Composites: Part B xxx (2013) xxx–xxx
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Composites: Part B
journal homepage: www.elsevier.com/locate/compositesb
Controllable synthesis of carbon coils and growth mechanism
for twinning double-helix catalyzed by Ni nanoparticle
Xian Jian a, Dingchuan Wang a, Hongyang Liu b, Man Jiang a, Zuowan Zhou a,⇑, Jun Lu a, Xiaoling Xu a,
Yong Wang a, Li Wang c, Zizheng Gong c, Mingli Yang d, Jihua Gou e, David Hui f
a
Key Laboratory of Advanced Technologies of Materials (Ministry of Education), School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, China
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
China Academy of Space Technology, Beijing 100094, China
d
Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, China
e
Department of Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, FL 32816, USA
f
Department of Mechanical Engineering, University of New Orleans, New Orleans, LA 70148, USA
b
c
a r t i c l e
i n f o
Article history:
Received 25 March 2013
Accepted 10 June 2013
Available online xxxx
Keywords:
A. Carbon fibre
A. Nano-structures
E. Chemical vapour deposition (CVD)
E. Heat treatment
a b s t r a c t
A simple approach using 90 nm Ni nanoparticles to tune the growth of carbon fibers is developed basing
on the bottom-up regulation in this study. By adjusting the preparing temperature and atmospheric composition, the nickel-containing catalysts with specific sizes and shapes are alternately prepared from
90 nm to 500 nm. The straight carbon nanofibers and three kinds of carbon coils (single-helix carbon
nanocoils, single-helix carbon microcoils and twinning double-helix carbon microcoils) with the coil
diameter ranging from 150 nm to 3 lm are achieved by using the as-prepared catalyst. The relationship
between the carbon coil and catalyst particle, and the growth mechanism for different kinds of helices are
discussed in details. The results suggest that catalytic anisotropy come from growing tendency and rate
of carbon deposition on the facets, edges and vertices of catalyst grain. The twinning structure exists in
each fiber of the double-helix carbon microcoils regardless of circular or flat fiber, which is separated by
the tip of the catalyst particle due to the different rate of carbon deposition at edge and vertex.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Since the discovery of carbon helix, the synthesis of carbon coils
involving fibers or tubes with helical diameter sized from 16 nm to
30 lm [1,2] has attracted significant attention, not only for fundamental scientific [3,4] interests, but also for their potential applications [5–7]. Though carbon nanofiber or tubes can strength some
matrix [8–12] such as multiscale syntactic foams [9], epoxy
[13,14], plastic laminates [15], polyurethane [16] and phenolic
composites [17], helical carbon fibers/tubes are also a good
strengthening fibers in composite materials [18–20]. Besides, carbon coils exhibit peculiar morphology-dependent electrical,
mechanical, magnetic and chemical properties, compared with
those of their bulk materials. However, for applications in electro-mechanical sensors, the morphological uniformity is critical
for achieving a stable piezoresistive behavior of carbon nanosprings. Besides, morphology control of the helical structure has
significant influence on chirality, which is essential for the electromagnetic wave absorbers [21]. Thus, the rational synthesis of special coil with high purity is of key importance for technological
applications.
⇑ Corresponding author. Tel./fax: +86 028 87600454.
E-mail address: zwzhou@at-c.net (Z. Zhou).
So far, two main strategies have been developed to synthesize
helical structures of carbon coils. one is the ‘‘catalyst-component’’
adjustment depending on the catalytic activity, and the other is the
‘‘growth-condition’’ method being governed by the chemical kinetics. Several helical carbon structures (double/single-helix, micro/
nano-coils, helical fibers/tubes, etc.) were obtained by adjusting
the component of metallic catalysts (Fe, Co, Ni, Cu, In or their alloys) [22–24]. The advantage of the catalytic activity methods is
the production with high pertinent characteristic, but the procedure of preparing the special catalyst seems quite complicated.
