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Carbon nanotubes as structural material and their application in composites
Composites: Part B 42 (2011) 2151–2157
Contents lists available at ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Carbon nanotubes as structural material and their application in composites T. Hayashi a,⇑, M. Endo a,b a b
Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-0911, Japan Institute for Carbon Science and Technology, 4-17-1 Wakasato, Nagano 380-0911, Japan
a r t i c l e
i n f o
Article history: Received 3 January 2011 Received in revised form 17 March 2011 Accepted 20 March 2011 Available online 11 May 2011 Keywords: A. Nano-structures E. Chemical vapor deposition (CVD)
a b s t r a c t In the present paper, we will introduce the synthesis and application of carbon nanotubes as filler for nanocomposite materials. Physical and chemical properties such as mechanical strength, thermal and electrical conductivity, chemical stability, density and affinity with the matrix vary a lot among different types of carbon nanotubes. We first will overview the main component of nanocomposite, carbon nanotube and its synthesis and purification, and introduce some of the nanocomposites and its applications that we have taken part in the development. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction In the present paper, we will introduce the synthesis and application of carbon nanotubes (CNT) as filler for nanocomposite materials. It is well known that there are many morphological variants in carbon nanotubes. For example, there are differences in tubular structure (e.g. straight, curled, branched, cup-stacked, herringbone), layer numbers (e.g. single, double, multi), layer crystallinities (e.g. well aligned, distorted), defects (e.g. topological, vacancy), surface chemistry (e.g. functional group, coating, oxidized) to list a few. Physical and chemical properties such as mechanical strength, thermal and electrical conductivity, chemical stability, density and affinity with the matrix vary a lot among different types of carbon nanotubes. Therefore, we need to strategically select suitable carbon nanotubes for each nanocomposite application. We will first overview the main component of nanocomposite, carbon nanotube and its synthesis and purification. Second, we introduce some of the basic studies and applications of nanocomposites that we have taken part in the development. Finally, we will summarize by pointing the challenges we are facing upon even broader range of application of carbon nanotube in nanocomposites. 2. Carbon nanotube as a fundamental component of nanocomposite Three major methods for the synthesis of carbon nanotubes are: arc discharge method [1], laser ablation method [2], and catalytic chemical vapor deposition (CCVD) method [3]. Nowadays, each of ⇑ Corresponding author. E-mail address: [email protected] (T. Hayashi). 1359-8368/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2011.05.011
these methods can, to some extent, selectively synthesize single walled carbon nanotube (SWCNT) and multi walled carbon nanotubes (MWCNT). However, for nanocomposite application, we need a large amount of nanotubes to prepare test pieces, and once nanotubes are applied in the actual product, manufacturing cost becomes a big factor. Since the most commonly used nanotube in nanocomposites is MWCNT, an efficient method that can synthesize MWNCT at low cost is required. For the moment, the only method that meets the above requirements is the CCVD method. The CCVD method uses nanometer sized metal catalyst particles to grow carbon nanotube, and is already used in the mass production of MWCNT. Recently, CCVD was also used to selectively synthesize SWCNT, double walled carbon nanotube, triple walled carbon nanotube, and so on (Fig. 1) [4]. This shows the robustness of the CCVD method that can be used for mass production, control of layer numbers and diameter of carbon nanotubes. There are several variants in the CCVD method. First one is the substrate method, which uses a substrate to support the nanometer sized catalyst particles (Fig. 2a). Catalyst particles can be sprayed or sputtered on the substrate that goes in a furnace with a regulated flow of hydrocarbons and an inert carrier gas. Depending on the temperature, the flow rate of the carbon source, and the reaction time, the thickness of carbon nanotubes can be controlled. The substrate method can be performed in batch basis, and is not totally suitable for mass production of MWCNT. In order to synthesize MWCNT in a nearly continuous manner, the floating catalyst method was developed (Fig. 2b) in the late 1980s [5]. In the floating method, catalyst solution and carbon source are fed into the vertical furnace from the top, and the resulting MWCNT can be collected at the bottom of the furnace. This method is widely used in industries for mass production of carbon nanotubes. Another widely used method is the fluidized bed method, which can be used to produce various carbon nanotubes such as SWCNT [6],
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(a)
(b)
(c)
(d)
Fig. 1. Carbon nanotube with various layer numbers: (a) single walled carbon nanotube, (b) double wall carbon nanotube, (c) triple wall carbon nanotube, and (d) quadruple wall carbon nanotube.
(a) Hydrocarbon
(c)
Furnace nanotube substrate
50 nm
(b)
(d)
Hydrocarbon + Catalyst Carrier gas
Furnace
10 nm Fig. 2. (a) In the substrate method, catalysts are placed on the substrate and nanotubes grow from the catalysts. (b) Floating method. Catalyst particles and carbon source are fed into a vertical furnace. Carbon nanotubes grow from floating catalysts. (c, d) Cup-stacked carbon nanotube prepared by the CCVD method.
