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The role of twist in dry spun carbon nanotube yarns
Carbon 96 (2016) 819e826
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Carbon journal homepage: www.elsevier.com/locate/carbon
The role of twist in dry spun carbon nanotube yarns Menghe Miao CSIRO Manufacturing, PO Box 21, Belmont, Victoria 3216, Australia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 August 2015 Received in revised form 9 October 2015 Accepted 10 October 2015 Available online xxx
Twist densification as a method of carbon nanotube (CNT) yarn manufacture has been widely studied. The load transfer between individual nanotubes in the twisted yarns is a result of the combined action of van der Waals attraction and the friction between the nanotubes, both of which increase in magnitude with the level of twist applied to the yarn during manufacturing. To isolate these effects, the twist in the initially twisted yarn was removed by untwisting to produce a twistless yarn. With the disappearance of compressional forces between CNTs originated from the presence of twist, load transfer between individual CNTs in the twistless yarn relies on the van der Waals contact established in the initial twisting stage. Despite the considerable decrease of nanotube packing density in yarn after the removal of twist, 80% of the specific strength and nearly 100% of the specific modulus and specific electrical conductivity of the CNT yarn were maintained. Crown Copyright © 2015 Published by Elsevier Ltd. All rights reserved.
1. Introduction Carbon nanotubes (CNTs) can be processed into yarns (also called fibres by some researchers) in several ways [1]. The method that has attracted the widest interest is the dry solid state spinning of CNT yarn from a vertically aligned array (forest) of multi-walled CNTs (MWCNTs) grown on a substrate. In the yarn production process, the vertically aligned carbon nanotubes are first drawn into an interconnected web [2]. The drawn web, which is extremely porous, has a rather low strength because of the weak CNTeCNT connection. Subsequent research has shown that twist insertion can densify this extremely porous CNT web into a tightly packed twisted yarn structure. The resultant twisted yarn possesses much higher strength than its parent CNT web [3]. The geometry of the twisted CNT yarn is very similar to that of textile yarns made from conventional fibres, such as cotton and wool fibres, but the number of CNTs in the yarn cross-section (hundreds of thousands) is orders of magnitude larger than the number of fibres in conventional textile yarns (typically 100). The insertion of twist to a yarn places individual fibres in approximate coaxial helix configuration (see Fig. 1(a)) and the fibres are pressed together because of the inward pressure generated by the tension in the helically disposed fibres. In a conventional textile yarn, interconnection between fibres relies on the fibreefibre friction that arises from the compression between fibres, which
E-mail address: [email protected]. http://dx.doi.org/10.1016/j.carbon.2015.10.022 0008-6223/Crown Copyright © 2015 Published by Elsevier Ltd. All rights reserved.
increases with the external tensile load applied to the yarn. At low twist levels, due to low fibreefibre compression and friction, the yarn failure mechanism is dominated by fibre slippage. At high twist, fibre slippage is largely prevented by high fibreefibre friction and thus the yarn fails due to fibre breakage. On the other hand, high twist reduces the contribution of fibre strength to the yarn strength due to fibre obliquity in the yarn. Therefore the maximum yarn specific strength is usually achieved at an intermediate twist level. This relationship is illustrated in Fig. 1(b). Twisted CNT yarn has a similar twistestrength relationship [4] as these conventional textile yarns. There is, however, a major difference in the mechanism of fibreefibre (CNTeCNT) interaction. Because of their nano-scale dimension, the van der Waals attraction (London dispersion force) between the nanotubes plays an important role in transfer of load between nanotubes in the yarns. Twistless CNT yarns can also be produced by liquid shrinking with or without twist insertion [5,6]. CNT yarns densified by polar solvent ethylene glycol [5] demonstrated a strength of 1.45 GPa, which is stronger than most twisted CNT dry spun yarns reported in literature [1]. Mechanical rubbing [7] produces a closely packed CNT yarn which is also quite strong. Both of these examples illustrate the significance of van der Waals forces between closely packed CNTs. A number of modelling studies have discussed the underlying mechanics of twisted CNT yarns with different levels of emphasis on the relative importance of twist-friction effect and van der Waals attraction [8e12]. The geometry of CNTs in twisted yarns is
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Fig. 1. Structureestrength relationship of conventional textile yarns spun from staple fibres, such as cotton and wool. (a) Fibre trajectories at different radii of an idealized twisted yarn. (b) Influence of twist on yarn strength. (A colour version of this figure can be viewed online.)
far from closely packed coaxial helices as is usually assumed in yarn mechanics modelling. Yarn cross-sections prepared by focused ion beam (FIB) milling [13,14] show that the voids between CNTs in twisted CNT yarns look far different from the regular shape and size that are expected of a closely packed CNT array [4]. The initial CNT packing density (or its opposite, porosity) of a twisted yarn depends on the level of CNT tension and the level of twist inserted during spinning. However, when an external load is applied to the twisted yarn, the CNT packing density can increase significantly, resulting in very high Poisson's ratio [4]. All these factors present practical difficulties for accurately modelling the CNT yarn structure and its mechanical behaviours. Twisting is a reversible process, i.e., the twist in a yarn can be removed by inserting a twist of the same amount but in the opposite direction (i.e., untwisting). When the twist in a conventionally spun textile yarn is removed by untwisting, the yarn returns to a loose fibre strand with virtually zero strength. As it will become clear later in this paper, when the twist of a twisted CNT yarn is removed by untwisting, the resultant yarn largely maintains its structural integrity and a major part of its strength. This provides an opportunity to examine the relative contributions of the twistderived friction and the van der Waals force between CNTs. The nanotube alignment, structural porosity and mechanical strength of a twistless CNT yarn may be utilized in a number of applications. The high porosity provides superior access for electrolyte to penetrate the yarn, which is beneficial for threadlike energy storage devices used to power wearable electronics [14e19]. The high CNT alignment due to the absence of twist provides a structure for high strength CNT composite materials [20].
2. Experimental 2.1. CNT forests and drawn webs The CNT forests used in our work were grown on silicon wafer substrates coated with a thermal oxide layer (100 nm) and iron catalyst coating (2.5 nm) using chemical vapour deposition (CVD) of acetylene (5%) in helium (700 sccm) at 680 C for 20 min [21]. The CNTs grown by this method were about 350 mm in length with an outer diameter of approximately 10 nm and an inner diameter of about 4 nm. A continuous web of CNTs was drawn from the forests.
2.2. Formation of twisted and twist-untwisted yarns Two methods were used to form twisted yarn and subsequent twist-untwisted yarns. One was based on a hand-operated twist tester and the other was based on a computer controlled miniature flyer spinning machine [1]. The two methods apply different levels of CNT densification to the yarn. To form a twisted yarn on a hand-operated twist tester, the drawn web was held at one end by the fixed jaw while twist was inserted into the web from the other end by manually driving the rotating jaw. To produce a twist-untwisted yarn, twist was inserted to the initial twisted yarn in an equal amount but opposite direction (untwisting) using the same twist tester, resulting in a twistless CNT yarn. In the second method, the miniature CNT yarn spinner was used to carry out both the twisting and untwisting operations. The twisting and winding operations are realized by two coaxial shafts rotating at differential speeds, causing the yarn to be twisted and wound simultaneously onto a yarn bobbin that is mounted on the twisting spindle. The flyer spinner was also used to form the twistuntwisted yarn by adding opposite twist to the initial twisted yarn. In the untwisting step, the initial twisted yarn was fed to the flyer spinner and the spindle was set to rotate in the opposite direction.
