Carbon nanotube yarn structures and properties

CHAPTER 7

Carbon nanotube yarn structures and properties Menghe Miao

CSIRO Manufacturing, Geelong,VIC, Australia

Carbon nanotubes (CNTs) have been shown to possess extraordinarily high mechanical properties [1, 2] combined with good electrical and thermal conductivity [3]. The challenge is to organize these nano-sized building blocks into macroscale structures that express similar properties. Without considering their detailed atomic structures, CNTs can be considered as nanoscale fibers that resemble the diameter of fibrils in plant and animal fibers such as cotton and wool. It is, therefore, a logical approach to align the CNTs in the form of a fiber or yarn that is expected to outperform their conventional textile counterparts [4].

7.1  CNT yarn geometry 7.1.1 Twist Twist insertion has been an important method to densify CNT webs drawn from vertically aligned CNT arrays (CNT forests) since it was first reported in 2004 [5].The method imitates the spinning of short textile fibers (staple fibers) into a continuous yarn, which has been traditionally used and is still the predominant method of staple fiber yarn production in the textile industry today. Twist is applied to a yarn by rotating one end of the yarn while it is wound on a bobbin. In the textile industry, twist is measured by counting the number of turns required per unit length (T, turns per unit length) to cancel out the twist applied to the yarn during spinning. In spinning mills, the twist applied to a yarn is set by tuning the ratio between the rotational speed of the spindle (revolutions per minute) and the throughput speed of the yarn (length per minute) set on the spinning machine. The geometry of twisted yarns is often represented by a series of coaxial helices (Fig. 7.1), a model first proposed by Gégauff in 1907 [7].The model has been widely adopted, sometimes with minor modifications, in yarn structural Carbon Nanotube Fibers and Yarns https://doi.org/10.1016/B978-0-08-102722-6.00007-9

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Fig.  7.1  Coaxial helix model of twisted yarns [6]. (Reprinted with permission from M. Miao, The role of twist in dry spun carbon nanotube yarns, Carbon 96 (2016) 819–826.)

mechanics analysis [8].When describing the helical path of a fiber in a yarn, we use the angle between the fiber and the yarn axis rather than the rising angle of the helix.The length of one turn of the helices is a constant (h) independent of the radial position of the fiber in the yarn, which is equal to the reciprocal of the twist of the yarn (T), h = 1/T. Consequently, the helix angle of a fiber varies according to its radial position in the yarn. At the center of the yarn (r = 0), the fiber helix angle is zero; and at the yarn surface (r = d/2, where d is the yarn diameter), the fiber helix angle is the maximum (θ in Fig. 7.1). The helix angle of fibers on the yarn surface is commonly referred to as the twist angle of the yarn. The twist angle of a yarn is related to its twist T and diameter d: tan θ = πdT. Although it is convenient to measure the twist level of a yarn by the number of turns per meter (TPM), the helical angle of fibers on the yarn surface should be used when comparing the degree of twist in yarns with different diameters. Clearly, for two yarns with the same TPM, a thicker yarn has a larger twist angle than a finer yarn.

7.1.2  Yarn diameter and linear density Measuring the diameter of a CNT yarn from SEM images has been a common practice in earlier research works [5, 9–11]. This method has some



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inherent shortcomings. Diameter measurement taken from random points on a short yarn specimen is often not representative of the yarn sample. It is especially problematic if the diameter measurement was taken on the fracture end of a specimen, outside of the tensile test specimen, or from another part of the yarn sample. Many CNT yarns and fibers have irregular cross-sectional shapes (e.g., yarns produced by liquid densification) and thus it is difficult to determine their diameter. Laser diffraction method is widely used for monitoring wire diameter [12]. This method has been used for measuring CNT yarn diameter [13– 15]. A laser diffraction system with multiple laser beams may be mounted on a tensile tester to measure the diameter of the tensile specimen at several points during tensile testing [14, 15] to allow the calculation of instantaneous stress and Poisson’s ratio. Fig. 7.2 shows how the diameter of twisted CNT yarns is affected by the strain applied to the yarns. The low twist yarn had an initial diameter of 33.1 μm. At 0.0536 axial strain, the yarn diameter contracted to 20.9 μm. This gives a huge Poisson’s ratio of 6.8, which is more than 20 times higher than common solid materials (Poisson's ratio is about 0.3). A very large disparity can appear when yarn diameter is used for calculating yarn tensile stress [14]. On the other hand, the diameter change of highly twisted yarns is much less dramatic, giving a Poisson’s ratio less than 1. Textile yarns, especially yarns spun from staple fibers, do not have well-defined cross-section boundaries although they are often approximated to a circular shape to simplify analysis. The variability of yarn cross section is associated with the random number of fibers along the yarn length, the irregularity of constituent fibers, and the imperfect condition in forming the yarn. Because porosity is inevitable in yarns produced from staple fibers, the application of a relatively small tensile load to a twisted yarn, which produces only a small change in the yarn length, can cause a significant decrease in yarn diameter. Generally speaking, it is difficult to make a precise determination of yarn diameter that is applicable to all downstream processes and specifications. For these reasons, textile technologists prefer to specify yarn size in terms of count number (traditionally used, expressed in length/ mass, e.g., in metric count, 1 Nm = 1 m/g) or linear density (preferred unit, expressed in tex, 1 tex = 1 g/km = 1 mg/m). Unlike diameter, linear density is an average value from a long length of yarn. CNT fibers and yarns are also porous and often have irregular and inconsistent cross-sectional shape within a sample. Measuring CNT yarn thickness by linear density has now become increasingly common. The ­linear

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High twist yarn

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Axial strain

0.08

0.1

0

(F)

0.02

0.04

0.06

0.08

0.1

Axial strain

Fig. 7.2  SEM images of yarns under different levels of axial strain. (A) Low twist yarn at zero axial strain; (B) low twist yarn at 0.0536 axial strain; (C) high twist yarn at zero axial strain; (D) high twist yarn at 0.061 axial strain; (E, F) yarn diameter and Poisson’s ratio at different axial strain levels [14]. (Reprinted with permission from M. Miao, J. McDonnell, L. Vuckovic, S.C. Hawkins, Poisson’s ratio and porosity of carbon nanotube dry-spun yarns, Carbon 48 (10) (2010) 2802–2811.)

density of a CNT yarn in tex can be determined by weighing a length of the CNT yarn sample using a microgram balance [14]. Alternatively, a Vibrascope [16] may be used to determine the linear density of a relatively short yarn sample directly.



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7.1.3  Nanotube packing density, bulk density, and porosity If all nanotubes are perfectly straight and aligned in one direction (a bundle of parallel cylinders), they can lie next to each other and the nanotube assembly achieves the maximum packing density. Fig.  7.3 shows a cross-sectional view of parallel cylinders closely packed in a hexagonal array. Using the parallelogram “unit cell” [14], we can find that the proportion of unfilled area (including spaces inside the nanotubes), or the minimum porosity of the closely packed yarn, is

ϕmin

π 1 − (d /D )2  =1− 2 3

(7.1)

where D (nm) is the outer diameter of the CNT and d (nm) is the inner diameter of the CNT. If the nanotubes are treated as solid cylinders, i.e., d = 0, Eq. (7.1) gives the minimum porosity of 9.3%, or the maximum packing density of 90.7%. If the space inside the nanotube is counted as voids in the yarn, the minimum porosity will be higher. For example, when d/D = 0.4, the minimum porosity is 23.8% [14]. The maximum number of nanotubes that can be packed in a 1 μm2 cross section (nmax, tubes/μm2) can be calculated from nmax =

2 3 × 106 3D 2

(7.2)

Unit cell

Fig. 7.3  Close hexagonal packing of parallel cylinders [14]. (Reprinted with permission from M. Miao, J. McDonnell, L. Vuckovic, S.C. Hawkins, Poisson’s ratio and porosity of carbon nanotube dry-spun yarns, Carbon 48 (10) (2010) 2802–2811.)

