Browsing by Subject "Carbon Nanotube"
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Item Atomic-scale investigations of multiwall carbon nanotube growth.(2010-06) Behr, Michael JohnThe fundamental processes of carbon nanotube (CNT) growth by plasma-enhanced chemical vapor deposition (PECVD) were investigated using a suite of characterization techniques, including attenuated total-reflection Fourier transform infrared spectroscopy (ATR-FTIR), optical emission spectroscopy (OES), Raman spectroscopy, convergent-beam electron diffraction (CBED), high-resolution transmission and scanning-transmission electron microscopy (TEM, STEM), energy dispersive x-ray spectroscopy, and electron energy-loss spectroscopy (EELS). It is found that hydrogen plays a critical role in determining the final CNT structure through controlling catalyst crystal phase and morphology. At low hydrogen concentrations in the plasma iron catalysts are converted to Fe3C, from which high-quality CNTs grow; however, catalyst particles remain as pure iron when hydrogen is in abundance, and produce highly defective CNTs with large diameters. The initially faceted and equiaxed catalyst nanocrystals are deformed by the surrounding CNT structure during growth. Although catalyst particles are single crystalline, they exhibit combinations of small-angle (~1-3 degree) rotations, twists, and bends along their axial length between adjacent locations. Fe3C catalyst nanoparticles that are located inside the base of well-graphitized CNTs of similar structure and diameter do not exhibit a preferred orientation relative to the nanotube axis, indicating that the graphene nanotube walls are not necessarily produced in an epitaxial process directly from Fe3C faces. Chemical processes occurring at the catalyst-CNT interface during growth were inferred by measuring, ex situ, changes in atomic bonding at an atomic scale with EELS. The observed variation in carbon concentration through the base of catalyst crystals reveals that carbon from the gas phase decomposes on Fe3C, near where the CNT walls terminate at the catalyst base. An amorphous carbon-rich layer at the catalyst base provides the source for CNT growth. These results suggest that what is required for CNT growth is a graphene seed and a source of decomposed carbon. Hydrogen atoms also interact with the graphene walls of CNTs. When the flux of H atoms is high, the continuous cylindrical nanotube walls are etched nonuniformly. Etch pits form at defective sites along the CNT, from which etching proceeds rapidly. It is determined that H etching occurs preferentially at graphene edges.Item Mesoscopic Distinct Element Method for Computational Design of Carbon Nanotube Materials(2017-07) Wang, YuezhouCarbon Nanotubes (CNTs) are hollow molecular cylinders conceptually formed by rolling single or multiple layers of graphene into tubes. CNT materials have become an attractive research subject during the last decades owning to the superior mechanical and electronic properties of individual CNTs. Developing applications, such as structural materials, supercapacitors, batteries or nanomechanical devices, depend on our ability to understand, model, and design the structure and properties of realistic CNT assemblies. Toward this goal, here we have applied a recently developed mesoscale computational method, titled the mesoscopic distinct element method (MDEM) that makes it possible to simulate the formation, stability, and mechanics of CNT aggregates and ultrathin CNT films. We first combine experiments and distinct element method simulations to understand the stability of rings and rackets formed by single-walled carbon nanotubes assembled into ropes. The obtained agreement validates MDEM and indicates that the stability of the experimental aggregates can be largely explained by the competition between bending and van der Waals adhesion energies. Next, we have considered the geometry and internal packing in twisted CNT ropes. Compared to the state of the art, MDEM accounts in a computationally tractable manner for both the deformation of the fiber and the distributed van der Waals cohesive energy between fibers. These features enable us to investigate the torsional response in a new regime where the twisted rope develops packing rearrangements and aspect-ratio-dependent geometric nonlinearities, in agreement with phenomenological models. Finally, we have performed MDEM simulations and developed an atomic-scale picture of the CNT network stress relaxation. On this basis, we put forward the concept of mesoscale design by the addition of excluded-volume interactions. Silicon nanoparticles are integrated into the model and the nanoparticle-filled networks present superior stability and mechanical response relative to those of pure films. The approach opens new possibilities for tuning the network microstructure in a manner that is compatible with flexible electronics applications. As a distinct direction, MDEM was explored for modeling the mechanics of nanocrystalline particles. Simulations that rely on the fitting of the peak stress, strain, and failure mode on the experimental testing of Au and CdS hollow nanocrystalline particles illustrate the promising potential of MDEM for bridging the atomistic-scale simulations with experimental testing data.