Surface and bulk electron irradiation effects in simple and complex glasses

A three oxide sodium borosilicate (BS3) and a complex, thirty oxide borosilicate glass (SON68) were irradiated with 2.3 MeV electrons to doses ranging from 0.15 GGy to 4.6 GGy at 350 K. The irradiated glasses were characterized using Raman and NMR spectroscopies, ToF-SIMS, AFM and microhardness to understand surface and bulk irradiation effects. Glass surfaces were observed to be depleted of the alkali atoms. The depletion depth depended on the dose and glass composition, reaching 660 nm and 500 nm on any vacuum facing surface for BS3 and SON68 at 4.6 GGy respectively. The alkali-depleted region was enriched in molecular oxygen and showed characteristics of phase separated glasses. In the bulk of the glass, Raman and NMR spectroscopies showed a silica network depolymerisation, a transformation of 4 to 3-coordinated boron and formation of non-bridging oxygen atoms on silicon and boron atoms. The hardness of the depolymerized glasses decreased by 20% (BS3) and 10% (SON68). Based on the experiments and theory, it is shown that electron stimulated desorption is the dominant surface depletion mechanism. These results show that the surfaces behave differently than the bulk of the glass. Therefore, surface related phenomena are expected to be dominant in TEM and other in-situ and ex-situ surface characterizations having depth resolution of a few hundred nanometers. Thus, sample size plays an important role in evaluating the radiation damage and one must be careful in extending the conclusions drawn from surface sensitive techniques to bulk irradiation effects in electron irradiated glasses.