Allison M. Beese

Bio-Inspired Carbon Nanotube–Polymer Composite Yarns with Hydrogen Bond-Mediated Lateral Interactions

Polymer composite yarns containing a high loading of double-walled carbon nanotubes (DWNTs) have been developed in which the inherent acrylate-based organic coating on the surface of the DWNT bundles interacts strongly with poly(vinyl alcohol) (PVA) through an extensive hydrogen-bond network. This design takes advantage of a toughening mechanism seen in spider silk and collagen, which contain an abundance of hydrogen bonds that can break and reform, allowing for large deformation while maintaining structural stability. Similar to that observed in natural materials, unfolding of the polymeric matrix at large deformations increases ductility without sacrificing stiffness. As the PVA content in the composite increases, the stiffness and energy to failure of the composite also increases up to an optimal point, beyond which mechanical performance in tension decreases. Molecular dynamics (MD) simulations confirm this trend, showing the dominance of nonproductive hydrogen bonding between PVA molecules at high PVA contents, which lubricates the interface between DWNTs.

Extraordinary Improvement of the Graphitic Structure of Continuous Carbon Nanofibers Templated with Double Wall Carbon Nanotubes

Carbon nanotubes are being widely studied as a reinforcing element in high-performance composites and fibers at high volume fractions. However, problems with nanotube processing, alignment, and non-optimal stress transfer between the nanotubes and surrounding matrix have so far prevented full utilization of their superb mechanical properties in composites. Here, we present an alternative use of carbon nanotubes, at a very small concentration, as a templating agent for the formation of graphitic structure in fibers. Continuous carbon nanofibers (CNF) were manufactured by electrospinning from polyacrylonitrile (PAN) with 1.2% of double wall nanotubes (DWNT). Nanofibers were oxidized and carbonized at temperatures from 600 °C to 1850 °C. Structural analyses revealed significant improvements in graphitic structure and crystal orientation in the templated CNFs, with the largest improvements observed at lower carbonization temperatures. In situ pull-out experiments showed good interfacial bonding between the DWNT bundles and the surrounding templated carbon matrix. Molecular Dynamics (MD) simulations of templated carbonization confirmed oriented graphitic growth and provided insight into mechanisms of carbonization initiation. The obtained results indicate that global templating of the graphitic structure in fine CNFs can be achieved at very small concentrations of well-dispersed DWNTs. The outcomes reveal a simple and inexpensive route to manufacture continuous CNFs with improved structure and properties for a variety of mechanical and functional applications. The demonstrated improvement of graphitic order at low carbonization temperatures in the absence of stretch shows potential as a promising new manufacturing technology for next generation carbon fibers.

Key Factors Limiting Carbon Nanotube Yarn Strength: Exploring Processing-Structure-Property Relationships

Studies of carbon nanotube (CNT) based composites have been unable to translate the extraordinary load-bearing capabilities of individual CNTs to macroscale composites such as yarns. A key challenge lies in the lack of understanding of how properties of filaments and interfaces across yarn hierarchical levels govern the properties of macroscale yarns. To provide insight required to enable the development of superior CNT yarns, we investigate the fabrication-structure-mechanical property relationships among CNT yarns prepared by different techniques and employ a Monte Carlo based model to predict upper bounds on their mechanical properties. We study the correlations between different levels of alignment and porosity and yarn strengths up to 2.4 GPa. The uniqueness of this experimentally informed modeling approach is the models ability to predict when filament rupture or interface sliding dominates yarn failure based on constituent mechanical properties and structural organization observed experimentally. By capturing this transition and predicting the yarn strengths that could be obtained under ideal fabrication conditions, the model provides critical insights to guide future efforts to improve the mechanical performance of CNT yarn systems. This multifaceted study provides a new perspective on CNT yarn design that can serve as a foundation for the development of future composites that effectively exploit the superior mechanical performance of CNTs.

The desmoplakin/intermediate filament linkage regulates cell mechanics

Desmoplakin connects desmosomal core components to intermediate filaments at sites of cell–cell adhesion. Modulating the strength of this linkage using desmoplakin mutants led to alterations in cell–substrate and cell–cell forces and cell stiffness as assessed by micropillar arrays and atomic force microscopy. Perturbation of the actin cytoskeleton leads to abrogation of these effects.

In situ scanning electron microscope peeling to quantify surface energy between multiwalled carbon nanotubes and graphene.

Understanding atomic interactions between constituents is critical to the design of high-performance nanocomposites. Here, we report an experimental-computational approach to investigate the adhesion energy between as-produced arc discharge multiwalled carbon nanotubes (MWCNTs) and graphene. An in situ scanning electron microscope (SEM) experiment is used to peel MWCNTs from graphene grown on copper foils. The force during peeling is obtained by monitoring the deflection of a cantilever. Finite element and molecular mechanics simulations are performed to assist the data analysis and interpretation of the results. A finite element analysis of the experimental configuration is employed to confirm the applicability of Kendalls peeling model to obtain the adhesion energy. Molecular mechanics simulations are used to estimate the effective contact width at the MWCNT-graphene interface. The measured surface energy is γ = 0.20 ± 0.09 J·m(-2) or γ = 0.36 ± 0.16 J·m(-2), depending on the assumed conformation of the tube cross section during peeling. The scatter in the data is believed to result from an amorphous carbon coating on the MWCNTs, observed using transmission electron microscopy (TEM), and the surface roughness of graphene as characterized by atomic force microscopy (AFM).

