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Mechanical and electrical properties of carbon nanotube ribbons
Chemical Physics Letters 365 (2002) 95–100 www.elsevier.com/locate/cplett
Mechanical and electrical properties of carbon nanotube ribbons Yan-Hui Li
a,*
, Jinquan Wei a, Xianfeng Zhang a, Cailu Xu a, Dehai Wu a, Li Lu b, Bingqing Wei c
a
c
Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China b Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180, USA Received 31 July 2002; in final form 7 September 2002
Abstract We have measured the YoungÕs modulus of long aligned carbon nanotube ribbons using a special stress–strain puller designed for whisker-like materials. The YoungÕs modulus of the graphitized carbon nanotube ribbons is about 60 GPa, which is 2.5 times higher than that of the as-grown ribbons. This suggests that the graphitization is an effective way to improve the mechanical properties of the ribbons. Most of the measured ribbons are semiconductor and their resistivities are in the range of 4.4–12:6 104 X cm. An interesting phenomenon appeared in one ribbon is that the electrical property of the ribbon changes from metallic to non-metallic with decreasing the temperature. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Carbon nanotubes (CNTs) have been predicted to possess exceptional mechanical [1–4] and unique electrical properties [5], which could have great potential applications as structural and intellectual materials. But too small individual CNTs make it difficult further exploit their various properties and applications, it has been the dream and challenge to prepare long CNTs or their macrostructures. Rinzler et al. [6] first prepared bucky paper con-
*
Corresponding author. Fax: +8601062782413. E-mail address: [email protected] (Y.-H. Li).
sisted of a free-standing mat of entangled singlewall CNTs and estimated their elastic modulus of 1.2 GPa [7]. Pan et al. [8] prepared very long aligned multiwalled CNTs and measured their YoungÕs moduli and the tensile strength to be 0:45 0:23 TPa and 1:72 0:64 GPa, respectively. Recently, Vigolo et al. [9] fabricated macroscopic fibers and ribbons of oriented CNTs through injecting nanotube dispersions into a flowing stream of polyvinylalcohol solution. The YoungÕs modulus of the fibers varied between 9 and 15 GPa. Electrical properties of the CNTs are another fascinating research field. They can be either metallic, non-metallic or insulating, depending on their diameter and chirality. Many efforts have been devoted to study the electrical properties of
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 4 3 4 - 3
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Y.-H. Li et al. / Chemical Physics Letters 365 (2002) 95–100
individual CNTs [10] and their macrostructures [9,11–13]. Ebbesen et al. [10] experimentally proved that individual CNT can either be metallic or non-metallic which was modified by the interlayer interactions of the CNTs. The resistivities of the bucky paper were also measured and they were metallic for the acid-treated CNTs, they changed from metallic behavior into non-metallic behavior after vacuum-annealing [11]. Ma et al. [13] fabricated a soft sinter of pure CNTs by hot-pressing under the conditions of 2273 K/25 MPa/Ar/1 h. The resistivity of the sinter was about ð2–3Þ 104 X cm and increased with decreasing temperature. Resistivity of VigoloÕs fibers [9] was about 0.1 X cm at room temperature and the fibers exhibited non-metallic behavior with decreasing temperature. In this Letter, we prepared long aligned CNT ribbons through self-organization and graphitized them at 2200 °C. The mechanical properties of the as-grown and graphitized ribbons were measured using a specially designed stress–strain puller. The electrical property of the as-grown ribbons was also determined using a four-point method.