2.3. Yarn porosity The definition of yarn porosity used in a previous paper [22] is adopted here. In deriving the density of single carbon nanotubes in the yarn, the linear density of the nanotube was calculated from a hollow tube model with a 10 nm outer diameter, 4 nm inner diameter [4] based on the wall density of 2.1 g/cm3 [23]. The apparent volume density of the nanotube was then derived from the calculated weight divided by the volume of a solid cylinder with a 10 nm outer diameter. This gives a nanotube volumetric density of 1.76 g/cm3 [22]. The yarn porosity 4 is defined as the volume fraction of spaces between the nanotubes and therefore can be derived from
4¼1
ryarn 1:76
(1)
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cm3) and can be calculated using the following equation [22]
2.4. Tensile testing Tensile tests on CNT yarns were conducted using a Chatillon® tensile testing machine. At least five specimens from each yarn sample were tested. A laser diffraction system was mounted on the side of the tensile tester for measuring the diameter of the CNT yarn specimen before applying tensile load. The yarn specimen was mounted on a paper frame cut with a 10 mm square window. Five measurements were taken simultaneously at five positions on the 10 mm yarn specimen, and the average was taken to represent the diameter of the yarn specimen [11]. The linear density of the CNT yarn sample in tex (1 tex ¼ 1 mg/ mm) was determined by weighing a 50 mm long yarn specimen on a Mettler Toledo Xp2U Ultra Micro Balance. Specific yarn strength (cN/tex), known as tenacity in the textile industry, was calculated from the breaking force divided by the yarn linear density. 2.5. Electrical conductivity A four-probe electrical resistance measuring device connected to a Hewlett Packard 4262A LCR Meter was used to measure the electrical resistances of CNT yarn specimens at a span length of 50 mm. A constant tension was applied to the yarn specimen during the measurement by hanging a 50 mg weight to each end of the specimen. The electrical conductivity s (S/m) was then calculated from:
s¼
l RA
821
(2)
where R is the measured electrical resistance (U), l (¼0.05 m) is the specimen length, and A is the specimen cross-sectional area (m2). Specific conductivity ssp of a CNT yarn is defined as the ratio between its electrical conductivity s and its volume density ryarn (g/
ssp ¼
l 107 R$T
S$cm2 $g1
(3)
where T is the yarn linear density in tex. 3. Results and discussion 3.1. Twisted and twist-untwisted yarns produced on twist tester Typical SEM images of twisted and twist-untwisted CNT yarns (alternatively known as false twisted yarns) are presented in Fig. 2. For twisted yarns (images a, c and e), a higher level of twist leads to tighter yarn structure (higher CNT packing density). This has been well understood from previous studies [3,4]. In comparison, all the twist-untwisted yarns (images b, d and f) showed considerably larger diameters than their parent twisted yarn counterparts. This means that the untwisting step has loosened the yarn structure. The average diameter, volume density and porosity of the twisted and twist-untwisted yarns at the three twist levels are compared in Fig. 3(a)e(c). All these structural parameters of the twist-untwisted yarn indicate a much looser yarn structure because the twist that holds the CNTs tightly in the initial twisted yarn has disappeared. Because CNT dry spun yarns contain a lot of voids, tensile stress (GPa or MPa) derived from the tensile load and yarn cross-sectional area does not reflect the actual load taken by the constituent CNTs. Besides, the yarn diameter can easily change during the yarn handling and testing [4]. To avoid these problems, we use yarn specific stress based on the tensile load (N or cN) and yarn linear density (tex), which is the standard method adopted by the textile industry. This approach is equivalent to expressing the yarn stress in GPa and then dividing by the yarn bulk density (g/cm3). If the bulk density of the yarn is unity (1 g/cm3), then the yarn specific stress in N/tex equates to the yarn tensile stress in GPa.
Fig. 2. SEM images of twisted and twist-untwisted CNT yarns. (a) Twisted yarn 5000 T/m. (b) Twist-untwisted yarn 5000 T/m. (c) Twisted yarn 10,000 T/m. (d) Twist-untwisted yarn 10,000 T/m. (e) Twisted yarn 20,000 T/m. (f) Twist-untwisted yarn 20,000 T/m. (T/m stands for turns of twist per metre of yarn.)
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b)
a)
c)
Fig. 3. Yarn diameter (a-top left), density (b-top right) and porosity (c-bottom) of twisted and twist-untwisted CNT yarns. (A colour version of this figure can be viewed online.)
Fig. 4 compares the tensile curves of two pairs of twisted yarns and twist-untwisted yarns at a low twist (4000 T/m) and a high twist (16,000 T/m). At 4000 T/m, the twisted yarn had a tenacity of 80 cN/tex. After removing the twist, the yarn strength was reduced to 36 cN/tex. Despite this significant reduction, the specific strength of the twist-untwisted yarn is still many times higher than that of a freshly drawn CNT web [4]. At the high twist level (16,000 T/m), the twisted CNT yarn had a tenacity of only 31 cN/tex because of over twisting. After the twist was removed by untwisting, the yarn strength increased significantly to 54 cN/tex. This strength can be attributed to the van der Waals attraction between CNTs in the twistless yarn. For the untwisted yarns, the effect of fibre inclination to yarn
strength in the twisted yarn is largely eliminated. The strength of the twist-untwisted yarn is mainly the result of van der Waals contact between the constituent CNTs introduced by the densifying action in the initial twisting operation. The higher the initial twist was inserted, the stronger the van der Waals action would be established. At 16,000 T/m, the van der Waals connection formed between CNTs would be much stronger than that formed at 4000 T/ m. As all CNTs are parallel to the yarn axis in the final untwisted yarn, the strength of the final untwisted yarn would increase with the increase of the van der Waals contact established during the initial twisting stage. So the twist-untwisted yarn at 16,000 T/m was much stronger than the twist-untwisted yarn at 4000 T/m. A detailed explanation will be presented in the Discussion section.
Fig. 4. Typical tensile curves of two sets of twisted and twist-untwisted yarns at 4000 T/m (left) and 16,000 T/m (right). (A colour version of this figure can be viewed online.)
