Carbon nanotube fibers for advanced composites

Carbon nanotube fibers for advanced composites Investigations into carbon nanotube fibers as not only structural reinforcement materials but also standalone or embedded strain/damage, thermal, atmospheric and biochemical sensors are driven by their high specific strength, stiffness, electrical and thermal conductivity, and extreme flexibility. With future applications in view, we have studied their load transfer mechanisms, coupled electrical and mechanical response, high strain rate behavior and adaptability to resin infusion. This article is intended to cover our research over the past couple years and to highlight relevant and interesting work performed by others in the area of carbon nanotube fiber mechanics and experimental characterization. Amanda S. Wu and Tsu-Wei Chou * Department of Mechanical Engineering and Center for Composite Materials, University of Delaware, Newark, DE 19716, USA * E-mail: chou@udel.edu The development and experimental characterization of continuous fibers comprised of carbon nanotubes (Fig. 1), held together primarily through van der Waals interactions and entanglements, have been the focus of significant research over the past decade. Initially, carbon nanotube fibers were produced through condensing and collating of a carbon nanotube/polymer dispersion 1. In this solution-spinning or wet-spinning process, carbon nanotubes are flow-aligned, resulting in orientation along the fiber axis. Carbon nanotube fibers produced using this procedure 1-4 are subject to defects and voids 4, possessing low strengths (0.1 0.2 GPa) and moderate to high elastic moduli (10 250 GPa). The incorporation of polymers as fiber fillers/ coatings 5-7 (e.g., polyvinyl alcohol or PBO) has led to improved load transfer between individual carbon nanotubes resulting in fiber strengths on the order of 1.5 4 GPa. The growth of long, highly-aligned arrays or forests of carbon nanotubes 8 has facilitated the drawing and spinning of carbon nanotube fibers 9,10. While this approach results in carbon nanotube fibers with moderate to high tensile strengths (0.1 up to 3 GPa) and a wide range of elastic moduli (2 300 GPa), the final fiber length is limited by the size of the carbon nanotube array. A third method in which carbon nanotube fibers are drawn directly from the chemical vapor deposition (CVD) furnace has resulted in the production of continuous carbon nanotube fibers 11,12 with tensile strengths ranging from 0.4 to 9 GPa and elastic moduli between 10 to 300 GPa. Note that the broad distribution of strength and moduli reported for carbon nanotube fibers are attributed not only to differences in their morphology and composition (carbon nanotube length, number of walls, twist angle, fiber density), but also due to fiber preparation and handling processes as well as differences in stress calculations (see Materials characterization section). Further improvements in carbon nanotube fiber strength are accomplished via post-processing condensing/twisting steps 13,14. An in-depth description of these processing methods and the challenges associated with each is provided by Lu et al. 15. Despite the challenges and expenses inherent in carbon nanotube fiber processing, their development remains a major topic of interest due to their potential for high strength 5,14,17, stiffness 17,18, thermal 19-21 and 302 JULY-AUGUST 2012 VOLUME 15 NUMBER 7-8

Carbon nanotube fibers for advanced composites REVIEW Fig. 1 CVD-spun continuous carbon nanotube fiber. Reproduced from Wu et al. 16 with permission from The Royal Society of Chemistry. electrical 3,12,22 conductivity as well as their flexibility 23 and capacity to translate the nanoscale properties of carbon nanotubes to the micro- and mesoscale. The strength and moduli of carbon nanotube fibers remain 1 to 2 orders of magnitude lower than those of individual carbon nanotubes (Fig. 2); however, carbon nanotube fiber strength is comparable to that of Kevlar fiber 24, Twaron 24 and A265 fiber 25, exceeding that of E-glass 26 - see Wu et al. 27. Furthermore, carbon nanotube fibers exhibit electrical conductivities ranging from 10 2 to 10 4 S/cm (Fig. 3) which are comparable or better than those of carbon fibers. Analytical and computational modeling efforts can provide insight into the many factors (e.g., carbon nanotube morphology, entanglements, electrical tunneling distance, etc.) influencing carbon nanotube fiber mechanical and electrical