- Email: [email protected]
Nafion nanocomposites
Organic Electronics 12 (2011) 2120–2125
Contents lists available at SciVerse ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Effect of the carbon nanotube type on the thermoelectric properties of CNT/Nafion nanocomposites Yongjoon Choi a, Yuhee Kim b, Sung-Geun Park a, Young-Gon Kim b, Bong June Sung c, Sung-Yeon Jang b,d,⇑, Woochul Kim a,⇑ a
School of Mechanical Engineering, Yonsei University, 262 Seongsanno, Seoul 120-749, Republic of Korea Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea c Department of Chemistry, Sogang University, Seoul 121-742, Republic of Korea d Department of Chemistry, Kookmin University, 861-1 Jeongneung-dong, Seoul 136-702, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 6 May 2011 Received in revised form 28 July 2011 Accepted 27 August 2011 Available online 13 September 2011 Keywords: Thermoelectric Carbon nanotube Nanocomposite Seebeck coefficient Power factor
a b s t r a c t The effect of different carbon nanotube (CNT) types on the thermoelectric performance of CNT/polymer nanocomposites was studied. Three different kinds of CNTs, single(SWCNTs), few- (FWCNTs) and multi-walled CNTs (MWCNTs), were effectively dispersed in an aqueous solution of Nafion. The electrical properties of the CNT/Nafion nanocomposites were primarily affected by the CNTs since the Nafion acts as an electrically nonconducting matrix, while the thermal conductivity of the nanocomposites was dominated by the Nafion mainly due to weak van der Waals interaction. In this way, electrical and thermal transport can be separated. In all three types of CNTs, both the electrical conductivity and Seebeck coefficient increased as the concentration of CNTs was increased. While the electrical conductivity depends on the type of CNT, the behavior of the Seebeck coefficient was relatively insensitive of the CNT type at high CNT loading. This indicates that high-energy-charges can participate in transport processes irrespective of the type of CNT. It is suggested that FWCNTs and MWCNTs are preferred over SWCNTs in CNT/Nafion nanocomposites for thermoelectric applications. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Much effort has been devoted to generating electricity from waste heat through the use of thermoelectric devices. In order to save waste heat energy with thermoelectric devices, they should be low cost while possessing a high thermoelectric figure of merit. The efficiency of thermoelectric devices is related to the thermoelectric figure of merit, ZT, defined as S2rT/k, where S denotes the Seebeck coefficient, r is the electrical conductivity, k is the thermal conductivity, and T is the absolute temperature. Good thermoelectric ⇑ Corresponding authors. Address: Department of Chemistry, Kookmin University, 861-1 Jeongneung-dong, Seoul 136-702, Republic of Korea. Tel./fax: +82 2 910 5768 (S.-Y. Jang), +82 2 2123 5816 (W. Kim). E-mail addresses: [email protected] (S.-Y. Jang), woochul@ yonsei.ac.kr (W. Kim). 1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2011.08.025
materials must have a high Seebeck coefficient for enhanced thermoelectricity, low electrical resistivity to minimize Joule heating, and low thermal conductivity to sustain large temperature gradient [1]. Typical thermoelectric materials have been inorganic semiconductors. However, some thermoelectric semiconductor materials (e.g., Te, Bi, Pb) are so toxic and expensive that their use on a large scale is not practical. To address this issue, organic materials could be an alternative candidate for thermoelectric materials because of their low cost and high processability. Polymers are intrinsically poor thermal conductors, which makes them ideal for use as thermoelectrics. However, their low electrical conductivity, Seebeck coefficient, and stability have hampered their use in thermoelectric applications. CNT/Polymer composite technology and the development of electrically conducting polymers with reasonable air stability have made it
Y. Choi et al. / Organic Electronics 12 (2011) 2120–2125
possible to bring organic materials into degenerate semiconductor or metallic regimes by incorporating relatively small amounts of conductive fillers [2]. Thus, developing polymer-based composites using thermoelectric and conducting fillers is a logical and promising strategy for creating a new class of thermoelectric materials with sufficient mechanical properties, including flexibility. In 2008, Yu et al. [2] fabricated carbon nanotube (CNT)/ PVAc composites and investigated their thermoelectric properties upon CNT addition. The researchers showed that the electrical conductivity can be dramatically increased by creating a network of CNTs in the composite, while the thermal conductivity and Seebeck coefficient remained relatively insensitive to the filler concentration. A thermoelectric figure of merit (ZT) greater than 0.006 was achieved with a CNT concentration of 20 wt.% at room temperature. The study by Yu et al. was the first investigation into the use of CNT/polymer composites as thermoelectric materials. In 2010, Kim et al. [3] reported a maximum ZT of 0.02 at room temperature for a polymer composite containing 35 wt.% SWCNTs. In the study, the main polymer matrix was copolymer latex containing vinyl acetate and ethylene. The stabilizer that was used to disperse the CNTs in water was PEDOT:PSS, a widely used conducting polymer, which was doped with DMSO. Electrically insulating gum Arabic (GA) also was studied for a comparison. Composites were made with two different CNTs (XM-CNT and SWCNT) at 2–15 wt.% CNT concentrations and 1:1–1:4 ratios of CNT to PEDOT:PSS; the structures were dried at room temperature and/or 80 °C. In the preparation procedure, CNTs were added to the conducting polymer as fillers to impart more electrical conductivity and ensure a high thermoelectric figure of merit. Meng et al. [4] and Yao et al. [5] fabricated composites consisting of a CNT network as the main matrix and polyaniline (PANI), a conductive polymer, as a coating additive to the CNTs. Meng et al. reported significantly enhanced Seebeck coefficients and power factors in the multi-walled CNTs (MWCNT)/PANI composites, which were several times larger than either of their individual components. The best ZT value obtained was 0.003 for a 15.8 wt.% MWCNT/PANI composite. Yao et al. fabricated single-walled CNTs (SWCNT)/PANI composites with a maximum ZT of 0.004 at 41.4 wt.% SWCNT. In the above cited studies, the effect of the CNT type on the thermoelectric performance of the CNT/polymer composite was not investigated. It has been known that different types of CNTs exhibit different electrical conductivities and Seebeck coefficients. At 300 K, the Seebeck coefficients of individual SWCNTs and MWCNTs are about 42 lV/K [6] and 82 lV/K [7], respectively. The Seebeck coefficients of SWCNT and MWCNT films [4,8] are comparable and have values of around 20–25 lV/K. With regard to electrical conductivity, metallic SWCNT rope exhibits values around 10,000–30,000 S/cm [9], while electrical conductivities of approximately 200–2000 S/cm have been obtained for individual MWCNTs [10,11]. For films or networks composed of CNTs, electrical conductivity becomes one or two orders of magnitude lower than that of individual CNTs and varies depending on the degree of aggregation. While the electrical conductivity of SWCNT films is about