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The relationships between twist and tensile properties of a range of twisted and the twist-untwisted yarns produced on the hand-operated twist tester are shown in Fig. 5. The breaking stress and the tenacity (specific stress) followed similar trends allowing for variability of CNT packing density in different yarns. The twistetenacity relationship of the twisted yarn in Fig. 5(b) is similar to the relationship of twisted CNT yarns produced on the Upspinner machine reported previously [4]. Like in Up-spinning, the forming yarn on the twist tester assumes a straight line and is under very little tension. The twisted yarn achieved its highest tenacity at a twist level of about 5000 T/m, or about 20 twist angle on the yarn surface. The tenacity of the twist-untwisted yarns followed a similar pattern of change as the twisted yarns but its peak value was reached at a higher twist level of around 8000 T/m. The maximum tenacity of the twist-untwisted yarn (at 8000 T/m) was more than 80% of maximum tenacity of the twisted yarn (at 4000 T/m). Further increase of degree of twist after the peak caused a decrease of tenacity for the twist-untwisted yarns, but not as much as the decrease of tenacity for the twisted yarn at high twist. At very high twist level (16,000 T/m), the twist-untwisted CNT yarn was more than 80% stronger than its parent twisted yarn. This increase can be attributed to the disappearance of the obliquity effect. Fig. 5(c) shows the influence of twist level on the specific modulus of the resultant yarns. The two sets of yarns reached maximum values at the same twist level of 4000 T/m. Prior to reaching the peak, the two curves almost overlapped with each other. At post-peak twist levels (8000 and 16,000 T/m), the twistuntwisted yarns showed higher specific modulus than the parent twisted yarns. As with yarn tenacity, the removal of twist in the
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yarn eliminated the obliquity effect on the specific modulus. Fig. 5(d) shows that the twist-untwisted yarns had lower breaking strain than their corresponding twisted yarns at all twist levels. Because the CNT yarns only utilized a small portion of the strength of the constituent CNTs, the CNT themselves elongate very little during the tensile testing of the yarns. Almost all the yarn elongation is a result of the yarn structural change under tensile load. For the twisted yarns, this means that the higher the twist, the larger the elongation. For the twist-untwisted yarns, although the twist is removed from the yarn, the CNT crimps (the waviness that is visible in the SEM images in Fig. 2) are mainly responsible for the yarn elongation. 3.2. Flyer-spun twisted yarns and twist-untwisted yarns When twist is applied and removed using the hand-operated twist tester as just discussed, the yarn is under very little tension except at the two ends of the yarn where the yarn is held by the jaws, so there is no compression force applied to the yarn other than the CNTeCNT compression introduced by the twisting action. Unlike the hand-operated twist-tester, when a yarn is formed on the flyer spinner [1], the yarn is under considerable tension as it passes over a series of tensioning pins, bends over the flyer guide and finally winds onto the yarn collection bobbin, as shown in Fig. 6(a). The yarn bend over the flyer guide, as shown in Fig. 6(b), is a sharp turn and the yarn is under a high compression force at the contact point. Consequently, the flyer-spun yarn has a more compact structure and the twistetenacity relationship is noticeably different from that of the yarns produced under low tension, such as the Up-spinner [4] and the hand-operated twist tester. When the
Fig. 5. Comparison of tensile properties of twisted and false twisted yarns produced on hand-operated twist tester. (A colour version of this figure can be viewed online.)
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Fig. 6. Flyer spun CNT yarns. (a) Schematic of the flyer spinning machine [1]; (b) CNT yarn bent over on the flyer hook during spinning. (A colour version of this figure can be viewed online.)
twisted yarn produced on the flyer spinner is untwisted using the flyer spinner, a similar level of tension is applied again to the yarn by these machine parts. Fig. 7 compares the diameter and tenacity (specific strength) of the flyer-spun twisted yarn and their corresponding twistuntwisted yarns. The diameter (and hence porosity because all yarns had constant linear density) of the flyer-spun twisted yarn decreased quickly with twist at the low twist range and then gradually settled into a low constant at high twist levels. The untwisted yarns had similar diameter (porosity) as their parent twisted yarns at low twist levels but started to show larger diameter (higher porosity) than their twisted yarn counterparts as the twist level further increased. At very high twist (15,000 T/m), the diameter difference had increased to about 25% (average diameter of 15.7 mm for the twisted yarns, 21 mm for the twistuntwisted yarns), translating into a volume density difference of about 45%. The specific strength (tenacity) of the flyer-spun twisted yarn reached its peak at about 15,000 T/m and then decreased slowly as twist level was further increased, as shown by the trend curve in Fig. 7(b). This relationship is similar to that reported by Sears et al. [13], which however did not include low twist yarns (5000 T/m). After untwisting, the low twist flyer-spun yarns showed about 15%e20% lower tenacity than their parent yarns according to the trend lines. In the textile industry, the variability of tensile strength of a yarn sample is usually expressed in terms of coefficient of variation (CV %, standard deviation as a percentage of the mean). High CV% is
associated with extremely weak spots in the yarn, which are responsible for yarn breakages during fabric production. The average CV% was 11.6% for the twisted yarns and 9.6% for the twistuntwisted yarns in this investigation. This indicates that some of the extremely weak spots in the initial twisted yarn might have broken during the untwisting step, similar to what is achieved by yarn clearing in the textile industry. The conductivity of CNT yarns is known to be affected by yarn porosity, which is in turn affected by the level of twist inserted into the yarn [22]. The electrical conductivity of the flyer-spun twisted and twist-untwisted yarns increased rapidly as the level of twist increased, as shown in Fig. 8(a). When the yarn conductivity is normalized against yarn density, the specific conductivity settles into a horizontal trend line, as shown in Fig. 8(b). The specific conductivity of both twisted and twist-untwisted yarn was scattered around a constant of about 550 S cm2 g1, irrespective of the level of twist applied. This is similar to what was reported previously for Up-spun twisted yarns [22]. The scattering of data can be explained by the variability of yarn linear density and porosity. Twist densification causes a dramatic change in yarn electrical conductivity, but this is mainly caused by the decrease of yarn diameter (cross-sectional area). When the yarn electrical conductivity is converted into specific conductivity, its value remains approximately constant irrespective of the changes in yarn construction and porosity. This is because the CNT bundles (referred to as splice in the earlier paper [22]) formed during web drawing play a predominant role in electrical conduction, rather than the contact between the bundles strengthened by twist insertion. So, either
Fig. 7. Flyer-spun CNT yarns. (a) Yarn diameter as a function of twist; (b) Yarn tenacity as a function of twist. (A colour version of this figure can be viewed online.)
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Fig. 8. Electrical conductivity of flyer-spun twisted and twist-untwisted CNT yarns. (a) Electrical conductivity as a function of twist; and (b) specific electrical conductivity as a function of twist. The upper and lower lines are one standard deviation from the average values. (A colour version of this figure can be viewed online.)
twisting or untwisting has little effect on the specific conductivity of the CNT yarns. 4. Discussion According to the idealized geometrical model in Fig. 1(a), a twisted yarn may be considered to consist of a series of concentric cylinders of differing radii. Each CNT follows a helical trajectory around one of the concentric cylinders. Because the number of twist turns per unit yarn length remains the same for all helices irrespective of their radii, the helix angle increases as the radial position of the helix going out from the yarn centre. This means that the nanotubes on the yarn surface follow a longer path than those in the yarn centre. In theory, the CNT in the exact centre of the yarn (radius ¼ 0) would assume a straight line. The greater the twist is in the yarn, the larger is the difference of CNT path length between the surface and the centre of the yarn. When the yarn twist is removed by untwisting, the CNTs at the yarn centre would still be straight but the CNTs on the yarn surface would have to buckle because of their longer lengths. These buckles are evident on the surface of the twist-untwisted yarns in Fig. 2. In reality, the CNTs in the initial twisted yarn do not exactly follow such ideal helix paths described in the yarn geometrical model, and instead they change their radius slightly when moving along the yarn length, a phenomenon known as fibre migration in the textile industry [24]. The yarn length will also increase a little when twist is removed so that the CNTs at the centre of the yarn would slip a little or break. These central CNTs would be of a very small proportion of the total number of CNTs in the yarn.
