Graphene consists of a monolayer of sp(2) bonded carbon atoms and has attracted considerable interest over recent years due to its extreme mechanical, electrical, and thermal properties. Graphene ...nanocomposites have naturally begun to be studied to capitalize upon these properties. A range of complex chemical and physical processing methods have been devised that achieve isolated graphene sheets that attempt to prevent aggregation. Here we demonstrate that the simple casting of a polymer solution containing dispersed graphene oxide, followed by thermal reduction, can produce well-isolated monolayer reduced-graphene oxide. The presence of single layer reduced-graphene oxide is quantitatively demonstrated through transmission electron microscopy and selected area electron diffraction studies and the reduction is verified by thermogravimetric, X-ray photoelectron spectroscopy, infrared spectrum, and electrical conductivity studies. These findings provide a simple, environmentally benign and commercially viable process to produce reduced-graphene oxide reinforced polymers without complex manufacturing, dispersion or reduction processes.
The ability to tailor interfacial shear strength for a particular fiber and resin system is critical to the development of composite materials that perform optimally in specific applications. One ...approach to tailor the interface is to introduce a secondary phase between the fiber and matrix, which can act to functionally grade the material properties and enhance load transfer across the interface. This approach has been applied in the past using nanowires, nanotubes, and whiskers and was demonstrated to significantly enhance interface performance. Unfortunately, these processes lack control over the interphase morphology to allow design of the interface for optimal properties. Recently, ZnO nanowires grown on the surface of carbon fibers have demonstrated more than a 110% increase in interfacial strength
1. Unlike other treatments, this interfacial reinforcement allows precise morphology control. Here, we develop the parameters for the growth of nanowires with varying lengths and diameters and study the influence of the nanowire’s morphology on the interfacial shear strength. ZnO nanowire arrays are grown on carbon fibers, with nanowire diameters ranging from 50 to 200
nm and lengths up to 4
μm. The interfacial shear strength with varying nanowire dimensions is shown to increase by up to 228%, ranging from 45.72 to 154.64
MPa. Unlike existing whiskerization approaches, it is shown that the tensile strength of the ZnO nanowire coated fibers remains constant throughout all growth procedures. The development of an interphase offering control over the interface strength and toughness will provide a means to produce multifunctional composites with optimized performance for multiple applications.
A novel functionalization method for aramid fibers is developed to enhance the bonding of a ZnO nanowire interphase grown on the fiber surface for interfacial strength enhancement. The nanowire ...interphase functionally grades the typically discrete interface and reduces the stress concentration between the fiber and matrix. The functionalization process is developed to improve the bonding between the ZnO nanowires and the aramid fiber and is validated through Fourier transform IR and X-ray photoelectron spectroscopy studies. Mechanical testing shows significant improvement in the interfacial shear strength with no decrease in the base fiber strength. This is the only technique found in the literature for the growth of a nanowire interphase on polymer fibers for structural enhancement without degrading the in-plane properties of the bulk composite. Furthermore, it is firmly shown that the functionalization process is a necessary condition for enhanced interfacial strength, demonstrating that ZnO nanowires strongly interact with carboxylic acid functional groups.
Composite materials can be enhanced by grafting a secondary material to a functional group on the surface of the reinforcing fibers to improve thermal, electrical or mechanical properties. Grafting ...secondary materials onto carbon fibers is often limited by the low reactivity of graphitic carbon and there is strong demand to create novel grafting methods with versatile functional groups. One desirable functional group is a carboxylic acid, which strongly interacts with many organic and inorganic materials. In this work, the surface of carbon fibers is functionalized by a reaction of naturally existing surface hydroxyl groups with isopropylidene malonate to graft terminal malonic esters, effectively creating a carboxyl functionalized surface. The reaction does not employ pre-oxidation to generate functional groups prior to grafting and is shown to preserve the tensile strength and morphology of the fiber. The surface functionalization is quantified by X-ray photoelectron spectroscopy, which shows that the relative surface coverage by carboxylic acid groups is increased from an initial 5.2% up to 9.2%. The effects of solvent, temperature, concentration and reaction time on the quantity of surface carboxylic acid groups are studied. This functionalization opens up new opportunities as a precursor reaction for further grafting reactions without sacrificing fiber strength.
