Influence of Carbon Nanotube Characteristics on Macroscopic Fiber

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Influence of Carbon Nanotube Characteristics on Macroscopic Fiber Properties Dmitri E. Tsentalovich, Robert James Headrick, Francesca Mirri, Junli Hao, Natnael Behabtu, Colin C. Young, and Matteo Pasquali ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10968 • Publication Date (Web): 22 Sep 2017 Downloaded from http://pubs.acs.org on September 24, 2017

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ACS Applied Materials & Interfaces

Influence of Carbon Nanotube Characteristics on Macroscopic Fiber Properties Dmitri E. Tsentalovich,† Robert J. Headrick,† Francesca Mirri,† Junli Hao,† Natnael Behabtu,† Colin C. Young,† Matteo Pasquali†* †

Department of Chemical & Biomolecular Engineering, Department of Chemistry, Department of Materials Science & NanoEngineering, The Smalley-Curl Institute, Rice University, Houston, TX 77005, USA * To whom correspondence should be addressed. Email: [email protected] Keywords: carbon nanotubes, fibers, fiber spinning, electrical conductivity, aspect ratio

ABSTRACT We study how intrinsic parameters of CNT samples affect the properties of macroscopic CNT fibers with optimized structure. We measure CNT diameter, number of walls, aspect ratio, graphitic character, and purity (residual catalyst and non-CNT carbon) in samples from 19 suppliers; we process the highest quality CNT samples into aligned, densely packed fibers, by using an established wet-spinning solution process. We find that fiber properties are mainly controlled by CNT aspect ratio and that sample purity is important for effective spinning. Properties appear largely unaffected by CNT diameter, number of walls, and graphitic character (determined by Raman G/D ratio) as long as the fibers comprise thin few-walled CNTs with high G/D ratio (above ~20). We show that both strength and conductivity can be improved simultaneously by assembling high aspect ratio CNTs, producing continuous CNT fibers with an average tensile strength of 2.4 GPa and a room temperature electrical conductivity of 8.5 MS/m, ~2-times higher than the highest reported literature value (~15% of copper’s value), obtained without post-spinning doping. This understanding of the relationship of intrinsic CNT parameters to macroscopic fiber properties is key to guiding CNT synthesis and continued improvement of fiber properties, paving the way for CNT fiber introduction in large-scale aerospace, consumer electronics, and textile applications.

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INTRODUCTION During the first decade after their early 1990s discovery, carbon nanotubes (CNTs) generated great interest as a revolutionary high-performance material; such interest was predicated on CNT singlemolecule properties - exceptional strength, and electrical and thermal conductivities. As methods for assembling CNTs into macroscopic samples were introduced throughout the 2000s, disillusionment set in, because macroscopic CNT materials (fibers and films) delivered just a fraction of the theoretical strength and even less of the theoretical transport properties, well-below the performance of metals, aramids, and carbon fibers.1-4 The 2010s have brought renewed applied interest in CNT materials, as strength, electrical conductivity, and thermal conductivity have been demonstrated on macroscopic fibers.5-8 Although advances in properties have clearly been enabled by progress in the synthesis of high-quality CNTs and the assembly of such CNTs into aligned and packed macroscopic fibers, there is only sparse information on the dependence of macroscopic properties on intrinsic CNT parameters such as CNT diameter, length, number of walls, graphitic character, and sample purity. A few studies have considered the effect of CNT characteristics such as aspect ratio on fiber properties;5,

9-12

yet, such studies have necessarily been

confined to CNTs coming from a single synthesis method and therefore do not account for the multitude of variations CNT samples that can be found. Moreover, fiber morphology was controlled to a limited extent in these earlier studies. Here, we use our established wet-spinning method to fabricate wellaligned, packed CNT fibers from 19 different commercial and laboratory sources. We determine that aspect ratio is the most important factor currently controlling strength and electrical conductivity. A multitude of CNT synthesis methods have been developed over the past 25 years, each having its own advantages and drawbacks.13 CNT production techniques with a capacity exceeding one gram per day include arc-discharge,14 laser oven,15 high-pressure carbon monoxide (HiPco),16 fluidized bed chemical vapor deposition (CVD),17 fixed bed catalyst CVD,18 catalytic gas flow CVD,2,

19-20

plasma enhanced

CVD,21 enhanced direct injection pyrolytic synthesis (eDIPS),22 and growth of vertically aligned CNT carpets (or forests) on substrates.23-24 Synthesis methods that produce highly crystalline, long CNTs with a small number of walls are the methods of choice for achieving high performance CNT materials. To date, 2 ACS Paragon Plus Environment