The chemical kinetic methods can be used to synthesize shapecontrolled carbon coils by choosing different carbon sources [24],
adding the substrate [25,26] or introducing magnetic/micro-wave
field [27,28], but the equipment is usually fussy and the procedure
is multi-step. In order to simplify the process to obtain special carbon coils, it is necessary to explore an effective method to combine
the catalytic activity with chemical kinetics.
Nickel is one of the typical catalysts for growing the helical carbon fibers (CFs). For example, pure Ni powder sized of 5 lm [29]
or 500 nm [30], activated carbon nanotubes supported Ni 0.8 lm
[31], nickel foam [32] and Ni plate [33] were used to prepare
helical CFs with an approximate fiber diameter of 500 nm. It can
be noticed that some carbon coils with similar sizes and diameters
were prepared using Ni catalyst with different particle size.
1359-8368/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.compositesb.2013.06.010
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X. Jian et al. / Composites: Part B xxx (2013) xxx–xxx
Nevertheless, few literatures provided reasonable explanations to
the relationship between the initial Ni catalyst size and the diameter of the CFs. In addition, it has been proposed that the driving
force for the coiling of vapor grown CFs is the presence of catalytic
anisotropy among the crystal facets of the catalyst grain. However,
the direct evidence of this presumption has not been reported yet.
In this work, we reported the size control of Ni catalyst by
chemical reduction and the rational synthesis of several different
carbon coils by catalytic chemical vapor deposition (CCVD) method
using acetylene as carbon source. The as-synthesized CFs could be
controlled into linear carbon nanofibers (CNFs) and carbon coils
including single-helix carbon nanocoils (SH-CNCs), single-helix
carbon microcoils (SH-CMCs) and twinning double-helix carbon
microcoils (TDH-CMCs). In addition, the existence of catalytic
anisotropy was experimentally approved and the expounded
growth mechanism for these carbon species was preliminarily
suggested.
2. Experimental section
For the synthesis of Ni catalyst, the mixed solution containing
20 mL of 0.8 mol/L (M) sodium hydroxide solution, 20 mL of 6 M
hydrazine hydrate aqueous solution and 1 g polyvinylpyrrolidone
(PVP-K30) was firstly prepared. Then 20 mL of 1 M nickel sulfate
aqueous solution was slowly added to the mixed solution. Finally,
Ni nanoparticles were prepared by chemical reduction with hydrazine hydrate in distilled water at 70 °C with magnetic stirring for
1.5 h. Ni nanoparticles, namely the black precipitates, were separated from the mother liquor by magnetic separation, and washed
several times with distilled water and acetone, respectively, until
the pH was 7. The obtained Ni nanoparticles, grey–black powder,
were dried in vacuum at 25 °C.
The Ni catalyst (30 mg/cm2) supported by graphite plate was
placed in the central part of a horizontal reaction tube (quartz,
24 mm i.d.), which was heated from the outside by an electrically
heated horizontal furnace. Then linear and helical CFs were prepared on the graphite plate using a gas mixture of C2H2, H2 and
N2 containing a small amount of thiophene as catalytic auxiliary.
The furnace was firstly flushed until 600 °C at the rate of 5 °C/
min under nitrogen, and it was unceasingly flushed from 600 to
750 °C at 3 °C/min under hydrogen. Acetylene along with thiophene vapor was allowed to slowly enter the furnace at a rate of
20 mL/min. Thiophene was introduced at a rate of about 80 mg/
min by bubbling acetylene through a vessel containing thiophene
at 40 °C. Meanwhile, the flow rate of H2 was fixed at 60 mL/min.
Nitrogen was used as the carrier gas at a given rate from 100 to
140 mL/min. The fluffy products of various CFs including linear
CNFs, SH-CNCs, SH-CMCs and TDH-CMCs were synthesized without further treatment.
The crystal structure of the catalyst particles and the helical CFs
was investigated using X-ray diffraction (XRD, Panalytical X’Pert
PRO diffractometer with Ni-filtered, the Netherlands). The size
and morphology analyses of the nickel particles and the CFs were
performed using scanning electron microscope (SEM, Fei, Quanta
200 and Inspect-F) with an accelerating voltage of 20.0 kV and
transmission electron microscopy (TEM, H-700H) at an accelerating voltage of 160 kV.