For the first time, carbon nanotube (CNT) forests are fully characterized as transversely isotropic continuum material. Each of the five independent elastic constants is experimentally obtained using ...a combination of nanoindenter-based uniaxial compression and shear testing, in situ SEM compression, and digital image correlation (DIC) of vertically and laterally oriented CNT microstructure columns. Material properties are highly anisotropic, with an axial modulus (165–275MPa) that is nearly two orders of magnitude greater than the transverse modulus (2.5–2.7MPa) and the out of plane shear modulus (0.8–1.6MPa). The Poisson’s ratios along three mutually orthogonal axes, measured directly by simultaneous in situ DIC evaluation of axial and transverse strain, are found to be similarly anisotropic (ν12=0.35, ν23=0.1, ν21=0.005). A Timoshenko beam model is then developed to accurately predict the critical buckling stress of the vertically oriented columns using a subset of these anisotropic properties and considering inelastic column buckling. These results show that the critical bucking stress of CNT microstructures vary predictably with geometry and that continuum models with appropriate material constants may be applied to analyze CNT microstructures and evaluate their stability for many applications.
Artificial hair sensors consisting of a piezoresistive carbon‐nanotube‐coated glass fiber embedded in a microcapillary are assembled and characterized. Individual sensors resemble a hair plug that ...may be integrated in a wide range of host materials. The sensors demonstrate an air‐flow detection threshold of less than 1 m/s with a piezoresistive sensitivity of 1.3% per m/s air‐flow change.
Predicting thermal responses of composite materials requires accurate input parameters derived from reliable thermal property characterization and kinetic models. Composite material properties and ...decomposition kinetics vary with temperature and heating rate. Typically, conventional kinetic models derived from thermogravimetric analysis data result in multiple sets of kinetic model parameters, which are difficult to implement into numerical simulations under widely varying temperature and heating rate conditions. Here, a methodology was developed to reliably predict decomposition processes of composite materials with a single (i.e., unified) set of kinetic model parameters. The unified kinetic model parameters for each of four different composite materials were used to accurately predict decomposition kinetics observed over the entire range of experimental temperatures and heating rates. Furthermore, this broadly applicable methodology may predict decomposition from limited data sets, and is expected to extrapolate reliably measurable data to experimentally challenging heating rates and temperatures.
A self-assembly method is developed for creating a thin shell of multiwall carbon nanotubes on aramid fibers. The fibers show resistive behavior and exhibit a gauge factor of approximately 1.6, which ...is competitive with existing foil strain gauges. The robust sensing package could be used in the development of embedded strain sensors in multifunctional composites.
While the signal quality of recording neural electrodes is observed to degrade over time, the degradation mechanisms are complex and less easily observable. Recording microelectrodes failures are ...attributed to different biological factors such as tissue encapsulation, immune response, and disruption of blood-brain barrier (BBB) and non-biological factors such as strain due to micromotion, insulation delamination, corrosion, and surface roughness on the recording site (1-4). Strain due to brain micromotion is considered to be one of the important abiotic factors contributing to the failure of the neural implants. To reduce the forces exerted by the electrode on the brain, a high compliance 2D serpentine shaped electrode cable was designed, simulated, and measured using polyimide as the substrate material. Serpentine electrode cables were fabricated using MEMS microfabrication techniques, and the prototypes were subjected to load tests to experimentally measure the compliance. The compliance of the serpentine cable was numerically modeled and quantitatively measured to be up to 10 times higher than the compliance of a straight cable of same dimensions and material.
Spatial mapping of strain fields within compressed carbon nanotube (CNT) array columns is achieved using digital image correlation (DIC) analysis of in situ scanning electron microscopy (SEM) image ...sequences. Full‐field displacement and strain maps are generated based upon the motion of the constituent CNTs, which serve as a traceable high‐contrast speckle pattern for DIC analysis. The deformation modes and CNT array buckling characteristics vary systematically as a function of column aspect ratio, including bending, crushing, and bottom‐up buckle accumulation behaviors. In spite of disparate appearing deformation modes, strain maps indicate that CNT array buckling consistently initiates at 5% local principal strain (ϵ2) for all columns. The ability to quantitatively assess the deformation modes and buckling behavior of CNT arrays at the nanoscale will enable their improved design for high‐strain electrical contacts, compliant thermal interfaces, force sensors, energy‐absorbing foams, or other applications.
Spatial mapping of deformation fields within CNT array columns is achieved using digital image correlation of in situ scanning electron microscopy (SEM) compression sequences. Strain is highly non‐uniform, with buckle formation occurring in localized regions of enhanced strain. A local buckling criterion of 5% principal strain is observed for all columns despite systematic column deformation variation as a function of aspect ratio.
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