Materials Sciences and Applications, 2013, 4, 453-457
http://dx.doi.org/10.4236/msa.2013.48055 Published Online August 2013 (http://www.scirp.org/journal/msa)
453
Enhanced Thermal Conductivity of Carbon Nanotube
Arrays by Carbonizing Impregnated Phenolic Resins
Dongmei Hu1, Hongyuan Chen1, Zhenzhong Yong1, Minghai Chen1, Xiaohua Zhang1, Qingwen Li1,
Zhen Fan2,3, Zhihai Fen g2,3
1Key Laboratory of Nano-Devices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sci-
ences, Suzhou, China; 2State Key Laboratory of Powder Metallurgy, Central South University, Changsha, China; 3National Key
Laboratory of Advanced Functional Composite Materials, Aerospace Research Institute of Materials and Processing Technology,
Beijing, China.
Received June 8th, 2013; revised July 9th, 2013; accepted July 18th, 2013
Copyright © 2013 Dongmei Hu et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
A carbonization method is reported to improve the thermal conductivity of carbon nanotube (CNT) arrays. After being
impregnated with phenolic resins, CNT arrays were carbonized at a temperature up to 1400˚C. As a result, pyrolytic
carbon was formed and connected non-neighboring CNTs. The pyrolysis improved the room temperature conductivity
from below 2 W/m·K up to 11.8 and 14.6 W/m·K with carbonization at 800˚C and 1400˚C, respectively. Besides the
light mass density of 1.1 g/cm3, the C/C composites demonstrated high thermal stability and a higher conductivity up to
21.4 W/m·K when working at 500˚C.
Keywords: Carbon Nanotube; Phenolic Resin; Pyrolysis; Thermal Conductivity
1. Introduction
Experimental and theoretical studies have shown that the
thermal conductivity of a carbon nanotube (CNT) is up to
several thousand W/m·K [1-4], showing great potentials
in the thermal management area. Many studies have been
reported on the use of dispersed CNTs as thermal con-
ducting fillers in a polymer matrix, whose thermal con-
ductivity can be significantly improved by 65% - 125%
even at a small loading of CNTs [5-7]. However, these
CNT/polymer composites still had a much lower thermal
conductivity than predicted, possibly due to the interfa-
cial resistances between the CNTs and matrix or between
contacting tubes, modification of the phonon conduction
in CNT by the matrix, formation of voids in composites,
and structural defects of CNTs, as briefly reviewed by
Marconnet et al. very recently [8].
Different from the random orientation of dispersed
CNTs, the aligned feature of CNT arrays offers another
solution to develop thermal management materials with a
unidirectionally high conductivity. The thermal conduc-
tivity of CNT arrays was reported to range from several
to hundreds of W/m·K [9-13]. However, the as-produced
CNT arrays were difficult to use due to the weak me-
chanical property in the direction perpendicular to the
alignment. In order to improve the mechanical and ther-
mal properties or to decrease the contact resistance of a
CNT array, metal filling and/or coating [14,15], polymer
impregnation using silicone rubber S160 [16], polydi-
methyl siloxane [17], or epoxy [8,18], covalent bonding
at contact interfaces [19], and graphitization inside the
array [20,21] have been designed.
The purpose of this study, based on the method of
polymer impregnation, is to develop C/C composite ther-
mal management materials by carbonizing phenolic res-
ins within multiwalled CNT arrays, and to investigate the
thermal conductivities at high working temperatures.
Carbonization converted the resins into pyrolytic carbon
and thus adhered non-neighboring CNTs. Owing to the
pathways to conduct heat between CNTs, the thermal
conductivity was significantly improved. For instance, by
using a carbonization temperature of 1400˚C, the thermal
conductivity was up to 14.6 W/m·K, nearly quintuple
that of the raw CNT arrays. The C/C composites exhib-
ited a higher conductivity of 21.4 W/m·K when working
at 500˚C. The composites had a low mass density (1.1
g/cm3) and were stable up to about 550˚C in air and
about 1000˚C in Ar, leading to possible applications at
extreme conditions.

