Mostafa Bedewy

Population Growth Dynamics of Carbon Nanotubes

Understanding the population growth behavior of filamentary nanostructures, such as carbon nanotubes (CNTs), is hampered by the lack of characterization techniques capable of probing statistical variations with high spatial resolution. We present a comprehensive methodology for studying the population growth dynamics of vertically aligned CNT forests, utilizing high-resolution spatial mapping of synchrotron X-ray scattering and attenuation, along with real-time height kinetics. We map the CNT alignment and dimensions within CNT forests, revealing broadening and focusing of size distributions during different stages of the process. Then, we calculate the number density and mass density of the CNT population versus time, which are true measures of the reaction kinetics. We find that the mass-based kinetics of a CNT population is accurately represented by the S-shaped Gompertz model of population growth, although the forest height and CNT length kinetics are essentially linear. Competition between catalyst activation and deactivation govern the rapid initial acceleration and slow decay of the CNT number density. The maximum CNT density (i.e., the overall catalyst activity) is limited by gas-phase reactions and catalyst-surface interactions, which collectively exhibit autocatalytic behavior. Thus, we propose a comprehensive picture of CNT population growth which combines both chemical and mechanical cooperation. Our findings are relevant to both bulk and substrate-based CNT synthesis methods and provide general insights into the self-assembly and collective growth of filamentary nanostructures.

Statistical analysis of variation in laboratory growth of carbon nanotube forests and recommendations for improved consistency

While many promising applications have been demonstrated for vertically aligned carbon nanotube (CNT) forests, lack of consistency in results (e.g., CNT quality, height, and density) continues to hinder knowledge transfer and commercialization. For example, it is well known that CNT growth can be influenced by small concentrations of water vapor, carbon deposits on the reactor wall, and experiment-to-experiment variations in pressure within the reaction chamber. However, even when these parameters are controlled during synthesis, we found that variations in ambient lab conditions can overwhelm attempts to perform parametric optimization studies. We established a standard growth procedure, including the chemical vapor deposition (CVD) recipe, while we varied other variables related to the furnace configuration and experimental procedure. Statistical analysis of 280 samples showed that ambient humidity, barometric pressure, and sample position in the CVD furnace contribute significantly to experiment-to-experiment variation. We investigated how these factors lead to CNT growth variation and recommend practices to improve process repeatability. Initial results using this approach reduced run-to-run variation in CNT forest height and density by more than 50%.

Measuring the lengthening kinetics of aligned nanostructures by spatiotemporal correlation of height and orientation

Owing to their inherent tortuosity, the collective height of vertically aligned nanostructures does not equal the average length of the individual constituent nanostructures, and therefore temporal height measurement is not an accurate measure of the genuine growth kinetics. We use high-resolution spatial mapping of alignment by small-angle X-ray scattering (SAXS) to transform real-time measurements of array height to the average length of the nanostructures. Applying this approach to carbon nanotube (CNT) forest growth transforms the kinetics from a sub-linear to a linear relationship with time, highlighting the potential for insights into the limiting growth mechanisms of CNTs and other one-dimensional nanostructures.

High-speed in situ X-ray scattering of carbon nanotube film nucleation and self-organization.

The production of high-performance carbon nanotube (CNT) materials demands understanding of the growth behavior of individual CNTs as well as collective effects among CNTs. We demonstrate the first use of grazing incidence small-angle X-ray scattering to monitor in real time the synthesis of CNT films by chemical vapor deposition. We use a custom-built cold-wall reactor along with a high-speed pixel array detector resulting in a time resolution of 10 msec. Quantitative models applied to time-resolved X-ray scattering patterns reveal that the Fe catalyst film first rapidly dewets into well-defined hemispherical particles during heating in a reducing atmosphere, and then the particles coarsen slowly upon continued annealing. After introduction of the carbon source, the initial CNT diameter distribution closely matches that of the catalyst particles. However, significant changes in CNT diameter can occur quickly during the subsequent CNT self-organization process. Correlation of time-resolved orientation data to X-ray scattering intensity and height kinetics suggests that the rate of self-organization is driven by both the CNT growth rate and density, and vertical CNT growth begins abruptly when CNT alignment reaches a critical threshold. The dynamics of CNT size evolution and self-organization vary according to the catalyst annealing conditions and substrate temperature. Knowledge of these intrinsically rapid processes is vital to improve control of CNT structure and to enable efficient manufacturing of high-density arrays of long, straight CNTs.