behavior. In addition to studies aimed at modeling the mechanical behavior of individual carbon nanotubes 52,53, load transfer mechanisms in carbon nanotube fibers have been examined on a single nanotube basis. In particular, research investigating the mechanics of carbon nanotube looping, entanglement and collapse 54,55, which are commonly found in carbon nanotube fibers, provides a foundation for future studies aimed at describing the mechanics of multiple carbon nanotubes interacting within a bundled fiber. Electrical conductivity of carbon nanotube fibers is significantly closer to that of individual carbon nanotubes due to nanotube-nanotube electron tunneling. Analytical modeling research describing the electrical tunneling behavior of straight 56 and wavy 57 carbon nanotubes embedded within a nonconductive polymer matrix provides a foundation for describing the electrical properties of polymer-infused carbon nanotube fibers. Despite the immense amount of modeling research performed on individual carbon nanotubes and carbon nanotube/polymer composites, there exists significant room for growth in the modeling of carbon nanotube fibers. In contrast to the lack of intensive modeling efforts aimed at understanding the micro and mesoscale behavior of carbon nanotube fibers, extensive research into their fabrication and quasi-static Fig. 2 Reported ultimate tensile strength (σ UTS ) and elastic moduli (E) of carbon nanotubes (black), solution-spun (blue), forest-spun (green) and aerogel-spun (red) carbon nanotube fibers and carbon fiber (violet). Reproduced and modified from Wu et al. 16 with permission from The Royal Society of Chemistry. characterization 15 has raised a great deal of excitement regarding their potential applications. This review will focus on recent advancements in the experimental characterization of carbon nanotube fibers with particular emphasis on the efforts of our research group in micromechanical modeling, fiber infusion, and high strain rate electrical and mechanical characterization. Morphology Recently, our group has evaluated both forest-spun and aerogel-spun carbon nanotube fibers. In aerogel-spun fibers, the effects of fiber JULY-AUGUST 2012 VOLUME 15 NUMBER 7-8 303

REVIEW Carbon nanotube fibers for advanced composites twisting (e.g., splits in the fiber and fiber diameter variations) are readily noticeable at lower magnifications; however, the carbon nanotubes appear to be highly entangled at the fiber surface (Fig. 4). Despite this, etching the fiber surface reveals a high degree of carbon nanotube alignment (Fig. 5). Carbon nanotube fibers depicted in Figs. 4,5 are provided by Dr. David Lashmore of Nanocomp Technologies, Inc. During fiber processing, inexpensive green fuel (i.e., grain alcohols and iron-based catalysts) is injected into a linear reactor furnace using hydrogen as a carrier gas. A loose suspension of single walled carbon nanotubes formed in a gas phase exits the furnace and impinges on a rotating anchor prior to collection. Post-processing involves drawing the carbon nanotube roving through an acetone bath and twisting. Fibers evaluated possess a linear density of 1.4 g/km and a diameter of 57.3 ± 5.9 μm 16. Forest-spun fibers provided by Prof. Yuntian Zhu of North Carolina State University, comprised of multi-walled carbon nanotubes, are similar in morphology, yet are produced with smaller diameters (20 50 μm) 45. Interfacial shear strength measurements performed by our group were carried out on a second set of forest-spun carbon nanotube fibers provided by Prof. Qingwen Li of the Suzhou Institute of Nano-Tech and Nano-Bionics with an average fiber diameter of 9.58 ± 0.63 μm 43 and a linear density of 0.034 g/km. These fibers consist of double- and triplewalled carbon nanotubes. Nanomechanics Carbon nanotube fibers consist of millions of individual carbon nanotubes held in place by a combination of nanotube-nanotube entanglements and van der Waals interactions 58. A concerted effort to isolate and better understand the load transferring mechanisms present within these fibers has been made by Lu and Chou 54,55. Carbon nanotube entanglements are not uncommon occurrences within carbon nanotube fibers (Fig. 4). In their work 54, single walled carbon nanotube entanglement is modeled by considering two connecting, self-folded carbon nanotubes. The geometrical characteristics of selffolded or looped carbon nanotubes, such as the critical nanotube length for self-folding