Dispersion and thermal conductivity of carbon nanotube composites

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journal homepage: www.elsevier.com/locate/carbon

Dispersion and thermal conductivity of carbon nanotube composites Shiren Wanga,*, Richard Liangb, Ben Wangb, Chuck Zhangb a

Department of Industrial Engineering, Texas Tech University, 2500 Broadway, Lubbock, TX 79409, USA High-Performance Materials Institute, Department of Industrial and Manufacturing Engineering, Florida State University, Tallahassee, FL 32310, USA

b

A R T I C L E I N F O

A B S T R A C T

Article history:

A mechanical method was used to shorten carbon nanotubes (CNTs) for improving disper-

Received 27 February 2008

sion without reducing their thermal conductivity. Single walled carbon nanotubes

Accepted 27 August 2008

(SWCNTs) were mechanically cut to produce short and open-ended fullerene pipes. These

Available online 2 September 2008

shortened SWCNTs were then used in polymer composites. Both atomic force microscopy and scanning electron microscopy characterizations suggested that nanotube shortening significantly improved CNT dispersion. Thermal conductivity of composites containing short CNTs were found to be much better than those containing pristine CNTs.  2008 Elsevier Ltd. All rights reserved.

1.

Introduction

Carbon nanotubes (CNTs) are molecular-scale tubes of graphitic carbon with outstanding properties, and are among the stiffest and strongest fibres known. The Young’s modulus of an individual single walled carbon nanotube (SWCNT) reaches as high as 1 TPa [1,2] and tensile strength of SWCNT ropes is about 63 GPa [3]. CNTs also demonstrate remarkable electronic properties and can be metallic or semiconducting dependent on their chiral structures [4]. Due to their exceptional properties, CNTs are very promising for the application of thermal management. However, CNTs tend to aggregate and cluster, resulting in poor dispersion and inferior properties. Shortened CNTs are expected to have richer chemistry and to be more easily dispersed. They are promising one-dimensional building blocks for constructing advanced nanoscale structures [5,6]. Many efforts have been directed toward cutting nanotubes to shorten the lengths and create open-ends. Smalley and co-workers initially developed a chemical method to cut long, tangled, purified nanotubes (1.1–1.3 nm in diameters) using concentrated, strong acids [7,8]. The same

technique was adapted to shorten CNTs (1.3–1.5 nm in diameters) produced by an electric arc-method [7,8]. Chen et al. found that the acid cutting technique may not be applied to small diameter nanotubes (0.7–0.8 nm in diameters), while small diameter nanotubes are more attractive for studying nanotube chemistry because of their expected higher chemical reactivity [7–9]. Other methods have also been tried. Smalley and co-workers demonstrated that chemical fluorination followed by pyrolysis provided the modified and cleaved fragments with broad length distributions [10,11]. The ultrasonic process is a typical method to disperse CNTs into a solvent and resin matrix. This process was also found to shorten CNTs [12]. However, ultrasonication created holes along the CNT sidewalls, resulting in ‘‘worm-eaten’’ damage [13–16]. Solid state processes such as grinding or milling with diamond balls can also shorten nanotubes, but these milling methods could damage sidewalls and create difficulty in the component separation [16,17]. Chen et al. developed an extending soft cutting method by grinding in cyclodextrins, which successfully avoided long-time ultrasonic processing or ultrasonication in strong acids and oxidants [18]. Lustig et al. reported a lithographical method to chop CNTs, which included

* Corresponding author. E-mail address: [email protected] (S. Wang). 0008-6223/$ - see front matter  2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2008.08.024

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lithography to place protective photo-resist patterns over the nanotubes and conducted reactive ion etching to destroy the unprotected parts of the nanotubes [18]. Zettl and co-workers reported a cutting method using a low-energy electron beam, which can cut the nanotubes either partially through creating hinge like geometries or fully through creating size-selected nanotube segments [19]. We have invented a mechanical method to precisely section CNTs for short lengths and open-ends [20]. In this paper, dispersion of these shortened CNTs is investigated and thermal conductivity of short SWCNT composites is also discussed.

2.

