Andre R. Muniz

Elastic properties of graphene nanomeshes

We study the elastic response of graphene nanomeshes based on molecular-statics and molecular-dynamics simulations of uniaxial tensile deformation tests. Elastic properties are determined as a function of the nanomesh architecture, namely, the lattice arrangement of the pores, pore morphology, material density (ρ), and pore edge passivation, and scaling laws for the density dependence of the elastic modulus M, M(ρ), are established. We find that, for circular unpassivated pores, M scales with the square of ρ. Deviations from quadratic scaling are most strongly influenced by pore morphology and, to a lesser extent, by pore edge passivation and temperature.

Effects of hydrogen chemisorption on the structure and deformation of single-walled carbon nanotubes

We report results of molecular-dynamics simulations for the effects of atomic hydrogen chemisorption on the structure and deformation of single-walled carbon nanotubes. Upon hydrogenation, the nanotubes expand and the degree of expansion depends on the hydrogen coverage. There is a critical hydrogen coverage, ∼25%–30%, that marks the onset of a structural transition associated with the sp2-to-sp3 bonding transition: at lower-than-critical coverage, sp2 C–C bonding dominates and nanotube swelling is negligible; at higher-than-critical coverage, however, sp3 C–C bonding dominates and radial and axial strains increase monotonically with coverage. This behavior is independent of nanotube chirality and diameter and of temperature.

On the hydrogen storage capacity of carbon nanotube bundles

An analytical model is presented to describe the effect of carbon nanotube (CNT) swelling upon hydrogenation on the hydrogen storage capacity of single-walled CNT bundles; the model is properly parameterized using atomistic calculations for the relationship between CNT swelling and the degree of hydrogenation as measured by the coverage of the CNTs by chemisorbed atomic H. The model generates experimentally testable hypotheses, which can be used to explain the lower H storage capacities reported for CNT bundles and the experimentally observed nonuniformity of hydrogenation of CNT bundles.

Hydrogenation effects on the structure and morphology of graphene and single-walled carbon nanotubes

A systematic computational study is presented of the effects of atomic hydrogen chemisorption on the structure and morphology of graphene layers and single-walled carbon nanotubes (SWCNTs). The study is based on a combination of classical molecular-dynamics (MD) and Monte Carlo simulations of structural and compositional relaxation of the hydrogenated surfaces, employing hydrogen distributions consistent with experimental observations and first-principles calculations. Results are reported for the strains induced on the graphene and the SWCNTs, as a result of sp2-sp3 bonding transitions due to atomic H chemisorption, and their dependence on the H surface coverage, Θ, over the entire range 0≤Θ≤1 and on nanotube diameters and chiralities. Detailed structural analysis of the relaxed hydrogenated surfaces demonstrates a tendency for clustering of hydrogenated and of nonhydrogenated sites; this leads to surface morphologies characterized by ripples, which consist of hills that form due to clustering of hydroge...

Tunable mechanical properties of diamond superlattices generated by interlayer bonding in twisted bilayer graphene

Using molecular-dynamics simulations of tensile deformation and shear loading tests, we determine the mechanical properties of superlattices of diamond-like nanocrystals embedded in twisted bilayer graphene (TBG) generated by covalent interlayer bonding through patterned hydrogenation. We find that the mechanical properties of these superstructures can be precisely tuned by controlling the fraction of sp3-hybridized C-C bonds in the material, fsp3, through the extent of chemical functionalization. The Young modulus and ultimate tensile strength weaken compared with pristine TBG with increasing fsp3, but they remain superior to those of most conventional materials. The interlayer shear modulus increases monotonically with fsp3.

Formation of fullerene superlattices by interlayer bonding in twisted bilayer graphene

Based on first-principles density functional theory calculations, we report a novel class of carbon nanostructures consisting of superlattice arrangements of caged fullerene configurations of various sizes embedded within planes of twisted bilayer graphene. Formation of these structures is the outcome of interlayer C-C bonding between pairs of graphene planes chemically modified with certain patterns of chemisorbed hydrogen and rotated with respect to each other by angles around 30°. A specific subclass of these nanostructures preserves the main features of the electronic structure of pristine single-layer graphene. Our study proposes possible functionalization strategies to systematically tailor the electronic properties of bilayer graphene.

