Molecular dynamics (MD) simulations of porous silica, in the density range 2.2 - 0.1 g/cm3, are carried out on a 41,472 particle system using two- and three-body interatomic potentials. Calculated results for fractal ...
Molecular dynamics (MD) simulations of porous silica, in the density range 2.2 - 0.1 g/cm3, are carried out on a 41,472 particle system using two- and three-body interatomic potentials. Calculated results for fractal dimension and small-angle neutron scattering data are in good agreement with neutron scattering experiments. Results for structural correlations reveal crossovers from the short- to intermediate range (< 8 Å) and fractal to large-scale regime (10 ~ 100 Å). The MD program simulations are carried out on distributed-memory MIMD computers using a domain-decomposition algorithm. The algorithm employs the linked-cell- list method and separable three-body force calculation. The force calculation is accelerated by the multiple-time-step method. The parallel algorithm is highly efficient (parallel efficiency = 0.974), as it involves only 3 % communication overhead.
Pressure-induced structural transformation from tetrahedral to octahedral coordination and the destruction of intermediate-range order (IRO) are studied in silica glass (a-SiO2) using the molecular-dynamics (MD) metho...
Pressure-induced structural transformation from tetrahedral to octahedral coordination and the destruction of intermediate-range order (IRO) are studied in silica glass (a-SiO2) using the molecular-dynamics (MD) method. Changes in the position and height of the first sharp diffraction peak (FSDP) in the static structure factor, bond lengths, coordination numbers, bond-angle distributions, and statistics of rings are investigated as a function of density. Modifications of the vibrational density of states and participation ratio are also discussed.
Molecular-dynamics simulations are performed to investigate structures, vibrational spectra, and fragmentation channels of silicon microclusters ranging in size from 32 to 52 atoms. Structural information is derived f...
Molecular-dynamics simulations are performed to investigate structures, vibrational spectra, and fragmentation channels of silicon microclusters ranging in size from 32 to 52 atoms. Structural information is derived from pair-distribution functions, bond-angle distributions, and the structure and statistics of rings. Molecular-dynamics simulation results for energetics suggest that 33, 39, 45 and 51 atom clusters are highly stable. These magic-number clusters have predominantly five and six membered rings. With an increase in “temperature”, most clusters tend to fragment by loosing one atom at a time. Vibrational densities of states of 32–52 atom silicon clusters show only minor deviations from the bulk behavior.
Using the molecular dynamics (MD) method, the nature of crystalline fragments in SiSe2 glass (a-SiSe2) at normal pressure, structural transformation and the loss of intermediate range order in SiO2 glass at very large...
Using the molecular dynamics (MD) method, the nature of crystalline fragments in SiSe2 glass (a-SiSe2) at normal pressure, structural transformation and the loss of intermediate range order in SiO2 glass at very large positive pressures, and the modification of SiO2 glass network at very large negative pressures have been investigated. Implementations of the MD algorithm on the Connection Machine and Intel iPSC/860 machine are discussed. We investigate the nature of tetrahedral-molecular fragments in a-SiSe2. MD simulations reveal that the glass consists of both edge-sharing and corner-sharing tetrahedra. Edge-sharing tetrahedra are the building blocks of 10–15 Å long tetrahedral fragments where two-edge-sharing tetrahedra are the nucleus and comer-sharing configurations provide the connecting hinges between the molecular fragments. Pressure-induced structural transformation in a-SiO2 is investigated with the MD method. At high densities, the height of the first sharp diffraction peak in the static structure factor is considerably diminished, its position shifts to larger wave vectors, and a new peak appears at ~2.85 Å−1. At twice the normal density, Si-O bond length increases, Si-O coordination changes from 4 to 6, and O-Si-O band-angle changes from 109° to 90°. This is clearly a tetrahedral to octahedral transformation, which is observed recently by Meade, Hemley, and Mao in their diffraction experiments using synchrotron radiation. MD simulations of porous silica are carried out in the density range 2.2 - 0.1 g/cm3. Internal surface area, pore surface-to-volume ratio, gyration radius, and fractal dimension are studied as a function of density. Simulation results are in good agreement with experimental results obtained by x-ray scattering experiments. The result reveals a crossover of the structural correlations between short- to intermediate-range (< 8 Å) and fractal- to large- scale-regime (10 ~ 100 Å). MD is a numerical approach which involves the solution of N
Oxidation of an aluminum nanocluster (252,158 atoms) of radius 100Å placed in gaseous oxygen (530,727 atoms) is investigated by performing molecular-dynamics simulations on parallel computers. The simulation take...