2121
2000 S/cm [12], that of MWCNT mat is approximately 10 S/cm [13]. Given the aforementioned findings, the CNT type would directly influence the thermoelectric performance of their composites. In this study, the thermoelectric properties of CNT/Nafion nanocomposite films with three different types of CNTs were investigated. The CNT/Nafion nanocomposites were prepared from aqueous solutions of CNT/Nafion and SWCNTs, few-walled CNTs (FWCNTs), and MWCNTs were employed in the structures. A water soluble polymer, Nafion, which also contains hydrophobic nature in its backbone, has exhibited good affinity with CNTs and provides enhanced solubility in an aqueous solution without additional surfactants. As a result, films on a substrate were readily fabricated by the doctor blading method. The electrical properties of the CNT/Nafion nanocomposites were primarily affected by the CNTs since the Nafion acts as an electrically non-conducting matrix, while the thermal conductivity of the composites was dominated by the Nafion matrix; electrical and thermal transport are separated. The electrical conductivity and Seebeck coefficient of the nanocomposites were measured as a function of the type and concentration of CNTs at room temperature. For all three kinds of CNTs, both the electrical conductivity and the Seebeck coefficient increased as the concentration of CNTs was increased. While the electrical conductivity was dependent on the type of CNT, the behavior of the Seebeck coefficient was relatively independent of the CNT type. This indicates that high-energy-charges can participate in transport processes irrespective of the type of CNT. 2. Experimental 2.1. Preparation of CNT/Nafion nanocomposites SWCNT (ASA-100F), FWCNT (CMP-310F), and MWCNT (CM-95) that are purchased from Hanwha Nanotech were purified according to the ‘‘Fenton Chemistry Method’’ reported by Wang et al. [14] Briefly, pristine CNTs were stirred in 1 N HCl and H2O2 (7/3 by weight) at 60 °C for 4 h. The resulting CNTs were filtered and washed with deionized water until the filtrated solution had a pH of 7. The pristine CNTs were heated for 4 h at 292 °C before the Fenton Chemistry purification in order to remove amorphous carbon impurities. The purity of the CNTs, as determined by thermogravimetric analysis (TGA), was >95%. The purified CNTs were mixed with a Nafion solution in deionized (DI) water (5% by weight, Aldrich). The solution was then briefly sonicated (30 min) and stirred overnight. The CNT/ Nafion composite solutions were cast onto glass plates that were cleaned by sequential washing with ethanol and acetone under sonication. The composite solution was then doctor-bladed to form the films. The thickness of the film is 2–5 lm. 2.2. Characterization The four-probe method was used to measure the electrical conductivities of the CNT/Nafion nanocomposites. A
2122
Y. Choi et al. / Organic Electronics 12 (2011) 2120–2125
Keithley 2400 was employed as a source meter. The Seebeck coefficient was extracted by applying a temperature gradient along the sample while measuring the thermoelectric voltage. Two T-type thermocouples were used to measure temperatures and voltages simultaneously. Two thermoelectric devices, one as a heat source and another as a heat sink, were used to create a temperature gradient across the samples. The measured Seebeck coefficient was calibrated due to the Seebeck coefficient of the Cu wire of the T-type thermocouple, which is 1.7 lV/K [15]. The thermal conductivity of the samples was extracted by measuring the thermal diffusivity using the laser flash technique (Netzsch LFA 457 MicroFlash) and the specific heat as measured with a differential scanning calorimeter (Netzsch Differential Scanning Calorimeter (DSC) 200).
3. Results and discussion For the preparation of the CNT/polymer composites, an effective dispersion of CNTs within the polymer matrix is quite a challenge since CNTs have poor solubility due to strong van der Waals interactions. Nafion, a water soluble perfluorosulfonated polymer, has been successfully used to solubilize CNTs in alcoholic solutions so as to form homogeneous composites [16,17]. The hydrophobic main chain of Nafion can wrap the surface of CNTs, while the polar side chain can provide solubility in polar solvents. A photo of a diluted SWCNT/Nafion composite (5 wt.% SWCNT in Nafion) solution in DI-water is shown in Fig. 1(a). A stable homogeneous dispersion of the composites is clearly evident. Stability was reasonably maintained at various SWCNT concentrations. Photos of aqueous SWCNT/Nafion composite solutions over a range of SWCNT/Nafion weight ratios (1/9–5/5) are shown in Fig. 1(b). The dispersion stability was intact after one month, indicating that the Nafion can efficiently solubilize a significant amount of CNTs for a long period of time. The aqueous dispersion and the solution stability of FWCNT and MWCNT-based composites were even more improved because SWCNTs have a stronger tendency toward self-aggregation [18–20]. CNT/ Nafion composite-based films were prepared using the doctor blade method, which is popular for micron-thick polymer film fabrication. The fabrication of crack-free CNT/Nafion composite films via a solution process has been demonstrated by Luo et al. [17]. The resulting films exhibited enhanced mechanical properties with strong flexibility and high electrical conductivity. A doctor-bladed SWCNT/Nafion composite film (5 lm thick) on a glass plate is shown in Fig. 1(c). The surface of the films was relatively smooth without noticeable cracks. Scanning electron microscopy (SEM) images of a 7 wt.% SWCNT/Nafion composite and a 50 wt.% SWCNT/Nafion composite are shown in Fig. 2(a) and (b), respectively. The SWCNTs were homogeneously dispersed within the Nafion matrix, which is indicative of effective wrapping of the SWCNTs by the Nafion in an aqueous solution. As the SWCNT/Nafion ratio was increased, more SWCNTs were observed in the SEM images (Fig. 2(a) and (b)), which indicates that the SWCNTs were homogeneously dispersed within the composite films.
Fig. 1. Photos of (a) a diluted SWCNT/Nafion (5 wt.% SWCNT in Nafion) solution in deionized (DI) water, (b) a range of SWCNT/Nafion composite (1/9–5/5 by weight ratio) solutions in DI water one month after preparation, (c) a SWCNT/Nafion composite film prepared via the doctor blade method. The film was relatively uniform with a thickness of 5 lm.
