What is so exciting about non-linear viscous flow in glass, molecular dynamics simulations of brittle fracture and semiconductor–glass quantum composites

Abstract

Results of recent research are reported in the areas of: (1) non-linear viscous flow in glass, (2) molecular dynamics of brittle fracture and (3) optical properties of semiconductor–glass quantum composites. In all three areas, a fundamental understanding of the underlying behavior of materials has emerged from key experiments or computer simulations. These are described here: (1) The non-linear viscosity studies have established the behavior of viscosity at high strain rates in silicate glasses. Results of measurements show marked shear thinning associated with changes in the glass structure at high shear rates. A strain-rate-dependent (SRD) viscosity equation is presented which describes well the behavior of silicate glasses. (2) Molecular dynamics computer simulations of amorphous and crystalline silica have examined brittle fracture under applied uniaxial strain. Their results begin to reveal the underlying atomic processes that take part in free surface formation and fracture. For example, an examination of the atomic dynamics at the crack tip reveals that cracks form by the coalescence of pre-existing voids in the glass structure and that fracture surfaces are formed as silicon ions are shielded from the surface by oxygen ions resulting in a predominance of oxygen ions on the fracture surface. The results also suggest a mechanism for the formation of mirror, mist and hackle regions in the fractography of fractured surfaces. (3) Experimental studies of the optical properties of semiconductors formed in a glass matrix were conducted to examine the effect of size confinement on the electronic bandgap of the semiconductors. The results reveal differences in behavior between direct-gap semiconductors (CdS, CdSe and CdTe) and indirect-gap semiconductors (Si and Ge). An analysis of the data shows that the expected blue shift in bandgap energy of direct-gap semiconductors that results from reduced crystal sizes saturates at very small sizes due to the mixing of states between conduction band side valleys and the central valley. This effect does not appear to dominate indirect-gap semiconductors allowing their size-induced bandgap blue shift to cover the entire range of visible wavelengths. This effect suggests the use of quantum confined indirect-gap materials such as Si and Ge in stacks of films with different bandgaps to produce solar cells with increased theoretical efficiency.