DOI: 10.1002/cphc.200700077

Mechanical Properties of Carbon NanomaterialsTakuya Hayashi,*[a] Yoong Ahm Kim,[a] Toshiaki Natsuki,[b] and Morinobu Endo[a]

Introduction

It is well known that carbon nanotubes, possessing the cylin-drical structure made of a seamless graphene sheet, are ex-pected to have outstanding mechanical properties such as ahigh tensile strength and a high elastic modulus. These proper-ties are the reason for considering carbon nanotubes to be anideal filler material for composite materials used in aerospacestructural elements and sporting goods. Some carbon nano-tube composites are already, applied for sporting goods, avail-able on the market. Research and development activities willlead to more sophisticated and optimized carbon nanotubefabrication and composite production processes, that willwidespread carbon nanotube composites to many applicationsfound in everyday life.

In this Minireview we, first, will briefly summarize the synthe-sis methods for carbon nanotubes, which are important forany application. Then, the mechanical properties obtainedboth theoretically and experimentally will be detailed. Finally,we will overview carbon nanotube composite properties ingeneral and we will detail applications that are based oncarbon nanotube composite materials mainly synthesized inour laboratories.

Fabrication of Carbon Nanotubes

There are three major methods used for fabrication of carbonnanotubes (CNTs). First, the catalytic chemical vapor deposition(CCVD) method[1] is suitable for mass production of CNTs. Thisprocess allows control over a wide range of tube diameters,layer numbers, and tube lengths. Nanosized metal particles areused as catalysts in this method (Figure 1), and the metal parti-cle size has an influence on the diameter of the resulting tube.The layer number depends on the amount of starting materialsfed during the synthesis. The length of the tube also dependson the amount of carbon source, but equally important is theactivity of the catalyst particle that determines duration of thegrowth of the tube. If there is an effective way to maintain thecatalyst active, the tube can, in theory, grow to any length thatis desired. A second CNT growth method is the laser ablationtechnique.[2] High-power laser pulses are applied to a carbontarget and a small amount of catalyst, which results in high-quality single-walled CNTs. Control over the CNT diameter, the

layer number, and the length obtained by this method isminor compared to the CCVD method. However, near-perfectsingle-walled CNTs with no defects can easily be obtainedusing the laser ablation technique. Third, the arc evaporationmethod is similar to the commonly known procedure used forsynthesizing fullerenes such as C60 and C70.

[3] Nanotubes werefirst found as a by-product of the C60 synthesis in the cathodicdeposit that is formed during arc discharge between carbonrod electrodes. The CNT in the cathodic deposit is multi-walled, and although its crystallinity is very high, no control ofthe diameter, the layer number, and the length of the tube canbe obtained. To produce a single-walled CNT, arc discharge be-

Carbon nanomaterials show a variety of interesting chemicaland physical properties. In this Minireview we focus on the me-chanical properties of carbon nanomaterilas with emphasis oncarbon nanotubes and their composite materials. We introduce

some recently developed components made of carbon nanotubecomposite materials and outline their importance for applica-tions in everyday life.

Figure 1. Growth of CNTs from a porous substrate with catalyst particles bythe CCVD method.

[a] Prof. T. Hayashi, Prof. Y. A. Kim, Prof. M. EndoFaculty of Engineering, Shinshu University4-17-1 Wakasato, Nagano 380-8553 (Japan)Fax: (+81)26-269-5208E-mail : hayashi@endomoribu.shinshu-u.ac.jp

[b] Prof. T. NatsukiFaculty of Textile Science and TechnologyShinshu University3-15-1 Tokida, Ueda 386-8567 (Japan)

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tween metal catalyst-containing electrodes is performed. Thesoot that is deposited in the chamber contains single-walledCNTs.

CCVD tubes usually contain a higher amount of defectscompared to the tubes produced by the other two methods atthe as-grown state, but the potential to control the CNT diam-eter, the number of layers, the crystallinity, and the length ismuch higher than with the other methods. By making use ofhigh-temperature heat treatment, it is possible to improve thecrystallinity of CNT to a near-perfect level (Figure 2). Therefore,the CCVD method has become mainstream among many CNTsynthesis methods. Despite its simplicity in the experimentalsetup, there are many parameters such as the catalyst, thetype of carbon source, the gas flow rate, and the furnace tem-

perature, to list a few, with influence on the synthesis of CCVDCNTs. Deep experience is needed for obtaining the desiredtype of CNT by the CCVD method.

