Carbon 172 (2021) 772e780
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Carbon
journal homepage: www.elsevier.com/locate/carbon
Research Article
Ultra-long carbon nanotube forest via in situ supplements of iron and
aluminum vapor sources
Hisashi Sugime a, b, *, Toshihiro Sato a, Rei Nakagawa a, Tatsuhiro Hayashi c, Yoku Inoue c,
Suguru Noda a, b
a
b
c
Department of Applied Chemistry, School of Advanced Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo, 169-8555, Japan
Waseda Research Institute for Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo, 169-8555, Japan
Department of Electronics and Materials Science, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, 432-8561, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 September 2020
Received in revised form
16 October 2020
Accepted 21 October 2020
Available online 24 October 2020
A carbon nanotube forest with a length of 14 cm grew with an average growth rate of 1.5 mm s1 and a
growth lifetime of 26 h. Several key factors to realize this unprecedented long growth such as catalyst
conditions, growth conditions in chemical vapor deposition, and reactor system were clarified. It was
found that the combination of the catalyst system of iron/gadolinium/aluminum oxide (Fe/Gd/Al2Ox) and
the in situ supplements of Fe and Al vapor sources at very low concentration was crucially important. A
cold-gas system, where only the substrate is heated while keeping the gas at room temperature, was
employed to suppress unnecessary reactions and depositions. The long carbon nanotube forest enabled
macroscopic measurements of the tensile and electrical properties of the carbon nanotube wires, and it
gave several important insights for industrial applications of the carbon nanotubes in the future.
© 2020 Elsevier Ltd. All rights reserved.
Keywords:
Vertically aligned carbon nanotube forest
Chemical vapor deposition
Growth termination
Gadolinium
Catalyst
1. Introduction
Over the several decades after the discovery of carbon nanotube
(CNT) [1], technology has progressed to a point where chemical
vapor deposition (CVD) enables the growth of large amounts of
CNTs on supports with fixed catalysts [2], or in gas phase with
floating catalysts [3]. Growth of CNTs in large scale is one of the
main challenges to utilize the attractive properties of CNTs in variety of applications [4]. When CNTs grow from the fixed catalysts
with a certain number density, especially for the base-growth
mode, the CNTs form vertically-aligned forests which are advantageous in terms of length and alignment of the tubes. After several
pioneering works on the growth of multi-wall CNT (MWCNT) forests [5e7], the forest growth of single-wall CNTs (SWCNTs) [8] was
reported, which was soon followed by the growth of 2-mm-long
SWCNT forest [9], To date, several groups have reported the growth
of centimeter-scale CNT forests, and the maximum length that has
been reached is ~2 cm using an iron-gadolinium (Fe-Gd) catalyst on
* Corresponding author. Department of Applied Chemistry, School of Advanced
Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo, 1698555, Japan.
E-mail address: sugime@aoni.waseda.jp (H. Sugime).
https://doi.org/10.1016/j.carbon.2020.10.066
0008-6223/© 2020 Elsevier Ltd. All rights reserved.
an aluminum oxide (Al2O3) support [10,11]. Nevertheless, with the
conventional approaches, there seemed to exist a ceiling preventing growth beyond ~2 cm. In the past, growth of a half-meter-long
individual CNT was reported with the tip-growth mode [12], but
the number density of the CNTs (~104 cm2, ten tubes per 1 mm)
was at least ~105 times lower than that of the forest growth
(~1 109e1013 cm2) [13,14]. Therefore, to utilize the advantage of
the productivity that comes from the high CNT number density, it is
necessary to realize a method that achieves longer CNT forests
while alleviating the growth termination. It is known that the
alignment disorder of the tubes [13,15,16] along with the increase
of the tube diameter [17,18] progresses before the growth termination, and these phenomena mainly come from the structural
change of the catalyst nanoparticles [19]. The catalyst nanoparticles
inherently exist closely to each other with high number density in
the base-growth mode, thus the influence of coarsening by sintering and/or by Ostwald ripening is much stronger than that of the
