Phase separation in the H2O-SiO2 system is examined in view of immiscibility in the alkali and alkaline earth silicates, critical parameters of which correlate with the charge and size of network modifier cations. Although the miscibility gaps of the H2O-SiO2 system have not been completely characterized, available data indicate a phase separation tendency greater than that of Li2O-SiO2, consistent with H+ being smaller than Li+. Extension of critical parameter correlations to H2O-SiO2 leads, however, to unrealistic predictions of critical composition due to neglect of cation/anion size asymmetry. To capture this effect, a new coulombic cell model is developed and combined with an asymmetric hard-sphere mixture model. The resulting equation of state predicts H2O-SiO2 critical parameters consistent with expected critical temperature and observed critical concentration. Suppression of the miscibility gap with pressure is explained as a consequence of silanol condensing into molecular H2O and increasing the background dielectric constant. The relevance of H2O-SiO2 phase separation to the geological sciences and glass technology is discussed.
Vincent McGahay received his BS in Materials Engineering (1985) and PhD in Materials Engineering (1992) from Rensselaer Polytechnic Institute. Upon finishing his doctorate, Dr. McGahay joined IBM’s Technology Products Division, predecessor of the IBM Microelectronics, in Hopewell Jct., NY, working in semiconductor process integration for high-end microprocessor development and manufacturing. In his time at IBM, Dr. McGahay participated in or led the introduction of on-chip multilevel Cu wiring technologies, low-k fluorinated and carbon-doped silica glasses, and ultra-low-k porous dielectrics. Upon IBM’s divestiture of its Microelectronics Division in 2015, Dr. McGahay joined GlobalFoundries where he led the definition and development of on-chip interconnect architectures for RF handheld and 5G infrastructure technologies. With the 2019 announcement of the transfer of GF’s Hopewell Junction facility to Onsemi, completed on January 1, 2023, Dr. McGahay’s work shifted to development of new image sensor and power management technologies. He has 47 publications, 12 as primary author, and is an inventor on 77 US patents.