Local Relative Density Modulates Failure and Strength in Vertically Aligned Carbon Nanotubes

Micromechanical experiments, image analysis, and theoretical modeling revealed that local failure events and compressive stresses of vertically aligned carbon nanotubes (VACNTs) were uniquely linked to relative density gradients. Edge detection analysis of systematically obtained scanning electron micrographs was used to quantify a microstructural figure-of-merit related to relative local density along VACNT heights. Sequential bottom-to-top buckling and hardening in stress-strain response were observed in samples with smaller relative density at the bottom. When density gradient was insubstantial or reversed, bottom regions always buckled last, and a flat stress plateau was obtained. These findings were consistent with predictions of a 2D material model based on a viscoplastic solid with plastic non-normality and a hardening-softening-hardening plastic flow relation. The hardening slope in compression generated by the model was directly related to the stiffness gradient along the sample height, and hence to the local relative density. These results demonstrate that a microstructural figure-of-merit, the effective relative density, can be used to quantify and predict the mechanical response.

Decoupled Control of Carbon Nanotube Forest Density and Diameter by Continuous-Feed Convective Assembly of Catalyst Particles

The widespread potential application of vertically aligned carbon nanotube (CNT) forests have stimulated recent work on large-area chemical vapor deposition growth methods, but improved control of the catalyst particles is needed to overcome limitations to the monodispersity and packing density of the CNTs. In particular, traditional thin-film deposition methods are not ideal due to their vacuum requirements, and due to limitations in particle uniformity and density imposed by the thin-film dewetting process. Here, a continuous-feed convective self-assembly process for manufacturing uniform mono- and multi-layers of catalyst particles for CNT growth is presented. Particles are deposited from a solution of commercially available iron oxide nanoparticles, by pinning the meniscus between a blade edge and the substrate. The substrate is translated at constant velocity under the blade so the meniscus and contact angle remain fixed as the particles are deposited on the substrate. Based on design of the particle solution and tuning of the assembly parameters, a priori control of CNT diameter and packing density is demonstrated. Quantitative relationships are established between the catalyst size and density, and the CNT morphology and density. The roll-to-roll compatibility of this method, along with initial results achieved on copper foils, suggest promise for scale-up of CNT forest manufacturing at commercially relevant throughput.

Growth of primary motor neurons on horizontally aligned carbon nanotube thin films and striped patterns

OBJECTIVE Carbon nanotubes (CNTs) are attractive for use in peripheral nerve interfaces because of their unique combination of strength, flexibility, electrical conductivity and nanoscale surface texture. Here we investigated the growth of motor neurons on thin films of horizontally aligned CNTs (HACNTs). APPROACH We cultured primary embryonic rat motor neurons on HACNTs and performed statistical analysis of the length and orientation of neurites. We next presented motor neurons with substrates of alternating stripes of HACNTs and SiO2. MAIN RESULTS The neurons survived on HACNT substrates for up to eight days, which was the full duration of our experiments. Statistical analysis of the length and orientation of neurites indicated that the longest neurites on HACNTs tended to align with the CNT direction, although the average neurite length was similar between HACNTs and glass control substrates. We observed that when motor neurons were presented with alternating stripes of HACNTs and SiO2, the proportion of neurons on HACNTs increases over time, suggesting that neurons selectively migrate toward and adhere to the HACNT surface. SIGNIFICANCE The behavior of motor neurons on CNTs has not been previously investigated, and we show that aligned CNTs could provide a viable interface material to motor neurons. Combined with emerging techniques to build complex hierarchical structures of CNTs, our results suggest that organised CNTs could be incorporated into nerve grafts that use physical and electrical cues to guide regenerating axons.