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In practice, the nanotubes in CNT yarns are not perfectly aligned and closely packed, and thus the yarn porosity is greater and the nanotube packing is lower than the above values, respectively. The real yarn porosity can be derived from the bulk density of the CNT yarn (ρyarn) and the density of the constituent nanotubes (ρcnt) ρ yarn (7.3) ϕ =1− ρ =1− ρcnt where ρ is the CNT packing fraction of the yarn. The average yarn density ρyarn can be calculated from the linear density and the average cross-sectional area of the yarn. The density of CNTs is strongly related to the nanotube diameter and number of walls [17]. The number of walls has a strong relationship with the diameter for chemical vapor deposition (CVD) grown nanotubes [18]. Fiber packing fraction in an assembly is affected by both the compression and the alignment between the fibers.The pressure P required to compress randomly orientated elastic fibers into a structure with a fiber packing fraction ρ follows the well-known van Wyk power relationship [19]. P = kE ρ 3

(7.4a)

where E is the Young’s modulus of the fiber and k is a proportionality factor to be determined by experiment. In a structure comprised of perfectly aligned fibers, almost no pressure is required to lay the fibers next to each other to obtain the maximum packing fraction. In practice, fibers in aligned structures such as yarns are not perfectly straight (i.e., with some level of waviness or crimp) and there are local misalignment between fibers. In such a case, the van Wyk relationship can be modified to [20]. P = kE ρ n

(7.4b)

The exponent n is greater than 3 and can be as high as 15 for highly aligned glass fiber rovings (no crimp). As the value of fiber packing fraction ρ is always smaller than unity, it is easier to densify aligned fibers than misaligned fibers. For yarns formed from textile fibers, some level of fiber misalignment is essential to achieve a self-locking structure. Fiber misalignment can be introduced during yarn formation due to twist insertion, fiber migration [21], fasciation [22], felting [23], and interlacing [24]. The pressure between staple fibers and the resulting interfiber friction are the primary forces that give a textile yarn its tensile strength.



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Unlike conventional textile fibers, van der Waals forces between nanotubes are of high importance to the strength of unbonded CNT yarn. The magnitude of the van der Waals force depends strongly on the distance between the nanotubes, which is a function of nanotube packing fraction in the yarn. 7.1.3.1  Twisted yarns Generally speaking, CNTs are not uniformly distributed in CNT yarns. The local nanotube packing fraction in a twisted CNT yarn can vary considerably. Sears et  al. [25] showed that nanotube packing fraction decreases from the center to the peripheral in twisted CNT yarns, as shown in the SEM images taken from focused ion beam (FIB) sectioned yarns in Fig. 7.4A. When studying the porous structure of CNT yarns, two types of voids may be distinguished, i.e., voids between nanotubes in the same bundle and voids between the bundles, as shown in Fig. 7.4B and C. Due to

Fig. 7.4  Nanotube packing density distribution in twisted CNT yarns. (A) SEM image of CNT yarn cross section, showing radial density distribution [25]. (B, C) SEM images showing CNT bundles and pores between CNT bundles [26]. (Panel (A) reprinted with permission from K. Sears, C. Skourtis, K. Atkinson, N. Finn, W. Humphries, Focused ion beam milling of carbon nanotube yarns to study the relationship between structure and strength, Carbon 48 (15) (2010) 4450–4456.; Panels (B and C) reprinted with permission from D. Zhang, M. Miao, H. Niu, Z. Wei, Core-spun carbon nanotube yarnsupercapacitors for wearable electronic textiles, ACS Nano 8 (5) (2014) 4571–4579.)

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the strong van der Waals force, CNTs tend to form bundles during growth and processing. Undensified CNT webs drawn from a CNT forest (Chapter 2) have an extremely high porosity, estimated to be in the vicinity of 99.97% [27]. The porosity of a twisted CNT yarn decreases with the degree of twist inserted into the yarn, as shown in Fig. 7.5. A highly twisted CNT yarn can have a packing fraction as high as 0.6 (a porosity of 0.4) [14]. This is similar to the fiber packing fraction of a highly densified textile yarns [28]. If the twist in a twisted yarn is removed, the yarn diameter can increase substantially, as shown in Fig. 7.6.The change of yarn porosity is believed to be mainly due to the widening of the voids between CNT bundles while voids within a CNT bundle do not change so significantly [6]. 1

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5000 t/m

(C)

60

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(D)

Fig.  7.5  CNT yarn bulk density and porosity. (A) Relationship between twist angle and yarn bulk density, (B) relationship between twist angle and yarn porosity [14], and (C, D) SEM images of FIB sections milled through CNT yarns with two levels of twist [25]. (Panels (A and B) reprinted with permission from M. Miao, J. McDonnell, L. Vuckovic, S.C. Hawkins, Poisson’s ratio and porosity of carbon nanotube dry-spun yarns, Carbon 48 (10) (2010) 2802–2811.; Panels (C and D) reprinted with permission from K. Sears, C. Skourtis, K. Atkinson, N. Finn, W. Humphries, Focused ion beam milling of carbon nanotube yarns to study the relationship between structure and strength, Carbon 48 (15) (2010) 4450–4456.)



Carbon nanotube yarn structures and properties

(B)

0.8

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Yarn density (g/cm3)

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145

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Fig. 7.6  (A, B) SEM images of twisted and twist-untwisted CNT yarns. (C, D) Relationship between twist/false twist and yarn density and porosity [6]. (Reprinted with permission from M. Miao, The role of twist in dry spun carbon nanotube yarns, Carbon 96 (2016) 819–826.)

7.1.3.2  Rub-densified yarns Rub-densified twistless yarn produced at different rubbing roller pressures showed different CNT packing density distributions across the yarn [29]. At low roller pressure, the yarn showed a high-density shell and a low-density core, as shown in Fig. 7.7A. Due to its very low density, we do not expect the core to make a significant contribution to the yarn strength. It was estimated that the core occupies about 40% of the total yarn cross-sectional area. The average density of the sheath could be estimated from the average yarn density (0.64 g/cm3) divided by the area proportion of the sheath (i.e., 0.6), giving a sheath density of 1.07 g/cm3. The low-density core can be eliminated by increasing the pressure between the rubbing rollers and by lowering the yarn tension, resulting in a ribbon-like yarn cross section with seemingly uniform CNT packing density (Fig. 7.7B). 7.1.3.3  Die-drawn yarns Cross sections of die-drawn yarns are shown in Fig.  7.8 [30]. The yarn diameter was controlled by adjusting the die diameter. As Fig. 7.9 shows,

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Fig. 7.7  SEM images of FIB milled rub-densified yarn cross sections. (A) Low-pressure rub-densified CNT yarn with a high-density sheath and a low-density core and (B) high-pressure rub-densified CNT yarn showing a uniform CNT packing density and a flat cross section [29]. (Reprinted with permission from M. Miao, Production, structure and properties of twistless carbon nanotube yarnswith a high density sheath, Carbon 50 (13) (2012) 4973–4983.)

30 µm

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30 µm

(B)

300 nm

(E)

30 µm

(C)

300 nm

(F)

30 µm

(D)

300 nm

(G)

300 nm

(H)

Fig. 7.8  Die-drawn CNT yarns. SEM images showing cross sections of CNT yarns with diameters of (A, E) 30 μm, (B, F) 35 μm, (C, G) 55 μm, and (D, H) 75 μm, respectively [30]. (Reprinted with permission from K. Sugano, M. Kurata, H. Kawada, Evaluation of mechanical properties of untwisted carbon nanotube yarn for application to composite materials, Carbon 78 (2014) 356–365.)

with the increase of die diameter, which means less compression during die drawing, the density of the resulting yarn decreased. Note that the yarns produced using the two small diameter dies (30 and 35 μm) showed similar yarn density as the high twist CNT yarns in Fig. 7.5A.



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Fig. 7.9  Yarn diameter and density as a function of die diameter, based on the data from Ref. [30]. (Source: K. Sugano, M. Kurata, H. Kawada, Evaluation of mechanical properties of untwisted carbon nanotube yarn for application to composite materials, Carbon 78 (2014) 356–365.)

7.1.3.4  Liquid-densified fibers Twistless liquid-densified fibers often demonstrate irregular cross-section shape and the shape usually changes along the fiber length. This makes it difficult to estimate the fiber porosity and the nanotube packing density. Liquid-densified yarns show more uniform nanotube packing density in the yarn cross section than twisted yarns. Qiu et al. [31] characterized diameters of the nanotube bundles and the ­inter-bundle pores using the longitudinal section SEM images of as-spun fibers (Fig. 7.10A, B, and E). The distribution of the bundle diameters was found to be similar to the diameter observed for inter-bundle pore diameters (Fig. 7.10D and E). Fig. 7.10F shows that the distribution of bundle size peaked at about 20–30 nm, while that of the pores reached a maximum around 30–40 nm. Cho et  al. [32] measured nanotube distribution in the cross section of acetone-densified CNT yarns drawn directly from a floating catalyst CVD furnace (Fig.  7.11 and Table  7.1). The as-spun acetone-densified yarn had a density of 0.24 g/cm3 and a porosity of 0.84, which indicates a rather low densification in comparison with the twisted yarns in Fig.  7.5. When the acetone-densified yarn was further treated with solvent 1-methyl-2-pyrrolidinone (NMP) or chlorosulfonic acid (CSA), the yarn density was more than doubled (Table 7.1) and the yarn porosity decreased to the level comparable to the twisted yarns with a 40-degree twist angle [14]. Nanotube flattening was found with the NMP and CSA

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Fig. 7.10  SEM images of a CNT fiber cut using a focused ion beam (FIB) showing the longitudinal (A, B) and radial (C, D) cross sections of a solvent-densified CNT yarn produced by the floating catalyst chemical vapor deposition method. (E) A SEM image at a higher magnification of a FIB-cut longitudinal section of a CNT fiber showing the inter-bundle pores and CNT bundles, and (F) histogram of the lateral bundle dimensions and pore dimensions [31]. (Reprinted with permission from J. Qiu, J. Terrones, J.J. Vilatela, M.E. Vickers, J.A. Elliott, A.H. Windle, Liquid infiltration into carbon nanotube fibers: effect on structure and electrical properties, ACS Nano 7 (10) (2013) 8412–8422.)



