CHANGES OF LEAD SILICATE-GLASSES INDUCED BY LEACHING

The structural differences in the surface region between freshly fract ured and leached silicate glasses containing 16.7, 18.8 and 44.4 mol% PbO, respectively were investigated by photoelectron spectroscopy (XPS ). The optical properties and the thicknesses of leached layers were d etermined by reflection measurements. The binding energies of the Ols signal components for untreated samples can be ascribed to non-bridgin g (NBO) and bridging (BO) oxygen and oxygen associated with lead as ne twork former (O-Pb). The binding energy of O-Pb was found to be 529.1 +/- 0.2 eV. For quantitative conclusions, relative XPS sensitivity fac tors were determined for oxygen, silicon and lead in these glasses. Th e experimental NBO/BO/O-Pb ratio found is in good agreement with the N BO/BO/O-Pb ratio calculated by use of the 'discrete bond model' (DBM). Leaching causes a removal of about 90% of the glass modifiers. The Ol s signal of silanol groups and other O-H bonds formed appears at a hig her binding energy as compared with the BO signal. Whereas the glasses with smaller Pb contents exhibit thin and well confined leached layer s, on the glass with the larger Pb content a structured thick layer is formed. Its thickness growth follows a root t-dependence expected for a diffusion controlled process. Ph at network forming sites is more s table with respect to leaching than network modifying Pb. After the re moval of Pb from network modifying sites, in addition to the formation of silanol and other O-H bonds a rebinding between Si tetrahedra is f ound resulting in an increased density of the leached layer. (C) 1998 Elsevier Science B.V. All rights reserved.