In the last 30 days

• Open/close Determination of the crystal-melt interface kinetic coefficient from molecular dynamics simulations

  J Monk et al 2010 Modelling Simul. Mater. Sci. Eng. 18 015004 Tag this article

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  The generation and dissipation of latent heat at the moving solid–liquid boundary during non-equilibrium molecular dynamics (MD) simulations of crystallization can lead to significant underestimations of the interface mobility. In this work we examine the heat flow problem in detail for an embedded atom description of pure Ni and offer strategies to obtain an accurate value of the kinetic coefficient, μ. For free-solidification simulations in which the entire system is thermostated using a Nose–Hoover or velocity rescaling algorithm a non-uniform temperature profile is observed and a peak in the temperature is found at the interface position. It is shown that if the actual interface temperature, rather than the thermostat set point temperature, is used to compute the kinetic coefficient then μ is approximately a factor of 2 larger than previous estimates. In addition, we introduce a layered thermostat method in which several sub-regions, aligned normal to the crystallization direction, are indepently thermostated to a desired undercooling. We show that as the number of thermostats increases (i.e., as the width of each independently thermostated layer decreases) the kinetic coefficient converges to a value consistent with that obtained using a single thermostat and the calculated interface temperature. Also, the kinetic coefficient was determined from an analysis of the equilibrium fluctuations of the solid–liquid interface position. We demonstrate that the kinetic coefficient obtained from the relaxation times of the fluctuation spectrum is equivalent to the two values obtained from free-solidification simulations provided a simple correction is made for the contribution of heat flow controlled interface motion. Finally, a one-dimensional phase field model that captures the effect of thermostats has been developed. The mesoscale model reproduces qualitatively the results from MD simulations and thus allows for an a priori estimate of the accuracy of a kinetic coefficient determination for any given classical MD system. The model also elucidates that the magnitude of the temperature gradients obtained in simulations with a single thermostat depends on the length of the simulation system normal to the interface; the need for the corrections discussed in this paper can thus be gauged from a study of the dependence of the calculated kinetic coefficient on system size.

• Open/close Modelling the role of size, edge structure and terminations on the electronic properties of graphene nano-flakes

  Amanda S Barnard and Ian K Snook 2011 Modelling Simul. Mater. Sci. Eng. 19 054001 Tag this article

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  The addition of graphene nano-flakes to the suite of materials for graphene-based nanotechnology requires a complete understanding of the relationship between shape, structure, properties and property dispersion. Due to the large number of configurational degrees of freedom, this is a very challenging undertaking, particularly if morphological ensembles contain a reasonable array of sizes, shapes (edges and corners) and edge/corner terminations. We report results of density functional tight-binding simulations of zigzag (ZZ) and armchair (AC) hexagonal graphene nano-flakes, with unterminated, monoyhydride or dihydride terminated edges and corners. We find that hexagonal nano-flakes with AC edges are most likely to be achievable experimentally, providing that sufficient H is present during synthesis (or processing) to facilitate dihydride edge and corner passivation, forming a circumference of sp ³ hybridized C atoms. This is significant, since the energy of the Fermi level and electronic density of states in the vicinity of the Fermi level are sensitive to the structural and chemical characteristics of the atoms around the circumference, which can be modified post-synthesis.

• Open/close Atomistic simulation and continuum modeling of graphene nanoribbons under uniaxial tension

  Qiang Lu et al 2011 Modelling Simul. Mater. Sci. Eng. 19 054006 Tag this article

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  Atomistic simulations are performed to study the nonlinear mechanical behavior of graphene nanoribbons under quasistatic uniaxial tension, emphasizing the effects of edge structures (armchair and zigzag, without and with hydrogen passivation) on elastic modulus and fracture strength. The numerical results are analyzed within a theoretical model of thermodynamics, which enables determination of the bulk strain energy density, the edge energy density and the hydrogen adsorption energy density as nonlinear functions of the applied strain based on static molecular mechanics simulations. These functions can be used to describe mechanical behavior of graphene nanoribbons from the initial linear elasticity to fracture. It is found that the initial Young's modulus of a graphene nanoribbon depends on the ribbon width and the edge chirality. Furthermore, it is found that the nominal strain to fracture is considerably lower for graphene nanoribbons with armchair edges than for ribbons with zigzag edges. Molecular dynamics simulations reveal two distinct fracture nucleation mechanisms: homogeneous nucleation for the zigzag-edged graphene nanoribbons and edge-controlled heterogeneous nucleation for the armchair-edged ribbons. The modeling and simulations in this study highlight the atomistic mechanisms for the nonlinear mechanical behavior of graphene nanoribbons with the edge effects, which is potentially important for developing integrated graphene-based devices.

• Open/close Focus section on graphene modelling: morphology, defect mechanics and growth

  V B Shenoy 2011 Modelling Simul. Mater. Sci. Eng. 19 050201 Tag this article

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  Graphene has received a lot of attention in the past few years for its superior electronic, optical, and thermal properties. Many of these properties depend on the geometrical structure of the flat crystalline membrane, and can be altered by deformation. Deformations can be caused by various factors, including thermal fluctuations, chemical functionalization as well as defects that arise during growth or processing. While there is a large body of modelling work on the electronic properties, the effect of defects and functional groups on morphology and the formation and structure of defects, ripples and wrinkles has not been as well studied or understood in comparison.

  This focus section invites papers that describe recent progress in the area of growth modelling, defect formation mechanisms, mechanics of rippling and wrinkling and the role of functional groups on morphology in graphene, the successes and also the fundamental challenges that yet remain. It is the intent of this focus section to give expression to the wide variety of different approaches applied to the study of these issues, as seen from the perspective of leading scientists in the field.