The electrical conductivities of the CNT/Nafion nanocomposites are shown in Fig. 3(a). The electrical conductivities of all of the composites increased as the CNT concentration was increased. These conductivities saturated at 30 wt.% CNT loading in Nafion. Note that even the lowest CNT concentration (10 wt.%) in the composites is much higher than the conventional percolation threshold of CNT/polymer composites ( a few %), thus the electrical conductivity at 10 wt.% CNT loading was not significantly lower than in the 30 wt.% CNT-loaded films (they were the same order of magnitude), which indicates the efficient percolation of charges. The optimized electrical conductivity of the CNT/Nafion composites (13 S/cm) was similar to the values obtained in a previous report (12.5 S/cm) [17], although the CNT loading in that study was 10 wt.% more than in our composites. Such a result indicates that our composites contained well-dispersed and efficiently interconnected CNTs within a matrix of Nafion. The SWCNT/Nafion composites exhibited relatively lower conductivities when compared to the FWCNT and MWCNT-based composites. For fully connected CNT networks, the electrical conductivity can be somewhat limited by the CNT–CNT contact resistance [21,22]. Taking
Y. Choi et al. / Organic Electronics 12 (2011) 2120–2125
2123
Fig. 2. Scanning electron microscopy (SEM) images of SWCNT/Nafion composites: (a) 7 wt.% and (b) 50 wt.% of SWCNTs in Nafion.
into account that the conductivity of individual CNTs is dependent on various factors such as the surface functionality, tube diameter, length, wall thickness, and bundle size, the conductivity of the composites will be governed by the electrical properties of the CNTs themselves and the contact resistance among the Nafion-wrapped CNTs. In the case of a SWCNT-based mat or composite films, the conductivity significantly varies depending on the ratio of metallic to semiconducting tubes. This is because the contact resistance between semiconducting and metallic junctions is significantly greater than that in metallic–metallic junctions [23]. Since our SWCNTs possessed a typical composition (1:2 for metallic:semiconducting) [23], the higher electrical conductivities of the FWCNT and MWCNT-based composites are not surprising and are consistent with the findings of other reports [24,25]. Based on Fig. 3(a), we deduce that a CNT concentration of 30% should be enough to obtain an optimum electrical conductivity. The highest observed electrical conductivity in this study was 13 S/cm from the 30 wt.% MWCNT/Nafion composites. The Seebeck coefficients of the CNT/Nafion nanocomposites with respect to the CNT concentration are shown in Fig. 3(b). All of the films, irrespective of the type and concentration of CNTs, exhibited p-type conduction, which means that the majority of charge carriers are holes. When charge carriers from the thermoelectric effect are transported in
Fig. 3. (a) Electrical conductivity [S/cm], (b) Seebeck coefficient [lV/K], and (c) power factor [W/m-K2] versus CNT weight percent.
conducting networks (CNTs in this study) within a non-conducting matrix, such as Nafion, the Seebeck coefficient of the composites should depend on the Seebeck coefficient of the networked materials. As mentioned previously, although the Seebeck coefficients of individual CNTs are dependent on the type of CNT (the Seebeck coefficients of the SWCNTs and MWCNTs are about 42 lV/K [6] and 82 lV/K [7], respectively), the Seebeck coefficients of SWCNT and MWCNT mats [4,24] are comparable to each other and are approximately 20–25 lV/K. The Seebeck coefficient is related to the transport of energetic charges, whereas the electrical conductivity is related to the transport of all mobile charges [2,3]. Charge transport in CNT mats is hindered by junctions such as metallic CNT-semiconductor CNT junctions, metallic CNT–metallic CNT junctions, etc. The fact that the Seebeck coefficients of CNT
2124
Y. Choi et al. / Organic Electronics 12 (2011) 2120–2125
mats are independent of the CNT type suggests that energetic charges transported regardless of the junction types in SWCNT mats. In our nanocomposites, behavior similar to that seen in CNT-based mats was observed. The Seebeck coefficients of the CNT/Nafion composites which contains >20 wt.% of CNTs ranged from 25–35 lV/K, and there was no significant difference among the types of CNTs (SWCNT, FWCNT, MWCNT) whereas the dependency on the types of CNTs was observed in the 10 wt.% CNT loaded composites. The CNT/Nafion nanocomposites have a different morphology from the CNT mats: Nafion-wrapped-CNTs are connected to each other in the composites, whereas the pure CNTs are connected in the mats. The fact that the Seebeck coefficients of CNT/Nafion composites with higher CNT concentration (20 wt.%) are independent of the type of CNT is an indicator that energetic charges have sufficient energy to transport through the Nafion-wrapped CNTs. This interCNT resistance should be even higher than that in CNT mats. Resistance due to metallic CNT-semiconductor CNT junctions should not be affected. Therefore, the Seebeck coefficient of CNT/Nafion nanocomposites should be independent of the CNT type. However, in the composites with 10 wt.% CNT loading, the higher inter-CNT resistance of the Nafion-wrapped CNT could influence to the transport of the energetic charges thus the Seebeck coefficients of the composites. The lower CNT loaded (10 wt.%) composites exhibited relatively reduced values of Seebeck coefficients; especially the SWCNT/Nafion (10 wt.%) displayed much lower values of Seebeck coefficient (0.9 lV/K) than other samples. This result indicates that both the connectivity of CNTs and the inter-CNT resistance determine the Seebeck coefficients of the composites. The power factors (S2r) of the CNT/Nafion composites are shown in Fig. 3(c). Since the Seebeck coefficients were almost independent of the CNT type and the electrical conductivities were dependent on the type of CNT at the high CNT concentrations, the composites exhibiting higher electrical conductivities possessed higher power factors. The power factor of the FWCNT/ Nafion composites was as high as 1 lW/m-K2, whereas one-tenth of this power factor was observed in the SWCNT/Nafion composites. The thermal conductivity of the 5 wt.% FWCNT/Nafion composites was determined to be 0.3 W/m-K at room temperature. The thermal conductivity of CNT mats is around 30 W/m-K [26], whereas that of polymer in general is around 0.2 W/mK at room temperature. This suggests that weak van der Waals bonding in the Nafion significantly contributes to thermal transport. It has been previously reported that the thermal conductivity of CNT/polymer composites is relatively insensitive to the CNT concentration [4,5]. CNT/Nafion thermal conductivity is dominated by the polymer matrix, whereas the electrical transport is dominated by the CNTs themselves. In this scenario, electrical transport can be separated from thermal transport. While the estimated thermoelectric figure of merit of 0.001 for the CNT/Nafion films is not comparable to that of inorganic materials at this time, the work presented here demonstrates that separating electrical and thermal transport should serve as a scheme to increase the thermoelectric figure of merit.
