Carbon nanotube reinforced composites: Potential and current challenges

Materials & Design Materials and Design 28 (2007) 2394–2401 www.elsevier.com/locate/matdes

Carbon nanotube reinforced composites: Potential and current challenges Amal M.K. Esawi *, Mahmoud M. Farag Department of Mechanical Engineering and, The Yousef Jameel Science and Technology Research Center (STRC), The American University in Cairo, 113 Kasr El Aini Street, P.O. Box 2511, Cairo 11511, Egypt Received 30 March 2006; accepted 25 September 2006 Available online 28 November 2006

Abstract The remarkable mechanical properties exhibited by carbon nanotubes have stimulated much interest in their use to reinforce advanced composites. Their elastic modulus is over 1 TPa and tensile strength is over 150 GPa, which makes them many times stiffer and stronger than steel while being three to five times lighter. In promoting their products, several manufacturers of sports equipment have advertised carbon nanotubes as reinforcements of some of their top of the line products. This paper evaluates the technical and economic feasibility of using carbon nanotubes in reinforcing polymer composites. It is concluded that carbon nanotubes can be used in conjunction with carbon fibers in a hybrid composite in order to achieve elastic modulus values in the range 170–450 GPa. As the sole reinforcing phase, carbon nanotubes present a viable option if elastic modulus values on the order of 600 GPa are desired. These conclusions are confirmed by a case study to select the optimum material for a tennis racket using the analytic hierarchy process. The discussion also shows that carbon nanotubes face several challenges, which need to be overcome before they can be widely used. They need to be produced in larger quantities at a lower cost, they need to be synthesized in longer lengths, and improved techniques are required to align and evenly distribute them in the matrix. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Polymer–matrix composites; Carbon nanotubes; Cost–benefit analysis

1. Carbon nanotubes 1.1. Background In view of their excellent properties, carbon nanotubes (CNTs) have been the focus of researchers’ interests since their discovery by Iijima in 1991 and have been labeled the ‘‘material for the 21st century’’ [1]. They are structures of nano-dimensions made up of rolled sheets of graphite. There are many ways in which a sheet of graphite is rolled up to form a tube: ‘‘zigzag’’, ‘‘chiral’’ and ‘‘armchair’’ are

*

Corresponding author. Tel.: +202 797 5786; fax: +202 795 7565. E-mail addresses: [email protected] (A.M.K. Esawi), mmfarag@ aucegypt.edu (M.M. Farag). 0261-3069/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2006.09.022

the names given to the different types. The tube can be single-wall (SWCNT) or multi-wall (MWCNT). Currently, both chemical and physical methods are used to synthesize carbon nanotubes. Electric arc discharge is the classic technique in which an arc is created between a graphite cathode and a graphite anode. By adding a catalyst (Co, Ni, Fe and Y powder), single walled tubes condense on the surface of the cathode. In laser ablation, carbon nanotubes form in the plume of carbon vapor evaporated by a laser from a graphite target held at 1200 °C. Adding metal dispersions to the target results in the formation of single walled tubes. Chemical vapor deposition (CVD), catalytic method, is the most common method in which a controlled reaction of the decomposition of a hydrocarbon gas (methane, carbon monoxide or acetylene) on a metal catalyst such as Ni, Fe or Co produces multi-

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walled carbon nanotubes. The catalytic tubes are cheaper, have higher purities (% nanotubes) and can be produced in kilogram quantities but – due to the presence of defects – their properties are much lower than the arc produced ones. The tubes produced by the three aforementioned techniques are randomly oriented. A variant of CVD: plasma enhanced CVD (PECVD) can produce aligned arrays of tubes with controlled diameter and length, but is more costly. The properties of carbon nanotubes are a mix of diamond and graphite: strong, thermally conductive like diamond; electrically conductive like graphite; and are light and flexible. Many potential applications have been proposed for carbon nanotubes: conductive polymers; energy storage and energy conversion devices;

Fig. 1. Modulus against density for various engineering materials [7]. Bubbles for CF, SWNT and MWNT were superimposed on the chart.