tip-growth mode. In addition, the effect of the subsurface diffusion
of the catalyst nanoparticles has been a serious problem [18,20,21],
since it causes the depletion of the active catalysts on the surface of
substrates resulting in the growth termination [18]. Therefore, it is
essential to establish a method that alleviates these problems
simultaneously, since even one factor can terminate the growth by
H. Sugime, T. Sato, R. Nakagawa et al.
Carbon 172 (2021) 772e780
A n-type Si (dopant: phosphorus, resistivity: 0.002e0.004 U cm)
with a size of 0.5 4 cm2 (thickness: 0.6 mm) was used as substrates. Al (5 nm), Gd (0.8 nm), and Fe (2 nm) were sequentially
deposited by a conventional radio frequency (RF) magnetron
sputtering apparatus under Ar atmosphere (~1 Pa). All the samples
were exposed to air between the depositions, which resulted in
partial oxidation of each metal layer. Fe (i.e. catalyst) was deposited
only in the center area of the substrate (0.5 1 or 0.5 2 cm2) by
covering other areas during the deposition to prevent the CNT
growth. The edges of the Si substrate were scratched by a diamond
scriber to have good electrical contact with the tungsten wires
(Fig. S1). Except for the growth experiment for the mass measurement, to enhance the growth rate, the center area of the substrate was also scratched to obtain the CNT forest that was
separated into multiple bundles so that the gas diffusion is
enhanced and the inter-bundle interaction is suppressed (Fig. S1).
schematic of the cold-gas CVD apparatus is shown in Supplementary data, Fig. S2. The substrate was fixed to a pair of electrodes
(tungsten wires) to apply voltage. The temperature of the substrate,
monitored by a pyrometer (CHINO, IR-CAT), was quickly ramped up
to 750 C in ~1 min by Joule heating with a direct current (DC, ~9 V,
~5 A) under CO2/H2/Ar flow. A portion of Ar (carrier gas) was supplied through the glass tubes at room temperature (~20 C) which
were filled with powders of ferrocene (Fc) or aluminum isopropoxide (AIP). After reaching 750 C, the temperature was maintained
for 2 min for catalyst pretreatment (reduction), and then C2H2 was
added to grow CNT forests. The standard gas condition during the
CNT growth was C2H2 (0.3 vol%)/CO2 (0.5 vol%)/H2 (5 vol%)/Fc (0.6
ppmv)/AIP (0.03 ppmv)/Ar (94 vol%). The total gas flow rate was
500 sccm for the growth longer than 70 min, while it was 1000
sccm for the growth to measure the mass change in 30 min. The
flow rate of each gas during the catalyst pretreatment (temperature
ramp) and the growth is summarized with a table in Supplementary data, Table S1. To investigate the effect of Fc or AIP, the growth
without these vapors was also carried out. The concentration of Fc
and AIP were estimated from the vapor pressures of Fc (0.6 Pa [23])
and AIP (0.02 Pa [24]) at 20 C. The growth behavior was recorded
by taking pictures at 1-min intervals using a digital camera. After
the growth period, all the gases except for Ar were turned off, and
the substrate was cooled down to room temperature.
2.2. Growth of CNT forests by cold-gas CVD method
2.3. Characterization of the as-grown CNTs
The growth of CNT was carried out with a custom-made “coldgas CVD” apparatus which we reported previously [22]. A
The as-grown samples were characterized by scanning electron
microscope (SEM, HITACHI, S-4800), transmission electron
deactivating the catalysts. In this report, we found several important conditions that realizes the breakthrough method for the
growth of long CNT forests of 14 cm.
2. Experimental
2.1. Catalyst preparation by sputtering
Fig. 1. (a) Pictures of the CNT forest growing with the cold-gas CVD method. The substrate temperature was set to 750 C. (b) CNT forest after 32 h growth and the corresponding
TG-DTA result. (c) Growth curves of the CNT forests with/without ferrocene (Fc) and/or aluminum isopropoxide (AIP) using Fe/Gd/Al2Ox catalyst (blue circle, red square, green circle,
and pink square), and with both Fc and AIP using Fe/Al2Ox catalyst (skyblue triangle). The inset shows the enlarged growth curves of the CNT forests in 70 min under different
growth conditions. (A colour version of this figure can be viewed online.)