Mechanical Properties of Carbon Nanotubes

Theoretical prediction of the mechanical properties of CNTswas, to our knowledge, first performed by Tomanek et al. in1993.[4] By using a Keating potential, the authors have shownthat the Young’s modulus of small single-walled CNTs can beup to 1.5 TPa, which surpasses materials well-known for a hightensile strength, such as steel strings, synthetic fibers, and soforth. Other groups have used different empirical and non-em-pirical methods and larger models and multiple layers to pre-dict various mechanical properties of CNTs,[5–10] which alsoshow that CNTs are indeed expected to be resilient materials.

Experimentally the mechanical properties of CNTs were de-termined by observing the vibrations of CNTs with a transmis-sion electron microscope (TEM).[11] By measuring the amplitudeof vibrating multi-walled CNTs, Treacy et al. have shown thatthe Young’s modulus is about 1.8 TPa. Wong et al. have usedan atomic force microscope (AFM) tip with anchored multi-walled CNTs to measure the bending force of the CNTs, andthey obtained an average value of 1.28 TPa.[12] Other experi-ments have shown similar or worse results depending on thesample and the experimental conditions.[13–15] The reason forthese deviations in the experimental values is the experimentalmethod itself and the nature of CNTs. Since CNTs are nanosizedmaterials, it is extremely difficult to manipulate and measuretheir mechanical properties. This makes it hard to reproducethe same experimental conditions for mechanical testing. Asdescribed above in the CNT production method section, themicrostructure of CNTs differs to a great extent depending onthe CNT synthesis method and the synthesis conditions. Evenin the same batch of synthesized CNTs, a distribution in length,layer number, diameter, and crystallinity is observed. Especially,the crystallinity of the tube wall greatly affects the mechanicalproperties of CNTs. Therefore, selecting different types of CNTsalso affects the experimental values, which results in the largediscrepancy among reported values. From the wide distribu-tion of Young’s moduli in various measurements, we can un-derstand the threshold values, which can give an indication forwhat we may expect from the CNT once it is applied in CNTcomposite materials.

Carbon Nanotube Composite Materials

Based on these theoretical and experimental studies, CNTs areregarded as the prime candidate for the next generation ofcomposite filler materials. With this perspective in mind manyresearchers in industry and institutions are strenuously seekingfor applications of CNT as filler materials in composite materi-als. As a result, there are already some commercialized prod-ucts on the market available. Figure 3 shows examples of suchproducts, which utilize our carbon nanotubes as filler materialin a three-phase composite. CNT fillers are expected not onlyto improve the mechanical properties of the composite, butFigure 2. Improvement of crystallinity after heat treatment (HHT) at 2800 8C.

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also to implement some extra functions such as electric con-ductivity, thermal conductivity, and electromagnetic shielding.For example, when added to transparent polymers, CNTs willimprove the conductivity without sacrificing much of the trans-parency of the polymer.

There are different kinds of composite materials thatdepend on the matrix material such as polymers, metals, andceramics, and so forth. The choice of diameter, layer number,length, crystallinity, and morphology of the CNTs are making iteven more complicated. Therefore, if there is a target applica-tion, we first have to try and find out which materials is mostappropriate and then we adjust the conditions for preparingthe composite material.

Xu et al. have prepared a thin-film epoxy composite includ-ing 0.1 wt% multi-walled CNTs as a filler and they showed thatthe elastic modulus improves up to 20%.[16] This could be oneof the most successful cases, since sometimes we need to addmore than 1 wt% to obtain the maximum Young modulus. Insome cases, for the improvement of the Young modulus andthe tensile strength in CNT composites, more than 10 wt% ofCNT were added.[17,18] In these cases, an increased amount ofCNTs in the matrix resulted in a decrease of the fracture strain,which indicates the strength of this method for designing ma-

terials with tailored characteristics. In many studies, CNTs usedas filler material were as-received samples, either purified oras-grown, from the CNT supplier.