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eDIPs, plasma enhanced CVD, fixed bed catalyst CVD, and fluidized bed CVD have produced CNTs with the lowest defect density and highest aspect ratio.5, 25 CNT fibers can be fabricated either by processing CNTs via wet-spinning from a CNT solution or by solid state spinning from an aligned CNT array,26-28 from entangled cotton-like CNTs,29 or directly from a CNT reaction chamber.2, 30 Wet spinning methods typically produce fibers with superior CNT packing and alignment but they require the dispersion of CNTs into liquids; preparation of metastable suspensions typically shortens CNTs, introduces defects, requires the use of surfactants, and yields relatively low CNT concentrations.31-32 On the other hand, solid-state spinning methods can be used to manufacture fibers comprising longer CNTs; however, the defect density of such CNTs cannot be controlled separately due to the difficulty of optimizing both CNT synthesis and CNT fiber spinning in one single operation unit. Chlorosulfonic acid (CSA) spontaneously dissolves CNTs at high concentrations without introducing defects or shortening CNTs.33-34 Because of CNT high aspect ratio, these solutions are liquid crystalline,33, 35-37

providing an inherent advantage in the manufacturing of aligned materials. Because defective CNTs

(low Raman G/D ratio) have poor solubility in strong acids,38 wet-spinning can only be used to process highly crystalline material, which is consistent with pursuing improved macroscopic electrical and thermal conductivity. Moreover, acid-spun CNTs are inherently p-doped, reducing or removing the need for a separate post-processing doping step. Therefore, wet-spinning from acid solutions is the best method to produce high purity, low defect density, well-ordered CNT fibers and has reached to date the highest levels of multifunctional performance in terms of combined strength and conductivity for continuous scalable manufacturing.5, 7, 39 However, processing must be optimized and tailored for each separate CNT source material with the key parameters influencing fiber quality being spin dope concentration (the CNT-CSA solution used to spin fiber), extrusion flow rate, spin-draw ratio, and choice of coagulant.5

RESULTS AND DISCUSSION CNT Source Material Evaluation and CNT Fiber Processing



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To select the best available sources of CNTs for producing high-performance fibers, we performed an extensive evaluation of available CNT materials produced by several different synthesis techniques from 19 worldwide CNT suppliers (see Table 1). Transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Raman spectroscopy, and extensional rheology were used to assess the CNT number of walls, diameter, carbon purity, highest Raman G/D ratio (at 514 nm, 633 nm, or 785 nm excitation wavelengths), and average CNT aspect ratio (more details on the characterization techniques can be found in Methods). Except for single walled carbon nanotubes (SWCNTs) from Raymor and the Canada National Research Council (CNRC), CNT samples with purities below 90 wt.% had not been purified by the supplier. Multiple CNT samples (or grades) were evaluated from several of the suppliers listed in Table 1; in these cases, Table 1 reports only data for the best samples (or grades). All SWCNT and double walled carbon nanotube (DWCNT) samples show a higher Raman G/D ratio (> 20) and higher purity (> 90 wt.%) than MWCNTs, which typically do not have G/D ratios greater than 10. Figure 1 shows the fiber spinning process used to produce CNT fibers from the source materials listed in Table 1. Even for the highest aspect ratio CNTs that have excellent graphitic character, the tensile strength of acid-spun CNT fibers is highly dependent on processing conditions and sub-optimal processing leads to poor fiber strength (Supporting Information, Figure S1). For each of the fibers reported here, we optimized dope concentration, filter pore size, spinneret diameter, extrusion speed, coagulation, and spin-draw ratio.


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Figure 1. Process flow for manufacturing acid-spun fiber from unstructured CNT material. Images of various stages of the process are shown next to the relevant process step.