• Open/close Phase-field models in materials science

  Ingo Steinbach 2009 Modelling Simul. Mater. Sci. Eng. 17 073001 Tag this article

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  The phase-field method is reviewed against its historical and theoretical background. Starting from Van der Waals considerations on the structure of interfaces in materials the concept of the phase-field method is developed along historical lines. Basic relations are summarized in a comprehensive way. Special emphasis is given to the multi-phase-field method with extension to elastic interactions and fluid flow which allows one to treat multi-grain multi-phase structures in multicomponent materials. Examples are collected demonstrating the applicability of the different variants of the phase-field method in different fields of materials science.

• Open/close Properties on the edge: graphene edge energies, edge stresses, edge warping, and the Wulff shape of graphene flakes

  Paulo S Branicio et al 2011 Modelling Simul. Mater. Sci. Eng. 19 054002 Tag this article

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  It has been shown that the broken bonds of an unreconstructed graphene edge generate compressive edge stresses leading to edge warping. Here, we investigate edge energies and edge stresses of graphene nanoribbons with arbitrary orientations from armchair to zigzag, considering both flat and warped edge shapes in the presence and absence of hydrogen. We use the second generation reactive empirical bond order potential to calculate the edge energies and stresses for clean and hydrogenated edges. Using these energies, we perform a Wulff construction to determine the equilibrium shapes of flat graphene flakes as a function of hydrogen chemical potential. While edge stresses for clean, flat edges are compressive, they become tensile if allowed to warp. Conversely, we find that edge energies change little (~1%) with edge warping. Hydrogenation of the edges virtually eliminates both the edge energy and edge stresses. For warped edges an approximately linear relationship is found between amplitudes and wavelengths. The equilibrium shape of a graphene flake is determined by the value of the hydrogen chemical potential. For very small (and large) values of it the flakes have a nearly hexagonal (dodecagon) shape with zigzag oriented edges, while for intermediate values graphene flakes are found with complex shapes.

• Open/close Extrinsic morphology of graphene

  Teng Li 2011 Modelling Simul. Mater. Sci. Eng. 19 054005 Tag this article

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  Graphene is intrinsically non-flat and corrugates randomly. Since the corrugating physics of atomically thin graphene is strongly tied to its electronics properties, randomly corrugating morphology of graphene poses a significant challenge to its application in nanoelectronic devices for which precise (digital) control is the key. Recent studies revealed that the morphology of substrate-supported graphene is regulated by the graphene–substrate interaction, thus is distinct from the random intrinsic morphology of freestanding graphene. The regulated extrinsic morphology of graphene sheds light on new pathways to fine tune the properties of graphene. To guide further research to explore these fertile opportunities, this paper reviews recent progress on modeling and experimental studies of the extrinsic morphology of graphene under a wide range of external regulation, including two-dimensional and one-dimensional substrate surface features and one-dimensional and zero-dimensional nanoscale scaffolds (e.g. nanowires and nanoparticles).

• Open/close A review of extended/generalized finite element methods for material modeling

  Ted Belytschko et al 2009 Modelling Simul. Mater. Sci. Eng. 17 043001 Tag this article

  Article References Full text PDF (578 KB)

  The extended and generalized finite element methods are reviewed with an emphasis on their applications to problems in material science: (1) fracture, (2) dislocations, (3) grain boundaries and (4) phases interfaces. These methods facilitate the modeling of complicated geometries and the evolution of such geometries, particularly when combined with level set methods, as for example in the simulation growing cracks or moving phase interfaces. The state of the art for these problems is described along with the history of developments.

• Open/close Morphologies of monolayer graphene under indentation

  Xu Huang and Sulin Zhang 2011 Modelling Simul. Mater. Sci. Eng. 19 054004 Tag this article

  Article References Full text PDF (639 KB)

  We employ a quasi-continuum method to characterize the deformation morphologies of monolayer graphene under indentation. The method involves casting the atomistic interaction potentials into the constitutive relations at the continuum level, thereby facilitating large-scale simulations with atomistic fidelity. Our simulations mapped out different morphological phases under indentation, depending on the indentation depth, the graphene–substrate adhesion strength and the graphene size. We carefully characterized the rippling phase and found that the wave number of the periodic rippling is dependent on the graphene–substrate adhesion strength and the graphene size, but not on the indentation depth. The simulation results provide new insights into the structural instability of graphene.

• Open/close A Lagrangian framework for analyzing microstructural level response of polymer-bonded explosives

  Ananda Barua and Min Zhou 2011 Modelling Simul. Mater. Sci. Eng. 19 055001 Tag this article

  Article References Full text PDF (3.77 MB)

  A cohesive finite element method (CFEM) framework for quantifying the thermomechanical response of polymer-bonded explosives (PBXs) at the microstructural level is developed. The analysis carried out concerns the impact loading of HMX/Estane at strain rates on the order of 10 ⁴–10 ⁵ s ⁻¹. Issues studied include large deformation, thermomechanical coupling, failure in the forms of microcracks in both bulk constituents and along grain/matrix interfaces, and frictional heating. The polymer matrix is described by a thermo-elasto-viscoelastic constitutive formulation, accounting for temperature dependence, strain rate sensitivity and strain hardening. The HMX crystals are assumed to be elastic. The CFEM framework allows the contributions of individual constituents, fracture and frictional contact along failed crack surfaces to heating to be tracked and analyzed. Digitized micrographs of actual PBX materials and idealized microstructures with Gaussian distributions of grain sizes are used in the analysis. The formation of local hot spots as potential ignition sites is primarily due to the viscoelastic dissipation in the matrix in early stages of deformation and frictional heating along crack surfaces in later stages of deformation. The framework is a useful tool for the design of energetic composites and the results can be used to establish microstructure–response relations that can be used to assess the performance of energetic composites.