was tested, by carrying out the ethanol runs in
the absence and presence of helium. Both yielded identical results
and the presence of helium did not seem to significantly affect the
process.
[0207] The preferred embodiments have been described by way of
example only. Modifications to these embodiments, further
embodiments and modifications thereof will be apparent to the
skilled person and as such are within the scope of the present
invention.
REFERENCES
[0208] The content of each of the following references is
incorporated herein in its entirety. [0209] 1. Koziol, K. et al;
High-Performance Carbon Nanotube Fiber; Science 318, 1892 (2007)
[0210] 2. Motta, M. S. et al; The Role of Sulphur in the Synthesis
of Carbon Nanotubes by Chemical Vapour Deposition at High
Temperatures; J. Nanosci. Nanotech. 8 1-8 (2008) [0211] 3.
Harutyunyan et al; Preferential Growth of Single-Walled Carbon
Nanotubes with Metallic Conductivity; Science 326, 116 (2009)
[0212] 4. Carbon Nanotubes; Ed: Jorio, Dresselhaus and Dresselhaus
Springer Verlag Heidelberg 2008 [0213] Kataura et al; Optical
Properties of Single-Wall Carbon Nanotubes; Syn. Met. 103 2555-2558
(1999)
* * * * *