as well as the critical effective loop width and length, are investigated by using both an exact theoretical model and an approximate theoretical model. Above a critical nanotube length (l c = 12.00 EI γ, where EI and γ are the bending stiffness and interfacial binding energy of the carbon nanotube, respectively), self-folding is energetically favorable due to the van der Waals interactions between different parts of the nanotube 54. The loop characteristics are governed by the carbon nanotube radius, rather than length, making this approach appropriate for describing entanglements between carbon nanotubes of varying lengths greater than l c 54. Theoretical modeling and atomistic simulations describing their tensile behavior show good agreement in their predictions of increased stiffness with applied force for these selffolded carbon nanotubes 54. Another aspect of carbon nanotube geometrical deformations affecting the mechanical behavior of carbon nanotube fibers is radial collapse 59 ; this phenomenon is driven by the variation of contributions from the bending strain energy and the van der Waals interaction energy between opposite walls of a single walled carbon nanotube to the total Fig. 3 Reported electrical conductivity (κ) vs. fiber density (ρ) of carbon nanotubes (black), solution-spun (blue), forest-spun (green) and aerogel-spun (red) carbon nanotube fibers and carbon fiber (violet). energy 55. According to atomistic simulations and continuum analysis performed by Lu et al. 55, collapse can occur in single walled carbon nanotubes with radii larger than 1.05 nm. For single walled carbon nanotubes with radii larger than 1.90 nm, the collapsed states are more energetically stable than the initial, undeformed states 55. Radial collapse gives rise to increases in the intertube contact area 55, which can enhance the load transfer efficiency between carbon nanotubes within a fiber. This improved understanding of the effects of carbon nanotube entanglements, folding and collapse on their mechanical behavior will result in more accurate descriptions of the state of carbon nanotubes and their contributions to carbon nanotube fiber strength. As atomistic computational simulations are met with advancements in processing power, modeling bundles of interacting carbon nanotubes with a view of analyzing and predicting carbon nanotube fiber behavior at the nano to microscale is on the horizon. Mechanical characterization Fiber strength/modulus Carbon nanotube fiber strength varies with carbon nanotube properties (length, walls, diameter, waviness, etc.) and morphology (twist angle, compaction, etc.). A more superfluous, but significant, factor giving rise to the broad range of reported strengths/moduli in the literature is the method used to calculate fiber strength. Three approaches are used and it is critical, when comparing properties, to use the appropriate strength value. Often, carbon nanotube fiber strength is the most impressive when stress (σ) is calculated in terms of a representative cross-sectional area which neglects voids and porosity between carbon nanotubes within the fiber. This method (eq. 1), which uses the fiber linear density (tex, typically reported in g/km), combined with an assumed density based 304 JULY-AUGUST 2012 VOLUME 15 NUMBER 7-8

Carbon nanotube fibers for advanced composites REVIEW Fig. 4 Scanning electron micrographs depicting an aerogel-spun carbon nanotube fiber processed with a 15º twist angle. on the type of carbon nanotubes used (typically ρ = 2.0 g/cm 3 for multiwalled carbon nanotubes), reports maximum failure load (F max ) per physical cross-sectional area and tends to yield the highest values due to low fiber linear density. F σ = tex ρ A method slightly more translatable to other fibers, but less accurate due to the assumed cylindrical cross-sectional area is presented in eq. 2, where A=πR 2. σ = A final approach (eq. 3) reports stress in terms of force per linear density (σ ); this has the broadest appeal in terms of comparing the strength of different types of fibers. F A F σ* = tex The first and third approaches require knowing the fiber linear density and involve the assumption that this linear density is constant along the fiber axis. The second approach assumes a constant cylindrical cross-sectional area. It is important to select the most appropriate and accurate method when reporting fiber strength and moduli. However, it is ideal, when possible, to report all values or to