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Carbon 172 (2021) 772e780
2 h 30 min, then kept at 2800 C for 1 h. The detailed temperature
profile was shown in Fig. S5.
microscope (TEM, JEOL, JEM-2100F), and Raman spectroscopy with
an excitation wavelength of 488 nm (Horiba, HR800). For the SEM
observation, a bundle of the CNTs was separated from the CNT
forest and was attached to the Si substrate with carbon tapes
(Figs. S3 and S4). For the TEM observation, some parts of the CNT
bundle were cut into small pieces and transferred to a Cu-grid to
observe several positions of the CNT bundles. Thermogravimetricdifferential thermal analysis (TG-DTA, Rigaku TG8120) of the CNT
forest was carried out at a ramp rate of 5 C min1 in air. The areal
mass of the CNT forests (mg cm2) was calculated from the area of
the substrates and the mass gain obtained by weighing the samples
before and after the CNT growth. The mass density (mg cm3) was
calculated by dividing the mass gain by the volume of the CNT
forests.
3. Results and discussion
3.1. Growth behavior of CNT forests
We first show a typical growth behavior of the CNT forest at
750 C for 32 h (Fig. 1a). A growth movie edited by arranging pictures taken every 1 min is also available in Supplementary data
(Movie S1). We employed a cold-gas CVD apparatus where only the
Si substrate was heated by Joule heating with DC while the temperature of the gas and the chamber wall was kept at near room
temperature [22]. The CNT forest in the glass tube chamber
(diameter: 5.5 cm) could be clearly observed in this apparatus as
only the Si substrate (shown with an arrow in Fig. 1a), which was
held and electrically connected to tungsten wires, was heated to
750 C. The CNT forest only grew in an area of 0.5 1 cm2 at the
center of the Si substrate where Fe was deposited (total substrate
area: 0.5 4 cm2, thickness: 0.6 mm, Fig. S1). Other areas of the Si
substrate were covered during the Fe deposition resulting in the
region with only Gd/Al2Ox on Si (without Fe). The CNT forests had a
packed morphology at the initial growth period. Then the top part
gradually split into several bundles as the growth progressed (after
~400 min). This split was caused by tiny differences of the growth
rates of the CNT forests which depended on the position of the
substrate. The side part of the CNT forest hit the glass chamber after
growing longer than ~5 cm in ~400 min. However, the growth at
the center part was not mechanically inhibited as the CNT bundles
were flexible. The final length of the CNT forest after 32 h growth
reached ~14 cm (Fig. 1b), and the TG-DTA showed that the amount
of impurities in the CNT forest, which were not burned during the
TG-DTA measurement, was under the detection limit, indicating
that unnecessary depositions did not occur (inset in Fig. 1b). The
growth lifetime was ~26 h which was unprecedentedly long
considering the rapid average growth rate of ~1.5 mm s1 in 26 h
(blue circle plot in Fig. 1c). As summarized in the inset in Fig. 1c, the
initial growth rate in 30 min was approximately 2 mm s1 under any
conditions investigated herein.
Supplementary data related to this article can be found at
https://doi.org/10.1016/j.carbon.2020.10.066.
We herein describe several key factors which are necessary to
realize such long and rapid growth. Firstly, we used Fe/Gd/Al2Ox
catalyst in which the addition of the small amount of Gd between
Fe and Al2Ox prolongs the catalyst lifetime significantly. It was
found in our previous report that Fe and Gd form an alloy (or oxide
compound), which prevents the structural change of the catalyst
nanoparticles in the lateral direction during the CNT growth by
decreasing the interaction between Fe and carbon [18]. In this
report, the forest growth terminated in ~20 min when Fe/Al2Ox
catalyst (without Gd) was used as shown in the growth curve in
Fig. 1c (skyblue triangle plot). Secondly, to enhance the growth rate,
the substrate surface was scratched with a diamond scribe as
shown in Fig. S1. By making sub-millimeter spaces, the diffusion of
the carbon feedstock was enhanced and the inter-bundle interaction was suppressed, realizing a similar growth condition to the
edge of the CNT forest. The initial growth rate (i.e. average growth
rate in 30 min) enhanced from ~1 mm s1 to ~2 mm s1 by the
scratching. We note that the spacing was not optimized in detail,
therefore further optimization of the spacing could realize faster
growth. Thirdly, by growing the CNT forest parallel to the chamber
axis (and the gas flow), the CNT forest was grown without the restriction of the chamber diameter (5.5 cm herein), which is
different from the conventional method where CNT forests grow
perpendicularly to the chamber axis. It is noteworthy that the
2.4. Fabrication of CNT wires and tensile and electrical
measurement
CNT wires with a length of ~14 cm was fabricated by passing the
CNT bundles through a hole that had a circular shape with a
diameter of 100 mm. The tensile test was carried out by a tensile
testing system (Shimadzu, EZ-L). At different positions of the wires,
a piece of ~10 mm was cut and fixed on a paper mount using a
cyanoacrylate adhesive. The gauge length was set to 10 mm. The
high wettability of the cyanoacrylate adhesive to CNTs ensured
good fixing of the entire CNT bundle to the test fixture. The crosshead speed was set to 0.1 mm min1 and the strain was
measured by a non-contact extensometer (Shimadzu, TRViewX).