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Table 1: Evaluation of CNTs produced by various manufacturers and production methods. CNT purity and highest Raman G/D ratio (514 nm or 633 nm) were either measured or specified by the manufacturer, unless otherwise noted. Aspect ratios were determined from extensional viscosity measurements25 for samples that were soluble in CSA. CNT Manufacturer or CNT Raman Purity Aspect Ratio Spun into Production Method Purified by Notes Supplier Type G/D Ratio (% Carbon) (L/D) Fiber? Teijin Aramid BV (AC 299) – 1 eDIPS DWCNT > 50 β 94 9610 Rice No Insufficient material amount β Teijin Aramid BV (AC 299) – 2 eDIPS DWCNT > 50 100 4400 Rice Yes Meijo Nano Carbon Co. – 1 eDIPS SWCNT > 50 β 88 6430 Unpurified No Not spinnable due to impurities Meijo Nano Carbon Co. – 2 eDIPS SWCNT 43 β 99 4350 Rice Yes Samsung Cheil – 1 FC-CVD DWCNT > 50 α 6310 Rice No Not spinnable due to impurities Samsung Cheil – 2 FC-CVD DWCNT > 50 α 98 5150 Rice Yes OCSiAl Group (Tuball) – 1 PE-CVD SWCNT > 50 β 86 3100 Rice No Not spinnable due to impurities β OCSiAl Group (Tuball) – 2 PE-CVD SWCNT > 50 97 1260 Rice Yes Purified by oxidizing for 48 hours OCSiAl Group (Tuball) – 3 PE-CVD SWCNT > 50 β 97 2310 Rice Yes Purified by oxidizing for 24 hours Raymor Industries Plasma torch CVD SWCNT > 50 β 75 > 2000 Supplier No Not spinnable due to impurities α UniDym Fixed bed CVD DWCNT > 50 96 4010 Supplier Yes CCNI – 1 Fixed bed CVD DWCNT > 50 α 94 2810 Supplier Yes CCNI – 2 Fixed bed CVD DWCNT > 50 β 98 2600 Supplier Yes Linde Group CVD SWCNT > 50 α 90 3520 Supplier No Insufficient material amount Canada Natl. Research Council Laser Oven SWCNT > 50 β 80 2400 Supplier No Not spinnable due to graphitic particles SWeNT – 1 Fluidized bed CVD SWCNT 30 α 97 4900 Supplier No Insufficient material amount α (CoMoCAT) SWeNT – 2 Fluidized bed CVD SWCNT 30 97 2800 Supplier Yes Rice University – 1 HiPco SWCNT 30 α 80 1780 Rice No Insufficient material amount Rice University – 2 HiPco SWCNT 11 α 98 440 Rice Yes Not spinnable due to impurities Nano-C Combustion synthesis SWCNT 30 α 97 450 Supplier No Not spinnable due to impurities α KH Chemicals Catalytic gas flow CVD SWCNT 20 80 2000 Unpurified Yes Unoptimized spinning Nanocyl Catalytic gas flow CVD DWCNT 20 β 90 < 200 Supplier No Not spinnable due to low aspect ratio TimesNano CVD SWCNT 25 β 95 N/A Supplier No Not soluble β Thomas Swan Elicarb CVD SWCNT 10 95 N/A Supplier No Not soluble Klean Carbon CVD SWCNT 10 γ 90 N/A Supplier No Not soluble CNano Catalytic gas flow CVD MWCNT 1β 95 N/A Supplier No Not soluble ‡ General Nano Carpet growth MWCNT 1 95 N/A Unpurified No Not soluble ‡Source: 40; α: 633 nm, β: 514 nm, γ: Provided by manufacturer Abbreviations: PE (Plasma-Enhanced), FC (Floating Catalyst), CCNI (Continental Carbon Nanotechnologies), SWeNT (SouthWest NanoTechnologies)


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Influence of Impurities on CNT Fiber Properties The presence of impurities can degrade both the electrical and mechanical properties of the fibers. Large particles (micron-sized) make continuous spinning essentially impossible as they disrupt flow and can clog the spinneret. Smaller-sized impurities reduce strength because they do not bear load while creating defects along the fiber and disrupting CNT alignment. The effect of impurities on electrical conductivity is less clear; however, impurities disrupt CNT alignment and increase junction resistance between CNTs. Unpurified CNTs typically contain catalyst and carbonaceous impurities; the residual catalyst content depends primarily on the CNT production method. For example, as-received AC 299 DWCNTs from Teijin contain 5.6 wt.% residual iron catalyst impurities, based on TGA. The presence of iron catalyst particles in AC 299 DWCNTs is apparent in the SEM image and the energy dispersive X-ray spectroscopy (EDS) spectrum in Figure 2a. After purification (see Methods), the EDS spectrum (Figure 2b) indicates a decrease from 2.24 wt.% to 0.06 wt.% iron, while TGA shows a decrease from 5.6 wt.% to 0.5 wt.% in residual mass, showing that most of the iron catalyst was removed. The peak in the EDS spectrum corresponding to chlorine is likely due to residual hydrochloric acid (HCl) from the purification process. Transmitted and cross-polarized light microscopy images of unpurified and purified AC 299 DWCNTs in CSA (Figure 2c-f) demonstrate that impurities have a significant effect on solution microstructure— although, the relative effect of catalyst versus carbonaceous impurities is unclear. In the transmitted light images, impurities are the small, circular particles dispersed throughout the solution; these particles are only visible in the unpurified solution. In the cross-polarized images, the liquid crystalline domains appear to be significantly smaller in the unpurified sample, showing that impurities are disrupting liquid crystalline domain growth (as expected). Indeed, at CNT concentrations above 0.5 wt.%, it is not possible to make a homogeneous solution with the unpurified sample, whereas the purified material yields homogeneous liquid crystalline solutions at concentrations above 0.5 wt.% CNT. Imperfectly dissolved CNTs in spin dopes cannot be processed into optimized fibers (in part because the fiber cannot be spun



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under tension, i.e., draw ratio higher than ~1); Figure S1c in Supporting Information shows one such suboptimally processed fiber,

Figure 2. (a) EDS spectrum of unpurified AC 299 DWCNTs and (b) EDS spectrum of AC 299 DWCNTs after purification by H2O2 and HCl treatments. The iron content in (a) corresponds to 2.24 wt. % whereas in (b) it is 0.06 wt. %. Figure insets are SEM images of the sample areas used to collect the EDS spectra. (c-e) Transmitted light microscopy images of 0.05 wt. % solutions of (c) unpurified AC 299 DWCNTs and (e) purified AC 299 DWCNTs. Cross-polarized light microscopy images of the (d) unpurified AC 2 ACS Paragon Plus Environment

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299 DWCNTs and (f) purified AC 299 DWCNTs, with the relative orientation of the polarizer and analyzer shown by the arrows on the top right of the images. Scale bars are 200 µm.

which is poorly aligned and has large variations in diameter along its length. Thus, purification is necessary for samples whose impurities exceed ~5 wt.%.



