Atomistic modeling of structure and mechanical properties of silica

Abstract

The first scientific question we tried to address is the underlying structural origin of the mechanical anomaly of silica glass, namely the negative pressure derivative of elastic moduli upon initial compression. We started from the cristobalite phase of silica (most analogous crystalline counterpart for silica glass in terms of the local disorder) and studied the mechanism of the α- to β-cristobalite phase transformation and the nature of disorder in the high temperature β-phase. We developed a unique method to characterize the atomic structures associated with the alpha-ring and beta-ring, constituting α- and β'-cristobalite, respectively. The latter is a new phase discovered here in our molecular dynamics simulations and first principles calculations. Our study revealed that the disorder in β-cristobalite can be characterized by a random distribution of alpha- and beta-rings on a crystalline lattice, analogous to spin disorder in magnetic materials or chemical disorder in solid solutions. We then tracked the population of structural motifs like alpha-rings and beta-rings to quantify the structural evolution of silica glass under compression. Our study revealed that the population of alpha-rings in silica glass increases with pressure due to localized reversible structural transitions similar to those observed in the α-β cristobalite silica phase transformation, therefore the elastic moduli decreases with pressure as α-cristobalite is characterized by a lower modulus than β-phase.

A fundamental understanding of the microscopic structure of glass and its response to external loading will enable predictive design of glasses with tailor-made properties and related benefits with respect to energy consumption and sustainability. The search for damage resistant glasses is of paramount interest today with the increasing demand for light-weight and durable glasses for applications in architecture, the automobile industry, telecommunications and touch screen displays, etc.

Last but not least, during the process of carrying out large-scale mechanical tests of bulk silica glass and amorphous silica nanowire, we identified the optimal range of parameters and proper sample preparation procedures to study the mechanical behaviors of such systems in MD simulations. Such test protocols can be extended to other vitreous systems and may replace some of the expensive and time-consuming mechanical tests required for developing damage resistant glasses.

The third scientific question we asked is how the plastic deformation mechanisms of pressure-quenched silica glass would be different from the pristine one (processed without pressure during quenching), given their very different elastic response to external loading. To this end, we carried out large-scale uniaxial tension test, V-crack tension test and nanoindentation test to systematically study the deformation mechanisms of silica glass quenched under different pressures. We observed a brittle to ductile transition in densified silica in all three types of mechanical tests and attributed structural origins of the enhanced ductility to the critical role of 5-fold Si coordination defects (bonded to 5 O neighbors) in facilitating shear deformation and in dissipating energy by converting back to the 4-fold coordination state during deformation. Our study showed that the pressure quenching route may provide a novel way to quench the 5-fold Si coordination defects, giving rise to enhanced ductility in densified silica glass.

The second scientific question is how to tune the mechanical anomaly of silica glass. Given that the localized reversible structural transition is the underlying mechanism, any means that can prevent such transitions would diminish or eradicate the mechanical anomaly completely. By using pressure-quenching or helium-stuffing to reduce the free volume needed for local structural transitions, the pressure dependent elastic moduli of silica glass changes from abnormal to normal with increasing the quenching pressure and the amount of helium atoms stuffed in the glass matrix. Our study showed that the microscopic structure of silica glass can be tuned in a controllable manner, so are the elastic properties.

An accurate knowledge of the microscopic structure of glass and its response to external loading is critical for enabling future breakthroughs in glass science and technology. Despite extensive research, characterizing the disordered structure of glass at the atomic level and probing the microscopic changes associated with the elastic or plastic deformation in glass is a formidable challenge for experiment. In this thesis work, we resorted to large-scale molecular dynamics (MD) simulation, complemented with first principles calculations when necessary, to directly probe the atomic level structural changes during mechanical tests.