The stress s (Pa) and strain ε is given by the following equations.
s¼
F
S
ε ¼
L1
L2
In these equations, F (N) is tensile force, S (m2) is cross-sectional
area of the CNT wire, L1 (m) is the measured length, and L2 (m) is
the gauge length. The Young’s modulus E (Pa) was calculated in the
range where the strain ε is less than 0.2% with the following
equation.
E ¼
s
ε
The specific strength ss (N tex1) and specific modulus Es (N
tex1) were calculated by the following equations where r (g cm3)
is the mass density of the CNT wires.
ss ¼
Es ¼
s
r
E
r
Based on these relationships, 1 tex ¼ 1 g km1. Therefore, the
dimension of N tex1 is the same as GPa g1 cm3. The mass of the
CNT wires was measured for a bundle with several-ten mm length
at a resolution of 0.1 mg using a microbalance (Mettler Toledo,
XP2UV). To assess the electrical properties of the CNT wire, the
edges of the CNT wires were fixed with Cu plates to apply DC
voltage. The electrical conductivity was obtained from the I-V
measurement with the two-probe method by fitting the I-V curves
linearly. For the ampacity measurement, the voltage was swept
until the CNT wire broke under the pressure of 1.5 103 Pa. The
annealing of the CNT wires under Ar at atmospheric pressure was
carried out in a carbon crucible. The temperature was ramped up in
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Carbon 172 (2021) 772e780
direction of the gas flow did not affect the CNT forest growth unless
it touched the chamber wall, as we confirmed that the same growth
occurred when the CNT forest grew perpendicularly to chamber
axis (and the gas flow). Last and most importantly, the CVD conditions were carefully customized. We employed the CO2-assisted
CVD [25] where 0.5 vol% of CO2 was added as a mild oxidant, similar
to that of the ppm-level H2O [9], to prolong the catalyst lifetime. In
addition, vapors of ferrocene (Fc, ~0.6 ppmv) and aluminum isopropoxide (AIP, ~0.03 ppmv) were supplied at extremely low concentration by passing a portion of Ar (carrier gas) through quartz
glass tubes at room temperature (~20 C), which were filled with
powders of Fc or AIP (Fig. S2). The in situ supplement of Fc was first
reported by Eres et al. to change the growth behavior [26], although
the mechanism had not been clearly discussed. In our previous
report, we also showed the effectiveness of supplying Fc during
growth for cold-gas CVD by obtaining a CNT forest of ~2 cm.
Considering the crucial effect of the subsurface diffusion of Fe into
Al2Ox layer on the depletion of Fe followed by the growth termination [18,20,21], the in situ supplement of Fc through gas phase is
effective to alleviate the depletion. It is reasonable that the necessary concentration of Fc was ppm-level (supplement without
heating Fc source) since the role of the supplement is to compensate for the amount of subsurface diffusion of Fe into Al2Ox which is
considered to be less than the initial amount of Fe deposited by
sputtering (nominal thickness of 2 nm). The effect of the Fc supplement was clearly observed in the growth curves without Fc with
AIP (green circle plot in Fig. 1c) and without neither Fc nor AIP (pink
square plot in Fig. 1c), both of which terminated in ~60 min. We
Fig. 2. Areal mass and mass density of the CNT forests grown with/without supplement of AIP during the catalyst pretreatment and/or the growth period (four different
conditions). “w/w” denotes the condition where AIP was supplied both during the
pretreatment and the growth period. “w/wo” denotes the condition where AIP was
supplied only during the pretreatment period. “wo/w” denotes the condition where
AIP was supplied only during the growth period. “wo/wo” denotes the condition where
AIP was not supplied. All the growth conditions are the same with the one in Fig. 1
except for the growth time of 30 min, the total gas flow rate of 1000 sccm, and the
scratching process of the center area of the substrates for the mass measurement. (A
colour version of this figure can be viewed online.)
