Molecular Dynamics Simulations of the Structure and Failure of Silicate Glasses

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New York State College of Ceramics at Alfred University. Kazuo Inamori School of Engineering.
Molecular dynamics simulations were used to study the atomic structure of silica glass fibers and bulk sodium silicate and sodium aluminosilicate glasses, both at rest and strained under tension until failure. The sodium silicate and sodium aluminosilicate glass samples were found to have an atomic structure that agreed well with the results of other computational work and experiments, but some small discrepancies were noted. A shift in the proportion of Qn species as well as a small number of five-coordinated silicon and triply-bridged oxygen, especially noted in large-scale simulation, suggested that the sample had an elevated structural temperature. Thus, the formed structure was found to be more analogous to a splat-cooled glass than a traditionally annealed sample. This work marks the first computational study wherein silica glass fibers were formed in their entirety. Fiber samples were formed in varying sizes and with several different cross-sectional geometries. Upon analysis, the fibers were found to have a distinct surface layer between 5.7 and 11Å thick, with a lower-density structure than the bulk, populated with unusually coordinated, or defect, species. Surface layer thickness was found to vary with sample size, but this dependence rapidly dropped off as system size was increased. When the silica glass fibers were strained, they broke at failure stress values of 12.21±0.04, 11.63±0.05, and 12.21±0.06 GPa, agreeing well with experiment, but at lower values of strain than expected. The failure of the fibers was initiated at the surface and the formed crack moved through the bulk of the material through to the other side of the fiber to break it in two. These cracks that caused failure were usually formed by bond breakages around three-coordinated silicon, triply-bridged oxygen, or bonds aligned with the strain axis. The sodium silicate and sodium aluminosilicate glass samples behaved in an unexpected manner under strain, stretching as if they were viscous rather than brittle solids. No clear cause for this behavior has been found.
Advisory committee members: James Varner, Nathan Mellott, William Lacourse, Walter Schulze. Dissertation completed in partial fulfillment of the requirements for the degree of Doctorate of Philosophy in Materials Science and Engineering at the Kazuo Inamori School of Engineering, New York State College of Ceramics at Alfred University