carbon nanocomposites

Materials Science and Engineering A 384 (2004) 209–214

Thermal properties of aligned carbon nanotube/carbon nanocomposites Qian-ming Gong∗ , Zhi Li, Xiao-dong Bai, Dan Li, Yun Zhao, Ji Liang Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China Received 20 February 2004; received in revised form 25 May 2004

Abstract Aligned carbon nanotube/carbon (Acnt/C) nanocomposites were fabricated with traditional chemical vapor infiltration (CVI) technology. Thermal conductivities of as-deposited and graphitized samples were tested by laser flash method. Results show that although only half the density and half the fraction of reinforcing element compared with carbon/carbon (C/C) composites in this work, the thermal diffusivity of Acnt/C is generally 3–5 times that of C/C composites, and the thermal conductivity of lower density Acnt/C (about 0.8 g/cm3 ) samples can reach 72.24 W/m K, about 12.31% higher than C/C composites with higher density (about 1.50 g/cm3 ). Heat transfer mechanism analyses show that the potential superiority of thermal conduction for Acnt/C can be ascribed to the long wrapped graphene layers of well-aligned carbon nanotubes, which can reduce phonon–phonon Umklapp scattering along the axial direction largely. The gap between predicted and measured thermal conductivity of Acnt/C suggests the demand for CNTs with virtually good thermal conducting capability and pyrocarbon with better properties by regulating CVI process. © 2004 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Chemical vapor infiltration; X-ray diffraction; Crystallite size; Phonons; Thermal conductivity

1. Introduction Carbon nanotubes (CNTs), which are brought by the one graphene layer (single-walled nanotubes or SWNTs) or many graphene layers (multi-walled nanotubes or MWNTs) wrapped tubular crystalline microstructure, have aroused great interesting in the research community since which have been discovered in 1991 because of their remarkable mechanical and electronic properties [1–3]. While up to now, because of their nanoscaled sizes, it is quite hard to directly measure their properties, thus the estimates are mainly relying upon computer simulations and calculations from indirect experiments. As far as mechanical properties are concerned, the axial Young modulus shows 0.45–5.0 TPa based on direct or indirect tests [4–9]. In addition to special electrical properties (metallic or semi-conducting character), field emission, optical character and hydrogen adsorption, CNTs exhibit unusually high thermal conductivity [10], i.e., the deduced longitudinal thermal conductivity of ∗ Corresponding author. Tel.: +86-10-62773641; fax: +86 10 62782413. E-mail addresses: [email protected], [email protected] (Q.-m. Gong).

0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.06.006

1800–6000 W/m K for a single carbon nanotube at room temperature (greater than that of diamond or graphite); and such value is much higher than carbon fibers (<600 W/m K) and ordinary carbon fiber reinforced carbon/carbon (C/C) composites (<200 W/m K) [11]. All the theoretically predicted and some direct tested rather good properties make CNTs the best promising candidate material for making high performance composites. In fact, besides being used as additives in nanoscaled molecular electronics, sensing and actuating devices, which are focused on their electronic properties, CNTs have been studied as reinforcement in CNT-based composites in numerous research works recently, in which different matrix materials, such as polymers, ceramics, resins and metals, etc., have been used [12–19]. But generally, they can only bring modest enhancements in strength compared with pure polymer matrix and have not been compared with carbon fiber reinforced corresponding composites. Although much lower in density and higher in strength, CNTs can not replace carbon fiber in many field immediately by virtue of unresolved hindrance of dispersion in matrix and the interface or bonding between matrix and reinforcement. As a result, aligned carbon nanotube/carbon (Acnt/C) nanocomposites were prepared by chemical vapor infiltration (CVI) technology to explore some properties of

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CNTs, especially thermal properties in this paper from the point of view that this project can avoid dispersion choke point by adding matrix carbon to fixed CNT-based preform and further more, microstructure of pyrocarbon and the interface between CNTs and pyrocarbon can be controlled by regulating deposition conditions. Most of all, by taking same process fabricated C/C composites as contrast, advantages in thermal conduction of CNTs could be investigated or evaluated and thermal conduction mechanism of carbon materials could be discussed further.

