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Investigation of microcombing parameters in enhancing the properties of carbon nanotube yarns
Materials and Design 134 (2017) 181–187
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Investigation of microcombing parameters in enhancing the properties of carbon nanotube yarns Yingying Yu a,b, Liwen Zhang b,⁎, Ozkan Yildiz c, Haotian Deng b, Changhao Zhao a,b, Philip D. Bradford c, Jianying Li a,⁎, Yuntian Zhu b a b c
State Key Laboratory of Electrical Insulation and Power Equipment, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China Department of Materials Science and Engineering, North Carolina State University, Raleigh 27695, USA Department of Textile Engineering and Chemistry and Science, North Carolina State University, Raleigh 27695, USA
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• A systematic study is conducted to understand how microcombing degree affects the structure and properties of CNT yarns. • The mechanical and electrical properties could improve with well-aligned packing structure due to microcombing effect. • On the other hand, over-combing will destroy the packing structure, resulting in lower mechanical and electrical properties.
a r t i c l e
i n f o
Article history: Received 6 May 2017 Received in revised form 15 August 2017 Accepted 16 August 2017 Available online 17 August 2017 Keywords: Carbon nanotube yarns microcombing effects mechanical properties electrical properties
a b s t r a c t Microcombing has been reported as a novel processing approach for reducing waviness and improving alignment of carbon nanotubes (CNTs), which effectively enhances the performance of materials made from CNT sheets. In this study, we have systematically investigated the effects of microcombing parameters on the properties of CNT yarns. It is found that the electrical and mechanical properties of CNT yarns first improved with increasing degree of microcombing and then degraded with over-combing. At the optimum degree of microcombing, the electrical conductivity, tensile strength, and Young's modulus of the CNT yarns were improved to 140%, 140%, and 230%, respectively, over those of uncombed yarns. The enhanced yarn properties were resulted from reduced nanotube waviness, improved CNT alignment and denser packing structure, which led to a more uniform yarn structure. On the other hand, over-combing degraded structural uniformity, resulting in lower electrical and mechanical properties. These observations are expected to help with future selection of microcombing parameters for producing high-quality CNT yarns and polymer-CNT composite yarns for superior electrical and mechanical properties. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Multi-walled carbon nanotubes (MWCNTs) with unique multiplelayer seamless cylindrical graphitic structure show extraordinary ⁎ Corresponding authors. E-mail addresses: [email protected] (L. Zhang), [email protected] (J. Li).
http://dx.doi.org/10.1016/j.matdes.2017.08.035 0264-1275/© 2017 Elsevier Ltd. All rights reserved.
mechanical [1,2], electrical [3,4], and thermal [5] properties. They show great potential for applications in aerospace, electrochemical devices, hydrogen storage, field emission devices, filters, sensors [6–9], etc. However, it has been a challenge to assemble nano-sized CNTs into macroscopic structures, such as CNT yarns [10,11], films [12,13], and buckypapers [14,15], while retaining the superior properties of individual nanotubes when assembling them into bulk structures.
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In 2002, a simple but effective way of making CNT yarns was reported by Jiang et al. [10]. They found that a continuous pure CNT yarn could be directly drawn from a super-aligned CNT array. However, the yarns made by this method usually have loose structures and exhibit low properties. Nevertheless, this inspired other following works in which twisting was applied to produce a more uniform and compact yarn structure in order to obtain better yarn properties [16–19]. Individual high aspect ratio CNTs are most often observed to be very wavy under high-magnification SEM. Good alignment and straightness are reported to be vital for improving mechanical and electrical properties of CNT yarns and films [12,20–22]. Many research works have been focused on reducing misalignment and waviness. Tran et al. [11] suggested a modified process of dry spinning CNT yarns, where a tension zone was introduced to improve the structure of CNT bundles in CNT web. This method improved the mechanical properties and showed a potential for further development of complex spinning processes. W. Liu et al. [23] coupled the spraying of a PVA solution with continuous winding of CNT sheets from an array onto a rotating mandrel, which increased the alignment and the volume fraction of CNT. Wang et al. [22] improved the winding method by adding a stretching process to reduce the waviness of CNT sheets before they were embedded into a polymer matrix. Q. Liu et al. [24] further increased the alignment of CNT sheets by stretching and pressing, obtaining CNT sheets with high density and outstanding mechanical properties. W. Xu et al. [25]reported a method involving a continuous production of a hollow cylindrical CNT assembly and its condensation on a winding drum to enhance the alignment and densification. The CNT products fabricated by these methods, which all aim at enhancing the alignment enhance and reducing the waviness, showed a significant improvement in their macroscopic properties. More recently, our group [26,27] developed another simple but effective approach, called microcombing, which uses the natural micro-sized teeth on the blade as “comb” to align CNTs. This process reduces waviness and defects in the CNT sheet structure. The dry CNT films and CNT/poly (vinyl alcohol) (PVA) films made by the microcombed CNTs dramatically improved both the tensile strength and the electrical conductivity to record values for dry CNTs and CNT/PVA composites. However, as mentioned in our early work [26,27], the microcombing parameters in our previous research were chosen arbitrarily in terms of number of blades, inter-blades distance and height offset. Systematic study is needed to understand how micro-combing degree affects the structure and properties of CNT assemblies. In order to understand the critical issues and underlying mechanisms of microcombing, in this work we systematically altered the number of microcombing blades and the distance between the adjacent blades while maintaining the blades at the same height to produce different degrees of microcombing. We used CNT yarns in this study instead of CNT sheets in our previous works [26,27]. We found that the electrical conductivity, tensile strength, and Young's modulus of the produced CNT yarns improved first with increasing degree of microcombing and then decreased. In other words, an optimum microcombing degree exists to achieve the best mechanical and electrical properties. While microcombing straightened CNTs and improved CNT alignment, it also caused damage to the yarn structure if the sheets were “over-combed”. We found there was a competition between better CNT packing and deterioration of the microstructural integrity. This new finding should be applicable to other CNT materials and can help with their future design and development. 