HIGH-ORDER FINITE VOLUME METHOD FOR SOLVING VISCOELASTIC FLUID FLOWS

Computational Fluid Dynamics (CFD) is widely used by polymer processing industries in order to evaluate polymeric fluid flows. A successful computational code must provide reliable predictions (modeling) in a fast and efficient way (simulation). In this work, a new approach to solve the governing equations of viscoelastic fluid flows is proposed. It is based on the finite-volume method with collocated arrangement of the variables, using high-order approximations for the linear and nonlinear average fluxes in the interfaces and for the nonlinear terms obtained from the discretization of the constitutive equations. The approximations are coupled to the Weighted Essentially Non-Oscillatory (WENO) scheme to avoid oscillations in the solution. The Oldroyd-B model is used to describe the rheological behavior of the viscoelastic fluid. The average values of the variables in the volumes are used during the resolution, and the point values are recovered in the post-processing step by deconvolution of the average values. The nonlinear system, resulting from the discretization of the equations, is solved simultaneously using a Newton-like method. The obtained solutions are oscillation-free and accurate, demonstrated by the application on a classic problem in computational fluid dynamics, the slip-stick flow.

Mechanical properties of hydrogenated electron-irradiated graphene

We report a systematic analysis on the effects of hydrogenation on the mechanical behavior of irradiated single-layer graphene sheets, including irradiation-induced amorphous graphene, based on molecular-dynamics simulations of uniaxial tensile straining tests and using an experimentally validated model of electron-irradiated graphene. We find that hydrogenation has a significant effect on the tensile strength of the irradiated sheets only if it changes the hybridization of the hydrogenated carbon atoms to sp3, causing a reduction in the strength of irradiation-induced amorphous graphene by ∼10 GPa. Hydrogenation also causes a substantial decrease in the failure strain of the defective sheets, regardless of the hybridization of the hydrogenated carbon atoms, and in their fracture toughness, which decreases with increasing hydrogenation for a given irradiation dose. We characterize in detail the fracture mechanisms of the hydrogenated irradiated graphene sheets and elucidate the role of hydrogen and the ex...

Uma nova metodologia para a simulação de escoamentos de fluidos viscoelásticos

Resumo: E proposta neste trabalho uma nova metodologia para resolucao das equacoes governantes de fluidos viscoelasticos, baseada no metodo dos volumes finitos, usando o arranjo co-localizado para as variaveis e malhas estruturadas. Sao utilizadas aproximacoes de alta ordem para os fluxos lineares e nao-lineares medios nas interfaces dos volumes, e para os termos nao-lineares que surgem da discretizacao das equacoes constitutivas. Nesta metodologia, os valores medios das variaveis nos volumes sao usados durante todo o procedimento de resolucao, e os valores pontuais sao obtidos ao final, atraves da deconvolucao dos valores medios. O sistema de equacoes discretizadas e resolvido de forma simultânea, pelo metodo de Newton. A metodologia e exemplificada para um problema classico em mecânica de fluidos computacional, o escoamento stick-slip, usando como equacao constitutiva o modelo de Oldroyd-B. As solucoes obtidas apresentaram boa precisao, sendo livres de oscilacoes mesmo em regioes de grandes gradientes das variaveis. Palavras-chave: Fluidos viscoelasticos, simulacao, metodo dos volumes finitos. A New Approach for Simulation of Viscoelastic Fluid Flows Abstract: In this work, a new methodology to solve the governing equations of viscoelastic fluid flows is proposed. This methodology is based on the finite-volume method with co-located arrangement of the variables, using high-order approximations for the linear and nonlinear average fluxes in the interfaces and for the nonlinear terms resulting from the discretization of the constitutive equations. In this methodology, the average values of the variable in the volumes are used during the resolution, and the point values are recovered in the post-processing step by deconvolution of the average values. The nonlinear equations, resulting from the discretization technique, are solved simultaneously, using the Newtons method. The solutions obtained are oscillation-free and accurate, as can be seen in the solution of the stick-slip flow, used as an illustrative example.

Nanoporous carbon superstructures based on covalent bonding of porous fullerenes

Abstract Highly porous and mechanically stable nanostructures are of great interest for applications in selective membranes, adsorbents, catalysts and sensors. In this study, we use Density Functional Theory calculations and Molecular Dynamics (MD) simulations to demonstrate the feasibility of a novel class of porous carbon-based nanostructures with uniform pore size distributions, formed by covalent bonding of porous fullerenes. Their corresponding mechanical and electronic properties are evaluated, and results show that they typically exhibit an outstanding mechanical strength and electronic behavior ranging from metallic to semiconducting, depending on the hybridization of the covalent interconnections and dimensionality. The efficacy of these materials as molecular sieves is also demonstrated using MD simulations of gas transport across the nanoporous structure. This combination of properties makes these nanostructures suitable for the development of novel porous functional materials with several potential applications.