2. Experimental In the process of aligned carbon nanotubes preparation, a xylene solution containing ferrocene was fed into a horizontal tubular furnace by a microfeeding pump, and then the mixed solution was carried into the furnace together with flowing argon and hydrogen. Under catalysis of ferrocene at 700–800 ◦ C for about 30 min, the aligned carbon nanotube preform was prepared. Preforms of carbon/carbon composites were prepared by stacking unidirectional long carbon fiber formed felt. The fiber used was typical commercial T700 type (PAN derived). The initial densities of Acnt/C and C/C composites were about 0.10 and 0.40 g/cm3 , respectively. The following densification process was performed in isothermal chemical vapor deposition (CVD) furnace by CVI technology at about 1050 ◦ C. Propylene, in the flowing stream of nitrogen as carbon resource, infiltrated and deposited on the substrate to fill the initial pores of the two preforms. Generally, graphitization process is one of the most efficient solutions to remove structure defects in carbon materials. In order to probe the effect of heat treatment temperature (HTT) on thermal properties of as-grown Acnt/C nanocomposites, graphitization of the resultant samples was performed at specific temperatures for 2 h using a graphite-resistance furnace operating in a high-purity argon atmosphere, but the heat treatment process at 3000 ◦ C lasted only 15 min in protecting samples from sublimation massively. The graphitization degree and the crystallite size of the preforms and the resultant composites were determined

by using X-ray diffraction (XRD) analysis (Philips, Model APD-10) using Cu K␣ radiation (0.15416 nm). All the block samples were conglutinated on the carrier with axial direction of CNTs perpendicular to the surface of carrier before testing. The position and widths of the peaks were calibrated with respect to silicon internal standard. The interlayer spacing (d0 0 2 ) and crystallite size (Lc ) were calculated from the Bragg and Scherrer equations, respectively [20] (Table 1). The graphitization degree were calculated from Maire and Merings equation [21]. Field emission scanning electron microscopy (LE–1350) with 0–20 kV accelerating voltage and high-resolution transmission electron microscopy (HRTEM; JEOL JEM 2010F, 200 kV) were used to investigate the as-grown and graphitized samples. Thermal conductivity was measured in both transverse and longitudinal directions by flash method through JR-2 instrument. The samples tested were washed in ultrasonic ethanol bath after being machined to the size of Φ = 10 × 4 mm. The corresponding formula is listed as follows [22]: λ = 418.68ραCp

(1)

where λ is thermal conductivity (W/m K), ρ is the density of the sample (g/cm3 ), α is thermal diffusivity (cm2 /s) and Cp is specific heat (cal/g ◦ C).

3. Results and discussion 3.1. Density variation and morphology observation The densities of Acnt/C nanocomposites and C/C composites changed from about 0.10 g/cm3 and 0.40 g/cm3 to about 0.80 and 1.50 g/cm3 , respectively, after 1–2 cycles of densification process. Fig. 1(a and b) shows the SEM image of as-grown Acnts and Acnt/C nanocomposites. The average size of the CNTs in Acnt perform is about 20–40 nm in diameter and 10–100 ␮m in length. After densification process, all the CNTs in the Acnt preform are coated with pyrocarbons. Straight and long MWNTs containing amorphous carbon layers are observed by TEM (see Fig. 2(a)). After graphitization process, distinctive variations occurred, i.e., the amorphous layer disappeared (Fig. 2(b)) and the interspaces between layers decreased. Compared with the PAN-derived

Table 1 Interlayer spacing of (0 0 2) planes (d0 0 2 ), graphitization degree (g, %) and stacking height(Lc ) from XRD profiles HTT/◦ C

As-deposited 1800 2100 2300 2500 3000

Acnt/C nanocomposites

C/C composites

d0 0 2 (nm)

g (%)

Lc (nm)

d0 0 2 (nm)

g (%)

Lc (nm)

0.3450 0.3464 0.3453 0.3460 0.3447 0.3402

−11.63 −27.91 −15.11 −23.26 −8.14 44.18

1.985 7.241 7.291 8.269 8.066 12.97

0.3481 0.3431 0.3421 0.3408 0.3389 0.3372

−47.67 10.23 22.09 37.21 59.30 79.07

1.247 5.691 6.395 8.140 9.779 17.62

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Fig. 1. SEM photographs of aligned carbon nanotubes (a) and Acnt/C nanocomposites (b).

carbon fibers, which are mainly composed of turbostratic carbon and there exists no clear orientation of (0 0 2) planes along the axial direction in corresponding HRTEM images [11] (the fibers in this work are same as one of those in Ref. [11]), the MWNTs exhibited well-ordered graphitic structure with long and straight (0 0 2) plane lattice along their axial direction. 3.2. Test results of thermal conductivity Fig. 3a and b represent the thermal diffusivity and thermal conductivity as function of graphitizing temperature from 1800 to 3000 ◦ C for Acnt/C nanocomposites and C/C composites, respectively. The same temperature dependence trend was observed in all samples. After heat treatment at 3000 ◦ C, thermal diffusivities increased from 0.19 and 0.051 cm2 /s for the as-deposited Acnt/C nanocomposites and C/C composites to 1.15 and 0.62 cm2 /s, respectively. While the corresponding thermal conductivities are similar with values of 72.24 and 64.32 W/m K due to density factor (Formula (1)). As a matter of fact, although the re-