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Influence of CNT Aspect Ratio on CNT Fiber Properties In order to evaluate the effect of CNT aspect ratio over CNT fiber properties, we spun fibers using CNT samples from seven different CNT manufacturers. Figure 3a shows that tensile strength scales with ~(L/d)0.9, close to the theoretical power law of 1 for friction-dominated failure.11, 41 These results are in agreement with other experimental5,

11-12

studies that have demonstrated that longer constituent CNTs

significantly increase CNT fiber strength. Although all samples include CNTs with various degrees of length and diameter polydispersity, the fiber strength scales linearly (R2 = 0.8705) with the average CNT aspect ratio, as predicted for fibers made out of monodisperse CNTs,41 indicating that either polydispersity is comparable among samples from various production methods (as recently established on a related set of CNT samples25), or polydispersity has a minor effect on strength scaling. The inconsistency in the correlation likely arises due to the inherent differences between each sample other than aspect ratio that are not easily controlled or captured in the plot, such as differences in purity, imperfect processing, or CNT type (see SI). Here, the highest performing CNTs (AC 299, 4400) produced acid-spun fibers with 2.4 GPa strength. We were able to purify AC 299 CNTs to yield higher aspect ratios (up to ~10,000) but in quantities that were insufficient for fiber spinning

Figure 3. (a) CNT fiber tensile strength versus aspect ratio and (b) CNT fiber conductivity versus aspect ratio for fibers made from SWCNTs and DWCNTs from different manufacturers. Error bars are the


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standard deviation from five or more measurements and on average we obtain 4%, 11%, and 4% error for aspect ratio, tensile strength, and electrical conductivity, respectively.

(< 50 mg), due to overall CNT supply limitations. The availability of larger quantities of these high aspect ratio (104) CNTs could lead to fibers with strength ~ 5 GPa assuming the same strength versus aspect ratio scaling continues to hold. Further extrapolation of the strength versus aspect ratio trend suggests that the predicted fiber ultimate strength (~30 GPa measured on 15 individual SWCNTs synthesized by arc discharge42) could be attained with CNTs of aspect ratio ~105. Interestingly and contrary to expectations, we find no increased processing difficulty as CNT aspect ratio increases and CSA has been previously shown to dissolve very long CNTs;38 thus, further improvements in CNT fiber strength are likely to continue as high quality CNT manufacturers increase the average length of their CNTs. Analogous to fiber tensile strength, electrical conductivity also scales with CNT aspect ratio as shown in Figure 3b. This is in agreement with experimental studies on transparent and conductive CNT films that have shown that thin film conductivity improves with CNT length.43-44 In fact, longer CNTs are expected to yield higher conductivity fibers due to fewer CNT junctions per unit fiber length. Influence of CNT graphitic character on CNT fiber properties To investigate if CNT graphitic character (crystallinity) affects fiber conductivity and strength, we measured CNT Raman G/D ratios and correlated these measurements to the fiber properties produced from those same samples. Raman G/D ratio (i.e. the ratio of maximum G band intensity to maximum D band intensity in a CNT Raman spectrum) provides a qualitative measurement of the level of defects in a CNT sample, with a higher G/D ratio indicating fewer defects (example shown in Supporting Information, Figure S2). For the Raman spectra taken with 514 nm and 633 nm excitation wavelengths, Figures 4a-b show that there was no discernible dependence of fiber electrical conductivity and tensile strength on Raman G/D ratio. However, both fiber mechanical and electrical properties seem to track with the G/D ratio at 785 nm laser wavelength (Figure 4c and 4d). Although such a trend may be rationalized in terms of defect-dependent intrinsic CNT properties, we find that both strength and conductivity 5 ACS Paragon Plus Environment


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normalized by aspect ratio are independent of the G/D ratio at 785 nm (Figure 5a and 5b). The apparent differences in trends for the G/D data at 785 nm are a reflection of the inherent physical characteristics of the CNTs. Indeed, our samples show a correlation between CNT aspect ratio and G/D ratio at 785 nm (Figure 5c). Because the CNT samples with highest aspect ratio used in this work happen to have large diameters (1.5-2.5 nm) and hence have electronic transitions that resonate with the 785 nm laser (while the smaller diameter, lower aspect ratio CNTs do not), we conclude that the apparent correlation between electrical conductivity/strength and G/D ratio at this excitation is due to the incidental relationship of G/D ratio and geometrical aspect ratio in these samples rather than their qualitative graphitic character. Therefore, for high-quality CNTs (G/D > 20) we cannot answer conclusively whether the G/D ratio may be a good indicator of macroscopic fiber conductivity or strength. Furthermore, this result suggests a lack of understanding and ability to characterize the relationship between CNT graphitic character and macroscopic properties and that there is a need for alternative methods for evaluating CNT quality.