2. Experimental Long CNT ribbons (100 mm) were prepared by heating oxidized acid-treated nanotubes at 100 °C [14]. The ribbons are about 50–140 lm wide, 4–40 lm thick and 100 mm long (Fig. 1a). SEM images show that the CNTs consisted of the ribbons align along the direction in length (Fig. 1b). The CNT ribbons from the same preparation batch were used in the experiments. Parts of ribbons were graphitized at 2200 °C for 2 h under Ar atmosphere with pressure of 0.5 MPa. The YoungÕs moduli of the as-grown and graphitized ribbons were measured using a specially designed stress– strain puller. A diagram of the stress–strain puller device is shown in Fig. 2, its principle was described in detail elsewhere [15]. In brief, one end of the sample is mounted on a movable rod suspended by two leaf springs and the other is fixed. A static, uniform force is applied to the sample via a coaxial electro-magnet acting on permanent magnets attached to the movable rod and dis-
Fig. 1. SEM image of a ribbon showing (a) its rectangular cross-section; (b) the alignment of the CNTs (arrow points to the length direction of the ribbon).
placements are measured capacitively. YoungÕs modulus, E, is given by the slope of a stress–strain curve: E ¼ Dr=De, where r is the uniaxial stress and e is the strain. In order to observe the structure change of the graphitized ribbons, the ribbons were embedded into an epoxy mould. After completely hardening of the epoxy with CNT ribbons, slices with thickness about 25–60 nm were cut using an ultramicrotomy (LKB-2088, ULTROTOME-V) with a diamond knife. The cutting planes were controlled parallel to the length of the ribbons. The slices were fixed on a holey copper grid and examined with a JEOL-2010 transmission electron microscope. Electrical properties of the as-grown ribbons were measured using a four-point resistivity method.
Y.-H. Li et al. / Chemical Physics Letters 365 (2002) 95–100
Fig. 2. A diagram of the stress–strain device and the mounted nanotube ribbon.
3. Results and discussion The force applied to the as-grown ribbon (Fribbon ) by magnets versus the displacement of the plates is shown in Fig. 3a. The Fribbon is given by the following equation: Fribbon ¼ Fmagnet KDd, where Fmagnet is the force applied by the magnets, K is the spring constant of the device ( ¼ 1.55 mN/lm), the ribbon is straight and there is no free displacement
97
of the rod, so Dd equals to the displacement of the ribbon and the force of KDd is produced by the spring. The device is designed to measure whiskerlike samples and produces a little force. So the ribbon only has a smaller displacement and cannot fracture. Although the data scatter in a broad scope, the linear relation between Fribbon and Dd can be regarded as the elastic deformation of the ribbon and can be used to determine the E of the ribbon roughly. A stress–strain plot, the slope of which is Y, is shown in Fig. 3b for the as-grown ribbon. The value of Y we determine for the asgrown ribbon is about 24 GPa. The result is much lower than the previous reported values [1,2]. The value is normal considered the formation mechanism of the ribbons. Firstly, the ribbons are prepared at lower temperature (100 °C). The driving force for making thousands of nanotubes aggregate into long ribbons is mainly van der Waals force. The as-grown ribbons formed by van der Waals force which can be dispersed in water also suggests the weak coalescent force existed in the ribbons. The other important factor affected the selforganization of the CNTs into ribbons is functional groups introduced by acid oxidation. Previous works reported that many functional groups, such as hydroxyl (–OH), carboxyl (–COOH) and carbonyl (> C@O), can be attached both on the surface and on the open tip of the CNTs by oxidation with acid [16,17]. The FT-IR spectra (Fig. 4) show that the peak at 1650 cm1 may be due to contaminating water and peak at
Fig. 3. YoungÕs modulus test results recorded on the as-grown ribbon with the following characteristics: L ¼ 2:40 mm, A ¼ 4200 lm2 . (a) The force applied to the ribbon versus the displacement of the ribbon. (b) The uniaxial stress applied to the as-grown ribbon versus the strain (elongation) of the ribbon. The slope yields a value for YoungÕs modulus of as-grown ribbon of 24 GPa.
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Fig. 4. FT-IR spectra of (a) as-grown CNTs; (b) as-grown CNT ribbons; (c) graphitized CNT ribbons.
