Chemical and physical characterization of porous materials prepared by leaching glasses in the system Na2O-Al2O3-B2O3-F

Abstract

The leachability of glasses in the system Na2O-Al2O3-B2O3-F with Al2O3 contents of about 50 wt% was investigated. It was found necessary to heat-treat the glasses at temperatures that assure not only phase separation but also formation of microcrystalline phases to prevent crumbling, swelling, and excessive restructuring of the glasses on leaching in hot dilute acetic acid solutions or hot distilled water. Chemical analysis showed that the leached glasses contain up to 96 wt% Al2O3 , based on ignited weight. Leaching rate studies show that the removal of the nonaluminous constituents by the hot leachant is diffusion-controlled. The surface areas and pore sizes of the alumina-rich porous skeleton that results on leaching are from ≈ 80 to ≈ 250 m2/g and from ≈ 6 to ≈ 15 nm, depending on heat treatment and leaching conditions. Visual examination and x-ray diffraction studies show that the heat treatment of the base glass induces phase separation and leads to the formation of crystalline phases such as Na3AlF6 and Na2Al2B2O7. The increases in density and thermal expansion of the base glasses with fluoride content on subsequent heat treatment are chiefly due to the formation of such crystalline phases.