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Polymer matrix nanocomposites (PNCs) incorporating high volume fractions (Vf in excess of 10 vol %) of aligned carbon nanotubes (A-CNTs) are promising for high-performance structural composite applications leveraging texture for multifunctionality and performance-to-weight ratios. However, to enable the manufacturing of scalable structures using A-CNT PNCs, nanoscale confinement and interfacial effects due to high A-CNT content in aerospace-grade polymer matrices need to be better understood. Here, we report the model-informed fabrication of high-Vf CNT PNCs to develop process–structure–property relationships, including a scaled film and laminate technique for A-CNT polymer laminates and the fabrication of microvoid-free and fully infused bis-(maleimide) (BMI) and epoxy PNCs with high packing densities (or Vf) of biaxially mechanically densified millimeter-tall A-CNT array reinforcement (1–30 vol % corresponding to the average inter-CNT spacings of ∼70 to 6 nm). A polymer infusion model developed from Darcy's law accurately predicts the time for resin to infuse into CNT arrays during capillary-assisted PNC processing, corroborated by experimental observations via X-ray microcomputed tomography and scanning electron microscopy showing that a diluted resin with ∼10× lower viscosity than a neat resin is required to obtain complete infusion into high-Vf A-CNT arrays (10–30 vol %). For each tested A-CNT volume percent, the cured PNCs maintain vertical CNT alignment and glass transition temperature, and the decomposition onset temperature remains constant for epoxy PNCs but increases by ∼8 °C for 30 vol % A-CNT–BMI PNCs compared to the neat resin. For both polymer matrix systems, a ∼2× increase in the axial indentation modulus for 30 vol % A-CNT PNCs compared to that of a neat resin is measured, and no significant change in the transverse A-CNT modulus is shown experimentally and via modeling, indicating that reinforcement with A-CNTs at higher Vf values leads to enhanced anisotropic mechanical properties. Through the process–structure–property scaling relationships established here, this work supports the development of next-generation structures comprised of nanomaterials with enhanced performance and manufacturability. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Polymer-derived carbon ceramics are highly desirable for lightweight, high strength, extreme environment material architectures, but their mechanical performance as a function of structure and processing is not currently understood and cannot be predicted. In this study, the mechanical behavior for bulk-scale pyrolytic carbons (PyCs) made via polymer pyrolysis of phenol-formaldehyde precursors is established as a function of heat treatment temperature and the resulting average nano- and meso-scale order and disorder via X-ray diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy. The PyCs exhibit crystallite evolution on both the atomic- and meso-scale for pyrolysis temperatures of 600 °C to 1000 °C, whereas only atomic-scale crystallite evolution is observed for pyrolysis temperatures of 1000 °C to 1400 °C. The measured Vickers hardness of the PyCs is observed to scale non-monotonically as a function of the pyrolysis temperature reaching a peak at -4 GPa for samples prepared at 1000 °C. New modeling results, based on the elastic constants of disordered graphite, indicate that this counter-intuitive Vickers hardness scaling, which is a decades-old open question, originates from the PyC inter-layer shear elastic constant and the crystallite aspect ratio evolution with processing temperature. PyCs studied here are shown to be the lightest super-hard materials, having Vickers hardness-to-density ratios that are comparable to super-hard carbides, oxides, nitrides, and phosphides. [ABSTRACT FROM AUTHOR]

Existing theories for quantifying the morphology of nanofibers (NFs) in aligned arrays either neglect or assume a simple functional form for the curvature of the NFs, commonly known as the NF waviness. However, since such assumptions cannot adequately describe the waviness of real NFs, errors that can exceed 10% in the predicted inter-NF separation can result. Here we use a theoretical framework capable of simulating ＞105 NFs with stochastic three-dimensional morphologies to quantify NF waviness on an easily accessible measure of the morphology, the inter-NF spacing, for a range of NF volume fractions. The presented scaling of inter-NF spacing with waviness is then used to study the morphology evolution of aligned carbon nanotube (A-CNT) arrays during packing, showing that the effective two-dimensional coordination number of the A-CNTs increases much faster than previously reported during close packing, and that hexagonal close packing can successfully describe the packing morphology of the A-CNTs at volume fractions greater than 40 vol%. [ABSTRACT FROM AUTHOR]

Here, we quantify the electron transport properties of aligned carbon nanotube (CNT) networks as a function of the CNT length, where the electrical conductivities may be tuned by up to 10 x with anisotropies exceeding 40%. Testing at elevated temperatures demonstrates that the aligned CNT networks have a negative temperature coefficient of resistance, and application of the fluctuation induced tunneling model leads to an activation energy of ≈14 meV for electron tunneling at the CNT-CNT junctions. Since the tunneling activation energy is shown to be independent of both CNT length and orientation, the variation in electron transport is attributed to the number of CNTCNT junctions an electron must tunnel through during its percolated path, which is proportional to the morphology of the aligned CNT network. [ABSTRACT FROM AUTHOR]

The effect of nanofiber proximity on the mechanical behavior of nanofiber arrays with volume fractions (Vf) from 1% to 20% was quantified via nanoindentation of an aligned carbon nanotube (A-CNT) array. The experimental results show that the indentation modulus for A-CNT arrays has a highly non-linear scaling with the CNT Vf, leading to modulus enhancements of up to ∼600× at Vf=20%. Modeling illustrates that the origin of the highly non-linear trend with Vf is due to the minimum inter-CNT spacing, which is shown to be more than an order of magnitude larger than the graphitic spacing. [ABSTRACT FROM AUTHOR]

