173 research outputs found
A Fluid Dynamics Calculation of Sputtering from a Cylindrical Thermal Spike
The sputtering yield, Y, from a cylindrical thermal spike is calculated using
a two dimensional fluid dynamics model which includes the transport of energy,
momentum and mass. The results show that the high pressure built-up within the
spike causes the hot core to perform a rapid expansion both laterally and
upwards. This expansion appears to play a significant role in the sputtering
process. It is responsible for the ejection of mass from the surface and causes
fast cooling of the cascade. The competition between these effects accounts for
the nearly linear dependence of with the deposited energy per unit depth
that was observed in recent Molecular Dynamics simulations. Based on this we
describe the conditions for attaining a linear yield at high excitation
densities and give a simple model for this yield.Comment: 10 pages, 9 pages (including 9 figures), submitted to PR
Crater formation by fast ions: comparison of experiment with Molecular Dynamics simulations
An incident fast ion in the electronic stopping regime produces a track of
excitations which can lead to particle ejection and cratering. Molecular
Dynamics simulations of the evolution of the deposited energy were used to
study the resulting crater morphology as a function of the excitation density
in a cylindrical track for large angle of incidence with respect to the surface
normal. Surprisingly, the overall behavior is shown to be similar to that seen
in the experimental data for crater formation in polymers. However, the
simulations give greater insight into the cratering process. The threshold for
crater formation occurs when the excitation density approaches the cohesive
energy density, and a crater rim is formed at about six times that energy
density. The crater length scales roughly as the square root of the electronic
stopping power, and the crater width and depth seem to saturate for the largest
energy densities considered here. The number of ejected particles, the
sputtering yield, is shown to be much smaller than simple estimates based on
crater size unless the full crater morphology is considered. Therefore, crater
size can not easily be used to estimate the sputtering yield.Comment: LaTeX, 7 pages, 5 EPS figures. For related figures/movies, see:
http://dirac.ms.virginia.edu/~emb3t/craters/craters.html New version uploaded
5/16/01, with minor text changes + new figure
Recommended from our members
Final Report LDRD 04-ERD-021
In this project, we performed experiments and simulations to establish constitutive models for plastic behavior and to determine the deformation mechanism of nanocrystalline materials at different grain sizes (<100 nm) and high strain rates (>10{sup 6}/s). The experiments used both laser-induced shocks and isentropic compression to investigate, for the first time, the high-strain-rate deformation of nanocrystalline Ni. Samples were characterized using transmission electron microscopy, nanoindentation, profilometry, and x-ray diffraction before and after loading. We validated constitutive models using both atomistic molecular dynamics and continuum simulations performed at the boundary of their current computational possibilities to match experimental scales
Coulomb Explosion and Thermal Spikes
A fast ion penetrating a solid creates a track of excitations. This can
produce displacements seen as an etched track, a process initially used to
detect energetic particles but now used to alter materials. From the seminal
papers by Fleischer et al. [Phys. Rev. 156, 353 (1967)] to the present [C.
Trautmann, S. Klaumunzer and H. Trinkaus, Phys. Rev. Lett. 85, 3648 (2000)],
`Coulomb explosion' and thermal spike models are treated as conflicting models
for describing ion track effects. Here molecular dynamics simulations of
electronic-sputtering, a surface manifestation of ion track formation, show
that `Coulomb explosion' produces a `heat' spike so that these are early and
late aspects of the same process. Therefore, differences in scaling are due to
the use of incomplete spike models.Comment: Submitted to PRL. 4 pages, 3 figures. For related movies see:
http://dirac.ms.virginia.edu/~emb3t/coulomb/coulomb.html PACS added in new
versio
Recommended from our members
Do grain boundaries in nanophase metals slide?
Nanophase metallic materials show a maximum in strength as grain size decreases to the nano scale, indicating a break down of the Hall-Petch relation. Grain boundary sliding, as a possible accommodation mechanisms, is often the picture that explain computer simulations results and real experiments. In a recent paper, Bringa et al. Science 309, 1838 (2005), we report on the observation of an ultra-hard behavior in nanophase Cu under shock loading, explained in terms of a reduction of grain boundary sliding under the influence of the shock pressure. In this work we perform a detailed study of the effects of hydrostatic pressure on nanophase Cu plasticity and find that it can be understood in terms of pressure dependent grain boundary sliding controlled by a Mohr-Coulomb law
Recommended from our members
Galaxies, Black Holes & Laboratories: Studies of interstellar medium materials in energetic environments
Recommended from our members
A viscoplastic micromechanical model for the yield strength of nanocrystalline materials
In this paper we present a micromechanical approach based on Fast Fourier Transforms to study the role played by dislocation glide and grain boundary (GB) accommodation in the determination of the plastic behavior of nanostructured materials. For this, we construct unit cells representing self-similar polycrystals with different grain sizes in the nanometer range and use local constitutive equations for slip and GB accommodation. We study the effect of grain size, strain rate and pressure on the local and effective behavior of nanostructured fcc materials with parameters obtained from experiments and atomistic simulations. Predictions of a previous qualitative pressure-sensitive model for the effective yield strength behind a shock front are substantially improved by considering strain partition between slip and GB activity. Under quasiestatic conditions, assuming diffusion-controlled mechanisms at GB, the model predicts a strain-rate sensitivity increase in nanocrystalline samples with respect to the same coarse-grained material of the same order as in recently published experiments
A feasibility analysis towards the simulation of hysteresis with spin-lattice dynamics
We use spin-lattice dynamics simulations to study the possibility of modeling
the magnetic hysteresis behavior of a ferromagnetic material. The temporal
evolution of the magnetic and mechanical degrees of freedom is obtained through
a set of two coupled Langevin equations. Hysteresis loops are calculated for
different angles between the external field and the magnetocrystalline
anisotropy axes. The influence of several relevant parameters is studied,
including the field frequency, magnetic damping, magnetic anisotropy (magnitude
and type), magnetic exchange, and system size. The role played by a moving
lattice is also discussed. For a perfect bulk ferromagnetic system we find
that, at low temperatures, the exchange and lattice dynamics barely affect the
loops, while the field frequency and magnetic damping have a large effect on
it. The influence of the anisotropy magnitude and symmetry are found to follow
the expected behavior. We show that a careful choice of simulation parameters
allows for an excellent agreement between the spin-lattice dynamics
measurements and the paradigmatic Stoner-Wohlfarth model. Furthermore, we
extend this analysis to intermediate and high temperatures for the perfect bulk
system and for spherical nanoparticles, with and without defects, reaching
values close to the Curie temperature. In this temperature range, we find that
lattice dynamics has a greater role on the magnetic behavior, especially in the
evolution of the defective samples. The present study opens the possibility for
more accurate inclusion of lattice defects and thermal effects in hysteresis
simulation
- …