Transformation of Na2O - AI2O3 - B2O3 glass into a material with interconnected pores

Abstract

The replacement of SiO2 in the ternary sodium-borosilicate system with AI2O3 produced clear glasses which include some droplet-like microphases. After heat treatment, phase separation occurred. The region in which glass formed with the Na2O content between 5 and 20 wt % was extended and the transition from glass to the crystalline state was gradual. In the leached state the porous skeleton showed only slight crystalline features which were identified as 2 AI2O3 · B2O3 while in the sintered state, well developed 9 AI2O3 · 2 B2O3 crystallisation was identified by X-ray diffraction, proving the presence of B2O3 compound in the porous insoluble skeleton. Specific surface areas of interconnected pores of between 143 and 282 m2/g and main pore radii between 2 and 7 nm are comparable to those of SiO2 porous glass. The high sintering temperature of AI2O3 based material (1500 °C for 30 min), compared with that of porous SiO2 (1250 °C for 3 min), is of interest.