Mar. 31/03/2020 14:00 SC12.01, FACULTÉ DES SCIENCES, Université de Montpellier.
ZHANG Zhen (L2C)
Fracture, Surface, and structure of silicate glasses: Insights from atomistic computer simulations
Understanding the structure and mechanical behavior of materials on the microscopic scales is crucial for the design of products with desired properties. This thesis focuses on obtaining microscopic insights into the properties, notably fracture, of oxide glasses which are among the most widely used materials in the world. To this end, we use state-of-the-art atomistic simulation techniques to investigate silica and sodium silicates, i.e., the prototypical compositions for many oxide glasses. Using large-scale molecular dynamics simulations, the dynamic fracture of the glasses is studied in depth. We show that the mechanical properties of the glasses are considerably more sensitive to the used interaction potential and simulation protocol than the structural properties. Fracture of silica glass is found to be pure bond rupturing at the crack tip, whereas fracture of Na-rich glasses is accompanied by the growth and coalescence of cavities. We also reveal that the nonaffine atomic displacement is the microscopic reason for the composition induced transition behavior in the stiffness of these glasses. It is found that the surfaces generated by the fracture are considerably rougher than the melt-formed surfaces and exhibit logarithmic-scaling at the nanoscale (≤ 10 nm). By using first-principles simulations, the vibrational and electronic signatures of some structural units that are abundant on the glass surface are identified. In addition, the ionicity and strength of various types of bonds are inferred from these simulations. Finally, we introduce a novel method to characterize the structure in liquids and glasses. Our analysis shows that these systems have a three-dimensional structure that is surprisingly ordered.
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