provide the information required for the reader to calculate stress according to his or her application 16. (1) (2) (3) Recently, our research group has evaluated both forest-spun 43,45 and aerogel-spun 16 carbon nanotube fibers under quasi-static tensile loading. In these reports, the statistical variation between individual carbon nanotube fiber specimens is commented on with respect to the fiber diameter measured via scanning electron microscopy. Fig. 6 demonstrates the variation in fiber specimen geometry within a group of 55 fiber specimens. Several studies have demonstrated the necessity of reporting specimen gage length when discussing mechanical behavior of carbon nanotube fibers. It was hypothesized that the fiber gage length will adversely affect strength due to the discontinuous fiber internal microstructure and lack of intertube bonding 14. At gage lengths below 8 mm, this adverse effect is apparent 14,17,60. However, at gage lengths above 10 mm, Koziol et al. 17 observed a significant decrease in fiber strength while Zhang et al. 60 observed little to no change in measured tensile strength. Despite conflicting results between these two studies, it is evident that reporting gage length and, when possible, performing evaluations at multiple gage lengths is ideal. The ASTM standard for single fiber evaluation (ASTM D3822) calls for fiber lengths of 10 mm or greater, however, this is often difficult to accomplish due to limited fiber supply. An important issue to note is that a fiber specimen of 8 mm in length is over 3 orders of magnitude longer than the carbon nanotubes which comprise the fiber. Due to the significant specimen-to-specimen variation, it is critical that a large number of fiber specimens be evaluated in order to fully assess the mechanical performance of a carbon nanotube fiber batch. By studying the factors affecting fiber strength measurement (e.g., JULY-AUGUST 2012 VOLUME 15 NUMBER 7-8 305

REVIEW Carbon nanotube fibers for advanced composites Fig. 6 Fiber diameter measured at ten locations along the length of each 7.9 mm long fiber specimen directly influences measured fiber strength. Reproduced from Wu et al. 16 with permission from The Royal Society of Chemistry. slip. Since these are rate-dependent phenomena, it is reasonable that the fiber exhibits rate-dependent mechanical behavior. A comparison of the quasi-static and high strain rate behavior of carbon nanotube fibers with other high performance fibers is provided in Fig. 7. Fig. 5 A focused ion beam (Zeiss Auriga FIB-SEM) was used to partially etch the surface of a carbon nanotube fiber and the exposed cut was imaged via scanning electron microscopy. Carbon nanotubes appear aligned at the focused ion beam cut cross-section of an aerogel-spun fiber 27. Reprinted with permission from 27. Copyright 2012, American Institute of Physics. gage length) and carefully considering the appropriateness of fiber linear density vs. cross-sectional area, it is possible to make accurate statements on the mechanical performance of carbon nanotube fibers. Rate dependent behavior The effect of loading rate on the tensile performance of aerogel-spun carbon nanotube fibers was evaluated using the modified Kolsky tension bar experimental method 61 for single fiber evaluation. This study 14 revealed that carbon nanotube fibers exhibit higher tensile strengths at increasing strain rates up to 1300 s -1. This finding is in agreement with previous studies 60 into the effect of applied strain rate which demonstrate improvements in tensile strength in the range 0 0.25 s -1. During fiber failure, individual carbon nanotubes must disentangle and Recoil compressive behavior Aerogel-spun fibers exhibited kinking upon failure under quasi-static tensile loading (Fig. 8) 16. This occurrence facilitated an estimation of the fiber compressive strength. Following the logic of Allen 62, we hypothesized that tensile failure at σ UTS, results in a compressive stress wave of the same magnitude which travels from the specimen grip toward the failed fiber ends. If the compressive strength of the fiber is lower than its tensile strength, then this compressive stress wave will result in localized kinking. By ranking experimental results according to failure load, it was possible to approximate compressive strength as the stress level at which kink formation began to occur. Thus, a compressive strength of 172 177 MPa was identified for the aerogel-spun fibers 16. A follow-up