Quasi-Exact-Constraint Design of Wind Turbine Gearing

In the past two decades the wind turbine industry has witnessed a considerable number of catastrophic accidents, many of which were due to gearbox failure. Ever increasing power ratings at decreased rotor speeds result in rotor torques of some million Nm. This imposes tooth loads and planet/pinion bearing loads on the order of a hundred tons within the first step-up stage. Such heavily loaded gearboxes, correctly (or rather innocently) designed according to the relevant codes, can be self-destructive. Due consideration should be given to the elastic environment in which the gears exist. Otherwise, appreciable, unsymmetrical/unequal elastic deformations in unwanted directions lead to gear tooth edge loading, in addition to overloading the bearing(s) near that edge. Designers of wind turbine gearing have in recent years identified several concepts and measures to be taken for counteracting the asymmetry of elastic deformations or mitigating their effects. In addition to giving a brief survey of such new design concepts, this paper suggests the use of selected types of curved-tooth cylindrical gears (so-called C-gears), primarily for their self-aligning capability; they allow four degrees of freedom (4-DOF), in contrast to the 3-DOF spur and helical gears and the 2-DOF double-helical gears. In addition, these gears offer a unique set of further advantages. When used in at least the most heavily loaded, first step-up stage, the design will be rendered quasi-exactly constrained; largely tolerant of misalignment due to elastic deformations, and the gearbox reliability should be improved, by design.Copyright

Real-Time Imaging of Self-Organization and Mechanical Competition in Carbon Nanotube Forest Growth

The properties of carbon nanotube (CNT) networks and analogous materials comprising filamentary nanostructures are governed by the intrinsic filament properties and their hierarchical organization and interconnection. As a result, direct knowledge of the collective dynamics of CNT synthesis and self-organization is essential to engineering improved CNT materials for applications such as membranes and thermal interfaces. Here, we use real-time environmental transmission electron microscopy (E-TEM) to observe nucleation and self-organization of CNTs into vertically aligned forests. Upon introduction of the carbon source, we observe a large scatter in the onset of nucleation of individual CNTs and the ensuing growth rates. Experiments performed at different temperatures and catalyst particle densities show the critical role of CNT density on the dynamics of self-organization; low-density CNT nucleation results in the CNTs becoming pinned to the substrate and forming random networks, whereas higher density CNT nucleation results in self-organization of the CNTs into bundles that are oriented perpendicular to the substrate. We also find that mechanical coupling between growing CNTs alters their growth trajectory and shape, causing significant deformations, buckling, and defects in the CNT walls. Therefore, it appears that CNT-CNT coupling not only is critical for self-organization but also directly influences CNT quality and likely the resulting properties of the forest. Our findings show that control of the time-distributed kinetics of CNT nucleation and bundle formation are critical to manufacturing well-organized CNT assemblies and that E-TEM can be a powerful tool to investigate the mesoscale dynamics of CNT networks.

Four degree of freedom liquid dispenser for direct write capillary self-assembly with sub-nanoliter precision.

Capillary forces provide a ubiquitous means of organizing micro- and nanoscale structures on substrates. In order to investigate the mechanism of capillary self-assembly and to fabricate complex ordered structures, precise control of the meniscus shape is needed. We present a precision instrument that enables deposition of liquid droplets spanning from 2 nl to 300 μl, in concert with mechanical manipulation of the liquid-substrate interface with four degrees of freedom. The substrate has sub-100 nm positioning resolution in three axes of translation, and its temperature is controlled using thermoelectric modules. The capillary tip can rotate about the vertical axis while simultaneously dispensing liquid onto the substrate. Liquid is displaced using a custom bidirectional diaphragm pump, in which an elastic membrane is hydraulically actuated by a stainless steel syringe. The syringe is driven by a piezoelectric actuator, enabling nanoliter volume and rate control. A quantitative model of the liquid dispenser is verified experimentally, and suggests that compressibility in the hydraulic line deamplifies the syringe stroke, enabling sub-nanoliter resolution control of liquid displacement at the capillary tip. We use this system to contact-print water and oil droplets by mechanical manipulation of a liquid bridge between the capillary and the substrate. Finally, we study the effect of droplet volume and substrate temperature on the evaporative self-assembly of monodisperse polymer microspheres from sessile droplets, and demonstrate the formation of 3D chiral assemblies of micro-rods by rotation of the capillary tip during evaporative assembly.