DWCNT [7], and MWCNT [8,9]. By carefully controlling the synthesis conditions in the CCVD method, we can produce another type of carbon nanotube such as the cup stacked carbon nanotube (CSCNT) (Fig. 2) [10]. It is also possible to synthesize high quality double walled carbon nanotubes (DWCNT) using the substrate method
(Fig. 3) [11]. High quality DWCNT bundle has a hexagonal packing, and after the purification and filtration process, we can obtain a flexible sheet of DWCNT (Fig. 3). DWCNT is thermally stable compared with SWCNT, and maintains its cylindrical structure up to 2000 °C in an inert atmosphere. When we raise the temperature
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(b)
(c)
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Fig. 3. (a) Cross sectional image of high purity DWCNT and its model (b). (c) Origami helmet made of DWCNT paper. (d) SEM image of DWCNT paper. (e) Cross sectional image of coalesced DWCNT and its model (f) [11,12].
above 2000 °C, DWCNT start to coalesce to form a larger tube (Fig. 3) [12]. Under certain conditions, only the outer layers of two DWCNT can coalesce, resulting in an interesting structure like a dual core DWCNT (Fig. 3). The thermal stability and mechanical property of DWCNT make it an ideal material for nanocomposites, and further development is under way. Purification of as-grown carbon nanotube is an important part for the application of carbon nanotube in nanocomposites. Usually, purification is performed combining multiple oxidation processes such as acid and heat treatment [11]. The advantage and disadvantage of using carbon nanotube in nanocomposite is the variety of choice of carbon nanotube. As previously stated, there are differences in tubular structure, layer numbers, layer crystallinities, defects, surface chemistry to list a few. Physical and chemical properties such as mechanical strength, thermal and electrical conductivity, chemical stability, density and affinity with the matrix vary a great deal among different types of carbon nanotubes. Even for the same tube, depending on the post synthesis treatment, some of the above mentioned properties can
Fig. 4. Transmission electron microscope image of twisty carbon nanotubes. Layer crystallinity is usually low in the as-grown state.
be modified. This is a great advantage in most applications, but can be a drawback in other cases because the required treatment may degrade the property that is required for a specific application. If one has some specific target applications, and can pick an appropriate type of carbon nanotube, length, purity, crystallinity, and surface treatment, it can result in an ideal nanocomposite with intended mechanical, chemical, and transport properties. However, inappropriate choice of CNT or treatment might not contribute to the improvement of the nanocomposite, and in the worst case, might even lower the properties of the nanocomposite. Mechanical properties and transport properties depend on the layer crystallinity of the tube walls. A well aligned, defect free layer is crucial for the optimal mechanical and transport properties [13,14]. Therefore, if there is a twisty, lower crystallinity CNT (Fig. 4), where layer structure is not well developed, high temperature heat treatment is required to improve the crystallinity of the layers. From the
Fig. 5. TEM image of a straight carbon nanotube with disordered layers. If the observation was performed in SEM, it should appear as perfect tube. But TEM image revealed that the layers do not have an ideal structure.
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CFRP compression test
Strength (MPa)
600 500 400 300 200 100 0
No CSCNT
CSCNT 12
CF:180g/m2 Resin content 38% Fig. 8. Compression test result of CFRP with CSCNT as the secondary filler compared with the reference CFRP [19].
Fig. 6. TEM image of heat treated carbon nanotube with layers non-parallel to the tube axis.
macroscopic viewpoint, CNT shown in Fig. 5 is considered to be a straight, ideal nanotube. However, looking closely at the microscopic structure, it is clear that the carbon layer is not well developed, and heat treatment is necessary to improve the layer structure. However, CNT with a microscopic structure shown in Fig. 6, does not have a continuous layer structure along the tube axis. Therefore, although the layer structure can be improved by heat treatment, the transport property along the tube axis may not be improved. If there is a deposition layer on the surface of CSCNT, heat treatment will help forming a straight aligned layer structure along the tube axis from the deposition layer, which will lead to improved transport properties. The length of the carbon nanotube is an important part for the performance of the nanocomposite, and it is expected that a longer tube should be used for better mechanical and transport properties. However, due to dispersion and entanglement problem, a long tube has the possibility of introducing voids in the matrix.
Therefore, shortened nanotubes are often used in nanocomposites for better dispersibility, compromising transport properties to a certain amount. The number of layers in the tube is another factor to be considered especially for thermal transport properties of nanocomposites. Since heat in a carbon nanotube is mainly transferred by phonon, increase in the layer number may lead to the increase in the phonon transfer channel [15]. The tube length should be long so that there are fewer connections between the tubes that will disperse the phonon. It is virtually impossible to connect the ends of adjacent carbon nanotubes, thermal transport property will degrade even if percolation is achieved. To address such issues, carbon nanotube with secondary growth nanotubes that result in a fox-tail, or branched carbon nanotube may be a good candidate material [16]. Surface treatment of carbon nanotube is the critical part for using nanotubes in nanocomposites, because the surface of carbon nanotube is usually hydrophobic, and the affinity between the carbon nanotube and the matrix material is key for the reinforcement of matrix by carbon nanotubes. Dispersion of carbon nanotube is equally important, and surface treatment can be a good way for good dispersibility. Therefore, surface treatment similar to the ones used in carbon fiber is applied [17]. The most common treatment is oxidation of carbon nanotubes into introduce defects to the surface
CF
CF
CF
CF CF
Fig. 7. SEM image of CFRP with cup stacked carbon nanotubes as the secondary filler. Large circles are the cross section of carbon fibers (denoted as CF), and white dots are cup stacked carbon nanotubes (denoted as CSCNT).
