Laser-induced formation of nanocarbons with tunable morphology directly on polymers

This project focuses on a new laser-based approach for bottom-up growth of nanocarbons directly on polymers, which is an attractive alternative to printing technologies currently used for flexible device manufacturing. The goal of this project is study the fundamental process-structure-property relationships underlying the formation of laser-induced nanocarbons, dubbed LINC for short, on polyimide films using both CO2 and fiber lasers. This process has the potential to become a disruptive technology owing to the unique combination of high rate and small resolution, combined with the versatility of the types of carbons that can be created in a direct-write fashion, including nanofibers, nanosheets and nanoporous carbon

Collective Mechanochemical Growth of Carbon Nanotubes.

Hierarchically ordered carbon nanotubes (CNTs) are promising for integration in high-performance structural composites, electrical interconnects, thermal interfaces, and filtration membranes. These and other applications require CNTs that are monodisperse, well aligned, and densely packed. Moreover, because more than 1 billion CNTs per square centimeter grow simultaneously in a typical chemical vapor deposition (CVD) process, understanding the collective chemical and mechanical effects of growth is key to engineering the properties of CNT-based materials. This dissertation presents tailored synthesis processes, characterization techniques, and mathematical models that enable improved control of the morphology of as-grown CNT “forests.” First, a comprehensive characterization methodology, combining synchrotron X-ray scattering and attenuation with real-time height kinetics, enabled mapping the spatiotemporal evolution of CNT diameter distribution, alignment and density. By this method, the forest mass kinetics were measured and found to follow the S-shaped Gompertz curve of population growth. Dividing a forest into subpopulations revealed size-dependent activation-deactivation competition. Additionally, in situ transmission electron microscopy (TEM) showed that the kinetics of CNT nucleation are S-shaped. Based on these findings, a collective growth model is proposed, wherein randomly oriented CNTs first nucleate then self-organize and lift-off during a crowding stage, followed by a density decay stage until self-termination when the density drops below the self-supporting threshold. Next, further X-ray data analysis enabled modeling the mechanics of entangled CNTs and proved that mechanical coupling is not only responsible for the self-organization into the aligned morphology, but is also an important limiting mechanism as significant forces ensue from diameter-dependent CNT growth rates. A custom-built CVD system was used for mechanical manipulation of growing CNTs, leading to insights that external forces modulate the reaction kinetics. Last, a mathematical model describing the synergetic chemical coupling among growing CNT micropillars predicted height variations, and enables the design of CNT catalyst patterns for improved uniformity. The insights in this dissertation contribute to the fundamental understanding of self-organized CNT growth, enabling improved manufacturing and metrology. The models and techniques for studying population behavior of nanofilaments may also be applied to other systems, such as inorganic nanotubes, nanowires, and biofilaments.PhDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/107179/1/mbedewy_1.pd

Modular assembly of a protein nanotriangle using orthogonally interacting coiled coils

Synthetic protein assemblies that adopt programmed shapes would support many applications in nanotechnology. We used a rational design approach that exploits the modularity of orthogonally interacting coiled coils to create a self-assembled protein nanotriangle. Coiled coils have frequently been used to construct nanoassemblies and materials, but rarely with successful prior specification of the resulting structure. We designed a heterotrimer from three pairs of heterodimeric coiled coils that mediate specific interactions while avoiding undesired crosstalk. Non-associating pairs of coiled-coil units were strategically fused to generate three chains that were predicted to preferentially form the heterotrimer, and a rational annealing proc ess led to the desired oligomer. Extensive biophysical characterization and modeling support the formation of a molecular triangle, which is a shape distinct from naturally occurring supramolecular nanostructures. Our approach can be extended to design more complex nanostructures using additional coiled-coil modules, other protein parts, or templated surfaces

Biomechanical Root Cause Analysis of Complications in Head Immobilization Devices for Pediatric Neurosurgery

Precise and firm fixation of the cranium is critical during craniotomy and delicate brain neurosurgery making head immobilization devices (HIDs) a staple instrument in brain neurosurgical operations today. However, despite their popularity, there is no standard procedure for their use and many complications arise from using HIDs in pediatric neurosurgery. In this paper, we identify biomechanical causes of complications and quantify risks in pin-type HIDs including clamping force selection, positioning and age effects. Based on our root cause analysis, we develop a framework to address the biomechanical factors that influence complications and understand the biomechanics of the clamping process. We develop an age-dependent finite element model (FEM) of a single pin on a cranial bone disc with the representative properties and skull thickness depending on age. This model can be utilized to reduce risk of complications by design as well as to provide recommendations for current practices

Scaling the Stiffness, Strength, and Toughness of Ceramic‐Coated Nanotube Foams into the Structural Regime

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/108652/1/adfm201400851-sup-0001-S1.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/108652/2/adfm201400851.pd

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

Diameter-dependent kinetics of activation and deactivation in carbon nanotube population growth

A B S T R A C T We reveal that the collective growth of vertically aligned carbon nanotube (CNT) forests by chemical vapor deposition (CVD) is governed by the size-dependent catalytic behavior of metal nanoparticles, which can be quantitatively related to the activation and deactivation kinetics of subpopulations of CNTs within the forest. We establish this understanding by uniquely combining real-time forest height kinetics with ex situ synchrotron X-ray scattering and mass-attenuation measurements. The growing CNT population is divided into subpopulations, each having a narrow diameter range, enabling the quantification of the diameter-dependent population dynamics. We find that the mass kinetics of different subpopulations are self-similar and are represented by the S-shaped Gompertz model of population growth, which reveals that smaller diameter CNTs activate more slowly but have longer catalytic lifetimes. While competition between growth activation and deactivation kinetics is diameter-dependent, CNTs are held in contact by van der Waals forces, thus preventing relative slip and resulting in a single collective growth rate of the forest. Therefore, we hypothesize that mechanical coupling gives rise to the inherent tortuosity of CNTs within forests and possibly causes structural defects which limit the properties of current CNT forests in comparison to pristine individual CNTs. Ó 2012 Elsevier Ltd. All rights reserved. Introduction The size-dependent catalytic behavior of metal nanoparticles [1,2] influences a broad spectrum of technologically important processes, such as the reforming of hydrocarbon