1220 cm1 is assigned to the carbon skeleton [18]. The signature of > C@O functional groups is evident at 1780 cm1 and –OH functional groups appear at 3430 cm1 in curve after oxidation (curve b). The increased strength of peaks and the new peaks appeared on the acid-oxidized sample suggested that oxidation with acid introduced great amounts of functional groups on the surfaces of CNTs compared with as-grown CNTs (curve a). The polycondensation may take place through the functional groups on the CNTs as shown in Fig. 5. In reaction, each CNTs can be seen as a monomer and they can form nanotube ribbons just like a typical polycondensation reaction in which water molecular are split out. The polycondensation may offer another driving force forming the ribbons and this may be proved indirectly by what the ribbons cannot be formed using unoxidized CNTs. Secondly, the nanotubes consisted of the ribbons are not continuously, each nanotubes is only several micrometer long. So the YoungÕs modulus of the as-grown ribbon is lower than the theoretical value of CNTs. In order to improve the strength of the ribbons, the as-grown ribbons were graphitized at 2200 °C for 2 h in Ar atmosphere (0.5 MPa). The crosssections of the ribbons decrease obviously after graphitization and the density of the ribbon increases from 1.1 to 1.5 g/cm3 correspondingly. Fig. 6a shows a plot of the force applied to the
Fig. 5. The diagram showing the driving force making CNTs aggregate into ribbons (the black arrows represent of van der Waals forces).
graphitized ribbons versus the displacement of the ribbon. When the Fribbon –Dd curve is transformed into a r–e curve (Fig. 6b), the value of Y, 60 GPa, for graphitized ribbon can be determined from the slope of r–e curve. The YoungÕs modulus of the graphitized ribbons is higher than that of the asgrown ribbons. High temperature graphitization of CNTs can reorder its layered structures and reduce the wall defects [19,20]. The CNTs used for preparing the ribbons come from decomposition of hydrocarbons using chemical vapor deposition method and have many impurities and inherent defects, which decrease mechanical property seriously. Graphitization of ribbons at 2200 °C cannot only reduce the defects on the surfaces of CNTs, but also weld the adjacent CNTs together through re-orgnization of carbon atoms on the outer layers of CNTs, so the YoungÕs modulus of the graphitized ribbons improved greatly. Fig. 7 shows the HRTEM images of the graphitized ribbons. It can be seen that the outer layers of the four CNTs were connected together, the HRTEM image was not so clear because the slices were covered with a layer of epoxy.
Y.-H. Li et al. / Chemical Physics Letters 365 (2002) 95–100
99
Fig. 6. YoungÕs modulus test results recorded on the graphitized ribbon with the following characteristics: L ¼ 2:55 mm, A ¼ 2000 lm2 . (a) The force applied to the ribbon versus the displacement of the ribbon. (b) The uniaxial stress applied to the graphitized ribbon versus the strain (elongation) of the ribbon. The slope gives a value for YoungÕs modulus of the graphitized ribbon of 60 GPa.
All of above results suggest that the interactions among the CNTs consisted of the ribbons become stronger relative to the as-grown ribbons, so the YoungÕs modulus of the graphitized ribbons is improved and 2.5 times higher than that of the asgrown ribbons. The electrical properties of five as-grown ribbons were measured using four-point resistivity (q) method. Four ribbons are semiconductor and have negative dq=dT from 4 to 300 K (Fig. 8a). The resistivities of the ribbons are in the range of 4.4–12:6 104 X cm at room temperature. Elec-
Fig. 7. HRTEM images of the connected CNTs. Four arrows point to the inner cavities of four CNTs.
On the other hand, the decrease of the crosssection and the increase the density indicate that the CNTs tend to compact tighter and the van der Waals forces between CNTs increase correspondingly. At this time the functional groups lost their bonding role because high temperature graphitization can eliminate the functional groups on the surfaces of the CNTs. This is strongly supported by the FT-IR spectra in Fig. 4 (curve c). It can be seen that most of peaks of the spectra for graphitized ribbons disappeared and the strength of –OH functional groups appeared at 3430 cm1 also decreased greatly.