sultant density of C/C composites is about 1.9 times that of Acnt/C nanocomposites and the fraction of carbon fiber in C/C composites is about two times that of CNTs in Acnt/C nanocomposites, the diffusivity of C/C composites treated at 3000 ◦ C is only about half of that of Acnt/C samples, or equal to that treated at 2300 ◦ C. Thus, to some degree, the results may have indicated the potential superiority of Acnt/C nanocomposites in thermal conduction compared with C/C composites. Since the quasi-three-dimensional graphite structure of MWNTs lies between graphite and SWNTs, the research about thermal properties of Acnt/C nanocomposites might help in understanding heat transfer mechanism in CNTs and accelerating utilization of their prominent thermal conduction properties. 3.3. XRD analyses In regards to ordinary carbon materials, graphitization degree or crystal size plays a dominant role in determining its properties, especially thermal properties. Therefore, XRD is

Fig. 2. HRTEM images of carbon nanotube in as-grown state (a) and graphitized state (b) (HTT3000 ◦ C).

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Fig. 3. The effect of heat treatment on thermal diffusivity (a) and thermal conductivity (b) of Acnt/C nanocomposites and C/C composites.

used to evaluate the effect of heat treatment. As for C/C composites, stacking height of graphite sheet (Lc ) increase linearly somewhat with the increase of thermal treatment temperature. But in contrast, Lc of Acnt/C is almost constant after HTT1800 ◦ C and a little higher after HTT3000 ◦ C. Correspondingly, graphitization degree (g, %) shows no regularity at all. This phenomenon might be ascribed to the following two factors: firstly, the Acnt perform should be mentioned. For the high alignment degree of pre-deposited Acnt preform, X-ray is parallel to the aligned CNTs during tests (see Section 2), i.e., parallel to (0 0 2) plane, no prominent (0 0 2) peak would appear for weak intensity of the diffracted X-ray could be received by inductor [23]; further more, even after HTT3000 ◦ C, the pre-deposited Acnt perform may still maintain good directionality (see Acnts curves in Fig. 4); secondly, as for Acnt/C nanocomposites, although on the one hand, discrepancy in thermal expansion coefficient would result in distortion or curvature of CNTs

during teat treatment, which could result in a little stronger received (0 0 2) diffraction intensity [23], on the other hand, the limited thickness of the pyrocarbon around CNTs will restrain the crystal size of graphenes in pyrocarbon as well, so that weak (0 0 2) peaks even after HTT3000 ◦ C would be inevitable. Further discussion about this subject would be performed in the forthcoming paper. It can be figured out that thermal conductivity of C/C composites increase almost linearly with the crystal size, while there is no such relationship among Acnt/C samples, which might be explained by different thermal conduction mechanism for the two composites. 3.4. Thermal conduction mechanism Actually, some previous work has fallen in full agreement with thermal conduction mechanism in carbon materials, i.e., researchers have proved the dominant role of

Fig. 4. XRD spectra of as-grown state, HTT1800 ◦ C (a) and HTT3000 ◦ C (b).

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phonons in thermal conductivity at all temperature judging by their two orders of magnitude higher Lorenz rato (κ/σT) compared with that of electron [24,25]. Consequently, Dybe formula (as follows) can describe the relationship between some parameters: λ=

1 1 C υl 3 v

(2)

while l is mean-free path (the distance that phonons propagation between two scatterings), Cv1 represents specific heat and υ means the lattice vibration velocity, it is generally constant for certain vibration mode. With respect to ordinary C/C composites, increase in Lc brings about larger mean-free path and less phonon–phonon Umklapp scattering. Correspondingly, thermal conductivity of C/C composites increased linearly with heat treatment temperature. Comparatively, although the apparent Lc is smaller and almost constant for Acnt/C naocomposites by XRD spectra, thermal conductivity of which increased non-linearly with thermal treatment temperature as well. Generally, the following factors in heat transfer should be mentioned: on the one hand, although the stacking height of (0 0 2) planes is almost invariable with heat treatment because of limited size of diameters of the coated CNTs, the in-plane size (La ) is fairly large because of their quasi-three-dimensional graphite structure (i.e., limited (0 0 2) stacking height and large in-plane size in axial direction) and large aspect ratio. As a result, phonons would propagate along the axial direction in aligned CNTs more favorably than along the same direction in carbon fibers, polycrystal graphite or ordinary carbon materials on account of significantly deduced phonon–phonon Umklapp scattering, especially for high crystalline SWNTs [24]; on the other hand, some researchers demonstrate ballistic heat conduction of CNTs which is far superior to thermal diffusion by studying thermal conductivity of nanotube-in-oil suspensions [26]; and deduced that the ballistic conduction must be associated with the large phonon mean-free path in CNTs, i.e., the larger the mean-free path, the less the loss of phonon’s energy by scattering, the stronger the ballistic conduction. Considering the larger size of graphitic CNT walls in axial direction and good alignment of all the CNTs in Acnt perform compared with disorderly oriented microcrystals in C/C composites, it should be natural for stronger ballistic conduction in Acnt/C nanocomposites, but such mechanism deserves further studying.

