Morphology and processing of aligned carbon nanotube carbon matrix nanocomposites

Intrinsic and scale-dependent properties of carbon nanotubes (CNTs) have led aligned CNT architectures to emerge as promising candidates for next-generation multifunctional applications. Enhanced operating regimes motivate the study of CNT-based aligned nanofiber carbon matrix nanocomposites (CNT A-CMNCs). However, in order to tailor the material properties of CNT A-CMNCs, porosity control of the carbon matrix is required. Such control is usually achieved via multiple liquid precursor infusions and pyrolyzations. Here we report a model that allows the quantitative prediction of the CNT A-CMNC density and matrix porosity as a function of number of processing steps. The experimental results indicate that the matrix porosity of A-CMNCs comprised of ∼1% aligned CNTs decreased from ∼61% to ∼55% after a second polymer infusion and pyrolyzation. The model predicts that diminishing returns for porosity reduction will occur after 4 processing steps (matrix porosity of ∼51%), and that >10 processing steps are required for matrix porosity <50%. Using this model, prediction of the processing necessary for the fabrication of liquid precursor derived A-CMNC architectures, with possible application to other nanowire/nanofiber systems, is enabled for a variety of high value applications.National Science Foundation (U.S.) (Grant CMMI-1130437

Aligned carbon nanotube carbon matrix nanocomposites

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2013.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (p. 129-141).Materials comprising carbon nanotube (CNT) aligned nanowire (NW) polymer nanocomposites (A-PNCs) have emerged as promising architectures for next-generation multifunctional applications. Enhanced operating regimes, such as operating temperatures, motivate the study of CNT aligned NW ceramic matrix nanocomposites (A-CMNCs). Here we report the synthesis of CNT A-CMNCs through the pyrolysis of CNT A-PNC precursors, creating carbon matrix CNT A-CMNCs. The CNT A-CMNC processing parameters were evaluated using an apparent density measurement, polymer re-infusion modeling, and CNT quality analysis, which elucidate the limitations of the processing parameters currently used to fabricate CNT A-CMNCs. Theoretical tools developed to help quantify and analyze the morphology of the CNTs in the A-CMNCs, and NWs in general, show that morphological parameters, such as NW outer diameter and inter-wire spacing, that are usually overlooked may have significant effects on the physical properties of NW architectures. Mechanical characterization of the CNT A-CMNCs illustrates that the presence of aligned CNTs can lead to an enhancement of > 60% in microhardness, meaning that the fabrication of high strength, high temperature, lightweight next-generation material architectures may be possible using the presented method. Finally, factors that influence the physical properties of CNT A-CMNCs, such as CNT waviness and the porosity of the carbon matrix, are identified, and since their effects cannot be modeled using existing theory, future paths of study that could enable their quantification are recommended.by Itai Y. Stein.S.M

Process-morphology scaling relations quantify self-organization in capillary densified nanofiber arrays

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

Exohedral Physisorption of Ambient Moisture Scales Non-monotonically with Fiber Proximity in Aligned Carbon Nanotube Arrays

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.National Science Foundation (U.S.) (NSF Grant No. CMMI-1130437)National Science Foundation (U.S.) (NSF Award Number ECS-0335765)United States. Army Research Office (contract W911NF-07-D-0004

OUT-OF-OVEN CURING OF POLYMERIC COMPOSITES VIA RESISTIVE MICROHEATERS COMPRISED OF ALIGNED CARBON NANOTUBE NETWORKS

The broader adoption of composite materials in next-generation aerospace architectures is currently limited by the geometrical constraints and high energy costs of traditional manufacturing techniques of PMCs such as autoclave and vacuum-bag-only oven curing techniques. Here, an in situ curing technique for PMCs using a resistive heating film comprised of an aligned carbon nanotube (A-CNT) network is presented. A carbon fiber reinforced plastic (CFRP) prepreg system is effectively cured via a single-side CNT network heater incorporated on the outer surface of the laminate without using an autoclave. Evaluation of the curing efficacy shows that composites cured by A-CNT film heaters can achieve degrees of cure that are equivalent or better than composites cured by an autoclave. This manufacturing technique enables highly efficient curing of PMCs while adding multifunctionality to finished composites.United States. Army Research Office (contract W911NF-07-D-0004)United States. Army Research Office (contract W91NF-13-D-0001)Kwanjeong Educational Foundation (Korea)National Defense Science and Engineering Graduate (NDSEG) FellowshipConselho Nacional de Pesquisas (Brazil) (Science without Borders Program)United States. Naval Sea Systems Command (contract N00024-12-P-4069 for SBIR topic N121-058