Fig. 8. Resistivity versus temperature of the ribbons with different cross-section: (a) 60 12 lm2 ; (b) 100 20 lm2 ).
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trical property of one ribbon presents an interesting phenomenon, which is similar to the results of single-wall CNT ropes [21]. It can be seen the qðT Þ of the ribbon increases linearly with temperature from 260 to 300 K, and positive dq=dT indicates the ribbon is metallic (Fig. 8b). This behavior changes when the temperature decreases below 260 K. The ribbon has negative dq=dT and is nonmetallic until the temperature decreases to 4 K. This result has never been reported on multiwall CNTs or their macrostructures. The reason why the ribbons have different electrical properties in the same preparation batch is still unclear. But it is already known that individual CNT can be either metallic or non-metallic and the bulk properties of the nanotubes vary significantly depending on their preparing method, perfection of structure, diameter and chirality. The ultimate electrical property of the ribbons may be integrated effects of densely aligned CNTs. Furthermore, we cannot rule out the possible effects of the structural defects and impurities on the electrical property.
4. Conclusions The YoungÕs moduli of the as-grown and graphitized CNT ribbons have been directly measured using a stress–strain puller. The binding force of the as-grown ribbons is offered mainly by van der Waals force and covalent band formed by polycondensation of the functional groups on the CNTs. So the YoungÕs modulus of as-grown CNT ribbon is only about 24 GPa. After graphitization the YoungÕs modulus was improved greatly and reached to 60 GPa. This suggests that graphitization may be an effective method to improve the mechanical property of the ribbons. The resistivities of the ribbons at room temperature are 4.4– 12:6 104 X cm. With decreasing the temperature most of the ribbons present semiconductive behavior. While one ribbon is metallic at the temperature between 260 and 300 K and changes into non-metallic bellow 260 K. This may be the integrating effects of the pristine nature of CNTs and interaction between CNTs and impurities.
References [1] B.I. Yakobson, C.J. Brabec, J. Bernholc, Phys. Rev. Lett. 76 (1996) 2511. [2] M.M. Treacy, T.W. Ebbesen, J.M. Gibson, Nature 381 (1996) 678. [3] E.W. Wong, P.E. Sheehan, C.M. Lieber, Science 277 (1997) 1971. [4] P. Poncharal, Z.L. Wang, D. Ugarte, W.A. de Heer, Science 283 (1999). [5] J.W. Mintmire, B.I. Dunlap, C.T. White, Phys. Rev. Lett. 68 (1992) 631. [6] J. Liu, A.G. Rinzler, H. Dai, J.H. Hafner, R.K. Bradley, P.J. Boul, A. Lu, T. Iverson, K. Shelimov, C.B. Huffman, F. Rodriguez-Macias, Y.-S. Shon, T.R. Lee, D.T. Colbert, R.E. Smalley, Science 280 (1998) 1253. [7] R.H. Baughman, C. Cui, A.A. Zakhidov, Z. Lqbal, J.N. Barisci, G.M. Spinks, G.