Graded pore size by ion exchange of phase-separated 6.5 Na2O center dot 33.5 B2O3 center dot 60 SiO2 glass

A phase-separated glass with the composition (in mol%) of 6.5 Na2O . 33.5 B 2O3. 60 SiO2 was ion-exchanged in molten KNO3 at temperatures in the range of 570 to 660 degrees C. At temperatures of 640 and 660 degrees C, the inte rdiffusion coefficient measured did not depend on the K2O concentration in the glass, while at lower temperatures, an increase in the diffusion coeffi cient with increasing K2O concentration was observed. The ion exchange and the subsequent heat treatment led to a change in the microstructure. In the bulk of the glass, where no K2O was detected, a fairly coarse microstructu re was observed. At the surface, a much finer rnicrostructure occurred. Wit hin the diffusion lever, from the surface to the bulk, a continuous coarsen ing was observed. When the K2O was subsequently re-exchanged against Na2O i n a second step, layers with a gradient in the pore size distribution were obtained, which were mechanically stable after leaching the boron-rich phas e.