3. Results and discussion
3.1. Preparation and characterization of Ni catalyst
Generally, catalyst plays an important role in the control of
helical carbon materials with desired category and morphology.
For example, three-dimensional spring-like carbon nanocoils are
obtained with high purity by the catalytic pyrolysis of acetylene
at 750–790 °C using a Fe-based catalyst, and the nanocoils have a
nanotube of 10–20 nm [34]. Besides, the carbon nanocoils with coil
diameters of 50–450 nm can be obtained using sputtered thin films
of Au and Au/Ni as catalysts [27]. Considering that Ni particle is one
of the typical catalysts for the mass production of carbon microcoils (CMCs), we prepared high-purity and monodispersed Ni
nanoparticles by the reduction of a nickel salt with hydrazine hydrate employing the surfactant of PVP to prevent agglomeration
of the nanoparticles. The effects of reaction temperature (Fig. S1
in Supporting information), NaOH concentration (Fig. S2) and reaction time for the preparation of nickel powders were systematically investigated. Spherical and relatively uniform Ni particles
(Fig. 1a) were obtained under the optimal condition that the molar
concentration of NaOH solution was 0.8 M at 70 °C with magnetic
stirring for 1.5 h. Particle-size distribution (Fig. 1c) of the obtained
monodispersed Ni nanoparticles was determined by software of
Nano Measurer [35] program based on the corresponding SEM
images. The average particle size was 90.12 nm with a relatively
narrow distribution. The TEM image of the as-prepared Ni nanoparticles placed in ethyl alcohol is shown in Fig. 1b. It could be seen
that sometimes Ni nanoparticles adhered to each other forming
into some larger Ni grains of about 150 nm, which might be due
to the ferromagnetism of them. The XRD pattern of the nanoparticles, shown in Fig. 1d, revealed an fcc Ni crystalline structure. In the
XRD profile, the three characteristic diffraction peaks corresponding
to the (1 1 1), (2 0 0) and (2 2 0) diffraction planes of only cubic Ni
phase over 40° were clearly observed, which was well in agreement
with the standard nickel diffraction pattern (ICDD, PDF file No. 01070-1849). The crystallite size of Ni for the most intense peak
(1 1 1) plane was determined from the X-ray diffraction data using
the Debye–Scherrer formula, and the average value was about
5.07 nm when the concentration of NaOH was 0.8 M. In addition, it
could be found that the particle size calculated from the XRD pattern
was rather smaller than that determined from the SEM image. The
results suggested that the spherical nickel particles contained a
number of ultra-fine crystals of about 5 nm, which agreed with the
TEM observation of Ni particles’ morphology.
3.2. Rational synthesis of CFs
The CMCs are usually prepared using typical nickel catalyst
such as Ni plate [33] and Ni powder with particle sizes from 0.5
to 5 lm [29–31]. Besides, Ni particles with a particle size below
50 nm [36] or about 500 nm [37] have also been chosen for the catalyst to prepare CMCs. It is can been seen that the Ni catalyst are of
various kinds and their original size does not determine the fiber
diameter of the corresponding carbon coil. The cause of this case
might be that it was difficult to obtain carbon nanocoils on Ni
nanoparticles below 100 nm. Similarly, we obtained linear CNFs
without helical shape using the as-prepared Ni nanoparticles as
the catalyst under the reaction conditions that the bath temperature of thiophene was 40 °C and the flow rate of C2H2, H2 and N2
was 20, 60 and 100 mL/min, respectively, at the reaction temperature of 660 °C for 3 min. A typical SEM image of the linear CNFs
(Fig. 2) displayed an almost uniform fiber diameter of about
100 nm. This was due to the presence of an almost uniform size
of catalyst particles and confirmed that the linear CNFs diameters
depended on the size of the Ni particles in chief. The linear CNFs
were straight or complexly curved, and interlaced with each other.