The helically disposed CNTs away from the centre of the twisted yarn exert an inward pressure that causes the CNTs to pack tightly. This compression increases the van der Waals force between contacting CNTs and leads to a friction force that suppresses slippage between CNTs when a tensile load is applied to the yarn. The high van der Waals interaction will bind parallel CNTs together into tight bundles (or ribbons) in the yarn. These bundle of parallel CNTs may be compared to the crystalline structure of molecule chains in polymer materials. When the twist in the initial twisted yarn is removed by the untwisting step, many of the tight CNT bundles (ribbons) survive in the resulting twistless yarn. Fig. 9(a) shows that CNTs are packed tightly in the twisted state; and Fig. 9(b) shows that tight bundles of parallel CNTs maintain their ribbon structure after the twist is removed from the yarn. These CNT bundles (ribbons) could provide the mechanism for load transfer in the twistuntwisted CNT yarn. A very interesting question was raised by one of the reviewers on the effect of removing twist in a solvent-densified CNT yarn, such as that reported in Ref. [5]. Due to the highly effective densification effect of the solvent, the twist required for spinning the CNT yarns was greatly reduced to <12 (estimated to be lower or close to the 2000 T/m data point in Fig. 3 in this paper) in comparison with the much higher twist required for dry spun yarns. The low twist inserted for solvent densified yarns was mainly to make the spinning process easier and to produce a round yarn crosssection. Under such circumstances, if the low twist in solventdensified yarn is removed later, I would not expect the yarn mechanical performance to change significantly, i.e., not more remarkable than what has been exhibited on the dry spun yarns at
Fig. 9. CNT bundles. (a) SEM image of twisted yarn 20,000 T/m. (b) SEM image of CNT bundles in twist-untwisted yarn (20,000 T/m).
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similar twist level discussed in this paper. However, it would be interesting to see experimental verification on this point. 5. Conclusions An investigation of twistless CNT yarns produced by a twistuntwisting process is reported. The van der Waals attraction between neighbouring CNTs in the form of tight CNT bundles established in the initial twisting stage is largely maintained in the resulting twistless yarn. Although the yarn porosity increases upon removal of twist, the resulting twistless CNT yarns preserves high levels of specific strength, modulus and electrical conductivity that closely match the parent twisted CNT yarns. This indicates that the main mechanism responsible for load transfer in twisted CNT yarns is the van der Waals attraction as a result of the contact established between CNTs by the densification effect of the initial twisting action. In the initial twisted yarn, the compression and friction between nanotubes generated by the helical geometry provides another load transfer mechanism, which is however substantially negated by the increased nanotube obliquity in the twisted yarn. Acknowledgement I would like to thank my colleagues Jill McDonnell and Colin Veitch for experimental assistance in this work. References [1] M. Miao, Yarn spun from carbon nanotube forests: production, structure, properties and applications, Particuology 11 (4) (2013) 378e393. [2] K. Jiang, Q. Li, S. Fan, Spinning continuous carbon nanotube yarns, Nature 419 (2002) 801. [3] M. Zhang, K. Atkinson, R.H. Baughman, Multifunctional carbon nanotube yarns by downsizing an ancient technology, Science 306 (5700) (2004) 1358e1361. [4] M. Miao, J. McDonnell, L. Vuckovic, S.C. Hawkins, Poisson's ratio and porosity of carbon nanotube dry-spun yarns, Carbon 48 (10) (2010) 2802e2811. [5] S. Li, X. Zhang, J. Zhao, F. Meng, G. Xu, Z. Yong, et al., Enhancement of carbon nanotube fibres using different solvents and polymers, Compos. Sci. Technol. 72 (12) (2012) 1402e1407. [6] X. Zhang, K. Jiang, C. Feng, P. Liu, L. Zhang, J. Kong, et al., Spinning and processing continuous yarns from 4-inch wafer scale super-aligned carbon
nanotube arrays, Adv. Mater. 18 (12) (2006) 1505e1510. [7] M. Miao, Production, structure and properties of twistless carbon nanotube yarns with a high density sheath, Carbon 50 (13) (2012) 4973e4983. [8] A.M. Beese, X. Wei, S. Sarkar, R. Ramachandramoorthy, M.R. Roenbeck, A. Moravsky, et al., Key factors limiting carbon nanotube yarn strength: exploring processing-structure-property relationships, ACS Nano 8 (11) (2014) 11454e11466. [9] J.T. Paci, Ao Furmanchuk, H.D. Espinosa, G.C. Schatz, Shear and friction between carbon nanotubes in bundles and yarns, Nano Lett. 14 (11) (2014) 6138e6147. [10] I. Beyerlein, P. Porwal, Y. Zhu, K. Hu, X. Xu, Scale and twist effects on the strength of nanostructured yarns and reinforced composites, Nanotechnology 20 (48) (2009) 485702. [11] X. Zhang, Q. Li, Enhancement of friction between carbon nanotubes: an efficient strategy to strengthen fibers, ACS Nano 4 (1) (2009) 312e316. [12] J.J. Vilatela, J.A. Elliott, A.H. Windle, A model for the strength of yarn-like carbon nanotube fibers, ACS Nano 5 (3) (2011) 1921e1927. [13] 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) 4450e4456. [14] D. Zhang, M. Miao, H. Niu, Z. Wei, Core-spun carbon nanotube yarn supercapacitors for wearable electronic textiles, ACS Nano 8 (5) (2014) 4571e4579. [15] K. Wang, Q. Meng, Y. Zhang, Z. Wei, M. Miao, High-performance two-ply yarn supercapacitors based on carbon nanotubes and polyaniline nanowire arrays, Adv. Mater. 25 (10) (2013) 1494e1498. [16] F. Su, M. Miao, Asymmetric carbon nanotube-MnO2 two-ply yarn supercapacitors for wearable electronics, Nanotechnology 25 (13) (2014) 135401. [17] F. Su, M. Miao, H. Niu, Z. Wei, Gamma-irradiated carbon nanotube yarn as substrate for high performance fiber supercapacitors, ACS Appl. Mater. Interfaces 6 (4) (2014) 2552e2560. [18] F. Su, M. Miao, Flexible, high performance two-ply yarn supercapacitors based on irradiated carbon nanotube yarn and PEDOT/PSS, Electrochim. Acta 127 (2014) 433e438. [19] F. Su, X. Lv, M. Miao, High-performance two-ply yarn supercapacitors based on carbon nanotube yarns dotted with Co3O4 and NiO nanoparticles, Small 11 (7) (2015) 854e861. [20] L. Zhang, M. Miao, Commingled natural fibre/polypropylene wrap spun yarns for structured thermoplastic composites, Compos. Sci. Technol. 70 (2010) 130e135. [21] C.P. Huynh, S.C. Hawkins, Understanding the synthesis of directly spinnable carbon nanotube forests, Carbon 48 (4) (2010) 1105e1115. [22] M. Miao, Electrical conductivity of pure carbon nanotube yarns, Carbon 49 (12) (2011) 3755e3761. [23] Q. Lu, G. Keskar, R. Ciocan, R. Rao, R.B. Mathur, A.M. Rao, et al., Determination of carbon nanotube density by gradient sedimentation, J. Phys. Chem. B 110 (48) (2006) 24371e24376. [24] J.W.S. Hearle, P. Grosberg, S. Backer, Structural Mechanics of Fibers, Yarns, and Fabrics, Wiley-Interscience, New York, 1969.
