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Advanced composites, such as those used in aerospace applications, employ a high volume fraction of aligned stiff fibers embedded in high-performance polymers. Unlike advanced composites, polymer nanocomposites (PNCs) employ low volume fraction filler-like concepts with randomly-oriented and poorly controlled morphologies due to difficult issues such as dispersion and alignment of the nanostructures. Here, novel fabrication techniques yield controlled-morphology aligned carbon nanotube (CNT) composites with measured non-isotropic properties and trends consistent with standard composites theories. Modulus and electrical conductivity are maximal along the CNT axis, and are the highest reported in the literature due to the continuous aligned-CNTs and use of an unmodified aerospace-grade structural epoxy. Rule-of-mixtures predictions are brought into agreement with the measured moduli when CNT waviness is incorporated. Waviness yields a large (similar to 10x) reduction in modulus, and therefore control of CNT collimation is seen as the primary limiting factor in CNT reinforcement of composites for stiffness. Anisotropic electron transport (conductivity and current-carrying capacity) follows expected trends, with enhanced conductivity and joule heating observed at high current densities. (C) 2009 Elsevier Ltd. All rights reserved.

Carbon nanotubes (CNTs) are a potential new component to be incorporated into existing aerospace structural composites for multifunctional (mechanical, electrical, thermal, etc.) property enhancement. Although CNT properties are extraordinary when measured individually, they tend to degrade by a large factor when integrated in system (often in polymer matrices). Mechanisms and effectiveness of nano-scale CNT implementation into macro-scale structural composites are not well understood. Non-mechanical aspects of these composites are the focus of this work. As a CNT hybridized fiber polymer composite, fuzzy fiber reinforced plastic (FFRP) is developed using a scalable fabrication method that achieves uniform CNT distributions for thermal and electrical conductive networks without requiring intensive mixing which can damage CNTs. At small CNT volume fractions (~0.5- 8% Vf), characterization shows significant enhancement in electrical conduction (x106-108) but limited enhancement in thermal conduction (x1.9). In addition, aligned-CNT polymer nanocomposites (A-CNT-PNCs) are being characterized as a representative volume element (RVE) of the FFRP. Experimentally obtained data on consistent A-CNT-PNC samples sets provide engineering knowledge and to achieve effective utilization of CNTs' multifunctional properties. Theoretical studies, both analytical and numerical, have been recently developed, suggesting interface effects may be a key to explaining the above limitations, including electron tunneling/hopping or phonon scattering at CNT-CNT and CNT-polymer interfaces. Multiple test techniques and property extraction methods for A-CNT-PNCs are developed and/or employed for cross-comparison. Applications of nano-engineered composites enhanced with CNTs can include lightning protection layers, electromagnetic interference shields, thermal management layers, and thermoelectrical sensor layers for airplane structures., Airbus Industrie, Massachusetts Institute of Technology (Richard and Linda Hardy Graduate Fellowship), Boeing Company, Empresa Brasileira de Aeronáutica, Lockheed Martin, Spirit AeroSystems (Firm), Textron, inc., Composite Systems Technology (Firm), Toho Tenax Co., Ltd., Massachusetts Institute of Technology (Nano-Engineered Composite aerospace STructures (NECST) Consortium)