Fig. 3. (a) Picture of 14-cm-long CNT bundle separated from the CNT forest grown for 32 h. (b) SEM and TEM images of the CNTs at each position shown with the dashed circles in
(a). (c) CNT diameter and the wall number at different positions which correspond to (a) and (b). (d) Side-view Raman spectra of the CNT forest and (e) corresponding IG/ID ratio at
each position. (A colour version of this figure can be viewed online.)
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Carbon 172 (2021) 772e780
Fig. 4. (a) SEM image of the CNT wire. The inset shows a cross-sectional image near the top part of the wire which is shown with an arrow. (b) Mass density of the CNT wire at
different positions. (c) Typical strain-stress curves of the CNT wire at different positions. (d) Tensile strength, (e) specific strength, (f) Young’s modulus, and (g) specific modulus of
the CNT wires at different positions. (A colour version of this figure can be viewed online.)
became significantly shorter (~15 h) resulting in the CNT forest with
a length of ~5 cm (red square plot in Fig. 1c). To further clarify the
effect of the AIP supplement, we investigated the areal mass and
the mass density of the CNT forests in 30 min growth under four
different conditions changing the timing of the AIP supplement
(during catalyst pretreatment and/or CNT growth). As shown in
Fig. 2, the growth results with/without AIP during catalyst pretreatment and/or growth period showed that the supplement of
note that the ppm-level of Fc was too small to initiate growth of the
CNT forests on Gd/Al2Ox layer, therefore no CNT forests grew even
at 750 C from areas where Fe was not initially deposited by
sputtering.
In the same way with Fc, we supplied AIP to suppress the
structural change of the catalyst nanoparticles for longer growth
lifetime. The growth rate decayed more rapidly without the AIP
supplement (with Fc supplement only) and the growth lifetime
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Carbon 172 (2021) 772e780
AIP during the growth period (“w/w” and “wo/w” in Fig. 2)
increased both the areal mass and the mass density of the CNT
forests compared to the conditions without AIP (“wo/wo” in Fig. 2).
Moreover, even lower areal mass and mass density were obtained
when AIP was supplied only in the catalyst pretreatment period
(“w/wo” in Fig. 2). This indicated that the decrease of the number
density caused by the structural change of the catalyst nanoparticles was suppressed by AIP supplement. Although the elucidation of the detailed mechanism is a future work, it was found that
the supplement of AIP clearly changes the growth behavior,
possibly influencing the structural change of the catalyst nanoparticles similar to that of the Fc supplement. We note that the
mass change of the samples during 30 min reaction (Fig. 2) without
carbon source (no CNT growth) was negligible indicating that the
mass change by Fc and/or AIP deposition is negligible. Since Fc and
AIP were supplied constantly during the growth, unnecessary reactions/depositions needed to be minimized, thus the cold-gas CVD
apparatus played an important role for the growth of long CNTs. As
we explained in our previous paper [22], “cold-wall CVD” is also
effective to suppress unnecessary reactions/depositions compared
with conventional hot-wall CVD by keeping the chamber wall at
low temperature. The advantage of the cold-gas CVD over the coldwall CVD is that it minimizes the unnecessary reactions/depositions more efficiently by making the heating zone even smaller.
For these reasons, satisfying these four conditions described above
are inevitable for the growth of the long CNT forests.
growth time, further optimization to alleviate both the depletion
and the structural change of the catalyst nanoparticles could
potentially lead to the ideal growth of the CNT forest without
spontaneous growth termination.