Impact of carbon nanotube length on electron transport in aligned carbon nanotube networks

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× 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 CNT-CNT junctions an electron must tunnel through during its percolated path, which is proportional to the morphology of the aligned CNT network.United States. Army Research Office (contract W911NF-07-D-0004)United States. Army Research Office (contract W911NF-13-D-0001)United States. Air Force Office of Scientific Research (AFRL/RX contract FA8650-11-D-5800, Task Order 0003)National Science Foundation (U.S.) (NSF Award No. ECS-0335765)United States. Dept. of Defense (National Defense Science and Engineering Graduate Fellowship

Three-dimensional elastic constitutive relations of aligned carbon nanotube architectures

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 ConsortiumUnited 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

Impact of morphology and confinement effects on the properties of aligned nanofiber architectures

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2016.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 151-168).The intrinsic and scale-dependent properties of nanofibers (NFs), nanowires, and nanotubes have made them the focus of many application-specific nanostructured materials studies. However, various NF morphology and proximity effects can lead to > 1000x reductions in the performance of NF-based material architectures, such as bulk materials and structures comprised of scalable aligned NF arrays. The physical and chemical origins of these effects, along with the concomitant structure property mechanisms of materials comprised of aligned NFs, are not currently known and cannot be properly integrated into existing theories. This originates, in part, from an incomplete understanding of the morphology of real NF systems, particularly in three-dimensions. Through experiments, theory, and multi-scale simulation, this dissertation presents a framework capable of modeling the stochastic 3D morphology of a relevant NF system, carbon nanotubes (CNTs), assembled into aligned CNT (A-CNT) arrays. New descriptions of the multi-wall A-CNT morphology demonstrate that the CNT tortuosity, quantified via sinusoidal amplitude-wavelength waviness ratio (w), decreases significantly from w ~/~ 0:2 to 0:1 as the CNT volume fraction (Vf) is increased from Vf ~ 1 to 20%. Using these new relations, a 3D stochastic morphology description is presented, and used to quantify the mechanical behavior of A-CNT arrays, A-CNT polymer matrix nanocomposites (A-PNCs), and A-CNT carbon matrix nanocomposites (A-CMNCs) via a mechanics analysis that was previously applied to carbon nanocoils. Focusing on deformations in the A-CNT axial reinforcement direction, torsion and shear deformation mechanisms, which are governed by the low ( 90% in the experimental A-CNT w regime, and are inferred to be the physical mechanisms responsible for the previously observed ~ 100x increase in the ACNT effective indentation modulus as Vf is increased from ~ 1 to 20%. In the case of A-PNCs, the polymer matrix effectively eliminates the torsion compliance contribution, so that the observed ~ 2x enhancement in the effective axial elastic modulus of A-PNCs as Vf is increased from ~ 1 to 20% is explained. The geometry of the graphitic crystallites that comprise the pyrolytic carbon (PyC) matrix of A-CMNCs is found to not evolve significantly at pyrolysis temperatures of 1000 to 1400°C, and crystallite size estimates from Raman spectroscopy reveal that the Tuinstra-Koenig correlation disagrees with the sizes measured by x-ray diffraction, suggesting a new amorphization transition crystallite size of 6 nm instead of 2-3 nm. In the case of A-CMNCs, CNT reinforcement is shown to lower the energy barrier (inferred through the pyrolysis temperature) for meso-scale self-organization of the graphitic crystallites of the PyC matrix, while having no effect on the PyC matrix on the atomic scale. Mechanical property analysis and modeling indicates that the aerospace materials selection criterion of the A-CMNCs can be enhanced to >8 GPa x (g/cm3)-2 at Vf >20% (experimentally we observe a value of ~ 5 GPa x (g/cm3)-2 at Vf ~ 10%). A-CMNCs introduced in this work have the potential to outperform state-of-the-art superhard materials, such as diamond (~/~ 7:8 GPa x (g/cm3)-2) and cubic boron nitride (~/~ 5:2 GPa x (g/cm3)-2). Using the structure-property prediction tools developed in this thesis, precise tailoring and prediction of application-specific performance of aligned NF based architectures is enabled, and specific new understanding of A-CNT systems is established. Future paths of study that enable the design and manufacture of several classes of next-generation materials are recommended.by Itai Y. Stein.Ph. D

Coordination number model to quantify packing morphology of aligned nanowire arrays

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