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sensors; field emission displays; replacing silicon in microcircuits; multilevel chips; probes for SPM (scanning probe microscopy). Recently, CNTs were considered for use as reinforcements in advanced composite materials in view of their high elastic modulus and strength compared to existing fibers (e.g. carbon and Kevlar) [1–3]. In that respect, being rolled up graphite sheets, their mechanical properties are expected to be equal to or greater than the value for a graphene sheet. Qualitative and quantitative TEM and AFM studies have been performed on individual tubes and have confirmed their superior strength and stiffness [2–6]. The Young’s modulus of a CNT is over 1 TPa. Its estimated tensile strength is over 150 GPa, i.e. they are a hundred times stronger than steel, though three to five times lighter. Fig. 1 is a plot of tensile modulus against density for various engineering materials [7]. Bubbles for SWNT, MWNT and CF were superimposed on the figure using data from [2–13]. The figure shows that CNTs occupy a region in the chart that is not previously occupied by any material; combining low weight and high modulus. Table 1 presents a comparison between the different types of commercially available carbon nanotubes. The 2006 prices range from $5/g for kg quantities of catalytic MWCNT to $800/g for high purity, select quality SWCNT. As may be seen from the table, the price of MWCNT is a function of purity and diameter with the higher purity and finer diameters being more expensive. Similarly, the price of SWCNT increases with increasing purity and also depends on the method of manufacture. With the current technology, the extraordinary properties that are often reported in the literature are only associated with the arcproduced defect-free nanotubes. Specific stiffness – being often of high technological importance in applications for which composite materials are considered – is plotted in Fig. 2 against price. With

Table 1 Comparison between different types of commercially available CNTs [9–13] Supplier

Product

Purity (% nanotubes)

Elastic modulus (GPa)

Diameter (nm)

Density (g/cc)

Pricea ($/g)

MER

Catalytic MWCNT

>90%

200–500

140

5–20

MER MER

Catalytic MWCNT Arc MWCNT

>90% 35%

1000–3700

35 13–50

MER Carbon solutions Carbolex

SWCNT AP-SWCNT

12 wt% nanotube 40–60%

1000+ 1000+

1.2–1.4 1.4

0.1 Powder 1.9 bulk 1.9 0.7 Powder 2.1 bulk 0.025 Powder 1.4

35–60 50

AP-SWCNT

50–70%

1000+

1.4

60–100

RFP-SWCNT

60–80%

1000+

1.4 (rope diam 20 nm) 1.4

1.4

250

P2-SWCNT

70–90%

1000+

1.4

1.4

400

SE-SWCNT Elicarb high purity SWCNT

70–90% 70–90%

1000+

1.4 <2 nm

1.4

800 180–360

Carbon solutions Carbon solutions Carbolex Thomas Swan a

Price depends on quantity purchased.

35–60 15–25

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1e+009

SWNT

CF

MWNT

Specific Modulus

1e+008

1e+007

1e+006

100000

10000

1000

0.1

1

10

100

1000

10000 100000 1e+006

Price (USD/kg)

Fig. 2. Modulus-to-weight ratio vs. price [7]. Bubbles for CF, SWNT, and MWNT were superimposed on the chart.

their very high modulus-to-weight ratio and high price, CNTs occupy the top right-hand corner of the chart. The analysis in this paper will be mainly concerned with the arc produced MWCNTs, which exhibit high elastic modulus and are moderately priced and, therefore, have a high potential of being used in reinforcing composites on a commercial scale.

(a) as the sole reinforcing phase (CNTRP), or (b) as an additional reinforcing phase in conjunction with carbon fibers (CF + CNT) in a hybrid composite. The above equation can be modified to estimate the different properties (Pc) of the resulting composites as follows: P c ¼ K 1 V 1 P 1 þ K 2 V 2 P 2 þ ½1  ðV 1 þ V 2 ÞP m

2. Carbon nanotube reinforced polymer 2.1. Comparison of CNTRP with CFRP The mechanical characteristics of CNT composites are not yet well established. In a recent paper [14], three models for predicting the mechanical properties of CNT/Al composites were presented and compared to experimental results. The models were based on three different strengthening mechanisms: thermal mismatch, Orowan looping and shear lag. The results of the latter model seem to agree very well with the experimental measurements of the elastic modulus of the composite. The model is, in fact, a special form of the rule of mixture in which: Ec ¼ K 1 V CNT ECNT þ ð1  V CNT ÞEm