Fig. 9. TEM image of CSCNT and its surface (inset). Note that graphene edges are exposed at the tube surface.
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Fig. 12. Tensile test results of CNT/Mg alloy composite with different CNT/Mg ratios [21]. Fig. 10. SEM image of a microgear made of CNT nanocomposite. A human hair is on the side to show how small the gear is [20].
so that open bonds can connect with the matrix polymer material. Functional groups can be added to the surface of carbon nanotubes, and coating carbon nanotubes with polymers is also an effective way to make good contacts between carbon nanotube and matrix materials [18]. When the tubes are entangled, viscosity of the matrix during preparation of nanocomposite becomes important. If the viscosity is too low, voids will remain in the space between the entangled carbon nanotubes, which results in the critical point of defect in the nanocomposites. Application of CNT nanocomposite to structural components is difficult at the moment due to the lack of mechanical strength, etc. This is because carbon nanotube is not long enough to reinforce large components, and the cost of a nanotube is prohibitively high for commercialization of structural components. Performance/ price ratio wise, CNT nanocomposites cannot compete with CFRP (carbon fiber reinforced plastics). Therefore, for structural applications, CNT is mainly used as secondary filler in CFRP to reinforce the resin that surrounds the carbon fibers. This way, the amount of resin used can be reduced and the resin itself is reinforced, which improves the compressive strength and shock damping characteristics of the composite. 3. Applications of CNT based nanocomposites Composites that use carbon nanotubes as secondary filler are already commercialized, and used in frames, cranks, handlebars,
front forks, etc. of road racing bikes from various manufacturers. The carbon nanotube composite contributes to further improve the durability, strength and shock absorption of such components. Usually, in such applications, MWCNT are used expecting the increase in compressive strength, and carbon fiber contributes to the tensile strength. Secondary filler CNT composites are also used in ice hockey sticks for durability and controllability. Another type of carbon nanotube, CSCNT, is also used as secondary filler for CFRP and a 20% increase in compressive strength was achieved (Fig. 7). Fig. 8 shows the compression test result with and without CSCNT as the secondary filler. As shown in Fig. 9, CSCNT has a characteristic microstructure where cup-shaped graphene layers are stacked to form a tubular shape [10]. Graphene layers are not parallel to the tube axis, and as a result, active graphene edges are exposed to the tube surface. These structural characteristics are considered to contribute to the compressive strength and affinity with the resin and coating materials. CSCNT dispersed in resin used for bonding composite sheets was found to improve the delamination of carbon fiber prepregs and carbon cloths by reinforcing the interface [19]. This is not exactly a secondary filler, but closer to a CNT nanocomposite application that is expected to gain popularity if the cost is acceptable for a specific application. As stated earlier, CNT based nanocomposites are suitable for small components such as small CNT reinforced plastic gear less than 1 mm in diameter (Fig. 10) [20]. It has high strength, thermal stability, shape stability, and moldability that surpass existing
Fig. 11. CNT/Mg alloy composite made by extrusion and the bolts made by the extruded CTT/Mg alloy composite (inset) [21].
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With carbon black
With MWCNT
65.5%
19.4%
Fig. 13. CNT/rubber composite unloaded after 10% compression at 200 °C, 70 h. Permanent set rate is shown in the image [22].
technology, and has less friction than conventional gears of the same size. These advantages make CNT nanocomposites especially useful in small mechanical components in MEMS (micro electro mechanical systems) and NEMS (nano electro mechanical systems). CNT/metal composites can be prepared by extrusion of the mixture of metal powder and CNT (Fig. 11) [21]. The present example is a CNT/Mg alloy composite that showed 20–30% increase in tensile strength (Fig. 12). This result shows that CNT can be used to reinforce metal composites. CNT/rubber composite is also a kind of CNT nanocomposite that is attracting much attention (Fig. 13) [22]. In this case, CNT serves as a spring and heat conductor to improve shape resiliency and thermal stability of the rubber component. Such material is considered to be useful in extreme conditions such as high pressure and high temperature as in oil mining. It was actually tested as a sealing material for the joints in pipes for drills, and withstood severe conditions that no other sealing materials were able to stand. This enables us to find more oil mines that are so deep under that conventional technology was unable to find. 4. Summary As shown in the present review, CNT nanocomposites are already being used in the fields of sporting goods, medical catheters and artificial joints, energy