2. Experimental 2.1. Growth of super aligned CNT (SACNT) arrays The SACNT arrays used in our experiment were grown by a chemical vapor deposition method. Vertically aligned multi-walled carbon nanotubes (MWCNT) were grown in a tube furnace via a modified version of the chlorine mediated chemical vapor deposition route. The aligned
CNT arrays were synthesized on a quartz substrate at 760 °C under 3 Torr with iron II chloride (FeCl2 anhydrous 99.5% VWR) as a catalyst and acetylene as the carbon precursors. The growth took 20 min under a flowing mixture of gases of acetylene (600 sccm), argon (395 sccm) and chlorine (5 sccm). The CNTs grown in this way usually have diameters in the range of 25 to 40 nm, and length about 1 mm [28, 29]. 2.2. Production of microcombed CNT yarns The CNT yarns fabrication and microcombing system is composed of three zones: sheet formation zone, microcombing zone, and twisting zone, as schematically shown in Fig. 1. One piece of SACNT array (16 × 76 mm) was connected to the stage, and then the CNT sheet was drawn out at a speed of ~ 9 mm/s. It went through pair(s) of blades with a mutual vertical offset of 9 mm, which was defined as the microcombing zone. The microfeatures of blades were shown in Fig. 2 with the teeth size of ~3 μm and teeth depth of ~0.3 μm. Compared to the teeth size, the diameters of the CNTs are much smaller. In this case, the blades can act as microcombs to straighten wavy CNTs. In the microcombing zone, blades were vertically mounted and equally spaced. To guarantee the excellent contact between the blades and the CNT sheets, the edges of these blades were alternately oriented upand-down. This configuration generated a contact angle of CNT sheets over the tip of each blade, as shown in the top inset in Fig.1. After microcombing, the CNT sheet was then narrowed down by passing through a V grove formed by a wire made into M shape, where the yarns were wetted by a condensing solution consisting of deionized water and ethanol with a volume ratio of 1:1 [30]. The condensing process can bring CNTs together and reduce the nanotube waviness to some extent [31]. Finally, the narrowed sheet was twisted by a spinning machine at a rotating speed of 1000 r/min to form the microcombed CNT yarns with twist per unit length 1.85 r/mm. 2.3. Characterization methods Scanning electron microscopy (SEM, Verios 460 L) was used to characterize the morphology of the CNT yarns produced in the experiments. Electrical conductivities of the produced CNT yarns were calculated from their electrical resistances, which were measured along the CNT alignment direction by a two-probe method using Agilent 34410A 6.5digit multimeter. The test samples were 100 mm long, and the two ends of samples were painted with silver electrodes by magnetron sputtering. The mechanical properties were measured by a Shimadzu EZ-S tensile testing machine with a load cell of 2 N and a crosshead speed of 0.5 mm/min [32]. The gauge length of the testing samples was 6 mm. The diameters of CNT yarns were measured by laser diffraction method and further verified by SEM. Nine yarns were tested for each group in these characterization processes. 3. Results and discussions 3.1. Total microcombing length (TML) The degree of microcombing can be altered by the number of microcombing blades, and the distance between the adjacent blades. In this work, nine groups of CNT yarns were produced using different microcombing parameters. For the first 5 groups, we increased the microcombing degree by increasing the number of blades (from 0 pairs to 4 pairs) while maintaining the inter-blade distance and the blade offset height. However, after adding four pairs of blades in the microcombing zone, the CNT sheet need to pass a long distance before forming yarns. Adding more blades will generate a greater CNT sheets area and larger tension in the CNT sheets, especially for the final blades. It will increase the probability of CNT sheets with a breakable weak segment when drawing them out from the array, which makes it harder to
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Fig. 1. Schematic the CNT yarns fabrication and microcombing system.
produce CNT yarns. Therefore, we then increased the degree of microcombing by decreasing the distance between the adjacent blade while maintaining the blade offset height. The relationship between the inter-blades distance and the degree of microcombing will be discussed in the following part. The microcombing parameters chosen in this work are listed in Table 1. In order to describe the degree of microcombing more quantitatively and consistently, we propose a new concept here, “microcombing length” (ML). As shown in Fig. 3, when the CNT sheet passed through the microcombing blade, there is a contacting length between the sheet and the blade on the blade edge. Although the edge was very sharp, we could assume the contact part to have a cylindrical curvature with a given radius. We define ∠OCA as the contact angle, and 2α as the wrapping angle of the CNT sheets over the tip of the blade. Obviously, ∠OCA + α = 90°. The length of arc AB shown in Fig. 3 represents the ML of each blade for the CNT sheet: _
ML ¼AB¼ 2αr where r is the radius of the assumed edge cylinder.
ð1Þ
From the trigonometry in Fig. 3, we have,
tanα ¼
CF FG
¼
CD þ DE þ EF FG
ð2Þ
In our setup, we have: DE≫CD ¼ EF ¼ GH
ð3Þ
Therefore, the Eq. (2) can be approximated as:
tanα ¼
CF FG
≈
DE FG
ð4Þ
DE is the offset height, which is a fixed value, h = 9 mm in our experiment (see Fig. 1). FG is the distance between adjacent blades, d, which we varied from 50 mm to 30 mm to change the degree of
Fig. 2. Microfeature of the edges of combing blades.
