STUDUIA UNIVERITATIS BABEŞ-BOLYAI, PHYSICA, SPECIAL ISSUE, 2003
SYNTESIS OF CARBON NANOSTRUCTURES
HEATING ASSISTED CCVD METHOD
BY
INDUCTION
D. Lupu 1, A.R. Biriş 1 , I. Mişan 1, G. Mihăilescu 1 , L. Olenic 1, S. Pruneanu 1,
A. Jianu 2, C. Bunescu 3, A. Weidenkaff 4, C. Diecker 4
1
National Institute for Research and Development of
Isotopic and Molecular Technologies, P.O. Box 700,
R-3400 Cluj-Napoca, Romania
2
National Institute of Materials Physics, P.O. Box
MG-7, RO-76900
3
Hochschule Wismar, PO Box 1210, D-23952,
Wismar, Germany
4
Institute of Solid State Chemistry, Universität
Augsburg, Universitätsstr. 1, D-86159 Augsburg,
Germany
Abstract
Experiments on the catalytic chemical vapour deposition
for the synthesis of carbon nanostructures are reported, both with
the outer furnace technique and with induction heating. The r esults
are analysed comparatively and the advantage of the induction
heating is revealed.
1. Introduction
The carbon-based materials and nano-scale sciences are envisaged as
major fields of the 21 st Century technology. The extensive work on carbon
nanotubes (CNT), started just after their discovery [1], resulted in an
exponential increase of the patent filings and publications [2]. Their
applicability has been investigated in various technological areas such as
aerospace, aeronautic, nanoelectronics, energy, medicine or chemical industry
in which they can be used as gas adsorbents, templates, actuators, composite
reinforcements, catalyst supports, probes, chemical sensors, nano-reactors.
The ability of the CNTs to immobilise biologically active substances [3-5]
opens new ways for bio-nanotechnologies.
The main challenge in working with carbon nanotubes is their
synthesis [6] and new techniques are examined in many laboratories to
develop methods able to lead to carbon nanotubes with controlled
characteristics and suitable for large-scale production. It is usually accepted
that the catalytic decomposition of hydrocarbons (CCVD) on solid (catalyst)
surfaces seems a viable method for large-scale production with low energy
consumption.
D. LUPU ET AL.
The use of the induction heating to reach high temperatures during the
laser synthesis of single-wall carbon nanotubes [7] pointed out to the ability
of this particular heating mode to control the temperature of the reaction even
at high temperatures, without significant heat transfer to the quartz tube
confining the reaction region. This made us to explore the possibilities of
coupling the induction heating (IH) with the CCVD method for the synthesis
of carbon nanotubes. By this CCVD-IH route, various types of carbon
nanostructures were indeed obtained [8], the attempts showing that it can be
used even for the vertical, floating catalyst method.
In this paper, the results obtained with outer furnace technique and
induction heating, keeping the same all the other reaction conditions
(hydrocarbon and carrier gas flow, reaction temperature and time) are
comparatively reported, trying to find out some possible advantages of the
CCVD-IH coupling.
2. The experimental set-up of CCVD with induction heating
The catalytic chemical vapor decomposition of hydrocarbons for the
synthesis of carbon nanofibers, single-wall nanotubes (SWNT), multi-wall
nanotubes (MWNT) is usually performed at temperatures over 500 oC, in a
Pressure gage
(0 , 10 bar)
Flow
meters
Hydrocarbon
Inductor Coil
Pressure gage
( -1 , 0 bar)
Gas
output
Gas
input
Catalyst
susceptor
Vacuum
Quartz tube
Hydrogen
( Hydrocarbo
99,9999% )
n
Carrier gas
Fig.1. Scheme of the experimental set-up for CCVD syntheses of carbon nanostructures
using induction heating
quartz tube containing the catalyst which may be displayed in a covenient
way to be kept at the reaction temperature. In our experiments, both heating
with an electric outer furnace (refered to as CCVD) and induction heating
(CCVD-IH) have been used. In the later, the outer furnace has been r eplaced
by the coils of an high frequency inductor working at 1.3 MHz as described in
SYNTHESIS OF CARBON NANOSTRUCTURES BY INDUCTION HEATING ASSISTED CCVD
Fig. 1. The catalyst is placed on the susceptor within the quartz tube – an
electric conductor of suitable shape which can be inductively heated.
Some peculiarities of the two heating modes need a brief review. The
heating of the catalyst (either as a powder in a ceramic boat or deposited on a
substrate) with an outer furnace is achieved through heat transfer by radiation
and convection from the hot tube walls. Conversely, in the induction heating
case, the thermal gradient is in the opposite direction: the heat transfer flows
from the inductively heated susceptor to the catalyst displayed on its surface.
The inductive heating (IH) occurs due to the confinement of the induced
currents and magnetic flux to a thin layer of an electric conductor subjected to
the high-frequency field- the skin effect. The thickness of this layer decreases
with the increasing frequency, at high frequencies the electromagnetic energy
beeing confined essentially to the surface [9] with the skin depth depending
on the frequency, relative permeability and electric conductivity – the
parameters on
which the selection of a suitable susceptor should depend on. Due to the
permeability, for example, the skin depth of magnetic steel increase from
0.014 mm below the Curie point to 0.55 mm above 780 oC. The power
absorbed by the IH heated body depends on the same parameters and on the
magnetic field. Because the catalyst involves metallic clust ers it may be
directly influenced by the high-frequency field with direct consequences on
the processes related to the CCVD occuring on its surface. These effects,
which are not present in the conventional heating mode, may ultimately result
in influences on the morphology and growth of the carbon nanostructures
synthesized by CCVD-IH. The experiments showed that the materials used for
the susceptor can be titanium, graphite, molybdenum or even iron.