Experimental

The SWCNTs used in this research were produced by Unidym Inc. Pristine SWCNTs were dispersed into water and stabilized with surfactant. Then the SWCNT suspension was passed through the nylon filter film kept in the bore of a resistive coil magnet with a magnetic field of 17.3 T. The SWCNT suspension was then pumped through the filter at a pressure of 138–172 kPa (20–25 psi) under the selected magnetic strength. After a desired amount of suspension passed through the filter, isopropyl alcohol was passed through the filter to wash the surfactant. The filter was then removed from the magnetic field and kept in a vacuum oven at 1.01 · 105 Pa. This vacuum process facilitated the drying of the buckypaper or SWCNT film. After drying, the filter was removed and the magnetically aligned nanotube buckypaper was achieved. The aligned buckypapers were layered-up and frozen with liquid nitrogen, and then these frozen aligned membranes were sectioned into desired lengths with an ultra-microtome instrument, which can cut nanotubes as short as 5 nm [20]. During the operation of ultra-microtome, feed rate was set to 50 nm per cutting movement, resulting in 50 nm-cut SWCNTs. Similarly, 200 nm-cut SWCNTs were achieved. The diameter and length distributions of both pristine and sectioned nanotubes were characterized by atomic force microscopy (AFM). The pristine and shortened SWCNTs were first dispersed into water with the aid of surfactant. A multi-step ultrasonic process was used to prepare the suspension. First the carbon nanotubes were processed with an ultrasonic processor 3000 (Misonix Inc.) under mild power (about 30 W/cm3) in a short time (15 min). Subsequently, the resultant supernatant suspension was carefully decanted and subjected to ultrasonic process in the aqueous bath ultrasonication (Branson 1510, 80 kHz) for 2 h. This multi-step process will minimize the damage to the CNTs and help to acquire accurate statistic analysis through dispersion in the water [15]. Then a drop of resultant suspension was transferred to the silicon wafer and allowed to dry in the air. Subsequently, the silicon wafer was soaked with iso-propanol to remove the surfactant and ready for AFM characterization. The AFM images were taken in tapping mode using a MultiModel II (Veeco Inc.). Nanotube lengths were determined using the nanotube length analysis module of SIMAGIS software. Short SWCNTs with average lengths of 246 nm and pristine SWCNTs were dispersed into epoxy resin. Both of them

were independently dispersed using the same procedures. First, SWCNTs were ground to black paste with mortar and pestle by adding three-drop acetone. Then the paste was left in the hood to evaporate the residual acetone. EPI-cure-W (based on the required curing ratio) was weighed and added to a 30 ml beaker. Subsequently, SWCNT paste was added to the EPI-cure-W agent when stirred. Then EPI-cure-W and SWCNT mixture were under cup-horn ultrasonic processing at 12 W for 30 min under ice cooling. The mixture of SWCNT and EPI-cure-W was then transferred to the epoxy resin diluted with acetone, resulting in a new mixture. Then SWCNT/EPI-W/Epoxy mixture was processed with cup-horn sonicator at the power of 12 W for 10 h under ice cooling. In the meantime, a magnetic stir bar was placed into the mixture and strongly stirred to help disperse nanotubes in the epoxy. The resultant mixture of SWCNT, EPI-W and epoxy, was left in the vacuum system for 10 h to remove residual acetone. When acetone was completely removed, the final mixture was cast into a metallic mould and cured under hot-press at 177 C for 2.5 h. Finally, the cured samples were also post-cured in the oven for another 2 h at 177 C and cooled to room temperature slowly, resulting in desired composite parts. The shortened SWCNTs were characterized with Raman spectroscopy and their dispersion was characterized with atomic force microscope (AFM) and scanning electron microscope (SEM). Short SWCNT-based composites were further characterized with a thermal conductivity analyzer (Model: Mathis Tci, C-Therm Technologies Ltd.).

3.

Results and discussion

3.1.

Characterization of sectioned SWCNTs

The pristine, 50 nm-cut and 200 nm-cut SWCNTs were dispersed into aqueous solution and characterized with AFM, respectively. The AFM results were further analyzed by an image software and the lengths were statistically quantified [20].The average lengths of shortened and uncut nanotubes are shown in Fig. 1. The pristine SWCNTs showed the average length of 580 nm while the length for the 50 nm-cut and 200 nm-cut carbon nanotubes showed the average lengths of 87 nm and 246 nm, respectively. Consequently, the

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Fig. 1 – Average length for uncut and shortened SWCNTs.

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ultra-microtome demonstrated effectiveness in cutting SWCNTs to produce desired nanotube lengths. Sidewall damage of shortened SWCNTs was compared to that of the pristine and acid-oxidized SWCNTs. Oxidized SWCNTs were achieved by dispersing SWCNTs into sulphuric acid (98%, Fisher Scientific Inc.) at 70 C for 2 h. Both acid-oxidized and microtome-cut SWCNTs were characterized with transmission electron microscopy (TEM), as shown in Fig. 2. The microtome-cut SWCNT showed a very smooth surface while acid-oxidized SWCNT showed an etching surface. Fig. 3 shows the results of Raman spectroscopy. D-band of the microtome-cut tube appeared almost the same as that of the pristine SWCNTs, but the acid-oxidized SWCNTs demonstrated an enlarged D-band. Since the D-band is proportional to the defects of the tubes, these results suggest minimal sidewall damage of the microtome-cut SWCNTs,

Fig. 2 – TEM images of (a) acid-oxidized and (b) microtomecut SWCNTs.

and significant devastation of acid-oxidized SWCNTs. Hence, strong acid oxidization is too harsh for nanotube functionalization.

3.2.