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The role of twist in dry spun carbon nanotube yarns Menghe Miao CSIRO Manufacturing, PO Box 21, Belmont, Victoria 3216, Australia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 August 2015 Received in revised form 9 October 2015 Accepted 10 October 2015 Available online xxx
Twist densification as a method of carbon nanotube (CNT) yarn manufacture has been widely studied. The load transfer between individual nanotubes in the twisted yarns is a result of the combined action of van der Waals attraction and the friction between the nanotubes, both of which increase in magnitude with the level of twist applied to the yarn during manufacturing. To isolate these effects, the twist in the initially twisted yarn was removed by untwisting to produce a twistless yarn. With the disappearance of compressional forces between CNTs originated from the presence of twist, load transfer between individual CNTs in the twistless yarn relies on the van der Waals contact established in the initial twisting stage. Despite the considerable decrease of nanotube packing density in yarn after the removal of twist, 80% of the specific strength and nearly 100% of the specific modulus and specific electrical conductivity of the CNT yarn were maintained. Crown Copyright © 2015 Published by Elsevier Ltd. All rights reserved.
1. Introduction Carbon nanotubes (CNTs) can be processed into yarns (also called fibres by some researchers) in several ways [1]. The method that has attracted the widest interest is the dry solid state spinning of CNT yarn from a vertically aligned array (forest) of multi-walled CNTs (MWCNTs) grown on a substrate. In the yarn production process, the vertically aligned carbon nanotubes are first drawn into an interconnected web [2]. The drawn web, which is extremely porous, has a rather low strength because of the weak CNTeCNT connection. Subsequent research has shown that twist insertion can densify this extremely porous CNT web into a tightly packed twisted yarn structure. The resultant twisted yarn possesses much higher strength than its parent CNT web [3]. The geometry of the twisted CNT yarn is very similar to that of textile yarns made from conventional fibres, such as cotton and wool fibres, but the number of CNTs in the yarn cross-section (hundreds of thousands) is orders of magnitude larger than the number of fibres in conventional textile yarns (typically 100). The insertion of twist to a yarn places individual fibres in approximate coaxial helix configuration (see Fig. 1(a)) and the fibres are pressed together because of the inward pressure generated by the tension in the helically disposed fibres. In a conventional textile yarn, interconnection between fibres relies on the fibreefibre friction that arises from the compression between fibres, which
E-mail address: [email protected]. http://dx.doi.org/10.1016/j.carbon.2015.10.022 0008-6223/Crown Copyright © 2015 Published by Elsevier Ltd. All rights reserved.
increases with the external tensile load applied to the yarn. At low twist levels, due to low fibreefibre compression and friction, the yarn failure mechanism is dominated by fibre slippage. At high twist, fibre slippage is largely prevented by high fibreefibre friction and thus the yarn fails due to fibre breakage. On the other hand, high twist reduces the contribution of fibre strength to the yarn strength due to fibre obliquity in the yarn. Therefore the maximum yarn specific strength is usually achieved at an intermediate twist level. This relationship is illustrated in Fig. 1(b). Twisted CNT yarn has a similar twistestrength relationship [4] as these conventional textile yarns. There is, however, a major difference in the mechanism of fibreefibre (CNTeCNT) interaction. Because of their nano-scale dimension, the van der Waals attraction (London dispersion force) between the nanotubes plays an important role in transfer of load between nanotubes in the yarns. Twistless CNT yarns can also be produced by liquid shrinking with or without twist insertion [5,6]. CNT yarns densified by polar solvent ethylene glycol [5] demonstrated a strength of 1.45 GPa, which is stronger than most twisted CNT dry spun yarns reported in literature [1]. Mechanical rubbing [7] produces a closely packed CNT yarn which is also quite strong. Both of these examples illustrate the significance of van der Waals forces between closely packed CNTs. A number of modelling studies have discussed the underlying mechanics of twisted CNT yarns with different levels of emphasis on the relative importance of twist-friction effect and van der Waals attraction [8e12]. The geometry of CNTs in twisted yarns is
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Fig. 1. Structureestrength relationship of conventional textile yarns spun from staple fibres, such as cotton and wool. (a) Fibre trajectories at different radii of an idealized twisted yarn. (b) Influence of twist on yarn strength. (A colour version of this figure can be viewed online.)
far from closely packed coaxial helices as is usually assumed in yarn mechanics modelling. Yarn cross-sections prepared by focused ion beam (FIB) milling [13,14] show that the voids between CNTs in twisted CNT yarns look far different from the regular shape and size that are expected of a closely packed CNT array [4]. The initial CNT packing density (or its opposite, porosity) of a twisted yarn depends on the level of CNT tension and the level of twist inserted during spinning. However, when an external load is applied to the twisted yarn, the CNT packing density can increase significantly, resulting in very high Poisson's ratio [4]. All these factors present practical difficulties for accurately modelling the CNT yarn structure and its mechanical behaviours. Twisting is a reversible process, i.e., the twist in a yarn can be removed by inserting a twist of the same amount but in the opposite direction (i.e., untwisting). When the twist in a conventionally spun textile yarn is removed by untwisting, the yarn returns to a loose fibre strand with virtually zero strength. As it will become clear later in this paper, when the twist of a twisted CNT yarn is removed by untwisting, the resultant yarn largely maintains its structural integrity and a major part of its strength. This provides an opportunity to examine the relative contributions of the twistderived friction and the van der Waals force between CNTs. The nanotube alignment, structural porosity and mechanical strength of a twistless CNT yarn may be utilized in a number of applications. The high porosity provides superior access for electrolyte to penetrate the yarn, which is beneficial for threadlike energy storage devices used to power wearable electronics [14e19]. The high CNT alignment due to the absence of twist provides a structure for high strength CNT composite materials [20].
2. Experimental 2.1. CNT forests and drawn webs The CNT forests used in our work were grown on silicon wafer substrates coated with a thermal oxide layer (100 nm) and iron catalyst coating (2.5 nm) using chemical vapour deposition (CVD) of acetylene (5%) in helium (700 sccm) at 680 C for 20 min [21]. The CNTs grown by this method were about 350 mm in length with an outer diameter of approximately 10 nm and an inner diameter of about 4 nm. A continuous web of CNTs was drawn from the forests.
2.2. Formation of twisted and twist-untwisted yarns Two methods were used to form twisted yarn and subsequent twist-untwisted yarns. One was based on a hand-operated twist tester and the other was based on a computer controlled miniature flyer spinning machine [1]. The two methods apply different levels of CNT densification to the yarn. To form a twisted yarn on a hand-operated twist tester, the drawn web was held at one end by the fixed jaw while twist was inserted into the web from the other end by manually driving the rotating jaw. To produce a twist-untwisted yarn, twist was inserted to the initial twisted yarn in an equal amount but opposite direction (untwisting) using the same twist tester, resulting in a twistless CNT yarn. In the second method, the miniature CNT yarn spinner was used to carry out both the twisting and untwisting operations. The twisting and winding operations are realized by two coaxial shafts rotating at differential speeds, causing the yarn to be twisted and wound simultaneously onto a yarn bobbin that is mounted on the twisting spindle. The flyer spinner was also used to form the twistuntwisted yarn by adding opposite twist to the initial twisted yarn. In the untwisting step, the initial twisted yarn was fed to the flyer spinner and the spindle was set to rotate in the opposite direction.