3.3. Tensile properties of the CNT wires
The advantage of growing long CNTs is that it is possible to
conduct macroscopic tensile tests with the continuous CNTs. To
investigate the tensile properties of the CNTs with different growth
period, we fabricated CNT wires without twisting (diameter:
30e80 mm) by passing the CNT bundles through a hole and carried
out the tensile tests at three different parts of the CNT forests (top,
middle, bottom) using a standard tensile test with a gauge length of
10 mm. Fig. 4a shows typical SEM images near the top of the CNT
wire (diameter: ~75 mm) with an enlarged image of the cross section of the wire in the inset corresponding to the part shown with
an arrow in Fig. 4a. The mass densities of the CNT wires at each
position, obtained from all the samples for the tensile and electrical
measurements (Figs. 4 and 5), were 0.23 ± 0.081, 0.17 ± 0.052, and
0.11 ± 0.049 g cm3 at the top, middle, and bottom, respectively
(Fig. 4b). The decrease of the mass density from the top to the
bottom was due to the decreased number of CNTs in the specimens
at the lower parts. Typical stress-strain (SS) curves of the CNT wires
at each position are shown in Fig. 4c. The tensile strength and the
specific strength at the top part show the highest values of
76 ± 18 MPa and 0.45 ± 0.11 N tex1, which both decreased at lower
parts (Fig. 4dee). The degradation trend from the top to the bottom
was also observed with the Young’s modulus and the specific
modulus; the top part showed the highest values of 6.2 ± 2.4 GPa
and 37 ± 13 N tex1 with decreasing values at lower parts
(Fig. 4feg). One of the main reasons for these degradations of the
tensile properties from the top to the bottom is the decrease of the
mass density (Fig. 4b), which stems from the decreasing number
density of the CNTs during the growth as shown in the SEM images
(Fig. 3b). However, the decrease of the specific strength and the
specific modulus, in which the effect of the mass density is subtracted, cannot be explained solely by the decrease of the mass
density. Since the microscopic defects did not increase (i.e. constant
IG/ID ratio in the Raman spectra, Fig. 3e), the increase of the wall
number (Fig. 3c) and the periodical buckling of the CNTs also
needed to be considered. The increase of the wall number resulted
in the stress concentration on the outer walls which lead to the
deterioration of the specific strength and the specific modulus [28].
As for the specific modulus, the periodical buckling of the CNTs as
shown in the SEM images in Fig. 3b resulted in the elongation
behavior of the CNT bundles, which lead to the lower specific
modulus especially at the bottom part.
Although much research has been done on the tensile properties
of individual CNT, due to the limitation of the CNT length, most of
them were carried out with a gauge length of several mm or less
[29e36]. The tensile strength has been reported as 10e80 GPa, and
the Young’s modulus reached a value of 1100 GPa. However, since
the graphene layers were only used in terms of the cross-sectional
area in the reports, the nominal strength and modulus, in which the
inner space of CNTs is also included for the cross-sectional area,
became significantly smaller: 0.8e60 GPa for the tensile strength
and 40e160 GPa for the Young’s modulus [28]. The gauge length
also had a significant effect on the tensile properties, since the
number of fatal defects, which are more prone to fracture,
increased as the gauge length increased. Recently, the tensile
strength and the Young’s modulus of several millimeter-long
MWCNTs using a standard tensile tester were reported to be
0.85 GPa and 35 GPa, respectively [37]. It was concluded that the
larger numbers of CNTs in the specimens lead to the increase of the
3.2. Characterization of the as-grown CNT forest
We characterized the as-grown CNT forests (32 h growth,
Fig. 1b) by SEM, TEM, and Raman spectroscopy. The longest parts of
the CNT forests were CNT bundles with a length of ~14 cm (Fig. 3a).
The SEM images with low magnification showed that the bundles
consisted of continuous CNT without obvious discontinuous parts
(Fig. S3). The detailed analysis of the CNT forest at different positions (as shown with dashed circles in Fig. 3a) by SEM showed that
the alignment of the CNTs became worse and worse gradually from
top to bottom part (Fig. 3b). This alignment disorder progressed
because of the gradual decrease of the active catalyst nanoparticles
resulting in the lower number density of the CNTs [13,19]. In
addition, the TEM images showed that the diameter increased from
5.4 ± 0.74 to 8.1 ± 1.7 nm (average ± standard deviation) and the
wall number increased from 2.0 to 3.8 ± 0.93, which are summarized in Fig. 3c. This continuous change of the CNT diameter is
similar to that of previous studies of millimeter-scale forest growth
[17,18], and thus solidifies the possibility of continuous growth of
the CNTs herein. The Raman spectra showed the G-band peak at
~1590 cm1 and the D-band peak at ~1350 cm1 with a constant
intensity ratio (IG/ID) of ~1.4 from top to bottom, which is a typical
value for MWCNT forests (Fig. 3dee) [11]. Compared to the previous
CNT forests, the behavior of the morphological change of the forests
and the diameter change of the individual CNT were generally the
same. The difference was that the ratio of growth rate to the rate of
structural change of the catalyst nanoparticles was much higher.