ð1Þ

where Ec, ECNT and Em are elastic modulus values of the composite, CNT and matrix, respectively, VCNT is the volume fraction of CNT, K1 is the CNT efficiency parameter, which depends on the aspect ratio of the CNTs, on the Em/ECNT ratio, on the poisson’s ratio of the matrix, cm, and on VCNT, as follows [14]: K 1 ¼ ð1  ðtanhðnsÞÞ=ðnsÞÞ

ð2Þ 1/2

where n = (2Em/(ECNT(1+cm) ln (1/VCNT))) . The factor K1 approaches 1 for large volume fractions of high aspect ratio CNTs. In our case, carbon nanotubes can be used in reinforcing polymer matrix composites in two ways:

ð3Þ

where V1 and V2 are volume fractions of phases 1 and 2, representing carbon nanotubes and carbon fibers respectively, P1 and P2 are properties of phases 1 and 2, representing carbon nanotubes and carbon fibers, respectively, Pm is property of the polymer matrix, K1 is the CNT efficiency parameter, which will be assumed equal to 1, K2 is the CF efficiency parameter, which is equal to 1 for continuous aligned fibers in the direction of alignment, as is the case here. The properties of the phases used in forming our model composites and in calculating their properties are given in Table 2. Table 3 gives the properties of different model composites as calculated according to Eq. (3) and using the values in Table 2. The maximum amount of reinforcing phase is assumed to be 65%. In the case of hybrid composites it

Table 2 Properties of materials used in forming the composites used in this discussion Material Epoxy (high strength laminate) [15] MWCNT (35% purity) [11] Carbon fibers [15]

Elastic modulus (GPa) 32 2000 240

Density q (g/cc)

Price ($/g)

1.84

0.01

1.9 1.9

15.0 0.175

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Table 3 Calculated properties of different model composites

Epoxy + 5%CNT Epoxy + 20%CNT Epoxy + 30%CNT Epoxy + 50%CF Epoxy + 55%CF Epoxy + 60%CF Epoxy + 65%CF Epoxy + 1%CNT + 64%CF Epoxy + 3%CNT + 62%CF Epoxy + 5%CNT + 60%CF Epoxy + 10%CNT + 55%CF Epoxy + 15%CNT + 50%CF a

Ec (GPa)

Density qc (g/cc)

Specific modulus (E/qc) (GPa)/(g/cc)

Cost Cca ($/kg)

130.4 425.6 622.4 136 146.4 156.8 167.2 184.8 220 255.2 343.2 431.2

1.843 1.852 1.858 1.87 1.873 1.876 1.879 1.879 1.879 1.879 1.879 1.879

70.75421 229.8056 334.9839 72.72727 78.16337 83.58209 88.9835 98.35019 117.0836 135.8169 182.6503 229.4838

2152.357 8579.429 12864.14 92.5 100.75 109 117.25 544.0714 1397.714 2251.357 4385.464 6519.571

The cost of the composite is calculated using Eq. (3) and taking into account the fact that arc produced material contains only 35 weight% CNTs.

is, therefore, assumed that the maximum combined volume fraction of the reinforcing phases is 0.65 with the matrix occupying a volume fraction of 0.35. The figures in Table 3 show that the maximum elastic modulus that can be obtained with carbon fiber reinforcement is 167 GPa. To increase the elastic modulus of a composite beyond this limit, CNTs have to be used either in excess of about 7%, in the case of epoxy + CNT composites, or as a replacement of carbon fibers, in the case of the hybrid composites. Fig. 3 is a plot of the cost of different composites as a function of their elastic modulus. The figure shows that composites become progressively more expensive with the addition of more reinforcing phases to increase their elastic modulus. Using carbon nanotubes as the only reinforcing phase is shown to be a more expensive alternative if a given elastic modulus can be achieved by other means of reinforcement. It is interesting to note that carbon nanotubes appear as a viable solution to achieving elastic modulus values of about 600 GPa. The line connects the most viable options for a required value of E, which will be used in the following cost/benefit analysis. 2.2. Cost–benefit analysis In applications such as aerospace and sports equipment, it has been shown that both high stiffness (E) and low density (q) represent major selection criteria [15]. Those two E vs. Cost

CF

CNT

properties are combined into one parameter, the specific stiffness (E/q), which can be used to measure technical performance of one material relative to another. In such cases, the cost–benefit analysis compares the additional cost (DC) of achieving better performance (Dc) in material 2 relative to material 1 as follows: The relative incremental cost (DC) is taken as: DC ¼ ðC 2  C 1 Þ=C 1