Polymer-derived pyrolytic carbons (PyCs) are highly desirable building blocks for high-strength low-density ceramic meta-materials, and reinforcement with nanofibers is of interest to address brittleness and tailor multi-functional properties. The properties of carbon nanotubes (CNTs) make them leading candidates for nanocomposite reinforcement, but how CNT confinement influences the structural evolution of the PyC matrix is unknown. Here, the influence of aligned CNT proximity interactions on nano- and mesoscale structural evolution of phenol-formaldehyde-derived PyCs is established as a function of pyrolysis temperature (Tₚ) using X-ray diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy. Aligned CNT PyC matrix nanocomposites are found to evolve faster at the mesoscale by plateauing in crystallite size at Tₚ ∼ 800°C, which is more than 200°C below that of unconfined PyCs. Since the aligned CNTs used here exhibit ∼ 80 nm average separations and ∼ 8 nm diameters, confinement effects are surprisingly not found to influence PyC structure on the atomic-scale at Tₚ ≤ 1400°C. Since CNT confinement could lead to anisotropic crystallite growth in PyCs synthesized below ∼ 1000°C, and recent modeling indicates that more slender crystallites increase PyC hardness, these results inform fabrication of PyC-based meta-materials with unrivaled specific mechanical properties., National Science Foundation (U.S.). Research Experience for Undergraduates (Program) (grant number DMR-08-19762), Massachusetts Institute of Technology. Materials Processing Center, United States. Department of Defense (National Defense Science & Engineering Graduate Fellowship (NDSEG) Program), Airbus Group, Boeing Company, Embraer, Lockheed Martin, Saab (Firm), ANSYS, Inc., Hexcel (Firm), Toho Tenax Co., Ltd. (MIT’s Nano-Engineered Composite aerospace STructures (NECST) Consortium)

Poly(urethane-urea) (PUU) has been infused into ultrahigh volume fraction carbon nanotube (CNT) forests using a heat-curable polymer formula. Polymer nanocomposites with carbon nanotube volume-fractions of 1%, 5%, 10%, 20%, and 30% were fabricated by overcoming densification and infusion obstacles. These polymer nanocomposites were nanoindented quasi-statically and dynamically to discern process-structure-(mechanical) property relations of polymerizing PUU in such densely-packed CNT forests. A 100× increase in indentation modulus has been observed, which is attributed not only to CNT reinforcement of the matrix, but also to molecular interactions in the matrix itself. Quasi-static elastic moduli ranging from 10 MPa–4.5 GPa have been recorded. Storage modulus for all materials is found to track well at loadings of 200 Hz, with little effect observed from increasing CNT volume fraction. Keywords: carbon nanotubes; polymer nanocomposites; polyurethane urea; self-assembly, United States. Army Research Office (Contract W911NF-13-D-0001)

Capillary-mediated densification is an inexpensive and versatile approach to tune the application-specific properties and packing morphology of bulk nanofiber (NF) arrays, such as aligned carbon nanotubes. While NF length governs elasto-capillary self-assembly, the geometry of cellular patterns formed by capillary densified NFs cannot be precisely predicted by existing theories. This originates from the recently quantified orders of magnitude lower than expected NF array effective axial elastic modulus (E), and here we show via parametric experimentation and modeling that E determines the width, area, and wall thickness of the resulting cellular pattern. Both experiments and models show that further tuning of the cellular pattern is possible by altering the NF-substrate adhesion strength, which could enable the broad use of this facile approach to predictably pattern NF arrays for high value applications., United States. National Aeronautics and Space Administration (Grant NNX17AJ32G)

For the first time, micro-scale digital image correlation (μDIC) is investigated for measurement of strain fields in hierarchical fiber-reinforced composites. The methodology is developed on an exemplary alumina fiber/epoxy composite laminate with aligned carbon nanotubes (A-CNTs) grown on fibers. Utilizing environmental scanning electron microscopy and nano-scale random speckle patterns, sufficient precision is achieved to detect the influence of the A-CNTs on the deformation field around the fibers. Debonded regions at the fiber/matrix interface with openings as small as 35 nm could be detected. μDIC could identify the propagation of the debonded region based on the non-linear increase of the opening. The image correlation uncertainty in the displacement analysis is estimated to be below 5 nm. The experimental results are validated by computational analysis performed on the region of interest. For this, an advanced model with two scales of reinforcement (microscopic fibers and nanotubes) and boundary conditions taken from the experiment is used. As verified by the model, A-CNTs are found to constrain matrix deformation in their longitudinal direction. Keywords: Digital image correlation (DIC); Scanning electron microscopy (SEM); Carbon nanotubes; Mechanical properties; Finite element analysis (FEA)