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Fig. 7.11  TEM images of cross sections of CNT bundles showing tube-to-tube spaces in (A) acetone-densified CNT yarn, (B) further treated in NMP, and (C) further treatment of (A) in CSA, showing nanotube flattening. All scale bars: 10 nm [32]. (Reprinted with permission from H. Cho, H. Lee, E. Oh, S.-H. Lee, J. Park, H.J. Park, et al., Hierarchical structure of carbon nanotube fibers, and the change of structure during densification by wet stretching, Carbon 136 (2018) 409–416.)

treatments. The increase of yarn density was accompanied by an increase of bundle size from 12 to 40–60 tubes/bundle.The bundle size was further increased to 86 tubes with the application of a 13% wet stretch. However, the yarn density showed a decrease from 0.74 to 0.69 g/cm3 as the stretch ratio was increased from 7% to 13%. Wang et al. [33] used a pair of calendar rollers to compress an initially solvent-densified yarn. The initial yarn was directly produced from a floating catalyst CVD furnace by passing the CNT yarn through a water or alcohol bath. Up to five passes of calendaring action consolidated the wet yarn into a ribbon-shape with substantially increased yarn density, estimated to be 1.3–1.8 g/cm−3, which may be the highest value reported in literature.

7.1.4  CNT alignment CNTs packed in fibers and yarns are far from ideally parallel-laid cylinders. The wavy (crimpy) configuration, misalignment, and bundling of CNTs in the forest and the drawn web can be observed from the SEM images in Fig. 7.12 [14]. The CNTs in the original forest in Fig. 7.12A have crimps but most of them are not entangled with each other. During web drawing, the CNTs are turned 90 degrees from the vertical direction in the forest to the horizontal direction in the newly formed web (Fig. 2.1A, Chapter 2). During this process, the CNTs inevitably interfere with each other and form loops, hooks, reversals, and crossings that constitute an entangled CNT network, as shown in Fig. 7.12B. By comparing the configurations of the CNTs in the forest and in the drawn web, it is clear that the tension

150

As-spun NMP NMP (7% stretch) CSA CSA (7% stretch) CSA (13% stretch) a

Yarn linear density (tex)

Yarn crosssection area (μm2)

Between-bundle distance (nm)

Tubes per bundle

Yarn density (g/cm3)a

Yarn porositya

0.1 0.12 0.14 0.13 0.11 0.11

417 208 243 218 148 160

61.8 64.9 39.8 46.1 40 33

11.7 44.6

0.240 0.577 0.576 0.596 0.743 0.688

0.85 0.64 0.64 0.63 0.54 0.57

64.2 85.8 3

Calculated values from reported yarn linear density/cross-sectional area and a CNT density of 1.6 g/cm (i.e., nanotubes treated as solid cylinders).

Carbon Nanotube Fibers and Yarns

Table 7.1  Solvent-densified yarn morphology [32].



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Fig. 7.12  Tortuosity and misalignment of CNTs in forest and drawn web. (A) SEM image of CNTs in vertically aligned CNT array (forest) and (B) SEM image of CNTs in a drawn web [14]. (C) Schematic of misaligned straight fibers (longitudinal view) and (D) schematic of a void formed between misaligned straight fibers (end view of schematic C). (Panels (A and B) reprinted with permission from M. Miao, J. McDonnell, L. Vuckovic, S.C. Hawkins, Poisson’s ratio and porosity of carbon nanotube dry-spun yarns, Carbon 48 (10) (2010) 2802–2811.)

applied to the CNTs during web drawing removed some of the crimps present in the CNT.The van der Waals force and the entanglement between the CNTs in the drawn web give the web its strength for the dry-spinning process. Unlike conventional staple or short fiber textile webs such as those of wool or cotton, the CNT web does not draft or stretch easily once formed. Under sufficient tensile load, the CNT web will break sharply instead of drafting apart through slippage between individual CNTs. This is because the constituent CNTs are strongly connected to each other by van der Waals and other interactions (e.g., amorphous carbon bridging) and thus do not slip readily. The tortuosities (the hooks, reversals, and crimps) and misalignment of the CNTs observed in the web will thus persist into the yarn while the straight CNTs will take up the tension applied to the yarn.The presence of crimps, hooks, and misalignment keep the CNTs apart from each other, resulting in voids between them, as illustrated in Fig. 7.12C and D. Enabling drafting of CNT strands (webs, yarns) has been considered an effective strategy for improving CNT alignment and thus the strength of the strands. This usually requires the use of a lubricating liquid or an u ­ ncured

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resin, resulting in a CNT nanocomposite material [34–38]. In some cases, the polymer matrix may be removed after the alignment treatment to obtain a pure CNT structure. As shown in Fig.  7.13, a relatively low draft (<30% of strain) can straighten many CNT segments, but it cannot remove the CNT reversals (hooks) and entanglements.

Fig. 7.13  SEM images of neat MWNT networks at strains of 0% (A), 10% (B), 20% (C), and 30% (D), and resin-treated MWNT networks at strains of 0% (E) and 40% (F). The strain direction in all images is lateral [38]. (Reprinted with permission from R. Downes, S. Wang, D. Haldane, A. Moench, R. Liang, Strain‐induced alignment mechanisms of carbon nanotube networks, Adv. Eng. Mater. 17 (3) (2015), 349–358.)



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Cho et al. [32] reported that CNT alignment can be improved by wet stretching of NMP- and CSA-infiltrated solvent densified CNT yarns. The SEM images of the CNT yarn surface in Fig. 7.14 show that the alignment of the main CNT bundles improved with increasing stretch ratio. However, there was no compelling evidence that CNT drafting was achieved as the stretch applied was quite small (up to 17%) compared with the draw ratio used in drafting staple fiber strands and drawing polymer filaments. In the direct spinning method based on the floating catalyst CVD process (detailed in Chapter 3), CNTs grown in a gas phase assemble into an

Fig. 7.14  SEM images of surface morphology of CNT fibers: (A) pristine, (B) NMP 0%, (C) NMP 7%, (D) CSA 0%, (E) CSA 7%, and (F) CSA 13%. Scale bars: 500 nm [32]. (Reprinted with permission from H. Cho, H. Lee, E. Oh, S.-H. Lee, J. Park, H.J. Park, et al., Hierarchical structure of carbon nanotube fibers, and the change of structure during densification by wet stretching, Carbon 136 (2018) 409–416.)

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initial low-density (aerogel state) entangled structure in the furnace. By lowering the concentration of nanotubes in the gas phase, through either reduction of the precursor feed rate or increase of carrier gas flow rate, the density of the CNT entanglement could be reduced and the CNT aerogel could be drawn by a ratio of up to 18 and wound at high speeds over 50 m/ min, as shown in Fig. 7.15A [39]. The increased winding speed improved

Fig. 7.15  Controlling the alignment of CNTs in direct spinning by diluting the concentration of CNTs in the reactor. (A) Schematic representation of the direct CNT fiber spinning process. SEM image of (B) nonoriented and (C) oriented CNT fibers [39]. (Reprinted with permission from B. Aleman, V. Reguero, B. Mas, J.J. Vilatela, Strong carbon nanotube fibers by drawing inspiration from polymer fiber spinning, ACS Nano 9 (7) (2015), 7392–7398.)



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the alignment of CNTs in the final yarn (Fig. 7.15B and C), resulting in an increase of yarn specific strength from 0.3 to 1 N/tex.