However, the morphological change of the forest, which possibly
stems from the structural change of the catalyst nanoparticles, was
not completely suppressed resulting in the decay of the growth rate
and the eventual growth termination (blue plot in Fig. 1c). By the
SEM observation of the CNT forest without AIP supplement
(Fig. S4), the morphological change of the forest was similar but
more rapid from top to bottom compared to the one with AIP
supplement (Fig. 3b) indicating the suppression of the structural
change of the catalyst nanoparticles by the AIP supplement. We
note that as the detailed optimization of the Fc and AIP flow conditions was not conducted this time, especially for the longer
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Carbon 172 (2021) 772e780
Fig. 5. (a) Electrical conductivity and (b) specific conductivity of the CNT wires (before annealing) at different positions. (c) SEM and TEM images and (d) Raman spectra of the CNT
wires at different positions after annealing at 2800 C for 1 h. Comparison of (e) electrical conductivity and ampacity, and (f) specific conductivity and scaled ampacity (ampacity per
CNT) at the middle of the CNT wires before and after annealing. (A colour version of this figure can be viewed online.)
3.4. Electrical properties of the CNT wires and annealing effect
fatal defects, which resulted in lower strength and modulus. This
explanation applies well with the lower values herein compared to
the ones measured using individual CNT [34]. Considering the
gauge length of 10 mm and the number of CNTs in the specimens at
an order of ~108, the obtained nominal tensile strength of 0.54 GPa
(¼ 0.45 N tex1 1.2 g cm3) and the nominal Young’s modulus of
44 GPa (¼ 37 N tex1 1.2 g cm3), measured at the top part, are
reasonable values. We note that the average mass density of
1.2 g cm3 for individual CNT was calculated from the CNT diameter
and wall number in TEM observation [38], Fig. S6.
In a similar manner to the tensile tests, we evaluated the electrical properties of the CNT wires. As observed in the tensile
properties, the electrical properties of the CNT wires showed the
same trend from the top to the bottom of the CNT forests. The top
part showed the highest electrical conductivity and the specific
conductivity of 220 ± 48 S cm1 and 680 ± 12 S cm2 g1, which
decreased at lower parts (Fig. 5a and b). The specific conductivity
was comparable with that of the millimeter-scale MWCNTs,
including 670 S cm2 g1 (0.9 mm in length and 10 nm in diameter)
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Carbon 172 (2021) 772e780
[39] and 230 S cm2 g1 (6 mm in length and 10e20 nm in diameter)
[40], reported so far. We note that in the electrical measurement,
since the perimeter CNTs in the bundle had a better connection to
the electrode than the inner CNTs, the perimeter CNTs dominantly
contributed to charge transport. By comparing the specific conductivity of the CNT wires herein (450e680 S cm2 g1) with the
reported values of the assembled CNTs, including yarns or sheets
which consist of millimeter-scale CNTs (440e1360 S cm2 g1
[41e43]), an important insight was obtained. The conductivity of
the assembled CNTs was generally considered to be governed by
the contact resistance between CNTs. However, the comparable
values revealed that the electrical conductivity of the assembled
MWCNTs was mainly governed by the conductivity of each CNT
rather than the contact resistance between CNTs.