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Table 1 Microcombing parameters of CNT yarns in different groups. Group number
Pairs of blades (n)
Blade distance (d/mm)
TML values
1 2 3 4 5 6 7 8 9
0 1 2 3 4 4 4 4 4
N/A 50 50 50 50 45 40 35 30
0 0.113r 0.227r 0.340r 0.454r 0.503r 0.564r 0.641r 0.742r
microcombing. Thus, Eq. (1) can be rewritten as: ML ¼ 2αr ¼ 2 arctan
! DE
h r ¼ 2 arctan r d FG
ð5Þ
The totally ML (TML) for all the blades in the microcombing zone is then the sum of the ML for each blade: h r TML ¼ n ML ¼ 2 arctan d
ð6Þ
With fixed offset height and the radius of the cylindrical contact surface, we can alter the degree of microcombing by changing the numbers of the blades and the distance between the adjacent blades. The resulted TML values for each group are listed in Table 1. It is evident that shortening the inter-blade distance leads to larger wrapping angle, which will contribute to a higher degree of microcombing. The radius is another factor in the microcombing process. As shown in Eq. (6), it can be concluded that smaller radius has a positive effect on the TML value. However, if the radius is too small, the sharp blade may cut the CNT sheets, which is not desired for the microcombing process. Thus, the radius can be selected as a variable to further optimize the microcombing process in future studies.
microcombing on the yarns' properties, which followed by the property deterioration with further microcombing. Fig. 4(c) presents the decreasing failure strain with increasing TML values, which is opposite to our previous work for microcombing CNT films in [26]. This is because the radial force in the yarns, which is developed by twist, provides additional friction to reduce CNT sliding during mechanical loading. The stress-strain curves of the samples are shown in Fig. 4(d). It is interesting that at zero degree of microcombing, the stress-strain curves have a gentle slope at the early stage. However, with the increasing degree of microcombing, the range of the gentle slope segment gets shorter and eventually disappears. The slopes of the strain-stress curves were analyzed and the minimum point in the slope is defined as the demarcation point of the gentle slope segment and the large slope segment. The monotone decrease in the percentage of gentle slope percentage was found with increasing TML value, with the percentage in the range of 89.89% and 100%. These gentle slope segments have been reported earlier on CNT yarns [11,30,33], but did not show in our previous work on microcombed CNT films [26]. This might be caused by the tighteningup or radial compaction of the yarns due to initial straightening of wavy CNTs. At low degrees of microcombing (including the uncombed ones), the CNTs should experience a tightening-up process under tensile deformation at the beginning. However, with increasing degree of microcombing, the CNTs become straighter in CNT sheet during the microcombing process and result in a more compact initial structure via twisting. Less tightening-up under initial tensile deformation will shorten the range of the gentle slope segment. The tightening-up process can be indicated from the trend of the yarns diameter, as shown in Fig.5. At low degrees of microcombing, the diameter of the yarns decreases with increasing TML value. This indicates the CNT bundles get straighter so that the yarns are more tightly packed. After the optimum point, the break of the CNT bundles becomes more significant with increasing TML value. The break of CNT bundles will induce more free and poorly unpacked ends inside the CNT yarns, resulting in looser packing structure and larger diameters of the yarns. 3.3. Effect of microcombing on electrical properties
3.2. Effect of microcombing on mechanical properties The mechanical properties for the produced CNT yarns are presented in Fig. 4.The strength and modulus of CNT yarns for the 9 groups of CNT yarns are exhibited in Fig. 4(a) and 4(b). Both the tensile strength and Young's modulus improved with increasing degree of microcombing, reaching a maximum of 408 MPa and 4.8 GPa, respectively, at TML = 0.564r, which were about 140% and 230% of those of the un-combed yarns. After reaching the peak values, both the tensile strength and Young's modulus start to decrease with further increasing the degree of microcombing. These results illustrate the effectiveness of
In our experiment, the resistances of the yarns in each group were measured, and their electrical conductivities were then calculated as:
σ¼
l πRΦ2
ð7Þ
where σ denotes the conductivity in S/m, R is the electrical resistance in Ω, l is the length of a CNT yarn in m, and Φ stands for the diameter of the yarns in m.
Fig. 3. Schematic of the cylindrical contacting surface between the CNT sheet and the microcombing blades.
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Fig. 4. Relationship between the mechanical properties and TML values of CNT yarns. (a) Relationship between strength and TML values for the 9 groups of CNT yarns. (b) Relationship between modulus and TML values for the 9 groups of CNT yarns. (c) Relationship between failure strain and TML values for the 9 groups of CNT yarns. (d) Stress–strain curves for TML = 0, TML = 0.113r and TML = 0.564r.
The measured electrical conductivities of the 9 groups of CNT yarns are presented in Fig. 6 as a function of TML values. These yarns exhibit a similar trend for electrical conductivity as seen for the mechanical properties. For the yarns without microcombing (TML = 0), they have an average electrical conductivity of about 5.0 × 104 S/m. When microcombing was applied, with increasing TML value, the yarns' electrical conductivities first increase, reaching the highest value for Group 7 (~7.3 × 104 S/m), where 4 pairs of microcombing blades were used and the blades distance was 40 mm, corresponding to a TML value of 0.564r. However, further increasing the degree of microcombing decreases the electrical conductivities of the CNT yarns. The electrical conduction should be significantly affected by the inter CNT bundle connections and defects in individual CNTs, which should also affect mechanical properties, as discussed in the next section.
Fig. 5. Relationship between yarns' diameter and TML values for the 9 groups of CNT yarns.
3.4. Effect of microcombing on yarn morphology The SEM images of the group 1,7 and 9 of CNT yarns are presented in Fig. 7. Fig. 7(a) shows the typical morphology of a CNT yarn without microcombing (TML = 0). Waviness and misalignments exist on the surface of the uncombed yarn, and the diameter varies along the yarn, indicating a non-uniform structure. With increasing degree of microcombing, the yarn structure was observed to reduce waviness and misalignment, resulting in a more uniform yarn structure. At TML = 0.564r (Group 7), the CNT yarns exhibited the most uniform structure, where microcombing effect reached its optimum (Fig. 7b). However, further increasing the degree of microcombing deteriorated the
Fig. 6. Relationship between the electrical conductivity and TML values for the 9 groups of CNT yarns.