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Figure 4. (a) CNT fiber tensile strength and (b) CNT fiber electrical conductivity versus Raman G/D ratio at 514 nm and 633 nm excitation wavelengths. (c) CNT fiber tensile strength and (d) CNT fiber electrical conductivity versus Raman G/D ratio at 785 nm excitation wavelength for fibers composed of SWCNTs and DWCNTs from different manufacturers. The Raman G/D ratio error bars are obtained by averaging the values from 8 different spectra for each material. The average errors for G/D ratios obtained with 785 nm, 633 nm, and 514 nm excitation are 14%, 21%, and 13%, respectively.



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Figure 5. CNT fiber tensile strength (a) and electrical conductivity (b) normalized by the aspect ratio of the constituent CNTs versus Raman G/D ratio at 514 nm, 633 nm, and 785 nm excitation. (c) Raman G/D ratio at 785 nm excitation versus CNT aspect ratio. SWCNT versus DWCNT/FWCNT fibers Although it is believed that the inner walls of few-walled CNTs (FWCNTs) do not carry load, their contribution to macroscopic electrical conductivity is less clear.45-49 Nonetheless, the strongest and most conductive CNT fibers produced to date have all been made out of either DWCNTs or FWCNTs,5, 7, 19, 5051

raising the question of whether it is simply a coincidence or whether features unique to FWCNTs

promote strength and conductivity. In order to determine whether SWCNT fibers have qualitatively different performance than DWCNT or FWCNT fibers, we produced fibers from five different SWCNT and DWCNT samples. Fiber processing conditions were optimized separately for each CNT sample; each sample was processed at least twice (starting from dope preparation) to ensure that the samples were spun as well as possible. The standard deviation of measured strength and electrical conductivity of the resulting fibers was 5 to 10 % of the average values. All fibers were spun from CNT samples with carbon purity greater than ~95 % and all the fibers were highly aligned as they were spun with draw ratios of at least 1.5. Such optimized fiber spinning for all the CNT samples guarantees that the variability in strength and conductivity across the fibers is primarily due to differences in the CNT intrinsic properties as opposed to differences in fiber morphology. Figure 6 shows plots of conductivity versus tensile strength for different SWCNT and DWCNT fibers. The correlation of fiber conductivity with strength is similar for both SWCNT and DWCNT fibers. The 8 ACS Paragon Plus Environment

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trend of improved conductivity with increasing strength results from the fact that both conductivity and strength scale with CNT aspect ratio. Properties of both SWCNT and DWCNT fibers follow the same mastercurve, demonstrating that fibers made from high quality CNTs can simultaneously attain high strength and conductivity regardless of whether they are composed of SWCNTs or DWCNTs. Our results show that

Figure 6. CNT fiber conductivity versus tensile strength for fibers made from SWCNTs and DWCNTs from different manufacturers. both DWCNTs and SWCNTs can be used to produce highly conductive CNT fibers (conductivity > 5 MS/m), despite prior research suggesting that DWCNTs are fundamentally more conductive than SWCNTs.45-46 Likely, these earlier literature conclusions stem from the historical availability of higher aspect ratio, highly graphitic FWCNT over SWCNTs. In fact, CNT suppliers such as Meijo Nano Carbon Co., SouthWest NanoTechnologies, and OCSiAl Group have developed very high aspect ratio, low-defect SWCNTs only recently, allowing the fabrication of SWCNT fibers with fiber properties that follow the same scaling laws as FWCNTs of similar quality and aspect ratio. In summary, CNTs characterized by low impurities, low defect density, and high aspect ratio should be synthesized for improved strength and conductivity of CNT fibers.



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Optimized Fiber Properties After spinning optimization, long, high quality, purified AC 299 DWCNTs yielded the fiber with the highest electrical conductivity and strength. This fiber exhibits a cylindrical, slightly dog-boned, morphology with large CNT superropes that propagate along the length of the fiber, as typically observed in solution-spun fibers (Figure 7a). The fiber cross-section (exposed by focused ion beam in Figure 7b) has relatively low porosity as the constituent CNTs are tightly-packed with relatively few voids visible by SEM, very similar to those previously reported with lower aspect ratio CNTs.5 Additionally, the CNTs are highly aligned along the length of the fiber which has a wide-angle X-ray diffraction (WAXD) azimuthal full-width half-maximum (FWHM) of 6.3° (roughly 3° narrower than our earlier fibers5), corresponding to a Herman orientation factor of 0.996 (Figure 7c). This results in an average fiber density of 1.5 ± 0.1 g/cm3 determined from SEM diameter measurements (20 ± 2 µm) in conjunction with linear density measurements performed with a microbalance (0.52 ± 0.01 tex, i.e., 0.52 ± 0.01 g/km). The average room-temperature DC conductivity value of AC 299 DWCNT fibers (measured by fourpoint probe over more than ten segments, each 200 mm long, from the same spool) is 7.7 ± 0.3 MS/m; the best fiber segment had a conductivity of 8.5 MS/m and a specific conductivity of 5500 Sm2/kg. Conductivity was measured periodically between March 2014 and May 2017 and showed no appreciable changes; hence,

Figure 7. (a) SEM of the AC 299 CNT fiber surface and (b) cross-section cut by focused ion beam, showing a partially dog-boned structure. (c) WAXD azimuthal scan of the AC 299 fiber, exhibiting a FWHM of 6.3° corresponding to a Herman orientation factor of 0.996.