Since an improvement of the mechanical propertise is ex-pected upon addition of CNTs to the composites, the affinityof CNTs with the matrix is crucial. Without strong interactionsbetween CNTs and the matrix material, mechanical propertiescan be even worse than for the material without CNTs. Sincethe CNT surface is, in most of the cases, the graphene planewithout much defects or active sites, some special treatment isneeded to strengthen the interactions with the matrix. Thesimplest way to improve these interactions is the oxidation ofthe CNTs. Since the edge side of the graphene sheet is muchmore active than the plane side, letting the graphene edgeappear at the surface of the CNT sidewall is a good way to im-prove the interaction with the matrix. By heat-treating CNTsamples in air, some part of the CNT sidewalls become thinner.By changing the heat treatment temperature and the durationof heat treatment, the rate of sidewall etching can be control-led (Figure 4). Another way to etch the CNT sidewalls is acid

treatment, which can have similar effects as the oxidationmethod. The difference is that some functional groups can beattached during the acid treatment that could be useful for im-proving the affinity with the matrix.

Surface functionalization of multi-walled CNTs by acid treat-ment followed by aminosilane treatment was performed andintroduced in rubber composites.[19] Surface-functionalizedCNTs improved the tensile strength and the elongation atbreak of the composite by 20 to 30% compared to the non-functionalized CNT/rubber composites.

The dispersion of CNT in the composite is also very impor-tant, because the entanglement of long CNT and the van derWaals interactions between the tubes result in CNT aggregatesand formation of lager clusters with many holes and rooms sothat high-viscosity matrix elements cannot penetrate. There-fore, insufficient dispersion of CNTs, leaving aggregated clus-ters uncoupled, will result in voids or vacancies in the matrix

Figure 3. Golf clubs and tennis rackets made of a material with CNT compo-nent. Part of club head uses CNT composite to improve the drive and en-large the sweet spot. Tennis rackets fully utilize CNT as a nanofiller to rein-force the composite and add durability to the racket.

Figure 4. CNTs oxidized at 640 8C for 15 min. The etching of the sidewall ofthe CNTs can be clearly seen, and the graphene edge is exposed.

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Mechanical Properties of Carbon Nanomaterials

that do not improve the mechanical properties of the compo-sites. To avoid the aggregation of CNTs, dispersions obaintedby kneading are useful for high-viscosity matrices. Dependingon the speed and time of kneading, the dispersion and thelength distribution of the CNTs change. Since the aspect ratioof the tube is an important factor for the mechanical proper-ties of the composites,[20] it is possible to optimize the lengthand dispersion of CNTs at a time. In a low-viscosity matrix, useof sonication or surface functionalization should be effective.However, the length of the tube should be adjusted by chemi-cal or mechanical processes such as ball milling (Figure 5) priorto application to the matrix.

There is another type of CNT, which is called the cup-stackedCNT.[21] Its morphology is like the stack of truncated cone-shape graphene layers, which has the edge of a graphenesheet exposed at the tube sidewall (Figure 6). Due to the ex-posed edge sites at the sidewall of the tube, cup-stacked CNTs(CSCNTs) are considered to have a good affinity to the matrix.Choi et al. have used the CSCNT to prepare the epoxy nano-composites and found that addition of 5 wt% CSCNT improvedthe Young’s modulus by 20~30%, and the friction coefficientby ~40%.[22]

For many applications such as aerospace structural elementsand sporting goods, CNTs are expected as an additive that re-inforces the matrix resin, while the main filler materials are stillconventional carbon fibers. This is because at the moment, thelength of the CNTs and the affinity to the matrix are not goodenough to be able to sustain the load and stress that is ap-plied in any macroscopic application. As an additive, multi-walled CNTs used in three-phase composites are known to im-prove the toughness of the matrix and they also improve thecompressive strength of the composite. It was also found thatCNTs in the matrix damp high-frequency vibrations that canoccur in some applications. The use of CNTs as nanofiller mate-rials in C/C composites have made it possible to produce light-er yet durable sporting goods such as road bikes and othercomponents.[23] In three-phase application, the composites are

usually provided as a prepreg or as woven fibers. To producethe shape of some elements, prepregs are usually laminated insome mold with the shape of the element. The lamination pro-cess in shape forming is responsible for the weak-spot in theout-of-plane strength or the strength between the carbonfiber plies. The effect of adding CSCNT as a nanofiller in C/Ccomposite laminates was reported by Yokozeki et al. and theyshow that the incorporation of CSCNTs improved the fracturetoughness by 40%.[24]

Figure 5. The broken end of CNTs after ball milling for 2 h. By using theright conditions, we can make tubes shorter.

