Tuning Network Connectivity of Silicate and Sodium Borosilicate Glasses by TiO2 for Enhanced Chemical Durability: Molecular Dynamics Simulation Investigations.

Abstract

Extensive molecular dynamics (MD) simulations were performed to disclose the beneficial aspects of TiO2 doping in SiO2 and sodium borosilicate (NBS) glasses. Significant amendment in short- and intermediate-range orders of glasses was captured by the radial distribution function, coordination number, bond/angle distribution profiles, structure factor, and probability of linking X-O-X' (where X and/or X' = Si, B, and Ti) structural motifs. Successively, the effect of microscopic structural modification on the macroscopic properties was analyzed in terms of mechanical strength, thermal stability, vibrational characteristics [(vibrational density of states (VDOS)], and chemical durability. The results show that Ti participates in the network chain in the form of TiO6 and TiO5 for the Ti-NBS glass whereas in the form of TiO6, TiO5, and TiO4 for the binary TiO2-SiO2 glass. The presence of TiO2 was found to strengthen the glass skeleton. However, the glass-transition temperature was also increased with Ti addition, which indicates increased hurdles during synthesis due to increased cross connections in the glass network with Ti doping. The computed results envisage enhanced chemical durability of Ti-added glasses. In addition, VDOS spectra showed network former-like characteristics of Ti in the glass network with significant contributions up to a vibration frequency of 800 cm-1. The strong binding of Ti-O-connected Na+ in the glass skeleton prevents Na+ migration toward the interface or bulk aqueous phase, which contributes to improved chemical stability of Ti-containing glasses. During contact with water, Na+ were less likely to leach out from glass to the aqueous solution during Ti doping. In addition, the increased fraction of stable ring structures (5m-7m) for Ti-NBS glasses than bare NBS glasses also supports the increased leaching resistivity of Ti-added glasses. Essentially, the elucidation of macroscopic glass properties has been provided in terms of microscopic understanding. The present findings will incite further MD simulations and experiments to disclose more interesting microstructures and dynamics due to the presence of TiO2 in glasses.