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fiber conductivity is stable for over three years in laboratory conditions. The AC 299 fibers had a mean tensile strength of 2.4 GPa ± 0.1 GPa (best value 2.5 GPa) and a mean Young’s modulus of 250 GPa ± 60 GPa (measured via Instron with 20 mm gauge length, see Supporting Information Figure S3 for representative stress-strain curves). The tenacity (specific strength) of the fibers was

1540 mN/tex

± 90 mN/tex

(best

value

1620 mN/tex)

and

the

specific

modulus

was

160 N/tex ± 40 N/tex. The strength and modulus of AC 299 fibers were measured independently with an electromagnetic tensile tester at the National Institute of Science and Technology (NIST). Representative mean strength and apparent Young’s modulus values measured at NIST were 2.2 GPa ± 0.3 GPa, and 196 GPa ± 28 GPa, respectively (obtained from 15 replicates at the 8 mm gauge length; Supporting Information Figure S4), within one standard deviation of the average values obtained at Rice University. The electrical conductivity of these CNT fibers is the highest reported to date and the specific conductivity of the best fiber (5500 Sm2/kg) is within 15% of copper (6500 Sm2/kg). Importantly, this conductivity is measured after stabilizing the fibers (thereby removing significant acid doping) and without any post-process doping with halogens or other materials. CSA processing results in sufficient residual acid within the fiber structure that acts as a strong and stable p-type dopant, likely because the constituent CNTs are filled with residual chlorine and sulfur compounds. For strength, AC 299 DWCNT fibers are second only to CNT ribbons densified by pressurized rolling51-52 and ~10 times stronger than copper (~70 MPa to ~250 MPa, depending on the alloy53-55), approaching the strength of carbon fiber – 3.5 GPa to 7 GPa (~2 to 4 N/tex) depending on the grade53. Figure 8 compares acid-spun CNT fibers with the best solid-state spun CNT ribbons,52 and with commercially available conductive fibers and wires including copper, metalized aramids, and nickel coated carbon fibers. Whereas the specific conductivity of CNT fibers does not yet exceed that of copper, both the specific conductivity and strength of the CNT fibers reported here are higher than metalized aramids and nickel-coated carbon fibers that are currently used as alternatives to copper-based products in electrical applications where light weight and high strength are critical (and even more so in applications where flexibility and wash resistance are needed, such as wearable electronics). 11 ACS Paragon Plus Environment


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Figure 8. Specific electrical conductivity versus strength of CNT fibers in this work (green diamond) compared to previous fibers produced by superacid spinning (blue diamonds),3, 5, 33, 6 densified solid-state spun CNT fibers (black square52 and yellow square30) and other conductive fibers / wires including copper (orange circle), metalized aramid (gray triangle), and nickel-coated carbon fiber (red triangle).56 Further CNT fiber property improvements could be obtained by the application of post-processing techniques such as doping50, 57 to improve electrical conductivity or chemical cross-linking to improve strength; however, substantial increase in fiber performance will depend on the availability of higher quality CNTs. Figure 9a shows that, over the course of the past decade, the electrical conductivity and tensile strength of fibers produced by the acid spinning process improved by well over an order of magnitude, from 0.5 MS/m and 0.1 GPa achieved in 2004 to 8.5 MS/m and 2.5 GPa reported here. Some of the increased fiber performance can be attributed to improved processing methods. Advances made from 2004 to 2009, such as using stronger solvents (CSA instead of sulfuric acid), attaining better control on the morphology of liquid crystalline solutions, applying tension during spinning, and using lower viscosity coagulants were the main reasons for the property improvement of fibers composed of relatively low aspect HiPco SWCNTs.33 Fiber properties in 2013 improved primarily because of the use of much longer (aspect ratio 2800 versus 400) and higher quality CNTs.5 Similarly, the multifunctional properties


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obtained in this study are again resultant from significant increase in CNT aspect ratio (4400 versus 2800) and quality.

Figure 9. (a) Electrical conductivity and tensile strength improvement of CNT fiber spun from superacids by Rice University over the past decade. (b) Comparison of tensile strength and electrical conductivity for CNT fibers spun or post-processed via CSA in the literature;6, 58 the CAS fiber was post-processed with CSA for densification after spinning from a CVD reactor.59 Notably, electrical conductivity and strength can be improved simultaneously by improving CNT quality and processing. processing.59

Figure 9b includes other fibers recently produced by CSA spinning6,

Fibers from Refs.6 and

58

58

or post-

are considerably weaker, likely because of their defective

structure; interestingly (and perhaps coincidentally), fibers produced by CSA post-processing and densification of yarns directly spun out of a CVD reactor fall approximately on the same mastercurve as the Rice CSA-spun fibers, although the direct-spun fibers likely comprise longer CNTs and appear less aligned. The data from Figure 9b can be found in Table S1. CONCLUSIONS This study shows that CNT intrinsic properties are key to improving the strength and conductivity of acid-spun CNT fibers. CNT fiber conductivity and strength scale linearly with CNT aspect ratio in the range tested here. We observe no systematic dependence of fiber properties on CNT graphitic character (as measured by Raman), number of walls, or purity, although CNT type certainly affects macroscopic 13 ACS Paragon Plus Environment