devices, conducting films, and still growing in a rapid pace. The challenge in further developing CNT nanocomposites is the quality of CNT and the art of CNT nanocomposite fabrication. There are many ways to produce CNT, such as CCVD, arc, laser vaporization, and template, to list a few. Every method, depending on synthesis condition, will result in different purity, crystallinity, layer number and diameter distribution. There are many companies that commercialize CNT, that are very much microscopically different. In the extreme case, a different batch of CNT from the same company is totally a different material. Scaling up the production of CNT is also difficult because it is very hard to keep the good synthesis parameter in the reaction chamber. This fact makes it difficult for the end users to use CNT, since users expect CNT to be exactly like the one they saw in textbooks, which in fact is different in real world. Therefore, end users should check the structure and characteristics of CNT that they have obtained so that they can adjust the composite preparation conditions accordingly. Since CNT is not long enough to substitute carbon fibers at the moment, further studies are required to fabricate long and continuous CNT at low cost. The art of CNT composite fabrication is in the surface treatment of CNT, and the choice of matrix materials. Since CNT is a nanometer-scale material and since surface structure may differ a great
deal, affinity with the matrix material is expected to be different from that of carbon fibers. CNT also tends to entangle, resulting in the formation of voids in nanocomposites. Although it may take time and resource, a better surface treatment and dispersion that suits CNT should be sought for further advancement and extended use of CNT nanocomposites. Acknowledgements This work was supported by the JSPS Grant-in-Aid Nos. 22510112 and 19002007, MEXT Regional Cluster Project, and JST Exotic Nano Carbons Project. References [1] Ebbesen TW, Ajayan PM. Large-scale synthesis of carbon nanotubes. Nature 1992;358:220–2. [2] Thess A, Lee R, Nikolaev P, Dai HJ, Petit P, Robert J, et al. Crystalline ropes of metallic carbon nanotubes. Science 1996;273:483–7. [3] Oberlin A, Endo M, Koyama S. Filamentous growth of carbon through benzene decomposition. J Cryst Growth 1976;32:335–49. [4] Jorio A, Dresselhaus MS, Dresselhaus G, editors. Carbon nanotubes. Springer; 2008. [5] Endo M. Grow carbon fibers in the vapor phase. Chem Technol 1988:568–76. [6] Li Y-L, Kinloch IA, Schaffer MSP, Geng J, Johnson B, Windle AH. Synthesis of single-walled carbon nanotubes by a fluidized-bed method. Chem Phys Lett 2004;384:98–102. [7] Dunens OM, MacKenzie KJ, Harris AT. Large-scale synthesis of double-walled carbon nanotubes in fluidized beds. Ind Eng Chem Res 2010;49:4031–5. [8] Hernadi K, Fonseca A, Nagy JB, Bernaerts D, Lucas AA. Fe-catalyzed carbon nanotube formation. Carbon 1996;34:1249–57. [9] Liu BC, Liang Q, Tang SH, Gao LZ, Zhang BL, Qu MZ, et al. Production of carbon nanotubes over pre-reduced LaCoO3 using fluidized-bed reactor. Chin Chem Lett 2000;11:1031–4. [10] Endo M, Kim YA, Hayashi T, Fukai Y, Oshida K, Terrones M, et al. Structural characterization of cup-stacked-type nanofibers with an entirely hollow core. Appl Phys Lett 2002;80:1267–9. [11] Endo M, Muramatsu H, Hayashi T, Kim YA, Terrones M, Dresselhaus MS. Buckypaper’ from coaxial nanotubes. Nature 2005;433:476. [12] Endo M, Hayashi T, Muramatsu H, Kim YA, Terrones H, Terrones M, et al. Coalescence of double-walled carbon nanotubes: formation of novel carbon bicables. Nanoletters 2004;4:1451–4. [13] Zang J-L, Yuan Q, Wang F-C, Zhao Y-P. A comparative study of Young’s modulus of single-walled carbon nanotube by CPMD, MD, and first principle simulations. Comput Mater Sci 2009;46:621–5. [14] Demczyk BG, Wang YM, Cumings J, Hetman M, Han W, Zettl A, et al. Direct mechanical measurement of the tensile strength and elastic modulus of multiwalled carbon nanotubes. Mater Sci Eng A 2002;334:173–8. [15] Aliev AE, Lima MH, Silverman EM, Baughman RH. Thermal conductivity of multi-walled carbon nanotube sheets: radiation losses and quenching of phonon modes. Nanotechnology 2010;21:035709. [16] Yang Jiao-Ping, Feng Qing-Ping, Chen Zhen-Kun, Fu Shao-Yun. Superiority of nanosized over microsized hyperbranched polymer second phase in modifying brittle epoxy resin. J Appl Polym Sci 2011;119:863–70. [17] Dresselhaus MS, Dresselhaus G, Sugiura K, Spain IL, Goldberg HA, editors. Graphite fibers and filaments. Springer; 1988. [18] Tasis D, Tagmatarchis N, Bianco A, Prato M. Chemistry of carbon nanotubes. Chem Rev 2006;106:1105–36. [19] Yokozeki T, Iwahori Y, Ishiwata S, Enomoto K. Mechanical properties of CFRP laminates manufactured from unidirectional prepregs using CSCNT-dispersed epoxy. Composites: Part A 2007;38:449–60.
T. Hayashi, M. Endo / Composites: Part B 42 (2011) 2151–2157 [20] Endo M, Hayashi T, Kim YA. Large-scale production of carbon nanotubes and their applications. Pure Appl Chem 2006;78:1703–13. [21] Shimizu Y, Miki S, Soga T, Itoh I, Todoroki H, Hosono T, et al. Multi-walled carbon nanotube-reinforced magnesium alloy composites. Scripta Mater 2008;58:267–70.