Flexible capacitive pressure sensors fabricated with nanocomposites were experimentally characterized and results compared with simulations from analytical modeling. Unlike traditional diaphragm silicon pressure sensors, the flexible nanocomposite sensors present a linear response over a large pressure range due to the changes in the volume of the dielectric that cause the pressure inside the dielectric cavity to change. Several devices with different geometries were used and the model results compare well with the experimental data. Sensitivities ranging from 2.5- 20 fF/kPa were obtained and the devices dynamic response show a dual behavior, i.e., response time of the sensor during a pressure decreasing step is lower than for a pressure increasing step. This comportment is explained by the viscoelastic behavior of the PDMS-based nanocomposites used for fabrication of the capacitive sensor electrodes., Fundação para a Ciência e a Tecnologia (FCT)

We demonstrate the layer-by-layer (LbL) assembly of polyelectrolyte multilayers (PEM) on three-dimensional nanofiber scaffolds. High porosity (99%) aligned carbon nanotube (CNT) arrays are photolithographically patterned into elements that act as textured scaffolds for the creation of functionally coated (nano)porous materials. Nanometer-scale bilayers of poly(allylamine hydrochloride)/poly(styrene sulfonate) (PAH/SPS) are formed conformally on the individual nanotubes by repeated deposition from aqueous solution in microfluidic channels. Computational and experimental results show that the LbL deposition is dominated by the diffusive transport of the polymeric constituents, and we use this understanding to demonstrate spatial tailoring on the patterned nanoporous elements. A proof-of-principle application, microfluidic bioparticle capture using N-hydroxysuccinimide-biotin binding for the isolation of prostate-specific antigen (PSA), is demonstrated., National Science Foundation (U.S.) (Award DMR-0819762)

The landmark theoretical properties of low dimensional materials have driven more than a decade of research on carbon nanotubes (CNTs) and related nanostructures. While studies on isolated CNTs report behavior that aligns closely with theoretical predictions, studies on cm-scale aligned CNT arrays (>10[superscript 10] CNTs) oftentimes report properties that are orders of magnitude below those predicted by theory. Using simulated arrays comprised of up to 105 CNTs with realistic stochastic morphologies, we show that the CNT waviness, quantified via the waviness ratio (w), is responsible for more than three orders of magnitude reduction in the effective CNT stiffness. Also, by including information on the volume fraction scaling of the CNT waviness, the simulation shows that the observed non-linear enhancement of the array stiffness as a function of the CNT close packing originates from the shear and torsion deformation mechanisms that are governed by the low shear modulus (∼1 GPa) of the CNTs., Massachusetts Institute of Technology. Nano-engineered Composite aerospace STructures (NECST) Consortium, United States. Army Research Office (Contract W911NF-07-D-0004), United States. Army Research Office (Contract W911NF-13-D-0001), United States. Dept. of Defense. National Defense Science & Engineering Graduate Fellowship Program

Ice protection systems (IPS) are critical components for many aerospace flight vehicles, including commercial transports and unmanned aerial systems (UAS), and can include anti-icing, de-icing, ice sensing, etc. Here, an IPS is created using nanomaterials to create a surface-modified external layer on an aerosurface based on observations that polymer nanocomposites have tailorable and attractive heating properties. The IPS uses Joule heating of aligned carbon nanotube (CNT) arrays to create highly efficient de-icing and anti-icing of aerosurfaces. An ice wind tunnel test of a CNT enhanced aerosurface is performed to demonstrate the system under a range of operating regimes (temperature, wind speed, water content in air) including operation down to -20.6°C (-5°F) at 55.9 m/s (125 mph) under heavy icing. Manufacturing, design considerations, and further improvements to the materials and systems are discussed., United States. Dept. of the Navy. Small Business Innovation Research (Contract N68335-11-C-0424), National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (Grant DMR-0819762)