The relative incremental performance index (Dc) is defined as: Dc ¼ ½ðE=qÞ2  ðE=qÞ1 =ðE=qÞ1

ð5Þ

where C1 and C2 are the costs and (E/q)1 and (E/q)2 the performance indices of materials 1 and 2, respectively. The cost/benefit can be measured as: ð6Þ

DC=Dc

Fig. 4 shows the change in the incremental (DC/Dc) at different levels of (E/q), for the most viable options discussed in Section 2.1. The peak in the curve of Fig. 4 shows the initial high cost–benefit of adding 1% CNT to the hybrid composite, the relative incremental cost of achieving additional incremental improvement in performance becomes lower as higher specific stiffness values are sought. 2.3. Case study – tennis racket The following simple case study is used to rank the suitability of the materials in Table 3 in making tennis rackets

CF+CNT

15000

Incremental Cost/Benefit vs. Specific

10000

Stiffness

5000 0 0

200

400

600

800

E (GPa)

Incremental Cost/Benefit

Cost ($/ kg)

ð4Þ

40 30 20 10 0 0

Fig. 3. Cost of different composites as a function of their elastic modulus according to Table 3. The line connects the most viable options for a given value of E.

100

200

300

400

Specific Stiffness (GPa)/(g/cc)

Fig. 4. Incremental (DC/Dc) at different levels of performance (E/q).

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following the analysis given in [15]. In a cost–benefit analysis, the cost is taken as cost of the material per unit mass while the benefit is considered to consist of two elements, power and damping. The power of a racket allows the delivery of faster balls with less effort and can be related to the specific modulus of the racket material. The damping is the ability of the racket material to reduce the vibrations in the strings after hitting the ball and thus reduce the possibility of the player developing tennis elbow. Materials with lower elastic modulus provide better damping. For the present analysis, the analytic hierarchy process (AHP) is used to rank the suitability of the materials in Table 3 in making tennis rackets. AHP is an approach to solving multi-criteria decision making problems that depends on pairwise comparison of alternatives with respect to the selection criteria [16]. The Criterium Decision Plus version 3.04 by InfoHarvest [17] was used to build a decision tree as shown in Fig. 5. The cost is taken to be directly related to the price of the material in $/kg, as shown in Table 3. The benefits side consists of power, which is taken to linearly increase with the specific modulus, and damping, which is categorized as high, medium and low for elastic modulus values less than 200 GPa, 200–300 GPa, and above 300 GPa, respectively. Various weights were allocated to the cost, power and damping and the resulting rankings are shown in Table 4. Table 4 shows that the material ranking is sensitive to the importance and weight allocated to each of the main variables: cost, power, and damping. Higher weight for the cost and less emphasis on power tends to favor the traditional Epoxy–CF composites. Epoxy–CF–CNT hybrid composites become more viable as the weight allocated to cost is reduced and the emphasis on power is increased. The epoxy–CNT composites become viable alternatives only at the two lowest weights for cost, and the consequent highest emphasis on benefits, with higher emphasis on power. The results of this case study are in agreement with Fig. 3, which shows that the most viable strengthening

option at relatively low elastic modulus values is carbon fibers (CF). Hybrid (CF + CNT) become viable at intermediate and high elastic modulus values. The highest elastic modulus values can only be achieved with 30% CNT reinforcement. 3. Challenges facing CNT-reinforced composites In the above analysis, it was assumed that the carbon nanotubes can be aligned and evenly distributed in the matrix of the CNT and CNT + CF hybrid composite. These issues are particularly problematic because of the small size scale of the CNTs which presents serious processing challenges. Another assumption is that the aspect ratio of the CNTs is high enough for the load to be efficiently transferred from the matrix to the carbon nanotubes. The following sections will examine such challenges and in addition will address the high cost issue of carbon nanotubes. 3.1. Fabrication of CNT and (CNT + CF) hybrid composites Many efforts have focused on incorporating nanotubes into polymer matrices [2,3,18]. In spite of the enormous interest CNTs have attracted as potential reinforcements for low weight structural composites, their performance so far has been inadequate. This is attributed by many experts to four issues: (1) the difficulty of dispersing the CNTs in the matrix due to the fact that they tend to stick together, (2) only small amounts of CNTs (1–5 wt%) have been used so far, (3) insufficient bonding at the nanotubes/matrix interface since CNT composites have been observed to have failed by either fracture at CNT/matrix interface or in the case of MWCNT pull out of the different layers of MWCNT [19], (4) the difficulty of aligning the tubes within the matrix. Although such issues have not yet been resolved, extensive efforts are underway to overcome them. For example, it

Fig. 5. Decision tree for AHP analysis.