2.3. Yarn porosity The definition of yarn porosity used in a previous paper [22] is adopted here. In deriving the density of single carbon nanotubes in the yarn, the linear density of the nanotube was calculated from a hollow tube model with a 10 nm outer diameter, 4 nm inner diameter [4] based on the wall density of 2.1 g/cm3 [23]. The apparent volume density of the nanotube was then derived from the calculated weight divided by the volume of a solid cylinder with a 10 nm outer diameter. This gives a nanotube volumetric density of 1.76 g/cm3 [22]. The yarn porosity 4 is defined as the volume fraction of spaces between the nanotubes and therefore can be derived from
4¼1
ryarn 1:76
(1)
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cm3) and can be calculated using the following equation [22]
2.4. Tensile testing Tensile tests on CNT yarns were conducted using a Chatillon® tensile testing machine. At least five specimens from each yarn sample were tested. A laser diffraction system was mounted on the side of the tensile tester for measuring the diameter of the CNT yarn specimen before applying tensile load. The yarn specimen was mounted on a paper frame cut with a 10 mm square window. Five measurements were taken simultaneously at five positions on the 10 mm yarn specimen, and the average was taken to represent the diameter of the yarn specimen [11]. The linear density of the CNT yarn sample in tex (1 tex ¼ 1 mg/ mm) was determined by weighing a 50 mm long yarn specimen on a Mettler Toledo Xp2U Ultra Micro Balance. Specific yarn strength (cN/tex), known as tenacity in the textile industry, was calculated from the breaking force divided by the yarn linear density. 2.5. Electrical conductivity A four-probe electrical resistance measuring device connected to a Hewlett Packard 4262A LCR Meter was used to measure the electrical resistances of CNT yarn specimens at a span length of 50 mm. A constant tension was applied to the yarn specimen during the measurement by hanging a 50 mg weight to each end of the specimen. The electrical conductivity s (S/m) was then calculated from:
s¼
l RA
821
(2)
where R is the measured electrical resistance (U), l (¼0.05 m) is the specimen length, and A is the specimen cross-sectional area (m2). Specific conductivity ssp of a CNT yarn is defined as the ratio between its electrical conductivity s and its volume density ryarn (g/
ssp ¼
l 107 R$T
S$cm2 $g1
(3)
where T is the yarn linear density in tex. 3. Results and discussion 3.1. Twisted and twist-untwisted yarns produced on twist tester Typical SEM images of twisted and twist-untwisted CNT yarns (alternatively known as false twisted yarns) are presented in Fig. 2. For twisted yarns (images a, c and e), a higher level of twist leads to tighter yarn structure (higher CNT packing density). This has been well understood from previous studies [3,4]. In comparison, all the twist-untwisted yarns (images b, d and f) showed considerably larger diameters than their parent twisted yarn counterparts. This means that the untwisting step has loosened the yarn structure. The average diameter, volume density and porosity of the twisted and twist-untwisted yarns at the three twist levels are compared in Fig. 3(a)e(c). All these structural parameters of the twist-untwisted yarn indicate a much looser yarn structure because the twist that holds the CNTs tightly in the initial twisted yarn has disappeared. Because CNT dry spun yarns contain a lot of voids, tensile stress (GPa or MPa) derived from the tensile load and yarn cross-sectional area does not reflect the actual load taken by the constituent CNTs. Besides, the yarn diameter can easily change during the yarn handling and testing [4]. To avoid these problems, we use yarn specific stress based on the tensile load (N or cN) and yarn linear density (tex), which is the standard method adopted by the textile industry. This approach is equivalent to expressing the yarn stress in GPa and then dividing by the yarn bulk density (g/cm3). If the bulk density of the yarn is unity (1 g/cm3), then the yarn specific stress in N/tex equates to the yarn tensile stress in GPa.
Fig. 2. SEM images of twisted and twist-untwisted CNT yarns. (a) Twisted yarn 5000 T/m. (b) Twist-untwisted yarn 5000 T/m. (c) Twisted yarn 10,000 T/m. (d) Twist-untwisted yarn 10,000 T/m. (e) Twisted yarn 20,000 T/m. (f) Twist-untwisted yarn 20,000 T/m. (T/m stands for turns of twist per metre of yarn.)
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b)
a)
c)
Fig. 3. Yarn diameter (a-top left), density (b-top right) and porosity (c-bottom) of twisted and twist-untwisted CNT yarns. (A colour version of this figure can be viewed online.)
Fig. 4 compares the tensile curves of two pairs of twisted yarns and twist-untwisted yarns at a low twist (4000 T/m) and a high twist (16,000 T/m). At 4000 T/m, the twisted yarn had a tenacity of 80 cN/tex. After removing the twist, the yarn strength was reduced to 36 cN/tex. Despite this significant reduction, the specific strength of the twist-untwisted yarn is still many times higher than that of a freshly drawn CNT web [4]. At the high twist level (16,000 T/m), the twisted CNT yarn had a tenacity of only 31 cN/tex because of over twisting. After the twist was removed by untwisting, the yarn strength increased significantly to 54 cN/tex. This strength can be attributed to the van der Waals attraction between CNTs in the twistless yarn. For the untwisted yarns, the effect of fibre inclination to yarn
strength in the twisted yarn is largely eliminated. The strength of the twist-untwisted yarn is mainly the result of van der Waals contact between the constituent CNTs introduced by the densifying action in the initial twisting operation. The higher the initial twist was inserted, the stronger the van der Waals action would be established. At 16,000 T/m, the van der Waals connection formed between CNTs would be much stronger than that formed at 4000 T/ m. As all CNTs are parallel to the yarn axis in the final untwisted yarn, the strength of the final untwisted yarn would increase with the increase of the van der Waals contact established during the initial twisting stage. So the twist-untwisted yarn at 16,000 T/m was much stronger than the twist-untwisted yarn at 4000 T/m. A detailed explanation will be presented in the Discussion section.
Fig. 4. Typical tensile curves of two sets of twisted and twist-untwisted yarns at 4000 T/m (left) and 16,000 T/m (right). (A colour version of this figure can be viewed online.)
