Investigation of alumino-silicate glasses by coupling experiments and simulations: Part II - radiation effects

Abstract A set of SiO2-Al2O3 Na2O CaO glasses with varying Si/Al and Na/Ca ratios was experimentally irradiated with Au4+ ions at 7.3MeV. Measurements by optical interferometry showed that, depending on the glass composition, the density could increase or decrease. Overall the higher the original quantity of non-bridging O, the greater the swelling observed. In order to explain the origin of these observations, some of the experimentally studied glasses were simulated by classical molecular dynamics and then subjected to a series of 4 keV displacement cascades or quenched at different rates. Comparisons of the density changes and atomic-scale modifications have allowed the authors to propose a phenomenological density change model. There appears to have been a competition between two processes with a first process where the vitreous structure “breathes” associated with the deposit of free volumes within the cascades that favours swelling. A second process is associated with changes in the local environments (creation of non-bridging O and of 3-coordinated O, creation of 5-coordinated Al, conversions between charge compensators and network modifiers) which act as brakes on the swelling. Depending on the relative dominance of these two processes, the glass may swell or contract.