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[22] Endo M, Noguchi T, Ito M, Takeuchi K, Hayashi T, Kim YA, et al. Extremeperformance rubber nanocomposites for probing and excavating deep oil resources using multi-walled carbon nanotubes. Adv Funct Mater 2008;18:3403–9.
Contents lists available at ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Carbon nanotubes as structural material and their application in composites T. Hayashi a,⇑, M. Endo a,b a b
Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-0911, Japan Institute for Carbon Science and Technology, 4-17-1 Wakasato, Nagano 380-0911, Japan
a r t i c l e
i n f o
Article history: Received 3 January 2011 Received in revised form 17 March 2011 Accepted 20 March 2011 Available online 11 May 2011 Keywords: A. Nano-structures E. Chemical vapor deposition (CVD)
a b s t r a c t In the present paper, we will introduce the synthesis and application of carbon nanotubes as filler for nanocomposite materials. Physical and chemical properties such as mechanical strength, thermal and electrical conductivity, chemical stability, density and affinity with the matrix vary a lot among different types of carbon nanotubes. We first will overview the main component of nanocomposite, carbon nanotube and its synthesis and purification, and introduce some of the nanocomposites and its applications that we have taken part in the development. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction In the present paper, we will introduce the synthesis and application of carbon nanotubes (CNT) as filler for nanocomposite materials. It is well known that there are many morphological variants in carbon nanotubes. For example, there are differences in tubular structure (e.g. straight, curled, branched, cup-stacked, herringbone), layer numbers (e.g. single, double, multi), layer crystallinities (e.g. well aligned, distorted), defects (e.g. topological, vacancy), surface chemistry (e.g. functional group, coating, oxidized) to list a few. Physical and chemical properties such as mechanical strength, thermal and electrical conductivity, chemical stability, density and affinity with the matrix vary a lot among different types of carbon nanotubes. Therefore, we need to strategically select suitable carbon nanotubes for each nanocomposite application. We will first overview the main component of nanocomposite, carbon nanotube and its synthesis and purification. Second, we introduce some of the basic studies and applications of nanocomposites that we have taken part in the development. Finally, we will summarize by pointing the challenges we are facing upon even broader range of application of carbon nanotube in nanocomposites. 2. Carbon nanotube as a fundamental component of nanocomposite Three major methods for the synthesis of carbon nanotubes are: arc discharge method [1], laser ablation method [2], and catalytic chemical vapor deposition (CCVD) method [3]. Nowadays, each of ⇑ Corresponding author. E-mail address: [email protected] (T. Hayashi). 1359-8368/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2011.05.011
these methods can, to some extent, selectively synthesize single walled carbon nanotube (SWCNT) and multi walled carbon nanotubes (MWCNT). However, for nanocomposite application, we need a large amount of nanotubes to prepare test pieces, and once nanotubes are applied in the actual product, manufacturing cost becomes a big factor. Since the most commonly used nanotube in nanocomposites is MWCNT, an efficient method that can synthesize MWNCT at low cost is required. For the moment, the only method that meets the above requirements is the CCVD method. The CCVD method uses nanometer sized metal catalyst particles to grow carbon nanotube, and is already used in the mass production of MWCNT. Recently, CCVD was also used to selectively synthesize SWCNT, double walled carbon nanotube, triple walled carbon nanotube, and so on (Fig. 1) [4]. This shows the robustness of the CCVD method that can be used for mass production, control of layer numbers and diameter of carbon nanotubes. There are several variants in the CCVD method. First one is the substrate method, which uses a substrate to support the nanometer sized catalyst particles (Fig. 2a). Catalyst particles can be sprayed or sputtered on the substrate that goes in a furnace with a regulated flow of hydrocarbons and an inert carrier gas. Depending on the temperature, the flow rate of the carbon source, and the reaction time, the thickness of carbon nanotubes can be controlled. The substrate method can be performed in batch basis, and is not totally suitable for mass production of MWCNT. In order to synthesize MWCNT in a nearly continuous manner, the floating catalyst method was developed (Fig. 2b) in the late 1980s [5]. In the floating method, catalyst solution and carbon source are fed into the vertical furnace from the top, and the resulting MWCNT can be collected at the bottom of the furnace. This method is widely used in industries for mass production of carbon nanotubes. Another widely used method is the fluidized bed method, which can be used to produce various carbon nanotubes such as SWCNT [6],
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(a)
(b)
(c)
(d)
Fig. 1. Carbon nanotube with various layer numbers: (a) single walled carbon nanotube, (b) double wall carbon nanotube, (c) triple wall carbon nanotube, and (d) quadruple wall carbon nanotube.
(a) Hydrocarbon
(c)
Furnace nanotube substrate
50 nm
(b)
(d)
Hydrocarbon + Catalyst Carrier gas
Furnace
10 nm Fig. 2. (a) In the substrate method, catalysts are placed on the substrate and nanotubes grow from the catalysts. (b) Floating method. Catalyst particles and carbon source are fed into a vertical furnace. Carbon nanotubes grow from floating catalysts. (c, d) Cup-stacked carbon nanotube prepared by the CCVD method.
