FLOW OF AIR, NITROGEN, AND HYDROGEN THROUGH POROUS-GLASS TUBES

A simple apparatus for obtaining diffusion data for porous tubes is described. It was used for determining the permeation rates of air, nitrogen, and hydrogen through porous 96% SiO2 glass tubes. To increase flow, these tubes were subjected to multiple etch/wash treatments using aqueous NH4F solutions at room temperature followed by successive hot washes in dilute mineral acid and distilled water. Further increases in permeation rates were achieved by subjecting the base glass used in the preparation of the porous glass to long heat treatments at its phase-separation liquidus prior to leaching and etching. Diffusion data show that the permeability of porous glass is not degraded on heating at temperatures up to 800-degrees-C, and that the separation ratios of the experimental glasses are in good agreement with values expected from Graham's law of diffusion. The role of heat treatment of the base glass, etch/wash treatments of the porous glass, and moisture in the gas stream are discussed.