This might be associated with the real-time shape of crystalline
metallic Ni particles with symmetric or asymmetric shape during
the growth of the CFs [38]. Since spherical catalyst particle has
symmetric planes on condition that they pass through any diameter of the Ni nanoparticles, and the anisotropic behaviors of the
catalyst do not show. Then straight CFs could be obtained using
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Fig. 1. (a) SEM and (b) TEM images, (c) size distribution and (d) XRD pattern of nickel particles obtained under the optimal condition that the concentration of NaOH solution
was 0.8 M at 70 °C with magnetic stirring for 1.5 h.
Fig. 2. SEM image of linear CNFs synthesized by CCVD route at 660 °C for 3 min
when the gas flow rate of C2H2, H2 and N2 was 20, 60 and 140 mL/min, respectively.
fine Ni particles with spherical shape, and curved CFs formed on
the asymmetric spheroidal Ni particles. From the early study
[39], linear CFs were synthesized using the catalyst containing Co
or Fe alloy. This is a supplementary method for the synthesis of linear CFs, differing from the noodle-like twin fibers obtained at
700 °C by Chen and Motojima [40], which might be potential and
effectual to prepare CFs networks.
As the basic unit of bulk materials, Ni nanoparticle has the advantage of forming the required Ni catalyst by the gas-adjusting approach. Three kinds of carbon coils, namely, SH-CNCs, SH-CMCs
and TDH-CMCs, were prepared using the as-prepared Ni nanoparticles by adjusting the gas flow rate of N2 at 140, 120 and 100 mL/min
respectively. The other experimental parameters were unchanged.
The bath temperature of thiophene was 40 °C, the flow rate of
C2H2 and H2 was 20 and 60 mL/min respectively, and the reaction
was carried out at 750 °C for 120 min. SEM images of the three kinds
of carbon coils are shown in Fig. 3. The SH-CNCs (Fig. 3a) mostly have
the fiber diameter of about 150 nm and the coil diameter of about
200 nm, and the coil pitch ranging from 200 to 400 nm. The morphology of SH-CNCs was analogous to the CFs obtained from the
interaction between the copper–nickel (7:3) and the ethylene/
hydrogen (4:1) at 600 °C by Kim et al. [41] Compared with the linear
fibers, the fiber diameters of SH-CNCs were generally larger. The
probable reason for this phenomenon was that the Ni particles
underwent a growing-up experience by agglomerating and/or
recrystallizing as the reaction temperature increased from 660 to
750 °C. The fiber diameters of CFs depend on the size of Ni particles,
thus the SH-CNCs have a little thicker fibers than linear CFs at
relatively higher temperature of 750 °C.
When the flow rate of N2 reduced to 120 mL/min, the SH-CMCs
(Fig. 3b) could be observed and they had a fiber diameter of
500–600 nm and a coil pitch of 2–3 lm. Different from conventional SH-CNCs, the coil pitch was generally quite large while the
coils still kept the regular helical forms. Besides, the SH-CMCs contain an abnormal interlayer of a wavy morphology in the middle
part of the fibers, which is similar to the spring-like carbon coils
with wave-form laces obtained by changing the composition and
the solid state conditions of the catalysts (71Fe–18Cr–8Ni–3Mo)
supported on ceramic nano-powders [34]. Then, it is suggested
that the SH-CMCs can be obtained just by controlling the flow rate
of the carrier gas at 120 mL/min in this case.
Conventional DH-CMCs are usually prepared by the catalytic
pyrolysis of acetylene using Ni powder of several micrometers in
diameter. For example, Kawaguchi prepared double helix regular
carbon microcoils with the catalyst of Ni powder having an average
diameter of 5 lm [42]. Here we designed the process of getting the
larger Ni particles by the bottom-up method, which is useful for
Please cite this article in press as: Jian X et al. Controllable synthesis of carbon coils and growth mechanism for twinning double-helix catalyzed by Ni
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The growth mechanism for the three kinds of helical carbon materials will be discussed in the next section.