Tailoring the Mechanical Properties of Carbon Nanotube Fibers

Abstract Performance and efficiency demands in industrial applications are pushing a need for carbon fibers that can outperform existing technologies. Fibers that incorporate carbon nanotubes (CNTs) to enhance specific mechanical properties are a promising route to addressing this need. Some of the major roadblocks to unlocking the full potential of macroscopic fibers based on CNTs are controlling and optimizing the shear interactions within and between CNTs, geometrical organization of the CNTs, and structural properties of the individual CNTs. Several approaches have been pursued in order to optimize the mechanical behavior of CNT fibers, including irradiation-induced covalent cross-linking, reformable or rehealable bonding, and optimized geometrical and structural fiber designs. These approaches are inspired by nature, which uses hierarchical bonding schemes in optimized orientations to tailor the mechanical properties of its materials to the needs and environment of specific organisms. In this chapter, these approaches for developing high-performance CNT fibers will be reviewed, and an outlook of their potential impact will be discussed.

Effect of directed energy deposition processing parameters on laser deposited Inconel® 718: External morphology

Through laser-based, directed energy deposition, single-track bead-on-plate clads of Inconel® 718 were deposited onto substrates of the same composition. Postprocessing analyses of the geometry of the single beads were made to assess the effects of changes in processing parameters. Laser power, travel speed, working distance, and initial substrate temperature were varied to alter the shape of the laser deposited material. The resulting geometries were analyzed through metallography and optical profilometry. This study concludes that laser power has the largest effect on bead width, and that working distance has the largest effect on bead height and angle of repose. Additionally, substrate preheating was found to amplify the effects of varying power on bead height and width. Empirical models were developed to describe the geometry of single beads based on chosen processing parameters. These models were compared to optical profilometry measurements for accuracy.

Effect of directed energy deposition processing parameters on laser deposited Inconel® 718: Microstructure, fusion zone morphology, and hardness

Single-bead, laser-deposited Inconel® 718 tracks atop substrates of the same composition were studied to ascertain the influence of laser power, processing speed, working distance, and substrate preheat on the fusion zone geometry, microstructure, and hardness. Modifying working distance encompassed both a change in powder flow distribution and beam diameter. Laser power and processing speed linearly affected fusion zone width and area, though laser power was found to have the most significant effect of all processing parameters. Preheating the substrates increased the width of the fusion zone by an average of 16% and led to a more uniform hardness throughout. The fusion zone cross-section was found to morph from semicircular to double-parabolic (wavy) with increasing laser power. This was attributed to surface tension induced Marangoni flow and the influence of surface-activated species on surface tension. The applicability of coupled parameters, including linear heat input and normalized enthalpy were i...

Absence of dynamic strain aging in an additively manufactured nickel-base superalloy

Dynamic strain aging (DSA), observed macroscopically as serrated plastic flow, has long been seen in nickel-base superalloys when plastically deformed at elevated temperatures. Here we report the absence of DSA in Inconel 625 made by additive manufacturing (AM) at temperatures and strain rates where DSA is present in its conventionally processed counterpart. This absence is attributed to the unique AM microstructure of finely dispersed secondary phases (carbides, N-rich phases, and Laves phase) and textured grains. Based on experimental observations, we propose a dislocation-arrest model to elucidate the criterion for DSA to occur or to be absent as a competition between dislocation pipe diffusion and carbide–carbon reactions. With in situ neutron diffraction studies of lattice strain evolution, our findings provide a new perspective for mesoscale understanding of dislocation–solute interactions and their impact on work-hardening behaviors in high-temperature alloys, and have important implications for tailoring thermomechanical properties by microstructure control via AM.Detrimental serrated plastic flow via dynamic strain aging (DSA) in conventionally processed nickel superalloys usually occurs during high temperature deformation. Here, the authors suppress DSA via a unique microstructure obtained using additive manufacturing and propose a new dislocation-arrest model in nickel superalloys.

Microstructure and Mechanical Properties of AM Builds

Abstract Additively manufactured metal components are subjected to heat and melting cycles that are vastly different from more conventional manufacturing processes whether cast, welded, or wrought. The selective melting of a small amount of material on a larger, relatively cool body, leads to solidification rates that are hundreds to thousands of time faster than observed in casting. This rapid solidification results in much different microstructures for AM parts versus traditional manufacturing, which in turn, results in differing mechanical properties for AM components. This study describes the microstructure, hardness, yield and ultimate tensile strengths, ductility, and fatigue strength of AM Inconel 625, Ti-6Al-4V, and Austenitic stainless steel. Comparisons are made for both microstructures and properties between traditional and additively manufactured components. Discussion is provided on the challenges of linking processing, microstructure, and the resulting material properties, and the high variation of AM properties that may occur even when using identical materials, machines, and processing parameters.