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

A phase-separated glass with the composition (in mol%) of 6.5 Na2O · 33.5 B2O3 · 60 SiO2 was ion-exchanged in molten KNO3 at temperatures in the range of 520 to 660 °C. At temperatures of 640 and 660 °C, the interdiffusion coefficient measured did not depend on the K2O concentration in the glass, while at lower temperatures, an increase in the diffusion coefficient 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 microstructure was observed. At the surface, a much finer microstructure occurred. Within the diffusion layer, from the surface to the bulk, a continuous coarsening was observed. When the K2O was subsequently re-exchanged against Na2O in a second step, layers with a gradient in the pore size distribution were obtained, which were mechanically stable after leaching the boron-rich phase.