Experimental and Theoretical Study on Small Gas Permeation Properties through Amorphous Silica Membranes Fabricated at Different Temperatures

The sol–gel method was applied to fabrication of amorphous silica membranes, which have different silica network sizes caused by control of the calcination temperatures. The effects of fabrication temperature on small gas (He, H2, Ne, NH3, CO2, N2, and CH4) permeation properties through silica membranes were evaluated quantitatively using modified gas translation (GT) model. A silica membrane fired at 550 °C showed He and H2 permeances of 8.6 × 10–7 and 5.5 × 10–7 mol m–2 s–1 Pa–1 with He/CH4 and H2/CH4 permeance ratios of 2350 and 1500 at 500 °C, respectively. The thermal stability was dramatically improved by the fabrication of deposited silica glass intermediate layer because N2 permeance showed slight change, and the membrane showed a H2/N2 permeance ratio above 100 even heat-treated at 750 °C. The estimated silica network size decreased from 0.385 to 0.347 nm when a membrane was fabricated at 750 °C, which was consistent with the trend in activation energy of gas permeation. H2 molecules were more permeable than Ne when passing through amorphous silica membranes despite their larger molecular size (H2, 0.289 nm; Ne, 0.275 nm), and the H2/Ne permeance ratios were approximately the same as the Knudsen ratio and were independent of the activation energy of Ne permeation, which was almost the same as that of H2 permeation.