With the modified CCVD-IH method, the activation of the catalyst
can be performed also at the desired temperature as for the CCVD with outer
furnace technique, by the control of the high-frequency generator at the
required conditions.
3. Comparative results obtained by CCVD and CCVD-IH
3.1 Hollow core nanofibers
With a Ti rod used as a susceptor, its surface was previously prepared
to obtain a thin layer of TiO 2 as the titanium oxide proved to be good as
support of metal catalyst [10] for the synthesis of carbon nanostructures. The
surface of the rod was oxidized electrochemically in a 3% aqueous solution at
constant voltage (20 V, DC) for 10 min. After that, nitrate solutios were
evaporated to obtain Fe:Co (1:1 mole ratio)/TiO 2 and the deposit was heated
in air at 400 oC for 1 h. Both for the CCVD and CCVD-IH synthesis, the
catalyst was previously activated “in situ” at 350 o C for 1 h, after which the
temperature of the rod was increased at 600 o C and an ethylene:hydrogen
D. LUPU ET AL.
mixture (4:1 volume ratio) at 0.35 bar was admitted in the quartz tube. After 1
h, the sample was cooled to the room temperature.
The carbonaceous products, collected by scraping from the Ti rod
were maintained in HCl 37% for 24 h, washed with distilled water and dried
at 150 oC for 1h.
The results reported comparatively in Fig. 2 for synthesis in exactly
the same conditions show some interesting differences. Hollow core
nanofibers of 20-50 nm o.d. (5-20 nm i.d.) many of them with catalyst
particles encapsulated at their tips are obtained by CCVD. On the other hand,
CCVD-IH results in significantly thinner nanofibers of 8-15 nm o.d. (3-5 nm
i.d.), without catalyst particles at the tip. This suggests a higher growth rate in
the IH case as the thin fibers grow faster than the thick ones [11]. Further
studies seem very interesting to elucidate if this might be a direct effect of the
induction currents.
a
b
Fig.2. TEM images of the carbon nanostructures obtained in the same reaction
conditions from ethylene:hydrogen (4:1) at 0.35 bar and 600 o C on Fe:Co(1:1)/TiO 2,
using: a) outer furnace and b) induction heating
3.2 Single wall nanotubes
Experiments on the synthesis of SWNT have been also performed,
using the Fe:Mo:Al 2O3 (1:0.2:16 weight ratios) reported in literature [12] to
be good for SWNT synthesis even without activation. The catalyst was
prepared as described [12] and 20-30 mg were introduced in a graphite
cylinder of 14 mm o.d. (1 mm wall thickness) as susceptor. The synthesis was
conducted at 800 oC with 12 ml/min methane flow and 80 ml/min purified
argon as carrier gas for 1 h after which the sample was allowed to cool at the
room temperature. Fig.39a,b) show images very similar to the bundles of
SWNT reported in [7]. They are more abundant in the case of CCVD-IH (Fig.
3b) as compared to the outer furnace technique (Fig. 3a). Raman studies will
SYNTHESIS OF CARBON NANOSTRUCTURES BY INDUCTION HEATING ASSISTED CCVD
be performed to obtain more detailed informations about the SWNTs
diameters and some possible differences between the two methods.
a
b
Fig.3. TEM images for as-prepared carbon nanotubes from methane/argon at 800 o C
on Fe:Mo:Al 2 O3 catalyst by: a) CCVD and b)CCVD-IH techniques
The average yield was 19-20% both for CCVD and for CCVD-IH but the
consumed energy was 2.5 kWh for CCVD aand only 0.6 kWh for IH.
3.3 Multi-wall nanotubes
With CCVD-IH, thin
hollow core nanofibers of 15-20
nm outer diameter, 5-6 nm inner
hollow core, (possibly MWNT),
were obtained from ethylene
(16% vol) in hydrogen flow (100
ml/min) at 600 o C with Ni/Al 2 O3
on molybdenum susceptor. The
catalyst
was
prepared
by
evaporating Ni and Al nitrates
on a rolled Mo foil. The TEM
image shown in Fig. 4 does not
allow a firm conclusion
about the identification as
MWNT and the work needs to be
Fig.4. TEM image of carbon
continued by HRTEM studies.
nanostructures obtained by CCVD-IH from
Recently it was reported
o
ethylene: hydrogen (16 % vol) at 600 C
[13] that high quality multi-wall
on Ni/Al 2O 3
carbon nanotubes are obtained from acetylene on Co 3 O4/MgO catalyst with
nitrogen as carrier gas. Preliminary results, within a cooperation with Prof. L.
D. LUPU ET AL.
Duclaux and Dr. Y. Soneda, show that in the same reaction conditions, better
MWNT are obtained with the CCVD-IH method [14], as revealed by
HRTEM. The work is now in progress to establish the suitable synthesis
conditions and to characterize the morphology and structure of the products.
It is worth to note that the energy consumed in this case, even using a short
electric outer furnace is 420 Wh for CCVD but only 175 Wh for CCVD-IH.
Conclusions
The induction heating can be used to synthesize all the main type
of carbon nanostructures (nanofibers, SWNT, MWNT) by the CCVD
method. This CCVD-IH method show some advantages as compared to
the outer furnace technique: 2-3 times lower energy consumption,
lowering of the overall time for a batch due to the fast heating and
cooling rates, versatility in the temperature control with much higher
temperatures available for synthesis and thermal annealing, reduced
extent of the heating zone. The influence of IH on the morphology,
structure and growth rate of the carbon nanostructures deserves further
studies.
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