4. Conclusions We investigated the thermoelectric properties of CNT/ Nafion nanocomposites using various types of CNTs. It was found that the electrical conductivities of the CNT/Nafion composites were primarily affected by the type and concentration of CNTs since the Nafion act as an electrically non-conducting matrix. For all three kinds of CNTs, both the electrical conductivity and the Seebeck coefficient increased as the concentration of CNTs was increased. While the electrical conductivity was dependent on the type of CNT, the behavior of the Seebeck coefficient was relatively insensitive of the CNT type. The Seebeck coefficient is related to the transport of energetic charges, whereas the electrical conductivity is related to the transport of all mobile charges [2,3]. Therefore, energetic carriers that contribute to the Seebeck coefficient would not see barriers such as the metallic CNT-semiconductor CNT junctions in SWCNT/Nafion nanocomposites at high enough concentration. The thermal conductivity of the composites was governed by the polymer matrix. As such, electrical and thermal transport can be separated. This study could prove quite useful in the design and synthesis of polymer composites for thermoelectric applications. Acknowledgements WK acknowledges financial support from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (grant numbers 20100000610 and 2010-0021487). SYJ thanks financial support by the research program 2011 of Kookmin University in Korea. References [1] M.S. Dresselhaus, G. Chen, M.Y. Tang, R.G. Yang, H. Lee, D.Z. Wang, et al., New directions for low-dimensional thermoelectric materials, Adv. Mater. 19 (8) (2007) 1043–1053. [2] C. Yu, Y.S. Kim, D. Kim, J.C. Grunlan, Thermoelectric behavior of segregated-network polymer nanocomposites, Nano Lett. 8 (12) (2008) 4428–4432. [3] D. Kim, Y. Kim, K. Choi, J.C. Grunlan, C. Yu, Improved thermoelectric behavior of nanotube-filled polymer composites with poly(3, 4ethylenedioxythiophene) poly(styrenesulfonate), ACS Nano 4 (1) (2010) 513–523. [4] C. Meng, C. Liu, S. Fan, A promising approach to enhanced thermoelectric properties using carbon nanotube networks, Adv. Mater. 22 (4) (2010) 535–539. [5] Q. Yao, L. Chen, W. Zhang, S. Liufu, X. Chen, Enhanced thermoelectric performance of single-walled carbon nanotubes/polyaniline hybrid nanocomposites, ACS Nano 4 (4) (2010) 2445–2451. [6] C.H. Yu, L. Shi, Z. Yao, D.Y. Li, A. Majumdar, Thermal conductance and thermopower of an individual single-wall carbon nanotube, Nano Lett. 5 (9) (2005) 1842–1846. [7] P. Kim, L. Shi, A. Majumdar, P.L. McEuen, Thermal transport measurements of individual multiwalled nanotubes, Phys. Rev. Lett. 87 (21) (2001) 215502. [8] T.D. Van, V.D. Dzung, T. Yamada, T.T. Bui, K. Hata, S. Sugiyama, Integration of SWNT film into MEMS for a micro-thermoelectric device, Smart Mater. Struct. 19 (7) (2010) 075003. [9] A. Thess, R. Lee, P. Nikolaev, H.J. Dai, P. Petit, J. Robert, et al., Crystalline ropes of metallic carbon nanotubes, Science 273 (5274) (1996) 483–487. [10] T.W. Ebbesen, H.J. Lezec, H. Hiura, J.W. Bennett, H.F. Ghaemi, T. Thio, Electrical conductivity of individual carbon nanotubes, Nature 382 (6586) (1996) 54–56.
Y. Choi et al. / Organic Electronics 12 (2011) 2120–2125 [11] H.J. Dai, E.W. Wong, C.M. Lieber, Probing electrical transport in nanomaterials: conductivity of individual carbon nanotubes, Science 272 (5261) (1996) 523–526. [12] W.J. Ma, L. Song, R. Yang, T.H. Zhang, Y.C. Zhao, L.F. Sun, et al., Directly synthesized strong, highly conducting, transparent singlewalled carbon nanotube films, Nano Lett. 7 (8) (2007) 2307–2311. [13] I.N. Mazov, V.L. Kuznetsov, S.I. Moseenkov, A.V. Ishchenko, A.I. Romanenko, O.B. Anikeeva, et al., Electrophysical and electromagnetic properties of pure MWNTs and MWNT/PMMA composite materials depending on their structure, Fullerenes, Nanotubes, Carbon Nanostruct. 18 (4–6) (2010) 505–515. [14] Y. Wang, H. Shan, R.H. Hauge, M. Pasquali, R.E. Smalley, A highly selective, one-pot purification method for single-walled carbon nanotubes, J. Phys. Chem. B 111 (6) (2007) 1249–1252. [15] A.W. Van Herwaarden, P.M. Sarro, Thermal sensors based on the Seebeck effect, Sens. Actuators 10 (3–4) (1986) 321–346. [16] J. Wang, M. Musameh, Y.H. Lin, Solubilization of carbon nanotubes by Nafion toward the preparation of amperometric biosensors, J. Am. Chem. Soc. 125 (9) (2003) 2408–2409. [17] C. Luo, X.L. Zuo, L. Wang, E.G. Wang, S.P. Song, J. Wang, et al., Flexible carbon nanotube-polymer composite films with high conductivity and superhydrophobicity made by solution process, Nano Lett. 8 (12) (2008) 4454–4458. [18] A. Hirsch, Functionalization of single-walled carbon nanotubes, Angew. Chem. Int. Ed. 41 (11) (2002) 1853–1859.
2125
[19] N. Karousis, N. Tagmatarchis, D. Tasis, Current progress on the chemical modification of carbon nanotubes, Chem. Rev. 110 (9) (2010) 5366–5397. [20] P. Singh, S. Campidelli, S. Giordani, D. Bonifazi, A. Bianco, M. Prato, Organic functionalisation and characterisation of single-walled carbon nanotubes, Chem. Soc. Rev. 38 (8) (2009) 2214–2230. [21] Z. Yao, C.L. Kane, C. Dekker, High-field electrical transport in singlewall carbon nanotubes, Phys. Rev. Lett. 84 (13) (2000) 2941–2944. [22] A. Buldum, J.P. Lu, Contact resistance between carbon nanotubes, Phys. Rev. B 63 (16) (2001). [23] Y.P. Sun, W. Wang, K.A.S. Fernando, Y. Lin, M.J. Meziani, L.M. Veca, et al., Metallic single-walled carbon nanotubes for conductive nanocomposites, J. Am. Chem. Soc. 130 (4) (2008) 1415–1419. [24] Y.K. Gun’ko, M.T. Byrne, Recent advances in research on carbon nanotube-polymer composites, Adv. Mater. 22 (15) (2010) 1672– 1688. [25] F.H. Gojny, M.H.G. Wichmann, B. Fiedler, I.A. Kinloch, W. Bauhofer, A.H. Windle, et al., Evaluation and identification of electrical and thermal conduction mechanisms in carbon nanotube/epoxy composites, Polymer 47 (6) (2006) 2036–2045. [26] J. Hone, M.C. Llaguno, N.M. Nemes, A.T. Johnson, J.E. Fischer, D.A. Walters, et al., Electrical and thermal transport properties of magnetically aligned single walt carbon nanotube films, Appl. Phys. Lett. 77 (5) (2000) 666–668.