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The relationships between twist and tensile properties of a range of twisted and the twist-untwisted yarns produced on the hand-operated twist tester are shown in Fig. 5. The breaking stress and the tenacity (specific stress) followed similar trends allowing for variability of CNT packing density in different yarns. The twistetenacity relationship of the twisted yarn in Fig. 5(b) is similar to the relationship of twisted CNT yarns produced on the Upspinner machine reported previously [4]. Like in Up-spinning, the forming yarn on the twist tester assumes a straight line and is under very little tension. The twisted yarn achieved its highest tenacity at a twist level of about 5000 T/m, or about 20 twist angle on the yarn surface. The tenacity of the twist-untwisted yarns followed a similar pattern of change as the twisted yarns but its peak value was reached at a higher twist level of around 8000 T/m. The maximum tenacity of the twist-untwisted yarn (at 8000 T/m) was more than 80% of maximum tenacity of the twisted yarn (at 4000 T/m). Further increase of degree of twist after the peak caused a decrease of tenacity for the twist-untwisted yarns, but not as much as the decrease of tenacity for the twisted yarn at high twist. At very high twist level (16,000 T/m), the twist-untwisted CNT yarn was more than 80% stronger than its parent twisted yarn. This increase can be attributed to the disappearance of the obliquity effect. Fig. 5(c) shows the influence of twist level on the specific modulus of the resultant yarns. The two sets of yarns reached maximum values at the same twist level of 4000 T/m. Prior to reaching the peak, the two curves almost overlapped with each other. At post-peak twist levels (8000 and 16,000 T/m), the twistuntwisted yarns showed higher specific modulus than the parent twisted yarns. As with yarn tenacity, the removal of twist in the
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yarn eliminated the obliquity effect on the specific modulus. Fig. 5(d) shows that the twist-untwisted yarns had lower breaking strain than their corresponding twisted yarns at all twist levels. Because the CNT yarns only utilized a small portion of the strength of the constituent CNTs, the CNT themselves elongate very little during the tensile testing of the yarns. Almost all the yarn elongation is a result of the yarn structural change under tensile load. For the twisted yarns, this means that the higher the twist, the larger the elongation. For the twist-untwisted yarns, although the twist is removed from the yarn, the CNT crimps (the waviness that is visible in the SEM images in Fig. 2) are mainly responsible for the yarn elongation. 3.2. Flyer-spun twisted yarns and twist-untwisted yarns When twist is applied and removed using the hand-operated twist tester as just discussed, the yarn is under very little tension except at the two ends of the yarn where the yarn is held by the jaws, so there is no compression force applied to the yarn other than the CNTeCNT compression introduced by the twisting action. Unlike the hand-operated twist-tester, when a yarn is formed on the flyer spinner [1], the yarn is under considerable tension as it passes over a series of tensioning pins, bends over the flyer guide and finally winds onto the yarn collection bobbin, as shown in Fig. 6(a). The yarn bend over the flyer guide, as shown in Fig. 6(b), is a sharp turn and the yarn is under a high compression force at the contact point. Consequently, the flyer-spun yarn has a more compact structure and the twistetenacity relationship is noticeably different from that of the yarns produced under low tension, such as the Up-spinner [4] and the hand-operated twist tester. When the
Fig. 5. Comparison of tensile properties of twisted and false twisted yarns produced on hand-operated twist tester. (A colour version of this figure can be viewed online.)
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Fig. 6. Flyer spun CNT yarns. (a) Schematic of the flyer spinning machine [1]; (b) CNT yarn bent over on the flyer hook during spinning. (A colour version of this figure can be viewed online.)
twisted yarn produced on the flyer spinner is untwisted using the flyer spinner, a similar level of tension is applied again to the yarn by these machine parts. Fig. 7 compares the diameter and tenacity (specific strength) of the flyer-spun twisted yarn and their corresponding twistuntwisted yarns. The diameter (and hence porosity because all yarns had constant linear density) of the flyer-spun twisted yarn decreased quickly with twist at the low twist range and then gradually settled into a low constant at high twist levels. The untwisted yarns had similar diameter (porosity) as their parent twisted yarns at low twist levels but started to show larger diameter (higher porosity) than their twisted yarn counterparts as the twist level further increased. At very high twist (15,000 T/m), the diameter difference had increased to about 25% (average diameter of 15.7 mm for the twisted yarns, 21 mm for the twistuntwisted yarns), translating into a volume density difference of about 45%. The specific strength (tenacity) of the flyer-spun twisted yarn reached its peak at about 15,000 T/m and then decreased slowly as twist level was further increased, as shown by the trend curve in Fig. 7(b). This relationship is similar to that reported by Sears et al. [13], which however did not include low twist yarns (5000 T/m). After untwisting, the low twist flyer-spun yarns showed about 15%e20% lower tenacity than their parent yarns according to the trend lines. In the textile industry, the variability of tensile strength of a yarn sample is usually expressed in terms of coefficient of variation (CV %, standard deviation as a percentage of the mean). High CV% is
associated with extremely weak spots in the yarn, which are responsible for yarn breakages during fabric production. The average CV% was 11.6% for the twisted yarns and 9.6% for the twistuntwisted yarns in this investigation. This indicates that some of the extremely weak spots in the initial twisted yarn might have broken during the untwisting step, similar to what is achieved by yarn clearing in the textile industry. The conductivity of CNT yarns is known to be affected by yarn porosity, which is in turn affected by the level of twist inserted into the yarn [22]. The electrical conductivity of the flyer-spun twisted and twist-untwisted yarns increased rapidly as the level of twist increased, as shown in Fig. 8(a). When the yarn conductivity is normalized against yarn density, the specific conductivity settles into a horizontal trend line, as shown in Fig. 8(b). The specific conductivity of both twisted and twist-untwisted yarn was scattered around a constant of about 550 S cm2 g1, irrespective of the level of twist applied. This is similar to what was reported previously for Up-spun twisted yarns [22]. The scattering of data can be explained by the variability of yarn linear density and porosity. Twist densification causes a dramatic change in yarn electrical conductivity, but this is mainly caused by the decrease of yarn diameter (cross-sectional area). When the yarn electrical conductivity is converted into specific conductivity, its value remains approximately constant irrespective of the changes in yarn construction and porosity. This is because the CNT bundles (referred to as splice in the earlier paper [22]) formed during web drawing play a predominant role in electrical conduction, rather than the contact between the bundles strengthened by twist insertion. So, either
Fig. 7. Flyer-spun CNT yarns. (a) Yarn diameter as a function of twist; (b) Yarn tenacity as a function of twist. (A colour version of this figure can be viewed online.)
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Fig. 8. Electrical conductivity of flyer-spun twisted and twist-untwisted CNT yarns. (a) Electrical conductivity as a function of twist; and (b) specific electrical conductivity as a function of twist. The upper and lower lines are one standard deviation from the average values. (A colour version of this figure can be viewed online.)
twisting or untwisting has little effect on the specific conductivity of the CNT yarns. 4. Discussion According to the idealized geometrical model in Fig. 1(a), a twisted yarn may be considered to consist of a series of concentric cylinders of differing radii. Each CNT follows a helical trajectory around one of the concentric cylinders. Because the number of twist turns per unit yarn length remains the same for all helices irrespective of their radii, the helix angle increases as the radial position of the helix going out from the yarn centre. This means that the nanotubes on the yarn surface follow a longer path than those in the yarn centre. In theory, the CNT in the exact centre of the yarn (radius ¼ 0) would assume a straight line. The greater the twist is in the yarn, the larger is the difference of CNT path length between the surface and the centre of the yarn. When the yarn twist is removed by untwisting, the CNTs at the yarn centre would still be straight but the CNTs on the yarn surface would have to buckle because of their longer lengths. These buckles are evident on the surface of the twist-untwisted yarns in Fig. 2. In reality, the CNTs in the initial twisted yarn do not exactly follow such ideal helix paths described in the yarn geometrical model, and instead they change their radius slightly when moving along the yarn length, a phenomenon known as fibre migration in the textile industry [24]. The yarn length will also increase a little when twist is removed so that the CNTs at the centre of the yarn would slip a little or break. These central CNTs would be of a very small proportion of the total number of CNTs in the yarn.
