Date of Award

1998

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics and Astronomy

First Advisor

Rajiv K. Kalia

Abstract

Large-scale parallel molecular-dynamics (MD) simulations are performed to investigate properties and processes in nanophase silicon carbide (n-SiC) and gallium arsenide (GaAs) under pressure. These simulations involve 10 6 atoms interacting via realistic interatomic potentials. Structure and sintering of crystalline n-SiC are also investigated with neutron scattering experiments performed at the Intense Pulsed Neutron Source Division (IPNS), Argonne National Laboratory. Both MD simulations and neutron scattering experiments indicate the onset of sintering around 1500K. During sintering, the pores shrink in size while maintaining their morphology: Their fractal dimension remains ∼2.4 and their surface roughness exponent is ∼0.45. The pair distribution functions and bond-angle distributions reveal that interfacial regions in n-SiC are amorphous whereas the interior regions of nanoclusters remain crystalline. The mean-squared displacements of atoms in the outer layer of nanoclusters is an order of magnitude higher than that in the interior regions of clusters in n-SiC. This indicates that the onset of sintering is primarily due to surface diffusion of atoms. The variations of the bulk and shear moduli with density, rho, is well described by the power law rhomu, and the value of mu is found to be ∼3.5. Various experiments on high-density silica and carbon aerogels also find the same power-law dependence. Structural phase transformation in crystalline GaAs is studied with a variable shape MD approach. The pair-distribution functions and bond-angle distributions indicate that the 4-fold coordinated zinc blende structure transforms to a 6-fold coordinated orthorhombic structure under a pressure of 22 GPa. The reverse transformation from the orthorhombic to zinc blende structure, observed at a pressure of ∼10 GPa, is in good agreement with experiments.

ISBN

9780599213555

Pages

81

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