Browsing by Author "Yakobson, Boris I"

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    Computational Modeling of Growth and Mechanical Behavior of Carbon-based Nanomaterials
    (2018-02-15) Luo, Ming; Yakobson, Boris I
    The design of future nanoelectromechanical systems (NEMS) requires new materials which do not conform to classical scale material models. Carbon nanomaterials, exemplified by graphene, a single layer of graphite, and carbon nanotubes (CNTs), rolled up graphene nanoribbon, have attracted great interests due to their extraordinary electronic, thermal, and mechanical properties, promising a rich variety of applications in future NEMS. In this work, two main problems relating to the application of these carbon nanomaterials are addressed: synthesis (including nucleation and growth) and interface mechanical behavior. Firstly, we investigated the nucleation process of CNT caps on specific metal catalyst from both thermodynamic and kinetic perspective. Despite the minor effect of vertices of the metal catalyst on distribution of pentagons of caps, the main factor that determines the formation energy of caps is still the interface energy between cap edge and metal catalyst. Edge-etching “reverse engineering” reveals that the nucleation barrier maxima happens before complete matureness of caps, thus the possibility to control the chirality of synthesized CNT through cap-catalyst matching is subtle. Another issue that affects the production of CNTs is the growth speed. We utilized Markov chain theory to explain and analyze the phenomena such as growth speed, chirality change and the early termination due to emergence of defects during the growth stage. From the Markov chain theory we explored the effects of energy barrier, flux rate and temperature on the growth of CNTs. Two interface mechanical phenomena were investigated in the second part. It is disclosed that recoverable covalent cross links between CNTs or graphene sheets result in a nanoscale friction due to bond ruptures, which is logarithmically dependent on the shear rate and temperature. This friction plays an important role to strengthen CNT bundles or other composites. Oscillatory motion between bilayer graphene sheets was also explored through molecular dynamics (MD). The tunable gigahertz oscillator could be implemented into NEMS as nanooscillator. Another nanoscale friction in this system due to dissipation of kinetic energy to heat was investigated through both MD and theoretical analysis of adiabatic process, which is discovered to be linearly dependent on the motion velocity. These reveal that nanoscale friction is significantly different from macroscopic friction.
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    Computational Study of Carbon-Based Low-Dimensional Materials on Structures, Properties and Applications
    (2016-08-30) Liu, Mingjie; Yakobson, Boris I
    Low-dimensional materials including both 1D and 2D scenarios exhibit unique properties distinguished from their bulk states. In this thesis, computational modeling of low-dimensional materials on their structures, properties and applications has been investigated. First-principles simulations are employed to investigate the following topics. First of all in the 1D scenario, a comprehensive study on carbyne-one dimensional carbon chain-from its structure to properties has been conducted, and the extreme mechanical performance and intriguing metal-insulator transition under tension has been demonstrated. The properties of proposed 1D boron nanostructures have also been investigated and a constant-tension structural transition between two boron phases has been revealed. Secondly, two examples for the energy application of low-dimensional materials have been presented. The first example contains the energy storage with graphene and its derivatives applied in Li-ion batteries as well as the examination on the lithium nucleation process on graphene. The second example is the exploration of the energy conversion with N-doped carbon materials as effective catalysts in electrochemical reduction of CO2. Lastly, the simplified model- jellium model- has been applied in carbon nanotube growth. The termination effect and the chiral selectivity in CNT growth have been investigated.
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    Computationally Modeling Strenthening Mechanisms in Carbon Nanotube Composites and Bundles
    (2019-01-30) Alred, John Michael; Yakobson, Boris I
    Carbon nanotubes (CNT) have extraordinary mechanical properties, but to take advantage of these properties in composites, bundles, and ropes requires strong bonding to achieve significant CNT-CNT or CNT-matrix load transfer. This work is a computational study examining strengthening CNT composites and bundles on the quantum, atomistic, and meso- scales. Density functional theory (DFT) and classical molecular dynamics (MD) are used to evaluate methods to improve bonding CNT-matrix crosslinking by the inclusion of dopants, defects, functional groups, and curvature. DFT and MD are also used to quantify the load transfer of CNT-CNT sulfur crosslinks. A coarse-grained (CG) technique for modeling CNTs on the mesoscale is extended to include nonconservative frictional forces which are parameterized to model crosslinking. This extended CG model is then used to predict the mechanical performance of CNT bundles on a much larger scale. In addition to utilizing these traditional computational material science methods, a general approach is developed for applying machine learning (ML) to predict the ground state electron and energy density of an atomistic system.
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    Nanomaterials Thermal Response and CNT Reinforced Polymer Composites: An ab initio Study
    (2016-04-20) Tsafack Tsopbeng, Thierry Tsafack; Yakobson, Boris I
    Research achievements, both on nanomaterials thermal response and on reinforced polymer composites, are compendiously submitted. Each of the 4 chapters begins with (1) a terse summary of the context, methodology and keys results; segues into (2) the necessity for and the state-of-the-art on the subject with which it concerns itself; then proceeds with (3) the unique contribution the present findings make and the directions the research takes. Each chapter is self-contained and can be perused independently. Chapter 1 covers the relationship between the hole density of boron monolayers and their thermal as well as mechanical properties. The triangular boron sheet (δ6) is found to possess 2.06 and 6.60 times graphene’s lattice and electronic ballistic thermal conductances. Hexagonal sheets such as α and δ5 are predicted to be roughly twice as stiff as graphene. Chapter 2 covers the meaning of the Debye temperature for bulk and low dimensional materials. Two new approaches, one based on the polarization and mode dependent heat capacities and the other based on the mode dependent Debye temperatures, converge to a more precise computation and understanding of the polarization dependent Debye temperature. Chapter 3 covers the thermal properties of carbyne showing and discussing its room-temperature heat capacity at constant volume being 1.6 times that of graphene, a negative coefficient of thermal expansion being 4 times that of graphene, and a much higher thermal conductivity than graphene nanoribbons. Chapter 4 covers the interaction of carbon nanotubes (CNTs) with DGEBA epoxy thermosets for the purpose of identifying strengthening mechanisms. Among doped (Si, B, N), defective (Stone-Wales, three nitrogen atoms surrounding one monovacancy, four nitrogen atoms surrounding one divacancy, monovacancy), functionalized (amine, hydroxyl, carboxyl, oxygen) and different size CNTs, Si-doped, a combination of oxygen and hydroxyl as well as smaller tubes exhibit the strongest indication for mechanical reinforcement.