Oxidation of an aluminum nanocluster (252,158 atoms) of radius 100Å placed in gaseous oxygen (530,727 atoms) is investigated by performing molecular-dynamics simulations on parallel computers. The simulation takes into account the effect of charge transfer between Al and 0 based on the electronegativity equalization principles. We find that the oxidation starts at the surface of the cluster and the oxide layer grows to a thickness of ∼28Å. Evolutions of local temperature and densities of Al and 0 are investigated. The surface oxide melts because of the high temperature resulting from the release of energy associated with Al-O bondings. Amorphous surface-oxides are obtained by quenching the cluster. Vibrational density-of-states for the surface oxide is analyzed through comparisons with those for crystalline Al, Al nanocluster, and α-Al2O3
Fracture in thin films of silicon nitride and alumina is investigated with large-scale multiresolution molecular-dynamics (MD) simulations on parallel computers. The simulations for alumina include dynamic charge tran...
Fracture in thin films of silicon nitride and alumina is investigated with large-scale multiresolution molecular-dynamics (MD) simulations on parallel computers. The simulations for alumina include dynamic charge transfer effects based on electronegativity equalization. Results for structural and dynamic correlations and the effects of temperature and orientation of film on fracture dynamics and morphology of fracture surfaces are presented.
This is a report of work in progress on 10 million atom Molecular Dynamics (MD) simulations of nanoindentation of crystalline and amorphous silicon nitride (Si3N4). Nanoindentation is used to determine mechanical prop...
This is a report of work in progress on 10 million atom Molecular Dynamics (MD) simulations of nanoindentation of crystalline and amorphous silicon nitride (Si3N4). Nanoindentation is used to determine mechanical properties of extremely thin films such as hardness and elastic moduli. We report load-displacement curves for several Si3N4 configurations using an idealized non-deformable indenter and analyze the local stress distributions in the vicinity of the indenter tip. Preliminary results for surface adhesion using Si3N4 for both tip and substrate are also reported.
Multiresolution molecular dynamics approach on parallel computers has been used to investigate fracture in ceramic materials. In microporous silica, critical behavior at fracture is analyzed in terms of pore percolati...
Multiresolution molecular dynamics approach on parallel computers has been used to investigate fracture in ceramic materials. In microporous silica, critical behavior at fracture is analyzed in terms of pore percolation and kinetic roughening of fracture surfaces. Crack propagation in amorphous silicon nitride films is investigated, and a correlation between the speed of crack propagation and the morphology of fracture surfaces is observed. In crystalline silicon nitride films, temperature-assisted void formation in front of a crack tip slows down crack propagation.
Parallel molecular dynamics simulations are performed to investigate dynamic fracture in bulk and nanostructured silica glasses at room temperature and 1000 K. In bulk silica the crack front develops multiple branches...
Parallel molecular dynamics simulations are performed to investigate dynamic fracture in bulk and nanostructured silica glasses at room temperature and 1000 K. In bulk silica the crack front develops multiple branches and nanoscale pores open up ahead of the crack tip. Pores coalesce and then they merge with the advancing crack-front to cause cleavage fracture. The calculated fracture toughness is in good agreement with experiments. In nanostrucutred silica the crack-front meanders along intercluster boundaries, merging with nanoscale pores in these regions to cause intergranular fracture. The failure strain in nanostructured silica is significantly larger than in the bulk systems.
Structural correlations at the Si(111)/Si3N4(0001) interface are studied using the molecular dynamics (MD) method. In the bulk, Si is described by the Stillinger-Weber potential and Si3N4 by an interaction potential w...
Structural correlations at the Si(111)/Si3N4(0001) interface are studied using the molecular dynamics (MD) method. In the bulk, Si is described by the Stillinger-Weber potential and Si3N4 by an interaction potential which contains two-body (steric, Coulomb, electronic polarizabilities) and three-body (bond bending and stretching) terms. At the interface, the charge transfer from silicon to nitrogen is taken from LCAO electronic structure calculations. Using these Si, Si3N4 and interface interactions in MD simulations, the interface structure (atomic positions, bond lengths, and bond angles) is determined. Results for fracture in silicon are also presented.
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