Contents lists available at SciVerse ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Effect of the carbon nanotube type on the thermoelectric properties of CNT/Nafion nanocomposites Yongjoon Choi a, Yuhee Kim b, Sung-Geun Park a, Young-Gon Kim b, Bong June Sung c, Sung-Yeon Jang b,d,⇑, Woochul Kim a,⇑ a
School of Mechanical Engineering, Yonsei University, 262 Seongsanno, Seoul 120-749, Republic of Korea Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea c Department of Chemistry, Sogang University, Seoul 121-742, Republic of Korea d Department of Chemistry, Kookmin University, 861-1 Jeongneung-dong, Seoul 136-702, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 6 May 2011 Received in revised form 28 July 2011 Accepted 27 August 2011 Available online 13 September 2011 Keywords: Thermoelectric Carbon nanotube Nanocomposite Seebeck coefficient Power factor
a b s t r a c t The effect of different carbon nanotube (CNT) types on the thermoelectric performance of CNT/polymer nanocomposites was studied. Three different kinds of CNTs, single(SWCNTs), few- (FWCNTs) and multi-walled CNTs (MWCNTs), were effectively dispersed in an aqueous solution of Nafion. The electrical properties of the CNT/Nafion nanocomposites were primarily affected by the CNTs since the Nafion acts as an electrically nonconducting matrix, while the thermal conductivity of the nanocomposites was dominated by the Nafion mainly due to weak van der Waals interaction. In this way, electrical and thermal transport can be separated. In all three types of CNTs, both the electrical conductivity and Seebeck coefficient increased as the concentration of CNTs was increased. While the electrical conductivity depends on the type of CNT, the behavior of the Seebeck coefficient was relatively insensitive of the CNT type at high CNT loading. This indicates that high-energy-charges can participate in transport processes irrespective of the type of CNT. It is suggested that FWCNTs and MWCNTs are preferred over SWCNTs in CNT/Nafion nanocomposites for thermoelectric applications. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Much effort has been devoted to generating electricity from waste heat through the use of thermoelectric devices. In order to save waste heat energy with thermoelectric devices, they should be low cost while possessing a high thermoelectric figure of merit. The efficiency of thermoelectric devices is related to the thermoelectric figure of merit, ZT, defined as S2rT/k, where S denotes the Seebeck coefficient, r is the electrical conductivity, k is the thermal conductivity, and T is the absolute temperature. Good thermoelectric ⇑ Corresponding authors. Address: Department of Chemistry, Kookmin University, 861-1 Jeongneung-dong, Seoul 136-702, Republic of Korea. Tel./fax: +82 2 910 5768 (S.-Y. Jang), +82 2 2123 5816 (W. Kim). E-mail addresses: [email protected] (S.-Y. Jang), woochul@ yonsei.ac.kr (W. Kim). 1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2011.08.025
materials must have a high Seebeck coefficient for enhanced thermoelectricity, low electrical resistivity to minimize Joule heating, and low thermal conductivity to sustain large temperature gradient [1]. Typical thermoelectric materials have been inorganic semiconductors. However, some thermoelectric semiconductor materials (e.g., Te, Bi, Pb) are so toxic and expensive that their use on a large scale is not practical. To address this issue, organic materials could be an alternative candidate for thermoelectric materials because of their low cost and high processability. Polymers are intrinsically poor thermal conductors, which makes them ideal for use as thermoelectrics. However, their low electrical conductivity, Seebeck coefficient, and stability have hampered their use in thermoelectric applications. CNT/Polymer composite technology and the development of electrically conducting polymers with reasonable air stability have made it
Y. Choi et al. / Organic Electronics 12 (2011) 2120–2125
possible to bring organic materials into degenerate semiconductor or metallic regimes by incorporating relatively small amounts of conductive fillers [2]. Thus, developing polymer-based composites using thermoelectric and conducting fillers is a logical and promising strategy for creating a new class of thermoelectric materials with sufficient mechanical properties, including flexibility. In 2008, Yu et al. [2] fabricated carbon nanotube (CNT)/ PVAc composites and investigated their thermoelectric properties upon CNT addition. The researchers showed that the electrical conductivity can be dramatically increased by creating a network of CNTs in the composite, while the thermal conductivity and Seebeck coefficient remained relatively insensitive to the filler concentration. A thermoelectric figure of merit (ZT) greater than 0.006 was achieved with a CNT concentration of 20 wt.% at room temperature. The study by Yu et al. was the first investigation into the use of CNT/polymer composites as thermoelectric materials. In 2010, Kim et al. [3] reported a maximum ZT of 0.02 at room temperature for a polymer composite containing 35 wt.% SWCNTs. In the study, the main polymer matrix was copolymer latex containing vinyl acetate and ethylene. The stabilizer that was used to disperse the CNTs in water was PEDOT:PSS, a widely used conducting polymer, which was doped with DMSO. Electrically insulating gum Arabic (GA) also was studied for a comparison. Composites were made with two different CNTs (XM-CNT and SWCNT) at 2–15 wt.% CNT concentrations and 1:1–1:4 ratios of CNT to PEDOT:PSS; the structures were dried at room temperature and/or 80 °C. In the preparation procedure, CNTs were added to the conducting polymer as fillers to impart more electrical conductivity and ensure a high thermoelectric figure of merit. Meng et al. [4] and Yao et al. [5] fabricated composites consisting of a CNT network as the main matrix and polyaniline (PANI), a conductive polymer, as a coating additive to the CNTs. Meng et al. reported significantly enhanced Seebeck coefficients and power factors in the multi-walled CNTs (MWCNT)/PANI composites, which were several times larger than either of their individual components. The best ZT value obtained was 0.003 for a 15.8 wt.% MWCNT/PANI composite. Yao et al. fabricated single-walled CNTs (SWCNT)/PANI composites with a maximum ZT of 0.004 at 41.4 wt.% SWCNT. In the above cited studies, the effect of the CNT type on the thermoelectric performance of the CNT/polymer composite was not investigated. It has been known that different types of CNTs exhibit different electrical conductivities and Seebeck coefficients. At 300 K, the Seebeck coefficients of individual SWCNTs and MWCNTs are about 42 lV/K [6] and 82 lV/K [7], respectively. The Seebeck coefficients of SWCNT and MWCNT films [4,8] are comparable and have values of around 20–25 lV/K. With regard to electrical conductivity, metallic SWCNT rope exhibits values around 10,000–30,000 S/cm [9], while electrical conductivities of approximately 200–2000 S/cm have been obtained for individual MWCNTs [10,11]. For films or networks composed of CNTs, electrical conductivity becomes one or two orders of magnitude lower than that of individual CNTs and varies depending on the degree of aggregation. While the electrical conductivity of SWCNT films is about