• Measurements of thermal diffusivity and conductivity of single-walled carbon nanotube (SWCNT) forests with biaxial mechanical densification. • Anomalous increase of thermal diffusivity with increase in volume fraction beyond threshold. • New thermal conduction path for densified forest of single-walled carbon nanotubes. Needs of a material for thermal management in reduced-sized electronic devices drive single-walled carbon nanotubes (SWCNTs) to be one of the most promising candidates due to their excellent thermal properties. Many numerical and experimental studies have reported on understanding thermal properties of the SWCNT for thermal device applications. In the present study, thermal diffusivity and conductivity of SWCNT forests in the axial direction with various volume fractions of SWCNTs were measured by laser flash analysis technique and the same properties of an individual SWCNT were derived. The volume fraction was controlled up to 25 vol% by biaxial mechanical densification which maintains SWCNT alignment. While the thermal diffusivity of SWCNT forests was almost constant, it increased when the volume fraction was higher than 17 vol%, suggesting that some SWCNTs, which did not serve as a thermal path in the case of lower volume fraction, were connected with the other SWCNTs. Through the increase of inferred thermal conductivity equivalent to an individual SWCNT after 17 vol%, we also realized that the enhancement of volume fraction of SWCNT forest diminished thermal boundary resistance in good agreement with the tendency inferred from the reported numerical data. [ABSTRACT FROM AUTHOR]

Here, we report the fabrication of aligned carbon nanotube (A-CNT)/conducting polymer (CP) heterostructures with both uniform conformal and periodic beaded polymer morphologies via oxidative chemical vapor deposition of poly(ethylenedioxythiophene). Periodic beaded CP morphologies are realized utilizing the Plateau-Rayleigh instability to transform the original uniform conformal film, yielding a beaded CP morphology with a >50% enhancement in specific surface area (SSA). Modeling indicates that this SSA increase originates from the internal volume of the A-CNTs becoming available for adsorption, and that these internal A-CNT surfaces, if they could be made accessible to electrolyte ions, could lead to >30% enhancement of specific gravimetric and volumetric capacitances of current state-of-the-art A-CNT/CP heterostructures.

Vertically aligned one-dimensional nanostructure arrays are promising in many applications such as electrochemical systems, solar cells, and electronics, taking advantage of high surface area per unit volume, nanometer length scale packing, and alignment leading to high conductivity. However, many devices need to optimize arrays for device performance by selecting an appropriate morphology. Developing a simple, non-invasive tool for understanding the role of pore volume distribution and interspacing would aid in the optimization of nanostructure morphologies in electrodes. In this work, we combined electrochemical impedance spectroscopy (EIS) with capacitance measurements and porous electrode theory to conduct in situ porosimetry of vertically aligned carbon nanotube (VA-CNT) forests non-destructively. We utilized the EIS measurements with a pore size distribution model to quantify the average and dispersion of inter-CNT spacing (Γ), stochastically, in carpets that were mechanically densified from [Formula: see text] tubes cm -2 to [Formula: see text] tubes cm -2 . Our analysis predicts that the inter-CNT spacing ranges from over 100 ± 50 nm in sparse carpets to sub 10 ± 5 nm in packed carpets. Our results suggest that waviness of CNTs leads to variations in the inter-CNT spacing, which can be significant in sparse carpets. This methodology can be used to predict the performance of many nanostructured devices, including supercapacitors, batteries, solar cells, and semiconductor electronics.

The promise of enhanced and tailored properties motivates the study of one-dimensional nanomaterials, especially aligned carbon nanotubes (A-CNTs), for the reinforcement of polymeric materials. While CNTs have remarkable theoretical properties, previous work on aligned CNT polymer matrix nanocomposites (A-PNCs) reported mechanical properties that are orders of magnitude lower than those predicted by rule of mixtures. This large difference primarily originates from the morphology of the CNTs, because the CNTs that comprise the A-PNCs have significant local curvature commonly referred to as waviness. Here we present a simulation framework capable of analyzing 10(5) wavy CNTs with realistic three-dimensional morphologies to quantify the impact of waviness on the effective elastic modulus contribution of wavy CNTs. The simulation results show that due to the low shear modulus of the reinforcing CNT 'fibers', and large ([Formula: see text]) compliance contribution of the shear deformation mode, waviness reduces the effective stiffness contribution of the A-CNTs by two to three orders of magnitude. Also, the mechanical property predictions resulting from the simulation framework outperform those previously reported using finite element analysis since representative descriptions of the morphology are required to accurately predict properties of the A-PNCs. Further work to quantify the morphology of A-PNCs in three-dimensions, simulate their full non-isotropic constitutive relations, and predict their failure mechanisms is planned.

The energy losses and geometric constraints associated with conventional curing techniques of polymeric systems motivate the study of a highly scalable out-of-oven curing method using a nanostructured resistive heater comprised of aligned carbon nanotubes (A-CNT). The experimental results indicate that, when compared to conventional oven based techniques, the use of an "out-of-oven" A-CNT integrated heater leads to orders of magnitude reductions in the energy required to process polymeric layered structures such as composites. Integration of this technology into structural systems enables the in situ curing of large-scale polymeric systems at high efficiencies, while adding sensing and control capabilities.

Here we present a study on the presence of physisorbed water on the surface of aligned carbon nanotubes (CNTs) in ambient conditions, where the wet CNT array mass can be more than 200% larger than that of dry CNTs, and modeling indicates that a water layer >5 nm thick can be present on the outer CNT surface. The experimentally observed nonlinear and non-monotonic dependence of the mass of adsorbed water on the CNT packing (volume fraction) originates from two competing modes. Physisorbed water cannot be neglected in the design and fabrication of materials and devices using nanowires/nanofibers, especially CNTs, and further experimental and ab initio studies on the influence of defects on the surface energies of CNTs, and nanowires/nanofibers in general, are necessary to understand the underlying physics and chemistry that govern this system.

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.