study by Zu et al. 63 reports an average compressive strength of ~416 MPa for forestspun carbon nanotube fibers 63. Despite to the lack of mechanical binder to prevent buckling and transverse motion of the carbon nanotubes, the forest-spun carbon nanotube fibers exhibit comparable compressive strengths to carbon fibers and Kevlar-49 (Table 1). Electro-mechanical behavior While reported electrical conductivities for individual carbon nanotubes vary significantly with carbon nanotube structure, geometry, processing method and annealing procedure 48, the potential for high electrical conductivity (> 10 5 S/m) based on that of individual carbon nanotubes is well-understood. This translates somewhat to the macroscale in continuous carbon nanotube fibers (10 2 10 4 S/m), albeit with losses due to tunneling effects (Fig. 3). Increases in fiber density and subsequent improvement in nanotube-nanotube electrical contact 306 JULY-AUGUST 2012 VOLUME 15 NUMBER 7-8

Carbon nanotube fibers for advanced composites REVIEW Table 1 Fiber compressive strengths measured using the tensile recoil method. Aerogel-spun CNT fiber 16 Forest-spun CNT fiber 63 P130 carbon fiber 64 Kevlar-49 62 σ C (GPa) σ UTS (GPa) E (GPa) 0.172-0.177 0.189 ± 0.052 9.16 ± 2.53 0.275-0.478 1.4 ± 0.04 66.0 ± 1.35 0.410 1.7-3.1 165-910 0.365 3.4 130 provide significant improvements in electrical conductivity along the fiber length. In carbon nanotube fibers, applied strain evokes carbon nanotube displacement/sliding on the microscale, as well as tensile deformation applied locally to individual carbon nanotubes. These responses give rise to piezoresistive behavior - applied tensile strains result in measurable changes in electrical resistivity across the fiber length (Fig. 9). We have also observed piezoresistive behavior under dynamic tensile loading (Fig. 10). The piezoresistivity of carbon nanotube fibers gives rise to their potential use as embedded strain sensors in composite materials. With this application in mind, electrical resistance change vs. strain is quantified by a gage factor (GF = ΔR R ε) of 3.03 4.37 for these aerogel-spun fibers 14. Carbon nanotube fiber composites Resin-carbon nanotube fiber interaction Zu et al. 43 recently measured the interfacial shear strength between forestspun carbon nanotube fibers and epoxy resin using the micro-droplet experiment, first proposed by Herrera-Franco and Drzal 65. In microdroplet experiments, the force required for tangential blades to displace a resin droplet at the surface of a suspended fiber a predetermined amount is measured using a balance (Mettler Toledo PB303-S) and displacement is recorded using a dual video camera setup 66. In the study performed by Zu et al. 43, the epoxy resin droplets partially infused the carbon nanotube fiber, resulting in a dry carbon nanotube core surrounded by a partially wetted interphase layer. During interfacial shear strength experiments, interfacial sliding between the dry carbon nanotube fiber core and the interphase layer occurred, resulting in measurement of the interfacial shear strength between the resin/carbon nanotube interphase and the carbon nanotube fiber core 43. This effective interfacial shear strength of 14.4 MPa 43 was measured for 9.58 ± 0.63 μm diameter carbon nanotube fibers comprised of double- and triple-walled carbon nanotubes. Deng et al. 45 performed single carbon nanotube fiber (forestspun) composite fragmentation tests 67 which revealed an interfacial shear strength of 11.6 20.2 MPa. These measurements are low in comparison with the interfacial shear strength of both sized and unsized T650 carbon fiber in an epoxy matrix (101.6 MPa and 50.5 MPa, respectively 68 ). We attribute this largely due to the partial wetting of the carbon nanotube fiber; however, this behavior drives current efforts into carbon nanotube fiber functionalization for enhanced adhesion with polymeric resins. The partial wetting of the carbon nanotube fiber has driven studies into improvement in fiber/matrix interaction to achieve full fiber wetting. Fig. 7 Tensile strength of E-glass 26, A265 25, Kevlar 129 24, Kevlar 24, Twaron 24 and chemically treated and stretched carbon nanotube fiber 14 under quasi-static and high strain rate loadings. Reprinted from 14. Copyright 2012, with permission from Elsevier. Fig. 8. Localized buckling/kinking observed in aerogel-spun fibers after tensile failure. Reproduced from Wu et al. 16 with permission from The Royal Society of Chemistry. JULY-AUGUST 2012 VOLUME 15 NUMBER 7-8 307