2121
2000 S/cm [12], that of MWCNT mat is approximately 10 S/cm [13]. Given the aforementioned findings, the CNT type would directly influence the thermoelectric performance of their composites. In this study, the thermoelectric properties of CNT/Nafion nanocomposite films with three different types of CNTs were investigated. The CNT/Nafion nanocomposites were prepared from aqueous solutions of CNT/Nafion and SWCNTs, few-walled CNTs (FWCNTs), and MWCNTs were employed in the structures. A water soluble polymer, Nafion, which also contains hydrophobic nature in its backbone, has exhibited good affinity with CNTs and provides enhanced solubility in an aqueous solution without additional surfactants. As a result, films on a substrate were readily fabricated by the doctor blading method. The electrical properties of the CNT/Nafion nanocomposites were primarily affected by the CNTs since the Nafion acts as an electrically non-conducting matrix, while the thermal conductivity of the composites was dominated by the Nafion matrix; electrical and thermal transport are separated. The electrical conductivity and Seebeck coefficient of the nanocomposites were measured as a function of the type and concentration of CNTs at room temperature. For all three kinds of CNTs, both the electrical conductivity and the Seebeck coefficient increased as the concentration of CNTs was increased. While the electrical conductivity was dependent on the type of CNT, the behavior of the Seebeck coefficient was relatively independent of the CNT type. This indicates that high-energy-charges can participate in transport processes irrespective of the type of CNT. 2. Experimental 2.1. Preparation of CNT/Nafion nanocomposites SWCNT (ASA-100F), FWCNT (CMP-310F), and MWCNT (CM-95) that are purchased from Hanwha Nanotech were purified according to the ‘‘Fenton Chemistry Method’’ reported by Wang et al. [14] Briefly, pristine CNTs were stirred in 1 N HCl and H2O2 (7/3 by weight) at 60 °C for 4 h. The resulting CNTs were filtered and washed with deionized water until the filtrated solution had a pH of 7. The pristine CNTs were heated for 4 h at 292 °C before the Fenton Chemistry purification in order to remove amorphous carbon impurities. The purity of the CNTs, as determined by thermogravimetric analysis (TGA), was >95%. The purified CNTs were mixed with a Nafion solution in deionized (DI) water (5% by weight, Aldrich). The solution was then briefly sonicated (30 min) and stirred overnight. The CNT/ Nafion composite solutions were cast onto glass plates that were cleaned by sequential washing with ethanol and acetone under sonication. The composite solution was then doctor-bladed to form the films. The thickness of the film is 2–5 lm. 2.2. Characterization The four-probe method was used to measure the electrical conductivities of the CNT/Nafion nanocomposites. A
2122
Y. Choi et al. / Organic Electronics 12 (2011) 2120–2125
Keithley 2400 was employed as a source meter. The Seebeck coefficient was extracted by applying a temperature gradient along the sample while measuring the thermoelectric voltage. Two T-type thermocouples were used to measure temperatures and voltages simultaneously. Two thermoelectric devices, one as a heat source and another as a heat sink, were used to create a temperature gradient across the samples. The measured Seebeck coefficient was calibrated due to the Seebeck coefficient of the Cu wire of the T-type thermocouple, which is 1.7 lV/K [15]. The thermal conductivity of the samples was extracted by measuring the thermal diffusivity using the laser flash technique (Netzsch LFA 457 MicroFlash) and the specific heat as measured with a differential scanning calorimeter (Netzsch Differential Scanning Calorimeter (DSC) 200).
3. Results and discussion For the preparation of the CNT/polymer composites, an effective dispersion of CNTs within the polymer matrix is quite a challenge since CNTs have poor solubility due to strong van der Waals interactions. Nafion, a water soluble perfluorosulfonated polymer, has been successfully used to solubilize CNTs in alcoholic solutions so as to form homogeneous composites [16,17]. The hydrophobic main chain of Nafion can wrap the surface of CNTs, while the polar side chain can provide solubility in polar solvents. A photo of a diluted SWCNT/Nafion composite (5 wt.% SWCNT in Nafion) solution in DI-water is shown in Fig. 1(a). A stable homogeneous dispersion of the composites is clearly evident. Stability was reasonably maintained at various SWCNT concentrations. Photos of aqueous SWCNT/Nafion composite solutions over a range of SWCNT/Nafion weight ratios (1/9–5/5) are shown in Fig. 1(b). The dispersion stability was intact after one month, indicating that the Nafion can efficiently solubilize a significant amount of CNTs for a long period of time. The aqueous dispersion and the solution stability of FWCNT and MWCNT-based composites were even more improved because SWCNTs have a stronger tendency toward self-aggregation [18–20]. CNT/ Nafion composite-based films were prepared using the doctor blade method, which is popular for micron-thick polymer film fabrication. The fabrication of crack-free CNT/Nafion composite films via a solution process has been demonstrated by Luo et al. [17]. The resulting films exhibited enhanced mechanical properties with strong flexibility and high electrical conductivity. A doctor-bladed SWCNT/Nafion composite film (5 lm thick) on a glass plate is shown in Fig. 1(c). The surface of the films was relatively smooth without noticeable cracks. Scanning electron microscopy (SEM) images of a 7 wt.% SWCNT/Nafion composite and a 50 wt.% SWCNT/Nafion composite are shown in Fig. 2(a) and (b), respectively. The SWCNTs were homogeneously dispersed within the Nafion matrix, which is indicative of effective wrapping of the SWCNTs by the Nafion in an aqueous solution. As the SWCNT/Nafion ratio was increased, more SWCNTs were observed in the SEM images (Fig. 2(a) and (b)), which indicates that the SWCNTs were homogeneously dispersed within the composite films.
Fig. 1. Photos of (a) a diluted SWCNT/Nafion (5 wt.% SWCNT in Nafion) solution in deionized (DI) water, (b) a range of SWCNT/Nafion composite (1/9–5/5 by weight ratio) solutions in DI water one month after preparation, (c) a SWCNT/Nafion composite film prepared via the doctor blade method. The film was relatively uniform with a thickness of 5 lm.