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structural integrity of CNT yarns, as shown in Fig. 7(c) and (d) as examples, where TML = 0.742r. This situation is referred to as over-combing. The enhancement in both mechanical and electrical properties could be explained by reduced CNT waviness, enhanced CNT alignment, and higher nanotube packing density after microcombing, as detailed below. During the microcombing process, the vertical force applied to the CNT sheets in the contact area can help the micro-sized teeth on the blades to comb into the CNT sheets. With a suitable degree of microcombing, the CNTs will get better straightness by passing through the micro-sized teeth, and the overall alignment will be enhanced. On the other hand, the tension in the CNT sheet, which is created by the frictional force and capstan effect [11], can help with reducing the waviness and to improve the alignment by stretching [34]. In addition, due to the decrease in waviness and misalignment, CNTs can get closer to each other, which increases their packing density and mutual Van der Waal's interactions between individual CNTs and CNT bundles. All of these factors will contribute to the observed improvement in mechanical properties of the CNT yarns. Although microcombing can improve the micro-packing structure by straightening the wavy nanotubes and enhancing their alignment, the deterioration of structural uniformity and integrity could be caused by over-combing. The break of the inter-bundle connection is the primary reason for structure deterioration, which may be attributed to a combination of the cutting effect and scraping of CNT bundle ends by the sharp blades. With increasing degree of microcombing, both the tension force along the CNT sheets and the vertical interaction force between the CNT sheets would increase. For the cutting effect, the overloaded interaction force might cut a partial layer of the CNT sheets, while the tension force could bring the CNT sheets into a tense status. Due to the small friction between the CNT [35], it is also possible that the CNT bundles are scraped off from the CNT bundle ends by the blades. When the CNT bundles are wavy, gentle microcombing process can straighten the bundles with a minor degree of scraping. However, when the degree of microcombing exceeds a critical value, the scraping effect will get more significant since most of the bundles are
straightened already. That will lead to a reduction in the contact length of the CNT bundles, and even to a break of the inter-bundle connection. These defect structures in the sheets will get captured in the yarns structures: peel-off and broken-down can be seen on the surface of CNT yarns. A more porous and looser structure was also observed, which was less desired for optimizing the yarns properties. The deterioration of structural uniformity and integrity of the sheets decreases the mechanical properties of CNT yarns. Under the current CNT synthesis techniques, CNT products show an inhomogeneous structure with varying diameters and chiralities, which introduce potential barrier on inter-tube electron transport [36]. Intertube resistance has reported to be two orders of magnitude higher than along nanotube axis [17], which means the inter-tube contact resistance is the dominant factor for the overall conductivity. In our work, the improvements in CNTs structure can lower the potential barrier between the CNTs and increase the inter-tube contact area, resulting in a higher probability for electrons to migrate between CNTs via hoping mechanism or tunneling conduction [37]. Maarouf [38]suggested that electron tunneling has a critical penetration depth of about 1.25 nm, which demonstrate tunneling conduction would happen when CNTs are close enough. Microcombing reduced the yarns waviness, which shortened the effective electron conductive length between two points along the longitudinal direction and thereby improved the yarn electrical conductivity. When the nanotube straightness, alignment, and yarn structural integrity reached an optimum, the yarns' electrical conductivity reached a maximum. At the higher degree of microcombing, the electrical conductivity of the yarns decreased, which was resulted from the deterioration of the yarn integrity: a) The break of inter-bundle connection will result in a cut-off of the electrons transfer route, which could significantly impede the electron transport between the CNTs [16,18]. b) Larger structural defect density could enhance the possibility of scattering and decrease the mean free path of electrons. c) Looser packing and more porosity structure of the yarns imply a higher average potential barrier/electrical resistance, which will make the electrons transfer process harder.
Fig. 7. SEM images of non-microcombed/microcombed CNT yarns. (a) SEM image under group 1 shows the morphology before microcombing; (b) SEM image of group 7 shows the morphology with best properties; (c)–(d) SEM image of group 9 shows the broken-down and peel-off on CNT yarns.
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4. Conclusion With increasing degree of microcombing (TML), the CNT yarn's electrical and mechanical properties first increased, reached a maximum value and the decreased again. At the optimum TML value, the strength, modulus and electrical conductivities of CNT yarns were improved to 140%, 140%, and 230%, respectively, as those of non-microcombed ones. The enhancement in those properties could be well explained by a more uniform structure: reduced CNT waviness, the enhanced CNT alignment and the higher nanotube packing density. On the other hand, over-combing will destroy these structural characteristics in some extent, and consequently lead to lower mechanical and electrical properties. Our findings here could help guide future design and development of microcombing parameters for producing CNT products superior in mechanical and electrical properties. Acknowledgement This work was performed in part at the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation (award number ECCS-1542015). The AIF is a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), a site in the National Nanotechnology Coordinated Infrastructure (NNCI). References [1] M.M.J. Treacy, T.W. Ebbesen, J.M. Gibson, Exceptionally high Young's modulus observed for individual carbon nanotubes, Nature 381 (6584) (1996) 678–680. [2] M.