Several hybrid architectures with aligned nanoscale fibers have been shown to provide inter- and intra-laminar reinforcement of fiber reinforced polymer composites. In one architecture, aligned carbon nanotubes (CNTs) grown on advanced fibers in a woven ply creates a ‘fuzzy fiber’ reinforced plastic (FFRP) laminate. Here the mechanisms of Mode I fracture toughness enhancement are elucidated by varying the type of epoxy and reinforcing CNT length experimentally. Reinforcement effects are shown to vary from reduced initiation toughness to more than 100% increase in steady-state fracture toughness, depending upon the multi-scale interlaminar fracture mechanisms. Fracture-surface morphology investigations using several techniques reveal that interlaminar toughness enhancement for an aerospace infusion resin is significantly less than that for a hand lay-up marine epoxy. Long (~20 micron) aligned CNTs toughens significantly (> 1 kJ/m[superscript 2] increase for marine epoxy) by driving the crack through tortuous paths around and through tows, whereas shorter CNTs produce less toughening (or even reduced toughness in aerospace epoxy), which is attributed to shorter pullout lengths and grown-CNT morphology differences. These findings reveal for the first time the multiscale nature of the composite ply interface, and the mechanisms at work at the chemical, nano, and micro scales that influence the macroscopic behavior. Extensions and future work are discussed, including preliminary results using the multifunctional attributes of the nanoengineered composite for structural health monitoring (SHM) concomitant with interlaminar fracture testing., United States. National Aeronautics and Space Administration (Space Technology Research Fellowship)

The average inter-wire spacing in aligned nanowire systems strongly influences both the physical and transport properties of the bulk material. Because most studies assume that the nanowire coordination is constant, a model that provides an analytical relationship between the average inter-wire spacings and measurable physical properties, such as nanowire volume fraction, is necessary. Here we report a continuous coordination number model with an analytical relationship between the average nanowire coordination, diameter, and volume fraction. The model is applied to vertically aligned carbon nanotube (VACNT) and nanofiber (VACNF) arrays, and the effective nanowire coordination number is established from easily accessible measures, such as the nanowire spacing and diameter. VACNT analysis shows that the coordination number increases with increasing nanowire volume fraction, leading the measured inter-CNT spacing values to deviate by as much as 13% from the spacing values predicted by the typically assumed hexagonal packing. VACNF analysis suggests that, by predicting an inter-fiber spacing that is within 6% of the reported value, the continuous coordination model outperforms both square and hexagonal packing in real nanowire arrays. Using this model, the average inter-wire spacing of nanowire arrays can be predicted, thus allowing more precise morphology descriptions, and thereby supporting the development of more accurate structure–property models of bulk materials comprised of aligned nanowires., Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (Contract W911NF-07-D-0004)

Author Manuscript 2011 July 22, Solid materials, such as silicon, glass, and polymers, dominate as structural elements in microsystems including microfluidics. Porous elements have been limited to membranes sandwiched between microchannel layers or polymer monoliths. This paper reports the use of micropatterned carbon-nanotube forests confined inside microfluidic channels for mechanically and/or chemically capturing particles ranging over three orders of magnitude in size. Nanoparticles below the internanotube spacing (80 nm) of the forest can penetrate inside the forest and interact with the large surface area created by individual nanotubes. For larger particles (>80 nm), the ultrahigh porosity of the nanotube elements reduces the fluid boundary layer and enhances particle–structure interactions on the outer surface of the patterned nanoporous elements. Specific biomolecular recognition is demonstrated using cells (≈10 μm), bacteria (≈1 μm), and viral-sized particles (≈40 nm) using both effects. This technology can provide unprecedented control of bioseparation processes to access bioparticles of interest, opening new pathways for both research and point-of-care diagnostics., National Institute of Biomedical Imaging and Bioengineering (U.S.) (Grant P41 EB002503), United States. Department of State (Fulbright Science and Technology Award)

Tailorable anisotropic intrinsic and scale-dependent properties of carbon nanotubes (CNTs) make them attractive elements in next-generation advanced materials. However, in order to model and predict the behavior of CNTs in macroscopic architectures, mechanical constitutive relations must be evaluated. This study presents the full stiffness tensor for aligned CNT-reinforced polymers as a function of the CNT packing (up to ∼20 vol. %), revealing noticeable anisotropy. Finite element models reveal that the usually neglected CNT waviness dictates the degree of anisotropy and packing dependence of the mechanical behavior, rather than any of the usually cited aggregation or polymer interphase mechanisms. Combined with extensive morphology characterization, this work enables the evaluation of structure-property relations for such materials, enabling design of aligned CNT material architectures., NECST Consortium, United States. Army Research Office (Contract No. W911NF- 07-D-0004), United States. Army Research Office (Contract No. W911NF-13-D-0001), United States. National Aeronautics and Space Administration (NASA Space Technology Research Fellowship Grant No. NNX11AN79H), National Science Foundation (U.S.) (Grant No. CMMI-1130437)