Enhanced Thermal Conductivity of Carbon Nanotube Arrays by Carbonizing Impregnated Phenolic Resins 454
2. Experimental
A floating catalyst chemical vapor deposition method
was used to grow the multiwalled CNT arrays [22], due
to the ability to produce long length CNTs. Scanning elec-
tron microscopy (SEM) and transmission electron mi-
croscopy (TEM) images show that the array height was
up to 1.8 mm and the tube diameter was >30 nm, see
Figures 1(a) and (b). As shown in Figure 1(c), the large
ratio between the Raman intensities of G and D peaks, as
well as the high 2D Raman peak, indicated that the CNTs
had high crystallinity. These features had both advan-
tages and shortages; the long length made the processing
easy and the perfect CNT structure allowed long phonon
mean free paths, while only the outermost layers of the
CNT played roles in heat transfers.
To prepare the C/C composites, aerospace-grade phe-
nolic resins (from Prof. T. Zhao’s group, Institute of
Chemistry, Chinese Academy of Sciences) were first in-
troduced into CNT arrays by a vacuum impregnation me-
thod at room temperature for 20 min. After this, the ar-
rays were cured at 100˚C for 6 h, at 120˚C for 2 h, and
finally at 180˚C for 4 h. Then the cured samples were
carbonized in argon for 2 h at a designed temperature
ranging from 200˚C to 1400˚C.
The thermal conductivity (κ) of C/C composites can be
calculated according to κ = αρCp, where α is the thermal
diffusivity, ρ the mass density, and Cp the specific heat
capacity. In the present study, a Laser Flash Apparatus
LFA-457 (Netzsch-Gerätebau GmbH, Selb, Germany)
and an STD-2960 thermal analyzer (TA Instruments,
New Castle, DE,USA) were used to measure α and Cp,
both in Ar atmosphere, while ρ was calculated according
to the dimensions and total mass.
3. Results and Discussion
By measuring the mass change after carbonization at
800˚C - 1400˚C, the weight fraction of CNT in the final
composites was calculated to be ∼30 wt%. This means
that the empty spaces between the CNTs were well im-
pregnated by the polymers, see Figure 2 where SEM
images of one pure CNT array and three carbonized C/C
composites (at 500˚C, 700˚C, and 1400˚C) are shown.
Notice that the CNT alignment was no longer straight
Figure 1. (a) SEM image of a 1.8 mm thick CNT array; (b)
TEM image of a CNT; (c) Raman spectrum for a raw CNT
array.
Figure 2. SEM images of a pure CNT array (a) and three
C/C composites carbonized at 500, 700, and 1400˚C (b)-(d),
respectively.
after the impregnation and kept to be curled after the
curing and carbonization, because the vacuum condition
usually applied a certain pressing on the composites.
The introduction of phenolic resins or carbonized py-
ro lytic carbon had strong influences on the thermal prop-
erties. Figure 3(a) shows the results of ρ and κ as func-
tions of carbonization temperature, and typical values of
α, ρ, Cp, and κ are also given in Table 1 for five selected
samples including a pure CNT array and phenolic matrix
as well. Generally, ρ decreased with increasing the tem-
perature because the pyrolysis reactions continuously
took place between phenolic resins by forming and re-
moving small molecules like H2O, CH4, H2, and CO
[23,24]. The sharp decrease in ρ started at a temperature
within 400˚C - 500˚C, indicating the beginning stage of
pyrolysis. When the temperature was as high as 700˚C -
800˚C, the mass loss became much smaller because the
evolution of CH4, H2, and CO nearly stopped [23]. Such
pyrolysis converted the glue-like phenolic matrix into
pyrolytic carbon, as shown in Figures 2(b) and (c). The
covalent cross-linking between the resins by pyrolysis,
and by curing as well, obvious enhanced the capacity to
conduct heat between non-neighboring CNTs, as re-
flected by the magnitude of α. For a pure CNT array α =
9.13 mm2/s and for a pure phenolic matrix α = 0.16
mm2/s. When they were hybrid, α became 1.2 mm2/s due
to a simple mixing rule, and, most importantly, higher
than 15 mm2/s after being carbonized at high tempera-
tures. This value was comparable to that of iron (23
mm2/s). The change in α played the key role in improv-
ing κ, so that κ was increased from <2 up to 11.8 - 14.6

Enhanced Thermal Conductivity of Carbon Nanotube Arrays by Carbonizing Impregnated Phenolic Resins 455
Figure 3. (a) Mass density and thermal conductivity of C/C
composites carbonized at different temperatures; (b) Ther-
mal conductivity as a function of test temperature for the
C/C composites carbonized at 800˚C.