-F. Yu, O. Lourie, M.J. Dyer, K. Moloni, T.F. Kelly, R.S. Ruoff, Strength and breaking mechanism of multiwalled carbon nanotubes under tensile load, Science 287 (5453) (2000) 637–640. [3] R. Andrews, D. Jacques, D. Qian, T. Rantell, Multiwall carbon nanotubes: synthesis and application, Acc. Chem. Res. 35 (12) (2002) 1008–1017. [4] H. Dai, E.W. Wong, C.M. Lieber, Probing electrical transport in nanomaterials: conductivity of individual carbon nanotubes, Nature 272 (5261) (1996) 523–526. [5] P. Kim, L. Shi, A. Majumdar, P.L. McEuen, Thermal transport measurements of individual multiwalled nanotubes, Phys. Rev. Lett. 87 (21) (2001) 215502. [6] R.H. Baughman, A.A. Zakhidov, W.A. de Heer, Carbon nanotubes–the route toward applications, Science 297 (5582) (2002) 787–792. [7] I.M.D. Rosa, F. Sarasini, M.S. Sarto, A. Tamburrano, EMC impact of advanced carbon fiber/carbon nanotube reinforced composites for next-generation aerospace applications, IEEE Trans. Electromagn. Compat. 50 (3) (2008) 556–563. [8] R.R. Salunkhe, J. Lin, V. Malgras, S.X. Dou, J.H. Kim, Y. Yamauchi, Large-scale synthesis of coaxial carbon nanotube/Ni(OH)2 composites for asymmetric supercapacitor application, Nano Energy 11 (2015) 211–218. [9] A. Ihsanullah, A.M. Abbas, T. Laoui Al-Amer, M.J. Al-Marri, M.S. Nasser, M. Khraisheh, M.A. Atieh, Heavy metal removal from aqueous solution by advanced carbon nanotubes: critical review of adsorption applications, Sep. Purif. Technol. 157 (2016) 141–161. [10] K. Jiang, Q. Li, S. Fan, Nanotechnology: spinning continuous carbon nanotube yarns, Nature 419 (6909) (2002) 801. [11] C.D. Tran, W. Humphries, S.M. Smith, C. Huynh, S. Lucas, Improving the tensile strength of carbon nanotube spun yarns using a modified spinning process, Carbon 47 (11) (2009) 2662–2670. [12] Q. Cheng, B. Wang, C. Zhang, Z. Liang, Functionalized carbon-nanotube sheet/ Bismaleimide nanocomposites: mechanical and electrical performance beyond carbon- fiber composites, Small 6 (6) (2010) 763–767. [13] Y. Wang, M. Li, W. Lu, Y. Gu, S. Wang, R. Sun, X. Zhang, Q. Li, Z. Zhang, Bio-inspired design and fabrication of an ultralight and strong nano-carbon gradient composite, Mater. Des. 107 (2016) 198–204.
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Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Investigation of microcombing parameters in enhancing the properties of carbon nanotube yarns Yingying Yu a,b, Liwen Zhang b,⁎, Ozkan Yildiz c, Haotian Deng b, Changhao Zhao a,b, Philip D. Bradford c, Jianying Li a,⁎, Yuntian Zhu b a b c
State Key Laboratory of Electrical Insulation and Power Equipment, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China Department of Materials Science and Engineering, North Carolina State University, Raleigh 27695, USA Department of Textile Engineering and Chemistry and Science, North Carolina State University, Raleigh 27695, USA
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• A systematic study is conducted to understand how microcombing degree affects the structure and properties of CNT yarns. • The mechanical and electrical properties could improve with well-aligned packing structure due to microcombing effect. • On the other hand, over-combing will destroy the packing structure, resulting in lower mechanical and electrical properties.
a r t i c l e
i n f o
Article history: Received 6 May 2017 Received in revised form 15 August 2017 Accepted 16 August 2017 Available online 17 August 2017 Keywords: Carbon nanotube yarns microcombing effects mechanical properties electrical properties
a b s t r a c t Microcombing has been reported as a novel processing approach for reducing waviness and improving alignment of carbon nanotubes (CNTs), which effectively enhances the performance of materials made from CNT sheets. In this study, we have systematically investigated the effects of microcombing parameters on the properties of CNT yarns. It is found that the electrical and mechanical properties of CNT yarns first improved with increasing degree of microcombing and then degraded with over-combing. At the optimum degree of microcombing, the electrical conductivity, tensile strength, and Young's modulus of the CNT yarns were improved to 140%, 140%, and 230%, respectively, over those of uncombed yarns. The enhanced yarn properties were resulted from reduced nanotube waviness, improved CNT alignment and denser packing structure, which led to a more uniform yarn structure. On the other hand, over-combing degraded structural uniformity, resulting in lower electrical and mechanical properties. These observations are expected to help with future selection of microcombing parameters for producing high-quality CNT yarns and polymer-CNT composite yarns for superior electrical and mechanical properties. © 2017 Elsevier Ltd. All rights reserved.
1. Introduction Multi-walled carbon nanotubes (MWCNTs) with unique multiplelayer seamless cylindrical graphitic structure show extraordinary ⁎ Corresponding authors. E-mail addresses: [email protected] (L. Zhang), [email protected] (J. Li).
http://dx.doi.org/10.1016/j.matdes.2017.08.035 0264-1275/© 2017 Elsevier Ltd. All rights reserved.
mechanical [1,2], electrical [3,4], and thermal [5] properties. They show great potential for applications in aerospace, electrochemical devices, hydrogen storage, field emission devices, filters, sensors [6–9], etc. However, it has been a challenge to assemble nano-sized CNTs into macroscopic structures, such as CNT yarns [10,11], films [12,13], and buckypapers [14,15], while retaining the superior properties of individual nanotubes when assembling them into bulk structures.