DWCNT [7], and MWCNT [8,9]. By carefully controlling the synthesis conditions in the CCVD method, we can produce another type of carbon nanotube such as the cup stacked carbon nanotube (CSCNT) (Fig. 2) [10]. It is also possible to synthesize high quality double walled carbon nanotubes (DWCNT) using the substrate method
(Fig. 3) [11]. High quality DWCNT bundle has a hexagonal packing, and after the purification and filtration process, we can obtain a flexible sheet of DWCNT (Fig. 3). DWCNT is thermally stable compared with SWCNT, and maintains its cylindrical structure up to 2000 °C in an inert atmosphere. When we raise the temperature
T. Hayashi, M. Endo / Composites: Part B 42 (2011) 2151–2157
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(b)
(c)
(d)
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Fig. 3. (a) Cross sectional image of high purity DWCNT and its model (b). (c) Origami helmet made of DWCNT paper. (d) SEM image of DWCNT paper. (e) Cross sectional image of coalesced DWCNT and its model (f) [11,12].
above 2000 °C, DWCNT start to coalesce to form a larger tube (Fig. 3) [12]. Under certain conditions, only the outer layers of two DWCNT can coalesce, resulting in an interesting structure like a dual core DWCNT (Fig. 3). The thermal stability and mechanical property of DWCNT make it an ideal material for nanocomposites, and further development is under way. Purification of as-grown carbon nanotube is an important part for the application of carbon nanotube in nanocomposites. Usually, purification is performed combining multiple oxidation processes such as acid and heat treatment [11]. The advantage and disadvantage of using carbon nanotube in nanocomposite is the variety of choice of carbon nanotube. As previously stated, there are differences in tubular structure, layer numbers, layer crystallinities, defects, surface chemistry to list a few. Physical and chemical properties such as mechanical strength, thermal and electrical conductivity, chemical stability, density and affinity with the matrix vary a great deal among different types of carbon nanotubes. Even for the same tube, depending on the post synthesis treatment, some of the above mentioned properties can
Fig. 4. Transmission electron microscope image of twisty carbon nanotubes. Layer crystallinity is usually low in the as-grown state.
be modified. This is a great advantage in most applications, but can be a drawback in other cases because the required treatment may degrade the property that is required for a specific application. If one has some specific target applications, and can pick an appropriate type of carbon nanotube, length, purity, crystallinity, and surface treatment, it can result in an ideal nanocomposite with intended mechanical, chemical, and transport properties. However, inappropriate choice of CNT or treatment might not contribute to the improvement of the nanocomposite, and in the worst case, might even lower the properties of the nanocomposite. Mechanical properties and transport properties depend on the layer crystallinity of the tube walls. A well aligned, defect free layer is crucial for the optimal mechanical and transport properties [13,14]. Therefore, if there is a twisty, lower crystallinity CNT (Fig. 4), where layer structure is not well developed, high temperature heat treatment is required to improve the crystallinity of the layers. From the
Fig. 5. TEM image of a straight carbon nanotube with disordered layers. If the observation was performed in SEM, it should appear as perfect tube. But TEM image revealed that the layers do not have an ideal structure.
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700
CFRP compression test
Strength (MPa)
600 500 400 300 200 100 0
No CSCNT
CSCNT 12
CF:180g/m2 Resin content 38% Fig. 8. Compression test result of CFRP with CSCNT as the secondary filler compared with the reference CFRP [19].
Fig. 6. TEM image of heat treated carbon nanotube with layers non-parallel to the tube axis.
macroscopic viewpoint, CNT shown in Fig. 5 is considered to be a straight, ideal nanotube. However, looking closely at the microscopic structure, it is clear that the carbon layer is not well developed, and heat treatment is necessary to improve the layer structure. However, CNT with a microscopic structure shown in Fig. 6, does not have a continuous layer structure along the tube axis. Therefore, although the layer structure can be improved by heat treatment, the transport property along the tube axis may not be improved. If there is a deposition layer on the surface of CSCNT, heat treatment will help forming a straight aligned layer structure along the tube axis from the deposition layer, which will lead to improved transport properties. The length of the carbon nanotube is an important part for the performance of the nanocomposite, and it is expected that a longer tube should be used for better mechanical and transport properties. However, due to dispersion and entanglement problem, a long tube has the possibility of introducing voids in the matrix.
Therefore, shortened nanotubes are often used in nanocomposites for better dispersibility, compromising transport properties to a certain amount. The number of layers in the tube is another factor to be considered especially for thermal transport properties of nanocomposites. Since heat in a carbon nanotube is mainly transferred by phonon, increase in the layer number may lead to the increase in the phonon transfer channel [15]. The tube length should be long so that there are fewer connections between the tubes that will disperse the phonon. It is virtually impossible to connect the ends of adjacent carbon nanotubes, thermal transport property will degrade even if percolation is achieved. To address such issues, carbon nanotube with secondary growth nanotubes that result in a fox-tail, or branched carbon nanotube may be a good candidate material [16]. Surface treatment of carbon nanotube is the critical part for using nanotubes in nanocomposites, because the surface of carbon nanotube is usually hydrophobic, and the affinity between the carbon nanotube and the matrix material is key for the reinforcement of matrix by carbon nanotubes. Dispersion of carbon nanotube is equally important, and surface treatment can be a good way for good dispersibility. Therefore, surface treatment similar to the ones used in carbon fiber is applied [17]. The most common treatment is oxidation of carbon nanotubes into introduce defects to the surface
CF
CF
CF
CF CF
Fig. 7. SEM image of CFRP with cup stacked carbon nanotubes as the secondary filler. Large circles are the cross section of carbon fibers (denoted as CF), and white dots are cup stacked carbon nanotubes (denoted as CSCNT).
