Enhanced Thermal Conductivity of Carbon Nanotube Arrays by Carbonizing Impregnated Phenolic Resins

doi:10.4236/msa.2013.48055

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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.

Email: [email protected]

Received June 8th, 2013; revised July 9th, 2013; accepted July 18th, 2013

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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