-G. Wallace, A. Mazzoldi, D.D. Rossi, A.G. Rinzler, O. Jaschinski, S. Roth, M. Kertesz, Science 284 (1998) 1340. [8] Z.W. Pan, S.S. Xie, L. Lu, B.H. Chang, L.F. Sun, W.Y. Zhou, G. Wang, D.L. Zhang, Appl. Phys. Lett. 74 (1999) 3152. [9] B. Vigolo, A. Penicaud, C. Coulon, C. Sauder, R. Pailler, C. Journet, P. Bernier, P. Poulin, Science 290 (2000) 1331. [10] T.W. Ebbesen, H.J. Lezee, H. Hiura, J.W. Bennett, H.F. Ghaemi, T. Thio, Nature 382 (1996) 54. [11] A.G. Rinzler, J. Liu, H. Dai, P. Nikolaev, C.B. Huffman, F.J. Rodriguez-Macias, P.J. Boul, A.H. Lu, D. Heymann, D.T. Colbert, R.S. Lee, J.E. Fischer, A.M. Rao, P.C. Eklund, R.E. Smalley, Appl. Phys. A 67 (1998) 29. [12] W.A. de Heer, W.S. Bacsa, A. Chatelain, T. Gerfin, R. Humphrey-Baker, L. Forro, D. Ugate, Science 268 (1995) 845. [13] R. Ma, C.L. Xu, B.Q. Wei, J. Liang, D.H. Wu, D.J. Li, Mater. Res. Bull. 34 (1999) 741. [14] Y.-H. Li, C. Xu, B. Wei, X. Zhang, M. Zheng, J. Liang, D. Wu, P.M. Ajayan, Chem. Mater. 14 (2002) 483. [15] M.J. Skove, T.M. Tritt, A.C. Ehrlich, H.S. Davis, Rev. Sci. Instrum. 62 (1991) 1010. [16] B. Li, Z. Shi, Y. Lian, Z. Gu, Chem. Lett. 7 (2001) 598. [17] Z. Jia, Z. Wang, J. Liang, B. Wei, D. Wu, Carbon 37 (1999) 903. [18] M.S.P. Shaffer, X. Fan, A.H. Windle, Carbon 36 (1998) 1603. [19] R. Andrews, D. Jacques, D. Qian, E.C. Dickey, Carbon 39 (2001) 1681. [20] L. Ci, B. Wei, C. Xu, J. Liang, D. Wu, S. Xie, W. Zhou, Y. Li, Z. Liu, D. Tang, J. Cryst. Growth 233 (2001) 823. [21] J.E. Fischer, H. Dai, A. Thess, R. Lee, N.M. Hanjani, D.L. Dehaas, R.E. Smalley, Phys. Rev. B 55 (1997) R4921.
Mechanical and electrical properties of carbon nanotube ribbons Yan-Hui Li
a,*
, Jinquan Wei a, Xianfeng Zhang a, Cailu Xu a, Dehai Wu a, Li Lu b, Bingqing Wei c
a
c
Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China b Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180, USA Received 31 July 2002; in final form 7 September 2002
Abstract We have measured the YoungÕs modulus of long aligned carbon nanotube ribbons using a special stress–strain puller designed for whisker-like materials. The YoungÕs modulus of the graphitized carbon nanotube ribbons is about 60 GPa, which is 2.5 times higher than that of the as-grown ribbons. This suggests that the graphitization is an effective way to improve the mechanical properties of the ribbons. Most of the measured ribbons are semiconductor and their resistivities are in the range of 4.4–12:6 104 X cm. An interesting phenomenon appeared in one ribbon is that the electrical property of the ribbon changes from metallic to non-metallic with decreasing the temperature. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Carbon nanotubes (CNTs) have been predicted to possess exceptional mechanical [1–4] and unique electrical properties [5], which could have great potential applications as structural and intellectual materials. But too small individual CNTs make it difficult further exploit their various properties and applications, it has been the dream and challenge to prepare long CNTs or their macrostructures. Rinzler et al. [6] first prepared bucky paper con-
*
Corresponding author. Fax: +8601062782413. E-mail address: [email protected] (Y.-H. Li).