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properties. However, we find that CNT samples with purity less than ~90 % are difficult to process continuously into highly-aligned, densely packed fibers. We also demonstrated that conductivity above 5.0 MS/m could be achieved with both SWCNTs and DWCNTs. The highest quality CNTs yielded continuous, cylindrical CNT fibers with an average strength of 2.4 GPa (1540 mN/tex, in the same range as the strongest continuous CNT fibers in the literature) and an average conductivity of ~7.7 MS/m; these properties are ~1.5-times higher than previously reported strength and conductivity.5 The insights presented here about the importance of high aspect ratio CNTs for producing high-performance CNT fibers can help CNT manufacturers and CNT fiber producers work together to continue to improve properties. Such a path to continued property improvement is critical because CNT fibers are finally within striking distance of copper, aluminum, and carbon fibers and could soon deliver on their promise of enabling high-performance multifunctional materials. METHODS CNT Purification. Commercially available CNTs produced from a variety of different CNT synthesis techniques were purchased or obtained directly from CNT manufacturers. CNT samples with greater than 10 wt.% metal catalyst/carbonaceous impurities were purified prior to acid processing. The purification procedure consisted of 24 hours of stir bar mixing CNTs in 30 % H2O2 at a ratio of 2 mL H2O2 per 1 mg CNTs and recovering the CNTs via vacuum filtration, followed by 24 hours of stir bar mixing the CNTs in 12 N HCl at a ratio of 1 mL HCl per 1 mg CNTs. Finally, the CNTs were neutralized with deionized water, frozen in liquid nitrogen, and dried overnight in a freeze dryer. CNT Characterization. CNT number of walls and average diameter was determined from highresolution TEM images acquired with a JEOL 2010* Transmission Electron Microscope. CNT samples with greater than 50 % single-walled CNTs were designated as SWCNT samples, while CNT samples *

Certain commercial equipment, instruments, or materials are identified in this work in order to adequately detail the experimental procedure. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the materials and equipment identified are necessarily the best available for this purpose 14 ACS Paragon Plus Environment

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with less than 50 % single-walled CNTs were designated as DWCNT or FWCNT samples. An FEI Quanta 400 ESEM FEG was used to perform EDS and take SEM images. Transmitted and polarized light microscopy images of CNT solutions in flame-sealed, 100 µm thick capillaries were taken with a Zeiss Axioplan optical microscope. TGA was performed on 3 mg to 5 mg CNT samples to determine the amount of metal catalyst impurities in each sample. The CNTs were loaded into the furnace of a TA Instruments Q-600 TGA/DSC. Industrial quality air was supplied to the furnace at a flow rate of 100 mL/min. The samples were equilibrated at 150 °C for 30 minutes and then heated to 850 °C at the rate of 2.5 °C/min. The temperature was kept at 850 °C for 60 minutes to ensure complete oxidation of the CNTs. Raman spectra were taken on CNT raw material with a Renishaw InVia Confocal Raman microscope at 514, 633, and 785 nm excitation wavelengths. Every test was repeated at eight different locations on the CNT sample and averages of the results were used to compare spectra between samples. The average aspect ratio of CNTs was obtained from extensional viscosity measurements, as described in a previous study.25 Briefly, CNTs were dispersed in CSA by stir bar mixing at concentrations ranging from 50 ppm to 1000 ppm and their extensional viscosities were measured with a Cambridge Trimaster Capillary Thinning Rheometer. Fiber Spinning. The key steps involved in manufacturing CNT fibers via wet-spinning5 are shown in the process flow diagram in Figure 1. The diagram highlights the various stages of the spinning process which include mixing CNTs in CSA at concentrations between 2 % and 6 % by mass to form a spinnable liquid crystalline solution (spin dope), filtering the dope to remove solid particles, extruding the dope through a spinneret with a diameter ranging from 65 µm to 150 µm into a coagulant bath of either acetone or water, and stretching the fiber during coagulation by collecting it on a drum rotating with a linear velocity greater than the extrusion velocity of the dope. High shear forces at the spinneret exit, along with the tension applied by the rotating drum, contribute to a densely-packed, highly aligned CNT morphology in the fibers. After spinning, fibers were stabilized by rinsing with room temperature water, annealing at 115 °C for 18 hours in an oven, and then washing in a 60 °C water bath for 3 hours to make sure that the



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room temperature conductivity of the fibers was stable prior to testing. Strongly adsorbed acid dopant is still present in the fibers since acid-spun fibers are always acid-doped as a result of the spinning process. Fiber Characterization. Fiber diameters were measured with either an FEI Quanta 400 ESEM FEG or with a Zeiss Axioplan optical microscope. Linear density was determined by weighing 10 cm – 20 cm lengths of CNT fiber on a Citizen microbalance. Mechanical properties were evaluated by performing tensile strength measurements at a strain rate of 10 mm/min on an Instron Model 1000 instrument with a 5 kg load cell. Fibers were epoxied to paper frames at a 20 mm gauge length. A custom-made 4-point probe connected to an HP 34401A multimeter was used to measure DC electrical conductivity of the fibers via 4-terminal sensing. Electrical and mechanical properties were measured on many sections of multiple meters of fiber.