Fig. 9. TEM image of CSCNT and its surface (inset). Note that graphene edges are exposed at the tube surface.
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Fig. 12. Tensile test results of CNT/Mg alloy composite with different CNT/Mg ratios [21]. Fig. 10. SEM image of a microgear made of CNT nanocomposite. A human hair is on the side to show how small the gear is [20].
so that open bonds can connect with the matrix polymer material. Functional groups can be added to the surface of carbon nanotubes, and coating carbon nanotubes with polymers is also an effective way to make good contacts between carbon nanotube and matrix materials [18]. When the tubes are entangled, viscosity of the matrix during preparation of nanocomposite becomes important. If the viscosity is too low, voids will remain in the space between the entangled carbon nanotubes, which results in the critical point of defect in the nanocomposites. Application of CNT nanocomposite to structural components is difficult at the moment due to the lack of mechanical strength, etc. This is because carbon nanotube is not long enough to reinforce large components, and the cost of a nanotube is prohibitively high for commercialization of structural components. Performance/ price ratio wise, CNT nanocomposites cannot compete with CFRP (carbon fiber reinforced plastics). Therefore, for structural applications, CNT is mainly used as secondary filler in CFRP to reinforce the resin that surrounds the carbon fibers. This way, the amount of resin used can be reduced and the resin itself is reinforced, which improves the compressive strength and shock damping characteristics of the composite. 3. Applications of CNT based nanocomposites Composites that use carbon nanotubes as secondary filler are already commercialized, and used in frames, cranks, handlebars,
front forks, etc. of road racing bikes from various manufacturers. The carbon nanotube composite contributes to further improve the durability, strength and shock absorption of such components. Usually, in such applications, MWCNT are used expecting the increase in compressive strength, and carbon fiber contributes to the tensile strength. Secondary filler CNT composites are also used in ice hockey sticks for durability and controllability. Another type of carbon nanotube, CSCNT, is also used as secondary filler for CFRP and a 20% increase in compressive strength was achieved (Fig. 7). Fig. 8 shows the compression test result with and without CSCNT as the secondary filler. As shown in Fig. 9, CSCNT has a characteristic microstructure where cup-shaped graphene layers are stacked to form a tubular shape [10]. Graphene layers are not parallel to the tube axis, and as a result, active graphene edges are exposed to the tube surface. These structural characteristics are considered to contribute to the compressive strength and affinity with the resin and coating materials. CSCNT dispersed in resin used for bonding composite sheets was found to improve the delamination of carbon fiber prepregs and carbon cloths by reinforcing the interface [19]. This is not exactly a secondary filler, but closer to a CNT nanocomposite application that is expected to gain popularity if the cost is acceptable for a specific application. As stated earlier, CNT based nanocomposites are suitable for small components such as small CNT reinforced plastic gear less than 1 mm in diameter (Fig. 10) [20]. It has high strength, thermal stability, shape stability, and moldability that surpass existing
Fig. 11. CNT/Mg alloy composite made by extrusion and the bolts made by the extruded CTT/Mg alloy composite (inset) [21].
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With carbon black
With MWCNT
65.5%
19.4%
Fig. 13. CNT/rubber composite unloaded after 10% compression at 200 °C, 70 h. Permanent set rate is shown in the image [22].