Dispersion of shortened SWCNTs

According to colloid chemistry, the attraction between two particles is proportional to the specific surface of the particles [21]. Particles with a large specific surface agglomerate easily compared to those with smaller specific surface. Generally, the specific surface of a typical SWCNT is about 1500 m2/g. Therefore, the SWCNTs tend to bundle together and uniform dispersion of SWCNTs is very difficult. In addition, Flory proposed the polymer solution theory about rod-particle dispersion [22,23]. The effect of rod length on thermodynamic properties increases as the concentration decreases. When anisotropic rod-like molecules are randomly dispersed in the isotropic solution, the allowable volume fraction of anisotropic molecules in the equilibrium is predominated by the rod aspect ratio. As rod aspect ratio is shortened, the volume fraction in the phase equilibrium is increased; therefore, the immiscibility gap is reduced as the aspect ratio is reduced. From this point of view, the short aspect ratio would facilitate dispersion of anisotropic rod molecules in the isotropic phase. Pristine SWCNTs and cut SWCNTs were dispersed in the aqueous solution with the aid of Triton X100. The dispersion effect was examined with AFM, and the diameters of the ropes were acquired by section analysis. The cumulative percentage of dispersed SWCNT diameter is shown in Fig. 4. Short SWCNTs were observed to have better dispersion than pristine SWCNTs. For example, nanotube ropes with diameter less than 4 nm account for 20% in pristine SWCNT

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Fig. 3 – Raman spectroscopy results of SWCNTs. (A) Pristine SWCNTs. (B) Shortened SWCNTs. (C) Oxidized SWCNTs by sulphuric acid.

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Thermal Conductivity ( W/m.K)

0.3 0.25 0.2 0.15 0.1 0.05 0 Neat resin

Pristine SWCNT composites

Acid-oxidized SWCNT composites

Chopped SWCNT composites

Fig. 6 – Thermal conductivity of 0.5 wt% nanotubes integrated composites. Fig. 4 – Cumulative percentage distributions of SWCNT rope diameters in aqueous solution.

suspension while they account for more than 60% in short SWCNT suspension. This improvement should arise from the reduction of the aspect ratio and agrees well with Flory’s theory. Dispersion of shortened SWCNTs in the polymer resin was also investigated by SEM (JSM-7401F). The SWCNTs/polymer composite samples were broken after immersion to the liquid nitrogen. The cross-sections of the epoxy composites were examined by SEM and are shown in Fig. 5. For the pristine SWCNT/epoxy composites, the SWCNTs tend to aggregate in large bundles. The phenomena of pullout can be observed. However, shortened SWCNTs were well dispersed in the epoxy resin and embedded in the epoxy matrix. Therefore, the mechanical cutting leads to improved dispersion and interfacial bonding in SWCNT composites.

3.3.

Thermal conductivity of short SWCNT composites

The thermal conductivity of neat epoxy resin and SWCNT composites were measured by a Mathis Tci thermal conductivity analyzer (C-Therm Technologies Ltd.), and the results are shown in Fig. 5. For the neat resin, the thermal conductivity is 0.18 W/m K at room temperature. The dispersion of 0.5 wt% pristine SWCNTs into epoxy resin did slightly increase the thermal conductivity. However, the acid-oxidized

SWCNT composites exhibit a less enhancement than pristine SWCNT composites due to the sidewall devastation. In contrast, the thermal conductivity of shortened SWCNT/epoxy composites increased 40% in comparison to the neat resin. Therefore, shortening considerably improved the SWCNT dispersion in the composites with minimum sidewall devastation, and also retained the thermal conductivity. Thermal transport in the CNT composites includes phonon diffusion in the matrix and ballistic transportation in the filler. The improvement of thermal conductivity in the short CNT composites may stem from the improved percolation because of better dispersion [24]. Park et al. calculated the effect of chemical functionalization on nanotube conductance and found that sidewall defects significantly disrupt the ballistic conductance near the Fermi level [25]. Hence, strong acid oxidization etched sidewall and affected the exceptional conductance of nanotubes, resulting in inferior conductance of oxidized SWCNT composites (Fig. 6).

4.

Conclusion

The precise sectioning of CNTs provides an effective way to shorten carbon nanotubes with controlled length and minimum sidewall damage. Shortened nanotubes were found to be easily dispersed into polymer matrices, and then effectively improved the percolation. The minimum CNT sidewall damage

Fig. 5 – SEM images of pristine SWCNT/epoxy and shortened SWCNT/epoxy composites. (a) Dispersion of pristine SWCNTs in epoxy resin. (b) Dispersion of shortened SWCNTs in epoxy resin.

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and improved percolation in short SWCNT composites led to an obvious improvement of thermal conductivity. Hence, this research suggests an effective way to improve dispersion of CNTs into polymer matrices and also retain perfect electronic structure of CNTs, resulting in desired functional materials.

Acknowledgements The authors acknowledge the financial supports from AFOSR and new faculty startup funding of Texas Tech University.

Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carbon.2008.08.024.

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