 Epoxy + 60%CF  Epoxy + 1%CNT + 64%CF  Epoxy + 30%CNT 10%/90% 9

70%/30%

 Epoxy + 65%CF

 Epoxy + 60%CF  Epoxy + 1%CNT + 64%CF  Epoxy + 30%CNT 15%/85% 8

70%/30%

 Epoxy + 65%CF

 Epoxy + 55%CF  Epoxy + 55%CF  Epoxy + 30%CNT  Epoxy + 65%CF  Epoxy + 65%CF  Epoxy + 1%CNT + 64%CF  Epoxy + 1%CNT + 64%CF 20%/80% 6 7

60%/40% 70%/30%

 Epoxy + 60%CF  Epoxy + 60%CF

 Epoxy + 55%CF  Epoxy + 55%CF  Epoxy + 65%CF  Epoxy + 65%CF  Epoxy + 1%CNT + 64%CF  Epoxy + 1%CNT + 64%CF 25%/75% 4 5

60%/40% 70%/30%

 Epoxy + 60%CF  Epoxy + 60%CF

 Epoxy + 55%CF  Epoxy + 55%CF  Epoxy + 65%CF  Epoxy + 65%CF  Epoxy + 1%CNT+ 64%CF  Epoxy + 1%CNT + 64%CF 30%/70% 2 3

60%/40% 70%/30%

 Epoxy + 60%CF  Epoxy + 60%CF

 Epoxy + 50%CF  Epoxy + 60%CF  Epoxy + 65%CF 50%/50%

 Epoxy + 1%CNT + 64%CF  Epoxy + 55%CF

Second Highest Power/damping

50%/50%

Cost/benefits

1

Weights Alternative

Table 4 Ranking of materials according to AHP

Top ranking materials

Third

Fourth

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has been reported that extruding the composite as a thin film and drawing the film prior to cooling can achieve alignment of CNTs in a polymer matrix [20]. The dispersion problem has been overcome to some extent by sonicating a suspension of nanotubes in a solvent and by the shear forces during melt processing of the mix [18,20]. Depending on the matrix, different solvents have been used to disperse the nanotubes. For example, acetone, DMF and methanol have been used to disperse CNTs into epoxy-based composites [21], whereas THF has been used with polystyrene [20]. However, it was observed that the solvents influence the mechanical performance of the composite [21] and therefore other dispersion techniques need to be investigated. It was also reported that a strong nanotube–matrix interface could be achieved by nanotube surface treatment, or by modifying the polymer to provide functional groups that can bond the nanotube and the matrix strongly together [22]. Another technique involves growing the CNTs as coils – using a reduced pressure catalytic CVD method – thus taking advantage of nano-mechanical interlocking and minimizing inter-tube slipping [21]. Researchers have also grown nanotubes from CF with the aim of enhancing the bonding between the CF and the matrix. For example, Thostenson et al. [23] grew CNTs directly on CF using CVD and thus forming a hybrid, multi-scale composite. The load transfer was found to improve as a result of local stiffening due to the nanotubes at the fiber/matrix interface. However, this technique is criticized by other researchers as not taking full advantage of the strength of CNTs [21]. More creative processing techniques are therefore needed. Because of the massive interest in nanotubes–polymer composites, it is anticipated that the current extensive work attempting to tackle the various issues mentioned here will lead to positive results in the near future. 3.2. Length issues Commercially available nanotubes are usually 0.5–5 lm long. In the design of conventional composites, it is well known that the fiber length has a major influence on strengthening and stiffening of the matrix. For effective load transfer, the fiber length has to exceed a certain critical length lc given by the following equation: lc ¼ rf d=2sc

ð7Þ

where rf is the ultimate or tensile strength of the fiber, d is the fiber diameter and sc is the fiber–matrix bond strength. If the fiber length is less than lc, the matrix cannot effectively grip the fibers and as a result they will slip. Bai and Allaoui [24] performed experimental investigations into the effect of the length of aggregates of MWNTs on the improvement efficiency of the mechanical and electrical properties of nanocomposites. Their study concluded that though MWNTs had a dominant role in improving electrical conductivity, they had only a moderate effect on mechanical efficiency. It should be stressed that the aggre-