7.2  Tensile strength of CNT fibers and yarns 7.2.1  Tensile testing conditions Many CNT researchers use engineering stress unit (MPa or GPa) to express CNT yarn strength and modulus. All CNT yarns are porous and even a low strain can cause the yarn diameter to decrease dramatically.We demonstrated how the strength of the same yarn could be stated as either 648 or 1630 MPa depending on whether the initial diameter or the instantaneous diameter of the yarn was used in calculating the yarn stress [14].This is why yarn-specific tensile properties have always been used in the textile industry.Yarn-specific stress is equal to the applied tensile force (in N or cN) divided by the yarn linear density in tex and expressed in N/tex or cN/tex (1 cN/tex = 10−2 N/ tex).The specific breaking strength of the yarn is known as tenacity. Dividing engineering stress by the yarn density gives the yarn-specific stress in GPa/ (g/cm3), which is equal to the value in N/tex. Tensile testing gauge length (distance between the two gripping points on the specimen) can have an important influence on the tensile test results. As a specimen always breaks at its weakest link, the longer the specimen, the weaker its weakest link becomes [40]. The effect of gauge length on yarn tensile strength is especially large when the gauge length is close to the fiber length. In other words, yarn strength tested at a gauge length slightly shorter than the fiber length would be significantly higher than that tested at a gauge length slightly longer than the fiber length. Most commonly used tensile gauge lengths for CNT yarn testing are 10 and 20 mm [10, 13, 14, 41], which are many times longer than the length of their constituent nanotubes. In one study, Zhang et al. [42] measured CNT yarn strength at gauge length between 1 and 20 mm. Surprisingly, the tensile strength showed a very weak dependence on the gauge length, as displayed in Fig. 7.16A.This was because the shortest gauge length (1 mm) was much longer than the length of the CNTs in the yarn. On the other hand, the strain rate (2 × 10−5–2 × 10−1 1/s) used in tensile testing showed a much greater impact on the tensile properties of dry-spun CNT yarns, as shown in Fig. 7.16B [42]. As the strain rate increased, both the strength (fracture stress) and the Young’s modulus increased while the elongation (fracture strain) displayed a decrease. This dependence of tensile properties on strain rate may be explained by the viscoelasticity of CNT yarns. At a low strain rate, the yarn is subjected to a gradual attenuation and

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Fig.  7.16  Influence of testing conditions on tensile properties of CNT yarns. (A) Gauge-length dependence of tensile strength for CNT yarns. (B) Stress-strain curves of CNT yarns at different strain rates. (C, D) SEM images taken at 100 μm away from the breaking ends for yarns tested under strain rate of (C) 2 × 10−1 1/s and (D) 2 × 10−5 1/s [42]. (Reprinted with permission from Y. Zhang, L. Zheng, G. Sun, Z. Zhan, K. Liao, Failure mechanisms of carbonnanotube fibers under different strain rates, Carbon 50 (8) (2012) 2887–2893.)

then a pullout before it breaks, while at a high strain rate, the yarn undergoes a sharp, elastic break. The SEM images taken 100 μm away from the breaking ends showed that the CNTs were wavy and entangled at high strain rates (Fig.  7.16C) but straight and well aligned at low strain rates (Fig. 7.16D).

7.2.2  Strength variability Weak spots in a yarn are caused by the existence of thin places and structural defects. Fig. 7.17 shows the variability of diameter and linear density of 50 mm long specimens taken consecutively from a CNT yarn [27] and the distribution of tensile strength [42] in 1-m yarn length.The mass irregularity (standard variation as a percentage of mean value) calculated from this set of data was 9.5%, which was better than a typical commercial textile yarn.



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Deng et al. [43] studied the strength distribution of a CNT yarn using a modified Weibull strength distribution model. The modulus, strength, and fracture strain of the yarn were in the ranges of 2–10 GPa, 82–490 MPa, and 0.03–0.12, with the average values of 4.6 GPa, 170 MPa, and 0.068, respectively. The modified Weibull model takes into account of yarn diameter distribution: m   h  σ   (7.5) F (σ ) = 1 − exp −d      σ d   where σd = σ0L−1/m, L is the yarn specimen length, d is the yarn diameter, h is a diameter dependent parameter, σ0 is the scale parameter (characteristic strength), and m is the Weibull shape parameter that is an indication of the scattering of strength distribution. A larger value for the Weibull shape parameter m indicates a narrower strength distribution (smaller scattering). The shape parameter m was found to be 4.1 and the diameter-dependent parameter h was 5.82. In comparison [44], the m values were 1.7 for CVDgrown multiwalled CNTs, 4.5 for Thornel-300 carbon fibers tested at 60 mm gauge length, and 5.12 for glass fibers tested at 5 mm gauge length. So the CNT yarn showed smaller scattering than those of multiwalled CNTs but larger than those of commercial carbon and glass fibers. Zu et  al. [45] produced CNT yarns using the twisting and solvent-densifying method [9]. The average strength, modulus, and strain to failure obtained from 50 tensile tests were 1.2 ± 0.3 GPa, 43.3 ± 7.4 GPa, and 2.7% ± 0.5%, respectively. They obtained a Weibull shape parameter m of 5.44, higher than that determined by Deng et al. [43].

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The oscillating nature of the rubbing action for producing rub-densified yarns raised a concern about the variability of its tensile properties. In all, 23 specimens taken randomly from a 5 m long rub-densified CNT yarn were tested [29].The Weibull scale parameter T0 obtained was 54.74 cN/tex with a shape parameter m of 9.72, higher than the values for the above twisted CNT yarns with and without solvent densifying.

7.2.3  Influencing factors There are a large number of nanotube, processing, and yarn constructional parameters that can affect the strength of CNT yarns. Many researchers compiled strength results from CNT yarns produced in different laboratories around the world in order to understand the contributions of individual parameters [4, 33, 46–49]. Unlike textile fibers and yarns, testing the mechanical properties of single CNTs is rather complicated and is thus rarely carried out; relatively small samples are usually used in yarn tensile testing; and testing conditions (e.g., gauge length) and yarn structural characteristics (e.g., yarn porosity, diameter, twist) are often not standardized. All these contribute to the difficulties of isolating the contribution of an individual parameter. Here, we summarize some of the conclusions made by different research groups, some of which may contradict one another. 7.2.3.1  Nanotube strength In the textile industry, there are well-established relationships between fiber strength and yarn strength. Fiber strength can be obtained by testing single fibers or fiber bundles. Fiber bundles used in testing usually contain about 1000 fibers held by a clamp at one end and are combed at the other end to align the fibers and to remove short fibers that are not held by the clamp. The position of the other clamp, which gives the testing gauge length, is usually from 3 to 9 mm for cotton which typically has a staple length of more than 25 mm. This means that the vast majority of the fibers are gripped at both ends during fiber bundle testing. The ratio between cotton fiber bundle tenacity and average single fiber tenacity is usually in the range from 0.426 to 0.52 [50]. The tenacity of a medium count cotton yarn (27 tex) is about 50% of the fiber bundle tenacity [51]. Experimentally determined tensile strength and Young’s modulus of single CNTs vary in a wide range between 11 and 150 GPa and between 200 and 1000 GPa, respectively.a These values (intrinsic strength) were calculated a

https://en.wikipedia.org/wiki/Mechanical_properties_of_carbon_nanotubes.



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including the hollow center of the CNT. Its engineering strength, i.e., when the hollowness is excluded, is lower. Some nanotubes can collapse into ­r ibbons and their engineering strength can approach the ultimate (intrinsic) strength as the void in the center diminishes. So far there has not been an experimentally established direct relationship between the strength of continuously made CNT yarn and the strength of its constituent nanotubes. Hill et  al. [52] measured the strength of CNT bundles taken from 3-mm-high CNT forests. The gauge length for the bundle test was 1 mm and the two ends of the bundle were secured using epoxy resin. It was estimated that 50% of the nanotubes in the bundle was continuous across the gauge length. The number of CNTs in the reported bundle test was in the order of a million, which is the same order of magnitude for a typical CNT yarn. The measured bundle strength had a high reliance on bundle size and method of densification.The highest specific strength and modulus were 1.8 and 88.7 N/tex, respectively, achieved on toluene-densified CNT bundles [52]. The measured bundle strength was thus one or two orders of magnitude lower than the values measured on single nanotubes reported by other researchers. Unfortunately, the tensile properties of single nanotubes used in Hill’s bundle test were not measured, making it impossible to establish a relationship between single nanotube strength and bundle strength. The reported CNT bundle strength of 1.8 N/tex is about twice the tenacity of typical CNT yarns reported by many researchers.This bundle-to-yarn strength ratio is similar to that between cotton fiber bundle and spun yarn from the textile industry [51]. 7.2.3.2  Nanotube length Several control experiments pointed to an increasing trend of yarn strength when longer CNTs were used. For example, Zhang et al. [13] reported that when CNT length was increased from 0.3 to 0.65 mm, the yarn strength improved from 0.3 to 0.85 GPa. When the CNT length was further increased to 1 mm, the average yarn strength increased to 1.9 GPa [53]. Fang et al. [41] grew CNT forests to a series of heights between 0.15 and 0.4 mm to spin yarns of similar diameter (5–7 μm) and surface twist angle (17– 21 degrees). The yarn strength was found to increase from 310 MPa for the shortest CNTs (0.15 mm) to about 420 MPa for the longest (0.4 mm). A less consistent increasing trend of yarn strength with increasing CNT length from 0.8 to 2.1 mm was reported by Ghemes et  al. [54]. A substantially low yarn strength at 1.4 mm was attributed to a thicker yarn sample. This generally positive influence of nanotube length on yarn strength shown