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In 2002, a simple but effective way of making CNT yarns was reported by Jiang et al. [10]. They found that a continuous pure CNT yarn could be directly drawn from a super-aligned CNT array. However, the yarns made by this method usually have loose structures and exhibit low properties. Nevertheless, this inspired other following works in which twisting was applied to produce a more uniform and compact yarn structure in order to obtain better yarn properties [16–19]. Individual high aspect ratio CNTs are most often observed to be very wavy under high-magnification SEM. Good alignment and straightness are reported to be vital for improving mechanical and electrical properties of CNT yarns and films [12,20–22]. Many research works have been focused on reducing misalignment and waviness. Tran et al. [11] suggested a modified process of dry spinning CNT yarns, where a tension zone was introduced to improve the structure of CNT bundles in CNT web. This method improved the mechanical properties and showed a potential for further development of complex spinning processes. W. Liu et al. [23] coupled the spraying of a PVA solution with continuous winding of CNT sheets from an array onto a rotating mandrel, which increased the alignment and the volume fraction of CNT. Wang et al. [22] improved the winding method by adding a stretching process to reduce the waviness of CNT sheets before they were embedded into a polymer matrix. Q. Liu et al. [24] further increased the alignment of CNT sheets by stretching and pressing, obtaining CNT sheets with high density and outstanding mechanical properties. W. Xu et al. [25]reported a method involving a continuous production of a hollow cylindrical CNT assembly and its condensation on a winding drum to enhance the alignment and densification. The CNT products fabricated by these methods, which all aim at enhancing the alignment enhance and reducing the waviness, showed a significant improvement in their macroscopic properties. More recently, our group [26,27] developed another simple but effective approach, called microcombing, which uses the natural micro-sized teeth on the blade as “comb” to align CNTs. This process reduces waviness and defects in the CNT sheet structure. The dry CNT films and CNT/poly (vinyl alcohol) (PVA) films made by the microcombed CNTs dramatically improved both the tensile strength and the electrical conductivity to record values for dry CNTs and CNT/PVA composites. However, as mentioned in our early work [26,27], the microcombing parameters in our previous research were chosen arbitrarily in terms of number of blades, inter-blades distance and height offset. Systematic study is needed to understand how micro-combing degree affects the structure and properties of CNT assemblies. In order to understand the critical issues and underlying mechanisms of microcombing, in this work we systematically altered the number of microcombing blades and the distance between the adjacent blades while maintaining the blades at the same height to produce different degrees of microcombing. We used CNT yarns in this study instead of CNT sheets in our previous works [26,27]. We found that the electrical conductivity, tensile strength, and Young's modulus of the produced CNT yarns improved first with increasing degree of microcombing and then decreased. In other words, an optimum microcombing degree exists to achieve the best mechanical and electrical properties. While microcombing straightened CNTs and improved CNT alignment, it also caused damage to the yarn structure if the sheets were “over-combed”. We found there was a competition between better CNT packing and deterioration of the microstructural integrity. This new finding should be applicable to other CNT materials and can help with their future design and development. 2. Experimental 2.1. Growth of super aligned CNT (SACNT) arrays The SACNT arrays used in our experiment were grown by a chemical vapor deposition method. Vertically aligned multi-walled carbon nanotubes (MWCNT) were grown in a tube furnace via a modified version of the chlorine mediated chemical vapor deposition route. The aligned
CNT arrays were synthesized on a quartz substrate at 760 °C under 3 Torr with iron II chloride (FeCl2 anhydrous 99.5% VWR) as a catalyst and acetylene as the carbon precursors. The growth took 20 min under a flowing mixture of gases of acetylene (600 sccm), argon (395 sccm) and chlorine (5 sccm). The CNTs grown in this way usually have diameters in the range of 25 to 40 nm, and length about 1 mm [28, 29]. 2.2. Production of microcombed CNT yarns The CNT yarns fabrication and microcombing system is composed of three zones: sheet formation zone, microcombing zone, and twisting zone, as schematically shown in Fig. 1. One piece of SACNT array (16 × 76 mm) was connected to the stage, and then the CNT sheet was drawn out at a speed of ~ 9 mm/s. It went through pair(s) of blades with a mutual vertical offset of 9 mm, which was defined as the microcombing zone. The microfeatures of blades were shown in Fig. 2 with the teeth size of ~3 μm and teeth depth of ~0.3 μm. Compared to the teeth size, the diameters of the CNTs are much smaller. In this case, the blades can act as microcombs to straighten wavy CNTs. In the microcombing zone, blades were vertically mounted and equally spaced. To guarantee the excellent contact between the blades and the CNT sheets, the edges of these blades were alternately oriented upand-down. This configuration generated a contact angle of CNT sheets over the tip of each blade, as shown in the top inset in Fig.1. After microcombing, the CNT sheet was then narrowed down by passing through a V grove formed by a wire made into M shape, where the yarns were wetted by a condensing solution consisting of deionized water and ethanol with a volume ratio of 1:1 [30]. The condensing process can bring CNTs together and reduce the nanotube waviness to some extent [31]. Finally, the narrowed sheet was twisted by a spinning machine at a rotating speed of 1000 r/min to form the microcombed CNT yarns with twist per unit length 1.85 r/mm. 2.3. Characterization methods Scanning electron microscopy (SEM, Verios 460 L) was used to characterize the morphology of the CNT yarns produced in the experiments. Electrical conductivities of the produced CNT yarns were calculated from their electrical resistances, which were measured along the CNT alignment direction by a two-probe method using Agilent 34410A 6.5digit multimeter. The test samples were 100 mm long, and the two ends of samples were painted with silver electrodes by magnetron sputtering. The mechanical properties were measured by a Shimadzu EZ-S tensile testing machine with a load cell of 2 N and a crosshead speed of 0.5 mm/min [32]. The gauge length of the testing samples was 6 mm. The diameters of CNT yarns were measured by laser diffraction method and further verified by SEM. Nine yarns were tested for each group in these characterization processes. 3. Results and discussions 3.1. Total microcombing length (TML) The degree of microcombing can be altered by the number of microcombing blades, and the distance between the adjacent blades. In this work, nine groups of CNT yarns were produced using different microcombing parameters. For the first 5 groups, we increased the microcombing degree by increasing the number of blades (from 0 pairs to 4 pairs) while maintaining the inter-blade distance and the blade offset height. However, after adding four pairs of blades in the microcombing zone, the CNT sheet need to pass a long distance before forming yarns. Adding more blades will generate a greater CNT sheets area and larger tension in the CNT sheets, especially for the final blades. It will increase the probability of CNT sheets with a breakable weak segment when drawing them out from the array, which makes it harder to
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Fig. 1. Schematic the CNT yarns fabrication and microcombing system.
produce CNT yarns. Therefore, we then increased the degree of microcombing by decreasing the distance between the adjacent blade while maintaining the blade offset height. The relationship between the inter-blades distance and the degree of microcombing will be discussed in the following part. The microcombing parameters chosen in this work are listed in Table 1. In order to describe the degree of microcombing more quantitatively and consistently, we propose a new concept here, “microcombing length” (ML). As shown in Fig. 3, when the CNT sheet passed through the microcombing blade, there is a contacting length between the sheet and the blade on the blade edge. Although the edge was very sharp, we could assume the contact part to have a cylindrical curvature with a given radius. We define ∠OCA as the contact angle, and 2α as the wrapping angle of the CNT sheets over the tip of the blade. Obviously, ∠OCA + α = 90°. The length of arc AB shown in Fig. 3 represents the ML of each blade for the CNT sheet: _
ML ¼AB¼ 2αr where r is the radius of the assumed edge cylinder.