3.3. Analysis and mechanism for carbon coils
Fig. 3. SEM images of three kinds of carbon coils: (a) SH-CNCs, (b) SH-CMCs, and (c)
typical DH-CMCs.
precise control the helical structure of carbon coils. Before the
growth of carbon coils, the as-prepared Ni nanoparticles were
heated under H2 at 700 °C for 1 h to make the Ni nanoparticles
with melting point less than 750 °C be melt and annexed by lager
Ni particles until the state of thermodynamic equilibrium. The uniform Ni particles with diameter of several micrometers formed
after the above heat treatment. Typical DH-CMCs (Fig. 3c) were obtained when the flow rate of N2 was 100 mL/min, and they had a
fiber diameter of 500–600 nm and a helical pitch of 1–3 lm. From
these observations, it is necessary to explain why the three kinds of
helical carbon species differ in filament diameter and helical pitch.
Amelinckx et al. proposed a formation mechanism for catalytically growing of helix-shaped graphite nanotube [43]. They reported the geometrical parameters of the resulting helix could be
expressed in terms of the extreme values VM (maximum velocity),
Vm (minimum velocity) of the extrusion velocity and of the inclination angle. The growth mechanism could also be sufficiently general to account for the variability in pitch of helices. About the
CFs, the size of its diameter depended on the particle size of the
catalyst, and the pitch and diameter of the coil were decided by
the extrusion speed of VM, Vm and the inclination angle. Then it
can be speculated that the direct cause for the discrepancies of
the three kinds of helical fibers is the differences of the size and
the anisotropy catalysis of the catalysts. Three kinds of carbon coils
can be rationally synthesized by adjusting the flow rate of N2 demonstrating that the flow rate makes huge effects on the catalytic
performance of Ni particles.
Nitrogen flow rate seems to be the only changed parameter during synthesizing, which results in different carbon coils under the
corresponding condition. However, the change of N2 flow rate is
not the essential cause, because N2 is non-active and does not take
part in the chemical reaction. Moreover, the N2 flow rate does
influence the quantity of carbon source and H2 for a certain period
of time. From the difference of the diameters of helical CFs, it can
be reasonably speculated that the fine Ni particles happen to form
larger agglomerates. For example, as the fine Ni particles of
100 nm have the volume of V0, they can grow up into larger particles sized of 150 nm (3.375V0) or 500 nm (125V0). Though it
can be noticed that hydrogen plays an essential role in the formation of helical CFs, it is difficult to in situ observe the changing process of the catalyst particles and the forming course of the carbon
coils. To elucidate the mystery, the effects of hydrogen on catalyst
particles were investigated and the results displayed that the fine
Ni particles changed not only their particles sizes, but also the exposed crystal facets and the topography after heat treatment under
H2 at 750 °C for 1 h as shown in Fig. 4. The fine Ni particles grew
into larger particles by recrystallization. Meanwhile, some concavity formed on the surface of the aggregative Ni particles highlighted by the small ellipse in Fig. 4a, and some of them had
polyhedral shape as shown in Fig. 4b. These changes might be ascribed to two reasons. One is the changes of surface energy under
H2. From the energy’s point of view, the Ni facets with lower surface energy tend to expose and wrap around the catalyst particle.
The other reason might be the existence of the reaction between
H2 and Ni, which causes forming the new species of NiHx. Then
the role of H2 during the growth of carbon coils can be summed
up in terms of three major points. Firstly, the flowing H2 adjusts
the rate of the reaction of C2H2 = 2C + H2, by diluting the concentration of the carbon source gas, and accordingly affects the growth
rate of the CFs. Secondly, hydrogen prevents the catalyst from poisoning, and reactivate the poisoned catalyst particles according to
this work. The main reaction are shown in Fig. 5, where NixCy is
some poisoned catalyst particles and the NimCn is the activated catalyst such as Ni3C [44]. Thirdly, hydrogen induces reconstruction of
the catalyst particles and control the exposed metal faces [45],
which influences the size, the exposed crystal facet and topography
of the catalyst particles [46] and the presence of H2 during synthesis was found to have a positive effect on CCF formation at given
condition [47].
