structural origin of dynamical heterogeneities in borosilicate glass
Open Access
- Author:
- Yin, Danqi
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- February 14, 2023
- Committee Members:
- John C Mauro, Thesis Advisor/Co-Advisor
Ismaila Dabo, Committee Member
John Mauro, Program Head/Chair
Seong H. Kim, Committee Member - Keywords:
- Borosilicate glass
relaxation
dynamical heterogeneity - Abstract:
- Dynamical heterogeneities give rise to spatiotemporal fluctuations yielding a broad distribution of relaxation rates in glass-forming systems. In order to better understand and control the relaxation process based on composition, it is essential to understand the structural origins of dynamical heterogeneities in glass-forming systems. This thesis first investigates the dominant factors governing dynamical heterogeneities in borosilicate glass. The dynamical heterogeneities of two alkali-(alumino)borosilicate glasses are investigated using the isoconfigurational ensemble method via molecular dynamics simulations. It is found that alkali ions drive high dynamical propensities in local spatial regions. The average number of bridging oxygens for both boron and silicon does not monotonically correlate with the corresponding calibrated dynamical propensity. The local alkali agglomeration affects the dynamical propensity of the alkali ion. The agglomeration of smaller alkali ions surrounding another alkali ion favors its high dynamical propensity, while the agglomeration of equal-sized or larger alkali ions favors its low dynamical propensity. The separation distance between the alkali ion and the alkali-modified boron or silicon atom affects the dynamical propensity of the alkali ion. Mixed-alkali environments perturb the relationship between the calibrated dynamical propensity of an alkali ion and its separation distance from its modified boron or its modified silicon atom. This thesis subsequently presents a theoretical framework that enables the statistical mapping of the tendency for synchronized positional rearrangements between any two structural species in the multicomponent supercooled liquid approaching the glass transition. The fundamental building blocks of the structural configuration of a multicomponent supercooled liquid are different types of structural units. Each type of structural unit is referred to as a structural species. It is established that the expected cage-breaking probability of a structural unit is proportional to the dynamical propensity of the structure calculated by the isoconfigurational ensemble method. In light of the cage-breaking probability of each structural unit, the synchronized positional rearrangements of structural units are interpreted as the simultaneous overcoming of kinetic activation barriers within a given duration. Based on this interpretation, a matrix is constructed that allows the quantitative mapping of the tendency for synchronized positional rearrangements between any two structural species in the multicomponent supercooled liquid approaching the glass transition. This established theoretical framework supports that compositional spatial heterogeneities result in dynamical heterogeneities and synchronized positional rearrangements can mitigate dynamical heterogeneities. The pairwise structural tendency of synchronized positional rearrangements for an alkali-borosilicate glass is investigated as an example. The investigated alkali-borosilicate glass has an alkali mole fraction that is greater than the mole fraction of boron, allowing for mixed network former effects, where boron speciation is energetically favored to be alkali-modified, whereas silicon speciation is entropically favored to be alkali-modified. The mapping of this alkali-borosilicate glass might imply that mixed network former effects could broaden relaxation time distributions. Considering mixed network former effects, the investigation of this alkali-borosilicate glass based on the established theoretical framework might also suggest that higher local concentrations of species that are energetically favored for alkali modification would lead to a longer regional relaxation time, however, the higher local concentrations of species that are entropically favored for alkali modification would lead to a shorter regional relaxation time.