We also characterized the CNT wires after annealing at 2800 C
for 1 h under Ar flow (the detailed temperature profile is shown in
Fig. S5). As shown in the SEM images in Fig. 5c, compared with the
bundles before annealing (Fig. 3b), CNT bundles with an even more
packed morphology were observed, which was most obvious at the
higher parts. In addition to the slightly straighter structure of the
graphene walls in the TEM images (Fig. 5c), Raman spectra showed
an obvious difference after annealing with an IG/ID ratio of ~10,
which is ~6 times higher than that before annealing (Fig. 3e). The
effect of this improvement in CNT crystallinity appeared in the
electrical properties of the CNTs. The electrical conductivity and the
ampacity of the CNT wires at the middle part increased from
140 ± 28 S cm1 and 6.7(±1.4) 103 A cm2 to 350 ± 47 S cm1 and
8.8(±1.2) 103 A cm2 (Fig. 5e), respectively. To subtract the effect
of the increased mass density of the CNT wires caused by annealing
(from 0.21 to 0.22 g cm3), the relationship between the specific
conductivity and the ampacity per CNT (scaled ampacity) were
summarized in Fig. 5f. We note that the mass density of 1.2 g cm3
was used for individual CNT to calculate the scaled ampacity as
mentioned in the previous section for the tensile properties. The
specific
conductivity and
the
scaled
ampacity
were
610 ± 7.3 S cm2 g1 and 4.0(±0.38) 104 A cm2 before annealing,
but it became 1600 ± 57 S cm2 g1 and 4.8(±0.24) 104 A cm2
after annealing. This significant increase in specific conductivity
was surely brought by the reduction of the defects which was
observed in the Raman spectra. Also, the more packed morphology
of the CNT wires in the SEM images (Fig. 5c) implied further
bundling by improved van der Waals interaction caused by high
graphitization of CNTs, which improved lateral electrical connection. It was concluded that the increase in the conductivity of the
CNT wire by heat treatment was due to the synergistic improvement of the conductivity of the CNT itself and the parallel conduction in the bundle. On the other hand, the improvement of the
scaled ampacity was not as significant as that of the specific conductivity possibly because the large defects, which became the
breaking point, could not be repaired by annealing at this temperature [44].
So far, many studies on the ampacity of short-range CNTs have
been carried out by in situ measurements using an electron microscope, and extremely high values of 107e109 A cm2 have been
reported [45e47]. However, macroscopic ampacity measurement
of millimeter-scale CNTs has not been reported. This study is the
first to report on the ampacity of the macroscale continuous CNTs
and it could contribute to the practical applications of the CNT
wires in the future. Under the high electrical current, CNTs can be
heated and evaporated, especially at the crystal defects (i.e. higher
resistance) due to Joule heating. Therefore, it is consistent that the
ampacity of the CNT bundles herein with the long measurement
length of 10 mm was significantly lower than that obtained by the
microscopic measurement, because the number of defects
increased. In addition, Kuroda et al. predicted that the CNT
ampacity decreases drastically as the length increases [48], because
the heat dissipation becomes worse for longer CNTs. Recently, a
neat p-doped CNT yarn with high specific conductivity of
2.6 104 S cm2 g1 has been reported with a scaled ampacity of
1.6 104 A cm2 [49]. Compared with this report, the CNT bundles
herein have a higher ampacity but an inferior specific conductivity,
possibly due to the high heat dissipation performance by the
continuous crystallinity.
4. Conclusions
A breakthrough method for growing a 14-cm-long CNT forest
with an average growth rate of 1.5 mm s1 and a growth lifetime of
26 h was developed. Several important factors such as the catalyst
conditions, the CVD conditions, and the reactor system were clarified. It was found that the combination of the catalyst system of Fe/
Gd/Al2Ox and the in situ supplements of Fe and Al vapor sources at
very low concentrations was crucially important for the long
growth. The cold-gas CVD apparatus was also shown to play an
important role in suppressing unnecessary reactions and depositions on the CNT forests. The long CNT forests enabled a
detailed investigation of the tensile and electrical properties of the
CNTs at different growth periods through macroscopic measurements. Several important insights, such as position dependence
and the governing factors for these properties, were obtained from
the measurements. These results will pave the way for developing
industrial applications of the CNTs in the future.
CRediT authorship contribution statement
Hisashi Sugime: Conceptualization, Validation, Formal analysis,
Investigation, Writing - original draft, Writing - review & editing,
Supervision, Project administration, Funding acquisition. Toshihiro
Sato: Validation, Formal analysis, Investigation, Writing - review &
editing. Rei Nakagawa: Validation, Formal analysis, Investigation,
Writing - review & editing. Tatsuhiro Hayashi: Validation, Formal
analysis, Investigation, Writing - review & editing. Yoku Inoue:
Validation, Formal analysis, Writing - original draft, Writing - review & editing. Suguru Noda: Validation, Writing - review &
editing.
Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by JSPS KAKENHI Grant Number
19K22090. The authors thank S. Enomoto at Kagami Memorial
Research Institute for Materials Science and Technology, Waseda
University for TEM observation. The authors also thank B. Chen of
Noda-Hanada lab at Waseda University for TG-DTA data
acquisition.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.carbon.2020.10.066.
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