As the reaction temperature increases from 600 to 750 °C under
H2, the Ni particles grow up to larger ones until they reach a stabilized state. Accordingly, the variable size and shape of the catalyst
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Fig. 4. SEM images of Ni particles after heat treatment under H2 at 750 °C for 1 h. (a) Some concavity formed on the surface of the aggregative Ni particles and (b) the district
of the Ni particle with polyhedral shape.
Fig. 5. The related reactions during the poisoned catalyst particles changing into
activated ones under the effects of hydrogen. where NixCy is some poisoned catalyst
particles, the NimCn is the activated catalyst such as Ni3C.
particles from spherical particle (100 nm) or special particle
(150 nm) to polyhedral micro-particle (500 nm) could be adjusted by N2 flow. Thus the three kinds of helical CFs can be rationally prepared with the same Ni nanoparticles just by controlling
the reaction temperature and the N2 flow. Fig. 6 depicts the formation of the linear CFs, SH-CNCs, SH-CMCs with lace and TDH-CMCs,
and the corresponding Ni particles. To lower their surface energy,
the small-sized nanoparticles tend to adopt spherical structures
and exhibit catalytic isotropy, resulting in the formation of linear
CFs. As the Ni particles growing up, their sizes become larger and
their shapes turn to be more regular. Then various carbon coils
formed on the corresponding nanoparticles of anisotropic catalysis.
Apart from the familiar morphologies of SH-CNCs and TDH-CMCs,
sometimes, SH-CMCs with lace grow on the asymmetric catalyst
grain such as the one composed of hemisphere and a hexahedron.
The suggestion will be verified in the future study.
It was reported that the driving force of curling and spiral coiling of the carbon coils was the catalytic anisotropies between catalyst crystal faces [30]. However, the catalytic anisotropy of the
catalyst grain is not well defined, and the origin of the catalytic
anisotropy is not well described yet. Boskovic et al. reported that
carbon flowed to the nanofiber on the surface of the catalyst particle rather than diffuse through the particle [48]. Then the catalytic
anisotropy mainly depends on the catalyst surface, leading to formation of various carbon coils with different cross section. In addition, Boskovic et al. observed that graphene layers were oriented
parallel to the facets of the catalyst particle, while the middle part
of the CNFs was composed of amorphous carbon corresponding to
the edge-zone of the catalyst particle. Shen et al. proposed a mechanism for the regular coiled carbon nanotubes [49], basing on their
mechanism analysis, a suggestion is put forward that the catalytic
anisotropy is the discrepancy of forming carbon atoms on the surface of the catalyst, and the catalytic anisotropy origins from the
atomic density of the catalyst surface and reflects in the form of
facets, edges and vertices of the catalyst grain. In general, the formation of carbon coils is often related to the existence of polyhedral catalyst grain no matter microcoil [50] or nanocoil [51]. In
order to analyze the polyhedral Ni compound particles, three kinds
of typical regular polyhedrons, namely octahedron, hexahedron
and dodecahedron, are sketched in Fig. 7a, b, and c, respectively.
The cuboid has the sectional plane of symmetry more or less. For
instance, the regular hexahedron divides into two comparative
parts by the diagonal plane as presented in the shadow tetragon
of Fig. 7b. Two main CFs (X and Y in Fig. 7e) grow bi-directionally
Fig. 6. Diagrams of the growth of four kinds of CFs on Ni particles with different size and shape: (a) small sphere (80 nm), (b) small regular polyhedron (150 nm), (c) quasiregular polyhedron (500 nm), and (d) regular polyhedron (500 nm).
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Fig. 7. Diagram of Ni particles with shapes of (a) octahedron, (b) hexahedron and (c) dodecahedron, and the DH-CMCs having cross sections of circle, rectangle and pentagon.