sisted of a free-standing mat of entangled singlewall CNTs and estimated their elastic modulus of 1.2 GPa [7]. Pan et al. [8] prepared very long aligned multiwalled CNTs and measured their YoungÕs moduli and the tensile strength to be 0:45 0:23 TPa and 1:72 0:64 GPa, respectively. Recently, Vigolo et al. [9] fabricated macroscopic fibers and ribbons of oriented CNTs through injecting nanotube dispersions into a flowing stream of polyvinylalcohol solution. The YoungÕs modulus of the fibers varied between 9 and 15 GPa. Electrical properties of the CNTs are another fascinating research field. They can be either metallic, non-metallic or insulating, depending on their diameter and chirality. Many efforts have been devoted to study the electrical properties of
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 4 3 4 - 3
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Y.-H. Li et al. / Chemical Physics Letters 365 (2002) 95–100
individual CNTs [10] and their macrostructures [9,11–13]. Ebbesen et al. [10] experimentally proved that individual CNT can either be metallic or non-metallic which was modified by the interlayer interactions of the CNTs. The resistivities of the bucky paper were also measured and they were metallic for the acid-treated CNTs, they changed from metallic behavior into non-metallic behavior after vacuum-annealing [11]. Ma et al. [13] fabricated a soft sinter of pure CNTs by hot-pressing under the conditions of 2273 K/25 MPa/Ar/1 h. The resistivity of the sinter was about ð2–3Þ 104 X cm and increased with decreasing temperature. Resistivity of VigoloÕs fibers [9] was about 0.1 X cm at room temperature and the fibers exhibited non-metallic behavior with decreasing temperature. In this Letter, we prepared long aligned CNT ribbons through self-organization and graphitized them at 2200 °C. The mechanical properties of the as-grown and graphitized ribbons were measured using a specially designed stress–strain puller. The electrical property of the as-grown ribbons was also determined using a four-point method.
2. Experimental Long CNT ribbons (100 mm) were prepared by heating oxidized acid-treated nanotubes at 100 °C [14]. The ribbons are about 50–140 lm wide, 4–40 lm thick and 100 mm long (Fig. 1a). SEM images show that the CNTs consisted of the ribbons align along the direction in length (Fig. 1b). The CNT ribbons from the same preparation batch were used in the experiments. Parts of ribbons were graphitized at 2200 °C for 2 h under Ar atmosphere with pressure of 0.5 MPa. The YoungÕs moduli of the as-grown and graphitized ribbons were measured using a specially designed stress– strain puller. A diagram of the stress–strain puller device is shown in Fig. 2, its principle was described in detail elsewhere [15]. In brief, one end of the sample is mounted on a movable rod suspended by two leaf springs and the other is fixed. A static, uniform force is applied to the sample via a coaxial electro-magnet acting on permanent magnets attached to the movable rod and dis-
Fig. 1. SEM image of a ribbon showing (a) its rectangular cross-section; (b) the alignment of the CNTs (arrow points to the length direction of the ribbon).
placements are measured capacitively. YoungÕs modulus, E, is given by the slope of a stress–strain curve: E ¼ Dr=De, where r is the uniaxial stress and e is the strain. In order to observe the structure change of the graphitized ribbons, the ribbons were embedded into an epoxy mould. After completely hardening of the epoxy with CNT ribbons, slices with thickness about 25–60 nm were cut using an ultramicrotomy (LKB-2088, ULTROTOME-V) with a diamond knife. The cutting planes were controlled parallel to the length of the ribbons. The slices were fixed on a holey copper grid and examined with a JEOL-2010 transmission electron microscope. Electrical properties of the as-grown ribbons were measured using a four-point resistivity method.
Y.-H. Li et al. / Chemical Physics Letters 365 (2002) 95–100
Fig. 2. A diagram of the stress–strain device and the mounted nanotube ribbon.
