Investigation of porous glasses based on sodium-borosilicate glass system

Abstract

This work investigated the development of porous glasses by making additions of zirconia (ZrO2) and zircon (ZrSiO4) to the sodium borosilicate glass system SiO2-B2O3-Na2O. Additions of Zr-based compounds were made in an attempt to yield more alkaline durable porous glasses compared to the silica-rich porous glass structures of the parent sodium borosilicate glass system. Glasses were fabricated using a high-temperature fusion process. X-ray diffraction (XRD) was used to confirm that the glasses were amorphous upon pouring from the melt. The glasses were characterised using differential thermal analysis (DTA) to identify important thermal events, including the glass transition temperature (Tg) and crystallisation temperature (Tx). The occurrence of amorphous phase separation was key to the formation of two-phase glasses and ultimately porous glasses. It was found that the quantity of sodium oxide (Na2O) in the glass composition played an important role in determining whether phase separation occurred via nucleation and growth or spinodal decomposition. Based on the DTA data, a heat treatment temperature of 650 °C was selected for three different durations (14, 24 and 63 hours). The heat-treatment caused the glasses to phase separate into two phases; a silica-rich phase and a sodium borate phase. The sodium borate phase coarsened as a function of heat-treatment time. Fourier transform infrared (FTIR) spectroscopy, together with XRD, was found to be effective as a means of comparing the phase separation and crystallisation characteristics. Glasses heat-treated for longer times showed some evidence of crystal formation. Having formed two-phase glass, acid leaching was used to remove the borate phase and create a porous structure. The leaching process had to be carefully controlled in terms of acid type, acid concentration, leaching time and leaching temperature. For all glasses, a post-leach alkali wash step was needed to remove trapped silica gel. The porous glasses comprised a silica-rich porous skeleton. Scanning electron microscopy (SEM) revealed classic interconnected porous morphologies. The most consistent and best-defined morphologies were observed for the zircon-containing glass. Energy dispersive X-ray (EDX) analysis confirmed the presence of zirconium (Zr) in the porous silica-rich skeleton. Mercury intrusion porosimetry (MIP) was used to characterise the pore characteristics and measure pore volume, pore size, pore distribution, total pore surface area, and bulk and apparent density. In general, the porous glasses exhibited sharp, unimodal pore distributions, but there was also evidence of micropores, believed due to residual silica gel. Mean pore size ranged from 40 nm to 200 nm for the different porous glasses studied. It was observed that mean pore size linearly increased with square root of the heat-treatment time. Total pore surface area increased with decreasing size of pores and ranged from 5 to 35 m2/g depending on glass composition and heat-treatment time. Alkaline resistance tests were carried out. Zircon- and zirconia-containing porous glasses are 3-4 times more alkali durable than the parent sodium borosilicate glass