4. Supplements Finally, it might deserve additional attention that if the thermal conductivity of the multi-walled nanotubes is taken as 1000 W/m K and thermal conductivity contributed by matrix pyrocarbon is neglected, thermal conductivity of the as-deposited Acnt/C nanocomposites would be at least 125 W/m K by simple “mixing law” considering CNTs ac-

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counted about 12.50%, which would be much higher than that of measured value for as-deposited Acnt/C samples. Similarly, although thermal conductivity of SWNTs has been estimated by classical molecular dynamics simulations with the Tersoff-Brenner potential for C–C interactions and resulted in very high values [24,25,27,28], i.e., 1800–6000 W/m K, the practical thermal conductivities were generally 30–250 W/m K for pure SWNT-formed mat or rope samples, which are equal to or lower than highly crystalline diamond or graphite more or less. At the same time, measurement of thermal conductivity of SWNTs related composites are always rather below the “law of mixtures” prediction even with a conservative estimate of the SWNT thermal conductivity [29]. The gap between measured value and expectation might be ascribed to the following reasons: one would be related to CNTs themselves, it has been postulate that graphene monolayers initially form scrolls and subsequently transform into multi-walled nanotubes through progression of defects [30]; these process may bring highly crystalline MWNTs or semi-crystalline MWNTs [31]. If the Acnt preform were mainly composed of semi-crystalline MWNTs, they would experience abrupt volume change through graphitization process. This physical transformation or loss of amorphous carbon indicates that a structural transformation from distorted fringes to straight graphene layers has to occur after the formation of a faceted plane. As a result, the faceted angle of the graphitized semi-crystalline might become larger than that of graphitized highly crystalline MWNTs, larger faceted angle would induce more phonon–phonon scattering and result in lower thermal conductivity. The aforementioned discrepancy indicates that Acnt/C composites with excellent thermal properties should be prepared from Acnt perform with high thermal conductivity. That is, CNTs with virtually good thermal conducting capability should be produced first. The other may be associated with the low density of the Acnt/C samples. Another reason might be that mismatch of the matrix and CNTs would bring about interfacial thermal contact resistance and result in a big drop in the effective thermal conductivity [32], which means that CVI process should be modulated to produce better structured pyrocarbon with better interface with CNTs.

5. Conclusions Aligned carbon nanotube/carbon nanocomposites would open the door of utilization of super high thermal conductivity of carbon nanotubes for the reason that although only half the density of C/C composites (1.50 g/cm3 ), the diffusivity of Acnt/C is generally three to five times that of C/C composites, but the gap will decrease with the increased thermal treatment temperature. The thermal conductivity of low density Acnt/C (about 0.8 g/cm3 ) samples can reach 72.24 W/m K, which is much higher than that of some SWNTs formed mats or ropes. X-ray diffraction

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analyses show that thermal conduction in C/C composites is completely limited by the crystal size (Lc ), while even though the Lc varied little for Acnt/C samples, their thermal conductivity increased sensitively with increase of thermal treatment temperature. It has been deduced that the nearly constant Lc should be ascribed to XRD testing method and difficulty in shrinkage of integral cylindrical structure during heat treatment, while the increased thermal conductivity should be the result that the size of well ordered graphitic ‘wall’ increases with heat treatment, which can reduce phonon–phonon Umklapp scattering. The gap between expected and measured thermal conductivity for Acnt/C nanocomposites suggests the need for CNTs with virtually good properties and pyrocarbon with better properties by regulating CVI process.

Acknowledgements Most of the work has been performed in the State Key Lab for Powder Metallurgy at Central South University and the authors would like to thank Prof. Xiang Xiong, Prof. Qizhong Huang, Prof. Hongbo Zhang and Mr. Jinlv Zuo for their help in preparing the samples. The financially support by the National Science Council of the People’s Republic of China is gratefully acknowledged.

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