3. Results and discussion The force applied to the as-grown ribbon (Fribbon ) by magnets versus the displacement of the plates is shown in Fig. 3a. The Fribbon is given by the following equation: Fribbon ¼ Fmagnet KDd, where Fmagnet is the force applied by the magnets, K is the spring constant of the device ( ¼ 1.55 mN/lm), the ribbon is straight and there is no free displacement
97
of the rod, so Dd equals to the displacement of the ribbon and the force of KDd is produced by the spring. The device is designed to measure whiskerlike samples and produces a little force. So the ribbon only has a smaller displacement and cannot fracture. Although the data scatter in a broad scope, the linear relation between Fribbon and Dd can be regarded as the elastic deformation of the ribbon and can be used to determine the E of the ribbon roughly. A stress–strain plot, the slope of which is Y, is shown in Fig. 3b for the as-grown ribbon. The value of Y we determine for the asgrown ribbon is about 24 GPa. The result is much lower than the previous reported values [1,2]. The value is normal considered the formation mechanism of the ribbons. Firstly, the ribbons are prepared at lower temperature (100 °C). The driving force for making thousands of nanotubes aggregate into long ribbons is mainly van der Waals force. The as-grown ribbons formed by van der Waals force which can be dispersed in water also suggests the weak coalescent force existed in the ribbons. The other important factor affected the selforganization of the CNTs into ribbons is functional groups introduced by acid oxidation. Previous works reported that many functional groups, such as hydroxyl (–OH), carboxyl (–COOH) and carbonyl (> C@O), can be attached both on the surface and on the open tip of the CNTs by oxidation with acid [16,17]. The FT-IR spectra (Fig. 4) show that the peak at 1650 cm1 may be due to contaminating water and peak at
Fig. 3. YoungÕs modulus test results recorded on the as-grown ribbon with the following characteristics: L ¼ 2:40 mm, A ¼ 4200 lm2 . (a) The force applied to the ribbon versus the displacement of the ribbon. (b) The uniaxial stress applied to the as-grown ribbon versus the strain (elongation) of the ribbon. The slope yields a value for YoungÕs modulus of as-grown ribbon of 24 GPa.
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Fig. 4. FT-IR spectra of (a) as-grown CNTs; (b) as-grown CNT ribbons; (c) graphitized CNT ribbons.
1220 cm1 is assigned to the carbon skeleton [18]. The signature of > C@O functional groups is evident at 1780 cm1 and –OH functional groups appear at 3430 cm1 in curve after oxidation (curve b). The increased strength of peaks and the new peaks appeared on the acid-oxidized sample suggested that oxidation with acid introduced great amounts of functional groups on the surfaces of CNTs compared with as-grown CNTs (curve a). The polycondensation may take place through the functional groups on the CNTs as shown in Fig. 5. In reaction, each CNTs can be seen as a monomer and they can form nanotube ribbons just like a typical polycondensation reaction in which water molecular are split out. The polycondensation may offer another driving force forming the ribbons and this may be proved indirectly by what the ribbons cannot be formed using unoxidized CNTs. Secondly, the nanotubes consisted of the ribbons are not continuously, each nanotubes is only several micrometer long. So the YoungÕs modulus of the as-grown ribbon is lower than the theoretical value of CNTs. In order to improve the strength of the ribbons, the as-grown ribbons were graphitized at 2200 °C for 2 h in Ar atmosphere (0.5 MPa). The crosssections of the ribbons decrease obviously after graphitization and the density of the ribbon increases from 1.1 to 1.5 g/cm3 correspondingly. Fig. 6a shows a plot of the force applied to the
Fig. 5. The diagram showing the driving force making CNTs aggregate into ribbons (the black arrows represent of van der Waals forces).
graphitized ribbons versus the displacement of the ribbon. When the Fribbon –Dd curve is transformed into a r–e curve (Fig. 6b), the value of Y, 60 GPa, for graphitized ribbon can be determined from the slope of r–e curve. The YoungÕs modulus of the graphitized ribbons is higher than that of the asgrown ribbons. High temperature graphitization of CNTs can reorder its layered structures and reduce the wall defects [19,20]. The CNTs used for preparing the ribbons come from decomposition of hydrocarbons using chemical vapor deposition method and have many impurities and inherent defects, which decrease mechanical property seriously. Graphitization of ribbons at 2200 °C cannot only reduce the defects on the surfaces of CNTs, but also weld the adjacent CNTs together through re-orgnization of carbon atoms on the outer layers of CNTs, so the YoungÕs modulus of the graphitized ribbons improved greatly. Fig. 7 shows the HRTEM images of the graphitized ribbons. It can be seen that the outer layers of the four CNTs were connected together, the HRTEM image was not so clear because the slices were covered with a layer of epoxy.