Fig. 8. SEM images of two kinds of TDH-CMCs with (a and b) circular and (c and d) flat cross sections.
from the symmetric catalyst grain and then entwine with each
other to form helical pattern of the conventional DH-CMCs. Similarly, two twinning fibers developed with the catalysis of a quarter
of the catalyst particles. The cross section of the DH-CMCs forms
according to the topography of the corresponding Ni particles. So
the circular (Fig. 7d) and rectangular (Fig. 7e) cross sections of
the carbon coils usually exist and are reported with adjusting some
reaction condition [52]. Besides, the carbon coils having pentagonal fiber are presented in Fig. 7f. This is in good agreement with
the experimental observation of the ruptured tip and of various
graphite coils [53].
Kawaguchi et al. suggested that each crystal plane of a Ni seed
had different catalytic ability for the growth of coiled fibers. Two
CFs began to grow in opposite directions from each pair of planes
(X and Y) on the Ni catalytic surface [42]. In the low magnification
image (Fig. 8a) of typical DH-CMCs, they seemed to be made up of
two circular fibers. The as-prepared regular TDH-CMCs could extend to 3–8 times of its original length as shown in Figs. S4 and
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X. Jian et al. / Composites: Part B xxx (2013) xxx–xxx
8b. The extension characteristic was better than conventional DHCMCs reported by Motojima et al. [33], but smaller than the superelastic carbon microcoils [54]. Interestingly, we found the TDHCMCs containing four small CFs as shown in Fig. 8b. The thick fiber
Y consists of two fibers (Y1 and Y2) as shown in the high magnification image. Besides, some obscure scoop channel was also found in
flat carbon filaments (Figs. 8c, d and S6). Thus, the growth mechanism proposed by Kawaguchi et al. [42] could not be applied to the
growth of twinning double helical fibers. We then proposed an
anisotropically carbon deposition on the facets, edges and vertices
of catalytic polyhedron as the growth mechanism for carbon coils.
The number of the branching fiber is mostly two [45] and three
[50], sometimes is four and six [53], which determined by the
anisotropical level of the catalyst grain. The catalyst particles had
the sectional plane of symmetry in terms of catalytic ability for
the growth of the fibers. Take the helical flat CFs as an example,
the fiber M and fiber N grew on each comparative part of the catalyst particle. Due to the different forming rate of carbon atoms
on the facet, edge and vertex of catalyst grain, two pieces of CFs
grew more quickly on the crystal plane than at the edge or vertex,
and the fibers twisted with each other leading to forming two coils.
The diameter of the coil depended on the diversity of each comparative part of the catalyst grain. Similarly, fibers M1 and M2 generate
on quarter part of catalyst grain and they lightly separate from
each other due to the smaller different rate of carbon deposition
at edge and vertex.
4. Conclusion
Linear CFs and three kinds of carbon coils were rationally prepared by gas-adjusting method using regular spherical Ni nanoparticles (90 nm) as catalyst obtained by liquid phase reduction with
hydrazine hydrate. The nano-scaled linear CFs often grow on
spherical Ni particle in the form of straight and curved shape. Three
kinds of carbon coils, namely, SH-CNCs, SH-CMCs and TDH-CMCs
are rationally prepared by controlling the flow rate of N2 ranging
from 100 to 140 mL/min. The morphological differences of the carbon species are caused by the characteristic of the real-time Ni catalyst, involving size, shape and external planar face. The catalytic
anisotropy reflects in the form of facets, edges and vertices of the
catalyst grain. The conventional DH-CMCs consisting of circular
and flat DH-CMCs in this work had the twinning structure, from
which a mechanism is proposed based on different growth rate
of carbon nanoparticles on the facet, edge and vertex of catalyst
grain. The mechanism is also useful for predicting the existence
of some novel carbon coils, such as DH-CMCs with pentagonal
cross section.
Acknowledgments
This work was financially supported by the National Natural
Science Foundation of China (Nos. 51173148 and 90305003) the
Special Research Fund for Doctoral Program of Higher Education
(No. 20060613004), the NBRP (Grant No. 2011CB606200), the
2011 Doctoral Innovation Funds of Southwest Jiaotong University
and the Fundamental Research Funds for the Central Universities
(Nos. 2010XS31 and SWJTU11ZT10).
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.compositesb.2013.
06.010.
7
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