Topological Principles of Borosilicate Glass Chemistry

Borosilicate glasses display a rich complexity of chemical behavior depending on the details of their composition and thermal history. Noted for their high chemical durability and thermal shock resistance, borosilicate glasses have found a variety of important uses from common household and laboratory glassware to high-tech applications such as liquid crystal displays. In this paper, we investigate the topological principles of borosilicate glass chemistry covering the extremes from pure borate to pure silicate end members. Based on NMR measurements, we present a two-state statistical mechanical model of boron speciation in which addition of network modifiers leads to a competition between
the formation of nonbridging oxygen and the conversion of boron from trigonal
to tetrahedral configuration. Using this model, we derive a detailed topological
representation of alkalialkaline earthborosilicate glasses that enables the accurate prediction of properties such as glass transition temperature, liquid
fragility, and hardness. The modeling approach enables an understanding of the microscopic mechanisms governing macroscopic properties. The implications of the glass topology are discussed in terms of both the temperature and thermal history dependence of the atomic bond constraints and the influence on relaxation behavior. We also observe a nonlinear evolution of the jump in isobaric
heat capacity at the glass transition when substituting SiO2 for B2O3, which can be accurately predicted using a combined topological and thermodynamic modeling approach.