The electrical conductivities of the CNT/Nafion nanocomposites are shown in Fig. 3(a). The electrical conductivities of all of the composites increased as the CNT concentration was increased. These conductivities saturated at 30 wt.% CNT loading in Nafion. Note that even the lowest CNT concentration (10 wt.%) in the composites is much higher than the conventional percolation threshold of CNT/polymer composites ( a few %), thus the electrical conductivity at 10 wt.% CNT loading was not significantly lower than in the 30 wt.% CNT-loaded films (they were the same order of magnitude), which indicates the efficient percolation of charges. The optimized electrical conductivity of the CNT/Nafion composites (13 S/cm) was similar to the values obtained in a previous report (12.5 S/cm) [17], although the CNT loading in that study was 10 wt.% more than in our composites. Such a result indicates that our composites contained well-dispersed and efficiently interconnected CNTs within a matrix of Nafion. The SWCNT/Nafion composites exhibited relatively lower conductivities when compared to the FWCNT and MWCNT-based composites. For fully connected CNT networks, the electrical conductivity can be somewhat limited by the CNT–CNT contact resistance [21,22]. Taking
Y. Choi et al. / Organic Electronics 12 (2011) 2120–2125
2123
Fig. 2. Scanning electron microscopy (SEM) images of SWCNT/Nafion composites: (a) 7 wt.% and (b) 50 wt.% of SWCNTs in Nafion.
into account that the conductivity of individual CNTs is dependent on various factors such as the surface functionality, tube diameter, length, wall thickness, and bundle size, the conductivity of the composites will be governed by the electrical properties of the CNTs themselves and the contact resistance among the Nafion-wrapped CNTs. In the case of a SWCNT-based mat or composite films, the conductivity significantly varies depending on the ratio of metallic to semiconducting tubes. This is because the contact resistance between semiconducting and metallic junctions is significantly greater than that in metallic–metallic junctions [23]. Since our SWCNTs possessed a typical composition (1:2 for metallic:semiconducting) [23], the higher electrical conductivities of the FWCNT and MWCNT-based composites are not surprising and are consistent with the findings of other reports [24,25]. Based on Fig. 3(a), we deduce that a CNT concentration of 30% should be enough to obtain an optimum electrical conductivity. The highest observed electrical conductivity in this study was 13 S/cm from the 30 wt.% MWCNT/Nafion composites. The Seebeck coefficients of the CNT/Nafion nanocomposites with respect to the CNT concentration are shown in Fig. 3(b). All of the films, irrespective of the type and concentration of CNTs, exhibited p-type conduction, which means that the majority of charge carriers are holes. When charge carriers from the thermoelectric effect are transported in
Fig. 3. (a) Electrical conductivity [S/cm], (b) Seebeck coefficient [lV/K], and (c) power factor [W/m-K2] versus CNT weight percent.
conducting networks (CNTs in this study) within a non-conducting matrix, such as Nafion, the Seebeck coefficient of the composites should depend on the Seebeck coefficient of the networked materials. As mentioned previously, although the Seebeck coefficients of individual CNTs are dependent on the type of CNT (the Seebeck coefficients of the SWCNTs and MWCNTs are about 42 lV/K [6] and 82 lV/K [7], respectively), the Seebeck coefficients of SWCNT and MWCNT mats [4,24] are comparable to each other and are approximately 20–25 lV/K. The Seebeck coefficient is related to the transport of energetic charges, whereas the electrical conductivity is related to the transport of all mobile charges [2,3]. Charge transport in CNT mats is hindered by junctions such as metallic CNT-semiconductor CNT junctions, metallic CNT–metallic CNT junctions, etc. The fact that the Seebeck coefficients of CNT
2124
Y. Choi et al. / Organic Electronics 12 (2011) 2120–2125
mats are independent of the CNT type suggests that energetic charges transported regardless of the junction types in SWCNT mats. In our nanocomposites, behavior similar to that seen in CNT-based mats was observed. The Seebeck coefficients of the CNT/Nafion composites which contains >20 wt.% of CNTs ranged from 25–35 lV/K, and there was no significant difference among the types of CNTs (SWCNT, FWCNT, MWCNT) whereas the dependency on the types of CNTs was observed in the 10 wt.% CNT loaded composites. The CNT/Nafion nanocomposites have a different morphology from the CNT mats: Nafion-wrapped-CNTs are connected to each other in the composites, whereas the pure CNTs are connected in the mats. The fact that the Seebeck coefficients of CNT/Nafion composites with higher CNT concentration (20 wt.%) are independent of the type of CNT is an indicator that energetic charges have sufficient energy to transport through the Nafion-wrapped CNTs. This interCNT resistance should be even higher than that in CNT mats. Resistance due to metallic CNT-semiconductor CNT junctions should not be affected. Therefore, the Seebeck coefficient of CNT/Nafion nanocomposites should be independent of the CNT type. However, in the composites with 10 wt.% CNT loading, the higher inter-CNT resistance of the Nafion-wrapped CNT could influence to the transport of the energetic charges thus the Seebeck coefficients of the composites. The lower CNT loaded (10 wt.%) composites exhibited relatively reduced values of Seebeck coefficients; especially the SWCNT/Nafion (10 wt.%) displayed much lower values of Seebeck coefficient (0.9 lV/K) than other samples. This result indicates that both the connectivity of CNTs and the inter-CNT resistance determine the Seebeck coefficients of the composites. The power factors (S2r) of the CNT/Nafion composites are shown in Fig. 3(c). Since the Seebeck coefficients were almost independent of the CNT type and the electrical conductivities were dependent on the type of CNT at the high CNT concentrations, the composites exhibiting higher electrical conductivities possessed higher power factors. The power factor of the FWCNT/ Nafion composites was as high as 1 lW/m-K2, whereas one-tenth of this power factor was observed in the SWCNT/Nafion composites. The thermal conductivity of the 5 wt.% FWCNT/Nafion composites was determined to be 0.3 W/m-K at room temperature. The thermal conductivity of CNT mats is around 30 W/m-K [26], whereas that of polymer in general is around 0.2 W/mK at room temperature. This suggests that weak van der Waals bonding in the Nafion significantly contributes to thermal transport. It has been previously reported that the thermal conductivity of CNT/polymer composites is relatively insensitive to the CNT concentration [4,5]. CNT/Nafion thermal conductivity is dominated by the polymer matrix, whereas the electrical transport is dominated by the CNTs themselves. In this scenario, electrical transport can be separated from thermal transport. While the estimated thermoelectric figure of merit of 0.001 for the CNT/Nafion films is not comparable to that of inorganic materials at this time, the work presented here demonstrates that separating electrical and thermal transport should serve as a scheme to increase the thermoelectric figure of merit.