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in control experiments is in general agreement with the fiber length-yarn strength relationship of conventional textile yarns [55]. However, when CNTs synthesized in different laboratories are compared, yarns spun from long CNTs made in one laboratory does not necessarily show higher strength than yarns from shorter CNTs made in another laboratory. For example, optimized yarns spun from 5 mm long CNTs in one laboratory exhibited a strength of 280 MPa [56], which was lower than the strength of CNT yarns produced from much shorter CNTs in other works [14, 41, 53, 54]. 7.2.3.3  Yarn diameter It is well understood in the textile industry that yarn production cost goes up dramatically as yarn count becomes finer. This is primarily because finer count yarns require more twists per meter and suffer higher end-down rate during processing due to their higher mass irregularity and lower breaking force. The effect of linear density (tex) on strength of textile yarns is generally considered to be negligible provided the yarns are of good evenness. A small negative effect on strength observed for a finer yarn can be attributed to the deterioration of yarn evenness, which leads to more weak spots in the yarn. Several CNT yarn researchers reported significant benefits on yarn strength by spinning finer yarns. Zhang et al. [13] produced yarns of different diameters using 650 μm long CNTs and found that the finer yarn with a diameter of 4 μm (0.85 GPa) was five times as strong as the thicker yarn with a 13 μm diameter (0.17 GPa). However, no details were given on other aspects of the two yarns, for example, whether the two yarns were spun to a similar twist angle. Ghemes et al. [54] reported decreases of both strength and modulus of twisted CNT yarns with increasing yarn diameter. However, in their experiment, the twist level (turns/m) was kept at a constant of 4000 turns/mm irrespective of the yarn diameter, which ranged from 10 to 60 μm. Fang et al. [41] carried out a controlled experiment on the influence of yarn diameter. Two sets of yarns were produced with different yarn diameters by varying the web width while twist angle was kept to an approximately constant level (~15 degrees).The strength was 430 MPa for a 5 μm diameter yarn but only 160 MPa for a 60 μm diameter yarn. Liu et al. [9] also reported a general improvement of strength for finer yarns, but reducing yarn diameter below 10 μm displayed a negative effect. Deng et al. [43] reported a statistical relationship showing a decrease of yarn strength (σ) with an increasing yarn diameter (d), σ = 22,617.5d−1.42.



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On the other hand, Zhao et al. [57] reported a general increasing trend of yarn strength as the yarn diameter increased, which was attributed to a lower radial pressure inside finer yarns. 7.2.3.4 Twist In textile spun yarns, fibers are held together by the fiber-fiber friction derived from the inward pressure generated by the coaxial fiber helices in the twisted yarn. At a low twist level, due to low pressure and low friction between the fibers, the yarn fails by fiber slippage. At a high twist, fiber slippage is largely prevented by high interfiber friction and the strain experienced by fibers differs greatly according to their radial positions in the yarn. High twist yarns fail mainly due to fiber breakage, first occurring at the outer ring which has the highest strain, and then propagating to the entire yarn [8]. In addition, high twist reduces the contribution of fiber strength to yarn strength due to fiber obliquity. Therefore, the maximum yarn strength is achieved at an intermediate twist level. Most researchers agree that CNT yarns follow a similar twist-strength (and modulus) relationship as textile spun yarns [14, 41, 46, 54] (Fig. 7.18A and B). Liu et al. [9] produced yarns by twisting and acetone densifying of webs drawn from CNT forests and found a decrease of CNT yarn strength and modulus with an increasing twist angle. As a CNT yarn without being twisted or densified in other ways has a very low strength, the complete relationship between twist and yarn strength/modulus should assume a parabolic shape. Liu et al. also demonstrated that the breaking strain of the yarn increased as a higher twist was applied, which is in general agreement with textile spun yarns. The explanation used in textile spun yarns, therefore, can be adapted to explain the relationship of CNT yarns by replacing the frictional force between fibers with van der Waals force between nanotubes [5]. Insertion of higher twist to CNT yarns provides greater densification effect and thus stronger inter-tube van der Waals forces. However, Zhao et al. [57] reported a double-peak relationship between twist and yarn strength. The second peak was attributed to CNT collapse due to high internal pressure in the yarn caused by very high twist. The collapse of CNTs would increase the contact between CNTs, which would allow the yarn to densify further. When twist in a twist-spun yarn is removed by applying opposite twist, the untwisted yarn retains most of its original strength [6]. The twist-untwist action is also known as false twist.The resulting false-twisted yarn had a larger diameter (Fig. 7.18C) but followed a similar twist-strength relationship as the

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Fig. 7.18  Influence of twist on yarn tensile properties. (A) Relationships between CNT yarn tenacity and twist angle in twisted yarns. (B) Relationship between specific modulus and twist angle in twisted yarn [14]. (C) Yarn diameter as a function of twist or false twist. (D) Yarn tenacity as a function of twist or false twist [6]. (Panels (A and B) reprinted with permission from M. Miao, J. McDonnell, L. Vuckovic, S.C. Hawkins, Poisson’s ratio and porosity of carbon nanotube dry-spun yarns, Carbon 48 (10) (2010) 2802–2811; Panels (C and D) reprinted with permission from M. Miao, The role of twist in dry spun carbon nanotube yarns, Carbon 96 (2016) 819–826.)

parent twisted yarns albeit at a somewhat lower level (Fig. 7.18D). Although the final false-twisted yarns were twistless, the nanotubes within each bundle in the yarn were still kept together by the van der Waals force that kept them together before the twist was removed. The bouncing back of yarn porosity with the removal of twist is mainly caused by the increase of the pore sizes between CNT bundles. The weakening effect of the porosity between CNT bundles is compensated partially by the increased alignment of CNT bundles in the final twistless yarn. 7.2.3.5  Spinning conditions The two spinning systems developed by CSIRO, the flyer-spinner and the up-spinner (see Chapter 2), were run in parallel for more than 2 years [4].



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There were several attempts to compare the properties of the yarns produced on the two systems. In general, the flyer-spun yarns tended to have a tighter structure than the up-spun yarns, which was attributed to the higher spinning tension in flyer-spinning. The elastic modulus of the flyer-spun yarns tended to be higher but the strain to failure tended to be lower. However, when the strength and modulus were converted into specific tensile strength (tenacity) and work-to-rupture (toughness), the differences between the yarns spun on the two systems started to disappear. Tran et al. [58] introduced a series of friction pins between the CNT forest and the spindle on the flyer spinning machine. These pins affected the spinning process in two ways, increasing the yarn tension and causing the twist to be inserted to the yarn in steps along the zones separated by the pins. The twist redistribution caused by the friction pins is known as twist blockage [59]. The higher tension increases the yarn density, leading to a more compact yarn (reduced yarn diameter) and higher stress-based strength and modulus as well as lower breaking strain. Tran et al. [58] also reported the benefits of using multiple narrow web strips, following the Siro-spun yarn method used for wool fiber spinning, as well as heat treatment of the CNT web during spinning. Siro-spun wool yarn spinning is an alternative to twofold wool yarns, which can be woven into fabrics without sizing because of lower yarn hairiness (protruding fiber ends) and higher resistance to fiber attrition [60].

7.2.4  Densification methods A number of methods have been used to densify CNT yarns, resulting in different yarn structures. The common parameter that can be used to quantify the level of densification achieved by these methods is yarn density, which directly affects voids and van der Waals force between nanotubes, and in turn affects the yarn strength. Fig. 7.19 plots the yarn density-tenacity relationships according to densification methods. Twist densification can provide a wide range of yarn density from 0.1 to 1.3 g/cm3 by changing the level of twist. However, the high yarn density obtained at very high twist is achieved at the expense of large nanotube obliquity, which has a negative effect on the yarn strength, as shown in Fig.  7.19A. By removing the twist from a twisted yarn (or by forming a twistless yarn directly using the false-twist method), the nanotube obliquity is eliminated, but this leads to loosening of the initially tight yarn structure, especially at high twist levels, and limits the highest density achievable, which is in turn reflected on the yarn tenacity (Fig. 7.19B).