Table 1. Thermal diffusivity (α), mass density (ρ), specific
heat capacity (Cp), and thermal conductivity (κ) for a pure
CNT array (referred as CNT), a phenolic matrix (referred
as phenolic), a hybrid material after the curing (referred as
C/C cured), and C/C composites carbonized at 500, 800,
and 1400˚C (referred as C/C 500, 800, and 1400), respec-
tively. All the values were measured at room temperature.
Sample α (mm2/s) ρ (g/cm3) Cp (J/g·K) κ (W/m·K)
CNT 9.13 0.18 0.73 1.2
Phenolic 0.16 1.58 0.79 0.2
C/C cured 1.2 1.4 0.83 1.4
C/C 500 4.85 1.28 0.49 3.04
C/C 800 14.2 1.11 0.75 11.8
C/C 1400 19.2 1.06 0.72 14.6
W/m·K only after a sufficient pyrolysis above 700˚C.
(Notice that Cp did not vary greatly while the decrease in
ρ drew back the enhancement of κ, and that for pyrolytic
graphite α = 1220 mm2/s.)
We thus suspect that a high level of graphitization ap
peared after the pyrolysis, and that the formation of
graphitized pyrolytic carbon allowed more efficient pho-
non transfers between the CNTs. It is confirmed by cal-
culating the intensity ratio between the G and D Raman
peaks (IG/ID) from the curves shown in Figure 4.
Owning to the abundant functional groups, the im-
pregnation of phenolic resins made it difficult to clearly
observe the G and D peaks, like the case for the C/C
composites carbonized at 400˚C. At higher carbonization
temperatures, it was possible to calculate IG/ID. The ratio
was about 0.4 at the start of pyrolysis at 500˚C, and in-
creased monotonically up to 1.07 and 1.3 after a suffi-
cient pyrolysis at 800 and 1400˚C, respectively. These
means the level of graphitization was improved by in-
creasing the carbonization temperature. Furthermore, it
was interesting to observe an enhanced 1620 cm−1 peak
for the C/C composites carbonized at 1400˚C. This peak
also indicated the existence of graphite crystals and was
Figure 4. Raman spectra for an untreated CNT array and
six C/C composites carbonized at 400, 500, 600, 800, 1000,
and 1400˚C, respectively.
Figure 5. Three TGA curves in air for a CNT array, a pure
phenolic matrix after being cured, and a C/C composite
sample carbonized at 800˚C, and one TGA curve for an-
other carbonized sample in Ar.
also observed in pyrolytic graphite [25].
The curing and carbonization of phenolic matrix wi-
thin CNT arrays provided the C/C composites with high
temperature properties. Figure 5 shows three thermo-
gravimetric analysis (TGA) curves for different samples
in air and one for the C/C composite in Ar. The weight
loss in air occurred mainly during 550˚C - 650˚C for the
pure phenolic matrix and the C/C composites, while the
loss in Ar was only of ∼35 wt% at 1000˚C. These mean
that the C/C composites can work at a wide range of tem-
perature. Furthermore, at high temperatures, thermally
excited phonons might also facilitate heat transfers. For
example, for the composites carbonized at 800 °C, κ in-
creased with the test temperature and saturated to ∼21
W/m·K above 350˚C, see Figure 3(b).
4. Conclusion
We report a carbonization method to improve the thermal

Enhanced Thermal Conductivity of Carbon Nanotube Arrays by Carbonizing Impregnated Phenolic Resins 456
conductivity of CNT arrays. Carbonization converted
phenolic matrix into pyrolytic carbon to form C/C com-
posites, and enhanced heat transfers between non-neigh-
boring CNTs. The thermal conductivity of <2 W/m·K for
a pure CNT array was significantly improved to 11.8 and
14.6 W/m·K with carbonization at 800˚C and 1400˚C,
respectively. The C/C composites showed high thermal
stability up to ~550˚C in air and up to ~1000˚C in Ar.
When working at 500˚C in Ar, the thermal conductivity
was up to 21.4 W/m·K.
5. Acknowledgements
This work is supported by the National Natural Science
Foundation of China (21273269), National Basic Re-
search Program (2010CB934700) by the Ministry of
Science and Technology, and International Science &
Technology Cooperation Project (BZ2011049) of Jiangsu
Province.
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