ð1Þ
From the trigonometry in Fig. 3, we have,
tanα ¼
CF FG
¼
CD þ DE þ EF FG
ð2Þ
In our setup, we have: DE≫CD ¼ EF ¼ GH
ð3Þ
Therefore, the Eq. (2) can be approximated as:
tanα ¼
CF FG
≈
DE FG
ð4Þ
DE is the offset height, which is a fixed value, h = 9 mm in our experiment (see Fig. 1). FG is the distance between adjacent blades, d, which we varied from 50 mm to 30 mm to change the degree of
Fig. 2. Microfeature of the edges of combing blades.
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Table 1 Microcombing parameters of CNT yarns in different groups. Group number
Pairs of blades (n)
Blade distance (d/mm)
TML values
1 2 3 4 5 6 7 8 9
0 1 2 3 4 4 4 4 4
N/A 50 50 50 50 45 40 35 30
0 0.113r 0.227r 0.340r 0.454r 0.503r 0.564r 0.641r 0.742r
microcombing. Thus, Eq. (1) can be rewritten as: ML ¼ 2αr ¼ 2 arctan
! DE
h r ¼ 2 arctan r d FG
ð5Þ
The totally ML (TML) for all the blades in the microcombing zone is then the sum of the ML for each blade: h r TML ¼ n ML ¼ 2 arctan d
ð6Þ
With fixed offset height and the radius of the cylindrical contact surface, we can alter the degree of microcombing by changing the numbers of the blades and the distance between the adjacent blades. The resulted TML values for each group are listed in Table 1. It is evident that shortening the inter-blade distance leads to larger wrapping angle, which will contribute to a higher degree of microcombing. The radius is another factor in the microcombing process. As shown in Eq. (6), it can be concluded that smaller radius has a positive effect on the TML value. However, if the radius is too small, the sharp blade may cut the CNT sheets, which is not desired for the microcombing process. Thus, the radius can be selected as a variable to further optimize the microcombing process in future studies.
microcombing on the yarns' properties, which followed by the property deterioration with further microcombing. Fig. 4(c) presents the decreasing failure strain with increasing TML values, which is opposite to our previous work for microcombing CNT films in [26]. This is because the radial force in the yarns, which is developed by twist, provides additional friction to reduce CNT sliding during mechanical loading. The stress-strain curves of the samples are shown in Fig. 4(d). It is interesting that at zero degree of microcombing, the stress-strain curves have a gentle slope at the early stage. However, with the increasing degree of microcombing, the range of the gentle slope segment gets shorter and eventually disappears. The slopes of the strain-stress curves were analyzed and the minimum point in the slope is defined as the demarcation point of the gentle slope segment and the large slope segment. The monotone decrease in the percentage of gentle slope percentage was found with increasing TML value, with the percentage in the range of 89.89% and 100%. These gentle slope segments have been reported earlier on CNT yarns [11,30,33], but did not show in our previous work on microcombed CNT films [26]. This might be caused by the tighteningup or radial compaction of the yarns due to initial straightening of wavy CNTs. At low degrees of microcombing (including the uncombed ones), the CNTs should experience a tightening-up process under tensile deformation at the beginning. However, with increasing degree of microcombing, the CNTs become straighter in CNT sheet during the microcombing process and result in a more compact initial structure via twisting. Less tightening-up under initial tensile deformation will shorten the range of the gentle slope segment. The tightening-up process can be indicated from the trend of the yarns diameter, as shown in Fig.5. At low degrees of microcombing, the diameter of the yarns decreases with increasing TML value. This indicates the CNT bundles get straighter so that the yarns are more tightly packed. After the optimum point, the break of the CNT bundles becomes more significant with increasing TML value. The break of CNT bundles will induce more free and poorly unpacked ends inside the CNT yarns, resulting in looser packing structure and larger diameters of the yarns. 3.3. Effect of microcombing on electrical properties
3.2. Effect of microcombing on mechanical properties The mechanical properties for the produced CNT yarns are presented in Fig. 4.The strength and modulus of CNT yarns for the 9 groups of CNT yarns are exhibited in Fig. 4(a) and 4(b). Both the tensile strength and Young's modulus improved with increasing degree of microcombing, reaching a maximum of 408 MPa and 4.8 GPa, respectively, at TML = 0.564r, which were about 140% and 230% of those of the un-combed yarns. After reaching the peak values, both the tensile strength and Young's modulus start to decrease with further increasing the degree of microcombing. These results illustrate the effectiveness of
In our experiment, the resistances of the yarns in each group were measured, and their electrical conductivities were then calculated as:
σ¼
l πRΦ2
ð7Þ
where σ denotes the conductivity in S/m, R is the electrical resistance in Ω, l is the length of a CNT yarn in m, and Φ stands for the diameter of the yarns in m.
Fig. 3. Schematic of the cylindrical contacting surface between the CNT sheet and the microcombing blades.
