Showing papers by "Fabrizio Cleri published in 2000"

Solid–liquid interface velocity and diffusivity in laser-melt amorphous silicon

Abstract: We studied the microscopic kinetics of the amorphous-liquid interface in supercooled laser-melt silicon by means of molecular dynamics computer simulations The interface velocity was obtained as a function of temperature by direct simulation of the interface motion in an amorphous-liquid model system The temperature dependence of the kinetic prefactor was extracted from the interface velocity function and compared to the values of self-diffusivity obtained from independent molecular dynamics simulations of bulk amorphous Si The kinetic prefactor for interfacial diffusion shows a distinctly non-Arrhenius behavior which is attributed to Fulcher–Vogel kinetics in the supercooled liquid

A stochastic grain growth model based on a variational principle for dissipative systems

Abstract: A stochastic model for the evolution of a cellular network driven by dissipative forces is presented. The model is based on a variational formulation for the dissipated power, from which we obtain an expression for the transition-rate generating function to be used in kinetic Monte Carlo simulations. The model canonical variables are the positions and velocities of the network vertices where cell walls meet. We apply such a model to the study of grain growth in two dimensions, in which the network represents a cross-section of a polycrystalline microstructure and the cell walls represent grain boundaries. The results of the stochastic grain-growth model for relevant statistical quantities are compared to deterministic model results and analytic theories.

Vibrational modes of graphitic fragments and the nucleation of carbon nanotubes

Abstract: We studied the nucleation mechanism of carbon nanotubes based on the hypothesis that the starting nanotube seed can be nucleated by rolling a small fragment of a graphite sheet (graphene) under thermal fluctuations. The energy barriers for rolling a graphene along different crystallographic directions are calculated from a tight-binding model,. We then estimate the relative weight of the large-amplitude fluctuations corresponding to low-frequency vibrational modes of graphene sheets of increasing size. Direct molecular dynamics simulation of the high- temperature fluctuation of a pair of parallel graphenes demonstrates that a nanotube closed at one end can spontaneously form. We discuss the combined effects due to: (a) the decrease of the energy barriers against rolling with increasing nanotube radius, and (b) the increase of random fluctuations with increasing size of the graphene sheet. The superposition of such effects may lead to a preferential range of nanotube diameters which could nucleate more abundantly than others.

Defect energetics, thermal stability and localized electronic states in carbon nanotubes

Abstract: Tight Binding molecular dynamics simulations have been performed on single wall carbon nanotubes, in order to evaluate thermal stability and the effect of the most relevant defects (the single vacancy and a Stone-Wales -SW- defect). The nanotubes are stable up to the graphite instability temperature. Both the considered defects have a large formation energy (E F (vac)=6.10 eV, E F (SW)= 5.55 eV).

A microstructure evolution model including dislocation plasticity

Abstract: We present a stochastic microstructure evolution model applicable to grain growth and its recent extensions, in particular relative to dislocation plasticity. The model is implemented by means of numerical simulations based on the velocity Monte Carlo algorithm. It describes the evolution of a two-dimensional microstructure by tracking the motion of triple junctions, i.e. the vertices where three grain boundaries meet. Grain boundaries can be modeled as straight or curved segments; the misorientation dependence of both grain-boundary energies and mobilities can be included, as well as grain rotation. We show simple examples of normal, abnormal and oriented grain growth. The model is already capable of dealing with a two-phase (liquid-solid) system, to simulate both grain growth and grain dissolution in the liquid. Finally, we report preliminary results of a recent extension of the model to include mechanical deformation from dislocation plasticity.

Identification of tetrahedrally coordinated atoms in supercooled liquid silicon

Abstract: An atomic-scale model of liquid silicon has been cooled from high temperatures down in the temperature range between the amorphous and the crystalline melting temperatures by nanosecond scale molecular dynamics simulations with the Stillinger-Weber potential. Tetrahedrally coordinated sites have been identified, in the supercooled liquids, by using a few structural order parameters. The local structure and the stability of these crystalline-like regions (c-type sites and clusters) have been characterized. These have been regarded as candidates for crystalline embryos.

Atomistic models of triple junctions and the origin of topological changes in microstructural evolution

Abstract: Triple junctions are crucial elements in microstructural evolution: for example, their mobility can be rate-limiting if lower than that of grain boundaries. However, very little is known about their atomic-level structure and properties. We studied the atomic structure of multiple-twin triple junctions in silicon, formed by the convergence of two {111} and one {221} symmetric-tilt grain boundaries. Molecular dynamics simulations with the Stillinger-Weber potential and constant-traction border conditions were performed on several triple junction configurations, obtained by different combinations of the three grain boundaries. All the configurations have a positive excess line energy, a measurable volume contraction and display regions of opposite, tensile and compressive, residual stress. Moreover, we tried to elucidate the role of triple junctions as being the seeds of the only microscopic events that can lead to topological changes in the microstructure. Such events, usually dubbed T1 and T2 in mesoscopic models, correspond to grain switching (in the Ashby-Verrall sense) and grain-disappearance events, respectively. We present preliminary results for the atomic-scale modelling of both classes of topological events and discuss the connection between atomistic and mesoscopic modelling of microstructural evolution.