Acknowledgments We thank E. A. Bengio, Ron ter Waarbeek, J. de Jong, M. Otto for useful discussions and J. H. Kim for additional tensile testing at NIST. We also thank Y. Talmon, L. Liberman, O. Kleinerman, and Y. Cohen for useful discussion and wide-angle X-ray diffraction data. We thank the following collaborators for providing CNTs: Marcin Otto (Teijin Aramids, AC 299), Takeshi Hashimo and Kei Takano (Meijo Nano Carbon), Lijie Ci (Samsung Cheil), Dmitri Aronov and Alexander Bezrodny (OCSiAl), Jens Kroeger (Raymor), Glen Irvin (Unidym), Ed Vega and Ken McElrath (CNNI), Sian Fogden (Linde), Benoit Simard and Chris Kingston (NRC Canada), Riccardo Prada Silvy (SWeNT), Ramesh Sivarajan (Nano-C), Lily Kim and Eun Hwa Hong (KH Chemicals), Julien Amadou (Nanocyl), Jiang Xiao (TimesNano), Marc Smith (Klean Carbon), Eva Yan (CNano), Joe Sprengard (General Nano). Research was supported by the Air Force Office of Scientific Research (AFOSR) grant FA9550-09-1-0590 and FA9550-15-1-0370, the


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Welch Foundation grant C-1668, and DOE award DE-EE0007865; R.J.H. was supported by a NASA Space Technology Research Fellowship (NSTRF14), grant number NNX14AL71H.

Conflict of interest The authors declare the following competing financial interest(s): M.P., D.E.T. and F.M. have a financial interest in DexMat, Inc., which is commercializing the technology reported in this publication.

Supporting Information Effect of processing on CNT fiber tensile strength, influence of CNT type, Raman spectroscopy of low and high defect CNTs, and representative stress versus strain data for strongest CNT fiber. The Supporting Information is available free of charge via the ACS Publications website at http://pubs.acs.org.



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Purified CNTs

Dissolution into Chlorosulfonic Acid

Spin Dope Loading

Further Processing

Collection

Coagulation

Winding Drum

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Extrusion

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2000

C

Intensity (a.u.)

11600 2 3 1200 4 5 800 6 3 µm 7 8 400 O Fe Fe 9 Fe Si 10 0 3 5 6 1 2 4 7 11 0 Energy (keV) b12 2000 C 13 14 1600 15 16 1200 17 18 19800 20 3 µm 21400 22 Si Cl O 23 Fe Fe Fe 0 24 0 3 5 6 1 2 4 7 25 Energy (keV) 26c d 27 28 29 30 31 32 33 f 34e 35 36 37 ACS Paragon Plus Environment 38 39 40

H2O2/HCl purified

Unpurified

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b 10.0 SWCNT

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SWCNT DWCNT 0.5

0

10 20 30 40 Raman G/D Ratio @ 785nm


Page 33 of 37

c

b

20

40

60

Raman G/D Ratio

80

10.0

35 G/D Ratio @ 785 nm

Conductivity / Aspect Ratio (kS/m)

Tensile Strength / Aspect Ratio (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 a 32 33 341.0 35 36 37 38 39 40 41 42 43 44 45 460.1 47 0 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0

785nm 0.1

0

20

633nm

514nm

40

60

Raman G/D Ratio


80

30 25 20 15 10 5 0

0

2000

4000

Aspect Ratio

6000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



SWCNT DWCNT

6.0

Slope = 0.8 0.6

0.1

1.0 Tensile Strength (GPa)


Page 34 of 37

Page 35 of 37

c

b

10 8

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 a10 11 12 13 14 15 16 17 18 19 20 21 22 5µm 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5µm

6

φ

4 2 0 0


50

100

150 200 φ(°)

250

300

350

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Specific Electrical Conductivity (Sm 2/kg)


Page 36 of 37

8000 Copper This Work

6000 Metalized Aramid

(5) Iodine Doped

4000 (5) Acid Doped

(6)

(30) (29) Miralon Yarn

2000 (32) (3) (33) 0

0

500

(52) (55)

(56) Nickel-Coated (59) Carbon Fiber 1000 1500 2000 2500 3000 Specific Tensile Strength (mN/tex)


3500

Page 37 of 37

a

b 10.0

1

0.1 2000

1.0

2005

2010 Year

2015

0.1 2020

10.0 Tensile Strength (GPa)


10


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Rice RIT Wollongong CAS

2017* 2013

2017

1.0

2017 0.1 0.01

2009

2015

2004 0.1 1 10 Electrical Conductivity (MS/m)


Influence of Carbon Nanotube Characteristics on Macroscopic Fiber

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