Non-destructive evaluation techniques can offer viable diagnostic and prognostic routes to mitigating failures in engineered structures such as bridges, buildings and vehicles. However, existing techniques have significant drawbacks, including poor spatial resolution and limited in situ capabilities. We report here a novel approach where structural advanced composites containing electrically conductive aligned carbon nanotubes (CNTs) are ohmically heated via simple electrical contacts, and damage is visualized via thermographic imaging. Damage, in the form of cracks and other discontinuities, usefully increases resistance to both electrical and thermal transport in these materials, which enables tomographic full-field damage assessment in many cases. Characteristics of the technique include the ability for real-time measurement of the damage state during loading, low-power operation (e.g. 15 °C rise at 1 W), and beyond state-of-the-art spatial resolution for sensing damage in composites. The enhanced thermographic technique is a novel and practical approach for in situ monitoring to ascertain structural health and to prevent structural failures in engineered structures such as aerospace and automotive vehicles and wind turbine blades, among others., Linda and Richard Hardy Fellowship, MIT-Spain/La Cambra de Barcelona

https://www.aiaa.org/ProceedingsDetail.aspx?id=5776, Vertically aligned carbon nanotubes (VACNTs) are placed between all plies in an aerospace carbon fiber reinforced plastic laminate (unidirectional plies, [(0/90/±45)2]s) to reinforce the interlaminar region in the z-direction. Significant improvement in Mode I and II interlaminar toughness have been observed previously. In this work, several substructural in-plane strength tests relevant to aerostructures were undertaken: bolt/tension-bearing, open hole compression, and L-shape laminate bending. Improvements are observed for the nanostitched samples: critical bearing strength by 30%, open-hole compression ultimate strength by 10%, and L-shape laminate energy (via increased deflection) of 40%. The mechanism of reinforcement is not compliant interlayer creation, but rather is a fiberstitching mechanism, as no increase in interlayer thickness occurs with the nanostitches. Unlike traditional (large-fiber/tow/pin) stitching or z-pinning techniques that damage inplane fibers and reduce laminate in-plane strengths, the nano-scale CNT-based ‘stitches’ improve in-plane strength, demonstrating the potential of such an architecture for aerospace structural applications. The quality of VACNT transfer to the prepreg laminates has not been optimized and therefore the noted enhancement to strength may be considered conservative. Ongoing work has been undertaken to both improve VACNT transfer and expand the data set., Massachusetts Institute of Technology (Nano-Engineered Composite aerospace STructures (NECST) Consortium)

Author Manuscript 2011 October 8, Recent advances in fabricating controlled-morphology vertically aligned carbon nanotubes (VA-CNTs) with ultrahigh volume fraction create unique opportunities for markedly improving the electromechanical performance of ionic polymer conductor network composite (IPCNC) actuators. Continuous paths through inter-VA-CNT channels allow fast ion transport, and high electrical conduction of the aligned CNTs in the composite electrodes lead to fast device actuation speed (>10% strain/second). One critical issue in developing advanced actuator materials is how to suppress the strain that does not contribute to the actuation (unwanted strain) thereby reducing actuation efficiency. Here, experiments demonstrate that the VA-CNTs give an anisotropic elastic response in the composite electrodes, which suppresses the unwanted strain and markedly enhances the actuation strain (>8% strain under 4 V). The results reported here suggest pathways for optimizing the electrode morphology in IPCNCs using ultrahigh volume fraction VA-CNTs to further enhanced performance., United States. Army Research Office (Grant W911NF-07-1-0452), National Institutes of Health (U.S.) (Grant R01-EY018387-02), United States. Multidisciplinary University Research Initiative