The helically disposed CNTs away from the centre of the twisted yarn exert an inward pressure that causes the CNTs to pack tightly. This compression increases the van der Waals force between contacting CNTs and leads to a friction force that suppresses slippage between CNTs when a tensile load is applied to the yarn. The high van der Waals interaction will bind parallel CNTs together into tight bundles (or ribbons) in the yarn. These bundle of parallel CNTs may be compared to the crystalline structure of molecule chains in polymer materials. When the twist in the initial twisted yarn is removed by the untwisting step, many of the tight CNT bundles (ribbons) survive in the resulting twistless yarn. Fig. 9(a) shows that CNTs are packed tightly in the twisted state; and Fig. 9(b) shows that tight bundles of parallel CNTs maintain their ribbon structure after the twist is removed from the yarn. These CNT bundles (ribbons) could provide the mechanism for load transfer in the twistuntwisted CNT yarn. A very interesting question was raised by one of the reviewers on the effect of removing twist in a solvent-densified CNT yarn, such as that reported in Ref. [5]. Due to the highly effective densification effect of the solvent, the twist required for spinning the CNT yarns was greatly reduced to <12 (estimated to be lower or close to the 2000 T/m data point in Fig. 3 in this paper) in comparison with the much higher twist required for dry spun yarns. The low twist inserted for solvent densified yarns was mainly to make the spinning process easier and to produce a round yarn crosssection. Under such circumstances, if the low twist in solventdensified yarn is removed later, I would not expect the yarn mechanical performance to change significantly, i.e., not more remarkable than what has been exhibited on the dry spun yarns at
Fig. 9. CNT bundles. (a) SEM image of twisted yarn 20,000 T/m. (b) SEM image of CNT bundles in twist-untwisted yarn (20,000 T/m).
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similar twist level discussed in this paper. However, it would be interesting to see experimental verification on this point. 5. Conclusions An investigation of twistless CNT yarns produced by a twistuntwisting process is reported. The van der Waals attraction between neighbouring CNTs in the form of tight CNT bundles established in the initial twisting stage is largely maintained in the resulting twistless yarn. Although the yarn porosity increases upon removal of twist, the resulting twistless CNT yarns preserves high levels of specific strength, modulus and electrical conductivity that closely match the parent twisted CNT yarns. This indicates that the main mechanism responsible for load transfer in twisted CNT yarns is the van der Waals attraction as a result of the contact established between CNTs by the densification effect of the initial twisting action. In the initial twisted yarn, the compression and friction between nanotubes generated by the helical geometry provides another load transfer mechanism, which is however substantially negated by the increased nanotube obliquity in the twisted yarn. Acknowledgement I would like to thank my colleagues Jill McDonnell and Colin Veitch for experimental assistance in this work. References [1] M. Miao, Yarn spun from carbon nanotube forests: production, structure, properties and applications, Particuology 11 (4) (2013) 378e393. [2] K. Jiang, Q. Li, S. Fan, Spinning continuous carbon nanotube yarns, Nature 419 (2002) 801. [3] M. Zhang, K. Atkinson, R.H. Baughman, Multifunctional carbon nanotube yarns by downsizing an ancient technology, Science 306 (5700) (2004) 1358e1361. [4] M. Miao, J. McDonnell, L. Vuckovic, S.C. Hawkins, Poisson's ratio and porosity of carbon nanotube dry-spun yarns, Carbon 48 (10) (2010) 2802e2811. [5] S. Li, X. Zhang, J. Zhao, F. Meng, G. Xu, Z. Yong, et al., Enhancement of carbon nanotube fibres using different solvents and polymers, Compos. Sci. Technol. 72 (12) (2012) 1402e1407. [6] X. Zhang, K. Jiang, C. Feng, P. Liu, L. Zhang, J. Kong, et al., Spinning and processing continuous yarns from 4-inch wafer scale super-aligned carbon
nanotube arrays, Adv. Mater. 18 (12) (2006) 1505e1510. [7] M. Miao, Production, structure and properties of twistless carbon nanotube yarns with a high density sheath, Carbon 50 (13) (2012) 4973e4983. [8] A.M. Beese, X. Wei, S. Sarkar, R. Ramachandramoorthy, M.R. Roenbeck, A. Moravsky, et al., Key factors limiting carbon nanotube yarn strength: exploring processing-structure-property relationships, ACS Nano 8 (11) (2014) 11454e11466. [9] J.T. Paci, Ao Furmanchuk, H.D. Espinosa, G.C. Schatz, Shear and friction between carbon nanotubes in bundles and yarns, Nano Lett. 14 (11) (2014) 6138e6147. [10] I. Beyerlein, P. Porwal, Y. Zhu, K. Hu, X. Xu, Scale and twist effects on the strength of nanostructured yarns and reinforced composites, Nanotechnology 20 (48) (2009) 485702. [11] X. Zhang, Q. Li, Enhancement of friction between carbon nanotubes: an efficient strategy to strengthen fibers, ACS Nano 4 (1) (2009) 312e316. [12] J.J. Vilatela, J.A. Elliott, A.H. Windle, A model for the strength of yarn-like carbon nanotube fibers, ACS Nano 5 (3) (2011) 1921e1927. [13] 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) 4450e4456. [14] D. Zhang, M. Miao, H. Niu, Z. Wei, Core-spun carbon nanotube yarn supercapacitors for wearable electronic textiles, ACS Nano 8 (5) (2014) 4571e4579. [15] K. Wang, Q. Meng, Y. Zhang, Z. Wei, M. Miao, High-performance two-ply yarn supercapacitors based on carbon nanotubes and polyaniline nanowire arrays, Adv. Mater. 25 (10) (2013) 1494e1498. [16] F. Su, M. Miao, Asymmetric carbon nanotube-MnO2 two-ply yarn supercapacitors for wearable electronics, Nanotechnology 25 (13) (2014) 135401. [17] F. Su, M. Miao, H. Niu, Z. Wei, Gamma-irradiated carbon nanotube yarn as substrate for high performance fiber supercapacitors, ACS Appl. Mater. Interfaces 6 (4) (2014) 2552e2560. [18] F. Su, M. Miao, Flexible, high performance two-ply yarn supercapacitors based on irradiated carbon nanotube yarn and PEDOT/PSS, Electrochim. Acta 127 (2014) 433e438. [19] F. Su, X. Lv, M. Miao, High-performance two-ply yarn supercapacitors based on carbon nanotube yarns dotted with Co3O4 and NiO nanoparticles, Small 11 (7) (2015) 854e861. [20] L. Zhang, M. Miao, Commingled natural fibre/polypropylene wrap spun yarns for structured thermoplastic composites, Compos. Sci. Technol. 70 (2010) 130e135. [21] C.P. Huynh, S.C. Hawkins, Understanding the synthesis of directly spinnable carbon nanotube forests, Carbon 48 (4) (2010) 1105e1115. [22] M. Miao, Electrical conductivity of pure carbon nanotube yarns, Carbon 49 (12) (2011) 3755e3761. [23] Q. Lu, G. Keskar, R. Ciocan, R. Rao, R.B. Mathur, A.M. Rao, et al., Determination of carbon nanotube density by gradient sedimentation, J. Phys. Chem. B 110 (48) (2006) 24371e24376. [24] J.W.S. Hearle, P. Grosberg, S. Backer, Structural Mechanics of Fibers, Yarns, and Fabrics, Wiley-Interscience, New York, 1969.