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Table 5 Critical length for various nanotube diameters Nanotube diameter (nm)

Observed average interfacial shear strength (MPa) [25]

Critical length lc from Eq. (7)a (lm)

30 70 90 130

90 20 40 18

10–25 105–260 67.5–170 217–540

a

Value depends on the nanotube strength (rf = 60–150 GPa).

gate length – rather than the individual tube length – was varied in their study. Table 5 gives estimates of the critical length, as calculated from Eq. (7), based on the interfacial bond strengths – measured by means of pullout experiments using the AFM – by Barber et al. [25] for MWNTs in a polymer matrix. Although researchers at the University of California have grown nanotubes up to 70 mm in length [26], and a worldrecord-length, 4 cm long SWNT, has been grown at the US Department of Energy Los Alamos National Lab [27], the length of the tubes available commercially only ranges from approximately 100 nm to several microns, depending on the growth process. Comparison with the calculated lengths in Table 5, shows that longer nanotubes than is currently available commercially are needed for effective strengthening and stiffening of composites. In addition, as with conventional fibers, the length of carbon nanotubes should be at least 15 times the critical length for them to be considered continuous and thus result in optimum stiffness [28]. The above analysis has assumed that the values for the average interfacial shear strength reported in [25] represent the strongest bond that can be achieved. However, if a stronger bond (>500 MPa) can be realized – perhaps by functionalizing the nanotubes before incorporating them in the matrix – then the critical length lc of nanotubes will be lower than calculated above (4.5 lm for smaller diameter tubes to 20 lm for larger diameter tubes). If, however, the CNT–matrix bond is weaker, the tubes need to be even longer. There is currently some interest in the development of advanced continuous fibers with nanoscale diameters [29], but more work is needed in this area.

cessing routes, which could be scaled up for commercial production, are needed if carbon nanotubes are to have wider applications. For application of carbon nanotubes in composites for which large quantities are required, the scaleup limitations of arc discharge and laser ablation make the cost of nanotubes-based composites prohibitive. Chemical vapor deposition is believed to offer the best potential for scaling up since the carbon source is a flowing gas [18]. In addition, the process tends to produce nanotubes with fewer impurities (catalyst particles, amorphous carbon and non-tubular fullerenes) compared to other techniques. Advances in the synthesis of CNTs continue to improve both their quantity and quality. For example, recently, researchers at EPFL [32,33] have managed to produce 100 g/day of purified carbon nanotubes using a rotary tube oven. However, growing a structurally perfect nanotube in large quantities is still not possible, though researchers anticipate that it is only a matter of time before this becomes a reality [31]. The current enormous efforts in using CNTs as reinforcements in polymer matrices make use of bulk quantities of relatively defective catalytically grown CNTs [30]. 4. Conclusions Although envisaged as reinforcing fibers for superstrong composites, carbon nanotubes face several challenges. They need to be produced in larger quantities at a lower cost, they need to be synthesized in longer lengths, and more efficient ways are needed to align and evenly distribute them in the matrix. Some research efforts are currently underway to overcome these challenges, but more needs to be done. At the current prices and with the available technology, carbon nanotubes can be added in small quantities to carbon fibers to form hybrid polymer–matrix composites. Such use allows manufacturers to produce components with higher performance at moderately higher prices. In fact, some companies are already producing sports equipment featuring small proportions of nanotubes for example a nanoengineered tennis racket has been produced by Babolat [34] and a carbon nanotubes hockey stick has been produced by Easton Hockey [35]. Acknowledgments

3.3. Cost of carbon nanotubes A wide variety of synthesis techniques have been used for yielding CNTs of different properties: size, aspect ratio, crystallinity, crystalline orientation, purity, entanglement and straightness [30]. Until now, it is not clear what the ideal CNT properties would be, as this depends on matrix and application. Researchers agree that one of the major obstacles to using CNT is cost [31]. Generally, the synthesis techniques used for making nanotubes are expensive. High quality/high purity carbon nanotubes currently cost $800/ g and even ones with defects and impurities (metal catalyst and amorphous carbon) cost $5–35/g. Efficient novel pro-

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