Figure 6. Structural model and TEM image of cup-stacked CNTs. The gra-phene edge is exposed at the side of the tube.

Figure 7. Photograph of the microcatheter with CNT (black) and withoutCNT (transparent). The black microcatheter uses CNT/nylon composite andthe other uses only nylon. The outer diameter is ~0.53 mm and inner diame-ter is ~0.43 mm.

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Applications of Two-Phase CNT CompositeMaterials

Here, we will present some of the CNT composite componentsthat were recently developed using CNTs prepared using ourgrowth methods. Figure 7 shows the microcatheter made ofCNT/nylon composites.[25] It was found that the inclusion ofCNTs greatly improves not only the elastic modulus but alsothe anti-thrombogenicity and blood coagulation resistivitycompared to a conventional pure nylon microcatheter. By in-corporating CNTs in nylon matrices, the elastic modulus in-creases from 820 to 1200 MPa. It was also shown that theCNT/nylon microcatheter shows a relatively high biocompati-bility compared to a pure nylon catheter. These characteristicsmake the CNT/nylon microcatheter extremely useful for drugdelivery in thin blood vessels.

CNT composites improve the formability of the component,which is important for small components such as the lens unit

of a cellular phone (Figure 8). In the lens unit, the precision ofthe lens holder determines the performance of the lens unitand consequently the image quality. When compared to thelens holder without CNTs, the CNT composite lens holder has amuch better resolution all the way from the center to theedge of the image (Figure 9). This is because CNT compositespossess a higher thermal conductivity that leads to a low-tem-perature gradient within the material and makes it easy to

Figure 8. Camera unit for a cell phone (a) and the lens holder made of aCNT composite (b).

Figure 9. Comparison of the resolution chart of a CNT composite lens holderand a conventional lens holder without CNTs. The improvement can beclearly seen, especially at the edge of the chart.

Figure 10. Bolts and nuts coated with CSCNT–polymer paint to improve vari-ous properties such as corrosion and impact resistance, and hardness. (Cour-tesy of GSI Creos Corp. , Japan).

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Mechanical Properties of Carbon Nanomaterials

keep the exact shape of the mold during the manufacturingprocess.

When CSCNTs are mixed in polymers, this mixture can beused as a coating material. Some bolts and nuts were coatedwith the CSCNT–polymer paint to improve the rust and corro-sion resistance, the impact resistance, the bending resistance,and the hardness (Figure 10). It was shown that by applyingthe CSCNT coating, the material becomes 15 times more corro-sion-resistive, 7 times harder, and a 4 times higher bending re-sistance compared to a conventional polymer coating. TheCSCNT–polymer paint enhances the durability of componentsused in ships and bridges, and as a consequence it can reducethe maintenance costs.

Conclusions

CNTs are expected to have a superior weight-to-strength ratiocompared to any other existing materials. However, upontaking advantage of the mechanical properties of CNTs, the mi-crostructure of CNTs must be taken into consideration. If theCNTs of interest have defective sidewalls, or an entangledshape, it is likely that the CNTs do not possess the mechanicalproperties that might be expected from theoretical studies onideal CNTs. In such cases, thermal treatment should be effec-tive to improve the alignment and the defect annealing, whichleads to an improvement of the mechanical properties of CNTs.

For the application of CNTs in composites, strong interac-tions of the CNTs with the matrix and good dispersion of CNTsin the matrix are the critical factors for bringing about the bestof the CNTs’ mechanical properties. By using CNTs in compo-sites, in addition to the excellent mechanical properties, wecan improve or implement other properties such as electricand thermal conductivity, or an electromagnetic shieldingfunction. These additional characteristics can add extra valueto the CNT composites, and can justify the additional costs in-curred by their use. For some applications such as aerospacestructural elements, three-phase CNT/C/C composite should bemore useful at the moment. Further research on CNTs andtheir composites will show that they are indispensable materi-als for many applications.

Acknowledgements

Part of this work was supported by grants-in-aid from MEXT (No.16201024, M.E.) and JSPS (No. 18710084, Y.A.K. , and No.1771096, T.H.).

Keywords: carbon · composite materials · nanomaterials ·nanotubes · mechanical properties

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5, 101.

Received: February 4, 2007

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