Reactions between water and vitreous silica during irradiation

Molecular dynamics simulations were conducted to determine the response of a vitreous silica surface in contact with water to radiation damage. The defects caused by radiation damage create channels that promote high H ⁺ mobility and result in significantly higher concentration and deeper penetration of H ⁺ in the silica subsurface. These subsurface H ⁺ hop between acidic sites such as SiOH2+ and Si-(OH)-Si until subsequent radiation ruptures siloxane bridges and forms subsurface non-bridging oxygens (NBOs); existing excess H ⁺ readily bonds to these NBO sites to form SiOH. The high temperature caused by irradiation also promotes the diffusion of molecular H ₂O into the subsurface, and although H ₂O does not penetrate as far as H ⁺, it readily reacts with ruptured bridges to form 2SiOH. These SiOH sites are thermally stable and inhibit the reformation of bridges that would otherwise occur in the absence of water. In addition to this reduction of self-healing, the presence of water during the self-irradiation of silica may cause an increase in the glass's proton conductivity.