Y.-H. Li et al. / Chemical Physics Letters 365 (2002) 95–100
99
Fig. 6. YoungÕs modulus test results recorded on the graphitized ribbon with the following characteristics: L ¼ 2:55 mm, A ¼ 2000 lm2 . (a) The force applied to the ribbon versus the displacement of the ribbon. (b) The uniaxial stress applied to the graphitized ribbon versus the strain (elongation) of the ribbon. The slope gives a value for YoungÕs modulus of the graphitized ribbon of 60 GPa.
All of above results suggest that the interactions among the CNTs consisted of the ribbons become stronger relative to the as-grown ribbons, so the YoungÕs modulus of the graphitized ribbons is improved and 2.5 times higher than that of the asgrown ribbons. The electrical properties of five as-grown ribbons were measured using four-point resistivity (q) method. Four ribbons are semiconductor and have negative dq=dT from 4 to 300 K (Fig. 8a). The resistivities of the ribbons are in the range of 4.4–12:6 104 X cm at room temperature. Elec-
Fig. 7. HRTEM images of the connected CNTs. Four arrows point to the inner cavities of four CNTs.
On the other hand, the decrease of the crosssection and the increase the density indicate that the CNTs tend to compact tighter and the van der Waals forces between CNTs increase correspondingly. At this time the functional groups lost their bonding role because high temperature graphitization can eliminate the functional groups on the surfaces of the CNTs. This is strongly supported by the FT-IR spectra in Fig. 4 (curve c). It can be seen that most of peaks of the spectra for graphitized ribbons disappeared and the strength of –OH functional groups appeared at 3430 cm1 also decreased greatly.
Fig. 8. Resistivity versus temperature of the ribbons with different cross-section: (a) 60 12 lm2 ; (b) 100 20 lm2 ).
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Y.-H. Li et al. / Chemical Physics Letters 365 (2002) 95–100
trical property of one ribbon presents an interesting phenomenon, which is similar to the results of single-wall CNT ropes [21]. It can be seen the qðT Þ of the ribbon increases linearly with temperature from 260 to 300 K, and positive dq=dT indicates the ribbon is metallic (Fig. 8b). This behavior changes when the temperature decreases below 260 K. The ribbon has negative dq=dT and is nonmetallic until the temperature decreases to 4 K. This result has never been reported on multiwall CNTs or their macrostructures. The reason why the ribbons have different electrical properties in the same preparation batch is still unclear. But it is already known that individual CNT can be either metallic or non-metallic and the bulk properties of the nanotubes vary significantly depending on their preparing method, perfection of structure, diameter and chirality. The ultimate electrical property of the ribbons may be integrated effects of densely aligned CNTs. Furthermore, we cannot rule out the possible effects of the structural defects and impurities on the electrical property.
4. Conclusions The YoungÕs moduli of the as-grown and graphitized CNT ribbons have been directly measured using a stress–strain puller. The binding force of the as-grown ribbons is offered mainly by van der Waals force and covalent band formed by polycondensation of the functional groups on the CNTs. So the YoungÕs modulus of as-grown CNT ribbon is only about 24 GPa. After graphitization the YoungÕs modulus was improved greatly and reached to 60 GPa. This suggests that graphitization may be an effective method to improve the mechanical property of the ribbons. The resistivities of the ribbons at room temperature are 4.4– 12:6 104 X cm. With decreasing the temperature most of the ribbons present semiconductive behavior. While one ribbon is metallic at the temperature between 260 and 300 K and changes into non-metallic bellow 260 K. This may be the integrating effects of the pristine nature of CNTs and interaction between CNTs and impurities.
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