4. Conclusions We investigated the thermoelectric properties of CNT/ Nafion nanocomposites using various types of CNTs. It was found that the electrical conductivities of the CNT/Nafion composites were primarily affected by the type and concentration of CNTs since the Nafion act as an electrically non-conducting matrix. For all three kinds of CNTs, both the electrical conductivity and the Seebeck coefficient increased as the concentration of CNTs was increased. While the electrical conductivity was dependent on the type of CNT, the behavior of the Seebeck coefficient was relatively insensitive of the CNT type. The Seebeck coefficient is related to the transport of energetic charges, whereas the electrical conductivity is related to the transport of all mobile charges [2,3]. Therefore, energetic carriers that contribute to the Seebeck coefficient would not see barriers such as the metallic CNT-semiconductor CNT junctions in SWCNT/Nafion nanocomposites at high enough concentration. The thermal conductivity of the composites was governed by the polymer matrix. As such, electrical and thermal transport can be separated. This study could prove quite useful in the design and synthesis of polymer composites for thermoelectric applications. Acknowledgements WK acknowledges financial support from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (grant numbers 20100000610 and 2010-0021487). SYJ thanks financial support by the research program 2011 of Kookmin University in Korea. References [1] M.S. Dresselhaus, G. Chen, M.Y. Tang, R.G. Yang, H. Lee, D.Z. Wang, et al., New directions for low-dimensional thermoelectric materials, Adv. Mater. 19 (8) (2007) 1043–1053. [2] C. Yu, Y.S. Kim, D. Kim, J.C. Grunlan, Thermoelectric behavior of segregated-network polymer nanocomposites, Nano Lett. 8 (12) (2008) 4428–4432. [3] D. Kim, Y. Kim, K. Choi, J.C. Grunlan, C. Yu, Improved thermoelectric behavior of nanotube-filled polymer composites with poly(3, 4ethylenedioxythiophene) poly(styrenesulfonate), ACS Nano 4 (1) (2010) 513–523. [4] C. Meng, C. Liu, S. Fan, A promising approach to enhanced thermoelectric properties using carbon nanotube networks, Adv. Mater. 22 (4) (2010) 535–539. [5] Q. Yao, L. Chen, W. Zhang, S. Liufu, X. Chen, Enhanced thermoelectric performance of single-walled carbon nanotubes/polyaniline hybrid nanocomposites, ACS Nano 4 (4) (2010) 2445–2451. [6] C.H. Yu, L. Shi, Z. Yao, D.Y. Li, A. Majumdar, Thermal conductance and thermopower of an individual single-wall carbon nanotube, Nano Lett. 5 (9) (2005) 1842–1846. [7] P. Kim, L. Shi, A. Majumdar, P.L. McEuen, Thermal transport measurements of individual multiwalled nanotubes, Phys. Rev. Lett. 87 (21) (2001) 215502. [8] T.D. Van, V.D. Dzung, T. Yamada, T.T. Bui, K. Hata, S. Sugiyama, Integration of SWNT film into MEMS for a micro-thermoelectric device, Smart Mater. Struct. 19 (7) (2010) 075003. [9] A. Thess, R. Lee, P. Nikolaev, H.J. Dai, P. Petit, J. Robert, et al., Crystalline ropes of metallic carbon nanotubes, Science 273 (5274) (1996) 483–487. [10] T.W. Ebbesen, H.J. Lezec, H. Hiura, J.W. Bennett, H.F. Ghaemi, T. Thio, Electrical conductivity of individual carbon nanotubes, Nature 382 (6586) (1996) 54–56.
Y. Choi et al. / Organic Electronics 12 (2011) 2120–2125 [11] H.J. Dai, E.W. Wong, C.M. Lieber, Probing electrical transport in nanomaterials: conductivity of individual carbon nanotubes, Science 272 (5261) (1996) 523–526. [12] W.J. Ma, L. Song, R. Yang, T.H. Zhang, Y.C. Zhao, L.F. Sun, et al., Directly synthesized strong, highly conducting, transparent singlewalled carbon nanotube films, Nano Lett. 7 (8) (2007) 2307–2311. [13] I.N. Mazov, V.L. Kuznetsov, S.I. Moseenkov, A.V. Ishchenko, A.I. Romanenko, O.B. Anikeeva, et al., Electrophysical and electromagnetic properties of pure MWNTs and MWNT/PMMA composite materials depending on their structure, Fullerenes, Nanotubes, Carbon Nanostruct. 18 (4–6) (2010) 505–515. [14] Y. Wang, H. Shan, R.H. Hauge, M. Pasquali, R.E. Smalley, A highly selective, one-pot purification method for single-walled carbon nanotubes, J. Phys. Chem. B 111 (6) (2007) 1249–1252. [15] A.W. Van Herwaarden, P.M. Sarro, Thermal sensors based on the Seebeck effect, Sens. Actuators 10 (3–4) (1986) 321–346. [16] J. Wang, M. Musameh, Y.H. Lin, Solubilization of carbon nanotubes by Nafion toward the preparation of amperometric biosensors, J. Am. Chem. Soc. 125 (9) (2003) 2408–2409. [17] C. Luo, X.L. Zuo, L. Wang, E.G. Wang, S.P. Song, J. Wang, et al., Flexible carbon nanotube-polymer composite films with high conductivity and superhydrophobicity made by solution process, Nano Lett. 8 (12) (2008) 4454–4458. [18] A. Hirsch, Functionalization of single-walled carbon nanotubes, Angew. Chem. Int. Ed. 41 (11) (2002) 1853–1859.
2125
[19] N. Karousis, N. Tagmatarchis, D. Tasis, Current progress on the chemical modification of carbon nanotubes, Chem. Rev. 110 (9) (2010) 5366–5397. [20] P. Singh, S. Campidelli, S. Giordani, D. Bonifazi, A. Bianco, M. Prato, Organic functionalisation and characterisation of single-walled carbon nanotubes, Chem. Soc. Rev. 38 (8) (2009) 2214–2230. [21] Z. Yao, C.L. Kane, C. Dekker, High-field electrical transport in singlewall carbon nanotubes, Phys. Rev. Lett. 84 (13) (2000) 2941–2944. [22] A. Buldum, J.P. Lu, Contact resistance between carbon nanotubes, Phys. Rev. B 63 (16) (2001). [23] Y.P. Sun, W. Wang, K.A.S. Fernando, Y. Lin, M.J. Meziani, L.M. Veca, et al., Metallic single-walled carbon nanotubes for conductive nanocomposites, J. Am. Chem. Soc. 130 (4) (2008) 1415–1419. [24] Y.K. Gun’ko, M.T. Byrne, Recent advances in research on carbon nanotube-polymer composites, Adv. Mater. 22 (15) (2010) 1672– 1688. [25] F.H. Gojny, M.H.G. Wichmann, B. Fiedler, I.A. Kinloch, W. Bauhofer, A.H. Windle, et al., Evaluation and identification of electrical and thermal conduction mechanisms in carbon nanotube/epoxy composites, Polymer 47 (6) (2006) 2036–2045. [26] J. Hone, M.C. Llaguno, N.M. Nemes, A.T. Johnson, J.E. Fischer, D.A. Walters, et al., Electrical and thermal transport properties of magnetically aligned single walt carbon nanotube films, Appl. Phys. Lett. 77 (5) (2000) 666–668.