Molecular mechanism of the expansion of silica glass upon exposure to moisture

Abstract

Molecular dynamics simulations show that the expansion of silica glass occurs by the presence of the hydroxyl (SiOH) groups present in the glass as opposed to intact water (H2O) molecules, providing an accurate molecular description of the experimentally observed volume changes in silica glass exposed to water. Using a robust and accurate reactive potential, the simulations show that the expansion is caused by the rupture of siloxane (Si–O–Si) linkages in the glass via reactions with water molecules, forming SiOHs. Such reactions remove smaller rings and form larger rings, with a decrease in the overall number of rings smaller than a prescribed large ring size in comparison to dry glasses. This change in ring structure overcomes the inherently stronger hydrogen bonding in the glasses containing SiOH in comparison to the glasses containing predominantly intact H2O molecules. This stronger H‐bonding of the SiOH also causes a shift to lower frequencies in the high‐frequency OH vibrational spectrum for the silanols, as shown in previous ab‐initio calculations. This introduces a question about assuming the lower frequency part of the high‐frequency peak is only due to intact H2O molecules. A slight decrease in volume occurred in the glasses containing the largest concentration of intact H2O molecules. There is no change in the ring size distribution between the H2O glasses and dry glasses. Rather, the slight decrease in volume in the H2O system is caused by a decrease in siloxane bond angles caused by the formation of H‐bonds between the H2O molecules and the glass O in the siloxane cages surrounding the H2O molecules.

Estimating Internal Stress of an Alteration Layer Formed on Corroded Boroaluminosilicate Glass through Spectroscopic Ellipsometry Analysis

“…When modifier ions in the glass network are exchanged with protons or water molecules, or some of the network formers are hydrolyzed, the resulting network becomes porous. ,, The presence of internal pores with small radius (mostly < 1 nm), which are filled with solvated ions or just water molecules if the sample is kept in an aqueous solution or become empty upon dehydration, may alter the local free energy and induce strains in chemical bonds in the local glass network . In an isolated system, such strains can be relaxed isotropically to a structure with the lowest free energy possible in a given condition.…”

“…7,38,39 The presence of internal pores with small radius (mostly < 1 nm), 7 which are filled with solvated ions or just water molecules if the sample is kept in an aqueous solution or become empty upon dehydration, may alter the local free energy and induce strains in chemical bonds in the local glass network. 40 In an isolated system, such strains can be relaxed isotropically to a structure with the lowest free energy possible in a given condition. However, in the case of a thin alteration layer, the bottom of its network is covalently bonded to the bulk substrate; thus, the relaxation would be anisotropic and may only occur partially.…”

Effects of thermal environment on dehydroxylation of porous silica preform in two‐step CVD synthesis

“…

Hydroxyl (-OH) is one of the main impurities in silica glass which has strong optical absorption near the infrared spectral region and greatly weakens the glass's optical performance. 1 To eliminate the hydroxyl impurities in silica glass, a two-step chemical vapor deposition (CVD) method is developed for fabricating silica glass ingot with low or controllable hydroxyl concentrations, which has drawn wide attentions in recent years. 2,3,4 The CVD synthesis of silica glass is mainly based on flame hydrolysis reaction, in which silicon-containing compounds (e.g., SiCl 4 , organosilicone, etc.)

…”