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By adjusting the diameter of the die, cold-drawn CNT yarns can achieve similar levels of density as that obtained by twist insertion. The reported yarn tenacity achieved at 1.1–1.2 g/cm3 (Fig. 7.19C) is similar to that of the twisted yarn at its optimum density of about 0.6 g/cm3 (Fig. 7.19A). Solvent-densified twistless yarns normally do not have regular cross sections and therefore, the yarn density is rarely reported. Cho et al. [32] presented results of NMP and CSA treated yarns subjected to stretching, as plotted in Fig. 7.19D. The reported yarn density was rather low (similar to the false-twisted yarns in Fig. 7.19B), but the yarn tenacity was very high. The dramatic improvement of yarn tenacity was attributed to the increased nanotube bundle size, bundle compaction, and alignment [32].



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In rubbing densification [29], roller pressure, roller speed, and yarn tension could influence the density and properties of rub-densified CNT yarns. The yarn tension was controlled by the speed ratio between the yarn take-up (collection) speed and the circumferential speed of the rubbing rollers. A high yarn take-up rate had a positive effect on the yarn specific modulus but its effect on the yarn tenacity was not significant. On the other hand, the yarn-specific modulus was increased by 56% when the yarn take-up ratio was increased from 1.00 (zero tensioning) to 1.03, which appeared to be the optimum tension level.

7.2.5  Post-spinning treatments Wang et al. [33] reported multifold improvement of yarn strength by repeatedly rolling a solvent-densified yarn. The resulting flat yarn showed a density estimated to be around 1.3–1.8 g/cm3 and an average strength of 4.34 GPa, which can be converted to a tenacity between 2.4 and 3.3 N/tex. Zhang et  al. [13] reported that “post-spin twisting” could dramatically increase yarn breaking stress from 0.81 to 1.91 GPa. During the post-spin twisting, a weight was hung on one end of the CNT yarn to provide tension in the axial direction while the other end of the yarn was twisted.The tension provided by the weight would increase the CNT packing density within the yarn. The yarn diameter reduced from 4 to 3 μm after the post-spin twisting, which represented a 44% reduction in yarn cross-sectional area. Similar benefit of post-spinning twisting was also reported by Ghemes et al. [54]. Liu et  al. [9] reported an 83% increase of yarn strength (from 0.6 to 1.1 GPa) by additional solvent-densifying treatment of twisted yarns. Less than a half of the increase could be attributed to the increase of yarn breaking force and the rest to the yarn diameter reduction (increase of yarn density). Li et  al. [61] used different solvents to densify low twist yarns (twist angle < 12 degrees) spun from CNT forests. In Fig. 7.20A, the solvents are sorted according to their dipole moments. The strength of the resulting yarns did not show a clear dependence on polarity.The fibers densified by highly polar but nonvolatile solvents (slow evaporation), such as N, N-dimethylformamide, dimethyl sulfoxide, and N-methyl-2-pyrrolidone, were 100–200 MPa stronger than those densified by volatile solvents, such as ethanol and acetone (fast evaporation). The fast vaporizing ethanol and acetone result in uniformly distributed pores in the yarn while low volatility solvents draw CNT in localized high-density areas with larger pores between them. Cho et al. [32] reported that wet stretching with NMP and CSA infiltration could significantly improve the specific strength of twistless

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CNT yarns produced by the floating catalyst CVD process (Fig.  7.20B). The improvement was attributed to the increased nanotube packing density caused by nanotube flattening and alignment. Irradiation by electron and ion beams has been studied as tools for engineering CNTs and strengthening CNT structures [62]. Electron-beam



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i­rradiation of single multiwalled CNTs, commonly performed in evacuated SEM or TEM chambers, can create between-wall bonds that increase the total breaking force [62, 63]. Single-walled CNTs in a “rope” can be linked to each other by a low dose of electron irradiation [64]. Theoretical modeling has shown that cross-links between CNTs can be formed with the participation of interstitial carbon atoms generated by irradiation, or more rarely by a radiation-induced chemical reaction between carboxyl groups [64]. Electron-beam radiation attenuates rapidly and thus is not a suitable large-scale treatment of CNT yarns. Gamma radiation, on the other hand, penetrates a great depth into carbon materials and can be adopted to treat large volumes of material uniformly. Miao et al. [65] reported that the gamma irradiation of CNT yarns in air significantly improved the tensile strength of the yarns. The improvement was much greater for tightly structured yarns (e.g., high twist yarns) than for loosely structured yarns. Sonic pulse tests also showed increased sound velocity and thus increased dynamic modulus of the CNT yarns as a result of gamma irradiation. X-ray photoelectron spectroscopic analyses on parent CNT forests showed that gamma irradiation treatment in air dramatically increased the concentration of oxygen in the CNT assemblies in proportion to radiation dose. This indicates that CNTs were oxidized under the ionizing effect of the gamma irradiation.The oxygen species are believed to contribute to the interaction between CNTs and thus improvement of the yarn mechanical properties.

7.3  Dynamic properties Tensile properties of materials measured on a tensile testing machine are dependent on the strain rate used in testing. For example, polymer fibers could increase their Young’s moduli by up to threefold as the straining rate was increased from 1% per minute to 100,000% per minute [66]. On the other hand, the modulus of linear-elastic fibers, such as the glass fiber, was not as affected by the straining rate used in testing [67]. Wang et al. [68] used a free-falling Hopkinson tension bar to measure the dynamic tensile strength of CNT yarns. The dynamic strength of CNT yarn could be 35% stronger than that obtained using a quasistatic test. This strengthening behavior could be expressed as a function of the strain rate using a simplified form of the Johnson-Cook model,

σ = σ 0 (1 + 0.0186lnε )

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where the strain rate ε represents the dimensionless strain rate based on ε0 = 1/s, and 0.0186 is a strain rate sensitivity coefficient. A pure CNT yarn tended to break at the weakest point, leading to unraveling of the yarn along the twist direction, while a polymer-infiltrated composite CNT fiber tended to break into several fragments. For pure CNT yarns, the strain rate effect arises not only from the rate dependence of millions of CNTs, but also from the interaction among CNTs. Irregular defects and voids in CNT yarns can be regarded as a factor in elevating the strength as the strain rate increases. The internal defect distributions could significantly influence the strain rate sensitivity. At the quasistatic loading rates, the failure strength was dominated by the strength of the weakest link, but the failure strength gradually got closer to the average material strength as the strain rate was increased to high levels. The tensile behaviors at sonic strain rates can be used to elucidate viscoelastic characteristics, which have been used to reflect some aspects of the interactions between molecules, fibrils, and fibers in polymer fiber materials [69]. In a perfect linear elastic material, the sonic velocity (c) in the material is related to its Young’s modulus (E) and mass density (ρ) by the well-known wave propagation equation c = √ ( E /ρ ) = E ′ , where E′ = E/ρ (N/tex) is the specific modulus of the material.The elastic modulus determined by the acoustic method is known as dynamic modulus or sonic modulus.The ratio between the sonic modulus and the quasistatic modulus (Es/Eqs) can be used as an indication of the internal friction in textile fibers and fiber-fiber friction in yarns [70, 71]. For a twist-spun CNT yarn, the degree of CNT orientation decreases as the twist in the yarn increases, resulting in a decrease of both dynamic and quasistatic moduli of the yarn, as shown in Fig. 7.21A [69]. The ratio ­between the dynamic modulus and the quasistatic modulus (Es/Eqs) followed a parabolic curve with a maximum at a twist angle of about 30–40 degrees, as shown in Fig. 7.21B.This can be explained by frictional slippage between CNTs in the yarn, which depends on both the intimacy of CNT-CNT contact and the freedom of CNT-CNT relative movement under load. As the twist increases, the contact force increases but the tendency of slippage decreases. The combined effect of the two factors plateaued at the intermediate twist level. A torque-free plied CNT yarn can be formed by twisting multiple singles yarns together in the opposite direction to the twist of the singles yarns. The CNT tortuosity (misalignment) in the final plied yarn is greater than that in the original singles yarns. This could result in decreases in yarn



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Fig. 7.21  Sonic and quasistatic moduli of twisted carbon nanotube yarns. (A) Sonic and quasistatic moduli of the twisted carbon nanotube yarns and (B) the ratio between sonic modulus (Es) and quasistatic modulus (Eqs) [69]. (Reprinted with permission from M. Miao, Characteristics of carbon nanotube yarn structure unveiled by acoustic wave propagation, Carbon 91 (2015) 163–170.)

