Porous glass membranes with an aligned pore system via stretch forming in combination with thermally induced phase separation

In this study, we report about a stretching process coupled with the phase separation of alkali-borosilicate glasses to produce porous glasses with different degrees of pore alignment. For this purpose two different initial glasses with the composition (wt %): 62.5SiO2 · 30.5B2O3 · 7Na2O and respectively 70SiO2 · 23B2O3 · 7Na2O were molten at 1500°C for 3 h and solidified in a casting mold. The resulting glass blocks were cut into thin plates with appropriate dimensions. These slides were clamped into a glass-stretching apparatus in which deformation and phase separation could occur simultaneously. The glass plates were stretched until breaking, thus having different elongation periods, i. e. phase separation periods. Different temperatures and tensile loads have been applied, thus having two independent parameters to influence the time until fracture. The elongated parts of the samples were cut off and underwent an acid and an alkaline treatment to remove the soluble components. The pore size distribution, the specific surface area and the microstructure of selected samples have been determined by mercury intrusion, nitrogen sorption and environmental scanning electron microscopy, respectively. Mercury intrusion porosimetry showed porosities up to 60% with pore sizes in the 100 nm range. The results were compared to those obtained for undeformed samples treated at comparable temperatures and times.