REVIEW Carbon nanotube fibers for advanced composites A method which has proven successful by Nanocomp Technologies, Inc. is discussed in the following section. Fiber infusion Prior to resin infusion, the aerogel-spun carbon nanotube fibers from Nanocomp Technologies, Inc. undergo a preprocessing treatment step involving an acetone rinse (to remove trace contaminants), immersion in a 50 % nitric acid solution and oven drying under vacuum. The oxygen containing functional groups 69 developed during this treatment process facilitate bonding between the epoxy resin and the carbon nanotubes. It is critical to ensure that fibers reach full saturation prior to curing; otherwise, under wetted fibers with high void content can result. To achieve this, fibers are submerged for 1.5 h under vacuum in a bath consisting of bisphenol-f epoxy resin and amine curing agent at an elevated temperature to lower the resin viscosity 27. After infiltration, fibers are cured while suspended to prevent resin pooling. Energy dispersive x-ray spectroscopy is performed on infused fibers to ensure that epoxy resin has fully infiltrated into the fiber core. A focused ion beam cut fiber contained 14.3 % oxygen at the fiber core (Fig. 11). By contrast, energy dispersive x-ray spectroscopy revealed that neat carbon nanotube fibers are comprised of greater than 98 % carbon. The nitric acid treatment has been shown to account for only 7.3 % oxygen at the fiber core, therefore, it is reasonable to conclude that the additional oxygen measured in infused fibers exists due to the epoxy addition. This improved infusion method of the carbon nanotube fibers will better facilitate their incorporation into polymer based composites. Opportunities for further study Modeling efforts Despite the impressive and extensive progress made in manufacturing carbon nanotube fibers, significant research is required prior to producing mesoscale materials able to utilize the unique properties of these fibers to their best advantage. In the short term, concentrated efforts into process modeling 70 can not only elucidate the mechanisms controlling carbon nanotube fiber drawing but also provide insight into potential avenues for improvements in fiber properties during the drawing and spinning phases. Along these lines, an improved understanding of the load transfer mechanisms and effects of potential cross-linking/functionalization can be gained through modeling carbon nanotube bundles of varying morphologies under different loading scenarios. Recently, Vilatela et al. 71 have developed a model predicting carbon nanotube fiber strength accounting for the length of individual carbon nanotubes, nanotube collapse, neighboring contact area and interfacial strength. This research provides insight into the effects of these parameters on fiber strength and a basis for a scientific approach to carbon nanotube fiber improvement. Researchers in our group are currently involved in efforts to model fiber strength while accounting for fiber twisting as well. Fiber characterization Basic fiber characterization (mechanical behavior 15 and coupled electrical/ mechanical response 14,16,72 ) has been accomplished using established methods for single fiber characterization 61,62. Current efforts aimed at Fig. 9 Electrical resistance of a carbon nanotube fiber changes with applied tensile strain under quasi-static monotonic and cyclic loading. Reproduced from Wu et al. 16 with permission from The Royal Society of Chemistry. Fig. 10 Electrical resistance increases proportionately with applied stress in carbon nanotube fibers loaded at a strain rate of 760 s -1. Reprinted from 14 by Wu et al. Copyright 2012, with permission from Elsevier. developing a fundamental understanding of fiber mechanics to facilitate the design and processing of carbon nanotube fiber based composites are underway. Kis et al., recently introduced a method for cross-linking individual carbon nanotubes to provide improvements in shear modulus 73. Couple this with their potential to serve as actuators 74, and one can conclude that the torsional behavior of carbon nanotube fibers is currently understudied. In an effort to bridge this gap, we recently measured the shear modulus of neat and epoxy-infused carbon nanotube fibers and investigated the electromechanical response of carbon nanotube fibers 308 JULY-AUGUST 2012 VOLUME 15 NUMBER 7-8