­tenacity, dynamic, and quasistatic moduli and modulus ratio (Es/Eqs), as shown in Table 7.2. The increase of the modulus ratio may be explained by the added friction between the three plies of singles yarns in the structure. On the other hand, gamma-irradiation treatment of twisted CNT yarn in air introduces CNT-CNT cross-linking, which increases the yarn sonic and quasistatic moduli and reduces the tendency of frictional slippage between the CNTs, leading to a decrease of the modulus ratio. Rub-densified CNT yarns have high degree of CNT alignment and intimate CNT-CNT contact because of the twistless yarn structure and the high CNT packing density, resulting in much higher quasistatic and sonic moduli and a very low modulus ratio as the nanotubes in the yarn are prevented from relative movement. Table 7.2  Dynamic and quasistatic tensile properties of CNT yarns [69]. Quasistatic tenacity (cN/tex)

Twisted singles yarn 3-ply twisted yarn Gamma-irradiated twisted yarn Rub-densified yarn

Specific modulus (N/tex) Quasistatic

Sonic

Modulus ratio (Es/Eqs)

74.3

14.8

37.5

2.5

50.5 87.7

8.1 26.1

22.4 58.1

2.8 2.2

72.9

56.7

69.4

1.2

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Carbon Nanotube Fibers and Yarns

7.4  Electrical conductivity Single-walled carbon nanotubes (SWNTs) may be either metallic or semiconducting depending on their chirality, i.e., how a single-layer graphene is wrapped into a cylinder (nanotube).The electrical resistivity through the same graphene layer in a MWNT is much lower than that between the coaxial layers (between walls) [3]. Resistivity in MWNTs can also be reduced by subjecting the CNTs to annealing [72] and by controlled defect creation that promotes cross-shell bridging. Contact resistance between nanotubes depends strongly on the atomic structure in the contact area and can vary by more than an order of magnitude.The optimal electronic transport between nanotubes occurs when the tubes are in atomic scale registry where atoms from one tube are placed on top of another [73]. Resistivity of CNT bundles formed in synthesis can vary greatly due to large differences in the structures of nanotubes and the electrical testing methods used. The electrical conductivity of MWNT-based macrostructures is generally lower than that of defect-free individual CNTs due to the presence of amorphous carbon and other impurities, which cause scattering and increase contact resistances [74, 75]. Chen et al. [76] reported the dependence of electrical conductivity of random and aligned CNT films on nanotube structure (wall number and diameter). The random films were made by dispersing CNT forest (500– 600 μm in length) in an organic solvent while the aligned films were produced by laying a CNT forest using a roller-press method. The two types of films showed similar conductivity and reached a peak when the average wall number was 2.7, corresponding to an average nanotube diameter of 5.4 nm. The appearance of the peak was attributed to the off-setting effects of increasing nanotube conductivity and decreasing CNT packing density of the films with the increase of nanotube wall number. Aliev et  al. [77] measured the temperature dependence of resistivity in different CNT assemblies including yarns (Fig. 7.22). Unlike the positive coefficient typically observed in metallic materials, all the MWNT assemblies showed a negative temperature coefficient of resistance (dR/ dT < 0), indicating their semiconducting characteristics.They proposed that although the longer overlap of aligned MWNTs could not greatly reduce the number of barriers to electron hopping process, alignment substantially reduced the electron pathway and the electrical resistivity. Typical values of room temperature electrical conductivity for CNT yarns spun from forests are between 1.5 × 104 and 4.1 × 104 S/m [5, 13, 27, 78–80]. A major reason for this wide spread of value is the difference in yarn



Carbon nanotube yarn structures and properties

1×10–4 9×10–5

1

8×10–5 2

10–4

3

10–5 4 10–6 1

10 100 Temperature (K)

Resisivity (W•m)

Resisivity (W•m)

10–3

(A)

171

7×10–5 1

6×10–5 –5

5×10

2

(B)

3

θ

4×10–5

4 10

100 Temperature (K)

Fig. 7.22  Temperature dependence of resistivity for different types of carbon nanotube assemblies: (A) MWNT sheet (densified), perpendicular to the pulling direction (1), randomly deposited buckypaper fabricated from the same CVD grown MWNT forest (2), densified MWNT sheet, parallel to the pulling direction (3), and SWNT (HiPCO) buckypaper (4). (B) Three-layer MWNT sheet (1), three-ply yarn (2), one-ply yarn with twist angle of 23 degrees (3), and one-ply yarn with twist angle of 29 degrees (4) [77]. (Reprinted with permission from A.E. Aliev, C. Guthy, M. Zhang, S. Fang, A.A. Zakhidov, J.E. Fischer, et al., Thermal transport in MWCNT sheets and yarns, Carbon 45 (15) (2007) 2880–2888.)

porosity, which is determined by the densification method used to produce the yarn. The conductivity of yarns with different levels of porosity should be compared based on specific conductivity. If T is the yarn linear density (tex), R is the measured electrical resistance (Ω) and l is the gauge length (m) used in measurement, the specific conductivity of the yarn in (S/m)/(g/cm3),or S∙m/tex, can be calculated from Ref. [27]

σ sp =

l × 109 RT

(7.7)

Miao [27] demonstrated the dependence of electrical conductivity on CNT yarn porosity.The conductivity for twisted yarns followed a linear relationship with the yarn density, as shown in Fig. 7.23A.When the electrical conductivity is normalized to specific conductivity, a very slow increasing trend was obtained (Fig. 7.23B). This indicates that the electrical contact between CNTs in a yarn is not significantly improved by twist insertion. As shown in Fig. 7.23C and D, when the twist in a twisted yarn is removed by applying opposite twist, the yarn diameter increased and the electrical conductivity decreased, but the specific conductivity of the yarn hardly changed. This is because the yarn diameter increase was almost all caused by the increased spaces between CNT bundles while the tube-tube contact within each bundle did not change significantly [6].

172

Conductivity (×104 S/m)

4 y = 4.3148x + 0.103 R2 = 0.9571

3 2 1 0

0

(A) Conductivity (×104 S/m)

8 7 6 5 4 3 Twisted False-twisted

2 1 0

(C)

0

5 10 15 20 Twist/false-twist (×103 T/m)

5

2

y = 0.0035x + 4.2412 R2 = 0.0529

1

Conductivity Specific conductivity

(B)

Yarn density (g/cm3)

25

6

3

0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Specific conductivity (×104 S.m/tex)

Conductivity (×104 S/m)

4

(D)

4 3 2 1 0

0

10

20

30

40

50

60

Specific conductivity (×104 S•m/tex)

Carbon Nanotube Fibers and Yarns

Twist angle (degrees) 8 7 6 5 4 3

Twisted False-twisted

2 1 0

0

5 10 15 20 Twist/false-twist (×103 T/m)

25

Fig. 7.23  Effect of yarn density on electrical conductivity (A) and effect of twist on specific conductivity (B) of CNT yarns dry-spun from forest, based on the data from Ref. [27]. Effect of twist and false twist on conductivity (C) and specific conductivity (D) of CNT yarns dry spun from forest, based on data from Ref. [6]. (Sources of (A and B): M. Miao, Electrical conductivity of pure carbon nanotube yarns, Carbon 49 (12) (2011) 3755–3761. Sources of (C and D): M. Miao, The role of twist in dry spun carbon nanotube yarns, Carbon 96 (2016) 819–826.)

Similarly, the specific conductivity of rubbing densified CNT yarns is not significantly affected by the rubbing conditions used in yarn production [29]. Table  7.3 compares the electrical conductivity of CNT fibers and yarns produced by different methods. Unfortunately, many authors do not provide specific conductivity or yarn density, so we cannot normalize the values for easy comparison. Generally speaking, yarns dry-spun from MWNT forests show similar conductivity and the differences between the values reported by different research groups could be largely explained by the difference in yarn density. Yarns directly spun from a furnace using the floating catalyst method have conductivity about one order of magnitude higher than those dry-spun from forests, mainly because the floating catalyst method produces mostly d­ ouble-walled carbon nanotubes (DWNT) or a mixture of SWNTs and MWNTs. The ­solution-spun yarns show a wide range of electrical conductivity as a result of the different types of CNTs used, from metallic to semiconducting.



Carbon nanotube yarn structures and properties

173

Table 7.3  Reported electrical conductivity of CNT yarns produced by different methods. CNT type

Densification method

Conductivity (104 S/m)

Refs.