technology, and has less friction than conventional gears of the same size. These advantages make CNT nanocomposites especially useful in small mechanical components in MEMS (micro electro mechanical systems) and NEMS (nano electro mechanical systems). CNT/metal composites can be prepared by extrusion of the mixture of metal powder and CNT (Fig. 11) [21]. The present example is a CNT/Mg alloy composite that showed 20–30% increase in tensile strength (Fig. 12). This result shows that CNT can be used to reinforce metal composites. CNT/rubber composite is also a kind of CNT nanocomposite that is attracting much attention (Fig. 13) [22]. In this case, CNT serves as a spring and heat conductor to improve shape resiliency and thermal stability of the rubber component. Such material is considered to be useful in extreme conditions such as high pressure and high temperature as in oil mining. It was actually tested as a sealing material for the joints in pipes for drills, and withstood severe conditions that no other sealing materials were able to stand. This enables us to find more oil mines that are so deep under that conventional technology was unable to find. 4. Summary As shown in the present review, CNT nanocomposites are already being used in the fields of sporting goods, medical catheters and artificial joints, energy devices, conducting films, and still growing in a rapid pace. The challenge in further developing CNT nanocomposites is the quality of CNT and the art of CNT nanocomposite fabrication. There are many ways to produce CNT, such as CCVD, arc, laser vaporization, and template, to list a few. Every method, depending on synthesis condition, will result in different purity, crystallinity, layer number and diameter distribution. There are many companies that commercialize CNT, that are very much microscopically different. In the extreme case, a different batch of CNT from the same company is totally a different material. Scaling up the production of CNT is also difficult because it is very hard to keep the good synthesis parameter in the reaction chamber. This fact makes it difficult for the end users to use CNT, since users expect CNT to be exactly like the one they saw in textbooks, which in fact is different in real world. Therefore, end users should check the structure and characteristics of CNT that they have obtained so that they can adjust the composite preparation conditions accordingly. Since CNT is not long enough to substitute carbon fibers at the moment, further studies are required to fabricate long and continuous CNT at low cost. The art of CNT composite fabrication is in the surface treatment of CNT, and the choice of matrix materials. Since CNT is a nanometer-scale material and since surface structure may differ a great
deal, affinity with the matrix material is expected to be different from that of carbon fibers. CNT also tends to entangle, resulting in the formation of voids in nanocomposites. Although it may take time and resource, a better surface treatment and dispersion that suits CNT should be sought for further advancement and extended use of CNT nanocomposites. Acknowledgements This work was supported by the JSPS Grant-in-Aid Nos. 22510112 and 19002007, MEXT Regional Cluster Project, and JST Exotic Nano Carbons Project. References [1] Ebbesen TW, Ajayan PM. Large-scale synthesis of carbon nanotubes. Nature 1992;358:220–2. [2] Thess A, Lee R, Nikolaev P, Dai HJ, Petit P, Robert J, et al. Crystalline ropes of metallic carbon nanotubes. Science 1996;273:483–7. [3] Oberlin A, Endo M, Koyama S. Filamentous growth of carbon through benzene decomposition. J Cryst Growth 1976;32:335–49. [4] Jorio A, Dresselhaus MS, Dresselhaus G, editors. Carbon nanotubes. Springer; 2008. [5] Endo M. Grow carbon fibers in the vapor phase. Chem Technol 1988:568–76. [6] Li Y-L, Kinloch IA, Schaffer MSP, Geng J, Johnson B, Windle AH. Synthesis of single-walled carbon nanotubes by a fluidized-bed method. Chem Phys Lett 2004;384:98–102. [7] Dunens OM, MacKenzie KJ, Harris AT. Large-scale synthesis of double-walled carbon nanotubes in fluidized beds. Ind Eng Chem Res 2010;49:4031–5. [8] Hernadi K, Fonseca A, Nagy JB, Bernaerts D, Lucas AA. Fe-catalyzed carbon nanotube formation. Carbon 1996;34:1249–57. [9] Liu BC, Liang Q, Tang SH, Gao LZ, Zhang BL, Qu MZ, et al. Production of carbon nanotubes over pre-reduced LaCoO3 using fluidized-bed reactor. Chin Chem Lett 2000;11:1031–4. [10] Endo M, Kim YA, Hayashi T, Fukai Y, Oshida K, Terrones M, et al. Structural characterization of cup-stacked-type nanofibers with an entirely hollow core. Appl Phys Lett 2002;80:1267–9. [11] Endo M, Muramatsu H, Hayashi T, Kim YA, Terrones M, Dresselhaus MS. Buckypaper’ from coaxial nanotubes. Nature 2005;433:476. [12] Endo M, Hayashi T, Muramatsu H, Kim YA, Terrones H, Terrones M, et al. Coalescence of double-walled carbon nanotubes: formation of novel carbon bicables. Nanoletters 2004;4:1451–4. [13] Zang J-L, Yuan Q, Wang F-C, Zhao Y-P. A comparative study of Young’s modulus of single-walled carbon nanotube by CPMD, MD, and first principle simulations. Comput Mater Sci 2009;46:621–5. [14] Demczyk BG, Wang YM, Cumings J, Hetman M, Han W, Zettl A, et al. Direct mechanical measurement of the tensile strength and elastic modulus of multiwalled carbon nanotubes. Mater Sci Eng A 2002;334:173–8. [15] Aliev AE, Lima MH, Silverman EM, Baughman RH. Thermal conductivity of multi-walled carbon nanotube sheets: radiation losses and quenching of phonon modes. Nanotechnology 2010;21:035709. [16] Yang Jiao-Ping, Feng Qing-Ping, Chen Zhen-Kun, Fu Shao-Yun. Superiority of nanosized over microsized hyperbranched polymer second phase in modifying brittle epoxy resin. J Appl Polym Sci 2011;119:863–70. [17] Dresselhaus MS, Dresselhaus G, Sugiura K, Spain IL, Goldberg HA, editors. Graphite fibers and filaments. Springer; 1988. [18] Tasis D, Tagmatarchis N, Bianco A, Prato M. Chemistry of carbon nanotubes. Chem Rev 2006;106:1105–36. [19] Yokozeki T, Iwahori Y, Ishiwata S, Enomoto K. Mechanical properties of CFRP laminates manufactured from unidirectional prepregs using CSCNT-dispersed epoxy. Composites: Part A 2007;38:449–60.
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