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185
Fig. 4. Relationship between the mechanical properties and TML values of CNT yarns. (a) Relationship between strength and TML values for the 9 groups of CNT yarns. (b) Relationship between modulus and TML values for the 9 groups of CNT yarns. (c) Relationship between failure strain and TML values for the 9 groups of CNT yarns. (d) Stress–strain curves for TML = 0, TML = 0.113r and TML = 0.564r.
The measured electrical conductivities of the 9 groups of CNT yarns are presented in Fig. 6 as a function of TML values. These yarns exhibit a similar trend for electrical conductivity as seen for the mechanical properties. For the yarns without microcombing (TML = 0), they have an average electrical conductivity of about 5.0 × 104 S/m. When microcombing was applied, with increasing TML value, the yarns' electrical conductivities first increase, reaching the highest value for Group 7 (~7.3 × 104 S/m), where 4 pairs of microcombing blades were used and the blades distance was 40 mm, corresponding to a TML value of 0.564r. However, further increasing the degree of microcombing decreases the electrical conductivities of the CNT yarns. The electrical conduction should be significantly affected by the inter CNT bundle connections and defects in individual CNTs, which should also affect mechanical properties, as discussed in the next section.
Fig. 5. Relationship between yarns' diameter and TML values for the 9 groups of CNT yarns.
3.4. Effect of microcombing on yarn morphology The SEM images of the group 1,7 and 9 of CNT yarns are presented in Fig. 7. Fig. 7(a) shows the typical morphology of a CNT yarn without microcombing (TML = 0). Waviness and misalignments exist on the surface of the uncombed yarn, and the diameter varies along the yarn, indicating a non-uniform structure. With increasing degree of microcombing, the yarn structure was observed to reduce waviness and misalignment, resulting in a more uniform yarn structure. At TML = 0.564r (Group 7), the CNT yarns exhibited the most uniform structure, where microcombing effect reached its optimum (Fig. 7b). However, further increasing the degree of microcombing deteriorated the
Fig. 6. Relationship between the electrical conductivity and TML values for the 9 groups of CNT yarns.
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structural integrity of CNT yarns, as shown in Fig. 7(c) and (d) as examples, where TML = 0.742r. This situation is referred to as over-combing. The enhancement in both mechanical and electrical properties could be explained by reduced CNT waviness, enhanced CNT alignment, and higher nanotube packing density after microcombing, as detailed below. During the microcombing process, the vertical force applied to the CNT sheets in the contact area can help the micro-sized teeth on the blades to comb into the CNT sheets. With a suitable degree of microcombing, the CNTs will get better straightness by passing through the micro-sized teeth, and the overall alignment will be enhanced. On the other hand, the tension in the CNT sheet, which is created by the frictional force and capstan effect [11], can help with reducing the waviness and to improve the alignment by stretching [34]. In addition, due to the decrease in waviness and misalignment, CNTs can get closer to each other, which increases their packing density and mutual Van der Waal's interactions between individual CNTs and CNT bundles. All of these factors will contribute to the observed improvement in mechanical properties of the CNT yarns. Although microcombing can improve the micro-packing structure by straightening the wavy nanotubes and enhancing their alignment, the deterioration of structural uniformity and integrity could be caused by over-combing. The break of the inter-bundle connection is the primary reason for structure deterioration, which may be attributed to a combination of the cutting effect and scraping of CNT bundle ends by the sharp blades. With increasing degree of microcombing, both the tension force along the CNT sheets and the vertical interaction force between the CNT sheets would increase. For the cutting effect, the overloaded interaction force might cut a partial layer of the CNT sheets, while the tension force could bring the CNT sheets into a tense status. Due to the small friction between the CNT [35], it is also possible that the CNT bundles are scraped off from the CNT bundle ends by the blades. When the CNT bundles are wavy, gentle microcombing process can straighten the bundles with a minor degree of scraping. However, when the degree of microcombing exceeds a critical value, the scraping effect will get more significant since most of the bundles are
straightened already. That will lead to a reduction in the contact length of the CNT bundles, and even to a break of the inter-bundle connection. These defect structures in the sheets will get captured in the yarns structures: peel-off and broken-down can be seen on the surface of CNT yarns. A more porous and looser structure was also observed, which was less desired for optimizing the yarns properties. The deterioration of structural uniformity and integrity of the sheets decreases the mechanical properties of CNT yarns. Under the current CNT synthesis techniques, CNT products show an inhomogeneous structure with varying diameters and chiralities, which introduce potential barrier on inter-tube electron transport [36]. Intertube resistance has reported to be two orders of magnitude higher than along nanotube axis [17], which means the inter-tube contact resistance is the dominant factor for the overall conductivity. In our work, the improvements in CNTs structure can lower the potential barrier between the CNTs and increase the inter-tube contact area, resulting in a higher probability for electrons to migrate between CNTs via hoping mechanism or tunneling conduction [37]. Maarouf [38]suggested that electron tunneling has a critical penetration depth of about 1.25 nm, which demonstrate tunneling conduction would happen when CNTs are close enough. Microcombing reduced the yarns waviness, which shortened the effective electron conductive length between two points along the longitudinal direction and thereby improved the yarn electrical conductivity. When the nanotube straightness, alignment, and yarn structural integrity reached an optimum, the yarns' electrical conductivity reached a maximum. At the higher degree of microcombing, the electrical conductivity of the yarns decreased, which was resulted from the deterioration of the yarn integrity: a) The break of inter-bundle connection will result in a cut-off of the electrons transfer route, which could significantly impede the electron transport between the CNTs [16,18]. b) Larger structural defect density could enhance the possibility of scattering and decrease the mean free path of electrons. c) Looser packing and more porosity structure of the yarns imply a higher average potential barrier/electrical resistance, which will make the electrons transfer process harder.
Fig. 7. SEM images of non-microcombed/microcombed CNT yarns. (a) SEM image under group 1 shows the morphology before microcombing; (b) SEM image of group 7 shows the morphology with best properties; (c)–(d) SEM image of group 9 shows the broken-down and peel-off on CNT yarns.
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