Notas técnicas.Tipos de asfaltos y su empleo en pavimentos

ABSTRACT: The idea of acoustic activation of surface diffusion is explored theoretically and in atomistic simulations. It is found that a substantial diffusion enhancement by surface acoustic waves is possible via (1) transient surface strain-induced modification of the diffusion barriers, (2) adiabatic variation in the surface temperature, and (3) dynamic coupling of the acoustic waves with vibrational states of adsorbed species. The approximate scaling laws describing the first two effects are established and verified in kinetic Monte Carlo simulations. The combined contribution of all three effects is studied in molecular dynamics simulations, and the conditions for the diffusion activation through the dynamic coupling are elucidated. The acoustic enhancement of surface diffusion provides an attractive alternative to thermal activation in thin film growth on heat-sensitive substrates. 1

Plume and Nanoparticle Formation During Laser Ablation

The processes that lead to material ejection when a solid sample is irradiated near and above the pulsed laser ablation threshold are discussed. Emphasis is placed on the thermal and mechanical mechanisms that occur during pulsed laser irradiation of metals and semiconductors. Distinctions are drawn between ultrafast-pulsed irradiation, which occurs under stress confinement, and shortpulsed irradiation, in which stress is released during the laser pulse. Similarly, the distinctions between the spallation and phase explosion regimes are discussed. Spallation is only possible when the time of the laser heating is shorter than the time needed for mechanical equilibration of the heated volume, while phase explosion can occur for pulses shorter than tens of ns. Nanoparticle formation can occur directly in the plume as the result of the decomposition of ejected liquid layers or a porous foam created by the phase explosion, as well as through condensation of vaporized atoms (enhanced by the presence of an ambient gas)

MOLECULAR DYNAMICS STUDY OF SHORT-PULSE LASER MELTING, RECRYSTALLIZATION, SPALLATION, AND ABLATION OF METAL TARGETS

ABSTRACT A hybrid computational model combining classical molecular dynamics method for simulation of fast nonequilibrium phase transformations with a continuum description of the laser excitation and subsequent relaxation of the conduction band electrons is developed. The model is applied for a systematic computational investigation of the mechanisms of short pulse laser interaction with bulk metal targets. The material response to laser irradiation is investigated in three regimes corresponding to the melting and resolidification of the surface region of the target, photomechanical spallation of a single or multiple layers/droplets, and ablation driven by the thermodynamic driving forces. The conditions leading to the transitions between the different regimes and the atomic-level characteristics of the involved processes are established

Atomistic modeling of the generation of crystal defects and microstructure development in short pulse laser processing of metals

Because of the extremely fast and localized energy deposition, short-pulse (pico- and femtosecond) laser irradiation of a metal target can induce ultrafast heating (1013‑1014 K/s) and melting within a surface layer of the irradiated target as thin as tens of nanometers. The shallow melt depth produced by the short-pulse laser irradiation and the high thermal conductivity of metals then lead to very high cooling rates (1011‑1012 K/s), strong undercooling and rapid resolidification. In this study, the melting and resolidification processes occurring under conditions of extreme heating and cooling rates are investigated in large-scale atomistic simulations performed with a computational model that combines the classical molecular dynamic method with a continuum description of the laser excitation of conduction band electrons, electron‑phonon coupling and electron heat conduction. The fast melting and resolidification cycle is found to be responsible for the generation of crystal defects, nonequilibrium phases and unusual microstructure in the surface region of the irradiated target. The kinetics of the resolidification process and the microstructure of the surface region are found to be defined by a competition between the epitaxial regrowth of the substrate and nucleation of crystallites within the undercooled melted region. The dependence of the final microstructure of the surface region on the irradiation conditions is discussed based on the results of the atomistic simulations. A special consideration is given to the conditions that result in the massive homogeneous nucleation and growth of the crystallites in a strongly undercooled surface region of the target. The generation of thin nanocrystalline surface layer with a high density of the grain boundaries, twins, and stacking faults suggests an effective method for highly localized surface hardening. In addition, the possibility of metastable phase generation is illustrated by an example in which the generation of metastable BCC‑Cu structure and the following collapse into a close-packed structure through BCC‑HCP transformation are observed

Multiscale modeling of laser ablation: Applications to nanotechnology

Abstract Computational modeling has a potential of making an important contribution to the advancement of laser-driven methods in nanotechnology. In this presentation a multiscale model for simulation of laser ablation and cluster deposition of nanostructured materials is discussed. A hierarchy of computational methods used to simulate different processes involved in laser ablation and film growth by cluster deposition is schematically illustrated i