Carbon nanotube fibers for advanced composites REVIEW knitting, etc.). Carbon nanotube fiber and preform infusion poses a significant challenge; with low fiber permeability and their complex fiber nanostructure, it is difficult to achieve complete wetting of a carbon nanotube fiber. Vilatela et al. 76 found that the polymer infiltrates between nanotube bundles...but not between nanotubes forming the bundles, in epoxy infused carbon nanotube fiber arrays. Despite this, rigorous infusion techniques 77 as well as methods to modify the carbon nanotube surface, creating functional groups to promote chemical bonding between a polymer resin and individual carbon nanotubes 27 have proven successful in producing composite fibers. Infiltration at the spinning step is a viable method for producing composite fiber 5-7 ; however, scaling this procedure up to produce composite lamina remains a challenge. Fig. 11 Focused ion beam etched composite fiber showing exposed carbon nanotubes at the cut surface and resin throughout. Reprinted with permission from 27. Copyright 2012, American Institute of Physics. to torsional loading with a view of developing these fibers as embedded sensors in composite materials 27. Evaluating the fatigue response of materials with structural and high performance applications is critical; the lifetime of a structure can be severely impacted by localized damage, which is difficult to detect. Their morphology, specifically their potential for relaxation and damping due to nanotube-nanotube interactions, makes carbon nanotube fibers interesting candidates for fatigue applications. Scale up and composites processing Due to their mechanical robustness, flexibility and size, carbon nanotube fibers can be processed using traditional preform fabrication methods (stitching, weaving, braiding - see Bogdanovich and Bradford 75, Future applications Carbon nanotube fibers possess moderate to high strengths and moduli, coupled with good electrical conductivity and very low density. These properties are translatable to the macroscale through the incorporation of carbon nanotube fibers into fabrics and composites. Several methods currently being explored include using the fibers as standalone woven, braided or knitted preforms, as z-pinning or stitching material and as interlayers in composites based on other preform materials (e.g., less expensive and nonconductive fiberglass). A major area for future development of carbon nanotube fibers involves their use as stand-alone or embedded sensors (strain and damage 13,14,16,78, thermal 20, atmospheric 79-83, bio 84, bio-electrochemical 22,85,86 ). In particular, their high strength 14,17, stiffness, thermal and electrical conductivity enable their use as functional reinforcements in composite materials (active heating/cooling 87, electrical conductivity/emi shielding - eg., CNT/epoxy composites 88 ), atmospheric anti-threat detection in UAVs). Recently, their electromechanical properties have garnered attention for carbon nanotube fibers as potential actuators and artificial muscles 73,89. Furthermore, carbon nanotube fibers can facilitate advanced preform processing due to their high flexibility 23. With many potential applications on the horizon, we anticipate that mass-production of carbon nanotube fibers with exceptional mechanical, electrical and thermal properties as well as development of scale-up preform processing capabilities will become increasingly important. Additionally, efforts into improving carbon nanotube fiber mechanical properties, through the use of functionalization for enhanced transverse interaction, will be of great importance in advancing this technology. Acknowledgements The authors gratefully acknowledge Nanocomp Technologies, Inc. (Dr. David Lashmore), North Carolina State University (Prof. Yuntian Zhu), Suzhou Institute of Nano-Tech and Nano-Bionics (Prof. Qingwen Li) and Tianjin University (Prof. Ya-Li Li) for material contributions to this effort. Funding for this research is provided by the U.S. Air Force Office of Scientific Research (Dr. Byung-Lip Lee, Program Director) and the National Research Foundation of Korea through the Korean Ministry for Education, Science and Technology (MEST). We would like to acknowledge our collaborators, Dr. Joon-Hyung Byun and Dr. Byung-Sun Kim of the Korea Institute for Materials Science, as well as Ms. Mei Zu and Dr. Weibang Lu for their helpful discussions. JULY-AUGUST 2012 VOLUME 15 NUMBER 7-8 309

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