Twist Twist Densified Twisted As-spun Twisted Twist + solvent Twist

3 1.7–4.1 5 4–7 4.16 4.3 4–9 4–8

[5] [13] [10] [6, 27] [81] [82] [9] [83]

As-spun As-spun As-spun Solvent + rolling

83 50 19 182–227

[84] [85] [86] [33]

Extrusion Extrusion Extrusion

0.1 50 0.067

[87] [88] [89]

Extrusion Extrusion Extrusion

0.8 83.33 290

[90] [91] [92]

Dry spun from forest MWNT MWNT MWNT MWNT MWNT MWNT MWNT MWNT Dry-spun from furnace SWNT/MWNT Flattened DWNT Flattened DWNT Mainly DWNT Solution-spun SWNT SWNT Argon grown SWNT MWNT SWNT SWNT

A number of treatments have been proposed to improve the electrical conductivity of CNT fibers and yarns, including thermal annealing [93], irradiation [94], acid treatment [95], iodine doping [96], and metallic coating [97, 98]. A detailed discussion on CNT yarn posttreatments is provided in Chapter 6.

7.5  Thermal conductivity CNTs are good thermal conductors along the length, but good insulators lateral to the tube axis. At room temperature, individual CNTs have a thermal conductivity of about 3000–3500 W m−1 K−1 along its axis [99, 100] and about 1.52 W m−1 K−1 in the radial direction.b In comparison, copper has a thermal conductivity of 350–400 W m−1 K−1 and polyacrylonitrile b

https://en.wikipedia.org/wiki/Carbon_nanotube#Thermal.

174

Carbon Nanotube Fibers and Yarns

(PAN)-based carbon fibers 7–60 W m−1 K−1 in the longitudinal direction and 0.5–1.2 W  m−1 K−1 in the transverse direction, respectively [101]. The thermal conductivity of a MWNT forest-drawn web was reported to be 50 W m−1 K−1 in the parallel direction and the thermal conductivity of its yarn was 26 W m−1 K−1 [77]. The nearly two orders of magnitude ­difference between the single tubes and the bulk materials suggests that thermal transport is largely dominated by the numerous highly resistive thermal junctions between tubes in the CNT bulk materials. Behabtu et  al. [92] reported that solution-spun CNT fibers had an average thermal conductivity of 380 ± 15 W m−1 K−1, which increased to 635 W  m−1 K−1 after iodine doping. Gspann et  al. [102] produced CNT fibers from the floating catalyst CVD method, which were treated by a 5% stretch during solvent evaporation to improve nanotube alignment. This process resulted in a yarn with a room temperature thermal conductivity as high as 770 ± 10 W m−1 K−1. Koziol et  al. [103] used a Veeco explorer AFM thermal probe setup to measure the thermal conductivity of as-spun CNT fibers produced by the floating catalyst CVD method and found that the thermal conductivity was the highest around room temperature (1255 ± 317 W m−1 K−1) and then decreased with increasing probe temperature.These values are about 1/3 of that of single CNTs and are much higher than high thermal conductivity metals such as copper and gold.

7.6  Outlook for CNT fiber strength An often asked question is whether the properties of the CNT fibers and yarns can be substantially further improved. As discussed in Chapter  8, a number of excellent essays [104–106] attempted to answer this question using different models of organizational geometries, mechanical behaviors, and load transfer mechanisms of the CNTs. Here, we take a survey on what have already been achieved in the utilization of intrinsic properties of polymers in textile fibers to cast a light on the prospects of future commercial CNT fibers and yarns. Table 7.4 compares the theoretical values of tensile properties for polymers and the corresponding values for commercial fibers made from these polymers.The theoretical specific strength (tenacity) and modulus, as summarized by Hongu et al. [107], were derived from ideal polymers comprising of fully orientated and infinitely long molecules. The theoretical values given in traditional unit g/d (gram per denier) in the reference have been converted to N/tex. Note that the theoretical values are converted using theoretical



Table 7.4  Theoretical and achieved fiber strength. Theoretical (N/tex) [107] Fiber

Tenacity

Fiber (N/tex)

Utilization (%)

Modulus

Tenacity

Modulus

Tenacity

Modulus

91 127 92 75 250

0.45 0.90 0.90 0.45 0.90

14.4 6.0 14.4 7.7 9.0

3.8% 3.2% 4.3% 2.6% 2.7%

15.8% 4.7% 15.6% 10.2% 3.6%

250 250 135 135 135 441

3.2 4.1 2.0 2.1 1.6 2.8

108.2 130 49.3 77.8 99.3 222

9.5% 12.2% 9.5% 9.9% 7.6% 6.3%

43.3% 52.1% 36.5% 57.6% 73.6% 50.3%

Commodity textile filaments 12.0 28.4 20.9 17.6 33.5

High-performance filaments [108] Spectra 1000 Dyneema purity Kevlar 29 Kevlar 49 Kevlar 149 Carbon fiber

33.5 33.5 21.2 21.2 21.2 44.1

Carbon nanotube yarn structures and properties

Cellulose (cotton) Polyamide Polyester Polyacrylonitrile Polypropylene

175

176

Carbon Nanotube Fibers and Yarns

density of the idea material (e.g., carbon crystal density = 2.266 g/cm3) while the values for commercial fibers are calculated using commercial fiber density (e.g., carbon fiber density = 1.8 g/cm3).The two columns on the right side of the table give utilization ratios, which are the values of commercial fibers as a percentage of the theoretical values in each row. Table  7.4 divides the fibers into two sets. The first set are commodity textile filaments for industrial applications, which are typically stronger than their short fiber (staple fiber) counterparts, except for cotton, which is a staple fiber consisting of over 90% cellulose.The cotton fiber strength and modulus in the table are based on St Vincent cotton, the strongest type of commercial cottons [109].Table 7.4 shows that only 2.6%–4.3% of the theoretical strength and 3.6%–15.8% of the theoretical modulus are utilized in these fibers. The second set of fibers, high-performance fibers, including Kevlar, Spectra, Dyneema, and carbon fibers, are extremely strong for high-value end uses, such as personnel ballistic protection and challenging engineering structures. They have utilized between 7.6% and 12.2% of their theoretical strength and between 36.5% and 73.6% of their theoretical modulus, much higher than the first set (commodity textile fibers). The reason why textile fibers achieve only a small fraction of their theoretical strength and modulus can be ascribed to fiber structural defects, such as practical limitations of molecular chain length and misalignment, and manufacturing defects. All these can be attributed to deficiencies of existing manufacturing technology and insufficient level of care taken during production. The high-performance fibers are typically manufactured from ultrahigh molecular weight polymers using sophisticated processing methods, such as gel spinning and super-drawing, to achieve very high level of molecular chain orientation. Commodity textile fibers are produced from lower molecular weight polymers using less sophisticated processes than the high-performance fibers, and they are likely to contain more defects than high-performance fibers by using less sophisticated processing methods to reduce production costs. The utilization of intrinsic polymer properties for commodity textile fibers is, therefore, considerably lower than that for the high-performance fibers. For the purpose of comparison, we will use the strength and modulus of monolayer graphene as the theoretical values for CNTs, which are generally agreed to be about 130 GPa and 1.0 TPa, respectively [110]. Based on the density of graphite (carbon crystal) 2.266 g/cm3, the theoretical specific strength (tenacity) and specific modulus for CNTs are 57.4 and 441 N/tex, respectively.



Carbon nanotube yarn structures and properties

177

Now, if future commercial CNT yarns can utilize their theoretical strength to the same level as the commodity textile fibers, they can be expected to achieve a specific strength in the range of 1.5–2.5 N/tex, which have already been reported by a number of research groups. If the utilization ratio can be increased to that of the high-performance fibers, future CNT yarns may be expected to reach specific strength between 4.3 and 7 N/tex and specific modulus between 161 and 325 N/tex.These values translate into 7.7–12.6 and 290–585 GPa, respectively, based on a density of 1.8 g/cm3. These strength and modulus are higher than the mechanical performance achieved by any commercially manufactured fibers.

7.7  Summary and future prospect CNTs possess very high mechanical properties combined with excellent electrical and thermal conductivity. Fiber and yarn are the most efficient structures known to researchers for utilization of these properties.The CNT fibers and yarns of today have utilized only a small fraction of the mechanical, electrical, and thermal properties of the constituent CNTs. The structural mechanics of conventional textile yarns is well understood and in many cases provides useful clues for understanding the relationship between the structure and properties of CNT yarns. The strong van der Waals force, the geometrical disorder of as-synthesized CNTs and the unique nanotube interfacial properties present challenges as well as opportunities for the research and development of higher performance CNT fibers and yarns. Maximizing densification and alignment of the CNTs are two widely used tools to improve the load transfer efficiency and conductivity between nanotubes in CNT fibers and yarns. Lessons from textile industry tell us that CNT fiber has a good prospect to become the strongest fiber we have ever had.

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